Space-Based Solar Energy
A very different approach to exploiting solar energyis to capture it in space and convey it to the Earth by wireless transmission.As with terrestrial capture of solar energy, it provides a source thatis virtually carbon-free and sustainable. Unlike terrestrial capture, however,a space-based system would not be limited by the vagaries of the day-nightcycle and adverse weather and thus could provide baseload electricity.
A substantial body of work, both analytical and experimental,has established the technical feasibility of wireless transmission of usefulamounts of power (Glaser et al, 1993 and 1997). Wireless transmission ofpower is similar in concept to information transmission by communicationssatellites, but at a higher intensity. Because the power beam is engineeredfor conversion to electricity at very high efficiency, useful amounts ofpower could be transmitted at intensities less than that of sunlight. Experimentaltransmissions of power in amounts up to 30 kW have been accomplished overshort distances (1.6 km) with conversion efficiencies in excess of 85%from incoming radio frequency energy into electrical energy.
Recent studies indicate that collection and transmissionof power from space could become an economically viable means of exploitingsolar energy within the next one to two decades (Mankins, 1998). A substantialmaturation of certain technologies is needed and, most importantly, thecost of launching material to space must be significantly reduced. Veryactive efforts are being pursued in the aerospace community to meet bothof these conditions.
Characteristics of the Space-Based Approach toSolar Energy
Solar radiation represents the basic source of energy,whether in space or on the Earth. In space, however, the maximum irradiance(or power density) is substantially higher than on the Earth, around 1360W/m2, and is virtually constant. This energy can be captured and convertedto electricity just as it can on the Earth and indeed as it is done routinelyto power spacecraft. Photovoltaic cells are the preferred means, althoughother approaches (such as heat engines) are possible. Wireless transmissionto receiving systems on the Earth provides the delivery mechanism. A depictionof this concept is shown in figure 1 (courtesy Nansen, 1995).
Figure 1 Concept of Space Based SolarEnergy Capture and Transmission
The elements of such a space-based solar energy systemwould include the following:
･ Satellites, in geosynchronous or other orbits,designed as large solar collector systems
･ Power conditioning and conversion components toturn the electricity gathered by the photovoltaic arrays into radio frequencyform
･ Transmitting antennas that form one or more beamsdirected from the satellite to the Earth
･ Receiving antennas on Earth that collect the incomingradio frequency energy and convert it into useful electricity. Such a deviceis termed a "rectenna" (for rectifying receiving antenna)
･ Power conditioning components to convert the directcurrent (DC) output from the rectenna to alternating current (AC) for localuse or for connection to the electricity transmission grid
The space-based approach introduces obvious complexitiesto the exploitation of solar energy. However, it has countervailing characteristicsthat may make it attractive as an option. In addition to the higher irradiancein space, as noted above, satellites operating in geosynchronous orbitsare illuminated over 99% of the year (there are short eclipse periods nearthe equinoxes). Importantly, space-based systems can, with the proper choiceof transmission frequencies, deliver power 24 hours a day, virtually independentof weather. Furthermore, since the incoming radio frequency energy canbe converted to electricity at high efficiency (85 % or more), the amountof land and electricity storage required for a unit of baseload power wouldbe modest in comparison with photovoltaic cells at the surface.
The Potential Supply of Space-Based Solar
The basic resource of solar energy available in spaceis virtually unlimited in comparison with human needs. Other limits would,of course, need to be considered. For example, energy-collecting platformswill be most useful if placed in geosynchronous orbit, the location fromwhich many communications satellites now operate. Certainly "realestate" in this unique orbit is finite, but most of the geosynchronousarc is useable and adequate to accommodate at least a few hundred platformseach of which might deliver from one to five gigawatts. The other key factoris land area on which to accommodate the rectennas. As with any mechanismfor exploiting an energy source so diffuse as solar, substantial areaswould be needed. The power yield from typical rectennas at low to middlelatitudes would be on the order of 30 MW per km2 with relativelylittle need for storage of electricity (Mankins 1998).
System and Technology Considerations
Space-based solar energy systems, in order to beefficient, need to operate at a relatively large scale, a simple consequenceof the physics of radio frequency energy propagation. Optimum architecturalapproaches and sizes have yet to be determined, but the best estimatesindicate that the on-orbit segments will likely be in the size range betweena few hundred MW to a few, perhaps up to ten, GW. A single on-orbit collectorcould serve a number of ground receivers that would probably be sized between100 MW and the order of a GW.
The most likely role for space-based systems is asalternative generating elements that would feed an electricity distributiongrid. There are, however, other attractive possibilities. One is to couplespace-based delivery of power to a hydrogen generation and delivery system.Nighttime power could be used to electrolyze water and generate, in effect,"solar hydrogen". This could be used in a variety of ways, asa fuel in the combustion sense, to power fuel cells, or to manufacturesynthetic transportation fluids. Another possibility would be to site rectennasnear hydroelectric installations and use the nighttime power to replenishthe reservoir and thus be able to serve higher loads during peak demandtimes.
As is the case with any solar source, space-basedenergy would not contribute to greenhouse gas emissions during operation.However, the manufacture of the components and the launching of these componentsto space would have some impact. A preliminary analysis (Yoshioka, 1998)indicates that the effective contribution of CO2 would, overa life cycle, be a little less than that involved in a nuclear fissionplant.
The high launch rate required to emplace a space-basedenergy system could affect the Erath's atmosphere. Analyses indicate thatthis can be addressed by shaping the launch trajectories to limit the persistenceof rocket exhaust products in the atmosphere.
The effects of transmission of power to the groundneeds to be assessed with regard to at least three factors:
(1) Influences on the atmosphere itself, particularlythe ionosphere on the way down. A rocket-borne experiment (Akiba, 1993)indicates that interactions between transmitted power and the atmosphereare slight and should cause no damage to the earth's ionosphere.
(2) Interference effects between the wireless powertransmission and communications or electronics equipment. This requiresfurther study, but a considered engineering view (Glaser, 1993) is thatsuch effects will be negligible.
(3) The effects of the transmitted beam on life forms.Most of the work to date has been based on transmission at microwave frequencies(2.45 GHz has been the most studied), with peak power density at the middleof the (gaussian) beam of 230 W/m2, approximately a quarterof that of sunlight. The receiving area would be protected and, with abuffer zone around the rectenna itself, the power density at the fenceline would be below the present limits for microwave exposure (for example,10 W/m2 at 5.85 GHz in Health Canada's Safety Code). Controlof the beam is assured by transmitting only when a pilot signal from theground commands the transmission. A brief summary of the safety issue wasarticulated by Glaser as follows:
"The fact that large populations have been exposedto microwave energy from communications, medical, radar and industrialprocesses for many decades and, more recently, from 250 million microwaveovens, without known demonstrated adverse effects on human health and theecosystem, suggests that properly controlled power beaming systems areunlikely to result in undesirable health and ecological effects".
As with any system handling major amounts of energy,safety will be a paramount consideration and will be given careful attention.
Rigorous cost estimates for space-based solar energycannot be developed without further study and without knowing the resultsof on-going technology development work. However, very preliminary estimates(Mankins, 1998) suggest that the cost target of 5 c/kWh for a mature systemis a reachable goal.
There are several important issues to be addressedin pursuing the implementation of space-based solar energy. Of these themost important are:
･ A number of key technologies require maturation.These include high performance solar collectors, low-mass space systems,and efficient wireless power transmission, among others
･ Optimum architectures for space-based solar systemsneed to be established
･ The cost of access to space needs to be substantiallyreduced
･ Orbital slots for collection platforms and frequenciesfor power transmission need to be obtained
･ Safety and environmental concerns require satisfactoryresolution
･ A satisfactory approach needs to be developed forthe introduction of new, large-scale, power provision systems into theglobal market place
The role space-based systems could play in providingenergy to the Earth has been extensively explored in the aerospace community,but is little known outside this community. However, other space-basedsystems have proven very successful in meeting human needs, for example,global telecommunications, navigation aids, and weather monitoring fromspace.
Space-based provision of energy to the Earth may,someday, be another example and, if successful, could eventually be:
･ among the most economical of renewable sources
･ virtually pollution-free
_ more sparing in the use of land than other renewablesources
･ capable of providing baseload electricity
･ delivered to locations of need anywhere on theEarth.
At present, no critical flaws have been identifiedin the basic approach.
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