Analysis of heat transfer characteristics for parabolic trough solar collector system with heat-pipe evacuated tube

Abstract: Heat pipe is an efficient heat transfer component. If the heat pipe and the trough concentrator with higher concentration ratio can be effectively combined, the operating temperature of working fluid can be increased obviously, thus achieving higher system thermal efficiency. Parabolic trough solar collector system with heat-pipe evacuated tube is designed in this paper to provide enough heat input to raise the temperature of an heat transfer fluid to around 200 ℃. The system consists of parabolic trough solar concentrator, heat-pipe evacuated tube (a heat collector element) and support structure. The heat-pipe evacuated tube with material of stainless steel consists of evaporator and condenser sections, and water is used as the working liquid. The evaporator section inside a glass envelope is about 45 mm in diameter with a special coating (selective coating) on the outside surface to provide the required optical properties. The selective coating has a high absorptance for radiation in the solar energy spectrum, and has low emittance in the long wave energy spectrum to reduce thermal radiation losses. The annulus space between the evaporator section and the glass envelope is under vacuum to reduce thermal losses. The condenser section of the heat pipe is inserted into the jacketed vessel and rejects heat to the heat transfer fluid. In order to analyze thermal efficiency and provide theoretical basis for collector system design, a one-dimensional heat transfer mathematical model was established. Using this model, heat transfer characteristics of the system were calculated and analyzed. The results indicate that the transient thermal efficiency of the parabolic trough solar collector system with heat-pipe evacuated tube is higher than 70%, and increases with the increase of the ambient temperature and direct normal solar insolation, and decreases with the increase of the heat transfer fluid temperature and wind speed. Working liquid in heat pipe is at saturated state when the system is running, and the temperature is affected mainly by temperature of absorber tube inner wall. The working liquid temperature in heat pipe increases with the increase of direct normal solar insolation, heat transfer fluid temperature and ambient temperature, and decreases with the increase of wind speed. The working liquid temperature in heat pipe is about 327.6-503.2 K under the given calculation condition. Operating pressure of heat-pipe evacuated tube is obtained by the operating temperature of working liquid, and is about 0.016-2.8 MPa in this paper. The working liquid temperature in this paper is within the optimal range with water as the working liquid in the heat pipe. The heat transfer characteristics of the system are influenced obviously by annulus pressure. Annulus pressures below 0.001 Pa have slight influence on the system performance. But under the annulus pressure above 0.001Pa, the heat loss of the collector increases significantly with the increase of annulus pressure. The system performance is also influenced by the type of gases in the annulus space. Heat loss of the collector is changed with the gases (air, hydrogen and argon) in the annulus space and heat transfer fluid temperature. An inert gas with a high thermal condition coefficient, such as hydrogen, results in the higher heat loss of the collector. The encouraging results in this paper will provide a fundamental reference for researching heat-transfer characteristics and optimization design and performance of parabolic trough solar collector system with heat-pipe evacuated tube.