Technology of PV modules

There are two types of PV modules: wafer based crystalline silicon and thin film modules (see box, cost implications for PV modules I – crystalline, on the opposite page).

The latter has an advantage in that the film can be applied very easily i.e. rolled out on a structure; it is also cheaper to manufacture and to install. It does, though, have a disadvantage as far as financing is concerned, in that only limited data on its performance is available.

Crystalline silicon technologies accounted for 90% of PV module production in 2006, slightly down from 92% in 2002. The remaining 10% comes from thin film technologies (Source, Solarbuzz, id).

PV modules are classified (or rated) by the power they produce under a specific set of standard test conditions. The most common rating used in Europe is the peak rating. The peak rating of a PV system is the sum total of the PV modules’ nameplate power under published standard test conditions (STC). The performance is then stated in Watt peak (Wp). This approach is not too different from that, for example, applied to wind turbines which are rated in mW to denote their peak performance.

Performance guaranty / degradation

One of the most positive aspects of PV technology is their reliability and low maintenance. Only 2.1% of the projects had a problem with modules and their cabling (Source: Gute Noten vom Handel für die boomende Solarbranche, Erneuerbare Energien 4/2005, page 53). With no moving parts, solar cells should be able to operate reliably for 25–30 years, with virtually no maintenance. The energy yield of PV modules can be substantially enhanced if put on tracking systems which follow the sun during the day and during the year (vertical and horizontal tracking – 20%-50%, depend-ing on location).

In PV module supply contracts, the manufacturer usually guarantees that in the first 10 years of the PV module’s operating life the performance of the PV modules, under STC, will not be lower than 90% of the specified minimum peak performance. These contracts typically accept that peak performance is subject to a general variance of +/- 5%, which means that the acceptable minimum peak performance would be 95% of the peak performance under STC.

In relation to the first 20 or 25 years of the PV module’s operating life, the guarantee is usually for 80% of the minimum peak performance. The reason for this model is that it is generally believed that the performance of the PV modules would decrease over time. However, long term studies have shown that after 10, or even 20 years, there is no evidence of general degradation and the performance generally remains within the tolerance laid down by the manufacturer.

It would be impracticable to check every module to be installed in a project for its compliance with the specified peak performance. Therefore, purchase agreements usually contain a clause – according to which the purchaser is entitled to have a certain percentage (usually less than 1%) of modules checked by their own engineers or an independent expert. It can also be agreed that the purchaser has the right to send its engineers to inspect the production facility and process.

Cost implications for PV modules - crystalline

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