Fuel celltechnology

How they work

What is a fuel cell?A fuel cell is an electrochemical device that converts hydrogen fuel directly into electricity and heat without combustion.

By the nature of its electrochemical reaction, a fuel cell can be more than twice as efficient as an internal combustion engine (approximately 60% compared to about 30%).

A conventional engine burns fuel to create heat and in turn converts heat into mechanical energy and finally electricity. A fuel cell produces electricity, water and heat directly from hydrogen and oxygen.

Conventional engine

Fuel → (combustion) → Heat → Mechanical energy → Electricity

Fuel cell

Fuel → (chemical reaction) → Electricity + Water + Heat

Heat

A number of fuel cells can be combined to form a fuel cell stack. The

The power output will depend on the size of the stack.

A stack may be used on its own with a direct hydrogen source, or integrated with a number of other components to provide an operational power system tailored for a specific application or market.

Construction of a fuel cell stack

To view an animation of the workings of a fuel cell, click here.

Sources of hydrogen

Fuel cells are fuelled by hydrogen, one of the most abundant elements on the planet. The hydrogen fuel can come from a wide range of sources:

Hydrogen can be produced through reformation of a range of hydrocarbon-based feedstocks such as natural gas, propane, methanol butane or propane (liquid petroleum gas or LPG).

Hydrogen can be derived from sustainable, carbon-neutral sources of methane such as biomass or land-fill gas.

Hydrogen can be produced through a process of electrolysis using renewable power sources such as wind or solar power.

Fuel cell typesFuel cell technology embraces a number of different fuel cell types with different characteristics and applications.

PEM fuel cells:

Efficiency ~55%

Operating temperature 80°C

PEM is considered to be the most promising of fuel cell types for mass market application and is being developed for stationary, portable and vehicle markets.

Proton Exchange Membrane (PEM) fuel cells are the most versatile of all fuel cell types. This versatility is due to their high power densities, modular construction, relatively low operating temperature (between 40°C and 100°C) and the solid polymer electrolyte.

The electrolyte used in PEM is poly-perfluorosulphonic acid and the electrochemical reaction at the anode and cathode is as follows:

Anode reaction: H2 → 2H+ + 2e-

Cathode reaction: O2 + 4H+ + 4e- → 2H2O

Fuel cell benefitsFuel cells of all types provide a range of benefits when compared to both conventional and other renewable technologies:

EfficiencyConventional technologies reach maximum efficiency at a single operating point. Fuel cells however achieve high efficiency both under partial loads and at full capacity, and have rapid load following capability.

This gives fuel cell technology a distinct advantage over other technologies in both stationary and automotive power applications, as they are capable of rapid start-up and immediate response to changes in demand.

Non-pollutingUnlike conventional technologies, when fuelled by hydrogen, fuel cells emit no pollutant by-products. Even when operating on hydrocarbon fuels, pollutant levels can be significantly lower.

With few moving parts, fuel cells are also a quiet source of power.

ModularityFuel cells are modular in construction and can be economic across a wide range of scales, with multiple units easily constructed to match demand, whereas conventional technologies are often least expensive and more efficient only at larger scales.

This modular feature also has advantages in the manufacturing process where operating efficiencies can be achieved by replicating a small number of core fuel cell stack designs to meet a range of power demands.

Power qualityThe modular design also mitigates against operations failure. The failure of a single cell or stack (in a group of more stacks) would only result in some loss of power rather than a complete shutdown, as is the case with other conventional technologies.

Power independenceFuel cells can be operated without the need for connections to an electricity grid infrastructure. This makes them a relatively cheap source of power for remote locations and applicable for portable uses.

Fuel flexibilityFuel cell systems can be tailored to accept a wide range of hydrogen-based fuels, from pure hydrogen through to heavy hydrocarbon feedstocks such as diesel. Systems can also be adapted for multi-fuel capability.

Capability as battery replacementUnlike batteries in which the reactants are contained within the case, fuel cells draw their energy from an external supply (hydrogen) which can be easily and quickly replaced.

Weight for weight, fuel cells contain around 1000 times the energy of a lead-acid battery and 200 times that of the latest lithium ion technology.

Advantages of PEM fuel cellsSome fuel cell technologies are only applicable to certain applications. PEM technology is suitable to a very wide range of applications for the following reasons:

RobustPEM cells contain a solid (rather than liquid) electrolyte and so are less sensitive to shock and vibration than other fuel cell types. As a result, PEM technology is better suited to portable and motive applications.

High power densityPEM fuel cells have a higher power density than all other fuel cell types and therefore can be more compact. As a result they are more suitable for applications where lower volume or weight are critical factors.

Lower cost of fabricationPEM fuel cells operate at relatively low temperatures (typically 80°C and lower). As a result, less exotic materials can be used making PEM cells more suitable for mass market applications.

Rapid load following capabilityPEM fuel cells can vary their output quickly to meet shifts in power demand and are suited for applications where quick start-up and high power quality is required. This responsiveness is important, not only for the demands of transport applications and the need for acceleration/deceleration etc. but also for stationary applications such a residential CHP (combined heat and power).

EfficientA PEM fuel cell system can exceed 90% overall efficiency when in CHP mode, with electrical efficiency typically around 50%. This compares to 35% efficiency for central electricity generation plants and around 20% for a typical automotive internal combustion engine.

Applications of PEM fuel cellsPEM fuel cells have the potential for application in all forms of power generation, including:

The first viable electric alternative to the internal combustion engine for vehicles, e.g. cars, motorbikes, buses, locomotives, forklifts, light aircraft and UAVs.

On board auxiliary power units (APUs) for land and air transportation.

Decentralised power generation for industrial and domestic applications.

Portable generation systems for domestic, industrial, military and maritime application.Small scale power packs for remote, unattended and military application.

General battery replacement/displacement.

Phosphoric acid fuel cells (PAFC): Efficiency ~55%

Operating temperature 180°-210°C

This was the earliest fuel cell technology to achieve commercialisation it remains a costly technology.

PAFCs are large and heavy compared to PEM cells.

Used in medium to high power stationary applications including CHP.

Alkaline fuel cells (AFC): Efficiency ~60%

Operating temperature 60°-90°C

The main disadvantage of AFC technology is its sensitivity to contaminants in the gas streams – carbon dioxide must be removed, adding to the scale and complexity of the system.

AFC technology is used in space applications.

Molten carbonate fuel cells (MCFC): Efficiency ~55%

Operating temperature 650°C

MCFC technology employs a liquid electrolyte which has a negative impact on the stability and life of the fuel cell components.

MCFC is in early development but promises high fuel-to-electricity efficiencies and the ability to consume coal-based fuels.

MCFC is being developed for natural gas and coal-based power plants for use in heavy industrial complexes and for CHP applications.

Solid oxide fuel cells (SOFC): Efficiency ~50%

Operating temperature 1000°C

SOFCs are constructed using high temperature ceramics and metals which can be costly.

SOFC technology is being designed and demonstrated for stationary applications, particularly for industrial and large-scale central electricity generating stations and CHP applications.

Stacks

Intelligent Energy’s fuel cell stack technology is designed to be both simple and cost-effective. Our stacks and systems have been designed from first principles and combine novel fluid and thermal management techniques and integrated humidification with metal bi-polar plate architecture. The result is both simpler and cheaper than competing stack technologies and eminently suitable for mass manufacture.

We have two distinct designs of PEM fuel cell each appropriate for specific power ranges and applications: air cooled (AC) technology suitable for lower power (e.g. up to 2kW) applications and our evaporatively cooled (EC) stacks for applications in the 1kW to 100kW range.

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Distributed generation of hydrogenProducing hydrogen from a range of fuels on-site, where it is required for refuelling or power provision has the advantage of allowing energy suppliers to make maximum use of their existing fuelling infrastructures. They are able to offer hydrogen to their customers without the need for the high capital expenditure associated with central generation and pipeline construction costs.This removes the need for the high capital expenditure associated with central generation and pipeline construction costs. Rather than replacing petrol stations with hydrogen refueling stations, or constructing thousands of miles of hydrogen conveyor pipelines, hydrogen will be generated at the point of dispensation from an existing fuel, such as diesel, natural gas or bio-fuel, using small reformer based hydrogen generators.

Distributed hydrogen generation and clean fuel technologies, such as those developed by Intelligent Energy, will ensure that the increasing demand for hydrogen can be met in a manner that is both incremental and cost effective.

Pictured below: Compact hydrogen generators

What is a fuel cell?A fuel cell is an “electrochemical device that produces electricity without combustion. The optimal energy carrier for fuel cells is hydrogen (which can be extracted from methanol, natural gas, water or petroleum products). When hydrogen is combined with oxygen (from air) it produces electrical energy. The conversion process is environmentally benign: only heat and water are emitted as by-products. Environmentally, hydrogen is the optimal energy carrier for fuel cells, because fuel cells that run on hydrogen have zero emissions.

By the nature of its electrochemical reaction, a fuel cell can be more than twice as efficient as an internal combustion engine. A conventional engine burns fuel to create heat and in turn converts heat into mechanical energy and finally electricity. A fuel cell produces electricity, water and heat directly from hydrogen and oxygen. Fuel cells are like batteries in that they are electrochemical devices, but unlike batteries do not need recharging and will continue to operate as long as they are provided with fuel (hydrogen).

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What is a fuel cell stack?A fuel cell stack is a number of fuel cells stacked together like a sandwich using bipolar plates (an anode and cathode combined in one). One advantage of fuel cell technology is that it is both scalable and modular. To achieve any desired power output, you only need to stack cells together.

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What is a fuel cell power system?A fuel cell power system is the complete set of components that integrate with the fuel cell stack so that electricity is produced. The fuel cell requires other systems to make it a complete power source, including air, fuel and control systems.

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What are PEM fuel cells?PEM is an acronym meaning Proton Exchange Membrane. In a PEM fuel cell the electrolyte is a proton (H+) conducting solid polymer membrane. They are also known as PEFC (polymer electrolyte fuel cell) or SPFC (solid polymer fuel cell).

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What are the advantages of PEM fuel cells?PEM fuel cells display the highest power densities of any of the fuel cell types, which makes them particularly attractive for transportation and portable applications where minimum size and weight are required. They contain no corrosive liquid electrolyte and can be robust in construction and are modular and scalable in design. They are low temperature fuel cells which usually operate below 100 degrees C. This means that unlike high temperature fuel cells such as solid oxide, which operate at ~600 degrees C they can be fabricated from cheaper, less exotic materials. The low temperature of PEM fuel cells can also be an advantage when low thermal signature is desired. PEM fuel cells also have the advantage of potential application across a very wide range; from portable power at a few watts to hundreds of kilowatts for vehicular and stationary power.

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What things can PEM fuel cells be used to power?PEM fuel cells can be used in portable electronic and electrical devices, such as laptops or power tools, to generate heat and power for domestic and distributed generation, to provide auxiliary and back-up power in a wide range stationary and transport applications and to provide propulsion power to motorbikes, cars, buses, submarines, unmanned vehicles and light aircraft.

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When will fuel cells be commercially available to consumers?PEM fuel cells are at the pre-commercial and early market entry stage today, poised to enter a wide range of markets, but their actual entry in to specific markets will depend on a number of factors. We are already seeing the introduction of fuel cells in a commercial context – the motive sector for example is now in the fleet trials stage of product role out, and fuel cell systems are being fielded for back-up power application in the telecoms sector.

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Where does the hydrogen come from to fuel PEM fuel cells?Hydrogen is an energy carrier not a naturally occurring fuel and must be produced from hydrogen containing feedstock. Hydrogen can be produced from an extremely wide range of sources, but most of the world’s hydrogen is presently produced by reformation of natural gas. Hydrogen is also commonly produced by the electrolysis of water. Electrolysis requires electricity, if that electricity is produced by renewable means; the hydrogen produced is as low carbon as it is possible to be.

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What about using hydrocarbon fuels to produce hydrogen - doesn’t this still produce greenhouse gas and other emissions? Why not just burn the natural gas?There are much less polluting emissions formed by the reformation of natural gas than by burning it, although carbon dioxide production is about the same. The key thing is the energy conversion at the point of power production. If the hydrogen is burned in an internal combustion engine there is no real benefit, if the hydrogen is fed into a PEM fuel cell, with its much higher efficiencies, the amount of carbon dioxide produced can be much less. Well-to-wheel, using hydrogen produced from natural gas to feed a fuel cell produces 30% – 50% less carbon dioxide than just burning the fuel to provide the same amount of energy.

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How safe is hydrogen?Hydrogen is no more or less dangerous than other flammable fuels, including petrol, LPG and natural gas. In fact, some of hydrogen’s properties actually provide safety benefits compared to petrol or other fuels, for example it dissipates very quickly and is difficult to ignite. However, all flammable fuels must be handled responsibly. Like petrol and natural gas, hydrogen is flammable and can behave dangerously under specific conditions. Hydrogen can be handled safely when guidelines are observed and the user has an understanding of its behaviour.

Over 50 million tons of hydrogen are produced every year and hydrogen’s safety record is excellent.