Design of Hetero-Core Fiber-Optic SPR Sensor for Reliable Detection of Geothermal Scale Deposition

Abstract

Scale formation caused by the precipitation of inorganic minerals is a major operational challenge in geothermal systems, compromising heat transfer and causing equipment fouling. This study is the first to report a surface plasmon resonance (SPR) sensor based on a hetero-core fiber-optic architecture for the real-time monitoring of geothermal scale formation. Its fiber-optic configuration enables highly reproducible sensor fabrication without manual cladding removal, ensuring consistent sensor performance. The reproducibility and refractive-index sensitivity of the proposed sensor are evaluated and compared with those of a conventional unclad-fiber sensor. The sensor exhibits significantly reduced signal variability and enhanced sensitivity. When tested through the laboratory-scale deposition of representative geothermal scale species, it produces clear, reproducible SPR peak shifts. Field tests at three geothermal sites demonstrate its practical advantages. Notably, silica-dominant scale species, which cannot be quantitatively evaluated using conventional fiber sensors, are successfully detected through SPR peak shifts under field conditions. In addition, scale deposition rates, which are evaluated within approximately 15 h using conventional gravimetric methods (∼8.7 mg cm⁻² h⁻¹), are assessed within only a few hours using the SPR sensor. Thus, the hetero-core fiber-optic SPR sensor is an effective platform for the rapid in situ monitoring of geothermal scale formation.