UFC 3-440-01
14 June 2002
CHAPTER 4
SYSTEM DESIGN
4-1
INTRODUCTION. This chapter presents the information required to complete
the solar energy system design.
4-2
COLLECTOR SUB-SYSTEM
4-2.1
Collector Specification
4-2.1.1
Collector Construction
4.2.1.1.1
Absorber Construction and Components. The solar collector absorber
surface normally has two separate components: the absorber plate and fluid
passageways. Many types of absorber designs have been used, such as parallel or
serpentine tubes bonded to the absorber plate and double plates rolled together and
bonded with hydrostatically expanded fluid passages. The method for bonding the
tubes, the circuit flow path, and the absorber surface properties are each critically
important to collector performance. The flow path geometry, cross-sectional area, and
flow rate determine the fluid pressure drop across the collector. This pressure drop
affects the flow distribution throughout the array. Methods used to bond the flow tubes
to the absorber plate include mechanical bonds (soldered, brazed, or welded),
adhesives, and mechanical encirclement. Flow tubes that have separated from the
absorber plates are a leading cause of poor performance for flat-plate collectors. It is
imperative that the bond be able to withstand the expected stagnation temperature of
the collector and the daily temperature variations to which the collectors are exposed.
Serpentine flow tubes and roll-bonded absorber plates can trap the heat transfer fluid in
the collector, which can freeze and burst the tubes or absorber plate. Some roll-
bonded absorbers have also been found to separate with time and cause flow problems
or short-circuiting within the fluid passageway.
4.2.1.1.2
Absorber Surface. The absorber plate surface is also an important factor
in the performance of the collector. There are two basic surface finishes, selective and
non-selective. Selective surfaces are typically finished with black chrome or black
nickel deposited film. Non-selective surfaces are usually finished with flat black paint
and can have as large a value of emissivity as they do absorptivity. Selective surfaces
have the advantage of absorbing the same amount of energy as the painted surface,
but they emit much less radiation back to the cover. Non-selective painted surfaces
have had numerous problems with fading, peeling, and outgassing. In contrast,
deposited metallic surface coatings have an excellent history for retaining their
properties with time. The most common absorber plate materials are copper, although
aluminum absorbers can still be found. Copper has shown the best success due to the
lack of thermal expansion problems with the attached copper flow tubes.
4.2.1.1.3
Collector Manifold. The collector manifold is the piping that branches
4-1
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