Net ZEB Case Studies - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 437

effectivethermalcapacitanceoftheroominteriorcontents(otherthanwall/
ceiling mass, which is slow responding) is 50 times that of the room air
(a typical value to account for furnishings), and the temperature difference
between outdoor and indoor air is 5 °C (say, 25 °C outdoor, and 20 °C
indoor), the assumption of air mixing well at 5 ACH means increasing the
inlet air by 4.5 °C, and decreasing the room air by 0.5 °C. This is achievable
with a convective heat transfer coefficient of 3 W/m 2 /K, ceiling and floor
areaforconvection,andameantemperaturedifferenceof4°C.For30ACH,
only about one-third of the required energy to warm up the incoming air
to the room temperature can be delivered. A mixing effectiveness ( ) is
introduced into the ventilation heat exchange formula. The decreases as
the ACH increases, and is assumed to be inversely linear with the ACH in
this case. The literature (Shaviv, Yezioro, and Capeluto, 2001) indicates that
30 ACH results in the highest potential savings. Hence, ACH ranging from
10 to 30 will be examined in this case study with the calibrated thermal
model. The cooling effect from natural ventilation is

(7.8)

where . is the exterior air temperature, and is the

average temperature of the three thermal zones. is the volumetric heat

capacity of air.
When natural ventilation is employed, heat transfer due to interzonal air
convection, infiltration, and mechanical ventilation is set to zero. The
natural ventilation heat loss calculated from Eq. (7.8) is assigned to the
three thermal zones according to the weights of their effective thermal
capacitances, which are equal in this case.

Radiosity Enclosure

In the radiosity enclosure, the workplane (0.9 m above raised floor) is used
for radiant heat transfer and daylight distribution calculations, replacing
the slab floor in the physical enclosure. The radiant heat absorbed/lost by
the workplane is added to the heat gain/loss of the control volumes of the
occupied space.

Transmittances for beam radiation of all windows are functions of the
incidence angle, while transmittances for diffuse radiation are constant,
but different for different window types. For daylighting windows, it is
assumed that 90% of the transmitted solar radiation (including daylight