MIL-HDBK-1003/19

4.4.1.3 Transmitted solar radiation. The symbols VT1, VT2, and VT3

represent the amount of solar radiation that is transmitted through one

square foot of vertical, south-facing solar aperture during a specific

one-month period for single, double, and triple glazed systems,

respectively. The corresponding annual sums are indicated by the symbols

QTA1, QTA2, and QTA3. In the general case for which the aperture is either

tilted or not south-facing, QS is used for the monthly sum and QSA

represents the annual sum. The units of all transmitted radiation quantites

are (Btu/ft2) per unit time.

4.4.1.4 Solar aperture absorptance. The solar aperture absorptance

([alpha]) is the fraction of transmitted solar radiation that is absorbed by

the passive heating system. The part not absorbed is lost back through the

glazing by reflection.

4.4.1.5 Absorbed solar radiation. The amount of radiation absorbed by

a passive solar heating system per square foot of aperture (S) is given by

the product of the transmitted radiation and the absorptance. In the

general case, for a one-month period, we have:

S = [alpha] [multiplied by] QS.

(Equation 4.9)

The units of S are (Btu/ft2) per unit time. The total amount of solar

radiation absorbed by a particular system (ST) is given by the product of

S and Ac (or Ap where appropriate) and has units of Btu per unit time.

4.4.1.6 Net load coefficient. The net load coefficient (NLC) is

defined as the amount of heat that would be required to maintain the air

temperature in a building -1deg.F above the outdoor ambient temperature for

a period of one day if no heat losses or gains were allowed through the

solar aperture. Thus the NLC, which is expressed in units of Btu/deg.F-day,

provides a measure of how effectively the nonsolar elements of a building

have been sealed and weatherstripped to reduce infiltration and insulated to

reduce heat loss by conduction. A procedure for obtaining a quick estimate

of the NLC will be presented in 5.1.1.

4.4.1.7 Load collector ratio. The load collector ratio (LCR) is the

NLC divided by the solar collection area (Ac), or, in the case of

sunspaces with tilted glazings, it is the NLC divided by the projected area

(Ap). The units of LCR are Btu/deg.F-day ft2 and the defining equation

is:

LCR

NLC/Ac

(Equation 4.10)

4.4.1.8 Total load coefficient. The total load coefficient (TLC) is

the sum of the NLC and the load coefficient of the solar aperture and as

such, provides a measure of the total building heat loss with no credit

taken for solar gains.

One of two values for the solar aperture conductance may be selected

depending on the application at hand. If the steady state aperture

conductance (Uc), expressed in Btu/hr-ft2-deg.F, is selected, then:

TLCs = NLC + 24 [multiplied by] Uc [multiplied by] Ac (Equation 4.11)

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