Scenario 4 represented if the island was to aim to be net energy positive – where all electricity consumed on the island is also generated there. The constant population demand profile for scenario 1 was used. The grid connection now blocks any imports from the mainland, but still allows for exports to be sold. After several different methods and technologies were tried, it was found that the most cost-effective way to meet the carbon reduction targets and provide enough energy to meet demands was to implement anaerobic digestion and biogas. This has been supplemented with 1.5MW of PV and 2.9MWh of Tesla Lithium-Ion batteries, as used in the other scenarios. The three second-hand Vestas turbines, have also been replaced by a single, larger 800kW turbine.
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Replacing the three older wind turbines, with one single turbine of a similar total rated capacity, greatly increased the power output from wind on the island. This is maybe partially due to the higher hub height of the larger turbine. The significance of the power output from the 800kW turbine is shown in the monthly electrical production chart, where the consistently high output throughout the year is made clear. The large wind turbine meets 63% of the islands energy requirements across the year, compared to the three Vestas turbines meeting 45% in scenario 1. The large 1.5MW PV installation helps make up the demand in summer months when wind output drops, contributing to 21.4% of the islands total energy produced. As the biogas is used for back-up in this scenario it only makes up 1.1%.
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