Roses and other cut flowers are always in season—if more popular perhaps on Valentine's Day—thanks to greenhouses. These hothouses provide a warm, safe environment in which all kinds of plants can thrive year-round. But what's good for the plants may not be good for the planet. It turns out that greenhouses aren't so green on the energy conservation front.
Most of those Valentine's roses in the U.S., along with cut chrysanthemums and carnations—the three most popular cut flowers worldwide—are grown in Colombia. Even near the equator, where the temperature and sunlight are already perfect for these flowers, growers still use greenhouses to reduce pest infestations and fungal infections as well as to control the amount of water plants get.
But in other climes, outdoor temperature extremes mean that greenhouses have to be heated or cooled. A prime example is the Netherlands. The summer sun raises the inside temperature so high that plants would die unless growers opened the greenhouse windows, which sends energy out the window—literally. And in places like northern Europe, sunlight is so scarce in the winter that greenhouses must be heated with growing lights, which takes energy.
But greenhouses are big business in the Netherlands; they account for nearly 80 percent of the energy used in agriculture there. The Netherlands also sells structural components for greenhouses to the U.S., and is one of the top three largest exporters of agricultural products in the world, along with the U.S. and France. Its exports include cut flowers, nursery stock and food plants such as tomatoes, cucumbers and sweet peppers—most greenhouse grown.
So it is perhaps no surprise that the Dutch are working on "greening" the greenhouse. Until recently, most efforts to improve them involved installing thicker insulation or blocking sunlight during summer months. But Piet Sonneveld, an agricultural engineer at Wageningen University in the Netherlands, and his colleagues designed a prototype greenhouse that creates an indoor environment ideal for the Netherlands's crops and saves energy in the process.
He designed a special removable film that blocks near-infrared radiation—a wavelength of sunlight that, unless filtered out, can overheat and damage plants during the summer. The film is just 80 microns thick (one micron equals about four one hundred-thousandths of an inch) but contains hundreds of layers of plastic polymers that filter out infrared light while allowing plant-nourishing visible light to pass through. In the wintertime, when heating is necessary, the film is removed.
But Sonneveld's team wanted to make their greenhouse energy-independent as well. Toward that end, the researchers used photovoltaic cells to convert some of the sunlight into electricity. To make the most of sunlight year-round, Sonneveld gave the greenhouse a specially curved roof. The curve focuses light on clusters of solar cells—six in each five- by 11-square-foot (0.5- by one-square-meter) section of the roof. The curve acts like a magnifying glass, concentrating the sunlight on the cells so that it is 30 times stronger. Motors powered by the solar cells in turn move large actuator arms to reposition the solar cells to follow the sun in its track across the sky, thereby achieving the most efficient angle year-round.
The prototype has been up and running since June. The solar cells now provide 98 percent of the energy needed, but Sonneveld hopes to up that to 100 percent by 2020. An energy-independent greenhouse could also be used in desert areas that lack electricity. Another plus: "Less ventilation keeps more [climate change–causing] CO2 [carbon dioxide] in the greenhouse and makes plants grow better," Sonneveld says.
Sonneveld's improvements could be a boon in the U.S., too, although the current trend here is larger greenhouses. The trade journal Greenhouse Grower tracks the top 100 greenhouse growers in the U.S. First on the 2008 list was Color Spot Nurseries, which has greenhouse space totaling 13.65 million square feet (1.27 million square meters). In its Fallbrook, Calif., location, 43 greenhouses are 58,500 square feet (about 5,400 square meters) each. In contrast, a typical European greenhouse is around 1,075 square feet (100 square meters), and Sonneveld's creation is just 107.5 square feet (10 square meters).
Greenhouses on this supersize scale would be too expensive to build using Sonneveld's design, which will cost an estimated $128 to $192 more per square foot, roughly twice what they cost now, according to Sonneveld.
But massive greenhouses still need to block the summer sun and the inside temperatures must be kept even all year. Terry Shores, maintenance manager for Color Spot's Fallbrook nursery, says his company uses layers of ultraviolet treated plastic. The ultraviolet treatment protects the plastic from being broken down by sunlight. Little blowers inside the greenhouse called puffer fans blow air through tubing nestled inside two layers of plastic. The fans run 24 hours a day to maintain a dead air space that helps insulate the greenhouse by keeping temperatures around 65 degrees Fahrenheit (18 degrees Celsius) and protects plants from temperature fluctuations. In the summer, the crew sprays on a white paint with a chalky consistency that is specially made to reflect sunlight but easy to remove. As the colder winter months set in they scrub it off with push brooms. Winters, at least in the San Diego area, are usually warm enough so that heaters only have to be used 10 percent of the time.
Estimates show that more than 80 percent of energy used for greenhouses goes toward such heating. In regions with cold winters like the Midwest, growers use heaters throughout the cold season to maintain a temperature at 65 degrees F. Just as heated air can seep out of houses, it can leak out of greenhouses, as well. At Neal Mast & Son in Grand Rapids, Mich., chief operating officer and owner Jim Mast says that heating with hot-water boiler systems accounts for the majority of the company's energy costs.
To defray these costs, Mast also uses fans to blow air through layers of plastic. This "air pillow" insulates walls and keeps heat in just as home insulation keeps heat in and thus minimizes the time the boilers have to run. Rather than painting roofs with layers of chalky white, he uses curtains made of aluminum and polyethylene that span the inside roof of the greenhouse and shades the entire floor area. The curtains are opened during the day to let in the sun, then drawn at dusk; Mast estimates that using this system saves 30 percent of energy daily.
For commercial growers, energy costs represent the third-largest expense after labor and stock, but figuring out how much energy greenhouses use is not so easy. Climate, size, construction materials and heating systems all determine the amount of energy expenditure. The University of Wisconsin–Madison has an online tool U.S. growers can use to figure out which energy-saving technologies would be most economical; it gives baseline energy costs, as well.
Scott Sanford, U.W.–Madison's outreach specialist, used the online tool to compare two, 23,400-square-foot (2,175-square-meter) greenhouses, one glass and the other plastic (double-polyethylene)—both materials commonly used to construct greenhouses. He used the most inefficient heater available and kept the greenhouses at 65 degrees F during the day and 70 degrees F (21 degrees C) at night to figure out how many therms of energy would be used in one year: He found that the glass greenhouse used 88,339 therms, whereas the double-poly burned 52,900 therms. (A therm is equal to 100,000 Btu and is the sales unit used for natural gas; a Btu, or British thermal unit, is the amount of heat required to raise the temperature of one pound of liquid water one degree Fahrenheit at a specified temperature, such as 39 degrees F, under a constant pressure of one atmosphere.)
The type of energy-saving materials chosen will depend on what's most cost-effective for growers. "In the greenhouse industry," says Sanford, "people will choose energy efficiency because keeping greenhouse costs low is the only way they will be able to stay in business."