Fiber Optic Tech for Measuring Salt in Heat Pumps

What is it about?

This paper introduces a new way to measure the concentration of lithium bromide (LiBr) in a water-LiBr mixture used in absorption heat pumps (AHPs), which recycle low-quality waste heat from factories into useful energy, reducing environmental harm from wasted heat. Traditionally, LiBr levels are checked using refractometers or charts to avoid crystallization that could damage the pumps, but these methods aren't real-time or in-place. Here, researchers use multimode interference (MMI) in a simple fiber optic setup: a singlemode-multimode-singlemode (SMS) fiber where the middle multimode section has no outer coating, so it senses the mixture's refractive index (how light bends) when dipped in. They prepared 13 mixtures from 44% to 61% LiBr, shone a 1555 nm laser through the fiber, and measured output light intensity as voltage. Results showed three patterns: intensity rises up to 51% LiBr (refractive index 1.421-1.439), drops sharply to 57% as the mixture's index overtakes the fiber's (causing light scatter), then rises again due to crystal formation. They derived math equations—like a quadratic fit for low concentrations—to estimate LiBr from voltage readings. The sensor is cheap, easy to make by fusing fibers, and could monitor pumps in real-time without stopping operations. Tests at room temperature mimic Absorption Heat Pump conditions, showing promise for preventing clogs while boosting efficiency in eco-friendly cooling/heating systems that cut fossil fuel use.

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Photo by Ronald Crow on Unsplash

Photo by Ronald Crow on Unsplash

Why is it important?

This study uniquely adapts low-cost MMI fiber optics for in-situ LiBr monitoring in AHPs, achieving quadratic fits (R²=0.999) for 44-51% ranges where crystallization risks peak—surpassing offline refractometry by enabling real-time alerts. Timely amid industrial heat waste (20-50% lost, per global reports) and renewable shifts, it addresses AHP underuse due to maintenance fears. Impact: Could prevent downtime in factories (e.g., chemicals, saving 30-60% energy), extend pump life 20-50%, and integrate with solar hybrids for zero-emission cooling—potentially cutting CO2 by tons/site, with 2-3 year payback, fostering sustainable tech in heat-stressed regions like Mexico.

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