14 June 2002
inhibitors (However, when using a non-toxic fluid, inhibitors should be avoided since
they require considerable maintenance and often become mildly toxic upon
degradation). Metallic erosion can occur in the system piping if excessive fluid
velocities occur. For the copper piping required for solar systems designed under this
guidance, a velocity limit of 5 feet per second is to be used. Maximum allowable fluid
velocities are dependent upon the type of metal used. Correct pipe sizing and analysis
of fluid flow paths should be used to avoid this problem.
b. Scaling. Scaling commonly refers to mineral deposits, such as calcium and
magnesium compounds, that collect and adhere to pipe interiors and equipment.
Scaling is promoted in systems by increased temperatures, high mineral concentrations
and high (alkaline) pH levels. The result of scaling is flow restriction, high fluid
velocities, and a decreased heat transfer rate. Scaling problems are most often
associated with poor-quality water supplies and can be avoided by proper analysis and
treatment of fluids to be used in the system.
c. Thermal Expansion. Differences between thermal rates of expansion for
dissimilar materials often cause problems throughout a solar system. This manual
addresses the thermal expansion issue for locations in the system where the most
a. Definition. The collector sub-system includes the collectors and support
structure, and all piping and fittings required to reach a common heat transfer fluid inlet
and outlet. For roof-mounted structures, this sub-system includes all components
above the roofline.
b. Solar Collectors
(1) Operation. A solar collector is a device that absorbs direct (and in some
cases, diffuse) radiant energy from the sun and delivers that energy to a heat transfer
fluid. While there are many different types of collectors, all have certain functional
components in common. The absorber surface is designed to convert radiant energy
from the sun to thermal energy. The fluid pathways allow the thermal energy from the
absorber surface to be transferred efficiently to the heat transfer fluid. Some form of
insulation is typically used to decrease thermal energy loss and allow as much of the
energy to reach the working fluid as possible. Finally, the entire collector package must
be designed to withstand ambient conditions ranging from sub-zero temperatures and
high winds to stagnation temperatures as high as 350 degrees F (177 degrees C).
(2) Collector Types. The three major categories that have been used most
often are flat-plate glazed collectors, unglazed collectors, and evacuated tube
collectors. A general description of each collector type and its application is given
(a) Flat-Plate. Flat-plate solar collectors are the most common type
used and are best suited for low temperature heating applications, such as service