Theoretical Performance of a Radioisotope Thermophotovoltaic
Transcription
Theoretical Performance of a Radioisotope Thermophotovoltaic
AIAA 2009-4655 7th International Energy Conversion Engineering Conference 2 - 5 August 2009, Denver, Colorado Theoretical Performance of a Radioisotope Thermophotovoltaic (RTPV) Power System David S. Wolford1, Donald L. Chubb (Distinguished Research Associate) NASA Glenn Research Center,21000 Brookpark Rd., Brook Park, OH 44135 An RTPV power system with a nominal output of 38W is being developed by NASA. As part of that program, a theoretical model of a planar thermophotovoltaic (TPV) system has been developed. Performance results from that model will be presented. The model uses experimentally determined optical and electrical properties of the major components (emitter, filter and photovoltaic array) of the system. One of the objectives of the model is to compare a system that uses a single optical cavity to one that has two optical cavities. Spectral emittance must be decreased as emitter and array size increase in order to maintain the high emitter temperature required for system efficiency. Several low vapor pressure metals as emitter materials will be modeled. Another objective is to determine the parasitic heat loss that occurs in the system. Discussion of these two objectives will be a major part of the presentation. I. INTRODUCTION Radioisotope thermophotovoltaic (RTPV) power systems have the potential for both high efficiency (>15% projected) and high mass specific power (6-7W/kg projected). In RTPV energy conversion, thermal energy from the General Purpose Heat Source (GPHS) is coupled to an emitter, which produces photons when heated to operational temperature (~1350K). The emitter material may be chosen and fabricated such that its spectral emittance favors photon emission in the short wavelength region (< 2 micron). The emitter and other hot components must be made of a material with very low vapor pressure in order to avoid sublimation losses and contamination of the other components. Photons emitted from the emitter impinge upon a tandem optical filter, consisting of a dielectric interference stack deposited on a semiconductor based plasma filter. The combined filter transmits about 80% of the useful photons (those with wavelengths < 2 microns) which are converted to electrical energy by a 1 Project Scientist, Photovoltaic & Power Technologies Branch, MS 302-1, AIAA Senior Member, [email protected] 1 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. photovoltaic (PV) array, while reflecting nearly 100% of the long wavelength photons back toward the emitter. The emitter and filter are separated by ~2mm, thus a significant fraction of these reflected photons are absorbed by the emitter, thereby recycling their thermal energy. The use of a selective emitter becomes less important with the use of a well designed optical filter. Directly beneath the tandem filter lies a low bandgap photovoltaic device, which absorbs and converts the transmitted photons directly into DC electrical energy. The energy source for this technology is the GPHS which contains plutonium dioxide fuel with a beginning of mission thermal output of about 240 watts. Heat is generated by natural radioactive decay of the 238Pu present in the oxide. RTPV has an estimated 80% of the initial power output remaining at the end of a 14-year mission life. The estimated loss is due to radioactive decay of the fuel and cell degradation effects from radiation damage. Losses due to contamination of the arrays from the hot side cannot be permitted. Radioisotope thermoelectric generators (RTGs) have traditionally powered spacecraft destined for the outer planets. RTGs have demonstrated excellent reliability, although their performance is in the 6% to 7% efficiency range1. NASA is interested in alternate thermal to electric conversion technologies that have higher efficiency and would enable more missions to be flown with the existing GPHS inventory. The Stirling Radioisotope Generator (SRG) technology has demonstrated excellent conversion efficiency (32-38%)2. NASA is vigorously pursuing SRG development for future space power needs. NASA and its industry partner Creare Inc. of Hanover, New Hampshire are currently engaged in a RTPV power system technology development program. RTPV is being developed as a small radioisotope power system (RPS) for the purpose of providing mission planners a suite of RPS options. It is to be used as a compact, long service, zero maintenance power source. Applications include remote sensing and communication installations as well as aiding the mobility of human and robotic exploration. Continuous power production is of particular value in locations where solar illumination is weak or interrupted for long periods (e.g. outer planets or earth’s moon). This effort follows prior work by Orbital Sciences Corp and others. Figure 1 is a drawing of the baseline 500 Wt RTPV converter proposed by A. Schock et al in 1996.3 2 Figure 1, two GPHS converter proposed in 1996 (ref. 3) In 2007 NASA reconfigured the baseline 500 Wt converter. It was decided to pursue RTPV in a configuration that used a single GPHS with a thermal input of about 240 Watts. This change was driven by the limited availability of GPHS fuel modules and SRG successes. Figure 2 is a drawing of the reconfigured converter. The initial vision of the reconfigured system was a converter with a single high emittance optical cavity at one end. The reconfiguration also presented the possibility of a system with two low emittance optical cavities located at opposite sides of the GPHS. The spectral emittance of the emitter surface is one parameter that can be selected in order to maintain the high core temperatures that produce high efficiency in the optical cavity .Figure 3 is a schematic representation of the single and double optical cavity systems. A system that uses two optical cavities at opposite sides of the GPHS will have a smaller parasitic heat loss than a single cavity system. 3 Housing End Plate and Waste Heat Radiator (2) Tandem Filter and 0.6 eV InGaAs MIM array (2) Canister Lid General Purpose Heat Source (GPHS) 240 watt Seal / Support Ass’y (4) Fuel Canister Multi-foil Insulation Housing Figure 2, 2007 single GPHS configuration II. DEPOSITION RATES A RTPV power system requires that the filter surface remains free from contamination in order to assure a long mission life. The high temperature components that have line-of-sight exposure to the filter are the side reflectors and the emitter. The side reflectors are a polished high temperature foil such as tantalum. The emitter can either be the fuel canister itself or a material applied to the fuel canister. The canister itself must be a metal to withstand the vibration requirements of the mission. The benefit of using the canister surface as the emitter is that the possibility of coating separation is eliminated. Very few metals have the required low vapor pressure this application requires. Additionally the material must be pure in order to prevent impurities from evaporating out. The emitter materials considered for this study are iridium (Ir), rhenium (Re), tungsten (W) and tantalum (Ta). Table 1 shows the rate of deposition for the materials. These deposition rates were calculated using the following expression developed in reference 4. 4 Figure 3, schematic of single and double cavity converters • d= S E F CE 8 mE pv (TE ) k BTE m/sec (1) This result assumes free molecular flow for the sublimating molecules. Appearing in equation (1) are the following quantities: FCE= view factor from the PV array to the evaporating material, FCE ≈ 1 αs= sticking coefficient ρE=density of evaporating material, kg/m3 mE= atomic mass of evaporating material, kg kB=Boltzmann’s constant = 1.3805 E-23 J/K TE= temperature of evaporating material, K 5 pv=vapor pressure, N/m2 In the calculations, it is assumed that FCE = 1 and αS = 1. Vapor pressure for the temperatures of interest are very low (< 10-10 torr) and precise experimental data is not available. As a result, conservative estimates of pv based on the available data at higher temperatures were used for the results in table 1. III. DESCRIPTION OF THE MODEL A theoretical model of a planar TPV system has been developed5 which allows for the evaluation and comparison of the different configurations. The equations were programmed in Mathematica 7. An energy balance that uses the optical and electrical properties of the components determines the equilibrium emitter operating temperature, TE. Radiation fluxes are determined by view-factors, thus two major assumptions are; 1) radiation intensities are isotropic (independent of angle) and 2) intensities are uniform over each area. It is also assumed that the temperature of the reflectors surrounding the optical cavities, Tb = TE the emitter temperature. A description of selected inputs and calculation methodology are given in the general order used in the model. This is an eight step process. 6 Figure 4, diagram of cold plate geometry used in the model (1) View factors of the components are the initial calculations. The separating distances and general dimensions are defined. Figure 4 is a diagram showing the dimensions of regions C1-C3 on the PV array side of the RTPV system. These components are directly cooled by phase change heat tubes coupled to lightweight radiators. C1 is a region central to the PV covered area. The effect of reflected radiation from reflectors that surround the cavity is negligible here. C2 is an area of PV arrays that interact more with reflectors that surround the sides of the optical cavity. C3 is a region around the active PV region that consists of highly polished gold plate. Gold is tolerated here due to direct cooling of the substrate; it is also an effective reflector of long wavelength radiation. It should be noted that region C3 represents a region present on current laboratory cold plates. For the configuration shown in figure 4, used for the purposes of this analysis, region C3 constitutes 33% of the cold plate area. Since components are to be custom fabricated for a deployment ready converter, an opportunity exists to minimize the inactive region and thereby enhance efficiency. Figure 5 is a schematic of the optical cavity showing the positions of the interacting elements of the optical cavity. The arrows indicate energy exchanges accounted for in the model. Figure 5, schematic of power flow in optical cavity model 7 The view factor calculation for the emitter to PV region C2 is shown. FE C2 = 1 AE cos AE AC 2 cos S2 E C2 dAC 2 dAE = 0.4999 (2) Where; θE, θC2, d and S are defined in figure 5. Additionally; AE= emitter area= 11 cm x 11 cm= 121 cm2 AC2= outer PV area = 9 cm x 9 cm – AC1 = 60.75 cm2 AC1 = inner PV area = 4.5 cm x 4.5 cm = 0.203 cm2 d = 0.2 cm (2) Radiation shield characteristics are then calculated. This models the performance of the multilayer foil insulation surrounding non-active portions of the system. It is used to determine the power loss through the shields. A twenty layer blanket is modeled. The emittance of the foils is given as 0.2 and the gap between the foils is 1 mm. Conductive loss for foil separators is not considered. For the single cavity system, shields are required on five sides of the GPHS. The two cavity system requires only four sides of the GPHS to have shields. As a result, the two cavity system has a smaller parasitic heat loss (3) Emittance data is input next. The temperature range for the data is 1500 to 1600 K for the polished emitter cases6. The emittance data for the rough surface tantalum emitter is calculated using 300 K reflectance data (ελ = 1 - ρλ ). Curve fits to the data are used in the model. Figure 6 shows the emittance of the different emitter materials. The four metals were chosen for their very low vapor pressure because it is essential that no material transfers by sublimation from the hot side to the cold side surfaces. In addition, very pure materials must be used in order prevent contamination from impurities that would reduce performance over a fourteen year service life. 8 1.0 rhenium , 0.9 iridium , 0.8 tungsten, 0.7 = 15.3261 = 43.324 polished tantalum , 0.6 emittance, = 11.4856 roughened tantalum , 0.5 -.521 -.602 -.694 = 217.349 = 14.636 -.948 -.491 0.4 0.3 0.2 0.1 0.0 0 2000 4000 6000 8000 10000 w avelength ( ), nm Figure 6, emittance of different emitter materials (4) Filter reflectance is entered in tabular form. The data shown is for a dual interference plasma filter fabricated for this program by Rugate Technologies. It consists of a 70 layer gallium telluride (GaTe) / yttrium fluoride (YF3) interference filter deposited on an indium phosphide (InP) plasma filter. GaTe filters are filters that are stable at operating temperatures of 150 ˚C, well above cold plate temperatures. Previous filters were known to be marginally stable at cold plate operating temperatures. Figure 7 shows the reflectance and transmittance data used for this model. 9 Figure 7, transmittance and reflectance of GaTe dual filter (5) A routine is used to give all data the same wavelength increment for subsequent calculations. (6) The quantum efficiency (QE) or external quantum yield is input for the PV devices. The data used is for PV devices provided to a Creare phase I NRA program in 2004 by Emcore Inc.7 Figure 8 shows an individual Monolithic Integrated Module (MIM) which contains a string of 25, 0.60 eV bandgap energy ( 2.07 μm), Indium Gallium Arsenide (InGaAs) junctions. The figure also shows measured QE data. The model assumes a 4 x 4 array of these modules. 10 Figure 8, measured QE of MIM and photo insert of MIM (7) Saturation current densities, series and shunt resistances and ideality factors are introduced for the PV arrays. Also the spectral responses are calculated for areas C1 and C2. (8) With all inputs and preliminary calculations complete the remaining optical cavity calculations are completed in a single routine. The emitter temperature is determined iteratively in approximately 15 steps. This is the model equivalent of the converter reaching thermal equilibrium. It is the optical properties of the components (emitter, filter, PV arrays, and shields) of the converter that determine the equilibrium emitter temperature. The following is an example of the five radiation transfer equations. qic1(λ) = (FC1E) qOE(λ) + (FC1b) qOb (3) Where; qic1(λ)= radiant energy reaching area C1, W/cm2 nm FC1E = view-factor from C1 to emitter qOE(λ) = radiant energy leaving the emitter, W/cm2 nm 11 FC1b = view-factor from C1 to reflector qOb = radiant energy leaving the reflector, W/cm2 nm Transfer equations are then combined with an overall energy balance, QGPHS = 240 W = QE(TE) + Qb(TE) + QP(TE) (4) Where; QGPHS = power input from GPHS, W QE(TE) = net power leaving the emitter, W Qb(TE) = net power leaving the reflector, W QP (TE) = parasitic power loss, W Additional parameters are; Initial temperature conditions, 1300 K hot, 300 K cold Cell bandgap energy = 0.6 eV Gold reflectance = 0.95 Reflector reflectance = 0.7 The radiant energy fluxes are used to calculate QE, Qb and Qp as a function of TE. The Q terms are then used in the energy balance, equation (4). An iteration on TE is made until the energy balance is satisfied. This yields the correct TE. Once all the radiant energies (as functions of TE and wavelength) are known, the PV electrical output can be calculated. IV. MODEL RESULTS The results reflect three varying parameters; emitter material, emitter roughness and number of optical cavities. The corresponding reflectances of the emitter and reflector are taken as ρλ = 1 - ελ. Table 1 shows selected model outputs for ten converter configurations. Part a) shows the results for single cavity systems. Part b) shows the results for double cavity systems. The 12 highlighted cells indicate the best results among the ten cases. The following definitions pertain to table 1. The thermal efficiency, ηTHERMAL = QE + Qb QGPHS (5) QGPHS = 240 W The cavity efficiency, ηCAVITY = QCAVITY QE + Qb QCAVITY = radiant power delivered to PV array that can be converted to electrical energy, 0 hc Eg hc = Planck’s constant *(speed of light in a vacuum) Eg = band gap energy of PV arrays = 0.6 eV The PV efficiency, ηPV = PELECTRIC QCAVITY PELECTRIC = electric power produced Parasitic heat loss = QP = QGPHS QE Qb = (1 The total efficiency, ηT = ηCAVITY ηTHERMAL ηPV = THERMAL PELECTRIC QGPHS 13 ) QGPHS Single Optical Cavity Configuration iridium, polished rhenium, polished tungsten, polished tantalum, polished Emitter temperature, K Material sublimation rate, nm/yr PV efficiency Cavity efficiency Parasitic heat loss, W Thermal efficiency Power out, W Total efficiency 1402 < 3.970 0.251 0.715 69.60 0.710 30.60 0.127 1364 < 0.410 0.253 0.731 62.30 0.740 32.77 0.137 1361 < 0.046 0.253 0.738 61.66 0.743 33.33 0.139 1396 < 0.055 0.253 0.730 68.28 0.715 31.66 0.132 Ta, 659 nm average roughness 1310 < 0.055 0.255 0.756 53.03 0.779 36.04 0.150 a) The single optical cavity configuration Double Optical Cavity Configuration iridium, polished rhenium, polished tungsten, polished tantalum, polished Emitter temperature, K Material sublimation rate, nm/yr PV efficiency Cavity efficiency Parasitic heat loss, W Thermal efficiency Power out, W Total efficiency 1279 < 3.970 0.240 0.660 34.58 0.856 32.52 0.136 1241 < 0.410 0.241 0.673 30.73 0.872 34.96 0.142 1239 < 0.046 0.242 0.682 30.48 0.873 34.62 0.144 1273 < 0.055 0.242 0.675 33.99 0.859 33.59 0.140 Ta, 659 nm average roughness 1191 < 0.055 0.244 0.697 26.01 0.892 36.35 0.152 b) The double optical cavity configuration Table 1, model results for 4 emitter materials V. CONCLUSIONS For all four emitter materials, the double cavity model yields a higher total efficiency and a lower operating temperature than a single cavity system does. It is the lower parasitic heat loss of the two cavity system that produces the larger total efficiency. Going from a single cavity to a double cavity design reduces unproductive shielding by about 30% while adding productive PV area. While the cavity efficiency is improved at higher emitter temperatures, the parasitic power loss is also higher. As stated earlier, if the gold reflector in area C3 is replaced with a PV array the efficiency will increase. 14 The highest total efficiency in this analysis is with a double optical cavity, the largest emittance and the lowest emitter temperature. Higher efficiency and lower emitter temperature resulting from large array area and an emitter with large emittance should be studied further for the purpose of optimization. Improved efficiency at lower operating temperatures will make two technology challenges less daunting. First, emitter sublimation rates decrease rapidly with decreasing temperatures. As a result, a 14 year lifetime is more easily achievable. Second, lower operating temperatures reduce the risk that the complex RTPV filters will degrade from the effects of high temperature over the 14 year mission life. Furthermore, the two optical cavity systems give redundancy since two separate power busses are available. In the event of a failure in one cavity the possibility of partial power output would exist. The critical parameter in these results is the emitter spectral emittance. Curve fits to experimental data were used in the model. A more accurate model will require spectral emittance data measured in thermal conditions similar to those expected in the converter. Additionally, advanced emitter technology to enhance emittance (e.g. textured surface8 and photonic crystal9) may be required for optimal efficiency. ACKNOWLEDGMENTS The authors wish to acknowledge the following business and institutional partners; Creare Inc., Rugate Technologies, General Atomics, Sandia National Laboratories. REFERENCES 1 Schmidt, G.R., Wiley, R.L., Richardson, R.L., and Furlong, R.R., “NASA’s Program for Radioisotope Power System Research and Development,” AIP Proceedings, Volume 746, Feb. 2005, pp 429-436. 2 Chan, J. et al, “Development of Advanced Stirling Radioisotope Generator for Space Exploration,” NASA/TM-2007-214806, May 2007 3 Schock, A. et al, “Modified Design of Radioisotope Thermophotovoltaic Generator to Mitigate Adverse st Effect of Measured Cell Voltage” Proceedings of the 31 Intersociety Energy Conversion Engineering Conference, IEEE Catalog No. 96CH35878, Vol. 2, edited by P.Chetty, et al, New Jersey, 1996, pp. 979-994 4 Scheiman, D. et al, “Emitter Evaporation Study in Space TPV Systems” TPV-8 Conference, Palm Desert, CA, November 20, 2008 5 Chubb, D.L., Fundamentals of Thermophotovoltaic Energy Conversion, Elsevier, First edition, 2007 , Chapter 6 and Appendix F. 15 6 Y.S Touloulian, D.P DeWitt, Thermal Radiative Properties; Metallic Elements and Alloys, Thermophysical Properties of Matter, Volume 7 (IFI Plenum, New York-Washington, 1970) pp. 291 (curve 3), 568 (curve 8), 676 (curve 14), 802 (curve 14). 7 Wilt, D. et al, “Progress in Radioisotope Thermophotovoltaic Power System Development” AIAA-20074771, 5th International Energy Conversion Engineering Conference and Exhibit (IECEC) , St. Louis, Missouri, June 25-27, 2007 8 DePoy, DM et al, “Thermovoltaic Spectral Control,” DOE publication LM-04K053, June 9, 2004 9 Celanovic, Ivan et al, “Two-dimensional tungsten photonic crystals as selective thermal emitters,” Applied Physics Letters, v 92, n 19, 2008 16
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