Development status and prospect analysis of tower solar thermal power generation technology in China

Beijing Yanqing Tower Experimental Demonstration Power Station, jointly designed and built by the Institute of Electrical Engineering of the Chinese Academy of Sciences and other units, successfully generated electricity for the first time in August 2012, with an installed capacity of 1MW. The heliocaptor consists of 100 sets with an area of 100m2, the heat collection tower is 118m high, and the cavity type heat absorber is adopted. The heat transfer medium is water/steam. The power station is the first MW-class tower solar power station independently developed, designed and built in China.

Delingha 50MW tower solar thermal Power Station Phase I 10MW project of China Control Group is the first commercial demonstration power station of tower power generation in China, officially put into operation in July 2013, the power station is divided into east and west towers of 5MW each, the mirror field is composed of 30,000 2m2 heliocorters. In order to promote the commercialization of the tower molten salt technology, the molten salt system transformation of the 10MW project is currently under way.

The first flight of energy saving Dunhuang 10MW molten salt tower power station started construction at the end of August 2014, is currently in the installation and commissioning period, the project with heat storage 15 hours design, if completed on schedule, will be China's first tower solar thermal power station with heat storage, its heliostat structure design is similar to Gemasolar power station, composed of 35 mirrors, According to the order of 5×7, a single heliostat lighting area of 120 square meters, a total of 1525 sets of heliostats.

3 Key Technologies

3.1 Heliostat and its control technology

Heliostat is the concentrating unit of the tower solar thermal power generation system, which accurately reflects the solar radiation to the heat absorber on the top of the concentrating tower through the tracking system. In order to ensure the normal and efficient operation of the whole system, the heliostat must have high reflectance and positioning accuracy. Because of its long-term exposure to the outdoor environment, it also needs to have certain corrosion resistance, wear resistance, deformation resistance, easy cleaning and other characteristics. The mirror field cost generally accounts for 40% to 50% of the investment cost of the entire tower solar thermal power station.

At present, research on tower heliostat mainly focuses on structural design and mechanical analysis of heliostat bracket, weather resistance of silver-coated mirror, combination of bracket and mirror, high-precision transmission system and control system, and optimization design of heliostat field [2].

3.2 Endothermic materials and equipment

In the tower solar thermal power generation, the solar receiver is the key to achieve power generation, which directly converts the solar energy captured, reflected and focused by the heliostat into available high temperature heat energy, providing a power source for the generator set, so as to achieve thermal power generation. According to the structure of the heat absorber, it is divided into tubular heat absorber and volumetric heat absorber. The heat absorption and heat transfer medium mainly includes water/steam, molten salt and air. At present, tubular molten salt heat absorber is widely favored by the industry.

The advantage of the tubular solar receiver is that it can receive the reflected and focused sunlight from the heliostat around the tower in a 360° range, which is conducive to the layout design of the heliostat mirror field and the large-scale utilization of solar energy. The internal structure design of the heat absorber, the research on the high temperature resistance and corrosion resistance of the metal material of the heat exchange tube, the research on the high temperature resistant selective absorbing coating material of the tube wall, the high temperature decomposition and corrosion of the molten salt in the heat absorber and the low temperature solidification are the key core technologies.

3.3 Heat transfer and storage technology

Molten salt is often used as heat storage and heat transfer medium in tower solar thermal power stations. The flow and heat transfer characteristics of molten salt are directly related to the design and arrangement of heat storage and heat transfer circulation system. The key technologies of molten salt heat storage and heat transfer include the development of low-temperature molten salt, the preparation of high-temperature heat storage materials, and the design and arrangement of molten salt heat transfer and heat storage system equipment [3,4].

In particular, the preheating and insulation, dredging and plugging technology of the pipeline system of the heat transfer and storage unit, the design and manufacture of the high-flow high-temperature molten salt pump that can work stably for a long time at a working temperature above 600℃, and the reliability of the whole molten salt system are still the focus of the research of the tower solar thermal power station.

4 Development prospect of tower solar thermal power generation in China