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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
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)
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 35:Alternative Powerplants
Parallel Hybrid
In the parallel hybrid, the engine and motor (run by battery) are mechanically connected to wheels and traction can be provided simultaneously by both the power units. When the engine is unable to meet the power requirement of the vehicle (such as under acceleration), energy from the battery supplements the vehicle demand. Engine is thus, subjected to transient demands and consequently fuel efficiency is poorer and emission penalties occur.
Mixed hybrid
In the mixed hybrid, an alternator run by the engine continues to charge batteries. The power to the wheels flow directly from the engine as well as from the batteries charged by the alternator simultaneously as in the parallel hybrids. Toyota Prius car, one of the most successful HEVs is a mixed hybrid vehicle.
Plug-in hybrid
Most cars run less than 50 to 60 km/day in cities. Thus, a near zero emission vehicle can be designed if a small size battery pack provides this range during city driving and when the vehicle needs to run more distance the IC engine drives the vehicle. The batteries are charged every day by the mains supply. Such hybrids are called as Plug-in- Hybrid Electric Vehicle (PHEV).
Figure
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, NO (^) x = 0.02 g/mile). HEV powered by diesel engine have obtained 25 % better fuel economy than the comparable diesel vehicle. The NO (^) x and PM emissions are lower by nearly^ 45 and 65 %, respectively. The diesel hybrids produce up to 50% less CO 2 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.
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 H 2 -O 2 fuel cell n = 2
F = Faraday constant = 96,485 Coulombs/ electron mol. At the other operating conditions, EMF of the fuel cell is,
where PH 2 , PO 2 , PH 2 O 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.
Table 7.
Theoretical EMF for Some Fuel Cells at reference
conditions
Fuel Reaction n E^0 , V
H 2 H 2 + 0.5 O 2 → H 2 O 2 1.
Methane CH^4 + 2O^2 →^ CO^2 +2H^2 O(l)^8 1.
Methanol
CH 3 OH (l)+ 1.5O2^ →^ CO^2 +2H^2 O
(l)
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/cm 2 , 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.
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 35:Alternative Powerplants
Figure 7.
Fuel cell efficiency
characteristics.
Factors Favouring Fuel Cell
Varieties of sources for generating hydrogen are possible to reduce dependence on crude petroleum Vehicles with zero emissions of CO, HC, NO (^) x and PM can be built. On board reforming of methanol, ethanol or hydrocarbon fuels can be employed to generate hydrogen. Thus, the existing fuel distribution network can be used. Fuel cells are more efficient under part load operation than the IC engines. These are not limited by the Carnot efficiency. At full power output, the internal resistance and concentration polarization losses increase resulting in the loss of fuel-cell efficiency and it drops close to that of the IC engines. Efficiency of fuel cell is compared with that of gasoline and diesel engines on Fig. 7.17.
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.
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- Very short
< 5kW, military, space Proton Exchange Membrane (PEMFC)
Polymer Electrolyte (H+)
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)
200 kW - MW, CHP and stand alone
Solid oxide (SOFC)
Solid doped Zr-oxide (O- )
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/cm 2 ). Hydrogen either can be directly stored on-board of vehicle or generated by steam- reforming of fuels such as methanol,
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 (
Module7:Advanced Combustion Systems and Alternative Powerplants
Questions
(7.1) Three different approaches are used to obtain charge stratification viz., spray jet controlled, wall controlled and flow controlled Discuss the differences in the duration of initial heat release rate ( 0 to 10%) and heat release rates towards the end of combustion ( 80 to 90%) with the three methods. What factors are responsible for these differences? (7.2) What type of emission control technologies may be employed to reduce HC and NO (^) x emissions in the DISC engines operating in the different air-fuel ratio regimes? (7.3) What are significant differences between CAI and HCCI engine systems from the point of mixture preparation and charge homogeneity? (7.4) What are main methods being studied to HCCI combustion in the diesel type engines? Discuss their merits and demerits. (7.5) Why is it not possible to build a practical engine to operate in HCCI mode in the whole speed and load range of a SI or CI engine? (7.6) Discuss why a HEV is more energy efficient than the vehicles powered by the conventional IC engine (7.7) Discuss why a HEV is more energy efficient than the vehicles powered by the conventional IC engine (7.9) Discuss why the energy efficiency of fuel cells at part loads is much higher than for the IC engines.
