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valuation of a Trombe wall system in a subtropical location
duardo Krüger ∗, Eimi Suzuki, Adalberto Matoskiepartment of Civil Construction, Universidade Tecnologica Federal do Parana, UTFPR, Campus Curitiba - Sede Ecoville, Rua, Deputado Heitor Alencarurtado, 4900, Postcode 81280-340 Curitiba, Brazil
r t i c l e i n f o
rticle history:eceived 22 April 2013eceived in revised form 22 May 2013ccepted 10 July 2013
eywords:rombe wall
a b s t r a c t
Trombe wall systems are based on the use of solar gains and the stack effect in winter, which takes placethrough an air gap between a glazed fac ade and a heat-absorbing wall with high thermal mass. The solarorientation of the wall and the vertical angle should allow solar gains in winter while minimizing thiseffect in summer. The correct operation of the system is based on the principles of heat storage fromdirect gains and of natural ventilation by stack effect, sometimes enhanced by forced convection. Withthe aim of analyzing the healing/cooling potential of a Trombe wall system, two test cells were built
3
hermal performancehermal monitoringest cells
with an internal volume of 5.4 m , one of them with a naturally ventilated Trombe wall attached to itand another one without it (reference test cell). Indoor temperature measurements were carried out incold periods of 2011 and during summer 2012. Results suggest a higher performance of the Trombe wallsystem relative to the reference test cell, which is more pronounced under cold conditions. In addition,an assessment of the seasonal benefits of the system was carried out by means of indoor temperaturepredictions of the two test cells for three subtropical locations.
. Introduction
The first to present the concept behind Trombe walls wasdward S. Morse, who patented it in 1881 [1]. Later, the Frenchngineer Felix Trombe and the French architect Jacques Michelntegrated Trombe walls as architectural elements in buildings andopularized the idea in the 1960s [2].
The Trombe wall system consists of an indirect solar passiveystem [3] where an outside glazed surface allows solar radia-ion as incoming visible light and near-visible shortwave radiationo warm up a high-mass heat storage wall, but preventing longave infra-red radiation to radiate back to the outside (greenhouse
ffect). Current design has added bottom and top air vents to thelazing frame to increase buoyancy through the “solar chimney”the air space between glazing and high-mass wall). In addition,he heat-absorbing wall, which normally is painted black, has twopenings at the bottom and at the top to allow air flow through ther-osyphon to the interior, which can be shut at night with dampers.
he system should be positioned at an elevation of the building fac-ng the equator, so as to have a greater irradiance potential in wintereriods (Fig. 1).
In winter, the solar heat is absorbed and stored in the high-massall for the night time period. The appropriate use of the air vents
f the wall can further increase air changes between chimney and
interior, as the stack effect is enhanced by incoming solar radiationthrough the glass. In summer, by closing the air vents of the heat-absorbing and storage wall with dampers, and thanks to a highersolar elevation, the warming effect is in great part neutralized. Fur-thermore, under such conditions, a cooling effect can take place. Asa means of enhancing the cooling effect from natural ventilationthrough the chimney it is also suggested that the lower and/or theupper air vent of the storage wall are used to extract warm air fromthe interior [4].
The spacing between wall and glazing is crucial for preventingheat losses through the glass. In addition, the flow rate, necessaryfor increasing convection through the air vents of the storage wallin the heating mode and for promoting more efficient natural ven-tilation in the cooling mode, was found to increase with gap width,due to a reduced frictional resistance. The chimney width is sug-gested to be from about 3–6 cm [5,6]. From CFD-based computersimulations of different Trombe wall models, Liping and Angui [7]recommend the use of an optimal ratio of air gap width to chimneyheight.
A driving force of the stack effect is the impinging radiationthrough the glass. In locations close to the Tropic, the use of bothsolar gains and increased stack effect for indoor ventilation can bebeneficial in winter while preventing direct solar gains throughthe glazing during the warmest hours in summer, with no need
of overhangs summer shading. Stasinopoulous [8] questions therelevance of solar gain systems through the equatorial side of abuilding, showing that if one considers the entire winter period, thesouth-facing (for the northern hemisphere) of a building receives
E. Krüger et al. / Energy and Buildings 66 (2013) 364–372 365
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during summer 2012. Indoor temperature was measured in themiddle of each test cell by means of HOBO T/RH H08-003-02 dataloggers, which were calibrated before field monitoring and havean accuracy of ± 0.4 ◦C. Data were recorded in 5-min intervals and
Fig. 1. Unvented Trombe wall (left); Vented Trombe wall in w
ess energy than a horizontal surface. Although this might apply toemperate, high latitude regions such as London (51.5 ◦N) and, to aertain extent, to Athens (38 ◦N), for locations closer to the equator,his might not be the case.
The motivation for the present study was to evaluate the ben-fits of a Trombe wall system both for cooling and heating in aubtropical location in high elevation (Curitiba, at 25◦ 30′ S, 49◦
0′ W, 910 m above mean sea level). Local climate is characterizedy a mild winter period and seasonal fluctuations. Solar irradiance
n winter solstice for this latitude for on equatorial, north-facingurfaces is about 20% higher than on horizontal surfaces. In sum-er, there is a reverse relationship: a horizontal roof would receive
pproximately 15 times more energy than a north-facing wall.
. Methods
The study was conducted from measurements in two 5.25 m3
est cells, which were specifically designed for testing the Trombeall system under subtropical conditions. The research comprised
hree steps: (1) construction of the test cells, (2) on site measure-ents and (3) indoor temperature predictions and assessment of
he seasonal performance of the system for three subtropical loca-ions.
.1. Building the test cells
The two test cells (a reference test cell and another with therombe wall system installed on a north-facing fac ade) have theame floor area (2.6 m2) and internal volume (5.25 m3) and are sit-ated side by side, with a 1.5 m distancing in between in ordero avoid overshadowing. The geometry and dimensions of the testells resemble the test cells used in a study carried out in Israelor testing a roof cooling system [9]. The internal height is 2.0 mfloor-ceiling) and the internal plan has the dimensions of 1.6 mor north-south and east-west facing walls (Figs. 2–4). The walls ofhe cells are made from 9 cm thick hollow concrete blocks with noxterior insulation layer which were left unplastered. The floor con-ists of a 10-cm thick concrete slab, again with no insulation. Theell’s roof is similarly built from a 10-cm thick concrete slab, whichas made impervious, with a 5-cm interior layer of expandedolystyrene insulation. Each cell has a wooden non-insulated door
ocated on the south wall. The equatorial elevation of the test cellith the Trombe wall system has an increased thickness of 24 cmhich was attained by using the two rows of the same concrete
locks that were laid flat and filled with concrete rubble to enhancehermal mass. Only the north-facing wall of the test cell with therombe wall was painted black, the remaining walls of both cellsnd the roof were left unpainted.
mode (center); Vented Trombe wall in summer mode (right).
Attached to the north-facing wall of the Trombe system isa 1.50 × 2.00 m2 aluminum window frame in light color. Doubleglazing was used, with the external glass layer with 4 mm of thick-ness and the internal glass 5 mm thick. The non-ventilated air gapbetween glass layers is 8 mm thick. The use of double glazing issuggested by Gan [4] as a means of reducing heat losses in winterand enhancing passive cooling in summer. The optimal air gap tochimney heigth ratio of 1/10, as suggested by Liping and Angui [7],was adopted: as the chimney is 2.0 m high, its width is 0.20 m.
Horizontal air vents were provided at the bottom and at the topof the frame with the same opening area of 1.40 × 0.15 m2, whichcould be opened/tilted/shut by a hinge.
Two large openings were made in the storage wall at two dif-ferent heights. The lower air vent is at 20 cm and the upper ventat 1.85 m above the floor and immediately underneath the ceil-ing. Each opening has an area of 0.90 × 0.20 m2. As dampers, twostyrofoam blocks were cut to fit the air vents of the storage wall.
The openings were numbered as follows: ‘air vent 1’ at the bot-tom of the aluminum frame; ‘air vent 2’ at the lower section of thestorage wall; ‘air vent 3’ at the top of the aluminum frame; ‘air vent4’ at the upper section of the storage wall (Fig. 5). Fig. 6 presentsboth prototypes located at the campus of the Technological Uni-versity of Parana, UTFPR, in Curitiba (25◦ 30′ S, 910 m above meansea level), Brazil.
2.2. Thermal monitoring
Measurements were carried out in cold periods of 2011 and
Fig. 2. Floor plan of the Trombe wall test cell.
366 E. Krüger et al. / Energy and Buildings 66 (2013) 364–372
Fig. 3. Sections AA/BB.
A and
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Fig. 4. Section A
ampled as hourly temperature data. The loggers were hung at
.5 m from the floor slab by a nylon string from the top of theell and wrapped in a thin aluminum foil to minimize long-waveadiant heat gains from the internal surfaces. Externally, a logger,
Fig. 5. Air vents of the Trombe wall system.
North Elevation.
covered with aluminum foil, was placed inside a 50 cm long PVCtube with 10 cm of diameter at 1.5 m from the ground, which cre-ated a naturally ventilated shield. For the summer measurements,a fully equipped, rooftop (at approximately 15 m from groundlevel) HOBO H21-001 weather station was employed for obtainingrelevant climate data: air temperature, relative humidity, windspeed and direction and global solar radiation. In addition, a tem-perature logger was hung in a mid distance from top in the solarchimney.
Results are presented here two-fold: (1) as normalized data for
clear-sky days for different operation modes of the Trombe wallsystem; (2) as summary tables with differences between indoorsand outdoors and those found between the test cells (control versusTrombe wall system).
Fig. 6. View of the test cells with the Trombe wall system to the left.
d Buildings 66 (2013) 364–372 367
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Fig. 7. Normalized indoor temperatures for the Trombe wall system during the cold
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.2.1. Cold periodThree operation modes were tested during the cold periods:Operation mode 1: air vents 1 and 3 shut, air vents 2 and 4 open
May 24th–29th 2011). Although the measurements were carriedut during the mid-season, ambient temperatures were consider-bly low throughout the monitoring (4.4–22.7 ◦C) as a result of aold front. This configuration is the recommended set-up for day-ime in cold periods, with the chimney serving as a greenhouseuring the day and with a permanent stack effect/air exchange withhe interior through air vents 2 and 4.
Operation mode 2: air vent 3 shut, air vents 1, 2 and 4 open (May1st–June 5th 2011). The same cold front was used for testing aecond configuration, with outdoor conditions very similar to therst setting (4.8–21.1 ◦C). In this second configuration, we testedhe possible benefit of enhancing air flow internally by letting warmir from the outside through the lower air vent of the frame duringhe day (with the drawback of allowing heat losses through the airet during the night).
Operation mode 3: the same as operation mode 1, but withampers in the storage wall openings between 6 pm and 8 amSeptember 19th–24th 2011). Outdoor conditions were somewhatigher than for the two previous modes (10.4–27.9 ◦C). The advan-age, in comparison to operation mode 1, is avoiding heat lossesuring night time, thorugh the two openings of the storage wall.
Comparisons are shown in terms of normalized data for a dailyycle, according to the procedure adopted by Pearlmutter andosenfeld [9]. Hourly temperatures for the three operation modesere normalized by multiplying temperature data of the test-cellith the Trombe wall system temperature by the ratio between
he average control cell temperature and the control cell temper-ture for the given period. For the normalization only clear daysere chosen out of each data series. The Year Day (YD) varied
ubstantially from the two first modes to the third, however max-mum solar irradiance for the three clear days was comparable10–590–770 W/m2 (global solar radiation data registered at theearest meteorological station - SIMEPAR).
The graph (Fig. 7) shows that in all three modes there is a timeag of about two hours relative to the external ambient tempera-ure cycle, which is due to the effect of thermal mass. Operation
ode 2 presents the most favorable conditions in terms of main-aining indoors warmer throughout the day while reducing thendoor diurnal temperature swing. Keeping the lower air vent ofhe frame open in order to provide an increase of the stack effect
s thus a viable strategy for the climate conditions analyzed. Theelective operation of the air vents in the storage wall (mode 3)id not suffice for promoting enough warming of the indoor spaceuring daytime.
able 1ndoor and outdoor conditions for each operation mode of the Trombe wall system during
Week-period Mode 1
Out Trombe Ctrl
Tmin (avg) 10.1 12.9 12.0
Tmax (avg) 18.6 22.6 21.4
Tavg 13.7 16.8 15.7
Diff (test-control) avg/std dev 1.1/0.60 1.9/0.45 0.6/0.50Diff (test-out) avg 3.1 4.6 2.4Diff (test-out) min 2.8 3.6 2.4
Clear day Mode 1
Out Trombe Ctrl
Tmin 4.2 8.6 7.0
Tmax 16.8 22.5 20.6
Tavg 8.8 13.9 12.1
Diff (test-control) avg/std dev 1.8/0.37 2.2/0.54 0.6/0.86Diff (test-out) avg 5.2 4.5 2.9Diff (test-out) min 4.5 3.7 2.3
spells for operation mode 1 (YD 149), operation mode 2 (YD 152) and operation mode3 (YD 262) over the background of the average outdoor temperature for the threedays.
Table 1 summarizes the observed effects for the whole week-periods and for the clear sky conditions under each configuration.
Operation mode 3 presents the smallest average difference tothe control test cell under both conditions (week averages and clearsky conditions). The highest difference to the control test cell isnoticed for operation mode 2, in both cases. With regard to temper-ature differences to outdoors, the week-period data set indicatesmode 2 as the best configuration, but during the clear day, mode1 shows a slightly higher temperature difference to the exterior(5.2 ◦C against 4.5 ◦C). Since the difference in average temperaturesbetween both cells for the week is significantly higher than for theclear day (about twice as much or 1.5 ◦C), operation mode 2 can stillbe considered as the best option.
2.2.2. SummerMeasurements in summer included the air temperature in the
chimney and had additional data from a nearby weather station.Four operation modes were tested:
Operation mode 1: all air vents shut (December 29th 2011–3rd
January 2012). Since no direct solar radiation reaches the chim-ney due to the high solar elevation near the summer solstice, byadopting this mode we avoid air flow from the outside throughthe chimney and cancel air changes with the interior. The major
the cold spells, comparatively to the control test cell (week-period and clear day).
Fig. 8. Normalized indoor temperatures for the Trombe wall system in summer foroperation mode 1 (YD 3), operation mode 2 (YD 8), operation mode 3 (YD 15) andoperation mode 4 (YD 24) over the background of the average outdoor temperaturef
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2.3. Seasonal performance of the system
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or the four days.
ffect from the system on indoor temperature conditions is due tohermal mass.
Operation mode 2: air vents 1 and 4 shut, air vents 2 and 3pen (5th–10th January 2012). This mode tests the extraction effecthrough the upper vent of the frame. The driving force is the outgo-ng air flow through the vent which is promoted by the stack effectn the chimney.
Operation mode 3: air vent 2 shut and all other vents open12th–17th January 2012). There is a stack effect in the chimneyhen external air rises and leaves the chimney through the upper
ent; an extraction of indoor air is attempted by opening the upperir vent in the storage wall.
Operation mode 4: air vents 2 and 4 from the storage wall shut,ir vents 1 and 3 from the frame open (19th–29th January 2012).he same strategy as above, but without extracting air from thenterior.
Again, comparisons are shown in terms of normalized data for aaily cycle (Fig. 8). In this case, the variation in Year Day (YD) wasmall, as measurements were conducted during the same month of
anuary. Maximum solar irradiance was a defining factor for choos-ng a clear day for analysis, all four days exhibited sunny to partly
able 2ndoor and outdoor conditions for each operation mode of the Trombe wall system in sum
Week-period Mode 1 Mode 2
Out Trombe Ctrl Out
Tmin (avg) 15.1 16.8 16.1 16.4
Tmax (avg) 23.3 24.9 26.8 25.2
Tavg 18.4 20.4 20.5 19.6
Diff (test-control) avg/std dev −0.1/1.1 0.0/0.8 −0.3/0.9 −0.3/1.2Diff (test-out) avg 2.1 2.5 1.7 2.3Diff (test-out) max 1.7 2.3 −0.1 2.5
Clear day Mode 1 Mode 2
Out Trombe Ctrl Out
Tmin 16.3 18.5 17.8 16.6
Tmax 27.1 29.5 32.9 24.9
Tavg 20.7 23.5 24.2 20.2
Diff (test-control) avg/std dev −0.7/1.5 −0.1/0.8 −0.5/1.1 0.04/1.0Diff (test-out) avg 2.8 2.2 1.0 2.4Diff (test-out) max 2.4 2.8 −0.5 4.1
dings 66 (2013) 364–372
cloudy sky, but with all maximum irradiance of about 1200 W/m2
(global solar radiation data registered at the weather station).From Fig. 7, the different operation modes overlap and confound
the analysis. There is a perceivable time-lag in operation modes 2and 4, comparatively to the other two modes. Whereas for mode 2it seems that the suction effect was not strong enough to promotea rapid heat removal from indoors (as under mode 3, where thecooling rate closely follows the decrease in outdoor temperature),for mode 4 the thermal mass played a major role in the observedtime-lag. The expected time lag under mode 1 (thermal mass effect)did not take place: in this case, the greenhouse effect in the chimney(with temperatures inside the chimney being the highest amongall operation modes – maximum temperature during the clear dayexceeding 37 ◦C; well above the 30–32 ◦C of the other modes) actedas an extra and relevant heat source.
Table 2 indicates that mode 3 is more advantageous than theothers particularly because under this operation mode the max-imum indoor temperature is slightly below outdoor’s, while thisdoes not occur in the other modes. Also noteworthy is the fact that,under mode 3, the difference between mean indoor and outdoortemperature is the smallest among the monitoring periods as wellas for the clear day. However, the average temperature difference tothe control cell is not quite evident and a closer inspection of Table 2will indicate that mode 1 yields the highest maximum temperaturedifference to the control cell on a clear day (3.4 ◦C against 2.6 ◦C).
For testing the significance of the average and maximum tem-perature differences to the outdoor conditions, Tables 3 and 4present the percent changes of the temperature difference in theTrombe wall relative to the control test over the temperature dif-ference found between the control test cell and outdoors, for eachoperation mode and for the minimum (cold period) and maximumtemperatures (summer). Such percent changes give a proxy of thepassive heating/cooling effect of the Trombe wall system relativeto the control cell. Operation mode 3 exhibits the greatest reduc-tions in terms of maximum temperatures in summer (Table 4).Even though the control cell also exhibited a reduced deviationto outdoor conditions under mode 3, the percent change betweenboth points to the cooling benefits of mode 3. In the cold period(Table 3), operation mode 2 presents the highest percent changesand therefore the strongest passive heating effect, thus confirmingobservations made before regarding the cold period.
The method of indoor temperature prediction can be used toestimate the performance of a given passive system under other
mer, comparatively to the control test cell (week-period and clear day).
Table 4Percent changes/passive cooling effect of the Trombe wall system in summer (highlighted the best configuration).
Mode 1 Mode 2 Mode 3 Mode 4
Test-ctrl(max)
Crtl-out(max)
% change(cooleffect)
Test-ctrl(max)
Crtl-out(max)
% change(cooleffect)
Test-ctrl(max)
Crtl-out(max)
% change(cooleffect)
Test-ctrl(max)
Crtl-out(max)
% change(cooleffect)
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Week-periods −1.8 3.5 52 −1.3 3.6
Clear-sky day −3.4 5.8 59 −1.7 4.5
limatic or geographic conditions, provided that results prove noto show significant dissimilarities and too great deviations fromhe base case [10,11]. For a seasonal assessment of the perfor-
ance of the system, a typical reference year was defined asase climatic conditions for analysis. This year was obtained fromata registered at the local meteorological station (SIMEPAR, ‘Sis-ema Meteorológico do Paraná’), located approximately 12 km fromhe site, and for a database corresponding to the 10-year period998–2007. The test reference year TRY was determined from theIMEPAR database, according to a standard procedure [12]. 2001as determined as the TRY for that period.
Curitiba (25◦ 30′ S, 49◦ 20′ W, 910 m above sea level) has a highlevation, which is responsible for the coldest winter among allrazilian capitals. The climate is humid subtropical and accord-
ng to Koeppen’s classification, the climate type is Cfb. Great dailynd seasonal fluctuations of the air temperature characterize locallimate.
Hourly predictions of the indoor temperature conditions in bothest cells were made based on following considerations:
There is an observable urban heat island effect at the monitor-ng site, relative to the meteorological site; the location presentseatures of a rural (or peri-urban) location, as the area borders therban outline of the city, the meteorological site, from which dataere used for the winter periods of analysis, is in the urban area.
his effect should be considered in temperature predictions for theinter period (for the summer period, onsite weather data were
vailable);The relevant variables for generating multiple regression equa-
ions for each operation mode were: the standard hourly dataollected at meteorological stations (air temperature ‘Ta’ and rel-tive humidity ‘RH’, wind speed ‘v’ and global solar radiation ‘Ig’),heoretical clear-sky global solar radiation ‘Ig(c-s)’ and solar eleva-ion ‘sol-elev’ [14] and, to account for the time lag due to thermal
ass, running hourly differences of the air temperature for fourifferent time steps: 1 h ‘Lag 1h’, 3 h ‘Lag 3h’, 6 h ‘Lag 6h’ and 16 h
Lag 16h’, which gives a proxy of the diurnal temperature rangeDTR’ [13].
.4. Predictive formulae
By means of multiple regression analysis, formulae were gener-ted for the hourly indoor temperature in the Trombe wall systemnder different modes, using hourly weather data and theoret-
cal solar data as independent variables with STATISTICA (datanalysis software system), version 8.0 (www.statsoft.com). As pre-iously mentioned the urban heat island (UHI) effect was takennto account for the cold period, as there is a substantial distance
−2.2 2.1 106 −2.0 4.5 45−2.6 2.1 125 −1.9 6.0 31
between the sites and differing morphology features between sites.For the cold period, the first step was to introduce a simple correc-tion of ambient temperature in order to account for the UHI effect, alinear regression between the site’s ambient temperature and thatof the meteorological station of SIMEPAR was carried out. A regres-sion formula was generated and the resulting formula for the indoortemperature added thus such an effect (Table 5).
In the same manner, a formula was developed for the controltest cell (Table 5). The data set for such predictions was longer thanfor the individual modes (428 cases against 140).
The only variable positively affecting heating in the Trombe wallsystem is the ambient temperature (added with the UHI effect) withPearson correlation coefficient (r) of 0.65. Variables, which reducethe heating effect, are the relative humidity (r = −0.59) and the 3-h time lag in air temperature (r = −0.81). Thus, the conjunction ofhigh humidity and rapid changes in air temperatures will have anegative effect on the passive heating potential of the system.
For the summer period, since there were temperature measure-ments in the solar chimney in each mode, two regressions werecarried out: one for the air temperatures inside the chimney andanother for the interior of the Trombe wall system (Table 6). Aspointed out above, outdoor variables were from the weather stationlocated a few meters from the test site.
In summer, the affecting variables in the cooling potential ofthe Trombe wall system are the ambient temperature (r = 0.86, inEq. (4), with the Chimney temperature) and the theoretical clear-sky global solar irradiance (r = 0.51, in Eq. (4), with the Chimneytemperature; and with a low r = 0.23, in Eq. (5), with the indoortemperature). Negatively affecting the indoor temperatures in theTrombe wall system are, again, the relative humidity and the timelag and, in addition, the measured incoming global solar irradiance(with a low r = −0.35, in Eq. (5), with the Chimney temperature –in this case the difference between theoretical and measured datais used in the equation).
2.4.1. Applying the predictive formulae to local climate dataThe formulae developed for the best operation modes in the
cold period (Eqs. (1) and (2)) and in summer (Eqs. (4) and (5))were applied to the local TRY, allowing an assessment of theseasonal performance of the Trombe wall system. Comparisonswere drawn between the expected indoor temperatures in the testcell with the Trombe wall system, relative to the predicted per-formance of the reference test cell with hollow concrete blocks,
in summer and winter. The 3 months of each season, i.e. fromsolstice to equinox, were used as periods of analysis. Resultsare presented in terms of indoor temperatures (average max-ima in summer, average minima in winter and their “absolute”
F(4,139) = 344.05;p < 0.0000;Std. error of estimate: 0.8 ◦C
Tctrl (hourly indoor temperature in thecontrol test cell in ◦C)
Tctrl = 3.892865 + 0.209145 ∗ v + 1.195966 ∗ T − 0.071631 ∗ RH −0.907196 ∗ Lag 3h (6)
R2 = 0.9431; adjusted R2 = 0.9428;
F(4,691) = 2864.6;
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with hot summers and cold and rainy winters (for Brazilian stan-dards). According to Koeppen’s classification, the climate type isCfa. The three locations, including Curitiba, are subtropical and lie
Table 7Indoor temperature predictions for Curitiba’s TRY for Tbase 24 ◦C (cooling) and 18 ◦C(heating) – temperatures given in ◦C.
ounterparts – period’s maximum and minimum) and as the sumf degree-hours for either cooling (summer) or heating (winter).he degree-hours procedure is a simplified, practical method foretermining cumulative temperatures over the course of a season.riginally designed to evaluate energy demand and consump-
ion, degree-hours are based on how far the average temperatureeparts from a pre-defined comfort level (‘Tbase’ or base tempera-ure). The sum of degree-hours accumulated in a day is proportionalo the amount of heating/cooling to keep a building within comfortonditions.
For both test cells, the average indoor temperature (meanetween both cells) was adopted as Tbase for the two periods,o that deviations to the mean value could be taken into account,xpressed in terms of cooling/heating degree hours, thus Tbase was4 ◦C for cooling and 18 ◦C for heating. Table 7 presents resultsbtained for Curitiba, using as input climate data from the local,pdated TRY. For an immediate assessment of the cooling poten-ial, the rationale was to see what improvements the test conditionsresent to the reference test cell (crtl-test) relative to a basic con-ition (ctrl-out). The heating potential was calculated according tohe relation test-ctrl/test-out.
In summer, indoor temperatures are above outdoor’s, as it woulde expected in a small test cell with limited air changes. The Trombeall system outperforms the concrete blocks test cell, exhibiting a
elative reduction in degree-hours for cooling of 33% to the lat-er. In winter, the relative reduction in degree-hours for heating isomewhat higher (36%), suggesting that the Trombe wall system is
lightly more advantageous as a winter strategy for such climate.his fact is also supported by the presence of average temperatureifferences between both test cells which are larger in winter than
n summer, meaning that the Trombe wall system is more capable
p < 0.0000;Std. error of estimate: 1.1 ◦C
of reducing cold stress in comparison to the concrete blocks testcell.
Apart from Curitiba, two other subtropical capital cities in thesouthern region of Brazil were considered for analysis: Florianópo-lis (27◦ 30′ S, 48◦ 30′ W, on the coast: Island of Santa Catarina); andPorto Alegre (30◦ S, 51◦ 10′ W, at sea level).
Florianópolis is located on the Island of Santa Catarina. The cli-mate is humid mesothermic without a dry season (Cfb, accordingto Koeppen) and is characterized by a high humidity, hot summersand mild winters. Porto Alegre has a temperate subtropical climate,
ithin a 5-degree latitude range, southern from Greater São Paulo,hich is crossed by the Tropic of Capricorn at 23◦ 26′ 16′′ S.
For each location, the corresponding test reference year (TRY)as used: for Florianópolis (TRY corresponding to year 1963) and
orto Alegre (TRY from 1954), from the database available atww.labeee.ufsc.br; for Curitiba, as mentioned, from an updated
ime series. Again, indoor predictions for both test cells (with therombe wall system and without it – reference test cell) wereerformed for winter and summer months. The same base tem-eratures used for Curitiba were applied to the other locations foregree-hour calculations (Tbase cooling = 24 ◦C, Tbase heating = 18 ◦C),o allow direct comparisons. Grouped results for Florianópolis andorto Alegre are presented in Table 8.
Among the three locations, Curitiba exhibits the highest thermaltress due to cold in winter and Porto Alegre the highest thermaltress due to heat in summer. For such subtropical locations, theres always a benefit of using the Trombe wall system, relatively tohe reference case (‘Ctrl’), with winter performance generally out-eighing summer performance (noteworthy is the fact that in all
ocations the heating output is higher than the cooling output).
ig. 9 illustrates those findings, showing that the highest potentialf adopting the Trombe wall system for both summer and winteronditions is for Curitiba than for the other locations. In addition,
ig. 9. Percent changes in cooling and heating degree-hours relatively to the refer-nce test cell (‘Ctrl’) for the three subtropical locations.
18.7 15.4 17.6 16.646%
under such climatic conditions, the discrepancy in percent varia-tions to the reference test cell between seasons is smaller.
3. Discussion
As observed by Liping and Angui [7], the stack effect, necessaryfor increasing air flow between the solar chimney and the interioris heavily dependent on radiation intensity. The best configurationin the cold period was with the air lets of the framing shut andwith the air vents of the storage wall open, favoring natural venti-lation between the solar chimney and the interior. The comparisonof the three possible operation modes on clear days showed that itsperformance excelled the others. The selective natural ventilation(only during daytime) proved not to be adequate for warming upthe indoor space and this is perhaps related to the lack of sufficientventilation of the indoor space during the day and to the fact that nothermal insulation has been applied to the remaining walls of thetest cell. Lebens [3] gives the example of the Odeillo houses, builtin 1967 in France, where the very massive concrete walls (600 mm)require 10–15 h for heat to travel through them. The Odeillo systemis based on natural circulation through the walls during the day andon the covering of the air lets with dampers during night time. Theremaining walls of the house are well insulated, which is not thecase of our test cell.
Findings from Balcomb and McFarland from 1978 (cited bySaadatian et al. [15] in a review of Trombe wall systems and per-formances) showed that the use of controlled vents (with dampers)does not significantly affect the performance of Trombe walls inmild climates. In severe climates, however, the use of selective ven-tilation and dampers during specific periods of the year, such asto prevent reverse air flow during night time, will bring a 10–20%increase in performance.
Summer results are in agreement to CFD studies performedby Hami et al. [16] testing air flow simulations in vented solarchimneys. The authors reinforce the importance of vents as relevantcontrol mechanisms both in heating and cooling a given building.From CFD simulations Hami et al. [16] conclude that the use of ventsthrough the glazing frame keeping the upper air vent of the stor-age wall shut reduces heat gain through the storage wall. Thosefindings were for Bechar (31◦37′ N, altitude 813 m), whose climaticand geographic features (in terms of latitude range) do not deviatemuch from our experimental site’s. We found that operation mode3, with the upper air vent of the storage wall closed while keeping
all other vents open, has the highest potential for indoor cooling insummer.
As pointed out by Saadatian et al. [15], Trombe walls can pro-mote heating and cooling of buildings, once they are properly
perated. For subtropical regions such as those evaluated in thistudy, the use of passive design tools such as Mahoney Tables [17]r Givoni’s building bioclimatic chart [18] will almost invariablyield recommendations for solar access in winter. This is a commonractice in the south of Brazil. The greatest benefit of the Trombeall system, however, is that it can promote solar heating from aorth elevation while not generating excess heat in summer with aubstantial cooling effect during this season. Since equatorial eleva-ions are already regarded as relevant in winter and this measure islready being reinforced by bioclimatic and energy efficiency stan-ards in Brazil [19–21], Trombe walls could easily fit to currentassive building design.
As pointed out before, the system could benefit from additionalhermal insulation for the building envelope which could preventhe stored heat (or cold) from traversing the non-equatorial walls,hus contributing to a higher efficiency.
. Conclusions
Air temperature measurements conducted in the Trombe wallystem showed small but significant differences among possibleperation modes of the system. The comparison to a standard build-ng without the system (control cell) yielded a high potential forooling and heating of the indoor space, which is consistent to liter-ture estimates of about 30% a year in reducing energy consumption6].
Particularly for the subtropical locations analyzed, the systemerformed reasonably well due to the high solar elevation in sum-er, which requires no need for shading devices or overhangs.
urthermore, since the winters are mild in this region, special fea-ures such as the insulation of the other walls and of the backside ofhe storage wall may not have affected too much the performancef the system.
The method of indoor temperature prediction for the regionithin a 5-degree latitude range yielded consistent results. Brazil’s
oldest capital, Curitiba, showed a more even balance betweenooling and heating performance.
Results indicated possible improvements of the system whichould be tested in future experiments: (1) to insulate the remainingalls of the Trombe wall test cell; (2) to insulate the backside of
he storage wall [according to Saadatian et al. [15], that reportedly
ould enhance the performance of the system by up to 56%]; (3) tose automated opening and shutting of the air vents of the framend of the dampers if the storage wall for testing different periodengths when the air vents are open/shut.
[
dings 66 (2013) 364–372
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