Heating systems are divided as follows: passive (see Ch. 5); active, which mostly use liquid solar collectors and storage tanks; combined.
Abroad, air heating systems are widely used, where building structures or special stone filling under it are used as batteries. In our country, the Physicotechnical Institute of the Academy of Sciences of the Uzbek SSR and TbilZNIIEP are working in this direction, but the results of the work are clearly insufficient and well-adjusted solutions have not been created, although air systems theoretically more efficient than liquid ones, in which the actual heating system is made of low-temperature panel-radiant or high-temperature with conventional heating devices. In our country, buildings with liquid systems were developed by IVTAN, FTI AN UzSSR, TashZNIIEP, TbilZNIIEP, KievZNIIEP and others and in some cases erected.
A large amount of information on active solar heating systems is given in a book published in 1980. Further, two individual residential buildings with autonomous solar heat supply systems developed and tested by KievZNIIEP are described: with a low-temperature panel-radiant heating system (a residential building in the village of Kolesnoye, Odessa region) and with a heat pump (a residential building in the village of Bucuria, Moldavian SSR ).
When developing a solar heating system for a residential building in the village. Kolesnoe, a number of changes were made to the architectural and construction part of the house (project by UkrNIIPgrazhdanselskstroy) aimed at adapting it to the requirements of solar heating: efficient masonry with insulation for the outer walls and triple glazing were used window openings; coils of the heating system are combined with interfloor ceilings; a basement is provided for placing equipment; carried out additional insulation attic and extract air heat recovery.
In terms of architecture and layout, the house is made on two levels. On the ground floor there is a front, common room, bedroom, kitchen, bathroom and pantries, and on the second - two bedrooms and a bathroom, an electric stove for cooking is provided. The equipment of the solar heating system (except collectors) is located in the basement; electric water heaters serve as an understudy of the system, which makes it possible to carry out a single energy input to the building and improve the comfortable qualities of housing.
Residential building solar heating system (Fig. 4.1) consists From three circuits: heat-receiving circulation and heating and hot water circuits. The first of them includes solar water heaters, a coil-heat exchanger of the storage tank, circulation pump and a tube-in-pipe heat exchanger to operate the system in natural circulation in summer. The equipment is connected by a system of pipelines with fittings, instrumentation and automation devices. A two-section coil heat exchanger with a surface area of 4.6 m2 for the heat carrier of the circulation circuit and a single-section heat exchanger with a surface area of 1.2 m2 for the hot water supply system are mounted in a storage tank with a capacity of 16 m3. The heat capacity of the tank with a water temperature of +45 °C provides a three-day heat requirement for a residential building. A tube-in-pipe type heat exchanger with a surface area of 1.25 m2 is located under the ridge of the roof of the house.
The heating circuit consists of two series-connected sections: panel-radiant with flow heating panels that ensure the operation of the system in basic mode with a water temperature difference of 45 ... 35 ° C, and vertically single-pipe with "Comfort" type convectors that provide peak system loads heating with a water temperature difference of 75 ... 70 ° C. Coils of pipes of heating panels are embedded in the plaster-finishing layer of round-hollow panels ceiling. Convectors are installed under the windows. The circulation in the heating system is incentive. Peak water heating is carried out by a flowing electric water heater EPV-2 with a power of 10 kW; It also serves as an understudy for the heating system.
The hot water circuit includes a heat exchanger built into the storage tank and a second instantaneous electric water heater as a closer and backup system.
During the heating period, the heat from the collectors is transferred by the coolant (45% aqueous solution of ethylene glycol) to the water in the storage tank, which is pumped to the coils heating panel and then returns back to the storage tank.
The required air temperature in the house is maintained by the automatic regulator PPT-2 by turning on and off the electric water heater in the convector section of the heating system.
In summer, the system provides for the needs of hot water supply from a heat exchanger of the "pipe in pipe" type with natural circulation of the coolant in the heat-receiving circuit. The transition to incentive circulation is carried out with the help of an electronic differential regulator РРТ-2.
The solar heating system of a four-room residential building in the village. Bucuria of the Moldavian SSR was designed by the Moldgiprograzhdanselstroy Institute under the scientific guidance of KievZNIIEP.
House - attic type. On the first floor there is a common room, a kitchen, a laundry room, an utility room, and on the second floor there are three bedrooms. AT ground floor a garage is located, a cellar is also a room for equipment of a solar heating system. With the house blocked outbuilding, which includes summer kitchen, shower, shed, inventory and workshop.
Autonomous solar heating system (Fig. 4.2) is a combined solar heat pump unit designed to meet the needs of heating (calculated heat loss of the house is 11 kW) and hot water supply throughout the year. The lack of solar heat and heat from the compressor of the heat pump installation is covered by electric heating. The system consists of four circuits: a heat-receiving circulation circuit, circuits of a heat pump installation, heating and hot water supply.
The equipment of the heat-receiving circuit includes solar collectors, a "pipe-in-pipe" heat exchanger and a storage tank with a capacity of 16 m3 with a heat exchanger built into it with a surface area of 6 m2. Solar collectors designed by KyivZNIIEP with double-layer glazing with a total area of 70 m2 are placed in a frame on the southern slope of the roof of the house at an angle of 55° to the horizon. 45 was used as a coolant. % an aqueous solution of ethylene glycol. The heat exchanger is located under the roof ridge, and the rest of the equipment is located in basement at home.
The compressor-condensing refrigeration unit AK1-9 with a heat output of 11.5 kW and a power consumption of 4.5 kW serves as a heat pump unit. The working agent of the heat pump installation is freon-12. Compressor - piston sealless, condenser and evaporator - shell-and-tube with water cooling.
The heating circuit equipment includes a circulation pump, heating devices of the "Comfort" type, and an EPV-2 instantaneous electric water heater as a closer and an understudy. The equipment of the hot water supply circuit includes a capacitive (0.4 m3) STD type water heater with a heat exchanger surface of 0.47 m2 and an end electric heater BAS-10/M 4-04 with a power of 1 kW. Circulation pumps of all circuits are of the TsVTs type, glandless, vertical, low noise, foundationless.
The system works as follows. The coolant transfers heat from the collectors to the water in the storage tank and to the freon in the evaporator heat pump. The vaporous freon after compression in the compressor condenses in the condenser, while heating the water in the heating system and tap water in the hot water system.
In the absence of solar radiation and the heat stored in the storage tank is used up, the heat pump unit is turned off and the heat supply to the house is carried out entirely from electric water heaters (electric boilers). In winter, the heat pump unit is in operation only at a certain level negative temperatures outside air (not lower than -7 °C) in order to prevent freezing of water in the storage tank. In summer, the hot water supply system is provided with heat mainly with the natural circulation of the coolant through a "pipe in pipe" heat exchanger. As a result of different modes operation, a combined solar heat pump plant allows saving heat of about 40 GJ/year (the results of the operation of these plants are given in Chapter 8).
The combination of solar energy and heat pumps was also reflected in the engineering equipment developed by TsNIIEP
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Rice. 4.3. circuit diagram heating systems in Gelendzhik 1 - solar collector; 2 - reheating heat exchanger with heat carrier from the condenser circuit of heat pumps; 3 - reheating heat exchanger with heat carrier from the heating network; 4 - condenser circuit pump; 5 - Heat pump; 6 - evaporator circuit pump; 7 - heat exchanger for heating (cooling) water in the evaporator (condenser) circuit; 8 - Heat exchanger for heating the source (raw) water; 9 - hot water pump; 10 - Battery tanks; 11 - solar circuit heat exchanger; 12 - solar circuit pump |
Heat supply project for the hotel complex "Privetlivy Bereg" in Gelendzhik (Fig. 4.3).
The basis of the solar heat pump installation is: flat solar collectors with a total area of 690 m2 and three serially produced refrigeration machines MKT 220-2-0 operating in heat pump mode. Estimated annual heat generation is about 21,000 GJ, including solar installation- 1470 GJ.
Sea water serves as a low-grade heat source for heat pumps. To ensure a corrosion-free and scale-free operation of the heating surfaces of collectors, pipelines and condensers, they are filled with softened and deaerated water from the heating network. Compared with traditional scheme heat supply from the boiler house attraction of non-traditional heat sources -
Sun and sea water, allows you to save about 500 tons of conventional units. fuel / year.
Another characteristic example of the use of new energy sources is the project of heat supply of a manor house with the help of
Solar heat pump installation. The project provides for year-round full satisfaction of the needs of heating and hot water supply of a mansard-type mansard house with a living area of 55 m2. The ground serves as a low-potential source of heat for the heat pump. The estimated economic effect from the introduction of the system is at least 300 rubles. per apartment compared to the traditional option of heat supply from a solid fuel apparatus.
Solar heating is a way of heating a residential building, which is becoming more and more popular every day in many, mostly developed, countries of the world. The greatest successes in the field of solar thermal energy today can be boasted in the countries of Western and central Europe. On the territory of the European Union over the past decade, there has been an annual growth in the renewable energy industry by 10-12%. This level of development is a very significant indicator.
solar collector
One of the most obvious applications of solar energy is its use for heating water and air (as heat carriers). In climatic regions where cold weather prevails, for comfortable living people are required to calculate and organize heating systems for each residential building. They should have hot water supply for various needs, besides, houses need to be heated. Of course, the best option here will be the application of the scheme where they work automated systems heat supply.
Large volumes of daily income hot water in the production process require industrial enterprises. An example is Australia, where almost 20 percent of all consumed energy is expended on heating a heat transfer fluid to a temperature not exceeding 100 o C. For this reason, in part of the developed countries of the West, and to a greater extent in Israel, North America, Japan and, of course, in Australia, the expansion of the production of solar heating systems is very fast.
In the near future, the development of energy will undoubtedly be directed in favor of the use of solar radiation. The density of solar radiation on the earth's surface is on average 250 watts per square meter. And this despite the fact that two watts per square meter is enough to meet the economic needs of a person in the least industrial areas.
The advantageous difference between solar energy and other energy industries that use fossil fuel combustion processes is the environmental friendliness of the energy received. The operation of solar equipment does not entail the release of harmful emissions into the atmosphere.
Selection of equipment application scheme, passive and active systems
There are two schemes for using solar radiation as a heating system for a home. These are active and passive systems. Passive solar heating systems - those in which the directly absorbing element solar radiation and the heat that forms from it is the structure of the house itself or its individual parts. These elements can be a fence, a roof, separate parts of a building built on the basis of a certain scheme. Passive systems do not use mechanical moving parts.
Active systems operate on the basis of the opposite home heating scheme, they actively use mechanical devices(pumps, motors, when using them, also calculate the required power).
The simplest in design and less costly in financial plan when mounting circuits are systems of passive action. Such heating circuits do not require the installation of additional devices for the absorption and subsequent distribution of solar radiation in the home heating system. The operation of such systems is based on the principle direct heating living space directly through the light-transmitting walls located on the south side. Optional feature heating is carried out by the outer surfaces of the house fencing elements, which are equipped with a layer of transparent screens.
To start the process of converting solar radiation into thermal energy they use a system of structures based on the use of solar receivers with a transparent surface, where the main function is played by the "greenhouse effect", the glass's ability to retain thermal radiation is used, due to which the temperature inside the room is increased.
It should be noted that the use of only one of the types of systems may not be entirely justified. Often a careful calculation shows that a significant reduction in heat loss and a reduction in the energy needs of a building can be achieved through the use of integrated systems. General work both active and passive systems by combining positive qualities will give the maximum effect.
A commonly used efficiency calculation shows that passive use of solar radiation will provide approximately 14 to 16 percent of your home's heating needs. Such a system will be an important part of the heat generation process.
However, despite certain positive traits passive systems, the main possibilities for fully meeting the needs of the building in heat, it is still necessary to use active heating equipment. Systems whose function is directly absorption, accumulation and distribution of solar radiation.
Planning and calculation
Calculate the possibility of installing active heating systems using solar energy(crystalline solar cells, solar collectors), preferably at the design stage of the building. But still, this moment is not mandatory, the installation of such a system is also possible on an existing task, regardless of the year of its construction (the basis for success is the correct calculation of the entire scheme).
Installation of equipment is carried out on the south side of the house. This location creates conditions for maximum absorption of incoming solar radiation in winter. Photocells that convert the energy of the sun and are installed on a fixed structure are most effective when they are mounted relative to the earth's surface at an angle equal to the geographical location of the heated building. The angle of the roof, the degree of turn of the house to the south - these are significant points that must be taken into account when calculating the entire heating scheme.
Solar photocells and solar collectors must be installed as close as possible to the place of energy consumption. Remember that the closer you build a bathroom and a kitchen, the less heat loss will be (in this case, you can get by with one solar collector that will heat both rooms). The main criterion for evaluating the selection of the equipment you need is its efficiency.
Heating solar systems active action are divided into the following groups according to the following criteria:
- The use of a backup circuit;
- Seasonality of work (throughout the whole year or in a certain season);
- Functional purpose - heating, supply hot water and combined systems;
- The heat carrier used is liquid or air;
- Applied technical solution for the number of circuits (1, 2 or more).
General economic data will serve as the main factor in choosing one of the types of equipment. A competent thermal calculation of the entire system will help you decide correctly. The calculation must be carried out taking into account the indicators of each specific premises where the organization of solar heating and (or) hot water supply is planned. It is necessary to take into account the location of the building, climatic natural conditions, the size of the cost of the displaced energy resource. correct calculation and good choice heat supply organization schemes are the key to the economic feasibility of using solar energy equipment.
Solar heating system
The most common heating scheme used is the installation of solar collectors, which provide for the accumulation of absorbed energy in a special container - a battery.
To date, the most widespread are double-circuit heating schemes for residential premises, in which forced system coolant circulation in the collector. The principle of its work is the following. Hot water is supplied from top point storage tank, the process occurs automatically according to the laws of physics. Cold running water is pumped into lower part tank, this water displaces the heated water collected in the upper part of the tank, which then enters the hot water supply system of the house to meet its household and heating needs.
For a single-family house, a storage tank with a capacity of 400 to 800 liters is usually installed. To heat up the heat carrier of such volumes, depending on natural conditions it is required to correctly calculate the surface area of the solar collector. It is also necessary to justify the use of equipment economically.
Standard set of mounting hardware heating system on solar radiation the following:
- Directly the solar collector itself;
- Mounting system (supports, beams, holders);
- storage tank;
- Tank compensating for excess expansion of the thermal carrier;
- Pump control device;
- Pump (set of valves);
- Temperature sensors;
- Heat exchange devices (used in schemes with large volumes);
- Heat-insulated pipes;
- Safety and control fittings;
- Fitting.
System based on heat-absorbing panels. Such panels, as a rule, are used at the stage of new construction. For their installation, it is necessary to build a special structure called a hot roof. This means that the panels must be installed directly into the roof structure, while using the roof elements as constituent elements equipment cases. Such an installation will reduce your costs for creating a heating system, however, it will require high-quality work on waterproofing the joints of devices and the roof. This way of installing equipment will require you to carefully design and plan all stages of work. It is necessary to solve many problems related to piping, placement of a storage tank, installation of a pump, adjustment of slopes. Quite a lot of installation problems will have to be solved if the building is not turned to the south in the most successful way.
In general, the project solar systems heating will be different from others in varying degrees. Only remain unchanged basic principles systems. Therefore, to provide an accurate list of the necessary parts for complete installation the entire system is impossible, since during the installation process it may be necessary to use additional elements and materials.
Liquid heating systems
In systems operating on the basis of a liquid heat carrier, ordinary water is used as a storage medium. Energy absorption takes place in flat solar collectors. Energy is stored in a storage tank and used as needed.
To transfer energy from the storage device to the building, a water-to-water or water-to-air heat exchanger is used. The hot water supply system is equipped with an additional tank, which is called the preheating tank. Water is heated in it due to solar radiation and then enters a conventional water heater. 
Air heating system
Such a system uses air as a heat carrier. The coolant is heated in a flat solar collector, and then the heated air enters the heated room or into a special storage device, where the absorbed energy is stored in a special nozzle, which is heated by the incoming hot air. Thanks to this feature, the system continues to supply the house with heat even at night when solar radiation not available.
Systems with forced and natural circulation
The basis of the operation of systems with natural circulation is independent movement coolant. Under the influence of rising temperature, it loses density and therefore tends to upper part devices. The resulting difference in pressure makes the equipment function.
On average throughout the year, depending on climatic conditions and latitude, the flux of solar radiation to the earth's surface ranges from 100 to 250 W / m 2, reaching peak values at noon at clear sky, in almost any (regardless of latitude) place, about 1,000 W / m 2. In conditions middle lane In Russia, solar radiation "brings" to the surface of the earth energy equivalent to about 100-150 kg reference fuel per m2 per year.
Mathematical modeling of the simplest solar water heating installation, carried out at the Institute high temperatures of the Russian Academy of Sciences, using modern software tools and typical weather data, showed that in real climatic conditions In central Russia, it is advisable to use seasonal flat solar water heaters operating from March to September. For an installation with a ratio of the solar collector area to the storage tank volume of 2 m 2 /100 l, the probability of daily water heating during this period to a temperature of at least 37 ° C is 50-90%, to a temperature of at least 45 ° C - 30- 70%, up to a temperature of at least 55 ° C - 20-60%. Maximum values the probabilities refer to the summer months.
"Your Solar House" develops, completes and delivers both with passive and active coolant circulation. Description of these systems can be found in the relevant sections of our website. Order and purchase is carried out through.
The question is often asked whether it is possible to use solar heating installations for heating in Russia. A separate article has been written about this - “Solar support for heating”
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Prepared by students of the B3TPEN31 Group
Solar heating systems are systems that use solar radiation as a source of thermal energy. Their characteristic difference from other systems low temperature heating is the application 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 passive, in which the building itself or its individual fences (collector building, collector wall, collector roof, etc.) serve as an element that receives solar radiation and converts it into heat.
Passive low-temperature solar heating system "collector wall": 1 - sun rays; 2 – translucent screen; 3 - air damper; 4 - heated air; 5 - cooled air from the room; 6 - own long-wave thermal radiation of the wall array; 7 - black ray-receiving surface of the wall; 8 - blinds.
Active
Solar low-temperature heating systems are called active, in which the solar receiver is an independent separate device that is not related to the building. Active solar systems can be subdivided:
by purpose (hot water supply, heating systems, combined systems for heat and cold supply);
by type of coolant used (liquid - water, antifreeze and air);
by duration of work (year-round, seasonal);
on technical solution schemes (one-, two-, multi-loop).
Classification of solar heating systems
can be classified according to various criteria:
by appointment:
1. hot water supply systems (DHW);
2. heating systems;
3. combined systems;
Type of coolant used:
1. liquid;
2. air;
By duration of work:
1. year-round;
2. seasonal;
According to the technical solution of the scheme:
1. single-circuit;
2. double-circuit;
3. multi-circuit.
Air is a widely used coolant that does not freeze over the entire range of operating parameters. When used as a heat carrier, it is possible to combine heating systems with a ventilation system. However, air is a low-heat-capacity coolant, which leads to an increase in metal consumption for the installation of systems air heating compared to water systems.
Water is a heat-intensive and widely available coolant. However, at temperatures below 0°C it is necessary to add antifreeze liquids. In addition, it must be taken into account that water saturated with oxygen causes corrosion of pipelines and apparatus. But the consumption of metal in water solar systems is much lower, which to a large extent contributes to their wider use.
Seasonal hot water solar systems are usually single-circuit and operate in the summer and transitional months, during periods with a positive outside temperature. They may have an additional source of heat or do without it, depending on the purpose of the serviced object and operating conditions.
Solar systems for heating buildings are usually double-circuit or, most often, multi-circuit, and different heat carriers can be used for different circuits (for example, aqueous solutions of antifreeze liquids in a solar circuit, water in intermediate circuits, and air in a consumer circuit).
Combined year-round solar systems for the purpose of heat and cold supply of buildings are multi-circuit and include an additional source of heat in the form of a traditional heat generator running on organic fuel or a heat transformer.
A schematic diagram of a solar heating system is shown in Figure 4.1.2. It includes three circulation circuits:
the first circuit, consisting of solar collectors 1, circulation pump 8 and liquid heat exchanger 3;
the second circuit, consisting of a storage tank 2, a circulation pump 8 and a heat exchanger 3;
the third circuit, consisting of a storage tank 2, a circulation pump 8, a water-air heat exchanger (heater) 5.
Schematic diagram of the solar heating system: 1 - solar collector; 2 - storage tank; 3 - heat exchanger; 4 - building; 5 - heater; 6 - understudy of the heating system; 7 - backup system of hot water supply; 8 - circulation pump; 9 - fan.
Functioning
The solar heating system operates as follows. The coolant (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 annular space of the heat exchanger 3 under the action of the pump 8 of the secondary circuit. Heated water enters the storage tank 2. Water is taken from the storage tank by the hot water supply pump 8, brought, if necessary, to the required temperature in the doubler 7 and enters the hot water supply system of the building. The storage tank is fed 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 by means of a fan 9 and, having heated up, enters the building 4. In the absence of solar radiation or a shortage of thermal energy generated by solar collectors, the work turn on backup 6.
The choice and layout of elements of the solar heat supply system in each case is determined by climatic factors, the purpose of the facility, the mode of heat consumption, and economic indicators.
Schematic diagram of a single-loop thermosyphon solar hot water system
A feature of the systems is that in the case of a thermosyphon system, the lower point of the storage tank should be located above the upper point of the collector and no further than 3-4 m from the collectors, and with pump circulation of the coolant, the location of the storage tank can be arbitrary.
1. Solar collectors.
The solar collector is the main element of the installation, in which the radiation energy of the Sun is converted into another form of useful energy. Unlike conventional heat exchangers, in which there is intense heat transfer 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 sunlight, the flux density of the incident radiation is at best -1100 W/m 2 and is a variable value. The wavelengths are in the range of 0.3 - 3.0 µm. They are much smaller than the intrinsic wavelengths of most absorbing surfaces. Thus, the study of solar collectors is associated with unique heat transfer problems at low and variable energy flux densities and relatively big role radiation.
Solar collectors can be used both with and without concentration of solar radiation. In flat-plate collectors, the surface that receives solar radiation is also the surface that absorbs radiation. Focusing collectors, usually having concave reflectors, concentrate the 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 due to solar radiation energy.
Flat-plate 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 need daily maintenance. Structurally, they are simpler than a system consisting of concentrating reflectors, absorbing surfaces and tracking mechanisms. Scope of solar collectors - heating systems of 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); coatings that are transparent with respect to solar radiation, located above the absorbing surface, which reduce convective and radiative losses to the atmosphere; thermal insulation of the reverse 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 - isolation; 3 - pipe with coolant; 4 - absorbing surface;
b) 1. surface that absorbs solar radiation, 2-channels of the coolant, 3-glass (??), 4-body,
5- thermal insulation.
Fig.2 Solar collector of sheet-pipe type.
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 - isolation.
1.2. Collector efficiency. The efficiency of a collector is determined by its optical and thermal efficiency. The optical efficiency ηо shows what part of the solar radiation that has reached the glazing surface of the collector is absorbed by the absorbing black surface, and takes into account the energy losses associated with the difference from unity of the glass transmittance and the absorption coefficient of the absorbing surface. For manifold with single glazing
where (τα) n is the product of glass transmittance τ and absorption coefficient α 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, taking into account the increase in reflection losses from the glass and the surface that absorbs solar radiation. On fig. 3 shows 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, which is different from the direct one,
Rice. 3. Correction factor for the reflection of sunlight from the glass surface and the black absorbent surface.
In addition to these losses in the collector of any design, there are heat losses in 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 over a certain time to the amount of radiation energy coming to it from the Sun over the same time:
where Ω is the collector aperture area; I - solar radiation flux density.
The optical and thermal efficiencies of a collector are related by the relation
Heat losses are characterized by total loss coefficient U
where T a is the temperature of the black surface that absorbs solar radiation; T about - ambient temperature.
The value of U can be considered constant with sufficient accuracy for calculations. In this case, substituting Qpot into 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 \u003d (T in + T out) / 2 - the average temperature of the coolant; F" - a parameter commonly 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 is almost independent of 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 in which the black surface that absorbs solar radiation is located is separated from the environment by a vacuum space, which makes it possible to significantly reduce heat losses to the environment due to heat conduction and convection. Radiation loss is largely suppressed by the use of a selective coating. 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. On fig. 9.10 shows examples of the design 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 coolant; 7 outer glass bottle; 8 vacuum
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