Air-heating collector design

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Design: Solar Heating of Buildings and Domestic Hot Water

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.

Job Summary

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

Solar System Cost Analysis

Additional Cost Items Related to Use of Solar Heating

Solar Air Collector System Design Summary

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