Evaluation of Energy and Water Utilization for Hybrid Solar-Driven Desalination Systems

Abstract

Inland arid regions facing prolonged periods of drought are implementing membrane-based treatment processes to augment potable water supplies from unconventional water sources. The objective of this study is to realize increased solar energy and water utilization in the desalination process through a hybrid membrane distillation concentrated solar power/photovoltaic (MD-CSP/PV) system for inland concentrate management and off-grid applications. The 37 m² CSP/PV trough produces between 40-222 kWh/day of thermal energy and 0.5-4.5 kWh/day of electrical energy, depending on season and local weather. The CSP/PV system includes two thermal storage vessels (68 L and 680 L) to offset seasonal impacts due to solar irradiance and ambient temperature fluctuations in the winter and summer. Thermal energy captured by the CSP/PV is directly supplied to an air gap membrane distillation (AGMD) system, producing up to 449 L of distilled water. Experiments were performed on the hybrid MD-CSP/PV system to integrate the staged thermal storage reservoirs for seasonal changes in operation, and flow regime controls were developed for uniform heat supply to the system. The developed flow regime controls mitigated temperature drops observed in the thermal storage system, and results indicated that mixing setpoints above the maximum MD evaporator inlet would reduce small temperature fluctuations observed during MD operation. System performance was evaluated under different MD circulation flow rates and with different MD module lengths. Specific thermal energy consumption (STEC), specific electrical energy consumption (SEEC), distillate production, and water vapor flux were analyzed as performance indicators. Results indicate that with increased MD circulation flow rates a tradeoff exists for higher STEC and greater distillate production rates independent of membrane area. Compared to shorter membrane modules, utilizing longer membrane modules resulted in less thermal energy utilization from the CSP/PV thermal storage system, lower STEC values, and more distillate production. Results highlight import design and operating considerations for integrating thermal desalination with solar energy resources in an operational environment.