Here is a new take on harnessing ocean thermal energy: a submersible vehicle that propels itself for thousands of kilometers by extracting heat from the ocean to melt an onboard reservoir of expanding wax. The vehicle is called a thermal glider.
Reported in a recent ME Magazine article, “Running Silent”, this autonomous underwater vehicle resulted from research by Dave Fratantoni at the Woods Hole Oceanographic Institute (WHOI) and Roy Watlington of the University of the Virgin Islands. The robot sub propels itself without a conventional power system by periodically melting and re-solidifying a special internal wax reservoir. Mechanical work is performed as the wax liquefies and expands and again as it cools and freezes, changing the bouncy of the glider.
The thermal glider design, which has been successfully demonstrated in a trail run of several months, makes ingenious use of ocean temperature gradients with depth. Warm water at the ocean’s surface melts the reservoir of wax, sinking the vehicle. At depth, the vehicle cools, and the wax solidifies and shrinks, increasing the vehicle’s buoyancy and sending it back topside. The gliders’ wings generate lift. So instead of just bobbing up and down in place, diving and surfacing induces forward motion. Control surfaces reminiscent of an airplane provide steering and control.
This mode of propulsion is so efficient that the robot vehicle’s operational life is limited by the longevity of the batteries that power its control and communications electronics. Webb Research, founded by Doug Webb, is collaborating on the project to reduce instrument power demand and extend the robot’s life. Fittingly, Doug Webb was the originator of the thermal glider concept in the 1980’s. Webb Research is also developing direct thermal energy conversion technologies to power onboard electronics indefinitely.
A fleet of autonomous ocean-powered gliders will eventually be used to study the waters in the subtropical gyre of the North Atlantic. This region is thought to be a key for assessing the ocean’s response to climate change.
More details on this technology are provided in the WHOI press release entitled, “Researchers Give New Hybrid Vehicle Its First Test-Drive in the Ocean.”