Energy from Space

Forecasts indicate that worldwide demand for new base-load electrical power generation capacity will continue to grow throughout the twenty-first century. However, evidence is mounting to show that the use of fossil fuels—coal, oil, and natural gas—and the resulting increase in greenhouse gas emissions may be leading to measurable global climate change. New energy technologies are needed to offset the future growth in fossil fuel use.

One concept that would provide power on a significant scale for global markets is energy from space. Earth is about 150 million kilometers (93 million miles) from the Sun. Sunlight constantly delivers 1,358 watts of energy per square meter of area to the part of Earth facing the Sun. However, by the time the sunlight reaches the surface of Earth, atmospheric filtering has removed about 30 percent of the initial energy even on a clear day. Moreover, the effects of the seasons and the day-night cycle further reduce the average energy received by an additional 90 percent. Often, the remaining 100 to 200 watts per square meter can be blocked completely by weather for days at a time.

A solar power satellite, by contrast, could collect sunlight in space and convert the energy into electrical current to drive a wireless power transmission system, which would in turn beam the power down to receiving antennas on Earth. Earth-bound receiving antennas could capture the transmitted energy—almost twenty-four hours a day, seven days a week—and deliver it to local electrical grids as base-load power. This approach could eliminate the need for extremely large solar arrays on the ground and dramatically expensive energy storage systems, but would require a number of new technological advances.

Details of the Concept of Energy from Space

Power Generation in Space.

Typically, photovoltaic arrays are used to generate power in space. These are solid-state devices that exploit the characteristic of semiconductors such as silicon to allow incoming photons to readily dislodge electrons, a process that creates voltage. Key measures of the effectiveness of these technologies are the specific power (e.g., watts produced per kilogram of solar array mass) and the efficiency (e.g., watts produced per square meter of solar array area). By the early twenty-first century, space solar power technology research programs were using concentrating lenses and multi-bandgap photovoltaic cells to achieve specific power levels approaching 200 watts per kilogram and efficiencies of almost 30 percent. These figures represent advances of more than a factor of three over the state-of-the-art technology of the late 1990s.

Wireless Power Transmission.

Power can be transmitted using either radio frequencies or visible light. In the case of radio frequencies, a wide range of devices could be used to generate the power beam, including highpower klystron tubes , low-power solid-state devices, and magnetrons, which are a type of vacuum tube providing in-between levels of power (and are also used in household microwave ovens). In all of these cases, a number of the devices would be arranged and operated in a lockstep phased array to create a coherent, collimated (parallel) beam of energy that would be transmitted from space to the ground. The efficiency of the transmitter can be as high as 80 percent or more. On the ground, a radio frequency power beam would be converted back to voltage by a rectifying antenna (also known as a "rectenna") operating at about 80 percent efficiency. Taking into account losses of the collimated beam at the edges of the rectenna and small levels of interference from the atmosphere, a power beam might generate about 100 watts per square meter on the ground on average.

The Challenge of Large Systems in Space.

A central challenge of space solar power is that of launching and building these exceptionally large systems in space. As of the early twenty-first century, the cost of space transportation ranged from $5,000 to $22,000 per kilogram of payload , launched to Earth orbit. In order to be economically viable, space solar power systems must be launched at costs of no more than $400 per kilogram. Such a dramatic improvement requires the development of a range of new technologies and new types of space transportation systems.

Base-load Solar Power Systems: Ground and Space.

The projected costs of base-load solar power using ground-based solar arrays are clearly dominated by the cost of the energy storage system needed to allow energy received during a clear day to be delivered to customers at night—or during several consecutive days of cloudy weather. These costs can be greater than $15,000 per kilowatt-hour of energy stored. In other words, to power a house using 2 kilowatts of power over five days of cloudy weather would require about $4 million to build the energy storage system alone. Conversely, the installation cost for a space-based solar power system (providing power for hundreds of thousands of homes) might be expected to range between $100,000 and $300,000 per home, for a comparable power-using home. This is still much greater than the cost of installing new fossil-fuel power-generating capacity, but the cost of a space-based solar power system could be as little as 1 percent of the cost of a comparable ground-based system.

History and Future Directions

In the 1970s the U.S. Department of Energy and the National Aeronautics and Space Administration (NASA) extensively examined large solar power satellite systems that might provide base-load power into terrestrial markets. From 1995 to 1997 and in 1998, NASA reexamined space solar power (SSP), with both encouraging technical results and cautionary findings concerning the economics of introducing the technology during the first two decades of the twenty-first century. As a result, from 1999 to 2000, NASA conducted the SSP Exploratory Research and Technology program, which refined and modeled SSP systems concepts, conducted research and development to yield "proof-of-concept" validation of key technological concepts, and laid the foundation for the creation of partnerships, both national and international. A number of innovative concepts and technology advances have resulted from these efforts, including a new solar power satellite concept, the Integrated Symmetrical Concentrator, and new technologies such as lightweight, high-efficiency photovoltaic arrays, inflatable heat radiators, and new robots for space assembly. Future technology efforts will focus on providing the basis for better-informed decisions regarding solar energy in space and related research and development.

see also Power, Methods of Generating (volume 4); Solar Power Systems (volume 4).

John C. Mankins

Bibliography

Glaser, Peter E., et al. "First Steps to the Solar Power Satellite." Institute of Electrical and Electronic Engineers (IEEE) Spectrum 16, no. 5 (1979):52-58.

Iles, Peter A. "From Vanguard to Pathfinder: Forty Years of Solar Cells in Space."Proceedings of Second World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, Austria (1998):LXVII-LXXVI.

——. "Evolution of Space Solar Cells." Solar Energy Materials and Solar Cells 68(2001):1-13.