Salinity gradient solar ponds - Solar Energy Sciences and Engineering Applications - page 206

the pond bottom by ropes to overcome buoyancy forces has proven to be a simple
and reliable method of heat extraction from the 3000 m 2 demonstration solar pond in
Pyramid Hill, Australia.
Recent investigations have shown that heat extraction from the gradient zone in
addition to the storage zone helps improve the overall efficiency of the solar ponds.
In-pond heat exchangers for heat extraction from the gradient zone are only effective
in case of small ponds. To extract heat from the gradient zone of large solar ponds,
a selective withdrawal method should be used. Here hot saline water from different
depths is extracted and is passed through a respective external heat exchanger where
the heat transfer fluid is preheated using the gradient zone (Andrews and Akbarzadeh,
2005; Leblanc et al., 2011; Yaakob et al., 2011).
7.3.4 Performance monitoring
For optimum operation and steady performance of a solar pond it is very important
to have a reliable monitoring system and procedures. Physical parameters such as
temperature, salinity profiles in the solar pond should be routinely monitored. It is
also very important to monitor the water clarity and for this measurement of turbidity
of water is essential, at the same time monitor the pH of water. Control of algae
growth is very critical to the efficient performance of a solar pond as discussed in the
earlier section. The temperature, density, turbidity and pH of the solar pond should
be monitored once every two weeks. Failure to do this will lead to a decrease in the
thermal efficiency of the solar pond. Temperature and salinity profiles help to examine
the stability of the gradient zone of the solar pond.
The simplest method of monitoring these parameters is by using a sample extrac-
tion device as shown in Figure 7.3.5 (top) (Malik et al., 2011). This sample extraction
device can be made from a 3.5 m long 25 mm diameter plastic tube. The sampling tube
is used to withdraw brine samples from different levels in the pond (guideline: 5 cm
intervals). Thermocouples are connected to the inlet of the tube such that the tem-
perature at the point in the pond from which the sample has been withdrawn can be
measured. An automated sampling system as shown with a schematic in Figure 7.3.5
(bottom) would be more practical for large solar ponds.
It is also useful to monitor the total global solar radiation incident upon a horizon-
tal surface near the pond surface to keep records of solar energy input for calculation
of the solar pond thermal efficiency. Inspection of temperature and salinity profiles is
a direct and simple way to locate the depth of the gradient zone with the surface zone
and storage zone, as well as the presence of any convective layers.
7.3.5 EEE (Energy, Environmental and Economic)
benefit evaluation
It is simple to calculate the thermal utilisation coefficient of the solar pond: it is defined
as the ratio of the useful heat extracted from the pond to the actual solar radiation
received by the pond. The rate of heat extraction from the solar pond and the tempera-
ture of the heat delivered should be monitored and recorded. So to accurately calculate
the thermal utilisation coefficient of a solar pond in operation, the total thermal energy
provided by the pond over a period of several months should be determined. Similarly