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       1.2.1.2 Linear Fresnel Reflector Systems

      Linear Fresnel reflector systems use a field of narrow long mirrors. The mirrors are rotated independently to concentrate solar radiation on a stationary receiver tube. The mirrors are either flat or curved that track the sun and focus solar radiation [Figure 1.3]. The fixed absorber tube allows for easier Direct Steam Generation (DSG). Due to direct DSG, the operating temperature for such systems can be larger. This system also works with line focusing concept.

Figure 1.4 CSP receiver concepts as used worldwide.

      One of the major advantages of such systems is the simple design of the reflector leading to lesser capital cost. Overall, Linear Fresnel CSP receiver system offers lowest start-up and maintenance cost. In addition, cheap and simple flat mirrors allow high reflectors density per square meter. However, linear Fresnel receivers have the lowest solar-to-electrical efficiency compared to other receivers due to high optical loss. Now 9% of all CSP plants opt for such collector concept [Figure 1.4]. As observed from the literature, there is a good amount of fundamental and academic research work going on to enhance the performance of the CSP system with linear Fresnel receiver (Desai and Bandyopadhyay, 2015; Mills et al., 2000; Morin et al., 2012; Xie et al., 2011).

       1.2.1.3 Central Receiver Plants

A circular array of large mirrors with sun-tracking motion (called heliostats) reflect direct sunlight onto an absorber system in central receiver. Such systems are also called power tower as the receiver is mounted on the top of a tower [Figure 1.3]. This is a point focusing system and the two axis tracking allows a higher concentration ratio and temperature. The receiver collects radiation as thermal energy and a HTF carries the energy from the receiver to the power block. Central receiver configuration allows high plant size due to its design. Currently, 25% of the installed CSP configurations are central receiver plants [Figure 1.4]. Research works on central receiver systems are reviewed and presented by Behar et al. (2013), Ho and Iverson (2013), and Avila-Marin (2011).

       1.2.1.4 Dish System

      In dish systems, dish-shaped parabolic mirrors are used as reflectors to concentrate and focus the solar radiation on a receiver [Figure 1.3]. The receiver is generally mounted above the dish. The receiver absorbs the thermal energy and transfers it to the Stirling engine or steam engine. Dish systems are also point focusing concept like solar towers. There were two thermal power plants with dish systems constructed [Figure 1.4]. In addition, very few works on dish systems are available in the literature (Zapata, 2015; Poullikkas et al., 2010; Affandi et al., 2015). Currently, dish systems have not been operational since the 1990s after the deployment of other configurations. The disadvantages of such systems are that they are expensive with a lower collection temperature.

1.3 Thermal Energy Storage in CSP

The intermittent and uncertain nature of solar energy are critical issues which cause a mismatch between source availability and energy demand. As a consequent, this results in ripples in the deployment and market penetrability of CSP technologies. Use of a backup system that uses nonrenewable energy is one of the solutions of such problems. In addition, another cleaner solution to this problem is use of a Thermal Energy Storage (TES) system. The TES system can store energy when solar radiation is abundant. The stored energy can be released during periods without the solar radiation. This results in uninterrupted electricity generation during nighttime or on a cloudy day. Since the last few years, TES has been an explicit and significant component of CSP plant with the ability to drive the turbine almost continuously. Thus, electricity can be generated continuously at night and during cloudy hours when sunlight is not available. In addition, CSP with TES provides utility-scale and dispatchable electricity to the power grid. Also, TES allows CSP plants to generate electricity during evening hours when electricity is highly valued, offering better cost effectiveness. The integrated TES system can also result in shorter start-up time of the absorber systems. The TES system can be installed after the receiver system as illustrated in the block diagrams of Figure 1.5.

Figure 1.5 Integration of thermal energy storage systems to a CSP plant.

      The possibility of integrating the TES system is one of the most distinct advantages of the solar energy field with CSP over other renewable energy fields. There are many storage technologies being developed that can be integrated into CSP plants. To accelerate the development of solar energy power generation in terms of economy and operation, an efficient and cost-effective TES system is important. Many earlier research works have studied the importance of TES on CSP and this results in an energy-efficient power plant (Guo et al., 2018; John et al., 2013; Prasad and Muthukumar, 2013; Adinberg, 2011; Powell and Edgar, 2012; Tamme and Laing, 2004; Tian and Zhao, 2013).

      Thermal energy storage concepts for CSP plants can be classified as active or passive systems. In an active TES system, the thermal energy storage material itself circulates through a heat exchanger. Active systems can be divided into direct or indirect TES systems. In the commercial CSP plants, only active TES systems are used. Commercial active direct thermal energy storage systems are molten salt systems and steam accumulators. A second medium is used for storing the thermal energy in an active indirect system. A heat exchanger is used to transfer thermal energy from HTF to the second medium.

Two different materials for heat storage and circulation are used for passive storage systems which are dual medium storage systems. The HTF passes through the thermal storage system only for charging and discharging a TES material. The TES material can store energy in terms of sensible or latent heat. Passive TES systems are still under major research and they have not yet reached the commercial CSP plant level. A brief discussion about different TES systems [Figure 1.6] as incorporated in commercial CSP follows below.

Figure 1.6 Reported integration of thermal energy storage systems in worldwide CSP plants.

      1.3.1 Active Two-Tank System

      In an Active two-tank system, one tank holds hot HTF and another tank holds cold HTF. The hot HTF and cold HTF do not interact with each other. Major advantages of such systems are there will be no interaction between the fluids. Thus, heat loss is less and temperatures of the fluids in the tanks are almost uniform with time and position. However, two separate tanks of equal volumes are needed for such systems and tanks of the walls are subjected to daily cycles of molten salts. Thermal CSP plants with two-tank-based molten salt system are well documented by Kelly and Kearney (2004) and Herrmann et al. (2004). This concept can be used as