by Linda Bertelsen
District Heating Stepping Stones

Focus on supporting a green transition does not necessarily mean that the only right path is to transition 100% from fossil fuels to renewable energy in one go. Nevertheless, carbon neutrality or CO2-negative emissions is the goal – and there should be a plan to reach it.

By Lars Gullev, Senior Consultant, VEKS

Published in Hot Cool, edition no. 6/2023 | ISSN 0904 9681 |


Transitioning a city from, for example, natural gas to green heat sources is a massive undertaking. This article argues that the goal of CO2 neutrality is right but that the path to it consists of a series of stepping stones, each contributing to the goal, with cities gradually reducing the use of fossil fuels over time. We must be careful not to let the dream of having the perfect system in a few years hinder us from taking important steps in the right direction as early as tomorrow, bringing us to our goal within the set time frame.

The primary goal must and should be to ensure a complete reduction in CO2 emissions. This can be achieved by transitioning from fossil fuels such as coal, oil, and natural gas to sustainable biomass, solar or wind-based electricity, industrial waste heat, or waste heat from waste incineration plants. A significant and essential first step is to transition smaller individual fossil-based systems to district heating partially based on coal, oil, or natural gas – as a starting point.

As mentioned, this article describes that just because you cannot transition to 100% CO2-neutral heat production right here and now, it does not mean you should do nothing.

Previous investments in fossil-based production facilities will not be wasted. They can be used as a step or steppingstone towards a green heat supply, as the historical development of the Danish district heating sector confirms.

For clarity, the expansion is divided into phases, but the phases can overlap. Today, through careful planning, we can gain early access to introducing sustainable heat sources, thus more rapidly displacing fossil alternatives. Finally, it demonstrates how Copenhagen has gone through this development – and provides a hint at the next stone.

Buildings are individually heated with fossil fuels, such as natural gas.

A range of buildings are individually heated with their small boilers using coal, oil, or natural gas as fuel.

An alternative to this individual solution is to base the building’s heat supply on a common heat center and a district heating network that distributes heat to individual buildings. Ideally, this heat center should be based on sustainable biomass, but the next best thing would be a heat center based on fossil fuels.

The decision-making basis for such a decision – FID (Final Investment Decision) – will involve comparing the total costs (TOTEX) for heating in the individual scenario with the corresponding costs in the district heating scenario.

For both solutions, both CAPEX (capital costs) and OPEX (operating costs) need to be calculated before a clear picture of TOTEX is obtained.

When district heating was significantly expanded in Denmark in the 1960s, the price difference between the light gas oil used in individual oil-fired boilers and the heavy fuel oil used in large boilers drove the development.

The reduction in OPEX by establishing district heating could finance CAPEX in the pipeline network and the central boiler, so TOTEX for the district heating solution was lower than TOTEX for the individual solution. The development of district heating in the 1960s in Denmark was thus market-driven – today, such growth can also be based on a political demand.

Utilization of surplus heat

Now, buildings are supplied with district heating based on the next-best solution – coal, oil, or natural gas – with better fuel utilization than the previous individual solution. This has created the opportunity to utilize local surplus heat resources in the district heating system. An option that was not present when individual buildings were solely responsible for heating.

A significant portion of the surplus heat can now be utilized, perhaps 4,000 hours per year. The remaining heat demand still needs to be covered by coal, oil, or natural gas-fired boilers.

This reduces CO2 emissions in two steps – first, from the individual scenario to the district heating scenario based on fossil fuels, and further through the utilization of surplus heat, which reduces the use of fossil fuels.

The fossil-based boilers now shift from units that cover the base load of heat demand to units that cover heat demand in peak load situations and when there is a need to activate reserve capacity due to interruptions in heat supply from surplus heat providers.

Expansion of the district heating network

There is still surplus heat available that cannot be utilized because the heat demand in the existing district heating system is limited. Therefore, there is now a basis for expanding the district heating system to utilize more surplus heat – thereby further reducing the use of fossil fuels, both from new customers converted to district heating and from fossil-fired boilers in the district heating system.

The fossil-based boilers continue to cover heat demand when heat deliveries from surplus heat providers are insufficient to meet heat demand.

Establishment of green heat sources

Now that all surplus heat has been utilized, the district heating network continues to expand with new customers. The immediate consequence is that the proportion of heat produced by fossil-based boilers increases. This is not good for CO2 emissions. In reality, this effect is reduced by the early introduction of green heat sources while expanding the network.

Therefore, it is now relevant to investigate the possibility of establishing a new production capacity to reduce heat production from fossil-based boilers. This could involve production units based on sustainable biomass or heat pumps, solar, electric boilers, surplus heat from CCS/U (Carbon Capture Storage/Utilization), hydrogen production, or other local options.

Again, we need to look at TOTEX, CAPEX, and OPEX for such production capacity before deciding to build something new.

If it proves economically attractive to establish new production capacity, it will likely be integrated into the “hierarchy,” with heat from surplus heat producers having priority, the new production unit having second priority, and finally, the fossil-fired boilers having third priority.

The fossil-based boilers continue to cover heat demand when heat deliveries from surplus heat providers and the new production unit are insufficient to meet heat demand. In the long term, they will only serve as peak and reserve loads.

The overall district heating system benefits significantly from the flexibility of these fossil-based boilers.

Copenhagen walked the talk!

Is the above brief description of the possible development of a district heating system just theory? No – if we briefly look at the district heating system in the western part of Greater Copenhagen, the historical development here has been as follows.

In the 1960s and 1970s, 19 local, independent district heating companies comprised the district heating system. The district heating supply was based on fuel oil or coal-fired boilers and small local waste incineration plants.

In the late 1980s, many district heating companies were connected to a central district heating transmission network (VEKS – www.veks.dk), which supplied surplus heat from large central combined heat and power plants (coal-fired) and large waste incineration plants to the local distribution networks.

At the same time, the small local waste incineration plants were closed, and the local coal-fired boilers were shut down. The central boilers in the local district heating companies were retained – over time, fuel oil was phased out and replaced with light gas oil. Some boilers were equipped with dual-fuel burners so that they could use both gas oil and natural gas.

Today, only 2-3% of the district heating demand in the western part of Greater Copenhagen is covered by production from oil and natural gas-fired boilers, while the remaining 97-98% is covered by surplus heat from combined heat and power (CHP) plants, which now use sustainable biomass, surplus heat from waste energy plants, and surplus heat from industry. The CHP plants were converted from fossil fuels to sustainable biomass between 2008 and 2020.

Today, the originally oil-fired boilers still have a significant function in the overall district heating system. Their role today is not to cover the entire heat demand but solely to serve as peak and reserve load when it is very cold – or when the large CHP plants or waste-to-energy plants cannot meet the heat demand. Over the next few years, large electric boilers will replace some of the larger oil-fired boilers.


If there had not been an expansion of district heating in the western part of Greater Copenhagen based on coal and oil-fired boilers 50-60 years ago, the foundation would not have been laid for a well-functioning, energy-efficient district heating system in the western part of Greater Copenhagen in 2025, which will be based on 100% CO2-neutral heat production. The next step is to reduce biomass consumption and utilize more surplus heat from future sources – such as CCS/U – more about this can be read in this article describing Copenhagen’s heat plan up to 2050. [Link to article here]

The Copenhagen system demonstrates the stepping stones used back then. Now, we need to move ahead, stepping on similar stones without building new fossil-based production capacity. So, starting the green transition by connecting fossil-based customers to a fossil-based district heating system can be the first step on the “green journey.” Let’s not sit around waiting for all conditions to be perfect; there are several stepping stones, and we gather knowledge with every step we take.

Meet the author

Lars Gullev
Senior Consultant, VEKS
“District Heating Stepping Stones – towards a sustainable heat solution” was published in Hot Cool, edition no. 6/2023. You can download the article here:
District Heating Stepping stones - towards a sustainable heat solution