Talk: Parabolic solar reflectors

== Paraboloid With Coincident Focus and Centre of Mass ==

Paraboloidal reflectors used as solar concentrators have to turn at 15 degrees per hour to follow the sun's motion in the sky. Ideally, the focal point, where the cooking pot or other energy collector is located, should not move. This means that the rotation axis must pass through the focus. If the rotation is driven by a low-power machine such as a clock, only a small torque is available. In order to rotate the reflector with this small torque, its centre of mass should be on the rotation axis. These two requirements can be simultaneously met if the paraboloid is such that its centre of mass and focal point coincide.

If the paraboloid is axially symmetrical and is made of material of uniform thickness, this coincidence occurs if the depth of the reflector, measured along the axis of the paraboloid from its vertex to the plane of its rim, is 1.8478 times its focal length. The radius of the rim is 2.7187 times the focal length. (The closeness of this number to "e", the base of natural logarithms, is just a chance coincidence, but can be a useful mnemonic.) The angular radius of the rim, seen from the focus, is 72.68 degrees.

If the cooking pot is held at the end of a fixed arm that enters the reflector along the line of the paraboloidal axis at noon, the rim of the reflector, which is rotating at 15 degrees per hour, will not strike the arm for more than four hours before and after noon. The device can therefore be used from before 8 a.m. until after 4 p.m. without any need for re-adjustment. Usually, this includes the whole part of the day when the sun is high enough in the sky for solar cooking to be practicable.<br />

Further information can be found in the [http://solarcooking.wikia.com/Focus-Balanced_Paraboloidal_Reflector Focus-Balanced Paraboloidal Reflector] page of this wiki.

[[User:DowenWilliams|DowenWilliams]] 22:24, May 25, 2010 (UTC) David Williams