Flat Holm Project, Bristol Channel

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The Island

Flat Holm Island 2Flat Holm Island is located in the Bristol Channel just five miles off the coast of Barry and Cardiff and is not connected to any mainland electicity grid system It is a tiny island which has a wealth of history and wildlife. Throughout its history the island has been a sanctuary for Vikings, Anglo-Saxons, silver miners, smugglers and cholera victims. There are various properties of historical interest around the island; including the Farmhouse, Fog Horn Building, Fog Horn Keeper’s cottage, Barracks, Lighthouse & various other ruined buildings, including wartime gun emplacements and a Victorian cholera hospital. Flat Holm is now a Site of Special Scientific Interest and a Local Nature Reserve. More information on Flat Holm Island.

Flatholm Farmhouse Pvs SunsetThe Project

The Flat Holm Project is a part of Cardiff County Council which operates to conserve the history and wildlife of Flat Holm. It employs six paid and four voluntary staff to run the Island and maintain the project vessel, the Lewis Alexander, which supplies the Island and runs trips for up to 45 passengers, from Barry.

Approximately 3,000 people visit the Island annually which can be scheduled public visits or self arranged group trips, tide and weather permitting of course. The Project also provides residential facilities for educational/working groups.

The Flat Holm project approached Wind & Sun for advice, design and installation of renewable energy systems on the island to benefit all those who visited and worked on the island.

Historically, the entire island’s power was supplied by several diesel generators and the different properties were not connected to each other. A battery and inverter system was in place at the Farmhouse to ensure 24hr power for essential loads such as communications and computer system but otherwise generators would only be run for limited periods in order to conserve fuel. 

Wind & Sun designed a renewable generation system to provide power for the island.

Flatholm Island Systems

This involved linking the Farmhouse, Workshops and the Fog Horn Keepers Cottage with underground 230Vac cables to form a ‘mini-grid’ and powering these properties using a large new battery store and powerful (13.5kW) 'Sunny Island’ inverter/charger system located at the farmhouse with the farmhouse diesel generator as back-up.

This battery bank is charged by two photovoltaic (PV) solar arrays, (one located on a freestanding support structure  by the farmhouse and one located by the fog horn keeper’s cottage) and by a 6kW wind turbine sited at the high point of the island where an existing redundant telecommunications tower was located.

The battery store, Sunny Island inverters, Sunny Boy inverters for the farmhouse PV array and other control gear is located in a new building close to the farmhouse & generator. Other inverters and control gear associated with the wind turbine and second PV array are located in an outbuilding near the fog horn keeper’s cottage.

The existing inverter/charger and battery store at the farmhouse was moved to the barracks classroom at the other end of the island for re-use with the generator there and a PV array on a pole mounted tracking system installed to charge the battery. See Flatholm Island Barracks System

The systems were designed 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 loads. The wind turbine and the PV arrays complement each other and the diesel generators are now only used as a standby in times of unfavourable weather.

Battery/Inverter System

Wind & Sun installed the battery/inverter systems, wind turbine and PV arrays in the summer of 2006.  Prior to this all materials were transported to site using the Flat Holm Project’s boat and taken up the cliff from the jetty. Groundworks such as cable trenching and laying together with construction of concrete foundations for the PV support frames had also been completed. Much of this time-consuming work was carried out by Flat Holm Project volunteers.

The system uses 3 Sunny Island SI-4500 bi-directional inverters connected as a single phase parallel cluster as the heart of the system to combine to give a high quality sine wave inverter with powerful overload capability (total of 13.5kW 30min rating). They include powerful battery chargers that ensures maximum battery lifetimes, energy management controller for loads and generators, and a generator management system.

All generation is connected on the AC side with the PV using Sunny Boy inverters and the wind turbine using a Windy Boy, so cable volt losses can be minimised. The Sunny Island cluster is connected to a battery bank rated at 60 VDC 916 Ah (@ C10 rate), comprising 15 x 4V Rolls Solar RB-4KS21PS batteries. This gives a storage capacity of over 27 kWh to 50% depth of discharge.

The Sunny Island inverters control the battery charging and the system is designed to tie in with the existing diesel generator which is only needed if loads exceed inverter capacity or if the batteries need recharging. 

The PV System

There are two separate PV arrays.  

Flatholm Farmhouse Pvs21. 4.95kWp located at the Farmhouse.  This consists of 2 sub-arrays comprising of 2 strings of BP Solar BP3165S 165Wp PV modules. (30 modules in total: 2 strings of 9 & 2 strings of 6). The modules are mounted on galvanised steel ground mount frames. These are connected using underground armoured cable to SMA Sunny Boy inverters (1 x SB-1700 & 1 x SB-2500) 


Flatholm Turbine Pv Foghorn2. 2.97kWp located at the Foghorn Keeper's Cottage  This consists of 2 strings of 9 BP Solar BP3165S 165Wp PV modules mounted on three galvanised steel ground mount frames and connected using underground armoured cable to SMA Sunny Boy SB-2500 inverter. 

The Wind System

Batchedflatholm TurbineThis is a Proven 6kW turbine on a 9m tower. An existing concrete foundation for a redundant telecommunications tower was utilised. This happened to be at the windiest point on the island and reusing the base helped with planning permission and reduced the amount of concrete that had to be transported to Flat Holm. The wind turbine, tower and telehandler (needed for installation) were transported to the island by landing craft.

The turbine is connected to an SMA WB-6000 Windy Boy inverter.  The Sunny & Windy Boy inverters are designed for ordinary AC grid connection. They  synchronise themselves with the grid voltage automatically when the sun comes up or wind blows. The power produced contributes to the consumer loads on the system with any surplus energy used by the Sunny Island inverters to charge the batteries. They 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 Sunny & Windy Boys 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.


Batchedflatholm Drilling Holes 1Batchedflatholm Landing CraftBatchedflatholm Telehandler Tower



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

Batchedflatholm Sunny Island InvertersBatchedflatholm Sunny Island Inverters Sunny Boys