A comprehensive review on humidifiers and dehumidifiers in solar and low-grade waste heat powered humidification-dehumidification desalination systems

Abstract

The solar and waste heat-powered humidification-dehumidification (HDH) desalination systems are gaining much significance owing to the alarming effects of global warming and water scarcity. Their performance depends on the efficiency of the individual humidification and dehumidification processes. To achieve maximum system efficiency, it is necessary to identify an effective combination of humidifiers and dehumidifiers under compatible operating conditions. Therefore, this study highlights the potential of various humidifiers and dehumidifiers in improving the efficiency of solar-powered and low-grade waste heat (HVAC system and PV panel) powered HDH systems that effectively utilize sustainable energy sources (solar and waste heat) to enable cleaner production of decentralized freshwater. Each material exhibits intrinsic beneficial property and their influence on system and process effectiveness are elaborated. Among humidifiers, cellulose was found to be influential owing to its inherent water absorption capacity. Similarly, due to the efficient latent heat extraction, the finned tube heat exchanger outperformed other dehumidifiers. By comparison, the combination of cellulose (34%) and finned-tube dehumidifier (56%) was effective in the HDH process due to the combination of efficient heat and mass transfer effects. Further, the higher heat capacity of water contributed to its dominant preheating (63.8%) compared to air (17.3%). It is inferred that waste heat-powered HDH offers the dual benefits of freshwater along with improved cooling effect (HVAC-HDH system) or electrical energy (PV-HDH system) while solar-powered systems produce only freshwater at a relatively high cost. Dual fluid preheating, closed circulation, enhanced wet area, optimum mass flow ratio across humidifier/dehumidifier, residual heat utilization, and adoption of thermal energy storage unit are identified to be significant factors in improving the HDH system performance. The scope of the potential system-and process-based improvements has been proposed to support increased efficiencies leading to cost-effective and continuous decentralized freshwater production.