4th International Conference On Building Energy, Environment

Combined Solar Chimney and Water Wall for Ventilation and Thermal Comfort

H. Wang and C. Lei

School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia

SUMMARY In order to improve the thermal comfort of an indoor space, a passive thermal comfort strategy combining a solar chimney with a water wall is introduced. A transient heat balance model (THBM) is adopted to evaluate the performance of this structure in terms of its ability to induce ventilation and provide thermal buffering. Calculations are performed for a representative winter weather condition in Sydney. The impacts of wind and glass panel thickness on the performance of the proposed system are both considered. The results show that the proposed system can effectively improve the thermal comfort level of the attached room by raising room temperature as well as providing considerable ventilation. In addition, wind can help increase the ventilation rate but will also reduce the room temperature. Compared to the thermal performance obtained with the 25mm-thick glass panels, a glass panel thickness of 19mm is preferred for the present application.

INTRODUCTION Ventilation and thermal buffering are two major design strategies to improve thermal comfort in an indoor space.

Ventilation comes into play when the indoor air turns stale and is necessary to be replaced by fresh air. One can either adopt mechanical ventilation strategies by forcing air circulation with fans or air conditioners, or apply natural ventilation strategies by opening windows or with the adoption of solar chimney and other forms of passive ventilation techniques. Natural ventilation has become a hot research topic all over the world because it is free of energy use and thus will not contribute to the increase of carbon footprint. Solar chimney stands out as a typical example of passive natural ventilation strategies for its effectiveness in inducing buoyancy-driven ventilation with solar radiation.

Unlike ventilation, the strategy of thermal buffering improves the indoor thermal comfort level by dampening the undesirable heat exchange between the indoor and outdoor environments. Water wall is a typical example of passive thermal buffering strategies. Standing between the indoor space and the outdoor environments, a water wall serves as a buffer zone against the undesirable outdoor weather conditions. In the meantime, the water wall works as a moderate space heater, especially at night, by releasing the solar heat stored during the day to the indoor space to slow down the cooling process of the room due to the relatively lower temperature in the ambient environment.

Though ventilation strategies, such as solar chimney, and thermal buffering methods, such as water wall, have both been widely acknowledged as effective, they have individual imperfections and thus are held back from realizing their full potential to achieve thermal comfort. For example, with solar radiation as the only heat source, a solar chimney is capable of providing ventilation during the day only, whereas after sunset the ventilation effect immediately ceases. In addition, in order to enhance the absorption of solar radiation, the

absorber wall of a solar chimney is usually painted black and thus will block lighting from entering the room. On the other hand, although a water wall stops the undesirable heat transfer between the indoor and outdoor environments and in the meantime provides sufficient lighting, it introduces excessive heat to the attached room if solar radiation is intensive, especially when no fan or air conditioner is used. It may also suffer from a significant deficiency in energy efficiency during the night due to heat loss to the ambient environment.

In order to combine the benefits of both ventilation and thermal buffering strategies while minimizing the negative impact of the individual strategies, a novel structure incorporating a water wall into a solar chimney is proposed. In this novel structure, a water wall takes the place of the absorber wall of a conventional solar chimney. During the day, the thermal mass of the water wall absorbs part of the solar radiation and acts as a thermal storage reservoir. After sunset, the heat stored in the water wall is released to the solar chimney to maintain ventilation. In this study, a transient heat balance model (THBM) is adopted to analyze the thermal performance of the integrated system. The effects of wind and glass panel thickness on the performance of the integrated system are also investigated.

METHODS 1. Model descriptionFigure 1 displays the integration of a combined window-sized

solar chimney and water wall system for a 3m×3m×3m

space. The dashed box in Fig. 1(a) shows the installation ofthe combined solar chimney and water wall structure, thedetails of which are shown in Fig. 1(b), with the blue regionrepresenting the water column and the glass panels beingshaded. The exposed area of the water wall is 1m2, throughwhich solar radiation enters the space. The thickness of thewater column and the air gap width of the solar chimney areboth 10cm. The red arrows in the figures indicate theexpected air flow path resulting from ventilation. An inlet forthe room is installed on the opposite wall to the proposedsystem and has the same dimensions as those of theopenings of the solar chimney. Material strength calculationsare performed to determine the required thickness of theglass panels. For brevity the detailed procedures are notshown here. According to the calculations, a thickness of19mm (3/4in) is chosen for all the glass panels in the design.

2. Problem formulationA 1D transient heat balance model (THBM) can be set up toaccount for the various heat transfer processes within themodel. As shown in Figure 2, the points representtemperature nodes (T) in the system while the linesrepresent thermal couplings between differentcomponents/nodes, which are denoted by distinctivesubscripts. Ta is the ambient temperature, and Tsky is the skytemperature, the radiation temperature of the ambientenvironment, which is given by (Swinbank 1963)

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Figure 1.(a) Schematic of the proposed thermal comfort strategy; (b) Dimensions of the combined solar chimney and water wall. (Not to scale).

Figure 2. Schematic of 1D transient heat balance model.

T

sky= 0.0552T

a

1.5 (1)

The subscript "g" denotes the glazing while "w1" and "w2" represent the glass panels bounding the water column. Tc, Tw and Tr denote the average temperatures of the air channel, water column and the room respectively. In order to account for transient heat conduction within the glazing and glass panels, several mesh points are inserted into each of these components, as shown in the boxes in Figure 2. Only the mesh points on the surfaces will be involved in calculating the heat transfer with its neighbouring components. Convection takes places at the interface between fluid and solid while radiation occurs between the neighbouring surfaces as well as between the ambient (Tsky) and the external surface of the glazing. The convective heat transfer coefficient between the glazing and the ambient environment due to wind is calculated by (McAdams 1985)

hwind

= 5.7 + 3.8V (2)

where V denotes wind velocity. Convective heat transfer

coefficients at other interfaces are derived from the corresponding Nusselt number correlations using h=Nu∙k/L, where k is the thermal conductivity of the fluid and L the characteristic length of the interface. Nusselt number is evaluated using the following expressions (Incropera & DeWitt) : For Ra<109,

Nu = 0.68 + 0.67Ra1/4( ) / 1+ 0.492 / Pr( )9/16é

ëêùûú

4/9

(3)

For Ra<109,

Nu = 0.825+ 0.387Ra1/6( ) / 1+ 0.492 / Pr( )9/16é

ëêùûú

8/27ìíî

üýþ

2

(4)

where Ra is the Rayleigh number and Pr is the Prandtl number. Surface temperatures used to compute the Rayleigh numbers are solved in an iterative manner until the difference between the current Nusselt number and that from the previous iteration is less than 1%. Radiative heat transfer coefficient between surface 1 and surface 2 is calculated by (Duffie and Beckman 2013):

ℎ1−2𝑟𝑎𝑑 = 𝜎 ∙ (𝑇1

2 + 𝑇22) ∙ (𝑇1 + 𝑇2) ∙ (1/𝜀1 + 1/𝜀2 − 1)−1 (5)

where σ=5.67×10-8W∙m-2∙K-4is Stefan-Boltzmann constant, and ε is surface emissivity. Air is assumed to be transparent to radiation while water and glass are not. According to Wu and Lei (2016b), water is assumed to bear an attenuation factor of h

w= 2m-1 such that its aborptance and

transmittance of radiation can be quantified as 1- e-hd

w and

e-hd

w respectively. For the glass panels in the presentdesign, according to the calculations done by Rubin (1985), a solar absorptance of 38.3% and a solar transmittance of 55.8% are used with only normal incidence considered.

Based on the thermal couplings shown in Figure 2, a set of energy balance equations with the average temperatures of system components as variables can be established. The ventilation rate of the solar chimney is obtained by solving the continuity and momentum conservation equations of the air flowing regions. The transient performance of the proposed system is obtained by solving the unsteady energy balance equations with an appropriate time step. For brevity, the actual equations solved in this study are not presented. Interested readers may find general equations relevant to the present problem in Munson et al. (1990).

RESULTS 1. Weather conditionsTo evaluate the performance of the proposed thermalcomfort strategy, calculations are performed for arepresentative and idealized winter weather condition ofSydney. The ambient temperature is assumed to follow asinusoidal pattern given below (Wu and Lei 2016a):

Ta(t) = T

0+ 0.5DT sin 2p(t - t

lag) / Pé

ëùû

(6)

where T0=285.5K is the daily average temperature, ΔT=15K is the daily temperature fluctuation, tlag=2h is the time lag of the ambient temperature relative to solar radiation, and

(a)

(b)

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P = 24h is the period of thermal cycles. Similarly, the

incident solar radiation is prescribed as (Wu and Lei 2016a):

I(t) =

Imax

sin 2pt / P( ) for (m-1)P<t £ (m-1

2)P

0 for(m-1

2)P<t £ mP

ì

íïï

îïï

(7)

where Imax=800W∙m-2 is the daily peak solar radiation and m is the sequence number of thermal cycles measured in days. Based on this definition, the beginning of the first half of each thermal cycle denotes the moment of sunrise, while the middle of each thermal cycle represents the moment of sunset. When solar radiation is present, heat sources are added to the glazing, glass panels and water body based on

their respective absorptivities of solar radiation。

Wind effect is also considered in this study. In addition to participating in the convective heat transfer at the external glazing, as described in Equation (2), wind near the outlet of the solar chimney and the room inlet affects the pressure distribution within the system. According to Butcher (2006), wind pressure can be characterised by

pw

=1

2rC

pV 2 (8)

where ρ is the air density, Cp is the wind pressure coefficient and V is the wind velocity. The room inlet is considered to be

located in the leeward side and thus its wind pressure coefficient is assumed to be -0.25 (Butcher 2006). On the other hand, the wind pressure coefficient for the outlet of the solar chimney is assumed to be -0.45 (Niemann and Höffer 2007). A constant wind velocity of 3.944m/s is taken as a reasonable simplification to the present problem formulation (Wu and Lei 2016a).

2. Spatial and temporal discretization testIn order to obtain the typical performance of the proposedstructure corresponding to the above-mentioned weatherconditions, THBM calculations are performed for severalthermal cycles until repeating results are observed betweentwo consecutive thermal cycles. Since thermal comfort isdirectly related to the ventilation rate and indoor temperature,the time history of the ventilation rate and indoor temperatureare monitored until the difference between the resultsobtained from two consecutive thermal cycles is reduced toless than 1%. Considering that the temporal and spatialdiscretization may have an impact on the accuracy of theresults, a mesh and time step dependence test is performedto determine the adequate resolution of mesh and time step.Two discretization schemes are considered: 5 meshnodes+15s and 9 mesh nodes+3.75s. The first numberrepresents the number of uniformly distributed mesh nodesacross the thicknesses of the glass panels (including theexternal glazing and the glass panels of the water wall), andthe second number represents the corresponding time step.In other words, the thickness of the glass panels is uniformlydivided into 4 segments in the first discretization scheme and8 in the second discretization scheme. Time steps for thesetwo discretization schemes are selected such that theFourier number (Fo) for these two configurations are kept thesame and that the stability requirement for the Forward-Time-Central-Space (FTCS) scheme (Fo≤0.5) is satisfied.

Figure 3. Performance comparison of two discretization schemes: (a) Ventilation rate; (b) Room temperature.

The stabilized results for these two sets of meshes and time steps are compared against each other to determine the adequacy of the mesh resolution and time step. The calculation results indicate that, starting from the fourth thermal cycle, the discrepancies between the results obtained in the current thermal cycle and that obtained in the previous thermal cycle are less than 1%, for both the ventilation rate and room temperature. The stabilized test results for these two discretization schemes are presented in Figure 3. As shown in the figure, a good agreement is reached for the test results obtained from these two mesh configurations. In this sense, the 5 mesh nodes+15s discretization can produce adequate results and thus will be adopted in the following calculations.

3. Calculation resultsAs shown in Figure 3, despite a slight dip in the ventilationrate due to the descending ventilation effect originated fromthe last few hours in the previous thermal cycle, theventilation rate produced by the system generally increaseswith solar radiation and reaches a peak in the first half of thethermal cycle. It is worth noting that there is a time lagbetween the peak of ventilation rate in the first half of thethermal cycle and the peak of solar radiation. Thisphenomenon may be attributed to the combined effect ofinstantaneous and non-instantaneous production ofbuoyancy due to solar radiation. When solar radiation isinitiated, the surfaces of the solar chimney absorbs some ofthe solar radiation and part of the absorbed energy isinstantaneously transferred to the adjacent air to generatebuoyancy effect. On the other hand, some of the solarradiation absorbed by the interior of the glazing or the waterwall will gradually be transferred to the surfaces of the solarchimney by conduction. Due to the time lag of conduction,

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the overall buoyancy effect peaks later than the solar radiation. After solar radiation is completely gone, heating of the solar chimney by the glazing and the water wall starts to take effect, bringing up the ventilation rate again with a second peak formed at around the middle of the second half of the thermal cycle. During this period, the decrease of the ambient temperature also contributes to the growth of ventilation rate by enhancing buoyancy effect in the solar chimney channel. As ventilation continues, the energy stored in the water wall keeps decreasing and the ventilation rate starts to go down when the water wall is unable to provide enough energy to maintain the ventilation rate. In the meantime, it can be observed that with the proposed system, although the room is open to the ambient environment, the room temperature is always higher than the ambient temperature, with the average temperature being 1.72K higher than the ambient. The maximum and the minimum temperatures in the room are more than 2.86K and 1.27K higher than that in the ambient environment. For a relatively cold winter weather condition considered here, the proposed system can thus improve the indoor thermal comfort level by increasing the room temperature as well as providing round-the-clock ventilation.

4. Wind effectTo distinguish the contribution of the system to the ventilationrate from the suction effect of wind blowing over the top ofthe outlet, calculations are performed for a case with zerowind velocity. The thermal equilibrium results are shown inFigure 4. It is clear in Figure 4(a) that, despite the effect ofwind, the proposed system alone is able to producesignificant amount of the ventilation. It can also be observedthat the ventilation rate for a no wind scenario experiencesmore fluctuations than that for a windy scenario. Thisphenomenon could be attributed to the suction effect of wind.

Figure 4. Performance comparison with or without wind: (a) Ventilation rate; (b) Room temperature.

Table 1. Optical properties of two glass thicknesses

Glass thickness(mm)

Solar absorptance

Solar transmittance Source

19 0.383 0.558 Rubin (1985) 25 0.456 0.488

Unlike the no wind scenario where the ventilation is solely caused by the buoyancy effect resulted from the heating of the solar chimney, when wind is present, the suction effect of wind also contributes to the overall ventilation rate produced by the proposed system. Thus, when solar radiation is not strong enough, or when the water wall is not able to provide enough heat to the solar chimney, wind steps in to maintain enough ventilation to smooth out the deficit caused by the solar chimney. For this reason, the ascending and descending trends of the ventilation rate under the influence of wind are not as obvious as those without wind. In the meantime, when there is no wind, due to relatively less air exchange with the colder ambient environment, the room temperature is higher than that with wind (refer to Figure 4b), which is preferable for the winter condition.

5. Effect of glass panel thicknessIn addition to its importance for the material strength andstructural stability, the thickness of glass panels also playsan important role in the thermal performance of thecombined solar chimney and water wall. As stated in Rubin(1985), for a specific type of glass, solar absorptanceincreases while transmittance decreases with its thickness.In other words, if a thicker glass panel is used in the designof the proposed system, a higher potential of solar heatabsorption by the combined solar chimney and water wall isanticipated. Considering that the heat absorbed by theproposed system can either be utilized to improve ventilationthrough the solar chimney or be released to the roomthrough the water wall as a means of space heating, theeffect of the glass panel thickness on the overall thermalperformance of the system is yet to be determined.In order to investigate the effect of the glass panel thickness,calculations are performed for the same system but with athickness of 25mm. The optical properties of this glassthickness are extracted from Rubin (1985) and are listed inTable 1, together with the optical properties of the glasspanels used in the original (reference) case. Calculationresults for the case with a glass thickness of 25mm areplotted in Figure 5 along with those of the original case.

As shown in Figures 5(a) and (b), although the increase of the glass thickness is supposed to enable the solar chimney and water wall to absorb more solar heat, the ventilation rate and room temperature during the daytime are in fact slightly lower than those with thinner glass panels. As for the water temperature shown in Figure 5(c), the case with thicker glass panels does not show a remarkable difference from the original case, and thus the increase in the glass thickness does not improve the thermal storage in the water wall. The decline in the thermal performance could be explained from the perspective of the energy attenuation. Since radiative energy attenuates along the thickness of the glass panels through absorption, the more radiation is absorbed, the less

radiative energy is allowed to pass through. In this sense, although the thicker glass panel has a higher overall solar

0.0050.0100.0150.0200.0250.0300.0350.0400.0450.050

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ate(

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time(h)

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Figure 5. Performance comparison of two glass thickness configurations: (a) Ventilation rate; (b) Room temperature; (c) Water temperature.

Table 2. Thermal comfort indicators obtained with two glass thicknesses

Glass thickness

(mm) (kg/s)

ACH Troom,avg

(K) Tmax

(K) Tmin

(K)

19 0.04 4.43 287.22 295.86 279.27 25 0.04 4.43 286.94 295.05 279.29

absorptance rate, the amount of energy reaching the inner surface of the external glazing and the outer surface of the first glass panel of the water wall is accordingly lower than that with the thinner glass. As a result, the heating rate of the inner surfaces of the solar chimney with thicker glass panels is less than that using thinner glass panels, which leads to the decrease in the production of buoyancy effect and eventually the reduction of the ventilation rate.

In addition, since the thicker glass allows less solar radiation to reach the room, the amount of residual solar heat absorbed by the room is also lower than that with the thinner glass, resulting in slightly lower daytime room temperature. Detailed comparison of the performance data obtained for these two glass thicknesses is presented in Table 2. It is clear that, with regard to the average ventilation rate over a full thermal cycle, these two configurations could achieve nearly identical performance and the ACH values obtained in either case are sufficient to meet the needs for typical residential ventilation. As for the room temperature, the system with the thicker glass panels can only increase the average and maximum room temperature by 1.44K and 2.05K respectively, compared with 1.72K and 2.86K when the thinner glass panels are used. However, for the minimum room temperature over the diurnal cycle, these two glass panel thicknesses do not make much difference at all.

DISCUSSION 1. Validation of the results

Although it has been widely accepted that THBM can be used to evaluate the performance of either a solar chimney or a water wall (for example, Ong and Chow 2003, Wu and Lei 2016b), rigorous validation should be undertaken to verify the adequacy of this technique in the study of a more complicated system, such as the one proposed here. Further effort will be made to validate the current results against CFD simulation and experiment. Considering that the THBM model discussed above is a simplified 1D model and carries the uncertainties of empirical equations in evaluating the heat transfer coefficients, head loss coefficients and wind pressure, discrepancies between the calculated results and the actual performance data are expected.

2. Comparison with solar chimney

Based on the results presented above, the proposed system can produce substantial ventilation for a regular-sized room. Although it could be anticipated that a conventional solar chimney has the ability to instantaneously introduce more ventilation with its highly absorptive wall, the ability of the proposed system to induce nocturnal ventilation is highlighted, especially for the places where ventilation during the night is sought after. In addition, considering that water wall in the current setting not only acts as a buffer to the heat loss at the windward side, but also help maintain the temperature of the indoor space with the heat collected during the day, the ability of the proposed system to reduce temperature swing and provide heating to the indoor space is highly appreciated. In the meantime, since in most conventional solar chimneys the absorbing wall is painted black or comprised of some opaque solar absorbing materials to maximize the absorption of solar radiation, daytime lighting is thus sacrificed. In contrast, since the exposed surfaces of solar chimney and water wall in the present system are made of glass panels and will not stop the sunlight from coming through, natural lighting is maintained for the indoor space during the daytime and this could be the most distinctive advantage over the conventional solar chimney designs.

3. Comparison with water wall

Compared with conventional water wall designs, the proposed system excels at making use of the heat released from the external glass panel as well as providing ventilation

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to the attached room. Although it is commonly known that water wall can act as a moderate space heater, the heat loss through the external glass panel is usually considered as a drawback to the system efficiency. In the proposed system, the water wall is combined with a solar chimney and thus the heat released from the water wall can be immediately reused to induce ventilation, and thus the overall energy efficiency of the system is effectively improved, especially at night.

4. Effect of windAs demonstrated in Figure 4, wind has a significant impacton the thermal performance of the proposed system.Although wind helps achieve a much higher ventilation rateand is effective in reducing ventilation fluctuations, it shouldalso be noted that the ability of the room to retain heat issacrificed by the high ventilation rate, which is a drawbackfor winter applications. Considering that the air change ratein a no wind scenario (2.2ACH) is adequate for residentialapplications, wind effect should be limited to stop the roomfrom being overcooled by excessive fresh air entering thespace. Possible solutions to overcoming the drawbacks ofthe wind effect include, but are not limited to:

• Installing wind breakers near the outlet of the solarchimney as well as near the inlet of the room;

• Reducing the size of the room inlet;

• Temporarily closing the solar chimney outlet and theroom inlet for the room to store heat.

5. Effect of glass panel thicknessAs shown in Figure 5 and Table 2, increasing the glass panelthickness from 19mm to 25mm in fact slightly deterioratesthe overall performance of the system. Although the overallsolar absorptance of the thicker glass panels is higher thanthat of the thinner glass panels, due to the decrease ofradiative energy reaching the inner surfaces of the solarchimney, the ventilation rate of the solar chimney does notbenefit from the increase of the glass panel thickness. Inaddition, due to the reduced solar transmittance, the residualsolar heat directly absorbed by the room is less for thethicker glass configuration, and as a result, the roomtemperature during the daytime is also lower. Therefore, it isnot advisable to replace the 19mm-thick glass with 25mm-thick glass for the present application.

CONCLUSIONS A novel passive thermal comfort strategy combining a solar chimney and a water wall is introduced. The performance of this system is calculated with a transient heat balance model. The effects of wind and glass panel thickness on the performance of the proposed structure are investigated based on the calculation results. A comparison between the proposed structure and a standalone solar chimney or water wall is discussed in terms of their ability to improve the thermal comfort level of the indoor space. Major conclusions drawn from the present study are as follows.

(a) The proposed system can effectively improve the thermalcomfort level by smoothing the diurnal temperature swing aswell as inducing ventilation.

(b) Wind effect is considerable. It increases the ventilationrate and also reduces the room temperature.

(c) Compared to the thermal performance obtained with the25-mm thick glass, a glass thickness of 19mm is preferredfor the present application.

(d) Compared with a conventional solar chimney, theproposed system provides daytime lighting as well asnocturnal ventilation. Compared with a conventional waterwall, the proposed system provides ventilation and alsohelps to improve the overall energy efficiency.

ACKNOWLEDGEMENT The current project is supported by the Australian Research Council through the Discovery Project grant DP170104023.

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CIBSE.

Duffie, J. A., & Beckman, W. A. (2013). Solar engineering of thermal processes: John Wiley & Sons.

Incropera, F., & DeWitt, D. Fundamentals of Heat and Mass Transfer, Wiley, New York, 1996.

McAdams, W. H. (1985). Heat Transmission, (1954). NY: McGraw Hill.

Munson, B. R., Young, D. F., & Okiishi, T. H. (1990). Fundamentals of fluid mechanics. New York, 3(4).

Niemann, H., & Höffer, R. (2007). Wind loading for the design of the solar tower. Paper presented at the Proc. 3rd Int. Conf. SEMC, Cape Town, South Africa.

Ong, K. S., & Chow, C. C. (2003). Performance of a solar chimney. Solar Energy, 74(1), 1-17. doi:10.1016/S0038-092X(03)00114-2

Rubin, M. (1985). Optical properties of soda lime silica glasses. Solar energy materials, 12(4), 275-288.

Swinbank, W. C. (1963). Long‐ wave radiation from clear

skies. Quarterly Journal of the Royal Meteorological Society, 89(381), 339-348.

Wu, T., & Lei, C. (2016a). CFD simulation of the thermal performance of an opaque water wall system for Australian climate. Solar Energy, 133, 141-154.

Wu, T., & Lei, C. (2016b). Thermal modelling and experimental validation of a semi-transparent water wall system for Sydney climate. Solar Energy, 136,

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