- Email: [email protected]
A study on a passively heated single-zone building using a thermal storage wall
Energy, Vol.14, Nos. 1-4, pp. 29-34, 1998 0 1998Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: SO960-1481 (98) 000433 0960-1481/98 $19.00+0.00
Pergamon
CI STUDY
ON CI PFISSIUELY
HEATED
SINGLE-ZONE
d THERNClL STORdGE IBDUL-JbBBdR Solar
Energy
Research
BUILDING
USING
WALL
N. KHlLIFCl
Center, Baghdad,
Jadiriya, Iraq
P,O.
Box
13026,
ABSTRACT a passively heated zone is predicted by calculating tho of each node in the system using a finite-difference The sensitivity of the zone air temperature to the change in some of the zone model. features is examined. Some of these parameters included; the thickness of the walls, controlled and constant and variable heat transfer coefficients, having using uncontrolled thernocirculation in a vented storage wall. 0 1998 Published by Elsevier Science Ltd. All rightsreserved.
The
thermal
transient
performance
temperature
KEY Passive
systems;
of
variation
WORDS thermal
storage
wall;
simulation;
solar
heating.
INTRODUCTION Passive system is one which uses the design features such as shading, orientation, insulation, thermal mass, etc., of the building to reduce or lleminrte the heating and of the zone. In contrast to the active system, passive system cooling requirements doesn't normally need any active devices (such as pump, blower, ltc,) to rchieve the heating or cooling process. One of the most common methods used in passive heating is the utilization of a massive wall for heat storage. The performance of the wall is affected by many factors such as the thickness of the wall and the media used for hert storage. The present study rims at examining analytically the performance of a zone heated by a thermal storage wall and how the zone temperature is affected by the change in some of the building features. The effect of the following parameters were examined: -The thickness of the thermal storage wall and other insulated walls of the zone. -The use of vented thermal storage wall with uncontrolled controlled and thrmocicrulation flow. -The use of auxiliary heat with the passive system. -The use of variable and constant interior and exterior heat transfer coefficients. To The and
rcheive the goal of the study a dynamic simulation computer program uses the hourly values of the diffuse and direct solar the ambient air temperature to predict the thermal performance
29
model is developed. radiation intensity of the zone.
30
A-J. N. KHALIFA DESCRIPTION OF THE MODEL
The zone is represented by a thermal network, as shown in Fig. 1, with each thermal conductance represented mathematically in the model. The mathematical equations are solved repeatedly at each time step for the period of the simulation. The program allows f o r the thermal r a d i a t i o n heat t r a n s f e r between walls of a zone and has the flexibility of using e i t h e r v a r i a b l e or constant convective and longwave r a d i a t i v e heat t r a n s f e r c o e f f i c i e n t s . The walls are connected to each other by surface nodes through a thermal r a d i a t i v e conductance and to the zone a i r by a convective conductance. Both are calculated at each time step.
\
LL&inf
-\
EUAwoUs
Tcmb~
Tomb
~OsoCw
Osot~
UAgrdI ,
UAgrd 2
Vents
/
\////
> / 7 / Tgrd
Fig.
I.
Building thermal network.
The program does not model the actual d i s t r i b u t i o n of the incoming s o l a r r a d i a t i o n to the various surfaces of the zone. Instead, a l l s o l a r r a d i a t i o n received by the zone is a l l o c a t e d to the e x t e r i o r surface of the thermal storage wall. The thickness of the thermal storage wall and i t s thermal p r o p e r t i e s can be varied so as to reduce or e l i m i n a t e the e f f e c t of the thermal storage. As a consequence, the wall can be l e f t to act as a surface where a l l the incoming shortwave r a d i a t i o n can be a l l o c a t e d . That avoides the need f o r d e t a i l e d modelling of the shortwave r a d i a t i o n inside the zone, which in most cases, can not be modelled accurately in a r e a l i s t i c room geometry. The thermal storage wall i t e s l f can be modelled with and without vents. In the former case a conductance due to t h e r m o c i r c u l a t i o n is calculated at each time step. For walls experiencing large temperature v a r i a t i o n s at the surface, such as the thermal storage w a l l , a node spacing of 50 mm gives accurate r e s u l t s (SERI-RES, 19B3). The thermal storage wall is divided i n t o f i v e slabs with a node at the cmntroied of each slab. The temperature of the glazing and that of the a i r between the storage wall and the glazing is represented by one node each. All walls, apart from the thermal storage w a l l , are assumed to store no heat. All zero capacitance elements are described by steady s t a t e equations. Likewise, a steady s t a t e equation is used to c a l c u l a t e the temperature at the i n t e r i o r and e x t e r i o r surface of the thermal storage wall and at a l l other opaque b u i l d i n g surfaces.
Study on a passively heated single-zone building
31
A f i n i t e difference method is used to calculate the tranmient temperaturm variation at each node. The natural convective h e a t transfer coefficients on the different i n t e r i o r surfaces of the building, including the thermal storage wall, and those on the different exterior surfaces o$ the building arm calculated by the empirical formulae given in the l i t e r a t u r e (see for example; Akberzadeh, 1982; Khalifa et a l . , 1989; Khalifa et a l . , 1990; Morck, 19861. The simulation is started by setting i n i t i a l system temperatures and describing the details of the building. While reading the hourly values of the solar radiation and the ambient temperature from the weather data f i l e , the temperatures and the heat transfer coefficients of the system are updated at each time step. The total heat loss coefficient of the buildung is estimated by adding up the heat loss coefficient of the zone components such as walls, f l o o r , roof, windows, i n f i l t r a t i o n . . e t c . That coefficient is then multiplied by the ambient and zone air temperature difference to obtain the total heat loss from the building for the specified time interval.
RESULTS AND DISCUSSION The study arm carried out for a singlm zone building which ham the details given in Table I. An annual set of hourly weather data for a typical cold weather (that of Kew/England for 1963/I764) is used. Each simulation is started by i n i t i a t i n g a l l system temperatures to the ambient temperature read from the weather data film. The s e n s i t i v i t y of the zone a i r temperature to the change in some of the zone features are shown in Figs. 2 and 3. Figure 2 shows the simulation results for the f i r s t eight days of November using a time step length o$ 900 seconds. Figure 2a shows the variation of the zone air temperature when a 350-mm-thick thermal storage wall is used. Periods of underheating and overheating can be noticed which corresponds mostly to the fluctuation in the ambient air temperature. Reducing the thickness of the storage wall to 200 mm (Fig. 2b) results in no significant change. Significant reduction in the magnitude and number of the temperature peaks is noticed, as shown in Fig. 2c, when the thermocirculation $1ow is controlled by preventing the circulation when the zone air temperature exceeds the set point temperature. The net contribution of the thermocirculation flow can thus be judged from the comparison of Figs. 2b and 2c. To eliminate the periods o$ underheating, a u x i l i a r y heat may be used. Figure 2d shows the situation when a controlled a u x i l i a r y heat is used. The effect of reducing the thickness of other insulated walls is shown in Fig. 2e. Some of the temperature peaks are compensated by the increase in the heat loss through t h e s e walls. It is worth mentioning that neither the thmrmocirculation flow nor the a u x i l i a r y heat has contributed to the overheating o$ the zone since their effects have been controlled and isolated during these periods. The s e n s i t i v i t y of the zone a i r temperature to the use of variable and constant heat transfer coefficients is investigated by the annual simulation of Fig. 3. In the f i r s t case, The radiative heat transfer coefficients on the i n t e r i o r surfaces and those due to wind and free convection on the exterior surfaces are calculated at each time step. For the second c a s e , the constant values of Table I are used through the annual simulation. A time s t e p lenght of 180 seconds is used in the simulation. The temperatures plotted in Fig. 3, however, are those representing the average values over 1800 seconds intervals and are meant only to show the general trend of the variation in the zone air temperature. To dsmonstarte the effcmt more c l e a r l y , the monthly integrated area above and below the set point temperature (°C hr)is shown in Fig. 3 for each case. I t was found that the zone a i r temperature predicted with constant heat transfer coefficients are lower than those predicted by the model with i t s variable h e a t transfer coefficients for most of the year. Furthermore, the time length of the overheating and underheating periods predicted by the model for each case are s i g n i f i c a n t l y d i f f e r e n t , especially for the summer months as for August for example where the difference is around a factor of 2.5. Such results highlight the importance of the accurate modelling of the heat transfer coefficients in building geometries. Figure 4 shows the monthly heating load predicted for the zone and the monthly solar energy collected by the thermal storage wall for the months of the year.
32
A-J. N. KHALIFA
40 : WITHOUT AUY HEAT i STORAGE WALL THICKN~S 2 0 ¢ s
WITHOUT AUX. BE3LT
STOKAGE WALL T1LICKNESS 35 cm 3 5 • UNCONTROLLED ll~R~5OCIRCULATION
UNCOKTROLLJ[D TSE/UdOC/HCULATION OTIaCH WALLS 95 ¢m OF INSULATION
"F
OTSER WALLS ~'$ c:m OF INSULATION +
30
+
+
+J+
1
+
~
+
+
+
+
~l'--t
+
+
~"
G
+
..FA + + + '4" + M II zoNz T ~ ' F . ~ T U R Z
ZONE TEMPERATURE
25
2
2
20 15 18
5__
i
-
Q
0 0
',4-
-I-
-i-
"4-"
"4-
q-
4'-
-F
tS
18
2
+
S
8
10
TIME
12
14
2
20 ~8 . . . .
WIT'HOOT AUZ. HE,AT STORAGE WALL I " ~ S
8
;0
+
+
+
12
t4
1G
+
+
....
,
.., .... , ....
18
2~
X10t
(MR)
, ..,
&UIL I ~ . ~ T - 2 0 0 0 { 1 - ( T r / T s e t
2 0 ¢~ CONTROLLED ~ I R C U I ~ T I O N OTBEH WALLS 25 om OF INSULATION
30
+
TIME
X101
(HR)
40
35
+
AMBIENT TI]fl~TURE
....
,...
,..
)]
STGiU ~ r WALL THICKNESS 2 0 m
CONTROLLED THJ[~JdOCIHCULATION OTBEX WALL~ 25 ¢~ OF INSULATION
+
+
+
+
.+
+
+
+
+
+
-I-
~
+
+
+
+
+
+
+
"4-
"4"
-'1-
÷
"4-
4"
+
÷
4-
+
+
+
'4-
-t-
+
2~ L" 10 5.~
~l~'~
i Cll ==~
C I .,UIBU~tT ~ T U R E
ir...J ......... 0
2
~'
G
8
10
TIrE
12
t'~
IS
. . . . . , .... , -,... , ......., AU~ I~_AT-ZOOO[I-(Tr/'Tset)] STOR,tG~ WALL l " ~ C l O q ~ s 2o o~ 35~. c o ~ m o u . ~ TUnOCmCULaTZO~ OT~I[H WAI.LS S ~ OF I~SULATION
~ 0
18
20
X101
(HR)
I
30~.
+
+
+
+
q-
+
+
4
2~
Simulation of 8 days s t a r t i n g f r o m the first of N o v e m b e r
---,
.....
2
T~,*m,z
; .................................. 6 e 10 12 I * 16 18 TIME (HR) X101
+
+
+
- a vs. b Effect of r e d u c i n g the thickness of the thermal s t o r a g e wall - b vs. c Effect of c o n t r o l l i n g the thermocirculation f l o w
,=,,
,o 2 1 \ i. ; Z 5 ::.-~
-t-
~e'" '2 " 4
"I-
i
+
+
iig TIME
Fig.
2-.
-,F
i I .4-
'.4-
- c vs. d E f f e c t of u s i n g an auxiliary heat - d vs. e Effect of r e d u c i n g the thickness of other w a l l s
-4"
i 2 ~i'"i6 ii3 (HR)"
XI01
Effect of C h a n g i n g some of the S i m u l a t i o n Parameters o n the Zone Air Temperature. S-day S i m u l a t i o n
Study on a passively heated single-zone building
:17i
7; TiiT;:i",i
i
' "' ~__I
t1 tl J.lll~
J ASO
FMAMJ
Conditions
I
' '
5 _
NOJ
i .|l
33
o~ the Simulatio~
Qaux = 6000 ( l - ( T r / ( T s e c - 0 . 5 ) ) ) . Controlled thezz~oci~culation.
,~. i,tJ~IAPI*'I ~1 I~I1t~
s t o r a g e w a l l t h i c k n e s s - 20 ~ *
~ez~al
I I ~"1 !.,lt. '4~g~'I l"'r;'Jtllll
Other w a l l s , 5 Cm o~ i~ulation. Time step = 3 m i n s .
N O J FH A H J J A $ 0 ~i¢ •
3 • g ~ f e c t of Using V a r i a b l e and C o n s t i u t ~ e a t Tra~fer
Coefficients
on the Zocie A i r
T e m p e r a t u r e and t h e Houthl 7 I n t e g r a t e d Area ( o C . h r ) above and below t h e S e t PoiDt, T e ~ e r a t u r e - Annual $ i ~ l a ~ i o u
Table I , ---.
Details
of the zone used in the s i m u l a t i o n .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wall
Material
k
Cp
j9
Thickness
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage w a l l Floor Other w a l l s
concrete concrete insulation
1.173 1,173 0,03
Blazing
window glass 0,76
B37 B37 1000
2243 2243 25
50-350mm 50mm 50-250mm
B400
1750
5mm
-Outside dimensions of the zone: Width = 2,50 m; Length = 3.00 m; Height = 2,50 m. -The constant heat t r a n s f e r c o e f f i c i e n t s used in the simulation: Convective on i n t e r i o r surfaces = 3,00 W/m2Ko Radiative on i n t e r i o r surfaces = 5.70 W/m=K. Combined (free+wind) on e x t e r i o r surfaces = 18,20 N/m=K. Combined (free+wind) on roof = 22.20 W/m=K. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
CONCLUSIONS 1, The s e n s i t i v i t y of heat t r a n s f e r c o e f f i c i e n t s significantly different
the zone a i r t e m p e r a t u r e to the use of c o n s t a n t end v a r i a b l e i s i n v e s t i g a t e d . The zone a i r t e m p e r a t u r e i s found to be for each case, A c t u a l ( v a r i a b l e ) heat t r a n s f e r c o e f f i c i n e t s
should be used in building simulations rather than constant values,
2, T h e r m o c l r c u l a t i o n through thermal s t o r a g e w a l l s mhould be the p e r i o d s of o v e r h e a t i n g of t h e zone, 3.
For
the
weather
conditions
to
minimize
storage w a l l s can not s a t i s f y
the
of the zone, A u x i l i a r y heat is required at
the
examined, thermal
d a i l y and seasonal heating requirments periods of underheating.
controlled
34
A-J. N. K H A L I F A xlO 2
M:o,h,;hoo't,og',ood'
x Monlhly solclr energy // Collected by the storcge wa[/x/
5
°-°
&
I
\ |
/ \o
.
/\
E3
x#
," \
// o
//
%%
14J
%
%x~ _ x l /
i O
I J
"~o,.~ ~ °
1 F
t
M
i A
I M
1 J
i J
I A
I S
0
Month
Fi~ .
4
"
The Bredicted Monthly ~ea~ing Load £or the Zone and the Monthly Solar EDergy Collected by the Thermal $~orage Wall - Annual Si~latlon
NOMENCLATURE
Cp, k N ~aux
Q.ol=, Q.°~.
Tamb, To.p Tgrd UA= UAQrdl UA~d= UA~o, UA.. UA. ~UA.°~.
Spesific heat (J/kgK) and thermal conductivity (N/mK) resp.. Node number. Auxiliary heat (W). Solar radiation absorbed by the glazing and the storage wall resp. (W). Ambient and gap air temperatures resp. (K). Ground temperature (K). Conductance of the glazing (W/K1. Conductance of the floor through the ground (W/K). Conductance of the floor through the walls (W/K). Conductance due to i n f i l t r a t i o n (W/K). Conductance between the thermal storage wall and zone air (W/K). Conductance between the thermal storage wall and gap air (W/K). Conductance through the walls of the zone (W/K). Density (kg/m~).
REFERENCES Akberzadeh,A.w W. Charters and D. Lesslie (1982). Thsrmocirculation characteristics of a Trombe wall passivi test c e l l . Solar Energy, 28, 461-468. Khalifa, A.J.N. and R. H. Marshall (1989). Natural and forced convection on i n t e r i o r building surfaces - prelimnary results. Inl Applied Energy Researchw pp. 249-257. Adam Hilger, Bristol and New York. Khalifa, A.J.N. and R. H. Marshall (1990}. Validation of heat transfer coefficients on i n t e r i o r building surfaces using a real-sized indoor test c e l l . Int. J. Heat and Mass Transfer, 33, 2219-2236. Morck, O. (1986). Simulation model validation using test cell data. Report prepard for the International Energy Agency, Task V I I I , Passive and hybrid solar and low inmrgy buildings. SERI-RES (1983). Solar Energy Research I n s t i t u t e - Residential Energy Simulator, version 1.0.
Pergamon
CI STUDY
ON CI PFISSIUELY
HEATED
SINGLE-ZONE
d THERNClL STORdGE IBDUL-JbBBdR Solar
Energy
Research
BUILDING
USING
WALL
N. KHlLIFCl
Center, Baghdad,
Jadiriya, Iraq
P,O.
Box
13026,
ABSTRACT a passively heated zone is predicted by calculating tho of each node in the system using a finite-difference The sensitivity of the zone air temperature to the change in some of the zone model. features is examined. Some of these parameters included; the thickness of the walls, controlled and constant and variable heat transfer coefficients, having using uncontrolled thernocirculation in a vented storage wall. 0 1998 Published by Elsevier Science Ltd. All rightsreserved.
The
thermal
transient
performance
temperature
KEY Passive
systems;
of
variation
WORDS thermal
storage
wall;
simulation;
solar
heating.
INTRODUCTION Passive system is one which uses the design features such as shading, orientation, insulation, thermal mass, etc., of the building to reduce or lleminrte the heating and of the zone. In contrast to the active system, passive system cooling requirements doesn't normally need any active devices (such as pump, blower, ltc,) to rchieve the heating or cooling process. One of the most common methods used in passive heating is the utilization of a massive wall for heat storage. The performance of the wall is affected by many factors such as the thickness of the wall and the media used for hert storage. The present study rims at examining analytically the performance of a zone heated by a thermal storage wall and how the zone temperature is affected by the change in some of the building features. The effect of the following parameters were examined: -The thickness of the thermal storage wall and other insulated walls of the zone. -The use of vented thermal storage wall with uncontrolled controlled and thrmocicrulation flow. -The use of auxiliary heat with the passive system. -The use of variable and constant interior and exterior heat transfer coefficients. To The and
rcheive the goal of the study a dynamic simulation computer program uses the hourly values of the diffuse and direct solar the ambient air temperature to predict the thermal performance
29
model is developed. radiation intensity of the zone.
30
A-J. N. KHALIFA DESCRIPTION OF THE MODEL
The zone is represented by a thermal network, as shown in Fig. 1, with each thermal conductance represented mathematically in the model. The mathematical equations are solved repeatedly at each time step for the period of the simulation. The program allows f o r the thermal r a d i a t i o n heat t r a n s f e r between walls of a zone and has the flexibility of using e i t h e r v a r i a b l e or constant convective and longwave r a d i a t i v e heat t r a n s f e r c o e f f i c i e n t s . The walls are connected to each other by surface nodes through a thermal r a d i a t i v e conductance and to the zone a i r by a convective conductance. Both are calculated at each time step.
\
LL&inf
-\
EUAwoUs
Tcmb~
Tomb
~OsoCw
Osot~
UAgrdI ,
UAgrd 2
Vents
/
\////
> / 7 / Tgrd
Fig.
I.
Building thermal network.
The program does not model the actual d i s t r i b u t i o n of the incoming s o l a r r a d i a t i o n to the various surfaces of the zone. Instead, a l l s o l a r r a d i a t i o n received by the zone is a l l o c a t e d to the e x t e r i o r surface of the thermal storage wall. The thickness of the thermal storage wall and i t s thermal p r o p e r t i e s can be varied so as to reduce or e l i m i n a t e the e f f e c t of the thermal storage. As a consequence, the wall can be l e f t to act as a surface where a l l the incoming shortwave r a d i a t i o n can be a l l o c a t e d . That avoides the need f o r d e t a i l e d modelling of the shortwave r a d i a t i o n inside the zone, which in most cases, can not be modelled accurately in a r e a l i s t i c room geometry. The thermal storage wall i t e s l f can be modelled with and without vents. In the former case a conductance due to t h e r m o c i r c u l a t i o n is calculated at each time step. For walls experiencing large temperature v a r i a t i o n s at the surface, such as the thermal storage w a l l , a node spacing of 50 mm gives accurate r e s u l t s (SERI-RES, 19B3). The thermal storage wall is divided i n t o f i v e slabs with a node at the cmntroied of each slab. The temperature of the glazing and that of the a i r between the storage wall and the glazing is represented by one node each. All walls, apart from the thermal storage w a l l , are assumed to store no heat. All zero capacitance elements are described by steady s t a t e equations. Likewise, a steady s t a t e equation is used to c a l c u l a t e the temperature at the i n t e r i o r and e x t e r i o r surface of the thermal storage wall and at a l l other opaque b u i l d i n g surfaces.
Study on a passively heated single-zone building
31
A f i n i t e difference method is used to calculate the tranmient temperaturm variation at each node. The natural convective h e a t transfer coefficients on the different i n t e r i o r surfaces of the building, including the thermal storage wall, and those on the different exterior surfaces o$ the building arm calculated by the empirical formulae given in the l i t e r a t u r e (see for example; Akberzadeh, 1982; Khalifa et a l . , 1989; Khalifa et a l . , 1990; Morck, 19861. The simulation is started by setting i n i t i a l system temperatures and describing the details of the building. While reading the hourly values of the solar radiation and the ambient temperature from the weather data f i l e , the temperatures and the heat transfer coefficients of the system are updated at each time step. The total heat loss coefficient of the buildung is estimated by adding up the heat loss coefficient of the zone components such as walls, f l o o r , roof, windows, i n f i l t r a t i o n . . e t c . That coefficient is then multiplied by the ambient and zone air temperature difference to obtain the total heat loss from the building for the specified time interval.
RESULTS AND DISCUSSION The study arm carried out for a singlm zone building which ham the details given in Table I. An annual set of hourly weather data for a typical cold weather (that of Kew/England for 1963/I764) is used. Each simulation is started by i n i t i a t i n g a l l system temperatures to the ambient temperature read from the weather data film. The s e n s i t i v i t y of the zone a i r temperature to the change in some of the zone features are shown in Figs. 2 and 3. Figure 2 shows the simulation results for the f i r s t eight days of November using a time step length o$ 900 seconds. Figure 2a shows the variation of the zone air temperature when a 350-mm-thick thermal storage wall is used. Periods of underheating and overheating can be noticed which corresponds mostly to the fluctuation in the ambient air temperature. Reducing the thickness of the storage wall to 200 mm (Fig. 2b) results in no significant change. Significant reduction in the magnitude and number of the temperature peaks is noticed, as shown in Fig. 2c, when the thermocirculation $1ow is controlled by preventing the circulation when the zone air temperature exceeds the set point temperature. The net contribution of the thermocirculation flow can thus be judged from the comparison of Figs. 2b and 2c. To eliminate the periods o$ underheating, a u x i l i a r y heat may be used. Figure 2d shows the situation when a controlled a u x i l i a r y heat is used. The effect of reducing the thickness of other insulated walls is shown in Fig. 2e. Some of the temperature peaks are compensated by the increase in the heat loss through t h e s e walls. It is worth mentioning that neither the thmrmocirculation flow nor the a u x i l i a r y heat has contributed to the overheating o$ the zone since their effects have been controlled and isolated during these periods. The s e n s i t i v i t y of the zone a i r temperature to the use of variable and constant heat transfer coefficients is investigated by the annual simulation of Fig. 3. In the f i r s t case, The radiative heat transfer coefficients on the i n t e r i o r surfaces and those due to wind and free convection on the exterior surfaces are calculated at each time step. For the second c a s e , the constant values of Table I are used through the annual simulation. A time s t e p lenght of 180 seconds is used in the simulation. The temperatures plotted in Fig. 3, however, are those representing the average values over 1800 seconds intervals and are meant only to show the general trend of the variation in the zone air temperature. To dsmonstarte the effcmt more c l e a r l y , the monthly integrated area above and below the set point temperature (°C hr)is shown in Fig. 3 for each case. I t was found that the zone a i r temperature predicted with constant heat transfer coefficients are lower than those predicted by the model with i t s variable h e a t transfer coefficients for most of the year. Furthermore, the time length of the overheating and underheating periods predicted by the model for each case are s i g n i f i c a n t l y d i f f e r e n t , especially for the summer months as for August for example where the difference is around a factor of 2.5. Such results highlight the importance of the accurate modelling of the heat transfer coefficients in building geometries. Figure 4 shows the monthly heating load predicted for the zone and the monthly solar energy collected by the thermal storage wall for the months of the year.
32
A-J. N. KHALIFA
40 : WITHOUT AUY HEAT i STORAGE WALL THICKN~S 2 0 ¢ s
WITHOUT AUX. BE3LT
STOKAGE WALL T1LICKNESS 35 cm 3 5 • UNCONTROLLED ll~R~5OCIRCULATION
UNCOKTROLLJ[D TSE/UdOC/HCULATION OTIaCH WALLS 95 ¢m OF INSULATION
"F
OTSER WALLS ~'$ c:m OF INSULATION +
30
+
+
+J+
1
+
~
+
+
+
+
~l'--t
+
+
~"
G
+
..FA + + + '4" + M II zoNz T ~ ' F . ~ T U R Z
ZONE TEMPERATURE
25
2
2
20 15 18
5__
i
-
Q
0 0
',4-
-I-
-i-
"4-"
"4-
q-
4'-
-F
tS
18
2
+
S
8
10
TIME
12
14
2
20 ~8 . . . .
WIT'HOOT AUZ. HE,AT STORAGE WALL I " ~ S
8
;0
+
+
+
12
t4
1G
+
+
....
,
.., .... , ....
18
2~
X10t
(MR)
, ..,
&UIL I ~ . ~ T - 2 0 0 0 { 1 - ( T r / T s e t
2 0 ¢~ CONTROLLED ~ I R C U I ~ T I O N OTBEH WALLS 25 om OF INSULATION
30
+
TIME
X101
(HR)
40
35
+
AMBIENT TI]fl~TURE
....
,...
,..
)]
STGiU ~ r WALL THICKNESS 2 0 m
CONTROLLED THJ[~JdOCIHCULATION OTBEX WALL~ 25 ¢~ OF INSULATION
+
+
+
+
.+
+
+
+
+
+
-I-
~
+
+
+
+
+
+
+
"4-
"4"
-'1-
÷
"4-
4"
+
÷
4-
+
+
+
'4-
-t-
+
2~ L" 10 5.~
~l~'~
i Cll ==~
C I .,UIBU~tT ~ T U R E
ir...J ......... 0
2
~'
G
8
10
TIrE
12
t'~
IS
. . . . . , .... , -,... , ......., AU~ I~_AT-ZOOO[I-(Tr/'Tset)] STOR,tG~ WALL l " ~ C l O q ~ s 2o o~ 35~. c o ~ m o u . ~ TUnOCmCULaTZO~ OT~I[H WAI.LS S ~ OF I~SULATION
~ 0
18
20
X101
(HR)
I
30~.
+
+
+
+
q-
+
+
4
2~
Simulation of 8 days s t a r t i n g f r o m the first of N o v e m b e r
---,
.....
2
T~,*m,z
; .................................. 6 e 10 12 I * 16 18 TIME (HR) X101
+
+
+
- a vs. b Effect of r e d u c i n g the thickness of the thermal s t o r a g e wall - b vs. c Effect of c o n t r o l l i n g the thermocirculation f l o w
,=,,
,o 2 1 \ i. ; Z 5 ::.-~
-t-
~e'" '2 " 4
"I-
i
+
+
iig TIME
Fig.
2-.
-,F
i I .4-
'.4-
- c vs. d E f f e c t of u s i n g an auxiliary heat - d vs. e Effect of r e d u c i n g the thickness of other w a l l s
-4"
i 2 ~i'"i6 ii3 (HR)"
XI01
Effect of C h a n g i n g some of the S i m u l a t i o n Parameters o n the Zone Air Temperature. S-day S i m u l a t i o n
Study on a passively heated single-zone building
:17i
7; TiiT;:i",i
i
' "' ~__I
t1 tl J.lll~
J ASO
FMAMJ
Conditions
I
' '
5 _
NOJ
i .|l
33
o~ the Simulatio~
Qaux = 6000 ( l - ( T r / ( T s e c - 0 . 5 ) ) ) . Controlled thezz~oci~culation.
,~. i,tJ~IAPI*'I ~1 I~I1t~
s t o r a g e w a l l t h i c k n e s s - 20 ~ *
~ez~al
I I ~"1 !.,lt. '4~g~'I l"'r;'Jtllll
Other w a l l s , 5 Cm o~ i~ulation. Time step = 3 m i n s .
N O J FH A H J J A $ 0 ~i¢ •
3 • g ~ f e c t of Using V a r i a b l e and C o n s t i u t ~ e a t Tra~fer
Coefficients
on the Zocie A i r
T e m p e r a t u r e and t h e Houthl 7 I n t e g r a t e d Area ( o C . h r ) above and below t h e S e t PoiDt, T e ~ e r a t u r e - Annual $ i ~ l a ~ i o u
Table I , ---.
Details
of the zone used in the s i m u l a t i o n .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wall
Material
k
Cp
j9
Thickness
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage w a l l Floor Other w a l l s
concrete concrete insulation
1.173 1,173 0,03
Blazing
window glass 0,76
B37 B37 1000
2243 2243 25
50-350mm 50mm 50-250mm
B400
1750
5mm
-Outside dimensions of the zone: Width = 2,50 m; Length = 3.00 m; Height = 2,50 m. -The constant heat t r a n s f e r c o e f f i c i e n t s used in the simulation: Convective on i n t e r i o r surfaces = 3,00 W/m2Ko Radiative on i n t e r i o r surfaces = 5.70 W/m=K. Combined (free+wind) on e x t e r i o r surfaces = 18,20 N/m=K. Combined (free+wind) on roof = 22.20 W/m=K. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
CONCLUSIONS 1, The s e n s i t i v i t y of heat t r a n s f e r c o e f f i c i e n t s significantly different
the zone a i r t e m p e r a t u r e to the use of c o n s t a n t end v a r i a b l e i s i n v e s t i g a t e d . The zone a i r t e m p e r a t u r e i s found to be for each case, A c t u a l ( v a r i a b l e ) heat t r a n s f e r c o e f f i c i n e t s
should be used in building simulations rather than constant values,
2, T h e r m o c l r c u l a t i o n through thermal s t o r a g e w a l l s mhould be the p e r i o d s of o v e r h e a t i n g of t h e zone, 3.
For
the
weather
conditions
to
minimize
storage w a l l s can not s a t i s f y
the
of the zone, A u x i l i a r y heat is required at
the
examined, thermal
d a i l y and seasonal heating requirments periods of underheating.
controlled
34
A-J. N. K H A L I F A xlO 2
M:o,h,;hoo't,og',ood'
x Monlhly solclr energy // Collected by the storcge wa[/x/
5
°-°
&
I
\ |
/ \o
.
/\
E3
x#
," \
// o
//
%%
14J
%
%x~ _ x l /
i O
I J
"~o,.~ ~ °
1 F
t
M
i A
I M
1 J
i J
I A
I S
0
Month
Fi~ .
4
"
The Bredicted Monthly ~ea~ing Load £or the Zone and the Monthly Solar EDergy Collected by the Thermal $~orage Wall - Annual Si~latlon
NOMENCLATURE
Cp, k N ~aux
Q.ol=, Q.°~.
Tamb, To.p Tgrd UA= UAQrdl UA~d= UA~o, UA.. UA. ~UA.°~.
Spesific heat (J/kgK) and thermal conductivity (N/mK) resp.. Node number. Auxiliary heat (W). Solar radiation absorbed by the glazing and the storage wall resp. (W). Ambient and gap air temperatures resp. (K). Ground temperature (K). Conductance of the glazing (W/K1. Conductance of the floor through the ground (W/K). Conductance of the floor through the walls (W/K). Conductance due to i n f i l t r a t i o n (W/K). Conductance between the thermal storage wall and zone air (W/K). Conductance between the thermal storage wall and gap air (W/K). Conductance through the walls of the zone (W/K). Density (kg/m~).
REFERENCES Akberzadeh,A.w W. Charters and D. Lesslie (1982). Thsrmocirculation characteristics of a Trombe wall passivi test c e l l . Solar Energy, 28, 461-468. Khalifa, A.J.N. and R. H. Marshall (1989). Natural and forced convection on i n t e r i o r building surfaces - prelimnary results. Inl Applied Energy Researchw pp. 249-257. Adam Hilger, Bristol and New York. Khalifa, A.J.N. and R. H. Marshall (1990}. Validation of heat transfer coefficients on i n t e r i o r building surfaces using a real-sized indoor test c e l l . Int. J. Heat and Mass Transfer, 33, 2219-2236. Morck, O. (1986). Simulation model validation using test cell data. Report prepard for the International Energy Agency, Task V I I I , Passive and hybrid solar and low inmrgy buildings. SERI-RES (1983). Solar Energy Research I n s t i t u t e - Residential Energy Simulator, version 1.0.