Nbsp; CALCULATION Heat supply systems using solar thermal collectors Methodical instructions for performing computational and graphic work for students of all forms of training in the specialty Power plants, power plants based on non-traditional and renewable energy sources CALCULATION Heat supply systems using solar thermal collectors: guidelines for performing computational and graphic work for students of all forms of training in the specialty Power plants, power plants based on non-traditional and renewable energy sources / A. V. CONTENTS 1. THEORETICAL PROVISIONS 1.1. Design and main characteristics of a flat solar collector 1.2. Basic elements and schematic diagrams of solar heat supply systems 2. STAGES OF DESIGN 3. CALCULATION OF HEAT FOR HEATING BUILDINGS 3.1. Basic provisions 3.2. Determination of transmission heat losses 3.3. Determination of heat consumption for heating ventilation air 3.4. Determination of heat costs for hot water supply 4. CALCULATION OF A SOLAR HEAT SUPPLY SYSTEM BIBLIOGRAPHY THEORETICAL PROVISIONS
Design and main characteristics of flat plate solar collector
A flat plate solar collector (SC) is the main element of solar heating and hot water systems. Its principle of operation is simple. Most of solar radiation falling on the collector is absorbed by the surface, which is "black" in relation to solar radiation. Some of the absorbed energy is transferred to the liquid circulating through the collector, and the rest is lost as a result of heat exchange with the environment. The heat carried away by the liquid is the useful heat that is either stored or used to cover the heating load.
The main elements of the collector are as follows: an absorbing plate, usually made of metal, with a non-reflective black coating for maximum absorption of solar radiation; pipes or channels through which liquid or air circulates and which are in thermal contact with the absorbing plate; thermal insulation of the bottom and side edges of the plate; one or more air gaps separated by transparent coatings in order to insulate the plate from above; and finally, a housing for durability and weather resistance. In fig. 1 shows cross-sections water and air heater.
Rice. 1. Schematic representation of solar collectors with water and air coolants: 1 - thermal insulation; 2 - air channel; 3 - transparent coatings; 4 - absorbing plate; 5 - pipes connected to the plate.
The transparent cover is usually made of glass. The glass has excellent weather resistance and good mechanical properties. It is relatively inexpensive and, with a low iron oxide content, can have a high transparency. The disadvantages of glass are fragility and high weight. Along with glass, plastic materials can also be used. Plastics are generally less prone to breakage, lightweight and inexpensive in the form of boggy sheets. However, it is generally not as weather-resistant as glass. The surface of the plastic sheet is easily scratched and many plastics degrade and turn yellow over time, as a result of which their transmittance to solar radiation decreases and their mechanical strength deteriorates. Another advantage of glass over plastics is that glass absorbs or reflects all the long-wave (thermal) radiation that falls on it from the absorbing plate. The heat loss to the environment by radiation is reduced more efficiently than in the case of a plastic coating, which transmits part of the long-wave radiation.
The flat collector absorbs both direct and diffuse radiation. Direct radiation causes a sunlit object to cast a shadow. Diffuse radiation is reflected and scattered by clouds and dust before reaching the earth's surface; unlike direct radiation, it does not produce shadows. A flat-plate collector is usually installed permanently on a building. Its orientation depends on the location and the time of year during which the solar power plant must operate. The flat collector provides the low-grade heat required to heat the water and heat the room.
Focusing (concentrating) solar collectors, including those with a parabolic or Fresnel concentrator, can be used in solar heating systems. Most focusing collectors use only direct solar radiation. The advantage of a focusing collector over a flat collector is that it has a smaller surface area from which heat is lost to the environment, and therefore, the working fluid can be heated in it to higher temperatures than in flat collectors. However, for the needs of heating and hot water supply, a higher temperature is almost (or not at all) irrelevant. For most concentrating systems, the collector must track the position of the sun. Systems that do not display the sun usually require adjustment several times a year.
A distinction should be made between the instantaneous characteristics of the reservoir (that is, the characteristics at a given moment in time, depending on the meteorological and operating conditions at that moment), and its long-term characteristics. In practice, the solar collector operates under a wide range of conditions throughout the year. In some cases, the operating mode is characterized by high temperature and low collector efficiency, in other cases, on the contrary, by low temperature and high efficiency.
To consider the operation of the collector under variable conditions, it is necessary to determine the dependence of its instantaneous characteristics on meteorological and operating factors. To describe the characteristics of the collector, two parameters are required, one of which determines the amount of absorbed energy, and the other determines the heat loss to the environment. These parameters are best determined by tests that measure the instantaneous efficiency of the reservoir over an appropriate range of conditions.
The useful energy removed from the collector at a given time is the difference between the amount of solar energy absorbed by the collector plate and the amount of energy lost to the environment. The equation that is applicable to the calculation of almost all existing flat collector structures is:
where is the useful energy removed from the collector per unit of time, W; - collector area, m 2; - coefficient of heat removal from the collector; - flux density of total solar radiation in the plane of the collector W / m 2; - the transmission capacity of transparent coatings in relation to solar radiation; - absorptive capacity of the collector plate in relation to solar radiation; - total heat loss coefficient of the collector, W / (m 2 ° C); - liquid temperature at the collector inlet, ° С; - ambient temperature, ° С.
Solar radiation falling on the collector at any time consists of three parts: direct radiation, diffuse radiation and radiation reflected from the ground or surrounding objects, the amount of which depends on the angle of inclination of the collector to the horizon and the nature of these objects. When the collector is tested, the radiation flux density I measured with a pyranometer installed at the same angle of inclination to the horizon as the collector. Used in calculations f-the method requires knowledge of the average monthly arrivals of solar radiation on the surface of the collector. Most often, reference books contain data on the average monthly arrivals of radiation on a horizontal surface.
The flux density of solar radiation absorbed by the collector plate at some point in time is equal to the product of the incident radiation flux density I, the transmission capacity of the transparent coating system t and the absorption capacity of the collector plate a... Both of the latter values depend on the material and the angle of incidence of solar radiation (i.e., the angle between the normal to the surface and the direction of the sun's rays). Direct, diffuse and reflected components of solar radiation enter the collector surface under different angles... Therefore, the optical characteristics t and a should be calculated taking into account the contribution of each of the components.
The collector is losing heat different ways... Heat losses from the plate to transparent coatings and from the top coat to the outside air occur by radiation and convection, but the ratio of these losses in the first and second cases is not the same. Heat loss through the insulated bottom and side walls of the collector is due to thermal conductivity. The collectors must be designed in such a way that all heat losses are as low as possible.
Product of the total loss coefficient U L and the temperature difference in equation (1) represents the heat loss from the absorbing plate, provided that its temperature is everywhere equal to the temperature of the liquid at the inlet. When the liquid is heated, the collector plate has a higher temperature than the temperature of the liquid at the inlet. it necessary condition heat transfer from the plate to the liquid. Therefore, the actual heat loss from the collector is greater than the product value. The difference in losses is taken into account using the heat rejection coefficient F R.
Total loss factor U L is equal to the sum of the loss coefficients through transparent insulation, the bottom and side walls of the collector. For a well-designed collector, the sum of the last two factors is usually about 0.5 - 0.75 W / (m 2 ° C). The coefficient of loss through transparent insulation depends on the temperature of the absorbing plate, the number and material of the transparent coatings, the degree of blackness of the plate in the infrared part of the spectrum, the ambient temperature and the wind speed.
Equation (1) is convenient for calculating solar energy systems, since the useful energy of the collector is determined from the temperature of the liquid at the inlet. However, heat loss to the environment depends on the average temperature of the absorber plate, which is always higher than the inlet temperature if the fluid is heated while passing through the manifold. Heat removal coefficient F R is equal to the ratio of the actual useful energy, when the temperature of the liquid in the reservoir increases in the direction of flow, to the useful energy, when the temperature of the entire absorber plate is equal to the temperature of the liquid at the inlet.
Coefficient F R depends on the liquid flow rate through the collector and the design of the absorbing plate (thickness, material properties, distance between pipes, etc.) and is almost independent of the intensity of solar radiation and the temperatures of the absorbing plate and the environment.
Basic elements and schematic diagrams of solar heating systems
Solar heating systems (or solar plants) can be divided into passive and active. The simplest and cheapest are passive systems, or “ solar houses", Which for the collection and distribution of solar energy use the architectural and construction elements of the building and do not require additional equipment... Most often, such systems include a blackened building wall facing south, at some distance from which a transparent covering is located. There are openings in the upper and lower parts of the wall that connect the space between the wall and the transparent covering to the interior volume of the building. Solar radiation heats up the wall: the air washing over the wall heats up from it and enters the building through the upper opening. Air circulation is provided either by natural convection or by a fan. Despite some of the advantages of passive systems, mainly active systems with specially installed equipment for collecting, storing and distributing solar radiation are used, since these systems improve the architecture of the building, increase the efficiency of using solar energy, and also provide greater opportunities for regulating the heat load and expand application area. The choice, composition and layout of the elements of an active solar heat supply system in each specific case are determined by climatic factors, type of object, heat consumption mode, economic indicators. The specific element of these systems is the solar collector; used elements, such as heat exchangers, accumulators, redundant heat sources, sanitary fittings, are widely used in industry. The solar collector converts solar radiation into heat, which is transferred to the heated coolant circulating in the collector.
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In addition to the basic elements described above, solar heat supply systems can include pumps, pipelines, instrumentation and automation elements, etc. A different combination of these elements leads to a wide variety of solar heat supply systems in terms of their characteristics and cost. On the basis of the use of solar power plants, the tasks of heating, cooling and hot water supply of residential, office buildings, industrial and agricultural facilities can be solved.
Solar power plants have the following classification:
1) by purpose:
Hot water supply systems;
Heating systems;
Combined installations for heating and cooling purposes;
2) by the type of coolant used:
Liquid;
Air;
3) by the duration of work:
Year-round;
Seasonal;
4) by technical solution schemes:
Single-circuit;
Double-circuit;
Multi-circuit.
The most commonly used heat carriers in solar heating systems are liquids (water, ethylene glycol solution, organic matter) and air. Each of them has certain advantages and disadvantages. The air does not freeze, does not pose major problems associated with leaks and corrosion of equipment. However, due to the low density and heat capacity of air, the dimensions of air installations, the power consumption for pumping the coolant is higher than that of liquid systems. Therefore, in most of the operated solar heating systems, preference is given to liquids. For housing and communal needs, the main heat carrier is water.
When solar collectors are in operation during periods from negative temperature outside air must either be used as a coolant antifreeze, or in some way to avoid freezing of the coolant (for example, timely drainage of water, heating it, insulating the solar collector).
Low-capacity solar heat supply systems that provide small remote consumers often work on the principle of natural circulation of the heat carrier. The water tank is located above the solar collector. This water is supplied to the lower part of the SC, located at a certain angle, where it begins to heat up, change its density and rise by gravity up through the collector channels. Then it enters the top of the tank, and its place in the collector is taken by cold water from its bottom. The natural circulation mode is set. In more powerful and efficient systems, the circulation of water in the solar circuit is provided by a pump.
Schematic diagrams of solar heat supply systems are shown in Fig. 2, 3, can be divided into two main groups: installations operating in an open-loop or direct-flow circuit (Fig. 2); installations operating in a closed circuit (Fig. 3). In installations of the first group, the coolant is supplied to solar collectors (Fig. 2 a, b) or to the solar circuit heat exchanger (Fig. 2 c), where it heats up and enters either directly to the consumer or to the storage tank. If the temperature of the heat carrier after the solar plant turns out to be below the set level, then the heat carrier is heated up in a redundant heat source. The considered schemes are mainly used in industrial facilities, in systems with long-term heat storage. To ensure a constant temperature level of the coolant at the outlet from the collector, it is necessary to change the flow rate of the coolant in accordance with the law of changes in the intensity of solar radiation during the day, which requires the use of automatic devices and complicates the system. In the schemes of the second group, the transfer of heat from solar collectors is carried out either through a storage tank, or by direct mixing of coolants (Fig. 3 a), or through a heat exchanger, which can be located both inside the tank (Fig. 1.4 b) and outside it (Fig. 3 c). The heated coolant enters the consumer through the tank and, if necessary, is heated in a redundant heat source. Installations operating according to the schemes shown in Fig. 3, can be single-circuit (Fig. 3 a), double-circuit (Fig. 3 b) or multi-circuit (Fig. 3 c, d).
Rice. 2. Schematic diagrams of direct-flow systems: 1-solar collector; 2- battery; 3-heat exchanger
Rice. 3. Schematic diagrams of solar heating systems
The application of this or that variant of the scheme depends on the nature of the load, the type of consumer, climatic, economic factors and other conditions. Considered in Fig. 3 schemes have found the greatest application at the present time, since they are distinguished by comparative simplicity and reliability in operation.
Stages of WORK PERFORMANCE
Settlement and graphic work consists of the following main stages:
1) Execution of the drawing "Building plan".
2) Selection of the thermal diagram of the heating system using solar collectors
3) Execution of the drawing "Scheme of heating and hot water supply using solar thermal collectors"
4) Calculation of the heating load (heating and hot water supply).
5) Calculation of the solar heating system and the share of heat load provided by solar energy f- method.
6) Execution of an explanatory note.
On the basis of the use of solar power plants, the tasks of heating, cooling and hot water supply of residential, office buildings, industrial and agricultural facilities can be solved. Solar power plants have the following classification:
The most commonly used heat carriers in solar heating systems are liquids (water, ethylene glycol solution, organic matter) and air. Each of them has certain advantages and disadvantages. The air does not freeze, does not pose major problems associated with leaks and corrosion of equipment. However, due to the low density and heat capacity of air, the dimensions of air installations, the power consumption for pumping the coolant is higher than that of liquid systems. Therefore, in most of the operated solar heating systems, preference is given to liquids. For housing and communal needs, the main heat carrier is water.
When solar collectors are operating during periods with negative outside temperatures, it is necessary either to use antifreeze as a coolant, or in some way to avoid freezing of the coolant (for example, by timely draining of water, heating it, insulating the solar collector).
Solar installations of hot water supply of year-round operation with a backup source of heat can be equipped with rural houses, multi-storey and apartment buildings, sanatoriums, hospitals and other objects. Seasonal installations, such as, for example, shower installations for pioneer camps, boarding houses, mobile installations for geologists, builders, shepherds usually operate in the summer and transitional months of the year, during periods with a positive outside temperature. They can have a duplicate heat source or do without it, depending on the type of facility and operating conditions.
The cost of solar hot water supply units can be from 5 to 15% of the cost of the object and depends on climatic conditions, the cost of equipment and the degree of its development.
In solar installations intended for heating systems, both liquids and air are used as heat carriers. In multi-circuit solar plants in different circuits, different coolants can be used (for example, in the solar circuit - water, in the distribution circuit - air). In our country, water solar installations for heat supply are prevalent.
The surface area of solar collectors required for heating systems is usually 3-5 times the surface area of collectors for hot water systems, so the utilization rate of these systems is lower, especially in summer period of the year. The installation cost for a heating system can be 15-35% of the cost of the object.
Combined systems can include year-round installations for heating and hot water supply, as well as installations operating in the mode of a heat pump and a heat pipe for heat and cold supply. These systems are not yet widely used in industry.
The density of the solar radiation flux arriving at the collector surface largely determines the heat engineering and technical and economic indicators of solar heat supply systems.
The density of the solar radiation flux varies during the day and throughout the year. This is one of the characteristic features of systems that use solar energy, and when carrying out specific engineering calculations for solar power plants, the choice of the calculated value of E is decisive.
As a design diagram of a solar heat supply system, consider the diagram shown in Figure 3.3, which makes it possible to take into account the peculiarities of the operation of various systems. The solar collector 1 converts the energy of solar radiation into heat, which is transferred to the storage tank 2 through the heat exchanger 3. The heat exchanger can be located in the storage tank itself. The circulation of the coolant is provided by a pump. The heated coolant enters the hot water supply and heating systems. In the event of a lack or absence of solar radiation, a back-up source of heat for hot water supply or heating 5 is switched on.
Figure 3.3. Solar heat supply system diagram: 1 - solar collectors; 2 - hot water storage tank; 3 - heat exchanger; 4 - building with underfloor heating; 5 - backup (source of additional energy); 6 - passive solar system; 7 - pebble battery; 8 - dampers; 9 - fan; 10 - flow of warm air into the building; 11- supply of recirculated air from the building
In the solar heating system, solar collectors of a new generation "Raduga" by NPP "Competitor" with improved thermal performance are used due to the use of a selective coating on a heat-absorbing stainless steel panel and a translucent coating made of extra strong glass with high optical characteristics.
The system uses as a heat carrier: water at positive temperatures or antifreeze during the heating period (solar circuit), water (second circuit underfloor heating) and air (third solar air heating circuit).
An electric boiler was used as a backup source.
Increasing the efficiency of solar supply systems can be achieved through the use of different methods accumulation of thermal energy, rational combination of solar systems with thermal boilers and heat pump installations, combination of active and passive development systems effective means and automatic control methods.
27.09.2019
Solar heating systems are systems that use solar radiation as a source of thermal energy. Their characteristic difference from other low-temperature heating systems is the use of a special element - a solar receiver, designed to capture solar radiation and convert it into thermal energy.
According to the method of using solar radiation, solar low-temperature heating systems are divided into passive and active.
Passive solar heating systems are called, in which the building itself or its individual enclosures (collector building, collector wall, collector roof, Figure 1) serve as an element that perceives solar radiation and converts it into heat.
In passive solar systems, the use of solar energy is carried out exclusively through the architectural and structural solutions of buildings.
In a passive system of solar low-temperature heating, a collector building, solar radiation, penetrating through the light openings into the room, falls into a heat trap, as it were. Short-wave solar radiation freely passes through the window glass and gets on the internal fences of the room, is converted into heat. All solar radiation entering the room is converted into heat and is able to partially or completely compensate for its heat losses.
To increase the efficiency of the collector-building system, large area light openings are placed on the southern facade, supplying them with blinds, which, when closed, should prevent losses with counter-radiation at night, and in a hot period, in combination with other sun-protection devices, overheating of the room. The inner surfaces are painted in dark colors.
The task of the calculation with this heating method is to determine the required area of the light openings for the passage of the solar radiation flux into the room, which is necessary, taking into account the accumulation to compensate for the heat losses. As a rule, the power of the passive building-collector system in cold period turns out to be insufficient, and an additional heat source is installed in the building, turning the system into a combined one. In this case, the calculation determines the economically feasible areas of the light openings and the power of the additional heat source.
Passive solar system of air low-temperature heating "wall-collector" includes a massive outer wall, in front of which, at a short distance, a radiant screen with shutters is installed. At the floor and under the ceiling, slot-like holes with valves are arranged in the wall. The sun's rays, passing through the translucent screen, are absorbed by the surface of the massive wall and converted into heat, which is transferred by convection to the air in the space between the screen and the wall. The air heats up and rises up, getting through the slot under the ceiling into the serviced room, and its place is taken by the cooled air from the room, penetrating into the space between the wall and the screen through the slot at the floor of the room. The supply of heated air to the room is controlled by opening the valve. If the valve is closed, heat accumulates in the wall mass. This heat can be removed by convective air flow, opening the valve at night or in cloudy weather.
When calculating such a passive low-temperature solar air heating system, the required wall surface area is determined. This system is also duplicated with an additional source of heat.
Active solar low-temperature heating systems are called in which the solar collector is an independent separate device that does not belong to the building. Active solar systems can be subdivided:
For active solar heating systems, two types of solar collectors are used: concentrating and flat.
Air is a widespread non-freezing coolant in the entire range of operating parameters. When using it as a heat carrier, it is possible to combine heating systems with a ventilation system. However, air is a low-heat heat carrier, which leads to an increase in metal consumption for the device of air heating systems in comparison with water systems. Water is a heat-retaining and widely available heat carrier. However, at temperatures below 0 ° C, it is necessary to add anti-freeze liquids to it. In addition, it must be borne in mind that water saturated with oxygen causes corrosion of pipelines and apparatus. But the consumption of metal in water solar systems is much lower, which greatly contributes to their wider application.
Seasonal solar hot water systems are usually single-circuit and operate in the summer and transitional months, during periods with a positive outside temperature. They can have an additional source of heat or do without it, depending on the purpose of the serviced facility and operating conditions.
The SVU solar water heating plant (Figure 2) consists of a solar collector and a heat exchanger-accumulator. A coolant (antifreeze) circulates through the solar collector. The coolant is heated in the solar collector by the energy of the Sun and then gives off thermal energy to the water through a heat exchanger mounted in the storage tank. The storage tank stores hot water until it is used, so it must have good thermal insulation. In the primary circuit, where the solar collector is located, natural or forced circulation of the coolant can be used. An electric or any other automatic backup heater can be installed in the storage tank. If the temperature in the storage tank drops below the set one (prolonged cloudy weather or few hours of sunshine in winter), the backup heater automatically turns on and heats up the water to the set temperature.
Solar heating systems for buildings are usually double-circuit or, most often, multi-circuit, and for different circuits, different heat carriers can be used (for example, in a solar circuit - aqueous solutions non-freezing liquids, in the intermediate circuits - water, and in the consumer circuit - air). Combined year-round solar systems for heat and cold supply of buildings are multi-circuit and include an additional heat source in the form of a traditional fossil-fueled heat generator or heat transformer. Schematic diagram solar heat supply system is shown in Figure 3. It includes three circulation circuits:
The solar heating system functions as follows. The heat carrier (antifreeze) of the heat-receiving circuit, being heated in the solar collectors 1, enters the heat exchanger 3, where the heat of the antifreeze is transferred to the water circulating in the shell space of the heat exchanger 3 under the action of the pump 8 of the secondary circuit. The heated water enters the accumulator tank 2. From the accumulator tank, water is taken by the hot water supply pump 8, is brought, if necessary, to the required temperature in the backup 7 and enters the building's hot water supply system. The accumulator tank is replenished from the water supply. For heating, water from the storage tank 2 is supplied by the pump of the third circuit 8 to the heater 5, through which air is passed with the help of the fan 9 and, when heated, enters the building 4. In the absence of solar radiation or lack of heat energy generated by solar collectors, into operation turn on the backup 6. The choice and arrangement of solar heat supply system elements in each case are determined by climatic factors, the purpose of the object, heat consumption mode, economic indicators.
Figure 4 shows a diagram of a solar heating system for an energy efficient eco-friendly home.
The system uses as a heat carrier: water at positive temperatures and antifreeze during the heating season (solar circuit), water (second underfloor heating circuit) and air (third solar air heating circuit).
An electric boiler was used as a backup source, and a 5 m 3 battery with a pebble attachment was used to accumulate heat for one day. One cubic meter of pebbles accumulates on average 5 MJ of heat per day.
Low-temperature heat storage systems cover the temperature range from 30 to 100 ◦C and are used in air (30 ◦ C) and hot water (30–90 ◦ C) heating and hot water systems (45–60 ◦ C).
The heat storage system, as a rule, contains a reservoir, heat storage material, with the help of which heat energy is accumulated and stored, heat exchange devices for the supply and removal of heat during charging and discharging of the battery, and thermal insulation.
Batteries can be classified according to the nature of the physicochemical processes occurring in heat storage materials:
The most widely used heat accumulators are of the capacitive type.
The amount of heat Q (kJ) that can be accumulated in a capacitive-type heat accumulator is determined by the formula
The most effective heat storage material in liquid solar heating systems is water. For seasonal accumulation of heat, it is promising to use underground reservoirs, rock soil and other natural formations.
Concentrating solar collectors are spherical or parabolic mirrors (Figure 5.), made of polished metal, in the focus of which a heat-receiving element (solar boiler) is placed, through which the coolant circulates. Water or non-freezing liquids are used as a heat carrier. When using water as a heat carrier at night and during a cold period, the system must be emptied to prevent it from freezing.
To ensure high efficiency of the process of capturing and converting solar radiation, the concentrating solar receiver must be constantly pointed strictly at the Sun. For this purpose, the solar receiver is equipped with a tracking system that includes a sun direction sensor, an electronic signal conversion unit, an electric motor with a gearbox for rotating the solar receiver structure in two planes.
The advantage of systems with concentrating solar collectors is the ability to generate heat with a relatively high temperature (up to 100 ◦ C) and even steam. The disadvantages include the high cost of the structure; the need for constant cleaning of reflective surfaces from dust; work only during daylight hours, and therefore the need for large batteries; large energy consumption for the drive of the solar tracking system, commensurate with the generated energy. These disadvantages restrain the widespread use of active low temperature systems solar heating with concentrating solar collectors. Recently, flat solar collectors are most often used for solar low-temperature heating systems.
A flat plate solar collector is a heat exchanger designed to heat a liquid or gas using solar energy. The area of application of flat solar collectors is heating systems for residential and industrial buildings, air conditioning systems, hot water supply systems, as well as power plants with a low-boiling working fluid, usually operating according to the Rankine cycle. Flat solar collectors (Figures 6 and 7) consist of a glass or plastic cover (single, double, triple), a heat-absorbing panel painted black on the sun-facing side, insulation on the back and a housing (metal, plastic, glass , wooden).
Any metal or plastic sheet with coolant channels can be used as a heat-absorbing panel. Heat-absorbing panels are made of aluminum or steel of two types: sheet-pipe and stamped panels (pipe in sheet). Plastic panels are not widely used due to their fragility and rapid aging under the influence of sunlight, as well as because of their low thermal conductivity. Under the influence of solar radiation, heat-sensing panels are heated to temperatures of 70–80 ◦ C, higher than the ambient temperature, which leads to an increase in the convective heat transfer of the panel to the environment and its own radiation to the sky. To achieve higher temperatures of the coolant, the surface of the plate is covered with spectrally selective layers that actively absorb short-wavelength radiation from the Sun and reduce its own thermal radiation in the long-wavelength part of the spectrum. Such designs based on "black nickel", "black chrome", copper oxide on aluminum, copper oxide on copper and others are expensive (their cost is often commensurate with the cost of the heat-absorbing panel itself). Another way to improve the performance of flat plate collectors is to create a vacuum between the heat absorbing panel and the transparent insulation to reduce heat loss (fourth generation solar collectors).
The principle of operation of the collector is based on the fact that it perceives solar radiation with a sufficiently high absorption coefficient of visible sunlight and has relatively low heat losses, including due to the low transmittance of a translucent glass coating for thermal radiation at operating temperature. It is clear that the temperature of the resulting coolant is determined by the heat balance of the collector. The incoming part of the balance represents the heat flux of solar radiation, taking into account the optical efficiency of the collector; the consumable part is determined by the recoverable useful heat, the total heat loss coefficient and the difference between the operating temperature and the environment. The perfection of a collector is determined by its optical and thermal efficiency.
The optical efficiency η o shows how much of the solar radiation that reaches the collector's glazing surface is absorbed by the absorbing black surface, and takes into account the energy losses associated with absorption in the glass, reflection and the difference in the coefficient of thermal emissivity of the absorbing surface from unity.
The simplest solar collector with a single-glass translucent coating, polyurethane foam insulation of the remaining surfaces and an absorber covered with black paint has an optical efficiency of about 85%, and the heat loss coefficient is about 5-6 W / (m 2 · K) (Fig. 7). The set of a flat radiation-absorbing surface and pipes (channels) for the coolant forms a single structural element- absorber. Such a collector in the summer in mid-latitudes can heat water up to 55–60 ◦ C and has an average daily productivity of 70–80 liters of water from 1 m 2 of the heater surface.
To obtain higher temperatures, collectors from evacuated pipes with a selective coating are used (Figure 8).
In a vacuum collector, the volume in which the black surface absorbing solar radiation is located is separated from the environment by an evacuated space (each element of the absorber is placed in a separate glass tube, inside which a vacuum is created), which makes it possible to almost completely eliminate heat losses to the environment due to thermal conductivity and convection. Radiation losses are largely suppressed by the use of selective coating. In a vacuum manifold, the coolant can be heated to 120–150 ◦C. The efficiency of a vacuum collector is significantly higher than that of a flat collector, but it also costs much more.
The efficiency of solar energy installations largely depends on the optical properties of the surface that absorbs solar radiation. To minimize energy losses, it is necessary that in the visible and near infrared regions of the solar spectrum, the absorption coefficient of this surface should be as close to unity as possible, and in the wavelength region of the intrinsic thermal radiation of the surface, the reflection coefficient should tend to unity. Thus, the surface must have selective properties - it is good to absorb short-wave radiation and reflect long-wave radiation well.
By the type of mechanism responsible for the selectivity of optical properties, four groups of selective coatings are distinguished:
A small number of known materials have intrinsic selectivity of optical properties, for example, W, Cu 2 S, HfC.
The most widespread are two-layer selective coatings. A layer with a high reflectance in the long-wavelength region of the spectrum, for example, copper, nickel, molybdenum, silver, aluminum, is applied to the surface, which must be given selective properties. On top of this layer, a layer is applied that is transparent in the long wavelength region, but has a high absorption coefficient in the visible and near infrared regions of the spectrum. Many oxides have these properties.
The selectivity of the surface can be ensured due to purely geometric factors: surface irregularities should be greater than the wavelength of light in the visible and near infrared regions of the spectrum and less than the wavelength corresponding to the intrinsic thermal radiation of the surface. Such a surface for the first of the indicated regions of the spectrum will be black, and for the second - specular.
Selective properties are exhibited by surfaces with a dendritic or porous structure with appropriate sizes of dendritic needles or pores.
Selective interference surfaces are formed by several alternating layers of metal and dielectric, in which short-wave radiation is quenched due to interference, and long-wave radiation is freely reflected.
According to the IEA, by the end of 2001, the total area of installed collectors in 26 countries, the most active in this regard, amounted to about 100 million m2, of which 27.7 million m2 falls on the share of non-glazed collectors, mainly used for heating water in swimming pools. The rest - flat glazed collectors and collectors with evacuated pipes - were used in hot water supply systems or for space heating. In terms of the area of installed collectors per 1000 inhabitants, the leaders are Israel (608 m 2), Greece (298) and Austria (220). They are followed by Turkey, Japan, Australia, Denmark and Germany with a specific area of installed collectors of 118–45 m 2/1000 inhabitants.
The total area of solar collectors installed by the end of 2004 in the EU countries reached 13.96 million m2, and in the world it has already exceeded 150 million m2. The annual increase in the area of solar collectors in Europe is on average 12%, and in some countries it is at the level of 28-30% or more. The world leader in the number of collectors per thousand inhabitants is Cyprus, where 90% of houses are equipped with solar installations (there are 615.7 m 2 solar collectors per thousand inhabitants), followed by Israel, Greece and Austria. The absolute leader in terms of the area of installed collectors in Europe is Germany - 47%, followed by Greece - 14%, Austria - 12%, Spain - 6%, Italy - 4%, France - 3%. European countries are the undisputed leaders in the development of new technologies for solar heating systems, but they are far behind China in terms of commissioning new solar installations.
Of the total area of solar collectors installed in the world in 2004, 78% were installed in China. The IED market in China has recently been growing at a rate of 28% per year.
In 2007, the total area of solar collectors installed in the world already amounted to 200 million m2, including more than 20 million m2 in Europe.
Today, on the world market, the cost of an IED (Figure 9), including a collector with an area of 5–6 m 2, a storage tank with a capacity of about 300 liters and the necessary fittings, is US $ 300–400 per 1 m 2 of a collector. Such systems are predominantly installed in single- and double-family houses and have a backup heater (electric or gas). When the storage tank is installed above the collector, the system can operate on natural circulation (thermosyphon principle); when installing a storage tank in the basement - on a forced one.
In world practice, the most widespread are small solar heating systems. As a rule, such systems include solar collectors with a total area of 2–8 m 2, a storage tank, the capacity of which is determined by the area of installed collectors, a circulation pump (depending on the type of heat circuit) and other auxiliary equipment.
Active systems big size, in which the storage tank is located below the collectors and the circulation of the coolant is carried out using a pump, are used for the needs of hot water supply and heating. As a rule, in active systems participating in covering part of the heating load, a back-up heat source is provided, operating on electricity or gas.
A relatively new phenomenon in the practice of using solar heat supply is large systems capable of meeting the needs of hot water supply and heating of apartment buildings or entire residential areas. Such systems provide for either daily or seasonal heat storage. Daily accumulation assumes the possibility of operating the system with the consumption of heat accumulated over several days, seasonal - over several months. For seasonal heat storage, large underground reservoirs filled with water are used, into which all excess heat received from the collectors during the summer is discharged. Another option for seasonal accumulation is soil heating with the help of wells with pipes through which hot water circulates from collectors.
Table 1 shows the main parameters of large solar systems with daily and seasonal heat storage in comparison with a small solar system for a single-family home.
Table 1. - Main parameters of solar heat supply systems Currently, 10 solar heat supply systems with collector areas from 2400 to 8040 m2, 22 systems with collector areas from 1000 to 1250 m2 and 25 systems with collector areas from 500 to 1000 m2 are operating in Europe. Below are the specifications for some of the larger systems.
Hamburg (Germany). The area of the heated premises is 14800 m 2. Solar collectors area - 3000 m 2. The volume of the water heat accumulator is 4500 m 3.
Fridrichshafen (Germany). The area of the heated premises is 33,000 m 2. The area of solar collectors is 4050 m 2. The volume of the water heat accumulator is 12000 m 3.
Ulm-am-Neckar (Germany). The area of the heated premises is 25000 m 2. Solar collectors area - 5300 m 2. The volume of the ground heat accumulator is 63400 m 3.
Rostock (Germany). The area of the heated premises is 7000 m 2. Solar collectors area - 1000 m 2. The volume of the ground heat accumulator is 20,000 m 3.
Hemnitz (Germany). The area of the heated premises is 4680 m 2. The area of vacuum solar collectors is 540 m 2. The volume of the gravel-water heat accumulator is 8000 m 3.
Attenkirchen (Germany). The area of the heated premises is 4500 m 2. The area of vacuum solar collectors is 800 m 2. The volume of the ground heat accumulator is 9850 m 3.
Saro (Sweden). The system consists of 10 small houses comprising 48 apartments. Solar collectors area - 740 m 2. The volume of the water heat accumulator is 640 m 3. The solar system covers 35% of the total heat load of the heating system.
Currently, there are several companies in Russia that produce solar collectors suitable for reliable operation. The main ones are the Kovrov Mechanical Plant, NPO Mashinostroenie and ZAO ALTEN.
Collectors of the Kovrov Mechanical Plant (Figure 10), which do not have a selective coating, are cheap and simple in design, are mainly focused on the domestic market. More than 1,500 collectors of this type are currently installed in the Krasnodar Territory.
The NPO Mashinostroyenia collector is close to European standards in terms of characteristics. The absorber of the collector is made of an aluminum alloy with a selective coating and is designed mainly for operation in two-circuit heating circuits, since direct contact of water with aluminum alloys can lead to pitting corrosion of the channels through which the coolant passes.
Collector ALTEN-1 has a completely new design and meets European standards, it can be used both in single-circuit and double-circuit heat supply schemes. The collector features high thermal performance, a wide range of possible applications, low weight and attractive design.
The experience of operating installations based on solar collectors has revealed a number of disadvantages of such systems. First of all, this is the high cost of collectors associated with selective coatings, increased transparency of glazing, evacuation, etc. A significant disadvantage is the need for frequent cleaning of glasses from dust, which practically excludes the use of the collector in industrial areas. During long-term operation of solar collectors, especially in winter conditions, there is a frequent failure of them due to the uneven expansion of the illuminated and darkened areas of the glass due to the violation of the integrity of the glazing. There is also a high percentage of collector failure during transportation and installation. A significant disadvantage of the systems with collectors is also the uneven loading during the year and day. The experience of operating collectors in the conditions of Europe and the European part of Russia with a high proportion of diffuse radiation (up to 50%) showed the impossibility of creating a year-round autonomous system hot water supply and heating. All solar systems with solar collectors in mid-latitudes require the installation of large-volume storage tanks and the inclusion of an additional source of energy in the system, which reduces the economic effect of their use. In this regard, it is most advisable to use them in areas with a high intensity of solar radiation (not less than 300 W / m 2).
In residential and administrative buildings solar energy is mainly used in the form of heat to meet the needs of hot water supply, heating, cooling, ventilation, drying, etc.
From an economic point of view, the use of solar heat is most profitable when creating hot water supply systems and in installations for heating water close to them in technical implementation (in pools, industrial devices). Hot water supply is needed in every residential building, and since hot water needs change relatively little during the year, the efficiency of such installations is high and they quickly pay off.
As for solar heating systems, the period of their use during the year is short, during the heating season, the intensity of solar radiation is low and, accordingly, the collector area is much larger than in hot water supply systems, and the economic efficiency is lower. Usually, when designing, a solar heating system and hot water supply are combined.
In solar cooling systems, the operating period is even lower (three summer months), which leads to long equipment downtime and very low utilization rates. Considering the high cost of cooling equipment, the economic efficiency of the systems becomes minimal.
The annual equipment utilization rate in combined heating and cooling systems (hot water supply, heating and cooling) is the highest, and these systems, at first glance, are more profitable than combined heating and hot water supply systems. However, if the cost of the required solar collectors and cooling system mechanisms is taken into account, it turns out that such solar installations will be very expensive and hardly economically profitable.
When creating solar heating systems, passive schemes should be used to increase the thermal insulation of the building and efficient use solar radiation coming through the window openings. The problem of thermal insulation must be solved on the basis of architectural and structural elements, using low thermal conductivity materials and structures. The missing heat is recommended to be replenished with the help of active solar systems.
The main problem of the widespread use of solar installations is associated with their insufficient economic efficiency in comparison with traditional heat supply systems. The cost of heat energy in installations with solar collectors is higher than in installations with traditional fuels. The payback period of a solar thermal installation T approx can be determined by the formula:
The economic effect of installing solar collectors in the areas of centralized energy supply E can be defined as income from the sale of energy during the entire service life of the installation minus operating costs:
Table 2 shows the cost of solar heating systems (in 1995 prices). The data show that domestic developments are 2.5–3 times cheaper than foreign ones.
The low price of domestic systems is explained by the fact that they are made of cheap materials, simple in design, and focused on the domestic market.
Table 2. - Cost of solar heating systems The specific economic effect (E / S) in the district heating zone, depending on the service life of the collectors, ranges from 200 to 800 rubles / m 2.
Heat supply installations with solar collectors in regions remote from centralized power grids, which in Russia constitute over 70% of its territory with a population of about 22 million people, have a much greater economic effect. These installations are designed to operate in an autonomous mode for individual consumers, where the demand for thermal energy is very significant. At the same time, the cost of traditional fuels is much higher than their cost in the areas of centralized heating due to transport costs and fuel losses during transportation, i.e., the regional factor r r is included in the cost of fuel in the Central heating region:
where r p> 1 and for different regions can change its value. At the same time, the unit cost of the C plant remains almost unchanged in comparison with the C tr. Therefore, when replacing Ts t by Ts tr in the formulas
the calculated payback period of autonomous installations in areas remote from centralized networks decreases by r p times, and the economic effect increases in proportion to r p.
In today's conditions in Russia, when energy prices are constantly growing and are uneven across regions due to transportation conditions, the decision on the economic feasibility of using solar collectors strongly depends on local socio-economic, geographic and climatic conditions.
From the point of view of uninterrupted supply of energy to the consumer, the most effective are combined technological systems that use two or more types of renewable energy sources.
Solar thermal energy can fully meet the needs for hot water in the house in the summer. In the autumn-spring period, up to 30% of the required energy for heating and up to 60% of the demand for hot water supply can be obtained from the Sun.
In recent years, geothermal heat supply systems based on heat pumps have been actively developing. In such systems, as noted above, low-potential (20–40 ◦ C) thermal water or petrothermal energy of the upper layers of the earth's crust is used as the primary heat source. When using the heat of the ground, ground heat exchangers are used, placed either in vertical wells with a depth of 100-300 m, or at a certain depth horizontally.
To effectively provide heat and hot water to decentralized low-power consumers, a combined solar-geothermal system has been developed at the IPG DSC RAS (Figure 11).
Such a system consists of a solar collector 1, a heat exchanger 2, a storage tank 3, a heat pump 7 and a heat exchanger well 8. A coolant (antifreeze) circulates through the solar collector. The heat carrier is heated in the solar collector by the energy of the Sun and then gives off thermal energy to the water through the heat exchanger 2, mounted in the storage tank 3. Hot water is stored in the storage tank until it is used, so it must have good thermal insulation. In the primary circuit, where the solar collector is located, natural or forced circulation of the coolant can be used. An electric heater 6 is also mounted in the storage tank. If the temperature in the storage tank drops below the set temperature (prolonged cloudy weather or few hours of sunshine in winter), the electric heater automatically turns on and heats the water to the set temperature.
The solar collector unit is operated year-round and provides the consumer with hot water, while the low-temperature underfloor heating unit with a heat pump (HP) and a heat exchanger well with a depth of 100–200 m is put into operation only during the heating season.
In the HP cycle, cold water with a temperature of 5 ◦ C descends into the annular space of the heat exchanger well and withdraws low-grade heat from the surrounding rock. Then the water heated, depending on the depth of the well, to a temperature of 10–15 ◦ C, rises along the central pipe string to the surface. To prevent heat backflow, the central column is thermally insulated from the outside. On the surface, water from the well enters the HP evaporator, where the low-boiling working agent is heated and evaporated. After the evaporator, the cooled water is again directed into the well. During the heating period, with constant circulation of water in the well, there is a gradual cooling of the rock around the well.
Calculated studies show that the radius of the cooling front during the heating period can reach 5–7 m. During the inter-heating period, when the heating system is turned off, there is a partial (up to 70%) recovery of the temperature field around the well due to heat inflow from rocks outside the cooling zone; it is not possible to achieve full recovery of the temperature field around the well during its downtime.
Solar collectors are installed based on the winter period of system operation, when the sunshine is minimal. In the summer, part of the hot water from the storage tank is directed into the well to fully restore the temperature in the rock around the well.
During the interheating period, valves 13 and 14 are closed, and with valves 15 and 16 open, hot water from the accumulator tank circulation pump It is pumped into the annular space of the well, where, as the descent proceeds, heat exchange with the surrounding rock occurs. Then the chilled water is directed back to the storage tank through the central insulated column. In the heating season, on the contrary, valves 13 and 14 are open, and valves 15 and 16 are closed.
In the proposed technological system the potential of solar energy is used to heat water in the hot water supply system and rocks around the well in the low-temperature heating system. Heat recovery in the rock makes it possible to operate the heat supply system in an economically optimal mode.
The sun is a significant source of energy on planet Earth. Solar energy is very often the subject of a wide variety of discussions. As soon as a project for a new solar power plant appears, questions arise about efficiency, capacity, investment volumes and payback periods.
There are scientists who see solar thermal power plants as a threat to the environment. Mirrors used in thermal solar power plants heat the air very strongly, which leads to climate change and the death of birds flying by. Despite this, in recent years, solar thermal power plants have become more widespread. In 1984, the first solar power plant went into operation near the Californian city of Cramer Junction in the Mohabe Desert (Figure 6.1). The station was named Solar Energy Generating System, or SEGS for short.
Rice. 6.1. Solar power plant in the Mohabe desert This power plant uses solar radiation to generate steam, which turns a turbine and generates electricity. The production of solar thermal power on a large scale is quite competitive. Currently, US power companies have built solar thermal power plants with a total installed capacity of more than 400 MW, which provide electricity to 350,000 people and replace 2.3 million barrels of oil per year. Nine power plants located in the Mohabe Desert have 354 MW of installed capacity. In other regions of the world, projects to use solar heat to generate electricity are also due to start soon. India, Egypt, Morocco and Mexico are developing related programs. Grants for their financing are provided by the Global Environment Protection Program (GEF). In Greece, Spain and the United States, new projects are being developed by independent power producers.
By the method of heat production, solar thermal power plants are divided into solar concentrators(mirrors) and sun ponds.
Thermal solar power plants concentrate solar energy using lenses and reflectors. Since this heat can be stored, such stations can generate electricity as needed, day and night, in any weather. Large mirrors - either point or line focus - concentrate the sun's rays to the point where water turns into steam, while releasing enough energy to turn the turbine. These systems can convert solar energy into electricity with an efficiency of about 15%. All thermal power plants, except for solar ponds, use concentrators to achieve high temperatures, which reflect the sun's light from a larger surface onto a smaller receiver surface. Typically, such a system consists of a concentrator, receiver, heat carrier, storage system and power transmission system. Modern technologies include parabolic concentrators, solar parabolic mirrors and solar towers. They can be combined with fossil fuel plants and, in some cases, adapted for heat storage. The main advantage of such hybridization and heat storage is that such technology can provide dispatching of electricity production, that is, electricity generation can be produced during periods when there is a need for it. Hybridization and heat storage can increase the economic value of the electricity produced and lower its average cost.
Some thermal solar power plants use parabolic mirrors that concentrate sunlight on receiving tubes containing a heat transfer fluid. This liquid is heated to almost 400 ºC and is pumped through a series of heat exchangers; this generates superheated steam that drives a conventional turbine generator to generate electricity. To reduce heat loss, the receiving tube can be surrounded by a transparent glass tube placed along the focal line of the cylinder. Typically, such installations include uniaxial or biaxial solar tracking systems. In rare cases, they are stationary (Fig. 6.2).
Rice. 6.2. Solar plant with parabolic concentrator Estimates of this technology show a higher cost of generated electricity than other solar thermal power plants. This is due to the low concentration of solar radiation, more low temperatures... However, with operational experience, improved technology and lower operating costs, parabolic concentrators may be the least expensive and most reliable technology in the near future.
A dish-type solar array is a battery of parabolic dish mirrors similar in shape to a satellite dish, which focus solar energy onto receivers located at the focal point of each dish (Fig. 6.3). The liquid in the receiver is heated to 1000 ° C and is directly used to generate electricity in a small engine and generator connected to the receiver.
Rice. 6.3. Disc type solar plant High optical efficiency and low start-up costs make mirror / motor systems the most efficient solar technology of all. The Stirling engine and parabolic mirror system holds the world record for the efficiency of converting solar energy into electricity. In 1984, Rancho Mirage in California achieved a practical efficiency of 29%. Thanks to modular design, such systems are the best option to meet the need for electricity both for autonomous consumers and for hybrid ones operating on a common network.
Tower-type solar power plants with a central receiver The tower-type solar power plants with a central receiver use a rotating field of heliostat reflectors. They focus sunlight onto a central receiver at the top of the tower that absorbs thermal energy and drives a turbine generator (Figure 6.4, Figure 6.5).
Rice. 6.4. Solar power plant of a tower type with a central receiver A computer-controlled biaxial tracking system sets the heliostats so that the reflected sunlight is stationary and always strikes the receiver. The liquid circulating in the receiver transfers heat to the heat accumulator in the form of vapor. The steam turns a turbine to generate electricity, or is directly used in industrial processes. Receiver temperatures range from 500 to 1500 ºC. By storing heat, the tower power plants have become a unique solar technology that generates electricity on a predetermined schedule.
Rice. 6.5. Solar power plant "Solar Two" in California Neither focusing mirrors nor solar cells can generate energy at night. For this purpose, solar energy accumulated during the day must be stored in heat storage tanks. This process naturally occurs in the so-called solar ponds (Fig. 6.6).
Rice. 6.6. Diagram of the solar pond device Solar ponds have a high salt concentration in the bottom water layers, a non-convective middle water layer in which the salt concentration increases with depth and a convection layer with a low salt concentration on the surface. Sunlight falls on the surface of the pond and heat is trapped in the lower layers of the water due to the high concentration of salt. High salinity water, heated by solar energy absorbed by the pond bottom, cannot rise due to its high density. It remains at the bottom of the pond, gradually warming up until it almost boils. The hot bottom "brine" is used day or night as a heat source, thanks to which a special turbine with an organic heat carrier can generate electricity. The middle layer of the sun pond acts as thermal insulation, preventing convection and heat loss from the bottom to the surface. The temperature difference between the bottom and the surface of the pond water is sufficient to power the generator. The coolant, passed through pipes through the lower layer of water, is fed further into a closed Rankine system, in which a turbine rotates to generate electricity.
Solar power plants of a tower type with a central receiver and solar power plants with parabolic concentrators work optimally as part of large, grid-connected power plants with a capacity of 30-200 MW, while disk-type solar power plants consist of modules and can be used both in stand-alone installations and in groups of general with a capacity of several megawatts.
Table 6.1 Characteristics of solar thermal power plants Solar parabolic concentrators are by far the most advanced solar energy technology and are likely to be used in the near term. Tower-type power plants with a central receiver, due to their efficient heat storage capacity, can also become solar power plants in the near future. The modularity of the poppet type units allows them to be used in smaller units. Solar power plants of a tower type with a central receiver and installations of a disk type allow achieving higher values of the efficiency of converting solar energy into electricity at a lower cost than power plants with solar parabolic concentrators. Table 6.1 shows the main characteristics of three options for solar thermal power generation.
Selective coatings
By the type of mechanism responsible for the selectivity of optical properties, four groups of selective coatings are distinguished:
1) own;
2) two-layer, in which the upper layer has a high absorption coefficient in the visible region and small in the IR region, and the lower layer has a high reflectivity in the IR region;
3) with a micro-relief providing the required effect;
4) interference.
A small number of known materials have intrinsic selectivity of optical properties, for example, W, Cu 2 S, HfC.
Selective interference surfaces are formed by several alternating layers of metal and dielectric, in which short-wavelength radiation is extinguished due to interference, and long-wavelength radiation is freely reflected.
Classification and basic elements of solar systems
Solar heating systems are systems that use solar energy as a heat source. Their characteristic difference from other low-temperature heating systems is the use of a special element - a solar collector, designed to capture solar radiation and convert it into thermal energy.
According to the method of using solar radiation, solar low-temperature heating systems are divided into passive and active.
Passive solar heating systems are called, in which the building itself or its individual enclosures (collector building, collector wall, collector roof, etc.) serve as an element that perceives solar radiation and converts it into heat (Fig. 4.1.1 )).
Active solar low-temperature heating systems are called in which the solar collector is an independent separate device that does not belong to the building. Active solar systems can be subdivided:
By appointment (hot water supply systems, heating systems, combined systems for heat and cold supply purposes);
By the type of coolant used (liquid - water, antifreeze and air);
By the duration of work (year-round, seasonal);
According to the technical solution of the schemes (one-, two-, multi-circuit).
Air is a widespread non-freezing coolant in the entire range of operating parameters. When using it as a heat carrier, it is possible to combine heating systems with a ventilation system.
Seasonal solar hot water systems are usually single-circuit and operate during periods with a positive outside temperature. They can have an additional source of heat or do without it, depending on the purpose of the serviced facility and operating conditions.
Solar heating systems for buildings are usually double-circuit or, most often, multi-circuit, and different heat carriers can be used for different circuits (for example, in the solar circuit - aqueous solutions of non-freezing liquids, in the intermediate circuits - water, and in the consumer circuit - air).
Combined year-round solar systems for heat and cold supply of buildings are multi-circuit and include an additional heat source in the form of a traditional fossil-fueled heat generator or heat transformer.
The main elements of an active solar system are a solar receiver, a heat accumulator, an additional source or transformer of heat (heat pump), and its consumer (heating and hot water supply systems for buildings). The choice and arrangement of elements in each specific case are determined by climatic factors, the purpose of the object, the mode of heat consumption, and economic indicators.
1. Solar collectors.
The solar collector is the main element of the installation, in which the energy of the sun's radiation is converted into another form of useful energy. Unlike conventional heat exchangers, in which there is an intense transfer of heat from one liquid to another, and radiation is insignificant, in a solar collector, energy is transferred to the liquid from a remote source of radiant energy. Without the concentration of the sun's rays, the flux density of the incident radiation is at best -1100 W / m 2 and is variable. The wavelengths are in the range of 0.3 - 3.0 µm. They are significantly less than the wavelengths of the intrinsic radiation of most surfaces that absorb radiation. Thus, the study of solar collectors is associated with unique problems of heat transfer at low and variable energy flux densities and a relatively large role of radiation.
Solar collectors can be used with or without concentration of solar radiation. In flat collectors, the surface that receives solar radiation is also the surface that absorbs the radiation. Focusing collectors, usually with concave reflectors, concentrate radiation incident on their entire surface onto a heat exchanger with a smaller surface area, thereby increasing the energy flux density.
1.1. Flat solar collectors. A flat solar collector is a heat exchanger designed to heat a liquid or gas using the energy of the sun's radiation.
Flat collectors can be used to heat the coolant to moderate temperatures, t ≈ 100 o C. Their advantages include the possibility of using both direct and scattered solar radiation; they do not require sun tracking and do not require daily maintenance. Structurally, they are simpler than a system consisting of concentrating reflectors, absorbing surfaces and tracking mechanisms. The area of application of solar collectors is heating systems for residential and industrial buildings, air conditioning systems, hot water supply, as well as power plants with a low-boiling working fluid, usually operating according to the Rankine cycle.
The main elements of a typical flat solar collector (Fig. 1) are: a "black" surface that absorbs solar radiation and transfers its energy to a coolant (usually a liquid); coverings transparent relative to solar radiation, located above the absorbing surface, which reduce convective and radiation losses into the atmosphere; thermal insulation of the return and end surfaces of the collector to reduce losses due to thermal conductivity.
Fig. 1. Schematic diagram of a flat solar collector.
a) 1 - transparent coatings; 2 - insulation; 3 - pipe with heat carrier; 4 - absorbing surface;
b) 1.surface that absorbs solar radiation, 2-coolant channels, 3-glass (??), 4-case,
5- thermal insulation.
Fig. 2 A sheet-tube solar collector.
1 - upper hydraulic manifold; 2 - lower hydraulic manifold; 3 - n pipes located at a distance W from each other; 4 - sheet (absorbing plate); 5- connection; 6 - pipe (not to scale);
7 - insulation.
1.2. Collector efficiency... The efficiency of a collector is determined by its optical and thermal efficiency. The optical efficiency η o shows how much of the solar radiation reaching the collector glazing surface is absorbed by the absorbing black surface, and takes into account the energy losses associated with the difference from the unit of the glass transmittance and the absorption coefficient of the absorbing surface. For collector with single glazing
where (τα) n is the product of the glass transmittance τ by the absorption coefficient α of the absorbing surface radiation at normal fall sun rays.
In the event that the angle of incidence of the rays differs from the direct one, a correction factor k is introduced to take into account the increase in losses due to reflection from glass and a surface that absorbs solar radiation. In fig. 3 shows the graphs k = f (1 / cos 0 - 1) for collectors with single and double glazing. Optical efficiency, taking into account the angle of incidence of the rays other than direct,
Rice. 3. Correction factor, which takes into account the reflection of sunlight from the glass surface and the black absorbent surface.
In addition to these losses in a collector of any design, there are heat losses to the environment Q sweat, which are taken into account by the thermal efficiency, which is equal to the ratio of the amount of useful heat removed from the collector for a certain time to the amount of radiation energy supplied to it from the Sun during the same time:
where Ω is the area of the collector aperture; I - solar radiation flux density.
The optical and thermal efficiency of the collector are related by the ratio
Heat losses are characterized by the total loss factor U
where T a is the temperature of the black surface that absorbs solar radiation; T about is the ambient temperature.
The value of U can be considered constant with an accuracy sufficient for calculations. In this case, substitution of Q sweat in the formula for thermal efficiency leads to the equation
The thermal efficiency of the collector can also be written in terms of the average temperature of the coolant flowing through it:
where T t = (T in + T out) / 2 is the average temperature of the coolant; F "is a parameter usually called" collector efficiency "and characterizing the efficiency of heat transfer from a surface that absorbs solar radiation to a coolant; it depends on the design of the collector and almost does not depend on other factors; typical values of the parameter F" ≈: 0.8- 0.9 - for flat air collectors; 0.9-0.95 - for flat liquid collectors; 0.95-1.0 - for vacuum collectors.
1.3. Vacuum collectors. In the case when heating to higher temperatures is required, vacuum collectors are used. In a vacuum collector, the volume containing a black surface that absorbs solar radiation is separated from the environment by an evacuated space, which significantly reduces heat loss to the environment due to heat conduction and convection. Radiation losses are largely suppressed by the use of selective coatings. Since the total loss factor in a vacuum collector is small, the coolant in it can be heated to higher temperatures (120-150 ° C) than in a flat collector. In fig. 9.10 shows examples of the constructive implementation of vacuum collectors.
Rice. 4. Types of vacuum collectors.
1 - tube with coolant; 2 - a plate with a selective coating that absorbs solar radiation; 3 heat pipe; 4 heat-removing element; 5 glass tube with selective coating; b - inner tube for supplying the coolant; 7 outer glass container; 8 vacuum
