What is the classification of solar thermal power generation and its systems?

What is the classification of solar thermal power generation and its systems?

The so-called solar thermal power generation is to use solar concentrators to gather solar radiation energy in a high density, obtain a high temperature working medium heat source of hundreds or even thousands of degrees through a heat conversion device, and then use a steam generator to use the thermal energy. It is a device that heats water to a state of superheated steam, drives conventional steam turbine generator sets, and performs conventional power generation. In addition, there is also a type of components that directly use solar radiation to heat semiconductor materials, or components composed of bimetallic sheets. When there is a temperature difference between the two materials, the electrons in the high-temperature material have higher kinetic energy, thereby causing electrons The so-called “thermoelectric power generation” is carried out by the physical phenomenon of current passing after the load is applied to the wire, resulting in the generation of electromotive force at both ends of the thermoelectric circuit.

The existing practical value of solar thermal power generation systems in the world can be roughly divided into several categories: trough line focusing system, tower surface focusing system, dish (Stirling) power generation system and solar thermal chimney power generation system.

(1) Grooved line focusing system: Using a long trough parabolic cylindrical focusing system, the solar radiation energy on both sides of the axis several times is focused on a small area on the focal line of the system to form solar radiation with high energy density It can be used to heat the heat-conducting oil or molten salt heat-carrying medium in the coke line pipeline: the high-temperature working medium is then transported to the steam generator to heat soft water to obtain superheated steam; finally, the superheated steam is used to drive the turbine to generate electricity The unit generates electricity. The trough parabolic focusing system is installed in an array in the east-west direction. It only needs to track the monthly sun displacement of about 8° in the north-south direction to achieve the purpose of focusing and heating the working medium. However, because the geometric concentration ratio of this type of system is not very high, the heating temperature of the focal line is generally only between 260°C and 570°C, and the conveying route of the working medium is too long, and the heat loss of the high-temperature working medium is large, so the thermal power generation efficiency Not very high.

What is the classification of solar thermal power generation and its systems?
Figure 1 Trough solar thermal power generation

(2) Tower surface focusing system: Using the reflective heliostat array composed of many plane mirrors around the tower, under the unified control of the sunlight tracking system, the solar radiation energy in a large area is fully concentrated to the sun at the top of the tower. On the receiver, the heated superheated steam drives the steam turbine to generate electricity. The heliostat of this type of system requires dual-axis tracking, the geometric concentration is relatively high, the temperature of the working medium can generally reach 500 ° C ~ 1000 ° C, and the conveying distance is relatively short, so the power generation efficiency of the system is much higher than that of the trough thermal power generation system. .

What is the classification of solar thermal power generation and its systems?
Figure 2 Tower solar thermal power generation

(3) Dish-type (Stirling) power generation system: It consists of a plurality of relatively independent parabolic spheres with automatic dual-axis tracking function, the dish-type reflective concentrating heat collector and the light-to-heat conversion device “combined into one” power generation system. The heat-carrying working medium of this kind of power generation device uses air, focusing and photothermal conversion device are integrated, the focusing ratio is high, the temperature of the heat-carrying working medium can be up to about 1500 ℃, and the system structure is compact, so the thermal power generation efficiency is higher than that of the tank type and tower systems.

What is the classification of solar thermal power generation and its systems?
Figure 3 Dish solar thermal power generation

According to the second law of thermodynamics, any thermal energy conversion process is accompanied by losses. It is impossible for any heat engine to absorb heat from a single heat source and convert it all into mechanical work; a part of the heat absorbed by a heat engine from a heat source must be transferred to a condenser lower than the temperature of the heat source. The efficiency (ηm) of an ideal heat engine is equal to the ratio of the mechanical work (W) output by the heat engine to the heat (Q1) supplied by the heat source to the heat engine:

In the formula: Q2 is the heat removed by the heat engine to the cold source;
T1 is the heat source temperature (K);
T2 is the cooling (condenser) source temperature (K).

Based on this, it can be seen that in order to improve the efficiency (ηm) of the solar thermal power generator, the heat source temperature (Q1) should be as high as possible, and the condenser temperature (Q2) should be as low as possible. In fact, for CSP systems, the so-called “cold source” (ie, the condenser) is the ambient temperature. The ambient temperature cannot be easily controlled to reduce the temperature according to human will: Therefore, to improve the efficiency of the solar thermal power generation heat engine, there is only one way out, that is, to increase the temperature of the heat source as much as possible. In solar thermal power generation, we can only obtain more solar radiation heat by improving the focusing ratio of the solar concentrator and the tracking accuracy of the system to the sun. However, as the temperature of the focal point increases, the physical properties and high temperature strength of the structural materials of the system are also required to be higher. In addition, there are also issues such as the cost and service life of the equipment, which must also be weighed and considered. To sum up, the existence of various mutually influencing and mutually restricting factors makes solar thermal power generation difficult to be widely used on a large scale. problem.

The efficiency (ηS) of the solar thermal power generation system is equal to the product of the solar thermal conversion efficiency (ηC) of the collector, the heat engine efficiency (ηm) and the power generation efficiency of the steam turbine generator (ηc), namely

Due to the discontinuous, unstable, periodic and regional influence factors of solar insolation, in order to ensure the normal needs of social production and people’s life, the solar thermal power generation system must be equipped with high-temperature working fluid energy storage devices with sufficient capacity or built into The hybrid power generation system uses conventional auxiliary energy when necessary to maintain the normal operation of the power generation equipment to ensure the user’s production and living electricity. This is a common problem faced by the above three solar thermal power generation systems.

(4) Solar thermal chimney power generation system: As we all know, when the sunlight is projected on the ground, in the place with forest vegetation, when the sunlight is sprinkled on the green plants, the green plants can process carbon dioxide and water into carbon water under the action of chlorophyll. compound and release oxygen for a calm and silent photo-chemical reaction. In desert areas without vegetation cover, the situation is completely different; the scorching sun scorches the earth to the point of scorching fire, and then the earth transfers heat to the air. A strong updraft soared to the sky: almost at the same time, the cold air layer in the low-temperature zone near the high latitudes, or the high-density cold air mass on the western plateau that the late sun had not had time to heat, keenly sensed the atmosphere. This subtle pressure difference change in the icy silence is a crisis poised to unfold, perhaps with the gentle flapping of a butterfly’s wings, this eerie balance of atmospheric pressure difference is then broken, and in an instant. The riot of the cold air mass was triggered, and the powerful cold air reacted quickly in an instant, roaring through the valleys and plains with a force of thousands of horses, and pounced on the “vacuum” vacancy left by the rising flow of the hot air mass; This forms what we call “cold current” or “wind”. Wind is a mass of air with strong air kinetic energy. People set up tall towers and large air propellers on the passages where the wind blows all the year round, use the power of the wind to drive the rotation of the blades, and convert the aerodynamic power into mechanical energy to drive the generator to generate electricity. This is what we call the use of meteorological energy. “wind power”. It should be said that wind power is also a form of solar “thermal” power generation. It uses the strong air kinetic energy generated by the large-scale horizontal horizontal movement of the air caused by the uneven thermal radiation of the sun on different latitudes and topography. In addition, there is a system that uses the power generated by the vertical and vertical upward movement of hot air to generate electricity. In order to obtain a continuous and continuous vertical updraft, this kind of system generally absorbs solar radiation energy by building a large-area heat-collecting glass greenhouse, so that the temperature and air pressure in the large-scale heat-collecting greenhouse increase, forming a high temperature. , high-pressure greenhouse effect; then the hot air in the collector warm shed passes through a vertically upward large-diameter hot chimney, using the insulating effect of the chimney cylinder and the atmospheric pressure difference naturally formed at the high and low ends of the chimney cylinder; guide the collector The warm hot air in the heat shed is directionally released, thereby generating a continuous dynamic pressure shock of strong hot air fluid, which drives the axial flow wind turbine placed at the bottom of the hot chimney, and drives the generator to generate electricity; this is the so-called “solar energy”. Thermal Chimney Power Generation”.

According to information, in 1981, the German government invested in a new 50kW solar thermal chimney power station in Spain. The thermal chimney of the power station is 150m high and 10m in diameter. The diameter of the plastic heat collecting and lighting greenhouse is 240m (covering an area of ​​about 68 acres). The power station has been running for 15,000 hours since it was put into use in 1982, and the operation rate is 95%. The reason for its shutdown is the restriction of the Spanish power grid, and the power generation is prohibited due to the sharp reduction of electricity consumption on weekends.

What is the classification of solar thermal power generation and its systems?
Figure 4 Solar thermal bacon power generation

Compared with the above-mentioned thermal power generation technologies, solar thermal chimney power generation has a series of advantages: First, it does not require high-precision sunlight tracking technology, complex equipment manufacturing process, high temperature resistant materials and expensive energy storage equipment; And special raw materials such as high-temperature thermal conductivity. Second, the requirements for solar resources have been reduced, because the heat collecting shed not only collects the direct light of the sun, but also effectively collects and utilizes the scattered light and diffuse reflected light of the sun, so the scope of application can be expanded to more cloudy and rainy days, but Tropical and subtropical rainforest areas with high ambient temperature and strong updrafts. Third, the system can achieve 24h continuous and efficient operation only by appropriately increasing the ground heat storage equipment in the heat collection shed. Fourth, it is safe and reliable. It only needs to maintain the only running component of the wind turbine generator set; neither cooling water nor special raw and auxiliary materials and human resources are required to maintain a long-term safe operation. Fifth, due to the low energy density of solar radiation reaching the earth’s surface, various solar power generation systems occupy a large area of ​​land, but the solar thermal chimney power generation system is mainly constructed of plastic or glass. Greenhouses, which are different from other photovoltaic-photothermal power generation projects, which are constructed with high-value materials and occupy land exclusively, can also be developed and utilized in other corresponding ways; for example, they can be used as places for the release of waste heat from production in certain factories. After comprehensive development and utilization, the solar thermal chimney power generation system will have better social and economic benefits. In summary, it can be considered that the solar thermal chimney power generation technology.