New York City Mayor Michael Bloomberg touted clean energy Tuesday at, aptly, the National Clean Energy Summit. He said his city has issued a formal request to companies for ideas on how to source electricity from the wind, sun, and waves. "Perhaps companies will want to put wind farms atop our bridges and skyscrapers, or use the enormous potential of powerful offshore winds miles out in the Atlantic Ocean," Bloomberg said, adding, "I think it would be a thing of beauty if, when Lady Liberty looks out on the horizon, she not only welcomes new immigrants but lights their way with a torch powered by an ocean wind farm." He also touted less-sexy smart power grids, increased transmission capacity, and carbon taxation. Bloomberg can't, of course, snap his fingers and create a Green Apple; plenty of obstacles remain, including potential expense, resident resistance, permitting, and the fact that less than 18 months remain in his mayoral term. Unfazed, Bloomberg declared, "When it comes to producing clean power, we're determined to make New York the No. 1 city in the nation."

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San Francisco Mayor Gavin Newsom got jiggy with eco-measures this week. He signed into a law a requirement that the city's taxi fleet be converted to low-emission vehicles by 2011; ordered all city departments to purchase 100 percent recycled paper and reduce overall paper use by 20 percent by 2010; and announced his support for a tidal-energy project in the San Francisco Bay, despite a recent study's conclusions that the project would be more expensive than it's worth. Newsom has proposed strict green-building standards for his city and will submit a carbon tax to voters; folks in don't-call-it-Frisco also live happily without plastic bags or toys containing bisphenol A and phthalates.

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The utility Pacific Gas & Electric this week became the first power company in the United States to sign a deal agreeing to purchase electricity generated by wave power. The wave-power farm that would generate said electricity is still years from completion -- not to mention government approval -- but securing a power buyer is seen as an important step. In the first phase of the project, slated for completion by 2012, eight specially equipped buoys located two and a half miles off the coast of Northern California will together generate up to two megawatts of power. If all goes as planned and the project is granted the necessary approvals, it could be expanded to produce up to 100 megawatts. In other renewables news, the largest photovoltaic solar array in the U.S. opened this week at Nellis Air Force Base near Las Vegas, Nev. The 140-acre farm is expected to generate up to 30,000 megawatt-hours of electricity a year, or about 25 percent of the base's power use.

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Tidal turbines in New York's East River halted temporarily for repairs

The company experimenting with underwater electricity turbines in New York's East River has found that the river's currents are powerful. Too powerful, if you want to get picky about it: the river sheared off several blades on the 20-foot-tall turbines, and has stressed the bolts on the replacement blades as well. In fact, Virginia-based Verdant Power has shut down the six turbines temporarily, for repairs and a bit of tweaking. "The only way for us to learn is to get the turbines into the water and start breaking them," says Verdant founder Trey Taylor. Adds Mollie Gardner, a geologist with the company, "the good thing is that there's more power in the East River than we thought." The other good news is that studies so far have shown that fish caught on sonar are smart enough to swim around the blades. Eventually, Verdant plans to install up to 300 turbines in the river, which could power 8,000 homes; meanwhile, a competitor has gotten a federal permit to try out the East River as well.

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Hello Umbra,

I just read a little bit about wave power and was wondering what you think the future potential is for this clean and never-ending energy source?

Peter Blomquist
Orlando, Fla.

Dearest Peter,

Wow. I'm a sucker for anything with a good name, like "LIMPET." There are fun technologies under development to harness the might of the sea as it swells up from the briny deep.

Crest easy.

Photo: iStockphoto

Wind hits the surface of the ocean and makes tiny ripples that over distance and time become waves. As contrasted with the tidal technologies we discussed last week, which use the tidal cycles to trap and release water, wave devices use the constant sloshing and swelling of the ocean to generate energy. Wave power technologies come in three broad categories: on shore, near shore, and offshore. And then three more categories: Buoyant Moored Device, Hinged Contour Device, and Oscillating Water Column. I'll give you just a few examples out of the truckloads I found.

Islay, Scotland, has an Oscillating Water Column called the LIMPET (an acronym), a 500 kW onshore wave-power generator. The LIMPET is a concrete bunker on the beach with a large central cavity, into which waves slosh. As water rises and falls within the cavity it displaces air, which exits and returns via a turbine-clogged tunnel. The spinning turbine is attached to a generator that, among other things, powers a Scottish bus -- here is a site with an animated show for you.

Offshore devices, in depths above 80 feet, include a long, segmented tube called the Pelamis, after a type of sea snake. The wave-induced gyrations of this Hinged Contour Device drive hydraulic pistons that send electricity to shore through cables on the sea floor. Then you have Buoyant Moored Devices, which look like signal buoys, with a central piston sloshing up and down as the sea swells; this mechanical stroking is connected to a generator as well. A wave farm would be a conglomerate area with several devices, much like a wind farm.

LIMPET in Islay, Scotland.

I read only promising information about wave power's capacities. A lot of development is occurring in Europe, where public enthusiasm and government commitment to renewables means funding and research support. Additionally, Western Europe happens to have bountiful wave-rich sites. Wave power is most promising around 40 to 60 degrees of latitude in both the Southern and Northern Hemispheres (there's a map here), particularly on the west coasts of Britain and the United States. I've seen one estimate that wave power could contribute 1 to 10 terawatts of energy worldwide -- the U.S. uses about 3 to 4 TW annually, and the world about 13 TW.

Interesting design problems with wave power include the destructive power of the salty sea -- corrosion and weather and storms and all. Benefits include the relative constancy of power availability, as contrasted with wind and tidal barrages. I'm running out of room, so I'll just say that one last drawback to wave power is that the near-shore ocean is now considered a recreational site full of beauty, not a working area, and so wave farms may meet the same resistance that has met many a wind farm. I eagerly await your letters on the subject.

Goodbyely,
Umbra

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Dear Umbra,

Much is made of wind, solar, geothermal, and even wave power, but why doesn't anyone talk about tidal power? It has more power than wind for the same turbine, without the eyesore of turbines, is totally renewable, and is predictable hundreds of years in advance. Yet nobody talks about it. What's wrong with tidal?

Chris Morrison
London, England

Dearest Chris,

The tides appear fruitful, but we disturb the ocean at our peril.

Do oceans hold the answer?

Photo: iStockphoto

One concern is that tides follow the lunar cycle, but human power use is linked to the solar cycle. There are two low and two high tides per day, broadly speaking, and their height varies depending on the positions of the moon and sun (tide height being a vertical measurement, not an up-the-beach measurement). Tidal amplitude -- the difference between high and low tide -- varies with coastal location, and 40 assorted planetary areas where water shoves itself into a bay or estuary have amplitudes suitable for tidal power generation. La Rance, France, is such a place, home to the oldest, largest, and practically only tidal barrage on Earth.

Jargon, ho!: A tidal barrage seems to be a pseudo-dam, with a substantial basin behind it, the sluice gates of which admit water as the tide rises. When the basin fills and a good "head" (differential between water height fore and aft) is established, the sluice gates shut, the water flows out through a turbine, and voila, les kilowatts.

The La Rance turbine spins as water goes both in and out, to maximize and consistentize power, and it has pumps that contribute toward the same end. To look at the numbers: La Rance's peak tidal amplitude is 13.5 meters, the barrage is 750 meters long, and it produces 600 million kilowatt-hours yearly, sufficient for 250,000 French domiciles.

The troubles with barrages include their incredibly high installation costs, which dwarf their low operating costs, give them long payback periods, and make them poor competitors in the current power market; the conundrum of intense but inconstant power generation, somewhat akin to wind; and the ecological interference inherent in blocking an estuary. Two sample eco-problems are: Estuaries themselves move over time as ocean sediment is removed and replaced, and animals and plants come and go from the estuary; both of these functions are impaired, with unpopular results.

The contemporary approach to tidal power (the La Rance barrage was built in the 1960s) tries to avoid ecological and financial limitations, and eschew the barrage, via development of underwater turbine systems resembling contained windmills. It is also called "marine current" technology, since it relies on underwater currents running at about 5 knots. These are already being tested in some areas, and according to some, look promising both for power and for not pureeing or otherwise badly disturbing animals. They do share the barrage's trouble of high installation costs and hence high power costs, and others believe this to be a death knell to all tidal power.

All that said, I don't know why no one talks about the highly interesting topic of tidal power. You could bring it up at summer barbecues yourself, you know.

Neaply,
Umbra

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