The effect of different working fluids and internal geometries on the thermal performance of heat pipes in evacuated tube solar collectors

Abstract

Evacuated tube heat pipe solar collectors produce hot water more efficiently than the flat plate type. It is observed from various studies that involve evacuated tube heat pipe solar collector that its thermal performance depends on various external and internal factors. The internal factors are primarily related to the performance of the heat pipe. It was assumed that the two elements that define the efficiency of the heat pipe are the working fluid and the internal geometry. No inclusive study exists that systematically investigated the combined effect of working fluid and internal geometry on the thermal performance of heat pipes in evacuated tube solar collectors. This investigation consisted of the design and testing of an evacuated tube solar collector comprising new modified heat pipes. Heat pipes containing inserts with particular profiles were tested with different working fluids. A multivariate polynomial regression analysis was conducted to validate the assumption that the merit number of the working fluid and the insert's surface area affect the efficiency. The regression analysis that resulted from the assumption that the merit number and the surface of the insert were the independent variables affecting the heat pipe’s efficiency had to be rejected due to poor/unacceptably low coefficient of determination (R2) result. Based on a newly thought assumption, that the boiling point temperature of the working fluid could be an independent variable affecting the heat pipe’s performance or efficiency, a new regression analysis produced very acceptable results. The results have shown that the surface areas of the insert in the heat pipe had an impact on the efficiency of the solar collector. This is possibly because of the enhancing of the heat transfer by increased convection between the surfaces of the insert and the vapour of the working fluid moving from the evaporator to the condenser. For example, with distilled water as the working fluid, the experiment on the conventional circular heat pipe without an insert produced an efficiency of 53.3%, while results on the single insert and the S insert show efficiencies of 59.1% and 64.3% respectively. A very good correlation with a R2= 0.99 was obtained when expressing the efficiency as a function of the boiling point of the working fluid for an evacuated tube heat pipe. It transpires from this correlation that the working fluid with a high boiling number has also a high thermal efficiency. The comparison between the measured and the predicted results using the final multivariate polynomial regression (equation 5.2) exhibits excellent accuracy in the prediction of the performance of an evacuated heat pipe solar collector with R2 = 0.98 and an average error of 1.1%. It may be of assistance in predicting the heat pipe’s efficiency for any untested insert’s profile, or working fluid’s boiling temperature or a combination of both.