Any body of water when it is warmed by the sun at the surface will tend to separate itself into thermocline gradients. The sharply defined layers prevents the mixing of the waters inside them. This leads to a leaching of nutrients needed for plant growth and so by mid spring in places like the north sea primary biological production has for the most part stopped. Deep Ocean Water (DOW), where these nutrients abound, represents 99% of the volume of the ocean and are just out of the reach of marine plants, separated by a thin skin. This is why the tropical oceans are crystal clear. There are almost no microscopic plants (micro algae) in the water to turn it green.

That is not to say that that the potential of seaweed isn’t huge. A company called Bio Architecture Lab based in Berkeley California has perfected an enzyme and process which liberates the sugar from alginate. This single substance represents 30% of the dry mass of seaweed. BAL’s method has an 80% efficiency rate of the theoretical yield. This is a huge advance and by their figures the harvesting of seaweed from 3% of costal waters would generate 60 billion gallons of biofuel.

The Tidal Irrigation and Electrical System opens up large areas of coastal ocean to macroscopic algae cultivation that would not normally grow commercially harvestable amounts of seaweed.  It does this by using tidal forces to syphon DOW into a lagoon where it is isolated from the surrounding ocean and creates a large bioreactor. It does this while producing food and gigawatts of electricity. (Please see the links on the right of this page for further details of the products of the system)

The new increase in efficiency in the processing of seaweed by BOL will result in a huge gain in production for the TIE System. The two technologies are made for one another.

The technology for creating stable off shore islands has been developed for the luxury home market in Dubai. The World and Palm projects have proven that stability is just a matter of engineering. The adaptation of the island creating technology to generating, food, fuel, electricity and fertiliser should be a small matter.

On another matter, one of the main concerns for the walls and the overall structure is that of tropical depressions (cyclones, typhoons and hurricanes) and tsunami. Any potential realisation of a TIE System will experience forces many thousands of times stronger in extreme events than it will face during normal day to day operations. Like any other structure, an architect will need to make a decision about how robust to make the structure versus what is the frequency of the event and the effects of the destruction of the structure on the surrounding environment. In the case of a TIE System the type and frequency of these hyper-events is dictated by the location of construction. However, it should be noted that in all tropical depressions the ocean itself rises around 10 meters. This could be used to a TIE System’s advantage. By keeping the profile low to the tidal maximum’s mark, once submerged, the TIE System’s structure would be protected from the most damaging waves and extreme tidal streams.

The other factor in the equation; the effect of a breach of a TIE System should be minimal. Certainly, if not returned to the nutrient rich deep oceanic water (DOW), any macroscopic algae will die off after a couple of weeks. Depending on what animals are being grown in the lagoon some will die. The vast majority of the life in the lagoon will probably be eaten by the animals living around the TIE System. Once the system is running again, by seeding the DOW with the desired species (and other methods of mariculture) normal production can resume.

The last thing to consider in this forum about extreme events on the walls of the lagoon of a TIE System is that a failure of one part of the structure will not lead to a failure of another part. If the pressure builds to any point on the walls to where it bursts, this will instantly start a tidal stream that will completely release the pressure that allows the overall system to work. It should only fail in one spot and that should be relatively easy to fix.

There are advantages to both systems but now a detailed analysis by Anna Stephenson at the University of cambridge has compared them and found that open air ponds are 16.8 times more efficient than perspex tubing (Energy and Fuels, DOI: 10.1021/ef1003123). In an article published in the New Scientist, it is pointed out that pumping through plastic tubes results in a higher energy cost per unit than traditionally sourced fossil diesel. However, this is not the case for open air ponds.

As Ms. Stephenson says, the major drawback to open air ponds is evaporation. So much water can be needed that ponds in many countries would be in direct conflict with traditional demands for water, if they want to produce enough biofuel to replace their domestic consumption. This is not an issue for the Tidal Irrigation and Electrical system because it does not use fresh water, it uses the ocean. Also, the tidal flux is being harnessed in a TIE System to do the pumping and deep ocean water is used as the both the fertilizer and source water for the open air pond, making it much more efficient than the land based ponds envisioned by Ms. Stephenson.

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?