MIL-HDBK-1003/19
The delivery efficiency is defined as the fraction of the solar heat
absorbed by the system that is actually delivered to the living space. For
direct gain systems, this quantity is always unity because the living space
is the absorber. For other systems, ed is always less than 1 and can be
increased by adding additional layers of glazing or employing a selective
surface. Both of these strategies decrease heat losses from the absorber
surface to ambient conditions. The delivery efficiency could also be
increased by decreasing the thickness of thermal storage walls. This
strategy, however, is not advisable because it can result in an offsetting
decrease in eu.
The utilization efficiency is the fraction of the heat delivered to the
building interior that is used to meet the building heat load. The
un-utilized heat must be ventilated to avoid overheating the living space.
The utilization efficiency therefore provides a useful measure of thermal
comfort and convenience. Systems having values of eu below 0.6 should be
avoided and values of 0.7 and above are advisable. The principal strategy
for increasing eu is to add more thermal storage mass. Thus, thermal
storage wall thickness may be increased and additional mass layers may be
added to direct gain or radiant panel buildings. In fact, the addition of
interior mass can be used to improve the utilization efficiency of any
passive heating system although the effect can presently be quantified only
for direct gain or radiant panel buildings.
A low utilization efficiency can also indicate that the solar aperture
is too large. If the annual heat to load ratio is fairly small, for example
0.2 or less, and the utilization efficiency is below 0.6, the aperture size
should be reduced. An excessively large aperture may yield good performance
in terms of energy savings, as indicated by low values of (QA/QL)a,
but may be uncomfortable and inconvenient as indicated by low values of
eu.
5.2.3 Worksheet for average maximum temperature during reference month.
A step by step procedure for estimating the average maximum room temperature
(assuming no heat is ventilated) during the reference month is presented in
Worksheet 8. The first step is to calculate QD, the solar energy
delivered to the living space. As specified on the worksheet, QD is the
product of [alpha] and Ac (Worksheet 3), ed (Worksheet 7), VTn/DD
(Worksheet 5), and DD, the heating degree days for the reference month.
Values of DD are tabulated in Appendix B for a series of base temperatures
in each included city.
The second step is to calculate the excess solar energy during the
reference month. The amount of solar energy utilized is given by the
product of eu and QD, so the excess heat (QE) is given by the product
of (1 - eu) and QD as indicated on the worksheet.
Next, the average room temperature (T) that would prevail in the living
space, if excess solar heat were ventilated, is calculated from the
empirical equation given on Worksheet 8; the solar heating fraction (SHF) is
available on Worksheet 7. The temperature increment without ventilation
([W-DELTA]TI) is then calculated by dividing the excess solar energy by
the number of days in the reference month and the DHC of the building. The
average maximum temperature in the living space without ventilation (Tmax)
is then obtained by summing T and [W-DELTA]TI.
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