What are HEVs?

 

Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle, resulting in twice the fuel economy of conventional vehicles. This combination offers the extended range and rapid refueling that consumers expect from a conventional vehicle, with a significant portion of the energy and environmental benefits of an electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles. The inherent flexibility of HEVs will allow them to be used in a wide range of applications, from personal transportation to commercial hauling.

 

Why HEVs?

 

Hybrid power systems were conceived as a way to compensate for the shortfall in battery technology. Because batteries could supply only enough energy for short trips, an onboard generator, powered by an internal combustion engine, could be installed and used for longer trips. In the old days, we thought that by biasing the system toward battery-electric power and operating on wall-plug electricity as much as possible, efficiency and emissions would then be about as optimal as we could hope for until better batteries came along. The natural conclusion of this concept was that, with better batteries, we probably would not need hybrids at all. But after 20 years of study, it seems that hybrids are taking center stage and electric vehicles are being used in niche market applications where fewer miles are traveled per trip or daily.

 

More efficient cars can make a big difference to society in terms of environmental benefits, and the serious deterioration of urban air has motivated regulators to require cleaner cars. Use of production HEVs will reduce smog-forming pollutants over the current national average. Hybrids will never be true zero-emission vehicles, however, because of their internal combustion engine. But the first hybrids on the market are cutting emissions of global-warming pollutants by a third to a half, and later models may cut emissions by even more.

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Topologies for Power Converters

Versatile circuit topologies can be found in power electronics for different applications. In terms of conversion form, four categories can be identified:

DC/DC CONVERTER

DC/AC CONVERTER(INVERTER)

AC/DC CONVERTER(RECTIFIER)

AC/AC CONVERTER(CYCLOCONVERTER)

Aiming at reducing switching losses and EMI of power converters, a lot of soft switching techniques are developed for above mentioned types of converters so that high effiency, small size, low weight can be achieved. DC/DC Converter

BUCK CONVERTER

BOOST CONVERTER

BUCK/BOOST CONVERTER

CUK CONVERTER

FORWARD CONVERTER (ISOLATED BUCK)

FLYBACK CONVERTER(ISOLATED BUCK/BOOST)

DC/AC Converter

VOLTAGE-FED INVERTER

CURRENT-FED INVERTER

AC/DC Converter An AC/DC converter is usually used as a front-end converter in a power electronics system. A front-end converter has to be friendly to the utility line, that means high power factor, low input current THD(total harmonic distortion, and low EMI emission. To meet more and more strict regulations, advanced AC/DC converters have been developed for single phase system and three-phase system.

SINGLE-PHASE BOOST PFC RECTIFIER

THREE-PHASE DCM BOOST PFC RECTIFIER

THREE-PHASE BOOST PWM PFC RECITIFER

THREE-PHASE BUCK PFC RECTIFIER

Motor Drive Control Issues

Many adjustable-speed drive (ASD) applications require medium- or high-bandwidth torque control in order to achieve adequate control performance. What this means is that if a drive system can function as a controllable source of torque, with fast dynamic response, it should be able to implement the desired motion control operation.

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Hydrogen and oxygen can be combined in a fuel cell to produce electrical energy. A fuel cell uses a chemical reaction to provide an external voltage, as does a battery, but differs from a battery in that the fuel is continually supplied in the form of hydrogen and oxygen gas. It can produce electrical energy at a higher efficiency than just burning the hydrogen to produce heat to drive a generator because it is not subject to the thermal bottleneck from the second law of thermodynamics. It's only product is water, so it is pollution-free. All these features have led to periodic great excitement about its potential, but we are still in the process of developing that potential as a pollution-free, efficient energy source (see Kartha and Grimes).

 

hydrogen fuel cell

 

Combining a mole of hydrogen gas and a half-mole of oxygen gas from their normal diatomic forms produces a mole of water. A detailed analysis of the process makes use of the thermodynamic potentials. This process is presumed to be at 298K and one atmosphere pressure, and the relevant values are taken from a table of thermodynamic properties.

 

Energy is provided by the combining of the atoms and from the decrease of the volume of the gases. Both of those are included in the change in enthalpy included in the table above. At temperature 298K and one atmosphere pressure, the system work is

 

W = PDV = (101.3 x 103 Pa)(1.5 moles)(-22.4 x 10-3 m3/mol)(298K/273K) = -3715 J

 

Since the enthalpy H= U+PV, the change in internal energy U is then

 

DU = DH - PDV = -285.83 kJ - 3.72 kJ = -282.1 kJ

 

The entropy of the gases decreases by 48.7 kJ in the process of combination since the number of water molecules is less than the number of hydrogen and oxygen molecules combining. Since the total entropy will not decrease in the reaction, the excess entropy in the amount TDS must be expelled to the environment as heat at temperature T. The amount of energy per mole of hydrogen which can be provided as electrical energy is the change in the Gibbs free energy:

 

DG = DH - TDS = -285.83 kJ + 48.7 kJ = -237.1 kJ

 

For this ideal case, the fuel energy is converted to electrical energy at an efficiency of 237.1/285.8 x100% = 83%! This is far greater than the ideal efficiency of a generating facility which burned the hydrogen and used the heat to power a generator! Although real fuel cells do not approach that ideal efficiency, they are still much more efficient than any electric power plant which burns a fuel.

 

Comparison of electrolysis and the fuel cell process

 

In comparing the fuel cell process to its reverse reaction, electrolysis of water, it is useful treat the enthalpy change as the overall energy change. The Gibbs free energy is that which you actually have to supply if you want to drive a reaction, or the amount that you can actually get out if the reaction is working for you. So in the electrolysis/fuel cell pair where the enthalpy change is 285.8 kJ, you have to put in 237 kJ of energy to drive electrolysis and the heat from the environment will contribute TDS=48.7 kJ to help you. Going the other way in the fuel cell, you can get out the 237 kJ as electric energy, but have to dump TDS = 48.7 kJ to the environment.