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By Robert Rapier on Feb 15, 2008 with no responses

Infinite Underwater Energy

I am always on the lookout for novel energy sources that don’t have obvious knockout factors. I have kept vaguely abreast of developments around tidal energy, and it always seemed like this should be a very attractive option for the energy needs of coastal communities. I was present at an alternative energy presentation last year where I heard that the primary problem is that the tides tend to tear the equipment up. It has to be able to withstand the worst conditions it could possibly see, and in the ocean that can be pretty bad.

While this is a serious problem for tidal energy, this morning I spotted a different variation of this concept. I have heard this mentioned before, but this story really goes into detail. I will provide a few excerpts, but the entire article is worth a read:

Oceans Eyed As New Energy Source

DANIA BEACH, Fla. – Just 15 miles off Florida’s coast, the world’s most powerful sustained ocean current – the mighty Gulf Stream – rushes by at nearly 8.5 billion gallons per second. And it never stops.

To scientists, it represents a tantalizing possibility: a new, plentiful and uninterrupted source of clean energy.

Florida Atlantic University researchers say the current could someday be used to drive thousands of underwater turbines, produce as much energy as perhaps 10 nuclear plants and supply one-third of Florida’s electricity. A small test turbine is expected to be installed within months.

From Oregon to Maine, Europe to Australia and beyond, researchers are looking to the sea – currents, tides and waves – for its infinite energy. So far, there are no commercial-scale projects in the U.S. delivering electricity to the grid.

Because the technology is still taking shape, it is too soon to say how much it might cost. But researchers hope to make it as cost-effective as fossil fuels. While the initial investment may be higher, the currents that drive the machinery are free.

Of course there is always an element of hype in these sorts of stories (I can be quite nit picky about the word “infinite”), and until they actually start to install these turbines, it is unclear whether there are serious knockout factors.

There are still many unknowns and risks. One fear is the “Cuisinart effect”: The spinning underwater blades could chop up fish and other creatures.

I would think that could be solved with a grid at the entrance to the turbines. That would increase pressure drop a little, but should be able to keep out the fish.

The field is growing, but there have been growing pains:

The Federal Energy Regulatory Commission has issued 47 preliminary permits for ocean, wave and tidal energy projects, said spokeswoman Celeste Miller. Most such permits grant rights just to study an area’s energy-producing potential, not to build anything.

The field has been dealt some setbacks. An ocean test last year ended in disaster when its $2 million buoy off Oregon’s coast sank to the sea floor. Similarly, a small test project using turbines powered by tidal currents in New York City’s East River ran into trouble last year after turbine blades broke.

One analysis showed that the overall potential is less than 10% of current electricity usage, but different areas are going to want to apply different solutions. Maybe ocean currents in Florida, but perhaps solar in Nevada and Arizona, and wind in Oklahoma.

Roger Bedard of the Electric Power Research Institute said an analysis by his organization found that wave- and tide-generated energy could supply only about 6.5 percent of today’s electricity needs.

I asked a colleague who has expertise in this area what he thought, and he replied:

Had not seen, but have seen similar before.

Issues:

- Power density goes with v^3, may need very big kit if velocities low (the ever reliable web says 1-2 m/s for Gulf Stream whereas tidal can be somewhat higher (2.5m/s) at peaks BUT is not constant, while Gulf Stream is, hence better load factor, no need for off design point operation)

- Power is force x velocity, need lots of mooring force to restrain if velocities low – obviously you would go for high velocity areas

- Conversion of big forces going slowly to little forces going fast is pricey (x100 gearboxes are expensive) but there are good solutions around, including hydraulics and lower speed up combined with non synchronous generation (Multibrid are doing this for wind).

- Near surface, you cannot do more than about 1MW per unit without getting into blade tip cavitation, likely not a problem in Gulf Stream (just go deeper)

- None of this is impossible, just needs smart people, good engineering and time – energy payback is likely to be good (wind turbines pay back in less than one year) since electric power is the other side of the Carnot efficiency – every kWh is displacing at least 2kWh of gas, more like 3 of coal (but you knew that).

There are lots of tidal devices around – speeds tend to be a bit higher and resource is near coasts. The Gulf Stream near US seems to be the highest speed ocean current, likely a good place to start development.

It will be interesting to watch this technology develop.