Agriculture Reference
In-Depth Information
7
7.1
Introduction
heat exchange surfaces with the exterior of
the greenhouse, building compact green-
houses, so that the proportion of sidewalls
is reduced to a minimum (area of sidewalls
versus greenhouse covered area) (Fig. 7.1).
Greenhouses with a high roof slope
have a larger roof area than those with a low
slope, which in turn involves a larger
exchange area resulting in greater heat
losses through the cover (Fig. 7.1). In a simi-
lar way, corrugated covering materials (for
instance, rigid polyester panels) increase
the exchange surface in relation to covering
materials that are flat.
The greenhouse temperature depends on
the energy balance. In order to avoid low
temperatures heat losses must be reduced
and heat supplies provided, taking into
account that when one of the energy bal-
ance components is altered, other compo-
nents, which should not be modified, may
be also altered.
When the natural energy supply (for
temperature increase) is not enough, we
must resort to artificial supply, by means of
heating.
7.2.2
Reduction of heat losses
per unit surface
7.2
Reduction of Heat Losses
This aspect must be considered when build-
ing the greenhouse. The reduction of heat
losses is achieved, mainly, by reducing the
heat exchange surfaces and the heat losses
per unit surface, using insulation materials
and windbreaks.
Besides limiting the exchange surfaces,
losses per unit of these surfaces must be
reduced as well. Radiation losses could be
limited using proper covering materials
(low transmission to far IR radiation, such
as thermal PE, for instance). Besides, radia-
tive losses can be decreased using thermal
screens deployed during the night. In cases
where there is a heating system, a manage-
ment regime that lowers the temperature of
the cover will decrease radiation losses
(because these losses increase with the tem-
perature of the cover).
7.2.1
Reduction of the exchange
surfaces
The reduction of greenhouse heat losses is
achieved, in the first place, by limiting the
