A few months ago we published an article about the current limitations on time-in-the-air (TITA) for electric UAV’s. In that piece, we focused on three alternatives to extend the amount of time that electric drones can remain in the air performing their mission. Tethered devices, solar power and in-the-field rapid battery replacements are the three viable options that operators have today.
All three solutions presented considerable extensions to TITA, but have limitations associated with replacing conventional and commonly used technology with new, and in some cases, cumbersome alternatives. Now though, the energy industry seems to be getting warmer (no pun intended) to the idea of Fuel Cell Power Systems (FCPS) and the many advantages that this technology, first proposed in 1838 by William Grove, bring to the table.
Today, restrictions imposed on small commercially available drones of flying times of 25 minutes or less (depending on number of rotors and/or load) are being challenged by a number of manufacturers both here in the USA, Europe and China using fuel cells.
For the purpose of this article we focused on two innovators in the field of fuel cells: Protonex, a wholly owned subsidiary of Ballard Power Systems (BLDP) that is based in Massachusetts, and MMC, headquartered in the province of Shenzhen, PR China. Both of these private companies are making significant inroads in the development and commercialization of FCPS for regular use in UAVs with missions which require longer time in the air.
FCPS come in many forms. Some are more suitable for the world of light, small drones. Others are specifically designed and manufactured for large, industrial or military uses.
Protonex has focused on the use of Proton Exchange Membrane (PEM) technology for small, light UAV applications weighing less than 20 lbs. For this category they offer the SBH UAV Power System which is a PEM fuel cell operating at relatively low temperatures of 60o to 80o C (140ºF to 175ºF). This alternative offer quick start-up times, generating 350 W at full capacity. The hydrogen is stored in a chemical hydride cartridge and liberated as the system requires, while the oxygen comes from the air. The fuel cell is hybridized with a battery to provide peak power required for launch or climbing. The system features power generation 2 to 3 times the specific energy of LiPo batteries.
For applications that require high payloads and are not heat sensitive Protonex offers PEM fuel cells running on compressed hydrogen with systems providing up to 1.4 kW of electricity. When properly integrated into a small fixed wing UAV, improvements over 5 times LiPo batteries have been demonstrated, including the 26 hour flight of the Ion Tiger by the Navy Research Laboratory (35 lbs including a 5 lbs payload). This technology is now making its way out of military systems and into commercial UAS.
Similarly, MMC has focused on the use of hydrogen as the main source of fuel for their power cells. MMC is currently manufacturing and distributing two models of fuel cells, HyDrone 1800 & HyDrone 1550 with endurances (TITA) of 4.5 hours and 2.5 hours respectively.
When we inquired about the possibility of using these alternative power sources for commercially available UAVs other than the MMC drone models, MMC officials clarified that their hydrogen fuel cells are designed for a wide range of popular commercial drones, both fixed-wing and multi-rotors, such as DJI M600, DJI M600 Pro and other heavy-payload drones.
The main advantage of fuel cells is the fact that they produce energy as required. That’s different from batteries, which simply store energy and release it on demand. Every fuel cell requires two components. One is the main body of the power-generating unit and the other is the fuel tank, being hydrogen or any other gas or liquid. However, since fuel cells use oxygen from the air for half the reaction, the energy density is much improved over batteries.
According to MMC, the fuel cell weight is in direct relationship to output power and the volume of fuel. In this case, hydrogen is directly proportionate to the requirements of flight. In other words, more fuel, more weight, hence more time in the air. LiPo batteries can store and release electricity, while the fuel cell is mainly there to generate electricity, so the ratio between weight and power generation is fixed. With fuel cells the tank size could be increased or decreased to fit a specific mission with the power generating unit staying the same.
For example, MMC’s HPS-1800 hydrogen fuel cell can be compared in terms of weight vs. output power against a lithium battery. The weight of the HPS-1800 is 9.2kg, and its power output is 1800W, the potential energy storage in the hydrogen tank is 4500Wh, so its energy density is 490Wh/kg, which is twice that of the LiPo battery.
When asked about the comparison between a fuel cell which requires an engine and a fuel tank, strikingly similar to a small internal combustion engine, both manufacturers were adamant that the reliability of the fuel cells was considerably better.
One key element of the advantage of fuel cells over internal combustion engines is the lack of moving parts and therefore complete absence of lubricants and a dramatic decrease in maintenance costs and unit replacement. While manufacturers of fuel cells are racing to lower weight and increase output, people on the battery side of the industry are skeptical fuel cells are the way of the future. According to some engineers we interviewed for this article and who asked to remain anonymous, the current state of LiPo battery technology is plagued with inefficiencies. Improvements to lithium batteries, which are possible with new materials, will make these batteries more efficient and comparable to fuel cells.
If we compare the energy density of today’s LiPo to the equivalent product a decade ago we see a steady improvement of about 7% a year, which is definitely not a revolution, but a slow evolution.
Another big problem with some fuel cells is that they generate a lot of heat, approximately equivalent to the electricity produced up to 1,000o C in some cases. Removal of the heat is an integration challenge in an industry where plastic is a large component of every small UAV. The use of delicate thermal or infrared sensors might be affected by this close source of temperature interference.
Even though fuel cells can increase time in the air considerably, the “silver bullet” that the UAV industry is expecting from the battery industry, is nowhere near the horizon.
Fuel cells, especially low-temperature PEMs, are very effective and efficient for certain applications and the extension of TITA is considerable. If the fuel cell manufacturers continue to mature this technology at the current rate, we could see a wave of small UAV products in the near future with a completely new set of capabilities. However, in order for these gains to be meaningful we need a regulatory environment that allows flights beyond visual line of sight (BVLOS).
In the meantime though, the push to make this technology viable will continue, and the interest in seeing that happen will be coming from multiple places throughput the industry.