High-Temperature Solar Thermal Energy Conversion Enhanced by Spectrally-Selective Metafilm Absorber under Concentrated Solar Irradiation

Abstract

Concentrating solar power, particularly parabolic trough system with solar concentrations less than 50, requires spectrally selective solar absorbers that are thermally stable at high temperatures of 400C above to achieve high efficiency. In this work, the solar-thermal energy conversion performance of a selective multilayer metafilm absorber with excellent thermal stability is characterized along with a black absorber for comparison by a lab-scale experimental setup that measures the steady-state absorber temperature under multiple solar concentrations. Heat transfer analysis is employed to elucidate different energy conversion modes and validate the solar-thermal experiment. The solar-thermal efficiency was measured to be 38% from the experiment and projected to be 86% without parasitic heat losses for the selective metafilm-on-Si absorber at the temperature of 336C under 9.1 suns, in comparison with measured 30% and projected 55% efficiency for the black absorber at a lower temperature of 266C under the same solar concentration. In addition, the metafilm absorber deposited on the flexible cost-effective stainless steel foil achieves the solar-thermal efficiency 57% at a steady-state temperature of 371C under 10 suns during the lab-scale experiment with losses, while a highest efficiency of 83% was projected under the same conditions for practical solar thermal applications. The results here will facilitate the research and development of novel solar materials for high-efficiency solar thermal energy conversion.

Enhancing solar-thermal energy conversion with silicon-cored tungsten nanowire selective metamaterial absorbers

“…To experimentally demonstrate the performance of the WNW-Al and SiNW-Al samples as selective solar absorbers, a laboratory-scale solar-thermal test with a custom-built setup consisting of a 1-kW solar simulator, a 1-ft 3 vacuum chamber, and optical filters and mirrors was conducted at selected concentrated solar irradiation from 1 up to 20 suns by different combinations of neutral density filters. A detailed description of the solar-thermal test apparatus can be found elsewhere ( Alshehri et al., 2020 ) and thus will not be discussed here. As depicted in Figure 4 A, a 2 × 2 mm 2 resistance temperature detector (RTD, OMEGA, F2020-100-B-100), which measures the absorber temperature up to 500°C, was adhered onto the backside of the nanowire absorber samples by thermal paste, which covered the entire back surface.…”

“… is the reflected solar irradiation by the front absorber surface, and is the radiated heat loss from top, bottom, and side surfaces (i.e., i = t, b, s) to the vacuum chamber wall at T ∞ = 20°C, where and are the area and total emittance of corresponding surfaces, respectively. The top surface is the nanowire absorber whose temperature-dependent total emittance is taken from Figure 2 C, whereas the side and bottom surfaces are, respectively, tungsten and thermal paste whose temperature-dependent total emittance was found from previous optical measurements ( Alshehri et al., 2020 ). is the conducted heat via RTD wires considered as useful heat gain during the solar-thermal test, and the experimental solar-thermal efficiency can be thereby defined as where R cond ( T ) is the conduction resistance of the RTD wires that is dependent on the absorber temperature T .…”

“… is the conducted heat via RTD wires considered as useful heat gain during the solar-thermal test, and the experimental solar-thermal efficiency can be thereby defined as where R cond ( T ) is the conduction resistance of the RTD wires that is dependent on the absorber temperature T . As cannot be directly measured, a commercial black absorber ( Acktar, n.d. ) with solar absorptance of 0.998 and total emittance around 0.93 characterized previously ( Alshehri et al., 2020 ) was used to calibrate the temperature-dependent conduction heat transfer via the RTD wires. The steady-state temperature of the black absorber was measured during the solar-thermal test for solar concentrations 1.1, 4.0, 6.4, and 13.3 suns (see Figure S4 in the Supplemental Information ) for calculating from the energy balance in Equation (3) , based on which the conduction resistance R cond ( T ) was fitted into a linear function of absorber temperature with .…”

“…Note that Q inc is the incident solar irradiation measured by a power sensor (Thorlabs, S310C) and the solar concentration factor can be calculated as CF = Q inc =ðA t G AM1:5 Þ. Q ref = ð1 Àa solar ÞQ inc is the reflected solar irradiation by the front absorber surface, and Q rad;i = A i ε i sðT 4 ÀT 4 N Þ is the radiated heat loss from top, bottom, and side surfaces (i.e., i = t, b, s) to the vacuum chamber wall at T N = 20 C, where A i and ε i are the area and total emittance of corresponding surfaces, respectively. The top surface is the nanowire absorber whose temperature-dependent total emittance is taken from Figure 2C, whereas the side and bottom surfaces are, respectively, tungsten and thermal paste whose temperature-dependent total emittance was found from previous optical measurements (Alshehri et al, 2020). Q cond is the conducted heat via RTD wires considered as useful heat gain during the solar-thermal test, and the experimental solar-thermal efficiency can be thereby defined as…”

“…Laboratory-scale photo-thermal conversion experiment with 1-sun illumination was conducted for a Cr grating structure that reached a high temperature of 80.7°C ( Seo et al., 2019 ). A novel laboratory-scale solar-thermal test platform with a broadband solar simulator was successfully developed to characterize the performance of a fabricated metafilm selective solar absorber under multiple solar concentrations up to 20 suns along with detailed heat transfer analysis ( Alshehri et al., 2020 ). So far, systematic laboratory-scale solar-thermal characterization of nanostructured metamaterial selective solar absorbers under variable solar irradiation is still rarely demonstrated.…”