Comparative review of membrane-based desalination technologies for energy-efficient regeneration in liquid desiccant air conditioning of greenhouses

Liquid desiccant air conditioning (LDAC) is an emerging technology able to maintain optimal growing conditions in self-sustained greenhouses powered by solar energy. However, the regeneration of the liquid desiccant (LD) is a bottleneck in LDAC. This study investigates six desalination technologies - membrane distillation (MD), reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), thermoresponsive (TR) solutions and electrodialysis (ED) - that may be employed for LD regeneration. The technologies are evaluated and compared based on criteria including achievable LD concentration, energy requirements, system efficiency, and availability of the technology. To date, only MD, RO and ED have been investigated for LDAC applications. These three technologies are not efficient for LDAC greenhouse applications. RO requires an applied pressure exceeding the maximum operating pressure of the membrane; ED requires multiple stages and second-stage desalination for adequate purification; and MD has high energy requirements. Energy efficiency of MD can be improved by employing feed temperatures >80 degrees C and using more selective solar collectors. Among the technologies that have not been tested for LD regeneration, Multistage NF and TR solutions have great potential for LDAC applications due to the expected high efficiency. High feed temperature MD, NF and TR solutions are in a preliminary stage and have been investigated only numerically, highlighting the possibility for future experimental studies.