Solar cells for extreme conditions in Aerospace applications

Drones will take over many of the tasks that helicopters and boats perform in the police, army, navy, coastguard and rescue services. They will also communicate their data to control centres wirelessly. For example, drones can be employed for crowd monitoring at public demonstrations, the tracking of suspects by the police, and for coastal defence surveillance and reconnaissance of enemy territory by the armed forces. They can also be very useful in search-and-rescue missions for people lost in open water on a hot summer day, or after an accident at sea.

Solar panels on drones

The amount of time that these functional drones can remain in the air has a major impact on their effectiveness, so the potential to recharge the batteries during their flight operation is of great added value. This can be done by integrating solar panels into the drone, which then receive sunlight and charge the battery. It can also be done by installing special high-intensity solar cells into the drone, to enable the transfer of energy by ‘power-beaming’ using an intense laser beam that is directed at the drone from land or a ship, and then converting this light energy into electricity.

Rapid internet

Another growing application area for solar cells in Aerospace is the networks of tens of thousands of LEOSats (Lower Earth Orbit Satellites) that are being rolled out to provide fast, wireless internet worldwide. Companies like Amazon and SpaceX are working hard on this. One of the driving forces of this development is the fact that only a small portion of the world’s population can receive high-quality internet (4G or 5G) via the cellular network and that access to broadband internet is a highly influential factor in the economic development of developing countries.

The original application

Space exploration (the ‘Space Race’ in the 1960s) was the first major driving force behind the development of silicon solar cells. Subsequently, multilayer thin-film solar cells, based on materials other than silicon, such as gallium arsenide (the compound semiconductors from groups III and V of the periodic table of elements), were developed and optimised for this application. These III-V PV cell architectures generally perform better in this application, due to their high conversion efficiency and low weight, but they are based on scarce raw materials and are much more expensive to produce.

The larger volumes of PV modules required for the LEOSat internet application, and the fact that costs must be kept low in order to make high-quality internet available to the masses, mean that silicon technology is viewed as the most suitable option for this growing market.

For the large-scale application of this concept, both the drones and the LEOSats need access to robust silicon solar cells with good yields (20+%) that can be produced cheaply. The electrical and geometrical design of these PV cells must have a free-form factor and therefore be easy to adapt. This means that the solar cells can easily be incorporated into a lightweight module system (the PV array) that, in turn, can be conveniently integrated into the drone or the LEOSat.

Key success factors for these solar cells and the arrays that contain them are:

• Mechanical robustness and semi-rigidity
• Resistance to high temperatures and a good radiation coefficient for effective self-cooling
• Resistance to UV radiation and space radiation found outside of the atmosphere
• Resistance to large and high-frequency temperature fluctuations
• The ability to self-repair (because maintenance or repairs during operations are clearly difficult, if not impossible)

TNO is working with various partners on highly innovative concepts for these application areas, making use of experience and capabilities in the design and modification of silicon cells and also of the back contact module technology, developed by TNO in Petten. This advanced module interconnection technology offers great advantages for the manufacturing of special designs and robustness in difficult operating environments and under exposure to extreme temperature effects.