Solar energy can be utilized through two different
systems, active and passive. The active system, as the name implies,
involves moving mechanical appliances along with necessary plumbing
and other accessories. The passive system utilizes a means of
collecting and storing for later use the heat from the sun's
rays with a minimum of mechanical help. For the majority of hobbyist
greenhouses, the latter system is undoubtedly the better.
Collection of the energy of the sun's rays,
in itself, can create some problems as we definitely want to
collect the maximum amount in winter when everything about us
is cold, and a minimum in the heat of summer. This can be accomplished
by so sloping the south-facing side or end of the greenhouse
as to take maximum advantage of the differences in angle of the
sun's rays during the winter and summer seasons. This angle,
at any given time, varies with the latitude of any given location.
And, for any given location, the angle will vary with the time
of year. For example, on December 22 or 23 the sun's rays at
noon at Juneau, Alaska, are very flat; while at noon at Philadelphia,
Pennsylvania they are at a steeper angle than at Juneau, and
are steeper still at Kerriville, north of San Antonio, Texas.
Six months later, there is a comparable difference between angles
at the above three locations, but in each case the angles are
steeper than in winter.
We can take advantage of these features in
designing solar greenhouses, first assuming that we have a location
where we can place the greenhouse free of shade from trees or
high buildings, and with either a side or an end facing south
or slightly southwest. The greater the orientation of the collecting
surface from south to southwest the greater becomes the reduction
in solar energy collected.
Let us assume that, as in my case at Kerriville,
the greenhouse is to be a modified lean-to extending lengthwise
from the south face of our house (figure 1). It has been determined
that the sun's rays, over a maximum period of time in the winter,
will strike a plane surface at or close to the perpendicular
with maximum transmittal of solar energy when the surface is
at a slope equal to the latitude of the location plus 15 degrees.
In Kerriville, at 30 degrees north latitude, the slope of the
south-facing glass would be 45 degrees. In Philadelphia, at 40
degrees north latitude, the required slope would be 55 degrees.
And in case anyone in Juneau, Alaska, at 58 degrees latitude,
were interested in a solar greenhouse, they would have to use
a slope of 73 degrees to the horizontal - almost vertical. Here
it must be emphasized that the computed slope is the angle of
the glass surface with the horizontal (figure 2, angle A), not
with the vertical (figure 2, angle C).
In the same way that the sun's rays strike
the sloping surface more or less perpendicularly in winter over
a maximum period of time so does the opposite occur in the summer
when the rays strike at a steep angle. This angle of impact is
such that part of the energy ricochets from the glass surface
leaving the balance to be transmitted into the greenhouse as
heat (figure 2).
The computed slopes given above create special
problems with the exception of Juneau. For example, if the 45-degree
slope needed for my greenhouse in Kerriville, which is about
16 feet long, had been carried to the wall of the house to which
it is attached, the top of the north wall would then be 16 feet
above the elevation of the toe of the slope at the south end.
At a 55-degree slope, as needed at Philadelphia, the top of the
rear wall would be nearly 23 feet above the toe elevation. For
the juneau location, we can almost say "The sky is the limit"!
With the toe of the sloping south end of the
greenhouse 16 feet from the rear north wall, and with an impractical
situation if I carried the slope from the south end to the top
of the rear wall, what to do for the roof became a problem. Fortunately,
there is a slight slope to the south at the rear of the house
so i excavated to bedrock - only a few inches below the surface
at the south end - and made the floor of the greenhouse at three
levels. Thi permitted a structure ofreasonable proportions with
a flat roof at a moderate slope from the rear wall to top of
the 45-degree south-facing slope (figure 1).
Installation of the flat-sloped roof created
another problem by exposing a substantial amount of roof to near-maximum
penetration of the sun's rays in summer. However, two nearby
oaks provide some shade and reduce the effect. An alternative
solution would have been to use a gable roof as in a free-standing
greenhouse with or without the roof ridge slanting downward toward
the south, and merge the sloping south face in with the roof
and sidewalls, with the glass of the sidewalls starting at bench
height. Tnis would, at least, reduce heat gains through the roof
in summer.
The approach to the collection of solar energy
for heating the free-standing type greenhouse is the same as
for the type just discussed. The same formula for determining
roof slope is applicable; however, the method of application
is somewhat different. Here again we assume that a desirable
site is available for the greenhouse with no trees, buildings
or other obstructions to cast shadows, and that the proposed
structure can be placed to take maximum advantage of the sun's
rays. Optimal orientation of a greenhouse of this type is with
the longer axis in the east-west direction; energy collection
efficiency decreases as clockwise rotation of the axis increases
to the west.
Soil temperatures below the frost-line are
quite uniform throughout the year and provide some warming during
winter and cooling in summer. Becasue of this, some site excavation,
with the use of the excavated soil as backfill around the foundation
walls, will pay off in the future although adding to initial
costs and labor. One can excavate to a shallow depth on a flat
site, and still have a higher wall by backfilling on the outside.
Depth of the underground portion of the structure
can be controversial. Too shallow a depth will mean some loss
of the insulative features of teh backfill. Too deep an excavation
will create varying shaded areas on the benches during the year.
Carrying the walls only to bench height might prove to be the
most satisfactory.
With completion of the concrete block walls,
we have a choice of two basic designs for the glass (or fiberglas)
enclosed portion of the greenhouse. We can either set the toe
of the roof slope directly on the block wall or carry a short
glass wall vertically to an eaves line from which the roof slope
will start. The growing space immediately above the benches will
be the deciding factor.
In the more northerly latitudes, as at Philadelphia
at 40 degrees north latitude, the 55-degree angle (or roof slope)
would provide 17 inches of clearance 1 foot from the toe of the
slope if the latter were at the top of the block wall. However,
at Kerriville's 30 degrees latitude, the 45 degrees would mean
sacrificing growing space. At Miami, Florida, the slope would
be even flatter - about 40 degrees. In these latter examples
the better solution is to carry the glass or fiberglas 3 feet
above the wall to the eaves line where the roof slope would start
at a height 5 1/2 feet above floor level, assuming 2 1/2 feet
bench height. In a location similar to that at Philadelphia,
the decision as to where the toe of the roof slope is placed
could well be at the choice of the owner.
Let us assume that the proposed greenhouse
is to have equal roof slopes on the north and south sides of
the ridge regardless of width, except for those narrower than
10-12 feet. maximum roof ridge height (at A in figure 3) above
the level of the toe of the roof slope for various slope angles
(at B in figure 3) at various latitudes and greenhouse widths
can be computed by the formula H=fD, in which H is the
maximum height of the roof ridge, f is a factor varying with
the slope angle, and D is the horizontal distance (B-C in figure 3) from
the toe of the slope to the point vertically below the ridge
(usually but not neccessarily equal to half their width for free-standing
greenhouses). The f factors are as follows: 0.84 for 40-degree slope
angles, 1.00 for 45-degree slope angles, 1.19 for 50-degree slope
angles, 1.43 for 55-degree slope angles, 1.73 for 60-degree slope
angles and 2.14 for 65-degree slope angles. In the design of
a narrower-width greenshouse, such as 8 feet, in southern Florida,
one might elect to carry the slope the full width of the greenhouse
to the north wall. Using D in the above formula as 8 feet, the height H of the
north wall would be 6 3/4 feet above the toe of the slope, at
Kerriville 8 feet and at Philadelphia about 11 1/2 feet. The
heights from floor to toe of slope in each instance would have
to be added to obtain the total height of each north wall. 
Reduction of the heights of the roof ridges
can be accomplished through use of gambrel roofs with the design
slope carried most of the way from toe to ridge with a flatter
slope for the balance of the distance to the desired ridge height.
This would reduce the surface area for collecting the solar energy
from the sun's rays in winter and provide a less desireable design
in summer for reducing penetration of solar energy resulting
in more heat intake when least desired.
For the assumed Philadelphia situation we
could set the toe of the 55-degree slope directly on the top
of the concrete block wall. Assuming again uniform roof slopes
on each side of the ridge and the proposed greenhouse to be 25
feet wide, the roof ridge would be about 20 feet above the floor
level with top of the wall at 2 1/2 feet. Increasing the width
by 10 feet to 35 feet would increase the floor to ridge height
to 27 1/2 feet. Decreasing the width to 15 feet would reduce
the ridge height to 13 feet from the floor. Gambrel roofs for
the two widest designs, at least, would undoubtedly be preferred.
If so, in each instance the lower slope would be the longer of
the two and at the desired angle.
Now that we have devised the means for collecting
the energy of the sun's rays, the next item is storage. This
can be accomplished as described in the article by Robert E.
Benson in the November 1982 issue (pages 1145, 1146) of the American
Orchid Society Bulletin. A second source of storage is water
contained in a vat, drums, or similar containers; unconfined
water free to drain will carry off rather than store heat.
If the solution of heat storage were limited
to providing a mass of rock, concrete or water as a thermal mass,
it would be a simple matter. However, it is not so simple, especially
in the southern part of the United States.
It is desireable to operate the greenhouse
within a night-day differential temperature range of 15-20 degrees
F. If the thermal mass is too great the heat carry-over into
night hours may not permit lowering of temperature to a desired
minimum without excessive use of the cooler. This can then be
aggravated by cloudy days when heat has to be applied to bring
the temperature to a required maximum. One possible solution
to this problem is the use of water as a thermal mass. Of all
the materials available for this use, water has the most flexibility
for change of mass (or volume) as required by seasonal changes
or even by the vagaries of the weather.
The problem of balancing thermal mass and
heat losses required some technical study of heat gains and losses
by various materail and the greenhouse itself. However, there
are several good books on teh market that will be of help. One
is The Solar Home Book by Bruce Anderson with Michael
Riordin, published by and available directly from Cheshire Books,
Church Hill, Harrisville, New Hampshire 03450. Well recommended,
it covers the subject (including greenhouses) in a most complete
and understandable manner, with plenty of tables and illustrations.
Of great help to the layman is the use of various technical problems
to be encountered in a design, with step-by-step solutions.
Another aid in conserving heat, in addition
to going underground with concrete block walls to at least bench
height, is double-glazing the greenhouse throughout. Probably
the most satisfactory way to do this is to use fiberglas sheeting
on the outside that has been treated to avoid 'blooming', the
gradual exposure of the fibers used in manufacturing the product
with resultant loss of light transmittal. An added advantage
is its impact resistance. The inner layer, on the inside of the
greenhouse frame, can be of glass if wod frame is used along
with a bit of ingenuity particularly for overhead work. An alternative
material, and one more commonly used, is translucent sheet plastic.
Unfortunately, polyethylene film, the type most readily available,
may have a short life of only a few months to 1 1/2 years with
no assurance as to time of year when deterioration occurs. The
bestare reported to be satisfactory for exterior work for 3-4
years. Vinyl and mylar have been used on the outside for over
4 years with no deterioration and probably could be expected
to last 6-8 years on the interior, but are difficult to obtain.
Montgomery Ward carries vinyl at this time but catalog information
is not too descriptive. Undoubtedly, the best material for the
inner liner over a trouble-free period of time would be the fiberglas
as used on the exterior. I can assure anyone who attempts to
use it on the interior, however, he will be faced with a frustrating
job. If the manufacturers could be induced to roll the material
for this phase of the job with the treated face on the outside
rather than on the inside as they do for outside application,
fiberglas would be the ideal although more costly material.
As a final note, no one should expect the
solar greenhouse to be the ultimate in carefree and economical
operation. The best that can be hoped for is a reasonable savings
in money and energy. Backup equipment for heating and cooling
is good insurance.
end