Driving Forces for Greenhouse Climate Control and Sustainable Energy Use

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.

Air 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 require heating, particularly at night. When the average maximum outside temperature is < 27°C, ventilation will prevent excessive internal temperatures during the day; however, if the average maximum temperature is > 27–28°C, artificial cooling may be necessary. The maximum greenhouse temperature should not exceed 30–35°C for prolonged periods. In temperate climates, heating and ventilation enable the temperature to be controlled throughout the year, while at lower latitudes the daytime temperatures are too high for ventilation to provide sufficient cooling during the summer. Positive cooling is then required to achieve suitable temperatures. The second important variable is humidity, traditionally expressed in terms of relative humidity. Relative humidity within the range of 60–90 per cent has little 64 GAPs for greenhouse vegetable crops: Principles for Mediterranean climate areas effect on plants. Values below 60 per cent may occur during ventilation in arid climates, or when plants are young with small leaves, and this can cause water stress. Serious problems can occur if relative humidity exceeds 95 per cent for long periods, particularly at night as this favours the rapid development of fungus diseases such as Botrytis cinerea. The increased interest in maintaining adequate transpiration to avoid problems associated with calcium deficiency has resulted in humidity being expressed in terms of the vapour pressure deficit (VPD) or the moisture deficit, both of which are directly related to transpiration. Maintaining the VPD above a minimum value helps to ensure adequate transpiration and also reduces disease problems. During the day, humidity can usually be reduced using ventilation. However, at night, unless the greenhouse is heated, the internal and external temperatures may be similar; if the external humidity is high, reducing the greenhouse humidity is not easy. Because of the global rise in energy prices, greenhouse energy use became a major research issue. With the recent increased interest in global warming and climate change, the use of fossil fuels is again on the political agenda and many governments have set maximum CO2 emission levels for various industries,

The challenge is to meet both needs: improved energy efficiency combined with an absolute reduction in the overall energy consumption and related CO2 emissions of the greenhouse industry.

including the greenhouse sector. There are two main ways to increase greenhouse energy efficiency: to reduce the energy input into the greenhouse system; and to increase production per unit of energy. The challenge is to meet both needs: improved energy efficiency combined with an absolute reduction in the overall energy consumption and related CO2 emissions of the greenhouse industry. Technological innovations must focus on energy consumption for the return to productivity, quality and societal satisfaction. There are a range of greenhouse system technologies which can be adopted by growers to improve climate control and energy use. However, there are numerous obstacles and constraints to overcome. The existing technology and know-how developed in north European countries are generally not directly transferable to Mediterranean climate countries; high-level technology is beyond the means of many growers due to the high cost compared with the modest investment capacity; and know-how from north European growers is often inappropriate for the problems encountered in South Africa. Where these technologies may be adopted, it is necessary to train and educate local growers. To this end, specific research and development tasks have been initiated by Universities with a Horticulture Department, Agricultural Research Institute and representative companies for European and other greenhouse companies. The issues addressed above concern the means and best practices by which local growers can alleviate the climategenerated stress conditions that inhibit the growth and the development of crops during a long, warm season in a sustainable and energy-friendly way. By; Sjaak Bakker of Wageningen UR Greenhouse Horticulture et al.