The following is a detailed explanation of the design method of an independent photovoltaic system in the form of an example, which includes a simple calculation formula.

For example, a photovoltaic off-grid project is built in Nanjing, Jiangsu. The total power consumption of the project is 4000W·h/d, and electricity is used all day long. The selected inverter efficiency is 90%, the number of continuous rainy days is 4 days, and the battery The depth of discharge is 70%, and the system voltage is 48V. Please design the power station according to user needs.

Solution:**1. Load and battery capacity matching design.**Step 1: The simple calculation formula of battery capacity is:

Battery capacity = (average daily power consumption of load × self-sufficiency days) / maximum depth of discharge

The formula does not consider the influence of various factors of the system. In the design process, the relevant factor coefficients must be included in the formula, and the battery capacity design is relatively complete. The complete formula is:

Battery capacity = (average daily power consumption under load × self-sufficiency days × discharge rate correction coefficient) / (maximum discharge depth × low temperature correction coefficient × inverter efficiency)

In the formula, the average power consumption of the load (A h) is not directly given in this example, but the system voltage in the question is 48v, which can be calculated according to P=U; calculate the system discharge hour rate, and then according to the selected battery manufacturer Supplied batteries are corrected for capacity at different discharge rates.

Average discharge rate=(Continuous rainy days×load working time)/Maximum discharge depth=(4×24h)/70%=137 hour rate

At this time, the load working time is taken as 24h, and the discharge rate correction factor can be taken as 0.8.

The second step: low temperature correction factor calculation. Usually, the capacity of lead-acid batteries is calibrated at 25°C. As the temperature decreases, the capacity at 0°C drops to approximately 90% of the rated capacity, and at -20°C to approximately 80% of the rated capacity. Therefore, the influence of the ambient temperature of the battery on its capacity must be considered. The annual minimum temperature in Nanjing is about -6~-4℃. According to Figure 1, at this temperature, the capacity of the battery will drop by about 10%.

The third step: data substitution type battery capacity, calculate the actual capacity of the battery

Battery capacity = (average daily power consumption under load × self-sufficiency days × discharge rate correction coefficient) / (maximum discharge depth × low temperature correction coefficient × inverter efficiency) = (4000 × 4 × 0.8) / 70% × 90% × 90% ×48=470A·h

**2. Determine the series-parallel mode of the battery**Each battery has its nominal voltage. In order to achieve the nominal voltage of the load operation, the batteries must be connected in series and parallel to supply power to the load. The number of batteries connected in series is equal to the nominal voltage of the load divided by the nominal voltage of the battery. The 24V/200A·h gel battery is used here.

Number of batteries in series = load nominal voltage / battery nominal voltage

Number of parallel batteries = total battery capacity/single battery capacity

Then the number of batteries in series in this question is 48V/24V=2; the number of batteries in parallel is 470A·h/200A·h=2.35, take 3.

To sum up: use 24V/200A·h-type colloidal battery, 2 batteries in series, 3 batteries in parallel, a total of 6 batteries to meet the system requirements, the connection method is shown in Figure 2.

**3. Design of solar cell modules and square arrays**(1) The azimuth and inclination of solar cell modules (or square arrays) have a direct relationship and influence on the power generation of photovoltaic power generation and the radiation intensity of sunlight, air quality, geographical location and other factors, so when designing photovoltaic power generation systems , the azimuth and inclination of solar radiation should be considered.

1) The choice of solar cell azimuth. In China, the azimuth angle of solar cells is generally selected in the south direction, so as to maximize the power generation per unit capacity of solar cells.

2) The choice of solar cell inclination. The most ideal inclination angle is to make the annual power generation of the solar cell as large as possible, and the difference between the power generation in winter and summer as small as possible. The optimal module inclination can be determined using formula software, calculations or by consulting Appendix B, or the solar cell inclination can be roughly determined according to the local latitude. For the specific determination method, please refer to Module 3. The latitude of Nanjing, Jiangsu is 320, and the best inclination angle is 37°.

(2) General design method of modules The basic idea of solar cell module design is to meet the electricity demand of the annual average daily load. The basic method for calculating the number of solar cell modules is to divide the energy (Ah) required by the load per day by the energy (Ah) that a solar cell module can generate in one day, so that the number of solar cell modules that the system needs to be connected in parallel can be calculated. Using these components in parallel can generate the current required by the system load.

The formula for calculating the number of components in series is

Number of series components = system working voltage × 1.43 / component peak working voltage

The coefficient 1.43 in the formula is the ratio of the peak working voltage of the solar cell module to the working voltage of the system.

The formula for calculating the number of parallel components is

The number of parallel components = the average daily power consumption of the load / the average daily power generation of the module

The average daily power generation of the module (A.h) = the peak working current of the selected module × the peak sunshine hours × the slope correction factor × the module decay loss correction factor × ……

In the formula, the peak sunshine hours and the slope correction factor refer to the actual data of the installation site of the photovoltaic power generation system; the module decay loss correction factor mainly refers to considering the factors such as module combination, module power attenuation, module dust cover, charging efficiency, etc. The conversion coefficient of loss is generally taken as 0.8; “…” is the coefficient of other influencing factors.

Average daily power consumption of load = load power × working hours per day / load working voltage

Solar cell array power = peak output power of selected battery modules × number of series connected battery modules × number of parallel connected battery modules

In this question, Yingli 250Wp components are selected, the peak current is 8.24A, the peak voltage is 30.4V, the open circuit voltage is 38.4V, and the short circuit current is 8.79A. Nanjing’s daily average peak sunshine hours is 3.94h, the module inclination angle adopts the optimal inclination angle, the correction coefficient is 1, and the inverter efficiency coefficient is 0.9, then

Number of series components = system working voltage × 1.43 / component peak working voltage = 48V × 1.43/30.4V = 2.26

The number of series components is 3.

The number of parallel components = the average daily power consumption of the load / the average daily power generation of the module

= Average daily power consumption of load/(component peak working current × peak sunshine hours × slope correction factor

×Component attenuation loss correction coefficient×other influencing factor coefficients)=(4000/48)/(8.24×3.94×1×0.8×0.9)=3.57

The number of parallel components is 4.

but

Total number of components = number of series components × number of parallel components = 3 × 4 = 12 pieces

Total power is 12 × 250Wp = 3000 Wp

**4. Selection of DC combiner box**Small-scale solar photovoltaic power generation systems generally do not need DC combiner boxes. DC combiner boxes are mainly used in medium and large-scale solar photovoltaic power generation systems, which are used to input and group the multi-output cables of the square array of solar cell modules. It is orderly and convenient for group inspection and maintenance. When the solar cell phalanx fails locally, it can be partially separated and repaired without affecting the continuous operation of the overall power generation system.

This project is realized by 3 series and 4 parallel battery components, and the actual combiner box with lightning protection function with four input and one output is selected as shown in Figure 3.

**5. Selection of photovoltaic controllers**The photovoltaic controller should determine the type of photovoltaic controller according to the system power, the system DC working voltage, the number of input channels of the battery array, the number of battery groups, the load condition and the special requirements of the user. Generally, the low-power photovoltaic power generation system adopts a single-channel PWM controller, and the high-power photovoltaic power generation system adopts a multi-channel controller or an intelligent controller with communication function and remote monitoring and control function.

Special attention should be paid to the selection of the controller: its maximum operating current must be greater than the short-circuit current of the solar cell module or square array and the maximum operating current of the load at the same time.

Select the controller of Bosin BX-C10048 model, as shown in Figure 4.

**6. Selection of photovoltaic off-grid inverters**The selection of photovoltaic off-grid inverters is generally based on the DC voltage determined by the design of the photovoltaic power generation system to select the DC input voltage of the inverter, determine the power and phase number of the inverter according to the type of load, and decide according to the impact of the load. Inverter rate headroom. The continuous power of the inverter should be greater than the power of the used load, and the starting power of the load should be less than the maximum impact power of the inverter. In the selection, it is also necessary to consider leaving a certain margin for the future expansion of the photovoltaic power generation system. In an independent photovoltaic power generation system, the selection of the system voltage should be determined according to the requirements of the load. In this example, the Solar B48P5K-1 inverter is selected, as shown in Figure 5.