Energy and Buildings 41 (2009) 615–621

Enhancement of natural ventilation in buildings using a thermal chimney

Kwang Ho Lee a,*, Richard K. Strand b

a University of California at Berkeley, Berkeley, CA, United Statesb University of Illinois at Urbana-Champaign, Champaign, IL, United States

A R T I C L E I N F O

Article history:

Received 6 October 2008

Received in revised form 2 December 2008

Accepted 21 December 2008

Keywords:

Thermal chimney

EnergyPlus

Parametric analysis

Cooling and heating potential

A B S T R A C T

A new module was developed for and implemented in the EnergyPlus program for the simulation and

determination of the energy impact of thermal chimneys. This paper describes the basic concepts,

assumptions, and algorithms implemented into the EnergyPlus program to predict the performance of a

thermal chimney. Using the new module, the effects of the chimney height, solar absorptance of the

absorber wall, solar transmittance of the glass cover and the air gap width are investigated under various

conditions. Chimney height, solar absorptance and solar transmittance turned out to have more

influence on the ventilation enhancement than the air gap width. The potential energy impacts of a

thermal chimney under three different climate conditions are also investigated. It turned out that

significant building cooling energy saving can be achieved by properly employing thermal chimneys and

that they have more potential for cooling than for heating. In addition, the performance of a thermal

chimney was heavily dependent on the climate of the location.

� 2008 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Energy and Buildings

journal homepage: www.e lsev ier .com/ locate /enbui ld

1. Introduction

As architects and engineers continue to search for better waysto improve both the indoor environmental quality and energyefficiency of buildings, cooling buildings using natural ventilationcontinues to be an approach that is considered highly desirableprovided that enough air movement can be provided and thatthermal conditions of the exterior environment can providecooling. One method for increasing the air movement through abuilding and thus potentially increasing the thermal benefits ofnatural ventilation is by implementing one or more thermalchimneys within a design. As illustrated in Fig. 1, a thermalchimney is a vertical shaft that utilizes solar radiation to enhancethe natural ventilation in buildings. This is achieved as a result ofthe fact that the solar energy causes a temperature rise as well as adensity drop in the air inside the thermal chimney. The drop in airdensity causes air within the thermal chimney to rise and beexpelled out of the top of the chimney. The air which leaves thethermal chimney is typically replaced with outside air that is firstdrawn through the building to provide natural cooling. Thisbuilding feature is typically composed of an absorber wall, an airgap and a glass cover with high solar transmissivity and is designedto maximize solar gain in order to increase the chimney effect andthus the air flow generated by the chimney.

* Corresponding author at: 3 Admiral Dr., #F269, Emeryville, CA 94608, United

States. Tel.: +1 217 419 6067.

E-mail address: kwhlee@berkeley.edu (K.H. Lee).

0378-7788/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.enbuild.2008.12.006

Due to the significance of thermal chimney systems, a numberof studies have been carried out thus far. Bansal et al. [1] developeda steady state mathematical model for a thermal chimney andassessed volumetric flow rate variations with regard to the solarradiation and discharge coefficients. This study concluded thatconsiderable amounts of air ventilation can be achieved with thethermal chimney. Ong [2] also developed a steady state model tosimulate a thermal chimney and validated the model againstexperimental data. Bansal et al. [3] proposed a system combining asolar chimney with a wind tower to further improve the naturalventilation in buildings. It is concluded from this paper that theventilation caused by the combination of a thermal chimney and awind tower was twice as high as that induced only by a wind toweralone. A detailed and dynamic model was developed by Marti-Herrero and Heras-Celemin [4], taking into account relativelydetailed mechanisms such as two-dimensional conductionthrough the absorber wall. The model is validated againstexperimental data from the literature. An analytical model is alsodeveloped by Miyazaki et al. [5] and is validated against CFD(Computational Fluid Dynamics) data. Using the verified model,reduced fan shaft power by a thermal chimney and heating andcooling load changes by the thermal chimney are evaluated.According to this study, the heating load can be reduced by 20%under Japanses climate, while the cooling load is increased by 12%.Harris and Helwig [8] used CFD (Computational Fluid Dynamics)modelling technique to evaluate the effects of inclination angle,low-emissivity wall and double glazing on the thermal chimneyventilation performance. Gan and Riffat [9] also sued CFD analysisto assess the thermal performance of the solar chimney for the heat

Nomenclature

Ai cross-sectional area of air channel inlet (m2)

Ao cross-sectional area of air channel outlet (m2)

Cd discharge coefficient

Cp specific heat of air (J/kg 8C)

g acceleration due to gravity (9.8 m/s2)

hgf convective heat transfer coefficients between the

glass and the fluid (W/m2 8C)

hi convective heat transfer coefficients between each

surface and the room air (W/m2 8C)

hwf convective heat transfer coefficients between

absorber wall and the fluid (W/m2 8C)

kair thermal conductivity of air (W/m 8C)

L total length of the thermal chimney (m)

m mass flow rate of the air (kg/s)

mi mass flow rate of each interzone air mixing (kg/s)

minf infiltration mass flow rate (kg/s)

msys air mass flow rate supplied from the air distribu-

tion system (kg/s)

qLWX long wave radiation from other surfaces (W/m2)

qSW short wave radiation from lights (W/m2)

qLWS long wave radiation from equipment (W/m2)

qcond conduction through the wall (W/m2)

qsol transmitted solar radiation (W/m2)

qconv convective exchange with the air (W/m2)

Qi convective internal heat gain (W)

Tf fluid temperature averaged over the entire length

of the thermal chimney (K)

Tfi inlet air temperature of the thermal chimney (K)

Tfo outlet air temperature of the thermal chimney (K)

Tg glass cover temperature (K)

Todb outdoor dry bulb temperature (K)

Tr room air temperature (K)

Ts surface temperature (K)

Tsi temperature of each surface (K)

Tsup supply air temperature from the air distribution

system (K)

Tw absorber wall temperature (K)

Tz zone temperature (K)

Tzone zone temperature (K)

T1 fluid temperature (K)

w width of the absorber wall (m)

x elemental length of the absorber wall (m)

Greek lettersag absorptance of glass cover

aw absorptance of absorber wall

b air volumetric coefficient of expansion (K�1)

eg emissivity of the glass cover

s Stefan-Boltzmann constant (5.67 � 10�8 W/m2 K4)

t transmittance of the glass cover

n kinematic viscosity of air (m2/s)

Fig. 1. Schematic diagram of thermal chimney.

K.H. Lee, R.K. Strand / Energy and Buildings 41 (2009) 615–621616

recovery. According to this paper, double or triple glazing shouldbe used to enhance the natural ventilation rate in cold weatherconditions. Hirunlabh et al. [7] performed both experiment andcomputational simulation for the thermal chimney under Bangkokclimate. Experimental results show that the solar chimney with

2 m height and 14.5 cm air gap induced the maximum naturalventilation rate. Mathematical model was validated againstexperimental data and turned out to be accurate for the long-term period investigation. Afonso and Oliveira [10] carried outexperiments to compare the ventilation performances betweensolar and conventional chimneys. As expected, it turned out thatsignificant increase in ventilation rate could be achieved. And also,a thermal model is developed and validated against experimentaldata in this study. Other mathematical models are also developedby many researchers such as Dai et al. [11], Pasumarthi and Sherif[12], Aboulnaga and Abdrabboh [13] and Awbi [14] to predict theventilation performances of thermal chimneys.

Most studies with the exception of that by Miyazaki et al. [5] didnot consider thermal load changes by the existence of a thermalchimney. The study by Miyazaki et al. [5] is done for Japaneseweather, indicating that the findings cannot be directly applied tothe U.S. In addition, none of the selected papers above integratedthe developed model with a building simulation software tool,indicating that it is not open to the users publicly. In this study, anew module was developed for the EnergyPlus program for thesimulation of thermal chimneys. This paper describes the basicconcepts, assumptions, and modeling algorithms used to developand implement a thermal chimney model in the EnergyPlusprogram. This new model allows building designers and engineersto predict the potential impact that a thermal chimney systemwould have and thus assess its potential effectiveness before abuilding was built. Using the new model, parametric analysis iscarried out to quantitatively look into the effects of four importantparameters on the overall thermal chimney performance. Inaddition, the potential energy impact of a thermal chimney is alsoinvestigated under three different weather conditions in the U.S.

2. Modeling algorithm description

The simulation program in which the thermal chimney modulewas implemented is EnergyPlus. A new thermal chimney modulewas developed separately and added to the existing EnergyPlusmodules. It was implemented at the air heat balance manager levelso that the enhanced infiltration rate caused by the thermalchimney is included in the interior zone air heat balance. This isdue to the fact that the infiltration object within EnergyPlus is alsoimplemented in the air heat balance manager. The interior zone air

K.H. Lee, R.K. Strand / Energy and Buildings 41 (2009) 615–621 617

heat balance in EnergyPlus automatically considers all theconvective internal loads, heat transfer due to infiltration, heattransfer due to air mixing between adjacent zones and the energyprovided by HVAC systems as follows [6].

XNi

i¼1

Q i þXNsurfaces

i¼1

hiAiðTsi � TzÞ þXNzones

i¼1

miCpðTzi � TzÞ

þminf CpðT1 � TzÞ þmsysCpðTsup � TzÞ ¼ 0 (1)

Due to the complicated phenomena taking place in the thermalchimney, several simplifying assumptions were made. Theseassumptions include:

� There is no variation in surface temperature for elements of thethermal chimney such as the glass cover and the absorber wall. Inother words, only one-dimensional heat transfer through theseelements is taken into account.� The inlet temperature of the air channel in the thermal chimney

is equal to the room air temperature.� Resistance to the air flow due to surface friction is negligible.� The discharged amount of interior air induced by the thermal

chimney is replaced by air at conditions equation to exteriorconditions.

The key output parameter in the thermal chimney model is theenhanced amount of natural ventilation rate caused by the presenceof a thermal chimney. In order to determine the enhancedventilation, the discharge air temperature from a thermal chimneyshould be calculated, which, in turn, should be computed based onthe information on the absorber wall temperature, glass covertemperature and the vertical air temperature distribution withinthe thermal chimney. Among them, surface heat balances for theabsorber wall and the glass cover are carried out using the existingalgorithm currently available in EnergyPlus. The surface heatbalance manager in EnergyPlus automatically calculates surfacetemperatures and convective heat transfer coefficients in each timestep by taking into account the conduction through the buildingconstructions, convection transfer to/from the air, solar radiationabsorption in the window and absorber wall and long wave radiationexchange among interior surfaces as follows [6].

qLWX þ qSW þ qLES þ qcond þ qsol þ qconv ¼ 0 (2)

On the other hand, the vertical air temperature distribution andthe resultant discharge air temperature of the thermal chimney arecomputed using the separate thermal chimney algorithm. Based onthe information on the surface temperatures of the glass cover andthe absorber wall and convective heat transfer coefficients of glasscover and absorber wall computed within EnergyPlus, the energybalance for the air inside the thermal chimney can be expressed as:

hwf ðTw � T f Þ ¼ hgf ðT f � TgÞ þmCp

w

dTf

dx(3)

where m is the total mass flow rate of the air (kg/s), Cp is the specificheat of air (J/kg 8C), w is the width of the absorber wall (m) and x isthe elemental length of the absorber wall (m).

Since the initial condition of inlet air temperature in thisdifferential equation is equal to the room air temperature (i.e. x = 0,Tf,i = Tr), the discharge air temperature, Tfo, can be evaluated bysolving the differential Eq. (3). This equation is rearranged intoEq. (4) for use in a finite difference solution.

hwp Tw �Tnþ1 � Tn

2

� �¼ hgf

Tnþ1 þ Tn

2� Tg

� �þmCp

w

Tnþ1 � Tn

Dx

(4)

where Tn and Tn+1 are air temperatures of nth and n + 1th sub-regions within the thermal chimney air channel, respectively andDx is the length of the absorber wall of each sub-region.

Based on the computed discharge air temperature, Tfo, usingfinite difference, the total air flow rate caused by the thermalchimney (m3/s), Q, can be finally evaluated from the followingexpression [1]:

Q ¼ CdAo

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2ðT fo � Tr=TrÞgL

ð1þ ArÞ2

s(5)

Ar ¼Ao

Ai(6)

where Cd is the discharge coefficient, Ao and Ai are the cross-sectional areas of air channel outlet and inlet (m2), respectively, Tfo

is the outlet air temperature (K), Tr is the room air temperature (K)and L is the total length of the thermal chimney (m).

The discharged amount of interior air from each zone caused bythe presence of the thermal chimney is assumed to be replaced byoutdoor air. As stated earlier, the air heat balance manager isconsidered to be the proper level to implement the separatethermal chimney module to correct the infiltration rate enhancedby thermal chimneys, since the infiltration is actually modeled inthe same level within EnergyPlus. This thermal chimney modelautomatically increases the infiltration rate to account for both theuser specified infiltration and the ventilation induced by thethermal chimney.

Finally, the heat gain and heat loss of each zone due to thethermal chimney can be determined from the following simpleexpressions:

Qgain ¼ mCpðTzone � TodbÞ (7)

Q loss ¼ mCpðTodb � TzoneÞ (8)

where Tzone and Todb are the zone and outdoor dry bulbtemperatures, respectively. Those heat gain and heat losses arealso included in the heat balance within EnergyPlus so that it canaccount for the thermal condition changes of the indoor air due tothe thermal chimney. EnergyPlus also accounts for any latenteffects caused by the thermal chimney in the moisture massbalance performed as part of the air heat balance.

3. Implementation into EnergyPlus

The uniqueness of this study is the implementation a modelinto EnergyPlus, which automatically performs all the detailedheat, air, surface and energy balances within the whole buildingincluding Eq. (1) and (2). In addition to a new modeling algorithmdescribed earlier, heat and air exchange between the adjacentzones, hourly solar transmittance through the glazing, internalheat gain from occupants, lighting and electric equipment, andtransient heat conduction through the building envelope aresimultaneously computed by EnergyPlus for the simulation of thethermal chimney performance. It also includes the conductive heatgain/loss of the space through the absorber wall, and surfacetemperature of the thermal chimney absorber wall, which is acrucial factor of thermal chimney performance, is also determinedfrom those heat balances. The effects of the air exfiltration andinfiltration taking place inside the thermal chimney on heat and airbalance calculations such as the surface temperature of theabsorber wall and the conductive heat gain/loss through the wallare taken into account. For these purposes, a separate thermal zoneshould be specified for the thermal chimney to make EnergyPlusautomatically consider the simultaneous heat and energy transfersabove. In other words, the true thermal chimney performancecannot be accurately predicted without those sophisticated heatbalance techniques within EnergyPlus.

Table 1Description of simulation conditions.

Conditions

Location Minneapolis, MN—cold

Spokane, WA—mild

Phoenix, AZ—hot

Run period Aug. 21st

Variables Height 3.5 m, 5 m, 6.5 m, 8 m, 9.5 m

Solar absorptance 0.25, 0.45, 0.65, 0.85, 1.0

Solar transmittance 0.25, 0.45, 0.65, 0.85, 0.92

Air gap width 0.15 m, 0.3 m, 0.45 m, 0.6 m, 0.75 m

Fig. 2. Effect of thermal chimney height.

K.H. Lee, R.K. Strand / Energy and Buildings 41 (2009) 615–621618

Since the primary function of the thermal chimney is theenhancement of natural ventilation, it is possible that thermalchimneys can increase the cooling load if the outdoor airtemperature is higher than the cooling set-point during thecooling period. Therefore, EnergyPlus allows the user to specifywhether the thermal chimney is turned on or off in each time stepthroughout the entire year so that the overheating of the space dueto the higher outdoor air temperature can be avoided.

In addition, as the fundamental concept of the thermal chimneyis to cause the updraft airflow for the natural ventilation using thesolar radiation, the flow direction does not vary. If discharge airtemperature from the top of the thermal chimney is less than roomair temperature, the thermal chimney will be automatically shutdown, since the outlet temperature should be greater than roomtemperature in order to cause the updraft airflow. In most cases insummer, the outlet air temperature on the top of thermal chimneysnecessarily exceeds room air temperature.

As a final step of the implementation into EnergyPlus, thevalidation process is briefly carried out to ensure that the modelcan properly predict the thermal chimney performance. Hirunlabhet al.’s experimental data was used for the validation process in thisstudy, since this study provides both conditions and results inrelative detail [7]. Experimental data for the air gap width of0.145 m and the chimney height of 2 m show the thermal chimneymass flow rate of 0.016–0.019 kg/s between 10 am to 3 pm onDecember 12th under Bangkok weather condition. The currentmodel shows the thermal chimney mass flow rate of 0.015–0.023 kg/s under similar conditions. The discrepancies are due todifferent weather conditions considered in those two studies.However, two data sets are in good agreement and thus the newmodel properly predicts the thermal chimney performance.

4. Parametric analysis

Using the newly developed thermal chimney model, theperformance of a thermal chimney can be predicted under variousconditions. Effects of each operating parameter can be also beinvestigated by changing each input variable one by one todetermine the sensitivity of the system performance to individualparameters. Those operating parameters include the on/offschedule of thermal chimney system, the chimney height, theabsorber wall solar absorptance, the glass cover solar transmit-tance, the convective heat transfer coefficient, the surfacetemperature of the absorber wall, the width of the air gap betweenglass cover and absorber wall, etc.

In this section, the parametric analysis looking into the effectsof four operating input variables affecting the natural ventilationrate is described. These four input parameters include: chimneyheight, solar absorptance, solar transmittance and air gap width.The analysis is performed on five different values of eachparameter and August 21st was chosen for the analysis sincesolar radiation can be sufficiently strong at that time of year. Inaddition, three different locations were chosen which have threedifferent climatic conditions within the U.S.: cold, mild and hotweather conditions.

The standard values of each variable were set at: 6.5 m forchimney height, 0.85 for solar absorptance of the absorber wall,0.85 for glass cover solar transmittance, and 0.3 m for the air gapwidth. When changing values for one of these variables for thisparametric study, the other variables were kept at those values.Table 1 shows the specifications of this parametric study.

A simple rectangular single-zone residential building waschosen in this study. Although it is a fictional building, all thebuilding constructions, glazings and internal heat gain schedulesare based on real buildings. It has the floor area of approximately200 m2 (20 m � 10 m) with 3 m ceiling height. The thermal

chimney is attached to the southern side of the building for themaximum solar radiation absorption. The internal heat gains fromlights and equipment were set at 5.4 W/m2 and two people wereplaced inside the building zone during the occupancy period. Inaddition infiltration rate was maintained at 0.25 ACH during thesimulation, which means that the ventilation enhancement due tothe thermal chimney is added to 0.25 ACH infiltration.

4.1. Effect of thermal chimney height

Fig. 2 presents the effect of thermal chimney height on theenhanced ventilation rate at the highest ambient air temperature.As the chimney height increases, the ventilation rate is alsoenhanced regardless of the locations (average increase rate of 74%).This is due to the fact that the longer chimney air channel providesa longer path for the convective heat transfer between the absorberwall and the air. As the height increases from 3.5 m to 9.5 m, theincrease rates of the ventilation flow are 76%, 72% and 75% inMinneapolis, Spokane and Phoenix, respectively.

Although the increase rates and the curves are similar amongdifferent climatic conditions as shown in Fig. 2, the actualventilation rates enhanced by the existence of the thermalchimney are different. Spokane showed the highest ventilationenhancement due to the thermal chimney. As illustrated in Fig. 2, itwas almost twice as high as that in Minneapolis. This is due to thedifferent weather conditions, especially different solar radiationintensities and ambient air temperatures. The maximum directnormal solar radiations during the simulation period were917 Wh/m2, 965 Wh/m2 and 209 Wh/m2 in Phoenix, Spokaneand Minneapolis, respectively, indicating that the solar radiation inSpokane was more than four times stronger than Minneapolis.Therefore, it would be concluded that the thermal chimneyperformance is largely dependent on the solar availability of thelocations as well as the chimney height itself and thus the climaticconditions should also be taken into account when designingthermal chimneys for building applications.

Fig. 3. Effect of solar absorptance of absorber wall. Fig. 5. Effect of air gap width.

K.H. Lee, R.K. Strand / Energy and Buildings 41 (2009) 615–621 619

4.2. Effect of solar absorptance

The absorber wall solar absorptance vs. ventilation enhance-ment profile is illustrated in Fig. 3. As the solar absorptance isenhanced, the ventilation rate also increases regardless of thelocation due to the fact that the absorber wall surface temperaturesignificantly increases with the solar absorptance (averageincrease rate of 48%). In Phoenix, the maximum absorber wallsurface temperature reaches 114 8C under solar absorptance of 1.0compared to 72 8C under 0.25 solar absorptance. This indicates thathigher solar absorptance should be used to achieve the highernatural ventilation in agreement with the common sense. As thesolar absorptance is improved from 0.25 to 1.0, the air flowincrease rates showed improvements of 57%, 42% and 45% inMinneapolis, Spokane and Phoenix, respectively. Similar to thecase of chimney height, enhanced ventilation rates due to thethermal chimney are different in each location. The ventilation ratein Spokane was approximately twice as high as that in Minneapolisin all solar absorptance values due to higher solar radiation andambient air temperature.

4.3. Effect of solar transmittance

Fig. 4 represents the enhanced ventilation rate with regard tothe glass cover solar transmittance. As the solar transmittanceincreases, the ventilation rate is also enhanced as shown in Fig. 4,since more solar radiation can be absorbed into the absorber wallunder higher glass solar transmittance (average increase rate of38%). As discussed in the previous section, higher absorber wallsurface temperature clearly increases the natural ventilation rate,indicating that higher transmittance is desirable in terms of thenatural ventilation enhancement. As the solar transmittance

Fig. 4. Effect of solar transmittance of glass cover.

increases from 0.25 to 0.92, the ventilation flow rate increasedby 40%, 38% and 36% in Minneapolis, Spokane and Phoenix,respectively. Likewise, it should also be noted that differentlocations produce different ranges and amounts of ventilation rateas can be seen in Fig. 4 due to different solar radiation intensitiesand ambient air temperatures. Compared to the other parametersdiscussed thus far, the solar transmittance turned out to have aslarge effect on the thermal chimney performance as the chimneyheight and the wall solar absorptance.

4.4. Effect of air gap width

Ventilation rate vs. air gap width profile is illustrated inFig. 5. As shown in this figure, the mass flow is slightly reducedby the higher air gap width, indicating that a smaller air gapenhances the natural ventilation flow rate. However, it turnedout that the increase amount or range is almost negligible andthat air gap width did not have as much of an effect on theperformance of the thermal chimney as the other designparameters described in the previous subsections. As the airgap width is reduced from 0.75 m into 0.15 m, the ventilationflow rate increased by only 4.7%, 1.9% and 2.4% in Minneapolis,Spokane and Phoenix, respectively.

Based on the discussions on the parametric study provided thusfar, the next part of this paper is to investigate the potential of athermal chimney to reduce both heating and cooling loads of thebuilding.

5. Cooling and heating potential

As a final step of this paper, additional simulations were carriedout to look into the heating and cooling potential of a thermalchimney. Two identical buildings were simulated at the same time:one with and one without a thermal chimney. All the otherconditions were kept the same as described in the parametricanalysis. The thermal chimney is controlled to reduce heating andcooling loads. During the heating mode, the thermal chimney isautomatically turned off when the ambient air temperature islower than the indoor setpoint temperature in order to avoid thebuilding to be overcooled by the enhanced infiltration. On theother hand, during the cooling period, it is also automatically shutdown when the ambient air temperature is higher than thetemperature setpoint for the prevention of the unnecessaryoverheating of the building. Since the annual simulation discussedlater in this subsection showed that thermal chimneys have morecapability for cooling than for heating, the single day simulation isperformed only for cooling. Figs. 6–8 illustrate the hourly coolingload variations on April 21st for Spokane and Phoenix and May 21stfor Minneapolis with and without a thermal chimney. Table 2

Fig. 6. Cooling load requirement with and without thermal chimney (Spokane).

Table 2Cooling load requirement (single day simulation).

Locations Cooling load without

thermal chimney (kWh)

Cooling load with

thermal chimney (kWh)

Minneapolis 19,692 13,112

Spokane 16,109 4,965

Phoenix 38,218 35,679

K.H. Lee, R.K. Strand / Energy and Buildings 41 (2009) 615–621620

summarizes the overall cooling load requirement to maintain theindoor temperature at 23 8C during the day.

The increases in the cooling load requirement around hours 12through 20 are due to the increase in the ambient air temperatureand the solar radiation during that time period. As can be seen inFigs. 6–8, the reduction of the cooling load due to the thermalchimney largely depends on the climatic conditions. The totalcooling load reduction in Spokane and Minneapolis were as muchas 69.2% and 33.4%, respectively, as summarized in Table 2,indicating that significant amounts of cooling energy saving can beachieved by properly employing thermal chimneys. Especially inthe morning time, there were no cooling loads in both locations if

Fig. 7. Cooling load requirement with and without thermal chimney (Phoenix).

Fig. 8. Cooling load requirement with and without thermal chimney (Minneapolis).

thermal chimneys were used. On the other hand, the cooling load isreduced only by 6.7% in Phoenix as shown in Fig. 7 and Table 2 dueto the fact that thermal chimney was turned off during the entireafternoon to avoid the overheating of the building. Since thethermal chimney induces the infiltration, the building can beoverheated by the ambient air infiltrating at high temperatures ifthe thermal chimney operates at that time. This indicates thatweather condition should be taken into account when determiningwhether or not a thermal chimney should be employed.

In order to investigate the heating and cooling potential ofthermal chimneys during the whole year, annual simulations werealso carried out for the three locations with and without thermalchimneys. Table 3 summarizes the cooling load requirements forthe annual simulation. As can be seen in this table, Phoenix hasmuch higher cooling load than the other two locations due to itshot climatic conditions. More importantly, significant amounts ofcooling energy can be saved by utilizing thermal chimneys. 20.4%,18.9% and 13.1% of the cooling load can be reduced in Spokane,Phoenix and Minneapolis, respectively. Unlike the single daysimulations discussed earlier, the differences in the reduction rateof cooling loads among each location are decreased. Morespecifically, the reduction rates in Spokane and Minneapolisdecreased from 69.2% to 20.4% and from 33.4% to 13.1%,respectively, due to the fact that the thermal chimneys weregenerally turned off during the winter and summer seasons toavoid the overcooling and overheating as opposed to the single daysimulation carried out only during the moderate period. However,the cooling load reduction in those locations is still significant,indicating that a thermal chimney has a significant potential tosave cooling energy as shown in Table 3. In addition, cooling loadreduction due to the existence of thermal chimneys is evenenhanced in the annual simulation compared to the single daysimulation in case of Phoenix as can be seen in Tables 2 and 3. Thiscan be due to the fact that other time periods other than April 21stselected for the single day simulation in Phoenix turned out to bemore suitable time for thermal chimneys.

On the other hand, the heating load requirement with andwithout thermal chimneys is summarized in Table 4 as well. Asexpected, Minneapolis has the largest heating load requirementamong the three locations because of its cold weather conditions,as opposed to Phoenix which has a relatively low heating load dueto the hot climate. In terms of the heating load reductions bythermal chimneys, they are relatively small compared to thecooling load reductions as shown in Table 4. Only 4.7%, 2.5% and4.7% of the heating loads can be reduced by the thermal chimney inMinneapolis, Spokane and Phoenix, respectively. This is due to thefact that the number of hours when the ambient air temperaturewas higher than the indoor temperature setpoint during the

Table 3Cooling load requirement (annual simulation).

Locations Cooling load without

thermal chimney (kWh)

Cooling load with

thermal chimney (kWh)

Minneapolis 4,602,496 4,002,917

Spokane 3,563,572 2,835,182

Phoenix 15,176,508 12,302,837

Table 4Heating load requirement (annual simulation).

Locations Heating load without

thermal chimney (kWh)

Heating load with

thermal chimney (kWh)

Minneapolis 12,323,645 11,743,705

Spokane 8,196,278 7,994,119

Phoenix 94,150 89,720

K.H. Lee, R.K. Strand / Energy and Buildings 41 (2009) 615–621 621

heating season was not frequent unlike the cooling season. If theambient temperature is lower than the indoor setpoint during theheating season, thermal chimneys are automatically turned off toprevent the overcooling of the building. Therefore, it can beconcluded that thermal chimneys have more potential for coolingthan for heating.

6. Conclusion

In this paper, the brief algorithm for the simulation of thermalchimneys in EnergyPlus is described, and parametric studies werecarried out to figure out the effect of each design parameter on thethermal chimney performance. Furthermore, the cooling andheating potential of thermal chimneys were also investigated forboth single day and annual simulations. The following conclusionswere drawn.

Chimney height, solar absorptance and solar transmittanceturned out to have more influence on the natural ventilationimprovement than the air gap width. The higher thermal chimneyswith greater absorber wall solar absorptance and the glass coversolar transmittance result in larger building natural ventilationenhancement in agreement with the literature.

More importantly, it turned out that thermal chimneys cansignificantly reduce the cooling energy consumptions in buildingsand that they have more potential for cooling than for heating asexpected. In addition, climatic conditions of particular locations alsohave significant impacts on the overall performance, indicating thatthe weather condition should also be taken into account whendetermining whether or not thermal chimneys should be used inparticular locations. However, the annual percentage savings for thethree different locations were fairly similar, indicating that thermalchimneys might be useful in locations that are not necessarilythought of as good locations for their application. Finally, thecomparison of the single day analysis against the annual simulationdemonstrates the need to go beyond steady state or even single dayanalysis. Only with an annual run can produce an accurate reading ofthe applicability of a particular technology, which makes theintegration of such models like the thermal chimney model into aprogram like EnergyPlus even more crucial.

Based on this study, future work that should be done includesthe experimental validation of the newly developed thermalchimney module and the environmental and economic assessmentof thermal chimney systems. Another possible improvement is toutilize the hot outlet air for heating, since the chimney outlet

temperature can be higher than the indoor air temperature duringthe heating period even if the ambient air temperature is lowerthan the indoor air temperature.

Acknowledgment

This material is based upon work supported by the Departmentof Energy National Energy Technology Laboratory under awardnumber DE-FC26_06NT42768.

Disclaimer: This report was prepared as an account of worksponsored by an agency of the United States Government. Neitherthe United States Government nor any agency thereof, nor any oftheir employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product,or process disclosed, or represents that its use would not infringeprivately owned rights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the United StatesGovernment or any agency thereof. The views and opinions ofauthors expressed herein do not necessarily state or reflect those ofthe United States Government or any agency thereof.

References

[1] N.K. Bansal, R. Mathur, M.S. Bhandari, Solar chimney for enhanced stack ventila-tion, Building and Environment 28 (1993) 373–377.

[2] K.S. Ong, A mathematical model of a solar chimney, Renewable Energy 28 (2003)1047–1060.

[3] N.K. Bansal, R. Mathur, M.S. Bhandari, A study of solar chimney assisted windtower system for natural ventilation in buildings, Building and Environment 29(1994) 495–500.

[4] J. Marti-Herrero, M.R. Heras-Celemin, Dynamic physical model for a solar chim-ney, Solar Energy 81 (2007) 614–622.

[5] T. Miyazaki, A. Akisawa, T. Kashiwagi, The effects of solar chimneys on thermalload mitigation of office buildings under the Japanese climate, Renewable Energy31 (2006) 987–1010.

[6] U.S. The, Department of Energy, EnergyPlus Engineering Reference Version 2.2,The U. S. Department of Energy, 2008.

[7] J. Hirunlabh, W. Kongduang, P. Namprakai, J. Khedari, Study of natural ventilationof houses by a metallic solar wall under tropical climate, Renewable Energy 18(1999) 109–119.

[8] D.J. Harris, N. Helwig, Solar chimney and building ventilation, Applied Energy 84(2007) 135–146.

[9] G. Gan, S.B. Riffat, A numerical study of solar chimney for natural ventilation ofbuildings with heat recovery, Applied Thermal Engineering 18 (1998) 1171–11187.

[10] C. Afonso, A. Oliveira, Solar chimneys: simulation and experiment, Energy andBuildings 32 (2000) 71–79.

[11] Y.J. Dai, K. Sumathy, R.Z. Wang, Y.G. Li, Enhancement of natural ventilation in asolar house with a solar chimney and a solid adsorption cooling cavity, SolarEnergy 74 (2003) 65–75.

[12] N. Pasumarthi, S.A. Sherif, Experimental and theoretical performance of a demon-stration solar chimney model–part 1: mathematical model development, Inter-national Journal of Energy Research 22 (1998) 277–288.

[13] M.M. Aboulnaga, S.N. Abdrabboh, Improving night ventilation into low-risebuildings in hot-arid climates exploring a combined wall-roof solar chimney,Renewable Energy 19 (2000) 47–54.

[14] H.B. Awbi, Design considerations for naturally ventilated buildings, RenewableEnergy 5 (1994) 1081–1090.