Featured Subject Matter Expert Research Corner
Credited with predicting the large-scale negative contribution of fossil fuel combustion to atmospheric carbon concentration in 1896, Svante Arrhenius and his collaborators shared a long realized need for alternatives to fossil fuels for global energy production . The significance of Arrhenius’ statement was not fundamentally understood by scientists until half a century later in the 1950s, when the increasing concern for the environmental impact of burning fossil fuels finally spread throughout both the scientific community and general public. This generated the first proposal to harness the most abundant energy source in our solar system, sunlight, locally in space by Peter Glaser in 1968 .
The concept of space solar outlined by Glaser involved two components that aligned with the then-existing two-fold structure of earth-space systems: 1) a space segment involving a large spacecraft utilizing photovoltaics for solar energy collection coupled with a transmission device, and 2) a ground segment to receive and convert incoming energy into a more readily storable and accessible form compatible with current energy systems. A depiction of one possible implementation appears in Figure 1 . Since its introduction by Glaser, many significant efforts to create and develop the technology that will allow for space solar energy harvesting and transmission have been made.
The functional components for solar power satellites have been separated into two primary forms for power beaming: optical and microwave; and two primary forms for collection: photovoltaics and solar thermal. Each transmission and collection scheme has its advantages and disadvantages. Radio frequency (RF) microwave power beaming involves the transmission of electricity through conversion of sunlight to a longer electromagnetic wavelength, usually within the microwave range. This longer wavelength allows for transmission that is less susceptible to atmospheric attenuation, but requires a larger transmitting and receiving apertures . Laser power beaming involves the transmission of electricity through conversion to monochromatic electromagnetic waves within the near-visible wavelength range. These shorter wavelengths of laser transmission allow for tighter beams over long distances that can be steered to specific receivers , and utilization of smaller transmission and receiver apertures relative to microwave transmission. Although this leads to smaller apertures, it also is subject to greater weather and atmospheric attenuation compared to microwave power beaming. Solar collection involving photovoltaics (PVs) consists of direct conversion of sunlight into electricity via the photovoltaic effect, in which electric current is generated within a solar cell when it is exposed to light. This method of collection has long been used for space applications as a result of its relative reliability and simplicity in implementation . Space solar thermal collection in principle would utilize concentrated sunlight to generate heat to allow for driving an electric generator via a heat transfer fluid. This method has a theoretical ability to operate at a high efficiency .
Many solar power satellite designs encompassing these collection and transmission schemes (predominantly photovoltaics for energy collection and microwave for transmission) have been proposed. These approaches to space solar would consist of a large set of system and design considerations. Thus, allowance must be made for individual limitations and strengths of these designs. As a result, there are several performance metrics to consider for each architecture: collection/transmission area-specific mass, mass-specific transmitted power, combined energy efficiency, temperature performance range and survivability, and serviceability of the proposed design. Research and development to improve these metrics is imperative, and will have benefits for essentially all space systems.
Collection/transmission area-specific mass is a metric of interest as most solar power satellite designs must accommodate large surfaces for solar collection and transmission. In the instance of microwave transmission, large transmission antenna apertures must be considered as the transmitter portion alone has ranged from 4 kg/m2 to 40 kg/m2 in many proposed solar power satellites . Mass specific power represents the mass required to output a given power level. This is most significant for economic modeling of space solar satellite designs and has been measured to be 4.5 W/kg in an environmentally tested element under simulated illumination of one sun (approximately 1368 W/m2) and 5.8 W/kg under simulated illumination of two suns (which simulates operation under solar concentration). The combined energy efficiency measures the ratio between the absorbed solar light and output power. A higher efficiency reduces the amount of heat generated during sunlight conversion. The highest reported sunlight-to-microwave conversion efficiency for a sandwich module in vacuum is 8 percent, recorded at the Naval Research Laboratory (NRL) in 2012 . Temperature range and space environment survivability are being explored through a recently launched space experiment  and work around the world points to paths to significant increases in efficiency  .
Two primary space solar power satellite architectures are perpendicular to orbital plane architectures and sandwich module architectures.
Perpendicular to Orbital Plane
The perpendicular to orbital plane architecture is a geosynchronous (GEO) satellite with separate collection and transmission surfaces. The solar collection surface rotates on an axis perpendicular to the sun and collects energy, which is redirected to a transmission antenna pointed at earth. This antenna would be capable of transmitting large amounts of energy to rectifying antennas at receiving stations on Earth . These surfaces are pointed independently of one another and are connected via a slip ring mechanism that allows for the transfer of current between these rotating structures.
Sandwich module architectures employ a modular approach to space solar. The sandwich module separates functions into three layers: solar energy collection, microwave conversion, and transmission of the microwave energy. These individual sandwich modules form part of a large phased array antenna. Many prototypes have been developed   , and recent work is illuminating performance in the space environment . There are plans to include additional sub-functions involved in the generation of a microwave signal and the transmission of the energy (DC power conversion, RF amplification, phase shifting, and output filtering). Figure 2 depicts the various layers of a sandwich module.
Researchers have also produced novel prototypes that enhance heat dissipation to increase the efficiency of solar energy to microwave conversion. These include alternatives to the “tile” design of traditional sandwich modules, like the “step” module design, in which additional area is provided for the dissipation of heat  . The recent launch of the photovoltaic radio-frequency antenna module flight experiment (PRAM FX) by the NRL is the first effort to characterize the solar to microwave conversion process in space . PRAM FX uses photovoltaics to collect solar energy and solid state electronics to create a 2.45 GHz microwave transmission. This flight experiment will provide vital ongoing thermal performance data that will inform future space solar satellite designs. Predecessor prototypes to PRAM FX developed by NRL can be seen in Figure 3. The traditional “tile” approach is on the left and the “step” approach is on the right. A 12-inch ruler is provided for scale.
Two example sandwich module architectures are the Solar Power Satellite by Arbitrarily Large Phased Array (SPS-ALPHA) and Modular Symmetrical Concentrator (MSC). These structures utilize similar modular elements for both optical and microwave transmittance apertures, removing the need for a large conducting rotating joint of historical designs . Another advantage of the modular approach is an economic one, with mass production of modular components likely reducing costs. Utilizing increased solar concentration could further diminish required system launch mass and cost.
Although significant advances have been made in the underlying technologies surrounding solar energy collection, power beaming, and developing architectures for implementation, the field of space solar is largely constrained by economic interests. These are dependent on the cost of implementation of technology, cost of access to space, and efficiency of solar energy captured in space. However, many improvements that address each respective area are being made. NASA and DoD are actively investing in and taking advantage of recent developments driving down launch costs and expanding access to space  . The emergence of companies such as SpaceX and Blue Origin, which aim to make space more accessible, has contributed to great progress towards reducing cost through reusability of rockets. SpaceX President Gwynne Shotwell reports a reduction in cost to “substantially less than half” from the ability to reuse boosters . Similarly, Blue Origin’s third New Shepard vehicle had logged six suborbital flights at the end of 2019 , illuminating the future of commercial reusable rocketry. Both achievements positively aid in reducing costs of access to space and implementation of space systems for tourism, industrialization, and extraterrestrial resource utilization.
Technology cost reductions have also been realized with the forays of many Silicon Valley space startups and their pioneering ideas regarding the “new space” age. Startups have generated spacecraft with capabilities and in quantities of previously unprecedented scale. By employing mass production techniques, Planet Labs, a notable emerging player in the new space industry, was able to establish a fleet of launched satellites exceeding 200 in February 2017 . Other emerging organizations have followed suit with the employment of mass production, driving the cost per unit mass of spaceflight hardware to low levels, within the range of $5000 per kilogram  . Combined with reductions in cost offered by new architectural approaches, these accomplishments drop the prospective price of electricity for solar power satellites further. The efficiency of solar energy capture has been improved by notable advances in solid-state electronics, development of lightweight materials, and clever power conversion strategies . These have enabled current technologies to achieve record-setting specific power. Research by the NRL, the California Institute of Technology, and Northrop Grumman for sunlight-to- microwave conversion modules have paved the path to increased amounts of power delivery to the ground per unit mass. Novel developments in optical power beaming technology utilizing fiber laser techniques and safety systems have also revived lasers’ potential viability as a method of power transmission for space solar  .
The world faces an abundance of immediate and long-term perils in the form of increasing population, energy demand, and the ever-growing environmental effects of climate change attributed to current energy sources. The culmination of the vision and efforts to create a solar power satellite capability promises a potential solution for a globally transmissible, clean, constant, and unlimited energy source.
 M. Maslin, Climate Change: A Very Short Introduction. Oxford, New York: Oxford University Press, 2014.
 P. E. Glaser, “Power from the Sun: Its Future,” Science, vol. 162, no. 3856, pp. 857–861, Nov. 1968.
 P. Jaffe et al., “Opportunities and Challenges for Space Solar for Remote Installations,” U.S. Naval Research Laboratory, Washington, D.C., Memo Report NRL/MR/8243–19-9813, Oct. 2019. [Online]. Available: https://www.researchgate.net/publication/337782857_Opportunities_and_Challenges_for_Space_Solar_for_Remote_Installations. [accessed 21 April 2020].
 A. W. Bett, F. Dimroth, R. Lockenhoff, E. Oliva, and J. Schubert, “III–V solar cells under monochromatic illumination,” in 2008 33rd IEEE Photovoltaic Specialists Conference, San Diego, California, May 2008, pp. 1–5, doi: 10.1109/PVSC.2008.4922910.
 M. A. Hamdy, M. E. Beshir, and S. E. Elmasry, “Reliability analysis of photovoltaic systems,” Appl. Energy, vol. 33, no. 4, pp. 253–263, 1989.
 W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys., vol. 32, pp. 510–519, 1961.
 J. O. McSpadden and J. C. Mankins, “Space solar power programs and microwave wireless power transmission technology,” IEEE Microw. Mag., vol. 3, no. 4, pp. 46–57, Dec. 2002, doi: 10.1109/MMW.2002.1145675.
 P. Jaffe, “A Sunlight-to-Microwave Power Transmission Module Prototype for Space Solar Power,” Doctoral thesis, University of Maryland, College Park, MD, USA, 2013.
 “NRL conducts first test of solar power satellite hardware in orbit,” May 18, 2020. https://www.nrl.navy.mil/news/releases/nrl-conducts-first-test-solar-power-satellite-hardware-orbit. [accessed 26 June 2020].
 K. Needham, “Plans for first Chinese solar power station in space revealed,” The Sydney Morning Herald, Feb. 15, 2019. https://www.smh.com.au/world/asia/plans-for-first-chinese-solar-power-station-in-space-revealed-20190214-p50xtg.html. [accessed 1 June 2020].
 E. Gdoutos et al., “A lightweight tile structure integrating photovoltaic conversion and RF power transfer for space solar power applications,” presented at the 2018 AIAA Spacecraft Structures Conference, Kissimmee, Florida, Jan. 2018, doi: 10.2514/6.2018-2202.
 “Satellite Power Systems (SPS) Concept Development and Evaluation Program Preliminary Assessment,” Technical Memorandum NASA-TM-81142 19800021341, Sep. 1979. Accessed: Jul. 10, 2020. [Online]. Available: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800021341.pdf.
 H. Matsumoto, “Research on solar power satellites and microwave power transmission in Japan,” IEEE Microw. Mag., vol. 3, no. 4, pp. 36–45, Dec. 2002, doi: 10.1109/MMW.2002.1145674.
 M. Mori, H. Matsumoto, N. Shinohara, and K. Hashimoto, “Solar Power Radio Integrated Transmitter (SPRITZ) Unit for SPS.”
 N. Shinohara, “Beam Control Technologies with a High-Efficiency Phased Array for Microwave Power Transmission in Japan,” Proc. IEEE, vol. 101, no. 6, pp. 1448–1463, Jun. 2013, doi: 10.1109/JPROC.2013.2253062.
 O. E. Maynard, “Solid State SPS Microwave Generation and Transmission Study,” NASA CR-3338, vol. 1, p. 230, 1980.
 K. Wiens, “Solar Power When It’s Raining: NRL Builds Space Satellite Module to Try,” News, Mar. 12, 2014. https://www.nrl.navy.mil/news/releases/solar-power-when-its-raining-nrl-builds-space-satellite-module-try. [accessed 10 July 2020].
 J. C. Mankins, “SPS-ALPHA: The first practical solar power satellite via arbitrarily large phased array,” 2012. [Online]. Available: https://www.nasa.gov/sites/default/files/atoms/files/niac_2011_phasei_mankins_spsalpha_tagged.pdf
 M. Wall, “NASA picks SpaceX, Dynetics and Blue Origin-led team to develop Artemis moon landers,” Space.com. https://www.space.com/nasa-artemis-moon-landers-spacex-blue-origin-dynetics-selection.html. [accessed 10 July 2020].
 L. Grush, “The Defense Department picks three companies to develop rockets for national security launches,” The Verge, Oct. 10, 2018. https://www.theverge.com/2018/10/10/17961832/defense-department-launch-service-agreement-ula-blue-origin-northrop-grumman. [accessed 10 July 2020].
 “SpaceX gaining substantial cost savings from reused Falcon 9,” SpaceNews, Apr. 05, 2017. https://spacenews.com/spacex-gaining-substantial-cost-savings-from-reused-falcon-9/. [accessed Jul. 10, 2020].
 J. Foust, “New Shepard sets reusability mark on latest suborbital spaceflight,” SpaceNews, Dec. 11, 2019.
 K. J. Ryan, “This Company Has the Largest Fleet of Orbiting Satellites in Human History. Here’s What It Plans to Do Next,” Inc, Dec. 08, 2017.
 P. B. de Selding, “Competition to Build OneWeb Constellation Draws 2 U.S., 3 European Companies,” SpaceNews, Mar. 19, 2015.
 C. Henry, “OneWeb scales back baseline constellation by 300 satellites,” SpaceNews, Dec. 13, 2018.
 P. Sprangle, B. Hafizi, A. Ting, and R. Fischer, “High-power lasers for directed-energy applications,” Appl. Opt., vol. 54, no. 31, p. F201, Nov. 2015, doi: 10.1364/AO.54.00F201.
 T. J. Nugent, Jr., D. Bashford, T. Bashford, T. J. Sayles, and A. Hay, “Long-Range, Integrated, Safe Laser Power Beaming Demonstration,” in Technical Digest OWPT 2020, Yokohama, Japan, Apr. 2020, pp. 12–13.
All Contributions from Paul Jaffe, Ph.D.
This State of the Art Report (SOAR) reviews the current state of a selection of novel, non-traditional, and/or emerging sources and technologies for harvesting, generating, and reusing energy. It offers synopses of new programs; summaries of significant technological breakthroughs and technology applications; highlights of outstanding developments; and impacts to the DoD.
Podcasts / Webinars
Wireless power beaming is the transmission of electrical energy without a physical link. In a wireless power transmission system, a transmitter device, driven by electric power from a power source, transmits power across space to a receiver device, which extracts power from the field and supplies it to an electrical load. The technology of wireless power transmission can eliminate the use of the wires and batteries, thus increasing the mobility, convenience, and safety of an electronic device for users. In this presentation, Dr. Jaffe will present the visions for power beaming and space solar, and delve into their technical, regulatory, and economic challenges and opportunities.