Monday, December 14, 2009

Powered Roadways

If it weren't for the battery problem, an electric vehicle could be a fairly reasonable vehicle today: electric motors are powerful, small, and cheap enough, etc. There are two parts to the battery problem, getting enough power and storing enough energy. Both can be solved by delivering power to the EV from the road. This has been considered many times before, and there are electric street cars that do it in Bordeaux.

The usual idea, however, is very expensive because
  • most sections of roadway or railway see little traffic and so the benefit of the high-cost infrastructure is spread over few vehicles.
  • the third rail power delivery system is made safe by expensive grade separations, fences, or electronic switching
  • the vehicles are very specialized and aren't built in large numbers.
So, to the extent that I'm advocating a new idea, it is... electrify urban highways. By which I mean, install a pair of metal strips flush against the paving in one of the lanes, such that ordinary cars aren't affected, but electric cars can lower a suitable pickup onto the strips and receive a few dozen kilowatts. Both the power and the energy storage demands on an electric vehicle's battery are dominated by the demands of highway operation.

Urban freeways are quite unusual as roads go.
  • They see quite a lot of traffic: The San Francisco Bay Bridge moves 270,000 vehicles a day across 10 lanes = 1 vehicle every 3.2 seconds over a 24-hour average.
  • They see quite a lot of the traffic: 24% of all traffic is on interstate highways, and I'll guess that most of that is on urban interstate highways.
  • They are short: in the entire US, there are just 15,300 miles of interstate highway in urban areas (same link)
  • They already have limited access. The safety hazards of a high-voltage electrical system out on the road are relatively minor compared to the existing vehicles using the roadway.
I don't have sources to verify the following claim, but I'm fairly sure that in many urban markets, most trips over 10 miles include some freeway. So, if the freeways in your urban area (e.g. Los Angeles or the Bay Area, or both) were electrified, you could make trips anywhere in that area in an electric car with a battery range of just 10 miles.

And, if all we want is 10 miles of range, with no need to deliver dozens of horsepower for minutes on end, existing battery technology is good enough.

The usual counterargument to big infrastructure is that it costs too much. However, electrifying freeways does not because the freeways are just not that big. Consider the Bay Area:
  • 300 miles of freeway
  • 7 million cars
If, over 10 years, we got to 10% of the cars being EVs with electric pickups, that would be 700,000 EVs, or, one for every 26 inches of freeway. Can 26 inches of roadway be upgraded for less than the cost of equipping an EV with a huge battery? Definitely.

There are many schemes for electrifying roadways, and some are quite complex. Although a simple pair of flush steel rails at 1000V is probably a reasonable implementation, it might be easier to sell to the public if the rails were safe enough to be touched by hand. This too is possible.

Imagine each rail is a hollow box, insulated on three sides, but with a nonmagnetic and top surface. Inside the box is a lightweight, magnetic, conductive cable (probably aluminum and steel), with a small gap between it and the underside of the top surface. The conductive top surface is broken at short regular intervals by an insulator. The bottom of the rail is probably a large conductor for moving electricity thousands of feet.

Without a magnet, the top surface is not electrified, and you can place your hand on it safely. The car's pickup would have a magnet which would ride on the strip, picking up the lightweight inner cable slightly, so that it contacts the top surface and conducts to the car.

Other variations would have a magnetic top surface with a flux gap, such that flux going from one side to the other shorts through the cable and picks it up that way.


  1. Rain, Mud, Snow, Ice, and Debris make ground sourced electric power difficult to do reliably. But an overhead wire and a grounding strip in the roadway routed along one lane of the interstate highway system could go a very long way toward reducing our country's diesel fuel usage. A single wire contacted with a pantograph is well developed technology. A grounding strip in the roadway is pretty safe because electrical potential remains low. If the connection is intermittent due to snow, ice, dirt, etc., perhaps the trucks can ride over the gaps using their battery power. On average, the ground wire will have some heating which should improve the ice and snow problem, but aggravate the mud and dirt problem. I wonder how bad the wear problem is for the sliding contact on the roadway. Our freeways see a LOT more vehicles than the typical rail route. I can imagine these things wearing out in weeks or months instead of decades.

  2. Perhaps I'm wrong, but my sense is that a pantograph on a car capable of reaching a wire that can clear a semi is going to be large enough to change people's conception of what a car is. And I suspect that the overhead of changing that conception is higher than the overhead of crud on the in-ground rails.