Energy From the Oceans

Ocean Thermal Electric Conversion

The oceans are a huge heat engine and have been absorbing solar energy for the last 4 billion years.

Temperature differences, caused by differences in insolation both in latitude and in depth.

More promising technology is OTEC (Ocean Thermal Energy Generation). This takes advantage of the fact that the ocean is an enormous heat engine.

This in fact is a "global solution" and represents a chance for the world to collaborate on various infrastructure and production facilities.

To wit:

On an average day, 60 million square kilometers (23 million square miles) of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil.

If less than one-tenth of one percent of this stored solar energy could be converted into electric power, it would supply more than 20 times the total amount of electricity consumed in the United States on any given day.

Physics of Heat Engines:

• efficiency = work done/energy input
• it can be shown that this is equivalent to
  efficiency (in %) = 1 - T1/T2 ; T1 < T2

  T is measured in Kelvins

• So, in principle any two reservoirs with different temperatures T1 and T2 can produce energy.

  Examples:

  • Boiling Water/Ice Water combination:
    eff = 1 - (273/373) = 27%

  • Solar Thermal Molten Salts/Water at Air Temperature combination:
    eff = 1 -(300/1000) = 70%
    
    
    
    Thermodynamic Constraints:
    • Systems are in equilibrium when they are at the same temperature

    • energy is conserved within a closed system

    • it is not possible to extract heat energy from a reservoir and perform work without transferring heat to a reservoir of lower temperature. In other words, all thermodynamic systems must tend towards equilibrium. Some energy goes towards performing work and some is lost as waste heat.

    To get the highest efficiency one wants to maximize the difference between T1 and T2 but in most natural environments nature doesn't really do this.

    Do this:

    

    

    

    Basic principle is that heat difference is used to condense a steam into a liquid then return it to be reheated.

    Since heat differences in the ocean will be smaller, then one must substitute ammonia for water as the working fluid.

    
    This 50-m high, 100-m diameter concrete facility is to generate 100 MW of electric power

    Example Calculations:

    • Surface Ocean temperature is 25 degrees C (298K)

    • At 500-700 meters depth the temperature is 5 degrees C (278K)

    • Efficiency = 1 - 278/298 = .067 (6.7%)

    • Power in cooling 1000 gallons of water per second by 2 degrees C is 32 MegaWatts (because water has such high heat capacity/storage)

    • using 6.7% efficiency would then yield 2 Megawatts (1/500 typical coal-fired plant)

    • But this is only for 1000 measly little gallons per second

    Possible Additional Benefits:

    • can also serve as a desalinization facility
    • Can produce mineral rich salts.
    • Spraying back of deep seawater onto the sea surface may help cultivate richer fishing grounds.
    • Can produce Hydrogen!

    OTEC Potential Sites:
    • Florida, Puerto Rico, and Hawaii
    • Indian Ocean
    • Northeast Australia, Indonesia and Mexico

    Above sites typically have thermal gradients higher then 22 degrees C

    Energy extracted comes from the cooling of the warmer water this is transferred to the ammonia which does the actual work of turning the turbine (as ammonia steam)

    Energy extracted proportional to the volume of water and the temperature it drops.

    Principal energy loss is when the warmer water meets the cooler water in the condenser.

    Review of OTEC (Ocean Thermal Energy Conversion)

    • Thermal gradients of greater than 22 C can be exploited and used as a heat engine

    • Energy is derived from cooling warm surface water to the temperature of the water at approximately 500-1000 feet depth.

    • The maximum surface temperature of ocean water is 25 C and its minimum value is of course 0 C

    • efficiency is then 1 -(273+0)/(273+25) = 1 - 273/298 = 7%

    • Energy is derived from the cooling water via transfer to a working fluid such as ammonia which when mixed with warm water vaporizes to steam and powers a turbine

    • Ammonia returns (condenses) to liquid when mixed with cooler water at depth and then the cycle repeats itself

    • Since the volume of water in the oceans is huge, the capacity in just the Gulf Coast Waters alone is several 10's of Giga

      More on OTEC

      Hawaii Facility