Richard Gilbert and Anthony Perl. Transport Revolutions: Moving People and Freight without Oil [2nd edition]. Earthscan and New Society Publishers, 2010.
As you’ve probably heard by now, the world is starting to run out of readily-accessible oil, and most rational predictions are that oil prices will continue to rise to reflect the increasing difficulty and risk involved in pursuing new supplies. How will that change our transport system? What kinds of change are needed? What technologies most urgently need research? And who will lead these changes?
Transport Revolutions: Moving People and Freight without Oil, is one of the most through analyses of this problem available to the general reader. While plenty of frightening books on this topic are available, Transport Revolutions is an essential counterpoint: it takes the kind of gentle and optimistic tone that you’d use to coax a suicidal friend off the ledge, or for that matter to pry anyone off of long-held but doomed opinions.
But the book goes further. The message is still that we need a revolution, but not quite the one that many of us have in mind.
Car-based thinking is so dominant in the US that attention has focused mostly on ways to solve the problem that let us still have lots of cars and not much limit on where we can drive them. These solutions include biofuels, diesel-electric hybrids, various kinds of battery technology, and hydrogen. One by one, Perl and Gilbert knock down all of them, predicting that each will develop but remain marginal to the real scale of the problem.
It looks pretty clear that the long-term answer is some form of electric motor, both because electricity can come from sustainable sources and because electric motors are highly efficient and give off little or no noise or pollution. But Perl and Gilbert argue that the crux of the problem is not sustainable power generation, but rather the means of storing energy for portable use; in other words, batteries. And the hard fact is that there just isn’t a battery, or even a sound technical basis to hope for a battery, that approaches the efficiency of internal combustion.
The crux of the crux is that petroleum has a very high energy density, which can be thought of as the energy stored divided by the mass required to store it. Batteries, by comparison, are just too heavy compared to the amount of energy that they can deliver, so when you put them in a mobile vehicle, they lose a lot of their efficiency to the work of transporting their own weight, and don’t have much left over to transport us or our cargo.
All this is carefully explained, and leads Gilbert and Perl to a striking conclusion: We will need to shift most of our mechanical transport to “grid-connected vehicles” (GCVs), vehicles -- like trolleybuses and electric rail lines – that can draw power from the grid continuously (and increasingly, return surplus energy back to the grid as well). At first, this may sound like a revolution in the direction of passenger transit, and much of it is. Gilbert and Perl call for high-speed rail replacing intercity aviation, growth of urban public transit, and so on.
But their vision goes further, to a network of “grid connected” roads, and a crucial moment, this vision turns into something that looks to the authors like Personal Rapid Transit (PRT).
Here’s the transition:
One [possible pathway toward implementation of a GCV-based land transport system] is via the plug-in hybrid car … Extensive operation of such vehicles could lead drivers to want more use of their electric motors. To facilitate this, governments or entrepreneurs could provide means of powering htem along major routes, accessible by appropriately equipped vehicles while in motion. When such en-route powering is sufficiently extensive, [electric vehicles] with only batteries and retract able connectors could prevail over plug in hybrids. As the grid-connection expands, the needf or off-grid movement would decline. Roads could be supplemented and even replaced by lower-cost guideway infrastructure. At the same time, vehicles would evolve to move only on the guideways. They would be as light as possible and, where appropriate, be assembled into trains. They would comprise PRT.
Another pathway could involve the evolution of public transport toward supplementation of or even replacement by PRT. This could be driven by PRT’s low energy cost and, perhaps even more, by its potentially low infrastructure cost. … An analysis for Corby, UK, compared costs of PRT and lighter ail. For similar initial investment, operating costs, and fare structure, PRT would carry almost twice as many passengers annually …
The second of these paths is almost certainly an illusion. You can do all kinds of comparisons of existing transit technology with proposed PRT technology, but none of these changes the physical fact that PRT requires carrying people in more vehicles, which means hauling around more metal per passenger. If higher oil prices caused a huge shift in demand to some form of automated transit, PRT would have to move more metal per passenger than conventional transit would do, and that would almost certainly be the decisive factor in its ultimate energy-efficiency. (An argument against this point would need to show that PRT is so dramatically lighter in weight than lightweight conventional transit [e.g. Vancouver’s SkyTrain] that it ends up moving fewer tons of metal per passenger. And this comparison would have to be against lightweight rapid transit alternatives. If anyone has made that argument, please point me to it.)
But the first path is probably worth contemplating at least as a thought experiment. Suppose we keep our current levels of car use, but gradually convert to grid connected cars. I guess we’re to imagine giant catenary systems above every street and highway, and something like trolley poles or pantographs on top of each private vehicle. However much they might love the freedom of the open road, motorists with hybrid vehicles would be motivated to connect to these catenaries at every opportunity, because their off-wire power options would be so expensive.
At that point, all we need is an automated system to combine cars into “trains” and we have PRT. I suppose we could arrive at that route, but this is so unlike the station-based PRT being proposed today that I’m not sure the term is even useful in understanding it.
[One nasty technical detail: grid-connected vehicles on tires need to route both directions of the electric circuit through the overhead catenary. That’s why trolley buses have two wires, while overhead-powered rail has only one. With two wires, you have to connect to them with poles, because the two sides of the circuit have to be kept apart. Trains, by contrast, are grounded through the rails and therefore need only one wire above. That means trains can meet this wire with a large horizontal structure -- called a pantograph -– which easily accommodates lateral motion. It’s a fine joke, really, by the technology gods: The power source that offers the most lateral flexibility works only on rails, which have the least need for it.
So grid-connected cars, if they run on tires, will need a power-supply system that supplies both current and ground from above – like the double-wire of trolleybuses. This is a fairly delicate technology even when all the drivers using it work for the transit agency, and even then the buses can’t pass each other. Will someone invent a system for grounding all of these private “trolley-cars” to the earth, maybe with some kind of continuous metal strip in the pavement that the car continues to touch? That would allow for zero lateral motion as well, so doesn’t really solve the problem. At times like these I’m so glad not to be an engineer!]
To sum up, I think we need to be reluctant to derive “PRT” from this path, because the result would be so unlike what anyone today means by PRT that we are only creating confusion by using the term.
I have delved on PRT because it’s a topic of interest on the blog, but the larger import of Gilbert and Perl’s book remains fascinating and timely. If the future lies in grid-connected vehicles, transit agencies should be getting out ahead by expanding trolleybus systems, and certainly not removing them.
Trolleybus infrastructure may be one of those investments that would make total sense in a world that correctly priced its carbon impacts, and other externalities. So long as the market is distorted, though, government simply has to override those market considerations in its own purchasing and planning decisions. Yet another reason that government, not just private industry, will need to drive this next “transport revolution.”