October 07, 2021

Automotive Hydrogen Powertrains: Could Hydrogen Internal Combustion Replace Fuel Cells?

Stratas Advisors

Fuel cell technology is gaining increasing attention due to its high potential to decrease transport emissions. However, hydrogen-powered electric vehicles are still facing multiple challenges that may hamper their large-scale penetration and competitiveness when compared with other zero-emissions options. Yet the possibilities for hydrogen are not only limited to fuel cells – in fact, liquid hydrogen combustion could offer viable solutions to many of the shortcomings of compressed hydrogen.

Storage Technology Improvements to Determine the Future of Hydrogen

Hydrogen storage is a key technology for multiple applications including transportation, portable power, and stationary power. While it contains the highest energy content per mass unit, its volumetric density is significantly lower than that of other fuels based on lower heating values, as shown in the figure below.

Comparison of Volumetric and Gravimetric Density for Transport Fuels

Notes: CNG – Compressed Natural Gas; LNG – Liquefied Natural Gas; LPG – Liquefied Petroleum Gas; HFO – Heavy Fuel Oil; H2 – Hydrogen

Source: Stratas Advisors, 2021

Currently available storage options typically involve large-volume systems for compressed hydrogen in gaseous form. This may pose significant challenges for light-duty fuel cell electric vehicles (FCEVs), which must rely on onboard large-volume and high-pressure compressed hydrogen storage in a way that a minimum driving range of 480 km (300 miles) is guaranteed. At the current state of technology, light-duty FCEVs would require an onboard hydrogen storage capacity of 5-13 kg to satisfy this driving range. Today, when compressed at a maximum pressure of 700 bar, a 125-liter tank can store up to 5 kg of gaseous hydrogen.

In contrast, liquid hydrogen has a higher density and fewer potential risks in terms of storage pressure, which makes it particularly suitable for large-scale transport and storage. Through liquefication, 5 kg of liquid hydrogen can be stored in a 75-liter tank. Because hydrogen liquefies at a temperature of -252.8°C (-423°F), however, storage vessels require sophisticated insulation techniques and cryogenic systems to minimize hydrogen boil-off. In fact, a major concern in hydrogen storage is the risk of hydrogen loss resulting from heat transfer.

With regards to hydrogen transport and distribution, super-insulated, cryogenic trailers offer the most economical solution to transport liquid hydrogen over long distances – in this respect, trucking liquid hydrogen via tanker trucks turns out more cost-effective than trucking gaseous hydrogen via tube trailers due to the higher amounts of hydrogen transported. When hydrogen pipelines are deployed, gaseous hydrogen is the preferred option because it has been proven to be technically feasible and may be blended into natural gas grids in order to amortize existing network infrastructure, while current methods for liquid hydrogen transport through pipelines demonstrate lower efficiency and higher risks of leakage.

No Winner-Takes-All Solution for Hydrogen Powertrains

There are today over 31,000 hydrogen-powered light-duty vehicles circulating globally – the majority of which run on fuel cell technology. Although the FCEV fleet has experienced a significant growth in select regions, particularly in East Asia (mostly in Japan), Europe (mostly in Germany), and North America (mostly in California, U.S.), it is still far from becoming the preferred zero-carbon transport option – for instance, in 2019 the global battery-electric vehicle (BEV) fleet amounted to almost 5 million.

Partly, the slow penetration of FCEVs is directly attributable to not only the costs of zero and low-carbon hydrogen production, but also the CAPEX associated with fuel cell system manufacturing. On top of overall total cost of ownership (TCO), dedicated filling infrastructure is still lacking in many parts of the world, thus generating investment uncertainty among final users. Provided that green hydrogen is utilized, however, FCEVs could virtually generate the lowest share of lifecycle greenhouse gas (GHG) emissions – in fact, well-to-wheel emissions are significantly lower for FCEVs than for BEVs because the latter depend on the power mix of the grid, which typically contains low shares of renewable electricity.

Besides its zero or minimal lifecycle GHG and local emissions, fuel cell technology allows for fast recharging and low reaction times when responding to load changes. In spite of this, the compression process of hydrogen leads to higher levels of efficiency loss, reaching only a maximum of 70% of overall efficiency.

Efficiency, however, is significantly lower for liquid hydrogen than for its gaseous counterpart. Tank-to-wheel efficiency for hydrogen combustion engines has been proven to reach a maximum of 45%, while well-to-wheel efficiency can drop up to 30%. In this respect, efficiency of combustion engines would be improved by engine downsizing at higher load conditions.

The table below provides an overview of the main characteristics for both compressed and liquid hydrogen, outlining the proven properties of fuel cell technology and hydrogen internal combustion engines (ICEs), respectively.

Specifications of Hydrogen Fuel Cell and Combustion Technologies

 

Compressed Hydrogen

Liquid Hydrogen

Gravimetric density

34 kWh/kg

34 kWh/kg

Volumetric density

1 kWh/L at 700 bar

2.5 kWh/L at -252.87°C

Efficiency

50-70%

38-45%

Lifetime

3,000 hours

10,000 hours

Payload/Weight

Fuel cell and hydrogen tank

Hydrogen tank

Refueling

15-30 minutes

15-30 minutes

Tailpipe emissions

None

NOx and PM emissions (reduced through SCR1)

Lifecycle GHG emissions

Zero if green hydrogen

Zero-CO2 combustion

Cost per hour of operation

USD 6,475 (fuel cell system)

USD 4,500 (combustion engine/transmission)

Notes: SCR – Selective Catalytic Reduction

Source: Stratas Advisors, 2021

Stratas Advisors’ Views

Lower efficiency and the persistence of tailpipe emissions of hydrogen internal combustion seem to have served as the main factors leading OEMs to lean towards fuel cell technology. At the same time, the higher volumetric density of liquid hydrogen offers multiple stimuli for technology development. Lower CAPEX can also serve as a major driver for the development of liquid hydrogen ICEs. In fact, hydrogen combustion is identical to current diesel ICEs, while CAPEX would be further reduced since hydrogen ICEs require less exhaust-treatment technology than diesel engines for NOx reduction.

Despite its many limitations, liquid hydrogen combustion could fill the existing gap left by fuel cells and BEVs. This is especially true for heavy-duty, off-highway segments with high-power requirements at harsh conditions, particularly in terms of heat and vibration tolerance. Hydrogen ICEs and liquid storage would be more suitable for the aviation and shipping sectors as well due to their high energy density requirements, though other challenges remain with respect to refueling infrastructure, onboard storage, and safety concerns.

Even where fuel cells and batteries would be technically and economically feasible, TCO for liquid hydrogen could become competitive thanks to the higher efficiencies reached at higher loads, supported by decreasing hydrogen prices. Liquid hydrogen could also play a role in decarbonizing heavy-duty fleets by deploying dual-fuel engines, especially in areas and conditions where hydrogen supply and infrastructure are still limited.

However, the typologies of hydrogen powertrain do not necessarily need to compete – rather, they should be complementary. Essentially, the rollout of both fuel cell technology and hydrogen combustion depends largely on hydrogen refueling infrastructure and the cost of hydrogen. In fact, the increasing commercialization and deployment of one technology is likely to drive down the costs of the other, bringing down the overall cost curve of hydrogen vehicles as a whole.

Utilizing liquid hydrogen combustion for certain applications in shipping, aviation, heavy machinery, and long-haul trucking, as well as increasing the share of vehicles running on liquid hydrogen dual-fuel engines, could thus significantly contribute to overall decarbonization goals in transport, providing complementary solutions to FCEVs and BEVs.

 


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