Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 126

• a storage zone at the bottom of the pond of ideally uniformly high dissolved salt

concentration that stores heat and is typically 1 m-3 m thick.

Heat stored at the bottom of the pond, is removed by hot brine being withdrawn from
the lowest storage zone of the pond by a pump passed through a heat exchanger and
then returned back to the lower of the storage zone. For power production applica-
tions that employ an organic Rankine cycle, the engine's condenser cooling water is
withdrawn from the top of the pond, from where it is passed through the condenser
before being returned back to the surface layers of the pond.
Non-convecting solar ponds are energy collectors with “built-in'' seasonal heat-
storage capabilities that can provide heat at temperatures in excess of 90 ◦ C with such a
large volume of inherent sensible thermal storage that heat can be collected in summer
and stored for use during.
The lower convecting zone (LCZ), is where the highest uniform salt concentration
occurs. In it the solar radiation will heat the highly-saline water but, because of its
high relative density due to its salt content, this heated water will not rise into the
lower salinity layers. Thus the heat is stored yet inhibited from being transferred by
convection. In order to establish a conventional non-convecting solar pond for power
production, it must have a large surface area (i.e. extend over several square kilo-
metres), and so vast excavations and site preparations are inevitably necessary: these
operations can usually account for more than 40% of the total capital cost of the non-
convecting solar pond. Construction of economically viable solar ponds requires the
ready availability of inexpensive flat land; accessibility to water; and an inexpensive
source of salt or brines (Norton, 1992).

4.3 CONCENTRATORS

4.3.1 Introduction
Solar energy is concentrated to produce higher output temperatures and/or reduce
collector cost by replacing an expensive absorber area with less expensive reflector.
Concentration ratio is the factor by which an intervening concentrating mirror, lens or
luminescent system increases the insolation flux on a solar energy absorbing surface.
A geometric concentrator ratio is the collector aperture area ( A a ) is divided by the
absorber surface area ( A R ) (Rabl, 1985):

A a
A r

C = (4.3.1)

4.3.2 Concentration systems
Thermal losses are dependent largely on the heat loss characteristics of, and propor-
tional to, absorber area. Insolation can be concentrated by a two-dimensional system
to a theoretical upper limit given by:

n
sin( ε/ 2)

C max,2 D = (4.3.2)