Concentrated Solar Power: Parabolic Troughs

Most of you know about Photovoltaic Solar plants, but not about solar thermal energy, whose concept was actually conceived with an ice cream machine by Augustin Mouchot in 1878. Now known as Concentrated Solar Thermal Power (CSP) Plants, the largest plant is located in the UAE, however, CSP plants are not widely implemented across different countries, primarily due to the high costs, which is exacerbated by the fact that it uses an intermittent energy source as the sun’s rays are only available during daylight hours. In the US in 2021, 4116 billion kilowatt-hours of electricity was generated at utility-scale power plants, while solar thermal power plants generated only 3 billion, about 0.07%. 

Like multiple renewable energy sources, there are quite a few technological variations used in CSP plants. All of them utilising mirrors to focus sunlight onto a central receiver that circulates a fluid. The most developed solar thermal energy technology is the parabolic trough system. These systems use long mirrors that are curved and so, are parabolically shaped. This focuses and reflects the majority of the sunlight that is incident on the mirrors, onto the receiver tubes, which is placed at the focal point.

Figure 1: Image illustrating how sunlight is reflected onto the focal point

Image link: https://www.sciencedirect.com/science/article/abs/pii/S0196890414005482

The heat-transfer fluid (HTF) flowing through these receiver tubes gets heated and is directed to a Rankine cycle. So at the heat exchanger, the HTF transfers the heat to water, evaporating it into steam, which is used to drive a turbine and so, run the generator to produce electricity. The heat transfer fluid is typically thermal oil, whose temperature can increase to 400ºC. Since this temperature is so great, its thermal efficiency ranges between 60 - 80%, significantly more than that of OTEC.

Figure 2: Schematic Diagram of a Parabolic Trough CSP system

Image link: https://www.sciencedirect.com/topics/engineering/parabolic-trough

With solar power towers, parabolic trough systems are the most implemented type of CSP because of its high efficiency and the fact that it also has minimal environmental impact because these plants do not take up significant amounts of space. Therefore, these systems are compatible with existing infrastructure and can be situated at the top of buildings to provide hot water or electricity. However, there are a few drawbacks to this technology, like the fact that it is slightly more costly because of the use of curved mirrors, however they are not as significant compared to the other types of CSP technology.