The path to emissions-free district heating in Denmark

Nearly two-thirds of Danish homes are heated through district heating, networks of pipes that distribute heat underground to warm buildings. As yet, two-fifths of the heating energy mix still comes from fossil fuels, with biomass making up the rest. But replace the thermal generators with heat pumps powered by renewable electricity and heating becomes emissions free

District heating plants in Denmark produce heat and electricity for 1.7 million households, 64% of all Danish homes. Some 40% of the energy mix in the country’s district heating system is, however, fossil fuels, mainly coal and gas. With a national goal to be fossil free by 2050, something has to change. The answer is to replace thermal generation with large capacity heat pumps of 20 and 150 megawatts (MW), powered by electricity from renewable sources of energy, says Siemens.

”We have the infrastructure already, it is fully paid for,” says Knud Brandelev, Copenhagen-based sales manager at the German manufacturing firm. “We now need to get electricity from wind turbines into the district heating system.”

Heat pumps powered by renewable electricity are highly efficient. They provide a coefficient of performance factor — the relation between the energy input needed to operate the heat pump and its energy output — of over three and in some cases up to four. In comparison, today’s energy mix of fossil fuels and biomass in the Danish district heating system has a factor of below one, says Siemens.

“In Denmark, we are really proud that around 45% of our electricity consumption comes from wind turbines. But if you look at total energy consumption, electricity, heat and transport, then it is just 8%,” says Brandelev. “This is not good enough.”

The key to a cleaner overall energy mix is to increase the role of renewables in provision of heat. “With more and more electricity coming from renewable sources we should use it in the best possible way, that is in the district heating sector, where you get a high rate of efficiency,” he states.

Biomass and waste water

A large number of Danish district heating plants have, in recent years, been converted to biomass-fired power plants. “Biomass is a transition stage solution,” says Brandelev. “It was fine to use when we decided to move away from coal, but now we must progress to the next stage. And then perhaps use biomass for something better, jet fuel for example.” He points out that when biomass is burned, carbon dioxide (CO2) is emitted into the atmosphere.

The first step is to replace the country’s remaining coal-fired power stations, which account for 13.7% of the energy mix, with heat pumps, then the gas-fired ones (22.9%) and finally those that run on biomass, says Brandelev. He cites Esbjerg and Aalborg as two cities where coal-fired plants still power district heating. “We think it is obvious, in both cases, to install heat pumps as the cities are close to the sea,” says Brandelev.

For large heat pumps to work in district heating, a heat source is needed such as waste water, seawater or air. Geographically, many of Denmark’s district heating plants are located almost perfectly, close to the sea, near a wastewater treatment plant or close to other sources of energy, such as surplus process heat. “If there is no access to sea water, then air or even groundwater can be used as an energy source,” explains Brandelev. The use of groundwater is increasing with more companies beginning to drill for warmer water, he adds.

Pay-back time

The volume of investment needed will depend on the size of the heat pump and the heat source used. The existing district heating infrastructure is already paid for. It is as good as new and will require very little work to make it fit for heat pumps, says Brandelev. Siemens has drawn up different scenarios using heat pumps of 20 MW, 50 MW and 150 MW, using sea water or waste water. Based on these, it concludes that the average pay-back time is 6.2 years. “Six years is not a long time, especially in the utility sector, which is used to a long-term planning horizon,” says Brandelev.

Converting district heating from fossil fuels to run on heat pumps powered by renewable electricity will essentially make the system emissions free. Based on data from 2016, Siemens estimates the changeover would stop 2.6 million tonnes of CO2 being pumped into the atmosphere a year. The Danish District Heating Association reckons that emissions were already lower than this in 2018 at 2.4 million tonnes, down from 3.5 million tonnes in 2008, because district heating in many areas has already switched away from coal.

Swedish example

The use of large heat pumps in district heating is nothing new. In Sweden, it has been the case for 30 years, supplying as much as 50% of Stockholm’s heating.

The oil crises in the 1970s forced European countries to look for new energy sources. Denmark became self-sufficient with oil and gas from the North Sea and decided to develop wind power. Sweden opted for nuclear power. Today around 80% of its electricity production comes from nuclear and hydro, with 11% from wind and 9% from biomass. As the 1980s dawned, with an abundance of electricity available and a desire to move away from oil in its heating system, Sweden plumped for heat pumps. Since then, Siemens has built 50 heat pump machines in the country, from Lund in the south to Örnsköldsvik in the north, including 13 in Stockholm “and they’re still running”, confirms Brandelev.

In most of the rest of Europe, however, “there is a big hole in our frame of reference” for heat pumps, says Brandelev, with interest only starting now to pick up. Nonetheless, he sees significant potential for large heat pumps in other European countries as they decide how to decarbonise their energy systems. He cites Eastern Europe, where district heating is widely used, but is still mostly run on coal.

“Germany is another ideal place because of the planned phase out of nuclear power, while coal-fired plants will be replaced with renewable energy and Germany has large cities with district heating,” says Brandelev. “I think Germany will see the same picture as in Denmark, just some years later.”

This story by Karin Jensen, originally appeared at FORESIGHT Climate & Energy and is a part of our series examining how to decarbonise heating and cooling systems