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ABSTRACT: The UK has ambitious plans to harness its available tidal stream resource, estimated at 95 TWh/year by The Crown Estate (2013). The economic viability of large-scale deployments will be largely governed by aspects of plant availability, including reliability. Using available information on environmental parameters of (pre-) consented sites across the UK, this paper explores subassembly target reliability levels for tidal stream devices. Reliability models of devices are investigated to establish the influence of environmental site conditions with regard to underlying subassembly failure rates and target reliability levels. Hence, a reliability-focussed perspective on the planned deployments is presented.
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1INTRODUCTION
With the potential to supply a quarter of UK's electricity demands, tidal stream technology is a growing sector (TCE, 2012). The world's first tidal stream array is under construction at Pentland Firth, Scotland placing the UK at the forefront of the industry. However, to allow the industry to be commercially viable, a levelled cost of energy (LCoE) of 10-20 p/kWh by 2020 and 5-8 p/kW by 2050 (Energy Technologies Institute, 2015) is envisaged as an industrial target. A significant contributor to LCoE is annual energy production which is highly dependant on the available resource at site and device performance metrics of reliability and availability (SIO, 2014). Numerous studies by the Energy Technologies Institute (ETI, 2015), Strategic Initiative for Ocean Energy (SIO, 2014) and CATAPULT (2015) discuss the significance of device availability levels to improve the momentum of the industry growth. This paper explores how differences in environmental site conditions and associated target reliability affect the expected availability levels.
1.1Reliability of tidal energy
According to ISO8402 (ISO, 1986), reliability is 'the ability of an item to perform a required function, under given environmental and operational conditions and for a stated period of time.' A Tidal Stream Device (TSD) continues to perform its function, i.e. generate electricity, until a 'failure' occurs. IEC 50 standards (IEC, 1990) define a failure as 'an event when a required function is terminated.' An important statistical measure required for reliability assessment is the failure rate. It is calculated as the ratio of the number of failures to the total observed time.
There is a lack of publicly available, field failure data for reliability calculations since the industry...