Photovoltaic Power Generation - Technology That Converts Solar Energy Directly Into Electricity

Photovoltaic power generation is a technology that directly converts light energy into electrical energy by using the photovoltaic effect at the semiconductor interface. It is mainly composed of three parts: solar panels (components), controllers and inverters, and the main components are composed of electronic components. The solar cells are packaged and protected after being connected in series to form a large-area solar cell module, and then cooperate with power controllers and other components to form a photovoltaic power generation device.

1. The development process of photovoltaic power generation

1.1 Early history
As early as 1839, the French scientist Becqurel discovered that light could generate potential differences between different parts of semiconductor materials. This phenomenon was later called the "photovoltaic effect", or "photovoltaic effect" for short. In 1954, American scientists Chapin and Pearson made a practical monocrystalline silicon solar cell for the first time at Bell Laboratories in the United States, giving birth to a practical photovoltaic power generation technology that converts sunlight energy into electrical energy.

After the 1970s, with the development of modern industry, the global energy crisis and air pollution problems have become increasingly prominent. Traditional fuel energy is decreasing day by day, and the harm to the environment is becoming more and more prominent. At the same time, about 2 billion people in the world do not have access to normal energy supply. At this time, the whole world is turning its attention to renewable energy, hoping that renewable energy can change the energy structure of mankind and maintain long-term sustainable development.

Solar energy has become the focus of people's attention due to its unique advantages. Abundant solar radiation energy is an important energy source, which is inexhaustible, non-polluting, cheap, and can be freely utilized by human beings. The energy of solar energy reaching the ground every second is as high as 800 megawatt hours. If 0.1% of the solar energy on the earth's surface is converted into electrical energy, the conversion rate is 5%, and the annual power generation can reach 5.6×1012 kilowatt hours, which is equivalent to 40% of the world's energy consumption. times. It is precisely because of these unique advantages of solar energy that after the 1980s, the types of solar cells have continued to increase, the scope of application has become increasingly broad, and the market scale has gradually expanded.

After the 1990s, photovoltaic power generation developed rapidly. By 2006, more than 10 megawatt-level photovoltaic power generation systems and 6 megawatt-level networked photovoltaic power plants had been built in the world. The United States is the first country to formulate a development plan for photovoltaic power generation. In 1997, the "Million Roofs" plan was proposed. Japan launched the New Sunshine Project in 1992. By 2003, Japan's photovoltaic module production accounted for 50% of the world's total. Four of the world's top 10 manufacturers are in Japan. The new German Renewable Energy Law stipulates the on-grid electricity price for photovoltaic power generation, which greatly promotes the development of the photovoltaic market and industry, making Germany the fastest-growing country in the world for photovoltaic power generation after Japan. Switzerland, France, Italy, Spain, Finland and other countries have also formulated photovoltaic development plans and invested huge sums of money in technology development and accelerated industrialization.

The average annual growth rate of photovoltaic modules in the world from 1990 to 2005 was about 15%. In the late 1990s, the development became more rapid. In 1999, the production of photovoltaic modules reached 200 megawatts. The efficiency of commercialized batteries has increased from 10% to 13% to 13% to 15%, and the production scale has grown from 1 to 5 MW/year to 5 to 25 MW/year, and is expanding to 50 MW or even 100 MW. The production cost of photovoltaic modules has dropped below $3/watt.

1.2 Status and Trends
In 2011, the global newly installed photovoltaic capacity was about 27.5GW, an increase of 52% compared with 18.1GW in the previous year, and the global cumulative installed capacity exceeded 67GW. Among the total installed capacity of nearly 28GW in the world, nearly 20GW systems are installed in Europe, but the growth rate is relatively slow. Among them, the Italian and German markets account for 55% of the global installed capacity growth, respectively 7.6GW and 7.5GW.

In 2011, the photovoltaic industry market demand in the Asia-Pacific region represented by China, Japan and India increased by 129% year-on-year, and its installed capacity was 2.2GW, 1.1GW and 350MW respectively. In addition, in the increasingly mature North American market, the newly installed capacity is about 2.1GW, an increase of up to 84%.

Among them, China is the country with the fastest growth in the installation of photovoltaic power generation in the world. In 2011, the installation of photovoltaic power generation increased by about 5 times compared with 2010. In 2011, the battery production reached 20GW, accounting for about 65% of the world. By the end of 2011, there were about 115 battery companies in China, with a total production capacity of about 36.5GW. Among them, there are 14 companies with a production capacity of more than 1GW, accounting for 53% of the total production capacity; 63 companies with a production capacity between 100MW and 1GW, accounting for 43% of the total production capacity; 4% of production capacity. The differentiated competition pattern of scale, technology and cost is gradually becoming clear. The shipments of the top ten domestic module manufacturers accounted for 60% of the total battery output.

In the next ten years, China's photovoltaic power generation market will shift from independent power generation systems to grid-connected power generation systems, including desert power plants and urban rooftop power generation systems. China's solar photovoltaic power generation stations have great potential. With active and stable policy support, by 2030, the installed photovoltaic capacity will reach 100 million kilowatts, and the annual power generation will reach 130 million megawatt hours, which is equivalent to saving more than 30 large coal power plants. The country will invest 20 billion yuan to subsidize the photovoltaic industry in the next three years. China's solar photovoltaic power generation has ushered in a new round of rapid growth and attracted more strategic investors to integrate into this industry.

In the first half of 2015, the country's cumulative photovoltaic power generation capacity was 19 million MWh.

On September 7, 2015, the first photovoltaic power generation project of Jiangsu Province was officially connected to the grid in Pukou District, Nanjing, and rural residents also used "green electricity". Next, photovoltaic power generation projects will be promoted in rural substations.

In November 2015, Lai'an County, Anhui Province fully launched the rural photovoltaic power generation project, and the photovoltaic power stations with an installed capacity of more than 60KW in 11 beautiful rural "shell villages" entered the bidding process. According to preliminary estimates, after grid-connected power generation, each village can provide 72,000KWh of clean electricity every year, and the village-level collective economy can increase income by more than 50,000 yuan.

From January to June 2015, the installed capacity of photovoltaic power generation in the country was 7.73 million kilowatts. By the end of June 2015, the installed capacity of photovoltaic power generation in the country had reached 35.78 million kilowatts.

Since 2013, the installed capacity of photovoltaic power generation has exceeded 10 million kilowatts for three consecutive years; by the end of 2015, the cumulative installed capacity of photovoltaic power generation had reached about 43 million kilowatts, surpassing Germany to become the world's number one. In addition, the photovoltaic industry is making efforts to "go out". According to data from the National Energy Administration, in 2015, the export volume of photovoltaic cells and components reached more than 25 million kilowatts, and the export value reached 14.4 billion US dollars.

2. The principle of photovoltaic power generation

The main principle of photovoltaic power generation is the photoelectric effect of semiconductors. When a photon irradiates a metal, its energy can be completely absorbed by an electron in the metal. The energy absorbed by the electron is large enough to overcome the Coulomb force inside the metal atom to do work, escape from the metal surface, and become a photoelectron. Silicon atoms have 4 outer electrons. If atoms with 5 outer electrons, such as phosphorus atoms, are mixed into pure silicon, it becomes an N-type semiconductor; if pure silicon is doped with atoms with 3 outer electrons, such as Boron atoms form a P-type semiconductor. When the P-type and N-type are combined, the contact surface will form a potential difference and become a solar cell. After sunlight irradiates the P-N junction, electric current will flow from the P-type side to the N-type side to form a current.

The photoelectric effect is an important and miraculous phenomenon in physics. Under the irradiation of electromagnetic waves higher than a certain frequency (this frequency is called the threshold frequency), electrons inside certain substances absorb energy and escape to form current, that is, photoelectricity.

Polycrystalline silicon is made into silicon wafers to be processed after ingot casting, ingot breaking, slicing and other procedures. Doping and diffusing a small amount of boron, phosphorus, etc. on the silicon wafer forms a PN junction. Then use screen printing to print the finely prepared silver paste on the silicon wafer to make a grid line, and after sintering, make a back electrode at the same time, and coat a layer of anti-reflective coating on the surface with the grid line, and the battery sheet is So far made. The battery slices are arranged and combined to form a battery assembly, which forms a large circuit board. Generally, the aluminum frame is wrapped around the module, the front is covered with glass, and the electrode is installed on the back. With battery components and other auxiliary equipment, a power generation system can be formed. In order to convert direct current into alternating current, a current converter is installed. After power generation, it can be stored in batteries or input into the public grid. The battery components account for about 50% of the cost of the power generation system, and the other 50% for current converters, installation fees, other auxiliary components, and other expenses.

3. Technical characteristics of photovoltaic power generation

3.1 Advantages of photovoltaic power generation
No matter from the perspective of the world or China, conventional energy is very limited. China's primary energy reserves are far below the world's average level, only about 10% of the world's total reserves. Solar energy is an inexhaustible renewable energy for human beings. It has sufficient cleanliness, absolute safety, relative extensiveness, long life and maintenance-free, resource adequacy and potential economy. It has an important position in the long-term energy strategy.

Compared with commonly used thermal power generation systems, the advantages of photovoltaic power generation are mainly reflected in:

3.1.1 No risk of depletion;
3.1.2 Safe and reliable, no noise, no pollution discharge, absolutely clean (no pollution);
3.1.3 The advantages of building roofs can be used without being restricted by the geographical distribution of resources; for example, areas without electricity, and areas with complex terrain;
3.1.4 Generate power locally without consuming fuel and erecting transmission lines;
3.1.5 High energy quality;
3.1.6 It is easy for users to accept emotionally;
3.1.7 The construction period is short, and the time spent in obtaining energy is short.

3.2 Disadvantages of photovoltaic power generation
However, the production of solar panels has the characteristics of high pollution and high energy consumption. Under the current conditions, it is reasonable to produce solar panels for domestic use, but a large number of exports is equivalent to polluting China and benefiting the world. According to statistics, To produce a 1m×1.5m solar panel, more than 40kg of coal must be burned, but even the most inefficient thermal power plant in China can use this coal to produce 130 kWh of electricity (generally, a 1mx1.6m solar panel can generate electricity in a year Above 250 kWh) -- that's enough to power a 2.2-watt light-emitting diode (LED) bulb for 30 years, calculated on a 12-hour day.

3.2.1 The energy distribution density of irradiation is small, that is to say, it will occupy a huge area;
3.2.2 The obtained energy is related to weather conditions such as four seasons, day and night, cloudy and sunny.
3.2.3 Compared with thermal power generation, the cost of generator will be high.
3.2.4 The manufacturing process of photovoltaic panels is not environmentally friendly.

4. Conversion rate of photovoltaic power generation

4.1 Monocrystalline silicon
Large-scale production conversion rate: 19.8-21%; mostly at 17.5%. The possibility of technological breakthroughs that increase efficiency by more than 30% is less likely.

4.2 Polysilicon
Large-scale production conversion rate: 18-18.5%; mostly at 16%. Like monocrystalline silicon, due to the limitations of the physical properties of the material, it is less likely to achieve a conversion rate of more than 30%.

4.3 Gallium Arsenide
The conversion rate of gallium arsenide solar cells is relatively high, about 23%. But the price is expensive, and it is mostly used in important places such as aerospace. There is basically no practical value for large-scale industrialization.

4.4 Films
Thin-film photovoltaic cells have the advantages of thinness, light weight, and good flexibility, and have a wide range of applications, especially suitable for building integrated photovoltaics. If the efficiency of thin-film battery components is almost the same as that of crystalline silicon batteries, its cost performance will be incomparable. Thin-film batteries prepared on flexible substrates have the advantages of being rollable and foldable, not afraid of falling, light weight, and good low-light performance, and will have broader application prospects in the future.

The conversion rate of amorphous silicon film is about 9%. The conversion rate of amorphous silicon is expected to be even higher.

4.5 Efficiency decay
After crystalline silicon photovoltaic modules are installed, they will be exposed to the sun for 50-100 days, and the efficiency will decay by about 2-3%. After that, the attenuation rate will be greatly slowed down and stabilized, with an annual attenuation of 0.5-0.8%, and the attenuation will be about 20% in 20 years. The attenuation rate of monocrystalline modules is less than that of polycrystalline modules. The attenuation of amorphous components is about lower than that of crystalline silicon.

Therefore, improving the conversion rate and reducing the cost per watt will still be the two major themes of the future development of photovoltaics. Either way, if the large-scale application can increase the conversion rate to 30%, and the cost is below 5,000 yuan per kilowatt (equal to hydropower), then mankind will have the most extensive and cleanest nuclear fusion power generation research before it succeeds. , the cheapest and almost unlimited reliable new energy.