Of the vast amount of energy arriving at our planet from our sun, a considerable amount ends up warming the surface of the oceans. The sea is characterized by a complex set of currents, mostly driven by the heating and cooling of the water. And one of the most prominent physical features of the ocean is the theromocline, the gradient of temperatures from the warm surface to the cold depths.
This month Jeremy Hsu writes about an interesting passive energy system that exploits these temperature differences to generate power for underwater devices .
The technology is rather closely guarded, but the general idea is pretty clear: specific chemical compounds phase change from solid to liquid to gas in the temperatures found in the ocean. The Seatrec system uses the energy from these transactions to generate electricity to charge batteries.
Basically, the buoy or other underwater device simply floats up or down (increasing and decreasing buoyancy), changing the temperature of the water. It’s that simple!
The current systems are available as add ons to extend the life of otherwise not rechargeable sensor buoys. This increases the amount of data that can be collected, and reduces the loss, and concomitant pollution, due to dead buoy batteries.
The company seems to be guarding the details closely, which makes sense. The whole deal depends on matching the exact chemical recipe with the thermocline to be inhabited. So, no, they don’t want to tell me their secret sauce.
I’m not an expert in this kind of thermochemistry, but I’m pretty sure that any given reactor will have a fairly narrow range of temperatures that work well. This means that a given generator will work best in specific locations in the ocean, and quite possible during specific time periods. Presumably, these ranges are wide enough to be useful.
But, I wouldn’t expect this system to work for anything but shallow water in the tropics or temperate zones. And I wonder if this system will work well in areas with major currents, especially upwelling or downwelling. And, of course, arctic or winter storms both cool the water and stir the surface reducing the thermocline.
Still, there are a lot of places where this technology will work well, and they are important areas like continental shelves and shores and lakes.
Another question I have is the timeline of this recharging. When the buoy changes depth, it takes time for the thermal energy to penetrate and for the chemical reactions to occur. So this is a pretty slow motion process: a slow graceful ascent or descent, patient absorption of heat, and a trickle of recharging power. Then slowly return to station.
This slow process is probably fine for buoys, which are very passive entities. But there has to be enough power generated to allow a lot of data collection in between recharging. The public materials don’t give much information about this duty cycle, but I have to assume that it works well enough to be useful. (And, of course, the alternative may be sending a ship to replace the battery or losing the buoy.)
Anyway, this is a fairly elegant technology, and I look forward to seeing how well it fares.
- Jeremy Hsu, These Underwater Drones Use Water Temperature Differences To Recharge, in IEEE Spectrum – Robotics, September 3, 2020. https://spectrum.ieee.org/automaton/robotics/drones/renewable-power-underwater-drones
- Sea Technology, Ocean Temperature Differences Enable Energy Harvesting, in Sea Technology, February 27, 2020. https://sea-technology.com/seatrec-sl1