The use of solar heat
The heart of a solar collector is the absorber,
which is usually composed of several narrow metal strips.
The carrier fluid for heat transfer flows through a
heat-carrying pipe, which is connected to the absorber
strip. In plate-type absorbers, two sheets are sandwiched
together allowing the medium to flow between the two sheets. Absorbers
are typically made of copper or aluminum.
Swimming pool absorbers, on the other hand,
are usually made of plastic (mostly EPDM, but also
of polypropylene and polyethylene), as the lower
temperatures involved do not require greater heat
capacity.
Heating and storage are united in a reservoir collector.
Arrays of reservoir collectors do not need
circulating pumps or regulating mechanisms, as the
drinking water is warmed and stored right in the
collector.
Highly efficient absorber surfaces
Absorbers are usually black, as dark surfaces
demonstrate a particularly high degree of light
absorption. The level of absorption indicates the
amount of short-wave solar radiation being absorbed
that means not being reflected. As the absorber
warms up to a temperature higher than the ambient temperature,
it gives off a great part of the accumulated solar energy
in form of long-wave heat rays. The ratio of absorbed
energy to emitted heat is indicated by the degree
of emission.
In order to reduce energy loss through heat emission, the most efficient absorbers have a selective surface coating.
This coating enables the conversion of a high
proportion of the solar radiation into heat,
simultaneously reducing the emission of heat.
The usual coatings provide a degree of absorption
of over 90%. Solar paints which can be mechanically
applied to the absorbers (with either brushes or
sprays), are less or not at all selective, as they
have a high level of emission. Galvanically applied
selective coatings include black chrome, black nickel,
and aluminum oxide with nickel. Relatively new is a
titanium-nitride-oxide layer, which is applied via
steam in a vacuum process. This type of coating
stands out not only because of its quite low
emission rates, but also because its production is emission-free
and energy-efficient.
Flat-plate Collectors
- Sketch of a flat-plate collector
A flat-plate collector consists of an absorber, a
transparent cover, a frame, and insulation.
Usually an iron-poor solar safety glass is used as a
transparent cover, as it transmits a great amount
of the short-wave light spectrum.
Simultaneously, only very little of the heat emitted
by the absorber escapes the cover (greenhouse
effect).
In addition, the transparent cover prevents wind and
breezes from carrying the collected heat away
(convection). Together with the frame, the cover
protects the absorber from adverse weather
conditions. Typical frame materials include aluminum
and galvanized steel; sometimes fiberglass-reinforced plastic
is used.
The insulation on the back of the absorber and on
the side walls lessens the heat loss through
conduction. Insulation is usually of polyurethane
foam or mineral wool, though sometimes mineral
fiber insulating materials like glass wool, rock wool,
glass fiber or fiberglass are used.
Flat collectors demonstrate a good price-performance
ratio, as well as a broad range of mounting
possibilities (on the roof, in the roof itself, or
unattached).
In order to reduce heat loss within the frame by
convection, the air can be pumped out of the
collector tubes. Such collectors then can be called
evacuated-tube collectors. They must be
re-evacuated once every one to three years.
Evacuated-tube collectors
- Sketch of a heat pipe collector
In this type of vacuum collector, the absorber
strip is located in an evacuated and pressure proof
glass tube. The heat transfer fluid flows through
the absorber directly in a U-tube or in
countercurrent in a tube-in-tube system. Several single tubes,
serially interconnected, or tubes connected to each other
via manifold, make up the solar collector. A heat
pipe collector incorporates a special fluid which
begins to vaporize even at low temperatures. The
steam rises in the individual heat pipes and warms
up the carrier fluid in the main pipe by means of a
heat exchanger. The condensed liquid then flows back
into the base of the heat pipe.
The pipes must be angled at a specific degree above
horizontal so that the process of vaporizing and
condensing functions. There are two types of
collector connection to the solar circulation
system. Either the heat exchanger extends directly into the
manifold ("wet connection") or it is connected to
the manifold by a heat-conducting material ("dry connection").
A "dry connection" allows to exchange individual
tubes without emptying the entire system of its fluid.
Evacuted tubes offer the advantage that they work
efficiently with high absorber temperatures and
with low radiation. Higher temperatures also may be
obtained for applications such as hot water
heating, steam production, and air conditioning.
How much energy does a solar collector provide?
- Graph of efficiency and temperature ranges of various types of collectors (radiation: 1000 W/m²)
The efficiency of a solar collector is defined as
the quotient of usable thermal energy versus
received solar energy. Besides thermal loss there
alwas is optical loss as well. The conversion
factor or optical efficiency h0 indicates the percentage of
the solar rays penetrating the transparent cover of the
collector (transmission) and the percentage being
absorbed. Basically, it is the product of the rate
of transmission of the cover and the absorption
rate of the absorber.
The specific costs of collectors are also important.
Evacuated-tube collectors are substantially more
expensive (at 511,29 - 1278,23 Euro /m² collector
surface) than flat-plate collectors (153,34 to
613,55 Euro /m²) or even plastic absorbers (25,60
to 102,26 Euro /m²). However, a good collector does
not guarantee a good solar system. Rather, all components
should be of high quality and similar capacity and strength.
source:http://www.solarserver.com
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