Pure passive solar design uses the sun’s
energy directly, without mechanical
intervention. In its simplest form, the sun
shining through a window directly heats
the space. Thermal mass within the
building can absorb some of the heat
and release it at night. Internal thermal mass reduces temperature
swings within a space. In a properly
designed passive solar system, thermal
mass absorbs solar energy during the day,
preventing the building from overheating,
and releases the energy at night. Thermal
mass is most effective when it can gain
energy directly from the sun. An ideal
thermal mass for passive solar heating has
high heat capacity, moderate conductance,
moderate density and high emissivity.
Additional cost is negligible if the material
is also structural or decorative. Concrete
and masonry are good thermal mass
materials. (Plaster, drywall, and tile are also
useful in this respect, but calculations are
needed to determine if they have sufficient
mass, as was done in the Halifax study.)
Passive solar design in single-family
residences shows that operational energy
can be reduced by 30 to 50 per cent
through window sizing and thermal mass
storage. A recent study of MURBs in
Sweden reported that operational energy
use in a heavy structure is only slightly
lower than in a similar, lightweight
structure.7 The additional energy used to
build the heavy structure outweighed its
operational advantage in a lifecycle analysis
of costs.
Mass is known to be able to reduce peak
cooling load when night temperatures are
cooler than day temperatures. Exterior and
interior masses cool down at night and
reduce peak cooling demand while also
delaying the time of the peak solar gain
during the day. However, the effectiveness
of thermal mass is proportional to the
allowable room temperature variation over
a day.