POWERING ELECTRIC UAVS Industry special – power systems in electric UAVs
Electrically-powered UAVs are increasingly popular for many applications – but how can they best be kept aloft and functional? This article reviews the electric power sourcing options available to UAV designers, and underlines the importance of effective power systems design.
Unmanned aerial vehicles (UAVs), or drones, are rather like Swiss Army Penknives in terms of their versatility; their applications are only limited by the imaginations of their designers and users. And their potential continues to grow as new technology advances add to their performance and functionality.
Accordingly, one survey gives the UAV market a projected CAGR of 16.4 %, from USD27.4 billion in 2021 to USD58.4 billion in 2026. This covers UAVs of all propulsion types, but those with electric motors form a significant proportion; another survey shows USD18.2 billion in 2020, reaching USD26.9 billion in 2026 with a CAGR of 5.7 %.
For users, electric UAVs offer the advantages of efficiency, reliability, reduced noise and thermal signatures, and precise control. Additionally, today’s climate change concerns make their Green attributes increasingly desirable. These factors often count for more than internal combustion engines’ very high power and energy densities.
And although their CAGR is below that for UAVs overall, their scope is not limited. While the smallest can be hand held, Boeing has developed a protype octocopter, equipped with custom batteries that power eight counter-rotating engines with six-foot-long blades . This ‘flying truck’ can carry up to 500 lb. of payload, and could potentially fulfil roles like quickly and inexpensively moving cargo out of warehouses, or shipping equipment to oil rigs or other remote industrial sites.
Meanwhile, these UAVs are of particular interest to the power electronics industry because they depend on batteries or fuel cells, as well as power electronic components and systems, for operation.
Irrespective of their application, electric UAVs are making ever-increasing demands on their batteries and power system components, to remain competitive and keep pace with their users’ expectations. Better flight times, range, and payload capacity are always sought after, along with faster communications for both data feedback and control.
Increasing payload imposes a double burden on UAV batteries; there is not only the additional weight, but payload items like cameras or communications equipment demand their own power. Yet simply adding battery capacity can be counterproductive, as the extra battery cells add more weight, while also consuming valuable payload space.
However, UAV designers have several paths they can follow, and decisions to make. Firstly, if they want to use a battery-based solution, there are different battery technologies to choose from, with more choices expected in the future. Alternatively, a design could use a fuel cell rather than a battery as the electrical power source. Finally, mechanisms are available to overcome the limitations of any onboard battery’s finite energy storage capacity.
The most commonly-used battery technologies in UAVs are lithium-based, as they have a higher energy density than the older nickel-based technologies. The two most popular lithium chemistries are lithium polymer (LiPo) and lithium ion (Li-ion). Lithium sulfur (LI-S) may offer another possibility; Li-S technologies are currently being developed which promise new breakthroughs in battery performance.
Li-S cells offer a substantial increase in gravimetric energy density, reduced costs, and improved safety prospects. However, the technology still has outstanding issues which are inhibiting its commercial development and benefit from Li-ion cells’ economies of scale. These issues are being addressed by LiSTAR , the Lithium-Sulfur Technology Accelerator; one of nine Faraday Institution projects which aim to place the UK at the forefront of global battery technology. The project is a collaboration, led by UCL, of seven university and eight industrial partners, each bringing unique capabilities to the development of Li-S batteries.
Fuel cells offer another power source possibility for UAVs. They can extend flight time endurance and range, thanks to their higher energy density. A further advantage is that they can be refueled extremely quickly, especially compared with battery charging times. They are also modular and therefore scalable, as cells can be simply stacked together to achieve the desired output.
Proton exchange membrane (PEM) fuel cells, in which the electrolyte is a proton-conducting solid polymer membrane, feature the highest power densities, making them ideal for UAV applications where size, weight and power (SWaP) are often highly constrained.
One practical consideration is that fuel cells require hydrogen fuel tanks - which must be included in the equation when considering the total weight and volume requirements imposed on the UAV. Also, fuel cells reach a maximum efficiency of around 60 %, compared with 90 % for lithium batteries . This is mainly due to the thermal management and other auxiliary subsystems needed for fuel cell stack operation.
Supplementing the onboard batteries’ energy storage capacity
Several methods of achieving this are possible. One possibility is to deploy a tethered drone, where power from the ground is carried up through the tether cable during normal operation. This means the UAV can remain uninterruptedly airborne for days, making them ideal for surveillance of borders or sensitive installations. If an operator spots something of interest through the UAV camera, they can remotely release the tether cable, allowing the vehicle to track the object of interest under its own battery power before being brought back to land.
UAVs can also recharge or replace their depleted batteries during their mission using a battery swapping technique. A typical swapping operation uses a ground recharge station for battery charging and replacing. It can be deployed on cell towers, rooftops, power poles, or standalone pylons. The station can be fed by large batteries, power lines or solar energy. In this way, small drones in a swarm can maintain a persistent surveillance mission through co-operatively-managed battery swapping.
Researchers at UC Berkeley have taken this concept a step further by proposing a system in which drone batteries are swapped in-flight. Other proposals for in-flight recharging depend on laser beam wireless charging, where a ground station transmits a laser beam to a UAV’s optical receiver.
UAV onboard electronics
The reality for many UAVs is that their flight time and performance depend ultimately on their onboard batteries. And, while the batteries’ performance relies much on their own technology and condition, it is also governed by the UAV’s battery management system (BMS) and associated electronics.
A battery management system will continuously monitor important battery parameters, while dealing with the varying power demands of different UAV operational modes, and optimizing the battery’s usage.
The battery management system may monitor battery voltage, current, temperature, state of charge, state of health and other parameters, and calculate additional information based on these. Some systems can report this data to an external device via a communications link.
In addition to managing the battery usage, the battery management system can also protect the battery during charging, safeguarding against conditions such as over-current or over-voltage.
UAVs also need internal power delivery networks to deliver the battery power to the motors, as well as cameras, communications equipment, and other payload devices. These networks, which include isolators, regulators, DC-DC converters, connectors, and cables, can be carefully designed to minimize I2R losses – ensuring as much battery power as possible reaches the supported loads rather than being dissipated as waste heat.