A key to making Power-to-X much more energy-efficient – and cost-effective – is district heating (DH). A key to making 100% carbon neutral DH is integrating excess heat from other sectors. Also, municipalities and governments are beginning to see that, together, heat and hydrogen can increase the speed and quality of the green transition.
By Hanne Kortegaard Støchkel, DBDH and Jannick Buhl, Danish District Heating Association
Report on integration of Power-to-X and district heating
This article is based on a comprehensive report on the integration of Power-to-X (PtX) and district heating (DH) in Denmark. The report is published by the Danish District Heating Association, the think tank Grøn Energi, COWI and TVIS.
PtX is closely tied to the green transition in the transport of goods by road, sea, and air and the production of carbon-neutral fertilizers for agriculture and carbon-neutral forms of products such as steel, plastic, and chemical products. This gives a rapidly increasing demand for green hydrogen in the coming year. This underlines the need to use green hydrogen where it is most needed and most valuable – and that is not for heating. For heating, much better alternatives exist already today.
In rural areas, this could be small, electric heat pumps. In more densely populated areas, the cheapest and most efficient solution is DH based on renewable heat source and reused surplus heat. Some argue that hydrogen should be used in households for heating, but it makes much more sense to combine PtX with DH and use the surplus from the hydrogen production for heating.
PtX processes involve energy losses in the form of heat, and infrastructure is needed to collect and use that heat. That is where DH systems come in (Figure 1).
Figure 1: PtX converts power into another energy source called ‘X,’ for instance, hydrogen, methanol, or aviation fuel. In energy conversions, a substantial part of the energy is converted into surplus heat – potentially to be utilized for DH.
A less known link between PtX and DH comes through the waste-to-energy (WtE) plants. The production of most e-fuels requires two things: green hydrogen and green carbon.
The hydrogen comes from electrolysis. Carbon, in the form of concentrated, green CO2, will become a challenge, but both WtE and biomass-based units for heat production are essential sites for collecting large quantities of CO2 for storage or use in e-fuels.
Don’t waste green energy
The forecasted electricity consumption for PtX is significant and so are the accompanying energy losses. Assessments on electrolysis show that usable waste heat makes up around 10-25 % of the energy input to the hydrogen plant. With energy efficiency becoming more and more important, energy losses on that scale must be investigated and converted into solutions.
Figure 2: Revenue for 20 MW electrolysis plant with a DH connection. In the report, the surplus heat at (70° C) is valued at 27 EUR/MWh during the winter and 20 EUR/MWh during the summer, whereas the surplus heat at 35° C is valued at 2 EUR/MWh in the analyses. The additional hydrogen revenue results from extra operating hours due to the DH revenue. Assumptions are described in the report.
Figure 3: The business sector needs green alternatives to replace fossil process heat. The report explains why surplus heat from PtX needs access to a DH system. Direct utilization for industrial processes and heating is difficult (top), whereas the integration can be made possible with district heating (bottom).
Faster and better green transition
Due to the climate challenges, we have to develop efficient solutions and implement them quickly. But risks and creating a good business case for hydrogen production are holding back PtX. By integrating PtX and DH, the owner of the hydrogen plant can sell the surplus heat to the DH company – generating revenue and increasing the competitiveness.
From the DH point of view, utilizing surplus heat fromhydrogen production can replace fossil-based heat production and provide energy for new DH areas currently heated by natural gas.
PtX plants require planning and cooperation, and potential use of surplus heat should be included as early as possible in the process.
Improving the business case for green hydrogen
The surplus heat from hydrogen production can contribute to the business case of electrolysis. If connected to a DH system, the heat has value and can be sold to generate revenue. The electricity price primarily sets The number of operating hours for the hydrogen plant is primarily set by the electricity price. Therefore, the revenue from the surplus heat is the additional effect of increasing the limit for when electricity prices become too high for hydrogen production. Figure 2 shows an example of the impact on the revenue of a hydrogen plant connected to a DH system.
District heating welcomes heat from Power-to-X
The heat from PtX is well suited for integration into a carbon neutral DH system. It could be a valuable green heat source with high temperatures, large volumes, and a high number of operating hours. This is why all the central DH systems in Denmark are investigating the possibilities for connecting to future PtX facilities. It is expected that all strategic energy plans and heat plans in these areas will discuss integration of PtX and DH.
A DH system will have several heat sources, and the value of the heat for PtX will depend on the other heat sources in the system. In the summer, heat is abundant and cheap, so the hydrogen plant will experience lower prices than during winter peak load. Other types of heat production may also depend on the electricity price. This means that the revenue of the surplus heat will vary hour-by-hour and season-by-season — something to include in the business case considerations.
Integrating heat and hydrogen benefits other sectors
PtX heat for DH promotes integration across sectors such as power, heating, transport, waste, industry, and agriculture. The results of this are increased energy efficiency, lower costs, and new possibilities.
An example of this is the challenges faced by industrial sites needing to convert from fossil-based to green process heat.
Direct electrification is one option, but if available, DH is often a competitive alternative. Figure 3 illustrates how waste heat from PtX can substitute fossil-based heat used in the industry if the heating infrastructure is or becomes available. This type of synergies is already known and used in Denmark, where WtE plants and DH systems are closely coupled and shows the power and potential of sector integration.
The recommendations in the report are grouped into four topics:
- The value of integrating DH and PtX is overlooked
- Investments in energy infrastructure are needed
- Development and demonstration – including a list of what is needed
- Planning, timing, and framework conditions}