End-loss and compensation for small linear Fresnel collectors in severe cold area

Abstract: The linear Fresnel solar concentrating collector has been widely used in the field of solar heat source utilization due to its compact structure, easy manufacture and low price. Because there is a certain distance between the primary mirror and the collector, the position of the sunlight after reaching the collector changes with the incident angle of the sun, and a part of the reflected light will run out of the collector end to form the end-loss. For the large linear Fresnel collector systems, the end-loss length accounts for a small proportion of the length of the collector, while for the small linear Fresnel systems, the end-loss length accounts for a large proportion of the length of the collector, it has a great influence on the heat collection performance of the collector, so it is particularly important to reduce the end loss. According to the geographical location, the end-loss length of a north-south linear Fresnel mirror field is difference. Based on this, aiming at the serious end- loss of linear Fresnel collector in severe cold area, taking the small linear Fresnel collector system in Hohhot area(north latitude 40.87°) as the research object, the end-loss of linear Fresnel collector placed along the horizontal north-south axis and its adjustment and compensation method were studied and verified by experiments in this paper. The experiment bench was developed on a building roof. The linear Fresnel experimental system was mainly composed of a primary mirror, a compound parabolic secondary mirror and a glass-metal vacuum heat collecting tube. After the light reflected by the plane mirror, part of the light was directly absorbed by the vacuum heat collecting tube, and the other part was absorbed by the vacuum heat collecting tube after the secondary reflection of CPC(compound parabolic collector). The change of end-loss in different seasons and different collector heights was analyzed and calculated theoretically, a method of adjusting reflectors to compensate end-loss was proposed. The focal spot length of collector at different time before and after end-loss compensation was captured by infrared thermal imager, and the change of instantaneous heat collection efficiency before and after reflectors adjustment was analyzed. The results showed that the end-loss length was mainly caused by the horizontal distance from the mirror field to the collector, the height of the collector, the solar azimuth angle and height angle. The end-loss length could be reduced by raise the height of the collector under the premise of avoiding the occlusion and shadow loss between the mirror elements. Meanwhile, the end loss could be reduced by increasing the angle between the mirror and the north-south horizontal plane according to the seasonal variation, and the angle should be kept within the theoretical range. The length of the end-loss decrease to about 1/10 of the collector length after the northern end of the mirror field was raised to 20°at summer solstice afternoon, and the instantaneous heat collecting efficiency increased to 65.9% at noon, which was 54.5% higher than that of before compensation. The change trend of the end-loss with time was the same as that of the sun altitude angle in the middle of the day, contrary to the change trend of the sun azimuth angle, the end-loss at noon was the largest, and the en-loss at morning and evening was the smallest. In the same period of time, the end-loss was the largest in winter, the smallest in summer, and almost the same in spring and autumn. The study provided a theoretical reference for reducing the loss-end length of small linear Fresnel concentrating collectors in severe cold area.