4th International Conference On Energy, Environment

Combined Solar and Water Wall for Ventilation and

H. Wang and C. Lei

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

absorber wall of a solar chimney is usually painted black and SUMMARY thus will block lighting from entering the room. On the other In order to improve the thermal comfort of an indoor space, a hand, although a water wall stops the undesirable heat passive thermal comfort strategy combining a solar chimney transfer between the indoor and outdoor environments and in with a water wall is introduced. A transient heat balance the meantime provides sufficient lighting, it introduces model (THBM) is adopted to evaluate the performance of this excessive heat to the attached room if solar radiation is structure in terms of its ability to induce ventilation and intensive, especially when no or air conditioner is used. It provide thermal buffering. Calculations are performed for a may also suffer from a significant deficiency in energy representative winter weather condition in Sydney. The efficiency during the night due to heat loss to the ambient impacts of wind and glass panel thickness on the environment. 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 In order to combine the benefits of both ventilation and raising as well as providing considerable thermal buffering strategies while minimizing the negative ventilation. In addition, wind can help increase the ventilation impact of the individual strategies, a novel structure rate but will also reduce the room temperature. Compared to incorporating a water wall into a solar chimney is proposed. the thermal performance obtained with the 25mm-thick glass In this novel structure, a water wall takes the place of the panels, a glass panel thickness of 19mm is preferred for the absorber wall of a conventional solar chimney. During the present application. day, the 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 INTRODUCTION solar chimney to maintain ventilation. In this study, a Ventilation and thermal buffering are two major design transient heat balance model (THBM) is adopted to analyze strategies to improve thermal comfort in an indoor space. the thermal performance of the integrated system. The effects of wind and glass panel thickness on the performance Ventilation comes into play when the indoor air turns stale of the integrated system are also investigated. 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 METHODS ventilation strategies by opening windows or with the 1. Model description adoption of solar chimney and other forms of passive Figure 1 displays the integration of a combined window-sized ventilation techniques. Natural ventilation has become a hot solar chimney and water wall system for a 3m×3m×3m research topic all over the world because it is free of energy space. The dashed box in Fig. 1(a) shows the installation of use and thus will not contribute to the increase of carbon the combined solar chimney and water wall structure, the footprint. Solar chimney stands out as a typical example of details of which are shown in Fig. 1(b), with the blue region passive natural ventilation strategies for its effectiveness in representing the water column and the glass panels being 2 inducing buoyancy-driven ventilation with solar radiation. shaded. The exposed area of the water wall is 1m , through which solar radiation enters the space. The thickness of the Unlike ventilation, the strategy of thermal buffering improves water column and the air gap width of the solar chimney are the indoor thermal comfort level by dampening the both 10cm. The red arrows in the figures indicate the undesirable heat exchange between the indoor and outdoor expected air flow path resulting from ventilation. An inlet for environments. Water wall is a typical example of passive the room is installed on the opposite wall to the proposed thermal buffering strategies. Standing between the indoor system and has the same dimensions as those of the space and the outdoor environments, a water wall serves as openings of the solar chimney. Material strength calculations a buffer zone against the undesirable outdoor weather are performed to determine the required thickness of the conditions. In the meantime, the water wall works as a glass panels. For brevity the detailed procedures are not moderate , especially at night, by releasing the shown here. According to the calculations, a thickness of solar heat stored during the day to the indoor space to slow 19mm (3/4in) is chosen for all the glass panels in the design. down the cooling process of the room due to the relatively lower temperature in the ambient environment. 2. Problem formulation A 1D transient heat balance model (THBM) can be set up to Though ventilation strategies, such as solar chimney, and account for the various processes within the thermal buffering methods, such as water wall, have both model. As shown in Figure 2, the points represent been widely acknowledged as effective, they have individual temperature nodes (T) in the system while the lines imperfections and thus are held back from realizing their full represent thermal couplings between different potential to achieve thermal comfort. For example, with solar components/nodes, which are denoted by distinctive radiation as the only heat source, a solar chimney is capable subscripts. Ta is the ambient temperature, and Tsky is the sky of providing ventilation during the day only, whereas after temperature, the radiation temperature of the ambient sunset the ventilation effect immediately ceases. In addition, environment, which is given by (Swinbank 1963) in order to enhance the absorption of solar radiation, the

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where V denotes wind velocity. Convective heat transfer (a) 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,

4/9 9/16 Nu = 0.68 + 0.67Ra1/4 /é 1+ 0.492 / Pr ù (3) () êë () úû

For Ra<109,

2 8/27 ì 1/6 é 9/16 ù ü (4) Nu = í0.825+ ()0.387Ra /1ê + ()0.492 / Pr ú ý î ë û þ (b) 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):

푟푎푑 2 2 −1 ℎ1−2 = 휎 ∙ (푇1 + 푇2 ) ∙ (푇1 + 푇2) ∙ (1/휀1 + 1/휀2 − 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 Figure 1.(a) Schematic of the proposed thermal comfort factor of -1 such that its aborptance and hw = 2m strategy; (b) Dimensions of the combined solar chimney and -hd water wall. (Not to scale). transmittance of radiation can be quantified as 1- e w and -hd e w respectively. For the glass panels in the present design, 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 Figure 2. Schematic of 1D transient heat balance model. 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 1.5 (1) air flowing regions. The transient performance of the Tsky = 0.0552Ta proposed system is obtained by solving the unsteady energy balance equations with an appropriate time step. For brevity, The subscript "g" denotes the glazing while "w1" and "w2" the actual equations solved in this study are not presented. represent the glass panels bounding the water column. Tc, Interested readers may find general equations relevant to the Tw and Tr denote the average temperatures of the air present problem in Munson et al. (1990). 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 RESULTS these components, as shown in the boxes in Figure 2. Only 1. Weather conditions the mesh points on the surfaces will be involved in To evaluate the performance of the proposed thermal calculating the heat transfer with its neighbouring comfort strategy, calculations are performed for a components. takes places at the interface representative and idealized winter weather condition of between fluid and solid while radiation occurs between the Sydney. The ambient temperature is assumed to follow a neighbouring surfaces as well as between the ambient (Tsky) sinusoidal pattern given below (Wu and Lei 2016a): 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) T (t) = T + 0.5DT siné 2p(t - t )/ Pù (6) a 0 ë lag û h = 5.7 + 3.8V (2) wind 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

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P = 24h is the period of thermal cycles. Similarly, the incident solar radiation is prescribed as (Wu and Lei 2016a): Ventilation rate 0.0415 (a) ì 1 (7) 0.0410 Imax sin2() pt / P for (m-1)P

temperature(K) 279 the solar chimney and the room inlet affects the pressure 276 distribution within the system. According to Butcher (2006), 0 3 6 9 12 15 18 21 24 wind pressure can be characterised by time(h) 9 mesh nodes+3.75s 5 mesh nodes+15s 1 p = rC V 2 (8) Ambient w 2 p where ρ is the air density, Cp is the wind pressure coefficient Figure 3. Performance comparison of two discretization and V is the wind velocity. The room inlet is considered to be schemes: (a) Ventilation rate; (b) Room temperature. 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 The stabilized results for these two sets of meshes and time solar chimney is assumed to be -0.45 (Niemann and Höffer steps are compared against each other to determine the 2007). A constant wind velocity of 3.944m/s is taken as a adequacy of the mesh resolution and time step. The reasonable simplification to the present problem formulation calculation results indicate that, starting from the fourth (Wu and Lei 2016a). 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 2. Spatial and temporal discretization test ventilation rate and room temperature. The stabilized test In order to obtain the typical performance of the proposed results for these two discretization schemes are presented in structure corresponding to the above-mentioned weather Figure 3. As shown in the figure, a good agreement is conditions, THBM calculations are performed for several reached for the test results obtained from these two mesh thermal cycles until repeating results are observed between configurations. In this sense, the 5 mesh nodes+15s two consecutive thermal cycles. Since thermal comfort is discretization can produce adequate results and thus will be directly related to the ventilation rate and indoor temperature, adopted in the following calculations. the time history of the ventilation rate and indoor temperature are monitored until the difference between the results 3. Calculation results obtained from two consecutive thermal cycles is reduced to As shown in Figure 3, despite a slight dip in the ventilation less than 1%. Considering that the temporal and spatial rate due to the descending ventilation effect originated from discretization may have an impact on the accuracy of the the last few hours in the previous thermal cycle, the results, a mesh and time step dependence test is performed ventilation rate produced by the system generally increases to determine the adequate resolution of mesh and time step. with solar radiation and reaches a peak in the first half of the Two discretization schemes are considered: 5 mesh thermal cycle. It is worth noting that there is a time lag nodes+15s and 9 mesh nodes+3.75s. The first number between the peak of ventilation rate in the first half of the represents the number of uniformly distributed mesh nodes thermal cycle and the peak of solar radiation. This across the thicknesses of the glass panels (including the phenomenon may be attributed to the combined effect of external glazing and the glass panels of the water wall), and instantaneous and non-instantaneous production of the second number represents the corresponding time step. buoyancy due to solar radiation. When solar radiation is In other words, the thickness of the glass panels is uniformly initiated, the surfaces of the solar chimney absorbs some of divided into 4 segments in the first discretization scheme and the solar radiation and part of the absorbed energy is 8 in the second discretization scheme. Time steps for these instantaneously transferred to the adjacent air to generate two discretization schemes are selected such that the buoyancy effect. On the other hand, some of the solar Fourier number (Fo) for these two configurations are kept the radiation absorbed by the interior of the glazing or the water same and that the stability requirement for the Forward- wall will gradually be transferred to the surfaces of the solar Time-Central-Space (FTCS) scheme (Fo≤0.5) is satisfied. chimney by conduction. Due to the time lag of conduction,

ISBN: 978-0-646-98213-7 COBEE2018-Paper235 page 707 4th International Conference On Building Energy, Environment the overall buoyancy effect peaks later than the solar Table 1. Optical properties of two glass thicknesses radiation. After solar radiation is completely gone, heating of Glass Solar Solar the solar chimney by the glazing and the water wall starts to Source take effect, bringing up the ventilation rate again with a thickness(mm) absorptance transmittance second peak formed at around the middle of the second half 19 0.383 0.558 of the thermal cycle. During this period, the decrease of the Rubin ambient temperature also contributes to the growth of 25 0.456 0.488 (1985) 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 Unlike the no wind scenario where the ventilation is solely starts to go down when the water wall is unable to provide caused by the buoyancy effect resulted from the heating of enough energy to maintain the ventilation rate. In the the solar chimney, when wind is present, the suction effect of meantime, it can be observed that with the proposed system, wind also contributes to the overall ventilation rate produced although the room is open to the ambient environment, the by the proposed system. Thus, when solar radiation is not room temperature is always higher than the ambient strong enough, or when the water wall is not able to provide temperature, with the average temperature being 1.72K enough heat to the solar chimney, wind steps in to maintain higher than the ambient. The maximum and the minimum enough ventilation to smooth out the deficit caused by the temperatures in the room are more than 2.86K and 1.27K solar chimney. For this reason, the ascending and higher than that in the ambient environment. For a relatively descending trends of the ventilation rate under the influence cold winter weather condition considered here, the proposed of wind are not as obvious as those without wind. In the system can thus improve the indoor thermal comfort level by meantime, when there is no wind, due to relatively less air increasing the room temperature as well as providing round- exchange with the colder ambient environment, the room the-clock ventilation. temperature is higher than that with wind (refer to Figure 4b), which is preferable for the winter condition. 4. Wind effect To distinguish the contribution of the system to the ventilation rate from the suction effect of wind blowing over the top of 5. Effect of glass panel thickness the outlet, calculations are performed for a case with zero In addition to its importance for the material strength and wind velocity. The thermal equilibrium results are shown in structural stability, the thickness of glass panels also plays Figure 4. It is clear in Figure 4(a) that, despite the effect of an important role in the thermal performance of the wind, the proposed system alone is able to produce combined solar chimney and water wall. As stated in Rubin significant amount of the ventilation. It can also be observed (1985), for a specific type of glass, solar absorptance that the ventilation rate for a no wind scenario experiences increases while transmittance decreases with its thickness. more fluctuations than that for a windy scenario. This In other words, if a thicker glass panel is used in the design phenomenon could be attributed to the suction effect of wind. of the proposed system, a higher potential of solar heat absorption by the combined solar chimney and water wall is anticipated. Considering that the heat absorbed by the Ventilation rate proposed system can either be utilized to improve ventilation 0.050 through the solar chimney or be released to the room (a) 0.045 through the water wall as a means of space heating, the 0.040 0.035 effect of the glass panel thickness on the overall thermal 0.030 performance of the system is yet to be determined. 0.025 In order to investigate the effect of the glass panel thickness, 0.020 calculations are performed for the same system but with a 0.015 0.010 thickness of 25mm. The optical properties of this glass thickness are extracted from Rubin (1985) and are listed in

ventilation rate(kg/s) ventilation 0.005 0 3 6 9 12 15 18 21 24 Table 1, together with the optical properties of the glass panels used in the original (reference) case. Calculation time(h) with wind no wind results for the case with a glass thickness of 25mm are plotted in Figure 5 along with those of the original case.

Room temperature As shown in Figures 5(a) and (b), although the increase of 300 (b) the glass thickness is supposed to enable the solar chimney 297 and water wall to absorb more solar heat, the ventilation rate 294 and room temperature during the daytime are in fact slightly 291 288 lower than those with thinner glass panels. As for the water 285 temperature shown in Figure 5(c), the case with thicker glass 282 panels does not show a remarkable difference from the temperature(K) 279 original case, and thus the increase in the glass thickness 276 does not improve the thermal storage in the water wall. The 0 3 6 9 12 15 18 21 24 decline in the thermal performance could be explained from the perspective of the energy attenuation. Since radiative time(h) energy attenuates along the thickness of the glass panels With wind No wind Ambient through absorption, the more radiation is absorbed, the less Figure 4. Performance comparison with or without wind: (a) radiative energy is allowed to pass through. In this sense, Ventilation rate; (b) Room temperature. although the thicker glass panel has a higher overall solar

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In addition, since the thicker glass allows less solar radiation Ventilation rate to reach the room, the amount of residual solar heat 0.0415 absorbed by the room is also lower than that with the thinner (a) 0.0410 glass, resulting in slightly lower daytime room temperature. Detailed comparison of the performance data obtained for 0.0405 these two glass thicknesses is presented in Table 2. It is 0.0400 clear that, with regard to the average ventilation rate over a full thermal cycle, these two configurations could achieve 0.0395 nearly identical performance and the ACH values obtained in 0.0390 either case are sufficient to meet the needs for typical

ventilation rate(kg/s) ventilation residential ventilation. As for the room temperature, the 0.0385 system with the thicker glass panels can only increase the 0 3 6 9 12 15 18 21 24 average and maximum room temperature by 1.44K and time(h) 2.05K respectively, compared with 1.72K and 2.86K when 19mm 25mm the thinner glass panels are used. However, for the minimum room temperature over the diurnal cycle, these two glass Room temperature panel thicknesses do not make much difference at all. 297 (b) 294 DISCUSSION 291 1. Validation of the results 288 Although it has been widely accepted that THBM can be 285 used to evaluate the performance of either a solar chimney 282 or a water wall (for example, Ong and Chow 2003, Wu and

temperature(K) 279 Lei 2016b), rigorous validation should be undertaken to verify 276 the adequacy of this technique in the study of a more 0 3 6 9 12 15 18 21 24 complicated system, such as the one proposed here. Further time(h) effort will be made to validate the current results against CFD simulation and experiment. Considering that the THBM 19mm 25mm Ambient model discussed above is a simplified 1D model and carries the uncertainties of empirical equations in evaluating the Water temperature heat transfer coefficients, head loss coefficients and wind 309 pressure, discrepancies between the calculated results and (c) 306 the actual performance data are expected. 303 300 297 2. Comparison with solar chimney 294 Based on the results presented above, the proposed system

temperature(K) 291 can produce substantial ventilation for a regular-sized room. 288 Although it could be anticipated that a conventional solar 0 3 6 9 12 15 18 21 24 chimney has the ability to instantaneously introduce more time(h) ventilation with its highly absorptive wall, the ability of the 19mm 25mm proposed system to induce nocturnal ventilation is Figure 5. Performance comparison of two glass thickness highlighted, especially for the places where ventilation during configurations: (a) Ventilation rate; (b) Room temperature; (c) the night is sought after. In addition, considering that water Water temperature. 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 Table 2. Thermal comfort indicators obtained with two glass during the day, the ability of the proposed system to reduce thicknesses temperature swing and provide heating to the indoor space is highly appreciated. In the meantime, since in most Glass ACH Troom,avg Tmax Tmin conventional solar chimneys the absorbing wall is painted thickness black or comprised of some opaque solar absorbing (K) (K) (K) (mm) (kg/s) materials to maximize the absorption of solar radiation, 19 0.04 4.43 287.22 295.86 279.27 daytime lighting is thus sacrificed. In contrast, since the 25 0.04 4.43 286.94 295.05 279.29 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 absorptance rate, the amount of energy reaching the inner maintained for the indoor space during the daytime and this surface of the external glazing and the outer surface of the could be the most distinctive advantage over the first glass panel of the water wall is accordingly lower than conventional solar chimney designs. that with the thinner glass. As a result, the heating rate of the inner surfaces of the solar chimney with thicker glass panels 3. Comparison with water wall is less than that using thinner glass panels, which leads to the decrease in the production of buoyancy effect and Compared with conventional water wall designs, the eventually the reduction of the ventilation rate. proposed system excels at making use of the heat released from the external glass panel as well as providing ventilation

ISBN: 978-0-646-98213-7 COBEE2018-Paper235 page 709 4th International Conference On Building Energy, Environment to the attached room. Although it is commonly known that (a) The proposed system can effectively improve the thermal water wall can act as a moderate space heater, the heat loss comfort level by smoothing the diurnal temperature swing as through the external glass panel is usually considered as a well as inducing ventilation. drawback to the system efficiency. In the proposed system, the water wall is combined with a solar chimney and thus the (b) Wind effect is considerable. It increases the ventilation heat released from the water wall can be immediately reused rate and also reduces the room temperature. to induce ventilation, and thus the overall energy efficiency of (c) Compared to the thermal performance obtained with the the system is effectively improved, especially at night. 25-mm thick glass, a glass thickness of 19mm is preferred for the present application.

4. Effect of wind (d) Compared with a conventional solar chimney, the As demonstrated in Figure 4, wind has a significant impact proposed system provides daytime lighting as well as on the thermal performance of the proposed system. nocturnal ventilation. Compared with a conventional water Although wind helps achieve a much higher ventilation rate wall, the proposed system provides ventilation and also and is effective in reducing ventilation fluctuations, it should helps to improve the overall energy efficiency. also be noted that the ability of the room to retain heat is sacrificed by the high ventilation rate, which is a drawback ACKNOWLEDGEMENT for winter applications. Considering that the air change rate The current project is supported by the Australian Research in a no wind scenario (2.2ACH) is adequate for residential Council through the Discovery Project grant DP170104023. applications, wind effect should be limited to stop the room from being overcooled by excessive fresh air entering the REFERENCES space. Possible solutions to overcoming the drawbacks of Butcher, K. (2006). CIBSE Guide A–Environmental Design: the wind effect include, but are not limited to: CIBSE. • Installing wind breakers near the outlet of the solar chimney as well as near the inlet of the room; Duffie, J. A., & Beckman, W. A. (2013). Solar engineering of • Reducing the size of the room inlet; thermal processes: John Wiley & Sons. • Temporarily closing the solar chimney outlet and the room inlet for the room to store heat. Incropera, F., & DeWitt, D. Fundamentals of Heat and Mass Transfer, Wiley, New York, 1996. 5. Effect of glass panel thickness McAdams, W. H. (1985). Heat Transmission, (1954). NY: As shown in Figure 5 and Table 2, increasing the glass panel McGraw Hill. thickness from 19mm to 25mm in fact slightly deteriorates the overall performance of the system. Although the overall Munson, B. R., Young, D. F., & Okiishi, T. H. (1990). solar absorptance of the thicker glass panels is higher than Fundamentals of fluid mechanics. New York, 3(4). that of the thinner glass panels, due to the decrease of radiative energy reaching the inner surfaces of the solar Niemann, H., & Höffer, R. (2007). Wind loading for the design chimney, the ventilation rate of the solar chimney does not of the solar tower. Paper presented at the Proc. 3rd benefit from the increase of the glass panel thickness. In Int. Conf. SEMC, Cape Town, South Africa. addition, due to the reduced solar transmittance, the residual Ong, K. S., & Chow, C. C. (2003). Performance of a solar solar heat directly absorbed by the room is less for the chimney. , 74(1), 1 17. thicker glass configuration, and as a result, the room - doi:10.1016/S0038 092X(03)00114 2 temperature during the daytime is also lower. Therefore, it is - - not advisable to replace the 19mm-thick glass with 25mm- Rubin, M. (1985). Optical properties of soda lime silica thick glass for the present application. glasses. Solar energy materials, 12(4), 275-288.

CONCLUSIONS Swinbank, W. C. (1963). Long‐ wave radiation from clear skies. Quarterly Journal of the Royal Meteorological A novel passive thermal comfort strategy combining a solar Society, 89(381), 339 348. chimney and a water wall is introduced. The performance of - this system is calculated with a transient heat balance model. Wu, T., & Lei, C. (2016a). CFD simulation of the thermal The effects of wind and glass panel thickness on the performance of an opaque water wall system for performance of the proposed structure are investigated Australian climate. Solar Energy, 133, 141-154. based on the calculation results. A comparison between the proposed structure and a standalone solar chimney or water Wu, T., & Lei, C. (2016b). Thermal modelling and wall is discussed in terms of their ability to improve the experimental validation of a semi-transparent water thermal comfort level of the indoor space. Major conclusions wall system for Sydney climate. Solar Energy, 136, drawn from the present study are as follows. 533-546.

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