The advancement in battery technology and in particular the increase of energy density (up to 250 Wh/kg) in the last decade has helped developed new devices that were not possible before.
Thanks to Li-ion chemistry, the full electric cars produced today are making huge inroads in the automotive market. Their success has transpired into the development of other more scaled down mobility products like Electric Quadricycles, Electric Bikes and a more recent invention, the Segway. Similarly, battery technology coupled with advancement in light materials is also reflected in modern airborne devices, at least at the micro-level.
In nearly all movies that depict future, levitation devices are a regular feature. From Back to the Future, to Terminator (Rise of the Machines), from Oblivion to Riddick, airborne transport has been inspiration for Science-fiction writers . Many of the devices/vehicles shown use a hovering mechanism as it allows the most versatility in flight dynamics.
In the last five years, quadcopter drones have cropped up in the market and sell like hot cakes particularly during the festive season. Their development has been possible because of progress in stiff lightweight materials, Light weight batteries and electronics. To put things into perspective, Carbon fibre layers (weight density 25 gms/m2 ) available today is lighter than A4 printer paper (80 gms/m2). For this reason these Carbon Fibre layers were used in Solar Impulse 2 (Solar Powered Aircraft).
More recently three toys that have captured imagination are BB-8 droid toy, Phantom Drone (with camera) and Festo Mechanical bird. All of them were made possible because of lightweight materials/ batteries and electronic stabilization system.
The question therefore is can this technology be up scaled to transport goods or humans in future?
Before a leap on speculation into the future is made, a better starting point would be observing the energy system of an existing micro drone to gain insight.
The specifications of a top selling drone are as follows:
Weight: 599 gms
Dimensions: 20.6 x 20.4 x 13.4 cm
The drone comes with a 2 x 750 mAh battery rated at 3.7 Volt that weighs about 34 gms. This equates to a pack density of 5.55 Wh/0.034 kg = 163.2 Wh/kg
Bear in mind this micro-drone battery energy density is slightly higher than the Energy density of a Tesla battery-pack. (141 Wh/kg)
The four motors on the drone are rated at 10 W consume the total energy in the battery pack in about 8 min (480 seconds). It should be noted that the battery-pack itself is simply an assembly of two batteries. It is devoid of any cooling or charge balancing system that are a feature of large battery packs and therefore has higher energy density.
An informative TED talk explaining the mechanics and the capabilities of micro drones can be seen in the video below:
The application of even the existing drone are numerous, few of them are listed below:
- Military reconnaissance
- Any kind of Ariel Surveillance (even used by farmers to find missing animals)
- Providing relief (food, medicine, first aid kits) in hard to reach areas
- Locating survivors inside buildings on fire
Nonetheless, if the role of drone has be to increased, further improvements will have to be made.
The following are challenges when scaling up to a level that allows human or large good transportation:
- Higher energy requirements for long flights
- Lower energy density availability at pack level
- Higher inertia and less manoeuvrability
- Safety and regulations
Scaling up the geometry does not increase the energy consumption linearly. When any flying object is scaled up, the lift increases by square power (as its dependent upon area) but the weight increases cubically (as it is dependent on volume). This means the energy required per unit mass to fly the object increases as geometry scales up. Furthermore, hovering mechanism with four blades is less energy efficient compared to single large blade. This reiterates again the higher energy requirements for any quadcopter drone that can replace helicopter.
Is the energy density requirement within reach of the existing technology? The answer is yes but it would take about 10 years increase the energy density up to 400 Wh/kg.
It is interesting to note that a human powered quadcopter has been fabricated and has achieved flight. Students from Maryland University were able to design a 36.2 kg (80 pounds) quadcopter that was able to achieve lift for several seconds. They were not able to win the Sikorsky prize which required hovering 3 meters above the ground for a time of one minute without drifting out of 10 meter box. Nonetheless this exercise provides extremely useful information.
A trained cyclist can generate up to 250 Watts for periods longer than a minute . The video above shows that with this power, hovering is possible. Very good athletes can maintain that power output for a period of several minutes. As can be seen in the video, with a power of around 250 Watts not only the person was being lifted but also weight of the quadcopter.
For those not looking to burn calories during their journey, the hover-board inspired quadcopters have been made but again they are more for recreation than anything else. From the above video it can be easily ascertained that 500 Watts would be sufficient energy for lifting up a person of average weight with a quadcopter that can weigh around 72 Kg. Off course putting aside human effort and replacing that with machine power means motor, batteries and hence added weight. And there lies the most fundamental design quandary, more lift or more weight, more weight more lift.
The most professional and serious effort so far in scaling up a Quadcopter has come from Ehang Inc. The Chinese company has recently released the Ehang 184, a twin bladed quadcopter that can hover with 100 kg payload for up to 2 hours. Although the effort should be lauded but there are several questions regarding the safety and performance. The drone is auto-piloted, the user only keys in the destination ( and select route if desired). Even with driver-less cars there is more a human issue than a technological one. It is unfathomable for humans to relinquish control in a fast moving vehicle. It will be a bigger leap of faith for automated flying machines. Nonetheless it is an effort in the right direction.
Recently an 18 rotor electric drone has been developed in Germany that is believed to be the worlds safest helicopter.
With big companies like Amazon and Google putting their weight behind Drone technology, advancement can be made leaps and bound. At present both the capital and the operational costs may not be low but they are bound to come down.
The important fact is that technology is here. Would this technology pave way for Sustainable transport in future? We will have to wait and see. However, Electric aviation as has been demonstrated by Solar impulse, Ehang 184 and Airbus Efan provides us the possibility of emission free transport that can be a key factor in combating climate change.
- Jump up ^ Wilson, David Gordon; Jim Papadopoulos (2004). Bicycling Science (Third ed.). The MIT Press. p. 44. ISBN 0-262-73154-1.
- Ehang 184
- Problems in scaling up Quadcopters