|
 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.
|
Solar Pool Heater Information Page