Heat Pumps

Another technology becoming more and more mainstream that uses solar energy is that of the heat pump. Heat pumps exist for a wide variety of applications, and take their heat from various sources such as the ground, water, or the air.

Air source heat pump

Heat pump principles

Think of a fridge - this is the easiest explanation of what happens inside a heat pump. You put a beer in a fridge and after a couple of hours...nice cold beer. Where has the heat gone? Put your hand at the back of the fridge. It's warm. It is emitting heat, and a heat pump works just like this, but in reverse. The heat in the outdoor air, or in the ground is removed, compressed to take it to a higher temperature, and then delivered to the central heating system.

Within every heat pump lives a compressor. Remember pumping up your bike tyres with a hand pump? Result - the pump gets hot. Because of the action of compressing the air, heat is emitted - another good concept to bear in mind when describing how heat pumps work!

The refrigeration cycle

Heat pumps use refrigerant (in the same way as a fridge!). Some refrigerants (such as R22) have now been outlawed due to their environmental impact. Common refrigerants used  are R407C and R410A. These refrigerants have a low boiling point (R410A boils at -48.5 degrees C) So when the temperature outside is minus 5, these substances are still boiling!

Heat pumps use these properties to their advantage

Refrigeration cycle

1. The process starts with the refrigerant as a liquid, under no pressure. As it passes through the heat source (air/ground loop/lake), the refrigerant boils (as long as the source is warmwer than minus 48 degrees!!

2. The bike pump effect! By compressing this gas, it becomes hot.

3. This high temperature, high pressure gas then condenses as it passes through the colder transfer point (typically a plate heat exchanger connected to your heating circuit) and emerges as a liquid. Because the heat energy comes from transferring the refrigerant from a gas to liquid state, the liquid at this stage is still at high temperature.

4. The high pressure, high temperature liquid is passed through an expansion device, taking the pressure, and in turn the heat out of the liquid. (Think of an aerosol can, which expands it's contents - if you leave you're finger on there for a long time, the nozzle gets cold.) And so you see, the principle is a fairly simple one.

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