HYDROGEN,PROPERTIES, HYDROGEN PRODUCTION BY ELECTROLYZERS, STORAGE,TRANSPORTATION, AND HYDROGEN INTERNAL COMBUSTION ENGINE FOR VEHICLE APPLICATION
vijay tharad
Director Operations at Corporate Professional Academy for Technical Training & Career Development
Hydrogen as a fuel for fuel cell electric vehicles
The transport sector is the most challenging with respect to transitioning to a 100% renewable society. It accounts for 14 % of global greenhouse gas emissions amongst economic sectors . Therefore, to achieve our goal of a fully renewable energy system, there must be a key focus focus on transportion.
Hydrogen has many applications, and many people see it as the clean fuel of the future when it is generated from water and returns to it oxidized. Hydrogen-powered fuel cells are increasingly seen as promising pollution-free sources of energy and are now being used in cars and buses. Furthermore, hydrogen is used in the chemical industry to produce ammonia for agricultural fertilizer (the Haber Bosch process) and cyclohexane and methanol, which are intermediates in the production of plastics and pharmaceuticals . Methanol is now also used as a fuel for transportation applications. Here we will focus on hydrogen as a fuel for fuel cell electric vehicles (FCEVs).
Properties of hydrogen
Gaseous hydrogen has some outstanding specifications compared to other fuel types, as can be seen in table 1.
? Petrol Methane Propane Hydrogen
Lower explosion limit (%, air) 15 5 2.1 4
Upper explosion limit (%, air) 8 15 9.5 75.6
Flash point?? ?C -20 -188 -104 -270.8
Lowest ignition energy mJ 0.8 0.3 0.25 0.017
Density (20? ?C, 1 bar) 0.7-0.78 kg/l 0.718 kg/m3 2.01 kg/m3 .089kg/m3
Boiling point? ?C 30-215 -161.5 -42 -252.7
Critical temperature? ?C -82.5 96.6 -239.3
Critical pressure bar 45 42.2 13
Diffusion coefficient cm2/s 0.16 0.12 0.61
Table 1: Fuel specifications
As can be seen in Table 1, hydrogen has a very wide flammability range (lower and upper explosion limit) compared to other fuels, at between 4% and 75%. The optimal combustion condition is a 29% hydrogen-to-air volume ratio. Detection sensors are almost always installed in hydrogen systems to quickly identify any leak and minimize the potential for undetected flames.
As mentioned above, hydrogen is the smallest known molecule. It has a low viscosity, which is why it is prone to leakage. In a confined space, leaking hydrogen can accumulate and reach flammable concentrations. Any gas other than oxygen is an asphyxiator in sufficient concentrations. In a closed environment, leaks of any size are a concern, as hydrogen is impossible for human senses to detect and can ignite in a wide range of concentrations in air. However, proper ventilation and the use of detection sensors can mitigate these hazards.
Hydrogen has the smallest ignition energy, much lower than that required for other common fuels. This means that small sparks can easily ignite it.
Hydrogen has high energy content by weight (density) but not by volume, which is a challenge for storage. In order to store sufficient quantities of hydrogen gas, it is compressed and stored at high pressures. As can be seen in Table 1, the critical pressure for gaseous hydrogen is 13 bar. For comparison, hydrogen is compressed to 350-700 bar in storage tanks in FCEVs. For safety, hydrogen tanks are equipped with pressure relief devices that prevent the pressure in the tanks from becoming too high?[4].
The easiest way to decrease the volume of a gas, at constant temperatures, is to increase its pressure. Thus, at 700 bar, hydrogen has a density of 42 kg/m3, compared to 0.089 kg/m3 under normal pressure and temperature conditions. At this pressure, 5 kg of hydrogen can be stored in a 125 liter tank?[5]. ?
As can be seen in Figure 1, the density of hydrogen highly depends on the temperature and pressure.
?Figure 1: Hydrogen density at different temperatures and pressures.?[5]
Due to its weight, hydrogen has a high diffusion rate, which results in rapid dispersion. This means that if a hydrogen cloud comes into contact with an ignition source in an open space with no confinement, flames will propagate through a flammable hydrogen-air cloud at several meters per second, and even more rapidly if the cloud is above ambient temperature [4].
Hydrogen can, nevertheless, be used as safely as other common fuels when simple guidelines are followed. This will be dealt with in the sub-section: Standards and regulations.
The application of hydrogen in the transport sector
Hydrogen can be used in three different ways in relation to transportation:
This paper will focus on FCEVs. There are two main reasons why FCEVs are superior to electric vehicles (EVs): 1) shorter refueling times; and 2) longer ranges?[6]. FCEV refueling times are only a few minutes, while an EV needs several hours fully recharge. FCEVs are more efficient than conventional internal combustion engines vehicles and produce no tailpipe emissions, only emitting water vapor and warm air. FCEVs use a propulsion system similar to that of electric vehicles, where energy stored as hydrogen is converted into electricity by the fuel cell. Furthermore, they are equipped with advanced technologies to increase efficiency, such as regenerative braking systems that capture the energy lost during braking and store it in a battery [7].
Today, most car manufacturers have opted for a solution that consists of storing hydrogen in gaseous form at high pressure. This enables the storage of enough hydrogen to allow a FCEVs to travel between 500 and 600 km between refuelings [8].
Hydrogen fuel technology has undergone a huge improvement in the last few years. In fact, there is significantly more energy and explosive potential in a gasoline fuel tank than in a hydrogen fuel cell tank. Furthermore, various sensors are emplaced on hydrogen-fuel cell vehicles to manage possible leaks, such as [9]:
An overview of the differences between gasoline/petrol cars, EVs and FCEVs can be seen in the table below.?
Car typeGasoline/petrolEV
Hydrogen purity
Hydrogen purity or quality is a term to describe the lack of impurities in hydrogen as a fuel gas. The purity requirement varies with the application. A hydrogen internal combustion engine can tolerate low hydrogen purity, whereas a hydrogen fuel cell requires high hydrogen purity to prevent catalyst poisoning. Impurities in hydrogen (even in the ppm and ppb range) have a severe effect on the performance of fuel cells. Thus, it is crucial to be able to detect any impurities before a fuel is used. An international standard (ISO 14687-2:2012) has been published to specify the impurities and levels that must be identified, as is shown in Table 3.
Hydrogen storage
Hydrogen can be stored physically as either a gas or liquid. It typically requires high-pressure tanks (350-700 bar tank pressure). Another possibility is the chemical storage of hydrogen, whereby it is stored on the surface of solids (by adsorption) or within solids (by absorption). The automotive application utilizes the physical storage of hydrogen [19].
The storage tanks in cars must withstand high pressures and be able to store hydrogen without any leakage. Several car manufacturers today use compressed hydrogen tanks in their cars, which are capable of 350 and 700 bars, depending on the automotive type (light duty/heavy duty).
As mentioned above, there are two main varieties of hydrogen fuel tanks. The most common hydrogen fuel tank for cars, trucks, buses and other vehicles is the compressed hydrogen gas tank. Most hydrogen fueling stations currently dispense compressed hydrogen and only a few carry cryogenic liquid hydrogen. This is because almost all car manufacturers have chosen to fuel their cars with compressed hydrogen gas. BMW is one exception, with their dual fuel “Hydrogen 7 automobile” that uses cryogenic hydrogen and gasoline.
One disadvantage of cryogenic hydrogen tanks in recent years has been that they can have boil-off problems. This means that the liquid hydrogen will, over time, find its way out of the tank and evaporate. This is only the case when the car is left alone in one place for a couple of weeks, e.g., an airport.? Furthermore, it is necessary to store cryogenic liquid hydrogen at a temperature below negative 253 degrees Celsius to maintain its liquidity. This demands a technologically advanced freezer system.
Compressed hydrogen fuel tanks are now made of carbon fiber composites or carbon fiber and metal alloys and composites. The inner line of the tank is a high-molecular weight polymer that serves as a hydrogen gas permeation barrier. The outer shell is placed on the tank for impact and damage resistance. A pressure regulator and an in-tank gas temperature sensor are located in the tank’s interior in order to monitor the pressure and temperature during the gas-filling process.
Compressed hydrogen gas tanks can have different interiors.
When the hydrogen is stored in the porous metal hydride material, the gas is released by adding a small amount of heat to the tank. The disadvantage of this is that metal hydrides are generally very heavy, which will cut down the range per liter of fuel in the vehicle.
The goal is to find a better way to store hydrogen that is not as costly as metal hydrides or related methods under development. Hydrogen tanks must be lighter, hold more volume and cost less than they presently do?[19].
Several studies have been conducted on material-based hydrogen storage to further improve storage potential. These studies have investigated metal hydride, chemical hydrogen storage and sorbent materials?[21]. Scientists and researchers are currently working on this issue and, as with many other technology-driven challenges, the future will most likely hold a variety of viable solutions.
Nevertheless, however, today’s hydrogen fuel tanks are safe due to several safety measures and requirements related to ATEX (European directives for controlling explosive atmospheres) approval requirements.
Standards and regulations
Hydrogen has a few unique properties that require special consideration, as described in the first section, Properties of Hydrogen. The same considerations apply as with any fuel; safe handling depends on knowledge of its particular physical, chemical and thermal properties and consideration of safe ways to accommodate these. Hydrogen, when handled with knowledge, is a safe fuel.
To ensure that hydrogen is handled properly, the International Organization for Standardization (ISO) is developing international safety standards; the Canadian Hydrogen Installation Code (CHIC), for instance, defines the requirements applicable to the installation of hydrogen equipment while the Society of Automotive Engineers (SAE) defines standards whereby the principal emphasis is placed on the transportation industry. Several standards for hydrogen applications have also been published during the last few years?[22]:
?Storage and transport:
There are international standards being developed specifically for both the stationary and portable storage of hydrogen, which is critical for ensuring safety in the hydrogen industry.
Designation Title
ISO 19884 Gaseous hydrogen – Cylinders and tubes for stationary storage
ISO 16111 Transportable gas storage devices – Hydrogen absorbed in reversible metal hybride
?
Safety:
Safety is a critical factor to be considered and is vital for satisfying community expectations and furthermore, to ensuring workforce and environmental health and safety.
Designation Title
ISO/TR 15916 Basic considerations for the safety of hydrogen systems
ISO 26142 Hydrogen detection apparatus – Stationary applications
Summary
Benefits of hydrogen as a fuel for fuel cell electric vehicles:
+ No tailpipe pollution
+ Non-toxic, generated from water and returning to water when oxidized
+ Range and refueling time is comparable to ICEs
+ 14 times lighter than air, rise and disperses rapidly
+ High energy content by mass
?
Drawbacks of hydrogen as a fuel for fuel cell electric vehicles:
- Low viscosity – difficulties in storing hydrogen
- Highly explosive and dangerous in closed environments
- Pressurized gaseous fuel requires special engines and infrastructure
- Low energy content by volume
?.
https://www.bmw.com/en/innovation/how-hydrogen-fuel-cell-cars-work.html
WHAT IS? HYDROGEN?
Representing 92% of the atoms in the universe (and 75% of its mass), hydrogen is the simplest and most common atom. It consists of a single proton and a single electron.
The dihydrogen molecule (H2), is often referred to as hydrogen. Now that we have cleared that out, and for the sake of simplicity we will keep referring to it as hydrogen.
THE DISCOVERY OF HYDROGEN
EARLY USES OF HYDROGEN
1800:
Half of the lighting gas is hydrogen
1878:
The inflatable balloon is filled with hydrogen
1852-1937:
Hydrogen, which is 11 times lighter than air, was also used in airships… until the 6th of May 1937, when the Hindenburg airship crashed in New Jersey. This tragic accident* along with the growing popularity of aircraft led to the end of airships’ use.
*This tragic accident wasn’t caused by hydrogen.
HYDROGEN AS A RAW MATERIAL
Following World War II, new uses of hydrogen as a raw material in the industry were found (petrochemistry, steel, food-processing…)
HYDROGEN: ENERGY OF THE SPACE INDUSTRY
New applications for hydrogen in space were found:
As a fuel, a mix of liquid hydrogen?is used to throw space rockets, such as Ariane 5 and Saturn V
As a reliable power supply system in space missions (Apollo, Gemini, Space Shuttle) through the development of compact fuel cell
NEW APPLICATIONS FOR HYDROGEN
Hydrogen as an energy carrier?
Produced from zero-emission or renewable electricity:
Hydrogen can then be stored, transported and used in all kinds of energy applications, such as fuel'
FOR ONE PURPOSE
ZERO EMISSIONS? ? ? ? ? ? ? ?ZERO COMPROMISE?
PRODUCING HYDROGEN USING AN ELECTROLYSER
As of today, hydrogen is mostly produced from natural gas which leads to tremendous CO2 emissions. Another way of producing hydrogen?is coming into being, that is electrolysis. A process through which hydrogen?can be produced from decarbonised or renewable electricity. But…
WHAT IS ELECTROLYSIS?
The principle of electrolysis is quite simple. A direct current is injected into an electrolytic solution, to separate the water molecules into hydrogen?and oxygen.
LET’S SEE HOW AN ALKALINE ELECTROLYSER WORKS
An alkaline electrolyser cell consists of two electrodes immersed in a bath of electrolytic solution. Strong direct currents are injected, and that’s it!
Hydrogen will appear at the cathode and oxygen will appear at the anode.
The electrolyser is made up of hundreds of cells stacked one on top of the other, and the whole thing is commonly known as a stack.
HOWEVER ALKALINE ELECTROLYSIS IS NOT
THE ONLY TECHNOLOGY USED.
THE MAJOR FAMILIES ARE:
PEM
Proton Exchange Membrane electrolysers
Alkaline
Alkaline electrolysers
AEM
Anion Exchange Membrane electrolysers
SOEC
Solid Oxide Electrolyser Cell electrolysers
Operating temperature - efficiency - maturity - deployment:
The general principle is the same: breaking up water molecules with electricity using different techniques:
PEM
The electrolyte solution is replaced by a solid polymer membrane.
The protons (H+) pass through it and form H2 at the cathode.
Alkaline
These are the electrolysers described above.
The electrolyte is an aqueous liquid solution. The hydroxide ions (OH-) flow through the membrane to form O2 at the anode.
AEM
Unlike PEM electrolysers, here it is the anions (H-) that pass through and form the H2 at the anode.?
SOEC
Here the electrolyte is in the form of a solid ceramic.
At the cathode, the hydrogen is separated from the oxygen ions (O2–), which pass through the ceramic membrane and form O2 at the anode.
THE MAIN DIFFERENCES:?
Each technique has its advantages and disadvantages.
PEM
Advantages: compact size, high current density, rapid production time-
Disadvantages: Use of Rare materials, high production cost, sensitive to bivalent ions (calcium and iron)?
Alkaline
Advantages: reliability, low production costs, not sensitive to bivalent ions-
Disadvantages: low efficiency, operation at low current density, bulky, long start-up time
AEM
Advantages: compact size, high current density, rapid production time, reliability, low production costs, not sensitive to bivalent ions?-
Disadvantages: less mature than other conventional techniques
SOEC
Advantages: high efficiency-
Disadvantages: requires high temperatures, low maturity level
HOW DOES A FUEL CELL WORK?
How does a fuel cell work?
Hydrogen can actually be used in fuel cells to generate electricity; for instance, in a vehicle. But…
WHAT IS A?FUEL CELL?
A FUEL CELL IS:
a magic box that turns hydrogen and oxygen into electricity!?
Well, not exactly. A fuel cell does in fact allow the production of electricity from hydrogen and oxygen. However, no magic tricks are involved in the process. Let’s see then how a PEM fuel cell works!?
First, let’s see what they’re made of. A fuel cell is made up of several layers: in the middle we have a membrane or an electrolyte separating the two electrodes, the anode and the cathode. The whole thing is then compressed between two flow plates.?
?
Hydrogen travels through the small channels in one plate, and oxygen through the other. The membrane in the middle has a secret, it acts as a filter that lets protons through but not electrons.?
Gaseous hydrogen arrives through small channels in the outer plate. When it reaches the anode, the latter’s catalytic power breaks it in two. Two protons and two electrons are formed. The protons travel across the membrane to the other side, where they attach themselves to the oxygen to form… water!?
The electrons can’t get through, remember? They have to take another route to reach the other side. It is this flow of electrons that produces an electricity that can be used to power an electric motor for any other application?.
REMEMBER! THERE ARE SEVERAL TYPES OF FUEL CELLS, NAMELY:?
PEMFC
Proton Exchange Membrane Fuel Cells
AFC
Alkaline Fuel Cells
SOFC
Solid Oxide Fuel Cell
Operating temperature – maturity – deployment:
However, these are only the main ones, there is also the MCFC?(Molten Carbonate Fuel Cells), the PAFC (Phosphoric Acid Fuel Cells) …
THE MAIN DIFFERENCES
To each technique its advantages and disadvantages.?
PEM fuel cells are highly adapted for mobility use. They are light, efficient and reactive.??
Alkaline fuel cells are the most mature and are even used by NASA in the Space Shuttle!?
But they are loosing ground to PEM fuel cells.?
The competition is fierce…?
SOFCs have excellent efficiency but require high temperatures. They are well suited for high-power applications.
Hydrogen Production: Electrolysis
Electrolysis is a promising option for carbon-free hydrogen production from renewable and nuclear resources. Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer. Electrolyzers can range in size from small, appliance-size equipment that is well-suited for small-scale distributed hydrogen production to large-scale, central production facilities that could be tied directly to renewable or other non-greenhouse-gas-emitting forms of electricity production.
How Does it Work?
Like fuel cells, electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in different ways, mainly due to the different type of electrolyte material involved and the ionic species it conducts.
Polymer Electrolyte Membrane Electrolyzers
In a polymer electrolyte membrane (PEM) electrolyzer, the electrolyte is a solid specialty plastic material.
Alkaline Electrolyzers
Alkaline electrolyzers operate via transport of hydroxide ions (OH-) through the electrolyte from the cathode to the anode with hydrogen being generated on the cathode side. Electrolyzers using a liquid alkaline solution of sodium or potassium hydroxide as the electrolyte have been commercially available for many years. Newer approaches using solid alkaline exchange membranes (AEM) as the electrolyte are showing promise on the lab scale.
Solid Oxide Electrolyzers
Solid oxide electrolyzers, which use a solid ceramic material as the electrolyte that selectively conducts negatively charged oxygen ions (O2-) at elevated temperatures, generate hydrogen in a slightly different way.
Solid oxide electrolyzers must operate at temperatures high enough for the solid oxide membranes to function properly (about 700°–800°C, compared to PEM electrolyzers, which operate at 70°–90°C, and commercial alkaline electrolyzers, which typically operate at less than 100°C). Advanced lab-scale solid oxide electrolyzers based on proton-conducting ceramic electrolytes are showing promise for lowering the operating temperature to 500°–600°C. The solid oxide electrolyzers can effectively use heat available at these elevated temperatures (from various sources, including nuclear energy) to decrease the amount of electrical energy needed to produce hydrogen from water.
Hydrogen Electrolyzers : Why They Matter for Sustainable Energy
Introduction to Electrolyzers
If solar power is defined by solar cells and wind production propelled by wind turbines, then the equivalent for green hydrogen production is the electrolyzer.
Put another way, an electrolyzer serves as “the building block of green hydrogen,” Plug President and CEO Andy Marsh told Bloomberg in July 2022.
B. Benefits of Electrolyzers
Electrolyzers that use renewables to power their hydrogen production – like wind, solar, hydroelectric power, or certain biofuels – churn out emissions-free production of green hydrogen.
Down the value chain, green hydrogen will prove increasingly crucial for the production of green ammonia and methanol, chemical compounds crucial for the future of more sustainable agricultural practices, chemical production, and seaborne shipping.
Beyond output value, electrolyzers can also be used for longer-term energy storage, producing hydrogen that is stored in pressurized vessels for later use, with “much higher storage capacity compared to batteries (small scale),” according to the alternative energy advocacy organization American Clean Power.
C. Components of Electrolyzer Stacks
Electrolyzers essentially function like fuel cells, but in reverse.
At the most basic level, water is provided to the electrolyzer stack, which looks like a multi-decker sandwich. The three layers in PEM electrolyzers utilized by Plug – called a cathode, an anode and a special membrane painted with a catalyst to separate the water into hydrogen and oxygen, using electricity to power the reaction. The hydrogen is then captured and stored for a variety of end uses.
D. Different Types of Electrolyzers
There are three different water-splitting electrolyzer types: Alkaline, Solid Oxide, and Proton Exchange Membrane, the latter of which is utilized by Plug.
Alkaline electrolyzers use water and a liquid electrolyte, generally either potassium hydroxide? or sodium hydroxide. They run at an internal temperature of just below 100°C. Solid oxide electrolyzers, sometimes also known as solid oxide electrolysis cells (SOECs), make use of a solid oxide ceramic electrolyte in producing hydrogen. Those run at internal temperatures of 500oC to 850oC.
And PEM electrolyzers utilize a proton exchange membrane functioning as a solid polymer electrolyte. When electricity runs through the stack during the water electrolysis process, the water splits into hydrogen and oxygen. Hydrogen then passes through the membrane and ejects on the cathode side. PEM electrolyzers run at ideal temperatures of 60oC to 80oC.
E. Electrolysis Process
The water electrolysis process utilized within Plug’s PEM electrolyzers is an electrochemical process that splits water into hydrogen and oxygen gases. At its simplest, the stack consists of a multi-decker sandwich of anodes and cathodes separated by proton exchange membranes coated in catalyst.
Here’s a simple explanation of how a PEM electrolyzer works:
Those aiming to produce green hydrogen, for which Plug is working to be a global-leading producer, make use of renewable energy sources like wind, solar, biomass, or hydro-electric power.
F. Factors Affecting Electrolysis Efficiency
While many factors determine the efficiency of the electrolysis process, temperature serves as a significant driving force.
This makes SOECs the most efficient electrolyzer fuel cell stack array according to most experts, but the extremely high temperature it requires to function also makes it the least versatile for that same reason. That’s because it takes more energy and time to power up to temperatures needed to carry out the electrolysis process. In other words, efficiency always comes within its own context and sits in the eye of the beholder.
Other key factors affecting electrolysis efficiency include electrolyte quality, electrical resistance of the electrolyte, the type of electrode material used, pressure applied for the current within the electrolyzer, varying types of separator and catalyst materials, and applied electricity voltage waveform.
A study published in the Journal of Electrochemical Science and Technology concluded that “concentration of [potassium hydroxide in] the electrolyte significantly affected the electrolyser performance” and that PEM electrolyzers “offer several advantages over traditional technologies including…higher production rates and more compact design,” in pointing to other electrolyzer efficiency factors.
G. Hydrogen Storage and Transportation
Hydrogen can be both stored and transported multiple ways.
For storage, while pressurized vessels are still useful for materially significant levels of storage, for larger amounts, underground caverns or ground-based storage tankers serve as the primary means of storing hydrogen.
The U.S. Department of Energy’s National Energy Technology Laboratory has also cited the prospect of storage in metal hydrates, surface-area absorption, and legacy natural gas pipelines as prospective hydrogen storage mechanisms. The agency states, however, that all of these except for the pipeline option are “still in the early stages of research and development, and as of yet it has not been shown to be energy or cost efficient.”
And for transportation, due to lack of pipeline infrastructure, the dominant method of moving hydrogen to market today is via cryogenic long-haul trucks holding liquefied hydrogen created via the liquefaction process.
And in order to facilitate that process, Plug produces two varying-sized liquefaction products for customers seeking to market their hydrogen, a more economical way of carrying the fuel than in its compressed gaseous state, due to its ability to superchill (at -253°C) and condense the gas to liquid. This enables hydrogen marketers to carry more of it per tanker.
Yet, the most efficient way to move hydrogen is via pipelines, only 1,600 miles of which exist today in the U.S. That compares to over 2 million miles of natural gas pipelines, with Plug CEO Andy Marsh calling for “a vastly more expansive [hydrogen pipeline] system” comparable to the gas industry in July 2022 congressional testimony.
For a more comprehensive review of the broader spectrum of storage and transportation options for moving green hydrogen from upstream electrolyzer to downstream storage point, check out the 2022 article published by Canadian researchers in the International Journal of Hydrogen Energy.
H. Industrial and Commercial Applications of Electrolyzers
One of the major current commercial applications for green hydrogen produced from electrolyzers is fueling stations, which can fuel forklifts, e-mobility, and larger vehicles.
Think of the fuel stations as akin to gas stations dispensing refined diesel or gasoline for vehicles or electric charging stations for battery-powered vehicles. Plug currently stands as a global leader in fueling stations, with customers using them to fuel over 60,000 fuel cell systems used in material handling operations worldwide.
An up and coming application for green hydrogen is Ammonia production. With 70% of all ammonia currently being made from natural gas, and the rest coming from coal, production under the status quo fuels climate change. The EIA has surmised that non-green ammonia is “twice as emissions-intensive as crude steel production and four times that of cement, on a direct CO2 emissions basis.”
But Plug, via its Proton Exchange Membrane (PEM) electrolyzers, can mitigate the greenhouse gas emissions currently embedded in ammonia production and make the vital food-making ammonia fertilizer more sustainable. With stakeholders pushing for industrial actors to green their production processes, Plug’s technology capably aids producers in decarbonizing the chemical compound into green ammonia.?
Another application for hydrogen after made from electrolyzers is power-to-gas.
And though still an emerging technology, at least compared to fueling stations which directly dispense hydrogen to customers, power-to-gas applications could be a major part of green hydrogen’s future and a way to make the electricity grid less greenhouse gas-intensive. The process involves injecting hydrogen into the natural gas pipeline as a refined synthetic methane.
I. Maintenance and Safety of Electrolyzers
Maintenance of electrolyzers, as with any industrial equipment, is important for ensuring their optimal functionality and safety.
H2 Tools, an information-sharing database built by the Pacific Northwest National Laboratory of the U.S. Department of Energy, has conveyed that “Proper and timely inspection and maintenance is key to ensuring safe system operation” and that “Reactive maintenance is generally unwise for equipment in hydrogen service.” The Laboratory further recommends a “systematic method” of maintenance which “should be documented and stewarded.”
H2 Tools added that the documentation for maintenance and inspection should “ include a description of any needed follow-up activity and the next scheduled inspection/maintenance activity,” recommending that U.S. Occupational Safety and Health Administration (OSHA) Standard 1910.19 should be the standard utilized for all electrolyzer maintenance.
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Beyond maintenance, H2 Tools has also stated that those overseeing electrolyzer operations should utilize a safety culture and that “goals [in that vein] should be clearly communicated and understood by all staff.” The U.S. Occupational Safety and Health Administration also has hydrogen workplace safety regulations on the books, holding companies accountable for the safety of their respective workforces.
J. Advancements in Electrolyzer Technology
Currently, electrolyzers are undergoing great change as they penetrate the market. PEM electrolyzers of the sort Plug uses, for example, are undergoing technological innovation to operate at increasingly higher and more efficient temperature ranges of between 120°C and 200°C.
The U.S. Department of Energy explains that high temperature electrolysis – often just called HTE by industry insiders – “results in very high electrical efficiencies and, hence, potential for low-cost hydrogen production when the steam is produced by either waste heat or a low-cost thermal energy generator.”
Another area of advancement within electrolyzer technology exists in the realm of generating green hydrogen through solar-powered electrolyzers.
In short, this involves utilizing solar technology to generate electricity, which then thereafter powers the electrolyzers producing green hydrogen. Plug’s technology stands ready to carry out that process.
Several such projects exist worldwide, primarily involving solar panel installations placed alongside electrolyzers to ensure maximum efficiency of both energy generation techniques.
K. Future of Electrolyzers
Electrolyzers have the potential to play a key role in the decarbonization efforts of countries and industries around the world. The ability to produce and store hydrogen allows it to be used for a growing number of applications and industrial processes including hydrogen fuel cells, power to gas to power, ammonia and methanol production processes, steel and cement manufacturing among others.
A number of countries have championed hydrogen as a method for meeting their sustainability targets, putting in place policies supporting hydrogen production growth. According to the Green Hydrogen Coalition’s 2nd Edition Green Hydrogen Handbook, as of 2022, Australia, Canada, Chile, the European Union and Japan all have national programs in place. Additionally, the United States’ Inflation Reduction Act includes a robust hydrogen program. India, too, has enacted a National Green Hydrogen Mission..
As countries and companies work together to grow electrolytic hydrogen production, the sky’s the limit on the future of electrolyzers.
L. Economic Considerations of Electrolyzers
One of the biggest barriers to entry for global adoption of electrolyzers, as it stands, is the cost of producing hydrogen from them.
Yet, the cost to produce green hydrogen is project to fall drastically with the passage of the Inflation Reduction Act, which provides producers with a decade of up to $3 per kilogram tax incentives to produce the fuel.
As of 2020, according to U.S. Department of Energy figures, the levelized cost to produce green hydrogen (often called Levelized Cost of Hydrogen, or LCOH) toggled between $4 to $6 per kilogram within PEM electrolyzers. This price point, without the tax incentives, can be achieved “assuming existing technology, low volume electrolyzer capital costs as high as $1,500 [per kilowatt] and grid electricity prices of $0.05 [per kilowatt hour] to $0.07 [per kilowatt hour].”
The cost primarily remains within that range, the DOE concluded, when coupled with both utility scale solar and onshore wind. Others, the DOE pointed out, put production costs at a range of $2.50 to $6.80 per kilogram “from a mix of renewable and grid feedstocks.”
Transportation costs, too, could ultimately drive up the price – speaking to the importance of building out more infrastructure as electrolyzers enable increased green hydrogen production.
The agency’s target range is $2 per kilogram, which means green hydrogen would be produced at a profit once it reaches that level with the new production tax credit in place created by the Inflation Reduction Act. The newly legally sanctioned credits equating to “60% of the average green hydrogen project’s cost,” The Wall Street Journal reported, pointing to financial services industry leader Goldman Sachs’ data.
With the newly codified tax incentives in place, the financial services firm Lazard put the LCOH of green hydrogen from PEM fuel cells at $1.68 to $4.28 per kilogram in an April 2023 report. Plug has a clear development roadmap to green hydrogen at a cost of $$1.50 per kilogram.
M. Electrolyzers and Energy Markets
The green hydrogen electrolyzer market will be worth over $120 billion by 2033, a new report by the consultancy IDTechEx has predicted. But to achieve that, many steps will need to be taken in the next decade, experts conclude.
“The industry expects capex to come down as manufacturing capacity increases and capabilities improve through greater levels of automation,” the report notes. “Performance also has a significant impact. For example, the more efficient a system is, the lower the energy consumption…Other key performance metrics for electrolyzer systems include operating lifetime, output pressure and purity, current and power density, start-up times, dynamic range, and minimum load levels.”
The law firm Norton Rose Fulbright, which advises clients on financing hydrogen projects and making them commercial, states that on a project-by-project basis the key to financial success is to “have a bankable offtake scheme.”
The firm points to ammonia as a case in point, an area in which Plug stands ready to supply its green hydrogen to make the product more sustainable. Norton Rose Fulbright also points to speciality vehicles, like forklifts, for which Plug serves as a global leader in providing fuel cells and hydrogen infrastructure.
Lastly, the firm called government support “essential to get the green hydrogen market off the ground,” with that support including “economic incentives and by providing clear and appropriate standards and regulation applicable to the transportation of hydrogen by pipeline, truck and ship.”
N. Environmental Impact of Electrolyzers
From an environmental standpoint, electrolyzers producing green hydrogen can enable significant greenhouse gas reductions, according to a 2022 study published in the journal Sustainable Energy and Fuels.
“Hydrogen powered by offshore wind results in a twentyfold (93–97%) reduction in GHG footprint compared to grey hydrogen and a reduction of 76–94% compared to blue hydrogen,” the study details. “Using solar PV for hydrogen production…equates to a 62–85% reduction compared to grey hydrogen and is in the same range as blue hydrogen (34% increase to 73% reduction in GHG emissions compared to blue hydrogen…).”
Relatedly, a 2019 study published in the journal of Applied Energy concludes that “hydrogen production via proton exchange membrane water electrolysis is a promising technology to reduce CO2 emissions of the hydrogen sector by up to 75%, if the electrolysis system runs exclusively on electricity generated from renewable energy sources.”
O. Performance Metrics of Electrolyzers
While many performance metrics exist for electrolyzers, two are particularly important: efficiency and durabili
y.
Many factors influence the performance metrics of electrolyzers’ efficiency level, which tallies an average of 61%, according to U.S. Department of Energy statistics.
They include electrolyte quality, electrical resistance of the electrolyte, the type of electrode material used, pressure applied for the current within the electrolyzer, varying types of separator materials, and applied electricity voltage waveform.
A 2017 study also concluded that “The efficiency of water electrolysis at 350°C/100 bars increased about 17%, compared with that at 80°C/1 bar,” meaning higher temperatures have demonstrably made electrolyzers more efficient.
And for durability, Plug’s PEM electrolyzer has a life expectancy of 80,000 hours, a world-leading figure.
P. Quality Control in Electrolyzer Production
Testing procedures in place within the electrolyzer production process ensure quality control in the final product. And Plug has taken a leadership role in quality control mechanisms, participating in the May 2021 International Meeting on Fuel Cell and Electrolyzer Quality Control hosted by the U.S. Department of Energy.
Robust certification standards for green hydrogen also exist, ensuring the product is as green as advertised and marketed, as well as safe.
One of the bodies overseeing safety is the U.S. Department of Energy’s Hydrogen Safety Panel, which maintains the H2 Tools website. In place since 2003, the Panel’s current members include those representing the public and private sectors. The agency has also published a 195-page Hydrogen Equipment Certification Guide, which acts as a domestic companion to the international International Organization for Standardization safety standards for “Hydrogen generators using water electrolysis - Industrial, commercial, and residential applications.”
Similarly, the Switzerland-based Green Hydrogen Organization also maintains a Green Hydrogen Standard seal of authority and certification for hydrogen production to ensure sustainable practices, while the International Renewable Energy Agency has laid out best practices for creating Green Hydrogen Certification.
Q. Regulatory Landscape for Electrolyzers
Electrolyzers are spreading globally, most robustly at the moment in the U.S. and East Asia. That said, 26 countries have either regulatory architecture or policy roadmaps in place to promote hydrogen production, the International Energy Agency’s “Global Hydrogen Review 2022” report has laid out.
In Plug’s home country, the United States, electrolyzers got a major win via the Inflation Reduction Act passed in 2022. The law gave a stamp of approval to the green hydrogen production market by implementing a new Clean Hydrogen Production Tax Credit, a ten-year tax credit (known as 45V) yielding up to $3.00 per kilogram of hydrogen production.
Marsh, of the legislation’s passage, stated that the “passage of the Inflation Reduction Act of 2022 represents our country’s commitment to creating a cleaner future with green hydrogen.”
R. Scaling Up Electrolyzer Technology
While electrolyzer technology is on the rise, manufacturing challenges still do exist to ensure the devices penetrate major regional and global markets. The number one challenge is cost.
A 2020 paper by The Electrochemical Society points to “both the upfront capital expense as well as the operating cost incurred over the product life.” This includes the “high cost of system and cell stack materials, which need to be durable for up to 100,000 [hours]” and “the lack of a robust supply chain for high volume manufacturing to further drive down cost through economies of scale.”
Yet, the paper also notes that PEM electrolyzer technology utilized by Plug is the furthest along and serves as a case study for a “relevant pathway for industrial scale hydrogen generation with tremendous opportunity for continuing cost reduction.”
While capital production costs are one barrier, the other is infrastructure for moving the product to market. As noted above, the biggest current barrier is lack of pipeline infrastructure to efficiently move green hydrogen to market. Increased infrastructure for carrying the product on the roads, too, is needed.
S. Yield Optimization in Electrolyzers
A crucial component to maximize the production value embedded in green hydrogen-producing electrolyzers, over the years, has been to tinker with them to optimize the yield of green hydrogen produced.
One study pointed out that the “efficiency of the PV-electrolysis system was optimized by matching the voltage and maximum power output of the photovoltaics to the operating voltage of proton exchange membrane (PEM) electrolyzers,” continuing, “The optimization process increased the hydrogen generation efficiency to 12% for a solar powered PV-PEM electrolyzer that could supply enough hydrogen to operate a fuel cell vehicle.”
Another way of tinkering is through the materials that become electrolyzer components.
The National Renewable Energy Laboratory is currently performing research on higher performance catalysts and “new materials for water splitting.” Two large-scale research projects remain ongoing in that vein within the Laboratory: the Liquid Sunlight Alliance Hub and the Hydrogen Advanced Water-splitting Materials Consortium.
But it’s catalyst materials that have been of particular interest for those studying optimization of green hydrogen electrolyzers.
“The number of catalyst materials studied for green hydrogen production has increased over the past decade, while the number of materials studied for use in hydrogen storage and fuel cell production has fallen, consistent with their relative levels of technical maturity,” a 2022 study published by the American Chemical Society details.
T. Zero-Emission Hydrogen Production with Electrolyzers
Green hydrogen made from electrolyzers could be a game-changer for climate change as the market for it flourishes globally. But with every great opportunity also comes challenges.
While the opportunity is bountiful, the challenge of truly hitting the jackpot comes down to the age-old infrastructure question.
“For green hydrogen to achieve commercialization as a carbon-free fuel, the industry must overcome the classic chicken-and-egg dilemma:? putting in place the needed infrastructure – including hydrogen transport, hydrogen vehicle refueling stations, and end users equipped to use hydrogen – before hydrogen production is fully scaled,” Leidos, a defense contracting and information services company, explained in a 2021 blog post. “There is less incentive to construct the supporting infrastructure and deploy end-user technologies if the hydrogen production facilities are not available and vice versa.” Other CHALLENGES include finding suitable places for placing electrolyzers, energy loss throughout the value chain, as well as water costs.
U. Techno-Economic Analysis of Electrolyzers
According to techno-economic analyses, which measure and evaluate forward-looking economic performances of assessed technologies, can achieve parity with fossil fuel-based hydrogen between 2025 and 2040.
And in comparing electrolyzer types to one another, the Fraunhofer Institute for Solar Energy Systems ISE has concluded that the total cost of ownership of a PEM electrolyzer totals one-third that of alkaline electrolyzer.
V. Utility-Scale Electrolyzers
For green hydrogen electrolyzers to achieve material scale, integration with the existing renewable energy utilities grid will be of utmost importance.
The Climate Council — funded by financial services companies such as Blackstone, Blackrock, Capital One, and J.P. Morgan, as well as the World Bank Group — opines that “Expanding renewable generation capacity to first and foremost deliver electricity to grids should be the priority for where wind or solar power are directed.”
Fortunately, doing so is feasible, according to a 2022 review article covering the integration of green hydrogen electrolyzers with solar and wind projects published by the journal Environmental Science and Pollution Research.
The article concluded that “authorities must enhance their energy budget to encourage green hydrogen” and that “increasing production would need developing hydrogen infrastructure, a massive effort that requires a solid plan and political backing.”
“Therefore, the authorities should collaborate with recognized firms in creating green hydrogen infrastructure to develop a strategic plan for green hydrogen’s success in the market and the expansion of such infrastructure,” the study concludes.
As explained above in “H,” power-to-gas applications can also serve as a means of linking green hydrogen to the electricity grid on the back end. Excess heat generated from the electrolysis process can be fed onto it as a zero-carbon fuel source.
Power Magazine reported that, as of 2019, “About 143 power-to-gas projects producing hydrogen and/or methane have operated since 1988 in 22 countries.” A list of all existing power-to-gas projects as of September 2019 can be seen here.
W. Value Chain of Electrolyzers
Electrolyzers do not come about by magic, despite their mystique. Instead, they require a bevy of resources to come to fruition.
X. Water Management in Electrolyzers
Management of water, as in the production of anything in an era of increasing water scarcity, is top of mind in utilizing electrolyzers. In that vein, Plug’s PEM electrolyzers make use of industrial water.
It differs from drinking water, used in industries such as smelting facilities, petroleum refineries, as well as the food, paper, and chemical industries. Plug utilizes a water filtration process to remove calcium, magnesium, and other mineral deposits that would poison the electrolysis process.
Over 910,000 liters of water are used to manufacture 75,000 kilograms of hydrogen per day. This equates to the water used on a large dairy farm, though is less than used to grow alfalfa or almonds en masse.
Importantly and most notably, the green hydrogen electrolysis process uses less water than all major electricity production methods besides geothermal.
Z. Conclusion
Electrolyzers are the bread-and-butter for the growth of green hydrogen and a more sustainable energy future, particularly in tough to decarbonize sectors.
And the future looks bright for the technology. Bloomberg has reported that the outlook looks bright for electrolyzers, comparing it to the “hockey stick”-like growth seen in the past decade for solar energy’s ascent.
“Measured by the amount of power the machines consume, worldwide electrolyzer sales doubled from 200 megawatts in 2020 to 458 in 2021,” the publication reported in 2022. “They’re expected to triple this year, reaching anywhere from 1,839 megawatts to 2,464 megawatts.”
The International Energy Agency has reiterated the hockey stick thesis, hypothesizing that after reaching a global electrolyzer capacity of 8 GW per year in 2022, that could spike to 134 to 240 GW by 2030.
BloombergNEF has estimated even higher at 242 GW, noting that it believes Plug will sit among a select few companies “leading the manufacturing” of electrolyzers during that period. “Cumulatively, around $130 billion will be spent on electrolyzers between now and 2030,” the research company explained, calling what lay ahead for green hydrogen “breakneck growth.”
Hydrogen-fueled internal combustion engines
The transportation system is heavily reliant on ICEs, which consume huge amounts of liquid fossil fuels in the form of diesel and gasoline. The energy conversion of petroleum fuel in ICEs via combustion releases various pollutants such as carbon monoxide (CO), nitric oxide (NOx), unburned hydrocarbons (UHCs), particulate matter (PM) and GHGs . Compared to gasoline engines, the high energy content of H2ICEs combined with their physical and chemical properties allow these engines to operate more effectively on excessively lean mixtures. Hydrogen has greater diffusivity, much lower ignition energy (0.02 MJ), and higher ignition temperature compared to the other fuels . Hydrogen possesses greater dispersion in air than gasoline, which is majorly accredited to the ease constitution of a uniform air/fuel mixture, and the rapid dispersion of hydrogen when a leak develops. Because of the low density of hydrogen, two main issues arise when it is used in ICEs. First, to give the vehicle the ability to drive an adequate range, a large storage volume is required. Second, the power output is mitigated by the energy density of the hydrogen/air mixture.
In recent decades, many investigations have been performed to find adequate substitutions for ICEs in transportation systems; however, none of those efforts have yet succeeded in challenging the dominant role of fossil-fueled ICEs. Nevertheless, innovative techniques have been proposed to improve ICEs efficiency and emission characteristics to meet the ever-rising challenges of environmental crisis and petroleum demand.
A hydrogen internal combustion engine vehicle (HICEV) is a type of hydrogen vehicle using an internal combustion engine. Hydrogen internal combustion engine vehicles are different from hydrogen fuel cell vehicles (which utilize hydrogen electrochemically rather than through combustion). Instead, the hydrogen internal combustion engine is simply a modified version of the traditional gasoline-powered internal combustion engine. The absence of carbon means that no CO2 is produced, which eliminates the main greenhouse gas emission of a conventional petroleum engine.
As pure hydrogen does not contain carbon, there are no carbon-based pollutants, such as carbon monoxide (CO) or hydrocarbons (HC), nor is there any carbon dioxide (CO2) in the exhaust. As hydrogen combustion occurs in an atmosphere containing nitrogen and oxygen, however, it can produce oxides of nitrogen known as NOx. In this way, the combustion process is much like other high temperature combustion fuels, such as kerosene, gasoline, diesel or natural gas. Therefore, hydrogen combustion engines are not considered zero emission.
A downside is that hydrogen is difficult to handle. Due to the very small size of the hydrogen molecule, hydrogen is able to leak through many apparently solid materials in a process called hydrogen embrittlement. Escaped hydrogen gas mixed with air is potentially explosive.
Hydrogen internal combustion engine development has been receiving more interest recently, particularly for heavy duty commercial vehicles. Part of the motivation for this is as a bridging technology to meet future climate CO2 emission goals, and as technology more compatible with existing automotive knowledge and manufacturing.
In September 2022, Kawasaki unveiled a hydrogen combustion engine developed using the same injector as the hydrogen Corolla, based on the Ninja H2.
In May 2023, Yamaha, Honda, Kawasaki and Suzuki received approval from Japan's Ministry of Economy, Trade and Industry (METI) to form a technological research association called HySE (Hydrogen Small mobility & Engine technology) for developing hydrogen-powered engines for small mobility.
In June 2022, Toyota revealed the progress of its efforts in the Super Taikyu Series at the ENEOS Super Taikyu Series 2022. They say cruising range was improved by approximately 20%, power output was improved by approx. 20% and torque was improved by approx. 30%. Also, Hydrogen suppliers are added and its transporting became more efficient to support the race. In July 2022, Isuzu, Denso, Toyota, Hino Motors, and Commercial Japan Partnership Technologies Corporation (CJPT) announced that they have started planning and foundational research on hydrogen engines for heavy-duty commercial vehicles with the aim of further utilizing internal combustion engines as one option to achieve carbon neutrality.
In August 2022, Toyota conducted demonstration run of GR Yaris H2, a special hydrogen-engine version of Toyota GR Yaris, during the ninth round of the World Rally Championship (WRC) in Ypres.
In May 2023, Toyota Corolla Sport which is equipped with liquid hydrogen engine entered the Super Taikyu Series race round 2 "NNAPAC Fuji SUPER TEC 24 Hours Race", and completed the 24 hours race. It was the first time that a car running on liquid hydrogen has entered a race anywhere in the world.
In June 2023, Toyota unveiled a hydrogen race car "GR H2 Racing Concept" built for 24 Hours of Le Mams.
Engine Efficiency
The thermal efficiency of an ideal Otto Cycle depends on the compression ratio and improves from 47% to 56% when this is raised from 8 to 15. Engines in practical vehicles achieve 50-75% of this, with about 60% is suggested as an unlimited-cost limit. However, a conference presentation by Oak Ridge claims that the theoretical efficiency limit is 100%, based on it being an open cycle engine and therefore not limited by Carnot efficiency. In comparison, the efficiency of a fuel cell is limited by the Gibbs free energy, which is typically higher than that of Carnot. The determination of a fuel cell's performance depends on the thermodynamic evaluation. Using hydrogen's lower heating value, the maximum fuel cell efficiency would be 94.5%.
The efficiency of a hydrogen combustion engine can be similar to that of a traditional combustion engine. If well optimized, slightly higher efficiencies can be achieved. The comparison with a hydrogen fuel cell is interesting. The fuel cell has a high efficiency peak at low load, while at high load the efficiency drops. The hydrogen combustion engine has a peak at high load and can achieve similar efficiency levels as a hydrogen fuel cell. From this, one can deduce that hydrogen combustion engines are a match in terms of efficiency for fuel cells for heavy duty applications.
Efficiency decreases for small internal combustion engines. A 67 ml 4-stroke engine converted to hydrogen and tested with a dynamometer at the best operating point (3000 rpm, 14 NLM (normal liters per minute), 2.5 times stoichiometric air/fuel ratio) achieved 520 W and 21% efficiency. In order to measure the vehicular efficiency an also converted similar 107 ml engine (Honda GX110 with best gasoline efficiency 26%) was installed in a lightweight vehicle and driven up known gradients while measuring speed and hydrogen flow. Calculations gave as results 3.5% to 5.9% average efficiencies and 7.5% peak efficiency. The consumption measured on a level road was 24 NLM/km at a speed of 25?km/h and 31 NLM/km at 43?km/h.
Adaptation of existing engines
The differences between a hydrogen ICE and a traditional gasoline engine include hardened valves and valve seats, stronger connecting rods, non-platinum tipped spark plugs, a higher voltage ignition coil, fuel injectors designed for a gas instead of a liquid, larger crankshaft damper, stronger head gasket material, modified (for supercharger) intake manifold, positive pressure supercharger, and high temperature engine oil. All modifications would amount to about one point five times (1.5) the current cost of a gasoline engine. These hydrogen engines burn fuel in the same manner that gasoline engines do.
The theoretical maximum power output from a hydrogen engine depends on the air/fuel ratio and fuel injection method used. The stoichiometric air/fuel ratio for hydrogen is 34:1. At this air/fuel ratio, hydrogen will displace 29% of the combustion chamber leaving only 71% for the air. As a result, the energy content of this mixture will be less than it would be if the fuel were gasoline. Since both the carbureted and port injection methods mix the fuel and air prior to it entering the combustion chamber, these systems limit the maximum theoretical power obtainable to approximately 85% of that of gasoline engines. For direct injection systems, which mix the fuel with the air after the intake valve has closed (and thus the combustion chamber has 100% air), the maximum output of the engine can be approximately 15% higher than that for gasoline engines.
Therefore, depending on how the fuel is metered, the maximum output for a hydrogen engine can be either 15% higher or 15% less than that of gasoline if a stoichiometric air/fuel ratio is used. However, at a stoichiometric air/fuel ratio, the combustion temperature is very high and as a result it will form a large amount of nitrogen oxides (NOx), which is a criteria pollutant. Since one of the reasons for using hydrogen is low exhaust emissions, hydrogen engines are not normally designed to run at a stoichiometric air/fuel ratio.
Typically hydrogen engines are designed to use about twice as much air as theoretically required for complete combustion. At this air/fuel ratio, the formation of NOx is reduced to near zero. Unfortunately, this also reduces the power output to about half that of a similarly sized gasoline engine. To make up for the power loss, hydrogen engines are usually larger than gasoline engines, and/or are equipped with turbochargers or superchargers. A small amount of hydrogen can be burned outside the combustion chamber and reach into the air/fuel mixture in the chamber to ignite the main combustion.
In the Netherlands, research organisation TNO has been working with industrial partners for the development of hydrogen internal combustion engines.
Hydrogen transportation engines
Two main hydrogen-based technologies have been employed to power vehicles: hydrogen fuel cell (HFC) , and hydrogen-fueled internal combustion engine (H2ICE) . The benefits of hydrogen FCVs are the high efficiency, the lack of harmful emissions (water vapor is the only emission which is harmless compared to the emissions created by the fossil fuels combustion, such as nitric oxide, nitrogen dioxide, carbon dioxide, and sulfur dioxide), they operate quietly, and are modular . FCVs use electrochemical reactions to produce electricity from hydrogen and oxygen. Alternatively, the benefits of H2ICEs are the reliance on a mature industry with a huge production infrastructure, the ability to offer “flex-fuel” to aid in the transitional period which could aid in the deployment of the hydrogen infrastructure, lower requirements for hydrogen compared to HFCs, ultra-low emissions, raised peak and part load efficiencies in comparison with conventionally fueled ICEs, and they are not dependent on rare materials . FCVs and battery-electric vehicles (BEVs) use rare materials which could limit the spread of these devices. FCs require platinum, which is already expensive and will increase in price as the demand raises. BEVs use rare earth elements that would be difficult to produce in large quantities. It is estimated that FCVs are capable of reducing greenhouse gas (GHG) emissions by 80% in the US in 2100 compared with that of 1990, and the country will become independent of gasoline fuel in the transportation system by the 2100s . An extensive amount of progress has been made in hydrogen ICE vehicles and hydrogen FCVs in the past century. The current hydrogen-fueled vehicles available on the market and some of the most innovative test engines have been compared to the most environmentally friendly fossil-fueled vehicles available today.
Understanding hydrogen combustion engines
Hydrogen combustion engines work by burning hydrogen in a conventional internal combustion engine, changed to manage the high-speed combustion of hydrogen. They run on similar principles as diesel engines but require specific technologies, such as specialized fuel injectors and ignition systems. These help to manage the distinct characteristics of hydrogen.?We believe that hydrogen combustion engines have the potential to offer a competitive total cost of ownership and could prove a great complement to other propulsion technologies.
Hydrogen combustion technology (VOLVO)
In March 2024, Volvo Group and Westport Fuel Systems?established a joint venture for high-pressure gas injection fuel systems (HPDI) for long haul and off-road applications.??
HPDI enables the world's trucking and off-road equipment manufacturers to address the challenges of meeting the regulatory requirements of?Euro 7?and the US EPA while offering end users affordable options that are powered by carbon neutral fuels like biogas, zero carbon fuels like green hydrogen and other renewable fuels. The HPDI fuel system consists of a fully integrated "tank to injector" solution, based on diesel technology.?
At the heart of the engine is a revolutionary patented injector with a dual concentric needle design. A small amount of pilot fuel (which can be HVO, or diesel fuel) is injected into the cylinder prior to the gas, to initiate the ignition resulting in a reduction of almost 97% of CO2 emissions, and only a small amount of NOx and particles, in-line with the existing Euro 6 and proposed Euro 7 emission regulations.?
We see hydrogen combustion engines as eminently suitable for long haul applications where there is limited access to, or time for, recharging, and refueling options are limited.?
Hydrogen combustion technology is a key element in the ongoing transition to net zero emissions that will support our customers' journey and investments in reducing their carbon footprint.?
Hydrogen dual engine from Volvo Penta
As with both transport and construction, hydrogen as a fuel for industrial and marine applications makes perfect sense. In this case we are talking about a dual-fuel solution which increases flexibility and helps to make today’s combustion engines hydrogen-ready in advance of the rollout of hydrogen production and refueling facilities. This same technique of using hydrogen as a combustion engine fuel will offer further options as the transition to fossil-free fuels continues.
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The benefits of hydrogen combustion engines
While both technologies use hydrogen, they do so differently. Hydrogen combustion engines burn hydrogen, producing power through combustion, like conventional engines. Fuel cells, on the other hand, use a chemical process to convert hydrogen into electricity.
In a hydrogen combustion engine, hydrogen is mixed with air and a pilot fuel (biodiesel for example) and ignited in the combustion chamber, causing an explosion that drives the pistons. These engines must manage the quick flame speeds and high temperatures of hydrogen combustion, which are achieved through advanced engine design, fuel injection technology and hydrogen-friendly materials.
Hydrogen-fueled internal combustion engines have considerable potential for increasing efficiency: the wide flammability limits and high flame velocity of hydrogen–air mixtures allow various load control strategies, and the high autoignition temperature allows increased compression ratios. Current research works on advanced hydrogen-fueled ICEs have focused on achieving brake thermal efficiency higher than 45% while keeping the levels of NOx emissions low. To accomplish this goal, modern hydrogen-fueled ICEs use direct injection (DI) fueling strategies. This results in increased volumetric efficiency and mitigates issues of knock, pre-ignition, and backfire, which are all negative effects associated with hydrogen port fuel injection. However, DI requires the precise control of air/fuel mixing during the compression stroke such that optimal fuel stratification can be obtained at ignition. The operation and widespread adoption of internal combustion engines running on hydrogen fuel pose several challenges, the most important of which are:
- Cylinder head adaptation for injectors;
- Turbo charger for lower pressure demand;
- Combustion irregularities;
- Early pre-ignition;
- Late pre-ignition;
- Knocking;
- Optimization of the mixture formation in the cylinder;
- Oil input into the combustion chamber;
-Piston rings and crevice volumes;
- Optimal compression ratio; Energies 2021, 14, 6504 21 of 26
- Optimizing injection strategies to improve engine efficiency, emissions, and power density;
- Load control strategies;
- Degradation and damage of piezoelectric injectors;
- Maximum reduction in adhesive wear and leakage between the needle and seat with age;
- Counteracting hydrogen diffusion into a dielectric shell or piezoelectric actuator;
- Delamination of dielectric epoxy coating;
- Piezoelectric surface blistering;
-Actuator cracking due to unwanted tensile loads;
- Sliding friction and wear of moving parts of injectors
- Durability of valves and spark plugs;
- Hydrogen slip into crankcase;
- High oil consumption.
The widespread introduction of hydrogen combustion engines into serial production has not yet begun, primarily because the hydrogen infrastructure that is required for all hydrogen-powered vehicles (i.e., fuel cell electric motors as well as hydrogen-fueled internal combustion engines) is underdeveloped. With the construction of H2 infrastructure planned and already underway in some markets, whether and how hydrogen-powered internal combustion engines can complement fuel cells to contribute to CO2-free propulsion systems are currently being discussed. Examples of arguments in favor of hydrogen-fueled internal combustion engines include low investment due to extensive use of existing production capacity and vehicle architecture to date as well as long service life. Furthermore, hydrogen-fueled ICEs can be integrated with electric motors in electrified powertrains (in hybrid powertrains). In addition to the advantages in terms of efficiency and driving range, this leads to attractive functional synergies and additional degrees of freedom in terms of design and operating strategies to be considered.
Emissions and efficiency
If the pilot fuel used to ignite the hydrogen is CO2-neutral, hydrogen combustion engines produce no CO2 emissions, as there's no carbon in hydrogen. The primary emission is water vapor, with some NOx emissions due to high combustion temperatures. These engines can be more efficient than gasoline engines but less so than hydrogen fuel cells. However, they can use much of the current engine technology and refueling infrastructure, making them an attractive choice
Challenges in hydrogen powered vehicles
Safety of hydrogen vehicles
Fuels with low density, high diffusion coefficient, and higher specific heat are safer. The higher specific heat alleviates the temperature mitigations for a given heat input . For a specific fuel, some characteristics like wider ignition limits and lower ignition temperature cause the fuel to become less safe by increasing the limits in which a fire could occur. Higher emissivity/temperature of the flame and high explosive energy make the fuel less safe because the fire will be more damaging
Hydrogen is very light (~6.9% density of air), 4 times as diffusive as NG, and 12 times as diffusive as gasoline; hence, a risk of combustion or explosion is minimal . Due to the non-toxicity characteristic of hydrogen, its leakage would not damage the environment. An explosion of hydrogen is very difficult to create, so it is much more likely that a quick burning fire would result if a hydrogen/air mixture experienced a spark. A safety advantage of hydrogen is that it blazes with little heat radiation, therefore only things immediately next to the flame would burn . The clear flame can also not sear skin at a distance due to its low thermal radiation. Hydrogen can burn in lower concentrations which causes safety concerns .
The US DOE has set parameters for storage and system safety. For permeation and leakage, the system must fulfill SAE J2579 for system safety; for toxicity, the system must meet applicable standards; and failure analysis must be conducted and evaluated for the system. The permeation and leakage tests are for the entire storage system, rather than each component or storage material. The toxicity criteria are regulated by government standards such as the EPA’s Toxic Substances Control Act Chemical Substance Inventory (TSCA Inventory) and the US Department of Labor Occupational Safety and Health Administration (OHSHA). The safety instructions cover the transport system, manufacture, certification and operation of vehicles, fuel dispensing, and end of life issues, which each must comply with applicable federal, state, and local standards. The onboard storage systems should comply with SAE J2579 and the United Nations Global Technical Regulation No. 13 and the applicable standards for the country that the vehicle is deployed.
Hydrogen storage tank
There are several levels of hydrogen storage that are required to develop a successful hydrogen economy: at production centers, at filling stations, onboard vehicles, and nationally as a strategic reserve. The storage of hydrogen is the most difficult challenge associated with the hydrogen economy. Before a hydrogen transportation economy can be built, an appropriate storage system must be developed for hydrogen powered vehicles. Due to hydrogen’s extremely low density, a huge onboard storage tank would be needed to transport the fuel. To mimic the 400 km range of a standard car, only 8 kg of hydrogen is required for an ICE or 4 kg of hydrogen for an FC . The challenge at hand is finding a material for the storage container that fulfills three requirements: high hydrogen density, fast release/charge kinetics with minimum energy barriers to hydrogen release and charge, and reversibility of the release/charge cycle at moderate temperatures (70–100°C) must be compatible with the FCs. The tank material must have strong chemical bonds and close atomic packing. The material also needs loose enough atomic packing to assist fast diffusion of gaseous hydrogen between the surface and the bulk. An adequate thermal conductivity of the material is required to hamper decomposition by the heat released upon hydrating . Investigations show that several materials can meet two of the requirements, but none have been found that fulfill all the necessary requirements. Whichever material is selected must also be cost effective, a practical weight, have an adequate lifetime, and meet safety requirements . The gravimetric and volumetric density in a storage tank material is very important in both mobile and stationary applications of gaseous hydrogen . Carbon nanotubes have been discovered to be a source of hydrogen storage, which spurred a massive influx of research devoted to nanostructures Various materials such as boron compounds ,chemical hydrides , carbon-based materials ,Mg-based alloys , and metal hydrides have been employed in hydrogen storage systems to achieve the best performance.
There are six different methods for storing hydrogen :
In the gaseous state, hydrogen has extremely low density (0.09 kg/NA m3), and very low boiling point (20.2 K); however, in liquid phase, the density of hydrogen is exceptionally high (70.9 kg/NA m3) . These properties make storing hydrogen very complicated in mobile application. The current method for storing hydrogen in transportation engines is as a liquid in cryogenic reservoirs (at 20 K) or as a gas in high-pressure cylinders (at up to 700 bar). Neither of these two methods are exceptionally efficient. Compressing the gaseous hydrogen uses up to 20% and liquefying hydrogen requires up to 40% of the energy content. Solid-phase hydrogen storage is compatible with electrolyzers and FCs. A porous material can absorb hydrogen molecules, but generally, an attractive capacity can only be obtained at cryogenic temperatures. The most commonly used material for solid-state hydrogen storage is interstitial metal hydride. Metals are the only reversible solid-phase hydrogen storage that have been commercialized, primarily for stationary applications.
Hydrogen transport and delivery
A successful hydrogen-based transportation system needs infrastructure to deliver hydrogen from production plants to refueling stations . The potential hydrogen delivery methods are compressed gas pipelines, cryogenic liquid trucks, and compressed tube trailers . Any combination of these methods could be utilized at varying stages of the hydrogen automotive transportation. Delivery distance and the number of stations are the crucial factors affecting the cost of delivering hydrogen . Pipelines could be used for transporting hydrogen farther distances, while trucks carry smaller amounts shorter distances. Due to the low density of hydrogen, transportation of liquid or pressurized hydrogen is inefficient . High-purity stainless steel piping with a maximum hardness of 80 HRB is preferred for pipelines . The lowest cost choice depends upon market and specific geographical characteristics. For the past 50 years, pipelines have been used to transport hydrogen. Currently, there are approximately 16,000 km of hydrogen pipelines throughout the world that transfer hydrogen to chemical plants and refineries . Although the capital cost of pipelines is high, their operation cost is low especially for compressor power. Liquid hydrogen could be transported on ships, railcars or trucks, but its operating cost is very high because of the electricity required for liquefaction (about 30–60% of the total liquefaction costs), which may leave a considerable carbon footprint. Distance is an important factor to select the hydrogen delivery method. Investments in the construction of pipelines of high-quality materials must be made due to the specific physical and chemical characteristics of hydrogen .
Hydrogen-only ICEs
Since the octane number of hydrogen is higher than that of gasoline fuel, it can be considered as a serious candidate to use in IC transportation engines. Some hydrogen properties, such as its higher diffusivity compared to gasoline and methane, lower ignition energy requirement, and higher flame velocity are desirable for spark ignition (SI) engines . Since hydrogen flame speed is approximately five times higher than gasoline and methane and ten-times higher than diesel, hydrogen-fueled SI engines can be run with lower cyclic variations .
Hydrogen in SI engines is used in one of the following ways :
Hydrogen also can be employed in a compression ignition (CI) engine. The generated power from a CI engine has been found to be twice that of the same engine worked in the premixed system . In hydrogen-based CI motors, an injector is used to inject high-pressure hydrogen into the cylinder . Therefore, not only the design of engine structure is important, but the design of the injector is crucial because the injection nozzle controls how the pressurized hydrogen is injected into the system . Hydrogen supply in CI engines has illustrated significant reductions in CO2, CO, HC and smoke levels that under optimum circumstances can reach as much as over 50%. An apparent effect of using high amounts of hydrogen in IC engines (high load conditions) is a sharp rise in heat release rate (and consequently the increased in-cylinder temperatures and high rates of NOx formation) and brake thermal efficiency .
used pure hydrogen in homogeneous charge compression ignition (HCCI) engine. To achieve hydrogen auto-ignition under the HCCI mode, a compression ratio of at least 16 was used. It was found that using a stoichiometric hydrogen–air mixture in the HCCI mode generates an extremely high combustion knock. Therefore, various methods such as leaning combustible mixture or EGR should be employed to reduce the knock effect.
In the pressure-boosted H2ICE structure, a hydrogen gas compression chamber is used to raise the intake-air/hydrogen pressure to maximize engine power density of ICEs. In this condition, the hydrogen pressure is increased to achieve hydrogen volumetric efficiencies closer to the gasoline fuel . Using liquid hydrogen is another method which does not require significant changes in conventional ICEs. In this system, liquefied hydrogen is injected to an expansion chamber to convert to a cold hydrogen gas and ultimately it is conducted to the combustion chamber. Using cold hydrogen decreases NOx emission as well as pre-ignition . Figu re 2 shows these two types of H2ICEs.
Figure 2. Schematic of (a) Pressure-boosted H2ICE (b) Liquid hydrogen internal combustion engine .
Currently, direct injection (DI) and PFI are the injection methods being investigated for H2ICEs. DI offers high efficiencies and controlled emissions, but the durability of DI injectors is low. Injection needs high pressure, which further limits the storage options. Hydrogen could be stored in cryogenic tanks in the liquid state and injection pressure are generated onboard, or compressed hydrogen is stored, which limits the tank capacity and onboard compression would negate the efficiency benefits of DI .
Numerous labs and organizations have been conducting research in direct injection for hydrogen fuels. The DOE program targeted demonstrating a peak brake thermal efficiency (BTE) of 45%, a part-load BTE of 31% with part load point set at 2 bar brake mean effective pressure (bmep) at 1500 rpm, and “Tier II Bin 5” emissions (which limit NOx emissions to 0.07 g/mile) (FreedomCAR and Fuel Partnership Plan -March 2006). The National Traffic Safety & Environment Laboratory (NTSEL) project aimed a peak power output of 100 kW from a 4-cylinder naturally aspirated engine with less than 0.05 g/kWh of NOx on a JE05 transient emission testing cycle.
Sopena et al. modified an SI gasoline-fueled ICE of a transportation vehicle (suitable for both interurban and city) to operate on hydrogen fuel only. The most important modifications were performed on the electronic management unit, gas injectors, inlet manifold, and oil radiator.
how do hydrogen car work
how does a hydrogen car work
Hydrogen engines continue to be one of the future bets of the automotive industry. Its operation has given it a series of advantages, keeping it afloat despite its failures. To this end, Toyota, BMW, Mazda, Hyundai, Ford and other brands have invested heavily in this technology. Hydrogen-using engines include internal combustion engines and fuel cell conversion engines. Many people don't know how does a hydrogen engine work and their respective advantages and disadvantages.
For this reason, we are going to dedicate this article to telling you how a step-by-step hydrogen engine works, what its characteristics are and its importance for the motor world.
How does a hydrogen combustion engine work?
These engines use hydrogen as gasoline. That is, they burn it in a combustion chamber to create an explosion (kinetic energy and heat). For this reason, conventional gasoline engines can be adapted to burn hydrogen in addition to LPG or CNG.
The operation of this engine is very similar to that of a gasoline engine. Hydrogen is used as fuel and oxygen as oxidant. The chemical reaction is initiated by a spark and a spark plug can produce a spark. Hydrogen has no carbon atoms, so the reaction is that two hydrogen molecules combine with one oxygen molecule, releasing energy and water.
The result of its chemical reaction is simply water vapor. However, hydrogen combustion engines do generate some emissions during their operation. For example, small amounts of NOx from the air and heat from the combustion chamber, or emissions from burning some oil through the piston rings.
As hydrogen is a gas, it is stored in a tank with a pressure of 700 bar. This is 350 to 280 times higher than normal car tire pressure. (2 to 2,5 bar). Although there are also cars that store hydrogen in liquid form at very low temperatures, as shown below.
Hydrogen combustion engines offer some interesting advantages over conventional combustion engines. For example, they can theoretically use very fine mixtures (Lambda close to 2). That is, they can use very little fuel to use all the incoming air and become very efficient.
Example of how a hydrogen combustion engine works
A good example of a hydrogen engine is the BMW 750hl, which came onto the market in 2000. Although it is actually a BMW petrol engine, it is also capable of burning hydrogen.
However, it has several drawbacks: First, it stores hydrogen in liquid form. This requires a very expensive tank made from materials from the aerospace sector to keep its temperature below -250oC. This can only be achieved within 12 to 14 days, during which time the hydrogen gradually evaporates and is safely released into the atmosphere. The second disadvantage is that by using hydrogen you lose a lot of power and efficiency. The later BMW Hydrogen 7 from 2005 partially solved these problems and increased the hydrogen pressure to 700 bar without keeping it cold.
Another good example is the Aquarius hydrogen engine. A fossil fuel engine developed by an Israeli company suitable for the use of hydrogen. The first functional version was introduced in 2014 and since then a revised and improved version has appeared. According to its developers, it can work without lubricating oil and has a gas exchange system to reduce NOx emissions.
In addition, the hydrogen internal combustion engine is light and has few parts, making it cheap to produce. It can be used as a range extender for electric vehicles or as a generator for the network.
How does a hydrogen fuel cell engine work?
Itss full name is a fuel cell converted hydrogen engine. Despite the word "fuel", they do not burn hydrogen. They use it to generate electricity through the reverse process of electrolysis. That's why they carry batteries for chemical reactions, like in a hydrogen combustion engine, where hydrogen is stored in tanks with a pressure of 700 bar.
It's just that instead of feeding it to the motor, it goes through the anode and cathode (like a battery) to the fuel cell. Once there, hydrogen gas (H2) passes through the membrane and breaks it down into two hydrogen ions. Hydrogen and two free electrons. These electrons pass from the anode to the cathode of the battery through an external circuit, creating an electrical current. The hydrogen ions produced combine with oxygen from the air to form water.
For this reason, a hydrogen fuel cell engine is zero emissions, since it does not produce NOx or the gases that are produced when burning oil like an internal combustion engine. The diaphragms used in these engines are made from platinum and are expensive. However, there is work to address this high cost. For example, at the Technical University of Berlin they have developed a ferroalloy that, if put into production, could greatly reduce costs.
Disadvantages of hydrogen engines
Advantages of hydrogen engines
Autonomy
The disadvantage of hydrogen engines is that their tanks or fuel cells must contain hydrogen at very high pressures. Thus, the supply point must also comply with the pressure of 700 bars that it supports.
This requires building a supply infrastructure to be able to refuel this type of vehicle. That said, it has the same issues as pure electric vehicles. However, the refueling operation is much faster than these, since it is the same as an LPG or GLC vehicle.
Cars currently equipped with hydrogen fuel cell engines have a range similar to gasoline. For example, the Toyota Mirai announced 650 km with a full battery, the Hyundai Nexo 756 km and the BMW iX5 Hydrogen 700 km.
Others like the Hopium Machina have announced a range of 1.000 km, although that figure will now have to be confirmed when it happens. In any case, autonomy is not as important as the battery, because refueling is much faster. The thing to keep in mind is the number of fuel points.
They're safe?
Brands have been working on this type of engine for years to improve their efficiency, reduce costs and, of course, make them as safe as those that run on fossil fuels.
In addition, the safety standards required by Europe, the United States and Japan are the guarantee of the safety of hydrogen-powered vehicles. Needless to say, Toyota touts that the Mirai's gas tank is tough enough to be bulletproof.
Will we see a day when all cars run on hydrogen? Time will show everything. It is clear that brands continue to invest and it has some advantages that make it a reasonable alternative to zero emission transport.
I hope that with this information you can learn more about how a hydrogen engine works, its characteristics, advantages and disadvantages.
Hydrogen Fuel Internal-Combustion Engines
To achieve the goals of low carbon emission and carbon neutrality, some urgent challenges include the development and utilization of low-carbon or zero-carbon internal combustion engine fuels. Hydrogen, as a clean, efficient, and sustainable fuel, has the potential to meet the abovementioned challenges. Thereby, hydrogen internal combustion engines have been attracting attention because of their zero carbon emissions, high thermal efficiency, high reliability, and low cost.
hydrogen internal combustion engine high efficiency low NOx emission
1. Opportunities for Hydrogen Internal-Combustion Engines
Environmental issues and global warming have become more prominent and critical in the past few decades. To solve these problems, the Paris Agreement reached a consensus and decided to attempt to slow the progress of global warming processes in December 2015, the goal of which being to control the increasing global rising temperature to within 2 °C in the 21st century. Therefore, many countries have been proposing and implementing carbon-reduction and carbon-neutrality strategies. At the 75th session of the United Nations General Assembly (September 2020), China proposed a double carbon target of peaking carbon dioxide emissions by 2030 and achieving carbon neutrality by 2060 and introduced a series of policies to promote the process of carbon reduction, creating a strong demand to decarbonize not only transport sectors, but also the power industry, incentivizing away from conventional carbon-based fuels and towards renewable energy sources. Hydrogen can be produced from several varieties of renewable energy sources and efficiently obtained through large-scale electrolysis. And after its reaction with oxygen, water is produced. Additionally, hydrogen combustion or electrochemical reactions can be used to generate thermal or electric energy as power sources for cars. Although hydrogen is less portable and has a lower volumetric energy density than liquid fuels, it has proven itself as having the highest mass-specific energy density among general fuels, such as gasoline, diesel, methanol, ethanol, and so on.
Up until now, hydrogen has been bridging the low-carbon economy and renewable energy, which suggests its key role in preventing global warming. Many countries have begun to produce hydrogen. According to the International Hydrogen Energy Commission’s statistics, 228 projects in the global hydrogen energy industry chain have been built, and more than 20 countries and regions, such as the United States, Japan, the European Union, South Korea, and New Zealand, have issued hydrogen energy-development strategies . In December 2021, China’s Ministry of Industry and Information Technology issued the Industrial Green Development Plan, a proposal to accelerate hydrogen energy technology innovation and infrastructure construction and promote the diversified use of hydrogen energy.
At present, the use of hydrogen energy in the power vehicle industry mainly includes fuel cells and internal-combustion engines. Hydrogen fuel cells are high-efficiency electrochemical devices that directly convert chemical energy into electric energy and only produce water as a by-product without any other harmful emissions. However, hydrogen fuel cells have a disadvantage in terms of their cost and service life, and the prices of fuel cell vehicles are still much higher than those of traditional vehicles . Based on the abovementioned reasons and combustion characteristics of hydrogen, it is attractive as a fuel for internal-combustion engines. Extensive literature studies have shown that hydrogen, as a fuel for internal-combustion engines, has a wider flammable range, the ignition limit range of which expressed by the air–fuel ratio is 0.14 to 10, the minimum ignition energy required is one-tenth of that of gasoline fuel, and the laminar flame velocity is more than six times faster than that of conventional fuels. Hydrogen also has a higher diffusion coefficient, lower ignition energy, and wider flammability limit, which, when compared with conventional fuels, result in better heat and mass-transfer characteristics, better lean-burn characteristics, and lower misfire rates, as can be seen in Table 1 .
Table 1. Physical and chemical properties of the different fuels '
The lower heating value of hydrogen is much higher than that of gasoline and natural gas. Combining faster laminar flame speeds, lean-burn characteristics, and higher spontaneous combustion temperatures makes hydrogen internal-combustion engines have a high thermal efficiency and potential knock resistance. Hydrogen has a shorter quenching distance, which is about one-third that of gasoline and methane. This affects crevice combustion and wall-heat transfer [3]. In addition, the wide flammability limit enables hydrogen-fueled SI engines for quality control such as diesel engines, rather than volume control at fixed fuel–air mixture conditions close to stoichiometric ratios, as is the case in regular gasoline engines. This ensures that hydrogen engines have a higher indicated thermal efficiency than gasoline engines .
Compared with fuel cells, hydrogen internal-combustion engines can take advantage of the mature industrial chain and technology of existing internal-combustion engines, and only need to optimize the fuel supply and injection system, turbocharger matching, lubrication system, and crankcase ventilation . Moreover, the purity of hydrogen is not strictly required; the byproduct of industrial hydrogen can be used, resulting in a lower user cost while fuel cells need high-purity, hydrolyzed hydrogen at the current stage and the cost is relatively high. Thus, hydrogen is an ideal alternative fuel for internal-combustion engines, which, in turn, assists the optimization and development of new energy technologies with hydrogen internal-combustion engines.
2. Research on Performance Improvement of Hydrogen Internal-Combustion Engines
Companies such as BMW and Ford Motor have been developing hydrogen fuel internal-combustion engine vehicles, and they have successfully demonstrated their excellent performance in terms of emissions and fuel economy . BMW tested a specially designed engine with an external and internal mixture formation system to study the effect of different injection strategies on hydrogen engine performance . The experimental results showed that the hydrogen engine, combining the external and internal mixture formation systems, can operate efficiently under partial load and lean-burn conditions. Stoichiometric mixture can be achieved even when operating at full load through external mixture formation or direct injection. For BMW’s operating strategy with a post-treatment catalyst to reduce emissions, using the lean mixture is only suitable for low-load engine conditions, while the stoichiometric mixture is suitable for high-load engine conditions. The average indicated pressure of the engine reached 1.8 MPa at 4000 rpm engine speed, which is higher than that of the basic gasoline engine.
A test for two BMW Hydrogen 7 Mono-Fuel demonstration vehicles was completed in 2008 . The two vehicles were tested at the FTP-75 cold-start as well as the highway drive cycle, respectively, achieving fuel economy performances of 3.7 kg of hydrogen per 100 km on the FTP-75 cycle and 2.1 kg of hydrogen per 100 km on the highway cycle. These results are, respectively, equivalent to 13.8 L per 100 km and 7.8 L per 100 km for gasoline fuel consumption at the FTP-75 cold-start and highway drive cycle. These emission results on the FTP-75 cycle showed that emission levels are inferior to 0.0008 g/mile of nitric oxide (NOx) emissions, 0 g/mile of nonmethane hydrocarbon (NMHC) emissions, and 0.003 g/mile of carbon monoxide (CO) emissions. These emission results are equivalent to the Super Ultra Low Emissions Vehicle (SULEV) emission levels, which are 3.9% NOx, 0% NMHC, and 0.3% CO.
Ford motor company built and tested the first production-ready vehicle, the P2000, with a hydrogen internal-combustion engine that could run without throttle on a lean mixture . The research team of Argonne National Laboratory evaluated several direct-injection hydrogen mixture formation strategies to reduce NOx emissions and achieve a higher thermal efficiency of the engine. The group carried out engine experiments under the speed range of 1000~3000 rpm and the average effective pressure range of 0.17~1.43 MPa . The results showed that the effective thermal efficiency (BTE) was more than 35% under about 80% of test conditions. There was a balance between wall-heat loss and other losses as a function of engine speed and load. Therefore, the peak effective thermal efficiency of 45.5% and NOx emission of 0.87 g/kW·h were obtained at 2000 rpm and BMEP of 1.35 MPa. However, NOx emissions increased with the increase in speeds and loads, which means that the mixture formation needs to be further optimized.
Due to the potential of hydrogen as a flexible energy carrier, the development projects of large hydrogen internal-combustion engines based on diesel engines have also begun to emerge in recent years. The group of National Traffic Safety and Environment Laboratory and Tokyo City University has developed a large (medium load) truck with a multi-cylinder, spark-fired, direct-injection hydrogen engine . The engine was developed for the project based on a four-cylinder diesel engine with a displacement of 4.73 L. A low NOx emission (0.7 g/kW·h), IMEP of 0.85 MPa, and indicated thermal efficiency (ITE) of 41% were obtained under the adopted combustion control strategy and the engine operating conditions. The torque was about 20% lower than that of the base diesel engine. The torque deficit of the hydrogen engine can be improved by boosting the intake air, but it is necessary to avoid pre-ignition and knock.
In order to achieve sufficiently low NOx emissions, high thermal efficiency and high torque without any post-treatment conditions. A study on large-scale hydrogen internal-combustion engines for stationary power generation was conducted in the Renewable Energy Research Center, National Institute of Advanced Industrial Science and Technology, Japan . Experimental studies were carried out by changing the piston design, adding a spark plug, and adding a direct-injection hydrogen injector on a single-cylinder diesel engine with a displacement of 1.3 L. In the absence of a post-treatment system, an extremely high EGR rate and intake boost with suitable hydrogen mixture formation strategies were used to achieve NOx emissions below 200 ppm. The ideal IMEP is above 1.35 MPa (140 Nm) at 1000 rpm, reaching the level of a benchmark diesel engine. Distinct from previous research on the injection strategy of the direct-injection hydrogen engine, it is proposed to set the injection pressure at a lower level through small-hole injection, which attempts to produce the stratification of the hydrogen mixture in the engine cylinder. Although low injection pressure and long hydrogen-injection time may lead to the increase in mixture inhomogeneity, there is a trade-off between the equivalent ratio and NOx emissions. In this study, lower NOx and higher ITE could be achieved when the global equivalent ratio was kept around 0.3. By analyzing the effect of EGR on combustion performance, it was found that the EGR rate had only a slight effect on combustion performance. No matter how large the EGR rate is, the indicated thermal efficiency, the average indicated pressure, as well as CA50 were essentially unchanged. However, increasing the EGR rate could significantly reduce nitrogen oxide emissions. In addition, it can be concluded that the inhomogeneity of the hydrogen mixture in the cylinder, results in robust combustion, which is not sensitive to the EGR rate. Experiments suggested that the maximum IMEP was 1.46 MPa, the engine NOx emission was less than 150 ppm, the boosting pressure was 175 kPa, the oxygen concentration of the intake air was 12.5 vol%, and the corresponding EGR rate was about 50%. To further improve IMEP and thermal efficiency without increasing NOx emissions, Atkinson/Miller cycles were used to attempt to delay the intake valve closing and exhaust valve opening to reduce the effective compression ratio and increase the effective expansion ratio. The IMEP eventually reached 1.64 MPa, NOx emissions were below 100 ppm, and the ITE was more than 50%.
In order to increase the power output and reduce NOx emissions, Verhelst’s research group studied an in-cylinder direct-injection hydrogen internal-combustion engine equipped with EGR and a turbocharger under lean-burn and stoichiometric mixture conditions . Comparing the performance of lean-burn without post-treatment and stoichiometric mixture conditions with both EGR and the post-treatment system, it was found that lean-burn combined with a turbocharger is the more effective method for achieving higher efficiency and lower NOx emissions. Clearly, to avoid abnormal combustion and unacceptable levels of NOx emissions, lean burns require higher boosting pressures for keeping the equivalent ratio enough low. Otherwise, lean-burn operation of the engine will inherently result in insufficient torque or power. On the other hand, a power output higher than 30% of gasoline can be achieved when selecting a supercharged stoichiometric mixture with EGR, but fuel economy is sacrificed by catalytic post-treatment with NOX removal.
By changing the hydrogen-injection timing, homogeneous mixture combustion, stratified combustion, and diffusion combustion can be realized in hydrogen internal-combustion engines. Toyota Motor Corporation conducted experimental research in a 2.2 L four-cylinder diesel engine equipped with a centrally mounted hydrogen injector, a toroidal shape combustion chamber, and a spark plug in the glow-plug position . The research investigated the high efficiency and low NOx of hydrogen combustion using a prototype high-pressure hydrogen injector (maximum 30 MPa). In addition, stratified combustion and spark-assisted diffusive combustion was investigated, and the results showed that the pressure-recovery effect by injection close to TDC and EGR effectively combined with stratified and diffusive combustion by high-pressure direct injection greatly improved the indicated thermal efficiency by approximately 3% compared with conventional homogeneous combustion. Furthermore, suppressing jet penetration and reducing cooling loss, a 52% ITE was achieved for a small engine.
The Indian Institute of Technology research groups developed the stoichiometric or over-stoichiometric mixture-formation strategies, including cooled EGR, turbocharging, and NOx removal catalytic post-processing, to achieve higher torques and prevent abnormal combustion and high NOx generation . Using unburned hydrogen as a NOx-reducing agent under stoichiometric conditions, a peak torque of 180 Nm was achieved at 3600 rpm with over 800 ppm NOx and a BMEP of approximately 0.9 MPa.
3. Port Injection and Direct Injection of Hydrogen
The hydrogen-injection methods of the hydrogen internal-combustion engine are divided into port fuel injection (PFI) and direct injection (DI) in the cylinder, but due to the small hydrogen density, the port injection will lead to a decrease in the intake efficiency, resulting in a significant decrease in power density. Direct injection in the cylinder can not only improve the intake efficiency and consequently result in a greater power density, but also avoid backfire compared with PFI, which can increase the power density by 38.4%. In addition, direct injection can also achieve a more flexible organization formation of the mixtures and, in turn, achieve a variety of combustion modes such as stratified combustion and homogeneous combustion or even diffusion combustion .
Due to gasoline and diesel fuel being liquid, injection causes little change in cylinder pressure. Therefore, the change in negative compression work due to fuel injection is negligible for gasoline and diesel fuel engines. However, for in-cylinder direct-injection hydrogen engines, hydrogen injection is generally carried out during the compression stroke, hydrogen will occupy a relatively large part of the cylinder volume, and the hydrogen injected has a large pressure, so it will cause an increase in compression pressure and negative compression work. However the negative compression work can be reduced by controlling the hydrogen-injection timing. Additionally, the thermal efficiency can be improved by optimizing the compression ratio and the phase of hydrogen injection. Compared with low-load uniform combustion, stratified combustion achieved by direct injection can achieve a high combustion constant volume degree, thus improving the engine efficiency. During engine operation, the combustion loss of port injection and direct injection is almost the same .
Compared with gasoline engines, the current port-injection and direct-injection hydrogen engines both have good effective thermal efficiency. Direct injection of hydrogen internal-combustion engines has more advantages in terms of power performance, fuel economy, and NOx emission, making it an ideal hydrogen-supply method. Compared with low-load uniform combustion, the stratified combustion can achieve a higher combustion constant volume, resulting in improved engine efficiency. The late injection strategy should be adopted to perform stratified combustion. Efficiency losses such as compression work, heat transfer to the coolant, and abnormal combustion should be reduced.
However, compared with direct-injection hydrogen engines, port-fuel-injection hydrogen engines have some disadvantages such as higher cooling loss, which results in low thermal efficiency and abnormal combustion (backfire, pre-ignition, higher burning velocity) leading to limited high-load operation. Direct injection is an effective method to overcome these disadvantages, but the combustion methods that enable both high efficiency and low NOx have not yet been thoroughly investigated.
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3 个月Alot of science talk so this is good?
Owner, Funk Auto Consulting Ltd
4 个月https://www.facebook.com/COBNMUSA/posts/pfbid02AYjNuTZnP3foWkBrNoVQWcaRrbUxtnfEijXEYDiEVGWDAY81TqYUwTDRKFj4Jzzvl
PhD (Prs.) Sustainable EV Ecosystem I RainMaker I Strategy Consulting I Decarbonization Strategy | Carbon Coach | Market Intelligence I Supply Chain I Automotive I Industrial I Academic
6 个月Very well explained Vijay, this is very insightful!
Retired
6 个月Insightful!