Published on November 13th, 2007 | by Stephanie Evans

PHEVs Revving the Hybrid Future

If you’re on the lookout for the Next Big Thing in green cars, you might be thinking about plug-in hybrids—especially if you’re of the opinion that conventional hybrids are “so last year.”

Plug-ins are something to ooh and ahh over while waiting for electric vehicles to go from on-again, off-again, back to on-again.

Bigger, Longer, Better

Plug-in hybrids are a bridge from conventional hybrids to fully electric vehicles. Also called PHEVs (plug-in hybrid electric vehicles), plug-ins are so named because their batteries can be charged from the grid. At the same time, they still contain a gas tank and an internal combustion engine—hybrid features distinguishing them from fully electric vehicles.

Because a gas tank complements the battery, PHEVs have a much longer range than electric vehicles, one that is comparable to conventional vehicles. Need to see your Aunt Maybelle in the next state over? No problem, just hop into your PHEV and go—if you find yourself running out of battery, just keep going on gas.

The significant difference between PHEVs and conventional hybrids lies in the larger battery, which enables a PHEV not Plug-In Hybrid only to operate in all-electric mode, but to operate in that mode for a significant distance. Even today’s full hybrids that can operate in all-electric mode only use it for extremely low speeds of around 10 mph, and therefore, only for very short times (though that makes rush hour a prime opportunity). All-electric operation is so limited because the batteries in conventional hybrids are too small to hold enough charge for more than a few feet of electric-only drive. PHEVs, on the other hand, use a larger battery so that all-electric mode can last for several miles—depending on the size of the battery, which can range from 10 to 70 miles.

The Best of All-Electric

There are several benefits to all-electric operation on the order of miles instead of feet:

Fuel Economy. Not only do you avoid gasoline consumption during all-electric drive–the gas-powered engine can operate closer to its peak efficiency. With a larger battery, the electric motor can handle the majority of low power needs, where the internal combustion engine is at its most inefficient and where the electric motor is remarkably efficient. Depending on your battery and your driving habits, you could be looking at a mileage on the order of 100 mpg. Pretty swell, eh?

Emissions. If the battery is sufficiently large and your trips are sufficiently short, you probably won’t be using your gas-powered engine very much. It won’t have to start up every time the light turns green or you pull out of a turn. Our conventional gas-electric hybrids article cites that controlling emissions is a challenge with hybrids because most pollutants are formed when the engine first starts up, which happens a lot in hybrids. However, PHEVs won’t be afflicted with this problem as severely, so emissions will be easier to control. You can expect that on many short trips, you won’t be producing any tailpipe emissions at all.

PHEVs, however, don’t necessarily emit less carbon dioxide—but they probably do. Even though all-electric drive merely diverts greenhouse gas emissions from tailpipe to power plant, don’t discount the power of economies of scale, centralized control, and energy source flexibility. Large power plants benefit from economies scale and rigorous pollution control. At the same time, using electricity instead of liquid fuel allows flexibility in the source of the power. The electricity could come not only from coal, but also nuclear, wind, water, and a combination of other sources. Even better, it could come from your very own rooftop solar panel! A car that can use electricity frees you from the grid and simultaneously provides you with another way to use the grid.

V2G Energy Transfers. Speaking of the grid, another interesting benefit of PHEVs is their potential to perform vehicle-to-grid (V2G) energy transfers. Since PHEVs are basically giant (but mobile) batteries, they can store energy from the grid during off-peak hours (the night) and then sell that energy back to the grid during peak usage hours (the day). Here’s a breakdown:

During the day, humans are a lot more active, so we use a lot more power.
At night we use much less power, and as a result, many power plants simple idle because it takes too much time to shut them down and start them up again.
Rather than wasting that power, the grid can charge up vehicle batteries at night, and those batteries can then return the power back to the grid, taking care of some of the high demand during the day.

Moreover, PHEVs can load balance. The electrical grid is always in perfect, incredibly delicate balance—there is never too much or too little electricity—and the exact amount you need is the exact amount produced. If that balance is upset, you get a brownout or a blackout . . . or worse. But the demand for electricity, termed the load on the grid, is constantly changing because your neighbor flicks on a light switch, and two milliseconds later the guy across the street flicks on his switch, and then two minutes later you turned off your computer. Just think about how much change is happening on your block. In your city. Across the country. Such volatility is the bane of an electric utility, and load balancing refers to efforts at smoothing that volatility out. PHEVs can do so by using electricity during dips in load and providing electricity during spikes.

Long Range. PHEV benefits thus approach those of electric vehicles, but because they’re still hybrids, they don’t suffer from the most serious shortcomings of all-electric vehicles: short range and long recharge times. Because a gas tank complements the battery, PHEVs have a much longer range than electric vehicles, one that is comparable to conventional vehicles. Need to see your Aunt Maybelle in the next state over? No problem, just hop into your PHEV and go—if you find yourself running out of battery, just keep going on gas.

Recharging. For the same reason, PHEV owners don’t need to worry about recharging their batteries either. You’ve had a long day at work, stumbled into the house, zoned out in front of the TV, and . . . next morning realize you completely forgot to charge the car. No problem—run on gas instead. It’s a better-looking day already.

Of course, the less driving you do on gasoline, the better, but the point is, you don’t have to rely on the battery alone—you’ve got backup.

Getting One

All right, you’re thinking, they’re better than conventional hybrids and electric cars, so where’s the nearest dealership? Sorry to say, but that’s the problem. PHEVs sound great on a conceptual level, but production vehicles are still a few years away (actually, some were produced in the past but they were discontinued). The problem, as with electric vehicles, is the battery technology. PHEVs demand more power and capacity, as well as resilience, in the face of deeper discharges. That is, the battery’s lifespan must not suffer unduly from frequent deep discharges that usually have significant negative impacts.

The good news is that engineers are almost there! Many major automakers have announced plans to dive headfirst into the PHEV pool—some are getting permission to do road tests, some have built concept cars, and some are keeping their plans pretty much under wraps.

Most manufacturers seem to be placing their bets on lithium-ion battery technology (in contrast to the nickel-metal hydride batteries used in many conventional hybrids today). Lithium-ion batteries have a very high power-to-weight ratio, which means they have great bang for their buck. It’s important to keep battery weight down, especially when we’re talking about so much capacity, because increased weight compromises efficiency (remember, small cars generally come with better fuel economies—because they’re lighter—and you’d be shocked at how quickly weight goes up with battery capacity). Lithium-ion batteries are also not too expensive and economies of scale will probably drive down their prices further. Lithium-ion batteries are also recyclable!

Conversion Options

So things are looking up, but you want a PHEV now—who wants to wait five years? For the impatient, the adventurous, and the do-it-yourselfers, there’s the conversion option—there are several commercial companies that provide conversion services. In general, they either replace the battery pack in an existing hybrid or add a supplemental battery pack to a conventional hybrid. Ideally, they would also reprogram the software to take advantage of the added capacity and manage the bigger battery pack—heat management, for example, is very important to maintaining lifespan as best you can and to charge-discharge cycles.

Note that most conversions are performed on Toyota Priuses and not on many other models, due to the drivetrain architecture. While a drivetrain discussion is outside the scope of this article, you are free to research this fascinating aspect of hybrids yourself. The Prius uses a series-parallel drivetrain, while most other hybrids (mild hybrids without all-electric ability that do not use Toyota’s technology) use parallel drivetrains that do not lend themselves easily to plug-in conversion. Series drivetrains are not usually found in passenger hybrid vehicles, but are, theoretically, amenable to plug-in conversion.

In addition to commercial companies, there is also an open-source community that provides specific instructions on performing conversions yourself. Their nuggets of wisdom can be accessed at the Electric Auto Association’s PHEV wiki.