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What is a heat-pump?

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If you have ever seen an air-conditioning unit stuck to the outside of a building, that is a heat-pump. Its moving heat from inside and making the outside hotter - it's pumping heat. Most modern units do more than that though, if its cold outside they can work the other way around and pump heat from outside (making outside colder) to inside (making inside warmer). Actually it's exactly the same thing it's just we humans sometimes have a little difficulty getting our heads around it!

We have to think of heat as being like any other commodity that can be easily moved around. If you are still not happy about the idea, think of what happens when you put some warm food in the fridge. The temperature of the food goes down as heat is extracted from it and then the fridge gets rid of the heat through the pipes at the back. The important thing to notice is the pipes are far hotter than the food was when it went into the fridge. The heat has been moved from a lower temperature to a higher temperature by the fridge's cooling system.

Now, because of the laws of physics (please take this on trust), it only requires a small amount of work to move a much larger amount of heat. That means we can actually make a profit and every 1kW of electricity we put into the fridge or heat-pump we get 3kW back. If that sounds too good to be true please look at websites that explain 'coefficient of performance' (COP) for fridges, freezers and heat-pumps. Another way of looking at it is to compare a heat-pump with a car engine. We are perfectly happy to accept that for every 3kW of heat we put into an engine we only get 1kW of power to the wheels (car engines are around 35% efficient). Now, physicists like to think of a heat-pump as a car engine running backwards so, for every 1kW we use turning the heat-pump-engine we get 3kW of heat out, it's just the reverse process. No physical laws have been broken by this but if you are still wrestling with the notion, have a look at the subject of Thermodynamics and the Carnot Cycle.

Previously I have said that electricity should not be used for heating but, if it is, I have to concede that this is one of the more environmentally friendly ways to do it. If, however, you could use a windmill or similar to drive the heat-pump, that really would be a great idea.