|
|
| Fig. 1: Energy density and energy per volume for selected fuels and energy storage media. [8] (Source: H. Stokowski) |
Climate change, driven by the emission of greenhouse gases (GHG) in a process of burning fossil fuels, emerges as one of the main challenges of the 21st century. There has been considerable political interest in transitioning to green sources of energy around the world, including ideas like The European Green Deal proposed by the European Commission. [1] While transitioning to an economy completely based on renewable sources of energy and abandoning fossil fuels seems like the target we should work towards, the switch should be gradual to avoid collapsing a well-established branch of the economy based on fossil fuels. In this text, I will discuss how using hydrogen can ease the switch in the context of using it as a fuel for cars.
The current fossil fuel industry is a considerable part of the economy of many countries. For instance, the United States produced about 6.22 × 109 barrels of oil in 2019. [2] Assuming the average price per barrel of $60, the entire production was worth about $400 billion. A similar calculation can be done for natural gas, for which the total production was 9.21 × 1011 m3 in 2019, or 3.14 × 1010 mmBTU. The average price in 2019 was $2.53 per mmBTU, hence, the generated market value can be estimated at around $80 billion. [2] Moreover, there are countries whose economies depend heavily on fossil fuels like, for example, Norway for which crude oil accounts for 35% of the total exports. [3] As a result, giving up on fossil fuels in a short period could have negative implications in certain parts of the world. Hydrogen is a type of fuel that could solve this challenge. While a complete switch to hydrogen as a source of energy, in a form of so-called Hydrogen Economy, seems unrealistic it is still a viable energy carrier for transportation. [4,5] H2 it is lightweight, can provide energy on demand through combustion engines or fuel cells with reduced emission of pollutants. At the same time, hydrogen can be currently produced from both crude oil and natural coal and, hence, sustain fossil fuel-dependent economies. [6] From the perspective of the next century, a gradual transition from using fossil fuels could be implemented to generate hydrogen from either biomass or by a direct splitting of water molecules in the photoelectrolysis or high-temperature electrolysis processes. [7]
If switching from the combustion of fossil fuels is unavoidable, hydrogen might be a good opportunity to make the switch gradual in the transportation industry, first by reducing emission and keeping the fossil fuels as source material. Next, as fossil fuel deposits become depleted, a gradual switch to other means of production can be easier for the industry. However, there are several details that have to be addressed to determine whether H2 is a suitable option as a fuel - how does it compare to other solutions, how can it help in fighting climate change, is it safe, and is it realistic to produce it on a mass scale.
Hydrogen atoms are the smallest ones in the periodic table and can form bonds with each other. These characteristics allow them to store energy and keep the mass of the fuel low compared to other substances that contain different atoms (like carbon in hydrocarbons). The lightweight of the fuel allows it to achieve extraordinarily high energy density, much higher than any other approach. Energy density and energy per volume of selected fuels and energy storage media is summarized in Fig. 1. The high energy density of hydrogen can be advantageous from the perspective of powering all sorts of green vehicles, compared to heavy lithium-ion batteries hydrogen has an advantage which potentially can lead to either lighter design or longer travel range. Moreover, fueling a hydrogen-based car is significantly faster than charging a large battery. The presented data might suggest that hydrogen is superior fuel, however, we have to discuss it in an appropriate context. First of all, it can be seen that the energy stored per volume is quite low for H2, this leads to several issues - hydrogen to be effectively used has to be either liquified or compressed which takes energy. Moreover, high-pressure tanks are often considered as not safe and prone to explosions. Secondly, the method of production and the effective decrease of GHG emission has to be discussed. Both of these will be discussed in the following sections.
Discussing hydrogen as a fuel for vehicles cannot be complete without considerations about the form of the fuel used and the type of container. As mentioned before, the energy per volume for this compound is very small, hence it has to be either liquified or pressurized. Liquefaction requires temperatures as low as -253°C and an extensive amount of energy, pressurizing is a more typical choice for car manufacturers. The downside of this approach lies in safety concerns, as the general public fears about hydrogen leaks causing vehicle fires and potential explosions during car collisions. To understand this topic it is worth to remember that hydrogen is 14 times lighter than air and dissolves in it very fast after any leak or puncture. [4] Comparison of the diffusion coefficient of hydrogen to other popular fuels is presented in Fig. 2. [9-11] Note that diffusion coefficient, or - the rate of the gas dissipating after the leak is much faster than traditional fuels. H2 is also lighter than air so it does not get trapped near the ground like, for example, gasoline vapor so it does not pose a large fire threat. Besides, modern hydrogen vehicles are equipped with state-of-the-art sensors and ventilation systems that allow detecting any leaks and shutdown valves to prevent them. Appropriate placing of the ventilation system allows reducing fire and explosion danger, at the same time leaks turn out to be not very dangerous as hydrogen diffuses to safe concentrations very fast. [12] Similar systems prevent explosions by fast venting the tank if necessary. [12] All of this prevention and strict regulations allowed Hyundai Nexo to earn the highest, 5-star rating in the Euro NCAP test in 2018, which included a series of crash tests. [13] Hydrogen leaks still have to be treated seriously because the gas is odorless and the flame is hardly visible to the naked eye but it seems that technology can help make hydrogen vehicles safe.
 |
| Fig. 2: Comparison of diffusion coefficients of hydrogen and other popular fuels. [9-11] (Source: H. Stokowski) |
The current yearly production of hydrogen is around 65-100 million tons, as it has been used for decades in other industries. [14] Hydrogen production is one of the main concerns in the scientific community because of its low efficiency. However, most of the criticism focuses on Hydrogen Economy, replacing energy lines with hydrogen delivery systems and converting between electricity and hydrogen two times - on the supplier side and on the customer side. [5,6] This is not viable. But in the context of powering cars, local production or small-scale transportation of hydrogen is not off the table. Currently, the most popular source of hydrogen is natural gas with 95% of H2 produced in the U.S. coming from this source. [4] The production relies on steam methane reforming, in which 4 moles of H2 are produced out of 1 mol of CH4 with a steam treatment. This means that the end-user can use about 1.14 MJ/mol of power from the resulting hydrogen, compared to 0.89 MJ/mol from the initial methane. There is no net gain though because reforming requires energy necessary to form steam and dive the reaction - total of ΔH0 = 257.3 kJ/mol. [15] The complete reaction can be described as follows:
The current steam reforming techniques are limited in terms of efficiency to about 70-80%, hence, burning hydrogen will be less efficient than burning natural gas directly. [15] Interestingly, the amount of CO2 produced in the steam reforming is exactly the same as one would get by burning the natural gas directly. From here, it can be seen that using methane directly would be more suitable economically and the only rationale for using hydrogen could be working towards controlled, local production if the collection of CO2 was possible. This way one could avoid burning natural gas in vehicles' internal combustion engines and releasing byproducts directly to the atmosphere.
Another option for H2 production is electrolysis, where water molecules are split by the electric voltage applied to a special cell. In this mechanism main criticism comes from low efficiency, the ratio of energy input to the total H2 energy delivered is about 1.59. [6] However, authors rarely discuss the process of photoelectrolysis, in which the electrolysis process is directly driven by absorbing light, similarly as photovoltaic cells generate electricity. [7] It turns out that the main factor to the low performance of the electrolytic approach is electrolysis itself, which accounts for a factor of 1.33 in the mentioned energy ratio. Replacing this step with a completely photoelectrolytic process would allow for a net energy gain in locally-driven photoelectrolytic centers once the technology is mature. Non-electric water splitting is also considered in a thermal process in nuclear reactors. [5] Achieving high efficiency and low-cost direct production of hydrogen seems reasonable when one of these technologies matures and is ready to replace the production based on fossil fuels. Working towards this goal can result in real zero-emission, hydrogen-based vehicles.
To complete the discussion we have to mention two ways of powering cars that can be considered and discuss whether or not they can help reduce GHG emission. One of them, which takes the lead in recent years is using hydrogen fuel cells. This approach is used in cars like Hyundai Nexo, Toyota Mirai, or Honda Clarity and allows for achieving zero-emission. These vehicles operate on electric engines, powered by an electrochemical reaction taking place in specially designed cells with the use of hydrogen and oxygen. The disadvantage is that fuel cells are relatively expensive, mainly because of the price of proton-exchange membranes used for their production. An alternative approach is burning hydrogen in an internal combustion engine, similar to today's gasoline or diesel motors. The major advantage of H2-powered units is that ideally there is no CO2 generated as a byproduct. Burning hydrogen in an oxygen environment should only yield water:
The reality is more complex because air contains other gases too, including nitrogen. As a result, nitrogen oxides (NOx), which are major greenhouse gases, can be created and pollute the atmosphere. However, studies show that burning hydrogen in a non-stoichiometric mix with air can significantly reduce the amount of nitrogen oxides generated down to less than 1 ppm. [16-18] This can be achieved by changing the air/hydrogen mix injected to the combustion chamber or exhaust gas recirculation. Currently, large interest is aimed towards combustion engines running on mixtures of hydrogen and methane, which can achieve the same goal with a proper ratio of H2 and CH4, however, the subject literature presents data confirming that the same results can be achieved with appropriate equivalence ratio for 100% hydrogen combustion. [19]
Hydrogen promise of a green economy has faced criticism because of the ideas of Hydrogen Economy but running the entire economy on hydrogen might not be feasible. However, some industries can still benefit from using it - the automotive sector could reduce GHG emissions by gradually increasing the share of H2 vehicles in the market while keeping the traditional fossil fuel sector from going bankrupt. From an economical standpoint, it seems more reasonable to try using natural gas instead. However, converting it into hydrogen can accelerate the development of the technology which could become true zero-emission once methods of production like photoelectrolysis or high-temperature electrolysis become mature. Recent technology advancements show that H2 cars are becoming safer and break to the industry mainstream. As a result, hydrogen might not dominate the green energy of the future, but certainly can make its contribution for the well-being of the planet while keeping our standard of life.
© Hubert Stokowski. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The European Green Deal," European Commission, COM(2019) 640 Final, December 2020.
[2] "BP Statistical Review of World Energy 2020," British Petroleum, June 2020.
[3] "Economic Survey: Economic Developments in Norway," Statistics Norway, June 2020.
[4] R. Hammerschlag and P. Mazza, "Questioning Hydrogen," Energy Policy 33, 2039 (2005).
[5] U. Bossel, "Does a Hydrogen Economy Make Sense?" Proc. IEEE 94, 1826 (2006).
[6] "Hydrogen Posture Plan," U.S. Department of Energy, December 2006.
[7] A. Fujishima and K. Honda, "Electrochemical Photolysis of Water at a Semiconductor Electrode," Nature 238, 37 (1972).
[8] K. Mazloomi and C. Gomes, "Hydrogen as an Energy Carrier: Prospects and Challenges," Renew. Sust. Energ. Rev. 16, 3024 (2012).
[9] E. L. Cussler, Diffusion: Mass Transfer in Fluid Systems, 3rd Ed. (Cambridge University Press, 2009).
[10] R. W. Elliott and H. Watts, "Diffusion of Some Hydrocarbons in Air: a Regularity in the Diffusion Coefficients of a Homologous Series," Can. J. Chem. 50, 31 (1972).
[11] I. Al Zubaidy et al., "Diffusion Coefficients of UAE Gasoline as Inputs to Some Environmental Transport or Risk Assessment," J. Clean Energy Technol. 1, 49 (2013).
[12] "Analysis of Published Hydrogen Vehicle Safety Research," National Highway Traffic Safety Administration, U.S. National Highway Traffic Safety Administration, DOT-HS-811-267, February 2010.
[13] "Hyundai NEXO," European New Car Assessment Programme, October 2018.
[14] E. Connelly, A. Elgowainy, and M. Ruth, "DOE Hydrogen and Fuel Cells Program Record," U.S. Department of Energy, 1 Oct 19.
[15] B. Sørensen, Hydrogen and Fuel Cells, 2nd Ed. (Elsevier, 2012).
[16] J. W. Heffel, "NOx Emission and Performance Data for a Hydrogen Fueled Internal Combustion Engine at 1500 RPM Using Exhaust Gas Recirculation," Int. J. Hydrog. Energy 28, 901 (2003).
[17] C. M. White, R. R. Steeper, and A. E. Lutz, "The Hydrogen-Fueled Internal Combustion Engine: A Technical Review," Int. J. Hydrog. Energy 31, 1292 (2006).
[18] S. Verhelst and T. Wallner, "Hydrogen-Fueled Internal Combustion Engines," Prog. Energy Combust. Sci. 35, 490 (2009).
[19] S. O. Akansu et al., "Internal Combustion Engines Fueled by Natural Gas-Hydrogen Mixtures," Int. J. Hydrog. Energy 29, 1527 (2004).