Tidal Stream Power Research Articles

Exploiting the temporal characteristics of tidal stream power for green ammonia production

Green ammonia, a promising zero-carbon energy storage vector, has never been produced using solely tidal stream energy despite its predictable characteristics. Combining tidal stream sites with a phase difference (tidal phasing) facilitates anticorrelation between these sites’ power profiles, enabling a more consistent power output. Green ammonia production benefits from a steady and predictable power input. The objective of this paper is to determine if nearby tidal sites have a large enough phase difference (anticorrelation) to reduce the cost of ammonia production. Moreover, producing green ammonia from underutilized or unutilized remote and grid-constrained tidal stream sites may enable viability of these sites. This paper is the first analysis of green ammonia production using solely tidal stream energy and the first analysis of tidal phasing for green fuel production. Two UK case studies are presented in i) the North Channel of the Irish Sea and ii) Orkney. There are tidal sites in each which have a phase difference (e.g. the Mull of Kintyre/Sound of Islay, and Graemsay/North Ronaldsay with correlation coefficients of 0.17 and −0.02 respectively), utilising this phase difference reduces the levelized cost of ammonia by 8% in the Irish Sea and by 12% in Orkney.

Performance Evaluation of Hydrostatic Transmission Systems in Tidal Energy Conversion

The growing need for renewable energy has brought oceancurrent energy’s promise as a consistent and ecologically benign source front stage. One can effectively harness this energy with tidal energy conversion systems (TECS). This work examined the performance of a hydrostatic transmission (HST) system in a prototype tidal stream power producing configuration. Comprising a turbine simulator, hydraulic transmission, and permanent magnet synchronous generator (PMSG) with torque, power, and speed sensors, the experimental setup Under different torque settings, tests were performed to assess if the system could separate submerged components from those above sea level and improve rotational speed. According to the findings, the HST system has rather poor efficiency, averaging about 22%, even as it delivers the intended speed gain. Efficiency loss results from elements including hydraulic fluid viscosity, internal wear, and shaft alignment. These results highlight the need of more study to maximize HST systems for improved dependability and performance in tidal energy uses.

Enhancing energy system resilience using tidal stream energy

During autumn 2021, high global gas demand led to a 400% increase in imported wholesale gas prices in the UK. This coincided with (i) an extended period of low wind resource, where wind turbines provided 60% less energy than typical levels for the time of year, (ii) low nuclear power availability and (iii) depleting domestic natural gas reserves. These simultaneous events led to UK wholesale electricity prices more than doubling. The 2022 military and political impact of Russia’s invasion of Ukraine has led to additional increases in imported fuel prices. These types of disruption have the potential to cause damage worth years of sector revenues [1]. In the future, energy resilience challenges must be overcome whilst also achieving net-zero to limit global warming to within 1.5 degrees Celsius above pre-industrial levels, whilst electricity demand at least doubles.
 This research investigates the role of tidal stream energy in enhancing energy system resilience during periods when the energy system is highly stressed, such as during autumn 2021 in the UK. The concept of resilience refers to the ability of the system to survive strong and unexpected disruptions and to recover quickly afterward [1]. This research is motivated by the acknowledgement that the commercial viability of marine power relies on the sector being able to explain how it can provide contributions beyond decarbonisation [2].
 We build on an energy system modelling case study of the Isle of Wight [3], using the EnerSyM-RC energy system model. Wind data between 2012 – 2020 is analysed to identify and characterise periods of low wind resource. These data form the inputs to the EnerySyM-RC modelling. The model then applies a simple brute force optimisation method to investigate the most suitable mix of solar PV, offshore wind, tidal stream and energy storage that enhances system resilience on the Isle of Wight.
 Initial results show that the adoption of tidal stream energy compliments solar and wind generation by reducing reliance on reserve energy (e.g. imported gas) during high-stress periods. Reduced reliance on imported energy is also achieved because of the distinct generation pattern of tidal power (i.e. 4 periods of power generation, each separated by slack tide, every day), which enhances the utilisation of local energy storage.
 References
 [1] Jasiunas J, et al., 2021, Energy system resilience – A review, Renewable and Sustainable Energy Reviews, 150:111476
 [2] Pacific Northwest National Laboratory, 2021, Grid Value Proposition of Marine Energy: A Preliminary Analysis, PNNL -31123, Technical report
 [3] Coles DS et al., 2023, Impacts of tidal stream power on energy system security: An Isle of Wight case study, Applied Energy, 334:120686

Impacts of tidal stream power on energy system security: An Isle of Wight case study

The new Energy System Model for Remote Communities (EnerSyM-RC) is implemented to quantify impacts from adopting tidal stream power alongside solar PV, offshore wind and energy storage in the Isle of Wight energy system. Based on scenarios with gross renewable energy generation matched to projected annual demand (equivalent to 136 MW mean power), installing 150 MW of solar PV, 150 MW of offshore wind, and 120 MW of tidal stream capacity enhances supply–demand balancing whilst also reducing the magnitude of maximum power surplus, both by 25% relative to the best performing solar+wind system. Tidal stream adoption also reduces total land/sea space by 33%. The economic viability of tidal stream capacity adoption is heavily dependent on the price of reserve energy; when the reserve energy price exceeds the average 2022 forward delivery contracts price (250 £/MWh), adopting tidal stream capacity reduces the levelised cost of whole-system energy relative to solar+wind systems. This tipping point, at which the whole-system levelised cost of energy is 92 £/MWh, occurs when the premium on tidal stream energy is outweighed by savings on reserve energy. In general these system benefits arising from tidal stream adoption are consistent over a range of different demand profiles, and in cases where gross annual renewable supply is oversized relative to demand.

Spatial and temporal variations of the flow characteristics at a tidal stream power site: A high-resolution numerical study

The upcoming commercial development of tidal stream power requires a profound knowledge of tidal streams. In this article, the spatial and temporal variations of the flow characteristics are investigated through numerical simulations, at fine spatial and temporal resolutions. The study highlights the necessity of local site characterisation prior to turbine installation, due to the high spatial variability of the flow characteristics over distances of less than 100 m. The high variability in space and time of the flow characteristics at the scale of a turbine and its impact on the blade angle of attack underscore the relevance of a good blade pitch control strategy. The study of extreme events in term of maximal flow velocity and variations in the flow direction gives insight for the structural dimensioning of future tidal stream power devices.

要素移動モデルを用いた潮流発電用相反転方式プロペラの流体解析

Tidal stream power generation as renewable energy is one of the energy-source candidates for realizing sustainable society. Counter-rotating-type propellers are utilized in the conversion of tidal stream kinetic energy to electric energy. In this type of propeller, the rotational torques of the front and rear propeller are counter-balanced, thus they have no reaction force. This feature enables to moor a power unit with a cable from a floating or sea-bed base. The posture and performance of a moored power unit is easily affected by tidal stream condition. To evaluate precisely the performance in such power unit, unsteady flow simulation using a propeller rotating model is studied. As a first step, the flow simulations were carried out under the condition that the power unit axis was aligned with the flow direction. Moving element model was employed as a propeller rotating model. At first, the grid convergence in the simulations was confirmed. Second, the rotational speeds of the front and rear propeller under the torque equilibrium were examined. Finally, the power coefficient was evaluated at various rotational speeds. The output ratios were calculated from the obtained power coefficients and were compared with Lee’s wind tunnel test results. The numerical output ratios almost agreed with the experimental ones.

Dispatchability, Energy Security, and Reduced Capital Cost in Tidal-Wind and Tidal-Solar Energy Farms

The global tidal energy resource for electricity generation is small, and converting tidal kinetic energy to electricity is expensive compared to solar-photovoltaic or land-based wind turbine generators. However, as the renewable energy content in electricity supplies grows, the need to stabilise these supplies increases. This paper describes tidal energy’s potential to reduce intermittency and variability in electricity supplied from solar and wind power farms while lowering the capital expenditure needed to improve dispatchability. The paper provides a model and hypothetical case studies to demonstrate how sharing energy storage between tidal stream power generators and wind or solar power generators can mitigate the level, frequency, and duration of power loss from wind or solar PV farms. The improvements in dispatchability use tidal energy’s innate regularity and take account of tidal asymmetry and extended duration low-velocity neap tides. The case studies are based on a national assessment of Australian tidal energy resources carried out from 2018 to 2021.

A review of the UK and British Channel Islands practical tidal stream energy resource.

This review provides a critical, multi-faceted assessment of the practical contribution tidal stream energy can make to the UK and British Channel Islands future energy mix. Evidence is presented that broadly supports the latest national-scale practical resource estimate, of 34 TWh/year, equivalent to 11% of the UK’s current annual electricity demand. The size of the practical resource depends in part on the economic competitiveness of projects. In the UK, 124 MW of prospective tidal stream capacity is currently eligible to bid for subsidy support (MeyGen 1C, 80 MW; PTEC, 30 MW; and Morlais, 14 MW). It is estimated that the installation of this 124 MW would serve to drive down the levelized cost of energy (LCoE), through learning, from its current level of around to below , based on a mid-range technology learning rate of 17%. Doing so would make tidal stream cost competitive with technologies such as combined cycle gas turbines, biomass and anaerobic digestion. Installing this 124 MW by 2031 would put tidal stream on a trajectory to install the estimated 11.5 GW needed to generate 34 TWh/year by 2050. The cyclic, predictable nature of tidal stream power shows potential to provide additional, whole-system cost benefits. These include reductions in balancing expenditure that are not considered in conventional LCoE estimates. The practical resource is also dependent on environmental constraints. To date, no collisions between animals and turbines have been detected, and only small changes in habitat have been measured. The impacts of large arrays on stratification and predator–prey interaction are projected to be an order of magnitude less than those from climate change, highlighting opportunities for risk retirement. Ongoing field measurements will be important as arrays scale up, given the uncertainty in some environmental and ecological impact models. Based on the findings presented in this review, we recommend that an updated national-scale practical resource study is undertaken that implements high-fidelity, site-specific modelling, with improved model validation from the wide range of field measurements that are now available from the major sites. Quantifying the sensitivity of the practical resource to constraints will be important to establish opportunities for constraint retirement. Quantification of whole-system benefits is necessary to fully understand the value of tidal stream in the energy system.

Resource Assessment of Tidal Stream Power in Pakiputan Strait, Davao Gulf, Philippines

During the last years, there are ongoing efforts on the development of tidal energy conversion systems in the Philippines. This study conducts tidal energy resource assessment in the Pakiputan Strait following a methodology outlined as stage 2a tidal resource assessment published by the European Marine Energy Centre (EMEC). The study assessed the preliminary results of the tidal velocities at Pakiputan Strait with a mean spring peak velocity (Vmsp) of 1.7m/s at 3m from the water surface from 15 days of continuous data collection using a seabed-mounted acoustic Doppler current profiler. This corresponded to an estimated Annual Energy Production (AEP) of 1350kWh/y for 1m2 of capture area of the generic device. Sensitivity analysis showed that the spatial distribution of hydrodynamic model results does not vary significantly with variations in certain input parameters. It further showed that a 10% decrease in the nominal value of Vmsp on-site led to a 15% decrease in the nominal value of AEP, while a 10% increase in the nominal value of Vmsp led to a 30% increase in the nominal value of AEP, assuming that the considered Vmsp still corresponded to the velocity distribution from observations. A static survey and the use of computational fluid dynamics modeling are recommended to further enhance the analysis of the study.

A standardised tidal-stream power curve, optimised for the global resource

Tidal-stream energy resource can be predicted deterministically, provided tidal harmonics and turbine-device characteristics are known. Many turbine designs exist, all having different characteristics (e.g. rated speed), which creates uncertainty in resource assessment or renewable energy system-design decision-making. A standardised normalised tidal-stream power-density curve was parameterised with data from 14 operational horizontal-axis turbines (e.g. mean cut-in speed was ∼30% of rated speed). Applying FES2014 global tidal data (1/16° gridded resolution) up to 25 km from the coast, allowed optimal turbine rated speed assessment. Maximum yield was found for turbine rated speed ∼97% of maximum current speed (maxU) using the 4 largest tidal constituents (M2, S2, K1 and O1) and ∼87% maxU for a “high yield” scenario (highest Capacity Factor in top 5% of yield cases); with little spatial variability found for either. Optimisation for firm power (highest Capacity Factor with power gaps less than 2 h), which is important for problematic or expensive energy-storage cases (e.g. off-grid), turbine rated speed of ∼56% maxU was found – but with spatial variability due to tidal form and maximum current speed. We find optimisation and convergent design is possible, and our standardised power curve should help future research in resource and environmental impact assessment.

Hydrodynamic Effects of Tidal-Stream Power Extraction for Varying Turbine Operating Conditions

Realistic evaluation of tidal-stream power extraction effects on local hydrodynamics requires the inclusion of the turbine’s operating conditions (TOC). An alternative approach for simulating the turbine’s array energy capture at a regional scale, momentum sink-TOC, is used to assess the impact of power extraction. The method computes a non-constant thrust force calculated based on the turbine’s operating conditions, and it uses the wake induction factor and blockage ratio to characterise the performance of a turbine. Additionally, the momentum sink-TOC relates the changes produced by power extraction, on the velocity and sea surface within the turbine’s near-field extension, to the turbine’s thrust force. The method was implemented in two hydrodynamic models that solved gradually varying flows (GVF) and rapidly varying flows (RVF). The local hydrodynamic effects produced by tidal-stream power extraction for varying the turbine’s operating conditions was investigated in (i) the thrust and power coefficient calculation, (ii) flow rate reduction, and (iii) tidal currents’ velocity and elevation profiles. Finally, for a turbine array that operates at optimal conditions, the potential energy resource was assessed. The maximisation of power extraction for electrical generation requires the use of an optimum turbine wake induction factor and an adequate blockage ratio, so that the power loss due to turbine wake mixing is reduced. On the other hand, the situations where limiting values of these parameters are used should be avoided as they lead to negligible power available. In terms of hydrodynamical models, an RVF solver provided a more accurate evaluation of the turbine’s operating conditions effect on local hydrodynamics. Particularly satisfactory results were obtained for a partial-fence. In the case of a fence configuration, the GVF solver was found to be a computationally economical tool to pre-assess the resource; however, caution should be taken as the solver did not accurately approximate the velocity decrease produced by energy extraction.

Tidal-stream power assessment—A novel modelling approach

An alternative approach for simulating turbine array energy capture, momentum sink-TOC, was developed to improve conventional methodologies for assessing tidal-stream energy resource. The method uses a non-constant thrust force coefficient calculated based on turbines operating-conditions and relates turbine near-field changes produced by power extraction to turbine thrust forces. Momentum sink-TOC was implemented in two depth-average complex hydrodynamic models to simulate an ideal turbine lay-out to perform tidal-stream energy resource assessment. The first model solves smooth and slow flows (SSF). The second model solves rapidly varying flows (RVF). Calculation of head drops across the turbine arrays and turbine efficiencies enabled estimation of further power metrics. Tidal-stream energy resource evaluation with a tidal fence indicates that a computationally economical pre-assessment can be adequately performed using an SSF solver. However, caution should be taken when using SSF solver due to the incapacity of the model to accurately solve velocity reduction due to power extraction.

The effect of bed roughness uncertainty on tidal stream power estimates for the Pentland Firth.

Uncertainty affects estimates of the power potential of tidal currents, resulting in large ranges in values reported for sites such as the Pentland Firth, UK. Kreitmair et al. (2019, R. Soc. open sci. 6, 180941. (doi:10.1098/rsos.191127)) have examined the effect of uncertainty in bottom friction on tidal power estimates by considering idealized theoretical models. The present paper considers the role of bottom friction uncertainty in a realistic numerical model of the Pentland Firth spanned by different fence configurations. We find that uncertainty in removable power estimates resulting from bed roughness uncertainty depends on the case considered, with relative uncertainty between 2% (for a fully spanned channel with small values of mean roughness and input uncertainty) and 44% (for an asymmetrically confined channel with high values of bed roughness and input uncertainty). Relative uncertainty in power estimates is generally smaller than (input) relative uncertainty in bottom friction by a factor of between 0.2 and 0.7, except for low turbine deployments and very high mean values of friction. This paper makes a start at quantifying uncertainty in tidal stream power estimates, and motivates further work for proper characterization of the resource, accounting for uncertainty inherent in resource modelling.

Posture Stabilization of Floating Counter-rotating Propeller Type Tidal Stream Power Unit

Posture Stabilization of Floating Counter-rotating Propeller Type Tidal Stream Power Unit

Preliminary Assessment of Tidal Stream Power Using 3-Dimensional Hydrodynamic Model in the Lembeh Strait

This paper provides a preliminary assessment of potential energy of tidal currents in the Lembeh Strait, North Sulawesi. Using two and three dimensional ocean hydrodynamic model, the assessment was done. A nesting method has been applied in order to simulate tidal currents in the Sulawesi Sea and the Lembeh Straits. Three dimensional simulation results at the Lembeh straits was validated using observation data, which are obtained from filed measurement campaign on 3 to 23 April 2016. Estimation of tidal current or tidal stream power was focused in the Lembeh Straits. The simulation results show power density during a tidal cycle can reach maximum value about 72.346,23 W/m2 at full moon condition. This amount of power is stored in the area between Sarena Island and Sulawesi Island. The result of tidal stream power estimation also reveals strong tidal currents that associated with maximum power occur during flood tide condition.

Acoustic noise measurement in counter-rotating propellers

The acoustic noise and flow condition of tandem propellers in a horizontal axis-type tidal stream power unit, which is composed of counter-rotating propellers and double-rotational armature-type generator without traditional stators, were experimentally investigated. The front and rear propellers rotate in opposite directions and drive the inner and outer armatures, respectively; the rotational torque of the front propeller coincides with the torque of the rear propeller. The front blade profile exerts a great effect on noise generation from the tandem propellers, but the effect of the rear blade profile is comparatively small. The sound pressure level is largely concentrated at low frequencies, and most predominant frequencies can be predicted by Hanson’s method. These predominant frequencies are mainly caused by the tip vortex shedding from the front and rear blades in the counter-rotating propellers. The vortex shedding from the front blade does not affect the rear blade. As a result, the acoustic noise is weakened as long as the diameter of the rear propeller is smaller than the diameter of the front propeller.

Profile: Laura Legnani

Laura Legnani was told at the start of her career not to worry about what she didn’t know, but to go and find out. From an airport roof, to tidal stream power, to residential towers, it is advice that has underpinned her approach to her work, she tells Jackie Whitelaw.

A numerical study on performance improvement of counter-rotating type tidal stream power unit

One of the forms of ocean energy, tidal stream energy is attracting attention as a sustainable, predictable and eco-friendly ocean energy source. Unique tandem propellers that can counter-rotate have been designed to generate electric power effectively from a tidal stream. This type of power unit has several advantages compared to the conventional unit with a single propeller and the increase in efficiency of counter-rotating type tidal stream power unit has already been proven. The counter-rotating type tidal stream power unit has a complex interrelationship between the front and rear propeller contrary to the conventional single propeller type tidal turbine. Such as, the no-load area on the front propeller, the diameter ratio of front and rear propeller and the rotational speed of each propeller, etc.. And the inlet condition of the rear propeller is very important to improve the efficiency. Thus, we have to consider these parameters to optimize tidal stream power unit. In this research, the numerical optimization algorithm with response surface method and adaptive single-objective optimization method are applied to obtain desirable propeller performance.

Performance of counter-rotating tandem propellers at oblique flow conditions

Tidal currents are a potential source of reliable and renewable energy. Various power generation systems have been proposed. The authors developed a counter-rotating type tidal stream power unit in which the tandem runners make the inner and the outer armatures counter-rotate. The unit is composed of tandem propellers with 3 front and 5 rear blades and has a promising advantage that the rotational torque between the front runner and the rear propellers, namely the inner and the outer armatures, is balanced in the unit. That is, this tidal stream power unit can be moored with only one cable and then its nose and tail will naturally line up in response to the tidal stream by yawing. In this paper, the performance of the propellers in oblique flow conditions were investigated experimentally and numerically. When the coming flow obliquely to the propeller by 30 degrees, the performance deteriorates by 20%.

Counter-rotating type tidal stream power unit: Excellent performance verified at offshore test

A counter-rotating type tidal stream power unit, composed of tandem propellers and a peculiar generator with double rotatable armatures, was provided for verification tests at offshore. The front propeller with diameter of 1 m has three blades, and the rear propeller with diameter of 0.95 m has five blades. The blade profiles were optimized with the blade element-momentum theory and CFD, and then covered with CFRP. The front and the rear propellers connect to inner and outer armatures in a synchronous type generator with net output 1.5 kW, whose efficiency was improved by heat pipes and the modification of the armature profiles. The shafts are equipped with mechanical seals to protect electric circuits from seawater. A rotational speed control system was also prepared not only to adjust the output but also to overcome the static friction torque due to bearings, slip rings and mechanical seals while starting-up. The power unit takes the maximum output with excellent efficiency at the relative tip speed ratio specified in the unit design. The unit can start-up after maintenance works and the output can be adjusted smoothly in the stream.