I've saved thousands of dollars by
building quality homemade greenhouses. In order to retain that
savings, I find simple and inexpensive ways to heat them. That
means I count on the sun to meet nearly all of my needs for
heat.Passive solar
energy is one of the most cost-effective ways of heating a
greenhouse and retaining heat during times when the sun isn't
shining. Of all the solar energy techniques, passive solar is
probably the oldest, least costly and most reliable.
The term passive means that
there are no active component in the system of energy
collection, storage and release. This means no pumps, fans,
heaters or louvers that require electricity or fuel. By design,
the system works all by itself without any help from you -
except initial design and construction.
Before we look at passive
methods of using solar energy, let's discuss four key principles
of thermal energy (heat) that will help to explain how these
solar heating systems work. Here they are in a nutshell.
- Energy is always trying to
move to a lower state; hot things cool down by giving off
their thermal energy to cooler things.
- Dark colored objects
absorb heat readily; light colored objects absorb heat much
more slowly.
- Dark colored objects emit
(give off) energy readily; light colored objects emit heat
much more slowly.
- We need good thermal mass
to store and release energy. The material must have high
density (heavy for its size) and low conductivity (doesn't
warm up or cool down quickly). Water, concrete, brick, rock
and moist soil are ideal to use as thermal mass.
Okay, let's dive in. Here are
two simple, effective and inexpensive designs that I use for
passive solar heating a greenhouse of mine. They both use scrap
material, so the cost of the system is my time and the fuel it
took to haul the materials.
The first design uses
six 55-gallon black steel drums to hold water. I fill each drum
to near the top, so that provides about 300 gallons of water to
act as a heat absorber for the greenhouse during the day. At
night, the drums release their thermal energy back into the
greenhouse slowly.
The drums are located on the
north wall where they get best light from the low angle winter
sun. The backs of the drums are painted white to reduce the
amount of heat they give off to the cold north wall. The black
sides face the south where they release heat to the plants I am
growing.
Water is one the most effective
materials for thermal mass. The most challenging aspect of water
is keeping it contained, and keeping it from freezing. No common
item will hold heat quite like water.
The second design is
55-gallon steel drums cut in half widthwise so each makes two
1.5 foot high miniature raised beds. The half-drums are arranged
in the greenhouse for growing taller plants like peppers and
tomatoes, and filled with soil. Since they are round, there is
always air space between the drums that help them warm up
quickly as they are surrounded on all sides by warm air during
the day.
The steel drums are burned
clean inside and out, and left to oxidize as they naturally
will. This provides a dark brown surface for thermal energy
absorption and emission. The challenge to using half-drums is
cutting them in half. I use a plasma torch, and this makes the
job a snap. A regular torch will work, or a circular saw with a
metal cutting blade would also work well.
Caution: the drums must
be free of flammable or explosive materials before you cut them.
I burn them out in a deep pit for a couple of hours to make
certain the flammable material is gone before I cut into them.
The drums are an excellent way
to retain heat in the greenhouse because they are filled with
moist soil. Next to water, I can't think of a better material to
use as thermal mass than moist soil. The rust colored drums emit
energy readily, but the moist soil doesn't give it up too
quickly.
