Solar thermal power generation is an important aspect of solar thermal utilization, which uses the collector to convert the solar radiation energy into heat energy and generate electricity through the thermal cycle process Since the 1980s, the United States, Europe, Australia and other countries have successively established different types of demonstration devices, which has promoted the development of thermal power generation technology. There are three types of solar thermal power generation systems in the world: trough line focusing system, tower system and dish system.

Classification of solar thermal power generation systems

1) Trough line focusing system

The system focuses the sunlight on the tubular receiver by using the parabolic cylindrical trough mirror, heats the heat transfer working medium in the tube, generates steam in the heat exchanger, and drives the conventional steam turbine to generate electricity

2) Tower system

The basic type of tower solar thermal power generation system is to use a group of heliostats that track the sun independently to focus the sun on a receiver fixed on the top of the tower to generate high temperature.

3) Disc system

The parabolic reflector / Stirling system is composed of a parabolic reflector composed of many mirrors. The receiver is at the focal point of the paraboloid, and the heat transfer working medium in the receiver is heated to about 750 ℃ to drive the engine for

4) Performance comparison of three systems

At present, only the trough line focusing system of the three systems has been commercialized, and the other two systems are in the demonstration stage, which has the possibility and Prospect of commercialization. All three systems can be operated by using solar energy alone or installed as a fuel mixing system.

So let’s follow Xiaobian to learn more about the trough line focusing system.

Trough solar thermal power generation system

The trough type solar thermal power generation system is fully known as the trough type parabolic reflector solar thermal power generation system. It is a series parallel arrangement of multiple trough type parabolic concentrator collectors to heat the working medium, generate high-temperature steam, and drive the steam turbine generator set to generate electricity.

1、 Working principle of trough solar thermal power generation system

  Trough solar thermal power generation system

Principle of trough solar thermal power generation system: a parabolic trough mirror collector bent in only one direction is used to focus the sunlight on the central tube located at the focal line, so that the heat transfer working medium (oil or water) in the tube is heated to 350~390 ℃, and then the heated heat transfer medium generates superheated steam through the heat exchanger, which drives the conventional steam turbine generator to generate electricity.

2、 Technology used in trough solar thermal power generation system

1. Medium temperature technology: heat transfer oil is used as heat carrying (molten salt) working medium to collect heat at < 400oC and generate steam for power generation through heat exchange.

  Trough solar thermal power generation system

2. High temperature technology: the mixed nitrate is used as the carrier to collect heat at ≤ 550oc and generate steam for power generation through heat exchange.

  Trough solar thermal power generation system

3. DSG direct steam technology: directly use water as working medium to generate steam for power generation through heating of collector tubes.

According to different technologies, the trough solar thermal power generation system has formed two different system forms: single loop system and double loop system.

  Trough solar thermal power generation system

3、 Composition of trough solar thermal power generation system

The trough solar thermal power generation system is composed of concentrating and collecting system, heat transfer system, heat storage and heat exchange system, power generation system, auxiliary energy system, etc.

  Trough solar thermal power generation system

1. Concentrating and heat collecting system: it is the core of the system and consists of a condenser, a receiver and a tracking device. There are mainly two types of receivers: vacuum tube type and cavity type; The tracking mode adopts one-dimensional tracking, including north-south, east-west and polar axis.

2. Heat transfer system: composed of preheater, steam generator, superheater and reheater. When the working medium of the system is oil, double circuits are adopted, that is, after the working medium oil in the receiver is heated, it enters the heat exchange system to generate steam, and the steam enters the power generation system to generate electricity. When water is directly used as working medium, the system can be simplified.

3. Power generation system: the basic composition is similar to that of conventional power generation equipment, but a special device is required for switching the working fluid between the receiver and the auxiliary energy system.

4. Heat storage and heat exchange system: the solar thermal power generation system must rely on the stored energy to maintain the normal operation of the system in the morning and evening or between clouds. There are three ways of heat storage: explicit, submersible and chemical.

5. Auxiliary energy system: at night or in rainy days, the auxiliary energy system is generally used for heating, otherwise the excessive heat storage system will cause the increase of initial investment.

4、 Heat transfer and exchange system

According to different heat transfer fluids, the trough collector heats the heat transfer fluid to different temperatures, generally about 400 degrees. Because the heat transfer pipeline of the trough solar thermal power generation system is very long, in order to reduce heat loss, thermal insulation materials shall be provided outside the pipeline and the pipeline shall be as short as possible; The long pipeline needs to be pumped to promote the circulation of heat transfer fluid. Try to reduce the pump power of heat transfer fluid. The heat transfer fluid can be phenyl ether mixture, pressurized water mixture, heat transfer oil and other liquids. The heat transfer mode can be direct heat transfer or phase change heat transfer.

The heat conducting liquid heats the water into steam of about 300 ° C through the heat exchanger. The steam drives the steam turbine to rotate and drives the generator to generate electricity. The heat exchanger has plate, tube and other structures. Clouds may block the sun. In order to ensure the stable operation of the system, there should be heat storage devices in the system, generally including high-temperature heat storage tanks and low-temperature heat storage tanks. For the heat transfer fluid that will freeze at low temperature, there must be an auxiliary heater to maintain the temperature of the heat transfer fluid to avoid freezing. If it is necessary to supply power when solar energy is insufficient, natural gas (or other energy) boilers shall be connected in parallel on the system to ensure the normal operation of the steam turbine.

5、 Heat storage system

As solar energy is affected by season, day and night and meteorological conditions, in order to ensure the stability of the heat source of the power generation system, it is necessary to set up a heat storage device.

The heat storage temperature is divided into low temperature, medium temperature, high temperature and high temperature: low temperature (100 ℃); medium temperature (100~500 ℃); high temperature (500 ℃); Extremely high temperature (about 1000 ℃).

The corresponding heat storage materials are hydrated salt, heat transfer oil, molten salt and zirconia fireball.

Heat storage mode: sensible heat storage, latent heat storage and chemical heat storage.

1. Sensible heat storage

Medium: water, oil, rock, sand, gravel, artificial alumina ball,

Features: low price, easy to get, small heat capacity.

2. Latent heat storage

Features: the latent heat of material is used for heat storage. The heat storage capacity per unit volume is large, and the heat storage device is expected to be miniaturized.

The heat storage medium shall have the following characteristics: it shall have several thousand reversible heat storage and release cycles; cheapness; Do not corrode containers.

3. Chemical heat storage

Features: large heat storage capacity, small volume of unit energy storage, light weight, chemical reactants can be stored separately.

The heat storage medium shall have the following characteristics: the heat storage and heat release reactions shall be reversible without side effects; Fast reaction speed; The reaction products are easy to separate and can be stored stably; cheapness; Reactants and products are non-toxic, non corrosive and non combustible; High reaction heat.

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