Power from space for a sustainable world

Acta Astronautica Vol. 40, No. 2-8, pp. 345-351, 1997 01997 International Astronautical Federation. Published by Elsevier Science Ltd Printed in Great Britain 0094-5765/97 $17.00 + 0.00

PII: soo94-5765(!n)oo117-3

IAF-96-R.2.03

POWER FROM SPACE FOR A SUSTAINABLE WORLD R. Bryan Erb+ Canadian Space Agency ABSTRACT This paper assesses the future energy needs of the world and examines options for sources that will be sustainable over generations from both supply and environmental points of view. It is demonstrated that for the supply of electricity, the preferred energy carrier, power from Space, i.e., solar energy captured in space and imported to the Earth, is likely to play a major role in providing base load energy. Energy from sustainably-grown biomass will play a modest role in base load supply, but requires too much land to provide the bulk of the energy in well-populated countries. Wind and solar energy captured on the Earth will provide useful supplies of peak load energy in many locations. A unique “distributed” characteristic of Space-Based Solar may permit access to this source with relatively low capital investments. That is, because of the wireless transmission link, part of the generating system is in space, and part is on the ground. Thus a potential user could finance and operate only the ground segment and buy ‘sunfuel”, in radio frequency form, from an (international) enterprise which owns and Operates the Space Segmerlt. 0 1997 httemationd AstronauticalFederation. publishedby Elsevier Science Ltd.

substantial body of literature available describing work conducted over several da&es’.

INTRODUCTION Power from space, i.e., solar energy captured in space and imported to the Earth

There are, of course, several energy sources which are sustainable. This paper

holds great promise as one sustainable means of powering our planet in the future. Furthermore, because essentially only electricity is imported, the environmental impact would be minimal, especially when compared to that caused by our present energy practices which are dominated by the combustion of fossil fuels. The possibility of importing power from space has been extensively studied and there is a very

’ The initial concept paper appeared nearly 30 years ago [l]. Extensive work by NASA and the U&d States Department of Energy was done in the late 1970s and this work is well summarized by Koomanoff (21. Important papers of that time include evaluations by other United States Government agencies [3,4]. US interest lapsed around 1980. but efforts wera carrfed on elsewhere [5,6]. A great surge of interest has come in the last few years and new developments are coming rapidly [7,8,9.10].

+ Assistant Director, Canadian Space Station Program, Associate Fellow, AIAA 345

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categorizes these sources, presents an overview of some of the key characteristics of the most important, and analyses these characteristics to identify situations in which each might find its most logical use in an energy future dominated by renewable

diminishes, whether because of supply limitations or concern over environmental impact.

OPTIONS

sources.

NEED

FOR ADDITIONAL, ENERGY

CLEAN

A sustainable supply of clean energy can come from one or more of several sources: l

The population of the Earth is growing rapidly, with most of this growth occurring in Developing Countries. The inhabitants of these countries understandably aspire to the energy-rich way of life enjoyed by the industrialized countries and these aspirations are motivating a strong demand for a much-expanded energy supply. The likely demand has been extensively studied and a top-level summary of the near-term projection from the World Energy Congress [ll] is shown in Figure 1.

FOR A SUSTAINABLE ENERGY SUPPLY

l

l

The Sun, i.e., various manifestations of solar energy The Earth, e.g., nuclear and geothermal energy Planetary gravitational effects, e.g., . tides.

It is the view of this author that only the solar option is capable of satisfying the worldwide need in perpetuity. Briefly: While nuclear fission is currently important as a source of electricity, indeed dominating in some countries like France, it faces major problems with public acceptance and is in decline, possibly permanently Nuclear fusion for electric power faces immense challenges, but might, if these challenges are overcome, provide a safe and sustainable source of energy toward the end of the next century

Figure 1. Growth and Regional Distribution of Global Energy Demand by the Year 2020 The world energy community recognizes that the environmental impact of the acquisition and use of energy is substantial and that lessening these impacts must be a goal for energy suppliers and users. The 1995 World Energy Congress in Tokyo [12] noted that, while a favorable supply of fossil fuels is available for 50 years or more, this should not be used as an excuse to postpone actions needed to achieve sustainability in the longer run. A longer range projection [13] of global energy demand is given in figure 2. This figure also shows the likely portion that will need to come from renewable sources as fossil fuel use

Geothermal energy is quasi-sustainable, but available at present only in locations that are especially favored, for example, Iceland, New Zealand, the Philippines, and some areas of the American West. If the technology for exploiting so-called “hot dry rock” could be rendered practical, vast supplies of geothermal energy could become available in many locations. This does not appear to be a near term prospect [14] Tidal energy resources are substantial, but very diffuse. A few tidal energy projects have been developed successfully, but relatively few locations are appropriate and it seems destined to be a specialized niche source.

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25

I)/ 20

0

Projected Total Energy /

d’

0f Required from Sustainable Sources

Permissible Fossil Use 5

-

Historical Energy Use

1900

2000

1950

2l.50

2100

Year

Figure 2. Global Energy Demand to the Year 2100 The sources which are unquestionably sustainable are those based on the energy flow from the providentially-placed fusion reactor we call our Sun. The Sun provides a virtually unlimited energy source which we can tap in various ways: “Solar direct”: i.e., solar energy captured directly by photovoltaic or thermal devices. Capture can be effected on the surface of the Earth or in space “Solar indirect”, variants:

with

two major

+ Photosynthetic, i.e., vegetation grown sustainably for energy use, commonly referred to as “modern biomass” to distinguish it from traditional use of fuel wood which is frequently not used in a sustainable manner

contributors in many locations. Hydro resources are rather well-exploited already and provide approximately 20% of the world supply of electricity. Possibilities for a major expansion are, however, limited. The use of wind is growing rapidly and stands to make a very significant contribution in favored regions. The other sources noted (wave, ocean thermal) are so diffuse as to be very difficult to harness. The balance of the paper will assess, for the electricity component of energy supplyZ, the relative merits of the following sources as major contributors: l

l

l l

+ Thermal/Mechanical, e.g., wind, hydro, ocean thermal, wave, and other manifestations of the solar energy which reaches the Earth. Of the sources mentioned above, solar direct and solar photosynthetic could have widespread applicability. Wind and hydro resources are large and will be substantial

Solar energy captured in space (SpaceBased Solar Power) Solar energy captured on the Earth’s surface (Solar Terrestrial) Biomass energy Wind energy

It is important to note, in any assessment energy supply, two distinctly different

* Electricity is the preferred energy carrier for most applications other than transportation. It seems likely that efforts to develop sustainable forms of energy will focus on electricity generation.

of

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classes of electric power services continuous or base load, and intermittent or peak load. Most important to industrial societies is a base load service, i.e., one available on a continuous basis, any hour of the day or night. Peak loads are typically met in one of two ways:

(1)

By generators which would be uneconomical to operate on a steady basis because of fuel cost or pollution

(2)

By intermittent sources which are available at the time the load occurs. For example, air condition power needs are large during the middle and later parts of the day when the sun is shining. Since that is the time when terrestrial solar power can be most readily generated, it satisfies, to some extent, this type of peak load need.

It is very costly, as will be shown later, to use an intermittent source to meet a base load need. Cf the sources mentioned above, solar photosynthetic and hydro resources can meet a base load need. Solar direct, if captured in space where the sun shines all the time, can also provide base load power. While not strictly a supply option, mention should be made of the notion of moving major quantities of electric power over intercontinental distances. A worldwide connection that could shunt power from areas of surplus to areas of need, including from the daylight side of the Earth to the night side, was proposed by Buckminster Fuller decades ago. The notion currently has its proponents [ 151 and may someday be a component of a robust global energy system. However, considering that the need for additional energy is very large, that it is predominantly in Developing Countries, and that such countries do not have established electricity grids3, worldwide interconnection is not considered an

’ Indeed. half of the world’s population access at all to electricity.

has no

alternative to new supplies. Rather, for Developing Countries at least, local or regional use must be matched by local or regional supply. In addition to distribution of electricity by means of a grid, other carriers may have a role. The most studied of these is hydrogen. To the extent that a renewable energy source is used for the electrolysis of water to generate the hydrogen, it is environmentally benign. Fuel cell technology allows the recombination of hydrogen with atmospheric oxygen to provide electric power and water. Hydrogen is also much more readily stored than is electricity. Using terrestrial solar or wind power to generate hydrogen as a storage and distribution mechanism may be applicable in some situations. There is, however, a significant energy penalty in the hydrogen generation and a further penalty in the fuel cell to convert back to electricity. These penalties and the need for substantial additional infrastructure suggest that this approach will be intrinsically costly. However, a careful analysis of the characteristics and cost of the renewable energyhydrogen-fuel cell approach should be undertaken. The thrust of this paper is to identify options suitable for supplying major amounts of power to serve the high density needs of an increasingly urban global population. A rather different class of service is needed for sparsely-populated or rural areas. In these situations only small amounts of power are needed, a few tens of kWehr per day. For such needs, terrestrial photovoltaic and conventional storage such as batteries will find major use and bring the benefits of electricity to many people. In the aggregate, however, such an energy supply will meet only a small part of the global demand indicated in Figure 2.

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CHARACTERISTICS OF THE MAJOR SUSTAINABLE ENERGY SOURCES To assess the relative merits of the various sustainable sources in making major contributions to world energy, the author offers the analysis which follows. Sustainable energy sources harvest energy that is arriving continually at the vicinity of the Earth as distinct from exploiting concentrated forms of energy like fossil fuels that have accumulated over millennia. These sources are very diffuse and thus the land area required for energy capture becomes a major determinant of the practicality of harvesting a particular energy form. The analysis considers the following as the major parameters: Land area required Cost of developing the facility Recurring costs, i.e. operation, maintenance and fuel Total cost of delivered power Whether the supply is continuous load) or intermittent

(base

A reference power plant size was assumed at 100 megawatts and plant life as 30 years. Costs were based on data from a number of

Table

*

1.

Characteristics

349

sources [16-231. Components of cost are: capital recovery (Cap. Rec.), fuel where applicable, and maintenance and operations (M&O). By way of reference, typical costs currently prevailing in the United States range from 2 to 21 cents/kWehr. A major market is expected for power that can be sold in the range of 10 cents/kWehr. The data for terrestrial solar and the scaling for the base load case follows the analysis of Stridtland [19] who studied an operating photovoftaic pilot plant in Austin, Texas, a relatively good location for terrestrial capture of solar energy. In sizing the arrays and storage, the analysis makes allowance for 24-hour operation in January, but does not add additional capacity for cloudy days. Strickland’s analysis is optimistic. Canough [23] shows that solar terrestrial for base load may require 12 times as much area as for peak load rather than 6 l/2 times as shown by Strickland. These figures should be considered as approximate and illustrative only. A summary of the analysis is given in Table 1 and the cost-area relationship is displayed graphically in Figure 3.

of Major Sustainable

Energy

Options

Actual capture area for incoming energy is 1.6 km’. The balance of the area is for a buffer zone whiih could be used as a “green belt” or for agricultural purposes.

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-

20,000

a Solar

Capit al Cost

Terrestrial (Base Load)

$1 kW,

10,000

-

l

Space- Based Solar Biomass 0 I

0

1

100

0

200

Land Area (km*) Figure 3. Capital Cost and Land Area for Various

Some general observations can be made on the basis of the analysis. Wind and solar energy captured on the Earth’s surface cannot provide a full solution to a sustainable energy supply unless storage is provided. Even inexpensive storage is not a solution since the major cost lies in the need to have a capture capacity several times larger than that needed to satisfy the daytime peak. Wind and solar energy captured on the Earth’s surface can provide essentially daytime energy under the right conditions. Incorporation of energy from such sustainable sources can be used as adjuncts to base load power supplies for up to 20 - 25% of the total energy needs [24]. The contribution might be somewhat more if a major part of the demand were for daytime purposes such

l

l

Energy

Sources

as air conditioning, a demand which follows, to some extent, the variation in solar input over the course of a day. Both solar energy captured in space and modem biomass can provide a base load power supply. Biomass energy requires a very large land area for sustainable production of fuel, land that is reasonably fertile and adequately watered.* Thus there will be direct competffion between food crops and energy crops for such productive land.

LAND USE IMPLICATIONS OF SUSTAINABLE ENERGY SOURCES A further analysis was done on the implications of the land needs of the various sources. This was done by examining typical populations served by an installation of the reference size (100 MW).

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Population projections for the middle of the next century indicate that the Earth will then be home to roughly 10 billion humans and the great preponderance of this population will be in the developing world. Making what are probably optimistic estimates of the per capita energy available to these populations and assuming half the total energy is provided as electricity, we arrive at approximately 0.25 kWe per person. Thus the reference plant producing 100 MWe or 100,000 kWe would serve a population of 400,000 in a typical Developing Country. For those options providing base load energy, the data from Table 1 indicates the land area required for such an energy capture is as follows:

Assuming a plausible range of population densities, say from 500 to 2000 persons per km*, the land actually occupied by the people ranges from 800 to 200 km*. If we compare the land needed for energy production with that occupied by the population over this range of population densities, we can arrive at the percentage of the land area which would be needed for energy production. These percentages are shown in Table 2 and graphed in Figure 4.

Percentage

For comparison, the current population densities of India and Bangladesh are shown. A biomass approach to energy in these two countries would require about 14% and 43% of their land areas respectively. Land commitments of the magnitude indicated are not likely to be made in densely-populated countries. Studies of the feasibility of biomass energy in the USA [18], a relatively sparsely populated country, indicate that shifting even 5% of agriculturally-productive land to energy crop use would be most challenging. Patterson [18] concludes that in the USA biomass use would be constrained to “well under 10% of the total US electricity supply”. Two key observations can be made based on this section of the analyses:

Space-Based Solar Power 4 km” Solar Terrestrial 26 173 Biomass

Table 2.

351

While sustainable use of biomass for energy is a worthy pursuit, it cannot be expected to provide a total energy solution except, perhaps, in relatively lightly-populated countries Since none of direct solar, biomass or wind offers a total solution to sustainable energy, Space-Based Solar Power stands to be the best choice for our energy future.

of Land Used for Energy

Production

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Of

L a n d

50 -I

Solar Terrestial 0 0

space-Ba!sadsolarPower I 1500

I 1000

500

Population

Density:

Persons per km*

Figure 4 Land Required for Various

FINANCING IMPLICATIONS OF SUSTAINABLE ENERGY SOURCES

Economically, costs for most sustainable energy sources are like those for a hydroelectric power facility - the dominant cost is for emplacement of the facility,

Hydra

I

%

Energy

Options

hence driven by the cost of financing. Also analogous to hydro installations, there is no fuel to purchase so operating costs should be low. However, as noted in earlier analyses [25], the cost of financing is so important that it is very difficult for renewable sources to compete with fossil plants based on present accounting. Figure 5, adapted from reference 25, charts the capital cost of both fossil and renewable sources and shows clearly the relatively low capital cost of the former, especially gas.

Sustainable energy sources have not made as much headway as their proponents would like, in large measure because of the relatively high capital cost when compared to fossil fuel plants.

z 8

1 2000

Wind

Solar Thermal

Solar

PV

1

1000 0 _

Figure

Base Load Sources

5.

Capital

-

Costs of Various

Peak Load Only Sources

Energy

Sources

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Space-Based Solar Power is unlike any other energy source in that its elements are

spatially “distributed”.

That is, because of

the radio frequency transmission link, part of the generating system is in space, and part is on the ground. While it would be perfectly feasible for a single entity to own and operate both the space and ground segments, this is not required. The distributed aspect of Space-Based Solar Power makes it feasible for one enterprise to own and operate the space segment and sell energy, in radio frequency form, to another enterprise which owns and operates the ground segment. This form of energy we might call “sunfuel” and it would become a commodity like other forms of energy. The concept of splitting ownership of the parts of a Space-Based Solar Power system

Table 3. Characteristics

The ability to partition the power supply as indicated could yield a distinct advantage if capital is scarce. In fact, the cost of installing the ground segment is on the same order as that for a gas-fired plant. If, as many anticipate, future energy accountin adds in external and environmental costs B, the advantage of Space-Based Solar Power would be substantial. Developing Countries are faced with multiple uses for their scarce capital. While they might like to select sustainable ’ There is general agreement that the pricing of energy should include its full cost. For fossil fuel sources, this would require adding in rather substantial costs for environmental impacts and for any external effects such as subsidies and the maintenance of supply routes.

353

has been addressed before by several authors, but only from the standpoint of a partitioning between industrial groups. For example, Collins [26] suggested that the aerospace community implement the space segment and the utility industry handle the ground segment. However, there may be a substantial economic benefit as well since the ground segment cost is the smaller part of the total system cost. This distributed system feature can be understood if we assess the economics of the space segment and the ground segment separately. The cost for the total SpaceBased Solar Power system as shown in Table 1 can be partitioned according the relative costs of the space and ground segments. This is shown in Table 3.

of Space-Based

Solar Power

energy sources that are benign to the environment, capital costs inhibit such choices. Financing only the ground segment of a Space-Based Solar Power system would make the initial investment tractable. Economically it would be somewhat like buying fossil fuel from abroad, except in this case, a form of electricity is being purchased and the local installation does the final conversion and conditioning. If financial capability increases, an ownership share could be taken in the space segment and recurring costs reduced. The observation from this section of the analyses is that Space-Based Solar Power systems offer an economic environmentally attractive option to countries lacking capital.

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SUMMARY

AND OUTLOOK

The broad conclusions that can be drawn from the foregoing discussions and simolified analvses are the followina: There is a major need for clean, sustainable energy and the world must move toward such sources and away from fossil fuels Nuclear energy sources, whether obtained by fission or fusion, will not provide the needed supply Geothermal energy and tidal energy, while useful, will be limited and sitespecific Solar energy captured on the Earth’s surface and wind can provide substantial daytime/intermittent energy under the right conditions and can be used as adjuncts to base load power supplies up to 20 - 25% of the total energy needs Base load supplies of electricity in a sustainable energy world will come from hydroelectric, modem biomass, and solar energy captured in space. It is the opinion of this author that by the end of the next century we must have a global energy system based on sustainable sources. The motivation for what amounts to a massive change in energy practices will be for reasons of environmental impacts in the next few decades, and by virtue of supply limitations in the longer run. The eventual mix of sources is likely to be something on the order of the following: l

Space-Based Solar Power

Hydroelectric . Wind 8 solar terrestrial l

l l

combined

Modem biomass Fossil and other miscellaneous

Considering that it takes roughly years for widespread substitution energy source for another, it is to begin now to move toward the solar options.

25 20 20 10

% % % %

25 % 50 to 75 of one important sustainable

A vast amount of work is needed to bring the necessary technology into being and to put in place the infrastructure for an energy source as different as Space-Based Solar Power. This could and should enoaoe the talents of the next generation of aerospace engineers. However, equally important is that many segments of the international community must be engaged in this enterprise - government officials and policy makers, bankers and investors, utility managers and constructors, and many others, all working for a sustainable energy future.

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APPENDIX

need to withstand wind, rain, earthquake and gravity forces.

A

THE CONCEPT OF SPACE-BASED SOLAR POWER FOR EARTH

Implementing systems to intercept solar energy in space will be challenging from several points of view, but the concept is straightforward. Four steps are involved:

A limitless supply of solar energy flows through space. Space-Based Solar Power for Earth refers to the process of acquiring and importing this energy for use on Earth. The potential advantages of acquiring the energy in space rather than on Earth are twofold: l

l

Capture solar energy in space and convert it to electricity Transform the electricity to radio frequency energy and transmit it to Earth

The flux of solar energy in space is significantly higher than on the Earth [27] and it can be captured virtually 24 hours a day, independent of clouds, rain or weather

Receive the radio frequency energy on Earth and convert it back to electricity Distribute the electricity grid or other means

Space-based energy capture systems can be very light-weight since they do not

.’

/

/

/--

/--

_0----

355

The concept Figure A-l.

via a utility

for such a system is shown in

------------~

/

8’

##’

8’ : i.

i

‘.

‘.

*--N_

_0--

.-

_’

.’

_0--

5-B

Figure

0-

r-‘

-____,--------

A-l.

Concept

for Space-Based

Very detailed studies carried out in the late 1970s [2] showed that Space-Based Solar Power costs would be competitive with other options for base load power under certain circumstances. The most important circumstances noted were the availability of low-cost transportation to orbit and of very

Solar

Power for Earth

large quantities of inexpensive photovoltaic cells. These circumstances did not prevail in 1980. However, great strides have been made since in various aspects of the needed technology and indeed large quantities of inexpensive photovoltaic cells are now produced. New assessments [Q] indicate

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that the delivery of power can be economical if the cost of transporting the building materials to space can be significantly reduced below the present levels. Furthermore, there is a growing acceptance of the premise that the cost of such transportation to space can indeed be reduced greatly and organizations around the world are working toward this end [28,29]. In addition to new technology, there has been substantial work on new architectures. The so-called “Reference System” of the 1970s involved giant platforms in geosynchronous orbit each providing 5 GW to a dedicated ground receiver. Current concepts stemming from a NASA study led by John Mankins [30] include very different approaches. For example, constellations of power-serving satellites in low and medium orbits would provide more modest amounts of power to each of a sequence of ground sites. Each ground station would derive a continuous flow of power, but from a sequence of satellites. Conceptually, such constellations would operate in the same manner as current space-based telecommunication systems. The premises of this paper are that the technology for Space-Based Solar Power will develop satisfactorily and that the costs of transportation to space will drop to a level that will support competitivelypriced power generation. Several recent studies [31,32] indicate that these are reasonable premises.

Office of Technology Assessment, Congress of the United States, August, 1961.

151

Dessus, B. and Pharabod, F., ‘Energy development and environment: What about solar energy in a long term perspective?“, Proceedings from SPS 91, Paris/Gif-Sur-Yvette, August 2730, 1991.

161

Alden, A. and Ohno, T., ‘A Power Reception and Conversion System for Remotely-Powered Vehicles’, Presented at the IEEE International Conference on Antennas and Propagation, University of Warwick, UK, April 4-7, 1989.

171

First Annual Wireless Power Transmission Conference, WPT 93, Center for Space Power, Texas A&M University, held at San Antonio, Texas, February 23-25, 1993.

[61

WPT 95, Second Wireless Power Transmission Conference, Kobe University, held at Kobe, Japan, October 16-19, 1995.

191

AIAA, “International Space Cooperation, Getting Serious About How”, April 1995, Report of the AIAA Workshop on International Space Cooperation, Kona, Hawaii, December 4-9, 1994

1101 Erb, FL Bryan, ‘Power From Space - The Tough Questions’, 45th Congress of the International Astronautical Federation, Paper IAF-95.R2.01, Oslo, Norway, October 2-6, 1995.

1111 World Energy Council, RoundUp, 15th WEC

REFERENCES ill

141 “Solar Power Satellites’,

Congress,

Glaser, Peter E., Power from the Sun: its Future, Science, 162, 657-666 (1966).

[21 Koomanoff, F.A., and Bloomquist, C.E.,

[I21

‘Electric Power From Orbit: A Critique a Satellite Power System”, National Research Council, National Academy Press, Washington, DC, 1961.

1992.

World Energy Council, RoundUp, 16th WEC Congress, Tokyo, Japan, 1995.

1131 Erb, R. Bryan, ‘Power From Space For The Next Century’, 42nd Congress of the International Astronautical Federation, Paper IAF-91-231, Montreal, Canada, October 5-11, 1991.

The satellite power system: concept development and evaluation programme, Section 1.2 of Solar Power Safellites, Ellis Horwood, New York, 1993. 131

Madrid, Spain,

of

[I41

Duchane, D., Progress in Making Hot Dry Rock Geothermal Energy a Viable Renewable Energy Resource for America in the 21st Century, IECEC, Washington DC, August 11-16, 1996.

47th IAF Congress [ 151

Pearce, Fred, The international Scientist, 8 July, 1995.

Grid, New

[ 1 S] Grubb, Michael, John Walker, et al, The

[27]

Smil, Vaclav, 1991.

[28]

Bekey, Ivan, ‘Access to Space’, 45th Congress of the International Astronautical Federation, Paper IAF-94V.1.515, Jerusalem, Israel, October 914, 1994.

Royal Institute of International Affairs, Emerging Energy Technologies: Impacts and Policy Implications, Billing and Sons Ltd., Worcester, England, 1992.

[ 171 Johansson,

Thomas B., Henry Kelly, Amulya K.N. Reddy, and Robert H. Williams, Editors, Renewable Energy: Sources for Fuels and Electricity, Island Press, Washington, DC, 1993.

357 General

Energetics,

Wiley,

[ 291 AIAAXEAS, ‘International Space Cooperation - From Recommendations to Action”, September 1996, Report of the AIAAICEAS Workshop on international Space Cooperation, Frascati, Italy, May 26-30, 1998.

[ 181 Patterson,

Walt, Power from Plants, The Global Implications of New Technologies for Electricity from Biomass, Eatthscan Publications, London, 1994.

[30]

Stan&, Michael L. et al, Space Solar Power, SAIC Report Number 98/1038 prepared for NASA, Contract NAS& 28565, December, 1995.

[ 3 I]

Leonard, Analysis Institute Fe, NM,

[32]

Nansen, Ralph, Sun Power: The Global Solution for the Coming Energy Crisis, Ocean Press, Ocean Shores, WA, October 1995.

[ 191 Strickland,

John K., Advantages of Solar Power Satellites Compared to Ground Solar Power, American Institute of Physics, 1994.

[20]

Flavin, Christopher, Windpower: But Growing Fast, World Watch, September/October, 1996.

Raymond S., ‘Net Present Value for Satellite Power Systems’. for Sustainable Futures, Santa 1992.

Small,

[2 1 ] Flavin, Christopher, and Lessen, Nicholas, Here Comes the Sun, World Watch, September/October, 1991. 1221 Mankins, John C., et al, Report of NASA Space Power Study to be published late 1996. 1231 Canough, Gay E., Space Solar Power vs. Terrestrial Solar Power, ETM Solar Works, Inc., 1996. White Paper for NASA study (Reference 22).

124 ] Grubb, M. J., “The integration of renewable electricity sources’, Energy Policy, September 1991. [25]

Erb, R. Bryan, “Power From Space - Can it Compete?‘, 45th Congress of the International Astronautical Federation, Paper IAF-94.R2.372, Jerusalem, Israel, October 9-14, 1994.

[26]

Collins, PC., “A method for utilities to assess the SPS commercially”, Proceedings from SPS 91, ParislGifSur-Yvette, France August 27-30, 1991.