Overview of Solar Photovoltaics

Introduction to Solar Photovoltaic Power Generation

Solar photovoltaic power generation is a way of generating electricity by utilizing the photovoltaic effect of a solar cell (a semiconductor device similar to a crystal diode) to directly convert the sun's radiant energy into electrical energy, and the energy converter of solar photovoltaic power generation is the solar cell, also known as a photovoltaic cell. When sunlight hits the solar cell (composed of two different types of homogeneous semiconductor materials of P and N types), part of the light is reflected, part of the light is absorbed, and part of the light passes through the cell.Absorbed light energy to stimulate the bound high energy state of the electron, resulting in electron - hole pairs, in the PN junction of the built-in electric field, electrons, holes and each other's movement (as shown in Figure 1-1), the N region of the hole to the P region, the P region of the electron to the N region of the movement of the light side of the solar cell so that a large number of negative charges (electrons) accumulated, and a large number of positive charges (holes) accumulated in the back light side of the battery. If a load is connected to both ends of the cell, a current will pass through the load, and when the light keeps shining, a constant current will flow through the load. A single solar cell is a thin slice of semiconductor PN junction, standard light conditions, the rated output voltage of 0.5 ~ 0.55 V. In order to obtain a higher output voltage and larger power capacity, in practical applications often connect multiple solar cells together to form a battery pack, or more battery packs to form a photovoltaic array, as shown in Figure 1-2. The output power of solar cells is random, and the output power of the same solar cell is different at different times, in different locations, with different light intensities, and in different installation methods.

1-1 Solar photovoltaic cell power generation principle

1-2 From Cells and Modules to PV Arrays

Benefits of Solar PV

The process of solar photovoltaic power generation is simple, with no mechanical rotating parts, no fuel consumption, no emission of any substance including greenhouse gases, no noise and no pollution; solar energy resources are widely distributed and inexhaustible. Therefore, compared with new power generation technologies such as wind power and biomass power generation, photovoltaic power generation is a renewable energy power generation technology characterized by the most sustainable development (the richest resources and the cleanest power generation process), and its main advantages are as follows.

(1) Solar energy resources are inexhaustible, the solar energy irradiated to the earth is nearly 6,000 times larger than the energy currently consumed by human beings, and solar energy is widely distributed on the earth, as long as there is light in the place where the photovoltaic power generation system can be used without the restriction of geography, altitude and other factors.

(2) Although on the earth's surface, different dimensions, differences in climatic conditions and other factors will cause uneven solar radiation, but because solar energy resources are available everywhere, can be close to the solution of power generation, power supply and electricity, do not have to be transported over long distances, to avoid long-distance transmission line investment and loss of electrical energy.

(3) Photovoltaic power generation is directly from photon to electron conversion, there is no intermediate process (such as thermal energy converted to mechanical energy, mechanical energy converted to electromagnetic energy, etc.) and mechanical movement, there is no mechanical wear and tear. According to thermodynamic analysis, photovoltaic power generation has a very high theoretical power generation efficiency, up to 80% or more, technology development potential.

(4) Photovoltaic power generation itself does not use fuel, the emission of greenhouse gases and other exhaust substances is almost zero, does not produce noise, and will not pollute the air and water, friendly to the environment. Will not suffer from the energy crisis or fuel market instability impact, solar energy is a real green renewable energy.

(5) The photovoltaic power generation process does not require cooling water, and the power generation device can be installed in the desert and the Gobi where there is no water. Photovoltaic power generation can also be easily combined with the roofs and walls of buildings to form a roof distributed or photovoltaic building integrated power generation system, which does not need to occupy the land separately and can save valuable land resources.

(6) Photovoltaic power generation has no mechanical transmission parts, simple operation and maintenance, and stable and reliable operation. A set of photovoltaic power generation system as long as there is the sun, battery components can generate electricity, coupled with the extensive use of automatic control technology, basically can realize unattended, low maintenance costs.

(7) photovoltaic power generation system performance is stable and reliable, long service life (more than 30 years), crystalline silicon solar cell life can be as long as 25 to 35 years. In the photovoltaic power generation system, as long as the design is reasonable and the selection is appropriate, the life of the battery can also be as long as 10 to 15 years.

(8) The structure of solar cell module is simple, small in size and light in weight, which is easy for transportation and installation. Photovoltaic power generation system construction cycle is short, and according to the power load capacity can be large or small, convenient and flexible, very easy to combine, expansion.

In addition, the most widely used in recent years is the distributed photovoltaic power generation system constructed by utilizing the roofs of various buildings and agricultural facilities as well as the roofs of family houses, which, in addition to the same advantages as those mentioned above, also has the following superiorities.

(1) Distributed photovoltaic power generation basically does not take up land resources, and can be used to generate and supply power in the vicinity, without or with fewer transmission lines, which reduces transmission costs. PV modules can also directly replace traditional wall and roof materials.

(2) Distributed photovoltaic power generation system is power generation and power coexistence when connected to the distribution grid. In the grid power supply is in the peak period of power generation, can effectively play the role of peak leveling, cut down the expensive peak power supply load in the city, can to a certain extent alleviate the tension of power consumption in local areas.

Disadvantages of Solar PV

(1) Low energy density. Although the sun's energy to the earth is extremely huge, but because the earth's surface area is also very large, and most of the earth's surface is covered by the ocean, the real solar energy can reach the land surface only to reach the earth's range of radiant energy of 10% or so, resulting in a unit area of the land can be directly access to the sun's energy is relatively small, usually to the sun's irradiance to express the surface of the earth's highest value of about 1.2kWh / m2, and the vast majority of areas and most of the sunshine time is lower than 1kWh / m2. m2, and the vast majority of areas and most of the sunshine hours are lower than 1kWh/m2. solar energy utilization is actually a low-density energy collection, utilization.

(2) Covering a large area. Solar energy density is low, which makes the photovoltaic power generation system will cover a large area, every 10kW photovoltaic power generation power covers an area of about 70m2, the average power generation power per square meter area is about 160W. With the promotion of distributed photovoltaic power generation and the maturity and development of photovoltaic building integrated power generation technology, more and more photovoltaic power generation systems can be utilized on the roofs and façades of buildings and structures, which gradually improves the shortcomings of photovoltaic power generation systems with a large floor space.

(3) Lower conversion efficiency. The most basic unit of photovoltaic power generation is the solar cell module. The conversion efficiency of photovoltaic power generation refers to the ratio of light energy converted into electricity. At present, the highest conversion efficiency of crystalline silicon photovoltaic cells in 21% or so, made of battery components conversion efficiency of 16% to 17%, amorphous silicon photovoltaic module conversion efficiency up to more than 13%. Due to the low photoelectric conversion efficiency, the photovoltaic power generation system power density is low, it is difficult to form a high power generation system.

(4) Intermittent work. On the surface of the earth, photovoltaic power generation system can only generate electricity during the daytime, but not at night, which is inconsistent with people's way of using electricity and habits. Unless there is no difference between day and night in space, solar cells can generate electricity continuously.

(5) Influenced by natural conditions and climatic factors. Solar photovoltaic power generation energy comes directly from sunlight irradiation, and the sunlight irradiation on the earth's surface by the natural conditions and climate has a great impact on the seasons of the year, the alternation of day and night, geographic latitude and altitude and other natural conditions, as well as cloudy and clear, rain, snow, fog or even changes in cloud cover will have a serious impact on the system's power generation status. In addition, the impact of environmental factors is also very large, especially airborne particles (such as dust, etc.) landed on the surface of the battery module, which will block some of the light irradiation, so that the conversion efficiency of the battery module is reduced, and power generation is reduced.

(6) Strong geographical dependence. Different geographic locations, so that the sunshine resources in various regions vary greatly. Photovoltaic power generation system only in the application of solar energy resource-rich areas is good, high return on investment.

(7) The system cost is high. The efficiency of solar photovoltaic power generation is relatively low, and so far, the cost of photovoltaic power generation is still higher than other conventional power generation methods (such as thermal and hydropower). This is also one of the main factors restricting its widespread application. However, we should see that with the continuous expansion of solar cell capacity and the continuous improvement of Solar-cell efficiency of cell chips, the cost of photovoltaic power generation system has dropped very quickly, and the price of solar cell modules has dropped from more than 10 yuan per watt in the past few years to about 2.5 yuan/W at present.

(8) The manufacturing process of crystalline silicon batteries is high in pollution and energy consumption. The main raw material for crystalline silicon batteries is pure silicon. Silicon is the second most abundant element on Earth after oxygen, mainly present in the form of sand (silica). The gradual transformation from sand to pure crystalline silicon with a content of over 99.9999% requires multiple chemical and physical processes, which not only consumes a large amount of energy but also causes certain environmental pollution.

Despite the aforementioned shortcomings and drawbacks of solar photovoltaic power generation, the gradual depletion of global fossil energy and the global warming and ecological environment degradation caused by excessive consumption of fossil energy pose a great threat to human survival. Therefore, vigorously developing renewable energy is one of the main measures to solve this problem.