Almost every day we receive a question similar to ‘what else do I need for my solar UAV?’ In this post we explain some of the typical components for a solar UAV Balance of System (BOS).
Increasingly, solar cells are being added to commercial drones/UAVs (unmanned aerial vehicles) to extend flight endurance. These solar drones may be intended for industry, agricultural, military or humanitarian use cases. Some examples include search and rescue, ISR (intelligence, surveillance, reconnaissance), inspections, and research.
The solar cells are added to the UAV either during the lay-up of the wings or as an after-market integration. When solar is added, several other components are required to ensure that available power from the solar is maximized.
The key components of the system include:
- Photovoltaic solar cells
- Battery management system (BMS)
- MPPT (maximum power point tracking) device
Ideally the solar cells should be high efficiency, lightweight and flexible to maximize power to weight, for ease of integration, and to preserve aerodynamics. Please see our blog post on integrating solar cells into a UAV .
A lithium polymer rechargeable battery or LiPo battery with polymer electrolyte is typically used for UAVs. This is primarily due to its lightness and high power density. Other attractive qualities are the small size, thinness, relative safety and cost effectiveness of this type of battery. LiPo batteries are also used in mobile phones and other mobile devices. Multiple cells of around 3.7-4.2V (fully charged) are connected in series to form the battery pack. LiPo batteries are sold in packs of 1S, 2S, 3S etc where the number represents the number of cells connected in series.
As the amount of sunlight hitting the solar cells changes, the load characteristic that will give the highest power transfer efficiency varies. A maximum power point tracker charge controller (MPPT) electronically varies this load to optimize power transfer. MPPT devices are usually integrated into a power conversion system such as a DC to DC converter. This DC to DC converter optimizes the match between a solar array (PV panels) and a battery bank. MPPTs are essential to convert a fluctuating DC output from the solar panels into a regulated DC voltage to charge the batteries, and to keep the solar panels running at their most efficient load point under changing conditions. A charge controller will step up the voltage to the correct amount to charge the battery.
Multiple MPPTs can be used to allow multiple solar modules to work at peak efficiency despite uneven shading or electrical mismatch. Additionally, if the battery is used to supply power to the UAV at night or at sunrise, the MPPT will help resolve voltage mistmatches.
Sometimes the Battery Management System (BMS) is part of the MPPT and sometimes it is stand alone. The BMS protects the battery from operating outside of its safe operating envelope. In addition, the BMS monitors the battery and can communicate with other hardware to report on the state of the battery. The BMS can monitor voltage, current, temperature, state of charge, as well as calculate number of cycles, charge current limit and total energy delivered.
Other components can be included in the BOS such as a Ground Control Station (GCS) which will be discussed in a future post.
Please contact us to discuss your solar UAV project: firstname.lastname@example.org