1. Annual Progress Report
J R Gates

2. General Summary
Attendance at the ISES one day event in Brighton, WREC conference in Brighton and Sustainable Building 2000 in Maastricht
Paper published in the WREC conference proceedings and presentation given
Paper accepted for the CIB conference in Maastricht
Contact has been made with, University of Dayton, University of Salford, South Bank University, University of South Australia, University of Nottingham, Netherlands Energy Research Foundation and Rubitherm
PCM web page
Foreign exchange students

3. Solar Thermal Storage With Phase Change Materials
Aim of the investigation:
To measure the performance of a phase
change energy storage system for the storage
of solar energy in buildings

4. Objectives
1. To review the state of the art on the use of phase change materials (PCM) for energy storage
2. To establish requirements and criteria for system configuration
3. To identify suitable phase change materials
4. To develop a PCM storage system using the criteria established in (2)
5. To evaluate the the theoretical performance
6. To construct a model of the PCM storage system
7. To measure and evaluate the performance of the PCM storage system

5. Statement of Problem
Residential buildings account for 27% of the UK’s final energy consumption (DTi, 1999)
Utilisation of available solar energy can reduce this energy demand but availability is unpredictable and therefore a form of thermal storage is required

6. PCMs
Inorganic
Organic

7. Desired Properties
Low cost, non-flammable, non-toxic, chemically stable, high latent heat of fusion, high thermal conductivity, low changes in volume due during phase change, low vapour pressure, low containment cost

8. Inorganic PCMs
Advantages - high heat of fusion, good thermal conductivity, cheap and non- flammable
Disadvantages - Corrosive to most metals, supercooling, phase decomposition and suffer from loss of hydrate

9. Organic PCMs
Advantages - chemically stable, suffer little or no supercooling, non-corrosive, non-toxic, high heat of fusion and low vapour pressure
Disadvantages - low thermal conductivity, high changes in volume on phase change, flammability

10. Containerisation
Macroencapsulation - large storage cylinders or pouches
Microencapsulation - powders or capsules

11. Containerisation - Basic Requirements
Strength, flexibility, temperature resistance, UV stability, good thermal conductivity, compatibility and stability with PCM

12. Containerisation - Problems
Large containers make it difficult to extract heat as PCM becomes a self insulating material
Compressive strength reduced when combined with concrete
Flexible containers are damaged during thermal cycling due to changes in volume
Organic PCMs react with most plastics
Inorganic PCMs need airtight containers to reduce hydrate loss

13. Previous Research - Weaknesses
Lack of long term testing for both proposed systems and PCMs
Lack of experimental data on melting points and heat of fusion

14. Mathematical Modelling
Difficult to analyse the heat transfer of PCM as it is a moving boundary problem
A PCM may consist of several layers a solid region a liquid region and a mushy region. Some models ignore the mushy region

15. Solar Collectors
Flat plate - able to utilise both direct and diffuse radiation, easily maintained, easy to construct, but temperatures limited to approximately 100°C
Evacuated tube - able to perform even during overcast weather, self cleaning and radiation is the only heat loss mechanism at work, but high in cost and can reflect more sunlight than flat plate collector

16. Solar System Selection
Thermo-Siphon
Indirect system
Open loop draindown system
Open loop drainback system

17. Thermo-siphon System

18. Indirect System

19. Open Loop Draindown System

20. Open Loop Drainback System

21. System Criteria Design life of 25 years
All the components used should either be recycled or easily recyclable
Able to reduce energy use and CO2 emissions on an all year round basis
Able to meet a given percentage of space heating load in the winter and a given percentage of hot water demand in the summer
The PCM must be easy to remove from the building once it is decommissioned, whereupon it can either be recycled or disposed of without adversely affecting the environment

22. System Criteria Contd
The PCM must not pose a threat to the environment or the inhabitants
The system should require low maintenance with a high proportion of this being able to be completed by the homeowner to reduce costs in use
The system must be suitable for installation in both new build and retrofit applications
Back up heating must be provided by an alternative energy source other than grid produced electricity
Should be so far as practicable be modular to reduce construction costs

23. Proposed System Combi-system PCM filled panels
drainback system with a PV driven water pump
solar collectors
storage tank
back up heating provided by gas fired condensing boiler or wood stove

24. Suspended Timber Floor

25. Concrete Floor

26. Typical System Layout

27. Advantages Panels can be recycled when the building is decommissioned
Panels will be marked with a resin identification code making recycling easier
System can be installed in both suspended timber and concrete floors
Long thin panels with a large surface area will facilitate heat storage and removal

28. Advantages Contd
The use of a PV powered pump will cease parasitic pump losses, pump speed and flow rate will be automatically modulated resulting in higher collection efficiencies and reduction in energy use and better protection against overheating of collectors in power cuts
High collector efficiencies should be produced due to low flow temperatures needed
Similar advantages to those offered by underfloor heating

29. Advantages Contd
Drainback system does not require electronic freeze protection valves, there is no anti-freeze levels to check reducing maintenance, no double walled heat exchanger is needed increasing efficiency, water returns back to storage tank when pumping stops, heat loss reduced by supplying heated water straight to the top of the storage tank which also aids thermal stratification
Capable of reducing C02 emissions and energy use all year round
Not reliant on back up heating from off peak electricity

30. Potential Disadvantages
Fire load
Plastic manufacture reliant on petroleum and to a lesser extent coal production
Additional dead load imposed by the system
Long pipe runs may result in low flow temperatures
If a wood stove is used there can be problem of storage and particulate pollution

31. Plastic Panels - Design Criteria
Good thermal conductivity
Thin wall section to aid heat extraction
Should be long and thin in shape to aid charge and discharge of PCM
Transparency (for lab model only)
Melting point above the highest potential flow rate temperature
Constructed from recycled material or material easily recycled

32. Plastic Panels Contd Unaffected by PCM
Fire resistant or self extinguishing
Ability to accommodate volume change during phase transition of PCM
Joints must be able to accommodate the thermal movement of the plastic itself and changes in volume in the PCM at phase transition

33. Potential Problems
High thermal movement
Low stiffness
Flammability

34. Potential Solutions
Thermal movement - careful material selection and detailing
Flammability - PVCu burns with great difficulty and is self extinguishing. Polyphenylene sulphide is self extinguishing, but will burn if a flame is present, charring without dripping or fire spread. Can also use fire retardants or anti oxidants although some lead to increased smoke emissions.

35. Future Work
Please see handout