The Passive Greenhouses

We are living in a great Greenhouse: our planet, the Earth. This sustainable system is almost closed, except the radiative energy received from the Sun. Understanding and respecting its principles is the only guaranty for a safe future of our species. The Greenhouse effect is a process by which a part of the incident solar energy is trapped by the lower atmosphere due to the action of some atmospheric gases, called greenhouse gases. A part of the Earth’s surface infrared radiation, which would be otherwise lost into the space, is redirected back by the greenhouse gases and the clouds, so the temperature of the lower atmosphere becomes higher than it would be if direct heating by solar radiation was the only warming mechanism (IPCC, 2007). This effect has a certain similarity with the overheating of a proper greenhouse, which works by isolating warm air inside a transparent building, by reducing the convection heat flow. The greenhouses were developed in order to cultivate plants under controlled conditions. They offer high productivity and efficiency, and remove much of the risks caused by the inappropriate weather and climate. A certain greenhouse surface can feed five to ten times more people than the same conventional agricultural terrain. That is why greenhouses are used from more than two millenniums, as we know from the Plinius the Elder reports on the culinary habits of the Roman emperor Tiberius, who was already aware of the vegetarian regimes’ advantages (Janick, 2007). His favorite cucumbers were planted in wheeled carts, put in the sun daily and then taken inside at night. The cucumbers were stored under frames or in special houses covered with oiled cloth or with sheets of selenite. The first modern greenhouse structures were built in the 13th century at the Vatican in Rome. Along centuries the greenhouses issued a powerful industry, which nowadays is supporting powerful economies, like in the case of the Netherlands. Although from the technological point of view greenhouses are well covered, they have to cope with the continuously increasing costs of the energy. Their main asset, the direct use of the solar energy, is not able to constantly ensure the temperature constraints demanded by plants, because of the hardly predictable weather conditions. That is why most greenhouses have to be connected to conventional energetic infrastructures: electricity, gas, warmed water, etc. Our purpose is to investigate a fundamental improvement of the greenhouse concept, with a huge potential to improve our lives: the Passive Greenhouses.