2000-01-09 – The OTEC problem

Ok, confession. I don’t know much about this, however… (Go here for detailed OTEC stuff about heat exchangers, closed vs. open-cycle, etc.)

OTEC needs the cold water from the deeps to act as a heat sink to extract heat from the warmer surface water. Basically a giant solar collector in the form of the surface water. The cold sink allows work to flow, hence the creation of electric. The system is almost free to run, but the setup is massively expensive, needing a pipe some 10 meters across and 1200 meters long – Vertically!  The water is sucked up by powerful pumps on the surface, causing an increase in the rate of flow and a drop in pressure. The wall thickness is obviously going to be a nightmare, as is construction costs as the pressure of the surrounding water will collapse the pipe unless it is rigid, and hence heavy.

Recently Kirk Bailey, a list contributor, has solved the problem by inverting the system. He believes putting the pumps at the bottom is the way to go. (Errata: Janyce Wynter & Phil Kopitske, amongst others have been discussing this bottom-feeding idea for years. Apologies to the real inventor!)

Kirk has, I believe, struck gold. By putting the pumps at the bottom he keeps it a positive pressure system. This means that the wall thickness can be negligible, and hence cheap. To reduce thermal flux from the warmer outside to the cold inside the wall can be a number of skins which would insulate the flow in the same way as a wetsuit protects a diver. It would also reduce the effect of a minor tear in the fabric. Only a steel cable would be needed to balance the lower pump downthrust against the buoyancy, allow towing and to supply power. Even this could be removed if the system had a variable buoyancy system and an, admittedly big, bank of batteries. This would allow the system to be detached in the event of a massive storm. Later a thermal imager would easily find the upwelling, and energy retrieval and electricity production could continue. In the event of a total loss of power, etc. the fans would cut, causing the device to surface for repairs. A transponder would allow easy location on the surface. In situ repairs would be infrequent due to the low tech of the system and the lack of marine activity at such depths. The noise and vibration would warn sea creatures and submarine traffic away. It is unlikely that a shark or other creature would either attack or otherwise damage the pipe, but these holes would be easily patched by blowing a new layer, mere microns thick, up the inside of the pipe. This would also reduce the friction caused by the inevitable marine fouling. The whole system could be shut down on rare occasions and inner fouled surfaces be totally removed if it were felt that it were needed, or the increased thickness of the pipe and it’s higher tensile strength (now layered with natural SeaCrete!) could allow higher pressure pumping than before.

We want/need a very thin but tough membrane, really. Any material will need replacing after a time anyway, so lets make it ultra cheap, and look to replace it after a year or two. A blown continuous piece of polyethylene (polythene) is the way to go. You can’t get cheaper, and the pressure differential is tiny over such a wide pipe. There is no seam to burst, and it has a low toxicity, high tensile strength, is pennies, is fairly neutrally buoyant and can be produced very quickly and easily with fairly low-tech kit.

All in all, Kirk’s idea is a winner. But it is not the whole story. Placing a set of pumps at the top also has an advantage in flow control and accessibility. Relying on just a lower pump would also require a thick pressure resisting pipe. For this reason I suggest a compromise. A large, slow, powerful pump at the bottom, with smaller, higher powered pumps at the top, with the flow pressures balanced so that the flow moves upwards fairly rapidly, but with only a small positive pressure. This would also further reduce the loss rate from any minor tears.

To recap, then. A slight positive pressure, easily contained by the high tensile plastic sleeve. Repeated layering will insulate and reinforce the pipe over the years, allowing higher flow rates and better thermal gradient at the surface. Very low cost to get running, and upgradeable as and when.

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