Extract from the PhD thesis of Dr Andrew John
Geens entitled "A Critical Appraisal of the use of Displacement
Ventilation in Commercial Buildings" May 2005.
The main aim of this thesis has been to investigate
the effectiveness of displacement ventilation in providing good
indoor air quality when used in conjunction with supplementary
cooling devices, and to assess alternative techniques for dealing
with higher heat loads by displacement ventilation alone. This
chapter presents the conclusions that may be drawn from the results
obtained, and recommendations for further research that will strengthen
and build on these conclusions.
1. When a chilled ceiling device is used
to provide supplementary cooling in conjunction with a displacement
ventilation system, the resulting downward convection plumes will
disrupt the buoyancy driven displacement flow, reducing ventilation
effectiveness and hence indoor air quality. Further, if the chilled
ceiling devices operate following a period of operation of the
displacement ventilation alone, the displaced "contaminated"
air is "dumped" or re-circulated back into the occupied
2. The "dumping" effect described
in 1 above is most pronounced when the chilled ceiling device
is of the chilled beam type. The chilled panel does not produce
such a strong effect, and with care over the construction of the
ceiling this effect can be minimised. It is unlikely in practice
that it can be eliminated completely and it is concluded that
displacement ventilation cannot be achieved in practice where
ceiling mounted supplementary cooling devices are used. Systems
currently in use and described as displacement ventilation with
ceiling mounted supplementary cooling devices are in fact mixing
ventilation systems delivered via displacement ventilation diffusers
with ceiling mounted supplementary cooling devices.
3. Textile diffusers show great potential
for use as displacement ventilation supply diffusers increasing
the number of buildings that can benefit from displacement ventilation.
However, care is required in their design to avoid problems of
noise generation. The high volume flow rates achievable with the
use of textile diffusers introduces the concept of a "high
flow" displacement ventilation system, in contrast to the
"standard flow" displacement ventilation currently utilised
for commercial building applications.
4. When used for displacement ventilation,
textile diffusers can be used for cooling loads up to 50 W/m2
without the assistance of supplementary cooling devices. At this
load, using PPD/PMV as indicators, thermal comfort is comparable
with a conventional displacement ventilation system assisted by
a supplementary cooling system in a 40:60 ratio.
5. The demonstrated ability of the textile
diffuser to create a relatively robust displacement flow regime
at high volume flow rates makes it particularly suitable for applications
where strong odours or levels of contamination are produced, and
where segregation of the occupants from the contaminants is required.
6. The use of a textile diffuser for displacement
ventilation significantly reduces the size of the draught zone
in front of the diffuser. This increases the effective useful
floor area, making the textile diffuser a better diffuser option
even when high flow rates are not required, or when supplementary
cooling devices are going to be used.
9.2 Further work
The use of textile diffusers for displacement
ventilation for cooling loads up to 50 W/m2 has been demonstrated
to have certain merits over the use of other diffuser types requiring
supplementary cooling devices in the ceiling. However these conclusions
have been based on physical modelling in one test room facility,
using instrumentation to measure temperatures and velocities.
These results have been converted to standard comfort indices
for comparison with other systems.
The work has clearly demonstrated the technical
feasibility of this application. However, the perceived benefits,
namely good air quality and low energy consumption whilst maintaining
thermal comfort, are deduced rather than measured. This means
that there is considerable scope for further work to remove uncertainty
in these areas.
The performance of the system in use in an operational
building needs to be monitored and assessed. A range of monitoring
techniques will be required to address the diverse criteria of
thermal comfort, air quality and energy consumption.
The question of thermal comfort of the occupants
requires objective and subjective treatment. The objective analysis
for comparison with Standards can be provided by the same physical
measurement techniques used in the experimental test room, as
all the equipment is readily transportable. The subjective analysis
to ensure occupant satisfaction can be achieved with the use of
questionnaire or interview based surveys of the occupants of a
building using this system. The occupants can also assess air
quality as part of an occupant survey, particularly where contaminants
are readily discernible by the occupants as is the case in buildings
where smoking is permitted.
Most office buildings do not have readily detected
contaminants, reducing the effectiveness of occupant surveys in
assessing air quality. In these buildings, instrumentation to
monitor a contaminant level such as carbon dioxide can be used
to assess the ventilation effectiveness. Alternatively tracer
gas techniques may be used.
The issue of energy consumption can be addressed
by analysing the actual energy performance of a building in use
over a full 12 month period. Alternatively, computational modelling
techniques are available to simulate the operation of a building
in terms of the energy performance of the environmental systems.
The only installation currently identified as
using this technique is serving a conference facility with a restaurant
area as identified in Section 7.8. In the absence of any other
opportunity this building could be used as the basis for this
proposed monitoring work. The air supply rates for this building
are higher by a factor of approximately two than those assessed
during the experimental work for this thesis, presenting an opportunity
to re-appraise the cooling performance of the technique. As the
building operator allows smoking in the building, it also presents
a good opportunity for assessing the segregation of smokers and
non-smokers. This could lead to applications in leisure and retail
buildings where customers smoke and staff need protection from
With the possibility of use in leisure and retail
buildings there is also scope for incorporating the textile diffusers
directly into the decorative finishes of the occupied spaces such
as within bar frontages or under bench seating.
However, the main application for displacement
ventilation is currently in commercial office buildings. Particularly
in respect of thermal comfort, the nature of occupancy is significant
and monitoring in other types of building would not be reliably
transferable. It is therefore important to have a textile diffuser
system installed and monitored in an office building.
Although the use of comfort indices indicated
comparable performance for the textile diffusers, the temperature
gradient in the room was on the limit of acceptability. By conducting
the tests in a test room set up for testing chilled ceilings,
the optimum performance of the system was not established, with
the test room scenario probably being the worst case. It is likely
that the best comfort conditions will be established when the
system is used without a suspended ceiling, taking advantage of
the thermal mass of the ceiling, with or without the use of night
cooling. This needs to be examined, ideally in an operational
building rather than a test facility, where the thermal mass may
be difficult to model.
The experimental work has addressed cooling
loads of the nature and magnitude of typical internal casual gains
(machinery, occupants and lighting) for a commercial office application.
No account has been taken of the problem of direct or indirect
solar gain. The nature and magnitude of these gains and their
impact on the occupied space can be significantly varied by the
design of the fabric of the building. Further work is required
to assess the relative merits of dealing with the issue of solar
gains in the design of the building, ie shading and thermal mass
or dealing with them through the use of air conditioning systems.
There may be scope for dealing with these additional gains with
the use of a "high volume" displacement ventilation
system, when some care is taken to minimise solar gains, particularly
Finally, further physical modelling is required
on a fine measurement grid to establish the nature of the discharge
profile in close proximity to the diffuser, and hence the size
of the draught zone for the diffuser.