Heat extraction from Non-Convective and Lower Convective Zones of the solar pond: A transient study

Highlights

• Proposed transient thermal performance model for heat extraction from NCZ and LCZ.

• Estimated instantaneous efficiency and average annual efficiency of the solar pond.

• Predicted optimum mass flux for limiting heat extraction condition.

• Transient analysis showed 47% improvement in the efficiency of solar pond.

Abstract

Heat extraction from the Non-Convective Zone (NCZ) or gradient layer and Lower Convective Zone (LCZ) of the solar pond has been investigated through one-dimensional finite difference transient model. Instantaneous efficiency of the solar pond is introduced and defined in this paper. The solar pond considered in the present study is assumed to have an in-pond heat exchanger for heat extraction. The rate of heat transfer is controlled by the mass flux of heat transfer fluid in this model. As in reality mass flux of heat transfer fluid is the simplest and most practical way to control the rate of heat extraction. In this model for an ideal situation it is assumed that the heat transfer fluid is initially at the local daily average ambient temperature and the in-pond heat exchanger has heat transfer effectiveness equal to unity. With these assumptions the model can predict the thermal performance of the solar pond with maximum heat extraction for a desired mass flux of the heat transfer fluid. This paper presents the comparison of the transient thermal performance of solar pond with heat extraction from LCZ alone and that with combined heat extraction from LCZ and NCZ. It is shown how the efficiency of the solar pond increases when heat is extracted from both NCZ and LCZ. The main objective of this study is to offer a simple method to predict transient thermal performance of a solar pond with heat extraction from NCZ and to estimate the mass flux of heat transfer fluid used in an in-pond heat exchanger for heat extraction from different layers of solar pond.

Introduction

A solar pond is a large body of saline water whose salinity increases with depth. These ponds are used as solar thermal energy collectors that can simultaneously store heat for long period, so they are suitable for sessional solar thermal energy storage. In case of fresh water ponds almost all the solar radiation that falls on the surface is absorbed by top 3 m of fresh water and this thermal energy is rapidly lost to the atmosphere through natural convection heat transfer. So the temperature of a fresh water pond never rises and is almost constant throughout the fresh water pond depth.

Practical heat extraction from Lower Convective Zone (LCZ) for different applications is very common. In past heat has been extracted from LCZ using two methods, in the first method hot saline water is extracted from the LCZ and passed through an external heat exchanger as used at Kutch in India (Kumar and Kishore, 1999) and Beith Ha’arava in Israel (Tabor and Doron, 1990). In the second method an in-pond heat exchanger is used for heat extraction from the LCZ as used at Pyramid Hill, Victoria (Leblanc et al., 2011) and Marshad in Iran (Jaefarzadeh, 2006). Tabor (Tabor, 1981) has discussed both of these methods in his review of solar pond technology, and stressed that both method have convenient practical merit. The main limitation to the full scale application of solar ponds in industry has been the low solar thermal efficiency and in many cases lowers temperature heat available (below 60 °C) as discussed by Leblanc et al., 2011, Andrews and Akbarzadeh, 2005. Researchers have proposed many different ways of improving the thermal performance of solar ponds, which include improving the water clarity (Gasulla et al., 2011, Wang and Seyed-Yagoobi, 1995, Malik et al., 2011), increasing the thickness of LCZ and reducing the thickness of Upper Convective Zone (UCZ) Wang and Akbarzadeh, 1982a, introducing an additional upper Non-Convective Zone (NCZ) Husain et al., 2012, putting an external reflector to reflect additional solar radiation into solar pond (Aboul-Enein et al., 2004), by integrating a solar pond with flat plate solar collectors (Bozkurt and Karakilcik, 2012), etc.

Researchers have proposed of extracting/utilizing the heat stored in the NCZ to improve the overall thermal performance of the solar ponds and have claimed to improve the solar pond efficiency by up to 50% using a steady state theoretical model (Leblanc et al., 2011, Andrews and Akbarzadeh, 2005, Yaakob et al., 2011).

To date all the studies have used steady state theoretical model and its unknown if similar improvement in the solar pond efficiency is achievable for real solar pond using a transient model. Present paper investigates the transient thermal performance of the solar pond with heat extraction from both NCZ and LCZ. Through this paper efforts have been made to further develop the one-dimensional finite difference model of the solar pond for predicting the mass flux of heat transfer fluid flowing through an in-pond heat exchanger in a solar pond. This new model can predict transient behaviour of such solar pond while neglecting the shading and wall effects. Effects on the temperature profile of the solar pond for different heat extraction rates from NCZ and LCZ have been discussed and the best mix of heat extraction has been presented.