Abstract:
The selling
price of electrical power varies with time. The economic viability of space
solar power is maximum if the power can be sold at peak power rates instead of
baseline rate. Demand varies roughly by a factor of two between the
early-morning minimum demand, and the afternoon maximum; both the amount of
peak power, and the location of the peak, depends significantly on the location
and the weather. New designs for a space
solar power (SSP) system were analyzed to provide electrical power to Earth for
economically competitive rates. The approach was to look at innovative power
architectures to more practical approaches to space solar power. A significant
barrier is the initial investment required before the first power is returned.
Three new concepts for solar power satellites were invented and analyzed. The
integral-array satellite had several advantages, including an initial
investment cost approximately eight times lower than the conventional design.
Introduction:
The
Solar Power Satellite is a concept to collect solar power in space, and then
transport it to the surface of the Earth by microwave (or possibly laser) beam,
where it is converted into electrical power for terrestrial use. The recent
prominence of possible climate change due to the “greenhouse effect” from
burning of fossil fuels has again brought alternative energy sources to public
attention, and the time is certainly appropriate to reexamine the economics of
SPS. In the analysis of the economics of solar power satellites to provide
electric power for terrestrial use, past analyses have typically assumed an
averaged (or "baseline") power pricing structure. In the real world,
price varies with location, season, and time of day; and the initial markets
for satellite solar electricity need to be selected to maximize revenue. It is
important to design the system to service the real-world electrical power
market, not to an unreal average-price model. The following criteria will have
to be used for a credible analysis of solar power satellite economic benefits
and rate of return:
Satellite power generation should fit electrical
demand profile
Satellite power generation should generate power at
the maximum selling price
Use actual data on electrical demand & price
Earth-Sun L2 Design details-:
The
space power system designed to be located Earth-Sun L2 will be radically
different from conventional GEO Space power concept Since the sun and Earth are
nearly the same direction, it can feature:
1.Integrated solar concentrator dish/microwave
transmission dish
2.Integrated solar cell/solid state transmitters
3.No rotating parts or slip-rings
Frequency: 30 GHz: efficiency is lower than 2.45 GHz,
but much tighter beam
4.transmitter diameter: 3 km
5.receiver diameter: 6 km
6.3 ground sites, receive 8 hours per day 33,000 16.5
meter integrated PV concentrator/transmitter elements
7.Concentrator PV efficiency 35%
Based on L'Garde designs
for inflatable microwave antennas, it should be possible to make a 16.5-meter
concentrator/antenna dish for 15 kg. The solar array/solid state power
amplifier array adds an additional 9 kg, for a mass of 24 kg per element .
Table: Super-synchronous Solar Power Satellite: Mass and power
Mass of mirror element
Similar to the design flown on the shuttle
Solar concentration ratio 50, focal plane area 4.28
square meters
Focal plane array mass is 9 kg
total mass per dish is 24 kg
PV power per dish is 100 kW
Total Mass
Inflatable PV
concentrator/transmitter elements mass 15 kg each (L’Garde design) PV mass 9 kg
each (50x concentration) Structural mass 500,000 kg
Total Mass 1,300 tonnes
At assumed transmitter efficiency
33% (today’s technology): 1 GW power output
At assumed transmitter efficiency 67% (future
technology): 2 GW power output
Disadvantages of Earth-sun L2 Solar Power Satellite
1. Size. The
satellite-Earth distance of 1.5 million km means that the physics of
diffraction demands a large size. This means that the initial cost will be
high.
2. Electrical generation
profile. The design produces power primarily during the night. For the existing
U.S. power market, the maximum power usage is during the day.
Fixed Geo synchronous Solar Power Satellite
One aspect of the
design remains extremely attractive: the absence of a rotary joint makes the L2
solar power satellite a design with no moving parts in geo synchronous orbit.
The baseline figure of merit for this design was to examine how the power
production profile fits with the demand (and price) profile for terrestrial
electrical power, assuming that the power is to "fill in" for a
ground solar power system. The satellite designed with the same design criteria:
maximum simplicity; no moving parts; mission is to power when ground solar
power is not available.A fixed microwave transmitter is permanently mounted on
a bifacial solar array, which can be illuminated from either side. The maximum
power dawn and at dusk, with zero power production at noon and at midnight .
This fills in for a hypothetical solar array on the ground, which produces
maximum power at noon and zero power at dawn and dusk. By employing a fixed
transmitter attached to the solar array, the power management and distribution
system size can be greatly simplified and reduced in mass. The difficulties
associated with power transfer from the array to the transmitter are minimized,
and the mass and cost of the SPS are reduced. The new SPS needs only gravity-gradient
stabilization to ensure that the transmitter remains pointed to the rectenna
site on the Earth. The solar array is now a simple flat structure to support
the photovoltaic solar cells. Since the array is designed to have cosine
illumination, a complicated structure is not required to point the arrays to
the Sun. Therefore further mass and cost savings may be realized. Over the
course of a day, the fixed array produces 64% of the energy of a tracking array
of the same size.
Analysis Using "Space Segment Model" Spreadsheet
The purpose of the Space Segment Model is to
evaluate the impact of technology and design choices on the mass, performance,
and cost of various solar power satellite (SPS) concepts using a common model.
Figure. Solar power satellite design with
fixed Figure. GEO
solar power satellite provides microwave transmitter (no moving parts). maximum power at 6 AM and 6 PM.
In the Input
worksheet, the user chooses various SPS concepts, architectures, and orbital
parameters. The chosen parameters are used in the various other worksheets
according to their purpose, and the relevant values are output to the summary
worksheet. The user may go to each specific worksheet to examine how performance
and cost characteristics are evaluated, and may make changes to these
worksheets. This allows the user to customize to some degree the SSM to fit the
SPS concept understudy. For the purposes of this study, the SSM provides more
analysis than required. Several of the worksheet calculations were not
applicable, such as market cities and interplanetary trajectory calculations.
For the applicable worksheets, namely Solar Collection, Power Management and
Distribution (PMAD), Power Transmission, Structure, and Propulsion, default
values were used in many cases, as they were appropriate to the concept under
study. When necessary, relevant values in the applicable worksheets were
altered to suit the concept under study. By selectively altering the SSM, the model
was used to determine the viability of the concept under study.
Figure. The Space Segment Model was used to perform a
first-order sizing of the concept. By inputting the desired values of SPS
concept, structure type, orbit type, power delivered, photovoltaic cell type,
transmitter frequency, etc results of system and subsystem mass and cost were
output to the Summary worksheet. Then relevant values in the appropriate
sub-system worksheets were altered to better reflect the proposed design. A
standard design was compared with the fixed design. The design considered is a
bi-facial solar array which would require two arrays with power scaled to
deliver 1 GW ground power scaled accordingly with values from the Solar
Conversion worksheet. Thin film solar cell arrays were assumed. Transmission frequency of 5.8 GHz was chosen.
Higher frequencies suffer from unacceptable atmospheric attenuation, and lower
frequencies require larger transmitter arrays and/or rectennas. When revising
the Space Segment Model to fit with the proposed concepts, each subsystem can
be modified to a certain degree, and some cannot be modified at all.
The differences between the baseline and
the fixed GEO systems are: Transmitter subsystem: no modification since it
is identical in both cases. It is sized according to power output and
frequency; other SPS variables have no effect on the transmitter. Solar
conversion subsystem: revised to be a bi-facial array, as opposed to a single
sun-tracking array. Attitude control and orbit maintenance: reduced since the
fixed SPS would be gravity-gradient stabilized. While station keeping cannot be
neglected in a thorough design, for a first-order sizing it may be assumed to
be negligible in terms of overall system mass and cost. Robotic subsystem:
responsible for the construction of the SPS in LEO, and so is not affected to a
large degree by the simplified system. Structure subsystem: a simple structure
was chosen in the Input worksheet, and thus mass or cost reductions cannot be
realized directly. However, it is assumed that both solar arrays can be fixed
to the one structure, so mass and cost savings are indirectly realized.
Telecommunications subsystem: ignored; negligible mass and cost as fraction of
total SPS. PMAD subsystem: The mass and cost of the cabling are eliminated,
since very little cabling is required. Also, since the transmitter is fixed to
the solar array, there is no need for a rotary joint. However, a sizeable
portion of the PMAD subsystem is attributed to the voltage converters, which
are necessary to transfer GW order levels of power to the transmitter.
Thermal subsystem: incorporated
throughout the SPS, and is evaluated in conjunction which the other subsystems.
Propulsion subsystem: required to move the SPS from LEO to GEO, is dependent
solely upon the overall SPS mass, and so is automatically calculated.
Integration and testing: automatically evaluated according to the other
subsystems. Listed below is a summary of relevant output values, followed by
the revised values generated by modifying the relevant values in the
appropriate sub-system worksheets. In table it is assumed that a bifacial solar
array can be produced at no additional cost or weight. Compared to the
baseline, total mass savings is 3%, but total cost reduction is nearly 10%. The
cost associated with an additional solar array is substantially greater than
the mass and cost savings realized through satellite design simplification. It
was anticipated that cost reductions from a simplified power management and
distribution (PMAD) system would be large, however, the PMAD system accounts
for only 2% of the overall system cost, regardless of it being a substantial
portion of the system mass. Therefore PMAD cost savings do not have a large
effect on the overall system cost, and consequently is unable to offset the
increased costs from the additional solar array. For the proposed concept to be
lower in cost than the baseline, it is necessary that the cost per watt of the
solar cells be reduced significantly. For example, a 50% reduction in thin film
photovoltaic cell cost, from $1/watt to $0.50/watt [9], would result in a
system cost equal to that of a single-array SPS. In other words, the previously
calculated savings of 3% mass reduction and 10% cost reduction would be
possible. The proposed SPS design could then become economically feasible (at
least according to a first-order calculation). Examination of the power price
profiles for candidate urban areas, indicated that the cosine power production
peaking at sunrise and sunset did not well match near-term power demand. Even
if a noon peaked solar generation is subtracted from the demand curve, the
power profile still does not perfectly match requirements. Much of the power is
produced when the power demand is very low (e.g., before 8 AM), and electricity
price is low. Since the power profile of the proposed design is not suited for
selling at peak demand, and much of the power produced will not be sold at peak
price, the higher energy cost per giga-watt-hour means that the design is not
economically feasible in the near-term compared to the base-line.
Fixed Design with integrated microwave transmitter: the "8 AM/4 PM" design.
If the design constraint of
a single array is relaxed, two arrays can be baselined, and the arrays can be
tilted outward to accommodate the actual demand peak (after subtraction of
solar) at 8 AM and 4 PM (or other times chosen to fit the peak demand). With
the addition of tilt, it is no longer true that the microwave beam is
perpendicular to the solar arrays. The backside of each solar array is in the
view of the Earth. A significant difficulty of the earlier design is the fact
that the initial size of the system requires an extremely high initial
investment. Due to the risk of the investment (market risk as well as
technical), such investment is unlikely to occur. The redesign of the solar
power satellite opens the possibility of integrating the solar array directly
to the micro-wave transmission [5,6]. By
placing solid-state microwave transmitters directly on the back of the solar
array, power management and distribution, as well as all voltage conversion, is
eliminated. Figure 9 shows the conceptual design for a satellite to deliver
maximum power at 8 AM and 4 PM, where the back
side of each array is an integrated microwave transmitter. This design
was presented at the third SERT technical integration meeting [13]. The
advantages of integration of the solar arrays and the transmitter are discussed
in reference [5] and [6]. By integrating solar array with the microwave
transmitter, the transmitter aperture becomes as large as the solar array area.
This results in a narrower beam. A narrow beam allows smaller rectenna areas,
thereby permitting much smaller solar power satellites. The smaller scale
reduces the initial capital investment. For "conventional" SPS
designs, the ratio of solar array area to the transmitter array area is
approximately a factor of 64.
Figure: notional
design for a solar power satellite to deliver peak power at 9 AM and 4 PM.
Very large
scale integration Each solar-array element incorporates microwave transistor
on reverse side Reverse side of solar
array acts as phased array antenna (Phase signal must be distributed to each
element) For the integrated design, the transmitter area equals the solar array
area. For the same power density on the ground, the minimum system size
decreases in power by a factor of 4 for the 4 PM/8 AM tilt design. The rectenna
area scales proportionately, and the minimum investment cost to first power
decreases. Overall, an integrated 4 PM/8 AM fixed delivers same peak power, but
2/π (64%) lower total energy than a fully tracking SPS. The power is delivered
at peak-power rates, not baseline power rates, resulting in two times higher
revenue per kW-hr. Thus, the integrated SPS delivers 27% more revenue at 30%
lower cost. The bottom
line is that the integrated SPS delivers power at 45% lower cost.
By reducing the size of the SPS to take
advantage of the narrower beam, an integrated SPS can be decreased in power by
factor of 4. This means that the cost to first power can be reduced by factor
of 5.7. Since the investment required to reach first return is the major
showstopper for the economic case for space solar power, this is a significant
improvement in the design.
Fixed Design
with integrated microwave transmitter: the "Slab" One-sided array
The 8 AM/4 PM
design has two leaves in a "dihedral" configuration. It is evident,
however, that the operation of the two leaves are independent of each other.
This brings up the possibility of making a solar power satellite with just a
single leaf: a "slab" design, with the solar energy incident on one
side and the power beamed out the other.
For the same power density on the ground, the minimum system size
decreases in power by a factor of 8 for a face-on solar array. This is even
better than the factor of 4 found for the 4 PM/8 AM tilt. The tilt of the
system can be chosen to provide power that is optimally adjusted to the peak
power requirements. For example, a tilt of 30 degrees could be used to provide
peak power at 2 PM. This matches the maximum power demand of urban areas in the
United States. This peak can be adjusted forward or back, subject to the
constraint that peaks at (or near) 6 AM and 6 PM are not possible, since these
would require the array to be edge-on to the direction of microwave beam.
Figure 10 shows a slab" one-sided array tilted to produce peak power at 2
PM.
Figure. "Slab" single-sheet
solar array, tilted to provide peak power at 2 PM.
In this design,
the 2 PM tilt is not a gravity-gradient equilibrium. Maintaining the tilt will
require
stabilization.
For example, a gravity-gradient boom could be deployed downward on a truss to
put thesystem into gravity-gradient stability.An alternate version would be to
orient the solar array horizontally, and to direct the beam at an angle,to
reach a receiver located at a slightly eastward latitude. The horizontal
orientation is an equilibrium inthe gravitational field, but weakly unstable
(this is the orientation of the ISS, for example). By aiming thepower beam
approximately 2500 km east, the “noon” power beak generation for the satellite
can be received at2PM.
Table .shows a space-segment model of the "slab"
design. The cost of the design is 64% of the cost of the conventional tracking
array. The conclusions of this analysis are:
1.Lower total energy, but power is matched to peak
demand
2.At the same size, system delivers 64% of the power at
64% of the cost
3.But power sells at 2 PM peak power rates, not baseline
power rates
4.minimum size can smaller by factor of 8
5.Eight times lower investment to first power
6.8 times more attractive
Power Transmission System:
Power
transmission from the satellite is made by 2.45 GHz micro-wave beam emitted
from the spacetenna, the antenna onboard the satellite. The beaming angle as
large as 60 degrees of this case makes this requirement more important than in
the case of the Reference System.
Table: Spacetenna
Characteristics
Electrical Characteristics
Frequency
|
2.45GHz
|
Beam
control
|
Retrodirective
|
Beam
scanning angle
|
+30
degrees (east-west)
|
+16.7
degrees (north-south)
|
|
Power
distribution
|
constant
|
Power
density
|
574W/m2
|
Max.
power density on ground
|
0.9mW/cm2
|
Input
power to spacetenna
|
16
MW
|
Transmitting
power
|
10
MW
|
Mechanical Characteristics :
Shape
and Dimension
|
132m
x 132m square
|
Mass
|
134.4
ton
|
Number
of Array module
|
88
|
Number
of subarray
|
1936
|
Number
of antenna elements
|
2,547,776
units
|
Number
of pilot receiver
|
7,744
units
|
Conclusions:
A space solar power
generation system can be designed to work in synergy with ground solar power.
Previous Space Solar Power architectures were designed to deliver 24-hour
power; this design constraint was relaxed. A non-tracking, integrated
solar/microwave Space Power system can be configured to match peak power demand
.The economic case for a solar power satellite is most compelling if the solar
power satellite can generate power that sells at peak, rather than average,
price. Data from New York and Boston were examined to determine when the peak
power prices occur. Several new designs for solar power satellites were
considered, in an attempt to maximize the amount of power produced at peak
rates.
