Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Measurement Results

The measurements are taken every 10 seconds by a data collecting device (type: Schuehle, MAC 19). Normally, they are saved as 5-minute average values. Other saving intervals are possible. For most of the measurement values, the system also records the minima and maxima over a 30-minute interval. These provide further information of the system's operation. Regular controls of the minima and maxima can detect measurement errors, which might be caused by faulty sensors. Incorrect average values can thus be singled out easily. Figure 5.1. and the Tables 5.1. and 5.2. provide an overview of the recorded data. The data logger records power (kW), volume flow (m³/h) and temperatures (°C) every 10 seconds, as well as the total operating hours every 2 seconds. All the values are saved every 5 minutes. 

Intensive Measurement In the course of the program Solarthemie, an intensive measurement phase of two years is a requirement. Thereby, system values are intensively monitored and evaluated. The objective of this detailed monitoring is twofold: on the one hand, it serves to optimize the plant operation and increase the system's efficiency, on the other, it helps to test the manufacturer's specifications in relation to the energy yield.

The pdf documents below show the most important measurement results of the first two years of the solar thermal system of Baden-Baden City Hospital. These include the usable solar energy, the solar fraction, the system's overall efficiency and the collector efficiency.

Specific daily sums of the radiation and useable solar energy (averaged over weekly sums) and weekly averages of the system’s efficiency (1st year of intensive measurement)
Specific daily sums of the radiation and useable solar energy (averaged over weekly sums) and weekly averages of the system’s efficiency (2nd year of intensive measurement)

System’s Efficiency

In the first year of measurement, a total of 139 439 kWh was transferred via the heat exchanger of the collector circuit to the buffer storage (QSP) (secondary circuit). This corresponds to an irradiation on the surface of the collector field (EITK) of 334 980 kWh (1 212 kWh/m²). The energy difference between EITK and QSP was either reflected at the collectors or emitted as thermal loss to the environment. In percentage terms, this amounts to a transfer of 41.6  % of the total irradiation to the loading of the solar storage (collector circuit efficiency). 

The usable solar energy from the solar thermal system (QSOL) i.e. the energy transferred from the solar reservoir to the potable water via heat exchanger, amounted to 123 716 kWh. The system’s efficiency thus lay at 37%.

In the second year of measurement, the irradiation on the surface of the collector field amounted to 340 393 kWh (3,4 kWh/m²). Out of these, 109 456 kWh were fed into the buffer storage (QSP). The system’s efficiency thus lay at 32%. This is slightly below the yield of the first year because the heat exchanger was calcified in August 2001, which shut the system down for two weeks in a row.

The pictures on the right show the daily averages calculated by the weekly sums of irradiation and usable energy as well as the system’s efficiency for both years. They clearly indicate that the solar yield reduces with decreasing radiation and rises with increasing radiation. Between August 2000 and July 2011, the system’s efficiency was 36.9% on average. The bottom pictures clearly show the effects of the calcified heat exchanger from August 2001.

When irradiation is low, the system’s efficiency strongly fluctuates. Even small amounts of stored energy can cause rather big fluctuations when having to be kept until the following week. The efficiency of a lengthy period in the winter lay at about 39 % on average.

Weekly sums of the useable solar energy, energy for warm water heating and the weekly averages of solar fraction in relation to the energy of the water used (1st year of intensive measurement)
Weekly sums of the useable solar energy, energy for warm water heating and the weekly averages of solar fraction in relation to the energy of the water used(2nd year of intensive measurement)

Heat Consumption and Solar Fraction

The figures on the side show the course of the solar fraction related to the energy demand for the warm water consumption. The bars show the weekly sums of usable solar energy of the solar thermal system and the corresponding energy demand for warm water consumption only, i.e. without taking circulation losses into account.

In winter, the solar fraction always ranges between 10 – 25%. In summer, this can go up to 90% on weeks with highest irradiation.

The average solar fraction of the amount of water used and circulation taken together was 41.4 % in the 1st year of intensive measurement and 35.8 % in the second. The solar fraction is relatively high, because the system is slightly oversized.

 

 

 

 

 

 

 

 

Further Information

Click on the pdf documents below for an overview of the results as well as more information on the energy guaranteed for the individual years.

 

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