Civil Engineering Reference
In-Depth Information
When ambient temperatures are mild, natural ventilation can be exploited to
dissipate excess heat from the building and to improve comfort. The feeling of
comfort improves with open windows and air movement; under natural ventilation
conditions, users can still feel comfortable at higher temperatures than in sealed
buildings (Brager and de Dear
1998a
).
In some seasons, natural ventilation can thus reduce the use of HVAC sys-
tems during the day (although it may not completely replace it in all climates).
Moreover, it can be beneficial during the night, when it can be used to cool down
the components of the building if temperature variations between day and night
are large enough.
Building design strategies and components that foster natural ventilation should
be implemented in new and refurbished buildings. Due to rising temperatures
around Europe (Rubel and Kottek
2010
), the latter are under particular stress,
especially in those countries where buildings were not designed for hot summers
(e.g. traditional homes with large glazing and small vents). Often, it is possible
to exploit natural phenomena such as wind forces and stack effect to activate air
flows in buildings through carefully placed vents, passive wind cowls and solar
chimneys. At a larger scale, an atrium can also be used to temper climate. Atria are
intermediate spaces, typically glazed, that rely exclusively on passive behaviour to
provide sufficient comfort conditions. Atria exploit direct solar gain in winter and
allow for abundant ventilation and shading in summer.
Natural ventilation alone may not always be sufficient to deliver the expected
air change rates and indoor air quality (especially in large buildings with high
occupancy rates), or it may not even be beneficial if ambient temperatures are
too high. This may happen, in particular, in temperate climates that experi-
ence extreme hot or cold conditions. It may then be necessary to develop hybrid
approaches, allowing for the use of natural ventilation when ambient conditions
allow, and of mechanical heating/cooling in peak periods of the year, when tem-
peratures outdoors are not comfortable or when natural drivers of ventilation are
too weak.
There are multiple motivations for the interest in hybrid ventilation:
• the likelihood of a positive response by occupants and a positive impact on
productivity;
• reduced environmental impact;
• increased robustness and/or lexibility and adaptability;
• commercial factors (the prospect of lower investment costs and/or operation
costs).
Hybrid ventilation is based on a different design philosophy and expectations
about its performance cannot be the same as for mechanical ventilation. Energy
performance targets and comfort requirements must be different. Cost compari-
sons between hybrid and mechanical systems should be done on a life-cycle cost
basis rather than simply on an initial capital cost basis, because of the different
design approach, and hence the different balance between initial, running, mainte-
nance and disposal costs (Abrams
1986
).
