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Valuable flexibility can be exploited by efficient heating and cooling systems in urban areas
In the 1920s, the Parisian heat network burned coal to provide steam to pre-heat the trains leaving Gare de Lyon station. Over time, the network grew into a district heating system that delivers heat to around 20% of the city. With its reliance on burning household waste, fossil gas and biomass to deliver very hot steam at greater than 200°C through old and leaky pipelines, however, the system is stuck in the past. By contrast, in Saclay, a suburb in southwest Paris and home to one of France’s most high-tech university campuses, a new vision for district heat is emerging. Clean, efficient and smart, it shows that heating (and cooling) networks can be a key piece of the puzzle of how to decarbonise our buildings. District heat can potentially utilise a huge amount of excess, ambient and renewable heat, especially in urban areas with high-density heat use. At the same time, it can benefit the electricity grid by providing much-needed flexibility through power-to-heat technologies and thermal storage.

CLEAN HEAT
In a district heating system like Paris’s main network, heat is often produced at a few central locations and then transported by pipelines to the users. Although not the most energy-efficient process, this works well when producing high-temperature heat from big plants burning fuels or when using waste heat from industry. Most clean, low-carbon heat sources, on the other hand, produce heat of lower temperatures, which would need to be upgraded to higher temperatures to transport over longer distances due to the losses that occur during distribution. It is therefore more efficient to use the heat as close to the source as possible. In Saclay, the local authorities opted for a decentralised approach that makes the most of the deep geothermal source they have available to provide heating and cooling. Low-temperature heat of approximately 30°C is pumped up and fed into a pipeline system that connects clusters of buildings, each with its own heat pump to upgrade the heat to the required temperatures (>60°C). One additional benefit: whenever the heat pumps produce heat, they also produce cold, which is fed back into the network and used for cooling processes. Over time, new locally available sources will be added to the network, such as waste heat from cooling processes in laboratories and a data centre. When fully operational, the Saclay district heating system aims to run on 60% renewable, waste and ambient heat. Another 36% will come from electricity used in the heat pumps, using fossil gas (4%) only in winter peak times. Compared to a system using gas boilers, the system’s CO₂ emissions are four-to-five times lower, and, even before the gas price spikes, users paid less for their heat.

STORAGE, FLEXIBILITY AND SMART MANAGEMENT
Storage is essential for clean energy systems to efficiently match demand with supply. District heat can contribute to this. Thermal energy storage, which usually entails putting hot water in a tank, pit or aquifer, is cheaper than storing electricity in batteries and gives heat networks the necessary flexibility to integrate renewable and waste-heat sources. For instance, systems can save the excess industrial or solar thermal heat available in summer for use in winter or can allow heat pumps to produce heat at times of the day when renewable electricity is plentiful and cheap saving it for when demand is highest. The Saclay system uses the heat retained by the well-insulated pipes and buildings as storage, providing valuable flexibility. Combined with the fully digitised and automated operation of a network relying on smart meters, heating and cooling production can be more easily matched with demand, maximising efficiency. With the planned addition of solar photovoltaics to the rooftops of the connected buildings, smart management and thermal storage will help make the best use of the electricity generated, running the heat pumps whenever the sun is shining and storing the heat until it is needed later in the day. As an added benefit of digitised operations, leaks and faults can be detected and fixed more easily, quickly reducing system losses and downtime.
THE CHALLENGE AHEAD
The Saclay system shows that clean, smart and efficient district heat can play an important role in decarbonising our buildings, allowing operators to tap into largely unused sources of renewable and excess heat while providing important flexibility and storage benefits by coupling the heat and electricity sectors. The Paris systems represent different generations of district heating. The old system is one of the few remaining that still uses steam as a heat carrier. The Saclay heat and cold exchange network, by contrast, is of the newest generation, combining low temperatures and efficient infrastructure with digitised and smart operation—and largely relying on heat that is renewable or would otherwise go to waste. Yet, this juxtaposition also shows the challenge to realise the great potential of district heat. There are hundreds of smaller and larger district heating systems across Europe that will need to be modernised and decarbonised. At the same time, many areas currently hooked up to a gas grid will have to shift to clean sources, including through newly built district heat. •

Sem Oxenaar is delving into the future of district heat in a three-part series. Keep an eye out for part two coming soonIf you have a thoughtful response to the opinions expressed here or if you have an idea for a thought leadership article regarding an aspect of the global energy transition, please send a short pitch of 200 words outlining your thoughts and credentials to: opinion@foresightdk.com.