Sustainable Energy NZ #8 – The Tides They Are A-Changin’ – the marine and waste energy resource

by Oliver Bruce on October 27, 2012

Welcome to the eighth post in the Sustainable Energy without the Hot Air – A New Zealand Perspective series. Today we’re crunching the numbers on marine and waste incineration potential in New Zealand. For the background to the work please our introductory post here. Also check out our earlier posts on the potential of hydro power,  geothermal and wind, and the summary on the big three. More recently we’ve dealt with solar and biofuels. Note: the units are in kWh/day/person – ie. if you ran a 40W lightbulb for 24 hours, it’d take ~1 kWh over the space of a day. We then divide it by person to give you a sense of the scale of the resource proportionate to the size of the population. Be sure to check out the methodology. For reference – we’re looking to replace around 55 kWh/d/p of energy currently generated by fossil fuels. 

The marine environment offers several possible renewable energy sources, notably wave and tidal energy. Wave energy systems have been studied by the Electricity Authority, and data here comes from their report [yeqtogu]. Feasible wave energy plants need wave energy greater than 20kW/m “close” (say 6km) to coast. New Zealand has 2000+ km of coast-line fulfilling these parameters, mostly on the west coast. Wave derived energies in the far south can be 60 to 80kW/m, which is impressive. That is approximately 86kWh/d/p for a 50% efficient wave generator covering half our available coastline. However, a reality check indicates that no such mechanism exists (so far wave generators have been built for survivability rather than efficiency) and many factors would constrain where wave generators could be built.

A fairly detailed analysis based on currently available technology has identified sites offering perhaps 2kWh/d/p and a maximum potential for perhaps 27kWh/d/p. While a number of prototype and early commercial plants have been deployed worldwide since 2009, this realistically still is best be described as an emerging technology with very substantial environmental and economic barriers to deployment.

What about tides, especially those huge currents going through Cook Strait? New Zealand has a limited tidal range so tidal pools and tidal lagoons have limited potential despite the technology’s very attractive features. Studies have identified sites in Cook Strait, Stewart Island and Cape Reinga which with existing technology might yield 0.4kWh/d/p. The Kaipara Harbour Tidal Scheme was approved in 2009 and will yield approximately 1.75 kWh/d/p of tidal based power when completed (according to the project developer Crest Energy) [8cugk6t]  An alternative study [9bm2evc] suggests there could be 86kWh/d/p extractable from Cook Strait, but this relies on stackable turbine technology that doesn’t exist yet and is a massive engineering challenge. A more realistic take on the 10 year potential for a row of turbines for around 8.6 kWh/d/p. The wide range in estimates emphasises how little is so far known about this technology.

Waste Incineration and Biogas

Currently energy from biogas supplies 0.5kWh/d/p, and there is obvious scope for increasing this with increasing landfill. At this stage a potential of 1kWh/d/p seems realistic. A novel waste use is wood-waste for domestic heating via pellet burners. There would appear to be a capacity for 2-3kWh/d/p.

Further Reading:

Once again, UCSD’s Tom Murphy has an excellent post on the global potential for tidal power (which at a global scale is decidedly wimpy).

{ 5 comments… read them below or add one }

Carol Cowan October 27, 2012 at 9:20 pm

(Off topic) – I just read your short story, Gareth. I enjoyed it. Especially the line “pausing for a pee, and another cup of tea” – Poetry!!

(On topic) Is there any chance of floating devices to generate power from waves/tides?

John ONeill October 27, 2012 at 10:21 pm

Waves- lots of tries, usually smashed to bits when Father Neptune acts up. One outfit named Pelamis was deployed off Portugal, but didn’t last long. Floating ones more common; some concrete chute-type wave magnifiers on shore, but have to be where there’s low tidal range.
Tides- couple of deployments in New York and Northern Ireland, but more like underwater windmills. Kaipara harbour scheme will be similar. A Swedish company had a concept for a kind of underwater glider that would swing back and forth on a cable like a kite, so there’d be faster water flow over the turbine it carried. Just vapourware so far I think. Anything in sea water has a hard life.

Phil Scadden October 28, 2012 at 8:43 am

Carol, I would strongly recommend MacKay’s little chapter on wave energy – both the potential and the challenge. http://www.inference.phy.cam.ac.uk/withouthotair/c12/page_73.shtml

Yes there are floating devices of various kinds but survivability and cost are big issues.

Thomas October 28, 2012 at 10:40 am

The power of the ocean to decompose man made materials is not to be underestimated. I am skeptical on wave energy systems. Areas with high wave energy content in the water are obviously areas where maintaining and establishing such structures will be particularly difficult, hazardous and costly. But tidal energy systems seem to have a better prospect.

noelfuller October 29, 2012 at 10:09 am

I have for some time wondered at the reaction of some Kaipara residents against the Tide turbine proposal. They are out of sight, below the draught of any vessel able to work the Kaipara bar, and turn at 6 rpm (7.8 m/sec at 25 metres diameter) which on the face of it is two slow to endanger their Maui dolphins (they might like the excitement!) let alone any small fish so the ecology argument does not hold up. Only the accepted objection to an overlong power reansmission cable seems to me to have had substance. I admit I have wondered if 200 turbines scattered along a 50 meter deep tide scoured trench might not wind up gathering sand thus becoming defunct as turbines. However, that is irelevant. Could it be that people who have long held in mind a view of their environment simply react against any proposed change to it, visual or not, regardless of use or need? That is a change that is man-made?

I wonder what they thought at the time of a proposal for a nuclear power station at South head? I had a minor part in monitoring the health of the research instruments – some locals valued the setup as a handy supply of stainless steel cable – free for the taking!

Of course I’m still awaiting the outraged public reactions to such tongue-in-cheek proposals as have recently been aired for the Firth of Thames :- ideas for Gareth’s next Aviator book perhaps? :)

Noel

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