Regulating the output characteristics of tidal current power stations to facilitate better base load matching over the lunar cycle

doi:10.1016/j.renene.2005.08.024

Renewable Energy

Volume 31, Issue 2, February 2006, Pages 173-180

https://doi.org/10.1016/j.renene.2005.08.024 Get rights and content

Abstract

To meet rising targets for renewable-derived electricity generation, wind power is currently the preferred technology. However, it is widely accepted that due to the stochastic nature of wind, planning restrictions and the finite availability of suitable sites there is an upper limit to the capacity that can be accommodated within the electricity network before power quality is affected. This paper demonstrates the potential of tidal energy to provide firm power and shows that limiting the capacity of the power generated provides base load supply without compromising power quality. This increases the capacity factor of the installed system, thus improving the economic viability and commercial competitiveness of tidal farms.

Introduction

Recent policy developments in the UK have favoured the development of renewable technologies [1], [2]. Targets of 20 and 40% for the UK and Scotland, respectively, have been set for the generation of electricity from renewable sources by 2020. The present trend is to develop renewable technologies that are stochastic in nature, principally hydro and land based wind power initially, then offshore wind power in the medium to longer term; this implies that target attainment will probably result in increased levels of vulnerability within the electrical supply network. This, in turn, will necessitate increased levels of energy storage, reserve plant and network control to prevent supply disruption and maintain power quality.

To minimise the risks associated with this, it would clearly be helpful if predictable renewable energy sources could be developed. By arranging for tidal power generation at well-phased locations, a near-continuous base load power supply should be achievable, which will impact positively on the integrity of the electrical network.

Section snippets

Development of tidal technology

Technology for the exploitation of marine currents is still in its infancy. At the present time, two systems are being investigated for commercial development: the oscillating aerofoil driving hydraulic accumulators [3]; and horizontal axis turbines evolved from wind power technology, with two prototypes installed off the coasts of Norway and England [4]. Tidal current turbines operate in a more determinate environment than wind turbines. Maximum current velocities can be predicted with

Design options

It is impossible at this time to predict the optimal configuration of future tidal stream energy conversion devices. However, comparisons with wind energy give some useful pointers. Tidal stream devices are likely to have a cut-in stream velocity, with a period of enforced idleness at slack water. While wind turbines have a cut-out speed to avoid damage in storms, this should not be necessary for tidal turbines given the predictable nature of the flow regime. Shut-down procedures would only be

Power output characteristics

The instantaneous power, P, available to a single tidal stream turbine is given by $P = \frac{1}{2} ρ A V^{3}$ where ρ is the fluid density, A the rotor swept area and V the velocity of the fluid stream. If the variation of V with time is assumed to be sinusoidal, P will vary as shown in Fig. 1 (upper profile), which covers a typical tidal half-cycle of about 6 h 12 min.

Fig. 1 also shows the variation of turbine power output, C_pP (lower profile), for arbitrary cut-in and rated stream velocities. This curve is plotted

Firm power

Tidal energy is unusual among renewable source technologies in that it offers ‘firm’ power, whereby the quantity and timing of power flows may be precisely predicted. By phasing suitably located tidal energy power stations, the aggregate power output, although not exactly constant, could match a substantial portion of base load. While this idea is not new [6], the implications for future resource planning are only now being appreciated. With large demands being placed on renewable energy

Conclusions

The aggregate outputs from a number of dispersed tidal current power stations can provide a base load. The magnitude of the delivered power can readily be increased by using hydraulic pumped storage to smooth fluctuations in the twice-daily cycle. The lunar cycle of spring and neap tides causes long-period variations in power output of a magnitude that may be accommodated by complementary sources of energy, or by energy storage. The predictability of tidal power output may be regarded as a

References (7)

DTI. Our energy future—creating a low carbon economy. UK Department of Trade and Industry, London, UK, URN 03/658;...
Scottish Executive, securing a renewable future: scotland's renewable energy, scottish executive, Edinburgh, UK, ISBN...
Trapp T, and Lomax C. Developing a tidal stream energy business. Proceeding of oceanology international, London, UK,...

There are more references available in the full text version of this article.

Cited by (69)

Exploiting the temporal characteristics of tidal stream power for green ammonia production
2024, Renewable Energy
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.
Numerical modelling of a 1.5 MW tidal turbine in realistic coupled wave–current sea states for the assessment of turbine hub-depth impacts on mechanical loads
2024, Ocean Engineering
This paper considers hub-depth impacts on mechanical loads for a tidal turbine operating in realistic coupled wave–current sea states. A novel medium-fidelity actuator-line CFD model for simulating tidal turbine non-steady hydrodynamic rotor load responses in the presence of turbulence, shear, and surface waves is developed. The model is validated using tank testing data from a lab-scale turbine. The validated model is then upscaled, to a power rating of 1.5 MW, and simulated in realistic wave–current conditions consistent with those of the MeyGen site. Mean torque and thrust are found to increase with turbine hub height, and the presence of waves is shown to increase mean torque and thrust values by up to 22% and 11%, respectively. The effect on standard deviations and maximum values for these variables is more pronounced, with increases of up to 2500% and 1700% in signal standard deviations, and up to 80% and 30% in maximum values for torque and thrust, respectively. The presence of longer period waves is also shown to reduce mean torque levels, while the corresponding standard deviations and maximum values remained relatively unchanged. In such circumstances, the turbine is operating with an undesirable combination of low-power and high-fatigue. Tidal turbine hub loading characteristics and sensitivities, in the context of the operational loads which subsequently enter the drivetrain and turbine support structure, are also analysed. The magnitude of out-of-plane rotor moments are found to increase with the hub height and wave height. Complex flow interactions are shown to play an important role in this context, leading to what is termed “wave-driven moment-type dominance” effects. Overall, both the rotor location and wave composition are found to significantly impact the turbine’s rotor mechanical load response.
A methodology for cost-effective analysis of hydrokinetic energy projects
2023, Energy
The cost-effective analysis (CEA) of hydrokinetic farms is typically based on simplistic assumptions regarding the performance and cost structure of hydrokinetic energy converters (HECs) and, in consequence, may lead to ill-informed decision-making. In this work, a novel approach to selecting the most appropriate combination of HEC and site within a coastal area is developed, with the accurate computation of the CEA parameters as the cornerstone. The approach, which is illustrated through a case study in the Shannon Estuary (W Ireland), encompasses four models, namely: (i) HEC-site selection model, (ii) energy production model, (iii) CAPEX model, and (iv) OPEX model. By avoiding simplistic assumptions, the proposed approach improves on current procedures and enables developers to accurately compute any cost-effective parameter of interest. In particular, operation and maintenance costs are considered, along with economies of scale, which are typically disregarded in existing procedures. Beyond the interest of the results of the Shannon case study, the approach can be implemented in other regions with potential for hydrokinetic energy conversion.
Effect of leading-edge tubercles on the hydrodynamic characteristics and wake development of tidal turbines
2023, Journal of Fluids and Structures
This paper describes an investigation into the effect of leading-edge tubercles on turbine hydrodynamic characteristics and wake development characteristics. The hydrodynamic performance and wake velocity distribution of a modified turbine—termed Bio-turbine—are tested by a channel test. The normalized mean velocity profile obtained by the large eddy simulation (LES) method is consistent with the experimental data, which indicates that the numerical method is credible in predicting turbine performance and wake turbulence. By carefully examining the wake velocity deficit, turbulence intensity variation, and evolution of the vortex structure, it is possible to discuss how the leading-edge tubercles affect turbine performance and wake instability at various tip speed ratios (TSRs). The findings demonstrate that Bio-turbine has superior energy conversion performance at high TSRs, but the power amplitude is larger and the instability is more prominent. Furthermore, the unique structure of the blade leading edge alters the pressure distribution and flow separation behavior of the blade suction side, resulting in an enlargement of the wake recovery area. As a result, greater emphasis should be placed on the design of turbines with leading-edge tubercles.
Impacts of tidal stream power on energy system security: An Isle of Wight case study
2023, Applied Energy
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.
Temporal complementarity of marine renewables with wind and solar generation: Implications for GB system benefits
2022, Applied Energy
Citation Excerpt :
These results are fairly consistent with the analysis presented here, as although wave and tidal do not outperform wind in all of the GB case studies, technology mixes including wave and tidal do outperform the current renewable technology mix in all cases. The benefits tidal stream power can provide with respect to supply–demand matching has been investigated in several studies [45,54–56]. In general, findings show that the cyclic nature of tidal stream power generation enables a greater amount of the demand to be met directly, reducing the annual shortfall in renewable power needed to meet demand.
Wave and tidal energy have the potential to provide benefits to power systems with high proportions of stochastic renewable generation. This is particularly applicable in combination with wind and solar photovoltaics, as the offsetting of these renewable resources results in more reliable renewable generation. This study utilises ten metrics to quantify the temporal complementarity and supply-demand balancing requirements of the energy mix in Great Britain, to investigate the potential magnitude of these system benefits. Wave and tidal generation profiles are created using historical resource data and hydrodynamic models. The results show that the inclusion of wave and tidal generation creates a renewable energy mix which is more available under multiple conditions: throughout a year of operation; at times of peak demand; for multiple consecutive hourly time periods; and at times when wind and solar generation are not available. Three regional case studies also show that the inclusion of marine energy allows for improved regional supply-demand matching, reducing instances of energy shortage and excess and potentially relieving transmission congestion at particularly constrained locations within GB. Finally, the implications of these findings are discussed in terms of GB wholesale market operation, system balancing and system security.

View all citing articles on Scopus

View full text