Previous concepts for solar power satellites have used conventional-technology photovoltaics and microwave tubes. The authors propose using thin film photovoltaics and an integrated solid state phased array to design an ultra-lightweight solar power satellite, resulting in a potential reduction in weight by a factor of ten to a hundred over conventional concepts for solar power satellites

Cull, Ronald C.

Landis, Geoffrey A.

English

NASA Technical Reports Server

N91-22179 '
Applications of Thin Film Technology
Toward a Low-Mass Solar Power Satellite
Geoffrey A. Landis* and Ronald C. Cull
NASA Lewis Research Center, Cleveland, OH 44135
i AbstractPrevious concepts for solar power satellites have used conventional-technology photovoltaics
and microwave tubes. We propose using thin-film photovoltaics and an integrated solid-state
phased-array to design an ultra-lightweight solar power satellite, resulting in a potential reduction in
weight by a factor of ten to a hundred over conventional concepts for solar power satellites.
Introduction
The concept of a Solar Power Satellite (SPS) to provide power for Earth was proposed in 1968
by Peter Glaser [1]. Glaser proposed to solve the energy crisis and provide abundant electrical
power for Earth by putting large (1-10 Gigawatt) solar collectors into geosynchronous Earth orbit,
and to transmit energy to the surface using a microwave beam. Solar power satellites based on this
concept were extensively analyzed in the period around 1978-1981 [2-4].
The solar power satellite concepts examined in the late seventies had two significant difficulties:
capital cost and weight.
In this paper we discuss the potential for two technologies currently under development to
considerably reduce the mass-to-orbit required for such a satellite power system: thin-film
photovoltaics and solid-state microwave electronics. It may be possible to design a solar power
satel'.ite to be constructed entirely by thin-film technology, consisting of thin (one to two micron)
active components on a plastic substrate, with the microwave phased-array components integrated
directly with the photovoltaics.
I,aessence, we propose discarding the "bridge-builder" mindset adopted by the initial designers
of a SPS system, and adopt a thin-film, integrated-circuit mentality: why can't a solar power
satellite be a single, integrated assembly deposited on a thin, lightweight substrate?
Thin film photovoltaics
Thin-film solar cells consist of thin (thickness -1-51a) films of photovoltaic material deposited
on a supporting substrate In the 1980's a considerable research program has been devoted to
development of thin-film photovoltaics for terrestrial power generation. Efficiencies over ten
percent have been achieved on amorphous silicon and copper indium diselenide thin-films, and
encouraging results _.hieved on other thin-film technologies such as CdTe and CulnS 2. Table 1
showsthe historic" i progress in efficiency of several of the thin-film materials over _he last few
years [5].
Because of J_e high optical absorption constant, for thin-film solar cells the active material may
be as thin P'. one to two microns, and hence the materials inherently have the potential to be
extremely ._ght. However, very little current research is aimed at depositing thin-film cells on
lightweight substrates, since most of the applications being currently considered are for terrestrial
applications, where weight is not important.
Preliminary results show that thin-film solar cells appear to be inherently radiation tolerant, and
may not require a glass cover for radiation protection 16]. They also are highly tolerant of small
damage areas, such as due to nficrorneteoroid or debris impact.
A conservative projection would be to project use of a 5% efficient thin-film cell on a 25 micron
thick Kapton substrate. This yields a photovoltaic blanket specific power of 1.7 kW/kg. An
optimistic projection might be 15% thin-film cell on a 7 micron thick Kapton substrate, leading to a
photovoltaic blanket specific power of 15 kW/kg. These nurnbers compare favorably to current
technology spacecraft solar arrays, e.g., 67 W/kg at the anay level for the flight-te¢,cd SAFE array.
*Sverdrup Technology, 2001 Aerospace Parkway. Brook Park, O1-I44135
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Solar cell cost was highlighted as one of the single most critical improvements needed for a SPS
in the NRC analysis. In addition to low mass, thin-film photovoltaics are also projected to have
considerably lower costs. Materials cost is reduced due to the reduced amount of materials required;
the cost of labor and assembly is reduced by the fact that large-area, inte_ated a._._emb!ies
dlr_.r'th, ,._,-,-.,A....A
..... a v ..... ,._ on the subshate sheet.
For typical existing systems the photovoltaic blanket weight is only about a quarter of the total
power system mass, with the array structure and power management and distribution (PMAD)
accounting for the remaining three quarters. This provides a powerful incentive to integrate the
PMAD elements directly to the solar array, and to design new array structures to take advantage of
the ultralight blankets.
Historical progress of thin-film solar cell efficiency. Experlmeni_lly
achieved efficiencies (at AMO, in %) as of 1978, 1983, 1988, and
projected values for future performance.
Material 1978 1982 1988 1990's
CdS/CuzS 7.3 8.2 9. 10
CuInSe 2 5.3 8.5 11.2 12
CuGaSe 2 - 4.6 12.5
CuInS 2 2.9 2.9 6.1 12.5
CdTe 4.1 8.4 8.6* 12.5
a-Si 4.4 8.1 9.0 11.5
CASCADES - - 12.5 17+
I
*(9.8% reported in May 1989)
Microwave electronics
Use of thin-film solar cells will reduce the satellite mass only if the mac° requiredfor the power
management and the microwave beaming system can be reduced as well. This may also be i
ach__evableusing solid state electronics. In the last ten years we have seen development of i
microwave integrated circuits, thin-film transistor_, and thin-film microwave rectennas
Solid-state electronics has developed considerably, and it is now reasonable to consider that the
rn_crowavesource for a SPS may be able to be manufactured from thin-fiim electronics.
Microwave rectifying antennas (rectennas) for receiving microwave power anclconverting it to
DC power have been demonstrated using thin-film techniques on a thin-plastic substrate [7]. Using
such a technology as a microwave source requires replacement of the GaAs diudes by appropriate!y
phased micruwave transistors.
Thin-film microwave electronics are a reasonable extrapolation of the union of two recent
developments: solid-state microwave electronics, and thin-film transistors. Microwave integrated
circuits are currently available which can operate in the gigaherz band proposed for SPS power
tran'_mission, and have been demonstrated by many different laboratories for operation at the tens
and hundreds of gigaherz [8].
Thin-film transistors have been developed for other applications such a_ _L_playscreens, where
many millions of devices can be integrated on a large-area sheet. Thin-film transistors are made
from amorphous silicon or polycrystalline silicon, and could in principal be manufactured integrated
onto the same substrate as the solar cells. Current technology only allows frequencies in the range
of kiloherz to at most megaherz [91,but this will likely increase with further research. Development
of gigaherz-speed thin-film transistors would allow the transmitter elements to be deposited on the
thin substrate at the same time as, and possibly using the same materials as, the thin-film solar cells.
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'I'iaeconcept for integration of the solar cell with the microwave oscillator and antenna is shown
in :;chematicin figure 1. A slightly more complex version, where the solar cell metallization is used
for the antenna in a "push-pull" configuration, is shown conceptually in figure 2.
.t..,v_Jj ',,'VAI.t,AUU*, tll_., O.k.,V_IUI3LAAK;IIL UA tlllll'llltll IIlLK, AOW_I.V_ tliJ, ll_.il31Ul_, _OilVl_lltlOllal [I|iCIOW_.,,e
solid-state integrated electronics would be able to perform the function at a cost considerably below
that of the microwave tube approach. Each element need operate only at a power level of a watt or
less, well within the capability of microwave integrat,;dcircuits.
An alternate approach to operation of the ;olid-state microwave source at even higher
frequencies is the use of superconducting electronics. Such technology is currently being
researched; for example, a superconducting circuit operating at 33-37 GHz has been developed at
NASA Lewis using thin-film YBaCuO at 77°K [10].
Micro-phased array distributed thin-film SPS
In an highly-integrated SPS design, microwaveoscillators and dipoleantennas are be integrated
directly on the plastic sheet with the solar cells, using phased-array techniques to steer the beam
back along a pilot beam generated at the receiving antenna on the Earth. Rather than a smaller
number of high-power microwave tubes, the integrated SPS will have billions of integrated
transmitters,each operati__gat a powerof no more than a watt. This integrationwouldeliminate the
power conditioningelements and the wiring used forpower distribution.
The proposal consists of the following elements:
--Total integration. Microwave transmitters are integrated directly at the solar cell level. No
wires or power management/distribution system is required.
--Thin-film technology. Lightweight photovoltaic films on a thin plastic substrate are used.
--Phast.d array technology. The antenna does not need to be physically "aimed" at the receiver.
The distributed thin-film SPS applies the integrated circuit approach to the satellite solar power
concept.
Table 2 shows a comparison of the mass of an integrated thin-film solar power satellite
compared to a baseline system. Using the "conservative" technology extrapolation the reduction in
weight is by more than a factor of ten; assuming a more advanced technology a reduction in weight
by over a factor of a hundred is achieved.
Table 2:
Mass Comparison
Baseline SPS (1980);
2.6 kg/kW Transmission and control
6.5 kg/kW Silicon Solar Array
0.6 kg/kW Power Conditioning
9.7 kg/kW
" Thin-Film SPS (1990'S): (5% efficient solar ceil on 25_ Kapton)
0.7 kg/kW Solar array + integrated transmitter
Thin-Film SPS (2000+): (15% efficient solar cell on 7l.t Kapton)
0.08 kg/kW Solar array, integrated transmitter
It is important to design new, low mass structures Jn order to reduce the structural mass of the
system proportionately to the photovoltaic and transmitter mass reductions. Many structural
designs for such a system are possible. Two, the "bicycle wheel" configuration (shown in figure 3)
and the "sphere" configuration (shown in figure 4), shown. The phased array microwave elements
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mean that the microwave antenna does not have to face directly toward the receiving station, as long
as the antenna is not edge-on to the receiver. The "bicycle wheel" concept uses centrifugal force to
place a thin circular membrane in tension. Since the size is large and the tension required _mal!, a
very low rotation rate, <<1 RPM, is sufficient to provide tension. Bracing cables to a central hub
provide, the. r_q,,;_it_. .... • ,.,¢ _! ..... :¢c .... ,r.................. -v,-,,,- _,,,lcnn. ,iecessary, a counter-rotating flywheel can
compensate for angular momentum.
Thin-film PV elements are light enough that high power to weight ratios may be maintained
even if the satellite does not track the sun. In the sphere configuration, the solar cell/microwave
transmitter elements cover the surface of an inflated sphere as in the Echo satellite. For a sphere
radius of many hundreds of meters the surface/v "ameratio is extremely low, and the gas pressure
i required to hold the form, and the associated leak rate, can be made small.F
Applications
The main application of a solar power satellite envisioned by Glaser and by most of the later
advocates of satellite solar power was to provide baseline electrical power for terrestrial use.
However, it is quite likely that some of the most importan,, applications, and certainly some of the
initial applications, will be in space. Existing power sources for use in space provide power at a
considerably higher effective price than terrestrial power sources, and a remote power station _tould
be able to serve several critical needs. Some of these are:
(1) Beamed power for lunar base night operation
(2) Inter-orbital ion-engine propelled transport spacecraft
(3) Power for Earth-orbital stations [11]
(4) Support for Mars missions and solar system exploration and exploitation.
Difficulties
;Sofar we have only discussed the potential advantages, and not the problems. The concepts
I outlined above have been schematic, not detailed engineering designs of how such a system could
be built, and many problem areas remain to be addressed. We will only briefly identify some of the
issues here, without attempting to detail all of the po_aibleapproaches.
A phased array system requires a pilot beam from the target to be directed to the microwave
source. The pilot beam need not be at the same frequency as the output beam. A pilot beam at, or
near, the same frequency as the power beam has the advantage of automatically correcting for
atmospheric effects. A disadvantage of a pilot beam at the output frequency is the difficulty of
distinguishing pilot beam from output (e.g., by polarization difference). Failure to adequately
isolate the pilot beam from the output beam would result in undes'-'rableself-stimulated oscillations
of the transmitter. One way to eliminate this problem would be to use a pulsed laser as the pilot
beam. Each laser pulse would be used to set the phase signal.
The issues involved with phase-conjugation of the reference beam, including the problem of
providing a reference clock signal and distributing the phase signal have not been addressed. This
could be done either with analog processing or with digital circuitry. The difficulty of this problem
is decreased if it is assumed that sigoificant amounts of integrated processing capability is available
at low cost.
If the elements are equipped with local oscillators, then the phase signal is only required to keep
the local oscillators in correct phase. If the system does not have local oscillators, or if the local
oscillators have pocr frequency stabilty, a continuous phase signal is needed.
Off-angle losses have not been discussed. The antenna elements will be tuned to radiate at best
efficiency at a given angle. Also, polarization issues have not been discussed. Finally the question
of a backplane for the microwave antennae has also not been addressed. The shorter the
wavelength used, the less difficulty this provides.
In general, all of the problem areas are amenable to engineering solutions. The important
question is whether resolution of the problem areas would unacceptably increase the complexity,
weight, or cost of the system,
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Conclusions
Thin-film photo,,oltaics and microwave solid-state devices have the potential to create a
revolutionary improvement in solar power satellite design, with possible improvements in power to
weight ratio of a factor of ten to a hundred. Thin-film ohotovoltaics alone could cause a significant
performance improvement, however, to take full advan'tage of the technologies being developed, we
have suggested design of a fully integrated photovoltaic/microwave system, where the phased array
microwave elements tu'e deposited integrally to the solar cells, eliminating all the power management
and distribution.
References
[1] P.E. Glaser, "Power from the Sun: Its Future," Science 162, 957-961 (1968).
[21 NASA/U.S. Department of Energy, Final Proceedings of the Solar Power Satellite Program
Review, April 22-25, 1980, Lincoln, Nebraska, Conf-80049!; Document N82-.22676 (NASA
TM-84183).
[3] National Aeronautics and Space Administration, Satellite Power System Concept
Development and Evaluation Program S),st_.m Definition Technical Assessment Report, prepared
for United Stated Department of Energy, DOE/ER/10035-03, Dec. 1980.
[4] United States Office of Technology Assessment, Solar Power Satellites, N81-77268,
August 1981.
[5] G.A. Landis,"Review of Thin-Film Solar Cell Technology and Applications for Ultra-Light
Spacecraft Solar Arrays," Space Photovoltaic Research and Technology Conference X, NASA
Lewis Research Center, Nov. 1990 (proceedings in press). Available as NASA TM-102017
(1989).
[6] J. Woodyard and G.A. Landis, "Rad:.ation Tolerance of Thin Film Solar Cells," Solar Cell_,
(to be published).
[7] W.C. Brown,"Status of Beamed-Power Transmission Technology and Applications at 2.45
Gigahertz," Space Power, Vol. 8, No. 3, 339-356 (1989).
[81 C. Adams, "Radar and GaAs," Military ancl Aerospace Electronics, May 1990, 27-29
[9] IEEE Transactions on Electron Devices, special issue on amorphous semiconductor devices,
Vol. 36. No. 12, Dec. 1989.
[10] K.B. Bhasin, et al., "Performance and Modelling of Superconducting Ring Resonators at
Millimeter-Wave Frequencies," 1990 IEEE MTT-S International Microwave Symposium, Dallas,
TX, May 8-10, 1990; available as NASA TM-102526 (1990).
[11] G.A. Landis, "A New Space Station Power System," Acta Astronautica, Vol. 17, No. 9
(1988).
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1
't
!
alqtelqrta elellleIlt
l oscillator I
input from
phase signal
Figure 1. Solar cell with integrated microwave antenna element
(conceptual diagram). The local oscillator receives a phase signal to
make the output signal coherent as part of a phased array beam.
current flow
current flow
ive)
Figure 2. Schematic diagram for integrated solar cell/transmitter
conmination in a push-pull configuration. The contact metallization of the
solar cells serves as the antenna element for the integrated microwave
transmitter. Complementary pnp and npn transistors receive the, ame
phase signal
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_111"
Figure 3 "Bicycle-Wheel" SPS Configuration
incident light Both sides of spher o covered
with integrated solar cell -
microwave transmitter
elements
illuminated
Figure 4: "Sphere" configuratian for SPS.
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Applications of thin film technology toward a low-mass solar power satellite

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