Hydrogen gas offers a compelling alternative energy source due to its numerous advantages, including being lightweight, easily storable, energy-rich, and much cleaner than fossil fuels—producing no harmful emissions. Consequently, it has wide-ranging applications in fields such as transportation, construction, power generation, and various industries. However, because hydrogen is highly flammable, its safe and ubiquitous use calls for dependable leak detection and purity verification methods. This demand has led to the advancement of trace-gas sensing technologies. Though various hydrogen detection methods exist, none have achieved optimal performance.
One noteworthy approach is tunable diode laser absorption spectroscopy (TDLAS), which has garnered much interest for gas detection. TDLAS presents several advantages, such as non-invasive measurement, real-time detection, high specificity, quick response times, cost-effectiveness, and the ability to measure multiple components and parameters simultaneously. The method is based on the principle that gases absorb light at particular wavelengths, resulting in a specific dark line in the absorption spectrum, referred to as the absorption line. By assessing how much laser light has been absorbed at this wavelength, the concentration of the target gas can be determined. However, measuring low levels of hydrogen using TDLAS presents challenges due to hydrogen’s relatively weak absorption in the infrared spectrum compared to other gases.
In response to this challenge, a team of researchers from Japan, led by Associate Professor Tatsuo Shiina from the Graduate School of Engineering at Chiba University, developed a groundbreaking method for accurately measuring hydrogen gas using TDLAS. The team included Alifu Xiafukaiti and Nofel Lagrosas from Chiba University, Ippei Asahi from the Shikoku Research Institute Inc., and Shigeru Yamaguchi from Tokai University’s School of Science. Their findings were published online on August 13, 2024, and subsequently appeared in Volume 180 of the journal Optics and Laser Technology on January 1, 2025.
“In this study, we achieved remarkably sensitive hydrogen detection by carefully regulating pressure and modulation settings within the TDLAS apparatus. We also introduced a calibration-free method that accommodates a wide variety of concentrations,” shares Prof. Shiina.
In the TDLAS setup, laser light travels through a pressurized gas cell known as a Herriott multipass cell (HMPC) that holds the gas being measured. The laser’s wavelength is modulated around the target gas’s absorption line at a specific frequency to eliminate external noise. The pressure in the HMPC plays a crucial role by affecting the width of the absorption line and the modulation parameters used in TDLAS.
The researchers meticulously examined the width of hydrogen’s most robust absorption line at various pressures. By conducting simulations, the team determined the optimal pressure conditions that yield a wider absorption line and identified the best modulation parameters within that line. Their calibration-free technique involved using the first harmonic of the modulated absorption signal to normalize the second harmonic based on their ratio, instead of relying solely on the second harmonic signal as is customary with traditional TDLAS systems. Additionally, they used a high-pressure gas cell filled with pure hydrogen as a reference to fine-tune the modulation settings of the laser signal.
As a result of this innovative approach, the researchers achieved accurate hydrogen concentration measurements across a broad detection range of 0.01% to 100%, equivalent to just 100 parts per million (ppm) at the lower end. The system’s performance also improved with longer integration times (the period in which light is allowed to be absorbed). At an integration time of 0.1 seconds, the lowest detection limit was 0.3% or 30,000 ppm, which improved to 0.0055% or 55 ppm at 30 seconds. However, extending the integration time beyond 30 seconds resulted in an increased minimum detection limit.
“Our system holds the potential to greatly enhance hydrogen detection for safety and quality assurance, thereby promoting a wider adoption of hydrogen fuel. For instance, it can be reliably employed to detect leaks in hydrogen fuel cell vehicles,” emphasizes Prof. Shiina, highlighting the possible applications of their study.
In conclusion, this pioneering technique could pave the way for a sustainable future and accelerate the adoption of hydrogen as an environmentally friendly fuel.
