What on earth is a heat pump?

There are two ways you can warm up a cold room. One is to create heat, as you would by running a traditional electric heater. The other is to move heat into the room from elsewhere, and the most efficient way to do that is by using a heat pump.

The principle

Understanding how a heat pump works means remembering two key things from your school physics lessons.

  1. Pressure affects the boiling point of a liquid.  Lowering the pressure means a fluid will turn into a gas at cooler temperatures, while raising the pressure means the fluid must be hotter before it can boil. This is why it’s easier to boil water on top of a mountain, where air pressure is low, than it is at sea level.
  2. A gas turning into a liquid will release heat energy and warm the environment, while a liquid turning into a gas does the opposite - sucking in heat energy and cooling the environment. (This is why sweating cools us down, as the liquid sweat evaporates!).

The combination of these two effects means that by controlling the pressure, we can make a liquid turn into a gas, or vice versa, whenever we want - warming or cooling the environment in the process.

How it works

A real-world heat pump is essentially a long circular tube where this process can take place. Part of the tube is inside the place you want to change the temperature of (heat it up or cool it down), and part is outside it. The tube is filled with a refrigerant, which can be any liquid that has the right blend of thermodynamic properties. Those properties will differ depending on the task, but commonly used refrigerants include ammonia, carbon dioxide, isobutane and hydrofluorocarbons.

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The refrigerant is circulated around the tube by a compressor, which it enters in a gaseous state at a lower pressure and temperature. The compressor increases the pressure of the gas. It then goes into the hot side of the tube where, thanks to the high pressure, it turns into a liquid, releasing heat energy into the environment in the process.

The refrigerant is then pushed through an expansion valve, which dramatically lowers the pressure. This drop in pressure begins the process of converting the refrigerant back into a gas, which is completed in the cool side of the tube. The energy it needs to do this is pulled out of the surrounding environment, cooling it. The cool, low-pressure refrigerant then passes back into the compressor, and the cycle starts again.

The hot side doesn’t even need to be that hot. When in liquid form, the refrigerant is so cold that it absorbs heat from its surroundings – even from air as cold as -15°C! By pumping the refrigerant round the system, the not-very-hot heat from the external environment can be converted into usable heat for the building or hot water system.

How to use it

There are lots of ways this system can be used. In a fridge, or air conditioning unit, heat is moved from inside to the outside. In a heating system, the reverse is true and heat energy is moved from outside to inside. It’s possible to build a system that’s reversible - releasing heat to the outdoors in the summer and capturing it in the winter.

Harvesting heat energy from the external environment, rather than creating it in place, makes heat pumps one of the most efficient forms of heating available. Putting just one unit of electrical energy in will produce up to five units of heat energy. It’s basically magic.


Photo: Kristoferb, cc