Intense discussion and debate swirl around how or whether to deploy hydrogen energy as a decarbonization strategy. Consensus is elusive partly because there are multiple sources (natural gas, wind power), multiple end uses (transportation, home heating), and thus multiple potential supply chains.
This article focuses on one use case: a district heating system that uses hydrogen as a fuel.1 A district heating system generates heat in a central location and then distributes it to local households, businesses, and industry. At every stage of the supply change, some useful energy is lost due to the second law of thermodynamics, and some greenhouse gases “leak” to the surrounding environment. Rigorous accounting of these energy and material flows is critical to good decision-making.
There are no commercial hydrogen energy systems that are coupled with district heating. But such a system can be characterized by what we already know about its important components.
Blue hydrogen is produced from natural gas using a process called steam methane reforming, where methane (CH₄) reacts with steam (H₂O) to produce hydrogen (H₂) and carbon dioxide (CO₂). The CO₂ generated during the process is captured and stored or utilized (CCUS) instead of being released into the atmosphere.
To deliver 100 joules of heat to a home or business requires 194 joules of natural gas in the blue hydrogen pathway, an efficiency of 51%. Most of the lost energy occurs in the steam reformation process that requires significant heat to raise the reaction temperature to 700 to 1000 °C. The extraction of natural gas involves energy losses via flaring and so-called fugitive methane emissions. The CCUS process is also energy-intensive.
How would a blue hydrogen system work in a district heating system? This technology would rely on the same natural gas infrastructure up to the outskirts of an urban area, where a natural gas combined heat and power plant (CHP) equipped with a CCUS technology generates both power and heat. The heat from the CHP system would be delivered directly to the district heating system, while the power generated would be used for driving heat pump plants that deliver additional heat to the district heating system.2
The conversion of natural gas into electricity is about 34%, but much of the resulting waste heat is captured and diverted to the district heating system. The electricity is used in a heat pump that uses ambient air as an energy source with an assumed coefficient of performance (efficiency) of 350% (3.5 units of heat for each unit of electrical energy used). Given these assumptions, the overall efficiency of the blue district heating system is 132% (100 joules of useful end use heat / 76 joules of natural gas).
Green hydrogen is produced from a “green” source of electricity such as wind or solar power via electrolysis, the chemical process that uses an electric current to split water (H₂O) water into its basic elements, H₂ and oxygen (O₂). To deliver 100 joules of heat to a home or business requires 144 joules of electricity in the green hydrogen pathway, an efficiency of 69%.
How would a green hydrogen system work in a district heating system? Assume again that electricity is used in a heat pump that uses ambient air as an energy source with an assumed coefficient of performance (efficiency) of 350%. The overall efficiency of the green system is 303% (100 joules of useful end use heat / 33 joules of green electricity).
A main takeaway is that a district heating system is much more energy efficient compared to using hydrogen directly as a heating fuel. The blue hydrogen district heating system is about 267% more efficient than the blue hydrogen alone. The major reason for the efficiency gap is the loss of energy in the hydrogen manufacturing process. Similarly , a renewable power-based district heating system is 440% more efficient than a green hydrogen-based heat supply system.3 It follows that the district heating systems would also have lower greenhouse gas emissions and lower consumer costs.
District heating is a mature technology with benefits such as higher energy efficiency, lower operation and maintenance costs, economy of scale, to name just a few. But it faces high initial costs, geographical limitations, and competition from legacy systems. In 2022 district heating provided about 9% of the global final heating need in buildings and industry.4 Policy that directs district heating to appropriate opportunities and removes regulatory barriers could help realize the energy and environmental benefits of district heating where feasible.
The energetics of hydrogen in district heating discussed here is consistent with a broader body of scientific evidence does not suggest a major role for hydrogen for heating in cost-optimal pathways and indicates higher system and consumer costs.5
1 Gudmundsson, Oddgeir, and Jan Eric Thorsen. “Source-to-Sink Efficiency of Blue and Green District Heating and Hydrogen-Based Heat Supply Systems.” Smart Energy 6 (May 1, 2022): 100071. https://doi.org/10.1016/j.segy.2022.100071
2 Gudmundsson and Thorsen, op.cit.
3 Gudmundsson and Thorsen, op.cit.
4 International Energy Agency, “District heating,” access September 16th, 2024, https://www.iea.org/energy-system/buildings/district-heating
5 Rosenow, Jan. “A Meta-Review of 54 Studies on Hydrogen Heating.” Cell Reports Sustainability 1, no. 1 (January 26, 2024): 100010, https://doi.org/10.1016/j.crsus.2023.100010