Annual Progress Report

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Annual Progress Report

• J R Gates

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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

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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

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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

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Statement of Problem

• Residential buildings account for 27 of the UKs
  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

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PCMs

• Inorganic
• Organic

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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

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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

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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

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Containerisation

• Macroencapsulation - large storage cylinders or
  pouches
• Microencapsulation - powders or capsules

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Containerisation - Basic Requirements

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

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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

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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

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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

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Solar Collectors

• Flat plate - able to utilise both direct and
  diffuse radiation, easily maintained, easy to
  construct, but temperatures limited to
  approximately 100C
• 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

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Solar System Selection

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

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Thermo-siphon System
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Indirect System
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Open Loop Draindown System
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Open Loop Drainback System
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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

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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

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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

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Suspended Timber Floor
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Concrete Floor
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Typical System Layout
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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

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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

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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

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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

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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

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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

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Potential Problems

• High thermal movement
• Low stiffness
• Flammability

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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.

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Future Work

• Please see handout