A plug-in hybrid is similar to a regular hybrid with both an internal combustion engine and an electrical motor and generator, but it has a battery charger and electrical charging port so that the car’s battery bank can be plugged into the electrical grid whenever the car is parked[1]. PHEVs generally have higher capacity (larger) batteries than regular hybrids.  A PHEV might be able to travel 10 to 40 miles on battery power alone, significantly reducing gasoline consumption for short trips.

Key issues for a PHEV include:

• The all-electric range: AER--the distance that the car can travel on battery power alone
• Fraction of Grid miles: the fraction of miles powered by grid electricity for typical driving habits
• Fraction of Grid energy: the fraction of energy supplied by the grid compared to the energy provided by gasoline. Once the fraction of grid energy compared to gasoline energy is determined, then the greenhouse gas and oil import reductions compared to non-plug-in hybrids can be estimated.

Fraction of grid miles. Some have suggested that a plug-in hybrid with 30 miles all electric range might be sufficient to replace most gasoline with electricity. Their reasoning might be based on the fact that an average passenger car travels 12,000 miles per year, which is an average of less than 33 miles per day. Others might point out that the average distance from home to work was 12.1 miles in 2001, so the average commute would be 24 miles (12 miles each way).

However, the real world is more complex. The worker might travel 24 miles most work days, but then takes a 100- or 200-mile trip on many weekends.  So the battery bank of a PHEV with 30 miles AER might cover 120 miles of gasoline-free commuting each 5-day work week, but then gasoline would be needed for 70 to 170 miles of weekend travel (assuming, of course, that the owner and his or her teenage children remember to plug-in the PHEV every night without fail!)

Kromer and Heywood at MIT [2] have summarized the results of several studies that looked at the average miles traveled each day in the US.  The graph below plots the highest and lowest estimates from the literature for the percentage of miles traveled using electricity as a function of the all-electric range for some future PHEV. For our hypothetical PHEV with 30 miles AER, the fraction of miles that would typically be powered only by the battery varies between 40% to 56%.  All the other miles traveled would be powered by gasoline.

Fraction of energy from electricity.  We also need to know the fraction of energy coming from the electricity grid to calculate greenhouse gases and oil consumption. Even though the PHEV might use battery electricity for 40% to 56% of the miles traveled, the percentage of electrical energy will be less than these percentages, since batteries plus an electric motor are more efficient than the internal combustion engine.  Thus the battery energy per mile provided to the motor while the PHEV is in the electric mode is less than the gasoline energy per mile required to run the internal combustion engine.

PHEV control strategy. Before we can estimate the percentage of energy from the grid, we need to know how the PHEV switches between the electric motor and the gasoline engine. One option is the charge depleting (CD) mode where the car runs exclusively on the battery until it is discharged to its lowest allowable state of charge, followed by a charge sustaining (CS) mode with the gasoline engine supplying the average power, similar to a conventional hybrid operation.

Blended CD mode. However, the pure CD mode is not practical, since the battery power would have to be large enough to supply all the vehicle acceleration and hill climbing capability. The weight and cost of such a high-power battery bank would be prohibitive.  A more practical option is the blended charge depleting mode, where the gasoline engine is used intermittently to provide peak power demand even during the CD mode, saving the battery for lower power portions of the driving cycle.

Elgowainy et al.[3] at the Argonne National Laboratory have analyzed the fraction of grid energy for the blended CD mode. For a PHEV with 30 miles AER, they estimate that approximately 17% of its energy would come from the grid and 83% would come from gasoline with typical American driving habits[3]. Their data were used in our model to calculate the fraction of energy coming from the grid and hence the fraction of energy supplied by gasoline.

[1]Some have suggested that in the future the batteries on HEVs or PHEVs might be charged by inductively coupling to a buried cable in the road while the vehicle is moving [return].

[2] Kromer, M.A.; Heywood, J.B. (2007) “Electric Powertrains: Opportunities and Challenges in the U.S. Light-Duty Vehicle Fleet,” MIT Laboratory for Energy and the Environment, Cambridge, Massachusetts.[return]

[3] Elgowainy A, Burnham A, Wang M, Molburg J, Rousseau A. “Well-to-wheels energy use and greenhouse gas emissions analysis of plug-in hybrid electric vehicles,” Argonne National Laboratory report ANL/ESD/09-2, February 2009 & SAE 2009-01-1309.

Initial Plug-in Hybrid Fuel Economy

There have been claims of extraordinarily high fuel economy (100 mpg to even 200 mpg) for PHEVs.  While no automobile company has yet to manufacture a PHEV, several after-market companies have retrofitted conventional hybrids with extra batteries, a charger and a power outlet.  Hundreds of Toyota Prius’s have been converted to plug-in HEVs. 

The Idaho National Laboratory is conducting a large test with 110 Prius PHEV’s now in their instrumented fleet. Some PHEVs did achieve over 100 mpg on short trips when most of the travel was powered by stored battery energy, and the gasoline engine was rarely turned on.  But over 500,000 miles of on-the-road travel under varying conditions, these 110 Prius PHEV’s have averaged approximately 51 mpg, not much better than a conventional (non-plug-in) Prius.

Google Inc. had a similar experience on a much smaller scale. They ran a structured driving test in 2008 with professional drivers over a road course  designed to simulate test driving conditions for the EPA ratings. Two Prius PHEVs averaged over 93 mpg in these long road tests, compared to 48 mpg for the standard Prius on the same course.

However, Google is also testing eight Prius’s converted to plug-in status, used by their employees in every day travel as corporate cars. The results of actual on-the-road testing, often with multiple drivers each day and very short trips, has shown only modest improvement in fuel economy. The non-plug-in Prius’s in their test averaged 42.1 mpg, while the PHEVs averaged 57.6 mpg. Google estimated that plugging in the Prius would save approximately $160 per year in fuel costs, with gasoline at $4.15/gallon and electricity at 10 cents/kWh. With the battery plug-in conversion costing over $10,000, it would take 60 years to pay back the investment.

Two caveats on these early mpg results:

• These retrofitted HEVs are not optimized for plug-in operation; for example, the added battery bank on most of these retrofits cannot accept regenerative braking energy (although the stock HEV battery bank does store braking energy). PHEV performance should improve when the automobile companies build cars specifically designed for plug-in operation.
• The mpg economy measure is not a complete benchmark for PHEV fuel efficiency.  Miles per gallon only accounts for the gasoline consumed, and does not reflect the electricity used to charge the batteries. This is particularly important for greenhouse gas calculations, for example, since most of the electricity used to charge batteries in the US comes from burning coal, which generates considerable greenhouse gas emissions. Thus a PHEV might achieve 100 mpg in terms of gasoline consumption, but might not reduce greenhouse gases compared to regular hybrids.

Ford Plug-in Fuel Cell Electric Vehicle (on a Ford Edge chassis)

 June 2010 Argonne National Laboratory Report on PHEVs

The Department of Energy’s Argonne National Laboratory has released a very detailed technical report [4] analyzing the energy consumption and greenhouse gas emissions from plug-in hybrid electric vehicles (PHEVs), including PHEVs powered by five fuels: hydrogen, E-85 (mixture of 85% ethanol and 15% gasoline) as well as gasoline and diesel. This report seems to undermine the Obama administration’s focus on gasoline- or ethanol-powered PHEVs and battery electric vehicle (BEVs) at the expense of hydrogen and FCEVs. Argonne assumes that hydrogen is used in fuel cell electric vehicles (PHEVs).  Since most if not all FCEVs also include batteries to provide added power and to recover and store electricity from regenerative braking, then all FCEVs can be plugged in to become PHEVs (see Ford fuel cell PHEV above).  So the question is not whether to build PHEVs, but what is the best power source to augment the batteries for longer range: the internal combustion engine or a hydrogen-powered fuel cell system?

Overall, the Argonne report shows that hydrogen is the best fuel to simultaneously cut greenhouse gases (GHGs) and oil consumption (points on the left indicate low oil consumption, and points toward the bottom of the graph indicate low GHGs):

The Argonne report exhaustively analyzed the impact of operating PHEVs in various parts of the country, since the type of electrical generators will impact the GHGs generated when electricity is used to charge the batteries. For the average US grid mix, plugging in an HEV will always INCREASE GHGs, as shown below. (the top lines for gasoline and diesel HEVs and PHEVs show that an HEV (zero AER or point on the far left always produces less GHGS than a PHEV, since the lines rise with increased all-electric range (AER): For example, a PHEV with 40 miles AER generates an average of 305 g/mile, while an HEV generates only 270 g/mile:  so plugging in increases GHGs; from an environmental perspective, it is better to operate the vehicle as an HEV instead of a PHEV. (The HEV has the added advantage of lower weight and lower cost since it does not require extra batteries to provide all-electric range.)

Note also on the graph above that the hydrogen made from biomass produces the lowest GHGs (green line on bottom of graph).  Using biomass to make ethanol (cellulosi ethanol) used in an internal combustion engine on a PHEV always produces more GHGs (blue line).  Therefore it is always preferable to convert biomass to hydrogen for use in a FCEV than to convert biomass to ethanol for an internal combustion engine in a PHEV.

 The Argonne report analyzed the impact of operating PHEVs in California, which has a lower carbon grid mix than the rest of the country.  As shown in this graph, even in California, hydrogen from biomass (lower green line) produces less GHGs than a BEV operating in California (gray square at 155 g/mile with dotted red circle.)

Note also on the graph above that a fuel cell PHEV with an AER of 30 to 40 miles operating on hydrogen made from natural gas (red line labeled “SMR” for steam methane reformer) has lower GHGs than a PHEV operating on E-85 using corn ethanol, and all fuel cell PHEVs operating on hydrogen made from natural gas (red lines) have lower GHGs than a gasoline or diesel PHEV of the same AER.

Finally, this graph shows that in Illinois, with a coal intensive electricity grid, a series PHEV with 40 miles AER like the proposed Chevy Volt would actually increase GHGs compared to a conventional (non-hybrid) gasoline car (GV on the graph) using the lowest cost electricity “smart charging):

[4] Source for all graphs: Amgad Elgowainy, J. Han, L Poch, M. Wang, A Vyas, M. Mahalik, A. Rousseau, “Well-to-Wheel Analysis of Energy Use and Greenhouse Gas Emissions of Plug-in Hybrid electric vehicles.”  available at http://www.transportation.anl.gov/pdfs/TA/629.PDF