The first four posts of the TCASE series were logically sequential — each post built on the conclusions of the previous one. Overall, I hope the TCASE will retain a sense of coherency, but at the same time, I don’t want to get too constrained in following a rigid structure. To be frank, I can’t plan the ‘storyline arc’ well enough at this stage to make that even half feasible, and besides, I want to the series to be responsive to topical debates (and keep each post to a digestible, bite-sized chunk of information).
So future offerings in TCASE will branch out to cover everything from examinations of different technologies/energy sources, case studies of actual real-world projects, evaluation of new policy decisions (such as Australia’s 2020 ERET), questions of build rates and constraints, cost/feasibility assessments, consideration of technology gaps and physical limitations, exposing spin and hype, limit analyses, thought experiments, etc. I certainly hope to continue to get ideas from the commenters on this blog, which collectively represent an enormous wealth of knowledge, experience and ideas. To me, this is a fine form of peer review and a great source of inspiration. Thanks BNC readers!
Today’s post offers a first look at ocean power — the mighty fist of Poseidon (mythologically and in reality) — harnessing the energy in waves (I’ll look at tidal energy separately). Wave power is a form of indirect solar energy — driven by fairly consistent oceanic winds, which whip up waves over hundreds or thousands of km of open ocean. This energy may be harnessed with the use of buoys, oscillating air columns, barrages and so on, with a conversion efficiency of ~30%. Waves are a linear energy resource — once you’ve tapped its energy, you need thousands more km of ocean to regenerate new waves, so the resource is measured in kW per linear metre (not metre-squared, like direct solar). Average annual wave power density range from 10-40 kW per metre in inshore regions to as much as 70 kW/m in highly energetic regions. Although it is somewhat more regular (‘available’) than wind (and with a higher power density), wave energy is not constant and will still require substantial back-up and/or energy storage. More technical documents here.
Carnegie corporation, an Australian wave power company, state that their CETO technology (which I will look at in detail in another post — it has some fascinating prospects) can generate 100 MW peak using an a 500 buoy system; so, 200 kW peak per undersea buoy. To date, however, the only commercially operating wave farm in the world is in Aguçadoura, Portugal, about a year ago — so let’s focus first on the energy potential of this technology.
The Aguçadoura wave farm has a peak capacity of 2.3 MW, and makes use of three linear snake-like ‘Pelamis‘ to convert the motion of the ocean surface waves into electricity. There are plans to add a further 25 Pelamis to increase its peak capacity to 21 MW. The Pelamis machines, moored 5 km offshore (too much energy is lost to bottom friction if they are placed in shallow water), are 150 m long, 3.5 m wide and weigh 700 tonnes. The are “… made up of connected sections which flex and bend relative to one another as waves run along the structure. This motion is resisted by hydraulic rams which pump high pressure oil through hydraulic motors which in turn drive electrical generators.” The machines face into the principal direction of the waves. The Portuguese farm is currently supported by a specific feed-in tariff of €0.23/kWh.
It is stated in that a 30 MW offshore wave farm would consist of 40 x 750 kW Pelamis machines, arranged in a 3-row lattice with a front of 2.1 km and a depth of 600 m. At a nominal wave power of 55 kW/m (i.e. a farm placed along a high quality Atlantic coastal resource), a single 750 kW unit will apparently yield 2.7 GWh/yr, which gives it a capacity factor in ideal conditions of 41% (this is 12 MWe average power for the 40-Pelamis farm, or 5.9 MW/km). The annual output of an AP1000 nuclear reactor rated at 1,154 MWe and run at 92% capacity factor would be 9,300 GWh. So, going by the manufacturer’s data, we would need to deploy 3,450 Pelamis machines to generate the equivalent yearly energy of one AP1000, arranged in an array 0.6 km wide and extending along 180 km of high-energy coastline. (Note that these are projected, not measured figures — the real world is often tougher)
Or, to put it in the build-rate context of TCASE 4, the hypothetical limit analysis of 680 MWe/day equates to 116 km of linear wave farm to be deployed per day (70 km2 total area), using 770,000 tonnes of steel and an approximately equal weight of ballast. Not only would that be a huge length of coastline to industrialise and isolate from shipping traffic, but the logistics of transmission connection (each of the 2,210 units/day would need to be hooked up) and ongoing maintenance (in often rough ocean conditions) would also be challenging. It is not clear how long each unit would survive before requiring replacement, but other wave technology has claimed to last 20 years.
In reality, no one is credibly imagining that wave power will be a majority component of a future sustainable energy mix, although a recent (highly speculative) paper suggested that 720,000 Pelamis-like units — 220 GWe average — could be deployed worldwide. Wave-power-poor areas, such as the coasts of the southeastern United States, northeastern South America, and southern Japan, where waves deliver only 10 to 20 kW/m, are unlikely to ever be exploited.
In a later TCASE post (not the next one), I will look more closely at the intriguing CETO electricity/desalination technology, consider some pitfalls, and speculate on its ultimate potential.