Solar power and wind energy specialists - UK

 Off-Grid Solar & Wind Power - Is The Site Suitable ?

Tree flagging - a sure sign of good wind speeds...

Off-Grid Power Systems - Site Survey Checklist

In 17 years of supplying off-grid solar & wind power systems, the single, most common issue we see for both solar & wind technologies is the incorrect choice of site.

The siting of a solar pv array is simpler than that of a wind turbine, and generally causes less planning issues. We've compiled the following check-list of the main points to be considered when selecting a site.

Siting Considerations - Off-Grid Solar PV

  • Latitude & Solar Irradiation - The closer to the equator, the more sun we get. Ironically, the hotter the solar panel, the less efficient it performs and so peak outputs are often seen on a cold, clear sunny day. When designing an off-grid system, the irradiation for the least sunny month of the year is used, as this will be the worst case scenario. Solar irradiation data for your location can be found here : Europe - Africa - World
  • Shadows / Shading - A complete 'no-no' with solar pv. Beware of long mid-winter shadows from tall trees or buildings. Shading has a massive detrimental effect on the output from a solar pv module and needs to be avoided wherever possible.
  • Angle of Tilt - Off-grid solar pv tilt angles are normally optimised for the worst sunlight months of the year. Generally speaking, the lower the sun is on the horizon in mid-winter, the steeper the optimum angle of tilt of the solar panels.
A rough rule of thumb for sites in the northern hemisphere (to optimise tilt angle for the winter) is to add approx 15 degrees to the latitude. For example, London is 51 and in the winter add 15. Thus, optimum winter tilt angle = 66 tilt from the horizontal.
Ideal orientation in the northern hemisphere is due south, and due north in the southern hemisphere.
  • PV Mounting Structure - The type of solar pv mounting structure will depend on the site and the optimum angle of tilt. Common solutions include ; tilted roof mounting, flat roof mounting, ground-mounting and post mounting.
Many larger off-grid solar pv systems are ground-mounted and smaller pv arrays are post-mounted, as these both allow ideal orientation and provide a steeper angle of tilt than most roofs.
Tracking mounts are also available, although it's often far cheaper & more reliable simply to increase the size of the pv array rather than introduce moving parts to the system.
  • Cable Run Distances - The pv array ideally wants to be sited within about 50m of the battery bank, although this all depends on the site and the pv array voltage. Systems can be designed to allow up to 200 to 300m of cable run, although long cable runs can be expensive.
  • Snow - If snow is to be expected regularly, a steeper tilt angle may be selected for the pv modules. Due to the dark surface of the solar module and the fact it's facing south on a tilt, when the sun shines the snow thaws and slides off fairly quickly.
  • Dust - In hot and sandy locations, dust may be a challenge and may need to be manually cleaned in the case of a sand-storm or dust-storm. Again, steeper tilt angles can help to reduce dust build-up.
  • Cleaning - In rainy locations, most solar pv modules will do a reasonable job of self-cleaning. However for some locations it will be necessary to arrange occasional cleaning of the pv glass. At the very least, we would recommend manual cleaning once per year, and ideal timing would be at the end of the summer, early winter.
  • Planning Permission - Generally speaking, in the UK, if the building is not in an SSSI or conservation area, and the solar pv panels are mounted flush to the roof, planning is not required. If you need any further information, contact your local council's planning department. Please also see the link below :

Siting Considerations - Small Wind Generators / Turbines

  • Average Wind Speeds - Sites with high average wind speeds are important for a wind turbine to work effectively. Generally speaking you would be looking for 5m/s or more to indicate a good wind resource.
  • Turbulence - Turbulence is the enemy of a wind turbine blade. Just like an aeroplane 'falls' out of the sky when it hits turbulence, the blade of a wind turbine looses lift and produces much less power.
  • Obstacles - Obstacles cause turbulence (now public enemy #1). A good distance is required away from buildings or trees. See diagram below.
  • Hills - If the turbine can be sited on the top of a smooth hill, the wind speeds will often be increased.
  • Mast Height - The higher the turbine, the better the wind speeds and the less turbulence will affect the blades.
  • Cable Run Distances - One of the issues with siting the turbine in the ideal wind spot is that cable runs can work out expensive. If the cable run needs to be a long one, it helps if the wind turbine puts out high voltages, brought down to the correct voltages by a control unit.
  • Ice - Most small-scale wind turbines can cope with ice, although in extremes may stop the turbine from working until it thaws.
  • Maintenance - Annual inspections must be planned in for a wind turbine. Every year the rotor of a wind turbine will do a massive number of revolutions and so it's important to ensure that everything is in tip-top shape and there has been no unusual wear or anything has come loose.
  • Neighbours & Planning Permission - One of the ways to upset the neighbours is to put a wind turbine up directly in their view. There are rules and regulations for off-grid wind turbine installations that UK councils publish, an example of which can be found below (link opens in new window).


Diagram showing windflow over an obstacle & turbulence - diagram courtesy of CAT

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