Fuel cell
A fuel cell is a galvanic cell, which converts the chemical reaction energy of a fuel continuously supplied and an oxidant into electrical energy. In the language fuel cell typically refers to the hydrogen-oxygen fuel cell.
A fuel cell is no energy storage, but a converter. The energy for power generation is supplied in chemically bonded form with the fuels.
- 9.1 Stationary Use 9.1.1 Operation
- 9.2.1 Road Traffic
- 9.2.2 Aviation
- 9.2.3 Space
- 9.2.4 Marine
Comparison
The production of electrical energy from chemical energy sources is usually carried out by incineration and recovery of the hot gases in a heat engine with a downstream generator. So only chemical energy by combustion in thermal energy and then into mechanical work is converted. Only from this current is generated in the generator. However, a fuel cell is likely to reach the transformation without the conversion into heat and energy and is therefore potentially more efficient. In contrast to an internal combustion engine ( ICE ) converts chemical energy directly into electrical energy and is not subject to the poor efficiency of VKM. The theoretical maximum useful work is limited solely by the free enthalpy of the chemical reaction and may thus be higher than in the coupling of a heat engine ( Carnot efficiency ) with a generator to produce electricity. Virtually also achieved with the combination of fuel cell and electric motor efficiency is higher than that of petrol or diesel engines. However, the effort required for the production and storage of energy should be considered with the entire chain of action. Well researched is the hydrogen-oxygen fuel cell.
History
A simple fuel cell was created in 1838 by Christian Friedrich Schönbein, lapped by two platinum wires in hydrochloric acid with hydrogen or oxygen and noticed an electrical voltage between the wires. A year later Schönbein published these results. Sir William Grove wrote in the same year a note about the so-called " batterisierte oxyhydrogen ". Together with Schönbein he recognized the reversal of the electrolysis and the generation of electricity in this phenomenon and applied these findings in several experiments with.
1870 Jules Verne wrote of the fuel cell:
" Water is the coal of the future. Tomorrow's energy is water that has been decomposed by electricity. The so- decomposed elements of water, hydrogen and oxygen, are secure for the indefinite future, the energy of the earth. "
Because of the invention of the electric generator, then called dynamo, by Werner von Siemens as "galvanic gas battery " designated invention waned. The dynamo was relatively simple and straightforward, in conjunction with the steam engine in terms of fuel and materials and was therefore preferred at this time of the complex fuel cell.
Special Events
The first prototype of a larger fuel cell powered vehicle was introduced in 1959 by Allis -Chalmers with a fuel cell powered tractor. The first productive inputs had the fuel cell in the American space technology of the 1960s. In the Apollo moon missions, it served as the most reliable supplier of energy. But on 11 April 1970, the rocket of the Apollo 13 mission with a crew of three after a problem-free start reaching the All, exploded one of the two oxygen tanks in the service module of the " Odyssey ", and two fuel cell had to be switched off.
Construction
A fuel cell is composed of electrodes separated by a semi-permeable membrane or an electrolyte ( ionic conductor ) are separated from each other.
The electrode plates / bipolar plates are usually made of metal or carbon nanotubes. They are coated with a catalyst, for example platinum or palladium. As an electrolyte, for example, dissolved alkalis or acids, Alkalicarbonatschmelzen, ceramics or membranes can be used.
The energy provides a reaction of oxygen with the fuel, which may be hydrogen, but can also consist of organic compounds such as methane or methanol. Both reactants are fed continuously over the electrodes.
The voltage supplied is theoretically 1.23 V for the hydrogen-oxygen cell at a temperature of 25 ° C. In practice, however, only voltages of 0.5-1 V ( experimentally above) reached. The voltage of the fuel, the quality of the cell and of the temperature. To obtain a higher voltage, several cells are connected to form a stack (English for ' stack ') in series. Under load, the electric and chemical processes cause a decrease in the voltage (not in the high temperature molten carbonate fuel cell, MCFC).
At the low temperature proton exchange membrane fuel cells ( Proton Exchange Membrane Fuel Cell, PEMFC, or Polymer Electrolyte Fuel Cell, PEFC ) has the structure as follows:
Types of fuel cells
Reversible fuel cell
A further development of the conventional polymer electrolyte fuel cells on hydrogen-oxygen basis is the reversible fuel cell ( en. reversible fuel cell RFC), which originally consisted of the combination of a hydrogen fuel cell with an electrolyzer. Newer models combine the internal combustion engine and the electrolysis process, to save weight and to reduce the complexity. Thus, reversible fuel cells are suitable as an energy converter for energy storage and for use as in battery systems.
Chemical Reaction
The principle of the fuel cell in 1838 by Christian Friedrich Schönbein based on the reaction
Invented. Many types of fuel cells today use this reaction as " cold combustion " for obtaining electrical energy.
An important example is the proton exchange membrane fuel cell ( PEMFC). Such a fuel cell typically uses hydrogen as an energy carrier and has an efficiency of about 60 %. Other designs operate with methanol or methane and generate hydrogen by steam reforming. The heart of the PEMFC is a polymer membrane (ie only for H ions) is permeable only to protons, the so-called proton exchange membrane (PEM ). The oxidant, usually oxygen from the air, is thereby spatially separated from the reducing agent.
Of the fuel, in this case hydrogen is catalytically oxidized at the anode, releasing electrons to protons. These pass through the ion exchange membrane to the chamber with the oxidizing agent. The electrons are discharged from the fuel cell and flow through an electrical device, such a light bulb, to the cathode. At the cathode, the oxidant, in this case oxygen, is reduced by absorption of the electrons to the anions which react directly with the hydrogen ions to form water.
Fuel cells with such a structure are called polyelectrolyte fuel cells, PEFC ( polymer-electrolyte fuel cell) or proton exchange membrane fuel cell, PEMFC ( Proton Exchange Membrane Fuel Cell). The membranes used are acidic electrolyte.
Redox reaction equations for a PEMFC:
There are also alkaline hydrogen fuel cells. However, they only work with high-purity hydrogen and oxygen. In them, the gases are introduced through porous, catalytically active electrodes in a basic solution.
The running there redox reactions are:
Electrical efficiency, cost, life
At the Institute for Energy Research at the Forschungszentrum Jülich following results were in 2003 for fuel cell systems obtained:
Modern fuel reach the practical operation at the moment (2012) up to 60% efficiency.
Cost and efficiency of a BSZ- system are determined not only by the fuel cell, but also by the auxiliary units (eg the BSZ- vehicle traction battery, electric drive ) and the cost of providing the BSZ- fuel. The basis of comparison should therefore comprehensive consideration of the active chains in motor vehicles on the basis of well-to -wheel form.
The following table shows performance, efficiency and cost overview for various conventional energy usages:
The life of a PAFC is between 40,000 hours for stationary and 5,000 operating hours for mobile systems ( 40,000 hours of continuous operation correspond to 1666 days or 4.6 years of continuous operation ). The life of a solid oxide fuel cell SOFC is still limited to a few months in production costs in the order of CHF 100,000 ( EUR 62,000 ) (Updated: March 13, 2006).
High temperature fuel cells can be connected in order to increase the efficiency of a micro gas turbine, so that they combine to achieve efficiencies of over 60%.
Energy
Applications
The first applications of fuel cells resulted in areas such as military and aerospace, where the cost played a very minor role and the specific benefits outweighed the cost advantages of diesel generators. Fuel cells are lighter than batteries and more reliable and quieter than generators. The low noise emissions and the ability to operate fuel cells after prolonged inactivity reliable, contributed to an often initially for military use, as well as an insert in in emergency power supplies. In addition, manufacturers of fuel cells are of the opinion that fuel cells can generate kinetic energy in various areas of more efficient in combination with an electric motor as internal combustion engines.
However, the particular strength of fuel cells is the high energy density, which explains the early interest of the military and aerospace to this technique.
Stationary use
The stationary field of application of a fuel cell system extends over a wide power range, from small systems with a capacity of two to five kilowatts of electricity - eg as domestic energy supply - the way to systems with several hundred kilowatts. Larger systems are used in hospitals, swimming pools or for the supply of small municipalities.
An electricity-generating fuel cell based Hyo- heating system ( " Hy " = Hydrogenium = hydrogen and "O " = Oxygenium = oxygen; mini- CHP = micro-CHP ) consists of several components. Ideally, the purchase of - as climate- neutral generated - Hydrogen produced with little effort a PEMFC ( polymer electrolyte membrane fuel cell ) is used. Until a ( bio-) hydrogen as fuel is available, but instead of fossil or biogenic methane (natural gas or even " biomethane "), an expensive and sensitive reformer unit is required. This converts the methane into hydrogen for direct operation of the fuel cell based Hyo- conditioning and in CO2 than gas. The second component is the fuel cell (FC ), with the result of the generation of electricity and heat used oxygen from the ambient air for the chemical process (oxidation of hydrogen supplied ). There are also the electrical power electronics and the associated regulation of plant management. To cover the thermal load peaks additional conventional natural gas-fired heat generators are usually installed.
For stationary application to all types of fuel cells are suitable. Current developments are limited to the SOFC, MCFC and PEMFC. The SOFC and MCFC have the advantage that - natural gas can be used directly as fuel gas - due to the high temperatures. The removal of hydrogen ( H2) from methane (CH4 ) of the gas pipeline network ( " reform process " ) in this case runs within the high temperature fuel cell ( HTFC ), which eliminates the need for a separate reformer when using methane. The working in the low-temperature range PEM fuel cell, however, required for methane used for the production of hydrogen reformer, a separate unit with a sophisticated gas cleaning stage, because the reformate largely of carbon monoxide ( CO) has to be freed. CO is formed at each reforming of hydrocarbons. CO is a catalyst poison in this BZ- type and would reduce both the performance and the lifetime of the fuel cell significantly.
During operation of the high-temperature SOFC and MCFC cells, the hot exhaust air to the sterilization of articles can be used. As an emergency power generator, they are unsuitable because of the longer start-up phase. A low-temperature PEMFC system, however, can contact in case of sudden emergency needs within fractions of a second automatically running.
Operation
In the stationary fuel cell application is currently available heat production compared with the production of electricity in the foreground. These systems are therefore usually operated heat demand led. This means that the system power is controlled by the amount of heat required, the electric power generated is fed into the public power grid. Stationary fuel cell systems are best operated at a low power modulation. Ideally, the base load heat demand is fully met by the FC CHP. (Heat) load peaks are covered by conventional heaters. In this way, the stationary fuel cell system operates at only a single constant load point. Thereby, the efficiency of the system be optimally designed. The life of a BZ is determined in first approximation by the number of start-stop cycles, as these show the most unfavorable effect on the catalysts inside.
For a PEM fuel cell with cathode closed rule is that it has been turned off on both sides - should be sealed - including oxygen- sided. This makes a new start, as necessary for the operation of moisture is retained, and may collect any harmful gases. If stored at temperatures below freezing to take place, the fuel cell must be completely dried to prevent damage from ice formation.
Mobile use
Road
Several automotive companies (including Volkswagen, Toyota, Daimler, Ford, Honda, General Motors / Opel ) research partly for twenty years with government support on automobiles whose fuel is hydrogen, and use the energy conversion fuel cells and an electric motor for propulsion. An example are the vehicles NECAR 1 to 5 NECAR and Mercedes -Benz F-Cell and the concept F125 from Daimler. The Swiss Hy-Light - vehicle moved in 2004 into the public eye. Currently, some MAN fuel cell city buses in Berlin go for the BVG in operation. At BMW, the fuel cell is not originally intended to produce electrical drive energy. The concept is here to continue to use an internal combustion engine, the fuel is hydrogen then, however, the stored liquid at very low temperatures. The concept vehicle for this is a type E68 ( 7 Series ) with a cryogenic hydrogen tank. The permanently in the tank evaporates, hydrogen is used as a gas in a fuel cell in order to ensure the power supply of the vehicle. Otherwise the gaseous hydrogen from time to time have to be blown off to the outside.
Trigger for the substantial efforts in research was particularly the Zero -emission act or the Zero Emission Vehicle Mandate ( ZEV ) in the United States, which provide that cars should drive future emission-free. For 2003, it was envisaged that 10% of all newly registered vehicles in California should be subject to this law. Shortly before, after massive pressure from the American automobile industry, the ZEV but was tilted, although it continues to be debated.
With the increased use of zero emission vehicles in urban areas and large cities to improve the local air quality is expected. A side effect would, however, that emissions are being relocated to the place of use of vehicles, where the hydrogen is produced, as far as this does not happen due to climate-neutral process. For hydrogen production, there are several options with different efficiency.
For the widespread use of mobile hydrogen applications simultaneous construction of hydrogen filling stations is required. The best way this is done by converting the energy to a hydrogen economy. For the transportation of hydrogen in vehicles with Pressure vessels are also other forms of hydrogen storage in question, such as metal hydrides or under high pressure and low temperature than liquid hydrogen. Energetically in mobile applications is the high energy requirement for compression (up to 700 bar) or liquefaction ( about -250 ° C) observed that significantly lowers the overall efficiency ( Well-to- Wheel) of vehicles with hydrogen storage.
Despite the high efficiency of the fuel cell is designed and the removal of waste heat to the relatively low temperature level of the PEM fuel cell of about 80 ° C as problematic, because in contrast to the internal combustion engine, the relatively cold gas ( water vapor) contains only a relatively small amount of heat. Accordingly, the aim is to raise the operating temperature of the PEM fuel cell to over 100 ° C in order to realize more powerful fuel cell cars with more than 100 kW can.
At temperatures below the freezing point of the start of the fuel cell may be impaired due to freezing water. It must be ensured that the electro-chemical reaction, in particular, the diffusion of the combustion gases, not by the formation of ice is prevented. That can be achieved for example by a suitable electrode structure. Several manufacturers have already 2003 and 2004 demonstrated that the freezing start of PEM fuel cells at temperatures of up to -20 ° C is possible; The start times are comparable with those of internal combustion engines.
The seriennah already available prototypes of smaller vehicles have to try to aim the size, weight and cost of the fuel cell and a suitable storage of hydrogen. For example, Daimler has unveiled vehicles of the A -Class and B- Class with fuel. In Hamburg and Stuttgart buses will be tested with hydrogen drive in normal line operation.
In the Cologne area two hydrogen buses RVK go of the " Phileas " the Dutch manufacturer Advanced Public Transportation Systems ( APTS ), in which the fuel cells from Ballard Power Systems Inc. produce 150 kW.
Since June 16, 2008 Honda provides a limited frame of the car FCX Clarity, which is operated exclusively with fuel cell technology.
Since 2007, go to the fleet of the Federal Ministry of Transport, the first car with fuel cell drive.
Also since about 2007, there are also hybrid bicycles and motorcycles with fuel cell drive.
Possible alternatives to direct storage of hydrogen are fuels such as ethanol, methanol or other hydrocarbons, one of which is obtained prior to use of the hydrogen by catalytic methods. These procedures shall, however, in no small measure by CO2 emissions to the environmental impact, which restricts the otherwise perfect environmental impact of the fuel cell. Ethanol and methanol can also be synthesized from carbon dioxide and water, but in turn, may be more energy consuming, the production of carbon dioxide, which is present in air in very small concentration. The efficiency of these methods depends on the catalysts whose best variants contain the expensive platinum. A wide use of platinum catalysts would also lead to further shortages and price increases of platinum.
The automaker Ford announced on 24 June 2009 announced that the work is set to fuel cells. Ford relies instead prefer to batteries and the electric motor. In December 2010, however, Ford explained that being worked internally to the fuel cell.
Hydrogen-powered prototypes of electric vehicles now have a range of up to 800 km.
Vehicle manufacturers Toyota, Nissan, Mercedes -Benz and Honda have now reduced the production cost for hydrogen-powered vehicles drastically. Toyota plans to use large series in Japan in 2015 in conjunction with many hydrogen filling stations in Japanese metropolitan areas.
In order to demonstrate the practicality of hydrogen drive, Mercedes -Benz has completed a circumnavigation of the globe successfully with several fuel cell vehicles, the B-Class. Already 200 series vehicles of this type have been delivered to customers in 2010.
Opel will manufacture from 2015, the first models powered by fuel cells in series and continue to build a nationwide infrastructure for hydrogen refueling stations parallel to the market launch.
Daimler will prefer the series production of fuel cell vehicles, contrary to the original plan by one year to 2014. The price is only about 20% higher than that of a vehicle with a combustion engine.
Aviation
Since mid-2005, fuel cells are also found in aviation. A first drone whose electric motors are powered by a fuel cell, launched in Yuma, Arizona. The DLR is currently working on the integration of fuel cell technology in the unmanned research aircraft HyFish which near Bern successfully completed its maiden flight in March 2007.
Elsewhere, too, research activities in aeronautics in progress. At the beginning of 2008, a converted Airbus A320 has been tested with a fuel cell as a backup system for the power supply on board in a test flight. As a positive side effect of the water produced can be used for on-board power supply, which reduces the take-off weight.
On 3 March 2008 Boeing plant for the first time a small plane, a Dimona Diamond Aircraft, with a hybrid drive: electric motor with lithium -ion batteries and fuel cells. After the rise with both energy sources at 1000 meters altitude the battery was disconnected and the pilot flew the first 20 minutes of the flight history with fuel cells. Developed the drive from Boeing Research & Technology Europe ( BR & TE) in Madrid with European industrial partners.
The first (public ) full flight (Start - Place Round - landing ) of a pilot controlled and exclusively driven by energy from the fuel cell aircraft took place in Hamburg on July 7, 2009. In the plane it was the motor glider Antares DLR -H2, with 20 meters span, the German Aerospace Center (DLR ) and the project partners Lange Aviation, BASF Fuel Cells and Serenergy ( Denmark), and in close cooperation with Airbus is designed and manufactured in 15 months.
Space travel
Fuel cells have long been used as an energy converter in space (Apollo, Space Shuttle ) is used.
The American space shuttle fuel cell used with a maximum continuous power of 3 × 7 kW of power to the orbiter. The accumulated water in the fuel cell could be used in life -support system.
Shipping
The world's first fuel cell boat was the Hydra, which was certified in 1999 by Germanischer Lloyd for public transport. For an alkaline fuel cell ( AFC) has been selected as this technology was readily available and can deal better for applications on the high seas with the salty sea air as the PEM fuel cells. In addition, the fuel cell system could also start at temperatures below the freezing point, since potassium hydroxide only freezes at about -77 ° C, and the efficiency of the AFC technology is still about 5 % higher than that of the PEM. The Hydra has an approval for twenty passengers and carried approximately 2,000 persons in 1999 /2000. The hydrogen is stored in the bow in metal hydride and sufficient for a two - day operation of 8 hours of operation.
Advantage of the storage of hydrogen in metal hydride storage is also very compact storage and the ability to preheat the fuel cell system during refueling already by the waste heat of the metal hydride to drive off after refueling at full power can.
The fuel cell system is based on the fuel cell stacks, and was a completely new design with lying among the stacks KOH reservoir ( drain system). The Hydra is since 2001 no longer in operation, but still exists in the Bonn area and has proved for the first time worldwide that it is technologically possible to drive with fuel cells, a passenger ship.
In submarines Germany is the only provider of a model series-produced fuel cell auxiliary drive. The HDW Kiel in cooperation with Siemens and the North Sea Emden supplies since 2005, the submarine class 212 with such a drive (AIP: air independent propulsion ) from. He makes about 300 kW ( 408 hp ) and allows a slow speed without the 1050 - kW diesel generator. Similarly, the submarine class has 214 ( from the same manufacturer ) fuel cells on board. In is currently under construction, the Spanish S-80 class, which also has an air-independent fuel cell propulsion. The first unit to be put into service as planned in 2013.
End of 2009, a molten carbonate fuel cell ( MCFC) was installed with 320 kW electrical power supply of the electrical system on the Norwegian offshore supply Viking Lady, to gain experience of ship operation.
Mobile telephony
With the increasing worldwide popularity of smartphones get the longest running time of battery plays a prominent role. However, this is limited depending on the degree of use to a few hours to days. Frequent travelers in particular are often forced to have to recharge their device. In order to reduce the dependence of the electrical outlet, various solutions are being sought. For example, the company Lilliputian Systems has developed portable fuel cells, with the help of smart phones can be charged on the go, and without the need for an electrical outlet several times. The launch was scheduled for 2012. The portable fuel cell equipped with a USB port and a tank of butane gas, which provides the necessary energy.