Solar energy can be used very effectively today in the home greenhouse, without being complicated and expensive; in fact, it is now possible to grow cool green crops in the greenhouse during the winter using solar energy, according to John H. Pierce, for 20 years a community college instructor in botany and horticulture and a former member of the staff of the New York botanical Gardens. He is also author of a very good book, "Greenhouse Grow How," published by Plants Alive Books.
Now that the other forms of energy have become expensive and limited, and research is underway to develop solar heat and power, he says, the next few years may well see the use of solar cells, photovoltaic cells, eutectic salt storage and optical collectors to trap solar energy to heat greenhouses.
Most of the current research and experimentation is with the active solar systems that involve outside sources of energy for pumps or blowers and sophisticated engineering for collection and storage. Passive systems, which need no outside source of energy, are being overlooked, he says.
Since the early '30s, people have been growing cool crops all winter in structures like a pit greenhouse, with the sun as the only source of heat energy. The principles involved can be applied to all types of home greenhouses.
"Get the heat in! Hold the heat overnight with the proper insulation! Any container filled with water, painted black and exposed to the sun will absorb heat.
"This last winter I supported the south bench in my 6-by-8-foot glass greenhouse with four 55-gallon oil drums, full of water and painted black on the sun side.
"The water, heated to a minimum of 130 degrees F, and night insulation kept the house above freezing. Adding black plastic bags full of water along the north wall increased the heat absorption to give an average winter temperature of 44 degrees F. This let me grow a delectable crop of lettuce, spinach, radishes, parsley and other herbs for salad greens all winter.
"It is a simple and inexpensive system for passive absorption of solar heat.
"Even better than the 55-gallon drums are square plastic containers painted black and stacked on their sides.
"A rule of thumb is that you need two gallons of water storage for every square foot of glass or polyethyiene, in order to get winter temperatures sufficient to grow salad greens in Canada.
"If you are building a new greehouse, by all means attach it to the residence on the south side as a lean-to. Stack your black water-containers against the solid wall of your hose. The lean-to then acts as a solar collector and will provide extra heat for the residence.
"Keep the wind off! Use wind-breaks of plant material or fencing to divert wind away from the greenhouse. Don't forget the wind-chill factor! When the wind blows at 10 miles per hour and the thermometer reads zero, the temperature is actually 22 degrees below zero.
"Keep the heat in at night! Last winter I used air-cap or bubble-pack polyethylene applied to the glass on the inside. On the outside, I used panels of Coraplex, a double-wall, plastic panel which admits 80 percent of the light.
"Over the Coraplex, I installed a roll-down thermal blanket of aluminum Mylar as a heat barrier. Grant you, there is the inconvenience of covering and uncovering the greenhouse every night, but the taste of fresh greens in the winter is worth it.
"There are a host of materials that will hold the heat is at night, and I have found that a double-covering helps to retain more heat. Don't let the cloudy winter day fool you into thinking there is no solar radiation. Often, the overcast sky diffuses light, so there may be as much as 14,000 foot candles of solar radiation."