Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Measurement Data Evaluation

By the end of 2011, the research program Solarthermie-2000 (initiated in 1993), its partial program 2 and successor program Solarthermie2000plus had seen to the establishment of more than 70 large-scale solar thermal systems all over Germany. Typically, these systems possess a collector field size of 100 m² and more. These prototypical systems are meant to demonstrate the technical and economical feasibility of active solar thermal systems, help in developing the technology further and set suitable standards for the systems’ circuits and dimensioning.

A scientific and technical service included in the program provides independent and competent consultation and support of solar thermal systems from the initial idea, via the realization unto the operational systems. Since 1999, the University of Applied Sciences Offenburg has been responsible for the monitoring of the demonstration systems in Southwest Germany. Chief tasks are:

  • suitability tests of objects for building solar thermal systems,
  • support of the project partners in the system’s planning, advertising and building,
  • conception and installation of measurement instruments,
  • data acquisition and analysis over several years,
  • monitoring and evaluation of the system’s operation,
  • analysis of failures and optimization proposals, and
  • know-how transfer.

The University of Applied Sciences Offenburg currently looks after ten solar thermal systems. Five of these (Freiburg (2x), Mindelheim, Singen and Baden-Baden) are exclusively used for domestic water heating. Another plant heats the swimming pool water in addition to the domestic water. One of the four most recent plants funded by Solarthermie2000plus feeds the solar heat into a local heating network; the other three feed into the heating system of the respective company/ school premises. There, the heat is used for space heating and cooling.

The following discussion and diagrams build on the most important results as well as the experience of the scientific and technical support of the systems provided by the University of Applied Sciences Offenburg.

The useable solar heat is the energy provided by the solar thermal system for heating the potable water.

Specific Usable Solar Heat

In 2013, the total amount of usable solar heat was 872 MWh for seven of the ten solar thermal systems monitored by the University of Applied Sciences Offenburg. This value relates to a collector surface of 2670 m2 and corresponds to an average of usable solar heat of 334 kWh/ (m²a). 

The diagram on the side shows the annual sums of the usable solar heat of each plant in relation to the collector surface. The diagram also shows the simulated data as an useful reference of the measured values. As one can see, there are large differences of the solar yield not only between the different solar thermal systems, the annual yields of systems themselves also fluctuate considerably. This can only be explained by the various factors involved in the generation of solar heat such as weather per se, warm water consumption, the system’s control, the operation of the conventional technology used as well as disruptions or failures of the solar thermal system.

For most of the years, Freiburg-Vauban gained high specific solar yields, sometimes as high as 650 kWh/(m2/a). 2002 and 2009 saw the system’s lowest yields, yet. In 2002, the system was not in use because of major refurbishment, which explains the low solar yield. In the ensuing years, the solar yield never reached the amount of the first two years again; the discharge performance could be increased by a better monitoring of the warm water used, yet, the heat exchanger had a lower performance in comparison to the initial one. In 2009, the system control failed for lengthy periods at a time and there was a complete system shutdown caused by leaky collectors. This is why there are no measurement data for the Student Village Freiburg-Vauban in 2010.

The solar yield of the first year at Singen remained below the energy warranty given, which is why the entire system was rebuilt in 2001. One of the chief adaptations made in the process was changing the connection of the potable water tanks, which used to be four parallel, to two parallel segments connected in series. The potable water tanks have since been divided into two pre-heating tanks and two storage tanks. In addition, the system’s control and the insulation of the solar tanks were improved. As a result, a considerably higher amount of usable heat could be harvested between 2002 and 2005. Various disruptions in 2006, 2008 and 2010 reduced the system’s performance. These disruptions did not occur in the following year so that the solar yield increased again. The solar yield of this system is also affected by the low solar irradiation and the reduced warm water consumption. 

In Baden-Baden, the solar yield had been decreasing until 2005 since start-up. Because of a calcification of the heat exchanger in the potable water circuit, the heat transfer to the potable water deteriorated. It was not until the rebuilding of the entire system in 2006 that the calcification could be reduced significantly and the solar yield rose again. In 2007 and 2008, the yield was low again, which was caused by reduced warm water consumption. In 2009, the consumption reached the initial level again and the yield could be increased thanks to a better handling of the calcification problem. There are no measurement data for 2010 largely due to the disrupted data acquisition.

At Waldbronn, problems with leaky heat exchangers caused a prolonged standstill in 2005 and 2006. After replacing the heat exchangers and re-commissioning of the pre-heating pools, the yield increased again considerably in 2007. In 2012 and 2013 the solar yield ist very low because of a long-term disorder of the system

The systems at Mindelheim and Freiburg Wilmersdorfer Straße ran smoothly with only few disruptions that did not affect the solar yield. The decrease in solar yield of these two systems is most likely due to reduced warm water consumption.

In sum, the highest yields, on average 587 kWh/m², were gained at Student Village Freiburg-Vauban. Most systems obtained their highest yield in 2003.

The measurement results of Holzgerlingen, Esslingen, Butzbach and Rottweil must be considered separately as we are dealing with a local heating network and solar cooling systems. Their yield cannot simply be compared to those of solar thermal systems for potable water heating.

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The system’s efficiency indicates how many percent of the solar irradiation is converted into usable heat.

System’s Efficiency

The diagram on the right shows the average annual system’s efficiency of the individual solar thermal systems next to the values estimated through simulation. It is evident that Freiburg-Vauban features a higher system’s efficiency than the other systems. This can be explained by the collector efficiency which stays high all year round. The annual system’s efficiency of the solar thermal systems used for potable water heating lay between 30 % and 52 %. Due to some malfunctions and reduced warm water consumption, the system’s efficiency sometimes fell below 30 % for some systems.

Holzgerlingen, Esslingen, Butzbach and Rottweil fell below 30 % on a regular basis or remained just above that level, which is chiefly due to the higher temperature level of these systems.

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The solar irradiation indicates the sum of radiation per square meter of collector surface over a period of one year.

Specific Solar Irradiation on Collector Surface

The diagram on the right shows that the solar irradiation lay between 1140 kWh/(m²a) and 1550 kWh/(m²a) throughout the years. All the systems reveal distinct peaks of irradiation in 2003 and 2011. 

2010, on the other hand, was the year of lowest solar irradiation for all the solar thermal systems. Singen generally obtained the lowest values whereas the other systems remained on roughly the same level. The azimuth of the collector field in Singen is 52° South, yet the irradiation is only slightly lower. 

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Specific Load indicates how many liters of potable water (60°C) are consumed per day in relation to one square meter of collector surface. One square meter of collector surface per 70 liters warm water consumption (60°C) per day.

Specific Load

The diagram on the side shows the system’s efficiency in relation to the average Specific Load. A higher specific load usually determines a higher system's efficiency. The Specific Load lies between 40 l/(m²d) and 70 l/(m²d).

The student village Freiburg–Vauban is an exception of this rule of thumb. There, the warm water consumption (and thus Specific Load) increased considerably than what was initially expected when planning it. 

Holzgerlingen, Esslingen and Butzbach cannot be evaluated regarding efficiency as the heat is not used for warm water generation. In sum, it was confirmed that the systems’ efficiencies decrease with Specific Load.

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The solar fraction represents the proportion of usable solar heat adding

Solar Fraction

The diagram on the side shows the solar fraction of the heating of the warm water used, in other words the amount of heat transferred to the local heating network or the system’s warm water circuit. 

This is usually between 30 % and 40 %, a typical ration for solar thermal systems that are used for pre-heating. The student village Freiburg-Vauban can be considerer an outlier in this regard. Its solar fraction is relatively small because of its small collector surface in relation to warm water consumption. 

This is also the case for Holzgerlingen and Esslingen when compared to the other solar thermal systems. The collector surface was clearly limited by the roof space available. The solar thermal system Butzbach is the smallest system but it features the highest solar fraction amongst the cooling systems because the size of the collector field was designed according to a recommended 3 m2/kW- cooling capacity. Its chillers work self-sustainingly with solar power.

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