Photovoltaic power generation refers to a form of power generation that uses the photovoltaic effect of semiconductor materials to directly convert solar radiation energy into electrical energy. Photovoltaic effect refers to the change in the distribution and concentration of carriers that can conduct current inside an object after absorbing light energy, resulting in the effects of current and electromotive force. This effect can be produced in gases, liquids and solids. The electric energy converted from solar energy can be directly stored in energy storage equipment for use when needed, or it can be connected to the grid according to the actual situation of electricity consumption, so that the solar power generation system and the large grid system can be connected to the grid.
Materials with a resistivity of 10-3~108Ω·cm are called semiconductors. The characteristics of semiconductor materials are that their conductivity and resistivity are very sensitive to the type and concentration of doped trace impurities, and they are sensitive to changes in external conditions such as temperature and light. Thermal, photosensitive and other characteristics. Commonly used semiconductor materials are: elemental semiconductors, such as silicon (Si), germanium (Ge), etc., compound semiconductors, such as gallium arsenide (GaAs), etc., and semiconductor materials doped or made into other compounds, such as boron (B) , Phosphorus (P), copper (In) and antimony (Sb), among which silicon is the most commonly used semiconductor material. The forbidden band width of semiconductors is relatively small. The valence electrons in the valence band can transition to the conduction band and become free electrons as long as they obtain a larger energy exceeding the band gap, and at the same time leave an empty energy level in the valence band. The rest of the trapped valence electrons in the valence band will come to occupy this empty energy level position, and thus the hole position will move, and its moving direction is exactly opposite to the moving direction of the valence electrons. The electrons in the conduction band and the holes in the valence band become two kinds of carriers in the semiconductor, and their charge polarity is opposite. The electrons are negatively charged and the holes are positively charged. In a pure semiconductor crystal, the excited electrons and holes are generated in pairs, which is called “intrinsic” excitation, and pure semiconductor crystals are also called “intrinsic semiconductors.”
At room temperature, only a very small number of electron-hole pairs in intrinsic semiconductors participate in conduction, and some free electrons will quickly recover and synthesize into a covalent bond electron structure when encountering holes, so they are non-conductive from the perspective of external characteristics. Both doping and defects can increase the electron concentration in the conduction band. After adding a small amount of pentavalent elements (such as phosphorus, arsenic, antimony, etc.) to silicon (or germanium) crystals, impurity atoms replace certain silicon atoms in the crystal lattice. Among its five valence electrons Four covalent bonds are formed with the surrounding silicon atoms, and the extra valence electron is outside the covalent bond. The impurity atom has a weak binding force to this extra valence electron, and it can be excited into free electrons at room temperature. . Because impurity atoms can donate electrons, they are called donor atoms. Such impurity semiconductors are called N-type semiconductors.
After doping a small amount of trivalent elements (such as boron, aluminum, indium, etc.) into the silicon (or germanium) crystal, the impurity atoms replace some of the silicon atoms in the crystal lattice, and its three valence electrons and the surrounding When the silicon atoms form a covalent bond, a hole is formed. At room temperature, these holes can attract nearby valence electrons to fill, making the impurity atoms become negatively charged ions. These impurity atoms are called acceptor atoms because they can absorb electrons. Such impurity semiconductors are called P-type semiconductors.
Regardless of whether it is an N-type semiconductor or a P-type semiconductor, although the doping concentration is extremely low, their semiconductor conductivity is much greater than that of the intrinsic semiconductor.
On a complete silicon wafer, different doping processes are used to form an N-type semiconductor on one side and a P-type semiconductor on the other side. We call the area near the interface of the two semiconductors a PN junction. After the P-type semiconductor and the N-type semiconductor are combined, there is a difference in the concentration of electrons and holes at their junction. Due to the difference in the concentration of free electrons and holes, some electrons will diffuse from the N-type region to the P-type region, and some holes will diffuse from the P-type region to the N-type region. As a result of their diffusion, the P region loses holes on one side, leaving negatively charged impurity ions, and the N region loses electrons, leaving behind positively charged impurity ions. The ions in the open-circuit semiconductor cannot move arbitrarily, so they do not participate in conduction. These immovable charged particles form a space charge zone near the interface of the P and N zones. The thickness of the space charge zone is related to the concentration of dopants.
After the space charge region is formed: due to the interaction between the positive and negative charges, an internal electric field is formed in the space charge region, the direction of which is from the positively charged N region to the negatively charged P region. Obviously, the direction of this electric field is opposite to the direction of carrier diffusion movement to prevent diffusion. On the other hand, this electric field will cause the minority carrier holes in the N region to drift to the P region, and make the minority tax in the P region flow to the electrons. Drift to the N zone: the direction of drifting movement is just opposite to the direction of the dispersion movement. The holes drifting from the N zone to the P zone supplement the holes lost in the P zone on the original interface, and the electrons drifting from the P zone to the N zone The electrons lost in the N zone on the original rice interface are supplemented, which reduces the space charge and weakens the internal electric field. Therefore, the result of drifting motion is to narrow the space charge region and strengthen the microscopic motion. On both sides of the junction surface of the P-type semiconductor and the N-type semiconductor, a thin ion layer is left. The space charge region formed by this thin ion layer is called a PN junction. The direction of the internal electric field of the PN junction points from the N area to the P area. Solar cells use light to excite a small number of currents to generate electricity through the PN junction.
When light irradiates the surface of the solar cell, part of the photons are absorbed by the silicon material, and the energy of the photons is transferred to the silicon atoms, causing the electrons to undergo transitions and become free electrons, which accumulate on both sides of the PN junction to form a potential difference. When the external circuit is turned on, under the action of the voltage, there will be a current flowing through the external circuit to produce a certain output power. The essence of this process is the process of converting photon energy into electrical energy. Solar cells can convert light energy into electrical energy as long as they are irradiated by sunlight or lights. Solar cells can generate electricity equivalent to 10% to 20% of the received light energy. Generally speaking, the stronger the light, the more electrical energy it emits. many. In a solar power generation system, the total efficiency of the system is determined by the photoelectric conversion efficiency of the photovoltaic module, the efficiency of the controller, the efficiency of energy storage equipment, the efficiency of the inverter, the efficiency of the transformer, the cable loss and the efficiency of the load. At present, the photoelectric conversion efficiency of large-scale and mass-produced solar cells is only 17% to 19%. Therefore, improving the photoelectric conversion efficiency of photovoltaic modules and reducing the unit power cost of photovoltaic power generation systems are the focus and difficulty of the continued development of photovoltaic power generation.
The basic working principle of the photovoltaic power generation system is to charge the energy storage device with the electric energy generated by the photovoltaic module under the sunlight, or directly supply power to the load when the load demand is met. If the sun is insufficient or at night, the energy storage device Under the control of the inverter, power is supplied to the DC load. For photovoltaic systems with AC loads, an inverter needs to be added to convert DC power into AC power. The application of photovoltaic systems has many forms, but the basic principles are similar. Other types of photovoltaic systems differ only in control mechanisms and system components according to actual needs.