Multiple Heat Sources

In the quest for a sustainable future, district heating emerges as a powerful ally to wind power, fostering efficient sector integration. As society seeks cost-effective green conversions for the heating sector, district heating stands out as the optimal driver, surpassing individual electrici-ty-based solutions. District heating offers flexibility and storage capabilities on both seasonal and daily scales, benefiting power and heating systems. Its true strength lies not in isolated components like heat pumps or electric boilers but in the seamless integration of infrastruc-ture, diverse heat sources, and intelligent operations.

By Hanne Kortegaard Støchkel, Researcher and Project Development Manager, DBDH
Lars Gullev, Senior Consultant, VEKS
Jesper Koch, Head of Analysis, Grøn Energi

Published in Hot Cool, edition no. 6/2024 | ISSN 0904 9681 |

Today, in the EU, 33-40% of our total energy consumption is used to heat our buildings and produce domestic hot water. In 2021, more than 75% of this energy consumption was based on fossil fuels such as coal, oil, and natural gas.

This situation is not sustainable, so the energy consumption for space heating must be reduced, and fossil fuel use must be converted to renewable energy. Electrification is often mentioned as the solu-tion, but it is not so simple if it must be done intelligently simultaneously. Here, district heating often will be the intelligent solution compared to the individual solutions.

Goodbye to silo thinking and hello to system integration

The simple solution can often be the biggest obstacle to the intelligent solution:

• with a sole focus on making the buildings more energy efficient, the consequence is that a more sustainable energy supply for space heating is not ensured.
• If the focus is solely on electrifying space heating, the opportunity to utilize large amounts of renewable energy and surplus heat from industry and data centres is lost. At the same time, there is an eminent risk of investing too much in expanding the power grid and increasing power production capacity.
• Massive expansion of power production capacity based on wind and solar requires system integration between the power system and district heating. That way, inexpensive excess power production can be cheaply stored in the district heating system. It is, therefore, neces-sary to leave the historical vertical silo way of thinking, where energy sectors are handled separately. Instead, we need to think horizontally to integrate sectors and use opportunities such as the co-production of heat and power (CHP), DH heat pumps, heat storage, and utili-zation of excess heat from hydrogen production.

System integration—also known as sector coupling—is only possible if strategic energy planning is prioritized. This is precisely what has happened with the latest Directive on Energy Efficiency EU2023/1791, which introduces an obligation for municipalities/cities with more than 45,000 inhabit-ants to draw up plans for heating and cooling.

The report “Optimal Electrification of Space Heating – Comparison of District Heating and Individual Solutions” – prepared by Grøn Energi in 2020, is admittedly a few years old and focuses on Danish conditions. However, the challenge addressed is still relevant both in and outside Den-mark.

How do we intelligently convert our space heating to renewable energy?

Electrification of space heating – three alternatives

If the heating of buildings is to be based on electricity, this can be done in three alternative ways:

• Direct electric heating, such as electric underfloor heating and electric radiators.
• Individual heat pumps in the individual homes (air/water or air/air).
• Electrification via district heating (large heat pumps).

But – as documented below – the two individual, electricity-based solutions challenge the power system:

• The low energy efficiency increases the need for further expansion of green power produc-tion. Electrification via electric heating increases the need, while district heating provides the lowest need for new, green power production, such as new offshore wind farms.
• The load on the power system is high, especially in winter. Electric heating has a significantly higher load on the power grid than district heating with large, electrically driven heat pumps.
• Electricity consumption is inflexible and does not match wind and solar power production; seasonal variation and faster variation on an hourly/daily level are challenges. The individual solutions put pressure on the power system when it is cold and the power production from wind and sun is low (e.g., in windless winter weather).

The report mentioned earlier in this article is based on a standard Danish house of 130 m2 with an-nual energy consumption for heating and domestic hot water of 18.1 MWh (see Fig. 1). The figure shows that district heating is the superior solution for heating the house.

Figure 1: Three ways to heat the same house. Electricity consumption for heating a standard house when heated respec-tively with electric heating, individual heat pump, and district heating partly (70%) based on a large, electrically powered heat pump. The curves are based on monthly averages.

Not surprisingly, electric heating consumes the most electricity because all the energy must come from electricity. The individual heat pumps consume less electricity, as part of the energy is obtained from the outside air. However, the outside air is cold in winter, so the heat pump is less effective when the heat demand is highest.

The large heat pumps in district heating can use a wide range of different heat sources, which are unavailable for individual heat pumps. These include heat sources with higher efficiency delivering a stable temperature all year round – e.g., groundwater, geothermal energy, or surplus heat from in-dustry. This means that the efficiency of the district heating heat pumps can also be high in winter.

Energy efficiency is relevant both to the need for expanding renewable power production—for exam-ple, in the form of new offshore wind farms—and to the expansion of the power grid. Without focus-ing on energy efficiency—reducing energy consumption—the green transition will simply be too ex-pensive for society.

There is a difference in how much the various alternatives strain the power grid in winter. Electric heating and individual heat pumps, respectively, consume 7—and 3-times more electricity than dis-trict heating when compared to the monthly average consumption for February (Danish conditions).

In addition, individual solutions are less flexible, as electricity consumption directly follows the current outside temperature and does not allow for district heating for storage and alternative heat produc-tion that is not based on electricity. Therefore, individual solutions can pressure the power system when it is cold, and the power production from wind and sun is low (e.g., windless winter weather).

The seasonal differences between power production and power consumption are also important. Fig. 2 shows numbers from the Danish example for the summer period (April-September) and the winter period (October-March). The results illustrate that district heating is also a better match for the sea-sonal wind and solar power production than electric heating and individual heat pumps.

Figure 2. Seasonal distribution of production from wind and solar power compared to power consump-tion for heating based on electric heating, individual heat pump, and district heating partly based on a large, electrically driven heat pump (from Danish analysis).

Significantly more electricity must, therefore be moved and stored if the electricity consumption is in individual homes rather than in large heat pumps in the district heating system where surplus heat production from the large heat pumps can be stored in large heat storages as Pit Thermal Energy Storages (PTES)

District heating’s electricity consumption has smaller seasonal fluctuations than that of electric heat-ing or individual heat pumps and thus fits better with electricity production from wind turbines. Gen-erally, space heating does not fit well with power production from solar cells, but district heating does fit better than individual heating.

Comparison of consequences – types of electrification

Sometimes, you come across the idea that heating in Denmark should be completely electrified. However, this is far from the most efficient solution, neither technically nor financially.

Why not?

The answer is partly given in Fig. 3, which shows the total Danish electricity and heat consumption in 2018. The heat demand is almost 100% larger than the electricity demand, and the figure clearly illustrates how a complete conversion to electricity-based heating is not realistic with the power sys-tem we have today.

Figure 3: Denmark’s energy consumption for electricity and heat, respectively, in 2018 (Source: Danish Energy Agency)

As the report and the arguments below illustrate, partial electrification via district heating will provide a much better synergy. Basically, the green conversion of electricity and heating is about creating cost-effective and robust systems that benefit the overall economy of society and customers. Here, renewable electricity from wind turbines and solar cells is not a goal in itself but a means on equal terms with other renewable energy sources, energy efficiency improvements, district heating infra-structure, technology development, and digitalisation.

Why district heating goes best with green electricity

Of the three ways of using electricity for heating, the district heating solution uses the least electricity. It is most compatible with a green transition, where power production from the sun and wind plays a major role.

Below are several arguments for why electrification of space heating is better via district heating and? gives a more effective green transition compared to individual electric heating and heat pumps.

• District heating ensures faster green conversion. District heating is already driving the green transformation of space heating.
  When a district heating company switches to renewable heat production, it immediately af-fects the heat consumption of all district heating customers. This happens, for example, through an ongoing replacement for greener and more efficient plants and fuels. It does not require an investment or conversion on the part of the individual heating customer, which is also why the conversion can go faster via district heating.
  District heating has the potential to implement the green transition quickly and as part of an overall integrated solution.
• District heating has a better seasonal match with power production from wind and solar and less strain on the electrical system. This means less need for network extensions and sea-sonal energy storage. There is less need for expansion of the power grids at both transmis-sion and distribution levels. Here, it also contributes positively that the district heating’s heat pumps can often be connected to a strong point in the electricity grid. Faster daily fluctuations in the power system are also handled better with electrification via district heating than with individual heat pumps and electric heating.
• High energy efficiency in district heating and lower electricity consumption in winter save in-vestments for the establishment of additional green power production.
• Only district heating’s large heat pumps can utilize heat sources such as excess heat, cleaned wastewater, groundwater, geothermal energy, and seawater. These energy sources are locally available, provide diversity in the energy supply, and do not fluctuate like wind and solar energy.

Summary

District heating is synergistic with wind power and delivers good sector integration. If the green con-version of the heating sector is to be cost-effective for society, district heating must be chosen as the driver instead of individual electricity-based solutions.

District heating provides access to flexibility and storage on both a seasonal and hourly/daily level, which benefits both power and heating systems.

District heating’s heat pump or electric boiler does not deliver this alone. Good sector integration comes from the overall combination of the district heating infrastructure, large heat pumps, good heat sources, electric boilers, other heat production, storage, and intelligent operation.

For further information please contact:
Hanne Kortegaard Støchkel, hks@dbdh.dk or Lars Gullev, lg@veks.dk