Solar energy as an alternative energy source: types and features of solar systems

In the last decade, solar energy as an alternative energy source has been increasingly used for heating and providing hot water to buildings. The main reason is the desire to replace traditional fuel with affordable, environmentally friendly and renewable energy resources.

The conversion of solar energy into thermal energy occurs in solar systems - the design and principle of operation of the module determines the specifics of its application. In this material we will look at the types of solar collectors and the principles of their operation, and also talk about popular models of solar modules.

The feasibility of using a solar system

A solar system is a complex for converting solar radiation energy into heat, which is subsequently transferred to a heat exchanger to heat the coolant of a heating or water supply system.

The efficiency of a solar thermal installation depends on solar insolation - the amount of energy received during one daylight hours per 1 square meter of surface located at an angle of 90° relative to the direction of the sun's rays. The measuring value of the indicator is kW*h/sq.m, the value of the parameter varies depending on the season.

The average level of solar insolation for a region with a temperate continental climate is 1000-1200 kWh/sq.m (per year). The amount of sun is the determining parameter for calculating the performance of a solar system.

The use of an alternative energy source allows you to heat a house and obtain hot water without traditional energy costs - exclusively through solar radiation

Installing a solar heating system is an expensive undertaking. In order for capital costs to be justified, an accurate calculation of the system and compliance with installation technology are necessary.

Example. The average value of solar insolation for Tula in mid-summer is 4.67 kV/sq.m*day, provided the system panel is installed at an angle of 50°. The productivity of a solar collector with an area of ​​5 sq.m is calculated as follows: 4.67*4=18.68 kW of heat energy per day. This volume is enough to heat 500 liters of water from 17 °C to 45 °C.

As practice shows, when using a solar power plant, cottage owners in the summer can completely switch from electric or gas water heating to the solar method

Speaking about the feasibility of introducing new technologies, it is important to take into account the technical features of a particular solar collector. Some start working at 80 W/sq.m of solar energy, while others need 20 W/sq.m.

Even in a southern climate, using a collector system solely for heating will not pay off. If the installation is used exclusively in winter when there is a shortage of sun, then the cost of the equipment will not be covered even in 15-20 years.

To use the solar complex as efficiently as possible, it must be included in the hot water supply system. Even in winter, the solar collector will allow you to “cut” energy bills for water heating by up to 40-50%.

According to experts, for domestic use, a solar system pays for itself in approximately 5 years. With rising prices for electricity and gas, the payback period of the complex will be reduced

In addition to economic benefits, solar heating has additional advantages:

1. Environmental friendliness. Carbon dioxide emissions are reduced. Over the course of a year, 1 sq.m of solar collector prevents 350-730 kg of waste from entering the atmosphere.
2. Aesthetics. The space of a compact bath or kitchen can be eliminated from bulky boilers or geysers.
3. Durability. Manufacturers assure that if installation technology is followed, the complex will last about 25-30 years. Many companies provide a warranty of up to 3 years.

Arguments against using solar energy: pronounced seasonality, dependence on weather and high initial investment.

General structure and principle of operation

Let's consider the option of a solar system with a collector as the main working element of the system. The appearance of the unit resembles a metal box, the front side of which is made of tempered glass. Inside the box there is a working element - a coil with an absorber.

The heat-absorbing unit provides heating of the coolant - circulating liquid, transfers the generated heat to the water supply circuit.

The main components of the solar system: 1 – collector field, 2 – air vent, 3 – distribution station, 4 – excess pressure relief tank, 5 – controller, 6 – water heater tank, 7.8 – heating element and heat exchanger, 9 – thermal mixing valve, 10 – hot water flow, 11 – cold water inlet, 12 – drain, T1/T2 – temperature sensors

The solar collector necessarily works in tandem with the storage tank. Since the coolant heats up to a temperature of 90-130°C, it cannot be supplied directly to hot water taps or heating radiators. The coolant enters the boiler heat exchanger. The storage tank is often supplemented with an electric heater.

Scheme of work:

1. The sun heats the surface collector.
2. Thermal radiation is transferred to the absorbing element (absorber), which contains the working fluid.
3. The coolant circulating through the coil tubes heats up.
4. Pumping equipment, a control and monitoring unit ensures the removal of coolant through a pipeline to the coil of the storage tank.
5. Heat is transferred to the water in the boiler.
6. The cooled coolant flows back into the collector and the cycle repeats.

Heated water from the water heater is supplied to the heating circuit or to water intake points.

When installing a heating system or year-round hot water supply, the system is equipped with a source of additional heating (boiler, electric heating element). This is a necessary condition for maintaining the set temperature

Solar panels in private homes are most often used as a backup source of electricity:

Types of solar collectors

Regardless of the purpose, the solar system is equipped with a flat or spherical tubular solar collector. Each option has a number of distinctive features in terms of technical characteristics and operational efficiency.

Vacuum – for cold and temperate climates

Structurally, a vacuum solar collector resembles a thermos - narrow tubes with coolant are placed in flasks of larger diameter. A vacuum layer is formed between the vessels, which is responsible for thermal insulation (heat retention is up to 95%). The tubular shape is most optimal for maintaining vacuum and “occupying” the sun’s rays.

Basic elements of a tubular solar thermal installation: support frame, heat exchanger housing, vacuum glass tubes treated with a highly selective coating for intensive “absorption” of solar energy

The inner (heat) tube is filled with a saline solution with a low boiling point (24-25 ° C). When heated, the liquid evaporates - the vapor rises to the top of the flask and heats the coolant circulating in the collector body.

During the condensation process, drops of water flow into the tip of the tube and the process repeats.

Thanks to the presence of a vacuum layer, the liquid inside the thermal flask is able to boil and evaporate at sub-zero street temperatures (down to -35 ° C).

The characteristics of solar modules depend on the following criteria:

• tube design – feather, coaxial;
• thermal channel device – "Heat pipe", direct-flow circulation.

Feather flask - a glass tube containing a plate absorber and a heat channel. The vacuum layer passes through the entire length of the thermal channel.

Coaxial tube – a double flask with a vacuum “insert” between the walls of two tanks. Heat transfer occurs from the inner surface of the tube. The tip of the thermotube is equipped with a vacuum indicator.

The efficiency of feather tubes (1) is higher compared to coaxial models (2). However, the former are more expensive and more difficult to install. In addition, in case of breakdown, the feather flask will have to be replaced entirely

The “Heat pipe” channel is the most common option for heat transfer in solar collectors.

The mechanism of action is based on placing an easily evaporating liquid in sealed metal tubes.

The popularity of “Heat pipe” is due to its affordable cost, ease of maintenance and maintainability. Due to the complexity of the heat exchange process, the maximum efficiency level is 65%

Direct flow channel – parallel metal tubes connected in a U-shaped arc pass through the glass flask

The coolant flowing through the channel is heated and supplied to the collector body.

Vacuum solar collector design options: 1 – modification with a heating central tube “Heat pipe”, 2 – solar installation with direct-flow coolant circulation

Coaxial and feather tubes can be combined with heat channels in different ways.

Option 1. A coaxial flask with “Heat pipe” is the most popular solution. In the collector, repeated heat transfer occurs from the walls of the glass tube to the inner flask, and then to the coolant. The degree of optical efficiency reaches 65%.

Diagram of the design of a coaxial tube “Heat pipe”: 1 – glass shell, 2 – selective coating, 3 – metal fins, 4 – vacuum, 5 – thermal flask with an easy-boiling substance, 6 – inner glass tube

Option 2. A coaxial flask with direct circulation is known as a U-shaped manifold. Thanks to the design, heat loss is reduced - thermal energy from aluminum is transferred to tubes with circulating coolant.

Along with high efficiency (up to 75%), the model has disadvantages:

• complexity of installation - the flasks are integral with the two-pipe manifold body (mainfold) and are installed entirely;
• replacement of single tubes is excluded.

In addition, the U-shaped unit is demanding on coolant and is more expensive than “Heat pipe” models.

Structure of a U-shaped solar collector: 1 – glass “cylinder”, 2 – absorbent coating, 3 – aluminum “case”, 4 – flask with coolant, 5 – vacuum, 6 – inner glass tube

Option 3. Feather pipe with the “Heat pipe” operating principle. Distinctive features of the collector:

• high optical characteristics - efficiency of about 77%;
• the flat absorber directly transfers heat energy to the coolant tube;
• due to the use of one layer of glass, the reflection of solar radiation is reduced;

It is possible to replace a damaged element without draining the coolant from the solar system.

Option 4. A direct-flow feather bulb is the most effective tool for using solar energy as an alternative energy source for heating water or heating a home. The high-performance collector operates with an efficiency of 80%. The disadvantage of the system is the difficulty of repair.

Design diagrams for feather solar collectors: 1 – solar system with a “Heat pipe” channel, 2 – two-pipe solar collector housing with direct flow of coolant

Regardless of the design, tubular collectors have the following advantages:

• performance at low temperatures;
• low heat losses;
• duration of operation during the day;
• the ability to heat the coolant to high temperatures;
• low windage;
• ease of installation.

The main disadvantage of vacuum models is the inability to self-clean from snow cover. The vacuum layer does not allow heat to pass out, so the layer of snow does not melt and blocks the sun’s access to the collector field. Additional disadvantages: high price and the need to maintain a working angle of inclination of the flasks of at least 20°.

Collector solar devices that heat the air coolant can be used in the preparation of hot water if they are equipped with a storage tank:

Read more about the operating principle of a vacuum solar collector with tubes Further.

Vodyanoy – the best option for southern latitudes

A flat (panel) solar collector is a rectangular aluminum plate covered on top with a plastic or glass lid. Inside the box there is an absorption field, a metal coil and a layer of thermal insulation. The collector area is filled with a flow pipeline through which the coolant moves.

The basic components of a flat solar collector: housing, absorber, protective coating, thermal insulation layer and fasteners. During assembly, frosted glass with a transmittance of the spectral range of 0.4-1.8 microns is used

The heat absorption of the highly selective absorbent coating reaches 90%. A flowing metal pipeline is placed between the “absorber” and the thermal insulation. Two tube laying schemes are used: “harp” and “meander”.

The process of assembling solar collectors that heat the coolant liquid includes a number of traditional steps:

If the heating circuit is supplemented with a line supplying sanitary water to the hot water supply, it makes sense to connect a heat accumulator to the solar collector. The simplest option would be a tank of a suitable container with thermal insulation that can maintain the temperature of the heated water. You need to install it on the overpass:

A tubular collector with a liquid coolant acts as a “greenhouse” effect - the sun's rays penetrate through the glass and warm up the pipeline. Thanks to tightness and thermal insulation, heat is retained inside the panel.

The strength of the solar module is largely determined by the material of the protective cover:

• ordinary glass – the cheapest and most fragile coating;
• strained glass – high degree of light dispersion and increased strength;
• anti-reflective glass – characterized by maximum absorption capacity (95%) due to the presence of a layer that eliminates the reflection of the sun’s rays;
• self-cleaning (polar) glass with titanium dioxide – organic contaminants burn out in the sun, and the remaining debris is washed away by rain.

Polycarbonate glass is the most impact resistant. The material is installed in expensive models.

Reflection of sunlight and absorption capacity: 1 – anti-reflex coating, 2 – tempered impact-resistant glass. The optimal thickness of the protective outer shell is 4 mm

Operational and functional features of panel solar installations:

• forced circulation systems have a defrosting function that allows you to quickly get rid of snow cover on the heliofield;
• prismatic glass captures a wide range of rays at different angles - in summer, the installation efficiency reaches 78-80%;
• the collector is not afraid of overheating - if there is an excess of thermal energy, forced cooling of the coolant is possible;
• increased impact resistance compared to tubular counterparts;
• Possibility of installation at any angle;
• affordable pricing policy.

The systems are not without shortcomings. During periods of solar radiation deficiency, as the temperature difference increases, the efficiency of a flat-plate solar collector drops significantly due to insufficient thermal insulation. Therefore, the panel module is justified in the summer or in regions with a warm climate.

Solar systems: design and operation features

The variety of solar systems can be classified according to the following parameters: method of using solar radiation, method of coolant circulation, number of circuits and seasonality of operation.

Active and passive complex

Any solar energy conversion system has a solar receiver. Based on the method of using the received heat, two types of solar complexes are distinguished: passive and active.

The first type is a solar heating system, where the structural elements of the building act as the heat-absorbing element of solar radiation. The roof, collector wall or windows act as a solar receiving surface.

Scheme of a low-temperature passive solar system with a collector wall: 1 - sun rays, 2 - translucent screen, 3 - air barrier, 4 - heated air, 5 - exhaust air flows, 6 - thermal radiation from the wall, 7 - heat-absorbing surface of the collector wall, 8 – decorative blinds

In European countries, passive technologies are used in the construction of energy-efficient buildings. Solar receiving surfaces are decorated as false windows. Behind the glass covering there is a blackened brick wall with light openings.

The elements of the structure - walls and ceilings, insulated with polystyrene from the outside - act as heat accumulators.

Active systems imply the use of independent devices not related to the structure.

This category includes the above-mentioned complexes with tubular, flat-plate collectors - solar thermal installations are usually located on the roof of the building

Thermosiphon and circulation systems

Solar thermal equipment with natural movement of the coolant along the collector-accumulator-collector circuit is carried out due to convection - warm liquid with low density rises upward, cooled liquid flows down.

In thermosiphon systems, the storage tank is located above the collector, ensuring spontaneous circulation of the coolant.

The operating scheme is typical for single-circuit seasonal systems. The thermosiphon complex is not recommended for use for collectors with an area of ​​more than 12 sq.m.

A non-pressure solar system has a wide range of disadvantages:

• on cloudy days, the performance of the complex drops - a large temperature difference is required for the coolant to move;
• heat losses due to the slow movement of liquid;
• the risk of tank overheating due to uncontrollability of the heating process;
• instability of the collector;
• difficulty in placing the storage tank - when installed on the roof, heat loss increases, corrosion processes accelerate, and there is a risk of pipes freezing.

The advantages of the “gravity” system: simplicity of design and affordability.

The capital costs of installing a circulation (forced) solar system are significantly higher than installing a free-flow complex. A pump “cuts” into the circuit, ensuring the movement of the coolant. The operation of the pumping station is controlled by a controller.

The additional thermal power generated in the forced-air complex exceeds the power consumed by the pumping equipment. System efficiency will increase by a third

This circulation method is used in year-round double-circuit solar thermal installations.

Advantages of a fully functional complex:

• unlimited choice of storage tank location;
• performance out of season;
• selection of optimal heating mode;
• safety – blocking of operation in case of overheating.

The disadvantage of the system is its dependence on electricity.

Technical solution of circuits: single- and double-circuit

In single-circuit installations, liquid circulates, which is subsequently supplied to water intake points. In winter, water from the system must be drained to prevent freezing and cracking of pipes.

Features of single-circuit solar thermal complexes:

• it is recommended to “fill” the system with purified, soft water - the deposition of salts on the walls of the pipes leads to clogging of the channels and breakdown of the collector;
• corrosion due to excess air in water;
• limited service life - within four to five years;
• high efficiency in summer.

In double-circuit solar complexes, a special coolant circulates (non-freezing liquid with anti-foaming and anti-corrosion additives), which transfers heat to the water through a heat exchanger.

Schemes of the design of a single-circuit (1) and double-circuit (2) solar system. The second option is characterized by increased reliability, the ability to work in winter and long service life (20-50 years)

The nuances of operating a dual-circuit module: a slight decrease in efficiency (3-5% less than in a single-circuit system), the need to completely replace the coolant every 7 years.

Conditions for work and efficiency improvement

It is better to entrust the calculation and installation of a solar system to professionals. Compliance with the installation technique will ensure operability and achievement of the declared performance. To improve efficiency and service life, it is necessary to take into account some nuances.

Thermostatic valve. In traditional heating systems thermostatic element rarely installed, since the heat generator is responsible for regulating the temperature. However, when installing a solar system, one should not forget about the safety valve.

Heating the tank to the maximum permissible temperature increases the performance of the collector and allows you to use solar heat even in cloudy weather

The optimal placement of the valve is 60 cm from the heater. When placed close, the “thermostat” heats up and blocks the supply of hot water.

Placement of the storage tank. The DHW buffer tank must be installed in an accessible location. When placed in a compact room, special attention is paid to the height of the ceilings.

The minimum free space above the tank is 60 cm. This gap is necessary for servicing the battery and replacing the magnesium anode

Installation expansion tank. The element compensates for thermal expansion during periods of stagnation. Installing the tank above the pumping equipment will cause overheating of the membrane and its premature wear.

The optimal place for the expansion tank is under the pump group. The temperature effect during this installation is significantly reduced, and the membrane retains its elasticity longer.

Solar circuit connection. When connecting pipes, it is recommended to organize a loop. The thermal loop reduces heat loss by preventing the release of heated liquid.

A technically correct option for implementing a “loop” of a solar circuit. Neglecting this requirement causes the temperature in the storage tank to drop by 1-2°C overnight

Check valve. Prevents “overturning” of coolant circulation. With a lack of solar activity check valve prevents the heat accumulated during the day from dissipating.

Popular models of solar modules

Solar systems from domestic and foreign companies are in demand. Products from manufacturers have won a good reputation: NPO Mashinostroeniya (Russia), Gelion (Russia), Ariston (Italy), Alten (Ukraine), Viessman (Germany), Amcor (Israel), etc.

Solar system "Falcon". Flat solar collector equipped with a multilayer optical coating with magnetron sputtering. The minimum emission capacity and high absorption level provide an efficiency of up to 80%.

Performance characteristics:

• operating temperature – up to -21 °C;
• reverse heat radiation – 3-5%;
• top layer – tempered glass (4 mm).

Collector SVK-A (Alten). Vacuum solar installation with an absorption area of ​​0.8-2.41 sq.m (depending on the model). The coolant is propylene glycol, the thermal insulation of a 75 mm copper heat exchanger minimizes heat loss.

Extra options:

• body – anodized aluminum;
• heat exchanger diameter – 38 mm;
• insulation – mineral wool with anti-hygroscopic treatment;
• coating – borosilicate glass 3.3 mm;
• Efficiency – 98%.

Vitosol 100-F is a flat solar collector for horizontal or vertical installation. Copper absorber with harp-shaped tubular coil and helio-titanium coating. Light transmission – 81%.

Approximate prices for solar systems: flat solar collectors – from 400 USD/sq.m, tubular solar collectors – 350 USD/10 vacuum flasks. Complete set of circulation system – from 2500 USD

Conclusions and useful video on the topic

The operating principle of solar collectors and their types:

Assessing the performance of a flat-plate collector at sub-zero temperatures:

Installation technology of a panel solar collector using the example of the Buderus model:

Solar energy is a renewable source of heat. Taking into account the rising prices for traditional energy resources, the implementation of solar systems justifies capital investments and pays off in the next five years if installation techniques are followed.

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