Space Solar Power Station/Satellite (SPS)

The concept of the SPS is very simple. It is a gigantic satellite designed as an electric power plant orbiting in the Geostationary Earth Orbit (GEO). It consists of mainly three segments; solar energy collector to convert the solar energy into DC (direct current) electricity, DC-to-microwave converter, and large antenna array to beam down the microwave power to the ground. The first solar collector can be either photovoltaic cells or solar thermal turbine. The second DC-to-microwave converter of the SPS can be either microwave tube system and/or semiconductor system. It may be their combination. The third segment is a gigantic antenna array. The power beam must be controlled accurately to less than 0.0005 degree. The SPS system is composed of a space segment and a ground power receiving site. The latter uses a device to receive and rectify the microwave power beam. The device is called rectenna (rectifying antenna). The rectenna system converts the microwave power back to DC power, and is connected to existing electric power networks.

In 2009 in Japan, ‘Basic Plan for Space Policy’ was published, on which ‘As a program that corresponds to the following major social needs and goals for the next 10 years, a Space Solar Power Program will be targeted for the promotion of the 5-year development and utilization plan.’ We need both technical advance and ‘surprise’ in the next space experiment based on the Basic Plan for Space Policy.

The SPS system has advantage in producing electricity with much higher efficiency compared with a photovoltaic system on the ground. Since SPS is placed in space such as the GEO there is no atmospheric absorption, the solar input power is about 30% higher density compared with the ground solar power density, and is available 24 hours (except for 70 minutes maximum during 42 days near the equinoxes as without affected by the weather condition. Solar flux is approximately 8 times higher in space than the long-term surface average on the ground if the insolation is 4 kWh/m²/day. For the terrestrial system, however, the efficiency, additional space due to the loss, and cost of the storage system should be taken into consideration in order to supply electricity 24 hours. This ratio would become higher depend on the efficiency since the 100 % efficiency is assumed in the storage system. The possible issues to be discussed on the SPS system are the microwave power beam impact on the existing communication networks and bio-bodies.

A huge and clean power source is to be developed for sustainable economic activities with a sufficient suppression of CO₂ emission. Only solar technologies can provide a huge and clean power source in the near future. The terrestrial photovoltaics, wind, geothermal, and other natural resources depend on the environmental conditions and are neither stable nor sufficient.

In the over-all SPS System, the output of the photovoltaic cell panel is converted to microwave, transmitted to the ground rectenna system, and converted back to DC. An aperture of a microwave transmitting antenna array can be designed with freedom of parameters such as the microwave operating frequency and the antenna element spacing. The dimension of the rectenna site on the ground is dependent on the transmitting antenna size and the beam (power) collection efficiency. Assuming 70% conversion rate in the space segment, 90% beam (power) collection efficiency, and 80% conversion rate in the ground segment, the estimated over-all efficiency from DC (output of the solar panel) to DC (output from the rectenna system) is approximately 50%.

The SPS space segment consists of solar cells, RF circuits and antennas, a sensor for the pilot signal, and a control unit for beam forming and retrodirectivity, and circuit power supply. The SPS system of 1 GW power plant has the following typical dimensions. The area of a solar cell panel is approximately 10 km² (2km x 5km) for production of 2GW DC power with the solar cell conversion efficiency of 15%. For the transmitting antenna array, one typical dimension will be 1km in diameter. According to the request of the beam collecting efficiency, an aperture distribution of the transmitting antenna is determined such as uniform profile or Gaussian profile. Assuming the antenna element spacing of 0.75λ=3.8cm at 5.8GHz, the radiator weight density of 2.69g/cc and 160 antenna elements, one could get 9.6 kg/ m² with this design approach.

In 2007, the IPCC (Intergovernmental Panel on Climate Change) concluded in the Fourth Assessment Report there is no doubt that human activities have caused the global warming. They estimate surface air warming in the 21st century) Best estimate for a “low scenario” is 1.8 °C with a likely range of 1.1 to 2.9 °C, 2) Best estimate for a “high scenario” is 4.0 °C with a likely range of 2.4 to 6.4 °C, 3) A temperature rise of about 0.2 °C per decade is projected for the next two decades for all scenarios. They conclude that we have to reduce the all long-lived greenhouse gases by 50%.

What is real cause of change of the global environment? Human activities or Human being itself? We have to consider the cause and solve the answer of social dilemma between the global environment and human activities. In this paper, we discuss a possibility of space environment use is most hopeful answer of the social dilemma. The SPS is best solution to reduce the CO2 emission and expand our sustainable humanosphere to the space.