Smart mobility - well-to-wheels efficiency of hydrogen vehicles

Michal Sura

Beside battery electric mobility, hydrogen mobility is considered as a further alternative for reducing CO2 emissions to reach carbon neutrality in 2050. At first sight, hydrogen looks like a fantastic energetic carrier. It is clean fuel because in the combustion process, the only waste produced is pure water. There is no CO2 released into the atmosphere. Water consists of hydrogen and oxygen, so there is plenty of hydrogen everywhere around us. It is possible to produce it in several ways, even from water, using electricity that can be produced from renewable energy sources such as wind or solar energy. So, why is hydrogen not widely used as fuel to power fuel cell electric vehicles (FCEVs) now? Simply because the devil is in the details.

We can see the interest in hydrogen is growing. Hydrogen is projected to play a very important role in the future of energy. We are witnesses of the emergence of a hydrogen economy. Hydrogen economics is an economy that relies on hydrogen as the commercial fuel for hydrogen vehicles, energy storage applications, heating, long-distance transport of energy, etc. We decided to do energy efficiency well-to-wheels analysis to see if hydrogen economics would be considered a viable option for the economical operation of FCEV.

Properties of hydrogen?

Hydrogen is odorless, tasteless, colorless and highly flammable gas. Hydrogen is one of the most abundant elements, but it is rarely found in its purest form. Hydrogen is almost always found as part of some other substance, such as water, natural gas, methanol, kerosene, or other hydrocarbons. Hydrogen is the lightest element in the periodic table. It has one proton in its nucleus and one outer electron.

• The energy density of hydrogen is 33.6 kWh/kg
• The energy density of methane (CH4) is 13.9 kWh/kg)
• The density of liquid hydrogen is 70 kg/m3 at -252.87°C
• The density of liquid methane (CH4) is 424 kg/m3?at -164°C
• The density of hydrogen is 0.089?kg/m3?(20??C, 0.1 MPa)
• The density of methane (CH4) is 0.718 kg/m3?(20??C, 0.1 MPa)
• The density of hydrogen is 42 kg/m3?(20??C, 70 MPa)
• The density of methane (CH4) is 305 kg/m3?(20??C, 70 MPa)

Large scale usage policy of hydrogen requires package, storage and transportation from the production site to the users.?

Production of hydrogen

The 48% of current hydrogen production is via steam reforming of natural gas (SR), 30% via petroleum fraction, 18% via coal gasification, and only 4% via electrolysis due to the still high cost of production (1)

Electrolysis is the only option for carbon emissions-free hydrogen production to achieve carbon neutrality by 2050. Hydrogen is possible to be produced by electrolysis of water. Electrolysis is the process of using electricity to split water into hydrogen and oxygen. Alkaline and PEM (Proton Exchange Membrane) electrolyzers are used for the industrial hydrogen production. Commercial alkaline electrolyzer systems have efficiency around 60% and are a mature technology for large systems. PEM electrolyzer systems have efficiency around 70%, they are more flexible and can be used for small decentralized solutions.?

Let’s take that efficiency of electrolysis is 70% and 5% are electricity transmission and distribution grid losses between the power plant and the electrolyzer (2) 0.7x0.95=0.665, so producing of hydrogen by electrolysis is 66.5% efficient.

0.7x0.95=0.665

Hydrogen package?

Energy is needed to compress hydrogen. Currently, hydrogen is typically compressed by a reciprocal compressor. There is information about a wide range of the amount of energy that is needed to compress hydrogen, likely due to the different types of compressors. There is possible to estimate that 2 to 4 kWh/kg of compression energy is needed to reach pressure 35 MPa. The energy density of hydrogen is 33.6 kWh/kg, it means that a high pressure hydrogen package (35 MPa) can be as good as 88%- 94% efficient. It is possible to store hydrogen by liquefaction, but there is even much more energy needed. Hydrogen may also be stored in metal hydrides, but metal hydride tanks are very heavy and store only a small amount of hydrogen, so this type of hydrogen packaging is not very suitable for automotive applications.?

The most efficient method for hydrogen packaging seems to be compression, and let’s suppose that the pressure of hydrogen is 90% efficient. 0.7 (H2 production) x 0.95 (transmission and distribution grid losses) x 0.9 (high pressure storage) = 0.5985. Efficiency from electric energy source to high pressure hydrogen package is roughly 60%.

0.7x0.95x0.9=0.5985

Hydrogen delivery

Hydrogen has very low volumetric energy density at standard temperatures and pressures. Hydrogen packaging requires significant energy when hydrogen is used as an energy carrier. We will consider only road delivery and pipeline delivery of hydrogen from a production site to customers as the most economical ways.

Road delivery of hydrogen

A 40-ton truck can carry at 20 MPa pressure only 320 kg of hydrogen and only 288 kg are delivered. There is possible to see energy needed for the road delivery of hydrogen and other fuels compared to their energy content, see below.

As we can see above, the efficiency of road delivery of compressed hydrogen by a truck is approximately 88% in a distance of 200 km.

Pipeline delivery of Hydrogen

Hydrogen has very low volumetric energy density, the flow velocity must be increased by over three times. There is needed 4.6 times more energy to move hydrogen through the pipeline compared to natural gas (4), see below.

It is obvious that only 80% of the hydrogen fed into a pipeline in Ukraine would arrive in Germany and only 70% to Spain. Transport of methane (natural gas) through pipelines is 3.5 times more efficient than transport of hydrogen.?

Types of hydrogen vehicles

Hydrogen can be used by fuel cell electric vehicles (FCEV) that use hydrogen fuel cells to power the vehicle's electric motor or by hydrogen internal combustion engine vehicle (HICEV) that uses an internal combustion engine.

Hydrogen internal combustion engine vehicle (HICEV)?

Hydrogen internal combustion engines have only 20-25% efficiency and low power output compared to fossil-fueled internal combustion engines. A good example of this is BMW Hydrogen 7. It was limited production hydrogen internal-combustion engine vehicle built from 2005-2007 by BMW. The car was powered by a 6.0-liter V12 engine, but it reached only 191 kW of power and its range was 201 km (5).?

Fuel cell electric vehicles (FCEV)?

FCEV uses a propulsion system similar to electric vehicle, but energy stored as hydrogen is converted to electricity by the fuel cell.

There are used Proton Exchange Membrane (PEM) fuel cells, they can achieve efficiency of about 50-60%. Let suppose that a PEM fuel cell has 60% efficiency. Electric energy produced by the fuel cell feeds a drivetrain (motor and power inverter) of 90% efficiency.

0.7x0.95x0.9x0.6x0.9=0.32319

Well-to-wheels efficiency of FCEV is only 32% if production, package and consuming hydrogen takes a place “in situ”, if there is necessary to transport hydrogen from a production place situated some 200 km away (88% efficiency), well-to-wheels efficiency would be 28%, but if there is transported hydrogen let say from Ukraine to Germany trough hydrogen pipeline (80% efficiency) well-to-wheels efficiency is only 25%. When there is necessary truck hydrogen delivery, because the hydrogen pipeline is situated far away from a consuming place, the well-to-wheels efficiency would be easily under 20%. Any compression of hydrogen needs energy, and it worsens overall well-to-wheels efficiency. You can see overall well-to-wheels efficiency of FCEV in the picture below.

Well-to-wheels efficiency of battery electric vehicles (BEV)

Transmission and distribution grid losses between the power plant and charging station are 5%. Efficiency of charging a battery is 90%. EV’s drivetrain (motor and power inverter) has 90% efficiency.

0.95x0.9x0.9=0.7895

Overall well-to-wheels efficiency of BEV is 79%. You can see overall well-to-wheels efficiency of BEV in the picture below.

BEV has roughly 2.5x better well-to-wheels efficiency than FCEV when it is filled up with “in situ” produced hydrogen. When hydrogen has to be transported by a road delivery or trough hydrogen pipeline, well-to-wheels efficiency will worsen even more.

1,https://www.mdpi.com/2077-0375/10/1/10/pdf

2, https://www.urso.gov.sk/data/att/c63/342.6c2274.pdf

3,https://www.hydrogen.energy.gov/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf

4, https://afdc.energy.gov/files/pdfs/hyd_economy_bossel_eliasson.pdf

5, https://en.wikipedia.org/wiki/BMW_Hydrogen_7