Energy exchange and heat accumulation mechanisms in greenhouse structures

Natural energy flows in a greenhouse

Thanks to the presence of a transparent enclosure (glass), a greenhouse is capable of accumulating a large portion of incoming solar radiation energy. Short-wave solar radiation penetrates through the glass relatively unhindered, depending on its thickness and surface cleanliness. Glass transmits electromagnetic radiation with a wavelength of 0.3–2.4 μm. This spectral region contains the largest part of solar energy, including the visible light range with a wavelength of 0.35–0.75 μm (see section 2.1). Almost half of the radiation consists of visible light, and the other half is infrared. _

When solar radiation hits a plant or a dark surface, it is converted into thermal energy with a wavelength of about 19 μm (the wavelength range for thermal radiation is 4–50 μm). Most materials, with the exception of certain types of organic glass, do not transmit this radiation; it is absorbed by the surface layer, heating it. |

Thermal energy received by various surfaces inside the greenhouse is absorbed by the materials, and some of this energy is reflected or transferred to the air, creating convection within the space. Thus, natural energy flows arise, seeking to bring the system to an equilibrium state. Thermal energy flows rush through the closed and transparent outer shell into the surrounding environment — into the air or into the soil. Thermal energy is also transferred to objects and materials with lower temperatures, i.e., thermal energy is accumulated by these materials.

Usually, it is necessary to reduce the propagation of heat flow towards the outside air and further into the surrounding space. More thermal energy enters the greenhouse than escapes, as a result of which the temperature inside rises, i.e., the "greenhouse effect" is provided for the plants.

3.2. "Passive" solutions based on the use of natural factors

Fig. 17 shows that thermal energy is transferred via radiation or convection. It is known that air rises when heated and sinks when cooled, which results in heat flows in the greenhouse and the residential house. If the physical principles behind the origin of these heat flows are known, the problem of heating can be solved without resorting to non-renewable sources (electricity, various types of fuel).

Below are several methods for accumulating solar energy in a greenhouse and creating reserves using special devices.

Greenhouse floor as a heat accumulator. Solar radiation, penetrating the greenhouse and falling on the dark stone surface of the floor, heats it. Part of the thermal energy, as a result of the resulting convection and thermal radiation, is transferred to the greenhouse space (mainly its internal air), and the heated air moves into the dwelling through open windows and doors. Part of the thermal energy (depending on the absorption capacity and heat-accumulating properties of the floor) is accumulated by the floor material. At night, the heat is transferred back to the dwelling and the greenhouse as a result of thermal radiation emitted by the floor materials and subsequent convection. 3.2.2. Greenhouse wall as a heat accumulator. Sunbeams, hitting the dark rear wall of the greenhouse built from stone, heat it. As in the previous case, thermal energy is transferred partly to the wall material and partly to the greenhouse air. The heated air rises and passes through an open hatch into the dwelling, where part of the thermal energy is accumulated by the floor, walls, and ceiling.

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Fig. 17. Natural energy flows in a greenhouse attached to an individual house on a cold sunny day (wavy arrows—long-wave thermal radiation)

Fig. 18. Thermal energy is accumulated in the mass of the floor. greenhouse space or dwelling.

At night, this thermal energy passes into the air of the dwelling and the greenhouse. The heat-accumulating greenhouse wall can also be equipped with a light-transmitting cover, ensuring more efficient collection of thermal energy and heating of the dwelling. Such a design solution is called a "solar wall".

A pool or chernozem as a heat accumulator. The essence of this inexpensive design solution is to ensure efficient heating of water or chernozem by appropriately orienting the solar collector. Since water has a good heat-accumulating capacity, it can absorb a larger amount of thermal energy, while chernozem absorbs correspondingly less.

Fig. 20. Thermal energy is accumulated in the mass of water or

“— pool with water; б — soil or thermal energy accumulator—

Using a swimming pool for heating a greenhouse attached to a private residential house is unfavorable. Water evaporation is accompanied by large heat losses and increases air humidity, which can lead to discomfort in living quarters. This solution can be applied if the swimming pool is equipped with a light-transmitting cover.

Insulated heat-accumulating devices. Similar to the methods described above, thermal energy can be stored in tanks with water, thermal reservoirs, or bags with Glauber's salt, etc. At night, the heat is transferred to the air in the room, and during the day — also to the air of the living quarters.

4? Fig. 21. Thermal energy is accumulated by the mass of individual bodies located in the greenhouse a — barrels with water, canisters, jars, etc.; b — stones laid close to the wall; c — stones laid loosely; d — bags with salt (heat accumulator operating on the basis of using the phenomenon of phase transitions)

Natural circulation (solar collectors and stone heat accumulator). During the day, the air is intensely heated by solar collectors located in the lower part of the greenhouse.

positioned under the floor, the air cools down. At night, the air heated in the solar collector rises upwards and pulls cooler air from the stone heat accumulator with it. Thus, natural air circulation arises, providing heat transfer from the solar collector to the heat accumulator or directly through a system of hatches.

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Fig. 22. Solar energy collector and energy storage system operating using natural circulation.

a — stone heat accumulator in the form of a pile of stones under the greenhouse floor; b — stone heat accumulator in the form of a pile of stones under the floor of the greenhouse and the apartment. Instead of a stone heat accumulator, one can use concrete slabs with cavities or other devices.

3.3. "Semi-active" engineering solutions based on the use of mechanical devices

With the help of fans and control devices, it is possible to control air flows more efficiently compared to systems operating using natural circulation. This is especially noticeable in cases where air must be moved over significant distances in a direction opposite to the natural one, for example, downwards or through a heat-accumulating device with high resistance, which can only be overcome by using a mechanical device. Such systems, which ensure the accumulation of solar energy in the greenhouse and the supply of heated air with the help of fans to a heat-accumulating device or to living rooms, are often called "semi-active" or "hybrid" systems in the terminology adopted in the USA.

Studies have shown the profitability of using fans, as this allows for accumulating large reserves of solar energy and using it accordingly. For example, English researcher Cedric Green, as a result of experiments and calculations, found that in such a system with one fan, with sufficient storage capacity, one can utilize approximately 30% more solar energy than in a system with natural air circulation without a heat accumulator.

A simple way to heat an apartment is to use air heated in the greenhouse with the help of an air heating system. During the heating period, fresh air for this system can be taken from the greenhouse, where the air is pre-warmed. In addition, warm air entering the apartment through doors, windows, or hatches can be cooled using heat distribution ducts located under the floor, and, thus, use the subfloor of the house as a heat accumulator.

Another way to heat an apartment is to direct the warm air with the help of an air cooling system further, for example, through the concrete slabs of the lower or intermediate floor of the house, as a result of which thermal energy can be accumulated directly in the structure, from where it is transferred to the living rooms at night, as well as the next day (if it is cloudy).

Several rational solutions using fans are described below.

Swimming pool or soil as a heat accumulator. Air heated during the day by solar radiation

Fig. 24. Heat is stored in a stone heat accumulator with the help of fans located under the floor:

• a — stone heat accumulator under the greenhouse, open air circulation;
• b — stone heat accumulator under the greenhouse and apartment, open air circulation;
• c — stone heat accumulator under the apartment floor, closed air circulation.

Fig. 25. Heat is accumulated in concrete slabs with internal cavities:

• a — greenhouse made of light structures, concrete slabs with cavities used for the apartment floor structure;
• b — massive wall in the greenhouse, apartment floor structure made of concrete slabs with cavities;
• c — solar (heat-accumulating) wall in the greenhouse, ceiling structure and apartment floor made of concrete slabs with cavities.

Principles of heat exchange systems in greenhouses

A stone heat accumulator under a greenhouse or an apartment. Warm air is forced through an air duct to the bottom of the stone heat accumulator, from where it passes independently between individual stones, cools down, and returns to the greenhouse.

In another solution, air from living rooms is drawn through a stone heat accumulator into the greenhouse, where it is heated and returned to the living rooms. Such a system functions well in late autumn and early spring, when the air humidity in the greenhouse remains low. During the period when the system needs active irrigation, problems may arise in the apartment due to high air humidity.

Heat is returned to the living rooms through radiation and convection. From the stone heat accumulator located under the floor of the greenhouse area, it enters the greenhouse with the help of fans. Warm air from the upper part of the greenhouse is blown through channels and openings in concrete slabs with cavities or in other greenhouse structures.

There are several alternative variants for the basic solutions described above; some of them are presented in Chapter 9.

Greenhouse space as part of an individual dwelling

In addition to functional and physical factors, the architectural and structural-technical solutions for greenhouses attached to individual houses largely depend on the type of building, for example, its size, number of stories, time of construction, etc. The choice of an effective solution is also significantly influenced by the features and nature of the surrounding environment — the shape and ruggedness of the terrain, surrounding buildings, and shading.

Greenhouse attached to an individual house. Figs. 26—29 show structural solutions for greenhouses attached to different types of individual houses.

In small one- and two-story individual houses, there are various options for greenhouse layout. Such greenhouses give residential houses an individual character; however, in terms of their design, they must be subordinate to the main architectural solution of the building.

The alternative solutions presented here reflect typical cases. However, there are other solutions, the choice of which is determined by specific circumstances.

Architecturally, a greenhouse is a natural extension of the building's forms, and it must be built taking into account the construction materials and color shades of the house. When joining a greenhouse to an existing individual house, the external design often causes difficulties, and it becomes necessary to seek the help of a specialist (architect). The greenhouse should not be an accidental extension, but a harmonious part of the individual house.

Greenhouses in multi-story buildings. Living in multi-story buildings, especially in new suburban neighborhoods, has turned out to be unfavorable for residents and has caused their dissatisfaction. This is due both to the shortcomings of the development area and apartment layout, and to the lack of comfort. Often apartments are cramped, the outdoor spaces adjacent to them, if any exist at all, are often small in size and insufficiently protected from the wind. Children's playgrounds often do not meet standard requirements. As a rule, there is a complete lack of functional rooms for young people and hobby rooms for adults.

The construction of multi-story buildings has ensured the efficient use of land plots. Practically always, this has come at the expense of deteriorating natural conditions and decreasing housing quality. Therefore, it is not surprising that many residents living in multi-story buildings feel like temporary tenants and dream of moving to small individual houses somewhere nearby. They associate living in small individual houses with achieving independence, family and personal well-being, the opportunity to apply their own efforts to improve living conditions, obtaining an individual yard, ensuring security, etc. For many, such hopes remain unattainable dreams, but they suggest that innovations are required in new residential areas, especially in those regions of Finland where about 30% of the country's population lives.

The construction of greenhouses attached to multi-story buildings should contribute to improving living conditions in neighborhoods. This provides the population with additional living space and gives residents the opportunity to engage in their favorite activities.

Greenhouses in new multi-story buildings. Many believe that greenhouses can only be created in rural areas, and in urban conditions this is applicable only to small houses. However, this is not the case. Greenhouses can also be arranged in apartments in multi-story buildings, where they form an individual or collective outdoor area where one can engage in crop production or spend free time. Some recommendations for implementing this idea are given below.

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Fig. 26. Structural solutions for greenhouses attached to small individual houses: a — new individual houses; 6—9 — solutions acceptable for both new and old individual houses

The greenhouse should be positioned on the sunny side of the house (see Ch. 4), where insolation is ensured primarily from February to November. The area of the green-

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Fig. 27. Structural solutions for greenhouses in various types of individual houses with dense site development house should be increased to dimensions larger than those of standard balconies. An area within 10—20 m? should be considered appropriate, so that it is possible to place a table and chairs, as well as allocate space for growing plants. When the living area is located at ground level, the greenhouse can be arranged partially recessed into the house. When designing greenhouses, it is necessary to take into account how the angles of incidence of sunlight on vertical and horizontal planes change, as well as the changes during day and night and across seasons. In summer, when the sun is high in the sky, it is necessary to provide shading and ensure a sufficient number of ventila- Fig. 28. Example in an individual house

More in the house at the same time

Fig. 29. Greenhouse extension to an old individual house

Fig. 30. A multi-story attic-corridor type house, in which wide balconies are converted into domestic greenhouses

Fig. 31. Greenhouses in a multi-story house with one- and two-story apartments tion hatches for air exchange both in the greenhouse and in the living rooms.

Most often, a greenhouse is a single-story room with a depth of 2—3 m. In some typical multi-story houses with duplex apartments, the height of the greenhouse can be quite large.

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Fig. 32. A multi-story house where the greenhouse is located on

In one successful solution, a narrower balcony is located along the wall adjacent to the bedroom or sauna rooms (on the top floor) and provides the desired shading in summer. The greenhouse functions:

as a solar energy accumulator and contributes to heating the apartment by capturing free solar heat and reducing heat loss. On a sunny day, when the apartment overheats quickly (in Finnish conditions this only happens in summer), the green- house effectively functions as an apartment cooler. If, in addition to ventilation transoms in the upper part, one ensures a connection with the cool side of the house (north-west — north — north-east), then it is possible to create air currents in which the exiting heated air draws in cooler air.

Fig. 33. A common courtyard area with a glass roof for multi-story residential buildings (cross-section and perspective)

One of the interesting solutions for equipping a greenhouse in a multi-story house is to place it on the roof of such a building. In this way, not only is proximity to nature achieved, but also the possibility of obtaining sunlight in densely built-up neighborhoods, as well as in residential buildings with unfavorable orientation. This solution is also fully applicable to old houses.

In the greenhouse, you can engage in both individual cultivation of plants on separate plots and collective plant growing in relatively large areas. Children can play in such a room all year round, especially in the cold season and in rainy weather. If the breakable glass in such rooms is replaced with unbreakable organic glass, at least in the most dangerous places, then such a greenhouse becomes safe and suitable.

The air temperature in the greenhouse is always higher than the outside air temperature. This feature can be used, for example, when equipping a ventilation system that supplies air to living quarters that has been pre-heated in the greenhouse, instead of supplying cold outside air directly. Thus, significant energy savings are achieved in a multi-story house, where more than half of the heat is often lost through ventilation devices. Fig. 33 shows an option for using a common courtyard of two or four houses, equipped with a glass roof, as a greenhouse. This space is not heated. All entrance doors open into the courtyard, where there is plenty of room for rest and various types of activities for the residents. Naturally, such conditions encourage communication between residents, since the covered courtyard unites them and involves them.

Greenhouses in old multi-story houses. Creating greenhouses in old multi-story houses requires only a transparent covering made of ordinary or organic glass (for example, acrylic or polycarbonate, see Ch. 6), installed in the place of the loggia's light opening. If the hermetically sealed parts and joint areas, such as railings and side walls, are sealed, this prevents the formation of drafts and ensures the creation of a small protected and illuminated room. The possibilities for using such a room are much better, and its service life is 2—3 times longer than that of a standard balcony. In such greenhouses, it is possible to grow transplants and decorative flowers in pots and containers, as far as the balcony dimensions allow.

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Fig. 35. Balconies are being expanded and equipped with a glass cover

5— 293 65 It is true that in many multi-storey buildings constructed in previous years, balconies are quite narrow, especially in terms of depth.

Creating such a glazed outdoor space does not require great expense — it can be built by oneself. In an already existing space, one only needs to install a protective cover made of transparent material and ensure sufficiently intensive ventilation. One of the advantages of such a glazed space is that mosquitoes remain outside on summer evenings. Protective nets are installed in front of ventilation openings, which most often remain open in summer. In many block-built multi-storey buildings, there are no apartments on the ground floor. A large greenhouse space can be set up on this floor, where residents can plant crops together or divide plots among themselves. Lightweight transparent greenhouse structures are sensitive to impact from stones, balls, objects falling from above, etc., so incidents of vandalism are possible here. However, such a possibility exists everywhere where conditions for reasonable leisure for young people have not been created.

by building greenhouses, one can achieve a significant improvement in the appearance of a residential neighbourhood with high-rise development. Often, block-built multi-storey buildings constructed in the last 15 years are quite monotonous and lack individuality. Placing greenhouses with window frames, protrusions, and plants in them, when well-executed, can noticeably improve the exterior appearance of buildings and breathe life into grey, dull houses.