Accurate determination of the total hemispherical emittance and solar absorptance of opaque surfaces at elevated temperatures

Abstract

a b s t r a c tIn many applications such as solar power conversion technologies, radiative heat transfer plays a significant role in the energy balance. For accurate performance predictions of solar power conversion devices the total hemispherical emittance and solar absorptance of surfaces need to be known with high accuracy. Often times the emittance of a surface is calculated from indirect spectral bidirectional or directional-hemispherical reflection measurements at room temperature which can significantly underestimate the total hemispherical emittance. Here, we report a simple steady-state calorimetric method to directly measure the total hemispherical emittance of opaque surfaces at elevated temperatures with a maximum experimental uncertainty of 5%. The method is further expanded to directly measure the solar absorptance of the surface at elevated temperature using a solar simulator. We show experimental total hemispherical emittance and solar absoprtance results for various surfaces such as machine polished copper and stainless steel and several spectrally selective solar absorbers. We describe a methodology to characterize solar absorbers and apply it to a cermet-based spectrally selective solar absorber.

A high-performance spectrally-selective solar absorber based on a yttria-stabilized zirconia cermet with high-temperature stability

“…However, to determine to what extent the solar absorptance is dependent on the operating temperature of the solar absorber and to obtain a more realistic value for the total-hemispherical emittance we use a previously developed steady-state calorimetric method Table 2 The estimated solar absorptance (solar spectrum (AM 1.5 direct + circumsolar) weighted integration of bidirectional reflectance spectra) and total-directional emittance (integration of bidirectional reflectance spectra weighted by the blackbody spectrum at 82 1C and 500 1C) of prepared solar absorbers before and after annealing at 600 1C for 7 days in vacuum (briefly described in the Experimental section). 19 For the most promising solar absorber sample (WNY-3), we measure a temperature-independent solar absorptance of B0.91 up to 500 1C (Fig. 8) which is close to the solar absorptance calculated from spectral bidirectional reflectance data shown in Table 2.…”

“…A previously introduced steady-state calorimetric method is used to determine the solar absorptance and total hemispherical emittance at elevated temperatures of up to 500 1C in a vacuum chamber. 19 A sample is attached to an electrical heater assembly and suspended in a vacuum chamber. The electrical input power required to maintain the sample at a steady-state set temperature is recorded.…”

“…11 In addition, using the single-angle bidirectional reflectance spectrum obtained at room temperature can considerably underestimate the emittance of a spectrally-selective surface at elevated temperatures compared to its true total hemispherical emittance. 19 Al 2 O 3based cermet solar absorbers with Pt as the light absorbing metal filler were suggested to have better thermal stability due to the super antioxidation property of the noble metal. 14,20 However, the use of a noble metal like Pt as filler particles for the cermet layer is rather expensive.…”

Semiconductor-based Multilayer Selective Solar Absorber for Unconcentrated Solar Thermal Energy Conversion

“…As these measurements are challenging using readily available equipment, the effectiveness of selective surfaces has been traditionally quantified by a single absorptance or reflectance spectrum, taken at room temperature. A recent work reported a procedure to obtain measurements of average solar absorptance and hemispherical thermal emittance of solar absorbers at operating temperatures 25 . Rather than performing this measurement, we instead determined device performance by measuring the stagnation temperature under solar insolation in field tests in Pasadena, CA.…”

Low Cost High Intensity LED Illumination Device for High Uniformity Laboratory Purposes

“…The proposed solution consists of four high power white LED arrays arranged in compact layout to be broadly applicable as multi-purposes workbench laboratory facility. The main application of the system is the measurement of absorptance and emittance in selective solar absorbers [33][34][35][36]. As described in the work [37], the observed difference in solar absorptance between optical and calorimetric measurements was due to a not perfect calibration of the adopted illumination system.…”