A partially static turbine first experimental results

Transcription

1 Renewable Energy 28 (2003) Technical note A partially static turbine first experimental results H. Grassmann, F. Bet, G. Cabras, M. Ceschia, D. Cobai, C. DelPapa Dipartimento di Fisica, Universita di Udine, Via delle Scienze 208, I Udine, Italy Received 27 December 2002; accepted 6 February 2003 Abstract Recently it has been shown in a fluidodynamic simulation, that a wing-profiled structure of rather small size placed in the vicinity of a wind turbine augments the power of the wind turbine. In this paper we present the first experimental results from a prototype Elsevier Science Ltd. All rights reserved. Keywords: Wind turbine; Shroud; Optimization; Cost of energy 1. Introduction In a flow of air, a field of low pressure forms over the bent surface of a sail or a wing. If one brings the wing close to a wind turbine, the pressure behind the propeller of the turbine decreases. In [1] we have simulated a wind turbine inserted in a shroud, which consisted of two wing-profiled rings. Due to the presence of the shroud, the power of the turbine doubled. The surface of the shroud in [1] was not much larger then the surface covered by the propeller. For conventional wind turbines it costs between 250 and 500 to cover one m 2 of airflow. If one is able to construct shrouds which cost per m 2 significantly less than this, a cheaper way of using wind energy would result. (For a more detailed discussion of the possible advantages of shrouded turbines see [2] and the references there.) Corresponding author. Tel.: ; fax: address: hans.grassmann@fisica.uniud.it (H. Grassmann) /03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)

2 1780 H. Grassmann et al. / Renewable Energy 28 (2003) In our simulation study [1] we were mostly concerned about the wing structure, and we paid little attention to the propeller. In order to save manpower and computing time, the propeller consisted simply of 36 two-dimensional blades without any profile. Such a propeller cannot be made in the real world. As we will discuss in the following, the properties of the propeller are of decisive importance for the performance of a shrouded turbine. The propeller therefore should be optimized for use in a shroud system. Such an optimization requires an effort, which would go beyond our actual possibilities. Therefore, in this paper we make use of not optimized propellers and explore whether their performance is well simulated by our program. 2. The prototype Our funding situation meant that privately owned wind turbines were used for our measurements, we could not use a wind tunnel, and we had no computer readable anemometer. We were therefore limited to performing relative measurements, comparing the performance of two wind turbines, which were mounted on a truck. The turbines are two Aero2gen from the British company LV motors. They have a diameter of 56 cm, corresponding to an area covered by the rotor of about 1/4 m 2. The shroud has the same structure as in our model in [1], two concentric wingprofiled rings, the outer ring has a diameter of 84 cm. (The cross-section of the air flow covered by the shroud is about twice as large as the one covered by the propeller only.) The shroud was constructed in fiberglass and wood by the company A. Sturli, Italy. The measurement procedure consists in first recording the output voltages of the two bare turbines at various speeds of the truck. Then the shroud is mounted around one turbine and the measurement is repeated. In this way the voltage of one generator with respect to the other one can be measured with a precision of about 4%. (But the wind speed itself is not known to better than about 10% and is therefore not used for our study). A photo of the prototype can be seen in Fig. 1. We operated the generators either as open circuits, in this case their output voltage is an approximate measure of their speed of rotation. Or we put various ohmic resistances as a load, in which case the output voltage measures the power produced by the generator, P = V 2 /R, with the power of the generator P, the voltage V and the resistive load R. 3. Fluidodynamic simulation As in [1], we used the program Star-CD for perfoming a fluidodynamic simulation of the prototype. The numerical simulation is performed on a cell-centered, unstructured mesh. The integration volume is composed of about 500,000 tetrahedric cells. Because of the symmetry of the problem, only one blade is simulated. For the numerical solution an upwind differencing scheme and the k-epsilon turbulence

3 H. Grassmann et al. / Renewable Energy 28 (2003) Fig. 1. truck. Prototype shrouded turbine (turbine 2) and bare turbine (turbine 1) for reference mounted on model are used. As convergence criterion we require the residuals to be less than 10 3 for the three velocity components and for the mass flow. 4. Measurements For the first measurement we used the Aero2gen turbine as delivered from LVM. The original blades of the Aero2gen are of very simple construction: the angle of inclination of the blades is constant over their radius, r. The simulation shows that consequently a large vortex behind the turbine is created. When one adds the shroud, this vortex strongly increases. As a result the shroud augments the power of the turbine by only 20% with these blades. Also, in the experiment we observe an increase of power by 20%. The measured output voltage of the generators is no better than 4%, which results in an error on the observed power of about 40% of its value. Also, the simulation may suffer from considerable uncertainties due to the presence of strong turbulent activity. Therefore one should not overemphasize the agreement between calculation and measurement. We simply conclude at this point, that the quality of the propeller blades is very important for the performance of a shrouded turbine. As a first step towards getting more suitable blades, we adopted a new design, analogous to the original blades, but it does have an inclination, which changes over

4 1782 H. Grassmann et al. / Renewable Energy 28 (2003) the radius of the blade, in such a way, that the apparent wind always has the same direction with respect to the blade. Based on this design, the rapid prototyping laboratory of Prof. C. Bandera of the University of Udine (with M.Zanzero and M.Felice), created a set of new blades for our turbine. We again measured the output voltages of the two generators at different truck velocities, once without a shroud present and once with the shroud mounted around turbine 2. Fig. 2 shows, as an example, the result of one of these measurements where the Fig. 2. Output voltage from generator 2 versus output voltage from generator 1. Above : turbine 2 without shroud. Below : shroud mounted at turbine 2.

5 H. Grassmann et al. / Renewable Energy 28 (2003) output voltage from turbine 2 with and without shroud is plotted versus the output voltage from turbine 1, turbine 1 serving as reference. Fig. 2 is from a measurement without ohmic load. Fig. 2 shows, that with the improved blades, the shroud increases the output voltage of the generator 2 by about 25%. Fig. 3 shows the increase in output voltage for different loads and without any load. We see that the voltage, and therefore the speed of rotation of the turbine, increases by 27% without any ohmic load. If the output terminals are connected by a resistance, the increase in voltage is 25% at high wind speeds, and up to about 40% at low wind velocities. This corresponds to an increase in power of 55% at high wind speeds and of 100% at low wind speeds. The power of the turbine as predicted by Star-CD is shown in Fig. 4, at a wind velocity of 5 m/s and at different speeds of rotation of the propeller, w. Fig. 4 also shows functions of the type b w 2, with two different values of b. They correspond to the power curve of some hypothetical generator. The combined system of propeller and generator will operate at the point where their respective power curves intersect. Fig. 4 can then be read in two different ways: given a certain wind velocity, the generator will increase its power by about 50% if b is small, and by about 100% if b is large. Or: given a certain generator, its power will increase by about 50% at higher wind speeds, and by 100% at lower wind speeds. From this, we conclude that the simulation program Star-CD gives a quite reasonable description of the prototype. The coupling between the low-pressure field of the turbine and the one created by the wings, as predicted by Star-CD, indeed exists. Since the shroud, which we constructed, results in an increase of power by 20% with poor quality propeller blades and of % with somewhat improved blades, it seems reasonable to expect, that with this shroud a further increase in power should be possible, once the propeller is optimized. Fig. 3. Increase in generator output voltage V2 due to a wing system, as function of V1. Generator 2 is operated without load (open circuit), and with a load of 72 or 110, respectively. A wind velocity of 8 m/s corresponds about to V1 = 53 V, and at 4 m/s we have about V1 = 18 V.

6 1784 H. Grassmann et al. / Renewable Energy 28 (2003) Fig. 4. Power absorbed by the propeller for different speeds of rotation, w, at a wind speed of 5 m/s, as calculated by Star-CD. Circles are for the bare turbine, squares for the same turbine with a shroud. The power of our generator is proportional to w 2 in the figure we indicate the power of two hypothetical generators as function of w: b w 2, The dotted line indicates a stronger generator of the same kind. 5. The Betz limit Betz has shown, that for reasons of energy and momentum conservation, the air flow must widen, when approaching a wind turbine, and that therefore a maximum 59% of the wind s energy can be extracted by an ideal turbine [3]. InFig. 5 we Fig. 5. Cross-section through the fluidodynamic model of the prototype. Five particles are tracked on their way through the device.

7 H. Grassmann et al. / Renewable Energy 28 (2003) show a side view of our device with several flow lines (simulation). Of course, our apparatus must also obey energy and momentum conservation, but it does so without causing a widening of the flow in front of the device, as can be seen in Fig. 5. This feature of our device is also of use when applying the same principle to a flow of water. One could extract the kinetic energy from a sea current by means of a propeller installed underwater [4]. This would be most feasible, where the currents are strongest, for instance between two islands. However, a conventional propeller might slow down and deviate the current. A device as described here could be inserted into the current, without disturbing it. 6. Conclusions Using a shroud system, which is about the same size as the propeller, we have increased the power output of a wind turbine by a factor of 55% at large wind velocities and 100% at small wind velocities, using a non-optimized propeller. This confirms the previous fluidodynamic simulation presented in [1]. The quality of the propeller is decisive for the performance of such a system. A dedicated program of optimization is needed for the propeller. This needs to be done next and it will require a rather extensive effort. We hope that this paper will help us to find partners in other laboratories and in industry. Acknowledgements A. Soldati and his collaborators A. Agosto, M. Ganis and S. Snidero have helped us with many fruitful discussions. Three of the students of the corso di laurea fisica computazionale have participated in some of the measurements: F. Barbarino, M. Citossi, F. Paladino. The CD adapco group has helped us on many occasions, in particular A. Massobrio. References [1] Bet F, Grassmann H. Upgrading conventional wind turbines. Renewable Energy 2003;28(1):71 8. [2] Frankovic B, Vrsalovic I. New high profitable wind turbines. Renewable Energy 2001;24: [3] Betz A. Wind-Energie und ihre Ausnutzung durch Windmuehlen, Oekobuch reprint, Staufen. [4] New Scientist 1998; 2139: 38.