Agriculture Reference
In-Depth Information
Fig. 7.1. Compact greenhouses have a lower proportion of sidewalls with respect to the greenhouse
ground area, limiting thermal losses. Square-shaped greenhouses have less perimeter and less sidewall
area than rectangular greenhouses of equal ground cover area. equally, greenhouses with a high roof
slope have a larger exchange surface and greater losses of heat through the cover.
Conduction and convection losses
decrease by minimizing the effects of the
exterior wind, for example protecting with
windbreaks, and locating and orienting the
greenhouse properly (Fig. 7.2). A reduction
in the 'thermal bridges' (points through
which heat, by conduction, escapes to the
exterior of the greenhouse through the struc-
tural elements which are good heat conduc-
tors) also limit the heat losses from the
structural elements to the exterior, but above
all the most efficient means are the use of a
double cover and thermal screens. For more
related details, see section 7.3.
Losses due to air renewal are another
type of heat losses. They can be limited by
means of good insulation of walls and vents,
to improve how airtight the greenhouse can
be, and by minimizing the effects of the
external wind. When a greenhouse is closed,
it is not possible for it to be perfectly airtight
as there is air infiltration through holes and
slits. The amount of infiltration depends on
the type of greenhouse and of the external
wind (Table 7.1).
Night heat losses due to leakage in a
closed low-cost greenhouse in Almeria are
10% of the total losses in the absence of
wind, and with wind velocities of 4 m s −1
they are more than 30% of the total losses
(López, 2003).
7.2.3 Total heat losses
Heat losses depend on the temperature dif-
ferences between the greenhouse and the
exterior. Radiation losses are predominant
in unheated greenhouses. Conduction and
convection losses are, proportionally, higher
in heated greenhouses.
With the aim of integrating the set of
losses through the walls, by conduction,
convection and radiation, the 'global heat
transfer coefficient' ( K ) is used (Table 5.2),
which is evaluated under standard condi-
tions of temperature, wind and cloudiness.
The amount of heat exchange between
the greenhouse and the exterior ( Q , excluding
the heat lost by air renewal, due to night leak-
age) is detailed in Chapter 5 (section 5.4).
Some authors increase the value of Q in
case of strong winds (Aldrich and Bartok,
1994), whereas others consider that the
 
Search WWH ::




Custom Search