Solar Water Panels
Solar hot water systems are environmentally friendly and can now be installed on your roof to blend with the architecture of your house. More than 1.5 million homes and businesses in the United States have invested in solar heating systems, and surveys indicate that over 94% of these customers consider the systems a good investment.
General Classification and Application of Solar Collectors
Solar collectors (panels) can be classified into two major categories based on the working fluid used to cool them. These two are “liquid” and “air”. Each of these two categories can then be sub-classified according to average temperature range over which they are intended to be used, and can be used effectively. These classifications are: “low temperature”, “medium temperature” and “high temperature”, with some overlap among the classifications depending on the construction of the individual collector model.
Low Temperature Collectors
Low temperature solar collectors are typically unglazed flat plate collectors, intended to operate at temperatures only 5 to 30 degrees above ambient temperature. Low temperature liquid collectors are used for swimming pool heating. With light glazing and enclosure, they are used as air collectors for agricultural low-temperature applications such as crop drying.
Plastic low-temperature collectors have been used widely for swimming pool heating. However, because of the deteriorating effects of ultra violet radiation and stagnation temperatures on some plastic solar collectors, metal collectors are being more widely utilized. Unglazed collectors with aluminum absorber plates and copper water passages appear to be most cost effective over the typical metal collector lifetime of 20 years or more. All-copper collectors for swimming pool heating also work well, but are generally more expensive for the same performance characteristics. Copper is preferred over any other metal for water passages because of its high conductivity and compatibility with water. Almost all other metals must be separated from direct contact with the water being heated by a heat exchanger, which seriously reduces the collector efficiency.
Medium Temperature Collectors
Medium temperature collectors typically are flat-plate collectors, enclosed in an insulated case, with one or two glazings. The intended temperature range of operation is from about 15 to 200 degrees F above ambient temperature. For the lower end of this range, single-glazed collectors with non-selective absorber plates are most cost effective. In the middle and high end of the range, selective collectors with one or two glazings become more cost effective. Typical applications include water heating, space heating and some medium temperature industrial heating uses.
High Temperature Collectors
High temperature collectors include some overlap from flat plate collectors in the medium temperature range, with selective absorber plates and heavy insulation, and may have temperature capabilities enhanced in installation by being mounted in a sun-tracking system. Many other variations of high temperature collectors include evacuated-tube flat plate types, parabolic dish reflector types, parabolic trough types and modified parabola types. Most high temperature collectors depend on some type of sun tracking, in one or two axes, for effective operation. Tracking collectors are used to a small extent for domestic water heating and space heating, but are limited in cost effectiveness and reliability by the complexities of the tracking mechanisms, where used. Other uses are highly specialized, such as in absorption cooling systems, and applications where steam must be generated, or very high temperatures are required with sacrifice of efficiency.
Parabolic and other types of focusing collectors do not respond to indirect solar radiation, and collect little, if any heat when the sun is obscured enough to prevent a clear shadow from being thrown. Conversely, flat plate collectors respond to radiation from all directions, and will collect diffuse radiation energy of as much as 30% of normal direct energy when there is no visible shadow.
Liquid Flat Plate Collector Design and Construction Liquid flat plate collectors can be further classified into two major sub-classifications, These classifications are: unglazed and glazed.
Unglazed Flat Plate Liquid Collectors
Unglazed liquid flat plate collectors are used almost exclusively for swimming pool heating. The only major components of a liquid flat plate unglazed collector are the absorber plate and the water passages. Since no insulation or glazing is needed there is no need for an enclosure.
Plastic versions require closely spaced thin-walled water passages because of the low thermal conductivity of plastics. This makes them tend to be susceptible to damage by relatively low water pressure and to abrasion and punctures. Flow rates required for swimming pool heating are 4 to 6 gallons per minute for a typical 40 square ft. collector, a condition which tends to aggravate the susceptibility of plastic collectors to failure. Some plastic collector designs are easier to install because of their lighter weight and flexibility, than are some metal collectors.
Unglazed metal collectors for swimming pools almost always have copper in contact with the water flowing through them. Many of them use aluminum for the absorber plate or fins, because of its lower cost relative to copper or other effective materials. Some are all copper, with very thin absorber plate or fins to reduce cost, but at the sacrifice of some efficiency. Water tubes may be soldered, brazed, snapped into grooves or inserted into spring-loaded extrusions.
Selective coatings are not used on properly designed unglazed solar collectors, because it serves no purpose. The temperatures of operation are low enough to make radiation losses negligible.
Desirable Design Features of Unglazed Collectors
Unglazed solar collectors have potentially high efficiency by comparison to any other type when each is used in its intended operating temperature range. The elimination of glazing and case reduces the losses of incident energy otherwise caused by absorption in and reflection by the glazing, and partial shading by the case. To ensure the availability of this high efficiency, the collector must have a high “fin efficiency”. The factors that make fin efficiency are the following:
A. High surface absorptivity. This is available through use of flat black paint with absorptivity of .95 or better, which can be maintained if a high quality weather resistant baked paint is used over the appropriate baked primer. Less expensive panels frequently omit both primer and baking, soon resulting in loss of absorptivity, and perhaps early onset of corrosion.
B. Optimum tube spacing. Wide spacing of tubes reduces collector cost, while close spacing increases cost, but improves efficiency. Fin efficiency drops rather fast as the tube spacing is increased above about four inches, depending on the thickness and thermal conductivity of the fin metal, and effectiveness of thermal bond. The highest quality, most cost effective collectors have sufficient spacing typically no more than 4 inches, with large area tight bonding to the water tubes.
Another highly desirable design and construction feature is secure attachment of headers to water tubes. High quality collectors have the tubes brazed or welded to the headers and supported mechanically by insertion into sockets extracted directly from the header metal by a “t-drill”. In addition to high level insurance against leaks, and breakage at joints and from wind vibration, this method of attachment provides smooth easily balanced flow and eliminates the possibility of eddy corrosion.
The high flow rates demanded for swimming pool heating make ample sizing of headers important. Properly sized headers will minimize pump energy demand, and will reduce installation plumbing costs. The best quality collectors typically have 1 1/2 inch nominal headers, permitting up to 400 square ft. of collectors to be connected header-to-header without parallel connections.
Desirable Design and Construction Features of Glazed Collectors
The statements above regarding fin efficiency and the factors that keep it high also pertain to glazed collectors, as do the remaining comments above on secure header attachment and proper sizing of headers. However, the flow rates required in glazed collectors are lower so smaller headers are permissible without degradation of quality. Serpentine arrangement of water tubes should be avoided in both glazed and unglazed collectors, in favor of parallel grid arrangement of tubes. Efficiency is reduced in serpentine collectors because the average collector temperature will be higher from the repeated passages through the collector. For an 8 tube collector, the pressure drop will be 8 times as high in a serpentine collector as in a parallel grid collector for the same flow rate and tube size. It is also extremely difficult to purge all air from a serpentine collector, which is essential to proper fluid flow and heat transfer. Conversely, no special air purging is needed for a parallel grid collector because the air will rise to the top header of its own accord to be purged automatically by the air vent.
Other important design and construction features are the following:
A. Glazing material. For longest life and maintained transmissivity, the most appropriate glazing material is tempered plate glass. Of the various grades of tempered plate glass, low-iron glass has the highest transmission and lowest reflection of sunlight. These properties result in significant increases in collector efficiency. The cost premium for low-iron glass is smaller than the increase in efficiency, so it is worthwhile.
B. Use of Plastics. Plastic glazing of various types is still used on some solar collectors to reduce weight and cost, but may reduce performance and lifetime. Plastics 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 will seriously reduce the collector efficiency. Plastic used in a collector may also result in limitations or restrictions of collector use in high fire-risk residential zones by local building and safety departments.
C. Insulation. Urethane or polyisocyanurate case insulation has become popular for solar collectors because it has a higher insulation value (“R”) per inch of thickness than does any other practical insulation material and is very east to handle. 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 effectiveness of the collector. Urethane and closely related products may be prohibited in collectors in fire hazard areas, because of their toxic fume production. When these materials are used in solar collectors, they should be used underneath a substantial blanket of other insulation material, such as binder-free fiberglass to reduce the hazard of exposure to high temperatures, and should have an intervening tight vapor barrier. When fiberglass is used, a larger thickness is required than would be needed for urethane or related products. For ease of handling, many fiberglass products contain resinous binder materials, but it has a less heat appearance, binder-free fiberglass should be used. Binder-free fiberglass has the full insulating properties of fiberglass with binders.
Case insulation is not the only important thermal insulation in a quality collector. The absorber plate and connecting tubing penetrating the enclosure must be thermally insulated from the case at all points of support. Heat losses can be severe if either the absorber or tubing touches the case or is supported through heat-conducting materials to the case.
D. Desiccants. Insulation must be kept dry or 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 this, quality collectors contain porous bags of silica gel desiccant to absorb the moisture. If the collector is properly sealed, it is not necessary to have access to the desiccant, as it does not require renewal. Desiccant is also required for the space between the glazings when two covers are used. Typically, the desiccant is contained in the hollow spacers separating the two glazing panes, 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 in either single-glazed or double-glazed collectors, it will become apparent through condensation of drops of water on the inner surface of the glass.
E. Enclosure. The enclosure is used to contain insulation, provide support for the absorber and glazing, and to protect the collector from heat loss due to wind, plus the important function of keeping moisture out of the insulation from rain and dew. Enclosures are made of an almost endless variety of materials and designs, including wood cases, aluminum extrusions with sheet aluminum back, galvanized steel, welded or formed, and even collectors without back covers. Whatever the case material and construction, it must be weather resistant, fireproof, durable, dimensionally stable, strong and completely and permanently sealed against moisture intrusion. As a general rule, the number of joints and seams should be minimized and completely sealed. Steel should be both galvanized and primed before painting and baking and paint should be tough and scratch-resistant. Aluminum should be used with caution in areas exposed to salt air or industrial pollution and smog in the air. Most top-quality collectors use enclosures of architectural anodized aluminum similar to those used for exterior windows.
F. Absorber coating. The absorber coating on both unglazed and glazed solar collector absorber plates must be stable and durable to withstand the weather exposure of unglazed collectors and the stagnation temperatures of glazed collectors without outgassing. For either type, the paint and primer must be baked, and must be a top quality material such as polyceram or epoxy. The most secure paint and primer bonding is obtained in high quality collectors by using an electrostatic painting process.
For collectors intended to operate in the upper end of the medium temperature range and in the high temperature range, selective absorber coating is worthwhile, because it reduces radiation losses significantly. The most effective selective coating available to date is black chrome, applied by a complex electroplating process over a nickel base. If applied on a material other than copper, the plating must be applied to both sides to avoid corrosion. Short cuts in the process to save materials or plating time have not been successful. Good black chrome plating on nickel base has proven stable and not susceptible to high stagnation temperatures or aging. Selective collectors are particularly cost-effective for large installations for water heating.