Space-Based Solar Power

Introduction

Fig. 1: Laser System with multiple receivers. [3] (Courtesy of the DOE)

Every hour, 4.3 × 10²⁰ J of energy from the sun reaches the Earth. Consider this in comparison with humanity's annual consumption of energy: 4.1 × 10²⁰ J. [1] More clean, renewable energy we could ever hope for resides in the giant orange star above. That said, 30% of sunlight energy is reflected by Earth's atmosphere. In conjunction with clouds and night time, this presents a problem for terrestrial solar panels. One way to circumvent these issues is to place our solar panels in space. As it is understood today, at a high level, satellites, equipped with solar panels, can collect energy from solar rays, store it in solar connectors, and beam it toward Earth.

Space-based solar power, though, is not a new concept. Dr. Peter Glaser proposed an idea in this area back in 1968, obtaining for a patent regarding methods of conversion of solar radiation to electrical power a couple years later; he wrote about the potential for transmitting microwave energy to Earth from a satellite system from space. [2] Why then don't we see space-based solar power in practice today? Well, a modicum of common sense will tell us that such systems are entirely impractical as they are understood today. Moreover, we will show, through a numerical investigation, the pitfalls, in terms of cost and danger, of laser-transmitting solar satellites.

Microwave Tangent

The focus on laser transmitting solar satellites only makes sense in the context of other systems. Another popular option is microwave transmitting solar satellites. While microwave systems obtain better efficiencies, they also require larger receiving equipment on land. The satellites are set up so that sunlight reflects off mirrors into its center, where the light is converted to microwave energy beamed to Earth. The terrestrial system consists of a rectified antenna capable of receiving and converting microwave energy into DC electricity. [2] The problems with this system are plentiful: production costs can reach the tens of billions, it will require dozens of launches, and the distance makes repair difficult, to name a few.

Technical Details

With that in mind, we shift to our alternative: laser-transmitting solar satellites. The appeal of laser systems is that they are much smaller, cheaper, and closer to Earth than microwave ones. However, they require very accurate beaming. The diffraction spo size d associated with a light wavelength λ, focusing lens diameter D, and a distance to the earth F is

Adjusting d upward to about 5 m to account for practical jitter, we obtain a required pointing accuracy of Δθ = 2 d/F = 2 × 5 m / 4.0 × 10⁵ m = 2.5 × 10^-6. [3]

Note that the parameter choice of D=400km means the beaming system operates in Low Earth Orbit (LEO) as opposed to Geosynchronous Orbit (GEO). In this way, the laser beam travel distance is 90 times smaller, and also that using the SpaceX Falcon 9, the permissible payload size is doubled. In terms of weight, we consider five major subsystems: solar reflector, solar collector, packaging container and utilities, diode pumped laser system, and focusing and beam director system. Respectively, these weigh:

The SpaceX Falcon 9 payload is 10,450kg, so the proposed laser system falls within its constraints. [3]

Let's now consider collection and conversion. The diode pumped laser mentioned above is 50% efficient. Its terrestrial system relies on multiple receivers (see Fig. 1) and a molten salt power generator capable of 70% efficiency. The product of these two gives an overall efficienc of 35%. So what's the catch?

Cost

It's no surprise that hoisting lasers in space to power the Earth is an expensive feat. However, only by examining the constituents of the system may we appraise it with confidence. The subsystems of interest are: launch vehicle, solar reflector, solar collector, packaging container and utilities, diode pumped laser system, focusing and beam director system, and ground receiver and power generation system. Respectively, these cost in millions of dollars:

This $5.0 × 10¹¹ is just for deployment! [3] Billions of dollars, at the least, are required for research and development toward a successful deployment. Therefore, it's hard to imagine such a product as a viable venture.

The Reality

Let's examine our technology in an extreme case: funneling the energy budget of the Earth (6.0 x 10²⁰ J y^-1) into the proposed laser spot size above. [4] We have:

6.0 × 1020 J y-1 365 d y-1 × 24 h d-1 × 3600 s h-1 × π × (0.1 m)2

=

6.05 × 1014 W m-12

This is 400 billion times the power density of solar radiation (1361 W m^-1). [5]

Imagine birds unknowingly intercepting these beams: POOF! What's more, clouds in the sky render it useless. And that's not even the worse part. Without realizing it, we've proposed a space weapon - one capable of destroying civilization as we know it. Such a power system is reminiscent of a doomsday device, or one of Dr. Doofenshmirtz's "inator" contraptions from the series Phineas and Ferb. It certainly doesn't belong above the Earth.

Conclusion

Overall, implementing this technology sounds like a great idea ... so long as costs come down by the tens of millions, there are no clouds in the sky, nothing flies in the skies, and no one with malicious intent sets their sights on it.

© Medhanie Irgau. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] N. N. Lewis et al., "Basic Research Needs for Solar Energy Utilization," U.S. Department of Energy, April 2005.

[2] J. L. Caton, "Space-Based Solar Power: A Technical, Economic, and Operational Assessment," U.S. Army War College, April 2015.

[3] A. M. Rubenchik et al., "Solar Power Beaming: From Space to Earth," Lawrence Livermore National Laboratory, LLNL-TR-412782, May 2009.

[4] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.

[5] S. K. Solanki et al., "Solar Irradiance Variability and Climate," Ann. Rev. Astron. Astrophys. 51, 311 (2013).