Energy From the Oceans

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

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. There will be a demonstration of this principle in class today.
  • Boiling water T = 373K
  • Ice Water T=273K
  • Liquid Nitrogen T=77K

• Efficiency of Boiling water and Ice water: 1 - (273/373) = 27%

• Efficienty of Ice water and LN₂: 1 - (77/273) = 72%

• What is efficiency of Boiling water and LN₂?

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 then their are material problems (containers melt, freeze, etc)

Typical Case:

• Coal-fired burner: T = 825K
• Cooling tower: T = 300 K
• efficiency = 1 - 300/825 = 64%

How this all works:

Exhaust steam is condensed back into liquid thereby decreasing its total volume by a factor of 1000

Therefore the work done by the pump is down by a factor of 1000 compared to if it had to pump steam directly back into the system

Heat Energy from the Ocean

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.

Example Calculations:

• Surface Ocean temperature is 25 degrees C (298K)
• At 1000 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

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

  Ocean Power also comes in 2 other forms:

  • Tidal Energy
  • Current Flow Energy

  The ocean is a huge reservoir for storing the energy of the sun that is incident on the earth. How huge is huge?

  Incident flux on ocean surface area is 10¹⁷ Watts or 0.1 Billion Billion Watts (its a large number)

  The oceans are a huge heat engine. Temperature differences, caused by differences in insolation both in latitude and in depth.

  • Equatorial waters warmer than higher latitude waters
  • surface layers warmer than deeper layers
  • This sets up an enormous circulation network
  • 

  Major currents are shaped by:
  • Temperature differences (driven mostly by tilt of earth's axis)
  • Prevailing wind patterns interacting with the surface waters (again driven mostly by tilt of earth's axis)
  • the rotation of the earth the Coriolis Force
  • shorelines of continental masses

  Tapping the Current for Energy:

  • Gulf current has 1000 times the flow of the Mississippi River(!)
  • Current averages about 5 mph
  • Density of water is higher as well
  • Its always there - no intermittency problem no need for energy storage
  • Build Turbines for underwater use
  • Anchor a foundation to the ocean floor
  • hundreds of miles long rigged with turbines
  • cables on ocean floor to shore deliver the electricity

  • An engineering challenge but there are few bad side effects from producing energy this way
  • Obviously the capital costs are huge in this case but this does represent a Large Scale Solution

  Tidal Energy from the Ocean

  Extracts energy from the kinetic energy of the earth-moon-sun system.

  Variations in water level along coastlines can be used to drive turbines technology is the same as low-head hydro power

  Vertical tides on US coast range from 2 feet in Florida to more than 18 feet in Maine

  To enhance efficiency of turbines driven by tidal currents, it is desireable to build a damlike structure across the mouth of a tidal basin in order to direct the flow to a turbine

  Turbines designed for work at both high and low tide (inflow or outflow)

  Intermittent tidal flow is major problem. Tidal facility produces about 1/3 the electrical energy of a hydro facility of the same peak capacity

  Two tidal plants in the world:

  • 1 MW facility on the White Sea in Russia (1969)
  • 240-MW on the Rance River, St. Malo France (1967) has 750 meter long dike to impound tides that can be as high as 13 meters (!)
    

  Proposed New Facilities:

  • Phillipines
  • Apsley Strait, Australia
  • A serious commercial venture
  • Large Scale production in the UK

  Potential Sites in the US

  • Alaska ( Cook Inlet)
    

  • Bay of Funday (US-Canadian Border; NE Coast of US) most favorable site in the World would produce about 30,000 MW in total (1/2 for the US)
    

  • A 20 MW demonstration plant has been built here
  • Locally (New England) this is a potential important source of power but on national scale is just a few percent of our (insatiable) need for power

  Bottom Line: There aren't many favorable sites in the world for tidal power and the estimated capacity is 50 times smaller than the world's hydroelectric power capacity.

  

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