Load calculations

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Design: Solar Heating of Buildings and Domestic Hot Water

MIL-HDBK-1003/13A

The term (mCp)c/(mCp)min is the ratio of the heat capacity flow rates

in the collector-tank heat exchanger. The subscript "c" refers to the

collector flow stream; the subscript "min" refers to whichever of the two

flowrates has the lesser value.

The ratio F'R/FR, line 6, Worksheet B, where F'R is the collector-tank

heat exchanger efficiency factor is calculated from the equation using the

factors previously developed. If there is to be no heat exchanger, then this

ratio equals 1.0 for air and liquid systems.

Typically, if a heat exchanger is used with an effectiveness of 0.7 as assumed

above and if the collector flow (mCp)c, is about the same as the other

(storage) flow, (mCp)min, as is often the case, then the ratio F'R/FR

is about 0.95 to 0.97 for a liquid system. This is not true for air systems

which must be calculated. See U.S. Dept HUD (1977). pages A-26 and A-27 for

further calculations of this parameter.

The next factor is

line 7 Worksheet B, where the bar refers

to an average value of

and subscript "n" refers to the value of

taken with the sun normal to the collector. This factor represents the

variations in transmittance and absorptance due to changes in the sun angle

during the day. Solar collectors are tested near solar noon, and there is a

reduction in transmission of insolation at high angles of incidence which

occur in early morning or late afternoon. This reduction (called incident

angle modifier) may be available from the manufacturer of the collector since

it is a parameter that is measured as part of normal ASHRAE 93-77 performance

test. If it is not available it may be taken as a constant as follows:

3.3 Load calculations - Worksheet C-l. Worksheet C-l is an aid to

calculating the space heating and DHW load for family housing. For other

buildings use conventional methods of calculating load; computer programs are

available for this. For existing buildings, heating load may be inferred from

fuel bills, if available, see Example 1 (Section 4.1); or the Btu/ft 2

degree-day (dd) method of Worksheet C-l may be used. Table 3-1 gives

estimated Btu/ft2-dd heat loss rates for various structural types used in

family housing. If net heat loss rate is based on amount of fuel used, load

is gross load and must be multiplied by furnace efficiency to get net heating

load.