Evaluation and conceptual design of triphenylphosphonium bromide-based deep eutectic solvent as novel thermal nanofluid for concentrated solar power

In a concentrated solar power (CSP) plant, an increase of heat transfer effect of the working fluid is a key deliverable which is usually obtained by enhancing its thermo-physical properties. The current work reports the synthesis of heat transfer fluids (HTF) based on deep eutectic solvents (DESs) consisting of a hydrogen bond donor (HBD), namely, triphenylphosphonium bromide, and a hydrogen bond acceptor (HBA), namely, ethylene glycol. Initially, the thermophysical properties, namely, density, viscosity, thermal conductivity (TC) and specific heat capacity were measured and compared with the conventional solvents. The properties were further enhanced by the dispersion of spherical $$\hbox {Al}₂\hbox {O}₃$$ nanoparticles in DESs. The alumina nanoparticles were found to have a negligible effect on the physical properties (density and viscosity) of the base fluid, thereby limiting the pressure drop and also the coefficient of friction. For their potential application as thermal fluids for CSP plants, the thermal properties of DESs and nanoparticle dispersed deep eutectic solvents (NDDESs) were measured within a temperature range of 25–60$$^\circ {\hbox {C}}$$. The TC of 1 wt% $$\hbox {Al}₂\hbox {O}₃$$ with the base fluid was around eight times higher than the base DES. It was found that the TCs of DES and NDDES were higher when compared to the commercial HTF, namely, Therminol VP-1. Eventually, the Aspen plus flowsheet was conceptualized to ascertain the steam generation rate and the overall heat transfer coefficient of these novel solvents. A combination of U-shaped for latent heat and shell and tube heat for sensible heat was employed in the flowsheet. The CSP scheme gave a steam generation rate of 1.7 kg h$$^-1$$ at 180$$^\circ \hbox {C}$$ with a corresponding DES flow rate of 1 $$\hbox {m}³\ \hbox {h}^-1$$.