Solar flat plate and concentrating collectors pdf
File Name: solar flat plate and concentrating collectors .zip
These devices are primarily used for active solar heating and allow for the heating of water for personal use. The use of these solar collectors provides an alternative for traditional domestic water heating using a water heater, potentially reducing energy costs over time. As well as in domestic settings, a large number of these collectors can be combined in an array and used to generate electricity in solar thermal power plants. There are many different types of solar collectors, but all of them are constructed with the same basic premise in mind.
A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating , but may refer to large power generating installations such as solar parabolic troughs and solar towers or non water heating devices such as solar air heaters.
Solar thermal collectors are either non-concentrating or concentrating. In non-concentrating collectors, the aperture area i. A common example of such a system is a metal plate that is painted a dark color to maximize the absorption of sunlight.
The energy is then collected by cooling the plate with a working fluid , often water or glycol running in pipes attached to the plate. Concentrating collectors have a much larger aperture than the absorber area.
The aperture is typically in the form of a mirror that is focussed on the absorber, which in most cases are the pipes carrying the working fluid.
Non-concentrating collectors are typically used in residential and commercial buildings for space heating , while concentrating collectors in concentrated solar power plants generate electricity by heating a heat-transfer fluid to drive a turbine connected to an electrical generator.
Flat-plate and evacuated-tube solar collectors are mainly used to collect heat for space heating, domestic hot water, or cooling with an absorption chiller. In contrast to solar hot water panels, they use a circulating fluid to displace heat to a separated reservoir.
The first solar thermal collector designed for building roofs was patented by William H. Goettl and called the " Solar heat collector and radiator for building roof ".
Flat-plate collectors are the most common solar thermal technology in Europe. The sides and back of the enclosure are typically insulated to reduce heat loss to the ambient.
A heat transfer fluid is circulated through the absorber's fluid passageways to remove heat from the solar collector. The circulation fluid in tropical and sub-tropical climates is typically water. In climates where freezing is likely, a heat transfer fluid similar to an automotive antifreeze solution may be used instead of water, or in a mixture with water. If a heat transfer fluid is used, a heat exchanger is typically employed to transfer heat from the solar collector fluid to a hot water storage tank.
The most common absorber design consists of copper tubing joined to a high conductivity metal sheet copper or aluminum. A dark coating is applied to the sun-facing side of the absorber assembly to increase it absorption of solar energy. A common absorber coating is black enamel paint. In higher performance solar collector designs, the transparent cover is tempered soda-lime glass having reduced iron oxide content same as for photovoltaic solar panels.
The glass may also have a stippling pattern and one or two anti-reflective coatings to further enhance transparency. The absorber coating is typically a selective coating, where selective stands for having the special optical property to combine high absorption in the visible part of the electromagnetic spectrum coupled to low emittance in the infrared one. This creates a selective surface , which reduces black body energy emission from the absorber and improves performance.
Piping can be laser or ultrasound welded to the absorber sheet to reduce damage to the selective coating, which is typically applied prior to joining to large coils in a roll-to-roll process.
A flat plate collector making use of a honeycomb structure to reduce heat loss also at the glass side too has also been made available commercially. Most flat plate collectors have a life expectancy of over 25 years. Evacuated tube collectors are the most common solar thermal technology in China and in the World. The vacuum that surrounds the absorber greatly reduces convection and conduction heat loss, therefore achieving greater energy conversion efficiency.
The absorber can be either metallic as in the case of flat plate collectors or being a second concentric glass tube "Sydney Tube". Heat transfer fluid can flow in and out each tube or being in contact with a heat pipe reaching inside the tube. For the latter, heat pipes transfer heat to the fluid in a heat exchanger called a "manifold" placed transverse in respect to the tubes. Glass-metal evacuated tubes are made with flat or curved metal absorber sheets same as those of flat plates.
These sheets are joined to pipes or heat pipes to make "fins" and placed inside a single borosilicate glass tube. An anti-reflective coating can be deposited on the inner and outer surfaces of such tube to improve transparency. Both selective and anti-reflective coating inner tube surface will not degrade until the vacuum is lost. This seal is cycled between ambient and fluid temperature each day of collector operation and might lead to failures in time. Glass-glass evacuated tubes are made with two borosilicate glass tubes fused together at one or both ends similar a vacuum bottle or dewar flask.
The absorber fin is placed inside the inner tube at atmospheric pressure. Glass-glass tubes have a very reliable seal, but the two layers of glass reduce the amount of sunlight that reaches the absorber.
The selective coating can be deposited on the inner borosilicate tube high vacuum side to avoid this, but heat has then to flow through the poorly conducting glass thickness of the inner tube in this case. Moreover, moisture may enter the non-evacuated area inside the inner tube and cause absorber corrosion in particular when made from dissimilar materials galvanic corrosion.
A Barium flash getter pump is commonly evaporated inside the high vacuum gap in between tubes to keep the internal pressure stable through time.
The high temperatures that can occur inside evacuated tubes may require special design to prevent overheating. Some evacuated tube collectors work as a thermal one-way valve due to their heat pipes. This gives them an inherent maximum operating temperature that acts as a safety feature. A longstanding argument exists between proponents of these two technologies. Some of this can be related to the structure of evacuated tube collectors which have a discontinuous absorbance area.
An array of evacuated tubes collectors on a roof has space between the individual tubes and a vacuum gap between each tube and its absorber inside, covering only a fraction of the installation area on a roof. If evacuated tubes are compared with flat-plate collectors on the basis of area of roof occupied gross area , a different conclusion might be reached than if the absorber or aperture areas were compared. The recent revision of the ISO standard  states that the efficiency of solar thermal collectors should be measured in terms of gross area and this might favour flat plates in respect to evacuated tube collectors in direct comparisons.
Flat-plate collectors usually lose more heat to the environment than evacuated tubes because there is no insulation at the glass side. Although several European companies manufacture evacuated tube collectors mainly glass-metal type , the evacuated tube market is dominated by manufacturers in China, with some companies having a track records of 15—30 years or more. There is no unambiguous evidence that the two designs differ in long term reliability. However, evacuated tube technology especially for newer variants with glass-metal seals and heat pipes still need to demonstrate competitive lifetimes.
The modularity of evacuated tubes can be advantageous in terms of extensibility and maintenance, for example if the vacuum in one heat pipe tube is lost it can be easily be replaced with minimal effort.
In most climates, flat plate collectors will generally be more cost-effective than evacuated tubes. Unglazed flat plate collectors are the preferred devices for heating swimming pool water. Evacuated tube collectors have less aerodynamic drag, which may allow for a simpler installation on roofs in windy locations. 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.
Flat plate collectors might be easier to clean. Other properties, such as appearance and ease of installation are more subjective and difficult to compare. Evacuated flat plate solar collectors provide all the advantages of both flat plate and evacuated tube collectors combined together.
They surround a large area metal sheet absorber with high vacuum inside a flat envelope made of glass and metal. They offer the highest energy conversion efficiency of any non-concentrating solar thermal collector,  but require sophisticated technology for manufacturing. They should not be confused with flat plate collectors featuring low vacuum inside.
Evacuated flat plate solar collectors require both a glass-metal seal to join the glass plate to the rest of the metal envelope and an internal structure to support such plate against atmospheric pressure. The absorber has to be segmented or provided with suitable holes to accommodate such structure. Joining of all parts has to be high vacuum tight and only materials with low vapour pressure can be used to prevent outgassing. Glass-metal seal technology can be based either on metallized glass  or vitrified metal  and defines the type of collector.
Different from evacuated tube collectors, they make use of non-evaporable getter NEG pumps to keep the internal pressure stable through time. This getter pump technology has the advantage of providing some regeneration in-situ by exposure to sunlight. Evacuated flat plate solar collectors have been studied for solar air condition and compared to compact solar concentrators.
These collectors are an alternative to metal collectors and are now being produced in Europe. Polymers are flexible and therefore freeze-tolerant and can employ plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing heat exchangers that lower efficiency.
By dispensing with a heat exchanger, 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 solar irradiance levels. Some early selectively coated polymer collectors suffered from overheating when insulated, as stagnation temperatures can exceed the polymer's melting point.
For this reason polypropylene is not often used in glazed selectively coated solar collectors. In areas where freezing is a possibility, freeze-tolerance the capability to freeze repeatedly without cracking can be achieved by the use of flexible polymers. Silicone rubber pipes have been used for this purpose in UK since Conventional metal collectors are vulnerable to damage from freezing, so if they are water filled they must be carefully plumbed so they completely drain using gravity before freezing is expected, so that they do not crack.
Many metal collectors are installed as part of a sealed heat exchanger system. Rather than having potable 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 down to a locally determined risk temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity 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.
Typically, polypropylene or EPDM rubber or silicone rubber is used as an absorber. Used for pool heating, it 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 less effective. A solar bowl is a type of solar thermal collector that operates similarly to a parabolic dish , but instead of using a tracking 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 moving a large mirror to track the sun as with parabolic dish systems. 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 at the sun does the light focus on one point. That is why parabolic dish systems track the sun. A fixed spherical mirror focuses the light in the same place independent of the sun's position.
Flat Plate Collector
A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating , but may refer to large power generating installations such as solar parabolic troughs and solar towers or non water heating devices such as solar air heaters. Solar thermal collectors are either non-concentrating or concentrating. In non-concentrating collectors, the aperture area i. A common example of such a system is a metal plate that is painted a dark color to maximize the absorption of sunlight. The energy is then collected by cooling the plate with a working fluid , often water or glycol running in pipes attached to the plate.
A Flat Plate Collector is a heat exchanger that converts the radiant solar energy from the sun into heat energy using the well known greenhouse effect. It collects, or captures, solar energy and uses that energy to heat water in the home for bathing, washing and heating, and can even be used to heat outdoor swimming pools and hot tubs. For most residential and small commercial hot water applications, the solar flat plate collector tends to be more cost effective due to their simple design, low cost, and relatively easier installation compared to other forms of hot water heating systems. Also, solar flat plate collectors are more than capable of delivering the necessary quantity of hot water at the required temperature. A solar flat plate collector typically consists of a large heat absorbing plate, usually a large sheet of copper or aluminium as they are both good conductors of heat, which is painted or chemically etched black to absorb as much solar radiation as possible for maximum efficiency. This blackened heat absorbing surface has several parallel copper pipes or tubes called risers, running length ways across the plate which contain the heat transfer fluid, typically water.
The flat-plate solar collectors are probably the most fundamental and most studied technology for solar-powered domestic hot water systems. The overall idea behind this technology is pretty simple. The Sun heats a dark flat surface, which collect as much energy as possible, and then the energy is transferred to water, air, or other fluid for further use. We partially discussed material choices and properties in Lesson 2. However, you are encouraged to look wider and review the current innovations in the flat-plate designs. For the discussion in this lesson, you will be asked to share what you found during your search and describe the state of the art materials that help increase collector performance. Flat-plate collectors are installed facing the equator i.
Concentrating Solar Collectors
Topic last reviewed: 10 April Sectors: Downstream, Upstream. Solar thermal technology can be used in the oil and gas industry to generate process heat or steam. Solar energy collectors transfer the solar energy to a process fluid typically water, oil or air , which is used directly e. Several different technologies can be used to heat a working fluid by sunlight. Solar thermal energy collectors are distinguished by their operating temperature low, medium or high and by their motion non-concentrating or concentrating.
Solar thermal collector
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Manuscript received May 14, ; final manuscript received November 17, ; published online December 23, Editor: Werner Platzer. Sobhansarbandi, S. June 1, Energy Eng.