Solar electric generating system resource requirements

S,lar Energy Vol. 23. pp. 255 261 Pergamon Press ltd.. 1979 Printed in Great Britain

SOLAR ELECTRIC GENERATING SYSTEM RESOURCE REQUIREMENTS R. C. EN(;ER't" and H. WEICHFL+ +

(Received 23 May 1978; revision accepted 6 April 1979) Abstract The potential consumption of materials, land, water, manpower, energy, and money by four proposed solar electric generating systems : a terrestrial solar thermal, a terrestrial photovoltaic, an orbiting solar reflector, and a satellite solar power system are analyzed. The evaluation demonstrated that, per megawatt of electricalgenerating capacity, the terrestrial solar thermal systemwould require lessmanpower, less energy of production, and less money than would the extra-terrestrial systems.

I. INTRODUCTION In view of the fact that the worldwide supply of fossil fuels is steadily diminishing, it is imperative that new sources of energy be developed. In an address before the nation, President Carter stated that "... we must start now to develop the new, unconventional sources of energy we will rely on in the next century[-4]". The experience of much of the nation during the 1977-1978 winter has accentuated this need. One of the proposed ways to solve the "energy crisis" is to use solar energy to generate electricity on a commercial scale. This is desirable because the sun provides energy without the need for mining and refining of fuel supplies, because solar energy is essentially inexhaustible, and because solar generated electricity is practically pollution free. However, the sun is not necessarily a perfect energy source. When compared to conventional methods of generating electricity, generation of electricity from solar energy is more expensive, requires larger generating plants (Ref. [3], pp. 1-2), and is hindered by the constant motion between the sun and the Earth. Several proposals have been made relative to the design of a commercial solar electric generating system. The primary purpose of this article is to present the results of an evaluation of the potential consumption of natural resources by four of these proposed systems: a terrestrial solar thermal (TST) system, a terrestrial photovoltaic (TP) system, an orbiting solar reflector (OSR) system, and a satellite solar power (SSP) system. 1.1. Scope of the research Each of the four systems listed above is evaluated with respect to its consumption of the following natural resources: materials, land, water, manpower, energy, and money. The usefulness of the results of the evaluation is limited by the fact that the four systems are at different stages of conceptual development. In t Captain, U.S.A.F.,Instructor of Physics, U.S. Air Force Academy. Lieutenant Colonel, U.S.A.F., Deputy Head, Department of Physics, Air Force Institute of Technology.

addition, the resource evaluation is limited by the uncertainty of some of the resource consumption data. In the future, as more accurate resource information becomes available, it will be necessary to reconsider the conclusions of the resource evaluation in light of the new information. 1.2. Assumptions It is assumed in the analysis that all systems, either proposed or evaluated, will be operational by the year 2000. It is furthermore assumed that the necessary mining, refining, and manufacturing facilities can be built in time to produce the tremendous quantities of materials needed for construction and operation of the systems. In addition, the analysis is based on the assumption that there will not be large scale social intervention, such as, for example, the environmentalist fight over the Alaskan oil pipeline, which would force system designers to adopt Significantly more costly construction and maintenance procedures. 1.3. Research approach The major task of the research effort was to collect data relative to the four systems evaluated. Every effort was made to compare the systems using the same basic assumptions. Ground rules were established for that purpose. Where necessary, modifications were made to existing designs, but only in those instances where it was believed to be in the best interest of the system being altered. A space transportation system was developed, based on work by NASA, and applied as fairly as possible to the OSR and SSP systems, the two systems using space components. In those cases where detailed system information was not available, independent Calculations were made based on available information. As a final step in the resource evaluation, all data and calculations were assembled, and the four systems were compared on the basis of their resource consumption per megawatt of electrical generating capacity. 2. SYSTEMSEVALUATED Before considering the results of the resource eva-

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R.C. ENGERand H. WEICHEL

luation, it is helpful to briefly describe the functions and major components of each system. In particular, it should be noted that the two terrestrial proposals can most nearly be termed "state-of-the-art" proposals. In fact, a prototype solar thermal plant is currently under construction near Barstow, California. A contract was recently awarded to the McDonnell Douglas Astronautics Co. for the $I00 m, 10,000 kW powerplant to be operational in 1980 or early 1981 (Ref. [15] p. 48). Finally, it should be noted that to take advantage of maximum available sunlight, the TST, TP, and OSR ground stations are assumed to be built in the southwestern portion of the United States. 2.1. Terrestrial solar thermal system ( T S T ) Information used in the analysis of the TST system was extracted primarily from a study by the Jet Propulsion Laboratory, California Institute of Technology (JPL)[3]. The TST is a central receiver type of plant with 6 hr thermal storage. Electricity produced from thermal storage is at only 70 per cent of rated capacity because of conversion and storage inefficiencies (Ref. [3] pp. 6-13). A central receiver plant uses sun-tracking mirrors to concentrate the sun's energy on receivers located on top of a central tower. Sunlight is used to heat a liquid contained in the receiver, The steam produced by this process is then used in a conventional steam Rankine generating plant (Ref. [3] pp. 4-22). Figure 1 is a sketch of a TST plant. t Air mass 1 is the mass of air in a vertical path through the atmosphere, above a sea level point on the Earth's surface.

For the resource analysis it is assumed that roughly 30 per cent of the land required for a solar thermal plant is covered with heliostats and that the average plant efficiency is 17 per cent (Ref. [3] pp. 6-13). Every 5 weeks the heliostat surfaces are cleaned (Ref. I-3] pp. 6-22), and to conserve water, dry cooling towers are used (Ref. [3] pp. 6-13). In dry cooling towers, fanforced air cools the liquid from the turbine generators as in an automobile radiator. 2.2. Terrestrial photovoltaic system ( T P ) The TP system selected for evaluation is based on the use of silicon solar cells supplemented with battery storage (Ref. [3] pp. 6-13). Primary source for data was a study by JPL[3]. The silicon cells are assumed to be 254 #m thick with an efficiency of 13 per cent at air mass 1 and at a cell temperature of 28°C.t The glass cover plates protecting the silicon cells are 76 #m thick (Ref. [3] pp. 4-36). The silicon cells are secured to tilted surfaces which are rotated twice a year. Tilted surfaces are used to compensate for the change in the inclination of the sun throughout the year. The TP system uses a concentration ratio of 2 : 1. This concentration ratio specifies the ratio of the total sunlight eventually reaching the solar cells to the sunlight received directly by the solar cells. Aluminium mirrors reflect sunlight onto the cells to achieve this concentration. A recent study indicates that the mass of silicon solar cells required to produce a given amount of electricity decreases as the concentration ratio increases, up to a limiting ratio of 2.4 : 1 (Ref. [1] pp. 35). To produce the desired concentration ratio,

Fig. 1. Terrestrial solar thermal (TST) system (Ref. [10] Fig. l(all.

Solar electric generating system resource requirements non-tracking asymmetric v-trough concentrators are used (Ref. [3] pp. 4-22). Figure 2 is a sketch of a TP plant. 2.3. Orbitin 9 solar reflector system ( OSR ) The OSR system selected for analysis is the system currently proposed by Ehricke and referred to as Soletta II. The analysis is based primarily upon information supplied by Ehricke[6-8]. The proposed system has both a ground and a space component. The ground component is similar to the TP system with the exception that it does not have the battery storage facilities. Instead, the system operates continuously by supplementing daytime sunlight with sunlight reflected from orbiting reflectors. Although it might be necessary to use some battery storage in order to levelize the electrical output of the plant, this factor is not considered in the analysis. A sketch of the OSR system is contained in Figure 3. The system consists of 1320 individual reflectors placed in 3 hr sun-synchronous orbits. Each reflector has an area of 8.73 km 2, a variable focal length, and a mass of 50-150 tons/km 2 (Ref. [6] pp. 26, 40 and 43). For this analysis, a mass of 100 tons/kin 2 is used. When focused on the Earth's surface, the image of the sun produced by each reflector has a diameter of 38.9 km. The orbital altitude of the reflectors is 4184 km. As currently proposed, the OSR system has a rated generating capacity of 74.2 GWe. This projection is based on the assumption that 776.6 km 2 ofsilicon cells, with a before-concentration efficiency of 13 per cent and a concentration ratio of 2 : 1, receive and process sunlight at an annual rate of 5 x 10 9 kWhr/km2-year.

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Of this incident energy, 2.15 x 109 kWhr/km2-year is provided directly from the Sun and 2.85 x 10 9 kWhr/km2-year is provided by sunlight reflected from the orbiting reflectors (Ref. ['6] p. 51). A total of five different orbits (Ref. [6] p. 431 are used in order to permit reflectors to simultaneously illuminate the terrestrial solar cells. Each reflector consists of several smaller reflector elements. The smaller reflector elements are made of sodium-coated kapton film. Sodium was selected because it is light, less costly, and has a reflectivity higher than aluminum. However, the coating must be applied in space in order to prevent the reflectivity from being destroyed by oxidation (Ref. [7] p. 35~. 2.4. Satellite solar power system ( SSP ) First proposed by Peter Giaser in 1968, the SSP system generates electricity by using large blankets of silicon solar cells placed in geosynchronous orbit. Sunlight intercepted by the solar cells is converted into electricity and then into microwave radiation. The microwave beam is then transmitted to an Earth receiving station located on low-value land or offshore. As a final step in the process, the microwaves are reconverted to electricity for commercial distribution at the ground station, commonly referred to as the rectenna. Figure 4 contains a sketch of the SSP system. Most SSP system design data was obtained from preliminary work done by the NASA, Johnson and Marshall Space Centers. Data relative to the ground portion of the SSP was taken from a study by JPL[3]t. Glaser uses silicon solar cells with an efficiency of 18 per cent IRef. [11] p. 574j. However, studies by JPL,

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Fig. 2. Terrestrial photovoltaic (TP) system (Ref. [12] Fig. l(bt).

258

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Fig. 3. Orbiting solar reflector (OSR) system. the Johnson Space Center, and the Marshall Space Center all have used lower efficiencies in their evaluations of the SSP system[3, 13, 14]. In fact, even though theoretical efficiency is 22 per cent (Ref. [ l] p.

8), there is some question as to whether 18 per cent efficiency will ever be obtained from silicon solar cells on a mass production scale. Conventional silicon solar cells in production volumes currently have an efficiency between 10 and 12 per cent (Ref. [14] IV.B.I.a.4,9). In order to be consistent with the other designs evaluated, the SSP solar cell efficiency is taken to be 13 per cent at air mass zero and 30°C. The 102/am thick solar cells are used in conjunction with v-trough type concentrators at a concentration ratio of 2 : 1 and have a plastic cover 25/am thick for radiation and micrometeoroid protection. It should be noted that solar cell thickness is one area where deviation between systems is tolerated. The 102/am cells modified for use in space will cost more, and the cheaper 254/am thick cells are probably adequate for terrestrial uses. However, a 254/am thickness places a large weight burden on the SSP system, and thus a 102/am thickness is required in order for space operations to be competitive (Ref. [3] pp. 4-36). The mirrors used for concentration are made of 6.35/am thick kapton, coated with aluminum, 2.54/am thick. The operating temperature of the silicon solar cells is assumed to be 100°C. Since a silicon cell of 16 per cent efficiency at 30°C has an efficiency of 10.3 per cent at 100°C (Ref. [14] IV.B.lb.6,7,8), therefore, by analogy, a 13 per cent efficient solar cell at 30°C has an efficiency of 8.37 per cent at 100°C. Assuming that 8.37 per cent is the operating efficiency of the silicon solar cells in the SSP system, the over-all efficiency is 4.8 per cent (Ref. [ 14] IV.A. 1.1). However, this low efficiency is offset by the fact that the amount of solar energy

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Solar electric generating system resource requirements

259

The energy of production figures found in Table 1 include the total energy required to construct, operate, and maintain each system. As will be discussed later in this paper, when dealing with an energy producing system, it is essential to ensure that it will produce more energy, in the long run, than will be consumed in its construction, operation, and maintenance. Thus, the energy of production becomes one of the most important methods of comparing competing systems. From the data in Table 1, it can be concluded that, of the four systems analysed, the TST system requires the least amount of production energy. The system requiring the most energy of production is the OSR system.

the material requirements of the respective systems. At first glance, it would appear that the TST system is the least desirable system because of its large material requirement. However, the real significance in the area of material requirements is not how much the material requirements weigh, but rather, how difficult the material requirements are to handle and produce. The TP and TST material requirements are the easiest to handle because all handling is done on Earth. In addition, although the TST system has the largest material requirement, over 50 per cent of this material requirement is cement, a material which is easy to handle and one that requires very little energy for production. In fact, as pointed out above, the TST system, while requiring more materials than the other three solar generating systems, requires the least energy of production. Thus, the large material requirement for the TST system should not be considered as a serious weakness of the system. On the other hand, when compared to the TST system, the SSP system requires only one fourth of the materials but over twice as much energy for production. This is primarily because the SSP system requires large amounts of aluminum, silicon, and liquid hydrogen. Production of each of these materials requires large amounts of energy. The OSR system fairs even worse because its liquid hydrogen requirement is nearly five times greater than that of the SSP system. This is due primarily to the extensive requirement for space maintenance in recoating reflector surfaces, and for space transportation to place the 1320 reflectors in their initial orbits. Finally, even though the TP system has the smallest material requirement, it has one of the highest energy of production requirements. This is attributable to its need for large quantities of aluminium and silicon. However, since weight is not a problem, it is possible that a heavier and less energy intensive metal could be substituted for the aluminium. Such a substitution is not as feasible for the OSR and SSP systems because of the requirement for lightweight structures to facilitate orbital insertion.

3.2. Material requirements

3.3. Manpower requirements

The reasons for the large differences in required production energy can be ascertained by considering

The terrestrial systems (TST and TP} require considerably less manpower for material acquisition,

available in synchronous orbit is six times greater than that available at the best location on Earth and approximately fifteen times greater when compared to a United States location with average weather conditions (Ref. [13] pp. 1-2). 3. RESULTSOF THE RESOURCE EVALUATION Table 1 is a summary of the results of the resource evaluation. A detailed discussion of the procedures used to determine the data found in Table 1 is contained in Ref. [10]. All data presented in Table I is normalized per MW of electrical generating capacity to facilitate comparisons between systems. Furthermore, for all systems except the OSR system, the resource analysis is based on a plant with a rated electrical generating capacity of 10.0 gigawatts (GWe). The OSR system is based on a 74.2 GWe capacity because this is the smallest scale upon which it can be efficiently produced. The results o f a JPL resource analysis for a gasified coal (GC) generating system are also included in Table 1. These results were not verified by the authors and may have been obtained using different assumptions than those used for the solar energy system analysis. However, to first order, the JPL results do permit a rough comparison between conventional and solar generating systems. 3.1. Energy of production

Table 1. Resource analysis summary't" Item/units Rated capacity (GWe) Material (metric tonne/MWe) Land (m2/MWe) Water ( 106 I/M We) Manpower (man yr/MWe) Energy of prod. (109 Btu/MWe) Cost ($106/MWe}

TST 10.0 6,368.5§ 108,000.0 27.0 57.0 95.0 6.0

TP 10.0 774.5§ 114,040.0 23.2 40.6 297.3 9.4

OSR 74.2 5,456.1§ 34,429.0 10.1 113.2 615.6 13.1

"I"Clarifying comments can be found in the Resource analysis[10]. :~Ref. [3] pp. 6-12, for a gasified coal (GC) generating system. § Excluding coal for backup power system, and other exclusions listed in Ref. [10].

SSP 10.0 1,473.4 108,000.0 0.01 76.8 197.6 14.1

GC:~ 10.0 105,420.0 117,000.0 15.0 39.6 2.1

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R.C. ENGERand H. WEICHEL

construction, operation, and maintenance than do the systems which utilize space components (OSR and SSP). This is because space operations require a large number of support personnel while ground operations generally do not. In addition, the space operations require a complete transportation and housing system, the manufacture of which requires still more personnel. 3.4. Land requirements Land requirements pose a problem for all of the systems, including the gasified coal system. The TST and TP systems have large land requirements in order to collect sufficient solar energy. The same land requirement exists for both the OSR and TP systems with the notable exception that because the OSR system receives 24 h/day illumination, each solar cell receives more sunlight per day than with the TP system. The SSP system must tolerate a large land requirement to provide a safety zone for the protection of people from the hazards of microwave exposure. Even the gasifiedcoal (GC) system has problems, requiring nearly 100,000 m2/MW just for the mining of coal over a 30 yr period.

that the system will generate during its lifetime. The time that it takes each of the four electrical generating systems to pay back the energy requirements listed in Table 1 is: 1. Terrestrial solar thermal system - 1.48 yr 2. Terrestrial photovoltaic system - 4.62 yr 3. Orbiting solar reflector system - 9.56 yr 4. Satellite solar power system - 2.53 yr The energy pay back times listed above were calculated by assuming that the energy consumption, for each system listed in Table 1, could have been used instead to generate electricity at the energy consumption rate of 1.05 x 104 Btu/kWhr (Ref. [2] A-I). It was then determined how long it would take each of the four systems to generate an amount of electricity equal to the effective amount that they consume during construction, operation, and maintenance. It is assumed that the TST, TP, and OSR systems produce electricity at a rate of 70 per cent of rated capacity (Ref. [3] pp. 1-3), while the SSP produces electricity at 85 per cent of rated capacity (Ref. [13] pp. 14-3).

4. ISSUESAFFECTINGTHE DEVELOPMENT OF SOLAR ELECTRIC GENERATING SYSTEMS 3.5. Water requirements The numbers in Table 1 describe systems that are Perhaps the only real advantage that the OSR and currently at different stages of development. Although SSP systems have over the TST and TP systems is in comparing many aspects of these systems, there are the area of water requirements. Because of the need to several uncertainties and environmental concerns that clean ground collectors, and in the case of the TST must be resolved before the systems evaluated can system, to cool the Rankine generating system, the become operational. Most notable of these are the terrestrial systems require more water. following : 1. The impact of varying weather conditions on the 3.6. Cost plant performance of all four systems must be more Strictly from a cost point of view, the TST and TP fully understood. systems are more desirable. Both the OSR and SSP 2. Techniques must be developed to mass produce systems must absorb significant space transportation solar cells at the desired cost of $0.50 per peak watt of and developmental costs for both the electrical ge- electrical output (Ref. I-3] pp. 1-6) and at the desired nerating and space transportation systems. For exa- thicknesses. mple, the cost of developing the SSP generating 3. The economic feasibility of space construction, system, per MW of electrical generating capacity, is orbital factories, and the construction of light-weight estimated at nearly forty times greater than the deployable structures must be established (Ref. I-3] pp. developmental costs of the TST generating system. 4-35). These figures are based on the assumption that the first 4. An entire space transportation system must be operational plant would absorb all of the developmen- developed. tal costs. Thus, such a figure differs from those of other 5. It must be demonstrated that man has the ability authors who amortize developmental costs over as to manufacture and assemble equipment in space (Ref. many as 100 operational plants. It would be possible to [13] A-5). build multiple copies of the various systems. However, 6. The environmental impact of prolonged micit is equally possible that after building just one plant, a rowave exposure on the Earth and its inhabitants, of decision would be made to discontinue production. In the projected ten or more shuttle flights per day in this analysis, the authors choose to take the most support of the power-satellite fleet, and of the manufconservative viewpoint and assume that only one copy acturing waste products resulting from system conof any system will ever be developed. struction must be determined (Ref. [5] pp. 61-63, [3] pp. 7-108). 3.7. Ener#y of production 7. The large size of the solar electric generating An electrical generating system should be an energy systems makes them uniquely vulnerable to sabotage, producer, not an energy consumer. What is meant by blackmail, and military attack which could result in this statement is that the electricity required to con- large scale blackouts. This problem must be antistruct, operate, and maintain an electrical generating cipated and avoided (Ref. [3] pp. 6-46). system should be less than the amount of electricity It should be noted that most of these technological

Solar electric generating system resource requirements

261

uncertainties and environmental concerns pertain to the extra-terrestrial systems and not to the terrestrial ones.

servative cost estimates of today, would be no cheaper than terrestrial methods.

5. CONCLUSIONS

1. Arthur D. Little, Inc. Evaluation of solar cells for potential space satellite power applications. NASA Contract ~NASA 9-15294. Cambridge, Massachusetts (1 June 1977). 2. Battelle Columbus Laboratories. Energy use patterns in metallurgical and nonmetallic mineral processing (phase 4 - energy data and flowsheets, high-priority commodities). Interim Report to the United States Bureau of Mines, Contract No. S0144093. Columbus, Ohio (27 June 1975). 3. Richard Caputo, An initial comparative assessment of orbital and terrestrial central power systems. JPL Final Report 900-780. Pasadena, California: Jet Propulsion Laboratory, California Institute of Technology, N7722612 (March 1977). 4. Jimmie Carter, National presidential address on energy. Dayton Daily News, 100 (14) (10 April 1977). 5. Barbara K. Ching, Space power systems - what environmental impact? Astronautics and Aeronautics 15, 60-65 (February 1977). 6. Krafft A. Ehricke, Cost Reductions in Energy Supply Through Space Operations. Paper to the 27th International Astronautical Congress, 6th International Cost Reduction in Space Operations Symposium II, Paper IAF-A-76-25. Rockwell International Corp., Downey, California (October 1976). 7. Krafft A. Ehricke, Space and Energy Sources. Rockwell International Corp., Downey, California (May 1977). 8. Krafft A. Ehricke, Space Industrial Productivity, New Options for the Future. Rockwell International Corp., Downey, California (July 1975). 9. Energy Research and Development Administration. Final Report of the ERDA Task Group on Satellite Power Stations. ERDA-76/148. Energy Research and Development Administration, Washington (November 1976). 10. R°lf C" Enger'S°lar Electric Generating System Res°urce Requirements and the Feasibility of Orbiting Solar Re.[lectors. Wright-Patterson AFB, Ohio: Air Force Institute of Technology, Ohio DDC #AD-A048908 (December 1977). ll. Peter E. Glaser, Evolution of the Satellite Solar Power Station (SSPS) Concept. J. Spacecraft and Rockets 13, 573 576 (September 1976). 12. S. R. McReynolds, Occupational safety and health impacts of the acquisition of materials and construction of two 1000 MWe solar power plants. Interoffice Memorandum. Pasadena, California: Jet Propulsion Laboratory, California Institute of Technology (26 May 1976). 13. NASA, George C. Marshall Space Flight Center. Solar Power System Engineering and Economic Analysis, Summary. NASA TMX-73344. National Aeronautics and Space Administration, Washington N77-15486 (November 1976). 14. NASA, Lyndon B. Johnson Space Ceater. Initial Technical, Environmental, and Economic El~aluation o/ Space Solar Power Concepts, Volume II - Detailed Report. NASA TMX-74310. Washington: National Aeronautics and Space Administration, Washington N77-16443 (31 August 1976). 15. Solar power concept chosen. Ariation Week and Space Technology 107, 48 (5 September 1977). 16. Private Communication with Dean Holcombe of the Dayton Power and Light Company (26 September 1977).

Concept development of the various systems is just beginning and little is known about the solutions to the many problems that remain. However, even if all of these technological hurdles are overcome, the fact remains that one 10 G W e generating plant would be roughly four times larger, in terms of electrical power production, than the entire Dayton Power and Light Company, an electrical company currently servicing all or parts of twenty-four counties in West Central Ohio[16]. The TST and T P systems could be built on scales smaller than 10 GWe. However, the m i n i m u m efficient sizes of the OSR and SSP systems are 74.2 G W e and 10 GWe, respectively. Thus, especially from a military point of view, large scale dependence upon a system such as the SSP or OSR for electricity could place the United States in an extremely vulnerable position in times of war. It is only necessary to recall the most recent New York blackout in order to understand the effect upon this nation of the elimination of all or most of its electrical power. Such a concern is particularly significant to the SSP system because, of the four systems evaluated, it is the only one that cannot function without its orbital component. Furthermore, a foreign nation could destroy an SSP orbital component without appearing to be at war with the United States by making the destruction appear to be the result of an accidental orbital collision (Ref. [3] pp. 6-47). Defense against such a threat would be difficult, if not impossible. 6. FINAL COMMENTS Today, the United States government is spending millions on research relative to the SSP system, the most publicized of the four systems evaluated. We are aware of the fact that continued research relative to solar electric generating systems would probably cause some ofthe members in Table 1 to change. We support this continued research effort relative to the terrestrial systems. However, it is the authors' opinion that considering the environmental concerns, the vulnerability questions, the need for technological advancements, and the resource requirements, it seems difficult to justify further commitment to the OSR and SSP programs. Studies by JPL, ERDA, and NASA, as well as this one, have all shown that, at best, the SSP system would be competitive with other ground solar systems like wind, geothermal, TST, and T P (Ref. [3] pp. 1-2; Ref. [14] II-3; Ref. [9] p. 7). Thus, it seems unreasonable that billions of dollars should be spent on very uncertain programs that, even if built at the con-

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REFERENCES