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 30-minute average values. Shorter saving intervals (< 30 mins) are possible; they are usually carried out to monitor the dynamic behavior of the system, e.g. during trial runs. 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 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 30 minutes. 

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 such a 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 2003.

Solar Radiation (EI1) in W/m², 2003

Solar Radiation

The diagram on the right 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 1070 W/m² and was measured in July.


Power of collector circuit (PKT) in kW, 2003

Power Delivered from the Collector Circuit to Heat Exchanger 1

The power delivered from the collector circuit to the buffer storage clearly related to the solar radiation. Days with bad weather are clearly recognizable as the system operated at lowest level or not at all. The maximum power is just below 80 kW and was measured in August.

Power of potable water circuit (PSV) in kW, 2003

Power Delivered to the Potable Water in Heat Exchanger 2

The power transferred to the potable water at the discharge heat exchanger relates to that of the amount of water used. By buffering the heat in the tanks, the potable water can be heated even at times with little or no solar radiation. The diagram shows that hot water is consumed from 5:30 am onwards and that from this point on heat is transferred from the solar thermal system to the potable water. From 2:30 pm onwards, the heat transfer is considerably lower. Heat transfer ceases around 10 pm. This is the time when hardly any warm water is used and the temperature of the potable water tanks is almost equal to those of the buffer storage. The maximum power of 90 kW was measured in June.

Amount of water used (VVV) in m³/h, 2003

Amount of Water Used

The data acquisition clock remains tuned to winter time, so that there is a detectably shift of one hour at the end March and October. Between 5.30 a.m. to 8 pm the flow volume of the amount of water used steadily decreases. On weekends, less water is required since the hospital service is reduced to basic medical care.

Top buffer storage temperature in °C (TPS1) 2003

Top Buffer Storage Temperature

In summer, the top buffer storage temperature stays at a high level almost throughout the entire day. Due to the large consumption after about 7 am, the tank temperature drops until it is heated again by the heat from the collector circuit. As it would be expected, the temperature is significantly lower in winter.

Bottom buffer storage temperature in °C (TPS2) 2003

Bottom Buffer Storage Temperature

In relation to the top buffer storage temperature, the bottom buffer storage temperature reveals clear temperature layers. It is only on a few days a year that the tanks are heated evenly. This is to say that the system and the tanks are well balanced so that the solar thermal system hardly ever comes to a standstill. As a result, the solar yield is used to the max. The maximum temperature was 83°C.

Intensive Measurement

In the course of the program Solarthermie-2000, 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 Mindelheim County Hospital. 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.

Mindelheim County Council 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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