INTEGRATING SOLAR CELLS INTO COMPOSITE MATERIAL an Opportunity for Electric Planes and Uavs

Home , Electric aircraft, NASA Centurion

“The future of aviation is electric”

Dr Susan Ying, President of the International Council of Aeronautical Science and former Director of Boeing Research and Technology.

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INTEGRATING SOLAR CELLS INTO COMPOSITE MATERIAL An Opportunity for Electric Planes and UAVs

CONTENTS EXECUTIVE SUMMARY

Executive summary...... 1 It is well established that adding solar cells to UAVs and manned electric aircraft is beneficial – the sun is an inexpensive, renewable, and plentiful What is holding back source of energy. So if the advantage of solar is clear, why don’t we see the solar aircraft industry?...... 1 solar aircraft in the market?

Electric aircraft revolution...... 2 Industry opinion is that a solution is required to enable drones to fly as long as needed. That solution has not been forthcoming. The approach The limiting factor has to date has been to glue solar cells on top of aircraft wings, but this is been battery technology...... 3 inefficient, adds weight, and is not particularly aerodynamic. Solar cells are fragile and so far there has been no way to integrate solar cells into Challenging the status quo...... 3 wing materials that was robust, inexpensive and light.

Solar cell composites - Praxis Aeronautics deduced – from its own experience in composite a solution...... 4 and aeronautical engineering – that what the industry needed was an aerodynamic solution that would enable solar cells to be shaped and From invention to reality...... 5 integrated into the product. The solution would need to make the cells flexible and able to efficiently absorb light. Praxis Aeronautics has Conclusion...... 6 developed a solution that meets these criteria.

About Praxis Aeronautics...... 6 WHAT IS HOLDING BACK THE SOLAR AIRCRAFT INDUSTRY?

The first manned solar aircraft flew in 1974. More than 40 years later, there are many examples of experimental solar aircraft and unmanned aerial vehicles (known as UAVs or drones), but they are yet to become a commercial reality. INTEGRATING SOLAR CELLS INTO COMPOSITE MATERIAL WHITEPAPER

Today’s UAVs are used by individuals through to large commercial operators for increasingly diverse applications, such as aerial surveying, agriculture and law enforcement. They also play a significant role in defence operations. Electric UAVs are used primarily because of their relatively lower cost, higher safety rating and simplicity of use. However due to battery limitations, electric drones have the major disadvantage of a flying time typically limited to around 1 hour.

A new technical approach is needed to enable industries such as Defence, Agribusiness, and the commercial Aerospace industry itself to push the boundaries of drone capabilities.

This paper discusses the historic problem, current thinking – and provides a solution to this problem.

EVOLUTION OF SOLAR AVIATION

ELECTRIC AIRCRAFT REVOLUTION 1974 SUNRISE 1 First solar UAV 1975 SUNRISE 2 The use of electric power for flight vehicles propulsion is not new. The first example was the 1976 SOLARIS hydrogen-filled dirigible La France in 1884. The 1978 First manned solar flight SOLAR 1 first solar-powered UAV was the Sunrise in 1974. 1979 SOLAR RISER 1980 GOSSAMER PENGUIN Today, the advantages of electric aircraft 1981 SOLAR CHALLENGER are clearly known: the cost of building and 1983 SOLAIR 1 maintaining electric aircraft is considerably less than petrol fuelled aircraft. Electric motors are 1990 SUNSEEKER both incredibly simple and efficient; an electric 1990 SOLAR EXCEL motor runs at 80-98% efficiency compared to 25- 1994 NASA PATHFINDER 50% efficiency for an internal combustion engine. 1996 SOLAR SOLITUDE 1996 ICARE 2 1998 NASA PATHFINDER PLUS 1998 NASA CENTURION 1998 SOLITAIR 2001 NASA HELIOS 2003 QINETIQ ZEPHYR 2005 SOLONG First 24+ hour UAV flight 2009 SOLAR IMPULSE 1 First 24+ hour manned flight 2010 AIRBUS ZEPHYR 7 2015 SOLAR IMPULSE 2 First circumnavigation of the earth by a piloted fixed-wing 2016 FACEBOOK AQUILA solar aircraft 1980-1993 HALSOL 1995-1998 MIKROSOL, PICOSOL, NANOSOL INTEGRATING SOLAR CELLS INTO COMPOSITE MATERIAL WHITEPAPER

THE LIMITING FACTOR HAS BEEN BATTERY TECHNOLOGY

By collecting energy from their environment, solar aircraft can greatly overcome the limitations of battery technology, extending range on average between 2 to 3 times longer than achieved with batteries alone. As solar cell and battery technology both improve, flight duration will increase. Aircraft that collect energy from their environment is the only way to achieve indefinite flight.

Praxis created its own test environment to measure the difference in battery life with and without solar input. See graph below.

PRAXIS PROOF OF CONCEPT - BATTERY VS SOLAR GROUND TEST

15.5 oltage Battery V 14 050 100 150 200 250 300 350 Run Time (minutes)

Battery Test Solar Test - Sunny Conditions Solar Test - Cloudy Conditions

CHALLENGING THE STATUS QUO

Because of the fragility of silicon cells, they have been mostly deemed unsuitable for use on electric aircraft. The current approach to manufacturing has been to redesign the cells using flexible Gallium Arsenide (GaAs) – an expensive solution that only partially addresses the problem.

Silicon solar cells of 24% efficiency cost approximately $3/watt. Single junction Gallium Arsenide cells of 26% efficiency cost approximately $100/watt. Multi-junction Gallium Arsenide cells of 32% efficiency cost in the order of US $200 - $300 per watt.

The work to produce lighter, flexible GaAs cells is valid and beneficial to the use of solar in electric aircraft but still does not address the problem of integration.

What has been holding back the development of solar aircraft and UAVs, beyond experimental stage, has been the failure to integrate the cells into the composite material. The approach to date has been to encapsulate the cells in plastic, which in itself has very little structural integrity, and then glue this to the top surface, which adds weight without any structural benefit. This resulting product is not robust or tough and while this is adequate for experimental aircraft, and to prove the validity of solar for range extension, it is not commercially viable. INTEGRATING SOLAR CELLS INTO COMPOSITE MATERIAL WHITEPAPER

SOLAR CELL COMPOSITES - A SOLUTION

The solution to commercially viable solar electric aircraft is not purely in the development of suitable solar technology but in how the solar cells are themselves incorporated into the aircraft structure.

Praxis has developed a novel process that encapsulates solar cells as a structural element of the composite material of a wing or fuselage. This process overcomes the problem of fragility in silicon cells, allowing Praxis to curve the cells without breaking them.

The Praxis approach is to make the composite itself perfectly optically clear so it serves as the integration material as well as the wing structure. This adds little or no weight because the cells become a structural element of the skin, replacing some of the structure that would be there in a non-solar wing. The result is a perfect aerodynamic surface, no different from a non-solar wing.

The Praxis solar wing has all the qualities of a non-solar composite wing - it is a perfect aerodynamic surface, tough, water-proof and scratch resistant, with minimal extra steps in manufacturing. The Praxis manufacturing processes are compatible with silicon and gallium arsenide and have demonstrable results, data, and quality standards.

FROM INVENTION TO REALITY

The robustness and increased flight duration offered by this integration technique makes it suitable for Defence applications such as intelligence, surveillance and reconnaissance (ISR). The composite can be retrofitted to current Small Unmanned Aerial Systems (SUAS), including Vertical Takeoff and Landing (VTOL) systems.

Increased flight duration and robust construction also means farmers will be able to monitor vast tracks of land, monitor crops and livestock or manage soil health. Public safety, fire control, civil disobedience and surveillance all represent applications that would benefit from the flexibility of a longer flight duration. INTEGRATING SOLAR CELLS INTO COMPOSITE MATERIAL WHITEPAPER

Longer flight duration would also benefit humanitarian challenges such as environmental and conservation management, deliveries of medical supplies to remote and isolated communities, and disaster relief efforts. These are just a few examples of the future of solar electric aircraft and UAVs.

The solar integration process can be used for a range of non-aeronautical applications including:

• equipment and cases, autonomous and manned vehicles (ground and sea) and habitat facilities such as communication and accommodation tents; • electric vehicles such as bus roofs and boats (surfaces and solid sails); • building surfaces.

CONCLUSION

The Praxis integration process results in a product that is robust, light and aerodynamic in surface for the toughest defence and commercial applications. Other integration techniques to date, while suitable for experimental aircraft, have not proven to be adequate for the rigours of real world applications. The Praxis approach of combining the cells within the structure of the composite is best practice in solar cell integration for electric aircraft and UAVs.

The process is also cost effective as the integration process adds little manufacturing complexity to the process.

ABOUT PRAXIS AERONAUTICS

Praxis Aeronautics was formed in 2016 to manufacture encapsulated solar cells for electric planes and UAVs by using its novel integration process to extend flying time, reduce costs and extend the range of applications across industry. The Praxis team comprises expertise in composite manufacturing, aeronautical and structural engineering, electronic and software engineering, maths and physics. The Australian company is based in Adelaide, South Australia, and provides its solar integration services and products to customers worldwide.