fuel cells Fuel cells are chemical power sources. They carry out the direct conversion of fuel energy into electricity, bypassing inefficient, high-loss combustion processes. This electrochemical device, as a result of highly efficient "cold" combustion of fuel, directly generates electricity.
Biochemists have established that a biological hydrogen-oxygen fuel cell is "built into" every living cell (see Chapter 2).
The source of hydrogen in the body is food - fats, proteins and carbohydrates. In the stomach, intestines, and cells, it eventually decomposes to monomers, which, in turn, after a series of chemical transformations, give hydrogen attached to the carrier molecule.
Oxygen from the air enters the blood through the lungs, combines with hemoglobin and is carried to all tissues. The process of combining hydrogen with oxygen is the basis of the body's bioenergetics. Here, under mild conditions (room temperature, normal pressure, aquatic environment), chemical energy with high efficiency is converted into thermal, mechanical (muscle movement), electricity (electric ramp), light (insects emitting light).
Man once again repeated the device for obtaining energy created by nature. At the same time, this fact indicates the prospects of the direction. All processes in nature are very rational, so steps towards the real use of fuel cells inspire hope for the energy future.
The discovery in 1838 of a hydrogen-oxygen fuel cell belongs to the English scientist W. Grove. Investigating the decomposition of water into hydrogen and oxygen, he discovered a side effect - the electrolyzer produced an electric current.
What burns in a fuel cell?
Fossil fuels (coal, gas and oil) are mostly carbon. During combustion, fuel atoms lose electrons, and air oxygen atoms gain them. So in the process of oxidation, carbon and oxygen atoms are combined into combustion products - carbon dioxide molecules. This process is vigorous: the atoms and molecules of the substances involved in combustion acquire high speeds, and this leads to an increase in their temperature. They begin to emit light - a flame appears.
The chemical reaction of carbon combustion has the form:
C + O2 = CO2 + heat
In the process of combustion, chemical energy is converted into thermal energy due to the exchange of electrons between the atoms of the fuel and the oxidizer. This exchange occurs randomly.
Combustion is the exchange of electrons between atoms, and electric current is the directed movement of electrons. If, in the process of a chemical reaction, the electrons are forced to do work, then the temperature of the combustion process will decrease. In FC, electrons are taken from the reactants at one electrode, give up their energy in the form of an electric current, and join the reactants at the other.
The basis of any HIT is two electrodes connected by an electrolyte. A fuel cell consists of an anode, a cathode, and an electrolyte (see Chap. 2). Oxidizes at the anode, i.e. donates electrons, the reducing agent (CO or H2 fuel), free electrons from the anode enter the external circuit, and positive ions are retained at the anode-electrolyte interface (CO+, H+). From the other end of the chain, the electrons approach the cathode, on which the reduction reaction takes place (the addition of electrons by the oxidizing agent O2–). The oxidant ions are then carried by the electrolyte to the cathode.
In FC, three phases of the physicochemical system are brought together:
gas (fuel, oxidizer);
electrolyte (conductor of ions);
metal electrode (conductor of electrons).
In fuel cells, the energy of the redox reaction is converted into electrical energy, and the processes of oxidation and reduction are spatially separated by an electrolyte. The electrodes and electrolyte do not participate in the reaction, but in real designs they become contaminated with fuel impurities over time. Electrochemical combustion can proceed at low temperatures and practically without losses. On fig. p087 shows the situation in which a mixture of gases (CO and H2) enters the fuel cell, i.e. it can burn gaseous fuel (see Chap. 1). Thus, TE turns out to be "omnivorous".
The use of fuel cells is complicated by the fact that fuel must be “prepared” for them. For fuel cells, hydrogen is obtained by conversion of organic fuel or coal gasification. Therefore, the block diagram of a power plant on a fuel cell, in addition to the batteries of a fuel cell, a DC-to-AC converter (see Chapter 3) and auxiliary equipment, includes a hydrogen production unit.
Two directions of FC development
There are two areas of application of fuel cells: autonomous and large-scale energy.
For autonomous use, specific characteristics and ease of use are the main ones. The cost of generated energy is not the main indicator.
For large power generation, efficiency is a decisive factor. In addition, the installations must be durable, do not contain expensive materials and use natural fuels with minimal preparation costs.
The greatest benefits are offered by the use of fuel cells in a car. Here, as nowhere else, the compactness of fuel cells will have an effect. With the direct receipt of electricity from fuel, the saving of the latter will be about 50%.
For the first time, the idea of using fuel cells in large-scale power engineering was formulated by the German scientist W. Oswald in 1894. Later, the idea of creating efficient sources of autonomous energy based on a fuel cell was developed.
After that, repeated attempts were made to use coal as an active substance in fuel cells. In the 1930s, the German researcher E. Bauer created a laboratory prototype of a fuel cell with a solid electrolyte for direct anodic oxidation of coal. At the same time, oxygen-hydrogen fuel cells were studied.
In 1958, in England, F. Bacon created the first oxygen-hydrogen plant with a capacity of 5 kW. But it was cumbersome due to the use of high gas pressure (2 ... 4 MPa).
Since 1955, K. Kordesh has been developing low-temperature oxygen-hydrogen fuel cells in the USA. They used carbon electrodes with platinum catalysts. In Germany, E. Yust worked on the creation of non-platinum catalysts.
After 1960, demonstration and advertising samples were created. The first practical application of fuel cells was found on the Apollo spacecraft. They were the main power plants for powering the onboard equipment and provided the astronauts with water and heat.
The main areas of use for off-grid FC installations have been military and naval applications. At the end of the 1960s, the volume of research on fuel cells decreased, and after the 1980s, it increased again in relation to large-scale energy.
VARTA has developed FCs using double-sided gas diffusion electrodes. Electrodes of this type are called "Janus". Siemens has developed electrodes with power density up to 90 W/kg. In the United States, work on oxygen-hydrogen cells is being carried out by United Technology Corp.
In the large-scale power industry, the use of fuel cells for large-scale energy storage, for example, the production of hydrogen (see Chap. 1), is very promising. (sun and wind) are dispersed (see Ch. 4). Their serious use, which is indispensable in the future, is unthinkable without capacious batteries that store energy in one form or another.
The problem of accumulation is already relevant today: daily and weekly fluctuations in the load of power systems significantly reduce their efficiency and require the so-called maneuverable capacities. One of the options for an electrochemical energy storage is a fuel cell in combination with electrolyzers and gas holders*.
* Gas holder [gas + English. holder] - storage for large quantities of gas.
The first generation of TE
Medium-temperature fuel cells of the first generation, operating at a temperature of 200...230°C on liquid fuel, natural gas or technical hydrogen*, have reached the greatest technological perfection. The electrolyte in them is phosphoric acid, which fills the porous carbon matrix. The electrodes are made of carbon and the catalyst is platinum (platinum is used in amounts on the order of a few grams per kilowatt of power).
* Commercial hydrogen is a fossil fuel conversion product containing minor impurities of carbon monoxide.
One such power plant was put into operation in the state of California in 1991. It consists of eighteen batteries weighing 18 tons each and is placed in a case with a diameter of just over 2 m and a height of about 5 m. The battery replacement procedure has been thought out using a frame structure moving along rails.
The United States delivered two power plants to Japan to Japan. The first of them was launched in early 1983. The operational performance of the station corresponded to the calculated ones. She worked with a load of 25 to 80% of the nominal. The efficiency reached 30...37% - this is close to modern large thermal power plants. Its start-up time from a cold state is from 4 hours to 10 minutes, and the duration of power change from zero to full is only 15 seconds.
Now in different parts of the United States, small combined heat and power plants with a capacity of 40 kW with a fuel utilization factor of about 80% are being tested. They can heat water up to 130°C and are placed in laundries, sports complexes, communication points, etc. About a hundred installations have already worked for a total of hundreds of thousands of hours. The environmental friendliness of FC power plants allows them to be placed directly in cities.
The first fuel power plant in New York, with a capacity of 4.5 MW, occupied an area of 1.3 hectares. Now, for new plants with a capacity of two and a half times more, a site measuring 30x60 m is needed. Several demonstration power plants with a capacity of 11 MW are being built. The construction time (7 months) and the area (30x60 m) occupied by the power plant are striking. The estimated service life of new power plants is 30 years.
Second and third generation TE
The best characteristics are already being designed modular plants with a capacity of 5 MW with medium-temperature fuel cells of the second generation. They operate at temperatures of 650...700°C. Their anodes are made from sintered particles of nickel and chromium, cathodes are made from sintered and oxidized aluminum, and the electrolyte is a mixture of lithium and potassium carbonates. Elevated temperature helps solve two major electrochemical problems:
reduce the "poisoning" of the catalyst by carbon monoxide;
increase the efficiency of the process of reduction of the oxidizer at the cathode.
High-temperature fuel cells of the third generation with an electrolyte of solid oxides (mainly zirconium dioxide) will be even more efficient. Their operating temperature is up to 1000°C. The efficiency of power plants with such fuel cells is close to 50%. Here, the products of gasification of hard coal with a significant content of carbon monoxide are also suitable as fuel. Equally important, waste heat from high-temperature plants can be used to produce steam to drive turbines for electric generators.
Vestingaus has been in the solid oxide fuel cell business since 1958. It develops power plants with a capacity of 25 ... 200 kW, in which gaseous fuel from coal can be used. Experimental installations with a capacity of several megawatts are being prepared for testing. Another American firm, Engelgurd, is designing 50 kW fuel cells that run on methanol with phosphoric acid as the electrolyte.
More and more firms all over the world are involved in the creation of fuel cells. The American United Technology and the Japanese Toshiba formed the International Fuel Cells Corporation. In Europe, the Belgian-Dutch consortium Elenko, the West German company Siemens, the Italian Fiat, and the British Jonson Metju are engaged in fuel cells.
Victor LAVRUS.
If you liked this material, then we offer you a selection of the best materials on our site according to our readers. You can find a selection - TOP about environmentally friendly technologies, new science and scientific discoveries where it is most convenient for you
Hydrogen fuel cells convert the chemical energy of the fuel into electricity, bypassing the inefficient, high-loss processes of combustion and the conversion of thermal energy into mechanical energy.
Description:
Hydrogen fuel cells convert the chemical energy of the fuel into electricity, bypassing the inefficient, high-loss processes of combustion and the conversion of thermal energy into mechanical energy. The hydrogen fuel cell is electrochemical the device as a result of highly efficient "cold" combustion of fuel directly generates electricity. The proton exchange membrane hydrogen-air fuel cell (PEMFC) is one of the most promising fuel technologies. elements.
A proton-conducting polymer membrane separates the two electrodes, the anode and the cathode. Each electrode is a carbon plate (matrix) coated with a catalyst. On the anode catalyst, molecular hydrogen dissociates and donates electrons. Hydrogen cations are conducted through the membrane to the cathode, but the electrons are given off to the external circuit, since the membrane does not allow electrons to pass through.
On the cathode catalyst, an oxygen molecule combines with an electron (which is supplied from the electrical circuit) and an incoming proton and forms water, which is the only reaction product (in the form of vapor and / or liquid).
Membrane-electrode blocks are made from hydrogen fuel cells, which are the key generating element of the energy system.
Advantages of hydrogen fuel cells compared to traditional solutions:
– increased specific energy intensity (500 ÷ 1000 W*h/kg),
– extended operating temperature range (-40 0 C / +40 0 C),
– absence of a heat spot, noise and vibration,
– cold start reliability
– practically unlimited energy storage period (no self-discharge),
– the ability to change the energy intensity of the system by changing the number of fuel cartridges, which provides almost unlimited autonomy,
– the ability to provide almost any reasonable energy intensity of the system by changing the capacity of the hydrogen storage,
– high energy consumption
– tolerance to impurities in hydrogen,
– long service life,
- environmental friendliness and noiseless operation.
Application:
– power supply systems for UAVs,
– portable chargers,
– uninterruptible power supplies,
– Other devices.
Nissan hydrogen fuel cell
Mobile electronics are improving every year, becoming more widespread and more accessible: PDAs, laptops, mobile and digital devices, photo frames, etc. All of them are constantly updated with new features, larger monitors, wireless communications, stronger processors, while decreasing in size. . Power technologies, unlike semiconductor technology, do not go by leaps and bounds.
The available batteries and accumulators to power the achievements of the industry are becoming insufficient, so the issue of alternative sources is very acute. Fuel cells are by far the most promising direction. The principle of their operation was discovered back in 1839 by William Grove, who generated electricity by changing the electrolysis of water.
Video: Documentary, Fuel Cells for Transportation: Past, Present, Future
Fuel cells are of interest to car manufacturers, and the creators of spacecraft are also interested in them. In 1965, they were even tested by America on the Gemini 5 launched into space, and later on the Apollo. Millions of dollars are invested in fuel cell research even today, when there are problems associated with environmental pollution, increasing greenhouse gas emissions from the combustion of fossil fuels, the reserves of which are also not endless.
A fuel cell, often referred to as an electrochemical generator, operates in the manner described below.
Being, like accumulators and batteries, a galvanic cell, but with the difference that active substances are stored in it separately. They come to the electrodes as they are used. On the negative electrode, natural fuel or any substance obtained from it burns, which can be gaseous (hydrogen, for example, and carbon monoxide) or liquid, like alcohols. At the positive electrode, as a rule, oxygen reacts.
But a simple-looking principle of action is not easy to translate into reality.
DIY fuel cell
Video: DIY hydrogen fuel cell
Unfortunately, we do not have photos of what this fuel element should look like, we hope for your imagination.
A low-power fuel cell with your own hands can be made even in a school laboratory. It is necessary to stock up on an old gas mask, several pieces of plexiglass, alkali and an aqueous solution of ethyl alcohol (more simply, vodka), which will serve as “fuel” for the fuel cell.
First of all, you need a housing for the fuel cell, which is best made from plexiglass, at least five millimeters thick. Internal partitions (five compartments inside) can be made a little thinner - 3 cm. For gluing plexiglass, glue of the following composition is used: six grams of plexiglass chips are dissolved in one hundred grams of chloroform or dichloroethane (they work under a hood).
In the outer wall, it is now necessary to drill a hole into which you need to insert a drain glass tube with a diameter of 5-6 centimeters through a rubber stopper.
Everyone knows that in the periodic table in the lower left corner there are the most active metals, and the high-activity metalloids are in the table in the upper right corner, i.e. the ability to donate electrons increases from top to bottom and from right to left. Elements that can, under certain conditions, manifest themselves as metals or metalloids are in the center of the table.
Now, in the second and fourth compartments, we pour activated carbon from the gas mask (between the first partition and the second, as well as the third and fourth), which will act as electrodes. So that coal does not spill out through the holes, it can be placed in a nylon fabric (women's nylon stockings will do). AT
The fuel will circulate in the first chamber, in the fifth there should be an oxygen supplier - air. There will be an electrolyte between the electrodes, and in order to prevent it from leaking into the air chamber, it is necessary to soak it with a solution of paraffin in gasoline (the ratio of 2 grams of paraffin to half a glass of gasoline) before filling the fourth chamber with coal for air electrolyte. On a layer of coal you need to put (slightly pressing) copper plates, to which the wires are soldered. Through them, the current will be diverted from the electrodes.
It remains only to charge the element. For this, vodka is needed, which must be diluted with water in 1: 1. Then carefully add three hundred to three hundred and fifty grams of caustic potassium. For electrolyte, 70 grams of caustic potassium are dissolved in 200 grams of water.
The fuel cell is ready for testing. Now you need to simultaneously pour fuel into the first chamber, and electrolyte into the third. A voltmeter attached to the electrodes should show from 07 volts to 0.9. To ensure continuous operation of the element, it is necessary to drain the spent fuel (drain into a glass) and add new fuel (through a rubber tube). The feed rate is controlled by squeezing the tube. This is how the operation of a fuel cell looks in laboratory conditions, the power of which is understandably small.
Video: Fuel cell or eternal battery at home
To make the power greater, scientists have been working on this problem for a long time. Methanol and ethanol fuel cells are located on the active development steel. But, unfortunately, so far there is no way to put them into practice.
Why the fuel cell is chosen as an alternative power source
A fuel cell was chosen as an alternative power source, since the end product of hydrogen combustion in it is water. The problem is only in finding an inexpensive and efficient way to produce hydrogen. The colossal funds invested in the development of hydrogen generators and fuel cells cannot fail to bear fruit, so a technological breakthrough and their real use in everyday life is only a matter of time.
Already today the monsters of the automotive industry: General Motors, Honda, Dreimler Koisler, Ballard demonstrate buses and cars that run on fuel cells with a power of up to 50 kW. But, the problems associated with their safety, reliability, cost - have not yet been resolved. As mentioned already, unlike traditional power sources - batteries and batteries, in this case, the oxidizer and fuel are supplied from the outside, and the fuel cell is only an intermediary in the ongoing reaction to burn the fuel and convert the released energy into electricity. “Burning” occurs only if the element delivers current to the load, like a diesel generator, but without a generator and diesel, and also without noise, smoke and overheating. At the same time, the efficiency is much higher, since there are no intermediate mechanisms.
Video: Hydrogen fuel cell car
Great hopes are placed on the use of nanotechnologies and nanomaterials, which will help miniaturize fuel cells, while increasing their power. There have been reports that ultra-efficient catalysts have been created, as well as fuel cell designs that do not have membranes. In them, together with the oxidizer, fuel (methane, for example) is supplied to the element. Solutions are interesting, where oxygen dissolved in water is used as an oxidizing agent, and organic impurities accumulating in polluted waters are used as fuel. These are the so-called biofuel cells.
Fuel cells, according to experts, can enter the mass market in the coming years
fuel cell- what it is? When and how did he appear? Why is it needed and why are they so often talked about in our time? What are its scope, characteristics and properties? Unstoppable progress requires answers to all these questions!
What is a fuel cell?
fuel cell- this is a chemical current source or an electrochemical generator, this is a device for converting chemical energy into electrical energy. In modern life, chemical current sources are used everywhere and are batteries for mobile phones, laptops, PDAs, as well as batteries in cars, uninterruptible power supplies, etc. The next stage in the development of this area will be the widespread distribution of fuel cells, and this is an undeniable fact.
History of fuel cells
The history of fuel cells is another story of how the properties of matter, once discovered on Earth, were widely used far in space, and at the turn of the millennium they returned from heaven to Earth.
It all started in 1839 when the German chemist Christian Schönbein published the principles of the fuel cell in the Philosophical Journal. In the same year, an Englishman, an Oxford graduate, William Robert Grove, designed a galvanic cell, later called the Grove galvanic cell, which is also recognized as the first fuel cell. The very name "fuel cell" was given to the invention in the year of its anniversary - in 1889. Ludwig Mond and Karl Langer are the authors of the term.
A little earlier, in 1874, Jules Verne, in The Mysterious Island, predicted the current energy situation, writing that "Water will one day be used as a fuel, hydrogen and oxygen, of which it is composed, will be used."
Meanwhile, the new technology of power supply was gradually improved, and starting from the 50s of the XX century, not a year passed without the announcement of the latest inventions in this area. In 1958, the first tractor powered by fuel cells appeared in the United States, in 1959. 5KW power supply for welding machine was released, etc. In the 70s, hydrogen technology took off into space: aircraft and rocket engines appeared on hydrogen. In the 1960s, RSC Energia developed fuel elements for the Soviet lunar program. The Buran program also did not do without them: alkaline 10 kW fuel cells were developed. And towards the end of the century, fuel cells crossed zero altitude above sea level - based on them, developed electricity supply German submarine. Returning to Earth, in 2009 the first locomotive was put into operation in the USA. Naturally, on fuel cells.
In all the beautiful history of fuel cells, what is interesting is that the wheel is still the unparalleled invention of mankind in nature. The fact is that in their design and principle of operation, fuel cells are similar to a biological cell, which, in fact, is a miniature hydrogen-oxygen fuel cell. As a result, man once again invented what nature has been using for millions of years.
The principle of operation of fuel cells
The principle of operation of fuel cells is obvious even from the school curriculum in chemistry, and it was he who was laid down in the experiments of William Grove in 1839. The thing is that the process of water electrolysis (water dissociation) is reversible. Just as it is true that when an electric current is passed through water, the latter is split into hydrogen and oxygen, so the opposite is also true: hydrogen and oxygen can be combined to produce water and electricity. In Grove's experiment, two electrodes were placed in a chamber into which limited portions of pure hydrogen and oxygen were supplied under pressure. Due to the small volumes of gas, as well as due to the chemical properties of the carbon electrodes, a slow reaction took place in the chamber with the release of heat, water and, most importantly, with the formation of a potential difference between the electrodes.
The simplest fuel cell consists of a special membrane used as an electrolyte, on both sides of which powdered electrodes are deposited. Hydrogen enters one side (anode) and oxygen (air) enters the other (cathode). Each electrode has a different chemical reaction. At the anode, hydrogen breaks down into a mixture of protons and electrons. In some fuel cells, the electrodes are surrounded by a catalyst, usually made of platinum or other noble metals, to aid in the dissociation reaction:
2H 2 → 4H + + 4e -
where H 2 is a diatomic hydrogen molecule (the form in which hydrogen is present as a gas); H + - ionized hydrogen (proton); e - - electron.
On the cathode side of the fuel cell, protons (passed through the electrolyte) and electrons (which passed through the external load) recombine and react with the oxygen supplied to the cathode to form water:
4H + + 4e - + O 2 → 2H 2 O
Overall reaction in the fuel cell is written as follows:
2H 2 + O 2 → 2H 2 O
The operation of a fuel cell is based on the fact that the electrolyte passes protons through itself (toward the cathode), but electrons do not. The electrons move towards the cathode along the outer conducting circuit. This movement of electrons is the electrical current that can be used to power an external device connected to the fuel cell (a load such as a light bulb):
In their work, fuel cells use hydrogen fuel and oxygen. The easiest way is with oxygen - it is taken from the air. Hydrogen can be supplied directly from a certain container or by separating it from an external source of fuel (natural gas, gasoline or methyl alcohol - methanol). In the case of an external source, it must be chemically converted to extract the hydrogen. Currently, most of the fuel cell technologies being developed for portable devices use methanol.
Fuel Cell Characteristics
they only operate as long as the fuel and oxidizer are supplied from an external source (i.e. they cannot store electrical energy),
the chemical composition of the electrolyte does not change during operation (the fuel cell does not need to be recharged),
they are completely independent of electricity (while conventional batteries store energy from the mains).
Fuel cells are analogous to existing batteries in the sense that in both cases electrical energy is obtained from chemical energy. But there are also fundamental differences:
Each fuel cell creates voltage in 1V. More voltage is achieved by connecting them in series. The increase in power (current) is realized through a parallel connection of cascades of series-connected fuel cells.
For fuel cells no hard limit on efficiency, as for heat engines (the efficiency of the Carnot cycle is the maximum possible efficiency among all heat engines with the same minimum and maximum temperatures).
High efficiency achieved through the direct conversion of fuel energy into electricity. If fuel is first burned in diesel generator sets, the resulting steam or gas turns a turbine or internal combustion engine shaft, which in turn turns an electric generator. The result is an efficiency of a maximum of 42%, more often it is about 35-38%. Moreover, due to the many links, as well as due to thermodynamic limitations on the maximum efficiency of heat engines, the existing efficiency is unlikely to be raised higher. For existing fuel cells Efficiency is 60-80%,
Efficiency almost does not depend on the load factor,
The capacity is several times higher than existing batteries
Complete no environmentally harmful emissions. Only clean water vapor and thermal energy are emitted (unlike diesel generators, which have polluting emissions and require them to be removed).
Types of fuel cells
fuel cells classified on the following grounds:
by fuel used
working pressure and temperature,
according to the nature of the application.
In general, there are the following fuel cell types:
Solid-oxide fuel cells (SOFC);
Fuel cell with proton exchange membrane (Proton-exchange membrane fuel cell - PEMFC);
Reversible Fuel Cell (RFC);
Direct methanol fuel cell (Direct-methanol fuel cell - DMFC);
Melt carbonate fuel cell (Molten-carbonate fuel cells - MCFC);
Phosphoric acid fuel cells (PAFC);
Alkaline fuel cells (AFC).
One of the types of fuel cells operating at normal temperatures and pressures using hydrogen and oxygen are elements with an ion exchange membrane. The resulting water does not dissolve the solid electrolyte, flows down and is easily removed.
Fuel Cell Problems
The main problem of fuel cells is related to the need for "packaged" hydrogen, which could be freely purchased. Obviously, the problem should be solved over time, but so far the situation causes a slight smile: what comes first - the chicken or the egg? Fuel cells are not yet advanced enough to build hydrogen plants, but their progress is unthinkable without these plants. Here we also note the problem of the source of hydrogen. Hydrogen is currently produced from natural gas, but rising energy costs will also increase the price of hydrogen. At the same time, the presence of CO and H 2 S (hydrogen sulfide) is inevitable in hydrogen from natural gas, which poison the catalyst.
Common platinum catalysts use a very expensive and irreplaceable metal in nature - platinum. However, this problem is planned to be solved by using catalysts based on enzymes, which are a cheap and easily produced substance.
Heat is also a problem. Efficiency will increase sharply if the generated heat is directed to a useful channel - to produce thermal energy for the heat supply system, to use it as waste heat in absorption refrigerating machines etc.
Methanol Fuel Cells (DMFC): Real Application
Direct Methanol Fuel Cells (DMFC) are of the highest practical interest today. A Portege M100 laptop running on a DMFC fuel cell looks like this:
A typical DMFC circuit contains, in addition to the anode, cathode and membrane, several additional components: a fuel cartridge, a methanol sensor, a fuel circulation pump, an air pump, a heat exchanger, etc.
The operating time, for example, of a laptop compared to batteries is planned to be increased by 4 times (up to 20 hours), a mobile phone - up to 100 hours in active mode and up to six months in standby mode. Recharging will be done by adding a portion of liquid methanol.
The main task is to find options for using the methanol solution with its highest concentration. The problem is that methanol is a fairly strong poison, lethal in doses of several tens of grams. But the concentration of methanol directly affects the duration of work. If earlier a 3-10% methanol solution was used, then mobile phones and PDAs using a 50% solution have already appeared, and in 2008, in laboratory conditions, MTI MicroFuel Cells and, a little later, Toshiba, obtained fuel cells operating on pure methanol.
Fuel cells are the future!
Finally, the fact that the international organization IEC (International Electrotechnical Commission), which defines industrial standards for electronic devices, has already announced the creation of a working group to develop an international standard for miniature fuel cells, speaks of the obvious great future of fuel cells.
Part 1
This article discusses in more detail the principle of operation of fuel cells, their design, classification, advantages and disadvantages, scope, efficiency, history of creation and modern prospects for use. In the second part of the article, which will be published in the next issue of the ABOK magazine, provides examples of facilities where various types of fuel cells were used as sources of heat and electricity (or only electricity).
Introduction
Fuel cells are a very efficient, reliable, durable and environmentally friendly way to generate energy.
Initially used only in the space industry, fuel cells are now increasingly used in a variety of areas - as stationary power plants, autonomous sources of heat and power for buildings, vehicle engines, power supplies for laptops and mobile phones. Some of these devices are laboratory prototypes, some are undergoing pre-series testing or are used for demonstration purposes, but many models are mass-produced and used in commercial projects.
A fuel cell (electrochemical generator) is a device that converts the chemical energy of a fuel (hydrogen) into electrical energy during an electrochemical reaction directly, unlike traditional technologies that use the combustion of solid, liquid and gaseous fuels. Direct electrochemical conversion of fuel is very efficient and attractive from an environmental point of view, since the minimum amount of pollutants is released during operation, and there are no strong noises and vibrations.
From a practical point of view, a fuel cell resembles a conventional galvanic battery. The difference lies in the fact that initially the battery is charged, i.e. filled with “fuel”. During operation, "fuel" is consumed and the battery is discharged. Unlike a battery, a fuel cell uses fuel supplied from an external source to generate electrical energy (Fig. 1).
For the production of electrical energy, not only pure hydrogen can be used, but also other hydrogen-containing raw materials, such as natural gas, ammonia, methanol or gasoline. Ordinary air is used as a source of oxygen, which is also necessary for the reaction.
When pure hydrogen is used as a fuel, the reaction products, in addition to electrical energy, are heat and water (or water vapor), i.e. no gases are emitted into the atmosphere that cause air pollution or cause a greenhouse effect. If a hydrogen-containing feedstock, such as natural gas, is used as a fuel, other gases, such as oxides of carbon and nitrogen, will be a by-product of the reaction, but its amount is much lower than when burning the same amount of natural gas.
The process of chemical conversion of fuel in order to produce hydrogen is called reforming, and the corresponding device is called a reformer.
Advantages and disadvantages of fuel cells
Fuel cells are more energy efficient than internal combustion engines because there is no thermodynamic limitation on energy efficiency for fuel cells. The efficiency of fuel cells is 50%, while the efficiency of internal combustion engines is 12-15%, and the efficiency of steam turbine power plants does not exceed 40%. By using heat and water, the efficiency of fuel cells is further increased.
In contrast to, for example, internal combustion engines, the efficiency of fuel cells remains very high even when they are not operating at full power. In addition, the power of fuel cells can be increased by simply adding separate blocks, while the efficiency does not change, i.e. large installations are as efficient as small ones. These circumstances allow a very flexible selection of the composition of equipment in accordance with the wishes of the customer and ultimately lead to a reduction in equipment costs.
An important advantage of fuel cells is their environmental friendliness. Air emissions from fuel cells are so low that in some areas of the United States they do not require special permits from government air quality agencies.
Fuel cells can be placed directly in the building, thus reducing the losses during the transportation of energy, and the heat generated by the reaction can be used to supply heat or hot water to the building. Autonomous sources of heat and power supply can be very beneficial in remote areas and in regions that are characterized by a shortage of electricity and its high cost, but at the same time there are reserves of hydrogen-containing raw materials (oil, natural gas).
The advantages of fuel cells are also the availability of fuel, reliability (there are no moving parts in the fuel cell), durability and ease of operation.
One of the main shortcomings of fuel cells today is their relatively high cost, but this shortcoming can be overcome soon - more and more companies produce commercial samples of fuel cells, they are constantly being improved, and their cost is decreasing.
The most efficient use of pure hydrogen as a fuel, however, this will require the creation of a special infrastructure for its generation and transportation. Currently, all commercial designs use natural gas and similar fuels. Motor vehicles can use ordinary gasoline, which will allow maintaining the existing developed network of gas stations. However, the use of such fuel leads to harmful emissions into the atmosphere (albeit very low) and complicates (and therefore increases the cost of) the fuel cell. In the future, the possibility of using environmentally friendly renewable energy sources (for example, solar energy or wind energy) is being considered to decompose water into hydrogen and oxygen by electrolysis, and then convert the resulting fuel in a fuel cell. Such combined plants operating in a closed cycle can be a completely environmentally friendly, reliable, durable and efficient source of energy.
Another feature of fuel cells is that they are most efficient when using both electrical and thermal energy at the same time. However, the possibility of using thermal energy is not available at every facility. In the case of using fuel cells only for generating electrical energy, their efficiency decreases, although it exceeds the efficiency of “traditional” installations.
History and modern uses of fuel cells
The principle of operation of fuel cells was discovered in 1839. The English scientist William Robert Grove (1811-1896) discovered that the process of electrolysis - the decomposition of water into hydrogen and oxygen by means of an electric current - is reversible, i.e. hydrogen and oxygen can be combined into water molecules without burning, but with the release of heat and electric current. Grove called the device in which such a reaction was carried out a "gas battery", which was the first fuel cell.
The active development of fuel cell technologies began after the Second World War, and it is associated with the aerospace industry. At that time, searches were conducted for an efficient and reliable, but at the same time quite compact source of energy. In the 1960s, NASA specialists (National Aeronautics and Space Administration, NASA) chose fuel cells as a power source for spacecraft of the Apollo (manned flights to the Moon), Apollo-Soyuz, Gemini and Skylab programs. . The Apollo used three 1.5 kW units (2.2 kW peak power) using cryogenic hydrogen and oxygen to produce electricity, heat and water. The mass of each installation was 113 kg. These three cells worked in parallel, but the energy generated by one unit was enough for a safe return. During 18 flights, the fuel cells have accumulated a total of 10,000 hours without any failures. Currently, fuel cells are used in the space shuttle "Space Shuttle", which uses three units with a power of 12 W, which generate all the electrical energy on board the spacecraft (Fig. 2). Water obtained as a result of an electrochemical reaction is used as drinking water, as well as for cooling equipment.
In our country, work was also underway to create fuel cells for use in astronautics. For example, fuel cells were used to power the Soviet Buran space shuttle.
Development of methods for the commercial use of fuel cells began in the mid-1960s. These developments were partially funded by government organizations.
Currently, the development of technologies for the use of fuel cells goes in several directions. This is the creation of stationary power plants on fuel cells (both for centralized and decentralized energy supply), power plants of vehicles (samples of cars and buses on fuel cells have been created, including in our country) (Fig. 3), and also power supplies for various mobile devices (laptops, mobile phones, etc.) (Fig. 4).
Examples of the use of fuel cells in various fields are given in Table. one.
One of the first commercial models of fuel cells designed for autonomous heat and power supply of buildings was the PC25 Model A manufactured by ONSI Corporation (now United Technologies, Inc.). This fuel cell with a nominal power of 200 kW belongs to the type of cells with an electrolyte based on phosphoric acid (Phosphoric Acid Fuel Cells, PAFC). The number "25" in the name of the model means the serial number of the design. Most previous models were experimental or test pieces, such as the 12.5 kW "PC11" model that appeared in the 1970s. The new models increased the power taken from a single fuel cell, and also reduced the cost per kilowatt of energy produced. Currently, one of the most efficient commercial models is the PC25 Model C fuel cell. Like model “A”, this is a fully automatic 200 kW PAFC type fuel cell designed for installation directly on the serviced object as an independent source of heat and electricity. Such a fuel cell can be installed outside the building. Outwardly, it is a parallelepiped 5.5 m long, 3 m wide and 3 m high, weighing 18,140 kg. The difference from previous models is an improved reformer and a higher current density.
| Table 1 Scope of fuel cells |
|||||||||||||||
|
In some types of fuel cells, the chemical process can be reversed: by applying a potential difference to the electrodes, water can be decomposed into hydrogen and oxygen, which are collected on porous electrodes. When a load is connected, such a regenerative fuel cell will begin to generate electrical energy.
A promising direction for the use of fuel cells is their use in conjunction with renewable energy sources, such as photovoltaic panels or wind turbines. This technology allows you to completely avoid air pollution. A similar system is planned to be created, for example, at the Adam Joseph Lewis Training Center in Oberlin (see ABOK, 2002, No. 5, p. 10). Currently, solar panels are used as one of the energy sources in this building. Together with NASA specialists, a project was developed to use photovoltaic panels to produce hydrogen and oxygen from water by electrolysis. The hydrogen is then used in fuel cells to generate electricity and hot water. This will allow the building to maintain the performance of all systems during cloudy days and at night.
The principle of operation of fuel cells
Let us consider the principle of operation of a fuel cell using the simplest element with a proton exchange membrane (Proton Exchange Membrane, PEM) as an example. Such an element consists of a polymer membrane placed between the anode (positive electrode) and the cathode (negative electrode) together with the anode and cathode catalysts. A polymer membrane is used as the electrolyte. The diagram of the PEM element is shown in fig. 5.
A proton exchange membrane (PEM) is a thin (approximately 2-7 sheets of plain paper thick) solid organic compound. This membrane functions as an electrolyte: it separates matter into positively and negatively charged ions in the presence of water.
An oxidative process occurs at the anode, and a reduction process occurs at the cathode. The anode and cathode in the PEM cell are made of a porous material, which is a mixture of particles of carbon and platinum. Platinum acts as a catalyst that promotes the dissociation reaction. The anode and cathode are made porous for free passage of hydrogen and oxygen through them, respectively.
The anode and cathode are placed between two metal plates, which supply hydrogen and oxygen to the anode and cathode, and remove heat and water, as well as electrical energy.
Hydrogen molecules pass through the channels in the plate to the anode, where the molecules decompose into individual atoms (Fig. 6).
Figure 5 () Schematic diagram of a proton exchange membrane (PEM) fuel cell |
|
Figure 6 () Hydrogen molecules through the channels in the plate enter the anode, where the molecules are decomposed into individual atoms |
|
Figure 7 () As a result of chemisorption in the presence of a catalyst, hydrogen atoms are converted into protons |
|
Figure 8 () Positively charged hydrogen ions diffuse through the membrane to the cathode, and the electron flow is directed to the cathode through an external electrical circuit to which the load is connected. |
|
Figure 9 () Oxygen supplied to the cathode, in the presence of a catalyst, enters into a chemical reaction with hydrogen ions from the proton-exchange membrane and electrons from the external electrical circuit. Water is formed as a result of a chemical reaction |
Then, as a result of chemisorption in the presence of a catalyst, hydrogen atoms, each donating one electron e - , are converted into positively charged hydrogen ions H +, i.e. protons (Fig. 7).
Positively charged hydrogen ions (protons) diffuse through the membrane to the cathode, and the electron flow is directed to the cathode through an external electrical circuit to which the load (consumer of electrical energy) is connected (Fig. 8).
Oxygen supplied to the cathode, in the presence of a catalyst, enters into a chemical reaction with hydrogen ions (protons) from the proton exchange membrane and electrons from the external electrical circuit (Fig. 9). As a result of a chemical reaction, water is formed.
The chemical reaction in a fuel cell of other types (for example, with an acidic electrolyte, which is a solution of phosphoric acid H 3 PO 4) is absolutely identical to the chemical reaction in a fuel cell with a proton exchange membrane.
In any fuel cell, part of the energy of a chemical reaction is released as heat.
The flow of electrons in an external circuit is a direct current that is used to do work. Opening the external circuit or stopping the movement of hydrogen ions stops the chemical reaction.
The amount of electrical energy produced by the fuel cell depends on the type of fuel cell, geometric dimensions, temperature, gas pressure. A single fuel cell provides an EMF of less than 1.16 V. It is possible to increase the size of the fuel cells, but in practice several cells are used, connected in batteries (Fig. 10).
Fuel cell device
Let's consider the fuel cell device on the example of the PC25 Model C model. The scheme of the fuel cell is shown in fig. eleven.
The fuel cell "PC25 Model C" consists of three main parts: the fuel processor, the actual power generation section and the voltage converter.
The main part of the fuel cell - the power generation section - is a stack composed of 256 individual fuel cells. The composition of the fuel cell electrodes includes a platinum catalyst. Through these cells, a direct electric current of 1,400 amperes is generated at a voltage of 155 volts. The dimensions of the battery are approximately 2.9 m in length and 0.9 m in width and height.
Since the electrochemical process takes place at a temperature of 177 ° C, it is necessary to heat the battery at the time of start-up and remove heat from it during operation. To do this, the fuel cell includes a separate water circuit, and the battery is equipped with special cooling plates.
The fuel processor allows you to convert natural gas into hydrogen, which is necessary for an electrochemical reaction. This process is called reforming. The main element of the fuel processor is the reformer. In the reformer, natural gas (or other hydrogen-containing fuel) reacts with steam at high temperature (900 °C) and high pressure in the presence of a nickel catalyst. The following chemical reactions take place:
CH 4 (methane) + H 2 O 3H 2 + CO
(reaction endothermic, with heat absorption);
CO + H 2 O H 2 + CO 2
(the reaction is exothermic, with the release of heat).
The overall reaction is expressed by the equation:
CH 4 (methane) + 2H 2 O 4H 2 + CO 2
(reaction endothermic, with heat absorption).
To provide the high temperature required for natural gas conversion, a portion of the spent fuel from the fuel cell stack is sent to a burner that maintains the reformer at the desired temperature.
The steam required for reforming is generated from the condensate formed during the operation of the fuel cell. In this case, the heat removed from the fuel cell stack is used (Fig. 12).
The fuel cell stack generates an intermittent direct current, which is characterized by low voltage and high current. A voltage converter is used to convert it to industrial standard AC. In addition, the voltage converter unit includes various control devices and safety interlock circuits that allow the fuel cell to be turned off in the event of various failures.
In such a fuel cell, approximately 40% of the energy in the fuel can be converted into electrical energy. Approximately the same amount, about 40% of the fuel energy, can be converted into thermal energy, which is then used as a heat source for heating, hot water supply and similar purposes. Thus, the total efficiency of such a plant can reach 80%.
An important advantage of such a source of heat and electricity is the possibility of its automatic operation. For maintenance, the owners of the facility on which the fuel cell is installed do not need to maintain specially trained personnel - periodic maintenance can be carried out by employees of the operating organization.
Fuel cell types
Currently, several types of fuel cells are known, which differ in the composition of the electrolyte used. The following four types are most widespread (Table 2):
1. Fuel cells with proton exchange membrane (Proton Exchange Membrane Fuel Cells, PEMFC).
2. Fuel cells based on orthophosphoric (phosphoric) acid (Phosphoric Acid Fuel Cells, PAFC).
3. Fuel cells based on molten carbonate (Molten Carbonate Fuel Cells, MCFC).
4. Solid oxide fuel cells (Solid Oxide Fuel Cells, SOFC). Currently, the largest fleet of fuel cells is built on the basis of PAFC technology.
One of the key characteristics of different types of fuel cells is the operating temperature. In many ways, it is the temperature that determines the scope of fuel cells. For example, high temperatures are critical for laptops, so proton exchange membrane fuel cells with low operating temperatures are being developed for this market segment.
For autonomous power supply of buildings, fuel cells of high installed capacity are required, and at the same time, it is possible to use thermal energy, therefore, fuel cells of other types can also be used for these purposes.
Proton Exchange Membrane Fuel Cells (PEMFC)
These fuel cells operate at relatively low operating temperatures (60-160°C). They are characterized by high power density, allow you to quickly adjust the output power, and can be quickly turned on. The disadvantage of this type of elements is the high requirements for fuel quality, since contaminated fuel can damage the membrane. The nominal power of fuel cells of this type is 1-100 kW.
Proton exchange membrane fuel cells were originally developed by the General Electric Corporation in the 1960s for NASA. This type of fuel cell uses a solid state polymer electrolyte called a Proton Exchange Membrane (PEM). Protons can move through the proton exchange membrane, but electrons cannot pass through it, resulting in a potential difference between the cathode and anode. Due to their simplicity and reliability, such fuel cells were used as a power source on the Gemini manned spacecraft.
This type of fuel cell is used as a power source for a wide variety of devices, including prototypes and prototypes, from mobile phones to buses and stationary power systems. The low operating temperature allows such cells to be used to power various types of complex electronic devices. Less efficient is their use as a source of heat and power supply for public and industrial buildings, where large amounts of thermal energy are required. At the same time, such elements are promising as an autonomous source of power supply for small residential buildings such as cottages built in regions with a hot climate.
| table 2 Fuel cell types |
||||||||||||||||||||
|
Phosphoric Acid Fuel Cells (PAFC)
Tests of fuel cells of this type were already carried out in the early 1970s. Operating temperature range - 150-200 °C. The main area of application is autonomous sources of heat and power supply of medium power (about 200 kW).
The electrolyte used in these fuel cells is a solution of phosphoric acid. The electrodes are made of paper coated with carbon, in which a platinum catalyst is dispersed.
The electrical efficiency of PAFC fuel cells is 37-42%. However, since these fuel cells operate at a sufficiently high temperature, it is possible to use the steam generated as a result of operation. In this case, the overall efficiency can reach 80%.
To generate energy, the hydrogen-containing feedstock must be converted to pure hydrogen through a reforming process. For example, if gasoline is used as a fuel, then sulfur compounds must be removed, since sulfur can damage the platinum catalyst.
PAFC fuel cells were the first commercial fuel cells to be economically justified. The most common model was the 200 kW PC25 fuel cell manufactured by ONSI Corporation (now United Technologies, Inc.) (Fig. 13). For example, these elements are used as a source of heat and electricity in a police station in New York's Central Park or as an additional source of energy for the Conde Nast Building & Four Times Square. The largest plant of this type is being tested as an 11 MW power plant located in Japan.
Fuel cells based on phosphoric acid are also used as an energy source in vehicles. For example, in 1994, H-Power Corp., Georgetown University, and the US Department of Energy equipped a bus with a 50 kW power plant.
Molten Carbonate Fuel Cells (MCFC)
Fuel cells of this type operate at very high temperatures - 600-700 °C. These operating temperatures allow the fuel to be used directly in the cell itself, without the need for a separate reformer. This process is called "internal reforming". It allows to significantly simplify the design of the fuel cell.
Fuel cells based on molten carbonate require a significant start-up time and do not allow to quickly adjust the output power, so their main area of application is large stationary sources of heat and electricity. However, they feature high fuel conversion efficiency - 60% electrical efficiency and up to 85% overall efficiency.
In this type of fuel cell, the electrolyte consists of potassium carbonate and lithium carbonate salts heated to about 650 °C. Under these conditions, the salts are in a molten state, forming an electrolyte. At the anode, hydrogen interacts with CO 3 ions, forming water, carbon dioxide and releasing electrons that are sent to the external circuit, and at the cathode, oxygen interacts with carbon dioxide and electrons from the external circuit, again forming CO 3 ions.
Laboratory samples of fuel cells of this type were created in the late 1950s by the Dutch scientists G. H. J. Broers and J. A. A. Ketelaar. In the 1960s, engineer Francis T. Bacon, a descendant of a famous 17th-century English writer and scientist, worked with these elements, which is why MCFC fuel cells are sometimes referred to as Bacon elements. NASA's Apollo, Apollo-Soyuz, and Scylab programs used just such fuel cells as a power source (Fig. 14). In the same years, the US military department tested several samples of MCFC fuel cells manufactured by Texas Instruments, in which army grades of gasoline were used as fuel. In the mid-1970s, the US Department of Energy began research to develop a stationary molten carbonate fuel cell suitable for practical applications. In the 1990s, a number of commercial units rated up to 250 kW were put into operation, such as at the US Naval Air Station Miramar in California. In 1996, FuelCell Energy, Inc. commissioned a 2 MW pre-series plant in Santa Clara, California.
Solid state oxide fuel cells (SOFC)
Solid-state oxide fuel cells are simple in design and operate at very high temperatures - 700-1000 °C. Such high temperatures allow the use of relatively "dirty", unrefined fuel. The same features as in fuel cells based on molten carbonate determine a similar area of application - large stationary sources of heat and electricity.
Solid oxide fuel cells are structurally different from fuel cells based on PAFC and MCFC technologies. The anode, cathode and electrolyte are made of special grades of ceramics. Most often, a mixture of zirconium oxide and calcium oxide is used as the electrolyte, but other oxides can be used. The electrolyte forms a crystal lattice coated on both sides with a porous electrode material. Structurally, such elements are made in the form of tubes or flat boards, which makes it possible to use technologies widely used in the electronics industry in their manufacture. As a result, solid-state oxide fuel cells can operate at very high temperatures, so they can be used to produce both electrical and thermal energy.
At high operating temperatures, oxygen ions are formed at the cathode, which migrate through the crystal lattice to the anode, where they interact with hydrogen ions, forming water and releasing free electrons. In this case, hydrogen is released from natural gas directly in the cell, i.e. there is no need for a separate reformer.
The theoretical foundations for the creation of solid-state oxide fuel cells were laid back in the late 1930s, when Swiss scientists Bauer (Emil Bauer) and Preis (H. Preis) experimented with zirconium, yttrium, cerium, lanthanum and tungsten, using them as electrolytes.
The first prototypes of such fuel cells were created in the late 1950s by a number of American and Dutch companies. Most of these companies soon abandoned further research due to technological difficulties, but one of them, Westinghouse Electric Corp. (now "Siemens Westinghouse Power Corporation"), continued work. The company is currently accepting pre-orders for a commercial model of tubular topology solid oxide fuel cell expected this year (Figure 15). The market segment of such elements is stationary installations for the production of heat and electric energy with a capacity of 250 kW to 5 MW.
SOFC type fuel cells have shown very high reliability. For example, a Siemens Westinghouse fuel cell prototype has logged 16,600 hours and continues to operate, making it the longest continuous fuel cell life in the world.
The high temperature, high pressure operating mode of SOFC fuel cells allows the creation of hybrid plants, in which fuel cell emissions drive gas turbines used to generate electricity. The first such hybrid plant is in operation in Irvine, California. The rated power of this plant is 220 kW, of which 200 kW from the fuel cell and 20 kW from the microturbine generator.
