Lecture 35: Advanced Combustion & Alternative Power in Hybrid EVs & Fuel Cells, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_1.htm[6/15/2012 3:09:31 PM] Module 7:Advanced Combustion Systems and Alternative Powerplants Lecture 35:Alternative Powerplants The Lecture Contains: ALTERNATIVE PROPULSION SYSTEMS HYBRID ELECTRIC VEHICLES (HEV) Main Components of HEV Types of HEVs FUEL CELL Fuel Cell Power Output Factors Favouring Fuel Cell Fuel Cell Types Energy Sources for Fuel Cell Prototype Fuel Cell Vehicles Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_2.htm[6/15/2012 3:09:31 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 35:Alternative Powerplants ALTERNATIVE PROPULSION SYSTEMS Following factors provided motivation for the development of alternative propulsion systems for road vehicles; Global warming - Reduction desired in the emission of greenhouse gas, carbon dioxide Control of urban air pollution Higher energy efficiency - to prolong availability of petroleum fuels as the crude reserves are diminishing Energy security – to be independent of import of energy from other countries The design of power unit of vehicles is governed by several factors e.g., type of available fuel/energy, economics of energy availability and environmental considerations. The following vehicle power plants have been under detailed investigations and some of them are already introduced in the market. Hybrid - electric propulsion Fuel cells Gas turbines Stirling engine Batteries for electric vehicle The hybrid electric and fuel cell vehicles hold a greater promise of practical application. Hybrid electric vehicles are built around the existing reciprocating IC engines and some vehicle models are already in market. The fuel cell vehicle is a zero emission vehicle and all the major auto- companies are pursuing its development as a future power plant. Hence, only these two are discussed here. HYBRID ELECTRIC VEHICLES (HEV) Motivating factors for HEV development are; Power required by vehicle to operate within cities may be around 4 to 7 kW although the rated engine power ranges from 25 to over 100kW. The engine thus, operates in the city under very low load conditions giving high fuel consumption and emissions. Small engine can be employed and operated at constant load and speed at the point of its maximum efficiency, and another propulsion system can take care of the transient operation. High vehicle fuel efficiencies are thus, obtained. Engine can be tuned to its lowest emissions at the operating load and speed point Emission control and exhaust after-treatment at steady engine load and speed operation is more efficient. Hybrid electric vehicle (HEV) allows achieving precisely this objective. The hybrid electric vehicle employs two different energy storage and two different propulsion systems: A conventional propulsion system like IC engine, and An on-board rechargeable electric energy storage system coupled with electric motor(s) Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_4.htm[6/15/2012 3:09:31 PM] Figure 7.14 Hybrid Electric Vehicle Systems. Already a few million HEVs are in use. The gasoline engines used on HEV run on Atkinson cycle to improve thermal efficiency with large reduction in pumping losses. The Atkinson cycle is implemented by late closing of intake valve (72 º to 105º after bdc) while keeping the expansion ratio close to 13:1. The power output of the engine is increased by supercharging. The fuel efficiency improvements of nearly 50% in city driving and 30% on combined city and highway driving have been obtained. HEVs have met the SULEV emission standards (NMHC = 0.01, CO = 1.0, NOx = 0.02 g/mile). HEV powered by diesel engine have obtained 25 % better fuel economy than the comparable diesel vehicle. The NOx and PM emissions are lower by nearly 45 and 65 %, respectively. The diesel hybrids produce up to 50% less CO2 than the gasoline engines and 30 to 35% less than the diesel engines making the diesel-hybrid more fuel efficient and environment friendly than the gasoline engine hybrid. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_5.htm[6/15/2012 3:09:31 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 35:Alternative Powerplants FUEL CELL Working Principle Fuel cell was invented in 1839 by Sir William Groves. It is an electro-chemical device, which continuously converts the chemical energy of fuel directly to electricity. The working principle of H2- O2 fuel cell is shown on Fig. 7.15. The fuel-cell has two electrodes made of porous material coated with platinum as catalyst. The electrodes are separated by a solid semi-permeable electrolyte. Hydrogen flows into fuel cell on catalytic anode and gives up an electron. Negatively charged oxygen at cathode attracts hydrogen protons through the solid electrolyte membrane. On cathode, hydrogen and oxygen ions combine to produce water. The electrons flow through external circuit producing current. Figure 7.15 Schematic of H2 – O2 fuel cell. Fuel Cell Power Output Open circuit standard EMF of fuel cell at reference condition is given by Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_5.htm[6/15/2012 3:09:31 PM] (7.1) where: = Gibbs free energy of formation for the reaction at reference condition of 298 K, and 1 atm n = no. of electrons per molecule of fuel e.g. for H2 -O2 fuel cell n = 2 F = Faraday constant = 96,485 Coulombs/ electron mol. At the other operating conditions, EMF of the fuel cell is, (7.2) where PH2, PO2, PH2O are the partial pressures in atm. Fuel cells can also use and operate directly on other fuels like methanol and methane Theoretical EMF of some fuel cell systems is given in Table 7.3 Table 7.3 Theoretical EMF for Some Fuel Cells at reference conditions Fuel Reaction n E0 , V H2 H2 + 0.5 O2 → H2 O 2 1.229 Methane CH4+ 2O2 → CO2+2H2O(l) 8 1.006 Methanol CH3 OH (l)+ 1.5O2 → CO2+2H2O (l) 6 1.214 Actual cell voltage is lower and is about 50 to 60% only of the theoretical EMF due to; Slow rate of chemical reactions Internal cell resistance As the current drawn is increased beyond about 0.7 A/cm2, the concentration polarization causes a further voltage drop. Typical fuel cell characteristics are shown on Fig. 7.16. The change in current and voltage efficiencies versus current drawn from the fuel cell are shown. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_7.htm[6/15/2012 3:09:32 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 35:Alternative Powerplants Fuel Cell Types Fuel cells are classified by the electrolyte used. The different types of fuel cell developed for various applications are given in Table 7.4. For vehicle application, the temperature of fuel cell operation and start up time are important. The PEM (proton exchange membrane) fuel cell has been accepted presently as best suited for vehicle application as it can be started in about 30 seconds and it operates at acceptably low temperatures. The PEM fuel cell consists of an electrolyte membrane in the form of a thin film of approximately 0.1 mm thickness made of sulfonated fluorocopolymer or an aromatic polymer. A typical automotive fuel cell stack consisting of 640 PEMFC developed 129kW peak power with continuous rating of 102 kW, weighed 100 kg and occupied 58 litres of space Table 7.4 Fuel Cell Types and their Characteristics Type Electrolyte Temperature of operation, ºC System Efficiency % HHV Start- up time, hours Power range and application Alkaline (AFC) KOH (OH-) 60-120 35-55 Very short < 5kW, military, space Proton Exchange Membrane (PEMFC) Polymer Electrolyte (H+) 20-120 32-45 < 0.01(30 seconds) 5 – 250 kW, High power density, automotive PAFC Phosphoric Acid (H+) 160 -220 36-45 1 -4 200 kW, CHP MCFC Molten carbonates (CO-3) 550-650 43-55 5 -10 200 kW - MW, CHP and stand alone Solid oxide (SOFC) Solid doped Zr-oxide (O- ) 850-1000 43-55 5 -10 2 kW - MW CHP and stand alone, High efficiency Energy Sources for Fuel Cell The following sources can supply energy to fuel cells Hydrogen Methanol Ethanol Hydrocarbon fuels, gasoline and diesel Hydrogen-oxygen fuel cell provides the highest EMF and power density (W/cm2). Hydrogen either can be directly stored on-board of vehicle or generated by steam- reforming of fuels such as methanol, Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_7.htm[6/15/2012 3:09:32 PM] ethanol and hydrocarbons. The purity of hydrogen is very important for operation and longer life of fuel cell as even small concentrations of carbon monoxide and sulphur are highly detrimental. The products of fuel reforming are to be cleaned to supply hydrogen to the fuel cell. Although in principle, methanol, ethanol, gasoline, diesel and other hydrocarbons can be reformed to supply hydrogen, but so far only methanol reforming on board has been successfully used. Direct methanol fuel cell (DMFC) where methanol is fed directly to the fuel cell for oxidation and generation of electricity, is another option being developed for automotive use. . Electrolysis of water using nuclear energy and the renewable solar, wind, hydro and wave energy is the other route to generate hydrogen. The electrolysis route appears to be a long term solution once the low cost renewable or nuclear power is available. On board storage of hydrogen is another important factor for commercial success of FCV. Hydrogen can be stored in the form of gas, liquid, metal hydrides as hydrogen or in chemically combined form such as methanol and NaBH4 (sodium borohydride). High pressure storage systems of hydrogen at 700 bars have been developed. The different methods of hydrogen storage on board are compared in Table 7.5. So far most FCV prototypes have however, used the high pressure (350 – 700 bar) hydrogen tanks. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture35/35_8.htm[6/15/2012 3:09:32 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 35:Alternative Powerplants Table 7.5 Comparison of Hydrogen Storage Methods for Fuel Cell Vehicles Name StorageTemp, ºC Storage Pressure, MPa Hydrogen by mass, % Volume,litre/ kg of H2 1 High pressure cylinder Ambient 35 to 70 2 to 4.5 40 -70 2 Liquid H2 - 252 Ambient 14 25 3 Fe-Ti hydride - 10 2.5 < 2 36 4 Methanol Ambient Ambient 12.5 10 5 NaBH4 Ambient Ambient 10.58 9.5 Prototype Fuel Cell Vehicles From the early FCV prototypes such as Necar-1 by DaimlerChrysler, considerable progress has been made in the fuel cell vehicle development. Necar-1 had a 50kW fuel-cell stack with 30 kW propulsion system, hydrogen storage capacity of about 2 kg at 300 bar, maximum speed of 90 km/h and a range of 130 km. Honda FCX vehicle built in 2004 is powered by a fuel cell stack of 86 kW, 4.3 kg of hydrogen is stored at 350 bar. It is a normal size car having 150 km/h maximum speed and 395 km range. The vehicle on US FTP cycle achieved fuel economy of 91.8 km/kg of H2. By the year 2006- 07, through development of more efficient fuel cells and 700 bar cylinder pressure storage systems the range of vehicles exceeding 500 km has been attained. Presently the cost of fuel cells is higher by a factor of 2 to 3 compared to gasoline engines of the same power output. Honda Co. believes that by the year 2018 the FCV could be produced at costs that are commercially viable. The FCV has varying impact on the CO2 emissions as it depends on the hydrogen generation process.. Obviously, if the hydrogen or methanol is produced from natural gas the CO2 advantage of FCV over the conventional IC engines is not significant. If methanol is produced from natural gas to provide fuel for the fuel cell, the effect on CO2 reduction in fact, is negative. The comparative CO2 emission scenario would again change when the IC engines are fuelled by the renewable fuels like ethanol or biodiesel.