Amazingly the
technology is inching forward. What is
forgotten in this item is that the dense deep ocean water affects a serious
pressure imbalance with the water piled on top that can also be tapped with
that vertical pipe. The pipe has to be
large enough but once the flow is established, it will actually rocket to the
surface to provide work energy as well as nutrients and possible heat exchange
as well.
I have posted
extensively on this concept and liked the use of a concrete shell reinforced by
basalt fiber. This also can be built on
site and lowered into the water column.
A fiber glass
pipe should allow a cheap proof of concept at least. I also expect to use ballast rings filled with
even fresh water to maintain neutral buoyancy and pipe stiffness. Just imagine an inner and an outer pipe with
fresh water in between. This could be
built in sections and locked together as it is lowered. Once to depth the bottom would then be opened
to allow flow to begin.
20,000 megawatts
under the sea: Oceanic steam engines
03 March 2014 by Helen Knight
Jules Verne mused about getting energy from heat in
the ocean (Image: Marc Pagani/Getty)
Jules Verne imagined this limitless power source in
Victorian times – now 21st-century engineers say heat trapped in the oceans
could provide electricity for the world
IF ANY energy source is worthy of the name
"steampunk", it is surely ocean thermal energy conversion.
Victorian-era science fiction? Check: Jules Verne mused about its potential in Twenty
Thousand Leagues Under the Sea in 1870. Mechanical, vaguely 19th-century
technology? Check. Compelling candidate for renewable energy in a
post-apocalyptic future? Tick that box as well.
Claims for it have certainly been grandiose. In
theory, ocean thermal energy conversion (OTEC) could provide 4000 times the
world's energy needs in any given year, with neither pollution nor
greenhouse gases to show for it. In the real world, however, it has long been
written off as impractical.
This year, a surprising number of projects are
getting under way around the world, helmed not by quixotic visionaries but by
hard-nosed pragmatists such as those at aerospace giant Lockheed Martin. So
what's changed?
It's possible that Verne dreamed up the idea for
OTEC to help out Captain Nemo, the protagonist of Verne's deep-sea yarn who
needed electricity to power his submarine, the Nautilus – it is the first
written mention of the idea. "By establishing a circuit between two wires
plunged to different depths, [it should be possible] to obtain electricity by
the difference of temperature to which they would have been exposed," Nemo
told his shipmate. Eleven years after the book was published, French physicist
Jacques-Arsène d'Arsonval proposed the first practical design for a power plant
that does exactly that. Instead of using wires, he used pipes to exploit the
temperature difference between the cold deep ocean and the warm surface waters
to generate steam energy.
The idea is brilliant. The ocean is a massive and
constantly replenished storage medium for solar energy. Most of that heat is
stored in the top 100 metres of the ocean, while the water 1000 metres below –
fed by the polar regions – remains at a fairly constant 4 to 5 °C.
To make energy from that heat difference, modern-day
systems pump warm surface water past pipes containing a liquid with a low
boiling point, such as ammonia. The ammonia boils and the steam is used to
power a turbine, generating electricity. Cold deep-ocean water is then piped
through the steam, causing the ammonia to condense back into a liquid, ready to
begin the cycle again (see
diagram). Steam-powered turbines drive nearly every coal and nuclear power
plant in the world, but their steam is produced by burning polluting coal or
generating long-lived nuclear waste. OTEC, by contrast, provides steam in a
clean and theoretically limitless way.
Electric ocean
That's in an ideal world. In reality, what the
ocean's thermal gradient gives, the equipment takes away. The main problem is
accessing the cold deep water: pumping the vast amounts of water needed requires
1000-metre-long pipes that are wide enough and strong enough to handle several
cubic metres of seawater per second for every megawatt of electricity produced.
Tally all the inefficiencies in the process and the theoretical efficiency of
an OTEC plant drops to a dismal 4 to 6 per cent.
Thanks to this and other factors, the process needs
a temperature difference of at least 20 °C between the surface and deep water
to work. Such conditions exist in a relatively narrow band around Earth's
equator that includes the tropics and subtropics (see
map).
Despite these constraints, the 20th century was
filled with fitful efforts to make OTEC work. The most ambitious of these, in
the 1970s, was sparked by an oil crisis, after which the US president Jimmy
Carter signed into law the production of 10,000 megawatts of electricity using
the technology by 1999. However, the price of oil then fell again, and
alternatives to petroleum sank once more to the bottom of the to-do list.
So when Lockheed Martin last year announced that it
would begin construction on a 10-megawatt plant off the coast of southern
China, the news was met with a marked lack of interest. We had been here
before.
A closer look, however, reveals that the project may
signal a sea change for OTEC. The time may finally have come for this
19th-century technology to become part of the 21st century's renewable energy
mix, thanks to a strange partnership of other renewables, the oil industry –
and perhaps even climate change.
Many calculations are changing. OTEC's efficiency
may be low, but since it uses seawater, which is abundant and free, it still
makes economic sense if done on a large-enough scale. Oil prices are unstable
and climate change is becoming an increasingly urgent driver of alternative
energy sources. The shortcomings of intermittent renewables such as wind and
solar energy, which only produce electricity when the sun is shining or the
wind is blowing, are still keeping these on the margins. By contrast, OTEC
plants can operate 24 hours a day, says Ted Johnson of Ocean Thermal Energy
Corporation, which plans to commercialise the technology. Round-the-clock power
means an OTEC plant could simply be plugged directly into a municipal grid to
replace fossil fuel power plants, without the adjustments and balances
necessary to integrate unpredictable solar and wind power.
But what use is that power if the equipment needed
to harness it costs more than the electricity it provides? Here, too, advances
have been made. Lockheed Martin borrowed techniques from bridge and
wind-turbine manufacturing – both of which use advanced fibreglass and resin
composites to make their ultra-light, ultra-strong materials – to design a cheap pipe that is strong and
flexible enough to withstand the stresses and strains of ocean currents.
Even better, it can be assembled on the ocean-surface platform of the OTEC
plant itself and gradually lowered in as it is made, eliminating the risk of
transporting the huge structure into position – and dropping it. A promising
OTEC project in the Bay of Bengal had to be scrapped in 2003, after engineers
building a 1-megawatt plant lost not only their first pipe but also its
replacement.
Then there are myriad lessons from the offshore oil
and gas industry, where it has become commonplace to operate in ocean depths
greater than 1000 metres. These have made equipment available for commercial
purchase that just 20 years ago would have needed to be designed from scratch.
Thanks to such developments, a 100-megwatt plant
would cost about $790 million to build, says Luis Vega, who researches OTEC at
the Hawaii Natural Energy Institute at the University of Hawaii at Manoa.
Taking the costs of building and running an OTEC plant into account, Vega
reckons the price of the electricity produced would come in at around 18
US cents per kilowatt hour, not far from US Department of Energy estimates
of 14 cents for coal with carbon capture and storage, and 14 to 26 cents for solar energy.
In this changed landscape, OTEC projects have begun
to pop up all over the world. Last year, a 50-kilowatt pilot OTEC plant began
operating on Kume Island in Okinawa, Japan. Meanwhile in Hawaii, Makai Ocean
Engineering is building a 100- kilowatt plant at its Ocean Energy Research
Center in Kailua-Kona on the Big Island. This year, Bluerise, a spin-out from
Delft University of Technology in the Netherlands, is planning to start
building a 500-kilowatt OTEC plant close to Curaçao International Airport in
the Carribbean. "These smaller islands are likely to be the first market,
as they are all suffering from a dependency on expensive imported fuels,"
says Remi Blokker, CEO of Bluerise.
But they won't be the last. Recent advances promise
to bring OTEC into the mainstream.
Various research groups have investigated the
possibility of combining OTEC with solar power. Paola Bombarda at the Polytechnic
University of Milan in Italy has modelled the output of an OTEC plant that uses
solar power to increase the temperature of the warm ocean water before it is
used to boil the ammonia. She found that even a low-cost solar collector – a
simple device that traps heat in lenses or tubes – could triple a plant's
daytime electricity output (Journal
of Engineering for Gas Turbines and Power, vol 135, p 42302).
Similar techniques could help plants in countries
that lie a bit too far north to rely on OTEC all year round, such as South
Korea. In the summer months, the temperature difference between the surface and
deep water around South Korea exceeds the all-important 20 °C minimum, but that
isn't the case in winter. So to make it work year-round, engineers at the Korea
Ocean Research & Development Institute (KORDI) in Goseong-gun are beginning
to modify a 20-kilowatt demonstration plant so that heat
from solar power, wind farms and waste incineration plants can
pre-heat the incoming surface water before it meets the ammonia.
An even better idea would be to combine OTEC with
another 24-hour power source. Hyeon-Ju Kim and his colleagues at KORDI are
looking to geothermal energy, which taps heat deep underground, to boost the
temperature of the seawater that boils the
ammonia in a combined "GeOTEC" plant. Such tweaks could
expand the "equatorial waistband" for productive OTEC plants by a
factor of two.
In light of these rapid developments, OTEC has
become promising enough that the prospect of its expansion has begun to ring
alarm bells among environmentalists. Concerns
have been raised by the US National Oceanic and Atmospheric Administration,
among others, about the risk of algal blooms forming as nutrient-rich,
bacteria-free water from the sunless depths is introduced to the hungry algae
in warmer, sunlit waters. But computer modelling suggests that as long as the
cold water is returned to the ocean at depths lower than 60 metres, the risk of algal blooms
should be minimal, says Vega.
To eliminate even this modest risk, London-based
Energy Island has patented a design for an OTEC plant in which the ammonia
vapour is no longer condensed into liquid at the surface but at depth. This
means nutrient-rich water would never need to be pumped up to the surface, says
founder Dominic Michaelis.
Another question being posed echoes previous
concerns about the large-scale take up of other renewables: does OTEC have
local and global effects on the environment, such
as changing global temperatures?
Happily, research suggests we can ramp up OTEC
production without affecting the ocean. Researchers at the University of
Hawaii's Ocean and Resources
Engineering department in Honolulu modelled the effect of widespread,
commercial-scale OTEC production on the seas, including the global thermohaline
circulation – the network of slow currents that transport deep water throughout
the oceans. They found that OTEC plants could safely extract the equivalent of
7 terawatts of electricity, or nearly 50 per cent of global energy consumption,
before they would have any noticeable effect on ocean temperatures (Journal
of Energy Resources Technology, vol 135, p 41202). However, the authors
acknowledge the difficulties of drawing strong conclusions about the
environmental effects of OTEC.
It is certainly a good time to add a new form of
renewable-energy generation to the mix, since climate change may have
unforeseen circumstances for some existing clean technologies. In July, the US
Department of Energy released a
report on the energy sector's vulnerability to climate change, which found
that higher temperatures could reduce the amount of fresh water available for
both hydropower generation and concentrated solar
power plants, whose superheated equipment requires water cooling.
By comparison, OTEC sweet spots don't appear to be
vulnerable to climate change, says Robert Thresher, a research fellow at the
National Renewable Energy Laboratory in Golden, Colorado. "Most of the
OTEC resources are along the equator, and you wouldn't expect the sea surface
temperature to dramatically change there," he says.
Out of the blue
Indeed, climate change might even increase the
global output for OTEC by expanding the OTEC-friendly zone: "As the oceans
warm with climate change, you might find warmer [surface] water further north
and south from the equator," he says. Though the idea has also been
proposed elsewhere, he hastens to add that this is "an intuitive
notion" that would need to be confirmed by rigorous modelling.
More problematic is the suggestion that the deep
oceans may have absorbed a great deal of the heat of climate change, which
could reduce the all-important temperature difference of surface and deep water
(New Scientist, 7 December 2013, p 34). However, according to research
published last year by Magdalena Balmaseda and colleagues at the European
Centre for Medium Range Weather Forecasts in Reading, UK, it is far from clear
where exactly that heat is going. "The heat absorption is not uniform in
space, depth and time," says Balmaseda (Geophysical Research
Letters, vol 40, p 1754).
Whether or not the warm equatorial waistband OTEC
relies on expands, the technology might not be limited to countries in the
tropics for much longer. At the Offshore Symposium in Houston, Texas, in
February 2013, SBM Offshore, which develops technology for oil exploration and
drilling, revealed that it has been investigating designs for a 10-megawatt
OTEC ship as a means of providing power to remote oil wells. OTEC plants become
more expensive the further they are built from shore, but ships, which are
cheaper to build, have no such constraints. OTEC ships could roam the seas in
search of spots with the best temperature ratios, tethering to submarine cables
to return power to shore.
Indeed, proponents of the technology believe the
future lies in OTEC ships that "graze" the oceans for electricity. To
get around the problem of delivering it to shore by submarine cables, the
electricity generated could be used in situ to split seawater into hydrogen and
oxygen, with the hydrogen stored in
fuel cells before being transported for use around the world. A
100-megawatt OTEC ship could produce 1.3 tonnes of liquid hydrogen per hour,
says Vega, albeit at a present cost of about three times what a barrel of oil
costs today. The hydrogen economy, after all, is still finding its feet.
Nonetheless, it appears, after all this time, that
Jules Verne may have been onto something. If anything, he was thinking too small.
Instead of a ship powered by the ocean, a fleet of ships may bring the ocean's
energy to the world. Steampunk indeed.
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