The Transformer – August 6, 2008

Plug-in hybrids:

A view from the grid

Vol. 5, No. 4 – August 6, 2008


Electricity-powered vehicles – particularly plug-in hybrid electric vehicles (PHEVs) — were in the news even before gas-pump prices shot above $4 per gallon. Toyota had already announced its intention to put a PHEV on the market in 2010, and GM, which just announced drastic rollbacks in its production of gas-guzzling models, and Ford won’t be far behind. Some small boutique customizers are selling PHEVs already.

PHEVs are a step beyond traditional hybrid vehicles such as the emblematic Toyota Prius. Much of their fuel comes directly from the electric grid (rather than a gasoline engine), and thus their extensive employment has significant implications for the electric power system.

This issue of The Transformer considers those implications. It draws on three sources:

  • A January 2008 presentation to the EUEC Energy and Environment Conference in Tucson, Ariz., by Jim Lazar, John Joyce and Xavier Baldwin. We thank them for permission to excerpt it.
  • An article in the December 2006 issue of Public Utilities Fortnightly by Letendre, Denholm and Lilienthal.
  • A July 2008 analysis by the Northwest Power and Conservation Council, “Impact of Plug-in Hybrid Vehicles on Northwest Power System: A Preliminary Assessment,” by Massoud Jourabchi.

‘Traditional’ hybrids and PHEVs

Increasingly common hybrid vehicles such as the Toyota Prius switch between gasoline-combustion engines and direct use of battery-stored electricity. The stored electricity powers the car at start-up and at relatively low speeds. The gas engine kicks in after a few miles and at speeds greater than about 25 miles per hour, running the vehicle and charging its relatively small batteries.

The hybrid’s advantage over a conventional car is threefold:

  • While in charging (higher-speed) mode, a hybrid’s gas engine runs steadily at its ideal efficiency since frequent speed and gear changes are unnecessary. It’s similar to being on cruise control.
  • When the car slows down, about half the braking energy is recaptured by the batteries.
  • In battery-power mode, the car runs on its stored electricity. The electric motor uses energy generated by a gas engine operating at maximum efficiency, which dramatically boosts miles per gallon, especially during periods of otherwise gas-gobbling acceleration.

Though the hybrid gets pretty good mileage, it is still basically powered by a gasoline engine. Plug-in hybrid electric vehicles, on the other hand, use gas only as a backup. Their primary fuel comes from standard wall sockets eventually connected to utility-size power plants that convert fuel to electricity more than twice as efficiently as car engines do. “Filling up at the plug” costs the gasoline-equivalent of about 45 cents per gallon. To take advantage of that cost savings, PHEVs are expected to come with larger batteries that allow them to go 30-50 miles on electricity alone before switching to gasoline.

Even compared to regular hybrids, PHEVs are a big environmental plus. According to the Council’s study, running on battery power generated by the mix of resources in the Western grid produces about one-third the per-mile emissions of gasoline-powered motoring. Even if the electricity came from a dirty coal plant, total emissions still would be only two-thirds of what a gas engine would produce! Of course, if the power comes from renewable resources, running on batteries computes to virtually zero emissions.

So PHEVs get great mileage, are cheaper to “fill up,” and substantially reduce global-warming emissions. Unfortunately, those bigger batteries are still very expensive and currently contain quite toxic components*.  Even with gas at $4 a gallon, the cars are not cost-competitive from a car-buyer’s perspective and probably won’t be for some time.

*Most hybrids use conventional lead-acid batteries that are too heavy to satisfy PHEVs’ greater storage needs. Lithion-ion batteries — like those used to power laptops — are being considered for PHEVs because of their lower weight-to-power ratio, but they are very expensive and also use toxic materials. A battery made from lithium-titanate, a much more common material, was recently tested for use in a utility-size, 2-megawatt system. Altair Nanotechnologies, however, says its technology is ideal for hybrid mass transit and PHEVs. All chemical batteries degrade over time from the chemical reactions that store and release power, and some take a long time to charge, though Altair tested its system with only 15-minute charge and discharge cycles.

So researchers are seeking alternatives. One involves using capacitors as batteries. Basically, a capacitor is two thin conducting sheets separated by a narrow insulating barrier. Positive and negative charges applied to the sheets are held in place by their attraction across the gap. By utilizing very large layered conductors folded into a small space, an enormous amount of electricity can be stored without the drawbacks of chemical batteries. Capacitors do not degrade over time and can be charged quite quickly. Finally, the raw materials are completely recyclable, non-toxic and far less costly.

Here comes the grid

But that’s not the only way to evaluate PHEVs. Let’s look at them from the perspective of the utility grid.

Utilities must make sure that generation supply exactly matches load second by second. When someone flips a switch, the power supply must rise or fall commensurately. The utility also must plan and instantly adjust for unexpected changes such as a power plant failing or a downed transmission line.

Utilities also deal with daily and seasonal load swings, and with intermittent renewable generation. They must have enough resources ready to meet peak needs on a hot summer afternoon when demand is twice its nighttime level. Keeping a lot of spare generation on hand to be used just a few hours a day is very expensive.

Growing percentages of intermittent wind power in a utility’s portfolio present another problem. Strong winds at night may generate more power than can be used when loads are low. And building transmission with enough capacity for the few high-wind hours is costly.

If a utility had a battery large enough to store unneeded energy produced at night (when power prices are low or there’s an overabundance of wind power), it wouldn’t need to keep so much spare generation and transmission capacity standing by for emergencies or peaks. But if not one big battery, how about thousands of smaller batteries — PHEVs — plugged in all over the grid? Most cars are parked about 90% of the time. All we have to do is plug them in and allow the utility to control when and how fast their batteries charge and discharge.

Day and night, night and day

Several scenarios flow from the confluence of PHEVs, wind power and smart grid technology.

Here’s a “normal” day scenario:

You wake up in the morning and jump into your PHEV, which was fully charged overnight with off-peak power that cost you about 5 cents per kilowatt-hour — equal to paying less than $1 per gallon of gas. You drive to work or to a transit park-and-ride and park in an electrical-outlet equipped garage, plug in your car and find it fully charged again when it’s time to head home. The daytime recharge cost you a bit more than the overnight charge, but you make the roundtrip entirely on electric power costing about half what you would have paid for a gas-fueled ride.

Here’s another kind of day:

Today will be hot, loads will be high and electricity will be more expensive than gasoline in the morning and afternoon. The grid sends your car a wireless signal telling it to use gasoline power for the morning commute to conserve your stored battery power.

You plug into the grid at work or at the garage. Over the next hour and a half, the grid draws all the stored energy from your batteries, crediting your account at the rate of 20 cents per kilowatt-hour — four times what you paid for charging it overnight. That also more than reimburses you for any gasoline you might have to buy while using your car that day.

At 10 a.m., a passing front boosts electricity production at your utility’s wind farm. The grid feeds the extra power to your PHEV, charging its batteries that much faster.

Then things get interesting:

Your PHEVs batteries are fully charged, ready to take you home, by 3 p.m. But at 4 p.m., a major generating plant fails. For the next hour, the grid uses your battery’s’ power as a form of spinning reserve, drawing on it to replace the failed plant’s power. It’s cheaper to pay you 20 cents per kilowatt-hour for your power than to maintain and start up a reserve generating plant. Your batteries are depleted when you leave work, so you drive home on gasoline.

As you drive, your batteries recharge to three-quarters capacity. You plug in at home around 5:30 p.m., and the grid draws down the last of your battery power before loads subside in the evening. Your account is again is credited at 20 cents per kilowatt-hour for this power.

Overnight, your car fully recharges on low-cost, off-peak power generated by higher nighttime wind and the high hydropower generation needed to help salmon migrate. (Salmon smolts tend to travel at night when temperatures are lower and when they are less visible to predators. Increased nighttime flows get them downstream more safely and generate more power.) You’re ready for another day.

Why bother?

The Council study assumes that by about 2030, 25% of the region’s cars and small trucks will be PHEVs. That’s about 2.5 million vehicles. The Letendre et al model assumes twice that many. Both studies find vehicles charged mainly during off-peak hours, with virtually no effect on peak use. No new generation or transmission wires would have to be built to meet the new demand. Rather, existing capacity would be used more efficiently — itself a cost savings. Total energy use would increase by about 500-1,000 average megawatts respectively, or 2.5-5% of regional load (assuming energy efficiency efforts hold load growth near zero).

The greenhouse-gas emission reductions would be enormous. Supplying power to 2.5 million PHEVs would increase grid CO2 emissions by about 4 million tons a year if supplied by natural gas-fired generation; less (or none) if supplied with renewables. At the same time, annual vehicle emissions would be slashed by about 12 million tons. The net reduction would be equal to closing down three 400-megawatt conventional coal plants.

So, according to the studies, PHEVs allow you to run your car on the equivalent of 45–cent gas, wean the country from dependence on imported oil, and combat global warming. PHEVs will increase electric loads by a few percent, but other utility benefits are quite substantial. Integrating thousands of PHEVs into the grid would save utilities enormous amounts of money since they wouldn’t have to keep large amounts of reserve generation on hand for emergencies and peak demand periods. They could easily integrate huge amounts of wind and other intermittent renewables at much lower costs. Finally, the need for hydropower generation adjustment (ramping up and down to follow changes in loads), on which our region depends for grid flexibility, could be reduced, making rivers friendlier to fish.

The National Renewable Energy Laboratory recently estimated that utilities could pass along savings of $1,000 to $4,000 per year to PHEV owners who allow remote control of their cars’ charging patterns. This benefit would easily overcome the extra costs of manufacturing PHEVs, making them more affordable to the average consumer. Letendre et al come up with similar figures: $499 to $3,285 per car, per year. (The higher savings accrue to cars plugged into 220-volt rather than standard 110-volt outlets. Greater voltage quadruples the speed of car battery charging or discharging.)

Making it real

The automobile and electric power industries can realize the benefits of spinning reserves and the ability to deliver hundreds of megawatts into the grid with some adaptations of existing electronics and communication networks. The components include:

  • A grid-control system that tracks the real-time cost of electric power, as well as system reserve requirements and the status of all generating units.
  • An automobile charging system that can use power line, cellular or wireless networks to report its state of charge and receive charge or discharge orders.
  • Plug-in locations at work, park-and-ride stations and at home to handle the power transfers.
  • A communication and accounting network linking the grid to millions of automobiles.

Cellular companies already track the minute-to-minute locations of more than 100 million wireless phones. They deliver calls, email messages, stock prices and sports scores to many of those phones and can bill their customers for the amount of service each phone actually uses. For prepaid phones selling for as little as $20 each, the companies keep track and communicate exact account balances in real time, automatically charging extra for roaming services where applicable.

Adapting this technology to communicate between the grid and millions of vehicles is no more complex than embedding a wireless phone in the vehicle. The communications/power transfer system can easily operate across utility service territories, allowing drivers to charge their cars at home using one utility and discharge at work to another.

Housing large numbers of PHEVs in a parking structure or other single location requires proper sizing of the facility’s electrical system infrastructure to accommodate the magnitude of power transfers. In addition, utilities will need to work with local governments to support residential street parking plug-in options and multi-family parking garages.


Smart meters, smart cars, smart utilities and smart consumers can work together to strengthen our economy, reduce pollution – particularly greenhouse gas emissions — and make our lives more comfortable.

Environmental advocates specializing in transportation issues will rightly note an 80-85% or larger cut in that sector’s global-warming emissions by 2050 (the amount indicated by the best consensus climate science) cannot be achieved without drastic reductions in individual vehicle miles traveled. Eventually, the car culture must give way to convenient mass transit, increased urban density, walking and bike riding.

In the interim, a wholesale switch to electric and hybrid-electric vehicles yielding average fuel efficiencies of greater than 100 miles per gallon must be considered. Some will argue that making cars more efficient and less polluting will reduce societal incentives to revolutionize transportation, but it’s hard to simply dismiss the potential economic and environmental benefits of a smart grid working with smart plug-in hybrid electric vehicles.

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