Of course, the potential for the use of fish-to-biodiesel for the Tidal Irrigation and Electrical System is huge. The introduction of filter feeding species could lead to a much greater energy capture for the entire system then by the sole utilization of macroscopic algae like kelp.

This does however create the nightmare scenario of giving every marine animal a monetary value as fuel. In the future will there be fishing mafias that strip entire ecosystems? Will the seas be subject to even further unsustainable practices in order to fuel our cars? The oceans already suffer from the tragedy of common ownership. The collapse is imminent in the majority of the world’s fisheries. This technology could inadvertently push them over the edge.

“What we found is that we can make ‘paper’ from an interwoven mesh of nanowires that is able to selectively absorb hydrophobic liquids–oil-like liquids–from water,” said Francesco Stellacci, an associate professor in the Department of Materials Science and Engineering and leader of the work.

Made of potassium manganese oxide, the nanowires are stable at high temperatures. As a result, oil within a loaded membrane can be removed by heating above the boiling point of oil. The oil evaporates, and can be condensed back into a liquid. The membrane–and oil–can be used again.

This is problematic for any potential large scale use in a TIE System unless the energy intensive extraction method can be incorporated into the bio-petroleum conversion process. However, this is an important technology for cleaning up oil spills and other environmental contamination.

Pumping enough water to make this worthwhile involves having the mechanical means to do it and the available excess energy to invest in the system. The technical hurdles are hard to underestimate as in a tidal lagoon of the scale of a proposed TIE System, it involves moving thousands of cubic meters of water per second. Also, the cost of artificial atoll walls are generally considered to be to the square of the height.  This means that careful analysis will need to be made of any potential site to see if the energy needed to invest in the system is available. If it isn’t then it makes no sense to build the additional height in to the atoll walls as the ocean water will not be pumped in. Dr. MacKay’s document points out that energy from wind turbines and even the national grid can be used for the purpose of pumping water in to or out of a lagoon to increase the output from a tidal source because the return of the investment is to the square of the cost and also that many tides are less than the natural tidal maximum. This is an ideal use for excess electrical capacity: for example during a windy night when the electricity generated by wind turbines would otherwise go to waste it can be used to pump water into or out of a tidal barrage and this potential energy can be released when the demand increases.

Dr. MacKay points out that it can also be used to a greater extent on the low ebb of the tide by pumping additional water out of the lagoon. The advantage of low tide over-pumping over high tide over-pumping is that in the high tide scenario the tidal barrage needs to be built to the height that one intends to store the water whereas in a low tide scenario no additional build cost is incurred.  

Over-pumping adds another layer of complexity to the cost benefit analysis of any Tidal Irrigation and Electrical System’s proposed location. Dr. MacKay has envisioned that this form of tidal barrage is connected directly to an electricity grid and so generates AC power. In my mind, this remains an open question as to the cost effectiveness of this use of the power. (see Electricity Infrastructure) Also, as the amount of energy generated by the OTEC subsystem compared to the simple tidal flow through turbines is so much greater in the OTEC and the amount of interference between the systems (see hydro) is an unknown, this is another area worth investigating. Nevertheless, this presents many intriguing possibilities and increases the flexibility and potential output of the TIE System. 

One question that springs to mind is; under what conditions would it be worthwhile to turn the OTEC system from passively siphoning deep ocean water to actively pumping it into the artificial atoll? Also, as the OTEC uses 2 parts warm to one part cold water what would be the effect of dumping the warm water component in to the lagoon as well? The OTEC’s efficiency would rapidly decrease but would this be offset by the increased efficiency due to over-pumping? To what degree?