• Artist impression of the Lemgo plant.
    Artist impression of the Lemgo plant.

By supplying two heat pumps, GEA is helping the city of Lemgo in Germany achieve climate neutrality by 2035.

The GEA technology will generate heat from the effluent of the city's wastewater treatment pla

GEA heat pumps use ammonia.
GEA heat pumps use ammonia.

nt, waste heat from a large combined heat and power (CHP) plant and from the River Bega.

The heat generated by the large heat pumps is sufficient to supply the historic city centre of Lemgo with low-carbon heat as far as possible. That's around 18 million kilowatt hours of heat per year, which saves 3,200 tonnes of CO₂ emissions per year.

The heat from the wastewater treatment plant's effluent alone can cover two thirds of the old town's heating requirements or 12 per cent of the district heating network's total demand. The operating strategy ensures that the heat pumps can operate constantly for up to 7,500 hours per year to maximize the benefits of the heat pumps.

The recovery of heat from the waste water of the municipal central sewage treatment plant in Lemgo forms an important part of the municipal district heating generation. The clean water at the sewage treatment plant outlet is highly suitable.

The treated wastewater has an average annual temperature of around 13° C. Due to the mixed water system, large quantities of water (equal to quantities of heat!) are available. More than 1.5 MW of source power is available for almost 8,000 h/a.

Furthermore, infrastructure such as a combined heat and power plant and district heating connection are available at the sewage treatment plant. Heat is only extracted downstream of the treatment stages at the outlet of the sewage treatment plant.

This enables the highest possible amount of heat to be generated with relatively little technical effort. The wastewater treatment process is also not affected by the heat extraction.

Upon commissioning, it was discovered that the water quality of the heat source from the sewage treatment plant did not meet the original purity expectations.

The resulting contamination of the evaporators meant that the "pure water heat pump" could no longer continue to operate.

The task now was to find a sustainable solution. Of the options considered, joint deliberations between Stadtwerke Lemgo and GEA rejected the possibility of an additional separating heat exchanger or the conversion of the heat pump and tube bundle evaporator with a complex self-cleaning system. The decision was made in favor of an additional filtration system, which was then implemented.

The fact that the GEA heat pumps are also operated with the natural refrigerant ammonia is another plus point in terms of environmental protection. Ammonia is an inorganic compound of nitrogen and hydrogen, lighter than air and easily soluble in water.

The COP (coefficient of performance) of an ammonia heat pump operating under typical conditions for a district heating network or for process heat below 100 °C, for example, is 40 per cent higher compared to synthetic refrigerants, which means 40 per cent fewer emissions, 40 per cent less energy and 40 per cent lower costs.

By 2028, more than 55 per  cent of the district heating required is to be generated from renewable sources. The Lemgo team is relying on a large-scale solar thermal system, another large heat storage tank with an integrated PtH and air heat pump.

These will be connected by direct cable to a wind turbine, which is also in the planning stage, in order to generate wind heat.

In addition, a wind power plant will contribute to hydrogen production - further investments are planned in a biomass boiler and a wood gasification plant for road side wood in order to achieve the goal of climate neutrality.

The project includes the GEA RedAstrum 2.0 which is now available with seven screw compressor types based on the successful GEA Grasso M and GEA Grasso LT series in a specific high-pressure design.

Plus, the GEA RedGenium heat pump which transfers its heat output to a liquid heat transfer medium and delivers flow temperatures between +55 and +80 °C for any industrial application requiring process heat or serving local or district heating networks.