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Summary of studies on space solar power systems of Japan Aerospace Exploration Agency (JAXA)
Acta Astronautica 59 (2006) 132 – 138 www.elsevier.com/locate/actaastro
Summary of studies on space solar power systems of Japan Aerospace Exploration Agency (JAXA) Masahiro Mori∗ , Hideshi Kagawa, Yuka Saito Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba-shi Ibaraki 305-8505, Japan Available online 19 May 2006
Abstract Japan Aerospace Exploration Agency (JAXA) has been conducting studies on space solar power systems (SSPS) using microwave and laser beams for years since FY1998 organizing a special committee and working groups. Current SSPS study undertaken by JAXA consists of three main subjects, SSPS concepts and architectures study, technology flight demonstration plan-making and major technology development. In SSPS concepts and architectures study, special committee and working groups which include 180 or more persons participate from industrial, administrative and academic sectors are organized. And some configurations of both microwave-based SSPS and laser-based SSPS have been studied. This paper presents the results of these study effort of JAXA and the most promising SSPS concepts, including their key technologies. © 2006 Elsevier Ltd. All rights reserved.
1. Introduction Japan Aerospace Exploration Agency (JAXA) has examined studies on space solar power systems (SSPS) using microwave and laser beams for years. The microwave-based SSPS (M-SSPS) are huge solar power systems that generate GW power by solar cells. The electric power is transmitted via microwave from the SSPS to the ground. The on-ground rectifying antenna would collect the microwave beam and convert it to electricity to connect to commercial power grids. In the laser-based SSPS (L-SSPS), a solar condenser equipped with lenses or mirrors and laser generator would be put into orbit. A laser beam would be sent to Earth-based hydrogen generating device. Fig. 1 shows the image of the commercial system of the L-SSPS and its ground hydrogen generating facilities. ∗ Corresponding author.
E-mail addresses: [email protected] (M. Mori), [email protected] (H. Kagawa), [email protected] (Y. Saito). 0094-5765/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2006.02.033
JAXA is proposing a roadmap that consists of a stepwise approach to achieve commercial SSPS in 20–30 years. The first step is tens of kW class Technology Demonstration Satellite to demonstrate microwave or laser power transmission. The second step is to demonstrate robotic assembly of 10 MW class large-scale flexible structure in space on ISS co-orbit. The third step is to build a prototype SSPS in GEO. The final step is to build commercial GW class SSPS in GEO. Fig. 2 shows the SSPS roadmap of JAXA. 2. A framework for SSPS study of JAXA Current SSPS study undertaken by JAXA consists of three main subjects, SSPS concepts and architectures study, technology flight demonstration and major technology development (see Fig. 3). In SSPS concepts and architectures study, special committee and working groups which include 180 or more persons participate from industrial, administrative and academic sectors are organized. In this study, a
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
Fig. 1. Concept of the laser-based SSPS.
major focus is on identifying system concepts, architectures and key technologies that may ultimately produce a practical and economical energy source. The results of this study effort are hereinafter described in detail. In the study of technology flight demonstration, system design of tens of kW class Technology Demonstration Satellite and conceptual study of 10 MW class demonstration system on ISS co-orbit are also conducted. In major technology development study, several key technologies which are needed to be developed in appropriate R&D roadmap are investigated. 3. Concept design of commercial SSPS In SSPS concepts and architectures study, system concepts and architectures of commercial type of M-SSPS and L-SSPS has been studied. 3.1. Microwave-based SSPS In case of M-SSPS, the solar energy must be converted to electricity and then converted to a microwave
133
beam in GEO. The on-ground rectifying antenna would collect the microwave beam and convert it to electricity to connect to commercial power grids. Several types of system configurations have been proposed before. From the past experiences of the conceptual design of the 1 GW class SSPS, it was clear that system with the mirrors and modularized unit which integrated solar cells and microwave power transmitters is promising. Fig. 4 shows the artist image of this type of M-SSPS, named 2003 JAXA Reference System. By using mirrors, the solar lights are directed to the energy conversion unit integrated solar cells and microwave power transmitters. Since solar cell panel and the Earth looking microwave antenna are two sides of the same modularized unit, length of power cables connected solar cells to the power transmitters will become short and this will reduce mass of whole system. And the modularized unit is also preferable because it is easy to assemble by robots automatically. Major hurdles in this power collection/transmission system are how to improve the efficiency of converting the solar energy to the electricity by the current major solar cells are in order of under 20% and how to exhaust heat of the system. Solar energy that is not converted to electricity will become heat and large area radiators will be necessary to discharge the heat. The key factor in designing systems is feasibility of thermal system. Another feature of FY2003 Reference System is the formation flying primary mirrors. This model has no center truss which connects to primary mirrors to energy conversion unit. When the large primary mirrors receive sunlight, the sunlight would produce buoyancy and horizontal force. If electrical thruster or some other way can cancel the horizontal force, the buoyancy can be used to fly the primary mirrors independently while maintaining proper relative position. This configuration has the merit that the rotary joint for control of the primary mirrors becomes unnecessary and the whole system becomes mechanically more stable and reliable. Since modularized unit which integrated solar cells and microwave power transmitter has great advantages, the possibility of thermal system design is unclear. So, the model which separated solar cell panels from microwave power transmitter was studied in FY2004 (see Fig. 5). The characteristics of this model is summarized as follows. • Solar panels are separated from microwave power transmitter (structurally connected), so the backside of solar panels and microwave power transmitter can be used as heat release surface.
134
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
Fig. 2. SSPS Roadmap of JAXA.
FY1998 ~ FY2004
FY2005
SSPS Concepts and Architectures Studyy Technology Flight Demonstration Plan making
10MW class Demonstration System 50kW class Engineering Test Satellite
• High voltage solar cell array • Thermal control system
Major Technology Development
• High efficiency magnetron • Direct solar pumping solid-state laser
microwave / laser energy transmission ground experiments
• Technologies for collimation mirrors • Robotic assembly / maintenance technology
Fig. 3. Framework for SSPS study of JAXA.
• Two primary mirrors are flying independently using solar radiation pressure as well as 2003 Reference System. • The size of solar cells depends on concentration ratio. In this model, concentration ratio is
limited to 2 or 3 with the object of solar cell temperature. However, concentration ratio can be higher with adoption of some wavelength selective film that could cut unusable light wavelengths.
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
135
Fig. 6. Image of L-SSPS (one base unit). Fig. 4. 2003 JAXA Reference System (M-SSPS).
3.2. Laser-based SSPS
Fig. 5. 2004 JAXA Reference System (M-SSPS).
• To limit mass of electric power cable to 1000 ton, the length of cable should be less than 1 km and the transmission voltage should be 4–5 kV. Weight saving of DC/DC converters is also necessary. Using this model as basis, we are carrying out examination from various viewpoints aiming at the cost minimum to build and maintain the systems. System size • Primary mirror: 2.5 km × 3.5 km, 1000 ton × 2. • Solar panels: 1.2 km ∼ 2 km (TBD). • Microwave power transmitter: 1.8 km ∼ 2.5 km (TBD). • Total weight: less than 10,000 ton.
In case of L-SSPS, solar condenser mirrors or lenses would be put into orbit to focus the Sun’s rays. The concentrated solar energy would then be sent to laser generator. The laser beam would be directly produced from the solar light using the direct solar pumping solidstate laser device. This laser beams would be collected on ground and used to produce hydrogen from seawater. The receiving/energy conversion station is settled on an ocean, and producing hydrogen can be stored and transported by ships to consumers. Based on the above concept, L-SSPS of Fig. 6 was considered. A base unit consists of solar condenser mirrors, radiators, laser generator, laser beam irradiator and support structures. Neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal is seemed to be the most qualified candidate for the laser medium of this system. Major parameters of the 10 MW class laserL-SSPS are as follows. • Primary solar condenser mirrors: 100 m × 100 m × 2. • Radiators: 100 m × 100 m × 2. The output of 1 GW class power could be obtained by connecting 100 base units vertically in series (see Fig. 7). In designing L-SSPS, conversion efficiency of the direct solar pumping solid-state laser and feasibility of thermal system are critical factors. Since solar concentration is of more than a few hundred times, heat removal is key issue to realize solar pumping laser systems. To improve the feasibility of thermal control problem, adoption of some wavelength selective film that could cut unusable light wavelengths has been considered as well as M-SSPS.
136
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
SSPS. The cost of power generation ($/kWh) is also calculated. Environmental impact model: Calculating carbon dioxide (CO2 ) emissions caused by the construction and operation of SSPS. Energy balance model: Calculating energy payback time and energy balance, based on the relationship between energy input during operation and construction and energy delivered during operation of SSPS.
Fig. 7. Image of L-SSPS (1 GW-class).
Feasibility of its ground facilities and production technology of hydrogen using laser beams have been also studied. Considering the expected the world’s growth in demand for hydrogen energy, L-SSPS have great potential in that SSPS could produce a lot of hydrogen from non-fossil-fuel sources. Both hydrogen generating systems with photocatalyst device and electrolytic ones have been examined. From the past experiences of this study, highefficient electric power generating technology using the solar cell which suited the wavelength of laser is promising. In L-SSPS, it is assumed that laser beam at a wavelength of 1.06 m would be transmitted from space to earth, but there is no photo-catalyst for water decomposition into hydrogen and oxygen at high efficiency by means of laser in the 1.06 m band at present. On the other hand, in case of using solar cell which suited the single wavelength of laser, the water electrolysis technologies at high efficiency has already established. When the solar cells which have the band gap suited to the wavelength of 1.06 m is used, converting the laser energy to the electricity with more than 70% would be theoretically possible. And conversion efficiency would become up with higher density of incoming laser beam. 3.3. Economic evaluation of SSPS 3.3.1. The creation of a life cycle model The life cycle model of SSPS was created. Based on various factors related to the construction and operation of SSPS, cost model, environmental impact model and energy balance model were created. Cost model: Calculating the total costs of power generation, based on construction and operational costs of
3.3.2. M-cost model The intent of M-cost model is to facilitate the cost calculations for constructing and operating M-SSPS. With the use of this model, technological sensitivity could be analyzed to identify key research issues that must be pursued in the future. These are the main cost considerations: (1) (2) (3) (4)
the space segment; the ground segment; launch expenses; maintenance expenses.
The space segment: This discussion considers the cost of manufacturing the 2004 Reference Model (1 GWclass: see Fig. 5). This model has five major parts: 1. 2. 3. 4. 5.
primary mirrors; solar panels; microwave power transmitter; radiator; support structures for all of the above.
The cost of solar panels relatively easy to calculate when the cost per unit area is known, and the total area required is known. In a similar way, it is not difficult to calculate the cost of the microwave power transmitter. Other parts are considered as structures and the cost is calculated by multiplying cost per unit weight by structures weight. The ground segment (Rectenna): Costs associated with the construction of the rectenna are segmented as follows: 1. microwave reception part; 2. support structure for it; 3. connection to the power grid on Earth. The cost of the microwave-to-DC conversion can be calculated by multiplying the cost per unit watt with the power requirement (say, 1 GW). The power requirement also establishes the area of the rectenna. Then costs to achieve this area (land
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
acquisition, construction cost, and so on) can be calculated. Launch expenses: Launch expenses can be calculated RLV (reusable launch vehicle) transportation cost plus OTV (orbit transfer vehicle) transportation cost. The RLV is used to transport material to low-orbit. The OTV (for example, an electric propulsion vehicle) is assumed to be used to lift the material from low-orbit to final geostationary orbit. Maintenance expenses: Maintenance costs of the space segment will be calculated as a fixed percentage of the construction costs of the space segment. Maintenance costs of the ground segment will be calculated as a fixed percentage of the construction costs of the ground segment. According to the above-mentioned cost method, the costs associated to build a 1 GW M-SSPS was calculated. As calculation results depend in good part on input parameter values, we have to thoroughly examine the suitability of them. On the one hand, it is hard to find the appropriate parameter value in the next 20–30 years. In this calculation, the input parameter values with a certain basis would be used. However, it is essential to conduct a review of parameter values in the future. In this calculation, it is assumed that the diameter of microwave power transmitter is 1.8 km and the concentration ratio is 2. In this case, the total mass of space segment is calculated to be 9907 ton and achieve the goals of “less than 10,000 ton”. A 1 GW M-SSPS would cost 1.18 trillion yen (=$10.7 billions) and the cost of power generation would be calculated to be 8.6 yen/kWh (=7.8 cents/kWh). The life of SSPS is assumed to be 40 years. The cost breakdown are following. The cost breakdown: • Space segment: 675.8 billion yen. ◦ Microwave power transmitter: 402.8 billion yen. ◦ Structures: 230.5 billion yen. ◦ Solar panels: 42.5 billion yen. • Launch cost: 297.5 billion yen. • Rectenna construction cost: 203.3 billion yen. 3.3.3. Environmental impact model SSPS is very friendly to the environment with respect to carbon dioxide emissions into the atmosphere. Refer to Table 1 to see how much carbon dioxide is emitted to produce 1 GW electric power. Even compared to wind power and nuclear power, SSPS releases less CO2 into the environment. SSPS is extremely clean source of energy.
137
Table 1 1 CO2 emissions of the SSPS project To To To To To
manufacture the space segment lift it into space manufacture the rectenna operate the space segment operate the rectenna
89,062 983,400 1,405,113 32,147 14,051
t-CO2 t-CO2 t-CO2 t-CO2 /yr t-CO2 /yr
Table 2 Energy payback time requires for SSPS Energy invested to manufacture the space segment Energy invested to lift it into space Energy invested to manufacture the ground segment Energy to keep the space segment operating Energy to keep the ground segment operating Total invested energy Total energy return on investment Energy return on investment Energy payback time
1682
GWh
2204
GWh
1706
GWh
117
GWh/yr
17 10,805 350,400 32.43 1.23
GWh/yr GWh GWh yr
Consequently, SSPS burden on the environment is this: to generate 1 kWh of electricity, 12.22 g of CO2 is released. 3.3.4. Energy balance model Calculating energy payback time based on the relationship between energy input during operation and construction and energy delivered during operation of SSPS. Energy payback time of power generated by combusting coal, oil, LNG and nuclear power is shorter than that of SSPS. SSPS achieves a higher payback ratio than various terrestrial solar power stations (Table 2). 4. Energy transmission to rover in lunar polar region In technology flight demonstration plan study, lunar orbit satellite which could transmit laser energy to rover in lunar polar region is investigated. The Moon exploration mission needs energy securement and transmission technology for manned moon presence and robotic activity. We examined mission/orbit/scale of systems/laser system which is suitable for laser energy transmission between the lunar-orbiting L-SSPS and the rover in lunar polar region (Fig. 8).
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M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
In FY2005, in addition to those, microwave and laser energy transmission ground experiments will be studied. 6. Summary and conclusions
Fig. 8. L-SSPS configuration on lunar orbit.
5. Major technology development In major technology development study, several key technologies which are needed to be developed in appropriate R&D roadmap are investigated. Some key elemental technologies are the following: • • • • • •
High-voltage solar cell array. Fiber type of direct solar pumping solid-state laser. High-efficiency magnetron. Thermal control technology. New technologies for collimation mirrors/lens. Control technology of large-scale flexible structure.
In this paper, JAXA’s current study status of SSPS was reviewed and SSPS roadmap of JAXA that includes the stepwise approach to realize commercial SSPS in 20–30 years was shown. Furthermore, the latest configuration of M-SSPS and L-SSPS was introduced and the technical problems were suggested. The results of economic evaluation were also shown. In the future, as the accuracy of this model is improved, it is necessary to establish the target value of each input parameter as ultimate power generation cost has competitiveness in costs (target cost = c8 kWh). Further reading [1] M. Oda, M. Mori, Stepwise development of SSPS; JAXA’s current study status of the 1 GW class operational SSPS and its precursor, 54th IAC, 2003. [2] M. Mori, H. Kagawa, Y. Saito, H. Nagayama, Current status of study on hydrogen production with space solar power systems (SSPS), Proceedings of the Fourth International Conference on Solar Power from Space -SPS’04- together with The Fifth International Conference on Wireless Power Transmission WPT5, July 2004. [3] Study of Space Solar Power Systems, JAXA Contractor Report, Mitsubishi Research Institute Inc., 2004.
Summary of studies on space solar power systems of Japan Aerospace Exploration Agency (JAXA) Masahiro Mori∗ , Hideshi Kagawa, Yuka Saito Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba-shi Ibaraki 305-8505, Japan Available online 19 May 2006
Abstract Japan Aerospace Exploration Agency (JAXA) has been conducting studies on space solar power systems (SSPS) using microwave and laser beams for years since FY1998 organizing a special committee and working groups. Current SSPS study undertaken by JAXA consists of three main subjects, SSPS concepts and architectures study, technology flight demonstration plan-making and major technology development. In SSPS concepts and architectures study, special committee and working groups which include 180 or more persons participate from industrial, administrative and academic sectors are organized. And some configurations of both microwave-based SSPS and laser-based SSPS have been studied. This paper presents the results of these study effort of JAXA and the most promising SSPS concepts, including their key technologies. © 2006 Elsevier Ltd. All rights reserved.
1. Introduction Japan Aerospace Exploration Agency (JAXA) has examined studies on space solar power systems (SSPS) using microwave and laser beams for years. The microwave-based SSPS (M-SSPS) are huge solar power systems that generate GW power by solar cells. The electric power is transmitted via microwave from the SSPS to the ground. The on-ground rectifying antenna would collect the microwave beam and convert it to electricity to connect to commercial power grids. In the laser-based SSPS (L-SSPS), a solar condenser equipped with lenses or mirrors and laser generator would be put into orbit. A laser beam would be sent to Earth-based hydrogen generating device. Fig. 1 shows the image of the commercial system of the L-SSPS and its ground hydrogen generating facilities. ∗ Corresponding author.
E-mail addresses: [email protected] (M. Mori), [email protected] (H. Kagawa), [email protected] (Y. Saito). 0094-5765/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2006.02.033
JAXA is proposing a roadmap that consists of a stepwise approach to achieve commercial SSPS in 20–30 years. The first step is tens of kW class Technology Demonstration Satellite to demonstrate microwave or laser power transmission. The second step is to demonstrate robotic assembly of 10 MW class large-scale flexible structure in space on ISS co-orbit. The third step is to build a prototype SSPS in GEO. The final step is to build commercial GW class SSPS in GEO. Fig. 2 shows the SSPS roadmap of JAXA. 2. A framework for SSPS study of JAXA Current SSPS study undertaken by JAXA consists of three main subjects, SSPS concepts and architectures study, technology flight demonstration and major technology development (see Fig. 3). In SSPS concepts and architectures study, special committee and working groups which include 180 or more persons participate from industrial, administrative and academic sectors are organized. In this study, a
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
Fig. 1. Concept of the laser-based SSPS.
major focus is on identifying system concepts, architectures and key technologies that may ultimately produce a practical and economical energy source. The results of this study effort are hereinafter described in detail. In the study of technology flight demonstration, system design of tens of kW class Technology Demonstration Satellite and conceptual study of 10 MW class demonstration system on ISS co-orbit are also conducted. In major technology development study, several key technologies which are needed to be developed in appropriate R&D roadmap are investigated. 3. Concept design of commercial SSPS In SSPS concepts and architectures study, system concepts and architectures of commercial type of M-SSPS and L-SSPS has been studied. 3.1. Microwave-based SSPS In case of M-SSPS, the solar energy must be converted to electricity and then converted to a microwave
133
beam in GEO. The on-ground rectifying antenna would collect the microwave beam and convert it to electricity to connect to commercial power grids. Several types of system configurations have been proposed before. From the past experiences of the conceptual design of the 1 GW class SSPS, it was clear that system with the mirrors and modularized unit which integrated solar cells and microwave power transmitters is promising. Fig. 4 shows the artist image of this type of M-SSPS, named 2003 JAXA Reference System. By using mirrors, the solar lights are directed to the energy conversion unit integrated solar cells and microwave power transmitters. Since solar cell panel and the Earth looking microwave antenna are two sides of the same modularized unit, length of power cables connected solar cells to the power transmitters will become short and this will reduce mass of whole system. And the modularized unit is also preferable because it is easy to assemble by robots automatically. Major hurdles in this power collection/transmission system are how to improve the efficiency of converting the solar energy to the electricity by the current major solar cells are in order of under 20% and how to exhaust heat of the system. Solar energy that is not converted to electricity will become heat and large area radiators will be necessary to discharge the heat. The key factor in designing systems is feasibility of thermal system. Another feature of FY2003 Reference System is the formation flying primary mirrors. This model has no center truss which connects to primary mirrors to energy conversion unit. When the large primary mirrors receive sunlight, the sunlight would produce buoyancy and horizontal force. If electrical thruster or some other way can cancel the horizontal force, the buoyancy can be used to fly the primary mirrors independently while maintaining proper relative position. This configuration has the merit that the rotary joint for control of the primary mirrors becomes unnecessary and the whole system becomes mechanically more stable and reliable. Since modularized unit which integrated solar cells and microwave power transmitter has great advantages, the possibility of thermal system design is unclear. So, the model which separated solar cell panels from microwave power transmitter was studied in FY2004 (see Fig. 5). The characteristics of this model is summarized as follows. • Solar panels are separated from microwave power transmitter (structurally connected), so the backside of solar panels and microwave power transmitter can be used as heat release surface.
134
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
Fig. 2. SSPS Roadmap of JAXA.
FY1998 ~ FY2004
FY2005
SSPS Concepts and Architectures Studyy Technology Flight Demonstration Plan making
10MW class Demonstration System 50kW class Engineering Test Satellite
• High voltage solar cell array • Thermal control system
Major Technology Development
• High efficiency magnetron • Direct solar pumping solid-state laser
microwave / laser energy transmission ground experiments
• Technologies for collimation mirrors • Robotic assembly / maintenance technology
Fig. 3. Framework for SSPS study of JAXA.
• Two primary mirrors are flying independently using solar radiation pressure as well as 2003 Reference System. • The size of solar cells depends on concentration ratio. In this model, concentration ratio is
limited to 2 or 3 with the object of solar cell temperature. However, concentration ratio can be higher with adoption of some wavelength selective film that could cut unusable light wavelengths.
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
135
Fig. 6. Image of L-SSPS (one base unit). Fig. 4. 2003 JAXA Reference System (M-SSPS).
3.2. Laser-based SSPS
Fig. 5. 2004 JAXA Reference System (M-SSPS).
• To limit mass of electric power cable to 1000 ton, the length of cable should be less than 1 km and the transmission voltage should be 4–5 kV. Weight saving of DC/DC converters is also necessary. Using this model as basis, we are carrying out examination from various viewpoints aiming at the cost minimum to build and maintain the systems. System size • Primary mirror: 2.5 km × 3.5 km, 1000 ton × 2. • Solar panels: 1.2 km ∼ 2 km (TBD). • Microwave power transmitter: 1.8 km ∼ 2.5 km (TBD). • Total weight: less than 10,000 ton.
In case of L-SSPS, solar condenser mirrors or lenses would be put into orbit to focus the Sun’s rays. The concentrated solar energy would then be sent to laser generator. The laser beam would be directly produced from the solar light using the direct solar pumping solidstate laser device. This laser beams would be collected on ground and used to produce hydrogen from seawater. The receiving/energy conversion station is settled on an ocean, and producing hydrogen can be stored and transported by ships to consumers. Based on the above concept, L-SSPS of Fig. 6 was considered. A base unit consists of solar condenser mirrors, radiators, laser generator, laser beam irradiator and support structures. Neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal is seemed to be the most qualified candidate for the laser medium of this system. Major parameters of the 10 MW class laserL-SSPS are as follows. • Primary solar condenser mirrors: 100 m × 100 m × 2. • Radiators: 100 m × 100 m × 2. The output of 1 GW class power could be obtained by connecting 100 base units vertically in series (see Fig. 7). In designing L-SSPS, conversion efficiency of the direct solar pumping solid-state laser and feasibility of thermal system are critical factors. Since solar concentration is of more than a few hundred times, heat removal is key issue to realize solar pumping laser systems. To improve the feasibility of thermal control problem, adoption of some wavelength selective film that could cut unusable light wavelengths has been considered as well as M-SSPS.
136
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
SSPS. The cost of power generation ($/kWh) is also calculated. Environmental impact model: Calculating carbon dioxide (CO2 ) emissions caused by the construction and operation of SSPS. Energy balance model: Calculating energy payback time and energy balance, based on the relationship between energy input during operation and construction and energy delivered during operation of SSPS.
Fig. 7. Image of L-SSPS (1 GW-class).
Feasibility of its ground facilities and production technology of hydrogen using laser beams have been also studied. Considering the expected the world’s growth in demand for hydrogen energy, L-SSPS have great potential in that SSPS could produce a lot of hydrogen from non-fossil-fuel sources. Both hydrogen generating systems with photocatalyst device and electrolytic ones have been examined. From the past experiences of this study, highefficient electric power generating technology using the solar cell which suited the wavelength of laser is promising. In L-SSPS, it is assumed that laser beam at a wavelength of 1.06 m would be transmitted from space to earth, but there is no photo-catalyst for water decomposition into hydrogen and oxygen at high efficiency by means of laser in the 1.06 m band at present. On the other hand, in case of using solar cell which suited the single wavelength of laser, the water electrolysis technologies at high efficiency has already established. When the solar cells which have the band gap suited to the wavelength of 1.06 m is used, converting the laser energy to the electricity with more than 70% would be theoretically possible. And conversion efficiency would become up with higher density of incoming laser beam. 3.3. Economic evaluation of SSPS 3.3.1. The creation of a life cycle model The life cycle model of SSPS was created. Based on various factors related to the construction and operation of SSPS, cost model, environmental impact model and energy balance model were created. Cost model: Calculating the total costs of power generation, based on construction and operational costs of
3.3.2. M-cost model The intent of M-cost model is to facilitate the cost calculations for constructing and operating M-SSPS. With the use of this model, technological sensitivity could be analyzed to identify key research issues that must be pursued in the future. These are the main cost considerations: (1) (2) (3) (4)
the space segment; the ground segment; launch expenses; maintenance expenses.
The space segment: This discussion considers the cost of manufacturing the 2004 Reference Model (1 GWclass: see Fig. 5). This model has five major parts: 1. 2. 3. 4. 5.
primary mirrors; solar panels; microwave power transmitter; radiator; support structures for all of the above.
The cost of solar panels relatively easy to calculate when the cost per unit area is known, and the total area required is known. In a similar way, it is not difficult to calculate the cost of the microwave power transmitter. Other parts are considered as structures and the cost is calculated by multiplying cost per unit weight by structures weight. The ground segment (Rectenna): Costs associated with the construction of the rectenna are segmented as follows: 1. microwave reception part; 2. support structure for it; 3. connection to the power grid on Earth. The cost of the microwave-to-DC conversion can be calculated by multiplying the cost per unit watt with the power requirement (say, 1 GW). The power requirement also establishes the area of the rectenna. Then costs to achieve this area (land
M. Mori et al. / Acta Astronautica 59 (2006) 132 – 138
acquisition, construction cost, and so on) can be calculated. Launch expenses: Launch expenses can be calculated RLV (reusable launch vehicle) transportation cost plus OTV (orbit transfer vehicle) transportation cost. The RLV is used to transport material to low-orbit. The OTV (for example, an electric propulsion vehicle) is assumed to be used to lift the material from low-orbit to final geostationary orbit. Maintenance expenses: Maintenance costs of the space segment will be calculated as a fixed percentage of the construction costs of the space segment. Maintenance costs of the ground segment will be calculated as a fixed percentage of the construction costs of the ground segment. According to the above-mentioned cost method, the costs associated to build a 1 GW M-SSPS was calculated. As calculation results depend in good part on input parameter values, we have to thoroughly examine the suitability of them. On the one hand, it is hard to find the appropriate parameter value in the next 20–30 years. In this calculation, the input parameter values with a certain basis would be used. However, it is essential to conduct a review of parameter values in the future. In this calculation, it is assumed that the diameter of microwave power transmitter is 1.8 km and the concentration ratio is 2. In this case, the total mass of space segment is calculated to be 9907 ton and achieve the goals of “less than 10,000 ton”. A 1 GW M-SSPS would cost 1.18 trillion yen (=$10.7 billions) and the cost of power generation would be calculated to be 8.6 yen/kWh (=7.8 cents/kWh). The life of SSPS is assumed to be 40 years. The cost breakdown are following. The cost breakdown: • Space segment: 675.8 billion yen. ◦ Microwave power transmitter: 402.8 billion yen. ◦ Structures: 230.5 billion yen. ◦ Solar panels: 42.5 billion yen. • Launch cost: 297.5 billion yen. • Rectenna construction cost: 203.3 billion yen. 3.3.3. Environmental impact model SSPS is very friendly to the environment with respect to carbon dioxide emissions into the atmosphere. Refer to Table 1 to see how much carbon dioxide is emitted to produce 1 GW electric power. Even compared to wind power and nuclear power, SSPS releases less CO2 into the environment. SSPS is extremely clean source of energy.
137
Table 1 1 CO2 emissions of the SSPS project To To To To To
manufacture the space segment lift it into space manufacture the rectenna operate the space segment operate the rectenna
89,062 983,400 1,405,113 32,147 14,051
t-CO2 t-CO2 t-CO2 t-CO2 /yr t-CO2 /yr
Table 2 Energy payback time requires for SSPS Energy invested to manufacture the space segment Energy invested to lift it into space Energy invested to manufacture the ground segment Energy to keep the space segment operating Energy to keep the ground segment operating Total invested energy Total energy return on investment Energy return on investment Energy payback time
1682
GWh
2204
GWh
1706
GWh
117
GWh/yr
17 10,805 350,400 32.43 1.23
GWh/yr GWh GWh yr
Consequently, SSPS burden on the environment is this: to generate 1 kWh of electricity, 12.22 g of CO2 is released. 3.3.4. Energy balance model Calculating energy payback time based on the relationship between energy input during operation and construction and energy delivered during operation of SSPS. Energy payback time of power generated by combusting coal, oil, LNG and nuclear power is shorter than that of SSPS. SSPS achieves a higher payback ratio than various terrestrial solar power stations (Table 2). 4. Energy transmission to rover in lunar polar region In technology flight demonstration plan study, lunar orbit satellite which could transmit laser energy to rover in lunar polar region is investigated. The Moon exploration mission needs energy securement and transmission technology for manned moon presence and robotic activity. We examined mission/orbit/scale of systems/laser system which is suitable for laser energy transmission between the lunar-orbiting L-SSPS and the rover in lunar polar region (Fig. 8).
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In FY2005, in addition to those, microwave and laser energy transmission ground experiments will be studied. 6. Summary and conclusions
Fig. 8. L-SSPS configuration on lunar orbit.
5. Major technology development In major technology development study, several key technologies which are needed to be developed in appropriate R&D roadmap are investigated. Some key elemental technologies are the following: • • • • • •
High-voltage solar cell array. Fiber type of direct solar pumping solid-state laser. High-efficiency magnetron. Thermal control technology. New technologies for collimation mirrors/lens. Control technology of large-scale flexible structure.
In this paper, JAXA’s current study status of SSPS was reviewed and SSPS roadmap of JAXA that includes the stepwise approach to realize commercial SSPS in 20–30 years was shown. Furthermore, the latest configuration of M-SSPS and L-SSPS was introduced and the technical problems were suggested. The results of economic evaluation were also shown. In the future, as the accuracy of this model is improved, it is necessary to establish the target value of each input parameter as ultimate power generation cost has competitiveness in costs (target cost = c8 kWh). Further reading [1] M. Oda, M. Mori, Stepwise development of SSPS; JAXA’s current study status of the 1 GW class operational SSPS and its precursor, 54th IAC, 2003. [2] M. Mori, H. Kagawa, Y. Saito, H. Nagayama, Current status of study on hydrogen production with space solar power systems (SSPS), Proceedings of the Fourth International Conference on Solar Power from Space -SPS’04- together with The Fifth International Conference on Wireless Power Transmission WPT5, July 2004. [3] Study of Space Solar Power Systems, JAXA Contractor Report, Mitsubishi Research Institute Inc., 2004.