Using minor temperature differences for power generation

[Elver Loho]

Since seawater has significant thermal inertia, would it be possible to create a floating platform, which uses the difference between air temperature and seawater temperature to generate electricity?

 

The generator could be solid state (Seebeck effect), a Stirling engine, or a gas turbine. One would have to reverse the generator for nighttime operation when the seawater is hotter than the air.

 

How would one go about calculating the maximum possible power generation given a temperature difference and the area, which it touches on both the hot and the cold side?

 

Solid state generators seem to have an efficiency of about 5%, Stirling engines 15% - 30%, and gas turbines even higher, although they would be a bigger engineering challenge. Since these things could work around the year and use the difference in stored heat between two materials with different thermal inertia, they could be more efficient in temperate climates than solar panels.

 

Looking at some seawater and air temperature graphs recorded at a latitude of 60 degrees, the daily temperature difference, on average, seems to be about 4-5 degrees Celsius. Given a generator efficiency of 5%, how many watts could one extract using a 10m by 10m floating generator platform?

[Enthalpy]

Hi Elver Loho, welcome here!

 

Oceans contain huge amounts of heat, but this is notoriously difficult to exploit, be the temperature difference with air or, more commonly, with the Ocean's depth, since water is cold already at 30m depth and its heat capacity is more concentrated than air's one.

 

The huge difficulty is the small temperature difference. Carnot's efficiency for a 10K difference around 300K is 10/300 or 3.3% as a theoretical maximum... But the machine must achieve to run in the first place! Imagine a gas turbine: the turbine stage would extract only 3% more work than the compressor takes, and since efficiency is 80% at each, the machine doesn't even rotate! The same holds for any thermal engine. The efficiencies you found are for healthy temperature differences, like 900K hot source and 300K cold sink.

 

The least bad thermal engines for small temperature differences evaporate a compressed liquid to expand the vapour, because the compression of the liquid needs much less work than the expansion of the bulky vapour gives, so the imperfect machine has a chance to run. Forget Seebeck, which is universally bad even under optimum conditions, and Stirling, which is heavy and inefficient as compared with turbines.

 

A lot of creativity was already invested in exploiting the Oceanic thermal gradient - just have a look at the many patents. I did try also. It's a damned hard topic, especially if you remember that seawater dissolves gas (which can't be compressed back to the liquid once evolved) and salts. So "energy is available" is not a new idea; "I know the previous attempts and have arguably found a better method" would be a welcome breakthrough.

 

How many watts, how difficult? A different answer could be: far more expensive than wind power, and we don't know how to do it, while wind power is existing technology.

[CaptainPanic]

All the comments that Enthalpy wrote are very helpful. I totally agree with his post.

 

So, practically your idea has many problems.

You cannot answer the question of how many Watts of power you can get. Speaking purely theoretically, you could extract all the heat in a matter of seconds, and a lot of power. However, then soon the temperature difference between water and air would become the same, and it would stop.

 

So, the question is how fast the sea and air can "replace" the water and air, to maintain the temperature difference. And that depends on things like the weather and the tides, and will be very specific for the location, and I cannot answer that here.

 

So, if you have a patch of ocean available for generating energy, I would agree with Enthalpy that you should build a wind turbine. It's also good for the bio-diversity.

[michel123456]

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