Volume 3 solar thermal systems components and applications 3 04 – low temperature stationary collectors Volume 3 solar thermal systems components and applications 3 04 – low temperature stationary collectors Volume 3 solar thermal systems components and applications 3 04 – low temperature stationary collectors Volume 3 solar thermal systems components and applications 3 04 – low temperature stationary collectors Volume 3 solar thermal systems components and applications 3 04 – low temperature stationary collectors
3.04 Low Temperature Stationary Collectors YG Caouris, University of Patras, Patras, Greece © 2012 Elsevier Ltd All rights reserved 3.04.1 3.04.1.1 3.04.1.2 3.04.1.2.1 3.04.1.2.2 3.04.1.2.3 3.04.1.2.4 3.04.1.3 3.04.1.4 3.04.1.5 3.04.1.6 3.04.1.7 3.04.1.8 3.04.2 3.04.2.1 3.04.2.2 3.04.2.3 3.04.2.4 3.04.2.5 3.04.3 3.04.3.1 3.04.3.1.1 3.04.3.1.2 3.04.3.1.3 3.04.3.1.4 3.04.3.1.5 3.04.3.1.6 3.04.3.2 3.04.3.3 3.04.3.4 3.04.4 3.04.4.1 3.04.4.2 3.04.4.3 References Introduction Flat-Plate Collectors Absorbers for Liquid FPCs Stamped absorbers Tube absorbers Roll-bond absorbers Organic absorbers Absorbers for Air FPCs Absorber Coating Cover Material for FPCs Back and Side Insulation for FPCs Enclosure or Casing for FPCs Evacuated Tube Collectors Optical Analysis Reflection and Transmission of Radiation Antireflective Coatings Absorption of Solar Radiation Transmittance–Absorptance Product Absorbed Solar Energy Thermal Analysis Steady-State Energy Balance of FPC Radiation exchange between glazing and sky hrc−a Convection exchange between glazing and ambient hcc−a Radiation exchange between absorber and glazing hrp−c Convection exchange between absorber and cover hcp−c Conduction back and edge exchange between absorber and ambient Overall thermal loss determination qloss Solar Collector Top Heat Loss Coefficient Ut Useful Energy Transferred to the Working Fluid Collector Heat-Removal Factor Collector Performance Determination Collector Efficiency Incident Angle Modifier Determination of Effective Thermal Capacity 103 103 104 104 104 106 107 108 108 111 113 113 116 120 120 125 126 127 128 129 129 130 131 132 132 132 133 134 134 138 141 141 143 145 146 3.04.1 Introduction The main part of a solar system is the solar collector, a device that absorbs solar radiation and converts it into heat Low-temperature stationary collectors are the most commonly used solar collectors They can supply heat at temperatures up to about 90 °C above ambient The advantages of these collectors include the lack of moving parts and the capability of collecting both direct and diffuse radiation They can be divided into two main categories: flat-plate collectors (FPCs) and vacuum tube collectors 3.04.1.1 Flat-Plate Collectors Partial components of flat-plate solar collectors, shown in Figures and 2, are as follows: • Absorbing plate It is suitably treated or painted to absorb as much as possible the incident solar radiation • Heat transfer area The area (tubes or channels) through which the absorbed energy is transferred to a fluid (liquid or air) Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00304-8 103 104 Components Transparent cover Liquid conduits Absorbing plate Thermal insulation Figure Cross section of a simple liquid flat-plate solar collector Transparent cover Absorbing plate Air ducts Thermal insulation Figure Cross section of a simple air flat-plate solar collector • Top cover(s) that are transparent to solar spectrum They are placed over the solar absorber surface to reduce convection and radiation losses to the atmosphere • Back and edge insulation It substantially reduces back and edge thermal losses • Enclosure or casing A supportive structure where the absorber is mounted on, together with the top cover and the back and edge insulation, thus forming an enclosure or casing The heart of any solar collector is the absorber, which usually consists of a plate with channels for the flow of the heat-removal fluid It is fabricated from metals (copper, aluminum, steel) or organic material A portion of incident solar radiation is absorbed, and the net produced heat (minus heat losses to ambient) is transferred to a gaseous or liquid heat transfer fluid Thermal insulation is placed on the back surface (not facing the sky) and edges of the absorber The front surface (facing the sky) is covered by sheet(s) that are transparent to solar radiation, placed at close distance Absorber, cover(s), and insulation are assembled into a supportive structure, thus forming an enclosure or casing 3.04.1.2 Absorbers for Liquid FPCs In liquid solar heaters, the liquid usually flows either in passages formed by tubes or in passages formed in metal sheets by stamping Almost all commercially available liquid heating solar collectors use parallel flow through the absorber The individual channels connect into headers at each end Wide spacing of channels reduces absorber cost, while close spacing increases cost, but improves efficiency Fin efficiency drops rather fast as the tube spacing is increased to >15 cm, depending on the thickness and thermal conductivity of the fin and effectiveness of the thermal contact The highest quality, most cost-effective absorbers have sufficient spacing typically no more than 15 cm According to the way of manufacturing, flat-plate absorbers can be classified as stamped, tube, roll bond, and organic ones 3.04.1.2.1 Stamped absorbers In large-scale industrial production, the most popular and cheapest way of absorber fabrication is to bond two metal sheets, usually steel, together The absorbers are formed by expensive machinery, and they are quite heavy and thermally inert Channels for the liquid flow are formed, usually on one sheet, by pressure A suitable pattern is pressed on the sheet so that all necessary flow passages are formed The two sheets are placed one over the other and are bonded across the channels, usually by in-line spot welding Furthermore, the two sheets are welded peripherally with electrical current During the process, a peripheral continuous seam is applied, as shown in Figure Stamped absorbers of a popular form are shown in Figure 4, ready for further treatment 3.04.1.2.2 Tube absorbers Another type of flat-plate solar absorber is the tube absorber It is widely fabricated by small and medium size industries, because for its fabrication there is no need of heavy machinery It consists of tubes attached or soldered to a fin or sheet Copper is the most popular material for the tubes and fins, because of its good thermal conductivity and corrosion resistance Low Temperature Stationary Collectors 105 Figure Fabrication of stamped steel absorbers Figure Stamped steel absorbers Figure Typical forms of tube absorbers The bonded-tube absorber is one of the earliest designs Liquid tubes are fastened to the sheet metal absorber by soldering, wiring, or other methods Tubes must be continuously bonded to the plate for adequate heat transfer However, an aluminum absorber with copper tubes attached by a forced fit combines the desirable features of copper with the economy of aluminum Copper tubes are usually welded, soldered, or clamped to copper or aluminum plates, as shown schematically in Figure The methods commonly used include the tube-in-strip method, tubes welded into headers, and finned tubes The most popular fabrication technique is the tube-in-strip method (Figure 6) Prefabricated parts of single tube-in-strip absorbers are commonly supplied in an overlapping fashion to enable small operators to make up solar collectors of their own designs The method of joining tubes to headers, finned or otherwise, is widely favored and may well be carried out in a small workshop without much expenditure It is best to set out the tubes on a frame to ensure correct alignment and grading of the headers A simple swage tool may be used to provide an overlapping joint, finished with high-grade silver solder Butt welds are prone to damage in transit and to leakage Often, a pipe is simply bent into a serpentine shape and welded onto the back of a flat sheet of metal Figure depicts two possible arrangements of serpentine tube collectors This design reduces slightly heat transfer efficiency but eliminates the possibility of header leaks and ensures uniform flow However, it also increases the pressure drop, and it is not suitable for a system using drain-down protection, because the curved flow passages cannot always be drained completely 106 Components Figure Tube-in-strip absorbers Figure Serpentine tube absorbers The technique of mechanical bonding (the absorber is crimped around the tubes) can be an effective means to attach the tubes to the sheet but there is a risk of poor mechanical sealing and then the thermal performance of the collector is greatly diminished Sometimes, it is used for the application of copper fins to copper tubes, but it must be applied with care A simple sheet metal folding machine may be used for this purpose This technique is widely used with hardened aluminum fins, but suitable protection must be applied at the interface between dissimilar metals The application of aluminum fins to copper tubing is best carried out by purpose-made and expensive machinery Lately, the use of laser and ultrasonic welding machines has been introduced by the industry, which improves both the speed and the quality of welds Fins or sheets are welded on risers, in order to improve heat conduction The greatest advantage of the ultrasonic technique is that the welding is performed at room temperature Therefore, deformation of the welded parts is avoided and the quality of the weld can readily be seen However, this technique leaves a line down the absorber, which diminishes slightly the blackened collecting area Figure illustrates a tube absorber welded by ultrasonic technique Laser welding provides a good seal between the absorber and the tubes without having the weak line associated with ultrasonic welding Figure shows the result of laser welding underneath the absorbing plate 3.04.1.2.3 Roll-bond absorbers The roll-bond technique, depicted schematically in Figure 10, lends itself to the production of low-cost absorbers This technique has been applied for many years to produce heat exchangers for refrigerators It is a well-developed application of mass production methods in the solar hardware field The majority of roll-bond manufacturers produce aluminum absorbers, while the same technique is applicable to copper and steel sheet metals The roll-bond process requires very thin sheet metals The production process starts with two sheets of metal that are thoroughly cleaned to remove the surface oxide film A silk screen process is applied to print the desired pattern of the cooling liquid channels onto the plate A special stop-weld ink is used to prevent bonding in the patterned area Next, the two sheets of metal are bonded together by a heating and pressure process After the adhesion, air pressure is applied to separate the metal sheets by inflation, where they have not been bonded because of the stop-weld ink pattern Thus, the channels for the heat transfer liquid are produced in the absorber It follows from this manufacturing procedure that, during operation, roll-bond absorbers cannot withstand high working pressure, which might lead to leakage of the absorber Roll-bond absorbers also deserve particular attention with respect to corrosion Low Temperature Stationary Collectors 107 Figure Tube absorber welded by ultrasonic technique Figure Tube absorber welded by laser technique Figure 10 Roll-bond absorbers 3.04.1.2.4 Organic absorbers In many low-temperature applications, such as pool and basin heating, unglazed and uninsulated flat-plate organic collectors are used Their main advantage is the much lower cost (about 10 times lower than common metal glazed collectors) In industrial production, a molten organic substance is molded in the form of channeled sheets, as shown in Figure 11 The substances used are colored black, so there is no need of painting However, for aesthetic reasons, many manufacturers produce sheets in a variety of colors, as shown in Figure 12 Sheets are produced in various lengthwise sizes, for example, 400 Â 30 cm They are assembled by the Figure 11 Organic absorber 108 Components Figure 12 Colored organic absorbers Figure 13 Module of unglazed collectors use of suitable fittings, which are bonded at the edges They are easily installed on roofs or terrains in a way of module assembling, as shown in Figure 13 The major components of liquid flat-plate unglazed collectors are the absorber plate and the water passages Since no insulation or glazing is needed, there is no need for an enclosure They require closely spaced thin-walled water passages because of the low thermal conductivity of plastics Their composition makes them susceptible to damage by abrasions and punctures Organic collectors are easier to install because of their lighter weight and flexibility, compared to metal collectors Some manufacturers also produce glazed collectors with organic roll-bond absorbers If they are intended to be used at higher tempera tures, they must be constructed by more expensive organic material with improved properties 3.04.1.3 Absorbers for Air FPCs Solar FPCs can be used to heat air or other gases, with satisfactory performance Because of the low heat transfer coefficients between absorber and air, some type of extended surface geometry is needed to counteract this problem Figure 14 shows a number of absorber designs for FPC solar air heaters that have been used with various accomplishments [1] Metal plates or thin corrugated metal sheets or fabric matrices may be used, in combination with selective or flat black surfaces Therefore, the principal requirement of a large contact area between the absorbing surface and the air applies for all types of absorbers 3.04.1.4 Absorber Coating The upper surface of the absorber that faces the sun must be suitably coated The type of coating plays a significant role in the performance of solar collectors and determines the absorbed fraction of incident solar energy Coatings must have high absorptance for radiation in the solar energy spectrum and long-wave emittance as low as possible, to reduce infrared (IR) thermal radiation losses Coatings are classified as flat black (nonselective) and selective Flat black paints consist of a pigment material (an organic binder that polymerizes during drying) and solvents that permit easy application of the paint film In drying, the solvent evaporates Low Temperature Stationary Collectors (a) Transparent cover (b) Absorbing plate Transparent cover Transparent cover (c) Absorbing surface Air flow passage Thermal insulation Plain sheet (d) Transparent cover Absorbing surface Air Air flow passage Thermal insulation Single corrugated sheet (e) Absorbing surface Air flow passage Thermal insulation V corrugated sheet Transparent cover Absorbing surface Air flow passage Thermal insulation Trapezoidal or square profile sheet 109 flow pas sage Thermal insulation Double corrugated sheet (f) Transparent cover Matrix er absorb Air flow Thermal insulation Matrix type Figure 14 Flat-plate solar air heater designs Figure 15 Flat black painting process in isolated space and the pigment and binder form a film of 1–3 mils thick Flat black coatings are applied as a common color painting The typical method of application is by spray gun, and all the work is done in chambers that are well ventilated or isolated, as in Figure 15 A flat black paint is a good absorber, but since the paint film is not selective at all, it has absorptance and emittance of 0.95–0.98 Until a few years ago, flat black paint was the most commonly used coating, because it is cheap and quite durable Lately, the mass production of good-quality, not expensive selective surfaces tends to dominate the solar thermal market The temperature of the absorbing surface in most stationary collectors is