Annual Progress Report

J R Gates

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

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

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

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

PCMs

Inorganic

Organic

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

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

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

Containerisation

Macroencapsulation - large storage cylinders or pouches

Microencapsulation - powders or capsules

Containerisation - Basic Requirements

Strength, flexibility, temperature resistance, UV stability, good thermal conductivity, compatibility and stability with PCM

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

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

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

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

Solar System Selection

Thermo-Siphon Indirect system Open loop draindown system Open loop drainback system

Thermo-siphon System

Indirect System

Open Loop Draindown System

Open Loop Drainback System

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

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

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

Suspended Timber Floor

Concrete Floor

Typical System Layout

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

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

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

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

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

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

Potential Problems

High thermal movement

Low stiffness

Flammability

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.

Future Work

Please see handout