Proceedings of the 37th National & 4th International Conference on Mechanics Fluid Mechanics Fluid Power Proceedings of the 37th International & 4th National Conference on Fluid andand Fluid Power December 16-18, 2010, IIT Madras, Chennai, India.
FMFP2010
FMFP10 - TM - 23
December 16-18, 2010, IIT Madras, Chennai, India FMFP2010________
PERFORMANCE MEASUREMENT OF A TWO-BLADED HDARRIEUS TURBINE MADE FROM FIBREGLASS REINFORCED PLASTICS (FRP) BLADES Agnimitra Biswas Lecturer (Contract), Deptt. of Mechanical Engg., NIT Silchar NIT Silchar, Assam, India
Rajat Gupta Prof. (Mech. Engg) & Dean (R & C), Deptt. of Mechanical Engg., NIT Silchar NIT Silchar, Assam, India
ABSTRACT Vertical Axis Wind Turbine (VAWT) has got many advantages, like simple in construction, omni-directional, selfstarting, low wind applications etc, which make VAWT a viable proposition for functions like pumping, irrigation, grinding etc. However, VAWT is not gaining popularity largely due to its low efficiency. In this paper, the power coefficient (Cp) of a twobladed H-Darrieus turbine made from Fibreglass Reinforced Plastic (FRP) blades was measured in a subsonic wind tunnel for height-to-diameter (H/D) ratios of 0.85, 1.0, 1.54 and 1.72. Three types of wind tunnel blockages, namely solid, wake and sidewall blockages were considered for measuring Cp. Cp increased with the increase of H/D ratio up to the maximum, and then decreased even if H/D ratio was increased. Thus, there was an optimum H/D ratio for which Cp was the maximum. And the maximum Cp of 0.267 was obtained at an optimum H/D ratio of 1.0. Keywords: Two-bladed FRP, tip speed
ratio, wind tunnel blockage, power coefficient, H/D ratio INTRODUCTION Wind energy is the most prolific alternative source of energy for power generation. The rise in the demand for wind energy is reflected in the increasing growth of wind-based energy systems all over the world. Global Wind Energy Council (GWEC) has predicted that the global wind market will be growing by over 155% from its current size to reach 240 GW of total installed capacity by the year 2012. According to the figures released by GWEC (GWEC, 2009), the five top countries in terms of installed wind power capacity are USA (35,159 MW), Germany (25,777 MW), China (25,104 MW), Spain (19,149 MW) and India (10,926 MW). Wind turbine is the heart of any wind-based energy system. Though most of the wind turbines in the present era are HAWT type, but VAWT has got definite role to play especially in low wind speed condition. The VAWT
H-Darrieus turbine and obtained a maximum Cp of 0.54 at a tip speed ratio of 2.5. Gupta et al. (Gupta et al., 2010) experimentally evaluated Cp with tunnel blockage of a twisted three-bladed airfoil shaped H-Darrieus turbine made from lightweight aluminium blades and obtained a maximum Cp of 0.15 at H/D ratio of 1.10. Howell et al. (Howell et al., 2010) experimentally investigated the performances of a two-bladed HDarrieus turbine and obtained a maximum Cp of 0.25. Fibreglass plastics as wind turbine blades would be popular in near future since it has better fatigue properties (McGowan et al., 2002). Further, it is light weighted having a density of almost 1.9 g/cc, which is less than aluminium (2.7 g/cc). In this paper, an attempt has been made to experimentally study the performance of a two-bladed H-Darrieus turbine made from Fibreglass Reinforced Plastic (FRP). The FRP is anisotropic in nature unlike aluminium or steel, and it contains reinforcement of high aspect ratio (ratio of length to thickness of the reinforcement) to enhance directional properties. The physical properties (Gardiner et al., 2002; Schmit and Kevin, 1998) of a typical FRP are given in table 1. The FRP selected for the present study was a composite of polyvinyl chloride (PVC) type thermoplastic, reinforced by fine glass fibres. The turbine was designed and fabricated in the department.
could be a viable proposition for smallscale applications in the remote areas of any developing country (Islam et al., 2005). But the major problem with VAWT is its low power coefficient. Thus, the challenge lies in designing a VAWT having high power coefficient. There are at present several VAWT models commercially available such as Savonius turbine, Darrieus turbine, H-Darrieus turbine etc. S.J. Savonius, a Finnish scientist, developed Savonius turbine initially in 1929 (Savonius, 1931). Savonius tested about 30 different models of the turbine in the wind tunnel. The best model showed a power coefficient (Cp) of 31%. The eggbeater Darrieus wind turbine was originally invented and patented by Georges Jean Marie Darrieus, a French aeronautical engineer, (Darrieus, 1931). H-Darrieus turbine was included in the same patent of 1931. The major problem with eggbeater Darrieus turbine or Savonius turbine is that of stall at increased wind speeds created by the blade moving out of the wind in the downstream side. This limits the speed that the advancing blade can propel the whole rotation forward. However, HDarrieus turbine is self-regulatory at all wind speeds (Islam et al., 2005). The H-Darrieus turbine has two or three blades designed as airfoils, which are attached vertically to the central shaft through support arms. The support to vertical axis helps the turbine maintain its shape. Only few works, mostly related to prototype testing, on HDarrieus turbine were reported in the literature. Roynarin et al. (Roynarin et al., 2002) studied theoretically the power curves for a small prototype of
Physical Properties Thermal conductivity Coefficient of thermal expansion Density Tensile strength
Values 0.05 W/mK 0.9 – 1.5 x 10-5 in./in./0F 1.9 g/cc 148.4 MPa
101.6 x 106 N/m Modulus of elasticity 1 x 106 psi 3 x 106 psi Table 1- Physical properties of FRP
diameter & 12 cm length, through which blades were connected to the central shaft of 1.5 cm diameter. The blades were moved through the screwed bolts inwards and outwards to change the overall turbine diameter (D) and fixed at different locations by nuts. Changing D, but keeping H constant created total four H/D ratios. A ball bearing supported the shaft at the base. The central shaft, bearing and base were made from mild steel. The turbine was tested in an open circuit subsonic wind tunnel having wind speed adjustable between 0 to 35 m/sec. The H/D ratios selected in the study were 0.85, 1.0, 1.54 and 1.72. The turbine rpm was measured by a digital tachometer having a least count of 1 rpm, and wind velocity was measured by a pitot static tube.
Compressive strength
DETAILED DESIGN OF THE MODEL The two-bladed H-Darrieus turbine is shown in Fig 1. The height (H) of the turbine is 20 cm and width of the blades is 5 cm with an angular twist of 300 at the trailing ends as shown in Fig 1. Selfstarting ability of the turbine was enhanced by the twists at the tips; since starting torque would be high as dynamic pressure is also high at the twist ends. The blades were mounted in such a way that the twists provided on the blades were symmetrical. The blades were mounted on the supports of mild steel screwed bolts of 5 mm
Fig.1 – Two-bladed H-Darrieus turbine
motor having rated rpm of 2890 that drove the fan. The motor had a starter for switching on and off the fan. The operating range of the wind tunnel was 0–35 m/s. The turbulence intensity was less than 1%. The brief description of the wind tunnel is given in (Gupta el al., 2006).
THE WIND TUNNEL The tests were conducted on an opencircuit subsonic wind tunnel available in the department as shown in Fig.2. The cross-sectional area of test section of the tunnel was 30 cm x 30 cm. The length of the test section was 3 meters. The blower section consisted of a three phase 15 kW
Fig 2 – Schematic diagram of subsonic wind tunnel
ε = φ + β +γ
ANALYSIS OF RESULTS The performance of a wind turbine can be expressed as the variation of power coefficient (Cp) versus tip speed ratio (TSR) at any H/D ratio. The wind tunnel blockage effect was taken into consideration. When an object is placed in a wind tunnel, the object creates blockage to the flow, and it increases the local free stream wind velocity in the test section. In wind tunnel testing, its effect is taken into consideration to determine the actual power produced by the turbine. The total factor is the sum of the velocity increment caused by wake blockage, solid blockage and also sidewall blockage (Blackwell et al., 1977; Pope and Harper, 1966). The total blockage correction factors for H/D ratios 0.85, 1.0, 1.54 and 1.72 are 33.35%, 37.53%, 53.17% and 58.59% respectively. In the present study, the following relations have been utilized –
C
p
φ =
β =
AF AF = 4 ATS 4H ′ W
(4)
qc − qu qu
(5)
2
qc Cd, u ª 1 § AF ·º = = «1+ ¨ ¸» Where, qu Cd, c «¬ 4¨© AS ¸¹»¼
(6)
γ=
6δ w AS 6δ w ncH = 4 ATS 4H ′W
(7)
λ =
u Rω = V1 V1
(8)
For the two-bladed H-Darrieus turbine, the variations of Cp with respect to tip speed ratio for four H/D ratios: 0.85, 1.0, 1.54 and 1.72 are plotted based on the experimental observations. The plots of Cp are shown from Fig.3 to Fig.6. It can be observed from Fig.3 that, at H/D ratio of 0.85, Cp increases with the increase of TSR up to the maximum and then decreases even though TSR is increased. The maximum Cp of 0.242 is obtained at 2.124 TSR. At H/D ratio of 1.0, the variation of Cp follows the same trend as
1 ρ A ( V 1 2 − V 22 ) R ω = 2 1 3 ρ AV free _ block 2 ( V 1 2 − V 22 ) R ω = 3 V free _ block
Vfree_block= Vfree (1+ε)
(3)
(1) (2)
previous case, which may be observed in Fig.4. And the maximum Cp of 0.267 is obtained at 2.214 TSR. Therefore, the maximum Cp has increased with the increase of H/D ratio. Figure 5 shows that at H/D ratio of 1.54, the maximum Cp of 0.099 is obtained at 0.837 TSR. Now, the maximum Cp has dropped since the last H/D ratio of 1.0; therefore, the optimum H/D ratio at which Cp is the highest is 1.0. At H/D ratio of 1.72, Fig.6, the maximum Cp is 0.064 at 0.793 TSR. Thus, the highest value of Cp of 0.267 is obtained at H/D ratio of 1.0. The highest Cp of 0.267 corresponds to a blockage correction factor of 37.53%, which is quite high. In spite of high blockage, the result is quite relevant in
that its Cp is higher than the Cp of HDarrieus turbine without blade twist (Howell et al., 2010). And after scaling– up this turbine, it could be used for small-scale applications especially in remote places where grid-connected electricity is a scarce.
H /D = 0 .8 5 0 .2 5 0 .2 4
Cp
0 .2 3 0 .2 2 0 .2 1 0.2 0
0 .5
1
1 .5
2
2 .5
T ip S p e e d R a t io
Fig. 3 – Variation of Cp with TSR for two-bladed H-turbine at H/D = 0.85 H/D = 1 .0 0 .2 7 0 .26 1
Cp
0 .25 2 0 .24 3 0 .23 4 0
0 .5
1
1 .5
2
2 .5
Tip Sp e e d R a tio
Fig.4 – Variation of Cp with TSR for two-bladed H-turbine at H/D = 1.0
H/D = 1.5 4 0 .12 0.1
Cp
0 .08 0 .06 0 .04 0 .02 0
0 .3
0 .6
0.9
1.2
Tip S pe e d Ra t io
Fig.5 – Variation of Cp with TSR for two-bladed H-turbine at H/D = 1.54
H/D = 1 .7 2 0.0 7
Cp
0.0 6 0.0 5 0.0 4 0.0 3 0
0 .4
0.8
T ip Sp e e d Ra tio
1.2
Fig.6 – Variation of Cp with TSR for two-bladed H-turbine at H/D = 1.72 ii) From the present investigation, it is also seen that the total blockage correction factor comprising of three components of solid, wake and sidewall blockages is quite significant in the performance measurement of the HDarrieus turbine with the highest Cp of 0.267 obtained for 37.53% blockage correction factor.
CONCLUSIONS Based on the investigation, the following conclusions have been drawn: i) Cp increases with the increase of H/D ratio up to the maximum and then decreases even though H/D ratio is increased. Thus, it can be concluded that there is an optimum value of H/D ratio for which Cp is the maximum. And from the present study, the maximum Cp of 0.267 is obtained at the optimum H/D ratio of 1.0.
iii) It can further be concluded that blades made from FRP will be a viable option since it is light in weight, strong and has significantly high power coefficient even for high blockage effect, which is quite comparable to conventional Savonius like VAWT.
δw Ȧ İ
ACKNOWLEDGEMENTS The authors acknowledge with thanks the support provided by Mr. Sudhir Deb, Mr. Manik Rajbangshi, Mr. Sashi Mohan Roy and Mr. Abdul Salam for their help.
wall correction factor circular frequency of the turbine total blockage correction factor
REFERENCES Darrieus, G.J.M., 1931. US Patent no. 1 835 018. Gardiner, C.P., Mathys, Z., Mouritz, A.P., 2002. Tensile and Compressive Properties of FRP Composites with Localized Fire Damage. International Journal of Applied Composite Materials 9 (6) 353–367.
NOMENCLATURE A turbine swept area (HD) AS blade planform area (ncH) c blade chord Cd,u drag coefficient uncorrected for blockage Cd,c drag coefficient corrected for blockage Cp power coefficient D overall turbine diameter H height of turbine H/ height of test section n total number of blades dynamic pressure qc corrected for blockage qu dynamic pressure uncorrected for blockage Vfree free-stream wind velocity uncorrected for blockage free stream wind Vfree_block velocity with total blockage correction factor V1 wind velocity on the upstream of turbine (m/sec) V2 wind velocity on the downstream of turbine W width of test section φ solid blockage correction factor β wake blockage correction factor γ sidewall blockage correction factor Ȝ tip speed ratio (TSR)
Global Wind Energy Council (GWEC), 2009. Global installed wind power capacity 2008/2009 Retrieved from http://www.gwec.net/fileadmin/documents /PressReleases/PR_2010/Annex%20stats %20PR%202009.pdf Gupta, R., Biswas, A., 2010. Performance measurement of a twisted three-bladed airfoil-shaped H-rotor, International Journal of Renewable Energy Technology 1 (3) 279-300. Gupta, R., Das, R., Sharma, K.K., 2006. Experimental study of a SavoniusDarrieus wind machine. In: Proceedings of the International Conference on Renewable Energy for Developing Countries, University of Columbia, Washington DC, USA. Howell, R., Qin ,N., Edwards, J., Durrani, N., 2010. Wind tunnel and numerical study of a small vertical axis wind turbine. Renewable Energy Journal 35 412-422. Islam, M., Esfahanian, V., Ting, D.S-K., Fartaj, A., 2005. Applications of Vertical Axis Wind Turbines for Remote Areas. In: Proceedings of 5th Iran National Energy Conference, Tehran, Iran. McGowan, J.G., Connors, S.R., 2000. Wind power: a turn of the century review. Ann Rev Energy Environ 25 147–197.
Pope, A., Harper, J. J., 1966. Low Speed Wind Tunnel Testing, John Wiley & Sons, Inc., New York. Roynarin, W., Leung, P.S., Datta, P.K., 2002. The performances of a vertical Darrieus machine with modern high lift airfoils. In: Proceedings of IMAREST conference MAREC, Newcastle, U.K. Savonius, S.J., 1931. The S-rotor and its applications. Journal of Mechanical Engineering 53 (5) 333–338. Sheldahl, R.E., Feltz, L.V., Blackwell, B.F., 1977. Wind tunnel performance data for twoand three-bucket Savonius rotors. Journal of Energy 2 160-164. Schmit, K., 1998. Fiberglass Reinforced Plastic (FRP) – a comparison to traditional metallic materials. Retrieved May 29, 1998 from http://www.fiberbond.com/do cs/FRPdesign.pdf
