Salinity gradient solar ponds - Solar Energy Sciences and Engineering Applications - page 195

Salinity gradient solar ponds
Abhijit Date & Aliakbar Akbarzadeh
School of Aerospace, Manufacturing and Mechanical Engineering,
RMIT University, Melbourne, Australia
7.1 INTRODUCTION
The most common form of solar pond is a salt-water solar pond. Salt water ponds
exist naturally in a variety of locations, the first ponds being discovered in Eastern
Europe at the beginning of the 20th century at a natural salt lake in Transylvania then
part of Romania. Most of the salt water ponds operated today, however, are artificial,
simulating natural solar ponds but taking advantage of engineering technologies to
advance their operation and application for practical purposes.
In the case of fresh water ponds all the solar radiation that falls on the surface is
absorbed by the top 3 meters 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.
A solar pond utilizes a large body of salinity gradient water to absorb the radiation
from the sun and store it in form of heat at the bottom. Figure 7.1.1 shows the schematic
of a salinity gradient solar pond, which consists of three regions. The cold upper layer
or Upper Convective Zone (UCZ) is a homogeneous thin layer of low salinity brine or
fresh water. The middle gradient layer or Non-Convective Zone (NCZ) has a salinity
gradient with salinity increasing from top of NCZ to the bottom of NCZ, this helps
suppress the natural convection heat loss. The bottom layer or Lower Convective
Zone (LCZ) has salinity close to saturation (high concentration brine) that absorbs
and stores solar radiation that reaches the LCZ in form of thermal energy. Out of the
100% of solar radiation that is incident on the surface of the solar pond, around 5%
is reflected back to the atmosphere; around 45% is absorbed by the water in the UCZ
and eventually is lost to the atmosphere by convection; around 20% is absorbed by
the water in the NCZ and eventually is conducted to the top UCZ and then lost to
atmosphere; around 25% is absorbed by the water in LCZ and the remaining 5% is
lost to the ground. Heat loss upwards in the pond from the storage zone is prevented
since natural convection currents in the gradient zone are suppressed. This suppression
and hence insulating effect occurs because of the density gradient present (Weinberger,
1964; Tabor, 1980).
Experiments show the formation of separate salinity/density gradient layers in the
NCZ (seen in Figure 7.1.2). When a particular layer of solution is heated its density
is slightly reduced, but remains higher than that of the layer above. Hence there can
be no movement upwards by the 'buoyancy' effect that drives natural convection in