Incident multiplier factor of vacuum solar collector tube

Vacuum tube solar water heaters, vacuum solar collector tube is an important part. From the component point of view, the vacuum solar collector tube is the difference between the vacuum tube solar water heater and the flat panel solar water heater. It is also this difference that has led to significant differences in the thermal performance of the two types of water heaters. For example, in the average heat loss coefficient, the average heat loss coefficient of the vacuum tube solar water heater is smaller than the average heat loss coefficient of the flat plate solar water heater; in the non-steady state efficiency curve, the non-steady state efficiency curve of the vacuum tube type solar water heater is a curve. The plate-type solar water heater's non-steady-state efficiency curve is a straight line 111; in terms of daily average efficiency, the average daily efficiency of the vacuum tube type solar water heater is generally lower than the average daily efficiency of the flat type solar water heater. As a cause of the above difference in thermal properties, it is apparent from the viewpoint of the components that the difference between the lighting surface and the heat absorber is different. Flat solar water heaters and heat absorbers are a flat surface; and vacuum tube solar water heaters and heat absorbers are cylindrical surface. If we further analyze their underlying causes, we will find that there is a big difference between the incident factors (cosine incident angle of sunlight) of the two, which is the main cause of the difference in thermal performance (except for the average heat loss coefficient) of the two. the reason. As the incident factor of the plate type solar water heater, when the location and the inclination angle are determined, its value mainly varies with the season and the time of the day; and for the incident factor of the vacuum tube type solar water heater, since the lighting surface is a cylindrical surface Therefore, its value is related to the incident light ray incident on the cylindrical surface in addition to the above factors. In addition, due to the arrangement of the tubes of the vacuum solar collector tube, the shade phenomenon between the tubes is also a unique phenomenon of the vacuum tube type solar water heater. We know that the incident factor plays an important and direct role in energy harvesting. The size and range of the incident factor will affect the heat collection performance of the water heater. This paper mainly discusses the distribution characteristics of the incident factors of a single vacuum solar collector tube, and compares it with the plate-type acquisition surface incident factor (ie, the solar incident factor of the flat panel solar water heater), and proposes the incident multiplication factor of the vacuum solar collector tube. concept. And using this concept it is easy to calculate the energy collected by a single tube.

1 Single tube incident multiplication factor 1. Average incidence factor at normal incidence In order to avoid the problem of shade between tubes, only one vacuum solar 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. A schematic of the vertical incidence of sunlight is given. Ignoring the influence of the outer tube on the refraction of the incident light (with little effect), there are 100,000 pieces of evenly distributed parallel beams perpendicular to the axis of the axis. The 9th day of the publication is 20-iaAcadco magnetic alElectronicPublish shot on the circle H surface. The cylindrical surface is The appearance of the inner tube i (the diameter is D (take 0.037m) is the incident angle of the ith light. It can be seen that the incident angle of each incident light is not the same, which will cause great inconvenience to the calculation. To simplify the calculation The concept of average angle of incidence can be obtained on the premise of equivalent energy collection, and if the average incidence angle is k, shading will occur.

For the shading criterion, the criterion is: when calculated by 卩: average incident angle ia = 50. 46 average incident factor cosia = 0.637. The average incident angle means that when calculating the energy of the tube under vertical incidence, the vertical The incident light of the tube axis 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.

1.2 Average Incidence Factor at Normal Incidence and Shading When the solar azimuth is large, there is a possibility of shade between tubes in adjacent vacuum tubes. Let the outer diameter of the inner tube be D, and the center distance of the D tube should be d. The collector tube should be placed to the south, and the tilt angle should be 0. Using the azimuth angle, when the shade phenomenon occurs, it is not the whole semi-cylindrical surface that receives sunlight, so there is a cover. The average incidence factor in the negative and the average incidence factor in the absence of shading should be different. Let the incident product factor of the shaded area in the semi-circular 13-like incident, for general incidence, use the general incident ray to project onto the cross-section of the tube first, so that the projected ray becomes the incident ray of the vertical tube axis. It can be processed according to the method of the previous section. Assuming that the angle between the general light and the cross-section of the tube is ig, the azimuth from (3) can be calculated as the angle formed by the cosig.cosig and cosia cylinders. By calculation, the positivity product can be obtained. The effect is equivalent to the average incidence factor under different shading angles when the entrance light of the flat solar water heater is normal incidence.

The factor cosi, therefore, defines cosigXcosia as the incident incidence of a single tube. The incident product factor of the vacuum solar collector tube is consistent with the flat plate species distribution characteristics. In addition, the incident factor of the blish% water absorber is average incident. The distribution of the factor with the shading angle. In the figure, the average incidence factor has a gradual increase within the shade angle of 050; in the 5090 interval, the average incidence factor gradually decreases; the average incidence factor of 90 is the same as the average incidence factor of 0, both are 0.637; the 90180 interval Internally, the average incidence factor drops rapidly until it reaches zero. This product 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.

A) CDSDSW) calculated the energy collected in the Beijing area for two hours and two to four days before the first four hours (8012:(1)). In addition, for the purpose of comparison, the energy collected by the flat lighting surface at the same inclination angle was calculated under the same lighting surface (the length is the same as the vacuum tube and the width is the same as the inner diameter of the inner tube of the vacuum tube). The energy collected by the two types of devices is shown in Table 1. It can be seen that the energy collected by the vacuum solar collector tube is always higher than 11% of the corresponding slab energy collection, which means that under the same lighting surface as a single vacuum solar collector tube, the energy collection performance of the vacuum tube must therefore not be the same. Under the lighting area, the energy harvesting performance of the vacuum tube solar water heater is better than that of the flat solar water heater. A comparison of the energy harvesting of two types of water heaters will be discussed in later articles. It is particularly important to point out here that with the incident multiplication factor, the calculation can be greatly simplified, so that the formula for calculating the energy harvesting is formally irrelevant to the position of the incident light on the cylindrical surface, ie, from a formal point of view, After the placement site and the inclination angle were determined, the collected energy was only a function of time (seasonal), which made it consistent with the plate's energy collection formulae in terms of form characteristics.

Table I Comparison of vacuum solar collector tube and corresponding plate energy collection vacuum solar collector tube / M plate / M ratio spring equinox summer solstice winter solstice 3 Conclusion (1) In a single vacuum solar collector energy calculation, the introduction of the concept of incident multiplication factor, It can make the expression of the formula more concise and the calculation more convenient.

When there is no shade between tubes, the incident multiplicative factor value of a single vacuum solar collector facing south is smaller and less volatile.

On the same day, in different regions, vacuum solar collector tubes with different dip angles will have the same daily distribution of the incident product factor as long as they have the same value of the dip angle-dips. Especially when the dip angle is equal to the dip angle, the value of the incident multiplication factor remains constant throughout the day, and the date may be different, and the values ​​may be different.

When the arrangement of the bundles of multiple vacuum solar collector tubes (such as a vacuum tube type solar water heater) appears shaded between the tubes, the incident product factor will change with the shade angle.

For the vacuum tube type solar water heater composed of multiple vacuum solar collector tubes, the incident product factor may be used regardless of whether there is shading between the tubes when calculating the collected energy. When there is a shading phenomenon, except that the value of the incident product factor changes according to the shade angle, the lighting area of ​​the collector tube will also change with the shade angle. In addition, vacuum tube solar water heaters are generally equipped with diffuse reflectors, which also have a great influence on the energy collection of vacuum tubes. Therefore, the energy collection of the vacuum tube solar water heater needs to consider a variety of factors, which is more tedious. This content will be described in a future article.

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