The really interesting part is that some of this energy appears to be free. If you're walking at 5.6 km/h and carrying a 29 kg load, the power input to the pack is 12.5 W. Humans operate at about 25% efficiency but when you measure the delta in energy consumption (using O2 consumption) it's only about 19 W, which is a lot less than you expect. It's not clear exactly what's going on but it appears that the sprung pack is giving the wearers some sort of biomechanical assistance--the gait is different when the wait is sprung than when it is locked in place on the pack.
Other interesting points in the paper:
- Soldiers carry an incredible amount of batteries. According to this article claims that soldiers in Iraq are carrying an average of 20 pounds of batteries each, almost a third of their load!
- The energy density of food vastly exceeds that of batteries. According to Rome et al., the density of food is 3.9x10^7 J/kg whereas zinc-air batteries are only 1.1x10^6 J/kg. Even factoring in the inefficiency of human energy production, eating and then running a generator is still a lot better than batteries.
Of course, 20 kg (let alone 38 kg) is a fairly heavy pack, and this is just for the power generating system. You've still got to carry the rest of your gear. But lots of devices need far less than 6 W, so you could probably get away with a lighter load and generator. If you could get this down to 10-15 pounds total, that would be pretty interesting.
Presumably, the load plate could be replaced with the actual rest of the stuff you're carrying anyway -- you only need the "extra" weight of the rest of the setup, which needn't necessarily weigh very much at all if it's all hollow carbon-fiber tubing or something.
Yeah, that's probably true. Though the balance of that might be a little weird, since a lot of packing a pack is dealing with the variable density of the materials, a problem which may be even worse if they bounce up and down....