Salinity gradient solar ponds - Solar Energy Sciences and Engineering Applications - page 198

In order to match the thermal output of a solar pond to the energy and temperature
requirements end application, it is very important that the temperature of the lower
convective zone is always maintained 3-5 ◦ C above the end use temperature. When
the LCZ temperature is much higher than the delivery temperature the heat loss will
increase and some economic penalties would increase.
7.2.2 Best site characteristics
Site selection is very important for easy construction and operation of a solar pond.
A potential site for solar pond should have easy access to salt or brine, low salinity
water, ample flat land, consistent soil to be used for building pond walls and the most
importantly the land should not be cultivable. The site should not be windy for most
part of the year, as high wind can disturb the stability of the pond. The sites for small
solar ponds should not be surrounded by buildings or tall trees. The site should receive
plenty of solar radiation as this will directly affect the performance of the solar pond.
However, it is still possible to build ponds that will operate well in high latitudes, with
increased area compensating for less available radiation per unit area of surface. The
performance of the solar pond would also depend on the local evaporation rates and
depth of the natural underground water table. High evaporation and a shallow water
table would make the solar pond performance drop as the heat loss to the atmosphere
and ground would increase. Heat loss to the ground water can be reduced by insulating
the floor of the solar pond, but this would add to the construction cost (Tabor, 1980;
Hull, 1989; Akbarzadeh et al., 2005).
An ideal site for solar pond would have free draining soil, free salt available nearby
to reduce costs, easy access to water, flat land to minimize earthmoving requirements,
easily compactable soil for structural stability, low prevailing wind speeds to minimize
wave-induced mixing and the depth of the top mixed zone, an environmentally accept-
able disposal method or recycling ability for closed-salt inventory balancing, dry soil for
good thermal insulation, high incident solar radiation for good thermal performance,
low evaporation to minimize the need for make-up water, soil with good cohesion for
forming stable walls for above-ground ponds, a low amount of wind-borne debris to
easily maintain cleanliness, a stationary or deep groundwater table to minimize heat
loss within the ground, most importantly proximity to end use application.
7.2.3 Performance and sizing
The thermal performance of a solar pond mainly depends on the absorption of solar
radiation in the layers of the ponds. Sun light attenuation as it passes through the top
layers of a solar pond puts an upper limit to the amount of solar radiation that can reach
the lower convective zone. Further, the amount of sunlight that can reach the lower
convective zone would decrease with an increase in turbidity, so it is very important
to maintain high water clarity in a solar pond. The more radiation that penetrates, the
higher the energy efficiency and operating temperature of the pond will be. In a well-
designed and set up solar pond, upward heat losses from the LCZ are small. Therefore
most of the solar radiation that gets through to the LCZ is stored there, apart from the
small amount lost by conduction to the ground. A well maintained solar pond with
a total depth of 3 metres, with 1 m deep LCZ (storage zone) would receive around