Optimising the Operation of Tidal Range Schemes

energies Article Optimising the Operation of Tidal Range Schemes Jingjing Xue , Reza Ahmadian * and Roger A. Falconer Cardiff School of Engineering, Cardiff University, Cardiff CF24 3AA, UK * Correspondence: [email protected] Received: 3 July 2019; Accepted: 23 July 2019; Published: 25 July 2019 Abstract: Marine renewable energy, including tidal renewable energy, is one of the less exploited sources of energy that could contribute to energy demand, while reducing greenhouse gas emissions. Amongst several proposals to build tidal range structure (TRS), a tidal lagoon has been proposed for construction in Swansea Bay, in the South West of the UK, but this scheme was recently rejected by the UK government due to the high electricity costs. This decision makes the optimisation of such schemes more important for the future. This study proposes various novel approaches by breaking the operation into small components to optimise the operation of TRS using a widely used 0-D modelling methodology. The approach results in a minimum 10% increase in energy output, without the inclusion of pumping, in comparison to the maximum energy output using a similar operation for all tides. This increase in energy will be approximately 25% more when pumping is included. The optimised operation schemes are used to simulate the lagoon operation using a 2-D model and the differences between the results are highlighted. Keywords: tidal range structure; tidal lagoons; marine renewable energy; optimisation of operation schemes; pumping 1. Introduction With improvements in environmental awareness globally, emission levels of CO2 are expected to decrease by reducing reliance on fossil fuels, and further development in renewable energy. The UK government aims to produce 15% of its total energy from renewable resources by 2020, which corresponds to approximately 35% of the UK’s electricity demand [1–3]. Marine renewable energy (MRE) is one of the emerging renewable energies being explored further. Currently, 0.5 GWh of commercial marine energy generation capacity is in operation and another 1.7 GWh is under construction, with most of this accounted for tidal range [4]. It has been suggested that the tidal range resources of the UK is able to deliver 25 GW theoretically [5]. A number of TRS have been proposed around the UK, particularly in the Severn Estuary and Bristol Channel, with these estuaries being located in the South West of the UK, as shown in Figure1. Swansea Bay Lagoon is one of these projects, which has been one of the world’s first tidal lagoon power plants [6]. It was granted planning permission by the UK Department of Energy and Climate Change (DECC) in June 2015 [7] and was positively supported by the independent Hendry Review of tidal lagoons, commissioned by the UK government and published in January 2017 [8]. However, the cost of electricity has been found to be an issue [9] and the UK government Business and Energy Secretary Greg Clark said that the £1.3bn project was not good value for money, despite claims to the contrary by the developers Tidal Lagoon Power [10]. There have been different values reported for the cost of electricity. The general figure quoted for the levelised cost of electricity (LCOE) is reported to be £150/MWh [11], while the consumer cost over the lifetime of the project is reported to be £25.78/MWh [12]. This reemphasises the need for further optimisation of tidal range structures to enable such projects to produce competitive energy costs. Energies 2019, 12, 2870; doi:10.3390/en12152870 www.mdpi.com/journal/energies Energies 2019, 12, 2870 2 of 23 Energies 2019, 12, x FOR PEER REVIEW 2 of 23 Figure 1. Lagoons proposals of tidal range structure ( (TRS)TRS) in the Severn Estuary and Bristol Channel (from GoogleGoogle Map).Map). TidalTidal range structure ( (TRS)TRS) creates creates an an artificial artificial head difference difference across the scheme and generatesgenerates energy usingusing thisthis head head di difference.fference. The The schemes schemes could could be be operated operated to generateto generate energy energy during during flood flood tide (floodtide (flood generation), generation), ebb tide ebb (ebb tide generation), (ebb generation) and on, bothand ebbon andboth flood ebb tidesand (two-wayflood tides generation). (two-way Onegeneration). of the key One aspects of the ofkey the aspects operation of the of operation such schemes of such is schemes the head is di thefference head difference at the time at when the time the schemewhen the would scheme be programmed would be programmed to start generating to start energy generating and when energy generation and when stops generation (further details stops on(further the operation details on of the TRS operation is provided of TRS in Sectionis provided2). Therefore, in Section TRS 2). Therefore can be operated, TRS can in be various operated ways in forvarious each ways type offor operation each type scheme, of operation i.e., for scheme, flood, ebb,i.e., for or two-wayflood, ebb generation,, or two-way and generation, this operation and willthis influenceoperation thewill basin influence water the level basin and water discharge level transferred and discharge between transferred the impoundment between the and impoundment open waters, and thereforeopen wate affrs,ecting and thetherefore energy affecting generated. the 0-D energy models generated. have been 0-D widely models used have in designingbeen widely TRS used and particularlyin designing in TRS initially and optimisingparticularly the in operationinitially optimising of such schemes the operation [13–16]. of Fundamental such schemes theoretical [13-16]. researchFundamental was carriedtheoretical out inresearch this field was by carried Prandle out in in the this 1980s field [17 by]. EightPrandle dimensionless in the 1980s parameters[17]. Eight weredimensionless defined to parameters provide a formulationwere defined for to theprovide design a formulation characteristics for and the energydesign calculations,characteristics using and fourenergy key calculations, assumptions using that afourffect key the energyassumptions predictions. that affect For example,the energy the predictions. energy generation For example, starts andthe stopsenergy at generation the same prescribed starts and minimum stops at the head, same i.e., prescribedηmin, which minimum can affect head, the energy i.e., ηmin output, which significantly. can affect Aggidisthe energy and output Benzon significantly. used a 0-D modelAggidis to evaluateand Benzon the energyused a (electricity)0-D model generationto evaluate in the relation energy to varying(electricity) trends generation in energy in demand relation [ 13to ].varying They optimised trends in drivingenergy headsdemand based [13] on. They the sizeoptimised and number driving of turbines,heads based which on variesthe size with and the number barrage andof turbines,/or lagoon which dimensions varies andwith characteristics. the barrage and/or These studieslagoon havedimensions demonstrated and characteristics. that the 0-D approach These studies has significant have demonstrated preliminary design that potentialthe 0-D forapproach all types has of tidalsignificant impoundments. preliminary design potential for all types of tidal impoundments. However, Angeloudis et et al. al. showed showed that that 0- 0-DD models models can can overestimate overestimate the the energy energy predictions predictions by byup upto 40% to 40% when when compared compared to toprediction prediction based based on on more more sophisticated sophisticated and and accurate accurate 2 2-D-D numerical models [18, [18, 1919]].. TheyThey concludedconcluded thatthat thethe 0-D0-D approach overestimation is relative to the size of the scheme. TheyThey suggested suggested that that 0-D 0 predictions-D predictions are only are reliable only reliable for design for optimisation design optimisation at the preliminary at the stagepreliminary and need stage to beand complemented need to be complemented by more sophisticated by more sophisticated 2-D models. It2- shouldD models. be noted It should that allbe threenoted studiesthat all havethree used studies constant have used driving constant and minimum driving and generation minimum heads generation throughout heads the throughout operation, i.e.,the operation, for all spring i.e. and, for neapall spring and flood and andneap ebb and tides. flood Based and ebb on thetides. authors’ Based extensive on the authors’ literature extensive review, Ahmadianliterature review, et al. [ 15Ahmadian] and Yates et al. et al.[15] [14 and] have Yates separately et al. [14] discussed have separately the concept discussed of variable the concept driving of andvariable minimum driving generation and minimum heads generation for each operation heads for period, each operation namely halfperiod, a tide, namely and highlightedhalf a tide, and the highlighted the potential improvements achievable by implementing variable driving and minimum Energies 2019, 12, 2870 3 of 23 potential improvements achievable by implementing variable driving and minimum generation heads. More recently, Angeloudis et al. also took advantage of a gradient-based method for the optimisation of flexible operation heads [16]. Research on using pumping to increase energy generation has been limited and was mainly carried out using a constant driving and minimum generation head throughout. Yates et al. used an unlimited pumping head and constant generating head to study the influence of turbining and pumping efficiencies

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