Design and Maintenance of Green House

Lesson 13 Solar Radiation in Greenhouses

Fig 13.1 Heating of greenhouse by solar radiation

(Source: http://www.hydroponics-simplified.com)

The light received by plants in a greenhouse has three qualities that growers are concerned with-intensity (brightness), quality (color whiteness) and the ability for light (amount of light received) to reach the plants. Plants respond to all three qualities. Light transmission into the greenhouse is the most important factor affecting plant growth and crop production. Solar radiation is required for plants to photosynthesize and produce flowers, fruits and vegetables. Without adequate light, plants grow slowly and are generally unable to produce the quantity and quality that will generate desired profits. There are many factors that affect both the amount of light (intensity) and the quality (color) of light transmission into a greenhouse. These are considered to be structural or environmental factors.

The first alteration which the greenhouse causes on the microclimate parameters is a decrease in available solar radiation (Fig. 13.2). On single-span greenhouses and on the greenhouse sidewalls of multi-span greenhouses, an important part of the penetrating light is lost through the sidewalls. Therefore, the use of reflecting surfaces on the north sides of greenhouses (in the northern hemisphere) contributes to an increase in the available light. Equally, the use of reflecting surfaces over the soil, to reflect the light not intercepted by the crop, allows for an increase in the light available for the crop.

 

Fig. 13.2 Radiation and energy balance in a greenhouse

13.2 TRANSMISSIVITY TO RADIATION

The fraction of global solar radiation transmitted inside the greenhouse is called as ‘greenhouse global transmissivity’. Maximizing the radiation inside the greenhouse is in fact a desirable objective in all latitudes, especially during the autumn and winter seasons.

At latitudes higher than 30°, from the equator, the natural decrease of solar radiation is the most important uncontrolled limiting factor for crop growth inside greenhouses, and thus it becomes imperative under such conditions to strive for the maximum possible intensity, duration and uniformity of radiation.

Factors affecting transmissivity inside the greenhouse:

1. The  climate conditions (cloudiness, mainly, which determines the  proportions of direct and  diffuse radiation);

2. The position of the  Sun  in the  sky (which will depend on  the  date  and  time  of day  and the latitude);

3. The geometry of the greenhouse cover;  

4. Its orientation (east–west, north–south);

5. The  covering material  (radiometric characteristics, cleanliness, water condensation   on its inner surface); and  

6. The structural elements and equipment inside the greenhouse which limit, due to shadowing, the available radiation inside.  

13.2.1 Effect of climatic conditions on light transmission   

On  clear   days,   when direct radiation predominates, the  average global  transmissivity (fraction of global  exterior radiation that  penetrates inside the greenhouse) must be  integrated as  an  average value  for  the whole greenhouse. This is because of the variability of radiation at different points throughout the greenhouse caused by differential shadowing of the structural elements of the greenhouse and of various pieces of installed equipment.

On completely cloudy days,   when all the solar radiation is diffuse (i.e. when there are no defined shadows) the distribution of radiation is more homogeneous inside a greenhouse.  The average instantaneous transmissivity of a certain greenhouse varies throughout the  day, according to the  position of the  Sun  in  the sky  and  the  characteristics of the  radiation; normally, on a sunny day, it slightly increases from  dawn until noon, and  decreases later until dusk. When talking about global greenhouse transmissivity, it is normally understood as the daily average transmissivity (proportion of daily accumulated radiation which penetrates inside the greenhouse with respect to the outside), to distinguish it from the instantaneous values.

It  is  important to  highlight the   notorious differences  that   exist,  from   the   point  of view   of  radiation  transmissivity, between single-span and  multi-span greenhouses (even  when spans have  the same  roof geometry) because of the shadows between spans (Fig. 13.3); consequently, transmissivity estimates obtained in  single-span greenhouses cannot be extrapolated to multi-span types.

Fig 13.3  Shadow effect  in multi-span greenhouses

(Source: Nicolas Castilla (2013) )

13.3 EFFECT OF GREENHOUSE ORIENTATION ON LIGHT TRANSMISSION

The greenhouse orientation influences the transmissivity, in autumn and   in winter, under clear sky conditions (when direct radiation predominates). A greenhouse oriented with the ridgeline running North-South will receive the most PAR radiation throughout the year.  In North –South oriented greenhouse, there will be two receiving areas of sunlight, 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 40° N, one need to reduce radiation because of the resulting high temperatures in the greenhouse.  The most PAR transmission into the greenhouse is needed 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 designer should not aim to maximize total yearly radiation but the radiation during the darker periods of the year.

The uniformity of radiation in east–west oriented  greenhouses (symmetrical with  a roof  pitch of  around 30°)  is  less  (on  clear days)  than in  north–south oriented green- houses, but  their transmissivity in autumn– winter is  higher, with differences of  more than 10%  of the outdoors daily global  radiation   around the  winter solstice. However, these differences in uniformity between multi-span greenhouses oriented east–west and north–south are attenuated by:  

• the greater the height of the greenhouse (3.5–4.0 m  at  the  gutters);

• the  lower the  span width; and  

• the radiation diffusion characteristics of plastic films  used nowadays.

Another important aspect to consider when orienting the greenhouse is the direction of the predominant winds, which may become a primary consideration in choosing one or other orientation. The wind has a strong influence on the structure as a result of its mechanical effects, and because it has an indirect influence on the greenhouse indoor microclimate and   energy balance. The wind increases the heat losses and the air infiltration leakage. Therefore, orientating the ridge parallel to the direction of the prevailing winds can, in certain cases, be advisable, but a reduction of ventilation must be expected.

The characteristics of the building plot (shape, slope, obstacles that generate shadows) may also limit the greenhouse orientation options.

13.4 INFLUENCE OF STRUCTURAL DESIGN

Roof slope is an important parameter affecting light transmission in greenhouse.  The maximum amount of light energy transmittance occurs when the glazing surface is perpendicular to the sun.  Essentially this happens only for a short time of the day.  The design of the greenhouse should be such that it will maximize the light energy entering through the roof of a greenhouse during the time of year when light is at a premium ( i.e period from October to March).  Glazing materials of different strength require supporting members at various spacings.  For wider spacing the individual structural support members will be heavier but produce less overall shadow than closely spaced supports.  Some glazings 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. 

13.5 INFLUENCE OF INTERIOR GREENHOUSE COMPONENTS AND SYSTEMS

With the advent of thermal screens, supplemental lighting and other greenhouse handling systems along with traditional overhead heating systems concern has been expressed for obstruction of PAR lighting which is caused by these overhead mounted components of the growing system.  Under bench heating and in-floor heating systems have reduced the number of overhead heating pipes necessary to meet the demand load.  Thermal screens which are installed and move gutter to gutter can reduce shading because the thermal screen shares the shadow pattern with the shadow caused by the structural gutter and does not add an additional shadow which is caused by the system which moves from truss to truss.  The grower must be concerned with the adoption of new practices which add significant overhead components.

13.6 INFLUENCE OF WEATHERING ON GLAZINGS

The design of sophisticated greenhouse glazing films nearly eliminate this problem. It is also true that not all waveband are attenuated the same over time of exposure of greenhouse glazings. There are distinct differences between the new and weathered film, particularly in the lower PAR region.

13.7 INFLUENCE OF CONDENSATION ON THE GLAZING

Condensation is found on most glazings 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 is 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.

13.8 OPTIMIZATION OF THE TRANSMISSIVITY

Daily transmissivity values above 70% in simple cover   greenhouses are very infrequent, because normally they range   between 55% (winter) and 70% (summer), whereas in double cover greenhouses they range between 50 and 60%.  The average reflectivity of a greenhouse ranges between 20 and 25%, and the absorptivity for both the cover and the structure ranges from 15% with a simple cover to 25% with a double cover.

At canopy level, the ‘radiation saturation   level’ has been   defined as the value above   which the radiation increments do not involve parallel increases of photosynthesis . This situation (widely studied in laboratory growth chambers) may occur in greenhouses during the high  radiation   months at  midday,  but   only   on  the leaves located on  the  upper strata of  the crop, exposed to higher radiation, whereas the  leaves of the  lower strata (shadowed by the  upper leaves) receive much less  radiation, and  are  far  from  the  saturation level. Therefore, considering the plant as a whole, it is not usual to achieve radiation saturation in species of edible vegetables, even under mediterranean conditions, and normally it does not seem justified to decrease the   radiation in the greenhouse for this reason. It might be necessary, however, to limit radiation for other reasons (e.g. to limit temperature in insufficiently ventilated greenhouses, for fruit quality considerations, to improve the colour   of the   product, or   to   reduce water stress).

The   anti-dripping effect   of  the   inner side    of   a   multilayer  plastic  film   (once located over  the  greenhouse) prevents the formation  of   thick   drops   (when  water vapour condenses on  the   film),   reducing transmissivity and   later   contributing  to water dripping on  the  crop, with negative effects  on plant health.

Washing the   plastic film   covers and restricting   greenhouse   white washing   as much as possible, together with a good selection of the plastic film, allow for a better availability of radiation inside the greenhouse.  Other measures, such as limiting the shadows of the super-structure and of the installed equipment (thermal screens, ventilator’s screens) and   the   outside windbreaks, are quite advisable.

The quality of radiation is affected by the soil particles deposited on the greenhouse cover,   limiting the PAR even   more than the IR radiation.

We must also consider those crop management techniques which optimize the use of radiation (intercepting it) inside the greenhouse:

• north–south  orientation  of the  crop  rows;  

• plant density;

• plant training;

• pruning; and  

• use of mulching.  

It is interesting to experiment with novel growing techniques, prior to their general adoption. In this   respect, it  is  important to  highlight the  potentially negative influence in  productivity of the  use  of white mulching in autumn–winter to  increase the  radiation intercepted by the  crop,  in unheated greenhouses under certain conditions, because of concomitant significant reductions in  root  temperature, both   in  crops grown in the  soil  or in artificial substrates.

REFERENCES

1. Nicolas Castilla, 2013. “Greenhouse Technology and Management.” Ediciones Mundi-Prensa, Madrid (Spain) and Mexico.PP 77.