Radiation and Energetic Analysis of Nanofluid Based Volumetric Absorbers for Concentrated Solar Power

Abstract

Recently, several publications gave attention to nanofluid based solar absorber systems in which the solar radiation energy is directly absorbed in the volume of the fluid. This idea could provide advantages over conventionally used surface absorbers regarding the optical and thermal efficiency. For the evaluation of this concept, a numerical approach is introduced and validated in this contribution. The results show that the optical efficiency of a volumetric absorber strongly depends on the scattering behavior of the nanofluid and can reach competitive values only if the particle size distribution is narrow and small. If this is achieved, the surface temperature and therefore the heat loss can be lowered significantly. Furthermore, the surface absorber requires very high Reynolds numbers to transfer the absorbed energy into the working fluid and avoid overheating of the absorber tube. This demand of pumping power can be reduced significantly using the concept of volumetric absorption.

Are Nanofluids Suitable for Volumetric Absorption in PTC-CSP Plants? An Exemplified, Realistic Assessment with Characterized Metal–Oil Nanofluids

“…The volumetric absorption concept itself appeared for the first time in a work by Taylor et al in 2009, which was about the use of concentrated sunlight for boiling heat transfer nanofluids, rather than an absorber surface and then transferring heat into a conventional base fluid, although earlier development ideas were reported by Tyagi et al in 2007 . For instance, Eggers et al explained the requirement of high Reynolds numbers for surface absorbers to transfer the caught energy into the HTF and avoid overheating of the absorber. Local overheating does not occur in volumetric absorbers, so pumping power can be reduced and the temperature difference between the surface absorber and the fluid would still be minimum and emissive losses reduced.…”

Optical and Transport Properties of Metal–Oil Nanofluids for Thermal Solar Industry: Experimental Characterization, Performance Assessment, and Molecular Dynamics Insights

“…The temperature rise per unit length is diminished for increasing flow rates under constant solar irradiation, but large flow rates are required, so that the turbulent regime is guaranteed for energy to be transferred from the surface to the HTF as it flows through the PTC in order to avoid local overheating. 17 This is particularly important in the case of the surface PTC, not only for radiative losses to be minimized but also because surface overheating promotes thermal stress of the receiver and degradation of the absorbing coating, whose lifespan decays exponentially with temperature. 56 The flow rate, v, in a typical surface PTC is ṽ= 2.6 l•s −1 (Re ≃ 300,000).…”

“…Additionally, if their absorption and scattering properties provide enough optical efficiency, ,− some nanofluids can be used as volumetric absorbers in parabolic-trough receivers for directly harvesting concentrated solar radiation . Eggers et al proved that the surface absorber requires very high Reynolds numbers to transfer the absorbed energy into the HTF and avoid overheating of the absorber tube. This demand for pumping power can be reduced significantly using volumetric absorption, as the temperature difference between the surface absorber and the fluid would be minimized and emissive losses consequently reduced …”

Direct Absorption Solar Collector: Use of Nanofluids and Biodegradable Colloids

“…The use of DASCs eliminates layers of tubes and absorber plates seen in conventional solar thermal technology, resulting in lower thermal resistance to solar thermal energy conversion. Moreover, the volumetric absorption reduces the exterior absorber temperature, and thereby the thermal losses [2,3]. Ultimately, higher system efficiency is achieved for the DASCs, as researchers have reported a 5%-20% efficiency increment when comparing nanofluid-based DASCs with conventional surface-based solar collectors [3][4][5][6][7][8][9][10][11][12].…”