Solar concentration mirrors in the outer solar system

From: wlm (wlm_at_EugeneFreenet.org)
Date: 06/19/04 

Date: Sat, 19 Jun 2004 04:41:50 -0000

Whenever settlement of the outer solar system is discussed in this
ng, it's always assumed that it will be done using fusion power to
provide energy, with an "edge of sunlight", beyond which solar
photovoltaics which cannot be used, around 3 A.U.

The edge of sunlight, commonly put at 3 A.U., could possibly be
extended to hundreds of times that by using extremely thin mirrors to
concentrate the attenuated solar light. The mirrors could be aluminum
a few tens of atoms thick, and an array of flat mirrors could be aimed
with a support structure to focus this for useful solar energy and/or
light.

To supply 1 gw. of electrical power at earth orbit using the best
photovoltaic cells available (~ 30% conversion efficiency), an
area of 2.38 x 10^6 square meters is required. For getting the
same electrical power at distances further from the sun, an area
of mirrors is required amounting to

   Area = A_e x Distance^2 / E

where

   A_e = area at earth orbit = 2.38 x 10^6 m2
   Distance = distance in A.U.
   E = efficiency factor based on the reflectivity of the mirror,
          and how well the light reflected hits the photovoltaics
          due to possible aiming problems or mirror deformation.

and

   Mass = Area x T x M_sp x Factor_s

where

   Mass = entire mass of mirror system (kg)
   T = thickness of mirror (m)
   M_sp = mass density of mirror material (kg/m3)
   Factor_s = factor for extra mass required for the support structure
                 for the mirrors.

Using an efficiency factor of 50%, a thickness of 40 nm, a mass
density of 5 gm/cm3 (= 5000 kg/m3) (I don't know the density of
aluminum off-hand so I used this), and a support structure factor of
2, one gets

                              distance area required mass required
                              (A.U.) (m2) (kg)

orbit of Pluto 35 5.83 x 10^9 2.33 x 10^6

current outer
edge of the Kuiper belt 70 2.33 x 10^10 9.32 x 10^6

furthest orbit of Sedna,
inner edge of the Oort cloud 900 3.86 x 10^12 1.54 x 10^9

These could be made from chunks of aluminum only 10, 16, and 83 meters
across respectively.

Problems

   erosion of the mirrors by dust and meteorites
   damage by the solar wind and/or cosmic rays
   reflectivity being obscured by dust particles or
      organic matter collecting on the surface
   solar sail effect which makes the mirror system
      (or what it is attached to) move
   manufacturing the huge mirrors in the first place

Solutions?

   Give the mirrors a static charge so they might have net mass
   collecting capability instead of mass loss.

   Attach the mirror system to its space settlement and orient it so
   that the settlement slowly spirals down toward a more desirable
   location closer to the sun.

Other possibilities

   Instead of using mirrors, use photovoltaic cells themselves.
   Assumes an extremely cheap way of manufacturing them of course.

   Have mirrors with a reflector made from organic materials
   (if there are any), which would make it lighter than an aluminum
   one and from more common organic elements. Or make a concentrating
   lense instead of a mirror.

Have there been any studies done on this subject? I think it's very
worthwhile looking at, given how much time and trouble we've had
getting controlled fusion power to work. Also a space settlement out
there in the Kuiper belt might want to save their deuterium for more
useful things like propulsion, self-defense, etc.

-------
WLM
wlm@EugeneFreenet.org
(in order to get the correct address, make "EugeneFreenet" into "efn")

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