WC Archive

Sure, intermitency is a problem. But here are two solutions (via The Energy Blog) at the Megawatt-hour level:

- Vanadium Redox Flow Batteries
(http://thefraserdomain.typepad.com/energy/2006/01/vandium_reflux_.html)

- Sodium-Sulfur (NaS) Batteries
(http://thefraserdomain.typepad.com/energy/2006/01/sodiumsulfur_na.html)

[sorry, couldn't figure out how to do links :( ]

In general these days they adjust flow with large stationary fuel cells. Unlike thier portable cusins stationary fuel cells are rather handy already and are selling like hotcakes.

Whoah Jamais! Wave and tide not 'intermittent'? (See below for the reason for the inverted commas.) I think not! The form of variability is different from wind and solar, but variable they are nonetheless. Tides are very predictable, but power from them definitely rises and falls. Wave, being a form of smoothed wind power, is also variable, though forecastable to some extent a few days ahead. I fail to see why these characteristics make them qualitatively different from wind and solar, where the accuracy of weather forecasts 24 hours ahead (the timeframe for planning power system capacity requirements at a managerial level) is pretty good and getting better. In fact, this 'differently variable' aspect of the various renewable forms enhances their impact when they are used in combination. See the work of Graham Sinden at Oxford University's Environmental Change Institute: www.eci.ox.ac.uk/lowercf/renewables/index.html. His research shows that you can significantly increase the overall penetration of variable renewables on a system when you have a diversified portfolio of them rather than 'all wind' or 'all wave'. Vive la difference!

I'm also going to take this opportunity to object violently to the use of the term 'intermittent' to describe the output of wind and solar. Intermittent is binary term: something that is either full on or full off is intermittent. Wind and solar are variable: their output varies between zero and rated capacity with the strength of the wind or sun. Nuclear power stations are intermittent: they run at full power or nothing. If a nuclear power station trips off-line suddenly (it happens, probably more often than you think) then you have a power network management headache of gigawatt proportions which makes dealing with wind power look like a cakewalk. Please do not use the term intermittent to refer to wind and solar again.

Thanks for the comment, Dr. E. To address your last bit first: "intermittency" is the term I've seen used extensively in renewable energy literature to refer to systems where output variability can drop to zero, and while I appreciate the more proper use you prefer, it's hard for me to hold back the tide (as it were) about terms-of-art in use in the industry.

That said, you're right of course, that tidal in particular has similar variability issues. While waves do change in size and strength, both the Electric Power Research Institute (which has done abundant work on wave power) and the various wave power start-ups emphasize the relatively constant flow of power from properly-placed wave power generators. Output is variable, but it (in principle) doesn't drop to zero.

Regardless, I'll add a note to the post pointing to the comments.

I'm well aware of the wide use of the word intermittency - I have to submit to uttering it myself a lot. I thought I'd start a crusade to re-establish proper English usage in this debate starting with the Worldchanging site. :)

Actually, there will be times when the output of wave generators is zero. While there are always waves, wave generators have similar issues to wind turbines in terms of cut-in/out conditions: there are times when wave heights are too low (analogous to the wind speed being below cut-in for a wind turbine), and also when the period is too small (when the waves are very steep - analogous to when wind turbines have to cut out when wind reaches a dangerous speed). This is illlustrated in Figure 5 of the Carbon Trust's report, which is freely available on their website (www.thecarbontrust.co.uk/carbontrust/about/publications/FutureMarineEnergy.pdf), though I will freely admit that it is not super-obvious to find. All that government money and they still can't organise a link from the press release to the report. You wouldn't think they were all McKinsey alumni...

I think I need to be clear that I am a very strong supporter of marine renewables (in fact part of my job is to lobby for them), but to over-state their advantages would be a dis-service to the sector, which still has a long way to go to prove itself and will disappoint people if there is too much hype. It's going to be at least a decade before these technologies are within striking distance of where wind power is now, and even that is not a done deal: we're talking about the sea here, which is a very hostile environment; and there is precisely one commercial-scale wave generator in the water, and one tidal stream generator of similar size. A lot of prototypes and excited venture capitalists does not make an industry capable of delivering GW - not yet, anyway.

Thanks for the caution, and for the link, Dr. E.