Constraints on solar power satellites are more ground-based than space-based, says study

Space-based solar power has been gaining more and more traction recently. The recent success of Caltech’s Space Solar Power Project, which demonstrated the feasibility of transmitting power from space to the ground, has been matched by a number of pilot projects throughout the world, all of which are hoping to tap into some of the almost unlimited and constant solar energy that is accessible up in geostationary orbit (GEO).

But, according to a new paper published in Acta Astronautica by a group of Italian and German researchers, there are plenty of constraints on getting that power down here to Earth—and most of them are more logistical than technical.

The goal of the paper was to attempt to calculate the maximum amount of power that could possibly be delivered by a constellation of space-based solar power (SBSP) satellites in geostationary orbit. To perform that calculation, the authors broke it down into two simple steps: how many satellites can there be in orbit, and how much power can each satellite effectively feed into the world’s electrical grid.

To calculate how many satellites could exist in orbit, the authors again broke the question down into four different “scenarios,” with tighter restrictions for each one. The only consistent constraint across all the scenarios was the Minimum Distance Angle (MDA), which is the calculated minimum angular separation between the satellites that would ensure there are no collisions or radio interference between the satellites. Granted, the MDA number they picked (.1°) already represented a relatively conservative estimate, with each satellite receiving an “aperture range” of 147 km, more than 10 times the size of the satellites themselves.

In the first scenario, they simply calculated how many could fit in orbit assuming no other constraints—that number was simple to calculate, given the .1° angle and 360° of a circle—in the first scenario, there could be 3,600 SBSP satellites in operation in GEO. The second scenario, which makes a lot of intuitive sense, is slotting in SBSPs around the satellites that already exist in GEO, giving them the same berth of that MDA clearance. This drops the number of potential satellites down to 2,509 satellites.

In the third scenario, Earth’s surface starts to come into play. To receive power from SBSPs, there must be a receiving antenna to collect the microwaves sent by the satellite, which is typically called a rectenna. The authors note that, at least for now, rectennas must be placed on land, not the ocean, and that land must be located within 30 degrees of the equator, where satellites sit above while in GEO. A further constraint is that the beam size itself must necessarily be larger at higher latitudes away from the equator, therefore requiring more land area for a rectenna in those spaces. With this constraint, the total number of SBSPs is limited to 1,771 stations.

As a final constraint in the fourth scenario, they limited the placement of rectennas to areas where human development has increased the population density of 3,000 people per square kilometer. The authors argue that this is a good proxy for having the necessary electrical infrastructure to capture and transmit the power beamed by the satellites. With this additional constraint in place, the number of potential satellites plummets to 364—89.9% lower than the original scenario.

Even with all those constraints, the satellites can still output plenty of power. Calculating their total power output is the same as doing so for their counterpart solar cells on Earth, and requires the area, the solar cell’s efficiency, the angle of incidence, and the amount of irradiance they are subjected to, which is relatively constant in GEO. The authors assumed a 10 km² area for the solar panels and an efficiency of 20%, both of which are reasonable assumptions given current technology. However, they then drastically decrease the amount of power calculated from what they are capable of collecting (272 GW per station) compared to what they estimate would be delivered to the grid with all of the losses during conversion and transmission (1 GW per station).

Even with that dramatic reduction, and even in the worst case restrictive scenario, the amount of power provided by these satellites is enough to cover 3% of the total global power usage. Certainly not a huge amount, but enough to make a significant difference in energy markets.

And, to be fair, all of the assumptions in the paper are extremely conservative. Disclaimer—I’m a huge SBSP fan, so I might be biased, but assuming a .3% conversion rate from the power collected in space to the power delivered to the grid is a huge leap with not a lot of justification for that large of conversion losses. Additionally, other assumptions, such as 10x the size spacing for an orbital clearance path and the assumption that rectennas can’t be built on water, or even in farm fields, with the appropriate infrastructure makes the case for SBSP much worse.

But, even with all those assumptions and constraints, the fact that this technology could one day provide even as much as 3% of the world’s electricity needs is impressive. It’s no wonder why so many companies and countries are looking more into it, and the authors certainly have the right idea by trying to quantify what the eventual benefit to humanity of SBSP might be.