MIL-HDBK-1003/13A
3.12 Air-heating collector design - Worksheet H. The design procedure for
air systems (Klein, Beckman, and Duffie, 1977) is very similar to that for
liquid systems - the same worksheets may be used. Figure 3-3 gives the
f-chart for this procedure. The procedure was developed using an air flow
heat capacitance rate of 2.87 Btu/hr-ft2 deg. F (about 156 SCF/hr-ft2).
The performance of systems having collector capacitance rates between 1.47
and 5.87 Btu/hr-ft2 deg. F can be estimated by multiplying the values of
FLAC {[(mcp)c/FRAC]/2.87}0.28 (Klein, Beckman, and Duffie,
1977). To calculate FR, see Duffie and Beckman (1974), Section 7.7. The
rock bed storage heat capacitance assumed was 19.6 Btu/ft2 deg. F. The
performance of systems with other storage capacities can be determined by
multiplying the dimensionless group FLAC by [([rho]VCpr/FRAC)/
19.6] -0.3 - (Klein, Beckman, and Duffie, 1977). The standard deviation of
the yearly f by this method from the computer simulated value was found to
be 0.017 (Klein, Beckman, and Duffie, 1977).
A comparison of the f-charts for the liquid and air systems indicates
that, for the same values of [FIAC] and [FLAC] the air system
outperforms the liquid system particularly for systems designed to
supply a large fraction of the heating load .... [Some reasons for this
behavior are:] the average collector fluid inlet temperature is lower
for the air system (and thus the collector efficiency is higher) than
that for the liquid system at times when . . . room air is circulated
through the collector and also because the more effective thermal
stratification achieved in rock storage results in lower temperature air
going to the collector. Also since no heat exchanger is required
between collector and storage, that inefficiency is avoided.
It cannot be concluded, however, that air heating systems perform better
than liquid systems. The overall collector efficiency factor, FR, is
ordinarily lower for air heaters. As a result [FIAC] and [FLAC]
are ordinarily lower and thus the performance of an air system may be
equivalent to or lower than that of a liquid system, all else being the
same. (Klein, Beckman, and Duffie, 1977)
Sections 2.1.1 and 2.5 of this document discuss the relative merits of sir
systems.
3.13 Worksheets.
A
Job Summary
B
Solar Collector Parameters
C-1
Load Calculations
C-2
Demand Calculations - Domestic Water Heater
D-1
Monthly Solar Collection Parameters
D-2
Fraction of Load Supplied by Solar Heat
E-l
Value of Fuel Saved
E-2
Present Worth Analysis
F
Solar System Cost Analysis
G
Additional Cost Items Related to Use of Solar Heating
H
Solar Air Collector System Design Summary
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