A heat pump functions a lot like a refrigerator: heat is transferred from the interior of a refrigerator to the outside via a refrigerant fluid - the same principle applies in a heat pump. However, heat pumps can be used for heating, cooling, and dehumidification. ‘Moving’ heat in this way is more energy-efficient than generating heat from scratch, such as by burning fossil fuels or biomass in boilers.
This happens in a heat pump via 4 steps, also shown in the illustration below:
Evaporation (of the refrigerant fluid): The heat from the source is transferred to the refrigerant fluid via heat exchangers. The heat is absorbed by the refrigerant fluid in the coil, and it evaporates.
Compression: The vaporized refrigerant is compressed, increasing its temperature and pressure - making it a high-energy, high-temperature gas.
Condensation: The hot, pressurized gas is condensed into a liquid and it releases heat – this is released to the desired ‘sink’ via heat exchangers.
Expansion: The now-liquid refrigerant is expanded, thus it evaporates. It is returned to the starting point, and the cycle is ready to repeat.
Depending on the desired function – heating or cooling – the greenhouse air can be a source or a sink for the heat moved by the pump.
Heating: Here the greenhouse is the heat sink. The heat sources can be: boiler or CHP(?), warmer outdoor air (in air-source heat pumps), geothermal heat stored in aquifers, solar thermal heat, or residual industrial heat.
Cooling: Here, the greenhouse is the heat source. Heat is released to heat sinks such as: cooler outdoor air, geothermal or underground aquifers (?).
Dehumidification: The humid air in the greenhouse goes into the heat pump and cools down as it transfers heat to the colder refrigerant fluid. The cooling causes the moisture content in the air to condense into water droplets, which can be collected, thus dehumidifying the air. The dehumidified, but also cooled air, must be warmed up to its initial temperature before releasing it back into the greenhouse.
The efficiency of a heat pump in heating/cooling is measured by its Coefficient of Performance (COP), which is the ratio:
[the heat energy supplied to the desired output (or) removed from the desired output]/[electrical input to the heat pump]
The efficiency can be affected by the type of heat pump and its capacity, and depends on the operating temperature level and the temperature difference between heat pump evaporator and condenser.
Contribution to energy balance and resource use of greenhouses:
The air conditioning capabilities of heat pumps can add or remove energy from the greenhouse as desired, in a controllable manner.
The possibility for controlled dehumidification reduces the requirement of natural ventilation for humidity control, and thus prevents undesirable release of CO2 to the outside air.
Possible steps towards sustainable, fossil-free greenhouses
By replacing heat-generation with heat-moving, heat pumps can significantly reduce the fossil-fuel usage in the greenhouse. Powering heat pumps with renewable and sustainable heat sources and sinks can lead to much more sustainable greenhouses.