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.
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].
Most studies have shown that heat pump dehumidifiers are a promising option with excellent water and energy savings. This method is especially attractive for closed greenhouses, facilitating the control of CO2 and humidity levels [68], [69]. Han et al. [70], [80] compared dehumidification options using a heat pump dehumidifier, an air–to–air heat exchanger, and an exhaust fan system in a commercial tomato greenhouse in Saskatchewan, Canada and showed the heat pump system to have the lowest overall energy consumption. At the same time, it was the most expensive approach due to its high electricity consumption. Campen et al. [81] concluded that heat pump dehumidifiers are not economical, unless used for space heating too. Chantoiseau et al. [71] and Migeon et al. [72] observed no sign of plant diseases by using a heat pump dehumidifier with 4 W/m2 energy consumption and found that the energy consumption was 3–8 times less than in the case of dehumidification through natural ventilation, depending on the outdoor conditions. In another study, Arbel et al. [74] indicated that the heat pump system was capable of energy savings of about 80% compared to natural ventilation. De Zwart [75] proposed an internal heat pump dehumidification system. The greenhouse air is cooled to around 14°C to condense some of the water vapor; then, using a second heat exchanger, the air is heated back to its original temperature. The results revealed that the heat exchanger required 2.2 times the latent heat extraction as cooling power. In other words, on average, condensing 20 g/(hm2) of water vapor content requires 30 W/m2 for cooling and 16 W/m2 for reheating the air that is inevitably cooled down during condensation.
Controlled condensation on a cold surface by using heat pump systems offers a much larger energy-saving potential than heat recovery exchangers. Heat pumps limit the energy consumption by recycling the inside air instead of heating the cold outside air. They supply the energy retrieved from water vapor condensation back to the greenhouse. Consequently, the main advantage of such a device is to minimize energy losses by re-using the energy extracted through condensation. In addition, heat pumps are easy to set up, operate, and maintain, and their effectiveness in controlling the humidity is independent of the external air conditions, making them suitable for high humidity greenhouses in the heating season. The main challenge in using heat pumps to replace natural ventilation for dehumidification is their high electricity consumption. The economic and environmental advantage of this approach depends on the price and source of electricity.
In terms of energy, the heat pump system is the most efficient method for greenhouse dehumidification in cold climates, followed by heat recovery exchangers and desiccant systems and then conventional ventilation systems. Fig. 16(b) illustrates the operating cost of each method, assuming natural gas and electricity prices of $2/GJ and $0.12/kWh respectively. The desiccant-based dehumidification system has the lowest operating cost, followed by the air-to-air heat exchanger, ventilation, and the heat pump system. Although heat pump systems are the most energy-efficient option, their operating cost is the highest, making them unpopular for greenhouse dehumidification, especially in cold climates. It should be noted that the operating cost of heat pumps is highly dependent on the price and source of energy. It is concluded that desiccant-based systems and air-to-air heat exchangers are promising dehumidification systems in greenhouses, considering energy consumption and operating cost.
A hybrid of heat pump and desiccant systems can also increase the energy efficiency and cost-effectiveness of humidity control in greenhouses.
Controlled condensation on a cold surface is widely used in the horticulture industry, usually done by heat pumps and air-to-air heat exchangers. Using heat pumps, less than 50% of the total energy consumption is spent on dehumidification, i.e., the removal of latent heat. The rest must be spent on sensible cooling of the air to reach the saturation point. In order to reduce energy losses, the energy extracted from the condensation process can be used to re-heat the supply air. The relative cost advantage of heat pumps over natural ventilation for dehumidification purposes is directly dictated by the energy costs, i.e., electricity vs. natural gas. Air-to-air heat exchangers are another type of controlled condensation mechanism which is preferred in cold climates. Additional energy efficiency enhancements can be achieved using heat recovery systems that recover energy from the warm exhaust air, decreasing the net heating demand in the greenhouse.