Page 177 - Biosystems Engineering

156 Cha pte r F o u r

the heating elements and tend to increase between the heated points
because of the presence of insulation on the bottom of the greenhouse
bench. Above the heating element, heat flows vertically toward the
substrate surface and is transferred into the air. After the heating
element has been switched off, the flow of heat gradually changes
direction and becomes vertical in the whole substrate. Heat will flow
upward or downward depending on substrate surface temperature.
The energy used by the heating system is a function of three
factors: geometry of the system, ambient temperature, and substrate
temperature. Thus, an increase in ambient temperature involves a
decrease in energy demand, whereas an increase in the root-zone tem-
perature will increase energy demand.
Heating-cable spacing is one of the geometric variables that most
affects the energy consumption of the heating system. Heat supply
decreases with the increase in spacing. Short spacings produce high
temperatures on the plane of installation of the heating cable. Conse-
quently, heat flows that are transferred to the lower part of the sub-
strate stay at the bottom of the substrate and are prevented from
going up to the root zone. This implies an increase in the energy at
short spacings because part of the heat transferred by the heating
cable does not arrive to the plane of reference. For wider spacings, the
heating-cable zone shows high temperatures, whereas temperatures
are lower in intermediate zones of the heating cable than in the bot-
tom layers of the substrate. Such temperature gradients cause an
ascent toward the root zone of the heat flows that initially headed to
the bottom of the substrate. The substrate acts as an accumulator,
capable of transferring the stored heat to the root zone.
Heat supply increases with the increase in heating-cable depth
because the distance from the heat source to the plane of reference of
temperature increases. However, a depth of installation very near the
surface may demand more heat because of the occurrence of high
temperature gradients between the heating cable and the environ-
ment, which produces high heat flows outside the substrate, with
subsequent energy losses. Moreover, part of the heat supplied by the
heating cable produces the heating of the bottom of the substrate,
where the temperatures achieved are near the reference temperatures.
As a result, the contribution to heating in the root zone is very poor.
The influence of the maximum operating temperature of the
heating cable is remarkable. The number of times the cable is switched
on decreases considerably with an increase in the maximum operat-
ing temperature of the cable; a high-temperature fluctuation occurs
in the substrate. The spacing between the heating cable and the refer-
ence plane has a direct effect on the number of times the system is
switched on. A shorter spacing results in more frequent switching
on, which in turn results in greater temporal uniformity of substrate
temperatures mainly because the heating cable operates at lower
temperatures. Moreover, an increase in the spacing between the