Technical Note by William Buttner, Ph.D. Manager, Hydrogen Safety Sensor Test Laboratory National Renewable Energy Laboratory Golden, CO 80401
Hydrogen is emerging as a fuel of the future. Unlike fossil fuels, hydrogen is not only clean (producing only water as a product), it can be produced from renewable resources. Already, hydrogen-powered fuel cell electric vehicles are commercially available to consumers albeit in a limited but growing number of markets. These hydrogen-powered vehicles have nearly double the fuel efficiency of conventional vehicles with internal combustion engines.
As with all fuels, including gasoline, there are risks associated with hydrogen. Flammability is the major concern. However, unlike gasoline, hydrogen releases will not form visible pools that slowly evaporate; instead, hydrogen is a light gas that immediately dissipates up and away in the open air. In an indoor environment, the dissipation of vapors (e.g., gasoline) and gases (e.g., hydrogen) may be impeded. As a light gas, hydrogen will rise, and in an indoor environment may accumulate at or near the ceiling. While this accumulation can be mitigated by ventilation, it is essential to overall system safety that personnel in the facility be alerted to the presence of hydrogen. However, hydrogen is an odorless, colorless gas that cannot be readily or directly detected by human senses. Fortunately, electronic sensors are available that can readily detect hydrogen.
There is an increased possibility of an unintended hydrogen release during maintenance of hydrogen-powered vehicles. Accordingly, regulations in the United States mandate that hydrogen vehicle repair facilities be equipped with hydrogen sensors. However, these regulations do not provide guidance on which hydrogen sensor should be used. In 2014, Toyota, KPA, and the National Renewable Energy Laboratory (NREL) partnered to develop and implement a modular approach to upgrade existing repair facilities to accommodate hydrogen vehicles. NREL’s Hydrogen Sensor Test Laboratory (http://www.nrel.gov/hydrogen/facilities_hsl.html) assisted KPA and Toyota to qualify hydrogen sensors for specific use in vehicle repair facilities. In this qualification process, several commercial sensors were evaluated by NREL for their ability to accurately detect hydrogen in a repair facility environment, and included an actual deployment in a Toyota repair facility. Sensors that met accuracy and stability specifications were thus qualified for this application. Toyota recommends that repair facilities being upgraded for hydrogen vehicles be equipped with sensors qualified for this application. This deployment has been ongoing for over a year, and the qualified sensors continue to meet expectations.
Hydrogen vehicles are designed with significant safety features to make it unlikely that there will be unintended hydrogen releases under normal consumer operation. However, during certain operations, such as during maintenance on the fuel system or the power train, the likelihood of a release increases. The hazards associated with such a release are minimized through appropriate facility designs, which include the use of sensors to alert personnel of the presence of hydrogen in the facility. It can thus be argued that hydrogen is as safe, if not safer, than existing fuels.
Dr. William Buttner has 35 years of experience in gas sensors and measurements. He is currently a senior scientist at the National Renewable Energy Laboratory (NREL) in Golden, CO. NREL is the premier research institute overseeing the Department of Energy commitment to the development of renewable energy as one approach to assure the energy security of the United States. Dr. Buttner has managed the NREL Hydrogen Sensor Test Laboratory (http://www.nrel.gov/hydrogen/facilities_hsl.html) since 2008. The NREL Sensor Laboratory is committed to the safe implementation of hydrogen as an alternative fuel.