The effect of light and temperature conditions on the microclimate of greenhouses

Illumination reaches its maximum value in spring and summer (11,000–40,000 lx) and its minimum in autumn and winter (100–11,000 lx). The ratio between solar radiation intensity, W/m², and illumination, lx, is determined by the factors listed above. For 60° N, 1 W/m² = 15–50 lx.

Obtaining a sufficient amount of light is a necessary condition for growing plants. Plants are capable of using only the visible part of the solar radiation spectrum (about

46% of the total incoming energy). Different plants require different levels of illumination to ensure optimal growth conditions (see ch. 7). The duration of the period during which energy is available is of decisive importance. In this regard, when designing a greenhouse, it is necessary to pay special attention to ensuring that light enters it in spring and autumn:

Monthly temperature fluctuations. At different times of the year, in various regions of Finland, significant fluctuations in outdoor air temperature are observed.

The best overview of temperature conditions in different months can be obtained based on averaged temperature data. For plant cultivation, the so-called stable data on temperature and solar radiation are the most important. This allows for the identification of periods during which the air temperature will be lower or higher than a known value, which provides information on the duration and probability of threshold temperature conditions. Table 2.6 provides data on weather condition stability, temperature, and solar radiation energy for the cities of Helsinki, Jyväskylä, and Sodankylä.

Table 2.5. Averaged temperature, °C, for the cities of Helsinki, Jyväskylä, and Sodankylä (1901–1960)

City January February March April May | June Helsinki —5.3 —5.9 —2.9 2.7 8.8 | 13.9 Jyväskylä —8.4 —8.6 —4.6 1.8 8.3 1 13.5 Sodankylä | —13.3 | —13.4 —8.9 —2.3 | 4.5 1.3

Continuation of Table 2.5 ` . r Sep-

_ City July | August tember | tember | November | December | Per year Helsinki 17.41 15.9] 1.2 5.7 1.2 1—9.6 5.00 Jyväskylä | 16.5 | 14.3 9.3 3.5 | —1.3 | 5.7 3.22 Sodankylä | 14.6 | 11.5 | 65.9 |—0.9 | —6.9 1—10.6 1 —0.74

Daily temperature fluctuations. The greatest difference between day and night temperatures is observed in the summer months, namely in June and July, and the smallest in December and January. The averaged daily temperature range for Helsinki in July is

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9 during the growing season is about 8°C (amplitude 3.79°C), and in December it is about 1°C (amplitude 0.36°C).

The same patterns in temperature fluctuations are found in the greenhouse. The daily temperature difference depends not only on the weather but also on the heat accumulation capacity of the greenhouse. The highest daytime temperature is observed in all months between 2 p.m. and 4 p.m., while the lowest occurs in winter between 7 a.m. and 9 a.m. and in summer between 3 a.m. and 5 a.m.

Wind speed. Table 2.7 provides data on wind speed for three regions of Finland at different times of the year. Wind speed varies within a fairly wide range depending on the terrain, being highest in autumn and winter. In the coastal areas of the country, wind speed is higher than in the interior.

Wind direction. The main wind directions in different regions of Finland change depending on the season. It is impossible to indicate the most general directions, if only because the change in speed and especially direction of the wind is strongly influenced by the terrain, vegetation, building density, etc. Therefore, to determine the prevailing wind directions, it is necessary to carry out studies directly at the site or in the given area over a sufficiently long period, or to conduct a survey of local residents.

Cooling effect of wind on the greenhouse. This effect is very significant and manifests itself in two ways:

with an increase in wind speed, the heat transfer process intensifies, especially from glass surfaces with poor thermal insulation (via convection). In strong winds, this factor can increase by 5 or even 10 times compared to calm weather.

air flows on the outer surface of the greenhouse create a pressure difference, which in turn leads to the emergence of local air currents in all areas of air leakage between the elements of greenhouse structures.

As wind speed increases, the amount of air escaping from the greenhouse also increases, which leads to greater heat losses.

Nature realizes the natural ability to absorb solar energy with the help of vegetation, which converts solar radiation energy into biomass as a result of a chemical reaction. During photosynthesis, soil water, carbon dioxide present in the air (carbon dioxide), and sunlight are transformed into carbohydrates, primarily with the help of chlorophyll contained in plant leaves, resulting in the release of oxygen into the atmosphere. Carbohydrates are the primary substances in plants that contain solar energy in a chemically bound form.

Carbohydrates are the building blocks of plants and contain energy resources for the nutrition of both humans and animals. As a result of the assimilation of these nutrients, energy is released, which is used for maintaining the necessary body temperature, muscle work, and the performance of the organism's multifaceted functional activities.

In the forests of Finland, woody biomass grows annually, the energy potential of which corresponds to approximately 17 megatonnes of coal equivalent. This is close to Finland's total energy resource requirements, which currently amount to about 25 megatonnes of coal equivalent. However, wood is required not only for energy production. The Finnish industry uses about 60% of the annually produced woody biomass. If the rest of the woody biomass, i.e., about 40%, could be used as energy resources, this would correspond to approximately 7 megatonnes of coal equivalent, or almost 60% of current oil imports.