Topics: Refurbishment of housing estates, Decentralised energy generation, Operational management & energy management, Solar heat, Optimisation of operations
Innovation: Gas engine heat pump combined with groundwater use optimised in operation, use of decentralised heating pumps and anodic oxidation for legionella protection investigated.


  • Prototype of a gas engine heat pump tested in the project
  • Facade insulation with vacuum insulation panels and resol rigid foam panels achieve very good thermal insulation
  • CO2-neutral energy supply not achieved because of problems with the gas engine heat pump
  • Decentralised heating pumps regulate the room-specific heating in accordance with needs
  • Evacuated tube collectors provide 20 % of the domestic hot water heating

A landmark project for the refurbishment of residential schemes is the Lilienstrasse Nord housing estate in Munich-Haidhausen. Additional storeys were added to the buildings, the thermal insulation significantly improved and it was intended to make the energy supply  CO2-neutral. Because the housing scheme cannot be supplied with district heating, the owner GWG decided to use a gas engine heat pump, including a peak-load condensing gas boiler, in combination with a solar thermal system. The refurbishment of the typical 1950s housing estate has now been completed.

Project context

Initial situation

The residential complex in the Munich quarter Haidhausen/Au built in 1955, meanwhile in need of refurbishment, consisted of four 3 and 5-storey buildings with cellars and unused top floors. The 149 existing apartments housed a total residential space of approx. 6,513 m², whereby the apartments were sized between 40 m² and 65 m². The apartments had two, three and four rooms.

Until now, the apartments were heated with individual coal or gas-fired stoves. Some apartments contained electrical heaters. Some apartments also had gas-fired central heating systems. The domestic water was also heated decentrally, primarily using gas-fired continuous-flow heaters.

Project aims and objectives

The objective of the project was to refurbish the four buildings to a primary energy requirement for heating and domestic water heating at least 50 % below the permitted value for a new building. The additional residual heat required is to be generated or compensated using renewable energy sources so that, in total, no additional CO2 emissions are released, thus guaranteeing a CO2-neutral energy supply. In the "Lilienstraße North Munich" project, the use of high-quality exergy is to be minimised (LowEx approach) throughout the entire energy chain, from generation and transport to application.

Modernisation, energtic renovation, addition of storeys

Small and very small apartments primarily oriented to one side with trapped rooms were combined to form larger apartments. The re-organisation of the layouts, the addition of balconies and wheelchair accessibility with lifts guarantee demand and future-oriented accommodation. The addition of one storey to each building, and a new building above the exit of the underground car part built as part of the project created an additional 15 rented apartments. The total number of apartments was reduced to 140.

The facades were clad with thermal insulation with a thermal conductivity of 0.022 W/mK. Vacuum insulation was used to achieve high thermal protection on the basement ceiling and on the road side, which only permitted 10-cm-thick insulation owing to an adjacent pavement. The new windows are triple-glazed in highly insulated frames.

Heat supply is optimized during the process

The heat supply of the four refurbished existing buildings and the new buildings now is provided by a heating centre built between house 33 and house 41. To generate heat, the GWG and the planners decided to combine a groundwater-coupled gas engine heat pump with a gas-fired condensing boiler for the peak loads together with a solar thermal system. The boiler will guarantee high supply temperatures of the heating circuits in the planning case, and to cover the peak load. This combination allows optimised operation of the gas motor heat pump. To reach the objective of a CO2 neutral residential complex, some of the heat is generated via a solar thermal tube collector array. It was envisaged that seventy per cent of the heat generation would be met by the heat pump.

A feature of the heat pump type is that it can utilise not only the groundwater but also the cooling water from the engine and the exhaust gas heat as a heat source via heat exchangers. This enables it to simultaneously supply heat efficiently at two temperature levels – for low-temperature space heating and domestic hot water heating, which is a great advantage compared with conventional heat pumps.

Three series-connected storage tanks buffer the generated heat. Priority for the feed-in is given to the solar thermal system, followed by the heat pump, since this works more efficiently than the gas condensing boiler. Electronically controlled high-efficiency pumps transport the heating water from the heating centre to the apartment blocks. A decentralised system regulates the room-specific heating in accordance with needs: a micro-pump is installed at the return for each radiator. It runs only when heat is required in the room. This solution does not require thermostatic valves, throttle valves and central heating circuit pumps. Thus, the system changes from “supply heating” with a central heating pump to “demand heating”. The opportunities of the decentralised pump system lie in a demand-oriented heat distribution and heat transfer, improvement of the control quality and a reduction in the pipe network resistance. This results in significant potential savings for electricity and useful heat. An fundamental advantage of this system is that the heating system is hydraulically equalised in every operating condition by design. Window contacts stop the pump as soon as a window is opened to prevent excessive ventilation losses. The entire control is achieved by servers.

With domestic hot water, anodic oxidation ensures the necessary legionella protection without an additional temperature increase.

Diagnosis, optimisation, evaluation

The compression heat pump driven by a gas motor used to transport the ground water was built specially for this project. At the start, problems were caused by a coupling that was too weakly designed. This was solved by making a stronger version. Nevertheless, the heat pump subsequently ran only half as much as was planned. This was due to excessive heating water temperatures before the compressor. The cause was not discovered until the middle of 2015: a return flow from the high-temperature storage tank to the heat pump, which was remedied by installing a check valve. However, there are still technical problems at the Otto engine–coupling–compressor interface. The prototype is therefore still susceptible to faults, combined with a high maintenance effort. As a result, the heat pump is unable to provide its assumed share of the heat generation. However, the device has impressed thermodynamically: when the heat pump ran properly it even saved more energy than had been previously calculated. Compared with gas condensing boilers, the gas engine heat pump operates 60% more efficiently in terms of the annual utilisation rate.

The system of anodic oxidation runs only periodically for a few hours per day. This is sufficient to prevent the growth of Legionella. However, because the system has not yet been listed and Munich City Council does not recognise it, the municipal housing association has to regularly carry out the legionella investigations prescribed in such a case. The fact that the technology works was shown after a temporary failure of the module and the associated legionella growth: when the unit was running again, the legionella portion fell back below the maximum permitted level without additional activities.

Economic efficiency

Overall, this is made possible on energy generation by using a custom-made gas motor heat pump which utilises ground water. The base heat generator is supported by a gas-fired condensing boiler system and a solar thermal collector system. A buffer storage tank charging system provides storage and hydraulic system separation. When distributing the heat, compact thermal insulation is emphasised, whereby the warm pipelines are combined in one thermal envelope. An anodic oxidisation system incorporates the LowEx approach for hot water heating. Thermal heat is transferred to the rooms via the heating surfaces by decentralised pump technology. The individual room control facilitates high control quality and will likely result in intensive user participation. Window contacts will restrict ventilation heat losses via inefficient ventilation.

Last Update: 8. April 2017

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