MIL-HDBK-1003/13A

3.5 Monthly solar collection parameters - Worksheet D-1. Figures for QL

total heat load per month, are transferred to Worksheet D-1. Solar

insolation, I, and slope factor, S, are obtained from Table 1-1 and Figure

3-2, respectively, for the location and latitude. If measured I for the

location is available for several years, then the average of this data should

be used. The slope factor corrects insolation data from the horizontal at

which the insolation data were taken, to the tilt angle of the collector. If

the tilt angle is latitude +/-15 degrees, then Figure 3-2 may be used for S.

For deviation from "latitude +/-15 degrees," see Duffie and Beckman (1974).

These calculations apply to south-facing collectors; no correction is needed

for collectors facing up to 10 degrees east or west of south. The air

temperature, Ta, is the average daily temperature taken from local records

or the Insolation Data Manual of The Solar Energy Research Institute (1980)

which is excerpted in Table 3-4. The factor (Tref - Ta) accounts for the

effects of ambient air temperature changes on collector heat losses. Then

the parameters FI and FL may be calculated. Special care should be taken

that the units are consistent, so that FI and FL will have units of

ft -2. All units in the tables are correct and consistent for entry

directly into the worksheets.

3.6 Fraction of load supplied by solar heat - Worksheet D-2. On Worksheet

D-2, first select an area of solar collector for study, based on experience,

similar design, or arbitrary size (a collector area approximately one-fourth

to one-half the floor area to be heated is a reasonable guess). Area is

multiplied by FI and FL factors from Worksheet D-1 and the product is

entered on Worksheet D-2. Then from Figure 3-1 pick off the values of f for

each set of values of ACFI and ACFL. Calculate average f =

[SIGMA]QLf/[SIGMA]QL. Then select other collector areas, larger or

smaller, and repeat above procedure so that a trend may be observed in the

following cost analysis calculations. Usually very sunny areas (I > 1800

Btu/ft2-Day) will have highest cost effectiveness at about f~0.75 and not

so sunny areas (I~1100 Btu/ft2-Day) at f~0.50.

Storage volume on Worksheet D-2 may be sized by rules of thumb for minimum

size. Minimum storage volumes are: one day's usage for DHW only and 1.8

gal/ft2 (collector) for space heating and DHW. (See Section 2.2 for sizing

storage.) Up to 2.5 days usage for DHW only has been recommended for family

dwellings without auxiliary heat for DHW. Up to 5 gal/ft2 has been used in

some installations for space heating and DHW. The parametric chart (Figure

3-1) has been developed using storage equal to 1.8 gal of water/ft2.

Results will not be greatly affected by moderate deviations (1.2 to 2.4) from

this value. For storage outside this range consult Beckman (1977), pages

66-67, for correction factors. Normally higher storage capacities will not

be cost effective. For liquids other than water the storage figures are

modified by multiplying by the ratio of specific heats:

Cp water/Cp liquid = 1.0/Cp liquid.

Minimum volume of rocks for air system storage is 0.8 ft3 per square foot

(sq ft) of collector (Section 2.2). If multiple cloudy days are a frequent

occurrence, then more auxiliary heat will be used than was planned; the

latter problem is relieved if larger storage is used. Consequently, if many

cloudy days are expected, then the high end of the "rules of thumb" for

storage sizing should be selected (Section 2.2). The cost of energy storage

may be calculated from Table 2-8, where, for the approximate size chosen, the

various elements of tank order-of-magnitude cost are listed in terms of

$/gal.

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