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 maxima and minima can detect measurement errors, which might be caused by a faulty sensor. 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 three years of the solar thermal system Freiburg Vauban. These include the usable solar energy, the amount covered by solar energy, the system’s overall efficiency and the collector’s 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 average of the system’s efficiency (2nd year of intensive measurement)

System’s Efficiency

In the first year of measurement, a total of 98047 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 185112 kWh or 1294 kWh/m² respectively.

In the second year of measurement, the irradiation on the surface of the collector field amounted to 180218 kWh respectively 1262kWh/m² (- 2.6 %). Out of these, 99079 kWh were fed into the buffer storage (QSP).

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 53 % of the total irradiation to the loading of the solar storage in the first year and of 55% (collector circuit efficiency) in the second.

The usable solar energy from the solar thermal system (QSV), i.e. the energy transferred from the solar reservoir to the potable water via heat exchanger, was 93036kWh (651 kWh/m² or 1.78 kWh(m²d)) in the first year and 88840 kWh (solar yield) (622 kWh/m² or 1.70 kWh/(m²d)) in the second. 

The system’s efficiency thus lay at 50.3 % in the first year and 49.3 % in the second.

The pictures on the side show the exact daily sums of irradiation and the usable solar energy as well as the system efficiency (averaged over the weekly sums and weekly averages). It is particularly noticeable that the system’s efficiency falls below 0% in the last weeks of December, which means that heat was transferred from the potable water circuit to the solar reservoir circuit (QSV < 0). This is caused by the fact that the potable water was preheated due to heat recovery before it went through the discharge heat exchanger. In effect, when little water was taken, the temperature of the potable water (at the heat exchanger) was higher than the temperature in the discharge circuit so that the potable water cooled down. However, the heat transfer does not have much of an impact. It only occurs when very little hot water is consumed and the buffer storage temperature is low while the preheating temperature is high (e.g. during term break in winter).

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. During the summer vacations, the solar fraction is relatively high, because of the low consumption and the simultaneous high irradiation; during term time, however, it decreases because of an increasing consumption when irradiation and thus usable solar energy are decreasing.

The usable solar energy from the solar thermal system (QSV), i.e. the energy that was transferred from the solar reservoir to the potable water by the heat exchanger, amounted to 93036kWh (651 kWh/m² or 1.78 kWh(m²d)) in the first year and 88840 kWh (622 kWh/m² or 1.70 kWh/(m²d)) in the second.

With the usable solar energy, supplied by the solar thermal system, 17.3 % of the required energy for the warm water could be covered in the first year of measurement, in the second year, the amount was at 17.1% (solar fraction of the energy required for warm water).

 

 

 

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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