Dear Muhammad Zeeshan

Generally, As the thermal form of solar, concentrated solar power (CSP) is ideally suited to storing solar thermally. Unlike the pile of coal or cavern-full of natural gas, the heat-storing salts used in solar thermal storage can be recycled daily within a tank like this for thirty or forty years.

Kind Regards

Qamar Ul Islam

24 hours charge and discharge? how much time for charging? and discharging?

Dear Muhammad:

At the first, and as you know:

Solar thermal storage (STS) refers to the accumulation of energy collected by a given solar field for its later use. In the context of this chapter, STS technologies are installed to provide the solar plant with partial or full dispatchability, so that the plant output does not depend strictly in time on the input, i.e., the solar irradiation. STSs are TES systems where the source of heat is provided by the solar field, capturing the excess of energy not directly converted into power or other useful utility. As such, most TES technologies known can be adapted and have been adopted in solar applications, in particular for power production. A detailed review on this topic can be found in the literature.

The selection of an STS is determined by a set of physical, chemical, environmental, and economic properties:

•Energy density of the storage material.

•Heat transfer and mechanical properties.

•Chemical compatibility and stability.

•Thermodynamic reversibility.

•Environmental impact.

•Thermal losses.

•Cost

To these properties one may add automation and control 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 always 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 temperature storage systems. Low temperature TES for heat storage for domestic application (heating or sanitary hot water) are in the range of 40–90°C. Medium temperature TES, up to 300°C, can be used, for example, in process heat applications and power production in Organic Rankine Cycles (ORCs). However, in solar 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 efficiencies 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.

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 chemical transformations in the experiment. According to the literature,1 there are three types of TES considering the process of storage and the corresponding medium:

•sensible energy storage;

•latent energy storage; and

•thermochemical storage.

Considering the mobility of the storage material, a classification applicable to latent and thermochemical heat can be drawn. Storage media that are allowed or forced to flow, e.g., through pipes or heat exchangers, are considered active storage systems. On the other hand, when storage media are confined permanently in a tank or container, the system is considered passive, also known as regenerators. With some exceptions, only liquid-type sensible heat STS can be used in the active configuration.

In general, HTFs do not store significant volumes of energy, nor they produce work; they only carry the energy from one element of the plant to the next. This is the case of mineral or synthetic oils that circulate along PTCs. In some cases, HTF work as storage material, as shown below for molten salts. Working fluids such as water or steam can, in certain cases, be used to collect and store the bulk of the energy stored. HTF connects thermally the storage with the rest of the plant, namely, the solar field and the power block.

Regarding the topology of the system, one or more tanks can be used to store the medium, being common the single-tank system, also known as thermocline tank due to the temperature distribution in the volume arising from density differences, and the two-tank system, where the medium flows from one tank to the other as the charge and discharge take place. Alternative configurations have been proposed such as the split tank and the cascade tank arrangement which, because of the additional 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.

This third type of TES relies on reversible endothermic chemical reactions 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 storage will also be determined by a process of cost optimization.

For solar plant without storage and without external inputs of conventional energy such as natural gas, the level cost of energy is a function of the capital investment 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, storing material, and the HTF; (2) the cost of the required heat exchangers; and (3) the cost of the space that the STS occupies.

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 suffice to establish their cost-effectiveness, which is determined by its performance (efficiency, 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.

For more details, you could benefit from this valuable article:

"Concentrating Solar Power"

By: José J.C.S. Santos, ... Marcelo A.

Also, you could benefit from this valuable article about your topic:

"Thermal Energy Storage in Solar Power Plants: A Review of the Materials, Associated Limitations, and Proposed Solutions".

@https://www.mdpi.com/1996-1073/12/21/4164/htm.

I hope it will be helpful...

With my best regards...