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. 

In addition to the electronic data acquisition, the various measurement instruments that are equipped with counters are checked once a week. Pressure readings are also taken at the collector and buffer storage circuits.

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 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 detect the change of radiation in relation to the change of seasons and the duration of radiation. The maximum radiation of 1236 W/m² and was measured in April.


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 178 kW and was measured in April.


Power of potable water circuit (PSV) in kW, 200

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. Between the end of July to the beginning of September, the heat exchanger was calcified which is why the diagram shows a power input of zero. The maximum power is just below 77 kW and was measured in March and July.

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. 

From 7 am to 2 pm, warm water is taken at with a decreasing flow volume. Peak consumption times are between 6 and 8 am. On weekends, less water is required since the hospital service is reduced to basic medical care. The same applies to Christmas time. The vertical blue line in August is due to the failure of a measuring point.

Top buffer storage temperature in °C (TPS11) 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 strong consumption between 7 and 10 a.m., 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 (TPS32) 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 hot all through. 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. 

In August, the temperature remained high even in the lower layers, which was due to the calcification of the heat exchanger. As a result, no heat could be discharged and the tank heated up to more than 100°C all day. 

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 of Baden-Baden City Hospital. These include the useable energy, the solar fraction, the system’s overall efficiency and the collector efficiency.

DATAPOOL provides access to our data server

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.

Baden-Baden City Hospital can be found on the Datapool pages under Solar Thermal Systems.

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

For a list of the measurement points, click here.

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