Ventilation for URBAN BUILDINGS.. Natural and Hybrid Ventilation in the Urban Environment March 2006

8/3/2019 Ventilation for URBAN BUILDINGS.. Natural and Hybrid Ventilation in the Urban Environment March 2006

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VENTILATION FOR URBAN BUILDINGS

Natural and Hybrid Ventilation in the Urban Environment

March 2006


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CALCULATION OF THE OPTIMAL OPENING....................43

VENTILATION CONFIGURATION....................................53

Introduction to Module 2

Natural and hybrid ventilation are used in urban buildings to provide good air

quality and acceptable thermal comfort while reducing energy consumption of

buildings.

Natural ventilation has many advantages compared to air-conditioned buildings;

however it remains an uncontrollable system and depends on the ambient

conditions.

Hybrid ventilation combines both natural and mechanical ventilation switching

from one mode to another according to outdoor and indoor parameters. Hybrid

ventilation may be a solution for naturally ventilated buildings in urban areas forextreme hot periods.

The urban environment offers disadvantages in the application of natural and

hybrid ventilation: the reduced wind speeds, the high ambient temperatures, the

outdoor pollution and the increased noise levels decrease the cooling potential of

natural and hybrid ventilation. The effective integration of both ventilation

systems in the building design requires a good understanding of the urban climatic

characteristics and the choice of the appropriate techniques in order to reduce the

buildings exposure to the ambient constraints. Several design assisted tools have

been developed for the design of naturally ventilated buildings. These include

deterministic and empirical models for the prediction of wind speed at specific

height in urban canyons and software for the calculation of optimal openings orairflows in naturally ventilated buildings.


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Alternative ventilation strategies can also be used in the urban environments to

enhance the airflows in naturally ventilated buildings. Their function is based on

the increased temperature difference of the inlet and exhaust air thus the increase

of the buoyancy stack pressure. These strategies are appropriate for urban sites

because the location of inlet and outlet of air is at high level, in an above roof

position where the noise levels are reduced, the pollutant concentration lower and

wind speed higher than inside the urban canyon.

Evaluation of the performance of natural and hybrid ventilation has been carried

out through experiments and monitoring of buildings in real urban canyons. The

assessment of both ventilation systems aims to identify the parameters that affect

the indoor air quality in naturally ventilated buildings, to present the advantages of

hybrid ventilation versus natural ventilation and to examine the parameters that

affect the operation of hybrid ventilation systems. It can be concluded that the

indoor air quality in naturally ventilated buildings depends on the air change rates,

the concentration of indoor pollutants, the use of buildings and the different

configurations of natural ventilation. Hybrid ventilation has high cooling potentialwhile keeping the energy consumption of buildings at low levels; it has advantage

over natural ventilation under windless conditions. The operation of the hybrid

system depends on various outdoor and indoor parameters: the canyon geometry,

the ambient air, the indoor pollutants concentration, the building leakage and the

control strategy.

The choice of the ventilation system of urban buildings and the application of

the appropriate design techniques should take into consideration the urban

characteristics while maximizing the natural forces in the urban environment,

optimizing the thermal comfort, saving energy and/or improving the indoor air

quality.

CHAPTER 1 Natural and Hybrid Ventilation inUrban Buildings

Learning objectives

After studying this section you should:


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1. Understand the principles of natural and hybrid ventilation systems used

in urban buildings2. Understand the function and control strategies of hybrid ventilation


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Introduction

This module presents the principles of natural and hybrid ventilation systems

that are used in urban buildings. Natural ventilation remains an uncontrollable

system depending on the ambient conditions i.e. wind direction and air velocity.

Often in urban sites, where wind flows are reduced due to the heat island effect,

natural ventilation is inadequate.

Hybrid ventilation system may be a solution for naturally ventilated buildings as

it combines the advantages of natural and mechanical ventilation. The system

combines both modes switching from one to another depending on outdoor and

indoor conditions; the optimum function of the system uses natural ventilation as

much as possible.

Both natural and hybrid ventilation systems use the outdoor environment to

create good indoor air quality and thermal comfort while reducing energy

consumption of buildings.

1.1 Natural Ventilation

Natural ventilation is caused by naturally produced pressure differences due to

wind, temperature difference or both. Natural ventilation is achieved by allowing

air to flow in and out of the building by opening windows and doors or specific

ventilation components like chimneys. The effectiveness of natural ventilation

depends on the wind speed and direction, temperature difference, the size and

characteristics of the openings. (See module 1)

The main configurations for natural ventilation in urban climate are the same as

for open area location: Single sided Cross ventilation Stack ventilation Combinations of these strategies and enhancement of the airflows make

them more suitable to urban climates

When it is used for free-cooling, natural ventilation can replace air-conditioning

systems for large periods of time during a year. The potential of natural ventilation

is related to the energy saved for cooling if natural ventilation is used instead of

cooling. However, urban environment presents disadvantages for the applicationof natural ventilation: lower wind speeds, higher temperatures due to the effect of

urban heat island, high levels of noise and air pollution.

1.2 Hybrid Ventilation

Hybrid ventilation is a two mode system combining of natural ventilation and

mechanical ventilation. (De Gids 2004) Mechanical ventilation is used when

natural driving forces cannot fulfill the required ventilation level, a case very often

met in the urban context (Figure 1.1).


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Figure 1.1: Definition ofHybrid Ventilation (De Gids, 2004)

Hybrid ventilation is a new ventilation concept that combines the best features ofnatural and mechanical ventilation at different times of the day or season of theyear. It is a ventilation system where mechanical and natural forces are combined

in a two mode system. The operating mode varies according to the season andwithin individual days, thus the current mode reflects the external environment

and takes maximum advantage of ambient conditions at any point in time

(Heiselbeg, 2002).

The main difference between conventional ventilation systems and hybrid systems

is the fact that the latter are intelligent systems with control systems thatautomatically can switch between natural and mechanical mode in order to

minimize energy consumption and maintain a satisfactory indoor environment

(RESHYVENT research programme, WP8)

The aim of the strategy is to reduce energy, cost and the environmental side

effects of year-round air conditioning while optimizing indoor air quality and

thermal comfort by combining the two modes of ventilation.

The operating mode performs according to seasons and depends on external

ambient conditions.

Figure 1.2 shows the combination of a balanced ventilation system with natural

ventilation in a dwelling. When the ambient conditions allow it, the dwelling is

naturally ventilated. In extreme weather conditions, natural ventilation is shut

down and the mode is switched to mechanical ventilation.

Figure 1.2: Example ofHybrid Ventilation in a Dwelling (RESHYVENT,WP8)


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1.2.4 Control Strategies for Hybrid Ventilation

The control strategy is a key role for the function of hybrid ventilation systems as

it switches from natural to mechanical mode depending on the driving forces and

the required airflows. Control strategies can be distinguished in spatial and

temporal.

Spatial Control

Spatial strategy is related to the design system and should be taken into

consideration at an early stage. It is linked with outdoor air entering the habitable

rooms living rooms and bedrooms, and being extracted from the service rooms

kitchen and bathrooms.

The aim of this strategy is

to control the air change rates that have an impact on the energyconsumption. These are linked to the presence of occupants since it islimited into the service rooms than into the habitable areas

to limit the diffusion of short term pollutants from the service rooms to thehabitable rooms.

The drawback of this strategy is the excessive increase of airflows rates into the

habitable rooms when high air rates are required in some of the service rooms.

Temporal Control

Temporal strategy is related to the presence of occupants, thus emissions from

metabolism and activities; and to the climatic conditions. It is linked to demandcontrol ventilation.

Flows can be controlled by:

Presence: movement, switching on/off lighting Metabolism: CO2 emissions, water vapour, odours Activities: cooking, shower (this applies mainly to residential buildings) Climatic conditions: outdoor/indoor temperature, wind speed and wind

direction

Temporal strategy is based mainly on control/monitoring of CO2, VOC,

humidity and temperature parameters.

Summary

Hybrid ventilation systems use both natural and mechanical ventilation aiming

at reducing energy consumption of buildings and enhancing indoor air quality and

thermal comfort. The optimized operation of the system uses natural ventilation as

much as possible; the mechanical part is used when due to ambient conditions

natural ventilation cannot provide adequate airflows.

The control strategy is a key parameter for the function of the system and may

be connected to the outdoor parameters, the indoor conditions or the usage of the


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building. Both natural and hybrid ventilation systems are influenced by the

outdoor climate and take maximum advantage of ambient conditions at any time.

1.3 Questions for self assessment

1. How hybrid ventilation is connected to outdoor parameters?

2. Which parameters that are connected to the presence of people can

be controlled using hybrid ventilation?

3. What are the benefits of hybrid ventilation compared to other

ventilation systems

Problem(s)

1. Describe using a sketch a simple hybrid ventilation scheme integrated in a

dwelling switching from mechanical to natural ventilation


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CHAPTER 2 Impact of the Urban Environment on

Natural and Hybrid Ventilation

Learning Objectives


1. Understand the conditions under which the urban environment presents

constraints to the use of natural and hybrid ventilation2. Be able to provide design solutions to enhance the use of natural andhybrid ventilation in urban buildings


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Introduction

The chapter aims to discuss the most important constraints of the urban

environment on the ventilation efficiency of natural and hybrid ventilation systems

and to suggest several solutions to overcome these barriers.

The urban environment presents disadvantages for the application of natural and

hybrid ventilation. Because of the specific urban characteristics, the potential of

natural and hybrid ventilation can be seriously decreased in the urban environment

mainly due to:

Reduced wind speeds High ambient temperatures due to urban heat island Increased external pollutant Increased noise levels

2.1 Wind speed

The urban wind pattern is complicated. Compared to the undistributed wind in

rural areas, wind in the urban context is characterized by irregular flows because

of the built landscape, building geometry, street orientation, arrangement of built

structures and streets. As a result, wind speeds within the urban canopy are usually

reduced in comparison with rural winds at the same height: the wind speed u at

any heightzis lower in the urban area, and much lower within the obstructed area.

As a result, wind induced pressure on building surface is also reduced.

Figure2.1: Wind Velocity and Wind Induced Pressure are Reduced inUrban Environment

[Pa]

4m/s

Length [m]

4m/s

4m/s

2.1.2 Direction of airflows with respect to canyon axis


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The wind distribution in urban canyons is determined by the prevailing airflow

direction with respect to the canyon axis. The following wind incidence angles can

be observed:

Perpendicular WindWhen the predominant direction of the airflow is approximately normal (say 20 degrees), to the long axis of the street canyon.

Parallel wind (say 20 degrees)When the airflow is along the canyon axis.

Oblique windWhen the airflow is at an angle to the canyon axis.

Additionally, different types of air flow regimes are observed as a function of

wind incident angle, building (L/W) and canyon (H/W) geometry. (See foundation

module)

2.1.3 Model to Predict Wind Speed

Knowledge of the air speed inside urban canyons is of high importance for

passive cooling applications and especially for hybrid and naturally ventilated

buildings. Various methods, simplified or detailed have been proposed to calculate

the wind speed inside a canyon. However, air flow in canyons is not always a

deterministic problem and prediction algorithms may not be appropriate for any

case. Additionally, the boundary conditions are difficult to be defined and are

rarely known. Thus, a complete methodology to predict and estimate wind speeds

in canyons should be a combination of deterministic and empirical methods.

This section describes a new model that predicts wind speed inside canyons at

any height above ground level. This model is an algorithm based on existing

experimental knowledge and has been developed within the framework of

URBVENT European Project (2000).This model operates as a function of the

geometrical characteristics of the canyon, the undisturbed wind speed and other

boundary conditions.

The inputs of the model are:

The orientation of the canyon

The geometrical characteristics (width, height and length of the canyon

without intersections) and

Undisturbed wind speed (wind speed and direction outside canyon).

The output is:

Wind speed value at any specific point inside the canyon which is definedby coordinates (x, y, z).

2.1.4 Description of the proposed model


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Figure 2.2: Flow-Chart ofThe Algorithms and The Empirical ValuesUsed in The Empirical Model forEstimating Wind Speed Inside Street

Canyons (Georgakis and Santamouris, 2005a).

Source: Final report of the URBVENT project, European Commission,

Directorate General for Research, Brussels, June 2004

The proposed model to predict wind speeds can be described by the following

chart:

2.1.5 Sequence ofCalculations

Based on the input data, the model carries out the following sequence of

calculations:

Calculations to check if a canyon situation exists

Aspect ratio (H/W): If the aspect ratio of the canyon (H/W) is greater than0.7 then there is a canyon situation. Otherwise the space between the buildingsis not a street canyon.

1. Aspect ratio

H/W>0.7Not a street canyon

Dominant end effects. Use wind

speed close to 0.5m/s

3.Wind speed

v>4m/s

2. Aspect ratio

L/W>20

5.Wind flow

along canyon

4.Wind flowperpendicular/

oblique to thecanyon axis

Use Exponential Law.Use Hotchikiss-Harlow and Yamartino-

Wiegand model.

Use empirical

values

Yes

No

Yes

Yes

Yes

Yes

No

No

No

No


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Calculations to check if there is wind circulation inside the urban canyon

Ratio of length and width between the buildings: the ratio of the buildinglength between main intersections and the width between buildings (L/W) is

greater than 20 then there is a wind circulation in the canyon and the

calculations continue. If the ratio L/W is less than 20 then, the end effects

dominate inside the canyon and extended experimental analysis indicated that

a wind speed value of 0.5 m/s could be considered as mean (Georgakis and

Santamouris, 2005b).

Calculations of wind speed

Consequently, if the wind speed outside the canyon is less than 4 m/s butgreater than 0.5m/sec and its direction is perpendicular or oblique to the

canyon, the values from Table 2.1 (Empirical Values) can be used.

Table 2.1: Empirical Values for Perpendicular/Oblique Canyon WindSpeed Inside the Canyon (Georgakis and Santamouris, 2005a)

Wind Speed Outside

The Canyon (U)

Wind Speed Inside The Canyon

Near The Windward Facade ofThe Canyon Near The Leeward

FacadeLowest Part Highest Part

U=0 0 m/s 0 m/s 0 m/s

0


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and

0

2

2 /1.0 zhz b= (2)

Where:Uo is the constant reference speed, (outside the canyon)

y is the height from the ground in which we want to calculate the upup is the calculated wind speed inside the canyonz2 is the roughness length for the obstructed sub-layer

zo is the aerodynamic roughness length

Typical values of zo are given by Oke, (1987) in Table 2.2.

Table 2.2: Typical Roughness Length Values zo, forUrbanized Terrain(Oke, 1987)

Terrain zo (m)

Scattered Settlement(farms, villages, trees, hedges) 0.2-0.6

Suburban

Low density residences and gardens

High Density0.4-1.2

0.8-1.8

Urban

High Density, < 5 story row and block buildings

Urban high density plus multi - story blocks

1.5-2.5

2.5-10

For example, for the centre of the city of Athens where the mean buildings height

is close to 30 meters, we considered 0z equal to the value 3.

If the wind incidence angle is perpendicular/oblique to the canyon mainaxis and the wind velocity is greater than 4 m/s, the following algorithms are

used:

In this case the proposed algorithms are based on the study of Hotchkiss and

Harlow (1973) and permit the calculation of the cross and vertical wind speed

components ( u ,v ). The algorithms consider incompressible flow, absence ofsources or sinks of vorticity within the canyon, and appropriate boundaryconditions for the simple two-dimensional rectangular notch of depth H and width

W.

The proposed algorithms are the following:

( ) ( )[ ] ( )kxkyekyek

u kyky sin11 +

=

(3)

and

( ) ( )kxeeyvkyky

cos

=

(4)


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Where

Wk /= (5)

( )kH2exp =

(6)

( )= 1/0ku (7)

Hzy = (8)

And 0u is the wind speed above the canyon and at point x=W/2, z=H.

The above-mentioned algorithms were tested and approved by Yamartino andWiegard (1986).

Additionally, the along canyon wind speed component, ( )zw is calculated by thefollowing equation:

[ ] [ ]0000 /)(log//)(log)( zzzzzzwzw rr ++= (9)

Where rw was the wind speed values measured outside the canyon at rz meters

above the ground and 0z was the surface roughness.

The horizontal wind speed inside the canyon is:

5.022 )( vuvh += (10)

The total wind speed inside canyon at any point ( x, y, z) is:

5.022

)( wvvht +=

(11)

2.2 Temperature Distribution

The temperature distribution in the urban canopy layer is greatly affected by the

radiation balance: the temperature of the canyon surfaces depend on the heat

exchange processes between the buildings surfaces and the environment through:

radiation, convection and conduction

Surfaces absorb short-wave radiation and absorb/emit long wave radiation

depending on:

Thermal and optical characteristics of the materials-materialsabsorptivity. This is defined by the albedo; the albedo being a measure of the

solar energy amount that is reflected by the surfaces. The use of high albedomaterials, thus high reflectivity results in cooler surface temperatures whereas


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low albedo implies higher surface temperatures since the larger amounts of

solar energy is absorbed (Santamouris, 2001).

Surfaces exposure to solar radiation and buildings orientation.Because of the cities characteristics, canyon geometry, buildings thermal

properties and anthropogenic heat rate ambient temperatures in the urban

contexts are higher than their surroundings. This results in the urban heatisland effect that is the best documented example of inadverted climate

modification (Oke, 1987).

2.2.1 Analysis ofThe Air and Surface Temperature in Urban Canyons

Analysis of temperature distribution in several deep urban canyons showed that:

The surface temperatures are higher than the air temperatures in urbancanyons.

Surface temperatures vary according to their orientation: Southerly, south-easterly and south westerly orientated facades during the day may result inhigher temperatures than northerly, north-westerly and north-easterly

orientated facades. Additionally, the surface temperature stratification

observed during the day period of the analysis was between 300C to 500 on the

South-East wall and 270C to 410C on the North-West wall.

Surface temperature depend on inclination and materials: Comparison ofthe maximum difference of daily temperatures of the building facades and the

surface temperatures of the street shows that at street level the temperature is

20C and 50 C higher than the lower and the highest parts of the canyon.

Horizontal surfaces during the summer period receive more solar radiation

than vertical ones. Additionally slabs used in the pedestrian streets absorb

more radiation than materials used fro the building walls

Surface temperatures vary according to the surface height: maximumsimultaneous difference of the two facades was up to 10-20C at the middle

and at the highest measured levels of the canyon.

It should be noted that the air temperature outside the canyon was during the

experimental period about 5C higher than the temperature inside the canyon. This

can be explained by the canopy geometry as most of the canyons that were used

were deep. Additionally, the street orientation prevented the solar radiation for a

long period and a very good airflow was observed due to the big aspect ratio

(H/W=3.3)Figure 2.1: Box-Plots ofTemperature Distribution in a Street Canyon:

a) Vertical Distribution in The Centre ofThe Canyon; b) WallsTemperature (Georkakis and Santamouris 2003).

1 2 3 4 523

25

27

29

31

Temperature[C]

utsde

canyon

At3.5m

At7.5m

At11.5m

At15.5m

25

30

35

40

45

South-West

facade

Ground North-Eastfacade

Tem

perature[C]


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2.3 OutdoorPollution

Outdoor air pollution is a serious limitation for natural and hybrid ventilation in

urban areas. The relative importance of different air pollutants and sources has

changed with time and culture in the different geographical areas. Nowadays, the

dominant sources of atmospheric pollution, in certain European cities, are motor

vehicles and combustions of gaseous fuels.

Outdoor pollution is associated with:

Poor indoor air quality in buildings if combined with inadequateventilation. Health problems may occur if a space is not properly ventilated

and indoors pollutants concentration is high.All pollutants can cause serious

health problems to the occupants like as respiratory and cardiovascular problems, dermal irritations, infections and intoxication that result in

occupants discomfort and to a poor quality life. High humidity levels indoors

may also provoke allergies and are linked to an increased concentration of

micro organisms, mould and bacteria. In developed countries concentrations of

indoor pollutants are very similar to those outdoors with the ratio of indoor to

outdoor concentration (I/O) falling in the range of 0.7-1.3

Poor life quality. If identical health problems are experienced by a bigpercentage of the inhabitants, then we could speak about the sick building

syndrome. Sick building syndrome is usually met in air-conditioned buildings

but also observed in naturally ventilated buildings. It is a frequent

phenomenon of all major cities of the western countries.

In the case of sickbuilding syndrome the peoples productivity is seriously affected.

Damages to buildings and historic monuments. They can causedeterioration of materials and serious damage to equipment resulting in loss of

services provided by the equipment or even release of harmful substances into

the environment.

According to World Health Organisation, the predominant outdoor pollutants are:

sulphur dioxide, nitrogen dioxide, carbon monoxide, ozone, particulate matter, and

lead. Guideline values are given for these pollutants (WHO 2000). Indoor

pollutants include environmental tobacco smoke, particles (biological and non-

biological), volatile organic compounds, nitrogen oxides, lead, radon, carbon

monoxide, asbestos, various synthetic chemicals and others.

2.3.1 Analysis of the outdoor & indoor pollutants concentration

Indoor air quality in developing countries is an extremely serious problem.

Experimental measurements of the indoor-outdoor pollutant transfer showed that

the indoor pollutants concentration depends on several parameters:

Outdoor pollutant concentration and ambient particle distributions

Buildings openings Faade air tightness Ventilation rates


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Wind speeds and wind angles

Figure 2.2: The Variation ofIndoor perOutdoorOzone Ratio as aFunction of: a) Air Changes perHour (CW Closed Window, POW

Partially Opened Window, OW Open Window); b) OutdoorConcentration.

Source: Natural ventilation in the urban environment, Series Editor M.

Santamouris

0.0

0.10.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350 400 450

Outdoor concentration, Co (ppb)

I/O at 0,6 (vol./h) Weschler et al. 1989I/O at 4,0 (vol./h) Weschler et al. 1989I/O Shair et Heitner 1974

I/O=1-exp(-0.027*Co.

)

I/O=exp(-0,379*Co,

)

I/O=1-exp(-0,027*Co.

)

0.0

0.10.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10 12

Air changes per hour, ach (h-1

)

OWPOWCW

Hayes1991

Weschler et al. 1989Iordache 2003

Monitoring of the main outdoor pollutants (SO2, NO2, CO, O3, suspended particle

matter, and lead) in large cities shows that:

The mean level of sulphur dioxide and lead are equal indoor and outdoor. Ozone and nitrogen dioxide react with the building material and the

building airtight ness: their concentration is lower indoors than outdoors when

the building is airtight.

The particle matter transfer depends on the particle size and the outdoorconcentration of the pollutant.

The indoor - outdoor pollutant transfer depends also on the windowsopening, thus the building air-tightness; in the case of closing windows the

transfer becomes more complex.

2.3.2 Ventilation strategies and guidelines to reduce exposure to outdoor

pollutants

Many guidelines have been developed to propose interactive design tools andventilation strategies for naturally ventilated buildings in polluted urban areas.

Pollution avoidance strategies in the building design that should be taken into

consideration include:

Location of vents on sheltered facades and positioning of central inlets at asufficient height from emissions.

Alternative solutions to natural ventilation as supply ventilation withfiltration or exhaust ventilation, filtration


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2.4 Noise Levels Ventilation Potential in Urban Areas

Noise is one of the most important considerations when designing openable

windows thus for the potential of natural ventilation in urban sites. Noise is often

considered as a restrictive parameter for the use of natural ventilation and the

selection of air-conditioning systems instead.

It is suggested that acceptable noise levels should not exceed 65 dB (A). High

levels of noise and continuous background noise may provoke sleep disturbance,

and have impact on peoples health, productivity and social life. Moreover, areas

with noise levels more than 75 dB(A) are considered harmful as these can lead to

hearing loss.

Studies showed that inhabitants of the most urban areas are exposed to high

noise pollution. Within the frames of the Fifth Programme it was stated that about

8% of the urban population is exposed to outdoor noise at a level 70 dB(A), while

11% is exposed at levels greater than 65 dB(A).

In the case of naturally ventilated buildings or buildings with hybrid ventilation,noise is an important parameter to consider. Window opening provides the

minimal sound insulation, thus many times noise exclusion and provision of

adequate ventilation consist of two controversial functions.

2.4.1 Noise Levels in Street Canyons and Potential ofNatural Ventilation

Several studies have been undertaken to estimate noise levels in urban areas and

their impact on the choice of ventilation strategies for buildings. Noise

measurements were undertaken in the canyon streets in Athens with aspect ratios

(height/width) varying from 1.0 to 5.0 and with a variety of traffic load, during

September 2001. The aim of the analysis is to assess the vertical variation of noiselevels along the building facades and the cooling potential of natural ventilation.

The analysis comprises two parts, measured data of daytime traffic noise and

simulations using a noise-level simulation package.

Part 1 - Measurement data: The study of the measured data uses a linear regression

analysis and shows that the noise levels in the urban areas depend on the canyon

geometry and the traffic density;

Attenuation of noise levels is noted with floor height above the street level

The maximum value of noise attenuation is almost entirely a function ofaspect ratio

Traffic density thus noise levels increase with reduced street width

Balconies may contribute to a noise reduction of 2-4 dB depending onfloor height.

Part 2 - Simulations: Three simulations were made and the simulation results were

compared to the measured data. For the study, different street widths were

assumed of 5m, 10m and15m respectively for a five storey building. The aim of

the analysis was to assess the impact of the balconies to the reduction of the traffic

noise levels at the four building levels: first floor, second, third and fourth floor

for the three different street widths. The results showed that:

The noise attenuation due to the balconies is around 2dB at first floor and


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more than 3dB for the top floor.

The geometry of the balcony, width and distance of the building faade aswell as the solidity of the front of the balcony has an impact on the noise

attenuation: for example narrower balconies do not have great impact on the

noise levels as larger balconies may have.

2.4.1 Analysis ofNoise Levels in Office Buildings

Within the frames of the Research programme Smart Controls and Thermal

Comfort (SCATS), surveys were undertaken in 25 office buildings of 5 European

countries to assess the use of natural ventilation and noise limitations in urban

areas. The results show that:

Noise levels of 55-60dB can be accepted, though greater tolerance isobserved in areas with open windows.

The noise attenuation at an open window is accepted at 10-15dB, thusoutdoor levels for acceptable comfort indoor conditions should be around

70dB.

Window design is very critical when providing natural ventilation: Specialglazing may contribute to a further 3-5dB reduction.

Figure 2.7: Potential forNatural Ventilation ofOffices as a Function ofStreet Width and Height Above the Street.

Configurations in which natural ventilation is possible are indicated (ok), as arethose in which it is ruled out (not ok). Between these two extremes is a region in

which there are possibilities for design solutions. Source Natural Ventilation in

the urban environment


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0 5 10 15 20 25 30 35 405

10

15

20

25

dB

Height above street (m)

Streetw

idth(m)

OK

NOT OKDesign

possibilities?

74 dB

70 dB

7276

68

2.4.2 Noise Control Strategies for Natural Ventilation

A number of noise control strategies for natural ventilation systems can be

implemented in the building design. It is suggested that:

With careful design, adequate airflow rates can be provided in buildingsfor good indoor quality and noise insulation

The use of hybrid ventilation can convert the natural ventilation strategy areal possibility in areas where noise levels have previously prohibited such

an approach.

SummaryThe urban characteristics influence the cooling potential and efficiency of

natural and hybrid ventilation: the reduced wind speeds, the high ambient

temperatures, the increased noise levels and the outdoor pollution are restrictive

parameters for the use of natural and hybrid ventilation in urban buildings.

Knowledge of the air speed in urban sites is of high importance for the

application of passive cooling techniques. The use of an algorithm that was

developed within the frames of the URBVENT research programme is suggested

for the prediction of wind speed in canyons at specific coordinates. Additionally,

the careful location of vents and inlets, the use of filtration or the replacement of

natural ventilation with hybrid ventilation offer solutions for naturally ventilatedbuildings that are located in high polluted areas.

Noise levels in urban areas depend on the canyon geometry and the traffic

density; additionally the building geometry i.e. number of floors, presence of

balconies can contribute significantly to the noise attenuation. However, hybrid

ventilation can be a solution for areas with increased noise levels: natural

ventilation should not be used in sites with noise levels higher than 74 dB.

2.5 Self assessment Questions


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1. Describe the sequence of calculations carried out by the algorithm

in order to predict wind speed inside urban canyons. In which case the

mean wind speed can be considered 0.5 m/sec?

2. Which parameters affect temperature distribution in urban areas?

3. Give the threshold lines of noise levels in urban areas where natural

ventilation can be applied

Problems

1. What are the consequences of badly selected and located air inlets in

naturally ventilated buildings in the urban environment? What is the

optimum location of air inlets?

2. The wind speed (Vo) is measured 4.2 m/sec at 15m above ground.

The orientation of the wind speed and urban canyon are given:

Canyon orientation = 50Wind (V0) incidence angle = 40It is also calculated that L/W >20. Explain why there is wind circulation

inside the urban canyon.

Assuming that z0 is equal to 3 and the roughness length is given by the

equation: 02

2 /1.0 zhz b= , calculate the wind speed up at the faade of the

fifth floor (at height 1.5m from floor level of the fifth floor)


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CHAPTER 3 Natural Ventilation Strategies to

EnhanceAirflows in Urban Environments

Learning Objectives


1. Understand the principles of the strategies that can be used to enhance

airflows in naturally ventilated buildings in urban areas

2. Use equations to calculate airflows in different ventilation strategies whenincreasing the buoyancy stack pressure


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Introduction

This chapter focuses on several alternative ventilation strategies and building

constructions that can be used to enhance airflows in naturally ventilated buildings

in urban areas.

The use of natural ventilation in urban environment should take into account the

lower wind velocity but also the noise and outdoor pollution. It is very important

to consider other techniques than windows to enhance airflow in buildings. In

many cases, the ventilation systems cannot rely on low-level inlets as these may

be close to external pollutant sources i.e. traffic.

3.1 Balanced stack ventilation

The balanced stack ventilation schemes use both inlets and exhausts of air at

different temperature: air is supplied in a cold stack (i.e., with air temperatures

maintained close to outdoor conditions through proper insulation of the stack) andexhausted through a warm stack. This strategy is met in the traditional Iranian

wind towers (bagdir) and the Arabian Asian wind catchers (malkaf) comprise

examples of balanced stack ventilation.

Figure 3.1: Top-down or balanced stack natural ventilation systemsuse high level supply inlets to access less contaminated air and to

place both inlet and outlets in higher wind velocity exposures.

Top-Down or Balanced Stack Ventilation

v ref

nlet

nternal

exhaust

w-inlet

w-outlet1 2

3

4

5 6

warm

stackcool

stack

The equation for the pressure loop for example through the second floor of the

figure above will be similar in form to the case of combined wind and buoyancy-

driven ventilation:

exhausternalinletws ppppp ++=+ int . (1)


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If air temperatures within the cold stack can be maintained close to outdoor

levels, the stack pressure is determined by the indoor-to-outdoor air density

difference and the height difference from the stack exhaust and the floor level inlet

locations is given by the equation:

( ) s o ip g z = (2)

Airflow through each floor level will, therefore, be identical to that expected in

the simpler single stack scheme if the airflow resistance of the supply stack (and

its inlet and outlet devices) is similar to that provided by the air inlet devices in

this scheme.

The driving wind pressure is determined by the difference between inlet and

exhaust wind pressure coefficients and the kinetic energy content of the approach

wind velocity

2/)( 2refexhaustpinletpw vCCp = . (3)

The high location of the inlet assures a higher inlet wind pressure and

insensitivity to wind direction. The location of the air-intakes in an above-roof

position where the pollutant concentration is lower than the street level and the

independency from air velocity and wind direction thus wind patterns remain

independent from surrounding buildings, make this strategy very attractive for the

urban areas.

Balanced stack systems have been used in the UK for, apparently, over a century

(Axley, 2001) although these commercially available systems have, until recently,

been designed to serve single rooms rather than whole buildings.

3.2 Passive Downdraught Evaporative Cooling

The passive downdraught evaporative cooling (PDEC) scheme is the same

scheme with the balanced stack ventilation system with the addition of

evaporative cooling to the supply stack. Traditionally, evaporative cooling was

achieved through water-filled porous pots within the supply air stream or the use

of a pool of water at the base of the supply stack. In more recent developments,

water sprayed high into the supply air stream cools the air stream and increases the

supply air density thereby augmenting the buoyancy induced pressure differences

that drive airflow.

In the loop analysis of thepassive downdraught evaporative coolingscheme, the

increased moisture content must be accounted for. Two height differences must

now be distinguished:za - the height above the room inlet location of the moist air

column in the supply stack and zb - the height of the exhaust above this moist

column.


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Figure 3.2: Passive Downdraught Evaporative Cooling StackVentilation.

The air density in the moist air supply column, s , will approach the saturation

density corresponding to the outdoor air wet bulb temperature more specifically,

experiments indicate these supply air conditions will be within 2 C of the wet

bulb temperature. Hence the loop equation describing the (time-averaged)

ventilation airflow in this system becomes:

( )inlet internal exhaust s wp p p p p + + = + (4)

where:

[ ]gzzzzp baiasbos )( ++= (5)

( )2

2r

exhaustpinletpw

vCCp

= . (6)

For a quantitative measure of the impact of this strategy, let us consider a case

similar to that one discussed above for wind and buoyancy induced natural

ventilation, but with a cool moist column height that equals the stack height of 10

m (i.e., mza 0 and mzb 10 ). If the outdoor air having the temperatures of 25

C and humidity of 20 % RH (i.e., with a density of approximately 1.18 kg/m3) is

evaporatively cooled to within 2 C of its wet bulb temperature (12.5 C), its dry

bulb temperature will drop to 14.5 C while its density will increase to

approximately 1.21 kg/m3 and relative humidity to 77%. If internal conditions are

kept just within the thermal comfort zone for these outdoor conditions (i.e., 28 C

and 60 % RH), using an appropriate ventilation flow rate given internal gains, then

internal air density will be approximately 1.15 kg/m3. Consequently the buoyancy

pressure difference that will result will be:


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3 3 3 2

kg kg kg m1.18 (0 m) 1.21 (10 m) 1.15 (0 10 m) 9.8 6.4 Pa

m m m ssp

= + + =

(7)

Without the evaporative cooling (i.e., with 10az and 0 mbz ):

3 3 3 2

kg kg kg m1.18 (10 m) 1.21 (0 m) 1.15 (10 0 m) 9.8 2.9 Pa

m m m ss

p = + + =

(8)

Thus, in this representative example, evaporative cooling more than doubles the

buoyancy pressure difference while, at the same time, providing adiabatic cooling.

3.2.1 Example

The School of Slavonic and East European Studies (SSEES) building of

University College London comprises an example where passive dowdraughtcooling is used. The building is located in a central area of London with high

ambient temperatures and increased noise levels and pollution. Initial studies

showed that natural ventilation strategy would not achieve acceptable thermal

comfort and the night time ventilation efficiency was reduced due to the urban

aspects. Therefore, the natural ventilation cooling capacity was enhanced with

chilled water coils for extreme weather conditions.

The air is entering the building to all floors through a three sided light well in

the centre of the building. The air is cooled by chilled water coils that are located

around the top of the light well. The air is exhausted through stacks on the curved

and double faade sides of the building (figure 3.3)

Figure 3.3: The School of Slavonic and East European Studies(SSEES) Building of University College London.Example ofPassiveDowndraught Evaporative Cooling (Source: IJV Volume 3, 4 March

2005)

3.3 Double Skin Faade

The double skin faade system consists of two glass skins placed in distance sothat air flows in the intermediate cavity. The ventilation of the cavity can be

neutral, fan supported or mechanical. Apart from the type of the ventilation inside


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the cavity, the origin and destination of the air can differ depending mostly on

climatic conditions, the use, the location, the occupational hours of the building

and the HVAC strategy. The glass skins can be single or double glazing units with

a distance from 15cm up to 2 metres.Usually, solar shading devices are placed

inside the cavity. (Haris Poirazis, 2004)

Potential advantages and disadvantages of the application of double skin facades,

typologies of the system and technical description of the systems components are

studied within the research programme BESTFACADE.

Compared to conventional office buildings with large glazed facades, this system

provides the follows advantages:

Thermal buffer zone that reduces heat losses and enables passive thermalgain from solar radiation

Solar preheating of ventilation air, thus reduced heating demands

Sound protection e.g. at locations with heavy traffic mainly during windowventilation Additional shading and protection of shading devices Energy savings if the design is well adapted to the climatic conditions Enables natural ventilation. Individual window ventilation is almostindependent of wind and weather conditions mainly during sunny winter days

and the intermediate season

Night cooling of the building by opening the inner windows

Potential disadvantages of double skin facades can be:

Poorer cross ventilation and insufficient removal of heat from the officesduring windless periods when ventilation is provided mainly by natural

ventilation

Overheating in the occupied spaces during the summer months Higher investment and cleaning costs Risk of noise-cross talk via the faade from one office to the other and/orfrom one level to the other.

It is very important the double skin faade to be well designed with the correct

type of ventilation and adapted to the climatic region; otherwise overheating or

energy inefficiency may be experienced.The space within the skin two skinsshould be well ventilated either by natural, mechanical or hybrid ventilation.

According to the origin and destination of the supplied air into the cavity the

double skin facades can follow into the following sections:

Outdoor air curtain, when the air comes from the outside and isimmediately rejected to the outside - Figure 3.3 (1)

Indoor air curtain, when the air circulates from the inside and returns to theinside- Figure 3.3 (2)

Air supply, where fresh air is supplied into the building throug the externalskin- Figure 3.3 (3)


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Air exhaust, the air is comes from the inside of the room and is exhaustedfrom the building faade - Figure 3.3 (4)

Buffer zone, the faade is made airtight. The cavity comprises a bufferzone between the internal and external skin. - Figure 3.3 (5)

Figure 3.3: Configurations ofDouble Skin Facades According to TheOrigin and Destination ofThe Supplied AirInto The Cavity

Source: Double Skin Facades for Office Buildings-Literature Review, Division of

Energy and Building Design, Department of Construction and Architecture, LundUniversity, 2004, Harris Poirazis

Double faades can be used for solar assisted stack ventilation or balanced stack

ventilation.

3.4 Solar-Assisted Ventilation

The solar assisted ventilation technique increases the difference between the

internal and external temperature by heating the air in the ventilation stack, thus it

increases the buoyancy-stack pressure. Solar energy is used to heat the air usually

stored into hot air solar collectors. This technique is very useful for naturally

ventilated buildings in urban areas where the wind speed thus the corresponding

airflows into the occupied areas are reduced.

The pressure losses for a solar collector depend on the inlet (pi) distributed (pd),and exit (pe) pressure losses:

edis pppp++=

(9)

Depending on the position of the control damper, ip or ep include the

control damper pressure losses. The stack pressure is:

zgTTTp ies = ]/1/1[00 , (10)

Where

Ti is the inlet air temperature of the collector, usually equal to the indoor

temperature,

Te is the exit temperature of the collector (Awbi 1998):


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)]/(exp[)/(/ vpeie qcBwHBATBAT += , (11)

2211 ww ThThA += (12)

21 hhB += (13)

Whereh1, h2 are surface heat transfer coefficients for internal surfaces of the collectorW/m

2KTw1, Tw2 are surface temperatures of internal surfaces of the collector (C)

w is the collector width (m)

H is the height between inlet and outlet openings (m)

e is the air density at exit, (kg/m3)

cp is the specific heat of air (J/(kg.K))

qv is the volumetric air flow rate. (m3/s)

Figure 3.4: SolarCollectorUsed as: a) Ventilator, b) Heater.

Tw1h1

Tw2h2

Ti

Te

Ti

Glass

Wall

Outdoor

airTo


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ventilation is supported by fans. Such fans may be installed either on stack ducts,

in walls or windows.

Summary

This chapter describes five alternative strategies that can be used in the urban

context to enhance airflows in naturally ventilated buildings and cope with

increased levels of noise and outdoor pollution. These techniques include the

following schemes: balanced stack ventilation, passive downdraught evaporative

cooling, double skin faade, solar assisted ventilation and fan assisted ventilation.

All strategies are based on the increased temperature difference of the inlet and

exhaust air thus the increase of the buoyancy stack pressure. When natural

ventilation cannot be provided by stack effect, then natural ventilation can be

supported by fans.

Usually in these techniques, the inlets of air are located at high level where

outdoor pollutant concentration is low and wind pressure is increased. Double skin

facades can also provide extra sound insulation in indoor areas.

3.6 SelfAssessment Questions

1. Describe 3 ventilation strategies that are used to enhance airflows in

naturally ventilated buildings and are based on the stack effect.

2. Why the use of the balanced stack ventilation system offers advantages

in urban areas?

3. Describe the main advantages that double skin facades offer as a

solution to the constraints of the urban environment

Problems


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1. Describe using sketches two methods by which natural ventilation can

be enhanced in the urban environment

2. Using the basic equations for the calculations of the buoyancy

pressure difference ps in the case of balanced stack ventilation and

evaporative cooling explain the increase of the stack pressure in the

second methodology


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CHAPTER 4 Evaluation of the Natural & HybridVentilation Potential in Urban Environments

Learning Objectives

After studying this section you should understand:

1. The potential of natural ventilation in urban areas2. The impact of natural ventilation on IAQ according to the external pollutants

concentration3. The potential of night ventilation in urban areas

4. The efficiency of hybrid ventilation systems in urban areas


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Introduction

This chapter evaluates the use of natural and hybrid ventilation in urban

buildings. The evaluation of the different ventilation configurations is based on

studies that have been carried out in real urban canyons with different

characteristics.

This section is split into 2 parts: the first part is focused on the potential of

natural ventilation and indoor air quality of urban buildings versus the external

pollutants concentration. This part also presents the night cooling capacity with

regard to the heat island effect and the reduced wind speeds in urban areas.

The second part describes the potential of hybrid ventilation in urban sites.

However, only a minor number of real urban buildings with hybrid ventilation

systems are known in the literature.

4.1 Natural Ventilation and IAQ

Indoor air quality was studied in several office and school buildings. All

buildings are naturally ventilated and the analysis showed that:

The air change rate is the determining factor for air quality in naturallyventilated buildings. The greater the supply of external air, the greater the

presence of external pollutant indoors in the urban environment.

Concentration of indoor pollutants (sulphur dioxide, nitrogen oxides,carbon monoxide and carbon dioxide) is higher in naturally ventilated

buildings in polluted urban areas than in air-conditioned ones. In some cases

though, indoor pollutants concentration may be higher in air-conditioned buildings if transfer of combustion products from heating boilers in the

occupied spaces occurs via the ventilation system.

Apart from the ventilation strategy used, the indoor air quality depends onthe use of the building. For example, in school buildingsthe indoor air quality

inside the classrooms is strongly related to the number of occupants and their

activities.

In the case of reduced wind speeds cross ventilation with two or more

windows into the occupied space can provide adequate airflows and betterindoor air quality. Cross ventilation seems more efficient than single sided

ventilation under calm conditions (very low wind speed).

Night ventilation can result in daytime temperature reduction up to 2.5Cunder free floating conditions and 1C under air conditioned operation in

buildings of the urban context. However, the cooling potential of night

ventilation for a specific building is a function of many parameters: the

building design and materials, climatic conditions, site layout, applied air flow

rate, efficient coupling of air flow with the thermal mass of the building and

assumed operational conditions.


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4.1.1 Evaluation ofNatural Ventilation and Night Cooling in Urban Areas

The performance of natural ventilation in the urban environment was assessed in

real urban canyons through field measurements. The measurements included

simulations of the air flow processes for ten different canyons in which wind

speed and temperature data were collected in a number of field measurements in

the framework of the POLISEuropean research project. The analysis focused on

single-sided and cross-ventilation configurations for a typical building zone with a

window opening in each canyon facade. Additionally the performance of night-

ventilation techniques in urban areas was simulated for the same reference

building, various simulations have been performed under controlled and free-

floating operation, when single-sided and cross ventilation are considered, during

the night period.

It has been found that:

The potential of natural ventilation techniques in urban canyons isseriously reduced with the decreased wind speed inside the canyons. Air flow

reduction may be up to 10 times than the flow that corresponds to undisturbed

ambient wind conditions.

The potential of night techniques is significantly reduced due to theincrease of air temperature and the decrease of wind velocity inside canyons

Figure 4.1: Air Change Rate for Single Sided and Cross Ventilated

Buildings in Ten Urban Canyons (Geros et al., 2001).

The effect of the increased ambient temperatures of London due to the heat

island effect on the effectiveness of stack night ventilation strategies for office

buildings were also studied by Kolokotroni et al. (2005).

Real air temperature measurements, carried out in London in 1999/2000 toquantify the London Urban Heat Island Intensity, were used to perform a

0.2 0.43.0 2.5 2.2

10.0

2.9 1.6 2.63.

12.6

17.8

34.6

4.2

34.9

60.0

52.9

27.4

69.3

10

20

30

40

50

60

70

80

Urban Canyon

Differenceofthea

irflow

(inACH)

Single Sided Ventilati

Cross Ventilation


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parametric analysis on the cooling demand and potential for night cooling

ventilation for typical offices. The study was applied by using a thermal and air

flow simulation tool specifically designed for London office in SE England. Two

representative weeks were studied, one with extreme hot weather and one with

typical hot weather in the centre of the London heat island as well as in a rural

reference site.

The results showed that the increased urban temperatures due to the heat island

effect can be taken into account for night cooling as they result in significant

deviations.

4.2 Evaluation ofHybrid Ventilation

Hybrid ventilation in urban areas is highly affected by a number of urban

parameters like as canyon geometry and layout, wind and temperature distribution

inside canyons, pollutant concentrations, external noise, and solar access.

Therefore, effective design of hybrid ventilation in urban buildings requires a

good understanding of the urban climate characteristics. However, a fewexperimental and theoretical studies on urban buildings with hybrid ventilation are

known in the literature.

Some measurements of hybrid systems were carried out in office and educational

buildings within the frames of the research programme Annex 35 (Hybrid

Ventilation in New and Retrofitted Office Buildings, IEA)

The buildings are located in urban areas with moderate levels of air and noise

pollution and the main ventilation strategy is stack effect with fan assistance. The

analysis shows that the strategy has very high cooling potential providing good

indoor air quality and thermal comfort. Additionally the building energy

consumption was kept in satisfactory levels.

Other monitoring in residential buildings in the streets canyons of Athens under

the RESHYVENT European project shows that under calm conditions with wind

speed lower than 0.5m/sec hybrid ventilation is more efficient than natural

ventilation with regard to ACH values.

4.2.1 Performance ofHybrid Ventilation Systems

The performance of two different hybrid ventilation systems was examined

within the research programme RESHYVENT: the first configuration comprised a

mechanical exhaust system and the second one a hybrid ventilation system thatwas developed for the purposes of the European project RESHYVENT. Two

demand controlled strategies were developed for the RESYVENT hybrid system,

the first one was based on occupant detection and the second one on CO2 levels.

The analysis included yearly simulations for four European climates (Athens,

Nice, Stokholm and Trappes) building and HVAC toolbox.

The analysis showed that both control strategies have better performance

regarding the indoor air quality and electrical consumptions of the fan compared

to the mechanical exhaust system because they optimize the use of natural

ventilation mode. Detailed description of the analysis of the two hybrid ventilation

systems is included in chapter 6.


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Summary

The evaluation of the natural ventilation performance in real urban canyons

shows that indoor air quality in naturally ventilated buildings depends on various

parameters like as: the air change rates, the concentration of indoor pollutants, the

use of buildings and the different configurations of natural ventilation. The

potential of natural ventilation and night cooling is reduced in urban areas due to

the increased ambient temperatures and the reduced wind speeds. However,

studies in the London area showed that the increased urban temperatures due to

the heat island effect can be taken into account for night cooling as they result in

significant deviations.

Limited data on real buildings with hybrid ventilation systems exists in the

literature. Studies on various urban canyons show that hybrid ventilation can have

high cooling potential providing good indoor air quality and thermal comfortalthough the urban constraints.

4.3 Self Assessment Questions

1. Which natural ventilation configuration is more appropriate in urban

buildings under calm wind conditions

2. What parameters affect indoor air quality in naturally ventilatedbuildings?

3. How the urban environment influences the cooling potential of night

ventilation

Problems

1. What are the advantages of hybrid ventilation in urban buildings

when compared a. to natural ventilation and b. to mechanical ventilation

2. Describe techniques to reduce peak air temperatures in naturally

ventilated buildings


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CHAPTER 5 A Methodology to Calculate The

Optimum Openings for Naturally Ventilated BuildingsLocated in UrbanCanyons

Learning Objectives


1. Be able to understand and describe the function and principles of themethodology developed for the best practice design of naturally ventilated

buildings in urban areas

2. Be able to use the methodology in order to calculate optimal openings innaturally ventilated buildings


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Introduction

When designing naturally ventilated buildings, it is very important to be able to

calculate optimal openings to achieve the required airflows for acceptable comfort

levels. Several tools have been developed to estimate the natural ventilation

potential and the passive cooling potential of urban buildings versus the climatic

characteristics of a location.

This chapter presents a methodology for the best practice design of naturally

ventilated buildings in urban canyons. This methodology was developed within

the frames of the research European programme URBVENT and is based on the

principle of a recurrent neural network model. The designer can use this tool for

the calculation of airflows in buildings, alternatively with a given database of

airflows; the tool can be used for the calculation of the optimum opening.

5.1 Description ofThe Methodology

The methodology is based on the principle of a recurrent neural network model;

it provides the designers with database of different parameters and then an

interpolation of the results is required.

For example, the tool calculates the airflows in naturally ventilated buildings

under specific:

- canyon characteristics

- openings of a building

- geometrical and operational characteristics of the building.

Alternatively, the designer can obtain a large database of airflows for buildings

configurations and search the database for the corresponding optimum openings.

Neural network model

A neural network model is based on establishment of empirical laws obtained

starting from an experimental data base. Practically this model can be seen as a

black box establishing the link between input variables which influence the

studied phenomenon, and an output variable corresponding to the value that we

seek to predict.

Figure 5.1: General Outline ofThe Model

INPUT

S

ACH

Neural

network


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5.2 Architecture Scenario and Databases

The models used in the methodology are based on two architectural

configurations: the single-sided ventilation scenario and the stack-induced

ventilation scenario. The scenarios were supplied by the Instituto de Engeharia

Mecnica and obtained from simulations on AIOLOS and COMIS softwares.

A database of air change rates is obtained through simulations using the validatedtools: AIOLOS and COMIS. Two databases have been created, one for single-

sided and another for stack induced ventilation by varying the size, location of the

openings, wind speed and temperature difference between the indoor and outdoor

environment. The databases are included into the library of the developed model.

Single Sided Ventilation

A matrix of 15 million values formed the database for single sided ventilated

rooms. The model is represented by a small room with one external opening on

one faade.

Figure 5.2: Single-Sided Ventilation Room

StackInduced Ventilation

A matrix containing 2.6 million values is used as the database for stack induced

ventilated rooms. The model is represented by the same small room shown in

figure 5.2, but it was inserted into a multi-storey building and the stack effect was

induced by a single external opening in the faade and a chimney linking the room

the roof of the building.

Figure 5.3: Multi-Storey Building with Stack Induced Ventilated

Rooms.

LW

M

h

X

H

Number of

stories : 1-5

Chimney

External

Opening

Room under

study


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.

5.3 Tool forCalculations ofACH orOpenings

5.3.1 Single Sided Scenario

In the case of a single sided ventilated room the tool can be used as follows:

Calculation of ACH

For the calculation of air change rates the network requires as inputs the values

of external temperature, the wind velocity, the room volume, the height of the

opening top of the window, the height of the opening bottom of the window and

the width of the window. After been trained according to correspondent values of

air change per hour for the rooms, it can simulate new inputs and predict the value

the air change rate per hour for the single sided ventilated room (figure 5.1,4).

Figure5.4: Architecture of the model of calculation of ACH for single sidedventilation.

Then a graphical interface was developed in order to make it easy for users to use

the model (figure 5.5).

Figure 5.5: Graphical Interface to Calculate ACH.

ACH

Neuralnetwork

Externaltemperature Wind speed

Roomvolume X, H, W


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Calculation ofThe Optimal Opening

For the calculation of optimal openings the network requires as inputs the values

of external temperature, the wind velocity, the room volume, the height of the

opening bottom of the window, the height of the opening top of the window and

the value of the Air Change Rate per hour. After been trained according to

correspondent values of air change per hour for the rooms, it can simulate new

inputs and predict the width of the window for the single sided ventilated room

(figure 5.6).

Figure 5.6: Architecture ofThe Model ofCalculation of W forSingleSided Ventilation.

Figure 5.7 Graphical Interface to Optimise The Opening.

W

Neural network

Externaltemperature Wind speed

Room

volume H, X ACH


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5.3.2 StackInduced Scenario

In the case of stack induced ventilation the tool can be used as follows:

Calculation of ACHThe techniques used are identical to these for single-sided ventilation:

For the calculation of the air change rate the network has inputs the values of

external temperature, the room volume, the height of the opening top, the height of

the opening bottom of the window and the width of the window, the diameter of

the chimney, the useful area and the floor level of the room in the building. Then

the network can predict the value the Air Change Rate per hour for the natural

ventilated room with stack effect.

Calculation of the Optimal OpeningFor the calculation of the optimal opening, the network has as inputs the values

of external temperature, wind velocity, room volume, the height of the opening

bottom, the height of the opening top of the window, the value of the air change

rate per hour, the diameter of the chimney, the useful area and the floor level of

the room in the building. Then the network can predict the width of the window

for the natural ventilated room with stack effect

Figure 5.8: Graphical Interface to Calculate ACH.


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Validation ofThe Software

The airflows that were predicted by the developed tool were compared with real

experimental data in order to compare the models accuracy.

The comparison was carried out for both architectural scenarios that are used in

the model; the single sided case and the stack induced configuration; the

calculated values are very close to the experimental values.

Therefore the model can be considered reliable and can be used by designers to

predict airflow rates or to size openings for naturally ventilated buildings in urban

areas.

Summary

The size of openings or the calculation of airflows is of high importance when

designing naturally ventilated buildings in urban areas. A methodology for the

best practice design of naturally ventilated buildings in urban canyons was

developed within the frames of the research European programme URBVENT. Itis based on the principle of a recurrent neural network model; it provides the

designers with database of different parameters and then an interpolation of the

results is required.

The methodology is based on two architectural configurations: the single-sided

ventilation scenario and the stack-induced ventilation scenario. The database of

the air change rates that is included in the tool is obtained through simulations

using the validated tools: AIOLOS and COMIS. For both architectural scenarios,

and inputs as the values of external temperature, the wind velocity, the room

volume, the height of the opening bottom of the window, the height of the opening

top of the window and the value of the air change rates, the network calculates the

optimum size of openings. Alternatively, when the size of openings is known the

methodology can be used for the calculation of the required airflows.

5.4 SelfAssessment Questions

1. Describe the principle on which the methodology for the best practice

design of naturally ventilated buildings is based

2.

3. Which two ventilation configurations the methodology uses?

4. What are the inputs required from the tool when calculating the optimal

openings of a building

Problems


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1. Describe using a sketch the procedure of the tool to calculate the airflows

in naturally ventilated buildings

2. Calculation of optimal opening of a naturally ventilated room (single

sided) using the software


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CHAPTER 6 Performance ofHybrid Ventilation in

Urban Environments Through Experimental Data

Learning Objectives


1. Be able to provide advantages or disadvantages of hybrid ventilation

systems against natural ventilation in urban buildings2. Describe principles and components of hybrid ventilation systems used in

urban buildings3. Understand the performance of hybrid ventilation systems versus indoor

parameters, outdoor parameters and different control strategies


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Introduction

This chapter presents the impact of the urban environment on hybrid ventilation

systems. The analysis is based on the results of experimental and theoretical

analysis in real urban buildings.

It consists of two parts: the first part gives the comparison among different

ventilation configurations: natural ventilation, hybrid ventilation and mechanical

ventilation through monitoring in real urban canyons. The comparison examines

the different methodologies in terms of air change rates and shows the advantages

of hybrid ventilation over natural ventilation under specific climatic conditions.

The second part describes two hybrid ventilation systems; the pilot system and the

RESHYVENT system that was developed during the unanimous research

programme. This part presents the performance of the systems against indoor

parameters, outdoor parameters and different control strategies.

6.1 Comparison ofHybrid Ventilation, Natural Ventilationand Mechanical Ventilation

Air-exchange rates and air exchange efficiency were monitored and compared

for different ventilation systems, under two specific urban canyons of Athens,

during summer 2002. The compared ventilation systems comprised natural

ventilation, infiltration, mechanical and hybrid systems.

Natural ventilation comprised single-sided and cross ventilation configurations.In

case of single-sided ventilation, openings were considered either, from the canyon

or, the rear canyon facade. Cross ventilation experiments were studied with two or

more openings placed at the front and back canyon side.

Mechanical systems comprised one or two fans in inlet or extract modes.Hybrid ventilation systems focused on fan-assisted natural ventilation, where

supply and extract fans were used to enhance pressure differences by mechanical

fan assistance. The fans were installed in the facades adjacent to the canyon or the

rear facades operating in inlet or extract mode in conjunction with natural

ventilation. The configurations that were monitored during the analysis are

described in the following table:

Figure 6.1: Hybrid Ventilation Systems Monitored During Summer2002 In Athens (Where (a) Refers To Canyon Faade And (b) To Rear Canyon

Faade)(Niachou et al., 2005)

1. Mechanical Exhaust (a) and

Natural Ventilation (b).

2. Mechanical Inlet (a) and

Natural Ventilation (b).

(a)(b) (a)(b)


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3.Natural Ventilation (a) andMechanical Exhaust (b).

4. Natural Ventilation (a) andMechanical Inlet (b).

5. Mechanical Exhaust (a) and

Natural Ventilation (a).

6. Mechanical Inlet (a) and

Natural Ventilation (a).

7.Natural Ventilation (a) and

Mechanical Exhaust (a).

8.Natural Ventilation (a) and

Mechanical Inlet (a).

9.Natural Ventilation with more than one

windows (a,b) and Mechanical Exhaust (b).

10. Mechanical Exhaust (a) and Natural

Ventilation with more than one windows (b).

11. Mechanical Exhaust (a,b) and

Natural Ventilation (b).12. Mechanical Exhaust (a), Mechanical Inlet

(b) and Natural Ventilation (b).

The experiment was conducted along the tracer gas method during which tracer

was injected inside the rooms, with internal fans used to homogenize its internal

concentration and when the internal fans were turned off the tracer gas decay was

measured.

(a)(b)(a)(b)

(a)(b) (a)(b)

(a)(b) (a)(b)

(b)(a) (b)(a)

(b)(a)(b)(a)


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Results & Conclusions

The comparison of the different ventilation systems showed that:

Natural cross ventilation results in higher ACH values than single-sidednatural ventilation.

Hybrid ventilation is associated with rather lower ACH than natural cross-ventilation, but slightly higher ACH under single-sided ventilation or calm

conditions. However this does not mean that hybrid ventilation may not be of

use during the summer days when natural ventilation is not an effective means

of cooling, either due to low winds or due to high ambient temperatures.

Hybrid ventilation has an advantage over natural under windlessconditions

Hybrid ventilation provides higher air-exchange rates in comparison withsingle-sided ventilation.In most cases, there is also an improvement relative to

natural cross ventilation.

Figure 6.2: Estimated Air Exchange Rates (h-1) for Natural (Single-Sided) And Hybrid Ventilation Experiments at A3 apartment, Under

Calm Conditions Based on Single-Zone (1,2) And Multi-Zone (3)Methods (Niachou et al., 2005)

6.2 Performance ofTwo Different Hybrid VentilationSystems

Within the RESHYVENT research programme, the performance of two hybrid

ventilation systems was assessed: the pilot ventilation system and the

RESHYVENT hybrid ventilation system.

The analysis of the systems efficiency was performed for different urban

situations, having aspect ratio (H/W) equal to 1, 1.5, 2, 2.5 and 3, and for climates

of eleven different European cities.

0

2

4

6

8

10

1 2 3 1 2 3

AirExchangeR

ates(h-1)

Natural Hybrid

min

75th percentile

25th percentile

average

max

Methdologies


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6.2.1 Pilot Ventilation System

The pilot ventilation system consists of:

Demand control strategy based on the indoor air quality levels. Differentcontrol functions have been introduced and are based either on the indoor CO 2or on the TVOCs concentrations.

Low-pressure system supported by wind and buoyancy: An asymmetricand non-ideal flow controller is used. Non-ideal means that no compensation

for other faade leaks is included in the flow controller model. Only the actual

characteristic of the controller is considered. The asymmetric controller has a

separate flow rate for negative flow-directions. The resulting flow is

temperature compensated according the relation of air densities at actual and

test conditions.

Balancing Supply and Exhaust: Two supply/exhaust fans have been usedwith a corresponding performance of 795m3/h at 0 Pa pressure difference. Thefans are installed on the zone external walls facing the rear and the front

canyon faade. The fans are reversible and they operate on both extract and

intake mode according to the specific case study.

Figure 6.3: A representation of the pilot hybrid ventilation systemwith two inlet/extract fans installed at the two external building walls.

An inverse operation of the fans is considered on the right photo(Niachou and Santamouris, 2005)

6.2.2 RESHYVENT Hybrid Ventilation System

The system consists of self-regulating air inlets, DC fan, motorized damper,

flow meter, central control unit, CO2 sensors and ductwork. The demand control

of the ventilation system is based on monitoring of CO2 in rooms. There is a CO2sensor and a self-regulating air inlet in each room. The self-regulating inlets are

usually positioned above windows. These inlets are able to maintain a constant

flow rate for the pressure difference across the facade higher than 1 Pa.

The hybrid ventilation system was simulated to operate when the CO2concentration in the apartment increased to 1200 ppm. The exhaust fan is used

when the air exhaust through the duct is lower than the demanded flow.

Supply fan

Exhaust fan

Supply fan

Exhaust fan Supply fan

Exhaust fan

Supply fan

Exhaust fan


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Pressure-dependent grilles of the system were also simulated in order to realize

better the canyon effect.

The pressure-dependent inlet grilles have the following characteristics, when the

pressure difference is above 1Pa:

Qv=CP

n

and C=25.96dm

3

/s per 1 Pa , n=0.50

The inlet grilles are considered full open between 0 Pa and 0.5 Pa. Above 0.5 Pa

the inlet grilles start to control and there is not longer a standard relation between

pressure and airflow. The ventilation system is examined either with natural or

hybrid exhaust mode. In the natural ventilation exhaust mode, the exhaust airflow

rate from the duct is affected by the natural driving forces. When the exhaust flow

rate through the duct is lower than the demanded flow rate, then the fan starts to

operate. A minimum of 21dm3/s is considered for the air exhaustion through the

duct.

Figure 6.4: Representation ofThe RESHYVENT Hybrid VentilationSystem For Moderate Climates (Niachou and Santamouris, 2005)

6.2.3 Results & Conclusions

The performance of the two hybrid system was assessed considering the following

parameters:

Canyon Geometry Canyon Layout Outdoor Urban Air Characteristics Indoor Pollutant Emissions Buildi

Documents

Ventilation for URBAN BUILDINGS.. Natural and Hybrid Ventilation in the Urban Environment March 2006