PASSIVE OFFICE BUILDING – THE EFFECT OF SHADING AND ... · normal angles, limits the choice of...

Central Europe towards Sustainable Building CESB10 Prague Advanced Building Design

PASSIVE OFFICE BUILDING – THE EFFECT OF SHADING AND THERMAL MASS

Ing. Kamil Staněk CTU in Prague, Faculty of Civil Engineering, Dept. of Building Structures, Thakurova 7,160 00 Prague, Czech Republic, [email protected]

Ing. Marek Ženka CTU in Prague, Faculty of Civil Engineering, Dept. of Building Structures, Thakurova 7,160 00 Prague, Czech Republic, [email protected]

Summary

The article concerns with summer operation of a passive office building. The importance of effective shading and is highlighted. The extent of shading vs. daylight level on a work desk is optimized. A detailed thermal model is used to evaluate the thermal comfort in the office space during a summer week. Different strategies of cooling are listed. The advantage of passive thermal accumulation in combination with cooling systems activating massive structures (TABS) is presented.

Keywords: shading, daylight, solar gains, thermal modeling

1 Passive office building during summer operation

The design of office buildings in passive standard for heating season has recently become a managed procedure. The key question to be answered is their summer operation with respect to the overheating risk. The building has to able to offer satisfactory internal environment while not exceeding the primary energy limit of 120 kWh/(m2.a). Solar radiation is considered to be one of the key factors – it is important but on the other hand rather unpredictable and unreliable source of daylight and thermal energy.

Fig. 1 Scheme of evaluation process of a passive office building during summer operation

The issue is demonstrated on a southeast oriented office building in Brno (currently in design) and is examined from four perspectives: (1) balancing shading and day-lighting; (2) optimization of active thermal mass; (3) natural ventilation strategy and (4) use of active cooling systems. The main design target is to minimize the energy need for active cooling and to ensure sufficient daylight level and acceptable indoor operative temperature during summer.

1


,1zoneT

,1zoneC

gQ

extT

extT

,2zoneT

office corridor

TABS

,V inT

TABS

,2zoneC

extT

dynamic modules of the respective structures with calculated surface temperatures

steady-state modules based on a specific heat flow rate in W/K

adiabatic boundary

1 2H

2 extH

VH

,1zoneT

extT

tbH

WIN struct

INT struct

EXT struct

gQ

desk min. 300 lux

redirecting sunlight

venetian blinds

mechventnatur

vent

2,76

m

5,1 m 6,4 m

height of the Sun21.8.

SE facade

Fig. 2 Schematic cross section of a typical office with its key features and RC representation of the

thermal model

A hot summer sunny day (21.8.) was chosen as a basic element out of which the modeled week was arranged by repeating it seven times in a row for evaluation the design scenarios. Solar radiation is based on clear sky data for Brno and the outdoor temperature is modeled as a sinus wave with its peak shifted to 4 pm (see appendix 2).

2 Shading and day-lighting

Southeast orientation of the main façade, where the morning sun hits the windows at nearly normal angles, limits the choice of shading devices capable of blocking the solar beam radiation and allowing a sufficient part of the diffuse radiation to enter the room. External venetian blinds are chosen as an effective shading device. A two-section slatted blind operated separately in the upper third and the rest of the window are designed.

All the components of solar radiation are considered individually (see appendix 3). Color, reflexivity and specularity of blinds are elective. Calculations in this article are carried out with white blinds. As an output, the amount of direct and diffuse radiation and the respective angles of incidence on the windows is calculated. Three major characteristics influencing the daylight comfort in the office are followed:

▪ the daylight level on the work desk being min 300 lx ▪ no direct radiation incident on the desk or monitor or hitting a sitting person and ▪ no glare in the 60° wide view angle from every working spot in the room

As a result an optimized setting of the blinds during the day was found (opt in Tab. 1, see appendix 3) and then used for thermal analysis.

2


3 Thermal gains, ventilation and active cooling

Internal and solar gains are the main factors influencing the thermal comfort in the office. Internal gains depend on the considered occupation pattern and used equipment. There is a little possibility to any change. The direct solar gain in Brno office surpasses the internal one for unshaded situation approximately twice. An appropriate design of shading devices enables to reduce it to less that 10% when still assuring acceptable light conditions inside. The total thermal gain is then 60% lower, not exceeding 30 kWh per m2 of the office floor area and a day (see appendix 4).

Several ventilation strategies of the office space are considered. The least energy demanding strategy assumes natural diurnal (need of fresh air) as well as nocturnal and weekend (need of precooling) ventilation. The second strategy combines diurnal mechanical ventilation by precooled fresh air and nocturnal natural ventilation. The third strategy is based on diurnal mechanical ventilation by precooled fresh air only. For thermal discharging of the structures another active approach is used – the thermal activation of the core of ceiling slabs made of reinforced concrete (TABS).

4 Thermal model

Dynamic multi-node thermal model is used to calculate mass, surface and internal air temperatures within the office space (see appendix 5). The resulting operative room temperature is used as a measure of occupancy well-being. Software WINDOW 5.0 is used to determine thermal and angular dependent optical properties of the glazing units. The direct and diffuse components of the solar radiation are treated separately. One dimensional finite volume method is used to calculate dynamic heat transfer within the structures. It enables incorporating TABS into selected nodes in the respective scenarios (case 3 in Tab. 1).

5 Results

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 126 132 138 144 150 156 1621617181920212223242526272829

00.511.5

0

200

400

600

800

1000

1200

2

com

fort

rang

e10

% d

issa

tisfie

d

ther

mal

gai

nstem

pera

ture

[°C

]

Qs,direct

[W] Qi [W] Q

vent [W] Q

s,strop [W] Q

s,příčky [W]

Fig. 3 The results of thermal analysis: Case 1 with optimal shading (case 1 opt in Tab. 1).

3


Tab. 1 The results for the selected cases. Energy for active

cooling [kWh/week] Operative room temperature

Top [°C] Discomfort when

Top > 27°C Case and blind set.

Direct solar + internal gains [kWh/week] vent TABS maximum daily increase hourdegrees hours

1 0 0 27,9 2,65 4,29 11,8 2 11,7 0 26,8 2,16 0 0 3

opt 6,8 + 40,8 11,7 33,1 25,2 1,80 0 0

1 0 0 28,6 2,68 25,8 39,4 2 11,7 0 27,6 2,20 3,84 15,6 3

0° 18,6 + 40,8 11,7 35,6 25,8 1,65 0 0

1 none 97,8 + 40,8 0 0 34,1 4,66 561 144 Case 1 Occupancy: full (6 persons); Ventilation: diurnal natural (6-17h) + nocturnal natural (23-6h)

+ weekend natural if Text < Tai with air change rate 2,0 h-1. Case 2 Occupancy: full (6 persons); Ventilation: diurnal mechanical with active precooling of the fresh

air (6-17h) + nocturnal natural (23-6h) + weekend natural if Text < Tai with air change rate 2,0 h-1. Case 3 Occupancy: full (6 persons); Ventilation: diurnal mechanical with active precooling of the fresh

air (6-17h); TABS: Mo-Fri, 0-8h, inlet temperature 19°C, flow rate 1,5 l/min, density of the pipes 4 m/m2.

opt Blind setting – assuring the conditions described in paragraph (2), for exact data see (0) in the appendix

0° Blind setting – inclination of the blinds is 0° from horizon in the working hours, during weekends the setting is identical with “opt”.

none Blind setting – no shading is applied at any time. Case studies showed that in well insulated buildings it is more difficult to dispose the excessive thermal gains. Therefore, an effective shading system proved to be the key element. In addition, to keep the operation truly passive, also a possibility of natural ventilation during the nighttime is needed (passive cooling). In Brno this is not secured, for this reason active cooling elements in combination with accumulation into massive structures are considered. Since the building is equipped with mechanical ventilation system for winter time operation, venting with precooled air during summer time is an easy option. Additionally, low potential cold from earth can be effectively used for massive ceiling activation in the night. This combination assures the thermal comfort is kept in desired range while the energy demand for cooling is still low. Furthermore, the activation of the accumulation mass offers non-simultaneous operation of the active systems which does not increase the peak output needed.

References

[1] EN ISO 13363-2. Solar protection devices combined with glazing. Calc. of total solar energy transmittance and light transmittance. Detailed calc. method. (2006)

[2] DUFFIE, J.A., Solar Engineering of thermal processes, Madison: Willey-Interscience. 1991. second edition. 199-205s. ISBN 0-417-51056-4

[3] EN ISO 7730, Ergonomics of the thermal environment - Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria (2006)

[4] LEHMANN et al. Application Range of Thermally Activated Building Systems TABS. Energy and Buildings 39 (2007), pp. 593-598.

4


Appendices

Appendix 1: Description of the evaluated office space

Tab. 2 Basic parameters of the evaluated office space.

Internal dimensions (length x width x height) 5,1 m x 6,4 m x 2,76 m

Floor area 32,6 m2

Air volume 90,1 m3

Working places (occupancy) 6 persons

floor area per 1 person 5,43 m2 / person

Orientation southeast (-59°)

Window area (glazing area) 13,2 m2 (10,0 m2)

window area related to floor area 0,40 m2 / m2

Solar heat gain coefficient (g, SHGC) 0,509 [-]

Visible light transmittance of the glazing 0,697 [-]

Thermal transmittance of the glazing 0,535 W/(m2.K)

frames 1,0 W/(m2.K)

windows total 0,76 W/(m2.K)

External wall area 6,9 m2

Flat roof area 37,6 m2

Thermal transmittance of the external wall 0,11 W/(m2.K)

flat roof 0,10 W/(m2.K)

Thermal transmittance – mean value 0,27 W/(m2.K)

Appendix 2: Climatic conditions

0

100

200

300

400

500

600

700

800

900

0:30

1:30

2:30

3:30

4:30

5:30

6:30

7:30

8:30

9:30

10:3

0

11:3

0

12:3

0

13:3

0

14:3

0

15:3

0

16:3

0

17:3

0

18:3

0

19:3

0

20:3

0

21:3

0

22:3

0

23:3

0

rad

iati

on

[W

/m2]

15

17

19

21

23

25

27

29

31

33

35

tem

per

atu

re [

ºC]

directional - SE difuse - SE horizont vertical SE ext. temperature

Fig. 4 Used diurnal behavior of clear sky radiation and exterior temperature.

5


Appendix 3: Shading and Daylight Model

Shading model

The performance model of venetian blinds is based on advanced numerical-analytical technique inspired. Presented model is more general. The shading effect results from separate approach to the individual components of solar radiation, according to Perez, and designed shading scheme. Inputs to the calculation are based on cross section dimensions: louver length, distance and inclination and its specular and diffuse reflectances. Model counts with two reflections (max. third on the glass) and distinguishes the front and the reverse side of the louver, as it is shown on. The output is adapted to the needs of the thermal model. It rests in the amount of direct and diffuse radiation and the respective angles of incidence on the windows.

Fig. 5 Scheme of the blind shading model

Daylight model

A 3D designer ArchiCAD was used for modeling of the office room. It was then imported to the Ecotect software. After assigning materials the room was evaluated using Radiance engine, which is executable directly from the Ecotect workspace. Various louvers positions were tested for every working hour during chosen summer sunny period. The aim was using as little angle inclinations and blind positions as possible to minimize the requirements for regulation. In the afternoon when the beam radiation does not fall on the façade any more, the lower part of the blinds is raised to enable an undisturbed view out of the surroundings. On the weekend the blinds are shut (80°).

6


Tab. 3 Optimal blind position

working day weekend

hour upper window lower window upper window lower window 0 - 4 0 80 0 80 5 - 7 45 80 80 80

8 30 30 80 80 9 - 12 30 60 80 80 13 - 14 0 raised 80 80 15 - 18 raised raised 80 80 19 - 23 0 80 0 80

Following figures demonstrate a similar level of daylight available on the working desks during the whole day.

Fig. 6 Daylight levels - time: 7:00, upper blinds: 30º, lower blinds: 30º

Fig. 7 Daylight levels - time: 10:00, upper blinds: 30º, lower blinds: 60º

7


Fig. 8 Daylight levels - time: 15:00, upper blinds: 0º, lower blinds: raised

Fig. 9 Daylight levels - time: 7:00, upper blinds: 0º, lower blinds: 45º (bad example)

Appendix 4: Solar and internal gains

0,25 0,25 0,25 0,25

0,46

0,18 0,05 0,04

0,0

0,2

0,4

0,6

0,8

unshaded blinds - 0º blinds - 45º blinds - 60º

[kW

h/m

2 o

ffic

e.d

ay]

1,0direct solar gains

internal gains0,43

kWh/m2day 0,30

kWh/m2day

0,71

kWh/m2day

0,29

kWh/m2day

Fig. 10 Internal and solar gains for Brno office room with regard to shading pattern.

8


Appendix 5: Thermal Model - Description

Internal environment

The internal environment is represented by two zones. The 1st zone represents the office space and it is modeled into detail. The 2nd zone represents the attached corridor and it is covered by a simplified model.

,1zoneT,1zoneC

gQ

extT

extT

,2zoneT

,V inT

,2zoneC

extT1 2H 2 extH

VH

,1zoneT

extT

tbH

gQ

Fig. 11 RC representation of the zone.

Thermal balance of the interior node representing zone 1 2W m :

a AIR EXTs EXTs EXTs INTs INTs INTs WINs WINs WINsg a s a s a

dTC Q h A T T h A T T h A T T

dt,int

int int ,int ,int int ,int ,int int ,int ,int

s

V a V in a zone tb a extH T T H T T H T T,int , 1 2 ,int ,2 ,int (1)

Thermal balance of the node representing zone 2

2W m :

zone ZONEzone g zone a ext zone a ext

dTC Q H T T H T

dt,2 ,2

,2 1 2 ,2 ,int 2 ,2 , T (2)

Submodel 1: External and internal opaque structures (EXT, INT struct)

The following equations (3) to (15) apply for the external opaque structures (structures separating conditioned internal space and external environment). These are the exterior wall (or window sill) and the flat roof.

9


1T

1C,intsT

iT

iC

nT

nC,s extT

1sR n sR 1i iR

1iT

1iC 1i iR

1iT

1iC

-1iq,intsq ,s extqiq 1iq1q nq

R1 2 n nR 1

Fig. 12 RC representation of opaque structures.

Thermal balance of the internal surface:

s

s a ss

T Tq h T T

R,int 1

,int int ,int ,int1

0 2W m where sR 1 1 2R1 (3)

and after rearranging we can express the surface temperature as

s as

R q h T TT

R h1 ,int int ,int 1

,int1 int

2

2 °C (4)

Thermal balance of the node 1:

s

s

T TdT T TC

dt R R,int 11 1 2

11 1 2

q1 2W m (5)

where sR 1 1 2R1 and R R1 2 1 21 2 1 2R (6) and after rearranging we can write

sT TdT T TC

dt R R R,int 11 1 2

1 11 1 2

2 q 2W m (7)

Thermal balance of the node i:

i i i i i

i i i i i

dT T T T TC

dt R R1 1

1 1iq

2W m (8)

where i i i iR R1 11 2 1 2R and i i i iR R1 1 2 1 2R 1 (9) and after rearranging we can write

i i i i ii

i i i i

dT T T T TC

idt R R R R

1 1

1 1

2 q 2W m

(10)

Thermal balance of the node n:

n s extn n n

n n n n s

T TdT T TC

dt R R,1

1nq

2W m (11)

where n n n nR R1 11 2 1 2R and n s nR 1 2R (12) and after rearranging we can write

n s extn n nn n

n n n

T TdT T TC q

dt R R R,1

1

2 2W m (13)

Thermal balance of the external surface:

n s ext

ext s ext ext s extn s

T Th T T q

R,

, , 0 2W m where n s nR 1 2R (14)


10


n n ext ext s ext

s extn ext

T R h T qT

R h,

,

2

2 °C (15)

Heat capacity of the node i:

i i iC c ix 2J m K (16)

Thermal resistance of the node i:

ii

i

xR

2m K W (17)

The equations (3) to (15) apply also for the internal opaque structures, where the equation (15) expressing the temperature of the external surface is replaced by the equation (4) since the respective structure is surrounded by the internal air on the both surfaces. The internal structures in this case are the internal partition walls and the internal floor slab.

Submodel 2: Glazing (WIN struct)

Windows are modeled into detail. The glazing units are treated as multi-layer transparent structures consisting of glass panes and gaps filled with inert gas. The frames are treated separately.

T1

1C,intsT

2T

2C

1T

3C,s extT

1 2R 2 3R1sR 3 sR

transq,3absq,2absq,1absq

reflq

solq

Fig. 13 RC representation of a triple glazing.

Thermal balance of the internal surface:

s

a ss

T Th T T

R,int 1

int ,int ,int1

0 2W m where sR R1 1 2 pane,1 (18)


pane as

pane

R h T TT

R h,1 int ,int 1

,int,1 int

2

2 °C (19)

Thermal balance of the node representing pane 1:

s

abss

T TdT T TC q ,1dt R R

,int 11 1 21

1 1 2

2W m (20)

where sR R1 1 2 pane,1 and pane gap paneR R R R1 2 ,1 ,1 ,21 2 1 2 (21)

and after rearranging we can write

sabs

pane pane gap pane

T TdT T TC q

dt R R R R,int 11 1 2

1 ,1,1 ,1 ,1 ,2

22

2W m (22)

11



abs

T TdT T TC

dt R R2 32 1 2

2 q1 2 2 3

,2 2W m (23)

where is expressed in R1 2 (21) and pane gap paneR R R R2 3 ,2 ,2 ,31 2 1 2 (24) and after rearranging we can write

abspane gap pane pane gap pane

T TdT T TC q

dt R R R R R R2 32 1 2

2 ,2,1 ,1 ,2 ,2 ,2 ,3

22 2

2W m (25)


s ext

abss

T TdT T TC q ,3dt R R

3 ,3 2 33

2 3 3

2W m

(26)

where is expressed in R2 3 (24) and s paneR R3 1 2 ,3 (27) and after rearranging we can write

s extabs

pane gap pane pane

T TdT T TC q

dt R R R R3 ,3 2 3

3 ,3,2 ,2 ,3 ,3

22

2W m (28)

Thermal balance of the external surface:

s ext

ext s ext exts

T Th T T

R3 ,

,3

0 2W m where s paneR R3 1 2 ,3 (29)


pane ext exts ext

pane ext

T R h TT

R h3 ,3

,,3

2

2 °C (30)

Heat capacity of the node representing pane i:

i pane i pane i pane iC c d, , , 2J m K (31)

Thermal resistance of the node representing pane i:

pane i

pane ipane i

dR

,,

,

2m K W (32)

Thermal resistance of the gap i:

gap i

gap igap i

dR

,,

,

2m K W (33)

Calculation of the heat absorbed on the panes is based on the amount of solar radiation incident on the external surface and absorptance coefficients of the panes derived from the software WINDOW 5.0. Individual components of the solar radiation are treated separately.

12


0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

0 10 20 30 40 50 60 70 80 90

refl

ecta

nce

, ab

sorp

tan

ce,

tran

smit

tan

ce [

-]

angle of incidence [°]

Tsol

Rf ,sol

abs1 (int)

abs2 (mid)

abs3 (ext)

Fig. 14 Angular dependant optical characteristics of the modeled glazing obtained from the

software WINDOW 5.0.

Submodel 3: TABS

Thermally active building system (TABS) in the presented study has a form of a floor slab made of reinforced concrete with integrated pipe system where a fluid can circulate in order to heat or cool the surrounding mass of the structure.

iT

iC layer i

fTfC

f iRfm

layer i-1

layer i+1

1i iR

1i iR

piped

pipet

d

pipeA

Fig. 15 RC representation of a thermally activated building system (TABS).

Thermal balance of the node representing circulating fluid:

ff struct f flow

dTC Q Q

dt , W (34)

Heat capacity of the fluid in the pipe: f fC m cf J K (35)

Heat flow from the structure of the pipe into circulating fluid:

struct pipe f i

f i

Q A TR

1T W (36)

Bulk convective heat flow within the circulating fluid:

f flow f flow f f out f inQ m c T T, , , , W (37)

using

f out f inf

T TT ,

2, (38)

we can rewrite (37) as

13


f flow f flow f f f inQ m c T T, , 2 , W (39)

It follows that the source term in (8) can be expressed using (36) as

struct

istruct

Qq

A

2W m (40)

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