When the sunlight passes through the atmosphere, only a small fraction of radiation reaches the ground:
- 30% is directly reflected into space,
- 20% is reflected by clouds,
- 6% is reflected by all different layers in the atmosphere,
- 4% by Earth.
Only 40% of sunlights may be used, about 350 W/sqm. Regarding these 350 W/sqm, between 5 and 15% may be converted into electricity, which provides a ratio between 17 and 52 W/sqm. Despite the current and future progress, this energy will therefore necessarily be limited.
Assuming that in the upper atmosphere, sun radiation is almost four times stronger than on the Earth, is permanent and does not depend on seasons or weather, several studies / projects are underway, to evaluate the feasibility of recovering these radiations beyond the atmosphere.
The International Academy of Astronautics, based in Paris, whose objectives are to promote the development of astronautics for peaceful purposes and to develop a series of studies and guidelines on many aspects of international cooperation (exploration and settlement of the solar system and beyond, space waste, small satellites, extra-vehicular activities, cheap scientific satellites missions, Lunar and Martian exploration…) has published a study whose findings indicate that "space programs could be implemented within 30 years, for the purpose of directly capturing solar energy from space, a cost-effective way to provide the energy needs of humans". The scheme developed by scientists around the world staged, orbiting power plants capturing sunlight before sending it back to the Earth. "This process would be technically possible within a decade or two, only based on existing technologies in laboratories".
The advantage of the system relies on the positionning of different satellites: orbiting above the Equator, they would receive a maximum and permanent sunlight (24h/24h). The principle is relatively "simple": putting into geostationary orbit satellites equipped with solar panels with adjustable articulated arms. Each solar panel should then retrieve the sunlight then reflect the captured energy to a master device. It would be in charge for converting energy into electricity and transmitting it to the ground thanks to a laser or a microwave antenna. Finally, ground equipments would be charged to recover these flows and to inject them into the grids.
According to John Mankins (past head of the concept at the NASA who led the study for the International Academy of Astronautics), "a pilot project, to demonstrate the capacity of this technology is quite possible thanks to low-cost launchers, currently in development". This statement should not obscure many issues to be solved in order to develop such a project, such as a management of space waste, a lack of targeted studies, final development costs, radiation, etc…
Two major projects are underway: an American project (developed by the NASA), which is moving towards a transmission to Earth with a system of microwaves and a European project (EADS / Astrium), which focuses on a laser beam technique to transmit solar energy to the Earth. Before placing the first devices in orbit, engineers will have to solve many technological problems but also financial, to be able to develop an economically realistic solution.
Safety is the main reason of concerns for engineers facing these projects, the words "laser and microwaves" scare public opinions…