Transient study of a solar pond under heat extraction from non-convective and lower convective zones considering finite effectiveness of exchangers

• Published in: Solar energy 2021-07, Vol.223, p.437-448
• Main Authors: Verma, Sunirmit, Das, Ranjan
• Format: Article
• Language: English
• Subjects: Convection; Entropy; Entropy generation; Forced convection; Free convection; Heat; Heat exchangers; Heat transfer; Heat transfer coefficients; Heat treatment; Ponds; Solar energy; Solar pond; Solar ponds (heat storage); Temperature profiles; Thermal conductivity; Time and space dependent effectiveness; Transient model; Transient performance; Two-zone extraction; Working fluids
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/j.solener.2021.05.072

•Dual zone extraction in transient state with variable thermofluidic parameters.•Time and space dependent effectiveness of exchangers within the solar pond.•Optimized size of exchanger pipe is proposed.•Conventional and real assumptions yield up to 69% deviation in annual efficiency.•Conventional and real assumptions also yield 9% deviation in entropy generation. The transient performance of a salt gradient solar pond, which experiences heat extraction from both gradient and storage regions has been studied for annual performance via accounting for the overall heat transfer coefficient across heat exchangers. The proposed method after reduced complexities is satisfactorily validated with theory and experiment-based results of pertinent literature. The novelty of the work lies in the fact that, the temperature drop across both non-convective and lower-convective zone exchanger surfaces has been accounted by using a local time and space dependent heat transfer coefficient. Here, it takes into the consideration free convection from the pond to the exchanger surface, and forced convection from the exchanger surface to flowing working fluid. Further, temperature variation of thermo-fluidic parameters of water involving, viscosity, density and thermal conductivity is considered. Detailed numerical investigation reveals that by neglecting this coefficient which is assumed in conventional studies, it can lead to significant errors in the prediction of transient temperature profiles in the pond and the exchangers. Calculations reveal an error of about +69% in the annual extraction efficiency and about −9% in entropy production if the conventional assumption is used as opposed to realistic technique presented herein. Interestingly, the present study reveals that there is an optimum radius of exchanger pipe in each of non-convective and lower convective zones that maximises the annual extraction efficiency. This work presents a useful analysis to assess multi zone extraction from solar ponds under transient state in a more practical and accurate manner.