Half baked ideas: mineral oil encapsulation, window film...

[markgwoodruff]

I'm currently working on a DIY Solar UPS using ultracapacitors (check out ultracapacitors.org). At the moment, it's based on two Harbor Freight 45W systems (6 15W panels) and 56 2600F 2.5V Maxwell BCAP0010 ultracaps. I've been seriously thinking of ordering some of the broken 6x6 cells from Ebay to build additional panels for it. Along the way, I've had some ideas which I haven't seen explored anywhere yet:

1. Mineral oil encapsulation: Keep the panels in a tank of mineral oil. Mineral oil doesn't conduct electricity, it keeps moisture out, and the thermal mass could keep the panels cooler and run more efficiently, especially if coupled with a heat exchanger to dump the extra heat.
2. Window tint: V-Kool makes some window tint (VK70) that rejects as much 94% of infrared (IRR) while still passing 70% of visible light (VLT). Might make up for the loss of light by keeping the panels cool. And if it was coupled with parabolic mirrors to concentrate the visible light...
3. Connect cells without soldering: Looks like soldering all those tabs is a pain. I only need 2.5V, so my thought was to connect the cells in parallel using a 1/8" aluminum sheet on the back and 1/8x1/4" aluminum bus bars on the front of the cells using springs to provide enough tension for good contact without breaking the cells. I also thought about bending the aluminum into clips and again holding them in place with pressure. Wouldn't work for panels on trackers, but for stationary panels, it seems a heck of a lot easier for DIY.

Thoughts?

Mark

[john p]

I'm currently working on a DIY Solar UPS using ultracapacitors (check out ultracapacitors.org). At the moment, it's based on two Harbor Freight 45W systems (6 15W panels) and 56 2600F 2.5V Maxwell BCAP0010 ultracaps.What are you hoping to power from this?That many ultra caps wont power much of anything for more than a minute or two1.Mineral oil encapsulation: Keep the panels in a tank of mineral oil. Mineral oil doesn't conduct electricity, it keeps moisture out, and the thermal mass could keep the panels cooler and run more efficiently, especially if coupled with a heat exchanger to dump the extra heat. But isnt keeping the panels under mineral oil going to stop most of the light falling on the panels.?2.Window tint: V-Kool makes some window tint (VK70) that rejects as much 94% of infrared (IRR) while still passing 70% of visible light (VLT). Might make up for the loss of light by keeping the panels cool. And if it was coupled with parabolic mirrors to concentrate the visible light...How you going to have the mirrors both track the sun and the panels?3.Connect cells without soldering: Looks like soldering all those tabs is a pain. I only need 2.5V, so my thought was to connect the cells in parallel using a 1/8" aluminum sheet on the back and 1/8x1/4" aluminum bus bars on the front of the cells using springs to provide enough tension for good contact without breaking the cells. I also thought about bending the aluminum into clips and again holding them in place with pressure. Wouldn't work for panels on trackers, but for stationary panels, it seems a heck of a lot easier for DIY.I think you will find this more difficult to achieve than you think??

[russ]

Like John noted -

1) Mineral oil encapsulation - might as well keep the cells in the shipping package and in the closet - production will be about the same

2) Window tint - the manufacturers would really be stupid to have overlooked something like this - if it worked which it doesn't.

3) Connection method - An improvement?

4) Ultra caps - short term storage - useless for this application

Russ

[Mike90250]

Better read up on "aluminum oxide", and how fast it forms. You will need conductive chem film treated aluminum buss bars, and even at that, they will likely need to be gold plated, to have the best possible contact, each pressure point has to be a low resistance or you loose cell power.

30% loss of visible light, to get a 15% decrease in heat, you will have to test this to see if it works out.

[markgwoodruff]

The caps would power my computer system (roughly 240 watts) for over two minutes, which is enough to handle most blackouts before it switches to battery.

Mineral oil is used as an optical coupler. There may not be much loss with a thin sheet of it between the cells and the glass, and it should more than make up for it by keeping the cells cool. We'll see.

Both Fresnel lenses and parabolic mirrors tend to fry the cells because they concentrate the infrared as well as the visible light. Removing much (90%+) of the infrared, concentrating the visible light, and keeping the cells cool...again, we'll see.

Lots of solar trackers out there. The entire assembly would move.

Not a lot of oxidation when aluminum is immersed in mineral oil. Tinned strips might work better though. There's a tradeoff. The ampacity of normal tabbing wire is too low for parallel connections.

[john p]

Tracker to move the panels and parabolic mirror is going to have to be reasonably heavy and strong so its not affected by wind, That is going to use a fair amount of that aprox 70w those panels will produce.

I have no idea what the expansion of mineral oil is when its hot ,but where does it expand to when it does?

[markgwoodruff]

Easy half baked enclosure

Baking pan and glass shelf joined with silicone. Fits 10 3x6 cells.

[john p]

I still cant understand how this mineral oil and baking pan and glass is going to work. As im understanding it you have the cells in a baking pan then imersed in mineral oil then a glass sheet sealed with silicone over that??
But the first problem i see is when the whole thing is at and angle facing the sun the oil will run to the bottom?? Second there have to be wires coming from the cells to a junction box(through the silicone?) I think you may find oil creep along the wires ? And silicone is not moisture proof.

[markgwoodruff]

I'll try to explain my thinking.

From what I've seen, a typical solar panel consists of a pane of tempered glass surrounded by aluminum frame backed with a plastic or aluminum panel. Inside are solar cells soldered in series with parallel bus wires encapsulated in UV-resistant silicone (SylGard) or EVA to protect them from moisture. The wires exit through a junction box in the back sealed with silicone.

My thought is to replace the encapsulant, frame and backing with with a sealed shell (a jelly roll sheet sealed to the glass with silicone) filled with mineral oil. The mineral oil both protects the cells from moisture and cools them by transferring the heat from the cells to the oil to the aluminum container.

Yes, there's the potential it will leak. The glass can be clamped to the shell; different types of gaskets can be used; ultimately, it it will fail, and when it fails the oil will leak out. If it lasts 20 years, I'd be completely happy. I also haven't been able to confirm that silicone caulk and mineral oil are compatible.

If it works, though, it's a heck of a lot cheaper and easier, especially at the DIY level.

Mark

[markgwoodruff]

Looks like some other people have had the same idea. I googled /"mineral oil" encapsulant solar/ and had a fair number of hits. I'll see if any of them followed through on it and what their results were if I can find out.

[john p]

I still see the major stumbling block being the oil will not "sit" on the cells but run down to the bottom of the container. I just cant see anyway you can stop that. oil is just one of those things that doesnt like to stay put on a sloping surface, it just likes to go downwards and is more likely to do so in the sun?? And have no idea what the heat conduction abilities of mineral oil are .?/ mabe they are very poor?? High power Transformers use oil in them.,but the oil they use is very nasty stuff not something you would want to be experimenting with.

[MadScientist]

I don't see major problems...

I would say the IDEA of using a liquid medium to cool photovoltaics is feasible. Even using something as cheap as Mineral Oil.
From what I have been able to find, mineral oil is transparent to the wavelengths that photovoltaics are most sensitive to (around 689 to 1240 nm). http://www.prenhall.com/settle/chapters/ch15.pdf
And as mineral oil has a higher heat capacity than air, it would also stand to reason that the oil will be more efficient at regulating that temperature.
The issues I do recognize are as follows:
Weight - An oil-filled photovoltaic cell array will be very heavy. Keep in mind that, although mineral oil has a relatively high heat capacity, you will still need some volume of oil to stabilize the temperature. If you use only a small amount of oil to cool the array, then the oil will get very hot, very fast, regardless of its heat capacity.
Temperature regulation - Photovoltaics are interesting in that they have to be heat balanced if you are to put them in series. The reason being is that the temperature of each cell - or cell region for that matter - will vary in its efficiency according to temperature. If it gets warm, it has a lower resistance. Cold, higher. So if you were to install an oil-cooling system, it would need to circulate very efficiently to keep the cells in the array thermally balanced. This means some sort of pump must be added to the system. (I'm talking about resistance according to temp, not energy production efficiency according to temp)
Cooling - Although oil can hold more heat than air, it will still need some means by which to 'dispose' of that heat. Otherwise you run into heat issues (as mentioned). This means that there would need to be an efficient heat-exchange system (remember, if a medium gathers heat and stores it efficiently, it will take more time to drain that heat).
Now, none of these problems are insurmountable, but it would be a matter of cost efficiency. Both energy costs (gain of efficiency/power as a result of this extra complexity) and in financial cost (how much will all this extra technology add to the cost of the system in both long and short term). But I do not see a problem with the idea.