Agriculture Reference
In-Depth Information
transpiration increases quickly raising the
humidity whereas temperature increases
more slowly, causing condensation on the
plants that favours fungal attacks. To avoid
this situation the heating set point must
be progressively increased before sunrise
(point A, Fig. 7.12). At sunset a similar proce-
dure is followed, for energy-saving purposes,
progressively decreasing the heating set
point from point C (Fig. 7.12).
In case of contradiction between the
ventilation control orders as a function of
temperature and humidity, priority control
by humidity is usually established.
For control of high temperature by ven-
tilation a proportional controller is normally
used. As the ventilation rate is difficult to
measure, a temperature set point is used
instead, corrected by the wind velocity and
the interior-exterior temperature difference
(Fig. 12.4). The vent opening percentage is
measured or estimated. Knowledge of the
wind direction allows for choosing which
vents to be opened: that is, the windward
vents (facing the wind) or the leeward vents
(opposite to the wind). Normally, the lee-
ward vents open first. Maximum and
minimum vent openings (in degrees) must
be pre-established, in case of storm, frost
or rain.
Temperature control
In the simplest systems, the user normally
fixes a temperature below which the heat-
ing system is activated (heating set point),
being common to use different set points
during the day and the night. In addition,
the user indicates a maximum temperature,
above which the vents open (ventilation set
point). Equally, the system can control water
evaporation equipment (fog, pad and fan) or
a shading screen. Nowadays, most systems
can adjust the set points for several inde-
pendent
periods
or
recalculate
them
periodically.
The temperature set point can be mod-
ulated as a function of other parameters,
such as radiation, increasing the set points
as radiation increases. It is usual to fix the
night set point temperature as a function of
the radiation of the previous day.
In thermosyphon heating systems, the
temperature of the heating pipes is usually
controlled independently from that of the
air, it being usual to maintain a minimum
pipe temperature, to achieve a leaf tempera-
ture higher than that of the air with the aim
of avoiding
Botrytis
(induced by water
condensation).
The presence of a thermal screen affects
the temperature set points. If soil or sub-
strate heating is available, besides air heat-
ing, they must be controlled in coordination
with each other. The management of the
screens can be done with a clock, by radia-
tion or by temperature. The opening of
screens must be gradual.
In exceptional cases (heavy frost) the
set points can be unreachable due to insuf-
ficient capacity of the existing heating sys-
tem. In these cases survival temperature set
points, lower than the usual are used.
Hot-water heating systems have consid-
erable thermal inertia. Therefore, their oper-
ation should be scheduled in advance, using
a proportional controller. In air heating sys-
tems the on/off control is used.
A typical example of the changes in heat-
ing and ventilation set point temperatures
is represented in Fig. 7.12, in thermosyphon
heating systems. The set point tempera-
tures change before sunrise. At sunrise,
Hygrometry control
If the greenhouse has a de-humidification
system, which is quite infrequent, it can be
activated with a humidity set point, to
decrease the humidity.
If the greenhouse does not have a
heating system the only way to limit the
humidity is to ventilate. When heating
is available, the humidity excesses can
be avoided by heating and ventilating,
although at a high energy cost. The humid-
ity set points are different during the day
and the night.
In some crops, such as tomato, this
simultaneous heating and ventilation is per-
formed every morning to decrease the RH
and avoid condensation on the plants.
The fog or pad and fan systems can be
activated when the hygrometry is low, nor-
mally during the daytime.
