Optical cavity for improved performance of solar receivers in solar-thermal systems

Abstract

Abstract:A principal loss mechanism for solar receivers in solar-thermal systems is radiation from the absorbing surface. This loss can be reduced by using the concept of directional selectivity in which radiation is suppressed at angles larger than the incident angle of the sunlight striking the absorber. Directional selectivity can achieve efficiencies similar to high solar concentration, without the drawbacks associated with large heat fluxes. A specularly reflective hemispherical cavity placed over the absorber can reflect emitted radiation back to the absorber, effectively suppressing emission losses. An aperture in the cavity will still allow sunlight to reach the absorber surface when used with point focus concentrating systems. In this paper the reduction in radiative losses through the use of a hemispherical cavity is predicted using ray tracing simulations, and the effects of cavity size and absorber alignment are investigated. Simulated results are validated with proof of concept experiments that show reductions in radiative losses of more than 75% from a near blackbody absorber surface. The demonstrated cavity system is shown to be capable of achieving receiver efficiencies comparable to idealized spectrally selective absorbers across a wide range of operating temperatures. Highlights:-A specularly reflective cavity placed over a solar absorber reduces radiative losses -Cavity performance is sensitive to cavity size and cavity-absorber alignment -Radiative loss reduction validated with proof of concept experiments -Losses from near blackbody absorber reduced by over 75% -Receiver efficiency shown to be comparable to idealized wavelength selective absorber

Enhanced absorption of thin-film photovoltaic cells using an optical cavity

“…1. Similar cavities have been shown to reduce radiative recombination emission losses in PV cells [21] and thermal emission losses in thermal systems [22,23]. Here, we show how it can also increase absorption in thin film PV cells.…”

The optical efficiency of three different geometries of a small scale cavity receiver for concentrated solar applications

“…The results showed that some improvement on the optical efficiency can be done during changing the dimensions of the interior vortex shape which can covered the dead area of the studied receiver. Enhancing the thermal performance of solar receiver through the focusing on the optical characteristics was tried by Weinstein et al [32]. In their research they tried to decrease the thermal losses, especially the radiative one, by applying the directional selectivity idea which leads to a reduction in the mentioned heat loss by 75% and as a result reaching high receiver efficiency for solar thermal applications.…”

Heat meets light on the nanoscale

“…Several possible approaches to achieve thermal emission with angular selectivity are illustrated in Figure 5. The first approach shown in Figure 5a is based on a reflective cavity with an aperture [142][143][144]. Thermally emitted photons propagating in the direction close to the normal to the emitter surface escape the cavity through the aperture.…”