Piezoelectric Generation in Roadways
Regarding my response on piezoelectric generation in roadways, Nick writes:
I had no idea they produced such a small amount of energy though I did realize they were expensive.
Well, I haven’t done the math, but just following the broad strokes associated with the conservation of energy, 80% of the chemical energy in gasoline is wasted as heat and mechanical losses in the drive train itself; the other 20% is converted to kinetic energy to move the car. Exactly how much is lost to compressing the road (and recoverable) as piezoelectricity? I don’t know, but it can’t be much.
I’m reminded of people who think they’re going to recapture the energy that is currently lost as heat in shock-absorbers — i.e., the vertical motion of a car as it moves along — via regenerative braking. That’s just as preposterous. You’re talking about a minute amount of energy. Note that shock absorbers don’t even get particularly hot. Terrible idea.
Actually, under some conditions, shock absorbers can get hot enough to thin out the shock absorber fluid thereby reducing the effectiveness of the shock absorbers. For that reason, some contain fluid that is less prone to temperature-induced viscosity changes.
But you’re right; as a source of energy, shock absorbers would not be practical.
Here is something proposed to convert waste engine heat into power:
http://nextbigfuture.com/2012/09/commercializable-thermoelectric.html
Even with that, I’m a bit skeptical. There is considerable heat rejected to the cooling system and exhaust, but if only a maximum of 20% can be recovered as power, it might not be very practical. I suppose that the waste heat could be used to air condition the car, using an absorption cycle, but the size and weight would probably make that impractical too.
Hi Craig
In the article I read about piezoelectric http://www.gizmag.com/piezoelectric-road-harvests-traffic-energy-to-generate-electricity/10568/
They were expecting to generate about 400 kilowatts per 1 kilometer of dual carriage way. Which I find hard to judge because there surely must be other factors to consider eg how many cars must pass over this 1 km stretch of dual carriage way to produce 400 kilowatts of electricity but it seems like a good idea it is just a pity it is such an inefficient system
Craig,
I agree with you that piezoelectrics are a terrible idea for roadways, and it’s unlikely that they will make a large impact.
However, I think that for walkways in heavily traveled areas, there is some promise for some renewable generation that might help people remain more fit.
If a subway system had sign sup saying “please feel free to move around, every step you take provides the city with carbon-neutral electricity… People would do a lot more milling around between trains; that would improve their health and yield cost-effective kWh’s from the install costs. It might be small, but it’s also cheap compared to solar panels (which also give very small amounts of energy per square meter).
The key would be installing the systems in extremely heavily traveled areas – like New York’s Grand Central Station or the footpaths on the Washington Mall.
The reason it’s a tragically stupid idea for roadways is that the energy that is converted can only be derived by adding additional energy demands on the vehicle. The roadway would have to depress slightly, requiring some elevation increase (granted it’s only a few mm, but it’s there nonetheless) with every rotation of the wheel – kind of like adding an incline to a treadmill. The additional energy that is required from the car is the only energy that could ever be available to be derived from the piezoelectrics… So you’re forcing a vehicle engine to exert extra power that you are then trying to inefficiently recover.
Not smart.
However… forcing human travelers to exert extra energy that you are then inefficiently recovering has multiple benefits, and is something that is probably cost effective in some situations – especially in northern climates, as the plastic walkway tiles would be less prone to thermal stress than concrete sidewalk slabs, so decreased maintenance costs over the years would be factored in to the total system cost/benefit analysis.
Still small potatoes, but I’d bet it would be more cost effective overall to put piezoelectric tiles on a New York City sidewalk than it is to put PV panels on a rooftop in one of New York’s suburbs. (Of course, as you know, I’m not really a fan of PV solar outside of the Sunny Southwest or high altitudes.)
Aren’t there some heat and mechanical losses between car and roadway that occur anyway (i.e., that don’t cause the car to move slightly uphill) and could be recaptured?