October 2009

Made for shade

The new OptiTrac Global Peak makes sure that partially shaded solar power plants operate efficiently

Dormers, chimneys or trees: you can't always prevent them from casting shadows on your PV plants. All shadows are not the same, and their impact on annual energy yield is often dramatically overestimated. With the newly developed OptiTrac Global Peak and a good planning process, the typical annual loss in yield is less than 2 percent.
The numbers that are currently published in the professional press are startling: shadows allegedly cause losses in yield of over 30 percent. Realistic or simply exaggerated? To answer this question, you first have to look at the period of time they are based on. Upon examination, the losses in yield claimed are only spot values that do not reflect the annual yield of a plant.
The reasons are diverse: Shadow is not only dependent on the location, size and type of the object that is casting it, but also on the weather, time of day and season of the year. Because there are hardly any shadows in indirect light and in the summer and at noon they are shorter than in the winter and in the morning or evening. In addition, shadows usually make a brief appearance and do not have a continuous impact on energy yield.

The impact of shadows is overestimated

The reassuring realization: the impact of shadows is often significantly overestimated. Especially during maximum yield periods, in the summer and at midday when the sun is high, they tend to have a minimal impact on well-planned PV plants. SMA has investigated the situation using extensive simulation calculations based on the example of a roof with a southern alignment and a dormer in its center (Fig. 1). Two variants were distinguished: one with heavy shade in which the PV modules went up to the dormer and another with slight shade and the recommended installation distance. Sensible system design, meaning strings that are separately operated for shaded and open modules, results in a maximum annual yield loss of 18.2 percent for more heavily shaded PV plants. The slightly shaded variant, however, only had a loss of 5.4 percent.

Fig. 1: Annual energy yield of a plant with slight and heavy shade when operated with a conventional MPP-tracker and OptiTrac Global Peak, relating to the yield of an open plant

MPP-tracker quality is the decisive factor

The impact of shadows is significantly lower than frequently claimed – but SMA has taken on the challenge of reducing it even further. An MPP-tracker has a decisive influence on the energy yield of a partially shaded plant. Its job is to continuously operate the PV-modules assigned to it at its maximum power point (MPP). If the individual modules in a string are shaded, the string's power curve suddenly shows several MPPs. Of course by definition, there is only one point of maximum power, but additional, local MPPs can exist somewhere on the curve (Fig. 2). The minimum voltage distances and power differences of individual MPPs depend on the number of shaded modules, the string length and the difference in radiation intensity between shaded and open areas. .

Fig. 2: Power curve of a module string in case of variations in shadowing

Globally successful: OptiTrac Global Peak

The inverter's MPP-tracker depends on always hitting the global MPP of the partially shaded string if possible and not "hovering" around a local maximum. The power of a local maximum can be significantly lower than that of the global maximum power. That is exactly the point at which the state-of-the-art OptiTrac Global Peak differs from traditional MPP-trackers. Because it reliably locates the global maximum power, OptiTrac Global Peak can use the energy of a partially shaded string almost completely (Fig. 3). The annual yield simulation of the two example plants provided convincing proof: the annual yield of the slightly shaded PV plant with OptiTrac Global Peak is only 1.3 percent lower than the open variant. And even the heavily shaded plant delivers only 2.9 percent less energy per year with OptiTrac Global Peak which equals a relative increase in yield of about 19 percent – or a reduction of the losses resulting from shadow by 84 percent.

Fig. 3: Energy yield on a cloudless day: Considerable increase in yields by OptiTrac Global Peak

Useful in individual cases: module inverters

Of course there is an alternative: using module inverters to integrate partially shaded generator areas. They facilitate individual MPP-tracking at the module level. However, this solution is only suitable for single modules that are heavily affected by shadows. For the others, the best solution is the proven string technology and an outstanding MPP-tracking. The extremely low differences in yield to the open plant in the comparative simulation are the impressive proof. The acquisition of the OKE technology platform for module inverters means that SMA has now created the prerequisites for offering a professional solution for single modules with heavy shade in the future and thus completing the SMA product portfolio on the low power end.

Your know-how remains important

The technology one deploys in the system remains to be essential and one factor still is the key to success: the expert design that you, in your capacity as qualified solar installation specialist, create. For detailed information on the optimal design of PV plants in partially shaded locations, see the SMA publication on "Shadow Management" that you can download  from the SMA website. You can use it together with the innovative products of SMA to find the best solution for every PV plant.