Saturday, 19 November 2011

Power from Ocean Currents


The New Scientist recently ran an article on Tidal Power technology. This form of renewable energy generation is of interest because tidal flows are predictable and reliable - flowing in and out twice a day, every day.


Thar's energy in them there ocean-currents-and-tidal-flows !

This is in stark contrast to wind turbines, who can be left standing idle in the middle of winter if an area of high pressure settle over the UK.

Tidal water flows, at just a few metres per second, are much slower than wind speeds on land - but water is some 800 times denser than air, so the potenial for energy generation is certainly viable.

Of course, one would be entitled to ask that if this energy source is so good, why hasn't it been exploited before? There are a number of reasons for this, including the expense of contructing prototype generators, the cost of bringing the power back to land via cables and, perhaps most importantly, the relative lack of sites where the speed of tidal flows are fast enough.

With water speeds of 3metrs per second, the wates around Orkney are a prime testing ground, with five new designs having been tested there recently.

However, turbines that can operate effectively in lower water speeds, perhaps down to 1 to 1.5 metres per second, are a much more attractive proposition and could be used in many more locations, thus generating a lot more power.

A number of UK companies are involved in this field. You can check out their various designs at their respective websites :

Marine Current Turbines have a traditional "wind turbine" approach.

Pulse Tidal use see-sawing hydrofoils.

Lunar Energy offer a ducted fan design.

Kepler Energy propose an unusual "lawnmover" bladed turbine.

Ocean Currents
The ocean currents around the world represent huge masses of water moving at up to 2metres per second. If generators could work effectively at these low speeds they may be able to tap into the resources of flows such as the Florida Current. Howard Hanson, from the Florida Atlantic University is involved in projects looking at the possibilities of generating energy from this water flow. He discusses the issue in an article in the Bulletin of the Americal Meterological Society. The article is fascinating and admirably frank about the challenges (past and present) of generating energy from ocean currents. Indeed it represents a refeshing change from the hype that one sees in the mainstream media (and sadly, to a degree in the New Scientist article)


Context
A study by the EU entitled "Oceans of Energy: European Ocean Energy Roadmap 2010-2050" gives some context for the energy generating capacity of the oceans when it states that ocean energy generation could represent a mere 0.3% of total demand by 2020, although it believes that it could be a more respectable 15% to total demand by 2050. You can find out more about the European Ocean Energy Centre here.

Critically, the EU report has the good sense to distinguish between generating power (measured on MegaWatts(MW) or GigaWatts(GW)) and actual amount of energy (meansured on TeraWattHours (TWh) per year).

In case that has left you with a touch of brainfreeze, Mega is a million, Giga is a thousand million, Tera is a million million.

To explain why this is important, consider a turbine and a small gas fired power station, both having a power output of 1MW. If the turbine operates only 10% of the time (or all the time at 10% of maximum output) then it will only generate 3.1TWh per year, whilst the gas fired station will generate 31TWh per year.

Dear reader, when you see a report that only talks about power, and not about energy, worry that the wool is being pulled over your eyes !

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