Development of a comprehensive transient model of energy capture and storage in solar ponds for use in thermal regeneration of draw solutes in forward osmosis.

Abstract

Salinity gradient solar ponds can be used to store heat by trapping solar radiation. The heat can then be employed to drive various industrial applications that require low-grade heat. In this study, a comprehensive finite difference transient model has been developed incorporating many processes that affect the performance of a solar pond to predict the hourly temperature distribution. A novel heat extraction method for salinity gradient solar ponds is then proposed. This method can be operated in batch or continuous modes. A comparison between the performance of two solar ponds of the same size (10,000 m2) in Adana (Turkey) and Ahvaz (Iran) is also presented. The heat extraction method entails brine removal from the Non-Convective Zone (NCZ) as well as the HSZ. The presented model incorporates the heat losses from the bottom and surface of the pond as well as the cooling effect imposed as a consequence of the replacement of extracted brine from each layer, and the supply of freshwater to the surface of the pond to maintain its inventory. The model can be employed to predict the performance of solar ponds of various dimensions for any given location. In the final part of this study, utilisation of solar thermal energy from salinity gradient solar ponds in forward osmosis (FO) is investigated. This study will present two novel processes for the regeneration of dimethyl ether (DME) and ammonium bicarbonate as a draw solutes in FO using thermal energy provided from a solar pond. The average daily volume of desalinated water produced using these processes and a solar pond of 10,000 m2 was determined. It is indicated that, a solar pond of such moderate size can drive a forward osmosis plant to provide a total of 5,210 m3 of potable water in the first two years of operation in the location considered in this study (Chabahar) if DME is used as the draw solute. The proposed process can provide freshwater at varying rates throughout the year and benefits from a very low electricity consumption rate of 0.46 kWh per cubic metre of desalinated water presenting a viable option for solar desalination. In case of ammonium bicarbonate, the product water contains small quantities of ammonia ions making it unsuitable for drinking purposes. Given that there are vast uninhabited coastal areas in many countries, particularly in the MENA region where there are high solar radiation rates, this method can contribute towards addressing the growing water scarcity