The main heat-technical parameters of solar greenhouses in the southern climate of Uzbekistan

. In the article, the main thermal and technical parameters of solar greenhouses in the climatic conditions of the city of Karshi, southern region of Uzbekistan, were studied. The results of the calculation of the heat load of solar greenhouses with different transparent coatings with a useful area of 100 m 2 in the conditions of the city of Karshi are presented. According to the calculation results, it was determined that the maximum heat load of the studied semi-cylindrical one-layer film sunken greenhouse is equal to Q he =50600 W (50.6 kW), and that of the two-layer film greenhouse Q he =29614 W (29.61 kW) is equal to.


Introduction
In the world, 20 percent of the total heat energy consumed in the sectors intended for the cultivation of agricultural products is spent on the heat supply systems of protected earth structures, i.e. greenhouses.Therefore, in the cost of vegetable products grown in greenhouses today, relative energy costs make up 60-70 percent [1].
In the Republic of Uzbekistan, one of the most important sectors of the economy, in the system of cultivation and processing of agricultural products, comprehensive measures are being implemented to increase energy efficiency, reduce energy capacity, and introduce energy-saving technologies in order to ensure high economic indicators.For example, in the Decision PD-4020 of the President of the Republic of Uzbekistan dated November 20, 2018 "On measures to create additional conditions for the development of greenhouse complexes," Placement taking into account the types of agricultural crops, adjacent infrastructure, and logistics facilities, the level of supply of energy resources of the regions and other factors affecting the efficiency of the greenhouse" important tasks are defined [2].As a solution to the above scientific and technical problems, the calculation of heating loads of greenhouses in stationary mode was studied.
One of the conditions for creating an optimal thermal regime in greenhouses is to maintain a constant temperature inside the structure, favorable for the growth of plants, and the second is to ensure that the thermal balance of the structure is equal to zero.The internal temperature of greenhouses and the heat balance of the structure are simultaneously related to heat-technical, meteorological, radiation, and structural parameters.The heat load of the heating system is an important energy parameter when creating the required temperature regime in greenhouses, and its accurate calculation is one of the main requirements for achieving energy efficiency.

Materials and methods
The analysis of the scientific and technical literature of the last years related to the research on the development of energy-efficient methods in the heating system of greenhouses shows that significant theoretical and practical results have been achieved in this field.Scientific research aimed at increasing the energy efficiency of the heating system of greenhouses and introducing energy and resource-saving modes is carried out by leading scientific and technical centers of the world, including Shandong Ruixu Agricultural Science and Technology Co., Ltd., Qingzhou Rainbow Modern Agriculture Development Co., Ltd.(China), Eco-culture (Russia), NatureSweet Tomatoes (USA), Venlo Projecten Holding B.V. (Netherlands), Dal Holding Investment Co., Ltd.(Turkey) are being conducted [7][8][9][10][11][12][13][14][15].
However, despite the positive results obtained, in order to save traditional fuel-energy resources in the heating system of greenhouses, scientific research on the development of an energy-efficient heating system of helio-greenhouses based on solar and biomass energy has not been carried out sufficiently.Therefore, the combined use of biomass and solar energy in greenhouses and thereby increasing the energy efficiency of greenhouses is of urgent scientific and practical importance.
As a result of the conducted research, the main thermal technical parameters of solar greenhouses located in the southern climate of Uzbekistan were established and presented in Table 1.When determining the heat load of a solar greenhouse, the construction, inventory, and useful area of the structure must first be determined.It is known that the surfaceconstruction surface is limited by the outer perimeter of the building base (F in , m 2 ), the surface-inventory area is limited by the internal perimeter (F ip , m 2 ) and the surface of the plant growing are considered useful area (F usef , m 2 ).

Results and Discussion
The main structural characteristic of the building, which is important in determining the amount of heat lost in greenhouses, is the wall (barrier) coefficient, which is determined by the following formula [3,4]: In this F en -the total surface of the part of the greenhouse fence (bounded by the environment), m 2 ; F is -inventory surface of the greenhouse, m 2 .
The value of the barrier coefficient for different structures of protected earth structures is in the range of 1.0...2.3.For example, it is equal to =1.0 for greenhouses, K en = 1.25 for glass block greenhouses, K en = 1.4 for semi-cylindrical film greenhouses, K en = 1.5÷1.7 and for recessed film greenhouses [4][5].
K en the coefficient depends on the geometric dimensions of the outer covering structure of the building and the shape of the cross section of the non-inventory surface.
The barrier coefficient for a semi-cylindrical greenhouse deepened by a distance h is calculated by the following formula [4]: For a semi-cylindrical film greenhouse, Where R is the radius of the semi-cylindrical part of the greenhouse, m; h -recessed height of the greenhouse wall, m; l -the length of the greenhouse, m.
The results of the calculation of the heat balance of the solar greenhouse are presented in Table 2.The coefficients used and adopted in the calculation of the solar greenhouse heat balance equations are presented in Table 3.To calculate the heat load of the greenhouse, the heat balance equation of the structure is created: Due to the change of metrological parameters during the day, it is difficult to calculate the exact value of the heat load.
Therefore, in practice, a simplified method that takes into account the maximum amount of heat loss is used.If the solar radiation is not taken into account in the night mode and the heat lost to the soil is considered to be zero, the heat load can be determined by the following formula [6][7].
Where Kthe heat transfer coefficient of the greenhouse wall (part of the barrier), W/(m 2 K); F -inventory surface, m 2 ; t in.tem.-air temperature inside the greenhouse, t in.tem.=15÷18 °С; t out.tem.-temperature of the external environment (atmospheric air), °C; The heat transfer coefficient (K) for a two-layer film greenhouse is equal to K=5.8 W/(m 2 °C) [4].
The proposed semi-cylindrical one-layer film sunken solar greenhouse in the conditions of the city of Karshi, the maximum heat load in the night mode is equal to:  .The heat load calculation results of different solar greenhouses with a useful area of 100 m 2 in the conditions of the city of Karshi are presented in Table 4.

Conclusion
As a result of experiments carried out in the climatic conditions of Karshi city, it was determined that the heating load of a greenhouse with a useful area of two layers of film 100 m 2 is around 18÷22.5 kW, that is, the specific heat load is equal to 180÷225 W/m 2 , and the heating load of a greenhouse with two layers of plastic coating is It was scientifically proven that it allows to reduce the fuel consumption by 1.73 times compared to the laminated coating .

Table 1 .
The main heat-technical parameters of solar greenhouses located in the southern climate of Uzbekistan.

Table 2 .
Calculation results of solar greenhouse heat balance (F usef = 100 m 2 ; in the climatic conditions of the city of Karshi, two layers of film are covered).

Table 3 .
The coefficients used and adopted in the calculation of the heat balance of the solar greenhouse.

Table 4 .
Heat load calculation results of solar greenhouses with different transparent coatings with a useful area of 100 m 2 in the conditions of the city of Karshi.