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

Energy Saving Greenhouses
J.C. Bakker
Eficient use of energy in greenhouses has been subject of research and development since the
first “energy crisis” in the early 1980s when the constriction in the supply of oil caused a significant
increase in energy prices (Bakker et al., 2008). This spike in energy cost occurred again
in 2008. Furthermore, the new European targets on reduction of CO 2 emission have resulted
in a renewed interest in innovative technologies to improve energy efficiency. The need for
energy cost reduction is critical in the greenhouse industry since energy forms a substantial
fraction of total production costs. For European conditions it has been calculated that annual
energy use for conditioning ranges from 10-30% depending on the different regions while
the absolute use differs from 1900 MJ m -2 (Finland), to 1500 MJ m -2 (The Netherlands), and
500-1600 MJ m -2 (southern France).
The increase of production per unit of energy (energy efficiency) can be achieved by reduction
of the energy use and/or improvement of production. In fact, all cultivation measures that
increase the production, such as improved irrigation, better nutrition, pest and disease control,
and better utilization of the available greenhouse area, also improve energy efficiency.
Focusing solely on energy efficiency without any focus on the absolute energy use may have
unexpected (and undesired from an environmental point of view) effects. Even in
Mediterranean areas, heating is used to obtain early production and a constant quantitativequalitative
yield, leading on one hand to a higher energy efficiency but at the same time also
to a higher absolute energy use. Also improved environmental control (e.g. more CO 2 supply,
additional lighting), intensified production schemes (Boulard, 2001), and use of cooling systems
to expand the growing period into months with high temperatures, all cause an increase
in energy consumption (De Pascale and Maggio, 2004). The same holds for the Northern
regions with respect to the use of artificial light up to very high levels (e.g. over 200 W m -2 )
in Finland (Olofsson et al., 2006) and over 100 W m -2 in The Netherlands (Fig. 1).
The major challenge is to find ways that meet both needs: improved energy efficiency combined
with an absolute reduction of the overall energy consumption and related CO 2 emission
of the greenhouse industry. In general the same objectives hold for Mediterranean and
Northern European regions: during fall/winter the objective is to maximize the radiation quantity
(either from natural light or artificial light) and minimize the energy loss; during the
spring/summer the objective is to reduce high temperatures.
In The Netherlands the target is even more ambitious than merely improved energy efficiency.
From 2020 and on, new greenhouses should operate almost without any fossil fuel and
even should be “energy producing.” This ambitious goal is based on the idea that greenhouses
are large solar collectors that collect almost 80% of all incoming solar radiation. For northwest
European conditions the yearly solar irradiation sum equals approximately 3.5 GJ m -2 ,
which is equivalent to the combustion value of 100 m 3 of natural gas (=100 NGE). Since the
incoming solar radiation represents about twice the energy used in the greenhouse itself, this
theoretically creates the possibility to use the greenhouse as a combined crop and heat producing
system.
To reach this ambitious goal, a research and development program has been started under
the name: “Greenhouse as Energy Source.” Although the ambitions are even higher, the
steps to be taken in this development process are still the same as before: maximum use of
solar energy, reduction of energy use in the greenhouse and replacement of fossil fuel by
other sustainable sources.
MAJOR STEPS IN THE
DEVELOPMENT OF ENERGY
EFFICIENT GREENHOUSES
Maximum Use of Solar Radiation
The first step in creating energy efficient greenhouses
is to maximize the use of incoming natural
radiation to maximize its positive impact on
production and to minimize additional heating
(Fig. 2). Improvement of the greenhouse as an
energy efficient solar collector is obtained by
improving the light transmission of its structure
and minimizing reflection losses through the
modification of the roof slope. In Southern
Europe, considerable enhancement of light
transmission has been reached by increasing
the roof slope from nearly zero, typical from
areas with low rainfall, to values close to 30º,
which has had a direct effect on crop response
in the winter time (Fig. 3). In North West
Europe, combined steps to improve the light
transmission of the greenhouse have led to
minimized and integrated construction parts,
e.g. minimal dimensions of gutters, the use of
Figure 1. High levels of artificial light in
greenhouses cause a significant increase
in the total energy use in northern and
southern regions.
Figure 2. The first step in creating energy
efficient greenhouses is to maximize the
use of the incoming natural radiation,
which has a positive impact on the production
and the reduction of the additional
heating power.
wide glass panels, and white coated frames to
limit the light interception.
Greenhouse covering materials can be roughly
classified into three groups: glass, film and hard
plastic sheets. Each cover has its own characteristics
with respect to light transmission for different
wavelengths and insulating value. For
energy efficiency and optimal use of solar radiation,
the transmission for visible light
(Photosynthetic Active radiation) and Infrared
radiation are important. For the cold period,
one should aim at materials that combine a
high transmission for visible light with a low
transmission for IR radiation combined with a
high insulation value. These materials enable a
CHRONICA HORTICULTURAE •VOL 49 • NUMBER 2 • 2009 • 19