The Passive Solar Greenhouse

Our most regular contributor reports on the perfect greenhouse
Don Tollefson, 599 California Avenue, Venice, California 90291, USA
Chamaerops No.32 Autumn 1998

Introduction: For most palm enthusiasts the primary objective is to attain a successful, outdoor, palm collection. One that looks as good on New Year's Day as it does any other day of the year. Since most palm species were not biologically intended to grow in a temperate climate, the first requirement in the development of such a collection is to grow the palm to the "optimum size" before planting it outside in the ground. Optimum, outdoor, planting size for most palms is as a substantially rootbound, five gallon. This size provides adequate mass to endure the cold of that first, and most difficult, upcoming winter. Any size smaller is a disservice to the collector, the collection and the palm.

The greenhouse advantage: Nothing facilitates growing palms quickly to the optimum, outdoor planting size like a greenhouse. Nothing! Estimates are, that in a temperate climate, a greenhouse can produce an optimum sized palm approximately eight to ten times faster then the same palm grown outdoors. This is not to mention the fact that most species capable of growing in a temperate climate cannot survive outdoors as young palms. With an effective greenhouse a grower can expect to grow a palm from a newly sprouted seedling to a robust, rootbound, five gallon size in about three years. Why then, are greenhouses so conspicuously absent from the arsenals of temperate climate palm enthusiasts? Primarily, because in a temperate climate, greenhouses are so extremely expensive to heat at night during the winter, that most palm enthusiasts are reluctant to have one.

Levelling the Playing Field: A greenhouse, effective for cold climate palm growing, can be unnecessary to heat at night or maintained with only a small amount of back up heat on cloudy days. How? By collecting maximum, passive solar heat per window size during the day and by providing (and containing) maximum, passive heat during the night. Always remember, the test comes during the coldest period of the winter. This requires the following:

A. Southerly exposure: Southerly exposure is the starting point in collecting maximum, daytime, solar heat. It's a simple formula. Southerly exposure is excellent. Easterly and westerly exposure is marginal. Northerly exposure is disastrous! It is during winter that greenhouses most need heat and during the winter most sunlight arrives from the south. Approximate south (in the northern hemisphere) is the direction of the sun at noon. To determine "the south", locate the sun at "solar noon". Solar noon is halfway in time between sunrise and sunset. Consult a local newspaper for these times. True south can also be located with a compass, but remember to account for the approximate 20 degrees difference between true south and magnetic south. The wide side (longest side) of the passive solar greenhouse must face south.

B. Avoid unnecessary glass via the optimum inclination of windows: Maximum winter "insolation" (the receipt of heat providing sunlight) is received by the windows being tilted at a 60 degree angle from the sun. This enables perpendicular sunlight penetration minimising any loss due to reflection. By contrast, vertical and horizontal glass surfaces reflect from 30 to 60% of their winter "insolation" due to reflection. With 60 degree glass inclination, the passive solar greenhouse receives as much insolation during the winter as any other combination of greenhouse configurations possible. Read my lips: NO UNNECESSARY GLASS. Extra glass is impermissible because unnecessary glass enables the unnecessary escape of an irrecoverable amount of night-time heat. For comparison purposes, a 4' x 4' sheet of single pane window glass allows the escape of the same amount of heat as an 8' tall by 40' long wall! Imagine the heat loss from an entire ceiling of unnecessary glass.

C. The depth of a passive solar greenhouse must not exceed 16 feet: Wide and shallow, not wide and deep. If a greenhouse exceeds more that 16 feet in depth, it will be difficult to obtain "abundant" winter heat. Abundant winter heat is necessary to exceed the heating needs of the greenhouse and thereby provide for proper ventilation and air circulation as well. If the greenhouse needs to be larger, make it wider, not deeper. Palms like heat the year around and they like heat even on cloudy days. In areas such as the North Mediterranean and the Pacific Northwest, this requirement is frequently thwarted by cloudy, overcast days. In general, cloudy days will heat a greenhouse only about 10% as well as a sunny day. If a shallow greenhouse requires only 1/5 as much sunlight to be adequately heated, it will still receive a substantial benefit on a cloudy day.

D. Optimum insulation: Any surface of the passive solar greenhouse that is not essential for sunlight collection must be insulated instead. The north wall is of no benefit for solar collection and insulating the north wall is essential. The east and west walls are of minimal solar benefit, and should also be insulated. Finally, if the front wall is tilted back at 60 degrees, winter, overhead sun exposure is of no additional benefit. Therefore, the ceiling can be insulated. In this situation, nearly 4/5 of the previous heat loss has been restricted merely by insulating the top, back and sides of the greenhouse. If this greenhouse previously lost its heat 1 hour after sunset, it will now require about 4 to 5 hours to lose that same heat. A substantial improvement.

Double pane glass or plastic: Double pane glass reduces insolation about 13%. Glass has an R (insulation) value of about R-0.6. This is equivalent to about 1/2" of wood. However, with a slight air space, the R value of double pane glass increases nearly threefold to R-1.6. The equivalent in wood requires about 1 1/2". Compared to standard R-11 wall insulation, double pane glass is only about 1/7th as effective. But in the example above, with double pane glass, the heat, reduced by only 13%, would now remain in the greenhouse for 10 to 12 hours instead of 4 to 5. A further, substantial improvement. Double pane glass can be quite beneficial if non-essential, solar collecting surfaces have been insulated.

Thermo shutters: Perhaps the easiest method available to maintain winter, night-time heat in the greenhouse is the use of thermo shutters (night insulation). Thermo shutters are merely insulation devices that are placed over the glass at night, and removed during the day. Their benefit is because the glazing necessary to generate adequate heat during the day also allows the escape of that same heat at night. Thermo shutters prevent the escape of much of that heat. Installing and removing them each day lends credence to the dictum, "passive solar requires active people", but it is an activity for active people which is well worth the effort. However, most growers are unwilling to utilise Thermo shutters. Fortunately, a properly designed, passive solar greenhouse will perform satisfactorily without them.

E. Thermal mass: Thermal mass is any material that in mass, acts like a storage heater. Thermal mass stores the heat from the day and releases the stored heat back during the night. Typical examples of high thermal mass materials are concrete, rock, brick, iron, soil and water. Large barrels filled with water are an excellent source of greenhouse thermal mass. The soil in the containers which hold the palms is a good form of thermal mass. A large cold frame with many large pots of soil will absorb more heat during the night than one with only a few. That's why a cold frame works better as the number of palms in containers in it increases. In order to be most effective, the pots should be black, and hit by direct sunlight. Accordingly, it is best to put the largest, potted palms in the front, southerly facing row so that they receive unimpeded, direct sunlight.

F. Venting: A passive solar greenhouse must be vented. The most practical vent openers are those operated by a wax cylinder. They require no electricity and cost about $50.00. The correct location for an exhaust vent is at the highest point of the greenhouse. Otherwise you will have a pocket of hot air. At the floor level you need intake vents. This enables the entry of fresh air containing carbon dioxide to permeate the greenhouse and perpetuate the growing cycle. Intake vents should be the same square footage as exhaust vents. This promotes convection.

G. Air circulation: Air circulation is beneficial. If you follow the above formula, your greenhouse will heat up quickly and on a normal day, voluminous amounts of excess, hot air will be exhausted through your ceiling vent via natural convection. The greater the convection, the greater the air circulation.

Summary: Regardless of your climate, you can have an effective, passive solar greenhouse, heated by the generosity of the sun. And it will bring you unbelievable success in terms of quickly growing palms to the optimum, plantable, substantially rootbound, five gallon size. Something that you probably, have never done before.

For a brochure on <:"The Diamond Lane Guide to Passive Solar Greenhouses"> Write to Don Tollefson, 599 California Avenue, Venice, California, USA. 90291. Phone +1 (310) 392-7806