.

Fig. 29 Brine supply and return temperatures averaged over the evaluated systems.
© FhG-ISE

Fig. 31 Monitoring the building services equipment with an App.
© Viessmann Werke GmbH & Co. KG, Allendorf

Fig. 32 Prototype of a lamellar heat exchanger.
© FhG-ISE

Fig. 33 Brine return temperature (minimum weekly average temperature) and average specific extraction capacity for 27 heat pump systems in the observation period from April 2009 to March 2010 in the “WP Effizienz” project.
© FhG-ISE
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Brine as a heat source

In the measured ground source heat pump systems, borehole heat exchangers or horizontal ground heat exchangers were mostly used as heat source systems. Special solutions (such as geothermal energy baskets) were the exception.

The systems with borehole heat exchangers show different characteristics in terms of the seasonal course of the brine temperatures than systems with horizontal ground heat exchangers. In Fig. 29, the average courses of the supply and return temperatures for all evaluated systems are depicted separately for these two types of heat source systems. In addition, the external temperature is depicted as a mean value of the different locations in Germany. The diagram shows that systems with borehole heat exchangers have on average considerably less fluctuating brine temperatures during the course of the year than systems with horizontal ground heat exchangers: the ground output temperature of the systems with borehole heat exchangers – here shown as the supply temperature – dropped to 4 °C during the core heating season and increased in the summer months to 13 °C. With the systems with horizontal ground heat exchangers, the brine supply temperature dropped at the beginning of the heating season to below the corresponding temperature for systems with borehole heat exchangers. It then exceeded this temperature once again from June when the upper earth layers heated up. This is reflected in the seasonal performance factors. In the summer months and at the beginning of the heating season, the systems with horizontal ground heat exchangers work on average with higher seasonal performance factors than the systems with borehole heat exchangers. The effect reverses in the core heating season and in spring. The greater provision of energy in winter compared with summer meant that the average annual performance factor for the systems with borehole heat exchangers was higher than the average annual performance factor for systems with horizontal ground heat exchangers. When viewed across the balancing period, the heat pumps with borehole heat exchangers were operated with an average supply temperature of 7 °C and the systems with horizontal ground heat exchangers were operated with a supply temperature of just under 4 °C. The average return temperature for the heat pumps was almost 4 K beneath the supply temperature.

The brine temperatures that occur during operation are dependent on a diverse range of factors. In the case of heat pumps with borehole heat exchangers, a comparison of the lowest weekly average temperatures for brine during the observation period from April 2009 to March 2010 shows that the individual systems differ by up to 8 K. With the “least favourable” system, the weekly average temperature for the supply (return) dropped during operation to 1 °C (–3 °C); with the “most favourable” system, on the other hand, it only dropped to 9 °C (5 °C).

When designing borehole heat exchangers in singlefamily homes, a parameter which is often used is the specific extraction capacity, which is the extraction capacity relative to the overall length of the borehole heat exchanger(s). The relevant guidelines (VDI 4640) stipulate three general reference values in accordance with the ground used by the heat pumps for space heating and domestic water heating: 20 W/m with poor subsoil, 50 W/m with normal bedrock and water-saturated sediment as well as 70 W/m for bedrock with high thermal conductivity. Fig. 33 depicts the measured brine return temperatures for individual systems relative to the respective average specific extraction capacity (determined from the measurement values for the annual energy extraction and the running period as well as the total length of the borehole heat exchangers). The bandwidth of the measured average extraction capacities ranges between 30 W/m and 64 W/m. On the one hand this shows that the systems with the highest brine temperatures in the heating season were operated with rather low extraction capacities. However, this also clearly shows the wide bandwidth of temperatures for those systems that had the same specific extraction capacities. It makes clear that in addition to the specific length of the respective borehole heat exchanger system, other factors also have a considerable influence such as the subsoil properties (mean temperature of the undisturbed soil, soil quality, occurrence of ground water), the borehole heat exchanger (in particular the filling) and the operation (flow regime, operational duration). Because no information about the ground was available in the project, the respective effects of the influencing factors cannot be investigated in more detail.

“WP Monitor”: Monitoring project with online display of measurement data

“WP Monitor” is the title of a current field test that is being conducted on heat pumps by Fraunhofer ISE. Around 100 heat pumps are being measured in the project, whereby around 50 systems are being taken over from the “WP Effizienz” project. The extension of the monitoring period makes it possible to generate a comprehensive database for long-term investigations. Measurement data can be collected across a period of up to six heating seasons, which for example improves the possibilities when investigating the ground as a heat source.

In contrast to the previous two projects, measurements are also being conducted on ground source heat pump systems that use a direct evaporator instead of a heat source circuit filled with a water-glycol mixture. In addition, capacity-controlled heat pumps are also now being investigated in line with market developments. The partners in the “WP Monitor” project include twelve German and Austrian manufacturers and an energy supplier.

All those interested in heat pumps have been given the possibility to follow the operation of real heat pump systems. For this purpose the project homepage  provides free access to the measurement data for anonymised heat pump systems. In addition to a description of the systems, all measurement parameters are shown as daily values and the most important energy and seasonal performance factors are shown as monthly values.

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