The tower solar thermal power generation system uses a large number of flat mirror arrays to reflect the solar radiation to the solar receiver placed on the top of the tower, heat the working medium to generate superheated steam, and drive the steam turbine generator set to generate electricity, thereby converting the solar energy into electricity. . Obviously, the greater the number of flat mirrors in the array, the greater the concentration ratio, and the higher the collector’s heat collection temperature.
The conceptual design principle system of the tower solar thermal power station is shown in Figure 1. The whole system consists of four parts: concentrating device, heat collecting device, heat storage device and steam turbine generating device.
- Condenser photon system
The concentrating device of a tower solar thermal power station is a large number of flat mirror arrays arranged in a certain arrangement. They are distributed around the tall central receiving tower in four quadrants, forming a huge mirror field, as shown in Figure 2. Obviously, the larger the design capacity of the power station, the larger the required mirror area and the larger the mirror field size. According to experience, the mirror field area required for a power generation of 100MW is about 2.43k㎡
The heliostat is the most basic optical unit in a tower solar thermal power station. It consists of a flat mirror, a mirror frame and a tracking mechanism. The plane mirror is mounted on the frame, and its tracking device drives the mirror to automatically track the sun instantly.
The schematic diagram of the heliostat structure is shown in Figure 3. Figure 3(a) is a flat reflector made of a film with good reflectivity, which is installed in a transparent film dome. Transparent films have high sunlight transmittance. The support frame of this heliostat is very light, so the electrical power consumption of the tracking mechanism can be small. Figure 3(b) is a glass back mirror using aluminum or silver as the reflective material. The mirror surface area of a heliostat is usually 30~40m”, which is composed of several small mirror surfaces. The mirror surface area of a large heliostat is about 100m. Since the heliostat is far from the receiver on the top of the tower, In order to prevent the sunlight from being too defocused after being reflected by the heliostat, so that more than 95% of the reflected sunlight falls on the receiver on the top of the tower, the mirror surface is generally a flat concave mirror with a small radian.
A large-scale tower solar thermal power station usually has thousands of heliostats in its mirror field, so it has a high concentration multiple. Usually 500~3000 times, the working temperature is above 350 ℃. Therefore, the tower solar thermal power generation system is also called a high temperature solar thermal power generation system.
The heliostat is one of the key components of the tower solar thermal power station and the main investment part of the power station. It occupies the main site of the power station, so it has strict requirements on the performance of the heliostat. The specific requirements are:
①High specular reflectivity
②Mirror flatness error is less than 16
③ The overall mechanical structure has high strength, and can resist the attack of typhoon level 8 during operation
④Stable operation
⑤ work around the clock
⑥Can be mass-produced
⑦Easy to install
⑧ Less maintenance and long working life.
- Central receiving tower
The central receiving tower, also known as the power tower, is the heat collecting device of the tower solar thermal power station. It consists of two parts: the solar radiation receiver and the high tower. The receiver is installed on the top of the tower, and the working medium conveying pipeline is arranged in the hollow tower body.
(1) Receiver
In the tower solar thermal power generation system, the receiver is the most critical core technology to realize the tower solar thermal power generation. It directly converts the solar energy captured, reflected and focused by the heliostat into high-temperature thermal energy that can be efficiently used, providing the generator set The required heat source or power source to realize the process of solar thermal power generation.
The receiver has vertical cavity type, horizontal cavity type and external light receiving type, which requires small size and high heat exchange efficiency. FIG. 4 is a schematic diagram of the structure of the receiver. The cavity-type receiver is a cavity with a certain opening size enclosed by many tube bundles. The sunlight is incident on the inner tube wall of the cavity from the cavity opening, and heat exchange is performed inside the cavity. Obviously, the heat loss of this cavity-type receiver can be minimized, and it is suitable for the modern high-parameter steam turbine power generation cycle. The external light-receiving type receiver is a cylinder with a certain diameter surrounded by many tube bundles, the heated surface is directly exposed, and the sunlight is incident on the surface for heat exchange. Compared with the cavity type receiver, the heat loss is obviously larger, but the receiver of this structure can more easily receive the reflected radiation of the heliostat on the edge of the mirror field, so it is more suitable for large-scale tower solar thermal power generation. system.
(2) Tower
In the mirror field of the tower solar thermal power station, there is a very high vertical tower. There are many heliostats around the tower, and a receiver is installed at the top of the tower. Sunlight is reflected by heliostats around the tower to a receiver on top of the tower. The working medium is sent from the ground through the pipeline to the receiver at the top of the tower for heating, and the heated working medium is sent back to the ground through the pipeline. All connecting pipes and control ties between ground and tower top receivers are routed along the tower.
At present, the vertical tower used in the tower solar thermal power station has two forms of reinforced concrete and steel frame structure. The height of the vertical tower is determined by the size of the mirror field. The larger the design capacity of the power station, the larger the scale of the mirror field and the higher the vertical tower. For example, the tower solar thermal power station built by the European Union in Sicily, Italy, has a mirror field covering an area of 35,000 m2 and a tower height of 55m.
- Thermal storage device
The heat storage device selects materials with good heat transfer and heat storage properties as the heat storage working medium. The heat storage device of a tower solar thermal power station is usually two unpressurized open heat storage tanks: one is a cold salt tank, the other is a hot salt tank, and mixed salt is used as the heat storage medium.
The cold salt in the cold salt tank is pumped to the receiver at the top of the tower, heated to high temperature by solar energy, and stored in the hot salt tank. During operation, the hot salt heats the water through the steam generator into superheated steam, drives the turbine engine unit to generate electricity, and then returns to the cold salt tank.
Usually the operating conditions of the mixed salt are close to atmospheric pressure, so the receiver is not under pressure, and it is allowed to be made of thin-walled steel pipes, which can improve the heat flux density of the heat transfer tube, reduce the overall size of the receiver, and reduce the radiation and convection of the receiver. heat loss, so that the receiver has a high absorption efficiency.
