“Nuclear is currently the only carbon-free energy source that can provide base load electricity — a characteristic crucial to reducing global greenhouse gas emissions.”
According to Rod Fujita, Director of Research and Development, Oceans Program for the Environmental Defense Fund, Energy From The Sea: Closer Than You Think.
He also coauthored a paper Revisiting ocean thermal energy conversion which concluded a Lockheed Martin/Reignwood Group project to develop a 10-megawatt power plant in southern China was the missing link in commercializing OTEC. As he says, “Several small OTEC plants have been built already, so we know that the concept produces net energy. . . But the costs associated with scaling OTEC up have been very uncertain, perhaps scaring off potential investors and customers up till now.
Conventional OTEC’s cost is driven by the size of the cold water pipe required to move large volumes of water to overcome the low thermodynamic efficiency of the process. With a delta T of about 21C the theoretical Carnot Efficiency is about 7% but practically it is less.
According to a design of Luis Vega and Dominic Michaelis for a First Generation 50 MW OTEC Plantship for the Production of Electricity and Desalinated Water, 264.6 m3/s (270,400 kg/s) of warm water; and 138.6 m3/s (142,300 kg/s) of cold water are moved to produce 53.5 MW. This facility employs 2,750 kg/s of anhydrous ammonia as the working fluid and with a density of 681.9 kg/m3 this equates to 4 m3/sec.
Dr. Fujita claims “OTEC is definitely not a panacea. Using large amounts of cold, nutrient rich water from the deep ocean in order to produce energy could have some very negative impacts, like killing sea life by sucking it into the intake pipe or creating algal blooms by discharging nutrient rich sea water into warm, nutrient-poor surface water. But these and other impacts can be prevented or mitigated.”
It is the movement of such a large volume of water that has the potential to negatively impact sea life, create algal blooms that produce dead zones in the ocean and potentially release CO2 into the atmosphere as the gas dissolved in the water under pressure is brought nearer the surface.
The best way to prevent this is to use a heat pipe as shown in Dominic Michaelis’ British Patent No. GB 2395754.
As Paul Curto, former NASA Chief Technologist points out, “if the condensing end of the heat pipe is exposed to a thousand feet or more of near freezing temperatures below the thermocline, no cold water pumping is required. The parasitic losses are cut in half. The costs for the cold water pipe are eliminated, along with the cold water return pipe and condenser pumps, the cleaning system for the condenser, and the overall plant efficiency approaches 85% of Carnot vs. about 70% with a cold water pipe.
The parasitic losses could be reduced as much as 50% and the complexity, mass (and cost) of the system reduced by at least 30%. The vast reduction in operating costs and environmental impacts would be worth investigation alone”
Relative to the above 53.5 MW plant, you would only move 4 M3/sec of working fluid in a close system and therefore would not impact sea life, create dead zones or release CO2.
Considering the current estimates for the amount of energy you can produce with such a system is 25 TW and the world operates currently on only 16TW then OTEC starts to look more and more like a renewable energy panacea.
Dr. Melvin Pruiett, Ph.D. in Physics and former guest scientist with Los Alamos National Laboratory has another patent application for a Heat Transfer For Ocean Thermal Energy Conversion system, that uses a heat pipe design to absorb heat in a heat exchanger near the ocean surface using the latent heat of evaporation and then deposits the latent heat of condensation in a deep ocean heat exchanger, using the cold seawater as a heat sink. The condensed liquid is pumped back to the ocean surface. The heat engine (turbine) and generator can be at the ocean surface, or it can be in deep ocean. By using a fluid that transfers heat by evaporation and condensation, much larger quantities of heat can be moved per kilogram of fluid than can be transferred by moving the same mass of seawater.
Dr. James Chung-Kei Lau, Ph.D. in Physics, 20 patents, has a patent application for an Ocean thermal energy conversion (OTEC) electric power plant, which uses CO2 as the working fluid, necessitates no water pumping because of the distributed nature of the evaporators and condensers. The design uses a heat pipe to communicate the working vapour between the evaporator and condensers.
Nagan Srinivasan; has a patent application for an Offshore Floating Platform With Ocean Thermal Energy Conversion System, in which heat is extracted from warm sea surface waters to vaporize a liquid working fluid and heat is rejected to cold water from lower depths of the sea to condense the vaporized working fluid. At least one turbine and power generator is disposed on the deck, at least one evaporator is disposed on the platform beneath the deck, and at least one condenser is disposed on the seabed or platform or keel tank a distance beneath the evaporator.
This too is a heat pipe design combined with an adapted oilfield type platform which the inventor claims can produce 1000 MW at a capital cost of 3billion USD.
By comparison the two 1200 MW nuclear reactors to be build at the Vogtle site in Georgia are expected to cost $14 billion or nearly twice as much.
With the OTEC plant there would be no fuel or waste cost. Decommissioning would be much simpler and a catastrophic failure would be far less of a catastrophe.
If CO2 was the working fluid it would be virtually benign.
The claimed shortcoming with the heat pipe design is the low pressure working fluid vapour inside a pipe subjected to the pressure of 1000 meters of water.
The writer has a patent application for an Ocean Thermal Energy Conversion Counter-current Transfer System that overcomes this by use of a coil inside the vapour channel acting against the external pressure. The coil returns the condensed working fluid and acts as a condensing surface upon which the vapour condenses. In the process the internal fluid absorbs part of the latent heat of condensation and returns this to the surface.
At the time this patent application was filed it was estimated the oceans could only produce about 5 terawatts of power because as more and more surface heat was transferred to the depths with OTEC the thermodynamic potential of the ocean would degrade. The counter-current system was developed to prevent this degradation and increase the potential of the ocean to produce power. Subsequently the same researcher has raised his estimate to 25 terawatts, which would be assured with the counter-current system shown below, which also counteracts the crushing forces exerted on the heat pipe.
With deference to Dr. Fujita, it is OTEC innovation that brings about an energy panacea that addresses the sea level problem 5 ways. By:
- Converting surface ocean heat to work, thus reducing thermal expansion.
- Moving surface heat to a depth of 1000 meters where the coefficient of expansion due to pressure and water temperature is half that at the surface.
- Converting liquid volume to gas by electrolysis to produce hydrogen as the energy carrier to bring power ashore.
- Some versions desalinate water for use on shore, and
- By sapping tropical surface heat that otherwise drives storms, which move tropical heat towards the poles causing the icecaps to melt.
As Dr. Pat Takahashi, Director Emeritus of the Hawaii Natural Energy Institute at the University of Hawaii and co-founder of the Pacific International Center for High Technology Research, recently posted, OTEC is the only energy option with any kind of chance for making a real contribution to the climate problem.