The UK has a significant tidal energy resource and according to studies tidal stream power can provide 1/5 of current UK electricity demand. The tidal stream energy resource in the UK waters is estimated to be 95TWh/year. However, only 18TWh/year of the tidal stream energy has been assessed as being economically recoverable with today’s technologies. At present, single devices are being tested and the industry is preparing for GWh production and multi-device arrays. Additionally, larger projects have already been approved such as the 10MW Sound of Islay Tidal Array project for which utility developer ScottishPower Renewables (SPR) has obtained consent and Andritz Hydro Hammerfest has been assigned as the device supplier.
Even though tidal current turbines are placed offshore and have similarities to offshore wind turbines in many aspects, a number of characteristics differentiate the approach needed regarding energy transmission and drivetrain design. Some of these characteristics are: predictable direction and speed of the tidal current, predetermined available area in a tidal channel, less swept area due to water high density, continuous underwater operation and smaller distances to shore.
To be more specific, the continuous underwater operation requires that the tidal system must work reliably with high availability and that onsite visits are reduced to a minimum since tidal devices are usually installed at locations with high tidal currents. In these locations, the windows of opportunity for onsite visits are relatively short (often less than one hour), which means that major operations need to be extremely quick or be able to continue in high tidal flows. Another aspect of tidal arrays that affect energy transmission is that they will be close enough to shore and so offshore substations and high-voltage subsea transmission can be avoided. Therefore, less equipment is offshore and consequently onsite visits can be reduced. In addition, it has been suggested that tidal energy developers can extend the availability of their systems by moving the power electronics from the nacelle to the shore. This could reduce onsite visits since the failure frequency of power converters can range from 0.15 to 0.25 failures per year per turbine based on data from onshore wind turbines. Locating the power electronics on land means that the generator has to be controlled using long subsea cables and therefore long distance converters are needed similar to the drives for the electrical submersible pumps (ESPs) used in oil offshore platforms.
The research in this project will focus on modelling full resource to wire dynamic models of tidal arrays in order to investigate and optimise their operation. For this reason realistic loss modelling is required, different generator technologies will be compared, site specific parameters such as flow reduction in the tidal channel will be added, different electrical layouts will be tested and harmonics will be investigated. In addition, Andritz Hydro Hammerfest will contribute to the project with industrial experience and data for verification. Cost models of each topology will be developed in the final stages of this research in order to investigate the cost-effective solutions. The aim of the study will not only target the development of mathematical modelling but will also provide the industry with guidelines and an understanding of the applicability of the different electrical layouts to specific locations and size of the array.