Solar Power from Space 
(SPS) - technical Card

We asked Dr. Bernasconi (responsible for research on space rigidizable and inflatable structures at Contraves - CH) to clarify some unclear points, on the topic of solar energy collection in space and of its transmission to Earth in the form of microwave beams. 


QUESTION. In order to answer the critic about the supposed danger of the SPS, experts say that the power of a microwave beam does not exceed 500 Watt/m2, while the sun irradiates more than 1000 W/m2. 

ANSWER. This is not exactly true. The power at the center of the microwave beam (according to DOE / NASA reference concept, and to the generally accepted agreement not to exceed these values), the intensity reaches 23 mWatt/cm2, i.e. 230 W/m2 and falls to 10 W/m2 at the edge of the rectenna. The solar constant in space is little less than 1400 W/m2. Due to reflections, atmospheric absorption, etc..., 700 W/m2 is a good maximum value for a cloudless sky, in the European temperate belt. 


Q. What does SPS energy have compared to that irradiated directly on the earth from the sun? 

A. It is monochromatic -- in the above example, the transmission is made at 2,45 GHz, i.e. at a wavelength little more than 10 cm, while the Sun emits on wavelengths from, say, 40 nm to 2 milimeters (and obviously beyond, but with negligible intensities in this context). It follows that the dipole field of the rectenna is "syntonized" with the transmission of the SPS and can convert the energy intercepted to current with an efficiency of 89%. 

Q. If the sun sends down, every blessed day, 1000 w/m2 of energy, is it not still more convenient - let's call it so - than the terrestrial photovoltaic?

A. Here we are confusing peak and average. A panel in orbit is always oriented and receives (except at eclipse) 1400 W/m2. Earth revolves around its axis and the maximum value (however inferior) is generally captured only at noon (without clouds). Also the photovoltaic cell works within a limited band, i.e. it works best at converting only a narrow band of wavelengths of light to electricity. If, in that bandwidth, the Sun pumps only 10% of its flow, the efficiency of the cell will not be able to exceed 10%, etc. Due to this fact, the most effective cells are in cascade, i.e. various overlapped materials (gallium, gallium arsenide, silicon...) so that each layer converts its slice of radiation. An evolutionary proposal foresees the use of millimetrical lasers, in order to transmit the energy from the appropriate photovoltaic SPS to be converted on earth, which would supply a little more energy, by converting the wavelength of the solar flow. In operation, the difference between rectennas and terrestrial photovoltaics is in dimensions (100 W/m2 vs. 10-20 W/m2 more or less) and in the thermal burden. 

To get energy for exosomatic or industrial use, it is associated with the introduction of an energy delta in the biosphere. Such a delta has two components: (i) the investment in order to realize power stations and to feed them with fuel and (ii) the limit of efficiency of the station. I add a third one, (iii) alterations of the albedo due to the plant itself. For instance, for a coal plant: 

  1. a value x in installed MJ/kW
  2. eta = 40%, i.e. a release of the 2,5 kW(th) for each useful kW (all the 2.5 kW must be included, since also the useful energy is eventually dissipated inside the biosphere) (source: David Criswell)
  3. ~ negligible, since the real density of such plants is very high (many kW / m2 of plant) 

Summing the three terms we arrive to an M relationship, that indicates the medium number of kW released by power plants in the biosphere for each useful supplied kW. 

In case of a photovoltaic system, not only (i) is somewhat high, but (iii) becomes very much large, because of the large surface necessary to obtain a useful kW. Also is to be considered the effect of the (often) great difference between the solar absorbivity of the original surface vs. the one of cells (> 0.8), the worst case being the desert. After that, the fact that the terrestrial photovoltaic converts solar energy that comes down naturally to the surface - and that does not burn coal or uranium - it becomes insignificant. For SPS: (i) is carried out, for the greater part, in space: only the rectenna demands an investment inside the biosphere; (ii) it is relatively small because eta = 75%, 1.3 kW/kW and (iii) is again small, in spite of a great used surface, because the rectenna consists of metallic batons that occupy a small percentage of the surface (etc... etc...).

As far as the possible danger, what it changes is the wavelength of the electromagnetic radiation. The level at the center of the beam was established according to safety norms for rectenna maintenance jobs, considering thermal effects. We also should consider, moreover, that, under the dipoles, the intensity diminishes by a factor 10, because the extracted energy... is no longer there!

Q. Does the SPS work when its cloudy? 

A. Sure, the Sun keeps on existing "behind clouds", and the microwaves pass through them.


Q. If I understand correctly, the SPS has an advanced yield, vs. terrestrial photovoltaics (and this justifies the greater expense to put a solar power station in orbit), essentially because it collects a continuous 1400 W/m2, and it sends this to Earth with an efficiency of nearly 90%. Moreover, reading again some of your papers, on this topic, I see that the construction materials of SPS plants could be of lunar origin. We would therefore have, from a terrestrial, environmental point of view, a very much lower weight: fewer installations on earth, for the same resultant energy, and lower weight on the budget of the Earths resources. But we are pushing, here, a little beyond the timid horizons of current space policy? As far as energy demand, whichever "solution" that predicts the reduction of consumption, as we have already analyzed many times, is in fact opposite to the continuation of the human development. It is however reasonable to take as a goal to get the necessary energy for the development of all humans, with the least possible pollution, and using the least possible resources of the Earth. Do you think that Space Photovoltaics totally match these requirements? 

A. The average consumption of energy per capita in post-industrial countries, in the '90s, is approximately 7 kW. Considering that, with the advent of the SPS, the entire economy would be converted to the electricity, the consumption per capita would go down around 3 kW. 
Notes: the 1998 yearly energy statistics of the European Commission reports a world-wide total production, in 1996, of 9234,4 Mtoe (Million Tonnes of Oil Equivalent). 

On the argument of the 21 century energy requirements, the following documents (amongst others) are available online: 

[MCB - TDF 2/2001 - 06/05/2001]
[The English version was revised by Ben Croxford]