© Manuel Lämmle | Fraunhofer ISE
Renewable energy in existing buildings
More heat pumps for apartment buildings
There are 3.3 million apartment buildings in Germany. According to the German Association of En-ergy and Water Industries, only around one in thirty of these is heated using heat pumps. Various research projects are showing how this can change.
So far, heat pumps have mainly been used in new buildings, where they now account for 57 per cent of heating systems. In older buildings, heating is still mostly provided by fossil fuels: of the 24 million heat gen-erators installed, over 78 per cent are fuelled by gas, oil or coal.
The challenge here lies not only in the heat pump technology itself, but also in the sometimes challenging task of integrating it into existing, older energy supply systems. With the heat transition mission in the 8th Energy Research Programme, the Federal Ministry for Economic Affairs and Climate Protection aims to reduce these barriers.
Research and development should contribute to the integration and standardisation of innovations in the heating and cooling sector and their broader application. Experts are already working on various projects to achieve these goals.
LowEx-Bestand: Solutions for renovated apartment buildings
What is the most efficient way to use heat pumps, heat transfer systems and ventilation systems when modernising the energy systems of apartment buildings? A team of scientists led by the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) addressed this question in the LowEx-Bestand project, together with heating and ventilation technology manufacturers and companies in the housing industry.
The researchers in the LowEx Bestand project conducted their investigations in various demonstration buildings. Here you can see such an apartment building in Adorf, Saxony, before renovation.
The renovated apartment building with a heat pump hybrid system in Adorf, Saxony.
In doing so, they were able to overcome a major obstacle to the use of heat pumps in existing apartment buildings: Contrary to widespread belief, it is often not the case that all radiators in all apartments have to be replaced or underfloor heating installed to achieve acceptable heat pump efficiency. If the building insulation has been renovated, the heating demand of the individual rooms is often significantly lower than when the heating system was planned. This means that the radiators are already rather oversized for the current heating demand. A room-by-room heating load calculation then shows which radiators need to be replaced in order to achieve the required heating circuit temperature.
In their investigations, the experts concentrated on so-called LowEx systems. In these systems, the temperature difference between the heating medium and the useful heat is low compared to conventional systems, which are mostly fossil-fuel based. This ensures greater efficiency. If heat pumps are integrated here, up to 50 percent of CO2 can be saved compared to the use of condensing boilers. Together, the partners tested their developments in various demonstration projects – for example in a neighbourhood in Karlsruhe-Durlach.
Karlsruhe-Durlach: heat supply in the neighbourhood
Although the existing residential area had already been renovated to improve energy efficiency in 1995, significant CO2 savings were still possible here before the start of the Smart District Karlsruhe-Durlach project. The residents of the neighbourhood still heated with conventional gas central heating systems. ‘It was not possible to supply district heating because the neighbourhood is not within the district heating network,’ explains project manager Manuel Rink from the municipal utility company in Karlsruhe.
Karlsruhe-Durlach is not alone in this: according to the German Association of Energy and Water Industries, only 15.2 per cent (as of 2023) of homes in Germany are currently heated by district heating. In addition to expanding climate-friendly heating supply via district heating networks, other low-carbon solutions are therefore needed. One option, as in the Karlsruhe project, is to plan the heating supply at the neighbourhood level – and to rely on heat pumps. To make this work in an energy-efficient way, the various technical systems, such as storage tanks, energy sources or units for heat transfer, must be networked with each other.
Heat pumps for densely built-up areas
Among other things, the partners in Karlsruhe have installed a heat pump that the experts from Fraunhofer ISE developed with the heating manufacturer Viessmann Climate Solutions in the HEAVEN project. The innovative thing about this system is that it uses both the ground and the outside air as a heat source. The thermal energy is supplied to the system alternately or simultaneously via brine-to-air heat exchangers and geothermal probe heat exchangers.
‘Heat exchange between the sources offers advantages: in summer, the heat from the outside air is stored in the ground. In winter, air evaporators tend to ice up. At positive ground temperatures, the energy required for defrosting can be obtained from the probe field,’ explains Dirk Ludwig, project manager for research and development at Viessmann Climate Solutions.
This heat pump is particularly interesting for use in densely built-up areas, where there is often little outside space available for geothermal probes, which are more efficient as heat sources than outside air. The multi-source heat pump makes it possible to combine them, thus ensuring greater efficiency and quieter operation compared to a pure outside air source.
One of the heat pumps used in Karlsruhe-Druchlach uses these photovoltaic-thermal collectors as its sole heat source.
Central heating system with combined heat and power plant, heat storage and gas boiler.
Here you can see the heat pumps used in the ‘Smartes Quartier Karlsruhe-Durlach’ project.
The second heat pump uses photovoltaic-thermal collectors as its sole heat source. Here, a solar air collector is integrated on the back of the PV modules. This cools the modules and increases their electrical efficiency.
40 per cent CO2 saved
The measures implemented in the Karlsruhe-Durlach smart neighbourhood project have reduced CO2 emissions by 40 percent compared to the baseline situation. The innovative heat pumps delivered good annual performance factors and could be integrated into the existing apartment buildings.
‘However, it has been shown that the heat pumps can only achieve economically competitive heat production costs in conjunction with the local electricity generators in the combined heat and power plant. We are investigating how we can further optimise this in the follow-up project launched in January 2024, among other things,’ says project manager Rink. In the project named EnEff:Stadt: SQ-Durlach II, experts at the Stutensee (near Karlsruhe) and Durlach sites are now working to optimise the plant technology and its integration into the energy system, as well as to develop standards. Under the leadership of the energy supplier Karlsruher Energieservice, they are developing digital methods for the planning, implementation and operation of heat pumps in existing neighbourhoods, among others.
