Thermal simulation of a passive solar house using a trombe-michel wall structure

Solar Energy, VoL 20, pp. 275-281, PergamonPress1978. PrintedinGreatBri~ain

THERMAL SIMULATION OF A PASSIVE SOLAR HOUSE USING A TROMBE-MICHEL WALL STRUCTURE P. OHANESSIANand W. W. S. CHARTERS Mechanical EngineeringDepartment, University of Melbourne,Parkville, Victoria, Australia (Received 9 September 1975; in revised form 22 February 1977)

Abstract--Since the construction in 1967 in France of the first house with a "Trombe Wall" there has been continuing interest on a world-widebasis of the potential of passive solar systems. The integration of the absorber as part of the buildingstructure can lead to substantial reduction in buildingcosts of active solar systems composed of absorbers, pumps, controls, etc.; and the simplicityof the system ensures that the structure will require no more maintenance than a conventional house. The prototype of this system was built in 1967 and later designs were constructed in 1974 in the French Pyrenees. There is a continuing programme of research and development into passive solar systems sponsored by the French Government through CNRS. At the University of Melbourne we have investigatedover the past 2 yr the feasibilityof applyingsuch a passive solar system to Australian conditions. We have studied by computer simulationthe thermal performance of the wall structure during the worst month of a Melbourne winter when equipped with a solar wall collector. This is essentially an adaptation of the French concept to conventional Australian building methods. It is interesting to note, that this type of system readily adapts itself to a modular construction approach with factory made solar walls pre-assembled and delivered complete for site installation. The solar contribution to the heating load in winter conditions appears to be of the order of 40 per cent, as predicted from the theoretical computer models and confirmedfrom the performance data published from the monitored French houses at Odeillo. INTRODUCTION

With the ever growing concern for energy conservation, a great deal of interest has recently been shown in the use of Solar Energy for the purpose of heating dwellings. At the moment the designs offered as possible configurations are many and varied[l]. These can however be conveniently divided into two classes; the so-called "active" and "passive" designs. The former utilizes relatively sophisticated equipment to absorb, transfer, release and/or store energy in the required manner. These, of course, tend to be expensive to install and are therefore suitable, that is cost effective, in locations that experience a harsh climate. "Passive" designs on the other hand, tend to be as simple as possible. They do this by incorporating the required thermal characteristics directly in the support structure. Examples of these include the "Odeillo" solar home by Trombe[2, 3], the solar roof pond home of Hay[4], and the "Zoneworks" Solar Houses[l]. Balcomb et al. in a recent publication[5] have studied the theoretical response of possible passive design configurations using as inputs the solar and weather data of Los Alamos (New Mexico) for September 1972 to August 1973. Their work lends support to the thermal efficacy and hence desirability of passive systems. For locations at medium to high latitudes (30° and above) such as Melbourne, it was felt that due to the increased winter solar input, a design utilizing a vertical surface facing the winter sun would be preferable. A decision was therefore made to investigate the theoretical performance of an "Odeillo" type design subjected to the winter climatic conditions experienced in Melbourne. The performance of the "Odeillo" model is compared with a conventional model identical to the above but with

the solar wall replaced by a conventional wall design, see Fig. 1. Appendix gives information on the modelling technique. "ODEILLO" DESIGN

The essential solar feature of the "Odeillo" design is the use of the North facing wall (Southern Hemisphere) as a solar collector, a storage facility and a heat transfer medium all in one. The efficiency of the collector is enhanced by placing single or double glazing in front of the outer face of the concrete slab; the latter painted a suitable dark colour. Heat is transferred into the internal environment by both conduction through the slab, and by a natural convection current in the air gap between the glass and the outer concrete face, fed by ventilation ports at the top and bottom of the slab, see Fig. 2. SOLAR RADIATION

The radiation on the North vertical surface will be a critical factor in the differential performance of the two model types, and whilst radiation on a horizontal and inclined surface is continuously monitored in Melbourne, it was felt that rather than incur possible inaccuracies in extrapolating the data to a North vertical surface, it would be preferable to measure the required radiation directly. This was done on the roof of the university for a specific design period of 28 days from 18 July 1974 to 14 August 1974; subsequently referred to as day numbers 1-28 in the results presented.

275

CONVECTION

Studies in the literature[6-9] on free convection between heated vertical planes deal mainly with theoretical (often computer) solutions for laminar flow, that is low

276

P. OHANESSIANand W. W. S. CHARTERS

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Fig. 2. Details of construction: Brick veneer with reflective insulation. Suspended timber floor with carpet and felt underlay. Double plaster partition wall, height of walls, windows 2.8 m. Rayleigh number flows, with either constant heat flux or constant temperature surfaces. As with most solar devices, the heat transfer process in the gap cannot be assumed to be either of the two cases, nor is it likely that the flow is laminar for the full height of the wall. To overcome the problem a sufficiently wide gap was assumed so that single vertical plate empirical turbulent correlations could be used. In comparing the convective heat input of the model with the published results from the "Odeillo" prototype, it was found that the empirical correlation consistently overpredicted the convective heat transfer. The correlation was therefore modified to more accurately simulate the "Odeillo" prototype.

DISCUSSIONOF RESULTS To represent the internal environment, an "Environmental" temperature is used, defined by the equation

TENv = 0.6 TaADIANT+ 0.4TAIR where TRADIANT is the mean internal surface temperature and TAm is the air temperature of the given

zone. Thus defined TENV may be used in a combined heat transfer coefficient for convection and radiation. The performance of the various models are presented in graphical form in Figs. 3-7. Details of the model configurations are set out in Table 1. (1) Solar house and conventional house (Figs. 3-5) The daily mean environmental temperature for the 4 zones of models $2 and C1 for the 28 day design period is shown in Fig. 3, together with ambient air temperatures and the integrated global solar radiation. It can be seen that for the entire 28-day period the solar model performs thermally significantly better than the conventional model. Figure 4 shows in greater detail, again for the 4 zones, the thermal behaviour for days 18-22; again one may note the higher temperatures obtained from the solar model. The performance of the individual zones is demonstrated in Fig. 5. It is significant that Zone 3 for the solar model (Zone 1 behaves in a similar way) has the highest temperatures of all and moreover, in the evening, the environmental temperature is very close to that required for thermal comfort (approximately 20°C).

277

Thermal simulation of a passive solar house

(3) Effect o/concrete slab thickness (Fig. 7) The effect of varying the slab thickness is shown in Fig. 7. It may be noted that the thinnest slab (12.5 cm) gives the highest evening temperatures. This is attributable to the higher internal surface temperature of the concrete slab. Figures 8 and 9 show the temperature fluctuations in the concrete for the three thicknesses 12.5, 25.0 and 50.0cm for a 24hr period on day 19. It is quite evident that the 50.0 cm slab is far too thick to allow effective heat conduction through the slab thus reducing the internal slab temperature by a significant amount.

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(4) Auxiliary heating The performance of models with the necessary auxiliary heating to maintain an internal air temperature of at least 20°C from 8 a.m. to 11 p.m. is summarized in Tables 2 and 3. It can be noted that savings in the order of 40

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(2) Effect o/glazing (Fig. 6) Figure 6 shows the differences in performance obtained with single and double glazing (refer to Table 1 for identification of the models)• It would seem from the plots that the performance is only marginally better with double glazing. This is most likely due to the relatively mild Melbourne winters and the poor transmissivity of typical Australian glasses.

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278

P. OHANESSIANand W. W. S. CHARTERS Table 2. Auxiliary heating supplied to 4 zones Model

Total for 28-day period M Joules

Daily mean

% of C2

2550 1700 1600 1680

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Thermal simulation of a passive solar house

279

Table 3. Auxiliary heating supplied to zone 3 Model

Total for 28-day period M Joules

Dily mean

% of C2

573 335 236 238

20.5 12.0 8.3 8.5

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280

P. OHANESSIANand W. S. S. CHARTERS

per cent can be achieved and the slab thickness of 25.0 cm would appear to be optimal for the situation considered. CONCLUSIONS

(i) The "Odeillo" wall model appears to work quite well under typical Melbourne winter conditions. (ii) The thermal advantage of double glazing seems to be minimal and it is therefore unlikely that double glazing is cost effective in moderate climate zones. (iii) Heat conduction through the slab is a critical factor in obtaining acceptable internal temperatures; the use of an overthick slab appears to be a positive disadvantage. (iv) With auxiliary heating a thickness of 25 cm seems to be near optimal for the particular case studied. (v) Energy savings of approximately 40 per cent are obtainable with such designs.

T.n

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r?_, Ol

At Ax TENv TAm TRADIANT

NOMENCLATURE temp. of ] th node at time n, °C temp. of i th node at time n + 1, °C temp. of (j - 1) th node at time n, °C temp. of (j + 1) th node at time n, °C thermal diffusivity, m2/sec. time increment, sec. nodal spacing, m. mean environmental temperature, °C mean air temperature, °C mean radiant temperature, °C

heating. Int. J. Heat and Mass Transfer 15, 2293 (1972). 7. J. R. Bodoia, and J. F. Osterle, The development of free convection between heated vertical plates. A.S.M.E.J. Heat Transfer 84, 40 (1962). 8. W. Elenbaas, Heat dissipation of parallel plates by free convection. Physica 9, 1 (1942). 9. C. Y. Warner and V. S. Arpaci, An experimental investigation of turbulent natural convection at low pressure along a vertical heated flat plate. Int. J. Heat and Mass Transfer 11, 397 (1968). APPENDIX

The thermal performance of systems with l~arge heat capacities Can only be described effectively by the unsteady transient thermal behaviour. Thus, in the heat transfer through heavy wall fabrics such as brick or concrete, the thermal diffusion equation, OT/Ot = a ~2T/Ox2 must be used. Analytical solutions to such equations are limited to problems with simple boundary conditions; for the house models investigated however, the effects of wind speed, ambient air temperatures and solar radiation are such as fo preclude analytical solutions. The equations however, can be approximated as finite difference equations and "solved" continuously on a digital computer. For simplicity an explicit form of the finite difference equation was used. As an example the heat transfer through the internal nodes of a fabric such as brick or concrete can be written as At

,

giving Tj~l+l aAt

,

/

2aAt\

REFERENCES

1. W. A. Shurchliffe, Solar Heated Buildings: A Brief Survey, 9th End. Cambridge, Massachusetts (1975). 2. F. Trombe, Heating by Solar Radiation. Director of the C.N.R.S. Solar Power Laboratory, C.N.R.S. Internal Report, B-1-73-100. 3. F. Trombe, et al., Characteristiques de Performance des lnsolateurs Equipant les Maisons a Chauffage Solaire du CNRS. Private Communication. 4. H. R. Hay, Energy, Technology and Solarchitecture. A. S. M. E. Mech. Engng, 95 (11) (1973). 5. J. D. Balcomb, et al. Simulation analysis of passive solar heated buildings--preliminary results. Solar Energy 19, 227 (1977). 6. W. Aung, L. S. Fletcher, and V. Sernas, Developing laminar free convection between vertical flat plates with asymmetric

with the stability requirement 2aAt _ 1 -(--~X)2>~O. For surface nodes the equations need to include any convective and/or radiative effects (including solar radiation where applicable). In order to obtain a starting temperature profile for the models, all nodes are set arbitrarily to 16°C and the environmental input for the first day are fed into the models until a similar temperature distribution at a given time on successive days is achieved. The environmental conditions for the following days are then inserted as inputs and the simulation proceeds automatically from that point.

Resumen--Desde la construccirn en Francia en 1967 de la primer casa con la "Pared de Trombe" ha habido un continuo interrs generalizado mundialmente por el potencial de los sitemas solares pasivos. La integraci6n de los colectores corot parte de la estructura edilicia puede traer una substancial reduccirn en los costos de los sistemas solares activos compuestos pot colectores, bombas, controles, etc.; y la simplicidad del sistema asegura clue la estructura no requerir~i mayor mantemimiento que el de una casa casa convencional. El prototipo de este sistema ha sido construfdo en 1967 y los disefios 61timos fueron construfdos en 1974 en los Pirineos franceses. Este es un programa continuo de investigacirn y desarrollo en sistemas solares pasivos patrocinado por el Gobierno francrs a travrs del CNRS. En la Universidad de Melbourne nosotros hemos investigado durante los dos (fltimos afios la factibilidad de la aplicaci6n de tal sistema solar pasivo alas condiciones de Australia. Nosotros hemos estudiado por simulaci6n de c6mputo el comportamiento trrmico de la estructura de la pared durante el peor rues del invierno de Melbourne cuando estfi equipada con un colector solar mural. Esta es esencialmente una adaptaci6n de la concepci6n francesa a los mrtodos edilicios convencionales australianos. Es interesante notar que este tipo de sistema se adapta r~ipidamente a la concepci6n de construcci6n modular donde la fabrica hace las paredes solares prearmadas y las envfa completas para la instalaci6n en su sitio. La contribuci6n a la calefacci6n en condiciones de invierno resulta ser del orden del 40% segfin 1o previsto de los modelos te6ricos de c6mputo y confirmado por los datos de comportamiento publicados de los registros de las casas francesas de Odeillo.

R~sumr----Depuis la construction en 1967 en France de la premirre maison avec un mur solaire type Trombe, un intrr~.t constant a 6t6 marqu6 sur le plan mondial pour les possibilitrs des systrmes solaires passifs. L'intrgration de

281 l'absorbeur en tant que partie de la structure d'habitat peut amener ~. une r6duction substantielle des cofits de construction de syst6mes solaires actifs compos6s d'absorbeurs, pompes, commandes, etc . . . . et la simplicit6 du syst~me assure que la structure ne demandera pus plus d'entretien qu'une maison traditionnelle. Le prototype de ce syst6me fut construit en 1967, puis d'autres constructions furent r6alis6es en 1974 duns les Pyren6es franqaises. II existe un programme suivi de recherche et de d6veloppement des syst6mes solaires passifs, subventionn6 par le gouvernement fran~ais par l'interm6diaire du CNRS. A l'Universit6 de Melbourne, nous avons fait de recherches, ces deux derni~res ann6es, sur les possibilit6s d'appliquer un tel syst~me solaire passif aux conditions existant en Australie. Nous avons 6tudi6 par simulation sur ordinateur le rendement thermique de la structure du mur, alors 6quip6e d'un capteur solaire mural, durant le mois le plus rude de l'hiver fi Melbourne. Ceci 6tant essentiellement une adaptation de la conception franqaise aux m6thodes de construction australiennes traditionnelles. Il est int6ressant de noter que ce type de syst6me s'adapte pr6cis6ment $ une approche modulaire de construction avec des murs solaires pr6-assembl6s fabriqu6s en usine et livr6s complets pour installation sur te site. La contribution du solaire dans la charge de chauffage, dans des conditions hivernales, pourrait ~tre de l'ordre de 40%, ainsi que pr6vu par les mod61es de calcul th6oriques faits sur ordinateur et confirm6 par les donn6es de rendement publi6es ~ partir des maisons pilotes franqaises d'Odeillo.