Controlled condensation is typically achieved through heat pumps where an electrically-driven refrigeration cycle removes water vapor from the air. There is excellent potential in employing a heat pump system for greenhouse air-conditioning based on its ability to perform the functions of heating, cooling, and dehumidification. The energy extracted during condensation can be re-used to reduce the net energy consumption. High energy saving potentials of recirculating the heat absorbed by the heat pump dehumidifier back to the greenhouse.
The heat pump operates in a closed cycle with a refrigerant. The refrigerant in the evaporator is at temperatures below the dew point of the air stream. As the humid air from the greenhouse passes through the evaporator, the temperature drops below the dew point and the moisture in the air undergoes a phase change. As a result, the air becomes dryer and colder. In the next step, the air passes through the condenser, absorbs heat, and as a result, becomes warmer. Latent heat released during moisture condensation is used as additional sensible heat for the greenhouse.
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The efficiency of a heat pump can be expressed in terms of the coefficient of performance (COP) (link), determined by dividing the desirable effect of the heat pump by the electrical power needed to run the heat pump The higher the COP, the more efficient the heat pump.
Although dehumidification using heat pumps is very useful, it is also energy-intensive. For example, for a greenhouse at 22°C and 80% RH, cooled to 5°C and 100% RH, the absorbed sensible and latent heats are nearly equal. Thus, only 50% of the power consumption of a heat pump goes toward dehumidification; the rest results in a cooling effect,which is not desirable, but unavoidable [65].
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The moisture content of humid air is removed by condensing it to liquid water by exposing it to a cold surface with a temperature lower than the dew point of the greenhouse air. The active dehumidification in the tool creates the cold surface by using a heat pump.
The greenhouse air is passed by forced ventilation through the dehumidification units - here, it first passes through the cooling unit and condenses - the water is recovered. The latent heat of condensation is regained by a heat pump, and the colder, drier greenhouse air is then passed through a hot surface to heat it back to the initial greenhouse temperature. The warmed, dry air is channeled back to the greenhouse.
The heat recovered can be used to immediately heat the dehumidified air, or stored, for instance in an aquifer or other forms of heat storage.
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The dehumidification capacity decreases rapidly at greenhouse air temperatures below 15°C and at low greenhouse humidity levels. Dehumidification capacity can be increased by increasing the heat exchange surface area per m2 of the greenhouse, though it could be an expensive measure.
Contribution to energy-efficiency and resource-use:
Mechanical dehumidification removes moisture from the air, and the condensed water can be recovered.
As the greenhouse air is inevitably cooled down to condense the moisture, energy is required to heat it up back to the initial greenhouse temperature, especially in cold climates/periods. Recovering and using the latent heat of water condensation for re-heating can reduce the energy use, though it comes with a significant consumption of electrical power.
Mechanical dehumidification reduces the dependence on external climate conditions, hence the greenhouse can be closed, conserving CO2.
Steps towards sustainable greenhouses:
The high electricity demand for mechanical dehumidifiers can be sourced from renewable sources.
Mechanical dehumidification facilitates closing of the greenhouse - this means that further energy savings can be achieved in colder climates/during colder periods by increased greenhouse insulation and increased use of energy screens.
A study compared different types of dehumidification in greenhouses and found the annual energy savings in a typical Dutch tomato greenhouse to be 225 MJ/m2 per year by using a cooling system to condense moisture and recover excess sensible and latent heat, and 250 MJ/m2 per year - by using a hygroscopic dehumidification system.