Concept design of a greenhouse cooling system using multi-stage nanofiltration for liquid desiccant regeneration

Liquid desiccant air conditioning (LDAC) has been proposed for greenhouse cooling. LDAC achieves lower temperatures than conventional evaporative cooling while also allowing closed air recirculation which has advantages over open ventilation. However, liquid desiccant regeneration is challenging and energy intensive. A novel multi-stage nanofiltration (NF) system is proposed as a regeneration method to enable both greenhouse cooling and irrigation in a closed water cycle. This concept study shows that, during winter and summer respectively, a 9-stage NF regenerator recovers 4.8 and 6.6 L of water per day per m2 of greenhouse floor area. Using commercial NF membranes with burst pressures of ∼ 40 bar, a 9-stage regenerator using CaCl2 could achieve a yearly COP of ∼ 12.4 and monthly greenhouse temperatures below 32 °C, almost independent of location. This improves greatly on LDAC using open evaporative regenerators, which only provides a COP of 0.3–0.5. Future research allowing NF membranes to operate with burst pressures of 55 bar could achieve the same COP by using just a 4-stage regenerator, making the system less expensive and more compact. Perspectives on the proposed design and implementation are discussed. This study shows the potential of the multi-stage NF regenerator to cool greenhouses in hot climates where it is difficult to produce crops during summer.