Ocean Thermal Energy Conversion

Ever since I was a little kid I have had a certain respect for the power of the sea. I used to sit with my granddad looking out over the ocean and he would tell me that I should never take things for granted when I was in the surf, that the sea could change in an instant and many people had lost their lives because they didn't respect the ocean enough. He would know, both him and my dad were commercial fisherman for a while and had seen their fair share of some of the sea's might.

It wasn't until many years later that I began to think that maybe the power of the ocean could be harnessed in some ways to provide us with energy. In many ways, the ocean is just a gigantic, constantly charging and discharging battery. It stores wind energy in the form of waves, and solar energy in the form of heat and even gravitational power in the form of tides. It then discharges these by crashing into land and by creating immensly powerful currents, so powerful that they influence the world's weather and stop many parts of the world from freezing over entirely.

The ocean is also a very difficult environment. As I learnt from my granddad, it can be highly unpredictable, but it is also very corrosive and provides an excellent environment for all forms of life, some more frustrating than others. Any ocean reliant energy technology, while having the potential to produce huge amounts of green power, is generally limited by the environment it is to be built in.

There are a few prominent technologies that harness power from different parts of the great ocean battery, these are, in no particular order,

• Wave Energy
• Tidal Energy, and
• Ocean Thermal Energy Conversion (OTEC)

I hadn't actually even heard of OTEC until a couple of days ago when I came across this article. To me it seems like a great idea, pretty much the ocean version of geothermal power. Unfortunately, for the reasons I mentioned above, the practicality of it all is very difficult and is made even more so by the fact that the parts of the world that are well suited to it, are also well suited to very unpredictable weather... like hurricanes!

Nonetheless, it made me wonder exactly how it all works and what the real challenges are. Basically, OTEC works by utilising the temperature difference between the deep, cold water and the warmer surface water in the ocean. Unfortunately, these temperature differences are not nearly as great as they are with hot rock geothermal (15-20 degrees vs 100-200 degrees!) although it does have the advantage of being truly renewable (unlike geothermal which has a lifetime of around 20 years). This small temperature difference means that the thermodynamic efficiency is very low, somewhere around 3%. This in turn makes the return on investment quite small even in the long run due to the high maintenance costs (to stop things rusting).

However the real power of OTEC is that by pulling up cold seawater (using an open cycle system, which also avoids using chemicals such as ammonia) you have the opportunity to create unique biosystems that would otherwise be unsustainable in the region. Concepts such as aquaculture for cold water environments in the tropics and chilled soil agriculture allow plants and animals that would otherwise not grow very well because of the heat.

Of course, playing god like this can have very serious consequences, but when you combine new foods with coal and oil free power, clean water (desalination is also possible using OTEC technology) and the possibility of exportable products (minerals and even hydrogen production are possible) then you have a very real opportunity to improve the quality of life for those living in small islands and other equatorial nations.

In the end I think that OTEC is not the most promising renewable energy technology available and this is perhaps the reason why there has been very little progress in developing it on a mass scale. There are however, a few good proposals of it in use on a small scale, for example military bases in Diego Garcia and Guam. As with all renewable and new technologies in general, all it needs is something to kick start it and get the research going!