PROS AND CONS OF OPEN AND CLOSED SYSTEM OTEC
OCEAN THERMAL ENERGY CONVERSION
Open and Closed System OTEC
, or Ocean Thermal Energy Conversion use the temperature difference between warm surface waters and deep, cold waters to create a pressure difference in a gas that can then be used to generate electricity.
Rather than boiling water to make steam, closed system OTEC uses one of several refrigerants that boil and create pressure when exposed to the warmth of surface waters, and condense and lose pressure when exposed to cold temperatures from deep water.
Once this pressure difference is created all that has to be done is to channel the high-pressure vapors through a turbine to drive a generator.
Open system OTEC is a bit different than closed system OTEC. In open system OTEC, cold water is used to reduce pressure in part of the system so much that warm surface water actually boils into a vapor at about 80 degrees Fahrenheit.
The water vapor travels from the high-pressure warm side of the system, through a turbine to drive a generator, and into the low-pressure cold side of the system where the vapor condenses into desalinated water. The big advantage of open system OTEC is that the desalinated water effluent has multiple uses, especially drinking water, and can increase economic efficiency by over 30%.
Other systems combine closed system OTEC with open system OTEC to form a Hybrid system that can offer some of the advantages of each system.
The essence of both open and closed system OTEC is that they bring cold water from the depths and use it with warm surface waters to generate energy.
ENVIRONMENTAL FRIENDLINESS -
Both open and closed system OTEC reach down about two thousand feet to the cold depths of the ocean. These systems will either involve power generating ships and power cables leading ashore, or they will be shore based facilities with large cold water intake pipes running along the ocean floor.
Maybe the largest environmental concern about OTEC is the possibility of upsetting the thermal balance of the oceans if these systems are adopted in large numbers. While wide scale use of this technology is probably decades in the future, humanity needs to assess any potentially negative impacts on our oceans.
For the present, the deep cold water that OTEC systems bring to the surface carries with it rich nutrients that create plentiful feeding grounds for near surface marine life. As OTEC systems create energy, they can also create some of the most plentiful fisheries known.
Our history suggests that we will more fully assess environmental impacts of OTEC as its use grows. How fast use of this resource grows depends largely on its ability to compete against other power generation technologies.
OTEC power has historically been expensive because low temperature differences mean low system efficiencies; which translates to a lot of money spent on equipment for a low power output.
The upsurge in fossil fuel prices, recognition of the full costs of fossil fuels (for example reclaiming land damaged by strip mining for coal), concerns about global warming, and energy security worries have caused investors to reexamine OTEC.
The focus now is on combining OTEC with things like desalination for fresh water, cold root crop production, building cooling, or other enterprises. Any of the combined usages of cold water brought from the depths can mitigate electrical power generation costs to make it economically competitive with conventional power generation.
As of this writing, open system OTEC seems to have some advantages over closed system OTEC. Open systems are simpler in design and have the added bonus of providing fresh water with multiple secondary uses that economize plant operation.
As discussed in this
World Energy Council OTEC Report
when all factors are considered, OTEC is competitive with fossil fuel electrical generation now. It’s only a matter of proving that to investors.
The report is full of OTEC information including diagrams of OTEC systems.
The most economical areas are those with high differences in temperature between surface waters and deep waters. These areas are found in the tropics in areas extending to about 20 degrees north or south of the Equator.
U.S. Department of Energy National Renewable Energy Laboratory OTEC Information
, the waters of the tropics absorb daily solar radiation equal in heat content to about 250 billion barrels of oil.
The theoretical practical generating potential of OTEC is estimated to be about 10 Terrawatts. That much installed capacity operating 24 hours a day would generate 87,600 Terrawatt hours of electricity annually.
How much electricity is that? Let’s look at the numbers.
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DOE 2004 Global Electrical Production Data
shows that we generated about 16,591 TWh that year.
[TWh: Terrawatt hours – One million Megawatt hours, or one billion kilowatt hours, or about 3.44 Trillion Btus.
Btu: British thermal unit; the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit.]
In other words, fully exploited OTEC could provide over five times more electricity than is currently used globally.
That’s not to say that it would be practical to do so, however, since most of the prime OTEC waters are in the middle of the ocean and we live on land, at least most of us.
It’s difficult to fully assess aesthetic aspects of OTEC until we see how the potential plays out.
Either open or closed system OTEC could be used in either onshore or offshore systems.
If the economy of size proves out as it usually does in power generation, it could be that OTEC will develop with just a few, large onshore facilities with a number of very large, buried pipes leading offshore to deep, cold waters.
But it could also happen that economics dictate that OTEC will work better on a number of sea-based platforms that combine OTEC with wind or wave generators. If that scenario proved out, we could see thousands of them bobbing around just offshore.
It’s really too early to tell.