New greenhouse roof

There are many greenhouse configurations, both in design and dimensions. No two greenhouses are exactly alike, if one considers the overall size, dimensions of length, width and height, number of bays, orientation, etc. The glazing is one most important of the component systems for the greenhouse. The selection of a covering is crucial for attainment of an optimal controlled environment, particularly relating to the solar radiation intensity and wavelengths. No covering material is ideal. Each will influence the plant microclimate parameters such as air humidity or carbon dioxide concentration in various ways. A glazing that is selected must help achieve the desirable end product.

The selection of a glazing and the design of the greenhouse system should not be mutually exclusive tasks. If the production of high quality plant products within efficient, cost-effective, controlled environment systems is the ultimate goal, then the design must consider
(1) the type of crop and its associated cultural demands,
and (2) the production system which will support the growth of the crop.

All other greenhouse systems should adequately support these two needs. The innovation presents a smart optical foil to conserve energy in a greenhouse as well as alternatives to “traditional” methods of glazing, photoperiod and illumination.



Some Perspective
The goal should be to provide the environment for the most optimum growth of the plant, or the highest quality final plant product, one that will demand the greatest return on investment. The plant response should remain dominant in all the discussions. Measurement and reporting procedures of the appropriate radiation wavelengths or waveband that are important to the growth and development of plants should become the focus.

Consider the concept of “more is better”. The plant growth rate will increase with additional light, but it does so at a decreasing rate. As more light is added, the unit return becomes less. Ultimately the growth curve levels off, meaning that more light energy will no longer provide any increase of plant growth rate.

In the evaluation of a particular glazing for a greenhouse application, it may be more valuable to determine the total amount of photosynthetic energy transmitted to the plant canopy during an extended period of time (day, crop cycle, season, or year), than to determine an instantaneous value.


Defining the Components of Radiation
Radiation from the sun can be described by its quantity and quality.
• The quality depends on the waveband of the light, as well as, the distribution and intensity of the wavelengths within the waveband.

• The quantity is the amount of energy within the radiation. This quantity can be measured as the number of photons of light [moles of photons] per square meter per second, µmol m-2 s-1, or as a total energy value of the light, Watts per square meter [W m-2].


Considerations for Designing and selecting a Covering System
Even when a material offers strength, consistency, durability, manufacturing quality control, and safety, other factors should be considered. These include the transmission of solar radiation and energy conservation, and how these interact with glazing/superstructure systems.

In general, radiation transmission is also influenced by the physical structure of the greenhouse. The factors include:
• angle and shape of the roof
• the number and width of spans (distance from gutter to gutter, if multi-span or ground to ground, if single span)
• height of end walls • length to width ratio of the structure, and
• compass orientation. The value of the hourly radiation transmission (and ultimately the total energy) is dependent upon numerous physical and time-dependent factors.

The following is a list of major factors which together modify the actual radiation transmitted.
• day of year and hour of day
• latitude • local weather conditions
• predominance of direct or diffuse solar radiation
• spectral quality or waveband of the radiation
• cover material properties (at installation and as affected in time by weathering, air pollutants, moisture condensation, and dust and dirt accumulation).


Greenhouse orientation
In northern latitudes single bay greenhouses can be oriented east-west to allow maximum light reception in the late fall, winter and early spring.

For multiple bay, gutter-connected greenhouses the orientation is usually north- south so that the shadows from the gutters track from west to east across the crop rather than shading the same areas all day. In either case, the rows of plants within the greenhouse should run north-south to optimize equal light to all plants throughout the day.


Influence of Greenhouse Orientation
Guidelines for 40oN latitude:
• Freestanding greenhouses should be oriented East-West in this latitude.
• Multi-bay, gutter connected or ridge and furrow greenhouses should be oriented North-South.


Free-standing greenhouses

Most textbooks will accurately state that a greenhouse e oriented with the ridgeline running North-South will receive the most PAR radiation throughout the year. This is a true, but misleading statement. The reason the statement is true is because with the sun traveling over a greenhouse with this orientation there will be two receiving areas, the East roof section (in the morning), and the West roof section (in the afternoon).

It is also true that the majority of the radiation enters the greenhouse through these two roof sections when the sun is high in the sky from April to October. The difficulty is that during this time of the year, at latitude 40o N, we need to reduce radiation because of the resulting high temperatures in the greenhouse. We need the most PAR transmission into the greenhouse from October to March when there is less light available because of the normally low sun angle in this latitude.

To achieve this goal, a free-standing greenhouse will receive more light with an East-West orientation during this critical period. Therefore the aim of the designer is not to maximize total yearly radiation but the radiation during the darker periods of the year.


Multi-bay, gutter connected or ridge and furrow greenhouses
In the orientation of multi-bay greenhouses structural components come into play. Although an East-West orientation allows more light to enter the greenhouse there are permanent shadows throughout the greenhouse caused by the structural members, particularly the gutter and thermal screen installations.

These permanent shadows remain in the same position and become wider or narrower depending upon the time of year. A North-South orientation, however, will cause the shadows to move from the west side of any gutter section in the morning to the east side of the gutter section in the afternoon.

This moving shadow pattern is more desirable for crop growth because there is no part of the crop subjected to shadow throughout the entire day. Some Dutch researchers have experimented with locating the aisles in the area of the shadow within an East-West oriented greenhouse, but that often causes serious problems with materials handling systems.

Newer growing systems such as movable bench systems, transportable bench systems and floor growing system also use up to 90% of the floor area. Permanent shadow patterns cause unequal growth and unequal growth in a moveable bench system, for instance, causes severe problems when it is time to sell and market the crop. It is mandatory with these systems that the entire crop be at a uniform maturity when it arrives in the headhouse or several paths of transport have to be provided and this is expensive in equipment and time consuming for the workers.


Influence of Structural Design
Roof slope is an important parameter in greenhouse design. The maximum amount of light energy transmitted occurs when the glazing surface is perpendicular to the sun. Essentially this happens only for a short time of the day, at best. The appropriate roof angle or slope of the greenhouse on December 22 in our latitude, 40oN, is approximately 68o.

This means that at 12 noon on December 22 the sun angle would be perpendicular to the roof of the greenhouse if it were at the 68o angle. On the other hand on June 21, the appropriate angle would be 12o.

The design objective is to maximize the light energy entering through the roof of a greenhouse during the time of year when light is at a premium, October to March. In actual current design practice a roof slope of 27o-30o is used. It is interesting that a slope of 1 in 2 equals a 27.5o angle which makes me feel that maybe the carpenters and builders decided the ‘correct’ roof slope and not the engineers or plant scientists.

Glazing materials of difference strength require supporting members at various spacing. For wider spacing the individual structural support members will be heavier but produce less overall shadow than closely based supports. Some glazing have less unit weight but the design of the greenhouse structural members should be essentially the same because the primary loads are live loads of wind and snow and the dead load of the glazing is small in comparison to the total loads experienced by the greenhouse.


Influence of Location
The graph below indicates the available solar radiation at 40oN throughout the year. On June 21 the radiation is 3 times that available on December 21. An alternative way to state this is that the available light for the winter crop in November, December and January is only one third of that available during the summer months. Note that the units of graph are Langley per day. To convert to Watt per square meter (W m-2), multiply by 0.484.


Solar Radiation Transmission
The main purpose of a greenhouse covering is to create an internal environment that is conducive to plant growth regardless of the external environment. Energy from the sun is transmitted through the transparent greenhouse covering to the plant where it drives the photosynthetic process of converting carbon dioxide and water to green plant matter and oxygen. The capability of the covering to transmit light in wavelengths useful to plants, of which only a portion is visible to the eye, is therefore extremely important.

The intensity of these wavelengths (400 - 700 nanometres) of photosynthetically active radiation (PAR) directly influences growth and development in green plants. Other non-visible solar radiation wavebands include ultra-violet (UV), infrared (IR) and Far-Red (FR) wavebands.

Radiation can be described by its wavelength or its frequency. Wavelength has units of meters, typically nanometers (nm) [one billionth of a meter] or micrometers (µm) [one millionth of a meter], frequency has units of cycles per second, and each can be used to describe the energy value and the quality of the photon of light. Wavelength and frequency are related by the constant c, the speed of light.

Frequency is equal to the speed of light multiplied by the wavelength. The energy of a wavelength of light is determined by its frequency or its wavelength. As the wavelength increases, the energy of the light wave decreases, and as the wavelength decreases, its energy increases. It is an inverse relationship. Thus short wavelength blue light has more energy than longer wavelength red light, and likewise ultraviolet radiation has more energy than infrared radiation.



Influence of Condensation on the Glazing
Condensation is found on most glazing and is useful at night for reducing energy loss for direct radiation to the sky from polyethylene glazed greenhouses which are not glazed with IR film. During the day, however, excess condensation can cause reduced PAR transmission and create localized disease potential if dripping occurs on the crop. Condensation between the two layers of polyethylene film can be reduced or eliminated by using outside air to supply the fan used to inflate and separate the two layers of film.

Air which is introduced into the space within the film envelope will always be warmed if it taken from outside. Warm moist air taken from within the greenhouse will be cooled when it enters the air envelope. The moisture will be condensed on the cooler surfaces causing build-up of moisture between the two layers. Although helpful from an energy standpoint it can be detrimental from a light transmission viewpoint.

Installing the inflation fans properly can completely overcome this problem. The use of IR films also is helpful in controlling condensation because the plastic film itself is usually at a higher temperature than conventional grade polyethylene greenhouse glazing.



The radiation spectrum contains various wavebands of interest Ultra-Violet or UV is the wavelengths less than 400 nm. These high energy wavelengths can cause skin damage [sunburn]. Visible light, which is based on the sensitivity of the human eye, is within the 380-770 nm wavebands, but based on its usefulness for plants it is within the PAR (400-700 nm) waveband. The “colours” of visible radiation to humans can be approximately divided into the following wavebands:


Red/Far-red ratio is important Infrared or IR are wavelengths greater than 770 nm and consist of a useful waveband for plants known as the Near Infrared or NIR which is 770-850 nm. Infrared wavelengths (770-1,400 nm) have the heating effect.

These can provide warming to a plant leaf surface, or to our skin. The PAR, Photosynthetically Active Radiation, is 400 to 700 nm waveband, which is the primary wavelength important for providing the energy for plant photosynthesis. Red: Far-red (R:FR) ratio consists of two narrow wavebands which influence plant growth responses. The leaf is designed to absorb nearly 95% of wavelengths between 400 – 700 nm, but only 5% of the 700-850nm waveband is absorbed. Of the remaining 95% of the 700-850 nm wavebands, ~45% is reflected, and 45% is transmitted.

In the table below, the quantity or intensity of the radiation from 400-800 nm is shown as the Photon Irradiance (first column) for a very clear day. The quality or distribution of the wavebands of radiation is shown as a percentage of each waveband of light. These are contrasted for direct sunlight, and for under dense leaf shade (Kendrick and Kronenberg, eds., 1986).


Definitions

Single-span -- an independent, single-bay structure, separate from adjacent structures.

multi-bay or gutter-connected -- construction where modular structural units are connected at the gutters to cover large ground areas.

lean-to -- structure which is attached to another building along its ridgeline.

over-wintering -- temporary, unheated structure for winter protection of hardy crops.

single-layer -- cover or glazing composed of one layer of rigid or flexible film material.

multi-layer -- cover or glazing consisting of two or more layers of rigid or flexible film materials.

air inflated -- separation of two layers of flexible film by sealing the edges and inflating withpressurized air.

roof slope -- angle of the face of the roof relative to the horizontal.

arch roof -- continuous curved roof face.

gable roof -- flat roof face.

truss -- structural framework used to support the roof.

purlin -- longitudinal members of the structural framework that support the glazing material on theroof.

bow or hoop -- pipe or tube framework used to support the glazing on an arched roof.

pipe or post or column -- vertical structural member which supports the gutters and end walls.

gutter -- water transport channel, supported by posts or columns, and providing attachment for the roof bows.

ridge -- peak or high point of the roof that spans the long length of the structure.

headhouse -- separate or attached building to the structure used as a preparation area.

Additional Index words. glazing, plastic film, structured plastic panels, photoperiod, natural photoperiod control, photosynthetically active radiation (PAR) directly, Photomorphogenesis, Photomorphogenic Radiation Energy Conservation For Commercial Greenhouses. 1989.




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