UE 12 - Guest lecture: Fossil free district heating

Question 1

Energy sources + sector coupling for climate-friendly district heating generation

What technologies do you know?

Answer 1

• Gas plants
  –> H2
  –> Natural gas + CCS
• Biomass plants
• Waste heat
  –> From industry, wastewater, water bodies and waste
• Large-scale heat pumps
  –> Ambient water body
  –> Waste heat (e.g. data centers)
  –> Geothermal energy
  –> Ambient air
• Power-to-Heat
  –> Electrode boiler
  –> Electric Boilers
• Geothermal energy

Question 2

What is missing?

Power-to-heat systems are comparable to “…” that work according to “…”. If there is a lot of surplus renewable electricity in the grid, it is used in the system to generate heat. If the heat load is low, “…” and thus supports sector coupling.

Answer 2

“large kettles (“Wasserkochern”)”

“the immersion heater principle (‘Tauchsiederprinzip’)”

“the renewable heat is stored in a heat storage for later use”

Question 3

What challenges do you think could arise in the use of P2H and heat storages in the context of district heating?

Answer 3

Electricity Market & Grid Constraints

• High electricity prices or taxes can make P2H uneconomical, especially if heat from gas or waste incineration is cheaper
• Grid capacity limitations may restrict how much electricity can be used during peak times.

Fluctuating Renewable Supply

• Is intermittent (‘unregelmäßig’) and weather-dependent, making heat production timing unpredictable.

Matching Heat Demand and Electricity Supply

• Heat demand is highest in winter, when electricity demand is also high and PV generation is low

Storage Sizing and Efficiency

• Thermal energy storage (TES) helps decouple electricity use from heat delivery — but:
  –> Large tanks take up space
  –> Long-term seasonal storage still faces efficiency losses and cost barriers

Question 4

What challenges do you think could arise in the use of large-scale heat pumps in district heating?

Answer 4

• High upfront investment costs
• Heat source availability and suitability
  –> Not all locations have accessible or adequate low-temperature heat sources (like rivers, sewage, or waste heat).
  –> Seasonal changes (e.g. colder rivers in winter) can affect efficiency and performance.
• Integration into existing heating networks
  –> Many existing district heating networks operate at high temperatures (80–120°C), while LSHPs work best at low to medium temperatures (up to ~70°C).
  –> Efficiency (COP) drops significantly when higher output temps are required.
• Electricity demand and grid impact
  –> Large heat pumps consume significant amounts of electricity, especially during winter.
  –> Could stress local electrical grids if not properly managed.
  –> Electricity prices may be volatile or unfavorable compared to gas or waste heat.
• Regulatory and planning complexity
  –> Large infrastructure projects face lengthy planning, permitting, and environmental approval processes.
  –> Rules around using ambient water, groundwater, or wastewater vary widely.

Question 5

What challenges do you think could arise in the use of biomass in the context of district heating?

Answer 5

• Sustainability and land use concerns
  –> Large-scale biomass demand can lead to: Deforestation, monoculture plantations, loss of biodiversity
• Competition for biomass resources
  –> Biomass is in high demand for: heat and power generation, biofuels, industrial use (e.g., steel or cement)
• Logistics and Infrastructure
  –> Biomass is bulky and heavy, requiring: frequent deliveries, storage facilities, transport infrastructure
• Air quality and emissions
  –> Biomass plants contribute to air pollution

Question 6

What is missing?

Geothermal energy: “…”

With geothermal energy, heat from underground can be harnessed with the help of various technologies. This requires drilling up to several kilometers into the ground. A major advantage of geothermal energy “…”. In addition, the underground can also be used as a seasonal heat store (aquifer storage).

Answer 6

“the constant source of heat underground”

“is the high number of operating hours and thus the year-round use of renewable heat in the base load”

Question 7

What challenges do you think could arise in the use of geothermal in the context of district heating?

Answer 7

• Geological uncertainty
  –> The biggest risk: You don’t know exactly what’s underground until you drill, there might be possible issues (insufficient temperatures, unfavourable rock formations)
• High upfront investment costs
• Long project development time
• Corrosion, scaling, and maintenance
  –> Geothermal fluids often contain dissolved minerals and gases (e.g., H₂S, CO₂, iron, silica).
  –> This can cause scaling and corrosion in pipes and heat exchangers.

Question 8

What challenges do you think could arise in the use of hydrogen in the context of district heating?

Answer 8

• Green H2 production has a low overall efficiency which leads to high production and fuel costs
• Infrastructure limitations
  –> Hydrogen needs dedicated pipelines, burners, and storage systems
• Technical performance of hydrogen boilers
  –> Hydrogen combustion produces very high flame temperatures, which can increase NOX emissions if not properly controlled