MIL-HDBK-1003/19
4.2.2 Importance of conservation measures. The fraction of the monthly
heating load of a building that can be met by passive solar strategies
depends on certain characteristics of the building design, and for double
glazed systems, which are by far the most common, on the ratio VT2/DD; the
details of the relationship between the solar heating fraction (SHF) and
VT2/DD will be addressed later in 4.4 and 5.1. For the present, it is
sufficient to know that the parameter VT2/DD provides an accurate measure of
the passive solar potential of a given climate during any selected month.
It follows that by considering the value of VT2/DD for each month in the
heating season, it is possible to assess the passive solar potential of the
climate-for the full annual cycle. One way to do this might be to average
VT2/DD over all months in the heating season, but that approach would
ignore the fact that it is more important to have high solar heating
fractions in cold months with high values of DD than it is in warm months
with low values of DD. The solution to this dilemma is to to determine the
degree day weighted average of VT2/DD as follows:
N
(VT2/DD)ave = [SIGMA] [(VT2/DD)m [multiplied by] (DDm/DDa)]
m=1
where the index, m, is the month number, N is the number of months in the
heating season, and DDa is the annual heating degree days. The quantity
(VT2/DD)ave provides the desired measure of the annual passive solar
potential of various climates. High values of (VT2/DD)ave are associated
with high values of SHF and conversely. It follows that in climates having
low values of (VT2/DD)ave, conservation measures such as insulation, storm
windows, weather stripping, etc., will be more important than in climates
having high values. If only a small portion of the building load can be
displaced with solar energy, then reduction of that load through the use of
conservation measures clearly becomes a top priority.
A map of the continental United States with contours of constant
(VT2/DD)ave is presented in figure 7. The values of (VT2/DD)ave on the
uppermost, middle, and lowest contours are 30, 50, and 90
Btu/deg.F-ft2-day, respectively. The three contour lines divide the map
into four climate regions that are referred to as mild (MI), moderate (MO),
harsh (HA), and very harsh (VH). General descriptions of these climate
regions and qualitative comments regarding regionally appropriate design are
presented in the next four subsections.
4.2.2.1 Mild climates. The mild climate region includes the southern
third of California and Arizona, small parts of the southern extremes of New
Mexico, Texas, and Louisiana, and most of the Florida peninsula.
In the mild region the winter heating load varies from small to nil and
in any case, there is plenty of sunshine available to meet whatever loads do
arise. Generally, the small heat loads can be displaced with inexpensive
radiant panels or direct gain systems having relatively small solar
collection apertures. However, summer cooling loads in this region can be
quite high, usually exceeding the winter heating load several times over.
It is therefore particularly important to assure that the incremental
cooling load associated with the passive heating system does not negate the
small savings realized during the winter heating season. The use of
defensive countermeasures such
18
Previous Page
