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
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 ,
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.
18.104.22.168 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
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.
22.214.171.124 Improving total system efficiency. It is convenient to think
in terms of improving the total system efficiency by increasing the