How Does a Heat Pump Work?
The concept of a heat pump is very broad and there are different ways to get hot from cold. A heat pump reverses the natural thermal energy flow of a difference in energy, to entropy.
Here is a great video by Ben Krasnow where he builds a heat pump by stretching elastic bands.
Heat pumps for generating domestic space heating and hot water use the vapour compression cycle. They work in the same way as the refrigerator in your kitchen or the air conditioning in your car. So what we are really asking is how does the vapour compression cycle work?
Physical Principles
Firstly, there are a few of physical phenomena that we need to understand:
The relationship between pressure and temperature is directly proportional, as pressure increases, so does temperature. For example, the handheld pump gets hot when you put air into your bicycle tyres, because you are doing work to increase the pressure inside the chamber. Alternatively, when you spray a deodorant can, it will get cold. You have released some gas, the pressure inside the can has decreased and therefore so has its temperature.
Latent heat is the energy required to change the phase of a refrigerant, without changing its temperature. For example, when you heat water in the kettle, energy initially goes into specific heat, which increases the temperature of the water. Once the water starts boiling, the temperature of the water stays the same, but the kettle is still putting energy into the water. This is latent heat; the energy is going into changing the water from a liquid into a gas.
Boiling point and pressure are directly proportional. Here is a video demonstrating that water can boil at room temperature when its pressure is reduced. You can control the temperature at which liquid evaporates or a gas condenses by manipulating its pressure.
Components of the Heat Pump
There are 5 main parts to a heat pump; the compressor, condenser, expansion valve, evaporator and the refrigerant.
The compressor increases the pressure of the refrigerant. This is very important because as the refrigerants’ pressure increases, so does its temperature, and its boiling point (saturation temperature).
The condenser is a heat exchanger that takes refrigerant as a gas, and removes energy so it condenses into a liquid. As the refrigerant changes from a gas to a liquid, it gives off its latent heat energy.
The expansion valve is just a small hole that creates a pressure drop. As the refrigerant goes through, its pressure decreases, as does its saturation temperature (boiling temperature).
The evaporator is a heat exchanger that takes refrigerant as a liquid, and evaporates it to a gas. The refrigerant has to absorb ambient heat in order to boil.
The refrigerant is a volatile fluid that the heat pump constantly cycles between liquid and gas. The heat pump moves heat from the evaporator to the condenser by storing it via the latent potential of the refrigerant.
How Does the Heat Pump Work?
The heat pump manipulates the boiling point of refrigerant to make it collect and reject latent heat at different points. The energy is stored in the refrigerant as latent heat.
The refrigerant enters the compressor at a low pressure, but with high enthalpy (energy). Because the refrigerant is a gas, it has lots of energy stored as latent heat. The compressor then increases the pressure of the refrigerant. Because its pressure increases so does its temperature and its saturation temperature (fancy word for boiling temperature).
The refrigerant now enters the condenser, which for air/ground to water heat pumps is where the heating system water is heated. The refrigerant is a gas so it has lots of latent energy, its pressure is high and its temperature is higher than the water in the plate heat exchanger. So naturally the heat is transferred from the refrigerant to the water. Now from the perspective of the refrigerant, the water is cold, so like on your car window in the winter, the refrigerant condenses into a liquid, and so ‘rejects’ its latent heat into the water. The heating system water goes off to your radiators and underfloor heating.
After the condenser the refrigerant is now at a high pressure and a liquid which means it has low enthalpy (energy). The next stage is the expansion valve, where the pressure of the refrigerant drops. Like when you spray a deodorant can, the pressure of the refrigerant drops, as does its temperature, and the saturation temperature (meaning it can now boil at a low temperature). The refrigerant is now in a state of low pressure, low temperature, and low enthalpy (energy).
The next stop is the evaporator. For an air source heat pump, this is the blue coil on the back of the outdoor unit, or the brine side plate heat exchanger in a ground/water source heat pump. This is where the heat is collected from the environment, and where the free energy comes from.
For an air source heat pump, imagine the outside air is -5ºC. The refrigerant enters the evaporator at -15ºC and because its pressure is low, it is close to its boiling point. Also the outside air is hot from the refrigerant’s perspective. The refrigerant starts to boil, absorbing latent heat in the process. Once the refrigerant has boiled, it leaves the evaporator at a low pressure but as a gas, which means it once again has lots of energy stored as latent heat.
The outside air goes into the evaporator at -5ºC and leaves at -15ºC. The heat pump has stripped 10º worth of heat out of the air. The refrigerant leaves the evaporator and goes on to the compressor and the cycle is repeated.
All the heat pump is doing is moving heat that already exists in the environment from one place to another. The evaporator always collects heat, and the condenser always rejects heat. A 4 way reversing valve is a component that controls the direction of refrigerant flow through the system. For an air-air heat pump (aka air conditioning), the indoor unit can be either the evaporator, or the condenser, depending on what position the 4 way valve is in. This means that heat pumps can all do heating and cooling. All heat pumps used in the UK have to have the 4 way valve inside so that they can reverse to complete an evaporator defrost. However, if your system is to comply with the MCS, it has to be set up as heating only. But non-MCS instals can take advantage of the 4 way valve and can provide space heating and cooling in your home.