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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy 3 Individual Technology Investment Evaluations 3-1 SYSTEMS INTEGRATION Systems integration is commonly utilized during the development phase of a product but is a vital consideration in the early space solar power (SSP) technology development work due to the large number of subsystems and their strong interaction with one another. Committee comments and recommendations are grouped into three areas: (1) improving technical management processes, (2) sharpening the technology development focus, and (3) capitalizing on other work. Improving Technical Management Processes As discussed in Section 2–1, NASA has allocated some of its available SERT funding to the development of an overall SSP concept and cost model that includes system mass and performance targets (Carrington and Feingold, 2000; Feingold, 2000; Mullins, 2000). This model, while not yet complete or independently validated, has been used as a tool to predict delivered baseload terrestrial power costs assuming various technology goals have been achieved. The committee endorses this methodology as one useful technique for assigning technology investment priorities and urges that its development continue as an indicator of the relative payoff from technology investments. The model is coarse at this time and must be expanded so that cost and mass targets can be allocated down to the lowest (component) level. Sensitivity studies, which the SERT program has begun, should be an integral part of such modeling, to quantify the impacts of departures from the nominal input values, many of which are, at this time, simply assumptions. Nominal values should be developed in consultation with acknowledged experts in the field. The modeling activity should also be extended to other near-term applications of SSP technologies. Integration, however, requires much more than the exercise of a modeling tool. An effective team process is necessary to coordinate actions and to ensure that the best possible decisions are made for SSP. NASA has established a working process for integrating the SERT effort. This program decision-making and organization process to integrate SERT technology working groups and task forces has been well coordinated and has succeeded in advancing the SERT program definition during the past 2 years. Figure 3-1 provides a schematic of the relationships and interactions among the various groups. The committee notes the exemplary degree to which NASA’s SERT project organization has been open and responsive to outside suggestions. That openness indicates a high degree of objectivity and confidence and should be encouraged in all continued work. However, three factors will emerge to force the SERT-type integration process to change in the future: (1) funding levels must increase
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy FIGURE 3-1 NASA’s SERT program integration process. NOTE: Figure reprinted in original form. SOURCE: Mankins and Howell, 2000a.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy in order for the currently planned program to become viable; (2) interaction with other government agencies, commercial entities (including the utility industry), and other organizations should increase; and (3) tough downselect decisions for various technologies and flight test hardware must be made, assuming that the increased funds will still be constraining. However, when NASA leaves the planning stage and undertakes serious technology development, the SSP program would be better served by adding a technology development process complete with specific goals, dates, and procedures to be followed. This clarification of the organization and decision-making approach is necessary to meet technology goals at critical program milestones and with efficient use of funds. The program should define each of the integration functions and clearly charge the responsible organizations with explicit responsibilities. Although advancements in research and technology are difficult to schedule, adequate funding should be provided to each organization to undertake these assigned responsibilities at a high level of quality and at a reasonable pace. The approach should be similar to major program organizations at a much later stage of the development cycle, because the program involves several NASA centers, outside agencies, and the international community. NASA should develop a written implementation plan for carrying out the work. Definition of a consistent process to adjudicate competing objectives is also necessary. It is almost inevitable that institutional considerations will sometimes be at odds with purely end-product goals, and it is better to address this likelihood with an objective decision process before positions stiffen within the SSP program and among the NASA centers. Recommendation 3–1–1: NASA’s SSP program should improve its organizational and decision-making approach by drawing up a written technology development plan with specific goals, dates, and procedures for carrying out technology advancement, systems integration, and flight demonstration. The SSP program should also establish a consistent process to adjudicate competing objectives within the program and specifically include timing and achievement of technology advances in robotics and space transportation in the roadmaps. Sharpening the Technology Development Focus Many SSP issues relate to manufacturing cost, not just performance. Cost estimation expertise is largely to be found in industry, not in the NASA laboratories or academia. Appreciable funding of and consultation with relevant industrial firms is required to gain authoritative costs to be used as future inputs to the integration model. Investment should also be made in high-payoff, high-risk manufacturing technologies to reduce the cost of all the components since many hundreds of duplicate pieces will be necessary. The committee notes that technology is available today to build an SSP system (i.e., solar electricity is currently generated in space, microwave power transmission has been successfully demonstrated terrestrially, and space transmission is in demonstration by Japan). However, such technology would be impractical and uneconomical for the generation of terrestrial baseload power due to the high cost and mass of the components and construction. The system would also have to be constructed and utilized by humans in LEO, not the GEO planned for future large SSP systems, which would be constructed or deployed autonomously. It would be completely unable to “close the business case” for its (presumably) commercial sponsors and their funding sources. The committee considers that the present SERT cost and performance targets for several of the technologies are beyond present credibility, particularly in solar power generation (SPG), space power management and distribution (SPMAD), and wireless power transmission (WPT). In addition, goals in structural mass and performance may be too stringent. The committee suggests further design studies in this area before additional technology investment is made. Substantial improvements in assembly, maintenance, and service are also essential for the success of this program. If successful, these technology advancements will have many other applications. However, the committee realizes that the SSP program as currently envisioned is planned to unfold in stages, with NASA leading demonstration projects at progressively higher power levels (100 kW, 1 MW, and 10 MW) provided that positive decisions are made at each milestone to make the investments necessary to continue with the program.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy Recommendation 3–1–2: The SSP program should allocate some of the technology development funds to the aerospace system level and component industries that can address manufacturing, assembly, and maintenance design and cost. This would include architecture, engineering, and electrical utility construction firms. An added benefit of broadening the industrial community involved in SSP development may be significant improvements to future baseline designs. The committee observes that the earliest of the commercial satellites, should the private sector decide to join the SSP program, will be a subscale power station, perhaps a repeat of the 10-MW NASA experiment with improved technology, followed by a private-sector decision to construct and operate a 100-MW pilot plant. Only then will deployment decisions be made for power plants of 1 GW or larger. Each of these several decisions will be based on economic as well as technological figures of merit appropriate to their particular time—not based on the limited understandings of today. Verification of SSP technology and the integration and testing of hardware and software are necessary before deployment of any SSP system. The SSP program’s investment in developing spacecraft integration and testing methodologies and processes should be expanded. The goal should be to make verification of the performance of the SSP satellite possible with a minimum of ground testing. A combination of two approaches may be needed to accomplish this objective: (1) new modeling methods that accurately predict on-orbit function and performance and (2) a new design method to adaptively accommodate errors in the predicted function and performance. The modeling approach may require the development of (1) high-resolution (possibly many millions of degrees of freedom) structural models (including the ability to predict damping in a zero-g environment), (2) complex, integrated structural-thermal-control performance models that include probabilistic sensitivity analyses, and (3) validated component-to-system verification test procedures. This effort should be supported by a complementary set of ground and flight experiment activities, the goal of which would be to establish the limits of the new modeling capabilities in predicting the zero-g performance of the SSP mechanical systems. The adaptive approach may require the development of self-healing structures, self-tuning adaptive control systems, autonomous robots, and failure-tolerant structural concepts. Some of these developments may be very far into the future, requiring rigorous, long-term investment before they will yield practical success. Recommendation 3–1–3: The SSP program should increase investments in developing spacecraft integration and testing so that the performance of SSP satellites can be verified with a minimum of ground or in-space testing. This may include the development of specialized integration, test, and verification methodologies for SSP spacecraft. Because there are so many competing designs for various technology flight demonstrations (TFDs), it was difficult for the committee to determine the accuracy of the SSP system and cost models used to evaluate trade studies for various technologies. The current models should incorporate one detailed concept definition, making it possible to evaluate the payoff of specific technology efforts within the broad functional systems areas of SPG and WPT, among others. This concept definition should also include assembly, checkout, and maintenance techniques. The model needs to clearly show where SSP technology investments branch off to the benefit of other missions, including both in-space and terrestrial applications. There was concern on the part of the study committee over the actual values used as technology metrics in the system and cost models. After discussion with NASA subsystem team leaders and systems integration personnel, the committee found the maximum and minimum values used seem to be especially suspect in areas such as structures and photovoltaics. Some performance metrics seem to be extrapolations that cannot be traced to the structural requirements specific to SSP platforms. Also, during (and after) presentations to the NRC committee, there were self-acknowledging errors in the models used to place priority on technical investments. This led the committee to be skeptical about the accuracy of the model input and the use of these metrics. There was further concern about the lack of comparison between assembly methods (i.e., fully autonomous, tele-robotic, or human) within the models. Technology metric input should be gathered from hardware builders, industry, academia, and other government agencies to improve the modeling effort. Input should include information on technical
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy performance, current state-of-the-art technology, forecasts for technology advancement, and system cost modeling. The expansion of the modeling should also include other concepts (in addition to terrestrial power supply) that will benefit from the SSP technology investments. Future comparisons in the models should include transportation costs and delivered performance on orbit, which will enhance decision making between alternative technology candidates. This seems to have been intended in some of the past SERT program trade studies; however, the committee believes that the principle is not sufficiently ingrained in the modeling effort. Recommendation 3–1–4: The SSP program should review its technology and modeling assumptions, subject them to peer review, and modify where indicated. A single SSP concept should be rigorously modeled, incorporating technology readiness levels and involving industry in conceptual design, as a means to improve the credibility of the model input and output but not to prematurely select a single system for ultimate implementation. Capitalizing on Other Work The committee realizes that a full-scale SSP system is greatly dependent on future space transportation technologies not yet defined. Such technologies include ground launch to low Earth orbit (LEO) followed by transportation from LEO to geosynchronous Earth orbit (GEO). NASA’s SSP program should identify and initiate detailed studies of the SSP program’s space transportation needs and requirements. This NASA study should include the space transportation needs and requirements of the SSP demonstrator elements in addition to the space transportation needs and requirements of related space systems that NASA may later embrace for other uses based on SSP technologies. Industry involvement is imperative in defining the means and estimating the programmatics of new and evolving space transportation systems to fulfill these needs. NASA’s technology development program, called the Space Launch Initiative (SLI), is currently charged with addressing the Earth-to-LEO launch issue. Coordination, however, is apparently not yet present between the SERT program and SLI. Any future SSP effort should work closely with SLI as future milestones and roadmaps are developed for both programs in order to provide realistic goals for future implementation. A similar program is also necessary to develop space transportation from LEO to GEO. NASA defines work in this area as in-space transportation and infrastructure, which, if undertaken, may be separate from NASA’s SLI initiative. These issues are discussed in further detail in Section 3.9. The SSP technology program presented to this committee (Mankins, 2000) relies on parallel technology development efforts undertaken in several NASA enterprises and has strong interagency and, potentially, international implications. It will be important to the SSP program, therefore, to have a strong advisory infrastructure to help oversee the technology development progress efficiently. Currently, the main advisory body for the SERT program is the Senior Management Oversight Committee (SMOC), consisting of members from NASA and industry. Various other research and technical working groups and a systems integration working group are also used throughout the program integration process to infuse expert knowledge into the program (see Figure 3-1). Upon evaluating the membership of the SMOC and, more important, the participation level of the individuals, the committee concluded that industry and government agency representation may be less than adequate to provide competent oversight and technical input. More external membership in these assessment teams and working groups should be brought together from industry and government in order to validate various technological metric inputs to the performance and cost models and to review progress at appropriate program milestones. Inputs from the Electric Power Research Institute (EPRI) and electric utilities will be of greater importance as the program proceeds. Government agencies, such as the Department of Defense (DOD), the U.S. Air Force (USAF), and the Department of Energy (DOE), should be involved, not just as liaison representatives but perhaps even as members of the SSP research team via the Intergovernmental Personnel Act or other cooperative programs. Such exchanges would not only foster cooperative activity among the agencies but also leverage all government investments in technologies applicable to SSP. It is also desirable, due to the breadth of SSP-related technologies, to establish advisory committees for specific technologies as well as the overall program. These advisory committees would be in addition to the research and technology working groups already used by the program.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy This addition of non-NASA experts in the various technical areas will assist in providing technical peer review of the SSP program’s research. Participation of others in the establishment of modeling metrics will also be extremely helpful to systems-modeling researchers by providing realistic and accurate technology parameters. As remarked on earlier, this is a major concern of the committee in areas such as solar arrays, SPMAD, WPT, structures, and robotics. Active representation of various aerospace manufacturing industries and the electric utilities will be imperative, especially as flight demonstration and implementation are approached. Recommendation 3–1–5: The current SSP advisory structure should be strengthened with industry (including EPRI and electric utility) representatives plus experts from other government agencies (particularly DOD/Air Force, DOE, and NRO) in order to validate technological and economic inputs into the performance and cost models. Also, due to the wide breadth of technologies related to SSP, the program should establish similar advisory committees for specific technologies in addition to the research and technology working groups currently utilized by the program. Various modeling techniques and methodologies have been previously developed for use in large space system integration and in cost, performance, and economic modeling. Such techniques do not seem to be used within the current SERT program or at least were not evident in the information provided to the committee. The SSP program should review previous modeling efforts and integrate successful methods, as appropriate. For example, it does not appear to the committee that the current SERT program has looked for lessons learned from modeling efforts used during the 1970s space solar power efforts (Hazelrigg, 1977) or considered other recent efforts in long-term, multiphase technology development (Hazelrigg and Greenberg, 1991; Hazelrigg, 1992). These methods are not endorsed by the committee but are mentioned only to provide examples of other modeling on which NASA might capitalize. The following sections address each of the major technical areas targeted in the organization of the SERT program. The discussion and recommendations include information on each technical area’s objective and scope as part of an SSP system, the current technical state of the art, necessary goals for economic competitiveness, challenges to be met by the NASA team, and recommended priorities for NASA’s SSP program activities. 3-2 SOLAR POWER GENERATION Objectives and Scope The solar power generation (SPG) subsystem of SSP consists of a large constellation of very large space solar photovoltaic (PV) arrays that convert the Sun’s solar energy into electricity. The PV arrays would be connected to transmitters that would beam down as much as several gigawatts of power to Earth via microwave or laser transmission. The PV array consists of a large number of PV panels that each mechanically support hundreds of individual PV cells. The panels are electrically and mechanically interfaced together via cabling and a rigid support structure to form the large PV array. A constellation of these larger arrays would form the SSP solar power generation system. Important figures of merit at the solar array level are specific power (watts per kilogram), cost (dollars per watt), areal power density (watts per square meter), and stowage volume (watts per cubic meter). For SSP, all four of these parameters are important, with complex trade-offs. For example, it can be argued that using increasingly higher-efficiency crystalline solar cells results in increased solar array cost in dollars per watt, but also gives increased specific power, areal power density, and stowage volume. On the other hand, if the paramount driver is PV array cost, as is the case in terrestrial PV applications, then lower-efficiency technology such as noncrystalline thin-film PV can provide advantages. At the PV blanket and cell level, the lower-efficiency (~10 percent versus 30 percent for crystalline cells) thin-film PV blanket can be 50–100 times cheaper than the crystalline cells. For relatively low power levels (1–20 kW), resulting thin-film PV arrays are expected to be at least 10 times cheaper, be 2–3 times lighter (2–3 times greater specific power), and have 3 times more stowage volume density than crystalline cell arrays, planar or concentrator, that produce the same power. However, an important disadvantage of thin-film PV technology will be much larger array size, 2–3 times larger than crystalline arrays, owing to the reduced cell efficiency. This in turn will increase array guidance, navigation, and control needs; structural mass and complexity; and cable
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy mass, especially for SSP. Because the SSP system will be very large (several square kilometers), a significant amount of cabling would be needed, tens if not hundreds of kilometers. To date, there has been no comprehensive PV array trade study for SSP. The baseline conversion technology indicated by the SERT program consists of array modules using high-efficiency crystalline solar cells under 8 times concentration. However, no studies were conducted to determine the support structure and electrical cabling that would be needed. Also, no consideration was given to the added support structure and guidance, navigation, and control system that would be needed to enable the Sun-pointing requirement of approximately 2°. Additional support structure would be needed to maintain flatness for concentrator arrays to maintain pointing tolerances, compared with planar unconcentrated thin-film arrays that suffer only a cosine loss. Current State of the Art Today’s state-of-practice solar arrays provide specific powers of 30–60 W/kg and utilize single-crystal, 14–27 percent efficient (under the space spectrum) silicon, gallium arsenide, or GaInP2/GaAs/Ge solar cells supported by rigid aluminum honeycomb or flexible polymetric substrates (Sovie, 2001; Oman, 2001). The cost for these arrays ranges from approximately $300 to $1,000/W (Marvin, 2001). Examples of state-of-practice arrays are the Boeing 601 10-kW array at approximately 50 W/kg and the International Space Station 260-kW array at 30 W/kg. Today’s state-of-the-art solar arrays utilize either 26.5 percent (and soon 27–28 percent) efficient three-junction GaInP2/GaAs/Ge solar cells supported by rigid aluminum honeycomb substrates or 17 percent high-efficiency silicon solar cells supported by a lightweight suspended mesh structure, resulting in specific powers of 70–100 W/kg and $500– $1,000/W (Sovie, 1999). Examples include the Boeing 702 15-kW array at 80 W/kg using three-junction cells and the ABLE Engineering Ultraflex Array at 100 W/ kg using high-efficiency silicon cells. The solar array design baselined by the SERT program for SSP is Entech’s stretched lens array (SLA) PV concentrator utilizing 28 percent efficient, three-junction solar cells under 8 times concentration via a silicon-based stretched Fresnel lens (Marvin, 2001). Measured performance for SLA at the module level is 378 W/kg but does not take into account the support structure mass required to assemble the modules into a practical array. Projected performance for a <20-kW SLA is less than 200 W/kg (O’Neill and Piszczor, 2001). However, most important, this projection does not include the additional structure mass required to maintain the 2° pointing accuracy over the several square kilometer array area of SSP. The specific power for the only full SLA solar array flown in space, aboard the Deep Space 1 mission, was 48 W/kg (Murphy and Allen, 1997). The projected specific power for next-generation, flexible thin-film PVs at the blanket level is approximately 2,700 W/kg for 9 percent efficient, three-junction amorphous silicon on 1-mil Kapton and 770 W/kg on 0.5-mil stainless steel (Guha et al., 1999). The high specific power of thin-film PVs derives from the deposition of ultrathin (<10 μm) layers of semiconductor absorber material on thin (0.5–3 mil) flexible polymer or steel substrate blankets. The semiconductor layers are typically deposited onto large polymer or steel sheets using low-cost evaporation, sputtering, or plasma-enhanced techniques. The leading thin-film candidates under development are amorphous silicon (a-Si) and polycrystalline copper indium gallium diselenide (CIGS) and its alloys. State-of-the-art efficiencies for large-area (0.5–1.0 ft2) a-Si are approximately 8–9 percent (space spectrum) and 6–8 percent (0.2 ft2) for CIGS on stainless steel (Reinhardt, 2001a). The present development goal for space-qualified thin-film PVs under ongoing Air Force research and development programs is 10–12 percent by 2002, 15 percent by 2005–2007, and 20 percent multijunction CIGS by 2010–2015, yielding projected solar-array-specific power levels of approximately 300, 450, and 600 W/kg for an array support structure areal density of 0.1 kg/m2. Technical Performance Goals Needed for Economic Competitiveness Successful development of an economically viable SSP will require substantial leaps in development of space solar array, PMAD, thermal control, wireless transmission, and launch technologies. However, improvements in PV solar array technologies alone will not enable SSP to be economically competitive with terrestrial utility electricity. The theoretical maximum solar cell conversion efficiency is between 50 and 60 percent for crystalline multijunction solar cells (Kurtz et al., 1997) and between 30 and 40 percent for amorphous and polycrystalline thin-film photovoltaics (Reinhardt, 2001a). Even for the case of 60 percent
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy efficient crystalline solar cells, the array specific power will be limited to values less than 500 W/kg. Assuming that the SSP solar array must produce 3 GW of power (to result in 1.2 GW to the ground), the mass of a 500-W/kg solar array alone will be 6×106 kg, approximately 241 times that of the space shuttle’s maximum cargo capability—clearly a formidable challenge. Even assuming the SERT program’s launch-to-GEO goal of $800/kg, the cost of launching the array alone would be $4.8 billion. In the case of thin-film PV, using even 40 percent efficient arrays at 1,200 W/kg would require approximately 100 launches of the current space shuttle (at maximum payload capacity) for the array to reach LEO. It is clear from this simple analysis that improvements in PV-based power generation technologies alone, even to theoretical efficiency limits, will not enable SSP to be economically viable for competitive baseload terrestrial electric power, regardless of solar array cost. Even if the solar array were free, the overriding factor is the cost of placing it in orbit. Challenges to Be Met As stated above, improvements in PV solar array technology alone will not enable SSP to be economically competitive with terrestrial utility electricity, even if the solar array were free and theoretical efficiency and mass performance levels were obtained. The committee believes that the greatest challenge for the SSP program is to develop more realistic and accurate system cost and performance models, including theoretical solar array, power management and distribution (PMAD), thermal control, and wireless transmission cost and performance parameters, that will allow the launch cost to be realistically quantified. The issue is not the future cost of PV solar array technology, because one day the terrestrial PV industry will reduce costs to a point competitive with utility electricity, but the cost to place the array in orbit. Considering the paramount challenge in technology development required for other SSP disciplines, and the approximate $200–$300 million invested annually in space and terrestrial PV development worldwide, the SSP program should make minimal investment in current PV technologies. It is important to note that there is actually little difference between space and terrestrial PV technologies. There is virtually no difference in the electrically active part of the PV cell that controls conversion efficiency, called the p/n junction. The only difference lies in packaging of the cell, a technology that requires no significant new development. Also, a more thorough trade study must be conducted to rule solar dynamic heat engines for power generation in or out. The option of using solar dynamic heat engines in the SERT program was briefly discussed; however, a comprehensive trade study has not been conducted. Solar dynamic options are presently 3–4 times heavier than conventional PV arrays, 20 W/kg versus 60–80 W/kg, respectively. Also, current solar dynamic options are roughly 10 times more expensive than PV arrays, $5,000/W versus $500/W, respectively. These data, along with the additional concern that solar dynamic requires very high solar concentration ratios and, hence, extremely accurate pointing of very large solar collectors, indicate that it may not be a good choice for SSP. Recommended Priority for Investment Considering the paramount challenge of reducing SSP power generation system mass to several orders of magnitude less than today’s PV solar array systems, concepts other than conventional crystalline PV, thin-film PV, or solar dynamic concepts need to be developed. Even if SSP PV-based solar array were free, the theoretical specific power for conventional PV arrays of 1,000 W/kg would result in a cost-prohibitive launch requirement. Revolutionary breakthroughs are required in solar-to-electric power generation technology offering system-specific power in the range of 2,500–10,000 W/kg. Considering the small SSP investment in PV solar array technology today (<$2 million) compared with the large national and international PV technology investment of more than $200–$300 million annually, the committee recommends that the SSP effort focus future investments on revolutionary high-risk, high-payoff solar-to-electric approaches that could result in system specific powers in excess of 2,500 W/kg. At present, SERT is not working on such a strategy. The U.S. government’s Small Business Innovation Research program could be one efficient avenue for exploring such high-payoff, high-risk technologies. Recommendation 3–2–1: The SSP program should focus future investment in solar power generation solely on next-generation, revolutionary, high payoff, high-risk concepts.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy Synergy with Other Programs The current SSP baseline power generation system is PV solar. PV is considered a pervasive spacecraft technology and important for all near- and far-term DOD, NASA, and commercial space programs, as well as for terrestrial electrical power applications. All performance improvements in solar array technology, such as increased efficiency and reduced mass and cost, will greatly benefit a multitude of future government and commercial space and terrestrial power systems. There is presently significant U.S. government and private industry investment in the development of technology for space and terrestrial photovoltaic cells, arrays, and systems. The NASA and Air Force R&D annual budgets for space PV are approximately $2 million and $5 million, respectively, with industry investing approximately $1 million of independent R&D in space PV. The DOE has an annual budget of approximately $75 million for PV technology development, supported by approximately $20 million from private industry. Japan and the European Union have also increased support for PV research significantly during the past few years. As stated in the previous section, the recommended priority for investment by the SSP program in power generation should be in revolutionary, extremely high-risk, extremely high-payoff solar-to-electric approaches that could result in system-specific powers in excess of 2,500 W/kg. Such investment would have tremendous synergy with future NASA and DOD SPG technology development. Both agencies have identified requirements for significantly higher levels of space platform electrical power for future missions, where the paramount goal is to provide the spacecraft payload increasingly greater power and mass budgets without increasing total spacecraft mass or cost. Any investment in next-generation, revolutionary, solar-to-electric conversion technology that results in increased solar array-specific power will benefit next-generation NASA and DOD spacecraft. Also, the SSP program should better capitalize on government and industry PV technology investment and participate in the U.S. government’s Space Technology Alliance1 studies that address government strategic planning and investment in space PV (Sovie, 1999). Further coordination with the National Center for Photovoltaics, which includes the DOE’s National Renewable Energy Laboratory and Sandia National Laboratories, should also be pursued in order to capitalize on other PV technology investments. Recommendation 3–2–2 [also 2–1–8]: NASA should expand its current cooperation with other solar power generation research and technology efforts by developing closer working relationships with the U.S. Air Force photovoltaics program, the National Center for Photovoltaics, industry, and the Space Technology Alliance’s Space Power subcommittee. 3-3 SPACE POWER MANAGEMENT AND DISTRIBUTION Objectives and Scope The scope of the space power management and distribution (SPMAD) subsystem starts with the power source (solar panels or heat engine) and ends with the major loads—the wireless power transmitters and, if used, the electric propulsion system. Most of this section is written assuming that the power source is photovoltaics. A short subsection near the end provides a discussion of the differences necessary in PMAD design if the power source is a heat engine using a thermodynamic cycle such as the Brayton (gas turbine) or Stirling. The basic starting point is the individual solar photovoltaic cell (also known as a solar cell). The solar cell can be large (1×3 in. for crystalline solar cells and up to 12×12 in. for thin-film solar cells), and many must be connected in series to establish the desired bus voltage. A collection of solar cells, their substrate, and the supporting structure is called a “panel.” A collection of panels with additional structure is called an “array.” We assume that the PMAD interface is at the panel level of integration. An essential element of PMAD is the cabling that collects the electricity and routes it to the major loads. Since the SSP system will be very large (several square kilometers) a large amount of cabling, tens if not hundreds of kilometers, is necessary and will account for a significant fraction of the mass of the system. The major load of this subsystem is, of course, the wireless power transmitters that beam the power to Earth (or to a receiving station in space). For the mi- 1 Currently the Space Technology Alliance consists of members from DOE, NASA, DOD, the Ballistic Missile Defense Organization, and the Defense Advanced Research Projects Agency.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy crowave systems (magnetron, klystron, or solid-state power amplifiers), the transmitters must be physically close together, and all the solar panel power must be concentrated at one point. For the laser power transmission option, the transmitters will be distributed over many smaller satellites, thus allowing the length of the power distribution lines to be much shorter. A secondary load might be an electric propulsion system used for station keeping, obstacle avoidance, and attitude control. Other major components of the SPMAD system are used for power conversion and conditioning. These components raise the voltage so that the power can be distributed with lower losses or less mass, or both, providing the exact voltages to the various loads. For some designs, the voltage may be converted from direct current (DC) to alternating current (AC) and then back to DC. Current State of the Art Most spacecraft have PMAD systems and have had them since the beginning of the space program. Thus, in principle, one could be built for the SSP system. However, it would be impractical due to excessive cost and mass. Both increased performance (higher efficiency, higher voltages, and higher temperature operation to reduce thermal control challenges) and reduced mass and cost of the individual components are necessary for a practical SSP PMAD system. The design and performance of state-of-the-art PMAD subsystems varies widely, depending on the specific application. Many satellites have extensive battery systems, which require battery charging and discharge circuitry and controls. Efficiencies are 60– 90 percent (Reinhardt, 2001b). For DC-to-DC inverters, the efficiency can be greater than 90 percent (Ashley, 2001). Masses of complete subsystems with cabling and battery chargers are 33–40 kg/kW (25–30 W/kg) (Reinhardt, 2001b). Cabling must be estimated and optimized separately for SSP because of the extreme lengths of the cable runs. Without including cabling and battery circuitry, the power density might be as high as 150–300 W/kg (Ashley, 2001). Costs are $50–$100/W (Reinhardt, 2001b; Ashley, 2001). Technical Performance Goals Needed for Economic Competitiveness The NASA subsystem team has adopted the following goals for this SPMAD subsystem: end-to-end efficiency greater than 94 percent, mass less than 2 kg/ kW (power density greater than 500 W/kg), and a cost of less than $300/kW (30 cents/W) (Mankins and Howell, 2000b). Challenges to Be Met System Design Optimization The mass and cost of the SPMAD subsystem form a major portion of the whole SSP system. For example, at 500 W/kg the PMAD mass would equal that of the solar array. If the solar array achieved a breakthrough performance goal of greater than $2,500/W, the SPMAD system would dominate the mass of the array. As a result, optimization of the design of the system will be important. This is a systems design issue and must have continuing attention to help guide investment priorities for the various hardware components of the whole SSP system. In other words, this optimization is not an R&D activity but rather a systems activity. Adequate resources are necessary to do it well, but it should not have to be an area of significant investment. Power Distribution Voltage and Insulation Power distribution voltages and insulation are areas of future research. The conductors will undoubtedly be aluminum, which is optimum. High voltages (tens of kilovolts) are desired to reduce transmission line losses and mass. Some design concepts even use bare wires (i.e., no insulation). Use of these high voltages will present challenges of electrical breakdown and performance of insulators, insulation, and switchgear. The power collection and distribution voltages are important system optimization variables. R&D investments in this area include (1) space effects on conductors (such as arcing and damage on bare wires from the plasma and particles resulting from possible meteoroid impact to the SSP system as a whole) and (2) types of insulation for insulated systems. These topics are important because they may limit the highest voltage, having a first-order impact on the system mass and cost. Electrical arcing and other breakdown limits are also key issues. Steady-state leakage of current can occur without arcing. Other solar system and spacecraft events such as solar storms, thruster firing, outgassing,
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy and possible cosmic ray or meteoroid impacts can cause potential damage. Recently, communications satellites in GEO have experienced serious arcing damage (Hoeber et al., 1998). As a result, select flight experiments at GEO may be necessary to ascertain optimum allowable voltage limits. Reduction of thermal heat rejection and radiation hardness of components are major challenges in PMAD component design. High-temperature operation and passive thermal control are goals. Adequate investment must be made in these areas to find the appropriate solutions. In-space experiments may need to be done, at GEO conditions, to demonstrate lack of breakdown and possibly lifetime issues. Power Conversion and Conditioning Power conversion and conditioning are important parts of the PMAD subsystem. These aspects of PMAD are major mass and cost drivers. The components needed include power transistors (probably thyristors), sensors, transformers, inductors, capacitors, and switch gear (transistors and circuit breakers). These components can be assembled into inverters to convert DC to AC (and vice versa) and to raise and lower the voltages. They also filter and regulate the output voltages to those required by the loads. These components have been an area for investment since electricity was discovered—initially for terrestrial power applications and, for the last 50 years, missiles and space. Total PMAD system efficiencies remain below 90 percent. Enormous investments have been made over many years by both the government and the private sector. Steady progress has been made; however, this is an appropriate area for investment by the SSP program in order to reach its goal of greater than 94 percent efficiency. A few percentage points of improvement in PMAD efficiency will translate to a larger percentage point reduction in SSP mass and cost, reduced system heat load, and thus fewer thermal management problems. However, large breakthroughs should not be expected but, rather, slow, incremental progress. Efficiencies may improve a few points, but factor-of-two improvements will not be seen. Thus, reductions in mass for this subsystem, other than by optimization, will be modest. This, however, is an area where investment in manufacturing techniques may provide dramatic reductions in cost. Space components, typically, are made in very small quantities and are subject to extensive testing. For any SSP system, there will be a need for millions of each item, so process improvement and mass production could result in substantial cost reduction. NASA typically does not invest in manufacturing technology, but for SPMAD, that investment may represent the largest payoff. Each subsystem should have mass, cost, and performance targets. The mass is also important because a significant portion of the overall system cost, approximately one-half, is incurred in launching all the mass to GEO. Heat Engine Usage Some of the SSP conceptual designs being considered use a heat engine, such as a Brayton or Stirling thermodynamic cycle, to convert solar heat to electricity. An advantage of such a system is that the sunlight can be concentrated at one, or several, points, and the power conversion and distribution (cabling) mass and costs can be reduced. Disadvantages are the large and heavy heat rejection radiators required, lifetime issues with moving parts, and the management of angular momentum of the rotating machinery for the Brayton option (the Stirling will undoubtedly have a reciprocating alternator). A system-level advantage of using a dynamic system is that the electrical generating system will directly produce AC power at a relatively high voltage. This will eliminate the inverter that is needed in the DC systems to convert DC to AC. Recommended Priority for Investment Recommendation 3–3–1: A major investment should be made in space power management and distribution. This should include efficiency improvements, mass reduction, and manufacturing techniques designed to reduce the cost of the components. Almost all of the manufacturing investment should go to companies that are already making such components for either ground or space applications. Synergy with Other Programs SPMAD is important for all space programs— whether they be NASA, DOD, or commercial efforts. Thus, investments made in this SSP program will benefit other users. In addition, PMAD R&D by others
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy The state-of-the-art thermal management system is arguably the Boeing 702 spacecraft’s deployable-loop heatpipe radiator, estimated to have a specific mass of about 4 kg/kW (Gerhart, 2001). Interestingly, the long-term (15–20 years) specific mass goal identified by the SERT program is 4 kg/kW. This figure greatly underestimates what is possible. A 4-kg/kW system would be far too heavy for systems larger than 25 kW. A new ultralightweight, flexible, deployable radiator has been developed by Creare, Inc., having a projected specific mass of ~1 kg/kW and areal mass density of 0.75 kg/ m2 using mechanical pumps and inflatable deployment structures presently under development.5 Considering the present minimal investment in next-generation thermal management technologies, the metrics of 1 kg/kW and 0.75 kg/m2 can be expected to achieve and remain the state of the art over the next 5–10 years. Assuming that the heat generated by the SSP PMAD system is actively dissipated by the thermal management system—that is, 100 percent of the waste heat passes through an active cooling loop—then the resulting mass and size for the thermal management system would be formidable. For example, assuming that 3 GW of electrical power must pass from the solar array through the SPMAD electronics to enable 1.2 GW to reach the terrestrial utility power grid (the solar array must be oversized to account for SPMAD and WPT transmission and collection losses) and assuming the PMAD system is 90 percent efficient, then the heat generated by the SPMAD system would be 300 MW. The required thermal management system mass and area to support PMAD alone would be 3×105 kg and 4×105 m2, respectively, requiring approximately 12 flights of the current space shuttle at maximum payload capacity to simply lift the PMAD components to LEO. However, it is noted that a significant reduction in thermal control mass could be achieved if the SSP PMAD system was distributed throughout the large area of the solar array, potentially enabling passive cooling via heat radiation directly out to space. Technical Performance Goals Needed for Economic Competitiveness Successful development of an economically viable SSP system will require large advances in development of space solar array, PMAD, thermal control, wireless transmission, and launch technologies. Improvements in thermal management technologies alone will not enable SSP to be economically competitive with terrestrial utility electricity. However, significant thermal management mass and cost savings could be achieved via use of heat pumps to increase radiator temperature. The size and mass of the radiator is determined by the maximum temperature of the radiator; the greater the temperature, the smaller and lighter the radiator. The 1-kg/kW and 0.75-kg/m2 performance metrics above are based on a rejection temperature of 120°C driven by today’s maximum PMAD operating temperature. It is projected that heat pump technology could be available in the 2010–2015 time frame, allowing an increase in radiator temperature to as high as 300°C, enabling a reduction in power density to 0.4 kg/ kW (Gerhart, 2001). The SERT program did not address the potential impact of future heat pump technology on thermal management performance. Challenges to Be Met As stated above, improvements in thermal management technology alone will not enable SSP to be economically competitive with terrestrial utility electricity. The committee believes that the greatest challenge to support SSP is to develop more realistic and accurate SSP system cost and performance models that include theoretical thermal management, solar array, PMAD, and wireless transmission cost and performance parameters, to be able to realistically quantify SSP mass, volume, cost, and launch challenges. Recommended Priority for Investment There is no single or simple solution for reducing the large size and mass of the thermal management system required to dissipate SSP PMAD system waste heat. In general, what is required is development of higher operating temperature PMAD electronics and thermal management components. To support higher operating temperatures, advanced heat pump development should be accelerated. Recommendation 3–8–1: The SSP program should focus efforts in thermal management and materials on revolutionary high-payoff, high-risk approaches having potential cost and mass savings not only for 5 Research was funded on this radiator through a U.S. Air Force Small Business Innovation Research program entitled “Freeze-Tolerant, Lightweight, Flexible Radiator,” Contract Number F29601– 98–C–0115.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy SSP-related activities but also for most other space technology development efforts. Recommendation 3–8–2: The goals of the SSP program in the thermal management and materials area should be re-evaluated, taking into consideration the current state of the art in thermal management and projections of technology advancement in the next 10–20 years. Synergy with Other Programs Thermal management is considered a pervasive spacecraft technology and important for all near- and far-term DOD, NASA, and commercial space programs. Thermal management is also critically enabling for all DOD, NASA, and commercial aircraft. Any performance improvements in thermal management technology, such as reduced specific mass (kg/kW) and areal mass density (kg/m2), will greatly benefit a multitude of future government and commercial space and aircraft systems. 3-9 SPACE TRANSPORTATION AND INFRASTRUCTURE Space transportation will be a vitally important aspect of a commercial-scale SSP initiative. Past studies have shown that a major portion of the total deployment cost for this program will be attributable to this one area (Davis, 1978). Two space transportation activities are now expected: (1) launch of the massive elements of an SSP system from Earth into LEO and (2) subsequent transfer of parts or partly assembled elements of an SSP system into the operational GEO location currently favored by the program because of its inherent orbital properties, lower levels of orbital debris, and room for assembly and maintenance activities. For an operational program, large reusable space launch vehicles are expected to serve the former needs, while highly efficient electric propulsion may be used for orbit transfer. Launch Vehicle Needs NASA demonstrated that in order to reduce costs for the system as a whole, vehicles and technology useful for continuous build and launch capability would be necessary. The scale of space launch activities necessary to place meaningful capacity for supplying Earth’s needs for electrical power was illustrated to the committee as follows (Olds, 2000):450 flights per year for 30 years of a launch vehicle capable of delivering 40 tons per flight or over 18,000 tons (40 million pounds) per year. This level of activity is 80 times that of current space launch activity. For example, eight flights per year of today’s space shuttle will produce useful mass in LEO of about 160 tons. If other space launch vehicles deliver half this amount, the multiplier of 80 is confirmed. If future studies and demonstrations indicate qualitative and cost viability of space power, two important questions will be the rate and the timing of penetration of the global market for this new source of electrical energy. If SSP is shown to be sufficiently economical to begin such a program, the demand to place these satellites may rapidly grow well beyond that indicated in the NASA SERT studies. Other large-scale uses of space by both the global private and public sectors may occur. These new uses are expected by many analysts to be contemporary with an SSP system and will multiply the levels of launch activity associated with it (Andrews and Andrews, 2001). No one can accurately forecast today if these huge increases in traffic to space will be realized. Similarly, although extrapolations and estimates may be made, no one can accurately determine, today, the costs of space transportation 30 and more years in the future. As a result, decisions on the viability of future space solar power efforts should not be based solely on the space transportation segment of the program. If today’s expendable launch vehicles persist for another four decades and if present forecasts made by their suppliers’ of an inelastic, or fixed-size, launch market prove to be accurate, continued high launch costs will render power from space uneconomical. If, however, new concepts and approaches for space launch are adopted and evolved and if new space launch traffic is attracted by lower costs and higher reliability, then costs will fall, perhaps quite dramatically. Future space transportation work within any near-term SSP program should be focused on the needs of a major NASA SSP demonstration project (i.e., MSC 3). Such a program will require significant space transportation. If a 10-MW full-scale demonstration is selected and if the specific mass is that of earlier NASA studies, the satellite may require 150 to 200 tons of mass placed into orbit. If the demonstrator satellite is to be placed into GEO, additional mass in LEO will be necessary. Extended experimental use of this demonstration unit
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy is likely, further increasing the space transportation needs of this program. Thus, the MSC 3 SSP demonstration satellite program may produce a space launch task approaching the magnitude of placing the International Space Station into orbit. During the same interval, NASA and others may embark on ventures in space as large or larger than this demonstration of power from space. These may include establishing permanent bases on the Moon, human exploration missions to the Martian moons or the surface of Mars, exploitation of asteroid resources, or public space travel. The NASA SERT program has established, for a commercially viable SSP system, a goal of $400/kg of mass as the allowable price for each of the two transportation legs—Earth to orbit (ETO) and in space (orbit to orbit). Determining whether this can be achieved is not within the scope of the present assessment; however, the committee feels that this goal may not be low enough to support a commercially viable SSP terrestrial power system. The costs of space transportation must be reduced below $800/kg if the present cost goals of the NASA SERT program are to be realized. Mass goals must also be tightened for other SSP subsystems since these will determine the size of the transportation task. Re-evaluation of these goals is important to SSP strategy because it will influence the technologies that should be pursued. Current space transportation goals are already driving near-term choices within the SERT program, creating the possibility that technology investment choices may be made based on goals that are not stringent enough in this area. Recommendation 3–9–1: The SSP program should re-evaluate its cost goals in the space transportation area. NASA should ensure that its space transportation technology development activities consider the revised SSP goals. In-Space Transportation Needs One important driver in establishing on-orbit SSP cost is in-space transportation (i.e., LEO-to-GEO transportation). The current SERT program has not yet prioritized investments in this area, although budget schedules for the program show increased activity in this area by 2006. Low-cost options for in-space transportation may require large advancements in technology—advances that may not occur within the next 5– 10 years without program stimulus. As a result, investments need to be made in this area, whether under the purview of the SSP program or as a separate NASA initiative. Applications in addition to SSP will be available for effective use of such new in-space transportation options. Technologies for in-space transportation such as electric, solar-electric, magnetohydrodynamic, ion, and solar-thermal propulsion could all be considered as possible candidates. Low cost and high rate of usage will be driving factors for SSP since multiple use of vehicles will be required. Preliminary cost analyses by the SERT program have also shown that costs related to ownership and use of in-space transportation, as well as launch vehicles, will have impact on the commercial viability of terrestrial power from space. Detailed trade studies of various in-space transportation concepts should be made along with satellite design and assembly concepts in order to establish lowest cost methods for placing SSP elements in GEO. A preliminary assessment of these needs and ideas to meet those needs was performed during the SERT program through a university-funded research grant. Again, as for launch vehicles, cost and mass goals must be tightened because they will influence the technologies that should be pursued. Recommendation 3–9–2: Detailed trade studies for various in-space propulsion options should be performed early in the program. Issues involving cost, packaging, mass, trip time, and risk assessment must be incorporated into the vehicle choice. Flight Test Demonstration Transportation Needs Although not dependent on major advances in space transportation within the next 5 years, planned SSP flight test demonstrations will require detailed assessment of needs in the area of space transportation. The committee understands that configurations have not yet been chosen for the future SSP technology flight demonstrations (TFDs). However, planning must begin now to determine if current launch vehicles will be adequate and if sufficient mass and volume are available on the shuttle or suitable expendable launch vehicles in the time frame anticipated. Other than very preliminary mass estimates for various TFD configurations, little systems work has been seen by the committee in the area of space transportation. Most space transportation analysis within the program has been concentrated on space transpor-
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy tation needs for the full-scale MSC 4 system. This is an important factor in determining the appeal of a commercial terrestrial SSP system; however, launch vehicle and in-space transportation needs for the early MSCs must be assessed during the planning stages. Recommendation 3–9–3: The SSP program should develop detailed space transportation requirements for all technology flight demonstrations (MSC 1, MSC 1.5, and MSC 3) that include data from studies on packaging, cost and mass estimates, and other important parameters. Opportunities for Synergy NASA’s efforts toward the new Space Launch Initiative are intended to reduce risk for NASA’s future needs, with funding of $290 million to NASA in FY 2001 and plans by NASA to expend up to $5 billion in this area during the next 5 years. However, nowhere in the descriptions of the SLI provided to the committee is power from space mentioned (Davis, 2000). Improved coordination is needed between NASA space transportation efforts and the future needs of the United States for space launch, including any future SSP program. Work should be conducted to explore the ramifications of other new markets and to describe the evolutionary paths that might bring this low-cost launch capability into operation. Information should be provided to SLI on the SSP program’s needs for future TFDs, including cost goals, optimal payload mass, packaging, launch rates, reliability needs, and scheduled need dates to achieve the SSP goals. Recommendation 3–9–4: The SSP program should begin discussions between its management and that of the NASA Space Launch Initiative, so that future milestones and roadmaps for both programs can reinforce one another effectively. This discussion should include specific information on SSP space transportation needs, including cost goals, timelines for deployment, optimal payload mass, packaging requirements, launch rates, and reliability requirements. NASA’s Advanced Space Transportation (AST) program (NASA, 1999) was created to achieve NASA’s goal of significantly lower space transportation costs. The program has initiatives in four major areas, including the support of long-term technology research in advanced chemical and nonchemical propulsion systems for use in space. Advancements in the areas of high-power electric propulsion (Hall and ion thrusters), cryogenic engines, spacecraft miniaturization, solar-thermal powered transfer, electrodynamic tether orbit transfer, and in-situ propellant utilization are expected under the program. Propulsion research is also being supported in the advanced concept areas of magnetic levitation, pulse detonation, beamed power, magnetohydrodynamics (MHD), fusion, antimatter annihilation, and breakthrough physics. Currently, there is little evidence of interaction between the SSP program and the AST program in developing new in-space transportation options. Other NASA efforts not labeled specifically within the AST program are also being made in electric and solar electric propulsion, including programs at several NASA centers and the Jet Propulsion Laboratory. Many of these efforts are in conjunction with academic institutions. Efforts in NASA also include advanced chemical and MHD propulsion, beamed energy and momentum propulsion, and fission and fusion propulsion. NASA co-sponsors, in conjunction with the University of Alabama in Huntsville, an annual joint conference on advanced space propulsion concepts6 from which current technology advances can be identified and the efforts of the academic community better utilized. Recommendation 3–9–5: The SSP program should encourage expansion of the current in-space transportation program within NASA and interact with its technical planning to ensure that SSP needs and desired schedules are considered. Currently, many other non-NASA investments are being made in orbit-to-orbit transportation research, including major programs within the Air Force and industry. There is little evidence of the SERT program leveraging these program investments or partnering with internal NASA space transfer technology programs. The Air Force Research Laboratory’s Propulsion Directorate currently has research programs in electric propulsion, solar thermal propulsion, pulsed 6 Proceedings for the last three meetings are available online at <http://www.eb.uah.edu/maglev/aspw/>. Accessed on August 16, 2001.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy plasma propulsion, and Hall effect thrusters.7 The U.S. Navy Research Laboratory’s Naval Center for Space Technology also supports research efforts in the areas of MHD, tether, and electric propulsion technology.8 Recommendation 3–9–6: The SSP program should increase coordination of industry, academic, and other NASA and non-NASA government investments in advanced in-space transportation concepts, particularly in the areas of electric, solar-electric, magnetohydrodynamic, ion, and solar-thermal propulsion. 3-10 ENVIRONMENTAL, HEALTH, AND SAFETY FACTORS Objective and Scope This section considers the environmental, health, and safety issues of SSP systems. Environmental factors include any significant effects on the space surrounding the satellite in orbit, on the media through which power is transmitted, and on the neighborhood of the receiving station. These effects generally include contamination of the environment in the form of communications interference, generation of debris in orbit, or possible effects on the atmosphere (i.e., power transmission to Earth receiving sites). Safety factors include any health hazards to Earth biota associated with power beaming, whether in the form of microwave or laser transmission. All of the concerns associated with environmental, health, and safety factors must be addressed early in the program, with particular emphasis on public awareness and public perception. If any of these risks are found to be of significant public concern, the public may be reluctant to host an SSP ground receiving station regardless of the economic competitiveness of SSP. Environmental, health, and safety risks may be minimized through appropriate technology investments and technology design guidelines. However, higher priority should be placed on these areas by the SSP program. In designing a full-scale SSP system, the environmental impact analysis must also include pollution that may occur during emplacement of the facility (production and launch of transport vehicles), as well as environmental effects during the operational phase. However, it is also essential to include, in the case of SSP for terrestrial power generation, positive effects such as decreased pollution in comparison with other forms of power generation (i.e., fossil fuels). The great appeal of terrestrial SSP, of course, is that, once in place and operating, its contribution to greenhouse gases in the atmosphere is zero. Current Issues Environmental, health, and safety issues are now recognized as essential concerns to be addressed as early in a program as possible. Some 60 percent of the effort in the early solar power satellite studies was devoted to investigating environmental and societal issues (Koomanoff, 2001). During the SERT program, a working group was assembled to look at environmental, health, and safety factors. Preliminary findings of this group show that the following are likely to be issues (Anderson, 2000): Lack of information on the risks associated with exposure to low-level microwaves, particularly long-term human exposure to low-level microwaves; Lack of data on effects of laser beaming options on humans; Spectrum allocation issues (international considerations); Orbital space allocation issues (in geosynchronous orbit); Land use availability for receiving systems for terrestrial SSP; Transmission interference concerns (both on orbit and on the ground); and Possible use of SSP systems as a weapon. The effects of power beaming are primarily a function of the frequency and the energy density of the beam. Currently, the frequencies and energy levels for SSP power transmission are estimated to be as follows: microwave, 5.8 GHz (5.2-cm wavelength) and 1 kW/ m2 power level; and laser, 1.03-mm wavelength and a yet-to-be-determined kW/m2 (note: design will be eye safe due to distributed transmitting sources). These values represent very low energy levels, and 7 More information on the Air Force Research Laboratory’s space propulsion programs is available online at <http://www.pr.afrl.af.mil/>. Accessed on August 16, 2001. 8 More information on the Naval Center for Space Technology’s programs in space propulsion is available online at <http://ncst-www.nrl.navy.mil/>. Accessed on August 16, 2001.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy in both cases, the beam will be designed to be quickly “turned off.” Microwave transmissions can be dephased, dramatically reducing energy density. Laser strengths will be planned only at eye-safe levels. Current design specifications for any future SSP system allow only low energy densities and design-safe standards to prevent further focusing of the beam. The center of the beam’s Gaussian distribution is planned for energy densities of about 23 mW/cm2, or about one-fifth the intensity of summer sunlight at noon (Mankins, 2000a; Moore, 2000). Additionally, any residual energy outside the rectenna’s protective fence would be far below current microwave safety standards, between 0.01 and 1 mW/cm2 (Moore, 2000). Most research in the area is focused on the effects on humans, animals, and biota of radiation from household devices, digital phones and other electronic equipment, and electric utilities (NIEHS, 1999). Little research has been performed at field levels specific to SSP application. However, research has been done on bees and birds exposed to microwave radiation at twice the dose expected for a creature flying through a typical microwave power transmission beam. Results to date indicate that there is no effect, at least on the animal’s directional flying ability (Koomanoff, 2001). Other testing has been performed on monkeys and is now under way with humans exposed to low-level microwave radiation. Results to date from this testing indicate that such exposure apparently does not render the subject sterile or result in cataracts or any other deleterious effects (Kolata, 2001). The larger problem may be preventing animals from roosting on the rectenna or damaging portions of the receiving site— issues that are already of concern to terrestrial solar power generation farms. Research was previously funded on the effects of microwave radiation on plants as well (Michaelson and Lin, 1987); however, only a small body of work has been performed or published in this specific area (Skiles, 2001). Results from the previous research have been inconclusive. Studies have shown that microwaves (i.e., 2.45 GHz) generally have an inhibiting effect on plant growth (Picazo et al., 1999) and that reduced growth rates were demonstrated for higher power densities (Urech et al., 1996). Other studies show that effects of radio frequency waves may vary with the stage of plant development (Magone, 1996) and that no significant differences between exposed and unexposed cells were seen for plants exposed to 41.6 GHz frequency (Gos et al., 1997). The SERT program is currently funding at least one experiment to determine the effects of microwave fields on alfalfa (Skiles, 2001). The committee believes that further studies in the areas of environmental, health, and safety are warranted. One of the topics currently under investigation is determining how much small transmitters such as cellular phones may interfere with aircraft avionics and communications. Results of these studies may indicate whether there is reason for concern about aircraft flying through an SSP power beam. Many different industries and government institutions are also performing research to determine if microwave transmitters can cause cancer. So far, results appear to show that no correlation exists, but public perception is driving cell phone manufacturers to change their designs even without clear scientific evidence. The SSP program may well be subject to the same sort of public relations requirements. The committee understands that the relevance of such cellular phone research to space solar power may be questionable. However, program managers must be aware that the long-term effects of SSP wireless power transmission on humans must be quantified before public acceptance is found. As mentioned previously, a major issue is the lack of research on the environmental, health, and safety effects of exposure to this WPT system. Technologists must be able to prove very limited risk during use. Interference effects of the chosen WPT method on satellites and other spacecraft in the vicinity of the SSP system should be investigated as well. Concerns are very real about the effect of the orbital debris environment on large space structures in LEO (NRC, 1995; NASA, 2000). The problem is not just the present orbital debris population, but the worsening evolution of this debris population, increased perhaps by SSP operations. The orbital debris pollution in both LEO and GEO will remain and worsen. Research is currently under way to model the orbital debris environment more accurately and predict its increase in time. In addition, testing of hypervelocity impacts on spacecraft materials is ongoing. Also, methods of collision avoidance are being developed to aid spacecraft in avoiding collision with mid- to large-sized debris that can be tracked. Technical Goals Needed for SSP Systems studies should be done to clearly show not only that the SSP system is safe but also that the overall
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy environmental impact of a space solar power system is a positive one. Although once in operation the SSP system essentially generates power with zero pollution, the pollution produced during the construction and deployment of an SSP system (i.e., multiple space launches) must be within acceptable levels. This study of balance would have to include, for example, any environmental pollution that occurs during emplacement of the facility, such as harmful exhaust from the large number of launches necessary to transport the components of the system to orbit (currently the SERT program estimates between 500 to 900 Earth-to-LEO launches, each with a 40-metric-ton capacity, for one 1.2-GW SSP facility, depending on the chosen design). Environmental effects of the SSP power beam on the atmosphere, on Earth biota, and on the space environment surrounding the SSP satellite will clearly have to be minimal in all respects. It is not sufficient to simply show scientifically that effects are likely to be minimal; it is essential to mitigate from the outset any and all perceptions that the system may be harmful or dangerous in any way to humans, animals, or the environment. Politics and public opinion will weigh in heavily on the ultimate viability of SSP systems. Issues of electromagnetic compatibility between various hardware components must also be treated during the course of technology development. As technology readiness is improved, studies should be performed on individual technologies to determine the level of electromagnetic compatibility between the SSP system components themselves and between the wireless power transmission system and external electronic hardware such as avionics, terrestrial wireless communications systems, communications satellites, and medical devices. It will also be essential to show that the system cannot be used as a weapon. Fail-safe features should be designed into the system to prevent pointing a full-strength beam anywhere but at the receiving site to which it is designated. Current plans are to keep beam density low. For the laser option, for example, a person standing in the center of the receiving site should be able to look up at the beam and experience no eye or skin damage. In addition, to assuage any fears that this technology could be reworked to be used as a weapon, the program should be international in structure, with full disclosure of information to all participating countries so that no strategic advantage can be gained by any one nation. Challenges to Be Met In terms of environmental, health, and safety factors, the greatest challenges for SSP are to develop a system with benefits that outweigh the costs and a high level of safety that can be proven to the public. These challenges can be addressed by funding near-term and continuing research on the topics listed in the following two sections at frequencies, power levels, and orbital locations specific to SSP. The lists are neither exhaustive nor prioritized. Environmental Challenges The following topics are areas of environmental research that must be investigated before deployment of a full-scale SSP system (by NASA, other appropriate organizations, or industry): Quantify the net environmental effect of having terrestrial SSP available in comparison with reliance on conventional sources of energy, including the polluting effects of building and launching SSP hardware and the possible deleterious effects of power beaming through the atmosphere. Develop construction methods for SSP systems that minimize the generation of additional orbital debris. Provide for efficient and final disposal methods for solar power platforms once their useful life is over; otherwise, they too will contribute to the orbital debris problem not just in LEO but in GEO as well (Anderson, 2000). Determine how SSP systems will alter the radiation environment around them and quantify the resulting impact in terms of orbital space allocation and interference effects at GEO. Assess power transmission side-lobe effects (energy transmitted outside the nominal beam diameter) and the impact on communications in the vicinity of the power beam as well as around the receiving station (ground based or in orbit). Perform additional research on dual use of rectenna sites for terrestrial solar power application, so that sufficient real estate can be found in all potential locations.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy Health and Safety Challenges The following topics are areas of health and safety research that must be investigated before deployment of a full-scale SSP system (by NASA, other appropriate organizations, or industry): Perform further research on the effects, especially long-term ones, of low-level microwave and laser radiation on Earth biota (humans, other animals, and plants). Design the SSP system so that use as a weapon is impossible under any circumstance. Establish public awareness and education outreach programs covering the benefits of SSP, the technology behind it, and the built-in safeguards. Such programs might include development of demonstration models (Pignolet, 2001), a children’s book on SSP, or op-ed pieces on space solar power. Recommendations Recommendation 3–10–1: The SSP program should expand its environmental, health, and safety team in order to review SSP design standards (beam intensity, launch guidelines, and end-of-life policies); assess possible environmental, health, and safety hazards of the design; identify research if these hazards are not fully understood; and consider legal and global issues of SSP (spectrum allocation, orbital space, etc.). One approach would be to involve an international organization such as the International Astronautical Federation Space Power Committee in such studies. Recommendation 3–10–2: Public awareness and education outreach should be initiated during the earliest phases of an SSP program to gain public acceptance and enthusiasm and to ensure ongoing support through program completion. Recommendation 3–10–3: The SSP program should collaborate more effectively with other NASA programs in space-related environmental, health, and safety and with external industry and government agencies currently performing research related to such issues. Funding should be increased to cover gaps in research, specifically in areas that overlap SSP-related technology. NASA should initiate research that evaluates the effects of microwave and laser wireless power transmission at levels planned to be utilized by a full-scale SSP system. Recommendation 3–10–4: Studies should be initiated to determine the effects of the electromagnetic incompatibility of wireless power transmission systems and avionics, terrestrial wireless communication systems, and SSP-related electronics. Synergy with Other Programs Research is currently being conducted at NASA and DOD on possible strategies to address orbital debris in both LEO and GEO (NSTC, 1995). These strategies include radar tracking and de-orbit stabilization, among others. Programs are also in place in fail-safe beam pointing technology and wireless power transmission. The Federal Communications Commission will be concerned about interference of radio frequency beams with current and planned communications systems and issues of spectrum allocation. Before deployment of a full-scale system, international issues in spectrum allocation, orbital space allocation, and orbital debris evolution must also be considered. Research is being conducted by the wireless communications industry, the federal government, and other interested parties to settle questions about whether microwave transmission can be harmful to humans. The Electric Power Research Institute currently has a substantial multidisciplinary program that evaluates all aspects of health, risk, and field management research in respect to electricity and power.9 This program includes efforts in epidemiology, toxicology, engineering, and other sciences. The National Institute of Environmental Health and Safety convened a working group on the effects of electromagnetic waves and currently has in place a program of public information and dissemination on the effects of electric and magnetic fields (NIEHS, 1999). Although frequencies and beam strength are different, inclusion of representatives from these organizations in any future environmental working groups would be beneficial. The SSP program should also capitalize on expert knowledge through 9 More information on the Electric Power Research Institute’s research in environmental, health, and safety issues of electric power is available online at <http://www.epri.com/>. Accessed on August 15, 2001.
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Laying the Foundation for Space Solar Power: An Assessment of NASA’s Space Solar Power Investment Strategy work with the Committee on Space Research established by the International Council for Science—in particular, its work on environmental, health, and safety issues. The group should also continue its work with the International Astronautical Federation Space Power Committee. 3-11 PLATFORM SYSTEMS Objectives and Scope This subsystem of the SSP system consists of all the additional systems that are needed to construct a complete, integrated spacecraft, including attitude control sensors (Sun sensors, star trackers, and gyros), flight computers, the telemetry and data handling system, the command system, communications, and so on. Current State of the Art All of the above-mentioned subsystems currently exist on most spacecraft. The state of the art is advanced, and further advances will be made independent of the SSP application. Technical Performance Goals Needed for Economic Competitiveness The aforementioned subsystems make up a negligible fraction of the mass and cost of the SSP system. Therefore, economic competitiveness is not affected by these subsystems. Autonomous robots will place the highest demand on space computing. Challenges to Be Met There are many improvements possible in various satellite subsystems technologies. However, any advancements will be driven by many other applications. Recommended Priority for Investment Recommendation 3–11–1: The SSP program should not expend any resources to develop advanced technologies for standard satellite subsystems (e.g., attitude control sensors, flight computers, telemetry and data handling systems, and communications systems). Synergy with Other Programs There are many other programs in NASA and DOD to improve the performance of these subsystems with which the SSP program could collaborate. REFERENCES Akin, Dave, and Russ Howard. 2001. 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Representative terms from entire chapter: