We developed a quadcopter able to continuously fly over 3 hours using solar energy, at last 7 times the commercial drones can, and 48 times to the same drone without solar power.
Nowadays, drones are mainly categorized into 2 types: a) fixed wings and b) copters. Fixed wings need a runway, but they consume less power. On the other hand, copters do not require runways because they have the convenience of taking off and landing vertically, yet copters consume more power than fixed wings. The copters’ market share is about 90% because they are very user-friendly. Therefore, we work on quadcopters.
Currently, the power source for a quadcopter is either liquid fuel or battery. The liquid fuel has the advantage of high energy density for long flight time, but it is explosive and very dangerous. In comparison, the battery is safe, yet its drawback is limited capacity, so it can only last for short flight time, typically less than 30 minutes. Neither of them reaches our goal, so we introduce a solar powered alternative.
Figure 1. Electrical connection of solar-powered drone
Figure 1 shows our electrical circuit for our solar-powered drone. First, the solar cells or battery provides power to the power module. Next, the power module converts the battery voltage to 5 volts for the flight controller, telemetry data link, RF receiver, and the GPS module. The flight controller sends the PWM signals to the ESC. Last, the ESC converts the DC current to the modulated current for the motors according to the commands from the flight controller.
The quadcopter only works when the motors can lift the total weight. The weight of our quadcopter is about 1.3 kg, including frame, motors, and electronics. The solar module consists of solar cells, lamination, and solar frame. A suitable weight-to-power ratio of the solar module enables the solar drone to fly well. Thus, delicate design and engineering are needed.
Figure 2. Motor property and solar power for the quadcopter
The purple curve in Figure 2 is the motor property, the relation between lifted weight and power. It indicates that the required input power is more than doubled to double the lifted weight. As more solar cells are equipped, the solar module weight increases, approximately linearly proportional to its power (green line). This will create a function range. Only when the solar module weight is lower than the motor-lifting capability can the solar drone fly. If the weight-to-power ratio is too large (orange line), the drone will be too heavy to fly. A low ratio (blue line) will allow for a very large drone size and payload because of its wide function range.
We had made some early solar modules, but the process was complicated and the product was too heavy. As a result, wefabricated new solar modules with 5-inch solar panels. For 5s voltage, there are 38 panels in series, 2 series in parallel, making the total 76 pieces; for 6s voltage, there are extra 5 panels in each series, and the total is 86 pieces.
We had experimented several combinations of solar module, frame size, and propellers on our drones. Our team members, Lan and Liao, reached a flight time of 2 hours and 20 minutes. My drone even flew for a longer time. I experimented it for 2 types of solar modules: 5s voltage and 6s voltage. With the 5s voltage, corresponding to 76 pieces of solar cells, the longest flight time is 2 hours and 25 minutes. With the 6s voltage, 86 cells, the record flight time is exhilarating: 3 hours and 28 minutes. Still, the flight time is highly affected by the weather, especially sunlight.
Furthermore, we found that we should also use a backup battery together with solar power although sunlight provided the most power for the quadcopter. If the drone is only solar-powered, its voltage, current, and power fluctuate because the drone adjusts its flight attitudes frequently and additional power and current are required to bring it back to balance. The power module was even burned out after one experiment. However, when a battery is connected, the voltage becomes stable. The solar cells are biased by the external voltage supplied by the battery. Current supplied by the solar module is then also fixed. The total current or power still fluctuates but is balanced by the battery.
Figure 3. Voltage-Time graph in one experiment
In one test flight, the voltage showed almost no change at around 30 to 40 minutes after takeoff. Then, the voltage dropped as the cloud came, but it increased later because of sunlight. This means our solar cells were charging the battery. It shows that the solar power is enough, and we expect the drone to fly as long as the sunlight is excellent.
After many experiments with different batteries, we learned that a larger backup battery does not necessarily give a longer flight time due to its increased weight. The lightest battery is charged the fastest, but the duration time under clouds is the shortest, and it can be easily overcharged, which is an issue of concern. Overcharge could be prevented by using a protection circuit, but unfortunately, it was not installed during oneflight. As a result, the battery exploded. Therefore, in a later experiment with another battery, a protection circuit was installed to prevent overcharge under strong sunlight, so we successfully achieved 3 hours 28 minutes of flight.
In summary, solar energy systems are developed for quadcopters. With solar energy only, dramatic fluctuation of voltage, current, and power occurs, so a backup battery is required to stably bias the voltage for the solar cell and the drone. The backup battery and the sunlight impact significantly on the flight time. Stable sunlight should allow the quadcopter to fly well for hours. Above all, we have achieved the record flight time of 3 hours and 28 minutes without using liquid fuel. The solar energy extends over 48 times of the flight time for using a battery only.
I would like to acknowledge the support and assistance for the experiments from Prof. Ching-Fuh Lin’s group at National Taiwan University.
The author of this text, Ta-Jung Lin, won a Student Award at the European PV Solar Energy Conference and Exhibition (EU PVSEC) 2018. With the support of LONGi Solar and Prof. Wen Zhong Shen from Shanghai Jiatong University, TaiyangNews organized a trip for the EU PVSEC Student Award winners to present their research at the CNPV 2018 Conference in X’ian, China and invited them to publish an article on their results on our website.
