The effect of the sun's angle of incidence on radiation intensity in greenhouses

Angles of incidence of sunbeams and radiation intensity

The solar altitude significantly affects the input of solar radiation. When the angle of incidence of sunbeams is small, the rays must travel through the thickness of the atmosphere. Solar radiation is partially absorbed, a portion of the rays is reflected by particles suspended in the air, and reaches the Earth's surface as diffuse radiation.

The altitude of the sun changes continuously as it transitions from winter to summer, as well as throughout the day. The angle of incidence of sunbeams reaches its maximum value at 18:00 (solar time). It is commonly said that at this moment the sun is at its zenith. At noon, radiation intensity also reaches its maximum value. Minimum values of radiation intensity are reached in the morning and evening when the sun is positioned low above the horizon, as well as in winter. However, in winter, slightly more direct sunlight reaches the ground. This is due to the fact that the absolute humidity of winter air is lower, and therefore it absorbs less solar radiation.

Fig. 37 shows the high values that radiation intensity reaches on a perpendicular surface oriented towards the sun, despite the fact that the acute angle of incidence of the sunbeams changes. The initial part of this curve quite accurately reflects the situation on a clear March day. The sun rises at 6:00 in the east and provides insignificant illumination to the eastern facade wall (only in the form of radiation reflected by the atmosphere). With an increase in the angle of incidence of the sunbeams, the intensity of solar radiation falling on the surface of the facade wall increases rapidly. At approximately 8:00, the intensity of solar radiation is already about 500 W/m², and it reaches its maximum value of approximately 700 W/m² on the southern facade wall of the building shortly before noon.

This is only true if the altitude of the sun above the horizon is the same (in summer and winter). (Note by Sci. Ed.)

5* 67 Fig. 36. Path of sunbeams as they pass through the Earth's atmosphere in winter (1) and summer (2)

Fig. 37. Influence of the solar altitude angle on solar radiation intensity in summer and winter. The perpendicular plane is oriented towards the sun

Fig. 88. Visible trajectory of the sun observed from Earth

Fig. 39. Visible trajectories of the sun and projection of the trajectory on the days of the vernal and autumnal equinox onto a horizontal plane

-7 Diffuse radiation #1 | | weather, 2 200 oO mu. em du 2 o. WEATHER 5 \5) № -77 y

10 20 30405 60 90 3 Solar altitude angle on the celestial sphere, degrees.

Always twice a year, the solar altitude angle on the celestial sphere turns out to be the same at the same azimuth angle values.

Fig. 39 shows the trajectories of the sun during its apparent motion around the globe in winter and summer on the days of the vernal and autumnal equinox. By projecting these trajectories onto a horizontal plane, a planar image is obtained, which makes it possible to accurately describe the position of the sun on the globe. Such a solar trajectory map is called a sun-path diagram or simply a solar map. Since the trajectory of the sun changes when moving from the south (from the equator) to the north, each latitude has its own characteristic solar map.

Reflection of solar radiation from the Earth's surface

In winter, a significant amount of additional solar radiation can be reflected from the Earth's surface onto vertical surfaces, such as the facade walls of buildings. Of the total amount of solar energy falling on a horizontal surface of the earth, up to 50—80%, depending on the cleanliness of the snow, is reflected from the snow cover. The uneven surface of the earth, vegetation remaining under the snow cover, etc., scatter most of the solar radiation. This means that only about half of the radiation falling on a horizontal surface is reflected and hits the surface of the facade wall. It can be calculated that as a result of reflection, the probability of utilizing solar radiation increases by approximately 25%. Such a gain is of significant importance, especially in early spring, when the solar altitude angle on the celestial sphere increases rapidly, and consequently, a greater amount of sunbeams will fall on and be reflected from the Earth's surface.

Snow is a natural thermal insulation: 30 cm of snow corresponds to a 5 cm thick layer of mineral wool. In spring, snow thaws first on the southern side, and therefore the surface area through which sunlight penetrates into the greenhouse increases (if frost on the glass thaws).

Fig. 40. Reflected amount of radiant energy that can be obtained through a perpendicularly positioned window when fresh snow has fallen on the ground (coefficient r=0.8) and when the ground surface is light (coefficient r=0.4)

_ Radiant energy, W/m2 8. |

Fig. 41. Design of a greenhouse in Lapland, developed by Prof. Rossi

In mid-winter In early spring

Former director of the Meteorological Research Institute, Professor Rossi, developed an interesting version of a greenhouse structure in Lapland. This solution optimally utilizes the climatic conditions of Lapland both in terms of accumulating solar energy (for heating) and from the perspective of protecting the greenhouse from wind and heat loss.

A good method for determining the insolation period of a greenhouse is as follows: imagine that you are standing in this greenhouse and looking clockwise from east to west and from the horizon upwards. By doing this, it is as if you are in the center of the celestial sphere and the greenhouse, and the view of the southern half of the sky opens up in front of you.

From autumn until spring, the sun rises and sets along such a semi-dome-shaped zone. On any given day during this period, it moves along the surface of this zone and is visible (in cloudless weather) from morning until evening. In Finnish conditions, the sun never shines directly from above, as is observed in southern countries near the equator (-E23.5° north and south latitude). However, due to the scattering of solar radiation, for example on a cloudy day, light enters the greenhouse interior from all sides, even directly from above!.

It is necessary that plants are exposed to sunlight daily for as long a time as possible, since the photosynthesis reaction does not occur if the illumination is too low. Most plants require a minimum solar illumination of 2000 to 3000 lx to ensure satisfactory growth conditions (see Ch. 7).

In mid-winter, such illumination values are achieved outdoors only at noon for approximately 1 hour, and often, due to a thick layer of clouds, even this is excluded. Only in February (October) are the desired average illumination levels reached for a sufficiently long time (approximately from 9 a.m. to 3 p.m.).

1: If the covering is light-transmitting, (Note by the scientific ed.) and Fig. 42: View of the southern half of the celestial sphere from the greenhouse in the absence of obstacles. Even in the case where part of the walls and ceiling 203041 are an obstacle, 50% of the southern half of the celestial sphere is exposed

E ‹ 22 _ y. kh. ra / RE U TE

Fig. 43. View from the greenhouse to the southern half of the celestial sphere 72 |

For growing plants, illumination is a more important factor than temperature; therefore, through the appropriate placement and shaping of such a greenhouse, it is necessary to guarantee that the greenhouse itself and especially the plants receive a sufficient amount of light energy. Sunbeams must penetrate through 1–2 layers of glass or polyethylene covering, so the intensity of sunlight entering the greenhouse interior is reduced by approximately 30 $. The surrounding environment also often has buildings and plants that create shade and thereby reduce the useful illumination provided by sunlight.

There are two reasons why it is not recommended to build greenhouses entirely out of transparent materials: firstly, on sunny days, too much radiant energy can accumulate in such a greenhouse, as a result of which the temperature rises there to an unacceptable level; secondly, light-transmitting materials are characterized by poor thermal insulation properties, due to which large heat losses can occur. |

To obtain a satisfactory final result, it is necessary to optimize a number of factors, for example, the orientation of the greenhouse, the size of the glazed area of the greenhouse shell, its shape and heat-accumulating capacity, and to minimize the shading of the greenhouse by the surrounding environment during the cold season. o o

This process is quite complex and requires computer assistance. Based on conducting automated data processing and taking into account practical experience, a "rule of thumb" (i.e., the best solution) can be formulated, according to which the area of the light-transmitting covering of the greenhouse should be such that half of the celestial sphere is exposed....

If the greenhouse is used mainly as a living space, the area of the light-transmitting covering can be slightly reduced. In this case, it is important to achieve a favorable temperature, i.e., a reduction in heat losses, since there is a tendency to use the greenhouse in autumn and spring evenings when the sun is already below the horizon. In this case, small plots for growing plants can be organized in the

Placement and orientation of the greenhouse

The most light-transmitting wall of the greenhouse is best oriented directly south, as the greatest height of the sun is always from the southern side, and the intensity of solar radiation here is maximum. More solar radiation falls on a vertical surface when the height of the sun is equal

Small deviations from a southern orientation do not have a significant effect on the gain of solar energy. Only when this deviation exceeds 45° from the southern direction does the amount of solar radiant energy entering the greenhouse decrease noticeably, especially during the heating season, when the sun is at an altitude of less than 10°, i.e., when it shines from the east-southeast direction.

If it is not possible to position the greenhouse in a southern direction, it is advantageous to orient it in an eastern direction, since the weather is most often clear in the mornings (clouds form most often during the day).

The importance of greenhouse orientation increases in cases where it has only one glazed wall. In cases where the side walls are also glazed, the deviation of the greenhouse orientation from the southern one.

It should be noted that an optimal orientation of the greenhouse to the south is not always possible. In new residential areas, the cramped nature of the plot or local planning regulations often become insurmountable obstacles, and in areas of dense development, neighboring buildings may shade the greenhouse. Obtaining sufficient light becomes a problem when the greenhouse is attached to an existing building. In such a case, one has to carefully choose an advantageous orientation of the greenhouse relative to the cardinal points, evaluate the possibility of its structural connection with the house taking into account architectural and functional features, as well as the possibility of shading by surrounding objects. In this case, it is necessary to strive to reach a compromise, providing, for example, for the construction of the greenhouse as an extension to an end wall or on both sides of the southern corner of an existing house.

Fig. 44. Various solutions for greenhouse construction when oriented in a southern direction.

Fig. 45. Residential buildings protect the greenhouse from the wind.

In addition to solving the lighting problem when placing a greenhouse, it is also necessary to always pay attention to its protection from the wind. Natural and artificial elevations of the surrounding terrain, existing and specially planted vegetation, as well as fences, walls, and auxiliary structures can serve as wind protection. The residential building itself, as a rule, effectively protects the greenhouse from northern winds. As a result of partially embedding the greenhouse into the southern facade wall or into the corner of the house, its protection is also provided to some extent in the lateral direction.

Fig. 46. Influence of orientation relative to cardinal points on the amount of solar radiation penetrating into the greenhouse: with single (1), double (2), and triple (3) glazing.

Φ 20 40 60 80 Angle of incidence of rays, degrees.

Amount of solar energy entering the greenhouse.

4.51. Angle of incidence of light. The amount of solar radiant energy entering the greenhouse depends not only on the direction of the sun's rays and the area of the light-transmitting surface, but also on the angle of inclination of the greenhouse's solar-transparent glass relative to the incident rays. The degree of cleanliness of the glass is also of significant importance: dirty glass reduces the intensity of radiation penetrating through it by up to 30%. The maximum amount of solar radiant energy passes through the greenhouse window glass when it is positioned perpendicular to the sun's rays. In addition, any glass reduces the intensity of solar radiation by approximately 10%. From the analysis of the data in Fig. 46, it follows that the angle of incidence of the sun's rays is not a very significant factor. Only when the angle of incidence of the rays deviates from the optimal direction of light passage through the glass (0°) by more than 50° does the amount of solar radiant energy entering the greenhouse begin to decrease noticeably. It is also necessary to take into account the influence of the deviation of the angle of incidence of these rays in the vertical and horizontal planes.

The orientation of the greenhouse light openings must be checked primarily in relation to the autumn or spring period, when the angles of incidence of the sun's rays are quite small. It is best to take measurements at the end of February, for example on February 21, when the sun's altitude is 11° (at 9 a.m.) and 19° (at 12 p.m.). In practice, this means that all angles of incidence of the sun's rays within the range of 90 to 60° are acceptable for the conditions of Finland.

Amount of solar energy entering the greenhouse. On certain days, the amount of solar energy entering through the greenhouse glass can be calculated using "a" dependencies according to known programs, or with the help of a sun path diagram or a solar radiation map.

Table 4.1. Solar energy intake into the greenhouse.

Amount of solar energy, W·h/m², in clear weather | in overcast weather

9.00—10.00 450 75 14.00—15.00 450 25 10.00—11.00 570 38 13.00—14.00 570 38 11.00—12.00 650 45 12.00—13.00 650 48. Total 3340 216

Table 4.1 presents data on the amount of solar energy entering Helsinki between 9 a.m. and 3 p.m. on February 21 through direct vertical double-glazed windows.

Shading factors. Usually, in the immediate vicinity of a greenhouse, there are structures, single trees, or a forest that create shade. Shading can be short-term, when the sun is behind a tree or structure for only a short period, or long-term, when the shade is created by a large building or forest. It is very important at what time of year or day the greenhouse remains in the shade. Nearby buildings or a forest represent insurmountable obstacles for light, as they cannot be influenced. There is only one possibility—to find a more favorable location for the greenhouse. Thus, the issues of placement and the structural design of the greenhouse must be coordinated with each other; otherwise, the plants will not be able to receive a sufficient amount of sunlight. Placement of the greenhouse in a less favorable location in terms of lighting is justified in cases where it is not intended for growing plants.

Residential house with greenhouse 0 NI (| m TI tu ch o EN 1 residential house NENNN _ from 3 to 15 h ria and |

: = \. s A S = \ || u ykh „^^ i « u kh iloy A. ch | A,. ShchO a - 7 Berez& ь = Solar map for „ee Residential house A \ Birch — 609 N.lat.

Fig. 47. Determination of shading from environmental objects using a shading zone diagram from 9 a.m. to 3 p.m. on February 21. Therefore, it remains to find a more favorable place for the greenhouse. Thus, the questions of placement and structural design of the greenhouse must be linked together, otherwise the plants will not be able to receive a sufficient amount of sunlight. Placement of the greenhouse in a less favorable location in terms of illumination is justified in cases where it is not intended for growing plants. |,

If the trees existing on the private garden plot shade the greenhouse, it may be advisable to perform their thinning. First of all, this applies to spruces, which create significant shading at any time of the year. The tops of pines can let through sunlight relatively satisfactorily. Deciduous species create almost solid shade in summer and autumn, however, the shading from them in winter and spring is insignificant. ... o

Fig. 48. Determination of greenhouse shading by a tree or building using a solar map -..

shade in summer and autumn, however, the shading from them in winter and spring is insignificant.. c... o

Determination of shading. From February 21 to October 21, the greenhouse receives the maximum solar radiation. In summer, it is better to create some shading (for example, from deciduous trees), as this reduces air overheating in the greenhouse. a VK

Method 1. Before placing the greenhouse, shading zones created by the environment are outlined for February 21 between 9 a.m. and 3 p.m., and their effect on the greenhouse is checked. At the same time, it is necessary to use a solar map!. With its help, the coordinates of the sun at different times of the day are clarified, and hourly shadows created by structures and trees are also examined. As a result, individual shading zones created by each object are obtained, based on which one can determine which of them will actually shade the greenhouse and how long this shading will last. Fig. 49. Determination of shading using a scale model and a lamp and the model of Prof. Gunnar Pleijel Method 2. A so-called shadow model is used together with a solar map. First, the shading angles are determined by drawing straight lines from the greenhouse to the outer outlines of trees and buildings in

' This is easier to do with the help of an insograph used in the USSR. (Note by scientific ed.) * Or "insolator". (Note by scientific ed.) vertical and horizontal directions, as a result of which vertical and horizontal angles of the shading zones are obtained. o

By combining the shadow model with a solar map and plotting the aforementioned "greenhouse — obstruction" straight lines on the shadow model on a horizontal plane, taking into account the corresponding vertical shading angles (on the shadow model) created by these obstructions, one can understand when the greenhouse will be in the shade. Based on this model, it is possible to determine in which months, days, and hours the greenhouse will be in the shade of a known obstacle, such as a tree or a building.

Method 3. First, a rough assessment of the shading created by the environment is performed directly on the site with the house, and the greenhouse is designed taking this into account. If the greenhouse is being set up in an existing building, then the change in the sun's coordinates and which trees and buildings shade the greenhouse at different times of the year are monitored. One should not cut down trees, except for spruces, which create excessively dense shade. In case of slight shading, it is advisable to build the greenhouse and test the effect of this shading under real conditions. It may turn out that such shading will be useful in the summer. |

Method 4. In densely built residential areas, determining shading from environmental objects and buildings using solar maps and shadow models is a very complex and time-consuming process. For a more efficient and rapid shading analysis, one can use a scaled-down building model and a lamp that simulates the sun, or direct sunlight, as well as a device developed by the Swedish researcher and architect Gunnar Pleijel. A needle protruding from the center of the device casts a shadow on a gridded scale, which can be used to obtain the sun's altitude and azimuth values. The sun's altitude indicates the time of year during the period under consideration. By changing the inclination angle of the model relative to the lamp simulating the sun, it is possible to determine the shading at different times of the year and at different times of the day. Experimental results characterizing shading in various cases can be recorded on corresponding sheets of paper and thereby find the optimal locations for the construction of buildings and greenhouses!, |

‚ Choosing the shape. The greenhouse is determined by the following factors: orientation of the greenhouse relative to the cardinal points; shading created by the environment; the main operational purpose of the greenhouse; ‚the possibility of connecting the greenhouse to a residential building in such a way that a single harmonious architectural whole is formed. The most characteristic period for using a greenhouse is from March to September, when the sun shines most intensely, starting from an altitude of about 30° (from March 21 to September 23)... _ o ‚ As mentioned above, approximately half of the southern sky should be visible from the greenhouse, or sunlight falling into the greenhouse at a 45° angle should reach the lower edge of its rear wall. The end walls should be fully or partially transparent if the greenhouse is oriented to the south or the deviation from this direction is insignificant. If these conditions are met, the most optimal and uniform illumination of the greenhouse is ensured. The useful floor area of the greenhouse can be increased by making the southern glazed wall inclined, as a result of which a well-lit area is formed. In cases where buildings and trees surrounding the house create strong shading or the house has a predominant orientation to the north, one of the end walls can be made completely closed, since sunlight enters the greenhouse to a negligible degree anyway. To ensure sufficient sunlight, it is advisable to make the pro-

. ' It is even simpler to do this | A: at NIISF. (Primak of nomov in solators of the MArchi system transparent roof or part of it, and also equip the rear wall with a light-reflecting surface or paint it with white paint. The same measures can be taken in cases where the aim is to make the greenhouse recessed into a residential building to increase its elongated area, as well as if it is necessary to ensure that sunlight enters the living rooms through the greenhouse. The roof of the latter should be partially or fully transparent:

The most advantageous in terms of illumination are the parts of the greenhouse located directly by the southern windows, therefore it is advisable to arrange long, but not too deep greenhouses. The depth also depends on the height — the greater the height of the greenhouse, the deeper the sunlight penetrates. It is recommended to choose a greenhouse depth in the range of 2—5 m.. o |

How to calculate the glazing area for an attached greenhouse

When designing greenhouses, any finished drawings and diagrams serve only as a guide for the agronomist. However, for the normal development of plants, it is important to observe the basic light transmission parameters of the structure. An acceptable option is considered to be a light opening whose size is equal to 50% of the entire area of the greenhouse enclosure.

Such a calculation allows the structure to be used both as a production area and as a glazed veranda. This planning solution is especially relevant for structures adjacent to buildings. If this proportion is maintained, sunlight continues to fully or partially penetrate into the living rooms located behind the greenhouse.