As discussed briefly here, the oceans have been absorbing solar energy for billion of years and essentially storing that. OTEC represents a viable, and truly scalable, mechanism for recovering that stored energy and powering the planet for centuries. While there are tremendous obstacles to overcome to realize this potential, we currently are doing very little to tap this resource.
Instead we are doing this Large Scale NONSENSE:
At some level, it seems fundamentally insane to keep digging up the Earth (usually via explosives)
to get resources for electricity and transport. Our continued dependence on coal for electricity on a world wide basis is a good example of this insanity, as this group of high school students in Australia attempts to inform us:
Why not try something different? Can we make things worse by doing so? Why not try to extract energy from the thermal differences in the world's oceans. As shown in the figure above, there are vast areas of equatorial waters that are amendable to being harvested to convert thermal energy into electricity.
This technology in fact is a "global solution" and represents a chance for the world
to collaborate on various infrastructure and production facilities. Such cooperation
is required because:
No one owns the world's oceans
The capital cost of harvesting this energy is enormous estimates
range from $4 to $10 a watt.
But the potential yield is also enormous:
On an average day, 60 million square kilometers (23 million square miles)
of tropical seas absorb an amount of solar radiation equal in heat content to
about 250 billion barrels of oil.
We use 85 million barrels a day 3000 times less than
this absorbed energy. Hmmm, seems like a solution.
The mechanism of electricity generation is based on simple principles of
thermodynamics that allow the ocean to be characterized as a giant heat
engine.
Any two reservoirs with different
temperatures T1 and T2 can produce energy.
Examples:
Geothermal example: 200C reservoir couple to surface water (20C)
eff = 1 - (293/473) = 38%
Solar Thermal Molten Salts/Water at Air Temperature combination:
eff = 1 -(300/1000) = 70%
To get the highest efficiency one wants to maximize the difference
between T1 and T2 but in most natural environments nature doesn't
really do this.
For example, here is the situation for OTEC:
Surface Ocean temperature is 25 degrees C (298K)
At 500-700 meters depth the temperature is 5 degrees C (278K)
Efficiency = 1 - 278/298 = .067 (6.7%)
Does this mean OTEC is silly due to low efficiency
hell no because the potential stored energy
is gigantic in the ocean. Its overall throughput that always matters in energy calculations:
low efficiency * semi-infinite resource = shitload of output
(note that the sun is a power plant operating at 0.7% efficiency)
To compensate for low efficiency, OTEC plants need to move a huge
volume of water. This is the technical/engineering challenge. You need
to net export electricity rather than spending more electricity to just run
the pumps!
Basic principle is that heat difference is used to condense
a steam into a liquid then return it to be reheated. But we would
need a working fluid that liquifies at a temperature of about 25 C.
Ammonia!
And there is plenty of ammonia (NH3) to use as the working
fluid.
Energy extracted comes from the cooling of the warmer water
this is transferred to the ammonia which does the actual work
of turning the turbine (as ammonia steam)
Energy extracted is directly proportional to the volume of water and the
temperature it drops.
Principal energy loss is when the warmer water meets the
cooler water in the condenser. The working fluid (Ammonia) turns to steam when mixed with warm surface water and then condenses back to liquid when mixed with cold water before it cycles through the system again.
Conceptual OTEC Turbine Design:
Scale is that a 50-m diameter foot print yields a 100 MW facility
Total available power in tropical waters. This is difficult to really
properly estimate.
one conservative study suggests 0.2 MW available per square km with out any environmental damage (due
to changed heat structure balance or large water discharges) Total yield is then 12 TW (enough electricity
for the whole planet)
However, studies by Avery ( a real optimist ) suggest that "mobile" floating islands that move
at 1/2 mile per hour could harvest OTEC at the rate of 5MW per square km (25 times higher than
previous estimate). Floating barges would migrate (slowly) from anchor point to
anchor point.
How about some artificial floating islands with solar cells and windmills overlaying
the OTEC infrastructure. Why the hell not? (functional stacking again)
Regardless of the overall potential yield, we must first construct
unit plants consisting of the basic components shown below.
We now need to do some basic engineering calculations to assess the required scales. Let's pretend that
someone else did those calculations that yields this calibration point -
To export 1 MW of electricity requires:
4 cubic meters per second of warm water "joined" with 2 cubic meters per second of cold water with a temperature difference of 20C.
Scaling this to a 100 MW plant:
400 cubic meters per second of 26 degree water "joining" with 200 cubic meters per second of 4 degree water
warm water flow requires a pipe diameter of 16 meters extending to a depth of 20 M. But in some cases, the pipe only needs to be at the surface.
cold water flows/pumped through an 11 meter diameter pipe extending to depths of 1000 M (this is an engineering/materials challenge)
Mixed water return requires 20 m pipe. Mixed water about 6 degrees higher and needs to be discharged to a depth of 60-100m.
So, large pipe/pump infrastructure is needed! This is the basic engineering limitation. What material can you use to make pipes of this diameter? You can not use concrete because the weight will crush the pipe. Steel reinforced pipes of this length are cost-prohibitive since no one makes them in any industry as there is no need, yet, for pipes
this big. Simple aluminum, like used in roadway culverts, will get crushed by the pressures in the ocean. We need something like carbon fiber for this task, and we are not yet at the point of mass production of carbon fiber meta structures. So until this problem is solved,
we can not yet deploy OTEC on the scale of stationary 100 MW plants.
This is unfortunate.
At the current time, OTEC facilities are much easier deployed at ON shore site as the diagram below demonstrates.
Sites similar to this are being deployed on some Island States (e.g. Hawaii, Puerto Rico, Okinawa) and have the additional value that they can also serve as a desalization facility. Indeed, small scale
OTEC systems currently have their best applicability to
Small Island Developing States
Clearly for large scale deployoment across the equatorial waters of the Earth, OTEC would require a massive ocean floor grid systems. This is also cost prohibitive which is why its important to develop a much better hydrogen distribution and storage system. 1 MW of OTEC electricity could produce 1.3 kg of Hydrogen (through electrolysis of
sea water) per hour. In principle, if we can figure out how to use
Hydrogen on a large scale as the carrier of electricity, then OTEC and fresh water production from OTEC may indeed give us our sustainable future.
Review of OTEC (Ocean Thermal Energy Conversion)
Thermal gradients of greater than 22 C can be exploited
and used as a heat engine
Energy is derived from cooling warm surface water to the
temperature of the water at approximately 500-1000 feet depth.
Maximum real world operating efficiency is 4%
Energy is derived from the cooling water via transfer to
a working fluid such as ammonia which when mixed with warm water
vaporizes to steam and powers a turbine
Ammonia returns (condenses) to liquid when mixed with cooler
water at depth and then the cycle repeats itself
Since the volume of water in the oceans is huge, the capacity
in just the Gulf Coast Waters alone is several 10's of Gigawatts
estimates
range from $4 to $10 a watt.
Basic principle is that heat difference is used to condense
a steam into a liquid then return it to be reheated. But we would
need a working fluid that liquifies at a temperature of about 25 C.
How about some artificial floating islands with solar cells and windmills overlaying
the OTEC infrastructure. Why the hell not? (functional stacking again)
Scaling this to a 100 MW plant:
