Drawbacks Overcome

"Ocean Thermal Energy Conversion (OTEC) is by far the most balanced means to face the challenge of global warming," Dr. Paul Curto, former NASA Chief Technologist.

OTEC is a promising renewable energy resource that has been neglected due to high capital cost and previously unresolved environmental issues.

Over 90 percent of the heat attributable to global warming has been absorbed by the oceans with the result sea levels are rising and icecaps are melting.

The first law of thermodynamics dictates that the increase in the internal energy of a system = heat supplied to the system - work done by the system.

Converting some of the 330 terawatts of excess energy the oceans have been accumulating annually to productive energy will limit the adverse effects of this buildup and provide the 30 to 60 terawatts of renewable energy the world will need by 2050.

OTEC is the only renewable energy technology available 24/7 that acts directly to reduce the temperature of the oceans, eliminates carbon emissions, and increases carbon dioxide absorption (cooler water absorbs more CO2), therefore it is has the potential to capture a significant share of the renewable energy market.

What has held OTEC back are high capital cost, environmental impact during installation and use, moderate power output, biofouling of heat exchange surfaces and the remoteness of the best sites from energy markets.

OTEC systems also operate in environments where hurricanes thrive and although the process saps the energy these storms feed on, they have been the demise of most of the prototypes that have been built.

It is estimated conventional OTEC can produce five terawatts of power at most due to massive transfers of surface heat to the depths which reduces thermodynamic efficiency and creates a potential to overturn the Thermohaline Circulation.

OTEC’s problems derive from the thermodynamic inefficiency of a Carnot cycle with small temperature differences between the cool and warm reservoirs.

To overcome this problem conventional methods pump massive volumes of warm and cold water to produce limited amounts of energy.

To produce 100 MW of power; 248 cubic meters of hot water and 138 cubic meters of cold water have to be circulated every second with the most recent designs.

Heat transfer occurs at the surface requiring the bringing of massive amounts of cold water to the platform deck to condense the working fluid.

These cold water pipes are 1000 meters long and have a diameter as great as 10 meters. The weight of such massive equipment mandates that the platforms that support them must also be great and therefore costly.

Marine life entrained in these massive flows is adversely impacted and the drop in pressure releases dissolved carbon dioxide into the atmosphere from the cold water.

GWMM OTEC uses a closed cycle which transfers heat in a heat pipe - by evaporation and condensation – which permits much larger and faster heat transfers per kilogram of fluid than can be obtained using water.

It then recycles the latent heat of condensation of the vapor in a counter-current flow arrangement which limits the amount of heat that is extracted from the surface as well as the amount of heat dumped to the depths.

This closed system overcomes the environmental problems inherent with conventional methods and greatly reduces the size and cost of the equipment required – the largest diameter pipes are in the range of two meters.

Vapor is condensed at depth in cold water limiting the biofouling problem and the absorption of the latent heat of condensation by the fluid returning to the surface occurs internally.

In many locations OTEC power can be produced within 5000 km of energy markets which can then be accessed using HVDC cabling with a conveyance loss of less than 10%.

Converting liquid volume to gas is one of the three ways GWMM OTEC addresses sea level rise – one of the greatest risks of global warming.

The other two are:
* Reducing thermal expansion of the oceans by converting heat to productive energy, and
* Moving liquid volume - in the form of desalinated water - to productive terrestrial use.

The durability of this system is enhanced by reduced volume and enhanced buoyancy and a dual gimbal design that allows the surface platform to ride out a storm while the heat pipe remains vertical.

There is also the potential for the submergence of this kind of platform in the face of oncoming storms or for it to operate permanently below the wave action.

GWMM OTEC is a solution to the problem of massive land use required by solar and wind arrays as well as their visual blight.

GWMM OTEC is scalable because the only limiting factor is the speed of return of the condensed low boiling point fluid to the surface.

(A pipe of .2 meters diameter can return 4x as much fluid as one of .1 meter)

To produce 50 MW of power requires the boiling of 4 M3 per second which can be replenished by a pipe of .1m diameter.

The dimensions of the rest of the system, except for the axial turbine, are little changed.

Because OTEC can operate at near 100% capacity, annual power production will average three times that of solar and wind per unit of power capacity.

Capital cost reduction is the principle requirement to make OTEC successful.

GWMM-OTEC reduces conventional costs by at least 30% and increases thermodynamic efficiency by 15% bringing this source of energy in line economically with fossil-fuel based power plants and nuclear energy.

The simplicity of GWMM OTEC platforms makes them 95% reliable over a 30 year life span and considering operational costs are minimal and fuel costs are non existent, GWMM OTEC is an extremely attractive alternative to solving the clean energy problem.