In Europe today, heat for urban areas is often generated in central plants and distributed through insulated pipes. While this concept has been in place for decades, economic and environmental considerations have increasingly focused attention on improving both the sustainability and efficiency of this practice.

“District heating (DH) networks are nothing new,” said RELaTED project coordinator Antonio Garrido Marijuan, from Tecnalia, in Spain. “But we have become more aware of the need to stop wasting energy. For example, the temperature at which heat is delivered needs to be reduced to avoid thermal losses.”

There is also growing awareness that DH networks could make more use of renewable and waste heat sources. This could improve energy efficiencies and cut costs for the consumer, while reducing Europe’s dependence on fossil fuels.

Developing ultra-low temperature thermal networks

RELaTED set out to develop and test novel technical solutions that could help DH networks to achieve these objectives by operating at ultra-low temperatures (ULTs) of 40 to 45 degrees Celsius. One of the trickier elements of the project was integrating different technologies to deliver the expected benefits.

“To deliver heat to consumers at 40 degrees instead of 60 degrees or even 100 degrees, for example, means using heat pumps to reboost the temperature when you need it,” said Garrido Marijuan. “And because of rising outdoor temperatures, we also needed to make sure that heat pumps could be used for cooling as well.”

The project also developed prototype DH substations that are bidirectional. This means that factories, for example, which generate waste heat, can send excess energy to the grid while receiving energy in return when needed.

The concepts developed for this ULT DH network were then tested at four sites. These included a large network in Belgrade (Serbia), a new urban development in Vinge (Denmark), an operational network in Tartu (Estonia), and a corporate network in Iurreta (Spain).

Direct energy savings while reducing heat losses

Garrido Marijuan said the project successfully demonstrated that DH networks can indeed transition towards more sustainable and energy-efficient models of operation. The pilots achieved direct energy savings by reducing thermal losses, while the injection of waste heat into the network provided a revenue stream for businesses as well as additional heating for consumers.

The DH operator in Estonia discovered that the investment made in upgrading the network was paid back in a mere two years. Temperature reductions achieved thermal savings in the range of 400 MWh/year for Tartu, which translates into a 20% reduction in heat loss.

The operator has plans to roll out the technology across its whole network, and some other regional administrations and large factories are in talks with the DH operator to develop new, energy-efficient DH network plans.

“Making such energy savings is critically important, because buildings account for about 40% of Europe’s energy consumption – and heating accounts for a large part of this,” he added. “Lower operational temperatures and bidirectional flows could also allow for a massive uptake of renewable energy and waste heat sources, in addition to reducing thermal losses.”

The next-generation ULT DH networks pioneered in the RELaTED project could therefore be a key part of Europe’s green transition. If this technology is replicated on a large scale across Europe, then the energy savings could be significant.

“This project is only a small step forward, but it is very much in the direction that we need to take DH systems,” said Garrido Marijuan. “To make energy savings, the operational temperature of DH networks will have to be lower. And we have shown what could be part of the solution to this.”