C a r b on D i o x i d e
E n r i ch m e n t
Fuels & Figures
C arbon dioxide (C O 2 ) enrichment enhances the growth and
production of ne arly every crop. G reenhouse operators a ll over
the world are keen to inject extra C O 2 into the growing
environment, but it is expensive bec ause a p art of the va lua ble
C O 2 g as le a ks to the outside. In hot w e ather when the greenhouse is vented, it's not fe asible to ma inta in a high C O 2 level.
C arbon dioxide is often produced by burning fossil fuel. In that
c ase, the he at generated during C O 2 enrichment is either
utilised or stored for l ater use. H ence, a he at stora ge tank
(buffer) has become a stand ard piece of equipment in countries
with cooler night temperatures.
C arbon dioxide enrichment requires a lot of economic considerations.
How much C O 2 is needed? How much is taken up by the plants?
How much is lost by venting? W hat is the effect? How much he at
can be stored? How much he at can be used at night? This article
presents some generic figures to estimate the costs and benefits of
C O 2 enrichment, and discusses fuels, burners and buffers. A
following article will look at the risk of noxious g ases accumulating
in relation to C O 2 enrichment from fossil fuels.
Article by Elly Nederhoff
50
Practical Hydroponics & Greenhouses
Figure 1. CO2-curve. Crop production (%) at various levels of CO2(ppm). The
production at the ambient CO2-level (calc. 340 ppm in 1985) is assumed 100%.
180
160
140
relative growth (%)
I
ncreased CO2 uptake means faster growth, and more or
bigger plant parts (leaves, branches, flowers, fruit), and
thus higher yields. The effect of CO2 enrichment has
been proven in hundreds of scientific experiments
performed worldwide over many years. The results of
about 60 experiments with greenhouse crops were collated in
one 'CO2 curve' (Figure 1). This figure shows the relative plant
growth or yield versus the average CO2 concentration. The
CO2 uptake (or yield) at ambient CO2 was taken as 100%. At
the time when the research was done (in the 1980’s), the
ambient CO2 level was 340 ppm. The band is wide because it
accounts for a range of crops and for the different conditions
during the many experiments. The curve shows that at a very
low CO2level (150 ppm), plant growth is about 30-40% lower
than at ambient CO2. Increasing the CO2 level from 340 ppm to
500 ppm increases the growth by 15-25%, while an increase
from 340 ppm to 700 ppm potentially increases plant growth by
30 to 40%. Beyond 700 ppm, the effect of more CO2 gets
smaller. Beyond about 1,000 ppm, the line is nearly horizontal,
meaning there is almost no further effect of more CO2. These
results are valid for CO2 enrichment during daytime.
120
100
80
60
40
20
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
CO2 concentration (ppm)
The wide band is due to variation between crops and to conditions. This graph is based on
data from about 60 publications of experiments worldwide in many greenhouse crops.
(Source: Nederhoff, 1994)
CO 2 depletion
In cold climates, CO2 consumption can be higher than the
CO2 influx. This happens on a bright and cold winter day, when
venting is minimal and no CO2 is supplied. The good light
conditions induce high photosynthesis rates, i.e. plants consume
a lot of CO2. If there is no fresh air coming in, and no CO2
enrichment, the CO2 level will drop below the ambient level.
This situation is known as CO2 depletion (Figure 2a). The CO2
uptake then virtually stops, and growth is dramatically reduced.
For instance, at 150 ppm CO2, the CO2 uptake rate (hence
growth rate) is reduced to about two-third of the rate at
350 ppm. An extremely low CO2 level of around 100 ppm will
completely prohibit CO2 uptake and growth. Depletion only
occurs at daytime, caused by photosynthesis (CO2 uptake which
requires light).
Flue gases from gas-fired boilers are used for CO2 enrichment.
CO 2 enrichment in winter
In cold weather, i.e. in the winter months, ventilation is usually low.
A relative small dose of CO2 can maintain the CO2 concentration
at an elevated level. As a result, the growth will increase
considerably over that period. If any produce is harvested and if
prices are high, CO2 enrichment in winter can be extremely
feasible. Obviously, the CO2 effect lasts only over the period it is
applied. As soon as the CO2 enrichment is stopped, for instance,
when more venting is needed in spring, the impact of
CO2 enrichment on the production tapers off and disappears. If
the bulk of the production falls in late spring and summer,
CO2 enrichment in winter has only a modest effect on the
annual monetary result.
CO 2 enrichment in summer?
In warmer weather when venting is needed, it is obviously harder
to elevate the CO2 level. It is very much an economic issue: how
much CO2 injection is profitable? Generally, it is always economic
to inject a relatively small amount of CO2, only to prevent
depletion. The amount of CO2 required for that is only the
amount that is taken up by the plants for photosynthesis, which is
A heat buffer enables CO2 enrichment during the day, while using the heat
at night. The green tank is a large heat buffer for a 5 ha greenhouse.
INSET: Four heat buffers on one property.
May/June 2004 51
Figure 2. CO2 concentration in the greenhouse.
(Source: [email protected])
around 5 g/m2/hour (in good light conditions). This 5 g/m2/hour
prevents depletion, which means it lifts the CO2 level from
below-ambient to ambient level. This has a strong effect on
production, as can be seen from the steep rise in the CO2 curve
(Figure 1). The costs are relatively low; there is no loss of CO2 to
the outside as long as the CO2 is around ambient level
(see Figure 2b).
Enrichment to higher CO2 levels has to be considered on
its merits. What are the costs and benefits? The costs for
CO2 enrichment depends very much on the amount of
ventilation. Here are some examples of CO2 loss in various
conditions.
Effect of ventilation
CO2 meter
Figure 2a: CO2 depletion (below ambient level), which causes CO2 inflow.
CO2
CO2 meter
Figure 2b: CO2 inside the greenhouse at ambient level.
Ventilation has a strong influence on the loss of CO2 gas. We
can distinguish three situations. The first is CO2 depletion
(discussed earlier), with the CO2 concentration in the
greenhouse lower than outside. Any ventilation will bring fresh
air (thus CO2) into the greenhouse (Figure 2a), and ventilation
is beneficial.
The second situation is when the CO2 level in the greenhouse
is equal to the level outside (around 360 ppm - Figure 2b).This
happens when the CO2 uptake is exactly compensated by
CO2 supply and/or influx of fresh air. In this situation, there is no
CO2 lost to the outside.
The third situation is with elevated CO2 levels due to
CO2 enrichment (Figure 2c). A part of the CO2 gas will leak
rapidly out of the greenhouse, and even more by venting. The
CO2 loss depends on the vent opening, wind speed and
CO2 level.
Here are some calculated examples: at 500 ppm CO2 and
ventilators closed, the loss of CO2 will be around 2-3 g/m2/h. With
the vents a little bit open, it will be around 10-25 g/m2/h or more.
At 900 ppm, with the ventilators closed, the CO2 loss will be
around 3-5 g/m2/h, with a bit of venting 15-50 g/m2/h, or much
more with higher wind speed and vents open.
Note that these are only ballpark figures. It is impossible to
be exact about the venting, because then we also need to be
precise about the type of greenhouse, type of vents, wind speed,
and other factors that affect the ventilation rate.
CO 2 demand (instantaneously)
CO2
CO2 meter
Figure 2c: elevated CO2 levels due to enrichment, with CO2 lost by ventilation.
52
Practical Hydroponics & Greenhouses
The CO2 demand equals the CO2 uptake by the plants, plus the
amount of CO2 lost by venting. The CO2 uptake depends on
the crop, the size of the leaf surface, and the growing conditions.
For our approximation, we consider greenhouse crops that have
large lush foliage. Their CO2 uptake varies from zero during very
poor conditions to about 5g CO2/m2/h under excellent light
conditions, and up to 8g CO2/m2/h under excellent conditions
combined with high CO2 levels (which is rarely achieved).
The figures for CO2 loss by venting, and the figures for
CO2 uptake by the plants, are added to determine the total
CO2 demand. The results are shown in Table 1. With increasing
CO2 level, the CO2 uptake increases a bit, while loss by
ventilation increases exponentially. For example, at CO2 levels
below ambient (e.g. 200 ppm), the CO2 loss by venting is
zero - the supply must only compensate the CO2 uptake, which
is 0-3 g/m2/h. At ambient CO2 level (350 ppm), the CO2 uptake
is 2-5 g/m2/h, depending on the conditions. Also in this case, the
CO2 loss by venting is zero, and the total CO2 demand is
between 2 and 5 g/m2/h. At a CO2 level of 500 ppm, the
CO2 uptake is 3-6 g/m2/h, while the loss by ventilation varies
from 2 to 25 g/m2/h. Hence, the total demand is between
5 and 31 g/m2/h. At higher CO2 levels and especially with more
venting or more wind, the CO2 loss becomes very high. Again,
note that these are only ballpark figures.
CO 2 supply rates
In the early 1980’s, when CO2 enrichment started in the
Netherlands, the advised supply rate was 5 g/m2/h. Nowadays,
that is considered a minimum supply rate. Table 1 shows that
5 g/m2/h is sufficient to maintain the CO2 level at around
ambient level. It compensates for the CO2 uptake and prevents
CO2 depletion. Under low light, CO2 uptake can be less than
5 g/m2/h, and a CO2 supply rate of 5 g/m2/h is able to lift the
CO2 level to 500 ppm or more, where the supply and demand
are in balance.
Nowadays, growers inject CO2 at a much higher rate, for
instance, around 20 g CO2/m2/hour, or even more in peak hours.
This supply rate is much higher than the CO2 uptake rate, and it
will increase the CO2 level and maintain it even when the vents
are a bit open. Such a high dosing rate is only feasible when the
CO2 is cheap, for instance, when a heat storage buffer is used,
which will be discussed later.
Table 1: approximated CO2 supply rates needed to maintain a certain CO2 concentration,
either with no venting (only air leakage) or with a little ventilation (ballpark figures
only). Note that a standard minimum supply rate is 5g/m2 /h or 50 kg/ha/h.
CO2 concentration
200ppm
350ppm
500ppm
900ppm
CO2 uptake by plants (g/m2 /h)
0-3
2-5
3-6
4-8
CO2 loss by leak (g/m /h)
*
0
2-3
3-5
CO2 loss by little venting (g/m2 /h)
*
0
10-25+
15-50+
CO2 supply rate (g/m2 /h)
0-3
2-5
5-9
7-13
CO2 supply rate (kg/ha/h)
0-30
20-50
50-90
70-130
CO2 supply rate (g/m2 /h)
0-3
2-5
13-31
19-58+
CO2 supply rate (kg/ha/h)
0-30
20-50
130-310
190-580+
2
without venting
with little venting
* CO2 depletion (CO2 level below ambient) occurs when there is no CO2 supply. In fact,
CO2 flows from outside into the greenhouse, and is immediately absorbed by the plants.
No CO2 is lost.
May/June 2004 53
CO 2 consumption (annually)
CO2 increases the production of nearly every crop
The annual consumption of CO2 depends strongly on the
CO2 strategy chosen: if enrichment takes place only in winter or
year-round; only early in the morning or all day; only while the
vents are closed or also during venting; at which level; etc. Some
generic figures from growers' experiences: an absolute minimal
annual consumption would be about 3 kg/m2/year. This can be
600 hours of 5g CO2 /m2/h. If this amount is spread over three
winter months, it will be nearly seven hours a day. If the greenhouse is not vented (in cold weather), the CO2 level will be
fairly high, and the effect of CO2 on the crop will be quite
substantial over that period. If any produce is harvested in that
time, and if the prices are good, the CO2 will give good returns.
However, the effect is restricted to the three months only. If the
3 kg/m2 CO2 is spread over the year by injecting a small dose
each morning, the effect will be diluted and less noticeable. The
yield effect of using 3 kg/m2/year is usually in the order of 3% at
maximum.
A somewhat more generous CO2 enrichment will use
10 kg/m2/year. If this is used in periods of little ventilation during
the six colder months, for example, the effect can be good over
this part of the year. On an annual basis, the yield increase may
be in the order of 10%.
More generous CO2 enrichment is common now for
growers using gas-fired boilers and heat storage. The
CO2 enrichment is then smartly integrated with heating at night.
CO2 is then a by-product and available in a larger quantity and
at a reasonable price. Some growers supply in the order of
25 kg/m2/year or more. The increase in annual production can
be in the order of 20-25%, depending on the CO2 control
strategy used.
Annual consumption and annual yield
When the vents are open, enrichment to a high CO2 level
becomes unfeasible.
The flue gases of 'clean' fuels can be readily used for
CO2 enrichment.
54
Practical Hydroponics & Greenhouses
Coincidentally, the annual yield increase roughly matches the
annual CO2 consumption: 3 kg/m2/year CO2 injection may give
3% yield increase annually; 10 kg/m2/year CO2 injection may give
around 10% yield increase annually; while generous CO2
enrichment using 25 kg/m2/year CO2 may give up to 25% yield
increase per annum.
For a feasibility analysis, it is best to distinguish different
periods with a certain CO2 regime, e.g. the winter months, or
summer months. Estimating the cost is a simple multiplication:
the dosing rate (g/m2/h) x number of hours per day x number
of days in that period. This will give the amount (grams) of
CO2 needed for that period. To calculate the costs of the CO2,
the grams are converted to kilograms, and multiplied by the
CO2 price per kg. If CO2 is produced by combustion of fuel, the
price of CO2 can be calculated from the fuel price.
To estimate the benefits, consider the production over the
period under consideration and calculate the relative production
increase (in %) over that period using Figure 1. Do not overestimate
the effect of CO2. If the CO2 is high for only three hours in the
morning out of a 12 hour day, expect only a quarter of the
potential yield increase. Also, enrichment in very dull weather
conditions has slightly less effect than enrichment in sunny hours.
In some ornamental crops, CO2 enrichment also enhances
the product quality, which is an extra effect on top of the yield
effect. The quality aspects are not considered in the CO2 graph.
CO 2 enrichment from flue gas
Combustion of fossil fuel produces flue gases that contain a lot
of carbon dioxide gas. For instance, flue gas from natural gas
combustion contains 10-12% CO2. When a suitable fuel is
combusted, the flue gases are very pure, consisting of only
carbon dioxide (CO2 ), water vapour (H2O), air compounds,
and only traces of other (harmless) gases. Such flue gases can be
used directly for CO2 enrichment without any treatment or
filtration. Provided the CO2 concentration is not elevated too
much, such enrichment does not harm plants or people.
However, if flue gas is used that contains traces of impurities,
these can accumulate in a greenhouse and reach dangerous
levels. This happens especially in a closed greenhouse. The key
noxious gases to be aware of are sulphur (sulphur dioxide,
SO2), ethylene (C2H2), and oxides of nitrogen (NOx).
Therefore, it is always important to fully understand the process
before embarking on the use of flue gases for CO2 enrichment.
The composition of the flue gases should be examined
regularly, and the grower should be vigilant of flaws in the
combustion process.
Suitable fuels
The key factors for combustion are the fuel and oxygen supply.
If the fuel contains any sulphur, it will be burned to sulphur
dioxide (SO2), which damages plants. Therefore, only
low-sulphur fuels are suitable for CO2 enrichment. These are:
propane, butane, premium kerosene (paraffin), low-sulphur
natural gas, and low-sulphur oil. Low-sulphur natural gas as well
CO2 enrichment is applied on a large scale in large greenhouse. The more light intercepted by the leaves the higher
the CO2 uptake (photosynthesis).
May/June 2004 55
as propane and butane are used extensively for CO2 enrichment
in greenhouses in many countries. LPG, which is a mixture of
propane and butane, may give a problem though. Propane and
butane require a different amount of oxygen for good
combustion. If the fuel:air ratio is ideal for either propane or
butane, it is not ideal for the other, and it may cause small
amounts of unwanted gases. Coal, wood-products and heavy oil
are unsuitable fuels for producing CO2, because their flue gases
contain many components that cause severe damage to plants
(and possibly humans too).
Incomplete combustion
Heat buffers are needed, even when space is at a premium.
Using a suitable fuel is the first prerequisite, but it is no
guarantee for toxic-free flue gas yet. Noxious gases can also be
created by a faulty combustion process. The main risk is
incomplete combustion, which is caused by insufficient oxygen
supply to the burning process. As a result, so-called unsaturated
hydrocarbons are created. The most dangerous one is ethylene
gas (C2H2) which, in extremely low concentrations, does a lot of
harm to plants. Another one is carbon monoxide gas (CO),
which is fatal for humans. A third gas that can be due to low
oxygen supply is nitric oxide (NO), which is harmful for plants.
Some burners can operate only on full capacity, and they are
switched off when the temperature is reached, and come on
again when the temperature drops. However, each burner
produces harmful gases just after ignition. It is, therefore,
important not to switch the burner on and off unnecessarily, and
also not to use the flue gases that are produced directly after
start-up. It is even better to use a burner with modulated flame,
which does not switch on and off so frequently.
During operation, it is very important to have the correct
fuel:air ratio to warrant complete combustion. Good regular
service and maintenance is paramount. There are ways to
monitor for incomplete combustion (to be discussed in a
following article), so that the CO2 enrichment can be stopped
automatically if any trouble occurs in the burner.
Small burners inside the greenhouse
Two main groups of burners can be distinguished: hot air
heaters, i.e. small burners that release hot air directly into the
greenhouse, and in contrast, large boilers or central heating
systems. Hot air heaters are located inside the greenhouse,
either suspended or free-standing. They can be atmospheric
burners using natural draw of ambient air, or fan-assisted
burners, which is more common for the somewhat larger types.
Some hot air heaters have a stack to discard the flue gases,
while many others release their flue gases directly into the
greenhouse. Both types provide no control over the flue gases.
Hot air burners releasing the flue gas directly in the
greenhouse can cause extremely high CO2 levels. Moreover, any
noxious gases in the flue gas will accumulate, too. The crop can
look really worn out and can also suffer long-lasting damage
after a night in such unfavourable conditions. Hot air systems are
often meant for heating, but are also used for CO2 enrichment.
However, CO2 is most required when it is light or even sunny. It
is often undesirable to add more heat. So, in practice, these hot
air heaters are far from ideal for CO2 enrichment.
56
Practical Hydroponics & Greenhouses
Boiler
A boiler or central heating system is located outside the
greenhouse, and the heat produced is ducted to the greenhouse
and distributed through a hot water pipe heating system. The
flue gases either go out through the stack, or are utilised for
CO2 enrichment, provided they are pure enough. A large fan,
called the CO2 unit, picks up the flue gas, pumps it into a
transport duct, pushes it to the greenhouse, and spreads it
through a network of plastic lay-flat ducts.
It is good practice to have a condenser between the boiler
and the CO2 unit. The condenser reduces the flue gas
temperature and takes the moisture out of the flue gases. In this
way, it extracts the last bit of energy and uses that for heating. In
the absence of a condenser, the CO2 unit will mix flue gases
with ambient air to lower the temperature of the flue gases.
Then, the CO2 unit must be much bigger. A drain pit in the
CO2 transport duct is necessary to collect the condensation
water from the flue gases. A boiler with a pipe heating system is
much better for CO2 enrichment than a hot air burner, because
the input of heat and of CO2 can be controlled separately.
These mini CO2 generators add a
small amount of extra CO2 to the air.
CO 2 and heat demand not synchronised
All burners produce heat and CO2 at the same time. This is not
ideal, because heat and CO2 are required at different times.
Obviously, heat is needed when it is cold (in winter, mostly at
night), whereas CO2 is required especially when it is sunny and
warm (always daytime, and especially in summer). It would be
handy if the CO2 produced during heating (at night) could be
stored until the next day. But this is not (yet) possible, at least
not in an economic and practical way. The other option is
storing heat for later use, and this is occurring more and more.
Heat storage tank (buffer)
Increasingly, modern glasshouses, especially the larger ones, are
equipped with a heat storage tank (or buffer). They are almost
exclusively found on properties with natural gas fired boilers
where the flue gases are used for CO2 enrichment, especially in
areas with low night temperature. In The Netherlands, 50% of all
glasshouses have a heat storage tank, while New Zealand has
around 15 heat buffers on greenhouse properties. Buffers are
also found in other European countries, Canada, USA, and
perhaps elsewhere too.
A buffer is an insulated steel tank filled with water. It is part
of a closed loop with the boiler, transport ducts and heating
spirals. Smaller tanks can lay horizontal or stand upright. The
larger storage tanks are upright tanks with a maximum height of
10 metres, and up to 8 metres diameter (500m3 volume). Even
bigger is possible. The top of the tank is filled with nitrogen to
allow expansion of the water.
Originally, the only purpose of a buffer was to store heat that
was produced during the day for CO2 enrichment, and to
release that heat during the night for heating the greenhouse.
Nowadays, in The Netherlands, heat buffers are also used for
energy management in a wider sense. The traditional buffer was
a so-called 'closed buffer', whereas the new system is called
'open buffer'. These two variations will be discussed in a
following article.
The terminology about buffers may need some clarification.
An old standing hot-air heater that can also be used for
CO2 enrichment.
A modern hot-air heater that is especially developed to
produce clean CO2.
May/June 2004 57
Growers will say that the buffer is 'full' or 'empty'. This has
nothing to do with the amount of water in the buffer, as the
buffer is always full of water. But 'empty' means that the water
in the buffer is at its minimum temperature, which is the
temperature of the return water. This is, for instance, 40oC in
summer and 55oC in winter. 'Full' means that it has reached its
maximum temperature, which is usually 95oC.
Size of the buffer
The first buffers in the 1980’s in The Netherlands, had a capacity of
25m3 water on a one hectare greenhouse area. In recent years,
most new buffers in The Netherlands are 100-150 or even 200m3
capacity per ha. In 2001, the average buffer size was 94m3 per ha.
The size of the buffer determines the amount of heat stored for
the night. In regions with very cold nights, it is useful to have a large
buffer, whereas in areas with mild night temperatures, a large buffer
leads to energy waste. In countries with fairly mild night temperatures (not far below zero), the buffers should be of a much smaller
size than those in The Netherlands.The size should be calculated
on the basis of heat and CO2 demand patterns.The amount of
heat that can be stored, and the amount of CO2 that can be
produced, are calculated in the example that follows.The examples
are for natural gas, because that is normally used for CO2
enrichment and heat storage.
Energy and CO 2 content of natural gas
To make calculations for a heat buffer, we need to know the
energy and CO2 content of natural gas. There are two figures
for energy content: the gross calorific value (GCV) and the net
calorific value (NCV). The difference between the GCV and
the NCV is about 10%. It is the amount of energy that
becomes available when a condenser obtains the energy out
of the water vapour in the flue gases.
Natural gas can have a different composition in different
places of the world. As an example, we use the characteristics
of natural gas in Australia and New Zealand. The GCV is
around 39 MegaJoule per m3 volume (MJ/m3), and the NCV
around 35 MJ/m3. Another characteristic is the amount of
CO2 produced by combustion of a fuel. For NZ natural gas,
this figure is 52.8 kg/GJ. This means that 1 GigaJoule (GJ) of
natural gas produces 52.8 kg CO2. Or expressed per m3 gas,
nearly 2kg CO2 is produced per m3 natural gas. By
comparison, natural gas in The Netherlands has about
10% lower GCV and NCV and also a slightly lower CO2 content.
Heat content of a buffer
The heat storage capacity of a heat buffer can easily be
calculated.The key factor is the heat capacity of water, which is
4.2 MJ/m3/oC.This means that input of 4.2 MegaJoule of energy
increases the temperature of m3 water by 1oC. If a heat storage
tank is, for instance, 125m3 and the temperature increases, say,
from 55 to 95oC, the amount of heat stored is:
125 x 40 x 4.2 = 21,000 MegaJoule (MJ), or 21 GigaJoule (GJ).
We have to consider that the boiler and the buffer are not
100% efficient: they lose about 10% energy each (it can be more
or less than 10%). Hence, the amount of energy to be burned
by the boiler is about 10% higher (23.3 GJ), and the amount that
can be retrieved later from the buffer is 10% less (19 GJ).
58
Practical Hydroponics & Greenhouses
Natural gas combustion and CO 2 levels
How much gas must be burned to obtain a certain CO2 flow?
The required amounts of CO2 (Table 1) can be used to
calculate how much natural gas needs to be burned. For
instance, the required CO2 supply rate of 2 g/m2/hour is first
converted to 20 kg/ha/hour. Then we use the fact that 1 GJ of
natural gas is equivalent to 52.8kg CO2 (see earlier). To
produce 20kg CO2/ha/hour requires combustion of 10m2 or
nearly 0.4 GJ of natural gas per ha per hour. To obtain the very
high CO22 dosing rate of 190 kg/ha/h, we need to burn
95 m3/ha/h or 3.6 GJ/ha/h. These and other data are also
shown in Table 1.
How much CO2 is produced when the buffer is completely
full? We take as a starting point the fact that 23.3 GJ of natural
gas must be burned to fill a buffer of 125m2. We saw that 1 GJ
natural gas produces 52.8kg CO2. Hence, 23.3 GJ is equivalent
to 1,230 kg CO2. If the greenhouse is, for instance, 2 ha, and we
assume there are eight hours in a day for CO2 enrichment, it
follows that the average CO2 production rate is 77 kg CO2 per
ha per hour (1,230 : 2 : 8 = 77). The table shows that 77 kg/ha/h
CO2 is sufficient to maintain a CO2 level of 900 ppm under
fairly good light conditions, if the greenhouse is closed (note
that this only happens at very low outside temperature!). With a
little bit of venting, this amount of CO2 will be enough to
maintain about 400 ppm or so. Obviously, if the windows are
opened further, more CO2 is lost and the CO2 level will drop.
Care for the environment
CO2 gas is a so-called 'greenhouse gas' and it contributes to
global warming and potentially climate change, too. Therefore, it
must be used very sensibly. Too generous CO2 enrichment
causes CO2 to escape unnecessarily out of the greenhouse into
the atmosphere. CO2 taken up by plants is fixated and kept out
of the atmosphere, which is very desirable (although this fixation
is only temporarily). Moderate CO2 supply, just to avoid
depletion, does not lead to CO2 loss and is, therefore, more
desirable. An even better solution is to utilise the CO2 that is
produced by other processes and that would otherwise be
emitted to the atmosphere.
beneficial for plants and unnecessarily expensive. For plants that
are sensitive to physiological damage (young plants, stressed
plants, sensitive species), the maximum setpoint should be quite
conservative, e.g. at 700 or 800 ppm CO2. Too high CO2 levels
cause partial closure of the pores in the leaves, which is not
good. Also, a higher CO2 concentration causes a higher risk of
accumulation of noxious gases that can be present in the
CO2 gas. Obviously, CO2 enrichment at night is absolutely not
useful, because there is no CO2 uptake at night.
Next article
A following article will look into noxious gases, and how they
can accumulate in the greenhouse.
Acknowledgements
With thanks to Ton Rijsdijk in The Netherlands
([email protected]). More information on greenhouse climate
control can be found on his website: www.cli-mate.net. Also,
thanks to Frank Hectors (Heko Ltd: [email protected],
021-978514), and Richard Hectors (RTF Ltd: [email protected],
021-550572) for practical information on heating installations.
About the author
Elly Nederhoff is a horticultural consultant for greenhouse
technology based in New Zealand. Elly can be contacted at:
Technolutionz Ltd, PO Box 521, Palmerston North, NZ
Ph: +64 (06) 355-4989 Fax +64 (06) 355-4969
Email: [email protected] ❧
VENTILATION PROBLEMS?
THERE’S AN AIR OF QUALITY BLOWING YOUR WAY
Settings for CO 2 enrichment
CO2 enrichment is normally controlled by the climate control
computer.The aim is that the benefits of CO2 enrichment must
outweigh the costs.This depends on the yield increase due to
CO2, on the price of the produce, as well as the price of
CO2. Moderate CO2 enrichment is sometimes more economic than
excessive enrichment. Some advanced programmes can calculate the
optimal CO2 level. Other programmes use some basic settings, for
instance, a high setpoint (e.g. 900 ppm) to be achieved when there is
no venting, and a low setpoint (e.g. 360 ppm) to be achieved at a
certain amount of venting. In between, the CO2 setpoint varies
proportionally.The CO2 level that can actually be achieved is limited
by the supply rate.Therefore, some growers don't use a CO2 setpoint
(target level), but use a setting for the rate of CO2 injection. A
recommended minimum supply rate is 5 g CO2/m2/h.
The setting should achieve the outcome that there is never
CO2 depletion, but also that not too much CO2 is wasted.
CO2 enrichment should not go beyond 1,000 ppm, as it is not
Our stirring fans are ideal for greenhouses, working areas,
dairies, chicken sheds & shearing sheds
STIRRING FAN AVAILABLE AS 19" OR 30"
• Distributes heat build up from roof to plants during winter
• Lowers greenhouse temperatures during summer
• Stops condensation on leaves and house surfaces
• Promotes growth and helps pollination
• Stops black spot and other diseases
• Allows air to hold moisture, creating a climate plants thrive in
AIR MOVEMENT SPECIALISTS
4 King Drive Horsham VIC 3400 Australia
Tel: (03) 5382 5688 Fax: (03) 5382 5465
Email: [email protected]
Web: www.smallaire.com.au
Manufacturers of Custom Made Air Moving Equipment Since 1974.
May/June 2004 59