U-Shaped Polymer Cladding and Hetero-Core Fiber Optic Sensors for Monitoring Scale Formation in Geothermal Brine

Abstract

In the development of fiber optic sensors for monitoring scale formation in geothermal brine, high sensitivity and easy handling are desirable features. These characteristics have already been successfully enhanced for U-shaped sensors with other applications. In this work, U-shaped fiber optic sensors for scale formation are reported. The sensors were easily fabricated by bending the exposed core of a multimode fiber optic. Two types of fiber optics, i.e., a polymer cladding silica fiber and a hetero-core structure fiber, were used in preparing the U-shaped sensors. The percentage of total internal reflection between water and the fiber core is affected by the high refractive index of the CaCO₃ scale formed on the surface. For laboratory measurements, the optical responses of the U-shaped fiber sensors with respect to calcium carbonate formation were investigated in a mixture of calcium chloride dehydrate and sodium hydrogen carbonate using a white-light source and a spectroscopic detector. The U-shaped fiber sensors were responsive to calcium carbonate formation on the sensor surface. Almost no change in the sensitivity was detected when comparing the U-shaped sensors with their straight counterparts. The transmittance changes were independent of the bending radius. Finally, the U-shaped polymer cladding fiber sensor was used to monitor the silica scale formation in geothermal brine at the Takigami geothermal plant.

Keywords:

• Fiber optic sensor
• geothermal brine
• scale formation

Acknowledgments

The authors would like to thank Idemitsu Oita Geothermal Co. Ltd. for their kind cooperation in our field work. We thank the members of West Japan Engineering Consultants, Inc. for the analysis of chemical parameters in geothermal brine. The authors also wish to thank the members of the West Japan Engineering Consaltants, Inc. and Kyushu University for their kind cooperation during our field work.

Additional information

Funding

This research was conducted as part of the New Energy and Industrial Technology Development Organization (NEDO) projects: Research and development of geothermal power generation technology/Development of technology for advanced use of geothermal energy/Development of chemical processing systems in order to properly utilize acid brine.