Net ZEB Case Studies - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 351

The energy balance of the roof is as follows.
(7.1)
where I is the incident solar radiation, A is the roof area, E PV is the rate
of electrical energy generation, E Thermal is the rate of thermal energy
collection, E Lost is the rate that absorbed solar energy is lost to the
surroundings, and ρ is the mean reflectance of the roof. For instance, for the
ÉcoTerra roof under sunny conditions at about solar noon, it is possible to
have about 1000 W/m 2 of incident solar radiation, of which nearly 6% (3
kW) can be converted to electrical energy and about 20% (12 kW) can be
converted to thermal energy.
The PV array is connected to the central utility grid; though the utility does
not currently pay a premium for its output. While the electricity generated
from the PV can be used in the house or sold to the grid, the thermal
energy output must be stored locally. The space heating demand is normally
minimal during periods of high solar gains from the passive solar gains
alone.Thus,thermalstorageisessentialforthethermalenergyoutputofthe
BIPV/T roof to be useful for space heating.
The heated air from the BIPV/T roof has three possible uses to reduce total
purchased energy of the house, as follows:
1. Space heating by charging the ventilated concrete slab (VCS) in the
basement. The basement slab, pictured in Figure 7.7 , is constructed of
corrugated metal decking with about 12.5 cm of concrete above it. Air is
injected into the channels at one end of the basement and it is
exhausted outdoors at the other end. Measured results have shown that
the system can store about 12 kWh (3.6 MJ) of thermal energy and
charge at a rate of about 2 to 3 kW during peak conditions. It should be
noted that the VCS can also be used for cooling, if it is discharged
during cool summer nights.
2. Preheating domestic hot water using an air-to-water heat exchanger.
3. Drying clothes in a conventional clothes dryer that was modified to
receive heated air in fan mode without electric heating.