Designing the longest range Electric Aircraft: The Solar Aircraft

hello and welcome many of you might be aware that the distance traveled by an electric aircraft at present is limited by the battery energy density so it is logical to look at solar energy as a means to extend the range particularly when there is the option to convert sunlight into electricity in just one step by means of photovoltaic and feed that electricity directly to the motor in this video we are going to explore solar aircraft we will look at the past models see where the technology stands and work out what the future holds for these emission-free transportation vehicles that run on nothing but sunlight and as a bonus we'll give you some calculation tips to design your own solar aircraft so watch this video till the end on this channel electric aviation we bring for you engineering videos on sustainable air transport subscribe to get all of our updates solar aerial vehicles have been around for a long time in fact the first unmanned solar aircraft called the astroflight sunrise had its maiden flight on the 4th of november 1974. at that point the best commercially available solar cells were only 10 percent efficient so an extremely lightweight aircraft was made that could run on just 500 watts of total power after that came along the moro solar rise followed by the solar one and the gossamer penguin the penguin could generate 600 watts of power it had an empty weight of just 30.8 kilograms the success of penguin prompted the development of solar challenger which was designed to be the first solar aircraft to cross the english channel in 1981 it successfully completed a 262 kilometer or 163 mile demonstration flight from france to england the solar challenger had a wingspan of 14.3 meters the large wing and stabilizer area allowed it to be covered with enough solar cells to generate a maximum of 3800 watts this power level was significantly higher than all its predecessors the solar challenger was also a much sturdier aircraft compared to the penguin with an empty weight of 90 kilogram and utilized nickel cadmium batteries for energy storage this goes to show that electric aircraft have been around for much longer than we think they were made even before the advent of lithium-ion batteries in the 1980s and the 90s solar aircraft presented an opportunity that got nasa interested this was the development of solar-powered atmospheric satellites the concept was designated the term household which was short for high altitude solar in 1995 the benefits of the health sole were realized by nasa's pathfinder it was an aircraft that could remain airborne for weeks or months on scientific sampling imaging and communication missions the pathfinder was followed by the pathfinder plus the centurion and later the helios all of them showed high endurance capability at high altitudes reaching levels where traditional propulsion systems would fail helios for example could reach altitudes of 96 000 feet solar aircraft have been around for a while now there was something we knew we could make but somehow the idea never gained traction in recent times global warming and sustainability has captured our consciousness research has shown that aviation is responsible for 2.5 percent of the total greenhouse gas emissions but its contribution to climate change is much higher in light of this a new wave of interest in solar aircraft began with an aim to cut down emissions the three technology enablers namely materials battery and solar cell technology all have progressed significantly in the last two decades with this in mind the solar impulse project for long range and long duration manned aircraft was initiated and to make its mark it was announced that this aircraft would circumnavigate the earth on nothing but solar energy this feat was eventually achieved in the summer of 2015 by solar impulse 2 the second operational aircraft of the project the solar impulse 2 designated si2 was an extraordinary aircraft it had a wingspan of 71.9 meters which was comparable to the a380 the world's biggest passenger airliner it had 17 248 photovoltaic cells that covered the top of the wings fuselage and tail plane over an area of 269.5 square meters the total pv output was rated at 66 kilowatts peak just to put that into perspective most houses have solar pv systems of just three to five kilowatts peak rating the batteries used in si2 are state of the art even by today's standard they were supplied by kokum and had an energy density of 260 watt hour per kilogram the total weight of the battery was 633 kilograms which was split in four modules of 41 kilowatt hour each giving a total pack capacity of 164 kilowatt hour this is almost the amount of battery capacity of two tesla model s cars however the most remarkable feature of the si2 were the four small propeller motors that had a peak power of just 13.5 kilowatt or 17 horsepower each thus giving the aircraft a total power of only 54 kilowatts this is lower than most light aircraft for example the susna 172 has an engine of 108 kilowatt rated power amazingly the solar impulse 2 with almost half the amount of power was able to take off with 2 000 kilograms of weight thanks to its large wingspan and the energy required by the aircraft to cover a unit distance was extremely low that is 180 watts per kilometer but one area in which the si2 wasn't great was the speed it was a slow moving aircraft with a crew speed of just 70 kilometers per hour or 43 miles per hour and had an extremely large turning radius looking at the data for the flights that it undertook the average flight speed never rose beyond 48 kilometers per hour and this one aspect went a long way in rendering the aircraft impractical because the si2 was designed for manned long endurance series of flights it needed to have life support systems these included the oxygen tanks that were used during high altitude cruise meals had to be stored for longer flights the cabin had systems for storage and heating of food a toilet had to be installed and to improve the factor of safety certain design parameters had to be increased for example the aircraft had a capacity to generate 340 kilowatt hour of energy every day but only 280 kilowatt hour of energy was utilized each day for higher safety margins the battery couldn't be drained to their full capacity and all this led to extra weight that had to be accommodated for this reason the aircraft had to scale up its wings as large as a passenger airliner so the questions that come to mind at this point are what if we don't design a solar aircraft to fly through a diurnal cycle but design it to fly long distances only during the day how small can we make it and how fast can it be the answer is we already have more sleeker solar aircraft which fly at much higher speeds and cover longer distances to name a couple we have the solar status and the sunseeker duo but before we discuss them let's go to the drawing board and learn the relationship between the sun the surface area of the aircraft and the power requirements one of the most interesting facts about solar energy is that the amount of it landing on the earth in just one hour is more than what all human beings consume during an entire year hence there's no question about its abundance on a clear day the amount of solar irradiance reaching the earth's surface is around a thousand watts per square meter in its strength while the amount of solar irradiance reaching just outside our atmosphere is 1361 watts per square meter so there's a reduction in the amount of power that eventually reaches us because of the atmospheric attenuation this includes the energy absorption by water vapor and ozone layer and greenhouse gases this also means that the higher a solar aircraft flies the more access it has to untrammel rays of sun it is obvious that the greater the surface of the solar aircraft the more energy we will be able to capture however this energy should be at par with the instantaneous energy being consumed by the aircraft to give it a long range to elaborate consider that we put solar cells on pivistral alpha electro that has a wing area of 10.5 square meter the highest amount of power we can get with the most efficient solar panels would be then 2415 watts or 2.4 kilowatts at ground level even during the cruise mode the alpha electro is consuming around 18.2 kilowatts the installation of solar cells on the wings would therefore only add just 8 to 10 minutes of flight time in the sunniest of days this calculation is based on one hour of flight time of the alpha electro which has a 21 kilowatt hour battery pack so in an aircraft like the alpha electro solar cells won't make much of a difference this is why we need an aircraft in which power consumption during cruise mode is comparable with the power generated by the solar cells and there's only one type of aircraft that can match the high surface area with low power consumption it is the powered sail plane or the power glider the two solar aircraft namely the solar stratos and sunseeker duo that were mentioned earlier are exactly that with just the solar cells added compared to the solar impulse 2 they are much more compact and can also go the distance the sun seeker duo can achieve speeds of 136 kilometers per hour or 84 miles per hour with a 20 kilowatt motor it has a 23 meter wingspan with an empty weight of 280 kg and a 8 kilowatt hour battery pack a total of 25 square meter area is covered by 22.8 efficiency solar cells giving it a peak power capacity of 5.7 kilowatts during its cruise when the speed is 72 kilometers per hour or 45 miles per hour the sun seeker duo consumes just 3 kilowatts at the maximum photovoltaic output it can reach up to 86 kilometers per hour this gives the sun seeker a minimum of six to eight hours of flight time on a sunny day even in mid latitudes we can now learn about a simple relationship between power and speed that is very useful in designing aircraft if the power and speed for one condition is known then this information can be used to determine power or speed for a second condition for example you'll find the max power and max speed of any aircraft in its specs and this can be used to determine the power utilized during the cruise mode that is if the crew speed is known the following relationship can be used note the cubic relationship between power and speed this shows that if you want to double the speed of the aircraft you will have to spend 8 times more power for more accuracy the efficiency of motors at different speed can also be taken into account the second calculation tip is for finding the maximum number of electricity units a pv system can generate during the day to do that one can simply multiply the peak capacity of the system in kilowatts by five so a three kilowatt pv system could yield a maximum of 15 units of electricity a word of caution here this thumb rule is more accurate for places close to the equator and up to 23 degree latitude for latitudes of up to 45 degrees a factor of 4.5 instead of 5 can be used at even higher latitudes the angle of inclination of the cells has a much stronger bearing on the output of the photovoltaic system an aircraft like the sun seeker can be made to fly autonomously it can be modified to deliver up to 120 kilograms of cargo with the autonomous piloting system installed and the best part is it can do that for no fuel cost at all and for distances of up to 300 kilometers but this is not just the only exciting prospect there is more the solar cells used in the above mentioned aircraft are high efficiency monocrystalline solar cells the peak efficiency of these type of solar cells is around 22.7 percent on the other hand the multi-junction solar cell available in the market for example the xjt prime by a spectrolab has 30.7 percent efficiency these cells can generate 27 percent more power thereby allowing not only higher speed but also longer range as they will be able to produce more electricity even in the non-peak cars at present these solar cells are expensive and are only used in satellites but their price is coming down and their efficiency is improving similarly with lighter more energy dense batteries the payload capacity can be further increased so with this information can you imagine what the next generation of more compact and efficient solar aircraft would look like can we design a tandem wing power glider with distributed propulsion i will be eager to hear your thoughts in the comments section and with this the video is concluded if you learned something from it please do consider giving it a thumbs up thank you for your attention

Channel: Electric Aviation

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Length: 15min 20sec (920 seconds)

Published: Thu Mar 18 2021