Supercharging the Fuel Cell

New electric-blower technology aims to add more air to enhance hydrogen fuel cell efficiency.

The power output of a fuel cell is proportional to the rate of air supply. (Aeristech)

Management of air has long been a salient aspect of vehicle design. It’s also vital for the operation of hydrogen fuel cells (HFC) as those propulsion systems continue to gain favor for commercial-vehicle use by a growing list of OEMs, including GM, Honda, Hyundai, Nikola, PACCAR and Toyota. Luke Read, head of engineering at U.K.-based Aeristech, is optimistic about the HFC’s re-emergence in the mobility space. The company aims to enhance fuel-cell efficiency through improved air management and electronic controls.

Aeristech’s integrated fuel cell compressor. The company has developed high speed motor and control technology to increase hydrogen fuel cell output. (Aeristech)

“Using electric motors to move air efficiently and effectively is what Aeristech is all about,” Read said. “The power output of a fuel cell is proportional to the rate of air supply, which effectively constrains the rate of reaction between the hydrogen and oxygen. So, the more air we can pump, the more power we get.” With the hydrogen at high pressure (typically 350-700 bar/5,000-10,000 psi), an efficient fuel cell needs to receive high volumes of air, typically using an electric blower or compressor. “That’s where the challenges arise,” Read said.

Aeristech has developed high-speed motor and control technology designed to significantly increase the output from an HFC by pressurizing the air supply. This, in turn, increases the rate at which the oxygen passes through the permeable membrane in the fuel-cell stack. The company’s motors also contain specific features to make them more compact and efficient, Read explained. They use solid copper busbars in their stators instead of traditional copper windings to improve performance. “So far, our designs have achieved pressures up to 3.25bar(g). Pressures are only limited by fuel cell capabilities, chiefly membrane integrity,” he noted.

Bespoke power electronics

The blue line in this graph indicates a typical map for an IC engine e-supercharger, with a red line showing the map for a fuel cell compressor. The combination of high pressure ratio and low mass flow required for the fuel cell are difficult to achieve at anything other than high compressor speeds. (Aeristech)
Luke Read: His company’s engineering innovation is all about using electric motors to move air efficiently and effectively. (Aeristech)

But HFC efficiency isn’t only about air. Read stressed the importance of applying comprehensive, bespoke power electronics to control the motor. “Rather than use a general design to fit several applications, we have power electronics which are designed specifically for a matching motor,” he told SAE’s Automotive Engineering. “In this way the design of both can be optimized for efficiency and cost-effectiveness. More importantly, we use power electronics architecture which separates the controls for torque and speed into two separate functions.”

The main limit on power electronics performance is the generation of excess heat. By splitting the controls into two, the switching losses are lowered “very significantly.” This reduces the amount of heat produced and simplifies transistor requirements, which can be standard silicon components as opposed to costly silicon carbide.

In testing, Read claimed that when pitted against a comparable competitor system, Aeristech’s controllers can handle 1.5 times the power using only two-thirds the number of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) of the same type. Tests showed that 12 MOSFETs handled 4.5 kW, versus 18 MOSFETs handling only 3 kW.

The reduced switching loads also create further headroom to extend the running speed of the motor, allowing it to spin faster. “The limitation is no longer the motor and controller, but the rotor dynamics and the bearing system, an area of specific development within Aeristech,” Read explained. He said that while such high speeds are not relevant for traction motors (which are ultimately linked to the road wheels), they are ideal for air pump applications such as hydrogen fuel cell compressors and e-superchargers.

The lower running temperatures facilitate improved packaging, making the company’s motors and controllers the most power-dense of their type, Read claimed. The Aeristech controlled compressors also deliver a constant air pressure regardless of variation in input voltage, so can be powered directly by the fuel cell without intermediate voltage regulation. “Because the output voltage from a cell fluctuates, a conventional electric compressor must resort to intermediate voltage regulation to provide constant pressure, which adds to system losses,” he said.

70% mass saving

Read said he believes that by using its in-house developed motor control technology, his is the first company to deliver motors that are “sufficiently cost-effective” for use in electric compressor applications. When compared to scroll or Roots-type systems, the Aeristech electric compressor provides the same boost pressure, while saving up to 70% of the mass on a full system basis including control unit, motor and compressor.

Typical high-speed motors would be unable to operate continuously at such high continuous power levels due to thermal management issues, Read noted: “We believe no other motor control arrangement is able to deliver these high pressures and similar high efficiency over long periods, whilst keeping volume costs low.”