Document: Report on visit to Kerrera of 22nd August 2007 with Mark Jennison, to
comment on the potential for community wind energy projects at different
scales, and assess proposed sites.
Client:
Isle of Kerrera Residents Association
Author:
Colin G Anderson
Date:
23 August 2007
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Scope & overview
This report covers a visit to Kerrera on 21st August 2007 with Mark Jennison of Real
Alternatives, whose company commissioned the work as an initial assessment of the
prospects for community wind energy on the island. At present the scale of wind project is
still to be determined and a primary aim of the visit was to assess key issues including wind
climate, grid access, and transport infrastructure.
During the visit we travelled between the ferry terminal and Lower Gylen on the main
island access track (see Figure 1), with a detour into the hills in the centre of the island
following the route of the new SHEPD 33kV overhead line: this crosses Kerrera to serve
Mull and ultimately Tiree. We assessed a potential turbine site at a high point (150m amsl)
at approximate grid ref NM811277 before traversing south towards Lower Gylen.
At Lower Gylen we examined an old farm track that leads north into the hills for suitability
as a site access route; we then looked for alternative turbine sites suitable for connection to
the 11kV single phase line that serves the island, in reasonable proximity to the main island
access track. A possible location was identified at NM802268 at 32m amsl, close to both
the track and the overhead line.
We finally returned by the main track via Upper Gylen and the east coast of the island back
to the ferry terminal. The survey therefore covered primarily the SE of the island, and areas
initially preferred for wind projects on the grounds of environmental acceptability. A more
complete survey would be recommended in due course, and the comments in this report are
confined to the parts of the island visited to date.
The overall conclusions from this preliminary visit are:
(a) the transport infrastructure on Kerrera would not be suitable for a large (eg Gigha-type)
windfarm project and the costs to upgrade it are likely to be disproportionate;
(b) it may be possible to locate a medium-scale (100-200kW) project at hilltop level with
optimal wind resource and connection to the new 33kV line if turbines capable of
winch-up erection are available and track access can be installed economically;
(c) opportunities may exist to site small grid-connected turbines at lower level in proximity
to the 11kV single-phase grid, where there is a favourable combination of access and
wind resource;
(d) further work should focus on identifying and costing solutions at the above scales, and
exploring the island for other suitable sites; wind speed measurements are also
recommended at the two sites already identified.
Wind resource
The situation of Kerrera in the Firth of Lorne generally suggests a favourable wind
resource, with exposure to prevailing south westerly winds. Wind turbines already operate
successfully on two other islands in this area, namely Luing and Gigha, and in both cases
prior wind surveys showed wind speeds in excess of 6m/s at 10m agl, on sites with sea
level elevation of 30-40m.
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Wind speed estimates for Kerrera from the DTI database range from 4.7m/s to 7.4m/s at
10m agl, as shown in Figure 2. There is strong local variation due to differences in terrain
height and the sheltering effect of the hills. On Kerrera the terrain is relatively rugged, and
micrositing is likely to be important, to avoid both sheltering and terrain-induced
turbulence. The best wind sites will be on hilltops exposed to all points of the compass.
The high-level site examined at NM811277 (Figure 1) has an excellent fetch in all
directions and is not terrain shadowed. The land in all directions is however quite rugged
so turbulence may be moderate or high, and will need to be measured as part of any wind
logging campaign. The DTI mean wind estimate for this location is 6.7-7.0m/s at 10m agl,
which seems reasonable from inspection of the site. This would translate to a highly
attractive speed in excess of 8m/s at 45m agl.
The above site is typical of the high ground in the southern part of Kerrera, and has the
significant advantage of proximity to the new 33kV SHEPD grid line. Access, however, is
a different issue and the infrastructure required to construct a project on the high ground
may be considerable, depending on project size. These points are discussed further below.
The predicted wind resource at lower levels around the coast is more modest: the flat land
south of Lower Gylen is accorded 5.2m/s at 10 agl by the DTI database, with slightly
higher figures on the coast further west. The topography of the island is not ideal for
coastal siting, as much of the coast is rugged and/or far from the necessary infrastructure.
The south coast has some flatter and accessible areas with good southerly exposure, but a
degree of sheltering due to the rugged promontories of Cnoc na Faire and Rubha Seanach.
As noted above a potential site was examined just to the south of Lower Gylen at grid ref
NM802268. This was a relatively flat outcrop at 32m amsl, though with relatively steep
sides to the north and east. Wind speed measurement would again be necessary to
determine the mean resource, and assess whether turbulence is an issue. There may be
other, similar, sites with better exposure and a more thorough search is recommended.
Grid access
Access to the SHEPD medium voltage network is potentially available (the significant issue
of transmission capacity notwithstanding) at 11kV or 33kV. In the former case the network
appears to be only single phase, at least in the areas of the present survey. This would
suggest suitability for small scale wind turbines suitable for single-phase connection, and it
is recommended that a survey be carried out of currently available turbines.
If it were possible to connect, say, a 5-15kW wind turbine to the single phase supply then
this might indicate a useful scale of project, using one or more such machines. For
economic reasons they would preferably be connected close to the 11kV line to minimise
additional electrical infrastructure. Alternatively it may be possible to upgrade the single
phase line to 3-phase by upgrading the cross-arms of the wood poles with an additional
conductor.
The 11kV line runs roughly alongside the main access track to the south of the island, via
Lower Gylen and on to Ardmore. Upgrading the line to 3-phase may be costly but would
introduce the potential for installing a larger wind turbine, in the 50kW class. There are a
few established products at this size, some optimised for installation in remote areas. If
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such turbines are considered, however, there is also the possibility they could be connected
instead to the 33kV distribution line.
The 33kV overhead line crosses Kerrera from east to west, but does not serve customers on
the island. It is instead the main feeder for Mull, and (via the Dervaig 33/11kV substation)
thereby Col and Tiree. The line surveyed for the present study is brand new and in fact still
awaiting completion when seen. The connection from undersea cable to overhead line on
the east coast is adjacent to the main track, from where it climbs steeply over the hills to the
west of the island.
Connection to the 33kV network would be ideal for a project on these hills, as described
above. The issue here, however, would be vehicle access, as there is no established major
track into the hills. It is understood that SHEPD used a combination of helicopter
deliveries and small all-terrain vehicles when constructing the 33kV line, which has been
done with almost no visible scarring of the hillside.
Locations elsewhere on Kerrera to the west (eg Barnabuck) or the north of the island were
not surveyed on this visit, and it has so far been presumed that they are also served only by
single phase 11kV. This needs to be confirmed, however, and a more detailed assessment
of the island grid network is recommended (ideally with access to the SHEPD grid map).
Vehicle access
Transportation requirements must take account of the capacity of (a) the main access track
on Kerrera, and (b) any new or upgraded access off the main track to the wind turbine site.
During the present survey the main track was followed from the ferry slipway to Lower
Gylen, a total distance of approximately 4.8km (see Figure 4). Detours were taken up the
hill to the potential turbine location at NM811277, and to assess an old farm track
branching off beyond Lower Gylen.
The main island track is relatively narrow with width of approximately 3m, and only partly
made. It is of a similar standard to a good farm track suitable for eg agricultural or light
commercial vehicles, but unlikely to accommodate a crane of any capacity or the
construction vehicles normally associated with windfarms. If a medium sized wind turbine
requiring crane erection were specified this track would have to be upgraded, typically to
3.5-4.0m width and with sufficient stone to accommodate the crane axle load.
Neglecting the issue of bends or gradients the cost of such an upgrade may rule out projects
below a certain size on economic grounds. The length of main track requiring upgrade
would be at least 3km, and the likely cost exceeding £100k. To this would be added the
cost of the access track to site, and assuming a hilltop location (as shown in Figure 4) a
further 800m+ of new track may be necessary, possibly adding at least £50k.
These figures are approximate, but unless a MW-scale project were envisaged for Kerrera
the cost of the track upgrade is likely to be disproportionate. The initial feeling is therefore
that a wind project on Kerrera would have to be designed around the use of the existing
main track. A survey of the whole island would, however, be needed to determine whether
there are sites with better access and where the above conclusion does not apply.
Assuming the existing main track is adequate, wind turbines would have to be sized to be
delivered and erected by agricultural and/or off road type vehicles. The hilltop site would
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still require a bespoke access, and two possible routes are shown in Figure 4. One of these
(mentioned above) has minimum length of 800m and follows the line of the 33kV overhead
line up the hill. The average gradient is 1 in 8 but steeper sections would probably be
involved, or a longer route required.
Another possibility to reach the hilltop is the old stone track that starts off the main track
about 400m west of Lower Gylen. This is quite steep over its initial length, up to 1 in 4 in
places, relatively narrow, and with steep drops in places. This track might be adequate to
upgrade to accommodate tractor-type vehicles, if width allows. Extending the track to the
above hilltop would also require about 800m of completely new track.
An alternative is to seek sites close to the main island track and served by the single-phase
11kV line. One possibility is the site surveyed at NM802268, in the vicinity of Lower
Gylen, and at sea level elevation of about 32m. In this case access is relatively good with
only a short run required from the main track to the site, and over flat terrain. Proximity to
the overhead line is equally good.
The above site would be of interest for a project at smaller scale, limited by the singlephase OHL capacity. The wind regime at this location is also less advantageous than on the
hilltop due to terrain sheltering, and there may be issues of turbulence. Nonetheless it does
look more typical of an accessible turbine site suitable for small-scale community
development. Other such sites may exist and a wider search is recommended.
Project scale comparisons
From the foregoing it is clear that a key question is that of appropriate project scale. The
most ambitious project would be to connect to the 33kV grid on a hilltop site, combining a
high installed generation capacity with the optimum wind resource. The main issues in this
case would be providing road access economically, and/or finding wind turbine technology
that can be delivered and erected without crane or other heavy vehicles.
In principle a multi-MW scale project could be built, large enough to justify the cost of a
major road upgrade, and assuming other aspects of infrastructure (ferries, public roads on
the mainland) were adequate. It is highly likely, however, that such a scheme would be
unacceptable on environmental grounds, and that its cost and scope are outwith the
aspirations of the local community.
Otherwise, a high level site connected at 33kV would have to be based on small/medium
sized wind turbines that can be installed by winch, such as Atlantic Orient and Vergnet.
Examples have already been installed in the UK and the 100kW scheme on Luing is worth
examination: it was designed to avoid the need for a crane, and to minimise concrete
requirements for the turbine foundations via the use of rock anchors. These principles may
be useful on Kerrera if this scale of project is considered.
Projects at lower elevation will have a lower wind resource, but potentially better access to
the main island track, and the 11kV grid. This currently limits the scope to single-phase
generators and the capacity of the overhead line, albeit there may be several sites around
the island where small turbines could be installed. It may then be worth considering the
possibility to install generators in the 6-15kW range with a total target of (say) 60kW. This
scale of project is, however, inherently expensive in terms of £/kW.
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One option may be to upgrade the single-phase line to 3-phase, allowing larger turbines to
access it. This is perhaps worth considering, though may be an unsatisfactory compromise
as the upgrade cost could still be considerable, while the 11kV line is mainly at lower
elevations and away from the main wind resource on the island (ie the hilltops).
Finally, domestic scale wind turbines may be suitable for Kerrera, connected directly to the
customer’s LV supply or in stand-alone mode. They should still be sited so as to achieve
reasonable efficiency, and not all properties on the island are necessarily well exposed to
the wind. It would be worthwhile identifying those that are, to assess the total potential for
this scale of generation.
A summary of possible options at different scale is given below. This is intended to direct
more detailed investigations, rather than prescribe any definite projects at this stage.
(a) Medium scale, 3-phase connected
A project located on an optimum high-level site and connected to the new 33kV line. Total
installed capacity (say) 100-200kW based on machines that can be winched up and
constructed using only agricultural-type vehicles. The route and cost of a new access track
up the hill would need to be identified early. Target installed cost to be £2000-2500/kW, or
roughly double that of modern windfarms. Commercial technology to be explored.
(b) Small scale, 11kV grid connected
This option to use commercial small-scale technology capable of connection to the 11kV
single-phase grid. Siting to be as close as possible to the overhead line, and in reasonable
proximity to the main island track. Probable turbine size limited to approximately 10kW
and inverter-connected to the supply. Good wind sites to be sought for a number of such
machines with target capacity of (say) 60kW. Economics to be investigated.
(c) Domestic scale, LV grid connected or stand-alone
Based on the most attractive available technology, small wind turbines located at the most
suitable properties and grid-connected inside the customer’s meter or in stand-alone mode
with eg heat store dump loads. The priority in this case to identify suitable properties with
a good wind resource.
Recommendations

Identify suitable medium-scale (>50kW) wind technology suitable for winch-up erection
and with minimal foundations. Assess construction requirements and total costs. A
review of the Luing and similar island projects may be instructive.

Carry out design & costing study for an access track up to the hilltop site, suitable to
allow the above medium-scale project to be built. Assess the cost of grid connection at
either 33kV or at LV (in the latter case assuming SSE own the HV assets).

Identify commercially available solutions for small-scale grid connected turbines in the
approximate range 5-15kW suitable for connection to single phase supply. Assess
infrastructure requirements and cost.
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
Begin wind speed measurements at preferred sites, eg the two already identified and
described above. The logging exercise should include mean wind speed, turbulence, and
wind direction.

Carry out a more comprehensive site search over the island in regard to all generation
options, taking account of likely environmental/landscape constraints.
CGA
23 August 2007
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Figure 2. Wind speed estimates at 10m agl from the DTI database.
Figure 1. Details of survey area and existing infrastructure.
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Figure 3. Views from vicinity of potential high level site looking (from top) towards
N, W, and SW. Note the proximity to the new SHEPD 33kV overhead line.
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Figure 4. Main road and access track survey from visit of 21-08-07. Grid squares are 1km.
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