Effective solar energy accumulation in modern greenhouse structures

In a long, south-facing greenhouse, uniform lighting is not as important as in a small greenhouse, where the area intended for growing plants is small and every corner must be utilized. In deep greenhouses, it is important to check how intensely the sun's rays penetrate; for narrow greenhouses, this is not as significant.

If any mass is used in the greenhouse to store solar energy (water bodies, stones, bags of salt, etc.), then a mandatory condition for the "useful work" of such masses is that solar radiation must enter the greenhouse, especially during the cold season, from 9 a.m. to 3 p.m. All this can be taken into account during design if solar charts are used.

Fig. 50. Influence of greenhouse covering on solar energy intake

Fig. 51. Some economical forms of greenhouses

5. Thermal energy storage

A conventional greenhouse or glasshouse with a large surface area of glazed enclosure does not store solar energy well, primarily due to its poor thermal insulation and sealing characteristics. Maintaining the temperature required for plant growth necessitates additional heating energy. For example, a conventional greenhouse or glasshouse operating year-round consumes approximately 10 times more energy than a small detached house.

A greenhouse recessed into a residential apartment has natural protection on one, and often three, sides. Heat losses in the living quarters are partially utilized by the greenhouse. With rational construction, the mass contained in the structure of the outer wall of the residential building can be used as a heat accumulator. The greenhouse offers favorable opportunities for accumulating and storing thermal energy, which is very important in the harsh northern climate of Finland.

Effect of increasing heat-accumulating mass. The effect of heat-accumulating mass on the air temperature in the greenhouse and on the utilization of solar energy can be easily identified by comparing two greenhouses. One of these greenhouses has no heat-accumulating mass, while in the second greenhouse, the heat-accumulating capacity is increased by constructing the back wall of the greenhouse from brick or concrete. In a greenhouse without heat-accumulating mass, there is a strong fluctuation in air temperature between day and night. The maximum temperature occurs at approximately 3 p.m., and the lowest in the early morning, before sunrise. When heat-accumulating mass is placed in the greenhouse, the difference between day and night air temperatures decreases significantly — the night temperature rises, and the daytime temperature decreases. At night, the heat-accumulating mass releases the heat stored during the day, and during the day, it serves as a cooler, i.e., it absorbs heat. This heat-accumulating mass would be better called a heat compensator. In a greenhouse with such a mass, the time to reach the maximum temperature shifted by 2–3 hours. A more uniform distribution of temperature in the greenhouse at different times of the day significantly improves the conditions for plant growth, and also increases its operational capabilities.

Placement of the heat compensator in the greenhouse and apartment. Solar radiation penetrates through the transparent covering of the greenhouse and hits the walls and floor, and in the case of directional radiation, the ceiling. The proportion of incoming solar energy absorbed or reflected depends mainly on the color of these surfaces. Dark surfaces absorb shortwave radiation well and reflect little; light-colored ones act in the opposite way; mirror surfaces reflect almost all the light falling on them.

Thermal energy generated from the absorbed solar radiation is partially transferred to the heat-accumulating mass, and the remaining part moves from the surface of the mass to other surfaces and from there via convection into the air, simultaneously raising the air temperature.

Table 5.1. Forms of heat transfer in a room

—ыы= I. Surface Convection Thermal radiation (shortwave), % Floor 46 54 Wall 40 60 Ceiling 30 70

Зарание вые зил] | ых РЕНН /^ ы ЕР, х / АЕ АИ

Fig. 53. Temperature in the "Aurinkopirttitalo" solar house greenhouse (solid line) and outside (dashed line) in Laihia in 1981-1982 (typical house designed by the Ecoso company)

Fig. 54. Influence of color on the absorption of shortwave solar radiation 1 — black; 2 — red and blue; 3 — yellow; 4 — white; 5 — mirror p 8? Reflected solar radiation is converted into thermal energy only if it hits the surface of an object or building structure. If the material receiving the reflected radiation is characterized by low heat-accumulating capacity or low thermal conductivity (e.g., furniture, wooden walls, etc.), it heats up very quickly, and the heat passes into the air, which can lead to overheating, which must be managed through ventilation.

The efficiency of direct solar radiation utilization can be increased in two ways:

by absorbing direct solar radiation and storing thermal energy directly in the mass upon which the sun's rays fall;

by diffusing solar radiation in all directions where there is some heat-storing mass.

Direct solar radiation. When using the first method, one identifies the areas of the greenhouse and the residential apartment that receive primary solar radiation during specific time intervals, for example, between 9 a.m. and 3 p.m. As auxiliary tools, one can use the shadow model developed by Gunnar Pleijel (see section 4.6.2), as well as insographics and insolators. On the floor and walls of the model, mark the areas that receive direct solar radiation during different months. It is advisable to cover these areas at the design stage with dark-colored materials that have good heat-storing capacity. Such a material must, in addition, have the ability to absorb thermal energy so that the largest possible portion of the heat accumulated on its surface can transfer into this mass, rather than into the air.

Diffuse radiation. Solar radiation entering the greenhouse or passing through it into the apartment can be diffused using a covering of appropriate glass or polyethylene, as well as by applying a rough white surface. From this surface, light is reflected in all directions and falls upon

other masses, by which it is absorbed. The advantage of this solution is that all masses are heated uniformly, and the temperature on their surface does not rise to such high values as on the surface of walls whose material absorbs solar radiation directly. The use of diffusing translucent coverings is also beneficial for plants, as light falls on them uniformly, and the sun's rays do not cause the plants to "burn." A disadvantage of using such coverings is that they are not completely transparent to light — the surrounding space is visible through them as if in a fog.

Placement of heat-storing masses. The dimensions of the areas in the greenhouse where any heat-storing mass can be placed are very limited. Most of the walls, and often the roof of the greenhouse, are equipped with a light-transparent covering, while the main part of the floor surface is set aside for growing plants. Therefore, for other purposes, only a small part of the floor surface and part of the back wall remain, which usually also contains windows and a door. These areas must be used as efficiently as possible for accumulating and storing thermal energy. To solve this task, it is necessary that direct solar radiation falls on the heat-storing masses placed in these areas during most of the daylight hours. Thus, these masses will perform the functions of solar energy accumulators and heat storage units. A mass placed in this way is called a primary mass. Depending on the properties of the primary mass, heat transfer from it occurs over a distance of up to 200 mm. From a secondary mass, which is not subjected to direct exposure to sunlight or mainly remains in the shade, heat transfer occurs over a distance of 80—100 mm. Therefore, the secondary mass requires a surface area 2—3 times larger than that of the primary.

When placing heat-storing masses and selecting materials, one must answer the following questions:

what needs to be provided with heat first — the apartment or the greenhouse room?

in what way can building materials used in the house's construction be utilized with maximum efficiency as heat accumulators?

how much space can be provided for placing the heat-storing mass, and how will this placement affect the use of the greenhouse area?

how can one ensure maximum efficiency when designing the structure of the partition wall, which is equipped with sliding heat-insulating covers on both sides?

and what are the costs of their implementation? how is this constructive solution implemented in practice, and will this structure serve for a long time? how does the chosen constructive solution affect the exterior appearance of the greenhouse and the apartment?

In Sweden, at Chalmers University of Technology, various systems were studied in which solar energy accumulated in a greenhouse or a glazed veranda was used to heat an apartment. Fig. 56 shows two alternative options for placing heat-storing masses. In the first option (system 1), such a mass is placed in both the greenhouse and the apartment; the blinds are lowered, and heat is accumulated in the mass placed in the greenhouse. The temperature in the apartment is maintained below 20°C, and the daily energy consumption is somewhat higher than in the second option (system 2), according to which the air heated in the greenhouse is subjected to forced circulation by means of fans through the existing

Fig. 55. Arrangement of heat-accumulating mass in the greenhouse and living rooms. 1--2— primary mass is placed in the greenhouse, and it is the first to be provided with heat; 3--4— primary mass is placed both in the greenhouse and in the living rooms; in this case, all rooms are provided with heat: 5--7 — secondary mass is placed in the apartment; the apartment and the greenhouse are provided with heat; heat transfer is carried out using mechanisms; 8 — secondary mass is placed in the greenhouse; the greenhouse is provided with heat in the first instance, heat transfer is mechanized; 9--10 — primary mass is placed between the greenhouse and the apartment; with the corresponding position of the movable thermal insulation shield, heat can be provided primarily either to the apartment or to the greenhouse. Daily energy consumption for heating, kWh.

0 2 4 6 8 10 12 14 16 18 20 22 24

Fig. 56. Research on apartment heating systems based on the use of stored solar energy. Research was conducted using the program "a!" (B/S) (Rötöböre, |

1— heat-accumulating mass is placed in the greenhouse and in the apartment, blinds are lowered; 2 — heat-accumulating mass is placed only in the apartment, heat transfer is carried out using fans, blinds are lowered

concrete slabs with cavities in the apartment. This ensures an intense transfer of thermal energy from the heated air to the material of the reinforced concrete slabs, as a result of which the air temperature in the apartment increases, although energy consumption decreases.

Heat-accumulating properties of various materials. The heat-accumulating capacity of building materials depends on their specific heat and temperature difference. As a general rule, it can be stated that the greater the density of a given substance, the greater its heat-accumulating capacity. Heavy substances, as a rule, are also characterized by good thermal conductivity.

Table 5.2. Heat-accumulating capacity and thermal conductivity of some materials:

Substance | Density | Heat-accumulating capacity | Thermal conductivity, W/(m·°C) | Relative capacity

Water | 1000 | 1.16 | 1.00 | 0.55 | 1.00

Steel | 7850 | 1.00 | 0.86 | 45.3 | 82.36

Natural stone | 2940 | 0.58 | 0.50 | 1.43 | 2.60

Concrete | 2300 | 0.53 | 0.46 | 1.65 | 3.00

Solid brick | 1800 | 0.46 | 0.40 | 0.66 | 1.20

Gravel, sand | 1600 | 0.37 | 0.32 | 0.39 | 0.70

Soil | — | 0.25 | 0.15 | 0.85 | 1.55

In small greenhouses where the space for placing heat-accumulating masses is limited, it is advisable to use substances with phase-change properties. Such products are available on the market in packaged form — bags a few centimeters thick that can be mounted into a wall, and within an apartment — also into the floor. The advantage of such materials is their small volume, due to their high heat-accumulating capacity (for example, 6 times greater than that of stone). Their disadvantage is their high cost. A more economical solution consists in using as heat accumulators such materials that simultaneously serve as building material for floor or wall structures. The only free heat-accumulating material is water. When using it, certain costs are incurred only for the installation of containers and equipment for a pool.

Influence of the heat-accumulating mass surface. The color of the primary heat-accumulating mass used has a decisive influence on the proportion of solar radiant energy absorbed by the material. For the secondary heat-accumulating mass, one of the most important factors is the structure of the surface layer, since a significant amount of the generated heat is transferred into it through the air (by convection). In this case, the rule remains valid: the rougher the surface, the more thermal energy will be transferred into it. Long-wave thermal radiation is well absorbed by almost all building materials.

Table 5.3. Ability of various materials to absorb short-wave and long-wave radiation

Substance | Short-wave solar radiation | Long-wave radiation | Substance | Short-wave solar radiation | Long-wave radiation

Water | 0.94 | 0.95—0.96 | Red brick | 0.55 | 0.92

Concrete | 0.60 | 0.88—0.97 | Sand | 0.82 | 0.90

Thickness of the heat-accumulating mass element. When using solar thermal energy accumulated during the day at night, the depth of heat penetration into the mass, for example, over 8 daylight hours, is determined by its thickness. Depending on the type of primary heat-accumulating material, its thickness in a building structure can vary from 150 to 250 mm. Structural elements such as floors and walls can be thicker if the building structures have good thermal conductivity. This ensures the achievement of better heat-accumulating capacity. Fig. 57 shows daily temperature fluctuations for concrete and brick structures. The figure shows how much mass is involved in the heat accumulation process.

4. Heat accumulators made of various materials. Traditional building materials. The heat-accumulating capacity of concrete and brick is not very high, yet these materials can still serve as primary mass, i.e., in general terms, work in the same way as natural stones. This is especially true for concrete, which has sufficient thermal conductivity and quite good heat-accumulating properties. Concrete is widely used in floor and wall structures. Concrete walls can be mass-produced in a factory in the form of blocks or cast monolithically on the construction site. The optimal thickness of a concrete wall is 200—250 mm for primary heat-accumulating mass and about 100 mm for secondary mass. For a brick wall, the corresponding thicknesses are 130—150 mm and 80 mm. In practice, this means that brick walls are laid to half the thickness of a brick, or the floor is constructed from concrete slabs 120 mm thick, which are then covered with brick laid on edge.

Water heat accumulators. Various containers previously used for other purposes can be used as such heat accumulators — kerosene and paint vessels, barrels, as well as inexpensive plastic or metal containers. The surface of such vessels must be dark to ensure good absorption of solar rays. By painting such containers with dark gray, black, and dark red corrosion-resistant paint (for example, "Teknos-Maalit" paints by "Kirje"), one can achieve an attractive appearance, which is important since the greenhouse is part of the living quarters. When placing containers with water in a greenhouse, it is important to remember that the largest possible part of their surface should be within the reach of solar rays. If large containers are used, it is necessary to ensure that air can flow around them from all sides, since the accumulated thermal energy must then be transferred back into the greenhouse. Water pools function well as...

4 - temperature fluctuations when changing day and night for concrete (a) and brick (6) 0.5 01 - 5 10 20 3 “ Thickness of heat-accumulating mass, cm x - 1.0 05 \. Fig. 58. Factory-made containers used for storing heat --1-- 1-3 tanks with a capacity of 10 l; 2— oil; 4 containers for paint, kerosene, etc. with a capacity of 200 l: 8 —

10 20 30 Thickness of heat-accumulating mass, cm `^ polyethylene bag; 4— bottle (3 l); 5 — canisters (5 l)

Solar energy accumulators in greenhouses

Solar energy accumulators. The solar radiation falling on them is absorbed almost completely if the inner surfaces of the pool are black. However, this causes problems associated with water evaporation, as this process is accompanied by the loss of a large amount of energy (water cooling occurs).

When 1 l of water evaporates, an amount of thermal energy is consumed sufficient for heating 100 l of water by 6°C. This amount of heat, in turn, is released during the condensation of humid air on cold surfaces. The appearance of moisture is unfavorable for the premises and necessitates ventilation, which is associated with heat energy losses. Therefore, it is recommended to equip such a pool with a transparent polyethylene cover to reduce heat loss and prevent water evaporation.

Using containers for heat accumulation

The most appropriate method for accumulating thermal energy in water is to use barrels filled with water, covered with transparent lids or polyethylene film to prevent water evaporation. The disadvantage of this solution is that the warmest region is at the top, and the coldest — at the bottom of the barrel, as a result of which the heat-accumulating capacity of the water will be used relatively poorly.

To utilize the heat coming from the barrels, various plants are placed on them, while free circulation of air and heat in the surrounding space must be ensured.

When using smaller containers as heat accumulators, temperature stratification of water is almost completely prevented. Moreover, smaller containers can be placed more compactly in the greenhouse, thereby saving useful space for recreation and growing plants.

The main feature of water heat accumulators is that heat is stored longer in large heat-accumulating containers, but it is transferred to different layers of the water mass more slowly. Small containers, due to their large heat-transfer surface, react quickly to changing conditions, which is useful, for example, in case of overheating. However, they quickly release the accumulated heat, which is highly undesirable on cold nights.

Chernozem as a heat accumulator. Using chernozem to accumulate heat

Fig. 61. A primary heat-accumulating wall, constructed from tin canisters, represents the cheapest and simplest way of heating the greenhouse. Unfortunately, this method is ineffective, as accumulating heat with the earth without using any devices yields worse results than could be expected based on the heat-accumulating capacity of the soil. The reason for this is the presence of plants in the greenhouse, which cover most of the area intended for their cultivation. Stone heat accumulators. These heat accumulators also proved to be relatively ineffective. A stone heat accumulator can be placed directly in the greenhouse by arranging natural stones, for example, against the back wall or under the greenhouse. When laying stones in front of the back wall, it is necessary to ensure that the thickness of their layer is small and that the sun's rays reach the surface of all stones. The stones can be placed over the entire surface of the wall using a mesh or embedded in the wall, which gives it a very beautiful appearance, however, all this requires significant labor.

, Fig. 62. Stone heat accumulator; stones are stacked against the back wall and attached Fig. 68. Relative change in air temperature in a concrete slab with structural cavities as a function of the length of these cavities. Volumetric flows: 200 m3/h=4 m/s; 100 m3/h=2 m/s; 50 m3/h=1 m/s

If several rows of stones are used, a fan must be used. In this case, washed stones should be laid so as to leave gaps between them for air passage, just as when placing a stone heat accumulator under the greenhouse floor. If such a heat accumulator is intended for short-term use, the size of the stones should be small. According to the experience of operating such a heat accumulator, it can be said that the best result in this case is provided by rounded stones with a diameter of 30—50 mm. The transfer of heat to the stones is still not very effective. Experience gained in Finland and Sweden has shown that only one-third or half of the stones (of the total number taken) participate in heat accumulation. Compared to concrete slabs that have structural cavities and ensure that 80—90% of the mass of the slabs participates in heat accumulation, the stone heat accumulator is relatively less efficient. |

Fig. 64. Experimental ZNEO house in Sheffield (England), architect Cedric Green. Cross-section and operating principle

1 — solar altitude angle in March; 2 — rising warm air in the greenhouse; 3 — horizontal air duct with thyristor control; 4 — vertical heat accumulators in the space between the windows; 5 — fan that draws in warm air and forces it through the floor heat accumulator into the greenhouse; 6 — direct solar radiation; 7 — return air; 8 — support for climbing plants that provide shade in summer; 9 — manually operated hatches

Hollow structures. To effectively transfer heat into a brick or concrete block wall with structural cavities, a fan must be used. In hollow concrete slabs, for example, at a length of 6 m, about half of the heat can be transferred to the concrete mass, depending on the warm air flow rate. The concrete is almost entirely involved in the heat accumulation process due to the relatively small thickness of the slab and the dense network of hol-