Flat-plate solar collectors are the most used type of solar panels today. When temperature below 90Â°C are adequate ,as they are for space and service water heating flat plate collector, which are the non concentrating type are more convenient. They are made in rectangular panels, from about 1.7 to 2.9 sq.m , in area, and are relatively simple to construct and erect. Flat plate can collect and absorb both direct and diffuse solar radiation. They are consequently partially effective even on cloudy days when there is no direct radiation. They can be glazed or unglazed. Freezing conditions may limit the efficiency of this kind of collectors.
Flat-plate collectors are in wide use for domestic household hot-water heating and for space heating, where the demand temperature is low. Many excellent models of flat-plate collectors are available commercially to the solar designer. A flat-plate collector is used in industrial systems either to supply low-temperature demands or to preheat the heat transfer fluid before entering a field of higher-temperature concentrating, collectors.
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Common flat-plate collectors use a liquid (water, glycolâ€¦) as the heating fluid.They involves a net of flow tubes (where the water or another heating fluid flows), a dark colour absorber plate (to absorb the sunlight heat) and a glazing surface. They also comprise an inlet and an outlet connection. The image below shows the interaction of these elements.
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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 of solar energy, (2) a transparent cover that allows solar energy to pass through but reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing. A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air heat. The more complex collectors are generally used in solar power plants where solar heat is used to generate electricity by heating water to produce steam which drives a turbine connected to an electrical generator. The simpler collectors are typically used for supplemental space heating in residential and commercial buildings. A collector is a device for converting the energy in solar radiation into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The solar energy striking the Earth's surface depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1,000 watts per square meter under clear skies with the surface directly perpendicular to the sun's rays.
A flat-plate solar collector is one of three main types of solar collectors, which are key components of active solar heating systems. The other main types are evacuated tube collectors and batch solar heaters (also called integrated collector-storage systems).
Flat-plate collectors are the most common solar collectors for use in solar water-heating systems in homes and in solar space heating. A flat-plate collector consists basically of an insulated metal box with a glass or plastic cover (the glazing) and a dark-coloured absorber plate. Solar radiation is absorbed by the absorber plate and transferred to a fluid that circulates through the collector in tubes. In an air-based collector the circulating fluid is air, whereas in a liquid-based collector it is usually water.
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Flat-plate collectors heat the circulating fluid to a temperature considerably less than that of the boiling point of water and are best suited to applications where the demand temperature is 30-90Â°C (86-158Â°F) and/or for applications that require heat during the winter months.
Air-based collectors are typically used for heating buildings and drying crops. Liquid-based may be glazed or unglazed. Glazed liquid collectors are the commonest type of solar collector for providing domestic and commercial water and for heating indoor swimming pools. Unglazed collectors are most often used for heating outdoor pools. A special type of unglazed collector called a perforated plate collector is used to preheat ventilation air for commercial buildings or, in some cases, for drying crops.
Flat collectors can be mounted in a variety of ways, depending on the type of building, application, and size of collector. Options include mounting on a roof, in the roof itself, or free-standing.
What is a flat-plate collector?
A typical flat-plate collector is a metal box with a glass or plastic cover (called glazing) on top and a dark-colour absorber plate on the bottom. The sides and bottom of the collector are usually insulated to minimize heat loss.
Flat-plate collectors are actually quite simple. They consist of an insulated metal box covered with a glass or plastic cover called glazing. Within the box, is a dark-colour absorber plate that is made out of a metal heat conductor and painted black to absorb and trap more heat. The absorber plate then transfers the heat to either air or liquid. Lying beside the absorber plate, are tubes that contain either water or air.
Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. 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 aluminium-because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminium. In locations with average available solar energy, flat plate collectors are sized approximately one-half- to one-square foot per gallon of one-day's hot water use.. They consist of (1) a dark flat-plate absorber of solar energy, (2) a transparent cover that allows solar energy to pass through but reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminium, steel orÂ copper, to which a black or selective coating is applied) often backed by a grid or coil of fluid tubing placed in an insulated casing with a glass or polycarbonate cover. In water heat panels, fluid is usually circulated through tubing to transfer heat from the absorber to an insulated water tank. This may be achieved directly or through aÂ heat exchanger. Most air heat fabricates and some water heat manufacturers have a completely flooded absorber consisting of two sheets of metal which the fluid passes between. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.]
There are a number of absorber configurations:
harp - traditional design with bottom pipe risers and top collection pipe, used in low pressure thermosyphon and pumped systems
serpentine - one continuous S that maximises temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role)
Completely flooded absorber consisting of two sheets of metal stamped to produce a circulation zone. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.
As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be whollyÂ polymer, or they may include metal plates in front of freeze-tolerant water channels made of silicone rubber. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing to use heat exchangers which lower efficiency. By dispensing with a heat exchanger in these flat plate panels, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be more efficient, particularly at low light levels. However, polymer collectors suffer from overheating when insulated, as stagnation temperatures can exceed the melting point of the polymer.Â For example, the melting point of polypropylene is 160Â°C, while the stagnation temperature of insulated thermal collectors can exceed 180Â°C if control strategies are not used.
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In areas where freezing is a possibility, metal collectors must be carefully plumbed so they completely drain down using gravity before freezing is expected so that they do not crack. Other collectors are part of a sealed heat exchange system, rather than having the potable water flow directly through the collectors. A mixture of water and propylene glycol (which is used in the food industry) can be used as a heat exchange fluid to protect against freeze damage down to a temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity only marginally, while the addition of an extra heat exchanger may lower system performance at low light levels.
A pool or unglazed collector is a simple form of flat-plate collector without a transparent cover. It is used for pool heating and can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become ineffective.Most flat plate collectors have a life expectancy of over 25 years.
Components of solar flat liquid collector
The majority of the flat plate collectors have five main components as follows:
A transparent cover which may be one or more sheet of glass or radiation transmitting plastic film or sheet.
Tubes, fins, passages or channels are integral with the collector absorber or connected to it, which carry the water, air or other fluid.
The absorber plate, normally metallic or with a black surface, although a wide variety of other material can be used with air heater.
Insulation, which should be provided at the back and sides to minimise the heat losses. Standard insulation material such as fiber glass or styro-foam are used for this purpose
The casing or container which encloses the other component and protects them from weather.
Types of liquid based flat solar collector
Glazed flat plate collector
Unglazed perforated plate collector
Evacuated tube collectors
Glazed flat-plate collectorsÂ
A solar collector that consists of an absorber without the glass covering of a glazed flat-plate collector. In North America Glazed flat-plate collectors are very common and are available asÂ liquid-basedÂ andÂ air-basedÂ collectors.Â Â These collectors are better suited for moderate temperature applications where the demand temperature is 30-70C and/or for applications that require heat during the winter months. TheÂ liquid-basedÂ collectors are most commonly used for the heating ofÂ domesticÂ andÂ commercialÂ hot water,Â buildings, andÂ indoor swimming pools. TheÂ air-basedÂ collectors are used for the heating of buildings,Â ventilation airÂ andÂ crop-drying.
Glazed flat-plate collector
[courtesy of Natural Resources Canadaan (RETScreen)
In this type of collector a flat absorber efficiently transforms sunlight into heat. To minimize heat escaping, the plate is located between a glazing (glass pane or transparent material) and an insulating panel. The glazing is chosen so that a maximum amount of sunlight will pass though it and reach the absorber.Â
, unglazed flat-plate collectors currently account for the most area installed per year of any solar collector. Because they are not insulated, these collectors are best suited for low temperature applications where the demand temperature is below 30Â°C. By far, the primary market is for heating outdoor swimming pools, but other markets exist including heating seasonal indoor swimming pools, pre-heating water for car washes, and heating water used in fish farming operations. There is also a market potential for these collectors for water heating at remote, seasonal locations such as summer camps.
Unglazed collectors are usually made of black plastic that has been stabilized to withstand ultraviolet light. Since these collectors have no glazing, a larger portion of the Sun's energy is absorbed. However, because they are not insulated a large portion of the heat absorbed is lost, particularly when it is windy and not warm outside. They transfer heat so well to air (and from air) that they can actually capture heat during the night when it is hot and windy outside.
Unglazed perforated plate collector
A type of solar collector that uses an industrial-grade siding/cladding perforated with many small holes at a pitch of 2-4 cm. Air passes through the holes in the collector before it is drawn into the building to provide preheated fresh ventilation air. Efficiencies are typically high because the collector operates close to the outside air temperature. Perforated plate collectors can be very cost-effective, especially when they replace conventional cladding on the building, because only incremental costs need be compared to the energy savings.
The most common application of this collector is for building ventilation air heating. Other possible components for this system are: a 20-30 cm air gap between the building, a canopy at the top of the wall that acts as a distribution manifold, and by-pass dampers so that air will by-pass the system during warm weather.
Another application for the perforated plate collector is crop drying. Systems have been installed in South America and Asia for drying of tea, coffee beans, and tobacco.
Evacuated tube collectors
Most (if not all) vacuum tube collectors use heat pipes for their core instead of passing liquid directly through them. Evacuated heat pipe tubes (EHPT's) are composed of multipleÂ evacuatedÂ glassÂ tubes each containing an absorber plate fused to aÂ heat pipe.Â The heat from the hot end of the heat pipes is transferred to the transfer fluid (water or anÂ antifreezeÂ mix-typicallyÂ propylene glycol) of a domestic hot water orÂ hydronicÂ space heating system in a heat exchanger called a "manifold". The manifold is wrapped in insulation and covered by a sheet metal or plastic case to protect it from the elements.
The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss to the outside, 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 very hot water is desirable, for example commercial process water. The high temperatures that can occur may require special system design to avoid or mitigate overheating conditions.
Some evacuated tubes (glass-metal) are made with one layer of glass that fuses to the heat pipe at the upper end and encloses the heat pipe and absorber 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 aÂ vacuum bottleÂ or flask) with the absorber and heat pipe contained at normal atmospheric pressure. Glass-glass tubes have a highly reliable vacuum seal but the two layers of glass reduce the light that reaches the absorber and there is some possibility that moisture will enter the non-evacuated area of the tube and cause absorber corrosion. Glass-metal tubes allow more light to reach the absorber and protect the absorber and heat pipe (contained in the vacuum) from corrosion even if they are made from dissimilar materials.
The gaps between the tubes may allow for snow to fall through the collector, minimizing the loss of production in some snowy conditions, though the lack of radiated heat from the tubes can also prevent effective shedding of accumulated snow
Evacuated tube collector Glass-glass evacuated tube
How does a liquid flat-plate collector work?
A flat-plate liquid collector works by transferring the heat gathered from the absorber plate to the liquid flowing through the tubes attached to the plate. A pump within the collector re-circulates this liquid throughout the tubes allowing it to absorb as much heat as possible.
These heat liquid as it flows through tubes in or adjacent to the absorber plate. The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house. Solar pool heating also uses liquid flat-plate collector technology, but the collectors are typically unglazed as in figure below.
What is the difference between the two forms of liquid systems?
There can be two different forms of liquid systems: the open-loop system, or the type that requires a heat-transfer fluid. The open-loop system is the most direct. The water is heated and then flows directly to the point of use. The heat-transfer method requires the use of an antifreeze liquid that is heated from the collector and then moved to a heat exchanger. The heat exchanger is connected to a storage tank which uses the heated anti-freeze to heat water. This seems like an extra, unnecessary step; but in actuality, allows to have more hot water readily available than the open-loop system. To save on energy bill, a solar-heated water system may be a good place to start.
The technique of heating water between 50-90Â°C through the abundantly available solar energy is fully recognized by now. However, the design pursuits are still revolving around various engineering techniques to produce a highly performing, reliable and durable solar water heating system. This is because; a well-designed flat plate collector can effectively lead to a cost-effective and durable heat supply mechanism, for a host of end-use applications in various sectors. A typical system incorporates tubes attached to the flat fins with a solar selective absorber coating in place to heat the water.
In the design under consideration, the collector efficiency has been vastly improved, so as to economise on the system cost as a whole. This effort is to be measured against a near-total focus on standardizing the material use and size by many others. So far, many systems based on the optimized solar collector design efficiency have been successfully operating under the actual field operating conditions in various locations in the country.
Amongst the prominent potential users are those involved with the production, installation and performance monitoring of the solar water heating systems.
Â Â The main use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family, or a situation in which the hot water demand is excessive due to frequent laundry washing.
Commercial applications include Laundromats, car washes, military laundry facilities and eating establishments. The technology can also be used for space heating if the building is located off-grid or if utility power is subject to frequent outages. Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water.
Unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather or can be drained easily during cold weather.
While solar collectors are most cost-effective in sunny, temperate areas, they can be cost effective virtually anywhere in the country so should be considered.
Â The amount of time required - usually in years - for positive cash flows to equal the total investment costs. This is often used to describe how long it will take for energy savings resulting from using more energy-efficient equipment to equal the premium paid to purchase the more energy-efficient equipment.vary widely, but for a well-designed and properly installed solar water heater, you can expect a simple payback of 4 to 10 years, depending on climate and utility costs. FSEC found that solar water heaters offer potential savings, compared to electric water heating, of as much as 50% to 85% in the water heating portion of the utility bill.
Flat plate water heating systems range in price from about $2,000 to $4,000 installed for residential systems (for 40 to 80 gallons per day usage), and $2,000 to $50,000 for commercial systems (for 40 to 1700 gallons per day usage). The following chart compares the percent of water heating energy that solar can provide in various cities for a 48-square-foot flat plate solar hot water system based on average water usage for four persons.
Â Flat plate solar water heating systems are available in most areas of the Indian States and many other countries.