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Research Papers

Development and Testing of a Kilohertz Solar-to-Acoustic Energy Converter

[+] Author and Article Information
Kuan Chen, Mohammed Albonaeem

Department of Mechanical Engineering,
University of Utah,
Salt Lake City, UT 84112

Yeongmin Kim, Nam Jin Kim, Sang Hoon Lim

Department of Nuclear and Energy Engineering,
Jeju National University,
Jeju 690-756, South Korea

Wongee Chun

Department of Nuclear and Energy Engineering,
Jeju National University,
Jeju 690-756, Korea
e-mail: wgchun@jejunu.ac.kr

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received May 3, 2016; final manuscript received September 21, 2016; published online November 10, 2016. Assoc. Editor: Carlos F. M. Coimbra.

J. Sol. Energy Eng 139(2), 021005 (Nov 10, 2016) (8 pages) Paper No: SOL-16-1205; doi: 10.1115/1.4034910 History: Received May 03, 2016; Revised September 21, 2016

A thermal-to-acoustic energy converter (TAC) was developed and tested to produce sound waves in the kilohertz range directly from solar energy. The converter consisted of a glass window and a small amount of steel wool in the shape of a disk sealed in an aluminum housing. A Fresnel lens and a chopper wheel with 60 holes in it were employed to generate a pulsed sunbeam of approximately 200 sun intensity as the heat source of the TAC. Various designs and techniques were tested to improve the sound amplitude and signal-to-noise ratio of the converter at high frequencies. Reduction in air volume, better cooling, and improvement in air tightness were found to be effective in enhancing the sound amplitude. A shockproof mount commonly used in radio studios to reduce microphone vibration was essential in noise reduction for the TAC at high chopper wheel rotations. The sound amplitude was found to rapidly decrease with the increase in pulse frequency of the sunbeam at low frequencies. The relationship between the decibel value and frequency of the generated sound waves was changed to linear for sunbeam frequencies above 1 kHz. This is the frequency at which the penetration of surface temperature fluctuations into the aluminum housing becomes comparable with the aluminum housing thickness. At a given frequency, the sound amplitude increased almost exponentially with the increase in solar flux intensity. To the best of our knowledge, the 3 kHz sound frequency measured in our experiments is by far the highest frequency produced by a solar-to-acoustical energy converter.

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Figures

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Fig. 1

Shockproof mount and acoustic insulation materials applied to the TAC

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Fig. 2

TAC and equipment on a cart

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Fig. 3

Noise signal of the microphone inserted in the TAC before implementation of noise-reduction techniques. (Only the gasoline-powered generator was on.)

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Fig. 4

Background noises when the microphone was inserted in the TA converter after implementation of noise-reduction techniques

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Fig. 5

FFT analysis of the noise signal in Fig. 4

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Fig. 6

Noise signal when the chopper wheel was rotating at 16.7 RPS. The RMS amplitude of the microphone signal was 0.040 V (sound decibel value = 66.6 dB).

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Fig. 7

Noise signal when the chopper wheel was rotating at 41.9 RPS. The RMS amplitude of the microphone signal was 0.183 V (68.8 dB).

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Fig. 8

Noise signal when the chopper wheel was rotating at 50.3 RPS. The RMS amplitude of the microphone signal was 0.208 V (69 dB).

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Fig. 9

FFT analysis of the noise signal in Fig. 8

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Fig. 10

Measured noise amplitudes of different levels of noise reduction

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Fig. 11

Measured thermoacoustic signals before implementing noise-reduction techniques

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Fig. 12

Thermoacoustic signal at pulse frequency = 241 Hz. (The abscissa and ordinate ranges of the enlarged plot are: abscissa [0.004, 0.016]; ordinate [−8, 8].)

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Fig. 13

Thermoacoustic signal at pulse frequency = 860 Hz. (The abscissa and ordinate ranges of the enlarged plot are: abscissa [0.004, 0.016]; ordinate [−8, 6].)

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Fig. 14

FFT analysis of the thermoacoustic signal in Fig. 13

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Fig. 15

Thermoacoustic signal at pulse frequency = 1000 Hz. (The abscissa and ordinate ranges of the enlarged plot are: abscissa [0.004, 0.016]; ordinate [−6, 6].)

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Fig. 16

FFT analysis of the thermoacoustic signal in Fig. 15

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Fig. 17

Thermoacoustic signal at pulse frequency = 2500 Hz. (The abscissa and ordinate ranges of the enlarged plot are: abscissa [2 × 10−3, 5 × 10−3]; ordinate [−4, 3].)

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Fig. 18

FFT analysis of the thermoacoustic signal in Fig. 17

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Fig. 19

Thermoacoustic signal at pulse frequency = 3000 Hz. (The abscissa and ordinate ranges of the enlarged plot are: abscissa [2 × 10−3, 5 × 10−3]; ordinate [−3, 2].)

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Fig. 20

FFT analysis of the thermoacoustic signal in Fig. 19

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Fig. 21

Measured sound amplitudes at different sunbeam pulse frequencies

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Fig. 22

The relationship between sound amplitude and pulse frequency for different solar fluxes

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