Through the use of a cam and piston or ratchet and flywheel mechanism, the motion of the heel might be converted to electrical energy through more traditional rotary generators. The efficiency for industrial electrical generators can be very good. However, the added mechanical friction of the stroke to rotary converter reduces this efficiency. A normal car engine, which contains all of these mechanisms and suffers from inefficient fuel combustion, attains 25% efficiency. Thus, for the purposes of this section, 50% conversion efficiency will be assumed for this method, which suggests that, conservatively, 17-34 W might be recovered from a ``mechanical'' generator.
How can this energy be recovered without creating a disagreeable load on the user? A possibility is to improve the energy return efficiency of the shoe and tap some of this recovered energy to generate power. Specifically, a spring system, mounted in the heel, would be compressed as a matter of course in the human gait. The energy stored in this compressed spring can then be returned later in the gait to the user. Normally this energy is lost to friction, noise, vibration, and the inelasticity of the runner's muscles and tendons (humans, unlike kangaroos, become less efficient the faster they run [Morton, 1952]). Spring systems have approximately 95% energy return efficiency while typical running shoes range from 40% to 60% efficiency [ETJ, 1995,Herr, 1996]. Volumetric oxygen studies have shown a 2-3% improvement in running economy using such spring systems over typical running shoes [Herr, 1996]. Similarly suggestive are the "tuned" running track experiments of McMahon [McMahon, 1984]. The stiffness of the surface of the indoor track was adjusted to decrease foot contact time and increase step length. The result was a 2--3% decrease in running times and seven new world records in the first two seasons of the track. Additionally, a reduction in injuries and increase of comfort was observed. Thus, if a similar spring mechanism could be designed for the gait of normal walking, and a ratchet and flywheel system is coupled to the upstroke of the spring, it may be possible to generate energy while still giving the user an improved sense of comfort (Figure 3). In fact, active control of the loading of the generation system may be used to adapt energy recovery based on the type of gait at any given time.
Figure:
Simple diagram showing two shoe generation systems: 1)
piezoelectric film insert or 2) metal spring with coupled generator
system.
Since a simple mechanical spring would not provide constant force over the fall of the heel but rather a linear increase (for the ideal spring), only about half of the calculated energy would be stored on the downstep. An open question is what fraction of the spring's return energy can be sapped on the upstep while still providing the user with the sense of an improved ``spring in the step'' gait. Initial mock--ups have not addressed this issue directly, but a modern running shoe returns approximately 50% of the 10J it receives during each compression cycle [ETJ, 1995] (such ``air cushion'' designs were considered a revolutionary step forward over the hard leather standard several decades ago). Given a similar energy return over the longer distance of the spring system, the energy storage of the spring, and the conversion efficiency of the generator, 12.5% of the initial 67 W is harnessed for a total of 8.4 W of available power.