Application of MPPT photovoltaic grid-connected system in household air conditioners

Solar energy is an inexhaustible clean energy. With global environmental pollution and energy crisis becoming increasingly serious, research on solar energy utilization is of great significance to alleviate the energy crisis, protect the ecological environment and ensure the sustainable development of the economy. At present, my country's photovoltaic technology is still far behind that of advanced countries in the world, and the operating cost of building a large independent photovoltaic power station is high. Therefore, it is urgent to develop a household photovoltaic grid-connected system. In addition, among household energy consumption, air conditioning consumes a lot of electricity in summer, and the load is peaked during the day, and the law of load change is very consistent with the output of solar cells. It plays a peak-leveling role for the city power supply system. Therefore, based on this feature of the air conditioner, this paper proposes a design idea of ​​combining household photovoltaic power generation system with air conditioning. Its working principle is to convert solar energy into electrical energy and send it to the air conditioner, which in turn powers the air conditioner. When the air conditioner is turned off, the electrical energy of the solar cell can also be fed back to the power grid, so that solar energy can be fully utilized. In the process of solar energy transfer, this paper introduces the maximum power point tracking method to keep the photovoltaic array at the maximum output power. This method is analyzed in detail in the article. There is not much research on solar air conditioning systems in China, but based on foreign market information, I believe that there will be broad application prospects in China in the future.

Working principle of photovoltaic power generation system

System composition

The main circuit of the system is shown in Figure 1.

For the entire system, the power consumption of the air conditioner can be supplied by both the photovoltaic array and the grid. The grid-connected inverter converts the DC power generated by the photovoltaic array into AC power with the same frequency and phase as the grid voltage to supply power to the air conditioner. During the day, when the AC power generated by the photovoltaic system exceeds the power required by the air conditioner, the excess is fed to the grid; at other times, when the AC power generated by the air conditioner is greater than the AC power generated by the photovoltaic system, the grid automatically provides supplementary power to the load.

Figure 1 is the main circuit diagram of the photovoltaic power generation system, which mainly consists of two parts. The first part is the Buck-Boost converter working in discontinuous mode, and the second part is the inversion process. In the photovoltaic power generation circuit, the Buck-Boost converter consists of S1, S2, Lp and D1. Its function is to shape the output current io in combination with maximum power point tracking. Li and Ci act as input filters to smooth the input current. The inverter output current iac serves as the input current to the air conditioner power supply circuit. Maximum power point tracking is completed based on the switching frequency modulation of S1.

In the Buck-Boost circuit, when S1 is turned on, S2 and D1 are turned off, and Lp stores energy. When S1 is turned off, D1 and S2 are turned on, and the energy stored in Lp and Cdc will be transferred to the output. During the switching period Ts, the switching of S2 is controlled by comparing the relationship between io and the given current i'ac,reg.

Lp acts as a buffer for temporary energy storage. If the output power of the photovoltaic array is higher than the power absorbed by the air conditioner, the energy stored in Lp will be transferred to Cdc through LS and D2 after S2 is turned off. The average voltage of Cdc is equivalent to half of the peak voltage of Vac. Cdc is used to correct i'ac,reg and i0. Therefore, the connection between Lp and Cdc plays a role in current shaping and reducing the voltage stress on Cdc in the process of transmitting energy to the charging circuit. In the inverter process, the inverter is composed of SA, SB, SC and SD. SA and SB are turned on in the positive half cycle of the AC voltage, and SC and SD are turned on in the negative half cycle of the AC voltage. When S2 and D1 are turned on, the current difference between L2 and is absorbed by C1, and L4 is used to filter out the high-frequency current components transmitted to the air conditioner to achieve a smooth sine wave output by the inverter.

Output characteristics of photovoltaic cells

In the photovoltaic power generation system, solar cells directly convert solar energy into electrical energy. The output of solar cells is determined by many factors, such as sunshine conditions, temperature, etc. In different environments, the output curve of solar cells is different, and the corresponding maximum power point is also different. The stronger the sunshine, the greater the power that the solar cell can output. The temperature is just the opposite. The higher the temperature of the solar cell itself, the smaller the power that the solar cell can output. Its output has nonlinear characteristics. Figure 2 shows the output characteristic curve of the solar cell. Figure 2 (a) is the curve of U, I and sunshine (S) of the solar cell when the solar cell temperature is 25℃. It can be seen from the figure that the power at any point on the curve is P=UI. Its value is related to U and I as well as sunshine (S), solar cell temperature, etc. It can be further known from Figure 2 (b) that since the working efficiency of the solar cell is equal to the ratio of the output power to the power projected on the solar cell area, in order to improve the working efficiency of this system, the solar cell must be operated at the maximum power point as much as possible, so that the maximum power output can be obtained with the solar cell with the smallest power possible. In Figure 2, points A, B, C, D, and E correspond to the maximum power points at different sunshine times.

Maximum power point tracking of solar cells

The maximum power point tracking of solar cells is to make full use of solar energy so that the solar cells always output the maximum power. The MPP tracking of solar cells is based on the modulation of the switching frequency S1. The PWM signal of switch S1 is modulated by a low-frequency signal. The model of the solar cell can be equivalent to the circuit in Figure 3.

Vg is the power source, rg is the internal resistance of the battery, both Vg and rg are affected by the intensity of sunlight and temperature, and the input voltage and input resistance of the converter are represented by Vin and rin. Assume 100% conversion efficiency.