Technological development trends in Solar‐powered Aircraft Systems

Renewable and Sustainable Energy Reviews 60 (2016) 770–783

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Technological development trends in Solar‐powered Aircraft Systems G. Abbe, H. Smith n Department of Aerospace Engineering, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, MK430AL, UK

art ic l e i nf o

a b s t r a c t

Article history: Received 2 June 2015 Received in revised form 6 January 2016 Accepted 13 January 2016

Key issues relating to the past, current and upcoming technologies of solar-powered aircraft are reviewed with the intent to assess the technology trend and offer a prediction for the next decade. The paper looks into the design issues comprising of structures, systems, propulsion, aerodynamics, and system integration for solar-powered aircraft. Additionally, the technological status which includes structural materials, photovoltaic systems, battery and power management systems in the case of solar aircraft, would be considered. Finally, technology trend issues would be assessed summarizing into a predictive outlook. The applicable technologies in the design of solar powered aircraft has evolved over the years, hence an overview is presented. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Solar-powered aircraft Renewable energy Technology trend Photovoltaic Battery Fuel cell

Contents 1.

2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 771 1.1. Basic Solar‐powered Aircraft Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 771 1.2. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 771 Historical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 772 Design issues/parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773 3.1. Aircraft Design Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773 3.2. Vehicle Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773 3.3. Aerodynamic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773 3.4. Propulsion Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774 3.5. Airframe Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774 Technologies for solar aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774 4.1. Platform Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774 4.2. Energy Source Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775 4.2.1. Crystalline wafers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775 4.2.2. Thin-film technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775 4.2.3. Multijunction cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 4.3. Energy Storage Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 4.3.1. Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 4.3.2. Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4.3.3. Super Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4.4. Power Management and Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.5. Propulsion Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.5.1. Motors/drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.5.2. Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 Technological trend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779

n

Corresponding author. E-mail addresses: g.e.abbe@cranfield.ac.uk (G. Abbe), howard.smith@cranfield.ac.uk (H. Smith). http://dx.doi.org/10.1016/j.rser.2016.01.053 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

G. Abbe, H. Smith / Renewable and Sustainable Energy Reviews 60 (2016) 770–783

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5.1. 5.2.

Technology Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779 Technological Endpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779 5.2.1. PV System Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779 5.2.2. Battery and Fuel Cell Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 5.3. Overall trends in Solar Aircraft Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781 5.4. Emerging Applicable Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781 5.4.1. Lithium Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781 5.4.2. Organic Photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781 5.4.3. Quantum dots and Polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782 5.4.4. Nanostructured Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782

1. Introduction By way of definition, solar-powered aircraft could be described as aerial vehicles capable of sustained level flight in the atmosphere depending solely on solar radiation impacting on its airframe as its primary energy source. The design and development of solar-powered aircraft has gained an increasing level of interest in the last 30 years. This has been amplified by concerted efforts towards environmentally benign vehicles and research into alternative renewable energy technology. The basic aim has not only been the environmental benefits, but also the capability of increasing the applicability of this class of aircraft by expanding the mission range and duration optimally (Fig. 1). The aim of this paper is to evaluate the current standpoint of solar aircraft technology and to provide a picture of the state of research in the most significant sub-system technologies. Solarpowered aircraft design is a combination of several multidisciplinary aspects which include aircraft structural design, propulsion system design, electrical system design, and power and control system design. The demanding process of integrating these systems together to achieve balanced efficiencies in aerodynamics, structural weight, and power conservation against predetermined mission profiles is important. Each sub-system requires an evaluation and design that is relative to mission requirements and in synergy with other sub-systems. Research in these fields have subsequently resulted in the production of small avionics devices, robust light-weight structural materials and higher output low power consumption electric components. 1.1. Basic Solar‐powered Aircraft Principle The basic principle has been to have solar cells cover a particular area of the aircraft, usually the wing and tail plane. When subjected to insolation, the cells convert solar radiation into electrical energy. The amount of energy that is produced depends

Fig. 1. Solar-powered aircraft (Solar Impulse) [1].

on factors such as the day of the year, the time of day, the inclination of the cells with respect to the sun, and the level of cloudcover. The electric power relay is done by a circuit board containing a programmable microchip. A Power Management and Distribution system on the circuit board ensures that the maximum amount of power is gained from the solar cells. The obtained power is primarily used for propulsion and to power the on board electronics. Surplus power is then used to charge the battery. When little or no solar power is available, the battery becomes energy source as depicted in Fig. 2. The evaluation of this class of aircraft from history, either manned or unmanned, gives an indication that any technological improvement on any sub-system has an overall impact on the design of the aircraft. Thus, developing higher efficiency solar cells or batteries would improve the general efficiency and performance of the aircraft. Subsequently, the introduction of high output low power consumption electrical components and higher efficiency propellers would achieve better power conservation and management. Furthermore, obtaining robust lightweight materials for structure and suitable aerofoil selection would enhance payload capacity, flight performance, and endurance of the aircraft. All of these are constantly being considered in the technology world of today. Many technologies have moved beyond theoretical and numerical analysis into actual production and application of miniature devices. These efforts in technological advancement, as can be seen today with the design of smaller but more efficient systems, continues to dictate the pattern in which solar aircraft are developed and operated. A critical study and analysis of this technological trend would prove a beneficial tool in predicting the future of solar-powered aircraft. 1.2. Methodology This paper is based on a survey of available data on past and current trends in research and development of solar-powered aircraft and solar energy technology in general. The author has assumed a direct correlation between the development and implementation of new technologies in the military/commercial domestic based applications and the aerospace application. The

Fig. 2. Basic solar-powered aircraft operating principle.

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Nomenclature CFD CD CL ERAST EV HALE HALSOL HEV MEM MPPT

Computational fluid dynamics Drag coefficient Lift coefficient Environmental Research Aircraft Sensor Technology Electric Vehicle High altitude long endurance High altitude solar aircraft Hybrid Electric Vehicle Microelectromechanical system Maximum Power Point Tracker

cited references are meant to indicate the state-of-the-art and not to be a pertinent representation of the on-going research. Although some basic design criteria such as power to weight ratio are applicable to most aircraft, some aspects are default in solar-powered aircraft such as the use of solar panels, electrical power storage and power management. It is also necessary to state here that the design criteria for this class of aircraft are constrained by some technological limitations of components. As a result, the trend in research and development of these technologies is a determining factor of the future of solar-powered aircraft.

2. Historical background The utilisation of solar energy dates as far back as before the 1700 s when rays from the sun were concentrated through a magnifying glass which generated high enough temperatures to start a fire. As at 1767, the world's first solar collector was built by Horace de Saussure which was later used for cooking [2]. By 1839, experiments by Edmund Becquerel had shown that exposure of electrolytic cells to light increased electricity generation also known as the Photovoltaic Effect [3]. This was the premier to solar cell technology. Subsequently, electrolytic cells were used to power steam engines for the next 3 degrades. Between 1873 and 1876, selenium was discovered by Willoughby Smith to possess photoconductive properties which proved that certain solid material could convert light into electricity without heat or mechanical parts [4]. As at 1880, Samuel Langley described the first selenium wafers solar cells. This was followed by advancements in material compositions to improve the photoelectric effect such as cadmium sulphide by Audobert and Stora in 1932 [5]. A major step in solar technology was the development of Photovoltaic (PV) technology in the United States in 1954. The silicon PV cell developed by Daryl Chapin, Calvin Fuller, and Gerald Pearson was capable of converting enough of the sun's ray energy to power domestic electrical equipment [5]. Thus PV cells were built with an initial efficiency of 4% and later improved to 11% [6]. This went on to see applications in space satellite technology and increased efficiencies up to 14% in 1959. At an average cost of power of $1000/W, most early PV applications were restricted to satellite systems [7]. As at 1964 the Nimbus satellite was launched by the National Aeronautics and Space Administration (NASA) and powered by a solar array of 470 W [8]. In 1974, the first solar powered aircraft, named Sunrise I, was built and flown [9]. It had 4096 cells of 11% efficiency and produced 450 W of power. It weighed about 12 kg and flew for 20 min at an altitude of 100 m before crashing in a sandstorm. The Sunrise II was built in the subsequent year which had 4480 solar cells of 14% efficiency and was capable of producing 600 W. It weighed less having the same wingspan and was more efficient but was

NASA PEMFC PMD PV Re RPM S SOFC UAV T W V

National Aeronautics and Space Administration Proton exchange membrane fuel cell Power Management and Distribution system Photovoltaic Reynold's number Revolutions per minute Wing surface area Solid oxide fuel cell unmanned aerial vehicle Thrust Weight Velocity

destroyed as a result of an uncontrolled high-speed dive [10]. Similarly, in Europe, was the development of the Solaris by Fred Milikty in 1976.This achieved a flight of a little over 2 min and an altitude of 50 m [11]. Further research into developing thin-film solar cells continued through to 1980 with success in the copper sulphide/cadmium sulphide cells of 10% efficiency [6]. In 1980 the Gossamer Penguin developed by Dr. Paul MacCready, became the first manned solar aircraft to demonstrate flight. In 1981 the Solar Challenger, was built also by MacCready. The aircraft flew with over 16,000 solar cells mounted on its wings which produced 2500 W of power with no energy storage devices [12]. Since then, the design space for solar powered aircraft has developed in both manned and unmanned directions. About the same time, Gunter Rochelt designed the Solair I which crossed the Channel in 1983 covered with 2499 solar cells and achieving 1800 W [13]. As at 1989, the Sunseeker designed by Eric Raymond as a glider was test flown. Further efforts subsequently went into the development of other aircraft such as the Icare 2 by Prof. Rudolf Voit-Nitschmann, O Sole Mio by Dr. Antonio Bubbico and Solair II in 1998 by Prof. Gunter Rochelt [14]. Of the unmanned solar-powered aircraft, the most famous are Pathfinder in 1995, Pathfinder Plus in 1998, Centurion, and Helios in 2001 which were High Altitude Solar aircraft (HALSOL) from the NASA Environmental Research Aircraft Sensor Technology (ERAST) program [15]. These aircraft, having a peculiar span loaded wing configuration of wingspan greater than 30 m, had the capability of reaching altitudes of about 96,000 ft utilising solar energy and relying on lithium batteries after sunset. The Helios was the most recent of the aircraft but was destroyed in 2003 due to structural failures during a test flight [16]. In Europe, achievements have been made in the design of high altitude, long endurance (HALE) aircraft. Notably is the Solitair in 1998 at the DLR Institute of Flight Systems, designed for yearlong endurance missions and equipped with adjustable solar panels [17]. Recent strides have seen unmanned aircraft such as the Solong, designed by Alan Cocconi, perform a 48 h endurance flight in 2005 and the British Qinetiq Zephyr in 2007 complete a 58 h flight [18]. All of these have verified the feasibility of continuous flight with solar energy as the main source of power. Most current and on-going technological achievements have been the design of the unmanned Sky‐Sailor by Noth [19], and the manned Solar-Impulse by Bertrand Piccard and Andre Borschberg [20]. In June 2008 the sky-sailor with a wing span of 3.20 m and weighing 2.5 kg achieved a solar powered flight of more than 27 h over 874 km. The Solar Impulse has a wingspan of 63.4 m covered with 12,000 solar cells, weighs 1600 kg, and has 400 kg of lithium batteries. In July 2010 this aircraft had achieved flight duration of 26 h and a maximum altitude of 28,600 ft [1]. As much as all these systems have pushed the current state of the art for both aircraft

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design and energy source technology, there continues to be a challenge to develop a solar-powered aircraft which is inexpensive, capable of perpetual flight across seasons and latitudes, and able to carry significant payloads.

3. Design issues/parameters In aircraft design, the re-evaluation or iteration of the design procedures, however cumbersome, is used to refine values and compare trade-offs. For solar-powered aircraft design, the power derived from the solar cells mounted on the upper wing surface and possibly the horizontal tailplane is directly proportional to the wing and tail areas. This in turn influences other aircraft parameters such as lift, drag, weight, and cruise velocity. All of these affect the overall performance of the aircraft; therefore, an alteration of any variable will initiate the re-evaluation of the other parameters. 3.1. Aircraft Design Analysis The design drive for the majority of the solar-powered aircraft to date has been exploiting the availability of solar energy from the sun for sustained endurance flights. Development of solarpowered aircraft has attracted several research and design centres over the decades. However, the major challenge of creating a design environment for the solar-powered aircraft has been in the integration of operational and systems requirements. In the design environment, the requirements are interlinked to the systems via a set of disciplines. These include aerodynamics, power and propulsion, structure, and control. A methodology for the conceptual design of solar-powered aircraft is presented by Brandt and Gilliam [21]. This provides a simple means for defining, evaluating, and sizing conceptual designs for high altitude long endurance solar-powered aircraft. The methodology was developed from conventional aircraft design and analysis techniques [22], incorporating the special characteristics and constraints for solarpowered aircraft into the design process. However, many assumptions on parameters such as the drag coefficient were made. Thus the results of this methodology may not provide optimal aerodynamic performance. Some design methods [23–25] are based on an assumption of a constant value for the aircraft power requirement, hence, they use power minimisation for cruise speed determination which leads to a speed less than the stall speed. Another analytical methodology is presented by Noth [19] for the conceptual design of solar aircraft. This methodology is based on power and mass balances during straight and level flight. It produces a relationship between the aspect ratio, wingspan and aircraft mass, and provides an opportunity to alter variables to achieve a feasible solution. Hence, the systems model represents a synthesis of disciplinary tools that provide a means to verify an aircraft meets its design requirements. 3.2. Vehicle Configuration A primary issue in the design of solar-powered aircraft is the determination of the overall layout of the aircraft. Selecting the most appropriate configuration of the airframe involves compromising between some essential characteristics such as stability, design simplicity, robustness of the structure, minimising weight, and maximising lift. From previous and current solar aircraft designs, it can be observed that the fuselage size is reduced to an absolute minimum and the wing being made the most prominent structure based on surface area. Another major criterion for configuration selection is in respect of achieving the stability of the aircraft with or without empennage.

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The conventional configuration, consisting of a main wing, tailplane and tail rudder, was often used that was mostly based on its understood simplicity and having an aft mounted empennage for stability. The canard configuration which consists of a main wing, tail rudder and a foreplane adds lift force during normal flight. Complications associated with lateral and longitudinal stability and wake disturbance are the drawbacks of this configuration [26,27]. Another novel configuration is the tail-less or flying wing configuration. This is intended to reduce the structural weight of the entire aircraft but requires a reflexed aerofoil section to provide longitudinal stability and trim. This results in undesired aerodynamic performance, lateral instability and a limited choice of suitable reflexed aerofoil sections [28,29]. Most of the current designs of solar powered aircraft have been aimed at achieving gradual flight manoeuvres to minimise power consumption. Hence, there has been the use of f two other distinct configurations namely the span loaded airframe and the Single/ Twin-boom configuration. The span loaded configuration utilises a low pitching moment airfoil and compensates for the pitching moment by the integration of a tail under the trailing edge of the primary airfoil. The Single/Twin-boom uses a main plane airfoil with a higher pitching moment and significantly low Reynolds number (Re) characteristics. However, this high pitching moment is counteracted by a boom cantilevered tail. 3.3. Aerodynamic Analysis An appropriate appraisal of the aerodynamic characteristics is required to predict the flight performance and power requirements of any aircraft. Firstly, the aerofoil selection from which the maximum lift coefficient value dictates factors such as maximum wing loading, minimum operation speed, and manoeuvre limitations [30]. This aspect of aircraft design is also influenced by environmental constraints such as the density of the air at various altitudes. The air viscosity effect, represented by the Re number, varies drastically depending on the flight condition and the size of the vehicle, whereas the air compressibility effect, represented by Mach number, strictly depends on the flight condition. A credible amount of research has been done on aerodynamic models including numerical lifting line [31], vortex lattice models [32] and applications of computational fluid dynamics (CFD) to solar powered aircraft [33,34]. According to Baldock and MokhtarzadehDehghan [35], the effect of the laminar boundary layer on aerofoil surfaces, combined with laminar separation and reattachment effects, has informed the development of numerical routines specifically designed for low velocity applications such as XFOIL. Findings from analytical codes such as this indicate that for a given coefficient of lift, aircraft fly at much lower chord Re numbers due to the reduction in density with increase in altitude. This results in a low energy content laminar boundary layer extending over most of the aerofoil surface. The effects of the adverse pressure gradient and the low energy content imply that there is a high tendency for boundary layer separation from the aerofoil surface to occur, adversely affecting its performance. Therefore it is important to consider the choice of aerofoil with respect to low Re numbers arising from reduced velocities with increase in altitude [35]. Another important measure of aerofoil performance is the relationship between coefficient of lift (CL) and coefficient of drag (CD). A maximum CL/CD ratio is desirable in terms of obtaining maximum range and mostly reflects the overall efficiency of the wing. However, for the power-critical application of solar-powered aircraft, the factor CL1.5/CD is important [28]. This maximum value gives a minimum power requirement for a given flight mode. This ensures maximum mission range or endurance by allowing the

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Fig. 3. Energy balance diagram [24].

vehicle to fly at a speed optimised to place the minimum demand on the power supplied by the solar cells. In order to achieve flight by solar power, the aircraft design combines a high aerodynamic glide number CL/CD, a large wing area and a low wing loading W/S [36]. 3.4. Propulsion Analysis The power required during different stages of flight and manoeuvres has been illustrated in an energy balance diagram in Fig. 3 as used by Reinhardt et al. [37]. The shaded section A1 represents the energy available during daylight hours and section A2 represents the energy required during hours of darkness. It follows that the energy balance is expressed as A1
EnergyStorageEfficiency ¼ 2A2 . This is a combination of the power system, propulsive system and power management system. Operational challenges depend on the power capacity and consumption and also requirements of an efficient power management system [27]. The power system basically includes the solar panels as the primary power generation source during insolation, and regenerative [38] or non-regenerative cells for storage and power source at night. The amount of power obtained from the solar cells is a factor of the level of irradiance which is dependent on variables such as geographic position, time of day, day of year, and cell orientation. An approximation of the daily energy per square metre can be calculated using values of the maximum irradiance and time of day depending on location and date [12]. The propulsive system is identical for most solar powered aircraft, in that it includes electrical motors with or without gear boxes and propellers for producing thrust. The power management system is for regulating the power to the loads. The selection of the propulsion system is dependent on the design environment. Most solar powered aircraft power system architecture depends on the operational time and location of the vehicle. 3.5. Airframe Analysis The design solar-powered aircraft airframe also presents major structural challenges. The structural behaviour of the vehicle is based on the physical stresses and displacements on the airframe. An appropriate analysis would include a prediction of the aeroelastic characteristics, mass balance and structural integrity [39]. In the design of the large and mostly manned solar-powered aircraft, wing design and propulsion systems have been the two main aspects of the vehicles that are driven by mission requirements, and consequently present the greatest opportunity for system improvements. A characteristic feature of these larger aircraft has been high aspect ratio wings which have lower induced drag and high L/D values, which support flights at lower speeds or at high altitudes of interest [40]. Trade-offs are made, however, because with a high aspect ratio platform comes high wing bending

moments, unfavourable dynamic structural responses, and large deflections. In terms of the propulsion, many current designs feature distributed propulsion, advanced propeller design, and a strong coupling between propulsion and flight controls. Accordingly, when considering new or revolutionary design concepts, these two areas should be of primary focus. As for the not so large manned and down to the small unmanned solar-powered aircraft, the air vehicles entire layout is assessed by deriving the power balance as a function of the aircraft size. By combining the conceptual mass models with the basic aerodynamic model for the various phases of flight, an appraisal is conducted to select most appropriate solutions based on the flight phase requirements [21]. Some components of the aircraft have predominant parameters which influence their structure and invariably the structure of the aircraft. For example, high-efficiency cells will provide a higher power, compared to thin film cells, but they will also constitute additional weight that must be supported. Due to their delicate nature and important thickness, their integration on the wing will be complicated. The selected cells will thus need to appropriately combine these properties.

4. Technologies for solar aircraft This section provides a brief overview of the key current technologies and the factors that affect/guide their applications. Based on available technologies, parameters such as solar cell efficiency and specific weight; energy storage system type, efficiency and density, and propulsion system efficiency are determined. 4.1. Platform Technologies The technical configuration of solar-powered aircraft depends strongly on the operational environmental conditions and mission profile, as mentioned earlier. A primary platform, either manned or unmanned, is selected based on aerodynamic concepts against intended payload mass in relation to power available against power consumed. For this class of aircraft, key aspects that drive the platform's technology are: a. b. c. d. e. f.

Structure and manufacturing aspects Energy Source Energy Storage Propulsion Avionics systems Operations

Solar-powered aircraft are gradually gaining advantage over fuel powered aircraft in terms of technology, environmental and operational compatibility. The majority of the essential technologies have developed to stages of advanced system integration. High-strength fabrics to minimise exterior airframe weight, thinfilm solar arrays, fuel cells and batteries for regenerative power supply, and light weight propulsion units are key technologies ready to make continuous flight a reality [41]. The combination of both photovoltaic systems and advanced energy storage systems ensures the provision of required power for perpetual day and night operation. However, drawbacks within the technologies for the airframe of aerodynamic systems do not seem to allow the design of solarpowered aircraft with payload masses beyond a few hundred kilograms in the near future. This is due to the efficiency of the systems and the mass of the structure required to support them. A comparison of the mass breakdown according to Ross [42] in

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Fig. 4. Relative mass breakdown.

Fig. 6. Best Research Cell Efficiencies [46].

Fig. 5. Sub-system Efficiencies.

Fig. 4, for a range of aircraft from commercial airliners, to typical fighter aircraft against solar-powered aircraft reveals that the sum of the structure and propulsion system comes up to about 40% of the maximum take-off weight for these conventional aircraft, whereas this value is about 85% for solar-powered aircraft. Designing this aircraft to be as light as possible will ensure enough mass allowance to accommodate more payload and equipment, as well as provide operators with the flexibility to change payload equipment to perform multiple tasks. According to the NASA Environmental Research Aircraft and Sensor Technology (ERAST) Program, one design opportunity has been the use of a mylar substrate. The solar cells could also be used as the outer aerofoil covering [43]. The forthcoming trend for this class of aircraft's key technology is an on-going development effort, where the focus is on accounting for all sub-systems from the conceptual design to have the highest possible efficiencies and lowest possible weight, as shown in Fig. 5. 4.2. Energy Source Technologies Direct conversion of sunlight into electricity in Photovoltaic (PV) cells is one of the three main solar active technologies, the two others being concentrating solar power (CSP) and solar thermal collectors for heating and cooling (SHC). PV technology is commonly known as a method for generating electric power by using solar panels composed of a number of solar cells to convert energy from the sun into a flow of electrons. Solar cells containing photovoltaic material produce direct current from sun light, which can be utilised to power equipment or to charge a battery. The first practical application of this technology was to power orbiting satellites and space craft. Basically from the 1970 s, research efforts have been seeking new low-cost processes for production and the use of low-cost materials for PV applications. There are three main classes of solar cells being produced today, thin film, single junction and multiple junction. Their principle of energy generation is theoretically the same but differs in material and manufacture. Materials presently used for PV include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/ sulphide. The National Renewable Energy Laboratory (NREL) presented a plot of compiled values of highest confirmed conversion

efficiencies for research cells show in Fig. 6, dating from 1976 to the present, for a range of photovoltaic technologies. The efficiency of thin-film laboratory cells has increased steadily in the last 20 years [44] and these technologies are predicted to improve with reduced cost in the next 10 years. There are two trends in solar cell development: bringing down manufacturing costs and boosting energy conversion efficiency, both of which are making solar energy much more affordable. A description of the PV cell classes is as follows: 4.2.1. Crystalline wafers Crystalline silicon (Si) wafers, still the most popular material for solar cells, have dominated 75% of the photovoltaic market in the past years [45]. They are reliable due to their simple, large area p–n junction design, thus having relatively high efficiency. However, this relates inversely with temperature at about 0.5% decline per degree Celsius. These cells are also expensive from a manufacturing standpoint. Sunpower, the world's largest producer of Si solar cells, has set records with its monocrystalline technology, recently achieving 24.2% efficiency. However, Sunpower is increasingly investing in its polycrystalline product lines, anticipating meeting rising demands with lower-cost manufacturing solutions. 4.2.2. Thin-film technology Over the past decade, "second generation" thin-film technologies have been developed that do not require costly crystalline silicon wafers and can be manufactured much more cheaply. These include devices based on a range of new inorganic semiconducting materials, as well as multi-junction amorphous (noncrystalline) silicon. 1) Amorphous silicon (a-Si:H) can be made from waste silicon from the computer chips industry. Thin-film cells are fabricated using techniques such as sputtering, physical vapour deposition and plasma-enhanced chemical vapour deposition. Thin-film photovoltaic solutions are gaining ground quickly and are expected to capture up to 30% of solar panel market share by 2014. The growing usage of thin-film cells also results from their better efficiency at higher temperatures when compared to crystalline Si cells. Its efficiency also decreases at lower rates depending on the type of material. 2) Highly absorptive cadmium telluride (CdTe) is another established thin-film material that lends itself to low-cost manufacturing techniques. It is typically bonded by cadmium sulphide (CdS) to create a heterojunction interface thereby increasing efficiency. First Solar, the leading developer and manufacturer of CdTe based solar panels, implemented procedures to recover the highly toxic cadmium in both manufacturing and panel

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recycling which is environmentally advantageous. CdTe are highly resistive electrically, and although their manufacturing cost is low, they may fall short of meeting the challenge of lessresistive copper indium gallium selenide/sulphide (CulnGaS, or CIGS) coatings. 3) CIGS is emerging as a premier thin-film material and might come to dominate the PV solar market. Lab efficiencies have reached nearly 20% and this performance can compete favourable against the monocrystalline Si-based solar cells if achievable in production panels. 4.2.3. Multijunction cells Multijunction cells are another high-efficiency solar energy solution. They capture and convert much broader light frequencies than other designs and can achieve efficiencies greater that 40%. These three-junction cells are typically made from gallium indium phosphide (GaInP), gallium arsenide (GaAs), and germanium (Ge) p–n junctions. The technology is on track to reach as high as 50% efficiency in the next 10 years. A major characteristic is that these multijunction cells do not lose efficiency at high temperatures. In the last decade, photovoltaic technologies have experienced an astonishing evolution that led to the increase of the efficiency of crystal-silicon solar cells up to 25% and of thin-film devices up to 19% as illustrated in Fig. 6. Recently, nano-technology, innovative deposition and growth techniques, and novel materials opened routes for reaching higher performances (multi-junction devices and other 3rd generation photovoltaics) and for developing very low-cost devices such as organic-based PVs. All these technologies face comparable fundamental issues related to the steps involved in the conversion of photon energy into electricity: photon absorption, charge carrier generation, charge separation, and charge transport. Both fundamental research and technical development are critical requirements for these technologies to become more efficient, stable, and reliable. 4.3. Energy Storage Technologies Advances in energy storage devices over the last 30 years have significantly affected the application of electric propulsion to aerial vehicles. This is due to the fact that the energy production is Table 1 Three classes of Energy Storage Category

Applications

Power quality

Discharge duration required

Transient Stability, Frequency Regulation Bridging power Contingency Reserves, Ramping Energy management Load Levelling, Firm Capacity

Seconds to Minutes Minutes to 1 h Hours

neither constant nor continuous. The major constraint associated with electrical propulsion of an aircraft is that the power sources have very low energy densities as such an energy storage device having a high energy density is desirable. Several methods of energy storage have been developed [47] but the selection of which is largely based on the intended application. According to NREL [48] the discharge duration is used as a basis for categorising energy storage application as shown in Table 1. Under the context of solar-powered aircraft, the specific energy (gravimetric energy and volumetric energy densities) and peak power density are the main determinants in the consideration of energy storage selection. Energy translates into achievable mission range within weight and space constraints while power translates into available torque and acceleration. This is as a result of requirements for providing power over long periods of time and at continuous discharge rates over these durations. Consequently, application to solar-powered aircraft falls under the Bridging power and Energy Management categories. Technologies for this application consist of several battery types, fuel cells and capacitors. 4.3.1. Batteries Batteries are classified either as primary or secondary type based on their electrochemistry. A primary battery is discharged completely and cannot be recharged. A secondary battery is rechargeable and provides power during the night when the primary source of power, the solar array, is unavailable. For solar aircraft application, the principal types of secondary batteries include the nickel–cadmium (NiCd), the nickel zinc (NiZn), Lithium ion (Li), nickel-metal-hydride (NiMH), and sodium–sulphur (NaS). Each of these types of batteries has satisfied various applications depending on its performance parameters such as energy density, cycle life and reliability [49]. A battery is rated in terms of its capacity being the total stores charge. The total battery energy, in watt hours, is the product of the capacity and voltage. The energy density (specific energy), in watt hours per kilogram, is important to the application to solar aircraft. Through the early 1990s, the best batteries available were nickel–cadmium or nickel-metal-hydride [50]. This was widely used during the first 30 years in the aerospace industry. NiCd batteries have high cycle life but have a low energy density. NiMH was developed to replace the NiCd cell but did not meet the expected mass, cycle life and size. Major improvement in technology performance was not realised until 1991 when lithium-ion batteries were released. Lithium-ion batteries were then modified to use a composite solid electrolyte. The resulting lithium-polymer batteries were introduced in 1996. Several characteristics of these batteries are included in Table 2. As stated previously, particular interest to aircraft applications is the energy density of the battery. By 2005, engineering process improvements had reached their limit and further improvements in energy density and cost-

Table 2 Characteristics of common batteries. Characteristic

Lead-acid

NiCd

NiMH

Li-ion

Li–Po

Li–S

Zn air

Energy density Energy/volume Power density Recharge time Cycle eff Lifetime Life cycles Nominal voltage Operating temperature Commercial use Cost per W h

33–40 W h/kg 50–100 W h/L 80–300 W/kg 8–16 h 82% – 300 2V Ambient (15–25 °C) 1900/1970 $0.17

40–60 W h/kg 50–150 W h/L 200–500 W/kg 1h 80% – 500 1.2 V  20 to 60 °C 1900/1950 $1.50

30–80 W h/kg 140–300 W h/L 250–1000 W/kg 2–4 h 70% – 500–1000 1.2 V  20 to 60 °C 1990 $0.99

160 W h/kg 270 W h/L 1800 W/kg 2–3 h 99.9% 24–36 mth 1200 3.6 V  40 to 60 °C 1991 $0.47

130–200 W h/kg 300 W h/L 2800 W/kg 2–4 h 99.8% 24–36 mth 4 1000 3.7 V – 1999 –

250–350 W h/kg 600 W h/L 2800 W/kg – 99.8% 24–36 mth 41000 3.7 V – – –

230 W h/kg 270 W h/L 105 W/kg 10 min – – 42000 1.2 V Ambient  25 °C – –

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reductions were introduced from the use of new cathode material. Compared to the early nickel–cadmium based cells of 30–80 W h/ kg, the lithium technologies offers drastic increases, potentially as high as 200 W h/kg [51]. While some battery chemistries are now used in vehicles, the main cell market is still into consumer electronics: phones, cameras, laptops etc. More recently, the advent of lithium–sulphur rechargeable batteries has pushed the envelope of energy density out to 350þ W h/kg [52], some sources citing as high as 400 W h/kg. Unfortunately, the lithium–sulphur batteries currently exhibit a cycle life of around 100 cycles. When implemented into an aircraft design environment, the energy density of the batteries, or other energy storage systems, becomes one of the primary indicators of technology level, and represents a major determinant of aircraft weight. 4.3.2. Fuel Cells Another method of energy storage applicable to solar-powered aircraft is the use of fuel cells; regenerative and non-regenerative. In the late 1950s and early 1960s NASA, in collaboration with some industries began development of fuel cell generators for manned space missions. Fuel cell systems are characterized by the energy conversion components. These include the fuel cell stacks, energy storage component, reactants and tankage as well as ancillary components. The regenerative system is considered to be highly efficient with an energy density potential two to three times greater than that of a secondary battery based system [38]. In 2007, NASA presented a feasibility study focused on “Solar Airplanes and Regenerative Fuel Cells” in which they evaluated the energy storage requirements for year-long continuous flight [53]. In Fig. 7, the regenerative system combines hydrogen with oxygen and the use of an electrolyser stack. Excess energy generated from the solar cells is stored and used to disassociate water molecules. Oxygen and hydrogen gases would then be accumulated in separate tanks under pressure. During the period of inactivity of the solar cells, the process would be reversed; hence the oxygen and hydrogen gases would be fed back through the

Fig. 7. Regenerative fuel concept.

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system to generate electricity and water as a by-product. The water is stored in a tank until sunrise when the process starts all over again. The non-regenerative system is the same but with the absence of an electrolyser. The premise of a fuel cell-based system is that oxygen and hydrogen are combined to produce electric power, heat and water. As long as these gases are supplied, the unit continues to produce power. There are currently several types of fuel cells under development, each having advantages, limitations and favourable applications as shown in Table 3. The two most promising fuel cell types for aircraft applications are the proton exchange membrane fuel cell (PEMFC) and the solid oxide fuel cell (SOFC). For either system the key for future implementation on aircraft is to increase the specific power (kW/ kg). PEMFCs are low-temperature devices offering quick start-up times, but requiring pure gaseous hydrogen fuel. Increasing the PEMFC's operating temperature will both improve tolerance to impurities and may improve the specific power for the system. PEMFC stacks produce a significant amount of heat that is difficult to dissipate or produce additional work, resulting in the need for liquid cooling to the higher potential specific power. The solid oxide system could be used as a stand-alone power source or, because of the high grade heat produced, combined with a turbine in a hybrid system to achieve high efficiencies. In contrast to PEMFC, SOFC operates with significantly more airflow through the stack which provides heat removal, eliminating the need and corresponding weight of a liquid cooling system. Both fuel cell types will require significant investments for incorporation into aircraft. Along with increases to the specific power, both operability and durability improvements will be required for flight applications. Although several different fuel cell systems are in existence today for aerospace applications, the PEMFC consists of an electrolyser technology which can be considered for flight operations. 4.3.3. Super Capacitors A capacitor is an electronic device that stores an electric charge, consisting of one or more pairs of conductors separated by an insulator. According to EPRI [54,55] capacitors have very rapid response time and provide stable transient voltage. The drawback is in their low energy density of less than 15 W h/kg which hinders their use in long timescale applications. They have a high power density of 4000 W/kg but a limited cell voltage of approximately 2.3 V. Capacitors considered for aerial vehicle applications are much larger than those found on common electronic devices, as such are referred to as ultracapacitors or supercapacitors. The super capacitors, (SC) or ultracapacitors considered here are also known as Electric Double Layer Capacitors (EDLC). Supercapacitors have a double layer construction consisting of two carbon electrodes immersed in an organic electrolyte. During charging, ions in the electrolyte move towards electrodes of

Table 3 Comparison of fuel cell technologies. Fuel cell type

Electrolyte

Operating temperature (°C)

Efficiency (%) Energy output

Polymer Electrolyte Membrane (PEMFC) Alkaline (AFC)

Proton exchange membrane

50–10

35–60

o 1–100 kW

Aqueous potassium hydroxide solution Liquid phosphoric acid Solution of lithium, sodium, and/or potassium carbonates Solid zirconium oxide stabilized with yttrium

90–100

60

10–100 kW

150–200 600–700

40 45–50

700–1000

60

400 kW 300 kW– 3 MW 1 kW–2MW

Phosphoric acid (PAFC) Molten carbonate (MCFC) Solid oxide (SOFC)

Application

Portable power, Transportation, Backup power, Distributed generation, Specialty vehicles Military, Space Distributed generation, Electric utility Distributed generation, Electric utility Auxiliary power, Distributed generation, Electric utility

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converter has an efficiency of about 92% and a power to mass ratio of 1 kW per 0.5 kg. 4.5. Propulsion Technology

Fig. 8. Power versus energy [59].

opposite polarity; this is caused by an electric field between the electrodes resulting from the applied voltage [56]. Consequently, two separate charged layers are produced. Even though the capacitors have a similar construction to batteries, their functioning depends on electrostatic action. No chemical action is required; the effect of this is an easily reversible cycle with a lifetime of several hundreds of thousands of cycles [57]. In practise, the cost and complexity of capacitors do not make them an attractive solution for solar-powered aircraft. However, efforts are being made to reduce their cost. Although they have impressive life cycle characteristics, capacitors suffer from accelerated degradation effects from parameters such as electrical aspects, temperature, vibrations, pressure and humidity [58]. A performance comparison of the energy storage technologies described here is illustrated on a Ragone plot in Fig. 8. As a result of the high charge and discharge rate, capacitors have larger power density attributes than fuel cells and batteries. However, fuel cells and batteries have larger energy densities but also have very low power densities due to their slow reaction kinetics. Supercapacitors tend to bridge this gap between capacitors and fuel cell and batteries, as they are considered to have the high energy densities attributable to batteries without conceding their high power densities as capacitors.

The power train of a solar-powered aircraft is configured to perform optimally in the absence of solar radiation, thereby relying on stored power. Having an electric propulsion architecture, maximising the system efficiency for night operation becomes a critical design point. The propulsion system comprising basically of the motor and propeller are required to operate at minimal power to sustain flight during this regime. Under this light load condition, the efficiencies of these components are a pointer of the technology status for that aircraft.

4.4. Power Management and Distribution

4.5.1. Motors/drives An electric motor is an electromechanical device that converts electrical energy into mechanical energy through the interaction of magnetic fields and current-carrying conductor, with the exceptions of piezoelectric and ultrasonic motors. An electric motor is rated in continuous horsepower (HP) while internal combustion engines are rated at peak. This describes the ability to apply a given HP continuously across the revolutions per minute (RPM) band of the motor. The desired result is that electric motors can deliver enormous amounts of torque at very low RPMs. Several types of electric motors exist today which include; AC induction motors, brush direct current motors, brushless direct current motors, and stepper motors. Electronically controlled motors have reached a high level of sophistication, with the power voltage ratings of the electronic switches being the principle present limitation. Significant development is preceding for aircraft applications as the major focus is to reduce weight, improve efficiency and minimise cooling requirements. Lightweight brushless DC motors with permanent magnets has been used in the most recently tested solar aircraft such as the Solar Impulse and Sky‐sailor due to the capability of giving high power for take-off and very high efficiency at low power during level flight.

Efficiently and correctly managing all flight situations, including climbing and descending, atmospheric changes, strong winds, and drops in solar intensity is highly complex. A Power Management and Distribution system (PMD) is required to provide and regulate the power needed to control a solar aircraft at all times. This has been included as a converter in Fig. 5 for simplicity. In order to sustain perpetual flight, adequate distribution of power from the solar cells and power storage system to the motor and on-board systems, an energy management strategy is required. PMD Technology has reached an advanced stage but is dependent on the understanding of power requirements for various applications. This implies that it is not a specific component, rather a combination of electronics, controls, conductors, and other systems. In view of this, it is difficult to estimate weight, volume, and complexity of the system as it applies to specific solar aircraft requirements. NASA was recently constructing an electrical systems test-bed that will integrate the various components in a configuration relevant to aerospace applications [60]. This enables the design and testing of various PMD configurations and approaches, as well as dynamics and control schemes for the entire system. Furthermore, the PMD might need to perform voltage conversions due to the fact that various power consuming components on the aircraft may operate within different voltage ranges. A survey of power management systems for use on high altitude solar powered systems indicated that a state of the art power

4.5.2. Propellers The use of propellers to generate thrust for subsonic flight dates to the beginning of powered flight as such the design and development is well understood to some extent. The purpose of a propeller is to convert the rotating power from the electric motor into thrust or propulsive force. On some solar aircraft, it provides propulsion to and from altitude across the entire mission profile of the aircraft. Early aircraft propellers were carved from solid or laminated wood with later propellers being fabricated from metal, plastic and composite materials. A propeller is characterized by the thrust and power coefficients, which depend primarily on the advance ratio, the blade Re number and, the prop geometry. They may be fixed or variable pitched. High pitched propellers provide greater thrust than lower pitched propellers at constant RPM. Additionally, larger diameter propellers require more power to produce thrust compared to smaller diameter propellers. However, the smaller propellers tend to be noisier. In the application to solar-powered aircraft, it is desirable to provide significant thrust, quietly and efficiently. Propeller propulsion has a high efficiency at low speeds; hence it is used on solar-powered aircraft which are characterised by low velocity flights. Advanced technology propellers of lighter weight and better performance with lower noise are available commercially today. This had been achieved through the use of improved composite materials, fabrication technology, and advanced computer analysis tools, in combination with new propeller concepts.

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Table 4 Technology trend of principal technologies solar aircraft. Commercial off-the-shelf technology

Intermediate technology

Based on commercially available products

Based on current government and industry R&D PV system Rooftop panels, off-grid remote applications, Use of cheaper, flexible, more efficient solar farms materials for solar cells Fuel cell system Automotive-derived PEM fuel cell stack Higher operating temperature PEM fuel cell stack; higher power densities Electric motors Automotive-derived permanent magnet Electric Motor with advanced cooling, electric motors low maintenance and more efficient design Electronics Automotive-derived power management and Higher temperature materials and distribution components, efficient design capability Battery system Automotive and telecommunications power High integrated electronics, improved and energy source and storage systems housing, optimised internal assembly

5. Technological trend In order to obtain a technological trend, three levels of technology were assessed which define the system level weight and efficiency goals necessary for their applications. These include the off-the-shelf technology, research-in-progress technology, and advanced technology as shown in Table 4. The first level is based on currently used and available commercial items. The second and third levels, representing the intermediate and advanced stages, are based on government and industry initiatives focused on specified research under development. The assumed technologies consist of new high power density fuel cells, high energy density batteries, superconducting motors, new materials and hardware, and innovative designs. Interestingly, none of the technologies in Table 4 is being driven primarily by solar-powered aircraft requirements. Although most of the initiatives in these areas are being pursued at government laboratories and institutions, the rapid advancement in energy source, storage and electric propulsion technologies is being achieved through Electric Vehicle (EV) and global renewable energy development programs. These have enabled more costeffective and capable solar-powered aircraft such as the Solar Impulse and may continue to do so in the future, however some challenges are present. 5.1. Technology Challenges The basic set up of a solar energy source system consists of PV cells interconnected to form a PV module which converts solar energy into DC electricity. The PV modules combined with a set of additional application-dependent system components such as batteries, inverters and electrical components form a PV system. The most important technical challenge that solar-powered aircraft is facing today is developing a system which is capable of not only harvesting enough energy for flight during the day, but to store enough energy during the day for sustained flight through the night until a new solar energy harvesting regime starts from sunrise the next day. Alongside this is the cost of some subsystems which hinders their commercialization. Cost is often discussed in terms of per-energy and per-power for different technologies. This narrows down to today's technological status in aerodynamics, solar cells, batteries, fuel cells, electrical propulsion systems and structural materials. Significant efforts have been made in developing each of these technological fields based on their intended applications. A major push has been from the automotive industry in the light of hybrid and electric-powered cars (HEVs) [61]. In terms of aerospace and aviation, research efforts include low Re number aerodynamics

Advanced technology Based on government and university lab tests New photoelectric material and polymer for ultra-high efficiency New type of fuel cell with different chemistry. Higher power densities, more efficient operation Superconducting electric motor with very efficient and light weight design High integrated electronics, new electro-mechanical components and new package design giving potentials for future energy increase on system level Optimised cell design, reduced sealing compound to support high energy and high intrinsic safety

leading to reduced L/D ratios, propulsion efficiency, solar cell structural flexibility, lightweight miniature avionics systems and sensors. The extreme fragility of solar cells, and keeping the interference between the solar cells and the wing aerodynamic profile to a minimum has been of importance. Designing costeffective batteries with high energy densities (W h/kg) and long lifespan has also been of consideration. Cost and durability are the major challenges to fuel cell commercialization; however, size and weight are key requirements in terms of aircraft application. A common issue with fuel cell designs is that the stack was designed for hydrogen–air operation for ground transportation applications. The target lifetime of these stacks is merely 3000 h. Considerations for using pure oxygen rather than air will result in some modifications of the gas flow field to accommodate the pure oxygen. This system has been tested on various applications especially small military UAVs [62]. The durability of these fuel cell systems for solar aircraft application has not been established. For sustained flight operation, fuel cell systems will be required to achieve the same level of durability and reliability of currently used batteries and be capable of functioning at temperatures outside the  35 to þ40 degrees Celsius range. The size and weight of current systems must be further reduced to meet the mass estimate requirements for solar aircraft. 5.2. Technological Endpoints Three specific fields of technology have been selected here as the main focus for solar-powered aircraft technology. This selection is based on their overall impact on the design outcome of the aircraft. These are PV Cell, batteries and fuel cells. Assessment of the technological endpoints is achieved in this paper according to two distinct factors; performance and cost. 5.2.1. PV System Technology According to the US Department of Energy [63], the cost of electricity derived from PV systems has dropped considerably 15 to 20 fold since the 1990s. The statistics also indicate that the system has been highly reliable for the last 20 years. Hundreds of the applications have proven to be cost-effective especially in remote scenarios. However, the fastest growing segment of the market globally is in the domestic applications such as roofmounted arrays on homes, commercial infrastructure and grid connection. There has been a slow but steady improvement in the cell efficiencies for the past 25 years. Conversion efficiency is one of the main performance indicators of PV cells and modules as can be seen over time in Fig. 9. This is the relationship between the produced electrical power and the

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amount of incident solar energy per second. Table 5 provides the current efficiencies of different commercial PV modules. The investment cost of PV systems is still relatively high, however decreasing rapidly as a result of technology improvements of volume and scale. Worldwide public expenditures for PV research and development have substantially increased over the past decade. This efforts span from material production to the manufacturing of modules and system components [65]. While the production costs vary among the different PV module technologies, these module-level cost differentials are less significant at the system level. Therefore, it is suggested that overall cost be assessed by application rather than for specific PV technologies. As shown in Fig. 10, it is assumed that cost-reductions for future PV systems will continue along the timeline curve [66]. This derives an adoption that PV system capital costs have decreased in the past at a learning rate of about 18%. For solar aircraft applications both the cost and durability of the solar cell modules are of concern. Even though the multi-junction and single junction crystalline cells have the highest performance, their applications toward solar-powered aircraft have disadvantages. Thin film solar cells on the other hand are most promising. They have the potential to be used as the aerodynamic surface of the aircraft and conform to its curves. Fig. 11 shows an increasing trend by a factor of 2 in efficiency of solar cells on tested solar aircraft over a span of 30 years. On a global scale, research and development investment on renewable energies jumped by 40% in 2010 [67]. The majority of the initiatives were stimulated by Government R&D. Solar system development has attracted most of the support, claiming approximately 40% of the total investments, thereby reflecting the pace of technological innovation in this field.

requirements as seen from Fig. 12. Current advancement in technology presents batteries with improved energy densities of 400– 600 W h/kg and exceeding 500 cycles at commercially viable recharge rates. Examples of these include the Planar Energy's solid-state lithium-ion battery, ReVolt technology's zinc-air battery and Sion Power's lithium sulphur battery. The Li–S technology provides rechargeable cells with a specific energy of over 350 W h/ kg, which is 50% greater than the currently commercially available rechargeable battery technologies over 600 W h/kg in specific energy and 600 W h/L in energy density are achievable in the near future.

Fig. 10. PV system capital cost-trend 1985–2011.

5.2.2. Battery and Fuel Cell Technology The development trends for batteries in the last century have been driven by auxiliary energy storage requirements and portable devices. More rapid development in recent years have been powered by the automotive and telecommunications industries

Fig. 11. Solar aircraft cell efficiency trend 1975– 2010.

Fig. 9. PV system efficiency.

Fig. 12. Battery energy density benchmark.

Table 5 Current efficiencies of different PV technology commercial modules [64]. Type

Wafer-based Single Crystalline Silicon Multicrystalline/Polycrystalline silicon Thin films Amorphous silicon Cadmium telluride (CdTe) Copper–Indium–Gallium–Selenium (CIGS) Thin film CuInGaSe2 GaAs GaInP2/GaAs Dye-sensitised solar cells

Typical module efficiency [%]

Maximum recorded module efficiency [%]

Maximum recorded laboratory efficiency [%]

12–15 11–14 5–7 8–10 9–11

22.7 15.3 9 10.5 12.1

24.7 19.8 12.7 16 18.2

10 18 24 3–5

40.7 41.1 8.2

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781

The development of this technology is also tied to the significant increase in the demand for consumer electronic devices to which lithium batteries are applicable. Fuel cells have been developed to generate electricity in stationary systems, and several different competing technology concepts have been developed. Current R&D focuses on the development of reliable, low-cost, high-performance fuel cell system components for transportation applications [68]. A high-energy density alternative to existing technologies is required to fill the increasing gap between energy demand and energy supply for aircraft applications. For example, the EO-310-XLE System [69] having an energy density of 450 W h/kg and a total system mass of 3.95 kg. The system also exhibits an output voltage range which eliminates the need for power conditioning between the fuel cell system and propulsion motor (i.e. DC/DC converter). Efforts are presently being made to develop electrodes with higher activity and selectivity, reduction of methanol crossover and decrease in system volume and weight. Researchers are also looking into total life cycle efficiency improvements. The objective is therefore reducing fuel cell weight and increase efficiency.

technology results in better performance in low and diffuse light conditions. By-pass diodes are connected across each cell, allowing the modules to produce power even when partially shaded [71]. Energy and power systems are basically sophisticated with the advent of more efficient microelectromechanical systems (MEMs), fuel cell technology for energy storage and alternative fuel. Proton exchange membrane (PEM) fuel cells now offer power densities equivalent to internal combustion engines. The thermoelectric characteristics of bismuth nanoparticles offer the potential for developing high efficiency, solid-state energy conversion devices that could further reduce their size and weight [72]. Sensors and avionics systems are reliable, smaller, and lighter and perform better. The introduction of smart materials is a combination of sensing, control and actuation functions into one entity thereby allowing adjustments to the surrounding environment and self-repair of damage. This provides opportunities for weight saving by replacing the conventional actuation and control devices previously used.

5.3. Overall trends in Solar Aircraft Efficiency

There are several technologies at laboratory/prototype stage which have the potential to offer superior performance.

The basic concept of a solar aircraft is a combination of systems which ultimately convert solar energy into electrical energy and then mechanical energy in order to do work. This work includes flight propulsion and powering on board avionics, sensors and electrical systems. All in all, the design optimisation for solarpowered aircraft has been to achieve low power loading while supporting high aerodynamic and propulsion efficiencies. Accessing the aircraft efficiency depends on the various system technology parameters such as propulsion, aerodynamic and structural efficiencies. Presently, nanotechnology for aerospace applications provides the opportunity to use advanced composite materials for airframe structural development. This brings about lighter and stronger components to withstand aerodynamic and inertia loading, and sufficiently portray good electrical and thermal conductivity characteristics. An example is the UNI-SOLAR triple-junction amorphous silicon solar cells used on the Zephyr aircraft [70]. This

5.4. Emerging Applicable Technology

5.4.1. Lithium Batteries Most notable are lithium–sulphur batteries and lithium–air batteries which have the highest theoretical energy density as shown in Fig. 13. Based on the observed development rates for battery technologies and the current challenges lithium–air face, practical commercial availability and application of these batteries are not expect before 2030. An energy density between 500– 1000 W h/kg at cell level is anticipated using an improvement factor of 2–3. The basis of this future prediction is from historical data on the ratio of change over time of theoretical and practical energy of other battery chemistries. 5.4.2. Organic Photovoltaics Organic photovoltaics (OPVs) are made from organic materials, which are diverse and versatile, offering endless possibilities for improving a wide range of properties. Organic molecules are

Fig. 13. Emerging battery technologies.

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cheap; they can have very high light absorbing capacity so that films as thin as several hundred nanometres would be sufficient for the purpose. Organic materials are compatible with plastic and other flexible substrates; and devices can therefore be fabricated with low-cost, high throughput printing techniques that consume less energy and require less capital investment than silicon-based devices and other thin-film technologies. The introduction of OPVs is an alternative to present-day p–n junction photovoltaic devices. 5.4.3. Quantum dots and Polymers Quantum dots are offering the possibilities for improving the efficiency of solar cells in at least two respects, by extending the band gap of solar cells for harvesting more of the light in the solar spectrum, and by generating more charges from a single photon. Solar cells based on quantum dots could theoretically convert more than 65% of the sun's energy into electricity, approximately doubling the efficiency of solar cells. A promising material in the manufacture of these cells is the Silicon quantum dot (Si QD) in dielectrics. The restriction of the dimensions of Si to less than the Bohr radius of bulk crystalline Si (less than 5 μm) results in an increment of the effective band gap, thereby increasing the cell efficiency. 5.4.4. Nanostructured Materials Nanomaterials such as nanowires and nanoparticles have unique applications in favour of photovoltaic devices. The nanometre size objects have very large surface areas per unit volume which offers the possibility to form very large interfacial area. The quantum confinement effect which is due to the nanosize presents a capability to design nanomaterials with various band gaps as shown in Fig. 14. As with the quantum dots, this presents an opportunity to tailor the effective band gap to meet efficiency requirements. This can not only be used to manufacture the solar cells but also the airframe of the aircraft. An example of this is the thin film carbon nanotubes and nanofibers as described in Fig. 15. These have been randomly oriented and magnetically aligned, becoming 10 times lighter but potentially 500 times stronger than steel [74]. Additionally, they conduct electricity like copper or silicon and disperse heat like brass and steel [74]. All these advantages offer the possibility to increase the efficiency of solar cells and modules and reduce their cost [75].

6. Conclusion Solar energy has a large potential to be a major fraction of a future carbon-free energy portfolio in aviation. However, technological advances and breakthroughs are necessary to overcome low conversion efficiency and high cost of presently available systems. A key conclusion is that the technology development

Fig. 15. Carbon nanotubes.

trend in solar aircraft systems can be assessed based on the current technology endpoint of individual systems with significant impact on the overall aircraft. These have been identified in this review to be the energy storage system and the energy source system. From a practical and technological point of view, improvements in energy storage technology can be achieved through the development of more robust devices. Having relatively long life cycles and increased specific power is a clear indication of an increase in the realisation of this technological strides. The level of sophistication involved in the design and manufacture of a power source for a specific application over the next few years may well approach the same degree of complexity as the devices they will power. Industry funded R & D is highly secretive and often new technologies are only known about in press releases before mass manufacture, or through patent applications. Estimating a date of technology readiness for various technologies would be very speculative and has not been attempted. Instead, only the technology with the greater theoretical performance has been used to model a future prediction. The efficiency of thin film photovoltaic cells which are desirable in solar aircraft applications are predicted to reach a commercial rating of 50% by the year 2030. Advanced development of nanomaterial technology is also predicted to be aviation certified in the next 20 years. Battery and fuel cell technology in terms of power and energy densities, safety, cost and lifetime is predicted to double by the year 2030. The advent of novel theoretical approaches, powerful computers, miniaturisation of transistors and electronic circuitry, innovations in high strength, light weight structural materials, and the persistent, pervasive support for a clean environment, all point towards a technically advanced solar-powered future. The ambitious goals of this class of aircraft are the catalyst for further improvements of advanced technologies. Summation of the contributions from each of these technological fields is likely to push aerospace further along this path.

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Fig. 14. Solar cells with polychiral carbon nanotubes [73].

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