Concentrating and heat collecting subsystem

One of the four components of a typical solar thermal power system. The concentrating heat collecting subsystem includes a concentrator, a receiver and a tracking device.

1. Concentrator

The concentrator is used to collect sunlight and concentrate it on a limited size surface to increase the solar irradiance per unit area, thereby increasing the working temperature of the heated working medium.

In theory, there are many ways of concentrating light, such as flat mirrors, curved mirrors and Fresnel lenses. However, in the solar thermal power generation system, there are two most commonly used concentrating methods, namely flat mirrors and curved mirrors.

The most representative way of concentrating light with a flat reflector is to use a multi-faceted flat reflector to gather sunlight to the top of a tall tower. Its concentration ratio is usually up to 100~1000, and the working medium in the receiver can be heated to 500~2000℃. There are three types of curved mirrors that constitute a high-temperature tower solar thermal power generation system, namely, one-dimensional parabolic mirrors, two-dimensional parabolic mirrors and hybrid plane-parabolic mirrors. One-dimensional parabolic mirrors are also called trough parabolic mirrors. The entire reflector is a parabolic trough, and sunlight is reflected and concentrated on a focal line by the parabolic trough. Its concentration ratio is 10-30, and the heat collection temperature can reach 400 ℃, forming a medium-temperature trough solar thermal power generation system. Two-dimensional parabolic mirror is also called disk parabolic mirror, the shape is a parabolic sphere surfaced by a parabola rotating 360″, so it is also called a rotating parabolic mirror. The concentration ratio of the two-dimensional parabolic reflector can reach 50~1000, and the focal temperature can reach 800~1000℃, forming a distributed high-temperature disk solar thermal power generation system.

In addition, there are linear and circular Fresnel lenses. The light-gathering ratio of the linear Fresnel lens is 3~50, and the light-gathering ratio of the circular Fresnel lens is 50~1000.

It can be seen from the research that different light-concentration and heat-collection methods have different light-concentration ratios and possible heat-collecting temperatures, and different tracking methods should be configured. For different ways of concentrating and collecting heat, the relationship between the concentrating ratio and the collecting temperature is shown in Figure 1. Obviously, the higher the concentration ratio, the higher the possible heat collection temperature.

Concentrating and heat collecting subsystem
Figure 1 – The relationship between the concentration ratio and the heat collection temperature

The concentrator is a key component in the solar thermal power generation system, and the human sunlight is first reflected to the receiver through it. The quality of its performance obviously affects the overall performance of the solar thermal power generation system.

Therefore, there are stricter requirements for it.

① Optical properties.

The higher the specular reflectance of the condenser, the better. The mirror surfaces currently used include glass or polymer plates with vapor-deposited silver or aluminum, and high-purity aluminum that is electrically or mechanically polished. They are all highly reflective, but if naturally exposed to sunlight, they are quickly oxidized, resulting in a dramatic drop in reflectivity.

Usually, the reflective surface can be protected by spraying a layer of transparent silica gel. Now most of them use glass back mirrors, that is, silver or aluminum is evaporated on the back of the glass mirror, and then sprayed with a multi-layer paint protective layer, or sealed and sandwiched between two layers of glass. This high-performance reflective surface has better use and protection properties.

In addition, all the mirrors used in the solar thermal power generation system, whether it is a flat mirror or a curved mirror, are exposed to atmospheric conditions, and dust is constantly deposited on the surface from the atmosphere, which greatly affects the performance of the reflecting surface. Therefore, how to keep the mirror surface clean is still one of the difficult problems faced by all concentrating and heat collecting technologies. Usually, mechanical cleaning equipment is used to clean the mirror surface regularly. Existing experience shows that this is the only effective and feasible method under the current technical conditions.

②Mechanical properties.

The mirror surface has good flatness. The line shape of the overall mirror surface has high precision, and the general processing error should not exceed 0.1.

The entire mirror surface and mirror body have high mechanical strength and stability, and can resist the blowing of strong winds.

The mirror surface and the protective film have strong adhesion.

③ Chemical stability.

The mirror surface has strong corrosion resistance.

2. Receiver

The receiver is a component that converts solar radiation energy into heat energy and transmits it to the working medium by receiving the focused sunlight. Here, the working medium is heated by solar radiation energy, turned into superheated steam, and then sent to the steam turbine through the pipeline. The main component of the receiver is the absorber. Its shape includes plane, point, line, and cavity structure. The surface of the absorber is often covered with a selective absorbing surface, such as the chemically treated metal surface, the surface composed of aluminum-molybdenum-aluminum and other multi-layer films, and the surface formed by spraying specific materials on the metal substrate by plasma spraying. These surfaces have a high absorption rate of sunlight and a low emissivity at the absorber surface temperature.

3. Tracking device

In order to reflect the solar radiation at all times of the day to the stationary receiver through the mirror surface, the mirror must be provided with a tracking mechanism. There are two tracking methods for solar concentrators, namely single-axis tracking and dual-axis tracking. The so-called single-axis or dual-axis tracking refers to the rotation of the mirror surface around one axis or two axes. Most of the trough parabolic mirrors are single-axis tracking, while the disc parabolic mirrors and the tower-type concentrating plane mirrors are both dual-axis tracking.

In terms of the way to achieve tracking, there are two ways of program control and sensor control. The program control method is to control the movement of the tracking mechanism according to the calculated sun motion law, and its disadvantage is that there is a cumulative error. The sensor control method is to measure the direction of the incident solar radiation instantaneously by the sensor to control the movement of the tracking mechanism. Its disadvantage is that it is difficult to find the correct orientation of the reflecting mirror under cloudy conditions. At present, the combination of the two is used for control, and the program control is the main method, and the instantaneous measurement of the sensor is used as feedback to correct the cumulative error of the program. In this way, stable and reliable tracking control of the mirror can be obtained under any weather conditions.

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