Agriculture Reference
In-Depth Information
The distribution circuit's pumps must
be insulated in the parts where heat trans-
mission is not required.
In greenhouses of up to 9 m wide, the
pipes fixed to the sidewalls may be all that
are needed (Fig. 7.7), but in wider green-
houses it will be necessary to install heating
pipes at one or more locations across the
span (ASAE, 2002).
When the natural circulation of the air
is not sufficient to achieve good uniformity
of air temperature, it will be necessary to
install fans to improve such air circulation
(see Chapter 9).
Losses by conduction/convection
Losses by conduction/convection are higher,
the greater the temperature difference between
inside and outside the greenhouse. They also
increase with wind velocity. Therefore, the
use of windbreaks may be of interest.
How airtight the greenhouse is, is fun-
damental. A very airtight greenhouse has a
limited leakage rate (see Table 7.1). Junctions
must be sealed. The 'thermal bridges' must
be eliminated.
On the other hand, if the greenhouse is
too airtight this may generate an excess of
environmental contaminants and humidity
and CO
2
depletion (if there is no CO
2
enrichment).
Energy economies
In Europe, heating expenses represent from
15 to 35% of the expenses of greenhouse cul-
tivation with a notable increase during the
last decades (Chaux and Foury, 1994b). To
decrease heating expenses without lowering
the temperature, cheaper sources of energy
are used. Also consumption is decreased
while improving the efficiency and reducing
the losses by radiation, convection, conduc-
tion and air leakage (Urban, 1997a).
During recent years, the efficiency of
boilers has been greatly improved. Energy
storage (as hot water) is used extensively in
the CO
2
and heating supply systems, when
the demands of both are not coincidental, as it
is very cost effective (Van de Braak, 1995).
Alternative sources of energy
In the context of energy costs (year 2004),
the majority of the alternative sources of
energy are not economically competitive.
Heat pumps
Heat pumps extract heat from a low tem-
perature source (cold source) to supply that
heat to a heating circuit. In efficient heat
pumps, temperatures above 55°C must not
be expected (Bordes, 1992).
Heat pumps can be used to dehumidify
the greenhouse environment but their use is
expensive.
Radiation losses
Geothermal and industrial hot waters
The greenhouse cover exchanges heat by
radiation with the sky (for calculations it is
accepted that the temperature of the night
sky is around 15°C lower than the outdoors
air). The clouds act as a screen, decreasing
radiation losses. In the south of France, with
clear winter skies, radiation losses may rep-
resent 60% of the total losses in an airtight
greenhouse (Urban, 1997a).
The geothermal gradient is approximately
1°C for every 30 m. Therefore, 1000-2000 m
must be perforated to obtain waters of
40-70°C (Urban, 1997a). These waters usu-
ally have a high concentration of salts and
are corrosive, being unavoidable-to-use heat
exchangers.
The facilities are expensive, although
the thermal energy is free, and the deprecia-
tion costs are high.
Losses by air leakage
These are more important the higher the
humidity and temperature differences
between the internal and the external air
and the less airtight the greenhouse is. The
wind increases heat losses by air leakage.
Cogeneration
The cogeneration of heat and electricity is
becoming popular, when the greenhouse
electricity consumption is high, because
