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 Solarthermie, 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 Mindelheim County 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 of Mindelheim County Hospital (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 of Mindelheim County Hospital (1st year of intensive measurement)

System's efficiency

In the first year of measurement, a total of 65 177 kWh (545 kWh/m2) 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 156 340 kWh (1 307 kWh/m²).

In the second year of measurement, the irradiation on the surface of the collector field amounted to 151 637 kWh (1 268 kWh/m²). Out of these, 64 614 kWh (540 kWh/m2) 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 41.7 % of the total irradiation to the loading of the solar storage in the first year and of 42.6% (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, amounted to 64 254 kWh (535 kWh/m²) in the first year of intensive measurement and 64 027 kWh (535 kWh/m²) in the second. 

The system’s efficiency lay at 41 % in the first year and 42.4 % in the second. Although, the radiation was less (-3%) in the second year of intensive measurement, the solar yield only reduced by 0.4 %, which explains the increase of the overall system’s efficiency.

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.

It is noticeable that the solar yield reduces with decreasing radiation and rises with increasing radiation. In winter, the yield is almost 0. This is caused by the fact that radiation on bad weather days can be so low that the collectors cannot deliver any usable energy.

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)
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 and circulation (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 and circulation (2nd year of intensive measurement)

Heat Consumption and Solar Fraction

The two diagrams on the side (top) show the solar fraction as related to the energy requirement for hot water consumption only (solar fraction supplied); the diagrams underneath show the solar fraction as related to the energy requirement for the hot water consumption inclusive of circulation loss (covering the amount of water used and the circulation.

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, inclusive of circulation loss. 

On average, the solar fraction for the amount of water used was 35,3 % in the 1st year of intensive measurement and 35,2 % in the second. 

The average solar fraction of the amount of water used and circulation taken together was 11,5 % in the 1st year of intensive measurement and 11,4 % in the second.

The fairly large difference between the two solar fractions is due to the high circulation loss. In comparison to the energy needed for heating the water for consumption (181 697 kWh and 182 599 kWh per measurement year), the energy required for covering the circulation loss (375 618 kWh and 381 561 kWh per year) is more than twice as much. Thus, the energy needed for circulation accounts for a consumption of more than 67 %. 

Most likely, the high circulation loss is determined by the warm water temperature of 66 °C required in hospital care. The high temperature of the preheating tanks is meant to guarantee that the entire potable water circuit is rinsed with hot water at 60 °C and is thus protected against legionella infections. DVGW regulations require that the reflow temperature must not be more than 5K (max) below the supply flow temperature. This is why the warm water must be constantly circulated at a rate of app. 43 m3/h, which is to say that the circulations pumps require even more energy.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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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