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394 CHAPTER 12 Concentrating Solar Power

such as the split tank and the cascade tank arrangement which, because of the addi-
tional complexity, enjoy currently a marginal penetration.
Sensible heat storage consists in the increase through heat transfer of the kinetic
energy of the molecules of the storage medium, which translates in a temperature
increase. Molten salts, synthetic oils, concrete, packed bed, or pressurized water
is the most common storage medium.
Latent energy storage makes use of the enthalpy difference of a given substance
between two physical states or phases. The use of phase change materials (PCMs) is
characteristic of this storage type. Sharma et al. provides a review on PCMs for
TES. 19
This third type of TES relies on reversible endothermic chemical reactions 20
fostered by the addition/retrieval of energy to/from a substance. Heat is the motive
source of the endothermic reactions, in contrast with other chemical energy storage
options such as water electrolysis to obtain hydrogen gas, which requires electricity.
The decision of providing a solar plant with a storage system responds almost
completely to economical criteria. In exceptional cases, flexibility of operation
and base load needs will influence the final design of the plant, but sizing of the stor-
age will also be determined by a process of cost optimization.
For solar plant without storage and without external inputs of conventional en-
ergy such as natural gas, the levelized cost of energy is a function of the capital in-
vestment and the operating costs and revenues. Investment may take place in
different instants of the life of the plant, but a great share will take place during
the construction of the plant.
The cost of STS depends on three main terms: (1) the cost of the containers, stor-
ing material, and the HTF; (2) the cost of the required heat exchangers; and (3) the
20
cost of the space that the STS occupies . Estimated costs of TES in solar plants with
commercial technologies are reproduced in Table 12.5.
What storage makes possible is mainly to increase the flexibility of the plant and
to run the equipment for longer hours at the nominal operating point. Flexibility or
dispatchability means that one can generate electricity not strictly when the sun
shines, but when electricity has more value. The economics of the plant improve
when the elements of the power block operate steadily, which reduces mechanical
and thermal fatigue typical cyclical operation.
Thus, it is important to notice that the cost of a storage technology does not suf-
fice to establish their cost-effectiveness, which is determined by its performance (ef-
ficiency, durability, etc.) within a specific plant design. Sizing and operation of STS
systems require the use of simulation tools to ensure that both the storage and the rest
of the plant behave within the technical and economical boundaries of the project.

19
A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change
materials and applications, Renew. Sustain. Energy Rev. 13 (2009) 318e345.
20
See footnote 9.