Notes from Windward: #68

Adding a Spun Parabolic Collector

setting up for some winter work

     As the fall rains drive home the point that winter's not far away, there are a handful of tasks that we're attending to so that we'll be able to continue our research through the winter months in spite of the inevitable snow and ice--work that goes well with a hot cut of tea by a toasty fire.

     While we're working with a variety of energy systems, our ultimate goal is to generate the bulk of our electricity using solar generated steam. The sun shines at an intensity of about a kilowatt of energy per square meter here, and being on the east side of the Cascades, we get about 200 clear days a year. But in order for all that solar energy to be useful, we need to be able to concentrate and collect it.

Lowering the pole and concrete into its hole

     There are a number of ways to do that, but given the delight we take in finding and reusing things other people view as waste or junk, one of our favorite resources are the obsolete C-band satellite dishes that still dot the countryside. Almost all satellite systems now use K-band frequencies in which the signal's wavelength is a third of that of C-band with the result that people can use the smaller dishes you see sprouting off of houses like some sort of modern mushroom. Since people spent a lot of money buying those three meter wide dishes, they're often reluctant to scrap the now-obsolete dishes, so we're enjoying developing a way that they can be reborn as solar collectors.

     To make the horizontal parabolic collectors that will be the work horses of our solar system, we're using the more common type of C-band dish--the type that has ten arms supporting an aluminum mesh. But now and then one comes across a dish made from spun aluminum. Since these can't be disassembled, we're looking at ways to use them to track the sun as a spherical collector. That's tricky because whereas a horizontal trough collector will peak out at about a 500°F, the spherical dishes can generate temps three or four times that--enough to melt whatever you put at the focal point into a puddle of smoking used-to-be.

using rocks to wedge the concrete into position

     The upshot is that incorporating one of these dishes into our solar steam plant will require mating it with one of the Butterfly control computers. The dedicated mini-computer will point the dish at the sun in order to heat the heat-transfer-fluid circulating through the heat collector mounted at its focal point, and then control the temperature by steering it slightly to one side whenever the collector starts getting too hot.

     One of our neighbors donated a spherical dish in support of our research, so we went over with the backhoe to remove the pole along with the mass of concrete that secured it in the ground. We could have invested a good deal of energy into busting up the concrete in order to reuse the 3" mounting pipe, but elected instead to just dig a larger hole roughly matching the shape of the concrete and secure that with a few more wheelbarrow loads of concrete.
removing the paint from the dish surface
in preparation for polishing

     One of this winter's key projects involves getting the Butterfly control computer up and running lots of small tasks such as tracking the sun. While December is a transition month with lots of wet, foggy days, we actually get a respectable number of bright, sunny days in January and February. While the days are short, the winter sun is still an important source of heat--a dish of this size will be able to capture between forty and fifty kilowatts of heat a day, even when there's a foot of snow on the ground.

     With the mount in place, we'll be able to mount the dish and work out the software needed to add another power source to our energy quilt.

Notes From Windward - Index - Vol. 68