NASA space-based solar-power satellite concept

Using space-based solar power to meet our energy needs on Earth is not a new idea. Dr. Peter Glaser of Arthur D. Little, Inc. described a space-based solar-power satellite concept in 1968, one year before Neal Armstrong walked on the Moon. Glaser proposed using microwave beams to transmit power back to Earth. Others have proposed using laser beams, but none of the space-based power concepts have gone anywhere in the last 45 years.

A practical space-based solar power satellite requires some technical advances, mainly in launch costs but also in other areas such as in-space fabrication, control of large space structures, and on-orbit maintainability. The biggest stumbling block, however, is not technical but economic. Despite the enthusiasm of advocates, the economic viability of space-based solar power has always been questionable, to say the least.

The prospects for space-based solar power may be dramatically improved, however, thanks to an emerging interest from the US Navy.

The US Naval Research Laboratory has been looking into space-based solar power for the last several years. The Navy’s interest in delivering energy from space is not surprising. The military often finds itself in special situations where it pays exceptionally high prices for energy. During World War II, for example, the Army was paying over $1000 a gallon to fly gasoline over “the Hump” into Burma. (Real 1940′s dollars, not today’s inflated greenbacks.) That’s comparable to what it costs to launch payloads into orbit today. The military continues to pay high prices today, in places like Afghanistan.

Prototype Hardware

Until recently, the NRL’s interest in space-based solar power has been mostly confined to paper studies. This week, however, the NRL announced the development of some prototype hardware.

Dr. Paul Jaffe, an aerospace engineer at NRL, has built and tested a module to capture and transmit solar power.

Dr. Paul Jaffe of the US Naval Research Laboratory

Jaffe has built two different versions of what he calls the “sandwich” module. In both designs, one side receives solar energy with a photovoltaic panel and the other side has an antenna to beam power away. Electronics in the middle convert the direct current to a radiofrequency.

Jaffe says his sandwich module is four times more efficient than anything done previously. His design also radiates heat radiate more efficiently, so the module can receive greater concentrations of sunlight without overheating. For the antenna, Jaffe partnered with Dr. Michael Nurnberger, an antenna expert at NRL.

Jaffe has designed the system to be operable no matter what the weather might be at the receiving antenna (rectenna) on Earth. “At 2.45 gigahertz,” says Jaffe, “you’ll get power in a monsoon.”

space-based solar-power modules developed at US Naval Research Laboratory

Jaffe envisions a solar-power satellite with a one-kilometer array of modules — nine times the length of the International Space Station. The modules would be launched separately, then assembled in space by robots similar to those now being developed by NRL’s Space Robotics Group.

Jaffe says he would to see “a demonstration mission, where you actually manufacture a whole bunch of these things and assemble them as an array in space to investigate some of the other challenges.”

Proposed Missions

In 2009, the NRL produced a report titled Space-based Solar Power: Possible Defense Applications and Opportunities for NRL Contributions.

In the report, NRL examined a number of possible missions for space-based solar power: providing electrical energy to forward operating bases, providing power to individual soldiers or vehicles in the field, powering distributed sensor networks, bistatic radar illumination, producing synthetic fuels, providing power to ships at sea, powering UAVs, and space-to-space power beaming.

Of these applications, space-to-space power beaming was considered the cheapest and easiest to realize. NRL estimated it could be implemented in just over two years for around $50 million. All of the other missions were estimated to cost over $10 billion and require more than five years of development. Providing power to individual soldiers and ground vehicles was judged to be infeasible with any foreseeable technology.

Providing electrical power to forward operating bases was judged to be the best military use of space-based solar power. Synthetic fuel production also appeared promising but would require some modifications to military fuels infrastructures.

Powering ships at sea was judged to be technically infeasible with microwave power beaming but possible with lasers (which were not examined in detail).

Bistatic radar illumination was judged to be feasible but expensive. Powering sensor networks, however, was judged to be infeasibly expensive.

Powering UAVs was judged to be feasible only if it was done in conjunction with the forward-base mission.

Forward Operating Bases

A forward operating base or FOB exists to support a small number of reconnaissance and surveillance teams and provide military power projection before the arrival primary forces. An FOB can support anywhere from 50 to 5000 people depending on its assigned tasks.

Forward operating bases need electrical power is a necessary commodity for the forward operating base. FOBs tend to be located in remote areas with access limited by terrain as well as enemy action. Resupply missions may require armed convoys or expensive (and possibly risky) airlift operations.

A typical forward operating base requires between 1 and 3 kiloWatts of electricity per person. A large FOB could require as much as 5-15 megaWatts. Generators account for about a third of all military fuel consumption in wartime.

In addition to powered the FOB utility grid, a space-based solar power system might be used to charge batteries or mechanical energy-storage devices or even to produce synthetic fuels.

Currently, the military locates generators away from command, control, and living areas due to heat and noise abatement. Similar restrictions would apply to power receiving antennas.

Space-solar power proposals usually limit power density to the equivalent of one Sun at ground level, for safety reasons. In theory, a 500-man forward operating base with medium tasking and no air strip could be powered by 500 square meters of microwave receiving antenna. In practice, limiting microwave power transfer to such a small area would be difficult, if not impossible. Much of the transmitted power would spill over into the surrounding countryside and might even be intercepted by opposition forces.

To maintain safe microwave power densities while avoiding spillover, an elliptical area of several square kilometers would be required. This may limit the applicability of space solar power to larger bases, major command posts, and supply depots.

Individual Soldiers / Ground Vehicles

Reducing the need for soldiers to carry heavy batteries is an attractive prospect. Batteries are troublesome because of their weight and the need to protect them from moisture, extreme temperatures, and other hazards. Batteries make up an 15% to 20% of a soldier’s 30- to 40-kilogram (65- to 90-pound) field pack. Supplying replacement batteries increases the demand on supply lines, and recharging batteries adds to the generator load at forward bases.

For these reasons, space-based solar power has been proposed as a means to recharge such batteries or to displace the need for them by providing power directly to the soldier.

Direct delivery of space-based solar power to individuals or vehicles seems problematic at best. At microwave frequencies of 1.5 to 15 GHz, safe power densities for continuous exposure are between 1 and 10 mW/cm2 (1-10 W per sq. ft.) according to IEEE standards. The FCC limits this exposure further, to a constant 1 mW/cm2.

Military radios require tens to hundreds of Watts while transmitting. Vehicles require tens of horsepower (tens of kiloWatts) while traveling at constant highway speeds and much more when accelerating or traversing rough terrain.

At microwave frequencies, it is extraordinarily difficult (if not impossible) to pinpoint individual users. This means most of the transmitted power would be wasted. Even worse, from a military viewpoint, it would be available for the enemy to use — for free!

As a result, direct delivery of space-based solar power to individual soldiers or vehicles, via microwave, is not feasible with near-term foreseeable technology.

Distributed Sensor Networks

Wireless distributed sensor networks (WDSNs) can perform a variety of surveillance functions with both military and civilian utility. One of the single largest limitations to their application is the availability of reliable power sources in remote locations.

WDSNs deployed in remote areas usually rely on batteries, which have limited life, or derive power from solar arrays or local sources that are not always reliable. Total energy requirements for sensor networks are low, tens of Watt-hours per day or less. Even the smallest space-based solar power system could delivery that level of power to small, field deployable rectennas with cross-sectional areas of approximately square meter, with safe microwave energy densities.

One attractive advantage of such systems is the ability to power buried or otherwise concealed sensors, which cannot rely on solar arrays. Additionally, the coherent microwave power beam could be used to autolocate sensor nodes that are field relocatable.

Wasted energy is still a problem, and spillover could be exploited by anyone with suitable receiving equipment who was aware of its presence. One possible solution might be to rely power through UAVs or other more proximate systems.

Bistatic Radar Illumination

Bistatic radars are systems where the radar illumination (transmitter) is located some distance from the receiver.

Bistatic radar systems outperform conventional monostatic radars in a number of tactical scenarios. They are especially good at countering anti-radiation missiles, retro-directive radar jammers, and stealth radar technologies. They are can implement processes such clutter-tuning that are impossible for monostatic radars.

Any space-based solar power satellite could provide bistatic radar illumination as a secondary function. As bistatic radar illuminators, space-based power satellites would have advantages over other satellites such as GPS or geosynchronous communication satellites. Their much higher radiated power would provide orders of magnitude higher illumination of the target, resulting in much better signal-to-noise ratio, detecting targets with smaller radar cross sections and minimizing the threat from surface jammers. Space-based power satellites could direct their illumination to specific areas of tactical interest, providing on-demand capability as an adjunct to the the primary power-transmission mission.

Synthetic Fuel Production

A space-based solar power system might be used to produce for a forward operating base or larger installation via a cracking or synthesis process. Liquid fuels currently comprise about 85% of the military’s total energy consumption, so this is an attractive alternative.

Hydrogen can be produced directly from water via electrolysis and used directly, as fuel, or as the first step in the synthesis of more complex, hydrocarbon fuels. Using hydrogen as a fuel entails non-trivial storage and transportation problems, and current engines are designed for use with more traditional hydrocarbon fuels.

Hydrocarbon fuels can be synthesized from molecular hydrogen and carbon monoxide in the presence of a catalyst at environmental temperatures between 150° and 300 °C using a process patented by NRL researchers Dennis Hardy and Timothy Coffey.

To supply the energy needed for synthfuel manufacturing, a space-based solar power system would require large rectenna arrays, comparable to those envisioned for terrestrial utility-grid applications (in the 100-MW class and up). This would likely limit its applicability to fixed-base facilities or, possibly, ocean-going vessels.

Ships at Sea

Space-based solar power could allow ships to stay at sea as long as their provisions held out, much like nuclear submarines or aircraft carriers. This application would almost certainly require laser power beaming and power densities above current safety levels.

Unmanned Air Vehicles

Solar-powered long-duration UAVs are payload limited due to the vehicle mass that must be devoted to energy storage, usually in the form of batteries. Energy storage is essential for nighttime flight and any time when solar energy is not available. Despite advances in battery technology, energy storage still comprises from 20% to 50% of total vehicle mass.

Space-based solar power could supplement local insolation as a power source, reducing the weight of batteries whch must be carried.

Tracking a small, moving target with the power beam would be a significant challenge. An easier application would combine the UAVS mission with the forward operating base scenario. In that case, the UAV would simply fly through the beam directed at the FOB, picking up a free charge without requiring ultraprecise pointing.

Satellite-to-Satellite Power Transmission

A space-based solar-power satellite in geosynchronous orbit could supplement or supplant the more traditional power sources on other satellites. Safety considerations for terrestrial power beaming would not apply, so power densities could be greater and rectenna arrays much smaller.

Power from a constellation of geosynchronous satellites would be nearly continuous, allowing satellites in Low Earth Orbit to operate without night-time disruptions.

As an alternative to microwave power beaming, a space-based solar-power satellite could use a laser to directly illuminate the solar arrays on existing satellites.

The development of space-to-space power beaming would aid the military is disaggregating satellite functions (currently a priority for the US Air Force, which wishes to reduce its dependence on large satellites). A space-based power satellite could support many different types of client payloads during its operational lifetime.

Written by Astro1 on March 18th, 2014 , Military Space

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