Ocean Energy

[Robert.Boston]

Add hydrokinetic to that mix please -- ocean waves, tides, and currents. Water has much higher energy density than sunlight or wind.

[Robert.Boston]

Robert.Boston: Wave might contribute, but not by much. Britain is very well endowed with wave power - it's ideally positioned right in the path of the waves of the Atlantic. Still, MacKay estimates the upper bound to be 4 kWh/day/person. That's about 2-3% of the needed amount. Tide is good stuff though, particularly because it's reliable. MacKay's estimates for Britain are here, he gets a maximum of about 8% of UK consumption. Britain is exceptionally well endowed with tidal power. We still need to find a way to store 80% of UK consumption for two or three weeks.

He's a smart man, but wrong on this. His calculations mistakenly consider waves to be linear fronts rather than having depth; furthermore, he assumes very poor conversion efficiency. The US West Coast, the British Isles, Portugal, South Africa, and several other regions could easily generate 100% of their total power requirements from ocean waves using technology that is currently under construction. It doesn't solve the problem of matching generation to load, however; we either need a lot of storage (including existing hydro and pumped hydro) or fossil generation.

[eledille]

Robert.Boston, this is really interesting. If you have any links to better info on the energy content of waves, then I would be very interested.

I don't immediately understand how the linear front idea can be wrong - is it not true that incoming waves only can be harvested once?

[Robert.Boston]

Robert.Boston, this is really interesting. If you have any links to better info on the energy content of waves, then I would be very interested.

I don't immediately understand how the linear front idea can be wrong - is it not true that incoming waves only can be harvested once?

At this point, details are still proprietary. Waves have depth, so the energy drawn off the top by a floating device is quickly replaced by energy from lower depths.

[RDoc]

I don't know anything proprietary about wave energy, but I did study oceanography at one point.

An ocean wave internally consists of long horizontal cylinders of water laid on top of each other rotating in opposite directions. Think of a stack of squishy cylinders all aligned horizontally with the top layer cylinders all rotating towards the direction of travel due to wind friction on the surface. The cylinders in the next layer down rotate the opposite way due to friction from the top layer, the next down opposite the ones above, etc. The lower ones have much less power than the top ones since water isn't much of a gear material.

Perhaps the notion is to extract some of that rotational energy.

[eledille]

RDoc: Aha, thanks.

Robert: As far as I can see, MacKay's estimate of total incoming Atlantic wave power to Britain uses the correct formula and is in agreement with published data. His wave formula gives total wave energy content, not just potential or kinetic. After extracting this energy, there is no energy left. This paper by Mollison gives the total wave energy crossing an optimally oriented line on Britain's west coast as 40 to 50 kW/m. Total energy content from waves travelling in all directions is 60-80 kW/m. There isn't any wave energy to speak of on the east coast (4-6 kW/m).

MacKay then assumes that only half of the coastline can be utilized (quite a substantial amount if you ask me), the wave machines are 50% efficient and that they must be arranged in a line and therefore only have access to the energy that crosses that line. 50% efficiency for an energy conversion system is very ambitious and *way* better than any wave machine made thus far. The result is 4 kWh per day per person. This is a substantial amount and I think this should be explored, particularly since wave is much more reliable than wind, but it's nowhere near enough.

Assuming a device that absorbs every bit of wave energy travelling in any direction can be made and deployed along the entire UK west coast, that still only yields 24 to 32 kWh per day per person. This is the absolute upper bound, completely unachievable in practice, and is about 20 to 25% of UK energy consumption, unless the published numbers for total wave energy are completely wrong.

The UK Department of Energy and Climate Change is less optimistic than MacKay, they estimate the UK wave resource at 50 TWh/year, which works out to about 2.3 kWh/d/p. They also note that the UK has 35% of Europe's total wave resource. 40 watts of incoming wave power per millimeter of shoreline is a truly impressive amount! The fundamental problem is that there is only 17 millimeters of western shore per UK citizen.

[dhrivnak]

There is a great book "Sustainable Energy Without all the Hot Air", yes the book is better than the title. The book is by David McKay and is free on-line at http://www.withouthotair.com/. Mr. McKay comes to a similar conclusion that wave power is unlikely to provide more than about more than 4 kw/person. What I like about the book is he goes into what it would truely take to transform to sustainable CO2 free power. And while possible it is very unlikely to come about without nuclear.

He is very pro, solar, wind, tidal, and other forms of sustainable power. But he also shows how challenging it will be to keep everything balanced.

[eledille]

dhrivnak: That's the book I'm linking to above I also think it's very good. Chapter F, here, explores the physics of wave power.

There are two big omissions in his book, though: The first is that he seriously underestimates the difficulty of backing up wind and solar. He considers outages lasting a few days, but the truth is that wind and solar frequently fall below 10% throughout entire continents and for weeks at a time. The second omission is osmotic power. There's an old thread on TMC about it. This technology might provide substantial amounts of reliable sustainable power. Only prototypes exist so far, but I think the future potential may be large. That does not mean that nuclear power can be ignored, of course.