Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Measurement Data Evaluation

The measurements described below are taken every 1-2 seconds by a data collecting device (type: HWH DL32). Normally, they are saved as 5-minute average values. Shorter saving intervals (< 5 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 1-2 seconds, as well as the total operating hours every 1-2 seconds. All the values are saved every 5 minutes. 

By analyzing the measurement data with a fairly high temporal resolution (average values over 5 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 side 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 1273 W/m² and was measured in July. The blue gaps in February and May result from a failure of the measurement system.

Power of collector circuit (PKT1) in kW, 2003

Power Delivered to the Potable Water at 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 196 kW and was measured in April.


Power of collector circuit (PKT2) in kW, 2003

Power Delivered to the Pool Water at Heat Exchanger 2

The diagram on the side shows the power transferred from the collector circuit to the pool (thermal) water at the heat exchanger of the pool water. Since the heat exchanger of the pool water and the heat exchanger of the potable water are connected in series, the collector liquid will have cooled down considerably, which is to say that less heat can be transferred to the pool water. At the beginning of 2003, this exchange was zero because the heat exchanger was broken and needed bypassing.

Solar power (PSOL) in kW 2003

Total Power Delivered to Potable Water and Pool Water

The solar fraction is the sum of all the different amounts of power absorbed by the pool and shower water. It is thus the total amount delivered by the solar thermal system as usable solar heat.





Top buffer storage temperature in °C (TPS1O) 2003

Top Buffer Storage Temperature

In summer, the top buffer storage temperature usually stays at a high level (75°C) throughout the entire day. The operation hours of the collector circuit are clearly visibly. They depend on irradiation. Other than in family homes or hospitals, the system operates with less buffer storage capacity since the showers are used continuously drawing warm water all the time.


Bottom buffer storage temperature in °C(TPS1U) 2003

Bottom Buffer Storage Temperature

In relation to the top buffer storage temperature, the bottom buffer storage temperature reveals clear temperature layers. Rarely, 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.



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 at the Bath and Wellness Center Waldbronn. 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.

The bath and wellness center Waldbronn 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 above.

For a list of the measurement points, click here.


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