MIL-HDBK-1003/19
4.4.1.19 Auxiliary heat requirement. The auxiliary heat requirement
(QA) is the amount of heat that must be supplied by a conventional back-up
heating system to maintain the building temperature at Tset for a
specified time period; the time period of interest usually has a duration of
one month or one year. If a building receives no solar heat, QA will
equal the building heat load whereas QA will be zero if the entire load is
met by solar energy. The auxiliary heat requirement is the bottom line
measure of passive solar heating performance.
4.4.1.20 Solar heating fraction.
The solar heating fraction (SHF) is
defined by the equation:
SHF = 1 - QA/QL
,
(Equation 4.19)
and is dimensionless.
4.4.2 Heat to load ratio nomograph. The primary design analysis tool
provided in these procedures is the nomograph for the annual heat to load
ratio, (QA/QL)a, presented in figure 23. In this figure, the quantity
(QA/QL)a is plotted as a function of the minimum monthly scaled solar
load ratio, SLR*, for a series of values for the city parameter (a). The
city parameter depends primarily on geographic location; tabulated values
are presented in the weather tables in Appendix B, which will be fully
explained in 5.1. The scaled solar load ratio is given by the relation:
SLR* = F [multiplied by] SLRm ,
(Equation 4.20)
where F is a system dependent scale factor that is tabulated along with G,
Uc, and other system-dependent parameters in Appendix A; a complete
explanation of Appendix A is included in 5.1. The quantity SLRm is the
minimum monthly solar load ratio for the building of interest at the
selected location; SLRm can easily be evaluated using data provided in the
weather tables.
Having obtained the heat to load ratio from figure 23, it is an easy
matter to calculate the annual auxiliary heat requirement as follows:
(QA)a = (QA/QL)a [multiplied by] (QL)a ,
(Equation 4.21)
where (QL)a is the annual effective building heat load.
4.4.3
System efficiencies.
4.4.3.1 Delivery efficiency. The delivery efficiency (ed) is defined
as the fraction of absorbed solar energy that is actually delivered to the
living space, or:
ed = QD/ST ,
(Equation 4.22)
where QD is the delivered energy.
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