experimental and numerical evaluation of a solar passive cooling system under hot and humid climatic...

Solar Energy Vol. 71, No. 1, pp. 71–80, 20012001 Elsevier Science Ltd

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EXPERIMENTAL AND NUMERICAL EVALUATION OF A SOLAR PASSIVECOOLING SYSTEM UNDER HOT AND HUMID CLIMATIC CONDITIONS

,´ ´ ´†JOSE RINCON* , NASTIA ALMAO* and EDUARDO GONZALEZ**´ ´*Laboratorio de Simulacion Computacional, Escuela de Ingenieria Mecanica, Universidad del Zulia,

Maracaibo, 4011-A-526, Zulia, Venezuela**IFA, Instituto de Investigaciones de la Facultad de Arquitectura, Universidad del Zulia, Maracaibo,

4011-A-526, Zulia, Venezuela

Received 5 August 1999; revised version accepted 5 November 2000

Communicated by ANDREAS ATHIENITIS

Abstract—The thermal performance of a solar passive cooling system (SPCS) under a hot and humid climateis experimentally and numerically evaluated. The experimental data were obtained from two full scale cells,with identical walls, but different roof configurations. One cell has a highly-insulated roof and the other has anSPCS incorporated consisting of a thermal mass (water), which is cooled by evaporation and long wavenocturnal radiation. The study was conducted taking into account the local climatic conditions of Maracaibo, atopical city located in Venezuela. The numerical evaluation was accomplished using the computational code‘EVITA’ which is based on the finite volume approach with high order bounded treatment of the convectiveterms. A PISO-like solution algorithm is used to solve the transient form of the continuity, momentum andenergy equations. It has been demonstrated experimentally and numerically that under a hot and humidclimate, it is possible to keep the indoor temperature below the outdoor temperature, using a passive coolingtechnique of a roof pond. The numerical results obtained using the model have demonstrated that thecomputational code used is a suitable cost-efficient alternative for the thermal performance evaluation ofSPCS. 2001 Elsevier Science Ltd. All rights reserved.

´1. INTRODUCTION previous studies. Almao de Herrera and Rincon(1993) have shown numerically that under local

In coastal, hot and humid regions of Venezuela,climatic conditions the higher heat gain occurs

like Maracaibo, it is impossible to achieve thermalthrough the roof, and that it is possible to reduce

comfort using only natural ventilation and thethe thermal load using an SPCS such as a roof

traditional construction housing system. This fact,pond. The concept of a roof pond as passive

besides the lack of building energy consumptioncooling and heating system was introduced by

regulations and the cheapest energy cost inHay and Yellot (1978), with the ‘skytherm’

America, have generated a dramatic increment insystem, in which the water was placed on the roof

the usage of mechanical air-conditioning systemsinto sealed plastic bags, and the cooling effect

(AC). As a consequence, 80% of the residentialwas obtained by nocturnal radiation. Clark and

energy consumption is due to mechanical cooling,Allen (1981), Clark (1989) and Sodha et al.

albeit most of the population cannot afford the(1981) have carried out commercial and ex-

energy cost of AC systems. This characteristicperimental applications of this technique under

demands the study of energy-efficient housinglocal climate conditions. Givoni (1981) and Gon-

alternatives, where internal conditions of thermal´zalez (1991) have tested massive roof and roof

comfort could be accomplished without, or with apond radiator performance, with movable insula-

minimal, consumption of fossil energy. The alter-tion, in small lightweight test cells. Givoni (1994)

natives should offer a more comfortable life tohas reported the efficiency of using fixed shade or

non-users of air conditioning, less energy con-floating insulation roof pond systems, under dif-

sumption to AC users and less environmentferent climatic conditions. In this work an SPCS

pollution.based on the cooling of a thermal mass by long

Different investigations using solar passivewave nocturnal radiation and by evaporation, with

cooling systems (SPCS) have been performed insolar control, has been selected as one of theviable alternatives to be analysed. Even though

† this technique is not new, it is important to assessAuthor to whom correspondence should be addressed.E-mail: [email protected] its thermal performance under the local climatic

71

´72 J. Rincon et al.

conditions and with the traditional housing designin Venezuela. On the other hand, computationalmodels have been used for the thermal evaluationof buildings using different approaches e.g. sim-ple resistance models or steady state models(Sodha et al., 1986). In this paper, the obtainedexperimental data is compared with the results ofthe two-dimensional computational model EVITA

´(Spanish acronym for Evaluacion de VIiendas´Termicamente Adaptadas) (Almao et al., 1998).

This model has been developed to simulate,evaluate and compare the transient thermal per-formance of a building section, incorporating, in aflexible way, the most usual SPCS along with awide range of boundary conditions and novelnumerical techniques.

To obtain the experimental data, two cells withidentical external walls but different roofs were

Fig. 1. Experimental cells.built. One of the cells, named the reference cell(CREF), has a well-insulated roof. The other cell,named the experimental cell (CEXP) has an openroof pond as roof, protected during the day with This represents the local traditional constructivetwo insulating panels, and exposed to the sky and system. The CREF cover has been conceived, byatmospheric air during the night. All relevant methodological reasons, as a very insulated ele-climatic conditions were recorded in the ment. Therefore, it is basically constituted by 25 cmmeteorological station, which is part of the ex- of expanded polystyrene covered by 5 cm of lightperimental platform. The comparison of the inter- concrete; followed by a white painted 3-mm-thicknal temperature measurements in both cells allows asphalt waterproof layer, (Fig. 2a). The CEXPto evaluate experimentally the efficiency and the roof (Fig. 2b) consists of a metallic reservoircooling potential of the SPCS, as well as to assess (galvanised steel plate, 1.2 mm thickness), whichthe use of the code as a precise tool to analyse is filled with water to 10 to 15 cm of height, withand compare different housing sections with its lateral walls thermally insulated with poly-incorporated SPCS. styrene. Two swinging insulating panels of poly-

styrene, covered by white painted glass fiber, are

2. EXPERIMENTAL PLATFORM

Two cells (reference cell and experimentalcell), a meteorological station and a data acquisi-tion system constitute the experimental platform.It has been designed to guarantee accurate andprecise measurements to characterise the com-parative thermal performance of the studiedSPCS. It allows evaluation of the system undernear real operation conditions; to show the SPCSand the CREF transient performance related to theexternal environmental conditions in real time;and to monitor the actual weather conditions atwhich the cells are exposed during the tests. Thetwo built cells have identical constructive charac-teristics except for the roof (see Fig. 1). Theinterior dimensions are 3.0 m 3 3.0 m 3 2.45 m.They have a reinforced concrete structure andtheir walls are constituted by nine-core red clayblocks of 0.15 m thickness, externally and inter- Fig. 2. (a, b) Roof details for the reference and experimentalnally plastered with cement and sand aggregate. cells.

Experimental and numerical evaluation of a solar passive cooling system under hot and humid climatic conditions 73

located 25 cm over the water surface to achieve SPCS alternatives, the performance of which willsolar control (closed during the day and open be presented in future work.during the night). Both cells have their facadesoriented in the main directions. Those facadeswithout windows and doors correspond to the east 3. COMPUTATIONAL MODELand west orientations. Horizontal and vertical

The computational model EVITA was mainlysolar radiation and other relevant climatic con-developed to solve transient, complex and mixedditions are the same for both cells. Fig. 3 showsboundary condition problems, which are cumber-the location of temperature and relative humiditysome to implement in most commercial CFDprobes for each cell, whose instant analogicalcodes. It is based on the finite volume method,signals are recorded by the data acquisition sys-non-staggered grid arrangement, and a boundedtem. Platinum Resistance 3-conductor thermome-high-order treatment for the convective termsters (PT-100) with a 0.18C precision were used to

´(Rincon and Elder, 1997). A PISO-like solutionmeasure surfaces, water, indoor air, globe andalgorithm is used to solve the unsteady continuity,ground temperatures. VAISALA HMW20UB typemomentum and energy equations for the conju-sensors were used to measure relative humidity,gated heat transfer problem in the rectangularwith a 62% precision (0–90% relative humidity),section of a construction. The grid is built suchand 63% (91–100% relative humidity). Signalthat it considers each material of the structure forsensors were taken each minute with a 30-channelwalls, roof and floor. The Boussinesq approxi-YOROGAWA HR 2300 register. The localmation for the treatment of the natural convectionclimatic variable measurements are registered inof the air contained in the interior of the section isthe same way, from the meteorological station. Inused. The code allows study of the dynamicorder to ensure that the experimental internalenergy performance of a system under transienttemperature (T ) can be compared with numericalinboundary conditions, and it provides detailedresults, the windows of each cell located in theinformation (temperature and velocity fields, asnorth and south facades were covered with poly-well as heat flux values through the buildingstyrene plates. In the CEXP roof design, provi-enclosure or any other given surface of interest) tosions were taken to be able to evaluate otherassess passive cooling building applications. Thisfact makes it different from other lower levelevaluation codes (resistance models), and fromother general purposes commercial CFD models.For SPCS research purposes it is more convenientto have a homemade code than a commercial one,where the user can hardly access the source codefor making important changes or adaptations tonew situations.

4. GOVERNING EQUATIONS

The equations that govern the conjugated prob-lem of fluid flow and heat transfer are thetransient two-dimensional form of continuity,momentum, and energy equations, applied to ano-isothermal viscous incompressible fluid withconstant physical properties. Basically, these con-servation equations can be written as a generaldifferential equation (Patankar, 1981), in thefollowing way:

→≠f]1 =? ( pVf) 5=? (G =f) 1 S (1)≠t

Fig. 3. Dimensions of the cells and locations of temperaturewhere f is the relevant dependent variable,and relative humidity probes. Solar passive cooling system

(left) and reference system (right). (velocity, enthalpy, temperature, turbulent kinetic


energy, etc.). G is the corresponding diffusioncoefficient related to the variable f under consid-eration, and S is the source term.

5. BOUNDARY CONDITIONS

Heat transfer fluxes are specified as boundaryconditions in all external surfaces, except for thefloor, where soil temperature is specified asknown. At the walls and roof surfaces, thespecified unsteady heat fluxes include absorbedsolar radiation, convection due to wind and long-wave radiative exchange between surfaces and thesky. For the passive cooling system implemented Fig. 4. Air ambient temperature, and global and diffusein the roof, heat flux by evaporation or condensa- irradiance, August 1996.tion from or toward the water surface is taken intoaccount. Such boundary conditions include thefollowing. 7. RESULTS AND DISCUSSION• Time dependent functions of the outdoor tem-

Hourly average experimental results of testsperature (dry bulb temperature) and globalaccomplished during 16 and 14 consecutive dayssolar irradiance on vertical and horizontalof the months of August 1996 and January 1997,oriented surfaces, obtained from the measuredrespectively, and the corresponding numericalhourly data (Almao de Herrera, 1994).results are presented. These periods were selected• Constant external wind convective heat trans-to carry out comparisons of cooling capacity offer coefficient, because the corresponding aver-the SPCS, when the climatic conditions are lessage daily values of wind velocity do not varyfavourable (August), or more favourablesignificantly during the day.(January). Figs. 4 and 5 show the hourly average• Hourly average values of relative humidity,values of global solar radiation, diffuse radiation,corresponding to the considered month or day.and air ambient temperature measurements, for• Constant thermal and optical properties of thethe aforementioned periods of time. Wind velocityconstruction materials.varies from 0.7 to 2.1 m/s in August, and from• Soil temperature values (1-m depth measure-0.9 to 3 m/s in January, the lower values corre-ment, during the experimental period).sponding to the nocturnal period. Relative humidi-ty varies from 70 to 85% in August, and from 63to 83% in January, the higher values corre-6. COMPUTATIONAL DETAILSsponding to the nocturnal period. Measured hour-

The calculation domain for the CREF consistsof a 30331 nodal points grid, and for the CEXP a30330 nodal points grid is used. They have beenadjusted to the physical configuration of theexperimental cells. The method of the tangentrecommended by Patankar (1981) is used tolinearise the heat transfer vs. temperature curve ofthe radiative and evaporative heat fluxes. In theanalysis of the CEXP, due to solar control, theconfiguration of the construction section dependson the considered time period (diurnal or noctur-nal). In order to use the same grid for bothperiods, during the night, the nodal points corre-sponding to the air over the water pond and to theinsulating panel are blocked. Details of the solu-tion method are thoroughly described in Almao et Fig. 5. Air ambient temperature, and global and diffuseal. (1998). irradiance, January 1997.


Fig. 6. Comparison of simulated (s) and measured (m) temperatures in the CREF and CEXP, August 1996.

ly values of these climatic variables are used as simulation day, when the transient problem be-input data by EVITA, except wind velocity for comes cyclical stationary. It can be observed thatwhich the diurnal average value was considered the trend of the simulation curves is similar to the(1.3 m/s and 1.9 m/s for August and January, experimental results, showing a very good quali-respectively). In Figs. 6 and 7, the hourly average tative prediction. When comparing January andof measured indoor temperature (T ) vs. time, for August performance, it can be observed that bothin

the months of August and January, are shown experimental and numerical curves descend |2 Kalong with those obtained by EVITA (at the same in January. This is the same temperature differ-position where the experimental probe is located), ence existing for daily average ambient tempera-for a two-dimensional section east–west oriented. ture and water temperature. Albeit the code wasThe numerical values correspond to the third conceived to make comparative studies, good

Fig. 7. Comparison of simulated (s) and measured (m) temperatures in the CREF and CEXP, January 1997.


Table 1. Daily average, maximum, minimum and amplitude of measured and simulated results corresponding to August

August 1996: experimental period from August 13th to 28th. V 5 1.3 m/sw

CREF CEXP Climatic variables

T T T T T T Relative T (8C)in,m in,s in,m in,s amb,m amb,s ground

(8C) (8C) (8C) (8C) (8C) (8C) humidity (at 1.0 m)

Avg 31.3 31.36 28.9 28.58 29.5 29.52 0.75 32.0Max 32.7 32.45 30.4 29.25 33.4 33.35 0.85 32.1Min 29.9 30.25 27.6 27.75 26.1 26.25 0.60 32.0Amp 2.8 2.2 2.8 1.5 7.3 7.1 0.25 0.1

agreement between simulated and experimental T ). A summary of the more relevant averagein

values was obtained. The experimental and nu- results is presented in Tables 1 and 2. Figs. 8 andmerical daily average indoor temperatures are 9 show the net heat transfer numerical resultspractically the same (see Tables 1 and 2), and the through walls, roof, water and floor CEXP sur-curves are almost coincident during the first 14 h faces. This balance indicates that in August,for the CEXP both in August and January. The always there is a cooling effect when the watermaximum deviation becomes 1.3 K (4%) in pond is uncovered (night period), but in JanuaryAugust, and 1.7 K (5%) in January, corre- the water gains heat during the first 3 h aftersponding to hours 19 and 18, respectively. The uncovering the water pond. This would indicateaverage absolute errors are 0.3 K and 20.1 K that, in this month, under clear sky condition, it is(CEXP and CREF, August), 0.4 K and 0.5 K convenient to uncover the water pond 3 h after(CEXP and CREF January). These differences are sunset. Table 3 shows a summary of the dailymainly due to the following. total heat transfer for the considered cases.

(a) Heat gains through north and south facades These results permit determination of the hour-have not been considered. ly thermal load reduction obtained with the SPCS(b) The external surface to interior volume with respect to the CREF, whose roof is practical-ratio in the simulation differs from the ex- ly adiabatic. As shown in Fig. 10, taking aperimental one (1.25 vs. 2.31, respectively). comfort temperature of 258C as reference, daily(c) The wind convection coefficient, h has average reductions of 44% (August) and 69%w

been taken to be the same for all external (January) were obtained numerically, and of 41%surfaces which is not really true. (August) and 66% (January) experimentally.(d) There is an important radiation contribution Another important parameter to be evaluated isover the south facade in January because of the the mean cooling potential (MCP) of the passivelocal latitude. system. This parameter was calculated using an(e) Clear sky was considered in the simulation experimental value of the global steady state heat

´and hence, the long wave cooling radiation has transfer coefficient (UA), obtained by Gonzalezbeen overestimated in both cells. (1997) in the CREF cell. The UA value used wasAdditionally, these figures also show that the 73.6 W/K and it is based on the external surface

thermal lag of the T in the CEXP, relating to the convection–radiation heat transfer coefficients.in

atmospherical air, is 6 h. This behaviour coincides Thus, using this value and the differences betweenexactly with the experimental results. On the other daily average indoor temperature of the CREFhand, these numerical and experimental results (T ), and daily average indoor temperaturein CREF

show that the convenient period for using external of the CEXP (T ), the mean cooling po-in CEXP

air ventilation for this SPCS goes from 19:00 h to tential per unit area of the passive system, was09:00 h (when outdoor temperature is lower than estimated as:

Table 2. Daily average, maximum, minimum and amplitude of measured and simulated results corresponding to January

January 1997: experimental period from January 14th to 27th. V 5 1.9 m/sw

CREF CEXP Climatic variables

T T T T T T Relative T (8C)in,m in,s in,m in,s amb,m amb,s ground

(8C) (8C) (8C) (8C) (8C) (8C) humidity (at 1.0 m)

Avg 29.6 29.03 26.6 26.28 27.5 27.45 0.71 29.2Max 31.2 30.05 28.2 26.85 30.2 30.15 0.83 29.2Min 27.9 28.05 25.2 25.65 24.9 24.85 0.63 29.2Amp 3.30 2.00 3.00 1.20 5.3 5.3 0.20 0.0


Fig. 8. Numerical values of heat fluxes through external surfaces, floor surface, water and ground (at 1 m depth) for the CEXP,August 1996.

¯ ¯MCP 5 (UA)(T 2 T ) /A . (2) removed heat, these values of cooling potentialin CREF in CEXP roof2turn out to be 465 Wh/m day (August) and 577

2The numerical values of mean cooling potential Wh/m day (January).2 2were 22.7 W/m K (August) and 22.5 W/m K

7.1. Computational code sensibility(January), while the corresponding experimental2values were 19.4 W/m K (August) and The effect of wind velocity and evaporation

224.0 W/m K (January). In terms of daily- over the SPCS cooling potential were selected to

Fig. 9. Numerical values of heat fluxes through external surfaces, floor surface, water and ground (at 1 m depth) for the CEXP,January 1997.


2Table 3. Numerical values of daily total heat transferred through walls, floor and roof, in terms of hourly energy, kJ /m

East West Floor Roof Roofa awall wall surface interior

CREF (August) 36.8 43.4 1.5 252.6 23.5CEXP (August) 157.0 173.4 17.1 2371.2 2318.2CREF (January) 27.2 28.7 29.1 248.9 22.9CEXP (January) 142.1 152.2 23.4 2336.3 2283.7CEXP (EV) (August 25th) 185.5 177.4 15.7 2415.4 2348.5CEXP (NEV) (August 25th) 166.6 159.4 15.7 2381.6 2307.4

a Roof surface denotes that the heat transfer has been evaluated in the surface while roof interior denotes the heat transferredthrough internal nodes located in the polystyrene (CREF) or in the water.

Fig. 10. Thermal load reduction percentage of the solar passive cooling system in relation to the reference cell.

show the computational code sensibility to certain In the second case, the wind velocity was variedparameter changes. A particular day (25-08-96) to study the convection heat transfer effect overwas selected for its simulation with measured the SPCS performance. Both of them were con-values as input data. In the first case, leaving fixed sidered under clear sky conditions and the respec-the wind velocity, evaporation was not permitted tive thermal performance is shown in Figs. 11 andand the heat flux through the water was evaluated. 12. It is important to notice that the obtained

Fig. 11. Predicted thermal performance of the SPCS with evaporation (EV) and with no evaporation (NEV).


Fig. 12. Comparison of simulated indoor and metal roof temperature for different wind velocities.

variations show how the numerical code responds faces in percentages from 41 to 66%, into the change of some variables. relation to an adiabatic roof cell.

• The average cooling potential of the studied27.1.1. Effect of the evaporation on the cooling SPCS, experimentally obtained is 19.4 W/m K2

2potential. A difference of 211.4 Wh/m day (August) and 24.0 W/m K (January), whichbetween the heat transfer through the water, when in terms of daily removed heat turn out to be

2 2evaporation was not permitted, was obtained (see 465 Wh/m (August) and 577 Wh/m2Table 3); and a difference of 215.8 Wh/m was (January).obtained during the night period. According to • Under clear sky condition, 16% of the totalthese results, it can be noted that 16% of the night SPCS cooling is due to evaporation.water-cooling is due to the evaporation under • Under clear sky condition, the cooling capacityclear sky condition, since the nocturnal radiation of the SPCS reduces as the wind velocityrepresents the maximum contribution to the total increases, however, the indoor temperaturecooling. For the indoor temperature this represents decreases indicating that the wind convectivea difference of 0.4 K, and the water temperature cooling effect of the walls is more importantfalls 1 K due to the evaporation, as shown in Fig. than the water heat gain by convection.11. • The SPCS experimental results have demon-

strated that the computational model EVITA7.1.2. Effect of the wind velocity on the coolingconstitutes a very useful tool for the study,potential. Nocturnal radiation cooling diminishesdesign and optimisation of energy-efficientwhen wind velocity increases due to heat gain byhousing.convection. Since nocturnal radiation cooling

prevails on the total cooling then, lower watertemperatures are obtained when the wind velocity

Acknowledgements—This work was financially supported bydecreases, as shown in Fig. 12. However, the CONDES (Scientific Development Council of the Universityindoor temperature decreases when the wind of Zulia).

velocity increases, even though the SPCS coolingcapacity has been reduced. This result indicatesthat, under clear sky condition the cooling effect REFERENCESof the walls by wind convection is more important

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