تجزیه و تحلیل عملکرد از یک طراحی جدید نوع کلکتور خورشیدی پر شده با لوله های U دو برابر

In order to eliminate the influence of thermal resistance between the absorber tube and the copper fin of the conventional evacuated solar collector, which would decrease the thermal performance of the evacuated tube collector, a filled-typed evacuated tube with single U-tube (SUFET) was presented in our previous work, in which the filled layer was used to transfer energy absorbed by the working fluid flowing in the U-tube. In this paper an improved filled-type evacuated tube with double U-tubes (DUFET) was presented by means of theoretical analysis and experimental study to increase the heat transfer area and then improve the heat transfer efficiency of collectors. The structure of filled-type evacuated tube with double U-tubes was given first, and then the thermal performance of filled-type evacuated tube with double U-tubes was studied in both theory and experiment. The experimental results showed the feasibility and validity of this design. Finally, a comparative study between SUFET and DUFET was given at the same working conditions.

The thermal efficiency of solar collectors is one of the most vital parameters of the solar collectors, and then the thermal performance of the solar collectors is the foundation for further spread of the solar thermal utilization. At present, the flat-plate solar collectors and the evacuated tube solar collectors are the main two kinds of solar collectors, which have been used in architectural field widely. However, evacuated tube solar collectors exhibit better performance than the flat-plate solar collectors, in particular for under high temperature operation. Furthermore, heat extraction manifold designs of single-ended evacuated tubes include simple fluid-in-glass and fluid-in-metal designs, such as heat pipes and a U-tube inserted into the tube. Several studies on the thermal performance of heat pipe solar collectors were presented in the literatures [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and [11]. The thermal performance of the heat-pipe solar collectors was studied in both theory and experiment [1], [2] and [3], in which the evaporator section of collector was consisted of heat pipes with two layers of 100-mesh stainless steel screen fitted. Furthermore, the optimum ratio of the heated length-cooled length of the pipe was also discussed. A new closed-loop, oscillating, heat pipe evacuated tube solar collector had been designed by Rittidech et al. [4] and [5], which had the advantages of corrosion-free operation and the elimination of the winter icing problem. Nkwetta et al. presented a combined low-concentrator augmented solar collector in an array of evacuated tube heat pipe solar collectors, and the optical performance evaluation of internal concentrated evacuated tube heat pipe collectors was reported. And then the performance of an evacuated tube heat pipe solar collector compared to a concentrated evacuated tube single-sided coated heat pipe absorber for medium temperature applications had been studied [6], [7] and [8]. In paper [9] and [10], the thermal performance of a thin membrane heat pipe solar collector and hybrid heat pipe solar collector/CHP system were studied to provide electricity and heating for a building. Vacuum environment was a crucial requirement in the heat pipe to obtain higher thermal efficiency [11]. In fact, maintaining a vacuum environment was very difficult due to the production of non-condensable gases in the heat pipe, especially, when the system was operating. However, the U-tube evacuated tube solar collectors could overcome the flaw, which was caused by the structure of heat pipe evacuated tube solar collector. Furthermore, the heat transfer of the U-tube evacuated tube was more effective due to the U-tube evacuated tube overcame the drawback that the thermal efficiency decreased with the increase of cycle numbers of working fluid [12]. The researches of the U-tube evacuated tube solar collectors were currently available. The thermal performance model of the evacuated tube solar collector with a U-shaped fluid channel was established in paper [13], besides that, the temperature distribution of fluid in the U-tube and the effect of the maximum shift of temperature from the central line of the U-tube are studied. The researchers gave a series of experiments in quasi-dynamic conditions to test the efficiency of the U-tube evacuated tubular collector following the standard EN 12975-2 [14]. Solar collector performance was studied numerically by Kim and Seo to find out the best shape for the absorber tube [15], the results showed that the U-tube collector is presenting better performance compared to other three signal-type collectors taking into account the influence of diffuse radiation and sunshade between the adjacent collectors. In order to enhance heat transfer from the absorber tube to the working fluid and reduce the heat loss of evacuated tube, Diaz developed mini-channel-based evacuated tube solar collectors. The results showed that the efficiency of the mini-channel tube can be improved approximately 5% compared to the standard U-tube collector [16]. The effect of the air gap between the absorber tube and copper tube on the thermal performance of the U-tube evacuated tube solar collector was studied by Ma et al. [17]. Furthermore, Liang et al. had studied the thermal performance of the filled-type evacuated tube with U-tube and the copper fin evacuated tube with U-tube by theoretical and experimental methods based on paper. The results showed that the thermal performance of the filled-type evacuated tube is 12% higher than that of the copper fin evacuated tube [18]. During the research, the authors found that the heat transfer area was a key factor for the efficiency of filled-type evacuated tube with U-tube (UFET). The thermal performance of UFET became higher along with the increment of heat transfer area under the same condition of quantity of flow. Therefore, a filled-type evacuated tube with double U-tubes (DUFET) is presented in this paper to improve the heat transfer efficiency of collectors. The rest of this paper is organized as follows. In Section 2, the description of theoretical modeling of the collector is given. In this part, the heat transfer model and analytic solution of dimensionless model are given after designing of the structure of DUFET. Section 3 constructs the experimental platform for thermal performance of evacuated tube. The test results and further discussion are presented in Section 4. Finally, Section 5 concludes the paper.

In this paper, theoretical analysis of the thermal efficiency of the filled-type evacuated tube with double U-tubes based on an experimental investigation is developed and accomplished. The energy balance equations of the working fluid are established in the flow direction. The results of the investigation may be summarized as follows. 1. The temperature difference between inlet and outlet increases along with increasing of the filled layer thermal conductivity, under the same solar irradiance. Furthermore, the temperature difference increases rapidly when λc goes up from 0.2 to 10, however, the growing tendency becomes slower after λc higher than 10. 2. When the solar irradiance is 900 W/m2 and the filled layer thermal conductivity reaches 100 W/(m K), the efficiency of DUFET is 0.8. 3. The efficiency of DUFET increases above 4% compared to the SUFET, under the same experimental condition.