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Buildings as thermal batteries in a transition towards a green energy system

Denmark is to be climate-neutral by 2050. This requires major changes in our utilities sector, and buildings will play a significant role in this context. Three PhD students at Aarhus University talk about their research to help Denmark meet targets in its climate plans.

[Translate to English:] ”Danmark er helt i front inden for forskning i fjernvarme. Med vores projekter vil vi demonstrere, at Danmark kan være et af de første lande, der på en økonomisk fornuftig måde udnytter den eksisterende boligmasse i fjernvarmesystemer som varmelagre," siger ph.d.-studerende Virginia Amato (tv). På billedet ses også Thea Broholt (midt) og Louise Christensen (th). Foto: AU Foto.

Danes love district heating. It has been in Denmark for more than 100 years, the district heating network is 60,000 kilometres long – one-and-a-half times around the Earth – and 64 per cent of all Danish households are currently supplied with district heating.

However, fossil fuels are the primary source of energy for district heating production. Only approx. 52% of the heat comes from renewable energy sources such as the sun, wind and geothermal sources.

Furthermore, 40 per cent of the overall energy consumption in Denmark is from heating.

If Denmark is to fulfil the government's climate action plan, with its target of climate neutrality by 2050, and a heating sector powered solely by green energy, the district heating supply will play a central role in phasing out fossil fuels.

Three PhD projects from Aarhus University could be a key element in the government's climate plan and in the work of solving this problem at district heating plants.

"Just as we use batteries to store surplus energy in the electricity grid, we want to use the thermal mass in buildings to store heat that can be regulated using an intelligent heating management system," says PhD student Louise Christensen, who is working on user behaviour and adaptation in relation to the new system.

Buildings as batteries

Heat can be stored in the building mass, furniture and even in the indoor air. Particularly heavy building materials such as concrete and bricks have high heating capacity, and a house could function as a thermal battery that can store heat from the district heating network.

In periods when energy consumption exceeds the production of green energy, it is necessary to power up the 'black' boilers. If it is possible to store green energy for times when energy consumption is high, it will be possible to avoid using fossil-based energy.

Therefore, the aim is to heat up buildings a little more before peak periods so that room heating can be turned off and we avoid using energy for heating during peak periods.

Louise Christensen has a master’s degree in building engineering within energy and indoor climate, as do her two PhD colleagues, Thea Broholt and Virginia Amato, who are both researching into developing a management system to achieve heating-flexible housing.

The aim is to make the district heating grid’s heating management system intelligent, flexible, environmentally friendly and affordable.

As things stand now, consumers regulate their space heating thermostats based on their own preferences for when a room is to be warm and how warm it is to be. This means that room heating consumption is more or less constant and therefore a foundation for peak loads, for example in the morning when many use hot water for showers.

Flexible heat consumption

However, in the future, district heating plants will not be able to use fossil fuels to cope with peak periods, so heat consumption needs to be more flexible to prevent peak loads from arising.

The purpose of a new heating management system is therefore to flatten out these loads, and the plan is to let an automatic system manage heating consumption for consumers.

In this system, occupants will simply define their personal comfort temperature interval, and then the intelligent management system will ensure that they are always comfortable with the most sustainable energy consumption.

"We’re used to setting our thermostat at a fixed, comfortable temperature, but in the future an intelligent system will adjust the temperature within a desired comfort range. By allowing this, we can use large amounts of data on individual heating requirements, indoor temperatures, weather and price signals to provide district heating plants with knowledge about how a given home should be managed to move energy demand away from peak-load periods," says Thea Broholt, who, through her research, is developing and programming the management system.

Smart management of energy consumption will ultimately reduce prices for consumers, because the district heating plant will regulate its prices according to the efficiency of the district heating system as a whole.

"The intelligent heating management system will minimise heating costs for the consumer by adding extra heat to buildings when the price of heat production is low, so that the stored heat in the building mass can keep the indoor climate comfortable without using energy when prices are high," says Thea.

Denmark at the forefront of research

Within the past ten years, research of the storage of thermal heat in buildings has progressed, but mostly at theoretical level. As a new initiative, the three PhD students will research in realising the theoretical potential in practice, and there is a need for concrete research results, so that the systems can be developed and tested, and later contribute to the green transition.

"Denmark is at the forefront of research in district heating. With our projects, we will demonstrate that Denmark could be one of the first countries to exploit the existing housing stock economically as heat storage in district heating systems to allow for increased integration of more sustainable sources of heat," says Virginia Amato.

“The research-based demonstration is still in its early stages, but it is expected that our projects will contribute to ensuring that smart management technology can be implemented on a large scale,” explains Virginia.

A fully integrated, intelligent management system requires many studies on various fronts. In addition to the smart management technology, it is also important to ensure satisfied and comfortable users.

"We need to take user preferences and behaviour into account in order to make the smart management system work. In fact, we expect that the users of the smart management system will either not notice it or will experience improved thermal comfort. However, demonstrating this will require experimental studies in people's homes, and these are already well underway," says Louise.


Louise Christensen’s project, ’The effect of building occupant preferences and behavior on model predictive control of space heating‘, is in collaboration with Flexible Energy Denmark and is financed by the Innovation Fund Denmark.

Thea Broholt’s project, ’Model Predictive Control of Residential Heating Systems in Practice’, is a part of the project Heat 4.0 and is financed by the Innovation Fund Denmark.

Virginia Amato’s project, ‘Model Predictive control of residential space heating systems’, is a part of the project PreHeat, partly financed by EUDP and in collaboration with the private company ReMoni.


The PhD projects will run to 2022.