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

Concentrating sola r power (CSP) has the potential to make significant carbon dioxide emission cuts by mitigating the intermittency of wind and photovoltaic power, therefore boosting their presence in our energy landscape. The low achievable energy conversion efficiency with CSP defines a l i m i t for its operational value whose improvement could accelerate the deployment of this technology. The use of nanofluids as heat transfer fluids (HTF) has been proposed for such a purpose, if the physical properties that rule convection heat transfer are properly enhanced . More recently, nanofluids have been proposed to be used not only as HTF but also as volumetric absorbers for direct sunlight harvesting in CSP plants with parabolic-trough collectors (PTC), if the absorption and scattering profile of nanofluids provides good optical performance. Here, we report on the design requirements for the achievement of the maximum working temperatures and assess the expected efficiencies for surface and volumetric collectors with Pd and Au nanoplate-containing nanofluids, prepared using the eutectic and azeotropic mixture of biphenyl and diphenyl oxide (the conventional HTF in CSP-PTC plants) as a base fluid. Their stabi l i t y , spectral extinction, density, dynamic viscosity, thermal conductivity, and specific heat have been characterized. The efficiency analysis is based on numerical models for both types of collectors and considers the actual characteristics of these collectors and the optical, rheological, and thermal properties of the nanofluid which has been characterized. Different case scenarios are compared.