At Earth's distance from the Sun, approximately 149,600,000 km, available sunlight is about 1366 Watts per square meter. Any Solar Power Satellite (SPS) has to be very large to collect significant amounts of energy, which it can then beam to Earth or another spacecraft via microwaves. An SPS closer to the sun would collect more energy, but microwave transmission uses long wavelengths and wouldn't work beyond geosynchronous orbit distance. Shorter wavelengths can be transmitted further. So what if we used optical wavelengths, or simply focussed the Sun itself into a beam, instead of allowing the Sun's rays to disperse? A lighthouse uses this idea, turning a normally diffuse light source into a powerful beam of light, using a Fresnel lens.

A Fresnel lens made of quartz would have a melting point of 1956 deg.Kelvin, but might begin to soften at temperatures as low as 1773 deg.K. Quartz also has a transmittance of about 90 percent, meaning that 10 percent of the sunlight might be absorbed and contribute to heating. It's emissivity is about 0.99, meaning that it will radiate that heat slowly. So how close can a Fresnel lens made of quartz get to the Sun before it melts?

0.1 * P = e * B * T^4

where,
P = total solar irradiance (Watts per square meter)
0.1*P = total radiation absorbed
e = emissivity
B = Boltzmann's constant
T = melting point of material.

This yields a value for maximum allowed irradiation of P = 5,546,936 Watts per square meter. Knowing that sunlight gets stronger closer to the sun, via the inverse square law of radiation, we realize then that the fresnel lens cannot be closer than 2.3472 million km before it gets too hot. That's about 5.868-times the radius of the Sun, or about 1/24th the distance from the Sun to the planet Mercury!

At this distance, the quartz Fresnel lens would transmit 90% of the Sun's rays, or about 4,992,243 Watts per square meter. To generate a one-Terawatt beam of sunlight, equal to 1/14th of the whole world's power consumption, would require a Fresnel lens about 505 meters in diameter!

Construction of such a device would require ALL control systems to be able to function normally above 1800 deg.Kelvin. Attitude control, gyros, actuators, populsion, maybe even onboard computers might all have to be constructed of ceramic materials (ceramic computer).

Now the catch: the focal point would be moving. The Fresnel lens would orbit the Sun; the target (SPS) would orbit the Earth; and the Earth would also be going around the Sun. In addition, different wavelengths would have different refraction angles and focal points, so there would be an aberration, or dispersed focus. Both the location of the focus, and its aberration would have to be controlled finely enough to allow collection of the amplified beam and conversion into RF for safe wireless transmission to Earth.

If it worked, such a system could increase the energy available at the focus, perhaps in the vicinity of Earth or the Moon, by 3500-fold or more!

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