What are the low-cost fuels for drones?

What are the low-cost fuels for drones?

Low-cost fuels include hydrogen, methane, gasoline and kerosene. Hydrogen can be produced at the lowest cost and can be stored for 4~5h UAV missions. Hydrogen fuel cells offer the highest conversion efficiencies because hydrogen electrodes produce the most efficient electrochemical processes. Hydrogen has the lowest density of most gaseous media, as listed in Table 1.

Table 1 - Various gas densities for the most ideal fuel cell designs widely used in drones
Table 1 – Various gas densities for the most ideal fuel cell designs widely used in drones

The density of air is strictly based on the percentage of nitrogen and oxygen in the air at a specific height above the ground. These levels vary with altitude. The oxygen content of the air decreases with the height of the ground. According to these discussions, the performance of oxygen-based fuel cells is strictly dependent on the percentage of oxygen in the air. Since UAVs usually operate at different altitudes, 2000~5000ft in the battlefield environment, the battery designer can choose to design an appropriate oxygen-free structure.

As mentioned before, a UAV drone is equipped with electro-optical sensors, infrared cameras, RF/mmWave systems, and a range of other sensors, and may take 1-4 hours to complete the mission objective within a specified time period. As long as the airborne side-looking radar is not deployed, the 3~5kw proton exchange membrane PEM fuel cell is enough for UAV unmanned aircraft. If side-looking radar is necessary for a given mission, a fuel cell with a capacity of 7.5kW is required. PEM-based fuel cells can maintain the same output power level for more than 4~6h.

Influence of cell parameters on the performance of PEM-based cells. PEM fuel cells with moderate power capacity are the most compact and efficient. PEM fuel cells include two porous electrodes separated by a polymer membrane. The electrodes can be separated by a porous silicon membrane, but the cost will be slightly higher. The membrane allows hydrogen ions (H+) to pass through, but it blocks the flow of electrons and gases. In other words, the fuel electrode acts as the anode and the oxidant electrode acts as the cathode. Hydrogen fuel flows along the anode surface and oxygen or air flows along the cathode surface. The catalyst helps the hydrogen atoms break down into protons and electrons. Electrochemical processes in batteries generate electricity directly through the oxidation of hydrogen. The chemical process takes place at around 80°C, and the batteries use a thin plastic sheet as their electrolyte. The by-products of such batteries are heat and water, and therefore, are harmless. The device is safe to manufacture and operates at moderate pressures, which increases power density, simplifies construction, and reduces manufacturing costs.

The open circuit voltage of a single cell is about 1V, the cells can be connected in series to form a stack to generate higher voltages, it can be stacked with several cells to meet the required power density and power output. The current rating of a fuel cell is a function of the size of the active surface area of ​​the cell. This means that the voltage and current density of the fuel cell can be varied by deploying in series or parallel configuration to obtain the required power characteristics of the fuel cell. Current cell designs using silicon membranes provide current densities approaching 200 mA/cm².

Such fuel cells were used to power Gemini and Apollo spacecraft in the 1960s and 1970s. These batteries are still widely deployed on space shuttles, a clear demonstration of their excellent mechanical integrity and reliable electrical performance over extended periods of time. Due to their compact size, improved reliability and cost-effectiveness in harsh operating environments, these fuel cells are best suited for military and aerospace applications, including UAVC, UUG, UUV, UAV, MRAP and counter-IED operations.

Read more: Military Aircraft Rechargeable Batteries