What is a Heat Pump and how does a Heat Pump work?

Modified on Tue, 18 Jul, 2023 at 12:32 PM

A heat pump is a type of “transport device” that transfers thermal energy from one location to another. 

Heat pumps typically work like a refrigerator, in other words, the same technology is applied but the function is reversed. Heat pumps extract heat from a cold environment which can then be used for Heating and Hot Water preparation.

They extract stored solar heat from the environment – either the air (e.g. outside air) or ground, water (e.g. ground water)– and transfers this, along with the operating energy (electrical energy), in the form of heat to the heating and hot water system.

Heat cannot transfer from a cold body to a warm body on its own. Rather, it flows from a body with a higher temperature to a body with a lower temperature (Second Law of Thermodynamics). For this reason, the heat pump must raise the temperature of the thermal energy extracted from the environment using electrical energy to a level suitable for heating and DHW preparation.

It is important to first understand that if we boil a substance it is changing state from a liquid to a gas / vapour, and in that process it will absorb thermal energy. If we condense a gas / vapour back into a liquid, thermal energy is released in that process. 

Inside a heat pump we use a substance called a refrigerant. We can change the boiling point of that refrigerant by changing its pressure. If we compress that refrigerant, we raise its boiling point and if we expand the refrigerant to a lower pressure, we reduce the boiling point of that refrigerant. By lowering the boiling point of the refrigerant, we can make that refrigerant boil at low temperatures. When refrigerant is being boiled, it is converted from a liquid to a gas and in that process, it will absorb thermal energy. 

If we then raise the pressure of that refrigerant, by passing it through a compressor, we will raise the boiling point of that refrigerant. Refrigerant in a high pressure gas state will then be able to condense back into a liquid at a higher temperature. In the process of that state change from gas to liquid, thermal energy is released.

The animation below shows the refrigeration cycle, in this example extracting thermal energy from the ground, but the same principles apply to extracting heat from the air or from a body of water. This example uses brine (water + antifreeze) pumped underground (Point 1) absorbing the thermal energy of the ground into the brine solutuon. 

This is then pumped to a heat exchanger called an evaporator (Point 2). A heat exchanger allows water to pass one side of it, and refrigerant the other side, without the two mixing together, only allowing thermal energy to pass across. The thermal energy in the brine is able to boil the refrigerant held in the Evaporator, even at low temperatures such as 2C. 

The boiled refrigerant, now in a gas / vapour state, is then compressed by the compressor (Point 3) which raises the refrigerant boiling point. That high pressure gas is then sent through another heat exchanger, this time called the Condenser (Point 4).

The job of the Condenser is to allow that refrigerant in a gas state to condense back into a liquid and in the process, release thermal energy across the heat exchanger into the water flowing around it. That water is then circulated around the home to radiators or a hot water cylinder (Point 5).

The refrigerant inside the Condenser (point 4) which is now a liquid, is then sent to the Expansion Valve (Point 6). The job of the Expansion valve is to pass a small controlled amount of high pressure liquid refrigerant through a tiny hole and in the process lowering the pressure of the liquid refrigerant on the other side of the Expansion Valve. This is similar to when an aerosol can is used, the propellant is expelled through a tiny hole. Now the refrigerant is able to pass back through to the Evaporator (Point 2) and the Refrigeration Cycle repeats itself.

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