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design criteria

designing professional greenhouses

Agrimec designs professional greenhouses: our experience allows us to analyse our customers’ needs so as to recommend the most suitable greenhouse, guaranteeing that it maintains the right temperature, humidity and light for your crops.

holding
the climatic parameters
A good greenhouse must be able to limit the sudden and excessive climatic variations (air and soil temperature, relative humidity) in relation to the ideal values.

A greenhouse which mostly uses solar energy to meet its energy demand, must:
  • Have a low summer and winter thermal load
  • Keep soil temperature constant
  • Use electric and thermal energy efficiently
  • Monitor the main environmental parameters which regulate plant growth: ambient temperature, relative humidity, light and CO2.
holding heat
To be able to meet the energy demand, a greenhouse needs to keep the soil temperature constant, lukewarm in the winter and cool in the summer, minimising heat losses from the ground. The soil in a greenhouse is the ideal place to store precious heat, even when not cultivated.
the main environmental parameters
ambient temperature
The ambient temperature can be monitored through an efficient ventilation system and, when possible, an adequate air conditioning system, avoiding values harmful for the crops.
how to limit heat loss
The greenhouse exchanges heat with the external environment in several ways:
By conduction through the ground
Entity: 10% of total losses.
For smaller greenhouses, edge effects can be quite significant. The soil itself loses heat by transferring it to the external perimeter environment. Recommended intervention: perimeter and/or underground insulation.
By upwards radiation
Entity: 25% of total losses.
The greenhouse has a transparent ceiling and therefore a certain amount of energy accumulated by the soil and plants is radiated towards the sky. The roof covering must therefore be transparent to incoming light and as opaque as possible to infrared radiation emitted by the soil. Polyethylene sheets are not very opaque to IR radiation, so it is advisable to install a reflective sheet (thermal screen).
Through air exchange
Entity: 25% of total losses.
Air exchange is necessary for temperature and humidity control. By exchanging the air, humid air is expelled from the greenhouse and replaced with drier air. It should be noted that air exchange carried out by opening windows or using air extractors results in the loss of a significant amount of thermal energy.

Recommended measures:
- more efficient air exchange when necessary;
- minimize heat loss due to unintentional leaks by improving the airtightness of the greenhouse.
By conduction and convection through the walls
Entity: 40% of total losses.

Recommended actions:
  • Pay attention to dimensional aspects that affect heat loss: it is better to have a single large greenhouse rather than several small ones;
  • With correct orientation: its axis must be north-south;
  • Limit heat loss surfaces by adjusting the arch profile and floor plan.
    Discover the solution adopted by Agrimec
  • The right height to control the greenhouse effect and the resulting rise in temperature, without exaggerating: a greenhouse that is too high is a wasteful investment.

    Recommended optimal height (H): between 3 and 4 m A tall greenhouse can create a cooler environment and a more usable space. It also involves slightly higher heating costs. Studies carried out on greenhouses (T. Foulard and H. Fatnassi, INRA-Unité URIH (France), “New models help optimize greenhouse design,” in FlowerTECH 2006, vol. 9/no. 6) have shown that the internal air temperature decreases as the height increases up to 4 meters and that after this height, the improvements become insignificant.
  • Limit discontinuities in roofing materials in order to reduce the incidence of thermal bridges.
  • Use a roof covering that guarantees excellent insulation: materials with a cavity. Agrimec recommends the use of double inflated sheeting.
relative humidity
This is a source of the main diseases, obliging greenhouse growers to intervene with fungicides and pesticides, often systemic, or else to resort to abundant air exchanges, which nullify the benefit of the greenhouse or heating effect, wasting a considerable amount of heat to the outside. Relative humidity in the air must exceed 60% to allow the stomates of the leaves to open up enough, for maximum absorption of CO2; too high RH values are detrimental as they restrict the transpiration capacity of the plant.
how to facilitate control of relative humidity
Managing humidity is a key factor in the fight against parasites. This is crucial to keeping branches and leaves dry during the night, when humidity and lower temperatures can form condensation on the plants and the onset of diseases.
air circulation
The use of anti-air-stratification appliances prevents the build-up of heat in the greenhouse vault and thus the formation of humidity pockets in the lower areas. Condensation can be limited to the simple movement of air to keep the leaves dry.
light
It is not easy to create a well-insulated environment and at the same time to attenuate the entry of light as much as possible. Light is essential for photosynthesis. A precise illuminance corresponds to a precise ideal temperature. This means that indiscriminately keeping high temperatures in the greenhouse is totally unwarranted. Too high of a temperature also leads to a drop in relative humidity and the consequent closing of stomates, namely suffocating the photosynthesis reaction. Day and night therefore have different illuminance levels and also different setpoint temperatures;
ideal growth of the plant (a bit of chemistry)
The plant develops properly when the two processes are balanced:respiration (absorbing O2 from the environment and releasing CO2) and photosynthesis (synthesis of sugar with the absorption of H2O e CO2 in the presence of light and release O2):

6H2O + 6CO2 + light energy -> C6H12O6 + 6O2
how to maximise brightness levels
Sodium-vapour lamps (HPS) were used in the past as lighting to grow plants: low cost, rather efficient, but without selection and therefore with high consumption and high power engaged.
CO2
Keep in mind that in winter the concentration of carbon dioxide inside greenhouses is constantly much lower than the outside atmosphere (300 ppm) due to limited ventilation. Photosynthesis is limited with too low a concentration. Most plants stop growing below 150 ppm. An artificially induced increase in CO2 percentage (carbon fertilisation) rising concentration to 1000 ppm, can promote growth up to 40%, affecting both quality (more strength, greater uniformity and resistance to diseases) and quantity (better yield, more luxuriant plants, greater size, number and weight of the products).
the greenhouse arc
agrimec structures are designed and built with a semicircular arc vault.
This choice, widely developed and tested through the years, carried the following advantages:
  • Response to wind: vertical lift and not cross thrust
  • Less dispersing surface
  • Greater service height at the same maximum height
  • Improved film inflation
  • Less film wear
  • Still good snow discharge
Agrimec recommends, when possible, to build a square greenhouse, which, for the same volume reduces dispersing surfaces to a minimum.