Development of a standardized procedure and a clear indicator to determine the functional capability of solar thermal systems for domestic hot water preparation and thermal solar combisystems

Brief description

The project objective is to develop a standardized procedure which can visualize the performance and function capability of a solar-assisted heat supply system in a clear performance indicator.

To date, operators, users or plumbers without in-depth knowledge can hardly determine if a solar thermal system shows a very good or relatively poor performance. As to PV systems, simple kWh counters and corresponding online portals are already sufficient to visualize and to compare their yields to be able to verify the function capability of the systems. In solar heat, a comparable instrument for performance evaluation is not available so far: due to the influence of auxiliary heating, regulation, energy storage and actual consumption, parameters which are easy to identify such as the fed solar energy are not informative. Many other parameters used in solar heat, such as saved conventional energy (as relative parameter indicated by fsave), solar utilization factor etc. depend from system, location and demand and cannot be compared with each other for different systems.

Therefore, it is not possible to compare the function capability of the overall heating system, neither on the basis of these parameters nor by comparing the performance of several systems among each other. In order to obtain a consistently applicable and comparable standard for the function capability, the development of a FSC based performance indicator is proposed. This indicator is determined within an automated function control procedure and specifies which output is actually delivered by the system – relative to a theoretically anticipated yield which is dynamically calculated according to the real boundary conditions, i.e. solar irradation and consumption.

The planned FSC based function control versus previous approaches is an innovation:

  • The procedure evaluates the performance of the overall system including auxiliary heating and not only the performance of the collector loop.
  • Therefore, not only the functioning of individual components is verified but also if the overall system works as specified. Since the procedure is based on an automatic yield rating, users, craftspersons and manufacturers receive the most relevant information: does the system work as intended and does it generate best possible savings?
  • The sensor technology shall be used which is available for the control system anyway (with potential moderate expansions, e.g. additional temperature sensors).
  • The requirements for precise installation of the sensors are low.
  • The FSC data can be evaluated via data logging in the controllers or in relevant web portals. Thus, existing structures are almost exclusively used.

In total, there are only marginally additional costs, relevant especially for small systems. This allows a broad or complete market presence. The FSC based performance indicator provides a consistent parameter to inform consumers about the proper operation and performance capabilities of the heat supply systems.

At the end of the project, the developed FSC based function control procedure shall be made available to the entire industry.

The planned performance indicator is based on the FSC procedure which was developed within the IEA SHC task 26 in order to compare solar thermal combisystems with various hydraulics and at various European locations. For this purpose, the “fractional solar consumption” (FSC) parameter was defined, indicating (under certain boundary conditions) the maximum possible solar percentage of the end energy demand for domestic hot water and space heating (see figure 1). Depending on insolation/location, collector area as well as collector positioning, only a fraction of demand can be provided on a solar basis in winter, whereas solar excess heat cannot be used in summer.

Basis for FSC parameter calculation (c)
Figure 1: Reference heat demand, solar irradiation as well as resulting intersections as areas; basis for FSC parameter calculation

The FSC value therefore depends on the collector area, location (incl. exposure) as well as the reference final energy demand, however, not on system design or similar. Based on the FSC, for each system type, a theoretically achievable fsave value can be identified via correlations. The correlations are, in excellent approximation, independent from collector area, location and reference heat demand but they represent, each, a certain system type only. Examples for such correlations are shown in figure 2.

Correlations between fsave and FSC (c)
Figure 2: Visualization of correlations connecting the dependence of saved conventional final energy fsave on FSC, for several solar combisystems from IEA SHC task 26

Project duration

10/2016 - 12/2020


Project partners

Coordination of the project:

  • Universität Kassel, Institut für Thermische Energietechnik, Fachgebiet Solar- und Anlagentechnik

Scientific project partner:

  • INES - Institut National de l'Energie Solaire

Participating companies:

  • Bosch Solarthermie GmbH, Wernau
  • Vaillant Deutschland GmbH & Co. KG, Remscheid
  • Viessmann Werke GmbH & Co. KG, Allendorf
  • RESOL – Elektronische Regelungen GmbH, Hattingen
  • SOLVIS GmbH, Braunschweig
  • WILO SE, Dortmund
  • Enertracting GmbH, Kassel

Associated partners:

  • Bundesverband der Deutschen Heizungsindustrie e. V.
  • BSW - Bundesverband Solarwirtschaft e.V.

The project is open to further industrial partners also after project kickoff.



The SolarCheck project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) based on a decision of the German Bundestag, via Project Management Jülich (PtJ) under grant number 0325870A/B. The authors would like to sincerely thank for their support and take responsibility for this publication.


Dr.-Ing. Harald Drück

Dr.-Ing. Harald Drück

Research Coordinator and Team Lead

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