MIL-HDBK-1003/19

5.2.1 System economics. The ratio of annual energy saved to capital

invested (E/C), in MMBtu/K$, is a useful economic parameter. The annual

energy saved is given by:

E = QN - QA

,

(Equation 5.18)

where QA is the annual auxiliary heat requirement from Worksheet 6 and

QN is the net annual load. The formula for net annual load is:

QN = NLC [multiplied by] DDa ,

(Equation 5.19)

where NLC is the net load coefficient from Worksheet 2 and DDa is the

annual heating degree days from Worksheet 5. Note that aperture losses

are not included in equation 5.19 so that the passive heating system is not

inappropriately credited with saving energy by meeting its own load.

The capital invested (C) is the total cost of the passive solar heating

system. The heating system cost depends on the design and on location

dependent costs for materials and construction. This parameter must be

estimated by the user.

Clearly, the E/C ratio can be increased by reducing the auxiliary heat

requirement and/or the system cost. Guidance for improving solar heating

performance is provided in the following section on system efficiency.

5.2.2

System efficiencies.

5.2.2.1 System efficiency worksheet for reference month. Worksheet 7

is provided for calculation of the system efficiencies during the reference

month (m) noted beneath the base temperature in the weather tables. The

reference month is the harshest month in the heating season, for a

particular base temperature, in that the associated value of VTn/DD is a

minimum.

In the first part of the worksheet, equations and blanks are provided

for calculating and recording the values of the effective total load

coefficient (TLCe) and the solar heating fraction (SHF). These two

quantities are then substituted into the equation for et that follows.

The second part of the worksheet merely provides a blank for recording

the value of the delivery efficiency (ed) that is tabulated for all

systems in Appendix B.

In the final part of the worksheet, the utilization efficiency (eu)

is calculated from the indicated formula.

5.2.2.2 Improving total system efficiency. It is convenient to think

in terms of improving the total system efficiency by increasing the

magnitude of its factors, ed and eu.

72

Integrated Publishing, Inc. |