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An estimated one million residential and 200,000 commercial solar
water-heating systems have been installed in the United States.
Although there are a large number of different types of solar water-heating
systems, the basic technology is very simple. Sunlight strikes and
heats an "absorber" surface within a "solar collector" or an actual
storage tank. Either a heat-transfer fluid or the actual potable
water to be used flows through tubes attached to the absorber and
picks up the heat from it. (Systems with a separate heat-transfer-fluid
loop include a heat exchanger that then heats the potable water.)
The heated water is stored in a separate preheat tank or a conventional
water heater tank until needed. If additional heat is needed, it
is provided by electricity or fossil-fuel energy by the conventional
water-heating system. By reducing the amount of heat that must be
provided by conventional water-heating, solar water-heating systems
directly substitute renewable energy for conventional energy, reducing
the use of electricity or fossil fuels by as much as 80%.
Today's solar water-heating systems are well proven and reliable
when correctly matched to climate and load. The current market consists
of a relatively small number of manufacturers and installers that
provide reliable equipment and quality system design. A quality
assurance and performance-rating program for solar water-heating
systems, instituted by a voluntary association of the solar industry
and various consumer groups, makes it easier to select reliable
equipment with confidence. Building owners should investigate installing
solar hot water-heating systems to reduce energy use. Before sizing
a solar system, water-use reduction strategies should be put into
practice.
There are five types of solar hot water systems:
Thermosiphon Systems. These systems heat water
or an antifreeze fluid, such as glycol. The fluid rises by natural
convection from collectors to the storage tank, which is placed
at a higher level. No pumps are required. In thermosiphon systems
fluid movement, and therefore heat transfer, increases with temperature,
so these systems are most efficient in areas with high levels of
solar radiation.
Direct-Circulation Systems.
These systems pump water from storage to collectors during sunny
hours. Freeze protection is obtained by recirculating hot water
from the storage tank, or by flushing the collectors (drain-down).
Since the recirculation system increases energy use while flushing
reduces the hours of operation, direct-circulation systems are used
only in areas where freezing temperatures are infrequent.
Drain-Down Systems. These
systems are generally indirect water-heating systems. Treated or
untreated water is circulated through a closed loop, and heat is
transferred to potable water through a heat exchanger. When no solar
heat is available, the collector fluid is drained by gravity to
avoid freezing and convection loops in which cool collector water
reduces the temperature of the stored water.
Indirect Water-Heating Systems.
In these systems, freeze-protected fluid is circulated through a
closed loop and its heat is transferred to potable water through
a heat exchanger with 80% to 90% efficiency. The most commonly used
fluids for freeze protection are water-ethylene glycol solutions
and water-propylene glycol solutions.
Air Systems. In this indirect
system the collectors heat the air, which is moved by a fan through
an air-to-water heat exchanger. The water is then used for domestic
or service needs. The efficiency of the heat exchanger is in the
50% range. Direct-circulation, thermosiphon, or pump-activated systems,
require higher maintenance in freezing climates. For most of the
United States, indirect air and water systems are the most appropriate.
Air solar systems, while not as efficient as water sytems, should
be considered if maintenance is a primary concern since they do
not leak or burst.
Types of Collectors
There are basically three types of collectors: flat-plate, evacuated-tube,
and concentrating.
A flat-plate collector, the most common type, is an insulated,
weatherproofed box containing a dark absorber plate under one or
more transparent or translucent covers.
Evacuated-tube collectors are made up of rows of parallel, transparent
glass tubes. Each tube consists of a glass outer tube and an inner
tube, or absorber, covered with a selective coating that absorbs
solar energy well but inhibits radiative heat loss. The air is withdrawn
(evacuated) from the space between the tubes to form a vacuum, which
eliminates conductive and convective heat loss. The vacuum also
helps them achieve extremely high temperatures (170°-350°
F); so they are appropriate for commercial and industrial uses.
Concentrating collectors are usually parabolic troughs that use
mirrored surfaces to concentrate the sun's energy on an absorber
tube (called a receiver) containing a heat-transfer fluid. They
provide hot water and steam, usually for industrial and commercial
applications.
Parabolic-trough collectors use curved mirrors to focus the sunlight
on a receiver tube (sometimes encased in an evacuated tube) running
through the focal point of the mirrors and can heat their transfer
fluid to as much as 570°F (299°C). Because they use only
direct-beam sunlight, parabolic-trough systems require tracking
systems to keep them focused toward the sun and are best suited
to areas with high direct solar radiation. Because they are particularly
susceptible to transmitting structural stress from wind loading
and require large areas for installation, parabolic-trough collectors
are usually ground mounted. For electrical generation or industrial
uses that require very high temperatures (greater than 392°F
[200°C]), a heat-transfer fluid such as oil is used, but depending
on the degree of danger of freezing, antifreeze or water is used
in the heat-transfer loop for domestic water-heating systems. Parabolic-trough
collectors generally require greater maintenance and supervision
and particularly benefit from economies of scale, so are generally
used for larger systems.
Low-, Mid-, and High-Temperature Collectors
The collectors can be low-temperature, mid-temperature, or high-temperature.
The glazed, flat-plate collectors most commonly used for commercial
or residential domestic hot water are classified as "mid-temperature"
collectors, generally increasing water temperature to as much as 160ÐF
(71°C). Flat-plate collectors consist of an insulated, weather-tight
housing or box, a clear glass or plastic cover glazing, a black absorber
plate, and a system of passages for the heat-transfer fluid to pass
through the collector. Special coatings on the absorber maximize absorption
of sunlight and minimize re-radiation of heat. Gaskets and seals at
the connections between the piping and the collector and around the
glazing ensure a watertight system.
Low-temperature collectors, which generally increase water temperature
to as much as 90°F (32°C), are less expensive because
they consist simply of an absorber with flow passages and have no
covering glass (glazing), insulation, or expensive materials such
as aluminum or copper. These collectors are less efficient in retaining
solar energy when outdoor temperatures are low, but are quite efficient
when outside air temperatures are close to the temperature to which
the water is being heated. They are highly suitable for swimming
pool heating and other uses that require only a moderate increase
in temperature and are most commonly used in warmer areas. For the
last several years, they have been the most frequently installed
collectors. In warm climates, low-temperature collectors are sometimes
used in hybrid systems that heat a pool in the winter and supplement
domestic water heating in the summer, when pool heating is not needed.
Large facilities or ones with quasi-industrial operations such
as laundries may be able to efficiently use more sophisticated high-temperature
collectors. Although they are also used in mid-temperature systems,
evacuated-tube collectors can be designed to increase water/steam
temperatures to as much as 350°F (177°C). They may use
a variety of configurations, but generally encase both the absorber
surface and the tubes of working fluid in a tubular glass vacuum
for highly efficient insulation. Evacuated-tube collectors are the
most efficient collector type for cold climates with low-level diffuse
sunlight. They can be mounted either on a roof or on the ground,
but they need to be protected from vandalism and can be damaged
by hail or hurricanes.
Solar Equipment Certification
The Solar Rating and Certification Corporation (SRCC) is an independent,
nonprofit trade organization that creates and implements solar equipment
certification programs and rating standards. SRCC certifies solar
thermal equipment that meets minimum standards jointly set by private
and public sectors. The compiled information is published in the Directory
of SRCC Certified Solar Collector and Systems Ratings, priced at $26.00.
The guidebook rates the performance, durability, and safety of solar
thermal collectors and systems. It also lists certified products and
consumer tips for suitable solar product selection. This and other
publications are available for downloading from SRCC Web
site.
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