What is a stand-alone PV system design?

The design work of an independent system starts from collecting meteorological data and calculating the load size, and then determining the specifications and capacity of the controller, inverter, battery and other equipment in the system, so that the equipment of the system can be perfectly matched and the equipment can work in the best working condition. , on the one hand to ensure the normal operation of the system, on the other hand to prolong the service life of the equipment, but also to ensure the normal use of the user.

What is a stand-alone PV system design?
Block diagram for the stand-alone PV system

The design principles of independent photovoltaic power plants are as follows:
1) The module should meet the daily electricity demand of the load under average weather conditions. The module design cannot consider fully charging the battery as quickly as possible. If fast charging is required, a solar module with a large power must be required. At the same time, the power generation of the module after fast charging will cause waste.

2) The module should meet the needs of the worst light season, and the output of the solar cell module should be equal to the average value of the annual load demand.

3) The design of the battery should ensure that the load can still work normally when the sunlight is continuously lower than the average value.

4) The number of self-sufficiency days of the system should be considered. Self-sufficient days is the number of days that the system can still work normally without any source of energy.

5) Before designing the system, try to visit the site to understand the installation location, so that the equipment layout and wiring can be designed reasonably.

Note: The number of self-sufficiency days is not the same as the number of consecutive rainy days. The number of self-sufficiency days is the time the user expects the system to work, while the number of consecutive cloudy and rainy days is a natural phenomenon, but the user can request that the number of self-sufficient days be equal to the local number of consecutive cloudy and rainy days.

The design content of the independent photovoltaic power generation system mainly includes two aspects:
1) Calculation and design of batteries and battery packs.

2) Calculation and design of solar cell modules and solar cell arrays.

Factors to consider when designing a stand-alone PV system:
1) Several parameters mainly considered in the system design are: load power (unit: w), continuous working time of the load per day (unit: h), the minimum effective sunshine hours in the area where the system is used, the longest continuous cloudy and rainy days in the local area, the local continuous Cloudy day interval factor.

2) Attenuation factor: On the premise of considering the above main parameters, it is also necessary to consider the overall efficiency of the system. The overall efficiency of an independent photovoltaic power generation system mainly considers component matching loss, solar radiation loss, system deviation from the maximum power point loss, and solar cell module attenuation loss. , cable loss, inclination angle and orientation loss, control system loss, battery attenuation loss and other factor losses on solar cell modules.
In practical projects, the output power of solar cell modules will be reduced by the influence of the external environment, such as the coverage of soil and dust, and the slow decay of module performance. The usual practice is to reduce the output power of the solar cell module by 10% in the calculation to consider the impact of the above unpredictable and unquantifiable factors. For the AC power generation system, the conversion efficiency of the AC inverter should also be considered, which is generally calculated at 10% power loss.

3) Coulombic efficiency: During the charging and discharging process of the battery, the battery will electrolyze water to generate gas and escape, and 5% to 10% of the current generated by the solar cell module will not be converted and stored but will be dissipated. We use The coulombic efficiency of the battery is used to evaluate this current loss. Therefore, it is necessary to increase the power of the solar cell module by 10% in a conservative design to offset the dissipation loss of the battery.

4) Discharge rate: Generally, the battery capacity can be corrected according to the capacity of the battery provided by the manufacturer at different discharge rates. For the battery capacity of the photovoltaic system with a slow discharge rate of 50~200h (hour rate), generally 105%~120% of the nominal capacity of the battery is taken, and the corresponding discharge rate correction coefficient is 0.95~0.8. The discharge rate of the photovoltaic system needs to be calculated, and its formula is
Average discharge rate h=(Continuous rainy days × load working time)/maximum discharge depth
For a photovoltaic power generation system with multiple loads, the load working time needs to be calculated by the weighted average method. The calculation method of the weighted load working time is:
Load working time = (Σ load power × load working time)/Σ load power

5) Ambient temperature: When the minimum temperature of the installation site is very low, the battery capacity required in the design should be larger than that at the normal temperature, so as to ensure that the system can provide the required energy even at the lowest temperature. Therefore, when designing, it is necessary to refer to the temperature and capacity correction coefficient in Figure 1, and incorporate this correction coefficient into the calculation formula to correct the capacity of the battery. Generally, the correction coefficient can be determined according to experience. The correction coefficient can be 0.95~0.9 at 0℃, 0.9~0.8 at -10℃, and 0.8~0.7 at -20℃.

What is a stand-alone PV system design?
Figure 1 The relationship between battery capacity and temperature

In addition, the low ambient temperature will also affect the maximum depth of discharge. When the ambient temperature is below -10°C, the maximum depth of discharge of shallow cycle batteries can be adjusted from 50% at room temperature to 35% to 40%. The maximum depth of discharge can be adjusted from 75% at room temperature to 60%.