Figure 4–1. Energy exchange between a greenhouse and the surroundings.
Environmental Control
SOLAR RADIATION
A greenhouse is built and operated to produce crops
and return a profit to the owner. In many areas of the
country, sunlight is the limiting factor in production,
especially during the winter; therefore, a greenhouse
should provide for optimum use of available
sunlight. The amount of sunlight available to plants
in a greenhouse is affected by the structural frame,
covering material, surrounding topography and
cultural features, and orientation of the greenhouse.
The amount of sunlight available outside is a
function of latitude, time of year, time of day, and
sky cover.
A greenhouse cover with high transmissivity for
solar energy can produce temperatures that are
higher than desired in the crop zone. Most surfaces
within a greenhouse have high absorptivities for
solar energy and, thus, convert incoming radiation
to thermal energy. Figure 4–1 shows energy
exchange for a greenhouse during daylight. Table
4–2 on the next page lists transmissivities of several
glazing materials for solar radiation and infrared
radiation from surfaces at about 80°F. Table 4–3 on
the next page lists absorptivities of several surfaces
for solar radiation and emissivities at about 80°F.
Transmissivity is the percent (in decimal form) of
solar energy transmitted when the sun’s rays strike
the surface at a right angle to the surface. Emissivity
is the ratio of the total radiation emitted by a body
to the total radiation emitted by a black body of the
same area for the same time period.