A moonbase concept with a solar thermal generator. Art by Kevin Hand. Image (c) Popular Science.

Space-Based Solar Thermal Power
Tech Level: 12

These generators are sometimes informally called solar boilers. Using solar thermal power for propulsion is discussed in the article on Solar Boilers, which is linked to at the end of this article.

Solar Thermal power is just entering the mainstream on Earth, in the form of large power stations comprised of dozens to hundreds of gimbaled mirrors which focus sunlight on a container or pipes containing a liquid medium. The liquid in these pipes is quickly superheated, in the case of water maybe even turned into steam, and then used to drive electrical turbines. In space, water is usually assumed to be the medium of choice, because it is easily transported in the form of ice, and it can be readily found in quantity on comets and in deposits on asteroids and the moon. However, other liquids have been used in real-life solar thermal applications, including oil and molten salts, which tend to have superior heat retention qualities.

Space-based versions of this scheme occasionally see the light of day in various proposals for space stations and moonbases, as well as the occasional mention in science fiction. With the success of solar thermal power stations on the ground a reality for over 20 years now, the use of these systems in space now seems much more plausible. Different configurations for such power plants can be found in the links at the end of this article.

In the case of space-based energy, traditional photovoltaic solar cells can provide a steady supply of wattage over the long term, but are not so good at supplying power in large spikes, as may occasionally be needed for some applications. Nuclear reactors can do this, but they are heavy and expensive, making their lift to stations and bases problematic. They can also become major safety hazards in the case of a breakdown. High-performance batteries and capacitors can provide large amounts of power at once, but may require long recharge times.

Solar thermal generators can fill this niche comfortably, however, by providing large bursts of power steadily with minimal long-term safety concerns, and relatively cheaply. Mirrors, pipes, and water pumps will likely remain more economically viable than plutonium or advanced composite flywheels for the foreseeable future.

The major change between ground-based solar thermal and its space-based cousin is that vacuum-environment solar boilers have to be a completely closed system. On Earth, water can be readily resupplied from outside sources. In space, the need for the water recycling to be as close to 100% as possible is paramount.

This means that after the solar-heated steam is used to drive the turbines, it must be re-condensed into water and cycled back into the pipes to be returned to steam once again. Vacuum environments do have a resource that can be of great help here: shadows.

Without an atmosphere to evenly distribute the heat, the temperatures of objects in shadow can plunge to hundreds of degrees below zero. Steam fed into pipes that pass into shadow in space will quickly recondense back into water. Specific shade units can be built around the pipes, or engineers can make judicious use of station or base design to provide the shadows instead.

The solar boiler scheme does have a number of disadvantages, such as its many complicated moving parts and the high-pressure cycling system that would need constant monitoring and maintenance.


In Print:

"The Green Side of the Moon" by Gregory Mone, Popular Science, January 2008, p.28

On The Web:

Solar Thermal Propulsion:

Earth-based Solar Thermal Power:

Article added 1/03/08