Chronica Horticulturae volume 49 number 2 ... - Acta Horticulturae

Figure 3. Increasing the roof slope to ca 30° increases the light transmission, crop production
and hence energy efficiency in southern Europe.
steps to create energy conservative greenhouses
(Fig. 5).
A thermal screen adds an additional barrier
between the greenhouse and its surrounding
and reduces both the convection and ventilation
loss. When movable, it has less impact on
the light transmission compared to fixed
screens or double covering materials. About
80% of the greenhouse area in The
Netherlands and 35% in France are equipped
with thermal screens. Theoretically, screens may
reduce the energy use by more than 35-40%,
depending on the material. In practice, movable
screens are closed only a part of the entire
period depending on the criteria for opening
and closing and due to the restrictions for closing
related to humidity and light. In commercial
practice the energy effects are about 20% but
the overall effect on energy efficiency is slightly
lower due to light reduction caused by the
screens.
Increasing the insulation value of the greenhouse
cover has a major impact on energy consumption
of the greenhouse. However, a major
disadvantage of most fixed insulating covers is
the reduction in light transmission and
increased humidity. Development of materials
that combine high insulation values with high
light transmission is one of the most challenging
issues. Under practical conditions, the
potential energy saving of double and triple
covering materials is almost never achieved,
since the grower tries to compensate for the
negative effects by increasing the dehumidification
of the greenhouse environment.
The Lexan ® ZigZag greenhouse covering
material is an example of a material that in
potential combines a high light transmittance
(80% for diffuse light) with a high insulation
value and a year round energy saving of 20-
25% when compared to single glass.
Unfortunately the large scale practical application
has not become successful due to additional
costs, the low overall economic benefit, and
problems with the production and modification
of the greenhouse construction. Promising
alternatives for the future are double side coated
Anti Reflex glass, combinations of micro V
treated glass or triple layer systems (Bot et al.,
2005). For example, double Anti Reflex glass,
which can more easily be used in combination
with current greenhouse constructions, combines
a high diffuse light transmission (82-86%)
with an energy reduction of 26%, leading to a
gain in energy efficiency of 40%.
maximum amount of solar (or “green”) energy
to enter the greenhouse (to be used for crop
production and temperature increase), while
the IR radiation loss from the greenhouse is
being restricted. Many film materials are suboptimal
for energy efficient greenhouses since
their IR transmission is high, except for the fluoropolymer
ethylenetetrafluoroethylene (ETFE)
membrane (Hemming, 2005).
To further improve the light transmission of the
materials, several antireflex coatings have been
introduced during the last decades to prevent
light reflection, which enables light transmission
to be increased by 5-6% (Fig. 4). Coatings
can also reduce the light reduction by condensation,
which is especially important with plastic
materials. Recent innovations in this field are
modifying the surface structures with e.g. micro
V, which is a promising technology for greenhouse
covers. However, the positive effect of all
these innovative technologies to improve
greenhouse light transmission can only have its
potential effect if the cover is regularly cleaned
since the transmission of the cover may significantly
be reduced by dust.
Reduction of Energy Use in the
Greenhouse
Minimize Energy Loss by Screens and Insulating
Materials.The major processes of energy loss in
natural ventilated greenhouses are: (1) convection
and radiation from the greenhouse cover,
and (2) thermal and latent heat transfer
through ventilation. Improved insulation and
reduced ventilation are therefore the first
Figure 4. To further improve the light
transmission of materials, several anti
reflex coatings have been introduced
during the last decades to prevent light
reflection, which enables light transmission
to be increased by 5-6%.
Minimize Energy Loss through Ventilation and
Latent Heat. On a year round basis, a major
fraction of the energy transfer from the greenhouse
to the environment is by natural ventilation.
Thus, many attempts to reduce the energy
input for greenhouses concentrate on the ventilation
process and its effects on heat- and
mass transfer (Baeza, 2007). During periods
with relatively low radiation and moderate
ambient temperatures, natural or forced ventilation
is generally used to prevent high humidity,
which is related to a significant (5 to 20%)
fraction of energy consumption. To reduce the
“humidity control related” energy consumption
several options are applied: higher humidity setpoints,
reducing the transpiration level of the
crop, or dehumidification with heat recovery
(Campen and Bot, 2002).
Although high humidity levels are generally
associated with increased risk of fungal diseases
and reduced quality (e.g. botrytis, blossom end
rot), increasing humidity may also be positive
for crop production and quality, under moderate
as well as under more subtropical conditions
(e.g. Montero, 2006). Increasing the humidity
level therefore is an effective way of increasing
energy efficiency.
Reduction of transpiration also has positive
effects on energy efficiency since lower transpiring
crops bring less water into the air and
therefore require less energy for humidity control
under low irradiation conditions. Two major
options show promising results: the use of antitranspirants
and reduction of the leaf area.
Antitranspirants, in combination with higher
CO 2 levels, may improve energy efficiency with
5-10% without effects on photosynthesis and
growth. Also controlled reduction of the leaf
area for crops with a high leaf area index like
pepper may reduce energy use without impact
on production. Halving the leaf area by removing
old leaves in tomatoes showed a reduction
Figure 5. A thermal screen adds an additional
barrier between the greenhouse
and its surrounding and reduces both the
convection and ventilation loss. When
movable, it has less impact on the light
transmission compared to fixed screens
or double covering materials.
ISHS • 20