How OTEC Technology Influenced LFTR

Several weeks ago, an engineer who has been involved in ocean-thermal-electric conversion (OTEC) technology wrote a piece in the Huffington Post quite favorable to thorium, based in large part on the recent article in WIRED magazine.

Patrick Takahashi: There is Something About Thorium

I enjoyed the article, and it caused me to reflect about my past enthusiasm about OTEC technology, and how at one point I thought it would play a large role in our energy future. It also caused me to reflect on how much my exposure to OTEC later influenced my concepts of LFTR and how it might be deployed and used.

For those who aren’t familiar with how OTEC works, let me give the briefest lesson in thermodynamics. Thermodynamics teaches us that wherever two “reservoirs” of energy exist, separated by a temperature difference, there is the potential to extract useful work. The efficiency at which that work can be extracted is directly proportional to that temperature difference.

I had just graduated from Utah State with a mechanical engineering degree when I picked up a copy of Marshall Savage’s book “The Millennial Project” and headed off to California for the summer to work on a rocket project before starting graduate school. “The Millennial Project” (TMP) is an incredibly ambitious text that argues for a step-by-step expansion of human presence into space, and it starts by saying we should build huge floating ocean colonies powered by OTEC.

OTEC utilizes those thermodynamic principles I previously mentioned by using the temperature difference between the cold waters of the deep ocean, where the temperature is only a degree or so above freezing, and the warm surface waters, where the Sun’s energy has brought the water temperature to about 80 degrees Fahrenheit. To use the expression we learned in thermodynamics for energy conversion efficiency, we first have to convert these temperatures from Fahrenheit to Kelvin. Let’s assume the deep cold water is 34 degrees Fahrenheit (274 Kelvin) and the warm surface water is 80 degrees Fahrenheit (300 Kelvin). The maximum efficiency that can be attained is (1 – (274/300)) = (1 – 0.9133) = 8.6%

So in the most perfect case, only 8.6% of the energy can be converted to work (or electricity) between these two sources. No problem, said the OTEC advocates, we have an unlimited amount of cold water and hot water! Let’s just pump more! So OTEC involved moving a LOT of cold water from the deep ocean to the surface. But the most exciting thing I learned in Savage’s book was that the deep ocean waters contained lots of nutrients, and so when they were brought to the surface they caused all kinds of ocean life growth to flourish.

This led to glorious depictions of floating ocean colonies, self-sufficient in energy, ecological oases of life and aquaculture, perhaps producing so much that they could be economically self-sufficient. It seemed so wonderful it was almost too much to believe.

I was motivated to act so I tried to read all that I could on OTEC and what state the technology was in. Of course, the real efficiencies that could be obtained by OTEC were substantially below that of the thermodynamic limit (which is pretty common in all real systems) but the geographic limitations of OTEC were much more severe than I thought.

This wasn’t exactly something you could park off the coasts of cities around the US and make power. You needed really warm water–tropical water–and so it was much better suited to the islands of the Pacific or the Caribbean. It seemed like an intriguing answer for them, but not very applicable to New York or Seattle. Ironically, the cold water was easy to find–cold ocean water can be found anywhere on Earth if you go deep enough, but the warm water was far less commmon.

So I started thinking about how to “artificially” make the water warmer. First I was thinking about huge floating solar thermal generators, that would focus the Sun’s energy on a concentrator and use the cold ocean water to achieve conversion efficiencies on the order of 50%. Such systems sounded great if the skies were clear, but how often would that be the case at sea? Furthermore, the areas needed to generate power were vast, just as they are for terrestrial solar power, but the ability to use the deep cold ocean water to achieve high efficiencies and to create these ecological oases discussed in Savage’s book seemed so compelling.

Looking back, I’m amazed it took me so long to connect two utterly compatible positions to one another.

I had known about LFTR technology for five years before I finally put two pieces together in my mind–the ability of LFTR to make high temperatures from fission and the profound value of the deep cold ocean water–in late 2005. It came about because I has been working in Mississippi with a church group doing post-Katrina cleanup and I was trying to figure out how to deploy LFTR in a way that would be impervious to weather and hurricanes and earthquakes and so forth.

It then occurred to me that the logical place to deploy a LFTR would be in a submersible, where the environment around the submersible stays nearly constant even as storms rage overhead or earthquakes rumble underneath. I went on to discover that I was not the first person to propose submersible power reactors, but I was the first to propose that they be LFTRs and take advantages of the profound operational improvements that LFTR allowed (very long run times between reactor shutdowns, easy changing out of fuel, etc)

This vision of fleets of LFTRs, parked underwater around the world, providing electrical energy to coastal cities via underwater HVDC cables and using the deep cold ocean water to cool the reactor (not directly of course) has consumed me ever since. I have also realized since then that the rejected heat of LFTR is eminently suitable to use for ocean desalination, making it possible that submersible LFTRs can be a huge source of fresh water for coastal communities.

This vision, which is still in development, owes its origins in large part to my initial interest in OTEC, and the directions of thought it led me. For that, I will always be grateful that I was exposed to that technology.