I just dropped the price of
the permaculture playing cards
for a wee bit.

 

 

uses include:
- infecting brains with permaculture
- convincing folks that you are not crazy
- gift giving obligations
- stocking stuffer
- gambling distraction
- an hour or two of reading
- find the needle
- find the 26 hidden names

clickity-click-click

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Jim Lea
Posts: 114
Location: Southern Sierra Nevada's
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I have remember that as sun passes through glass (as in a solar box oven) the wave length changes in such a way that when it bounces off the bottom of the box it cant easily escape back out of the glass. Trapped to some extent.
Here is my question to anyone who might know...
Many plans for solar heat use a double or triple pane system to "insulate" the inside to bump up enternal temps. Well insulated houses do the same thing. Makes sense. But as the sun passes through the first pane it changes wave lenght right? How does it make it through the 2nd and 3rd pane to heat the box? I know it works, I'm just looking for an explaination.
Anyone know?
Jim
 
Bob Ramlow
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Location: Amherst, Wisconsin
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During the early days of the modern solar water heating era many different collector designs were tried. Some were double glazed. Testing showed that in nearly every case that it was not worth it. The only case where double glazing is worth it is in batch heaters. Batch heaters are simply uninsulated tanks inside an insulated box with glass on one side. In these collectors the domestic water is heated and stored in the collector. To minimize heat loss, double glazing is used. On regular flat plate collectors we do not have to have super insulation because the heat is not in the collector very long. The sun shines in, the absorber plate gets hot, the solar fluid takes the heat into the building right away and they work at very high efficiencies.

Bob
 
Jim Lea
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Location: Southern Sierra Nevada's
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Great explaination Bob. Thanks for taking the time to do so.
Do you have any idea if it is worth it when building a solar oven. I live totally off grid and would benifit from using an oven to save on propane.
If there is little benifit it would save some$ in construction costs.
On another note, Im looking into hydronic for home some heating leaning toward a drip down system as freezing is an issue here. Should be a fun adventure. Other heating will be Rocket mass (if your interested) Can't wait.

Thanks again,
Jim
 
Jim Lea
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Bob! Didnt realize who responded to my post. Welcome.
 
Bob Ramlow
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Same deal with the solar oven, single glazed is fine.
 
Jim Lea
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Thanks Bob. I was not sure if the higher target temps would make a difference.
Jim
 
Morgan Morrigan
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Location: Verde Valley, AZ.
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there is a new technique that uses offset reflective glazes to trap, and reflect light to bounce around inside for electric cells, would prob work for thermal too.

think of mirror strips, about half inch wide, facing to the inside, and offset so that they optimize for non zenith trapping.
 
Jim Lea
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Interesting Morgan, any links you can share?
Jim
 
Morgan Morrigan
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Had to find it at home, this appears to be a new site, old one was prettier...

http://www.prismsolar.com/technology.php

(old header)
Prisms Unique HPC Film

Prism Solar HPC (holographic Planar Concentrator) FilmHPC Film is the core component of Prism Solar’s Holographic Planar Concentrator™ (HPC) technology, which enables solar modules to generate the same yield as conventional modules while using 50-75% less photovoltaic material.

Reducing the amount of expensive silicon in a solar module greatly lowers material cost. It also results in manufacturing savings through reduced assembly and processing requirements.

HPC Film consists of several gelatin-on-PET plastic layers. Each film is imprinted with a holographic optical element. Together, these elements diffract the wavelengths (400-1100nm) that can be converted to energy by solar cells.

HPC Film diffracts usable wavelengths of sunlight to increase efficiency of solar cells

Prism Solar panel employing HPC FilmHPC film diffracts usable wavelengths of sunlight and guides them to the solar cells.

The usable energy is guided to the cells, resulting in up to a 3X concentration of energy per unit area of photovoltaic material.
and



Waveguide tech
http://www.nature.com/ncomms/journal/v2/n10/full/ncomms1528.html



From an Ars Technica article

Solar energy is looking better and better in the light of public reaction to Japan's recent travails. Current technology works, and it works pretty well. But to get vast amounts of power, you need to cover a fair patch of terrain in solar cells. This is because the conversion efficiency of solar cells is not even close to the maximum we'd expect based solely on thermodynamics.

Efficient solar cells are close to a reality, having reached 40 percent efficiency or better. Unfortunately, there is a small fly in the ointment: the cells are really expensive to make, and sometimes involve rare materials. One answer is to make the cell smaller and focus sunlight from a wider area on to it. Solar collectors are not new, but the standard way of making collectors involves moving parts that track the sun. Much better to do it with waveguides.

There have been earlier attempts to use waveguides to make a solar concentrator. Indeed, based on purely thermodynamic considerations, waveguide-based solar concentrators should outperform mirror-based systems by quite a margin. In practice, they don't, due to a fundamental limitation that comes about from the physics used to construct them.
Building a waveguide-based solar concentrator

Waveguide-based solar concentrators make use of two very cool properties of matter. First, when an atom or molecule absorbs a photon of light, it will often emit it again at a lower frequency. So, you put in blue light and you get out green light. This means that we can use atoms to absorb light from the sun, which comes from a wide range of directions over the course of the day, and have it re-emitted under controlled circumstances. This is where our second cool property helps us out.

We think of excited atoms and molecules as emitting light of their own volition, but the direction—and to some extent the color—of the emitted light depends strongly on the environment. This is all due to the fact that light is emitted into modes. Which mode light is emitted into is purely random, provided all the modes have the same number of photons in them.

But the number of modes available is something that we can engineer. For instance, a waveguide can be thought of as having a high density of modes, because the small number of waveguide modes are reinforced by their own interference—the modes look as if they have a lot of photons in them because of the constructive interference caused by photons interfering with themselves. This makes it highly probable that a nearby molecule will emit into the mode of the waveguide.

This preferential emission into the waveguide is exactly the physics that researchers who make waveguide-based concentrators rely on. They place their absorbing molecules on top of a waveguide. Sunlight excites the molecules, which then emit a slightly redder wavelength of light that travels along the waveguide to the solar cell. Essentially, you can imagine making a plate glass window which, instead of transmitting the light that hits it, redirects it all to exit along the edges. This can be a very bright light indeed.

It all sounds good, but it doesn't work. Typically, the light is concentrated by a factor of two to ten, which is close to nothing. Worse than that, the performance gets worse as the illuminating sunlight gets stronger. The reason the concentrator fails is that anything that can emit light can also absorb it. In other words, any of the molecules that are waiting for sunlight to excite them can instead absorb light that is in the waveguide. For pretty fundamental reasons, it seemed that this was never going to be beaten.
Take your principles and shove 'em where the sun don't shine

Remember I said that when an excited molecule wants to emit it has to emit into a mode? Well, the same is true for absorption: a molecule has to absorb light from a mode as well. So what happens if the light mode ceases to exist where the molecules are? Or, more precisely, the light in that mode must have zero intensity? Simple, no absorption.

This is exactly what the researchers have done. The waveguide is exactly the same: a strip of high refractive index material with a thin coating of a low refractive index material. The absorbing molecules sit in a layer on top of the low refractive index material.

To change the way that the modes behave, the researchers change the thickness of the absorber material so that it looks a bit like a repeating staircase. This thickness changes the behavior of the light in the waveguides. Essentially, the steps change where the high intensity points in the waveguide modes fall. This ensures that each thickness emits into modes that have low intensity in the remaining areas.

The matching of molecules to particular modes of the waveguide ensures that absorption of the guided light is minimized, and allows much higher concentration of light to be reached. Or at least that is the idea. The researchers showed that they could increase the transmission efficiency of waveguides based on these structures by some 20 percent. However, they did not actually show much in the way of improvement in solar concentration.

However, part of this is due to the size of their test sample. Their simulation results really shine, estimating that, for very large structures with geometric concentrations better than 100, the structured waveguides really start to win. However, the ultimate performance, even simulated, is not that spectacular, because the actual concentration factor (that is, illumination intensity out divided by illumination intensity in) is only about 25.

No doubt this will be improved by optimizing the structure of the absorber layer. But as the authors point out, there are limits to that, because the waveguide mode structure and the patterns will eventually repeat, leading to unwanted absorption. It seems likely that this will find use in fields like space applications where you really need solar cells, but can't afford the space and weight of inefficient cells.

Nature Photonics, 2011, DOI: 10.1038/NPHOTON.2011.236
 
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