Ann Vole (annvole) wrote,
Ann Vole

Heliostat trough solar-heat collector idea

A heliostat is a mechanism designed to track the movement of the sun so the sun's rays always point to the same spot. This allows for a concentrated spot for solar heating which is needed to get the most amount of heating with the minimum of materials or the minimum of heat loss from exposed hot parts of the collector. The sun's angle changes vertically only a little each day due to the tilt of the earth but the angle changes drastically in the horizontal every day from east to west. A trough collector uses a parabolic mirror to point at a line rather then a single spot and then a pipe is placed on that line to gather the heat. This means that the heliostat only has to move the trough collector in one of the two angles. I figure that the smaller seasonal movement is a better thing to track because if the heliostat breaks down, it will still be close enough for a few days so I choose a horizontal trough collector. Most trough collectors are "U" shaped mirrors but they focus fairly precisely to the line so the mirror must be moved. If you move the mirror further from the focus point, the mirror gets closer to the bottom of that "U" and thus closer to being flat and not focusing the rays as much. This could be enough that the pipe can be moved instead of the mirror. This could allow for a giant wall of mirror that is fixed in place to focus on a pipe that moves a bit though the seasons. The problem happens when the sun is on an angle, the focus point now moves closer to the mirror so the pipe may have to be moved closer to the mirror in the mornings and evenings and further away at noon. Wind can be a problem for large walls so I thought of designing the wall like a Fresnel lens. A Fresnel lens is a lens that is broken up into circles so the excessive material between the two outside angles of the lens can be removed making the lens relatively flat and light compared to a normal lens. Instead I want to make the trough of mirror to be in strips of flat mirror that are aimed to the right angle but each strip is stacked directly on top of each other so the wall can be vertical and the breaks in the mirror where the angle changes can provide vents for the wind to pass through reducing wind pressure on the wall of mirrors. I figure with three pipes, pre-heat ones on top and bottom and main heat collecting one in the middle, temperature sensors (likely thermocouples) on the four points in the system of pipes (beginning, end, and the two connections between the three sections of pipe) can tell me if the pipe is too close or to far (comparing heat gain of outside pipes to inside pipe) or if it needs to move up or down (comparing the heat gain of the top pipe compared to the bottom pipe)


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