Heating/Cooling: Heat Pumps

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.

Open

A schematic showing the heat pump cycle with the 4 steps

There are different greenhouse applications for heat pumps:

• Heating: here a heat pump is used to upgrade low-grade (low temperature) heat to high-grade (high temperature) heat to be able to heat the greenhouse with a high temperature heating network. In this way low grade energy from e.g. a geothermal source, outside air, an aquifer, solar applications or residual industrial heat can be used for greenhouse heating.

• Cooling: Instead of heating, the cold side of the heat pump can be used to cool the greenhouse. The extracted heat from the greenhouse can be reused for periods with a heat demand by storing it in the warm side of an aquifer (for use in winter) or returning it to the geothermal source, or dicharged to the outside air.

• Dehumidification: the cold surface of the heat pump can be used do dehumificat the greenhouse air. The humid air cools down and the moisture condenses when the temperature drops below the dewpoint. If the temperature of the dehumidified and cooled air drops below the heating setpoints, it must be heated before releasing it back to the greenhouse.

The efficiency of a heat pump in heating/cooling is measured by its Coefficient of Performance (COP):

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.