Beneath Aarhus, more than 2,000 kilometres of district heating pipes wind their way through the city, heating almost 330,000 homes. Above ground, large cooling machines, server rooms and industrial plants release vast amounts of excess heat into the air every year. That energy could instead flow into the pipes and become part of the district heating system – but today we are only exploiting a fraction of the potential. 

In a new project, computer scientists and engineers from Aarhus University are developing algorithms that can analyse vast amounts of data from the district heating network. With these algorithms, the researchers can identify where surplus heat can replace costly energy production, where it can flow directly into thousands of Aarhus households, and how it can make its way through the tangled network of pipes and storage facilities so that not a single drop is wasted.

“We need to take a holistic view, unlocking the possibility to plan where energy creates the most value and thereby use surplus heat far more intelligently. Denmark has one of the most extensive district heating networks in the world, which enables us to conduct full-scale research that creates direct value in the everyday lives of the city’s residents,” says Massimo Fiorentini, Associate Professor at the Department of Civil and Architectural Engineering, Aarhus University.

“The main innovation of this project is that we are looking at the entire thermal network, not just individual parts. By zooming out, we can adapt methods that are originally designed for social networks to solve this fascinating engineering problem thus optimizing the energy flow in the whole network." Says Davide Mottin, Associate Professor at the Department of Computer Science, Aarhus University. 

The project - Recycling waste heat: Graph-based Algorithms for Spatio-temporal Optimization of Thermal Networks - is funded by the Villum Synergy programme, which supports collaborations between computer science and other areas of research.

From data to hot water in the shower

In the project, the Aarhus University researchers are working with the municipally owned utility company Kredsløb, which runs the city’s district heating and manages the vast underground pipeline network. Kredsløb has already mapped several areas with potential for using surplus heat and is providing the researchers with detailed operational data. The project will develop models that take real-world complexity into account, such as heat consumption variability, energy losses through pipes and capacity limitations of generation and storage facilities.

The knowledge gained from the project will be turned into a concrete tool to help Kredsløb – and eventually other utilities in Denmark – make better decisions about where and when it is most beneficial to use surplus heat from industry to ensure it doesn’t go to waste.

“This is an example of research that can create very tangible value for citizens. When they switch on the radiator on a cold winter morning, the heat will come from energy that would otherwise have been lost in industry. When the water hits their skin in the shower, it will be the result of our algorithms finding new ways for the energy to flow through the city’s pipes and storage facilities. That makes the project incredibly motivating for us,” says Steffen Petersen, Professor at the Department of Civil and Architectural Engineering, Aarhus University, collaborating investigator.

A digital map of the city’s potential

According to calculations by the Confederation of Danish Industry and Rambøll, there are around 3,000 gigawatt-hours of surplus heat from industry and businesses in Denmark every year. At present, we use only about 1,200. The rest corresponds to heating for 100,000 households, lost year after year – which could instead support our ambitions for the green transition.

“If we want to meet our climate targets, it is obvious that we should make use of the resources we already have. Developing solutions that allow us to identify and exploit, within the district heating system, economically viable surplus heat from our industries, is a clear low-hanging fruit,” says Massimo Fiorentini.

The researchers therefore plan to develop a digital map that can show where surplus heat is located, how it can be connected to the district heating network, and how it can be distributed to residents.