What are fuel cells?
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent.Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied.
Why Fuel Cells?
The rising urgency for alternative, non-polluting sources of energy has given prominence to newer, cleaner sources. While technologies like wind and solar have been around for a while now and have rapidly risen in popularity and installations, they have their fair share of problems with intermittency being a major stumbling block. Offering clean, continuous and stable power while harnessing a rich energy source, fuel cells offer a reliable solution for mission-critical power.
Every fuel cell has two electrodes, one positive and one negative, called, respectively, the anode and cathode. The reactions that produce electricity take place at the electrodes. Every fuel cell also has an electrolyte, which carries electrically charged particles from one electrode to the other, and a catalyst, which speeds the reactions at the electrodes.
Parts of a Fuel Cell
Source: US Department of Energy
Fuel Cell Stack
The fuel cell stack is the heart of a fuel cell power system. It generates electricity in the form of direct current (DC) from chemical reactions that take place in the fuel cell. A single fuel cell produces enough electricity for only the smallest applications. Therefore, individual fuel cells are typically combined in series into a fuel cell stack. A typical fuel cell stack may consist of hundreds of fuel cells. The amount of power produced by a fuel cell depends upon several factors, such as fuel cell type, cell size, the temperature at which it operates, and the pressure at which the gases are supplied to the cell. Learn more about the parts of a fuel cell.
The fuel processor converts fuel into a form useable by the fuel cell. If hydrogen is fed to the system, a processor may not be required, or it may be needed only to filter impurities out of the hydrogen gas.
If the system is powered by a hydrogen-rich, conventional fuel, such as methanol, gasoline, diesel, or gasified coal, a reformer is typically used to convert hydrocarbons into a gas mixture of hydrogen and carbon compounds called "reformate." In many cases, the reformate is then sent to another reactor to remove impurities, such as carbon oxides or sulphur, before it is sent to the fuel cell stack. This process prevents impurities in the gas from binding with the fuel cell catalysts. This binding process is also called "poisoning" because it reduces the efficiency and life expectancy of the fuel cell.
Some fuel cells, such as molten carbonate and solid oxide fuel cells, operate at temperatures high enough that the fuel can be reformed in the fuel cell itself. This type is called internal reforming. Fuel cells that use internal reforming still need traps to remove impurities from the unreformed fuel before it reaches the fuel cell.
Both internal and external reforming release carbon dioxide, but less than the amount emitted by internal-combustion engines, such as those used in gasoline-powered vehicles.
Current Inverters and Conditioners
Current inverters and conditioners adapt the electrical current from the fuel cell to suit the electrical needs of the application, whether it is a simple electrical motor or a complex utility power grid.
Fuel cells produce electricity in the form of direct current (DC). In a direct current circuit, electricity flows in only one direction. The electricity in your home and workplace is in the form of alternating current (AC), which flows in both directions on alternating cycles. If the fuel cell is used to power equipment using AC, the direct current will have to be converted to alternating current.
Both AC and DC power must be conditioned. Power conditioning includes controlling current flow (amperes), voltage, frequency, and other characteristics of the electrical current to meet the needs of the application. Conversion and conditioning reduce system efficiency only slightly, around 2%–6%.
Heat Recovery System
Fuel cell systems are not primarily used to generate heat. However, because significant amounts of heat are generated by some fuel cell systems—especially those that operate at high temperatures, such as solid oxide and molten carbonate systems—this excess energy can be used to produce steam or hot water or to be converted to electricity via a gas turbine or other technology. These methods increase the overall energy efficiency of the systems.
Types of Fuel Cells
Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications.
Sources: University of Cambridge
Polymer Electrolyte Membrane (PEM) Fuel Cells
Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high-power density and offer the advantages of low weight and volume, compared with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fuelled with pure hydrogen supplied from storage tanks or on-board reformers.
Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80°C (176°F). Low-temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO.
PEM fuel cells are used primarily for transportation applications and some stationary applications. Due to their fast start-up time, low sensitivity to orientation, and favourable power-to-weight ratio, PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.
A significant barrier to using these fuel cells in vehicles is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the hydrogen on-board as a compressed gas in pressurized tanks. Due to the low-energy density of hydrogen, it is difficult to store enough hydrogen on-board to allow vehicles to travel the same distance as gasoline-powered vehicles before re-fuelling, typically 300–400 miles. Higher-density liquid fuels, such as methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline, can be used for fuel, but the vehicles must have an on-board fuel processor to reform the methanol to hydrogen. This requirement increases costs and maintenance. The reformer also releases carbon dioxide (a greenhouse gas), though less than that emitted from current gasoline-powered engines.
Direct Methanol Fuel Cells
Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode.
Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cells because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline.
Direct methanol fuel cell technology is relatively new compared with that of fuel cells powered by pure hydrogen, and DMFC research and development is roughly 3–4 years behind that for other fuel cell types.
Alkaline Fuel Cells
Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F)
AFCs' high performance is due to the rate at which chemical reactions take place in the cell. They have also demonstrated efficiencies near 60% in space applications.
The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can affect this cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost.
Cost is less of a factor for remote locations, such as space or under the sea. However, to effectively compete in most mainstream commercial markets, these fuel cells will have to become more cost-effective. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours, something that has not yet been achieved due to material durability issues. This obstacle is possibly the most significant in commercializing this fuel cell technology.
Phosphoric Acid Fuel Cells
Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The chemical reactions that take place in the cell are shown in the diagram to the right.
The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.
PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide because carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell's efficiency. They are 85% efficient when used for the co-generation of electricity and heat but less efficient at generating electricity alone (37%–42%). This is only slightly more efficient than combustion-based power plants, which typically operate at 33%–35% efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell.
Molten Carbonate Fuel Cells
Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide (LiAlO2) matrix. Because they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.
Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85%.
Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs do not require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.
Molten carbonate fuel cells are not prone to carbon monoxide or carbon dioxide "poisoning" —they can even use carbon oxides as fuel—making them more attractive for fueling with gases made from coal. Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulphur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen.
The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.
Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Because the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50%–60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80%–85%.
Solid oxide fuel cells operate at very high temperatures—around 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.
SOFCs are also the most sulphur-resistant fuel cell type; they can tolerate several orders of magnitude more of sulphur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This property allows SOFCs to use gases made from coal.
High-temperature operation has disadvantages. It results in a slow start-up and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation and small portable applications. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.
Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 800°C that have fewer durability problems and cost less. Lower-temperature SOFCs produce less electrical power, however, and stack materials that will function in this lower temperature range have not been identified.
Regenerative Fuel Cells
Regenerative fuel cells produce electricity from hydrogen and oxygen and generate heat and water as by-products, just like other fuel cells. However, regenerative fuel cell systems can also use electricity from solar power or some other source to divide the excess water into oxygen and hydrogen fuel—this process is called "electrolysis." This is a comparatively young fuel cell technology being developed by NASA and others.
Advantages of Fuel Cells
Fuel Cells present a very clean energy source utilizing the potential of hydrogen which has the highest specific energy (energy/unit mass). They produce water as a by-product and have a zero carbon footprint unless reformation of fossil fuels is involved in which case a small amount of carbon di-oxide is released.
Since fuel cells are electrochemical in nature, the energy efficiency is much higher as compared to most other prevalent renewable energy sources i.e. a better portion of the input is converted into useful energy.
Installation simplicity and Operational Ease
Fuel cells are easy to install. They have no moving parts, require very little space and produce no vibration or noise. Consequently, there is very little maintenance involved.
Disadvantages of Fuel Cells
Hydrogen Supply Chain
Hydrogen, as said before, is the most prevalent element on the earth but is never found in isolation. Having high specific energy, it has been identified as a potential replacement for conventional transport fuels and an energy carrier.
The production of hydrogen has always been an expensive process but hydrogen is released as a by-product in the production of caustic soda; biomass; refineries; captive plants for fertilizer, soap, glass plants and sugar industries. Reformed hydrogen is sourced from hydrocarbons like natural gas and LPG.
However, caustic soda plants are the biggest source of hydrogen at this point. The production of one tonne of caustic soda generates 28 kg of hydrogen. India produces 5.5 million tonnes of alkali out of which 70% is caustic soda. This translates to an electricity production capacity of about 1.3 billion units from the hydrogen generated.
Typically the hydrogen generated in caustic soda plants is used inefficiently for captive consumption. If this is made available with a proper supply chain mechanism, it can effectively be used for power production in myriad applications. A secure supply of hydrogen is very crucial to determining the success of a fuel cell installation.
50% of the operational cost of a fuel cell comes from storage and transportation. Even if hydrogen is easily available, its storage and transportation pose major hurdles. Hydrogen needs to be stored under a pressure of 150 baras per standards in India. However there is possibility that this standard might change in future to 300 bar as is the international standard. In the future as market matures and more players come in, the cost might go down. We have seen this happen in the case of LPG and CNG in the past.
Lifetime of components
The fuel cell stack, which is the most important component of the system, has a life of about 8000 hours. If we assume that a site needs a backup for, let’s say, 8 hours a day, it means that a stack life is about three years after which the stack has to be replaced. The cost of fuel cell stack represents between 30-40 per cent of the system’s capital cost and is the only maintenance cost incurred. In comparison diesel generator sets require maintenance every 500 hours and its comprehensive annual maintenance cost is INR 12-14 / hour.
This means that in a 20 year time period, which is the life of the balance of systems, the cells have to be replaced six or seven times.
Fuel cells offer a convenient solution for standalone power in off-grid and power-deficit areas for captive consumption in mission-critical applications. They don’t require much space and produce continuous, reliable power contingent upon the availability of hydrogen nearby.
Fuel cells are modular and the capacities can be extended easily by adding more stacks. They can operate continuously to meet base-load power needs or, intermittently to provide peak power during times of high demand.
Backup power requirements, in India, are addressed mostly with the use of diesel generators. Diesel generators involve a lower capital outlay up-front but have to be maintained and serviced multiple times during their lifetime. Add to that the harmful environmental effects of using diesel and the imminent price escalation and the total lifecycle costs shoot up tilting the economics in favour of all alternative sources including fuel cells. Specific areas where fuel cells are already being implemented as backup sources include telecom tower sites.
One of the biggest upsides of fuel cell technology is that it has the potential to replace traditional methods of powering automobiles. Indeed, Tata motors have launched a pilot bus, with support from the Government of India’s Department of Scientific and Industrial research under the Technology Department & Demonstration programme.
Hydrogen is stored in compressed form and combines with oxygen to produce electricity and charge a battery which is used to power the vehicle’s motor. This mechanism involves a fuel cell with gross peak power of 114 HP, coupled with a motor with the peak power output of 250 HP with motor speed from 600 rpm to 2100 rpm and torque of 1050 Nm at 800 rpm. The maximum speed of the bus is 70 kmph and gradeability is 17%, which is very suitable for city application.
Batteries have been used in material handling equipment for the past 100 years at least. While their efficiencies have improved drastically over the last few years, lead acid batteries still only have a lifetime of a maximum to 5-7 years.
Indeed, this is one area where fuel cells have increasingly found applications in. Ballard power systems designs and manufactures fuel cell stacks for use in material handling market and has tied up with Plug Power Inc., which holds the largest share of the North American Class 1, 2 and 3 clean energy lift truck market. They also have agreements with other leading OEM’s, lift truck dealers and equipment leasing companies.