{"id":10143,"date":"2015-07-14T10:43:24","date_gmt":"2015-07-14T17:43:24","guid":{"rendered":"http:\/\/cafe.foundation\/blog\/?p=10143"},"modified":"2015-07-15T06:49:35","modified_gmt":"2015-07-15T13:49:35","slug":"solar-impulse-tougher-looked","status":"publish","type":"post","link":"http:\/\/cafe.foundation\/blog\/solar-impulse-tougher-looked\/","title":{"rendered":"Solar Impulse – Tougher than it Looked"},"content":{"rendered":"

The record-breaking flight from Nagoya, Japan to Kalaeloa, Hawaii by Andre\u2019 Borschberg in Solar Impulse had more difficulties than were immediately apparent to even the most involved observer.\u00a0 During the first climb out of Nagoya, some of the cells mounted behind each motor experienced overheating, now attributed to over-insulation.\u00a0 High-density foam packed around the cells was apparently too effective for even the freezing temperatures at altitude and caused the batteries to become warmer than desired.<\/p>\n

\"Location<\/a>

Location of Solar Impulse batteries – one pack behind each motor. \u00a0That’s the combined weight, each individual pack being 158,25 kilograms or 348,15 pounds<\/p><\/div>\n

As explained on the Solar Impulse web site<\/a>, \u201cThe energy collected by the solar cells is stored in lithium polymer batteries, whose energy density is optimized to 260 [Watt-hours per kilogram]. The batteries are insulated by high-density foam and mounted in the four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 633 [kilograms] (1,393 pounds), or just over a quarter of the aircraft\u2019s all-up weight.<\/p>\n

\u201cIn order to save energy, the aircraft climbs to 8,500 meters (27,887 feet) during the day and descends to 1,500 meters (4,921 feet) at night.\u201d<\/p>\n

This sawtooth profile allowed the airplane to climb in the sunshine and descend in darkness, using less power and conserving the cells so Solar Impulse could greet the dawn with at least partially charged batteries.<\/p>\n

\"What<\/a>

What the public could view during the flight, among other data<\/p><\/div>\n

BatteryBro.com<\/a><\/em>, a commercial operation which wholesales 18650 cells has a technically astute writeup by Anton Doos, one of the group’s cofounders, about the batteries on board HB-SIB.<\/p>\n

\u201c<\/strong>Energy storage represents the plane\u2019s primary inefficiency, but also is a very achievable goal to improve upon. [Bertrand] Piccard has this to say on the state of battery technology for aircraft:<\/p>\n

\u201c\u2018The batteries we have used have been specially developed for optimal maintenance but are still 10 times heavier than kerosene. I would imagine that we are still 20 years away from attaining parity on this point.\u2019\u201d\u00a0 CAFE Blog readers would love to see a more optimistic projection, but even modest improvements in energy density and pack weight are welcome.<\/p>\n

For instance, Solar Impulse 1 used Kokam batteries with an energy density of 243 Watt-hours per kilogram.\u00a0 Solar Impulse 2 flies with Kokam batteries\u00a0 with an energy density of 260 Wh\/kg., a modest gain, but significant out over a dark ocean. \u00a0Both aircraft used battery housings designed and built by Air Energy<\/a>, a German firm.\u00a0 The four battery packs were housed in the same insulating materials used in the cockpit to protect the pilot from temperature extremes.<\/p>\n

\"Pouch<\/a>

Pouch cells as installed in Solar Impulse 2<\/p><\/div>\n

Commenting that the four 8-kilowatt motors are operating at a level that does not allow for errors, the site explains the choice of Kokam pouch-type cells used in the Solar Impulse.<\/p>\n

\u201cThis solar-powered aircraft Solar Impulse uses four main lithium-ion batteries, each containing 70 lithium-polymer cells. It does not use a commodity cell like the 18650 that Tesla uses. There are several reasons, foremost being\u00a0weight. An 18650 battery is in a steel housing, and a lithium-ion polymer battery like what is in this aircraft or in a cell-phone is housed in a soft, thin, flexible material. That is why nearly all drones, RVs, and electric planes use polymer (soft-packs). The polymer pack used by Solar Impulse is completely custom-built.\u201d<\/p>\n

Battery University<\/a>, another on-line resource, has a brief history of pouch-type cells as used on Solar Impulse.\u00a0 Its main benefit is weight savings, but the lack of a metal enclosure as in a cylindrical cell requires the use of some form of protective housing for the pouch or pouches.\u00a0 Despite this the pouch cell attains 90-95 percent packaging efficiency, the highest among battery packs.\u00a0 No standard pouch cell exists like an 18650 battery \u2013 each manufacturer designing its own version.<\/p>\n

The batteries operate at a high voltage range of 210-304.5 Volts DC, which must generate some heat of its own.\u00a0 Batteries have a total capacity of 38 kilowatt-hours. (Corrected July 15.)<\/p>\n

Solar Impulse\u2019s blog says their new Kokams are \u201cparticularly revolutionary,\u201d in having a new chemical composition that has improved the oxidation issue which ages batteries and causes them to lose efficiency.\u00a0 \u00a0This technology is two years ahead of the industry, according to the blog, and \u201callows Solar Impulse to have batteries able to guarantee 2\u2019000 flight hours, versus 500 in HB-SIA\u2019s case.\u201d \u00a0To gain energy density, Kokam added Monoflourethylencorbonat solvent developed by Solvay Chemicals.<\/p>\n

The 70 cells in each battery pack don\u2019t work well in extreme heat or extreme cold, and the insulation made by Bayer Material Science is intended to help maintain an optimum temperature of around 25\u00b0C (77\u00b0F \u2013 or around room temperature) inside the motor gondolas.\u00a0 This involves an active heating mechanism to counter the effects of low temperatures at altitude, -40.5\u00b0C (-40.9\u00b0F) at 28,000 feet and an even more grueling -44.4\u00b0C (-48\u00b0F) at 30,000 feet, which the airplane exceeded on at least one climb.<\/p>\n

According to Solar Impulse, each battery pack is an independent element, but a balancing controller can transfer power between cells in case of motor failure.\u00a0 Each battery is fed by a particular solar panel associated with a given motor.<\/p>\n