Experimental investigation of U-Pipe evacuated tube solar collector: thermal performance enhancement methodology

Abstract

In this study thermal performance comparison of U-Pipe evacuated tube solar collector (ETC) vs heat pipe ETC (HPETC) was investigated experimentally. ETC's are available in either heat pipe or u-pipe configurations. In this work, a custom made U-Pipe ETC design is studied alongside commercially available HPETC setup. The copper U-pipe design was developed to fit an Apricus mini-ETC and maximize heat extraction from the collector tubes. Collection performance of the U-pipe ETC design was compared to that of a HPETC configuration in a custom tandem collector natural convection setup over several days with and without thermal storage media. Results from testing without thermal storage media showed U-pipe ETC achieved higher peak water temperatures, maximum of 31⁰C, than that of heat pipe, maximum 29⁰C, resulting in 13% efficiency enhancement of U-pipe ETC compared with HPETC. The results also showed more rapid heating/cooling in the U-pipe setup. To further enhance the thermal performance of the collectors, several forms of phase change materials were investigation for their potential use as either heat transfer media, or thermal storage material in the context of the low-medium operating temperature of the ETC. The PCM's considered in this study are ternary eutectic nitrate salt (HITEC), tritriacontane paraffin wax (PCM72) and erythritol. In order to overcome the low thermal conductivity and further enhance specific heat capacity, various weight concentrations of multi-walled carbon nanotubes (MWCNT) are added. Additionally, to insure even distribution of MWCNT and consistent properties of the HTF, triethanolamine (TEA) is proposed to be incorporated as a dispersant. The samples were each tested in a Thermogravimetric Analyzer (TGA) and Differential Scanning Calorimeter (DSC) to analyze their thermal properties. To assess homogeneity of the MWCNT dispersion, scanning electron microscopy was also performed. Performance enhancement to both the U-pipe ETC and HPETC setups was investigated by integrating PCM72 as thermal storage and silicone oil as heat transfer medium. Results of this experiment showed peak temperatures for fin and water tank of the U-pipe as ETC 47⁰C and 38.7⁰C respectively, whereas heat pipe showed values of 60⁰C and 36.7⁰C for max fin and water temperatures respectively. Further performance enhancement to the U-pipe ETC system was investigated through integration of PCM72 with added 1wt%MWCNT nanoparticles. Peak temperatures recorded for this experiment were 63.8⁰C and 43.3⁰C for the U-pipe fin and water tank respectively. The results from this study show that the U-Pipe setup was able to achieve higher water temperatures than that of the HPETC setup. Incorporation of PCM thermal storage allowed for slower heat release in the water, extending useful duration, however internal temperatures were not high enough to fully realize latent heat capability. Testing with nanoparticle enhanced PCM thermal storage exhibited higher internal temperatures, reaching closer to full utilization of latent heat storage capability.