By Bill Moore
In the 1970s the concept of Ocean Thermal Energy Conversion or OTEC was proven technically feasible, but not economic. Oil and gas prices were just too low.
Now like a long-submerged submarine, OTEC is again resurfacing. With oil prices headed north of $60 a barrel, what wasn't economically practical just fifteen years ago is starting to look attractive again -- especially to original pioneers like Hans Jurgen Krock with the University of Hawaii .
Joining him is NREL's Terry Penney, himself an early OTEC researcher and proponent, with whom I was having dinner in Philadelphia a few weeks back. Appropriately, we were having seafood and the discussion gravitated, naturally enough, to making power from the difference between the warm surface waters of the tropics and the much colder waters 300 meters down. The concept has been around for more than a century, first being attempted off Cuba in the late 1920's. Unfortunately, a storm carried away the deep water pipe that was to bring up the cold water and the experiment never recovered.
Penney, who is a senior manager at NREL, worked on the Energy Department's original technology demonstration project in Hawaii, then moved on to SERI, which eventually became the National Renewable Energy Laboratory in Golden, Colorado.
The original research facility is still operating in Hawaii, but not on ocean thermal energy. Instead it uses electricity generated by Hawaiian Electric Company to draw up chilled seawater through a 55-inch diameter pipe. Using reverse osmosis, the operators purify the "ancient" water and bottle it for shipment to Japan and elsewhere. According to Dr. Krock, it is the islands' largest export dollar-wise. The facility also utilities the nutrient-rich waters to run an adjacent aquaculture operation, a vestige of the original 70s government program.
But the promise of using the ocean's unfathomable store of solar thermal energy has remained, until now, mostly an illusive dream for a handful of visionaries like Penney and Krock. Today, that's all changed.
"The economics have turned around completely," Krock told me via a voice-over-internet connection, "and in fact, it is now economically viable to look at this resource for power production and fresh water production."
Krock explained the basic concept of ocean thermal energy conversion, which works on the principle of a simple heat engine.
"You have a warm water reservoir, which is the surface of the ocean, the surface layer which is warm because the sun shines on it; and then you have the deep water, which is cold because the sunshine doesn't get down that far. The last time that water saw the light of day was in Antarctica or in the Arctic Ocean someplace and so you have this difference in temperature. In thermodynamic terms you can run a heat engine if you have a warm reservoir and a cold reservoir. And a heat engine is simply a very conventional system where you boil a working fluid and you run the working fluid through a turbine and the turbine is connected to a generator and you make electricity. After it runs through the turbine, it gets in touch with the cold water and it condenses back. So you have vapor production in the turbine and the condenser at the end. It's very simple conceptually and it simply uses the fact that heat flows to cold to run this engine."
One of the by-products of the condensation is fresh water, if the system is open-cycle, meaning the working fluid is seawater. In a closed-cycle system, which typically uses ammonia as the working fluid, fresh water is not produced, but the size of the turbine can be significantly smaller, reducing capital costs.
Because of the price of oil and still relatively low interest rates, Krock believes the time is ripe for the re-emergence of OTEC which is why he started Ocean Engineering and Energy Systems or OCEES, one of a handful of companies laboring to launch commercial OTEC systems in the next few years.
Another key factor in the OTEC revival story is the development of a new technology known as the Kalina-cycle, which Krock said is superior to the approaches he and Penney investigated in the 1980s.
He explained that while the Kalina-cycle is a closed-cycle system, it uses two working fluids -- ammonia and seawater -- instead of just one.
"It's sort of a thermodynamic trick that you can play on the working fluid if you have a dual working fluid and you can get more net power out the same difference in temperature than you can with just a single working fluid. It's a detail of thermodynamics that would take a little while for me to explain...," he said. "It is a proven technology and is superior to the one that is before.
"There have also been improvements in our ability to access the deep water because the oil companies have deep water installations down to two thousand meters and more in the Gulf of Mexico, for example. So the technology for access to the deep water has improved."
Krock also noted that there have been improvements in other materials needed to make the process feasible.
"We know better how to make floating platforms. We know better how to make large cold water pipes, and all manner of other improvements."
In addition to these improvements, he said that "we are better able to integrate the multiple products that can be supplied by this OTEC process... fresh water, aquaculture, the cold water air conditioning and hydrogen production."
And here is the newest wrinkle in OTEC: hydrogen. Imagine hectares (or acres) square mini-islands gently drifting with the currents in the tropical belt of warm water around the earth's equator, roughly between Hawaii in the north and Samoa in the south. 24-hours a day they produce both electricity and fresh water, but they are also 3000 miles from the nearest mainland.
What do you with the water and power? Make hydrogen from it, is Krock's vision. The hydrogen could then be compressed into storage and periodically shipped to receiving centers in Asia or the Americas. Atlantic drift-plants could supply Africa, the Americas and Europe.
And here's an interesting angle. Both the hydrogen and the oxygen that results from electrolyzing water could be used to fuel commercial rocket launches from the platforms, which because of their position on the equator are ideal locations for orbital insertions, using the earth's own spin to help boost bigger payloads are use less fuel to lift missions into space.
Krock strongly emphasized that, "the resource that we are tapping into, the ocean thermal heat storage... the ocean here... is the only resource large enough to be able to replace oil. And we are running out of oil and this is really the only thing that has the hope of replacing oil. It's bigger than wind. It's bigger than photovoltaics. It is, in fact, the biggest energy resource in the world."
Although the optimal area for the deployment of OTEC power-islands lies in a 40 degree wide band around the planet's middle, it is, according to Krock, an area equivalent to all the earth's landmass.
While onshore installations like the one in Hawaii have their place in providing island communities with power, water, air conditioning and aquaculture, OCEES believes the real potential is offshore. The limiting factor for onshore is the size and length of the pipe needed to reach deep, cold water. Offshore production requires relatively short pipes that can be much larger in diameter that drop straight down below the platform.
Krock said he is confident that we can now built 100 megawatt plants and he can foresee the day when 500 megawatt and 1000 megawatt (1 gigawatt) plants will be possible.
Because the resource is far out into the ocean, far away from any national political entity, it isn't under the jurisdiction of any particular nation.
"So countries such as Switzerland or others could go out there and be completely self-sufficient in energy by having their own energy supply in the tropical zone on the high seas far outside anybody‘s two hundred mile economic zone."
Global Warming's Benefit
Krock explained that the solar energy stored in the world's oceans is what drives the planet's weather and that a single category five hurricane generates more energy in a day than all mankind uses in a year. This may be the only benefit of global warming, providing even more warm water from which to produce power.
"The ocean has increased in temperature by about point six degrees. That extra amount of heat that is in the ocean that has been stored in there over, say, the last forty years; that amount of heat, that amount of energy is enough to run all of humankind's energy requirements for the next five hundred years... just the extra."
I asked Dr. Krock about two potential drawbacks to OTEC: environmental disruption and susceptibility to storm damage. He explained that his team has carefully looked at the first issue, environmental disruption, and determined that there would be none despite bringing up hundreds of millions of gallons of water a day to run the facility, because the water could be shunted back down to a level in the ocean where it would be neutrally buoyant.
As to the question of tropical storms like typhoons or hurricanes and the risk they might pose for offshore OTEC platforms, he explained that these storms form outside of a tropical zone which extends approximately 4-5 degrees above and below the equator. Platforms operating within this narrower belt won't have to worry about these powerful storms and the damage they might cause, though he does plan to engineer for such contingencies.
Unlike the illustration above that uses propellers to drive the plant, Krock's concept for moving the "grazing" OTEC mini-islands would rely on two intriguing systems: thrust vectoring and ocean current "sails". An OTEC plant generates a great deal of thrust from the uptake and expulsion of seawater, which can be directed to gradually move the platform in a desired direction. The 1000-feet stand pipe below the plant is like an inverted mast on a sailing ship. Sensors can detect the direction of the current at various depths, allowing the deployment of underwater "sails" that could also be used to passively steer the plant.
"There is nothing better than working with nature," Krock commented. "This is simply a model on a human scale of the world's hydrological cycle." When compared to other renewable energy sources such as wind and biomass, he calls the heat energy stored in the ocean as the "elephant in the room".
Krock envisions a plant made of floating concrete that is five square acres in size and could include fish processing facilities, ocean mineral mining and refining and the aforementioned rocket launch pad. An earlier Lockheed design was circular, measured some 100 meters in diameter and would generate 500 megawatts of electric power.
"This is a transformation of endeavors from land to the ocean. The world is 70 percent oceans, 30 percent [land]... which we have used up to a large extent. The only major resource we have left is the ocean. This is a mechanism to utilize the ocean."
"We do not have the luxury of waiting far into the future because I am sure you have read peak oil is coming... Unless we do this now, a transformation of this magnitude takes time. We have to allocate at least 50 years to do this, but that means we have to start now, because in fifty years we won't have the luxury of having another energy source to let us do the construction for these things.
"The United States is the best placed of any country in the world to do this," he contends. "The United States is the only country in the world of any size whose budget for its navy is bigger than the budget for its army."
It's his contention that this will enable America to assume a leadership position in OTEC technology, allowing it to deploy plants in the Atlantic, Caribbean and Pacific, but he offers a warming.
"If we are stupid enough not to take advantage of this, well then this will be China's century and not the American century."
Krock is currently negotiating with the U.S. Navy to deploy first working OTEC plant offshore of a British-controlled island in the Indian Ocean -- most likely Diego Garcia though he wouldn't confirm this for security purposes.
He is also working with firms in Britain and Netherlands and will be headed to China for talks with the government in Beijing.
"The Chinese know very well that they cannot build there futures on oil," he stated, noting that China's is investing large sums of money in a blue water navy. "The United States will be playing catch-up in this technology. We're here. We're willing to do it. We're doing it with the Navy." He expects to put his first plant to sea sometime in 2008 after constructing it, mostly likely, in Singapore.
"We simply have to look at the all the alternatives [to conventional fossil fuels and nuclear power] and this is, hands down, the only alternative that's big enough to replace oil."