Evaluating the performance of solar thermal plants is of great interest for operators and it provides a basis for optimizations. A tool for this task is SunPeek, which met with great interest at the recent IEA SHC Solar Academy webinar in November. The open-source software automates the performance assessment of collector fields in operation by comparing target and measured power output. SunPeek is available free of charge for scientific and commercial use. Philip Ohnewein, one of the SunPeek developers and researcher at the Austrian institute AEE INTEC, introduced the software and answered a lot of questions. The graphic above illustrates how cleaning the collectors can increase the power output as demonstrated using SunPeek. The webinar was organized by Task 68 on Efficient Solar District Heating Systems. The recording and the presentations are available online: https://www.iea-shc.org/solar-academy/webinars
“SunPeek is a powerful tool that can be used in two ways: Firstly, for guarantee procedures for specific time intervals, such as plant commissioning, and secondly, for ongoing assessment of the plant performance during plant operation”, explained Ohnewein. The use of SunPeek is not geographically restricted. It can be utilized by system operators worldwide.
The performance assessment with the software is very reliable, because it is based on the Power Check method, which is described in the ISO 24194 standard. This standard has been developed by an international group of scientists and practitioners, incorporating experience gained from over 100 Danish solar district heating plants. The supplier of the collector field and the customer, for example a municipal utility company, can therefore rely on the SunPeek software to check and verify the performance of the plant in operation.
“SunPeek is a great service tool for project developers of large collector fields. In the future, we plan to use it to check our performance guarantee agreements between different stakeholders who are part of EPC contracts or eventually to contribute to our periodic performance checks during the operation phase”, Pierre Delmas, CTO and co-founder at the French company NewHeat, was quoted as saying in Ohnewein´s presentation.
When adding a new plant to SunPeek, there are only a few steps necessary to successfully execute the Power Check: First, you describe the installation and select the collector model from the SunPeek collector database or define a custom collector. Then you upload the monitoring data set with sampling at intervals of five minutes or less. SunPeek supports all data in tabular format, such as common CSV data. The third step is sensor mapping. This involves assigning the sensor to the correct input slot for the calculation algorithms. Once the safety factor has been specified, the requirements for the ISO 24194 Power Check are met and all further calculation steps run automatically.
SunPeek generates figures and a PDF reports that illustrates the results of the target/measured comparison. The online demo version of SunPeek (https://demo.sunpeek.org/) already contains an open data set of a real-world installation, so that new users can try out the software straight away.
Comparison of target and measured power within a few seconds
The following approach is crucial for the performance assessment. The software selects time intervals from the monitoring data where the operation of the system was not affected by shading, very cloudy sky or significant collector temperature changes. "SunPeek can automatically carry out the ISO 24194 Power Check. According to the standard, for a valid check we need at least 20 operating hours with stable, specific operating conditions," explained Ohnewein. The automated process of data quality check and data selection saves the plant operator enormous amounts of time for assessing the plant performance.
SunPeek automatically calculates the target power output according to the provisions of the standard. This calculation is based on the collector efficiency parameters, as well as the solar irradiance, ambient and collector temperatures as measured on site. If the measured power matches the target power within a given tolerance, the system is running as expected and the performance has been verified, e.g. between the customer and the system supplier.
However, one important aspect still needs to be agreed between the customer and supplier: the “safety factor”, specifying the tolerance and accounting for effects such as heat losses from pipes and measurement uncertainties. It is usually around 0.90 to 0.95 and results in a reduction in the expected output.
Additional questions during the Q&A session
Ohnewein confirmed that the software is currently being applied to about 10 to 15 real-world large solar thermal plants and their historic monitoring data. “We are currently working on improving the automation of SunPeek to enable the software to run in a long-term, ongoing mode parallel to plant operation”, he added.
Ohnewein also explained the difference between SunPeek and Polysun. “Polysun is simulation software primarily aimed at the planning and design of new solar plants and energy systems. SunPeek, on the other hand, comes into play when the solar plant is commissioned and operational, providing insights into how well the system is performing.”
When asked "have you incorporated failure analysis with k-out-of-m configuration?", Ohnewein answered: "We have not tried this specific analysis. However, we recognize the potential of using SunPeek and ISO 24194 analysis methods for failure detection and ongoing plant monitoring and we are planning further steps in this direction."
Ohnewein also announced the upcoming publication of a guide document for the application of the ISO 24194 Power Check with SunPeek. The guide aims to make the ISO standard method more accessible and clarifies areas that leaves room for interpretation. The document will be available for download in the first quarter of 2025 here: https://task68.iea-shc.org/publications
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