HOW DOES A HEAT PUMP WORK?

The most familiar form of heat pump is the domestic refrigerator. Here, heat is extracted from the cabinet to keep food fresh and the extracted heat is expelled through the radiator grill at the back of the unit. In this case the heat is merely a waste product. In the heat pump, we utilise this heat, and put the "cold part" outside.

To make this more understandable, imagine that the "ice box" of your refrigerator is fitted outside with a fan blowing over it, and the hot grid from the back is placed inside a house. The "ice box" will attempt to freeze the outside air and, if we stopped the fan from running, freezing of the unit would actually occur. But the passing air will constantly warm up the very cold "ice box". The temperature of the flowing air will actually be reduced very slightly.

So we are extracting heat from the air, heat which ends up as heat in the radiator grill, available to warm the house. In every case, the useful heat delivered to the house will be greater than the energy required to drive the heat pump itself. So we have extracted "free" heat from the air.

Here is an alternative simple way of looking at the principle;-

Let us consider an example of an opposite scenario:- If an electric kettle element was left outside and switched on, then the heat would be rapidly absorbed by the air. This would be a continuous loss of heat energy to the air. The heat would be dispersed and the air would hardly heat up.

Back to our heat pump, if, conversely, the element was colder than the air, then the world would be warming it up, we are therefore absorbing, hence gaining energy.

This principle can be hard to grasp, since it is not an immediately obvious process. The diagram below may help the penny drop.

Extracted heat (2.5kW) + Power input (1kW) = The useful heat output (3.5kW)

The details of the inner workings of a heat pump are not dealt with here but if you are intrigued .... they involve the evaporation and condesation of a working fluid called the Refrigerant, but you don't really need to know this for it to work!