Determination of the heat losses of insolation passive solar heating systems

. The article considers the determination of heat losses of insolation passive solar heating systems. The calculation expressions for determining heat losses in a building with a direct solar heating system through glazing are given and the total heat loss of the room is considered as the sum of two components: heat losses of the southern wall with a light opening, as variables depending on the area of the light opening; heat losses through other fences (western, eastern and northern walls, floor, ceiling).


Introduction
The search for non-traditional and renewable energy sources for involvement in the fuel and energy balance is an urgent task of the world energy industry.
Of the non-traditional and renewable energy resources in the conditions of the Republic of Uzbekistan, solar energy is the most promising, the energy potential of which is 98.5% of renewable energy sources combined [1][2].
Converting the energy of solar radiation into thermal energy in order to use the latter for heat supply of low-temperature consumers is currently the most developed direction for the use of solar energy.
In the conditions of our Republic, the relevance of using solar energy in heat supply systems as a source of low-grade heat is due to the fact that at present, out of all the heat energy generated in 2000 (46.4 million Gcal), 50,4% was spent for low-temperature needs of the population ( 33.6%) and household sector (16.8%) [1][2][3][4][5][6].
Reducing the amount of traditional fuel and energy resources used for winter heating of residential and generalized buildings by 10% due to the use of solar energy allows saving more than 1 million tce. and thereby reducing the release to the environment of more than 2,5 million tons of CO2 equivalent.
World experience in the use of solar energy shows that one of the effective systems for heat supply to low-temperature consumers is passive heating systems, which are distinguished by their simplicity, primarily from the point of view of a constructive solution.
Depending on the accepted scheme of heat supply to the heated premises, they are distinguished by the type of passive solar heating system: with insolation (i.e.direct), indirect and isolated perception, and transfer of heat from solar radiation [3].The sun's rays penetrate the heated premises through window openings (usually oversized) and heat the internal enclosures of the premises, which become radiation receivers and heat accumulators, despite the highest thermal efficiency, a number of disadvantages inherent in insolation systems should be noted, such as the instability of the thermal regime, the need to use auxiliary devices that reduce additional heat loss at night, thermal and light discomfort during the day.
With indirect heating, the flow of solar radiation does not penetrate directly into the premises but is absorbed by radiation receivers protected by translucent fences, which, as a rule, are heat accumulators.
The disadvantages of indirect passive solar heating systems are low thermal efficiency, which is the result of relatively large heat losses from the outer surface of the wall, which combines the functions of solar radiation receivers and heat accumulators, and the limited intensity of heat transfer from the ray-absorbing surface of the radiation receiver (heat accumulator) to the heated room.
Insulated systems are characterized by the fact that the heat required to maintain the set temperature in the premises is perceived by solar thermal collectors located outside the building and accumulated in batteries, which are also located outside the heated premises.
There are passive solar heating systems that are based on the principles of operation of both passive and isolated systems.In this case, the solar thermal collector is placed on the southern vertical wall, and the thermal accumulator is combined with the blocking of the flow and at the same time acts as a radiator of a panel-radiant heating system [3][4][5].
The disadvantages of isolated passive solar heating systems, as well as indirect ones, are relatively low thermal efficiency, due to the limited intensity of transfer from the collector to the accumulator and from the heat accumulator to the heated room by the airflow circulating under the action of gravitational pressure, as well as the complexity of the operation, associated with the need to place the battery is below the level of the room, and the collector is even lower than the level of the battery, and the loss of temperature difference in the elements of the thermal chain collector-heat accumulator-heated room.

Materials and methods
These shortcomings of the considered types of passive solar heating systems can be substantially eliminated if the receiver of the solar collector and heat accumulator is combined with a heater and installed inside a heated room near a light opening.With such an arrangement and placement of elements of a passive solar heating system, heat losses from the surface of the solar radiation receiver-heat accumulator, as well as radiation from it, are not lost to the environment but are transferred to the heated room as useful energy.The amount of solar energy penetrating into the heated room through the light opening can be increased by additionally illuminating it with flat solar radiation reflectors hinged to the base of the considered light opening.
An analysis of the operating experience of such systems shows their undoubted advantage over other types of passive solar heating systems [6][7][8][9][10].Thus, the actual contribution of solar energy in the annual heat balance, having passive solar heating systems of the type under consideration, in the climatic conditions of the state of New Mexico, USA ( Undoubtedly, it is of great practical interest to determine the potential capabilities of these systems in the climatic conditions of our Republic.Existing methods for calculating passive solar heating systems [3] to a large extent allow solving this problem.However, it should be noted that the main feature of passive solar heating systems in general, and the type under consideration in particular, depends on the choice of the optimal design solution for the building as a whole and the combination of the main elements of the heating system in it.Such a solution can only be obtained using analytical models that take into account the non-stationary of the thermal regime of the building, caused by the irregularity of the nature of external thermal disturbances (the arrival of solar radiation, ambient temperature, wind direction, and speed, etc.).An important feature of the analytical solution to such problems is the possibility of obtaining an engineering method, which is extremely necessary at the stage of predesign development of passive solar heating systems of the type under consideration.
In connection with the foregoing, the purpose of the study is to establish the optimal parameters of insolation passive solar heating systems, the collector solar radiation receiver of which combines the functions of a heat accumulator and inside a house heater, installed on a windowsill inside a heated room, a light aperture equipped with a hinged transformable flat reflector from the outside, providing its maximum efficiency and, on this basis, the development of scientifically substantiated initial data for their experimental design and construction in the climatic conditions of Uzbekistan.
Heat loss in a building with direct solar heating through glazing can be quite significant.The heat transfer coefficient through single glazing is 5.6 W/(m 2 •K), with double glazing it drops to 3 W/(m 2 •K).If the air temperature in the interlayer between the wall ("massive wall" system) and the glass is +35℃, and the outside temperature is -15℃, then the amount of heat lost in 1 hour is 280 W/m 2 with single-layer glazing, and 150 W/m 2 with double glazing.However, double glazing also reduces solar transmittance, which is 0.85 for single glazing and 0.72 for double glazing.Thus, if the solar radiation that has reached the absorbing surface of the solar receiver is 425 W/m 2 with single glazing, and 360 W/m 2 with double glazing.Double glazing can only be used taking into account climatic conditions.It is necessary in areas with a cold climate, but not necessarily in mild winters.In any case, it must be borne in mind that a decrease in solar income is observed only in the daytime, while a decrease in heat loss occurs constantly.
During solar radiation, selectively transparent protection in the form of glazing should be used as much as possible, but after sunset, in order to reduce heat loss, it is advisable to use transformable opaque thermal insulation for the glazed surface.
Hundreds of different solutions have been proposed and demonstrated [1].For a direct solar heating system, the simplest is to use heavy curtains with tightly closed tops to cut off the convective heat flow between the curtain and the glass.Outdoor shutters, sliding or hinged, are very effective.They must be thermally insulated (at least 50 mm polystyrene or equivalent) and airtight.One ingenious solution uses a vacuum cleaner fan to fill the space between two layers of glass with small spherical beads (about 5 mm in diameter) of polystyrene.They are sucked back in the morning and kept in a container during the day.The system is very effective but short-lived.
An unusual system designed in (New Mexico, America) by Steve Baer uses large folding shutters.Their inner surface is separated by aluminum so that when the shutters cover the glass, the protective effect is increased due to the additional reflection of radiation from the glass.
The most widely used variant of passive solar heating systems with an insolated volume is the greenhouse.It can be considered a modified version of the "massive wall" system, where the usual distance between the glass and the wall, equal to 100-120 mm, is increased to 2 m.This room can be used as a greenhouse -for growing plants, but it also serves as a source of heat for the room located behind it, due to either convection or delayed heat transfer through the wall.The operation of this system is very similar to the operation of the "massive wall" system.
A greenhouse is not the only form of a system with an insolated volume.It can be a glazed porch or veranda, or some kind of mixed option.In some small buildings of institutions, the foyer is used as an isolated space.In fact, it can be any room where greater temperature fluctuations are possible than in residential premises.
In insolation passive solar heating systems with direct heat input, as the name implies, the sun's rays passing through the glazing are absorbed by the surface of the inner fence and the mass of all fences is a heat accumulator.A distinctive feature of insolation passive solar heating systems compared to others is that the heat loss of a heated room is a function of the surface area of the light opening through which the sun's rays enter the room [2][3][4][5][6][7][8][9][10].

Equations and mathematics
Therefore, the total heat loss of room ( hl Q ) is considered as the sum of two components: the heat loss of the southern wall with a light opening, as variables depending on the area of the light opening; heat losses through the rest of the fences (western, eastern and northern walls, floor, ceiling) [3] as constants ( In turn, the value s hl Q is the sum of the heat losses through the opening ( s s hl Q , ) and the rest of the blind fences of the southern wall ( The calculated values  In accordance with the results of calculations to determine the constant component of the heat loss of an experimental object with an insolation solar heating system (Table 1 The dependence of 0 q on the ratio   The heat losses of the experimental object per day ((d) depending on the actual ambient temperature based on (8) and ( 9) can be determined from the expression . 47 (10) The heat loss of the experimental object per day depending on the actual ambient temperature is shown in figure 2.

Conclusions
Calculated expressions and results show that the determination of heat losses in a building with a direct solar heating system through glazing and the total heat loss of the room is considered as the sum of two components: heat losses of the south wall with a light opening, as variables depending on the area of the light opening; heat losses through other fences (western, eastern and northern walls, floor, ceiling).

Q
, in(2), in turn, are determined from the expressions

2 m
for the experimental object is shown in figure1.

Fig. 1 .FF
Fig.1.Dependence of the specific thermal characteristic of the heated room   o q on the ratio

Fig. 2 .
Fig.2.Thermal losses of the experimental object per day depending on the actual ambient temperature.

Table 1 .
Calculation of the constant component of heat loss in the experimental room of the experimental object with a passive solar heating system at