The calculated payback times therefore include a 10% annual increase in the price of electricityhttp://www.bwea.com/pdf/small/mid-wales-microwind.pdf

The calculated payback
times therefore include a 10% annual increase in the price of electricity charged to consumers.

The calculated payback
times therefore include a 10% annual increase in the price of electricity charged to consumers.

Caveats
The development of domestic roof-mounted wind turbines has only just begun. The two turbines
investigated here are at the post-development stages and are gearing up for mass production. This
means that prices for installed turbines are currently far in excess of the target price. Work done on
payback times is performed using the final target price, which is probably some 18 months away
for Swift, and perhaps 12 months away for Windsave.
There is currently no mechanism for getting paid for electricity exported to the grid, or for claiming
Renewable Obligation Certificates (ROCs). The control panels of both turbines display indicative
values of kWh produced, but this is not sufficient for DNOs to credit electricity payments or claim
ROCs. This greatly decreases the cost-effectiveness of the turbines, although it should be pointed
out that the environmental benefits of increasing locally generated electricity still exist. To increase
the cost-effectiveness of the turbines, a method of energy storage, such as an electric immersion
heating element in a water tank, should be used. This will help ensure that all of the production of
the wind turbine is matched by the load of the household. This caveat is probably less of an issue
for businesses where there is likely to be some level of 24hr loading. Work done on payback times
therefore assumes that the output of the turbine is completely matched by the load. Where an
immersion element is not available, this assumption is stretched and the payback times should be
viewed accordingly.
This report is written in the climate of unsettled but increasing energy prices. DTI statistics
catalogue a rise in electricity prices by 11.2% from March 2004 to March 2005. To suggest that
electricity prices will not rise in the future would be contrary to the projected increase in worldwide
demand for gas (the single largest component of the electricity generating fuels in the UK). Indeed,
prices for gas have risen even faster than electricity prices over the last year. The calculated payback
times therefore include a 10% annual increase in the price of electricity charged to consumers.
This document is likely to be revisited several times as development of turbine technology
continues, and the electricity prices will be reviewed as part of the update.
The payback time of a turbine will be influenced by any grant support available. At the time of
writing, Swift was accredited for Clear Skies grant funding of £1500. Windsave was undergoing the
accreditation process, and the assumption has been made that the application for accreditation will
be successful. The grant funding has not been included in calculation of payback times, as the
cost of installation has been estimated to be roughly equivalent to the available grant.
The final important point when considering payback times for any wind turbine is the windspeed
at its location. There is an online resource available to determine average windspeeds at a given
location to the nearest square kilometre (http://www.dti.gov.uk/renewables/technologies/windspeed/
online.html). Although this should not be used for detailed study, it suggests that a windspeed of
4.5-6ms-1 is typical for an urban site at 10m hub height. There is not much data on whether the
siting of a turbine on a roof reduces yield due to turbulence or increases it due to a “wind flow
enhancement” process. Given the above, a windspeed of 5.5ms-1 has been selected as the basis for
payback times.
Windsave has yet to produce a power curve for their turbine. Without this information it is
impossible to determine payback times. It is hoped that this will become available by Autumn 2005.
For this version of the document Swift is the only turbine for which payback times have been calculated.