High-performance absorbers for solar thermal applications

Energy has emerged as one of the most significant and universal concerns society will face in this century. Currently, the world relies on fossil fuels to produce 86% of its energy. However, the combustion of fossil fuels generates greenhouse gases, a major factor responsible for global warming. Because concentrated solar thermal (CST) systems offer a pathway to a cost-competitive alternative to our traditional fossil-fuel-fired power plants, they have recently regained interest from the green community, investors, and utility companies. A common CST configuration, the parabolic trough, is illustrated in Figure 1(a). In a parabolic trough, sunlight is concentrated bymirrors onto an absorber tube running along the focal point of the parabolic mirrors. The absorber tube is coated with a spectrally selective material to maximize solar absorption and minimize thermal emission from its surface.1 A cross section of the absorber tube is shown in Figure 1(b). The absorbed sunlight warms a heat transfer fluid (HTF), which flows inside the absorber tube. The HTF is then driven through a heat exchanger to produce steam that can be used in an industrial process or converted into electrical energy in a turbine. The latter process is thermodynamically limited by the Carnot efficiency and is thus strongly dependent on the maximal achievable temperature of the HTF. The working temperature is currently limited by the cracking temperature of the synthetic oil working fluid. Alternative working fluids such as molten salts are being investigated to allow for operation at higher temperatures (∼720K). However, to achieve these higher temperatures, better absorber coatings need to be developed which have lower thermal losses than commercially available cermet coatings.3 If we consider the thermal radiation from a black body at 720K, the emission intensity peaks around 4μm (Figure 2). If we compare this to the solar spectrum, we observe little spectral overlap. This is exactly what is being exploited in a solar Figure 1. (a) Schematic of the parabolic trough configuration.2 (b) Cross section of the coated absorber tube. T: temperature.