Sunlight through ocean

Thermal energy from the deep

The current focus on extracting energy from the oceans has been on wave or tidal devices, but as E&T discovers there is huge potential in energy derived from the thermal extremes in oceans.

Sunlight falls on the world's oceans and almost all of that energy is captured within the top 300ft of water. You only have to consider the amount of power that could be generated from that energy in a land-based solar plant, so it is no surprise that since the early 1900s engineers have been attempting to mine this rich resource.

In tropical regions this so-called mixed layer is warmed to around 30°C, a temperature that is constant 24 hours a day, 365 days a year. But it is what is happening below that has interested engineers and provides a huge potential energy source. The deeper you travel in the oceans, the colder the water becomes. When you reach depths of around 3,000ft, the temperature has plummeted to a bone-chilling 4°C.

At this depth, the ice-cold water, formed from melting ice in the Polar regions, does not mix with the warmer top layer providing a fairly constant environment. And it is this 26°C temperature change that allows electricity to be generated using Ocean Thermal Energy Conversion (OTEC). The OTEC process uses this temperature difference to operate a heat engine that generates electricity.

These conditions occur primarily in the tropical regions but still amount to more than 23 million square miles. According to a recent report from the US Department of Energy (DOE), if floating power plants were positioned throughout the tropical region a staggering ten million MWe could be generated. In their calculations each plant would generate 200MWe and would be placed 20 miles apart.

With the cost of oil spiralling and looking to remain high for the foreseeable future, allied with the continued drive to cut carbon emissions, the focus has once again fallen on ocean thermal energy. One company at the forefront of the chase to develop a commercial size facility is Lockheed Martin, a company better known for stealth fighters and satellites.

"OTEC uses an existing and proven process to employ small temperature gradients over 40ºF to drive an engine," Alan Miller, of Lockheed Martin says. "That is the same thermal cycle that's in a normal steam power plant, so there is nothing magical about it, but it is using energy in the ocean so there is a huge renewable energy source.

"You can use the difference in temperature to drive the heat engine, but you need large, really large water flows to extract the energy and generate commercially significant quantities of logistic energy."

The OTEC technology can basically be divided into two configurations - open or closed cycle - although it is possible to develop a hybrid system.

In the closed-cycle OTEC system, warm seawater vaporises a working fluid, such as ammonia, flowing through a heat exchanger (evaporator). The vapour expands at moderate pressures and turns a turbine coupled to a generator that produces electricity. The vapour is then condensed in another heat exchanger (condenser) using cold seawater pumped from the ocean's depths through a cold-water pipe. The condensed working fluid is pumped back to the evaporator to repeat the cycle. The working fluid remains in a closed system and circulates continuously.

In an open-cycle OTEC system, warm seawater is the working fluid. The warm seawater is 'flash'-evaporated in a vacuum chamber to produce steam at an absolute pressure of about 2.4 kilopascals (kPa). The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity. The steam exiting the turbine is condensed by cold seawater pumped from the ocean's depths through a cold-water pipe. If a surface condenser is used in the system, the condensed steam remains separated from the cold seawater and provides a supply of desalinated water.

History of OTEC

In 1881, Jacques Arsene d'Arsonval, a French physicist, was the first to propose tapping the thermal energy of the ocean. But it was not until 49 years later that Georges Claude, a student of d'Arsonval's, built an experimental open-cycle OTEC system at Matanzas Bay, Cuba.

The system produced 22kW of electricity with a low-pressure turbine. In 1935, Claude constructed another open-cycle plant, this time aboard a 10,000-ton cargo vessel moored off the coast of Brazil. But both plants were destroyed by weather and waves, and Claude never produced net power from an open-cycle OTEC system.

Then, in 1956, French researchers designed a 3MWe open-cycle plant for Abidjan on Africa's west coast. But the project was scrapped with the advent of inexpensive hydroelectric power in the region. The action moved over to the US in the 1970s when the Natural Energy Laboratory of Hawaii was established.

In 1979, the first 50kWe closed-cycle OTEC demonstration plant went up at NELHA. Known as Mini-OTEC, the plant was mounted on a converted US Navy barge moored approximately 2km off Keahole Point. The plant used a cold-water pipe to produce 52 kWe of gross power and 15 kWe net power.

In 1980, the DOE built OTEC-1, a test site for closed-cycle OTEC heat exchangers installed on a converted US Navy tanker. Tests identified methods for designing commercial-scale heat exchangers and demonstrated that OTEC systems can operate from slow ships with little effect on the marine environment. A new design for suspended cold-water pipes was validated at that test site.

In 1981, Japan demonstrated a shore-based, 100-kWe closed-cycle plant in the Republic of Nauru in the Pacific Ocean. This plant employed cold-water pipe laid on the sea bed to a depth of 580m. Freon was the working fluid, and a titanium shell-and-tube heat exchanger was used. The plant surpassed engineering expectations by producing 31.5kWe of net power during continuous operating tests.

Later, tests by the DOE determined that aluminium alloy can be used in place of more expensive titanium to make large heat exchangers for OTEC systems. At-sea tests by DOE demonstrated that biofouling and corrosion of heat exchangers can be controlled. Biofouling does not appear to be a problem in cold seawater systems. In warm seawater systems, it can be controlled with a small amount of intermittent chlorination (70 parts per billion per hour per day).

In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment. This broke the record of 40,000 watts set by a Japanese system in 1982.

The current state

"OTEC is not a new process," Miller explains. "But it has been known since Claude's day that the single biggest challenge is the cold water pipe. So it is no surprise that the challenges for a commercially significant OTEC full-sized plant at 100MW is the cold water pipe, which needs to be 10m in diameter."

Lockheed Martin is developing a concept for fabricating and deploying a cold water pipe to be used in the extraction of thermal energy from the ocean.

To obtain the temperatures and flows necessary to run a commercial OTEC cycle, the cold water pipe itself is significant in size at a scale of roughly 10m in diameter and 1,000m.

Lockheed Martin's innovative process and construction aims to reduce previous risks associated with the loss of the cold pipe during deployment and in surviving the harsh environment for the life of the plant.

Using state of the art composite materials and an in-situ fabrication approach, Lockheed Martin's design should overcome the greatest technical challenge any floating OTEC system has faced over the years, attaching a survivable cold water pipe.

"When mentioning this in conversation, I just say simply that you could take the house that I grew up in and drop it inside the cold water pipe. Then, being a former New Yorker, the other physical reference point that I make is that the pipe needs to be about four times as high as the Empire State Building."

Lockheed are not the first to be thinking about cold water pipes. There have been pipes consisting of rigid cylinders joined by flexible joints, compliant material membranes that are stiffened by ring stiffeners. People have proposed stockade pipe where you take pipe in smaller manufacturable sizes and join them together into a ready-assembled device and use as your cold water conduit.

The team have settled on a fibreglass composite one-piece cold water pipe fabricated and deployed directly off the platform. "Currently we have completed a proof of principal and demonstration at small scale - 18in - and that was successful at the end of 2008. Currently we are engaged in scaling up the process to a 13ft internal diameter."

The challenges faced by any cold water pipe are threefold: deployment, survivability and sustainability. By manufacturing the pipe on the platform itself they have done away with any deployment issues. In the area of survivability, since the 1970s a lot of advances have been made in low-cost composites manufacturing and they are exploiting those modern methods of fabricating large composites structures.

The preliminary analysis indicates that no fatigue failure is expected for a cold water pipe that is rigidly connected to the OTEC platform. The analysis carried out includes a full spectrum of sea states that the platform is expected to be exposed to in its 50-year life.

"Finally, in terms of scalability, the flexible mould approach scales OK to large sizes and we don't have any issues about scalability," Miller says. "Obviously we can demonstrate at a larger scale, but there is nothing inherent in the process that will not scale."

The cold water pipe is constructed with a double wall sandwich, outer face sheets and inner face sheets. In between them there is a lightweight material that does not have the same mechanical properties as the facing sheets have. Such composite sandwiches are commonly used in applications such as commercial airliners.

"We started off with elements of this core that are called planks," Miller explains. "So these are partial circumference regions of finite length. What we then do is assemble those core planks into a complete core ring. The core ring is in fact discrete core pieces, but then the outside and the inside of the core are the face sheets that are essentially continuous down the length. The only interruptions are the change in fabric rolls."

In choosing the actual material, Lockheed carried out a national quantitative trade study in which they explored quantitatively what it would cost if it were made of any of the four primary candidate materials - fibreglass, carbon-fibre deposit, steel and high-density polyethylene. "Fibreglass came out to be the lowest cost, with carbon not far behind, but we know that in carbon fibres we will get some galvanic issues," Miller says. "So all things being equal the research sent us in the direction of fibreglass.

"Both fibreglass and carbon-fibre deposit met all the requirements. Very briefly, the principle requirements that the design has to meet is first of all external pressure; it's a suction pipe so right up near the platform you have actually got half an atmosphere of negative pressure that means you are putting half an atmosphere net external pressure on a 33ft diameter pipe.

"The second principle driver is WIM (wave-induced motion): platforms rocking back and forth attached to the cold water pipe that would just as soon sit there and mind its own business, so you can generate reasonable levels of strain especially at the interface areas, between the platform pole and pipe." So with all that considered, fibreglass came out on top.

Lockheed say that they could have an OTEC platform up and running by 2013, but as with all new technologies it won't be cheap. Funding is being sought from the DOE, and future work is likely to be dependent on that outcome. Whatever Lockheed decides on the future of its OTEC project, someone will eventually tap the vast resources of the tropical oceans.

OTEC News [new window]

National Institute of Ocean Technology (NIOT) [new window]

World Energy Council: Survey of Energy Resources 2007 [new window]

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