NRCC passive solar test facility performance of a mass-wall unit

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NRCC passive solar test facility performance of a mass-wall unit

Barakat, S. A.

Sex

m

NATIONAL RESEARCH

DIVISION

RESEARCH

NRCC PASSIVE

SOLAR TEST FACILITY

UNIT

S

OZ tawa May 1984

NRCC

PERFORMANCE OF A M A S H J A L L UNIT

by

S.A. Barakat

Experimental data f o r t h e mass--wall u n i t of t h e passive solar t e s t f a c i l i t y af t h e D i v i s i o n of B u i l d i n g Research are p r e s e n t e d f o r

two

seasons. The e f f e c t s of solar g a i n and b u i l d i n g thermal load on t h e

performance of the m a s s w a l l system as a passive s o l a r h e a t i n g d e v i c e are examined.

A comparison was made between the performance o f the mass-wall system

and an equivalent: d i r e c t - g a i n system. Although t h e r n a s ~ w a l l system u t i l i z e d less sotar e n e r g y than a d i r e c t - g a i n system, it resulted in a

marginally l o w e r space heating requirement, d u e to the h i g h e r thermal

resistance values of the mass-wall system. Analysis indicated that a

d i r e c t - g a l n system w i t h a t r i p l e g l a z e d window

would

1. INTRODUCTION

The NRC

and a m a s s w a l l unit. Construction d e t a i l s of all u n i t s are g i v e n i n

Reference 1. Reference 1 a l s o describes the data acquisition system,

p r e s e n t s the method

used

factors, and discusses t h e uncertainties assocfated wfth these factors.

The "mass-wall" unit of the passive s o l a r t e s t f a c i l i t y c o n s i s t s of a

s o u t h and a north room.

It

light-mass direct-gain U n i t

1,

wall

inside the south w i n d o w . The wall, of solid concrete b r i c k s , i s 305 mm

thick and has rwo 406 x 76 mm vent h o l e s at both t o p and bottom. Before t h e

1981J32 heating season, additional insulation was _{added to t h e w a l l s and} c e i l i n g , t o demonstrate t h e effect of reducing the h e a t loss on t h e mass

w a l l performance.

In

90 m gap between the window and t h e mass wall for part of t h e 1981/82

s e a s o n . The unit can b e operated in t w o modes. In Mode

1,

while

door

d o o r .

Data gathered f o r the mass-wall unit between October

1 and

1980/81 and

1981182

seasonal performance factors (1).

Performance data for t h i s unit in Modes 1 and 2 are given in Table 1.

S i n c e t h e i n s u l a t i n g c u r t a i n caused a reduction in the room h e a t loss

coefficient (UA) of only 4 2 , data far t h e 1981/82 season for t h e u n i t

with

and w i t h o u t t h e curtain were combined and are p r e s e n t e d

for

changes in t h e r a t i o of h e a t i n g load to s o l a r gatn are examined, and t h e performance of the mass-wall system is compared to that of a direct-gain system.

2. P E R P O W C E FACTORS

1

The following are the main factors used to d e s c r l b e the performance o f

the units. Derails on these and other parameter calculations are g i v e n in

Reference

I.

( I )

This i s t h e p a r t i o n of t h e solar g a i n t h a t contributes to the

reduction in the purchased h e a t i n g requirement. I t i n c l u d e s t h e s o l a r gain used t o offset heat l o s s e s , p l u s t h e p o r t i o n s t o r e d in

the thermal mass and used to offset lasses a s _{a l a t e r time.}

_It

does n o t i n c l u d e the excess gain that must be vented to prevent

room temperature from exceedfng a preset maximum, nor any g a i n

u t i l i z e d to o f f s e t a d d f t i o n a l losses caused by a rise

In

temperature above the thermostat s e t t i n g .

(2) Reference heat loss

( 3 )

GtR

This

is

calculated at thermostat s e t t i n g (reference heat loss). ( 4 ) Mass-gain r a t i o ,

MGR

T h i s is the r a t i o of the thermal c a p a c i t y of the unit t o the

average hourly s o l a r gain.

It

characteristics of the unit.

(5) Solar utilization factor,

us

This

( 6 ) Purchased heating fraction, F h

This is t h e f r a c t i o n of the n e t heating load of the b u i l d i n g that is supplied by t h e h e a t i n g system,

( 7 ) N e t effective gain,

QNE

This is

defined

and the reference heat loss of t h e mass-wall system. The net

effective gain is a measure of t h e n e t c o n t r i b u t i o n of t h e passive system t o the space heating requirement.

'En comparing t h e performance of various passive systems, it is m o s t r e a l i s t i c to compare t h e i r n e t effective gain, s i n c e i t is referenced as a f i x e d base (an adiabatic wall).

3. RESULTS

3.1 Effect of Interzone Circulation

The

in

f o r

1

performance i s almost the same f a r both modes

CGLR

8 ;

It~mperature in t h e south r o o m d i d not exceed 23.6D6. Thus,

little

r o a m in thermal combination (Mode

2)

the thermal performance ather than a slight reduction in maximum

temperature. The u s e f u l solar gain, In this case, is c o n t r o l l e d by the

characteristics of t h e mass wall itself and does n o t depend on the heating load or the thermal s t o r a g e of the b u i l d i n g .

This conclusion is not apparent from the 1981/82 results; t h e solar

utilization was l o w e r f o r Mode 2

(GLR

0.43)

(GLR

0.6,

from t h e outer

wall surface

The average outdoor temperature was - 3 . 0 ° C for Mode 2 and - 1 . 4 " C f o r

3.2 Effect

of

S i n c e additional insulation w a s added t o the e n v e l o p e of

all

in

1981182, t h e e f f e c t of r e d u c i n g the building load ( h i g h e r

GLR)

the two h e a t i n g seasons. A reduction in building load d o e s not have a significant effect on the u s e f u l gain, p r o v i d e d t h a t dumping of excess h e a t

is not n e c e s s a r y . For example, comparing t h e d a t a in Table 1 f o r the s o u t h

room in t h e two heating seasons shows t h a t the o n l y effect of l o a d reduction

is an increase in t h e maximum temperature of t h e south room

m ode

1980/81

and 0.45 f o r

1981182).

gain

fraction of the heating requiremenr as i n d i c a t e d by a r e d u c t i o n in purchased

heaLing f r a c t i o n from

0.66

0.7)

(GLR = 1 . 0 4 ) .

3 . 3 Mass Wall Effectiveness

The n e t effective gain of t h e mass wall, d e f i n e d as _{t h e d i f f e r e n c e}

between the u s e f u l solar g a i n and t h e reference losses throwh the m a s s w a l l

system, is given in Table 2 f a r both heating seasons. To c a l c u l a t e the r e f e r e n c ~ loss of the mass wall, the Loss coefficient of t h e wall system i s obtained by subtracting t h e calculated less coefficient of a l l other

e n v e l o p e components

from

system was _{calculated as 3.5 W/K compared t o 8 . 9}

WIK

.

Under the operating conditions described, the mass wall has a n e t e f f e c t i v e g a i n i n all cases. In o t h e r words, i t perform b e t t e r than a

p e r f e c t l y i n s u l a t e d

wall.

This

3 . 4 Performance Comparison Between

Mass-Wall

the mass _{w a l l measured i n 1980181 and 1981182 is compared to t h e p r e d i c t e d}

performance of an equivalent: direct-gafn unit. The d i r e c t s a i n performance

is estimated using the u t i l i z a t i o n f a c t o r curves g i v e n in Reference 2 f o r

t h e same mode and h e a t i n g season as for t h e mass-wall system.

Table 3 gives v a l u e s of the s o l a r utilization factor and t h e n e t effective gain for the mass wall unit and t h e e q u i v a l e n t direct-ga-ln unit. As might

be

p o r t i o n of the solar g a i n . Under t h e c o n d i t i o n s specified, b o t h the d i r e c t -

gain and the mass-wall systems make

a

of additional s o l a r energy was utilized to offset the a d d i t i o n a l window

losses.

In all

consumption. The difference is small, however, a rnaximrrn of 42 kW*h f o r t h e

gain has a

large

example gf ven in Reference

1).

draw conclusions.

The

not. This overheating

consideration (MGB = 4.7 to 5.1) as g i v e n in Reference 3 .

I n general, under the conditions in t h e t e s t f a c i l i t y , t h e mass wall

performed s l i g h t l y better than a d a u b l e g l a z e d window because of t h e e x t r a insulation of the mass-wall assembly and a l s o because, in the direct-gain

unir, t h e window area was n o t optimrn. The results should d i f f e r f o r e t h e r conditions ar o t h e r glazings. F o r example, f o r a d i r e c t - g a i n unit in Mode 1 ( s o u t h room only) with t r i p l e g l a z e d south w i n d o w (II =

1.8

~ / m k

SC = 0.81) and the same amount of thermal mass as t h e masswall u n i t

(GLR = 0.49

and MGR =

1980J81

calculated to be 91 kW*h, which is higher than t h a t o f t h e mass wall. A direct-gain system with a window of b e t t e r thermal p r o p e r t i e s than d o u b l e g l a z i n g ( t r i p l e g l a z i n g or '"super windowm' ) would, therefore, result i n a lower purchased heating requirement t h a n t h e mass-wall system. A mass

w a l l may be considered as complementary t o a d i r e s e g a i n system.

It

be

used instead of a d d i t i o n a l windows when an increased s o l a r contrPbatiwn is

requtted. The m a s s w a l l component would t e n d t o reduce t h e p o t e n t i a l f o r overheating and d e l f v e r s o l a r h e a t in the e a r l y e v e n i n g , thereby s a t i s f y i n g

a g r e a t e r p o r t i o n of

t h e

thermal

d a y l i g h t and view. They should perhaps be regarded as a t y p e o f s o l a r

collector wall and compared w i t h

walls

4 .

CONCLUSIONS

The

these c o n d i t i o n s , t h e following conclusions can be drawn:

(1) A mass-wall system produces a n e t effective gafn to the apace. That means that the u s e f u l solar galn exceeds rhe system l o s s e s or that t h e

mass-wall system performs better than a perfectly i n s u l a t e d wall.

(2) A mass-wall system without night i n s u l a t i o n (or

wirh

i n s u l a t i o n ) has a much lower utilization of solar gain than a d i r e c e g a i n system of the same MGR. However, due to its higher i n s u l a t i o n

value compared to a d o u b l e g l a z e d window, i t : could r e s u l t in a higher net eEfective gaPn.

( 3 ) A direct-gain system with triple-glazed windows r e s u l t s in a _{higher net} e f f e c t i v e gain or lower purchased h e a t i n g requirement than a m a s - w a l l sys tern.

REFERENCES

1, Barakat, S.A.

NRGC Passive Solar

Reduction, B u i l d i n g Research N o t e 214, D i v i s i o n of Building Research,

National Research

Council

2. Barakat, S . A . , and R.M. Sander. Utilization of Solar Gains Rrough

Windows f u r H e a t i n g Houses, Building Research N o t e 184, Division of

B u i l d i n g Research, National Research Council Canada, Ottawa, 1982.

3. Barakat, S.A. NRCC Passive Solar T e s t Facility, Performance of Direct Gain U n i t s , B u i l d i n g Research Note 215, D i v i s l o n of Building Research,

maim r n ? Q

d o d

e

E >

2

'

w

L - 5

d ;

_-

64

s

+

s

v-

-

?5

5

-

> =

-VE+

~

u3q9

4 m 4

Table

2

Ref, S o l a r Useful N e t e f f

.

loss gain gain gain

Node Days ~ m / ~ n i t

kW

kW

kW

kW

1980/#l

51

k o n s i d e r i n g south window o n l y o f t h e unit Gall n o r t h window g a i n is considered u s e f u l ) .

Table 3

Mass Wall (Measured) Direct Gaia (Calculated)

Net: eff. N e t c f f . ~ o l J ~ n i t g a i n g a i n Mode

GLR

ns

kW -h

MGR

0.54

37

1.04

18

_5-33

1 7