Numerical and experimental analysis of a salt gradient solar pond performance with or without reflective covered surface

Abstract

An experimental salt gradient solar pond having a surface area of 3.5 × 3.5 m² and depth of 2 m has been built. Two covers, which are collapsible, have been used for reducing the thermal energy loses from the surface of the solar pond during the night and increasing the thermal efficiency of the pond solar energy harvesting during daytime. These covers having reflective properties can be rotated between 0° and 180° by an electric motor and they can be fixed at any angle automatically. A mathematical formulation which calculates the amount of the solar energy harvested by the covers has been developed and it is adapted into a mathematical model capable of giving the temporal temperature variation at any point inside or outside the pond at any time. From these calculations, hourly air and daily soil temperature values calculated from analytical functions are used. These analytic functions are derived by using the average hourly and daily temperature values for air and soil data obtained from the local meteorological station in Isparta region. The computational modeling has been carried out for the determination of the performance of insulated and uninsulated solar ponds having different sizes with or without covers and reflectors. Reflectors increase the performance of the solar ponds by about 25%. Finally, this model has been employed for the prediction of temperature variations of an experimental salt gradient solar pond. Numerical results are in good agreement with the experiments.

Introduction

A salt-gradient solar pond (SGSP) which consists of three distinct zones: an upper convecting zone (UCZ), a non-convecting zone (NCZ), and a lower convecting zone (LCZ), is an inexpensive solar energy collection and storage system for low-temperature heat-sources. The concept of the solar pond appears very simple: a body of water harvests the incident solar energy and stores it for a long period of time. This long-term store provides an alternative for conventional energy-source [1], [2], [3].

Solar ponds have been studied by many researchers because of their excellent heat collection and storage performances. There have been considerable theoretical and experimental studies [4], [5], [6], [7], [8] on SGSPs. Many experimental solar ponds [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have been constructed, instrumented and operated, and various numerical models [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30] have been developed for analyzing SGSP performance in the literature.

Theoretical studies have concentrated on modeling and predicting solar pond performance. Early studies of these types of works [31], [32], [33] were performed on one-dimensional models that did not account in a detailed way for the dynamical thermal interactions between the solar pond and the surrounding soil. Later researchers [34], [35] developed more detailed models which account for the two-dimensional interactions within the soil. Recently, studies on non-stable solar ponds have appeared [6], [36]. Kanayama et al. considered a cylindrical solar pond, 44 m in diameter. They used average hourly values of the horizontal solar radiation, and represented the monthly average temperature variation by a sine curve evaluated from the monthly average ambient temperatures in their simulations. Kayali et al. have developed empirical functions for air and soil temperatures obtained from meteorological data and used them for the calculations of the temperature variations in a rectangular type of solar pond [37], [38]. Hawladar [39] did similar studies on the performance of the solar ponds operating at different latitudes and under different physical and operational conditions and used hourly ambient temperatures obtained from meteorology stations in his simulations. He also described the influence of the extinction coefficient on the effectiveness of solar ponds in his other study [40].

This paper reports the summary of the results of the numerical modeling developed for a salt gradient solar pond having covers used for preventing heat loss and reflecting the sun light into solar pond and how these covers affect performance of the solar pond. An experimental salt gradient solar pond with a surface area of 3.5 × 3.5 m² and depth of 2 m has been built for supplying hot water to a leather workshop on the campus area of the Vocational College of Isparta/Yalvaç. A cover system for the surface of the pond was designed and used firstly to reduce the thermal energy loses from the top to air during night-time and to increase the thermal efficiency of solar energy harvesting during daytime. Simulations have been carried out using the mathematical formulation, which calculates the amount of the solar energy harvested by the covers, adapted into the model developed in [37] to see the affect of the covers on the performance of the solar pond. In order to get the best performance from the covers, one of them is kept at a fixed position and the other is rotated it from 0° to 180° to following the sun. This system is controlled by an electric motor and these covers have insulation and reflection properties. Analytical soil and air temperature functions used in the theoretical model simulations were developed using meteorological data obtained from the local meteorological station of Isparta region. Results obtained from theoretical and experimental studies showed that these kinds of salt gradient solar ponds can be used as a source for the warm water required at a leather workshop and for domestic applications. It was also found that covers have little effect on thermal losses during nights, but they are very effective for increasing the daytime harvesting performance.