SR article

Greenhouse fundamentals

ABSTRACT: Air temperature is the most important parameter for the growth of plants or vegetables. A transparent cover allows short wave solar radiation to enter but reflects parts of the long wave radiation. This causes the greenhouse effect and leads to a higher temperature inside the greenhouse. The inside temperature is depends on, if there are no cooling and/or heating systems applied, the ambient air temperature, the solar radiation, the overall heat transfer coefficient, the covering material and the wind velocity.

For crop production there are three kinds of greenhouse systems: cooling, heating and both of them. These systems are divided in active and passive systems. For the Kenyan, and especially around Siaya, the only system needed is a cooling system. Examples of active cooling systems are forced ventilation, evaporative, earth air tunnel and misting. Examples of passive cooling systems are natural ventilation and shading.

Greenhouses are considered as a semi-permanent structure and are designed for a service lifetime of 10 to 25 years, dependent on the usage of high value equipment. Stress limits should be considered in the design process, just like the ultimate limit states like loss of load bearing capacity due to breakage, shakiness, fatigue, excessive tensions and deformations.

Another consideration is the orientation of the greenhouse. The two main parameters for the orientation are light transmission and wind directions. There are some basic rules about the orientation: above 40⁰N latitude the ridge of the greenhouse should be running from east to west, so that low angle light during the winter can enter through the side of the greenhouse instead of the fore or aft side, which has a smaller surface. Below 40⁰N latitude the ridge should be orientated from north to south, since the angle of the sun is much higher. Gutter connected and multispan greenhouses (different greenhouses built side-to-side) should always be orientated north to south to avoid shadows in the surrounding greenhouses. For the lay-out there are also different options. The beds can be orientated longitudinal, peninsular or cross benched. All bed arrangements should have easy access to the crops. The advantage of a peninsular arrangement with regard to cross benching is that one uses the maximum growing area. This is directly connected to the production capacity and thus the profits of the greenhouse. Another option for the lay-out is the decision to grow in ground beds or raised benches. A raised bench has easy access for planting, spraying and harvesting but is less suited for high growing crops.

The next important parameter for the greenhouse construction is the availability of some basic resources like water and electricity. A greenhouse requires a bigger water supply than natural farming due to the increased temperature. Therefore water availability is essential. The quality of the water should be tested as well. River water could carry diseases, ground water could be salty. These factors could influence the success of the greenhouse and one should be aware of this before building the greenhouse. Besides the water supply, some greenhouses may need electrical power for active temperature regulating systems and water pumps. The electricity usage relies on the size of the greenhouse, a small one of 500m2 uses around 15kW and a big one of 8.000 – 12.000 m2 uses up to 145kW. Agriculture is an industry with great potential for renewable energy, many farms already proved the viability and effectiveness of autonomous electrification.

The choice between different shapes and designs should be based on the climate it’s operating in. criteria like maximum amount of insulation and covering the maximum area for the minimum costs resulted in different designs. Greenhouses are divided in arch shaped roofs or tunnel greenhouses (standard arch, Quonset and gold frame) and peak shaped roofs (standard peak, mansard, vinery house and Venlo house). Sometimes, greenhouses are combined in multispan structures. These are commonly used to cover big areas. Some advantages of an arched shape roof are its all-metal structure with low to medium complexity, its high transmittance of light and its good resistance to winds. Unfortunately they are costly, are difficult to ventilate in a multispan construction and have no roof ventilation. An peak shaped roof is constructible with inexpensive material, has a low to medium complexity, has the possibility for both side-ventilation and roof-ventilation and has good facilities for draining water. On the other hand they have a larger number of elements and therefore more shading inside the greenhouse. And it needs internal support elements, which hinder the location of crops.

As mentioned, the climate is very important for the design of the greenhouse. Dry tropical or desert greenhouses should be protected against strong winds and should have good ventilation. Subtropical desert and Mediterranean greenhouses needs to have good temperature control for both heating and cooling. Humid tropical greenhouses often endure heavy rain. Plastic cover protects good against this rain. It should also have open or roll-up sides with incest mesh to make ventilation possible. For climate temperatures it’s important that the greenhouse has a decent and efficient heating and cooling system, since the temperatures vary a lot. The cold areas need a strong greenhouse which is capable of carrying snow. Heating and insulation is also very important, because the temperatures are often very low.

If all these criteria are considered one can think about the construction of the greenhouse. The loads are very important and have a direct relation to the safety of the farmers working inside. The main loads to be considered are: permanent load (weight of the structure and covers), permanently-present installation load (all the equipment installed on the structure), wind load on the sides, snow load (not relevant for a Kenyan greenhouse), crop load (some constructions support the crops by wiring for example), concentrated vertical load (people performing maintenance), incidentally-present installation load (temporary mobile equipment).

Greenhouse farming is a capital-intensive cultivation method, taking in mind all the investment costs, operation costs and maintenance costs, when it’s compared to growing crops traditionally. Therefore the economic viability is important to investigate before building the greenhouse. The cost analysis of a greenhouse is dependents on the following factors: initial investment for the structure, heating and cooling systems. Furthermore, during operation it will bring costs as well: energy costs of the temperature regulation systems, annual maintenance costs (seeds, preparation soil, irrigation, labor, repairs). Lastly the lifetime and its salvage value is important as well. The return of investment can be based on these costs and the expected revenues from selling the crops.

Reference

Cepeda, P., MacCleery, B., Molina, A., Lugo, E., Ponce, P. (2014), Greenhouse design and control, accessible via: http://www.crcnetbase.com.tudelft.idm.oclc.org/doi/pdfplus/10.1201/b17391-4