The Tidal Irrigation and Electrical System

For the last several months I have had a series of constructive conversations with Dr. Chris J C H Watkins about the TIE System and I intend to write more about these discussions later. One piece of information in particular has caught my imagination; “Enhancing Electrical Supply by Pumped Storage in Tidal Lagoons” by Dr. David J. C. MacKay. Nicknamed by Dr. Watkins as over-pumping, the principle behind the idea comes from the fact that the amount of energy able to be extracted from a tidal barrage is increased by the square of the flux. So in a hypothetical tidal barrage that produced one megawatt for a one meter tidal flux, if the flux increased to two meters it would produce four megawatts – or if the tidal flux was increased to three meters the same tidal lagoon would produce nine megawatts. Of course, this is a gross simplification of the principle.

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?

Tags: hydroelectric, over-pumping, tidal

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