Ocean thermal energy conversion
or OTEC, as the term implies, is a method of converting surface ocean heat to
Figure 1 to right is a diagram of
the working principle of the system.
On the ocean surface heat
generates storms that cause water to expand leading to sea level rise and moves
towards the poles where it melts the ice caps.
With devices similar to the
ones used to keep heat from frying the workings of a computing device (a heat
pipe per Figure 2 to right), surface heat can instead be moved to an ocean depth
of 3000 feet, where the coefficient of expansion of ocean water is half that of
the tropical surface (Figure 3) and where the heat is no longer available to
produce storms, melt icecaps and would take 250 years to return to the surface.
estimated the oceans have the capacity to produce as much energy as is
currently derived from fossil fuels with these systems.
MIT thesis also shows OTEC
has the highest capacity of all energy sources and the lowest levelized cost of
the renewables, including nuclear. (Figure 4)
The best locations for
producing OTEC power (the reds and oranges of Figure 5) are remote enough from
shore it is necessary to convert the electrical energy generated to an energy
carrier like hydrogen to bring it to market.
Using the electrolysis process
developed by a team from Lawrence Livermore Laboratories to produce this
hydrogen, as much as 79 billion metric tons of atmospheric carbon dioxide would
be sequesters annually, accumulating acidity in the oceans would be neutralized
and when the hydrogen is converted back to energy the equivalent of 600 gallons
of water annually for every person on the planet would be produced.
This transfer of ocean volume
to land would replicate the events of 2010 that saw sea levels decline by 5 mm
but more importantly the necessity to pump aquifers to provide this 16 trillion
kilograms of water would be negated.
OTEC can mitigate many of the
problems the world faces as a consequence of global warming by converting heat
that causes thermal expansion to work, diminishing the power of storms that move
heat to the poles, moving heat to regions of diminished coefficient of
expansion, and converting ocean volumes to the energy currency hydrogen that is
necessary to move offshore generated power to market.
OTEC was first proposed in 1881 by
the French physicist and engineer Arsene d’Arsonval and all subsequent efforts
have focused on the cold water pipe design, where cold deep sea water is pumped
to the ocean surface.
Cost and a fear of environmental
drawbacks have been the principal impediments to the advancement of this
Costs are driven by the need in this
design for large pipes to bring cold water near the surface to condense the
working fluid once it has passed through the turbine to produce power.
To produce 100 MW power a 1000 meter
long cold water pipe of approximately 10 meters diameter is required. These are
costly as is the infrastructure required to support them. And the movement of
large volumes of water in these pipes can be detrimental to marine life. Plus
the fact the cold water brought to the surface contains pressurized dissolved CO2
that is released into the atmosphere with the reduction in pressure and the
increase in water temperature.
The improvements of the
proposed heat pipe design are:
The piping can be one order of
magnitude smaller. Instead of the 10 meter diameter cold water pipe referred
to above for a 100MW plant using a cold water pipe, United States Patent
Application 20070289303 of Melvin Prueitt uses a heat pipe, referred to in the
application as a heat channel pipe, with an internal cross sectional area of
just one square meter.
Smaller pipes and supporting
infrastructure lead to a commensurate decline in the cost of the system.
The parasitic losses of the system
are reduced because of the ability to move a greater volume of heat in a
working fluid vapor rather than in water. (With the heat pipe, cold water
doesn’t move; the working fluid moves to the depths and is pumped back again.)
The embodiment of U.S. patent
8,484,972 to James Lau requires no sea water movement with respect to either
the evaporator or the condenser because the working fluid is sufficiently
distributed within either heat exchanger to achieve a predetermined amount of
heat energy transfer.
Marine life is not impacted by the
movement of vapor or the returning working fluid, which cycle in a closed
CO2 remains dissolved
in sea water as no water is pumped up to the surface.
Tropical cyclones require surface
waters to a depth of 50 meters to be at least 26.5oC before they
can form. The heat pipe is the fastest way to remove heat approaching that
threshold to deeper water away, or lessen the intensity of storms that have
managed to form.
A unit of heat at a depth of 1000
meters produces less sea level rise than the same unit on the surface, because
at depth the coefficient of expansion of sea water is half that of the
A wide-spread use of OTEC, and in
particular of the heat pipe design, has a number of positive features:
In the long run it will be more
productive for the developed countries to provide emerging nations with energy
that mitigates the climate problem than to pay them reparations that will have
limited remedial impact on their environment.
OTEC can effectively air condition
the planet by converting heat to work and moving exponentially more to the
relative safety of deeper water. The use of a heat pipe replicates the
conditions that precipitated the global warming hiatus, only more so: The
trade winds move surface heat to depths of about 100 to 300 meters.OTEC using
a heat pipe would triple this relocation; making it that much more difficult
for the relegated heat to return.
Flooding and sea level rise would
be reduced by the relocation of heat to a region of reduced coefficient of
Moving heat away from the surface
also saps the energy of tropical storms that bring with them the dual threats
of wind and low pressure driven storm surge.
Implemented on a massive scale
OTEC would moderate atmospheric temperatures the same way the warming hiatus
is believed to have been brought about by moving heat into the ocean deep, and
in so doing marine and coastal ecosystems would benefit from the reduction in
thermal stratification that cuts phytoplankton off from the nutrients they
need to survive. Phytoplankton are the base of the ocean food chain and the
lungs of the planet as they provide half of the oxygen we breathe by consuming
CO2 in the ocean.
All use of fossil fuels can be
replaced by OTEC, which in turn mitigates sea level rise and storm surge, and
effectively stops global warming.
The design and build out of the
infrastructure that could save coastal communities and island nations
represent an enormous economic as well as social opportunity.
There are also anticipated
spin-offs from a transition to this remedial energy form, such as desalinated
water or water reconstituted from hydrogen electrolyzed at sea to bring OTEC
power to shore and, hypothetically, recovered dissolved mineral wealth from
OTEC is one of the best rationales
for the “hydrogen economy”, i.e., a system where energy is delivered using
The recent IPCC SpecialReport on
Sources and Climate Change Mitigation found that taking into account the
lifecycle of various means of producing electricity, ocean energy was the second
lowest generator of CO2/kWh.
Hydro was the best performer by this measure but has limited capacity, and hydro
power can have severe environmental effects including flooding and large scale
relocations of populations.
Nuclear power is often touted as the best environmental option but it produces
twice as much CO2 as well as twice as much waste heat as energy
produced and both of these end up in the oceans.
OTEC on the other hand converts some
of the damaging heat in the ocean to productive use and the electrolysis process
can sequester CO2.
Paul Curto, former chief technologist with NASA, has posted, OTEC is by far
the most balanced means to face the challenge of global warming. It is also the
one that requires the greatest investment to meet its potential. It is a most
intriguing answer that can save us from Armageddon.
Few can argue that is a goal we
should be endeavoring to attain.