CPC solar thermal collector for winter heating and its optical efficiency evaluation method

Abstract

The present invention discloses a CPC type solar collector for winter heating and its optical efficiency evaluation method, including a bracket, a collector tube and a CPC. The solar collector is placed along the east-west direction, and the relationship between the incident angle θ' of sunlight and the inclination angle #imgabs0# of the CPC is as follows: #imgabs1# where γ s is the solar hour angle, and it is stipulated that solar noon is 0°, positive values are taken in the morning and negative values are taken in the afternoon, and each hour corresponds to 15°. The solar altitude angle α s = arcsin(cosλ·cosδ·cosγ s + sinλ·sinδ), λ is the local latitude, the solar declination δ = 23.45·sin(360·(284 + n) / 365), n is the number of days calculated from January 1st, and the inclination angle #imgabs2# is the angle between the symmetry axis of the cross-section of the CPC when tilted and the vertical line of the horizontal plane. The beneficial effects of the present invention are mainly applied to winter heating, maximizing high-efficiency operation (condensing effect > 90%) during the period of 9:00 - 15:00 in winter solar time, and ensuring a higher concentration ratio and a larger opening. On the premise of ensuring a high photothermal conversion efficiency, the cost is further reduced.

Description

Technical Field

[0001] The invention relates to the field of solar energy utilization, and in particular to a CPC type solar thermal collector for winter heating and an optical efficiency evaluation method thereof. Background Art

[0002] Solar collector is a device that efficiently converts sunlight into heat energy. It can be used to provide hot water, heating, desalination, food fermentation, industrial steam, and even thermal power generation. It is widely used. At present, mature solar collectors mainly include flat plate collectors, all-glass vacuum tube collectors, heat pipe vacuum tube collectors and compound parabolic concentrators (CPC). CPC can make sunlight reach the receiving surface after multiple reflections within the specified receiving angle range, with less light loss, especially without considering tracking the sun. The effective collection of sunlight can almost reach the theoretical value, and it has stronger competitiveness in the field of solar medium and low temperature heat collection applications. The ideal CPC concentrator must be in contact with the surface of the receiver. However, in practical applications, in order to avoid conduction heat loss, as well as the damage caused by contact during installation and long-term gravity deformation of the concentrator, a certain gap margin must be left in the design, but the energy loss caused by the gap reaches 5% to 20%.

[0003] One solution proposed by Winston et al. is to use a truncated CPC and a W-shaped cavity to form a concentrator, which can achieve a higher theoretical concentration ratio (U.S. Patent No. 230137), but the W-shaped cavity is not easy to process in the subsequent process. Huang Hulin's team at Nanjing University of Aeronautics and Astronautics also mentioned that a large number of vacuum, non-tracking concentrators have low concentration efficiency and poor economic performance, making them difficult to promote; in 2009, the team designed a concentrator with two parabolas rotated 3° toward the center, which increased the receiving half angle, and the efficiency did not change too much due to seasonal changes (patent CN200910035270.5), but it reduced the concentration ratio and increased the cost per unit area.

[0004] The widespread use of CPC requires further optimization of the balance between light-to-heat conversion efficiency and cost, as well as the reflector bracket materials, installation ease, etc. For these issues, in summary, the following issues are less considered or have not yet been overcome: 1. How to comprehensively consider key parameters such as the diameter of the collector tube, the receiving half angle, the inclination angle, and combine them with the latitude of the area of ​​use, the season of use, the sunshine time, etc.; 2. How to adopt appropriate interception ratio and reflector opening to reduce costs while keeping the concentration ratio unchanged. Summary of the invention

[0005] The problem to be solved by the present invention is that it is difficult to strike a balance between the existing CPC light-to-heat conversion efficiency and the cost. To this end, a CPC type solar collector for winter heating and an optical efficiency evaluation method thereof are provided.

[0006] The technical solution of the present invention is: a CPC type solar collector for winter heating, comprising a bracket, a heat collecting tube and a CPC, the solar collector is placed in the east-west direction, and the relationship between the incident angle of sunlight θ′ and the CPC inclination angle φ is as follows: θ′=arctan(cosγ s / tanα s )-φ, where γ s The solar hour angle is defined as 0° at noon, positive values ​​are taken in the morning, and negative values ​​are taken in the afternoon. Each hour is equivalent to 15°. The solar altitude angle α s =arcsin(cosλ·cosδ·cosγ s +sinλ·sinδ), λ is the local latitude, solar declination δ = 23.45·sin(360·(284+n) / 365), n is the number of days from January 1, and the inclination angle φ is the angle between the symmetry axis of the cross section and the vertical line of the horizontal plane when the CPC is tilted.

[0007] The diameter of the heat collecting tube in the above scheme is 50mm to 90mm.

[0008] The minimum spacing between the collector tube and CPC in the above scheme is 20 mm to 50 mm.

[0009] The CPC cross section in the above scheme has an opening range of 350 mm to 850 mm and a height of 250 mm to 1250 mm.

[0010] The interception ratio of CPC in the above scheme is 0.5-0.8.

[0011] The optical efficiency evaluation method of a solar thermal collector equipped with CPC for winter heating comprises the following steps: using a ray tracing method to calculate the optical efficiency of CPC at different incident angles, wherein the calculation process of the ray tracing method is as follows: (a) giving the initial position and direction of all light rays; (b) for each light ray, judging whether it intersects with a heat collecting tube, if so, proceeding to step (e), otherwise proceeding to step (c); (c) judging whether it intersects with a reflector, if so, proceeding to step (f), otherwise proceeding to step (d); (d) the light escapes and cannot be absorbed by the heat collecting tube, calculating the escape position, and terminating the calculation; (e) calculating the intersection position of the light ray and the heat collecting tube, and terminating the calculation; (f) calculating the intersection position of the light ray and the reflector, determining the direction and position of the light ray after being reflected by the surface through the law of reflection, generating a reflected light ray, and continuing to step (b); according to the calculation of the ray tracing method, the total number of light rays to be traced is M, and the number of light rays C that reach the surface of the heat collecting tube after 0 to 3 reflections is counted, then the incident angle θ i The focusing effect under oblique incidence is η = C / M. Considering the cosine loss under oblique incidence, the actual focusing effect is η t =C / M*cos(θ i ), calculate the incident angle of sunlight on the CPC cross section at different times of the winter solstice, summer solstice and spring and autumn equinoxes, and calculate the actual focusing effect at different times based on the incident angle. The actual focusing effect at different times is weighted averaged, with the weight being the local solar radiation intensity at different times, thereby estimating the average optical efficiency of CPC.

[0012] The beneficial effect of the present invention is that it is mainly used for winter heating, maximizing the high-efficiency work during the winter solar time period of 9:00-15:00 (focusing effect > 90%), and ensuring a higher focusing ratio and a larger opening, further reducing costs while ensuring a higher light-to-heat conversion efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the relationship between the incident angle of CPC and solar time under different times and placement modes of the present invention (inclination angle φ = 31.5°);

[0014] Figure 2 Schematic diagram of the relationship between the incident angle of CPC and solar time at different times (inclination angle φ = 41.5°);

[0015] Figure 3 It is a schematic diagram of the CPC cross-sectional structure of the present invention;

[0016] Figure 4 Schematic diagram of CPC cross-sectional structure with different receiving half angles of the present invention;

[0017] Figure 5 It is a schematic diagram of the CPC interception of the present invention;

[0018] Figure 6 is the optical efficiency of the CPC of the present invention at different incident angles;

[0019] Figure 7 It is a schematic diagram of the working condition of the CPC of the present invention when it is less than the half acceptance angle. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with the accompanying drawings.

[0021] When using solar energy for winter heating, the winter working hours and incident angle must be taken into account. When the incident angle of sunlight exceeds the CPC receiving half angle, the efficiency will quickly decay to 0. Take Ma'anshan as an example (31.5° north latitude), when the collector is tilted at 31.5° (the tilt angle is the angle between the collector tube and the ground when placed north-south, and the tilt angle is the angle between the CPC cross-section symmetry axis and the vertical line of the horizontal plane when placed east-west), that is, the tilt angle = local latitude: (1) To make the north-south collector work from 9:00 to 15:00 in winter, the receiving half angle needs to be greater than 45°; (2) To make the east-west collector work from 9:00 to 15:00 in winter, the receiving half angle needs to be greater than 30°. The CPC concentration ratio is equal to the inverse of the sine value of its receiving half angle. Therefore, the smaller the receiving half angle, the higher the concentration ratio and the higher the efficiency. And the smaller the receiving half angle, the larger the CPC opening, and the lower the unit area cost. However, too small a receiving half angle will seriously restrict the working hours in winter. It is necessary to comprehensively weigh key parameters such as the diameter of the collector tube, the opening of the reflector, the receiving half angle, the inclination angle, the interception ratio, etc. In order to maximize the efficiency of light-to-heat conversion, it is very important to determine the key parameters of the CPC collector through a large number of simulations and experiments based on local data (latitude, sunshine hours):

[0022] 1. Determination of receiving half angle and inclination:

[0023] Taking Ma'anshan (31.5° north latitude) as an example, it can be calculated (see relevant literature for specific formulas) that when the collector inclination is 31.5° (the inclination is the angle between the collector tube and the ground when placed north-south, and the inclination is the angle between the CPC section symmetry axis and the horizontal plane vertical when placed east-west), that is, when the inclination = local latitude, the relationship between the solar incident angle of the CPC section and the solar time at the winter solstice, summer solstice and spring and autumn equinox is as follows: Figure 1 shown.

[0024] The total efficiency of north-south placement is slightly higher than that of east-west placement, but the concentration ratio of north-south placement is lower than that of east-west placement, and the receiving half-angle required for east-west placement is smaller than that of north-south placement, the opening is larger, the concentration ratio is larger, and the relative cost is lower, so we adopt the placement of CPC collectors in the east-west direction. Considering that our CPC collectors are mainly used for winter heating, in order to reduce costs, the receiving half-angle can be further reduced to obtain a higher concentration ratio and a larger opening. The efficiency of winter light-to-heat conversion can be further improved and the cost can be reduced by increasing the inclination angle and sacrificing part of the efficiency in summer. Taking Ma'anshan as an example, after increasing the inclination angle by 10°, that is, when the inclination angle = 41.5°, it can be found that the receiving half-angle that meets the winter 9:00-15:00 work can be reduced to 22.5°, which is conducive to further reducing costs. Therefore, we choose the receiving half-angle = 22.5°. For cities at other latitudes, the same method can be used for design to determine the appropriate working hours, inclination angles and receiving half-angles.

[0025] Specifically, the sunlight incident angle of the CPC cross section is related to the CPC inclination angle. The relationship between the sunlight incident angle θ′ of the CPC cross section and the CPC inclination angle φ is as follows:

[0026] Place the things θ′=arctan(cosγ s / tanα s )-φ

[0027] North-South placement θ′=cotα s ·sinγ s / (cosφ+cotα s ·cosγ s ·sinφ)

[0028] Among them, γ s The solar hour angle is defined as 0° at solar noon, with positive values ​​in the morning and negative values ​​in the afternoon, which is equivalent to 15° per hour.

[0029] Sun altitude angle α s =arcsin(cosλ·cosδ·cosγ s + sinλ·sinδ);

[0030] λ is the local latitude;

[0031] Solar declination δ = 23.45 sin(360 (284 + n) / 365)

[0032] n is the number of days from January 1;

[0033] The inclination angle φ is the angle between the cross-section symmetry axis and the vertical line of the horizontal plane when the CPC is tilted. When the incident angle of sunlight on the CPC cross section is greater than the half-angle of acceptance of the CPC, the CPC cannot focus the light on the collector tube and thus cannot work. The example of Ma'anshan is just for the convenience of understanding.

[0034] 2. Overall design of CPC collector

[0035] Solar collectors are mainly used for medium and low temperature heating. We believe that the diameter (inner diameter) of the collector tube should be between 50mm and 90mm, and the minimum distance between the collector tube and the reflector should be between 20mm and 50mm. Considering the cost, winter working hours and efficiency, the receiving half angle should be between 15° and 30°, and the inclination angle should be between the local latitude and the local latitude + 25°, which can be adjusted according to the actual situation.

[0036] The minimum spacing between the collector tube and the reflector is based on the diameter, thickness and margin of the vacuum glass tube during installation. For collector tubes with larger diameters, considering the difficulty of actual engineering installation, a vacuum glass tube with a larger diameter is generally required, and the gap is larger. However, too large a spacing is prone to light leakage and loss of efficiency, so a 20mm to 50mm spacing is selected after comprehensive consideration. The low temperature of solar energy is generally 80℃ to 250℃.

[0037] Through analysis of economy, optical efficiency, and thermal collection efficiency, some scholars have found that when the collection temperature is less than 80°C, a CPC collector with a receiving half-angle greater than 30° should be selected; when the collection temperature is between 80°C and 120°C, a CPC with a receiving half-angle of 20° to 30° should be selected; when the collection temperature is between 130°C and 50°C, a CPC with a receiving half-angle of 10° to 20° should be selected; when the collection temperature is greater than 150°C, a CPC with a receiving half-angle of less than 10° should be selected.

[0038] The heat collecting tube can be in the form of vacuum glass tube, heat pipe, U-shaped tube, etc. It is also possible to add absorption fins to the heat collecting tube to further improve the absorption. Figure 3 shown.

[0039] The involute and parabola of CPC satisfy the standard CPC trajectory equation respectively.

[0040] The opening range of CPC section is 350mm~850mm, and the height is 250mm~1250mm. In order to reduce the cost, CPC can be cut to a certain extent. Under the premise that the concentration ratio does not change much, the cut ratio range is selected to be 0.5~0.8. The schematic diagram of the section of CPC is as follows Figure 5As shown. The opening range of the 3CPC section is 350mm~850mm, and the height is 250mm~1250mm. This range is calculated by different receiving half angles and different collector tube diameters and gaps. For example, the CPC with a receiving half angle of 30°, a collector tube diameter of 60mm and a gap of 20mm has an opening of 425mm and a height of 489mm. The CPC with a receiving half angle of 22.5°, a collector tube diameter of 90mm and a gap of 35mm has an opening of 855mm and a height of 1244mm.

[0041] 3. Evaluate CPC optical efficiency

[0042] The optical efficiency of CPC at different incident angles was calculated using the ray tracing method to estimate the average optical efficiency. Figure 6 shown.

[0043] The optical efficiency of CPC at different incident angles is calculated using a ray tracing method. The calculation process of the ray tracing method is as follows: (a) the initial position and direction of all light rays are given; (b) for each beam of light, determine whether it intersects with the heat collector, if so, proceed to step (e), otherwise proceed to step (c); (c) determine whether it intersects with the reflector, if so, proceed to step (f), otherwise proceed to step (d); (d) the light escapes and cannot be absorbed by the heat collector, the escape position is calculated, and the calculation is terminated; (e) the intersection position of the light and the heat collector is calculated, and the calculation is terminated; (f) the intersection position of the light and the reflector is calculated, the direction and position of the light after reflection from the surface is determined by the law of reflection, the reflected light is generated, and step (b) is continued; according to the calculation of the ray tracing method, the total number of light rays for ray tracing is M, and the number of light rays C that reach the surface of the heat collector after 0 to 3 reflections is counted, then the incident angle θ i The focusing effect under oblique incidence is η = C / M. Considering the cosine loss under oblique incidence, the actual focusing effect is η t =C / M*cos(θ i ), calculate the incident angle of sunlight on the cross section at different times of the winter solstice, summer solstice and spring and autumn equinoxes, and calculate the actual focusing effect at different times based on the incident angle. Take the weighted average of the actual focusing effect at different times, and the weight is the local solar radiation intensity at different times (find relevant literature or data), so as to estimate the average optical efficiency of CPC.

[0044] The schematic diagram of the working condition when the receiving angle is smaller than the receiving half angle is as follows Figure 7 shown.

[0045] The interception ratio is defined as the ratio of the height of the CPC after interception to the height of the CPC without interception. The interception ratio is generally 0.5 to 0.8. The main purpose of interception is to reduce costs. The upper part of the CPC reflective surface is almost vertical, and the focusing effect of this part of the mirror is very poor. After a large part of the area is intercepted, the effect on the focusing ratio is small, which can reduce material consumption and save costs. When the interception ratio is greater than 0.5, the focusing ratio decreases very slowly. Only when the interception ratio is less than 0.5 will the focusing ratio decrease rapidly. Considering the system optical efficiency and cost, it is more appropriate to choose a focusing ratio of 0.5 to 0.8. Figure 7 It shows that when the incident angle of sunlight is close to the receiving half angle, the efficiency of CPC is the highest, close to 100%.

[0046] The advantages of the compound parabolic concentrator are: the concentrator can reflect solar radiation within the receiving half-angle range to the collector tube, so there is no need to track the sun, saving the cost of tracking the sun; we designed a certain distance between the vacuum collector tube and the CPC concentrator, which can reduce contact heat loss, facilitate installation and debugging, and avoid extrusion caused by collision and deformation. This CPC is mainly used for winter heating. Taking Ma'anshan as an example, we designed the CPC receiving half-angle for this area to be 22.5° and the concentration ratio to be 2.61, which not only maximizes the high-efficiency work during the 9:00-15:00 solar time in winter (concentration effect>90%), but also ensures a higher concentration ratio and a larger opening, further reducing costs while ensuring a high light-to-heat conversion efficiency. In addition, by adjusting the inclination angle, the CPC can also work in other places in the country where there is a need for winter heating.

Claims

1. CPC type solar collector for winter heating, including bracket, heat collecting tube and CPC, characterized by: The solar collector is placed in the east-west direction, and the incident angle of sunlight θ′ is related to the CPC inclination angle The relationship is as follows: θ′=arcta Among them, γ s The solar hour angle is defined as 0° at noon, positive values ​​are taken in the morning, and negative values ​​are taken in the afternoon. Each hour is equivalent to 15°. The solar altitude angle α s =arcsin(cosλ·cosδ·cosγ s + sinλ·sinδ), λ is the local latitude, solar declination δ = 23.45·sin(360·(284+n) / 365), n is the number of days from January 1, and the inclination It is the angle between the symmetry axis of the cross section and the perpendicular line of the horizontal plane when the CPC is tilted.

2. The CPC type solar collector for winter heating as claimed in claim 1, characterized in that: The heat collecting tube has a diameter of 50 mm to 90 mm.

3. The CPC type solar collector for winter heating according to claim 1, characterized in that: The minimum spacing between the heat collecting tube and CPC is in the range of 20 mm to 50 mm.

4. The CPC type solar collector for winter heating according to claim 1, characterized in that: The CPC cross section has an opening range of 350 mm to 850 mm and a height of 250 mm to 1250 mm.

5. The CPC type solar collector for winter heating according to claim 1, characterized in that: The intercept ratio of the CPC is 0.5 to 0.

8.

6. The optical efficiency evaluation method of CPC type solar collector for winter heating according to any one of claims 1 to 5, characterized in that: The following steps are involved: The optical efficiency of CPC at different incident angles is calculated using a ray tracing method. The calculation process of the ray tracing method is as follows: (a) the initial position and direction of all light rays are given; (b) for each beam of light, determine whether it intersects with the heat collector, if so, proceed to step (e), otherwise proceed to step (c); (c) determine whether it intersects with the reflector, if so, proceed to step (f), otherwise proceed to step (d); (d) the light escapes and cannot be absorbed by the heat collector, the escape position is calculated, and the calculation is terminated; (e) the intersection position of the light and the heat collector is calculated, and the calculation is terminated; (f) the intersection position of the light and the reflector is calculated, the direction and position of the light after reflection from the surface is determined by the law of reflection, the reflected light is generated, and step (b) is continued; according to the calculation of the ray tracing method, the total number of light rays for ray tracing is M, and the number of light rays C that reach the surface of the heat collector after 0 to 3 reflections is counted, then the incident angle θ i The focusing effect under oblique incidence is η = C / M. Considering the cosine loss under oblique incidence, the actual focusing effect is η t =C / M*cos(θ i ), calculate the incident angle of sunlight on the CPC cross section at different times of the winter solstice, summer solstice and spring and autumn equinoxes, and calculate the actual focusing effect at different times based on the incident angle. The actual focusing effect at different times is weighted averaged, with the weight being the local solar radiation intensity at different times, thereby estimating the average optical efficiency of CPC.

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