Comparison of Propane (R290) and Carbon Dioxide (CO₂, R744) as refrigerants for electric buses
Choosing the right refrigerant plays an important role with regard to energy efficiency, safety, and environmental compatibility of HVAC systems in electric buses. The following comparison highlights the differences between the two natural refrigerants—propane (R290) and carbon dioxide (CO₂, R744)— compared in different categories, including energy efficiency, safety requirements, environmental impact, and long-term economic-effiency.
Category | Propane (R290) | Rating Propane (1–5) | Carbon Dioxide (CO₂, R744) | Rating CO₂ (1–5) | Energy Efficiency in E-Buses |
Environmental Impact | GWP direct 0.02 / actual climate impact including degradation products ≈ 10 → degradation products: methane (CH₄), ground-level ozone (O₃) (source: Hodnebrog et al.) | 4 | GWP of 1 / minimal ecological footprint. | 5 | CO₂ systems can save up to 65 kWh per 100 km compared to conventional electric heating components. |
no ozone damage | 3 | no ozone damage | 3 | Both refrigerants are ozone-friendly and meet basic environmental requirements. | |
Energy Efficiency | Additional losses due to double indirect systems; less efficient in heat pumps for city buses. | 2 | Direct systems minimize losses; savings of up to 65 kWh/100 km possible. | 5 | CO₂ heat pumps significantly reduce energy consumption and enable substantial operational cost savings. |
Flammability | Highly flammable; safety class A3. Requires extensive safety measures (e.g. gas detectors, ventilation) | 2 | Non-flammable; safer and easier to handle. | 5 | The handling of CO₂ in bus depots and vehicle halls, especially in enclosed spaces is much safer |
Toxicity | Non-toxic, can cause asphyxiation in high concentrations. | 4 | Non-toxic, can cause asphyxiation in high concentrations. | 4 | Both refrigerants require basic safety measures in case of leakages. |
Operating Pressure | Low operating pressure; easier to handle, but requires a more of complex systems. | 4 | High pressure, requires special pressure-resistant components. | 3 | Each refrigerant has specific technical requirements, without one refrigerant being fundamentally superior. |
Availability & Costs | Low purchasing costs; but safety measures increase overall cost. | 3 | Higher initial costs, but more long term efficiency due to lower energy and maintenance costs. | 4 | CO2 pays for itself in the long term through low energy and maintenance costs, |
Safety Aspects | Flammability requires elaborate intensive measures; use in depots particularly challenging. | 2 | Safe in depots; low infrastructure and training requirements. | 4 | CO₂ is a safer choice, especially in in closed rooms such as bus depots. |
Use in Public Transport | Limited use in city buses; mainly found in pilot projects. | 3 | Established and widely used; proven in electric city buses for over 10 years. | 5 | CO₂ is standard in electric city buses, particularly in Europe. |
Origin and Production | Derived from fossil fuels (oil, natural gas); significant CO₂ emissions during extraction and processing | 2 | Industrial by-product; no additional extraction of fossil resources required. | 5 | CO₂ is a sustainable option, as it is a by-product that is repurposed in a meaningful way. |
Secondary Environmental Effects | Used as a propellant gas (e.g. in spray cans); potentially harmful if disposed incorrectly or released uncontrolled. | 2 | Meaningful use of a waste product; helps reducing environmental impact. | 5 | CO₂ supports circular economy principles and helps reducing environmental impact. |
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Conclusion
With all the latest data considered, CO₂ (R744) remains the clearly superior choice for climate systems in electric city buses. Especially in the fields of energy efficiency, safety, and sustainability, CO₂ sets new standards, while propane performs significantly worse due to its environmental impacts and its lower efficiency.
Simulation results: CO2 versus propane
- For safety reasons, the use of propane as a refrigerant requires heat pump systems with indirect heat transfer to the air.
- Additional heat transfer steps reduce efficiency and limit the usable temperature range.
- These disadvantages are avoided in CO2 heat pumps by means of direct heat transfer to the air.
