Solar Plane Takes Flight

DN Staff

October 18, 2010

9 Min Read
Solar Plane Takes Flight

In 2012, a manned aircraft powered by 11,628 photovoltaiccells covering 2,200 sq ft of wing space will circumnavigate the globe - if aglobal engineering team can make it light enough.

Called the SolarImpulse, the first prototype of the aircraft flew an entire daily solarcycle, including nearly nine hours of night flying, in July.

"During the whole of the flight, I just sat there andwatched the battery charge level rise and rise. Sitting in a plane producingmore energy than it consumes is a fantastic feeling", says Andre Borschberg,CEO and co-founder of the Solar Impulse project.

"Flying by night using solely solar power is a stunningmanifestation of the potential that clean technologies offer today to reducethe dependency of our society on fossil fuels," says Bertrand Piccard,initiator and president of Solar Impulse.

The engineer heading up the structural design of the SolarImpulse is Peter Frei, who graduated from the Swiss Federal Institute ofTechnology (ETHZ) in mechanical engineering with specialization in fluiddynamics and lightweight structures. Frei also is a former Swiss fighter pilot.

Reporting to Frei are a small group of staff engineers and alarge group who work for corporate partners. They include Claude Michel, seniorvice president at Solvay and head of the Solvay-Solar Impulse Partnership. TheSolvay Group is one of the three major funding partners for the aircraft and thecurrent prototype features a profusion of new materials developments fromSolvay companies such as Solvay Advanced Polymers in Alpharetta, GA, and SolvaySolexis of Milan, Italy.

"In terms of technical constraints, the question is verysimple," Michel told Design News. "Weneed a surface area of photovoltaics that delivers enough energy to put intothe air a certain amount of weight. This is what we call the equation of the SolarImpulse. One square meter in photovoltaics delivers 30 watts, and with that 30watts, we can put 8 kg (17.6 lb) in the air. This is our target, and it is aneveryday obsession."

Carbon Honeycombs

One of the most interesting lightweighting solutions is ahoneycomb structure sandwiched between two carbon fiber foils that function aswing and stabilizer spars. The honeycomb is a lightweight paper structureimpregnated with a TorlonAI10 polyamide-imide polymer via a dipping process. The result is a compositestructure that possesses excellent mechanical properties, including strength,torsion, flexion and vibration. Sections of the wing that are 66-ft long can beeasily handled by a few people.

Torlon, originally developed by Union Carbide, functions as aglue in the honeycomb structure. It features the highest strength and stiffnessof any thermoplastic up to 525F and has outstanding resistance to wear, creepand chemicals, making it ideal for severe environments.

Another interesting development is a centering cylinder forthe landing system. It was initially designed in aluminum and is being replacedwith Ixef polyarylamide 1022.

Ixef compounds are typically highly loaded with glass (50 to60 percent) and possess the tensile and flexural strength similar to many castmetals and alloys at ambient temperature. Ixef 1032 (60-percent glass fiber)exhibits typical values of 280 MPa at 73F.

The internal cylinder component is machined to its finaldimension from an extruded rod of Ixef 1022. The external part remains aluminumdue to impact requirements. The shaft is made from PrimoSpireSRP due to its intrinsic modulus and its machinability. PrimoSpireself-reinforced polyphenylene (SRP) is one of the stiffest and strongestunreinforced plastics available. Even without fiber reinforcement, PrimoSpireSRP delivers tensile properties that are comparable to those of many reinforcedplastics. The added benefits are lighter weight and no loss of ductility.

Radelpolyphenylsulfone is used for the rear attachment legs in the landing systemassembly due to its high deformation capability.

Many other plastics from Solvay Advanced Polymers are usedfor other parts, more than 6,000 all together, such as bolts, screws, nuts and washersin the HB-SIA prototype to minimize weight.

Wing Structure

High-performance plastic films play an important role instabilizing the wings of the Solar Impulse.

"The wings are somewhat flexible," says Michel. "Their shapeis quite different on the ground than in the air. We can use films to alleviatethis."

Solef PVdF-based laminate is used to cover the ailerons,elevators, rudder, airbrakes and spoilers because of its mechanical strength atlow thickness (15 microns). It will be used for the entire airplane wings ofthe next version of the Solar Impulse, called HB-SIB, if Solvay can provide thematerial in 3m (9.8 ft) width. The development partners for the new technologyare ParkinsonTechnologies of Woonsocket, RI, for the film extrusion and with Jehier of Hutchinson, France, for the lamination.

Here's what the 60m (197 ft) wing looks like:

  • Spars are made of honeycomb-carbon fibers that support120 ribs, which profile the shape of the wing;

  • An encapsulated photovoltaic system is on top; and

  • A laminated PVdF film is on the underside.

It may surprise some design engineers how stiff that systemis. The deflection, defined as the difference between ground and flightpositions, at the wing tips is less than 0.5M (19.7 inch).

The film that protects the photovoltaic (PV) cells from theharsh environment is made from Halarethylene chlorotrifluoroethylene (ECTFE).

Both plasma and corona treatments have been studied in orderto increase the adhesiveness of the fluorinated films to the epoxy layer, theencapsulation material used by Solar Impulse. The thickness of the filmproduced by Ajedium of Newark, DE, was reduced to 17 microns, resulting inabout 35 percent weight savings with acceptable electrical performance.

Solvay Solexis is looking for more ways to save weight. Asolution with a single product replacing both the glass-reinforced epoxy layerand the top film is under evaluation.

A PVdF-based adhesive tape that is both UV-transparent andUV-resistant is used to close gaps between the solar cells.

The fuselage is made from carbon fiber rods encapsulatedwith polyester film. The name of the supplier of the polyester film was notmade public. Decision, aSwiss boat builder, is the composite materials specialist for the SolarImpulse, and Bruhlmeier Modellbau,also Swiss, manufactures composite parts.

Insulating Foams

Polyurethane foams are used to make the cockpit of the airplanelight, but also stable, while providing insulation.

"We're trying not to use any watts for heating or airconditioning," says Michel. The pilot must be protected from temperatures thatcan go as low as -40C (-40F), and the batteries must be kept above 15C (59F).

Solvay Fluor's third-generation blowing agent Solkane 365 provideshigh gas phase lambda (thermal conductivity) value and foam dimensionalstability/compressive strength. Foam insulation is used for the cockpit, fourmotor gondolas and the wing tips. These pieces are produced by forming bigblocks and then machining the eggshell-shaped wing tips out of them.

A new partner providing materials technologies for the SolarImpulse is BayerMaterialScience (BMS) of Leverkusen, Germany.

Lightweight polyurethane foam from Puren GmbH of Uberlingen, Germany, used in the enginecowling, cockpit cladding and the wings is based on raw materials from BayerMaterialScience. The cockpit windows are made of thin but strong Makrofolpolycarbonate film.

The bar is rising for the next version of the Solar Impulseboth for structural requirements and materials' technologies.

One interesting possibility is the use of carbon nanotubes,which are described as the strongest and stiffest materials available in termsof tensile strength and elastic modulus, respectively.

BMS says they are nearly as strong as steel, but half asheavy. They also conduct electric current as effectively as copper.

Bayer MaterialScience operates a CNT plant in Leverkusenwith an annual capacity of 200 metric tons. BCC Research forecasts that globalsales of carbon nanotube-reinforced plastics will grow at an average annual rateexceeding 55 percent over the next five years for electrical/electronic,transportation, structural and other applications.

The second prototype is scheduled to fly around the world infive stages, each lasting five days, traveling at an average speed of 70 km/h(43.5 mph).

Improvements in battery technology will play a criticalrole.

"In order to fly at night, we need to store energy that iscaptured during the day," says Michel. "There are 400 kilos (882 lb) ofbatteries and this is the chief bottleneck in the system."

Electrical energy harvested by the solar cells is stored inlithium batteries, where PVdF is used as a binder for both electrodes. The PVdFalso boosts the electrochemical stability of the cells, allowing repeatedcharging and discharging cycles at a high rate, without deterioration ofbattery performance.

The original design of the plane in 2004 was made with Libatteries having an electric density of 180 Wh/Kg. The four 100 Kg batteries ofHB-SIA (the current prototype) have an electric density of 220 Wh/Kg.

Solvay says that a recently developed grade of PVdF offersvery high adhesion to the electrodes, potentially contributing to the weightreduction of batteries for HB-SIB (the 2013 flight aircraft).

Carbon nanotubes could significantly improve batteryperformance, but officials at Bayer MaterialScience declined specific comment.

New PV System

The original design of the plane was made with monocrystallinesilicon photovoltaic cells, having a thickness of 180 microns and an energyefficiency of 18 percent. The PV system is now made of 11,628 mono crystallinesilicon cells whose thickness is below 150 microns for an energy efficiency of22 percent after encapsulation.

Solar Impulse originally used small cells with both frontand back electrical contacts from RWE Solutions (now Azur Space Solar PowerGmbH). The Solar Impulse engineering team is now using large cells (125 x 125mm) from Sun Power (that are now onthe prototype). These have back contacts, higher efficiency, and need lesselectrical connections due to their larger size.

For the design and development phases of Solar Impulse,Solvay performed more than 2,000 hours of non-linear finite element analysis(FEA) with ABAQUS on the thermo-mechanical behavior of several modules underdifferent conditions. Linear simulation is used to model metallic structures.

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