Providing a small island community with an economic, convenient, and reliable electricity supply can be a major challenge. Traditionally, islanders in the developed world have installed diesel generators and depended on fuel deliveries from a mainland depot. But diesel engine maintenance is expensive, fuel costs always seem to be rising – and there is a noise and pollution problem that people who cherish their natural environment would rather avoid. Most islands have a valuable wind resource, many have lots of sunshine and free-running rivers or streams. Such plentiful flows of natural energy act as a strong incentive to generate renewable electricity, and when several different energy sources are available it makes good sense to consider a hybrid system and distribute the electricity using an island mini-grid.
Such systems are still ‘stand-alone’ in the sense of being unsupported by large conventional electricity grids. So are mini-grids serving isolated communities on the mainland. Their major advantage compared with individual stand-alone systems for each user is that integration of various energy sources with different daily and seasonal peaks can provide a more consistent, reliable and economic supply for a whole community. Although backup diesel generators are generally still needed to ensure a reliable 24-hour service throughout the year, they can be started up for short periods only when necessary – and then run hard and at high efficiency.
The Isle of Eigg, 6 x 4 km in extent, is one of the jewels of the Inner Hebrides. Lying off the west coast of Scotland to the south of Skye, it has an equable climate thanks to the Gulf Stream, a generous wind resource, lots of sunlight in summer, a few streams, and just under a hundred inhabitants. Like many Scottish islands, Eigg has a harsh history behind it, including 19th-century depopulation and more recent absentee landlords, but in 1997 funds were raised to purchase the island and set up the Isle of Eigg Heritage Trust to manage it for the inhabitants and their wonderful environment. Determined to update their electricity supply from reliance on ageing diesel generators to a modern ‘ green ’ alternative, they raised capital grants totalling £1.6m for a hybrid system comprising PV, wind, and hydroelectric power, with diesel back-up.8,9 Early in 2008 all 37 households and 5 businesses on Eigg were connected to the new island grid, achieving celebrity status for a state-of-art renewable energy system that is
providing inspiration to other island communities in Scotland and around the world.
Eigg’s system is illustrated in Figure 5.22, which summarises the generation and consumption of electricity. A key feature is that all generators and loads are interconnected by an island-wide AC grid. Transmission is at 11kV for long cable runs and at 230 V for short ones (from the PV and diesel generators), with transformers inserted where necessary. Power sources that generate DC (the wind turbines and PV) feed into the grid via inverters. An advanced load management system monitors the balance between supply and demand, bringing in the diesel generators when necessary, and controlling energy flow to and from the battery banks via a set of bidirectional inverter-chargers. The batteries, PV and wind turbines are the only DC components; homes and businesses are supplied with 230V AC. Grid frequency is set by the inverter-chargers, or by the diesel generators when they are running. We now comment further on the various items:
■ 10kWp of PV It may be surprising to see a substantial PV array included because Scotland is hardly noted for its sunshine! However the Hebridean islands have a better sunshine record than the mainland, where higher mountains tend to increase cloud formation and precipitation. Eigg, at latitude 57 °N, has plenty of sunlight in the summer months, with up to 18 hours between sunrise and sunset in June, so PV can make a valuable contribution when wind and hydropower tend to be at their lowest. In this system the output from 60 PV modules, connected in 6 strings of 10, is converted to 230 V AC by adjacent inverters.
■ 24 kWp of wind energy. A group of four wind turbines, each rated at 6kWp, is sited at one of the island’s windiest locations. Although wind turbines are generally rated in kilowatts at a standard high wind speed, we have used kWp units in the figure to emphasise that they rarely operate at full output – even though the months October to April are highly productive on an island subject to Atlantic storms. In fact the Eigg wind turbines make a valuable contribution throughout the year. Their DC outputs are inverted and transformed to 11 kV for transmission.
Figure 5.23 PV on Eigg (Wind & Sun Ltd); windpower on Eigg (Eigg Electric).
100kWp of Hydropower. The most powerful contributor to the renewable energy portfolio is a new run-of-river water turbine rated at 100kWp supplied by a substantial stream (there are also two much smaller pre-existing turbines in other locations rated at 9 and 10 kWp, not shown in the figure). However on a small island the flow of streams closely follows current rainfall and tends to be intermittent
and seasonal. Hydroelectric generation on Eigg is therefore variable, much stronger in winter than summer, with an average value far lower than the nominal ratings of the turbines.
■ 2 x 80kWp ofdiesel. Twonewdieselgeneratorsprovideback-upto ensure 24-hour service throughout the year. In an average year the renewable sources are expected to provide over 95% of total electricity demand, so the total diesel contribution is small. Typically, the generators are run hard for short periods to boost – charge the battery bank on days when the renewables are unable to meet the full load demand. They generate power at 230 V AC.
■ Load management. A comprehensive hybrid system of this kind, involving various energy sources and domestic and business loads, justifies a sophisticated control system. Its aim is to make the most of available renewable generation, deciding between the various sources in times of surfeit, ensuring that the battery bank is neither overcharged nor over-discharged, transmitting electricity efficiently to the various loads, and bringing in the diesel generators when necessary.
■ 12 x 5kWp inverter-chargers. Arranged as four 3-phase clusters, each with its own battery bank, the bidirectional inverter-chargers are at the heart of the system. When the renewable generation is insufficient to meet demand they take energy from the batteries and invert it to augment the AC supply. When generation exceeds demand they rectify the AC and charge the batteries. If the batteries are fully charged and excess energy is being generated, the inverters raise the frequency, and additional ‘opportunity’ loads such as heaters in community buildings (not shown in the figure) detect the increase and switch on automatically. If there is still surplus energy, the frequency is increased further and the various generators respond by backing off to prevent battery overcharging.
■ 4 x 53 kWh battery bank. The batteries are arranged in four 48 V banks and located in the power house with the inverter-chargers and diesel generators. The banks are normally kept above 50% state-of – charge (SOC) to prolong their life. The quoted total capacity is therefore half the full nominal capacity of 424kWh and equates to approximately one day’s electricity usage on the island. Additional days of storage are not needed in this case because of the diesel back-up.
■ Households and businesses. 37 households and 5 businesses were initially connected to the island grid and supplied at 230V AC.
Figure 5.24 The battery banks, and some of the main inverter-chargers (Wind & Sun Ltd).
Householders agreed to limit peak demand to 5 kWp each, businesses to 10kWp. All consumers are provided with an energy meter to monitor the amount of electricity being used. The islanders have adapted well to the new system and are far better informed about electricity usage and energy conservation than most people on the mainland.
Although the Eigg electricity supply is not especially strong in PV it is an excellent example of a modern hybrid system. The PV component, being essentially modular, may be increased in the future to provide more summer electricity. In any case, the principles of design and implementation are of widespread relevance, even though the relative contributions from PV wind, hydro and diesel back-up power are bound to vary from one island system to another.