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.
