Comparison of the thermo-hydraulic performance and the entropy generation rate for two types of low temperature solar collectors using CFD

In this work, a comparison of the thermo-hydraulic performance and the entropy generation rate for two different types of low temperature solar collectors: flat plate solar collector (FPC) and water-in-glass evacuated tube solar collector (ETC), is addressed. The absorber area for both solar collectors were considered to be equal for a reliable comparison. The operation of the solar collectors was simulated under different volumetric flow rates and solar radiation values for the state of Guanajuato in Mexico. The volumetric flow rate for both collectors ranged from 1 to 9 L/min. The variation of the solar radiation was based on: (1) the solar radiation taken from several experimental tests reported elsewhere, (2) the month with the lowest average solar radiation in one year, (3) the average solar radiation of one year and (4) the month with the highest average solar radiation in one year. The buoyancy effects were considered in the CFD simulations using the Boussinesq approximation (BA) model. The distribution profiles of temperature, pressure, and velocity inside the tubes of the solar collectors, along with the local entropy generation rate distribution due to heat transfer and the fluid viscosity, are shown in detail. The results show a better thermal performance for the solar water-in-glass evacuated tube collector (ETC) than for the flat plate solar collector (FPC) at low flow rates (under 3.0 L/min). The outlet temperature reached is similar in both collectors for volumetric flow rates higher than 3.0 L/min. The analysis of the entropy generation rate shows that the generation due to the transfer of heat is higher for the ETC than for the FPC, and this contribution is up to 10% of the total entropy generation rate; on the other hand, the generation rate due to the fluid viscosity is higher for the FPC than the ETC at high volumetric flow rates (above 3.5 L/min), however, this contribution is negligible. Finally, the total entropy generation rate is higher for the FPC than the ETC at low volumetric flow rates (below 3.0 L/min) and this is increased if the solar radiation increases.