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 Volume 3 solar thermal systems components and applications 3 04 – low temperature stationary collectors

© 2012 Elsevier Ltd All rights reserved

References

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 1 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 3 doi:10.1016/B978-0-08-087872-0.00304-8 103

Transparent cover

Thermal insulation Figure 1 Cross section of a simple liquid flat-plate solar collector

Transparent cover

Air ducts

Absorbing plate

Thermal insulation Figure 2 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

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 3 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

Figure 3 Fabrication of stamped steel absorbers

Figure 4 Stamped steel absorbers

Figure 5 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 5 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 7 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

Figure 6 Tube-in-strip absorbers

Figure 7 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 8 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 9 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

Figure 8 Tube absorber welded by ultrasonic technique

Figure 9 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
30 cm They are assembled by the

Figure 11 Organic absorber

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 tempera­tures, 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

Matrix absorber

Air flow passage

Thermal insulation

Plain sheet

Air flow passage Thermal insulation Single corrugated sheet

Air flow pas sage Thermal insulation Double corrugated sheet

Air flow

profile sheet

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 <100 °C (373 K), while the equivalent temperature of the sun is ∼6000 K The great portion (98%) of the extraterrestrial solar radiation lies in the range of 0.2–3.0 μm, while 99% of the long-wave radiation of a blackbody at 200 °C lies at wavelengths <3.0 μm So a perfect coating for solar absorber surface should have absorptance α = 1 for solar spectrum and emittance ε = 0 for long-wave radiation The characteristics desired for an ideal coating surface are as follows: α = ε = 1 for wavelengths <4 μm and α = ε = 0 for wavelengths >4 μm A surface with these ideal properties is called ‘selective’, because of its selective behavior for those two discrete radiation wavelength ranges Unfortunately, materials with these properties do not exist in nature Virtually, all black materials have high solar absorptance and also high IR emittance Thus, it

is necessary to manufacture selective materials with ideal or very close to ideal properties Selective coatings should have the following physical properties [2]:

• They must have high absorptance for solar spectrum in the range of 0.2–2.5 μm and low emittance for spectrum >2.0 μm

• The spectral transition between the region of high absorptance and low emittance must be as sharp as possible

• The opto-physical properties of the coating must remain stable under long-term operation at elevated temperatures, repeated thermal cycling, air exposure, ultraviolet radiation, and other conditions

• Adherence of coating to substrate must be good

• Coating should be easily applicable and must be economical

The most commonly used selective surfaces are thin layers of metal oxides that are deposited by electrolysis or in vacuum on the polished metal absorber plate Typical selective surfaces consist of a thin upper layer, which is highly absorbent to solar radiation but relatively transparent to long-wave thermal radiation, deposited on a surface that has high reflectance and low emittance for long-wave radiation Lately, low-cost mechanically manufactured selective solar absorber surfaces have been developed

Blackbody spectrum at

6000 K (aproximation

of Solar spectrum)

Blackbody spectrum at

Ideal selective surface Real selective surface

Selective coatings can be categorized into six distinct types [3]: (1) intrinsic, (2) semiconductor–metal tandems, (3) multilayer absorbers, (4) metal–dielectric composite coatings, (5) textured surfaces, and (6) selectively solar-transmitting coating on a blackbody-like absorber Intrinsic coatings use substances having intrinsic properties that lead to the desired spectral relevance Semiconductor–metal tandems are highly absorbing for solar radiation because of the semiconductor band gap and have low long-wave emittance as a result of the metal layer Multilayer absorbers use multiple reflections between layers to absorb light Metal–dielectric composites (cermets) consist of fine metal particles in a dielectric or ceramic host material Textured surfaces present high solar absorptance because of multiple reflections among porous dendritic, or needle-like, microstructure For low-temperature applications, solar-transmitting and high-IR-reflecting coatings on a blackbody-like absorber are also used Solar selective surfaces can be fabricated by the following major techniques [1]: (1) vacuum evaporation, (2) vacuum sputtering, (3) ion exchange, (4) chemical vapor disposition, (5) chemical oxidation, (6) dipping in chemical baths, (7) electroplating, (8) spraying, (9) screen printing, and (10) brush painting method During recent years, much of the progress has been based on the implementa­tion of vacuum techniques for the production of fin- and sheet-type absorbers The chemical and electrochemical processes were readily taken over from the metal finishing industry The vacuum techniques are, nowadays, mature, characterized by low cost and have the advantage of being less environmentally polluting than the wet processes A typical structure of commercial tandem selective absorber is shown in Figure 17 The substrate could be any material used in solar energy collection, usually metal or glass The second layer is an IR-reflecting low-emittance layer, usually a copper-deposited layer, which reflects back the long-wave radiation of the substrate There is no need of this layer if the substrate is metal The third layer is the selective absorbing surface, usually made of nickel, chrome, or copper oxides Finally, the fourth antireflective layer improves the optical performance as it decreases reflectance losses of the absorbing layer It also has the function of a protective film and is made typically of dielectrics with a graded refractive index

Today, technology produces selective surfaces in large ribbon rolls, as shown in Figure 18, ready for further elaboration, that is, welding on tubes Furthermore, selective thin surfaces are offered in ribbon rolls of self-adhesive thin-film metal sheets, as shown in

Antireflective layer

IR reflecting layer Substrate

Absorbing layer

Figure 17 Typical selective absorber structure

Table 1 Absorptance and emittance of selective surfaces

Copper, aluminum, or nickel plate with CuO coating 0.8–0.93 0.09–0.21

Black copper (BlCu-Cu2O:Cu) on Cu substrate 0.97–0.98 0.02

Figure 18 Commercial selective metal roll sheet

Figure 19 Self-adhesive selective thin metal roll sheet

3.04.1.5 Cover Material for FPCs

Covering or glazing is essential for the prevention of absorber thermal losses to ambient The glazing should allow as much as possible incident solar irradiation to arrive at the absorber and reduce as much as possible the upward heat losses As upward heat losses occur by convection and long-wave radiation, covering must reduce both of them The presence of cover(s) prevents convection losses by shielding the absorber from ambient air The perfect shielding for long-wave radiation happens when a reflective cover is used However, a thermally opaque material acts in the same way Absorber thermal losses cause an increase in the cover temperature and subsequently a loss of heat to ambient by radiation and convection

Glass has been widely used to glaze solar collectors because it can transmit as much as 92% of the incoming solar irradiation [1] Also, being thermally opaque it absorbs around 88% of the absorber long-wave radiation and reflects back the rest Glass has very good mechanical and physical properties, and withstands perfectly time aging under ambient conditions Some drawbacks of common glass are that it is usually heavy and vulnerable to breaking by hail or stones However, the use of tempered glass surpasses the last disadvantage Tempered or toughened glass is glass that has been processed by controlled thermal or chemical treatments, which create balanced internal stresses, to increase its strength compared with normal glass Of the various grades of tempered plate glass, low-iron glass has the highest transmission and lowest reflection for solar radiation These properties result in significant increase in collector efficiency, so the cost premium for low-iron glass is smaller than the increase in efficiency Coatings that are antireflective to solar spectrum, and surface texture (e.g., prismatic), can also improve transmission significantly In Figure 20, a prismatic textured (at the internal surface only) tempered glass cover of a commercial collector is shown Also, coatings that are reflective to thermal radiation reduce thermal radiation losses when applied to the internal glass surface (absorber side)

Figure 20 Internally prismatic tempered glass, at work

Polymeric materials also indicate high solar transmittance, but because almost all of them have transmission bands in the thermal radiation spectrum, they may allow a substantial portion (as high as 40%) of the absorber long-wave radiation to pass through Furthermore, polymers can sustain smaller temperature limits and deteriorate easily Only a few types of polymers can withstand the sun’s ultraviolet radiation for long periods Polymers inside a well-sealed collector may deteriorate rapidly and will outgas, depositing a haze of condensed oily liquid on the inside surface of the glazing Such haze may seriously reduce the collector efficiency However, they are not broken by hail or stones, have increased strength, less weight than glass, and in the form of thin films, they are completely flexible In some double-glazing designs, one layer of glass is used along with a layer of thin polymer underneath

The effect of dirt and dust on collector glazing must be quite small, and the cleansing effect of an occasional rainfall is usually adequate to maintain the transmittance within 96–98% of its maximum value [1]

The presence of one transparent cover reduces absorber thermal losses by convection and radiation As radiation loss is almost eliminated with the use of selective coatings, the cover contributes almost exclusively to the suppression of convective loss Further suppression could be achieved if two or more covers are placed However, the presence of more covers decreases essentially the transmitted solar radiation, due to reflection and absorption, and makes the structure much heavier An alternative solution would

be to maintain vacuum or very low pressure between absorber and cover, but when speaking for FPC designs, insuperable difficulties appear Vacuum maintenance is almost impossible, and requirements for material strength are very high However, for other collector designs (evacuated tube), this concept is widely applied, as will be mentioned later

Convection loss could be as well inhibited if the air between absorber and cover remains stagnant For free convection, this means that buoyant forces must be less than friction forces This is the case of enclosed air in narrow cavities Thus, convection loss could be prevented if a honeycomb-type transparent cellular structure is placed between the absorber and the outer cover, as shown schematically in Figure 21 However, such a transparent insulation material (TIM) reflects a greater part of the incoming radiation than a simple glass cover, thus preventing solar radiation from reaching the absorber, and also increases the cost A cellular structure also increases the thermal conductivity between absorber and cover A TIM that transmits well solar radiation, is opaque in the thermal radiation, and has low thermal conductivity could be ideal for a solar collector Solar transmittance and heat loss coefficient are the two parameters used for the characterization of a TIM Various prototypes of transparently insulated FPCs have been fabricated and tested in the last decade [4, 5] Figure 22 shows a cutout of TIM used in a collector Low-cost and high-temperature­resistant TIMs have been developed so that the commercialization of these collectors becomes feasible A comparative study of TIM cover systems shows that honeycomb systems excel over other systems [6] TIM covers presently available (e.g., small-celled

Transparent co

ver

Transparent insulation Absorber

Figure 21 Honeycomb structure for absorber convective loss suppression

Figure 22 Commercial transparent insulation material over collector’s absorbing surface

polycarbonate honeycomb TIM covers) offer good possibilities for their application where the typical working temperatures are between 50 and 80 °C Recently, the cost of optimized honeycomb covers made of Mylar and Lexan runs into $9 and $7 m−2, respectively [6, 7]

3.04.1.6 Back and Side Insulation for FPCs

Insulation plays a significant role in curbing heat loss due to conduction in a solar collector Various types of insulation can be used

in collectors Polyurethane chlorofluorocarbon (CFC)-free case insulation has become popular for solar collectors because it has a higher insulation value than any other practical insulation material and does not deteriorate with humidity However, it must be used in solar collectors with great care An otherwise well-designed solar collector will experience stagnation temperatures that will cause the insulation of this type to outgas and rapidly destroy the efficiency of the collector Another solution is the use of hardened glass wool or mineral wool, which are temperature and fire resistant, although they are very sensitive to humidity Insulation must

be kept dry or else it loses all or most of its insulating value When the collector is assembled, the air trapped inside will contain moisture, which eventually will condense and become soaked into the insulation To prevent it, quality collectors contain porous bags of silica gel desiccant to absorb the moisture Typically, the desiccant is contained in the hollow spacers separating the glazing and the absorber, and small holes on the surface of the spacers facing the space between the panes permit the trapped air to contact the desiccant If desiccant is not used, it will become apparent through condensation of drops of water on the inner surface of the glass A cheaper solution is to create small holes at the bottom (base) frame, so that rain water is very difficult to enter and any formed condensation can come out by evaporation Usually, a thin reflective aluminum foil is adhered at the absorber side of the back insulation The absorber must not touch the foil It must be mounted in suitable sockets keeping a distance of 0.5–1.5 cm away from the foil, so that the foil acts as a reflector (radiation shield) to thermal radiation emitted by the absorber

Case insulation is not the only important insulation task in a quality collector The absorber plate and connecting tubing penetrating the enclosure must be thermally insulated from the case at every point of the support Heat paths from the warm sections of the collector to the basic structure must be eliminated The supports for the cover, for instance, must be insulated not only from the absorber surface but also from glass and the air spaces Heat losses can be severe if either the absorber or tubing touches the case or is supported through heat-conducting materials to the case

Vacuum insulation materials, which are recently developed for other applications such as building insulation or insulation for stoves and boilers, could also be used With vacuum insulation materials, it should be possible to reach thermal conductivity values between 5 and 10 times lower than for ordinary insulation materials, depending on the temperature range Vacuum insulation panels consist in general of a base material that is placed in a volume surrounded by gastight foils [8] The vacuum inside these panels has a key function due to the fact that the thermal conductivity of an insulation material depends mainly on the heat conduction of the gas inside the material By evacuation, the conductivity of the composite structure will be reduced The base material is a kind of silicon acid with a very small pore size It is produced under low pressure and packed in panels, covered with a gastight foil [8] However, this material is still expensive and its use in solar collectors is not yet cost-effective

3.04.1.7 Enclosure or Casing for FPCs

The absorber, the top cover, and the back and edge insulation must be mounted together on a supportive structure, forming thus an enclosure or casing The enclosure serves to contain insulation, provides support for the absorber and glazing, and protects the collector from heat loss to ambient Furthermore, it has the important function of keeping moisture from rain and dew out of the insulation Enclosures are made of a variety of materials and designs but are usually made of galvanized steel (bonded or formed) or

of aluminum profiles with a back aluminum thinner sheet Whatever the case material and construction, it must be weather resistant, fireproof, durable, dimensionally stable, strong, and completely sealed permanently against moisture intrusion As a

general rule, the joints and seams should be minimized and completely sealed Aluminum should be used with caution in areas exposed to salt air, industrial pollution, or smog in the air Most top-quality collectors use enclosures of anodized aluminum similar

to those for exterior windows Adequate clearance (e.g., around the glass cover) and proper gaskets must be provided for the expansion of various collector components Provision must be made for expansion and contraction of the cover plate material, because its expansion coefficient is quite different from that of the framework These requirements are generally met by using a U-shaped, extruded rubber gasket held in place within a metal trim strip A silicone rubber is an excellent choice because this material is very weather resistant The frame should be designed to cast almost no shadow on the absorber plate, and the aperture area should be at least 85% of the gross area Sealing compounds and gaskets should be capable of withstanding thermal cycling and stagnation temperatures without outgassing

An enclosure made of anodized aluminum profiles, with a polyurethane back and side insulation, is shown in Figure 23 The adhered reflective aluminum foil (malformed) is also shown The sockets for the absorber support are made from hard rubber They are incorporated into the insulation and are shown as knobs (indicated by arrows) Figure 24 shows, on the left side, enclosures made of formed galvanized steel plates and, on the right side, the glass wool insulation, covered by thin aluminum foil

An entire collector structure is shown in Figure 25, where different parts are indicated separately Flat Plate Collectors dominate today’s solar thermal market, with higher quality issue the full face absorber type, which has reduced thermal losses and greatest absorber to enclosure ratio Some other collector types must also be mentioned, such as the transpired air collectors and multipass types

Transpired air collectors are quite simple structures for heating purposes in buildings A perforated blackened metal sheet is placed at close distance, in front and across a building wall A fan sorbs ambient air, which passes through the perforation holes, and

is heated and distributed inside the building, as shown in Figure 26

Multipass solar collectors are another type, dedicated for different applications [9–11] In Figure 27, a two-pass solar air heater is shown schematically This design doubles the heat transfer area and improves the performance for elevated air temperatures Another two-pass liquid collector is shown schematically in Figure 28 The first liquid layer (2) is made of transparent material with ducts where a transparent heat transfer liquid passes through and functions as a preheater, absorbing the heat loss of the absorber (3) It has been shown that this collector type outperforms when inlet temperature is kept low (near ambient temperature) [12, 13] Therefore, it is suitable for once-through systems

Typical flow rate for liquid FPC is 0.01–0.05 kg s−1m−2 and for air FPC it is 0.01 m3 s m

Figure 23 Enclosure made by aluminum profiles and plastic joins

Figure 24 Galvanized steel enclosures (in stock) and fiber glass wool insulation plates

Aluminum absorbing surface with bonded copper tubes, underneath Figure 25 Typical commercial flat-plate collector

Thermal insulation Figure 27 Schematic diagram of a two-pass air heater

3.04.1.8 Evacuated Tube Collectors

The previously mentioned concept, that is, the evacuation of the space between the absorber and the outer cover, can be easily applied on tubular collector designs The vacuum glass tube technology is extremely mature, from the well-established production of fluorescent lamps, special scientific apparatus, and other products Thereby, a collector with evacuated space between absorber and cover could be created, if an absorbing finned tube is placed inside a glass tube, which is then welded at its edges to the glass and finally the tube is evacuated and sealed Because of differential expansion of glass and finned tube, it is essential to have only a single welding area, or withstanding bellows; otherwise, the glass tube will be broken This vulnerability has been surpassed by mainly two types of configurations: the single-glass type and the twin-glass type (also called Dewar or Sydney type)

The ‘single-glass type’ consists of a glass vacuum tube with an inside mounted flat absorber The absorber tube may have U-shaped forms or concentric forms, as in Figure 29 The type shown in Figure 29(a) has a heat pipe absorber with a single glass-to-metal seal Heat pipe is a hermetically sealed tube that contains a small amount of vaporizable fluid (e.g., methanol) When the tube is heated, the liquid evaporates and condenses at a colder chamber (condenser, heat sink section), transferring heat with great effectiveness (thousands of times greater than that of the best solid heat conductor of the same dimensions), because of the latent heat of condensation To ensure that the liquid flows back to the heated tube, the heat pipe contains a wick or is tilted (or both) With a proper tilt, gravity returns the condensed fluid back to the evaporating region, so there is no need of capillary wick and thereby it functions only in one direction In an evacuated tube collector (ETC), a sealed copper pipe (heat pipe) is bonded to

a copper absorbing fin, usually selective, that is mounted inside the evacuated glass space A small condenser is attached to the top

of the heat pipe and is inserted into a thermally insulated heat exchanger duct, at the top of the solar collector system As the absorber is heated, the heat pipe liquid boils and hot vapor rises toward the condenser A heat transfer liquid (usually water or water–glycol mixture) flows through the duct and cools the condenser This heat transfer liquid delivers its heat to storage and/or

to load through a heat exchanger The whole process is shown schematically in Figure 30 The maximum operating temperature

of a heat pipe is the critical temperature of the vaporizable fluid used Since no evaporation/condensation above the critical temperature is possible, the thermodynamic cycle interrupts when the temperature of the evaporator exceeds this critical temperature Thus, the heat pipe offers inherent protection from overheating and freezing This self-limiting temperature control

is a unique feature of the heat pipe collector Also, heat pipes have lower heat capacity than ordinary liquid-filled absorber tubes, thus collecting solar energy more efficiently by minimizing warm-up and cool-down losses A heat pipe ETC unit is shown in

Figure 31

In the second configuration (Figure 29(b)), liquid flow is ‘down and back’ through a U-shaped tube Two glass-to-metal seals are formed at the same edge of the tube, so attention must be given to the edge geometry to allow absorption of differential expansions Also the two flow streams must be thermally decoupled as much as possible and the solution is to split the fin into two parts [14] The third configuration (Figure 29(c)) has a single glass-to-metal seal and heat is extracted through concentric tube geometry The heat transfer fluid enters the inner tube (inlet) and turns back through the outer tube (outlet) In a multi-glass-tube collector configuration, inlets and outlets are connected to separate top headers

Condenser

Glassseal

Evaporator

Liquidcondensate

Concentric tube

Intlet

Glass seal

Selective coating

Vacuum space

Selective coating

Vacuum space

Barium getter

Figure 30 Schematic diagram of heat pipe operation process

Thermal insulation Condenser

Heat transfer fluid Evacuated tube

Heat pipe tube Absorber

Figure 31 Heat pipe evacuated tube collector

The ‘twin-glass ETC’ (Dewar) type consists of two glass tubes, usually made of borosilicate glass (commercially known as SCHOTT or PYREX), as shown schematically in Figure 32 The outer tube is transparent allowing solar radiation to pass through with minimal reflection The two tubes are fused together at the top, and the air contained in the space between the tubes is pumped

Figure 32 Schematic diagram of twin-glass evacuated tube

out, thus creating a vacuum jacket The inner tube is coated outside (vacuum-side surface) with a selective absorbing coating The advantage of this design is that it is made entirely of glass, thus leakage losses are avoided Another big advantage of the twin-glass ETC is its ability to passively track the sun This feature gives a more consistent output than any other collector over the whole day It

is also less expensive compared to the single-glass configuration [15] Heat can be extracted if a heat transfer fluid fills directly the inside Dewar space and turns back through a concentric tube, but a breakage of glass tube will result in loss of fluid and failure of an entire collector array A safer solution is to insert a fin-and-tube absorber inside the Dewar space, in good thermal contact with the inner glass; for this, the fin must be rolled into a cylindrical form to provide a ‘spring’ fit when inserted In Figure 33, two configurations of twin-glass ETC with fin-and-tube absorber are shown: a U-tube type (a) and a heat pipe type (b)

The vacuum (10−5 torr) of a tubular evacuated solar collector has to be maintained during the 25+ years life of the device It has been found that a number of evacuated solar collectors face the problem of vacuum degradation due to poor sealing techniques Therefore, highly reliable vacuum seals are key quality criterion as the seals withstand the thermal stress and temperature shocks To absorb material outgassing due to the high operational temperature, the vacuum is maintained through

a barium getter (as in old radio tubes) inserted in the collector tube During manufacture, this getter is exposed to high temperature, which causes the bottom of the evacuated tube to be coated with a pure layer of barium This barium layer actively absorbs any CO, CO2, N2, O2, H2O, and H2 outgassed during storage and operation, thus helping to maintain the vacuum The barium layer also provides a clear visual indicator of the vacuum status The silver-colored barium layer will turn white if the vacuum is lost The dose of barium must be calculated for the targeted life cycle of the system A final remark is that system stagnation reduces the life expectancy of tubes

A complete collector panel consists of a large number of individual tubes positioned in parallel rows and connected to separate header pipes (U-tube or concentric tube type) or to a manifold header (heat pipe type) The headers are mounted in a well-insulated box that reduces the heat loss The number of tubes depends on the heating needs of every individual application A typical ETC panel used for hot water production is shown in Figure 34 Several manufacturers produce ETC panels with an added diffuse or compound parabolic reflector (CPR) of low concentration ratio, underneath the tubes CPRs are nonimaging concentrators They are capable of reflecting the incident solar radiation to the absorber, within the wide limits of the acceptance angle They are trough-shaped with two sections of a truncated parabola facing each other They can accept incoming radiation over a relatively wide range of incident angles [1] By multiple reflections, any radiation that is entering the aperture, within the collector acceptance angle, strikes the absorber surface at the bottom of the structure, as shown schematically in Figure 35 For stationary collectors mounted in CPR, big acceptance angles are used to enable the collection of diffuse radiation, at the expense of a lower concentration ratio A CPR with low concentration ratio (e.g., C = 1.5) collects two-thirds of the available diffuse solar radiation [16] An ETC commercial panel with CPRs is shown in Figure 36

One of the principal advantages of conventional vacuum tube collectors is that the wind can pass between the tubes; however with a reflector, increased wind loading is inevitable It is also very important to verify that the reflectors are very tightly connected to minimize rattles

Like FPCs, ETCs collect both direct and diffuse radiation However, their efficiency is higher at low incidence angles This effect tends to give ETC an advantage over FPC in day-long performance ETCs are more breakable by hailstones, although some manufacturers construct the outer tube of extremely strong transparent borosilicate glass that is able to resist impact from hailstones of up to 25 mm in diameter [17] Another disadvantage is their difficulty of snow rejection If snow is accumulated over and between glass tubes, it does not melt easily, because of their negligible heat loss and the collector will not be able to capture solar energy

(a)

(b)

Figure 33 Twin-glass evacuated tube collector with fin-and-tube absorber: (a) U-tube and (b) heat pipe

Figure 34 Typical evacuated tube collector panel

Figure 35 Light trapping by evacuated tube collector with compound parabolic reflector to the back

Figure 36 Typical evacuated tube collector panel with a low-concentration compound parabolic reflector

3.04.2 Optical Analysis

Generally, the term ‘optical’ refers to how visible electromagnetic radiation (light) is propagated through various mediums Hereafter, it will be used for the description of how various parts of a solar collector behave to solar radiation spectrum (0.2–4 μm) This behavior in the transmission, reflection, and absorption of solar radiation is important for the determination of collector performance The transmittance, reflectance, and absorptance of transparent materials depend on the thickness, extinction coefficient k, and refractive index n of the material The refractive index of a medium is specified as the ratio of the radiation velocity

in vacuum to that in the medium The extinction coefficient is a proportionality constant related with the per unit distance absorption of radiation through a medium Physically, k and n depend on the wavelength of the radiation However, for common glazing materials (glass and plastics), they are assumed to be independent of the wavelength, considering their mean values for the solar spectrum Of significant importance is the effect of radiation polarization Polarization describes the orientation of wave oscillations in space Transverse electromagnetic waves such as solar radiation exhibit polarization In a uniform isotropic medium,

→

→ solar radiation waves may be described as a superposition of sinusoidally varying electric E and magnetic B field plane waves, aligned perpendicular to one another and to the direction of propagation For polarization description, it is sufficient to specify the behavior of the

When radiation of intensity I contacts a transparent (or translucent) medium, a part Ir of it is reflected, a part Iα is absorbed, and a part I
is transmitted Reflectance r, absorptance α, and transmittance
are defined as the ratios r = Ir/I, α = Iα/I,
= I
/I, and according

to the first law of thermodynamics r + α +
= 1

3.04.2.1 Reflection and Transmission of Radiation

Fresnel, Snell, and Stokes have established the principles governing the radiation transmission through smooth, homogeneous transparent mediums with no internal scattering When a radiation beam is passing from a medium 1 with refractive index n1 to another medium 2 with refractive index n2, then at the interface of the two mediums a part is reflected and the rest passes into medium 2, subjected to direction change The ratio of the intensity of reflected radiation Ir to the intensity of incident radiation Ii is defined as surface reflectance r = Ir/Ii

Considering the case of a plane interface as in Figure 38, if the incident radiation forms angle 1 with the normal to the plane, called angle of incidence, then the reflected radiation forms an equal angle at the same incident plane, defined by the incident beam and the surface normal The following relationships apply for the parallel r║ and perpendicular r reflectance components, relative ┴

to the plane of incidence:

Figure 38 Reflection and refraction of smooth surfaces

For unpolarized radiation, which is the case of natural solar radiation, the total reflectance is equal to the average of the two components:

Ir r⊥ þ r∥

Ii 2 The transmitted radiation is deflected by an angle 2 on the same plane of incidence and is related to 1, n1, and n2 by the relation

n2 sin 1 For normal incidence, 1= 2 = 0 and the reflectance is

∞ X

‖;r ¼ ð1 − r‖ Þ2 þ ð1 − r‖ Þ r þ ð1 − r‖ Þ r þ ⋯¼ð1 − r‖ ‖ ‖ Þ2ð1 þ r þ r þ ⋯Þ ¼ ð1 − r‖ ‖ ‖ Þ r‖ ½6

i ¼ 0 P∞

where series i¼0 r∥ 2i tends to 1=ð1− r2‖ Þ, so

The transmitted radiation becomes thus partially polarized, because of the different values of
║,r and
┴,r:

Figure 39 Ray trace for transmission and reflection of one nonabsorbing cover

an infinitesimal path dx, at a point inside a medium, is proportional to the intensity of radiation I at that point and the path length:

dI

I where dI is the infinitesimal absorption or the incremental decrease of the radiation intensity, k is a proportionality coefficient, the absorption or extinction coefficient, which is assumed to be constant in the solar spectrum, and x is the distance that radiation travels Because there are fewer photons that pass through the path than those that are entering it, the intensity change

is actually negative The solution of the differential equation [10], for an overall traveled distance X, is obtained by integrating both sides

where I0 is the entering radiation and IX the transmitted radiation after traveling the distance X The transmittance
a due

to absorption in a path length X is by definition
a= IX/I0 For a cover with thickness L, any traveled distance X can be expressed

as X = L/cos(2), where 2 is the beam deflection angle, as in Figure 38 Then from eqn [11],

− kL cosð2Þ

For glass, the extinction coefficient k takes values from 4 m−1 (water white glass) to 34 m−1 (high-iron glass)

Taking into account the transmittances due to reflection and absorption, a similar analysis, as previously for the combined transmittance
, reflectance ρ, and absorptance α, for each polarization component yields the following relations (perpendicular polarization component):

1 0.9 0.8 0.7

Transmittance due to reflection for 1 cover

Figure 40 Angular dependence of transmittance of glass cover systems with kL = 0.05

Figure 41 Angular dependence of transmittance of polycarbonate cover systems with kL = 0.03

1

0.8

0.6

Single cover 0.4

−3.868 84 1.511 85 –

−0.017 91 1.548 39 2.991 74

−7.155 28 3.633 05

1.517 and extinction coefficient is 28 m−1 [19]

cover systems

The previous analysis applies only to the beam component of solar radiation However, the radiation incident on a collector consists also of diffuse sky radiation and radiation diffusely reflected from the ground While the preceding analysis can be applied directly to beam contribution, the transmittance cover systems for diffuse and ground-reflected radiation must be calculated by integrating the transmittance over the appropriate incidence angles with an assumed sky model In general, the angular distribution

of sky and ground-reflected radiation is unknown The calculation can be simplified by defining equivalent angles that give the same transmittance as the result of integration for diffuse and ground-reflected radiation [20] The integration of the transmittance over the appropriate incident angle, with an isotropic diffuse radiation model, leads to an equivalent angle of incidence θsky for diffuse sky radiation:

All angles in relations [22]–[24] are expressed in degrees

Figure 42 Angular dependence of normalized transmittance of common cover systems