Applied Energy 86 (2009) 1823–1835

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Applied Energy

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Hydrokinetic energy conversion systems and assessment of horizontal and verticalaxis turbines for river and tidal applications: A technology status review

M.J. Khan a,*, G. Bhuyan a, M.T. Iqbal b, J.E. Quaicoe b

a Power System Technologies, Powertech Labs Inc., Surrey, BC, Canada V3W 7R7b Faculty of Engineering & Applied Science, Memorial University, St. John’s, NL, Canada A1B 3X5

a r t i c l e i n f o

Article history:Received 13 August 2008Received in revised form 23 February 2009Accepted 24 February 2009Available online 1 April 2009

Keywords:Renewable energyTidal currentRiver streamHydrokinetic technologyDuct augmentation

0306-2619/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.apenergy.2009.02.017

* Corresponding author. Tel.: +1 604 590 6634; faxE-mail addresses: jahangir.khan@powertechlabs.co

powertechlabs.com (G. Bhuyan), tariq@mun.ca (M(J.E. Quaicoe).

a b s t r a c t

The energy in flowing river streams, tidal currents or other artificial water channels is being considered asviable source of renewable power. Hydrokinetic conversion systems, albeit mostly at its early stage ofdevelopment, may appear suitable in harnessing energy from such renewable resources. A number ofresource quantization and demonstrations have been conducted throughout the world and it is believedthat both in-land water resources and offshore ocean energy sector will benefit from this technology. Inthis paper, starting with a set of basic definitions pertaining to this technology, a review of the existingand upcoming conversion schemes, and their fields of applications are outlined. Based on a comprehen-sive survey of various hydrokinetic systems reported to date, general trends in system design, duct aug-mentation, and placement methods are deduced. A detailed assessment of various turbine systems(horizontal and vertical axis), along with their classification and qualitative comparison, is presented.In addition, the progression of technological advancements tracing several decades of R&D efforts arehighlighted.

� 2009 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18232. Hydrokinetic energy conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824

2.1. Conversion schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18242.2. Terminologies for turbine systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18252.3. Areas of application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826

3. Technology survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826

3.1. Survey methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18263.2. Analysis of survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1827

4. Horizontal and vertical axis turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828

4.1. Rotor configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18284.2. Duct augmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18304.3. Rotor placement options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1831

5. Technical advantages and disadvantages of horizontal and vertical turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18326. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1832

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1833Appendix A. List of surveyed technologies (in alphabetic order). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1833References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1833

ll rights reserved.

: +1 604 590 8192.m (M.J. Khan), gouri.bhuyan@.T. Iqbal), jquaicoe@mun.ca

1. Introduction

The process of hydrokinetic energy conversion implies utiliza-tion of kinetic energy contained in river streams, tidal currents,or other man-made waterways for generation of electricity. This

1824 M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835

emerging class of renewable energy technology is being stronglyrecognized as a unique and unconventional solution that fallswithin the realms of both in-land water resource and marine en-ergy. In contrast to conventional hydroelectric plants, where anartificial water-head is created using dams or penstocks (forlarge-hydro and micro-hydro, respectively), hydrokinetic convert-ers are constructed without significantly altering the natural path-way of the water stream. With regard to ocean power utilization,these technologies can be arranged in multi-unit array that wouldextract energy from tidal and marine currents as opposed to tidalbarrages where stored potential energy of a basin is harnessed.While modularity and scalability are attractive features, it is alsoexpected that hydrokinetic systems would be more environmen-tally friendly when compared to conventional hydroelectric and ti-dal barrages.

In addition to worldwide interest, recent initiatives by NorthAmerican entities have also seen a greater momentum [1–4].Resource and technology assessment by EPRI in US [5], BC Hy-dro/Triton [6] and NRC in Canada [7] have given newer perspec-tives of North America’s tidal current energy potential. While anumber of projects are being actively pursued, notable progresshas been made in Bay-of-Fundy (Nova Scotia) and in Puget Sound(Washington) [8,9]. Recently (2003–2007), preliminary investiga-tions on the use of hydrokinetic technologies for in-land waterresources have been conducted by organization such as, US Depart-ment of Energy [10], EPRI [11], Idaho National Laboratory [12], andNational Hydropower Association [13]. In response to interestsfrom a number of project developers, US Federal Energy RegulatoryCommission (FERC) has stated this technology as of tremendouspotential [14]. Also, the US congress has endorsed the Energy Inde-pendence and Security Act of 2007 (the ‘‘EISAct” [15]) bringing fur-ther encouragement to the development of this technology. At thesame time various projects and proposals are in place within anumber of jurisdictions in North America ([16–20]).

While the enthusiasm in this field is obvious, skepticism ontechnological viability is also prevalent. In addition to several fun-damental inquiries (resource availability, definition of technolo-gies, field of application, etc.), a number of technology-specificquestions (such as, what converter type is best suited, whetherduct augmentation is worth attempting, how to place a turbinein a channel) are continuously being put forward. In this paper,based on a comprehensive technology survey, the approach of anumber of technology developers as well as R&D institutions are

Fig. 1. Outline of a hydrokinetic energy converter system [37].

analyzed in light of the questions above. Discussions on perfor-mance analysis and modeling issues are beyond the scope of thiswork and will be addressed through separate publications (suchas, [21]). While a complete converter system may incorporate var-ious important sub-systems (such as, power electronics, anchoring,and environmental monitoring, Fig. 1), this work mostly deals withthe front-end process of hydrodynamic-to-mechanical powerconversion.

2. Hydrokinetic energy conversion

Being an emerging energy solution, there exists noticeableambiguity in defining the technology classes, field of applications,and their conversion concepts. This section aims at elaborating onthese issues in consultation with the existing literature and presenttrends.

2.1. Conversion schemes

The energy flux contained in a fluid stream is directly depen-dent on the density of the fluid, cross-sectional area, and fluidvelocity cubed. In addition, the conversion efficiency of hydrody-namic, mechanical, or electrical processes reduce the overall out-put. While turbine systems are conceived as prime choices forsuch conversion, other non-turbine approaches are also being pur-sued with keen interest. A brief description of ten (10) interrelatedconcepts categorized in two broader classes (turbine/non-turbine)is given below:

� Turbine Systems– Axial (Horizontal): Rotational axis of rotor is parallel to the

incoming water stream (employing lift or drag type blades)[22].

– Vertical: Rotational axis of rotor is vertical to the water sur-face and also orthogonal to the incoming water stream(employing lift or drag type blades) [23].

– Cross-flow: Rotational axis of rotor is parallel to the watersurface but orthogonal to the incoming water stream(employing lift or drag type blades) [24].

– Venturi: Accelerated water resulting from a choke system(that creates pressure gradient) is used to run an in-built oron-shore turbine [25].

– Gravitational vortex: Artificially induced vortex effect is usedin driving a vertical turbine [26].

� Non-turbine Systems– Flutter Vane: Systems that are based on the principle of

power generation from hydroelastic resonance (flutter) infree-flowing water [27].

– Piezoelectric: Piezo-property of polymers is utilized for elec-tricity generation when a sheet of such material is placedin the water stream [28].

– Vortex induced vibration: Employs vibrations resulting fromvortices forming and shedding on the downstream side of abluff body in a current [29].

– Oscillating hydrofoil: Vertical oscillation of hydrofoils can beutilized in generating pressurized fluids and subsequent tur-bine operation [30]. A variant of this class includes biomi-metic devices for energy harvesting [31].

– Sails: Employs drag motion of linearly/circularly movingsheets of foils placed in a water stream [32].

At present, various turbine concepts and designs are beingwidely pursued (Fig. 2) while the non-turbine systems (Fig. 3)are mostly at the proof-of-concept stage (with some exceptions

Fig. 2. Example of turbine systems: (a) Free FlowTM [22]; (b) KoboldTM [23]; (c) AtlantisstromTM [24]; (d) HydroVenturiTM [25]; (e) Neo-AerodynamicTM [26].

Fig. 3. Example of non-turbine systems: (a) OCPSTM [27]; (b) EELTM [28]; (c) VIVACETM [29]; (d) SeasnailTM [30]; (e) Tidal SailsTM [32].

M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835 1825

[30]). Therefore, the former type of devices are given due attentionas they hold promise for deployment in the near future.

2.2. Terminologies for turbine systems

The term ‘Hydrokinetic Turbine’ has long been interchangeablyused with other synonyms such as, ‘Water Current Turbine’ (WCT)[19,33], ‘Ultra-low-head Hydro Turbine’ [34], ‘Free Flow/StreamTurbine’ (implying use of no dam, reservoir or augmentation)[35], ‘Zero Head Hydro Turbine’ [33,36], or ‘In-stream Hydro Tur-bine’ [11]. For tidal applications, these converters are often termedas Tidal In-stream Energy Converter (TISEC) [5] or simply ‘TidalCurrent Turbine’. For rivers or artificial waterways the same tech-nology is generally identified as ‘River Current Turbine (RCT)’, ‘Riv-er Current Energy Conversion System’ (RCECS) [37], ‘River In-stream Energy Converter’ (RISEC) [11], or in brief,‘River Turbine’.Other common but somewhat misleading identifiers include‘Watermill’, ‘Water-wheel’, or even ‘Water Turbine’ [33].

Fig. 4. Conventional hydro versus hydrokin

In a 1981 US Deportment of Energy report [34], this class oftechnology has been defined as ‘Low pressure run-of-the-river ul-tra-low-head turbine that will operate on the equivalent of lessthan 0.2 m of head’. A more recent (2006) assessment by this orga-nization [10] has classified these devices as ‘Low Power/Unconven-tional Systems’ that may use hydro resources with less than 8 feethead. As indicated in Fig. 4, the USDoE report uses the hydropowerpotential and working hydraulic head of a potential project as mea-sures of technology classification. This also indicates that the con-ventional hydroelectric plants use higher head and/or capacity insharp contrast to the unconventional low-head/hydrokineticschemes.

In keeping with the present norms [5,10–12,35] and adopting aconcise term, the word ‘Hydrokinetic’ is used here. While otherterms may deem suitable for application-specific cases (river, arti-ficial channel, tidal, or marine current), this approach envelopes abroader spectrum where all kinetic energy conversion schemesfor use in free-flowing/zero-head hydro streams are considered.

etic energy conversion schemes [10].

1826 M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835

2.3. Areas of application

Two main areas where hydrokinetic devices can be used inpower generation purposes are, (a) tidal current, and (b) riverstream. Ocean current represent another potential source of oceanenergy where the flow is unidirectional, as opposed to bidirectionaltidal variations. In addition to these, other resources include, man-made channels, irrigation canals, and industrial outflows [22,38].While all hydrokinetic devices operated on the same conversionprinciples regardless of their areas of application, a set of subtledifferences may appear in the forms of design and operational fea-tures. These include,

� Design– Size: In order to achieve economies of scale, tidal current tur-

bines are currently being designed with larger capacity (sev-eral MW). River turbines on the other hand, are beingconsidered in the range of few kW to several hundred kW[5,19].

– Directionality: River flow is unidirectional and this eliminatesthe requirement for rotor yawing. In tidal streams, a turbinemay operate during both flood and ebb tides, if such yaw/pitch mechanism is in place.

– Placement: Depending on the channel cross-section, a tidal orriver current turbine may only be placed at the seafloor/riv-erbed or in other arrangements (floating or mounted to anear-surface structure). This arises from a multitude of tech-nical (power generation capacity, instrumentation) and non-technical (shipping, fishing, and recreational boating)constraints.

� Operation– Flow characteristics: The flow characteristic of a river stream

is significantly different from tidal variations. While the for-mer has strong stochastic variation (seasonal to daily), thelatter undergoes fluctuations of dominant periodic nature(diurnal to semidiurnal). In addition, stage of a stream mayhave diversely varying profile for these two cases.

– Water density: The density of seawater is higher than that offreshwater. This implies, lesser power generation capacity fora tidal turbine unit when placed in a river stream. In addition,depending on the level of salinity and temperature, seawaterin different location and time may have varying energycontent.

– Control: Tidal turbines are candidates for operating underforecasted tide conditions. River turbines may not fall intosuch paradigms of control and more dynamic control sys-tems may need to be synthesized.

– Resource prediction: Tidal conditions can be almost entirelypredicted and readily available charts can be used in coordi-nating the operation of a tidal power plant. For river applica-tions, forecasting the flow conditions is more involved andmany geographical locations may not have such arrange-ments. For a hydrokinetic converter, the level of power out-put is directly related to flow velocity (and stage). Eventhough volumetric flow information is available for manylocations, water velocity varies from one potential site tothe other depending on the cross-sectional area. Therefore,unless a correlation between flow variations and sitebathymetry is established, and turbines are operated accord-ingly, only sub-optimal operation can be achieved.

� End-use– Grid-connectivity: While tidal current systems may see large-

scale deployment (analogous to large wind farms), hydroki-netic converters used in river streams may become feasiblein powering remote areas or stand-alone loads. Depending

on how the technology evolves, this type of alternativeschemes may also fall within the distributed generation sce-narios in the near future. Bulk power generation throughtidal power plants are expected in longer time horizons. Itis expected that these technologies will face similar networkintegration challenges as wind power systems and will takeadvantage of higher resource predictability [39].

– Other purposes: Hydrokinetic turbines can potentially be usedin conjunction with an existing large hydroelectric facility,where the tailrace of a stream can be utilized for capacityaugmentation (i.e, resource usage maximization) [10,19]).Direct water pumping for irrigation, desalination of seawater,and space heating are other potential areas of end-use.

3. Technology survey

In order to aid the advancement of hydrokinetic conversiontechnologies and develop suitable solutions to various relevantproblems, it is important to identify the current status of this fieldof engineering and research. A survey that provides insight into thehistorical perspective and also indicates the industry trends can bevery useful in that regard. As part of this work, a comprehensivetechnology review has been conducted and most of the majorschemes reported to date have been considered. This survey essen-tially overlaps the authors’ previous work [37], complements a setof more recent reports published by EPRI [5], Verdant Power [19],and Powertech Labs [20], and identifies subtle advancement incontrast to some previous reviews [34,40].

3.1. Survey methodology

The survey conducted in this work not only identifies commer-cial systems, but also accommodates various R&D initiativesundertaken in the academia. As indicated in Appendix A, total ofseventy six different devices and schemes were analyzed. Due toavailability of limited information for many devices, mostly theprimary conversion hardware and their peripherals (rotors, ducts,placement method in a stream, etc.) are evaluated.

The information gathered along the process is organizedthrough the following headings:

� Application: In the previous section, various areas of applicationfor hydrokinetic devices have been identified. This discussion iscarried forward into the survey by categorizing the potentialuse of a given device into (a) tidal current (for tidal and oceancurrent resources) (b) river stream (for free-flowing/zero-headrivers), and (c) multi-application (river, tidal, and other applica-tions). While the information disseminated through the relevanttechnology developer, research institute, or public-domain docu-ment has been the basis of this classification, several ambiguouscases have been considered as ‘Multi-application’.

� Technology type: In light of the discussion presented earlier, all ofthe 76 devices or concepts have been attributed to one of the ten(10) conversion schemes. However, further division into ‘tur-bine’ or ‘non-turbine’ systems has not been carried out.

� Duct: Ducts are engineered structures that elevate the energydensity of a water stream as observed by a hydrokinetic con-verter. Considerations for these devices is of high significanceprimarily because of two opposing reasons (a) potential to aug-ment the power capacity and hence reduce the cost of energy (b)lack of confidence as far as their survivability and design/dem-onstration are concerned. In this survey, attempts were madeto identify whether a given scheme is considered for duct aug-mentation (unknown cases were identified separately) or not.

Fig. 6. Use of ducts and applications.

M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835 1827

� Placement: The method of placement of a hydrokinetic device, inrelation to a channel cross-section, is a very significant compo-nent for two basic reasons:– The energy flux in the surface of a stream is higher than that

of a channel-bottom. In addition, this quantity takes diversevalues depending on the distance from the shore and chan-nel-geography. Therefore, water velocity has a highly local-ized and site-specific three-dimensional profile and rotorpositioning against such variations will dictate the amountof energy that can be effectively extracted.

– Competing users of the water stream (recreational boats,fishing vessels, bridges & culverts, etc.) would essentiallyreduce the effective usable area for a turbine installation[19,20]. In this work, three classes of mounting arrangementsare considered: (i) BSM – Bottom Structure Mounted (Fixed)(ii) FSM – Floating Structure Mounted (Buoyant), and (iii)NSM – Near-surface Structure Mounted (Fixed). Each of thedevices or schemes has been assigned to one of these meth-ods, whereas unknown systems are identified separately.

In addition to the aspects mentioned above, each of the R&D ini-tiatives is observed for its present status of development and chro-nology of progression. The summary of these assessments aregiven in the following section.

3.2. Analysis of survey

Although a number of novel concepts have emerged recently,hydrokinetic energy conversion has mostly seen advancements inthe domain of axial (horizontal) and vertical axis turbine systems.The significantly higher number of initiatives and several commer-cial/pre-commercial deployments have brought these systems atthe forefront this emerging industry.

The commercial systems (existing/discontinued) mostly repre-sent several small-scale river turbines employing inclined [41–44] and floating [45,46] horizontal axis turbines. These systemswere developed for remote powering applications in various coun-tries (Sudan, Peru, etc.). However, the current market-status ofmany these devices is unknown.

One interesting observation derived from the survey is that agreat number of technology developers and researchers view theirinitiatives as solutions for a wide spectrum of applications, beyondriver or tidal applications only. Reflecting the lesser level of resourceavailability, the number of technologies being developed specifi-cally for river applications is less than that of tidal energy systems.

Fig. 5. Use of ducts and c

The present trend clearly indicates that the area of multiple applica-tion (such as, river, tidal, artificial waterways, dam tailrace, andindustrial outflows) is of high importance, as these technologiescan probably be tailored to suit resource-specific needs.

In addition to realizing various rotor concepts, considerationsfor incorporating duct augmentation to these systems is a very sig-nificant aspect of this technology’s overall advancement. As shownin Fig. 5, vertical axis systems are given more emphasis for sucharrangements, whereas significant portion of axial-flow turbinesare considered for non-ducted application.

Regardless of the field of application (river, tidal or others), ductaugmentation has naturally seen lesser share of consideration(Fig. 6). This arises from the fact that most of the turbine conceptsare still at the R&D level, whereas ducts are peripherals to suchsystems.

Placement of a turbine system, in relation to a given open-chan-nel, is another field of progression where basic design (structuralstrength, floatation, and anchoring) and feasibility studies (surviv-ability, provisions for competing users, etc.) are being investigated.As seen in Fig. 7, most vertical axis turbines are being consideredfor either floating (FSM) or near-surface (NSM) placements. Onthe contrary, about one-third of the axial turbines are consideredfor seabed/riverbed installations. Other concepts have indicatedearly stage plans on their placement methods, which needs to bere-evaluated as these systems attain further advancement (seeFig. 8).

From applications point of view, river turbines have beendesigned and developed for either floating or near-surface

onversion schemes.

Fig. 7. System placement and conversion schemes.

Fig. 8. System placement and applications.

1828 M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835

arrangements. On the contrary, many tidal turbines are being con-sidered for placement at the bottom of the channel. This reflectsthe constraints imposed by other competing sea users (shipping,fishing, and other usage) as well as design challenges associatedwith large floating or near-surface-fixed structures, especially inharsh sea conditions.

While both vertical and axial turbines have long been consid-ered as primary choices for hydrokinetic energy conversion, anumber of unconventional concepts (such as, vortex induced vibra-tion, and piezoelectric conversion) have appeared recently. Severalearly river turbine prototypes were deployed and operated fromlate 1970s to late 1990s [41,45] until these were eventuallydecommissioned. Various non-turbine concepts (namely, oscillat-ing hydrofoil and piezoelectric conversion) had gained good atten-tion in the past [28,30,47]. However, their present status ofdevelopment is unknown. Analyzing the modern day history ofhydrokinetic energy conversion, it can be clearly noticed that thepresent decade has so far seen the greatest level of research anddevelopment initiatives. These efforts have enveloped a multitudeof technological concepts as well as diverse fields of applicationswhere hydrokinetic technologies may prosper in future.

4. Horizontal and vertical axis turbines

At the present state of this technology, both horizontal and ver-tical axis turbines are key contenders for further research, develop-

ment, and demonstration (RD&D) initiatives [20]. In addition toaiming for specific applications (such as, tidal currents or riverstreams), a great number of development efforts are directed to-ward realizing solutions that may serve both of these areas. Ductaugmentation is another area, which apparently did not find muchsuccess in the wind energy domain. However, it is perceived as acritical element to hydrokinetic conversion concepts.

In this article, an attempt is made to shed light on many of theseissues using qualitative and broad observations. This article, how-ever, does not attempt to indicate superiority of one option againstthe other. Rather, observations of generic nature are provided forthe reader and these may appear useful depending on the scopeand nature of any RD&D effort in this domain. The following dis-cussions focus on rotor configurations, duct augmentations, andplacement schemes, followed by a qualitative discussion on vari-ous technical advantages and disadvantages of these options.

4.1. Rotor configurations

As discussed in Section 3, hydrokinetic energy conversion mayemploy either rotary turbo machinery or can use non-turbineschemes. While the former class (turbine system) encompassesvarious classical rotary technologies, the latter group (non-tur-bine system) is mostly based on various unconventional concepts.Such schemes include, oscillating hydrofoil [30], vortex inducedvibration [29], piezo polymer conversion [28], and variable geom-etry sails [32]. Presently, most of these technologies are either attheir proof-of-concept stage or being developed as part-scalemodels. On the other hand, rotary turbine systems employinghorizontal, vertical, or cross flow turbines are occupying most ofthe discussion. A broad survey of existing and discontinuedRD&D initiatives are explored and classified in various maturitygroups (from ‘concept’ to ‘commercial’) in Fig. 9a. It should benoted that many of the ‘commercial’ systems, as shown in the fig-ure, employ inclined horizontal axis turbines and probably nolonger exist in the market.

In Fig. 9b, percentages of the turbine systems among all thestudied RD&D efforts (76 systems) are shown. It can be seen thathorizontal and vertical axis turbines consist of the greater share(43% and 33%, respectively). Although this result is not surprising,the point of interest is that vertical axis systems are seeing re-newed interest, especially when the wind energy industry haseffectively discarded this technology.

Fig. 9. General technology status of hydrokinetic turbine technologies.

M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835 1829

The choice of turbine rotor configuration requires consider-ations of a broad array of technical and economical factors. As anemerging field of energy conversion, these issues become evenmore dominant for hydrokinetic turbines. A general classificationof these turbines based on their physical arrangements is givenin Fig. 10. This list is by no means exhaustive, and many of the con-cepts are adopted from the wind engineering domain.

Fig. 10. Classification of turbine rotors.

Fig. 11. Horizontal axis turbines.

Based on the alignment of the rotor axis with respect to waterflow, three generic classes could be formed (a) horizontal axis, (b)vertical axis, and (c) cross flow turbines. The horizontal axis (alter-nately called as axial-flow) turbines have axes parallel to the fluidflow and employ propeller type rotors. Various arrangements ofaxial turbines for use in hydro environment are shown in Fig. 11.

Inclined axis turbines have mostly been studied for small riverenergy converters. Literature on the design and performance anal-ysis could be found in [33,48,49]. Information on several commer-cial products utilizing such topologies is available in [42–44,50].Most of these devices were tested in river streams and were com-mercialized in limited scales. The turbine system reported in [50]was used for water pumping, while the others [42–44] were pro-moeted for remote area electrification. It is however not clearwhether these latter devices are still being commercialized.

Horizontal axis turbines are common in tidal energy convertersand are very similar to modern day wind turbines from conceptand design point of view. Turbines with solid mooring structuresrequire the generator unit to be placed near the riverbed or sea-floor. Reports and information on rigidly moored tidal/river tur-bines are available in [22,34,51–55]. Horizontal axis rotors with abuoyant mooring mechanism may allow a non-submerged gener-ator to be placed closer to the water surface. Information on

Fig. 12. Vertical axis turbines.

N/AN/A16%

3%

Yes33%

Nc64%

No36%

Yes48%

Fig. 13. Reported consideration for duct augmentation for (a) horizontal axis and (b) vertical axis turbines.

Fig. 14. Augmentation channel classification.

1 A measure of extracted power against the theoretical fluid power consideringfree-stream/unducted water velocity.

1830 M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835

submerged generator systems can be found in [56,57] and that ofnon-submerged types are presented in [35,58].

The cross flow turbines have rotor axes orthogonal to the waterflow but parallel to the water surface. These turbines are alsoknown as floating waterwheels. These are mainly drag based de-vices and inherently less efficient than their lift based counter-parts. The large amount of material usage is another problem forsuch turbines [33,35,59]. Darrieus turbines with cross flowarrangements may also fall under this category.

Various arrangements under the vertical axis turbine categoryare given in Fig. 12. In the vertical axis domain, Darrieus turbinesare the most prominent options. Although use of H-Darrieus orSquirrel-cage Darrieus (straight bladed) turbine is very common,examples of Darrieus turbine (curved or parabolic blades) beingused in hydro applications is non-existent. In publications suchas, [35,60–68] a wide array of design, operational and performanceissues regarding straight bladed Darrieus turbines are discussed.The Gorlov turbine is another member of the vertical axis family,where the blades are of helical structure [36,69,70]. Savonious tur-bines are drag type devices, which may consist of straight orskewed blades [62,63,71].

Hydrokinetic turbines may also be classified based on their lift/drag properties, orientation to up/down flow, and fixed/variable(active/passive) blade pitching mechanisms. Different types of ro-tors may also be hybridized (such as, Darrieus–Savonious hybrid)in order to achieve certain performance features.

4.2. Duct augmentation

Augmentation channels induce a sub-atmospheric pressurewithin a constrained area and thereby increase the flow velocity.If a turbine is placed in such a channel, the flow velocity aroundthe rotor is higher than that of a free rotor. This increases the pos-sible total power capture significantly. In addition, it may help toregulate the speed of the rotor and impose lesser system designconstraints as the upper ceiling on flow velocity is reduced [72].Such devices have been widely tested in the wind energy domain.Terms such as, duct, shroud, wind-lens, nozzle, concentrator, dif-fuser, and augmentation channel are used synonymously for thesedevices. Discussions on duct augmentation in river/tidal applica-tions can be found in [34,72–74]. A survey conducted with seventysix hydrokinetic system concepts show that around one-third ofthe horizontal axis turbines are being considered for such arrange-ments. On the contrary, vertical axis turbines are being given moreattention when it comes to duct augmentation. Almost half of thestudied systems consider some form of augmentation scheme to beincorporated with the vertical turbine (see Fig. 13).

The ducts for horizontal axis turbines mostly take conical shapes(for operation under unidirectional flow) as opposed to vertical tur-

bines where the channels are of rectangular cross-section. This im-poses a design asymmetry and subsequent structural vulnerabilityfor the former type. The lesser number of duct augmentation beingconsidered for horizontal axis turbines can be attributed to this is-sues. These results only indicate accumulated experience andunderstanding of duct augmentation options for horizontal andvertical axis turbines, as perceived to date. It is believed that furtherRD&D on this area will go hand in hand with turbine development.

A simplified classification of various channel designs are given inFigs. 14 and 15. A simple channel may consist of a single nozzle, cyl-inder (or straight path) with brim or diffuser. In a hybrid design, allthree options may be incorporated in one unit. Test results on anumber of hydrodynamic models can be found in [72,73] and anexample shape is given in Fig. 15a. This work has reported a maxi-mum velocity increase factor of 1.67 (i.e, power coefficient1 in-creases 4.63 times). In [74] various hybrid models with rectilinearpaths are experimented (Fig. 15b). Diffusers with multi-unit hydro-foils (Fig. 15c) are also possible when higher efficiency is required.A straight model with a brim (Fig. 15d) may have a velocity amplifica-tion factor of 1.32. Analytic and test results of various rectilinear dif-fuser models (Fig. 15e) can be found in [75,76]. It has been found that,a diffuser with an inlet and brim performs the best in this category.Information on various annular ring shaped diffuser models(Fig. 15f) can be found in [34,77]. In [34], it has been shown that apower coefficient as high as 1.69 is possible, exceeding the Betz limitof 0.59.

Each of these models come with unique set of performancemerits and design limitations. For instance, the hybrid types per-form better at the expense of bigger size (as high as 6 times the ro-

Fig. 15. Channel shapes (top and side view).

Fig. 16. Turbine mounting options.

M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835 1831

tor diameter). The annular shapes also perform very well whenhydrodynamic shapes are optimally designed. Nevertheless, de-tailed investigation on optimal size, shape and design is still an un-solved problem.

4.3. Rotor placement options

While the type of rotor to be deployed and duct augmentationto be incorporated are of paramount importance, placement ofthe system in a channel also deserves due attention. A turbinemay incorporate bottom structure mounting (BSM) arrangementwhere the converter is fixed near the seafloor/riverbed. Also, tur-bine units may operate under variable elevation if a floating struc-ture mounting (FSM) is devised. The last option is to mount theconverter with a structure that is closer to the surface (near-sur-face structure mounting, NSM).

The technology survey conducted as part of this work indicatesthat axial-flow turbines are given almost equal consideration forthe three options outlined above (Fig. 16). However, more than half

N/A3%

BSM37%

FSM33%

NSM27%

Fig. 17. Percentage of turbines considered for various placeme

of the vertical axis turbines are being considered for near-surfaceplacement. This probably arises from the fact that this option allowsthe generator and other apparatus to be placed above the water le-vel. However, at the present state of this technology, there is no cleardirection on the most attractive option. Several subtle aspects thatcan be observed in this regard are highlighted below (see Fig. 17):

� Energy capture: The energy flux in a river/tidal channel is highernear the surface. This suggests that the FSM option is the bestoption as long energy extraction is the prime concern. In con-trast, the BSM method allows only sub-optimal energy capture.Also, energy capture using the NSM scheme would see fluctuat-ing output subject to variations in river stage or tide height.

� Competing users: While placing a turbine at the surface of achannel seems attractive, competing users of the water resourcemay object to such arrangement. Fishing, shipping, recreationalboating, and many other activities may leave the BSM or NSMmethods as the only option. Floating structures are still possiblebut these need to be placed closer to the shore where energyresources may appear limited.

� Construction challenge: Experience of floating structure designfor energy harvesting is limited. In contrast, knowledge in civilengineering domain for bottom mounted structures (e.g,bridges, offshore oil and gas platforms) are quite abundant.

� Footprint: Any trenching, piling, or excavation at the riverbed/oceanfloor may become subject to environmental scrutiny.Floating or near-surface structures appear more permissible inthis context.

� Design and operational constraints: Depending on where a tur-bine is to be placed various power conversion apparatus (gener-ator, bearing, gearboxes, and power conditioning equipment)would require special design considerations such as, water seal-ing, lubrication, and protection. Also, variation of water velocity

N/A12%

BSM8%

FSM28%

NSM52%

nt arrangements (a) horizontal axis and (b) vertical axis.

1832 M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835

and stage will impose operational constraints. Due attention isalso required to address the challenges associated with severstorm conditions, especially for the near surface and floating-type systems.

The areas of application will have specific repercussions on useof duct augmentations devices and corresponding placementschemes. For instance, tidal and marine current turbines work un-der the natural events of daily tide flow and seasonal ocean currentvariations, respectively. River turbines operate under the influenceof varying volumetric water flow through a river channel subject tovarious external factors such as, channel cross-section, rainfall, andartificial incidences (such as, transportation, upstream dam open-ing, etc.). River water is less dense than seawater and therefore ithas lower energy density. Siting is more stringent in river channelsas the usable space is limited and river transportation may furtherconstrain the usability of the sites. There could also be varyingtypes of suspended particles and materials (fish, debris, rock, ice,etc.) in river and sea channels depending on the geography of asite. It remains to be seen, how these factors will affect the design,operation, and commercialization of various turbine concepts.

5. Technical advantages and disadvantages of horizontal andvertical turbines

It is worthwhile to investigate the opportunities and challengesassociated with various hydrokinetic turbine systems, especiallywhen this sector of energy engineering is mostly at the designand development phase. Of particular interest is a review of bothhorizontal and vertical axis configurations with regard to theirtechnical merits and drawbacks. In this section these two configu-rations will be studied further.

Vertical axis turbines, especially the straight bladed Darrieustypes have gained considerable attention owing to various favor-able features such as:

� Design simplicity: As an emerging technology, design simplicityand system cost are important factors that may determine thesuccess of hydrokinetic turbine technology. In contrast to hori-zontal axis turbines where blade design involves delicatemachining and manufacturing, use of straight blades make thedesign potentially simpler and less expensive.

� Generator coupling: For hydrokinetic applications, generator cou-pling with the turbine rotor poses a special challenge. In the hor-izontal axis turbines, this could be achieved by a right-angledgear coupling, long inclined shaft or underwater placement ofthe generator. In vertical axis turbines, the generator can beplaced in one end of the shaft, allowing the generator to beplaced above the water surface. This reduces the need and sub-sequent cost in arranging water-sealed electric machines.

� Flotation and augmentation equipment: The cylindrical shape ofthe Darrieus turbine allows convenient mounting of various cur-vilinear or rectilinear ducts. These channels can also beemployed for mooring and floating purposes [72]. For axial-flowturbines, ducts can not be easily used for floatation purposes.

� Noise emission: Vertical turbines generally emit less noise thanthe horizontal turbine concepts due to reduced blade tip losses[78]. Subject to further research and investigation, this mayprove to be beneficial in preserving the marine-life habitat.

� Skewed flow: The vertical profile of water velocity variation in achannel may have significant impact on turbine operation. In ashallow channel, the upper part of a turbine faces higher veloc-ity than the lower section. Vertical turbines, especially the oneswith helical/inclined blades are reportedly more suitable foroperation under such conditions [79].

The disadvantages associated with vertical axis turbines are:low starting torque, torque ripple, and lower efficiency. Dependingon their design, these turbines generally possess poor starting per-formance. This may require special arrangement for external elec-trical, mechanical, or electromechanical starting mechanisms. Theblades of a vertical turbine unit are subject to cyclic tangentialpulls and generate significant torque ripple in the output. Cavita-tion and fatigue loading due to unsteady hydrodynamics are otherconcerning issues associated vertical turbines. Axial-flow turbineson the other hand, eliminate many of these drawbacks. In addition,various merits of such rotors are:

� Knowledgebase: Literature on system design and performanceinformation of axial type rotors is abundant. Advancements inwind engineering and marine propellers have significantly con-tributed to this field. Use of such rotors have been successfullydemonstrated for large scale applications (10–350 kW), espe-cially for tidal energy conversion [52].

� Performance: One key advantage of axial type turbines is that allthe blades are designed to have sufficient taper and twist suchthat lift forces are exerted evenly along the blade. Therefore,these turbines are self-starting. Also, their optimum perfor-mance is achieved at higher rotor speeds, and this eases theproblem of generator matching, allowing reduced gear coupling.

� Control: Various control methods (stall or pitch regulated) ofaxial type turbines have been studied in great details. Activecontrol by blade pitching allows greater flexibility in over speedprotection and efficient operation [52].

� Annular ring augmentation channels: Annular ring type augmen-tation channels provide greater augmentation of fluid velocity asthese systems allow concentrated/diffused flow in a three-dimensional manner [34]. The circular shape of the propellerrotor’s disc permits the use of this type of duct, which is not pos-sible for vertical axis turbines.

The major technical challenges encountered with axial type ro-tors are: blade design, underwater generator installation andunderwater cabling. While different types of rotors come with un-ique features, only extensive theoretical understanding, experi-mental validation, and design expertise would allow selection ofan ideal system. As the industry matures, greater insight into var-ious rotor systems will be available.

6. Conclusions

In this paper, the state of the hydrokinetic energy conversiontechnologies has been revisited with an emphasis on indicatingthe current trends in research and development initiatives. Whilethe initial discussions encompassed various definitions and classi-fications, the core analysis has been undertaken based on a com-prehensive literature survey. The major conclusions that can bederived from the discussions presented earlier are:

� Except for some early commercial systems (small-scale remotepower generation from river streams), most of the technologiesare at the proof-of-concept or part-system R&D stage.

� A number of novel schemes (such as, piezo-electric, biomimeticand vortex-induced-vibration) have surfaced in recent times, inaddition to the continued progress on classical hydrokineticenergy conversion approaches (vertical, axial-flow turbines,etc.).

� In the presence of a wide variety of terminologies attributed tothe fundamental process of kinetic energy conversion fromwater streams, the term ‘Hydrokinetic’ energy conversion canbe used as long as sufficient caveats are given for diverse fields

M.J. Khan et al. / Applied Energy 86 (2009) 1823–1835 1833

of application such as, rivers, artificial channels, tides, and mar-ine currents.

� In addition to the specific focus on river or tidal current conver-sion, strong emphasis is given to technologies that may serveboth of these areas as well as other potential resources (suchas, man-made canals, irrigation channels, and industrialoutflows).

� While both axial and vertical axis turbines are being developedfor hydrokinetic energy conversion, considerations for ductusage have seen higher preference for the latter class.

� Various options for turbine placement with respect to a channelcross-section (bottom, floating, or near-surface/fixed) are beinggiven almost equal emphasis. However, axial turbines aremostly being considered for placement at the bottom of a chan-nel, whereas vertical turbines are being designed for either float-ing or near-surface mounting arrangements.

� Recent technological advancement and project-developmentinitiatives clearly indicate a rejuvenated interest in the domainof hydrokinetic energy conversion.

As the hydrokinetic technologies evolve over time, new solu-tions emerge, and old concepts resurface/disappear, the reviewpresented in this work may need to be re-evaluated. However,the major observations made in this work may still appear usefulin identifying the technology trend being followed in this field ofenergy engineering. To conclude this discussion, it can be statedthat hydrokinetic energy technologies are emerging as a viablesolution for renewable power generation and significant research,development, and deployment initiatives need to be embarkedupon before realizing true commercial success in this sector.

Acknowledgement

Funding contributions from NSERC and AIF is dulyacknowledged.

Appendix A. List of surveyed technologies (in alphabetic order)

1. Alternative Hydro Solutions Ltd., ON, Canada2. Amazon AquachargerTM, Marlec Engineering, UK3. AquanatorTM Atlantis Energy, Australia4. Atlantisstrom, Germany5. Bangladesh Univ. of Engg. & Tech, Dhaka, Bangladesh6. BioPower Systems, Australia7. Brazil-prototype (cross flow), Center of Research in Electrical

Energy - CEPEL, Brazil8. Brazil-prototype (ducted axial), Department of Mech. Engg.

from the Univ. of Brasilia UNB, Brazil9. CADDET Centre for Renewable Energy, UK.

10. Clean Current Power Systems Inc., BC, Canada.11. Cross Flow TurbinesTM, Coastal Hydropower Corporation,

Canada.12. CurrentTM, Hydro Green Energy, LLC, TX, US.13. Cycloidal TurbineTM, QinetiQ Ltd., UK.14. EELTM, OPT Ocean Power Technologies Inc., US.15. EnCurrentTM, New Energy Corporation Inc., Canada.16. EvopodTM, Oceanflow Energy, Overberg Ltd., UK.17. EXIMTM, Tidal Turbine Sea Power, Sweden.18. Free FlowTM, Verdant Power LLC, US.19. Gentec VenturiTM, Greenheat Systems Limited, UK.20. Gorlov- Amazon demonstrations, Miscellaneous.21. Gorlov TurbineTM, GCK Technology Inc., US.22. Gravitation water vortex power plantTM, ZOTLOETERER,

Austria.23. School of Engineering, Griffith University, Australia.

24. HammerfestTM, Hammerfest Strøm AS, Norway.25. HarmonicaTM, Tidal Sails AS, Norway.26. HydraTM, Statkraft, Norway.27. Hydrokinetic GeneratorTM, Kinetic Energy Systems Corpora-

tion, FL, USA.28. Hydro VenturiTM, Hydro Venturi Ltd., UK.29. Impulsa TurbineTM, UNAM Engineering Institute, Mexico.30. Inha University, South Korea.31. ITDG-Guba,Sudan; Supported by ITDG, UK.32. Jack RabbitTM, Ampair, UK.33. Kobold turbineTM, Ponte di Archimede S.p.A., Italy.34. Memorial Univ. of Newfoundland, NL, Canada.35. Miscellaneous Demonstration projects.36. Munich University of Technology, Germany.37. Neo-Aerodynamic converterTM, Neo-Aerodynamic Ltd. Com-

pany; TX, USA38. Neptune Proteus Tidal Power PontoonTM, Neptune Renewable

Energy, UK39. Nihon University, Japan40. Northern Territory University, Darwin N.T., Australia.41. OCPSTM, Arnold Cooper Hydropower Systems, USA42. Open Hydro TurbineTM, OpenHydro Group Ltd., UK43. OptimsetTM, Optimset, ON, Canada44. PEEHRTM, Rua Lúcio de Azevedo,Lisboa, Portugal45. Pole Mer BretagneTM, Pole Mer Bretagne, France46. Pulse GeneratorTM, Pulse Generation Ltd.,UK47. RiverStarTM, Bourne Energy Pvt. Ltd.; Malibu, CA48. RotechTM, Tidal Turbine Lunar Energy Limited, UK49. Russian cross flow turbine Russian cross flow turbine50. Rutten Company, Belgium51. ScotrenewablesTM, Scotrenewables Tidal Turbine (SRTT), UK52. SeaFlowTM, Marine Current Turbines Ltd., UK53. SeasnailTM, Robert Gordon University, UK54. StringrayTM, The Engineering Business (EB), UK55. SwanturbinesTM, Swanturbines Ltd., UK56. TGL turbineTM, Tidal Generation Ltd., UK57. Thropton TurbineTM, Thropton Energy Services, UK58. Tidal FenceTM, Blue Energy International, BC, Canada59. Tidal Stream GeneratorTM, Tidal Hydraulic Generators Ltd.

(THGL), UK60. Tidal StreamTM, J A Consult, UK (Tidal Stream Turbine)61. TidelTM, SMD Hydrovision, UK62. TocardoTM, Teamwork Technology, NL63. TransverpelloTM Germany64. Tyson TurbineTM, Australia65. Underwater Electric Kite, US66. University College London, London UK67. University of British Columbia, Canada68. College of Engineering, University of Buenos Aires, Argentina69. Department of Mech. and Manu. Eng., University of Mani-

toba, Canada70. University of Southampton, UK71. Uppsala University, Sweden72. Vertical Axis Ring Cam Turbine, Edinburgh University, UK73. VIVACETM,Vortex Hydro Energy LLC; Ann Arbor, MI, USA74. Wanxiang Vertical Turbine Harbin Engineering University

(HEU), China75. Wild Water Power, Canada76. WPI Turbine- Water Power Industries, Norway

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