A heat pump is a device that can provide heating, cooling and sanitary hot water for residential, commercial and industrial applications.
It transforms energy from the air, ground and water to useful heat. This transformation is done via the refrigerant cycle.
Let's take the example of a ground source heat pump.
A heat pump always has an outdoor heat source and an indoor outlet. Outdoor sources can be ambient air, exhaust air, groundrock, groundwater, water etc. The energy from these sources is infinite and therefore renewable. This energy makes up about 75% of the energy that is used to drive the heat pump.
The fluid in the underground pipes absorbs the heat from the ground. The outdoor heat exchanger, the evaporator, uses the thermal energy from the outdoor source to boil the refrigerant (the liquid in the heat pump) and turns it into a gaseous state.
Key to understand this: The ground has a stable temperature of around 10-12°C throughout the year. This temperature is enough to heat the refrigerant because it has a very low boiling point. This means that it only needs a very low temperature to heat up.
Then, the refrigerant arrives at the heart of a heat pump: the compressor. The compressor compresses the refrigerant - which is in a gaseous state - to a high pressure, which leads to a rise in temperature.
Key to understand this: High pressure heats up gas (think of a bicycle pump that heats up when you are using (pumping) it)
To drive the compressor, additonal energy is needed: from electricity, gas or thermal energy. This makes up 25% of the total energy needed to run the heat pump. If green electricity is used - e.g. by means of photovoltaics - then a heat pump is using 100% renewables and therefore CO2 neutral.
On the discharge side of the compressor now hot and highly pressurised vapor passes through the second heat exchanger, called the condenser. This heat exchanger allows the refrigerant to release the heat into the heating system for the house (air blower, floor heating or radiators), as a result the refrigerant then condenses, i.e. the refrigerant moves from gaseous into liquid state.
The indoor outlet can be an air system (as the typical air conditioner units) or a hydronic (water-based) system, where the heat pump is connected to a floor-heating system or radiators. For the provision of sanitary hot water, the indoor unit (also) exists of a hot water storage tank of which the content can go up to several hundred litres.
The condensed refrigerant then passes through a pressure-lowering device, the expansion valve. The now low-pressure liquid refrigerant then enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils. From thereon the cycle starts again.