Only solar cells that use the same technology, come from the same manufacturer and are of the same type should be wired in parallel to each other. The current of parallel-connected solar cells is cumulative, but the voltage remains the same as for only one cell. Inasmuch as manufacturing tolerances can induce discrepancies between the characteristic curves of individual solar cells at the same insolation, the maximum power of parallel-connected solar cells is always somewhat lower than the aggregate maximum power of the individual cells. This so-called mismatch loss in parallel-connected cells can be minimized by using solar cells whose MPP voltage VMPP is as similar as possible.
Like series-connected solar cells, parallel-connected solar cells are subject to critical operating states that need to be managed. To this end, it is necessary first to determine the effect of one parallel-connected cell being shaded. Under certain circumstances, in such cases the shaded solar cell can be operated in the first power quadrant (see Figures 3.13 and 4.16), i. e. in the forward direction, and can thus have the effect of an appliance.
This state poses the greatest hazard for shaded cells in cases where the entire module is at open-circuit voltage, such that the voltage generated by the non-insolated cells is nearly the same as the open-circuit voltage. In such a case, the shaded cell receives power from all insolated cells adjacent to it (see Figure 4.26). The results of an investigation of this scenario with crystalline silicon solar cells are illustrated in Figure 4.27,which displays the characteristic curves of the 1 kW/m2 insolation cells and the dark characteristic curve for a fully shaded cell. The operating point for the shaded cell in such a case is the intersection of the shaded cell and insolated cell curve.
Parallel connection of an insolated and non-insolated solar cell, each of which exhibits a cell temperature of 25 0C, results in operating point A1, which is a completely benign situation since the resulting reverse current in the shaded cell is far below ISC-STC. However, it is safe to assume that in an actual PV system a solar cell at 1 kW/m2 insolation will be around 30 0C higher than a shaded cell. Based on this assumption, the reverse current for operating point A2 will be lower still. But for parallel-connected
1 + І2 + ІЗ + ■■■ + !n-1
shaded or defective solar cell operating as load
shaded cells with an unlimited number of 1 kW/m2 insolation solar cells with 25 °C cell temperature, the resulting operating point A3 will initially present reverse current that is around half of short-circuit current ISC at STC power output. This is clearly shown in the solar cell equivalent circuit (see Figure 3.12), since a reverse current induces a voltage drop at the series resistance RS, such that the diode voltage in the equivalent circuit diagram is lower than at open-circuit current.
In the interest of allowing for manufacturing tolerances between solar cells, a second curve (blue) was inserted for the shaded cell, which under the same current exhibits only 95% of the voltage of a normal cell. This results in the attendant operating points B1, B2 and B3 under otherwise identical conditions, whereby the reverse current that arises in such cases is likewise lower than at ISC-STC. Hence an unlimited number of solar cells of the same type can be parallel connected without difficulty, in terms of reverse current and under a shading condition.
solar cell with only 0.95VOC
If the relevant module is under load, the voltage via the parallel-connected cells is lower, and the current in the shaded cell is far lower. The module current decreases by approximately the total amount of the current previously input by the now shaded cell, thus in turn reducing module power, albeit to a far lesser degree than if all cells are series connected.
The worst-case scenario here is an imperfect short circuit in a cell, where the current from all remaining intact cells is fed into the short circuit, thus heating up the residual resistance in the failed cell. The best way to safeguard against this rare, but by no means impossible, malfunction scenario is to parallel-connect a maximum of around three or four cells, thus obviating the potentially catastrophic effect of limited power loss in the failed cell secondary to low voltage. In the worst – case scenario, cell calefaction will provoke a saturated short circuit (contact sweating) or open circuit (wire meltdown), both of which events will permanently reduce power but will probably not knock out the module affected.
Series connection of groups of nZP parallel-connected modules in a matrix arrangement engenders higher voltages that are less sensitive to local shading than is the case with series connection of all solar cells. However, in order to avoid shading-induced damage, it is still necessary to realize the measures described in Section 4.2.2, such as using cooled bypass diodes that are rated for 1.25 ■ nZP ■ ISC current in a group of 12-24 solar cells.