M. G. Lima, A. Joyce, J. Milhazes, D. Loureiro*

INETI, Department of Renewable Energies, Campus do Lumiar INETI, 1649-038 Lisbon, Portugal
Corresponding Author, scientific. comm@eurosun2008.org

Abstract

A Salt Gradient Solar Pond (SGSP) is a salt water basin that collects and stores solar energy. These devices rely on the existence of a non-convective zone (NCZ) that functions as a transparent thermal insulation zone, created by a salt gradient. Salinity and temperature gradients in this zone can give rise to double diffusive problems that can decrease the insulation properties of this zone.

The stability of this zone is thus crucial in a SGSP.

Stability control, analysis of energy extraction, device efficiency and maintenance strategies are determinant for the correct performance of the SGSP. The implementation of these strategies can be expensive and not sufficient to prevent instability problems. This paper intends to give a contribution to the maintenance problem presenting a new concept of a SGSP utilisation: The Dual Solar Pond (DSP)

Keywords: Solar Pond, stability, maintenance.

1. Introduction

A Solar Pond is a salt water basin that collects and stores solar thermal energy. These devices are based on the prevention of free convection, either by increasing the solution’s viscosity, or by the establishment of a salinity gradient that opposes the adverse temperature gradient, induced by solar radiation absorption. Most existing Solar Ponds are based on this last type. The working of a Salt Gradient Solar Pond settles in the stability of its non convective zone, where salinity and temperature gradients are established and where physical instabilities may occur as a result of double diffusion phenomena. There are several models that analyse the behaviour of this zone. These models allow the control and prevention of any perturbations capable of triggering an instability process. For the study here presented it will be considered a simplified structure of a SGSP with two zones: the gradient non convective zone (NCZ) and the storage zone (SZ) as it is depicted in Figure 1. It was considered that the Storage Zone is nearly saturation in terms of concentration (25 % in salt) and that the top of the gradient zone has a concentration near of that of sea water (3%).

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Presently Solar Pond studies focus on stability control of the non convective zone, analysis of energy extraction, device efficiency and maintenance strategies as in the works of [1], [2], [3], [4], [5], [6] and others.

Applications of Solar Ponds are very important especially for Mediterranean countries and go from greenhouse or livestock structures heating, aquaculture heating, salt purification and water desalination processes and even electricity production, using Rankine cycle engines. All these applications are an opportunity to economic development of near coast regions using local resources (seawater, salt and solar energy) and promoting employment. Concerning environment these solar applications could achieve an integrated way to the mitigation of GEE with low environmental impacts.