Solar Impulse HB-SIA, Switzerland

Solar Impulse is a solar plane intended to run solely on solar energy. The €70m project is being promoted by Swiss balloonist Bertrand Piccard. A project feasibility study was carried out by the École Polytechnique Fédérale de Lausanne (EPFL), the official scientific advisor of the project, in Switzerland in 2003, and the first prototype of the aircraft, HB-SIA, was unveiled on 26 June 2009.

Solar Impulse HB-SIA has a large wingspan and a non-pressurised cabin. It is much lighter than conventional aircraft, with the large wingspan layered with solar cells to supply energy to the engines. It is a one-pilot aircraft that is capable of taking off autonomously.

The test flights and the first night flight of the prototype are scheduled for 2010. The major objective of HB-SIA is to complete a day-night-day flight of 36 hours, completely propelled by solar energy. The test flights will aim to optimise and adjust the balance between energy consumption, weight, performance and controllability.

The final solar aircraft, a two-seater HB-SIB with a pressurised cabin, will be built in 2011 to make long-haul, non-stop flights. The final aircraft will go on a 20-to-25-day world tour with five stopovers for changing pilots. With pressurisation, the aircraft will be able to gain a maximum altitude of 12,000m.

Solar Impulse design

The HB-SIA has a 63m wingspan and weighs 1,600kg. The long wingspan provides space for 10,748 solar cells that absorb solar energy. The fuselage of the solar aircraft is very small in comparison to the wingspan.

The aircraft is constructed from composite carbon-fibre materials using a sandwich structure to make the aircraft light. The upper surface of the wings is layered with encapsulated solar cells, while the undersides are covered with flexible film. There are 120 carbon-fibre ribs placed at 50cm intervals throughout the frame between these two layers, which provide the aircraft with an aerodynamic shape.

Solar power

Solar Impulse uses solar energy instead of jet fuel. The solar energy is then converted to different forms of energy at various stages of the flight.

The aircraft receives energy equivalent to 1,000W for each square metre of wing surface. Over the day it averages at 250W. It features 200m² of photovoltaic cells and a 12% total efficiency of the propulsion chain. With this energy, each engine achieves an average 8hp of light power.

Solar energy is converted to electrical energy in the photovoltaic cells, batteries and motors. The system has chemical energy inside the batteries, potential energy when gaining altitude, kinetic energy when gaining speed, mechanical energy through propulsion system and thermal energy through the various losses due to heat and friction.

Energy capture and storage

The energy is captured by 11,628 ultra-thin monocrystalline silicon cells layered on the upper surfaces of the wings and horizontal stabiliser at the rear side of the aircraft.

Each cell is 150-microns thick and chosen for its efficiency, lightness and flexibility. These solar panels will generate electricity during the day to propel the plane. The extra energy required for the night flight will be stored in the lithium polymer batteries.

The maximum energy density is 220Wh/kg while the accumulators for the night flight weigh 400kg which is a quarter of the total mass of the plane. Therefore, a successful night flight of HB-SIA requires maximising aerodynamic performance and optimising the energy chain.

HB-SIA propulsion

The aircraft has four pods, each comprising a motor, a polymer lithium battery consisting of 70 accumulators and a management system that controls temperature and charge / discharge. The thermal insulation conserves the heat radiated by the batteries and keeps them functioning even at the extreme temperature of -40°C, encountered at 8,500m.

Engine

HB-SIA carries four propeller engines, which are powered by 100kg lithium batteries. The twin-blade propellers have a 3.5m diameter. Each engine has a maximum power of 10hp and is equipped with a reducer that limits the rotation of each propeller to between 200rpm and 4,000rpm.

Cockpit

The Solar Impulse cockpit accommodates a single pilot. The size and weight of the batteries limit the size of the cockpit. The instrumentation panel acts as a power status summary indicator that shows revolutions per minute and temperature for the four engines. This enables the pilot to monitor the condition of the flight in two key parameters – bank angle and side slip.

The cockpit includes a series of slider bars, which display the condition of batteries or energy accumulators. The cockpit also features environmental control and life support systems that erase the carbon dioxide and humidity generated by the human body.

Construction

Once the test flights are complete, the final Solar Impulse aircraft, with a wingspan will be 80m, will be constructed in 2011.

The batteries of the final aircraft are expected to be lighter and more efficient. The Solar Impulse HB-SIB is scheduled to make a world tour in 2012 near the equator, primarily in the northern hemisphere.

Key players

Solar Impulse is the combined effort of a group of firms. Solvay, the international chemicals and pharmaceuticals group, brings its expertise in plastics and polymers technology to the HB-SIA project, while Omega's electromechanical system has been used to test the aircraft's energy system. It will also develop instrumentation for landing.

Swisscom is developing tools to permit communication between the aircraft and mission control from anywhere around the world. Financial support for the project has come from Deutsche Bank, Solvay and Omega.

Technical expertise has been provided by EPFL, the European Space Agency and Dassault Aviation. Dassault provides expertise in aeroelasticity and flight command, safety and systems reliability.