Future Energy Sources 1.3 Energy from the Ocean

I would like to add as possible energy sources:

A. Cultivation of oil producing green algae - genetically engineered.

B. Using the geothermal heat of seafloorspreading.

RHABE

Hi RHABE thank you for the suggestions, I believe these fall under technologies that are already on the Future Energy Sources list that I am using to generate these discussions. If you can think of anything that you would like me to include on the list for future discussion please don't hesitate to drop me an e-mail.

I thought I remember hearing a few years ago about an offshore power station that would generate by harnessing the incoming and outgoing tide - somewhere off the coast of California. I don't remember ever following up to see if it ever started operation or not. I think I remember Popular Mechanics or maybe Popular Science doing a few articles about tide harnessing technology and mentioned a few stations be considered or constructed.

My concerns for this type of power generation would be that it would likely require a huge capital expense and then the payback would turn out to be a measley few MW of power. I think that is why the big power companies are staying away from the ocean - too much outlay for too little electricity return, and don't mess up environmentally or you are basically screwed!

My basic problem with tidal energy is that

it is extracting energy from the rotational momentum of the earth.

Meaning that it will slow down and the days will get longer prematurely.

I also have a problem with geothermal,

in that it is increasing the normal flow of heat from the interior.

Premature cooling of the earths core?

Wave energy, on the other hand, is derived from wind, which derives from solar.

Anything solar derived doesnt upset anything,

as long as the albedo of the planet is maintained.

Guest wrote: "My basic problem with tidal energy is that it is extracting energy from the rotational momentum of the earth. Meaning that it will slow down and the days will get longer prematurely."

I disagree with this statement. The energy extracted from tidal currents would have been transferred to Earth's surface as friction in any case. So even if the tidal currents were the cause of Earth's rotation slowing down (which they are not), it would make no difference.

The cause of the slowing rotation is the heaping of both coastal waters and Earth's crust, which has an angular lead if viewed relative to the Moon's orbital direction. This shifts Earth's gravitational vector relative to the Moon somewhat, causing Earth to transfer some angular momentum to the Moon and losing some herself.

Jorrie

Jorrie

I agree that the sorts of tidal ocean currents that we might be able to harness as a source of energy are a relatively small part of the total. Surely, however, the the 'heaping' you refer is primarily in the molten core and has an associated current both in the core and in the crust; returning to the intended point, presumably it is the energy losses associated with this flow (and in the flexure of the crust) that determine the phase of the gravitational vector.

As a equally academic exercise, I note that the secondary losses, such as could be used for tidal generation, are largely resonant effects*. In this case, increasing the damping could actually reduce the total energy lost in the tidal movement. This is certainly the case for estuarine tidal energy schemes; plus they tend to ameliorate flooding. The potentially negative aspect is changes in the environment due to reduced tidal flow.

Out of purely idle curiosity, do you happen to know whether increasing the flexure/flow losses of the dominant movements would increase or reduce tidal slowing?

Regards

Fyz

Hi Fyz,

I more or less agree with your assessment that the oceanic tide friction is a secondary effect as far as the rotation rate of Earth is concerned - and shouldn't it largely balance out - an equal amount of 'eastward friction' than 'westward friction'?

I think that the resonant effects are significant on large scales, not on estuarine scales. I have not tried to compute the resonant frequency of any estuary, but my guess is that it is far too short for the ~12.5 or 25 hour periods of the Moon's tidal effect.

Lastly, the dominant movements would be in the mantle and crust, where damping is probably no on! So my guess is that whatever we do in the fast estuarine tidal flows makes no difference to the rotation of Earth at all. It may however be ecologically disturbing the system...

Regards, Jorrie

Hi Jorrie

Mostly we agree, but as to "balancing out" of the (admittedly trivial) estuary component - not yet. My preferred "method of analysis" for a quasi-isolated Earth-moon system would be to consider in terms of the total Kinetic energy loss, and balance the equations to maintain total angular momentum (taking into account the changes in both Kinetic and potential energy). Viewed like this, we can easily see that it doesn't matter what is the mechanism or the timing of the energy loss - all that matters is whether the effect is to increase or diminish the total dissipation due to tidal activity.

This is not to say that this gives an idea directly as to the global answer for the effect of an estuary.

Another viewpoint would be to look at the time evolution of the water height (and so its gravitational effect on the Earth's momentum) and compare it with the estuary-free height. Additional delay here would certainly tend to increase losses, up to a maximum at about 6 hours.

Coming back to the effects of using the water's kinetic energy: I think that the local loss to estuarine resonance would actually reduce as we damp the motion using tidal power. If, on the other hand, we simply allow the water to flow upstream and dam the stream for the return flow, we would expect the slowing to increase.

Trivial effects, maybe, but mildly enlightening just the same?

Regards

Fyz

Hi again Fyz

I think my view of "balancing out" is a bit more global than just estuary components. I'm thinking of e.g., the Atlantic Ocean in roughly east-west resonance with the Moon. The oscillation produces highs on alternate sides every 6h25m or so. The effect on Earth's rotation will have the same oscillatory period and should largely average out.

I believe VLBI position measurements of distant radio sources confirm that the slight daily ripples in Earth's rotation speed correlate with tidal periods. I have to look for the reference...

Regards, Jorrie

Hi Jorrie

I'm probably misreading what you say - and so a bit perplexed. I would expect the peaks to be on opposite sides of the globe at the same time (i.e. separated by about 12-hours 26 minutes), with the troughs about 6-hours 13-minutes separated from the peaks.

The proportionate loss (potential energy-change that is converted to heat) would be indicated by the phase difference w.r.t the moon being overhead.

I would expect the largest ripples in the rotation to have semi-lunar-monthly period, as these would be caused by the synchrony between the sun and moon.

Best regards

Phys

Hi again Fyz. You wrote: "I'm probably misreading what you say - and so a bit perplexed. I would expect the peaks to be on opposite sides of the globe at the same time (i.e. separated by about 12-hours 26 minutes), with the troughs about 6-hours 13-minutes separated from the peaks."

This is more or less what I tried to say (not very clearly, I must confess).

In the case of an Atlantic resonance of about 12 hours period: if there's a high in the UK at 00.00 UT, there would be a low at the US east coast. At just after 06:00 UT, it would be low tide in the UK and the US east coast would have the high tide. At about 12:00 UT, the cycle starts again.

My point is that such an oscillating system would not transfer notable angular momentum to the Earth on average. One must remember that the Moon-Sun tidal gravity vector pulls the water both ways (east and west), so I don't think one can favor one direction above the other.

You also wrote: "I would expect the largest ripples in the rotation to have semi-lunar-monthly period, as these would be caused by the synchrony between the sun and moon."

I agree. The diurnal and semi-diurnal ripples are evidence of oceanic tides that largely cancel out over a day. There obviously must also be a semi-lunar period component in the ripples (also canceling out?)

Hope we are nearer to agreement now, but this is really a stimulating exchange of views!

Regards, Jorrie

Hello again Jorrie

I think that we agree that any momentum of (in this case) the water that is reconverted to potential energy will have no effect on the Earth's rotation.

What I've been trying to address is the effects of the losses. Assume that an estuarine bore typically loses some proportion of its kinetic energy between running up and running down the estuary. The lost energy needs to come from somewhere; locally, it's the raising and lowering of the sea. More globally, that comes from the rotational kinetic energy of the Earth-moon system - there is no other suitable source; this is exactly the same as for the bulge in the spheroid of the Earth, the only differences are the sizes of the basic effects and the proportional losses.

To summarise my view - without losses, the Earth will maintain a shape that exerts no accelerational pull on the moon. It is the losses that cause the orientation of the bulge to vary from being parallel/orthogonal to the Earth-moon axis, and that causes an outward movement of the moon and a slowing of the Earth. (I am of course ignoring gravitational waves, which are negligible for this system).

To me, this seems inescapable. It may be that I've missed something - if so, please point me to the escape hatch.

Regards

Fyz

Hi again Fyz:

You wrote: "It may be that I've missed something - if so, please point me to the escape hatch."

It's me that needs the escape hatch! You're obviously right - there can be no losses is the system without having a source. What confused me is that the main tidal currents run both ways (actually mostly circular) and that friction vectors would cancel out. Obviously the summation of the friction loss remains and needs energy...

Fortunately the influence of my mistake is tiny, I hope!

Regards, Jorrie

Thanks Jorrie

No harm done - a minor drop in the Beeblebrox coefficient is always a good thing, however temporary.

Perhaps the point I failed to make is that it's not just the momentum that makes the difference - it's the time delay before running back that means that the water stays high for long enough to break the symmetry about the Earth-moon axis - so it's gravity that has the direct effect on the Earth's rotation. In principle the relative velocity (water to ground) in the Estuary could be higher on the way back than on the way in (possible, if the bulk of the water is great enough, but unusual) - the KE would then be lost at a later time. Returning to the original issue, this was why I said that damping on the way in would improve the situation - because less water would flow up the estuary to disturb the symmetry.

Regards

Fyz

You are correct that wholesale development of geothermal energy will increase the rate of interior cooling. But the greatest change we could envisage would be less than a percent of the total cooling. Even supposing that we could effect a significant change - the first observable effect of interior cooling would be at the poles, and we'd be rather relieved if that would slow the melting of the ice caps.

Turning to longer-term speculation: hopefully, should mankind last another million years, we'll find less troublesome ways to generate energy and geothermal will be abandoned. This will be long before there is any significant effect.

In any event, I feel our responsibility is to get through the next hundred or so years, without leaving any greater mess at the end of the period than we currently find ourselves in. All the proposed low-carbon methods can contribute towards this - provided that the actual implementation is low carbon, of course, and provided that they do not rapidly generate side-effects that are as bad or worse than the problem we are trying to solve - and that could include quite a number of Chernobyl-sized disasters (though that's not necessary, and I'd rather it didn't happen).

Fyz

_________________________

The excellent can be the enemy of the good

You're right physicist, I too, strongly feel that geothermal energy is a safe and more importantly proven, bet. Just look at Iceland, its minting money from geothermal power production. I read that a few smelting(Al, Cu) companies are or already have moved plants to Iceland due exactly this reason.

Oil and gas industry is digging deeper these days and HPHT(High Pressure High Temperature) zone of earth's belly for oil, my point-geothermal energy extraction can pretty much be done any where in the world. All hardware required for these works are already available, and getting better by the day too!

I agree that energy from oceans plays a signicant role as oceans cover 70% of earth suface it will be the one of the best renewable energy as it is largest collector and reatainer of sun's vast energy and this is equal to 16000 nuclear plants and there are many types to capture this power such as wave energy,tidal energy,otec,etc..but nowadays the most important and developed technology is OTEC,which utilizes the temperature to produce the power and the cost for this project is comparably high and there are some risky jobs are also involved as in cases of tsunami and etc..so researchs in u.s are going on to develop this ocean energy to use the enromous energy that available in the oceans.

so i hope that the researchs will make the ocean enregy as one of the vast future energy

I find it interesting that the loss of the earths rotational energy due to tidal energy harvest is discounted by some of you. You do not get energy from nothing. Collecting tidal energy is the harvesting of rotational energy that has been converted to potential energy by the moons offsetting the rotational axis of the earth from it's center of mass as the moons gravity pulls at it. When you harvest that potential energy it can no longer be returned to rotational inertia and you have forever reduced the earths rotational energy. Not only that, but on a large enough scale you can begin to convert rotational energy into vibration as when a car tire is out of balance. Minor problems when you consider the mass of the earth - but when you consider that you can not reverse it I would suggest no large scale tidal energy projects should be done without actually knowing the long term results. Yes the earth is slowing on it's own, but that does not me we should too, in a few years from now a 25 hour day might have far more impact than global warming does now.

Guest - it can help to look at numbers:

I'm not discounting the side effects in principle. In theory, if we could optimise the arrangements so that we extracted all the power that could possibly be gleaned from the oceanic part of the tides, we might increase the earth's rate of slowing by something like 1% of the present value (and even that is probably a gross overestimate). The reason that it is so small is that we can only utilise the difference between oceanic movement and that of the remainder of the earth.

So, we are looking at < 20-us per century. This means that, over a period of 5-million years, such efforts would add 1-second to the rotational period. Not a bad trade-off in the present circumstances, I think. And we could certainly afford to continue using this source for several million years. Of course, extracting all possible oceanic tidal energy would involve engineering on an unimaginable scale, and no-one as yet has any concept of how this might be done.

Another way to look at this would be to estimate the effect on kinetic energy of the Earth if we were to extract all of our power needs from the Earth's rotation. It's quite a simple sum, and all the numbers are readily available on Wikipedia. I'm just suggesting this as a sanity check for anyone who distrusts my numbers - on the basis that it's often more convincing to work these things for yourself.

Add to that that the situation for most methods of tidal energy gathering is quite different - in general our efforts do not increase the total energy loss - they merely divert it from stone-breaking, estuarine erosion etc. into electrical energy available for our use. Some of them may even reduce the total energy loss, because they damp out resonances to the point that the total loss is reduced. (So we might indeed be getting "something for nothing" in your terms)

So, do I worry that tidal energy conversion will cause problems with global slowing? Not really, and neither should anyone else.

However, each scheme will have effects on the local environment - and some would do more overall harm than good; but that just means that each proposal needs to be properly assessed on its individual merits.

Fyz

I find it interesting that the loss of the earths rotational energy due to tidal energy harvest is discounted by some of you. You do not get energy from nothing.

This is a reasonable point so I did a little research on how much energy we consume and found that the worldwide annual energy from fossil fuels is around 0.5 Zj.

The kinetic energy stored in a rotating sphere is

Kinetic Energy = ½ Iω²

ω = Angular Velocity = v / r

I = Solid Sphere = 2/5 m / r²

Therefore substituting we get

KE = ½ x 2/5 (m/r²) (v / r)² = 1/5 mv²

Therefore for earth the total kinetic energy stored in its rotation is

KE_{Earth Rotation} = 1/5 5.97 x 10²⁴ Kg x 465.11² ms^-1 ≈ 260 x 10²⁷ j

So comparing this with the amount of energy we consume each year at 0.5 Zj which is 500 x 10¹⁸ j give us an annual consumption of

KE_{Usage Rate} = Energy_Used ÷ KE_{Earth Rotation}

KE_{Usage Rate} = 500 x 10¹⁸ ÷ 260 x 10²⁷

KE_{Usage Rate} ≈ 2 x 10^-9

or

0.000002% each year

Lets now work out how much the day is going to be extended for each year. Firstly we need to calculate the ratios of the old rotational velocity compared to the new therefore

KE₂/KE₁ = (1/5 mV₂²) / (1/5 mV₁²) = V₂²/V₁²

V₂/V₁ = SQRT(KE₂/KE₁) = f

Rather than solving the equation at this point I have assigned the symbol f to represent the ratios of the velocities. This is because the f is so close to being 1 that it would disappear in a normal calculation and it is difficult to display equations with the CR4 editor. Therefore

v = s/t

(s/t₂) /s(t₁) = f

s/t₂ = fs / t₁

1/t2 = f / t1

t₂ = t₁ / f

Now since t1 is equal to the length of the current day we can say that the difference in the length of the day is

Day_Difference = t2 – t1 = t1/f –t1 = 24 Hr x 1/f - 24 Hr

Day_Difference = 24 Hr x 1/f – 24 Hr

Day_Difference = 24 x 1 / √KE₂/KE₁ - 24

Day_Difference = 24 x 1/√1.00000002 - 24

Day_Difference ≈ 864 μs

So there you have it if we used the earths rotation to replace all the energy we get from fossil fuels we will be increasing the length of the day by less than 1 ms each year or a length of a year by 315 ms

Now that's if all the energy comes from the rotation of the earth but I believe some of it will come for the moons orbital energy so it will probably be less that this. In any case adding a second to the length of a year every 3 or four years would do far less damage to the environment that warming it by the 3-6°C that is currently predicted.

NOTE I have included all the steps in the calculations so that others can follow and check for themselves that the figures are accurate.

Hi Masu

Thanks for doing the sums on the energy. I think you added an order of magnitude when transcribing from 2E-9 to 0.000002%, so the annual change would be 31.5-ms, rather than the 315-ms you suggested. (There were also a few difficulties with the way some formulae actually appeared in the post, but these did not affect the results)

That is as nothing compared with the size of my miscalculation*. I found that I'd written down the daily rate of the earth's slowing instead of the annual rate, so my figure should have been 7-ms per century, and not the rather small figure I gave.
*I checked when I saw that the discrepancy between your figure and the numbers I gave were much larger than the order of 1000:1 that I might have thought plausible.)

I think that both our numbers - after these corrections - could well be in the right order (yours would be much closer than mine, of course); if so, the implication is that, even in principle, true tidal energy could provide no more than a small fraction ( << 0.2% ) of man's energy usage. (Wave-power is something else again)

A fine/final point - you suggested that we could be taking energy from the moon as well. That's not the case, as the increasing tidal loss will further repel the moon (it's a matter of relative direction of motion); so it's potential energy is increasing**. Given conservation of momentum, and the relative moments of inertia (~90:1), the difference will be rather small, I think.
**Without checking, I think the kinetic energy reduces by half the amount the potential energy increases

Fyz

"I think you added an order of magnitude when transcribing from 2E-9 to 0.000002%, so the annual change would be 31.5-ms, rather than the 315-ms you suggested."

You are absolutely correct, somewhere along the line I managed to drop a zero. That makes it adds 86.4 μs to the day or 31.5 ms to the year for every year of use. So the odd second every thirty years just doesn't really give us any sort of problem. I think the natural variations in the earths orbit are greater than this so I really can't see it ever causing us anywhere near the problem that burning fossil fuels will.

This sort of error is why I try and put in as much of the working as possible and is so easy to do when working with numbers like this. The big hassle is having a calculator that works with scientific notation and then working in engineering notation.

When I was a cadet engineer I saved up and purchased a HP-25 calculator and have always loved how it worked. It was brought up on another thread recently and I am seriously thinking of writing a windows application that mimics the HP-25 with a few enhancements. I am thinking of displaying the entire stack, an option to use SI prefixes for multipliers and being able to cut from the calculators output and then pasting directly into documents. It would certainly reduce the number of errors of the kink I made in the previous post.

It will be a couple of months before I have time to do this but what do you think, is it worth the effort?

I agree about reverse Polish notation - saves a lot of hassle and errors. And, at least you missed out on the ln/exponential bug that was on the first HP35 (it cost my employers 3-month's salary, we had to share it initially, and I'm told it was only released in 1972, but I could have sworn I had one a bit earlier than that).

I believe there are a number of free calculators that support RPN. I haven't tried Calculator.net, that is supposed to be a RPN extension of the standard windows calculator, because it says it's 4.5-GByte. I suspect it's "just" 4.5 MByte - still bloatware, but it's probably the template that's at fault. There's a web version by someone called Dario Alpern - supposedly complex numbers from the off, which would be useful if it could handle complex exponentials (it can't). Then there are a number of versions with minimal charges: the cost is low, but the risk that you're just wasting your time is high.

I did think about doing a macro in the open-office spread-sheet, but somehow there's always something more interesting or useful to go at.

So, yes, it's a great idea - but could be more time-consuming than you imagine to do everything you'll find you want.

Fyz

Using heat tubes to use the heat from the heat vents to drive turbines or Stirling engines. 700 degree water from the vents and using long insulated heat tubes with the right evaporative medium and you could generate enough pressure to drive a good sized turbine.

Using the temperature difference to drive a Stirling engine would work. 700 degree water pushed up through an insulated pipe to one of the really cold layers - hot and cold - ideal for a Sterling engine.

Of course there are a few issues: The unique life forms that depend on the hot water and nutriments - warming the cool layers and cooling the hot layers. Maybe we shouldn't even care - we don't know anything about most of them anyway - maybe don't care either?

I know that there are Sterling engines that operate on as little as 20 degree differences in sea water temperatures. But look at 700 degrees difference!

It's quiet possible that you could manufacture a thermal vent by drilling a bore hole then fracturing the surrounding rock with explosives. If it works then you can bypass all the problems associated with messing up the ecosystem around existing ones. Then again I can guarantee there would be a fringe group that would have some sort of grumble about it.

I realize it is important that we look at the whole picture and minimize the damage we are doing to the environment but no matter what we do there will be some sort of trade off. The idea is to minimize the damage and maximize returns.

Australia is in the midst of the worst drought since records began and it's believed to be at least a once in a thousand year event. As a result Sydney's water supply is at a dangerously low level and unless something happens we will run out of water in mid 2008. The last dam and reservoir was completed in the late 1950s and when it was completed the planners believed that Sydney's water supply would be adequate till the beginning of the 21^st century. There were plans to build a further dam and reservoir but the usual fringe groups managed to cause the project to stagnate for half a century. Guess what the 21^st century has arrived, there is no new dam and Sydney is running out of water. The government has been forced to react with a fast tracked desalination plant but even this has people complaining because it means the loss of something like a dozen trees. All I can say is get real folks which is worse 4.5 million people running out of water of a few trees in somebody's back yard?

A similar sort of problem came up with a wind farm that was planned to reduce the need for expanded use of coal fired plants. Again the fringe groups complained that it might hurt some of the birds that occasionally flew around the area of the proposed farm. The result was the developers got fed up with the delays and went overseas. Now we are going to end up burning more coal to make up for the shortfall. So what is worse, the possibility of a few dead birds or a few hundred thousand tonnes of CO₂ and other pollutants being vented to the atmosphere.

Engineers have made some mistakes in the past and there have been some catastrophic disasters but they pail into insignificance when you talk about Sydney running out of water. No matter what we do there will be an impact on the environment and minimizing that impact is important but we have to accept that there will be some damage. If we sit around and do nothing on the whim of it might be bad we are doomed to extinction.

I realize this thread has bee idle for quiet some time now but I just saw a program on television about CETO technology that is being developed by an Australian company to harness wave energy.

Basically the system consists of a system of flotation devices that are anchored to fixed points on the sea bed and tied so that they sit just below sea level. As waves move over the flotation devices they produce massive fluctuations in the restraining force that is used to hold the flotation device in position. This fluctuation is harnessed by using it to pump sea water to an on shore hydroelectric power generating plant. The water is then either returned to the ocean or desalinated and used as drinking water.

Very neat, simple, reliable and the areas required to generate the water and power for major cities is remarkably small.

Let's hope the scaling all works out and that it is economic as well.