Mobile Wave Power: Ship-Based Energy Offers Renewable Solutions0

Dave Levitan | Thu Nov 17 2011 |

Ocean waves carry a lot of energy. And all of that energy, clean and relatively consistent, looks pretty good to a world that is continuing to search for ways to ramp up generation of electricity from carbon dioxide-free sources.

As a result, dozens of devices designed to harness the energy in ocean waves have sprung up in recent years, from massive buoys to snake-like metal chains, all of them sharing a need for that bane of renewable energy developers everywhere: the transmission line.

With offshore technology of the wave or wind variety, not to mention tidal and other fringe possibilities, running a cable from a device back to shore represents one of the biggest economic stumbling blocks. Enter ship-based wave power.

Andre Sharon, an engineering professor at Boston University and Director of the Fraunhofer Center for Manufacturing Innovation, has come up with an elegant way around the transmission line problem—by putting wave energy harvesters on board a boat.

Many prototypes are being tested, but none have yet to be deployed on a large scale.

“It seems like ocean energy is sort of an untapped resource,” Sharon said. “[There are] a few isolated cases of prototype deployment, but compared to wind and solar it’s not very mature.”

Wave harvesting uncovered

Sharon’s idea works like this: each day, a barge with several wave harvesters—basically large bobs hanging off the side of the boat—heads out to sea. At a certain distance from shore, it drops anchor, lowers its harvesters, and sits still for a while. The bobs rise and fall with every passing wave, while the barge acts as a (relatively) stationary reference point. It is that relative motion that creates the electricity; and instead of sending it along an expensive transmission line, the electricity is stored on board the boat in large battery systems.

After 20 hours at sea, the barge heads back to the dock with fully charged batteries and plugs into a device known as an inverter—this converts the stored direct current electricity into alternating current, which is what the power grid needs. Because the power is stored in the batteries, it can be sent to the grid whenever demand is highest, say in the evening when people come home from work and switch appliances and televisions on.

With so many wave power devices in development—many prototypes are being tested, but none have yet to be deployed on a large scale—there is a novel sort of appeal to the only one that sits on board a boat (a patent on the idea is pending).

Paul Jacobson, an expert on wave energy with the Electric Power Research Institute who is not involved in the idea’s development, said it could circumvent substantial permitting requirements for ocean structures.

“I’m curious about the energetics of travel to and from station versus energy captured on station,” Jacobson said. “Travel distance would likely be greater on the East Coast than on the West, because of water depth near the respective coasts and the prevailing westerly winds.” Still, he said, “it’s an interesting concept.”

Price point predicament

The primary challenge with this idea, as with any new energy avenue, is economic. Can the entire system—boat, battery, harvester, inverter, dock space, incidental costs—be built and maintained at a cost that delivers affordable power?

Each boat would contribute enough to power more than 600 homes.

Sharon assumed early on that the storage of the power would be the prohibitive cost; after all, reliable and cheap electricity storage has long been sought for renewable energy and continues to exist only on limited scales. Sharon’s colleague, John Briggs, set about doing the math on every conceivable source of electricity storage he could think of: he estimated costs and logistics of flywheels, molten salt storage, gravity-based systems, and numerous battery technologies.

“Ultimately it turned out that you had to use batteries.” The reason for this is efficiency of the storage mechanism: turning the energy into something like compressed air, or the heat in molten salt, leads to serious conversion losses when it turns back into electricity for use. Storing it in a battery, though, saves much more of the original energy taken from the ocean wave.

After all the calculations, Briggs and Sharon settled on lithium-ion batteries—you know them as the batteries in that new Nissan LEAF you just bought—with sodium-sulfur batteries close behind.

The lithium-ion system actually could make this competitive with various other technologies. Sharon and Briggs say with an optimal configuration—six boats per dock, 20 hours per trip to sea—they can get electricity at 15 cents per kilowatt-hour. This puts it in the range of some solar power, and cheaper than estimates for offshore wind (though none yet exists in the United States). Each boat would contribute about 73,000 MW-hours of electricity over a ten-year period, enough to power more than 600 homes.

Notably, though, the capital costs for a real buildout of this idea could be substantial. According to data from Sharon, the batteries alone would cost close to $10 million for a single boat; that Nissan LEAF battery pack can store 24 kilowatt-hours of power, while the barge-based battery needs to hold 20 megawatt-hours, or more than 800 times as much.

I’ve spent a lot of time researching technologies, and I haven’t seen anything even approaching this.

The energy harvesting devices adds another $4 million, and with the cost of the inverter and the barge itself, along with daily tugboat service, maintenance, dock space rental, and mooring, costs rise quickly. However, many of these components do have long lifespans; Sharon calculates a ten-year, per-barge cost of between $11 and $12 million.

Logic and logistics

There are technical challenges, along with the economic issues. Waves are a certain size, and they can differ in various locations; the boat and wave harvesters need to be certain sizes relative to that wavelength, or the energy generated won’t be up to snuff. Getting these parameters right is crucial.

The idea does have further advantages over other renewable technology, however, including the ability to provide electricity only when it is most needed. Sharon also points out that these boats could serve dual purposes: “You’re going to stay out there for 20 hours, you might as well do commercial fishing as well.”

Also, not having a permanent ocean structure carries more benefit than simply avoiding the need for transmission.

“Let’s say on the average day you need to have a certain thickness of metal, a certain strength, for your average day of waves,” Briggs said. “You may have to make that 100 times stronger to survive the worst storm that you’re ever going to see…. We don’t have to do that. We can leave the system in port [during a storm]. We don’t have to build it as heavy, and durable and as absolutely bulletproof as the other people do, and it’s less expensive as a result.”

For the moment, the idea exists only in tiny scale models riding waves inside a laboratory tank, but Sharon said there has been interest from various parties that may provide funding to build a prototype. No particular timeline on this happening is available at the moment, but the idea’s progenitors have high hopes. “I really struggled to think about why somebody didn’t think of this sooner,” Briggs said. “I’ve spent a lot of time researching different technologies, and I haven’t seen anything even approaching this.”

Illustration by Jon Contino