MIL-HDBK-1003/13A
where f
=
fraction supplied by solar (Worksheet D-2 for month selected)
To
=
collector outlet temperature
Ti
=
collector inlet temperature
G
=
flowrate in lbm/hr ft2 collector (use 10.0 lbm/hr ft2)
QL
=
heat load, Btu/mo (Worksheet C-1)
Cp
=
specific heat of fluid = 1 Btu/lbm deg. F for water
[theta]
=
hours of useful sun in day - use 5 hours (winter), 6 hours
(summer)
No
=
number of days in month
Ac
=
area of solar collector
If a DHW only system is used, then temperature rise may be added to ground
water temperature to obtain actual collector outlet temperature. For space
heating/DHW systems, the minimum collector outlet temperatures will be the
temperature of the water returning from the room heat exchanger plus the
temperature rise through the collector. In general, the storage tank bottom
temperature is added to temperature rise to obtain actual collector outlet
temperature.
3.9 Solar system cost - Worksheet F. Worksheets F and G may be used to
convert all costs of the solar installation into cost/ft2 collector. Since
costs can differ significantly for space heating/DHW compared to DHW only,
two separate columns are shown. Recent manufacturer's data are best for
computations, but Tables 3-6 and 3-7 may be used as representative prices
(based on data as of December 1984). Contractor profit is indicated as 20%
and is included in tables; another figure may be used if warranted. Solar
collector costs are also given in Table 2-6. Total system cost estimate is
transferred to Worksheet A.
3.10 Additional costs - Worksheet G. Worksheet G is a convenient checklist
to collect costs associated with converting to solar energy. In new building
designs, good insulation, weatherstripping, etc., will be called for to save
energy, even if solar heating is not adopted; thus, the solar system should
not be burdened with these costs in new buildings. Costs are summed and
divided by collector area, then cost is transferred to Worksheet F.
3.11 Sizing the heat exchanger for space heating.
According to Klein,
Beckman, and Duffie (1976):
The dimensionless parameter [epsilon]LCmin/UA, has been found to
provide a measure of the size heat exchanger needed to supply solar heat
to a specified building. For values of [epsilon]LCmin/UA less than
1.0, the reduction in system performance due to too small a heat
exchanger will be appreciable. Reasonable values of
[epsilon]LCmin/UA for solar space heating systems are between 1 and
3 when costs are considered. (This design method has been developed)
with [epsilon]LCmin/UA equal to 2.0.
Cmin is heat capacity flowrate, which is the lesser of the two heat
capacity flowrates in the load heat exchanger; [epsilon]L is effectiveness
of load heat exchanger and UA is overall heat loss coefficient of building
times the building area.
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