(Published on May 2008 – JEC Magazine #40)

 

MASAHIKO SUZUKI HIDETO TANIGUCHI ASSISTANT GLOBAL ENERGY CORPORATION

 

Small wind turbines must meet safety, low-noise and high efficiency requirements at low wind velocities, too. Various studies have been carried out in Japan and abroad. Most of these, however, are just additional work on blades made by using common design techniques that follow the Blade Element/ Momentum Theory (BEM). Therefore, we decided to investigate blade shape starting with basic notions such as the leverage principle, the action-reaction law and Bernoulli's theorem, in order to address requirements for wind turbines used in urban environments.

 

Wind generators

There are two main types of wind generators as a function of the direction of their axis. The most common type of large-scale wind generators are horizontal axis (propeller type), where the blade axis is parallel to the ground. In vertical-axis generators, where the blade axis is perpendicular to the ground, wind direction control is unnecessary (Figure 1).

 

 

Wind turbines can also be classified into drag or lift type. The H- Darrieus model, for example, is a typical lift-type VAWT.

 

However, the wind power coefficient of VAWT generators is considered to be lower than horizontal-axis models. Typical VAWTs have slim blades attached with four or five components. They are designed to increase the wind power generation efficiency as rotor speed increases by reducing the blade surface area and, therefore, the drag. But beyond a certain rotor speed, if the number of blade elements increases, air resistance increases with acceleration. In addition, when you factor in the load on the generator, a VAWT can stall easily, because each blade transmits only a small amount of torque to the axis.

 

Vertical-axis Bellshion blades

In a VAWT using Bellshion-type blades, the double-vaned blade is designed to raise wind power generation efficiency by generating more lift through increased sweep at optimum rotor speed. Several vertical-axis Bellshion blade prototypes were manufactured and tested for increased generation efficiency.

 

Power generation efficiency of the wind turbine A wind turbine’s power is proportional to the swept area and the cube of wind speed. The theoretical maximum amount of power that a machine can extract from the wind is expressed by the Betz coefficient, which is 16/27, or 59.3%. The higher a wind turbine’s rotor efficiency, the higher the wind power coefficient (Cp).

 

Power P [W] P = Cp x (1/2) x ρ x A x V3 Wind power coefficient: Cp Air density: ρ (kg/m3) Swept area: A (m2) Wind velocity: V (m/s)

 

The wind turbine power (P) has been shown to be equal to the torque (N-m) x the angle of rotor speed (rad/s): P= torque x angle of speed.

 

This expresses the electric power converted by the generator from the wind turbine’s power. The greater the wind speed, the more power is transferred to the wind turbine shaft (axis) via the rotor, and the more electricity that can be generated from the wind turbine.

 

Characteristics of the Bellshion VAWT The Bellshion blade prototype tests helped to establish the conditions necessary for efficient extraction, as described below.

 

• Winglet (wing end stall avoidance): A standard straight blade can lead to stalling of the tip due to lack of lift at the tip. The Bellshion blade is a winglet which bends the end of the wing inward to keep the tip from stalling.
• Inarch(ring)-type arm (wing arm strength): When the vertical-type (H-Darrieus) wing is made larger, the rotor speed changes with wind velocity, creating a fictitious force and resulting in fatigue load on the root of wing arm. The Inarch(ring)-type wing arm method is adopted to solve this problem (Figure 2).
• Increased blade sweep area (for high torque output and the right rotor speed)

 

For the vertical-axis Bellshion model, the unit blade length is reduced, the most suitable chord length is taken, a sweep area is determined, and lift power is calculated.

 

It will not stall easily, even at higher torque and if it is subjected to load. Also, increasing rotor speed to the right extent can maximize the wind force efficiency (Figure 3).

 

 

Performance of the vertical-axis Bellshion blade

Wind tunnel test

A wind tunnel test was carried out at the Ashikaga Institute of Technology’s synthetic research centre to validate the blade’s performance (Figure 4). An open wind tunnel with a 1.04 x 1.04 m wind exit and variable speed was used. Wind velocity measurements were carried out using a Betz-type manometer + pito pipe. The torque test was carried out at the Ushiyama lab (inverter motor type). Test wind velocities were 4, 6, 8, 10, 12, and 14 m/s. The characteristics of the wind turbine tested were as follows: radius, 0.4 m; blade length, 0. 8 m; number of blade elements, two; swept area 0.64m; (radius x 2x blade lenght).

 

 

The maximum wind power coefficient is reached at a tip speed ratio of about 1.7, and exceeds Cp 0.3 in wind velocities higher than 12 m/s. An examination of the vertical-axis Bellshion blade’s performance in the wind tunnel confirmed that a tip speed ratio of 1.7 generates more power output (Figure 5).

 

 

Tab. 1: Measurements
Wind speed (m/s)	Rotor speed (rpm)	Output power (W)	Wind power coefficient (Cp)
4	163	5.974	0.250
6	253	22.26	0.278
8	320	55.29	0.292
10	412	109.6	0.297
12	499	192.8	0.302
14	554	313.9	0.310

 

Field testing

Field tests were carried out at the Global Energy Tochigi laboratory in the following conditions: test wind turbine radius, 1.0 m; blade length, 2.5 m; number of blade elements, two; swept area 5m²

.

 

The wave pattern of the wind velocity and the rotor speed is almost the same, as seen in figure 6, confirming that the blade is able to cope instantly with the strength and direction of wind. While in the wind tunnel test it reached maximum power output at a tip speed ratio as low as 1.7 for a wind power coefficient of about 0.3, the field test results confirmed that a wind power coefficient of more than 0.4 could be obtained at a tip speed ratio of 2.0. Wind power coefficients of more 0.4 are difficult to obtain, even with large high-efficiency wind generators of the propeller type.

 

Tab. 2: Bellshion performance measurements (field test)
Time	Average for minute							Maximum for minute
	Wind speed [m/s]	Tip speed [m/s]	Tip speed ratio	Retor speed [rpm]	Voltage [V]	Electric current [A]	Output power [W]	Wind speed [m/s]	Rotor speed [rpm]	Voltage [V]	Electric current [A]	Output power [W]
14:30	4.1	8.1	1.98	77	52.7	1.60	82.3	5.8	108	53.3	2.97	158.3
14:31	4.7	10.2	2.19	98	53.5	2.61	136.7	5.7	118	54.4	4.28	232.8
14:32	2.9	4.8	1.63	46	52.1	0.58	26.5	3.9	80	53.0	1.60	84.8
14:33	4.0	7.5	1.88	72	52.4	1.43	71.7	5.5	89	52.8	2.10	110.9
14:34	6.5	12.6	1.94	121	55.6	5.12	292.5	8.1	149	58.2	8.38	535.1
14:35	5.9	14.0	2.36	133	56.3	4.24	243.6	8.8	208	59.4	6.89	406.5
14:36	4.6	9.7	2.11	92	53.4	2.28	122.1	6.0	113	54.2	3.62	196.2
14:37	4.6	10.0	2.18	96	53.4	2.46	130.8	5.8	114	54.0	3.73	201.4
14:38	5.5	11.6	2.12	111	54.5	3.87	212.9	7.2	138	56.8	6.74	382.8
14:39	6.1	12.8	2.10	122	55.4	4.73	264.4	7.3	133	56.4	6.19	349.1
14:40	5.2	11.0	2.11	105	54.1	3.23	175.6	7.0	124	55.1	5.09	280.5
14:41	5.7	12.2	2.15	117	54.8	4.08	226.9	7.1	134	56.5	6.42	362.7
14:42	5.3	11.6	2.19	111	54.5	3.62	198.1	6.4	128	56.0	5.51	307.4
14:43	4.2	8.8	2.10	84	53.4	2.04	107.4	6.1	119	54.7	4.32	236.3
14:44	3.2	5.9	1.86	57	52.3	0.83	41.2	4.3	64	52.4	1.02	53.4

 

Field testing of vertical axis Bellshion blade in multiple-stage configuration