To replace the inverters of your PV plant can make sense not only when the old components already show defects. New components can noticeably increase the yield of a photovoltaic system. Therefore, an investment in repowering can pay off relatively quickly – depending on the age of the system – as the feed-in tariff of subsidised plants can compensate for a large part of the investment costs. This is particularly interesting if you want to operate the plant for as long as possible after the subsidy period. A conversation about inverter repowering with the greentech repowering experts Michael Strübing, Key Account Manager, and Johannes Liebich, Head of Engineering & Technical Advisory.

Owning a subsidised PV system with a relatively high feed-in tariff, should I generally replace my inverters after about 15 years?

Johannes Liebich: Of course, this depends primarily on how the system performs, whether you are satisfied with the yields and how you are going to proceed with your plant after the subsidy period. In larger old plants, usually central inverters are installed. In this case, it is important to check whether spare parts are available on the market to ensure the continued operation of the plant in the event of defects. We have made the experience, that many manufacturers have withdrawn from this sector. Correspondingly, spare parts can be rare.

Michael Strübing: Also, when considering a continued operation after the subsidy period you need to take different requirements for PV systems and their components today into account. For example, many older inverters cannot fulfil the conditions of the current grid connection regulations. Therefore, the only option for continued operation may be a replacement with newer models from the supplier or – if the supplier is no longer on the market – with new manufacturers. In any case, we recommend inverter repowering during the subsidy period, even if the system is to continue operation after the subsidy period. This is because the higher revenues resulting from the earlier feed-in tariffs often compensate for the costs incurred – this is especially true for older plants if they only have a short remaining subsidy term. Then inverter repowering projects can pay for themselves after only about two to three years.

How much additional yield can be generated with new inverters in an older existing plant?

Michael Strübing: Of course, we can’t make a general statement. Among other things, it depends on how old the system is, and which components have been installed so far. But we are talking about expected yield increases of between 3 and 5 percent, sometimes even more. In one case, we were able to achieve an additional yield of 9.1 percent with a 1.12 MW plant.

Johannes Liebich: Therefore, solid planning is always important in advance. It makes the technical requirements and the economic potentials transparent for the operator or investor. Within the framework of a sensitivity analysis, we show what the repowering will cost for the customer and how it will pay off eventually. In doing so, we run through various scenarios and include possible deviations: What happens, for example, if instead of the assumed 3 percent, only 2.5 percent yield increase are achieved? Or if the costs of the project turn out to be 10 percent higher – is the investment still profitable? By this, the customer has a solid basis of information for his decision.

What has to be considered when exchanging the inverters?

Michael Strübing: First, central inverters are still installed in many older systems, but they are used less today. If there are no suitable updated models of a system available, we usually replace them with string inverters. In the technical implementation, this change results in one or two challenges. In terms of future O&M and servicing costs, however, we consider this to be the economically better option in the long term, even if the changeover is considerably more expensive.

What challenges are you talking about? Do you have an example?

Johannes Liebich: For cost reasons, it usually makes more sense to use the existing infrastructure and modify it if necessary. This means, for example, that we usually install string inverters in existing central inverter stations and adapt existing interfaces to other plant components accordingly. With the central inverter, for example, the main DC cables of the PV generator are usually connected directly to the inverter. This is not possible when replacing with string inverters. They have several inputs for the respective single module strings. Therefore, we usually place a “reversed” generator junction box in front of the string inverters here, which distributes the bundled power from the PV generator evenly to the string inputs on the inverter.

What happens after the repowering?

Johannes Liebich: After the implementation, which we accompany closely as a service provider, we also want to verify the success of the project. Therefore, a comprehensive evaluation is carried out after about one year of operation with the new components. If the inverters were completely replaced, this is not very easy, as a suitable reference is missing, and fluctuating irradiation and temperature conditions make an analysis difficult. Innovative evaluation methods such as a power curve analysis are suitable for this. They prepare detailed and adjusted yield data, thus providing an objective basis for evaluating the measure.

Michael Strübing: From an operational management point of view, the plant operator does not only benefit from the fact that new components also come with new guarantees and give the plant a rejuvenating cure, so to speak. He also profits from the fact that failures and corresponding repair costs are reduced in the long term. Additionally, longer maintenance intervals can also be agreed upon with the plant operator, which lead to reduced operational management costs.

Thank you very much for this interview.