characteristics of a highly efﬁcient propeller type small wind...

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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. 2-13-1, Kita-otsuka,

Toshima-ku, Tokyo 170-0004, Japan

Received 17 December 2004; accepted 24 July 2005

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 2005 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

Renewable Energy xx (2005) 1–12

www.elsevier.com/locate/renene

0960-1481/$ - see front matter q 2005 Elsevier B.V.. All rights reserved.

doi:10.1016/j.renene.2005.07.008

* Corresponding author. Tel.: C81 3 5907 6421; fax: C81 3 5961 6424.

E-mail addresses: [email protected] (T. Matsushima), [email protected] (S. Takagi), muroya22@ntt-

f.co.jp (S. Muroyama).

http://www.elsevier.com/locate/renene

T. Matsushima et al. / Renewable Energy xx (2005) 1–122


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].

PZ1

2rAn3ðr : 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 1900 [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.

Diffuser

Wind flow

Low atmospheric pressure

Wind turbine

Fig. 1. Schematic cross sectional view of a diffuser and wind speed increase mechanism.

T. Matsushima et al. / Renewable Energy xx (2005) 1–12 3


2.1. 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 20 m!10 m analysis

space for the simulations. In these simulations, we used the thermo-hydrodynamic analysis

software program, I-DEAS [10]. 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.



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: 1 m

L: 2–4 m

T: 0.1–0.5 m

q: 0–128

2.2. Simulation results

Fig. 4 shows some examples of analysis of wind speed distribution when wind speed (n)

is 5 msK1. 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. 5 shows the wind speed ratio when the main body length L was changed and when

qZ48, TZ0.1 m and nZ5 msK1. 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 whenDZ1, LZ2,

TZ0.1 m and nZ5 msK1. 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 nZ5 msK1. 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 0.1 m, the wind speed ratio remained essentially constant.

3. Characteristics of field trial device

3.1. Field trial device

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

4.5

4.26.9

7

8

5.4.555

5.0

5.1

5.0

5.0

5.1

4.5

3.93.6

3.3

3.3

3.94.24.85.7

7.84.

4.5

4.5

4.23.93.63.34.24.85.7

7.8

5.4

4.5

5.45.0

5.05.0 5.4

8.4

4.5

4.5 5.04.23.94.8

7.2

6.95.4

Unit : ms–1

Unit : ms–1

Unit : ms–1

diffuser

diffuser

diffuser

(b) L=2 m

(c) L=3 m

(a) L=1 m

Fig. 4. Example of analysis results.Simulation was done at a wind speed n of 5 msK1. Dimensions of the diffuser

were DZ1 m, qZ48, and TZ0.1 m.



length LZ2 m, entrance diameter DZ1 m, expansion angle qZ48, and flange length

TZ0.1.

A five-blade propeller type wind turbine was installed in the diffuser to make a field

trial device (rotor diameter: 950 mm, rated power: 62 W at 8 msK1). Diffuser was made of

aluminum frame and a 0.5 mm thick polyester sheet on it to lighten the weight. In addition,

a tail unit 0.4 m high and 1.0 m long was installed at the top and bottom at the rear of the

diffuser to make the diffuser follow the wind direction.

1.0

1.2

1.4

1.6

1.8

2.0

Main body length L (m)

Win

d sp

eed

ratio

s

D=1, =4 degrees, T=0.1 m

0 1 2 3

Fig. 5. Relation between main body length L and wind speed ratio. Wind speed ratios are calculated based on an

outside speed of 5 msK1.

1.00 121082 4 6

1.2

1.4

1.6

1.8

2.0

Expansion angle (degree)

Win

d sp

eed

rat

ios

D=1 m, L=2 m, T=0.1 m

Fig. 6. Relation between expansion angle q and wind speed ratio. Wind speed ratios are calculated based on an


1.0

1.2

1.4

1.6

1.8

2.0

0 0.1 0.2 0.3 0.4 0.5

Flange length T(m)

Win

d sp

eed

ratio

s

D=1 m, L=2m, =4 degrees

Fig. 7. Relation between flange length T and wind speed ratios. Wind speed ratios are calculated based on an




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. 10 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.

3.2. 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

Conventionalwind turbine

Field trial device

Fig. 9. External view of field trial device installed at test site.

Shuntresistor

Shuntresistor

200AhVRLA

Data loggerPropeller typeanemometer

Field trial device

Dummyload

wind speedwind direction

Controller

Controller

voltage

current

Conventionalwind turbine

Fig. 10. Experimental apparatus.



power up until 2:00 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 12:00 pm the energy production of the conventional wind turbine was generally

00 2 4 6 8 10 12 2 4 6 8 10

5

10

15

Timea.m. p.m.

0

5

10

15

0

2.5

5.0

Win

d Sp

eed

(ms–1

)O

utpu

tpow

er (

W)

Out

putp

ower

(W

) Conventional wind turbine

Field trial device

12

(1) Wind speed

(2) Output power

Fig. 11. Changes in wind speed and output power characteristics over time (2002/10/19).

Time

Ene

rgy

prod

uctio

n (W

h)

0

1

2

3

4

Rat

io o

f th

e en

ergy

prod

uctio

n

a.m. p.m.

0

1

2

3

4

Field trial device

Conventional wind turbine

Ratio of energy production

0 2 4 6 8 10 12 2 4 6 8 10

Fig. 12. Changes in energy production characteristics and ratios over time (2002/10/19).



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 1.75 times at 5:00 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 1.65 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. 15 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

0.0

2.5

5.0

Win

d sp

eed

(ms–1

)

Timea.m. p.m.


Field trial device

0 2 4 6 8 10 12 2 4 6 8 10

0

5

10

15

20

Out

put p

ower

(W

)

0

5

10

15

20

Out

put p

ower

(W

)

(1) Wind speed

(2) Output power

Fig. 13. Changes in wind speed and output power characteristics over time (2002/11/16). 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 5, 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

00 2 4 6 8 10 12 2 4 6 8 10

1

2

3

Time

Ene

rgy

prod

uctio

n (W

h)

0

1

2

3

4

5

6

Ene

rgy

prod

uctio

n ra

tios

a.m. p.m.

Field trial device


Ratio of energy production

Fig. 14. Changes in energy production characteristics and ratio over time (2002/11/16). Both wind turbines were

fixed in the same wind direction.

00 1 2 3 4 5 6 7

20

40

60

80

Wind speed (ms–1)

Out

put p

ower

(W

)

Conventional wind turbineField trial device

Fig. 15. 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.



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

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[3] Solar Energy Utilization Handbook, p.55, Japan Solar Energy Society.

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Department of Aeronautical Engineering, Technion, T.A.E. report 32, (1963).

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