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, Wolford@nasa.gov
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
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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
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5
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15
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7
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8
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9
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