Toyota Mirai fuel cell engine

Fuel cell – What is it and how does it work?

in Renewable Energy by

During the 19th century, William Grove found that the energy produced by simple chemical reactions could be harnessed and exploited. His work led to the creation of the first alkaline one-kilowatt fuel cell system in the 1950’s. The power producing the potential of fuel cells was quickly adopted by NASA, which used them in the Gemini, Apollo, and space shuttle programs. Since their use in the 1960’s space programs, fuel cells have been improved and are becoming commercially available in products such as cars, computers, cell phones, residential and small business power generators, and large-scale stationary power generators. Fuel cells are also potential power providers for computers, credit card processing centers, jails, cellular towers, mining equipment, entertainment complexes, communication centers, navigation equipment, airports, road signs, defense installations, urban transit buses, and even vacuums.

Fuel cell
Fuel cell diagram

Fuel cells rely on a relatively simple chemical reaction to generate energy. While there are different kinds of fuel cells, all with their unique characteristics, the general principle is the same across the board. A fuel cell is composed of an electrolyte sandwiched in between two electrodes. Electrodes are charged, usually metal plates. The electrolyte is the substance that hydrogen protons freely pass through as they move between electrodes. Electrolytes are made of different materials, and they are what usually give fuel cells their distinctive names.

The production of energy begins when a stream of hydrogen molecules is forced against the first electrode, called the anode, which is negatively charged. The anode forces the hydrogen molecule to split into protons and electrons. The protons are pulled through the electrolyte directly to the other electrode, called the cathode, which is positively charged. The electrons, having taken a different path around the electrolyte, are captured and used as an electrical current. These electrons then rejoin the protons in the cathode where they are both exposed to oxygen. The hydrogen and the oxygen combine to form pure potable water and some heat. Often, through a process known as cogeneration, this waste heat is captured and utilized in the heating and cooling of the facility where the fuel cell is located.

Fuel Cell parts

The fuel cell is only one part of a complete standard fuel cell system. There are three main parts. These three main parts are the fuel reformer, the fuel cell stack, and the power conditioner.

  1. The fuel reformer, usually through a process called steam reformation ( a process that creates carbon emissions ) isolates the pure hydrogen from a hydrocarbon fuel ( methane, ethanol, propane, natural gas, etc. ) This hydrogen, although pure, is sometimes referred to as dirty hydrogen due to the way it is created. This hydrogen is then put into the fuel cell stack.
  2. The fuel cell stack is any number of fuel cells stacked together to increase the produced energy output. The energy created by the fuel cell stack is in the form of direct current.
  3. The power conditioner, or the inverter, is the mechanism that inverts the direct current produced by the fuel cell into alternating current. The inverting of direct current into alternating current is required for most power applications to work.

Why are fuel cells attractive as alternative energy generators?

Fuel cells are attractive as energy generators because of their:

  • Cleaner non-combustive operation. Fuel Cells emit no particulate matter and almost no NOx and SO2. While fuel cells still have some substantive CO2 emissions, they are only 45% that of coal generation and 47% the amount emitted from the production of energy using fossil fuels.
  • Higher efficiencies when compared to combustion driven generators. Before even using cogeneration technologies in conjunction with the fuel cell, the cell is about 50%, and as much as 65%, efficient. Using cogeneration can boost efficiencies to as high as 90%. Compared to the 12-15% efficiency of most internal combustion engines, fuel cells are much more efficient.
  • Ability to deliver reliable, uninterrupted power. In fact, a fuel cell in an integrated power supply system can provide 99.9999% reliability.

These are all welcome benefits in a time of unreliable polluting energy.

Types of fuel cells

There are various kinds of fuel cells. The basis of these multiple identities is founded on the difference in substance that is used as the electrolyte. This variation in electrolyte leads to different efficiencies, several heat outputs, and, therefore, preferred use in many applications. It is unlikely that one type of electrolyte will dominate all technology sectors. Instead, each type will probably fill specific technology niches depending on their relative advantages and disadvantages.

PEM fuel cell stack
PEM fuel cell stack

– Proton Exchange Membrane fuel cells (also known as PEM), which use a moist polymer membrane electrolyte, are the most common type of fuel cell. There are four basic elements of a PEM Fuel Cell. PEM cells run at cooler temperatures, perhaps as low as 41 degrees F, with average temperatures of about 80 degrees F that make them ideal for portable fuel cell technologies and small-scale residential applications. Their compact size, relatively low weight, quick start up, and ability to vary output to meet sudden shifts in power demands also make PEM fuel cells appealing to those looking to apply it in portable and smaller devices, light duty vehicles, and as replacements for rechargeable batteries. Their adverse qualities are high production costs, full maintenance, and repair requirements. These are problems not limited to PEM cells.

– Phosphoric Acid fuel cells (PA fuel cells) are another major type of fuel cell and are currently installed in over 200 systems globally. PA fuel cells, which use an immobilized liquid form of phosphoric acid as its electrolyte, have higher operating temperatures of around 200 degrees C. This warmer temperature makes cogeneration, using the steam produced as a byproduct, a somewhat more appealing option for those employing this type of fuel cell. However, this higher temperature also restricts the use of PA cells to stationary applications. Personal electronic devices, such as mobile phones, laptops, or even cars, would most likely not use PA fuel cells.

– Molten Carbonate fuel cells are a third type of fuel cell. They use an immobilized liquid form of an alkali carbonate mixture as an electrolyte. Molten Carbonate cells have even higher operating temps of around 650 degrees C. These cells are currently fueled by a wide variety of fuels including pure hydrogen, carbon monoxide, natural gas, propane, methane, marine diesel, and stimulate coal gasification products. Stationary applications of these cells have been successfully demonstrated in Japan and Italy.

– Solid Oxide (SO) fuel cells, which utilize a solid ceramic yttria stabilized zirconia electrolyte, have a much higher operating temperature of 980 degrees C or more. For this reason, SO fuel cells are most often used in the major high-power applications such as industrial and large-scale electrical generating stations. This type of fuel cell has the highest efficiency at about 60%. Because of their high heat output, when cogeneration is used with SO fuel cell efficiencies can reach as much as 90%.

Direct Methanol Fuel Cell DMFC
Direct Methanol Fuel Cell (DMFC)

– Other types of fuel cells include: Alkaline fuel cells, which were used by the NASA space program (with efficiencies of 70%); Direct Methanol fuel cells, which draw the hydrogen out of the methane without the assistance of a fuel reformer; and Regeneration fuel cells, which utilize a solar-powered electrolyzer and their own waste water to form a self-contained regenerative system. Today, fuel cells are used commercially or in researches.

Where do we get all of the hydrogen necessary to power fuel cells?

It is estimated that the typical household with a fuel cell system with a 40% efficiency, using 24 kWh per day, will need 21,500 ft3 of hydrogen a year to generate energy. That is roughly 60 ft3 a day.

100 kW automotive fuel cell
100 kW automotive fuel cell

Where is all of this hydrogen going to come from?

Hydrogen is the most abundant element in nature. However, it is usually not found on its own but instead in conjunction with other elements. For this reason, it is necessary to separate hydrogen out of substances that contain hydrogen. The most common substance thus far has been the hydrocarbon natural gas. Currently, the US uses more than 3.2 trillion ft3 of hydrogen per year, mostly derived from reforming natural gas. While using natural gas in a fuel cell is better than using it in a combustion engine, it is still not renewable or emission free. A source of pure non-reformed hydrogen is needed to become entirely renewable and emission-free. Research on a low cost and efficient way to obtain large amounts of hydrogen has been going on for some time now. While it is true that fuel cells have evolved with the help of non-renewable fossil fuels, it is imperative that we discover a way to produce pure (and therefore clean) hydrogen. An infrastructure, which can deliver and transport massive amounts of hydrogen safely and efficiently must also be developed. The following are some examples of the current techniques for capturing pure hydrogen.

Pem fuel cell stack endplates
Pem fuel cell stack endplates

There is four basic methods of obtaining pure (not extracted from fossil fuel hydrocarbons) hydrogen. The methods are: thermochemical, electrochemical, photoelectrochemical, and photobiological.

  1. Thermochemical technologies use a steam reforming process to produce hydrogen from fuels such as gasified biomass, natural gas, coal, methanol, and gasoline. This includes processes that obtain hydrogen from switchgrass grown on marginal land.
  2. Electrochemical techniques rely on the electrolysis of water to produce hydrogen. This is nothing more than passing an electrical current through the water.
  3. Photoelectrochemical Processes split water, and hence produce hydrogen, by illuminating a water-immersed semiconductor with sunlight. Clean hydrogen (hydrogen not derived from fossil fuels) is most commonly produced using photoelectrochemical processes through the exploitation of solar panels. The solar panels produce electricity to electrically split water into hydrogen and oxygen through a process known as electrolysis. However, the cost of solar panels does not make this process economically competitive with the production of hydrogen from fossil fuels. It is perhaps better to just use the electricity straight from the solar panels, rather than using it to make hydrogen that will be used in fuel cells to make electricity.
  4. Photobiological methods use the natural photosynthetic activity of bacteria and green algae to produce hydrogen. This process is sometimes called biophotolysis. The University of Hawaii has built a processing plant with a development scale bioreactor.

There are also some researchers that are using wind turbines to make electricity, which is then used to electrolyze (split) water, releasing pure hydrogen.

Why aren’t more fuel cells being used today?

The main reason for the lack of use of fuel cells today is that fuel cells are relatively very expensive. The cost of producing and using fuel cells is still high because of the lack of mass production. Like any other young technology, it must be fine-tuned and then mass-produced before it becomes a more economical option for the consumer.

How do fuel cells affect the environment?

Fuel cells, when powered by pure, clean hydrogen not acquired through the reformation of fossil fuels, are total emission free. The only products of the cell, besides electric current, are heat and pure potable H2O. Through cogeneration of the heat into steam and regeneration of the water using the process of electrolyzation, even these byproducts are utilized. In the case that they are not utilized, there is still no harm done to the surrounding environment.

The reality, however, is that pure hydrogen is rarely used except for in laboratory studies. Steam reformers are often used to isolate the hydrogen out of hydrocarbon fuels. Steam reformation is the process of combining steam with a hydrocarbon to isolate the hydrogen. The resulting CO2 emissions are lower than those from a combustion engine. Steam reformation emits zero to small amounts of NOx and SO2.

Even though the use of fossil fuels such as natural gas and propane is not totally emissions free and not renewable, the use of these fuels to power fuel cells is cleaner than the use of combustion engines and will allow for the continued improvement of fuel cells. Fuel cells are more environmentally sound than conventional energy technologies. Below is a comparison of the amount of pollution produced by fuel cells versus conventional energy technologies.

Toyota Mirai fuel cell engine
Toyota Mirai fuel cell engine

– Fuel Cell Engine vs. Gasoline Engine

In the US, passenger vehicles alone consume 6 million barrels of oil a day. If 10,000 vehicles were to be powered by fuel cells, oil consumption would be reduced by as much as 6.98 million gallons per year. According to the Department of Energy, if 10% of automobiles in the US were powered by fuel cells, 60 million tons of CO2 would be kept from entering the atmosphere. Furthermore, massive amounts of other greenhouse gasses such as NOx and SO2 would be reduced as well as a lessened demand for the harmful drilling of oil amidst finely tuned fragile ecosystems.

– Fuel Cell Generation vs. Standard Coal and Oil Generation

Compared to the emissions created by conventional stationary power plants, fuel cells are much less emissive. Solid Oxide (SO) fuel cells emit less than coal and oil generators.

– Fuel Cell Generator vs. Diesel Generator

Compared to the emissions created by the most commonly used form of micropower, the diesel generator, fuel cells are much cleaner for the environment. Diesel generators produce enormously large amounts of NOx and particulate matter, far more than fuel cells almost zero emissions of these two climate-changing substances.

Almost all the well-known auto manufacturers plan for FCEV development and commercialization.

Why don’t we put a fuel cell on the moon and beam the energy back to Earth?

Although almost 1/3 of the moon’s thin atmosphere is composed of hydrogen, the idea of creating electricity that will have to be beamed 384,400 kilometers to be used is problematic at best. By current technology, electricity travels along transmission cables that are inefficient and very expensive: high voltage cables can cost thousands of dollars per mile, which is part of why there are people on earth that cannot get electricity. Even if Bill Gates were to decide to fund such an operation, there exists no transmission cable that could maintain a current for the distance between the earth and the moon. Since even the best cables experience power losses of about 10% just between the standard earth-bound generating plant and its earthling customer, the electric current would never make the journey to the moon and back to the earth. Also, terrible tangling of the cables would occur due to the incongruent orbital and rotational patterns of the earth and the moon. It would be much more efficient and cost-effective to produce energy using fuel cells. But maybe in the future, we can wirelessly transmit the hydrogen energy from the moon to the earth.

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