Let's shed light on the nature of sunlight. If you find errors or have
concerns please contact me so together we may better understand terms like
transmittance, emittance, absorbance, conductance, solar heat gain, and
net solar heat gain and low-e coatings. Our outer atmosphere
receives about 1.4 Kw/m^2. However less than 1Kw/m^2 make it to the
surface of our planet. Our eyes are only sensitive to a narrow range of this light
energy. The three main categories of light are defined by their
wavelength. They are ultraviolet, visible and infrared.
A large portion of this IR is made
right here on the earth's surface. Visible and ultraviolet light are
absorbed by dark surfaces and emitted as infrared radiation. The long
wave infrared light is then reflected back by clouds, CO2 and other
gasses. The phenomenon is commonly called the greenhouse effect.
ACTIVE and PASSIVE solar heating systems depend on three basic
properties of radiant energy. ABSORPTIVITY (a) is the fraction of incident light absorbed by a
surface. REFLECTIVITY(p) is the fraction if light reflected at the
same angle of incidence. It is the reciprocal of emissivity.
TRANSMITTANCE (t) is the fraction of incident light at a specified
wavelength that passes through a material so the transmittance of
visible light may be high while the transmittance of IR may be low.
a + p + t = 1
EMISSIVITY is the relative ability of a material to emit radiant energy
from the surface at a given temperature. A black body has an emissivity
of 1. A pure reflector has an emissivity of 0. Now that we understand
the basic properties of radiant energy we can explore practical
applications of the greenhouse effect.
SOLAR HEAT GAIN measures the rise in thermal energy due to sunlight. The
Solar Heat Gain Coefficient SHGC is a fraction of solar radiation
transmitted and or absorbed through a window or glazed door. For
uncoated windows this fraction is normally less than .8. For low-e glass
the SHGC is normally between .2 and .4. Lowering the SHGC may seem like
a strange way of increasing the net heat gain since it blocks IR coming
from the outside of a window, however it also blocks IR radiation from
leaving the inside of this solar greenhouse. Unfortunately we can
not have our cake
and eat it too.
The Solar Heat Gain Coefficient for common uncoated window glass
is .84 BUT this does not mean 84 % of the energy from the sunlight stays
inside the sunspace or solar collector. 16 % of the IR heat radiation
will pass back out so only 44% of the incident solar radiation or
INSOLATION is trapped by a greenhouse. Heat is also lost by conductance
through the glass. Common thermopane glass has an R value of 2. Argon
filled thermopane has an R rating of 3. Evacuated thermopane may have R
values as high as 12 BUT maintaining a leek proof vacuum inside
thermopane is difficult and expensive.
LOW-E COATINGS contain extremely thin layers of metal or metal
compounds that reflect specific wavelengths of light such as UV and IR
radiation. Visible light is allowed to pass through low-e coating and
the transmitted visible light is changed into long wave IR radiation
that cannot easily pass back out. A theoretical black body emits 100%
and a perfect mirror emits 0 % radiation. Low-e coatings are designed to
block IR and UV radiation and allow the transmission of visible light.
If a low-e coating lowers the emissivity to .01 99% of the IR radiation generated within a structure will remain in the
NET SOLAR HEAT GAIN is the average solar heat gain over a specified
period of time. Active solar heating systems maximize the net solar heat
gain. The balance of sunlight and insulation and
thermal mass has to be just right to make passive solar heating systems
practical in cold climates.. In most cases a
backup heating system and thermal shades are recommended. Even if the
net heat gain is positive the temperature fluctuations in a 100% passive
heated dwelling may be outside the comfort range of most people. BUT an
active solar hot water system will always be practical.
Sunlight is an intermittent resource so its energy must be stored for
times when there is no light available. IR radiation may be blocked with
low-e coatings BUT heat may also be lost through conduction. Argon
filled thermopane only has an R factor of 3. This means that the heat
loss through an 8x8 Argon filled thermopane with the outside temp of
20*F and an inside temp of 70*F would be Q = A x U x (T1-T2) = 1,056
BTU/hr or 12,672 BTUs / 12 hrs. This sunspace is isolated from the house
to prevent heat loss through the glazing. A fan is used to pump heated
air into the house where it is stored.
THERMAL SHADES may be used to reduce the heat loss at night. A 20,000 BTU solar
heat gain through a 64 ft^2 thermopane window is good BUT without
thermal shades the net heat gain could be less than 10,000 BTUs. Always
isolate the area of heat collection
from the area of heat storage to minimize heat loss. Active solar heating systems do this
automatically. Passive solar heated houses are a step in the right
Combing active solar heating systems with passive solar heating systems
could solve a lot of problems.
The North side of the house may have too many windows to be energy
efficient during the winter months, but remember we want our house to be
practical and esthetically pleasing all year long. The Solar
Greenhouse on the south side of the house provides extra heat as well as a place for plants.
If you decide to automate the heat collection process and isolate the
area of heat storage a differential controller may be used to regulate a
pump or fan and bring heat where you need it when you need it. Remember
a pump or fan only consumes a fraction of the energy that it collects. <
br>Differential controllers provide a bridge between the intermittent
nature of sunlight and the need for a dependable supply of heat and hot