Abstract
Solar collectors absorb solar energy have been used in different applications. Among the different types of solar collectors available, evacuated tube heat pipe solar collectors remain the most convenient type of collector. The shortage of energy supply results in persistent focus on the performance improvement of this system. The main purpose of this work is to produce a thermal resistance network model for evaluating the impact of the geometry insert in evacuated tube heat pipe solar collectors. An evacuated tube solar collector with a heat pipe containing an I-section geometry insert was used for the investigation.
A numerical analysis was performed by use of a thermal resistance network model. Five different working fluids namely water, methanol, acetone, toluene, and ethanol were considered. Data were recorded for a duration of seven hours in Durban from the solar station at Mangosuthu University of Technology (Sauran Sta Station at Umlazi, South Africa). The data were recorded for seven hours under cloudy weather conditions. Those data were combined with values from books and previous works to calculate the thermal resistances and efficiencies of the solar collector with and without insert.
This study reveals the following: introducing an insert decreased the total thermal resistance, which resulted in improved efficiencies. For example, when water was used as a working fluid, the total thermal resistances without and with an insert were 1.52 K/W and 1.49 K/W, respectively. The efficiencies of water without and with an insert were observed to be 67.01% and 68.2%, respectively at 9:00 a.m. The rate of useful heat transfer also increased from 622.46 KW to 633.54 KW. Water was observed to have higher efficiencies than the other working fluids with and without a geometry insert.
The trend of the obtained experimental and numerical results was congruent. However, the experiment results obtained with ethanol did not match the trend of its numerical results. This deviation was due to reasons such as assumptions and variance in fluid properties. However, the thermal resistance network model is deemed appropriate to be used for performance prediction of the heat pipe with an insert.