New sensor paves way to low-cost sensitive methane measurements

"Agricultural and waste industries emit significant amounts of methane," said Mark Zondlo, leader of the Princeton University research team that developed the sensor. "Detecting methane leaks is also critical to the oil and gas industry for both environmental and economic reasons because natural gas is mainly composed of methane."

In The Optical Society (OSA) journal Optics Express, researchers from Princeton University and the U.S. Naval Research Laboratory demonstrate their new gas sensor, which uses an interband cascade light emitting device (ICLED) to detect methane concentrations as low as 0.1 parts per million. ICLEDs are a new type of higher-power LED that emits light at mid-infrared (IR) wavelengths, which can be used to measure many chemicals.

"We hope that this research will eventually open the door to low-cost, accurate and sensitive methane measurements," said Nathan Li, first author of the paper. "These sensors could be used to better understand methane emissions from livestock and dairy farms and to enable more accurate and pervasive monitoring of the climate crisis."

Building a less expensive sensor Laser-based sensors are currently the gold standard for methane detection, but they cost between USD 10,000 and 100,000 each. A sensor network that detects leaks across a landfill, petrochemical facility, wastewater treatment plant or farm would be prohibitively expensive to implement using laser-based sensors.

Although methane sensing has been demonstrated with mid-IR LEDs, performance has been limited by the low light intensities generated by available devices. To substantially improve the sensitivity and develop a practical system for monitoring methane, the researchers used a new ICLED developed by Jerry Meyer's team at the U.S. Naval Research Laboratory.

"The ICLEDs we developed emit roughly ten times more power than commercially available mid-IR LEDs had generated, and could potentially be mass-produced," said Meyer. "This could enable ICLED-based sensors that cost less than USD 100 per sensor."

To measure methane, the new sensor measures infrared light transmitted through clean air with no methane and compares that with transmission through air that contains methane. To boost sensitivity, the researchers sent the infrared light from the high-power ICLED through a 1-meter-long hollow-core fiber containing an air sample. The inside of the fiber is coated with silver, which causes the light to reflect off its surfaces as it travels down the fiber to the photodetector at the other end. This allows the light to interact with additional molecules of methane in the air resulting in higher absorption of the light.

"Mirrors are commonly used to bounce light back and forth multiple times to increase sensor sensitivity but can be bulky and require precise alignment," said Li. "Hollow core fibers are compact, require low volumes of sample gas and are mechanically flexible."

Measuring up to laser-based sensors To test the new sensor, the researchers flowed known concentrations of methane into the hollow core fiber and compared the infrared transmission of the samples with state-of-the-art laser-based sensors. The ICLED sensor was able to detect concentrations as low as 0.1 parts per million while showing excellent agreement with both calibrated standards and the laser-based sensor.

"This level of precision is sufficient to monitor emissions near sources of methane pollution," said Li. "An array of these sensors could be installed to measure methane emissions at large facilities, allowing operators to affordably and quickly detect leaks and mitigate them."

The researchers plan to improve the design of the sensor to make it practical for long-term field measurements by investigating ways to increase the mechanical stability of the hollow-core fiber. They will also study how extreme weather conditions and changes in ambient humidity and temperature might affect the system. Because most greenhouse gases, and many other chemicals, can be identified by using mid-IR light, the methane sensor could also be adapted to detect other important gases.