Improving the energy efficiency of the solar heat supply system of buildings based on vacuum collectors

. Currently, many residential buildings and businesses use solar water heating systems. This is an economical and reliable type of hot water supply. Heating water for domestic purposes or heating using solar energy is a natural and simple method of saving energy and preserving fossil fuel reserves. The use of renewable energy sources is relevant for all countries of the world. The article deals with the issue of improving the energy efficiency of the solar heat supply system of buildings based on vacuum collectors with natural circulation of the coolant.


Introduction
The sun is the most powerful renewable energy source for our planet: the amount of energy falling on the earth's surface from the sun per day exceeds global consumption per year. Solar energy can be used both for electricity generation and for heating and lighting residential and industrial premises, water heating. Being an environmentally friendly renewable energy source that does not emit carbon dioxide and does not depend on fossil resources, solar energy contributes to the diversification of energy sources, improving energy efficiency and saving money and resources. There are various technologies for converting and using solar energy. If solar cells and concentration stations are used for electricity generation, then passive solar energy is used for lighting and heating rooms, heating water.
Photovoltaic systems, as well as solar heat supply systems (STS) have received largescale practical application for energy and heat supply of residential and public buildings [1][2][3][4]. In 2021, the total area of installed solar collectors (SC) as part of various STSs in the world amounted to 715 million m 2 [5]. SCs have a different design. They are divided into flat, tubular evacuated, collectors without glazing and air solar collectors [1].
In world practice, flat and tubular evacuated solar collectors (figure 1) are most widely used. If flat solar collectors (PSC) first appeared in 1909, then the tubular evacuated solar collector (TVSC) was developed more than fifty years later -in 1963 [6].
PSCs have been comprehensively theoretically and experimentally studied for more than one century, the main results of these studies are given in [7]. At the same time, based on the generalization of experience in experimental design, construction and operation of SST with PSK [8]. regulatory documents and design manuals were developed.
However, despite such an impressive global volume of TVSC implementation, many issues of designing and building STS based on them remain unresolved to this day, since the current regulatory documents and manuals for the design of STS do not contain sufficient information on their application and calculation. This is explained by the fact that TVSCs have found their wide application in the world mainly for heat supply of small objects, for which they are selected according to approximate specific indicators in the form of ready-made factory-made installations, without performing any calculations and developing circuit solutions and project documentation.
The aim of the work is to increase the efficiency of solar collectors through the use of natural circulation.

Main part
A solar collector is one of the components of a solar system, a device for collecting thermal energy from the Sun, carried by visible light and near infrared radiation. Unlike solar panels that produce electricity directly, the solar collector heats the coolant material.
One of the main components of the solar system is a solar collector. The main element of the collector is the absorber, which is a plate made of copper or aluminium, blackened on one side using a special technology. In fact, this blackening when viewed "by eye" may have a bluish tint, but the ability to absorb the required spectrum of solar radiation from such a surface is many times higher than when covering the plate with the blackest of all possible paints or pigments. In addition, the blackened surface must necessarily be matte.
On the reverse side, copper tubes are attached to the plate, through which a coolant passes -water or antifreeze. The larger the area of contact of the tubes with the surface of the plate, the more fully the energy collected by the plate is transferred to the heat carrier. It is also necessary to ensure unconditional contact and reliability of the entire contact area of the plate and tubes, for which they are connected, as a rule, by welding or high-temperature soldering (about 600 ° C).
The rest of the collector consists of a housing with thermal insulation and a protective coating (usually tempered glass is used), it provides protection from hail, small stones, branches. And also passes the necessary spectra of solar radiation and reduces the reverse transmission of the reflected part of solar radiation back.
Since the coolant has a very high temperature, it cannot be directly fed into the heating batteries or into the hot water tap. Such a coolant is fed into a heat exchanger, which acts as a heat accumulator.
Solar collectors of various types allow obtaining thermal energy, which is primarily used for the preparation of hot water, which is especially important in the summer period of the year, when there is maximum solar activity and maximum consumption of hot water. In addition, in some cases, when constructing combined boiler plants, heat from solar collectors can be partially used in various heating systems, for example, when operating a boiler plant during transitional periods of the year, in areas with high solar activity. This approach makes it possible to significantly increase the efficiency of the boiler plant as a whole.
Solar collectors of various types allow to receive thermal energy, which is primarily used for the preparation of hot water, which is especially important in the summer period of the year, when there is maximum solar activity and maximum consumption of hot water. In addition, in some cases, during the construction of combined boiler houses, heat from solar collectors can be partially used in various heating systems, for example, when operating a boiler house during transitional periods of the year, in areas with high solar activity. This approach makes it possible to significantly increase the efficiency of the boiler plant as a whole.
Hot water supply is the most common type of direct application of solar energy. A typical installation consists of one or more collectors in which the liquid is heated in the sun, as well as a tank for storing hot water heated by means of a coolant. Even in regions with relatively little solar radiation, for example in Northern Europe, the solar system can provide 50-70% of the demand for hot water. It is impossible to get more, except with the help of seasonal regulation. In Southern Europe, a solar collector can provide 70-90% of the hot water consumed. Heating water with the help of solar energy is a very practical and economical way. While photovoltaic systems achieve an efficiency of 10-15%, thermal solar systems show an efficiency of 50-90%. In combination with wood-burning stoves, the domestic need for hot water can be met almost all year round without the use of fossil fuels.
The task of the solar collector is to accumulate solar energy in the modules of tubes and metal plates installed on the roof of the building, painted black, in order to absorb radiation as much as possible. They are enclosed in a glass or plastic case and in order for them to catch the maximum sunlight, they are tilted to the south. It turns out that the collector is a miniature greenhouse that accumulates heat under a glass panel. Since the solar is distributed over the entire surface, the collector must have a large area.
In TVSC with direct heat transfer to water, vacuum tubes are connected to a storage tank ( fig. 1). From the heat exchanger circuit, water flows directly into the tubes, heats up and returns. Such systems are also called thermosiphon. The advantages of these systems include the direct transfer of heat to water without the participation of other elements. Thermosiphon systems work on the principle of natural convection, when warm water rises. In thermosiphon systems, the tank must be located above the collector. As the water in the collector tubes heats up, it becomes lighter and naturally rises to the top of the tank. The cooler water in the tank flows down into the tubes, thus circulating the entire system. Such a system has a minimum hydraulic resistance. The most technologically advanced are collectors with U-tubes ( fig. 2). In them, the copper tube is bent and forms an individual circuit through which the coolant fluid is driven. This ensures the fastest possible heating of the coolant and the highest efficiency per unit area. The main element of solar collectors of the last third type is a thermostable (heat pipe)a closed copper pipe with a small content of low-boiling liquid ( fig. 3). The operation of high-tech vacuum tubes is based on the simple principle of a heat pipe, which is a hollow copper rod soldered at both ends with an expansion at the top. Inside it is a non-toxic liquid (inorgatic). When the liquid is heated to the boiling point, it boils and in the vapour state rises to the upper part -the tip (condenser), the temperature at which can reach 250-380 °C. And there it condenses, giving off heat. And the condensate flows down the walls of the tube and the process is repeated. The heat pipe is inserted into the glass pipe and fixed between two aluminium fins. The shape of the ribs is such that the area of their contact with the heat pipe and the inner surface of the vacuum tube is maximum. The internal cavity of the heat pipe is evacuated, so this liquid evaporates even at a temperature of about 30 °C. At a lower temperature, the tube "locks" and additionally retains heat. The movement of the fluid, completely due to the internal forces of the system, is usually classified as natural convection.
Under conditions of free or natural convection, heat transfer to or from the body leads to the formation of temperature and density gradients in a resting liquid if its temperature is not equal to the body temperature. Unlike forced convection, in which the velocity of a liquid is determined by external forces, with free convection, the movement of a liquid occurs under the action of lifting (Archimedean) forces, which are associated with changes in temperature and density in the liquid itself and which cannot be accurately determined. As with forced convection, fluid movement under the action of lifting forces can be both turbulent and laminar. However, under conditions of free convection, the boundary layer has zero velocity not only at the solid-liquid interface, but also at its outer boundary, remote from the body. The movement of the liquid can be directed upwards, as in the case of heating air from a flat plate, or downwards, as in the case when a cold cylinder is placed in a container with stationary warm water.
Considering the nature of the origin of the driving force, it should be noted that if  is the density of a cold undisturbed liquid, and  is the density of a more heated liquid, then the lifting force acting per unit volume in the gravitational field is equal to ( -)g, where g is the acceleration of gravity.
The density difference can be expressed depending on the thermal coefficient of volumetric expansion of the liquid , which is equal to It follows from this that the temperature does not depend on either the heat transfer coefficient or the shape of the body, but is completely determined by the ambient temperature.
Natural water circulation occurs and develops in the presence of small gradients of static pressure, determined by the difference in hydrostatic pressures formed due to the different density of water in the environment and elements of the heat exchanger and circulation system. The use of natural circulation provides a number of important advantages: low energy consumption for their own needs, the absence of an incentive for the movement of cooling water and, as a result, the lack of automation, control and management tools, simplification of the schemes of condensing plants and increasing their reliability. It is also necessary to note a decrease in the level of vibroacoustic radiation associated with the operation of mechanisms in the circulation route.
According to the layout conditions or due to a change in the spatial position of the object on which the heat exchanger is installed, the latter can often occupy an inclined position. This affects the intensity of heat exchange during natural circulation of water, as well as in the case of viscous-gravitational flow in conditions of forced circulation. In a horizontal pipe with a viscous-gravitational flow, heat exchange proceeds more intensively than in a vertical one.
At the same time, when the pipes are tilted, the projection of the height of the column of heated water on the vertical decreases, and, consequently, the driving pressure of natural circulation. The result of this should be a decrease in the speed of water flow and the intensity of heat exchange.

Conclusions
In this paper, the development of technical solutions for the implementation of the use of solar collectors for hot water supply was considered .It can be concluded that the development of technical solutions for the implementation of the use of solar collectors for hot water supply of buildings is promising and economically profitable. It has also been shown that it is possible to increase the efficiency of solar collectors by using natural circulation.