On the possible techniques to cool the condenser of seawater greenhouses

Seawater desalination via air humdification–dehumidification has been recently proposed for incorporation into agricultural greenhouses to produce fresh water while maintaining suitable environment for crop growth in hot coastal regions. Dehumidification by cooling the moist (humidified) air is the key to a successful application of the proposed seawater greenhouse (SWGH) desalination. In this paper a theoretical study of possible cooling techniques for the condenser of SWGH is presented. Possible candidate techniques for cooling the condenser are to apply evaporative cooling for surface seawater, make use of a cooling machine to cool the condenser coolant in a closed loop, or to utilize deep seawater as a condenser coolant. Theoretical assessment of these techniques is conducted and the results are reported. Applying evaporative cooling reveals that there is a limitation on the relative humidity of the ambient air which depends on the ambient air dry-bulb temperature. Considering a 6 K temperature difference between the moist air and the evaporatively cooled surface seawater, for the heat transfer process in the condenser, the fresh water production starts at ambient air relative humidities below 54% for an ambient air dry-bulb temperature of 45°C. This limit on the ambient air relative humidity decreases to 48% and 40% for the ambient air dry-bulb temperatures of 35 and 25°C, respectively, at the same temperature difference for the condenser heat transfer process. A measure to permit applying evaporative cooling for a wider range of ambient relative humidities is to increase the temperature of air entering the condenser. A modification on SWGH design in order to realize this option has been recommended. In case deep seawater is used to cool the condenser, both the cooling capacities of cooling machines and the corresponding flow rates of deep seawater have been estimated for different condenser inlet and outlet temperatures.