NOTICIAS

Figure 1. Schematic of a typical CSP system showing solar field with concentrators, a heat exchanger (red-highlighted blocks) to transfer heat to thermal energy storage system, and a power block for electricity generation (Reprinted from Zhang et al. with permission from Elsevier, Copyright 2013)

Given the low energy density and intermittent nature of the solar resource, an important design parameter for CSP plants is the solar multiple (SM). SM relates the size of solar field to the energy demand of the power cycle at its design point, as follows (Goel et al., 2020): Due to the seasonal and daily variations of DNI, it is typical to have SM > 1.0 along with a sizable thermal storage unit to ensure operation even when the direct DNI input is below the design value for full-load plant operation. As a result, CST power plants are typically designed with 6–12 h of integrated storage capacity and SMs in the range of two to three (Mehos et al., 2015). The collector of a CSP system consists of two main components: the concentrator and the receiver. The concentrator can be designed as either a point focus or line focus system (achieving a higher concentration ratio). The receiver, in turn, is located at the focal point (or line) to absorb as much of the concentrated DNI as possible and, in the case of a CST system, transfer it to a thermal fluid passing through it. The line focus and point focus types of collectors have been deployed at commercial scales in four different types, as shown in Fig. 2 (Mehos et al., 2017), which are as follows:

1. Linear Fresnel Collector: Linear Fresnel reflector technology uses a field of long narrow mirrors to concentrate the DNI on a single stationary receiver with one or more receiver tubes (Zhu et al., 2014). This technology, although not fully mature, provides a cheaper alternative to parabolic trough technology given the simpler design. However, the capital cost savings with linear Fresnel reflector technology come at the cost of reduced optical efficiency compared to parabolic trough technology (IRENA, 2012).
2. Central Receiver (Power Tower): Central receiver power tower technology uses an array of mirrors (also referred to as heliostats) that can individually concentrate the DNI onto a single receiver mounted at the top of a tower, where the solar energy is absorbed by the working fluid. The central receiver power tower plant can be imagined as an approximation of a huge parabolic dish with each heliostat representing a small section of the parabolic dish and capable of independently tracking the sun.
3. Parabolic Dish Collector: Parabolic dish collectors are stand-alone systems that use mirrors mounted on a parabolic dish and focus solar DNI onto individual receivers mounted at the focal point of the dish. The niche for this technology is the flexibility that a sterling engine—or a novel sodium thermal electrochemical converter (Gunawan et al., 2020)—can either be mounted on the receiver directly for generation of power or the heat transfer fluid from the receiver could be used to operate the heat engine independently like the other technologies (Pandey et al., 2022).
4. Parabolic Trough Collector: A parabolic trough collector power plant consists of a solar collection field that is made up of rows of reflective mirrors with a parabolic cross section. The parabolic reflector focuses the sunlight to a line along the length of the trough, where a receiver tube carrying the heat absorbing fluid is placed (Sun et al., 2020). Typically, the parabolic mirrors can track the sun’s path along a single axis.

Figure 2. Schematic of concentrated solar technologies (Reprinted from the International Energy Agency, 2014)

A comparison of the four technologies is provided in Table 1. According to SolarPACES, the installed capacity of parabolic trough collector CSP/CST systems in July 2021 was ∼ 4,650 MW. This was closely followed by central receiver power tower technology at ∼ 1,330 MW (IEA, 2019; Lilliestam et al., 2021). Table 2 provides the breakdown for installed and under-construction systems by the type of heat transfer fluid used. CSP/CST technologies are often marred by their own set of limitations that indicate room for future research. One such limitation is the daily and seasonal variability in DNI. The intermittency of the solar resource, from variations in DNI, has been proven to be quickly addressable by equipping the CSP/CST system with a thermal energy storage (TES) unit. With a TES unit, the solar energy collected during daylight hours can be used to heat the storage medium, from where heat can subsequently be extracted as needed using a second heat transfer cycle. TES units provide additional benefits, including dispatchable high-value energy, operating reserves, reliable system capacity, and the ability to ramp operations rapidly. TES systems though appealing suffer limitations, such as storage capacity and form, string time, special structural requirements, energy releasing efficiency, and operation time (Mohd et al., 2008; Faisal et al., 2018; Saha et al., 2020). Another limitation with the current state of the art of CSP/CST technologies is the achievable temperature of the heat transfer fluid. Molten salts and synthetic oils are currently the most extensively used heat transfer medium. Although the use of salts provides the benefit of an additional absorption of latent heat as the salts change phase from solid to liquid, the use of salts is inhibited by the fact that they quickly degrade at temperatures of ∼ 500°C (Mehos et al., 2017). Further research is needed to identify salts that will retain their stability at temperatures > 800°C as well as alternative materials like solid particles and sCO₂. Another limitation associated with the CSP/CST systems is the degradation in performance of collectors from soiling, dust accumulation, and mirror erosion (Wu et al., 2020; Niknia et al., 2012). They have a direct impact of the ability of collectors to concentrate DNI. While soiling and dust accumulation rates depend on the location and the frequency of high winds, sandstorms, etc., associated with any arid region, mirror erosion is a secondary phenomenon resulting from the impact of abrasive particles on the mirror surface resulting in a reduction in mirror reflectance. CSP/CST systems can play an undeniable role in the future due to their ability to attain high temperatures by concentrating solar energy. However, this heavily depends on improvements in dispatchability via integration of TES, attaining higher heat transfer fluid temperatures and achieving significant cost reductions in the solar collector field for all the CSP technologies. Furthermore, complementary improvements in the power conversion system are also needed to remove the constrains of a steam-Rankine power cycle.