In the first article, we talked about the increasing labor and energy cost for commercial greenhouses along with possible solutions which can be implemented at greenhouse design and greenhouse construction stage. We have already discussed the building side of commercial greenhouses and we will talk about greenhouse structures and ventilation/cooling systems in this article:
Several things must be considered carefully when selecting a greenhouse structure, including:
- Building code requirements
- The crops to be grown
- The growing systems to be used
- The level of automation to be employed
All these issues have a significant bearing on the selection of the greenhouse structure and will influence the various choices that have to be made, i.e.:
- Bay width and length
- Gutter and/or truss height
- Type of greenhouse ventilation to be used
- Type of glazing to be used
It is especially important that the height of the truss is considered carefully at this stage. With growing tables, hanging basket systems, irrigation booms, grow lights and shade curtains now becoming standard equipment in most greenhouses, 20 to 22 feet truss height is often needed to accommodate these systems. Additionally, the large air volume in Commercial greenhouses with this gutter height provide much improved climate control possibilities.
The selection of the glazing materials for a greenhouse depends basically on the same factors as those for the structure, although financial considerations usually play a larger role for this component. For example, there is a substantial difference in the cost of a greenhouse roof covered with double layer polyethylene film and a roof covered with (tempered) glass. Personal preference and/or experience of the grower often play a role as well in choosing a glazing material. When selecting the glazing, several factors need to be taken into consideration, including light transmittance, life expectancy, energy values, drip control, flammability and greenhouse maintenance requirements.
Greenhouse ventilation and cooling systems:
The selection of the appropriate greenhouse ventilation system for the facility will be determined by factors such as crops to be grown, location and orientation of the greenhouse, type of greenhouse, cost of the equipment, etc. Many growers are now looking at natural ventilation as an alternative to the more commonly used forced ventilation systems. Ever increasing electric energy costs, better understanding of the actual function of greenhouse ventilation and improved environmental control systems all contribute to this tendency. The emergence of the Open roof greenhouse further points in that direction, although this type of greenhouse is not a good choice for vegetable growing facilities.
For vine crops such as tomatoes, peppers and cucumbers a combination of forced and natural ventilation may provide the best solution. In this case air can be forced into the greenhouse underneath the rows of plants, creating an upward draft through the crop.
For most vegetable crops the inclusion of vertical airflow fans will provide additional control over the greenhouse environment.
In case insect netting is included in the design, the ventilation system must be designed accordingly and compensate for the additional flow resistance.