A picture is worth a thousand words: please open the first two links below, «Solar Tower» and «Rectenna», to view them side-by-side. (Right-click “Open in New Window”.)

The Solar Tower is a proven technology: a 195 meter tall version had been running successfully in Manzanares, Spain for years. To understand how the Solar Tower works, please view the animation of the one kilometer tall version to be built in Australia, at the «Solar Tower Video» link below.

The Rectenna is the Earth-based collector for the microwave laser (maser) that will beam down the energy collected by the Solar Power Satellite stationed in geostationary orbit. Essentially, a rectenna is a metal grid made of dipole antennas interconnected with diodes to direct the current. It is highly efficient, converting about 85% of the microwave radiation into DC power. The remaining 15% is converted into heat, and in the NASA version, it is dissipated into the atmosphere.

In a nutshell, what I propose is to put the rectenna grill just under the greenhouse glass of the Solar Tower: the glass being transparent to both sunlight and microwaves. Not only would that cut the collector area in half, but the waste heat of the rectenna would add energy to the convection-accelerated air already being forced through the Solar Tower turbines (synergy).

Although 100% of the microwave radiation is absorbed by the rectenna grid, more than 90% of the ambient sunlight will go on through; allowing, as NASA has proposed, the area underneath the grid to be used for agriculture or pastoral purposes. This is because the optimum frequency for the maser has been determined to be 2.45 GHz – which is an 8.1 meter wavelength; meaning the dipole antennas of the rectenna can be spaced far apart, indeed: about ¼ wavelength.

The most controversial aspect of Solar Power Satellite design has been the potential danger from the microwave power transmission laser. However, it has been well studied and documented from at least the time of Gerard O’Neill’s “The High Frontier” (‘70’s), and the current scientific consensus is that the danger is highly overblown and within acceptable limits. See the «Solar Space Satellite» link below for a synopsis (chapter 4.2: Safety).

I realize that this is an obvious idea, and I’m surprised that I can’t find it mentioned anywhere else. But in order for it to catch on, it has to be mentioned somewhere first…so why not the Halfbakery?