Fuel Cells |
|
|
An elegant method of generating electricity is to react oxygen and a fuel across an electrolyte. Such a device is called a fuel cell. Unlike the generation of electrical energy by combustion very little or no heat is produced There is no conversion of chemical energy into heat and the process is therefore not subject to the effect of Carnot's equation which has a limiting effect upon the actual electricity production efficiency of normal heat engines. Before dealing with the fuel cell it is appropriate to review some of the present methods of electricity production which make use of the battery in one form or another. These fall into two categories: - Non re-usable cells, for example, the so called dry batteries. - The various forms of rechargeable cell using such combinations as lead-acid or nickel-iron. It is the wet cell which is of particular interest to us in the consideration of fuel cells. The conventional wet cell comprises plates of lead and lead compounds immersed in an acidic electrolyte, or alternatively combinations of nickel and iron plates in an alkaline electrolyte. With the wet cell, when charging current is passed through the plates and electrolyte, chemical changes occur in the lead and lead compounds corresponding to the quantity of electricity fed into the cell. In addition, gases, i.e. hydrogen and oxygen, are given off by the plates. Investigations were therefore directed to the possibility that if oxygen and hydrogen were somehow fed back into the plates of the cell the result should be the generation of an electric current within the cell, and the produc tion of water as a by-product; essentially the reverse of charging a wet cell.
Classification and Types of Fuel Cells There are three main considerations governing the classification of fuel cells. Firstly, by temperature of operation, that is High, Medium or Low: - High temperature cells include those having molten salt electrolyte at temperatures in the region 590 to 650 0C. - Medium temperature hydrogen-oxygen cells which operate around 200 0C. - Low temperature cells, which are those operating up to the boiling point of an aqueous electrolyte. Secondly, by electrolyte type; this can be either an Acidic or an Alkaline system, in either solid or liquid form. Thirdly, classification by fuel: - Gaseous, for example hydrogen. - Liquid, for example alcohol. - Solid. Hydrogen/Oxygen Fuel Cells By far the most common fuel cell is the hydrogen/oxygen system which is available in low-, inter mediate- and high-temperature forms with a working temperature from 900C to 1 0000C depending on the electrolyte used. Reaction Mechanism The overall chemical reaction in the fuel cell utilising hydrogen and oxygen is the reaction 2H2 + O2 = 2H2O. The electrode reactions are: H2= 2H+ + 2e (Hydrogen electrode : anode) 1/2O2 + H2O = 2OH-- 2e (Oxygen electrode : cathode) 2H+ + 2OH-= 2H20 (Water as waste product) The water formed must not be allowed to pollute the electrolyte and it is necessary therefore that it be removed by being absorbed by the fuel gases passing through the cell and eventually being condensed out. The sensible heat of this water can be used in local space heating applications. It is therefore possible to use this type of cell in moving vehicles, there being no storage capacity required for waste products. It is, however, necessary to provide for storage space for the reactive gases, preferably in a liquid form. The combination of the hydrogen/oxygen cell with an electrolyser would give a system for storing electricity, the gas from the electrolyser being stored under pressure or liquified, then re-combined when required to provide the necessary electrical energy A typical hydrogen/oxygen cell consists of two porous metal electrodes with electrolyte being constantly recirculated between their inner faces. The porous electrodes have a matrix of coarse pores with the side facing the electrolyte being coated with a layer of material having finer pores. The reactant gases are under sufficient pressure to displace the liquid electrolyte from the coarse porous structure of each electrode but not from the fine pores, the generation of electrical energy in the fuel cell taking place by means of the electrical ionisation reaction. The voltage available per cell is in the region of 1 volt with the available current being from 250 to several thousand amps per square metre of electrode area. Hydrogen/oxygen fuel cells are available in four main types. - Aqueous Acid, where the electrolytes are either diluted sulphuric acid or phosphoric acid. The overall electricity production efficiency presently achievable is 40%. - Alkaline, where potassium hydroxide or saturated carbonate/bicarbonate solutions are used as the electrolyte. Operating temperatures are low and electrical conversion efficiencies of up to 30% can be attained. - Molten Carbonate, where a mixture of alkali carbonates and aluminates is kept at a tempera ture of 650 0C. Higher temperature operation, up to 750 0C, improves the power density of the cell but increases the liability of corrosion. Fuel conversion efficiencies can be as high as 60%. - Stabilised Zirconium, where solid zirconium oxide at 1 000 0C is used as the electrolyte with conversion efficiencies approaching 60%. These cells have very low capacities, approxi mately 100 W, and a very short useful life. However this cell is in the development stage and may be attractive in future since waste heat is produced at a high temperature. Use of Air Instead of Pure Oxygen ln space or submarine applications pure oxygen is the normal choice. Additional difficulties to be expected when using air is not too severe. These include slightly lower theoretical voltage, a higher degree of polarisation on load, pumping losses and some carbonation of the alkaline electrolyte. It can be said that air may possibly replace oxygen would a rather lower performance be tolerated. Use of Hydrocarbon Fuels Some new difficulties arise when attempts are made to use hydrocarbon fuels such as methane, propane and butane in fuel cells in place of hydrogen. For example, hydrocarbons are not so reactive electrochemically as hydrogen in this application and it is therefore usually necessary to raise the operating temperature well above that used by the simple hydrogen/oxygen cells. The reaction products contain C02 which will rapidly carbonate an alkaline electrolyte, thus necessitating a molten electrolyte at a high temperature. Acid electrolytes may be used but with obvious additional corrosion problems. In addition carbon dioxide produced during the reaction cannot easily be separated from the fuel gases whereas by using hydrogen alone the reaction product, steam, is easily removed by condensation. Finally, impurities which may be present such as sulphur in some of the commercially available hydrocarbon fuels may have to be removed to avoid the poisoning of reactive catalyst surfaces of the electrodes. Indirect Fuel Cells Practical fuel cells have to rely upon coal, oil or natural gas. Coal can be converted into fuel gas by means such as the Lurgi gasifier, while fuel gases are produced both at the well-head of oil boreholes and as refinery by-products. Waste gas from sewage works and other sources can also be used as the primary fuel feedstock. The hydrocarbons would be reformed to a mixture of hydrogen and carbon monoxide with further reaction to carbon dioxide. For steam reforming of natural gas the heat required for the endothermic reaction of methane could be provided by the waste heat produced by the fuel cell. In most fuel cells it is important to reduce to a minimum the carbon monoxide levels as this will adversely affect the fuel cell life time. An efficient water gas converter is therefore an essential feature. Fuel Cell Research and Development Three approaches towards large stationary fuel cells are in progress: these use phosphoric acid, molten carbonate or solid oxide as the electrolyte. Two distinct markets can be identified. The first of these is CHP of several kilowatts electrical and heat output using natural gas as fuel. The second is multi-megawatt units for power stations using natural gas or naptha as fuel. 12 kW to 40 kW units have been produced in the USA, based on the phosphoric acid electrolyte capable of producing electrical power at 40% efficiency with another 40% of the heat input being recovered to produce low temperature hot water at 85 0C. The Molten Carbonate Fuel Cell was investigated by the former CEGB during the 1960s. Main pro blems concerned the deterioration of the electrodes and the high vapour pressure of the carbonate melt leading to rapid loss of electrolyte. The programme was abandoned when the intolerance of the system to sulphur was discovered. Interest revived in the USA and work is being carried out by the Institute of Gas Technology. Electrode deterioration is much reduced but electrolyte loss and sulphur poisoning still need further R & D work. The system has the attraction of a potentially high electrical efficiency. Some of the difficulties of corrosion, fuel gas purity and electrolyte loss can be overcome by the use of a solid electrolyte. Fuel cell technology has yet to be proven in long term operation. However there are now several types in operation, ranging from a few kilowatts electrical output to several megawatts. Projected costs at the moment are up to £1 500/kWe, with an anticipated reduction to about £500/kWe if technical targets are achieved and market expansion occurs. Economic analysis of fuel cell based CHP shows that key R & D targets are to reduce capital and maintenance costs and increase electrical efficiency. This latter should aid in lowering capital costs and reduce the dependency of the economic case on heat recovery. Consideration of the present and anticipated electrical performance of a practical range of CHP systems shows that, in the longer term, fuel cells could well out-perform heat engine alternator systems. In addition, they may offer unobtrusive and effluent free operation. Electrical efficiencies upwards of 60% are achievable in principle. Apart from electro-chemistry, other important aspects will be reforming heat transfer, fluid flow, heat recovery, high temperature construction and process controls. For more information
contact Mearsecroft
|