Selecting Solar for High Altitude Long Endurance Aircraft

WHITEPAPER Selecting Solar for High Altitude Long Endurance Aircraft Contents Introduction... 1 HALE vs Satellite... 1 Unique Performance Variables... 1 Angle of Incidence... 3 Applicable Performance Metrics... 4 Technology Landscape... 5 Comparison of Technologies... 8 HALE Advantages Cost Increased bandwidth Continuous persistent overhead coverage Maintenance and upgrade capability Disadvantages Unproven High power requirements Large footprint Introduction In the 1990 s, NASA s development of the Helios and Pathfinder family of high altitude aircraft demonstrated the potential for solar cells to transform the wing from a passive mechanical component into a primary power source with minimal impact on aerodynamics. Recent improvements in electric propulsion technology and wireless connectivity are now driving strong commercial interest in solar power for aircraft capable of continuous stratospheric operation. This paper will provide an overview of the different solar technologies available and the unique requirements during high altitude operation. HALE vs Satellite Many question why designers choose to build HALE aircraft instead of using more traditional means of continuous communication and surveillance provided by satellite constellations. Both have their advantages and disadvantages, but the answer ultimately comes down to cost. HALE aircraft can potentially provide a significantly cheaper means of communication especially in areas where network infrastructure is lacking or non-existent. However, the HALE aircraft market is unproven. There are both technical and operational hurdles, but if they can be overcome, HALE aircraft could prove valuable. Unique Performance Variables Solar performance at high altitudes will be dictated primarily by temperature and weather, spectral irradiance, and angle of incidence. Temperature and Weather MIL-HDBK-310 and the 2009 Schlatter Atmospheric Composition and Vertical Structure paper are excellent references for temperature, wind, and air density conditions at high altitudes. The temperature and winds are fairly consistent above 60 kft (20 km). Diurnal temperature changes are flat with only slight variations occurring seasonally. Polar regions generally exhibit more stable conditions than equatorial airspace. page 1

The low air density not only results in poor lift, but also poor thermal conductivity. Thermal energy transferred by solar radiation could result in temperatures on the trailing edge of the wing that are 20-40 deg above ambient. Altitude Lowest Recorded Temperature Highest Recorded Temperature km ft C F C F 14 45.9-77 -107-30 -22 16 52.5-87 -125-35 -31 18 59.1-88 -126-35 -31 20 65.6-87 -125-31 -24 22 72.2-85 -121-29 -20 24 78.7-86 -123-33 -27 Data from MIL-HDBK-310 Temperature Coefficient All solar cells increase power output as temperature decreases. However, the degree to which the power changes is a direct result of the temperature coefficient. It is important to consider that coefficients for materials may vary over wide temperature ranges. Spectral Irradiance The solar irradiance spectrum encompasses light wavelengths from UV into the near IR (300 1300 nm). Figure 1 shows an example of the spectrum. Most HALE aircraft plan to operate at altitudes between 50 80 kft (15 24 km). This region of airspace is located within the ozone and above any clouds. Aircraft will benefit from unobstructed solar irradiation as well as reduced levels of harmful UV and other atmospheric radiation. The spectrum will vary in intensity depending on the time of year, day, and angle of incidence of the light to the solar material. Figure 1. Spectral irradiance will vary based on altitude, time of day, and angle of incidence Modeling Solar Spectrum Irradiance The Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) predicts clear sky spectral irradiances. This model is used to predict solar conditions and was used to generate figure 1. It is used by the National Renewable Energy page 2

Lab (NREL) and other research organizations and is useful for predicting solar power output at high altitudes. For more information on SMARTS go to: www.nrel.gov/rredc/ smarts/ Spectrum and Quantum Efficiency Solar efficiency is measured as the difference between the integral of the solar spectrum and the quantum efficiency (QE) of a specific solar cell. All solar cells have properties that allow them to absorb certain portions of the light spectrum which vary based on the materials used to construct the solar cell. A QE curve is a solar cell specific graphical representation of the spectral response. The differences can be further illustrated using a spectral efficiency chart. Ultimately, spectral efficiency is needed to determine the variance in solar cell performance with changes in time of year and day. Figure 2. QE and spectral response curves determine power output as spectrum changes Angle of Incidence The angle at which light is captured by the solar cell, or angle of incidence, not only occurs as an effect based on the time of year and day, but also as a result of aircraft bank angle. Ideally, the resulting power loss would follow the law of cosines. However, solar material can vary widely where it begins to deviate from the cosine curve based on the manufacturer. Figure 3. Ideally solar cells would closely follow the law of cosines page 3

Applicable Performance Metrics The photovoltaic industry is dominated by large players manufacturing product for large scale utility, commercial and residential power generation markets. The dominant performance metric is efficiency. However, there are other metrics and solar technologies that are more relevant to the unique environment in which HALE aircraft will operate. Power-to-Weight To stay aloft at high altitudes for long periods of time weight must be minimized. Since the solar panels are usually integrated into cantilevered wings, the weight of the solar can lead to a further increase in the weight of the aircraft due to increased structural requirements. Therefore, the power to weight ratio of the solar technology being evaluated should be a primary consideration. The total weight of the solar not only includes weight of the cells themselves, but also the protective packaging needed and the addition of any structural support for the cells to survive the expected operating environment. Power-to-Area Minimizing the area needed for solar is another important consideration and can be measured using the power-to-area ratio. Power-to-area should be computed at the array level and not the just the cell level. At the array level, packing factor will determine the wing area required to achieve a certain power level. Packing factor can be calculated using a variation of the Atomic Packing Factor Packing Factor Packing factor significantly contributes to array level power-to-area. The ability and willingness of the solar cell manufacturer to provide customized sizes and shapes of solar cells and cell assemblies to ensure maximum utilization of available area are important here. Mass and Area Ratios 300 Alta Devices Dual Junction Watts per square meter 200 100 Traditional GaAs Advanced c-si c-si CIGS Alta Devices Single Junction Figure 4. Mass and area ratios are great metrics for HALE aircraft 0 Data is for fully encapsulated modules or submodules and includes bypass diodes and interconnect ribbons. 0 500 1000 1500 Watts per kilogram page 4

Ease of Installation Ideally the chosen solar technology must not impose additional constraints on the aircraft design or the manufacturing process. Solar cell technologies can vary greatly in this regard. Thickness, mechanical flexibility and ability to withstand common manufacturing processes, such as vacuum forming, are major factors to be considered. Thinner solar cells are typically easier to integrate because they are more flexible and more resistant to breakage. Cells can be encapsulated with a variety of plastic films and even spray-on clear-coats. Other factors include the ability to provide a smooth surface for optimal aerodynamics while maintaining high power-to-weight and power-to-area ratios. Flexible solar cells allow easier integration Durability and Reliability HALE aircraft operate in harsh environments. Extreme temperatures, vibration and flexing are common. Impact and abrasion are possible during takeoff, landing and altitude transition. Finally, the solar must be able to last months without maintenance. Many HALE designs seek to employ higher bus voltages in order to reduce the weight of wiring and use more efficient propulsion. Higher voltages can result in specific failure modes in the solar panels which may not appear during low voltage bench testing. Finally, if the solar sub-modules are to be adhered to the aircraft surfaces, it is extremely important that the adhesion system be thoroughly qualified. The encapsulation system used to protect the cells must be similarly qualified. It is important to select a technology vendor that is willing to invest in needed testing and to provide relevant data on an ongoing basis as the UAV design evolves. Price It is important to focus on the total value of the purchase. Many manufacturers only supply cells which must then be interconnected and protected by the HALE manufacturer or a third party. Additionally, many will not and cannot provide integration expertise or testing data/capabilities which will again add costs elsewhere. Cost is a good metric, but lifecycle cost is even more important.. Technology Landscape There are over twenty photovoltaic technologies being actively pursued by various manufacturers and research groups. Broadly, they can be divided into wafer-based and thin-film types. Here we discuss only those technologies which can readily be produced in a form suitable for aircraft integration. Relevant wafer-based technologies are crystalline silicon (c-si) and gallium arsenide (GaAs) that are thinned post-process. Relevant thin-film technologies are amorphous silicon (a-si), copper-indium-galliumselenide (CIGS) and thin gallium-arsenide (thin-gaas) based photovoltaics. page 5

Wafer Based Technologies Crystalline Silicon Solar Cells (c-si) The vast majority of existing solar cells and manufacturing capacity is of crystalline silicon, typically made from 6-inch size wafers. The typical efficiency of this technology is approximately 17% (up to 20% for some newer technologies) as measured under standard test conditions. These wafer-based cells are generally about 0.2 mm thick, are brittle and must be encapsulated in relatively thick laminates for adequate environmental protection which can increase weight by a factor of 3 or more. Production of these cells is dominated by large manufacturers who may be reluctant to support the relatively low volumes required for aircraft development. The chief advantages of crystalline silicon are its technology maturity and good efficiencies under standard conditions. Wafer based solar cells are highly susceptible to cracking unless they are properly protected. Gallium Arsenide Solar Cells (GaAs) First developed in 1970, gallium arsenide solar cells offer higher efficiencies (22-30%) than crystalline silicon. Like c-si, wafer based GaAs solar cells are brittle. Moreover, GaAs wafers are about 100 times more expensive than c-si. As a result, GaAs cells have been used almost exclusively in space applications and most suppliers have moved to multi-junction production. Multi-junction allows higher efficiencies (26-35%), but at even higher costs than single-junctions. Production is controlled by very few suppliers with low annual volumes. Multi-junction GaAs solar cells can provide the highest efficiencies available, but they have very high costs. Lightweight Thin Film Solar Cells Thin-film solar cells utilize much thinner layers of semiconductor material, a hundred times thinner than the wafers discussed above. Potentially, these cells can be made into very thin and lightweight panels. We discuss these next. Copper Indium Gallium Selenide (CIGS) A popular thin-film technology is copper-indium-gallium arsenide (CIGS). Flexible CIGS cells can be fabricated on thin metal foils and offer large area efficiencies of up to 13% (under standard conditions). They are extremely sensitive to moisture and page 6

air, however, and must be encapsulated in a hermetically sealed package in order to provide a satisfactory lifetime. This package increases product weight and reduces options for integration. They also have relatively low energy yield in high temperature and low light conditions. However, integration can be problematic due to moisture sensitivity. Some technologies are flexible, but require heavy insulating material. Amorphous Silicon (a-si) Lightweight amorphous silicon (a-si) solar cells are technologically mature, inexpensive, relatively insensitive to moisture and air, easy to integrate and can provide good energy yields in real world conditions. However, their conversion efficiency is relatively low (about 8%) and it can be difficult to find sufficient surface area on an aircraft to mount enough of these cells to meet power needs. Thin Gallium Arsenide (Thin GaAs) Thin GaAs is a relatively new technology that offers the high conversion efficiencies (25%-35%) and high energy yield of traditional GaAs solar cells, but in a lightweight and flexible format. This technology can offer power to weight ratios exceeding 1 kw/kg as well as the high conversion efficiencies needed in situations where available real estate is limited. Not as sensitive to air or moisture, these cells are also easier to integrate than CIGS cells. They are manufactured by specialty companies that are able to be more responsive to customer needs in terms of customized geometries or electrical configurations. Thin GaAs cells are available in smaller sizes than traditional GaAs solar, which can result in superior packing density especially in smaller aircraft. Example of poor packing factor page 7

Comparison of Technologies Technology Power-to-Weight Power-to-Area Packing Factor Durability/ Reliability Thin Film Ease of Integration Alta Devices (Thin GaAs) High High High High High Medium Thin GaAs (multi-junction) High High Low High Low High CIGS Medium Low Medium Low Medium Low a-si Low Low High Medium High Low Wafer Based c-si Low Medium Medium Medium Low Low GaAs (multi-junction) Medium High Low Medium Low High Conclusion Thin GaAs solar is well suited to meet the unique environmental conditions and mission requirements of HALE aircraft. Particularly, Alta Devices manufactures a highly configurable single junction thin GaAs solar cell that has the potential to also provide high volume and low price within 5 years. Ultimately, solar cell price and performance will be key to the HALE industry s ability to grow. There are many technical hurdles solar and HALE aircraft manufactures must solve in the coming years, but if they can be overcome society will see an expansion of the information age to even the most remote locations. Price ABOUT ALTA DEVICES Alta Devices (www.altadevices.com) holds the world record for single-junction solar cell efficiency (28.8%). It manufactures ultralightweight gallium-arsenide based solar cells in its Sunnyvale, California factory and provides application engineering support to OEM customers worldwide. Alta Devices can be contacted at info@altadevices.com. 545 Oakmead Parkway, Sunnyvale, CA 94085 www.altadevices.com 2016 Alta Devices. All rights reserved. AnyLight is a trademark of Alta Devices. WP SOLAR-FOR-HALE 201610-001-EN page 8