A school building reclad with thermosyphoning air panels

Solar Energy Vol. 52, No. 1, pp. 49-58, 1994 Printed in the U.S.A.

0038--092X/94 $6.00 + .00 Copyright © 1993 Pergamon Press Ltd.

A SCHOOL B U I L D I N G R E C L A D WITH T H E R M O S Y P H O N I N G AIR PANELS S. N. G. LO,** C. R. DEAL,** and B. NORTON** *PROBE: centre for Performance Research On the Built Environment, Department of Buildingand Environmental Engineering,University of Ulster, NewtownabbeyBT37 0QB, Northern Ireland, * *Property Services Department, Essex County Council, Chelmsford CM 1 1LB, England Abstract--Thermosyphoning air panels (TAPs) were installed as part of the refurbishment of a school building in south-east England. Design,construction, and operation are described. Their long-term thermal performance was monitored extensively.Annual contributions to the heating load were 1970 kWh for the 23 m2 of south-facingTAPs, and 498 kWh for the 27.3 m: of east-facingTAPs during the 1989/90 heating season. The economic viability of the installation is discussed.

1.2 The building: Nazeing County Primary School Nazeing County Primary School, owned by Essex County Council, lies at a latitude of 51 °44'N, a longitude of 0°01 'E, and is 30 m above mean sea level in the village of Lower Nazeing, 26 km due north of central London. The school's 181 pupils, aged between 5 and 12 years old, are taught by 11 teachers. The building occupies a total floor area of 1631 m 2. Nine of the ten classrooms are heated by fan convector heaters. The periods of occupation were from 8:00 A.M. until 3:30 P.M. Monday-to-Friday with occasional use during the evenings and at weekends. The majority of classrooms have south-facing glazing. The east, south, and west facades of the building overlook playgrounds and playing fields, and remain free from any obstructions that may impinge upon the collection of solar energy. The school, built in 1958, represents a typical lowcost, rapidly guilt, modular lightweight curtain-walled building of that era. Unfortunately, due to inadequate detailing during construction, and failure of sealants, the timber walls of such buildings have deteriorated rapidly. Thus Essex County Council (in common with many other local authorities in the United Kingdom) had a refurbishment programme for all such schools. By taking advantage of the opportunity presented by refurbishment, and incorporating the thermosyphoning air panels (TAPs) into a conventional curtain-wall cladding system, their additional initial cost was minimised.

1. I N T R O D U C T I O N

1.1 Refurbishment solarization Modifications to existing buildings are likely to provide the major proportion of the benefits that passive solar heating can offer in the short term, as new buildings constitute only a very small proportion of the existing stock. Many commercial and public organisations, seeking to improve the working environments within their premises, find that refurbishment of an existing building is often both less expensive, and more convenient than the acquisition of new premises. However, a more pressing reason for the refurbishment of buildings in the United Kingdom is failure of ageing glazing and curtain wall cladding systems. This premature deterioration has been attributed to the widespread sue of then (1960s) newly available materials and techniques in the absence of appropriate standards, controls, or the necessary expertise [1]. The introduction of the curtain wall cladding technique enabled buildings to be constructed with lightweight, nonload-bearing external walls suspended from structural frameworks. The performance of many of these curtain walling systems depended on the integrity of sealants, none of which had a guaranteed life in excess of twenty years [ 2 ]. The failure of these sealants and subsequent water penetration had led to internal rotting and structural damage. The energy substituted by many passive solar features may recover their initial investment well within their operational lifetimes. However, the length of these payback periods are usually unacceptable to all but public bodies. Thus it is in the buildings (mostly school buildings) operated by such organizations that passive solar design tbr the nondomestic sector has flourished in the United Kingdom [3].

1.3 Thermosyphoning air panels The TAP [4], patented originally in 1881 [5], in the form shown in Fig. 1, relies solely on natural convection to transfer solar heat gain. It is an example of a passive solar feature that is an integral element of a building which, without mechanical systems, harnesses solar energy. As a TAP may be decoupled thermally from the building, generically it falls within the "isolated gain" group of passive solar features [ 6 ]. A more controllable heat gain, combined with an inherent

*Member ISES. Address correspondence to B. Norton. 49

S. N. G. Lo, C. R. DEAL,and B. NORTON

50

GLASS PLATE WOODEN SUPP CORRUGATED ABSORBER PLATE THE MORSEPASSIVESOLAR HEATIN(3AND VENTILATINGUNIT PATENTED IN 1881.

Fig. 1. The Morse Passive Solar Heating and Ventilating Unit patented in 1881.

avoidance of summer overheating, are the primary advantages of isolated gain when compared with other passive solar features. As the heat input is almost instantaneous, such solar walls are thus more appropriate for buildings that are occupied mostly during the daylight hours. Heat losses during nongain periods are low, as the insulated collector is isolated from the heated space. A TAP operates in the same manner as the nat-

ural convection mode o f a Trombe-Michel wall. However, a TAP's absorber is made of metal, usually alum i n u m or steel, (in contrast to the heat-storing masonry of a Trombe-Michel wall) and it is insulated to inhibit conductive heat transfer to, or from, the TAP to the building: To prevent any unwanted heat output, either the inlet or outlet vent may be closed: thermosyphoning then ceases, or, a passage to an external vent

~ Fig. 2. Details of vertical cross-section through thermosyphoning air panel (TAP).

RN

Thermosyphoning air panels

51

RIVETDAMPERASSEMBLYTO REAR --~J]J~--- - -

UPPER INSULATION PANEL - -

OF PANELUSINGPIANOHINGE /"//i~ AND LOCATEHANDLETHROUGH / 7 J PANELSECURINGONTOLNSIDEOF / l " J DAMPERTRAYWITH RIVETS / J J (DAMPER ASSEMBLY MUST BE,,/ I I

DAMPERISTOOVERLAPPANEL/

l ~

FIT SOFTDASKET ON ALL SEALING SU~CES~

/

DAMPERASSY.~

LOWER INSULATION PANEL - -

NON RETURN VALVE.TEDLAR SHEET

111

TYPE 200SG-t,OTR TAPED TO REAR OF LOWER PERFORATED SECTION.

]]I JAI

.._._..__./ FIX TO FRAME USIND N'8 SELF TAPPING SCREWS. {OUTER SKIN HOLES ARE TO BE BLANKED USING RUBBER CAPS

I

PERFORATED PATTERN 7

25ram

F

L2S~ LOWERINSULATIONPANEL SECTION Fig. 3. Fabrication o f insulating back pane] o f T A P .

is opened, inducing a buoyant ventilative flow. Commercial development of the TAP to date has been comparatively small, but it has proved to be popular with some enthusiasts in the United States [7] and Italy [ 8]. Also, in Italy, TAPs form part of the proprietary Barra-Constantini natural-convection dualpass solar air heating system [9]: the solar heated air from the TAP passes through, and relinquishes heat to, a hollow masonry floor. The latter system has also been installed in three dwellings in England and two

in Scotland [10]. However, unlike a Trombe-Michel wall, the Barra-Constantini TAP may be decoupled from the remote heat store.

1.4 TAP design and operation Detailed drawings of the construction of the TAPs are shown in Figs. 2 and 3. There were two different modes of collector operation intended to be activated manually by each class teacher as required. In the heating season, during periods of insolation, the air

Fig. 4. Internal view of TAP.

52

S. N. G. Lo, C. R. DEAL,and B. NORTON

from the classroom was drawn through a low-level grille, circulated by buoyancy forces between the absorber plate and the insulated back-panel, and expelled via a high level outlet grille back into the classroom. These inlet and outlet grilles can be seen in Fig. 4. For the given system geometry, dimensions, and materials properties, the flow rate of the solar-heated air was dependent upon the solar radiation incident upon the collector panel. Thus, during the summer months when heating from the TAPs was not required, the upper return grille into the classroom was closed and the solarheated air exhausted out via a slot through the adjacent transom to a weather-proof louvre. This operation also reduced the risk of overheating, which could damage the absorber surface a n d / o r the glazing. Thirty-six TAPs, with a total glazed area of 101 m 2, were installed. Views of the east facade before and after refurbishment are shown in Fig. 5; the TAPs are a near-indistinguishableintegral part of the reclad curtain wall. A more detailed close-up of the refurbished south

facade is shown in Fig. 6. In the majority of cases, the TAPs represented only 35% of the external glazed wall area of each classroom or office. This minimized the reduction in daylighting experienced prior to refurbishment as recalled by the occupants. The collectors first began to contribute to the auxiliary space-heating demand from the beginning of the 1988-89 heating season, in October 1988.

1.5 Monitoring schedule A total of seventy-three type-T copper/constantan thermocouple temperature sensors, five solarimeters, and two heat-meters were installed to monitor TAP performance. Three of the solarimeters measured the global radiation on the vertical plane of the east, south, and west facades, and the remaining two solarimeters measured global and diffuse radiation on the horizontal plane. A heat-meter was installed in the main plant room in order to measure the total heat input into the

Fig. 5, The east facade of the school before (a) and after (b) refurbishment.

Thermosyphoning air panels

53

Fig. 6. The south facade of the school after refurbishment.

school, a n d a more sensitive heat-meter"was located in a south-facing classroom to record the space heating r e q u i r e m e n t of two adjacent classrooms. During the installation of the m o n i t o r i n g equipment, each insulating panel h a d to be part-dismantled in order to affix thermocouples within the collector a n d to the back of the absorber plate. Seventy-three sensors were employed as shown in Table 1. The signals from all of these sensors were recorded a n d stored by two "standalone" data-loggers. They were programmed to sample all of the sensors every l0 rain a n d store the average of these six readings every hour. A total of thirty days

data could be stored before the logger was interrogated a n d the data transferred to floppy disc for further analysis. The solar fraction, thermal efficiency, and the total c o n t r i b u t i o n to the overall heating load from the TAPs could then be determined. Short-term high-sample-rate monitoring of the T A P flow rate at four second intervals, with the average recorded every 5 rain, was u n d e r t a k e n over a range o f t e m p e r a t u r e differences between the panel inlet a n d outlet using hot-wire a n e m o m e t e r s . This i n s t r u m e n t had a resolution of_+ 0,01 ms -1 a n d an accuracy of + 0.1 ms -l .

Table 1. Schedule of sensors and their distribution throughout Nazeing County Primary School Number of TAPS and orientation Room no. Classroom 1 Classroom 2 Classroom 3 Classroom 4 Classroom 5 Classroom 6 Classroom 7 Classroom 8 Classroom 9 Classroom 10 Library Secretary Headmistress Staff Room Assembly Hall Staff Lavatory Entrance Lobby Utility Room Plant Room

East

South 1

2

3 2 1 3

West

Absorber outlet

Absorber plate

Room

1

1

2

2

2

4 4 1 2 1 1 l 1

4 4 2 2 1 1 1 1

1 1 1

1 1 1

1 1

2

2

1

1 1

1 1

1 1

25

26

1

1 1 1 1

12

Solarimeter orientations Collector inlet

2

2 2 2 2

14

Temperature sensors

10

East

South

West

Horiz.

1 1

2 2

3 3 l 1 l 1 1

4

18

1

2

Heat meters

54

S.N.G. Lo, C. R. DEAL,and B. NORTON

/

/,,

:,

/ /I

1500

,~

:1

30-

J

i,

i \

-400

~

-300

20-

200

m

I 0

I 2

I 4

I 6

I 8

I 10

I 12

I 14

I 16

I 18

I 20

I 22

bo,~ Trne (h~n)

Fig. 7. Diurnal variation of incident insolation and the mean values for five south-facing TAPs for their inlet and outlet temperatures recorded on the 24th October, 1989.

and 24th October), were 675 Wm -2 and 650 Wm -2. Figure 7 illustrates the mean inlet and outlet temperatures and total insolation on the vertical plane of the five south-facing collectors on one of these days. Figures 8 and 9 illustrate the mean inlet and outlet temperatures and total incident insolation for east-facing and west-facing TAPs. Comparison of daily inlet/ outlet temperature differences, (Fig. 10), shows, as expected, how the east- and west-facing TAPs produced their greatest solar contributions in the morning and evening, respectively. Also, lower peak output temperatures were achieved by TAPs on both these two facades than for the south-facing collectors. The total monthly heating output from the TAPs on both the south and east facades are shown in Figs. 11 and 12, respectively. Overall monthly efficiencies are shown in Fig. 13 and the monthly solar fractions using contributions from both south and east facades are shown in Fig. 14.

The correlation coefficients between the linear variations of both insolation and temperature difference with TAP flow rate were 0.918 and 0.906, respectively. This relationship enabled the mass flow rate through each TAP to be calculated for periods when direct measurements of mass flow rate were not taken. This also allowed the calculation of the total energy contribution from all of the TAPs. It was shown that a minimum temperature of 5.3°C provided by 275 Wm -2 for an internal air temperature difference of 22°C at an ambient temperature of 16°C was required to initiate any thermosyphon air-flow. 2.

OPERATION

AND

RESULTS

Two different days in October 1989 have been selected to illustrate the range of temperature differences that were being experienced in the TAPs on each facade. The peak hourly levels of insolation on a southfacing vertical plane during each of these days, (20th

50

700

- 5OO I~

~ 30-

~-~"'~:

- 400

- 300

2O-

.~

- 200 10- 100

O

I 0

I 2

I 4

I 8

110

I 12

I 14

I 16

I 18

I 20

I 22

0

Loca'rrm ex~s)

Fig. 8. Diurnal variation of incident insolation and the mean values for two east-facing TAPs for their inlet and outlet temperatures recorded on the 24th October, 1989.

Thermosyphoning air panels

55

- 50O

- 400

- 300

I

- 200

.,.';

o

I 0

I 2

I 4

I 6

'"'i" 8

I 10

I 12

- 100

~,.,

I 14

I 16

'1 18

I 20

I 22

0

Fig. 9. Diurnal variation of incident insolation and the mean values for two west-facing TAPs for their inlet and outlet temperatures recorded on the 24th October, 1989.

12-11-IO--

i

WW F ~

7 e

4

3

~

m

m

mml

21

23

t~mn~ Fig. 10. Diurnal variation of incident insolation and the mean TAP inlet and outlet temperature differences for the south, east, and west facades recorded on the 24th October, 1989.

•

Msr

I I Tolal n~n(hly e ~ BE Total montNy ~

o o ~ ~om TA.~ on ooulh b ~ on louth t ~ g vedk~d fmcade

Apr

| |

NOV

Dec

Jin

~b

Months

Fig, 11. Annual variation of the monthly useful energy contributed by the south-facing TAPs and monthly incident insolation in the period February, 1989 and February, 1990.

56

S . N . G . Lo, C. R. DEAL, and B. NORTON

I I T°ml m°nthlybs°liti°n °n eMt fad~ vertl~ ficade

M|r

Apt

Nov

Dec

Jan

i

Feb

Months

Fig. 12. Annual variation of the monthly useful energy contributed by the east-facing TAPs and monthly incident insolation in the period February, 1989 and February, 1990.

I m Mone~ya~leecy ~' TAPeon ~

f ~ d e 11

25

f

20 ¸

i

15-

!

10-

MJr

Apt

NoV

DG¢

Jan

Fab

Months

Fig. 13. Annual variation of the monthly solar energy thermal conversion etficiencies of the TAPs on the south and east facades in the period February, 1989 and February, 1990.

0.1

i

0.08

i

0.00-

!

Mmr

Apr

Nov

Dec

.Jan

Fib

Mor,thl

Fig. 14. Annual variation of the monthly solar fractions of the TAPs on the south and east facades in the period February, 1989 and February, 1990.

57

Thermosyphoning air panels Table 2. Auxiliary fuel costs and annual solar heating contribution at Nazeing County Primary School (10/10/89-24/4/90) South facade Annual energy contribution from TAPs on each facade (kWh) Number of TAPs supplementing electricity on each facade Number of TAPs supplementing gas on each facade Overall cost of fuel supplemented by TAPS on each facade (p/kWh) Annual financial contribution TAPs on each facade (£)

East facade 498

1970

2

2

10

12

2.22

2.10

43.7

10.47

Teachers often failed to actuate the deflector allowing heat gains from the TAPs into the classrooms. This was due to: (a) the effects of operating the damper mechanism, upon the TAP flow rate, was not consistent from one panel to the next; (b) newly-employed teachers were unaware of the operational principles of the TAPs; and (c) heat delivery was not immediately tangible.

On the east and south facades there were 14 and 12 TAPs installed, respectively. Two TAPs on each of these facades were supplementing either electric convector heaters or electric radiant heaters, with the remaining TAPs displacing space heating provided by the primary gas-fired heating system. During 1989 the cost of electricity and gas at Nazeing County Primary School was 6.31 p / k W h and 1.4 p / k W h , respectively. In order to obtain the overall cost of the two displaced auxiliary fuels, the number of panels that supplemented gas were considered together with those that supplemented electricity on each facade. For the south facade, ten TAPs supplementing gas at 1.4 p / k W h in combination with two TAPs supplementing electricity at 6.31 p / k W h , were equivalent to twelve TAPs displacing auxiliary fuel requirements at 2.22 p / k W h ; (see Table 2). The originally installed continuous lightweight film damper proved to be too obstructive to the low airflow rates through the TAP to permit optimal heat delivery. In addition to this flow restriction, the damper occasionally adhered to the back of the absorber plate as a result of static charges in the film; this could obstruct the flow path completely. The air gap between the insulation and the absorber should have been 60 m m but in practice this varied from 35 m m - 6 5 mm, and the above flow restriction phenomena occurred most often in collectors that had particularly narrow channel depths.

2.1 Occupant interaction with TAPs There is an unfortunate dichotomy between the widely perceived need for unobtrusive passive solar

features (both in terms of operation and architectural harmony), and the possible optimization of system performance that ensues from sympathetic conduct of occupants. This was illustrated clearly by the influence of the occupants on the TAPs. Some of the teachers at Nazeing County Primary School did not fully understand the TAPs operation; many of the TAPs were obstructed by posters and bookcases. More often than not, the teachers were never fully aware of the function of the only moving part in the system, the damper. Perhaps only energy-saving measures that reduce fuel bills that the occupants are directly responsible for, are likely to promote behaviour that optimises system performance. 2.2 Calculation of simple payback periods The simple payback calculation has value only for the purposes of comparison. The contribution to the auxiliary heating system from the TAPs on the south facade during the 1989/90 heating season from October 10th, 1989, to April 24th, 1990, was 1970 kWh. Based upon ten TAPs supplementing gas at 1.40 p / kWh and two TAPs supplementing electricity at 6.31 p / k W h , this represents an annual contribution of £43 70p, which gives a simple payback period of 46 years (Table 3 ). The economics of the panel installation depended upon the type of auxiliary heating fuel used. If the panels were used to supplement electric heating, economic projections would depend on the extent to which energy conversion costs will reflect concomitant carbon dioxide production. At present in the United Kingdom, the cost of substituted electrical energy depends on the

Table 3. Simple payback periods for TAPs based upon four different economic scenarios Scenario (overcost related to replacing a simple insulated panel with a TAP):

Cost (£)

Payback period (years)

A. Total overcost of 36 TAPs installed as part of a demonstration project.

£14,010

259

B. Total overcost of 36 TAPs as installed on future replication.

£5,994

111

C. Total overcost of 14 TAPs on East facade, on further replication.

£2,331

223

D. Total overcost of 12 TAPs on South facade, on further replication

£1,998

46

S. N. G. Lo, C. R. DEAL, and B. NORTON

58

applicable tariff regime. At an "off-peak" tariff rate (with a fuel cost of 4.44 p / k W h ) , the payback period for 12 TAPs installed on a south facade is reduced to 23 years. Similarly, if electricity at the on-peak tariff was being supplemented, the payback for 12 TAPs on the south facade is further diminished to 16 years. The latter is commensurate with the likely m i n i m u m durable life of the system. 3. CONCLUSIONS The consistent occupancy patterns observed at Nazeing County Primary School were compatible with the diurnal performance characteristics of the TAPs, though any excess solar gains from the TAPs could not be stored to offset any auxiliary heating requirements the following day. The total annual space-heating contribution from all of the TAPs was 2468 kWh providing an increase in the annual solar heating fraction of 6%.

Acknowledgments--This project was supported financially by the Directorate-General for Energy of the Commission of the European Communities, Brussels, Belgium. Significant contributions were made by the architectural staffof Essex County Council and by Mr. A. C. Lyall, now of MAP Ltd. The contributions to the project of fabricators, installer, and the teachers and pupils of the school are gratefully acknowledged.

REFERENCES

1. Anon., External cladding. Building Refurbishment and Maintenance, September/October, 67-81 (1986). 2. K. Endean, Curtain repairs. Building Refurbishment and Maintenance, May/June, 52 (1986). 3. B. Norton and R. A. Hobday, Passive solar schools in the U.K.: Features employed currently and their operation, International Journal of Ambient Energy. 11, 60 ( 1990 ). 4. B. Norton, S. N. G. Lo, and R. A. Hobday, Thermosyphoning air panels, Advances in Solar Energy. 7, 475 (1992). 5. E. S. Morse, U.S. Patent no. 246, 626 ( 1881 ). 6. B. Norton, Solar energy thermal technology, SpringerVerlag, Heidelburg, Germany (1992). 7. D. F. Malloy, Solar wall collector plans review, Urban Solar Publishers, MA, U.S.A., 1987. 8. G. Scudo and De Pascali P., Climate responsive cladding elements, Proceedings of the 2nd European Conference on Architecture. December, 1989, Paris, France, 313-315 (1987). 9. O. A. Barra, G. Artesi, T. Costantini, G. Costanzo, L. Franceschi, and E. P. Carratelli, The Barra-Costantini solar passive system experimental performance, Proceedings of the International Congress of Building Energy Management. May 12-16, 1980, Portugal, 143-151 (1980). 10. P. D. Johnson and D. A. Hobbs, A passive solar heating facility for existing houses, Final report for project no. SE/594/83. Wimpey Laboratories Ltd, Hayes, England, for the Commission of the European Communities, September, 1986, Brussels, Belgium ( 1986 ).