These crops were grown surrounded by massive “fruit walls”, which stored the heat from the sun and released it at night, creating a microclimate that could increase the temperature by more than 10°C (18°F).
Later, greenhouses built against the fruit walls further improved yields from solar energy alone. It was only at the very end of the nineteenth century that the greenhouse turned into a fully glazed and artificially heated building where heat is lost almost instantaneously — the complete opposite of the technology it evolved from.
Picture: fruit walls in Montreuil, a suburb of Paris.
The modern glass greenhouse, often located in temperate climates where winters can be cold, requires massive inputs of energy, mainly for heating but also for artificial lighting and humidity control.
According to the FAO, crops grown in heated greenhouses have energy intensity demands around 10 to 20 times those of the same crops grown in open fields. A heated greenhouse requires around 40 megajoule of energy to grow one kilogram of fresh produce, such as tomatoes and peppers. [source – page 15] This makes greenhouse-grown crops as energy-intensive as pork meat (40-45 MJ/kg in the USA). [source]
Dutch-style all-glass greenhouses. Picture: Wikipedia Commons.
In the Netherlands, which is the world’s largest producer of glasshouse grown crops, some 10,500 hectares of greenhouses used 120 petajoules (PJ) of natural gas in 2013 — that’s about half the amount of fossil fuels used by all Dutch passenger cars. [source: 1/2]
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