Motivation and initial situation

The aim of social housing to create affordable living space results in significant cost pressure when
constructing buildings. At the same time, there are requirements to construct energy-efficient and
resource-conserving buildings and to make a positive contribution to the quality of life of residents
through the living and open spaces created. The project in question is located in precisely this area of conflict: Within a tight budget, the aim was to create a residential building that makes a  contribution to a climate-neutral city, creates affordable living space and at the same time offers residents a high quality of life.

The residential complex at Käthe-Dorsch-Gasse 17 (project name "Wientalterrassen") was one of the two winning projects for the Käthe-Dorsch-Gasse area in a competition for developers, held by ÖBB Immobilienmanagement GmbH and wohnfonds_wien in 2018, in which, among other things, a climate-friendly energy concept was required and the use of natural gas was excluded. A district heating connection was out of the question for the site in Vienna's 14th district, which is bordered to the north by the ÖBB railroad line, as district heating is not available in this area. Under the leadership of the property developer WBV-GPA („Wohnbauvereinigung für Privatangestellte"), a large residential construction project with 295 subsidized rental apartments including some additional facilities such as the "All in Penzing" generation center has been planned on the approximately 10,500 m2 building site since 2018 and was implemented between autumn 2020 and autumn 2022.

Project content and objectives

The overarching objective of the "Sozial100%Erneuerbar" project was to implement a 100% renewable heating and cooling supply for the residential complex at Käthe-Dorsch-Gasse 17 ("Wientalterrassen") under the conditions of social housing while achieving a high level of living comfort.

Project goals in detail:

• Implementation, monitoring and optimization of an integrated heating and cooling supply system that has not yet been implemented in this combination, particularly in social housing. This system consists of the following elements: Geothermal energy with regeneration of borehole fields, heating and cooling via thermal activation of building components, free cooling, wastewater heat recovery, a photovoltaic system to cover the electricity demand of the building services systems, regeneration of borehole fields via solar thermal collectors and an asphalt collector;
• As a special feature, implementation and performance monitoring of an asphalt collector (for regeneration of the borehole field and to mitigate the urban heat island effect);
• Achieving a high multiplicability of the system for comparable construction projects, highlighting challenges and optimization potential;
• Informing and involving residents through suitable information channels/materials;
• Evaluation of the acceptance of the energy system and the user behavior of the residents.

Results and conclusions

As described above, the energy supply system was successfully implemented and put into operation
in November 2022. Particular highlights of the project were the significantly lower energy costs
compared to a district heating supply and the largely high level of satisfaction among residents, which was confirmed by the two-part post-occupancy evaluation (online survey, personal interviews). Individual points of criticism (such as a room control unit that was sometimes perceived as counterintuitive and was replaced in the second year of operation) did not lead to general dissatisfaction with the energy supply system. Thermal comfort was rated as very high, particularly in the summermonths. The project therefore met the requirements of a flagship project.

The property developer WBV-GPA pursued a comprehensive information strategy for residents right
from the start. The information materials developed in the project, in particular the two explanatory videos, were well received. The information provided at the time the residents moved in (in fall 2022) was very comprehensive.

The asphalt collector, which was installed in October 2022 and is the first of its kind in Austria, was put into operation for the first time in May 2023. The effect of the asphalt collector in terms of reducing the "urban heat island" effect and the contribution to the regeneration of the borehole fields were measured. A cooling effect of up to 10 °C was demonstrated on the surface of the asphalt collector (compared to a reference surface) by thermographic measurements. The contribution of the asphalt collector to borehole field regeneration was low during the first two operating periods, but this was partly due to an unfavorable control strategy. Further research should be carried out to optimize the control strategy and improve the performance of the research prototype of the asphalt collector.

Another special feature of the energy supply system is the implementation of a wastewater heat
recovery system for water heating. This system uses the entire wastewater flow (gray and black water) for heat recovery. The domestic hot water heat pump coupled with this system achieved an annual coefficient of performance of 3. After heat recovery, the average inlet temperature of the domestic hot water heat pump was 10.7 °C in 2024. The system has proven itself well in operation, but in the course of the energy monitoring it became apparent that the air heat pump installed to cover the losses from the hot water circulation by using the waste heat from the technical center was not dimensioned large enough. A replacement is currently being planned.

Comprehensive energy and comfort monitoring began in January 2023, meaning that the monitoring results from two operating periods are available in full at the time of reporting, on the basis of which various conclusions can be drawn regarding the efficiency of the systemoperation to date and possible optimization potential. There was potential for optimization, for example, in terms of reducing the length of heating operation, improving the control strategy of the three heat pumps (for space heating) or changing the flow rates in the continuously supplied heating circuits of the apartments.

The monitoring of thermal comfort in nine selected rooms (in nine different apparments) shows
relatively balanced indoor temperatures throughout the year. In the ground floor apartments included in the comfort monitoring, the temperatures during the heating period were between 22 and 26 °C, in the apartments with high solar input on the top floor they could reach up to 28 °C (no data is available on the use of the standard solar shading in these apartments). The relative humidity levels were between 20 and 50 % during the heating period. Low humidity levels could be counteracted by lower flow temperatures and a lower base air exchange rate of the forced ventilation system during the heating period.

The heat balance of the two borehole fields was not balanced during the first two monitoring periods (more heat extraction than heat feed-in), although the heat deficit was reduced in the second year of operation. In 2024, 68 % of the extracted heat was regenerated, with the extraction of heat from the apartments through free cooling making the largest contribution to probe field regeneration (67.3 %), followed by the thermal solar collectors (31.6 %) and the asphalt collector with a relatively small share of 1.2 %. Based on the target of complete regeneration, 42.8% of the heat extracted was regenerated by free cooling of the apartments in the first year of operation (2023) and 45.7% in the second year of operation (2024).

Monitoring of the temperatures of the adjacent soil (onemeasured value per month) began in January 2024.

Outlook

The extensive technical monitoring of the energy and comfort parameters will be continued for a full third year of operation by an experienced external company. The main task during this period will be to ensure an (almost) balanced operation of the borehole fields by changing the control strategy, as well as to install a functioning and efficient system to cover the heat losses of hot water circulation and to identify further smaller optimization potentials.

Once this extensive monitoring has been completed, the operator of the heating system will use the monitoring data to ensure the balance in operation independently, with external support being provided if necessary. 

The monitoring of the asphalt collector's performance parameters should also continue after the end of the project "Sozial100%Erneuerbar". The long-term monitoring of the asphalt collector should enable a more precise analysis of the causes of the asphalt collector's relatively low performance to date.

There are also plans to implement further areas with asphalt collectors in follow-up projects at other locations. This could also involve varying key design parameters such as the thickness and composition of the asphalt collector layers. Other fundamental points to be addressed would be an optimized integration into the overall system and an improved control strategy.

Project management

Schöberl & Pöll GmbH

Project or cooperation partners

• Wohnbauvereinigung für Privatangestellte Gemeinnützige Gesellschaft mit beschränkter Haftung (WBV-GPA)
• AIT - Austrian Institute of Technology GmbH

Schöberl & Pöll GmbH
Lassallestraße 2/6-8
A-1020 Vienna
Tel.: +43 (1) 726 45 66
E-mail: office@schoeberlpoell.at
Web: www.schoeberlpoell.at