What’s a heat pump?
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, sun and water to useful heat. This transformation is done via the refrigerant cycle.
THE REFRIGERANT CYCLE
Let’s take the example of a ground source heat pump.
Evaporation
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.
Compression
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.
Condensation
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.
Expansion
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.
Air source heat pump
Air source heat pumps use the ambient energy in outside-air or exhaust-air for heating, cooling and preparation of hot water. They can be installed as compact units entirely inside or outside the house (so called mono-bloc). Split systems consist (Fig. 1) of one unit inside the building and one outside. Heat is commonly distributed inside the house by a hydronic distribution system or by air using fan coils or a ducted ventilation system. Recent technical developments allow for efficient use in almost all climatic regions.
Air source heat pump
Air source heat pumps use the ambient energy in outside-air or exhaust-air for heating, cooling and preparation of hot water. They can be installed as compact units entirely inside or outside the house (so called mono-bloc). Split systems consist (Fig. 1) of one unit inside the building and one outside. Heat is commonly distributed inside the house by a hydronic distribution system or by air using fan coils or a ducted ventilation system. Recent technical developments allow for efficient use in almost all climatic regions.
Water source heat pumps
Water source heat pumps use energy stored in ground, surface or sea water. Where ground water is easily available it is accessed by two drillings (Fig. 2). One is used as a water source, the second is used to reinject the water into the ground. The heat pump extracts heat from the water and makes it available for heating, cooling and preparation of hot water. Heat is commonly distributed inside the house by a hydronic distribution system or by air using fan coils or a ducted ventilation system. Water source heat pumps profit from particularly high efficiency due to excellent temperature characteristics of water as energy carrier.
Fig 2: Water-Water heat pump system (open loop)
Ground source heat pumps
Ground source heat pumps use energy stored in the ground for heating, cooling and preparation of hot water. They extract heat from the ground either by a vertical (Fig. 3) or horizontal collector (Fig. 4). Heat is commonly distributed by a hydronic distribution system or by air. Ground source heat pumps can be operated efficiently by employing the consistent temperature level of the ground.
Fig 3: Ground coupled heat pumps system (vertical drilling)
Fig. 4: Ground coupled system (horizontal collector). Same principle is used for direct expansion systems (DX)