Hydrokinetic Energy: Here, There But Not Everywhere

The news about a tidal power station to be built in the waters surrounding New York City might have gotten some people a little worked up.  It all sounds perfect: a clean energy technology that doesn’t ruin anyone’s view and doesn’t seem to have any big environmental impacts. Build a few thousand underwater turbines off the coast of the United States and presto…we'll be free from coal and living la vida low carbon.

Well, not quite.

Tidal power is but one of the cobbles that could pave the road to energy independence, and a pretty obscure one at that.  As a whole, electricity generated from tides, waves, currents and free-flowing rivers is categorized as hydrokinetic energy: the harnessing of the energy of moving water.

Hydrokinetic energy gets little attention compared to its renewable energy cousins like solar and wind power, but studies estimate that the amount of hydrokinetic energy that could be realistically harnessed is about 10 percent of national energy demand (PDF).  That’s a significant number, especially for coastal and riverside cities and towns.

So let’s take a brief look at the two main hydrokinetic technologies – in-stream and wave power – and the potential they bring to America’s clean energy future.


In-Stream (Tidal and River)

"In-stream" power has little in common with your grandparents' version of hydropower.   Instead of damming a river to capture its potential energy, or building expansive dykes at the mouth of a coastal bay to harness the flow of the tides, in-stream hydrokinetic power relies on free-flowing currents that spin underwater turbines.  Some initial estimates indicate that stream-based hydrokinetic energy could generate enough power to fulfill the electricity needs of about 23 million homes.

Tidal power, in particular, has a major advantage over traditional power plants and remotely-located wind and solar installations: proximity to customers.  For example, building tidal power plants that harness the massive amounts of water that surge through New York City’s or San Francisco’s narrow tidal straits, or that take advantage of large tidal ranges along the Maine coast, could provide electricity for adjacent cities and towns without requiring long transmission lines.

Inland cities and towns along the nation’s rivers can also benefit from electricity generated from free-flowing waters.  The powerful Mississippi River, for example, has long been the apple of many hydrokinetic developers' eye.  With the Southeast United States' limited onshore wind energy prospects, in-stream power installations could help to fill the renewable energy gap in some parts of the region.



Wave energy is the Holy Grail of hydrokinetic energy.  Annually, the earth’s waves can provide more than five times the electricity the world uses in a year.  But the question is how can we capture that staggering amount of energy?

Numerous experimental technologies are being tested to harness the energy from waves at the ocean’s surface, but so far no commercial wave power farms have been installed in U.S. waters.  (The world’s first commercial wave farm was built offshore of Portugal, and operated for just two months before mechanical problems and financial troubles halted the project.)  In the United States alone, it’s conservatively estimated that 44 million homes could be powered by wave energy.

The technology emerging to harness this energy takes many forms.  In some cases a floating buoy converts the energy in the rise and fall of the passing waves into electricity. Another technique is to use the rising and falling wave water to fill a chamber that compresses air to drive an electric generator. In other cases a machine that looks like a giant sea snake made up of several different segments heaves and sways on the surface of the waves, driving hydraulic pumps to power an electric generator.


The federal government has demonstrated increasing interest in these emerging technologies, most recently by awarding $37 million to 27 different marine and hydrokinetic projects.  Forward-looking renewable energy developers are also taking note – there are currently 143 approved federal permits for hydrokinetic pilot projects around the country.

A look at a map of those 143 projects reveals several regional patterns.  In the Northeast, tidal projects dominate, while the West Coast and Hawaii are home primarily to wave energy ventures.  From the Puget Sound up through Alaska, tidal power projects are common, while the Mississippi and Ohio Rivers are dotted with over 100 different hydrokinetic prospects.

Like politics, hydrokinetic power’s best potential is local.  If these and other pilot projects lead to actual installations, Electric Power Research Institute (EPRI) conservatively estimates that the United States could be generating 13,000 megawatts of wave and stream-powered electricity by 2025 (PDF).  While that’s less than half of the amount of wind energy currently installed in the U.S., it’s more than the 10,000 megawatts projected for conventional hydropower (namely, dams) by that same year.

So why isn’t hydrokinetic energy as far along as solar or wind?  Because, as a National Renewable Energy Laboratory researcher noted, “It’s much more difficult to do things underwater than on dry land.  The water tears stuff apart. There’s fish, rust, fouling … all kinds of problems.”

Permitting is also difficult because neither federal nor state authorities have much experience reviewing these technologies.  In particular, the environmental impacts of hydrokinetic power projects aren’t yet fully understood.  While the lack of greenhouse gas emissions is a significant benefit, the potential for fish strikes, noise pollution and other possible alterations to aquatic ecosystems demands more research that will allow permitting authorities to make informed decisions.  And of course, money from anyone other than well-funded energy prospectors is tough to come by when you're talking about unproven technologies that can require millions of dollars for a single pilot project. That means even more public and private research and development funding is badly needed.

The New York City tidal turbines, and other hydrokinetic pilot projects across the country and around the world, may well usher in commercial projects that would help to power our cities and towns.  But any feasible plan to switch from fossil fuels to renewables acknowledges that a mix of different sources of energy is essential; there is no silver energy bullet.  Add tidal, stream and wave power to the mix for coastal and river communities, combine it with solar, wind, geothermal and other renewable sources, and we may be on our way towards that 100 percent renewable goal.