Ocean thermal energy conversion (OTEC) uses
the temperature difference between cooler deep and warmer shallow or surface ocean waters to run a
heat
engine and produce useful work, usually in the form of electricity. OTEC is
a base load electricity generation system, i.e. 24hrs/day all year long.
However, the temperature differential is small and this impacts the economic feasibility
of ocean thermal energy for electricity generation.
Systems may be either closed-cycle or open-cycle.
Closed-cycle engines use working fluids that are typically thought of as refrigerants
such as ammonia
or R-134a. These
fluids have low boiling points, and are therefore suitable for powering the system’s
generator to generate electricity. The most commonly used heat cycle for OTEC
to date is the Rankine cycle using a low-pressure turbine.
Open-cycle engines use vapour from the seawater itself
as the working fluid.
OTEC can also supply quantities of cold water as a
by-product. This can be used for air conditioning and refrigeration and the
nutrient-rich deep ocean water can feed biological technologies. Another
by-product is fresh water distilled from the sea.
OTEC theory was first developed in the 1880s and the first
bench size demonstration model was constructed in 1926. Currently the world's
only operating OTEC plant is in Japan, overseen by Saga
University.
History
Attempts to develop and refine OTEC technology started in
the 1880s. In 1881, Jacques Arsene d'Arsonval, a French physicist,
proposed tapping the thermal energy of the ocean. D'Arsonval's student, Georges
Claude, built the first OTEC plant, in Matanzas, Cuba in 1930. The
system generated 22 kW of electricity with a low-pressure turbine. The
plant was later destroyed in a storm.
In 1935, Claude constructed a plant aboard a 10,000-ton cargo vessel moored
off the coast of Brazil. Weather and waves destroyed it before it could
generate net power. (Net power is the amount of power generated after
subtracting power needed to run the system).
In 1956, French scientists designed a 3 MW plant for Abidjan, Côte
d'Ivoire. The plant was never completed, because new finds of large amounts
of cheap petroleum made it uneconomical.
In 1962, J. Hilbert Anderson and James H. Anderson, Jr.
focused on increasing component efficiency. They patented their new
"closed cycle" design in 1967. This design improved upon the original
closed-cycle Rankine system, and included this in an outline for a plant that
would produce power at lower cost than oil or coal. At the time, however, their
research garnered little attention since coal and nuclear were considered the
future of energy.
Japan is a major contributor to the development of OTEC
technology. Beginning in 1970 the Tokyo Electric Power Company
successfully built and deployed a 100 kW closed-cycle OTEC plant on the
island of Nauru.
The plant became operational on 14 October 1981, producing about 120 kW of
electricity; 90 kW was used to power the plant and the remaining
electricity was used to power a school and other places. This set a world
record for power output from an OTEC system where the power was sent to a real
(as opposed to an experimental) power grid. 1981 also saw a major development
in OTEC technology when Russian engineer, Dr. Alexander Kalina, used a mixture
of ammonia and water to produce electricity. This new ammonia-water mixture
greatly improved the efficiency of the power cycle.In 1994 Saga University
designed and constructed a 4.5 kW plant for the purpose of testing a newly
invented Uehara cycle, also named after its inventor Haruo Uehara. This cycle
included absorption and extraction processes that allow this system to
outperform the Kalina cycle by 1-2%. Currently, the Institute of Ocean Energy,
Saga University, is the leader in OTEC power plant research and also focuses on
many of the technology's secondary benefits.
The 1970s saw an uptick in OTEC research and development
during the post 1973 Arab-Israeli War, which caused oil prices to triple. The
U.S. federal government poured $260 million into OTEC research after President
Carter signed a law that committed the US to a production goal of 10,000 MW of
electricy from OTEC systems by 1999.
In 1974, The U.S. established the Natural Energy Laboratory of Hawaii
Authority (NELHA) at Keahole Point on the Kona coast of Hawaii.
Hawaii is the best US OTEC location, due to its warm surface water, access to
very deep, very cold water, and high electricity costs. The laboratory has
become a leading test facility for OTEC technology. In the same year, Lockheed
received a grant from the U.S. National Science Foundation to study OTEC. This
eventually led to an effort by Lockheed, the US Navy, Makai Ocean Engineering,
Dillingham Construction, and other firms to build the world's first and only
net-power producing OTEC plant, dubbed "Mini-OTEC" For three months
in 1979, a small amount of electricity was generated.
Research related to making open-cycle OTEC a reality began
earnestly in 1979 at the Solar Energy Research Institute (SERI) with funding
from the US Department of Energy. Evaporators and suitably configured
direct-contact condensers were developed and patented by SERI (see ). An
original design for a power-producing experiment, then called the 165-kW
experiment was described by Kreith and Bharathan (, and ) as the Max Jacob Memorial Award Lecture. The
initial design used two parallel axial turbines, using last stage rotors taken
from large steam turbines. Later, a team lead by Dr. Bharathan at the National
Renewable Energy Laboratory (NREL) developed the initial conceptual design for
up-dated 210 kW open-cycle OTEC experiment. This design integrated all
components of the cycle, namely, the evaporator, condenser and the turbine into
one single vacuum vessel, with the turbine mounted on top to prevent any
potential for water to reach it. The vessel was made of concrete as the first
process vacuum vessel of its kind. Attempts to make all components using
low-cost plastic material could not be fully achieved, as some conservatism was
required for the turbine and the vacuum pumps developed as the first of their
kind. Later Dr. Bharathan worked with a team of engineers at the Pacific
Institute for High Technology Research (PICHTR) to further pursue this design
through preliminary and final stages. It was renamed the Net Power Producing
Experiment (NPPE) and was constructed at the Natural Energy Laboratory of
Hawaii (NELH) by PICHTR by a team lead by Chief Engineer Don Evans and the
project was managed by Dr. Luis Vega.
In 2002, India tested a 1 MW floating OTEC pilot plant near
Tamil Nadu. The plant was ultimately unsuccessful due to a failure of the deep
sea cold water pipe. Its government continues to sponsor research.
In 2006, Makai Ocean Engineering was awarded a contract from
the U.S. Office of Naval Research (ONR) to investigate
the potential for OTEC to produce nationally-significant quantities of hydrogen
in at-sea floating plants located in warm, tropical waters. Realizing the need
for larger partners to actually commercialize OTEC, Makai approached Lockheed
Martin to renew their previous relationship and determine if the time was ready
for OTEC. And so in 2007, Lockheed Martin resumed work in OTEC and became a
subcontractor to Makai to support their SBIR, which was followed by other
subsequent collaborations
In July 2011, Makai Ocean Engineering completed the design
and construction of an OTEC Heat Exchanger Test Facility at the Natural Energy Laboratory of Hawaii.
The purpose of the facility is to arrive at an optimal design for OTEC heat
exchangers, increasing performance and useful life while reducing cost (heat
exchangers being the #1 cost driver for an OTEC plant). And in March 2013,
Makai announced an award to install and operate a 100 kilowatt turbine on the
OTEC Heat Exchanger Test Facility, and once again connect OTEC power to the
grid.
Currently Operating OTEC Plants
In March 2013, Saga University with various Japanese
industries completed the installation of a new OTEC plant.[18] Okinawa
Prefecture announced the start of the OTEC operation testing at Kume Island on
April 15, 2013. The main aim is to prove the validity of computer models and
demonstrate OTEC to the public. The testing and research will be conducted with
the support of Saga University until the end of FY 2014. IHI Plant Construction
Co. Ltd, Yokogawa Electric Corporation, and Xenesys Inc were entrusted with
constructing the 100 kilowatt class plant within the grounds of the Okinawa
Prefecture Deep Sea Water Research Center. The location was specifically chosen
in order to utilize existing deep seawater and surface seawater intake pipes
installed for the research center in 2000. The pipe is used for the intake of
deep sea water for research, fishery, and agricultural use.[19] The plant
consists of two units; one includes the 50 kW generator while the second
unit is used for component testing and optimization. The OTEC facility and deep
seawater research center are open to free public tours by appointment in
English and Japanese. Currently, this is the only fully operational OTEC plant
in the world.
In 2011, Makai Ocean Engineering completed a heat exchanger
test facility at NELHA. Used to test a variety of heat exhcange technology for
use in OTEC, Makai has received funding to install a 100kw turbine.
Installation will make this facility the largest operational OTEC facility,
though the record for largest power will remain with the Open Cycle plant also
developed in Hawaii.
In July 2014, DCNS group partnered with Akuo Energy
announced NER 300 funding for their NEMO project. If successful, the 16MW gross
10MW net offshore plant will be the largest OTEC facility to date. DCNS plans
to have NEMO operational within four years.
Thermodynamic efficiency
A heat engine gives greater efficiency when run with a
large temperature
difference. In the oceans the temperature difference between surface and deep
water is greatest in the tropics, although still a modest 20 to 25 °C. It is
therefore in the tropics that OTEC offers the greatest possibilities. OTEC has
the potential to offer global amounts of energy that are 10 to 100 times
greater than other ocean energy options such as wave power.
OTEC plants can operate continuously providing a base load
supply for an electrical power generation system.
The main technical challenge of OTEC is to generate
significant amounts of power efficiently from small temperature differences. It
is still considered an emerging technology. Early OTEC systems were 1
to 3 percent thermally efficient, well below the theoretical
maximum 6 and 7 percent for this temperature difference. Modern designs allow
performance approaching the theoretical maximum Carnot
efficiency and the largest built in 1999 by the USA generated 250 kW.
Cycle types
Cold seawater is an integral part of each of the three types
of OTEC systems: closed-cycle, open-cycle, and hybrid. To operate, the cold
seawater must be brought to the surface. The primary approaches are active
pumping and desalination. Desalinating seawater near the sea floor lowers its
density, which causes it to rise to the surface.
The alternative to costly pipes to bring condensing cold
water to the surface is to pump vaporized low boiling point fluid into the
depths to be condensed, thus reducing pumping volumes and reducing technical
and environmental problems and lowering costs.
Closed
Closed-cycle systems use fluid with a low boiling point,
such as ammonia
(having a boiling point around -33 °C at atmospheric pressure), to power a
turbine to
generate electricity. Warm surface seawater is
pumped through a heat exchanger to vaporize the fluid. The expanding
vapor turns the turbo-generator. Cold water, pumped through a second heat
exchanger, condenses the vapor into a liquid, which is then recycled through
the system.
In 1979, the Natural Energy Laboratory and several
private-sector partners developed the "mini OTEC" experiment, which
achieved the first successful at-sea production of net electrical power from
closed-cycle OTEC.The mini OTEC vessel was moored 1.5 miles (2.4 km) off
the Hawaiian coast and produced enough net electricity to illuminate the ship's
light bulbs and run its computers and television.
Open
Open-cycle OTEC uses warm surface water directly to make
electricity. The warm seawater is first pumped into a low-pressure container,
which causes it to boil. In some schemes, the expanding steam drives a
low-pressure turbine attached to an electrical generator. The steam, which has
left its salt and
other contaminants in the low-pressure container, is pure fresh water. It is
condensed into a liquid by exposure to cold temperatures from deep-ocean water.
This method produces desalinized fresh water, suitable for drinking
water, irrigation
or aquaculture.
In other schemes, the rising steam is used in a gas lift
technique of lifting water to significant heights. Depending on the embodiment,
such steam
lift pump techniques generate power from a hydroelectric
turbine either before or after the pump is used.
In 1984, the Solar Energy Research Institute (now
known as the National Renewable Energy
Laboratory) developed a vertical-spout evaporator to convert warm seawater
into low-pressure steam for open-cycle plants. Conversion efficiencies were as
high as 97% for seawater-to-steam conversion (overall steam production would
only be a few percent of the incoming water). In May 1993, an open-cycle OTEC
plant at Keahole Point, Hawaii, produced close to 80 kW of electricity during a
net power-producing experiment. This broke the record of 40 kW set by a
Japanese system in 1982.
Hybrid
A hybrid cycle combines the features of the closed- and
open-cycle systems. In a hybrid, warm seawater enters a vacuum chamber and is
flash-evaporated, similar to the open-cycle evaporation process. The steam
vaporizes the ammonia
working fluid of a closed-cycle loop on the other side of an ammonia vaporizer.
The vaporized fluid then drives a turbine to produce electricity. The steam
condenses within the heat exchanger and provides desalinated
water
Working fluids
A popular choice of working fluid is ammonia, which has
superior transport properties, easy availability, and low cost. Ammonia,
however, is toxic and flammable. Fluorinated carbons such as CFCs and HCFCs are not toxic or
flammable, but they contribute to ozone layer depletion. Hydrocarbons too are
good candidates, but they are highly flammable; in addition, this would create
competition for use of them directly as fuels. The power plant size is
dependent upon the vapor pressure of the working fluid. With increasing vapor
pressure, the size of the turbine and heat exchangers decreases while the wall
thickness of the pipe and heat exchangers increase to endure high pressure
especially on the evaporator side.
Land, shelf and floating sites
OTEC has the potential to produce gigawatts of electrical
power, and in conjunction with electrolysis,
could produce enough hydrogen to completely replace all projected global fossil
fuel consumption. Reducing costs remains an unsolved challenge, however. OTEC
plants require a long, large diameter intake pipe, which is submerged a
kilometer or more into the ocean's depths, to bring cold water to the surface.
Land-based
Land-based and near-shore facilities offer three main
advantages over those located in deep water. Plants constructed on or near land
do not require sophisticated mooring, lengthy power cables, or the more
extensive maintenance associated with open-ocean environments. They can be
installed in sheltered areas so that they are relatively safe from storms and
heavy seas. Electricity, desalinated water, and cold, nutrient-rich seawater
could be transmitted from near-shore facilities via trestle bridges or
causeways. In addition, land-based or near-shore sites allow plants to operate
with related industries such as mariculture
or those that require desalinated water.
Favored locations include those with narrow shelves
(volcanic islands), steep (15-20 degrees) offshore slopes, and relatively
smooth sea floors. These sites minimize the length of the intake pipe. A land-based
plant could be built well inland from the shore, offering more protection from
storms, or on the beach, where the pipes would be shorter. In either case, easy
access for construction and operation helps lower costs.
Land-based or near-shore sites can also support mariculture
or chilled water agriculture. Tanks or lagoons built on shore allow workers to
monitor and control miniature marine environments. Mariculture products can be
delivered to market via standard transport.
One disadvantage of land-based facilities arises from the
turbulent wave action in the surf zone. OTEC discharge pipes should be placed in
protective trenches to prevent subjecting them to extreme stress during storms
and prolonged periods of heavy seas. Also, the mixed discharge of cold and warm
seawater may need to be carried several hundred meters offshore to reach the
proper depth before it is released, requiring additional expense in
construction and maintenance.
One way that OTEC systems can avoid some of the problems and
expenses of operating in a surf zone is by building them just offshore in
waters ranging from 10 to 30 meters deep (Ocean Thermal Corporation 1984). This
type of plant would use shorter (and therefore less costly) intake and
discharge pipes, which would avoid the dangers of turbulent surf. The plant
itself, however, would require protection from the marine environment, such as
breakwaters and erosion-resistant foundations, and the plant output would need
to be transmitted to shore.
Shelf based
To avoid the turbulent surf zone as well as to move closer
to the cold-water resource, OTEC plants can be mounted to the continental shelf
at depths up to 100 meters (330 ft). A shelf-mounted plant could be towed
to the site and affixed to the sea bottom. This type of construction is already
used for offshore oil rigs. The complexities of operating an OTEC plant in
deeper water may make them more expensive than land-based approaches. Problems
include the stress of open-ocean conditions and more difficult product
delivery. Addressing strong ocean currents and large waves adds engineering and
construction expense. Platforms require extensive pilings to maintain a stable
base. Power delivery can require long underwater cables to reach land. For
these reasons, shelf-mounted plants are less attractive.
Floating
Floating OTEC facilities operate off-shore. Although
potentially optimal for large systems, floating facilities present several
difficulties. The difficulty of mooring plants in very deep water complicates
power delivery. Cables attached to floating platforms are more susceptible to damage,
especially during storms. Cables at depths greater than 1000 meters are
difficult to maintain and repair. Riser cables, which connect the sea bed and
the plant, need to be constructed to resist entanglement.
As with shelf-mounted plants, floating plants need a stable
base for continuous operation. Major storms and heavy seas can break the
vertically suspended cold-water pipe and interrupt warm water intake as well.
To help prevent these problems, pipes can be made of flexible polyethylene
attached to the bottom of the platform and gimballed with joints or collars.
Pipes may need to be uncoupled from the plant to prevent storm damage. As an
alternative to a warm-water pipe, surface water can be drawn directly into the
platform; however, it is necessary to prevent the intake flow from being
damaged or interrupted during violent motions caused by heavy seas.
Connecting a floating plant to power delivery cables
requires the plant to remain relatively stationary. Mooring is an acceptable
method, but current mooring technology is limited to depths of about 2,000
meters (6,600 ft). Even at shallower depths, the cost of mooring may be
prohibitive.
Some proposed projects
OTEC projects under consideration include a small plant for
the U.S.
Navy base on the British overseas territory island of Diego
Garcia in the Indian Ocean. Ocean Thermal Energy Corporation
(formerly OCEES International, Inc.) is working with the U.S. Navy on a design
for a proposed 13-MW OTEC plant, to replace the current diesel generators. The
OTEC plant would also provide 1.25 million gallons per day of potable water.
This project is currently waiting for changes in US military contract policies.
OTE has proposed building a 10-MW OTEC plant on Guam.
Bahamas
Ocean Thermal Energy Corporation (OTE) currently has plans
to install two 10 MW OTEC plants in the US Virgin Islands and a 5-10 MW OTEC
facility in The Bahamas. OTE has also designed the world’s largest SDC plant
which was planned for a resorted in The Bahamas, which will use cold deep
seawater as a method of air-conditioning. Unfortunately, this project was
postponed due to schedule delays.
Hawaii
Lockheed Martin's Alternative Energy Development
team has partnered with Makai Ocean Engineering to complete the final design
phase of a 10-MW closed cycle OTEC pilot system which will become operational
in Hawaii in the
2012-2013 time frame. This system is being designed to expand to 100-MW
commercial systems in the near future. In November, 2010 the U.S. Naval Facilities Engineering
Command (NAVFAC) awarded Lockheed Martin a US$4.4 million contract
modification to develop critical system components and designs for the plant,
adding to the 2009 $8.1 million contract and two Department of Energy grants
totaling over $1 million in 2008 and March 2010. This effort was canceled when
the Navy determined that the system was not viable.
Hainan
On April 13, 2013 Lockheed contracted with the Reignwood
Group to build a 10 megawatt plant off the coast of southern China to provide
power for a planned resort on Hainan island. A plant of that size would power several
thousand homes.The Reignwood Group acquired Opus Offshore in 2011 which forms
its Reignwood Ocean Engineering division which also is engaged in development
of deepwater drilling.
Japan
Currently the only fully operational OTEC system is located
in Okinawa Prefecture, Japan. The Governmental support, local community
support, and advanced researched carried out by Saga University were key for
the contractors, IHI Plant Construction Co. Ltd, Yokogawa Electric Corporation,
and Xenesys Inc, to succeed with this project. Work is being conducted to
develop a 1MW facility on Kume Island requiring new pipelines. In July 2014
more than 50 members formed an international organization to study the
development of an Ocean Energy research center on Kume Island and work towards
the installation of larger deep seawater pipelines. The companies involved in
the current OTEC projects, along with other interested parties have developed
plans for offshore OTEC systems as well.- For more details, see "Currently
Operating OTEC Plants" above.
United States Virgin Islands
On March 5, 2014, Ocean Thermal Energy Corporation (OTE) and
the 30th Legislature of the United States Virgin Islands (USVI) signed a
Memorandum of Understanding to move forward with a study to evaluate the feasibility
and potential benefits to the USVI of installing on-shore Ocean Thermal Energy
Conversion (OTEC) renewable energy power plants and Seawater Air Conditioning
(SWAC) facilities. The benefits to be assessed in the USVI study include both
the baseload (24/7) clean electricity generated by OTEC, as well as the various
related products associated with OTEC and SWAC, including abundant fresh
drinking water, energy-saving air conditioning, sustainable aquaculture and
mariculture, and agricultural enhancement projects for the Islands of St Thomas
and St Croix. The Honorable Shawn-Michael Malone, President of the USVI Senate,
commented on his signing of the Memorandum of Understanding (MOU) authorizing
OTE's feasibility study. “The most fundamental duty of government is to
protect the health and welfare of its citizens," said Senator Malone. "These
clean energy technologies have the potential to improve the air quality and
environment for our residents, and to provide the foundation for meaningful
economic development. Therefore, it is our duty as elected representatives to
explore the feasibility and possible benefits of OTEC and SWAC for the people
of USVI.”
Related activities
OTEC has uses other than power production.
Desalination
Desalinated water can be produced in open- or hybrid-cycle
plants using surface condensers to turn evaporated seawater
into potable water. System analysis indicates that a 2-megawatt plant could
produce about 4,300 cubic metres (150,000 cu ft) of desalinated water
each day. Another system patented by Richard Bailey creates condensate water by
regulating deep ocean water flow through surface condensers correlating with
fluctuating dew-point temperatures. This condensation system uses no
incremental energy and has no moving parts.
Air conditioning
The 41 °F (5 °C) cold seawater made available by
an OTEC system creates an opportunity to provide large amounts of cooling to
industries and homes near the plant. The water can be used in chilled-water
coils to provide air-conditioning for buildings. It is estimated that a pipe 1
foot (0.30 m) in diameter can deliver 4,700 gallons of water per minute.
Water at 43 °F (6 °C) could provide more than enough air-conditioning
for a large building. Operating 8,000 hours per year in lieu of electrical
conditioning selling for 5-10¢ per kilowatt-hour, it would save
$200,000-$400,000 in energy bills annually.
The InterContinental Resort and Thalasso-Spa on the
island of Bora
Bora uses an OTEC system to air-condition its buildings. The system passes
seawater through a heat exchanger where it cools freshwater in a closed loop
system. This freshwater is then pumped to buildings and directly cools the air.
In 2010, Copenhagen Energy opened a district cooling plant
in Copenhagen, Denmark. The plant delivers cold seawater to commercial and
industrial buildings, and has reduced electricity consumption by 80 percent.
Ocean Thermal Energy Corporation (OTE) has designed a 9800 ton SDC system for a
vacation resort in The Bahamas.
Chilled-soil agriculture
OTEC technology supports chilled-soil agriculture. When cold
seawater flows through underground pipes, it chills the surrounding soil. The
temperature difference between roots in the cool soil and leaves in the warm
air allows plants that evolved in temperate
climates to be grown in the subtropics.
Dr. John P. Craven, Dr. Jack Davidson and Richard Bailey patented this process
and demonstrated it at a research facility at the Natural Energy Laboratory of
Hawaii Authority (NELHA). The research facility demonstrated that more than 100
different crops can be grown using this system. Many normally could not survive
in Hawaii or at Keahole Point.
Japan has also been researching agricultural uses of Deep
Sea Water since 2000 at the Okinawa Deep Sea Water Research Institute on Kume
Island. The Kume Island facilities use regular water cooled by Deep Sea Water
in a heat exchanger run through pipes in the ground to cool soil. Their
techniques have developed an important resource for the island community as
they now produce spinach, a winter vegetable, commercially year round. An
expansion of the deep seawater agriculture facility is currently under
construction next to the OTEC Demonstration Facility and will be completed in
2014.
Aquaculture
Aquaculture is the best-known byproduct, because it
reduces the financial and energy costs of pumping large volumes of water from
the deep ocean. Deep ocean water contains high concentrations of essential
nutrients that are depleted in surface waters due to biological consumption.
This "artificial upwelling" mimics the natural upwellings that are
responsible for fertilizing and supporting the world's largest marine
ecosystems, and the largest densities of life on the planet.
Cold-water delicacies, such as salmon and lobster, thrive
in this nutrient-rich, deep, seawater. Microalgae
such as Spirulina, a health food
supplement, also can be cultivated. Deep-ocean water can be combined with
surface water to deliver water at an optimal temperature.
Non-native species such as salmon, lobster, abalone, trout, oysters, and clams can be raised in
pools supplied by OTEC-pumped water. This extends the variety of fresh seafood
products available for nearby markets. Such low-cost refrigeration can be used
to maintain the quality of harvested fish, which deteriorate quickly in warm
tropical regions. In Kona, Hawaii, aquaculture companies working with NELHA
generate about $40 million annually, a significant portion of Hawaii’s GDP.
The NELHA plant established in 1993 produced anaverage of
7,000 gallons of freshwater per day. KOYO USA was established in 2002 to
capitalize on this new economic opportunity. KOYO bottles the water produced by
the NELHA plant in Hawaii. With the capacity to produce one million bottles of
water every day, KOYO is now Hawaii’s biggest exporter with $140 million in
sales.
Hydrogen production
Hydrogen can be produced via electrolysis
using OTEC electricity. Generated steam with electrolyte compounds added to
improve efficiency is a relatively pure medium for hydrogen production. OTEC
can be scaled to generate large quantities of hydrogen. The main challenge is
cost relative to other energy sources and fuels.
Mineral extraction
The ocean contains 57 trace
elements in salts and other forms and dissolved in solution. In the past,
most economic analyses concluded that mining the ocean for trace elements would
be unprofitable, in part because of the energy required to pump the water.
Mining generally targets minerals that occur in high concentrations, and can be
extracted easily, such as magnesium. With OTEC plants supplying water, the only cost
is for extraction. The Japanese investigated the possibility of extracting uranium and found
developments in other technologies (especially materials sciences) were
improving the prospects.
Political concerns
Because OTEC facilities are more-or-less stationary surface
platforms, their exact location and legal status may be affected by the United Nations
Convention on the Law of the Sea treaty (UNCLOS). This treaty grants
coastal nations 3-, 12-, and 200-mile (320 km) zones of varying legal
authority from land, creating potential conflicts and regulatory barriers. OTEC
plants and similar structures would be considered artificial islands under the treaty, giving them
no independent legal status. OTEC plants could be perceived as either a threat
or potential partner to fisheries or to seabed mining operations
controlled by the International Seabed Authority.
Cost and economics
For OTEC to be viable as a power source, the technology must
have tax and subsidy treatment similar to competing energy sources. Because
OTEC systems have not yet been widely deployed, cost estimates are uncertain.
One study estimates power generation costs as low as US $0.07 per
kilowatt-hour, compared with $0.05 - $0.07 for subsidized wind systems.
Beneficial factors that should be taken into account include
OTEC's lack of waste products and fuel consumption, the area in which it is
available, (often within 20° of the equator) the geopolitical effects of petroleum
dependence, compatibility with alternate forms of ocean power such as wave
energy, tidal energy and methane hydrates, and supplemental uses for the
seawater.
Thermodynamics
A rigorous treatment of OTEC reveals that a 20 °C
temperature difference will provide as much energy as a hydroelectric plant
with 34 m head for the same volume of water flow. The low temperature
difference means that water volumes must be very large to extract useful
amounts of heat. A 100MW power plant would be expected to pump on the order of
12 million gallons (44,400 metric tonnes) per minute. For comparison, pumps
must move a mass of water greater than the weight of the Battleship Bismark, which weighed 41,700
metric tons, every minute. This makes pumping a substantial parasitic
drain on energy production in OTEC systems, with one Lockheed design
consuming 19.55 MW in pumping costs for every 49.8 MW net electricity
generated. For OTEC schemes using heat exchangers, to handle this volume of
water the exchangers need to be enormous compared to those used in conventional
thermal power generation plants, making them one of the most critical
components due to their impact on overall efficiency. A 100 MW OTEC power plant
would require 200 exchangers each larger than a 20 foot shipping container
making them the single most expensive component.
Variation of ocean temperature with depth
The total insolation received by the oceans (covering 70% of the
earth's surface, with clearness index of 0.5 and average energy retention of
15%) is: 5.45×1018 MJ/yr × 0.7 × 0.5 × 0.15 = 2.87×1017
MJ/yr
We can use Lambert's
law to quantify the solar energy absorption by water,
,Since the intensity falls
exponentially with depth y, heat absorption is concentrated at the
top layers. Typically in the tropics, surface temperature values are in excess
of 25 °C (77 °F), while at 1 kilometer (0.62 mi), the
temperature is about 5–10 °C (41–50 °F). The warmer (and hence
lighter) waters at the surface means there are no thermal convection currents. Due to the small
temperature gradients, heat transfer by conduction
is too low to equalize the temperatures. The ocean is thus both a practically
infinite heat source and a practically infinite heat sink.
This temperature difference varies with latitude and season,
with the maximum in tropical, subtropical and equatorial waters. Hence the tropics are
generally the best OTEC locations.
Open/Claude cycle
In this scheme, warm surface water at around 27 °C
(81 °F) enters an evaporator at pressure slightly below the saturation pressures causing it to vaporize.
Where Hf is enthalpy of
liquid water at the inlet temperature, T1.
This temporarily superheated
water undergoes volume boiling as opposed to pool boiling in conventional
boilers where the heating surface is in contact. Thus the water partially flashes
to steam with two-phase equilibrium prevailing. Suppose that the pressure
inside the evaporator is maintained at the saturation pressure, T2.
The low pressure in the evaporator is maintained by a vacuum pump
that also removes the dissolved non-condensable gases from the evaporator. The
evaporator now contains a mixture of water and steam of very low vapor
quality (steam content). The steam is separated from the water as saturated
vapor. The remaining water is saturated and is discharged to the ocean in the
open cycle. The steam is a low pressure/high specific
volume working fluid. It expands in a special low pressure turbine.
This enthalpy is lower. The adiabatic reversible turbine
work = H3-H5,s .
Actual turbine work WT = (H3-H5,s)
x polytropic efficiency
The condenser temperature and pressure are lower. Since the
turbine exhaust is to be discharged back into the ocean, a direct contact
condenser is used to mix the exhaust with cold water, which results in a
near-saturated water. That water is now discharged back to the ocean.
H6=Hf, at T5.
T7 is the temperature of the exhaust mixed with cold sea
water, as the vapour content now is negligible,
The temperature differences between stages include that
between warm surface water and working steam, that between exhaust steam and
cooling water, and that between cooling water reaching the condenser and deep
water. These represent external irreversibilities that reduce
the overall temperature difference.
The cold water flow rate per unit turbine mass flow
rate,
Closed Anderson cycle
Developed starting in the 1960s by J. Hilbert Anderson of
Sea Solar Power, Inc. In this cycle, QH is the heat
transferred in the evaporator from the warm sea water to the working fluid. The
working fluid exits the evaporator as a gas near its dew point.
The high-pressure, high-temperature gas then is expanded in
the turbine to yield turbine work, WT. The working fluid is
slightly superheated at the turbine exit and the turbine typically has an
efficiency of 90% based on reversible, adiabatic expansion.
From the turbine exit, the working fluid enters the
condenser where it rejects heat, -QC, to the cold sea water.
The condensate is then compressed to the highest pressure in the cycle,
requiring condensate pump work, WC. Thus, the Anderson closed
cycle is a Rankine-type cycle similar to the conventional power plant steam
cycle except that in the Anderson cycle the working fluid is never superheated
more than a few degrees Fahrenheit. Owing to viscous effects,
working fluid pressure drops in both the evaporator and the condenser. This
pressure drop, which depends on the types of heat exchangers used, must be
considered in final design calculations but is ignored here to simplify the
analysis. Thus, the parasitic condensate pump work, WC,
computed here will be lower than if the heat exchanger pressure drop was
included. The major additional parasitic energy requirements in the OTEC plant
are the cold water pump work, WCT, and the warm water pump
work, WHT.
The thermodynamic cycle undergone by the working fluid can
be analyzed without detailed consideration of the parasitic energy
requirements. From the first law of thermodynamics, the energy balance for the
working fluid as the system is
where WN = WT + WC
is the net work for the thermodynamic cycle. For the idealized case in which
there is no working fluid pressure drop in the heat exchangers,
Subcooled liquid enters the evaporator. Due to the heat
exchange with warm sea water, evaporation takes place and usually superheated
vapor leaves the evaporator. This vapor drives the turbine and the 2-phase
mixture enters the condenser. Usually, the subcooled liquid leaves the
condenser and finally, this liquid is pumped to the evaporator completing a
cycle.
Environmental impact
Carbon dioxide dissolved in deep cold and high pressure
layers is brought up to the surface and released as the water warms.
Mixing of deep ocean water with shallower water brings up
nutrients and makes them available to shallow water life. This may be an
advantage for aquaculture of commercially important species, but may also
unbalance the ecological system around the power plant.
OTEC plants use very large flows of warm surface seawater
and cold deep seawater to generate constant renewable power. The deep seawater
is oxygen deficient and generally 20-40 times more nutrient rich (in nitrate
and nitrite) than shallow seawater. When these plumes are mixed, they are
slightly denser than the ambient seawater.Though no large scale physical
environmental testing of OTEC has been done, computer models have been
developed to simulate the effect of OTEC plants.
Hydrodynamic Modeling Work
In 2010, a computer model was developed to simulate the
physical oceanographic effects of one or several 100 megawatt OTEC plant(s).
The model suggests that OTEC plants can be configured such that the plant can
conduct continuous operations, with resulting temperature and nutrient
variations that are within naturally occurring levels. Studies to date suggest
that by discharging the OTEC flows downwards at a depth below 70 meters, the
dilution is adequate and nutrient enrichment is small enough so that 100
megawatt OTEC plants could be operated in a sustainable manner on a continuous
basis.
Biological Modeling Work
The nutrients from an OTEC discharge could potentially cause
increased biological activity if they accumulate in large quantities in the photic zone.
In 2011 a biological component was added to the hydrodynamic computer model to
simulate the biological response to plumes from 100 megawatt OTEC plants. In
all cases modeled (discharge at 70 meters depth or more), no unnatural
variations occurs in the upper 40 meters of the ocean's surface. The
picoplankton response in the 110 - 70 meter depth layer is approximately a
10-25% increase, which is well within naturally occurring variability. The
nanoplankton response is negligible. The enhanced productivity of diatoms
(microplankton) is small. The subtle phytoplankton increase of the baseline
OTEC plant suggests that higher-order biochemical effects will be very small.
Environmental Impact Studies
A previous Final Environmental Impact Statement (EIS) for
the United States' NOAA from 1981 is available, but needs to be brought up to
current oceanographic and engineering standards. Studies have been done to
propose the best environmental baseline monitoring practices, focusing on a set
of ten chemical oceanographic parameters relevant to OTEC. Most recently, NOAA
held an OTEC Workshop in 2010 and 2012 seeking to assess the physical,
chemical, and biological impacts and risks, and identify information gaps or
needs.
Technical difficulties
Dissolved gases
The performance of direct contact heat exchangers operating
at typical OTEC boundary conditions is important to the Claude cycle. Many
early Claude cycle designs used a surface condenser since their performance was
well understood. However, direct contact condensers offer significant
disadvantages. As cold water rises in the intake pipe, the pressure decreases
to the point where gas begins to evolve. If a significant amount of gas comes out
of solution, placing a gas trap before the direct contact heat exchangers may
be justified. Experiments simulating conditions in the warm water intake pipe
indicated about 30% of the dissolved gas evolves in the top 8.5 meters
(28 ft) of the tube. The trade-off between pre-dearation
of the seawater and expulsion of non-condensable gases from the condenser
is dependent on the gas evolution dynamics, deaerator efficiency, head loss,
vent compressor efficiency and parasitic power. Experimental results indicate
vertical spout condensers perform some 30% better than falling jet types.
Microbial fouling
Because raw seawater must pass through the heat exchanger,
care must be taken to maintain good thermal conductivity. Biofouling
layers as thin as 25 to 50 micrometres (0.00098 to 0.00197 in) can degrade
heat exchanger performance by as much as 50%. A 1977 study in which mock heat
exchangers were exposed to seawater for ten weeks concluded that although the
level of microbial fouling was low, the thermal conductivity of the system was
significantly impaired. The apparent discrepancy between the level of fouling
and the heat transfer impairment is the result of a thin layer of water trapped
by the microbial growth on the surface of the heat exchanger.
Another study concluded that fouling degrades performance
over time, and determined that although regular brushing was able to remove
most of the microbial layer, over time a tougher layer formed that could not be
removed through simple brushing. The study passed sponge rubber balls through
the system. It concluded that although the ball treatment decreased the fouling
rate it was not enough to completely halt growth and brushing was occasionally
necessary to restore capacity. The microbes regrew more quickly later in the
experiment (i.e. brushing became necessary more often) replicating the results
of a previous study. The increased growth rate after subsequent cleanings
appears to result from selection pressure on the microbial colony.
Continuous use of 1 hour per day and intermittent periods of
free fouling and then chlorination periods (again 1 hour per day) were
studied. Chlorination slowed but did not stop microbial growth; however
chlorination levels of .1 mg per liter for 1 hour per day may prove
effective for long term operation of a plant. The study concluded that although
microbial fouling was an issue for the warm surface water heat exchanger, the
cold water heat exchanger suffered little or no biofouling and only minimal
inorganic fouling.
Besides water temperature, microbial fouling also depends on
nutrient levels, with growth occurring faster in nutrient rich water.The
fouling rate also depends on the material used to construct the heat exchanger.
Aluminium
tubing slows the growth of microbial life, although the oxide
layer which forms on the inside of the pipes complicates cleaning and leads to
larger efficiency losses. In contrast, titanium tubing
allows biofouling to occur faster but cleaning is more effective than with
aluminium.
Sealing
The evaporator, turbine, and condenser operate in partial
vacuum ranging from 3% to 1% of atmospheric pressure. The system must be
carefully sealed to prevent in-leakage of atmospheric air that can degrade or
shut down operation. In closed-cycle OTEC, the specific volume of low-pressure
steam is very large compared to that of the pressurized working fluid.
Components must have large flow areas to ensure steam velocities do not attain
excessively high values.
Parasitic power consumption by exhaust compressor
An approach for reducing the exhaust compressor parasitic
power loss is as follows. After most of the steam has been condensed by
spout condensers, the non-condensible gas steam mixture is passed through a
counter current region which increases the gas-steam reaction by a factor of
five. The result is an 80% reduction in the exhaust pumping power requirements.
Cold air/warm water conversion
In winter in coastal Arctic locations,
the delta T between the seawater and ambient air can be as high as 40 °C
(72 °F). Closed-cycle systems could exploit the air-water temperature
difference. Eliminating seawater extraction pipes might make a system based on
this concept less expensive than OTEC. This technology is due to H. Barjot, who
suggested butane as cryogen, because of its boiling point of −0.5 °C
(31.1 °F) and its non-solubility in water. Assuming a level of efficiency
of realistic 4%, calculations show that the amount of energy generated with one
cubic meter water at a temperature of 2 °C (36 °F) in a place with an
air temperature of −22 °C (−8 °F) equals the amount of energy
generated by letting this cubic meter water run through a hydroelectric plant
of 4000 feet (1,200 m) height.
Barjot Polar Power Plants could be located on islands in the
polar region or designed as swimming barges or platforms attached to the ice cap. The
weather station Myggbuka at Greenlands east coast for example, which is only
2,100 km away from Glasgow, detects monthly mean temperatures below
−15 °C (5 °F) during 6 winter months in the year.
SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
For affiliation:
No comments:
Post a Comment