Solar selective volumetric receivers for harnessing solar thermal energy

Abstract

Given the largely untapped solar energy resource, there has been an ongoing international effort to engineer improved solar-harvesting technologies. Toward this, the possibility of engineering a solar selective volumetric receiver (SSVR) has been explored in the present study. Common heat transfer liquids (HTLs) typically have high transmissivity in the visible-near infrared (VIS-NIR) region and high emission in the midinfrared region, due to the presence of intramolecular vibration bands. This precludes them from being solar absorbers. In fact, they have nearly the opposite properties from selective surfaces such as cermet, TiNOX, and black chrome. However, liquid receivers which approach the radiative properties of selective surfaces can be realized through a combination of anisotropic geometries of metal nanoparticles (or broad band absorption multiwalled carbon nanotubes (MWCNTs)) and transparent heat mirrors. SSVRs represent a paradigm shift in the manner in which solar thermal energy is harnessed and promise higher thermal efficiencies (and lower material requirements) than their surface absorption-based counterparts. In the present work, the “effective” solar absorption to infrared emission ratio has been evaluated for a representative SSVR employing copper nanospheroids/MWCNTs and Sn-In2O3 based heat mirrors. It has been found that a solar selectivity comparable to (or even higher than) cermet-based Schott receiver is achievable through control of the cut-off solar selective wavelength. Theoretical calculations show that the thermal efficiency of Sn-In2O3 based SSVR is 6–7% higher than the cermetbased Schott receiver. Furthermore, stagnation temperature experiments have been conducted on a laboratory-scale SSVR to validate the theoretical results. It has been found that higher stagnation temperatures (and hence higher thermal efficiencies) compared to conventional surface absorption-based collectors are achievable through proper control of nanoparticle concentration.