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Hybrids Plus PHEV Toyota Prius conversion with PHEV-30 (30 mile all-electric range) battery packs

A plug-in hybrid electric vehicle (PHEV) is a hybrid vehicle with batteries that can be recharged by connecting a plug to an electrical power source. Plug-in hybrids have characteristics of both conventional hybrid electric vehicles and of battery electric vehicles. While PHEVs are usually passenger vehicles, they can also be commercial passenger vans,[1] utility trucks,[2][3] school buses,[4] scooters,[5] and military vehicles.[6] PHEVs are sometimes called grid-connected hybrids, full hybrids, gas-optional hybrids or GO-HEVs.

As of 2007, the cost for electricity to power plug-in hybrids during all-electric operation in California has been estimated as less than one fourth the cost of gasoline.[7] PHEVs also have the potential to help reduce air pollution as well as dependence on petroleum and help mitigate global warming by producing less greenhouse gases than conventional vehicles. Plug-in hybrids use no fossil fuel during their all-electric range to the extent that their batteries are charged from renewable energy sources. Other potential benefits include improved national energy security, fewer fill-ups at the gas station, the convenience of home recharging, opportunities to provide emergency backup power in the home, and the potential for vehicle to grid applications.[8]

As of April 2007, plug-in hybrid passenger vehicles are not yet in production. However, Toyota[9] and General Motors[10] have announced their intention to introduce production PHEV automobiles. Conversions of production model hybrid vehicles are available from conversion kits and conversion services. The most prominent PHEVs on the road in the U.S. are conversions of 2004+ Toyota Prius hybrid cars, which extend their electric-only range and add plug-in charging.

Terminology

A plug-in hybrid's all-electric range is designated by PHEV-(miles) or PHEV(kilometers)km representing the distance the vehicle can travel on electric power alone. For example, a PHEV-20 can travel twenty miles without using its internal combustion engine, or about 32 kilometers, so it may also be designated as PHEV32km.[11]

History

Further information: History of plug-in hybrids

Hybrid vehicles were produced beginning as early as 1899 by Lohner-Porsche. Early hybrids could be charged from an external source before operation. However, the term "plug-in hybrid" has come to mean a hybrid vehicle that can be charged from a standard electrical wall socket.

The July 1969 issue of Popular Science featured an article on the General Motors XP-883 plug-in hybrid. The concept commuter vehicle housed six 12 volt lead acid batteries in the trunk area and a transverse-mounted DC electric motor turning a front-wheel drive.[12] The car could be plugged into a standard 110 Volt AC outlet for recharging.

In 2003, Renault began selling the Elect'Road, a plug-in series hybrid version of their popular Kangoo, in Europe. It was sold alongside Renault's "Electri'cite" electric-drive Kangoo battery electric van. The Elect'Road had a 150 km range using a nickel-cadmium battery pack and a 500 cc, 16 kilowatt liquid-cooled gasoline "range-extender" engine. It powered two high voltage/high output/low volume alternators, each of which supplied up to 5.5 kW at 132 volts at 5000 rpm.[13] The operating speed of the internal combustion engine — and therefore the output delivered by the generators — varied according to demand. The fuel tank had a capacity of 10 litres and was housed within the right rear wheel arch. The range extender function was activated by a switch on the dashboard. The on board 3.5 kilowatt charger could charge a depleted battery pack to 95% charge in about four hours from 220 volts.[14] Passenger compartment heat was powered by the battery pack as well as an auxiliary coolant circuit that was heated by the range extender engine. Renault discontinued the Elect'Road after selling about 500, primarily in France, Norway and the UK, for about 25,000 euros.[13]

Lithium-ion battery pack, with cover removed, in the CalCars plug-in hybrid converted Toyota Prius

In September 2004, the California Cars Initiative (CalCars) converted a 2004 Toyota Prius into a prototype of what it calls the PRIUS+. With the addition of 130 kg (300 lb) of lead-acid batteries, the PRIUS+ achieved roughly double the gasoline mileage of a standard Prius and can make trips of up to 15 km (10 miles) using only electric power. The vehicle, which is owned by CalCars technical lead Ron Gremban, is used in daily driving, as well as a test bed for various improvements to the system.[15]

On July 18, 2006, Toyota announced that it "plans to develop a hybrid vehicle that will run locally on batteries charged by a typical 120 volt outlet before switching over to a gasoline engine for longer hauls."[9] The next major update to the Toyota Prius is said to use lithium ion batteries.[16] Toyota’s fuel economy target for the upcoming next-generation Prius has been reported to be 40 kilometers/liter (2.5 l/100km, or 94 mpg US.)[17]

On November 29, 2006 GM announced plans to introduce a production plug-in hybrid version of Saturn's Greenline Vue SUV with an all-electric range of 10 miles. GM announced that they will be introducing plug-in and other hybrids "for the next several years."[10]

In January 2007, GM unveiled the Chevrolet Volt, which is expected to initially feature a plug-in capable, battery-dominant series hybrid architecture which they are calling E-Flex.[18] Future E-Flex plug-in hybrid vehicles may use gasoline, diesel, or hydrogen fuel cell power to supplement the vehicle's battery. General Motors envisions an eventual progression of E-Flex vehicles from plug-in hybrids to pure electric vehicles, as battery technology improves.[19] General Motors presented the Volt as a PHEV-40 that starts its engine when 40% of the battery charge remains, and which can achieve a fuel economy of 50 mpg (4.7 l/100 km), even if the vehicle is not plugged in.[20]

On February 28, 2007, The United States Department of Energy released a draft of a plan to accelerate the development and deployment of plug-in hybrid vehicle technology.[21] Specific areas of concern were lithium-ion batteries, power electronics, and electric motors. On May 22, 2007, five research projects were selected to receive US$ 19 million to further the development of technologies related to PHEVs, such as electric motor power inverters.[22]

Technology

Powertrains

The Chevrolet Volt is a series hybrid.
Further information: Hybrid vehicle drivetrains

PHEVs are based on the same three basic powertrain architectures as conventional hybrids:[23]

Series hybrids use an internal combustion engine (ICE) to turn a generator, which in turn supplies current to an electric motor, which then rotates the vehicle’s drive wheels. A battery or capacitor pack, or a combination of the two, can be used as a buffer of sorts to store excess charge. Examples of series hybrids include the Renault Kangoo Elect'Road, Toyota's Japan-only Coaster light-duty passenger bus, DaimlerChrysler's hybrid Orion bus, the Chevrolet Volt concept car, and many diesel-electric locomotives. With an appropriate balance of components this type can operate over a substantial distance with its full range of power without engaging the ICE. As is the case for other architectures, series hybrids can operate without plugging in as long as there is liquid fuel in the tank.[24]

Parallel hybrids, such as Honda's Insight, Civic, and Accord hybrids, can simultaneously transmit power to their drive wheels from two distinct sources—for example, an internal-combustion engine and a battery-powered electric drive. Although most parallel hybrids incorporate an electric motor between the vehicle's engine and transmission, a parallel hybrid can also use its engine to drive one of the vehicle's axles, while its electric motor drives the other axle. The Audi Duo plug-in hybrid concept car is an example of this type of parallel hybrid architecture. Parallel hybrids can be programmed to use the electric motor to substitute for the ICE at lower power demands and to substantially increase the power available to a smaller ICE than would normally be used, either mode substantially increasing fuel economy compared to a simple ICE vehicle.[25]

Series-parallel hybrids have the flexibility to operate in either series or parallel mode. Hybrid powertrains currently used by Ford, Lexus, Nissan, and Toyota, which some refer to as “series-parallel with power-split,” can operate in both series and parallel mode at the same time. At present, most plug-in hybrid conversions of conventional hybrids utilize this architecture.[26]

Modes of operation

Regardless of its architecture, a plug-in hybrid may be capable of charge-depleting and charge-sustaining modes. Combinations of these two modes are termed blended mode or mixed-mode. These vehicles can be designed to drive for an extended range in all-electric mode, either at low speeds only or at all speeds. These modes manage the vehicle's battery discharge strategy, and their use has a direct effect on the size and type of battery required:[27]

Charge-depleting mode allows a fully charged PHEV to operate exclusively (or depending on the vehicle, almost exclusively, except during hard acceleration) on electric power alone until its battery state of charge is depleted to a predetermined level, at which time the vehicle's internal combustion engine or fuel cell will be engaged. This period is the vehicle's all-electric range. This is the only mode that a battery electric vehicle can operate in, thus their limited range.[28]

Charge-sustaining mode is used by production hybrid vehicles (HEV) today, and combines the operation of the vehicle's two power sources in such a manner that the vehicle is operating as efficiently as possible without allowing the battery state of charge to move beyond some predetermined narrow band. Over the course of a trip in a HEV the state of charge may fluctuate but will have no net change.[29] The battery in a HEV can thus be thought of as an energy accumulator rather than a fuel storage device. Once a plug-in hybrid has exhausted its all-electric range in charge-depleting mode, it can switch into charge-sustaining mode automatically.

The discontinued Renault Kangoo Elect'road operates in blended mode.

Blended mode is a type of charge-depleting mode normally employed by vehicles which do not have enough electric power to sustain high speeds without the help of the internal combustion portion of the powertrain. A blended control strategy typically takes more miles to use stored grid electricity than a charge-depleting strategy.[30] The Renault Kangoo and some Toyota Prius conversions are examples of vehicles that use this mode of operation. The Electri'cité & Elect'road versions of the Relant Kangoo were charge-depleting battery electric vehicles; the Elect'road had a modest internal-combustion engine (ICE) which extended its range somewhat. 2004+ Toyota Prius conversions can only run without using the ICE at speeds of less than about 42 mph due to the limits dictates by the vehicle's powertrain control software, which resides in the Hybrid Vehicle Electronic Control Unit (HV ECU). However at speeds above this electric power can still be used to displace gas fuel thus improving the gas mileage when operating in this mode, generally doubling the miles per gallon.

Mixed mode is a particular trip in which a combination of the above modes are utilized.[31] For example, a PHEV-20 Prius conversion may begin a trip with 5 miles of low speed charge-depleting, then get onto a freeway and operate in blended mode for 20 miles, using 10 miles worth of AER at twice the miles per gallon. Finally the driver might exit the freeway and drive for another 5 miles without the ICE until the full 20 miles of AER were exhausted. At this point the vehicle would revert back to a charge sustaining-mode for another 10 miles until the final destination is reached. Such a trip would be considered a mixed mode, as multiple modes are employed in one trip. This contrasts with a charge-depleting trip which would be driven within the limits of a PHEV's all electric range. Conversely, the portion of a trip which extends beyond the AER of a PHEV would be driven primarily in charge-sustaining mode, much as a conventional hybrid.

Batteries

PHEVs typically require deeper battery charging and discharging cycles than conventional hybrids. Because the number of full cycles influences battery lifetime, battery life may be less than in traditional HEVs which do not deplete their batteries as deeply. However, some authors argue that PHEVs will soon become standard in the automobile industry.[32] Design issues and trade-offs concerning battery life, capacity, heat dissipation, weight, costs, and safety need to be solved.[33] Advanced battery technology is under development.[34][35][36] Battery life expectancy is expected to increase.[37]

The cathodes of some early 2007 lithium-ion batteries are made from lithium-cobalt metal oxide. This material is expensive, and cells made with it can release oxygen if its cell is overcharged. If the cobalt is replaced with iron phosphates, the cells will not burn or release oxygen under any charge. The price premium for early 2007 hybrids is about US$ 5000, some $3000 of which is for their NiMH battery packs. At early 2007 gasoline and electricity prices, that would break even after six to ten years of operation. The hybrid premium could fall to $2000 in five years, with $1200 or more of that being cost of lithium-ion batteries, providing a three-year payback.[38]

Conversions of production hybrids

15 lead-acid batteries, PFC charger, and regulators installed into WhiteBird, a PHEV-10 conversion of a Toyota Prius
Further information: Electric vehicle conversion

Conversion of an existing production hybrid to a plug-in hybrid typically involves increasing the capacity of the vehicle's battery pack and adding an onboard AC-to-DC charger. Ideally, the vehicle's powertrain software would be reprogrammed to make full use of the battery pack's additional energy storage capacity and power output.

Many early plug-in hybrid electric vehicle conversions have been based on the 2004 or later model year Toyota Prius.[39] Some of the systems have involved replacement of the vehicle's original Ni-MH battery pack and its electronic control unit. Others, such as Hymotion as well as builders of the CalCars Prius+, and the PiPrius, piggyback an additional battery back onto the OEM battery pack, this is also referred to as Battery Range Extender Modules (BREMs).[40] This has been referred to as a "hybrid battery pack configuration" within the electric vehicle conversion community.[41] Early lead-acid battery conversions by CalCars demonstrated 10 miles (15 km) of EV-only and 20 miles (30 km) of double mileage blended mode range.[15]

EDrive Systems use Valence Technology Li-ion batteries and have a claimed 40 to 50 miles of electric range.[42] Other companies offering plug-in conversions or kits for the Toyota Prius include Hymotion, Hybrids Plus, and Manzanita Micro.

The EAA-PHEV project was conceived in October of 2005 to accelerate efforts to document existing HEVs and their potential for conversion into PHEVs.[43] The Electric Auto Association-PHEV "Do-It-Yourself" Open Source community's primary focus is to provide general information to curious parties and detailed conversion instruction to help guide experienced EV Converters through the process. Many members of organizations such as CalCars and the EAA as well as companies like Hybrids Plus, Hybrid Interfaces of Canada, and Manzanita Micro participate in the development of the project.

Potential advantages

Fuel efficiency

A 120 km (70 mile) range HEV-70 may annually require only about 25% as much gasoline as a similarly designed HEV-0, depending on how it will be driven and the trips for which will be used.[8]

A further advantage of PHEVs is that they have potential to be even more efficient than conventional hybrids because more limited use of the PHEV's internal combustion engines may allow the engine to be used at closer to its maximum efficiency. While a Prius is likely to convert fuel to motive energy on average at about 30% efficiency (well below the engine's 38% peak efficiency) the engine of a PHEV-70 would likely operate far more often near its peak efficiency because the batteries can serve modest power needs when the combustion engine would run well below its peak efficiency.[citation needed] The actual efficiency achieved depends on losses from electricity generation, inversion, battery charging/discharging, the motor controller and motor itself. If the efficiencies of these additional stages multiply to yield 60% (this is an estimate) and the engine peak efficiency is 38%, the engine should remain in use until its efficiency drops below 25%.[citation needed]

Fuel economy claims for PHEV depend crucially on assumptions about the amount of driving between recharges. If this is less than battery capacity and the engine is not used, no gasoline is used and the MPG is infinite. As trip length increases, PHEV fuel economy approaches an asymptote MPG which is lower than for a HEV or conventional car of the same class because of the additional tire drag caused by PHEV battery weight. As of June, 2007 PHEV claims almost never cite the distance or type of driving between recharges. (e.g. RechargeIT.org) Consequently, the fuel economy numbers cited by advocates and repeated in the press have been based on unknown driving range.

Greenhouse gas emissions

Another potential advantage of PHEVs is a predicted reduction in carbon emissions should PHEV use become widespread. Increased drivetrain efficiency results in significant reduction of greenhouse gas emissions, even taking into account energy lost to inefficiency in the production and distribution of grid power and charging of batteries. A study by the American Council for an Energy Efficient Economy (ACEEE) predicts that, on average, a typical American driver is expected to achieve about a 15% reduction in net CO2 emissions compared to a regular hybrid, based on the 2005 distribution of power sources feeding the US electrical grid. Additionally, for PHEV’s recharged in areas where the grid is fed by power sources with lower CO2 emissions than the current average, net CO2 emissions associated with PHEVs will decrease correspondingly.

The same study predicts that in areas where more than 80% of grid-power comes from coal-burning power plants, local net CO2 emissions will increase.[44] However, given the global nature of problems associated with CO2 emissions, specifically those related to global warming, localized increases in CO2 emissions are not considered a significant problem if global CO2 emissions are decreased.[11]

Operating costs

George W. Bush (right) is shown the PHEV Mercedes-Benz Sprinter van in the U.S. Postal Service

In California, as of 2006, the cost to plug in at night is equivalent to US$ 0.75 per gallon of gasoline,[7] whereas gasoline sells for over $3 per gallon. The cost of electricity for a Prius PHEV is about $0.03/mi (US$ 0.019/km), based on 0.26 kilowatt hours per mile and a cost of electricity of $0.10 per kilowatt hour.[45][46] Current PHEV conversions install a higher capacity battery than common hybrids like the Toyota Prius in order to extend the range. This lowers the energy cost per mile because just US$ 1.00 worth of electricity from the wall (at $0.09/kW·h) is sufficient to drive the same distance as a gallon of gasoline. During 2007, many government and industry researchers will focus on determining what level of all-electric range is economically optimum for the design.

Vehicle-to-grid electricity features

PHEVs and fully electric cars may allow for more efficient use of existing electric production capacity, much of which sits idle as operating reserve most of the time. This assumes that vehicles are charged primarily during off peak periods (i.e., at night), or equipped with technology to shut off charging during periods of peak demand. Another advantage of a plug-in vehicle is their potential ability to load balance or help the grid during peak loads. This is accomplished with vehicle to grid technology. By using excess battery capacity to send power back into the grid and then recharge during off peak times using cheaper power, such vehicles are actually advantageous to utilities as well as their owners. Even if such vehicles just led to an increase in the use of night time electricity they would even out electricity demand which is typically higher in the day time, and provide a greater return on capital for electricity infrastructure.[11]

In October 2005, five Toyota engineers and one Aisin AW engineer published an IEEE technical paper detailing a Toyota-approved project to add vehicle to grid capability to a Toyota Prius.[47] Although the technical paper described "a method for generating voltage between respective lines of neutral points in the generator and motor of the THS-II (Toyota Hybrid System) to add a function for generating electricity," it did not state whether or not the experimental vehicle could be charged through the circuit, as well. However, the vehicle was featured in a Toyota Dream House, and a brochure for the exhibit stated that "the house can supply electricity to the battery packs of the vehicles via the stand in the middle of the garage," indicating that the vehicle may have been a plug-in hybrid.[48]

In November 2005, more than fifty public power leaders from across the nation met at Los Angeles Department of Water and Power headquarters to discuss plug-in hybrid and vehicle-to-grid technology. The event, which was sponsored by the American Public Power Association, also provided an opportunity for association members to plan strategies that public power utility companies could use to promote plug-in hybrid technology. Greg Hanssen and Peter Nortman of EnergyCS and EDrive attended the two-day session, and during a break in the proceedings, made an impromptu display in the LADWP parking lot of their converted Prius plug-in hybrid.[49]

From September 25 to 27, 2006 the California Air Resources Board held a Zero Emission Vehicle symposium that included several presentations on V2G technology.[50] In April 2007, Pacific Gas and Electric showcased a PHEV at the Silicon Valley Leadership Alternative Energy Solutions Summit with vehicle-to-grid capability, and demonstrated that they could be used as a source of emergency home power in the event of an electrical power failure.[51] Regulations intended to protect electricians against power other than from grid sources would need to be changed, or regulations requiring consumers to disconnect from the grid when connected to non-grid sources will be required before such backup power solutions would be feasible.[52]

Disadvantages

Disadvantages of plug-in hybrids include the additional weight and cost of a larger battery pack. The fuel economy increase for a PHEV are highly dependent upon the way a vehicle is used (its duty cycle) and the opportunities to recharge by connecting to the electrical grid.

The study by the ACEEE predicts that widespread PHEV use in heavily coal-dependent areas would result in an increase in local net sulfur dioxide and mercury emissions, given emissions levels from most coal plants currently supplying power to the grid.[53][54] Although clean coal technologies could create power plants which supply grid power from coal without emitting significant amounts of such pollutants, the higher cost of the application of these technologies may increase the price of coal-generated electricity dramatically. The net effect on pollution is dependent on the fuel source of the electrical grid (fossil or renewable, for example) and the pollution profile of the powerplants themselves. Identifying, regulating and upgrading single point pollution source such as a powerplant—or replacing a plant altogether—may also be more practical. From a human health perspective, shifting pollution away from large urban areas may be considered a significant advantage.

Battery disposal is another life-cycle consideration. Nickel-metal hydride and lithium-ion batteries can be recycled; Toyota, for example, has a recycling program in place under which dealers are paid a US$ 200 credit for each battery returned.[55] However, plug-in hybrids typically use larger battery packs than comparable conventional hybrids, and thus require larger resource flows.

Commercialization

The number of US survey respondents willing to pay US$4,000 more for a plug-in hybrid car increased from 17% in 2005 to 26% in 2006.

Interest in plug-in hybrids increased in 2006 to such a level that the architecture was included as an area of research in President George W. Bush's advanced energy initiative and mentioned in his 2007 State of the Union Address. After hearing an explanation of PHEVs, 49% of consumers surveyed in 2006 said they would consider purchasing one. That is about the same level of interest as standard hybrid technology.[56]

Patent encumbrance of NiMH batteries

In 1994, General Motors acquired a controlling interest in Ovonics's battery development and manufacturing, including patents and trade secrets controlling the manufacturing of large nickel metal hydride (NiMH) batteries. In 2001, Texaco purchased GM's share in GM Ovonics. A few months later, Chevron acquired Texaco. In 2003, Texaco Ovonics Battery Systems was changed to Cobasys, a 50/50 joint venture between Chevron and Energy Conversion Devices (ECD) Ovonics.[57] Large-format NiMH batteries were commercially viable and ready for mass production, but Chevron and other oil-related interests suppressed the technology to forestall the introduction of plug-in hybrids.[58]

Due to the rapid deterioration of NiMH batteries when fully charged and discharged, PHEV vehicles based on NiMH are not cost effective. The cost of battery replacement is many times higher than the cost of the electricity the stored by the battery over its lifetime.[citation needed]

See also

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References

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External links

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