Think big! Large heat pumps for climate-neutral district heating networks

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February 16, 2024

Pilot projects with Vattenfall in Berlin and MVV in Mannheim show us new ways forward

 

Take electricity, ambient heat, and proven technology and generate more usable heat from them. To heat a house, for example, or an entire housing estate. Or for industrial purposes. This is precisely the principle and the appeal of heat pumps. 

How exactly does a heat pump work? 

Put simply, it uses electricity to raise the temperature of a low-temperature heat source to a higher, more usable level. This is how considerably more heat can be generated with the same amount of electricity than by the direct conversion of electrical energy into heat, as is the case with electric heating. The heat can then be used directly on-site or stored if needed, or it can be distributed via a heating network. If the electricity used to operate the heat pump comes from renewable sources, the heat is generated in a completely carbon-neutral way, because no fossil fuels like coal, oil, or gas were used. 

The way large industrial heat pumps work is essentially the same as the models used in homes. They work in a similar way to refrigerators, although unlike refrigerators, the aim is of course to generate heat rather than cold. They can extract heat from a heat source and raise it to a higher level using a compressor, condenser, evaporator and a special refrigerant.

Siemens Energy has been building large heat pumps based on this principle in Finspang, Sweden, since the 1980s. So far, they’ve been used mainly in Scandinavia. About 50 units have been delivered to date, with a total of more than six million cumulative operating hours. The ongoing development of the pump components and modern refrigerants have ensured that the efficiency of the heat pumps has increased significantly in recent years. Today large heat pumps play an increasingly important role in decarbonizing the district heating networks, that is, in zero-carbon heat generation.

The first district heating networks were built in the U.S. in the 1870s, including in New York. In Germany, a centralized supply of hot water and heat to households started being delivered at the beginning of the twentieth century. The heat energy can reach temperatures of up to 100° Celsius and is supplied by an underground pipeline system. This means that residential buildings supplied with district heating don’t need their own heating system. There are currently about 1,400 district heating networks in Germany, and they account for roughly nine percent of the heat supply.

Germany: More district heating, fewer emissions

District heating networks also account for less than ten percent of the supply worldwide. In Europe, the landscape is very heterogeneous: The share of district heating varies between less than one percent – for example, in Belgium, Spain, and Ireland – and up to 50 percent in the Scandinavian and Baltic countries. 

In Germany, changes to the Building Energy Act (Gebäudeenergiegesetz) came into force on January 1, 2024, with new requirements for the future climate-neutral heat supply for residential buildings. The Act specifies that district heating networks are an important building-block of the "heating transition." However, this will only be feasible if the central generation of heat is CO2-free – and large heat pumps make this possible.

How does the heat get into the grid?

Around two-thirds of the power plants currently used to "fire" district heating networks in EU countries generate both electricity and heat (combined heat and power: CHP). The proportion of fossil fuels used to generate thermal and electrical energy is correspondingly high: Natural gas is the dominant energy source at about 30 to 40 percent, and coal is still used in more than one-quarter of the plants. Other fuels include biomass and biofuels, and waste incineration plants also supply thermal energy for district heating networks in some places today.

As an alternative or supplement to these sources, large modern heat pumps can provide large quantities of heat for feeding into the networks with little use of green electricity. Siemens Energy is currently involved in three very different projects.

CONNECT4: Heat pump quartet at Vattenfall in Berlin

Four large heat pumps will begin operating on the site of a former coal-fired power plant in German’s capital Berlin over the next few years. Along with Berliner Wasserbetriebe (BWB) and Berliner Stadtreinigung (BSR), Vattenfall Wärme will invest over 200 million euros in sustainable energy and heat generation at the site.

The large heat pump system from Siemens Energy – with an average thermal output of 75 megawatts – will use the purification stage of the Ruhleben sewage treatment plant as a heat source. Depending on weather conditions, the water temperature there is between 13 and 27° Celsius. The heat pump cools the wastewater to a temperature as low as 15° Celsius and uses the energy obtained to supply district heat in a temperature range from 80° to 95° Celsius. Around 45,000 households will be supplied with climate-safe district heating after commissioning. Vattenfall expects annual savings of approximately 50,000 tons of CO2 emissions, which is roughly equivalent to the average emissions from 36,000 mid-range cars over the same period.

Everything in flow: MVV uses heat from the Rhine

In October 2023, MVV Energy AG commissioned Europe's largest river heat pump in the German city of Mannheim. The municipal energy and heating supplier uses the river water of the Rhine as a source to generate climate-neutral district heating for the equivalent of 3,500 households. The thermal energy from Germany's largest river will be sufficient to generate this energy for Mannheim's district heating network, even in winter when water temperatures are in the single-digit range.

The river water is fed underground to the heat pump system. Thermal energy is extracted there via the heat exchanger and fed into the closed refrigerant circuit. Water from the return flow of the district heating network can be heated from 60° up to 99° Celsius. It’s then either fed directly back into the network or temporarily stored in a heat storage tank with a capacity of approximately 1,500 megawatt hours.

Turning cold into hot

A large heat pump from Siemens Energy will also be used at Vattenfall's cooling plant at Potsdamer Platz in the future. The waste heat from a refrigeration system will be harnessed with the help of a high-temperature heat pump and fed into the city's heating network.

More information on this topic can be found in a recent study by Agora Energiewende and Fraunhofer.