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Paul D O’SullivanLecturer,Dept of Process Energy & Transport
Engineering,Cork Institute of Technology, Cork, Irelandpaul.osullivan@cit.ie | Adam O’DonovanResearcher,Dept of Process Energy & Transport
Engineering,Cork Institute of Technology, Cork, Irelandadam.odonovan@mycit.ie | Michael D MurphyLecturer,Dept of Process Energy & Transport
Engineering,Cork Institute of Technology, Cork, Irelandmichaeld.murphy@cit.ie | Maria KolokotroniProfessor, Theme Leader for Resource Efficient Future Cities,Department of Mechanical Aerospace and Civil Engineering,Brunel University,
LondonMaria.kolokotroni@brunel.ac.uk |
Ventilative cooling coupled with exposed thermal mass is widely accepted as an
important strategy for reducing summer overheating in non-domestic buildings.
Extended monitoring has shown that naturally ventilated buildings typically use
less than 50% of the corresponding energy consumption of air conditioned
buildings and assessment of ventilative cooling
techniques in Europe have shown they may contribute highly to reducing the
cooling needs of buildings (Kolokotroni et al, 2008)
and be an effective tool for tackling climate change adaptation in existing
buildings. Furthermore, increased ventilation rates can also lead to improved
work performance. Recently, focus for market activation in the construction
sector has shifted towards dealing with the overhaul of the existing building
stock. The Irish National Energy Efficiency Action Plan 2013–2020 report has
identified refurbishment of existing public sector buildings as a key focus.
The report states that there are over 10,000 existing public sector buildings
in Ireland. In responding to these external drivers Cork Institute of
Technology in Ireland (CIT) have recently completed a pilot project/research testbed,
zero2020, for the low energy retrofit of their existing 29,000 m² teaching
building constructed in 1974. The retrofit pilot project covered 1.5% of the
total building floor area and is shown in Figure 1. At both concept and
design stage there were no guidelines within the Irish context upon which to
base performance targets to achieve a near zero energy building (NZEB) through
retrofit.
Figure 1. External facades; existing
1974 building (left) and retrofitted zero2020 test-bed building (right) where
red indicates the location of the thermal comfort study and yellow indicates the
location of the ventilation rate performance experiments.
The design proceeded along a simple
strategy of firstly, ensuring compliance with the environmental specification
for occupant comfort and secondly to achieve the best fabric and energy
performance subject to constraints imposed by budgets and retrofit/structural
limitations. The final solution consisted of design and installation of a
structurally independent external envelope solution. This resulted in U-values
for opaque element 3 times better than current regulations and glazing U-values
6 times better. Further details of the design and specification of the retrofit
solution can be found in (O’Sullivan et al. 2013). The objective of this
article is to present measured performance of the retrofitted single sided
natural ventilation system that utilises a purpose provided slot louvered
opening (see Figure 2). Ventilation rate performance along with objective and subjective
thermal comfort performance of the retrofit building have been experimentally
investigated.
For most enclosed spaces in the existing
building (left of Figure 1) the ventilation system is based on single sided top hung pivoting
window sections. There is generally one opening window per structural grid. In
the retrofit space fenestration system, the ventilation module uses a flush
faced external louvre with individual air inlet sections with 2 ventilation
sections per structural grid. On the internal side of the slot louvres there
are automated high level insulated doors and manual low level insulated doors
providing different control mechanisms. The installed slot louvre system has a
net 50% free open area for airflow and overall structural opening dimensions
are 0.30 m (w) x 1.60 m (h) with a net opening area of 0.102 m²
(e.g. in a single cellular office space there are 2 openings at low level and 2
openings at high level in the test space). Each of the ventilation openings has
17 airflow slots across the louvre bank. The overall thermal transmittance
performance of this unit including doors and linear transmittance is 0.84 W/m²k.
The new fenestration module resulted in an overall opaque/transparent area
ratio reduction of 20%. Unwanted ventilation through adventitious openings has
also been greatly reduced. The retrofit envelope air permeability was tested in
accordance with BS EN 13829:2001. The envelope achieved an air
permeability of 1.76 (m³/hr)/m² at 50Pa building pressure. The existing
structure was measured as 14.77 (m³/hr)/m². In order to quantify the
actual range of ventilation rates achievable 38 tracer gas concentration decay
tests were completed as part of an experimental field study during summer 2013.
The results are summarised in the following section.
Figure 2. Ventilation system
configurations, from left to right: existing building window (CS.01), Retrofit
space no ventilation scenario (RS.01), bottom manual louvre only (RS.02), top
automated louvre only (RS.03), both louvres opened (RS.04).
To investigate ventilation performance in
cooling mode in the retrofitted space an isolated, 6.0 m²west
facing, first floor cellular office (highlighted in yellow in Figure 1) employing
single sided ventilation was used to measure ventilation rates under various
boundary conditions. A similar space in the existing building was used for
comparative purposes. The field tests were completed in accordance with the
procedures set out in ASTM E741-11. Details can be found in (O’Sullivan et al.
2014). All ventilation rate values presented have been calculated using the
decay regression technique. Figure 3 presents boxplot distributions of results from all tracer gas tests
completed under each of the different operating configurations in Figure 2.
Figure 3. Boxplot distributions of
measured mean ventilation rates grouped according to configurations in Figure 2
above (no of tests shown for each, mean ACH shown in red, median shown as bar)
The results show that for a similar spread
of boundary conditions the existing control space (top hung window, CS.01) has
consistently higher time-averaged ventilation rates with a mean value of 4.2 h-1
and standard deviation of 1.5 h-1. In the retrofit space the
full height configuration had the best performance profile with a mean value of
3.8 h-1 and standard deviation of 1.0 h-1
indicating a slightly more concentrated spread of results. Based on the
guideline values for indoor air quality classification in BS EN 13779:2007
both the existing control space and retrofit space time averaged ventilation
rates can be classified as IDA1 (High). Ventilation rate values were on average
lower for the low level RS.02 & high level RS.03 configurations although
there were individual instances of values above 5.0 h-1.
Figure 4. Percentage time exceedance
of long term index reference values during extended cooling period in 2013 (Monthly
95th percentile Tex values
shown in each month).
As well as ventilation rate performance the
potential risk to overheating was also evaluated for each ventilation
configuration. Indoor air temperature is often used as an index for evaluating
long term risk of overheating in buildings. The extent of exceedance of
acceptable conditions is often based on the percentage of annual occupancy
hours with indoor air temperatures above a reference exposure threshold value
compared to a maximum acceptable percentage exceedance (i.e. 5% of hours above
25 °C for CIBSE, 1% above 28 °C for BRE). To investigate long term performance
of the ventilative cooling system, indoor air
temperatures were recorded in the single cell retrofit office (highlighted in
yellow in Figure 1) during an extended period of warm summer conditions (May to
October 2013). A range of different configurations were employed in the office
during this time. Figure 4 presents monthly binned percentage of hours’ exposure for different
reference threshold value. There was some overheating present during July, even
with some night cooling in place but in general conditions were acceptable
according to the criteria proposed. It should be noted that the percentage will
reduce when the full annual hours are factored in.
In order to evaluate the thermal perception
potential of the thermally decoupled low energy space and ventilative
cooling system a subjective thermal comfort study was designed and carried out
in May 2015 (O’Donovan et al, 2015). The study evaluated all four of the
retrofitted ventilation configurations shown in Figure 2. The study gathered feedback
from 35 participants (10 females, 25 males) as to thermal environment during
controlled tests in the open plan seminar room of the building. This is a 42 m²
first floor, north facing room employing single sided ventilation (see red in Figure 1).
Participant feedback on perceived thermal state during the four tests was
gathered using two standardised questionnaires based on ISO 10551.
Objective continuous environmental data was
also gathered in order to calculate predicted response of participants for each
test using the Predicted Mean Vote (PMV) index. Table 1 below indicates the ISO 7730
categories used in this comparison where the values presented are for cooling
season performance only. The study was designed to evaluate the capabilities of
all ventilation configurations to provide thermal comfort in a simulated
overheating scenario, where before each test the seminar room was preheated to
26°C (±1°C).
Table 1. Categories of a thermal
environment.
Category | PMV | to(°C) |
A | -0.2 < PMV < +0.2 | 24.5 ± 1 |
B | -0.5 < PMV < +0.5 | 24.5 ± 1.5 |
C | -0.7 < PMV < +0.7 | 24.5 ± 2.5 |
Figure 5 presents predicted (objective) and subjective frequency of thermal vote for each configuration. Configurations RS.02 (high level only) and RS.03 (low level only) experienced the largest percentage of neutral responses (40%, 43%) with mean thermal sensation votes of −0.40 and −0.49 respectively (Category B). The full height opening configuration (RS.04) had a mean vote of -1.06 putting it outside all ISO7730 thermal environment categories. Subjectively no configuration tested achieved Category A (see Table 1).
Figure 5. Results thermal comfort
study showing objective and subjective thermal sensation votes for retrofitted space
ventilation configurations shown in Figure 2 (Vel
= Indoor air velocity, Tgl = Indoor Globe
Temperature, Text = External Air Temperature, RH = Relative Humidity).
Overall good ventilation rates were
achievable in the retrofit space with the new purpose provided slot louvred openings operated using a single sided ventilation
strategy. Although there was a reduced envelope temperature difference due to the
improved thermal performance of the building resulting in weaker buoyancy
forces compared with the existing building the large opening height of the
RS4.0 configuration seemed to ensure comparable performance. The thermal
comfort study suggests that using high level only openings or low level only
openings provided the most satisfactory thermal environments. It is therefore
possible to achieve effective ventilative cooling
using certain configurations but care must be taken when low external air
temperatures are present using large openings that provide air flow directly to
the occupied zone resulting in potential overcooling and local thermal
discomfort.
Kolokotroni, M., and Warren, P., (2008), Building AdVent:
Building Advanced Ventilation Technological examples to demonstrate
materialised energy savings for acceptable indoor air quality and thermal
comfort, Indoor Air 2008, Copenhagen, Denmark, August.
O’Sullivan, P.D., and Kolokotroni, M., (2014), Time-averaged single
sided ventilation rates and thermal environment in cooling mode for a low energy
retrofit envelope, International Journal
of Ventilation, vol. 13, 153–168.
O’Donovan, A., Saint-Omer, A., Serafin, G., Murphy, M.D., O’Sullivan, P.D., (2015),
Thermal comfort performance evaluation of a ventilative cooling system in a low
energy retrofit. IMC32 2015, Belfast, Northern Ireland, September.
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