Duct Extension Effect on the Primary Conversion of aWave Energy Converter "Backward Bent Duct Buoy"
Yasutaka IMAI*1, Kazutaka TOYOTA*1, Shuichi NAGATA*1 andMohammad A.H. Mamun*1*1

Institute of Ocean Energy, Saga University,Honjo 1, Saga, 840-8502 Japan

The primary conversion efficiency of a "Backward Bent Duct Buoy” (BBDB) wave energy
converter is measured in two and three dimensional tank experiments. The effect of duct extension onthe primary conversion efficiency is investigated by attaching 15cm and 45cm ducts to the normalmodel. The experimental result shows that the duct extensions reduce the primary conversion
efficiency because they reduce the pitching motion of the BBDB.
Key Words : Backward Bent Duct Buoy, wave energy, primary conversion

The goal of this paper is to investigate the primary conversion
1. BACKGROUND
Because carbon dioxide emissions have been restricted, fossil
fuel-based energy generation is being converted to renewableefficiency of the BBDB. The effect of the buoy shape on theprimary conversion efficiency was also investigated.

energy generation. Equipment to utilize wind, tidal, wave,geothermal and solar energy are under investigation.
We have been studying the "Backward Bent Duct Buoy(BBDB)" wave energy converter [1-4]. The BBDB wasproposed by Masuda in 1986 [5]. Fig 1 illustrates the concept ofa BBDB. A BBDB consists of an L-shaped duct, a buoyancy
module, an air chamber, a turbine and a generator. Water comes
into the duct through a rear opening and pushes the air in the airchamber. This high-pressure air turns a turbine, generating
electricity.
Wave energy is converted to electricity in two steps. First,Fig.1 BBDB Concept

wave energy is transformed into pneumatic energy. This stage iscalled the primary conversion. Next, the pneumatic energy ischanged into electric energy, and this stage is called thesecondary conversion.

2. MODEL EXPERIMENT
Fig2 shows the dimensions of the model, which was madefrom an aluminum frame and an acrylic board. The model was

This buoy has some advantages. First, the primary conversion

0.85m long and 0.78m wide, and had a 0.35m draft. To maintainefficiency is high. Second, in some frequency ranges, the drift

the buoy’s level in the water, 37.5kg of ballast was loaded intoforce becomes negative. Usually, the wave comes from this side,

the buoyancy module. Two ultrasonic sensors are installed in theand the floating body drifts downstream. However the BBDB

air chamber to measure the water level. A pressure sensor wasdrifts upstream in some frequency ranges [6], reducing thealso installed in the air chamber. The sampling interval wasmooring force and mooring cost.

20Hz.
The motion of the model was measured with image

E-mail: imai@ioes.saga-u.ac.jp

processing, using images taken at a sampling interval of 10Hz.
The motion of a marker was converted to the motion around thecenter of gravity.

The primary conversion efficiency is investigated in two- andthree dimensional tank experiments. Table1 shows the geometryof the tanks. The model was moored in the center of the tanks,
and the incident waves, with amplitudes of 2.5-3.0 cm, weremade using a wavekmaker.

SagaUniversity
KyushuUniversity

Table 1 Tank geometry
LengthWidth
18.0 m
0.8 m

Water Depth
1.0 m

65.0 m

7.0 m5.0 m

The primary conversion efficiency is defined asη=EoutEin(1)

where Ein is the energy flux of the incident wave andEout is the energy flux of the air. Ein is defined as

Ein =Fig.2 Model dimensions

1
ρ gζ i 2 C g B
2

Eout =

To investigate the effect of the body shape, the duct wasextended by 15cm and 45cm. the duct extensions were attachedS T∂  η ( t ) + η2 ( t ) p (t )  1
 dt (3)∫T 0∂t 2
to the normal body (Fig.3).
whereFig.3 Extended Duct Models2(2)ρdensity of waterggravity

ζiamplitude of the incident wave

Cggroup velocity

BBBDB widthSarea of the air chamberT

period of incident wave

P(t)pressure in the air chamber

η1(t), η2(t)water level in the air chamber

Fig.4 Primary Conversion Efficiency

3. RESULTS

4. CONCLUSION

Fig. 4 shows the incident wave amplitude, internal
water level, pressure and conversion efficiency. The
horizontal axis is defined as the incident wave length (λ) /body length (L). Extending the duct reduces the pressureand, as a result, the primary conversion efficiency is alsoreduced.

The effect of duct extension on the primary conversion
efficiency of a BBDB was investigated in two- andthree-dimensional tank experiments. The results showsthat the duct extension reduces the primary conversion
efficiency because it reduces the pitching motion of theBBDB.

Fig. 5 shows the amplitude of motion. The amplitude isnon-dimensionalized, using the incident wave amplitude.
This figure shows a large pitching motion is required toachieve a high efficiency.

Fig.5 Motion amplitude[4] Toyota K., et al.(2006). "Study on Motion ofREFERENCES

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