Save with Solar Hot Water
(Page 3 of 6)
Evacuated-tube collectors use rows of parallel, transparent
glass tubes, which are coated with a selective finish. A
vacuum inside the tubes helps the collectors achieve
temperatures between 170 and 350 degrees, which makes
evacuated tube collectors ideal for high temperature water
applications. Batch collectors are simple systems that use
one or more black-painted tanks enclosed in a
well-insulated, glass-topped box. In this design, the
collector and storage tank are combined.
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Flat-plate collectors generally perform most consistently
for residential applications, yielding temperatures in a
range of 140 to 160 degrees. Thermal performance ratings
can be used to compare the efficiency and economics of
various flat-plate solar collectors (see Sources, Page 80).
To maximize their efficiency, collectors need unobstructed
southern solar exposure between the hours of 9 a.m. and 3
p.m., so site them accordingly. The collectors can be
mounted on a building's roof or walls or set on a frame on
the ground.
TYPES OF SYSTEMS
Water heated by the collectors may be moved through the
house with pumps (active systems) or by natural convection
(passive systems). Batch and passive open-loop systems
(which use natural convection to circulate fluid) require
little maintenance but are vulnerable to freezing. More
common active systems that use pumps and sensors to
regulate fluid flow are generally more efficient and
freeze-resistant, but require more maintenance.
In a direct or open-loop active system, a
pump (regulated by an electronic controller, an appliance
timer or photovoltaic panel) circulates heated domestic
water throughout the entire system, from the collectors to
the heat-storage tanks. With fewer components and no heat
exchanger to lose heat, direct active systems tend to be
more efficient. They can be powered by photovoltaic modules
or a simple AC pump and controller. However, because these
systems circulate water instead of antifreeze, they are
vulnerable to freezing and are not recommended in climates
that experience freezing temperatures.
Indirect or closed-loop active systems are
the most freeze-resistant. They circulate a heat-transfer
fluid (most commonly a nontoxic propylene glycol and water
mixture) that transfers heat from the collectors to potable
water held in storage tanks (the antifreeze also keeps the
collectors and exterior piping from freezing). Like direct
active systems, they can be powered by photovoltaics, too.
They have more components and are more complex.
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