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Heat pump

A heat pump does what its name implies: it moves heat from one point to another. There are several physical effects used to create heat pumps. The three most common are gas compression, gas/liquid phase change, and the thermo-peltier effect. Refrigerators are the commonest domestic appliance making use of a heat pump. Refrigerators making use of all three types of heat pump can be found. Heating systems which make use of heat pumps are also available. Finally gas compression heat pumps using the Stirling cycle are commonly used to liquefy air in order to produce liquid nitrogen, oxygen, argon, etc. for industrial purposes.

In a Carnot heat engine, e.g. a steam engine, we are usually interested in producing work (e.g. electrical energy) out of heat. The Second law of thermodynamics makes that rather difficult and puts severe restrictions on the efficiency of such a process. Only part of the heat available, say, from burning fuel can be transformed into useful work, and the rest must be dumped in a cold reservoir (e.g. the river).

In a heat pump the desired product is heat. There is no restriction on transforming work (electricity) into heat. That means that in an electrical heater 100 joules (watt seconds) of electricity will give 100 joules of brand new heat. In a heat pump we can do more. We can use the electrical power to run the heat pump to move existing heat about.

A phase change heat pump employs a liquid with a low boiling point, once typically freon (CFC), now generally liquid ammonia, or occasionally the less corrosive propane or butane. This liquid requires energy (called latent heat) to evaporate, and it drains that energy from its surroundings in the form of heat (in the same way that sweating cools the body). When the vapour condenses again, it releases the energy, again in the form of heat.

First the liquid's pressure is lowered by an expansion valve on the side that is to cool down, forcing it to evaporate and extract heat from its surroundings. The gas is then pumped to the other side (the compressor) where it is compressed into a liquid, causing it to release its heat.

The result is that on the end of the pump where the heat is dumped, we get the heat that was pumped from one side to the other plus the amount of heat that corresponds to the electrical power we used to run the engine (100 joules per second).

How much heat we can pump depends on the difference in temperature between where we pump from (outside) and where we dump it (inside). The colder outside the less we can pump. If the pump is based on the phase change principle, in very cold weather the machine stops working when the outdoor part, the condenser, freezes. In these conditions a simple electric heater works better unless the phase change heat pump is replaced by a more suitable type such as a gas compression device.

When comparing the performance of heat pumps, it is best to avoid the word "efficiency", as it has many different meanings. The term "coefficient of performance" or COP is used to describe the ratio of heat output to electrical power consumption. A typical heat pump has a COP of about three, whereas an electric heater has a COP of just one.

Another advantage of heat pumps is that they can be operated in reverse to cool down the room. This makes them a useful component of air-conditioning systems.



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