Thermal performance of ionic liquids for solar thermal applications

Experimental investigations were carried out to evaluate thermophysical properties, i.e. density, viscosity, heat capacity, and thermal conductivity and high temperature forced convection behavior of N-butyl-N,N,N-trimetylammoniumbis(trifluormethylsulfonyl)imide ([N4111][NTf2]) ionic liquid (IL) for its potential application in concentrated solar power (CSP). Results from the experiments will be useful in assessing the potential of using ILs for solar thermal collectors. Experimental results show that thethermal conductivity was slightly decrease with the increase of temperature; vary from 0.124 to 0.121 W/m K for a change in temperature of 283-343 K, strong temperature effect on the viscosity of IL was observed and maintain an exponential relationship with the temperature; heat capacity increases linearly with temperature measured from 298 to 618 K. Forced convection performance of the IL was studied in a circular tube under both in laminar and turbulent conditions. Although the heat transfer coefficient of the IL was found to be lower compared to the De-Ionized (DI) water, its thermal stability and other attractive properties may make it a viable candidate for solar collector use. Our experimental results also established that Shah's equation and Gnielinski's equation can predict forced convection performance of IL for both the laminar region and turbulent region respectively.