Climate change

April 13, 2010

TCASE 9: Ocean power II – CETO

Filed under: Emissions Reduction, Renewable Energy — Barry Brook @ 3:42 pm

In TCASE (thinking critically about sustainable energy) #5, I analysed a currently deployed technology for harnessing wave energy – the Pelamis device. If you haven’t read TCASE 5 then please do so now, since it explains some of the basic physical properties of wave energy, the extend of the global resource, etc. In writing the following post, I’ll consider this to be assumed knowledge.

CETO, named after a Greek sea goddess, has been developed by Carnegie Wave Energy (an Australian company), and is described in detail on their website. It is based on a submerged, underwater buoy-like device, anchored to the sea floor, which pumps water to shore at high pressure (6,400 kPa). Read more about the technology here.

The list of advantages of CETO on the website are worth citing here, as they provide a useful target for analysis. Main ones are:

  • 60% of the world live within 60km (40 miles) of a coast, removing transmission issues.
  • Waves are predictable days in advance making it easy to match supply and demand. (Wind is predictable hours in advance at best.)
  • CETO units are designed to operate in harmony with the waves rather than attempting to resist them. This means there is no need for massive steel and concrete structures to be built.
  • CETO wave farms will have no impact on popular surfing sites as breaking waves equate to areas of energy loss. CETO wave farms will operate in water deeper than 15 metres in areas where there are no breaking waves.
  • CETO is the only wave energy technology that produces fresh water directly from seawater by magnifying the pressure variations in ocean waves.
  • CETO contains no oils, lubricants, or offshore electrical components. CETO is built from components with a known subsea life of over 30 years.
  • Wave energy can be harnessed for permanent base load power and for fresh water desalination. The ratio of electrical generation to fresh water production can be quickly varied from 100% to 0% allowing for rapid variations in power demand.
  • CETO uses a great multiplicity of identical units each of which can be mass produced and containerised for shipping to anywhere in the world.

For these reasons, wave power is certainly among the most attractive of the range of possible renewable energy technologies. Unfortunately, it is also one of the most nascent in its development cycle (along with engineered geothermal systems, which is probably even further behind — I’ll cover this in a later TCASE post). Wave power is also up against one of the most hostile environments that any man-made structure has to endure — the salty (corrosive) and capricious (exposed to occasional very high energy events) marine environment.

CETO is certainly an innovative technology: it appears to overcome some of the shortcomings of the Pelamis device, such as a reduced bulkiness with more modular construction and deployment possible, lower vulnerability to storm damage due to anchoring at 25 m, and an added bonus of providing a neat method for reverse osmosis desalination using mechanical rather than electrical energy. The latter seems to be its biggest selling point, as I explain below.

The anticipated output for CETO wave power is given on page 10 of this senate submission. At deep-water wave resource sites with 90% availability (i.e. times when the units can generate some power, even if below their peak), capacity factors are stated to be 40%, which agrees with experience from the Pelamis device. For instance, if all of Australia’s estimated resource of 572 GW could be harnessed, the annual yield would be 1.6 million GWh (we’re talking about theoretical limits here, not what is actually likely to happen).

Carnegie’s strategic plans look towards building a 50 MW peak facility using an a 300 buoy system at 180 kW peak per undersea buoy. The technical documentation says that 2 m wave conditions with a 6 sec period, a single CETO unit can produce 80 kW of power (i.e. 45% capacity factor) or 11 litres/sec of fresh water. Each CETO unit will extract 15 — 20 % of the wave’s incident energy, and they’ll be arrayed a few units deep.

The news story here, citing a CETO official, claims a 50 MW plant could produce 25 MW of electricity and 50 GL of fresh water per year. From the technical data (table 3.7), you can work out that if the 50 MW unit was dedicated just to desalination, it could output ~90 GL/year. Actually, at 11 l/sec average output, each unit would produce 350 ML of fresh water per year (on average), and a 300-unit field’s output would be 104 GL/year. So, a figure of 90 to 105 GL/year seems for a 50 MW plant seems to be right, if the technical report is to be believed.

Their first ‘commercial’ wave farm, however, is a more modest venture, at 5 MW peak (2 MW average at a 40% capacity factor). It is due to begin operating outside of Perth, Western Australia in 2011, at a cost of $50 million. That works out to be a capital cost of $25,000 per kW of average power. Assuming a loan for capital at 6% interest, repaid over 20 years, and O&M costs of 1-2 c/kWh, this works out to be a wholesale cost of electricity of ~28 c/kWh. If successful, a 50 MW plant is planned for 2013.

Even if they can achieve a sharply declining cost curve after their first-of-a-kind plant, it’s a long stretch to imagine them expanding to 1,495 MW (peak)  in the subsequent 9 years. Carnegie’s forecast is $7.5 billion for the 1,500 MW, which is $12.5 billion per average GWe, so a projected better than halving of total cost compared to the initial 5 MW demonstration plant. That sort of price reduction with scale-up actually seems conceivable to me — I just doubt their capacity to raise the capital without massive subsidisation (WWF is asking for special wave-power feed-in tariffs — in Ireland, Portugal and the UK they average 25 to 35 c/kWh).

Still, the desal costs look impressive — indeed, almost too good to believe. The 100 GL/year Port Stanvac RO desalination plant in Adelaide will cost $1.83 billion in capital costs, and then has to pay for its electricity. A direct-cost scaling-up of the 5 MW CETO plant to 50 MW, which is anticipated to produce ~100 GL/year (see above), is $500 million (a slightly lower figure is cited here), with no electricity costs (or greenhouse gas emissions). Am I missing something here?? If this fresh water production rate from CETO can be confirmed, then this seems like the ideal use of this technology, high-cost electricity be damned. Comments on this point would be appreciated.

WWF recently produced a glossy document which stated that we should be aiming for 12,000 MW of wave energy by 2050. They claim:

Wave energy is an ideal source of baseload power – it is highly predictable and reliable, particularly along the southern coastline of Australia where regular storms in the Southern Ocean deliver constant swells to the shoreline. Analysis indicates that waves from which CETO generates electricity exist over 97.5% of the time, making it a baseload resource.

They also say:

Carnegie forecasts that by 2020, approximately 1,500 MW of CETO wave energy capacity could be installed along the southern coastline of Australia, contributing around 4% of Australia’s forecast electricity needs – emissions-free. To achieve this, the combined area occupied by CETO wave energy facilities is less than 1,000 ha (3.2 km2).

There are two obvious sleights-of-hand here.

First, ‘availability’ is quite different to capacity factor. A 40% capacity factor (if that is achieved consistently – probably only in the high-energy locations) means sometimes it is delivering close to 100% of rated capacity, sometimes close to 0%, sometimes 10%, sometimes 70% etc. Cumulatively, it averages 40%, but the actual output is variable and not able to be controlled. Thus, as Peter Lang has explained in his many posts, to ensure system reliability, you have to have back it up — at least to the minimum system-wide output of a geographically dispersed deployment. It could conceivably be considered ‘baseload’ if you chose to rate the plants according to a 1 m wave height (see figure here), but that would blow out the costs of average power enormously compared to the already high costs cited above, and would also lead to massive ‘dumping’ when there were large swells across large stretches of coastline.

So, CETO power may be ‘predictable’ (many hours in advance), and even ‘reliable’ (i.e., there will always be some waves, generated some power, so the 97.5% figure cited above might be strictly true but not relevant), but ultimately, wave power is still variable and fickle, and therefore not suitable for cost-effective ‘baseload’ electricity (or dispatchable uses, such as intermediate or peaking power).

Second, with the area occupied figure (the claim of 1,000 ha for 1.5 GWe peak).  My calculations, as outlined in TCASE 5, suggest a requirement of 115 linear km of coastline, with the buoy-field occupying an area of about 700,000 ha. Even if a CETO facility is twice as efficient at trapping wave energy than Pelamis (a generous assumption), this figure is still 2 to 3 orders of magnitude larger than the Carnegie claim. Why the huge difference? I speculate that they’re using a statistical trick here that is similar to the Mark Jacobson’s habit of stating the area of occupied by wind farms by only counting the actual physical displacement of the turbine towers when packed together. You can do a similar thing with the 6.8 billion people on Earth and find that they can all fit, cheek-by-jowl, within the Isle of Man. Maybe so, but it’s hardly reflective of reality and is a silly and disingenuous distortion of the facts.

Still, it’s tough to pick apart the details of spin vs reality, especially from news stories and media releases (I emailed CETO to ask if they had any available operating data on their demo plant, especially with regards to annual output per unit, but they never replied). So if any of you have alternative interpretations to me on these data, I’d like to hear it.

Conclusion (preliminary): CETO is not cost-effective for electricity generation, it cannot be considered ‘baseload’ (it is certainly not dispatchable), and its lifespan in trying marine conditions is highly uncertain. BUT, its potential for low-cost, high-output mechanical desalination (based on high-pressure pumping to an onshore facility) seems enormous, if the preliminary reports can be confirmed.


Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a free website or blog at

%d bloggers like this: