by Brian Betz
Solar is well adapted to many different types of loads. Water pumping in remote locations has been popular for years. Another great solar application is for greenhouse ventilation fans. Typically the most ventilation is needed when the sun is shining brightly. Solar panels powering fans work together to exhaust hot air from the greenhouse. This is a perfect solar direct application. Backwoods Solar has been selling Sugar Mountain industrial fans produced in Sandpoint Idaho just up the road from our headquarters. The fans are designed, assembled and boxed in an off-grid facility powered by micro-hydro and solar. These fans offer brushless motors, incredible efficiency and great reliability. A solar direct fan system can pay for itself in electricity savings in as little as 2 years in some applications.
In a greenhouse during hot times of the day, it is customary to cycle the air once every one to two minutes. One air exchange per minute should keep the greenhouse within 8F of the outside air temperature. To figure how much ventilation is needed you need to know the cubic feet of the greenhouse. If your greenhouse has few openings for natural ventilation, you should plan on fans to do all your exhaust venting. So a greenhouse that is 10’ wide and roughly 10’ tall and 20’ long would equal 2000 cubic feet. A single F16 fan with a 100 watt solar panel would exchange the air every minute.
Backwoods Solar carries the full line of Sugar Mountain Fans. They currently come in 3 sizes and are designated by blade diameter. F16 list price is $320 and has a maximum of 2000cfm at 70 watts. The F20 list price is $420 and has a maximum of 3000cfm at 100 watts. The F24 list price is $599 and has a maximum of 4000cfm at 150 watts. Use a solar panel 10-20% larger than the power draw listed, wire to the fan and let it keep your greenhouse cool.
For large greenhouses multiple fans can be used. Let’s do an example of a large greenhouse of 100’ x 25’x 15’h or 45,000 cubic feet. For this larger greenhouse we will use the F24 fan. At maximum output of 4000cfm we need 11 fans if we want to maintain the exchange of once per minute. The fans can make use of 150 watts each, but still offer excellent output around 130-140 watts. A 60 cell, 280 watt solar panel is perfectly suited to power 2 of these fans. So a system for a greenhouse may have 5-6 of these solar panels. Placement of the fans is important. Don’t push the air toward your prevailing wind. Use your prevailing winds to help with your ventilation. Also, you may need/want some fans blowing air out and some fans blowing air in on the opposite wall.
Moving some air at night is a more difficult system and requires batteries. The Sugar Mountain Fans will operate well off a 24v battery bank. Please contact Backwoods Solar for help in designing a system where constant air movement is required.
Off-grid aqua-ponics or hydro-ponics systems are also possible. DC pumps can often be used in addition to the Super Fans for ventilation. Backwoods would look at these systems on a case-by-case basis. Contact Backwoods Solar for a custom design.