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392 CHAPTER 12 Concentrating Solar Power

• Thermodynamic reversibility
• Environmental impact
• Thermal losses
• Cost

To these properties one may add automation and control 13 and health and safety
requirements as factors that influence the performance of STS when integrated in a
given solar plant. Efforts made toward enhancement of a particular property often
results in trade-off situations. For instance, effort in heat transfer improvements
brings about normally a cost increase. Thus, designing and operating STS is not al-
ways an off-the-shelf pick-up process.
The classification of STS is generally done considering two characteristics: the
nominal or maximum temperature, which affects the maximum overall efficiency of
the system, and the nature of the energy storage process regarding the materials and
equipment used.
As for the temperature, TES can be classified as low, medium, and high temper-
ature storage systems. Low temperature TES for heat storage for domestic applica-

tion (heating or sanitary hot water) are in the range of 40e90 C. Medium

temperature TES, up to 300 C, can be used, for example, in process heat applica-
14 15
tions and power production in Organic Rankine Cycles (ORCs). However, in so-
lar applications where electric power is the goal, discharge temperatures must

remain above 400 C for Rankine cycles and 600 C for Brayton cycles to overall ef-
ficiencies be economically attractive. Thus, not only the solar field collectors should
ensure high output temperatures (HTF temperature leaving the solar field), but also
HTF and storage materials should interact such that energy and exergy losses during
the various heat transfer processes are minimized and their working life guaranteed,
as explained later in the chapter. Table 12.4 summarized the temperature range
achieved and sought to be achieved by several solar field technologies. 16
Regarding the nature of the STS, two main elements constitute and define these
storage systems, namely, the HTF, and the storage material. Depending on the
configuration, they can be in direct contact, separated, or be the same.
The catalog of storage media increases with ongoing research, and an exhaustive
enumeration is out of the scope of this chapter. In general, media are classified by
their operating state (solid, liquid, gas, or a combination) and the physical and

13
See footnote 12.
14
R. Tamme, T. Bauer, J. Buschle, D. Laing, H. Mu ¨ller-Steinhagen, W.-D. Steinmann, Latent heat stor-
age above 120 C for applications in the industrial process heat sector and solar power generation, Int.

J. Energy Res. 32 (2008) 264e271.
15
R. Chacartegui, L. Vigna, J.A. Becerra, V. Verda, Analysis of two heat storage integrations for an
Organic Rankine Cycle Parabolic trough solar power plant, Energy Convers. Manag. 125 (2016)
353e367.
16
See footnote 12.