Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Measurement Data Evaluation

The measurements described below 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.

The data logger records the following parameters: power given in kW, volume flow given in m³/h and temperatures given in °C every 10 seconds, as well as the total operating hours every 2 seconds. All the values are saved every 5 minutes.

In addition to recording the measured values via data logger, a heat meter with two monitoring stations for the temperature (TRW1, TRW2) is connected on the side of the potable water of the discharge heat exchanger (WT2).

The impulse signal for measuring the volume flow is obtained from a volume counter VSV by an amplifier for doubling the impulse. Independent of the data acquisition, this volume counter indicates the yield of the solar thermal system through the logger. It serves to both verify and control the solar yield recorded by the data logger.

By analyzing the measurement data with a fairly high temporal resolution (average values over 5 and 30 minute intervals), it is possible to assess the system’s performance and identify problems. On the basis of the malfunctions identified through these analyses, the solar thermal system was modified substantially after start-up. 

Carpet-Plots for the Visualization of Large Data Volume

Data collected over long periods of time are graphically visualized through carpet plots. In this diagram, the individual days are plotted side by side along the x-axis. The measurement points collected over the course of one day are put along the y-axis. The height is indicated by color coding. Below, the crucial measurement points are shown as carpet plot, which illustrates how these can be used for the data analysis and error search. The diagrams below date from 2004.

Solar Radiation (EI1) in W/m² 2004

Solar Radiation

The diagram below shows the solar radiation on the collector surface (EI1) over the course of one year. It is easy to distinguish the times without solar radiation and days of bad weather by the blue gaps. It is also easy to see the change of radiation in relation to the change of seasons and the duration of radiation. The maximum radiation of 1250 W/m² was measured in June.

 

Power of potable water circuit (PSV) in kW, 2004

Power Delivered to the Potable Water in Heat Exchanger 2

By buffering the heat in the tank, the potable water can be heated even at times with little or no solar radiation. The diagram shows that between 8 and 9 am hot water is consumed; afterwards heat from the solar thermal system can be used for the potable water again. Late in the evening and at night, the heat transfer is next to none. The maximum power was measured at 118 kW in May.

Amount of water used (VSV) in m³/h, 2004

Amount of Water Used 

The students’ summer vacations are easily recognizable, lasting from the end of July until the beginning of October; so are the Christmas vacations and weekends. In these periods, the demand is clearly lower, which indicates a smaller number of students present. Over the course of the day, there is a clear morning peak of the amount used between 7 and 12 am. The evening peak is not as clearly indicated. It usually starts around 6 pm and lasts well past midnight. In the afternoon, the consumption decreases detectably. The changeover to daylight saving time can be seen in the shift of consumption by one hour.

Buffer storage temperature 3 up (TPS3) in °C, 2004

Top Buffer Storage Temperature 

The top buffer storage temperature indicates when the loading of the buffer storage begins. In the summer, this happens at 8:15 am at the earliest. Late in the evening, the storage is usually heavily discharged, such that there is no more discharge possible and the temperature is kept until charging is resumed. The summer vacations can be detected quite clearly. The buffer temperature is at top level almost the entire day since there is hardly any heat discharged during that time. In comparison to the beginning of the year, the tank temperature is clearly lower at the end of the year. This is caused by the removal of the surplus heat recovery of the boiler. The water that reaches the discharge heat exchanger is colder as it is not preheated anymore.

Bottom buffer storage temperature (TPS2) in °C 2004

Bottom Buffer Storage Temperature 

The students’ summer vacations also have an impact on the bottom buffer storage temperature. In this period the temperatures do not fall below 17°C. During term time, the buffer storage is rarely fully charged, so that the temperature rise above 30°C.This indicates that the system was oriented on an average energy consumption during the summer vacations; for the rest of the year, it remained well below the capacity level. This shows that the system is used at capacity level and thus runs more efficiently compared to others.

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 three years of the solar thermal system Freiburg Vauban. These include the useable energy, the solar fraction, the system's overall efficiency and the collector efficiency.

Access our Data Sever via DATAPOOL

Via DATAPOOL you can access selected data of this solar thermal system. You can choose from various illustrations like line charts, carpet plots, or scatter plots.

The student village Vauban can also be found on the Datapool pages under Solar Thermal Systems.

The individual measurement points of the data pool can be found in the diagram on the side or here.

For a list of the measurement points, click here.

 

 

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