Experimental analysis of the temperature and concentration profiles in a salinity gradient solar pond with, and without a liquid cover to suppress evaporation

Abstract

Solar ponds offer an effective way to collect and store incident solar radiation, making them an attractive alternative to photovoltaic systems for applications which require low-grade heat to operate. If these ponds are to be implemented successfully, then a more complete understanding of the mechanisms and phenomena governing their behaviour is required. Evaporation has been shown previously to be the dominant mode of heat loss from the pond surface, and the fresh water that would need to be added to maintain the pond’s inventory could potentially add significantly to operating costs. To this end, an experimental unit was constructed to examine and observe the behaviour of a salinity gradient solar pond (SGSP) before and after covering the pond with a thin layer (0.5 cm) of paraffin, with the aim of eliminating evaporation. The unit was run for 71 days in Nasiriyah, Iraq. This is the first study to attempt to completely eliminate the harmful effects of evaporation on solar pond performance using a liquid layer. The layer successfully eliminated the significant evaporation observed from the uncovered pond and crucially, while the salinity gradient through the non-convective zone remained substantially intact over the course of the study, the temperature profile became approximately uniform throughout the entire pond after about 50 days. This behaviour has significant implications for the construction of the pond, as it may mean that if evaporation can be largely suppressed, the salinity gradient may not be necessary for the pond to capture and efficiently store heat. Furthermore, the effects on evaporation of different climatic factors such as relative humidity, wind speed, ambient temperature and solar radiation were considered by analysing data measured on-site and longer-term meteorological data. The results showed that ambient temperature, solar radiation and humidity have a significant correlation with the evaporation rate; and their impact varies seasonally. A more comprehensive multiple regression analysis showed that ambient temperature has the highest impact on evaporation, while the effect of the incident solar radiation is insignificant. Such insights are vital in the design and siting of solar ponds, and can be used to minimise evaporative losses.