Photovoltaic systems have different scales and application forms. For example, the scale of the system ranges from a few watts of solar garden lights to MW-level solar photovoltaic power plants. Its application forms are also diverse, and can be widely used in many fields such as household, transportation, communication, and space applications. Although the scale of the photovoltaic system is different, its composition structure and working principle are basically the same.

The solar power generation system consists of solar battery packs, solar controllers, and batteries (groups). If the output power is AC 220V or 110V, an inverter is also required. The role of each part is:

(1) Solar panels: Solar panels are the core part of the solar power system and the most valuable part of the solar power system. Its function is to convert the sun’s radiation capacity into electric energy, or send it to the storage battery for storage, or drive the load to work.

(2) Solar controller: The function of the solar controller is to control the working state of the entire system, and to protect the battery from overcharge and over discharge. In places with large temperature differences, a qualified controller should also have the function of temperature compensation. Other additional functions such as light control switch and time control switch should be optional for the controller;

(3) Batteries: generally lead-acid batteries, nickel-hydrogen batteries, nickel-cadmium batteries or lithium batteries can also be used in small and micro systems. Its function is to store the electrical energy generated by the solar panel when there is light, and then release it when needed.

(4) Inverter: In many occasions, 220VAC, 110VAC AC power supply is required. Because the direct output of solar energy is generally 12VDC, 24VDC, 48VDC. In order to provide electrical energy to 220VAC electrical appliances, it is necessary to convert the DC power generated by the solar power system into AC power, so a DC-AC inverter is required. In some occasions, when multiple voltage loads are needed, a DC-DC inverter is also used, such as converting 24VDC electrical energy into 5VDC electrical energy (note that it is not a simple step-down).

The design of photovoltaic system includes two aspects: capacity design and hardware design.

Before proceeding with the design of the photovoltaic system, it is necessary to understand and obtain some basic data necessary for calculation and selection: the geographic location of the photovoltaic system site, including location, latitude, longitude and altitude; the meteorological data of the area, including the monthly total solar energy Radiation, direct radiation and scattered radiation, annual average temperature and maximum and minimum temperature, the longest continuous number of rainy days, maximum wind speed, hail, snow and other special meteorological conditions.

The design of the battery includes the design calculation of the battery capacity and the series-parallel design of the battery pack. First, the basic method of calculating battery capacity is given.

(1) Basic formula

I. The first step is to multiply the daily power consumption required by the load by the number of self-sufficient days determined according to the actual situation to get the preliminary battery capacity.

II. In the second step, divide the battery capacity obtained in the first step by the maximum allowable depth of discharge of the battery. Because the battery cannot be completely discharged in self-sufficient days, it is necessary to divide by the maximum depth of discharge to get the required battery capacity. The selection of the maximum depth of discharge requires reference to the performance parameters of the battery selected for use in the photovoltaic system, and detailed information about the maximum depth of discharge of the battery can be obtained from the battery supplier. Generally, if you are using a deep-cycle battery, it is recommended to use 80% depth of discharge (DOD); if you are using a shallow-cycle battery, it is recommended to use 50% DOD. Assume

The calculation formula of battery capacity BC is:

BC=A×QL×NL×TO／CCAh(1)

In the formula: A is the safety factor, which is between 1.1 and 1.4;

QL is the daily average power consumption of the load, which is the working current multiplied by the daily working hours;

NL is the longest continuous rainy days;

TO is the temperature correction coefficient, generally 1, if it is above 0°C, 1.1 if it is above -10°C, and 1.2 if it is below -10°C;

CC is the depth of discharge of the battery, generally 0.75 for lead-acid batteries and 0.85 for alkaline nickel-cadmium batteries.

Below we introduce the method to determine the battery series and parallel connection. Each battery has its nominal voltage. In order to reach the nominal voltage of the load, we connect the batteries in series to supply power to the load. The number of batteries that need to be connected in series is equal to the nominal voltage of the load divided by the nominal voltage of the battery.

The basic idea of the solar battery module design is to meet the annual average daily load power demand. The basic method of calculating solar modules is to divide the energy required by the load per day (ampere hours) by the energy that a solar module can generate in a day (ampere hours), so that the system needs to be connected in parallel. The number of components, using these components in parallel can generate the current required by the system load. By dividing the nominal voltage of the system by the nominal voltage of the solar cell module, the number of solar cell modules that need to be connected in series can be obtained. Using these solar cell modules in series can generate the voltage required by the system load. The basic calculation formula is as follows:

Number of modules connected in parallel = average daily load (AH)/daily output of modules (AH) Number of modules in series = system voltage (V)/module voltage (V)

The above are all uncorrected formulas. The following formulas are for reference

Solar cell array design:

Ns=UR/Uoc=(Uf＋UD＋Uc)/Uoc(2)

Where: UR is the minimum output voltage of the solar cell array;

Uoc is the best working voltage of solar cell modules;

Uf is the floating voltage of the battery;

UD is the diode voltage drop, generally 0.7V;

UC is the pressure drop caused by other factors.

Parallel number of solar modules Np

Before determining NP, we first determine the calculation method of its correlation quantity.

①Convert the solar radiation amount Ht at the installation site of the solar cell array into the average daily radiation hours H under the standard light intensity (see Table 1 for the daily radiation amount):

H=Ht×2.778/10000h (3)

In the formula: 2.778/10000 (h·m2/kJ) is the coefficient that converts the daily radiation into the average daily radiation hours under the standard light intensity (1000W/m2).

②Daily power generation of solar cell modules Qp

Qp=Ioc×H×Kop×Cz(Ah) (4)

In the formula: Ioc is the optimal working current of the solar cell module;

Kop is the slope correction coefficient (refer to Table 1);

Cz is the correction coefficient, which is mainly the loss of combination, attenuation, dust, charging efficiency, etc., generally 0.8.

③The shortest interval between two groups of longest continuous rainy days Nw. This data is a unique feature of this design. The main consideration is to supplement the loss of battery power during this period. The battery capacity Bcb to be supplemented is:

Bcb=A×QL×NL (Ah) (5)

④The calculation method of the number Np of solar cell modules in parallel is:

Np=(Bcb＋Nw×QL)/(Qp×Nw)(6)

The expression of formula (6) means: the number of solar battery groups connected in parallel, the amount of electricity generated in the shortest interval between the two consecutive rainy days, not only for load use, but also to make up for the battery during the longest continuous rainy day Loss of power.

(3) Power calculation of solar cell array

According to the number of series and parallel of solar cell modules, the required power P of the solar cell array can be obtained:

P=Po×Ns×NpW (7)

Where: Po is the rated power of the solar cell module.

A really good designer should specifically consider the following factors:

1. Where is the solar power system used? What is the solar radiation situation in this place?

2. What is the load power of the system?

3. What is the output voltage of the system, DC or AC?

4. How many hours does the system need to work every day?

5. In case of rainy weather without sunlight, how many days does the system need to supply power continuously?

6. What is the starting current when the load is purely resistive, capacitive or inductive?

7. The number of system requirements.

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