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Citation:
M. Grabbe, K. Yuen, A. Goude, E. Lalander and M. Leijon
"Design of an experimental setup for hydro-kinetic energy conversion"
The International Journal on Hydropower & Dams, 2009, Vol. 16, Issue 5,
pp. 112-116
URL: http://www.hydropower-dams.com/
Design of an experimental
setup for hydro-kinetic
energy conversion
M. Grabbe, K. Yuen, A. Goude, E. Lalander and M. Leijon, Uppsala University, Sweden
A hydro-kinetic energy project has been underway in Sweden since 2000, and an in-stream prototype setup
for experiments at a site in a Swedish river is now in progress. The system comprises a vertical axis turbine
and a directly driven permanent magnet generator. Methods and choices used in designing the system are
described here. The turbine and generator are evaluated based on measurements and CFD simulations of
conditions at the site for the experimental setup.
T
he kinetic energy of water currents could make
a significant contribution to global electricity
production, provided suitable technical solutions are developed. As reported in the last issue of
H&D [Acker, 20091], research and development in the
marine and tidal current energy sector is becoming
more intense and several projects are reaching the
final stages of their first full-scale demonstration facilities. There are also projects looking at utilizing the
kinetic energy in rivers [Khan, Iqbal and Quaicoe,
20082].
Many tidal turbine designs, for example the 1.2 MW
SeaGen deployed in Strangford Lough, Northern
Ireland, by Marine Current Turbines [Fraenkel, 20073]
and the Free Flow System turbine deployed in New
York City’s East River by Verdant Power, resemble
stout, submerged wind powerplants, which seems a
reasonable starting point for exploiting the kinetic
energy of a fluid with a density 800 times higher than
that of air. At the same time, strong efforts are being
made to find other turbine concepts suitable for hydrokinetic energy conversion. More than one-third of the
76 tidal technologies surveyed by Khan et al [20094],
are vertical axis (cross-flow) turbines.
Research in the area of marine current energy has
been underway at Uppsala University since 2000. It is
believed that the functionality and survivability of a
marine energy system demands simplicity and robustness. Thus, the focus has been on the development of a
system with few moving parts, to limit the need for
maintenance. The concept is based on a vertical axis turbine, directly coupled to a permanent magnet synchronous generator. With no blade pitch mechanism and no
gearbox, the need for oil and grease is minimized. The
system can be placed on the bottom of the ocean or a
river where it would be protected from strong waves and
floating debris. The system is shown in Fig. 1.
Several energy converters may be interconnected in
a ‘farm’ configuration. The output from each unit will
be rectified and connected to a common DC-bus
before the aggregate output is inverted and transformed to grid specifications. This would require one
diode rectifier bridge for each unit, but only one
inverter and transformer to deliver power from all the
energy converters on the common DC-bus to the grid.
Previously, a prototype generator for hydro-kinetic
energy conversion has been constructed and used to
validate experimentally the finite element tool used to
design the generator [Thomas, et al, 20085].
Furthermore, the interaction between the turbine and
the generator has been studied [Yuen et al, 20096;
Lundin et al, 20097], and methods to evaluate the
influence of struts [Goude, Lundin and Leijon, 20098]
and vertical velocity profile [Goude, Lalander and
Leijon, 20099] on the performance and structural
mechanics of the turbine have been developed. The
next major step is to construct and deploy a full system for experiments in real conditions. A possible
research site has already been identified in a river in
Sweden [Lalander and Leijon, 200910]. In this article,
the design methods that have been developed so far
are described and applied to that site, with a focus on
the turbine and generator.
1. Methods
A fundamental idea within the project has been to
adopt a system approach. Thus, the work is necessarily interdisciplinary, and while methods can be developed for the various parts of the system, the whole
system must be evaluated in the context of the characteristics of the particular site at hand.
1.1 Characterization of sites
Fig. 1. Artist’s
impression of a fivebladed cross-flow
turbine on a gravity
foundation. The
generator is recessed
in the foundation.
112
In the area of characterizing the kinetic energy potential of a site, research has had a focus on tidal
resources. River sources have, however, also gained in
interest over the last few years. Although much is
common for the two, the site characterization differs;
where tidal resources can be calculated knowing the
amplitude of the tidal wave, river resources can be
characterized knowing the flow.
Hydropower & Dams
Issue Five, 2009
Water current measurements are often scarce, especially measurements made specifically for characterizing the hydro-kinetic potential of a site. Surface
observations for navigational purposes and river discharge data are more common, but the accuracy and
stringency of such data are often not sufficient to carry
out a reliable resource assessment.
The variation of current speed along a channel can
be obtained using numerical models with flow data
and high-resolution bathymetry data as input. In the
project at Uppsala University, a two-dimensional
hydrodynamic model (Mike21 Flow model FM) is
used for this purpose. The model is capable of calculating the velocity and water depth along the entire
channel. With the use of the model, the effects and
power output of turbines can also be estimated
[Lalander and Leijon, 200910].
water current changes, the rotation speed should be
changed proportionally, so as to maintain approximately the same tip-speed ratio. At the same time, the power
absorbed by the turbine will change proportionally to
the cube of the water velocity.
It should be recognized that maximizing the power
absorption of the turbine is not necessarily the same
thing as maximizing the power output of the whole
system, nor is it the same thing as minimizing cost per
kWh produced. It is, however, a reasonable guide in
designing the generator. Depending on site-specific
conditions and other design constraints, one may opt
to limit the power absorption at higher velocities, to
achieve a high degree of utilization [Clarke et al, 2006
20]. Such an approach could have a strong impact on
the economic feasibility of any renewable energy project [Leijon et al, 200321].
1.2 Turbine design and modelling
2. Experimental setup
Design and modelling of vertical axis turbines is a
complex task where the engineer is faced with many
design choices which will affect the turbine performance and structural strength. A number of authors
have presented numerical models of vertical axis turbines, but it appears that there is a lack of experimental data for comparison and validation of the models.
Pioneering work in offshore experiments with vertical
axis turbines has been carried out in Italy [Coiro et al,
200511] and Japan [Kiho, Shiono and Suzuki, 199612],
but it would still be desirable to have more detailed
data of the forces and moments on each blade.
In this study, a double multiple streamtube model
[Paraschivoiu and Delclaux, 198313; Paraschivoiu,
200214] is implemented for turbine design. The model
is based on experimental data for lift and drag coefficients obtained from Sheldahl and Klimas [198115].
Tip corrections resulting from a finite aspect ratio are
made, as described by Paraschivoiu [200214].
Dynamic stall is modelled using the Gormont method
[Gormont, 197316] with the modifications of Massé
[198117] and Berg [198318]. To incorporate the dynamic stall model with the finite aspect ratio corrections,
dynamic stall is calculated on the reduced angle of
attack obtained by the tip corrections, and the induced
drag is recalculated with the new value of the lift coefficient. Corrections to take into account the effect of
struts are made, using the method described by
Goude, Lundin and Leijon [20098], and the vertical
velocity profile was included using the method of
Goude, Lalander and Leijon [20099]; however the
expansion model was not taken into account, to make
the model compatible with the strut corrections.
A site was chosen where a full-system experimental
setup could be deployed, including a turbine, generator, and necessary power electronics for control and
power conversion to grid specifications. The purpose
of this is to prove the concept, to validate simulations,
and to learn more about the conditions for and characteristics of this type of energy conversion system.
An ideal site for research may differ from a site for
commercial electricity production, as the purpose of
an experimental setup is to take measurements under
various conditions rather than to produce electricity at
a competitive price. Guidelines in finding a site have
been related to issues such as variations in current
velocity, depth and width of the waterway, but also
practical issues such as proximity to the university,
other uses of the water, and availability of a site for a
monitoring station on land.
2.1 Söderfors in the Dal river
The kinetic energy potential of the Dal river has been
studied. Close to its outlet in the sea, about 1 km
downstream of a hydropower station in the town of
Söderfors, a suitable site has been identified, see Fig.
2 and the photo on p114. The channel is about 100 m
wide at the site with a depth of 6 to 7 m. Current measurements and water depth readings at the site were
made using a 1200 kHz ADCP from RD Instruments
over a 30 day period and compared with hourly flow
data from the upstream hydropower plant. A linear
relationship between velocity and flow was then
obtained.
1.3 Generator design and modelling
Today, design tools based on the finite element
method are standard for modelling rotating electrical
machines. The cable-wound permanent magnet generators presented here are modelled using an in-house
FEM software based on the program ACE [200119]. A
recently calculated machine designed for 5 kW, 10
rpm, and with 120 poles has shown good correlation
with experimental results [Thomas, et al, 20085].
A generator designed for this application is most
strongly characterized by low rotation speeds, in the
order of 10 rpm, and a wide operating range in terms of
speed and power. The design point has been a typical
operation point rather than the upper limit that the term
‘rated power’ tends to denote. A low current density and
a low load angle have been sought at the design point,
allowing for high over-loads. As the velocity of the
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Issue Five, 2009
Fig. 2. The channel
downstream from
the Söderfors hydro
plant. The velocity
data have been
generated using
Mike21.
113
View of the site at
Söderfors.
This linear relationship was used to extrapolate
velocity data for a wide range of flows. By using discharge data from between 2004 and 2008, the velocity distribution for five years at the chosen site could be
found. The annual velocity distribution, averaged over
the five years, is shown in Fig. 3. As the Figure shows,
the velocity is mostly within 0.4 and 1.4 m/s. For turbine simulations, a vertical profile was required. This
was obtained by averaging a velocity sample over 24
hours. The resulting vertical profile is shown in Fig. 4.
Fig. 3. Annual
water speed
distribution at
the selected site,
averaged over
five years.
Fig. 5. Cp versus tip-speed ratio for the selected turbine, using
the velocity profile in Fig. 4. Strut losses are included. A
maximum Cp value of 0.35 is obtained at tip-speed ratio 3.5.
2.2 Turbine for experimental setup
The first constraint for the turbine is the available depth
at the considered site. A five-bladed turbine with a height
of 3.5 m and a 3 m radius was considered suitable, leaving ample clearance from the bottom and the surface.
The number of blades and solidity affects various issues
such as blade chord length, and thus the structural
integrity of the blades and the weight of the machine.
The turbine (blade profile NACA0021, 0.18 m chord)
was simulated according to section 3.2 for water velocities of 0.4 to 1.5 m/s. The simulated power coefficient,
Cp, in Fig. 5 was calculated using the measured velocity profile in Fig. 4, where the turbine was assumed to
be located between 1 m and 4.5 m below the surface.
The value was normalized against the incoming energy of the flow, which was obtained by integrating the
incoming velocity profile over the turbine area.
The variations in maximum Cp caused by flow
velocity, as seen in Fig. 6, were calculated with homogeneous velocity profiles as the velocity profile only
had a small impact on the turbine performance.
However, the method developed to take the velocity
profile into account [Goude, Lalander and Leijon,
20099] is of importance when considering the load distribution and structural mechanics of the turbine. The
decrease in maximum Cp at low velocities can be
explained by a decrease in the Reynolds number of the
system, which increases the drag coefficient and lowers the stall angle.
2.3 Generator for experimental setup
in Söderfors
The site and turbine described above are fairly well
Fig. 4. Velocity
profile at the
selected site: 24 h
average.
114
Fig. 6. Maximum Cp for the selected turbine at different water
current velocities.
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Issue Five, 2009
Fig. 8. Electricity
production at
different speeds
during a year.
Table 1: The three evaluated generators and their
annual output together with the selected turbine.
Design voltage (V)
Machine length (mm)
Annual output (MWh)
A
40
125
24.4
B
60
183
24.9
C
80
241
24.4
Table 2: Design data for the three generators with a
design voltage of 40, 60 and 80 V,at 5 rpm.
Inner diameter (mm)
Outer diameter (mm)
Air gap (mm)
Number of poles
Magnet height (mm)
Magnet width (mm)
Winding factor
Conductor area (mm2)
Number of coils per slot
Slot opening width (mm)
Slot depth (mm)
Power factor
B in air gap* (T)
B in stator tooth* (T)
* at no-load
1835
2000
7.5
120
13
36
9/8
16
6
4.0
59.2
1.0
0.84
1.64
matched by the prototype generator used by Yuen et al
[20096]. However, the generator was slightly
redesigned to make better use of the low velocities
available in Söderfors.
As discussed by Yuen et al [20096], iron losses dominate at lower speeds, and these iron losses are proportional to the volume of iron in the stator. The amount
of iron in the stator can be reduced (while maintaining
the frequency) by, for example, making a shorter
machine. As a result, however, this will also reduce the
voltage. A lower voltage results in a higher current for
a given speed and power, thus resulting in higher copper losses at higher water current speeds. Table 1
shows the length and voltage at 5 rpm for three generators with a cross-sectional geometry based on that of
the prototype in the paper by Yuen et al. Details of the
generators are shown in Table 2. The generators are
simulated with a purely resistive load and frictional
losses in bearings are not considered at this point.
3. Results
In Fig. 7, the maximum power absorption of the turbine has been matched with the operation of the generators at different current velocities. We can see that
generator A has a better efficiency at low speeds, while
generator C has a better efficiency at high speeds.
In Fig. 8, the generator efficiencies are combined
with the Cp-curve and the water velocity distribution
(average for 2004 to 2008). The results show that B
gives a slightly better annual energy yield, though the
difference is small, see Table 1.
A lower limit of 0.4 m/s has been used, as the velocity is very seldom lower and this will not influence the
total power production. At high speeds, operation at
best tip-speed ratio, and thus maximum Cp, has been
assumed up to 1.5 m/s. Above that, the turbine may be
stall-regulated or stopped to reduce loads.
4. Discussion
The main purpose with this experimental setup, as
mentioned previously, is to prove the concept, validate
the simulation tools and gain valuable experience in
operating the unit in various conditions. Hence, the
proposed generator and turbine combination described
here should not be seen as optimized yet. On the contrary, validating the simulation tools with experimental results is believed to be an essential foundation for
moving forward and improving the system performance in ways that will be realizable in practice.
Aspects for possible improvements that have not
been discussed here include integrating the foundation
and support structure in such a way that not only
makes installation and retrieval operations easier, but
also improves the performance of the turbine. Another
topic of great interest is to develop blade profiles
which are specifically designed for hydro-kinetic
energy conversion.
◊
Acknowledgments
The Swedish Centre for Renewable Electric Energy
Conversion is supported by Statkraft AS, The Swedish Agency
for Innovation Systems (VINNOVA), The Swedish Energy
Agency (STEM) and Uppsala University. The work reported
on here was also supported financially by Vattenfall AB,
Ångpanneföreningen’s Foundation for Research and
Development, The J. Gust. Richert Foundation for Technical
Scientific Research, Östkraft’s Environmental Fund and CF’s
Environmental Fund. The numerical program Mike21 was used
with permission from Stefan Ahlman at the DHI Group in
Sweden. The flow data was provided by Vattenfall AB and
water level readings are from Fortum AB.
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Fig. 7. Efficiency for the three generators: in black, the
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turbine Cp, as shown in Fig. 6.
Hydropower & Dams
Issue Five, 2009
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M. Leijon
M. Grabbe was awarded his MSc degree in Engineering
Physics at Uppsala University, Sweden, in 2006. He is
currently working towards a PhD in the field of tidal energy at
the Division of Electricity at Uppsala University.
K. Yuen was awarded her MSc in Engineering Physics at
Uppsala University, Sweden, in 2001. In 2006 she returned to
the University to work towards a PhD in renewable energy
conversion.
A. Goude was also awarded his MSc in Engineering Physics
at Uppsala University, Sweden, in 2007. Currently, he is
working on his PhD with a focus on turbine simulation and
modelling.
E. Lalander obtained her MSc in Oceanography at
Gothenburg University, Sweden, in 2006. She is currently
working on her PhD in renewable energy conversion at
Uppsala University.
Prof. M. Leijon received his PhD in 1987 from Chalmers
University of Technology, Gothenburg, Sweden. From 1993 to
2000, he was Head of the Department for High Voltage
Electromagnetic Systems at ABB Corporate Research,
Västerås, Sweden. In 2000 he became Professor of Electricity
at Uppsala University, Sweden. He received the ‘John Ericsson
medal’ from Chalmers in 1984, the ‘Porjus International Hydro
Power Prize’ in 1998, the Royal University of Technology
‘Grand Prize’ in 1998, the Finnish academy of science ‘Walter
Alstrom prize’ in 1999, and the 2000 Chalmers ‘Gustav
Dahlén medal’. He received both the Grand Energy Prize in
Sweden and the Polhem Prize in 2001 as well as the Thureus
Prize 2003. He is a member of IEEE, IEE, WEC, CIGRE and
the Swedish Royal Academy of Engineering Science.
The Swedish Centre for Renewable Electric Energy
Conversion, Department of Engineering Sciences, Uppsala
University, Box 534, SE-751 21, Uppsala, Sweden.
Hydropower & Dams
Issue Five, 2009