Agriculture Reference
In-Depth Information
conduction between the soil surface
and its deep layers are important.
We call convection the energy transport
•
Water transpiration by the plants is a
particular form of evaporation, which
allows plants to cool down and decrease
their temperature.
Water condensation in a greenhouse
•
by a fluid in the direction of flow, or
between a static surface and a fluid. It is
of crucial importance in a greenhouse.
The air in contact with the heating
pipes is heated by conduction and,
once heated, moves by convection,
heating the rest of the greenhouse 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.
Water evaporation consumes a lot of
•
starts at the colder spots, normally on
the greenhouse cover. Condensation
in the internal surface of the cover
affects the light transmission and
improves the insulation conditions of
the cover.
All bodies with temperatures above
•
−273°C emit energy from their surface
in the form of electromagnetic radia-
tion. This radiant energy is transformed
into thermal energy if it impacts a body
that absorbs it.
The radiation received by a body can be
•
energy, as water goes from the liquid state
to a gas. We call the latent heat of vapouri-
zation the energy needed to evaporate
1 kg of water, at 20°C, and its value is 2445
kJ kg
−1
of evaporated water. If there is no
change of state (from liquid to vapour) the
heat required to increase its temperature
by 1°C (specific heat of water) is low, of
the order of 4.2 kJ kg
−1
of water.
The condensation of the air water vapour
•
reflected, absorbed or transmitted
through it. The greenhouse covering
materials must be transmissive to solar
radiation.
The heat exchanges by air renewal in
•
the greenhouse are very important for
the energy balance. The humid air con-
tains more energy than dry air at the
same temperature, as it incorporates
the energy used for the evaporation of
the water. The renewal of the internal
air, which is usually warmer and more
humid, by external air (cooler and drier)
involves a great loss of energy from the
greenhouse.
•
is the inverse phenomenon to evapora-
tion and releases a large amount of
energy. Condensation only occurs if the
partial pressure of water vapour reaches
a value known as 'saturation pressure'.
The dew point is the temperature below
•
which condensation occurs.
