GM Quantifies CO2 and Fuel Consumption Reductions Via E-REVs And PHEVs, As Compared To Conventional Hybrids

by Jack Rosebro

Energy sources, paths, storage media, and propulsion systems available or in development. “FCEV” refers to all fuel cell vehicles, including E-REVs and fuel cell hybrids. Adapted from Tate et al. (2009). Click to enlarge.

General Motors has released a white paper that evaluates the CO₂ reduction potential of extended-range electric vehicles (E-REVs) as well as plug-in hybrids (PHEVs), in combination with multiple vehicle charging scenarios, as compared to conventional hybrids. The paper was presented by authors Ed Tate and Peter Savagian at last month’s SAE 2009 World Congress in Detroit.

In the paper, the GM team broke down CO₂ and fuel consumption reduction potentials into several categories:

• The introduction of E-REVs and PHEVs to existing grids, displacing petroleum use;
• The influence of charging availability and consumer behavior on CO₂ reduction;
• Changes in power grid mix as well as vehicle stock; and
• The impact of selective and voluntary consumer behaviors on CO₂ reduction.

Using 2008 estimates from the US Energy Information Administration

(EIA) as well as the Organization of Petroleum Exporting Countries (OPEC), Tate and Savagian note that global primary energy use is projected to rise by 50% over the next 22 years, with worldwide vehicle population increasing by 60% over the same period of time. Per capita vehicle ownership is expected to rise by one-fourth during this time—from 12% to 15%—with private vehicle ownership functioning as “both a means to, and a dividend of, economic development.”

Taking note of pending vehicular greenhouse gas (GHG) regulations such as California’s Assembly Bill 1493 (Pavley), the authors observe that GHG emissions from conventional vehicle powertrains are already regulated to some extent by fuel economy regulations in many countries, and that GHG regulations would function as additional de facto fuel economy regulations, as “the use of fuel and the production of CO₂ are simply different measures of the amount of combustion” of fuel. To simplify compliance, they argue, “one regulation is sufficient to control both measures.”

CO₂ Displacement Through Vehicle Electrification

“Electrification” is defined in the paper as referring to the relative increase in the electrical content and magnitude of a vehicle’s motive power as compared to the amount of energy that is derived from petroleum-based energy sources.

In a previous paper (The Electrification Of The Automobile: From Conventional Hybrid to Plug-In Hybrids To Extended-Range Electric Vehicles, SAE 2008-01-0458), Tate, Savagian, and Michael Harpster (also of GM) had evaluated the potential for petroleum displacement as well as emissions reductions, using vehicles similar to those in the current study.

However, that work had considered scenarios involving only one battery pack charge per vehicle per day. The current paper expands this work to several scenarios that involve multiple charge opportunities per driving day, and advocates a well-to-wheel analysis of vehicle-based CO₂ emissions, rather than just tailpipe emissions, to inform several questions:

• What kind of plug-in vehicles will enable CO₂ reductions most effectively?
• What is the most effective charging infrastructure for such vehicles?
• What are the best test procedures and measures of merit for plug- in vehicles?

The study compared petroleum and CO₂ displacement opportunities of three types of vehicles: PHEVs, E-REVs, and conventional full hybrids, on the assumption that they would be mid-sized vehicles equivalent to a 2009 Chevrolet Malibu sedan and operated in a variety of regional driving patterns using a variety of regional electrical power source profiles. Charge-sustaining fuel economy for all vehicles is assumed to be 36 miles per US gallon. The vehicles are defined as

follows:

• Hybrid Electric Vehicle (HEV): A 2.4L four-cylinder gasoline engine and a 1.8kWh, 30kW battery pack provide motive power through GM’s two- mode FWD hybrid system (earlier post). However, all motive power, including that from the battery pack, ultimately comes from the combustion of liquid fuel.

• Plug-In Hybrid Electric Vehicle (PHEV): A 2.4L four-cylinder gasoline engine and a 2 to 16 kWh (2, 4, 8 and 16), 53kW battery pack provide motive power through GM’s two-mode FWD hybrid system. Motive power comes from the battery pack when available and when power and speed requirements are below 53kW and/or 56 MPH. Once the battery pack is depleted to a given state of charge (SOC), the vehicle operates as a conventional hybrid vehicle. The range in battery pack capacity allows for evaluation of the effect of battery pack size on liquid fuel consumption.

• Extended-Range Electric Vehicle (E-REV): A 1.4L four-cylinder gasoline engine and a 2 to 16 kWh (2, 4, 8 and 16), 120kW battery pack provide motive power through GM’s E-REV propulsion system. When the battery pack is depleted to a given state of charge (SOC), the vehicle operates as a conventional hybrid vehicle. There are no speed or power constraints that force the engine to run; fuel is burned only after the battery pack is depleted.

Study Methodology

The researchers began by comparing behavioral models of the three vehicles with trip information from the 2001 National Household Travel Survey (NHTS) of US travel behavior, conducted by the Department of Transportation (DOT). The behavioral vehicle models profile energy consumption at the fuel tank, the battery pack, and the charging station, by defining:

• Consumption of electrical energy in charge-depleting mode;
• Fuel energy used, if any, during charge-depleting mode;
• Consumption of fuel in charge-sustaining mode;
• Battery pack capacity;
• Charger power rate;
• Charger efficiency;
• Restrictions on charging time; and
• Restrictions on charging opportunities.

The models assume that plug-in vehicles start each day at full charge.

NHTS data includes trip distances, travel times, travel speeds, trip origin locations, and trip destination locations for approximately 50,000 vehicles over more than 130,000 trips, and evaluates vehicles which move as well as vehicle that remain parked during the travel day. Data was collected at a rate of once per minute per vehicle. NHTS data shows that most cars are left at home during the weekday; the vehicles that are used to commute to work are obvious candidates for recharging after their commute.

Three charging scenarios were evaluated:

• Charging exclusively at home, between the hours of 9 PM and 9 AM;
• Charging at home, as above, plus unrestricted charging at work; and
• Unrestricted charging at home and at work, as well as other opportunities.

Each charging scenario was broken down into three subsets, reflecting charging rates of 1.1, 3.3, and 6.6kW, (at potentials of 110, 220, and 400V, respectively) and assuming 90% efficiency for all chargers.

Data on primary power used to fuel electrical grids was taken from current and projected status of the EPA’s Emissions and Generation Resource Integrated Database (eGRID). In general, the study uses a 2005 EPA estimate of California average electric generation emissions (0.32 kg of CO₂ per kWh); however, the EPA estimates that such emissions vary by region from 0.23 to 0.89 kg of CO₂ per kWh.

Results

Within any given driving profile, reductions in fuel consumption and

CO₂ production are primarily influenced by battery size, powertrain architecture, and charging scenario. If a relatively modest battery pack capacity is employed and combined with a restrictive charging scenario, PHEV and E-REV fuel consumption and CO₂ reduction potential (as compared to a conventional hybrid) are roughly equal to one another.

An aggressive battery pack size coupled with an aggressive charging scenario can, however, reduce either plug-in vehicle architecture’s fuel consumption by as much as half. As battery size and charging opportunities increase, the rate of reduction of fuel consumption and CO₂ production begins to favor an E-REV architecture.

Tate and Savagian also looked at the role of the electrical power grid in terms of projected peak power draw from a given population of vehicles as well as the total projected energy use by those vehicles while charging. Using the median charging rate of 3.3kW, for example, the peak expected power consumption of an 8kWh battery pack is 813 watts, which the authors equate to “a 50-inch plasma television set or a few high-wattage light bulbs.”

Peak grid loading is primarily affected by both charging locations and charging time restrictions. For example, the peak grid loading of a 4kWh E-REV that is charged only at home with a 1.1kW charger will be similar to peak grid loading if the same vehicle is charged with a 3.3kW charger at both home and work, even though the total amount of charge to the battery pack will be very different.

The authors conclude that charging scenarios are at least as influential as battery pack size when considering the ability of a given plug-in vehicle to optimize reductions of fuel consumption and well-to-wheel

CO₂ emissions.

“If publicly available data sets that provided summary data similar to the NHTS [data] with second-by-second velocity information were available,” they add, “detailed simulations could be performed instead of the behavioral models used here.”

Several additional conclusions were drawn from an analysis of the data:

1. Fuel consumption and CO₂ production of both PHEVs and E-REVs are lower than that of equivalent full hybrids, with expected fuel savings of around 70% and CO₂ reduction of around 40%, assuming a grid power mix similar to that of the current California grid, and assuming that the vehicle is charged daily and driven an average of 100 miles or less per day.

2. Fuel consumption and CO₂ production of E-REVs—particularly those with larger battery packs—are lower than that of equivalent plug-in hybrids. Assuming a battery pack size of 8kWh for both vehicle types, E-REV fuel consumption is 22% lower and CO2 output is 8% lower than that of a PHEV, with 3.3 kW charging twice a day and a California-type grid power mix.

3. Although the impact of PHEVs and E-REVs on grid loading is expected to be minimal, peak loading can be minimized by making charging widely available at work. Assuming that drivers are also charging at home, this is expected to further reduce a given vehicle’s fuel consumption and CO₂ production.

4. As the electrical grid shifts to renewables, grid-based CO₂ emissions related to vehicle operation per mile traveled will decline, further decreasing well-to-wheel CO2 emissions from both PHEVs and E- REVs. A consumer preference for green and renewable electricity programs would leverage carbon emissions even further.

Resources

• Tate et al (2009) The CO₂ Benefits of

  Electrification: E-REVs, PHEVs and Charging Scenarios. (SAE 2009-01-1311)

• Tate et al. (2008) The Electrification of the Automobile: From Conventional Hybrid, to Plug-in Hybrids, to Extended-Range Electric Vehicles. (SAE 2008-01-0458).

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