ARTICLE IN PRESS. Research and Development Headquarters, NTT FACILITIES, INC , Kita-otsuka, Toshima-ku, Tokyo , Japan

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1 Renewable Energy xx (2) Characteristics of a highly efficient propeller type small wind turbine with a diffuser Toshio Matsushima*, Shinya Takagi, Seiichi Muroyama Research and Development Headquarters, NTT FACILITIES, INC , Kita-otsuka, Toshima-ku, Tokyo 17-4, Japan Received 17 December 24; accepted 24 July 2 Abstract We studied the improved effects a diffuser had on the output power of small wind turbine systems, aiming to introduce these systems to radio relay stations as an independent power supply system. A frustum-shaped diffuser was chosen from an economical standpoint and wind speed distribution. The effect the diffuser s shape had on the wind speed was analyzed by simulation and showed that the wind speed in the diffuser was greatly influenced by the length and expansion angle of the diffuser, and maximum wind speed increased 1.7 times with the selection of the appropriate diffuser shape. The wind speed in the diffuser was fastest near the diffuser s entrance. We conducted field tests using a real examination device with a diffuser and confirmed that the output power of the wind power generator increased by up to 2.4 times compared to that of a conventional turbine. Moreover, it was confirmed that the diffuser was especially useful where the wind direction was constant. q 2 Elsevier B.V.. All rights reserved. Keywords: Wind turbine; Diffuser; Wind speed; Output power; Energy production 1. Introduction In recent years, the need to protect the global environment has seen the use of clean energy systems being extended into the telecommunications [1]. A good example of this is * Corresponding author. Tel.: C ; fax: C addresses: matsus29@ntt-f.co.jp (T. Matsushima), takagi2@ntt-f.co.jp (S. Takagi), muroya22@nttf.co.jp (S. Muroyama) /$ - see front matter q 2 Elsevier B.V.. All rights reserved. doi:1.116/j.renene.2.7.8

2 2 T. Matsushima et al. / Renewable Energy xx (2) 1 12 the introduction of stand-alone power supply systems for sites such as radio relay stations in mountainous areas where commercial power cannot be supplied [2]. Wind-solar hybrid systems are desirable as stand-alone power supply systems in such applications, in terms of steady power generation and stable power supply. The introduction of wind power devices to the hybrid systems is beneficial for obtaining more power, because wind power devices can generate power continuously throughout the day, so long as they receive wind energy. So, an improvement in the output power generation of wind-solar hybrid systems is desirable for this particular application. The energy (P) generated by a wind turbine is proportional to the swept area (A) of the turbine and the third power of the wind speed (n), as follows [3]. P Z 1 2rAn 3 ðr : specific gravity of airþ (1) Therefore, enlarging the swept area (A) or increasing the wind speed (n) can effectively increase the output power. In particular, since, the output is proportional to the third power of the wind speed, increased output will be obtained even with a slight increase in wind speed. One idea for increasing wind speed is the attachment of a diffuser to a wind turbine. This idea was proposed in the middle of 19 [4 6]. Recently, there have been reports on the construction of large-scale wind turbine prototype systems with diffusers in New Zealand [7]. The application of this kind of diffuser to small-scale wind turbines has also been tried [7 9]. Grassmann [9] has analyzed the pressure distribution around the propeller of a small wind turbine with wing-profiled ring diffusers and reported the test results of an increased output voltage on that turbine. However, he did not analyze the wind speed distribution around the propeller, the wind speed in the diffuser nor describe the relationship between the increase in wind speed and the diffuser s shape. This relationship is important and must be analyzed because the diffuser s shape is directly related to the improvement in output power generation and must be designed accordingly. Moreover, the actual increase in power generation by a real wind turbine has not been measured and reported. In this study, a diffuser with a simple external frustum view was chosen from the economical and ease of processing standpoints, and the relation between the diffuser s size and wind speed was clarified. We constructed a real examination device by fitting a diffuser to a propeller-type wind turbine and examined the effects it had on output power generation, in outside field tests. We report the results and field test data obtained from these tests. 2. Wind speed simulation in the diffuser Fig. 1 shows the propeller-type wind turbine surrounded by the diffuser that we studied in this report. In this examination, the diffuser shape was selected as a simple frustum, taking a cost suppression in the manufacturing process at practical use.

3 T. Matsushima et al. / Renewable Energy xx (2) Wind flow Low atmospheric pressure Wind turbine Diffuser Fig. 1. Schematic cross sectional view of a diffuser and wind speed increase mechanism Simulation method Wind speed in the diffuser was simulated, varying the external dimensions of the diffuser. Fig. 2 shows the shape of the diffuser, and Fig. 3 shows the 2 m!1 m analysis space for the simulations. In these simulations, we used the thermo-hydrodynamic analysis software program, I-DEAS [1]. I-DEAS is a three-dimensional design aid system developed by SDRC Co. in the United States and mainly used in the manufacturing of items such as automobiles, aircraft, and home electric appliances. Fig. 2. External view of the diffuser. Fig. 3. Analysis space.

4 4 T. Matsushima et al. / Renewable Energy xx (2) 1 12 In the simulations, after the diffuser was set in the space, a uniform amount of wind was sent from the inflow inlet toward the outlet. We simulated the speed of the wind passing through the diffuser, varying the diffuser s main body length (L), entrance diameter (D), its expansion angle (q) and flange length (T). Simulations were conducted on the diffuser without a wind turbine in it. The diameter (D) of the entrance was selected to be 1 m, taking the rotating diameter of a small propeller-type wind turbine into consideration. Simulation parameters were as follows; D: 1m L: 2 4 m T:.1. m q: Simulation results Fig. 4 shows some examples of analysis of wind speed distribution when wind speed (n) is ms K1. From these analyses, we found that a diffuser can influence wind speed and that the wind speed is highest at the entrance of the diffuser and lowest at the rear of the diffuser outlet. Analysis also showed that longer the main body, the higher the wind speed, and that maximum wind speed can be obtained along the inside of the diffuser near the entrance, regardless of the length of the diffuser. As the wind speed shows the maximum value at a point inside the entrance of the diffuser, the wind speed at this point was selected and its dependency on each of the parameters was analyzed. Fig. shows the wind speed ratio when the main body length L was changed and when qz48, TZ.1 m and nz ms K1. From this figure, we find that an increase in the L initially raised the wind speed ratio, but that as the L became larger the wind speed ratio gradually approached a constant value. Fig. 6 shows how the expansion angle q affected the wind speed ratio when DZ1, LZ2, TZ.1 m and nz ms K1. The wind speed ratio increased more steeply in angles of less than 48, reaching a maximum when expansion angle q was 68, and decreased at angles of more than 68. We analyzed the relationship between flange length T and the wind speed ratio, when DZ1, LZ2 m,qz48 and nz ms K1. Fig. 7 shows the results. The initial 1.4 wind speed ratio without flange increased to 1.7 when a flange was attached. However, it was found that flange length T had little effect on the increase of the wind speed ratio: for all values of T greater than.1 m, the wind speed ratio remained essentially constant. 3. Characteristics of field trial device 3.1. A field trial device was constructed on the basis of the above results. Fig. 8 shows the dimensions of the diffuser that was used in the field tests: main body

5 T. Matsushima et al. / Renewable Energy xx (2) 1 12 (a) L=1 m diffuser Unit : ms 1 (b) L=2 m diffuser Unit : ms 1 (c) L=3 m diffuser Unit : ms 1 Fig. 4. Example of analysis results.simulation was done at a wind speed n of ms K1. Dimensions of the diffuser were DZ1 m, qz48, and TZ.1 m. length LZ2 m, entrance diameter DZ1 m, expansion angle qz48, and flange length TZ.1. A five-blade propeller type wind turbine was installed in the diffuser to make a field trial device (rotor diameter: 9 mm, rated power: 62 W at 8 ms K1 ). Diffuser was made of aluminum frame and a. mm thick polyester sheet on it to lighten the weight. In addition, a tail unit.4 m high and 1. m long was installed at the top and bottom at the rear of the diffuser to make the diffuser follow the wind direction.

6 6 T. Matsushima et al. / Renewable Energy xx (2) 1 12 Wind speed ratios D=1, =4 degrees, T=.1 m Main body length L (m) Fig.. Relation between main body length L and wind speed ratio. Wind speed ratios are calculated based on an outside speed of ms K1. Wind speed ratios D=1 m, L=2 m, T=.1 m Expansion angle (degree) Fig. 6. Relation between expansion angle q and wind speed ratio. Wind speed ratios are calculated based on an outside speed of ms K1. Wind speed ratios D=1 m, L=2m, =4 degrees Flange length T(m) Fig. 7. Relation between flange length T and wind speed ratios. Wind speed ratios are calculated based on an outside speed of ms K1.

7 T. Matsushima et al. / Renewable Energy xx (2) Fig. 8. Dimensions of diffuser applied for the field trial device. Fig. 9 shows an external view of the device. A conventional five-blade propeller type wind turbine of the same type as that used for the device was set up at the same test site to compare their characteristics. Fig. 1 shows the makeup of the experimental apparatus. The output power from the two wind turbines is stored in a lead-acid battery and the excess energy is consumed by a dummy load. A propeller type anemometer was used for the measurement of wind speed and direction. Wind speed, wind direction, and output power were measured by a data logger at 1-s intervals Experimental results and discussion Fig. 11 shows typical data obtained for the wind speed and output power on the field trial device and the conventional wind turbine. Both devices had largely the same output Conventional wind turbine Fig. 9. External view of field trial device installed at test site.

8 8 T. Matsushima et al. / Renewable Energy xx (2) 1 12 Propeller type anemometer wind speed wind direction Data logger voltage current Conventional wind turbine Shunt resistor Shunt resistor Controller Controller Dummy load 2Ah VRLA Fig. 1. Experimental apparatus. power up until 2: pm, but from that time the field trial device delivered larger output power than the conventional wind turbine. Fig. 12 shows changes in energy production and the energy production ratio over time. Until 12: pm the energy production of the conventional wind turbine was generally Wind Speed (ms 1 ) Outputpower (W) Outputpower (W) (1) Wind speed (2) Output power Conventional wind turbine a.m. p.m. Time 12 Fig. 11. Changes in wind speed and output power characteristics over time (22/1/19).

9 T. Matsushima et al. / Renewable Energy xx (2) Energy production (Wh) Conventional wind turbine Ratio of energy production Ratio of the energyproduction a.m. p.m. Time Fig. 12. Changes in energy production characteristics and ratios over time (22/1/19). larger than or equal to that of the field trial device. However, the proportion of energy that was generated by the test device increased to reach a maximum of 1.7 times at : pm. Total energy production of the field trial device for the entire day was 1.16 times that of the conventional wind turbine. A larger energy production than this, however, had been expected on the basis of the simulation results obtained. We next focused attention on how the wind-following performance affected output power. Both wind turbines were fixed facing the direction where frequency distribution of the wind speed was high, and their output power was measured. The results obtained (Fig. 13) show the power from the field trial device was larger than that from the conventional wind turbine for the entire day. Energy production and the energy production ratios are shown in Fig. 14. Fixing both wind turbines increased the superiority of the energy production ratio of the field trial device to over one and raised the energy ratio to a maximum of 2.44 times, and in terms of total energy production for the entire day, the output of the field trial device was 1.6 times than that of the conventional wind turbine. These results indicate wind-following performance is a problem affecting the energy production characteristics of the field trial device. That is, when the wind direction changes frequently over a short period, it is difficult for the device to quickly and correctly adjust itself to the new direction of the wind. In these weather conditions, therefore, the device may not make effective use of wind energy. From a visual evaluation of both turbines, when the wind direction changed frequently, the conventional turbine adjusted itself more than did the field trial device, suggesting a relationship between this and the above-mentioned energy production ratio. Fig. 1 shows the output power measured for both wind turbines after fixing them in the direction where the frequency distribution of the wind was high. At each wind speed, the actual output power of the field trial device was from 3 to 4 times larger than that of

10 1 DTD T. Matsushima et al. / Renewable Energy xx (2) 1 12 Wind speed (ms 1 ) Output power (W) Output power (W) (1) Wind speed (2) Output power Conventional wind turbine a.m. p.m. Time Fig. 13. Changes in wind speed and output power characteristics over time (22/11/16). Both wind turbines were fixed in the same direction. the conventional turbine, in the wind speed conditions over 3 m/s. Consequently, we are convinced that the application of the diffuser is useful for the improvement of the output power from a wind turbine, when wind speed and direction are stable. From our numerical simulation, the maximum wind speed ratio and output power increase in the trial device are calculated to be 1.7 and, respectively. Therefore, the measured value was somewhat lower than expected. One possible reason is that the simulation was done on a diffuser without a wind turbine in it, and in the field tests, wind flow into the diffuser may have been affected by the presence of the wind turbine propellers. Another possible reason is the frequent and rapid changes in wind speed and direction. In these conditions, the generator may not give an optimal performance. 4. Conclusions We evaluated a wind turbine fitted with a diffuser with the aim of improving the turbine s output power characteristics. We used thermohydrodynamic analysis software to simulate the effect of the diffuser parameters on the wind speed, and evaluated the turbine

11 T. Matsushima et al. / Renewable Energy xx (2) Conventional wind turbine Ratio of energy production 6 Energy production (Wh) Energy production ratios a.m. Time p.m. Fig. 14. Changes in energy production characteristics and ratio over time (22/11/16). Both wind turbines were fixed in the same wind direction. Output power (W) Conventional wind turbine Wind speed (ms 1 ) Fig. 1. Power curve measured in the field tests. characteristics using a field trial device. The following results were obtained for a wind turbine with a diffuser. 1 We ascertained the effect on wind speed for each of the diffuser parameters (main body length L, entrance diameter D, expansion angle q and flange length T). Results showed that the parameters were able to increase the maximum wind speed in the vicinity of the diffuser entrance by around 1.7 times.

12 12 DTD T. Matsushima et al. / Renewable Energy xx (2) The fitting of a diffuser improved the power curve and increased the energy production of the wind turbine. A maximum energy production ratio of around 2.4 times was obtained by collecting wind energy in the turbine. 3 The diffuser is useful at sites where the wind direction is comparatively steady, by setting the turbine in the direction of the wind. References [1] Ueno H, Serada T. Wind turbine generation system & photovoltaic power generation system. NTT Power Build Facil J 22;39(231):42. [2] Tanezaki S, Kuramoto M, Yamanaka T. Construction of stand-alone power system using photovoltaic power generation. NTT Power Build Facil J 2;37(22): [3] Solar Energy Utilization Handbook, p., Japan Solar Energy Society. [4] Glauert H. The elements of airfoil and airsrew theory. Cambridge: Cambridge University Press; 199. [] A. Kogan, A. Seginer. Shrouded Aerogenerator Design Study II, Axisymmetrical shroud performance, Department of Aeronautical Engineering, Technion, T.A.E. report 32, (1963). [6] Oman RA, Foreman KM. Cost effective diffuser augmentation of wind turbine power generators Grumman aerospace corporation, Bethpage, New york Second workshop on wind energy conversion systems, Washington, D.C., June [7] Bet F, Grassmann H. Upgrading conventional wind turbines. Renew Energy 23;28:71. [8] Ohya Y, Toritani T, Sakurai A, Inoue M. Part 2 Development of high-performance wind turbine system by wind-lens effect The 24th wind energy symposium 22. [9] Grassmann H, Bet F, Cabras G, Ceschia M, Cobai D, DelPapa C. A partially static turbine first experimental results. Renewable Energy 23;28:1779. [1] Russell R, Louie J. Recent advances in thermal/flow simulation: integrating thermal analysis into the mechanical design process, 11th IEEE Semi-Therm, 199.