The 2011: IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation – Ocean Energy, states, “The resource potential for OTEC is considered to be much larger than for other ocean energy forms . . . Among ocean energy sources, OTEC is one of the continuously available renewable resources that could contribute to base-load power supply.”
It then cites a 2007 article, A Preliminary Assessment of Ocean Thermal Energy Conversion Resources, by Gérard Nihous, University of Hawaii, that calculated the steady-state OTEC power potential at about 44,000 TWh/yr or 5 terawatts (TW).
In 1998 a team of NYU researchers lead by physicist Martin Hoffert concluded, the Earth’s atmospheric carbon dioxide content cannot be stabilized without a tenfold increase in carbon-emission-free power generation over the next 50 years.
In 1998 the world produced 1.5 TW of carbon-emission-free power and 15 years later that output has barely increased to just over 2 TW; far short of Hoffert’s goal of 15 TW and increasing at a pace that assures his goal will go unattained.
Dr. Hoffert stated in an email to this writer, he is a fan of OTEC but considers its potential too small to meet his objective.
The 2007 analysis of Nihous was based on a simple one-dimensional time-domain model of the thermal structure of the ocean. In more recent work he has used a three-dimensional approach that has upped his estimate to 30 TW, though he suggests the environmental effects at that level would probably be unacceptable. Taking the environment into account he suggests less than 10 TW is probably the limit.
Professor Hoffert identified one of the environmental effects as a potential to overturn the Thermohaline circulation due to the dumping of massive amounts of heat to the depths with OTEC on a massive scale.
Dr. Rod Fujita, of the Environmental Defense Fund has pointed to two others, “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.”
When algal blooms die they eutrophy the water column to produce a dead zone.
About one-third of all human-generated carbon emissions have dissolved in the ocean. If nutrient-and carbon dioxide rich cold water is brought to the surface to produce OTEC power some of the gas will come out of solution and return to the atmosphere as the pressure drops.
The final problem with conventional OTEC is cost, which is driven by the large diameter of the pipes required to move large masses of water.
The Ocean Thermal Energy Conversion Counter-Current Heat Transfer System addresses each of these issues.
First by reducing the size of the piping required to move heat by one order of magnitude. The system uses a heat pipe design, similar to a Liebig condenser. A heat pipe is the most efficient way to move heat by phase changes of the working fluid and the Liebig condenser is one of the oldest and simplest forms of laboratory condenser. It consists of a glass tube down which vapor flows surrounded by a glass envelope through which cooling water flows to induce condensation of the vapor in the internal tube.
In the Counter-Current Heat Transfer System a heat pipe of 1000 meters is the means of conveyance of the vapor and the 800 meters of ocean beneath the Thermocline is the cooling medium.
Heat Pipe Liebig Condenser
This is a closed system requiring minimal pumping of water therefore the impact on marine life is negligible.
The perceived shortcoming with the heat pipe design has been the thickness of the pipe required to withstand a pressure of 1000 meters of water acting on a cavity containing essentially a vacuum as the vaporized working fluid condenses. The thicker the pipe the slower heat transfers through its wall.
The proposed system overcomes this problem with two coil condensers within the vapor channel, which not only strengthen the pipe wall, similar to an inflated bicycle tube acting on a tire, they facilitate condensation by introducing the condensing medium into the column. The bottom coil circulates cold water within the condensed working fluid to reduce its temperature to that of the surrounding water – 4oC. The chilled working fluid is then pumped through the second coil, above, which condenses the descending working fluid and warms the ascending fluid by absorbing the latent heat of condensation. This warmed fluid then enters segmented sections surrounding the vapor channel to be returned to the surface in a break tank arrangement. These sections not only strengthen the pipe, they induce counter-current heat flow which maximizes the heat transfer of the system.
This counter-current flow limits the impact on the Thermohaline circulation and maximizes the oceans potential to produce power.
By overcoming OTEC’s capacity, environmental and cost issues the Ocean Thermal Energy Conversion Counter-Current Heat Transfer System realizes the renewable energy potential of five times the output of the world’s producing oil fields.