Table Of Content' .
IAF-02-R.4.06
Future Photovoltaic Power Generation
for Space-Based Power Utilities
Sheila Bailey, Geoffrey Landis, Aloysius Hepp,
and *Ryne RaffaeIIe
NASA Glenn Research Center, MS 302-1 ,
Cleveland, OH 44135
*Rochester Institute of Technology,
Rochester, NY 14623
53rdI nternational Astronautical
Congress
-
The World Space Congress 2002
10-19 Qct 2002/Houston, Texas
For permission to copy or republish, contact the International Federation, 3-5 Rue Mario-
Nikis, 75015 Paris, France
This is a preprint or reprint of a paper intended for presentation at a
conference. Because changes may be made before formal
publication, this is made available with the understanding that it will
not be cited or reproduced without the permission of the author.
IAP-02-R.4.06
FUTURE PHOTOVOLTAIC POWER GENERATION
FOR SPACE-BASED POWER UTILITIES
Sheila Bailey, Geoffrey Landis, Aloysius Hepp, and “Ryne Raffaelle
NASA Glenn Research Center, MS 302-1, Cleveland, OH 44135
*Rochester Institute of Technology, Rochester, NY 14623
[email protected]
ABSTRACT
This paper discusses requirements for large weight gigawatt (GW) space power generation.
earth orbiting power stations that can serve as Investment in solar power generation
central utilities for other orbiting spacecraft, or technologies would also benefit high power
for beaming power to the earth itself. The military, commercial and science missions.
current state of the art of space solar cells, and a These missions are generally those involving
variety of both evolving thin film cells as well solar electric propulsion, surface power
as new technologies that may impact the future systems to sustain an outpost or a permanent
choice of space solar cells for high power colony on the surface of the moon or mars,
mission applications are addressed. space based lasers or radar, or as large earth
orbiting power stations which can serve as
central utilities for other orbiting spacecraft, or
INTRODUCTION as in the SERT program, potentially beaming
power to the earth itself.
Starting with Peter Glaser’s initial 1968
proposal’, many people have discussed use of A significant risk element for any satellite
the satellite solar power system [SSPS] as a power system is the photovoltaic array. This
means of supplying energy to the Earth to was identified twenty years ago in the National
replace fossil fuel sources. The recent Research Council (NRC) review of SSPS2 as
prominence of the “greenhouse effect” from one of the most critical areas where
burning of fossil fuels has again brought extrapolations from current technology in terms
alternative energy sources to public attention, of cost and performance were made. That
and the time is certainly appropriate to analysis is still true today.
reexamine the SSPS.
A recent NASA program, Space Solar
Power Exploratory Research and Technology SPACE AND GROUND SOLAR POWER
(SERT), investigated the technologies needed to A first step toward space solar power will
provide cost-competitive ground baseload be to demonstrate multi-megawatt power
electrical power from space based solar energy production with ground-based solar arrays.
conversion. This goal mandated low cost, light Proponents of SSPS often disparage the
Copyright 0 2002 by fhc Amcrican institute of Acronautics and Astronautics, inc. No copyright is asscrtcd in thc Uuitcd Statc undcr Titlc 17, U.S.
Code. Thc U.S.G ovcrnment has a royalty-frcc licensc to cxcrcise all rights undcr thc copyright claimcd hcrcin for Govcrnmcntal purposcs. All othcr
rights arc rescrvcd by thc copyright owncr.
potential use of ground-based solar energy, the most significant statistic in this time period,
possibly considering ground-based systems as a however, is the shift in use of photovoltaic
competitor. Nothing could be further from the systems from only 15% grid connection in 1991
truth: ground-based and satellite-based solar to 55% grid connection in 2001. Off-grid use
power are complementary technologies, and dropped from 60% to 44% in that same time
satellite-based solar power will only be period.
economically viable if terrestrial power is also Governments worldwide have encouraged
viable. This synergy and diversity in space the development of the photovoltaic industry
and terrestrial photovoltaics was recently with new manufacturers in India, Korea and
discussed3 at the 200 1 Space Photovoltaic many other developing countries. In addition
Research and Technology conference. Europe, Japan and the United States are also
Experience with ground-based solar power encouraging both domestic production and
is a necessary step to assess the technology, domestic markets. Governments have sought to
define and trouble-shoot the manufacturing address the relatively high capital start-up costs
technologies, and move photovoltaics along the of the photovoltaic industry by identifying and
learning curve to low-cost production. guiding opportunities between producers and
Many ground sites exist in the US. with over investors, clarifying barriers, instilling
300 clear days per year. Flat-plate confidence in consumers, and improving
photovoltaic systems will also provide interactions with other industries that have
significant power during overcast days. The solved similar problems.
difficulty with terrestrial solar power is that it The U. S. Photovoltaic Industry published a
.’
provides power only during the daytime. Solar Electric Power roadmap in April of 200 1
Ground-based solar power could be viable for a The roadmap goals were to achieve flexible
significant fraction of the power needs of the high-speed manufacturing to create a 200 MW
United States due to the fact that the peak loads factory by 2020. The incremental needs were
are typically in the da~time.~A n additional listed as a 5-fold reduction in module
advantageous feature of terrestrial photovoltaics manufacturing costs by 2010, then a 10-fold
is the short construction lead-time required and reduction by 2020 with a 40-fold increase in
the ability to add capacity in small, modular module manufacturing by 2020. Required
increments. This allows photovoltaic actions to achieve these goals were identified as
installations to avoid the uncertainty of designing of lower-cost module packaging,
forecasting power requirements far in advance, developing high-volume, high-throughput, high-
and also allows rapid progress along the learning efficiency cell processes and moving from
curve. company-specific equipment manufacturing
In the ten year interval between 1991 and toward equipment design that can be transferred
2001 there was a six-fold increase in worldwide to and used by more than one manufacturer.
terrestrial total sales and a cost decrease in Both industry-government and industry-
modules from $6rW to $4rW and a decrease in university interactions were seen as critical to
installed system cost from $12/W to $ S N . reaching these goals. All of these issues are also
There was also a shift, and in some cases a relevant for future SSPS needs.
merger, from the four largest suppliers in 1991, Current photovoltaic module production is
ARCO/Siemens, Solarex, Sanyo, and Kyocera, about 400 MW(pwkjyear,a nd increasing by
to Sharp, Kyocera, BP Solar, and Siemens/Shell about 40% per year, as seen in Figure I.
as the four largest suppliers in 2001. Perhaps
World PV shipments
400
350
300 . . .
250 -. . -~ .
n
p.
g o o .. .. . -. .. . .. . . .
I
150
100
50
0
1980 1985 year 1995 '000
lggo
Figure 1: Growth in world photovoltaic array shipments, 1980-2001
Cumulative production of tens to hundreds of tolerant and have the potential for being
Gigawatts will be required for photovoltaics to manufacturable on thin, lightweight substrates.
reach the technological maturity required for Such materials could be ideal for space use.
finalizing a SPS design.
Several earlier papers have reviewed the
Having gained valuable photovoltaic progress of photovoltaic techn~logy.~N,A~S A
experience with solar energy, when the solar recently formed a technology review committee
generation market share begins to saturate that consulted m-ission plaming ofEces, solar
demand for peak power, utilities will begin to cell and array manufacturers, and research
search for a solar-energy alternative that laboratories to assess solar cell and array
provides continuous power. technologies required for future NASA Science
missions, beyond 2006. The resulting draft
evaluation of the state of the art of solar cells
PV TECHNOLOGIES
and arrays and their potential evolution over the
As yet it is still too early to chose a
next five years cross-referenced to the mission
technology for SSPS. Among the photovoltaic
needs was presented at the 29th IEEE
technologies, many different approaches are
Photovoltaic Specialist Conferezce in May,
still in consideration. One of the leading flat-
2002.* Areas were highlighted where
plate photovoltaic approaches is the use of
investments in research and technology
thin-film photovoltaic materials such as
development are required to meet these mission
amorphous silicon, cadmium telluride, and
requirements. There is also an evaluation of the
copper indium diselenide. Coincidentally, such
supporting infrastructure required to sustain a
thin-film materials are inherently radiation
Copyright 8 2002 by ihc American insiituic of Acronautics ana Astronautics, Inc. No copyright is asscrted in thc Unitcd Starc undcr Titic 17, U.S.
Codc. Thc US.G ovemmcnt has a royalty-frcc licensc to cxercisc al! rights undcr thc copyright claimed hercin for Govcrnmental purposcs. A11 othcr
rights are rcscrvcd by thc copyright owncr.
viable research and technology development organichnorganic solar cells may also be of
program within NASA, while assessing those interest in the future.
developments in the commercial sector, which Single-crystal solar cells have been used to
would require very limited NASA involvement. produce electrical power on almost all space
Solar cell and array manufacturers provided satellites since 1958. Their scalability,
both data on current and near term commerciai reliability, and predictability, have made solar
goals and also insight into future development
photovoltaics the prime choice for spacecraft
requirements specifically for NASA missions. designers. Cell efficiencies have grown from
Prosam managers provided mission details and 8%, Air Mass Zero (AMO), on Vanguard I, the
the impact on power system needs. There has world’s first solar powered satellite, in 1958 to
also been a recent discussion of terrestrial solar 14.2% efficient crystalline Si cells on the
cells, past, present, and f~ture.~ International Space Station (ISS). In the last
Current state of the art decade there has been a transition to today’s
-27% efficient multi-junction cells,
Crystalline silicon dominates (-90%)
GaInP/GaAs/Ge, that are commercially
today’s terrestrial market. Even though
available fiom several vendors. It is likely that
-lOOpm of crystalline Si is required for the
production efficiencies will exceed 30% in a few
indirect 90% absorption of light versus -lpm
years. Higher efficiency cells and arrays are
of GaAs, a direct conversion semiconductor, for
almost always a benefit. Higher efficiency
the same light absorption, the historical
results in smaller arrays that can be used to
development of silicon technology and high
generate the same ainount of power as less
quality material for the electronics industry has
efficient arrays, or makes higher power levels
insured the dominance of crystalline Si.
feasible for a given configuration. These smaller
However, if we look to the future of crystalline
arrays require less volume when stowed for
Si it is recognized that 50% of the cost of a
launch and present less obscuration to sensors.
module is due to the cost of processed silicon
Traded against this benefit are other parameters
wafers. In the past the PV industry has relied
such as cost, specific power, and special needs
on reject material fiom the semiconductor
such as high temperature, low intensity low
industry that was available at low cost. This
temperature (LILT) and radiation resistance,
dependence on the semiconductor industry has
but the trade tends to favor more efficient cells.
led to cycles of low supply and high cost
material. The feed stock supply of‘ silicon is Tables 1 and 2 Below list the current
currently a significant issue. New technologies status of cell efficiencies for AM1.5 and
are hoping to replace the Czochralski grown AM03. Notice that the conversion fiom
100 kg ingots with poly-crystalline, ribbon, or AM1.5 io AM0 is -.88 f .01 for the thin film
*
thin-film crystalline production. Each of these cells and -.95 .01 for the high efficiency
techniques has yielded lower efficiencies than crystalline silicon and 111-V triple junction cell.
crystalline Si. However, over time, the
efficiencies have been steadily improving.
Approximately 10% of today’s terrestrial
market is in thin film materials such as
amorphous silicon, crystalline and amorphous
heterostructures, copper indium diselenide
(CIS), cadmium telluride (CdTe), and dye-
sensitized cells. Other types of
Module Efficiency (%) Area (cm2)
Global AM1.5
c-Si 22.7 778
multi-c-Si 15.3 1017
CIGSS 12.1 365 1
CdTe 10.7 4874
a-Si/a-Sua-SiGe 10.4 905
photochemical 4.7 141.4
-
~ ~ ~~ ~ ~ ~
Table 2. AM1.5 Efficiencies for Modules
- . -
FUTURE SOLAR CELL DEVELOPMENT a 40% cell. Cost reduction would be achieved if
III-V multi-function solar cells these cells could be grown on a silicon rather
than a germanium substrate since the substrate
Research in the III-V multi-
is -65% of the cell cost. The advent of a new
junction solar cells has been focused on
competitor to the space cell manufacturing
fdx-icati~ge ither !2ttice-mism2t,tched materials market in i998 and other factors combined to
-
with optimum stacking bandgaps or new lattice reduce space cells costs by 40% of their 1997
matched materials with optimum bandgaps. In cost in 2002. Several possible cell structures for
the near term this will yield a 30% commercially future III-V devices are illustrated in Figure 2.
available space cell and in the far term possibly
Lattice mismatched cell Quadjunction cell Triple junction on Si
-
Efficiency -30% Efficiency -35% Efficiency 30%
Figure 2. Proposed structures for 111-V tandem cell development
The problem areas with projected m-V cell large-area fi-esnel lens is used to concentrate
development include the material growth sunlight onto a smaller solar array. The
difficulty of the InGaAsN 1.05 eV bandgap proposed fresnel lens has a diameter of ten to
material, minimizing defect growth in lattice fifteen meters, and is proposed to be supported
mismatched material, and current limiting in the by an datable structure. The advantage of
Ge subcell. Longer-term projects in the area of this structure is that the photovoltaic array is
multi-junction III-V cells would include the much more expensive than the fresnel lens
potential of growing these cells on a low cost material, and hence a low-cost plastic film can
cermic subsnate and the possibility of substitute for a large area of array.
efficiency enhancement by nano-structures.
The difficulty of this approach is that solar
With a recurring interest in terrestrial
cells do not work well at elevated
concentrators, both the space and terrestrial
temperatures." Analysis shows that the use of
communities have common goals for high
active cooling for the focal plane array is, in
efficiency III-V cells.
general, too expensive to be cost competitive.
To make this approach work, therefore, will
High-temperature cells require development of a new solar cell
technology that retains high performance at
In one of the proposed design for a solar
elevated temperatures. Such photovoltaic
power satellite, the "Power Tower" concept, a
technology is being considered for NASA
missions such as the Solar Probe." Possible
semiconductor technologies that could improve thin film cells of less than 15 to 20 % efficient
high-temperature performance include Gap, would not be cost effective except for certain
and nitride-based solar cells. While such applications which might involve a high
solar cells have not yet been developed, radiation environment, or a stowage volume
analysis of high temperature photovoltaic problem in the launch configuration, or perhaps
iec’moiogy is the subject of a research effort at a unique spacecraft coniiguration. Current
NASA Glenn. terrestrial thin film programs will benefit the
space community as manufacturing techniques
are improved bringing the small area cell
Thin film cells
efficiencies in Table 1 closer to the large area
Thin film cells require substantially less
modules in Table 2. The Space community
material and have promised the advantage of
requires that thin film cells must be produced
large area, low cost manufacturing. However,
on a lightweight substrate due to the mass
space cell requirements dictate a more
penalties imposed in launching. The best thin
complicated trade space. Until recently the
film cells to date have required processing
focus in space cells has been on efficiency
temperatures in excess of 600°C, which
rather than cost. In a several billion-dollar
prohibit the use of current polyimide
spacecraft the cell cost is relatively small at
substrates. Research has focused both on
even a thousand dollars per watt, which is
finding high temperature tolerant substrates and
approximately the current array cost. This has
on reducing the processing temperature of the
primarily been true for spacecraft with power
thin film cell itself while maintaining the higher
needs from a few hundred watts to tens of
efficiencies. A low cost flexible substrate
kilowatts. However, deployment of a large
would also benefit the terrestrial community by
earth orbiting space power system will require
replacing the expensive and fragile heavy glass
major advances in the photovoltaic array
structures.
weight, stability in the space environment,
Clearly, the ability to increase thin film cell
efficiency, and ultimately the cost of
efficiencies would impact both the terrestrial
- production and deployment of such arrays.
and space cell communities. Semiconductor
The development of large space power
quantum dots are currently a subject of great
systems, and a host of other proposed space
interest by both commupities. This is mainly
missions, will require the development of viable
due to their size-dependent electronic
thin film arrays. The specific power required is
structures, in particular the increased band gap
almost 40 times what is presently available in
and therefore tunable opto-electronic
commercial arrays. Whde high efficiency ultra
properties. A quantum dot is a granule of a
lightweight arrays are not likely to become
semiconductor material whose size is on the
commercially available anytime soon, a6vances
nanometer scale. These nano-crystallites
in thin film photovoltaics may still impact other
behave essentially as a 3-dimesional potential
space technologies (Le., thin film integrated
well for electrons (i.e., the quantum mechanical
power ~upplies’~a)n d thus support a broad
“particle in a box”). To date these nano-
range of missions in the next decades. Mission
particles have been primarily limited to sensors,
examples include micro-air vehicles, dirigibles,
lasers, LEDs, and other opto-electronic devices.
ultra-long duration balloons (e.g. Olympus),
However the unique properties of the size
deep space solar electric propulsion (SEP)
dependent increase in oscillator strength due to
“Tug” Array, Mars SEP Array, and Mars
the strong confinement exhibited in quantum
surface power outpost. Spacecraft systems
dots and the blue shift in the band gap energy of
studies which consider the system level
quantum dots are properties that can be
implications of increased array area indicate that
exploited for developing photovoltaic devices
that offer advantages over conventional band gap energy allows for engineering an
photovoltaic^.^^ The increased oscillator ensemble of quantum dots in a size range that
strength of the quantum dots will produce an will capture most of the radiation from the
increase in the number of photons absorbed and terrestrial and space solar energy spectrum, see
consequently, the number of photogenerated Figure 3.
carriers. On the other hand, the biue shift in the
Additional phc ons accessible
via quantum
C )tS
rT
I
. , I : I I I I
0 500 1000 1500 2000 2500
Wavelength (mm)
Fi-we 3. Air Mass Zero Spectrum showins additional photons accessible with a quantum dot cell
structure
There have been several proposed ordered array of quantum dots within the
methods to improve solar cell efficiency intrinsic region of a p-i-n solar cell (see Figure
through the introduction of quantum dots. 4)-
One of the main methods is to produce an Ohmic contacb
Fimre 4. Proposed quantum dot solar cell structure
The overlap of the discrete wavefunctions energy bands or "mini-bands." By adjusting the
associated with the electronic states of the dot size and spacing, a device can be
individual do-cs will produce narrow electronic manufactured such that these mini-bands will lie
energetically between the valence and
conduction bands of the host semiconductor, or, CONCLUSIONS
in other words, within their bandgap. The Clearly both research and manufacturing
quantum dots in an intermediate bandgap solar development is required to fulfill the promise of
cell can be thought of as an array of light-weight, low-cost photovoltaic power,
semiconductors which are individually size-
whether on the ground or in space. Significant
tuned for optimal absorption at a desired region
strides have been made in many areas, but many
of the solar energy emission spectrum. This is
more areas will be addressed in the future.
in contrast with a bulk material where photons
Whether in twenty years, or more, there will be
are absorbed at the band gap and energies above
viable choices for generating power and we will
the band gap where the photogeneration of
need them.
carriers is less efficient. In addition, bulk
materials used in solar energy cells suffer from
reflective losses at energies about the band gap,
whereas for individual quantum dots reflective
losses are minimized. REFEREIVCES
Two additional desirable features of I P.E. Glaser, “Power from the Sun: Its
quantum dot solar cell behaviour are the Future,” Science Vol. 162, 95’7-961 (1 968).
expected superior radiation resistance of such U.S. Office of Technology Assessment, Solar
devices and the independence of conversion Power Satellites, (1981).
efficiency with temperature. To a first
S. Bailey, R. Raffaelle, and K. Emery. “Space
approximation the energy levels of quantum dot ’
and Terrestrial Photovoltaics: Synergy and
structures are temperature independent. In fact
Diversity” Progress in Photovoltaics, Vol.I O,
thermal energy assists in populating those
NO. 6,399-406 (2002).
levels. This implies a greater thermal stability
G. Landis, “Peak Power Markets for Satellite
in contrast to a normal pn-junction solar cell.
Solar Power,” paper IAC-02-R.3.06, to be
Unfortunately, it is difficult to estimate the
presented, 53rd International Astronautical
potential temperature range due to the
Congress/2002 World Space Congress, Houston
temperature dependence of other cell
TX, 10-19 October (2002).
components.
’
“Solar Electric Power”, the US Photovoltaic
Both dye-sensitized cells and other
Industry Roadmap, produced and printed by
organic/inorganic hybrid cells may be of future
the U.S. photovoltaics industry, facilitated by
interest. Both efficiency and stability are issues
the National Center for Photovoltaics, April
in both dye-sensitized and polymeric cells.
(200 1).
However, research efforts in those areas have
been rewarding. A current assessment of space G: Landis, S. Bailey and M. Piszczor,
qualification characteristics was presented at a “Recent Advances in Solar Cell Technology,” J.
recent meeting.’’ While presenting significant Propulsion and Power, Vol. 12, No. 5, 835-841,
challenges, the small amount of needed material Sept-Oct. 1996. Paper AIAA-95-002’7.
and the ability to produce these cells on a S. Bailey, G. Landis and D. Flood,
variety of substrates make them attractive for ”Photovoltaic Space Power: Status and Future,”
future systems. paper AIAA-98-1053, 36th Aerospace Sciences
Meeting & Exhibit, Reno hi,J an 12-15 1998.
Copyright 8 2002 by thc American Institutc of Acronautics and Astronautics, Inc. No copyright is asscrtcd in the Unitcd Statc under Titlc 17, U.S.
Code. The US. Government has a royalty-frcc liccnsc to cxcrcisc all rights undcr thc copyright claimcd hcrcin for Govcrnmcntal purposes. Ali othcr
rights arc rcscrved by thc copyright owncr.
