The scale and application forms of photovoltaic systems vary, such as large system scales ranging from 0.3 to 2W solar courtyard lights to MW level solar photovoltaic power plants. Its application forms are also diverse, and it can be widely used in various fields such as household use, transportation, communication, and space applications. Although the scale of photovoltaic systems varies, their composition, structure, and working principle are basically the same. This article will briefly introduce the structure of photovoltaic systems and focus on their power calculation methods.

 

1. Composition of photovoltaic system

 

Figure 1 is a typical schematic diagram of a photovoltaic system supplying DC loads.

Figure 1 DC load photovoltaic system

Figure 2 Schematic Block Diagram of Photovoltaic Power Generation System

Several main components in photovoltaic systems:

 

 

1. Photovoltaic module array: It is a core component of a solar photovoltaic system that is composed of solar cell modules (also known as photovoltaic cell modules) in series or parallel according to system requirements, converting solar energy into electrical energy output under sunlight irradiation.

 

 

2. Battery: It stores the electrical energy generated by solar cell modules and releases the stored energy to meet the energy demand of the load when there is insufficient sunlight or at night, or when the load demand exceeds the electricity generated by the solar cell modules. It is an energy storage component of a solar photovoltaic system. At present, lead-acid batteries are commonly used in solar photovoltaic systems. For systems with higher requirements, deep discharge valve regulated sealed lead-acid batteries, deep discharge liquid absorbing lead-acid batteries, etc. are usually used.

 

3. Controller: It regulates and controls the charging and discharging conditions of the battery, and controls the solar cell components and the electrical energy output of the battery to the load according to the power demand of the load. It is the core control part of the entire system. With the development of the solar photovoltaic industry, the functions of controllers are becoming increasingly powerful, and there is a trend to integrate traditional control parts, inverters, and monitoring systems. For example, AES's SPP and SMD series controllers integrate the above three functions.

4. Inverter: In a solar photovoltaic power supply system, if there is an AC load, an inverter device must be used to convert the DC generated by the solar cell module or the DC released by the battery into the AC power required by the load.

 

The basic working principle of a solar photovoltaic power supply system is to charge the battery or directly supply power to the load under the control of the controller under the illumination of sunlight. If the sunlight is insufficient or at night, the battery will supply power to the DC load under the control of the controller. For photovoltaic systems containing AC loads, It is also necessary to add an inverter to convert direct current into alternating current. The application of photovoltaic systems has various forms, but their basic principles are similar.

 

2.0 Calculation method for solar cell module power

 

The capacity of silicon solar panels refers to the power WP generated by flat panel solar panels. The power value of solar power generation depends on the power H (WH) consumed by the load for 24 hours. The capacity P (AH) consumed by the load for 24 hours is determined by the rated power supply of the load and the power consumed by the load for 24 hours. Taking into account the average daily sunshine hours and the impact of rainy and cloudy days, the working current IP (A) of the solar array is calculated.

 

Based on the rated power supply of the load, the nominal voltage of the battery is selected, and the number of batteries connected in series and the floating charging voltage VF (V) of the battery are determined by the nominal voltage of the battery. Taking into account the temperature rise voltage VT (v) caused by the temperature rise of the solar cell and the voltage drop VD (V) caused by the reverse charging diode P-N junction, the working voltage VP (V) of the solar cell array can be calculated. The working voltage IP (A) and working voltage VP (V) of the solar cell array are calculated, The power generation power WPW of the flat panel solar panel can be determined, thereby designing the solar panel capacity. Based on the designed capacity WP and the working voltage VP of the solar cell array, the number of series and parallel blocks of the silicon battery panel can be determined.

 

The specific design steps for solar cell arrays are as follows:

 

1. Calculate the consumption capacity P of the load for 24 hours.

 

P=H/VH - Electricity consumed by the load within 24 hours (WH, watts ˙ Hour)

 

V - Rated power supply of load

 

2. Select the daily sunshine hours T (H).

 

3. Calculate the working current of the solar array.

 

IP=P (1+Q)/TQ - according to the surplus coefficient during rainy and cloudy periods, Q=0.21~1.00

 

4. Determine the float charging voltage VF of the battery.

 

The individual float charging voltages of cadmium nickel (GN) and lead-acid (CS) batteries are 1.4-1.6V and 2.2V, respectively.

 

5. Solar cell temperature compensation voltage VT.

 

VT=2.1/430 (T-25) VF

 

6. Calculate the working voltage VP of the solar cell array. VP=VF+VD+VT, where VD=0.5~0.7 is approximately equal to VF

 

7. Solar cell array output power WP, flat panel solar panel.

 

WP=IP × VP

 

8. Based on the table of VP and WP in the silicon battery flat panel combination series, determine the standard specifications for the number of series blocks and parallel groups. In addition, AC systems or grid connected systems also need to consider inverter conversion efficiency, other power losses, etc.

 

3.0 Example

 

Taking 100W output power and 5 hours of daily use as an example, the following calculation method is introduced:

 

1. Firstly, the watt-hours consumed per day (including inverter losses) should be calculated:

 

If the conversion efficiency of the inverter is 90%, then when the output power is 100W, the actual required output power should be 100W/90%=111W; If used for 5 hours per day, the power consumption is 111W * 5 hours=555Wh.

 

2. Calculate the solar panel:

 

Calculated based on a daily effective sunshine time of 6 hours, taking into account charging efficiency and losses during the charging process, the output power of the solar panel should be 555Wh/6h/70%=130W. 70% of it is the actual usage power of the solar panel during the charging process.