Hydrogen Fuel Cell Vehicles

Hannu Jääskeläinen

DRAFT manuscript under development—Please do not cite or quote!
This paper is still being edited or under review. The final version may be significantly different.

This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.

Introduction

The question often arises “Are there viable alternatives to the diesel engine?” Figure 2, adapted from a study by Chrysler Corporation [403], shows that there are few powerplant alternatives that can meet the efficiency of a direct injection diesel engine.

[SVG image]
Figure 2. Energy conversion efficiencies of several powerplants

It should be noted that Figure 2 understates the peak brake thermal efficiency (BTE) of the diesel engine. Light-duty diesel engine peak BTE is typically around 42%. Commercial heavy-duty diesel engines have a peak BTE of about 43% while the US DOE SuperTruck program has shown that greater than 47% BTE is possible with changes to the engine only while adding technologies such as waste heat recover can further increase the BTE above 50% [3083]. Low speed diesel engines have even higher BTE in the range of 55%.

Fuel Cells

Historically, Sir William Grove is credited with creating the first fuel cell in 1839. However, he could not produce enough power to compete with other sources of power available in his time [395]. In the 1930s, approximately 100 years later, Francis Bacon made significant engineering advances in fuel cell technology. By 1959, after 27 years of research and development, he was able to produce a 5 kW fuel cell system that powered a fork-lift truck. Since this modest beginning, fuel cells have been further developed and used in various military applications. They are used to provide power for life-support systems aboard space shuttles, power homes and businesses, and as propulsion systems for vehicles.

Fuel cells are electrochemical devices using hydrogen and oxygen to produce electricity. Their by-product is water and they reject heat as a result of the chemical conversion process. Unlike batteries, which must be periodically recharged, fuel cells can produce power as long as they are supplied with fuel and oxidant.

At the heart of the fuel cell is a solid electrolyte consisting of proton-conducting plastic foil, proto-exchange membrane (PEM), as shown in Figure 15. This foil is coated with a platinum catalyst and an electrode made from gas permeable graphite paper. Graphite bipolar plates, in which fine gas channels are milled, are positioned on both sides of the catalyst. Hydrogen is fed through the channels on one side of the membrane (solid electrolyte), and air flows through the channels on the other side of the electrolyte. The hydrogen side is designated the anode (negative polarity) since the platinum catalyst essentially ionizes the hydrogen molecules at the anode into negatively charged electrons and hydrogen ions (protons), which migrate through the electrolyte toward the cathode. Oxygen flows through the second electrode, where it combines with hydrogen forming water vapor which eventually exits from the powerplant. The result is an electrical voltage between the negative terminal of the anode and the positive terminal of the cathode. Stacks of electrodes sandwiching proton exchange membranes are used to generate the power required for a given application.

[schematic]
Figure 15. Functional schematic of the fuel cell

An alternative to the proton exchange membranes is the solid oxide technology, used mostly for high power applications such as industrial and large scale power generating plants. Solid oxide fuel cells (SOFC) use ceramics instead of either liquid or dry electrolytes. Operating temperatures may reach 980°C (1800°F) and thermal efficiencies nearing 60% are quite possible. At one time, many observers felt that SOFCs would be a viable alternative for automotive applications.

Beyond its relatively simple concept, the fuel cell still faces many challenges before it could be considered a commercially viable powerplant for automotive use. An important issue to be resolved is which fuel will most likely be the energy source for future fuel cells. Hydrogen appears to be an obvious choice and, theoretically at least, there is an abundance of water to produce hydrogen fuel by electrolysis.

[SVG image]
Figure 16. Fueling the fuel cell—options and implications

However, the process of deriving hydrogen from water by electrolysis is energy-intensive and seems to defeat the purpose of economical conversion of energy for transportation. One of the most economical ways to produce hydrogen is by steam/methane reforming with the methane provided by natural gas. This is in fact the primary means by which industrial hydrogen is produced on a large scale. The process does produce CO2 emissions from the carbon contained in methane.

Some feel that the most environmentally- and economically-sound way to provide hydrogen fuel is to use hydroelectric or solar power in the electrolysis process; only then will it be environmentally responsible and, possibly, economically viable. Therefore, as illustrated in Figure 16, producing hydrogen for future fuel cell powered vehicles will require further development [396].

Another choice, obtaining hydrogen from methanol, will require onboard fuel refining using a device known as reformer. Among the advantages of using methanol, the prospect of low CO2 emission (estimated at 30% less than today’s conventional internal combustion engines) must be the most attractive. It is followed closely with the fact that methanol is produced from natural gas which is available in large quantities in many areas of the world. In addition, methanol can be made from renewable sources such as organic waste or timber residues. Even though a methanol distribution infrastructure is currently non-existent, it could be distributed through other existing networks such as filling stations as we know them today.

The least desirable choice of fuel for the fuel cell is gasoline. An onboard refining process is also required using a more complicated reformer than that of methanol. To balance this major negative, gasoline infrastructure should be considered one of the more significant advantages for this fuel. However, gasoline fuel as we know it may have to change drastically to encourage the formation of large quantities of hydrogen from a reasonably sized reformer.

While there has been was a great deal of excitement around fuel cells in the 1990s/early 2000s, this sentiment has been moderated by the many challenges. Fuel selection, reformer functionality and efficiency, fuel handling and infrastructure, safety, environmental concerns, toxicity, ground water contamination, fuel efficiency, performance, and consumer acceptance are just a few of the topics holding both challenge and promise for this concept.

Beyond the wider infrastructure issues is the challenge of fuel cell durability, cost and vehicle range. Table 2 illustrates the status for an urban bus application reported in 2013 [3090]. One way to improve the range of a fuel cell vehicle is to operate it as a PHEV where electricity from the fuel cell is supplemented by electricity from the grid.

Table 2
DOE/FTA performance, cost, and durability targets for FCEB units
ParameterJuly 2013 Status2016 TargetUltimate Target
Bus lifetimeyears/miles12/500,00012/500,000
Power plant lifetimeahours13,000 (single bus)18,00025,000
Bus availability%69% avg.8590
Fuel fillsper day1 (< 10 min)1 (< 10 min)
Bus costb$1,000,000600,000
Roadcall frequency (bus/fuel cell system)miles between roadcalls2,728/11,0433,500/15,0004,000/20,000
Operation timehours per day/days per week20/720/7
Scheduled and unscheduled maintenance costc$/mile0.750.40
Rangemiles250 avg. (buses plugged in over-night to maximize range – i.e., PHEV. Range ~160 mi w/o PHEV feature)300300
Fuel economymiles per gallon diesel equivalent6.08 as PHEV88
a The power plant is defined as the fuel cell system and the battery system.
b Cost is projected to a production volume of 400 systems per year. This production volume is assumed for analysis purposes only and does not represent an anticipated level of sales.
c Excludes mid-life overhaul of power plant.

Commercialization Status. On the light-duty side, there has been limited commercial availability of FCVs since 2008 by Honda and Daimler. By the mid-2010s, a number of automobile manufacturers claimed that they will start commercial production of fuel cell vehicles in the 2015-2020 time frame [3091][3220]. These vehicles include the Hyundai Tucson Fuel Cell SUV that became available in 2014 in Southern California, the Toyota Mirai scheduled to go on sale in California in late 2015 and the Honda FCV Concept scheduled to be launched in Japan in 2016. GM and Honda are also cooperating on a vehicle intended to be launched around 2020. However, the price of these vehicles remains very high relative to comparable non-fuel cell vehicles. In 2014, Toyota reported that its fuel cell passenger car would cost around 7 million yen (about US$70,000) when it goes on sale in Japan in 2015. In 2015, the cost of the Hyundai Tuscon was reported to be 85 million won (about US$78,000) [3221]. Even with significant subsidies, these vehicles will appeal to a very limited number of users.

Hydrogen refueling infrastructure is critical for light-duty fuel cell vehicles. In early 2015, it was reported that there were 51 hydrogen refueling stations worldwide with a number of additional stations scheduled to be opened later that year [3221]. Table 3 summarizes the estimated number of hydrogen stations that will be available by 2020.

Table 3
Estimated number of hydrogen refueling stations by 2020
CountryHydrogen Refueling Stations
Japan2015: 45
2016: 100
Germany2015: 14-16
2016: 50
South Korea2015: 11
2020: 100
North America2015: 12 (11 in California)
2020: 115 (100 in California)

On the heavy-duty side, in 2014 there were 7 on-road fuel cell vehicles available on the market: three tractors (i.e., for tractor trailer combinations), three transit buses and a shuttle bus [3220].

###