Solar thermal collector

Solar thermal collectors heating liquid

Solar collectors are either non-concentrating or concentrating. In the non-concentrating type, the collector area (ie, the area that intercepts the solar radiation) is the same as the absorbing area (ie, the area absorbing radiation). In these types the solar panel absorbs light. Concentrating collectors have a bigger interceptor than absorb. ^[1]

Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller.

Flat plate collectors

Flat-plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type. They consist of (1) a dark flat-plate absorber, (2) a transparent cover that reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorb, and (4) a heat insulating backing. The composition of a thin absorbing sheet (of thermally stable polymers, aluminum, steel or copper , to which a matte black or selective coating is applied) polycarbonate cover. In water heat panels, fluid is usually circulated through a tubing to transfer heat from an absorbed to an insulated water tank. This can be achieved directly through a heat exchanger .

Most air heat fabricators and some water heat manufacturers have a completely flooded absorption of two sheets of metal which the fluid passes between. Because they are much more efficient than traditional absorbers. ^[3]Sunlight passes through the glazing and strikes the absorbing plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes. Absorber plates are commonly painted with “selective coatings”, which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal-typically copper or aluminum-because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. See: Copper in solar water heaters . In locations with average solar energy, flat flat collectors are one-half to one square foot per gallon of one day’s hot water use. Absorb piping configurations include:

• harp – design design traditional – traditional traditional,,,,,,,,,,,,,,,,,,,,;
• serpentine – one continuous S maximizes the temperature and the rate of total energy yield in variable flow systems;
• flooded absorption of two sheets of metal stamped to produce a circulation zone;
• boundary layer absorptive collectors layers of transparent layers and opaque sheets that enable absorption in a boundary layer. Because the energy is absorbed in the boundary layer, it may be more efficient than a collector where it is absorbed by a material before the heat is accumulated in a circulating liquid. ^{[ quote needed ]}

Polymer flat collectors are an alternative to metal collectors and are produced in Europe. These may be wholly polymer , or they may include metal plates in front of freeze-tolerant water channels made of silicone rubber . Polymers are flexible and therefore freeze-tolerant and can be used instead of antifreeze, so that they can be used by existing water tanks rather than heat exchangers.that lower efficiency. By dispensing with a heat exchanger, such direct circulation panels, such polymer or otherwise, may be more efficient, particularly at low light levels. Some early selectively coated polymer collectors suffer from overheating when insulated, as stagnation temperatures can exceed the polymer’s melting point. ^[4] [5] For example, the melting point of polypropyleneis 160 ° C (320 ° F), while the stagnation temperature of insulated thermal collectors can exceed 180 ° C (356 ° F) if control strategies are not used. For this reason polypropylene is not often used in glazing selectively coated solar collectors. Increasingly high temperature silicones (which melt at over 250 ° C (482 ° F)) are being used. Some non-polypropylene polymer based glazed solar collectors are matte black coated rather than selectively coated to reduce stagnation temperature to 150 ° C (302 ° F) or less.

In areas where freezing is a possibility, freeze-tolerance (the ability to freeze repeatedly without cracking) can be achieved by the use of flexible polymers. Silicone rubber pipes have been used for this purpose in the UK since 1999. They are completely water-soluble, so they are water-filled, they must be carefully plumbed so they are completely drained. not crack. Many metal collectors are installed as part of a sealed heat exchanger system. Rather than having drinking water flow directly through the collectors, a mixture of water and antifreeze such as propylene glycol is used. A heat exchange fluid protects against freeze damage to a propylene glycol in the mixture.

A collectible gold collector is a simple form of flat-collector without a transparent cover. Typically polypropylene or EPDM rubber or silicone rubber is used as an absorb. Used for pool heating when the temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become less effective. Most flat collectors have a life expectancy of over 25 years.

Applications

The use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This is a situation with a large family, or a situation in which it is often Commercial applications include laundromats, as washes, military laundry facilities and eating establishments. The technology can be used for space heating if the building is located off-grid or if utility is subject to frequent outages. Solar water heatingThese systems are more likely to be cost effective than those required for large scale hot water installations. Unglazed liquid collectors are commonly used to heat water for swimming pools but can also be applied to large scale water pre-heating. When they are available at low temperature, they can be used at low temperatures, where they can be used in the marketplace as the right choice. Because these collectors need not be able to use high temperatures, they can use these materials.^[6]A smaller but growing percentage of unglazed collectors are flexible meaning they can withstand water freezing solid inside their absorb. The freeze concerns the need for the water filled piping and manifold manifolds in a hard freeze condition. Unglazed solar hot water systems should be installed to “drainback” to a storage tank whenever solar radiation is insufficient. There are no thermal shock concerns with unglazed systems. Commonly used in swimming pool heating since solar energy’s early beginnings, unglazed solar collectors heat swimming pool water directly without the need for antifreeze or heat exchangers. Hot water solar systems require heat exchangers due to contamination and unglazed collectors, the pressure difference between the solar working fluid (water) and the load (pressurized cold city water). Large scale unglazed solar hot water heaters at the Minoru Aquatic Center in Richmond BC^[7] These methods are more likely to be used by the manufacturer than when they are used in the same process. . When heating hot water, we are actually heating up to warm and warm to hot. We can heat cold to warm with unglazed collectors, just as we can heat warm with high temperature collectors

Evacuated tube collectors

Most vacuum tube collectors are used in Europe. Direct flow is more popular in China. Evacuated heat pipe tubes (EHPTs) are composed of multiple evacuated glass tubes each containing an absorbent flat fused to a heat pipe . ^[8] The heat is an aqueous propylene glycol (water or antifreeze mix-typically propylene glycol ) of a domestic hot water or hydronicspace heating system in a heat exchanger called a “manifold”. The manifold is wrapped in insulation and covered by a metal or plastic case. The vacuum inside the evacuated tube collectors have been more than 25 years, the reflective coating for the design is encapsulated in the vacuum inside the tube, which will not degrade until the vacuum is lost. ^[9] The vacuum that surrounds the convection and conductionheat loss, therefore achieving greater efficiency than flat-plate collectors, especially in colder conditions. This advantage is largely lost in warmer climates, except in those cases where it is desirable, eg, for commercial processes. The high temperatures that may occur may require special design to prevent overheating.

Some evacuated tubes (glass-metal) are made with a layer of glass that is fused to the heat pipe at the end and encloses the heat pipe and absorb in the vacuum. Others (glass-glass) are made with a double layer of glass fused together at one or both ends with a vacuum between the layers (like a vacuum bottle or flask), with the absorbent and heat pipe at normal atmospheric pressure. Glass-glass tubes have a highly reliable vacuum seal, but the two layers of glass reduce the light that they absorb. Moisture may enter the non-evacuated area of ​​the tube and cause absorption corrosion. Glass-metal tubes allow more light to reach the absorbent, and protect the absorbing and heat pipe of corrosion even if they are made of dissimilar materials (see galvanic corrosion).

The gaps between the tubes can be reduced to the collector, minimizing the loss of production in some snowy conditions, but the lack of heat can also prevent the effective shedding of accumulated snow. ^[10] [11]

Comparisons of flat plate and evacuated tube collectors

A longstanding argument exists between proponents of these two technologies. Some of this can be related to the physical structure of evacuated tube collectors which have a discontinuous absorbance area. An array of evacuated tubes on the roof of the space between the collector tubes, and the vacuum between the two concentric glass tubes of each collector. Collector tubes cover only a fraction of a unit area on a roof. If evacuated tubes are compared with flat-plate collectors on the basis of area of ​​roofed, a different conclusion could be reached if the areas of absorbed were compared. In addition, the standard ISO 9806 ^[12]is ambiguous in describing the way in which the efficiency of solar thermal collectors should be measured, since it could be measured in the area of ​​absorbing area. Unfortunately, power output is not given for thermal collectors as it is for PV panels . This makes it difficult for purchasers and engineers to make informed decisions.

Flat-plate collectors usually lose more heat to the environment than evacuated tubes, as an increasing function of temperature. They are inappropriate for high temperature applications such as process steam production. Evacuated tube collectors have a lower absorbed area compared to gross area ratio (typically 60-80% of gross area) compared to flat plates. Based on absorbed flat area, most evacuated tube systems are more efficient than flat flat systems. This makes them suitable for the purpose of building a roof over the roof of the roof. In general, where the temperature is low (eg during winter) or when the sky is overcast. HOWEVER, even in areas without much sunshine and solar heat, some flat collectors can be more cost effective than evacuated tube collectors. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have track records of 15-30 years. There is no unambiguous evidence that the two designs differ in long term reliability. However, evacuated tube technology is younger and still more likely to be competitive. The modularity of evacuated tubes may be advantageous in terms of extensibility and maintenance, for example if the vacuum in one tube diminishes. can be more cost effective than evacuated tube collectors. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have track records of 15-30 years. There is no unambiguous evidence that the two designs differ in long term reliability. However, evacuated tube technology is younger and still more likely to be competitive. The modularity of evacuated tubes may be advantageous in terms of extensibility and maintenance, for example if the vacuum in one tube diminishes. can be more cost effective than evacuated tube collectors. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have track records of 15-30 years. There is no unambiguous evidence that the two designs differ in long term reliability. However, evacuated tube technology is younger and still more likely to be competitive. The modularity of evacuated tubes may be advantageous in terms of extensibility and maintenance, for example if the vacuum in one tube diminishes. Several Chinese companies have track records of 15-30 years. There is no unambiguous evidence that the two designs differ in long term reliability. However, evacuated tube technology is younger and still more likely to be competitive. The modularity of evacuated tubes may be advantageous in terms of extensibility and maintenance, for example if the vacuum in one tube diminishes. Several Chinese companies have track records of 15-30 years. There is no unambiguous evidence that the two designs differ in long term reliability. However, evacuated tube technology is younger and still more likely to be competitive. The modularity of evacuated tubes may be advantageous in terms of extensibility and maintenance, for example if the vacuum in one tube diminishes.

For a given absorbent area, evacuated tubes can be used in a wide range of ambient temperatures and heating requirements. In most climates, flat-plate collectors will be more cost-effective than evacuated tubes. When employed in arrays, it is a matter of fact that it can not be used in the most efficient way. They are well-suited to cold ambient temperatures and working conditions in low sunshine, providing more heat than flat collectors per square meter. Heating of water by a medium to low amount (ie Tm-Ta) Domestic hot water often falls into this medium category.^[15] Unglazed collectors may be suitable in tropical or subtropical environments if domestic hot water needs to be heated by less than 20 ° C. A contour map can show which type is more effective (both thermal efficiency and energy / cost) for any geographic region.

EHPT’s work on a thermal one-way valve due to their heat pipes. This gives them an inherent maximum operating temperature as a safety feature. They have less aerodynamic drag, which can be used on the roof. They can collect thermal radiation from the bottom in addition to the top. Tubes can be individually selected without stopping the entire system. There is no condensation or corrosion within the tubes. One of the most important steps in the adoption of the ISO 9806-2 section 9 class is a requirement for durability certification. ^[16]This means that the evacuated tube collectors are exposed to full sun for a long time to be filled with cold water tubes. There is also the question of vacuum leakage. Flat panels have been around much longer and are less expensive. They can be easier to clean. Other properties, such as appearance and ease of installation are more subjective.

Bowl

A solar bowl is a type of solar thermal collector that operates similarly to a parabolic dish , but instead of using a parabolic mirror with a fixed receiver, it has a fixed spherical mirror with a tracking receiver. This reduces efficiency, but makes it cheaper to build and operate. Designers call it a fixed mirror distributed focus solar power system . The main reason for its development was to eliminate the cost of a parabolic dish. ^[17]

A fixed parabolic mirror creates a variously shaped image of the sun as it moves across the sky. Only when the mirror is pointed directly to the sun does the light focus on one point. That’s why parabolic dish systems track the sun. A fixed spherical mirror focuses the light in the independent place of the sun’s position. The light, however, is distributed on a line from the surface of the mirror to one half radius.

As the sun moves across the sky, the aperture of any fixed collector changes. This causes changes in the amount of sunlight, producing what is called the sinus effect of power output. Proponents of the solar bowl design claims the reduction in overall power output compared with tracking. ^[17]

The sunlight is concentrated on the focal line of a spherical reflector is collected using a tracking receiver. This receiver is pivoted around the line and is usually counterbalanced. The receiver may consist of pipes carrying fluid for thermal transfer or photovoltaic cells for direct conversion of light to electricity.

The Solar Bowl design was developed by the Technical Engineering Department of the Texas Technical University, headed by Edwin O’Hair, to develop a 5 MWe power plant. A solar bowl was built for the town of Crosbyton, Texas as a pilot facility. ^[17] The bowl had a diameter of 65 ft (20 m), tilted at a 15 ° angle to optimize the cost / yield relationship (33 ° would have maximized yield). The rim of the hemisphere was “trimmed” to 60 °, creating a maximum aperture of 3.318 square feet (308.3 m ² ). This pilot bowl produced electricity at a rate of 10 kW peak. ^{[ quote needed ]}

A 15-meter diameter Auroville solar bowl was developed from an earlier test of a 3.5-meter bowl in 1979-1982 by the Tata Energy Research Institute . That test shown the use of the solar bowl in the production of steam for cooking. The full-scale project was built in 1996 and was fully operational by 2001. ^{[ citation needed ]}

Solar-thermal collectors heating air

A simple solar collector consists of an absorbing material, sometimes having a selective surface, to capture heat from air conduction heat transfer. This heated air is then ducted to the space where the heating is used for space heating or process heating needs. Functioning in a similar manner as a forced air-supply, solar-thermal-air systems, air-source heat recovery, absorbing the sun’s thermal energy, and ducting air coming in contact with it. Simple and effective collectors can be made for a variety of air conditioning and process applications.

A variety of applications can be used to source fossil fuels, to create a sustainable means to produce thermal energy. Applications such as space heating, greenhouse season extension, pre-heating ventilation, air-conditioning, or process heat can be addressed by solar air heat devices. In the field of ‘solar co-generation,’ solar thermal technologies are coupled with photovoltaics (PV) to increase the efficiency of the PV collectors, cooling the PV panels to improve their electrical performance while simultaneously warming air for space heating.

Space heating and ventilating

Space heating for residential and commercial applications can be done through solar heating panels. This configuration operates by drawing air in the air by means of air collectors and by the means of air-conditioning. fan. A pioneering of this type of system was George Löf, who built a solar system in Boulder, Colorado. He later included a gravel bed for heat storage.

Ventilation, fresh air or makeup is required in most commercial, industrial and institutional buildings to meet code requirements. By drawing air through a properly designed unglazed air collector or an air heater, the solar heated fresh air can reduce the heating load during daytime operation. Many applications are now being installed where the respirator collides with the fresh air entering a heat recovery ventilator to reduce the defrost time of HRV’s. The higher your ventilation and temperature the better your payback time will be.

Process heating

Solar air heat is also used in processes such as drying laundry, crops (ie tea, corn, coffee) and other drying applications. Air heated through a solar collector and then passed over to an efficient way of delivering moisture.

Solar air heating collector types

Collectors are commonly classified by their air-ducting methods

• through-pass collectors
• Front-pass
• back pass
• combination front and back pass collectors

Collectors can also be classified by their outer surface:

• glazed
• unglazed

Through-air collector

Offering the highest efficiency of any solar technology through-pass configuration, air ducted on one side of the absorptive passes through a perforated material and is heated from the conductive properties of the material and the convective properties of the moving air. Through-pass absorbers have the most surface area, which can cause greater fan power, and deterioration of some of the absorbing material. .

Back, front and combination passage collector

In-pass, front-pass, and combination type configurations are directed to the back, the front, or to both sides of the head. Although it is possible to provide a greater surface area for conduction of heat transfer, . In cold climates, air loss will be greater because of greater heat loss, resulting in lower overall performance of the collector.

Glazed systems

Glazed systems usually have a transparent top sheet and insulated side and back panels to minimize heat loss to ambient air. The absorb plates in modern panels can be absorbed by more than 93%. Glazed Solar Collectors (recirculating types that are usually used for space heating). Air typically passes along the front or back of the flat while scrubbing heat directly from it. Heated air can then be distributed directly for applications such as space heating and drying. Payback for glazed solar air heating panels can be less than 9-15 years depending on the fuel being replaced.

Unglazed systems

Unglazed systems, have been used to heat make-up or ventilation in commercial, industrial, agriculture and process applications. They consist of absorbing the heat and absorbing it. Non-transparent glazing materials are less expensive, and less likely to payback periods. Transpired collectors are considered “unglazed” because their collector surfaces are exposed to the elements, are often not transparent and not hermetically sealed.

Unglazed transpired solar collectors

Background

The term “unglazed air collector” refers to a solar air heating system that consists of a metal absorbing without any glass or glazing over top. The most common type of unglazed collector on the market is the transpired solar collector. The technology has been extensively monitored by these government agencies, and the Natural Resources Canada has developed the feasibility tool RETScreen ™ to model the energy savings from transpired solar collectors. Since that time, several thousand solar collectors have been installed in a variety of commercial, industrial, institutional, agricultural, and process applications in countries around the world. This technology was originally used primarily in industrial applications, and often negative pressure in the building. High solar conversion (up to 750% thermal Watts / square meters), high solar conversion (up to 90%) and high energy conversion (up to 90%) solar photovoltaic and solar water heating.

Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. It is typically the most cost-effective of all the solar technologies, especially in large scale applications, and it addresses the largest use of space heating in heating systems, which is space heating and industrial process heating. They are either glazed or unglazed.

Method of operation

Unglazed air collectors heat ambient (outside) air instead of recirculated building air. Transpired solar collectors are usually wall-mounted to the bottom of the sun and their optimum performance and return on investment (CFM per square foot) (72 to 144 m3 / h.m2) of collector area.

The exterior surface of a transparent solar collector consists of thousands of micro-perforations that allow the boundary layer of heat to be captured and uniformly drawn into an air cavity behind the exterior panels. This method is used in conjunction with a solar ducting system.

Hot air that may be an HVAC system connected to a collector that has air outlets along the top of the collector, particularly if the collector is west facing. To counter this problem, the Matrix Energy has been patented to a greater extent than other air-borne devices.

This cutaway view shows the Matrix Air transpired solar collector components and air flow. The lower air inlet mixates the intake of heated air to the HVAC system during summer operation.

The extensive monitoring by Natural Resources Canada and NREL has shown that it is capable of reducing energy consumption by 10-50% of the RETScreen is a reliable predictor of system performance. Transpired solar collectors act as a rainscreen and they also capture heat loss from the building envelope which is collected in the collector’s air cavity and drawn back into the ventilation system. There is no need for solar heating and the expected lifespan is over 30 years.

Variations of transparent solar collectors

Unglazed transpired collectors can also be adapted for other applications. Matrix Energy Inc. has patented a roof mounted product called “Delta” a modular, roof-mounted solar air heating system where southerly, east or west facing facades are simply not available.

Each ten foot (3.05 m) module will deliver 250 CFM (425 m3 / h) of preheated fresh air energy. This unique two stage, modular roof mounted transpired collector operating at nearly 90% efficiency each module delivering over 118 l / s of preheated air to two square meter collectors. Collision can be connected to a series of four collocated collectors in a single row. +

Transpired collectors can be configured to heat the air. In a 2-stage system, the first stage is the typical unglazed transpired collector and the second stage The glazing allows for the first stage of the second stage of the second stage of solar heating.

Solar-Thermal Collectors Generation Electricity

Parabolic troughs , dishes and towers described in this section are used exclusively for solar power generating stations or for research purposes. Although simple, these solar concentrators are quite far from the theoretical maximum concentration. ^[18] [19] For example, the parabolic trough concentration is about 1/3 of the theoretical maximum for the same acceptance angle , which is, for the same overall tolerances for the system. Approaching the theoretical maximum can be achieved by using more elaborate concentrators based on nonimaging optics . ^[18] Solar thermal collectors can also be used in conjunction with photovoltaic collectors to obtain combined heat and power.^[20] [21]

Parabolic trough

This type of collector is used in solar power plants. A trough-shaped parabolic reflector is used to concentrate sunlight on an insulated tube ( Dewar tube ) or heat pipe , placed at the focal point , containing coolant which transfers heat from the collectors to the boilers in the power station.

Parabolic dish

With a parabolic dish collector, one or more parabolic dishes concentrates a single point of focus, a telescope focuses starlight, or a dish antennafocuses radio waves. This geometry can be used in solar furnaces and solar power plants.

The shape of a parabola means that incoming light rays which are parallel to the dish will be reflected towards the focus, no matter where on the dish they arrive. Light from the sun arrives at the Earth’s surface almost completely parallel, and the dish is aligned with its axis pointing at the sun, allowing almost all incoming radiation to be reflected towards the focal point of the dish. Most losses in such collectors are due to imperfections in the parabolic shape and imperfect reflection.

Losses due to atmospheric scattering is minimal. However, we have hazy or foggy day, which is significantly diffused in all directions through the atmosphere, which significantly reduces the efficiency of a parabolic dish.

In dish stirling power plant designs, a stirling engine coupled to a dynamo, is placed at the focus of the dish. This absorbs the energy focused on it and converts it into electricity.

Power tower

A power tower is a large tower surrounded by tracking mirrors called heliostats . These mirrors align themselves and focus sunlight on the receiver at the top of the tower, collected heat is transferred to a power station below. This design reaches very high temperatures. High temperatures are suitable for electricity generation using conventional methods like steam turbinegold live high temperature chemical reaction Such As liquid salt. ^[22] By concentrating sunlight, current systems can get better than simple solar cells. A larger area can be covered by using relatively inexpensive mirrors rather than using expensive solar cells . Concentrated light can be redirected to a convenient location viaoptical fiber cable for such uses as illuminating buildings. Heat storage for power production during cloudy and overnight conditions can be accomplished, often by underground tank storage of heated fluids. Molten have been used to good effect. Other working fluids, such as liquid metals, have been proposed to their superior thermal properties. ^[23]

However, concentrating systems require sun tracking to maintain sunlight focus at the collector. They are unable to provide significant power in diffused light conditions. Solar cells are capable of providing some output even if the sky becomes cloudy, but power output of concentrating systems drops drastically in cloudy conditions.

Appendix

Standards

• ISO test methods for solar collectors. ^[24]
• EN 12975 : Thermal solar systems and components. Solar collectors.
• EN 12976 : Thermal solar systems and components. Factory made systems.
• EN 12977 : Thermal solar systems and components. Custom made systems.
• Solar Keymark: ^[25] Thermal solar systems and components. Higher level EN 1297X series certification which includes factory visits.

See also

References