Space Power Technological, Economic and Societal Issues in Space Systems Development Volume 8 Number 4 1989 —carfax— publi/hing company
SPACE POWER Published under the auspices of the SUNS AT Energy Council EDITOR Andrew Hall Cutler, Space Studies Institute Space Power is an international journal for the presentation, discussion and analysis of advanced concepts, initial treatments and ground-breaking basic research on the technical, economic and societal aspects of large-scale, space-based solar power, space resource utilization, space manufacturing, and other areas related to the development and use of space for the long-term benefit of humanity. Papers should be of general and lasting interest and should be written so as to make them accessible to technically educated professionals who may not have worked in the specific area discussed in the paper. Editorial and opinion pieces of approximately one journal page in length will occasionally be considered if they are well argued and pertinent to the content of the journal. Submissions should represent the original work of the authors and should not have appeared elsewhere in substantially the same form. Proposals for review papers are encouraged and will be considered by the Editor on an individual basis. Editorial Correspondence: Dr Andrew Hall Cutler can be reached by telephone at (602) 327-9205 or 322-2997, and his address is 4717 E. Fort Lowell, Tucson, AZ 85712, USA. Dr Cutler should be consulted to discuss the appropriateness of a given paper or topic for publication in the journal, or to submit papers to it. Questions and suggestions about editorial policy, scope and criteria should initially be directed to him, although they may be passed on to an Associate Editor. Details concerning the preparation and submission of manuscripts can be found on the inside back cover of each issue. Business correspondence, including orders and remittances to subscriptions, advertisements, back numbers and offprints, should be addressed to the publishers: Carfax Publishing Company, P.O. Box 25, Abingdon, Oxfordshire 0X14 3UE, United Kingdom. The journal is published in four issues which constitute one volume. An annual index and titlepage is bound in the December issue. ISSN 0883-6272 Cover: An artist's concept of the enhanced configuration of the permanently manned Space Station, produced by Rockwell International. The enhanced configuration includes an upper and lower keel for attaching external payloads, a 50 kW solar dynamic system mounted on the ends of the transverse boom, a servicing bay and a co-orbiting platform (not pictured). Reproduced by courtesy of NASA, Washington, DC, USA. © 1989, SUNSAT Energy Council
Volume 8 Number 4 1989 Papers from the IAF International Conference on Space Power, Cleveland, OH, USA, 5-7 June 1989 ENERGY STORAGE 3-1. Hamid Torab. Optimal Design of Thermal Energy Storage for Space Power 415 3-4. Kotaro Tanaka, Yoshiyuki Abe, Katsuhiko Kanari, Osami Nomura & Masayuki Kamimoto. Advanced Concepts for Latent Thermal Energy Storage for Solar Dynamic Receivers 425 3-6. David F. Pickett. Advanced Nickel-Hydrogen Batteries 435 3-8. Lisa L. Kohout. Cryogenic Reactant Storage for Lunar Base Regenerative Fuel Cells 443 SOLAR DYNAMIC POWER FOR SPACE 4-5. Adrian Tylim. Structural Configuration Options for the Space Station Freedom Solar Dynamic Radiator 459 4-8. Masashito Oguma, Shintaro Enya, Yuichiro Asano, Hiroshi Muramoto & Nobuhiro Tanatsugu. Development of a Deployable Film Type Radiator 469 NUCLEAR SPACE POWER TECHNOLOGIES 5-6. R. Ponnappan, L. I. Boehman & E. T. Mahejkey. Transient Characteristics of a Gas-loaded Liquid Metal Heat Pipe with a Long Adiabatic Section YT1 ADVANCED SOLAR SPACE POWER SYSTEMS 7-5. Oktay Yesil. A Space-based Combined Thermophotovoltaic Electric Generator and Gas Laser Solar Energy Conversion System 493 SPACE POWER MISSION APPLICATIONS I 9-3. Af. Joseph Cork & James A. Turi. Galileo and Ulysses Missions Safety Analysis and Launch Readiness Status 509 Title-page and Contents, Volume 8 527
Volume 8 of Space Power contains the proceedings of the IAF International Conference on Space Power — IAF-ICOSP89. Much of Volume 9 will contain papers submitted after the original deadline
3-1. Optimal Design of Thermal Energy Storage for Space Power HAMID TORAB Summary Thermal energy storage (TES) has been utilized in most thermal systems where temporal variations in the quantity of energy available do not coincide with the load demand. Encapsulated phase change material (PCM) thermal energy storage may be used to reduce the total volume and mass of the heat rejection system in pulsed space power supplies. The storage system receives waste heat generated during high power sprint operation. The stored energy will then be dissipated into space using a radiator during the non-operational period. Since the non-operational period is much longer than the operational period, the use of a storage system allows the radiator to be made significantly smaller. This study is concerned with optimization of high temperature TES using encapsulated PCM. The PCM considered in this study is lithium hydride. The heat transport fluid is assumed to be lithium. The goal of this optimization is to minimize the volume of the TES for a given operating condition. Introduction Themal energy storage is used in thermal systems which exhibit temporal variations in the quantity of energy available that do not coincide with thermal load demand. The flexibility of the energy utilization system and the continuity of its function can be greatly increased by using an efficient method of thermal energy storage. Packed beds provide an effective means of energy storage. The energy density of packed beds may be greatly enhanced if a phase change material is utilized. These storage systems can be used to reduce the size of the heat rejection system in pulsed space power (PSP) supplies. Waste heat is generated during the high-power sprint operation of PSP supplies, and should be dissipated into space via the radiator. Utilization of a thermal energy storage system under certain conditions could significantly reduce the size of the radiator. Using this concept, the waste heat generated during high power sprint mode operation is rejected to the thermal storage and thus into space via a radiator over the longer non-operational period of the orbit. By using thermal energy storage, the dissipation time can be increased by one order of magnitude. The total radiator capacity needed for waste heat dissipation into space can then be reduced drastically as compared to the case where no thermal energy storage is used. The addition of a thermal energy storage system will be advantageous Hamid Torab, Associate Professor, Department of Mechanical Engineering, Gannon University, Erie, PA 16541, USA.
only if the total space needed for the storage system and the radiator is smaller than the space needed for the radiator of a system with no thermal energy storage. Due to their large interfacial transport areas, packed beds are considered to be one of the most efficient methods for direct thermal energy storage. In a packed bed, a fluid is used to exchange heat with the storage material by forced convection as it passes through the void area of the bed. The main objective in the design of thermal storage for a space-power system is to maximize the energy storage density. This can be accomplished by utilizing a phase change material as the storage medium, encapsulated in spherical containers. Estimated sprint power levels required for space-power systems are very high, with projections in the multi-megawatt range. However, these high power levels are required in a pulsed mode. Due to the pulsed nature of the power required thermal energy storage may be employed to reduce the size and mass of power system components. The thermal energy storage receives the rejected heat from the power conversion system during sprint power operation. During the remaining period of the orbit, which is usually much longer than the sprint power mode period, the stored heat is dissipated into space. In launch packages that are limited by volume, utilization of thermal energy could prove to be very effective under certain conditions. In this study, lithium hydride is considered as the PCM. This is mainly due to its superior heat storage properties and its melting point, which falls well within the required range (Morris et al., 1986). Lithium is considered as the heat transport fluid. The goal of this study is to develop an optimal design procedure for a space-power TES. To do this, it is necessary to understand the dynamic behavior of the TES. The main parameters are the energy storage density and the minimum temperature of the storage medium. The storage medium utilizes the sensible heat of the liquid and solid phases and, more importantly, the latent heat of fusion. The maximum temperature of the storage unit is controlled by the reactor coolant temperature, which is assumed to be 1300°C in this study. A total orbit time of 6000 s is assumed with a sprint period of 600 s. Thus, the non-operational period is considered to be nine times longer than the sprint period. Background Packed beds have been utilized as thermal energy storage devices for industrial waste heat recovery, solar energy systems, electrical power generation, etc. Most of the literature in this area has been on the investigation of sensible heat storage systems. More recently, however, PCMs are utilized to increase the energy density. When the PCM is encapsulated in spheres, the studies related to flow and heat transfer in packed beds are directly applicable. To understand and develop an accurate model representing the heat storage process in a PCM storage tank, other topics related specifically to encapsulated PCMs have also been investigated. These areas are related primarily to the melting of the PCM and its encapsulation. In this section, a brief overview of studies which are directly related to the application of packed beds as thermal storage systems are presented. The structural aspects of packed beds have been the subject of several studies. Among these is the work by Haughey & Beveridge (1969), where the effect of packing of the particles on void fractions has been studied. Galloway et al. (1957) studied the flow and heat transfer characteristics of various regular packings of uniform spheres and the effect of tank diameter over particle diameter ratio D0/Ds on mean void
fraction. The variation of mean void fraction e with D0/Ds is also demonstrated by the data of Beavers et al. (1973) and Clark & Beasley (1984). Flow characteristics in porous media have been studied by several researchers. Whitaker (1966) and Greenkorn (1981) demonstrated that the nonlinear behavior of flow in porous media was due to internal effects in the flow. Ergun (1952) developed a nonlinear model for flow in terms of a friction factor. This was later modified by MacDonald et al. (1979) to include geometry-dependent parameters. The heat transfer coefficient in packed beds has been extensively studied. Gamson et al. (1943) and Galloway & Sage (1970) developed correlations to calculate the overall heat transfer coefficient. Yagi & Kunii (1957) combined the effect of thermal conduction between particles and radiation into a stagnant effective thermal conductivity at no net fluid flow. Packed bed energy storage models can be generally divided into single and separate phase models for one- or two-dimensional cases. A summary of studies on modelling of packed beds for thermal energy storage using sensible heat can be found in Beasley & Clark (1984). In the area of latent heat thermal storage, several studies have been conducted on melting and freezing of PCM. Longwell (1958) used a graphical method for obtaining a numerical solution to Stefan-type problems involving a moving boundary which can be described in terms of our space coordinate. Murray & Landis (1959) improved the previous methods for the solution of one-dimensional heat conduction problems with melting and freezing. Sutherland & Grosh (1961) considered superheating and subcooling in calculating the temperature distribution before phase change occurs, during the transient process of phase change, and after steady-state conditions. Tao (1967) developed a numerical method and graphs of generalized solutions for the moving interface problem of freezing a saturated liquid inside a cylinder or a sphere. Solomon (1980) developed simple equations for the evaluation of the melting time of phase change material having a slab shape and a convective boundary condition. Marianowski & Maru (1977) studied latent heat thermal energy storage systems above 450°C. They placed emphasis on the choice of the salts. Lou (1983) proposed to add some nucleating agents to the salts to improve their poor nucleating properties which result in supercooling of the liquid salt hydrate prior to freezing. Torab & Beasley (1985) employed a finite difference method to study the dynamic response of a packed bed of encapsulated PCM. Ji (1986) developed two transient models for both sensible and latent thermal energy storage. Both models involve modeling of the phase change material as a conduction problem with sensible and latent heat energy storages and include consideration of the temperature gradients in both phases. Torab & Chang (1988) used a one-dimensional model to stimulate the performance of high temperature thermal energy storage for space power systems. Analysis and Modeling The model presented in this study is based on the constant temperature approach. The following coupled set of equations are the governing equations representing the temperature changes of the fluid and PCM. These equations are one-dimensional and for separate phases, i.e. they differentiate between temperatures of fluid and PCM at each point (Torab & Beasley, 1985). For fluid,
For PCM, when it is in the solid phase, The above equations are approximated using fully-implicit finite difference equations. It turns out that the PCM equation becomes explicit for PCM temperature or quality when it is written in finite difference form. Here, i and n refer to the location of the node under consideration and the time step, respectively. Using the finite difference equation for the PCM in the finite difference form of Equation (1) yields the equation for the fluid temperature as Equations (8)—(12) are a set of simultaneous equations, the solution of which provides the temperatures of the fluid, PCM, and the quality of the PCM at each time step. This set of equations is tri-diagonal in matrix form. The scheme used in this study restricts the size of the time step so that only one node changes phase at a time. This time step is found using iteration.
Heat Transfer Coefficient The following correlation is used for the heat transfer coefficient. This correlation was developed by Galloway & Sage (1970) with constants determined from measured data by Beasley & Clark (1984). The Biot number effects on axial thermal dispersion are determined by correcting the heat transfer coefficient as recommended by Bradshaw et al. (1976). The effect of intra-particle conduction is to reduce the effective heat transfer coefficient in the following manner. Effective Thermal Conductivity For effective thermal conductivity, a correlation developed by Yagi & Kunii (1957) is used Optimization Procedure Particle size and relative dimensions of the storage tank are the main variables influencing the performance of the TES. In general, the total volume of the thermal storage unit is directly related to the total energy storage capacity. However, for a given storage capacity, the total volume of the TES can be minimized by choosing optimal values for particle size and relative dimensions of the storage system. Although the model used to simulate the performance for the TES is one-dimensional, two- dimensional effects exist within a packed bed. The wall effect creates higher void fractions and decreased resistance to fluid flow in the non-wall region. The nonuniformity in flow distribution is most severe at low values of △/>, and a minimum value of this variable must be set to avoid the flow non-uniformity. This minimum value will serve as one of the constraints for the optimization problem. In addition, to minimize the wall effect of flow and void fraction distribution, the ratio of the bed diameter to the particle diameter should satisfy Also, in general, particle sizes of less than 1 cm would be considered impractical. The above mentioned restriction on pressure drop, relative dimensions of TES and particle size form the major set of constraints in optimal design of the TES. In addition to these constraints the geometric constraints due to the launch package must also be considered. Results and Discussions Figures 1 and 2 represent the simulation results for temperature distribution in a TES
temperature distribution to determine the total energy stored in the system. This temperature distribution and therefore the total energy in the TES is influenced by particle size and relative dimensions of the TES. Depending on design constraints the designer will then choose the optimal design variables to maximize the energy density in the TES. The designer may also consider using different PCMs in the same TES to increase the energy density. Nomenclature a Interphase surface area per unit volume A U/\t/2/\x Bf Biot number c Fluid specific heat Effective liquid specific heat cs Effective solid specific heat C D/(l+M0 D ha/St/PCs Do Diameter of vessel containing PCM-laden spheres Ds Diameter of spheres containing PCM E D/3/S-4t/(l+/3/St) f0 Fluid temperature at inlet fi Initial fluid temperature /2 Initial PCM temperature h Heat transfer coefficient Effective heat transfer coefficient hl{ Latent heat of fusion Effective thermal conductivity of quiescent bed K* Axial effective thermal conductivity Kt Fluid thermal conductivity Km Average PCM thermal conductivity L Length of the bed Nu Nusselt number hD/Kt Pr Prandtl number Ap Pressure drop Re Reynolds number t Time △ t Time step T Fluid temperature Tt Inlet fluid temperature T" Fluid temperature of node i and time step n V Fluid velocity △ x Spatial increment Greek Symbols a Quality P K^/^Pc^x2^ E Void fraction p Fluid density Pi Density of liquid PCM T \ + 2A + B+2N~E 0 Temperature of the PCM 0* Melting temperature of PCM
REFERENCES Beasley, D.E. & Clark, J. A. (1984) Transient response of a packed bed of thermal energy storage, International Journal of Heat and Mass Transfer, 27(9), pp. 1659-1669. Beavers, G.S., Sparrow, E.M. & Rodenz, D.E. (1973) Influence of bed size on the flow characteristics and porosity of randomly packed beds of spheres, Journal of Applied Mechanics, 40, p. 655. Bradshaw, P.J., Klein, S.A. & Close, D.J. (1976) Packed bed thermal storage models for solar air heating and cooling systems, Journal of Heat Transfer, 98(2), p. 336. Ergun, S. (1952) Fluid flow through packed columns, Chemical Engineering Progress, 48(2), p. 89. Galloway, L.N., Komarnicky, W. & Epstein, N. (1957) Effect of packing configuration on mass and heat transfer in beds and stacked spheres, Canadian Journal of Chemical Engineering, 35, p. 139. Galloway, T.R. & Sage, B.H. (1970) A model for the mechanism of transport in packed, distended, and fluidized beds, Chemical Engineering Science, 25, pp. 495. Gamson, B.N., Thodus, G. & Hongen, U.A. (1943) Heat, mass and momentum transfer in the flow of gas through granular solids, Transactions of the American Institute of Chemical Engineering, 39(1), p. 1. Greenkorn, R.A. (1981) Steady flow through porous media, American Institute of Chemical Engineers Journal (AIChE), 27(4), p. 529. Haughey, D.P. & Beveridge, A.S.G. (1969) Structural properties of packed beds—a review, Candian Journal of Engineering, 130. Ji, S.H. (1986) Transient response of a packed bed for both sensible and latent heat thermal energy storage, Ph.D. Dissertation, The University of Tennessee, Knoxville, TN, USA. Longwell, P.A. (1958) A graphical method for solution of freezing problems, American Institute of Chemical Engineers Journal, 4(1), p. 53. Lou, D.Y.S. (1983) Solidification process in a glauber salt mixture, Solar Energy, 30(2) p. 115. MacDonald, I.F., El-Sayed, M.S., Mow, K. & Dullien, A.L. (1979) Flow through porous media—the Ergun equation revised, Industrial Engineering Chemical Fundamentals, 3, p. 199. Marianowski, L.G. & Maru, H.C. (1977) Latent heat thermal energy storage systems above 450°C, Proceedings of the 12th IECEC, 779090, pp. 555. Morris, D.G., Foote, J.P. & Olszewski, M. (1986) Development of Encapsulated Lithium Hydride Thermal Energy Storage for Space Power Systems, Oak Ridge National Lab Report No. ORNL/TM-10413. Murray, W.D. & Landis, F. (1959) Numerical and machine solutions of transient heat conduction problems involving melting or freezing. Part 1—method of analysis and sample solutions, Transactions ofASME, May, pp. 106. Solomon, A.D. (1980) On the melting time of a simple body with a convection boundary conduction, International Journal of Heat and Mass Transfer (letters section), 7, p. 183. Sutherland, J.E. & Grosh, R.J. (1961) Transient temperature in a melting solid, Transaction of American Society of Mechanical Engineers, November, p. 410. Tao, L.C. (1967) Generalized numerical solutions of freezing a saturated liquid in cylinders and spheres, AIChE Journal, 13(1), p. 165. Torab, H. & Beasley, D.E. (1985) Dynamic response of a packed bed of encapsulated
PCM, Proceedings of the 20th Intersociety Energy Conversion Engineering Conference, Miami Beach, FL, p. 313. Torab, H. & Chang, W.S. (1988) High temperature thermal energy storage with space power systems, ASME Winter Annual Meeting, Analysis of Time-dependent systems (AES), 5. Whitaker, S. (1966) The equations of motion in porous media, Chemical Engineering Science 21, p. 291. Yagi, S. & Kunii, D. (1957) Studies of effective thermal conductivities in packed beds, AIChE Journal, 3(3), p. 373.
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3-4. Advanced Concepts for Latent Thermal Energy Storage for Solar Dynamic Receivers KOTARO TANAKA, YOSHIYUKI ABE, KATSUHIKO KANARI, OSAMI NOMURA & MASAYUKI KAMIMOTO Summary The present paper discusses latent thermal energy storage (LTES) systems designed for a closed Brayton cycle (CBC) dynamic power generator. Two new LTES concepts are proposed. The first is the addition of volumetrically variable fins which remain unwetted by the phase change materials (PCM). The use of such fins can reduce mechanical stress on the containment walls. The second is encapsulation of the PCM within a submicron-sized porous ceramic structure. Considerable weight reduction can be achieved by introducing such ceramic-PCM composite systems. A thermal and stress analysis on melting and solidification has been performed. In addition, we cover results from an experiment on void formation and from preliminary cyclic performance tests on receiver LTES components. Introduction A space solar receiver is one of the key components of the solar dynamic power systems (SDPS) used in future space development programs. Energy from sunlight falls on the receiver and is stored in one of the various forms to be used later as necessary. Latent Thermal Energy Storage (LTES) is the most promising energy storage technique because of its high storage density and high efficiency. However, present state-of-the-art LTES systems are large and heavy, massing nearly half of the complete space solar receiver. The specific mass of recent receiver designs has been evaluated to be in the range 50-100 kg/kWe [1,2], mainly due to the weight of the thick metallic containment canisters. If one looks at the components of the conventional receiver recently designed for the 25 kWe space station closed Brayton cycle (CBC), less than 20% of the total weight is associated with the PCM itself, while 40% is from the containment canisters [1], This indicates that the weight of the receiver could possibly be reduced significantly by introducing some new LTES concepts. The authors have conducted systematic screening of PCMs in the temperature range 473-1773 K, evaluating more than 1800 kinds of material [3,4]. Candidate materials for space use are mostly fluoride salts, such as lithium fluoride (LiF), calcium fluoride (CaF2), magnesium fluoride (MgF2) and their eutectic mixtures. The melting temperatures of the selected PCMs match the inlet temperature of the CBC Kotaro Tanaka, Yoshiyuki Abe, Katsuhiko Kanari, Osami Nomura & Masayuki Kamimoto, Energy Materials Section, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan. Paper number IAF- ICOSP89-3-4.
gas turbine. The abovementioned fluoride salts have high latent heats of fusion and relatively high densities, yet have low thermal conductivity (of the order of 1 W m-1 K-1 and large volumetric changes upon melting (10-30%). Due to the latter, small separated containment canisters with thick walls are required in construction of the LTES. In this paper, two new LTES methods are proposed. An advanced receiver configuration is also presented. Encapsulated Thermal Energy Storage Principle of Stress Relaxation One of the serious problems encountered when PCMs are encapsulated in hard canisters is the generation of a void due to volumetric changes of the PCM upon melting. Most of the candidate fluoride salts mentioned above are known to have large volumetric changes upon melting, and thus pose difficulties of this sort. The formation of voids affects both melting and solidification heat transfer processes, and induces mechanical stress on the containment walls. A typical example of phase change behavior within a canister is illustrated in Fig. 1. Void placement under 1 g and microgravity conditions is shown in Figs 1(a) and 1(b), respectively. When heat is added, the liquid in the melted region expands at the beginning of the phase change. In both cases, the liquid is trapped inside a solid section and cannot expand into the void(s), resulting in mechanical stress on the wall. Eventually, a deformation and/or rupture of the canister may occur. This problem can be resolved by thickening the walls of the containment canister, but this also significantly increases the system's weight. The principle of our new method, which relaxes the stress, is illustrated in Fig. 2. The inside of the containment canister is divided by the addition of volumetric variable fins which remain unwetted by the PCM. Under such conditions, it is expected that melting will also occur preferentially along the fins, creating channels from the liquid up into the void(s). The liquid phase can thus be transferred to the center of the canister via such channels. This not only relaxes the mechanical stress but also enhances the heat transfer rate. If the fin material is wetted by the PCM, the liquid phase permeates the inside of the fin structure and then solidifies. In such a case, the gap is not created. It is thus essential that the correct materials are selected for the fin. Fortunately, we obtained a good material: carbon is not wetted by fluoride salts and has a high thermal conductivity and low density. Moreover, it resists corrosion by fluoride salts even at elevated temperatures. Carbon cloth or carbon felt has been the most promising form of the material. A cylindrical canister with a carbon fin and the canister after sealing by electron-beam welding are shown in Figs 3 and 4, respectively. Each canister is 20 mm in diameter, 30 mm in height, and has molybdenum walls (of thickness 0.8 mm). Visualization Experiment In order to obtain experimental information on phase changes inside the containment canister, an in situ visualization experiment using X-ray computer tomography (CT) was performed. X-ray CT is effective for continuous observation of the curved upper surface of the liquid (the meniscus) inside the metallic canister. The experimen-
tai apparatus is shown in Fig. 5. The electric heater was made of carbon cloth since it has good X-ray transmissivity. Temperature was measured with a platinum-rhodium thermocouple located just under the containment canister. The maximum operating temperature was approximately 1500 K. The outer cylindrical vessel (made of aluminum) was 150 mm in diameter and about 200 mm in height. An X-ray scan of the general view is shown in Fig. 6. An example of the continuous visualization results of the solidification process is shown in Fig. 7. A lithium fluoride and calcium fluoride eutectic mixture (80.5-19.5 mol%) was placed within the open carbon canister. The melting temperature was first stabilized at 1070 K. At this temperature, the salt is a liquid (see Fig. 7, first picture). From this setting, the input power of the heater was gradually decreased. Snapshots of the changing meniscus (intervals 90 s) as the salt underwent solidification are shown in Fig. 7. The last picture shows the frozen PCM at 1010 K. By examining the results (though accurate estimation is difficult), the volumetric change of this eutectic salt can be calculated to be 24%. This measured value almost agrees with the data presented in Ref. [1]. A cross section of the cylindrical canister plus carbon fin is shown in Fig. 8. The white semicircular area indicates the solid lithium fluoride while the interior black area indicates the void. The outer circular dashed line delineates the electric heater. This
photograph is of a preliminary test at room temperature; an experiment investigating the effects of fins under elevated temperatures is scheduled for the near future. In order to measure the mechanical stress caused by volumetric expansion, an experiment with a salt melting at a low temperature—such as LiNO3—is also planned.
On the other hand, we are also developing a computer program to simulate moving boundary problems in three dimensions. This project will investigate phase changes in three dimensions both under 1# and microgravity conditions, including the formation of voids. A detailed theoretical analysis of mechanical and thermal stress caused by repetitive melting and solidification processes is also in progress. Our rough estimation of the LTES system with carbon fins indicates a reduction of
the specific mass of about 20% (55 kg/kWe). We expect to use the abovementioned numerical simulations to further optimize the configuration of the LTES system and minimize the specific mass of the receiver. Composite Type Thermal Energy Storage Composite Materials Design and development of the solar receiver for future space missions will require innovative concepts that achieve higher efficiency and lighter weight. We are engaged not only in development of conventional receivers such as the metallic containment canisters with PCMs as described above, but also the development of advanced LTES systems. Our particular interest is in incorporating the PCMs in a submicron-sized porous structure. In this system, the distribution of voids can be controlled. Porous media can also provide the PCM with a continuous conduction path resulting in an increase in effective thermal conductivity. Moreover, the direct absorption of concentrated sunlight minimizes temperatures gradients. System weight is also reduced. The composite LTES storage materials proposed in this study use porous silicon carbide (SiC) for the structure and fluoride salts (LiF and MgF2) as the thermal storage media. The thermal conductivity of SiC is so high that further thermal devices are unnecessary. Simple configurations for the composite system also improve long life reliability. A scanning electron micrograph (SEM) of the SiC-LiF composite material is shown in Fig. 9. The weight fraction of LiF is 0.35. We also constructed a high temperature SiC/MgF2 composite, which could be combined with an advanced gas turbine with a 1500 K inlet temperature. Thermal stability verification on these composite materials has been performed. The results revealed the SiC porous structure is not damaged after several tens of melting and solidification cycles. However, we realized that a very serious drawback is the evaporation of fluoride salts from the composite surface. Some technical approaches are under evaluation to prevent evaporation. Thermal analysis of the
composite materials has also been performed using differential scanning calorimetry (DSC). Latent heats of fusion were determined for both fluoride salts and their composite materials. It is clear that the latent heats of the composite materials agree well with the calculated values if one assumes that the porous volume is full of PCM. No appreciable supercooling is observed. The experimental data and the effect of porosity, pore size and composition of the molten salts is presented in detail in Refs [5] and [6]. Composite Type Receiver A preliminary analysis has been performed to evaluate the feasibility of the composite materials for a space solar receiver. The thermodynamic properties of the SiC-LiF composite are listed in Table I. The density and the heat capacity of the composite materials are estimated on an additive basis. Thermal conductivity is estimated assuming that Lichtenecker's equation [7] applies. The latent heat of fusion used is from our measured data [6]. A conceptual design for the composite receiver for a CBC system is illustrated in Fig. 10. Two heat exchanger tubes are coupled with each segment of the sector-shaped composite material. He-Xe circulating gas flows inside the outer tube and returns to the inner tube. Concentrated incident sunlight is directly absorbed on the inner side surface of the composite segments. Preliminary calculations indicate using a composite receiver of SiC/LiF results in a specific mass of about 50 kg/kWe. This is a weight reduction of about 30% compared to using LiF/canister receivers (the present space station design). Moreover, by assuming a CBC cycle efficiency of 40% at 1500 K [8], the SiC/MgF2 composite receiver achieves a specific mass of less than 40 kg/kWe. It is noteworthy that the high effective thermal conductivity of the composite material improves the efficiency of the LTES as well. He-Xe Loop Test A space solar receiver for a CBC must be designed to provide a constant supply of energy to the helium-xenon working fluid during both insolation and eclipse periods. An experimental apparatus for preliminary cyclic tests of LTES components is under construction. One of the main objectives of the tests is to define operating character-
istics of encapsulated and composite LTES systems under heat cycle conditions corresponding to an actual orbit. Figure 11 shows a diagram of such a system loop. In the tests, radiation input energy is provided by electric heaters which are separated into several parts to simulate the calculated axial distribution of solar flux inside the receiver cavity. The energy flux transferred from the heater to the LTES components is about 8 kW of which circulated heat transfer fluid removes 5 kW thermal energy continuously. Designed inlet and outlet temperatures are about 800 K and 1100 K respectively. The temperature distribution is measured by thermocouples
while thermal stress is observed with the aid of an optical instrument through windows. The detailed design has been completed and construction is nearly done. Conclusion Two new LTES concepts to be applied to CBC space solar receivers have been discussed. The addition of carbon fabric fins inside the containment canister not only enhances heat transfer but also relaxes mechanical stress caused by volumetric change of the PCM. This method offers the potential of reducing the specific mass of the receiver by about 20%. For an advanced type of space solar receiver, composite materials with silicon carbide and fluoride salts (LiF, MgF2) have been developed. Thermal stability and thermal properties have been confirmed experimentally. These composite uncanistered LTES systems are expected to be used in higher temperature applications. Receivers using SiC/LiF or SiC/MgF2 composite materials as the LTES have the possibility of achieving specific masses of approximately 50 kg/kWe or 40 kg/kWe, respectively. This results in an expected weight reduction of 30-50% as compared to receivers using conventional canisters. REFERENCES [1] Boyle R.V., Coombs, M.G. & Kudija, C.T. (1988) Solar dynamic power option for the space station, Proceedings of the 23rd IECEC, Vol. 3, p. 319. [2] Kesseli, J.B. & Lacy D.E. (1987) Advanced solar receiver conceptual design study, Proceedings of the 22nd IECEC, Vol. 1, p. 162. [3] Tanaka, K., Kanari, K., Kamimoto, M., Abe, Y., Takahashi, Y., Sakamoto, R. & Ozawa, T. (1987) Preliminary examination of latent heat-transfer energy storage materials; third report, screening of eutectic mixtures over a range from 200°C to 1500°C, Bulletin of the Electrotechnical Laboratory, 51(7), p. 19 (in Japanese). [4] Tanaka, K., Abe, Y., Takahashi, Y., Kamimoto, M. & Tanatsugu, N. (1988) Latent thermal storage for solar dynamic power system, Proceedings of the 23rd IECEC, Vol. 3, p. 63. [5] Takahashi, Y., Abe, Y., Sakamoto, R., Tanaka, K., Kanari, K. & Kamimoto, M. High temperature fluoride composites for latent thermal storage in advanced space solar dynamic power system, Proceedings of the 24th IECEC, August, Washington, DC. IEEE. [6] Takahashi, Y., Abe, Y., Tanaka, K. & Kamimoto, M. (1989) Latent heat of thermal storage materials for space use at high temperature, Proceedings of the 2nd ATPC, September, Sapporo, Japan. [7] Lichtenecker, K. (1926) The electrical conductivity of periodic and random aggregates, Phys. Z., 27, p. 115. [8] Strumpf, H.J., Coombs, M.G. & Lacy, D.E. (1988) Advanced space solar receivers, Proceedings of the 23rd IECEC, July, Denver, CO, Vol. 3, p. 357, ASME.
3-6. Advanced Nickel-Hydrogen Batteries DAVID F. PICKETT Summary Development of nickel-hydrogen batteries has progressed to the point where the nickel-hydrogen battery is the secondary power source of choice for almost all satellite systems within the USA and it is also making inroads into European Space Agency programs. The technology has emerged in the form of two basic cell designs, one developed by COMSAT and the other by Hughes Aircraft under US Air Force sponsorship. Thus far, most of the spacecraft using this technology have been geosynchronous (GEO) commercial communications satellites. Nickel-hydrogen batteries are soon scheduled for a low Earth orbit (LEO) launch on the Hubble Space Telescope and on space station in the 1990s. There are a number of improvements in the nickel-hydrogen cell which indicate that its current LEO cycle life-performance can be extended by five times and its specific energy in GEO can be doubled or tripled. Background Work started in the USA in nickel-hydrogen batteries at COMSAT Laboratories in 1971 [1, 2, 3]. Soon afterwards, results of studies appeared on the feasibility of intergrating this type of battery, with its required pressure vessel, into a spacecraft [4]. A few years later both the US. Navy and Air Force demonstrated its feasibility in flight experiments [5, 6]. The first launch of a commercial spacecraft having nickel-hydrogen batteries (INTELSAT V) occurred in 1983 [7]. Since that time, there have been launches of several GEO commercial communications satellites. A history of flight experience up to 1988 has been given by Miller [7]. Initial development of the nickel-hydrogen cell proceeded with two different concepts: one sponsored by COMSAT aimed at use in GEO missions [3] and the other sponsored by the US Air Force and Hughes Aircraft with LEO missions as a goal [4], Neither design has been limited to consideration for only LEO or GEO missions; in other words, either cell is interchangeable for other orbits depending on thermal, weight, mission lifetime and other conditions. In the COMSAT design, electrode leads are run along the edges of the electrode stack and an asbestos separator is used. The pressure vessel is coated on the inside with Teflon. A Ziegler seal is used at electrode terminals [3]. In the Hughes/Air Force version, electrodes are designed in a ‘pineapple slice' configuration so that electrode leads can run through the center of the stack, allowing very short distances between the electrode stack and pressure vessel wall (0.050 in.) [8]. The separator of choice is Zircar, a zirconium oxide cloth manufactured by the Zircar Products Corporation in Florida, New York. The pressure vessel's David F. Pickett, Hughes Aircraft Company, Space and Communications Group, El Segunda, CA 90245, USA. Paper number IAF-COSP89-3-6.
inside walls are flame-sprayed with zirconium oxide. The electrode terminal seal is of a Teflon compression type. Two types of stack orders are used: one in which the positive electrodes are placed back to back (called back-to-back) with the separator, then negative, with the gas screen following; and another, where positive electrodes are followed by the separator, then the negative electrode, then the gas screen (called recirculating). This sequence is repeated throughout the stack. A comparison of the two designs is given in Fig. 1 and Table I. Other cell/battery designs have emerged recently such as the bipolar [9, 10] and the common pressure vessel design [11], but the only flight experience or commitment for flight to date has been with the COMSAT or Hughes/USAF design.
State-of-the-Art All launches up to 1989 have used the COMSAT design cell. The first commercial launch of a spacecraft using the Hughes/USAF design was accomplished in March, 1989. There are several more launches of spacecraft which will use this cell design (or a modification of this design) scheduled for the near term, including a LEO mission (Hubble Space Telescope) which is scheduled for this year. More launches are expected as the LEO database is expanded. A concerted effort is underway, with USAF and USN sponsorship, to expand this database [12,13]. Limited databases are available from Hughes Aircraft [14], Martin Marietta [15], RCA [16] Eagle-Picher [17] and other US aerospace contractors. Data ranges from 6500 to 10000 cycles at an 80% depth of discharge [14] to in excess of 32 900 cycles at a 15% depth of discharge [17]. The temperature range is 10 to 25°C. NASA is also sponsoring LEO testing in support of space station [18]. There have been several modifications/improvements to the Hughes/USAF cell during the past several years. These were made under USAF [19], NASA [20] and private sponsorship [21]. As originally designed, the 3.5 in. (8.9 cm) diameter pressure vessel had a capacity limitation of about 60 ampere-hours (Ah). By lengthening the pressure vessel and including another series stack, the capacity can be increased to about 90 Ah; however, additional pressure vessel design is needed to keep the burst/operation pressure ratio in excess of safety margins required (>3). Another approach is to increase the pressure vessel diameter to 4.5 in. (11.4 cm). Use of the 4.5 in. diameter individual pressure cell (IPV) is considered a better alternative to the risk of development and successful demonstration of a common pressure vessel cell (CPV) [11], A 4.5 in. IPV cell with a single stack has a capacity range from 90 to 160 Ah. Recently capacities up to 220 Ah have been achieved using a tandem stack arrangement [22]. Recent Advances in the Technology Even though the capacity of nickel-hydrogen cells has been improved by the above innovations, the more significant issues such as improvement of cycle life and specific energy remain. In the case of cycle life, it has long been established that the life limiting component in a nickel-hydrogen cell is the nickel oxide electrode [23] in spite of advances that have been made at Bell Telephone Laboratories [24], USAF laboratories [25] and in private industry [26]. Probably the most significant breakthrough in cycle life improvement of nickel-hydrogen cells has been reported by Lim & Verzwyvelt in an effort at Hughes Aircraft that has been ongoing under NASA LeRC sponsorship for the past few years [27]. They were able to increase the cycle life to over five times that of state-of-the-art with a simple adjustment of electrolyte concentration (26% vs 31% KOH). A summary of their data from tests using boiler plate cells is shown in Fig. 2. The results are currently being re-evaluated using flight design hardware. Results to date are shown in Fig. 3 [28]. It is hoped that sufficient data from these tests will be available prior to finalization of the space station cell design so that this technology can be utilized. Nickel-hydrogen battery design and development for the space station has been initiated by Ford Aerospace [18]. Present requirements for the battery are 41000 cycles (five years) at a depth of discharge of 35%. To support the 75 kilowatt (kW) average user load (100 kW peak) modules with five batteries, 90 cells at 81 Ah (3.5 in. cell diameter) are the baseline, with 30 cell orbital replacement units. Studies are
aimed at improvement of the Inconel 718 pressure vessel and optimization of the components and thermal interface. Boiler plate cell tests are being performed for evaluation of electrodes. Two cell manufactures have been selected. Cell qualification is scheduled for 1991 and battery qualification for mid-1992. In other developments, attempts toward improving specific energy through use of
lightweight substrates for nickel oxide electrodes have been under way at NASA LeRC for the past few years and have been initiated at Hughes under NASA contract NAS 3-22238. Britton has recently described studies using substrates from several sources: Fibrex from National Standard, Metapore nickel felt from Soropec and machined sintered nickel fiber mat from Nippon Seisen [29]. A weight reduction of 40% in nickel electrode weight appears possible. Using a 1.1 C charge, 1.37 C discharge regime, 4534 cycles at 100 % depth of discharge have been recorded. At present it is uncertain if the lightweight substrate technology can be used for LEO applications, but results thus far suggest that specific energies approaching 80 watt-hours per kilogram (Wh/kg) for batteries in geosyncrhonous orbit may be achievable with successful implementation of these substrates and other weight reduction measures. Comparison to Other Technologies Other technologies in competition with nickel-hydrogen batteries for spacecraft energy storage systems include regenerative fuel cell systems, sodium-sulfur batteries and nickel-cadmium batteries. Solar dynamic systems and nuclear power systems have particular advantages/disadvantages over electrochemical/photovoltaic systems, but are not yet used extensively in the USA. They will not be addressed here. One of the most recent comparisons of regenerative fuel cell systems with nickel-hydrogen and sodium-sulfur batteries has been made by Taenaka et al. in USAF sponsored effort performed at Hughes Aircraft [30]. They concluded that the energy storage system specific energy (which includes the dedicated solar array, thermal menagement, power electronics, etc.) projected for a regenerative fuel cell system (~31 Wh/kg) in a mid- to high-altitude orbit satellite is intermediate between that presently attainable with flight-qualified nickel-hydrogen batteries (~24 Wh/kg) and those projected for the new sodium-sulfur technology (>53 Wh/kg) (system) or >100 Wh/kg at the battery only level). For large power systems (>2000 W) the advanced nickel-cadmium battery is not competitive with the nickel-hydrogen type, but in cases where power levels less than 1 kW are required, nickel-cadmium batteries are more weight and volume efficient and cost less than nickel-hydrogen batteries, and will continue to be used in small science satellites. However, if the cycle life projections made by Lim & Thaller [31] are valid, advanced nickel-hydrogen cells will have five times the cycle life. Conclusions In the past 18 years since their conception, nickel-hydrogen batteries have made large inroads into the space economy storage area, virtually replacing nickel-cadmium cells for all power applications greater that 1.5 kW. This trend is not restricted to the USA, but is catching hold in Europe [32] and Japan [33] as well. Advanced nickel-cadmium cells are expected to be used in small science satellites for some time. With improvements, regenerative fuel cells will probably be used in some mid-altitude and higher orbits since their specific energy becomes attractive in these cases. Because of their high specific energy and moderate cycle life, sodium-sulfur cells will replace nickel- hydrogen cells for some applications in the mid- to late-nineties. However, if lightweight nickel substrates prove feasible, nickel-hydrogen batteries can offer some formidable competition.
REFERENCES [1] Giner J. & Dunlop, J.D. (1975) Journal of the Electrochemical Society, 122, p. 4. [2] Dunlop, J.D., Giner, J., Ommering, G. van & Stockel, J. (1975) US Patent 3,867,199. [3] Dunlop, J., Stockel, J. & Ommering, G. van (1984) Sealed metal oxide-hydrogen secondary cells, Proceedings of the 9th International Power Sources Symposium, Brighton, 16-19 September, pp. 315-329. [4] Levy, E. et al. (1974) Nickel Hydrogen Energy Storage for Satellites, US Air Force Technical Report AFAPL-TR-74-111, final report under Air Force contract F33615-74-C-2064, November. [5] Betz, F. (1977) The NTS-2 nickel-hydrogen battery, The 1977 Goddard Space Flight Center Battery Workshop, NASA Conference Publication 2041, November, pp. 489-498. [6] Harsh, W. (1977) Air force nickel-hydrogen experiment, ibid., pp. 499-505. [7] Miller, L. (1988) The NiH2 battery system: a space flight applications summary, Proceedings of the 23rd IECEC, Denver, CO, pp. 489-492 (ASME). [8] Adler, E., Standnick, S. & Rogers, H. (1980) Nickel hydrogen battery advanced development program status report, Proceedings of the 15th IECEC, Seattle, WA, August, pp. 1891-1896 (AI A A). [9] Cataldo, R.L. et al. Test results of a 60 bolt bipolar nickel-hydrogen battery, Proceedings of the 22nd IECEC, Philadelphia, PA, August, pp. 873-877 (AIAA). [10] Lenhart, S. et al. 1988) Test results on a 75 Ah bipolar nickel-hydrogen battery, Proceedings of the 23rd IECEC, Denver, CO, August, pp. 379-384 (ASME). [11] Adler, E. & Perez, F.A. (1986) Design considerations related to nickel-hydrogen common pressure vessel battery modules, Proceedings of the 21st IECEC, San Diego, CA, August, pp. 1554-1159 (ACS). [12] Badcock, C.C. & Haag, R.L. (1985) Nickel hydrogen low earth orbit life testing, The 1985 Goddard Space Flight Center Battery Workshop, NASA Conference Publication 2434, November, pp. 399-410. [13] Donley, S.W., Hill, C.A. & Manichiello, A.A. (1988) Nickel Hydrogen Cell Life Test Plan—A Low Earth Orbit Test, November (Test plan prepared by the Aerospace Corporation). [14] Levy, E. Jr., (1986) US. Air Force design nickel-hydrogen cells—flight status and recent improvements, Proceedings of the 21st IECEC, San Diego, CA, August, pp. 1537-1540. [15] Fuhr, K.H. (1987) Failure analysis of 3.5 inch, 50 ampere-hour nickel-hydrogen cells undergoing low Earth orbit testing, Proceedings of the 22nd IECEC, Philadelphia, PA, August, pp. 889-893. [16] Shiffer, S.F. & Kaden, W.M. Testing of NiH2 battery cells for use in low earth orbits, ibid., pp. 885-888. [17] Coates, D.K. & Barnett, R.M. (1988) Nickel-hydrogen cell life testing, Proceedings of the 23rd IECEC, Denver, CO, August, pp. 483-488. [18] Ommering, G. van, Chawathe, A.K. & Haas, R.J. (1989) Battery design and development for the space station Freedom power system, Proceedings of the Fourth Annual Battery Conference on Applications and Advances, California State University, Long Beach, CA, January. [19] Taenaka, R.K. (1988) Common Pressure Vessel Nickel-Hydrogen Battery Development Program, Final Report under USAF Contract F33615-82-C-2213, Report No. AFAPL-TR-87-2101, December.
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