Experimental and numerical investigation of volumetric versus surface solar absorbers for a concentrated solar thermal collector

A low-profile concentrated solar thermal collector (<15 cm in height) was proposed and investigated to demonstrate its potential to deliver heat energy in the range of 100-250 °C. We use both experimental and numerical methods to investigate of the effect of modifying the absorber in this collector. As such, a volumetric absorber (consisting of a multi-walled carbon nanotube nanofluid contained within a glass tube) was compared against a conventional surface absorber (consisting of a black chrome-coated copper tube). The experimental and computational fluid dynamics (CFD) results were found to be in good agreement for the thermal efficiency of these two receivers. The analysis revealed that the vacuum-packaged volumetric receiver had an efficiency of 54% and 26% operating at 80 °C and 200 °C, respectively. This lower than a vacuum-packaged black chrome-coated receiver, which had an efficiency of 68% and 47% in the same concentrator, operating at the same temperatures, respectively. [Note that commercial linear concentration systems typically have efficiency in the range 44-57% at 200 °C.] The inferior performance of the volumetric receiver was found to be due to higher reflective optical and radiative heat loss from the surface of glass tube. Overall, this study reveals that the proposed low-profile collector design is suitable for utilisation in industrial and commercial heating applications, but that volumetric absorbers will require anti-reflective and good selective coatings to be competitive with surface absorbers. If these challenges can be overcome, nanofluid receivers may have a cost/manufacturing advantage since glass-to-glass vacuum sealing is easier to achieve than metal-to-glass