MIL-HDBK-1003/19
season, a passive solar heating system continues to function although the
increased solar elevation angle reduces the radiation flux transmitted
through the glazings, particularly if an overhang is employed. However, all
heat delivered to the building during the cooling season is unwanted and
must be removed either by ventilation or by evaporative or vapor compression
cooling systems. A poorly designed passive heating system can significantly
aggravate the summer cooling load of a building.
In the design procedures, the sum of all unwanted heat delivered to a
building by the passive heating system will be referred to as the
incremental cooling load. This is clearly an important parameter because it
represents the cooling penalty associated with various passive solar
designs.
4.2
General climatic considerations.
4.2.1 Characteristic weather parameters. All of the discussion in this
section is based on two weather parameters that, in certain combinations,
may be used to characterize climates with respect to the potential
effectiveness of conservation and passive solar measures in reducing energy
consumption for space heating.
The first of these important parameters is the heating degree days,
which is represented by the symbol DD and has units of deg.F-day. In these
procedures, DD is calculated by summing the difference between the base
temperature and the outside ambient temperature over each hour in the time
period of interest and dividing the result by 24 hr/day; all negative terms
are omitted from the sum. The base temperature is the thermostat setpoint
adjusted to account for the presence of internal heat sources; the time
period of interest is usually one month or one year. This method of
calculating DD differs from the American Society of Heating, Refrigerating
and Air-Conditioning Engineers (ASHRAE) approach and was selected because it
yields better accuracy when applied to the analysis of passive solar
buildings. Furthermore, the hourly data required for such a calculation is
available in the Typical Meteorological Year (TMY) data base that is used
consistently throughout these procedures (from Input Data for Solar Systems
and Generation of Typical Meteorolgical Years for 26 SOLMET Stations). The
heating degree days is an important weather parameter because the amount of
heat lost from a building during a particular time period is directly
proportional to DD, i.e., if a building is moved from one location to
another having twice as many degree days, the heat loss from the building
will double.
The second important weather parameter is VT2, the amount of solar
energy transmitted through a vertical, south facing, double glazed aperture
during a specific time period. The V in VT2 stands for vertical, the T
indicates transmitted radiation, and the 2 represents the two glazing
layers. The parameter VT2 is important because it quantifies the solar
resource available for passive space heating.
In the following sections, combinations of VT2 and DD will be used to
characterize climates with regard to the relative importance of conservation
and passive solar measures for reducing auxiliary heat consumption in
buildings.
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