The design calculation of solar photovoltaic power supply system is mainly based on the actual situation on site. In order to meet the energy requirements, suitable solar cell arrays and battery capacities are selected under the conditions of sunlight conditions and environmental temperature at the system setting location, and all equipment in the system is matched to ensure the rationality and applicability of the system. A complete solar photovoltaic power system needs to consider various factors for design, such as electrical performance design, thermal design, mechanical structure design, etc. For independent power systems used on the ground, the most important thing is to determine the solar cell array and battery size according to usage requirements to meet the needs of normal operation.
The overall design principle of solar photovoltaic power supply system is to determine the minimum amount of solar cell modules and battery capacity while ensuring the satisfaction of electricity demand. Through technical and economic analysis, the number of solar cell modules and battery capacity are reasonably determined, including requirements for safety, reliability, and many other aspects. The design of system configuration mainly considers two factors:
Select different system components based on load requirements, environmental parameters, and electrical parameters of solar photovoltaic power system components.
The data that needs to be determined mainly includes: sunlight radiation at the installation site, sunlight radiation on the inclined surface of the solar cell array, environmental temperature parameters, system voltage, load energy demand, maximum and average discharge current, controller, inverter regulation characteristics and parameters, characteristic parameters of solar cell components and batteries, system power supply reliability and power supply availability.
The spectrum and intensity of solar radiation on the ground solar cell array are influenced by atmospheric thickness (i.e. atmospheric mass), geographical location, climate and meteorology, terrain and features, etc. The energy of solar energy varies greatly in a day, month, and year, and even the total annual radiation varies greatly between years.
The photoelectric conversion efficiency of solar cell arrays is influenced by the temperature, solar intensity, and battery voltage fluctuations of the battery itself, which can change throughout the day. Therefore, the photoelectric conversion efficiency of solar cell arrays is also a variable. The battery pack also operates in a floating charging state, and its voltage varies with the changes in square grid power generation and load power consumption. The energy provided by the battery is also affected by temperature.
The solar cell charging and discharging controller is made of electronic components, which also require energy consumption. The performance and quality of the components used also affect the magnitude of energy consumption, thereby affecting the efficiency of charging.
The power consumption of the load also depends on the purpose, such as communication relay stations, unmanned weather stations, etc., which have fixed equipment power consumption. However, some equipment such as lighthouses, navigation lights, civil lighting, and household electricity consumption often have changes in power consumption. Therefore, the design of solar power systems requires consideration of multiple and complex factors. The characteristic is that the data used is mostly from previous statistics, and the measurement and selection of each statistical data are important. The design of solar cell arrays and battery power systems should not only focus on efficiency, but also ensure high reliability of the system. The solar radiation energy data of a specific location, based on the information provided by the meteorological station, is used for designing solar cell arrays. These meteorological data need to be averaged over several years or even decades of accumulation.
1. Selection of solar photovoltaic cells
The household small-scale solar photovoltaic power supply system can be configured with solar cell power and battery capacity based on a simple calculation method of sampling local sunshine time and geographical environment. Calculate the configuration battery capacity, solar cell power, and inverter power based on the electricity consumption of each household.
1.1 Design of lead-acid batteries
When designing the capacity of lead-acid batteries, in addition to considering the number of days that lead-acid batteries can be used in continuous rainy and cloudy days, it is also necessary to consider that each time the lead-acid battery is discharged, it should not be fully discharged at 100% depth, and a portion of the capacity should be left. This is beneficial for the conversion of internal energy in lead-acid batteries, and the service life of lead-acid batteries is relatively extended. However, it is also important to note that the larger the capacity of lead-acid batteries, the more self discharge they will have. If the capacity is chosen too much, it will consume a portion of electrical energy and increase unnecessary investment; If the capacity is chosen too small, the excess electricity cannot be stored when the electricity consumption is low. Therefore, we need to fully consider the number of consecutive rainy days.
1.2 Power of solar cells
The power of the solar cell must be appropriately greater than the actual power consumption by 100W, because on the one hand, it is necessary to consider charging efficiency and line losses, and on the other hand, it is necessary to consider that lead-acid batteries can still provide electricity for three days of load use in the event of continuous cloudy and rainy days. Solar cell array design:
2.2.1 Number of solar cell modules in series NC
By connecting a certain number of solar cell modules in series, the required operating voltage can be obtained. However, the number of series connections for solar cell modules must be appropriate. If the number of series connections is too small and the series voltage is lower than the float charging voltage of the battery, the square array cannot charge the battery. If there are too many series connections and the output voltage is much higher than the float charging voltage, the charging current will not increase significantly. Therefore, the optimal charging state can only be achieved when the series voltage of solar cell modules is equal to the appropriate float charging voltage.
The float charging voltage of the battery is related to the selected battery parameters, and should be equal to the maximum working voltage of the selected battery cell at the lowest temperature multiplied by the number of batteries in series.
2.2.2 Number of parallel solar cell modules Np
Before determining Np, we first determine the calculation method for its correlation.
a. Convert the daily solar radiation Ho at the installation site of the solar cell array into the average daily radiation hours H under standard light intensity.
H=Ho * 2.778/10000 h
In the formula: 2.778/10000 is the coefficient that converts daily radiation energy into the average daily radiation hours under standard light intensity (1000W/m2).
b. The daily power generation Qp of solar cell modules.
Qp=Io * H * Kop * CoAh
In the formula: Io is the optimal working current of the solar cell module; Kop is the slope correction coefficient; Co is the correction coefficient, mainly for the losses of combination, attenuation, dust, charging efficiency, etc., generally taken as 0.8.
c. The shortest interval of N1 between the two longest consecutive rainy days is mainly considered to replenish the lost battery power during this period. The battery capacity Bah that needs to be supplemented is:
Bah=A * Qp * Nl Ah
d. The calculation method for the parallel number Np of solar cell modules is:
Np=(Bah+N1 * Ql)/(Q0 * N1)
The number of parallel solar cell packs generates electricity within the shortest interval between two consecutive rainy and cloudy days, not only for load use, but also to make up for the battery’s loss of electricity during the longest continuous rainy and cloudy day.
Based on the number of series and parallel connections of solar cell modules, the required power P of the solar cell array can be obtained:
P=Po * Np * NC W
In the formula, Po is the rated power of the solar cell module.
3. Selection of batteries
The energy storage device of the solar cell power supply system is mainly the battery. Batteries paired with solar cell arrays typically operate in a float state, and their voltage varies with changes in the array’s power generation and load consumption. Its capacity is much larger than the amount of electricity required by the load. The energy provided by the battery is also affected by the ambient temperature. In order to match with solar cells, it is required that the battery has a long working life and simple maintenance.
The capacity of a battery is crucial for ensuring continuous power supply. Within a year, the power generation of the square array varies greatly from month to month. In months when the power generation of the square array cannot meet the electricity demand, it needs to be supplemented by the electrical energy of the battery; In the months when the electricity demand exceeds, the excess energy is stored by the battery, so the insufficient and excess value of the square array power generation is one of the basis for determining the battery capacity. Similarly, the load electricity during continuous rainy and cloudy days must also be obtained from the battery. So, the power consumption during this period is also one of the factors determining the battery capacity.
Therefore, the capacity of the battery BC=A * QL * NL * T0/C0 Ah
In the formula: A is the safety factor, taken between 1.1 and 1.4; QL is the average daily power consumption of the load, which is the working current multiplied by the daily working hours;
NL is the longest continuous rainy day;
T0 is the temperature correction coefficient, generally taken as 1 above 0 ℃, 1.1 above 10 ℃, and 1.2 below -10 ℃;
C0 is the discharge depth of the battery, usually 0.75 for lead-acid batteries and 0.85 for alkaline nickel cadmium batteries;
There are many types of batteries that can be used in conjunction with solar cells. Currently, there are three widely used types: lead-acid maintenance free batteries, ordinary lead-acid batteries, and alkaline nickel cadmium batteries. At present, lead-acid maintenance free batteries are mainly used in China. Due to their inherent “maintenance free” characteristics and less environmental pollution, they are very suitable for reliable solar power systems, especially unmanned workstations; Ordinary lead-acid batteries are mainly suitable for use in situations with maintenance capabilities or low-end situations due to their frequent maintenance and high environmental pollution; Although alkaline nickel cadmium batteries have good low-temperature, overcharge, and over discharge performance, they are only suitable for special occasions due to their high price.
In practical use, we use sealed valve regulated maintenance free lead-acid batteries instead of traditional open type batteries because sealed valve regulated maintenance free lead-acid batteries have the following advantages compared to traditional open type batteries:
(1) The sealing degree is high, and the electrolyte will not flow easily, so the battery can be placed horizontally;
(2) The electrode grid of sealed valve controlled maintenance free lead-acid batteries adopts antimony free lead alloy, and the self discharge coefficient of the battery is very small;
(3) The positive and negative plates of the battery are completely surrounded by isolation plates, and the effective substances are not easily detached, resulting in a long service life;
(4) The volume of sealed valve controlled maintenance free lead-acid batteries is smaller than that of older batteries, but their capacity is higher than that of older open type batteries;
(5) The battery does not require any liquid replenishment during long-term operation, and does not produce acid mist or gas during use, with minimal maintenance workload;
(6) The internal resistance of the battery is very small, and the high current discharge characteristics are good.
4. Selection of transformer system
The selection of inverters in transformer systems needs to consider the following factors:
(1) Input voltage and waveform. Select the input DC size and output waveform based on the size of the battery used to provide electricity in the solar photovoltaic power supply system, as well as the parameters of the main load.
(2) Output power. Generally, it is necessary to consider the need to increase electrical appliances in the future and choose inverters with higher output power. Inverters that work under low loads for a long time are beneficial for their operation.
(3) Function. In addition to overload, undervoltage, and reverse protection functions, it should also have intelligent management and control functions for the charging and discharging capacity of solar cells, which is conducive to protecting the battery and extending its service life.
(4) Reliability. Inverters are one of the core and durable components of the entire solar photovoltaic power supply system. When choosing an inverter, one should not be tempted by cheap prices. It is necessary to choose an inverter that is easy to operate, of good quality and durability, in order to improve the credibility of the entire solar photovoltaic power supply system.
5. Security guarantee
For the safety issues of household solar power supply systems, we have taken lightning protection, lightning protection, and grounding protection measures.
Site lightning protection method: lightning protection strip. Lay the metal conductor along the top contour of the protected object and maintain an appropriate distance to eliminate lightning charges and avoid direct lightning strikes.
Lightning protection on the DC side: The controller should have lightning protection inside;
System grounding protection: The grounding resistance of the lightning protection system should comply with the requirements of DL/T620-1997 “Overvoltage Protection and Insulation Coordination of AC Electrical Equipment” (generally not greater than 10 Ω); The grounding system of the line should comply with the requirements of DL/T62-1997 “Grounding of AC Electrical Equipment” and the technical requirements of DL499-92 “Technical Specification for Rural Low Voltage Power” (generally not greater than 4 Ω).
6. Precautions for installation and use of solar power sources
(1) The array board should be installed in areas without tall buildings, trees, power poles, or other obstructions from sunlight and wind.
(2) The battery that comes with the solar panel array should be charged to its rated capacity before the first use, and should not be overcharged or discharged.
(3) Pay attention to regular maintenance work.