PHASE CHANGE THERMAL ENERGY STORAGE

A thesis submitted in partial fulfillment of the

requirements of the University of Brighton

for the degree of Building Surveying

 

May 1997

Abstract

For many years, all major conservation bodies have recognised the need to conserve our precious fuel and power resources. The steady increased consumption of fuel and power causing carbon dioxide omissions to soar has resulted in ozone layer depletion with scientific evidence of global warming. As a result, ways of utilising renewable fuel and power sources are developing with solar energy providing one sustainable alternative to traditional ways of heating buildings.

 

The problems with solar heating is the need to store solar energy during the day for use at night or when the need arises. Phase change thermal energy storage provides a feasible means of storing this heat energy.

 

This study aims to provide an appraisal of developments within the phase change thermal energy field identifying key players responsible for initial investigation and research into the subject. Research programs undertaken by state universities and national laboratories together with important material discoveries are detailed, providing the reader with a background knowledge of the history into phase change materials. A range of commercially available latent heat energy storage devices and building systems is described fully with illustrated information to support this work. The results from an Investigation of a well insulated house incorporating latent heat thermal energy technology storing renewable heat sources over a winter season is included. The results detail the potential cost and fossil fuel saving that can be made by the introduction of thermal energy storage to existing and new house designs.

i

 

Acknowledgments

 

I would like to thank the following people who gave so freely their time and assistance, during the preparation and writing of this dissertation.

 

I am indebted to my supervisor, Dr. Kenneth Ip, Department of Construction, Geography and Surveying for both constructive suggestions, beneficial help and advice, without which, it would not have been possible to produce this dissertation.

 

Special thanks to Dr. George Lane of Dow Chemical Ltd., Michigan, U.S.A and Dr. John Tomlinson of Oak Ridge National Laboratory, Tenessee, U.S.A for their time and guidance throughout my research development period and for an insight to leading research in latent heat thermal energy storage.

 

Thanks to Dr. Richard Austin of Tes Systems limited for his time given during analysis of my calculations and all technical information sent with relation to my research without which this dissertation could not be executed.

 

Finally a thanks to all family for their support and help throughout my final year of study at the University of Brighton.

 

 

TABLE OF CONTENTS

 

Pages

Abstract . . . . . . . . i

Table of Contents . . . . . . ii-iii

List of Tables . . . . . . . iv

Acknowledgments . . . . . . v

 

1.0 Introduction . . . . . . 1-4

 

2.0 Principles of Latent Heat Energy Storage . . 5-11

in Buildings

 

3.0 Development of Phase Change Thermal Storage

3.1 Introduction . . . . . . . 12-15

3.2 NASA Space Program . . . . . . 15-16

3.3 PCM For Heating and Cooling Buildings . . . 16

3.4 Further Government Interest . . . . . 22-23

3.5 National Science Federation PCM Research Program . . 23 -24

3.6 Gluaber’s Salts Research . . . . . 24-26

3.7 Paraffin Wax PCM Research . . . . . 26-27

3.8 West Germany . . . . . . . 27-28

3.9 Denmark . . . . . . . 28

3.10 Sweden . . . . . . . 29

3.11 France . . . . . . . . 29-30

3.12 United Kingdom . . . . . . 30-31

3 13 Japan . . . . . . . . 31-32

 

4.0 Heat Storage PCM for Residential Buildings

4.1 Introduction . . . . . . . 33

4.2 Dow Chemical PCM Research program . . . 33-37

4.3 Paraffins . . . . . . . 37-39

4.4 Fatty Acids . . . . . . . 39

4.5 Salt Hydrates . . . . . . . 40

 

 

5.0 PCM Thermal Storage Applications

5.1 Introduction . . . . . . . 41-42

5.2 Sol-Ar PCM Tile . . . . . . 42-43

5.3 Calor Alternative Energy . . . . . 44-47

5.4 Cristopia . . . . . . 48

5.5 Pennwalt PCM Pellets . . . . . . 48-50

5.6 Enerphase Panels . . . . . . 51-52

5.7 Rodwall PCM Energy Storage System . . . 53

5.8 O.E.M Heat Battery . . . . . . 53-56

5.9 Tes Systems Term AC TH30 System . . . 56-58

5.10 Wax Impregnated Wall Board . . . . . 58-66

 

6.0 Phase Change Thermal Storage Investigation

6.1 Introduction . . . . . . . 67-68

6.2 Standard Domestic Model . . . . . 68

6.3 Construction . . . . . . . 69-71

6.4 U Values . . . . . . . 71

6.5 Heat Distribution System . . . . . 73

6.6 Phase Change Thermal Storage System . . . 73

6.7 Assumptions within calculations . . . . 74-75

6.8 Case Study 1 . . . . . . . 75-88

6.9 Case Study 2 . . . . . . 88-91

6.10 Case Study 3 . . . . . . . 92-97

 

7.0 Conclusion

7.1 Case Study 1 . . . . . . . 98

7.2 Case Study 2 . . . . . . . 99

7.3 Case Study 3 . . . . . . . 99-100

7.4 Phase Change Materials . . . . . 100-102

7.5 Commercial PCM Thermal Energy Storage Systems . . 102-103

 

8.0 Recommendations for Future Research . . 104-105

Bibliography

Appendices

Appendix A U Value Calculations

Appendix B Tes Systems Term AC TH30 Technical Literature

Appendix C Amcor Solarion 2000 Calculations

Appendix D Off Peak Electricity Calculations

Appendix E BP PV Solar Collector Calculations

Appendix F BRE Standard House plans

Appendix G Contact Addresses

 

 

List of Figures, Tables & Graphs

Chapter 2.00

Figure 1.0 Phase Change Material Water diagram

Figure 2.0 Phase Change Heat Energy Storage Flow Chart

 

Chapter 4.00

Figure 1.0 Development Flowchart for Phase Change Thermal Storage Systems

Figure 2.0 Families of Phase Change Heat Storage Materials

 

Table 1.0 Phase Change Materials Physical Properties

Chapter 5.00

Figure 1.0 Sol-ar Tile PCM Thermal Storage Diagram

Figure 2.0 Calortherm PCM Thermal Storage Diagram

Figure 3.0 Cristopia Plastic Sphere PCM Thermal Storage Diagram

Figure 4.0 Pennwalt PCM pellets Thermal Storage Diagram

Figure 5.0 Enerphase PCM Panel Thermal Storage Diagram

Figure 6.0 Rodwall PCM passive Thermal Storage Panel Diagram

Figure 7.0 O.E.M PCM Energy storage eat Battery Diagram

Figure 8.0 Tes Systems PCM Thermal Storage Diagram

Figure 9.0 Results from RADCOOL PCM Wax Impregnated wall board

Figure 10.0 Summary of PCM Devices.

Chapter 6.00

Figure 1.0 Cross Section through Domestic Model

Figure 2.0 Tes Systems PCM Thermal Storage Detail

Figure 3.0 Amcor Solarion 2000 trade Information

Figure 4.0 Amcor Solarion 2000 trade Information

Figure 5.0 Schematic Diagram of Hybrid Heating System

Figure 6.0 BP PV Solar Collector Trade Information

Figure 7.0 BP PV Solar Collector Trade Information

 

Graph 1.0 Amcor Solar collector analysis for SE England

Graph 2.0 Amcor Solar collector analysis for Israel

Graph 3.0 Amcor Solar collector analysis for Spain

Graph 4.0 Amcor Solar collector analysis for Australia

Graph 5.0 Amcor Solar collector analysis for U.S.A

Graph 6.0 Amcor Solar collector analysis for China

Graph 7.0 Amcor Solar collector analysis for New Zealand

Graph 8.0 BP PV Solar collector analysis

 

 

1.0 Introduction

 

For many years, all major conservation bodies have recognized the need to conserve our precious fuel and power resources. The steady increased consumption of fuel and power causing Carbon Dioxide (CO2) omissions to soar, has resulted in ozone layer depletion with scientific evidence of world global warming.

 

The United Nations Intergovernmental panel on climate change consisting of over 300 leading scientists stated at the 1992 conference on climate change that , to freeze the level of CO2 omissions by the year 2005, a reduction of 60% of all fuel used will have to be made1.

 

This represents a considerable change to the way fuel is used at present with over 50% of fuels being used for operating and heating building of which 60% of omissions is caused by heating and lighting residential properties2. As a result, considerable funding is being put forward for the development of renewable energy used in both domestic and commercial buildings. Additionally, Department of Environment is presently drawing up policies to conserve fuel and power within all future building developments.

 

 

 

Renewable Energy Developments

 

For the last 10 -15 years, the research into latent heat materials for storing thermal energy has evolved rapidly3. Through lengthy research program’s carried out by many European countries including USA and Japan leading the developments, latent heat storage has been scientifically proven to store renewable energy for heating dwellings and commercial buildings4.

 

The reason for this research and development is in response to the need for more energy conscious forms of heating, rapidly reducing the effects of global warming. Other such developments in renewable energy include solar collection, biomass and geothermal energy forming a foundation for the future energy needs.

 

Out of all the various heat storage developments, latent heat storage was found to be the most favorable storage medium due to its large capacity to store heat5.

 

The main obstacle of any solar collecting device designed to provide heating for a dwelling is the need to store the heat energy collected for use at a later stage. Latent heat energy storage provides one practical means of storing collected solar energy during the day for use at night or whenever the need arises.

 

The latent heat energy storage systems comprising of Phase Change Materials (PCM), have been of particular interest to many scientists within the solar energy

 

field, due to the large specific latent heat capacity properties, compared with other construction materials previously considered.

This can be demonstrated by comparing concrete with a heat storage capacity of 0.784 kWh/m3 with a typical PCM holding between 38 to 105 kWh/m3 of energy during phase change transition6. Therefore energy storage systems incorporating PCM comprise of significantly smaller volumes when compared to alternative materials storing only sensible heat.

 

This study aims to provide the reader with an understanding of PCM latent heat technology for supplying a viable storage system when used in conjunction with renewable energy heat sources for domestic heating demands.

 

The study will describe the history of PCM technology identifying key scientist and leading researchers within the initial stages of development, together with an overview of government funded research taking place within various countries.

 

A chapter has been dedicated to the principals of Phase Change Latent Heat Storage for heating domestic dwellings, providing the reader with a knowledge of the theory

behind the authors area of chosen research.

 

An appraisal of various PCM thermal storage design applications is contained within the study, detailing systems that are presently being applied for storing renewable energy sources for heating a wide range of building types including low energy houses.

 

Critical analysis of a selected PCM Latent Heat Storage system currently available on the open market and used within various building types in several countries will be examined. The investigation aims to analyze the chosen PCM Latent Heat Storage system using various alternative, renewable energy sources for heating a typical domestic house.

 

 

References to Chapter

1. Association of Conservation of Energy Briefing notes(1994) No.13 Climate change pp 1. paragraph 1.

2. Association of Conservation of Energy Briefing notes (1994) No.13 Climate change , pp 1. paragraph 4.

3. Lane. G A, (1983), Volume 1, Latent Heat Storage: Background and Scientific Principals ,U.S.A CRC Press, pp2.

4. Lane. G A, (1983), Volume 1, Latent Heat Storage: Background and Scientific Principals, U.S.A CRC Press, pp2.ch.3.

5. A. Abhat, (1983), University of Stuttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4 pp313.

6. Goulding. J.R,(1992) Energy Conscious Design, Commission of the European Communities,Architectural Press, pp61.

 

 

 

2.0 Principles of Latent Heat Energy Storage in Buildings

Before continuing, the author felt it was essential for the reader to understand the concept of Phase Change Latent Heat Theory for storing heat energy within domestic properties. To do this without proving the concept mathematically, a simple example using water as the Phase Change Material (PCM) can be considered.

" Latent heat is the large quantity of energy which needs to be absorbed or released when a material changes (Phase) from the solid to the liquid termed fusion (ice melting to water) or from the liquid to the solid state (water freezing) termed crystallization. These phase changes take place at constant temperature and for certain materials the process of melting and freezing can be repeated over an unlimited number of cycles with no change to there physical or chemical properties".

 

[Richard Austin (1994) - TES Systems limited (UK) trade literature,p2]

The latent heat phase change energy storage and discharge can be illustrated for water over different temperature bands. There are two temperature zones where latent heat storage and discharge occurs. When water is in a crystallized state (ice), then subjected to a heat source causing the temperature to rise resulting in melting,

the material through latent heat will hold the capacity to store heat (See Figure 1.0).

Alternatively, water in a liquid state cooled to the point of crystallization

(0 degrees centigrade) will discharge heat.

This process is similar at the other phase (100 degrees centigrade) with boiling resulting in heat storage and condensing resulting in heat discharge.

Latent heat storage and discharge for water at 100 degrees centigrade is scientifically termed Latent heat of vaporization and heat storage and discharge at 0 degrees centigrade is termed Latent Heat of Fusion.

For the purposes of this study only Latent heat of fusion will be considered.

Any material holding the characteristics to provide Latent heat are termed Phase Change Materials (PCM).

These PCM can therefore be used to store and discharge heat energy from solar collectors, photovaltaic cells, wind turbines and any other source of renewable heat sources. Additionally these materials can be applied to store heat energy from conventional heat generating sources including off peak electricity.

This process can be achieved by applying the following procedure:-

Firstly, it is necessary to identify a suitable PCM with melting and freezing

range within the temperature band for the proposed heating system operating temperature.

Figure 2.0 demonstrates the procedures of latent heat thermal energy storage for heating a property using an under floor heating system incorporating PCM within the concrete floor construction.

The core working temperature of the concrete floor slab is approximately 29 degree centigrade, providing an ambient (room air) temperature between 20 to 21 degree centigrade.

The PCM selected should therefore have a fusion (melting) temperature of 29 degree centigrade.

Heated water supplied from say a solar collector, passes through the under floor heating coils resulting in heat dissipated to the floor slab through radiation causing the concrete slab and PCM to increase in temperature.

The temperature will continue to increase until a core temperature of 29 degree centigrade is reached. This stage of heat storage is termed sensible heat storage.

Once the core temperature of the concrete floor construction and PCM reaches 29 degree centigrade, the PCM changes state (Phase change) from crystallization to

fusion (melts from a solid to a liquid). During this temperature, the PCM stores considerable amounts of what is termed latent heat (heat required to be absorbed to change state).

This process is termed the "charge period" during which the PCM absorbs heat energy.

The storage of heat can continue for a period until the PCM becomes saturated (can no longer store any further heat energy).

The PCM will remain charged, containing stored heat energy as long as a temperature of 29 degree centigrade, or greater, is maintained.

Once the charging period is complete, providing a sufficient quantity of stored heat energy within the PCM to supply heating to the dwelling over a given period, the under floor heating system is switched off.

This causes the core temperature of the floor construction and PCM to fall due to heat loss into the cooler ambient space.

Once the charged PCM falls below 29 degree centigrade, the PCM changes state causing the material to crystallize (changes from a liquid to a solid), a process termed discharging.

During this change in state, large quantities of (latent) heat are required to be discharged to allow the material to change from a liquid to a solid.

Therefore the stored heat within the PCM discharges at a constant rate over a long period due to thermal mass of the concrete and the considerable quantity of heat stored during charging.

Therefore the under floor heating system is supplying heat energy to the ambient space by the PCM discharging stored heat alone, as opposed to the under floor heating coils.

For certain PCM detailed within this study, the process of phase change can be repeated over an unlimited number of cycles with no change to their physical or chemical properties.

This concept discussed above can be applied to a wide range of applications and heat collecting sources due to the considerable number of PCM with various different fusion temperatures to suite particular design criteria as discussed in later chapters of this study.

 

 

 

3.0 Development of Phase Change Thermal Storage

 

3.1 Introduction

 

The application of Phase Change Materials (PCM) to provide a means of thermal storage has been practiced for many years. The principal of ice thermal storage for retaining items at a constant low temperature is quite old. The use of ice as a PCM was one of the first systems to be used1. Ice houses were built incorporating large blocks of ice cut from frozen rivers in the winter season and contained within sawdust insulation to provide ice for use in the spring periods.

 

The food industry have used the principals of thermal storage to maintain food produce at a controlled low temperature during transportation and storage. These systems named cold accumulators or cooling plates comprised of flat metal containers filled with frozen low temperature melting salt mixtures2.

 

Frozen low temperature melting salts have also been used within plastic containers by pharmaceuticals and throughout the hospital industry to keep chemicals and medicines at a controlled temperature during storage3. The main PCM employed for low temperature applications were inorganic salt mixtures, such as Sulphates of Magnesium and Sodium, Potassium, Sodium Ammonium, Calcium, or Magnesium Chlorides4.

 

Once scientists realized the potential of Phase Change Technology in low temperature situations, research was carried out to find PC materials with

higher melting temperatures for heating applications.

 

At this stage the scientist were more focused on the possibility of using these materials for general consumers, as opposed to technological advanced systems. Early research included hot plates for hotelier’s and restaurant owners for storing food during serving customers5, hot bottle applications and survival clothing5. One such system invented by Bowen in the late 1940 ‘s was a therapeutic pad containing Sodium Metaborate as the thermal medium6.

 

A similar system was developed and manufactured by D.E.Truelock comprising of two devices base on the same principals but incorporating different PCM. The general concept was to provide a product that when activated released the stored heat.

 

The first product released comprising of a cool pack was developed for use by sports doctors and hospital institutions to reduce the level of swelling and bruising to injured persons involved in an accident. The product comprised of a two part plastic bag; one compartment containing a low temperature PCM stabilized in a supercool state; the other a nucleating material. The device is activated when the seal separating the two materials is ruptured causing the two materials to mix resulting in crystallization, releasing all the stored energy.

 

Similarly the heat pack comprising of the same system but was impregnated with PCM at a higher melting point. The hot packs have been used widely in the survival field including the military divisions together with explorers exposed to prolonged periods on cold weather.

 

The ‘hot bottle’ invention founded by Crooker7 and Othmer8 has been in wide use for many years and was developed as a form of thermos flask. The systems comprised of Sodium Acetate PCM and Thiosulfate mixed with finely divided sand and containing small particles of pebbles and metals with a metal cavity surrounding the bottle.

 

The bottle , when shaken cause the PCM to scrap along the surface of the cavity wall releasing the nucleating materials and resulting in crystallization. By experimenting with a number of PC materials, Othmer developed a flask which heated babies milk to the recommended temperature and was in wide use soon after being placed on the market.

 

Inventors soon realized the potential of PCM technology being applied to heat the human body when exposed to prolonged periods of cold weather. The first invention comprised of a suit with a heated back pack developed by Mauleos and Davy in 19659. The suit specifically designed for use by divers, explorers and aviators contained Lithium Hydride PCM with water circulating round the suit by a heat exchanger.

 

Further products were brought onto the market for survival activities including rescue bags containing sponge rubber impregnated with an in-organic salt Hydrate PCM with a melting point between 28 - 45 degrees centigrade10.

 

3.2 NASA Space Program

 

Recognizing the potential use of PCM to reduce the thermal loads and provide greater comfort controls, the scientist involved in the space program established a potential use of such materials, and later applied various PCM within the program11.

 

The scientists were particularly interested in the systems potential for maintaining temperature control.

 

One of the first PCM used in the Apollo 15 space program was to control the heat load released from the computers together with other electrical equipment. The second use of PCM technology was introduced within the solar electric power system due to the heat storage characteristics.

 

The scientist’s carried out considerable research to establish the correct PCM with four main candidates being Lithium Nitrate Trihydrate, Acetamide, Methyl Fumarate and Myristic Acid.

 

Further PCM were incorporated on the Apollo 15 Luna Rover Vehicle11 to stabilize

 

the heat dissipated through the signal processing unit, drive control electronics and Luna communications relay unit. The PCM comprising Paraffin mixture, stored excessive heat generated by the three systems and released the heat stored via movable insulating devices allowing the heat to dissipate by radiation11.

 

3.3 PCM For Heating and Cooling Buildings

 

Although considerable research had been carried out with various products manufactured, the system had not been in wide use in the building industry.

Several attempts have been made with the earliest developed in the 1920’s together with a number of sample house types constructed in the late 1940’s and 1970’s. The systems employed were considered successful although no development was created from the early examples due to the lack of funding and confidence.

 

The following will detail some of the early innovative PCM thermal storage systems to give the reader a knowledge of the history behind the development up until the modern day.

 

3.3.1 Dr. Maria Telkes , United States of America

 

From carrying out considerable research into the potential materials and systems for PCM thermal storage, Dr. Maria Telkes recognized the potential of PCM to be employed to heat buildings11.

 

In 1946, the proposed trial house named The Dover House was to be constructed on a estate in Dover Mass, 5 miles from Boston, USA.

 

The house comprised of a single storey, two bedroom, five room house of 135 m2. floor area, 23m long and one room deep. Thermal radiation was collected by 18 solar collectors comprising of thin gauge galvanized absorber plates, painted black arranged behind double 1.2 x 3.0m glazing panels. Heat generated from these panels were passed along a duct via a fan to three heat storage bins situated either side of the rooms.

 

The heat storage bins contained five gallon drums filled with Glauber’s Salt. With a total 21 tonnes of PCM, the system had the potential heat storage capacity of 4.7 million Btu (11MJ).

 

Glauber’s Salts holding the capacity to store 12 days heating load would provide a sensible heat store between room temperature at 32 degrees centigrade melting point.

 

The house was completed in 1948 at a cost of $20,000 with the solar heating appliance accounting for $3,000.

 

The site was selected by Telkes for its yearly sunlight hours being above average for the area guaranteeing maximum use of the solar system.

 

The first two years proved the system to be highly successful even during periods of up to seven days with overcast weather.

 

The system ran successfully, providing a comfortable internal environment at approximately 21 degrees centigrade requiring no use of any secondary back up heating system.

 

A similar residential house was later designed and built using similar principals to Dr. Maria Telkes design. The house built by Lawrence Gardshire12 in New Mexico comprise a two storey 102 m2. floor area with collector glazing located in the roof structure. Unlike Telkes design comprising PCM bins located at the ground floor level, the 5 gallon storage steel cans were located next to the collectors within the roof space. The only difference in the make up of the PCM containers was the addition of Borex to the Gluaber’s Salts to act as a nucleator.

 

A later project was drawn up by Dr. J.W.Hodgins and Dr. T.W.Hoffman at the Royal Military College in Canada, to construct a residential property using PCM heat storage technology13. Dr. Maria Telkes was appointed to design the PCM thermal storage vessels due to her previous experience in the PCM field.

 

The house was completed in 1959, comprising 111 m2 floor area spread over two

storeys, incorporating 12 tonnes of nucleated Gluaber’s Salts, thickened with Sodium Silicate, and containing Chromate corrosion inhibitor. Similar to the previous design

 

the PCM was contained in steel cans 100 mm x 600 mm long, arrange vertically.

 

3.3.2 PCM Heat Pump Experiment

 

During the 1950’s when PCM were being experimented for use in solar heating applications, the Joint Heat Pump Committee of the Association of Edison Illuminating Companies and Edison Electric Institute, USA, were carrying out test to introduce PCM within a heat pump system14. The study was based around using Disodium Hydrogen Phosphate Dodecahdrate (DSP) as the storage medium.

 

The heat pump system incorporated a series of tinned steel cans containing DSP PCM. The cans were immersed in water as a heat transfer medium. The system were found to be of little success with the PCM not being completely reversible through fusion and crystallization dramatically reducing the storage capacity.

 

Other problems included corrosion of the steel cans and irregular disposition of heat due to irregular flow between cans. The project was promptly halted in 1954 due to

 

the number of reoccurring problems 14.

 

From the previous research development of the Joint Heat Pump Committee of the Association of Edison Illuminating Companies and Edison Electric Institute, USA, Dr. M Telkes found a method of nucleating the DSP PCM by constantly maintaining

 

a source of NA2HPO4.H2O crystals in the PCM tank. The system was later Patented by Dr. Maria Telkes and later Mr. C.S Herrick and Mr.T.L Etherington of General Electric and was in wide use throughout the USA.

 

3.3.3 Off-Peak Electricity Storage

 

The use of PCM to provide a form of Off-peak electrical heat storage began in 1955. The system using a sensible storage medium containing Sodium Hydroxide PCM held the potential for 400,000 Btu heat storage. The system charged by Off-Peak electricity was tested in a number of utility employees homes of Philadelphia Electric Co 15.

 

3.3.4 The First Large PCM Research Program

 

From the previous research and trial tests carried out in the field of Phase Change Thermal Storage, the University of Pennsylvania recognized the need for a structured

research program to overcome the many problems previously discovered. In 1971, the research program was executed by a team of three leading scientists in the latent heat field with funding by the National Science Foundation, USA.

 

The research aim was to discover a number of organic and In-organic PCM suitable for heating and cooling applications, together with detail analysis of the PCM cost, economic analysis and storage capacity.

 

The study was based around the following materials:-

 

q Salt Hydrates

q Organic Eutectics

q Clathrate Hydrates

q Organic - Inorganic Eutectics

 

The results showed that K2HPO4.6H2O (In-organic PCM) and certain paraffin waxes (Organic PCM) were favorable for air conditioning applications. The two most promising PCM for space heating were Zn(NO3)2.6H2O and Ca(NO3).4H2O (Both In-organic PCM) 16.

 

3.4 Further Government Interest

 

The Year 1973 was an important landmark for the development of PCM

Latent Heat technology and research.

 

During the Arab - Israeli war in 1973 and the consequent Arab Oil Embargo made it very evident that the future supply of fuel may be easily disrupted due to political events and the depletion of a major fuel source. This prompted numerous governmental organizations to research into alternative and renewable energy sources.

 

The main area of technological advancement was in solar energy, waste heat recovery, and heat storage17.

 

Research in general was carried out in the UK, USA, Germany and France being heavily focused on Solar Energy due to the lack of advancement in the field as a whole.

 

It was from this time that latent heat storage research become an important part of the renewable energy program with considerable funding being spent in the development of thermal storage technology.

 

3.5 National Science Federation PCM Research Program

 

As a result of the need for research into thermal storage systems grew, the

United States of America pioneered the first large scale research program funded by the National Science Federation. Dow Chemical (Leading scientific group in the PCM field at that time) were awarded to carry out the research comprising of approximately 20,000 potential PCM tests.

 

Due to the size of the research project additional consultant scientists were involved from Canada to join the operation. The research used previous developments of the University of Pennsylvania to set up a database of all the past PCM latent heat sources.

 

From the original 20,000 PCM tested, only 1% were selected for further research being considered as potential candidates.

 

The materials selected were a number of Congruent-melting salt hydrates and common organic materials 18.

 

3.6 Gluaber’s Salts Research

 

During the research activities of Dow Chemical to establish a database of potential PCM for heating applications, Dr. Maria Telkes (University of Delaware) continued to research into ways of improving the longevity of Glauber’s Salts. One of the key

problems with this form of PCM was its ability to separate the Anhydrous Sodium Sulphate on melting, resulting in reduced thermal capacity over a series of freeze-thaw cycles.

 

Tests carried out using various gelling agents and wood pulp for thickening Glauber’s Salts dramatically increased the materials life span although reduction in heat storage capacity was still being experienced over time (1000 freeze-thaw cycles)18.

 

Further research was being carried out by Dr. S.B Marks of the University of Delaware to try and resolve the situation. His research was based around Glauber’s

 

Salts mixed with Attapulgite Clay to act as a gelling agent. The test concluded that although increased heat storage cycles were reported, the reduction in Latent Heat Storage was still occurring 19.

 

Dr. S.B Marks continued his research using a number of other forms of gelling agents resulting in establishing Polymeric Polycarboxylic Acid dramatically

increasing the heat storage capacity longevity 18.

 

A Dr. P.G Rueffel at Saskatchewan Minerals Corporation, USA, interested in the concept of using Glauber’s Salts as a PCM carried out a series of tests using a Peat

Moss matrix to absorb the PCM. The results found the peat forming a fibrous network that absorbs PCM created an ideal gelling agent resulting in dramatic thermal storage longevity potential18.

 

Other alternatives to this have been researched by Boardman Energy Systems, USA19. There PCM mixture comprises of Gluaber’s Salts mixed with Ordinary Portland cement providing a suspension of the two materials during pouring and curing. The results showed that over 2000 test cycles resulted in no thermal storage reduction.

 

The system is chemically stable (safe) and provides great potential as a PCM thermal storage system for building applications. Further analysis of cheap Salt Hydrates as PCM thermal storage was conducted by the University of Delaware, USA 19. An experimental full size house was constructed, named the "Solar One" experimental

 

solar house within the grounds of the University. Due to the development of various systems, resulted in educating governmental bodies in PCM technology. In 1976 Dow Chemical research group was given additional funding to continue there valid research. The project aim was to increase the data on potential PCM as to test them in sub scale storage units. Since then Dow Chemical have been approached by other organizations providing private funding 20.

 

3.7 Paraffin Wax PCM Research

 

During Dow Chemicals original survey to establish potential PCM out of the 2000 sampled, Paraffin wax PCM being an organic substance, relatively inexpensive and large supply was found to be a potential storage material for the building industry.

 

Dr. John Bailey and other associates at the North Carolina State University decided to research this area further seeing that Dow Chemical were concentrating there efforts on various Salt Hydrates and other in-organic PCM22. There research was centered around Aluminum Honeycomb thermal capacitors incorporating a paraffin wax (N-eicosane) with a sharp melting point of 36 degrees centigrade to make the monitoring less complicated.

 

In 1974, further studies into this potential PCM were carried out by Dr.T.R Galloway at the Lawrence Livermoore Laboratory. The study was based around using Paraffin

 

Wax PCM in three different heating applications; bulk PCM tank with plastic heat exchange coil; steel cans filled with the PCM surrounded by water inside a tank and large encapsulated disks of wax surrounded by water in a tank 21.

 

The USA research institutions have mainly concentrated on in-organic PCM with research into organic PCM being more thoroughly investigated within Europe.

Although the research centers in the United States have been investigating PCM thermal storage technology for a considerable length of time compared to countries, significant advances have been made in Europe and the Far East.

 

The most significant advances made in PCM research has been in Japan with Sweden, West Germany, France Italy and the USSR contributing to research in the PCM heat energy storage field.

 

3.8 West Germany

 

Scientists researching PCM thermal storage technology in Germany have provided extensive charts and tables of PCM physical and thermal properties, materials for construction and insulation properties.

 

The main research has been centered around Paraffin’s, Salt Hydrates and Clathrates

considered to be the most favorable PCM for application below 100 degree centigrade.

 

Research groups at the University of Stuttgart have carried out considerable research on several organic and Salt Hydrates candidates. The test storage systems incorporate Paraffin’s, Fatty Acids and Salt Hydrates sealed in finned tube containers with heat exchangers coupled to heat pipes 21.

 

A private company in West Germany carried out research on the BBC Solar House to examine the different performance of several PCM. The Solar House was heated by a heat pump using water as the storage medium. The researchers decided to change the storage medium first with ice then Paraffin (m.p.18 to 25 degrees centigrade) over a period of three heating seasons22.

 

The experiment proved that Paraffin provided the greatest efficiency and also increases the heat pump COP from 2.6 to 3.3. Also paraffin only required half the volume needed for water providing greater cost efficiency.

 

3.9 Denmark

 

A research group in Denmark have carried out considerable research into both in-organic Salt Hydrates and Organic PCM’s. There research concentrated on Gluaber’s

Salt, Sodium Carbonate Decahydrate, Sodium Acetate Trihydrate and Disodium Phosphate. The aim of the research is to extent the chemical analysis of these materials.

 

3.10 Sweden

 

Considerable advancement of PCM storage technology is taking place at Studsvik Energitenik A B headed by Prof.H.Hedman 23. The research is concentrated around the application of Salt Hydrates, using air as the heat transfer fluid. The PCM systems, encapsulated in metal capsules, was built and tested in a sub scale model and proved more efficient than water tanks and rock beds presently used for passive solar heating in domestic houses in Denmark. A full scale demonstration house was later built incorporating the Energitenik A B PCM thermal storage technology.

 

A joint project involving three large Swedish construction forms was carried out to investigate the potential of incorporating PCM within the building services24.

 

The project involved constructing three different heating systems. The first with a

standard hot water district heating system; the second using forced hot air heating and Off-Peak power changed to store a PCM encapsulated in sealed linear polyethylene pipes; and the third, a similar system to the second, but with air-cooled solar collectors as the primary heat source.

 

3.11 France

 

For many years, France has been carrying out research into PCM thermal storage technology and through research at the Center National de la Recherché Scientifique

 

Laboratory (CNRS) in Nice, have constructed several buildings using there technology 24.

 

The two structures comprise of a solar house for residential purposes and a greenhouse for rose production near Nice. There system uses PCM ‘Clairolithe’ consisting of CaCI2.6H2O mixed with a nucleating agent encapsulated into cells forming part of the wall structure. The CNRS is also developing the research into PCM Paraffin wax, by testing systems incorporating metal mixtures to increase the heat transfer characteristics Scientists from the Center Scientific et Technique du Batiment in Paris have patented devices incorporating PCM with melting .points of 15 to 40 degrees centigrade to be used in air conditioning applications 24.

 

3.12 United Kingdom

 

The British Research Establishment, Garston, Watford have for many years carried out research on the subject of PCM thermal storage technology.

 

A.K.R.Bromley, E.M.McKay and J.P Wilkins form the Thermal Storage Study Group and have carried out research into ice-thermal storage and latent heat storage for heating applications. The group have published several papers on there developments and are regular contributors to the CIBSE National Conference discussing developments in the field of thermal storage25.

 

There has been considerable use of PCM technology in the ice-storage field in the UK with large organizations including and Baltimore Cristopia [in partnership with France]supply systems worldwide.

 

A small team of scientists at the University of Salford, carried out research on Hydrated Disodium Phosphate and Calcium Chloride Hexahydrate together with Cardiff University, in Wales, developed PCM technology using Paraffin’s as the PCM storage medium26.

 

3.13 Japan

 

Japan commands a leading role along side the United States of America in PCM thermal storage technology having research groups study the area for many years with strong government backing. The Japanese leading area of research is in the field of PCM thermal storage heating systems with many initial designs later manufactured.

 

The materials found to be most successful during there research by Mitsubishi Electronic Corp. and Tokyo Electric Power Co. were Hydrated Nitrate Salts, Phosphates, Fluorides and Calcium Chloride 27. Research up to the 1970’s was focused on the PCM thermal heat storage although more recent advances have been concentrated in cooling applications.

 

In the early 1980’s, several new organizations including Nippon Pillar Packing Co. patented a Calcium Bromide Hexahydrate PCM with Nucleating agents. Other’s such as Kureha Chemical Industry Inventors used the research of Gluaber’s Salts PCM for room heating and cooling using Gypsum and other lightweight aggregates to stabilize the PCM28.

 

Considerable research and development was carried out on a number of Nitrate Eutectics for room temperature applications. Yoneda and Takanashi of the Department of Industrial and Engineering Chemistry of Science University of Tokyo27 developed a series of models using a simple heat exchanger to monitor the PCM system.

 

Initial results concluded that the system was very efficient, but problems occurred when linked up with a Solar Collector resulting in considerable reduction in efficiency.

 

The United States of America later carried out further research into the Japanese project, established the inaccuracies, and later patented the system27.

 

 

References to Chapter

1. Lane. G A, (1983), Latent Heat Storage: Background and Scientific Principles, Volume 1, U.S.A CRC Press, PP. 9.

2. McGuffey. O.S, (1947), U.S Patent 2, 416,015

3. Shepard, J.C, (1957), US Patent 2, 800,454

4. Tomlinson.J,(1989), New Materials for Thermal Storage, Oak Ridge National Laboratory,Tenessee,HI-89-38No.4,pp1931..

5. Lane. G A,(1983), Volume 1, Latent Heat storage: Background and Scientific Principles, U.S.A CRC Press, pp10.

6. Bowen. C.T.,(1949),U.S Patent 2,595,328

7. Crooker. H.L,(1928), British Patent 309,24

8. Othmer. D.F,(1939), U.S Patent 2 ,220,777

9. Mauleos. M.G. and Desy. J.J, U.S Patent 3,182,653

10. Feldman. J.E.,(1965), U.S Patent 2,515,298 1950

11. Frysinger.G.A & Sliwkowski.J,(1987),Phase Change Material Storage Assisted Heating Systems, University of Deleware,PH-87-5,No.4,pp516.

12. Ghoneim.A.A. & Klein.S.A, (1991), Phase Change Materials Analysis, Solar Energy, Vol.47, No.3 pp239.

13. Lane. G A,(1983), Volume 1, Latent Heat Storage: Background and Scientific Principles, U.S.A CRC Press, P.P. 15

14. Bromley.A.K.R & McKay.E.M,(1994) Incorporating Phase Change Materials into the Building Fabric, CIBSE Conference 1994,pp101.

15. Brandsetter.A & Kaneff.S,(1990),Materials and Systems for Phase Change Thermal Storage, Australian National University paper, pp.1462.

16. Bromley.A.K.R & McKay.E.M,(1994) Incorporating Phase Change Materials into the Building Fabric, CIBSE Conference 1994,pp.104.

17. Lane. G A,(1983), Volume 1, Latent Heat Storage: Background and Scientific Principles,U.S.A CRC Press, pp 18

18. Lane. G A,(1983), Volume 1, Latent Heat Storage: Background and Scientific Principles, U.S.A CRC Press, pp 19

19. Schroder.J & Gawron.K,(1981),Latent Heat Storage, Philips GmbH Forschungslaboratotium Aachchen, West Germany, Energy Research Conference paper 5, pp.109.

20. A.Abhat, (1983) University of Suttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4 , pp320,

21. Neeper.A.D,(1989), Potential Benefits of Distributed PCM Thermal, American Solar Energy Society 1989 conference report, Los Alamos National Laboratory, New Mexico, pp11.

22. Lane. G A, (1983), Volume 1, Latent Heat Storage: Background and Scientific Principles, U.S.A CRC Press, pp 27

23. Lane. G A, (1983), Volume 1, Latent Heat Storage: Background and Scientific Principles, U.S.A CRC Press,pp 29

24. Brandsetter.A & Kaneff.S,(1990),Materials and Systems for Phase Change Thermal Storage, Australian National University paper, pp.1471.

25. Bromley.A.K.R & McKay.E.M,(1994) Incorporating Phase Change Materials into the Building Fabric, CIBSE Conference 1994,pp.105

26. Marshall. R.H.,(1992), Phase Change Heat Energy Storage Materials, report 644/SEU 205, University of Cardiff, Wales.

27. Lane. G A, (1983), Volume 1, Latent Heat Storage: Background and Scientific Principles, U.S.A CRC Press, pp 31.

 

 

 

4.0 Heat Storage PCM for Residential Heating

 

4.1 Introduction

 

It is critical when selecting a potential PCM candidate for residential heating applications to screen the materials under three initial components. These are as follows:-1

 

1. A Heat Storage substance that undergoes a solid-to-liquid phase transition within the desired operating temperature range and where in the bulk of the heat added is stored as the latent heat of fusion.

 

2. A containment for the storage substance.

 

3. A heat exchanging surface for transferring heat from the heat source to the heat storage substance.

 

There are many secondary factors to be taken into consideration when developing a latent heat storage system. Figure 1 demonstrates the various stages of development and areas where attention needs to be concentrated. The flow chart shown in figure 1.0 illustrates the need to identify not only a suitable PCM but also compatibility

with construction materials and suitable choice of heat exchanging design.

 

4.2 Dow Chemical’s PCM Research and Development Program (USA)

 

Dr.George A. Lane, a senior research specialist together with other associates of Dow Chemical, Michigan, USA, carried out research of over 20,000 potential PCM for use in heating applications. The criteria for selection for further study included melting point, phase diagram, toxicity, stability, corrosivity, flammability, safety, availability and cost.

 

Test carried out on over 20,000 PCM resulted in approximately 200 materials found suitable for further research and then eventually 40 PCM were established as true potential compounds. Within the 40 PCM selected, several held the correct criteria to be used for heating buildings. These are as follows:-

 

1. Paraffin wax - Organic compound

2. Fatty Acids - Organic compound

3. Salt Hydrates - In-organic compound

 

Physical Properties for a selection of the 40 PCM are shown in table 1. Figure 2.0

illustrates the two main families of PCM and the forms of latent heat materials found

within each family of materials.

 

4.3 Paraffins

 

Paraffins consisting of a waxy consistency at room temperature is grouped in the organic family. The substances are made up of straight chain hydrocarbons with 2-methyl branching groups near the end of the chains2. The Paraffin’s are separated into two main sub groups, even chained (n-Paraffin) and odd chained (iso-Paraffin). Whether a paraffin falls into an even chain or odd chain group is dependent upon the content of alkanes within the substance (ranging from 75 - 100%) 3.

 

The melting point of Paraffins is directly related to the number of Carbon Atoms within the material structure with alkanes containing 14-40 C-atoms possessing melting points between 6 and 80 degrees centigrade. These are termed pure paraffins and should not be confused with Paraffin waxes. Paraffin waxes contain only 8-15 carbon numbers with lower melting points than pure paraffins at 2 - 45 degrees centigrade 4.

 

When paraffins reach there melting points an allotropic modification takes place with the material being soft and plastic with individual crystals being needle shaped5.

 

Additionally a second allotropic modification occurs below melting point forming a brittle hard structure (similar to that of a section of unlit candle) with disc shaped crystals 6.

 

Paraffins form an ideal PCM candidate for residential heating applications due to there large temperature range and there various forms of structure allowing specific paraffin’s to be selected for a certain temperature range. The material, being of an organic compound is cheap and in huge quantity. The storage capacity is relatively high compared with other compounds, plus the materials are proven to freeze without supercooling (The entire material content changes phase resulting in maximum thermal capacity without any segregation over longevity).

 

See table 1 for temperature melting point data, thermal capacity and volume.

 

The greatest advantage of using paraffins over other compounds is there melting points of a selection of paraffin types are around 22 - 24 degrees centigrade, ideal for residential heating applications.

 

4.4 Fatty Acids

 

Fatty acids being of a organic compound have similar melting ranges and heat of fusion values to organic paraffin substances. The material CH3 [CH2]2nCOOH can be considered for the application for residential heating applications although the cost of the materials is approximately 2 - 2.5 times greater than that of Paraffins 7.

Other disadvantages is there being only several fatty acids within the melting and freezing point between 20 - 30 degrees centigrade band 8.

 

4.5 Salt Hydrates

 

Salt Hydrates PCM has been one of the most researched latent heat storage material with numerous trails and sub-scale tests carried out on this compound as described in previous sections of this study. The main reason for this is due to the considerable large melting points of a range of Salt Hydrates between 0 - 120 degree centigrade providing numerous thermal storage applications 9 . The material comprises M.nH2O where M is an inorganic compound holding the capacity to contain a high volumetric latent storage density 10.

 

The main problems experienced in the past thermal storage applications using Salt-Hydrates was due to the fact that most Salt-Hydrates melt incongruently (e.g. they melt to a saturated aqueous phase and a solid phase which is generally of a lower hydrate of the same salt)10.Due to this density variation, the solid phase settles out and collects at the bottom of the containment device termed decomposition.

 

The problems of decomposition together with the Salt Hydrates poor nucleating properties results in what is termed supercooling of the liquid salt-Hydrate prior to freezing. This must be overcome by the addition of a suitable nucleating agent that

have a crystal structure similar to that of the parent substance10.

 

Products have later been manufactured taking into consideration for the need of nucleating agents within the Salt-Hydrate thermal storage system which have proved very successful.

 

References to chapter

1. Abhat A.,(1983)University of Suttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4 pp313

2. Neeper.A.D,(1989) Potential Benefits of Distributed PCM Thermal, American Solar Energy Society 1989 conference report, Los Alamos National Laboratory, New Mexico, pp5.

2. Neeper.A.D,(1989) Potential Benefits of Distributed PCM Thermal, American Solar Energy Society 1989 conference report, Los Alamos National Laboratory, New Mexico, pp7.

4. Stovall.T.K,(1989), Activities to Support Wax-Impregnated Wall Board Concept, U.S. Department of Energy,Thermal Energy Storage Research Activities Review, New Orleans,Louisiana, March 17 1989,pp10

5. Lane.G.A,(1980), Low Temperature Heat Storage with Phase Change Materials, The International Journal of Ambient Energy, Volume 1, No.3 ,pp159.

6. Lane.G.A,(1980), Low Temperature Heat Storage with Phase Change Materials, The International Journal of Ambient Energy, Volume 1, No.3,pp160.

7. Gawron.K,(1981), Latent Heat Storage,Philips GmbH Forschungslaboratotium Aachchen, West Germany, Energy Research Conference paper 5, pp105.

8. Brandstetter.A, Kaneff.S,(1991), Materials and Systems for Phase Change Thermal Storage paper, Energy Research Centre, Australian National University, Canberra, pp1461.

9. A.Abhat,(1983) University of Suttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4 , pp317, para.2.

10. Lane.G.A,(1991),Phase Change Material for Energy Storage Nucleation to Prevent Supercooling, Solar Energy Materials and Solar Cells 27 135-160, pp4.

 

 

 

4.0 Heat Storage PCM for Residential Heating

 

4.1 Introduction

 

It is critical when selecting a potential PCM candidate for residential heating applications to screen the materials under three initial components. These are as follows:-1

 

1. A Heat Storage substance that undergoes a solid-to-liquid phase transition within the desired operating temperature range and where in the bulk of the heat added is stored as the latent heat of fusion.

 

2. A containment for the storage substance.

 

3. A heat exchanging surface for transferring heat from the heat source to the heat storage substance.

 

There are many secondary factors to be taken into consideration when developing a latent heat storage system. Figure 1 demonstrates the various stages of development and areas where attention needs to be concentrated. The flow chart shown in figure 1.0 illustrates the need to identify not only a suitable PCM but also compatibility

with construction materials and suitable choice of heat exchanging design.

 

4.2 Dow Chemical’s PCM Research and Development Program (USA)

 

Dr.George A. Lane, a senior research specialist together with other associates of Dow Chemical, Michigan, USA, carried out research of over 20,000 potential PCM for use in heating applications. The criteria for selection for further study included melting point, phase diagram, toxicity, stability, corrosivity, flammability, safety, availability and cost.

 

Test carried out on over 20,000 PCM resulted in approximately 200 materials found suitable for further research and then eventually 40 PCM were established as true potential compounds. Within the 40 PCM selected, several held the correct criteria to be used for heating buildings. These are as follows:-

 

1. Paraffin wax - Organic compound

2. Fatty Acids - Organic compound

3. Salt Hydrates - In-organic compound

 

Physical Properties for a selection of the 40 PCM are shown in table 1. Figure 2.0

illustrates the two main families of PCM and the forms of latent heat materials found

within each family of materials.

 

4.3 Paraffins

 

Paraffins consisting of a waxy consistency at room temperature is grouped in the organic family. The substances are made up of straight chain hydrocarbons with 2-methyl branching groups near the end of the chains2. The Paraffin’s are separated into two main sub groups, even chained (n-Paraffin) and odd chained (iso-Paraffin). Whether a paraffin falls into an even chain or odd chain group is dependent upon the content of alkanes within the substance (ranging from 75 - 100%) 3.

 

The melting point of Paraffins is directly related to the number of Carbon Atoms within the material structure with alkanes containing 14-40 C-atoms possessing melting points between 6 and 80 degrees centigrade. These are termed pure paraffins and should not be confused with Paraffin waxes. Paraffin waxes contain only 8-15 carbon numbers with lower melting points than pure paraffins at 2 - 45 degrees centigrade 4.

 

When paraffins reach there melting points an allotropic modification takes place with the material being soft and plastic with individual crystals being needle shaped5.

 

Additionally a second allotropic modification occurs below melting point forming a brittle hard structure (similar to that of a section of unlit candle) with disc shaped crystals 6.

 

Paraffins form an ideal PCM candidate for residential heating applications due to there large temperature range and there various forms of structure allowing specific paraffin’s to be selected for a certain temperature range. The material, being of an organic compound is cheap and in huge quantity. The storage capacity is relatively high compared with other compounds, plus the materials are proven to freeze without supercooling (The entire material content changes phase resulting in maximum thermal capacity without any segregation over longevity).

 

See table 1 for temperature melting point data, thermal capacity and volume.

 

The greatest advantage of using paraffins over other compounds is there melting points of a selection of paraffin types are around 22 - 24 degrees centigrade, ideal for residential heating applications.

 

4.4 Fatty Acids

 

Fatty acids being of a organic compound have similar melting ranges and heat of fusion values to organic paraffin substances. The material CH3 [CH2]2nCOOH can be considered for the application for residential heating applications although the cost of the materials is approximately 2 - 2.5 times greater than that of Paraffins 7.

Other disadvantages is there being only several fatty acids within the melting and freezing point between 20 - 30 degrees centigrade band 8.

 

4.5 Salt Hydrates

 

Salt Hydrates PCM has been one of the most researched latent heat storage material with numerous trails and sub-scale tests carried out on this compound as described in previous sections of this study. The main reason for this is due to the considerable large melting points of a range of Salt Hydrates between 0 - 120 degree centigrade providing numerous thermal storage applications 9 . The material comprises M.nH2O where M is an inorganic compound holding the capacity to contain a high volumetric latent storage density 10.

 

The main problems experienced in the past thermal storage applications using Salt-Hydrates was due to the fact that most Salt-Hydrates melt incongruently (e.g. they melt to a saturated aqueous phase and a solid phase which is generally of a lower hydrate of the same salt)10.Due to this density variation, the solid phase settles out and collects at the bottom of the containment device termed decomposition.

 

The problems of decomposition together with the Salt Hydrates poor nucleating properties results in what is termed supercooling of the liquid salt-Hydrate prior to freezing. This must be overcome by the addition of a suitable nucleating agent that

have a crystal structure similar to that of the parent substance10.

 

Products have later been manufactured taking into consideration for the need of nucleating agents within the Salt-Hydrate thermal storage system which have proved very successful.

 

References to chapter

1. Abhat A.,(1983)University of Suttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4 pp313

2. Neeper.A.D,(1989) Potential Benefits of Distributed PCM Thermal, American Solar Energy Society 1989 conference report, Los Alamos National Laboratory, New Mexico, pp5.

2. Neeper.A.D,(1989) Potential Benefits of Distributed PCM Thermal, American Solar Energy Society 1989 conference report, Los Alamos National Laboratory, New Mexico, pp7.

4. Stovall.T.K,(1989), Activities to Support Wax-Impregnated Wall Board Concept, U.S. Department of Energy,Thermal Energy Storage Research Activities Review, New Orleans,Louisiana, March 17 1989,pp10

5. Lane.G.A,(1980), Low Temperature Heat Storage with Phase Change Materials, The International Journal of Ambient Energy, Volume 1, No.3 ,pp159.

6. Lane.G.A,(1980), Low Temperature Heat Storage with Phase Change Materials, The International Journal of Ambient Energy, Volume 1, No.3,pp160.

7. Gawron.K,(1981), Latent Heat Storage,Philips GmbH Forschungslaboratotium Aachchen, West Germany, Energy Research Conference paper 5, pp105.

8. Brandstetter.A, Kaneff.S,(1991), Materials and Systems for Phase Change Thermal Storage paper, Energy Research Centre, Australian National University, Canberra, pp1461.

9. A.Abhat,(1983) University of Suttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4 , pp317, para.2.

10. Lane.G.A,(1991),Phase Change Material for Energy Storage Nucleation to Prevent Supercooling, Solar Energy Materials and Solar Cells 27 135-160, pp4.

 

 

 

5.0 PCM Thermal Storage Applications

 

5.1 Introduction

 

The Phase Change Thermal Storage Materials field is littered with the bones of organizations who developed products, marketed them, and went out of business. Inevitably, they were not thorough enough in the chemistry, not diligent enough in the design, not endowed enough in resources, or not committed enough in vision.

 

Dr.G.A.Lane of Dow Chemical, Michigen, U.S.A, when ask by the author for the main reasons as to why these first PCM products failed on the commercial market summarized his answers in the following:-

 

r Poor choice of PCM, often employing chemicals that were not congruent melting by basic physicochemical definition, therefore a reduction in thermal storage capacity existed over a number of fusion/chrystalisation cycles resulting in PCM segregation.

 

r Poor choice of encapsulation leading to corrosion or leakage.

 

r Inappropriate design, resulting in products incompatible within existing construction.

 

During the last 12 - 15 years, encapsulated phase change heat storage products have become available which are better suited for integration into building structures. Building modules are now being designed around these encapsulated PCM which is

 

considered by many leading scientist within the field to be a necessary step to the success of this technology.

 

This chapter provides an appraisal of the more successful commercial products brought into wholesale production during the last 12- 15 years. These products are correctly in use within a wide range of building structures ranging from sports halls, offices to domestic houses in countries including France, Germany, U.S.A and Japan.

 

5.2 Sol-Ar-Tile

 

The Sol-Ar-Tile produced by Ar-Lite Panelcraft ltd, Michigan, U.S.A, comprises of two PCM pouches cast in the center of fiberglass-reinforced polymer concrete tiles1. The PCM pouches comprise of Gluaber’s salt mineral chemical providing 220Kcal (880 BTU) latent heat storage capacity.

 

The tiles come in two main sizes measuring 2.96 x 5.99 x 0.032 meters and 5.99 x 5.99 x 0.032 meters weighing approximately 20 Kg (See Figure 1.0).

 

This product has been widely used within passive solar houses with typical applications including floor construction within sun rooms, ceilings, bench tops and window seats.

 

5.3 Calor Alternative Energy

 

Calor Alternative Energy are manufactures of PCM technology products with offices in U.S.A comprising of a partnership with Calmac Manufacturing corporation with main head courters located in Kent, England headed by Mr.G.K.Harris 2.

 

The company manufacture Calortherm tubes comprising of Polypropylene encapsulance 1 meter in length and either 36 mm or 50mm in diameter. These tubes are filled with a variety of proven PCM chemicals depending on the desired melting temperature and application. The tubes are colour coded and numbered in relation to the PCM contained within the product.

 

The number is in direct relation to the melting temperature (i.e. Calortherm 31 contains a PCM which melts at 31 degree centigrade). The company presently produce 7 different tubes as follows (See Figure 2.0).

 

Calortherm 7 Tubes

 

Filled with a gelled formulation of Na2SO4.10H20,KCI, which melts at 7.5 degree centigrade with working temperature range of 2.5 to 20 degrees centigrade. This provides a heat storage capacity of 0.144 kWh for the 50mm tube. The suggested use of this product is for off-peak coolness storage medium to replace on-peak air conditioners or chillers.

Calortherm 18 Tubes

 

Filled with a gelled formulation of Na2SO4.10H20/NaCI composition, which melts at 18 degree centigrade with working temperature range of 10 - 30 degree centigrade. This provides a heat storage capacity of 0.172 kWh for a 50mm diameter tube. Typical applications for the Calortherm 18 include incorporation within the construction of sun rooms forming part of a passive solar house design.

 

Calortherm 31 Tubes

 

Filled with a gelled formulation of Na2SO4.10H20 composition, which melts at 31 degree centigrade with working temperature range of 20 - 50 degree centigrade. This provides a heat storage capacity of 0.25 kWh for a 50mm diameter tube. Suggested applications for this product include active solar heating design situations.

 

Calortherm 48 Tubes

 

Filled with a gelled formulation of Na2S3O2.5H2O composition, which melts at 48 degree centigrade with working temperature range of 35 - 55 degree centigrade. Typical applications for this product include heat pump output storage.

 

Calortherm 70 Tubes

 

Filled with a gelled formulation of Na4P2O7.10H20 composition, which melts at 70 degree centigrade with working temperature range of 40 - 80 degree centigrade. Typical applications for this product include off-peak heating design solutions.

 

5.4 Cristopia Plastic Spheres

 

Cristopia and Stocris, situated in Valbonne, France, for the last 16 years have produced PCM filled spheres developed by J.Patry and C.Lenotre3. The "boulet d’energie" (energy balls) comprise of a mixture of hydrated salts and organic materials sealed in a polyethylene or polypropylene casing. The products come in a range of phase transition temperatures ranging from 8 - 64 degrees centigrade and measure 77mm in diameter.

 

The prime application for the PCM spheres are for filling large heat storage cylinders with a 3.0 meter diameter cylinder capable of containing 2,500 Kg of heat storage spheres (See Figure 3.0).

 

5.5 Pennwalt PCM Pellets

 

Pennwalt Corporation, Paris, manufacture and distribute PCM pellets for use within the solar heating field4. The product comprises of roughly spherical shaped pellets

measuring 63 mm diameter capsule coated with a tough polymeric outer skin containing a variety of PCM to suite particular applications. The company currently manufacture four PCM thermal storage capsules comprising the following:-

 

(i) Paraffin wax with melting point temperature 47 degree centigrade.

 

(ii) Na2SO4.10H20 in-organic compound with melting point temperature 32 degree centigrade.

 

(iii) CaCl2.6H2O in-organic compound with melting point temperature 27 degree centigrade.

 

(iv) Na2SO4.10H20/ Na2CO3.10H20 in-organic compound with melting point temperature 22 to 24 degree centigrade.

 

Typical applications for Pennwalt pellets include tanks storing solar collector water storage and bedding into construction materials such as concrete floor construction and precast concrete block work to improve thermal storage capacity for passive solar collection (See Figure 4.0).

 

 

5.6 Enerphase Panels

 

Dow Chemical Co., Michigan, U.S.A, manufactures and distributes a PCM thermal

storage panel to be used within the passive solar application field5.

 

The panels measuring 550 x 350 x 55mm thickness comprises of CaCl2.6H2O in-organic compound PCM encapsulated within a seamless low density polyethylene structure by rotational molding process (See Figure 5.0).

 

The panel, with a melting temperature of 27 degree centigrade contains 8.4Kg of PCM comprising of a thermal storage capacity of 386.4Kcal (1533 BTU) latent heat. The panel has been carefully designed to form a rigid composite panel incorporating 15 V-shaped transverse troughs, forming a total of 96 structural bonds providing considerable panel strength.

 

Typical recommended applications for the Enerphase panels include incorporation within timber stud or masonry cavity wall construction behind a single glazed screen for direct solar collection. Alternatively, mounted within the sun room behind glazing within residential buildings.

 

The manufactures have estimated that 50% of the heating load can be supplied by 25 -30 Enerphase panels for a typical house in southern U.S.A.

 

5.7 Rodwall PCM Energy Storage System

 

Sunwood Energy Products based in Harrisburg, Vaginia, manufacture a modular passive solar system incorporating heat storage, heat distribution, insulation and shading. The single unit measuring 2.08m x 1.07m x 20 mm thickness contains a series of PCM filled rods comprising a total heat storage capacity of 4940 Kcal (19,600 BTU) (See Figure 6.0).

 

The rods actuated by automatic controls, rotate, exposing either a black, sun

absorbing surface in the heat-storage mode, or a reflective surface covered with

 

insulation for periods of heat retention during night time and heat rejection in

summer. Heat from discharging rods is circulated into the room by fans through air grills installed into the wall finish. The rods are covered externally by a single glazed panel to optimize solar collection.

 

5.8 O.E.M Heat Battery

 

O.E.M. Products Inc. based in Dover, Florida, manufactures and distributes a PCM thermal storage battery comprising of a nonmetallic bulk heat storage tank filled with Glauber’s salts6. The system uses a hydrocarbon oil to charge or extract heat from the PCM by fluid being pumped to the bottom of the tank through a ‘Christmas tree’ distribution system and bubbling up through the PCM, collecting at the top (See Figure 7.0). 53

 

During the operation the PCM crystallizes and sinks to the bottom of the tank, and in doing so blocks the lowest fluid outlet holes resulting in added pressure which causes the higher distributors to open.

 

The PCM freezing process continues in an ascending manner until the entire PCM tank is frozen. During the charging period this process is reversed, resulting in the distributor ports opening in descending order. Storage and discharge is achieved through passing water or glycol heat transfer fluid through copper heat exchange coils near the top of the tank, immersed within the PCM.

 

Typical heat storage capacities for this system range between 60,000 to 436,000 Kcal (237,000 to 1,730,0000 BTU).

 

Typical heat sources for the O.E.M. Heat Battery include solar heat, waste heat from air-conditioning plant, off-peak electricity heated water applications.

 

5.9 Tes Term-AC Thermal Storage Systems

 

Tes Systems Ltd, with offices in England and Switzerland, manufacture PCM thermal storage panels for use within timber and concrete floor construction for both domestic, commercial and industrial premises (See Figure 8).

 

The product comprises of a mineral salt composition encapsulated in conically shaped high density polypropylene casing, with a melting point around 29 degree centigrade. The capsules measuring 25mm in height and 225 x 1100mm length have

capacity to store 1789 kJ/m2 heat energy.

 

Typical application include incorporation within in-situ concrete floor construction containing a wet or dry under-floor heating system. This system is particularly

 

favorable where free heat source or renewable energy can be exploited.

 

5.10 Wax Impregnated Wallboard Concept

 

On March 15, 1989, Dr.J.J.Tomlinson of the Oak Ridge National Laboratory, Tennessee, presented research into the implementation of wax impregnated wallboard for passive solar applications to the United States Department of Energy Thermal Storage conference held in New Orleans, Louisiana 7.

 

The presentation focused on the application of Phase Change Thermal Storage to reduce the peak-power demand and down sizing of air conditioning plant for cooling of residential building in the United States.

 

Researchers at the Oak Ridge National Laboratory recognized the potential of PCM plaster board having a significant impact on the utility peak due to more than 7

 

billion square meters of plaster board produced annually in the United States of America alone. Additionally, the scientists recognized the advantages of moderating temperature swings and enhancing comfort within homes8.

 

This research area was founded by Dr.Ival Salyer at the Dayton Research Institute in

1982. Dr.Salyer’s research focused on a blend of n-paraffin carbon chains of different lengths below and above 18 carbon atoms [K-18], which melts and freezes almost exactly at 25 degree centigrade 9.

 

Dr. R.J Kedl and Dr.T.K.Stovall of the Engineering Technology Division, ORNL, Tennessee, carried out considerable research into ways of impregnating the PCM into the plasterboard without altering the adhesion of paint and joint compound 10. The researchers established two successful methods of incorporating the wax PCM into the plasterboard material.

 

The first method uses small 3mm diameter pellets of high-density polyethylene (HDPE) to hold the wax. The gypsum wallboard material occupies the void space surrounding the pellets. The second method immerses conventional wallboard into containers of melted wax so that the wax is absorbed into the pores of the wall board. Sub scale tests concluded that up to 30% of composite weight of the PCM can

be absorbed in less than 10 minutes.

 

Further research was carried out to prevent the PCM wallboard becoming a severe fire hazard. Tests carried out identified that limiting the amount of PCM to 15% to 20%, and subsequently treating the plasterboard with an insoluble fire retardant minimized the risk of combustion 11.

 

Figure 9.0 shows the physical properties for wallboard as measured by Oak Ridge National Laboratory 12.

 

In August 1996, Dr. Helmut E. Feustel heading the Indoor Environment Program at Lawrence Berkeley National Laboratory, University of California carried out a study of the benefits of PCM wallboard using a computer modeling program 13.

RADCOOL, a thermal building simulation program based on the finite difference approach, was used to numerically evaluate the latent storage performance of treated wallboard.

 

Simulation results for a living room with high internal loads and weather data for Sunnyvale, California, showed significant reduction of room air temperature when conventional wallboards were replaced with PCM-treated plasterboard. Results of using double thickness PCM wallboard concluded that air temperatures could be controlled within the comfort limits without the need for any mechanical cooling 13.

 

Test results carried out on a plaster board sample measuring 0.0125 x 1.22 x 2.44 meters containing 15% phase change material held approximate storage

 

capacity of 277 Wh (93 Wh/m3). Simulation results for a house with 370 m2 of 15% PCM treated wallboard (170 m2 floor area), held the storage capacity of more than

34 kWh, or 9 ton-hours storage capacity eliminating the need for any compressor cooling 14.

 

Dr. Helmut E. Feustel concluded from his research that PCM-treated wallboard holds the potential to convert light buildings into thermally heavy constructions. In Californian climate with large diurnal swings, the proposal can keep residences comfortable for most of the year without applying any means of mechanical cooling and by using night time ventilation to discharge the latent storage of the wallboard, is considered a viable preposition to reducing depletion of fossil fuels and CO2 omissions 15.

 

Additionally the treated wallboard can be coupled with a hydronic loop facilitating continuos discharge of thermal energy storage without ‘dumping’ the energy back into the conditioned space 15.

 

Wax impregnated wallboard although mainly focused on applications in the United States holds many cost and energy conservative applications in the United Kingdom. The material can play a role in maintaining ambient air temperature levels within buildings where comfort limits vary considerably requiring the use of air conditioning plant equipment. Phase Change storage materials can be specified in

 

relation to there melting point range to be applied to specific temperate conditions.

Such applications may comprise the following where sudden fluctuations in casual gains occur.:-

 

r Lecture theaters or large assembly halls within educational establishments

 

r Leisure centers (sports halls) where users perform various levels of activity producing large fluctuations in heat gains

 

r Cinemas, Indoor sports stadiums with large volumes of spectators and

various forms of theaters that experience sudden influxes of occupancy

 

r Office environments where increasing amounts of electrical equipment and computers are used

 

The PCM-treated wallboard hold the potential to be applied in applications where specific ambient air temperatures require to be controlled. Application for PCM-treated wallboard hold potential use in the following internal environments were strict control of temperatures are required have been identified as follows:-

 

r Storage rooms containing medicine or chemicals that need to be maintained within certain temperature bands.

 

r Food storage areas where temperatures require to be maintained below certain temperatures.

 

r Electrical plant or computer rooms can be temperature controlled without the need for air-conditioning plant to prevent overheating.

 

r Hospitals and operation theaters where specific comfort conditions are required to be maintained for the well being of patients can reduce the need for air-conditioning plant equipment.

 

Figure 10.0 Summary of PCM Devices

 

Product

Dimensions

Charging Heat Source

Applications

       

Sol-Ar-Tile

2.96 x 5.99 x 0.032 m

Direct solar radiation

1. Floor construction within sun rooms and conservatories

     

2. Bench tops and window seats.

Calortherm Tubes

1.0 x 0.05 m diameter

Low temperature hot water

1. To be used within a heat storage tank for charging by low temperature hot water.

Cristopia Plastic Spheres

0.077 m diameter

Low temperature hot water

1. To be used within a heat storage tank for charging by low temperature hot water

Pennwalt PCM Pellets

0.063 m diameter

Radiated heat from adjoining construction materials

1. Impregnation into a wide range of construction materials including concrete floors & ceilings, block work, insulation’s.

Rodwall Panels

2.08 x 1.07 x 0.02 m

Direct Solar radiation

1. Installation within external brick cavity wall construction.

2. Installation within external timber framed facade.

3. Installation within roof construction.

 

 

 

Product

Dimensions

Charging Heat Source

Applications

       

O.E.M

1.5 x 1.5 x 2.0 m

Hydrocarbon Oil

1. Providing heat energy storage for domestic heating applications.

Tes Term AC TH30 PCM

1.1 x .225 x 0.025 m

Under floor heating coils

 
     

1 Providing heat energy storage for domestic heating applications.

Wax Impregnated Wallboard

2.44 x 1.22 x 0.0125 m

Warm Air

1. Heat Storage medium for domestic applications.

2. Maintaining confort conditions within domestic and other building types.

Enerphase Panels

0.55 x 0.35 x 0.055 m

Direct Solar radiation

1. Installation within external brick cavity wall construction.

2. Installation within external timber framed facade.

3. Installation within roof construction.

Contact Addresses :-

 

 

 

Reference to chapter

1. Kohler, J. and Lewes, D.,(1983),Phase Change Products for passive homes, Solar Age Magazine, P.65,

2. Calortherm products, Calor Alternative Energy, 14 Johns Road, Meopham, Kent.

3. Dr.G.A.Lane,(1983), Solar Heat Storage, Latent Heat Materials,

U.S.A CRC Press, Volume II, pp136.

4. Dr.G.A.Lane,(1983), Solar Heat Storage, Latent Heat Materials, U.S.A CRC Press, Volume II,pp138.

5. Enerphase, Dow Chemical Research Co, Central Research Center, 1776 Building, Midland,U.S.A

6. Dr.G.A.Lane,(1983), Solar Heat Storage, Latent Heat Materials, U.S.A CRC Press,Volume II,pp148.

7. Kedl, R.J, Stovall, T.K,(1989), Activities to support of the Wax- Impregnated Wallboard Concept, Oak Ridge National Laboratory, Tennessee

8. Stetiu,C, Feustel, H.E,(1996), Phase Change Wallboard as an alternative to compressor cooling in Californian residences, Lawrence Berkeley National Laboratory, University of California.

9. Stetiu,C, Feustel, H.E,(1995), Simplified Numerical Description of Latent Storage Characteristics for Phase Change Wallboard, Lawrence Berkeley National Laboratory, University of California,pp4

10. Stetiu,C, Feustel, H.E,(1995), Simplified Numerical Description of Latent Storage Characteristics for Phase Change Wallboard, Lawrence Berkeley National Laboratory, University of California,pp5.

11. Stetiu,C, Feustel, H.E,(1996), Phase Change Wallboard as an alternative to compressor cooling in Californian residences, Lawrence Berkeley National Laboratory, University of California,pp3.

12. Dr.J.J.Tomlinson ,(1992), What are the benefits of including latent storage in common wallboard, Conf.920801/7, Oak Ridge National Laboratory, Florida, U.S.A, pp4.

13. Stetiu,C, Feustel, H.E,(1995), Simplified Numerical Description of Latent Storage Characteristics for Phase Change Wallboard, Lawrence Berkeley National Laboratory, University of California,pp6.

14. Stetiu,C, Feustel, H.E,(1996), Phase Change Wallboard as an alternative to compressor cooling in Californian residences, Lawrence Berkeley National Laboratory, University of California,pp9.

15. Stetiu,C, Feustel, H.E,(1995), Simplified Numerical Description of Latent Storage Characteristics for Phase Change Wallboard, Lawrence Berkeley National Laboratory, University of California,pp4.

 

 

 

6.00 Phase Change Thermal Storage Investigation

 

6.1 Introduction

 

This chapter details the results of an investigation for the use of a Phase Change Thermal Energy Storage System to store both conventional and renewable energy for supplying heating to a typical domestic house.

 

The aims of the numerical investigation is to provide the reader with data illustrating the potential use of Phase Change Thermal Storage when applied to a number of forms of heating. The results from the investigation will provide clear evidence of both fossil fuel and cost savings by the application of simple PCM Thermal Heat Energy Storage design solutions currently available on the open market.

 

The investigation is based on a numerical analysis incorporating published data and calculation procedures detailed within the Chartered Institute of Building Service Engineers (CIBSE) guide Code A & C together with formulae and figures provided by trade manufacturers. All calculations and trade literature used within the calculation procedures can be found within the appendices of this study.

 

The investigation will detail the results for a chosen phase change thermal storage system storing heat energy from the following heating designs:-

 

Case Study 1 - Off-peak Electric under floor heating design.

 

Case Study 2 - Active Solar collection panels coupled to a under floor wet heating system .

 

Case Study 3 - Photo voltaic Solar Collector Cells coupled to an electric under floor heating system.

 

6.2 Standard Domestic Model

 

The domestic model used within the numerical analysis has been based on the British Research Establishments (BRE) standard house, consisting of an average sized detached house in the United Kingdom.

 

The chosen domestic model comprises of a two storey 3 bedroom house

orientated on a North-South axis with a total gross internal floor areas of 102 m2.

 

6.3 Construction

 

The domestic model used for the numerical analysis has been based on a well insulated property similar to that of a low energy house design. It was considered that the simulated results shown should represent that of a house design that is energy conscious, conserving both fuel and power supply.

 

Secondly, through the Authors research into conservation of fuel and power, it was evident that the current regulations and standards of house construction is due to change considerably during the early part of the next century to enable governments to abide with the CO2 omission reductions laid out by the European Commission on Global Warming.

 

The construction comprises the following:-

 

Roof:- Floor:-

13mm plasterboard ceiling finish. 20mm Quarry Tiles

300mm Mineral fiber insulation 75mm sand :cement Screed

Roofing felt underlay 100mm Thermal insulation batts.

Plain clay tiles 25mm Tes TH30 System

150mm concrete base

300 mm Hardcore

 

Walls:- Windows:-

102.5mm clay brick work outer leaf. Double glazed low-E glass

50mm cavity.

75mm Wallmate Insulation.

150mm concrete block work.

13mm Plaster wall finish.

 

6.4 U Value Calculations

 

The U Values calculated for the domestic model comprise the following:-

 

q Roof 0.11 W/m2 K

q Walls 0.23 W/m2 K

q Floor 0.29 W/m2 K

q Windows 1.9 W/m2 K

 

Figure 1.0 shows cross section through construction.

 

Please refer to Appendix B for U Value calculations

 

6.5 Heat Distribution System

 

The proposed heating system will comprise of an under floor warming system comprising of a series of flexible heating coils laid within the Term-AC TH30 Phase Change Thermal Storage System.

 

6.6 Phase Change Thermal Storage System

 

The Tes Systems Term-AC TH30 Phase Change Thermal Storage System has been selected for the energy storage medium from the radiant heat coils located in the under floor heating system. The TH30 system will be incorporated within the concrete floor construction on both storey levels to allow optimum heat energy storage capacity. See figure 2.0 for typical construction detail.

 

The PCM product comprises of a mineral salt composition encapsulated in conically shaped high density aluminum foiled back polypropylene casing, with a melting point around 29 degree centigrade. The capsules measuring 25mm in height and 225 x 1100mm length have capacity to store 1789 kJ/m2 heat energy per unit.

 

6.7 Assumptions made within Calculation Procedure

 

6.7.1 Casual Gains

 

The numerical analysis has not taken into consideration of heat gains from occupants, electrical equipment, solar gain or any other form of casual gain likely to be experienced within a typical dwelling.

 

6.7.2 Design Temperatures

 

The steady state heat loss calculations for the house located in the UK has been based on an outside design temperature of -1 Degree centigrade with inside ambient temperature of 21 degree centigrade.

 

The calculation of steady state heat loss for analysis within other counties has been based on outside design temperatures as published within the CIBSE Guide Code A.

 

6.7.3 Solar Radiation

 

All solar radiation figures for various countries have been based on published data found within CIBSE Guide Code A.

 

 

 

6.7.4 Considerations within Calculation Procedures

 

Tes Systems Term AC TH30 PCM formulas used within the calculation take into consideration that 60% of the heat source (heating coils) is absorbed into the PCM. The remaining 40% is radiated into the rooms space. Therefore it was necessary during the calculations procedure to determine the 40% radiated heat to be equal to that of the steady state heat requirement to maintain inside comfort conditions during charging.

 

6.8 Case Study 1

 

6.8.1 Introduction

 

Considerable research and development into the field of active solar collection is rapidly evolving due to issues related to the conservation of fuel and power and global warming. Therefore the author felt it may be of interest to carry out a series of calculations applying Phase Change Material Technology with a solar collector currently on the open market.

 

Therefore a series of calculations have been carried out using the Amcor 2000 series

solar collectors coupled with the Tes Term-AC TH30 PCM System based on the standard BRE domestic house type (See calculations in appendix C).

 

 

Due to the collectors efficiency being largely dependent upon the magnitude of solar radiation, the author felt it may be of interest to demonstrate the systems performance over a number of different latitudes to fully ascertain the systems energy saving benefits and commercial viability.

 

The countries were carefully selected with winter outside design temperatures below 10 degree centigrade with comparatively high solar radiation in comparison to UK published figures.

 

The countries considered where as follows:-

 

q United Kingdom

q U.S.A.

q Israel.

q Australia.

q China

q Spain

q New Zealand

6.8.2 Amcor Solar Collectors

 

The solar collector comprises of a grid of 13mm copper waterways with black chrome finished copper fin stripes welded to sides resulting in a solid black solar collecting surface integrating a series of copper pipes.

 

The absorber plates are covered by a 3.2mm single glazed pane of glass with approximately 90% transmitance. Aluminium foil and insulation backing is located behind the absorber plates to minimize heat loss and encased within an aluminium frame.

 

See Figure 3.0 & 4.0 for trade literature.

 

6.8.3 Heating System Design

 

During the winter heating period, times will exist when the solar collectors efficiency will fall during periods of prolonged overcast weather resulting in the need for a standby system to provide additional heating. Therefore for the purposes of this experiment, a conventional gas fired heating system will be considered to form the hybrid system. A typical installation of the hybrid system is illustrated in Figure 5.0.

 

6.8.4 Analysis of Results

 

The results for SE England demonstrates that although solar systems are more viable within warmer counties with comparatively higher solar irradiance, the proposed PCM thermal storage system coupled with solar collectors can still be successful within the United Kingdom (See Graph 1.0).

 

By incorporating Tes Systems Term AC TH30 PCM thermal storage system throughout both ground and first floor areas coupled with 8 Amcor solar collectors based on 10 hours charging, can provide 50% of all winter heating needs for a standard house in the South East of England.

 

By employing a hybrid system in the form of a back up gas fired heating system as shown in figure 5.0, provides a very viable option to reduce both fuel cost and reduce CO2 omissions. The quantity of 8 Amcor solar collectors will cover an area of approximately 20 m2. The proposed solar collectors may be mounted on a pergola or timber framed stand within the garden, within a sun room conservatory, or mounted on the main roof.

 

The calculations for SE England shown on Graph 1.0 illustrate that total heating requirements for a typical house type can be provided by the use of Tes PCM thermal storage system incorporating 17 Amcor solar collectors. This represents a saving in heating load of approximately 24,750 kWh per winter season.

 

Results for the same house in Eilat, Israel demonstrates the systems greater potential in areas that receive greater levels of solar irradiance. The calculations show that the PCM thermal storage system hold the capacity to store sufficient levels of heat energy to provided 50% of the heating demand using only 6 Amcor solar collectors (10,350 kWh).

 

An additional 6 Amcor solar collectors provides suitable energy requirements to meet the total heating demand for the property, with the PCM storing approximately 20,700 kWh over the winter heating season.

 

From analysis of the results, it can be seen that the Amcor solar collector coupled with the Tes Systems PCM thermal storage system works to the greatest efficiency in countries that receive high levels of solar irradiance with mild (above -0 degree centigrade) winter design temperatures. This is shown by comparing U.S.A data with New Zealand for example. The solar irradiance are similar to one another although the winter temperature differences reduce the efficiency of the system, resulting in U.S.A figures using a further 4 solar collectors due to higher heat losses.

 

The results demonstrate that the systems are extremely viable in countries including Israel, New Zealand and Australia where total heating demands can be achieved by 11 Amcor solar collectors with the PCM storing approximately 20,000 kWh heat energy during the winter season, incorporated within the floor construction. This represents a solar panel area space covering 27 m2. A hybrid system comprising of a

 

convention heating system coupled with 6 Amcor collectors with the PCM storing 10, 500 kWh heat energy can providing 50% of total heating demand.

 

The results of various countries demonstrates that although greater numbers of solar collectors are required in colder climates, the Tes Systems PCM holds the capacity to store sufficient levels of heat energy even in colder climates within the UK. Therefore this system is commercially feasible in a great number of countries providing significant cost and more importantly fossil fuel reductions attributing to a plausible alternative for supplying and storing renewable energy sources.

 

6.8.5 Installation Costings

 

A simple cost analysis for heating system installation for house in SE England has been carried out using figures provided by manufacturers trade literature as follows:-

 

q Amcor Solarion 2000 Model AM2131G Size 1.9 x 1.3 m. = £150 + VAT

q Tes Term AC TH30 PCM with under floor heating system = £30 +VAT/m2

q 200mm thickness concrete floor construction = £35 + VAT/m2

 

Item Total Cost

17 Amcor Solar Collectors £2,550

Tes Term AC TH30 PCM

with under floor heating system £3,060

200mm thickness concrete floor construction £3,570

 

Total materials cost (exc. Labour) = £9,180 +VAT (1997 Costs)

 

6.9 Case Study 2

 

6.9.1 Introduction

 

The Tes Systems Term AC TH30 PCM thermal storage product with its capacity to store heat for later use can be used to store off peak electricity during the night for heating during the remaining hours of the day. The system represents a considerable cost saving to conventional electricity fueled heating systems for domestic or other forms of buildings.

 

6.9.2 Heating System Design

 

The proposed heating system design for case study 2 can comprise of an electric under floor heating system. The floor construction can comprise a series of electric

 

heating coils laid between the Tes TH30 Term AC PCM system. The floor can then be overlaid with cement:sand screed finishing. The electric under floor heating system can be connected up to the 3 fase electricity mains supplying off peak electricity.

 

Electric heating systems as described above are commonly used within frail elderly homes, requiring the need for 24 hours heating, due to the potential hazards of gas fired heating systems. Additionally, electric heating is commonly used for houses located out of town due to the significant cost for connecting up mains gas supply. Therefore there is a considerable market for the use of such a latent heat thermal storage system fueled by off peak electricity.

 

6.9.3 Analysis of Results

 

The calculations reveal that the PCM thermal storage system provides a potential saving of 55% in cost due to the sole use of off peak electricity to provide heat for the remaining hours of the day.

 

The results from the calculations can be shown as follows (See appendix D):-

 

Heating System

Total Output

required for

winter season

Electricity Rate

(Pence/kWh)

Cost for winter

heating (£)

       

Term AC TH30 under

floor heating system

19,820

2.7

578

       

Term AC TH30 under

floor heating system

with temperature

control

16,409

2.7

443

       

Standard electric under

floor heating system

4,786

11,62

16,409

2.7

7.3

139.56

916.36

1,056

       

 

Further costs savings may be achieved by monitoring the temperature within the room due there being periods where the 40% radiated heat from the floor during the charge period may exceed heating requirements. Such periods may exist during milder periods or at times when the building is receiving high solar or other incidental gains.

 

With suitable temperature control, the PCM thermal storage system may provide a total saving in the region of 65% of all heating cost. For a standard house this may represent a saving in heating cost in the region of £600.00 per winter season.

 

6.9.4 Installation Costings

 

A simple cost analysis for the electric under floor heating system installation incorporating the Tes Systems PCM for a house in SE England as follows:-

 

q Tes Term AC TH30 PCM with under floor heating system = £30 +VAT/m2

q 200mm thickness concrete floor construction = £35 + VAT/m2

 

Item Total Cost

 

Tes Term AC TH30 PCM

with under floor heating system £3,060

200mm thickness concrete floor construction £3,570

Total materials cost (exc. Labour) = £6,630 +VAT (1997 Costs)

 

6.10 Case Study 3

 

6.10.1 Introduction

 

A set of calculations have been carried out to analyze the use of Photo Voltaic cells coupled with the Tes Systems Term AC TH30 PCM thermal storage system providing heating to a standard domestic house (See Appendix E).

 

6.10.2 Heating System Design

 

The PV solar collector chosen for the study consist of a product BP Solar systems manufacture as detailed within figure 6.0 & 7.0.

 

The unit measuring 0.53 x 1.118 m in length provides a peak power output of 75 watts and minimum output of 70 watts.

 

6.10.3 Analysis of Results

 

To supply any form of heating to a property, a significant number will have to be incorporated into the design as illustrated in graph 8.0.

 

Approximately 70 BP solar collectors will have to be coupled to the Tes Systems PCM thermal energy storage design to provide 50% of all winter heating requirements. This represents a solar collector surface area 40 m2.

 

The Tes systems Term AC TH30 PCM thermal storage system incorporating Approximately 100 BP solar collectors will supply 75% of all winter heating

for a standard sized house in SE England. To achieve this the Term AC TH30 PCM thermal storage system requires to store approximately 12,000 kWh of heat energy during the winter season with a solar collector surface area 60 m2.

 

Total heating requirements can be supplied by 140 BP solar collectors with total energy stored by the Term AC TH30 PCM in the region of 23,800 kWh The BP solar collectors will cover an area of 83 m2

 

6.9.4 Installation Costings

 

A simple cost analysis for using the BP solar charging the Tes Systems PCM for a house in SE England to provide 50% heating is as follows:-

 

q Tes Term AC TH30 PCM with under floor heating system = £30 +VAT/m2

q 200mm thickness concrete floor construction = £35 + VAT/m2

q BP Solar model BP275 = £423+VAT

 

Item Total Cost

BP Solar model BP275 £29,600

Tes Term AC TH30 PCM

with under floor heating system £3,060

200mm thickness concrete floor construction £3,570

Total materials cost (exc. Labour) = £36,230+VAT (1997 Costs)

 

 

 

7.00 Conclusion

 

7.1 Case Study 1

 

From the analysis it can be concluded that the use of amcor or any other similar form of solar collectors combined within a heating system incorporating Tes Systems PCM has considerable potential as a viable alternative renewable heating system.

 

The heating system proposal put forward in case study 1 must be considered as the most simple to operate and environmentally sustainable in comparison to the remaining case studies examined. The calculations have clearly shown that such a system holds the potential for applications not only within warmer climates receiving greater solar radiation during the winter season but also UK climates. The PCM and solar panels require minimal maintenance (cleaning solar panel glazing) throughout the design life therefore requiring little and running cost attention during.

 

One of the systems greatest advantages over say PV cells is the issue of design life. The heating system if operated correctly will remain operational throughout the design life without any need for replacement of any parts including the PCM. Therefore the PCM and solar collectors hold the potential for reuse after the design life of the house expires.

 

7.2 Case Study 2

 

The case study analysis for coupling off peak with PCM demonstrates a viable system with considerable cost benefits over existing electric heating systems. This system is particularly suited to large old age peoples homes that consume large amounts of electricity throughout the winter heating season that cost considerably. This system demonstrating cost reductions in the region of up to 65% shows clearly the potential use of PCM thermal energy storage.

 

7.3 Case Study 3

 

Results from the use of PV cells coupled with the PCM is considered to be an unfeasible alternative heating system design. PV cells clearly demonstrates that converting solar energy into electricity to supply heating results in massive energy losses. This has been proven within case study 1 concluding that solar collection for heating application is far more successful if this energy from the sun is collected directly.

 

The design life for PV cells ranges between 10-30 years on average, therefore a requirement exists to replace the PV cells several times throughout the houses design life of say 50 years resulting in massive cost increases.

 

Perhaps in later years with the increase development in the field of PV cells and technology as a whole, such a heating system proposed within case study 3 may become feasible.

 

7.4 Phase Change Materials

 

7.4.1 Paraffins

 

It can be concluded that Paraffins can be considered as a suitable PCM candidate for storing conventional or renewable free heat source. Scientific experiments and studies have proved the materials capacity to store large volumes of heat during phase change transition without resulting in super cooling over a number of freeze thaw cycles compared to many in-organic materials.

 

The organic material comes in various forms with a wide variety of melting points resulting in a material group which is very versatile allowing specific materials to be chosen with regard to there melting point temperatures.

 

The material being organic in nature is comparatively cheap compared with in-organic chemicals, therefore resulting in a cheaper material with high specific heat capacity.

 

It is considered that there is still a need for further research into the actual PCM storage system design due to the hazards of combustion and potential residue during liquid state.

 

The wax impregnated wallboard is considered to be an ingenious way of storing heat to reduce the air conditioning loading therefore conserving fuel and power. The system has a wide variety of applications as detailed within the main report with considerable potential use within the UK. The use of HDPE encapsulated has shown to be a more successful system as opposed to the direct immersing of the Paraffin wax. This is due to the greater control on the quantity of PCM impregnated and the proven containment of PCM using HDPE capsulant.

 

7.4.2 Fatty Acids

 

Unlike Paraffin’s , although within the same organic material group, the variety of suitable PCM with temperature melting bands suitable for heating applications is much smaller. The main two candidates within this group are Capric Acid [mpt. 31.5 degree centigrade] and Caprilic Aid [mpt. 16.5 degree centigrade]. Part of the reason for the limited use of these PCM is due to the problems of giving off bad smelling odor. However, with a suitable incapsulant this problem could be tackled, resulting in an in-expensive, chemically stable storage medium, ideal for storing heat for residential heating.

 

7.4.3 Salt Hydrates

 

Salt hydrates, part of the in-organic PCM chemical group must be considered as the most successful PCM, widely used in commercial thermal energy storage systems. In terms of heating domestic properties, salt hydrates provide the most wide range of materials within a temperature melting range of between 18.5 & 36.4 degree centigrade. These temperature bands provide a large scope to designers allowing the flexibility for using a number of heating system designs.

 

The problems of the materials ability to super cool resulting in eventual segregation and depleting heat capacity has been overcome as result of many years research carried out using various nucleating agents stabilizing the compound. Therefore, with correct design, salt hydrates provide one of the most versatile and flexible compounds for storing heat energy for domestic heating applications within the PCM field.

 

7.5 Commercial PCM Thermal Energy Storage Systems

 

From the appraisal of past PCM energy storage systems to present commercially available models, it can be concluded that there are a number of important factors to be taken into consideration during any design process.

 

One of the must important factors that was overlooked by many early PCM energy storage designs was the analysis of the material and its behavior over a number of freeze thaw cycles. The methods of material analysis carried out by Dow Chemical Group in Michigan, U.S.A, represented a logical, thorough examination of all potential PCM candidates by testing each material over 1,000’s of phase change cycles equivalent to twice the design life of the PCM storage system.

 

Another important consideration during the design process of PCM thermal energy storage systems is the testing of the encapsulant or outer casing holding the PCM in suspension. Numerous PCM thermal energy storage devices have failed on the commercial market due to corrosion or decomposition of the containment device.

 

From the research into various past and present PCM systems on the market, it can be concluded that the use of high density polyurethane [HDPE] coating is the most successful encapsulant used.

 

 

 

8.00 Recommendations for Further Research

 

From the research into commercial PCM thermal energy storage devices there is a considerable number of passive and active PCM solar collection systems available. An analysis can be carried out on a similar house design as selected in this study applying passive solar PCM thermal energy collectors. An investigation can be carried out on a range of these systems to compare efficiency and cost together with the compatibility into existing and new build construction . From research carried out by the author, there is currently a number of trial houses built in California using Dow Chemicals active PCM solar collection devices that are being monitored by the University of Delaware.

 

From research, there is considerable developments into experiments on Wax Impregnated wall board carried out by Oak Ridge National Laboratory, Tennessee and Los Alamos University, New Mexico. Current research is centered around controlling comfort temperatures within domestic homes in the United States to reduce the need of air conditioning plant. This same material can be applied to store heat energy within domestic homes in the UK.

 

Research into commercially available Calortherm PCM technology can be undertaken. Areas of research may include hot water storage by Calotherm thermal energy storage tubes or equally PCM thermal storage for domestic heating using Calortherm thermal storage tanks.

 

Further research may be undertaken on the Tes Systems TH30 PCM energy storage system incorporating a heat pump design or other free renewable energy resources such as wind turbine electricity generation to heat energy for domestic heating.

 

 

 

Bibliography

 

1. Association of Conservation of Energy Briefing notes (1994) No.13 Climate change.

2. Lane. G A, (1983), Volume 1, Latent Heat Storage: Background and Scientific Principals ,U.S.A CRC Press.

3. Lane. G.A,(1983), Solar Heat Storage, Latent Heat Materials, Volume II, U.S.A CRC Press

4. A. Abhat, (1983), University of Stuttgart, Low temperature latent heat thermal energy storage ,Solar Energy Journal, Vol.30, No.4.

5. Goulding. J.R,(1992) Energy Conscious Design, Commission of the European Communities.

6. Tomlinson.J,(1989), New Materials for Thermal Storage, Oak Ridge National Laboratory,Tenessee,HI-89-38 No.4.

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8. Ghoneim.A.A. & Klein.S.A, (1991), Phase Change Materials Analysis, Solar Energy, Vol.47, No.3.

9. Bromley.A.K.R & McKay.E.M,(1994) Incorporating Phase Change Materials into the Building Fabric, CIBSE Conference 1994.

10. Brandsetter.A & Kaneff.S,(1990),Materials and Systems for Phase Change Thermal Storage, Australian National University paper.

11. Schroder.J & Gawron.K,(1981),Latent Heat Storage, Philips GmbH Forschungslaboratotium Aachchen, West Germany, Energy Research Conference paper 5.

12. Neeper.A.D,(1989), Potential Benefits of Distributed PCM Thermal, American Solar Energy Society 1989 conference report, Los Alamos National Laboratory, New Mexico.

13. Marshall. R.H.,(1992), Phase Change Heat Energy Storage Materials, report 644/SEU 205, University of Cardiff, Wales.

14. Stovall.T.K,(1989), Activities to Support Wax-Impregnated Wall Board Concept, U.S. Department of Energy,Thermal Energy Storage Research Activities Review, New Orleans,Louisiana, March 17 ,1989.

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19. Calortherm products, Calor Alternative Energy, 14 Johns Road, Meopham, Kent.

20. Enerphase, Dow Chemical Research Co, Central Research Center, 1776 Building, Midland,U.S.A

21. Kedl, R.J, Stovall, T.K,(1989), Activities to support of the Wax-Impregnated Wallboard Concept, Oak Ridge National Laboratory, Tennessee

22. Stetiu,C, Feustel, H.E,(1996), Phase Change Wallboard as an alternative to compressor cooling in Californian residences, Lawrence Berkeley National Laboratory, University of California.

23. Stetiu,C, Feustel, H.E,(1995), Simplified Numerical Description of Latent Storage Characteristics for Phase Change Wallboard, Lawrence Berkeley National Laboratory, University of California.

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27. Lane G.A, (1976) Macroencapsulation of Heat Storage Phase Change Materials for use in Residential Buildings, ERDA Thermal Energy Storage Program Information Exchange Meeting, Cleveland,Ohio.

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29. Morrison,D.J, (1978) Effects of Phase Change Energy Storage on the Performance of Air and Liquid Based Solar Heating Systems.

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31. Misra A.K, (1987) Fluoride Salts and Container Material for Thermal Energy Storage Applications, NASA Lewis Central Research Center, Cleveland,OH.

32. Tomlinson J.J,(1985) Clathrates and Conjugating Binaries; New Materials for Thermal Storage, ASHREA Transaction HI-85-38 No.4.

33. Lane G.A, (1980) Low Temperature Heat Storage With Phase Change Materials, International Journal of Ambient Energy, Vol. 1, Number 3.

 

 

 

The Dow Chemical Company Shape Energy Resources

1776 Building 6530 Corporate Drive

Midland Indianapolis

Michigan Indiana

U.S.A U.S.A

Dr. George A. Lane Michael Farmer

Senior PCM Research Specialist Licensed PCM technology

Tel: 001-517-636-0292

Fax:001-517-638-9716 Fax: 001-317-261-5904

 

Calortherm PC Thermal Storage Lawrence Berkeley Laboratory

14 Johns Road University of California

Meopham Berkeley

Kent CA 94720

England U.S.A

DA13 OLP

Mr. Ken Harris Dr. Helmut E. Feustel

Licensed PCM technology PCM Research Specialist.

Tel: 01474-812949 Tel: 001-510-486400

 

Los Alamos National Laboratory Oak Ridge National Laboratory

Los Alamos Engineering Technical Division

New Mexico Oak Ridge

87545 Tennessee

Dr. Donald A. Neeper Dr. John J. Tomlinson

PCM Research Scientist PCM Research Scientist

Tel: 001-505-6675061 Tel: 001-423-4811182

 

 

Institut fur Kernenergetik und Australian National University

Energiesysteme Energy Research Centre

University of Stuttgart Canberra

Stuttgart Australia

Germany

Dr. A.Abhat Dr. A. Brandstetter

PCM Research Scientist PCM Research Specialist

Tel: 0049-711-1210 Tel: 0061-6-2495111

 

University of Cardiff Tes Systems Ltd.

Wales 

United Kingdom

Dr. R.H. Marshall Mr. Richard Ostin

PCM Research Scientist Licensed PCM Technology

Tel: 01222-874060 Tel: 01344-845540

Fax: 01344-805554

 

Amcor Ltd BP Solar

98 Yigal Alon street. Winch Road

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King’s Lynn

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