4.2.3 Solar availability. As previously discussed, the parameter VT2
provides a measure of the availability of solar radiation as a space heating
resource during a specified time period. If VT2 were evaluated for the
duration of the winter heating season the result would provide some
indication of the potential of the site for passive solar heating
applications. However, it is more important to have high solar availability
during the colder months of the heating season than during the warmer
months, and the straight summation involved in evaluation of VT2 does not
reflect this fact. A better measure of the effective solar availability is
obtained by taking the degree day weighted average of the monthly VT2s that
occur during the heating season as follows:
VT2ave = [SIGMA] [VT2 [multiplied by] (DDm/DDa)]
A map of the continental United States with contours of constant
VT2ave is presented in figure 8. The contours are defined by VT2ave
values of 30, 25, 20, and 15. The four contours divide the map into five
regions that are labeled most sunny (MS), very sunny (VS), sunny (SU),
cloudy (CL), and very cloudy (VC). These five regions cut across the four
principal climate regions defined in figure 7 and form subregions that are
related to the appropriate size of solar apertures. As a general rule, the
sunnier subregions of a particular principal climate region should have the
larger solar apertures.
The ideal climate for passive solar applications is one in which high
solar availability coincides with a large heat load; large apertures are
appropriate in such a climate. In the continental United States, the best
climates for passive solar design lie in the subregion formed by the most
sunny and harsh climate regions. Solar apertures should be relatively
small in the mild climate region because the heat load is small, and
relatively small in the very harsh region because solar availability is low.
Some general comments on the solar regions defined in figure 8 are presented
126.96.36.199 Most sunny region. This region is limited to the desert
southwest and includes major parts of Nevada, Arizona, and New Mexico.
Subregions in which the most sunny region overlaps the harsh region are
ideal for passive solar heating because of the coincidence of a substantial
heating load and excellent solar availability. The most sunny/moderate
subregion is also quite good for passive solar heating.
188.8.131.52 Very sunny region. The very sunny region forms a complex
crescent that bounds the most sunny region. It forms a large, very
sunny/harsh subregion in which passive solar applications are very
184.108.40.206 Sunny region. The sunny region forms a still larger crescent
about the very sunny region, and includes parts of Florida, Alabama,
Georgia, South Carolina, North Carolina, and Virginia. The sunny area cuts
completely across the country from North to South and forms subregions with
all four principal climate zones. A broad range of passive solar designs is
viable across these subregions.
220.127.116.11 Clouds region. The cloudy region also traverses the country
from north to south and forms four types of subregions among which many
passive designs are feasible. Parts of the Pacific northwest, the Midwest,
and the eastern seaboard are included in the cloudy region.