Capturing the sun's power – an age-old dream for mankind. A dream that may be fulfilled, if not tomorrow then perhaps the day after. In any case, EADS Astrium Space Transportation has taken up the challenge and plans to have power plants in orbit in less than 50 years.
At first glance, the object that attracts attention in the greenish light of a huge hall at EADS Astrium Space Transportation in Bremen brings to mind a little radio-controlled toy car. Except that this innocuous-looking yellowish green object about 20 centimetres long, topped by a large silvery corolla, could presage a veritable revolution. It was developed on the initiative of EADS Astrium Space Transportation together with five young engineers at a Swiss start-up called FiveCo. The energy that enables it to move with ease at the rate of 2.6 centimetres per second is supplied not by a battery but by a laser located more than 250 metres away. At the centre of the corolla, photovoltaic cells convert the powerful laser beam into electric current to power the micro motors – and even a miniature camera.
In the future the energy source for the laser need not necessarily be the German power grid but the sun itself. With its Solar Power Initiative (SPI), which was presented in September 2003, and is supported by ESA and financed by the city of Bremen, EADS Astrium Space Transportation intends to demonstrate that the sun could be used as a gigantic power plant – with sufficient energy for several millennia.
The concept itself is simple. It involves placing in geostationary orbit a generator that would collect solar energy and transmit it in concentrated form to the Earth, where fields of photovoltaic cells would pick it up. Either of two – needless to say wireless – technologies could be used to transfer energy to the ground: microwave – the preferred option of Japanese researchers – or laser, the option chosen by EADS. "There are several reasons why we prefer the laser option," explained Frank Steinsiek, head of the SPI project. "The space structures necessary for this are 50 times smaller, and the laser beam allows greater energy concentration and avoids lateral dispersion over very long distances. There are also no negative effects on electronic communications or navigation systems in the vicinity. In addition, the effects a continuous microwave beam could have on the atmosphere are regarded critically.
Another remaining problem is directing the laser beam at its target. For two years, EADS Astrium Space Transportation worked on a solution in cooperation with the University of Kaiserslautern. The result is a technology which allows the laser beam itself to transmit position recognition data in addition to energy proper – rather like carrier frequencies of radio waves. The receiver is correspondingly equipped with sensors that continually determine its position in relation to the transmitter so as to ensure that the photovoltaic cell panel is always set at right angles to the laser beam. It is precisely this positioning function that the corolla on the miniature vehicle at Bremen performs.
Nevertheless, technical obstacles remain to be overcome before solar energy can be 'tamed'. Indeed, according to Hans-Jörg Heidmann, who is responsible for future projects at EADS Astrium Space Transportation, it will be at least half a century before we can expect to have one-gigawatt power plants stationed 36,000 kilometres above us.
By that time, the performance of the lasers has to be considerably improved and to move from five watts for the experiment in Bremen to several kilowatts and finally to the megawatt scale. In addition, the material must be suitable for transport into space. In 2012, for the first time energy could be transmitted by laser from space to the Earth, via the European Columbus module on the International Space Station (ISS). The basic technical principles – highly precise alignment of the laser with receiving equipment on the ground and the laser's suitability for use in space – are to be demonstrated in this way. In addition to the transfer of energy, the transmission of data, such as experimental data to the ground station, will be possible.
Alongside improving the laser's performance, the researchers also have to solve the problems posed by the negative effect of clouds. Laser beams are not particularly effective in a hazy atmosphere. One solution could be relay stations in the upper atmosphere (at an altitude of around 25 km): these would intercept the beam and direct its energy to Earth via either microwaves (not affected by clouds) or simply by cable! However, to avoid such difficulties, another possibility would be to install the receivers in meteorologically favoured regions of the globe like South Europe or North Africa. Incidentally, the photovoltaic cells of the receivers need to be much more sensitive for laser beams than for normal sunlight in order to bring the efficiency of the entire energy transmission chain up to 40 percent – which is comparable to nuclear energy. Only then are competitive production costs of five Euro cents per kilowatt hour possible.
As for the orbital generator technology, this largely remains to be developed – particularly energy collection technology. A 10-gigawatt generating unit would require a reflector with a diameter of 10km. The converter linked to this would have the twofold task of concentrating light energy and eliminating thermal energy – ultimately it is the sun's light, not its heat, that needs to be exploited. To do this, a complex system of radiators would have to be used to dissipate the heat.
But that is not all, since transportation of the equipment into space also has to become more cost-effective. On the basis of current designs, a 400-kilowatt laser weighs twelve tonnes. Even disregarding direct proportionality, extrapolation to 10 gigawatts results in mind-boggling figures. No less so is the prospect of transporting reflectors with a surface area of 78.5 km² to an altitude of 36,000 km – even if researchers solve the problem of developing materials that can be folded for transportation with a launcher. Achieving a production capacity of 500 gigawatts (with around 50 orbital units) would require hundreds of launches – which is unaffordable at present-day launch costs. For this reason, studies into new and reusable transportation systems, such as ESA's Future Launcher Preparatory Program (FLPP), could play an important role in the transfer of solar energy to Earth.
Meanwhile, deep inside its Bremen hangar, the miniature vehicle goes about its business unaware of the hope it has awakened among scientists and engineers.
A potential power generation scenario could be based on the combined utilization of natural solar energy on Earth and laser energy from the Earth's orbit. Using an existing solar cell installation with an existing capacity as the point of departure, two methods are conceivable as alternatives for boosting capacity:
On the one hand an enlargement of the solar cell surfaces, which is not entirely unproblematic from the technology and meteorology points of view, and on the other a retention of the existing cell surfaces, but with the natural solar irradiation of these cells being supplemented with laser energy from outer space.
An increase in the energy capacity would be achieved by additional orbital energy platforms being brought into operation, one after the other. The increased efficiency when converting laser energy into electricity would produce economic advantages if this technology were to be used on an extensive scale. The broadly fanned out laser beam from outer space does not pose a danger to human beings or other living creatures.
