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).

Figure 1. (a) Schematic of the parabolic trough configuration.² (b) Cross section of the coated absorber tube. T: temperature.

In a parabolic trough, sunlight is concentrated by mirrors 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 selective absorber coating. An ideal absorber coating at 720K, illustrated in Figure 2, behaves as a perfect absorber—α(λ)=ε(λ)=1—for wavelengths shorter than the cutoff wavelength, λ=2.24μm, to optimize light absorption, and it suppresses thermal emission—α(λ)=ε(λ)=0—for longer wavelengths to minimize losses through infrared emission.

Figure 2. Normalized spectral power density of a black body (BB) at 720K and the solar spectrum (air mass 1.5, AM1.5). The spectral absorptivity is shown for an ideal absorber at 720K.

Our research group has optimized non-periodic multilayer stacks as solar selective absorbers.^4,5 The metals inside the stack act as absorbers and the dielectrics as optical spacers, creating interference effects that enhance absorption in a desired spectral range. These effects in metal-dielectric multilayer stacks can be used to achieve sharp spectral tuning. Recent progress in nanofabrication has created new opportunities to design sub-wavelength structures that could help achieve even higher spectral selectivity. Therefore, we have studied surfaces consisting of periodic, nanoscale V-grooves coated with metal-dielectric stacks. The structure is illustrated in Figure 3.

Figure 3. A V-groove grating with sub-wavelength period a=300nm is coated with a metal-dielectric stack.¹

This approach combines impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. Figure 4 shows the modeled spectral selectivity for grooves coated with stacks consisting of 5, 7, 9, and 11 layers. The spectral selectivity closely approximates that of the ideal absorber at 720K.

Figure 4. Spectral absorptivity at normal incidence for molybdenum (Mo) V-grooves with grating period a=300nm coated with 5-, 7-, 9-, and 11-layer stacks with layers of Mo, TiO₂, and MgF₂.

Our optimal solar selective coatings are predicted to have thermal emissivity below 5% at 720K while still absorbing >90% of the incident light. In our recent publication,¹ we explain how changes in the angle of the V-grooves can be used to tailor the spectral selectivity to significantly increase the efficiency of solar thermal systems. We predict that these structures would be able to reduce thermal emission by about 50% compared to commercially available coatings. To verify our modeled results, we are currently working on a vacuum emissometer to measure the spectral behavior of the proposed structures at various temperatures. This will also enable us to acquire accurate optical data at elevated temperature, which can then be used to reiterate the optimization process. If the experimental results confirm the predicted spectral behavior, these structures will be excellent candidates for the next generation of absorber coatings.