Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 224

If deeper layers of a common pond or lake are heated by the sun, the heated
water rises up to the surface since warm water has a lower density than cold wa-
ter. The heat supplied by the sun is returned to the atmosphere at the water sur-
face. This is why, in most cases, the mean water temperature approximately
equals ambient temperature. In a solar pond heat transmission to the atmosphere is
prevented by the salt dissolved in deeper layers, since, due to the salt, water den-
sity at the bottom of the pond is that high, that the water cannot rise to the surface,
even if the sun heats up the water to temperatures that are close to the boiling
point.
The salt concentration of the different layers must thus increase with increasing
depth (Fig. 5.33). In a first phase, this ensures stable water stratification. The up-
per, almost salt-less layer only acts as transparent, heat-insulating cover for the
cooling, heat-storing deeper layers at the pond bottom /5-41/.

Solar radiation Solar radiation

Solar pond Solar pond

10 13 Density in g/cm³ 30 60 90 Temperature in °C

Fig. 5.33 Density (left) and temperature gradient (right) of a solar pond

To ensure stable stratification of a solar pond, with increasing depth the tem-
perature increase must not exceed density increase (i.e. salt content). This is why
all relevant parameters must be continuously monitored in order to take appropri-
ate measures (e.g. heat withdrawal, salt supply) in due time.
To achieve the utmost collector efficiency, a high portion of the solar radiation
must reach the absorption zone. Yet, this can only be achieved, if the top layers
are of sufficient transmission capability.
During the operation of a solar pond, the transmissivity, the salt content and the
temperature must be regularly monitored. The timely course of these parameters
must be measured from the water surface to the ground in order to determine the
heat quantity that can be withdrawn from the pond or to determine the measures to
maintain the respective required salt concentration and the water quality (preven-
tion of turbidity due to particulate matter, algae or bacteria).
Diffusion ensures permanent equalisation of the salt concentration in a solar
pond which is even intensified by wave motion due to wind near the surface. This
is why salt needs to be withdrawn from the surface water and added to deeper
layers. For this purpose surface water is evaporated in separate flat basins (sa-
lines). Subsequently, the extracted salt is added to deeper zones.

Heat exchangers. Basically, there are two methods to withdraw heat from a solar
pond.