Phase change cooling

A phase change material (PCM) is one which changes its state from solid to liquid when subjected to heating and vice versa when cooled. Water is the obvious example. When it changes from a solid (ice) to a liquid it absorbs large amounts of heat before it shows any increase in temperature. A PCM which changes its state at temperatures around the range for thermal comfort is ideal for moderating temperature in buildings. One such material is sodium sulphate and its variant, Glauber's salt, which is a decahydrate of sodium sulphate. Glauber was

Figure 3.4 Phase change cooling. Fan draws warm air from the room which passes over phase change tanks cooling in the process as PCM changes from solid to liquid

a German Chemist born in 1604, yet another case of an early discovery waiting centuries to find its true 'vocation'. It changes from a solid to a liquid at around 28°C, absorbing large amounts of heat and thus cooling the air in its vicinity. The reverse operation creates heat when the PCM returns to the solid.

A variation on the theme of phase change cooling has been produced by researchers at Nottingham University Institute of Building Technology. The underlying principle still uses a chemical heat sink to soak up the heat in the air and pump cool air into a building. It is a highly energy-efficient system, using only a fraction of the energy consumed by conventional air conditioning. It is a system particularly suited to temperate climes where a few degrees of cooling can achieve comfort temperature.

The system invented by David Etheridge and David Rae draws daytime warm external air by fan over an array of fluid filled heat pipes. The pipes conduct heat to storage modules containing a solid PCM. The PCMs located in the ceiling void absorb the heat as they slowly melt during the day, providing cool ventilation air (see Fig. 3.4).

During the night the opposite occurs. Shutters to the outside air are opened and the fan reverses direction to draw the cool air over the PCMs, causing the material to solidify. The heat generated in the process is dumped outside the building (see Fig. 3.5).

The whole operation works on the principle that latent heat is stored in the PCMs. Their temperature hardly changes throughout the cycle since it is their latent heat capacity which brings about the change in air temperature.

The system is capable of being fine-tuned to suit specific circumstances by adding chemicals which change the melting point of the PCM.

The researchers claim that this method of cooling is more agreeable to the occupants than conventional air conditioning, which often produces zones of excessive cooling whilst, at the same time, excluding the possibility of receiving natural ventilation via open windows. The Nottingham system is not affected by being supplemented by extra natural ventilation.

Perhaps the greatest virtue of the system in the context of the Kyoto Protocol is that its energy costs are merely one sixteenth those of conventional air conditioning with obvious CO2 emission benefits.

Nighttime operation

Figure 3.5 Phase change cooling. At night the fan reverses direction and the external vents open. Cool air is drawn over the PCM which cools and solidifies

Nighttime operation

Figure 3.5 Phase change cooling. At night the fan reverses direction and the external vents open. Cool air is drawn over the PCM which cools and solidifies

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