Article by: Riccardo Battisti

How to decarbonise existing district heating and cooling networks, in line with the intermediate and final targets of Article 26 of the European Efficiency Directive?

The ENABLE DHC project, supported within the LIFE Clean Energy Transition programme, is developing investment plans for decarbonising 9 networks in 7 countries, through a massive use of renewable energy sources, waste heat recovery and low-temperature heat supply. The detailed descriptions of the 9 case studies, as well as of the planned intervention measures, are available here. Now, let’s go deeper into some of the concerned systems…

Geothermal heat pumps replacing gas

The district heating system in Acqui Terme (Italy), which started operation in 2009, supplies heat by two cogeneration units (CHP) in parallel and three boilers normally activated when the heat demand exceeds the CHP capacity. The system does not feature thermal storage. In 2023, the system generated 34.8 GWh of thermal energy, primarily using natural gas (5.8 million Sm³), resulting in approximately 11,500 tons of CO₂ emissions. The 8.9 km long (12.8 km including connections) network serves 180 substations, mainly residential units and some tertiary buildings, and the total heat sold to consumers amounts to 34.2 GWh/y.

The Enable DHC project is carrying out a feasibility assessment of integrating two water-to-water heat pumps fed by a local low-enthalpy geothermal resource at around 40 °C, as well as a storage tank to optimise the use of water extracted from the geothermal wells. Another analysis will address the possibility of utilizing the wastewater from the thermal hot springs of the city.

Phase out coal soon

In Ljubljana (Slovenia), a district heating network includes a 277 km hot water line and a 6 km steam pipeline.

The total heat sold in 2023 was 979,213 MWh, of which 817,054 MWh was hot water and 162,159 MWh was steam. The production is mainly based on cogeneration (66.8%), boilers (30.7%), reduced production (2.1%) and waste heat (0.4%). This last contribution comes from a pharmaceutical factory close to the production units, and the amount of energy recovered is around 3,300 MWh/year. The fuels used for generating heat are coal (20,83 TJ), biomass (827 TJ) and natural gas (78,726,000 Nm3), featuring an overall primary energy factor of 0.771 and total GHG emissions of 364.000 t CO2-equivalent per year.

Plans for the future include shutting down the remaining coal powered units, probably between 2026 and 2030, and converting one of them to operate on biomass and building a waste incineration plant, that will also be connected to the district heating network. Additional upgrading measures to be analysed within ENABLE DHC are related to the integration of renewables, mainly heat pumps, either from river water or wastewater treatment plants.

Energy efficiency first

The third case brings us to the city of Poltava, in Ukraine, where a district heating infrastructure currently serves approximately 95% of buildings, for a total of 3,374 structures, broken down by heated area as follows: 80.8% residential buildings, 11.2% public institutions, and 8.0% commercial units. 91 decentralised power plants deliver a thermal capacity of 891.5 MWth. In addition to conventional energy generation facilities, the system also includes two biomass boilers with a cumulative installed capacity of 1.5 MWth and a solar thermal installation with a capacity of 30 kWth, dedicated specifically to supplying heat to administrative buildings. Waste heat recovery has been initiated, though it remains limited to approximately 1.12 MWth. Nevertheless, there is an untapped potential for further exploitation of waste heat, identified at 2.81 MWth. Annually, the total thermal energy generated by the existing infrastructure reaches approximately 756 GWh.

Natural gas remains a significant energy input for the existing district heating network and the greenhouse gas emissions from current operation stand at about 304,700 t of CO₂-equivalent per year.

Being a quite old system, the path of this network towards decarbonisation focuses on efficiency first through several measures, namely pipeline replacement, implementation of frequency

converters on existing equipment, substantial reduction of water leakages and heat losses, comprehensive reconstruction of boiler houses and heating substations, installation of advanced control devices and automation systems at 8 central heating stations and 107 individual heating substations, as well as the deployment of cogeneration units with an electrical capacity of 4.93 MWel.