Harvesting is not necessarily a free energy source, but for some applications it may be the best choice compared to the alternatives.
Energy harvesting is a very attractive power-source option, as at first it seems to offer the promise of never-ending “something for nothing.” In some cases, it delivers on that promise, and has been used successfully in vibration-powered bridge monitors, for example. In other cases, the cost/benefit ratio is not attractive (although the “buzz” factor is high) as the cost is not zero and the benefits are marginal: think of shoes with embedded components which transform “step energy” into power, or even bicycles with regenerative brakes which add weight, bulk and cost, but yield very little recovered energy.
However, there’s one place where harvesting may truly become a winning (or winnable) proposition: powering implanted medical devices, and some advanced researchers are looking within the body to power devices such as pacemakers and other devices. Most pacemakers presently use primary (non-rechargeable) cells with five-to-ten-year life spans, after which surgery is needed for replacement. (Note that the alternative of using rechargeable secondary cells with contact-free wireless recharging sounds like a viable and so is a preferred alternative, but there are major medical and technical issues which are still barriers to widespread adoption.)
A research team at the Thayer School of Engineering at Dartmouth College worked with UT Health San Antonio (part of the University of Texas) and developed a new way to build a piezo-based harvesting transducer for these medical devices. They used a combination of thin-film energy-conversion materials with a minimally invasive pacemaker mechanical design, as detailed in their paper “Flexible Porous Piezoelectric Cantilever on a Pacemaker Lead for Compact Energy Harvesting.” They harnessed the kinetic energy of the lead wire which is attached to a beating heart, and then converted that energy into electricity to continually charge the batteries (Figure 1).
Figure 1 (a) Schematic of the porous piezoelectric-energy harvester; (b) flexible porous PVDF-TrFE thin film; and (c) scanning electron microscope image of the cross-section of the thin film. (Source: Thayer School of Engineering at Dartmouth College)
The power-generating material is a polymer piezoelectric film called polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). The team created a basic dual-cantilever structure which wraps around the pacemaker’s wire lead, and Initial testing was done using a mechanical shaker to simulate the motion of the myocardium and the corresponding deformation of a pacemaker lead. Maximum output was 0.5 V at 43 nA at 1 Hz, a little over 20 nW. They were able to increase the power output by a little over 80% by adding a small “proof mass” of 31.6 mg to the tip of the dual-cantilever tip; the added mass enabled a larger bending curvature, resulting in higher electrical output from the harvester. Further, multiple harvesters can be paralleled for greater output current. [The researchers note that today’s ultra-low-power implantable biomedical devices require about 0.3 μW for cardiac-activity sensing, 10 to 100 μW for pacemakers, 100 to 2000 μW for cochlear implants, and 1 to 10 mW for neural recording.]
Harvesting may not be the low- or no-cost source for energy that some have said it would be, but there are situations where it may be the right solution due to application priorities and constraints. Powering of implanted medical electronics may turn out to be one of these areas where the challenges are outweighed by the genuine benefits.
Have you ever used energy harvesting in a commercial product? What were the most memorable lessons learned by the harsh reality of experience – both good and bad?
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.