Article originally printed in Flower Tech Volume
5, No. 7, 2002.
Wind and the difference in temperature between
the inside and outside of a greenhouse have varying effects on natural
ventilation depending on the time of the year.
Wherever greenhouses are located, from Norway in
the Artic to Australasia, almost all will require cooling at some time
during the year to prevent the internal temperatures becoming too high
and causing damage to plants. In greenhouses used to produce commercial
crops, this cooling is almost always provided by ventilation, a process
by which the warm internal air is replaced by cooler external air.
Although ventilation is viewed primarily as a way
of keeping greenhouses cool, it is also very important in removing the
water vapour transpired by plants and so ensures the greenhouse humidity
remains at an acceptable value.
Ventilation is also an important means of replacing
carbon dioxide (CO²) used by plants in photosynthesis. In a
ventilated greenhouse during bright sunlight the CO² concentration
in a crop canopy can be reduced below 300 parts per million (ppm) compared
to the 360 ppm in the external air. Without ventilation to provide
a continuous supply of CO² the concentration would become much lower
and plant growth would be reduced. However, when CO² is added
to enrich the greenhouse atmosphere to concentrations above 360 ppm, ventilation
is a disadvantage as it removes CO², which will have cost the grower
money.
Methods of air exchange
Air exchange can be achieved by placing fans along
one greenhouse wall to extract internal air. This is replaced by
external air entering through openings in the opposite wall. This
enables the rate of air exchange to be accurately controlled and gives
reasonable control over the air flow through and over the plant canopy.
The main disadvantage of fan ventilation lies in the cost of the electricity
required to run the fans.
The most common form of ventilation uses windows
or ventilators that open in the roof and in some greenhouses also in the
sidewalls. The exchange of air through these openings occurs naturally
because of the external wind and the difference in temperature between
the inside and outside of the greenhouse; consequently this form of ventilation
is known as natural ventilation.
When fan ventilation is used the airflow can be
predicted from the number and capacity of the fans being used, provided
the inlets have been correctly designed and do not present a high resistance
to air flow. Also, the nature of the airflow, most importantly in
the vicinity of the crop, can be predetermined to a reasonable extent.
Unfortunately, with natural ventilation this is not so straightforward.
However, we know that the rate of air flow depends on the following three
factors:
1. area of the open
ventilators
2. internal-external
temperature difference
3. wind
Comparisons
The total area of the ventilators in the roof
slopes and possibly also in the greenhouse walls, is frequently quoted
as a percentage of the ground area covered by the greenhouse. This
provides a simple way of comparing the ventilation of different greenhouses.
Larger ventilator areas will give higher rates of air exchange and so provide
temperatures closer to those outside the greenhouse. Values for the
area of ventilators in glass and film plastic covered greenhouse are generally
between 10 and 25%, although there have been greenhouses with ventilator
areas of 33%. The widely used Venlo glasshouse has a ventilator area of
between 15 and 20%.
The Opening Roof type of greenhouse that has appeared
in recent years provides a ventilator area in the region of 90%.
In sunny conditions the temperature inside a greenhouse
is almost always higher than outside and air exchange can be created by
opening the ventilators. The warm internal air is less dense than
the cooler external air and so is more buoyant. This creates an exchange
of air through an open ventilator as warm air leaves via the upper part
of the opening and cooler air enters through the lower part. The
rate of air exchange depends on the temperature difference between the
inside and outside air. However, the relationship is not linear i.e.
the rate of air exchange is not doubled if the temperature difference is
double; the rate of air exchange depends on the square root of the temperature
difference as shown in Figure 1. This means that when the temperature
difference is small, a small change will produce a greater change in air
flow than when the temperature difference is large.
Wind Speed
Wind produces ventilation by creating a pressure
distribution over the greenhouse surface. This creates inflow through
ventilators in high pressure regions and outflow in low pressure regions.
The rate of air exchange produced over a period of time is proportional
to the mean wind speed. However, the wind does not occur at a constant
speed but as gusts with speeds that fluctuate in time. These speed
fluctuations cause changes in pressure at individual ventilators and also
at points with a single ventilator. As a consequence there are fluctuations
in the flow as air alternately enters and leaves individual ventilators
or parts of a ventilator. It is a characteristic of the wind that
greater fluctuations in the speed of gusts occur when the mean wind speed
is high than when it is low. In practice it is found that if the
wind speed is doubled, the ventilation rate will also double without any
change in ventilation area.
Rate of Exchange
In most real situations both wind and temperature
difference will contribute to the actual rate of air exchange. Theoretical
analysis of greenhouse ventilation, with both wind and temperature difference
making contributions, show that the influence of temperature difference
is only apparent at low wind speeds. Analysis of ventilation experiments
has shown that if ‘Windspeed x Windspeed ÷ Temperature Difference’
is greater than 1, wind driven ventilation will be the main mode of ventilation.
If the value is less than 1, then temperature difference provides the main
mode of ventilation. When calculating this quantity, the wind speed
is measured in metres per second and the temperature difference in °C.
In summer, when the ambient temperatures are high,
the aim is usually to keep the greenhouse as cool as possible. The
temperature difference is then likely to be less than 2-3°C.
Consequently wind will be the main driver of ventilation when the wind
speed is higher than 1.5m/s. However in winter, especially at high
latitudes, while the greenhouse temperature will be typically 15°C,
the ambient temperature can be very low. If it is 0°C, then the
influence of temperature difference will dominate unless the wind speed
exceeds nearly 4 m/s.
Therefore, wind is the main weather factor influencing
greenhouse ventilation in summer, whereas the influence of temperature
difference becomes more important in winter.
