Transport: The Hard-to-Decarbonize Sector
Transport accounts for about one-third of final energy consumption in most industrialized economies, and it remains overwhelmingly dependent on petroleum fuels. In Austria, the transport sector consumed 361 PJ in 2020—virtually all from oil-derived fuels.
Decarbonizing transport is therefore essential to any serious climate strategy. But which technology pathway makes the most sense from a physics standpoint?
Efficiency: The Key Metric
When comparing propulsion technologies, the critical metric is tank-to-wheel (TTW) efficiency: what fraction of the energy stored in the vehicle actually propels it forward?
Internal Combustion Engines (ICE)
The Otto cycle (gasoline) and Diesel cycle are inherently limited by the Carnot efficiency:
$$\eta_{Carnot} = 1 - \frac{T_{cold}}{T_{hot}}$$
For typical combustion temperatures, theoretical maximum efficiency is around 60-65%. Real-world engines achieve:
- Gasoline ICE: 20-25% average TTW efficiency
- Diesel ICE: 25-35% average TTW efficiency
- Hybrid ICE: 30-45% (through regenerative braking and optimized operation)
Most of the fuel’s energy is lost as waste heat through the radiator and exhaust.
Battery Electric Vehicles (BEV)
Electric motors are not heat engines and are not subject to Carnot limits. Modern permanent magnet synchronous motors achieve:
- Motor efficiency: 90-95%
- Inverter efficiency: 95-98%
- Transmission efficiency: 95-98%
- Overall TTW efficiency: 85-90%
BEVs also recover significant energy through regenerative braking, further improving real-world efficiency.
Hydrogen Fuel Cell Vehicles (FCV)
Fuel cell vehicles use hydrogen to generate electricity onboard, which then powers an electric motor. The key component is the Proton Exchange Membrane Fuel Cell (PEMFC):
- PEMFC efficiency: 50-60% (under typical driving conditions)
- Motor and inverter: 90-95%
- Overall TTW efficiency: 45-55%
While less efficient than BEVs, FCVs are significantly more efficient than ICE vehicles and offer advantages in range and refueling time.
Hydrogen Internal Combustion (H₂-ICE)
Some manufacturers have developed hydrogen-burning internal combustion engines. Examples include the BMW Hydrogen 7. These engines:
- Are subject to the same Carnot limitations as conventional ICE
- Achieve TTW efficiency of 20-35%
- Produce only water vapor emissions
- Can use existing manufacturing infrastructure
Well-to-Wheel Analysis
TTW efficiency tells only part of the story. Well-to-wheel (WTW) analysis includes the entire energy chain:
BEV Pathway
- Electricity generation (varies: 30-90% depending on source)
- Grid transmission: 92-95%
- Battery charging: 90-95%
- Motor efficiency: 85-90%
Total WTW efficiency: 60-80% (for renewable electricity)
FCV Pathway (Renewable Hydrogen)
- Electricity generation: 85-90% (renewable)
- Electrolysis: 60-80%
- Compression to 700 bar: 85-90%
- Distribution: 90-95%
- Fuel cell: 50-60%
Total WTW efficiency: 25-35%
The efficiency gap is significant: BEVs require roughly half the primary energy of FCVs for the same distance traveled.
The Power Demand Question
A hydrogen refueling station presents unique challenges. Consider a station serving 100 vehicles per day, each requiring 5 kg H₂:
- Daily hydrogen demand: 500 kg
- Energy content: 500 × 33.3 = 16,650 kWh
- If produced onsite via electrolysis (70% efficiency): 23,800 kWh/day
- Average power demand: 990 kW
- Peak demand (during busy hours): ~3.3 MW
For comparison, a typical fast-charging station serving 100 BEVs daily might require only 500-800 kW average power.
Hydrogen Properties and Handling
Hydrogen’s physical properties create engineering challenges:
| Property | Value |
|---|---|
| Molecular weight | 2.016 g/mol |
| Density at STP | 0.089 kg/m³ |
| Boiling point | -252.9°C (20.3 K) |
| Energy density (LHV) | 120 MJ/kg |
| Volumetric energy | 10.8 MJ/m³ (gas at STP) |
| Flame speed | 265-325 cm/s |
| Flammability range | 4-75% in air |
Hydrogen’s extremely low density means that even at 700 bar pressure, compressed hydrogen tanks contain far less energy per volume than gasoline tanks. This explains why FCVs like the Toyota Mirai, Hyundai ix35, and Honda Clarity require large, heavy tanks to achieve acceptable range.
Real-World Vehicle Comparisons
| Parameter | BEV (Tesla Model 3) | FCV (Toyota Mirai) | ICE (VW Golf) |
|---|---|---|---|
| Energy consumption | 15 kWh/100km | 1.0 kg H₂/100km | 6.0 L/100km |
| Tank-to-wheel efficiency | ~85% | ~50% | ~25% |
| Refueling time | 20-40 min (fast) | 3-5 min | 3-5 min |
| Range | 350-500 km | 500-650 km | 700-900 km |
Where Each Technology Fits
The physics suggests a natural division:
BEVs are optimal for:
- Personal vehicles with predictable daily driving
- Urban delivery vehicles
- Short-to-medium range applications
- Any application where overnight charging is feasible
FCVs are optimal for:
- Heavy-duty trucks and buses
- Trains on non-electrified routes
- Marine vessels
- Applications requiring rapid refueling
- Very long range requirements
Neither technology currently offers a complete solution for:
- Long-haul aviation
- Shipping (very large vessels)
- Some industrial processes
In the next installment, we examine the thermal sector—heating and cooling—where even more dramatic efficiency gains are possible.
Technical data from HyCentA Research Center, Graz University of Technology, and manufacturer specifications