Space Power Resources, Manufacturing and Development Volume 11 Numbers 3-4 1992
SPACE POWER Published under the auspices of the Council for Social and Economic Studies EDITOR Andrew Hall Cutler, Space Engineering Research 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 Owen Gwynn, Mars Center for Exploration, Moffet Field CA. Praveen K. Jain, Northern Telecomm, Ottawa, Ont., Canada Dieter Kassing, ESTEC, The Netherlands Fred Koomanoff, DoE, Washington, D.C. Mikhail Ya. Marov, Academy of Sciences, Moscow, Russia 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 Gillian Pierce, Oxford University, UK Vladimir Prisnyakov, Dniepropetrovsk State University Tanya Sienko, NASDA, Tsukuba, Japan Ray A. Williamson, OTA/US Congress, USA Space Power: Resources, Manufacturing and Development 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) 622-6074, by Facsimile at (602) 795-0949 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. Detailsconcerning 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 © 1992, SUNSAT Enerev Council
SPACE POWER Volume 11, Number 3-4,1992 F.P. Chiaramonte andJ.D. Taylor. Phase Change Energy Storage For Solar Power Systems 195 David R. Criswell. Inexpensive and Environmentally Safe Solar Power for Earth from Bases on the Moon 209 M. Crutchik and J. Applebaum. Solar Radiation on a Catenary Collector 215 M. Duchet, L. Cabaret, A. Laurens andJ.C. Miscault. Space Power Supply Networks Using Laser Beams * 241 A. Arismunandar & P. DuPuis. Energy in ASEAN: An Outlook Into the 21s1 Century* 251 Peter E. Glaser. A Perspective on Power from Space for Use on Earth** 265 N. Kaya, H. Matsumoto and R. Akiba. Rocket Experiment METS Microwave Energy Transmission in Space + 267 Christian Koenig. The Legal Regime Governing the Exploitation of Natural Resources on the Moon * 275 Yuri Kruzhilin. Laser Prospects for SPS and Restoration of the Ozone Layer 283 Y. Kuroda, M. Nagatomo and P. Collins. Japanese Perspectives from Space for Earth 299 GeofferyA. Landis and Ronald C. Cull. Integrated Solar Power Satellites: An Approach to Low-Mass Space Power 303 Leonid Latyshev. Systems Analysis of Global Power Problem* 319 Glenn A. Olds. Energy Transmission re: Remote Sites Key to Economic Development for the Arctic and Developing Regions** 323 V. Prisniakov. The Ecological Situation and C.I.S. Scientists' Work on the SPS Problem** 325 V. V. Rybakov and A. P. Smakhtin. Frequency Range For Power Transmission by an Electromagnetic Beam* 329
Johann Spies. GSEK — Global Solar Energy Concept Environmental and Social Consequences1"1" 335 Marcel Toussaint. A First Stage of Experimentation On the Route to SPS 11" 337 Peter Kaloupis, Peter E. Nolan and Andrew H. Cutler. Martian Resource Utilization I. Plant Design And Transportation Selection Criteria 343 Authors Index 377 t Also presented at Power from Space '91, held in Gif-Sur-Yvette, France, August 1991. tt Also presented at the Earth Summit, Rio de Janeiro, June 1992
Phase Change Energy Storage For Solar Dynamic Power Systems F. P. CHIARAMONTE AND J. D. TAYLOR + SUMMARY This paper presents the results of a transient computer simulation that was developed to study phase change energy storage techniques for Space Station Freedom (SSF) solar dynamic (SD) power systems. Such SD systems may be used in future growth SSF configurations. Two solar dynamic options are considered in this paper: Brayton and Rankine. Model elements consist of a single node receiver and concentrator, and takes into account overall heat engine efficiency and power distribution characteristics. The simulation not only computes the energy stored in the receiver phase change material (PCM), but also the amount of the PCM required for various combinations of load demands and power system mission constraints. For a solar dynamic power system in low earth orbit, the amount of stored PCM energy is calculated by balancing the solar energy input and the energy consumed by the loads corrected by an overall system efficiency. The model assumes an average 75 kW SD power system load profile which is connected to user loads via dedicated power distribution channels. The model then calculates the stored energy in the receiver and subsequently estimates the quantity of PCM necessary to meet peaking and contingency requirements. The model can also be used to conduct trade studies on the performance of SD power systems using different storage materials. Nomenclature Cs specific heat of the solid kWe kilowatt, electric m mass of the phase change material N number of some parameter Q energy power T temperature t time At time interval u specific internal energy X liquid fraction AX change in the liquid fraction r] efficiency k latent heat of fusion + NASA Lewis Research Center, 21000 Brookpark Rd., Cleveland, OH 44135 USA.
Subscripts and Superscripts a accumulation d dissipated 1 load min minimum orb orbit ref reference s net solar to receiver 1 first segment in worst case scenario 2 second segment in worst case scenario 3 third segment in worst case scenario * melting point Introduction Solar dynamic (SD) power systems are an attractive option for future space exploration and utilization. These systems offer higher efficiency, lower launch mass, and significant life cycle cost savings, compared to current photovoltaic power systems [1]. Consequently, SD power systems in the 100 kW range have been considered for the Space Station Freedom’s (SSF) growth configuration. Multiple SD power modules would be used to achieve this power level. The SD system proposed for SSF would use the latent heat of fusion from a phase change material (PCM), stored during the sunlit portion of an orbit, for use during the eclipse periods [2]- This paper presents an energy balance analysis technique for a solar dynamic power system that will determine the stored energy in the PCM as a function of time. The approach models the receiver as a single node, and it is assumed that the receiver phase change material remains at a constant temperature. The modeled solar dynamic system in low earth orbit was subjected to realistic loads and the analysis uses a combined overall thermal efficiency for the receiver, heat engine and power distribution system. It assumes that the engine power varies to account for coarse load changes and that the unused power due to instantaneous load changes is dissipated through a parasitic load resistor. A block diagram of the SD power system is shown in Figure 1 (P. 204). This paper will discuss the analytical model used in the program, and the methods by which the mass of the phase change material, and the minimum required liquid fraction (reserve capacity) of the PCM are computed. Results comparing the state of the PCM for the Brayton and the Organic Rankine power systems, subjected to an average load demand of 75 kWe, will be made. Analytical Model The governing equation for a single node receiver storage system takes into account (1) the solar power to the receiver from the concentrator, 0 s, (2) the loads
Since the load demand influences the energy accumulation rate, the heat engine output power will vary with these changing load requirements. One technique to vary engine output power is to adjust the amount of working fluid in the system by using a sophisticated fluid inventory system. Engine output power would be tailored to follow coarse load changes. Rapidly changing load demands would be accommodated by using a parasitic load to dissipate excess power. During the insolation (sunlit) portion of the orbit 0 s is assumed to be a constant and is calculated from knowing the solar constant and the concentrator efficiency. Also, in equation (1) is assumed constant and includes the receiver, the heat engine and the power management and distribution (PMAD) efficiencies. £),, as previously mentioned, can vary and is a user input to the program. Defining in terms of its specific internal energy [3], equation (1) can be rewritten as: Assuming the PCM will remain in the two-phase region (and therefore isothermal), the liquid fraction, X, can be calculated and is a useful method of monitoring the energy level of the salt. It is related to u in the following manner [31: Equations 2 and 3 are used together to determine the liquid fraction of the salt at any instant in time. In the event that the salt has attained its maximum amount of latent energy, X=l, while in the sun portion of the orbit, the excess energy, Qd, is dissipated through the parasitic load. The salt, then, remains at T*. This scenario may occur if the load demand is less than the power available or if there exists a system failure downstream of the SD power unit. A block diagram illustrating this energy balance is shown in Figure 2 (P. 204). Referring to equation 1, the accumulated energy rate, , will be positive or zero provided that 0/ri does not exceed Qs during the sunlit period. Hence, u and X increase over time and the PCM is storing energy. When X attains the value of 1, the PCM is fully liquid (u is constant), and 0a will be zero. From Figure 2 it is then assumed that the PMAD control logic monitoring the accumulated power level,a, would begin to dump power, , whenX=l. Conversely, during the eclipse period with 0s=0, the specific internal energy, u, and the liquid fraction, X, decrease over time depleting the stored energy. It is not expected that the PCM will be fully solid (X=0) for any length of time.
Program Description The transient behavior of the phase change material is described by the first order differential equation, equation 2. Euler’s method [4] is used to numerically integrate this equation to provide an updated value for the specific internal energy, u. Equation 3, then, determines the liquid fraction X. A time step of 1 min is used. If the PCM becomes liquid, X=l, all power is, then, diverted to the parasitic load. A constant 95 minute orbit is assumed; comprised of 36 minute and 59 minute eclipse and insolation periods, respectively. Mass Quantity of the Phase Change Material The amount of receiver mass required for the two phase mixture is related to the accumulated energy and its latent heat by [3]: In addition to the normal loads, the energy stored in the PCM must be capable of accommodating peak load demands and contingency requirements. A hypothetical worst case scenario is shown in Figure 3. This scenario is divided into three segments: first, an isolation period with peaking (49 min at 60 kW and 10 min at 112.5kW); second, an eclipse period with peaking (10 min at 112.5kW and 26 min at 75kW); and third, a contingency period (95 min at 37.5 kW) [5]. The contingency requirement is defined as one half power for one complete orbit. The accumulated energy is found by [6,7]: The value for Qa determines the maximum value of latent energy necessary (in the worst case) for the PCM with its associated power system. By inserting equation 5 into equation 4, the required mass is readily attained. The Brayton power cycle, using a PCM of 67 mole percent lithium fluoride, LiF, and 33 mole percent magnesium fluoride eutectic, MgF2, requires a mass of 1725 kg. The Organic Rankine cycle, using a PCM of lithium hydroxide, LiOH, requires a mass of 2320 kg. The respective latent heats are 913 kJ/kg for the Brayton PCM, and 872 kJ/kg for the Rankine PCM [8]. The efficiencies were 0.25 for the Brayton system and 0.2 for the Rankine system. Using the worst case scenario of Figure 3 (P. 205) the liquid fraction is tracked as shown in Figures 4 and 5 for Brayton and Rankine systems respectively. The liquid fraction, X, must be equal to 1 (one) entering the eclipse period. Figures 4 and 5 reveal the accumulated energy is being extracted at three different load rates
slopes in each curve. By the end of the contingency orbit, all the latent energy has been extracted and the PCM is all solid, X=0. Minimum Required Liquid Fraction (Reserve Capacity) The minimum liquid fraction of the PCM normally occurs at the end of an eclipse period. The amount of excess receiver capacity at this time is determined by the contingency requirement of one half power for one complete orbit. The energy storage system must have a sufficient reserve capacity to meet this contingency demand if required. The minimum fraction of the stored energy which is its minimum liquid fraction is: The time t = 95 minutes corresponds to the end of a typical orbit and the beginning of the contingency orbit as illustrated in Figures 4 (bottom P. 205) and 5 (top P. 206). This would normally be the time at which the minimum liquid fraction would occur. At this point, X is equal to 0.537 for both the Brayton and Rankine systems. Therefore, because of the contingency requirement, the thermal energy storage system has about twice the amount of PCM that is necessary for normal operation in this case study. Results The computer simulation was used to compare the transient PCM behavior of the Brayton and the Organic Rankine power systems. Each power system was subjected to a typical 48 hour load profile as shown in Figure 6 (bottom P. 206), which was adjusted to have a 75 kW mean value. Although the minimum liquid fraction of the PCM was 0.537, it was initially assumed that the PCM was completely solid, X=0, for comparative purposes. Transient periodic responses in the liquid fraction are shown in Figures 7 and 8 for the Brayton and Rankine systems respectively. Steady periodic profiles are reached in the sixth orbit for the Brayton system and the eleventh orbit for the Rankine system. The following is an analytical derivation whose results were compared with those of the numerical approach. This discussion will initially focus on examining the different rates of change of the liquid fraction between LiOH for the Rankine and LiF/MgF2 for the Brayton systems. From equation 2 the rate of change of internal energy is:
Also from equation 3, du/dt can be defined in terms of the liquid fraction: As the load profile was averaged (at 75 kW ), 0] is assumed a constant for this analysis. Using equation 12, the Brayton system has a liquid fraction increase of 0.508 during the sun portion and a loss of 0.425 during the eclipse portion of the orbit. This yields a net liquid fraction orbital gain of 0.083. Similarly, the Rankine cycle has a liquid fraction increase of 0.461 during the sun portion and a loss of 0.414 during the eclipse; hence a net orbital gain of 0.047. When the minimum liquid fraction for a given orbit reaches a value of 0.537, it can be assumed that the thermal energy storage response has achieved its steady periodic profile. The number of orbits required to reach steady state, 1%^, is defined as:
These analytical results agree with data from the numerical simulations shown in Figures 7 and 8 (P. 207). In general, it can be observed from equation 11 that the rate of change of liquid fraction, dX/dt is directly proportional to the net thermal power into the receiver, ( /r]); however, inversely proportional to the PCM mass, m, and latent heat of fusion,X. Although the Brayton PCM has a higher positive rate dX/ dt in the sun period and a higher negative rate during eclipse than the Rankine PCM, the duration of the sun portion is greater than that of the eclipse period. Therefore, the Brayton PCM has a larger net gain of liquid fraction per orbit before reaching the steady periodic profile. The rate of change of the liquid fraction is related to the energy accumulation rate in the receiver. Combining equations 1 and 12 results in: Before reaching the steady periodic profile, the amount of stored energy per orbit for the Brayton system is: For the Organic Rankine cycle the amount of energy stored per orbit before reaching the steady periodic profile is: For the Rankine cycle (LiOH) the number of orbits required is:
Equation 15 clearly reveals that the Brayton system stores 37% more energy per orbit than the Organic Rankine system before achieving the steady periodic profile. Conclusion This program simulates the phase change energy stored in a proposed PCM for a Space Station Freedom solar dynamic power system. The power system is modeled using a single node receiver and uses an overall efficiency for the receiver, heat engine and power distribution systems. The thermal energy storage material is assumed to remain in the two- phase region at all times. This simulation assumed a 95 minute orbit; comprised of 36 minute eclipse and 59 minute insolation periods. Given, (1) a load profile, (2) a net power to the receiver, (3) PCM properties [8], and (4) an overall efficiency, this transient model will compute the liquid fraction of the PCM and any excess power dissipated through the parasitic load. Using the representative peaking and contingency requirement [6,7], it is found that only about one-half of the available latent heat is used under normal load requirements. The rate of change of the liquid fraction, dX/dt, was also computed for each system. Using this in combination withXmin, the number of orbits required for the liquid fraction to reach a steady periodic profile was calculated. The analytical results compared well with the numerical results in this regard. This program can be used to conduct trade studies on the performance of a specific SD power system using different storage materials. It can also be used to analyze different power systems (i.e. heat engines) with their respective phase change materials as was done in this report. In addition, this simulation can be used as a tool to develop load management control strategies. As an example of the latter, for a given load profile, if the liquid fraction, X, is greater than one at the end of sun period, then the controller would add more loads to force X back to unity at the onset of eclipse. The goal is to minimize the use of the parasitic load, illustrated in Figure 2. REFERENCES [1] Calogeras, J.E., Dustin, M.W., and R. R. Secunde (1991) “Solar Dynamic Power for Earth Orbital and Lunar Application,” presented at the 26 Intersociety Energy Conversion Engineering Conference, Boston, MA, August 4-9, 1991, Vol. 1, pp. 274-283. [2] Kerslake, T.W., (1991) “Experiments with Phase Change Thermal Energy Storage Canisters for Space Station Freedom,” presented at the 26 th Intersociety Energy Conversion Engineering Conference, Boston, MA, August 4-9,1991, Vol. 1, pp. 248-261.
[3] Duffie J.A., and Beckman W. A., (1980) Solar Engineering of Thermal Processes (New York, Wiley) pp. 342-348. [4] Carnahan, B., Luther, H. A. and Wilkes, J. O. (1969) Applied Numerical Methods (New York, Wiley) pp. 341-352. [5] Johnson Space Center, (1984) “Space Station Reference Configuration Description.” [6] Lewis Research Center, (1985) “Reference System Concept for Space Station Brayton Power System,” Data Book 2. [7] Lewis Research Center, (1985) “Reference System Concept for Space Station Organic Rankine Power System,” Data Book 3. [8] Tye, R. P., Bourne, J. G. and Desjarlias, A. O., (1976) “Thermal Energy Storage Material Thermophysical Property Measurement and Heat Transfer Impact,” NASA-CR-135098. [9] Lawrence, J. T., (1984) “Preliminary Power Profiles for 6 and 8 Man Space Station IOC Configuration,” Johnson Space Center.
digitized by the Space Studies Institute ssi.org
Inexpensive and Environmentally Safe Solar Power for Earth from Bases on the Moon DAVID R. CRISWELL+ Lunar Power System (LPS) The latest Space Commission Report to the President of the United States (1991), a recent NASA (1989) task force on energy from space, and the acting Associate Administrator of NASA (Cohen 1991) all recommend immediate studies of bases on the moon to provide electric power to Earth. Lunar power bases should be the next major goal in space. Figure 1 shows the general features of the Lunar Power System (LPS). Pairs of power bases on opposite limbs of the moon convert dependable solar power to microwaves. The Earth stays in the same region of the sky as seen from a given power base. Thus, pairs of bases can beam power dependably to collectors, called rectennas, on Earth. Rectennas, simply specialized types of TV antennae and electric rectifiers, are the major cost element. Figure 1 greatly exaggerates the sizes of the orbital reflectors (<lkm diameter) and rectennas (0.4 to several kilometers diameter). In the late 1970s the Department of Energy estimated the cost of rectennas. LPS power from rectennas will cost a fraction of a cent per kilowatt-hour when receiving beams with an intensity of 20% of sunlight or 250 Megawatts per square kilometer. To power the United States the U.S. rectennas would occupy only 5% of the land area now devoted to the production and distribution of electricity in the United States. The cost of power increases as beam intensity decreases. For a beam intensity equal to approximately 1% of the intensity of sunlight, or one-fifth the leak allowed from a home-microwave oven, the power would cost approximately 10 cents/ kilowatt- hour. A mature pair of lunar bases can broadcast several thousand gigawatts of power, approximately the level of electric power now generated on Earth. The mature LPS uses very large segmented antennae on the moon, approximately 50 to 100 kilometers diameter as perceived from Earth, to direct power to Earth. Each large antenna is composed of tens of thousands of fixed, billboard-sized screens made of lunar materials. These large, segmented antennae reduce the stray power around a beam to a tiny fraction of the safe levels specified in guidelines in the United States. LPS can be environmentally neutral, even if 50 times more electric power is provided to Earth than is now used on Earth. LPS can safely provide both low and high power beams. Rectennas in isolated regions could receive more intense beams * Director, Institute of Space Systems Operations, University of Houston, TX 77204-5502, USA.
and output very low cost power. Energy intensive industries such as refining of primary materials, recycling wastes, or production of synthetic fuels could take place in small regions remote from human populations, even in biological deserts. LPS can provide net energy to Earth (Criswell 1991) and do so at lower costs than any alternative. Each lunar power base consists of thousands of power plots. Each power plot consists of a field of solar collectors, primarily thin-film photovoltaic cells, microwave transmitters, and the billboard-sized microwave screen reflectors. Each base projects thousands of separate power beams. Depending on the power needs at Earth the beams are turned down or cut off to one rectenna and reformed to feed power to another. The bases are described in the technical literature (Criswell and
Waldron 1991 a and b, as well as 1990) and in popular publications (Shiner 1990, Chaikin 1991/92). LPS will certainly include microwave relay satellites, in moderate altitude, high inclination orbits about Earth, that redirect LPS beams to receivers on Earth that cannot directly view the power bases. The mature system will likely include some power storage, probably 3 to 6 hours. The mature LPS will include sets of photovoltaics built across the limb of the moon from each power base. These sets provide steady power output except for 3 hours during a full eclipse of the moon by the Earth. Mirrors orbiting the moon can reflect additional sunlight to the bases. Even without these enhancements LPS will provide cheaper and more reliable power than solar installations on Earth. LPS components can be repaired or improved without shutting down power transmission. Many types of power plots can be used. The LPS can be modified while on-line because power addition does not occur on the moon. Rather, the subbeams from the separate power plots add together in space, between the moon and the Earth, to form the final power beams. LPS Demonstration and Production The LPS is practical because we can send small, mobile factories to the moon to build most of the power plots and bases from the lunar soil. The LPS can be fully demonstrated as part of a manned research base on the moon. The LPS demonstration would add approximately 20 % to the cost of a large base. This demonstration could pay for itself and the research base by delivering approximately 200 GWe-Yr of energy to Earth. LPS is built from common lunar resources of the moon that are well understood. Industry is now able to design and then provide the solar cells, microwave circuits, microcomputers, rectennas, and automated means of production. Now is the time to merge these worldwide abilities with America’s ability to go to the moon and work there. We fully understand how to design and cost LPS. Laboratory and field demonstrations can begin immediately on production of glass components and solar cells appropriate to lunar conditions. The design and demonstration of robots to assemble LPS components and construct the power plots can be done in parallel. Unmanned lunar landers can be used to confirm the moon as a suitable platform for beaming power to Earth. The radio transmitters of two or more unmanned lunar landers can be coordinated to send very low power test beams from the moon to Earth. Full-scale beaming can be demonstrated by using large radar installations on Earth that were established in the 1980s to track intercontinental ballistic missiles. These existing radars can project full-scale microwave beams to prototype reflector satellites deployed from the Space Shuttle. The prototype reflector satellites could redirect the test beams back to full-scale rectennas on Earth.
Both manned and unmanned launch systems can deploy facilities to low Earth orbit to demonstrate the in-space production of solar arrays and microwave- integrated circuits. Space Station Freedom can be the assembly and test point for the operational reflectors that are too large to be deployed directly from the space shuttle or other launch systems. Two-Planet Wealth LPS can grow by 2050 to meet most of the energy needs of the world. Global wealth can grow far beyond the levels possible with present depleting, polluting, irregular, and more capital-intensive sources of energy. LPS energy can be used to restore the environment. Coal and oil will be used primarily in high-value petrochemicals rather than burned as fuels. Industry will use common terrestrial resources to meet people’s material needs. Dependable, inexpensive energy from the rectennas will stimulate regional economies and expand international trade. LPS will establish a permanent two-planet economy between the Earth and the moon and provide dependable access to solar energy. REFERENCES [1] Chaikin, A. (1991/1992) Shoot for the Moon, Air & Space, pp. 42-51, Smithsonian Institute, Washington, D.C., U.S.A. December/January. [2] Cohen, A. (1991) Human Exploration of Space & Power Development, Proc, of SPS 91: Power from Space, the 2nd International Symposium, Paper 1.1, 5 pp., Paris, France. [3] Criswell, D.R. (1991) Terrestrial & Space Power Systems: Life-cycle Energy Considerations, Proc. “SPS 91 : Power from Space”, the Second International Symposium, Paper al.2, 9 pp., Paris, France. [4] Criswell, D.R. & Waldron, R.D. (1991a) International Lunar Base & Lunar-based Power System to Supply Earth with Electric Power, Proceedings 42nd Congress of the International Astronautical Federation, October 5-11, 1991 15 pp. Paper # IAA-91-699, Montreal, Canada.. [5] Criswell, D.R. & Waldron, R.D. (1991b) Results of Analyses of a Lunarbased Power System to Supply Earth with 20,000 GW of Electric Power, Proc, of “SPS 91: Power from Space”, the 2nd International Symposium, Paper a3.6, 11pp., Paris, France.
[6] Criswell, D.R. & Waldron, R.D. (1990) Lunar System to Supply Solar Electric Power to Earth, Proc, of the 25th Intersociety Energy Conversion Engineering Conf., Eds: P.A. Nelson, W.W. Schertz & R.H. Till, Vol. 1, Aerospace Power Systems, pp. 72-76, Space Power Systems 2-Session 2.2 American Inst. ofChem. Engineers, 345 East 47th St., NY, NY 10017, U.S.A. [7] NASA (1989) Lunar Power System : Summary of Studies for the Lunar Energy Enterprise Task Force, Appn. B-4, (pp. 84-96) in Report of NAS A Lunar Energy Enterprise Case Study Task Force, NASA TM 101652, 178 pp., July. [8] Office of the President of the United States (1991) America at the Threshold: America’s Space Exploration Initiative, Synthesis Group, 144 pp. &64pp. appendix. U.S. GPO, Washington, D.C. 20402, U.S.A. (Note Architecture IV) [9] Shiner, L. (1990) 300 Billion Watts, 24 Hours a Day, Air & Space, pp. 69-75, Smithsonian Inst., Washington, DC, U.S.A. June/July.
digitized by the Space Studies Institute ssi.org
Solar Radiation on a Catenary Collector M. CRUTCHIK? AND J. APPELBAUM^ Summary A tent-shaped structure with a flexible photovoltaic blanket acting as a catenary collector is presented. The shadow cast by one side of the collector on the other side producing a self shading effect is analyzed. The direct beam, the diffuse and the albedo radiation on the collector are determined. An example is given for the insolation on the collector operating on the martian surface for the location of Viking Lander 1 (VL1). Introduction Missions to the lunar or martian surface will require electric power. A power supply that requires little installation time, being light weight and stowable in a small volume can be accomplished by a photovoltaic (PV) array. A tent-shaped structure with a flexible PV blanket for solar power generation is proposed in [1], Fig.l. The array is designed with a self-deploying mechanism using pressurized gas expansion. The structural design for the array uses a combination of cables, beams and columns to support and deploy the PV blanket. The shape of the PV blanket is determined by an optimization between reduction in the cable tension and increase in blanket area. Under the force of gravity a cable carrying a uniformly distributed load will take the shape of a catenary curve, fc(y), with respect to the y- z plane, Fig. 2. The catenary constant k can be determined using the condition fc(O) = H and solving iteratively. However, when the blanket is fairly taut, the load may be assumed uniformly distributed along the y-axis and the catenary curve may be approximated by a parabola [2], i.e., 1 t Faculity of Engineering, Tel-Aviv University 69978, Israel 2tt National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH 44135. Current address: Tel-Aviv University, Faculty of Engineering, Tel-Aviv 69978, Israel. This work was done while the author was a National Research Council — NASA Research Associate at NASA Lewis Reseaech Center. Work funded under NASA Grant NAGW — 2022.
Because of the shape of a catenary-tent-collector, a self-shading effect occurs on one of its sides (side B in Fig. 2). In this article we analyze the shadow shape and area. Based on these results, the beam insolation on the collector is calculated. We also determine the diffuse and the albedo insolation on the collector. An example for the planet Mars is given in Appendix B. Shadow Calculation The catenary-tent-collector is self shading. The size of the shadow and the side which is shaded depend on the sun position. In general, both sides of the collector will be alternately shaded in a given day if at sunrise The azimuth angles are measured from true south positively in a clockwise direction. In days when eq. (3) is not satisfied, only one side of the collector is shaded at all the time. In this paper we analyze the shape and size of the shadow cast on the catenary-tent-collector facing in a south-north direction. The results can be generalized for an arbitrary oriented tent by replacing the sun azimuth angle ys by the difference between the solar and the collector azimuth angles, i.e.
Case (i): P < L, P < D x y In this case the solar rays penetrate the collector surface and the shadow on the collector takes the shape NTEExM as shown in Fig. 4. Point E is a point where the ray penetrates the collector, and point T is a point where the ray is tangent to the collector. In order to calculate the shaded area, the components of the points E and T must be determined. Since the catenary function fc(y) is hyperbolic, the solution for the component yE is obtained numerically from the solution of the following equation (see Appendix A):
Solar Radiation Calculation With the results developed in the previous section, the beam irradiance in W or W/m2 -day on the catenary-tent-collector can be determined. The diffuse and albedo components will be added to the beam to get the global irradiance and insolation. A north-south facing catenary tent collector will be considered. A generalization to any arbitrary oriented tent may be obtained using eq. (4). Beam Irradiance The beam irradiance on both sides of the tent depends on the self-shading condition of eq. (3). In calculating the irradiance on an unshaded side we resort to Fig. 8. The beam irradiance P, in watts (W), on an unshaded side is given by:
Integrating eq. (37) in the interval [0,D], we obtain the beam irradiance, in W, on the unshaded side of the collector, i.e., i.e., the beam irradiance on a catenary collector is equivalent to that of a flat plate collector MNMsNs. This conclusion is also valid for any non-flat shape collector. The beam irradiance on a collector that is partially shaded can not be obtained by multiplying the incoming beam irradiance with the factor (1-e) as for a flat plate collector since the beam irradiance on the collector is not uniform along the y axis of the collector, i.e., the angle 0 varies with y. Using eqs. (33), (35) and (36), the variation of the beam irradiance, in W/m2, along the collector is: Substituting these results, we obtain the beam irradiance on the partially shaded side of the collector:
In Winter, side A (Fig. 2) always remains unshaded, therefore the beam irradiance PbA on this side is given by eq. (38) or eq. (39). Side B may be partially shaded and the beam irradiance PshbB is given by eqs. (45), (46) or (47) where the collector azimuth is 180°, therefore, ys is replaced by (ys-180°). The total beam irradiance on the catenary tent collector in winter is: In summer, early in the morning and late in the afternoon, side A is partially shaded, therefore, the beam irradiance P^g is given by eq. (38) with the azimuth s-180°, i.e., thus the beam irradiance on both sides of the tent collector for early morning and late afternoon hours is: Diffuse Irradiance The irradiance on a concave collector is equivalent to the irradiance on a flat plate collector of the same aperture. This applies to direct as well as to indirect irradiances. Therefore, the difference irradiance Pd on the catenary is given by
Since the diffuse irradiance is independent of the orientation of the collector (for isotropic skies), eq. (53) applies also to the other side of the tent. Albedo The albedo can be determined by using the expression: Since the ground reflected irradiance is independent on the orientation of the tent, eq. (57) applies also to the other side of the tent.
Example The example refers to a north-south facing catenary tent collector deployed on the Martian surface [6] at the location of Viking Lander VLI (Latitude - 22.3°N, Longitude - 47.9°W) and in an autumn day Ls = 200° [6], For this time of the year eq. (3) does not apply and, therefore, only side B will be partially shaded during the day. The dimensions of the catenary collector are: D = 3m, H = 2M and L = 1.5m. Since fc(0) = H, this results in k = 2.53 and the catenary equation is given by: Figure 10(a) shows the shadow shapes on side B of the catenary collector for 0 = 22.3°N for different hours of the day. The shadow calculations are based on section 2. For comparison, the shadow shapes for 0 =32° are shown in Fig. 10(b). As expected, the self-shading effect is more pronounced for higher latitudes. It is interesting to note that for 0 =22.3°N, the shadow effect is quite small during the noon hours. In summer, the shadow will be even less. The insolation of the catenary-tent-collector for 0 =22.3°N, Ls=200° and albedo al=0.22 is shown in Table 1 based on radiation data at VLI [6]. The table shows the beam, diffuse and the albedo insolations on sides A and B in kWhr-day and kWhr/ m2-day. As expected, the beam insolation on side B is lower by 59.6% than on one side A. It is interesting to note that the diffuse insolation comprises 46.6% of the global insolation. This characteristic is typical for Mars, a place where the atmosphere consists mainly of dust particles.
Conclusions The article analyses the performance of a catenary-tent-collector [1] (a flexible blanket that falls freely on both sides of a central support). This kind of collector has characteristics (portability and simplicity) that are desirable for solar power plants on other planets. Because of its shape, there is a self-shading effect that must be taken into account in the solar radiation calculation. Therefore, the shape and area of the shadow on the collector is calculated and used in the determination of the beam radiation. The diffuse and albedo radiation were also calculated to determine the global radiation on the collector. The numerical example is based on solar radiation data on Mars. Nomenclature A Collector area, [m2] Ash shaded area of a collector, [m2] Aush unshaded area of a collector, [m] D length of the collector, [m] FA G view factor of area A with respect to ground FA s view factor of area A with respect to sky FdA s view factor of incremental area dA with respect to sky Gb direct beam irradiance, [W/m2] Gdh diffuse irradiance on a horizontal surface, [W/m2] Gh global irradiance on a horizontal surface, [W/m2] H height of collector, [m] k catenary constant L collector width, [m] Ls areocentric longitude of the sun (for Mars) Pal albedo irradiance, [W] Pb direct beam irradiance on an unshaded side of a collector, [W] Pshb direct beam irradiance on a partially shaded side of a collector, [W] PbA, PbB direct beam irradiance on an unshaded side A and B of a collector, respectively, [W] PshbA> PshbB direct beam irradiance on a partially shaded side A and B of a collector, respectively, [W] Px, Py x and y components of a shadow length, [m] Q insolation, [kWhr-day] q insolation, [kWhr/m2-day a sun altitude yc collector azimuth ys sun azimuth Ysr sun azimuth at sunrise o solar declination angle e characteristic angle of the collector 6 angle between solar ray and the normal to the collector % relative shaded area
0 local latitude w solar hour angle REFERENCES [1] A.J. Colozza, (1991) “Design, Optimization, and Analysis of a SelfDeploying PV Tent Aray,” NASA Contract Report 187119, June. [2] F.B. Beer, E.R. Johnston, (1967) Mechanics for Engineers: Static and Dynamics McGraw-Hill, New York, pp. 258-273. [3] J. Appelbaum, J. Bany, (1979) “Shadow Effect of Adjacent Solar Collectors in Large Scale Systems” Solar Energy, Vol. 23, pp. 497-507. [4] J. Bany, J. Appelbaum (1987) “The Effect of Shadowing on the Design of Field of Solar Collectors” Solar Cell, Vol. 20, pp. 201-228. [5] H.C. Hottel, A.F. Sarofim, (1967) Radiative Transfer, McGraw Hill, New York, pp. 25-39. [6] J. Appelbaum, D.J. Flood, (1990) “Solar Radiation on Mars,” Solar Energy, Vol. 45, pp. 353-363.
Appendix A Calculation of Points E and T Calculation of Point E and T for case (i) Point E Point T Calculation ofyuandxEfor case (ii)
Appendix B Using the parabolic approximation for fc(y) greatly simplifies the mathematical calculation, the error is small and the results are very similar to the catenary case. The procedure for calculating the points for the shadows are the same as in Appendix A but now fc(y) is the parabolic approximation (eq. (2)), i.e.,
Space Power Supply Networks Using Laser Beams M. DUCHET,L. CABARET, A. LAURENS, J.C de MISCAULT* Summary This paper describes laser-based space power supply networks which could perhaps be feasible within several decades. These networks could be used to supply energy to satellites in low earth orbit (LEO) or geostationary earth orbit (GEO), either to meet extra energy requirements or to allow a shift in orbit. In addition, they could be used to transmit energy towards the Earth or lunar bases, as well as many other space power applications. Also described are the different types of lasers which could be used in such networks, and the advantages of these networks in terms of satellite design. Introduction Following are four examples of laser based space power supply networks that could perhaps be feasible within several decades. Future power needs in space The concept of a space power supply using laser beams appears very promising to supply energy to satellites, for their excess internal needs or to maintain their orbit or to change it, as well as to supply power to lunar bases, to transmit energy from space to Earth or for many other space applications. A study by EUROSPACE 1 indicates that space energy needs will show considerable growth after 1995. Description of different networks 1 Low orbit laser supply network without relay mirrors Figure 1 describes a low orbit laser supply network without relay mirrors designed mainly to supply power to low earth orbit (LEO) or geostationary earth orbit (GEO) satellites or lunar bases from lasers in LEO. The LEO laser satellite comprises large solar panels or light concentrators. It must be located at an average altitude of about 1000 km in order to avoid the residual atmospheric braking effect and therefore provide the required long life span. * Laserdot, route de Nozay, 91460, Marcoussis, France M. Toussaint-Eurospace, 16 bis Avenue Bosquet, 75007 Paris, France, J.P. Gex-Sylarec, route de Nozay, 91460, Marcoussis, France.
2 Low orbit laser supply network with relay mirrors In the previous network each laser acts independently. To improve system efficiency and redundancy, and to avoid sun eclipses, relay mirrors can be used as shown in Figure 2, the different lasers being interconnected. In this case the power generated from several lasers is directed successively towards several relay mirror satellites. These relay mirrors select a part of the beam and direct it via pointing optics towards all space object which need power. It is worth noting that for LEO receivers the dimensions of the laser beam receiving panels are smaller than equivalent solar panels. Indeed not only can the irradiance of a focused laser beam be much greater that the one given by solar irradiance but the power conversion effiency is greater when appropriate monochromatic laser wavelengths are used. We can therefore predict that the braking effect related to the LEO receiving satellite is generally smaller than that for conventional satellites. In addition the receiving panels’ weight is reduced.
3 Geostationary orbit laser supply network with relay mirrors The third system shown in Figure 3 is a geostationary orbit laser supply network with relay mirrors mainly supplying GEO satellites. Direct GEO-Earth transmission can also be considered in this case. This network can be coupled to an LEO network. 4 Geostationary supply network with ground-based laser The fourth system is a geostationary supply network using ground-based lasers. To cope with local weather conditions which interfere with ground to space transmissions, several lasers must be used, each at a different location and with unrelated weather (for example, on different continents).
The diagram in Fig. 5 shows an example of this system, including relay minors, laser and pointing optics. Optics and pointing requirements Good laser transmission efficiency assumes that the diameter DE of the emission optics is approximated by the formula: DE = 2,44 kxZ/DR, with DR the receiver diameter, X the wavelength and Z the distance between emitter and receiver (Fig. 6). For the following calculations X has been set at 0.8 pm. For example, in the case of Fig. 1, with Z = 7,500 km and DR=3 m, DE is equal to 4.9 m. In the case of Fig. 2, the diameter of the relay mirror is about 4 m, and the other parameters remain the same. In the case of Fig. 3 with Z = 40,000 km, the relay mirror diameter is about 9 m (in this case DE = DR). For an LEO satellite supplying energy to a lunar base, the diameter of the optics on the satellite would be 10 m, and the diameter of the energy collector on the moon’s surface would be 75 m. This would provide a convenient and permanent power supply for a lunar base, even at “night”. Pointing accuracy (at 3 r.m.s) on the order
of k/DE/2 is required. For DE = 10 m and k = 0.8 mm, k/DE/2 = 40 nanoradians. By using adaptive optics, such accuracy appears feasible in the near future. The same conclusion applies to the large relay mirrors, based on the used of segmented phased mirror technology 2. Ref. 3 provides further details about these aspects. Laser systems in space Today, based on efficiency, technology and power criteria, four types of lasers can be considered for use in these networks: • laser diode pumped solid-state lasers, • phased arrays of semiconductor lasers, • solid-state solar pumped lasers, • free electron lasers (FEL). This order also corresponds to the most likely timetable for using these lasers in space applications. In the very short term, laser diode-pumped solid-state lasers are the only lasers that can emit a near diffraction limited beam, to an average power of about one kilowatt, (although with weak overall efficiency). Moreover, the emission wavelength (about 1 pm) requires the use of photovoltaic cells specially designed to obtain good conversion efficiency (compound cells). The state of the art in the domain of high power semiconductor lasers arrays is still not sufficiently advanced to enable energy transmission in the short term. Indeed, these arrays must be phase- matched in order to obtain a beam in fundamental space mode. Today, many laboratories are working on these questions; the best reported performance to date is 16 W in pulsed mode 4. If the phased-array laser diode technology succeeds in the near future, and if significant power levels are obtained on the fundamental mode, this type of laser would replace laser diode-pumped solid state media. Total efficiency could be multiplied by six or more and the system complexity would be reduced in consequence. With directly solar pumped lasers, the conversion efficiency must be greater since there is a direct photon-photon conversion process. This laser technology should therefore take on more importance in space applications. Nevertheless, the efficiency obtained with these lasers today is rather weak, due to the fact that absorption spectra of the amplifier media studied are not well matched to the solar spectrum: sharp lines or badly centered compared with this spectrum. For example, the use factor of sun energy is only 3% for iodine laser (t-C4F9I gas) at 1.3 pm. With solid state materials like Nd:YAG, the very numerous absorption lines distributed in the visible spectrum would give good overall efficiency; the absorp-
tion efficiency for a 5900 K black body reaches 11% with Nd:YAG and 26% with codoped Nd:Cr:GSGG matrix 5. The experiments carried out with Nd:YAG have given promising results (1.5% overall efficiency, emitted power > 60 W) but this is still insufficient, due to the thermal effect of the pumping process and the accuracy of solar pumped laser medium coupling. Today the sun-pumped laser appears very promising, in particular due to the availability of a new type of solid-state laser medium. For example, the Alexandrite or Ti:Al2O3 crystals have an absorption band well centered on the sun spectrum, a high quantum efficiency and a thermal conductivity better than most other materials. Within this context solid state solar pumped lasers promise higher efficiency. Note that the use of a Ti:Al2O3 crystal might be questionable because such sources require high concentration collector. Finally even though little work has been done on solar pumped lasers, their development for power transmission in space seems very promising. Compared to other candidates, their advantages include: • the photon-photon conversion process, which allows the use of reflectors which are lighter and smaller than solar panels; • a direct conversion which eliminates additional energy consumption through intermediate processes; • overall efficiency fairly competitive with other laser candidates (sun-laser efficiency ratio: 5 to 10%); • a less complex and more rugged technology; • an emission band (0.7 -1 pm) well matched to the quantum efficiency curve of Si or GaAs cells. In conclusion, sun-pumped laser research and development phases should focus on the following points: • optimization of the conversion efficiency of sunlight to laser light; • thermal issues; • geometrical amplifier design able to increase conversion efficiency and lessen thermal effects; • effective heat removal; • elimination of useless solar flux; • research to find the best crystal.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==