1. Introduction
The periodic rise and fall of sea levels are attributed to the gravitational interaction between the sun and moon combined with the rotation of the earth. This motion carries a predictable and reliable source of energy since the relative motion of these bodies occurs at an astronomical time scale. Large-scale tidal power plants that close off a portion of the shore and act as sea-based dams have proven to be effective in converting this energy into usable power. These dams incur large capital costs as very large and sturdy foundations are required in closing off any part of the sea or bay.
Tidal stream turbines (TSTs) provide an alternative in harnessing energy from marine currents as they can be used in the open sea without the need to enclose large areas of maritime space. However, these devices are still more expensive [
1] than other forms of renewable technology. Horizontal axis tidal turbines (HATTs) are currently the dominant device type [
2], with multiple developers in the pre-commercial and commercial phase of implementation.
As of the time of writing, Scotrenewables SR2000 [
3] deployed in Orkney, UK, is the world’s most powerful turbine operating on a full commercial scale. Two 16 m tidal turbines with a shared floating platform are capable of producing 2 MW at a rated speed of 3 m/s. Another project in Orkney, Atlantis Meygen [
4], is one of the largest planned full commercial scale tidal turbine array with four AR1500 single-rotor turbines, each capable of producing 1.5 MW. Atlantis is also set to test the AR2000, a single 20–24 m diameter turbine, which should be capable of producing 2 MW at speeds of more than 3 m/s [
5]. The Nova M100 [
6] is a smaller 9 m turbine with a rated capacity of 100 kW at a rated speed of 2 m/s. Three M100 turbines are deployed in the world’s first fully-operational grid-connected tidal turbine array in Shetland, UK.
Outside the UK, the Sabella D10 [
7] is a 10 m diameter 6-bladed turbine capable of producing 1 MW at a speed of 4 m/s. The device is set to be the first marine current turbine that will provide electricity to the French energy network. The Verdant Power Roosevelt Island Tidal Energy [
8] project in the US is a tidal array project aiming to deploy thirty Verdant Gen5 5 m diameter turbines with capacities of 35 kW at a speed of 2.5 m/s. Back in 2006, Verdant successfully demonstrated the operation of a grid-connected array with six of their Gen4 full-scale turbines, with 9000 turbine-hours of operation [
9].
While success has been achieved in commercialisation, the technology generally faces the challenge of being highly site-specific as existing designs are geared toward current velocities greater than 2 m/s [
10,
11], while most of the world’s oceans have velocities less than this value. Less energetic flows have less than half of the extractable power from most northern UK sites (
). Nonetheless, there is a considerable increase in the number of exploitable sites, while lower structural loading resulting from a less energetic flow should translate to lower cost of manufacturing, materials, operation, and maintenance.
The south, west, and east coasts of the UK are included in the less energetic current category, with current velocities of less than 2 m/s [
12,
13]. Deployment of tidal energy devices are planned to harness energy in the tidal channel of Ria Formosa, Portugal, although the same challenge is faced as the site is limited to a current velocity of 1.4 m/s. Initial deployment tests were performed showing underutilisation of the small-scale Evopod turbine [
14,
15], with a maximum output below 25% of the rated power output [
16].
Less energetic flows dominate South East Asia [
17], with current velocities less than 2 m/s. The San Bernardino Strait in the Philippines has been identified as a potential tidal energy site, with current velocities reaching upto 4.5 m/s [
18] near the southern tip of the Capul islands. However, the country is still characterised by flow speeds of less than 2 m/s, with most areas reaching current velocities of 1.4 m/s [
19,
20]. Malaysia has current velocities reaching up to 1.2 m/s [
21], which limits the possible operation of current TST designs to near cut-in speeds, leaving the turbine underutilised. Other tropical countries such as Brazil and Mexico also have similar flow conditions. Brazil has slightly better conditions, with peak current velocities reaching a range of 2–2.5 m/s, although median speeds still remain at 1.1 m/s [
22].
The Yucatan channel in Mexico is a passageway that has current velocities with an upper bound of 2 m/s [
23,
24], but much of the passing current is limited to below 1.4 m/s [
25]. Nonetheless, continuous operation is possible since the Yucatan current is a constant marine current as opposed to the periodic tidal current present in the previously mentioned sites. A similar marine current is found near Taiwan, with an average flow speed of less than 1.5 m/s [
26]. The Kuroshio current has been identified as a potential tidal energy resource for the country, leading to the research and development of the floating Kuroshio turbines [
27,
28,
29].
Reduction in capital cost is needed to make these sites more viable for tidal technology, and may be achieved through downsizing and utilisation of cheaper materials. Additional ways to reduce cost are possible and involves designing TSTs that would have an optimal operation under a less energetic flow, with velocities ranging from 1 m/s to 1.5 m/s. TST rotor designs will typically have a low tip-speed ratio (TSR) operating range, where maximum power is delivered at lower values (TSR < 4), leading to large torque production.
Currently, researchers and developers in Taiwan have been working on a floating turbine that can harness energy from the Kuroshio. Towing tank tests have shown success with a peak generation of over 400 W for a 2 m diameter turbine [
29]. However, the design still achieves maximum output at low TSRs (TSR ≈ 4). This will require large generators and complex power take-off mechanisms to bring up the rotational speed. The new design methodology hypothesises that enabling operation at higher TSRs (TSR > 6) in less energetic flow conditions would lead to savings since it would require smaller generators due to reduced torque requirements.
Large torque production is not possible in less energetic flows, but the amount of power output may be comparable by utilising higher angular speeds (Power = torque × angular speed). Direct-drive or at least a less complex power take-off mechanism could be employed with a faster rotational speed. This would allow for further reductions in cost. Smaller diameter rotors with m provide a benefit of further increasing rotational speed in addition to reducing capital cost.
A design methodology for horizontal axis tidal turbines operating in less energetic currents is presented. The study outlines the design process and its application to two different aerofoil-specific blade geometries that operate at different TSR ranges. A steady-state performance filtering is undertaken to filter through different blade variations before evaluating the performance of the optimised blade geometry operating under a velocity profile obtained from two sites in Asia and America.
The analysis of the resulting blade geometries serves as an investigation of the hypothesised benefits of designing blades that operate at higher TSRs in less energetic currents according to (a) lower torque production at a (b) reasonable decrease in power output. The study also investigates how far the TSR location of the maximum may be pushed towards higher values as a result of the blade alterations.
4. Conclusions and Future Work
A tidal turbine design process for less energetic currents has been presented. The best performing blade for each aerofoil-specific blade geometry was selected using the two set objectives of achieving a high maximum and high TSR location. The twist distribution was determined to have a more significant effect, and the best variation was found to be the base twist distributions as published by Bahaj and Doman.
The simulations show reasonable performance for high-TSR blades with a slight decrease in power. The hyperbolic chord distribution () was the best low-solidity variation for the NACA 638xx blade with a TSR location at 7.75 and a maximum of 0.405. The maximum value is roughly 8% lower than the maximum of the original Bahaj blade, but the 33% reduction in torque requirements and the significantly higher angular speed, can reduce the costs as smaller generators and direct-drive power take-off may be employed. The reduced chord-length variation of the original Doman blade gave the best performance for the low-solidity NREL blade. Similarly, the maximum of the low-solidity variation was lower by about 5% compared to the original blade. Cost reduction can be achieved, although it may not be as large as the NACA blade since the reduction in torque is only 15% with a lower increase in angular speed .
The quasi-unsteady simulations show benefits for the high-TSR blades as load variations are minimised, with both low-solidity blades resulting in an averaged reduced torque and thrust variation of 30% and 6% decrease, respectively. However, additional analysis must be performed to quantify fatigue since the reduced loads are accompanied by an increase in the number of cyclic loads, with the NACA blades rotating 1.35 times faster than the original NACA blade.
The design methodology may be expanded to accommodate more blade variations and may be done programmatically with as a parameter to change the chord and twist distributions. Additional parameters to alter blade geometry may also be added. Pool filtering with a conditional multi-objective decision model provides a simple decision method to select an optimised blade. Other objectives and constraints may be added to ensure that output blades are technically and economically feasible. These objectives and constraints may include cost-effectiveness, minimal deflection, cavitation, etc.
The blade-element momentum method is limited to quasi-unsteady analysis. Expansion to unsteady analysis with added mass methods should improve the accuracy of the simulations. The output of the single-parameter BEM resulted in highly erratic output for some of the simulations. This can be fixed by adding more constraints in convergence among others. Further increasing the temporal resolution to 0.01 s may improve the accuracy for quasi-unsteady simulations, but additional mathematical corrections must be included as erratic values also occur at very high TSRs (TSR > 10) during steady-state simulations.
Wave–current interactions should also be considered. The current method of computing velocities at every time step already allows for the incorporation of changing velocities from wave–current interactions. The azimuthal coordinate system allows for vertical velocity contributions. Additional data on wave-induced velocities are needed to identify conditions in less energetic currents.