[Slugsie (OP)]

Frequently whilst stumbling around he interwebs (damn you StumbleUpon, you've sucked away so much of my spare time) I come across sites touting various energy recovery systems that are supposed to herald a revolution in green energy, but something always strikes me as being off. However as I'm not hugely well versed in these matters I'm never quite sure if my objects are valid or not.

Basically the systems usually come in one of two forms:
(1) 'Active' surfaces that generate electricity when something passes over them - e.g. a floor that generates power through the pressure of people walking across it using some form of piezoelectric crystals or similar
(2) Wind turbines along roads that collect power from the displacement of air by passing vehicles

With both systems my objects are the simple principle 'You don't get something for nothing.' With (1) it strikes me that each person (for example) walking across that surface would have to exert slightly more force than they would walking across a normal floor, thus they have to burn a few more calories (not necessarily a bad thing mind you), thus you will eat more, thus more food needs to be grown and transported etc etc. With (2) it seems that the turbines will cause the air to move less and any following vehicles will have to work harder to punch through the air, thus consuming more fuel amongst other problems.

It just seems to me that whilst whoever owns and operates such systems will undoubtedly be getting power out, the general population passing through will have to be putting in even more power than they normally would, and that there will overall be a net loss i.e. more extra power going in than is generated on the way out. Thus it would actually be more efficient to just directly link a crappy petrol powered engine directly to a generator and harness that power directly.

I know I'm probably grossly simplifying things, but am I basically right or wrong?

TIA

[CliffordK]

You are right that there isn't any true "free energy".

I was thinking that many systems such as Shock Absorbers are designed to dissipate energy.  However, with your shoes.  While it is nice to have squishy soles, you may get some of the energy lost in compressing the soles as they spring back.

There are new "exercise shoes" that are supposed to be walking on sand.  Sand has the give, but doesn't have the spring back, and thus can take significantly more energy to walk on.  Perhaps theses systems are similar to the sand, absorbing energy, but not springing back.

Enough people in the USA could use a couple fewer pounds, that it may not be a bad idea anyway. 

[Slugsie (OP)]

Thanks for the reply.

I think my main point is that whilst there will be a localised power generation effect, the overall nett effect is that more energy will need to be generated, and that energy will likely be in the form of a fossil fuel, and thus overall the system is in fact less green than it was before.

[Geezer]

I believe you are correct. Cars and people convert chemical energy into work, so the more work that has to be done, the greater the expenditure of energy (the conversion process is usually nowhere near 100% efficient.) The human walking case requires the person to do extra work.

The car/windmill case is a bit less obvious, because you could say that the work done to produce the turbulence was going to waste anyway. The counterargument is that if the vehicle was more aerodynamic, there would be less free energy available.

Both of these cases do dump heat energy into the environment which is a byproduct of the chemical conversion. That essentially is "free" because the systems have to dissipate it in order to work properly. If we could recover that thermal energy effectively, that would be free. We can do that sometimes with the heat people contribute to buildings for example.

Unfortunately, at the moment, it's not really practical to convert "low grade" heat into a more useful energy source like electricity.

[CliffordK]

The car/windmill case is a bit less obvious, because you could say that the work done to produce the turbulence was going to waste anyway. The counterargument is that if the vehicle was more aerodynamic, there would be less free energy available.

You have heard about the faster-than-wind propeller powered car?

http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/

However, ordinarily propellers on a car would be a bad idea. 
Power Generating shock absorbers would probably be functional, but likely would not be a large amount of energy.
http://www.technologyreview.com/energy/25274/?a=f
The article suggests a 1.5 to 6% fuel savings (most for heavy off-road vehicles).