Thermodynamic and economic feasibility of solar ponds for various thermal applications: A comprehensive review

Abstract

Solar energy technologies and their applications are as relevant today as they were in 1950s. A comprehensive review of the most recent studies having numerous references to the past research works on the thermodynamic and economic feasibility of solar ponds is presented in this communication. Thermodynamic models for the performance analysis of solar ponds and its validation with the experimental results by a number of researchers are highlighted. Expressions for estimation of energy and exergy efficiencies of all three zones of salt-gradient solar pond are presented. Need of the evaluation and revival of solar pond technology for continuous supply of large quantity of solar thermal energy for low temperature applications has been advocated. Some of these applications are domestic and industrial process heating, heating of building and greenhouse, refrigeration and air-conditioning, desalination and salt production, agriculture and aquaculture, and power generation. Usefulness of the alternative technology of salt-gradient solar ponds such as solar gel ponds, equilibrium solar ponds, and shallow solar ponds has also been incorporated. It has been explored that solar ponds have a great potential of saving a large quantity of energy from degradation as well as exergy destruction due to the use of ‘energy based on fossil fuels and electricity’ for the purpose of usual low temperature applications. In this way, solar ponds will prove to be helpful in minimizing the present alarming ecological problems and energy shortage by substituting a large fraction of high grade energy consumption.

Introduction

High cost of the solar thermal system is due to lower thermodynamic efficiency, and higher cost of separate solar radiation collection and solar thermal storage systems. It has become essential to build up more economical solar thermal device which can collect as well as store solar energy simultaneously for further applications. Solar pond is a typical example of such solar thermal device among various solar energy systems. The concept of the solar pond is not novel. Research on solar ponds has been conducted in a number of countries. Solar ponds have been successfully built and operated in Europe, USA, Australia and developing countries mostly on an experimental basis and for demonstration purposes in the last 50 years. But the successful technology development process based on energy from fossil fuels and other conventional energy sources accelerated the growth of economy all over the world in 20th century. Less attention towards the commercialization of solar ponds technologies has been paid. Now, time has come to revive the study of solar ponds when there is need of energy supply from non-polluting and renewable energy sources for sustainable development of the world.

It has been established that the solar ponds have all potential with the annual collection efficiency in the range of 15–25 % to perform well for all locations and can supply adequate heat even in regions near the Arctic Circle. The capacity of long term storage which can supply sufficient heat for the entire year is a major attractive characteristic of solar ponds. It has been found that the solar ponds of the area of the order of 1000 m² or further are more cost effective than flat plate collector with higher efficiency as their cost per square meter is much less than that of a flat plate collector. It is suitable for space heating, to warm swimming pools and greenhouses, and for a range of industrial processes of low grade heat applications. Applications in the agriculture and rural areas such as crop drying, dairy plants, water desalination and salt production, and power generation using organic fluid Rankine Cycle have proved to be successful. The prospect of solar ponds to power absorption chillers for air-conditioning in hot and humid climates, especially in the Arabian Gulf regions, has also been explored. The concept of combining a salinity gradient solar pond with a chimney to produce power in salt affected areas and other hybrid applications are also being examined [1], [2].

A series of useful reviews of the theory and applications of solar ponds have been compiled by Tabor and Weinberger (1981), Nielsen (1988) and Hull et al. (1989) as reported by Duffie and Beckman [2]. Energy analysis of solar ponds have been performed and reported by many researchers. A review on solar ponds by Kaushika [3] covers historical development of solar ponds since 1902, their applications, thermal modeling of salt-gradient solar ponds including analytical and numerical models. Brief economics of solar ponds considered by Wittenberg and Harris (1981) and Sheridan (1982) are also incorporated here. Velmurugan and Srithar [4] have reviewed various designs of solar ponds, prospects to improve performance, factors affecting performance, mode of heat extraction, theoretical simulation, and measurement of parameters, economic analysis and applications. El-Sebaii et al. [5] have reviewed types of solar ponds, energy extraction methods from solar ponds and its some applications.

Recently, performances of solar thermal systems are being studied by researchers on the basis of second law of thermodynamics (i.e. exergy analysis) in addition to energy analysis based on the first law [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Energy efficiency is not sufficient to evaluate the actual health and overall efficient utilization of energy transfer from and or within any thermodynamic system. Exergy analysis has been found as an effective tool of thermodynamic analysis and optimization. It is felt that the true location and quantity of exergy destruction or irreversibility in the process of solar/solar thermal energy transfer is to be explored. Effective utilization of available solar energy may reduce the size of solar energy collection device reducing the cost of the solar thermal system. At the same time, the energy efficiency and performances of solar thermal systems will also improve.

Review on exergy analysis of solar ponds is scant in the literature. In this communication, the authors have presented a comprehensive review of thermodynamic analysis based on energy as well as exergy. It has also been tried to present the theoretical and experimental studies, utility and economic feasibility of salt-gradient solar ponds for various applications in addition to a brief description of the design and development, principle of working of solar ponds, etc. for further ready reference and research. Brief appraisal of an alternative technology of salt-gradient solar ponds such as solar gel ponds, equilibrium solar ponds, and shallow solar ponds has also been incorporated in the present paper.