Solar energy accumulation in greenhouse and residential structures

Hollow slabs can be used to construct a wall between a greenhouse and an apartment, or a greenhouse floor directly on a heat-insulating surface, as well as in an apartment where, for example, slabs already having thermal insulation on the underside are used. In addition, intermediate and upper floor bases in stone buildings can also be made of hollow concrete slabs.

One such design solution was implemented at the University of Sheffield (England) for the experimental ZNEO house (Solar Heated Experimental Dwelling). In this house, heated by solar energy, there is a greenhouse occupying the entire south wall of the one-story house. As soon as the air temperature in the greenhouse rises above what is required for crop growth, the air from there is transferred via fans to heat accumulators. The power of the fans, equipped with thyristor regulators, is 2.30 W. Bottles, bricks, and small stones are used as heat accumulators. From them, heat is transferred to the living rooms by radiation and natural convection. Based on tests, it was concluded that the system functions most effectively if it is equipped with fans and heat accumulators; in a system equipped with fans and heat accumulators, energy savings amounted to 78%, while in a similar solution without the mentioned equipment, it was 49% (these figures are given in comparison with a standard residential house);

the additional costs for the construction of the greenhouse, the purchase and installation of fans, and the equipment of heat accumulators are fully recovered within nine years of the house's operation;

the only drawback of this method is the low level of noise created by the operation of the fans.

Determination of heat consumption and selection of heat accumulator sizes. "Rule of thumb." When designing the placement of heat-accumulating masses and assessing the need to create heat reserves, one can use the methodology that provides for

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. Air temperature fluctuations in the greenhouse with changes in heat capacity: for 80 l of water/m² of transparent cover (a), 100 l of water/m² of transparent cover (b), and l of water/m² of transparent cover (c) the application of the "rule of thumb." In essence, this methodology is quite crude; however, it is convenient and prevents major errors. The final verification of the project can be carried out after this using model "a" (automatic data processing) or by using previously gained experience, for example, regarding the selection of sizes and the method of heat transfer in heat accumulators. Issues of solar radiation accumulation in accordance with the "rule of thumb" were considered earlier (see ch. 4). The dimensions of the required heat-accumulating mass can be determined by calculating its specific heat capacity for a given greenhouse. This will show how many liters of water or other heat-accumulating mass (in terms of water equivalent) fall per 1 m² of transparent cover surface area. It is assumed that light rays penetrate the greenhouse without obstruction.

Comparison of various solutions made it possible to identify dependencies, on the basis of which it was established that for the conditions of Finland, the following volumes of primary accumulating mass per 1 m² of transparent cover are optimal: 0.1–0.2 m³ of water, 0.3–0.5 m³ of bricks, and 0.6–1.2 m³ of stones.

Features of heat-accumulating masses and calculation models

The quantity of heat-accumulating mass says nothing about the shape of the heat accumulator itself, nor about the dimensions of a single accumulator. It is not at all the same thing, for example, whether 1000 l of water are placed in five barrels with a volume of 200 l each or in 2 thousand jars with a volume of 0.5 l each. The use of the same volume of water would lead to completely different results!

Fig. 66. Using the ZPTET program to study the ZNEO experimental house in Sheffield.

Determination of heat consumption using the "ak" model. If the building characteristics and local climatic conditions are known, for example, temperature, solar radiation, shading, etc., then measurements can be started in accordance with the construction plan.

If it is necessary to obtain accurate information about the thermal regime of the greenhouse and the apartment, experiments must be conducted at small time intervals (usually 1 h). Every hour throughout the year, the following should be determined:

• angles of incidence and intensity of solar rays;
• outdoor temperature;
• surface upon which solar radiation falls;
• amount of heat generated;
• directions of heat transfer;
• temperature obtained in different places, etc.

It is quite clear that this process will be so time-consuming that the assistance of an “ac” model becomes simply necessary.

In various countries, programs for the “a” model have been developed, for instance, the VK!$ program, or the ERTET calculation program described below, which is quite convenient due to its visual clarity. The application of the 5PTET program for studying the experimental ZNEO house will also be shown below.

Grid method and energy storage

Grid method. This method is a very effective and simple way of performing the necessary heat circulation calculations. It is based on the fact that thermal equilibrium corresponds to the equilibrium state of current in an electrical circuit. External weather conditions (mainly the amount of solar energy and temperature) change continuously.

Using special electronic devices, images of walls, the ceiling, and heat-storage mass are obtained, as well as so-called sink points, which represent locations for temperature measurement. Once we have data on the building’s characteristics, as well as some information about weather conditions, we can proceed with the experiment.

There are two ways to conduct measurements:

1. Build an electronic model—a circuit on which the experiment is then conducted.
2. Enter a program in digital form for a small computer or a multi-functional computing device and perform calculations.

In both cases, results are obtained in the form of temperatures measured every hour at the control points. If they are not satisfactory, the experimenters will have to modify the design by adding or subtracting the corresponding values. The principles of this method are used in the ERTEG program, which allows for the use of a small computer or a multi-functional computing device.

Creation of stored thermal energy reserves. The most significant factors determining the heat loss of a greenhouse are as follows:

large air exchange requirements due to the use of a large glass covering area that effectively accumulates solar energy.

Optimizing heat balance and ventilation of greenhouses

The problems are partially related to one another, so it is desirable to find ways to solve them simultaneously. The heat balance of south-facing double-glazed windows during the heating season for the Etelä-Suomi province (Southern Finland) is negative. This means that the amount of solar radiant energy entering through the windows will be greater than the amount of heat escaping through them to the outside.

If the greenhouse structure were airtight and excess heat could be fully collected for storage, the greenhouse would remain warm throughout the entire plant growing season, from mid-March until the end of October. In practice, however, this is impossible because, firstly, it is impossible to achieve complete airtightness of greenhouse structures and, secondly, air must be exchanged to ensure that plants receive carbon dioxide, and the greenhouse must be ventilated to reduce humidity (see Ch. 7).

Methods for reducing heat loss in a greenhouse

Reducing the surface area of the transparent covering. Since the transparent covering is the weakest element of the greenhouse envelope in terms of its thermal insulation characteristics, efforts are made to reduce its surface area where it is less important for sunlight penetration. This is not difficult to do, as a greenhouse is always built as an extension to a residential house or another building, such as a barn, so the greenhouse is protected and receives some amount of heat from this structure.

By burying part of the greenhouse into the residential building or making one or both end walls as solid insulated walls, almost complete protection can be provided from two or even three sides. As a result, heat loss is significantly reduced. Depending on the shape and, above all, the depth of the greenhouse, the roof can also be made partially or fully enclosed. During the design process of the opaque and transparent parts of the building, it is very important to keep in mind the factors ensuring the intake of a sufficient amount of sunlight into the greenhouse (see Ch. 4), as well as its sufficient illumination (see Ch. 7).

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Fig. 67. Heat balance of a south-oriented window in Helsinki during the heating season

Increasing the thermal insulation capacity of the transparent covering. The thermal insulation capacity of a greenhouse's transparent covering can be increased in two ways:

• improve the window design to enhance the thermal insulation capacity and airtightness of the architraves, frames, and especially the window panes;
• cover windows at night with thermal insulation materials.

Improving window insulation. For the construction of the transparent enclosure of a greenhouse, one can use gl-.

Choosing design solutions for greenhouse glazing

When building a greenhouse, it is recommended to use glass or plastic (see clause 6.1.5) in the form of double glazing, the thermal insulation capacity of which is significantly higher than that of traditional single-glass designs. Admittedly, such products, classified as thermal insulation glass, are expensive, and likely for this reason, in some places, enclosures with a single layer of glass must be used.

Accumulated experience indicates that a hermetic window design with a single layer of glass results in lower heat loss than a loose design with two layers of glass. However, the main problem with a single-glass window design is not related to heat loss, but to the effect of condensation of water vapor on the inner surface, as a result of which the window becomes opaque, loads occur, especially on the lower parts of wooden frames, and in winter, window icing is observed.

If the greenhouse occupies the entire southern outer wall of a building, the iced-over greenhouse windows give it a rather unsightly appearance. Icing can also occur in windows with two layers of glass, but this happens much less frequently and, as a rule, only part of the window is covered with ice.

Application of movable thermal insulation means

With the help of movable thermal insulation means, one can significantly improve the thermal insulation properties of windows. As such means, one can use thermal insulation shades, curtains, shutters, or beads made of polyester plastic. These means are used for the wall structure between an apartment and a greenhouse when it is necessary to improve the thermal regime in a living room, or in greenhouse windows on the inside or outside, or in the space between the panes, when it is necessary to ensure a gain in the heat balance directly for the greenhouse.

Movable thermal insulation means can be implemented in three ways:

• in the form of rigid shutters and doors;
• in the form of freely moving shades, felt covers, curtains, or roller blinds;
• in the form of beads made of expanded polystyrene or similar materials.

Fig. 68. Diagrams of greenhouses with single and double glazing

Rigid shutters and doors are best suited for relatively small openings. It is most convenient to open them by sliding, since in this case, they require little space and are the least visible. The airtightness of such structures is a criterion for functionality; otherwise, their thermal insulation capacity will not be utilized. External shutters are, as a rule, inconvenient, as they are difficult to operate due to the climatic conditions in Finland.

Fig. 69. Rigid adjustable thermal insulation devices a — principle of a folding design; b — principle of a louver design

Flexible design means, such as covers, blinds, slat shades, or sliding screens, are inferior in thermal insulation capacity but more convenient. They function as additional thermal insulation material and, in addition, create shade. If the airtightness of these means is ensured, especially at the bottom and sides, and overlapping joints are implemented, their thermal insulation effect will correspond to the use of a window with an additional layer of glass.

The specified covers can also be equipped on the outside of the greenhouse, in which case their operation in the climatic conditions of Finland would be difficult. The authors recommend using internal design solutions due to their ease of use, despite the fact that higher thermal insulation results can be achieved with external structural thermal insulation solutions.

In the USA, a system has entered the market in which polyurethane (styrox) beads are used as thermal insulation material. These beads are fed by air into the space between two panes inserted into window frames in the wall at night or in cloudy, cold weather. The system is fully automated, and the consumer can decide for themselves when thermal insulation is more important than sunlight for the greenhouse and the entire apartment. When using this solution, the thermal insulation capacity of the wall with the window is slightly improved. A disadvantage of the system considered is its high cost.

Thermal insulation of the greenhouse base. Heat loss through the greenhouse floor has proven to be much lower than through the external enclosure. This is due to the fact that the soil temperature is higher than the outside air temperature. For example, in Southern Finland at a depth of 2 m, the soil temperature in January is -4°C. The soil temperature under the greenhouse is even higher, and if careful thermal insulation of the base is performed, the soil under the greenhouse begins to function as a long-term heat accumulator.

This solution has both advantages and a disadvantage. The ability of a greenhouse to accumulate heat increases; however, as heat is transferred to the soil beneath the greenhouse, it returns from it very slowly. When considering daily temperature fluctuations, where the heat-accumulating mass of the floor is insulated from the soil, a more favorable solution can be found for the heating season in Finland. In this case, the thickness of such a mass layer should be within 20 cm for concrete in areas that receive sunlight for the greatest part of the daylight hours. Insulation, for example, a 10 cm thick Styrox sheet, is placed on compacted sand, which at the same time will serve as a base for placing the heat-accumulating mass.

In both cases, good thermal insulation of the greenhouse plinth and the implementation of anti-frost measures when constructing shallow foundations remain necessary prerequisites.

o, Reducing the cooling effect of wind. Wind has a strong cooling effect on a greenhouse because it creates pressure differences that increase heat loss, and also intensifies heat transfer from the greenhouse enclosure through convection, u

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Fig. 70. Various types of thermal insulation devices; interior installation Fig. 71. Thermal insulation system with Styrox beads fed by compressed air from a storage unit Fig. 72. Temperature distribution a — under a greenhouse (research by a Canadian specialist); 6 — in the surface layers of the soil in Southern Finland

Protection of greenhouses from wind impact

The cooling effect of wind depends on its speed. The higher the wind speed, the greater the pressure difference and heat transfer from the surfaces of the buildings, which is especially noticeable for glass covering.

Therefore, it is necessary to protect the greenhouse from wind. From the north side, where the coldest winds blow, the greenhouse is protected by a residential house.

To protect against winds from other directions, one can use:

• tree planting;
• low structures (hedges, lattice canopies — pergolas) that block the wind.

At the same time, it is necessary that they do not obstruct the passage of sunlight into the greenhouse.