March 2010

"Impossible" is not an option

Tailor-made plant configuration and maximum yields: with Optiflex both are possible.

Exactly 71 modules should go on the roof - both installers and plant designers are faced with tasks such as this regularly. Besides the look which is increasingly important for the operator, there are some other reasons to select a certain number of modules during plant design: homogeneity of the generator, omission of obstacles or simply financial aspects.

Such specifications are regularly a real challenge for the designer, as acknowledged by Product Manager Stefan Zanger. "Once I have selected an appropriate module, unfortunately it is not self-evident that the exact amount of modules required can be connected to the inverter." Indeed Multistring devices such as the Sunny Boy 5000TL from SMA already belong to the market leaders for design flexibility. "We wanted to make it even better, above all for challenging plants", says Zanger. "Therefore we have created Optiflex and offer designers unrivalled flexibility in the configuration of highly efficient plants as a result."

The general rules of connection

The principle challenge during design is associated with the basic rules of electrical connection. With several modules in series, the module voltage adds together, while the electric current remains the same. Module strings connected in parallel, however, increase the electric current while the voltage remains the same. Especially important: in order that several module strings can be connected in parallel at all, they must all have the same length and therefore the same voltage.

Conventional design allows less scope

These basic rules set really tight limits on the design of the PV generator with conventional inverter concepts: the minimum and maximum possible string lengths result from the input voltage range of the inverter and the thermal characteristics of the modules. The maximum possible number of strings that can be connected in parallel results from the ampacity of the inverter input and the MPP-current of the module. It is also clear that with, for example, two strings connected in parallel, the number of modules in the generator can only be increased by two - the voltage and therefore the number of modules must always remain identical for all strings. Prime numbers in the number of modules are impossible for the same reason, as soon as more than one string is required. "These few rules often make it impossible to connect a specific number of modules to an individual inverter", Zanger explains the dilemma. "When it does work out, then not necessarily with the most efficient MPP voltage for the inverter." In the event of "surplus" modules, one often requires an additional, smaller inverter. Alongside the additional installation cost, the efficiency of this is mostly lower and therefore reduces the economic efficiency of the plant.

Extra flexibility with Optiflex

These basic rules set really tight limits on the design of the PV generator with conventional inverter concepts: the minimum and maximum possible string lengths result from the input voltage range of the inverter and the thermal characteristics of the modules. The maximum possible number of strings that can be connected in parallel results from the ampacity of the inverter input and the MPP-current of the module. It is also clear that with, for example, two strings connected in parallel, the number of modules in the generator can only be increased by two - the voltage and therefore the number of modules must always remain identical for all strings. Prime numbers in the number of modules are impossible for the same reason, as soon as more than one string is required. "These few rules often make it impossible to connect a specific number of modules to an individual inverter", Zanger explains the dilemma. "When it does work out, then not necessarily with the most efficient MPP voltage for the inverter." In the event of "surplus" modules, one often requires an additional, smaller inverter. Alongside the additional installation cost, the efficiency of this is mostly lower and therefore reduces the economic efficiency of the plant.

Tailor-made and highly efficient

Optiflex allows almost all module numbers to be included - without "surpluses" and with a single, highly efficient inverter. In addition, the generator voltage can be ideally tailored to the inverter's efficiency curve. Since even in the event of an exactly prescribed number of modules, Optiflex still offers variation possibilities for the string length in most cases. "This additional degree of freedom is the decisive advantage - even over classic Multistring devices", explains Stefan Zanger. "Only when the generator voltage is ideally tuned, the inverter works with maximum efficiency for most of the time." With a planning example he makes the advantages of Optiflex even more clear: "Assume that exactly 71 modules should be on the roof. Then four strings each with 14 modules on the larger and one string with 15 modules on the smaller input. But also other string lengths are possible on the main input - from 12 up to 16 modules. The voltage of the larger partial generator can therefore be set precisely."

Well-prepared for partial shadowing

Not least Optiflex offers the best conditions for the handling of partially shaded PV generators. In this case the smaller input with its own MPP tracker is able to be used exceptionally for the separate connection of partially shaded modules. The unavoidable loss in yield is therefore clearly reduced.

All sorts of possibilities

All in all, Optiflex is a great step forward when the design of cost-effective PV plants with high yields and various module configurations is the issue. "After string technology and the invention of the Multistring inverter, SMA can again set the direction here", Stefan Zanger is certain. "The advantages of Optiflex are so obvious - they will convince everyone."