Power Take-Off System for a Subsea Tidal Kite - LCA report

Rapport, 2018

Executive summary

The marine renewable energy technology company Minesto has developed and patented the Deep Green ocean energy power plant, where power is generated by a turbine that is attached to a wing moved like a kite by the water current. It can operate at ocean currents less than 2.5 m/s, which adds a new ocean energy potential to the market (Minesto, 2018a). The PowerKite project received funding from the European Union’s Horizon 2020 research and innovation programme and was launched to enhance the structural and power performance of the power take-off (PTO) of Deep Green.

The environmental impacts of the technology are also assessed by the PowerKite project (WP 6). This Life Cycle Assessment (LCA), was carried out at the Environmental System Analysis division at Chalmers University of Technology, Gothenburg, Sweden, with Minesto as the main data provider. LCA is a well-established tool to assess a range of environmental impacts of a technical system (Baumann & Tillman, 2004). This initial LCA is intended to provide first indications of the environmental performance of the Deep Green Utility (DGU) tidal current power plant. This study is designed so results may guide and influence the design of hardware and operational procedures of the power plant as well as provide a benchmark compared to other electricity power generation technologies. Deliverables of the PowerKite project use a prototype design as starting-point and some of the conclusions will therefore not fully represent the potential of the Deep Green technology.

At the time of writing, the first DGU power plant is being installed in Holyhead Deep in the waters west of Holyhead, Wales. A prospective model has been assessed based on the initial plans for the Holyhead site with an array of 24 kites, four tidal marine substation (TMS), PTO to grid cables, and an onshore workshop and grid substation. The generator rated capacity of the kites assessed is 500 kW, with assumed power productions ranging from 1 to 2 GWh/year per kite, corresponding to a total installed capacity of 12 MW and a capacity factor between 23% and 46%. Since the output might increase depending on the location (installing in a site with higher flows), an additional scenario reflecting a more favourable tidal site has been assessed with 18 kites with a rated capacity of 750 kW and a 3 GWh/yr average power output, corresponding to a total installed capacity of 13.5 MW and a 46% capacity factor. In a continuous ocean current, Deep Green can operate at a capacity factor in the range of 70-95%. Downtime is then only due to maintenance, not tidal cycles.

This cradle to grave LCA of the DGU power plant includes material resource extraction, processing, component manufacturing, power plant construction, operation, electricity distribution, maintenance, dismantling, waste management with recycling, and transports. The function assessed is one (1) kilowatt hour electric energy (kWhe) delivered to the end consumer. All environmental impacts are calculated based on this functional unit.

The resulting impacts of the DGU power plant is in range with other renewable technologies in the impact categories land occupation, non-renewable energy demand, global warming potential (GWP), freshwater eutrophication, freshwater ecotoxicity, and particulate matter formation. Our results indicate that there are no major concerns in terms of impacts from the DGU in relation to the aforementioned categories. It is well known that fossil fuel combustion technologies in general have a substantially worse environmental performance than renewable energy technologies in these categories. (Nuclear power has not been compared). An exception is the impact on terrestrial land occupation where in some cases PV, hydropower, and land based wind power, are performing less well. Sea area occupation might be a more relevant issue to assess for the DGU power plant but it is not included in this study as it is still debated how to account for this.

The total GWP impact of the Holyhead site, including grid distribution losses and emissions, ranges between 22 to 50 g CO2 eq/kWhe, depending on different scenarios and uncertainties in the system. Assuming the same array in a continuous ocean current would result in 14 to 18 g CO2 eq/kWhe. For the favourable site scenario, the GWP impact is 20 g CO2 eq/kWhe. These results indicate that DGU power plant emissions are in the same range as other ocean energy technologies, with reported ranges for off-shore wind power from 15 to 105 g CO2 eq/kWh (Uihlein, 2016) or 11 to 20 g CO2 eq/kWhe (Hertwich et al., 2014). Significant contributors to the GWP are the frequent replacements of the tether, emissions from offshore vessels used for construction and maintenance as well as the concrete and steel material production for the gravity base foundations.

Another important indicator that has been derived is the energy return on investment (EROI). It describes the relationship between energy generated and energy required throughout the life cycle of the plant. The energy required includes non-renewable and renewable sources as well as chemically bound energy in plastic materials made from fossil carbon resources. The power plant lifetime divided by the EROI yields the energy payback time. The estimate for EROI at the Holyhead site was found to be between 4.6 to 8.7, which can be compared with that of the wind power plants ranging between 6.1 to 33.5 (Kubiszewski, Cleveland, & Endres, 2010). This corresponds to an energy payback period of 3 to 6 years for the DGU power plant. The major contributor to this energy demand is the maintenance phase, especially the manufacturing of the tether replacement parts, and the diesel used in on-site ships during construction and maintenance.

When examining the contributions from individual processes it is evident that three main activities namely diesel combustion, steel production and utility electricity generation contribute significantly to a range of impact categories. The diesel combustion refers to fuel combustion mainly for construction and maintenance offshore vessel trips. Impact from steel production is directly connected to the amount of steel that is needed in components including replacement needs during maintenance. Emissions from utility electricity generation is mainly due to the use of fossil fuel technologies in the UK electricity mix, in this system mainly consumed by material production.

To improve the environmental performance of the DGU power plant system assessed in this LCA, the results points to that focus should be put foremost on a high capacity factor; less material-intensive kite foundation and mooring system; efficient offshore vessel utilisation during construction and maintenance, and possibilities of using alternatives to diesel fuel; lower material requirements, mainly steel, while not reducing component life-times; investigate possibilities to extend the lifetime of the tether and using recyclable materials; and strive for high recycling of steel and copper.

Since the Deep Green technology is still at a very early stage of development, improvements of its technical and environmental performance are expected. This LCA concludes that the environmental performance of the DGU power plant is in the same range as other renewable technologies. The environmental performance of DGU technology is likely to improve significantly with the development of the technology as, according to Arvesen and Hertwich (2012), there are strong economies of scale for wind turbines with power ratings up to 1 MW. Other possible gains from upscaling would be increasing the array (adding more kites), likely reducing common parts needed per kWhe, as well as more efficient component manufacturing from large scale implementation of the technology.