Solar Power

An attractive source for power on the Moon is the Sun. Solar is particularly inviting in the polar regions where mountain tops are available that have solar views 75% to 99% of the time. The sun light is not diminished by an atmosphere and is never blocked by clouds. It is harsh and unrelenting.

From the surface of the Moon, the Sun appears to move slowly across the sky making a full cycle every 29 days. On rare occasions it is eclipsed by the Earth for a few minutes.

During the local summer at the poles, the Sun simply circles the sky very near the horizon. Occasionally, it may be blocked by mountain tops or local instillations such as other power collectors. During the dark local winter at the poles, a sunlit region is never more than 30 miles away and those special sunlit mountain tops are much closer.

Near the lunar equator, the Sun is visible only one half the time. The 14 day lunar nights are a major problem for power generation and are a major factor in setting a polar location for the first lunar settlement.

There are basically two types of solar power, Solar Dynamic (SD) and Photovoltaic (PV).

SD uses a heat cycle to drive a piston or a turbine which connects to a generator or dynamo. Two popular cycles for Solar Dynamic are Brayton Cycle or Stirling Cycle. Solar Dynamic systems employ a large reflector to focus sunlight to a high concentration to achieve a high temperature for the heat cycle to operate at highest possible efficiency.

PV uses semiconductors (e.g. Silicon or Gallium Arsenide) to directly convert sunlight photons into electric potential. Commonly know as “Solar cells”

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Lasers versus Microwaves

The ideal frequency to use for lunar power beaming has not been definitively established. Microwave frequencies of 2Ghz and 5GHz have been proposed. But lasers are an alternative. In the lunar environment it is a vacuum, so absorption by atmospheric water vapor and other gases are not a factor, allowing greater choice of frequencies than for beaming power down to Earth.

Pros and cons:

Rectenna versus PV array

Microwave rectenna converts microwave to DC at about 90% efficiency, compared to about 50% efficiency for PV conversion of laser to DC.

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Thermal Problems

The laser method suffers from major thermal problems versus microwave. Rejecting the large amount of waste heat will require large heavy thermal radiators which will add significantly to the launch costs.

Transmitter Antenna Problems

Microwave antenna, depending on frequency, will need very large aperture transmitter antenna, and even larger diameter rectenna array. A laser would be much smaller and more compact, and so would the receiving PV array

Overall Comparison

At this time no definitive work has been published to show whether the larger radiators of the laser system would outweigh the larger antennas of the microwave system.

Lunar Solar power

Once lunar development is under way, one potential major lunar export could be lunar-produced energy. With in-situ manufactured solar panels on the lunar surface, more power could easily be produced than is needed for lunar activities; this power could be beamed via microwave or laser systems to where it's needed in near-Earth space, or to Earth itself. Such a system has been described in detail by David Criswell as the solution to Earth's future energy problems.

Advantages of the Lunar Solar Power System:

No need to launch massive solar panels into orbit from Earth or Moon once the initial lunar manufacturing is established; only minimal launch requirements to build the system out.
Provides stable base line for synchronizing and precisely targeting power beams
All the usual advantages of solar power satellites, over normal Earth sources of energy

Disadvantages:

the lunar surface rotates under the Sun, just as Earth does, so the LSP solar panels would only be in sunlight for half the lunar month.
significant technology development is needed on automating production of solar panels from lunar soil, and related space manufacturing issues.