Experimental and Numerical Assessments of the Effects of Vacuum and Solar Film on the Performance of a Low Concentration Eccentric Solar Collector

Abstract

Low concentration collectors are usually recommended for water heating and refrigeration systems. The literature reveals lack of information on numerical modeling, experimental data, and thermal performance of eccentric evacuated double tube solar collectors. This study is focused on eccentric solar collectors since this arrangement allows adequate concentration for achieving relatively high temperatures while having small size and occupying less space. The effects of the vacuum in the annular space and reflective film on the enhancement of working fluid heating and overall thermal performance were also assessed. An in-house numerical code using finite volume method was used to discretize the conservation equations and the predictions were validated by experimental results obtained from an experimental rig that was constructed and instrumented for outdoor tests. The experiments were performed in the city of Campinas-Brazil during the autumn season. The investigated versions of the eccentric double tube solar collector include a version with a reflective film and vacuum, a second version with a reflective film but without vacuum, a third arrangement without a reflective film but with vacuum and finally a version without both a reflective film and vacuum. The results showed that the version with reflective film and vacuum demonstrated high efficiency achieving 89%. The lowest efficiency of 42% was achieved by the version without both reflective film and vacuum. The comparative analysis of the four versions shows that the incorporation of reflective film increases the collector efficiency by 28%, while the vacuum increases the efficiency by about 1.3%.

Thermal evaluation of geometrical innovations in concentric and eccentric evacuated single and double solar collectors

“…To determine the eccentricity (focus point of re ection) in the eccentric models (E-WR-WV, E-WR-WTV, E-WTR-WV and E-WTR-WTV) the same formulation and models developed by Teles and Ismail [22] were used. While the concentration in the value of 3.46 was obtained based on Du e and Beckman [24].…”

“…The sky temperature was estimated by the correlation developed by Swinbank [27]. The detailed formulation was based on the study by Teles and Ismail [22].…”

“…The parametercan be the variables u, v, T and P of the conservation equations and n is the index of the iteration order. The home build code was constructed in MATLAB and the numerical code used is similar to that used by Teles and Ismail [22].Grid tests were conducted for the cover and absorber domain. The most relevant test was the one related to the results.…”

“…A high collector length-to-diameter ratio, inclination angle, and circumferential heat input lead to a high natural circulation ow rate. The effects of vacuum and solar lm in a low-concentration eccentric collector were investigated [22]. The eccentric collector with both re ective lm and vacuum reached a maximum e ciency of 89% and a minimum of 42%.…”