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The experts assessed the solar thermal systems before starting the monitoring.
© Zentrum für Innovative Energiesysteme
Solarthermie2000 and Solarthermie2000plus funding programmes

Annual solar yield for the best, worst and a typical system relative to the yield during the first year of operation.
© Zentrum für Innovative Energiesysteme

Large-scale solar thermal systems in long-term testing

Can solar thermal plants achieve a consistently high yield for more than ten years? In order to answer these and other questions, scientists from Düsseldorf University of Applied Sciences have surveyed the operators of 76 large-scale solar thermal plants and monitored ten of these systems for one year. All systems were funded at the turn of the millennium as part of the Solarthermie2000(plus) programme.

The investigations have shown that the vast majority of the solar collectors achieve a consistently high efficiency even after more than ten years of operation. The yield from six of the ten monitored systems has averaged over 90% of the solar yield for the first year of operation since the start of operations. With a mean value of 112%, one system has for many years even exceeded the yield value of the first year of operation. This is a plant operated by a municipal utility company. This sells the heat to the connected customers. For this reason, the operators monitor the system permanently and ensure optimum operation.

Two systems ran completely error-free during the monitoring. The others showed weaknesses, especially in regard to aspects that are not specific to solar thermal energy. In reference to this, project manager Professor Dr. Mario Adam from the Centre of Innovative Energy Systems (ZIES) at Düsseldorf University of Applied Sciences says: “Serious losses in the performance of the solar thermal plants only occurred when incorrect modifications were made, loads decreased or significant ageing phenomena and operating errors were not identified or remedied. Examples of these include small or reduced flow rates caused by leaks or defective pumps, or errors in the control system.”

Professional servicing influences solar yield

In addition to the one-year monitoring, the scientists also surveyed the operators of the 76 solar thermal systems. The response rate was around 60%. The experts received information on the operation, annual operating expenses and satisfaction of the users. Around three-quarters of the operators have their systems regularly serviced. However, no direct correlation can be made in this regard to the systems’ condition, as faults occur just as frequently in systems that are serviced as in systems that are not serviced. This would suggest that in some cases the servicing has been inadequate. Almost 70% of the operators rate their satisfaction with the solar thermal system as good to very good, but only 42% would recommend solar thermal technology. The respondents claim that this is due, among other things, to the lack of profitability.

“On the whole, the project shows that solar thermal systems are technically capable of delivering a consistently high solar yield for 20 years and longer,” concludes Adam. “Two things are generally required to ensure good operation in the long term and thus a permanently high solar yield: an interested operator and system monitoring or high-quality servicing to ensure early fault detection and yield control.”

A detailed final report and further information on the “Long-term study on the operation and performance of large-scale solar thermal systems from the Solarthermie2000 and Solarthermie2000plus programmes” are available on the ZIES website.



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