Effects of working fluids on the performance of a bi-directional thermodiode for solar energy utilization in buildings

A series of experiments were conducted to investigate the effects of different working fluids on the behavior and performance of a bi-directional thermodiode. The thermodiode was made up of two rectangular loops mounted between a collector plate and a radiator plate. The loops were filled with a working fluid for effective heat transfer when the thermodiode was forward biased. Five different working fluids were tested with thermal conductivity values ranging from 0.1 to 0.607 W/m-K, thermal expansion coefficient values ranging from 2.54 × 10^-4 to 1.43 × 10^-3 1/K, and kinematic viscosity values ranging from 6.5 × 10^-7 to 1 × 10^-4 m²/s. The thermodiode was heated by a radiant heater consisting of 88 halogen lamps that generated a heat flux of about 10³ W/m² on the collector surface. Experimental results indicated that the onset time for natural convection to be induced throughout the diode system did not differ considerably when different working fluids were used. On the other hand the required fluid temperature differences in the loops for the onset of throughflow were quite different and depended strongly upon the viscosity and other properties such as thermal expansion coefficient and specific heat of the working fluid. Of the five fluids tested, water and low-viscosity silicon oil had the highest heat transfer rate. An analytical model was developed to predict and analyze the steady operation of the diode system when different working fluids are used.