Ocean Thermal Power: Why Hasn't It Come of Age? – Part One

by Stuart Burns on May 9, 2011

Style:    Category: Green, Product Developments

We wrote a piece last month entitled OTEC’s Zero-Carbon Emissions Too Good To Be True? Maybe Not on what we had assumed was an obscure if interesting technology-in-waiting called Ocean Thermal Power. In essence, the technology relies on small temperature differences between warm surface tropical waters at 26-28 degrees Celsius and 4-degree cold waters at depths of 1000 meters or more, to drive a heat pump that generates electricity and potentially a host of other benefits. The article stimulated a number of online comments and a wave (pun intended) of offline communication, principally from supporters of the technology generally along the line of “this is a technically and economically viable technology that is just not getting the coverage it deserves.”

Well, always happy to go against accepted thinking, we at MetalMiner rather agreed that on the face of it, this technology had a lot to commend it and a follow-up article analyzing some the objections critics have raised and exploring the technological and economic challenges was probably in order.

The objections are broadly in two forms: the first is how such small temperature differences and low levels of thermodynamic efficiency can ever be economically viable, and the second is around the challenges inherent in the design and construction of the thermal power plants themselves. Inevitably our evaluation will be general in nature, as we don’t have the room for detailed mathematical analysis, but plenty has been done and we can point any interested parties to those qualified to carry on the discussion in more depth. The question of efficiency immediately illustrates the interconnectedness of the issues. While it is fair to say if the raw materials are free and unlimited warm and cold water does it matter that the efficiency is only 3 percent? Yes is the answer, because to generate 100 Mw of electrical power, the latest designs of OTP would require the movement of a vast amount of water – 248 cubic meters per second of warm water and 138 cubic meters per second of cold water, according to a paper by J. Hilbert Anderson, the doyen of the fledgling OTP industry. The greater the volume of water that needs to be moved, the larger and more capitally expensive the plant and equipment would inevitably be — and therefore the greater the capital cost. As a side bar, and to clarify Anderson’s credentials, it should be said he played a key role in the Manhattan Project by designing a compressor fast enough to separate uranium hexafluoride at a time when no one else thought it possible. In the 70s, he and his son Jim designed the first ever geothermal hot water plant. While it was an extraordinary achievement to succeed on the first try with new power plant technology, the even more extraordinary part was the cost: the 10-MW plant was built for the same cost-per-kilowatt as nuclear plants of the time that were 80 times larger using established technology. It has taken someone with Anderson’s tenacity to stay the course with OTP when officialdom preferred to throw money at pork barrel projects like wind and solar.

But let’s get back to the technology, shall we?

(Check back in Wednesday, 5/11 for Part 2.)

–Stuart Burns

{ 1 comment… read it below or add one }

1 Jim Baird May 9, 2011 at 11:34 am

The major problem with OTEC is the fact most proponents are fixated on moving massive volumes of cold water to the surface rather than much smaller volumes of spent vapours, which have driven a turbine to produce power, into the depths to be condensed.

e.g. Conventional OTEC requires the movement of 150 m3/s of cold water to produce 50MW.

To produce this power about 2,750 kg/s of anhydrous ammonia is boiled and has to be replaced i.e. returned to the surface after it has been condensed in a deep water condenser.

2750kgs of ammonia is 4 cubic meters worth which is a 3700% gain in efficiency and a huge reduction in infrastructure cost.

In a counter-current heat transfer system, which is closed, you would negate the environmentally impacts of OTEC and maximize the potential output by limiting the amount of surface heat extracted and the amount of heat dumped into the depths. Both of which would negative consequences for Thermohaline circulation.

You also greatly reduce the size of the infrastructure needed to produce power which as you point out equates to cost.

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