a new concept of solar power satellite

Acta Astronautica 60 (2006) 153–165www.elsevier.com/locate/actaastro

A new concept of solar power satellite: Tethered-SPSSusumu Sasakia,∗, Koji Tanakaa, Ken Higuchia, Nobukatsu Okuizumia,Shigeo Kawasakib, Naoki Shinoharab, Kei Sendac, Kousei Ishimurad

aThe Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara,Kanagawa 229-8510, Japan

bResearch Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611-0011, JapancKanazawa University, 2-40-20, Kodatsuno, Kanazawa, Ishikawa 920-8667, Japan

dHokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan

Received 6 June 2005; accepted 18 July 2006Available online 2 October 2006

Abstract

Tethered solar power satellite (Tethered-SPS) consisting of a large panel with a capability of power generation/transmissionand a bus system which are connected by multi-wires is proposed as an innovative solar power satellite (SPS). The powergeneration/transmission panel is composed of a huge number of perfectly equivalent power modules. The electric power generatedby the solar cells at the surface of each module is converted to the microwave power in the same module. Since the modulesare controlled by the bus system using wireless LAN, no wired signal/power interfaces are required between the modules. Theattitude in which the microwave transmission antenna is directed to the ground is maintained by the gravity gradient force.The tethered panel is composed of individual tethered subpanels which are loosely connected to each other. This configurationenables an evolutional construction in which the function of the SPS grows as the construction proceeds. A scale model of thetethered subpanel can be used for the first step demonstration experiment of the SPS in the near future.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Solar power satellite; Space tether; Microwave power transmission

1. Introduction

Since the NASA/DOE study of the solar power satel-lite (SPS) in the 1970s, various types of the SPS havebeen proposed in Japan, the United States, Europe, andRussia. Typical examples are summarized in Table 1.The NASA/DOE reference model has a simple con-figuration consisting of single large solar array panel,a microwave transmitting antenna, and a rotary jointconnecting the panel and the antenna, which is quite

∗ Corresponding author. Tel.: +81 42 759 8330;fax: +81 42 759 8464.

E-mail address: [email protected] (S. Sasaki).

0094-5765/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.actaastro.2006.07.010

similar to the Glaser’s original idea [8]. But the ref-erence model has technical difficulties in the rotaryjoint mechanism and power collection. The rotary jointmechanism requires a power transmission mechanismof GW level through a movable contact. There is nopractical technology for the rotary joint mechanismwithout a serious power loss. The mechanism is essen-tially fragile because one-point failure could lead to atotal loss of the SPS function. For the power collec-tion, the power transmission line of GW level from thesolar array panel to the microwave transmitter requiresa huge amount of conductor or a super-conductionsystem to avoid a serious Joule loss, but both are non-practical for SPS. A system with movable reflective

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154 S. Sasaki et al. / Acta Astronautica 60 (2006) 153–165

Table 1Concept of typical SPS

Reference NEDO grand SPS2000 [3] Sun tower [4] Sail tower [5] Integrated NASDA 2001system [1] design [2] symmetrical model [7]

concentrator [6]

Organization NASA/DOE NEDO ISAS NASA ESA NASA NASDA (JAXA)Year 1979 1992 1993 1995 1999 2001 2001Power 5 GW 1 GW 10 MW 250 MW 450 MW 1.2 GW 1 GWOrbit GEO GEO LEO MEO GEO GEO GEOConfiguration Single rectan-

gular solararray panel,circular diskantenna

Two rectangu-lar solararray panels,circular diskantenna

Triangularprism, solararray on theupper twopanels, trans-mitter on thelower panel

Tree-liketower, mod-ular structurefor powergeneration,circular diskantenna

Flower-liketower, sun-tracking sailmodules forpower genera-tion, circulardisk antenna

Two clamshellcondenser mir-rors, separatedpower genera-tor and trans-mitter

Two primarymirror, twosecondary mir-rors, sand-wich panel

Bus power Yes Yes Yes Yes Yes Yes NoRotary joint Yes Yes No Yes Yes No NoRotating light-condensermirror

No No No Fixed condenser No Yes Yes

GEO: geo-synchronous orbit; MEO: medium earth orbit; LEO: low earth orbit.

mirrors combined with the power generation/trans-mission panel (sandwich panel) has been proposed asa potential configuration to overcome the difficulties inthe NASA/DOE reference model. In this configuration,both the rotary joint mechanism and the high-currenttransmission line can be deleted. By condensing thesunlight on the power generation/transmission panel,the size of the panel can be reduced. However, the sys-tem combined with the light-reflector and the powergeneration/transmission panel requires complicatedconfiguration and highly challenging technologies forthe attitude control and stabilization of the rotatinglarge mirror, and another challenging technology forheat rejection from the power generation/transmissionpanel if the light is condensed. Furthermore, the mov-able system even without the power transmission stillhas a fatal problem to be damaged mechanically by asingle point failure. One of the reasons why the pastSPS concept has not been accepted as a realistic energysystem for more than 30 years since Glaser’s first ideais due to lack of technical feasibility and robustness.Many SPS models have advertised the achievable costcompetitive with that of the existing energy system, butinvestors have no confidence in the cost analysis be-cause it is based on the highly challenging technologiesor fanciful schemes.

As an opposite approach to the past efforts, we haveinvestigated a simple, technically feasible, and practi-cal configuration SPS which consists of a large power

Fig. 1. Artist conception of Tethered-SPS.

generation/transmission panel suspended by multi-tether wires from a bus system above the panel. Anartist conception of the tethered solar power satellite(Tethered-SPS) is shown in Fig. 1. This concept wasfirst developed by a study team organized by Institutefor Unmanned Space Experiment Free Flyer (USEF)in 2001 and 2002 [9]. The essential technologies re-quired for this concept are the deployment of the longtether of 10 km scale and the large panel of km scalein orbit. The basic parts of theses technologies havebeen already verified in space. Space tether has beendeployed up to 20 km 3 times in the 1990s [10]. The

S. Sasaki et al. / Acta Astronautica 60 (2006) 153–165 155

panel deployment in km scale has not been demon-strated yet, but the solar array panels of 4.6 m × 32 mon the International Space Station were successfullydeployed in 2000. As is described later, the size of aunit panel of the Tethered-SPS is 100 m × 95 m, whichis achievable in the near future based on the currenttechnologies. The attitude is stabilized automaticallyby the gravity gradient force in the tether configurationwithout any active attitude control. Since this systemhas no mechanism to track the sun for the powergeneration, the total power efficiency is 36% lowerthan that for the NASA/DOE reference model or othersun-pointing-type SPS even when the solar cells areattached to both upper and lower sides of the panel.However, the simple configuration resolves almost allthe technical difficulties in the past SPS models. Norotation mechanism in a large-scale makes this systemhighly robust and stable. Since the light condenser isnot used for the power generation, a large-scale powergeneration area of km size is required, but the heatgenerated in the panel can be released into the spacewithout any active thermal control. The power genera-tion/transmission panel consists of perfectly equivalentmodules, which will greatly contribute to the low-costproduction, simple testing, and high-quality assurance.Another innovative feature of the module is cablelessinterface with the bus system and other modules usingwireless LAN system, which leads to simple deploy-ment, integration, and maintenance. The Tethered-SPSis an assembly of equivalent miniature Tethered-SPSelements. This configuration makes it possible to verifythe function of the SPS phase by phase during the in-tegration. In most of the past SPS models, the conceptof the phased construction has not been considered,but is very important for the large space infrastructure.The miniature tethered element, a part of the practicalSPS, can be used for the demonstration experimentin the near future. This strategy gives a straightfor-ward scenario for the evolutional development fromthe demonstration model to the commercial SPS in thetechnology road map.

2. Tethered-SPS

Fig. 2 illustrates the concept of the Tethered-SPSwhich is capable of 1.2 GW power supply maximumand 0.75 GW average on the ground. It is composed of apower generation/transmission panel of 2.0 km×1.9 kmsuspended by multi-wires deployed from a bus systemwhich is located at 10 km upward. The panel consistsof 400 subpanels of 100 m×95 m with 0.1 m thickness.Each subpanel has 9500 power generation/transmission

Power generator(upper plane)

Power transmitter

1.9 km

10 km

(lower plane)Microwave

2.0km

Bus system

Fig. 2. GW-class Tethered-SPS.

Wireless signal interface with bus system

Solar cellMicrowave circuit

Microwave transmitting antenna(solar cell also attached)

Fig. 3. Concept of power generation/transmission module.

modules of 1 m × 1 m size. In each power module, theelectric power generated by the solar cells is convertedto the microwave power and no power line interface ex-ists between the modules. Fig. 3 shows the concept ofthe power generation/transmission module. The powermodule has thin film solar cells both on the upper andlower planes. The microwave transmitting antennas areon the lower plane. The module contains a power pro-cessor, microwave circuits, and their controller. Eachmodule transmits a microwave power of 420 W maxi-mum. The power conversion efficiencies for the solar


Table 2Summary of Tetherred-SPS

Configuration Power generation/transmission panel suspended by 441 wiresPanel size 2.0 km × 1.9 km × 0.1 mTether wire length 10 km approx.

Total weight 20,000 MTPanel 19,000 MTBus 1000 MT

Subpanel Power generation/transmission panel suspended by four wiresSize 100 m × 95 m × 0.1 mTotal number/panel 400

Structural unit panelSize 10 m × 1 m × 0.1 mTotal number/subpanel 950

Module Power generation/transmission capabilityPower generation 490 W maximumPower transmission 420 W maximumSize 1 m × 1 m × 0.1 mTotal number/subpanel 9500

MicrowaveFrequency 5.8 GHzOutput power 1.2 GW maximum, 0.75 GW on average

cells and the DC to RF converter are assumed to be35% and 85%, respectively. The weight of the moduleis 5 kg or the specific power of the module is 0.08 W/g(12 g/W). These values are beyond the existing tech-nologies by factor two for the power conversion effi-ciencies and approximately 10 times for the specificpower, but are considered to be realizable in 20–30 yearsbased on the potential progress of the photovoltaic andmonolithic microwave integrated circuit (MMIC) tech-nologies. There is no wired signal interface between thepower modules. The control signal and frequency/phasestandard for each module are provided from the bussystem by the wireless LAN. The system characteris-tics of the 1 GW-class Tethered-SPS are summarized inTable 2.

3. System analysis

3.1. Construction

The tethered subpanel is composed of a 100 m×95 msubpanel (47.5 ton) suspended by four wires connectedto a bus system (2.5 ton). The 100 m × 95 m panel0.1 m thick is regarded as a solid panel with a flatnessrequired for the phase control in the microwave powertransmission. The subpanel consists of 950 structuralunit panels of 10 m×1 m×0.1 m. The overall construc-tion scenario is illustrated in Fig. 4. The structural unit

panels are folded in a package of 95 m × 10 m × 10 mwhich is a unit cargo transported from the ground to thelow earth orbit by reusable launch vehicles (RLV). Thecargo is transferred to the orbit transfer vehicle (OTV)in the low earth orbit around 500 km and transportedto the geo-synchronous orbit. Delta-V required for thetransportation is 4500 m/s. To avoid the degradationof the solar cells by the trapped energetic particles inthe radiation belt, the cargo is contained in a radia-tion shield vessel. If we use a 200 MT OTV equippedwith an electric propulsion of 80 N thrust, the cargo istransferred to the geo-synchronous orbit in 4 months.The tethered subpanel is deployed automatically in thegeo-synchronous orbit. After the function test of thetethered subpanel is completed, it is integrated tothe SPS main body. Docking assistant robots whichare manipulated from the ground control center will berequired for the integration. Since the SPS main bodyis an assembly of the functional units, the SPS func-tion of the main body can be verified any time duringthe construction phase from the low power to the fullpower.

3.2. System dynamics

The gravity gradient force for the tethered subpanel is40 g per wire in the geo-synchronized orbit. The gravitygradient force for the complete Tethered-SPS is about


Transportation from low earth orbit to geo-synchronous orbit

95m100m

10km10 km

Power generator

Practical Model

Bus

(upper plane) 1.9km

Power transmitter(lower plane)2.0km

Microwave

Integration of sub-panel assisted by robot

Automatic deploymentof sub-panel

Unloading

Cargo transfer from RLV to OTV

OTV

RLV

Transportation from groundto low earth orbit

Fig. 4. Construction scenario of Tethered-SPS.

150 N, while the sunlight pressure to the 2.0 km×1.9 kmpanel is 17 N at maximum. Based on a quasi-staticanalysis considering the solar light pressure, it hasbeen shown that the variation of the attitude is largestalong the pitch axis, but is still less than 0.14◦ [11].The inclination of the subpanel due to the temperaturedifference between the wires is less than 1◦ for a tem-perature difference of 30 ◦C in case of Kevlar wire.Since the temperature difference between the tetherwires is estimated less than 30 ◦C and the pointingcapability of the microwave transmission for each sub-panel is assumed to be ±5◦, the inclination control ofthe subpanel will not be required. However, if the panelcontrol is required for some other reason, the inclina-tion can be easily controlled by adjusting the length ofthe wires of each tethered subpanel. This technique isusually used for the surface shape control of the pri-mary mirror of the large telescope at the astronomicalobservatories.

3.3. Power generation

Since the lower plane of the power genera-tion/transmission panel is always faced to the earthby the gravity gradient force, the power generation bythe solar cells on the upper and lower panels varieswith the local time as the sun angle changes. The aver-age solar power to the panel is 64% of the maximumpower, but this number is practically 60%, considering

1400

1200

1000

800

400

600

200

00 4 8 12 16 20 24

Local Time (Hour)

Sol

ar P

ower

Den

sity

(W

att/m

2 )

Sun-pointing in spaceTethered-SPS in spaceGround (June)Ground (Dec)

Fig. 5. Variation of power generation with local time.

the operational limit of the output voltage of the solarcells and the antenna area in the lower plane. Since theaverage solar power on the ground is about 10–15% ofthat in orbit, this system is still able to collect the solarpower by 4–6 times more than the ground solar powersystem, as shown in Fig. 5. While the incident power istoo low to drive the microwave circuits near the local


1000

100

10

1

0.1

0.01

Pow

er D

ensi

ty (

mW

/cm

2 )

0 0.5 1 1.5 2 2.5 3

Radius (km)

Uniform10 dB Gaussian

Fig. 6. Power density profiles on the ground for the flat and 10 dBGaussian distribution for 2 km size transmitting antenna at 5.8 GHz.

time at 6 and 18 h, the generated power is used to heatthe circuits to keep the operational temperature of themodule.

3.4. Power transmission

The profile of the microwave intensity for the trans-mitting antenna has been premised 10 dB Gaussiancommonly in the past and current SPS models to con-centrate the microwave beam power in the main lobe.For the 10 dB Gaussian taper, the main lobe contains95% of the total power, while it has about 87% powerfor the flat distribution. The peak power of the side lobefor the flat distribution is larger by approximately 10 dBthan the 10 dB Gaussian distribution. The flat distribu-tion requires a larger rectenna area and evacuation area,approximately twice as compared with that for the 10 dBGaussian taper to collect the same power. However,the flat distribution has more important advantages. Byadopting the flat power distribution, we can use per-fectly equivalent modules, which ensures low-cost massproduction. In case of the 10 dB Gaussian taper config-uration, an active thermal control will be required forthe higher power modules near the center of the trans-mitting antenna. For the flat distribution, we need nothave any active thermal control for all power modulesas described in Section 3.5. Fig. 6 shows the power den-sity profiles for the flat and 10 dB Gaussian distribution,which are derived from the calculation by Dickinson[12]. It shows that a rectenna of 4 km in diameter isenough to collect the 90% power contained in the mainand the first side lobes for the flat distribution. In that

case, the peak of the power density is 1000 W/m2 at thecenter of the microwave beam, 4.4 times higher than thatof the NASA/DOE reference model, but is still less thanthe power density of the sunlight. The non-linear inter-action of the high-power density microwave beam withthe ionospheric plasma is the major concern which willbe studied in the demonstration experiment described inSection 4.2.

3.5. Thermal analysis

All the heat generated in the system in orbit mustbe radiated into space for any energy system in space.The amount of the heat radiation depends on the surfacearea, its thermal and optical properties, and the temper-ature. In the present configuration in which the panelis composed of the perfectly equivalent and thermallyisolated modules, the thermal analysis for one moduleis sufficient to show the feasibility of the total system.The equilibrium temperature for the module is calcu-lated from the Stefan–Boltzmann’s law. Since both sidesof the module are mostly covered by the solar cells, thecoefficients, 0.8 and 0.7 are used as the typical valuesfor the solar absorptance and emittance, respectively.If we assume that the efficiency of the solar cell arrayand the conversion efficiency from DC to RF are 0.35and 0.85, respectively, the equilibrium temperature isapproximately 10 ◦C maximum, which is well withinthe operating temperature of the commercial parts usu-ally below 80 ◦C. Fig. 7 shows the temperature varia-tion of the upper and lower planes in a day calculatedby a simplified thermal model. In the calculation, it isassumed that heaters are activated when the solar radia-tion to the panel is minimum near the sun angles at 90◦or 270◦.

4. Demonstration experiment

A scale model of the tethered subpanel, a part of thepractical SPS shown in Fig. 4 is used for the demon-stration experiment. The basic concepts of the powergeneration/transmission module and tether configura-tion which play an essential role in the Tethered-SPS areverified in the demonstration experiment. Evolutionalrelationship from the demonstration experiment to thepractical SPS is shown in Fig. 8.

4.1. Demonstration experiment on the ground

A demonstration experiment on the ground usinga balloon has been investigated as a precursor ex-periment to the demonstration experiment in orbit.


80

60

40

20

0

-20

-40

-600 60 120 180 240 300 360

Sun Angle (degree)

Tem

pera

ture

(C

)

Lower Plane

Upper Plane

Fig. 7. Temperature variation of the upper and lower planes in a day.

Balloon

8m

4m x 4m

1km

Demonstration on ground Demonstration in orbit

Direct evolution Direct evolution

100m95m

Integration

10km

10km

2.0km

1.9km

Unit of practical model Practical model

17.6m 16m

30m

Fig. 8. Evolution from the demonstration model to the practical model.

The configuration of the experiment is shown in Fig. 9.A super-pressure balloon of 1000 m3 capacity can lift anexperimental system of 800 kg. The balloon is mooredto the rectenna site on the ground. The length of themooring cable is 1 km. It takes about 1 h to deploy andto retract the system, which makes it possible to con-duct the experiment in a day. The experiment time forthe microwave power transmission will be 2–4 h perday. A tethered power generation/transmission panelof 4 m × 4 m which is a 1

4 scale model of the demon-stration model in orbit is used for the experiment. Thetransmitting power is 17.5 kW. If a rectenna of 30 mdiameter is prepared, approximately 7 kW power isobtained on the ground. This experiment is regarded asa systems test to verify the function and operation ofthe space and ground segments for the demonstrationexperiment in orbit.

4.2. Demonstration experiment in orbit

The most important subject in the SPS technologiesis the verification of the power transmission from theorbit to the ground. The concept of the demonstrationexperiment is shown in Fig. 10. The main objectives ofthe experiment are:

(1) demonstration of the microwave beam control pre-cisely to the rectenna on the ground from the largepanel antenna in orbit,

(2) evaluation of the overall power efficiency as an en-ergy system,

(3) demonstration of the electromagnetic compatibil-ity with the existing communication infrastructure,and

(4) study of the operational procedure of the SPS.


Balloon, 1000 m3

12.5 m diameter

Bussystem

Power generation/transmission panel4m x 4m, 800kg

Microwave

8m

Wire1km

Rectenna

Fig. 9. Demonstration experiment using a balloon on the ground.

H2A rocket, an advanced type, is considered as thelaunch vehicle for the demonstration experiment. Thesubrecurrent orbit at an altitude of 370 km (47 revolu-tions per 3 days) is selected to compromise the require-ments from the microwave power density on the groundand orbit maintenance operation against the air drag.Fig. 11 illustrates the configuration of the experimentalsystem. The system characteristics are summarized inTable 3. The attitude of the system is stabilized by thegravity gradient force of the H2A second stage as anend mass and the power generation/transmission panelwhich are connected by a truss and four tether wires.The truss is used to support the bus system at the centerof the gravity to operate a propulsion system to keep theorbit. The truss also gives a tension to the wires underthe low-gravity gradient force for the 30 m scale tether.The truss is peculiar to the demonstration experimentand will not be used for the subpanel of the practicalSPS. The panel consists of 88 foldable structural unitpanels. The structural unit panel is 0.8 m × 4 m wide

SPS Demonstrator

Ionosphere

Microwave Power Beam

EMC MonitorIonosphere Monitor

Pilot Beam

Rectenna

Fig. 10. Outline of the demonstration experiment in orbit.

Second Stage (3700kg)

Truss

Bus

Orbital Motion

Panel (13000kg)

17.6mEarth

16m0.1m

7.7m

30 m

10.7 m

x

y

z

o

Fig. 11. Configuration of the experiment system.

and 0.1 m thick. The 80 structural unit panels are usedfor the power transmission, while the other eight areblank panels to stabilize the yaw motion. The structural


Table 3Summary of demonstraction experiment for Tetherred-SPS

Configuration Power generation/transmission panel suspended by four tether wiresPanel size 17.6 m × 16 m × 0.1 mTether wire length 30 km approx.

Total weight 18,100 kgPanel 12,600 kgOthers 5500 kg

Structural unit panelSize 4 m × 0.8 m × 0.1 mTotal number/panel 80

Module Power generation/transmission capabilityPower generation 55 W maximumPower transmission 700 W maximumSize 0.8 m × 0.8 m × 0.1 mTotal number/structural unit panel 5

MicrowaveFrequency 5.8 GHzOutput power 280 kW high-power mode, 28 kW to lower-power mode

unit panel contains five power generation/transmissionmodules of 0.8 m × 0.8 m. There is no electrical wireinterface between the power modules. The cylindricalbus system has a control and data management systemand a propulsion system to keep the orbit. Either a hy-drogen thruster system or an ion engine is consideredfor the propulsion system. Since the large panel inter-rupts the direct communication between the bus systemand the ground station, a communication relay systemwill be installed in the part of the panel.

For the microwave power transmission, two kindsof microwave system have been investigated. One isthe combination of a phase control magnetron (PCM)and integrated antennas with a phase shifting capabil-ity composed of semiconductors. In this configuration,the microwave power of the PCM typically more than1 kW is divided into the 625 antenna elements in eachpower module. Another system is the combination ofa local oscillator and active integrated antennas (AIA)in each power module. In this configuration, the sourcesignal is amplified by 625 sets of the AIA up to 700 Win total. The former-type module, consisting of a thinfilm solar array, batteries, a power control unit, a high-voltage converter, a PCM, and integrated antennas, hasbeen designed. Fig. 12 shows the configuration of thecomponents in the power module. The antenna panel isattached to the bottom of the wave guides, while theelectrical components are installed on the wave guides.Fig. 13 illustrates the block diagram of the module.The microwave circuit is designed to control the di-rection of the beam ±10◦ from the normal line of the

panel. The total microwave power injected from thepower generation/transmission panel is 280 kW. The mi-crowave power transmission will generate the powerdensity above 100 W/m2 for 20 km and above 10 W/m2

for 60 km in the ionosphere, as shown in Fig. 14. Theinteractions between the high-power density microwaveand the ionospheric plasma can be studied on a largescale. They are observed by ionosphere sounding in-struments both from the spacecraft and the ground. Thepower density on the ground is estimated as 0.25 W/m2,which is well below the standard for public exposure(10 W/m2). Since the power density is too low to berectified by a single antenna combined with a Schottkydiode, it is necessary to use a parabola to concentratethe microwave power [13]. If we have a rectenna area of500 m in diameter, the output power from the rectennawill be more than 10 kW.

The analysis of the microwave beam pattern trans-mitted from the panel indicates that the misalignmentand the gap between the structural unit panels need tobe less than 5◦ and 10 mm, respectively, from the stand-point of the allowable beam divergence and avoidanceof the unnecessary radiation [14]. The analysis of thedynamic motion of the system in orbit has been con-ducted to find out the orbital and structural conditionsto satisfy the requirements from the microwave beam-ing. The results of the computer simulation have shownthat the pitch (pendulum) motion is within 3.5◦ for theorbit eccentricity less than 0.01 [11], which is in the ca-pability of the beam control (±10◦). Based on the lin-ear elastic analysis, the characteristic frequency of the


Fig. 12. Configuration of the components in the power generation/transmission module. Right picture shows the scale model of the module.

PCM PowerSupply

1600W 1100W 1000W

Wave GuideP C MIntegrated AntennaArray with Phase Shifter

700W

Microwave PowerTransmission toGround

Frequency and PhaseSynchronization withBus System

Pilot Signalfrom Ground

Pilot SignalAntenna

Pilot Signal ProcessingCircuit

Pilot SynchronizingCircuit

Controller2300 W

Batteries

Power ControlUnit

55W

50W

2000W

Solar CellsLANAntenna

Fig. 13. Block diagram and power diagram of the module.

400

350

300

250

200

150

100

50

00.1 1 10 100 1000

Power Density (Watt/m2)

Hei

ght (

km)

Fig. 14. Height profile of the microwave power density.

oscillation of the panel is less than 10 Hz for the realis-tic range of the stiffness of the panel, truss, and wires.Since the feedback frequency for the beam control of the

microwave circuit is expected to be more than 100 Hz,the direction of the beam can be well controlled regard-less of the dynamic motion of the panel. The structuralunit panels are latched to each other in two dimensionsto keep their gaps within 10 mm.

The thermal analysis is conducted for the powermodule composed of the PCM, batteries, integratedantennas, and the other electric parts. The analysisresults show that the temperature is maximum at thePCM, but can be kept below 100 ◦C for the intermit-tent operation if a thermal capacitor is combined withthe PCM.

The orbit analysis shows that the fuel of 500 kg isrequired for two years mission if we use the hydrazinethruster system. The dominant force to the system isthe air drag force to the H2A second stage used asthe end mass. The electric propulsion system can bemore beneficial considering the amount of the fuel andattitude disturbances during the operation, but additional


M E T 0

M E T 1

M E T 2

M E T 3 ~

M E T 3 0M E T 3 1 ~ Power transmission experiment

Experiment checkout

Orbit adjustment

Panel deployment

Truss deploment

Bus system checkoutEarth pointing

LaunchOrbit (370km altitude)Despin

nadir

nadirflight

microwave

Standby

Fig. 15. Sequence of mission operation in the demonstration exper-iment.

solar array paddles extended from the bus system willbe required to supply the electric power to the electricpropulsion system.

The sequence of the mission operation is shown inFig. 15. After separation from the H2A rocket at thealtitude of 370 km, the experimental system is automat-ically controlled into the earth-pointing attitude. Firstthe bus system and folded structural unit panels are de-ployed from the H2A second stage by extending thetruss and tether wires. Then the folded structural unitpanels are deployed from the bus system by anothertruss. Finally, the folded structural unit panels are ex-tended to the 16 m × 17.6 m panel. The microwavetransmission experiment will be started approximately1 month after the launch when the possibility of theelectric discharge in the high-voltage system is com-pletely excluded. During the 1 month, evacuation ofthe fuel from the H2A second stage, checkout of theexperimental system, and orbit adjustment to the sub-recurrent orbit are conducted.

The experiment sequence is shown in Fig. 16. Forthe first 2 min, the microwave beam at 10% of the

full power is transmitted to the ground. The onboardcomputer controls the beam direction without the pilotsignal from the ground. When the demonstrator passesover the rectenna site, the microwave beam at the fullpower is transmitted to the rectenna for 16 s guided bythe pilot signal from the rectenna site. The beam direc-tion is changed in ±10◦ from the normal line of thepanel to target the rectenna. After the full power oper-ation, the power transmission at 10% of the full poweris performed for another 2 min.

5. Summary and conclusion

Tethered-SPS in which a power generation/transmission panel is suspended by multiple wires froma bus system above the panel has been investigatedin a conceptual study level. This concept has severaladvantages, as summarized below:

(1) Since the attitude is stabilized automatically by thegravity gradient force, no active attitude control isrequired.

(2) There is no moving structure, which makes the sys-tem highly robust and stable. Especially one-pointfailure mode peculiar to the rotary mechanism isexcluded.

(3) The tethered system is composed of equivalenttethered subpanels, which enables the phasedconstruction and leads to easy integration andmaintenance.

(4) The subpanel consists of equivalent power enera-tion/transmission modules, which enables low-costmass production.

(5) There is no wired signal/power interface betweenthe modules, which leads to easy deployment ofthe subpanel.

(6) Active thermal control is not required becauseof the flat distribution of the transmitting power(10 dB taper configuration will require the activethermal control for the modules near the center ofthe power generation/transmission panel).

(7) A scale model of the tethered subpanel can be usedfor the demonstration experiment on the groundand in orbit in the near future, which assures theevolutional scenario for the SPS development fromthe initial demonstration to the commercial SPS.

Especially, (3)–(7) are the innovative points peculiarto the concept of the Tethered-SPS. On the other hand,the Tethered-SPS has several disadvantages in the per-formance as compared with the past SPS model thathas a sun-pointing capability for the constant power


Beam transmissioncontrolled by anonboard computer

Beam transmissioncontrolled by a pilot signalfrom the ground (±10˚)

Beam transmissioncontrolled by anonboard computer

-2 min

370 km

Receiving Antenna

900 km 900 km

+2 min+8 sec0-8 sec

Fig. 16. Sequence of microwave power transmission experiment.

0 2 4 6 8 10 12 14 16 18 20 22 24

Peak

50 Tethered-SPS

ReservoirHydropower

(6000x104kW)

Petroleum

Middle

Hydropower & Geothermal PowerBase

Local Time

LNG & Other Gas

Coal

Nuclear Power

Pumping-upPower

Fig. 17. Combination of the power supplies to meet with the diurnal variation of the power consumption in Japan. The power from the 50Tethered-SPS (60 GW maximum) is superimposed.

generation. They are summarized as:

(1) The power efficiency is 64%, or practically 60%,as compared with the sun-pointing-type SPS, even

if the solar cells are attached to both sides of thepower generation/transmission panel.

(2) The power generation changes 100% with the localtime as the sun angle changes.


(3) A larger size rectenna is required for the flat distri-bution of the transmitting power as compared withthat for the tapered distribution.

The lower power efficiency of the Tethered-SPS re-sults in higher cost of the power supply. However, thecost of the power from the Tethered-SPS could be stilllower than the sun-pointing SPS that requires highlychallenging technologies or non-practical technologies.Actually, the cost for the non-existing technologies suchas the rotary joint or the large rotating mirrors cannotbe estimated. The change of the power supply with thelocal time for the Tethered-SPS can be accepted forthe commercial system in the mixture of varieties of thepower resources on the ground, especially in the ini-tial phase of commercialization with a ratio of the SPSpower to the total less than 30%. Fig. 17 shows a com-bination of the power supplies to meet with the diurnalvariation of the power consumption in Japan. The powerfrom 50 Tethered-SPS (60 GW maximum) is superim-posed on the same chart for reference. SPS can sub-stitute for the power generated by LNG or petroleum,which greatly contributes to the reduction of CO2. Therectenna size for the flat distribution is relatively largerthan that for the tapered one, but the actual size is notso large as compared with the past GW-class SPS us-ing a transmitting antenna typically with 1 km diame-ter, because the Tethered-SPS has a larger transmittingantenna by approximately twice than the past SPS. Incase of the current study model, a rectenna 4 km in di-ameter can collect 90% of the total power for the flatdistribution.

As a conclusion, the Tethered-SPS is a highly prac-tical SPS model, with a number of advantages in theproduction, integration, construction, and operation ascompared with the past SPS models. This model canbe used as a realistic reference model to evaluate thecost and CO2 load as a future energy system for humansociety, because the technologies employed in this con-cept are all achievable in the near future. However, thecurrent study still remains at initial conceptual stage.Further studies are required to confirm the technical

feasibilities more extensively, especially on the tech-nologies for the microwave transmission and the con-struction of the large system in orbit.

References

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[11] K. Ishimura, M. Wada, N. Takai, Stability analysis of thespace solar power system stabilized by gravity gradient torque,Proceedings of the 22nd ISAS Space Energy Symposium, 2002,pp. 71–75.

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[13] Y. Fujino, K. Ogimura, A rectangular parabola rectenna withelliptical beam for SPS test satellite experiment, IAC-04-R.2.02,55th International Astronautical Congress, Vancouver, Canada,4–8th, October 2004.

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a new concept of solar power satellite

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