UFC 3-440-01
14 June 2002
types of metals used in solar systems. System piping, which is usually copper,
expands at a different rate than the aluminum structure.
e. Collector Sub-Systems (Lessons Learned)
(1) Collectors. The single glazed, flat-plate, selective surface collector has
proven to be the most reliable and best suited for service water heating needs.
Although reflector systems are sometimes advocated to increase the insolation on a
collector, they can seldom be justified because they must be cleaned, adjusted, and
maintained, and can add a large capital expense. Similarly, strategies involving
seasonal collector tilt adjustment are to be avoided. Problems also have arisen with
evacuated tube collectors due to thermal expansion and improper fluid flow. The
interior construction quality of flat plate collectors remains an issue. Problems such as
poor absorber plate/fluid path bonding and improper allowance for absorber plate
expansion have been observed. Some collectors have not performed as advertised
due to atypical flow rates used during testing and degradation of collector components.
Outgassing from insulation and binder materials also remains an issue.
(2) Arrays. The most common problem with collector arrays is that they do
not achieve balanced flow. Shading of the collectors by other collectors and nearby
objects must be avoided. Some systems have experienced leaks because thermal
expansion was not considered, or improper design methods were used in allowing for
thermal expansion.
(3) Array Support. Most support structure problems have been associated
with material maintenance and aesthetics rather than structural integrity.
F-5.
STORAGE SUB-SYSTEM
a. Definition and Operation. The intermittent nature of solar energy
establishes a need for a sub-system capable of storing energy for 1 to 2 days. The
most common method of doing this for an active solar system is through the use of a
water-filled tank that obtains thermal energy from the collector loop either directly or
through a heat exchanger. The water from the storage tank then functions as a source
of preheated water to an auxiliary heater or boiler that adds the necessary energy to
raise it to the required temperature. In some cases, the storage medium may be
heated above the required temperature, and a mixing valve can be used to reduce the
storage fluid to the desired temperature before it reaches the load. The systems
discussed in this manual assume a storage requirement of approximately 1 day.
b. Storage Media. The most effective and trouble-free storage medium is
water. For this reason, systems discussed in this manual will assume water-based
storage.
F-6.
TRANSPORT SUB-SYSTEM
a. Purpose. The fluid transport sub-system is required to maintain efficient
transport of thermal energy from the collectors to the storage tank. Fundamental
F-9
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