SSPS : Space Solar Power System

USEF has been studying Space solar power system (SSPS) as an alternative future energy resource under a support of METI (The Ministry of Economy, Trade and Industry) and the other related agency for the past several years. The study has covered from basic laboratory testing level to the practical power plant level. The activities have been supported by working committee. Participants of the committee were selected from people of wide range of backgrounds. They are experts for respective technologies working for universities, industries and research organization.

DEVELOPMENT PLAN

Fig.1 shows proposed milestones roadmap to future commercial SSPS. - 1st step   100kW demonstration - 2nd step  10MW demonstration - 3rd step   250MW demonstration - 4th step   1GW Commercial system operation

Fig.1 Milestones Roadmap

SSPS CONCEPT

The Working Committee has investigated a simple, technically feasible, and practical configuration SSPS which consists of a large power generation/transmission panel suspended by multi-tether wires from a bus system above the panel. The upper surface of the generation/transmission panel is covered with solar cells, and the lower surface mounts transmitting phased array antenna elements and solar cells. An artist concept of the tethered SSPS is shown in Fig.2.

Fig.2 Artist conception of the SSPS

The attitude is automatically stabilized by the gravity gradient force in the tether configuration. So the antenna surface of the panel is always oriented towards the earth without any active attitude control. No moving structure in a large scale makes this system highly robust and stable. The power generation/transmission panel consists of identical modules, which greatly contributes to the low cost production, testing, and realizing high quality. Another innovative feature of the module is the application of wireless LAN system for the signal interface among those panels, which leads to the reliable deployment, integration, and maintenance.    Fig3 shows concept of the "Baseline type" SSPS which is capable of 1.6GW power supply maximum and 1GW average on the ground. It is composed of a power generation/transmission panel of 2.6x2.4 km suspended by multi-tethers deployed from a bus system which is located at 10 km upward. The panel consists of sub-panels of 100 mx100 m with 0.1m thickness. Each sub-panel consists of module panels 1 mx1 m size. There is no wired signal interface between module panels. The control signal and reference signal for each module panel are provided from the bus system by the wireless LAN.    The power generation of the baseline type SSPS varies with the local time as the sun angle changes as shown in Fig.4. The average power amounts to 64% of the maximum power.

Fig.3 Baseline type concept                Fig.4 Baseline type electrical power

A derivative concept ("Derivative type") is investigated to improve the average power amounts. The derivative type SSPS is shown in Fig.5. Two rotating flat mirror is attached to the bus to reflect the solar light towards the generation/transmission panel, making the average power amount to 95% of the maximum power.

Fig.5 Derivative type concept

LIFECYCLE CO2 EMISSION

The lifecycle carbon dioxide emission for the operational SSPS has been estimated. The result indicates that the carbon dioxide emission from the operational SSPS per unit of energy generated is almost the same as from nuclear power system and much less than fossil fuel power system, so the SSPS is one of the most effective alternative technology for further CO2 reduction in electric power generation. (Fig.6)

Fig.6 CO2 Emission Intensity