Outside temperatures are lower than the ideal band and the house needs to be heated. Ideally there should be no ventilation of the house, as this will exhaust hot air from the house and introduce more cold air, which would increase the heating requirement. But at the same time there will be a humidity build-up in the house due to the transpiration of plants. In this case the best trade-off is to have a small ventilation rate to remove the humidity from the house, but heat the air as it enters the house. The air passing through the house will absorb the humidity and exhaust it as the air is exhausted.
In this scenario it is economically feasible to enrich the air with carbon dioxide to concentrations above ambient levels (350ppm).
The outside temperature is now within the ideal band of the plant. But the temperature inside the greenhouse will be higher than outside, because heat will accumulate in the greenhouse. If the house is ventilated at a moderate rate, the accumulated heat and excess humidity will be removed, creating the ideal conditions for plants inside. Enriching the air with carbon dioxide might still be economically feasible. But a lot more carbon dioxide will have to be added to the house, because a lot of it will be exhausted.
When the outside temperature exceeds the ideal for the plant, ventilation alone is no longer sufficient. Firstly, there would need to be a very high ventilation rate of the house in order to remove all the accumulated heat. At best, this will bring the inside temperature down to the outside temperature. To get the temperature down to within the ideal band, the incoming air would need to be cooled as well. Using an evaporative cooling system will cool the air as it enters the house. In this scenario it would no longer be economically feasible to enrich the air with carbon dioxide due to the massive volume of air that will pass through the house. Limiting the radiation energy entering the house by means of a reflective screen will help to keep the temperature down.
A summary of the actions required to maintain ideal conditions in the house for the various scenarios is given in Table 2. It should be obvious that there is a vast difference in the approach to handle the three scenarios. When a climate control system is chosen, one of the most important questions to answer will be which of the three scenarios will be prevalent for what duration of the year. But to be more accurate, RH and solar radiation also need to be considered.
Table 2: Actions required to keep the ideal temperature inside a greenhouse for different temperatures outside the house.
Figure 2 depicts annual average temperatures for the globe. If a location’s annual average temperature is lower than the ideal temperature band for a specific plant, the primary focus of the greenhouse operation at that location will be to heat up the inside of the house. If the annual average temperature for a location is the same or higher than the ideal band, the primary focus will shift towards ventilation and cooling to remove heat and cool the inside down.
Figure 2: Average annual temperatures.
Figure 3 depicts the total annual solar radiation energy received for the globe. Solar radiation received at the surface of the air is a function of latitude and the composition of the earth’s atmosphere above the surface. The further away (North or South) a location is from the equator, the less radiation it will receive. The higher the moisture content and cloud cover, the less radiation a location will receive. Looking only at latitude, one would expect the tropical regions to receive the most radiation. But because of the high moisture content and cloud cover in the tropics, the dry regions just North and South of the tropics receive the most solar radiation. Greenhouses situated at locations, which receive a high amount of solar radiation, would require a lot of ventilation during summer to remove all the excess heat that is streaming into the house. Greenhouses situated at locations that do not receive such high amounts of solar radiation would require much lower ventilation rates during the summer, because the heat build up in the house is not so much.
Figure 3: Total solar radiation received.
An ideal climate is required inside the greenhouse throughout the year. Therefore, one shouldn’t only look at average values, but also at seasonal variations and extremes. When all the variables are considered, four climatic regions can be identified as far as greenhouse production is concerned. They are cold, temperate, dry and tropical regions. A description of the typical climatic conditions for each of the regions is given in Table 3. The latitudes where each of the regions can typically be found is also included in the table. It is however possible to get locations outside the specified latitudes that will fit the description.
Table 3: Description of the typical climatic conditions by region.
When the information from Table 1, Table 2 and Table 3 is combined it can be seen that distinct climate control systems are required to obtain effective and economical climate control in greenhouses exposed to different outside climatic conditions.