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By Gerard Wynn
LONDON Dec 13 (Reuters) - The performance of offshore wind is below expectations in the world's leading market, Britain, suggesting the emerging technology may deliver slightly less power than planned at least in its early stages.
Britain has put great faith in offshore wind as a technology to help it meet 2020 renewable energy targets, alongside Germany and Denmark.
Delivery below expectations is a warning over adoption of the technology which is already one of the most expensive options, and has implications for developers' profits.
Load factor measures how intensively a power plant is used and is calculated as the quantity of electricity generated over a period of time, expressed as a percentage of its maximum generating capacity.
The load factor of intermittent renewables such as wind and solar power is usually less than conventional fossil fuel power given they depend on variable fuel sources.
A study by renewable energy consultancy Garrad Hassan in 2003 for Britain's then Department of Trade Industry estimated what load factors offshore wind farms could expect in a ramp-up stage which the country is now going through. (See Chart 1)
Actual performance from 2007 through 2010 was consistently below that forecast, with an uptick last year partly as a result of higher wind speeds. (See Chart 2)
Load factors this year drifted lower again. (See Chart 3)
The chief executive of Britain's energy watchdog Ofgem, Alistair Buchanan, last month reported that offshore wind load factors were about a tenth below expectations, in a presentation to the Chartered Institution of Building Services Engineers.
"Load factor 10 percent below plan in ramp-up stage. Expect delivery of GW (gigawatts) at the low end," he said.
Chart 1: goo.gl/4Q4RI (page 28)
Chart 2: link.reuters.com/zan64t
Chart 3: link.reuters.com/cen64t
Chart 4: goo.gl/4GyLm
Britain has the world's largest offshore wind capacity, with 1.85 GW installed as of June across 15 operational wind farms. That compares with total European installed capacity of 4.3 GW.
The country is targeting a central range of up to 18 GW by 2020 under in its "Renewable Energy Roadmap" published last year, which would correspond to nearly a fifth of the country's net electricity production.
In the development of that capacity, the country has run a number of leasing rounds for areas of the seabed with the first round starting in December 2000 and the second in July 2003.
In January 2010, successful bidders were announced for the third round totalling up to 33 GW in capacity if developed.
Garrad Hassan's report, "Offshore wind: Economies of scale, engineering resource and load factors", sought to "provide a supported opinion on the likely load factors for offshore wind power in the UK Round 1 and 2 offshore wind farms".
Government studies have since cited it as a reference.
The report calculated a range of load factors from 33 to 38 percent, for wind speeds ranging from 8.5 to 9.5 metres per second (See Chart 4).
Actual annual average load factors in fact only passed 35 percent for the first time last year, according to the Department of Energy and Climate Change's (DECC) "Digest of United Kingdom Energy Statistics 2012".
For previous years they were in the range 26-30 percent, or 28-35 percent depending on whether they are measured including or excluding turbines under construction.
More recent data show that load factors this year dipped back below the bottom end of the Garrad Hassan range.
A small variation in wind speeds significantly impacts the amount of electricity produced yet speeds have met expectations.
Historical data show that average British wind speeds have averaged around 9 metres per second since 2001, in the centre of the Garrad Hassan projections.
Those observed wind speeds are averaged for the whole of the UK, implying higher values for the superior offshore resource.
Other factors that will impact load factor include: downtime (as a result of technical faults and servicing); wake losses (from turbines caught in the wind "shadow" of others); and electrical transmission losses.
One possible culprit for the lower-than-expected load factors are wake losses.
UK-based renewable energy developer Renewable Energy Systems (RES) estimates that these have been under-estimated as a result of applying onshore wind modelling techniques to offshore farms.
"Research carried out on operational data of offshore wind farms has shown that the actual wake losses for large arrays of turbines are greater than what the industry standard onshore wake models predicted," the company said in a presentation to an offshore wind conference last year.
RES reported actual wake losses several percentage points higher than as modelled using the "onshore method".
It estimated actual wake losses for the Horns Rev offshore wind farm off Denmark at 12.4 percent compared with a modelled 8.3 percent, and similar discrepancies for Denmark's Middelgrunden farm (10 percent versus 6.2 percent) and the Lillgrund farm between Denmark and Sweden (23 percent versus 14.7 percent).
Garrad Hassan's 2003 report did not explain or reference its estimate for average British offshore wind farm wake losses of 8 percent of output.
It appears, then, that wake losses are one candidate culprit for lower-than-expected load factors.
The resulting handicap is by no means a deal breaker for offshore wind, but the technology is already far more expensive than most rival power generation technologies and must meet steep cost cuts to gain wider acceptance, where lower-than-expected performance matters. (Reporting by Gerard Wynn)