Hobby Greenhouses 101 Michael S. Caron Extension Assistant Professor Utah State University Points of Traditional Greenhouses May be used as season extenders or yearround Are designed more for plant production on a larger scale Are usually actively climate controlled They are designed for maximum light penetration to the crop and not for heat retention Hobby Greenhouses Cold frame Include cold frames High tunnels Small traditional greenhouses Solar greenhouses Have all the same needs and problems as commercial greenhouses High Tunnel Small Traditional Greenhouse 1
Adapted for Solar Points of Solar Greenhouses They are designed for heat accumulation and storage to reduce or eliminate artificial inputs They are generally very small Not used commercially They can be passive or active systems Usually used for season extension Basic Principles of Solar Solar Design Solar Heat Absorption Solar Heat Storage Insulation Ventilation Solar Designs The basic idea is to orient the glazing to maximize solar gain and insulate the other walls This is in contrast to traditional greenhouse designs Traditional designs maximize light penetration into the greenhouse No significant insulation value Basic Designs For cold winters, north latitudes, or year-round steep north roof pitched to the highest summer sun angle vertical north wall for heat storage 40-60 sloped south roof glazing vertical kneewall high enough to accommodate planting beds and snow sliding off roof end walls partially glazed for added light For cold winters, mid-latitudes, or year-round vertical north wall for heat storage 45 sloped south roof glazing vertical kneewall end walls partially glazed for added light Less heat storage than above 2
Solar Heat Absorption Most critical factors are Position/location to the sun Type of glazing used Glazing should face true south However if true south is not an option, staying within 15-20 o of true south will still provide about 90% of the solar gain Southeast orientation may provide better soil gain in spring Sun s Path Summer/Winter Solar path at 40 north latitude Solar Azimuth for Salt Lake City Glazing Slope A general guideline for slope of the glazing is to add 10-15 o to your latitude So for our region at about 40 o N latitude we would take 40+10 or 15 to get our best glazing angle of 50-55 o. Solar Heated Greenhouse Select Location Carefully North wall Reflective material on wall Thermal curtains South wall -glazing Can insulate north wall with straw before placing collectors Heat collection system Collects reflective solar heat 3
Longwood Solar Greenhouses Solar Heat Storage Rocks Brick or concrete-filled blocks Water Phase-change materials Rocks and Brick Only about the first 4 inches store heat Need about 80 lbs/ft 2 for season extending Or 180 lbs/ft 2 for year round growing Or 3 ft 2 of 4-inch thickness per ft 2 of glazing A typical 6 x 12 solar greenhouse will have around 110 ft 2 of glazing Needs 19,800 lbs of rocks Rocks and Brick So approximately 10 tons of rock is needed Where will this be put? Rocks should be 1 to 1½ diameter The bottom line is it takes a lot of rock or brick Phase-change Materials Phase change materials are chemical mixtures either liquid or solid based on temperature Their advantage is that they absorb heat during the day And release heat at night Lots of it if they crystallize Hold 5-14 times more heat than water 4
Conserving Heat Repair broken glass or holes in the plastic covering Close holes under the foundation of plastic covered houses Have the heating system checked to make sure it operates at peak efficiency Insulation Insulate greenhouse areas that are not glazed Walls, floors, ceiling Foundation may be insulated Tight doors Many insulation materials need to stay dry Not easy in a greenhouse Home heating with Greenhouse There is a contradiction between the use of a greenhouse to grow plants and the use of it as a solar collector for heating the house To provide heat for a home, a solar collector needs to be able to collect more heat than what plants can tolerate Much of the heat that enters into a greenhouse is used for evaporating water from the soil and from plant leaves resulting in little storage of heat for home use A home heat collector should be sealed to minimize the amount of heat loss. Greenhouses, however, require some ventilation to maintain carbon dioxide for photosynthesis and prevent excess humidity Glazing Materials There are 3 main types of glazing materials: Glass Plastic Films Structured plastic sheets (rigid panels) Fiberglass-reinforced plastic (FRP) Acrylic Polycarbonate 5
Glass Glass Easy to break Lower humidity than plastic houses Air leaks around the panes Little insulating value 0.9 R value for single pane More intense management to maintain environment Bright and hot direct light Plastic Film Polyethylene (poly) Least expensive Greenhouse type lasts 3-5 years Lower light transmission esp. if double Very tight, can cause humidity problems Least structure required Used as inflated double-layer for heat conservation and wind resistance single layer poly Squirrel-cage fan double layer (double) poly Structured Sheets Acrylic Fiberglass-reinforced plastic (FRP) Polycarbonate Acrylic (Plexiglass) Flexible Highly flammable Double wall available Excellent light transmission Long life Good UV resistance with inhibitors 6
Polycarbonate Acrylic Low flammability Flexible Shatter-proof Yellows with age, use laminated sheets with UV Inhibitor on one side Single (corrugated), Double and Triplewall available Lasts at least 10, often to 20 years Fiberglass-reinforced plastic (FRP) Corrugated for strength diffuse light UV Inhibitors on one side Flammable can have retardants added Single-layer corrugated only 8-10 year life if cleaned periodically Yellows and gets brittle as it ages Can result in very poor light levels Fiberglass 7
Valuable Online Resource Ventilation A greenhouse designed to collect heat must also vent it when it become excessive Solar chimney may be used Opening windows or vents May be active or passive External shading may be needed Points to Remember The main idea is to orient greenhouse to maximize south exposure Location and design should allow maximum winter sun While minimizing summer sun Rocks, water and other materials may be used to store heat It takes a lot of heat storage Points to Remember Conserve heat whenever possible Insulate, seal, insulate Greenhouses are poor choices to heat a home Greenhouses need ventilation Even in winter There is a wealth of online information about solar greenhouses Other Any greenhouse needs consideration for Irrigation Drainage Utilities Fertilization Pest Management Temperature Control Humidity Control Benches or other growing area Questions? 8