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Ocean Thermal Energy Conversion

Discussions of conventional and alternative energy production technologies.

Re: Energy from Ocean

Unread postby Tanada » Fri 01 Oct 2010, 12:44:25

RankineCycle wrote:Ocean thermal electricity generation is in most cases a losing battle, the temperature difference is so low that the work required to pump the water is greater than the work that can be extracted from the differences in temperature of said water.

I've done an idealized analysis on this. It will only work at all where the cold and warm water bodies are close to one another. The turbine efficiency is still horribly low (less than 5%) and must use a low-boiling point working fluid like R-134a.

Using ocean water as a source or sink for a heat pump, however, is a more reasonable proposition.


You are making machinery do a lot of un-necessary work the way you figured out this problem.

Think of it this way, you put a large diameter pipe from 3000 feet depth up to your shore lagoon facility. Have the pipe attach to the low end of the artificial lagoon you are using with a sealed structure. Enclose the lagoon from the open ocean with a dike. Pump the water in the lagoon over the dike into the surface waters. As you reduce the water level in the impounded lagoon the pressure goes down and creates a differential at the mouth of the pipe, the cold bottom water then flows up the pipe to replace the volume you are pumping out into the surface waters. Because of natural forces you only have to use enough energy to pump water as high as the top of the dike, nature abhors a vacuum and will replace the volume you remove through the only available pathway, up the deep pipe to the lagoon at the surface.

If that seems too complex for you, you can also run the system in reverse. Pump warm surface waters over the dike into the lagoon. What happens? The water level in the lagoon will go up until the pressure exceeds the density differential between the warm surface water and the cold bottom water. That is not actually very much, a few inches of elevation will do it, after which time the warm surface waters will be forced down the pipe to the cold bottom level. Put your thermocouples around the bottom of the pipe where the warm water exits and all you need are surface pumps pulling water into the impounded lagoon. For max efficiency use insulated pipe so that the waters maintain temperature all the way to the other end.

I prefer pulling the bottom water up because it is nutrient rich and when you dump it into the surface waters you will draw in all sorts of plankton, which in turn will bring in fish and other life. Either way let nature do the heavy lifting of filling the partial vacuum you create, don't do all the work mechanically.
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Re: Energy from Ocean

Unread postby anador » Fri 01 Oct 2010, 14:55:05

I never considered that.

That is definitely superior to the what I had always pictured at the coast.

I always imagined a sort of double sided dike separating a large basin, the bottom elevation of which is the level of lowest low tide.

As the tide comes in and fills the basin it must pass through holes in the "dike" fitted with turbines, then as the tide goes out the water flows over the same turbines in reverse generating power twice and each tide.

The water can be pumped to raised lagoon "battery" tanks that release the water in a constant and regular stream and provide constant power between tides.

Depending on the area of the lagoon it could produce a significant amount of power, but use alot of land
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Re: Energy from Ocean

Unread postby RankineCycle » Fri 01 Oct 2010, 23:59:14

I filled a 5-gallon bucket with water, stuck a 1/4-inch diameter polyethylene hose at the very bottom and started a siphon. I brought the other end of the hose up to just below the water level in the bucket, and it dribbled out ever so slowly. So yes, if I've interpreted your suggestion properly a system such as that could work and cut the pumping power down from the pure mass-flow-rate*gravity*height requirement I used in the calculation.

I did, however, neglect pipe friction (still in 3000 m of pipe), heat exchanger friction, imperfect heat exchange, etc., which become a big deal at those low temperature difference/high mass-flow rates. Though I will change the page and mention that the water column pressure may be used and m-dot*g*h for the whole pipe length is not necessarily required.

Another alternative method I have seen mentioned recently is to pump the heat transfer fluid vapor to the bottom, condensing it there, and returning the liquid back to the surface. Most of the heat is transferred in latent form (when the vapor condenses), so the flow rate can be lower, pipes smaller, the whole deal.

A utility-scale OTEC plant would require serious engineering, everything would have to be optimized just to generate an energy yield when turbine inlet/outlet temperature difference is just a few degrees. Pipes would have to be sized to impart minimal friction, insulation would have to be impeccable, heat exchange surfaces very large, the ocean water contact surfaces would have to be cleaned often to ensure that things stay optimized, and the turbine could not let a single expanding vapor molecule go to waste.
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Re: Energy from Ocean

Unread postby Outcast_Searcher » Sat 02 Oct 2010, 03:11:10

All this is great stuff with fantastic promise. Just like solar, geothermal, etc.

Isn't the base issue cost and available resources?

I have little doubt that we COULD, as a species, convert to a largely green energy society in, say, 30-50 years -- IF we would cooperate and pursue it like our existence depended on it. (As it very well may).

However, trying to convince people of that, get people to think long term, get people to cooperate, get people to make meaningful sacrifices to provide the HUGE needed resources -- good luck getting that to happen before the problem is obvious to all and IN OUR FACE.

By then, I shudder to contemplate the amount of pain it will take to fix it.
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Re: Energy from Ocean

Unread postby RankineCycle » Sat 02 Oct 2010, 10:26:52

Well the cost of renewables (especially OTEC) is only an issue because, from an energy standpoint, we are trying to convert lead into gold. In other words, trying to convert dilute sunlight and whimsical winds into that always-on, invisible does-it-all servant we call electricity, or corn stalks and garbage into liquid fuel good enough to burn in a modern computer-controlled engine to keep us rolling in our sanctuaries-on-wheels.

If we used energy (and other resources) according to the level of quality required (Use the lowest quality necessary to complete the task), the cost would be lower. But it would often mean reverting to such arcane things as clotheslines, bicycles, houses oriented to use solar heat, compost piles, sawtooth roofs instead of electric lights and mechanical fans, placement of industries to use each other's waste heat and materials, etc.

I think many people see that as a submission to nature, a reversion of the progress which is measured by the domination over nature. The second law of thermodynamics doesn't care, however.
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Re: Energy from Ocean

Unread postby sparky » Tue 21 Dec 2010, 04:12:05

.
Ever heard of the Carnot cycle

http://en.wikipedia.org/wiki/Carnot_cycle

It calculate the theoretical efficiency of a power thermal cycle as directly proportional to the difference in temperature.
It is the founding principle in power generation , proven for two hundred years of the industrial revolution .
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OTEC revisited

Unread postby prajeshbhat » Mon 09 Jan 2012, 05:25:39

http://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion

Ocean thermal energy conversion

Ocean Thermal Energy Conversion (OTEC) uses the difference between cooler deep and warmer shallow or surface ocean waters to run a heat engine and produce useful work, usually in the form of electricity.

A heat engine gives greater efficiency and power when run with a large temperature difference. In the oceans the temperature difference between surface and deep water is greatest in the tropics, although still a modest 20 to 25 °C. It is therefore in the tropics that OTEC offers the greatest possibilities. OTEC has the potential to offer global amounts of energy that are 10 to 100 times greater than other ocean energy options such as wave power. OTEC plants can operate continuously providing a base load supply for an electrical power generation system.


http://www.youtube.com/watch?v=JepAHqlOgx8&feature=related

The idea is to pump cold ocean water at 5 °C from the depths of 1000 m or more and use it to create a temperature differential of 20 °C with the surface ocean water which is at 25 °C. This is enough to run a turbine using a low boiling point working fluid like ammonia. The conversion efficiency is 1%.

Here's my question. Why cant they use ice? In countries like Canada and Russia, ice is plentifully available which can be used as heat sink. For the heat source, there is always the underground ( at the depths of 3-4 m underground, the temperature is consistently 15-20 °C). This way you get a temperature differential of 40 °C or more. So you get twice the efficiency. And there is no need to pump several millions of gallons of cold water from the depths of 1000 m under the sea every day So the cost would be significantly less. I know that right now both Canada and Russia are enjoying the Oil money. But what about in the future?
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Re: OTEC revisited

Unread postby Keith_McClary » Tue 10 Jan 2012, 01:42:31

prajeshbhat wrote:Why cant they use ice? In countries like Canada and Russia, ice is plentifully available which can be used as heat sink. For the heat source, there is always the underground ( at the depths of 3-4 m underground, the temperature is consistently 15-20 °C). This way you get a temperature differential of 40 °C or more.
Underground is only 10C in southern Alberta. This is used for geothermal heating (using heat pumps) but it is expensive to drill in pipes to extract it.

Ice or snow may be plentiful for a couple of months a year, but is spread out and would be expensive to haul to your heat engine.
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Re: OTEC revisited

Unread postby prajeshbhat » Tue 10 Jan 2012, 07:13:27

OTEC doesn't seem to be a bright idea to begin with. You don't have to pump water from 1000m under the sea to get a 20 °C temperature differential. A simple solar water heater (black metal pipe placed inside an evacuated glass tube will give you a slightly higher temperature differential.

Here is a video from a company that claims they can tap this small temperature differential to generate electricity

http://www.youtube.com/watch?v=B_M8x7WttAA
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Re: OTEC revisited

Unread postby SeaGypsy » Tue 10 Jan 2012, 07:22:29

Pumping water up 1000 meter up through water is vastly different an equation than 1000 meters up above water. I was thinking about the differential example of Alberta. With 10c below ground year round, how much of the year would there not be a significant difference between that and the air temperature?
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Re: OTEC revisited

Unread postby prajeshbhat » Tue 10 Jan 2012, 07:35:00

SeaGypsy wrote:With 10c below ground year round, how much of the year would there not be a significant difference between that and the air temperature?


The air temperature will be much higher than the temperature just a few feet under ice surface.

But if a pipe placed under a large heap of ice can reach a temperature of -40 °C, then we have temperature differential of 50 °C.
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Re: OTEC revisited

Unread postby SeaGypsy » Tue 10 Jan 2012, 08:04:51

Is anyone building anything to make this usefull?
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Re: OTEC revisited

Unread postby mstraub » Tue 10 Jan 2012, 10:39:46

Seagypsy,

There are people in the process of building very useful OTEC projects. Take a look at the Bahamas, an American company has deals in place to build 2 commercial OTEC plants. The Bahamians are excited to cut their fossil fuel costs, have reliable power (their ancient fossil fuel plants can't handle the demand), and enjoy the millions of gallons of clean water produced in an OTEC plant.

You can read up on the Bahamas deal here...
http://www.theonproject.org/2011/the-ba ... =mscomment

The same company also has a deal for a Sea-Water District Cooling system at a new mega-resort in the Bahamas. It's an OTEC system using cold sea water in an air-conditioning system. It drastically cuts the energy costs of the resort, and also provides clean drinking water as a byproduct.

OTEC does some serious good in tropical regions, and many people are starting to take notice.
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Re: OTEC revisited

Unread postby prajeshbhat » Thu 12 Jan 2012, 06:12:56

Check out this video at around 1:50. The guy uses ice to rotate a turbine.

http://www.youtube.com/watch?v=2japP5d0qCI

So basically, a powerful heat sink is just as good as a heat source.
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Re: OTEC revisited

Unread postby SeaGypsy » Thu 12 Jan 2012, 06:27:39

Interesting, I had heard of some early stage experimental projects, but nothing upscale or commercial yet.
Here in Oz we just had a major setback to geothermal, with the collapse of the first serious wells due to granite fractures caused by thermal shock. I know the Philippines is looking into OTEC, besides wave and tidal and wind, they have ultra deep ocean just offshore on the Pacific side.
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Ocean thermal energy conversion

Unread postby Graeme » Sat 15 Jun 2013, 20:40:40

Ocean thermal energy conversion

Ocean Thermal Energy Conversion (OTEC) uses the temperature difference between cooler deep and warmer shallow or surface ocean waters to run a heat engine and produce useful work, usually in the form of electricity. However, the temperature differential is small and this impacts the economic feasibility of ocean thermal energy for electricity generation.

The most commonly used heat cycle for OTEC is the Rankine cycle using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia or R-134a. Open-cycle engines use vapour from the seawater itself as the working fluid.

OTEC can also supply quantities of cold water as a by-product. This can be used for air conditioning and refrigeration and the fertile deep ocean water can feed biological technologies. Another by-product is fresh water distilled from the sea.[1]

Demonstration plants were first constructed in the 1880s and continue to be built, but no large-scale commercial plants are in operation.


Hainan[edit]
On April 13, 2013 Lockheed contracted with the Reignwood Group to build a 10 megawatt plant off the coast of southern China to provide power for a planned resort on Hainan island.[25] A plant of that size would power several thousand homes.[26][27] The Reignwood Group acquired Opus Offshore in 2011 which forms its Reignwood Ocean Engineering division which also is engaged in development of deepwater drilling.[28]


Cost and economics[edit]

For OTEC to be viable as a power source, the technology must have tax and subsidy treatment similar to competing energy sources. Because OTEC systems have not yet been widely deployed, cost estimates are uncertain. One study estimates power generation costs as low as US $0.07 per kilowatt-hour, compared with $0.05 - $0.07 for subsidized wind systems.[34]
Beneficial factors that should be taken into account include OTEC's lack of waste products and fuel consumption, the area in which it is available,[citation needed] (often within 20° of the equator)[35] the geopolitical effects of petroleum dependence, compatibility with alternate forms of ocean power such as wave energy, tidal energy and methane hydrates, and supplemental uses for the seawater.[36]
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Re: Ocean thermal energy conversion

Unread postby Graeme » Sat 15 Jun 2013, 20:48:51

And it just so happens that an article about this topic was published yesterday!

Ocean Thermal Energy Conversion Technology And Its Possibilites

The ocean absorbs energy from the sun, stores it, and then releases it slowly. Sounds like a prescription for meeting the world’s energy needs, since the ocean is the largest feature of our planet. But can ocean energy be tapped in a way that doesn’t create more problems than it solves?

That’s the promise behind a recently announced deal between Lockheed Martin and Reignwood Group, a resort developer based in Beijing. The two companies will develop a 10-megawatt power plant using ocean thermal energy conversion (OTEC) technology in waters off southern China’s Hainan Island. Construction is expected to be completed in 2017.


Why the delay?

If OTEC is so great, why has it been in development since the late 1800’s but never commercialized? One reason is that there are only so many suitable sites – conventional OTEC plants need to be onshore yet close to deep water, and the surface water must be quite warm. Another reason is that the capital costs are high while the energy yield is low – the process isn’t very efficient. And yet another reason is that it is technologically challenging to lay a pipe long enough to reach deep water – and keep it there.

Despite these challenges, scientists and engineers have remained intrigued enough with OTEC’s possibilities to keep working on it for all of these decades. The potential sites have been expanded by using floating platforms, as with the plant Lockheed Martin is building, minimizing pipe length. Other innovations allow OTEC to extract energy from smaller temperature differentials. Efficiency has been increased through the use of better heat exchangers and advanced materials. Pipe manufacturing, laying methods and maintenance have all vastly improved. And while OTEC remains quite costly, the rising price of oil has made OTEC and other forms of renewable energy much more competitive in recent years, as our paper outlines. These factors make me think that OTEC may be on the verge of commercialization.

In fact, in our paper we concluded that the project in southern China was the missing link in commercializing OTEC. Several small OTEC plants have been built already, so we know that the concept produces net energy and all of the other benefits described above. But the costs associated with scaling OTEC up have been very uncertain, perhaps scaring off potential investors and customers up till now.


earthtechling

Please read comments at end.
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Re: Ocean thermal energy conversion

Unread postby diemos » Sat 15 Jun 2013, 21:02:30

Graeme wrote:Demonstration plants were first constructed in the 1880s and continue to be built, but no large-scale commercial plants are in operation.


and there's your giant blinking neon sign of a clue that let's you know just how practical this technology is.
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Re: Ocean thermal energy conversion

Unread postby Graeme » Sat 15 Jun 2013, 21:37:15

As I said, please read links in comments esp on Blue Revolution here and here. I found over 90,000 links on this topic in google scholar. Here's one:

Estimates of global Ocean Thermal Energy Conversion (OTEC) resources using an ocean general circulation model

Global Ocean Thermal Energy Conversion (OTEC) resources are assessed for the first time with an ocean general circulation model (OGCM). Large-scale OTEC operations are represented with fluid sources and sinks of prescribed strength in global (4° × 4°) MITgcm simulations. Preliminary steady-state (time-asymptotic) results show similarities, but also significant differences with earlier one-dimensional (1-D) studies. It is confirmed that global OTEC resources are likely limited by OTEC flow effects on the stability of the vertical oceanic thermal structure. Such a limit is several times greater in a full three-dimensional context, however, with an estimated maximum annual OTEC net power production of about 30 TW. The significant OTEC flow rates corresponding to maximum net power output would result in a strong boost of the oceanic thermohaline circulation (THC). In contrast to simple 1-D analyses, the present simulations of large-scale OTEC operations also show a persistent cooling of the tropical oceanic mixed-layer. This would be balanced by a warming trend in the higher latitudes, which may practically limit OTEC deployment to smaller flow rates than at maximum net power output. An annual OTEC net power production of about 7 TW, for example, could be achieved with little effect on the oceanic temperature field.


sciencedirect

Apparently, James Baird has applied for a patent for his system.
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Re: Ocean thermal energy conversion

Unread postby ian807 » Mon 17 Jun 2013, 12:28:38

Oddly, this idea could work, but not as most people envision. Free energy, in this case, is defined by temperature differentials between surface and deep water. There's just not much there.

In contrast, an array of closed cycle heat engines, each with one end dropped down to a deepwater volcanic vent to take advantage of the differential between boiling hot water and the surrounding cold water might generate enough power to be worth the trouble.

Of course, the problems of loss don't go away. Volcanic vents aren't conveniently located around coastlines. Even if you cranked up the voltage, it would be tough to deliver that power where it's needed.
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