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The recent IEA-EBC Annex 62 State of the Art Review report recently defined Ventilative Cooling (VC) as, ‘The application of ventilation flow rates to reduce the cooling loads in buildings. Ventilative Cooling utilizes the cooling and thermal perception potential of outdoor air. The air driving force can be natural, mechanical or a combination’ [1]. Within Annex 62, the role of Subtask C was to analyse and evaluate the performance of real VC solutions as well as investigate design methods and tools adopted using similar criteria and methods. In doing this the objective was to identify lessons learned and develop recommendations for design and operation of VC as well as identifying barriers for the application and functioning of VC. In total 15 case studies were included in the Annex 62 project [2]. This article gives an overview of the characteristics and lessons learned of investigated case studies in Annex 62. A final project report is due to be published containing a detailed summary of the case studies along with a book of case study brochures. Further details are available at http://www.venticool.eu.
Table 1. Building Type, size and year of completion for all case studies.
No | Country | Building | Type | Year (New or Refurb) | Floor Area m² | Strategy |
01 | IE | zero2020 | Office | 2012(R) | 223 | Natural |
02 | NO.1 | Brunla Primary school | Education | 2011(R) | 2500 | Hybrid |
03 | NO.2 | Solstadbarnehage | Kindergarten | 2011(N) | 788 | Hybrid |
04 | CN | Wanguo MOMA | Residential | 2007(N) | 1109 | Mechanical |
05 | AT.1 | UNI Innsbruck | Education | 2014(R) | 12530 | Hybrid |
06 | AT.2 | wkSimonsfeld | Office | 2014(N) | 967 | Hybrid |
07 | BE.1 | Renson | Office | 2003(N) | 2107 | Natural |
08 | BE.2 | KU Leuven Ghent | Education | 2012(N) | 278 | Hybrid |
09 | FR | Maison Air et Lumiere | House | 2011(N) | 173 | Natural |
10 | IT | Mascalucia ZEB | House | 2013(N) | 144 | Hybrid |
11 | JP.1 | Nexus Hayama | Mixed Use | 2011(N) | 12836 | Natural |
12 | JP.2 | GFO | Mixed Use | 2013(N) | 399000 | Hybrid |
13 | PT | CML Kindergarden | Education | 2013(N) | 680 | Natural |
14 | UK | Bristol University | Education | 2013(R) | 117 | Mechanical |
15 | NO.3 | Living Lab | Residential | 2014(N) | 100 | Hybrid |
The average
elemental U-value for all 15 case studies is 0.35 W/m²K, which appears
high but there is a large spread in individual values, with an average standard
deviation across all elements of 0.27 W/m²K.Six of the case studies can be
classified as having heavy or very heavy thermal mass according to ISO 13790:2008.
Good air tightness is a recurring feature of most case studies with the average
Air Change Rate (ACR) from infiltration at 1.13 h-1, ranging from 0.51 to 1.85 h-1. Table 2 gives some insight into the design
influences for each of case studies in urban or rural surroundings.
Table 2. Design Influences (R denotes Rural; U denotes Urban; *denotes residential).
Country | Building | Surroundings | Lower Initial costs | Lower Maintenance Costs | Lower Energy Costs | Reducing Solar Loads | Reducing Internal Loads | Reducing External Noise | High Internal noise propagation | Elevated Air Pollution | Avoiding Rain Ingress | Insect Prevention | Burglary Prevention | Reduced Privacy | Air Leakage |
IE | zero2020 | R | H | M | H | H | L | L | L | L | M | L | H | M | M |
NO.1 | Brunla School | R | H | H | H | L | M | L | L | H | M | L | L | L | H |
NO.2 | Solstadbarnehage | R | L | L | H | L | L | L | M | H | L | L | L | L | H |
CN | Wanguo MOMA* | U | H | M | H | H | L | L | L | L | M | L | M | L | H |
AT.1 | UNI Innsbruck | U | H | H | H | M | L | M | L | L | M | L | L | L | H |
AT.2 | wkSimonsfeld | R | H | H | H | M | L | L | L | L | L | L | L | L | M |
BE.1 | Renson | R | L | M | L | H | H | H | L | L | L | L | L | L | L |
BE.2 | KU Leuven Ghent | U | H | L | H | H | H | L | L | L | M | L | L | L | H |
FR | MAL* | U | M | M | L | H | M | L | L | H | L | L | M | L | M |
IT | Mascalucia ZEB* | R | H | M | H | H | L | L | L | L | L | L | M | L | M |
JP.1 | Nexus Hayama | R | M | M | H | H | L | L | L | L | M | H | H | M | M |
PT | CML Kindergarden | U | H | L | L | M | M | L | L | L | M | M | M | M | L |
JP.2 | GFO | U | H | M | L | L | L | L | L | L | L | L | L | L | L |
UK | Bristol University | R | H | H | H | L | H | L | M | L | M | M | H | L | L |
NO.3 | Living Lab* | U | L | L | H | H | M | L | M | L | H | L | L | L | H |
Information
on the recommended aperture area when sizing natural and hybrid VC systems is
critical for the building designer. For almost all case studies, natural
ventilation was adopted as either the sole source of VC or as part of a wider
strategy, with, for example, natural supply and mechanical exhaust. It is
generally beneficial to identify possible dimensionless parameters that provide
a characterization of the system, thus allowing for similarity investigations
across multiple different systems. Owing to this and the inherent importance of
the ventilation opening geometry to the delivered airflow rate, a parameter
calculated as the percentage opening area to floor area ratio, or POF, was
obtained for each case study. The opening area used is the maximum available
geometric opening area and does not incorporate the flow effects of the opening.
We see a large spread of values for the case studies. 65% of buildings had POF
values less than 4%. There seems to be no correlation with building category.
The two highest values are from climates with hot summers. Two of the lowest
three values are from fully naturally ventilated offices. Natural ventilated
buildings had a POF of 3.6% while hybrid buildings had a POF of 4.6%, or 6.0%
when the Italian case study is included. Although several building regulations impose
a minimum floor to opening area ratio of 5% there is a generally accepted rule
of thumb for designers when sizing openings at the concept stage of 1-3%. The
low end of this range appears inadequate when compared with these case studies.
A range of 2-4% seems more reasonable.
Table 3 gives an indication as the VC
strategies used for all case studies. Most control strategies for occupied
periods used the internal zone temperature and an external temperature low limit
as controlling parameters in ventilation strategies. The overall range of indoor
set-point temperatures were observed to be between 20-24°C where the mean
internal air temperature set-point was around 22°C. The range of low
temperature limits for outside air was between 10-18°C, with a mean external
low temperature limit set-point of around 14°C. Around 54% of the case study
buildings had a manual override switch or allowed occupant-controlled
ventilation during occupied hours as part of their typical occupied control
strategies. All-natural ventilation case studies allowed a form of occupant
interaction with the ventilation system while 60% of hybrid systems allowed
occupant interaction with the ventilation system. For systems that controlled
depending on relative humidity an average set-point of 60% was observed. There
were differing ranges of acceptability depending on whether the VC system was
mechanical or natural. 69% of the case studies investigated incorporated a
night ventilation strategy as well as an occupied ventilation control
strategy. Typically, night ventilation strategies had different control
parameters than ventilation strategies during occupied hours. The night
ventilation strategies incorporated typically had a set-point for the zone as
well as a limit on the properties of the air brought into the building also.
The range of internal temperatures used for night ventilation strategies was
between 15-23°C while the low limits on the external air temperature were
between 10-18°C. Night ventilation was also dependent on the presence or
absence of rain and wind speeds above a certain value. Typically, the wind
speed had to be below 14m/s or 10m/s respectively and with no rain for night
ventilation systems to operate. In cases where relative humidity was the
control parameter night ventilation would not be activated unless the relative
humidity was below 70% for a given zone.
Table 3. VC Strategies in all Case Studies.
Country | Building | VC Strategies | ||||||||
Natural Driven | Mech. Supply Driven | Mech. Exhaust Driven | Natural Night Ventilation | Mech. Night Ventilation | Air Conditioning | Indirect Evap. Cooling | Earth to Air Heat Exch. | Phase Chang eMaterials | ||
IE | Zero2020 | X | X | |||||||
NO.1 | Brunla Primary school | X | X | |||||||
NO.2 | Solstadbarnehage | X | X | X | X | |||||
CN | Wanguo MOMA | X | X | X | X | |||||
AT.1 | UNI Innsbruck | X | X | X | ||||||
AT.2 | WkSimonsfeld | X | X | |||||||
BE.1 | Renson | X | X | |||||||
BE.2 | KU Leuven Ghent | X | X | X | ||||||
FR | Maison Air et Lumiere | X | ||||||||
IT | Mascalucia ZEB | X | X | X | ||||||
JP.1 | Nexus Hayama | X | X | |||||||
JP.2 | GFO Building | X | X | X | ||||||
PT | CML Kindergarden | X | X | |||||||
UK | Bristol University | X | X | X | ||||||
NO.3 | Living Lab | X |
All case studies completed performance evaluations involving various different
measurement campaigns. Each case study adopted different approaches and
investigated different phenomena. This included ventilation rate measurements,
thermal comfort studies, analysis of internal thermal environments,
investigation of the performance of specific solutions such as displacement
ventilation, chimney-stacks, hybrid systems, cross flow ventilation etc. More
information on specific studies can be found at http://www.venticool.eu. The project
identified that in order to assess the minimum performance of the VC strategy one
cooling season of internal air temperatures data should be obtained. This data
should then be compared with an overheating risk criteria.
Two static thresholds were chosen with each case study quoting the percentage
of hours exceeding this value. Table 4 presents a
selection of results from the case studies.
Table 4.Preliminary results of VC performance evaluation.
Country | Building | Summer Design Values | Overheating criteria / note | % Occ hrs above threshold | Occ hrs | ||
Te | Ti,o | 28°C | 25°C | ||||
IE | zero2020 | 26.0 | 25.0 | Ti<28°Cfor 99% occ hrs | 0.7 | 5.5 | 2600 |
NO.1 | BrunlaSchool | 25.0 | 26.0 | Ti> 26°C | 0.0 | 0.0 | 2600 |
NO.2 | Solstad | 25.0 | 24.0 | Ti> 26 | 0.0 | 0.0 | 2860 |
AT.1 | UNI Innsbruck | 34.0 | 27.0 | Ti< 26 for 95% occ hrs | 1.1 | 16.2 | 2600 |
AT.2 | wkSimonsfeld | 34.5 | 24.0 | Ti> 26 zone / T > 29 gallery | 0.0 | 5.0 | 3250 |
JP.1 | Nexus Hayama | 26.0 | 26.0 | Ti< 28 for 99% occ hrs | 1.0 | 40.0 | 8736 |
PT | Kindergarden | 30.0 | 26.0 | 80% acceptability 99% hr occ | 2.6 | 16.0 | 3640 |
UK | Bristol Uni | 26.0 | 25.0 | Adaptive TC Model | - | - | 2600 |
BE.2 | Renson Builing | - | - | Tz < 26°C for 95% of hocc Tz < 28°C for 99% of hocc | 0.3 | 11.4 | 2600 |
BE.1 | KU Leuven Ghent | - | 25 | Ti < 26°C for 95 % hr occ | 0.3 | 5.1 | 1560 |
NO.3 | Living Lab | 25 | 26 | Ti> 26 | 24 | 2.6 | 832 |
Designing a building to incorporate VC can be challenging and may require a
lot of detailed building information. While each challenge was different the main
key lessons were as follows:
·
Detailed building simulation is important when
simulating VC strategies. Most case studies analysed highlighted the need for
reliable building simulations in the design phase of a VC system. This was
considered most important when designing for hybrid ventilation strategies
where multiple mechanical systems need harmonization. Some studies also said
that simulating the window opening in detail was important.
·
Customisation may be an important factor in designing
a VC system. In order to ventilate certain buildings, it may be necessary to
design custom components. Some case studies highlighted the need to have custom
design systems that were specific to country regulations and the use of a
building or space. Some consideration should also be given to the clients’
expectations around specific issues like rain ingress and insect prevention.
·
VC systems were considered a cost-effective and energy
efficient in design by most case studies, but particularly with naturally
ventilated systems. It was indicated that designing with the integration of
manual operation and control was important, particularly in a domestic setting.
While systems may be designed to have high levels of comfort, IAQ and
energy performance, achieving this was difficult. All case studies emphasised
that monitoring a buildings performance post occupancy is important if not
essential in building performance optimisation. While some key lessons were
more specific than others the following general observations were made:
·
Engaging with the building owners or operators as soon
as possible is integral to guaranteeing building performance for IAQ, comfort
or energy savings. For some case studies this specifically meant educating or
working with the facilities operator or manager for the building, for others it
meant educating the building occupiers themselves.
·
It was suggested by some that this engagement should
occur already in the design stage.
·
VC in operation is generally a good option. Case
studies comment on the reduction of overheating and improvement of comfort
conditions in the buildings that used outside air. However, correct maintenance
and calibration of the systems is integral to maintaining performance.
·
Some case studies highlighted the need to exploit the
outside air more with lower external air control limits during typical and
night-time operation. Others suggested that exploiting the thermal mass of a
building was key. However, it was noted that care must be taken with
considering these low temperatures as some case studies, particularly in cold
climates observed more incidences of overcooling than overheating.
·
Each brochure includes lessons learned in a dedicated
section at the end of the brochure.
More
details related to specific lessons learned can be found in the final summary
reports for IEA-EBC Annex 62 at http://www.venticool.eu.
In the last two decades, the use of VC has been slowly increasing. The best
contemporary designs combine natural ventilation with conventional mechanical
cooling. When properly designed, and implemented, these hybrid approaches maximize
the VC potential while avoiding overheating during the warmer months. Yet,
despite the potential shown in the case studies presented here, and other
existing examples, the potential of VC remains largely untapped. The
characteristics of each case study appeared unique due to the need for the
approach to respond to a specific climate, the building usage, morphology and
client criteria. Hybrid systems are the most common type of system for VC and
the use of mechanical fans to compliment a passive system should be strongly
considered where possible. A combination of automated and manual control seemed
to be the most adaptable and reliable solution to providing a system that
worked well with its users satisfied with its operation. The use of simulation
in the VC system design phase can reduce the uncertainties that are usually
associated with natural ventilation systems. A POF value in the region of 2–8%
was recorded and choosing a value on the larger end of this range at the
concept design stage may be appropriate.
The authors would like to acknowledge all IEA Annex 62 participants that provided a case study for the project. Further details are available at http://www.venticool.eu. This article is based on a paper presented at the 38th AIVC - 6th TightVent & 4thVenticool Conference, 2017 “Ventilating healthy low-energy buildings” held on 13-14 September 2017 in Nottingham, UK.
[1] M. Kolokotroni, P. Heiselberg, L. Pagliano, J. Han, R. Bokel, P. Holzer and A. Belleri, “IEA EBC Annex 62 Ventilative Cooling - State of the Art Review,” Aalborg University, Aalborg, 2015.
[2] P. D. O'Sullivan, A. O'Donovan, G. Zhang and G. C. d. Graca, “Design and performance of ventilative cooling: a review of principals, strategies and components from International case studies,” in AIVC2017, Nottingham , 2017.
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