20 December 2007

BRE report FB17: Ampair comments part 2

(continued from previous post on the BRE urban micro wind turbine assessment)

In calculating electricity generation BRE make the careful statement “the conditions under which the the [manufacturers’] power curves were obtained is, in most cases, uncertain, it is expected that they were obtained in wind tunnels or using free standing mast-mounted turbines”. Herein lies my biggest quibble with the FB17 methodology as BRE should have put this in much bigger letters and written an additional caution, namely:


At the moment to use all wind turbine manufacturers’ power curves to predict electricity generation as if they were equal in quality is to introduce a large error especially as (to the best of my knowledge) of the three systems analysed only the Ampair 600 is an open field power curve. The reason for the difference is that in a wind tunnel one can hold the wind speed absolutely constant for the measurement period, and the wind direction is necessarily constant, and the turulence can be reduced to a minimum. This means that the turbine system performance is not degraded by a variety of tracking errors irrespective of whether they are mechanical in nature or electrical in nature. (This unfortunate situation will change as very rigorous standards for generating power curves are being introduced, but it is so important a quibble that we are disappointed with BRE for not emphasising their predicament).

Once this quibble is out of the way the actual values for electricity production in an urban environment are of approximately the right order of magnitude (at least for the Ampair). But the net electricity generation is different than the gross electricity production. This is because BRE have assumed (or not even stated the assumption) that the turbines are not importing electricity. Again for some systems this is an unwise assumption as the annual standby power consumption can be of the same order of magnitude as the annual production leading to a net generation of approximately zero, or even negative. For an Ampair system the extra money we put in to a high quality inverter pays off in very low standby powers so for Ampair the BRE assumption is valid.

The energy production figures are not corrected for any wind quality issues. As BRE themselves state this is a known issue and it is too early to try and make such adjustments.

The CO2 payback results appear sound. From an Ampair perspective it is pleasing that the Ampair pays back CO2 more quickly than the other turbines. We believe that this would be even more noticeable if the power curves were put on a level playing field and if the electricity import issue was accurately modelled.

The lifecycle costings contain an important error regarding the actual cost of System 3 (RD Swift). Obtaining pricing for this system has been notoriously difficult for the last two years and so it is unsuprising that BRE have used the only public domain data they could locate, that of £3.5k as provided by bettergeneration.co.uk who are not an RD Swift distributor and who are themselves misled. Unfortunately it is very misleading and a more accurate cost would have been about £7k. Whilst RD Swift are working hard to reduce both price and cost and certainly aspire to much lower than £7k the other two systems are being evaluated on their costs in the market today and once again a level playing field should have been applied.

The other issue with lifecycle costings is the maintenance and longevity one discussed earlier. We think that a quality turbine should aim to last on average 15 years with perhaps a five yearly on demand maintenance visit (i.e. four visits: once to install; two for maintenance; and one to remove), and a lower quality turbine should aim to last on average 10 years with four visits in total (at reduced intervals). We don’t mean lower quality in a pejorative sense: it may be more cost effective to make a cheaper turbine with a shorter life.

These two costing problems contaminate the finncial payback calculations sufficiently that one cannot make any further comments.

Further research is of course required and the proposed list is a sensible one. Overall I think this FB17 report from BRE is a pretty good assessment of the aspects of urban microwind and look forward to more pieces of the puzzle being slotted into the jigsaw in due course. Whether one thinks urban microwind is a good idea is a different thing and one we'll comment on separately.

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