greenhouses I shade net I hydroponics I aquaponics
All greenhouse cultivation systems, regardless of geographic location, comprise fundamental climate control components; depending on their design and complexity, they provide more or less climate control, and condition to a varying degree plant growth and productivity.
Driving Forces for
Greenhouse Climate Control and Sustainable Energy Use
A
ir temperature – as well as solar radiation and air relative humidity – is one of the most important variables of the greenhouse climate that can be controlled. It conditions not only crop development and production but also energy requirements, which can account for up to 40 per cent of the total production costs. The majority of plants grown in greenhouses are warm-season species, adapted to average temperatures in the range 17–27°C, with approximate lower and upper limits of 10 and 35°C. If the average minimum outside temperature is < 10°C, the greenhouse is likely to
10 Undercover farming I
July/August 2021
require heating, humidity exceeds The challenge is to meet particularly at night. 95 per cent for long both needs: improved energy periods, particularly When the average maximum outside at night as this favours efficiency combined with temperature is an absolute reduction in the the rapid development < 27°C, ventilation overall energy consumption of fungus diseases such will prevent as Botrytis cinerea. The and related CO2 emissions of increased interest in excessive internal the greenhouse industry. temperatures during maintaining adequate the day; however, transpiration to if the average maximum temperature avoid problems associated with calcium is > 27–28°C, artificial cooling may be deficiency has resulted in humidity being necessary. expressed in terms of the vapour pressure deficit (VPD) or the moisture deficit, The maximum greenhouse temperature should not exceed 30–35°C for prolonged both of which are directly related to transpiration. periods. In temperate climates, heating Maintaining the VPD above a minimum and ventilation enable the temperature value helps to ensure adequate to be controlled throughout the year, transpiration and also reduces disease while at lower latitudes the daytime problems. During the day, humidity can temperatures are too high for ventilation usually be reduced using ventilation. to provide sufficient cooling during the However, at night, unless the greenhouse summer. Positive cooling is then required is heated, the internal and external to achieve suitable temperatures. The second important variable is humidity, temperatures may be similar; if the external humidity is high, reducing the traditionally expressed in terms of relative greenhouse humidity is not easy. humidity. Relative humidity within the range of 60–90 per cent has little 64 GAPs Because of the global rise in energy for greenhouse vegetable crops: Principles prices, greenhouse energy use became a major research issue. With the recent for Mediterranean climate areas effect increased interest in global warming and on plants. Values below 60 per cent may occur during ventilation in arid climates, or climate change, the use of fossil fuels is again on the political agenda and many when plants are young with small leaves, governments have set maximum CO2 and this can cause water stress. emission levels for various industries, Serious problems can occur if relative
I Volume 18 No 4
