Outfit Your Greenhouse with a Solar Electric Fan Chris Greacen Words: 941 Seeds planted in our greenhouse have made the miraculous transformation to small green starts poking optimistically through the potting soil. As these plants grow they will need sun and warmth and water. To grow strong and prevent mold and mildew, plants also need a supply of dry, carbon dioxide rich air from outside. In fact, greenhouse growers recommend 150 air changes a day, with an air change every couple minutes in the hottest time of the day. For our greenhouse, a solar powered fan was the solution to adequate greenhouse ventilation. There's something clean and wholesome about sunlight powering the movement of air in a solar greenhouse. Also, since our greenhouse is several hundred feet from utility power, a solar powered fan was the cheapest ventilation available. When it's bright and sunny the fan blows like crazy. At these times the greenhouse needs the most ventilation because of the heat, and because the plants are active making oxygen and evaporating moisture. When light levels are low, the fan barely turns, but this is great too, because the plants' metabolism is down, and the greenhouse needs to preserve heat. Sizing An air change every couple of minute! Seems like an awful lot. To figure out the size fan you need, first find the volume of air in your greenhouse. Ours is about 15 feet by 20 feet, and an average 15 feet tall. Multiply the dimensions together Ä ours is 4500 cubic feet. For an exchange of air every five minutes, we'd need a fan that blows 900 cfm (cubic feet per minute). We chose a 16 inch fan in a 20 inch housing, driven by an efficient DC permanent magnet motor. Under full sun, powered by an 18 Watt photovoltaic panel, it is rated at 1000 cfm. If this isn't enough, another panel, doubling the voltage, will more than double the cfm. Installing the Fan I mounted the fan in a 20 inch by 20 inch 2x4 framed hole above the door on the west wall of the greenhouse. On the east wall at ground level is a vent for incoming air. It's ideal to have a low "intake" and higher exhaust vent to keep air from forming stagnant stratified layers and to take advantage of the draft formed by air heated in the greenhouse. In fact, with no fan at all, vents in this configuration would help create a gentle breeze. Orient the vents to make use of the prevailing wind direction. In our area wind blows most from east to west, therefore we designed the exhaust vent on the west side. To seal off the vents at night to keep in the heat, build a door for the vent, or use a solar vent opener (available from greenhouse suppliers). Put screens over the vents to keep the bugs out. Look Maw! No batteries! There's no battery in this system. This is a good thing. Batteries are usually the weak link in renewable energy systems. Anytime you can leave out the battery, your system will be more reliable and longer lived. In this case we're interested in ventilating only when the sun is shining, and blowing more when the sun is brighter, so there's no need for energy storage. To make the system work well in low light, though, a bit of fancy electronics called a Linear Current Booster (LCB) is needed. In low light, a photovoltaic panel produces power at a voltage/current combination which is poorly matched with the current and voltage the motor needs to begin turning. To get useful power out of the panels in low light, the LCB (model 3T) "trades" the panel's higher voltage for higher current needed to run the motor. It runs slower than in full sun, but without the LCB, it wouldn't run at all. Be sure to mount the LCB so that you can access the small "tuning" potentiometer on the back. You'll need to diddle this knob when you set it up so that the LCB does its job most effectively electrically matching your fan and photovoltaic panel. Include a switch on the panel's positive output so that you can turn on and off the fan. Figure out how much wire you'll need and consult wire tables (you'll find a set on page 32, Home Power #18) to determine which gauge to use. I used . Solder all connections to prevent corrosion (incidentally, there's also a great article on soldering in HP #18). Finally, install the photovoltaic panel. I made an adjustable rack out of angle iron. With a drill press I drilled holes which allow the angle of the panel to be adjusted with the changing seasons. Since we live at around 48ø latitude, so I made the rack so that it could be set at 48ø (spring and fall), 48ø - 23ø = 25ø (summer), and 48ø + 23ø = 71ø (winter). The magic 23ø comes from the tilt of the earth's axis. Cost You should be able to find 18 Watt Solarex panel for around $235, the fan for $105, and the LCB-3T for $80. For about $10 less you can get a non-tunable LCB-3 which will also work, but won't allow you to add a second series panel. Access Author: Chris Greacen, POB 520 Ashland OR 97520 ù 916-475- 3179 Fans, panels & LCBs: Bob'O Schultze, Electron Connection, POB 203, Hornbrook, CA 96044 ù 800-945-7587; Steve & Elizabeth Wiley, Backwoods Solar Electric Systems, 8530 Rapid Lightening Creek Rd., Sandpoint, ID 83864 ù 208-263-4290; Alternative Energy Engineering, P.O. Box 339, Redway, CA 95560 ù 800-777-6609