Isle of Foula, Shetland Isles

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The Island

Foula Island1Foula is Britain's most remote inhabited island.  It is situated in the Atlantic Ocean approximately 20 miles to the west of the Shetland mainland. The island currently has a population of 25 residents. Travel to the island is by sea or air and is completely dependent on suitable weather conditions.

It is an island of crofting townships, breathtaking sheer cliff drops, and a wealth of wild flowers and wildlife.

For more information on Foula visit

Image1Further details of the project here:

The Project

Foula June 028The island of Foula is not connected to any mainland electricity grid system. In 1987 a community electricity scheme was constructed, comprising a 3.3kV island grid which linked diesel generators, a wind turbine and a hydroelectricity scheme to the island’s properties.

This scheme gradually fell into disrepair and has undergone a major refurbishment, funded primarily through grants.

The transformers and control system have been replaced; a new photovoltaic (PV) array has been installed; battery storage and associated inverters have been integrated into the system; and the hydro generator system is due to undergo completer refurbishment with the addition of a second pipeline.

The project has received funding assistance from: Shetland Islands Council, Shetland Enterprise, Highlands and Islands Energy Company, European Regional Development Fund, Energy Saving Trust, Big Lottery Fund. This is a three phase network with all generation and consumption at 400/230 Vac. In order to transmit power around the island network with minimal cable sizes whilst reducing power losses over the distances involved a number of transformers are used to first step-up power from the generation to 3.3kV and then down again for consumption.

Foula LayoutBefore refurbishment, the entire island's power was supplied by one of the two diesel generators which operated between approximately 7.20am and 00.30am.








Typical island load would vary between 12kW and 22kW during that period.

The renewable generation has been specified to optimise energy output around the year. The battery storage system manages the discrepancies between periods when power is available and when it is required by the island load. The hydro generator will provide the bulk of the island’s baseload during the winter months, supplemented by small amounts of energy from the PV array. Before the hydro generator is recommissioned, and during the summer months, the island will still rely on the diesel generators, but the battery storage system and PV array will help to minimise the diesel generator run time. During the summer, the energy from the hydro generator and the PV array will help to reduce diesel fuel consumption. Because of the northerly latitude of Foula long hours of daylight are experienced over summer months so output from the PV system is expected to be very significant. The system provides 24-hour power and the battery storage has been sized to supply a small overnight load, which is assumed to comprise fridges, freezers, central heating pumps and a few lights.

Load controllers within properties will allow any surplus hydro energy to be employed by water or space heaters when the batteries become fully charged. This new electrical system for Foula was designed by Econnect Ventures who worked with Wind & Sun Ltd to design the battery, inverter & PV system.

Econnect Ventures have provided Foula Community Electricity Scheme operating results for November 2007.

Foula Map2Geographical layout of Foula's electrical network










Battery/Inverter System

Batchedfoula BatteriesWind & Sun designed and installed the battery/inverter system and PV array and installed it in November 2006 and January 2007. The system uses 9 Sunny Island SI-4500  bi-directional inverters as the heart of the system to combine to give a high quality sine wave inverter with powerful overload capability (total of 40kW 30min rating). They include high performance battery chargers that ensures maximum battery lifetimes, energy management controller for loads and generators, and a generator management system. Further information on the inverters (data sheets, sample systems and operation manuals can be found on the SMA website.)

All generation is connected on the AC side with the PV using Sunny Boyinverters, so cable volt losses can then be minimised. Three 3-phase clusters of inverters are used with each cluster connected to it’s own battery bank rated at 60 VDC 916 Ah (@ C10 rate) and comprising 15 x 4V Rolls Solar RB-4KS21PS batteries. This gives a total storage capacity for the whole system of over 80 kWh to 50% depth of discharge.

Batchedfoula SunnyislandsThe Sunny Island inverters control the battery charging and the system is designed to tie in with the existing diesel generators. One of these generators is selected at a time and can be controlled from the Sunny Island.

The generators have a so called ‘droop characteristic’ which means they reduce their frequency analogously to the load and vary their output voltage versus the apparent power. Generators are only needed if loads exceed inverter capacity or if the batteries needed recharging.

PV System

Batchedfoula Schoolpv1Batchedfoula Schoolpv2The 19.2kWp PV array consists of 6 sub-arrays each comprising of 2 strings of 20 BP Solar BP380S 80Wp PV modules. (240 modules in total). The modules are mounted on a galvanised steel ground mount framework designed and constructed by Malakoff Ltd (the Shetland based main contractors for the electrification project). Malakoff also mounted the PV modules ready for wiring by Wind & Sun. A large concern during the design process was to ensure that the entire system would last a long time. Foulaexperiences extreme weather (173mph winds have been recorded) so the PV modules were chosen for being the strongest available. Malakoff undertaking the mechanical installation ensured that their knowledge of the local conditions resulted in a robust solution and a smooth construction. Since Foula is so remote the simple logistics of getting materials to site safely and on time is significant, the wet peat ground conditions necessitated many tons of concrete alone, so this was no means feat!

Batchedfoula June 025Each of the six 3.2kWp PV sub-arrays is connected via underground armoured cable to an SMA Sunny Boy SB-3000 inverter.

These Sunny Boy inverters are designed for ordinary AC grid connection. They  synchronise themselves with the grid voltage automatically when the sun comes up. The power from the PV  contributes to the consumer loads on the system. Any surplus energy  is used by the Sunny Island inverters to charge the batteries for later use.

The Sunny Boy inverters are set to Off-Grid mode to enable their power output to be controlled by the Sunny Island inverters. If there is too much renewable power being input to the system and the batteries become fully charged the Sunny Island inverters raise the grid frequency. As the grid frequency rises to 51Hz, the PV inverters  start to reduce their output linearly, until at 52Hz they  are outputting no power. This removes the need for conventional charge controllers and ensures optimal battery charging.

Batchedfoula Display2Batchedprojects FoulacontrolThe PV array is located outside the island primary school and community hall.  A display is fitted at the entrance so everyone including visitors to the island can see exactly how well the PV's are working. 

The entire system is monitored using a Sunny Control Plus unit and all the inverters are connected to this via RS485 cabling. An analogue telephone modem is included which allows performance to be monitored remotely. This is invaluable for system set-up, diagnosis and control.

The graph below shows how the system operates in practice as recorded using the Sunny Control Plus.

Foula Operation 22 03 07

The system was fully commissioned at the beginning of March 2007 and already the islanders not only have continuous power ( instead of the previous 17 hours per day) but are noticing considerable savings in diesel fuel use. Since diesel has to be shipped in by ferry (and often the weather is too bad for the ferry to run for up to 3 weeks on end) this of huge value.

Photos below show the layout of the island power station.  Sunny Boys are on the left hand side and Sunny Islands are on the right.  In the middle is the power centre control cabinet where the control switch gear and protection are located and cables are marshalled.

Foula Feb 2009 WorkstationFoula Workstation1Foula Sunnyboys

After long delays due to planning constraints and weather, the hydro turbine was installed and commissioned in 2008 and is now keeping up with the island load during the winter months.

Batchedhydro TurbineThe hydro turbine is rated at 15kW and obtains it's water supply from both the Ouvrafandal Loch on the hill above the turbine and the Creyg Burn using a new pipeline. The Burn supply only works after it has been raining (which is quite often on Foula!) whilst the Loch provides storage for a more consistent supply.

During the summer when insufficient water is expected, the PV's will be working at their best so it is hoped that diesel generator running time will be minimal.

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