Environmental Engineering Reference
In-Depth Information
from the high temperature tank flows through a heat exchanger, where it
generates steam for electricity production. The fluid exits the heat exchanger
at a low temperature and returns to the low temperature tank.
Two-tank direct storage was used in early parabolic trough power plants
at the Solar Electric Generating Station I and at the Solar Two power tower in
California, as discussed later in this chapter. The two trough plants used min-
eral oil as the heat transfer and storage fluid; the Solar Two power tower used
molten salt.
Molten Salt as Heat Transfer Fluid
Using molten salt at a solar field and in a TES system eliminates the need
for expensive heat exchangers. This concept allows a solar field to operate at
higher temperatures than systems using other common heat transfer fluids
such as oils. Due to the elimination of heat exchangers and the reduction of
heat transfer fluids, the use of molten salt as a heat transfer fluid substan-
tially reduces the total cost of a TES system.
Unfortunately, molten salts freeze at relatively high temperatures—about 120
to 220°C (250 to 430°F). This means that special care must be taken to ensure
that the salt does not freeze in the solar field piping. The Italian research labora-
tory, ENEA, and Sandia National Laboratories in the United States are currently
developing new salt mixtures with the potential for freeze points below 100°C
(212°F) to make molten salt much more manageable as a heat transfer fluid.
Two-Tank Indirect Storage
The two-tank indirect system functions in the same way as the two-tank
direct system, but different fluids are used for heat transfer and storage. This
system is used in plants where the heat transfer fluid is too expensive or
not suited for use as the storage fluid. The storage fluid from a low tem-
perature tank flows through an extra heat exchanger, where it is heated by
the high temperature heat transfer fluid. The high temperature storage fluid
then flows back to the high temperature storage tank. The fluid exits the heat
exchanger at a low temperature and returns to the solar collector or receiver,
where it is heated back to the high temperature. Storage fluid from the high-
temperature tank is used to generate steam in the same manner as the two-
tank direct system. The indirect system requires an extra heat exchanger
and this adds cost to the system and decreases the overall TES efficiency.
This system will be used in several parabolic power plants in Spain and has
also been proposed for several U.S. plants that will use organic oil as the
heat transfer fluid and molten salt as the storage fluid. Later in this chapter,
Figure 7.11 illustrates Abengoa Solar's Solana plant in which a two-tank indi-
rect storage system is used with oil as the heat transfer fluid and molten salt
as the storage material.
