AskPablo: Wave Power

wave.jpgAugust 13th’s article on desalination received a lot of great feedback. On reader informed me of a technology that uses wave power to pump sea water, at high pressures, through a reverse-osmosis filtration system, using virtually no fossil fuel-based energy. He also informed me that the same technology is being used to pump seawater uphill into large storage tanks. When electricity is needed the water is run back downhill and through a turbine generator. Brilliant! But how much energy can we get from the waves and how do we go about figuring that out? Well, that’s what I’m here for. Read on to find out…

The oceans are unimaginably huge, covering a majority of the earth’s surface. If you have ever surfed you have a great appreciation for the power of waves, not only as they crash down on you, but as the relentlessly batter the shore. The energy contained in waves is renewable and abundant. You could even call it a sort of solar energy since the sun creates wind and wind creates waves… It is hardly imaginable that any human activity could have an impact on the role of waves in the ocean ecosystems, yet breakwaters and other man-made structures are a good example. Some people would rightfully raise a caution flag regarding harnessing wave power because capturing too much may dissipate waves in some areas to such a degree as to disrupt marine ecosystems that depend on the churning of nutrients. On the other hand harnessing wave power could help dissipate its energy to protect sensitive coastline or human infrastructure. This issue is obviously one that needs to be addressed at a local level but in general wave power would have a less harmful effect on sea life than mercury emissions from a coal-fired power plant.
There are several technologies out there for capturing wave energy. One uses an articulating bus-sized machine that generates energy as waves lift and lower the center and pistons drive hydraulic fluid through turbines. Another technology is shore-based and channels the incoming wave energy into a chamber which compresses air and drives several fans. The type of device that is used in the water desalination and water pumping operations consists of a weighted float mounted atop a hydraulic piston. As waves come in they raise the float to their crest and it then falls back down to the trough in between the waves before being raised again. The amount of energy that can be obtained from this device depends on the weight of the float, the wave height, and the wave period.
Let’s assume that the float weighs 100 kg, the waves are 2 m high, and the wave period is 10 seconds. A much heavier float can be used but, since an object at rest wants to remain at rest, it would provide diminishing returns. Think of an oil tanker and a small rowboat; in a light chop the oil tanker would remain steady while the rower might get seasick. Smaller units also means that you can deploy more of them in a given area…
So, if a 100 kg weight gets raised and lowered 2m the amount of energy required can be found by the gravitational potential energy equation (PE = m x g x h, where m is mass in kg, g is the gravitational constant, and h is the height in meters). In our case this would be 1960 Joules (100 kg x 9.8 m/s2 x 2 m). Since the waves come along every 10 seconds we get 196 J/s (1960 J / 10 s), or 196 Watts. Let’s assume that, due to various losses of energy, the generating efficiency of this device is 50%. This means that we get 98 W of electricity from each device, or more than enough to run a strong light bulb. In one day this system would generate 8,467.2 kWh (98 J/s x 24 h/d x 60 m/h x 60 s/m) of electricity.

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Now let’s assume that you put one of these devices every 5 meters just off of Ocean Beach in San Francisco, from Seal Rocks to Mussel Rock, a distance of 12,000 m. This would amount to 2,400 units (12,000 m / 5 m), which could generate a total of 20,321,280 kWh per day, equivalent to a 846.7 MW plant. Of course you could stagger the units and have one every meter, and you could conceivably have several rows as well. Let’s assume one unit per meter and five rows, or 60,000 units. This could generate 508,032,000 kWh per day, or as much as a 21.168 GW plant (or 21 nuclear power plants).
Of course this is just a quick calculation based on consistent wave heights and wave intervals. In reality there would be a lot of variability with higher generation on some days and almost none on others. The cost of such a system would also be a major factor; not just the design, permitting, and installation, but also frequent maintenance due to the highly corrosive marine environment. It is a fun idea and certainly deserves further study.
Pablo Päster, MBA
Sustainability Engineer
9/17 – Google Map added
9/17 – According to a wave power specialist there would probably be a further 50% cut in efficiency, but my average wave hight was underestimated at 2m. So the final power potential should be somewhere between my original result and half of my original result.