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Anno 2020 Numero 6

Gardening
June 2020

Green and water spray for defence against the summer heat

Plants offer various “ecosystem” services, including natural defense against high temperatures, especially in summer. Through the plants’ evapotranspiration, the water contained in the soil is vaporized into the air producing more coolness. At the same time, particular interest is raised by the so-called “air factory”, a sort of glass greenhouse structured to draw air from outside and return it purified and refreshed after contact with the plant essences contained in the greenhouses

Climate change is turning our summer seasons warmer and warmer. This is proven by the fact that the sunniest seasons are all concentrated in the year 2000. The record summer is that of 2003, which recorded damages of around EUR 13 billion and the premature death of at least 3,500 people in Europe.  In Italy, the summer of 2019 was the third hottest in the last 60 years, with five heatwaves, the most severe at the end of June. Heatwaves are those where the temperature remains high (above 29°C) for several consecutive days, with intense sunlight and little or no ventilation.

A particularly serious effect that affects urban centers is that of the “heat islands”, which means that the temperature in the city center areas is significantly higher (even + 10°C) than in outlying and rural areas. The trend of the temperature, analyzed from the periphery and proceeding towards the center, is similar to a Gaussian with the highest peak in the city center. This is where a micro-climate is formed in which the temperature rises in the afternoon and reaches its maximum at night when the buildings and paved roads return the heat stored during the hours of sunshine. The Albedo index shows the fraction of solar radiation reflected by a body. The value ranges from zero to one: value 1.00 indicates total reflection. Depending on the materials and surfaces, the index lies between: 0.10 - 0.35 for concrete; 0.03 - 0.18 for bitumen and gravel; 0.15 - 0.18 for trees; 0.25 - 0.30 for turf; 0.50 - 0.90 for a white wall.

 

Health risks

Excessive heat can affect people’s health, especially when the body temperature control system is affected. If the external temperature exceeds that of the skin, the only defense is sweating because sweat, evaporating, causes cooling. If sweat is insufficient, for example, because of high humidity, the body temperature rises, which can affect vital organs. Illness symptoms can be mild, such as cramps, edema, fainting, and dangerous, such as heatstroke (body temperature exceeds 41°C) with fatal consequences for the most fragile people.  Hopefully, Covid-19 will lose virulence with summer; on the other hand, as we have seen, excessive heat can be a health hazard. There are undoubtedly individual behaviors that can reduce the risk, such as wearing appropriate clothing, avoiding exposure to sunlight, drinking at least 2 liters of water a day, avoiding alcohol and sugary drinks, etc... There are, however, other solutions of more general scope and able to limit excess temperatures; solutions that can be of help both outdoors and indoors. This is particularly true of greenery and water misting systems, i.e., the combination of nature and technology. They both cool through the evaporation of water.

 

The green

Plants provide various ecosystem functions. These include the natural defense against high temperatures. The water in the soil is absorbed by the plants, and through the process of evapotranspiration from the leaf apparatus is vaporized into the atmosphere. The transition from the liquid to the vapor state occurs with energy absorption (2.27 MJ/kg water) and, therefore, takes heat from the environment resulting in cooling. In turn, the soil water is purified of pollutants through the microbial activity of bacteria living in the soil. Other important plant environmental activities are reducing CO2 emissions in the atmosphere (one of the greenhouse gases responsible for climate change), thanks to chlorophyll photosynthesis, and the purification of some air pollutants.

Therefore, it is vital to have a lot of greenery in the city, where air pollution is higher and more likely to have high temperatures, increasing the lawn area and the number of trees planted. It is estimated that 1,000 m2 of a good turf cover provide 2,500-3,000 liters of water back into the air by evapotranspiration on a sunny day. The drop in temperature that the turf can create is 5°C compared to bare soil and 15°C compared to an asphalted surface.  To achieve these results, it is necessary to keep the carpet in good condition by carrying out proper maintenance. When the temperature exceeds 30°C, photosynthesis slows down, and the plant is subjected to stress. It is therefore advisable, in addition to the standard water treatment, to make two watering for a few minutes twice a day, during the hottest hours.  The water reaches the ground, evaporates quickly, lowering the temperature, and the vegetation keeps its strength. 

It is time to try to conceive a new city model in which greenery is considered a fundamental urbanistic component. The creation of urban forests, for example, would allow mitigation of temperature extremes, reduction of “heat islands”, improvement of air quality, carbon, and storage sequestration. Interventions on buildings are essential.  The creation of green roofs and entire green walls (vertical green) would not only lead to a drop in temperature on sunny days, both outside and inside the buildings. Still, it would also represent a biological filter against pollution.

 

The air factory

The plants’ ability to purify the air is at the basis of the “Air Factory” project, carried out on the studies of Prof. Stefano Mancuso, a neurobiologist at the University of Florence and director of the International Laboratory of Plant Neurobiology (Linv) and the startup Pnat (Project nature), an interdisciplinary team of architects, botanists, agronomists ... There is much concern about the air quality of our cities, ignoring or underestimating the fact that the air inside buildings is more polluted than the air outside. The air factory is a glass greenhouse in which plants are placed. The air is sucked inside the case and put back into circulation purified and refreshed.   The plants and the soil perform the purification function as they can absorb pollutants. This capacity is higher for broadleaf plants, such as Ficus, Palma areca, Photus, Kentia, Philodendron, Sterlizia, etc... Plants must be kept in good health, and for this reason, proper maintenance, i.e. automated irrigation, and proper artificial lighting, plays an important role. The prototype of the air factory was first tested in Florence in the former tobacco factory. There are several possibilities for applying this solution:  workshops, shopping malls, supermarkets, and individual homes. Small “greenhouses” (0.5 - 1 m3) could perform the same function inside single apartments, without energy consumption and also bringing a note of color and embellishment.

 

Water atomization

In urban areas, to fight high temperatures, fountains are used, as water is always a heat mitigator.   However, technology offers a more effective solution in the form of water atomization systems. With warm, dry air, they can create comfortable microclimatic conditions within their range.

There are two types of atomization systems: low pressure (BP) atomization, carried out with pressure values of a few bars producing coarse drops, is used in horticultural greenhouses but is unsuitable for environmental cooling;  high-pressure atomization (HP), with pressure values ranging from 50 to 150 bar producing drops with a diameter of fewer than 10 microns.  A sort of mist in which the micro-drops in contact with warm, dry air immediately evaporate and cool. 

The system is also called evaporative cooling.  The lowering of the temperature in the affected area can reach 10 - 12°C. The lower the relative humidity of the air, the higher the cooling effect. The installations mainly concern gardens, parks, play areas, terraces, gazebos, etc., but also workshops and industrial sheds.

A high-pressure atomization system is composed of a motor-pump unit for high pressure, a distribution pipeline, and a system of water atomization (normally nozzles) and mist dispersion (fans).  It is necessary to have a three-phase electrical socket for the supply of the electric motor and a drinking water supply tank. Water decalcification systems can be installed. The pump is equipped with a timer to control its operation with adjustable working and pause times. Electronic management of the system is possible with automatic adjustment of power and flow rate according to temperature and air humidity. The installation can be done with columns anchored to the ground, with arches, or other solutions. Running costs are reduced as water and electricity consumption are extremely low.

by Pietro Piccarolo

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