It has been a tough couple of months for Boeing. Its 787 has been grounded since January 16 by the Federal Aviation Administration because of problems with its onboard lithium-ion (Li-ion) batteries. In one incident a fire took place aboard a Japan Airlines 787 shortly after it landed at Boston’s Logan International Airport. It took firefighters nearly 40 minutes to put out the fire in the aircraft's rear auxiliary power unit (APU).
The 787 has two 32-volt Li-ion battery packs. One is located forward in an equipment bay below the cabin floor near the front passenger door where it provides power for aircraft start-up and ground operations such as towing and refueling.
The second Li-Ion battery pack is located aft in another electronics bay. It starts up the APU and in case of a power failure provides backup power for flight instruments in the cockpit. The APU is a small jet engine for lighting and air conditioning that is used when the main engines are switched off. Once the main engines are started, the APU is no longer required, although it can provide an emergency source of electric power in the event of engine failure.
Although lithium cells need lots of careful attention when charging and discharging (although in this case data from the flight recorder retrieved from the airplane shows that its APU battery did not charge beyond its design limit of 32 volts) a lithium-based solution was selected because it offers much higher energy density (by weight and volume) than any other available battery chemistry. It is precisely because this energy density is so high that there's a lot of stored energy in a small volume, and a failure such as an internal short can result in huge current flows and subsequent fires or even explosions.
In a growing number of applications supercapacitors − similarly known as ultracapacitors or electrochemical double layer capacitors (EDLCs) − are either replacing or sharing the load with Li-ion batteries. Like their smaller capacitor cousins they are electrostatic, not electrochemical devices. Supercapacitors have replaced Li-ion batteries in many hybrid urban buses, some power tools and for wind turbine blade feathering and blade pitch angle systems, where batteries are not reliable enough because of the operational temperatures involved. And last September a Toyota TS030 Hybrid won the fifth round of the FIA World Endurance Championship in Sao Paulo, Brazil using a 3.4 liter V8 gasoline engine and a supercapacitor energy recovery unit, which fed energy to the rear wheels.
Supercapacitors are now specified for these and other applications because they are reliable, completely maintenance free and have a long shelf life at temperatures up to 70 degrees Celsius. Unlike batteries, they can be fully discharged for transport, their behavior is more predictable and they are more tolerant of faults and damage.
The amount of energy supercapacitors can hold is related to the surface area and conductivity of their electrodes. Commercial supercapacitors can achieve an energy density of 10-20 Watt-hours per kilogram (Wh/kg), much greater than the energy density of a conventional capacitor but still much lower than the energy density reached by Li-ion batteries, which is about 32 Wh/kg.
With 100 percent reliability at temperatures from -40°C to 70°C, extremely long lifespans (over a million cycles, as compared to 10,000 for conventional batteries), unsurpassed ruggedness and durability why not consider supercaps in passenger aircraft APU operations?
Here’s why: Li-ion chemistry is particularly attractive because it packs more power in a smaller size, and is therefore lighter than more traditional battery designs — a factor that was key to Boeing’s strategy of building a lightweight aircraft (the 787 is made almost entirely of carbon fiber, replacing traditional, and heavier, aluminum). Light weight reduces an airline’s fuel bill; fuel efficiency is one of the 787's main sales points. As we just noted above current supercapacitors fall short of Li-ion batteries in terms of energy density, and energy density is a measure of how big or heavy a battery or capacitor needs to be to store a particular amount of energy. Put another way right now supercapacitors need to be much larger than batteries to hold the same charge.
In case you were wondering, re-charging supercaps for aircraft use shouldn’t be an issue. In hybrid cars or buses supercapacitors quickly capture energy from braking and then can use that energy either to provide a short burst of power during acceleration or to dramatically reduce the use of fuel or battery drain in an electric/hybrid system.
Capturing energy from re-generative braking should work well in aircraft, too, since an aircraft has to quickly reduce its speed from 150 to about 20 miles per hour when it lands, dissipating several hundred kWh of energy during braking. In fact, a start-up company called eXtreme Capacitor Inc. plans to use Carbon Nanotubes (CNTs) in supercapacitors to recover that energy and possibly use it for engine-less taxiing at airports. eXtreme is targeting a very high power-to-weight ratio of 250 Watt-hours per kilogram for its supercapacitors, compared to lithium cells’ 128 watt-hours per kilogram, to overcome this major objection to capacitors as energy storage devices.
eXtreme’s development goal gives us a glimpse as to what’s next in supercapacitor technology. Since for capacitors storage capacity is proportional to the surface area of the electrodes, increasing the surface area will increase the capacitance of the supercapacitor. Carbon nanotubes have excellent porosity, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. One square centimeter of conductive plate when coated with nanotubes is said to have a surface area of about 50,000 cm2.
Another startup, FastCAP Systems out of MIT has unveiled a novel nanotube-enhanced version that is said to store twice as much energy and deliver about 10 times as much power as a conventional supercapacitor can. The FastCAP product also uses tiny carbon nanotubes, each one thirty-thousandth the width of a single human hair. According to the company the regular shape and alignment of the nanotubes makes a very efficient and high surface area electrode, and its process for making the nanotube electrodes yields an increased capability to withstand voltage.
There is also a body of research work looking at a hybrid combination of a supercapacitor electrode working against either an anode or, more commonly, a cathode consisting of a rechargeable Li-ion battery. This "lithium ion capacitor (LIC)" type of device should enable almost all of the charge residing on the capacitor-type electrode to be utilized on discharge. Compared to the standard supercapacitor the LIC will have a higher output voltage. It should also have a higher power density and be safer in use than Li ion batteries, and without thermal runaway issues.
So don’t be surprised if the 797, or whatever the next generation passenger aircraft is called, eschews Li-ion batteries altogether in favor of supercapacitors.