Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)17

The Development of Space
Solar Power System Technologies

TAKANORI NARITA*1TOSHIHIRO KAMIYA*1

KEIJI SUZUKI*1

KENICHI ANMA*2MAYUKI NIITSU*3NOBUHIKO FUKUDA*4

Mitsubishi Heavy Industries (MHI) is developing technology in partnership with national
research institutions to realize a space solar power system that is expected to be a source of clean,safe and sustainable solar energy. Development has focused on two key technologies for thesystem: wireless power transmission technology and large-scale space structure technology. Thisarticle describes the status of research and development and test results for these technologies.

|1. Introduction
A space solar power system (SSPS) is a power-generation plant that uses the sunlightabundant in space as a clean, sustainable energy source. The research and development phaseconducted jointly by national research institutes and universities indicates that SSPS is a potential
solution for energy demands and for dealing with climate change. Moreover, wirelesspower-transmission technology, originally developed for SSPS, is receiving interest in land-based
systems, including wireless battery charging for electric vehicles and power transmission to remoteislands and isolated locations.

|2. Space Solar Power System
An overview of a SSPS is shown in Figure 1.

Figure 1

An overview of the space solar power system*1

Space Systems Engineering Department, Space Systems Division, Aerospace Systems*2

Engineering Manager, Space Systems Engineering Department, Space Systems Division, Aerospace Systems*3Manager, Space Systems Engineering Department, Space Systems Division, Aerospace Systems*4Group Manager, Space Systems Division, Aerospace Systems

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
18The system uses photovoltaic cell arrays in geostationary orbit 36,000 km above the surfaceof the Earth to generate power. The power is converted into a microwave or laser beam aimed atreceiving stations on Earth. By directing the beam, power can be collected safely by a rectenna andconverted into electricity for commercial power grids. To ensure its safety, there would berestricted access to the rectenna and the airspace over it. In the event of an on-orbit system
malfunction, a redundant beam controller would prevent the beam from hitting areas other than therectenna. In space, solar energy could be collected continually, regardless of weather and time ofthe day, enabling stable, efficient power generation. The SSPS targeted for commercial launch in2030 is capable of a power output equivalent to a 400-MV thermal plant unit and able to supply
electricity at a price similar to that of publicly supplied power (Table 1).
Table 1 The projected development roadmap|3. Wireless Power-transmission Technology Development: PAC andParallel Methods(1)

Principles
The SSPS would generate a microwave beam transmitted wirelessly to Earth. Apower-generation/transmission unit in geostationary orbit would consist of several tens of millionsof panels. Generating a microwave beam using so many panels requires power-transmission
otechnology with high-precision phase control at a target level of <10 for economic viability andsafety (Figure 2).

Figure 2

The purpose of power-transmission-control technology

Mitsubishi Heavy Industries is developing two transmission-control technologies: theposition and angle correction (PAC) and parallel methods. In the PAC method, the unique position
gap of each panel is estimated by comparing the reaching-phase angle of pilot-signal from Earth,and the microwave beam phase angle is corrected accordingly (Figure 3). In the parallel method,
microwave phase modulation occurs at each panel. When the phase-modulated microwave isMitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
19compared to a reference signal at the rectenna, the resulting phase deviation is equivalent to themodulation frequency output. Based on this principle, the phase deviation for each panel is detectedindividually and corrected (Figure 4).

Figure 3 The basic principle of the PAC method

Figure 4(2)The basic principle of the parallel method

Development of fundamental test units
To verify the basic performance of the PAC and parallel methods, MHI developed afundamental test unit in 2010 as part of a research facility at the Research Institute for Sustainable
Humanosphere, Kyoto University.
The PAC test unit includes a pilot-signal transmitter, phase detector, and control computer(Figure 5). The pilot-signal transmitter sends an encrypted 2.94533-GHz signal using
spread-spectrum techniques. The pilot signal is received via two antennas that simulate
power-generation/transmission panels on the phase detector, after which, signal decoding, downconversion, and I/Q demodulation are performed. Finally, the control computer processes the signal
phase deviation recorded between the two antennas (Figure 6).
The parallel method test unit includes a microwave transmitter, monitor, and control
computer (Figure 7). The microwave transmitter modulates a 5.8-GHz microwave to 100 and 130
kHz and sends the modulated waves via two antennas that simulate power-generation/transmission
panels. The monitor receives the modulated waves, compares them with a reference signal, andperforms I/Q demodulation. The control computer detects the phase deviation of each antenna bysignal processing using the Fourier transform based on the I/Q modulation data (Figure 8).

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)20

Figure 5

PAC method test unit

Figure 6

A functional diagram of the PAC method test unit

Figure 8

A functional diagram of the parallel method test unit

(3)

Figure 7

The parallel method test unit

Test results
In the PAC method test unit, the detection accuracy of the pilot-signal phase deviation wasanalyzed by placing two antennas that simulated power-generation/transmission panels at ±1.4omm relative to the reference (Figure 9). The results showed an accuracy of ≤1 (Figure 10).
For the parallel method test unit, the phase deviation of the two antennas was detected byplacing two antennas that simulated power-generation/transmission panels at ±0.7 mm relative toothe reference (Figure 11). The resulting accuracy of the parallel method was also ≤1 (Figure 12).
(4) Conclusion
To verify the basic performance of the PAC and parallel methods as
power-transmission-control technologies, the phase deviation detection accuracy of each methodowas evaluated. The test results indicated that both methods produced accuracies of ≤1 , confirmingthat phase deviation can be corrected at this tolerance level. Therefore, both methods werevalidated as effective power-transmission technologies, as the tests demonstrated phase control
obelow the 10 target level.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
21Figure 9

The PAC method test configuration

Figure 10

The PAC method test results

Figure 11

The parallel method test configuration

Figure 12

Parallel method test results|4. Large-scale Space Structure Technology Development: InflatableStructures(1)

Structure concept
The SSPS requires inflatable structure technology, in which the structural component iscompact and collapsible for launch and expands to full size in orbit. Inflatable structure technology
requires structural expansion in space and post-expansion curing.
Mitsubishi Heavy Industries proposed an expansion method in which a folded structure of
uncured composite material is expanded by external forces and further inflated with gas fed to its
interior and a curing technique in which a chain-reaction curing process is triggered by the heatgenerated by the initial curing using UV irradiation and heat on part of the expanded structure
pre-impregnated with a chain curing polymer (CCP).1 The curing would not require heating of theentire structure, reducing the mass associated with curing.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
22(2)Test overview
Under a 2010 contract with the Japan Aerospace Exploration Agency (JAXA), structural
expansion and curing were tested on an inflatable tube prototype composed of carbon fiber and aCCP.
The prototype was created by forming carbon fiber impregnated with a CCP into acylindrical inflatable tube, which was folded into its storage position. To simulate structural
expansion using external force, a wire attached to one end of the folded tube was pulled to expand
the tube. After the expansion, the tube was filled with gas to complete inflation. Then, artificial UV
light was applied to the exterior of the tube to initiate the chain-reaction curing process. Heatgeneration from the chain-reaction curing was monitored with a thermograph (Figure 13).
(3) Test results
Chain-reaction curing and complete curing of the inflatable tube were confirmed (Figure 14).
The thermographic images indicated that chain-reaction curing progressed from the bottom to thetop of the tube (Figure 15). From the results, some new technical issues were targeted for furtherresearch.
(4) Conclusion
Structural expansion and post-expansion curing were demonstrated using an inflatable tube
prototype composed of carbon fiber and a CCP, confirming its applicability.

Figure 13

Inflation and curing test overview

Figure 14
The inflatedtubepost-curingFigure 15

The thermographic images|5. Conclusion
Space solar power systems are regarded as a promising future energy source to solve energy
and climate-change issues. Mitsubishi Heavy Industries has been developing key elemental
technologies for the SSPS, including wireless power transmission and large-scale space structures.
With more ground and space-based demonstrations, in addition to ongoing technology
improvements, MHI is pursuing the realization of SSPS.

Reference
1.

Hayashi, S. et al., Development of Smart Polymer Materials and its Various Applications, Mitsubishi
Heavy Industries Technical Review Vol. 41 No. 1 (2004) p.62-63