30.08.2021
Today, electric batteries are considered to be the technology of tomorrow and their diffusion is on a stark rise. However, concerns still remain regarding their safety: short-circuits or thermal runaways are common issues attributed to this kind of energy storage system.
As explained in a previous article in this series, we, at Pie Aeronefs SA choose lithium-ion polymer batteries to power the first Swiss all-electric race aircraft: the UR-1. For maximum safety, our battery system is equipped with a fire protection system and a thermal management system, which both prevents fires and protects the aircraft in the event that the thermal management fails.
In this article, we answer the following questions: what is a thermal runaway? In what ways may Lithium-ion polymer batteries be hazardous when handled incorrectly? How does Pie Aeronefs SA overcome these hazards to ensure maximum safety?
There are two main elements that may increase the risk of fire in a Lithium-ion polymer battery:
When the battery overheats or overreacts, the pressure inside the battery increases to the point where the container can no longer sustain it. Once the battery explodes, the contact of the lithium it contains with the moisture of the air creates an instant chemical reaction, causing a fire.
For this reason, it is imperative to have an effective fire protection system on electric vehicles. This system, explained by Ghiji et al. (2020) in their review of Lithium-ion polymer batteries, is “considered at the cell, battery, module, pack, system and enclosure levels”.
Electric aircraft must be managed by software control systems. For example, the Battery Management System (BMS) controls the output and monitors the health of the batteries.
As batteries are flammable, this management system includes firewalls to reduce the fire hazard and to take control of the correct functioning of the batteries.
The BMS or "battery management system" is the main interface which links the batteries, the motor and the pilot. It is described by Gabbar and his colleagues (2021) as such: “BMS uses measurements to estimate state charge (SOC), state of health (SOH), depth of discharge (DOD), and the operational key parameters of the cells/battery packs”.
The TMS or "thermal management system" monitors the temperature of the electric system, including the batteries. If it detects any abnormal behavior or potential overheating, it is able to shut off the system completely or partially, depending on the situation.
"Nothing is lost, nothing is created, everything is transformed" - Antoine Lavoisier, 1789.
This basic principle describing chemical reactions is also true for energy. When, for example, you brake with a car, you transform your kinetic energy (speed) into calorific energy (heat). This energy transfer happens when the brake pads press against the brake disc, creating friction and heat. Then, this energy is stored in the disc as heat. It is important to remove this heat from the disc in order to keep the brake functioning. This is done through a cooling mechanism.
The hotter a material, the more energy it contains. The cooler, the less energy it contains. So if you place something cold next to something hot, the cold material will absorb excess energy from the hot material which will, in turn, lose energy. It is by this process that hot materials can be cooled down.
This process, called cooling, is a physical reaction we can use to remove thermal energy from an element.
When you use an electric system, energy is lost in the form of heat. Sometimes, the system creates more heat than it loses. It is for this reason that computers are equipped with a fan, or cars with a radiator, for example.
Regarding the cooling of lithium-ion batteries, there are four main possible methods: air cooling, direct liquid cooling, indirect liquid cooling, and fin cooling.
As every lithium-ion battery might be designed differently and for different purposes, it is necessary to select an appropriate cooling system adapted to the situation to ensure safety.
Chen and colleagues (2016) put forward a list of advantages and disadvantages to the different cooling methods and conclude the following:
As mentioned, every electric aircraft needs a BMS in order to run the electric motor. This vital system often integrates a TMS. In our UR-1, the TMS is able to monitor the batteries’ temperature and cut-off the system when a threatening situation arises.
Currently, our system is only able to shut-off the whole set of batteries when issues in a single battery arise. A future system upgrade will, however, allow our TMS to isolate a single battery and keep energy supply coming to the motor.
The indirect liquid cooling allows us to run a closed circuit of coolant around the components we need to cool down.
Once the coolant gets warm, we bring the liquid to the bottom of the aircraft, where we installed two cooling plates. The liquid will then be cooled down by the air around. This closed-loop circuit allows us to keep all our heat-producing elements at the best functioning temperature and prevents any thermal runaway.
If the TMS and the cooling system are not sufficient to prevent a thermal runaway, we need to be equipped with a physical system which prevents any structural damage to the aircraft in case of a fire. For this reason, every battery is enclosed in a special protective proprietary ceramic composite sandwich, which acts as a firewall between each battery.
Besides the vents that allow eventual gases to vacate the aircraft through open pipes in the lower wing skin, our ceramic composite sandwich has the important role containing any eventual fires, thus preventing it from spreading to other batteries.
There are some regulations about this kind of fire protection system. The Federal Office of Civil Aviation guidance principles state that this kind of system has to withstand at least 5 minutes at 1200°C with no structural failure. We are proud that ours can withstand 15 minutes with no noticeable degradation of performance.
With all of these protective systems, we are confident about the safety of our UR-1 Swiss electric aircraft during flight and racing.
