Concentrated solar power
Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) generation is a system that converts solar energy into thermal energy and generates power through thermal–work conversion. CSP systems generate solar power by using mirrors or lenses to focus a large area of sunlight onto a small area. The concentrated light is then used as heat or as a heat source for a conventional power plant (solar thermoelectricity).
CSP is often compared to photovoltaic solar (PV) since they both use solar energy. However, CSP can more easily store energy during the night, making it more competitive with dispatchable generators and baseload plants. A CSP plant can incorporate thermal energy storage, which stores energy either in the form of sensible heat or as latent heat (for example, using molten salt). CSP has more in common with thermal power stations such as coal, gas, or geothermal, which enables these plants to continue to generate electricity whenever it is needed, day or night.
The CSP operation principle is similar to that of a magnifying glass, with which many youngsters have experimented. Light is concentrated on a heat absorber that contains a heat transfer fluid (HTF) that is heated to temperatures between 600°C and 1200°C, depending on the technology. As a consequence of these high temperatures, thermodynamic energy conversion efficiencies are high: Carnot efficiencies are theoretically about 66% and 80% for these two temperatures, respectively.
Based on different concentration modes for the CSP generation system, the CSP generation normally can be divided into linear Fresnel reflector, parabolic trough collector, central receiver (also known as solar tower), and parabolic dish. In the first two types, light is concentrated on a linear receiver, and these two types are therefore denoted as line-focus systems, with a maximum 2D concentration ratio of 210× (Twidell and Weir, 2015). The receiver typically is a steel tube inside an evacuated glass container for isolation. Typical generated temperatures are ~400°C with thermal oil as working fluid (Pitz-Paal et al., 2004), while conversion efficiencies range from 8% to 18%. In the other types, light is concentrated to a point receiver and has a maximum 3D concentration ratio of 46,000× (Twidell and Weir, 2015), with typical temperatures in the receiver of ~800°C (parabolic dish) and 600°C–1200°C (central receiver), with molten salts or thermal oils as working media (Pitz-Paal et al., 2004). Efficiencies range from 20% to 40%, which come at the expense of needing more accurate Sun tracking. Recent reviews of CSP are provided by Gil et al. (2010), Teske et al. (2016), Gauche et al. (2017), Tasbirul Islam et al. (2018), and Fernández et al. (2019).
Those that have already reached the commercial application level are mainly concentrating solar tower and parabolic trough types. CSP enjoys the advantages of comparatively mature techniques, low power generation costs, and minor impacts on the power grid; thus it has been deemed the most promising among various renewable energy power-generation modes.
Central receiver systems
Central receiver systems (also called solar tower or power tower) consist of a large number of two-axis tracking mirrors (heliostats) each with a surface of 20–200 m2 and a heat exchanger (receiver) located at the top of a central tower. Central receiver systems are considered to have a large potential for mid-term cost reduction of electricity compared to parabolic trough technology, because they can achieve higher temperatures, resulting in more efficient steam cycles or ultimately higher exergy cycles using gas turbines at temperatures above 1000°C to further increase efficiency and throughput.
