A numerical study to predict the energy and exergy performances of a salinity gradient solar pond with thermal extraction. (15th November 2017)

• Record Type: Journal Article
• Authors: Njoku, Howard O.; Agashi, Boniface E.; Onyegegbu, Samuel O.
• Is Part Of: Solar energy. Volume 157(2017)
• Page Start: 744
• Page End: 761
• Publication Date: 2017-11-15
• Subjects: Solar ponds -- Energy and exergy efficiencies -- Second law analysis -- Extraction efficiencies; Solar energy -- Periodicals; Solar engines -- Periodicals; 621.47
• DOI: 10.1016/j.solener.2017.08.079
• Languages: English
• ISSNs: 0038-092X

Abstract

Highlights: Solar pond energy and exergy performances are investigated. Energy and exergy flows are determined with a 1D numerical model of the pond. Zonal and pond storage, extraction and overall energy and exergy efficiencies are computed. Energy and exergy storage efficiencies are highest in the NCZ and least in the UCZ. Energy efficiencies are at least an order of magnitude higher than exergy efficiencies. Abstract: An investigation of the performance of a salinity gradient solar pond (SGSP) on the basis of energy and exergy considerations is presented. Unlike existing exergy studies, the transient 1-D numerical model developed in this study incorporates thermal extraction from the SGSP's lower convective zone (LCZ). Hence the efficiencies of the energy/exergy extractions were defined and determined in addition to the conventional efficiencies presented in existing studies. As is characteristic of solar thermal devices, irreversibilities were immense in all the zones, leading to the very low exergy efficiencies. The highest irreversibilities occurred in the upper convective zone, pointing to its need for prioritized attention in efforts aimed at improving pond performances. Most of the energy/exergy input to the solar pond is stored in the LCZ, though the exergy efficiencies of the non convective zone are comparable with those of the LCZ. A positive correlation was found to exist between the performance of solar ponds and prevailing heat extraction rates. Extraction energy and exergy efficiencies were obtained respectively as 63.0% and 3.2% for the LCZ, and as 19.4% and 1.0% for the entire pond, for a thermal extraction mass flux rate of 0.0003 kg m −2 s −1 . At this thermal extraction mass flux rate, the exergy efficiencies were maximum in the pond considered, whereas no maxima in energy efficiencies were obtained. This shows that an exergy analysis is required for realistic appraisals of SGSP performance. Hence, it's use is recommended in such studies in future.