This series of FAQs is looking at various technologies being used to power robots. The first FAQ focused on “battery options for mobile robots.” Following this look at fuel cell-powered robot platforms will be FAQs considering “where do supercapacitors fit in robots?” and “running robots on ambient energy.” This FAQ will begin by looking at the use of hydrogen fuel cells to power robots and drones, move to advances in being able to efficiently transport the hydrogen cylinders needed to supply the fuel, and consider how robots are being developed to refuel other robots with hydrogen to power fuel cells autonomously. It will close with a look at possible liquid-fueled fuel cells with twice the output voltage and higher power densities than hydrogen fuel cells.

Hydrogen fuel cells can provide a high energy density and eco-friendly power source for robots and drones. In a fuel cell, the hydrogen fuel is split into a positive hydrogen ion and an electron at the hydrogen electrode. The positive hydrogen ions pass through an electrolyte membrane to the oxygen electrode, producing water and electricity. Since the oxygen needed for the electrochemical reaction is obtained from the atmosphere, only hydrogen fuel is needed to power the fuel cell.

Battery capacity is limited by the sizes and numbers of cells used. It can fail to provide longer flight times for drones after a certain weight crossover is reached where the added batteries are much heavier than the actual payload, and the batteries are simply carrying their own weight. Fuel cells can be a more efficient solution for large drones and longer flight times. The same applies to other mobile platforms such as underwater autonomous vehicles (UAVs). For example, one analysis comparing annual operation and utilization efficiency for battery-powered versus fuel-cell-powered drones found that fuel-cell-powered drones can be about half the overall cost while delivering increased productivity.

However, as discussed below in the section on “Transporting hydrogen robot fuel,” there are still significant logistical hurdles to overcome to enable the widespread use of hydrogen to fuel drones and other robots.

Hydrogen fuel cells can support long-duration drone flights, which enable new use cases. For example, long-range hydrogen-powered drones can deliver humanitarian relief to remote locations. These eight rotor drones are powered by a 2.6kW DP30 fuel cell and can fly at speeds up to 80 kilometers per hour for two hours without refueling when fully loaded. Other potential applications include inspection of photovoltaic plants or long-distance power lines.

A key to developing hydrogen fuel cell-powered drones is the miniaturization of all components in the powertrain from the fuel cell stack to the power converters used in the power distribution system. The DP30 power pack has two main power buses; one for the rotor motors and one for the fans and digital control circuitry. High density, high efficiency, and dc-dc converters convert the DP30 output voltage (that varies from 40Vdc to 74Vdc) to tightly regulated voltage rails; 48Vdc, 12A for the rotor motors of the drone, plus a 12Vdc, 8A output to the stack controller board and fans.

Several efforts are underway to make the refueling of hydrogen fuel cell-powered robots and drones less costly and less complex. The following are examples of a valve that enables the transportation of full cylinders of hydrogen to support drone operations and an automated hydrogen refueling station for logistics robots in warehouses.

Transporting hydrogen robot fuel

The Pressure Tech CV414, a high-pressure, TPED, and DoT rated valve enabling customers to transport full cylinders of hydrogen to power their UAVs is now available from Intelligent Energy. It is fully certified for use throughout Europe and the US.

 

Without a valve like the CV414, the transportation of full, UAV compatible hydrogen cylinders is prohibited, which in turn has a significant impact on the time and cost required to complete an operation. This valve removes this issue and opens doors for many operational applications and service providers to realize the full potential of using fuel cell technology to power their UAV fleets.

The new valve was developed by specialist company Pressure Tech Ltd. and is a self-closing cylinder valve for high-pressure gas systems, offering users a quick and simple disconnect feature. When connected, it provides a continual supply of gas to the system. If a cylinder refill is required, the low torque design means it is easy for users to disconnect the valve to isolate the gas supply, even under high pressure. In contrast, the valve remains attached to the gas cylinder. The TPED certification allows its use on applications involving the transportation of pressure equipment up to 350 bar (5,075 psi).

With the new valve, Intelligent Energy joins DMI to approve the United States Department of Transportation for transportation of full hydrogen cylinders. DMI was one of the first companies to receive such approval from the U.S. DoT. DMI also has approval from the Korean Gas Safety (KGS, the Korean standard) and Transportable Pressure Equipment Directive (TPED, the European standard) and is currently obtaining approval from Standards Australia.

Robots to refuel hydrogen-powered robots

The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) is sponsoring development work on a robotic hydrogen fueling technology for mobile robots and other motive power systems. The short-term goal is to increase the efficiency and reduce the time needed to refuel hydrogen-powered vehicles in industrial environments such as warehouses. According to NREL, every second saved in a warehouse for refueling robots saves over $1,000 annually in operational costs in medium-sized facilities.

Plug Power and Rensselaer Polytechnic Institute’s Center for Automation Technologies & Systems have completed a feasibility study for NREL’s envisioned robotic fueling station, which demonstrated the ability of the system to independently connect the fuel cell to the fueling station without human intervention. The robot refueling station employs computer vision, sensors, robotic manipulation of the fueling nozzle, and remote controls.

Next, the team will tackle the challenge of automotive fueling. In comparison, robots refuel indoors in a controlled environment and with well-defined fueling systems. Automotive fueling is more complex for several reasons. It occurs in variable outdoor environments with highly variable lighting and other conditions, cars do not have standard receptacle locations, and the vehicles are not automated. Still, they are driven by people, which adds a dimension of uncertainty.

Liquid fuels for higher performance

Liquid-fueled fuel cells may provide an alternative to hydrogen. For example, direct borohydride fuel cells (DBFC) can double the voltage of conventional hydrogen fuel cells and provide correspondingly higher output power levels. DBFCs could power unmanned underwater vehicles (UUVs), drones, and, eventually, electric aircraft, all at a significantly lower cost than hydrogen. These fuel cells could also serve as range-extenders for current battery-powered electric vehicles.

A single-cell DBFC with an operating voltage of 1.4V, double that obtained in conventional hydrogen fuel cells, and peak powers approaching 1 watt/cm2, was built by a team at McKelvey School of Engineering at Washington University St. Louis. Doubling the voltage would allow for a smaller, lighter, more efficient fuel cell design, translating to significant gravimetric and volumetric advantages when assembling multiple cells into a stack for commercial use. Their approach is broadly applicable to other classes of liquid/liquid fuel cells. But commercial deployments are still relatively far in the future.

The key to the development of commercially viable DBFC designs is reducing or eliminating side reactions. The majority of efforts to achieve this goal involve developing new catalysts. For now, DBFCs face significant hurdles in terms of adoption and field deployment.

Summary

Hydrogen fuel cells are beginning to be deployed in drones and logistics robots. There are several keys to the development of hydrogen fuel cell-powered drones, including: miniaturization of all components in the powertrain from the fuel cell stack to the power converters used in the power distribution system; the ability to transport the hydrogen fuel cells used by drones to the launch sites; and the ability to efficiently (and automatically) refuel hydrogen-fueled logistics robots. Each of these challenges is at some stage of being met. At the same time that hydrogen fuel cell technology is being commercialized, competitive, higher energy, alternatives are under active development. The use of fuel cells to power drones, logistics robots, UUVs, and even aircraft, is expected to expand in the future.

References:

Autonomous Hydrogen Fueling Station, Plug Power
Engineers develop new fuel cells with twice the operating voltage as hydrogen, ScienceDaily
Hydrogen is a highly productive energy source for drones, Doosan Mobility Innovation
World’s first commercialized hydrogen fuel cell power pack for UAVs, Vicor