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- Apr 08, 2018 -

Analysis of Potential and Prospects of Solar Power

The utilization of solar energy in China is promising. At present, the scale of the solar energy industry has ranked first in the world and is an important producer of solar photovoltaic cells. Although the foundation for the large-scale development of photovoltaic power generation in China has been laid, there is still room for improvement in technology. This article discusses the solar power potential and its prospects by discussing solar power generation technologies such as flat photovoltaic cells and arrays, concentrating photovoltaic power generation equipment, trough converging thermal power generation systems, and tower concentrating thermal power generation systems. Analysis, and preliminary pointed out the main direction of China's solar power development in the future.

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The status quo of solar power generation When the issue of energy is increasingly becoming a bottleneck restricting the economic development of the international community, more and more countries are beginning to develop solar energy resources and seek new impetus for economic development. As a renewable new energy source, solar energy has attracted increasing attention. From the perspective of energy supply security and clean utilization, countries around the world are taking the commercialization and utilization of solar energy as an important development direction. The EU, Japan, and the United States will focus their energy supply security after 2030 on renewable energy such as solar energy. It is estimated that by 2030, solar power will account for more than 10% of the world's electricity supply and 200% by 2050. Large-scale development and use of solar energy will occupy a place in the entire energy supply. China is rich in solar energy resources, and its use of solar energy has broad prospects. At present, China's solar energy industry has ranked first in the world and is an important producer of solar photovoltaic cells. China's photovoltaic power generation industry started in the 1970s and entered a period of steady development in the mid-1990s. The output of solar cells and modules has steadily increased year by year. After more than 30 years of hard work, we have ushered in a new stage of rapid development. Driven by the “Guangming Project” pilot project and the “Transmission to Township” project and other countries' projects and the world photovoltaic market, China's photovoltaic power generation industry has developed rapidly. By the end of 2007, the cumulative installed capacity of photovoltaic systems nationwide reached 100,000 kilowatts, with more than 50 companies engaged in solar cell production, solar cell production capacity of 2.9 million kilowatts, and annual solar cell output of 1,188 megawatts, exceeding Japan and Europe. A complete industrial chain composed of raw materials production and photovoltaic system construction has been preliminarily established. In particular, significant progress has been made in the production of polysilicon materials, breaking the annual output of 10,000 tons and breaking through the bottleneck of solar cell raw material production. It laid the foundation for the large-scale development of photovoltaic power generation in China. At present, the promulgation and implementation of the “Renewable Energy Law” in China provides a policy guarantee for the development of the solar energy utilization industry; the signing of the Kyoto Protocol, the introduction of environmental protection policies and the commitment to the international community will bring opportunities to the solar energy utilization industry; China The adjustment of the energy strategy has enabled the government to increase its support for the development of renewable energy, which has created opportunities for the development of the solar power industry. The main technologies of solar power The main forms of solar power include: flat photovoltaic cells and arrays, concentrating photovoltaic power generation equipment, trough converging thermal power generation systems, tower-type concentrating thermal power generation systems, and trough condenser heat collection Power generation technology and hybrid power systems. Flat Panel Photovoltaic Cells and Arrays are Now Available in Commercial Flat Panel Photovoltaic Cells.

Use monocrystalline or polycrystalline silicon cell technology. Usually a single battery module is connected in series to form a battery string, and several battery strings are then parallel-connected and packaged to make a solar panel. The installed battery capacity of each solar panel is 150Wp to 200Wp, that is, under ideal conditions (vertical sunlight, ambient temperature does not exceed 25°C, the illuminance DNI value reaches a Class I or Class II higher region index), and its DC peak value Power generation capacity is 150W to 200W. Under normal circumstances, in order to ensure power generation, the installed capacity of solar panels is greater than the expected use of capacity. Under normal conditions, the installation capacity needs to be designed to use 1.3 to 1.5 times the capacity. In Europe and the United States, solar panels are mainly used in independent residential power generation, with installation capacity generally ranging from 3kWp to 5kWp; or large-scale public buildings or commercial building roof or curtain wall power generation, and its installation capacity is usually 100kW to 1000kWp. This type of solar power generation is called Building Integrated Photovoltaic (BIPV). When the flat panel type photovoltaic power generation system supplies power to the DC load, the panel array is directly supplied with the load power after being assembled by the junction box (box); when used together with the traditional AC system, the DC power source is assembled and then generated by the inverter to meet the AC voltage frequency. Single-phase or three-phase AC power is imported into the user's power system. The solar arrays are laid in accordance with planned power generation capacity to form a large-scale flat-panel photovoltaic power generation system, and large-scale photovoltaic power plants can also be built. According to the empirical data of large-scale flat-panel photovoltaic power plants built abroad, the fixed-panel photovoltaic power generation technology requires a space of 3.5 acres per MW of installed capacity, approximately 21 mu of land. At present, the largest flat-panel photovoltaic power plant has a scale of no more than 5MW.

 

Flat-panel photovoltaic power generation systems mainly include solar panels, DC protection and collection systems, inverters, AC protection and switching systems, power generation measurement, and infrastructure. If it is a large-scale grid-connected power plant, DC lines, AC lines, and booster stations must also be considered. The total photo-electric conversion efficiency of the plate type photovoltaic power generation system is approximately 16% to 18%. In this system, in order to increase the power generation efficiency of sunlight, a single-axis or two-axis tracking system can be used to increase the time for direct sunlight to increase the power generation. The single-axis tracking system can increase power generation by about 25%, and the dual-axis tracking system can increase power generation by about 40%. Since the tracking system needs to drive the panel to rotate according to the sun's azimuth angle, the footprint will double.

According to the current price of crystalline silicon raw materials in the international market, the unit cost of the flat panel photovoltaic power generation system is approximately 20,000 yuan to 50,000 yuan per kW of installed capacity. If a megawatt flat panel type photovoltaic power plant is built, the line cost will be greatly increased. Taking into account the contaminant loss, DC loss, inverter loss, and line loss at the panel site of the battery module caused by climate factors, the integrated investment cost per kilowatt of electricity generated by the flat panel photovoltaic power generation system is approximately 35,000 to 40,000. yuan. The flat panel type photovoltaic power generation system has a simple structure, a low technical content, and convenient installation and construction. Since the price of crystalline silicon materials has fallen, its cost has been declining. However, the power generation efficiency is low, transportation is inconvenient, and maintenance is not convenient. For example, after the sand or snow is formed and the surface of the battery board is blocked, it takes a long time to clean and affect the power generation efficiency. Once the surface of the battery board forms a partial “spot” effect, This will cause the obstructed battery module to be damaged by excessive heat, resulting in permanent loss. At the same time, if flat-panel photovoltaic power generation technology is used to build large-scale photovoltaic power plants, the installation and line construction time will be significantly extended, affecting the investment return period. In addition, the flat-panel photovoltaic power generation system mainly relies on a large amount of crystalline silicon, the cost depends on the price of international crystalline silicon materials, raw materials are mainly in the hands of a handful of countries, and only domestic processing companies have strategic risks.

Concentrating photovoltaic power generation equipment

Concentrating Photovoltaic Technology (CPV) is a large-scale photovoltaic power generation technology that has been rapidly developed in recent years. It is mainly used in grid-connected solar photovoltaic power plants of megawatt and above. Compared with the flat-panel photovoltaic power generation technology, the main reasons for its popularity are its economical nature, short construction period, small footprint, easy maintenance, and less demand for site leveling than flat-panel photovoltaic power generation systems. The core technologies for CPV system power generation are "Multiple-Junction Cell" and "Fresnel Lens". At the same time, high-precision dual-axis solar azimuth tracking technology and hydraulically-driven CPV module-to-date system are used. Concentrating light of a large area on the surface of a battery with a small area can fully exert the conversion efficiency of the photovoltaic cell and generate more electricity than the direct sunlight on the surface of the cell. In laboratory conditions, a 6-inch flat-panel battery can generate 2 to 3 watts of electricity, while a multi-junction cell with the same area after focusing by a Fresnel lens can produce 1000 watts of electricity.

 

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According to the current CPV system that has been put into commercial use in the world, its optical-electric conversion efficiency exceeds 30%. According to the latest installation of the CPV system in the United States, the average investment cost per watt of power generation is about 3 to 4 US dollars, that is, the comprehensive investment cost per kilowatt of electricity generation is about 30,000 to 35,000 yuan. If domestic production is achieved, then The cost can be gradually reduced to 20,000 yuan per kilowatt of electricity generation. The following table shows the major technical indicators of high-concentration solar photovoltaic power generation equipment produced by a foreign company. From the table, we can see that the 53 kilowatts of power generation equipment has an annual power generation of 145'242 kWh (under a type of high-quality sunlight conditions) with a life expectancy exceeding 25 years. The individual CPV units mainly include "Fresnel condensers", multi-junction photovoltaic cells and cell structure supports. Fresnel mirrors are used to focus incoming sunlight onto its focal point, installing a small area photovoltaic cell assembly at the focal point, and combining the lens and battery into a single unit by the support.

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Several units form a module (see the figure below). A CPV system includes a CPV module, an infrastructure, a hydraulic dual-axis drive mechanism, an illumination and wind speed sensor, an automatic control system, a DC line and an inverter, and a grid control and protection. At present, the largest capacity of CPV power generation equipment is 53kW AC power generation. CPV Power Generation Equipment • One megawatt of power generation capacity covers an area of 4 to 6 acres, approximately 30 acres. Suitable for flat and open areas with extremely high sunlight and high illuminance. Taking the United States as an example, starting from the Los Angeles area until California is the best and best place for solar energy resources in the continental United States, the annual power output of the CPV technology is about 25% higher than the flat panel technology.

The concentrating photovoltaic power generation equipment has high photo-electric conversion rate, strong ability to withstand the impact of the climate, low requirements for site leveling, convenient scale-up, lower investment cost, low dependence on semiconductor materials, and short installation period for easy implementation Return on investment. At the same time, the concentrating photovoltaic power generation technology costs and equipment concentration are relatively decentralized, and it is easy to realize local assembly, and it is also convenient to realize local industrial production. The strategic risk is relatively small. However, the basic construction of the system requires high requirements and completely relies on the installation of large-scale machinery. The technical level of the installation and construction team and operation and maintenance personnel requires high standards and requires specialized system debugging from time to time.

Trough Condensation Power Generation System

Trough technology is currently the most mature technology in Concentrated Solar Power (CSP). In large-scale desert solar power applications, trough technology is the earliest used technology, and more and more shows its operational and cost advantages. At present, more than 400 MW trough systems are in operation in the world, and 350 MW are under construction. The trough system in planning and design is about 7 GW. Trough solar thermal power plants include heat collection and power generation. The power generation section is the same as conventional steam power generation. The heat collection section mainly includes parabolic trough mirrors, heat receivers, single-axis tracking control systems, and collectors infrastructure. There are currently three main solar thermal power plant structures: the simplest is to generate electricity only in sunlight, the other includes a set of heat storage devices, and the third type is the aforementioned hybrid power system. To build a 100MW solar trough thermal power plant, the parabolic trough requires an area of about 2,883.388 mu, including a full-tank solar thermal power plant with 7 hours of heat storage. It will take up an area of 5,706 mu. The parabolic trough continuously tracks the sun during the day, reflecting the sunlight to the receiving tube mounted at its focal point. The design of the receiving tube enables it to maximize the collection of solar energy with as little loss as possible. The medium fluid used for heat transfer is circulated in the receiving tube and heated to approximately 750. F (400°C). In the solar energy collection field

Next to the ground, the heated heat medium is heat exchanged to generate steam to drive the conventional steam turbine to generate electricity. Thermal energy can be stored in tanks containing molten salt, so it can also generate electricity in the absence of sunlight, so the operation of a solar thermal plant can be partially dispatched by the grid. In addition, steam can be generated through a solar thermal field and combined with an existing circulating steam turbine power generation system to form a hybrid power generation system, thereby reducing the consumption of fossil fuels and reducing emissions.

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The schematic diagram of the tank type condensing thermal power generation system


Taking the SOLANA solar power station in Arizona, USA, the world’s largest trough solar thermal power system currently under construction as an example, the total scale is 288 MW. The power generation capacity of this power station is through conventional steam turbines, compared with the farmland occupied by the power plant. Reduce the use of water by about 85%. The power plant "solar field" covers 3 square miles and includes 2700 trough collectors. The collector size is about 25 feet wide, about 500 feet long and about 10 feet high. Its heat storage device guarantees 6 hours of sunless power generation. It is reported that the project is expected to put into production in 2011. APS (Arizona Power System) will acquire 100% of its electricity. While providing solar power, SOLANA Solar Power Station also wants to provide modern technology tourism and sightseeing services.

Tower Concentrating Thermal Power System

Tower technology is also a type of GSP. By distributing the array of heliostats arranged in a ring around the light tower, the sunlight is focused and reflected to the receiver mounted on the top of the tower. The heat transfer medium in the receiver absorbs the radiant energy reflected from the highly concentrated heliostat and converts it into heat energy. The heat energy is further converted into steam to drive the turbine to drive the generator to generate electricity. The heat energy generated is also stored in the molten salt tank. Because the temperature is higher during the tower thermal cycle, its overall light-electricity conversion efficiency can reach 25%.

The largest tower thermal power plant currently under construction is located in Spain with a PSMW installed capacity of 20MW and an area of approximately 1,415 acres. Before the PS20, the PS10 was put into commercial operation in 2007. Its installed capacity is 11MW, including 624 heliostats, each mirror area of 120m2, controlled by their own separate daylight tracking control system, the sun will be reflected to the top of the tower receiver, condenser tower 115m high. The PS10 generates 24GWh of electricity annually.

Under the premise of site conditions, it is possible to generate steam through the solar thermal collector field beside the existing conventional thermal power plants, and with existing circulating steam turbines.

The power generation system combines the formation of a hybrid power system to reduce the consumption of fossil fuels

Consumption, reduce emissions.

in conclusion

From the comparison of photovoltaic power generation and solar thermal power generation technology, photovoltaic power generation has a high level of technology maturity, and has been successfully put into commercial operation with large installed capacity. The scale of construction and investment is flexible and can be expanded in stages, but due to the use of DC power generation, AC inverter and Network operation mode, power quality can not be reliably guaranteed, and due to the self-protection function of the inverter equipment, when the network failure requires power support can not be scheduled, so it is suitable for near consumption or for grid peaking, especially for seasonal and time Stronger civilian and commercial electricity; Photo-thermal power generation has higher photoelectric conversion efficiency than photovoltaic power generation, and the scale can reach the scale of small-to-medium-sized conventional thermal power plants, with a scale-cost advantage because it uses conventional power generation methods on the grid side and The traditional thermal power plant is the same, and it can be used as the power supply for the Internet. In the presence of site conditions, the heat energy generated by the conversion of light and heat can be combined with the original thermal power, and some of the coal heat or oil heat can be replaced by light and heat to form a hybrid power generation system. Emissions from small thermal power plants may be seriously planned and successfully implemented. Save some small thermal power plants, it continued efficiency of the turbine generator set.

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The main problem of CSP is the large scale of investment, which must reach a certain scale in order to realize the benefits of investment. The penetration of technology is lower than that of photovoltaic power generation, which requires water resources, long construction period, and the system's flexible expansion performance is inferior to photovoltaic power generation systems. Photovoltaic power generation technology, flat-panel photovoltaic power generation is suitable for ordinary residential or

Peak-hour electricity is used for public buildings and commercial buildings; concentrating photovoltaic power generation equipment is suitable for large-scale online power plant construction.