Executive summary: Didn’t work as well as I had hoped. Lowered the temp in my tank by about 4 degrees F – and it’s a pretty small tank.
As a long-overdue followup to my climate-controlled terrarium, i.e. the actual climate control part, I bonded heat sinks to both sides of a Peltier device (thermoelectric “heat pump”) embedded in a Lexan terrarium cover. The Peltier looks like a thin ceramic pad about 1″ square. When electricity is applied to it, one side gets hot and one side gets cold. Each heat sink also has a fan (just like a CPU) to efficiently move the hot/cold off the sink and into the surrounding environment.
This lid is cut for a small terrarium (actually a 5-gallon Betta tank) I’m using to winter over the stuff that requires a dormancy period. Apply the lid, apply the power, and watch it become Instant Arctic inside the tank, right? Wrong…
It turns out the inside temperature doesn’t drop very much at all – I was averaging a drop of maybe 4degF, applying 12V @ 3A to the Peltier and both fans running. Although the Peltier can produce a maximum delta-T of almost 80degC, it just can’t pump a lot of heat very fast, and air (no matter how hard you blow it past a heatsink) is a good insulator that doesn’t give up its heat that easily. My guess is that between the less-than-ideal mating of the lid and tank (you want to have *some* airflow to the outside, right?) and conduction through the Lexan and thin glass walls, heat is just pouring back in from the outside too fast for one small Peltier chip to keep up with. Maybe with some good insulation this will be a viable approach, but it doesn’t seem feasible for most real-world terraria (in direct sunlight, etc.)
Possible improvements:
The idea is to keep the “wintering” terrarium in e.g. my nice warm bedroom without having to do something like try to rig it to sit halfway out an open window, heat the entire outdoors and freeze my ass off. During winter, an alternate approach would be to stick the tank in a *cold* (quite possibly below freezing some days) area, like the unheated porch, and reverse power to the Peltier so that the warm side is inside the tank. Unfortunate fact is, Peltiers do a much better job generating heat than pumping it around, and at 3A being cranked through it, will be generating a fair amount (about as much as an average computer CPU). On the bright side, electric heating is by definition 100% efficient (P = I*V = I^2*R = V^2/R)…so you could just look at it as having a teeny tiny electric space heater going.
I was also using one of the smallest and cheapest Peltier chips available – only 30mm on a side, max power handling of ~ 38W, about $9 from AllElectronics. There are certainly beefier ones. Except that the power they suck increases exponentially with size, and I don’t feel like feeding it enough juice to drive a window A/C (because then I could just use a window A/C). Maybe it’s time for a proper insulation experiment…
Humidity control redux
Lest you think science is all about trying things that do not work :) a revised approach to humidity control turned out very well. If you’ll remember, I was using an aquarium air pump and Coke bottle full of water to humidify the tank. Problem with this is, it adds humidity (relatively) slowly, and works primarily by splashing water all the hell over your tank where it eventually evaporates. Anyway, I found a relatively cheap ($18) ultrasonic fog generator from Artistic Delights and sank it into the bottle in place of the bubbler. Instead of just splashing water all over (when properly covered of course!) and making constant noise, this atomizes it into a dense fog that disperses throughout the volume of the tank, raising it to ~ 95%RH in minutes. Not that any of the stuff in my tank is this picky, but with no fans/etc. blowing your air around, the fog will naturally sink toward the bottom of the tank, probably allowing you to leave the lid off your terrarium for extended periods (for maintenance, pruning, whatever) without stressing your plants. I highly recommend this approach over the air pump.
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