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Extracting Power from Seawater without Nuclear Fusion

A 20°C Temperature Difference is All You Need to Generate Megawatts of Electricity

Every once in a while, I find it refreshing and stimulating to stop thinking about circuits and to ponder where we will get all of the electricity to power those circuits in the coming years. During a recent trip to the big island in Hawaii, I got a reminder. My wife and I visited NELHA, the Natural Energy Laboratory of Hawaii Authority, located between the ocean and the Kailua-Kona airport, and took a half-day “Friends of NELHA” tour, conducted by the organization’s Executive Director Candee Ellsworth (an ocean biologist and zoologist), which included a wholly unexpected but powerful reminder that it was again time to ponder the future of power generation.

NELHA and the associated HOST (Hawaii Ocean Science and Technology) Park was founded by Dr. John Piña Craven, a former chief scientist for the US Navy’s Special Projects Office with a resume like James Bond. (Look him up.) NEHLA was founded on a simple premise: stick a few very long pipes into the ocean to bring up ancient, cold seawater from 3000 feet deep and put a few more pipes into the ocean to draw in warm surface seawater, supply the water at low cost to a host of commercial enterprises, and let them find useful and profitable things to do with these inexpensive liquid resources in combination with Hawaii’s abundant solar energy, supplied daily by a giant fusion reactor located 93 million miles away (the Sun).

Over the course of 44 years, since the HOST Park was started in 1974, a wide assortment of enterprises established themselves in the park to use these resources. Several are aquaculture startups. There are corporate farms for seahorses, lobsters, jumbo shrimp, abalone, octopi, plankton, and algae; a nursery for baby clams; and two bottlers that desalinate and market the deep ocean water.

And then, there’s Makai Ocean Engineering. Founded in 1973 as a general ocean engineering firm, Makai has been working on OTEC (ocean thermal energy conversion) since the 1970s. OTEC’s goal is to tap some of the enormous amount of solar energy stored as heat in warm ocean surface water and convert it into electricity. No, not with nuclear fusion but with simple thermodynamics and a Rankine cycle heat engine.

In 1979, Makai developed the “Mini-OTEC,” the world’s first net-positive, power-producing OTEC electrical plant. Mini-OTEC was installed on a US Navy barge moored several kilometers off the coast of the island of Hawaii. Although it didn’t produce much electricity, only about 15 kilowatts, Mini-OTEC proved that the OTEC concept could be realized.

Makai’s OTEC testbed located in HOST Park became operational in 2015. It’s capable of generating about 100 kilowatts of electricity, which it can supply to the Hawaii power grid. However, the testbed’s real purpose is not energy generation. It is research. Makai uses the OTEC testbed to study the design and configuration of various OTEC heat exchangers, three of which appear in Figure 1.

Figure 1: Makai Ocean Engineering’s OTEC testbed located next to the Kona airport in Hawaii can generate 100 kilowatts of electricity from warm and cold seawater drawn directly from the ocean. (Image credit: Steve Leibson)

Makai’s OTEC testbed is designed in a modular fashion so that heat exchangers can be easily swapped in an out for testing. Three vertical, hot-side heat exchangers appear on the left side of the tower in Figure 1. A fourth heat exchanger, used for cooling, appears on the right side of Figure 1 in a horizontal position.

There’s a strange connection between the use of ocean thermal energy to generate electricity and the effect that makes Zener diodes work. Both phenomena were studied by Clarence Melvin Zener, an American physicist who wrote papers about both. Zener published a paper in 1934 that predicted a breakdown effect, which would be named for him by none other than the famous/infamous William Shockley in 1950 while he was studying early semiconductor p-n junction diodes at Bell Labs. Zener’s paper had predicted a possible reverse-bias breakdown of a semiconductor diode due to electron tunneling across a heavily doped p-n junction from the n-doped to the p-doped side when sufficient reverse-bias voltage was applied.

OTEC: Power Source of the Future?

More than 40 years later, in 1976, Zener published an article in the “Bulletin of the Atomic Scientists” titled “Solar Sea Power” that discussed converting ocean thermal energy into electricity. As a preface to the article, Zener wrote:

“The tropical oceans collect and store vast amounts of solar energy; research in converting this energy to electrical power is already well advanced, and the prospects are promising that costs can be reduced to make it competitive with power produced by conventional means; what the United States needs now is a more intensive program to demonstrate the economic feasibility.”

His 8-page article carefully makes the case for OTEC power generation and compares the costs for an OTEC power plant against those for a nuclear plant. Zener’s article concludes:

“Success in the development [of OTEC generating plants] as described above would enable us to place OTEC plants in the Gulf of Mexico, and have the power transmitted into the Gulf states at a cost lower than for power generated by nuclear plants of the type now being built. How far this power could economically penetrate into the rest of the United States would depend partly upon just how low the cost of the OTEC plants can be reduced, and partly upon the continued rise of nuclear plant costs above that expected by normal inflation.”

Zener’s prescient conclusion preceded the Three Mile Island incident – which effectively shut down the development of new nuclear power-generating plants in the US, by slightly more than three years.

So, by now, we should have OTEC generating plants all along the United States’ southern coasts and ringing the Hawaiian Islands, right? We don’t. Something didn’t happen – at least not yet. That something goes to the heart of the Rankine-cycle heat engine that forms the heart of OTEC power generation. (See Figure 2.)

Figure 2: The Rankine-cycle OTEC heat engine uses one heat exchanger to boil a working fluid (anhydrous ammonia) using the heat from warm seawater drawn from the ocean’s surface. The heat transforms the liquid ammonia into a high-pressure vapor that spins a turbine/generator combo. A second heat exchanger then condenses the ammonia vapor back into a liquid using cold ocean seawater. (Image credit: Makai Ocean Engineering)

The Rankine-cycle OTEC heat engine uses one heat exchanger (also called a boiler) to vaporize a working fluid (anhydrous ammonia) using warm seawater at 25°C from the ocean’s surface. The resulting high-pressure vapor spins a turbine/generator combo to generate electricity. A second heat exchanger then condenses the ammonia vapor back into a liquid using cold, 5°C seawater drawn from deep in the ocean. There are not many moving parts in this Rankine-cycle heat engine: three seawater pumps and a turbine/generator.

Looks simple, right? To me, it looks a lot simpler and safer than a power plant driven by nuclear fission or by an always-ten-years-in-the-future nuclear fusion plant with its near-impossible magnetic containment of hot plasmas. After all, with an OTEC plant we’re just dealing with seawater between 25°C and 5°C and ammonia, which has been used in air conditioning systems for more than a century. What could be simpler?

If Mini-OTEC became operational in 1979 and the pilot plant became operational in 2015, then why don’t we see a rush to build OTEC plants all over the place? Zener’s 1976 article predicted that we might see a 25-megawatt module built and tested as soon as 1981 and a testing of a 100-megawatt demo plant built from four 25-megawatt modules as soon as 1983.

None of that happened. Why not?

My first thought was that it’s really hard to suck ocean water up from 3000 feet below the surface of the ocean. Our Friends of NELHA tour dispelled that guess. The pipe into the ocean works just like a soda straw in a glass of water. Block the top of the straw with your finger, stick it into a glass of water, and remove your finger. Immediately, water at the bottom of the glass will fill the straw until it reaches the level of the water in the glass. It takes little effort to draw water from the bottom of the glass to the top of the straw. Air and water pressure do most of the work for you. The cold-water pipelines at the HOST Park work the same way.

Salty Water = Tough Engineering Challenge

So if it’s not the cold water supply, what’s the problem? It’s the extreme salinity of the cold water interacting with the metal in the heat exchanger. Salty water is extremely corrosive. Want corroboration? Every time I’ve visited a coastal resort and gone into a bathroom located near the shore, I’ve seen that every metal surface is rusty. Salty water, even salty water vapor, is extremely corrosive. Seawater from the ocean’s depths is even more so because it’s saltier.

The HOST Park water-delivery pipes effectively deal with the corrosion issue through the use of plastic piping. However, plastic pipes are poor heat conductors. The material used for the heat exchanger needs to conduct heat well and it needs to resist corrosion caused by extremely salty water.

The best material for heat exchangers is currently titanium, which is relatively rare and therefore very expensive. Makai Ocean Engineering is currently designing a 110 megawatt plant for Japan. According to the Friends of NEHLA presentation, if the heat exchangers for this plant are made entirely of titanium, that one plant will consume approximately 7% of the world’s proven titanium resources. This is not a scalable proposition and that’s why you don’t see OTEC generating plants ringing Hawaii or lining the southern California and Gulf coasts. We need a more scalable design for the heat exchangers to make OTEC practical, and that was Makai’s purpose in building the OTEC testbed: to test different heat-exchanger designs.

Either we find a way to use less titanium in the heat exchangers, or we need to find another suitable material that’s a lot less expensive. I did some quick Googling after the tour and discovered, as I’d guessed, that carbon nanotubes are a possible contender for this role. After all, aren’t carbon nanotubes pretty much the future answer to every electrical and thermodynamic engineering problem? It seems that way to me. We’ll just need to see if they work here.

Finally, much of Zener’s decades-old paper is now out of date due to unforeseen technological progress and unexpected effects from climate change. For example, Zener predicted that no new electrical generation facilities fueled by natural gas would be built because of the scarcity of natural gas back then. You can’t blame him for failing to foresee the effects of fracking, which drove down the cost of natural gas. Then there was Three Mile Island followed by Chernobyl to leave a very bad taste for nuclear power in everyone’s mouth. Fukushima didn’t help matters.

There’s another anachronism located at the beginning of Zener’s article. He wrote:

“A [100-megawatt plant would have] only one-tenth of the capacity of a modern nuclear or fossil fuel power plant. More power would be obtained by having more plants rather than larger plants. Such plants would have to be spaced about 10 miles apart in a limited region so surface waters would not be cooled too much.”

Back in 1976, it would have been very unlikely that Zener could or would have foreseen that warming ocean and gulf waters would be of great concern, as they have become in recent years due to their observed effect on amplifying hurricanes and tropical storms.

Is this truly an opportunity to kill two birds with one stone? Could we really use OTEC power-generating plants to free parts of the world from needing fossil fuels for power generation by tapping into the free energy stored in warm ocean waters while, at the same time, helping to tame super hurricanes and typhoons?

Food for thought.

 

References

Clarence Zener, “Solar Sea Power,” Bulletin of the Atomic Scientists,” January, 1976, pages 17-24.

One thought on “Extracting Power from Seawater without Nuclear Fusion”

  1. Interesting concept. You might want to re-think titanium. It may certainly true that in the 1970s’, one would have used 7% of the world supply of titanium to create a 100MW heat exchanger. However, today, and based on Google search, it is the 9th most abundant metal in the earth’s crust. I believe with the proper economic incentives, one can create all of the titanium needed to build 100MW titanium heat exchangers — all over the world.

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