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.