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Projektinfo – Detailed information on energy research

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During the course of the renovation, the roof spaces of the three- and five-storey apartment buildings were converted into flats and furnished with thermal solar collectors and PV modules.
© Ingrid Scheffler
Renovation residential complex
Projektinfo 09/2017

Site plan (after renovation)
© GWG München

Summary: Installed building technology
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Gas engine heat pump supplies renovated residential complex

The energy efficient renovation of apartment buildings from the post-war years was aimed at achieving a CO2-neutral energy supply in conjunction with the use of renewable energies. The core element of the new heating system is a gas engine heat pump that uses groundwater as the heat source. This not only supplies the low-temperature space heating but simultaneously heats the domestic hot water efficiently. However, during the monitoring period the prototype failed to contribute to the heat supply to the planned extent. This was due to structural and regulatory problems.

The residential complex with a total of 149 apartments, which is owned by the GWG municipal housing association and is situated in the Munich district of Haidhausen, has not changed significantly since it was constructed in the 1950s. The primary energy consumption exceeded 300 kWh/m2p.a. Through the renovation, the GWG was aiming to reduce the primary energy consumption for space and DHW heating to less than half the value permitted at the beginning of the project for a new-build scheme in accordance with the 2007 German Energy Savings Ordinance (EnEV 2007). It was intended that renewable energies would generate or compensate for the residual energy required. It was also decided to equip tenants with simple and robust home automation systems that do not require active participation.

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.

Since the old buildings were originally heated with coal or gas via individual ovens, a heating centre had to be newly built. It is located next to the inner courtyard together with the new underground car park.

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Low-Ex: Thermodynamically optimised heating

Exergy describes the amount of energy that can be completely converted into usable work. Mechanical and electrical energy are energy forms with high exergy content. Thermal energy can only be converted partially into work or into another energy form. The closer the temperature of the thermal energy is to the ambient temperature, the lower is its exergy.

The approach of low-exergy systems (LowEx) is to use energy forms that have as little exergy as possible for heating and cooling, such as waste heat or environmental heat, and which thus work in a primary energy-efficient manner. A prerequisite for using such low-temperature heat is an energy efficiently optimised building envelope.

If environmental heat is not directly available at a temperature level that is sufficient for heating, a heat pump can help. Although this requires additional exergy (for example in the form of electrical energy), it causes the amount of generated heating energy to be several times higher than the amount of primary energy used.

LowEx building technology

The building complex was unable to be supplied with district heating. To generate heat, the GWG and the planners therefore 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. 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.

The heat pump system consists of five interconnected hydraulic circuits. The primary circuit uses a groundwater well as the energy source. Via a heat exchanger an intermediate circuit is integrated that supplies the evaporator for the heat pump by means of a speed-controlled feed pump. The compressor then compresses the refrigerant in the condenser, which decouples the recovered heat and feeds it to a low-temperature circuit (maximum 50 °C). Most of the heat is used directly from there. The other part serves as a return line for the high-temperature circuit (60-70 °C). This is fed from the waste heat from the engine (cooling water and exhaust gas). The engine in turn drives the compressor via a direct coupling, which means that the heat pump and the engine always provide heat at the same time. The internal cooling circuit for the engine is always maintained at a constant operating temperature. The decoupled temperatures depend on the requirements made by the building control system. The compressor can be operated with a partial load as well as with a 20% variable speed.

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. An 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.

Projektinfo 09/2017:
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Addresses

Energy concept and Monitoring
Fraunhofer IBP

Prototype heat pump
BLZ Geotechnik Service GmbH

Energy concept
Ebert-Ingenieure GmbH

Legionella protection
Hydrosystemtechnik GmbH

Link

Annex 50
Research project LowEx stock