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I'm building an LED grow light for an, um, indoor vegetable garden, and I'm trying to figure out what dimensions I can get away with for the aluminum sheeting I'm going to use for the mounting plate. I've been trying to use this heat transfer calculator, but unfortunately the physics is a little above my head.

I will be using 11 high intensity LEDs (data sheet), wired in series running at 700 mA. Due to build constraints, I plan to use an aluminum sheet that's 12" x 5", 1/16" thick (0.30m x 0.13m, 1.5mm thick). So that works out to a surface area of 60 [sq in], but the manufacturer recommends at least 9 [sq in] per LED (thus 99 [sq in] total for this project), so I'm significantly under that.

TL;DR - Here's the question - I have an aluminum sheet 12" x 12", 1/16" thick, if I cut it in half, and then use thermal epoxy to bond one half on top of the other, would that sufficiently increase the heat transfer capacity of my heatsink? Sorry if this is a dumb/obvious question, I really should have paid more attention in physics class :)

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  • \$\begingroup\$ LED data sheet \$\endgroup\$ – ignorantslut Apr 14 '12 at 18:08
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    \$\begingroup\$ About the TL;DR: No. What you need is to increase surface area of the heatsink. If possible, have some forced airflow through the surface. Take a look at for example this computer heatsink. See the countless ribs? Well you should try to do something like that. Try looking for large heatsinks with flat bottom and try using thermal epoxy to glue them to the aluminium sheet you're planning to use. You could also try to find some old computer processor heatsinks, they'll work nice too. \$\endgroup\$ – AndrejaKo Apr 14 '12 at 18:23
  • \$\begingroup\$ In the worst case, try cutting the half of the sheet into smaller sheets and gluing them at a 90 degree angle to the other half of the sheet. Due to low thickness, this could be difficult. \$\endgroup\$ – AndrejaKo Apr 14 '12 at 18:24
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    \$\begingroup\$ New tag proposal: 'marijuana' :) \$\endgroup\$ – m.Alin Apr 14 '12 at 20:04
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    \$\begingroup\$ I hope you enjoy your... uh... "tomatoes". \$\endgroup\$ – Connor Wolf Apr 14 '12 at 20:35
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Cutting the Alu in half and sticking it to the back will do almost nothing to help. What you need to do is increase the surface area of the Alu.

If you're stuck for space, then you have a couple of options. If you have space in the vertical direction, you might try just bending the alu so it's not as wide and fits into the shape of the available space. However, you do still need to make sure the heat can get to the outside world. There needs to be some kind of air flow, preferably over the alu.

Bent Alu

If there is a fairly constant trickle of water to the plants, then you can use this to extract a lot of heat from the LEDs. Instead of using a sheet of Alu, use some box section. Seal up the ends and attach a push-fit pneumatic connector at each end. Pump the water supply through this, and you should be good to go. Even a tiny flow of water should be enough, but it probably needs to flow fairly constantly, not just once a day.

Alu box section

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    \$\begingroup\$ You speak as if you've done this before! (hah ha, just kidding.) +1 for mentioning that cutting the Al and gluing it back on itself is stupid. The rest of the answer is good too. \$\endgroup\$ – user3624 Apr 14 '12 at 20:53
  • \$\begingroup\$ Thanks. Yes, I have done this before, but for another application. We were looking at how much air flow would be needed to keep cool 80 small solenoid valves which were embedded in alu. The answer was hardly any. \$\endgroup\$ – Rocketmagnet Apr 14 '12 at 21:02
  • \$\begingroup\$ I also like this answer, especially the use of water cooling; if possible...brilliant! If that is not possible [this heat sink][1] is double your 12 inches. [1]ebay.com/itm/… \$\endgroup\$ – rdivilbiss Apr 14 '12 at 22:16
  • \$\begingroup\$ @rdivilbiss Thanks. That's exactly the kind of heatsink I was looking for to illustrate the third suggestion! \$\endgroup\$ – Rocketmagnet Apr 14 '12 at 22:35
  • \$\begingroup\$ awesome diagrams, thanks for taking the time to help me out :) \$\endgroup\$ – ignorantslut Apr 15 '12 at 17:31
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You don't say why you would cut the sheet in half. Anyway, the two half sheets attached to each other will have the same heat capacity as the sheet as a whole. Heat capacity is the product of the sheet's mass and the specific heat capacity of aluminium, which is 0.91 J/(g K). So no matter how you cut and fold your sheet, as long as the weight is the same its heat capacity remains the same.

What you can alter though is the heat transfer capacity. That determines how easy the sheet will exchange heat with the environment. There are three ways of heat transfer:

convection
That's heat exchange through a fluidum flowing over the sheet's surface, typically (forced) air flow. The larger the contact surface the more heat can be exchanged with the air, so cutting in half and gluing the two parts together effectively halves the convection exchange! Keep the surface as large as possible, and as "aerodynamic" as possible; the better the air can flow over the sheet the more heat it will absorb. Mounting the two half panels on top of each other isn't bad, provided that you have air flowing between them. If they're close together you'll need forced air flow (fan), because the air resistance will prevent air passing.

conduction
The most effective for low temperatures. Thermal resistance for conduction is often a few orders of magnitude smaller than for convection. Mount the sheet on a good thermal conductor, like another piece of metal.

radiation
Is more effective at higher temperatures, but you can increase radiation losses by painting the sheet a matte black.

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