Design of an experimental setup for hydro-kinetic energy conversion

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http://uu.diva-portal.org

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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/

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T

he kinetic energy of water currents could make a significant contribution to global electricity production, provided suitable technical solu- tions 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 facil- ities. 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 hydro- kinetic 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 robust- ness. 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 trans- formed 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

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.

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.

Fig. 1. Artist’s impression of a five- bladed cross-flow turbine on a gravity foundation. The generator is recessed in the foundation.

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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 dis- charge 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 calcu- lating 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].

1.2 Turbine design and modelling

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 coeffi- cients 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.

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

water current changes, the rotation speed should be changed proportionally, so as to maintain approximate- ly 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 pro- ject [Leijon et al, 2003²¹].

2. Experimental setup

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.

Fig. 2. The channel downstream from the Söderfors hydro plant. The velocity data have been generated using Mike21.

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This linear relationship was used to extrapolate velocity data for a wide range of flows. By using dis- charge 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.

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, leav- ing 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 homo- geneous 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 low- ers the stall angle.

2.3 Generator for experimental setup in Söderfors

The site and turbine described above are fairly well Fig. 5. Cpversus tip-speed ratio for the selected turbine, using the velocity profile in Fig. 4. Strut losses are included. A maximum Cpvalue of 0.35 is obtained at tip-speed ratio 3.5.

View of the site at Söderfors.

Fig. 3. Annual water speed distribution at the selected site, averaged over five years.

Fig. 4. Velocity profile at the selected site: 24 h average.

Fig. 6. Maximum Cpfor the selected turbine at different water current velocities.

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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 dom- inate at lower speeds, and these iron losses are propor- tional 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 cop- per 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 con- trary, 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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Table 1: The three evaluated generators and their annual output together with the selected turbine.

A B C

Design voltage (V) 40 60 80

Machine length (mm) 125 183 241

Annual output (MWh) 24.4 24.9 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) 1835

Outer diameter (mm) 2000

Air gap (mm) 7.5

Number of poles 120

Magnet height (mm) 13

Magnet width (mm) 36

Winding factor 9/8

Conductor area (mm²) 16

Number of coils per slot 6

Slot opening width (mm) 4.0

Slot depth (mm) 59.2

Power factor 1.0

B in air gap* (T) 0.84

B in stator tooth* (T) 1.64

* at no-load

Fig. 8. Electricity production at different speeds during a year.

Fig. 7. Efficiency for the three generators: in black, the generators alone; and in grey, the generators together with the turbine Cp, as shown in Fig. 6.

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A. Goude E. Lalander

K. Yuen

M. Leijon M. Grabbe

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.