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Comment by bumby | Hacker News Reader

I don't work in the industry, but I've often wondered if the allure of wave energy comes from the fact that the theoretical energy available is proportional to the density of the fluid. So a turbine driven by water would have a much higher theoretical power capacity than one in the air for the same fluid speed.

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Quick note, confusingly throughout this thread people have used the phrase "wave energy" as essentially a synonym for "tidal energy", they are separate technologies sometimes talked about under the combined category of ocean energy or marine hydrokinetics.

Increased power density is indeed one allure of wave energy vs wind. In the same way that wind is concentrated solar energy, studying the mechanisms and origins of waves show they are essentially a form of concentrated wind energy. So if the difficult design challenges are solved, there is potential for greater energy per unit of structural material which some believe could be associated with competitive cost of energy.

Other allures of wave energy include that it is fairly consistent, with some seasonal variations, and its decoupled from solar or wind which decreases the chance that all renewables resources aren't producing at the same time. Of course this is talking about wave energy where oscillations occur with a period of approximately 4-18 seconds rather than tidal with several periods per day, and for these periods of time flow is essentially unidirectional. Tidal energy, like the link in this thread, often uses underwater turbine but wave energy devices rarely use this design.

Thanks for clarifying! So are the true wave energy designs the variants that use vertical oscillations rather than propeller designs?

Wave energy would be any type of machine that uses the motion of individual waves or swells, rather than tides.

There is a wide range of ideas, most are at small scale, early stage, and no winner has emerged the same way that wind energy has seen the 3 bladed turbine win out.

Wave energy devices exist using almost any type of power take off mechanism - direct drive electric generators, linear generators, compressed air, and hydraulic actuators. Some may even use propellers in water but that's not as common since direction is always changing.

> the allure of wave energy comes from the fact that the theoretical energy available is proportional to the density of the fluid.

But the power generated by a turbine is proportional to the cube of the air/fluid velocity..... and the wind normally moves much faster than tidal flow.

Right, that's why I wanted to make sure to add the caveat about fluid velocity. But the density of water is almost 1000x that of air so there's a large benefit to water at lower fluid speeds. A rough back-of-the-envelope calculation shows the crossover is when airspeed is about 9.2x the speed of the water flow under perfect theoretical conditions.

It's also worth noting that when dealing with centrifugal devices, the realized power never actually follows the power law. It's usually degraded quite a bit due to linear losses. A common power used in air systems is 2.1 rather than the cube; using that value, the airspeed must be almost 24x the water velocity. I've never personally seen anything greater than 2.7 (the higher numbers, IMO, tend to be used as a sales pitch rather than an engineering decision), but again, I don't work in these particular systems.

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