Design and Maintenance of Green House 3(2+1)

Lesson 5 Criteria for Design and Construction of Greenhouse

5.2 CRITERIA FOR THE DESIGN OF PLASTIC FILM GREENHOUSES

Greenhouse design is very much influenced, in practice, by the local climate and the latitude of the site, and in many cases is limited by the availability of materials for the construction. No design is perfect, thus it is necessary to prioritize in each case, the criteria to follow, these being:

i.   The maximization of the light

ii.  Minimizing, if possible, the structural elements to avoid shadows

iii. Ensuring good insulation which decrease the heat losses and

iv. Affordable costs.

The physical and mechanical properties of the covering materials and their availability limit the options while building a greenhouse, so there is a certain trend among growers to build traditional greenhouses.

Relative to plastic-film greenhouses, the most important aspects to achieve is as below.

Besides the proper structural resistance to the wind, but also to other predictable loads (snow, crops which are trained to hang, auxiliary equipment), the greenhouse must be built in such a way that the plastic film will remain well fastened, airtight, and without wrinkles, to avoid breaks caused by the wind. It must, as well, be easy to change the film. For this, the fastening system must be simple and efficient. The increasing costs of mounting the film and the plastic materials have favoured the use of special films with several years durability. For longer durability, if possible, the structural elements susceptible to heating up by solar radiation which are in contact with the plastic film must be insulated, because excess temperatures contribute to shortening the shelf life of the plastic film.

When arcs or metal frames are used, the separation between them will depend on the predictable loads (wind, snow), normally does not exceed 3 m.

The greenhouse must be airtight, to prevent night cooling in those climates in which low night temperatures are expected, as well as to prevent undesirable leakage of CO. A proper ventilation system is needed, with airtight vents. The entrance of water from rainfall must be avoided.

Its volume must be large enough, not only to obtain a higher thermal inertia, but also to allow for crops that are trained to grow up high supports, and proper movement of the inside air necessary for natural ventilation. The unitary volume of the greenhouse is the quotient between the greenhouse inner volume (m³) and the area that it covers (m²), being equivalent to the average height.

Collection of rainfall water by means of gutters, for its later storage and use for irrigation, is not only of interest in areas of low rainfall, but also because the excellent quality of rain water makes it especially valuable for soil less cultivation, a technique for fast growth. The gutter must be 4 cm larger than the diameter of the drainpipe and must have a slope of 1% to avoid overflows (the minimum slope must be higher than 0.2% in any case). The drainpipes must have a cross-section of 7 cm² for each 10 m² of cover area that is to be drained, which caters for rainfall intensities of up to 75 mm h^-1.

To avoid water dripping over the crops from condensation on the inner surface of the cover, it is important to build the greenhouse with roof angles greater than 26° such angles also allow snow to run off the cover), and have an appropriate collection system, or to use anti-dripping plastic film. In unheated greenhouses, where climate control is quite limited, the slope of the roof becomes relevant to avoiding condensed water dripping from the roof cover; roofs with ogive shape might be of interest (Fig. 5.2.1).

Likewise, as a general rule, the greenhouse must maximize solar radiation transmission, at least in winter (when it is lower), for which proper roof geometry and orientation are fundamental.

Fig 5.2.1 Some common type of curved roof greenhouses

(Source: Nicolas Castilla, 2013)

5.3 DESIGN CRITERIA IN AREAS WITH A MEDITERRANEAN CLIMATE

The most limiting climate conditions for greenhouse cultivation in Mediterranean climates are:

i.   Low night temperatures in winter;

ii.  High daytime temperatures;

iii. High ambient humidity at night and low values during the day; and

iv. CO depletion during the day.

Therefore, it is especially necessary to achieve efficient ventilation, which allows for alleviation of the thermal excesses and extreme humidity, and prevents CO deficiency. Depending on the type of greenhouse and climate conditions it is advisable that the ventilation area is up to 30% of the ground area of the greenhouse. The increasing use of insect-proof screens in the vents, to avoid or limit the entrance of insects, decreases the efficiency of ventilation. Collection of rainfall water must also be a priority. In the low-cost type greenhouses, the general problem of condensed water dripping is aggravated in flat-roof greenhouses, inducing serious plant protection problems as it facilitates the development of diseases.

Thermal losses must be limited by choosing a suitable cladding material and making it as airtight as possible. Night heating may be necessary for the crop, during the critical winter months but its economic profitability is questionable in many cases.

5.4 DESIGN CRITERIA IN HUMID TROPICAL CLIMATES

The high rainfall during the whole year or during the rainfall season (which induces high RH), the stability of the temperatures (high during both the day and the night) throughout the year, and the solar radiation (which may be excessive in some cases), are the most outstanding characteristics of humid tropical climates.

As a consequence, in these greenhouses protection against the rainfall must prevail (the greenhouse umbrella effect) and there should be efficient permanent ventilation (with vents frequently equipped with screens to prevent the entrance of insects), as well as a good height and sufficient resistance to withstand strong hurricane winds which are usual in such climates. (Figure 5.4.1) shows some of the solutions for humid tropical climates. Achieving a compromise between these requirements, at a low cost, is not easy.

Fig 5.4.1 Greenhouse structures used in tropical regions

(Source: Nicolas Castilla, 2013)

5.5 GREENHOUSES FOR OTHER CLIMATE CONDITIONS

In dry desert climates, the extreme temperature values are more acute than those experienced in Mediterranean climates, and the ambient humidity is notably lower, the winds being frequently loaded with sand and with very low water content.

In these conditions, high ventilation capacity and efficiency is a priority (with the possibility of tightly closing the vents), and there is possibly a need for humidification systems (if the evapotranspiration of water is insufficient) to decrease the temperature and increase the RH (oasis effect).Preventing thermal

losses at night is necessary (so choice of a proper cladding material and enough

Sealing are important) to avoid the need for night heating. The structural resistance to the wind is fundamental and the collection of rainfall water for irrigation is normally desirable.

Under cold climate conditions, the greenhouse effect must be enhanced and, normally, the maximum solar energy collection (interception) should be reached with proper roof geometry and cladding material as well as optimized greenhouse orientation. Limiting thermal losses is always desirable (using proper cladding material, thermal screens and being as airtight as possible) .

Frequently, the insulation measures to reduce thermal losses imply a decrease in available solar radiation (the double wall decreases the transmission, the thermal screens generate shadows even when folded) so it is not easy to obtain a compromise solution which must be based on profitability criteria in each specific case. In these cold climates, the obvious choice between multi-span and single-span greenhouses is clearly for the first type. Heating is a must, not just during the winter months, and ventilation is necessary during the season of high radiation.

In some cases, greenhouse cladding with a screen (permeable to air and water) aims at achieving a windbreak effect, a shading effect, or plant protection (limiting the access of pests), when the natural thermal conditions are adequate for crop growth and, therefore, a greenhouse effect is not pursued.

Reference:

1. Nicolas Castilla, (2013). “Greenhouse Technology and Management” Ediciones Mundi-Prensa, Madrid (Spain) and Mexico.PP.73-75