Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 454

or concrete as storage materials. Concrete, for example, is chosen because of its low
cost, availability throughout the life of the plant and easy processing. Moreover,
concrete is a material with high specific heat, good mechanical properties (espe-
cially when subjected to compression strain), thermal expansion with a coefficient
close to one of steel, and high mechanical resistance to cyclic thermal loading.
When concrete is heated, a number of reactions and transformations take place
which influence its strength and other physical properties: resistance to a compres-
sion strain at 400 ◦ C is about 20% lower than its value at ambient temperature; the
specific heat decreases in the range of temperature between 20 ◦ C and 120 ◦ C; and
the thermal conductivity decreases between 20 ◦ C and 280 ◦ C (Gil et al., 2010).
Resistance to thermal cycling depends on the thermal expansion coefficients of
the materials used in the concrete. To minimize such problems, a basalt concrete
is sometimes used. Steel needles and reinforcement are sometimes added to the
concrete to prevent cracking. At the same time, by doing so, thermal conductivity
is increased by about 15% at 100 ◦ C and 10% at 250 ◦ C. Another material that can
be employed is rock, which is a more inexpensive TES material costwise, even if its
physical and mechanical properties are not as good as concrete (see Table 14.4.2).

• Liquid media : liquids (mainly molten salts, mineral oils and synthetic oils) are

more commonly used for sensible thermal storage than solids. One important
design consideration of a liquid storage system is the need to maintain a separa-
tion between the colder fluid and the warmer fluid. There are mainly two ways
of separating temperatures: two-tank systems and stratified thermal storage tanks
(thermocline tanks). Two-tank systems separately store the hot and cold fluid in
different tanks; as the hot tank is being filled, the cold is being emptied and vice
versa. In this way mixing is avoided but a higher storage volume is required com-
pared to the stratified thermal storage tanks. In fact, in a thermocline tank liquid
medium maintains natural thermal stratification because of density differences
between hot and cold fluid. Thermal stratification ensures the existence of sepa-
rate volumes of liquid at different temperatures inside the tank and the temperature
gradient occurs in a small portion of the tank height, the so-called thermocline.
Significant volume and cost reductions are generally achieved with respect to the
two-tank configuration. The requirements of this type of TES are that the hot and
cold fluids have to be supplied in different parts of the tanks in order to limit fluids
mixing: the fluid enters from the upper part of storage during charging, and the
cold fluid has to be extracted from the bottom part during discharging. In any
case a minimum mixing of hot and cold fluids takes place. For this reason, an
accurate design of the geometry of inlet and outlet ducts is necessary to limit fluid
velocity. As regards the shape of the tank, a slim storage container is desirable to
improve thermal stratification (Dincer and Rosen, 2002). However, the optimum
value of the ratio between the tank height and diameter cannot be determined
whatever the techno-economic optimization of the power plant.

In addition to storage tanks based on sensible heat storage media, there are other
TES solutions employing latent heat storage media . In fact, thermal energy can be
stored almost isothermally in some substances as latent heat of phase change, as
heat of fusion, exploiting solid-to-liquid transition, heat of vaporization, exploiting
liquid-to-vapour transition, or even solid structure change (transition from amorphous