A greenhouse (also called a glasshouse) is a building where plants are grown. These structures range in size from small sheds to very large buildings. A miniature greenhouse is known as a cold frame.
A greenhouse is a structure with different types of covering materials, such as a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming visible solar radiation (for which the glass is transparent) from the sun is absorbed by plants, soil, and other things inside the building. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. In addition, the warmed structures and plants inside the greenhouse re-radiate some of their thermal energy in the infra-red, to which glass is partly opaque, so some of this energy is also trapped inside the glasshouse. However, this latter process is a minor player compared with the former (convective) process. Thus, the primary heating mechanism of a greenhouse is convection. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature drops considerably. This principle is the basis of the autovent automatic cooling system. Thus, the glass used for a greenhouse works as a barrier to air flow, and its effect is to trap energy within the greenhouse. The air that is warmed near the ground is prevented from rising indefinitely and flowing away.
Although there is some heat loss due to thermal conduction through the glass and other building materials, there is a net increase in energy (and therefore temperature) inside the greenhouse. Greenhouses can be divided into glass greenhouses and plastic greenhouses. Plastics mostly used are PEfilm and multiwall sheet in PC or PMMA. Commercial glass greenhouses are often high tech production facilities for vegetables or flowers. The glass greenhouses are filled with equipment like screening installations, heating, cooling, lighting and may be automatically controlled by a computer.
Uses Greenhouses protect crops from too much heat or cold, shield plants from dust storms and blizzards, and help to keep out pests. Light and temperature control allows greenhouses to turn inarable land into arable land, there by improving food production in marginal environments.
Because greenhouses allow certain crops to be grown throughout the year, greenhouses are increasingly important in the food supply of high latitude countries. One of the largest greenhouse complexes in the world is in Almeria, Spain, where greenhouses cover almost 50,000 acres (200 km2). Sometimes called the sea of plastics.
Greenhouses are often used for growing flowers, vegetables, fruits, and tobacco plants. Bumblebees are the pollinators of choice for most greenhouse pollination, although other types of bees have been used, as well as artificial pollination. Hydroponics can be used in greenhouses as well to make the most use of the interior space.
Besides tobacco, many vegetables and flowers are grown in greenhouses in late winter and early spring, and then transplanted outside as the weather warms. Started plants are usually available for gardeners in farmers' markets at transplanting time. Special greenhouse varieties of certain crops such as tomatoes are generally used for commercial production.
The closed environment of a greenhouse has its own unique requirements, compared with outdoor production. Pests and diseases, and extremes of heat and humidity, have to be controlled, and irrigation is necessary to provide water. Significant inputs of heat and light may be required, particularly with winter production of warm-weather vegetables.
Because the temperature and humidity of greenhouses must be constantly monitored to ensure optimal conditions, a wireless sensor network can be used to gather data remotely. The data is transmitted to a control location and used to control heating, cooling, and irrigation systems.
Cucumbers reached to the ceiling in a greenhouse in Richfield, Minnesota, where market gardeners grew a wide variety of produce for sale in Minneapolis. ca. 1910
19th Century Orangerie in Weilburg, Germany The idea of growing plants in environmentally controlled areas has existed since Roman times. The Roman emperor Tiberius ate a cucumber-like vegetable daily. The Roman gardeners used artificial methods (similar to the greenhouse system) of growing to have it available for his table every day of the year. Cucumbers were planted in wheeled carts which were put in the sun daily, then taken inside to keep them warm at night. The cucumbers were stored under frames or in cucumber houses glazed with either oiled cloth known as "specularia" or with sheets of selenite (a.k.a. lapis specularis), according to the description by Pliny the Elder.
Giant greenhouses in the Netherlands The first modern greenhouses were built in Italy in the 13th century to house the exotic plants that explorers brought back from the tropics. They were originally called giardini botanici (botanical gardens). The concept of greenhouses soon spread to the Netherlands and then England, along with the plants. Some of these early attempts required enormous amounts of work to close up at night or to winterize. There were serious problems with providing adequate and balanced heat in these early greenhouses. Today the Netherlands has many of the largest greenhouses in the world, some of them so vast that they are able to produce millions of vegetables every year.
â€˘ The French botanist Charles Lucien Bonaparte is often credited with building the first practical modern greenhouse in Leiden, Holland to grow medicinal tropical plants. â€˘ Originally on the estates of the rich, with the growth of the science of botany, greenhouses spread to the universities. The French called their first greenhouses orangeries, since they were used to protect orange trees from freezing. As pineapples became popular pineries, or pineapple pits, were built. Experimentation with the design of greenhouses continued during the Seventeenth Century in Europe as technology produced better glass and construction techniques improved. The greenhouse at the Palace of Versailles was an example of their size and elaborateness; it was more than 500 feet (150 m) long, 42 feet (13 m) wide, and 45 feet (14 m) high.
In the nineteenth Century the largest greenhouses were built. The conservatory at Kew Gardens in England is a prime example of the Victorian greenhouse. Although intended for both horticultural and nonhorticultural exhibition these included London's Crystal Palace, the New York Crystal Palace and Munichâ€™s Glaspalast. Joseph Paxton, who had experimented with glass and iron in the creation of large greenhouses as the head gardener at Chatsworth, in Derbyshire, working for the Duke of Devonshire, designed and built the first, London's Crystal Palace. A major architectural achievement in monumental greenhouse building were the Royal Greenhouses of Laeken (1874â€“1895) for King Leopold II of Belgium.
â€˘ The Netherlands has some of the largest greenhouses in the world. Such is the scale of food production in the country that in 2000, greenhouses occupied 10,526 hectares, or 0.25% of the total land area of the Netherlands.
â€˘ Since 2000, technical innovations include the "closed greenhouse", a completely closed system allowing the grower complete control over the growing process while using less energy. Floating greenhouses are used in watery areas of the country.
â€˘ The Netherlands has around 9,000 greenhouse enterprises that operate over 10,000 hectares of greenhouses and employ some 150,000 workers, efficiently producing â‚Ź4.5 billion worth of vegetables, fruit, plants, and flowers, some 80% of which is exported.
Greenhouse ventilation Ventilation is one of the most important components in a successful greenhouse. If there is no proper ventilation, greenhouses and their plants become prone to a myriad of problems.
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Ventilation serves four major purposes within the greenhouse: Regulating the temperature Ensurance of plenty of fresh air to photosynthesize Good ventilation prevents pest infestations Encouraging important pollination within the greenhouse In greenhouses recirculation fans can be used in parallel or series ventilation.
Basic mechanism â€˘ The Earth receives energy from the Sun in the form UV, visible, and near IR radiation, most of which passes through the atmosphere without being absorbed. Of the total amount of energy available at the top of the atmosphere (TOA), about 50% is absorbed at the Earth's surface. Because it is warm, the surface radiates far IR thermal radiation that consists of wavelengths that are predominantly much longer than the wavelengths that were absorbed. Most of this thermal radiation is absorbed by the atmosphere and re-radiated both upwards and downwards; that radiated downwards is absorbed by the Earth's surface. This trapping of long-wavelength thermal radiation leads to a higher equilibrium temperature than if the atmosphere were absent.
• The solar radiation spectrum for direct light at both the top of the Earth's atmosphere and at sea level • The incoming radiation from the Sun is mostly in the form of visible light and nearby wavelengths, largely in the range 0.2–4 μm, corresponding to the Sun's radiative temperature of 6,000 K. Almost half the radiation is in the form of "visible" light, which our eyes are adapted to use. • About 50% of the Sun's energy is absorbed at the Earth's surface and the rest is reflected or absorbed by the atmosphere. The reflection of light back into space—largely by clouds— does not much affect the basic mechanism; this light, effectively, is lost to the system.
• The absorbed energy warms the surface. Simple presentations of the greenhouse effect, such as the idealized greenhouse model, show this heat being lost as thermal radiation. The reality is more complex: the atmosphere near the surface is largely opaque to thermal radiation (with important exceptions for "window" bands), and most heat loss from the surface is by sensible heat and latent heat transport. Radiative energy losses become increasingly important higher in the atmosphere largely because of the decreasing concentration of water vapor, an important greenhouse gas. It is more realistic to think of the greenhouse effect as applying to a "surface" in the mid-troposphere, which is effectively coupled to the surface by a lapse rate. • Within the region where radiative effects are important the description given by the idealized greenhouse model becomes realistic: The surface of the Earth, warmed to a temperature around 255 K, radiates long-wavelength, infrared heat in the range 4–100 μm. At these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation are more absorbent. Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers. To maintain its own equilibrium, it re-radiates the absorbed heat in all directions, both upwards and downwards. This results in more warmth below, while still radiating enough heat back out into deep space from the upper layers to maintain overall thermal equilibrium. Increasing the concentration of the gases increases the amount of absorption and re-radiation, and thereby further warms the layers and ultimately the surface below.
• Greenhouse gases—including most diatomic gases with two different atoms (such as carbon monoxide, CO) and all gases with three or more atoms—are able to absorb and emit infrared radiation. Though more than 99% of the dry atmosphere is IR transparent (because the main constituents—N2, O2, and Ar— are not able to directly absorb or emit infrared radiation), intermolecular collisions cause the energy absorbed and emitted by the greenhouse gases to be shared with the other, non-IRactive, gases. • The simple picture assumes equilibrium. In the real world there is the diurnal cycle as well as seasonal cycles and weather. Solar heating only applies during daytime. During the night, the atmosphere cools somewhat, but not greatly, because its emissivity is low, and during the day the atmosphere warms. Diurnal temperature changes decrease with height in the atmosphere.
Earth Talk: Building a Green Economy • From the Editors of E/The Environmental Magazine • Dear EarthTalk: What does it mean when one
uses the phrase, “building a green economy?” I’ve heard it repeated a few times lately and would like to have a better understanding of the concept.
– Rosie Chang, Islip, NY • The phrase “building a green economy” means different things to different people, but in general it refers to encouraging economic development that prioritizes sustainability—that is, working with nature and not against it in the quest to meet peoples’ needs and wants—instead of disregarding environmental concerns in the process of growing the economy. The primary way governments around the world are trying to “green” their own economies today is by increasing investment in—and, by extension, creating jobs in—industries on the cutting edge of non-polluting renewable forms of energy, such as solar and wind power. • President Obama has repeatedly invoked his vision of a green economy as a tool for helping the U.S. lift itself out of recession and position itself as an economic powerhouse in a carbon-constrained future.
• The American Recovery and Reinvestment Act (ARRA) of 2009, the $787.2 billion stimulus package that Congress signed into law in 2009, was chock full of provisions to boost renewable energy, energy efficiency and environmental restoration initiatives. Examples include $4.5 billion to convert government buildings into high-performance green buildings, $8.4 billion for investments in public transportation, and tens of billions of dollars more for research into new technologies to amplify existing efforts. ARRA also earmark $11 billion for the implementation of the “smart grid,” a new approach to power distribution that will bring more clean energy sources into the mix and promote energy efficiency.
• Of course, Americans aren’t the only ones bent on building a green economy. During the 1980s and 1990s, while the American government was largely asleep at the wheel on environmental issues, countries such as Denmark, Germany, Spain and Japan were already busy investing in wind and solar research and implementation. And while these nations’ ongoing efforts are nothing to sneeze at, economists point out that what is most needed is action on the part of the world’s fastest growing economies— China and India.
• A recent report by the consulting firm McKinsey & Company found that China—which surpassed the U.S. as the world’s largest generator of greenhouse gases three years ago—has great potential for building a green economy over the coming decades. According to McKinsey, by 2030 China could reduce its oil and coal imports by up to 40 percent and its greenhouse gas emissions by 50 percent by investing upwards of 1.5 trillion yuan ($220 billion in U.S. dollars) per year in both existing and new green technologies. China has begun to see the light with regard to reducing emissions, increasing energy efficiency and embracing renewable alternative energy, but it has yet to make significant financial commitments, which will be key to both warding off catastrophic climate change and building a truly global green economy.
• CONTACTS: ARRA, www.recovery.gov; McKinsey & Company, www.McKinsey.com. SEND YOUR ENVIRONMENTAL QUESTIONS TO: EarthTalk®, c/o E – The Environmental Magazine, P.O. Box 5098, Westport, CT 06881; firstname.lastname@example.org m. E is a nonprofit publication. Subscribe: www.emagazine.com/subs cribe; Request a Free Trial Issue: www.emagazine.com/trial.