Raaphorst, M.G.M.

English

Wageningen Yield 

185 
Conditioning in an Organic Greenhouse 
 
M.G.M. Raaphorsta 
Wageningen UR Greenhouse Horticulture 
P.O. Box 20 
2665 ZG Bleiswijk  
The Netherlands 
 
Keywords: energy, pest control, CO2, cooling, dehumidification  
 
Abstract 
Organic growers in the Netherlands have to cope with ambitious goals for the 
emission of CO2, which in practice requires that less fossil fuel is used for heating. 
With conditioning, the greenhouse air can be cooled and/or dehumidificated 
mechanically, so that excess summer heat is collected and can be stored in aquifers 
for reuse with the help of a heat pump in winter. The required energy can be 
provided through green electricity instead of fossil fuel for heating.  
Experiences with closed greenhouses have shown that mechanical cooling 
lowers the need for ventilation, thereby decreasing the CO2 emission and pest 
pressure. In addition, conditioning can give a more balanced indoor climate and 
allows for a better management of humidity, which greatly reduces the chance of 
outbreak of Botrytis and similar diseases. Wageningen UR Greenhouse Horticulture 
is carrying out a dedicated research with an organic grower who operates a 
conditioned greenhouse. In this project, the main objective is to find out in what way 
the advantages of cooling are helpful, or can be improved for organic production, in 
view of the very strict requirements about application of plant protection chemicals.  
Conditioning an organic greenhouse may give higher yield and a better 
control of diseases and insects, but not enough references are available to give a 
significant comparison.  
 
INTRODUCTION 
Organic greenhouses in the Netherlands emit more CO2 per unit of produce than 
conventional greenhouses (Kramer et al., 2000; Spruijt-Verkerke et al., 2002). Although 
the energy use per unit of organic produce has decreased in the last decade (Raaphorst, 
2009), organic growers still need to make more of an effort to show that their produce can 
be more environmentally friendly than conventionally-produced flowers and vegetables. 
That is why BiJo, a greenhouse company growing organic vegetables in the Netherlands, 
started a conditioned greenhouse. With conditioning, the greenhouse air can be cooled 
and/or dehumidified mechanically, so that excess summer heat is collected and stored in 
aquifers for reuse with the help of a heat pump in winter. In this way, the required energy 
can be produced using green electricity instead of fossil fuel for heating. 
Mechanical cooling lowers the need for ventilation, thereby decreasing pest 
pressure. In addition, conditioning can give a more balanced indoor climate and allows 
for a better management of humidity, which greatly reduces the chance of outbreak of 
Botrytis and similar diseases. Both these factors can be particularly helpful in organic 
production, in view of the very strict requirements for application of plant protection 
chemicals. In a greenhouse where CO2 fertilization is applied, this would have the 
additional advantage of decreasing CO2 emissions.  
Wageningen UR Greenhouse Horticulture, in cooperation with GreenQ, carried 
out a research project with BiJo. The use of energy, the effects on the indoor climate and 
the impact on the produce were the main topics. 
 
 
                                                 
a marcel.raaphorst@wur.nl 
Proc. First IC on Organic Greenhouse Hort. 
Eds.: M. Dorais and S.D. Bishop 
Acta Hort. 915, ISHS 2011 
186 
MATERIALS AND METHODS 
The greenhouse of BiJo consists of several sections. The smallest and newest 
section is a 2.5 ha venlo-type greenhouse where fruit vegetables (tomato, sweet pepper 
and cucumber) are grown. These crops need to be heated, and the average air temperature 
is maintained at 18 to 20°C. If the greenhouse air temperature increases to a level that is 
higher than necessary for the growth (22–27°C, depending on the global radiation rate), or 
if the relative humidity of the greenhouse air exceeds 85%, the greenhouse is cooled or 
dehumidified with an air conditioning device. This air conditioning device can also 
support the low temperature (40°C) heating pipes with the distribution of heat in the 
greenhouse. The hose pipes can be moved vertically up and down, depending on whether 
heating or cooling is required. 
The largest and oldest section is a 7 ha wide-span greenhouse, where green 
vegetables that do not need much heat are grown. This section can be heated with a 
relatively low water temperature (±20°C), using heat exchangers with fans and large hose 
pipes. This section cannot be cooled or dehumidified mechanically, but only uses the low 
temperature heat that is harvested in the conditioned section with fruit vegetables (Fig. 1). 
Heat and cold are provided by three heat pumps which use green electricity. A 
boiler that can be heated with bio-oil is installed as a backup and is used for heat supply 
during peak demand. 
CO2 for the enrichment of the greenhouse air, to improve the photosynthesis rate, 
is bought from a supplier because an electric heat pump does not produce any CO2. The 
bio-oil burner can produce CO2, but CO2 from bio-oil is more expensive and gives a 
higher risk of polluting the air in a semi-closed greenhouse. 
All the climate and energy data were collected from the climate computer (Priva 
Intégro) and analyzed using pivot tables (Microsoft Excel). 
 
RESULTS AND DISCUSSION 
Conditioning had a direct impact on the CO2-concentration, the humidity and the 
energy use of the greenhouse. Indirectly it affected yield, quality and pest control. 
 
Temperature 
A 24-h average temperature that exceeds 25°C can negatively affect the anthesis 
of fruit vegetables like tomato (Sato and Peet, 2005). Cooling the greenhouse with an air 
conditioning device can avoid problems with the anthesis. However, with a moderate 
outdoor climate near the coast of the Netherlands and a standard unconditioned 
greenhouse, the 24-h average temperature rarely exceeded 25°C. In fact, the temperature 
of conditioned greenhouses hardly differed from unconditioned greenhouses in the 
Netherlands. 
 
CO2-Concentration 
If the greenhouse was kept closed during the night, the CO2-concentration 
exceeded the range of the CO2-sensor (3000 ppm). This was partly caused by the CO2 
emission of the crop and mainly caused by the emission of decomposition gases from the 
soil and the organic fertilizers. Because very high levels of CO2 and other decomposition 
gases have been shown to damage the crop (de Visser et al., 2004), the vents were opened 
to remove these gases if the CO2-concentration was higher than 2000 ppm. 
In greenhouses without active cooling possibilities, the vents had to be opened 
during periods of high global radiation and a high outside temperature. The CO2-
concentration in the greenhouse could not be kept much higher than the outside 
concentration. Five hundred ppm is a common CO2-concentration in a greenhouse with 
CO2-enrichment and wide open ventilators. In a conditioned greenhouse, the ventilators 
rarely have to be wide open, so that the CO2-concentration in the conditioned greenhouse 
can increase to a level of 800–1000 ppm. This high CO2-concentration during high global 
radiation is supposed to give a 12% higher photosynthesis level than at a level of 500 ppm 
(Nederhoff, 1994). 
187 
The achieved CO2-concentration in the conditioned organic greenhouses of BiJo 
during the summer days was between 600 and 1000 ppm. This means that not all the 
possibilities of a conditioned greenhouse with increased CO2-concentration were 
achieved. 
 
Humidity Level 
In a conditioned greenhouse, it is possible to keep the humidity at almost any 
desired level. The cold water in the air conditioning device can have a temperature of 
±10°C. While a dew point of 10°C corresponds with an absolute humidity of 9.4 g m-3 
and the greenhouse air is kept at a temperature of 18°C (saturation vapor density is 15.4 g 
m-3), the relative humidity of the injected greenhouse air can be kept at 
9.4/15.4*100%=61%. This is much lower than necessary. The air conditioning device at 
BiJo has a minimum airflow of 30 m3 m-2.h, so at a greenhouse temperature of 18°C and 
an absolute humidity of 11.6 g m-3 (RH=85%), the dehumidification level can be 
30*(11.6-9.4)=66 g m-2 h-1, which is more than twice as much as the crop evaporation 
during the night.  
1. CASE 1: Low Global Radiation. If no cooling but only dehumidification is needed, 
the air conditioning device needs a higher water temperature than 10°C. This means that a 
large volume of greenhouse air is cooled and reheated for only a small quantity of 
dehumidification. Dehumidification with outside air would need less energy for reheating. 
The only advantages of dehumidification with an air conditioning device are the harvest 
of heat and the maintenance of a high CO2 level in the greenhouse. At BiJo, these 
advantages were mostly insignificant, thus dehumidification generally took place with a 
small opening in the energy screen and the ventilators in combination with additional 
heating. 
2. CASE 2: High Global Radiation. With a high global radiation, the greenhouse needs 
to be cooled. Cooling with outside air lowers the humidity more than cooling with an air 
conditioning unit. A very low relative humidity (RH) of the greenhouse air increases the 
risk of water stress in the plants. During periods with high global radiation, the RH in the 
BiJo greenhouse was only lower than 60% accidentally. The average RH during radiation 
higher than 800 W m-2 was 82% in all the cooled sections. If these sections had been 
cooled with outside air, the average RH would have been ±67%. 
 
Distribution of Humidity 
The distribution of the absolute humidity is affected by the way of 
dehumidification. With cucumber and tomato, the cooling and dehumidification took 
place at the bottom of the crop during the daytime. Figure 2 shows that the absolute 
humidity during the daytime averaged 2.5 g m-3 lower at the bottom of the crop than at 
the top of the crop. During the nighttime, hardly any dehumidification took place through 
the hose pipes and the absolute humidity was then higher at the bottom of the crop 
because the crop was the source of evaporation. With sweet pepper, the hose pipes were 
generally near the top of the crop, so the absolute humidity was higher at the bottom of 
the crop. These differences and fluctuations increased the need for more humidity sensors 
in a conditioned greenhouse. 
 
Pest Control 
In a completely closed greenhouse, it is possible to prevent insects and diseases 
from entering the greenhouse. However, it seems to be impossible to keep organic 
greenhouses completely closed because accumulation of CO2 and decomposition gases 
needs to be avoided using air exchange. Further, dehumidification of the greenhouse air 
with outside air generally needs less energy than dehumidification with an air 
conditioning device. Therefore, the ventilators at the BiJo greenhouse were opened almost 
every day. Although only a reduced number of insects can enter the greenhouse if the 
ventilator opening remains small (Körner and Jakobsen, 2006), several insects and 
diseases were found in 2009 and 2010 in the crops of the conditioned greenhouses. 
188 
After insects or diseases have entered the organic greenhouse, their populations 
must be controlled with biological products. Many predators that are used to control 
arthropods are less tolerant of low humidity than their prey. Many diseases only thrive 
under conditions of a very high humidity. While a conditioned greenhouse makes it 
possible to control the humidity, pests and diseases should be controlled as well. 
No quantitative measurements of the occurrences of pests and diseases were made 
in the conditioned organic greenhouse of BiJo. Also, references to an organic 
unconditioned greenhouse were not included in the research. Still, insects and mites 
seemed to be more easily controlled in a conditioned greenhouse if the right pest control 
means were also introduced. Also, hygienic measures during crop handling were still 
necessary to control diseases. 
 
Yield 
A higher CO2-concentration and a controlled humidity should increase the yield. 
The yield at BiJo has been relatively high in comparison with unconditioned organic 
greenhouses, but too few figures of organic crop production in the Netherlands are known 
to make conclusions of a significant yield improvement. It might be possible that the yield 
improvement in comparison with other organic greenhouses is also caused by the 
freshness of the soil. Many organic growers have to cope with nematodes and soil fungi 
after a few years of growing fruit vegetables in the same soil. 
 
Energy Use 
The use of heat in the conditioned greenhouse is more or less the same as in an 
unconditioned greenhouse. From January to July 2010, the heat demand at BiJo was 800 
MJ m-2. The main difference in the energy use at BiJo was that most of the heat was 
produced in a heat pump. The coefficient of performance (COP) of the heat pump 
averaged 6.5. This means that with 1 kWh of electricity, 6.5 kWh of heat was produced. 
Only electricity from renewable sources was purchased. Further, a bio-oil boiler was used 
during peaks in the heat demand. Because heat from bio-oil was much more expensive 
than from a heat pump, the use of the bio-oil boiler was kept to a minimum. 
 
CONCLUSIONS 
Conditioning an organic greenhouse may give higher yields and better control of 
diseases and insects, but not enough references are available to give a significant 
comparison. 
When neither extra CO2 nor harvested heat is needed, the most energy efficient 
way of dehumidification is with open ventilators. Dehumidification with an air 
conditioning device needs electricity for the fans and more heat to reheat the cooled and 
dried air. 
Conditioning in The Netherlands mainly gives advantages for the CO2-
concentration and the humidity in the greenhouse. A lower temperature is rarely needed 
for fruit vegetables in the Netherlands. 
Cooling from the bottom of the crop gives a different distribution of the absolute 
humidity than dehumidification from the top of the crop as seen with the ventilators. Thus 
conditioned greenhouses that can dehumidify from both levels need more humidity 
sensors than conventional greenhouses. 
 
ACKNOWLEDGEMENTS 
This research was supported through the program “Greenhouse as energy source” 
(Kas als energiebron), an initiative of the Ministry of Agriculture and Productschap 
Tuinbouw. The author also would like to thank the members of the board of directors of 
BiJo for their cooperation and Herbert Stolker of GreenQ for his advice. 
 
 
 
189 
Literature Cited 
de Visser, P., Gorissen, T. and van der Zweerde, W. 2004. Gasemissie na toediening van 
organische meststoffen in de glastuinbouw: risico's op plantschade in gesloten kassen 
[Gas emission after provision with organic fertilizers in the greenhouse horticulture: 
Risks of plant damage in closed greenhouses]. Plant Research International. 
Wageningen, Nederland. (in Dutch) 
Körner, O. and Jakobsen, L. 2006. A thrips pest pressure model for greenhouse climate 
control. Acta Hort. (ISHS) 718:407–414. 
Kramer, K.J., Ploeger, C. and van Woerden, S.C. 2000. Biologische glasgroenteteelt – 
Tussenrapportage bedrijfseconomische en milieukundige aspecten 1998–1999 [The 
cultivation of fruit vegetables in organic greenhouses - Report of business economic 
and environmental views]. Project 1211, Proefstation voor Bloemisterij en 
Glasgroente, Naaldwijk, Nederland, 44p. (in Dutch) 
Nederhoff, E.M. 1994. Effects of CO2 concentration on photosynthesis, transpiration and 
production of greenhouse fruit vegetable crops. PhD. Thesis, Wageningen 
Agricultural University, Wageningen, Nederland. 213p. 
Raaphorst, M. 2009. Ontwikkeling in de energie-efficiëntie in de biologische 
glasgroenteteelt 1998-2008 [Development of energy effciency in the cultivation of 
fruit vegetables in organic greenhouses]. Wageningen UR Glastuinbouw. Bleiswijk. 
(in Dutch) 
Sato, S. and Peet, M.M. 2005. Effects of moderately elevated temperature stress on the 
timing of pollen release and its germination in tomato (Lycopersicon esculentum 
Mill.). J. of Horticultural Science and Biotechnology 80:23–28. 
Spruijt-Verkerke, J., Schoorlemmer, H., van Woerden, S., Peppelman, G., de Visser, M. 
and Vermeij, I. 2002. Duurzaamheid van de biologische landbouw [Sustainability of 
organic agriculture]. p.110, Report number 328, Praktijkonderzoek Plant en 
Omgeving, Lelystad. (in Dutch) 
 
 
 
 
Figures 
 
 
 
 
 
Fig. 1.  The heat pump provides the fruit vegetables with cold and heat. The harvested 
summer heat is stored in an aquifer before heating the leaf vegetables in the 
winter.  
 
 
190 
 
 
 
 
 
 
 
 
 
 
 
 
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Fig. 2.  Cycle mean of the difference in absolute humidity (g m-3) between the top and the 
bottom of the crop in four conditioned sections. Cycle mean is between 1 April 
2010 and 5 June 2010. 


Conditioning in an organic greenhouse

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