Experimental investigation of energy and exergy performance of square and circular solar ponds

摘要

Solar energy is a free, clean and widely available renewable energy. Various kinds of solar systems provide scope for several studies on energy and exergy analysis. Solar ponds are one of applications for collecting and storing solar energy simultaneously and are usable in regions with readily available land, much sunlight, and access to low-priced salt. In the present work, an experimental investigation of the energetic and exergetic performance of two prototype solar ponds with square and circular cross sections has been carried out. The solar ponds were built at Bafgh city, a city near Yazd in the central part of Iran. The cross sectional area and depth of both prototype solar ponds are 3 m~2 and 1.5 m, respectively. During 11 months of energy storage operation, a data acquisition system was used to measure the temperature variations along the height of the solar ponds and to capture the development of thermal stratification. Data collection is done with 10 min intervals. Thermal analysis of an experimental solar pond is usually a complicated task due to variations of many involved factors such as solar radiation, thermophysical properties of operating fluid, climatic changes etc. The energy performances of the solar ponds under investigation are carried out by modeling the solar incident radiation on the ponds surface, calculating energy exchange within each zone and thermal energy losses to the environment. Although the data are collected for almost one year of operation, however, the energy analysis is performed for some warmer months of the year during which the ponds are in the charging or energy storing period. An exergy balance model is also developed to investigate the exergetic performance of solar ponds. Monthly averaged exergy efficiencies are obtained for different regions of the ponds and compared with the corresponding energy efficiencies. The monthly average temperature of ambient is considered as the reference environment temperature for the exergy analysis. It was demonstrated that the circular solar pond has superior thermal performance than the square one. Stored water in the storage zone of the circular solar pond reaches to a higher temperature in comparison with the square one. It is attributed to the different shading effect in the two solar ponds. The maximum energy efficiencies for NCZ and HSZ of circular solar pond are found to be 17.25% and 25.8%, respectively while the corresponding values for the square solar pond are 17.39% and 23.65%. Maximum exergy efficiency for NCZ and HSZ are 0.86% and 2.44% for circular solar pond and 0.71% and 1.91% for the square solar pond.