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Innovative Fuel Cell Plus Standard DC Modules Yield Compact, Long-Flying Drone

Think drone, and you are likely to think of their use of lithium-based batteries with their best-in-class power density as assessed by weight and volume. But it is conventional in-the-box thinking to assume that small drones are restricted solely to chemical-based battery power. This thought process is disproven by a small drone from Doosan Mobility Innovation (DMI) of South Korea that is powered by a pressurized tank of hydrogen feeding a fuel cell (Figure 1).

Figure 1: This compact drone from Doosan Mobility Innovation is powered by hydrogen fuel rather than a conventional battery for power. (Image source: Doosan Mobility Innovation)

This drone is not a one-off, custom research project or advanced experimental unit. Instead, the Doosan DS30 is a commercially available drone already in use for projects such as inspecting huge solar-panel installation, or waterways such as lakes and rivers and delivering medical supplies including automated external defibrillators (AEDs) to remote locations.

This compact octocopter (eight rotors) measures just under 2 × 2 meters by 750 millimeters (mm) high and weighs 21 kilograms (kg) with its 10.8-liter tank (Figure 2). For transport, it can fold into a case measuring less than a meter on each side.

Figure 2: The DS30 is an eight-rotor drone measuring about two meters square that folds into a one-meter square configuration. (Image source: Doosan Mobility Innovation)

The company’s data shows that due to the favorable energy density of hydrogen of about four to five times that of a lithium-based battery, along with the associated fuel cell, its drone has a flight time of about 120 minutes with a 5 kg (maximum) payload, and can travel up to 80 kilometers (km). As with batteries, refueling is simple: when the lightweight carbon fiber tank runs empty (Figure 3), it is replaced with another one in a few minutes, minimizing ground time. The tank size is modest: just 435 mm long with a diameter of 225 mm, and is pressurized to 350 bar.

Figure 3: The 10.8-liter hydrogen tank is made of lightweight carbon fiber and operates at a 350 bar pressure. (Image source: Doosan Mobility Innovation)

Fuel cells, updated

The basic specifications tell only part of the story, as it is the fuel cell which is the starting point for the innovative design’s overall performance (Figure 4). Although fuel cell technology has been in use for decades and was even used on the Apollo moon mission, Doosan has devised a more efficient, lighter cell using a proprietary design and materials.

Figure 4: The principle of the fuel cell is simple, but recent advances in configuration details and materials have improved efficiency and lowered weight. (Image source: Doosan Mobility Innovation)

The fuel cell occupies a small portion of the overall power pack (Figure 5), taking hydrogen from the inserted tank and combining it with oxygen to generate electricity.

Figure 5: The fuel cell fits alongside the hydrogen tank as part of the power pack. (Image source: Doosan Mobility Innovation)

The complete DP30 powerpack of the DS30 drone is rated at 2.6 kilowatts (kW) continuous/5 kW peak power output. It weighs 12.34 kg with the 10.8-liter tank installed.

From raw fuel cell to regulated rails

Of course, having raw fuel cell DC output alone is not enough. That output voltage has to be regulated to provide clean and stable DC rails for the rotor motors as well as the electronics.

Here’s where the Doosan engineers faced the classic design decision: for which of the many required functions of a design do you choose an innovative, custom design versus a suitable standard component? The reality is that it often makes the most sense to innovate only in those areas where it will make a difference. Here, it’s primarily the fuel cell power unit. Then, using high-performance off-the-shelf components where possible, designers can minimize design unknowns, risk, integration issues, time-to-market delays, and other unwanted or unforeseen “surprises.”

For the DS30 drone, the fuel cell has a wide-ranging and variable open-circuit voltage (OCV) of 40 to 74 volts. From this, the electrical power pack provides two main power distribution networks (PDNs): one to supply power (48 volts at 12 amperes (A)) to the drone’s eight rotor motors, plus a 12 volt, 8 A output to the controller boards and cooling fans. To achieve high efficiency and high energy density in the PDN, Doosan chose three standard products from Vicor Corp. (Figure 6):

Figure 6: The power subsystem from the fuel cell has two main branches: one for the rotors, and one for the control boards and cooling fans. (Image source: Vicor Corp.)

For the rotors, buck-boost regulators accept the output from the two hydrogen fuel cell stacks to provide a stable, regulated 48 volt output. Two 32.5 mm × 22 mm Vicor PRM48AF480T400A00 buck-boost regulators are configured in parallel to supply the 12 A required by the rotors (Figure 7).

Figure 7: Two Vicor PRM48AF480T400A00 buck-boost regulators in parallel power the rotor motors. (Image source: Vicor Corp.)

For the stack controller boards, a Vicor PRM48AH480T200A00 22.0 mm × 16.5 mm lower power regulator is used to deliver a regulated 48 volt output at 4.17 A (Figure 8). This is followed by a PI3546-00-LGIZ 10 × 10 mm 48 volt to 12 volt zero-voltage switching (ZVS) buck regulator (Figure 9).

Figure 8: A Vicor PRM48AH480T200A00 is used to “pre-regulate” the DC rail before the final stage regulator. (Image source: Vicor Corp.)

Figure 9: The final regulator of the power to the controller board and fans is the Vicor PI3546-00-LGIZ. (Image source: Vicor Corp.)

As a result of these DC-DC regulator choices, the fuel cell power system offers a DC input to rail output efficiency of 96.6% with a loss of just 23.4 watts. A DIY solution would be hard-pressed to do better, would take longer, and would require careful performance evaluation.

Conclusion

Successful product innovation often requires a blend of leading-edge advances, unconventional thinking, and persistence, all combined with the use of standard off-the-shelf products where they offer the performance needed. By doing so, the design team can focus on system development, debug, and integration issues while removing as many functional blocks as possible from the list of concerns.

References and Specifications

1 – Doosan DS30 Drone: https://www.doosanmobility.com/en/products/drone-ds30/

2 – Doosan DP30 Power Pack: https://www.doosanmobility.com/en/products/powerpack/

3 – Doosan Hydrogen Tank: https://www.doosanmobility.com/en/products/hydrogen-tank/

4 – Vicor: http://www.vicorpower.com/resource-library/case-studies/doosan

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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