Dehumidification: hygroscopic

Hygroscopic dehumidification for greenhouses can be done using hygroscopic fluids or chemicals called ‘desiccants’, which adsorb water molecules from the greenhouse air. As the moist greenhouse air comes in contact with the hygroscopic material, the water vapour is adsorbed, releasing the latent heat of vaporisation. Thus, along with removal of moisture from the air, the latent heat is converted to sensible heat, which is released to the greenhouse.

Desiccant materials can be of two types: liquid (e.g. calcium chloride, lithium bromide and lithium chloride) and solid (e.g. zeolite, titanium silicates, activated silica gel).

For liquid desiccants, moist air from the greenhouse is brought in contact with a concentrated desiccant solution to absorb the moisture - the liquid desiccant solution thus becomes diluted as more water is absorbed over time. To regenerate the concentrated desiccant solution, the water can be desorbed from the solution to an air stream using low-grade heat. Hygroscopic dehumidification using liquid desiccants can be implemented in the greenhouse using a pad-and-fan kind of system with hygroscopic brine solution (such as CaCl₂) running over the pad instead of water, which makes the air passing through the pad less humid. The liquid hygroscopic media are highly concentrated salt solutions (bromides, chlorides, etc) that have to be pumped between the absorbing surface and the re-conditioner situated outside the greenhouse. These liquids are expensive and may cause severe problems to the environment if a system failure or leakage occurs.

Solid desiccants are more environmentally-friendly, non-corrosive, non-flammable, and less expensive than liquid desiccants. They are not chemically reactive with moist air. Solid desiccant dehumidifiers can be implemented as a slowly rotating desiccant wheel consisting of an adsorption section (where mosit air moves through the desiccant to be dehumidified) and a desorption section (where a stream of heated air removes the adsorbed water from the desiccant). Solid-desiccant systems are relatively less-studied but promising alternatives for energy-efficient dehumidification of greenhouses.

Compared to mechanical dehumidification, hygroscopic dehumidification does not lose efficiency at low greenhouse temperatures and humidity. However, a significant amount of heat is needed for regeneration of the desiccant.

Contribution to energy balance and resource use of greenhouses:

On the one hand, the hygroscopic dehumidification process reduces the need for ventilation in the greenhouse while also converting the latent heat to sensible heat and releasing it to the greenhouse air - this can reduce the heating demand in the greenhouse. On the other hand, the desiccant will need to be recharged periodically, which demands a large energy input for regeneration.

The reduced need for ventilation leads to a smaller loss of CO₂.

Possible steps towards sustainable, energy-efficient greenhouses

The high heat requirement for the regeneration of the desiccant is a huge limitation in implementing hygroscopic dehumidification - its viability could be increased by using solar heat or industrial heat.