The average incidence factor at normal incidence To avoid the problem of shading between tubes, only one vacuum solar collector tube is discussed here. In general, the incidence factors of the incident light on the surface of the tube are different at different locations and at different times. It is also very tedious to accurately describe the variation law of the incident factor with time and space. However, when we look at it from another perspective, we can get a useful revelation. That is, first consider the case where incident light is incident perpendicular to the tube axis (ie, normal incidence), and then consider the conversion of general incident light into vertical tube axis incidence. Ignoring the effect of the outer tube on the refraction of incident light (with minimal effect), there are 100,000 parallel light rays that are evenly distributed and perpendicular to the axis of the cylinder. The cylindrical surface is the outer surface of the inner tube, and its diameter is D (0.037m); ii is the incident angle of the ith light. It can be seen that the incident angle of each incident ray is different, which will cause great inconvenience to the calculation. In order to simplify the calculation, the concept of average incidence angle can be obtained on the premise of equivalent energy collection. Let the average incidence angle be ia, then there is cosia=QP20cosidiQP20di(1) calculated: average incidence angle ia=50146b, average incidence factor cosia=01637. The average incidence angle means that when calculating the energy of the tube under normal incidence, The incident light of the vertical axis of the tube is incident at the average incident angle, and thus the collected energy is calculated, which is the same as the energy actually given to the tube surface at the time of normal incidence.

Of course, the average incident angle here can only be applied to the vertical axis incidence. Therefore, we also discuss the situation at the time of general incidence. Average incidence factor for normal incidence and shading When the solar azimuth angle is large, there is a possibility of shade between tubes in adjacent vacuum tubes. Let the inner diameter of the inner tube be D, the center distance of the tube be d, and the collector tube should be placed to the south, and the inclination angle should be H. Using the azimuth angle U as the shading criterion, the criterion is: When |U|>tan-1( When dPD) is 2-1sinH, shade will occur. When shading occurs, since the entire semi-cylindrical surface is not already receiving sunlight, the average incident factor when there is shading and the average incident factor when there is no shading should be different. The angle formed by the shading area on the semi-cylindrical surface is the shading angle. Through calculation, the average incident factor at different shading angles can be obtained when sunlight is incident vertically. Figure 2 shows the distribution of the average incidence factor with shading angle. In the figure, in 0b50b shading angle, the average incidence factor has a gradual increase; in the 50b90b interval, the average incidence factor gradually decreases; 90b average incidence factor and 0b average incidence factor are the same, both are 0.637; 90b180b interval Internally, the average incidence factor drops rapidly to zero. This distribution is in line with the actual situation. In addition, the regression function formula with the change of the average incidence factor with the shading angle can be obtained. (2) is the regression equation obtained after polynomial regression: cosia=016400 319083@10-3A-314437@10-5A2-114458@10-7A3 519225@10-10A4 (2) where a is the shading angle. 113 Incidence multiplication factor at ordinary incidence For general incidence, the general incident light can be projected onto the cross-section of the tube first. In this way, the projection light becomes the incident light of the vertical axis, and the method of the upper section can be used. Dealt with. Let the angle between the general light and the tube cross section be ig, then there is sinig=|sinHtans(1 tan2U)-cosH|1 tan2U tan2s(1 tan2U), (3) where H is the inclination of the collector tube; s is the height of the sun Angle; U is the azimuth of the sun. From equation (3), cosig can be calculated. The effect of cosig and cosia product is equivalent to the incident factor cosi of the flat solar water heater, so cosig@cosia is defined as a single tube incident multiplicative factor. The value of this multiplier factor will change at any point and shade angle. In order to compare with the incident factor of the flat panel, the following only discusses the incident multiplication factor of the vacuum solar collector tube without shading, and the effect of the shading will be discussed separately.

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