Trough type solar concentrating heat collection system with flat secondary reflector and design method thereof

Abstract

The present invention discloses a trough-type concentrating heat collection system with a flat secondary reflector and a design method thereof. The heat collection system comprises: a large-opening trough-type concentrator and a novel heat collection tube, the novel heat collection tube is composed of a metal heat absorbing tube, a glass shell and a flat secondary reflector, the center point of the metal heat absorbing tube deviates from the focus of the original large-opening trough-type concentrator and moves downward, the lowest end point of the metal heat absorbing tube reaches point N, intercepting the sunlight passing through the lower end of the metal heat absorbing tube, the metal heat absorbing tube and the glass shell are not concentric, the flat secondary reflector is located in the upper half of the glass shell, at the upper end of the metal heat absorbing tube, and reflects the light passing through the upper end surface of the metal heat absorbing tube and not being reflected back to the metal heat absorbing tube. The present invention intercepts sunlight to the maximum extent and improves the optical efficiency; at the same time, a heat absorbing tube with a smaller diameter is used, the external radiation area is small, and the thermal efficiency is high.

Description

Technical Field

[0001] The invention belongs to the technical field of solar energy concentrating and heat collecting, and in particular relates to a trough type concentrating and heat collecting system with a flat secondary reflector and a design method thereof. Background Art

[0002] Solar energy is undoubtedly the largest renewable energy source that can be developed on Earth. The forms of solar energy concentration utilization are divided into dish, tower, trough and linear Fresnel. Trough power generation technology is the most mature, accounting for more than 90% of the solar concentrating thermal power generation market, and is also the only commercial solar thermal power generation product.

[0003] Large opening and high concentration ratio are currently the most effective methods to increase the heat collection temperature and reduce investment costs. The traditional large opening high concentration solar collector system consists of a large opening trough concentrator and a collector tube, which uses a larger diameter collector tube to intercept solar light and improve optical efficiency. However, the larger diameter collector tube leads to a large external radiation loss area and low thermal efficiency. Therefore, in order to improve optical efficiency and reduce external radiation loss, it is urgent to design a collector tube that can intercept sunlight but has a small diameter to improve optical efficiency and reduce investment costs. Summary of the invention

[0004] To solve the above problems, the present invention discloses a trough-type concentrating heat collection system with a flat secondary reflector and a design method thereof, which improves optical efficiency, reduces external radiation loss, and ultimately improves the thermal efficiency of the system.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] A trough-type solar concentrator and heat-collecting system with a flat secondary reflector comprises: a large-opening trough-type concentrator and a heat-collecting tube, wherein the heat-collecting tube is composed of a metal heat-absorbing tube, a glass shell and a flat secondary reflector, wherein the metal heat-absorbing tube is located inside the glass shell and is not concentric with the glass shell, and the flat secondary reflector is located in the upper half of the glass shell, at the upper end of the metal heat-absorbing tube.

[0007] Furthermore, the center point O′ of the metal heat absorption tube deviates from the focus O of the large-opening trough concentrator, moves downward by a distance L to reach point O′, and the lowest end point of the metal heat absorption tube reaches point N, which is used to intercept the sunlight passing through the lower end of the metal heat collecting tube; the distance between the flat plate secondary reflector and the focus O of the large-opening trough concentrator is h, which is used to reflect the light that passes through the upper end surface of the metal heat absorption tube and is not reflected back to the metal heat absorption tube; the difference between the radius R of the metal heat absorption tube and the downward moving distance L is less than the h from the flat plate secondary reflector to the primary reflector; the sum of the radius R of the metal heat absorption tube and the downward moving distance L is less than the radius of the glass shell.

[0008] Furthermore, the width K of the flat secondary emission mirror is larger than the diameter of the metal heat absorption tube and smaller than the diameter of the glass shell.

[0009] The design method of the trough type solar concentrator system with a flat secondary reflector comprises the following steps:

[0010] Step 1: Determination of the lowest point N of the collector tube:

[0011] The distance L from the center point O' of the metal heat absorber to the focus O of the large opening trough concentrator 1 It is expressed as:

[0012]

[0013] Where: f is the focal length of the large-opening trough concentrator, is the edge angle, M is the opening width of the large-opening trough concentrator, α is the maximum error angle, and R is the radius of the metal heat absorption tube;

[0014] Step 2: Calculate the width K of the flat secondary reflector mirror and the distance h from the flat secondary reflector mirror to the focus O of the large opening slot concentrator:

[0015]

[0016] Step 3: Optimization of geometric parameters:

[0017] The geometric parameters obtained in the first and second steps, as well as the geometric parameters of the collector tube and the optical parameters of the material are input into the SolTrace software to establish a geometric model. The geometric parameters L, h, and K are adjusted to maximize the solar heat flux density on the surface of the collector tube, thereby obtaining the optimal geometric model.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The new collector tube intercepts the sun's rays to the maximum extent, improving the optical efficiency. At the same time, it adopts a smaller diameter heat-absorbing tube, which has a small external radiation area and high thermal efficiency.

[0020] 2. Flat secondary reflectors are easy to manufacture, have high thermal efficiency and low cost;

[0021] 3. While increasing the concentrator opening and improving the concentration ratio, the optical efficiency is ensured and the construction cost is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0023] Figure 2 This is a design principle diagram of the novel heat collecting tube of the present invention.

[0024] Figure 3 This is the heat flux density distribution diagram on the 70mm collector tube.

[0025] Figure 4 This is the heat flux density distribution diagram on the 80mm collector tube.

[0026] Figure 5 DNI = 400-1000W / m 2 thermal efficiency. DETAILED DESCRIPTION

[0027] A trough type solar concentrator system with a flat secondary reflector according to the present invention is as follows Figure 1 As shown, it includes: a large opening trough concentrator 1 and a new type of heat collecting tube 2. The new type of heat collecting tube is as follows Figure 2 As shown, it is composed of a metal heat absorbing tube 2-1, a glass shell 2-2 and a flat secondary reflector 2-3. Figure 1 As shown in the figure, the center point (O') of the metal heat absorber tube deviates from the original focus (O) of the large-opening trough concentrator and moves downward by a distance L to reach point O'. The lowest end of the metal heat absorber tube (close to the end of the large-opening trough concentrator) reaches point N, intercepting the sunlight passing through the lower end of the metal heat absorber tube. Figure 2 As shown, the flat secondary reflector is located in the upper part of the glass shell, at the upper end of the metal heat absorbing tube, with a distance h from the focus (O) of the large opening slot concentrator and a width K. It reflects the light that passes through the upper end face of the metal heat absorbing tube and is not reflected back to the metal heat absorbing tube.

[0028] like Figure 1 As shown, the design of the new collector tube is as follows:

[0029] Step 1: Determination of the lowest point N of the collector tube:

[0030] The purpose of moving the collector tube down a distance of L is to intercept the sunlight passing through the lower end. Therefore, the distance L from the center point (O') of the metal heat absorber tube to the focus (O) of the large opening trough concentrator can be expressed as:

[0031]

[0032] Where: f is the focal length of the large-opening trough concentrator, is the edge angle, M is the opening width, α is the maximum error angle, and R is the radius of the metal heat absorber tube.

[0033] Step 2: Solution of geometric parameters K and h of flat secondary reflector mirror:

[0034] The first step is to determine the location of the collector tube, and then design the flat-plate secondary reflector. The purpose of the flat-plate reflector is to reflect the sunlight that is not intercepted by the collector tube. The light reflected by the edge of the large-opening trough concentrator reaches the center point (O′) of the metal heat absorption tube after being reflected by the flat-plate secondary reflector. The sunlight is perpendicular to the surface of the collector tube, with small cosine loss and high optical efficiency. The width (K) of the flat-plate reflector and the distance (h) to the focus (O) of the large-opening trough concentrator can be obtained by the following relationship:

[0035]

[0036] Step 3: Optimization of geometric parameters:

[0037] The geometric parameters obtained in the first and second steps, as well as the geometric parameters of the collector tube and the optical parameters of the material are input into the SolTrace software to establish a geometric model. The geometric parameters L, h, and K are adjusted to maximize the solar heat flux density on the collector tube surface, thereby obtaining the optimal geometric model.

[0038] The following is a specific example to verify the explanation.

[0039] The opening width of the primary parabola reflector is M = 8000 mm, and the edge angle The focal length f=2384 mm, the maximum allowable error angle α=0.5°, the tracking parameters adopt the LS-3 system, the optical parameters are shown in Table 1, and the geometric parameters obtained according to the above design method and design principle are shown in Table 2.

[0040] Table 1 Optical parameters

[0041]

[0042] Table 2 Geometric dimensions of K, L, h

[0043]

[0044] The corresponding heat collector system in Table 2 is studied, and the heat flux density distribution on the corresponding heat collector is obtained as follows: Figure 3 and 4 As shown, the horizontal axis is the circumferential position of the metal heat absorption tube (starting from the lowest point), and the horizontal axis is the heat flux density distribution on the surface of the metal heat absorption tube. Figure 3 and 4 As shown in the figure, the heat flux density on the surface of the heat collector is symmetrically distributed, and the maximum value is at the lowest end of the heat collector. Figure 3 As shown in the figure, the average heat flux density (E ave ) is 28.3kW / m 2 ,like Figure 4 As shown, the corresponding average heat flux density (Eave ) is 25.8kW / m 2 The optical efficiency η is calculated according to the following formula: opt :

[0045]

[0046] The calculation results show that when the diameter of the metal heat absorber tube is 80mm, the optical efficiency of the system is 81%; when the diameter of the metal heat absorber tube is 70mm, the optical efficiency is 77.9%.

[0047] Molten salt (60NaNO 3 +40NaNO 3 ) as the heat transfer medium, and use Fluent to calculate the thermal efficiency, and the results are as follows Figure 5 When DNI=1000W / m 2 When DNI = 400-1000 W / m 2 The average thermal efficiency of 80mm circular collector tubes is 69.1%, and the average thermal efficiency of 70mm circular collector tubes is 67.9%.

[0048] Finally, it should be noted that the above is only a preferred embodiment of the invention and is not intended to limit the invention. Although the invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the invention shall be included in the protection scope of the invention.

Claims

1. A design method for a trough type solar concentrator solar collector system with a flat secondary reflector, the trough type solar concentrator solar collector system with a flat secondary reflector comprising: A large-opening trough concentrator and a heat collecting tube, wherein the heat collecting tube is composed of a metal heat absorbing tube, a glass shell and a flat secondary reflector, wherein the metal heat absorbing tube is located inside the glass shell and is not concentric with the glass shell, and the flat secondary reflector is located in the upper half of the glass shell, at the upper end of the metal heat absorbing tube; the center point O' of the metal heat absorbing tube deviates from the focus O of the large-opening trough concentrator, moves downward by a distance L to reach point O', and the lowest end point of the metal heat absorbing tube reaches point N, which is used to intercept the sunlight passing through the lower end of the metal heat absorbing tube; the distance between the flat secondary reflector and the focus O of the large-opening trough concentrator is h, which is used to reflect the light passing through the upper end surface of the metal heat absorbing tube and not absorbed back to the metal heat absorbing tube; the difference between the radius R of the metal heat absorbing tube and the downward moving distance L is less than the h from the flat secondary reflector to the primary reflector; the sum of the radius R of the metal heat absorbing tube and the downward moving distance L is less than the radius of the glass shell; the method comprises the following steps: Step 1: Determination of the lowest point N of the collector tube: The distance L1 from the center point O' of the metal heat absorber to the focus O of the large opening trough concentrator is expressed as: Where: f is the focal length of the large-opening trough concentrator, is the edge angle, M is the opening width of the large-opening trough concentrator, α is the maximum error angle, and R is the radius of the metal heat absorption tube; Step 2: Calculate the width K of the flat secondary reflector mirror and the distance h from the flat secondary reflector mirror to the focus O of the large opening slot concentrator: Step 3: Optimization of geometric parameters: The geometric parameters obtained in the first and second steps, as well as the geometric parameters of the collector tube and the optical parameters of the material are input into the SolTrace software to establish a geometric model. The geometric parameters L, h, and K are adjusted to maximize the solar heat flux density on the surface of the collector tube, thereby obtaining the optimal geometric model.

2. The design method of the trough type solar concentrator system with a flat secondary reflector according to claim 1, characterized in that: The width K of the flat secondary emission mirror is larger than the diameter of the metal heat absorption tube and smaller than the diameter of the glass shell.

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