PHASE CHANGE MATERIALS – BACKGROUND INFORMATION

This post was initially written to provide background information for this article on the potential benefits of moving air conditioners to off peak power. (published by RenewEconomy on 21 Sept 2012) The article argues that that gradually moving air conditioners to off peak would allow most of the proposed power system upgrades to be deferred for a long time.  The use of phase change materials (PCMs) to store cold or heat would allow this conversion to be done without reducing people's comfort levels.

PCMs are finding a markets as a compact way of storing heat or cold.  This post provides background information on the characteristics and uses of PCMs.  

DEFINITIONS:

Phase:  Many materials can exist in more than one phase.  For example, water can exist in the solid, liquid or gas phase.  Some materials can also exist as different crystal phases.

Phase change: The change from one phase to another.  For example, the melting of ice to liquid water.  Phase changes can be driven by changes in temperature and/or pressure.

Phase change material (PCM): A mixture or pure substance that can change phase.  The term is often restricted to materials that change phase over a narrow temperature range.

Latent heat: The heat that has to be added (or taken out) in order for the phase change to take place. It is the high value of many latent heats that make PCMs so useful for storing heat and cold.  For example, one kWh of heat will melt 10.8 kg of ice or raise the temperature of the same weight of liquid water by about 80 deg C.

Heat flow: The movement of heat from one place to another. 

Thermal inertia:  If something needs a lot of heat (or cold) to change its temperature, it has high thermal inertia.  For example, a 10,000 litre tank of water has more thermal inertia than a 100 litre tank.  Traditional buildings in hot dry areas are often built with thick, heavy walls to give high thermal inertia so that the temperature inside the house doesn’t vary as much as it does outside.  Using PCMs to provide thermal inertia reduces building weight as well as concentrating the thermal inertia within the comfortable temperature range.  (Note that, for keeping a building cool, the PCM must get cold enough to freeze at some time during the day.)

Heat pump: Heat pumps are used in most refrigerators, air conditioners and some hot water systems to “pump” heat from one place to another. This Wikipedia diagram shows how a heat pump works:

Refrigerant gas is compressed in the compressor (4). The hot compressed gas then flows to the condenser (1) where it loses heat and condenses to liquid.  The liquid then flows through an expansion valve (2) to the evaporator (3) where it absorbs heat and returns as cool, low-pressure gas to the compressor.  In effect, heat is pumped from the evaporator to the condenser. The condenser can be used for heating and the evaporator for cooling.  Reverse cycle air conditioners can be used for heating or cooling.  The flow of refrigerant is reversed so that the condenser becomes the evaporator.

Heat pump COP (Coefficient of performance): The ratio between the heat pumped and the energy consumed by the pump.  COP drops as the difference between condenser and evaporator temperature increases. Doubling the temperature difference will approximately halve the COP.  The “COP” for a simple bar radiator is always 1.0 no matter what the temperatures are.  

DESIRABLE PROPERTIES OF PCMS:

Desirable properties include:

1.          The phase change occurs slightly below the desired temperature for cooling, above for heating.

2.          Phase change occurs within a narrow temperature range.

3.          Low cost.

4.          Stability.  Can be used indefinitely over a wide range of temperatures without deteriorating.

5.          Tasteless and odorless.

6.          Neither flammable nor toxic.

7.          Can be safely stored in a wide range of materials without causing corrosion problems. 

WHAT PCMS ARE COMMERCIALLY AVAILABLE?

A wide range of PCMs are commercially available.  For example, UK company PCM Products produces a range of PCMs based on the solid/liquid phase change.  Melting points range from -100 to 885 deg C. Their products include PCMs based on organic mixtures, salt mixtures, and salt solutions. PCM products also produces a range of PCMs based on the solid to solid phase change.

Development of commercial PCMs is difficult.  Most commercial PCMs don’t have all the desirable properties listed above.  Choices may involve compromises. 

The liquid/gas phase change is rarely used to store energy because of the volumes required to store the gas.  The exceptions are cases where it is environmentally and economically acceptable to discharge the gas to atmosphere.  For example, liquid nitrogen is commonly used to store cold when temperatures as low as -196 deg C are required.

Dry ice (solid CO₂) uses a solid/gas phase change to store cold at -78.5 deg C.

HOW MUCH DO PCMS COST?

The cost of PCMs varies considerably because of the different raw materials used in their manufacture.  Indicative data supplied by Australian manufacturer PCP Australia  (Jan 2012) suggests it would cost $70 to buy enough of their PC25 product to store one kWh of heat.  By contrast, the equivalent cost for their PC17 product would be about $150.  PC17 costs more because the mix contains bromides instead of chlorides. (PC25 melts at 25 deg C.  About 15 litres of either product is required to store one kWh of heat or cold.) 

APPLICATIONS:

Both the PCM Products and PCP Australia websites provide information on ways in which PCMs are used and how PCMs can be incorporated into various materials.  Smart Planet provides a wider view of the possibilities.

Example: Using PCMs to allow air conditioning to move to off peak power:

In countries such as Australia, peak power demand is driven by the very high power demand from air conditioners during very hot days. This problem could be avoided if the heat pumps on enough air conditioners were switched from “on-demand” power to some form of “controlled power” (Off-peak power.)  Switching to controlled power allows power suppliers to control demand by turning air conditioner heat pumps on and off. 

Switching air conditioners to controlled power would only be acceptable to the community if people’s comfort is not affected.  Using PCMs to store heat or cold while the heat pump is running would be one logical approach to protecting comfort.  PCM storage is attractive because relatively little space is required.  Approaches that might be considered include:

1.          Leaving existing air conditioners unchanged and installing PCM based products within the building rooms.  Products such Dupont Energain Thermal Mass Panels might be used to do this.  One disadvantage of this approach is that the panels will continue to heat/cool the building when this is not needed.  (Think of a house that is empty while the occupants are at work.)  Another disadvantage is that the building may need to be kept hotter or colder than desirable while heat/cold is being stored. OR

2.          Locating the heat/cold storage outside of the in an insulated storage tank.  The air conditioner system would be modified so that the heat pump pumps heat/cold to the storage tank.  Cold (or hot) water from the tank is then pumped to the rooms for temperature control.  This PCB Australia circuit shows one way that this could be done. The tank contains water and the PCM. The water in the tank would be physically separated from the PCM to avoid contamination. 

Apart from solving the peak power problem, the use of PCM storage in air conditioning systems saves money for the consumer by allowing:

1.          Low-cost off-peak power to be used to run the heat pump.

2.          Average power consumption to be reduced.  (The compressor would be scheduled to run at times of the day when outside temperatures are closer to the storage temperature.  (See “Heat pump COP” above re the benefits of doing this.)  This link gives average monthly maximum and minimum temperatures for various Australian locations and times of the year. The differences between maximum and minimum temperatures are sufficient to make the power savings significant. 

Solar hot water heating may provide a better source of heat than heat pumps in some locations.

DISCLAIMER:

John Davidson is not associated with any organization that profits from sale PCMs.