• Lazyweb, I also want a Solar Energy Diode

    by  • February 19, 2008 • Cool Tools, Hexayurt • 4 Comments

    This is a bit more complex to describe. Here’s how it works.

    Take two disks of glass, one black, one clear. Let us say they are 4″ in diameter, of indeterminate thickness.

    Now take a ring, say 1″ thick, 4″ in diameter, made from a strong insulator. Separate the two disks with this ring, enclosing some air.

    Now evacuate the air, leaving a vacuum gap between the two glass disks, except at the edge of each disk, where the ring holds them apart.

    Now place this ring into, say, the top of a highly insulated box. Let us say the black side of the “Solar Energy Diode” faces into the box.

    Infrared energy will pass through the clear disk, hit the black disk, be converted into heat, and then trapped inside the box by the vacuum gap between the black and clear disk. In this way, incoming solar energy can be trapped inside of the box, with only a little energy escaping back through the black disk, vacuum, and clear disk into the surrounding area.

    In the converse case, the Solar Energy Diode is turned around, black disk out, making heat flow more easily in the other direction. In this orientation, black body radiation from the items in the box flows through the clear disk, through the vacuum gap, and then strikes the black disk, being converted into heat.

    An exercise for the interested reader: can a half-silvered mirror (a one-way mirror) be used to make this Solar Energy Diode more effective?

    Couple with a solar funnel to create an effective solar oven, or with a solar funnel, pointed at the night sky, with the disk in the “black-side-out” condition, to create an effective solar refrigerator.

    Who’s going to manufacture???

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    Vinay Gupta is a consultant on disaster relief and risk management.


    4 Responses to Lazyweb, I also want a Solar Energy Diode

    1. Troy
      May 20, 2008 at 4:34 am

      Hey there! I think the issue you’d run into here is the lack of thermal mass to store the heat. Even with the box extremely well insulated, I believe a box full of water would be able to store more heat than a box full of air. And since you’d ultimately want to do something with that energy, water is easier to move around than air (direct hot water, radiant heating, flash steam to drive a turbine, etc.)

      At that point you’ve essentiually got a solar thermal water heater. There are a number of people making them with evacuated tubes…something like this:


      I commented to my fiance this morning about how much I wish there were stacks of hexayurts ready to go in China right now. Not as much as you, I’m sure…

      I enjoy reading your blog…keep the great ideas coming!

    2. May 20, 2008 at 10:32 am

      Thanks. The problem with water is it would boil. Maybe heavily salted water? A box of rocks?

      I dunno.

      And, yes, on the hexayurts for china. Yes. They have plenty of factories making the right materials but the connection isn’t made yet.

    3. May 21, 2008 at 7:19 pm

      Transfer fluid does not have to be air or water. It could be a vegetable oil or other liquid which can withstand high temperatures. In the 1920s, Charles Greeley Abbot built a fine parabolic trough oven that used walnut oil as the transfer fluid. He was able to bake bread in the oven on top of Mt Palomar, CA.

    4. Troy
      May 22, 2008 at 4:24 pm

      Good point on the oils. I always like the relative simplicity of this design for home heating, that uses waste motor oil:


      Lots of the commercial solar thermal farms use some type of oil, and the homemade ones often just use water with radiator fluid added, or straight radiator fluid.

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