Space Power Resources, Manufacturing and Development Volume 10 Number 2 1991
SPACE POWER Published under the auspices of the Council for Social and Economic Studies EDITOR Andrew Hall Cutler, NASA Space Engineering Center, The University of Arizona ASSOCIATE EDITORS Roger A. Binot, European Space Agency, The Netherlands Eleanor A. Blakely, Lawrence Berkeley Laboratory, USA Richard Boudreault, Consultant, Montreal, Canada Lars Broman, SERC, Sweden Gay Canough, Extraterrestrial Materials, Inc., USA Lucien Deschamps, Paris, France Ben Finney, University of Hawaii, USA Josef Gitelson, Academy of Sciences, USSR Peter Glaser, Arthur D. Little, Inc., USA Aven Gwynn, Mars Center for Exploration, Moffet Field CA. Praveen K. Jain, Northern Telecomm, Ottawa, Ont., Canada Dieter Kassing, ESTEC, The Netherlands Mikhail Ya. Marov, Academy of Sciences, USSR Gregg Maryniak, Space Studies Institute, USA Michael Mautner, University of Canterbury, New Zealand Makoto Nagatomo, ISAS, Japan Mark Nelson, Institute of Ecotechnics, USA John R. Page, University of New South Wales, Australia Geoffrey Pardoe, Brunel Science Park, UK Tanya Sienko, NASDA, Tsukuba, Japan Ray A. Williamson, OTA/US Congress, USA Space Power is a quarterly, international journal for the presentation, discussion and analysis of advanced concepts, initial treatments and ground-breaking basic research on the technical, economic and societal aspects of: large-scale spaced-based solar power, space resource utilization, space manufacturing, space colonization, and other areas related to the development and use of space for the benefit of humanity. Papers should be of general and lasting interest and should be written so as to make them accessible to technically educated professionals who may not have worked in the specific area discussed in the paper. Editorial and opinion pieces of approximately one journal page in length will occasionally be considered if they are well argued and pertinent to the content of the journal. Submissions should represent the original work of the authors and should not have appeared elsewhere in substantially the same form. Proposals for review papers are encouraged and will be considered by the Editor on an individual basis. Editorial Correspondence: Dr. Andrew Hall Cutler can be reached by telephone at (602) 322- 2997, by Facsimile at (602) 326-0938 and by mail at 4717 East Fort Lowell, Tucson, AZ 85712, USA. Dr. Cutler should be consulted to discuss the appropriateness of a given paper or topic for publication in the journal, or to submit papers to it. Questions and suggestions about editorial policy, scope and criteria should initially be directed to him, although they may be passed on to an Associate Editor. Details concerning the preparation and submission of manuscripts can be found on the inside back cover of each issue. Business correspondence including orders and remittances for subscriptions, advertisements, back numbers and offprints, should be addressed to the publisher: The Council for Social and Economic Studies, 6861 Elm Street, Suite 4H, McLean, Virginia 22101. The journal is published in four issues which constitute one volume. An annual index and title-page is bound in the December issue. ISSN 0883-6272 © 1991, SUNSAT Energy Council
Volume 10, Number 2, 1991 M. Hoffert, S. Potter, M. Kadiramangalam & F. Tubiello. Solar Power Satellites: Energy source for the Greenhouse Century?1 131 Lyle M. Jenkins. Countermeasures for Mitigating the Effects of Global Environment Changes1 153 Charles L. Owen. Project Phoenix: Confronting Global Warming with Solar Power1 157 Albert A. Harrison & Joshua Summit. How "Third Force" Psychology Might View Humans in Space 185 Dana Rotegard. Fusion, Cold Fusion, and Space Policy? 205 Robert C. Richardson, III. Prospects for Inexpensive Space Transportation1 217 Geoffrey A. Landis & Joseph Appelbaum. Photovoltaic Power Options for Mars 225 S. Ortner & S. Krause. Heat Transfer Predictions for a New Heat-Pipe Latent-Heat-Storage Receiver Element for Solar Dynamic Space Power Systems 239 t Papers presented at SPS '91: Power from Space, a meeting sponsored by the Societe des Electiciens et Electroniciens, and the Societe des Ingeneurs et Scientifiques de France; organized by Lucien Deschamps; and held at 1'Ecole Superieure d'Electricite in Gif-Sur- Yvette. SPACE POWER
University of Arizona / NASA Space Engineering Research Center for the Utilization of Local Planetary Resources Extraterrestrial Resources: Stepping Stones to the Stars Innovations in space technology are needed if our dream of settling other planets in the solar system is to become reality. The idea of "mining" and processing resources on another planet to support man's presence there and his travels in the solar system may help make this dream possible. The UA/NASA Space Engineering Research Center, a national center for space engineering, research, and educaton, seeks out the means to make habitation on Earth's moon and other planets affordable by using the resources found there. Research at the Center focuses on making useful products—water, fuels, building materials—from materials (and energy) that occur naturally in near-Earth space. Offering numerous research opportunities for students interested in the space resources field, the Center also serves as a meeting ground where government agencies and private sector companies can create strategies for establishing industries in space, and develop the hardware to do the job in the severe environments near-Earth planetary bodies present. Research at the Center emphasizes: ♦ Investigating methods of producing oxygen and hydrogen for rocket propellant and other products from Lunar and Martian materials ♦ Developing artificially intelligent control and communication systems for remote materials-processing plants designed to operate autonomously ♦ Defining the conditions necessary to optimize the production of such materials while minimizing energy consumption ♦ Searching the skies and tracking near-Earth asteroids that may also provide valuable resources for space exploration and development Student Research: The Center offers abundant research opportunity, in which both graduate and undergraduate students participate extensively, as well as course work in numerous engineering and science departments. Ongoing areas of student research include: ♦ Fuel for space exploration. The Center is exploring ways to extract oxygen from materials found on the Moon, Mars, and asteroids that could then be used in the production of chemical propellants for spacecraft. ♦ Oxygen from Carbon Dioxide. The Center has produced a pilot plant engineering demonstration system: oxygen production from a simulated Martian atmosphere. ♦ Building Materials. After Lunar soil has been mined for its oxygen, for example, it can be processed into bricks, beams, metals and ceramic/composite materials from which habitable structures and a variety of useful products can be constructed. Education Programs: Students at the Center are also actively involved in reaching out to the surrounding community, providing elementary, middle, and high school students the opportunity to work on its experiments, a hands-on approach to student involvement directed toward attracting the nation's youth to technical careers. Publications: The Center provides information services, at no charge, to interested individuals and organization. For a complete list of available publications and further information on the Center, contact: Dr. Terry Triffet Space Engineering Research Center 4717 East Fort Lowell Rd. / Tucson, Arizona 85712
Solar Power Satellites: Energy Source for the Greenhouse Century? MARTIN I. HOFFERT, SETH D. POTTER, MURALI N. KADIRAMANGALAM & FRANCESCO TUBIELLOf SUMMARY Energy is needed to produce wealth, and an increasing world population will need increasing amounts of energy to improve its standard of living. Through the use of a carbon cycle model, it is shown that continued reliance on fossil fuels will cause a global greenhouse warming. An energy-CO 2-economics model is used to project future demand for fossil-fuel-generated energy. When this demand is compared with the fossil fuel use that is permissible if a global warming is to be avoided, a shortfall in energy becomes evident. Terrestrial photovoltaics, nuclear fission, nuclear fusion, and the solar power satellite (SPS) are examined as means of making up this energy shortfall. On comparing these technologies, the SPS appears to be the most feasible means of providing the required energy and preventing a global warming. Laser, 2.45 GHz, and 35 GHz SPS technologies are intercompared, and results indicate that the 2.45 GHz technology remains the most feasible SPS option. Introduction Anthropogenic CO2 emissions, predominantly from fossil fuel combustion, have increased the CO2 concentration in the atmosphere from a pre-industrial value of about 270 parts per million (ppm) to about 350 ppm today. Climate models predict that the Earth will warm by 1.5 to as much as 4.5 °C due to a doubling of CO2 in the atmosphere. Possible consequences of a greenhouse effect range from a rise in the sea level, causing flooding of lowlands, to a shift in weather patterns. Many schemes have been suggested to decrease the amount of CO2 being put into the atmosphere, including injecting the CO2 emissions into the ocean,1 afforestation, fertilizing the Antarctic waters to increase carbon uptake, and some more esoteric schemes such as preventing the melting of the polar ice caps and changing the Earth's surface albedo by injecting aerosols into the atmosphere. Some of these schemes are extremely energy intensive, others are only curative in nature, and some may have disastrous "side-effects" on the climate system. The only reliable path toward preventing a global warming is the reduction of fossil fuel combustion over the next few decades and the next century. 1 Department of Applied Science, New York University, 26-36 Stuyvesant Street, New York, New York 10003, USA.
The standard of living (gross national product per capita) that can be developec and maintained in a country is closely tied to the amount of energy per capita i consumes. With most of the population expansion of the world taking place in the third-world, these developing and under-developed nations will attempt to increas< their standard of living through the next decade and well into the next century. Thi: will be done mostly by the combustion of fossil fuels, which will add to the emission: and the greenhouse effect. Thus, there are two conflicting needs in the work energy economy: (1) the energy needs of expanding populations seeking a bettei standard of living, which could create an upward trend in fossil fuel combustion am CO2 emissions; and (2) the need to limit global warming, which would require < downward trend in fossil fuel combustion and CO2 emissions. It is first necessary tc calculate the shortfall in energy supply that occurs because of these two factors tha drive the world energy economy in two different directions, which is the main theme of this paper. This paper goes somewhat beyond this and investigates the choices tc make up this shortfall in energy supply. The alternative energies considered here include terrestrial solar photovoltaics, nuclear fission, fusion, and solar powei satellites. The Greenhouse Effect For approximately the past 200 years, the economic development of the industrialized nations has been fueled by increasing amounts of fossil energy sources. In recent years, there has been increasing concern about the possible environmental effects of the emission of carbon dioxide resulting from fossil fuel combustion. The
increasing level of CO2 emission is well documented and is depicted in Figure 1. The increase of CO2 emissions has been most drastic during the past 50 years and now stands at approximately 6000 million metric tons of carbon per year.2 It is likely that unless non-fossil fuel energy sources are developed soon, the rate of emissions will continue to rise. The consequence of this anthropogenic CO2 emission is an increase in the CO2 concentration in the atmosphere. All fossil fuel emissions do not directly contribute to an increase in concentration, because there is complex carbon cycling through several so called "reservoirs" in the Earth system, namely the atmosphere, the biosphere, the ocean, and the fossil fuel inventory. This "carbon cycle" controls the CO2 concentration in the atmosphere by processes involving chemical equilibrium between the ocean and the atmosphere. Global climate models (GCM's) predict that a doubling of atmospheric CO2 concentration will lead to an increase in global mean temperature between 1.5 and 4.5°C due to the greenhouse effect. The greenhouse effect can be understood better by examining the radiation budget of the Earth. A small fraction of the short wavelength electromagnetic radiation emitted by the Sun is either absorbed or reflected to space by the Earth's atmosphere. The rest of the short wavelength radiation makes it to the Earth and is absorbed and converted to heat. The Earth loses heat both by evaporation (the hydrologic cycle) and by radiating the heat as long wavelength infrared radiation. The atmosphere warms due to solar and Earth radiation, which it radiates both to space and to the Earth. This radiation warms the Earth. CO2 is one of the main gases which traps the heat in the atmosphere. The atmosphere behaves like a greenhouse that traps sufficient heat to enable the growth of plants. Figure 2 shows global-mean temperature change for the period 1861 to 1989 relative to the average for 1951 to 1980, and is obtained from the assessment of the Intergovermental Panel on Climate Change (IPCC).3 This global-mean temperature is a combination of the temperature measured over land and sea and is smoothed. The graph shows that the Earth has warmed by about 0.5°C in the last 130 years. Most of the warming occured between 1910 and 1940 and then after 1975. 1990 was the warmest year ever4 and the warmest five years before that were in the 1980's. It is uncertain whether the warming of the last century can be attributed to the natural variability or the greenhouse gases. An unambiguous greenhouse warming may not become apparent for several more years. However, our investigations of the effect of continued reliance on fossil fuels show that it is likely to become quite significant during the next century. What would be the consequences of such a warming? Many climatologists have written extensively about the consequences of a global warming and we provide a summary here. One of the primary effects of global warming is expected to be the drying of continental interiors, with significant negative consequences for agriculture. For example, Hansen, et. al.,5 using the Goddard Institute of Space Studies Global Climate Model (GISS GCM), suggest that a doubling of atmospheric CO2 concentration would cause hot dry conditions in much of the western United States, Canada, and major parts of central Asia. The increased warming could also cause a melting of glacial ice, which may lead to an increase in sea level and lowland flooding in many parts of the world. Many authors
have suggested that the western Antarctic ice sheet could collapse, resulting in a major sea-level rise. It has been suggested by numerous authors that there is a correlation between energy consumption and gross national product per capita. This issue was considered by us in some detail and in this paper it is assumed that gross national product is a good indicator of the standard of living. Data for gross national product per capita, population, and energy consumption per capita are from Reference 6. These data sets were aggregated into nine groups of countries in accordance with the groupings developed by Edmonds and Reilly.7 The following nine groups, comprising the entire world, were used: 1. United States (US) 2. Western Europe and Canada (WEC) 3. Japan, Australia, and New Zealand (JANZ) 4. Eastern Europe and the Soviet Union (EUSSR) 5. Centrally Planned Asia (AC) 6. Middle East (ME) 7. Africa (AFR) 8. Latin America (LA) 9. South and East Asia (SEAS) The data shows that for both GNP per capita and energy consumption per capita, region 1 had the largest value and region 9 had the smallest value. The disparity in the standard of living and the respective energy consumption is quite large, with a ratio of values for these regions of 39:1 and 59:1 respectively. In order compress the range for plotting, the values for the nine regions were normalized to U.S. = 1, and then the logarithm (to the base 10) was taken. A least squares straight line fit was calculated for these log normalized values (see Figure 3). In doing this fit, data for
the nine regions were weighted by their population. Figure 3 shows the log normalized energy consumption per capita versus the log normalized GNP per capita. The data points lie close to the line, demonstrating a correlation between energy consumption per capita and GNP per capita. Carbon Cycle Modelling If the world is to satisfy its growing energy needs through fossil-fuel-generated energy, it is necessary to predict the resulting climate change (if any), in order to determine if non-fossil fuel alternatives should be developed, and, if so, to what extent they will have to be deployed. The way we have done this is by determining a limit on the allowable fossil fuel combustion (or emission) that will be permissible when the greenhouse constraint is imposed. A useful tool in this endeavor is a carbon cycle model. Such a model calculates the change in atmospheric CO2 concentration that results from the emission of CO2 into the atmosphere (e.g., from fossil fuel combustion). Wigley has devised an elegant carbon cycle model (henceforth referred to as the forward model) that he has inverted.8 The inverse
model calculates the emission of CO2 that would lead to a given concentration. Since fossil fuel energy production relates directly to emission of CO2, and atmospheric concentration of CO2 relates to global warming, these models can be used to relate fossil fuel energy production to global warming. The forward model can thus be used to predict the globally averaged temperature change that would result from a given emissions (or energy) profile. The inverse model can be used to give an emissions (or energy) profile that would cause a given temperature change. Wigley's work is based on earlier work by Maier-Reimer and Hasselmann.9 Atmospheric concentration of CO2 is given by the following convolution integral: C(t) = atmospheric CO2 concentration E(t) = Rate of CO2 emissions G(t) = impulse response (Green's) function t = time u = variable of integration. Maier-Reimerand Hasselmann calculated G(t) for three different pulse injections of CO2. Wigley's models interpolate between these three impulse functions as the total CO2 emissions range between the values of the three impulse injections. Total CO2 emission is given by: In this integral, t = 0 refers to the beginning of anthropogenic CO2 emissions (in the year 1770). The impulse response functions are of the form: where aj are numerical coefficients and tj are decay times. The fraction of emissions that remains permanently in the atmosphere is given by a0. Note that Equation 3 represents the form of each of the three impulse response functions. Thus, the models use a total of 15 different a's and 12 different t's. The impulse response function represents the ability of the oceans to take up CO2 from the atmosphere. Primary electricity was subtracted from the total energy requirement in conventional fuel equivalent to obtain fossil fuel use for the year 1986 (Reference 10). The total CO2 emissions for 1986 were then considered and the small contribution from where
cement production was subtracted.11 The conversion factor from CO2 emissions to fossil fuel generated energy production was obtained by taking a ratio of the fossil fuel consumption to the CO2 emissions. The year 1986 was used because it was the most recent year for which data was available. The relationship between atmospheric concentration and global warming was based on an equation from Berner, et. al.,12 and is given by: where C(t) = atmospheric CO2 concentration t years from today C(0) = atmospheric CO2 concentration today n = rate of temperature change in degrees per year s = climate sensitivity in degrees Celsius. Note that here, the climate sensitivity is the temperature change that results from the CO2 concentration increasing by a factor of e, not 2. In our modelling, the best estimate of 2.5°C for CO2 doubling was used? To obtain s for Equation 4, this was divided by ln(2) to give 3.6°C. For the inverse model, n was specified as an input, CO2 concentration was calculated from Equation 4, emission was calculated by using the inverse of Equation 1, and then emission was converted to energy. Here, C(0) is the concentration for the year 1990. The results are plotted in Figure 4. For years before the present, the concentration is based on observed data, so that the graph shows implied fossil fuel energy production. For 1991 and beyond, the graph shows the maximum allowable fossil fuel energy production which would limit global warming to the rates indicated. Note that in order to hold the globally averaged temperature to its 1990 value, fossil fuel energy production must immediately be cut by approximately two-thirds, and then must continue to decrease over the next century. This scenario confirms Wigley's emission results for the case in which the future CO2 concentration is held to its 1990 value. If we allow a 1°C per century global warming, then fossil fuel energy production must immediately be cut by approximately one-third, but it can then be allowed to slowly increase over the next century. These energy scenarios show a discontinuity at the year 1990 because we are now using fossil fuels at a rate consistent with a global warming greater than 1°C per century. By comparing the slope of the 2°C per century curve with the slope of the curve prior to 1990, it is seen that at current rates of fossil fuel use, a considerable amount of global warming is likely during the next century. Of the three curves in Figure 4, only the 2°C per century scenario represents an energy scenario that can be achieved without an immediate and drastic energy production decrease. Setting strict limits on global warming thus results in some rather contrived and unrealistic fossil fuel energy production profiles. It would therefore make more sense to set limits on fossil fuel energy production and then examine the effect on climate. This was done using Wigley's forward model, with an input fossil fuel use that varies by a fixed percent per year. To accomplish this, Equation 4 was solved for n; this temperature increase rate was then integrated over time to give a temperature difference from 1990. Thus,
where n(t) = temperature increase rate during year t in °C per year s = climate sensitivity in °C C(t) = CO2 concentration at the midpoint of year t C(t-l) = CO2 concentration at the midpoint of year t-1. Figure 4. Allowable annual fossil fuel energy production calculated for the three global warming scenarios indicated in the graph. Prior to 1990, energy production is inferred from observed atmospheric CO2 concentration. Equation 5 is a result of applying Equation 4 to a single year and then solving for n; thus, the right side of Equation 5 can be thought of as being divided by one year. The units of both sides of Equation 5 are therefore consistent. The temperature difference from 1990 for year t, called AT(t), is given by: The value of AT(t) has been initialized at 0 for the year 1990. Figure 5 shows AT(t) for 1990-2100 for five fossil fuel use (and thus emission) scenarios. These scenarios include a two percent and one percent increase as well as a two percent
and one percent decrease of the fossil fuel emission each year from the 1990 value, and a scenario in which the emission remains constant at the 1990 value. Since we are applying a steady-state temperature model to a transient scenario, our results for temperature increase are somewhat higher than they should be for the larger fossil fuel use increases. For fossil fuel use changes that are small positive, zero, or negative, steady state and transient results are similar. It must be kept in mind, however, that all of our results are somewhat optimistic, since only the effect of CO2 emission is considered in this model. However, CO2 is not the only greenhouse gas that results from fossil fuel use. Thus, increases in fossil fuel use should be avoided, as they will result in several degrees of global warming by the year 2100, as seen in Figure 5. If fossil fuel use in the years after 2100 rises at one or two percent annually, then temperature will continue to rise. If fossil fuel use is held constant at 1990
levels, then a little over 1°C of temperature increase will occur by the year 2100, but the increase will eventually level off. If fossil fuel fuel use decreases, the temperature increase during the 21st century will be fairly small, and temperature will eventually level off to its pre-industrial value. Keeping in mind the somewhat optimistic nature of these models, a 1% annual decrease in fossil fuel use was chosen as a basis for comparison with projections of future world energy needs. This 1% annual decrease can be thought of as a constraint on fossil fuel use that is necessary to avoid a serious alteration of the Earth's climate. Projection of Future World Energy Demand In order to compare constraints on fossil fuel energy production with world energy demand, a projection was done using a computer model. The model used here is the Oak Ridge long term global energy-CO2 model, developed by Edmonds and Reilly7 of the Institute for Energy Analysis of Oak Ridge Associated Universities; it provides assessments of the CO2 emissions due to fossil fuel use, as well as a host of vital information regarding the world economy's many possible scenarios.13 The computer model divides the world into nine different regions mentioned earlier, based on their energy resources and reserves, economic and technical compatibility, social similarities, and geographic proximity. Five benchmark years are chosen for the projections, which start in 1975: 2000, 2025, 2050, 2075, 2100. For each of these periods, the model sets up a balance between total energy demand and energy supply, giving as a result the projected CO2 emissions. This is done through four interacting modules: Supply, Demand, Balance, and Emission. The Supply module works on such data as the price of extraction of the primary fossil fuels: oil, coal, and gas; and the prices of transportation, refining, and production of electricity from the above. These are used to forecast a market price for each of the benchmark years and for each of the regions. In addition, production of energy through terrestrial solar, nuclear, and hydroelectric power is calculated. The Demand module uses such information as population growth and technology improvement parameters to forecast regional and global GNP, and, therefore, the energy demand. If the projected total supply and demand for a particular period do not match, the Balance module perturbs the initial input prices until global balance is obtained. The calculation of carbon emissions, once the production of fossil fuels is known, is carried out by the Emission module through the use of fuel-burning coefficients. The four modules work on a pre-specified set of data, which are often specified for each of the six forecast periods and for each of the nine areas mentioned above, and that can be changed with the aid of a computer editor. A default set of such data, contained in the model version we have been using, has permitted us to perform a basic run of the program and to obtain valuable information. In particular, we were interested in world energy demand as well as GNP per capita projections. The results are shown in Figures 6 and 7. In Figure 6, energy demand (in gigajoules per year) is plotted versus time (years), starting from 1990. We have plotted the total demand for fossil fuels (oil, gas, and solids), together with the
projected total energy demand. The gap between total energy demand (which reaches about 2.5 x 1012 gigajoules per year in 2100) and fossil fuel demand corresponds to the use of nuclear, terrestrial solar, and hydroelectric sources. Figure 7 shows GNP per capita, in thousands of 1975 US $, versus time in years, starting from 2000, for each of the nine regions, together with the world average GNP per capita. The abbreviations on the horizontal axis refer back to the aggregates of countries in the section on Energy and GNP. The numbers on the bar graph refer to per capita GNP values of the appropriate group of nations for the year 2100. Well into the twenty-first century, the Middle East seems to have the largest per capita GNP, while South and East Asia has the lowest value of about US$ 11,000 per capita; world average (referred to as TOT) is about US$ 29,000 per capita. The disparity between the wealthiest and poorest groups of nations reduces from a factor of 39 to a factor of 12 between now and the next century.
Constraints on World Energy At this point in time, world energy is constrained by three factors, namely the environment, the resources, and demographics and economics together. While energy policy should be formulated within the constraints imposed by these factors, more often than not, in most countries energy policy has been dictated by market forces. The fact that fossil fuel resources are limited and that alternative energy development should be a priority was recognized in the brief interlude following the oil shock of the 1970's. With the advent of the mid 80's came the era of a glut of cheap oil and the alternatives were tossed away. One may probably even speculate that some potentially major conflagrations could have been avoided if reliance on Middle East oil was done away with in the last three decades. Numerous studies have examined the world energy problem from the viewpoint of the finite resources of both fossil and nuclear energy, and we only wish to state that it is a key issue, but is beyond the scope of this paper.
In the earlier sections of this paper, we have tried to investigate two of the three constraints, namely the environmental and economic. The economic argument is closely linked to the demographics issue. The population of the world is expected to increase significantly during the 21st century. Most of this increase will occur in the developing nations. Currently, some of these nations lag behind the developed nations by more than an order of magnitude in both per capita energy consumption and per capita GNP. In order to improve the standard of living of the developing nations and provide for an increasing population, world energy production will have to increase greatly. If this energy is produced by fossil fuel combustion, the atmospheric concentration of CO2 will increase, leading to global warming due to the greenhouse effect.
In the previous sections, it was pointed out that in order to avoid greenhouse warming, a one percent per year reduction of CO2 emissions will have to be put in place almost immediately. This reduction in CO2 emissions translates into a cap on the amount of fossil fuel combustion permissible, and this is shown in Figure 8 (the curve is labelled allowable fossil fuel use). Also shown in Figure 8 is the total fossil fuel demand that would be necessary to fuel the economy of the world. When these two curves are compared, a shortfall in energy becomes apparent and this shortfall will have to be made up for with the use of non-fossil fuel energy sources. Non-Fossil Energy Alternatives The alternative energy sources to fossil fuel combustion considered in this study are terrestrial solar photovoltaics (PV), nuclear fission, nuclear fusion, and solar power satellites (SPS). Solar energy available at the Earth's surface, when averaged over the day/night cycle, the seasons, the different latitudes, the 50% average cloud cover of the Earth, and even some attenuation in clear air, is perhaps one-tenth of that available in space. Great progress in conversion efficiency and costs of solar cell modules has been made in recent years. The highest efficiency achieved by a non-concentrating experimental cell was 30%,14 while 10 to 12% efficiency for large operational modules seems to be typical. The cost of electricity generated by photovoltaics stands at 30 cents per kWh, about a factor of five more than the cost of power generated by conventional utilities. The major problem of terrestrial PV is not the installed cost of the solar cell modules but storage cost and the intermittent nature of solar insolation. The storage battery that is most preferable for terrestrial photovoltaics is a lead-acid battery designed for deep discharge,15 and at this point is the best technology available. Costs of the lead-acid battery are almost 400 times the average price of the solar cells per kWh. Furthermore, the best of these are limited to about 1500 charge-discharge cycles. Consequently, battery lifetime is less than the lifetime of the PV modules themselves requiring that they (the batteries) be replaced approximately every four years. Since these batteries are expensive to begin with, it is likely that energy storage is the economic bottleneck for terrestrial solar energy. Hubbard16 points out that a major problem with PV is the fluctuations in the power output due to sudden changes in the cloud cover. Furthermore, during overcast sky conditions and the night, there would be no power produced, and these problems, Hubbard feels, might lead to problems in the interfacing of PV with the electrical grids. Nuclear fission power has the advantage of being a well-developed technology that is already being used. However, a great deal of controversy remains as to the safety of nuclear power plants. Waste disposal as well as security concerns about the production and shipping of ever-increasing amounts of fissionable material are also issues that remain to be solved. Most of the fissile material that is used in conventional nuclear reactors is an oxide of 235U. The abundance of 235U in natural uranium is about 0.7 % and the rest is mainly 238U. Thus it is apparent that with a limited supply, conventional reactors (henceforth referred to as burners) cannot be utilized to make up the shortfall of energy and hence, breeder reactors will have to
be used if nuclear energy is to become the alternative to fossil fuels. Breeders convert the 238U to 239Pu (plutonium) which then undergoes fission to produce energy. A term commonly used by designers of breeders is the concept of doubling time. Waltar and Reynolds17 define reactor doubling time as "the time required for a particular breeder reactor to produce enough fissile material in excess of its own fissile inventory to fuel an additional reactor", and this time is on the order of 15 to 20 years. Thus, a breeder program akin to the French would already have to be in place if breeder reactors are going to make up the energy shortfall in the future. This is because most countries are exploiting their uranium resources in burners, and by the time breeders would be deployed, sufficient amount of 235U would not be available to breed the 238U. Nuclear fusion power has several advantages over fission. There is no danger of melt-downs, radioactive wastes are short-lived, and the deuterium or tritium fuel can be extracted from sea water. Furth18 reports that the ratio of fusion output
power to heating input power (called Q) must be about 30 for an economical power reactor, and that the power levels of fusion experimentshave become inconveniently high as Q has increased. Figure 9 shows the output power of fusion experiments versus the input power, where Q is approximately 1 in recent experiments. A commercial reactor would have an output of perhaps a gigawatt, which Furth predicts will come on line around 2020 and will be the largest engineering endeavor ever undertaken. Nuclear fusion thus will have to be a large-scale operation, and may be many years away from being feasible. After almost 40 years of research worldwide, with huge research budgets (for example, the U.S alone spends about $400 million each year), scientific and technical breakthroughs will have to be made before fusion power becomes a reality. In light of the above discussion, it appears that the most promising alternative that could make up the energy shortfall in the decades to come is the solar power satellite (SPS). The SPS is a satellite in geostationary orbit (GEO), 35,800 kilometers above the equator, which collects solar energy on arrays of photovoltaic panels and beams the energy to Earth using either microwaves or lasers. The energy is received on the Earth and is routed to users by electric power lines. The advantages of the SPS are that it uses proven technologies, and does not produce greenhouse gases or nuclear waste. Solar Power Satellite Technology Options Three SPS technologies are compared, which differ in their means of generation and transmission of power. These designs include a 2.45 GHz system similar to the NASA-DOE SPS reference design, a 35 GHz system, and a laser system. The mass of the reference design is about 50,000 metric tons for a 5 GW power level (see for example reference 19). Recent studies conducted at NASA's Langley Research Center have compared mass estimates for advanced laser power generation and beaming systems.20 On the basis of their comparison, we conclude that a laser SPS would be several times the mass of an SPS that would use microwaves. Thus we do not examine the laser option, though it might be pointed out that recent successes in concentrators might be able to reduce the mass involved.21 The Space Studies Institute's study19 investigated the feasibility of construction of the SPS using lunar materials. The reference design uses transmission through the atmosphere's microwave window at a wavelength of approximately 12 cm (2.45 GHz). The diffraction pattern for a 2.45 GHz beam was calculated assuming a quadratic aperture, and is shown in Figure 10. A rectenna large enough to capture 88% of the energy would have dimensions of approximately 9.7 x 9.7 km. An exclusion boundary set at a distance where the microwave intensity tapers off to 0.1 mW/cm2 would be a distance of approximately 6.5 km from the central maximum (assuming that the SPS delivers 6 gigawatts of power to the surface of the Earth). A question might be raised as to the value of this calculation, as extremely detailed calculations, designs, and simulations were done during the NASA-DOE SPS program. However, this calculation was done as a benchmark to compare it with the 35 GHz case.
At the frequency of the 35 GHz atmospheric window, for a given amount of power transmitted, the amount of land needed would appear to be less as compared to the 2.45 GHz case. This would make building rectennas near populated areas (where the power is to be used, but where less land is available) more feasible. Using the same 6 GW transmitted as in the case of 2.45 GHz, the diffraction pattern was calculated for reception at the equator, and is shown in Figure 11. Note that the width of the 88% capture area has decreased to 0.7 km (i.e., by a factor of 2.45/35), so that only about l/200th as much land is apparently needed. However, due to the increased height of the secondary diffraction maxima, the 0.1 mW/cm2 exclusion zone would be a distance of about 3.3 kilometers from the center. Thus, the savings in land is not as large as anticipated, though it might be possible that using an optimized gain taper might reduce the land area in the 35 GHz case. Furthermore, the assumption that the same 6 GW can be concentrated into a
smaller land area may be environmentally unsound, since the peak microwave intensity would increase from 23 mW/cm2 (for 2.45 GHz) to 4600 mW/cm2. If the size of the SPS were scaled down so that the peak intensity of the 35 GHz beam was 23 mW/cm2, then the power leaving the transmitter would have to be restricted to about 32 MW, because of which more SPS units would be needed to make up the shortfall, so that there would be no savings in land area. However, if the demonstration of SPS technology is the goal that is set forth, 35 GHz technology might be considered as an additional option, since the units would be smaller and might more easily be financed and constructed. However, microwave lenses in space have been suggested as a means of increasing the effective aperture of the SPS transmitting antenna.19 This would allow for smaller rectennas, while retaining the 2.45 GHz frequency. The long-term feasibility of 35 GHz power transmission is limited by rain, cloud, and atmospheric attenuation. For a typical temperate zone rainfall rate of 5 mm/hour, the transmission efficiency is only 17% (our calculation was based on Ref.
22, page 39, Figure 29). Similarly, atmospheric transmission at 35 GHz is approximately 90 to 95% (our calculation was based on Ref. 22, page 39, Figure 28a). The overall power link efficiency will be lower at 35 GHz at this point in time as both DC-to-RF and RF-to-DC conversion efficiencies are lower than at 2.45 GHz. With time, 35 GHz technology will probably approach 2.45 GHz efficiencies, but the attenuation problem cannot be done away with. Thus, either the total SPS generating capacity would have to be increased, and a ground based energy storage system developed, or additional rectennas would have to be constructed. The latter proposal would involve building more than one rectenna for each SPS unit, so that the beam can be redirected on rainy days. This would involve additional land area, added complexity in the SPS units themselves, as well as additional transmission losses along the ground as power is transmitted from a rectenna located where rain is not falling to users located where rain is falling. Thus, further development of 35 GHz power transmission does not appear to be desirable for SPS, although it may have space-to-space or Earth-to-space applications and other potential applications. The NASA-DOE SPS study was terminated in 1980, with the National Research Council estimating that the cost of an individual SPS would be very large. Since then, several important studies on the feasibility of constructing an SPS using lunar materials have been conducted by the Space Studies Institute.19 In addition, SSI has also conducted several experiments on mass-drivers, telerobotic assembly, manufacturing lightweight composites from lunar composite materials, and is in the process of designing a lunar probe to investigate the availability of water on the Moon. In addition, the strides made in solar cell efficiencies and concentrators can reduce the mass of the solar panels on an SPS by 50% and the overall mass by 20%. Power beaming technology today is much more mature than it was ten years ago. Carbon fiber composites developed for the space programs might further reduce the mass of the SPS. As SSI has demonstrated, the transportation costs to LEO can almost be eliminated if the SPS is built of lunar materials. In the short-term, CO2 emission reduction treaties should be signed. Such treaties should be linked to the adoption of alternative energy sources by the signatories to displace their fossil fuel consumption. The world can either continue to pump CO2 into the atmosphere and wait and watch the globe warm or can cooperatively pool its resources together to secure a peaceful and prosperous future for its citizens. As we have demonstrated, the time to act is now! Conclusions While the economy of the world can become somewhat less energy-intensive through conservation, the increase in the world's population, especially in developing nations, will necessitate extensive increases in energy production in the coming years. Climatological studies show that if this is done through continued reliance on fossil fuel combustion, a global greenhouse warming of several degrees is likely to result by the end of the 21st century. Therefore, new energy sources must be developed. Those considered in this paper include terrestrial photovoltaics, nuclear fission, nuclear fusion, and the solar power satellite (SPS). Of these
technologies, only the SPS produces no nuclear waste, requires no technological breakthroughs, and does not rely on costly energy storage techniques. Development of SPS should begin immediately through internationalcooperation to prevent global warming, and the efforts should be concentrated on 2.45 GHz as the frequency at which power is beamed to the Earth. Acknowledgements We thank Ken Caldeira for helpful suggestions and comments, Helen Jones for help with the manuscript, and Malini Kadiramangalam for translation. REFERENCES [1] Hoffert, M.I., ET. AL., (1979) "Atmospheric Response to Deep-Sea Injection of Fossil-Fuel Carbon Dioxide," Climate Change, Vol. 2. [2] Boden, T.A., KANCIRUK, P., and Farrell, M.P., (1990) Trends '90: A Compendium of Data on Global Change, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA, August. [3] Houghton, J.T., Jenkins, G.J., and Ephraums, J.J., editors, (1990) Climate Change: The IPCC Scientific Assessment, Cambridge University Press, Cambridge, UK. [4] Stevens, W.K., (1991) "Separate Studies Rank '90 As World's Warmest Year," New York Times, pgs. Al and D21, Thursday, January 10. [5] HANSEN, J., ET. AL., (1981) "Climatic Impact of Increasing Atmospheric Carbon Dioxide," Science, Vol. 213, Number 4511. [6] Ehrlich, P., Ehrlich, A., and Holdren, J., (1977) Ecoscience: Population, Resources, Environment, W.H. Freeman and Company, San Francisco, CA, USA. [7] EDMONDS, J., AND Reilly, J.M., (1985) Global Energy: Assessing the Future, Oxford University Press, Oxford, UK. [8] Wigley, T.M.L., (1990) "A Simple Reversible Carbon Cycle Model," August, manuscript submitted to Climate Monitor. [9] Maier-Reimer, E., and Hasselmann, K., (1987) "Transport and storage of CO2 in the ocean - an inorganic ocean-circulation carbon cycle model," Climate Dynamics 2, pgs. 63-90. [10] 1986 Energy Statistics yearbook, (1988) United Nations Department of International Economic and Social Affairs, Statistical Office, New York. [11] MARLAND, ET. AL., (1989) Estimates of CO2 Emissions from Fossil Fuel Burning and Cement Manufacturing..., Oak Ridge National Laboratory, Environmental Sciences Division, Publication No. 3176, Oak Ridge, Tennessee, USA, May. [12] Berner, R.A., Lasaga, A.C., and Garrels, R.M., (1983) "The Carbonate-Silicate Geochemical Cycle and its Effect on Atmospheric Carbon Dioxide Over the Past 100 Million Years," American Journal of Science, Vol. 283, pgs. 641 - 683, September.
[13] EDMONDS, J., AND REILLY, J.M, (1986) T/ie IEA/ORAULong-Term GlobalEnergy-CO? Model: Personal Computer Version A84PC, Institute for Energy Analysis, Oak Ridge Associated Universities, Environmental Sciences Division, Publication No. 2797, Washington, D.C., U.S.A., December. [14] POOL, R., (1988) "Solar Cells Turn 30," Science, Vol. 241, Number 4868, pgs. 900-901, 19 August. [15] STARR, M.R., AND Palz, W., (1983) Photovoltaic Power for Europe: An Assessment Study, D. Reidel Publishing Company, Dordrecht, Holland, for the Commission of the European Communities. [16] Hubbard, H.M., (1989) "Photovoltaics Today and Tomorrow", Science, Vol. 244, Number 4902, pgs. 297-304, 21 April. [17] Waltar, A.E., AND REYNOLDS, A.B., (1981) Fast Breeder Reactors, Pergamon Press, Elmsford, New York, USA. [18] FURTH, H.P., (1990) "Magnetic Confinement Fusion", Science, Vol. 249, pp. 1522-1527, 28 September. [19] Solar Power Satellite Built of Lunar Materials, (1985) Final Report of a study conducted by Space Research Associates, Inc., for Space Studies Institute, Princeton, New Jersey, USA, 21 September. [20] LEE, J.H., (1988) "Solar-Pumped Laser for Free Space Power Transmission," from Free Space Power Transmission, NASA Conference Publication 10016, NASA Lewis Research Center, Cleveland, Ohio, USA, March. [21] Cooke, D., Gleckman, P„ Krebs, H„ O'Gallagher, J., Sagie, D. and WINSTON, R., (1990) “Sunlight brighter than the sun," Correspondence in Nature, Vol. 346, p. 802, 30 August. [22] BROOKNER, E., editor, (1977) Radar Technology, Artech House, Inc., Dedham, Massachusetts, USA.
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Countermeasures for Mitigating the Effects of Global Environment Changes LYLE M. JENKINS1 SUMMARY Increased concern over the effects of global climate change and depletion of the ozone layer has resulted in support for the Global Change Research Program. Research to understand Earth system processes is critical, but it falls short of providing ways of mitigating the effects of change. Options and alternatives need to be developed. Space-based concepts for environmental countermeasures should be considered in addition to Earth-based actions. Definition, analysis, demonstration and preparation of mitigation technology provide a basis for policy response if global change consequences are severe. Background and Problem Definition A broad public awareness of the potential for global climate change and for the depletion of the stratospheric ozone layer has been the result of current media attention to issues long troubling the scientific community. These issues have been addressed in the intensive research now underway to monitor, model, and predict the course of environmental change. This activity has been particularly focused in the U.S. Global Change Research Program [1]. The process to establish the rate and magnitude of change will take some time to reach a level of certainty that will support consensus on those mitigating actions which are very costly. By the time a consensus is reached, active response may be imperative. Policy making responses can be categorized into prevention, adaptation, and engineeringcountermeasures[2]. Prevention methods should be used as extensively as economically possible. Adaptation is the ultimate response to change as it has been throughout the history of the world. The primary concern must be the rate of change to be accommodated. As insurance against rapid change, countermeasures options and alternatives for mitigation of the effects of change should be brought to a state of readiness for early implementation. Both Earth-based and space-based countermeasures should be analyzed to assess benefits, costs and risks of implementation. Research and testing of countermeasures concepts should be undertaken to reduce the risks inherent in instituting an intervention program. Formulation of these programs should parallel ongoing research into the Earth's environmental processes to reduce the reaction time in commencing required countermeasures. t Technology Project Manager, NASA Johnson Space Center, Houston, Texas, USA.
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