The previous article introduced the determination of series and parallel number of solar cell module. But in practical situations, it is necessary to revise the theoretical results considering the actual situation to obtain more correct results.
(1) Reduce the output of solar cell modules by 10%. In actual operation, the output of solar cell modules will be reduced by environmental factors. Dirt, dust coverage and slow decay of module performance can reduce the output of a solar module. The usual practice is to reduce the output of the solar cell module by 10% in the calculation to account for the above-mentioned unpredictable and unquantifiable factors. This can be regarded as an engineering safety factor to be considered when designing a solar LED lighting system. And because the operation of the solar LED lighting system also depends on the weather conditions, it is necessary to evaluate these factors, so there is a certain margin in the design, so that the solar LED lighting system can operate stably and safely for a long time.
(2) Increase the load by 10% to cope with the coulombic efficiency of the battery. During the charging and discharging process of the battery, the lead-acid battery will electrolyze water and generate gas to escape, which means that a part of the current generated by the solar cell module will not be converted into electrical energy and stored but will be dissipated. Therefore, it can be considered that a part of the current must be used to compensate for the loss of the battery, and the coulombic efficiency of the battery can be used to evaluate this current loss. Different batteries have different coulombic efficiencies. Generally, it can be considered that the battery has a loss of 5% to 10%. Therefore, it is necessary to increase the power of the solar cell module by 10% in a conservative design to compensate for the dissipation loss of the battery.
Considering the above factors, the solar cell module design formula must be revised. Divide the daily load by the coulombic efficiency of the battery, which adds to the daily load and actually gives the true load that the solar module needs to bear; the attenuation factor is multiplied by the daily output of the solar cell module, which takes into account the reduction in the daily output of the solar cell module caused by environmental factors and the attenuation of the module itself, and gives a conservative estimate of the solar cell module output under practical conditions.
When carrying out the design calculation of the solar cell module, the design calculation of the solar cell module is based on the month with the lowest irradiance when the load remains unchanged throughout the year. If the working condition of the load is changed, that is, the demand for power by the load is different every month. Then the best method to take in the design is to calculate according to different seasons or each month, and calculate the maximum number of solar cell modules required. Usually in summer, spring and autumn, the power output of solar cell modules is relatively high, and winter is relatively small. However, the load demand may also be relatively large in summer, so in this case, it is inaccurate to use the annual average or a certain month for the design calculation. Because the number of solar cell modules required to meet the load demand of each month is different, the required solar cell modules for that month must be calculated according to the load required for each month. The maximum of these is the number of solar modules required in a year. For example, the required solar cell module power is calculated to be Wp1 in winter, but only Wp2 may be required in summer (Wp1>Wp2). However, in order to ensure the normal operation of the solar LED lighting system throughout the year, it is necessary to install larger-capacity solar cell modules, that is, solar cell modules with a power of Wp1 are selected to meet the needs of the annual load.
The series-parallel connection method of solar cells is shown in Figure 1(a). But this connection method has its drawbacks. Once one of the solar cells is damaged, open circuited or shaded, it is not the power of one solar cell that is lost, but the entire string of solar cells will be rendered useless. This is particularly serious when the number of solar cells connected in series is large. In order to avoid this situation, a hybrid (or mesh connection) method can be used, as shown in Figure 1(b). In this way, even if a few solar cells fail (such as shaded ones), the entire output of the solar cell module will not be seriously lost.