Agriculture Reference
In-Depth Information
5
5.1
Heat Transfer
surface and its deep layers, and the thermal
losses through the greenhouse structure.
See Appendix section A.4.1.
Two bodies at different temperatures
exchange energy in the form of heat, which
flows from the hotter body to the colder
body. There are three main modes of heat
exchange: (i) conduction; (ii) radiation; and
(iii) convection (with or without change of
state).
The air renewal of the greenhouse
involves a mass transfer, notoriously affect-
ing the greenhouse energy balance.
5.1.2
Convection
Convection without phase change
Convection is the energy transport by a fluid
in the same direction as the flow or between
a static surface and a fluid. The thermal
exchanges by convection involve the dis-
placement of matter. They take place mainly
in fluids and induce their movement.
Convection is of the 'forced' type when
it is provoked by an external mechanical
action (e.g. air fan). 'Natural' convection is
driven by density differences, derived from
temperature differences, which generate the
fluid movement (for instance, the warm air
weighs less than the cold air and rises up).
Convection exchanges have great
importance in the greenhouse. The air in
contact with the heating pipes is heated by
conduction and, once heated, moves by
convection, heating the rest of the green-
house air. This warm air contacts the plants
and then heats the plants by conduction.
When it is windy, the greenhouse cover
loses a lot of heat because of active (forced)
convection. See Appendix section A.4.2.
5.1.1
Conduction
Conduction is the energy transport from
molecule to molecule inside a body, solid or
fluid. By conduction the energy is trans-
ported through a media at rest, which is not
flowing in the direction of the energy trans-
port (Fig. 5.1).
Conduction is the only mode of heat
propagation inside a solid, or between two
solids in physical contact, at different tem-
peratures. The energy flows from the higher
to the lower temperature. The rate of tem-
perature change along the distance (
d
) is
called the thermal gradient.
The main heat changes by conduction
in a greenhouse take place between the soil
