Co2

We breath o2 and breath out co2. Plant are the opposite. They grow faster under a higher concentration of co2. You can get a co2 tank and regulator and put it on a timer. I have never used co2 this way so I'm a little ignorant of how much to use.
What I use runs off a propane bottle and is basically a pilot light that burns when my lights are on. I bought it at the local hydroponic shop. It raises the temperature in my room about 10 degrees. Its kinda bad but when you are in a co2 enriched environment the plant actually does better at higher temps. 85*+.

Here is a co2 generator that burns propane.