Canada is a major producer of greenhouse tomatoes, peppers and cucumbers in North America. Greenhouse vegetable production is energy intensive. Growers have been installing thermal screens/curtains in glasshouses for years.
Flower growers have been using screens in double-polyethylene-skinned houses for decades. The practice is increasingly being adopted by vegetable growers in plastic houses. It appears that the technology will become standard equipment for professional vegetable growers in the next few winters regardless of glazing type. This overview is for those still considering energy screens.
CHARACTERISTICS OF THE CANADIAN CLIMATE
Ontario, British Columbia and Quebec all have significant greenhouse vegetable industries. All three have unique climates (see table 1). For comparison, a few other regions containing significant greenhouses are included.
High and low temperature extremes indicate the size of boiler and cooling that will be required for each climate as “delta T,” or the ability to achieve a temperature change compared with ambient. Average high and average low temperatures are typically a monthly average for the most extreme month. Note that temperature extremes can vary significantly from averages. Investments should not be based upon extremes but there should be contingencies in place for dealing with them.
Heating degree-days and cooling degree-days are used by designers to compare different regions. This table represents average degree data from the past three years. You can see that the Leamington grower area requires about 25 per cent more heating than the delta area in B.C. and that the Montreal region requires 25 per cent more heating than Leamington. More data than three years would provide conditions that could be considered “typical.
Annual degree data can be used to evaluate year-to-year fuel consumption and efficiency improvements. That is, one can only compare energy use performance year to year if comparing to the year’s specific winter weather and unit fuel cost.
The climate of British Columbia in the delta area west of Vancouver is remarkably similar to that found in southwestern Holland. As a result, greenhouse cultural practices and equipment can closely follow the experience that has been generated in the Dutch climate. Most greenhouses in British Columbia, not surprisingly, are glass.
In Quebec and Ontario, where the climates are more extreme, double-glazed houses are predominant. More recently, glasshouses are gaining favour with growers who seek higher light transmission and can manage energy costs with screens.
Higher light transmission in climates with warm, bright conditions will have cooling implications. What does not always reveal itself in climate data is weather. Monthly temperature data will not show that the emergence from winter in western Ontario presents growers with bright blue, clear skies. Night temperatures may still be rather low but this sudden, high radiation creates large diurnal temperature swings and significant challenges for growers.
Screen selection and operation will be dependent on the local climate conditions and represent a tool for helping to manipulate the greenhouse climate and soften temperature extremes...
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