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!

Geothermal Power in Australia

This week saw me attend 'Geothermal Energy - Current Projects and Future Directions', a presentation hosted by the Brisbane chapter of the Institute of Engineers (Society for Sustainability and Environmental Engineering). The presentation included two points of view; one from GeoDynamics, an Australian company that (in conjuction with Origin Energy) has recently completed 'proof of concept' for energy production using 'hot rock' geothermal, the other from a government sponsored research and development centre (in conjuction with UQ), the Queensland Geothermal Energy Centre of Excellence (QGECE).

Geothermal power can be generated using one of two methods. The first, traditional, method generates energy from hydrothermal systems, where fluids circulate through rock fractures where high heat flow is present, typically near active tectonic plate boundaries. Hydrothermal systems are generally used for direct applications and not electricity generation as they may be located in unstable areas and have problems with maintaining sufficient fluid to generate power over time.

The second method, which was discussed in the presentation, involves pumping fluid deep into the ground (3km and below), extracting heat from rocks before retrieving it above ground for use in a heat exchanger. The rocks underground can be very hot, upwards of 200ºC, due to the decay of radioactive material and the kilometers of insulation above them. This provides an excellent source for baseload energy.

Obviously, using the aptly named 'hot rock' (method two) process has complications, not the least of which is having to drill more than 3km underground. But never fear, we have the technology! The oil and gas industry has been developing deep drilling techniques for decades now and it is possible to buy equipment for drilling a long way down. Another problem is finding rocks that are hot enough and over a large enough area. In this case, we are very lucky here in Australia as there are huge regions containing rocks with temperatures greater than 250ºC!

As a side note, while geothermal power is technically 'renewable' it takes many thousands of years for these rocks to heat up once they have been cooled from extracting the heat for energy. Fortunately for us, to reduce the temperature by around 40 degrees (resulting in an approximate 50% loss of efficiency) would take between 25 and 50 years and that would occur over a relatively small area. Given this is a huge resource (albeit out in the middle of nowhere for the most part), investment in geothermal power makes a lot of sense for Australia.

The process to extract energy from hot rocks involves drilling an 'injection' well down into the hot granite approximately 3-5km below the surface. This granite has natural horizontal fractures (due to horizontal compression). These are expanded over a wide area by pumping high pressure fluid into them and forcing the fractures open further. Then a second (and possibly more) 'production' well is drilled to the same depth within the expanded fracture region. Once this is established, fluid is pumped down the injection well, heated by the rock below and retrieved up the production well(s), passed through a heat exchanger and sent back down again. Since the heat of the rocks is so high, the heat exchanger produces enough energy to drive a turbine which in turn generates electricity.

Anyway, back to the presentation. Gerry Grove-White, the Managing Director of Geodynamics, described the process that their business had been through to come to the proof of concept milestone they achieved in early 2009. Their journey was not without difficulty. Gerry described some of the difficulties they had, for example, obtaining equipment as a 'small consumer' in the world of mining, or that there was actually a surprisingly large amount of water under very high pressure (to the order of 70MPa!) down there. This was alongside understanding many of the risks involved in hot rock geothermal power. Things like the ability to create a horizontal fracture, using air condensing rather than Australia's precious water resources, how to manage multiple production wells and whether corrosion (due to radioactive material or otherwise) would be an issue.

Despite all of this, the project has continued with only one major setback - a well blowout in April 2009. This was a fairly major incident although, as Gerry explained, it is not entirely uncommon incident in the oil and gas industry. While the reason for the blowout has not been resolved, there doesn't seem to be any specific concerns regarding the continuation of the project, however there will be some delays in reaching a commercial demostration plant (of the order of 6-12 months).

Gerry finished off with a very interesting note about energy distribution in GeoDynamics' business model. Obviously, having power generated so far from its end consumers creates problem for distribution. One ingenious solution to this was to (particularly in the initial stages) provide the power to consumers willing to use it on or near the site. Examples given for this were data centres and high quality silicon producers, both of which use large amounts of power (which could become very costly when an ETS is introduced) and could be co-located with the plant.

Next up was Professor Hal Gurgenci from the QGECE and it was time to put our theory hats on. It really was quite amazing to hear the difference in the two presentations. One focused on the practicalities of creating a business and the process required to actually create energy (and hence money) from it, while the other was devoted to the theory behind creating a more efficient cycle. Professor Gurgenci described the difference between the standard rankine cycle (commonly used in steam turbines), and a theoretical supercritical cycle using CO₂ as the fluid for heat extraction.

Unfortunately, my memory of 2^nd year thermodynamics failed me and I spent a large section of his presentation trying desperately to follow what was going on! One point I did catch onto though, was the necessity for dry rocks to allow the supercritical cycle to work. Given what Gerry had said regarding the high pressure liquid underground it got me wondering whether there may have been a disconnect between the academic world and the real world as is so often the case. On the plus side however, Professor Gurgenci did present some excellent points on increasing the efficiency of the entire process using concentrated solar power to further heat the fluid once it reaches the surface before going into the heat exchanger.

All in all I thought it was an excellent presentation from both speakers. For someone who knew very little about the entire hot rock geothermal process it was very well explained and was, for the most part, taken from an engineering point of view.

Sources:

Geoscience Australia

GeoDynamics Limited

'Direct use of Geothermal Energy: Opportunities for Australia', GEOCAT #65454, Geoscience Australia

'Electricity Generation from Geothermal Energy in Australia', GEOCAT #65455, Geoscience Australia

'The Power Beneath Our Feet', Tim Flannery, Sydney Morning Herald, April 2005