Fig. 5: Distribution of the monthly heat supplied to the local heating network
© ZAE Bayern
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Solar thermal heat can be used into December

The measurements started when the system commenced operation in 2007. One of the aims was to ensure that the measurement and control technology used for regulating the system was linked to the measurement technology used by the accompanying scientific research as closely as possible. This will also enable measurement data to be recorded once the accompanying research has been completed. Although the solar radiation in the 2008/2009 operating year was around 3% less than in the previous year, the system still managed to supply 11% more heat from solar energy to the storage tank (Fig. 3). The winter in the first operating year was relatively mild and the heat pump was not yet operating because of faults in the control program. In the second measurement year, its use enabled an additional 117 MWh of solar energy to be drawn from the storage tank. If the average load state of around 42% of the nominal load and the irregular operation of the thermal heat pump are taken into account, the mean coefficient of performance of the two heating periods of 1.52 fully accorded with the expectations for these operating conditions. Solar heat is directly used until into December (Fig. 5). The absorption heat pump then cools the storage tank even more. In 2008, the tank’s direct use started in February. In 2009, this point in time was delayed until into April because of the weather. When the problem with the stratified charger is solved, its direct use can be extended by several weeks. The reason why a high proportion of district heating is used directly is because the municipal utility company prescribes that the district heating return line must be cooled to 50 °C. Therefore theoretically, even more solar heat could be used for energy-related purposes.

System’s potential not yet exhausted

In the summer of 2006, Stadtwerke München converted its district heating network in the Am Ackermannbogen area from steam to hot water. This meant that the scientists had to completely rethink the overall concept in the middle of the planning phase. They were nevertheless able to successfully master this challenge as well as problems with the collectors. However, anomalies are still occurring with the stratified charger as with the controls. It is only when these are solved that a solar fraction of 50% is realistic.


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Stadtwerke München GmbH