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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