The sunlight absorption by solar cells is more at high altitudes than on the ground or earth stations. This makes solar systems suitable for powering small aircrafts
B.S. Sastry and B. Ramana
Elektra One—single-seat solar-powered aircraft by SolarWorld (Image courtesy: http://questpointsolarsolutions.com)
Single- and two-seater aircrafts are now flying in Germany, China and Switzerland. In fact, Germany and USA are planning a whole line of aircrafts powered by solar cells. For propulsion, these aircrafts rely on electric engines (brushless DC motors)—the motors extensively used in the defense and aerospace industries in India.
The Li-ion batteries are charged by solar cells, which, in turn, supply electrical energy to the electric engines. High-efficiency solar cells are now available with extra concentrates. Also, solidstate designs that have the poten-tial to deliver three times the energy density at less than half the cost per kWh of the present batteries will be available shortly. With this development, some of the manufacturers are planning four-seater aircrafts.
High-efficiency solar cells
A solar power system converts the sunlight into electricity by using photovoltaic (PV) cells. To improve the efficiency of PV cells, concentrating solar power systems use lenses or mirrors.
The advantage of using solar power systems for aircrafts is that the sunlight absorption by solar cells is higher at high altitudes than on the ground or earth stations. These planes are ideally suited for a country like India which receives abundant solar energy throughout the year. The average solar energy per unit area outside the earth’s atmosphere is ten times that available on the earth’s surface.
Some emerging technologies that can considerably improve energy utilisation efficiency include multijunction cells, optical frequency shifting, multiple-exciton-generation cells, multiple-energy-level cells, hot-carrier cells, concentrating PV systems and hybrid PV systems.
PV cells having quantised band structure—such as quantum wells and quantum dots—can theoretically be as efficient as 60 per cent. In fact, leadselenide (PbSe) quantum dots have already demonstrated this level of efficiency in laboratories.
Electric propulsion systems
The electric engines are brushless DC motors incorporating high-energy magnets. High-energy magnets are made of neodium-iron-boron or samarium-cobalt, which reduces the motor size while increasing its efficiency.
Hall effect sensors are used for electronic commutation of the motors. Speed control is through a conventional PID controller. All the electronics is enclosed in the motor casing and the motor, along with the controls, is connected through a circular connector.
Electric engines are used in two-seater small planes, miniature helicopters and unmanned aerial vehicles. Light-weight lithium-ion batteries—the energy source—are charged by solar cells mounted on the wings of the aircraft.
The advantages of using brushless DC motors are:
1. Broader speed range
2. Operation in harsh environments and at high altitudes
3. Lower mechanical losses than brush-type motors
4. Extra long life
5. Low current consumption
6. On account of electronic commutation, a brushless system is immune to low humidity and arcing.
The servo amplifiers used to drive the brushless DC motors provide great flexibility for precision motor control with various types of feedback transducers.
While selecting the brushless DC motor, its size, weight and operating range need to be taken into consideration. The operating range torque vs revolutions per minute (rpm) graphs are given by the manufacturer. Since these motors operate at a very high rpm, suitable gear ratios are to be incorporated in the motor.
It is a common practice to incorporate two-three stages of epicyclic gear trains to drive the propellers of the aircrafts.
Li-ion batteries are rechargeable batteries for portable applications. These feature one of the best energy densities, no memory effect and slow loss of charge when not in use.
The advantages of using Li-ion batteries for aircrafts are:
1. Compared to other types of secondary batteries, these are very lightweight and have much higher energy densities.
2. A wide variety of shapes and sizes are available for installation in the aircraft.
3. Higher open-circuit voltage than lead-acid and Ni-Cd batteries. This increases the amount of power that can be transferred at a lower current.
4. Self-discharge rate of approximately 5-10 per cent per month compared to over 30 per cent per month in other types of batteries.
5. Components of the battery are environmentally safe.
6. Since solar-powered aircrafts fly at high altitudes where the temperature is low, these do not degrade and last for a long time.
Efforts are underway to improve the power density, recharge cycle and other characteristics of Li-ion batteries.
Ready to take off?
Advances in high-efficiency solar cells and high-energy rare earth magnets of brushless DC motors are contributing significantly to solar-powered electric aircrafts. Li-ion batteries that drive the brushless DC motors with significant power-to-weight ratio are also a major enabler.
Regenerative flight techniques are being used to recharge the batteries during certain modes of flight. In this approach, a propeller using symmetrical blade sections is used as a turbine to recharge the stored energy when the aircraft encounters an updraft. At high altitudes, the energy available from vertical atmospheric motion within a thermal can exceed the available solar power by a factor of ten or more.
The knowledge levels of system integration of multidisciplinary technologies, viz, flight control, flight management systems and landing gear, are available indigenously. It’s time for the Indian industry to take advantage of these skills.