Repelling Radiant Energy

Repelling Radiant Energy

For bedding plant growers who grow every month of the year, shading, cooling and ventilating prevent greenhouses from overheating and creating a stressful environment for the plants.

The greenhouse was developed for plant protection against local climatic extreme pressures such as wind, rain, snow and insects. The greenhouse structure is the starting point for controlling the climate or for creating a climate that is tolerable and ideal for plants. If one is growing year-round most anywhere in North America, there are certain months when overheating is a problem. The plants being grown need to be protected from a too stressful environment. Ventilating and/or cooling must be used to drive the temperature down.

From a practical perspective, the first step in the cooling of a greenhouse is to ventilate it or exchange the captured heat with fresh, outdoor air. For example, let the wind extract the hot air or install exhaust fans in the greenhouse to extract the hot air. If and when the ventilating capacity becomes limited and too stressful a climate is created, an additional mode of cooling a greenhouse is generally used. Most commonly, that would be with shading to reduce the extent to which solar radiation heats the greenhouse or the plants themselves. It could also be with either evaporative cooling, or in the rare case, air conditioning.

Generally, the goal is to drive the leaf temperature down rather than to reach a specific light intensity. Of course, there are some under-canopy/shade-loving plants that can’t handle light that is too intense as a result of the accompanying heat. A main point to consider with shading is it is most efficient to keep excess radiation away from the greenhouse before it comes in. Therefore, shading over the top of the greenhouse is more efficient than shading inside the greenhouse. Depending on greenhouse climate priorities and economies, it is common to use a dual-purpose shade/energy curtain or an inside curtain which is less costly because the greenhouse structure protects it.

When choosing a method of shading, the ideal strategy is to have something retractable. That way, you shade when it gets too hot, but not during the hours when too-high temperatures are not a problem and also when available light is a growth-limiting factor.

If you are using a fixed shade, shade paint or chalk, you are reducing the available light around the clock or over the period it’s applied. If light quantity is always excessive, this poses no problem. This can be less than ideal because there may be hours in the morning and evening when temperature is not a limiting factor and the greatest possible photosynthesis is desired. From a material selection viewpoint, it is advantageous to select reflective materials rather than ones that absorb light.

Ludvig Svensson is engaged in shading efficiency experiments with an engineering research group at North Carolina State University. Over the years, the company has worked with several of the top growers in orchids and bromeliads. Orchids need less light and lower temperatures, so shading becomes more important.

“I think most of the large orchid growers in the U.S. and Canada have discovered that the warm weather climate is critical to productivity and quality, and most can relate the cost of cooling these high value plants in terms of dollars and cents,” says Kurt Parbst, president of Ludvig Svensson. 

Trends To Watch

Some growers may be overshading their crops, according to research by horticulturists and engineers.

“My thoughts on this, speaking of retractable shades, are not to abandon a level of shade which will have strong impact on the greenhouse temperature, but have the discipline not to shade in terms of covered hours,” Parbst says. “I do not think putting a very light shade in a greenhouse is very useful, as it will have a smaller impact in the ability to cool the greenhouse.” Parbst points out the additional benefits of shading: worker comfort and irrigation (evaporation) management.

“If a high-value crop is grown in warm weeks or months, satisfy the crop’s light and temperature needs to a level that results in a satisfactory plant,” he says. “Then try to make a comfortable environment and one that does not rapidly dry out.”

Another trend is recognition of the value of the right climate. Investment trends are going away from adding more, simple acreage to planning efficient processes and selecting higher-performing materials, because timing and cost controls are more critical at this point in the market.

Guidelines For Designing With Natural Ventilation

1. Maximize airflow through the greenhouse at night when the air temperature is the lowest. This will help drop the average temperature of the crop and also remove heat absorbed by the structure and concrete.

2. Topography, landscaping and surroundings should redirect airflow into the greenhouse and give inlets maximum exposure to breezes.

3. Maximize the area of openings exposed to breezes, or orient long façade with vent openings to prevailing wind direction, if one exists.

4. Locate vents on opposite sides of a building rather than adjacent sides, which requires airflow to change direction.

5. Locate the openings in the sidewalls at the correct height, which is the location of the plants, whether it is the bench or floor.

6. Maximize the vertical distance from the sidewall inlet to the roof vents to maximize the stack effect. This means putting the sidewall inlets as low as practical and building a greenhouse as tall as is practical.

7. Inlet and outlet openings of nearly equal area maximize airflow.

Strategizing For Strength

The best way to navigate this selection process is to map out the ideal climate for the crop that is being grown on a month-by-month basis for all of the months that the greenhouse is planted, Parbst says. He recommends growers rank each month into priority levels according to which months hold the most valuable crops. Focus on these months first to satisfy the climate needs and what the local climate throws at you in terms of extremes. Continue through the year, trying to eliminate the climate extremes that would prevent you from growing a crop of satisfactory quality. If the months when the greenhouse is overheated happen to be months of high value, apply the right level of cooling technology to ensure success.

There are special considerations for natural ventilation, however.

“Many growers prefer natural ventilation because it is the lower-cost method,” Parbst says. “It is, however, less predictable than mechanical ventilation. Wind and thermal buoyancy forces generate airflow through the greenhouse. The effect is dictated by available wind and the arrangement and location of ventilation openings. Wind varies in speed and direction both on a daily and seasonal basis. Airflow caused by thermal forces is greatest when the height from inlet to the outlet is maximized and when there is a big difference between the indoor and outdoor temperatures. This is why tall greenhouses are generally cooler than the older, shorter greenhouses.”

Readers interested in more of the subjects of shading, cooling and ventilating production greenhouses will find Svensson’s Web site, www.ludvigsvensson.com, helpful.

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