Environmental Engineering Reference
In-Depth Information
20-25% of the radiation incident upon the pond's surface. After accounting for losses
to the ground, in practice around 15-20% of the incoming radiation is available for
extraction to an application, with the heat delivered 40 to 50 degrees above the local
daily average temperature (Weinberger, 1964).
Thermal energy output capacity of a solar pond further depends on the surface
area and depth of the storage zone. For a given application with a known heat load the
size of a solar pond that can meet the load can be approximated through the following
steps, for example the heat load is 3000 GJ/year, (for this approximation it is assumed
that the solar pond is clear and well maintained):
1
Find the annual solar energy incident per square metre on a horizontal surface for
the proposed solar pond site, for example 6 GJ/m
2
/year.
2
Now estimate the solar energy that will reach the storage zone and be available
for extraction, divide the annual solar energy value found in the previous step by
4 and 8, for example 6/4
1.5 GJ/m
2
/year; 6/8
0.75 GJ/m
2
/year.
=
=
3
Now estimate the solar pond surface area, divide the heat load by the frac-
tion of solar radiation that is available for extraction as estimated from
the previous step, for example solar pond surface area
2000 m
2
;
=
3000/1.5
=
4000 m
2
.
area
=
3000/0.75
=
The size of a real pond to supply the heat load will depend upon the brine clarity
(transparency) and the rate of heat extraction. For continuous heat extraction the pond
will be larger, while rapid peak load heat extraction the pond will be smaller. Thus
the important design parameters will include: information on ground thermal con-
ductivity, requiring site specific figures for soil thermal conductivity and permeability;
pond configuration requirements, requiring knowledge of pond site characteristics;
local weather features, including long-term weather and solar data; and pond depth
characteristics.
The temperature and quantity of the heat storage in a solar pond depends upon and
can be estimated by knowing the salinity gradient of the pond, transparency and the
depth of the pond. The depth determines such factors as maximum pond temperature,
heat losses to the surrounding soil and atmosphere, and temperature decay time for the
storage zone. There are lot of assumptions to be made, and the experience of practical
solar pond operations provide a very useful guide for design sizing purposes.
A solar pond with a deep storage zone (typically of the order of 2 to 5 m) will store
a large quantity of heat for a long time. Heat losses will be lower and the collection
and storage efficiencies of the pond will be high. In contrast, a shallower storage zone
(typically of the order of 1 to 2 m) can readily attain higher temperatures (since there
is not so much thermal mass in storage), but will then have higher heat losses to the
air and ground and a shorter storage capacity. Hence the design of a solar pond would
mainly depend on the application.
7.2.4 Liner, salt and water
Liners are a very important component of a solar pond as they prevent saline water
from leaking into the soil underneath the pond and endangering the purity of the
aquifer. Hot brine leaking out from a pond will carry away with it salt and heat, which
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