Life cycle assessment of an offshore electricity grid interconnecting Northern Europe
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There is a growing demand for increased electricity transfer capacities between the countries surrounding the North Sea. The increased capacities will enable easier integration of intermittent renewable energy sources, decrease the need for balancing power, increase power trade and competition, and increase security of supply across the region. Interregional offshore grid connections are required if large scale deployment of deep sea, far from shore offshore wind energy in the North Sea is to take place. The WINDSPEED research project has resulted in proposals of realistic scenarios for large scale deployment of offshore grid and wind energy in the North Sea. In this study the environmental impacts of an interregional meshed offshore grid as proposed by WINDSPEED have been assessed. Environmental impacts of the offshore wind farms, which may be connected to the grid, have been included in the assessment as well, completing the system boundaries.The methods used to quantify the environmental impacts are process-based life cycle assessment (LCA), input-output assessment (IOA) and tiered hybrid LCA, with main focus on the results of the latter. Four offshore grid scenarios have been assessed, with and without offshore wind farms connected. The offshore grid is primarily composed of 450 kV HVDC technology for long distance transmission, based on the HVDC cables used in the NorNed connection. Wind farms are deployed far from shore (requiring much sea transport and long distance grid connections) and at an average of 43.9 meters depth (requiring large bottom-mounted foundations for the wind turbines). These requirements make the environmental impacts of deep sea, far from shore offshore wind energy substantially higher than for both close to shore offshore wind energy and onshore wind energy.The environmental assessment of the interregional meshed offshore grid found that the largest contribution to environmental impacts is from manufacturing and installation of HVDC cables. Sea transport required for installation of components and operation and maintenance contributes between 5 and 25 percent to most impact categories. The electrical equipment (converters, breakers and switchgear) required by the grid has a quite varying contribution, from almost none to some impact categories to about 35 percent to climate change impact. The environmental assessment of the deep sea, far from shore offshore wind energy, finds that the largest contributors to environmental impacts are the wind turbines. But the other components required deep sea foundations, offshore grid and sea transport for installation, operation and maintenance makes the environmental impacts caused by it around twice as high as for onshore wind energy installations. Total climate change impacts were found to be 42.9 g CO2-Eq/kWh; the grid is responsible for 11, foundations 31 and sea transport 9 percent of that. The largest impacts of deep sea, far from shore offshore wind energy as compared to other relevant energy sources are to the impact categories freshwater ecotoxicity, human toxicity and metal depletion. The impacts to these categories are many times larger, up to almost 20 times, compared to other relevant fossil fueled energy sources. The impacts to the other impact categories are substantially lower.The results indicate that the environmental impacts caused by an interregional meshed offshore grid in the North Sea are substantial; it needs to be considered an important part of an environmental assessment of deep sea, far from shore offshore wind energy. On the other hand, the environmental costs are probably not so high that they outweigh the potential benefits of such offshore grid connections. It may in fact lead to net environmental gains because of a decreased demand for fossil balance power. As for large scale deployment of deep sea, far from shore offshore wind energy the environmental benefits as opposed to relevant fossil alternatives are obvious, but, including the significant disadvantages of intermittent energy supply and high monetary costs, overall gain to society is harder to predict.