Energy Harvesting Gets Big—And Small

Large-scale energy harvesting isn't new. The Swiss have run their whole country on melted snow-water running downhill for more than a century. Nearly half a million Hollanders live in a province called Flevoland, which was completely under water until 1930. Much of that work was done by the wind. A few weeks ago, I visited a saltworks near my home on San Francisco Bay. For over 100 years, sun and wind enabled commercial salt harvesting in the region. For millennia before that, the Ohlone people obtained their salt in much the same way, but on a smaller scale.

In fact, one could argue that the first farmers and pastoralists were really harvesters of solar energy. If you're looking for energy-harvesting technology, there are Web sites describing vertical-shaft windmills in Persia in the ninth century CE. They also describe water-wheels in India (fourth century BCE), Greece (first century BCE), and China (first century CE).

In a way, burning coal or petroleum is a form of energy harvesting—harvesting cretaceous sunlight, if you will, but there's the rub. It's getting continually more expensive to capture and exploit that ancient sunshine. This has led to fresh interest in energy sources other than fossil fuels, from reliable old wind and sunshine, to waves and tides, to all the little bits of energy that typically goes to waste—from the rumbling of a railroad car to the security lighting that discourages nighttime crime.

The buzz these days is all about collecting millijoules (mJ) of energy wherever they can be found (which is more like gleaning or scavenging than harvesting), but even that isn't so new. My grandparents had a mantelpiece clock that wound itself by capturing changes in atmospheric pressure.

Megascale Harvesting
The latest wind turbines have rotor diameters spanning 160 feet, with the hub 250 feet above ground level (Fig. 1). An intriguing development in the engineering of these behemoths is their use of ultracapacitors. One critical function that must be dealt with is feathering the blades when winds get too strong (Fig. 2).

Blade-pitch motors could be battery-driven, but battery maintenance involves sending workers up the towers in fields of hundreds of towers— an expensive proposition in terms of steeplejack (one who climbs tall structures to do repairs) pay scales and liability insurance. Ultracapacitors last much longer than batteries and can tolerate greater temperature extremes, making them a more cost-effective energy storage device for blade-pitch control.

You may know all about photovoltaic solar. But let's look at what Sandia National Laboratories is doing with parabolic troughs, which use curved mirrors to focus sunlight on a tube that runs the length of the trough. Oil runs through the tube to heat it up, and it's then sent to a heat exchanger to generate steam for a conventional power plant.

There are some big troughs out there. The largest are in the Mojave Desert, near Barstow, Calif. Nine plants ranging in size from 14 to 80 MW generate 354 MW at peak output (Fig. 3). A middle-sized plant may comprise 10,000 modules, where each module has 20 mirrors.

What's new with these plants is mirror-alignment improvement. Sandia's latest work involves a pole with five cameras positioned along it--one for each of the four rows of mirrors and another to vertically center the pole with the trough module (Fig. 4). The actual images are then matched to a mathematically generated ideal image, and the mirrors are adjusted in real time to coax them into the ideal alignment.

Microscale Harvesting
The real excitement in energy harvesting is on the microscale. All sorts of schemes can recover wasted energy, from photovoltaic cells on your laptop that recharge your battery overnight (using energy from continuously burning overhead lights) to monitors for bearing-wear in industrial machinery, trucks, and rail cars—monitors that power themselves from the very vibrations they monitor.

In the latter case, the microgenerators are made either from what I call "spring-mounted-slugs-inside-a-coil" or piezoelectrics. What's new are advances in the fabrication of materials. Another approach is represented by Advanced Cerametrics' (ACI) viscous suspension spinning process (VSSP), which adapts rayon-fiber technology for making ceramic fibers. As ACI explains it, the precursor of rayon is cellulose dissolved in caustic soda and water. This is mixed with a slurry of piezoelectric ceramic spun through a spinneret and baked.

A typical single piezoelectric fiber composite bimorph (PFCB) can generate voltages in the range of 400 V p-p with some forms reaching outputs of 4000 V p-p. In practice, with a 30-Hz vibration, ACI's piezo fibers have taken just 13 seconds to produce 1 J. If you're rusty on MKS units, one joule is the kinetic energy in a mass of 1 kg moving at 1 m/s.