Presentation on theme: "Fuel cells and hydrogen storage"— Presentation transcript:
1Fuel cells and hydrogen storage By Reza EnayatollahiStudent No:095442
2IntroductionDue to the high cost of energy, and the growing concern of global warming, a new effort has begun to find sources of energy that are cheap and don't harm the environment. Alternative sources of energy such as solar power, wind power and hydro power are all effective in their own ways, but they're not viable as a source of energy for vehicular transportation. One source that's drawing interest for this is hydrogen. Hydrogen is an extremely clean-burning fuel, and it's also the most abundant element in the universe. It does have some drawbacks, though.
3Introduction Now I am going to explain topics below: 1)what is a fuel cell?2)how do fuel cells work?3)types of fuel cell?4)fuel cell application?5)difficulties of using fuel cell?6)hydrogen storage?7)types of hydrogen storage?
4What is fuel cell?Fuel cell is a device that produce electricity with a chemical reaction, like batteries but with a deference and that is :fuel cells does not need recharging . As long as hydrogen and oxygen are supplied as fuel , fuel cells can produce electricity . Each fuel cell consist of two electrode, one negative and one positive, called respectively , cathode and anode , also an electrolyte and catalyst. Which the reaction that produce electricity take place at the electrode and , and electrolyte carries the charged particles from one electrode to the other one.
5What is fuel cell And catalyst speeds the reaction at the electrode. In fact fuel cells are devices that produce electricity through a chemical reaction , and produce heat and water as byproduct , without noise and environmental pollution.
8How do fuel cells workfuel cells are using a chemical reaction to produce electricity, half of this reaction takes place at the anode, when hydrogen enters to fuel cell , at the anode it separates to electrons and protons by this reaction:Anode half reaction: 2H2 -> 4H+ + 4e-electrons pass a wire as a current of electricity to the cathode , and protons move through electrolyte to the anode .second half of reaction takes place at the cathode ,protons and electrons from the anode are combine with oxygen from air and produce water and heat by
9How do fuel cells work This equation: Cathode half reaction: O2 + 4H+ + 4e- -> 2H2OSo the overal raction is:Overall reaction: 2H2 + O2 -> 2H2O
11Types of fuel cellThere are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by their operating temperature and the type of electrolyte they use. Some types of fuel cells work well for use in stationary power generation plants. Others may be useful for small portable applications or for powering cars. The main types of fuel cells include:
13Types of fuel cell PEMFC: Proton exchange Membrane (PEM) fuel cells, also known as Polymer exchange membrane fuel cells typically operate on pure hydrogen fuel. The PEM fuel cell combines the hydrogen fuel with the oxygen from the atmosphere to produce Water, heat (up to 90°C) and electricity.PEM Fuel cells typically utilize platinum based catalysts on the Anode to split the Hydrogen into positive ions (protons) and negative electrons. The ions pass through the membrane to the cathode to combine with oxygen from air.
14Types of fuel cellThe electrons must pass round an external circuit creating a current to rejoin the H2 ion on the cathode
15Types of fuel cellPEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons.Compared to other types of fuel cells, PEMFCs generate more power for a given volume or weight of fuel cell. This high-power density characteristic makes them compact and lightweight. In addition, the operating temperature is less than 100ºC, which allows rapid start-up. These traits and the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for automotive power applications.
16Types of fuel cellOther advantages result from the electrolyte being a solid material, compared to a liquid. The sealing of the anode and cathode gases is simpler with a solid electrolyte, and therefore, less expensive to manufacture. The solid electrolyte is also more immune to difficulties with orientation and has less problems with corrosion, compared to many of the other electrolytes, thus leading to a longer cell and stack life.One of the disadvantages of the PEMFC for some applications is that the operating temperature is low. Temperatures near 100ºC are not high enough to perform useful cogeneration. Also, since the electrolyte is required to be saturated with water to operate
17Types of fuel celloptimally, careful control of the moisture of the anode and cathode streams is important.a single fuel cell produces only about 0.7 volts. To get this voltage up to a reasonable level, many separate fuel cells must be combined to form a fuel-cell stack. Bipolar plates are used to connect one fuel cell to another and are subjected to both oxidizing and reducing conditions and potentials. A big issue with bipolar plates is stability.
19Types of fuel cell SOFC: A solid oxide fuel cell is an electrochemical device that converts fueled hydrogen directly into electricity in the presence of heat. The process is driven by the flow of oxygen ions from a cathode to an anode through an electrolyte. When these ions combine with hydrogen from the fuel, electrons are released to an external circuit. This process is replicated many times in the fuel cell, in arrays or stacks.
22Types of fuel cellA solid oxide fuel cell (SOFC) uses a hard ceramic electrolyte and operates at temperatures up to 1,000 degrees C .High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.So they can use light hydrocarbons like :methane , propane and butane .Chemical reaction in the reformer is:CnH2n+2+(n/2)O2nCO+(n+1)H2
23Types of fuel cellWith the introduction of a small amount of air with the fuel, the inherent high temperature of the process reforms the .fuel, producing the needed hydrogen as well as COA mixture of zirconium oxide and calcium oxide form a crystal lattice, though other oxide combinations have also been used as electrolytes. The solid electrolyte is coated on both sides with specialized porous electrode materials.There is two possible design for SOFC s : tubular design and planar design , as shown in two next image , respectively:
26Types of fuel cellAFC:Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecrafts. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F)
27Types of fuel cellThe processes that take place in the fuel cell are as follows: 1. Hydrogen fuel is channeled through field flow plates to the anode on one side of the fuel cell, while oxygen from the air is channeled to the cathode on the other side of the cell. 2. At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons. 3. The positively charged hydrogen ions react with hydroxyl (OH-) ions in the electrolyte to form water. 4. The negatively charged electrons cannot flow through the electrolyte to reach the positively charged cathode, so they must flow through an external circuit, forming an electrical current.
28Types of fuel cell5. At the cathode, the electrons combine with oxygen and water to form the hydroxyl ions that move across the electrolyte toward the anode to continue the process.Anode Reaction: 2 H2 + 4 OH- => 4 H2O + 4 e-Cathode Reaction: O2 + 2 H2O + 4 e- => 4 OH-Overall Net Reaction: 2 H2 + O2 => 2 H2O
30Types of fuel cellOne characteristic of AFCs is that they are very sensitive to CO2 that may be present in the fuel or air. The CO2 reacts with the electrolyte, poisoning it rapidly, and severely degrading the fuel cell performance. Therefore, AFCs are limited to closed environments, such as space and undersea vehicles, and must be run on pure hydrogen and oxygen.On the positive side, AFCs are the cheapest fuel cells to manufacture. This is because the catalyst that is required on the electrodes can be any of a number of different materials that are relatively inexpensive compared to the catalysts required for other types of fuel cells.
31Types of fuel cell MCFC: Molten Carbonate Fuel Cells (MCFC) are in the class of high-temperature fuel cells. The higher operating temperature allows them to use natural gas directly without the need for a fuel processor.MCFCs work quite differently from other fuel cells. These cells use an electrolyte composed of a molten mixture of carbonate salts. Two mixtures are currently used: lithium carbonate and potassium carbonate, or lithium carbonate and sodium carbonate. To melt the carbonate salts and achieve high ion mobility through the electrolyte, MCFCs operate at high temperatures (650ºC).
32Types of fuel cellWhen heated to a temperature of around 650ºC, these salts melt and become conductive to carbonate ions (CO32-). These ions flow from the cathode to the anode where they combine with hydrogen to give water, carbon dioxide and electrons. These electrons are routed through an external circuit back to the cathode, generating electricity and by-product heat.Anode Reaction: CO32- + H2 => H2O + CO2 + 2e-Cathode Reaction: CO2+ 1/2O2 + 2e- => CO32-Overall Cell Reaction:H2(g) + ½O2(g) + CO2 (cathode) => H2O(g) + CO2 (anode)
34Types of fuel cellHigh-temperature MCFCs can extract hydrogen from a variety of fuels using either an internal or external reformer. They are also less prone to carbon monoxide "poisoning" than lower temperature fuel cells, which makes coal-based fuels more attractive for this type of fuel cell. MCFCs work well with catalysts made of nickel, which is much less expensive than platinum. MCFCs exhibit up to 60 percent efficiency, and this can rise to 80 percent if the waste heat is utilized for cogeneration. Currently, demonstration units have produced up to 2 megawatts (MW), but designs exist for units of 50 to 100 MW capacity.
36Types of fuel cell PAFC: Phosphoric Acid Fuel Cells (PAFC) were the first fuel cells to be commercialized. they have improved significantly in stability, performance, and cost. Such characteristics have made the PAFC a good candidate for early stationary applications.The PAFC uses an electrolyte that is phosphoric acid (H3PO4) that can approach 100% concentration. The ionic conductivity of phosphoric acid is low at low temperatures, so PAFCs are operated at the upper end of the range 150ºC–220ºC.
37Types of fuel cellThe charge carrier in this type of fuel cell is the hydrogen ion (H+, proton). This is similar to the PEFC where the hydrogen introduced at the anode is split into its protons and electrons. The protons migrate through the electrolyte and combine with the oxygen, usually from air, at the cathode to form water. The electrons are routed through an external circuit where they can perform useful work. This set of reactions in the fuel cell produces electricity and by-product heat.Anode Reaction: 2 H2 => 4 H+ + 4 e-Cathode Reaction: O2(g) + 4 H+ + 4 e- => 2 H2OOverall Cell Reaction: 2 H2 + O2 => 2 H2O
39Types of fuel cellThe PAFC operates at greater than 40% efficiency in generating electricity. When operating in cogeneration applications, the overall efficiency is approximately 85%. Furthermore, at the operating temperature of PAFCs, the waste heat is capable of heating hot water or generating steam at atmospheric pressure.The high efficiency of the PAFC when operated in cogeneration mode is one advantage of this fuel cell type. In addition, CO2 does not affect the electrolyte or cell performance and can therefore be easily operated with reformed fossil fuel. Simple construction, low electrolyte volatility and long-term stability are additional advantages.
40Types of fuel cell DMFC: The technology behind Direct Methanol Fuel Cells (DMFC) is still in the early stages of development, but it has been successfully demonstrated powering mobile phones and laptop computers—potential target end uses in future years.DMFC is similar to the PEMFC in that the electrolyte is a polymer and the charge carrier is the hydrogen ion (proton). However, the liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The hydrogen ions travel through the electrolyte and react with
41Types of fuel celloxygen from the air and the electrons from the external circuit to form water at the anode completing the circuit.Anode Reaction: CH3OH + H2O => CO2 + 6H+ + 6e-Cathode Reaction: 3/2 O2 + 6 H+ + 6e- => 3 H2OOverall Cell Reaction: CH3OH + 3/2 O2 => CO2 + 2 H2O
43Types of fuel cellInitially developed in the early 1990s, DMFCs were not embraced because of their low efficiency and power density, as well as other problems. Improvements in catalysts and other recent developments have increased power density 20-fold and the efficiency may eventually reach 40%.One of the drawbacks of the DMFC is that the low-temperature oxidation of methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which typically means a larger quantity of expensive platinum catalyst is required than in conventional PEMFCs. This increased cost is, however, expected to be more than outweighed by the
44Types of fuel cellconvenience of using a liquid fuel and the ability to function without a reforming unit.One other concern driving the development of alcohol-based fuel cells is the fact that methanol is toxic. Therefore, some companies have embarked on developing a Direct Ethanol Fuel Cell (DEFC). The performance of the DEFC is currently about half that of the DMFC, but this gap is expected to narrow with further development.
45Types of fuel cellThese cells have been tested in a temperature range from about 50ºC-120ºC. This low operating temperature and no requirement for a fuel reformer make the DMFC an excellent candidate for very small to mid-sized applications, such ascellular phones and other consumer products, up to automobile power plants.
49Fuel cell applicationEach of the fuel cell types currently being developed or manufactured has features that make it particularly attractive for certain applications. For example, the greater efficiency and higher temperature operation of MCFCs and SOFCs make them more amenable to large stationary power generation where high grade waste heat can be utilized to heat water or air or to provide cooling.Conversely, the lower operating temperature fuel cells like PEMFCs and PAFCs are particularly well suited for transportation applications where the heat is neither usable nor desirable.
50Fuel cell problemsTo use hydrogen as a source of energy we have some problemmost important problems are:1) cost2)Storage and safety
51Cost:Many of the component pieces of a fuel cell are costly. For PEMFC systems, proton exchange membranes, precious metal catalysts (usually platinum), gas diffusion layers, and bipolar plates make up 70 percent of a system's cost.Also producing pure hydrogen is costly, because we have to extract it from water an hydrocarbons.
52Fuel cell problems Storage and safety Hydrogen, which is the lightest, most abundant element in the universe, also has the lowest density of any element in the universe. For hydrogen to be stored in an efficient, cost-effective way, it needs to be stored under pressure or kept at a cold enough temperature that it turns into a liquid. Hydrogen is also extremely flammable and burns with a pale blue flame that's almost invisible. The good thing about hydrogen's low density, however, is that if it leaks, it quickly diffuses into the air--unlike gasoline, which will form a pool at the site of the leak.
53Hydrogen storageHydrogen has the highest energy content per unit of weight of any known element. It is also the lightest element. As a result, it is characterized by low volume energy density, meaning that a given volume of hydrogen contains a small amount of energy. This presents significant challenges to storing the large quantities of hydrogen that will be necessary in the hydrogen energy economy.
54Hydrogen storageA critical challenge for transportation applications is balancing the need for a conventional driving range (>480 km) with the vehicular constraints of weight, volume, efficiency, safety, and the cost of on-board hydrogen storage systems. A second set of challenges for transportation applications relate to durability over the performance lifetime of on board storage systems.
55Hydrogen storage Hydrogen Storage Today Today, hydrogen for transportation applications is compressed and stored in high-pressure metal and composite storage tanks. Hydrogen is also stored by cooling it to its liquid form and containing it in super-insulated tanks.
56Hydrogen storage Gaseous storage (compressed hydrogen gas) Using a compressed gas storage system is probably the most straightforward option at this time. The National Renewable Energy Laboratory (NREL) found that compressed hydrogen gas offers the simplest and least expensive method for onboard storage of hydrogen. Compressed hydrogen gas storage uses technology similar to that used for compressed natural gas, with stainless steel, aluminum or composite cylinders. The refilling time of compressed hydrogen tanks is also similar to that of gasoline tanks.
57Hydrogen storageHydrogen, however, requires more volume for the same energy equivalent amount of natural gas. One way to increase the fuel stored in the container is to increase pressure, but this requires more expensive storage containers, increasing compression costs and entails investigation into safety issues. Lower pressures, while lessening these concerns, would mean taking up more vehicle space. In addition, hydrogen has a tendency to leak because of its small size. Seals and valves on the containers need to be designed to prevent leaks. If a fuel cell vehicle is stored in a closed garage, hydrogen that has leaked out could accumulate and increase the risk of fire or explosion.
58Hydrogen storage Liquid storage Liquefied hydrogen (LH2) does not have the high weight penalty seen with compressed hydrogen, but it is still bulkier than gasoline storage. As with compressed hydrogen, liquid hydrogen storage takes advantage of similar technologies used in liquid natural gas storage. A drawback to this method of hydrogen storage is that the process to liquefy hydrogen is energy intensive.
59Hydrogen storageHydrogen's low boiling point requires excellent insulation of storage containers; otherwise, left for a period of time, the storage tanks could become depleted. Maintaining the extreme cold temperatures of LH2 during refueling and onboard storage currently poses a significant technical challenge.
60Future Storage Technologies Current research on future storage technology includes:• Metal hydride technology uses metals and metal alloys to adsorb hydrogen under moderate pressure and temperature, creating hydrides. A metal hydride tank contains a granular metal, which adsorbs hydrogen and releases it with the application of heat. The heat may be supplied as excess heat from a fuel cell.
61Conventional high capacity metal hydrides require high temperatures (300°-350°C) to liberate hydrogen, but sufficient heat is not generally available in fuel cell transportation applications.Chemical hydride slurries or solutions can be used as a hydrogen carrier or storage medium. The hydrogen in the hydride is released through a reaction with water. Chemical hydride systems are irreversible and require thermal management and regeneration of the carrier to recharge the hydrogen content.
62An essential feature of the process is recovery and reuse of spent hydride at a centralized processing plant. Research issues include the identification of safe, stable, and pumpable slurries, and the design of the reactor for regeneration of the spent slurry.Carbon nanotubes are microscopic tubes of carbon, two nanometers (billionths of a meter) across, that store hydrogen in microscopic pores on the tubes and within the tube structures. Similar to metal hydrides in their mechanism for storing and releasing hydrogen, they hold the potential to store a significant volume of hydrogen.
63However, the amount of storage and the mechanism through which hydrogen is stored in these materials are not yet well-defined.