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					             Justification for Space Solar Power
        Space solar power would be the largest engineering project in human
history. It merits an explanation from the perspective of human history. It is
justified both because human existence now depends on it and because there is
ultimately no realistic alternative. Both reasons are related to a world population
approaching 10 billion. Population growth is a tangible indication that the present
period in human history is unique.

Population and Agriculture
       Population growth rate can be expressed as the time for it to double. The
20 minute doubling time for bacteria is constant. The doubling time for humans is
not. Between the initial spread from Northern Africa 50-100 thousand years ago
until the start of agriculture about 10,000 years ago doubling took 30-50 centuries.
As long as migration provided new resources, the population spread until it
covered the entire globe.
       The Earth’s surface is 30% land, of which about 13% is arable2, 3. The
average world population density is now about 5 people per acre of arable land.
For arable land to sustain the population, people must use it effectively.
Agriculture began 10,000 years ago with recognition that the value of land, or any
other natural resource, depends on how people use it. Between the start of
agriculture and 2000 years ago the doubling time steadily decreased to an average
of 13-19 centuries as people learned the art of organized civilization.

        By 350 years ago the doubling time was an average of 2-4 centuries.
During the past 350 years it reached about 50 years1. The most rapid growth, and
decrease in average individual share of natural resources, coincides with the
industrial revolution. The cause and effect relationships among population,
natural resources, and individual productivity are too complex to be obvious.
They appear in the diversity of ways in which people are productive.
        A bar graph in decreasing order individual productivity, or GDP/person, in
the 150 most populous nations shows how people’s income depends on what they
do to earn it. The length of each bar is divided into three segments according to
the fractions of the work force in agriculture, manufacturing, and other
occupations, respectively. The different widths show that individual productivity
increases as occupations become more diverse.

        The agriculture work force contributes the fraction of the GDP/person
shown by the wide bottom segment of each bar. Unlike other sectors, there is little
correlation between a nation’s agricultural effort and its total productivity. The
land produces an average of $575/person regardless of the wealth of the country.
The total agricultural production of a nation depends more on the productivity of
the land than on the number of people who farm it.4

Population and Manufacturing
        Significant increases in the productivity of land have followed
developments by agriculture science during 10,000 years of agriculture. However
farmers increase their individual productivity mostly by farming more land. This
is possible because manufacturing provides tools designed for that purpose.
      The manufacturing work force contributes the fraction of the national
income per person shown as the of the middle width center portion of the bars.
The work force in areas other than agriculture and manufacturing contributes the
narrow top portion. The relation of productivity of agriculture is fundamentally
different from manufacturing. Agriculture involves the use of a specific natural
resource, arable land. Manufacturing enhances the use of whatever resources are
available, including agriculture.
      Industrialization, the start of manufacturing, was the second major human
departure from passive dependence on natural resources. It refers to activities that
rely on power to increase the value of natural resources, including transportation
as well as manufacturing. It began over three centuries ago in Europe with the
invention of heat engines which provide power equivalent to the muscle power of
thousands of humans at a small fraction of the cost. At best, a human can exert a
half kilowatt of power for a minute, or so. A large steam engine can power an
entire factory through an array of belts, pulleys, and gears. This achieves large
economies of scale. Factories centralize the manufacturing work force and
eliminate trades that are no longer competitive. Power becomes the infrastructure
of productivity if its cost is low in comparison with total productivity.
        Electric power grids appeared a little over a century ago. Although they
originated to provide light during nighttime, the broader potential value of
decentralized power was well recognized. It is simpler to transmit power by wires
than belts and pulleys. It makes power accessible at any location and simplifies
inventions to replace workers by more productive machines. This increases
productivity throughout the economic system. It also causes major changes that
affect the labor force. A few people require new skills and/or more knowledge.
The knowledge and skills of others becomes obsolete. The role of electric power
is to mediate power from different energy sources and distribute it broadly to
        The high initial cost of industrial equipment requires a different branch
economics to produce the initial capital. The people who provide the capital
generally retain control of the enterprise. Balancing the relative reward for
education, knowledge, ingenuity, labor, and risking wealth on investment remains
an unresolved political problem. Broader more transparent participation in
investing might decrease cultural resistance to capitalism. Whatever the role of
government, financing large projects must follow essentially the same economic
principles of differed payment.

World Energy Supply
        Humans are organized as sovereign nations for good reasons. The mixture
of available natural resources is different at every location. Every geographic
region must find the combination of ways to use its resources that best suits its
circumstances. Some circumstances are malleable, some are not, and others
change only with difficulty. A significant part of the differences in productivity
and individual income in different nations stems from cultural adaptability that is
properly the choice of the people.
        Until the 20th century nations exerted control over natural resources by
capturing control of territory that contained them. World War II marked a major
change away from this form of imperialism toward recognizing resources as the
property of the regions where they occur. The 20th century also marked the
growth in the importance of energy as a national resource and the recognition that
energy sources are finite and not inexhaustible.
        Electric power obeys the same economics whatever the source. The
revenues must cover the cost at a low price. Economics causes the most valuable
deposits and/or sites of significant energy to be discovered, exploited, and
exhausted first. The quality now changes significantly during the amortization
period of a major facility. New terrestrial sources are inevitably less valuable
than the sources or sites they augment or replace.
        Mineral energy sources, coal, oil, gas, and uranium, were the dominant
energy sources of the 20th century. Their impact on the environment is one of
many impacts caused by the demands of a large population. The serious question
is whether low cost of energy from renewable sources is sustainable when all
costs are properly valued. It is already clear that the future cost of mineral fuels
must increase. Over 99 percent of the energy available for use by humans on
Earth originates as solar energy. Only a small fraction of that energy is
recoverable. The demand by humans is now large enough that plans for the future
should consider the ultimate supply as well as its disposition on Earth.
       Renewable Terrestrial Power Resources (in petawatts, 10 W)
        Solar Power                                          173.
        Earth core                                             0.2
        Tides                                                  0.001
       Total input                                          173.2 PW
        Reflected albedo                                     52.0
        Thermal radiation                                   121.2
          Surface heating                    80.
          Evaporation - precipitation        38.
          Air circulation                      3.0
          Photosynthesis                       0.18
       Total output                                         173.2
          World total consumption (2000)       0.017

        Radiation by the sun is by far the greatest source of renewable terrestrial
power. A large fraction of this radiation is reflected as the albedo. The majority is
re-radiated to space within hours as thermal radiation. The atmosphere retains
another large portion as water vapor and clouds for an average period of weeks
before returning it to Earth as precipitation.
        Hydroelectric power is the result of the evaporation-precipitation cycle.
It is by far the largest source of renewable power exploited thus far. It uses the
fraction of the precipitation that is retained at high elevations. It is a renewable
source in the sense that a particular installation can deliver power for as long as
the climate at that site provides precipitation. Like mineral deposits, the best
hydroelectric sites are exploited first.
        As the result of a spate of dam building hydroelectric sites provided nearly
a fourth of U.S. power in the mid 20th century. These dams used the best sites.
The impetus for new dams subsided. The added demand due to population growth
reduced the hydroelectric share to about 7% of the total. The total world
production of hydroelectric power, about 300 GW, is less than 4 ppm of the total
evaporation-precipitation energy.
        Renewable energy sites share two disadvantages. Only a small fraction of
the total resource can be recovered and the value of new sites decreases as the
best sites are exploited.

        The ability to generate electricity is only one attribute that qualifies an
energy source to supply power to the grid. The laws of physics (nature), the laws
of economics (culture) and the laws of government (regulation) each play a role.
Physics determines whether and when an energy source can produce power.
Economics determines whether that power is useful. At their best, regulations
minimize the conflict between physics and economics to guarantee power to
consumers at a low price. The major conflict occurs because electric power
supply and demand can only occur simultaneously. Power sources can supply
power only at the time-of-use.
        A power grid operator requires access to enough power to meet the peak
demand with high reliability. The equipment of a power provider must have high
enough utilization to achieve a reasonable return on the investment in expensive
equipment. A regulator must have a plan that matches these requirements at a low
cost to the consumer. The plan must satisfy the requirements hour-by-hour, day-
by-day, and season-by-season. In an era of increasing cost of energy resources a
plan must also satisfy the requirements over the long amortization period of a
major facility. The hour-by-hour requirements are difficult for sources with
variable or unreliable timing and/or that do not match the demand.

        Hourly demand falls in three classes based on their burden on power
suppliers at different times of day. Off-peak power during the late night hours has
low value due to low demand. Peak power accounts for most of the consumption
through the middle of the day. It has high reliability, high value and the lowest
price. The steam engine sources that traditionally supply the peak demand have a
limited range over which their output is adjustable within a day, its maximum
spinning reserve. These power plants, called base power providers, are designed
to have a spinning reserve that roughly matches the peak demand. This generally
leaves a peaking power to be provided by high value sources at a high price. In
recent decades this power has been provided by gas turbine engines.

       Seasonal demand by different user classes shows more clearly how
power is used. The classifications give clues to how future power utilization
might change. The change depends critically future population growth and on
how much non-electric power use shifts to electric power.
       The seasonal power demand has two peaks. A summer peak lags the
summer solstice by about six weeks. The peak is attributed to cooling by air
conditioners. The lag is consistent with thermal inertia of the heat capacity of the
Earth. The peak that coincides with the winter solstice is smaller because most
heating is provided by fossil fuels.

        Residential demand accounts for most of the variability in demand and
its correlation with air conditioning. A correlation between the air conditioning
and labor saving devices in a residence with individual productivity probably
exists even though it is indirect and hard to demonstrate.
        Commercial demand involves most of the same people as residential
demand in larger buildings with different kinds of equipment. A part of the
difference in variability is the smaller heat exchange rate by the smaller surface-
to-volume ratio of larger buildings.
        Industrial demand affects a smaller number of people and has a more
direct connection between power and productivity.
        Government demand for power is mostly by municipal governments for
water supply, sewerage disposal, and public lighting. This has little seasonal
        Transportation might well consume much more electric power as the
cost of petroleum continues to increase. This could occur directly through mass
transit and/or indirectly through electrolytic fuels such as hydrogen. Applications
of electric power that are not time sensitive could increase the value of off-peak
power and power from unreliable sources, such as wind and solar.
        Conservation can and should play an important role, but the forces that
are likely to increase the demand for power appear much greater than the
opportunities to conserve. For example, much of what appears to be profligate use
of light is stimulated by the low price of off-peak power that must be consumed
regardless of price.

World Energy Demand
        Humans can exert up to about a half kilowatt for a minute, or so. A
machine can exert the power of a thousand men continuously with the advantages
in skill of a well designed tool. Consumption in developed nations averages a
kilowatt per person continuously. A world average consumption of a half
kilowatt/person is a reasonable minimum expectation. A conservative working
estimate of the minimum future demand is 5000 GW.
        The correlation between individual productivity and electric power
consumption is clear. The vertical bars in the following 3-dimensional bar graph
are populations of geographic regions5. The base of the diagram plots the GDP
per person as a function of electric power consumption per person. The diagonal
line across the base has a slope of $25,000 of individual GDP per kW of electric
power per person. For retail electric power at a cost of 10 cents per KWh, the
average national income is about 28 times the total expenditure for electric power.

        At a cost less than 4% of GDP, electric power qualifies as infrastructure
for productivity, even if some uses are frivolous. The figure shows that the bulk of
the world population consumes a much smaller quantity of electric power per
person than the U.S. and Europe.

Future demand for electric power
        In a long view, unsustainable population growth is now driving events.
The effect of infrastructure on productivity depends on human ingenuity. A
credible plan depends on the logical arithmetic. About a dozen terrestrial energy
sources deserve consideration. Only a fraction of these qualify in any particular
region. A comparison of terrestrial solar power with that transmitted to Earth from
a geostationary orbit in space is an illustration.

         Space solar power provides a constant 1.37 GW/km2 except for a 2 hour
eclipse period at midnight of each equinox. At 40° north latitude, the center of the
zone of greatest human productivity, there is no terrestrial solar power for 9-16
hours every day depending on the season. Under a clear sky it is a factor of 3-5
less than space solar power depending on the season. It decreases with path length
through the atmosphere and density of gases and particles that absorb or scatter
light. This decreases the average radiant power by another factor of 3-8. The fact
that the decrease is variable and unpredictable is more important than the amount.
Terrestrial solar power is not feasible without extensive backing by other sources.
Unreliable power can reasonably be expected to bear the cost of any decrease in
utilization of reliable sources.
         None of this proves that space solar power is feasible. It is even more
difficult to prove that it is not feasible. The correct question to consider is “What
conditions would make space solar power feasible?” This remains to be shown.

Notes and references
1. Joel E. Cohen, How Many People Can the Earth Support, W.W. Norton & Company, New
York, 1995
2 Reasons for the pattern of origin and spread of global agriculture derived from the anthropology
of different people and the agriculture they developed is presented and documented by Jared
Diamond, Guns, Germs, and Steel, W.W. Norton, Inc, 1999
3. Single species crops reduce bio-diversity and marginal farmland causes more soil erosion than
more productive land. Farming land more marginal than that now being cultivated might cost
more in erosion and lost bio-diversity than would be gained. Dennis A. Avery, Hudson Institute
4. Note that to feed their large population, Chinese farms produce several times more per acre than
the U.S. by growing several crops per year.
5. Data for the different geographic regions is a weighted average of individual nation taken from
the 150 largest nations. In the U.S. Central Intelligence Agency Factbook 2004

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Edward J. Bair
Department of Chemistry
Indiana University
Bloomington, Indiana 47405
Last revised 7/6/08