Development of an ocean-current turbine for the Kuroshio current Katsutoshi Shirasawa1*, Junichiro Minami1, Hidetsugu Iwashita2 and Tsumoru Shintake1

1Okinawa Institute of Science and Technology Graduate University, Okinawa 904-0495, Japan 2Graduate School of Engineering, Hiroshima University, Hiroshima 739-8527, Japan

AbstractOceancurrentsareapromisingsourceofsustainableenergybecausetheflowofwaterprovidesregularandpredictableenergy.Japanisinasuitableloca;onforharnessingthepowerofoceancurrentsbecausetheKuroshiooceancurrentrunssteadilyneartheJapaneseseaside.Thecurrentflowisapproximately500mdeepand100kmwidewithaflowspeedof1-1.5m/s.Inordertoharnessthekine;cenergyofmarinecurrents,weproposeanovelocean-currentturbine.Theturbinesaremooredtotheseabedandfunc;onlikekitesinthewaterflow.Tooperatesuchturbinesinthemiddlelayerofamarinecurrent,itisnecessarytocanceltheresul;ngrotortorque.Therefore,ourturbineisdesignedwithafloatatitstopandacounterweightatitsboHom.Owingtobuoyancyandgravita;onalforce,theturbinebodymaintainsitsaItudestablybycancelingtherotortorque.Inotherwords,buoyancyandgravityacttogetherasarigh;ngmoment.Anotheradvantageofworkingfarfromtheseasurfaceisthelackofinfluencefromwavesandwind,especiallyinatyphoon.Asafirststep,weconstructedaprototypeturbineandconductedtowingexperimentsinordertoconfirmthefloatandcounterweightconfigura;on.

Blade Torque (T)

Buoyancy (F)�

Gravity (G)�

Counterweight�

Rotor Blade�

Float�

Nacelle�

Ocean Current 1-1.5 m/s �

(a) Side view� (b) Back view�

Roll (θ)�

Major Diameter�

Minor Diameter�

Experimental verification of a floating ocean-current turbine with asingle rotor for use in Kuroshio currents*

Katsutoshi Shirasawa a, *, 1, Kohei Tokunaga b, Hidetsugu Iwashita b, Tsumoru Shintake a

a Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa 904-0495, Japanb Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan

a r t i c l e i n f o

Article history:Received 17 March 2015Received in revised form2 January 2016Accepted 6 January 2016Available online 28 January 2016

Keywords:Renewable energyOcean currentOcean-current turbineOcean energy

a b s t r a c t

Ocean currents have excellent potential as future renewable energy resources. In order to harness thekinetic energy of marine currents, we propose a new ocean-current turbine. In general, ocean currentshave sufficiently large cross sections. Thus, the turbines are moored to the seabed and function like kitesin the water flow. In the future, turbines will be installed approximately 100 m deep to avoid the in-fluence of surface waves; this is especially important during typhoons. To operate such turbines in themiddle layer of a marine current, it is necessary to cancel the resulting rotor torque. Therefore, ourturbine is designed with a float at its top and a counterweight at its bottom. Owing to buoyancy andgravity, the turbine maintains a stable position. We describe towing experiments carried out to confirmthe float and counterweight configuration and show that the results verify hydrostatic stability andelectric power generation for the proposed turbine.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

There are several different oceanic energy forms: wave, marinecurrents, tidal currents, and thermal energy. Many studies arecurrently being carried out in order to realize commercial operationworldwide [1,2]. Ocean currents are also a promising source ofsustainable energy because the flow of water provides regular andpredictable energy. On the other hand, research and developmentare difficult technically and potentially costly. However, the de-mand for renewable energy sources has increased since theFukushima nuclear power plant disaster in 2011.

Japan is in a suitable location for harnessing the power of oceancurrents because the Kuroshio ocean current runs steadily near theJapanese seaside [3]. The Kuroshio current is a strong ocean currentin the western North Pacific Ocean. The current flow is approxi-mately 500mdeep and 100 kmwidewith a flow speed of 1e1.5m/s[4e6]. This appears to be a rather slow flow, but it is sufficient forgenerating electricity because the density of water is 800 timeshigher than that of air. The Kuroshio current has a power densityequivalent to that of a wind flow at 9e14 m/s. Moreover, the

Kuroshio current is an energy resource with only small fluctuationsin flow, regardless of the time of day or the season. As mentionedabove, an ocean-current turbine has many advantages for powerproduction; these include:

! Stability

Ocean currents flow constantly.

! Availability

The volume of the water flow is very large.

! Predictability

The flow speeds and paths have been investigated thoroughly.

! No visual impact

The turbine operates beneath the water surface.Tidal-current turbines have technologies in common with

ocean-current turbines. At the European Marine Energy Centre(EMEC), several tidal-current turbine projects are currently un-derway [7]. Most of these projects have adopted horizontal-axisturbines mounted onto the seabed. This means that, even though

* Fully documented templates are available in the elsarticle package on CTAN.* Corresponding author.

E-mail address: katsutoshi.shirasawa@oist.jp (K. Shirasawa).1 Since 2012.

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Renewable Energy

journal homepage: www.elsevier .com/locate/renene

http://dx.doi.org/10.1016/j.renene.2016.01.0350960-1481/© 2016 Elsevier Ltd. All rights reserved.

Renewable Energy 91 (2016) 189e195

Sinker �

Mooring Line�

Ocean Current�

Sea Level�

Turbine�

�100 m�

Seabed�

Over 500 m�

Float�

Nacelle�

Counterweight�

Pitch�

hHps://en.wikipedia.org/wiki/Kuroshio_Current

Mo+va+on•  Japanissuitabletoharnessthepoweroftheoceancurrent.

•  KuroshioOceanCurrentrunssteadilynearJapanseaside.

•  Technicalissues–  Stabilityofafloa;ngbody.

•  Canceloutarotortorque.

•  Proposalofnewocean-currentturbine–  WestartedR&Din2012.

PrototypeTurbine

EnergyFarm

KuroshioCurrent Principle

TowingExperimentatSea

Reference

Advantages

•  Stability–  Oceancurrentsflowconstantly.

•  Availability–  Thevolumeofthewaterflowisverylarge.

•  Predictability–  Theflowspeedsandpathshavebeeninves;gated

thoroughly.

•  Novisualimpact–  Theturbineoperatesbeneaththewatersurface.

MeasurementResult

Ocean-currentturbinesystem

Rotor diameter 2 m

Number of blades 3

Speed of flow 1~1.5 m/s

Rated generator output 1 kW

Cp (max.) 0.42 (TSR=4)

Float volume 0.1 m3

Counterweight( in air) 50 kg

Conclusion

•  Wehaveproposedanewocean-currentturbine.–  Floatandweightconfigura;onhasworkedvery

well.•  Wehavedevelopedtheprototypeturbineand

performedtowingtestinthesea.•  Output　Ave.300W(Peak400W)

–  goodagreementwiththeexpectedvalue.

–  Mooredfloa;ngbodyhasshowedhighbodystability.

•  Mooringexperimentusingascalemodelhasbeencarriedoutwithacircula;ngwaterchannel.

ParametersofthePrototypeTurbine

Scalemodel�

Float390g(Buoyancy760gf)

Nacelle+Blade350g

Weight520g

Scalemodel

Dia.250mm

Scale-modelTestatCircula+onWaterChannel

Measuredvalue�

Rotor diameter 1.46 m

Number of blades 3

Tip speed ratio 4

CP (max.) 0.42

CT (max.) 0.63

Airfoil NACA642-415

Width〜100km,Depth〜500m,Speed1〜1.5m/s

Rotordiameter80m,1Turbine:Output3MW@1.5m/s

TowingTankExperiment TurbineDesign OutputPowerThrustForce