MIL-HDBK-1003/19
Direct gain buildings have a delivery efficiency of unity because the
living space itself is the solar collector. Thermal storage walls, on the
other hand, absorb energy on their outer surface and deliver heat to the
interior by conduction through a masonry medium or by convection through
water. Radiant panels must radiate and convect heat to the interior
subsequent to absorption on the outer surface whereas TAPs convect heat to
the interior through vents provided for that purpose. Regardless of what
transport mechanism is involved, all passive solar systems except direct
gain have delivery efficiencies less than one because part of the absorbed
energy is lost back out through the glazing before it can be delivered to
the interior. The delivery efficiencies of all passive solar systems
addressed in this document are tabulated in Appendix A.
4.4.3.2 Utilization efficiency. The utilization efficiency (eu) is
the fraction of delivered solar energy that provides useful heat. The
defining equation is:
eu = QSQD ,
(Equation 4.23)
where:
QS = QSL - QA ,
(Equation 4.24)
is the utilizable solar heat. Systems with low utilization efficiencies are
to be avoided because delivered solar energy that is not utilizable must be
vented to avoid overheating the building. Typically, direct gain systems
will have relatively low utilization efficiencies although overheating can
be kept within acceptable limits by sizing the aperture properly and
providing adequate thermal storage mass.
4.4.3.3 Total efficiency. The total system efficiency (et) is the
fraction of absorbed solar energy that ultimately provides useful solar
heat, or:
et = QS/ST
,
(Equation 4.25)
which is equivalent to:
et = ed [multiplied by] eu
(Equation 4.26)
Thus, et depends on the efficiencies of delivery and utilization, and is
an excellent measure of solar heating potential.
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