What is seen
Here’s the start of a recent article at Gizmag about a Mexican inventor’s scheme to harvest energy from passing cars.
Over the years, various researchers have developed systems in which the weight transferred through cars’ wheels onto the road – or through pedestrians’ feet onto the sidewalk – is used to generate electricity. These systems utilize piezoelectric materials, which convert mechanical stress into an electrical current. Such materials may be effective, but they’re also too expensive for use in many parts of the world. That’s why Mexican entrepreneur Héctor Ricardo Macías Hernández created his own rather ingenious alternative.
In Macías Hernández’ system, small ramps made from a tough, tire-like polymer are embedded in the road, protruding 5 cm (2 in) above the surface. When cars drive over them, the ramps are temporarily pushed down.
When this happens, air is forced through a bellows that’s attached to the underside of the ramp. That air travels through a hose, and is compressed in a storage tank. The stored compressed air is ultimately fed into a turbine, generating electricity.
What is unseen.
I’m not the first person to make this point about this type of scheme, but what Senor Macías Hernández’ system does is to steal a small bit of energy from each car that passes it. Ultimately, his system is powered by whatever is powering the cars – and petroleum is still a pretty safe bet.
Further, Macías Hernández’ system generates electricity through the rather cumbersome process of burning the refined petroleum in small-scale internal combustion engines (at 25 – 30% efficiency) to propel the cars over an air pump in the pavement (and I’d like to see how efficient that process is) to compress air to drive an electric generator.
Mr. Rube, meet Mr. Goldberg.
I think several factors probably go unseen in this picture.
1. There’s a cost imposed on the drivers of the cars. It’s probably so small that it would be difficult to measure but, still, it’s there. Otherwise, there’d be no energy to harvest.
In effect, there’d be a barely visible tax imposed on drivers using roads with this system in place. It wouldn’t be difficult to make a reductio ad absurdum argument showing a measurable result. What would happen to your car’s fuel economy if you had to drive over one of these air pumps every block or so?
2. What are the construction and maintenance costs of the recovery system? Unfortunately, the Gizmag article doesn’t give any figures for expected construction costs (despite calling it a "low-cost system") nor does it address the cost of maintaining the system.
Once the politicians have got a Green buzz on from sponsoring such a project, will they still be willing to vote for maintenance money after the buzz is gone?
How long will it take to pay back the capital investment? What will the cost be for a megawatt-hour generated from the compressed air? How will that cost compare to the market price for electricity?
3. What’s the opportunity cost of this system? Since it’s built into roads, my guess is that city, state or national governments (the typical road-builders) will be the agents building this thing. What else might be done with the taxes that would fund the building of Senor Macías Hernández’ system?
If a government agency wants to generate & sell electricity, doesn’t it make more sense for it to build a generating station and burn the petroleum directly instead? Of course it does.
Note: I have no problem with people producing petroleum nor with people burning it. The unseen problem here is not the use of petroleum in itself.