[RedDevil]

Originally Posted by Blue Angel

... The very act of compressing a car's suspension system requires energy, because dampers generate heat (resist motion). The more we reduce suspension movement the less energy we waste. ...

That's close but the culprit is the damping, not suspension travel per se.

Compressing a car's suspension system require energy both to overcome the spring load and the resistance from the shock absorbers.
The spring will release most of that energy when the wheel moves down again. The damper will resist the outgoing movement and waste more energy doing that.

The reason reducing suspension movement wastes less energy is by and large just because of the shock absorbers. Lose the shocks, then the suspension can travel without adding (much) resistance.
The Australian solar racers had a working suspension system with springs, just no dampers.

Obviously, that compromises safety and comfort. Having no shocks is no option for real cars. That's why Ferrari, Audi and many others use shocks with variable resistance; low when dealing with minor bumps (where the shocks added friction would reduce comfort) and stiffer when the road or the handling roughens.
By operating the shocks in a smart way they can prevent energy from getting wasted instead of trying to gain some of the wasted energy back.

Just varying the rigidity irrespective of the actual movement of the wheel and car is a bit crude.
Those systems should switch rigidity quickly enough (maybe by using a solenoid switch in the damper fluid channels) to dampen just those movements that would increase body roll etc. while allowing for the adverse movements.
That would be a kind of holy grail; better comfort, lesser resistance, better handling all marching together while no outside power is required (apart from operating the switch).

A full active suspension would go even further than that; actively pulling the wheels in at the upstroke and pushing them down on the recurve, such a system could work as a propulsion system on its own as long as the road is choppy. But it would be a very expensive way of propulsion.

Going back to the original idea of reducing travel in the suspension system; that's what stiffer springs would achieve.
Unfortunately, stiffer springs always come with bigger rims, wider tires with tiny sidewalls, tons of carbon decals and >2000W audio sets, making it hard to measure any gain in FE.
And maybe the stiffer springs would also require stiffer shocks.

Another approach is to make sure the wheels are as light as possible. While the spring load stays the same (as the rest of the car keeps the same weight) the lighter wheels make that lighter shocks may provide enough damping where they would underdampen the OEM wheels.

Maybe combining the two would be best; e.g. stiffer springs, lighter wheels, original shocks travelling less. Obvoiusly such a setup would reduce the effect of regenerative shocks, though the combination might still work.

My favourite remains the solenoid operated shocks; while not producing power they would reduce resistance to an extent that cannot be topped by regen shocks, and improve other characteristics to boot. Building the logic to operate that solenoid and its fail safe system make it a project for the experts only.