The vital aspect in human history today is communication, navigation, climate and environmental monitoring, military reconnaissance and surveillance etc. Till now, satellites are being used for these purposes. But satellites require a high operating cost, they cannot be launched easily and they cannot be recovered easily. Moreover, they cannot be moved to the desired places easily. The geosynchronous satellites require the altitude of about 36000 kilometers to orbit around the earth which renders them very costly to use. The low altitude satellites pass the intended area of focus very quickly thus giving a very small working window.
So, in order to have such a device which could be self-launched, and operated in the stratospheric region. The one which could achieve a high altitude, and have a long endurance time. In addition which could be recovered to ground quickly and easily for maintenance. A device which could be moved easily to different regions. So one may think of an Unmanned Aerial Vehicle (UAV). But such a UAV which can be used for high altitude and long endurance, such a propulsion system which has a regenerative energy property is necessary. So, it may be a Solar Powered UAV (Unmanned Aerial Vehicle).
Solar powered UAV (unmanned air vehicles) are the UAVs that operate using solar energy. This can be done using fuel cells or rechargeable batteries. A solar powered UAV can operate by charging the solar cells or rechargeable batteries with the help of solar energy acquired from the sun. This can save the fuel cost in the UAV. Today there are 11000 UAVs operating in different countries. Although these drones provide numerous beneﬁts like surveillance in enemy areas without the worry of a pilots life or the ease of operating the aircraft from the ground, they lack in performance due to power restrictions. They consume a lot of fuel which costs more and is also harmful for the environment.
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The performance of UAV cannot be increased by simply adding more area for the fuel. We know that lift is inversely proportional to weight. So if we even do manage to increase the fuel tank size in the UAV then the extra fuel added will only increase the size of the vehicle. That extra fuel will just be used to generate lift which is reduced by the extra weight. By using solar cells or batteries we can minimize the refueling costs. Today most of the UAVs run on different types of fuels. Some UAVs like quadcopters and octocopters operate on batteries but they have very low life. These also need continuous recharging to function. This means that they cannot be used for long distance missions. However solar powered batteries can help solve this problem.
The solar powered UAVs can operate in a number of ways. The basic working principle of solar powered UAVs is the same. It collects energy from the solar radiations coming from the sun and uses it to charge the fuel cells or the rechargeable batteries. It works with the energy of the sun and uses the energy from fuel cell when it receives no solar energy. The energy from the batteries or the fuel cells can help the UAV operate at night time or on a cloudy day. The performance of such air vehicles can also be enhanced by using special batteries that can store more amount of energy. Implementing higher density batteries such Li-Su battery can increase the UAVs time of ﬂight thus increasing its endurance.
Manufacturing Solar Powered UAV
Efﬁciency of the UAV depends upon the systems used for producing the vehicle. For a highly efﬁcient UAV, items used for its manufacturing must be strong, light-weight and the fuel cells used store maximum amount of energy. This can help it operate without depending upon solar energy all the time. The solar powered UAV should be designed in such a way that it uses some amount of the solar energy for immediate ﬂight and stores the rest of the energy for later use. The collecting of energy is also a major factor in designing the drone. There are two different methods for this purpose.
One method is using solar panels. Solar panels are constructed using special silicon chips that absorbs the suns rays and convert them into electricity. It stores the energy from the rays into a battery and also uses it to generate electricity. This electricity can be used for ﬂying a UAV. The solar panels can be set around the UAV according to desire to get maximum exposure to the sun. The solar powered UAV uses the solar energy for immediate use as well as charges the battery for use when there is no solar energy.
The other method of designing the solar powered UAV is by designing the entire air vehicle structure with a conductive material that absorbs the suns rays and uses some of it for immediate ﬂight and the rest of it to charge the battery. This method does not involve installing solar panels at speciﬁc places of the vehicle. Instead the entire solar powered UAV structure is composed of a conductive material that absorbs the solar energy and uses it for operation.
This helps in storing more amount of energy because the entire structure absorbs energy. Due to this each and every part of the structure helps in charging the battery. The solar panels are ﬁxed at only some parts of the entire vehicle which means that they battery is only charge using the solar energy at those points. But when the entire structure is made up of a conductive material then the battery can receive energy from the whole airframe which can charge the battery even faster.
Both these designs can be used to produce long endurance solar powered UAV but currently all these UAVs are in testing stage and no large scale UAVs are produced out of these designs. Even today many companies are working on producing long endurance solar powered air vehicles. A few of these companies are Airbus, Lockheed Martin, Boeing and AeroVironment/NASA. A few programs are under production related to such solar powered UAV. NASA in this case is leading from the front. Many of its products are underway and it is doing a lot of research in this ﬁeld. NASA has also developed some initial test drones that work on solar energy. NASA Pathﬁnder and Pathﬁnder Plus are the two aircrafts that work completely on solar power using solar cells to save energy. They were designed as a part of NASA’s ERAST program which means Environmental Research Aircraft and Sensor Technology. These are only prototypes and are not used in the ﬁeld because they have very low range. These use solar panels to store energy and use it to ﬂy.
AeroEnvironment designed another drone called NASA Helios. ERAST program designed it as the fourth and final air vehicle. They were designed as solar cell powered air vehicles and were to be used for high altitude, long term aircrafts to be used as atmospheric satellites. These satellites were to be used for atmospheric research and communication systems. Helios was designed from the NASA Centurion and Pathﬁnder aircrafts.
The Helios aircraft was an ultralight ﬂying wing aircraft with a wing span of 75 meters. Its use was for high altitude long range ﬂights.Unfortunately the prototype could not be put into large scale production as the UAVs design was not aerodynamically stable. It suffered a crash during a test ﬂight which revealed that the aircraft was not acceptable for wide scale production.
Despite this setback, NASA still works on producing better and long endurance aircrafts that will be able to replace the fuel consuming UAVs. Other companies are also working on developing high functioning UAVs. They are working for such UAV’s that are low cost and can ﬂy at high altitudes for a long period of time. Scientists are working on producing good quality solar panels that can charge the fuel/solar cells or rechargeable batteries at a higher rate. The Pegasus UAV and QinetiQ Zephyr are some examples of the lightweight solar powered drones designed by companies other than NASA.
Today most of the solar powered UAVs developed are in testing stage and there is no large scale functioning vehicle. But innovations in technology can help in designing a solar powered UAV. The UAV technology combined with solar cells can be a powerful instrument for civilian and commercial applications as well as reshaping the future of aviation in all ﬁelds.
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