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Highlight Story - Stadtwerke Giessen, Germany
Green district heating for Giessen: PowerLahn project with large heat pumps from Johnson Controls
Stadtwerke Giessen (SWG) is setting an innovative example for the heating transition with the PowerLahn project. At its heart is a government-subsidized iCHP plant that combines three Sabroe DualPAC water-to-water heat pumps from Johnson Controls with two combined heat and power (CHP) units and a power-to-heat (P2H) plant. The aim is to achieve maximum efficiency, flexibility, and CO₂ reduction in district heating supply – commissioning is planned for mid-2026.
The heat pumps use the thermal energy of the Lahn River to feed around 29,000 MWh of emission-free heat into the grid each year – enough for around 3,900 households. With a COP of 3.9 and only 115 kg of ammonia (R717) as a refrigerant, they offer high efficiency and future-proofing. Their compact design (7 m long, 17 t weight) makes them ideal for large-scale use.
In summer, the heat pumps take over the base load, while in winter the CHP units kick in and generate an additional 50,000 MWh of heat and 47,000 MWh of electricity – with an overall efficiency of 95%. The P2H plant converts surplus green electricity into heat and stabilises the power grid.
With PowerLahn, SWG saves over 7,700 tons of CO₂ annually and demonstrates how local environmental heat can be efficiently integrated into existing infrastructures. The project is considered a model for sustainable district heating and offers both ecological and economic advantages – also for other municipalities.
Facts and figures about the CHP plant:
Component l: Sabroe DualPAC heat pumps:
- Output: 3 x 1,774 kWth
- Guaranteed operating period: April to October
- Emissions-free heat extraction: up to 29,000 MWh
- Apartments supplied: around 3,900
- CO2savings (compared to a gas boiler): 7,767 tCO2/a
Components ll + lll: CHP units and P2H module:
- Thermal output: 2 x 4,726 kWth
- Electrical output: 2 x 4,507 kWel
- Guaranteed operating period: September to April
- Heat generation: 49,995 MWh
- Electricity generation: 46,934 MWh
- Homes supplied (heat): approx. 6,800
- Average households (electricity): approx. 21,300
- CO2 savings (compared to turbine plant): 2,736 t CO2/a