6152nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

Cooling of outdoor spaces by means of evaporative-cooling ceramic-pillars

D. Lanceta, F. Manteca, C. Martín, D. Martínez, F. Serna, J. LlorenteRenewable Energy Centre (CENER), Spain

The principle of evaporative cooling using porous mate-rials is nothing new and goes well back into ancient times. Even nowadays porous containers are used to have fresh water available in hot and dry places like Spain or EgyptEvaporative cooling systems may be classified into di-rect and indirect. Direct evaporative cooling introduces moisture into the supply air. Indirect systems produce cooling without adding moisture to supply air by using a heat exchanger between two separate airflows. Achieve-ment of evaporative cooling of the air stream has been effected by passing air through damp pads (Hicks, N. G. et al. 1991), passing it through falling water (Gillet, A.C., et al. 1991), or spraying into the air stream (Bow-man, N. et al. 1997).The aims of this project, besides the adaptation of an evaporative cooling system to the specific architecture of a building, are to bring the evaporative cooling sys-tems closer to the general public.

2. DEVELOPMENT

2.1 Facility descriptionFigure 1 represents the floor of the Spanish pavilion for the International Exhibition in Zaragoza 2008. The building has a great trapezoidal-shape roof covering both the pavilion itself and the outdoor spaces where plenty of ceramic pillars suggest the idea of a passable artificial wood where visitors can wait in the shadow to enter the precincts. It is in this space, delimited by a rectangle in Figure 1, where the evaporative cooling pillars object of the current study would be placed. These false pillars are apparently the same as the structural ones on the out-side and they consist of a hollow ceramic pillar being continuously moisturized. As water evaporates on the ceramic it reduces its temperature which in turn cools the air forced to circulate inside it. The pillars are ap-proximately 11 meters high and have an external diam-eter of 0.28 meters. The water openings which moisturize the inside surface of the ceramic are located on the upper part. This inner part is moisturized during a certain time by a continuous film descending due to gravity (Fig. 3).Once the moisturizing process is over, air is impelled out through the space between the ceramic and the structural core (Fig. 3).

ABSTRACT

This document presents the theoretical study of evapo-rative cooling designed for the outdoor spaces in the Spanish pavilion of the coming International Exhibition in Zaragoza 2008. The system designed used some ce-ramic pillars located in the external spaces of the build-ing, as means of heat exchanger, causing the tempera-ture to drop as well as an increase of humidity in the air-flow going through. The result of the study concludes that by using the sys-tem designed, a considerable local cooling effect is ob-tained as well as fulfilling the objectives of architectoni-cal integration, being it also a spreading and didactic system suitable for such an event as an International Exhibition.

1. INTRODUCTION

Primary energy consumption continues to grow franti-cally on a worldwide base. This growth is partly due to the greater use of air-conditioning systems in buildings. Due to the low energy price, solar and ventilation protec-tion measures have been partially consigned to powerful HVAC systems capable of coping with high demands.In order to reduce the energy needs associated to these systems, it is necessary to count on good architecture, adapted to the environment and the needs of the users. So before deciding on a conventional air-conditioning system, the possibility of incorporating a passive cool-ing system has to be considered, knowing that the main heat sinks and dis-sipating ways in passive cooling are :Radiative cooling: the heat sink being the sky and the heat transfer mode radiation.Ground cooling: the main heat sink being the earth and the heat transfer mode conduction.Evaporative cooling: the heat sink being air (the wet bulb depression) and the heat transfer mode evaporation.Ventilative cooling: the heat sink being the air and the heat transfer mode convection.In the building context, traditional dwellings in dry cli-mates have been granted with passive elements. The role of trees, greenery and water has always been ap-preciated. The construction of great walls and water cooling by evaporation were some of the strategies.

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When the pillar has lost enough moisture so that it af-fects its operation, the air flow is stopped and the inter-nal surface of the pillar is once again moisturized.This operation produces a cooler and more moistened air than initially which is driven to the lower part of the pillar and from there to the visitors through some grilles on the ceramic (Fig. 2).

Figure 1. Floor of the Spanish Pavilion Expo 2008.

Figure 2. Distribution system for cooled air.

Figure 3. Sections of pillar.

2.2 Mathematical modelA mathematical model has been made starting from the physical processes prevailing in the behaviour of the system. This model establishes an energy balance in the ceramic wall of the pillar which temperature varies along the day as the result of the energy balance. Solving the equation system, several data of interest are obtained such as the temperature of the inner air when leaving the pillar and the flow rate of evaporated water among others.

2.2.1 RadiationThe term radiation reflects the radiative exchange due to temperature difference between the system environment and the external surface. This heat exchange only makes reference to long wave radiation so the sun radiation is not included, because the cooling pillars are in the shad-ow during most of the day as they are under the pavilion roof projection. This heat radiation is considered as:

outerceramicambceramicrad ATTQ )( 44 −= σε (1)

radQ : Radiation Heat Transfer (W).

σ : Stefan-Boltzmann constant.ceramicε :

Emissivity of ceramic pillars.

ambT : Ambient Temperature.(K)

ceramicT :Temperature of ceramics pillars. (K)

outerA :Area of outer surface (m2).

2.2.2 External convectionTo estimate convective exchange of the pillar outer sur-face, equation (2) for crossed flow on circular cylinder is used (Incropera, F. P. and De Witt, D. P. 1996).

(2)Nu D_outer : Nusselt number for outer surface.Re D_outer : Reynolds number, outer diam-

eter.Pr: Prandtl number.PrS: Prandtl number for surface.

Finding the value for the convection coefficient in equa-tion (2), heat transference through the external part of the ceramic takes the value shown on equation (3).

)(_ ceramicambouterouterouterconv TTAhQ −= (3)

outerconQ _

:Convective heat transfer (W) through the external part.

houter :Convective heat transfer coefficient in the outer surface (W/m2K).

Aouter : Outer Area (m2).

2.2.3 Internal convectionThe inner flow driven inside the pillar exchanges heat and moisture with the internal part. Assuming a fully developed turbulent profile (both hydrodynamically and thermally) in smooth pipe, the coefficient for heat transfer can be determined by equation (4). As the ge-ometry of the pipe is not circular, the hydraulic diameter of the section is used instead (Incropera, F. P. and De Witt, D. P. 1996).

σε

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(4)Nu Dh_innerr : Nusselt number for hydraulic diameter of

inner section.

Re D_h_inner: Reynolds number, hydraulic inner diam-eter.

The heat exchanged between the air flow and the ceram-ic results from integrating, over the whole pipe, the en-ergy balance on a small length of pipe. Considering that the ceramic temperature does not vary greatly within the domain and that the flow is developed, wall temper-ature value and local convection coefficient are taken as constants. The amount of heat exchanged by convection in the inside takes the value of equation (5).

(5)

innerconvQ _

:Convective heat transfer (W) in the internal part.

m : Mass flow rate (kg/s)

cp: Specific heat (J/kg K)

houter : Convective heat transfer coefficient in the inner surface (W/m2K).

Ainner : Inner Area (m2).

2.2.4 EvaporationThe mass transference coefficients for evaporation are calculated from the convection coefficients using Chilton-Colburn’s analogy (eq. (6)) (Incropera, F. P. and De Witt, D. P. 1996).

(6) hm: Convective mass transfer coefficient (m/s)

Le: Lewis number.

This way, the transference of water vapour from the sur-face (evaporation) is calculated in the same way as the convective heat transference. To estimate the heat loss associated with evaporation, the evaporated water mass flow is multiplied by the latent heat of vaporization. The heat loss from the outer surface results in equation (7), and equation (8) reflects the exchange from the inner one.

(7)Qevap_outer : Evaporative heat transfer (W) in

the internal part.

hm_outer: Convective mass transfer coeffi-cient (m/s) in the external side

f_wet_outer: Partial moisture coverage param-eter in the outer side.

Ainner : Inner Area (m2).ρ h20_sat : Density of water vapour in ambient

conditions (kg/m3).ρ h20_amb : Density of water vapour in ambient

conditions (kg/m3).

(8)Qevap_inner: Evaporative heat transfer (W) in the internal part.

V : Volumetric flow rate (m3/s).

f_wet_inner: Partial moisture coverage parameter in the inner part.

hm_inner: Convective mass transfer coefficient (m/s) in the internal side

Lvap : Latent heat of vaporization (J/kg).

2.2.5 Global balanceThe addition of all heat flows on the ceramic determines the evolution of its average temperature within time. Its variation rate is calculated with equation (9).

(9)mceramic: Mass of ceramic (kg).cp_ceramic: Specific heat of ceramic(J/kg K)

2.3 Moisturizing the ceramic2.3.1 Moisturizing flow rateThe moisturizing of the pillars is performed by an in-ternal spray system which lets water out on the upper part to form a film which goes down the internal part due to gravity. As ceramic is a porous material, it retains part of the water going down its surface by capillarity and therefore increasing its water content. The amount of water absorbed by the ceramic is a function of its moisturizing time so the flow needed for such moisture shall be the minimum one guaranteeing the existence of a film going down the whole surface.This flow depends among other things on the geometry and the nature of the moistened surface, of the surface tension in the liquid and of the existing mass flows. Part joints can influence the film behaviour (Ponter et al. 1967), (Stainthorp and Allen 1967) and (Watanabe et al. 1975).Due to the complexity of the problem, the estimations by means of equations for the minimum moistened flow tend not to be accurate. The most reliable thing is to perform tests on the final location. According to the previously mentioned bibliogra-

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phy, the interval one can practically assure the mini-mum moistened flow will be within is as follows:

(10)Using the data for the internal perimeter of real geometry, P = 0.85 m, the resulting interval for mass flow is as follows:

(11) With the greater flow within the interval, 0.34 kg/s per pil-lar, the existence of an even film is almost guaranteed. Great values such as this one enhance the probability to have good moisturizing, increasing water expense on the other hand.

2.3.2 Moisturizing timeThe moisturizing time for the ceramic should be enough for it to absorb sufficient water to allow the correct op-eration of the system. On the other hand, too long times will result in excessive water waste as when the ceramic is saturated it absorbs less water from the water film.The problem is similar to the drying process for solids although the purpose is different. Figure 4 shows the typical curve for drying solids.

Figure 4. Typical curve for drying solids.

After a short and transitory initial period to balance with the external temperature in the solid (line AB), there is a period of constant drying speed (lineBC). In this period, the water speed movement inside the solid is faster than the one of evaporation on the surface so that the whole external surface is moisturized and it is the evaporation speed which limits water loss. Point C is the critical moisture content, and from then on the surface can no longer remain saturated in some areas and the evapora-tion speed starts to depend more and more on the inter-nal speeds of moisture movement. For the good operation of the evaporative-cooling sys-tem, it has to work on the BC area of the graph as greater evaporation speeds result in a bigger drop on the tem-

perature of the air to be treated. The closer to this critical moisture content point, the greater the amount of water absorbed by time unit will be during the wetting process.An accurate experimental adjustment of the system will be made based on this information so that the wetting times needed are minimized without affecting the cor-rect operation of the system.

2.4 Pressure lossThe aim of this section is to calculate the pressure loss the air suffers when going through the system. Deter-mining this pressure is necessary, among other things, to take the measures of the appropriate fans to supply the required air flow.The problem of pressure loss is a complex one with many variables involved. In the case of knowing the ge-ometry, pressure drop between two points is a function of the following variables:

),,,( lvfP µρ=∆ (12)By virtue of Buckingham´s Pi Theorem (Kundu, P. K. and Cohen, I. M. 2004), these variables can be related in the following way:

(13)ΔP: Pressure drop (Pa).ρ: Density (kg/m3).ν: Velocity (m/s).D: Diameter (m).μ: Viscosity (N s/m2).

In equation (13), the dependence of Reynolds number is an unknown function. In order to obtain a quick approx-imation to the system pressure loss curve, it has been as-sumed that Reynolds number has little dependence and therefore the function has been taken as a constant. This way the equation becomes equation (14) and only the value for the constant K remains unknown.

Kv

P=

∆2

21 ρ

(14)A CFD code has been used to determine this value. This software was used to solve numerically the mass and mo-mentum conservation equation, using the standard k-ε tur-bulence model (Launder, B. E. and Spalding, D. B. 1974).In order to perform the simulation properly, the domain was divided into three parts (approximately 700k cells each part), calculating the loss coefficient for each of them.The mouth part in the air pipes has a little conventional geom-etry due to the constructive characteristics of the pavilion roof. This geometry might show an important load loss (Figure 5-left).

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6192nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

The straight part of the pillar is basically a ring-section conduction. To secure the ceramic part to the internal structural pillar, some fittings had to be included, partially occupying the section for air transit. These fittings appear at the beginning of each ceramic part, being a periodical obstacle for the internal air-movement. (Fig. 5-middle).The pressure loss at the external air outlet grille has to be added to the pressure drop mentioned up to now. Fig-ure 5-right shows a detail of the geometry considered:

Figure 5 - Mouth, fittings and outlet grille of the pillar.

From the results obtained for the different sections, a constant for the whole system is estimated as the addi-tion of the K´s at each section. The result obtained is:Table 1 - Loss coefficients predicted by CFD simulations.

Section Air Flow Velocity (m/s)/ /Flow Rate (m3 /hour) Re K ΔP (Pa)

Mouth 5 / 814 50112 1.90 27.5Pillar 5 / 814 50112 9.86 151.0Outlet 5 / 814 50112 1.76 26.9Total 5 / 814 50112 13.52 205.4

Substituting internal velocity by volumetric flow rate, the resulting pressure loss curve is plotted in Figure 6.

Figure 6 - System pressure loss.

3. RESULTS

Using the theoretical model developed, the behaviour of an evaporative pillar has been calculated during a sum-mer day with high temperatures in Zaragoza (Spain).

In order to have an interval to place the possible operation range of the system, two simulations have been performed:The aim of the first simulation is to establish the highest level for system operation. To do so, some very idealis-tic assumptions have been made which will hardly hap-pen in the system in a continuous way: ceramic has been considered to be always moisturized both internally as on the outside and a high average speed for internal air flow has also been assumed (5 m/s).

Figure 7 - Operation of the system during a day (upper limit).

2. The second simulation establishes the inferior limits for the system to operate. The values for the parameters chosen determine poor system operation to establish the lowest performance level. It is assumed that the exter-nal face of the ceramic completely lacks moisture on its surface and the average air blow speed is low (3 m/s).

Figure 8 - Operation of the system during a day (lowest limit).

4. CONCLUSIONS

Starting from an idea which tries to join architectonic integration and comfort together in outdoor spaces, this theoretical study intends to quantify the operation of a system which was originally designed in an intuitive way.Despite the fact that other evaporative cooling systems such as water micronization do not need such a com-

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plicated structural installation, architectonical elements have been thought to be used in this innovative way.The aim of the evaporative cooling pillars is also to be an interaction element for visitors, getting the evapora-tive cooling principles close to the general public in an enjoyable way.Monitoring the real operation of the pillars will serve to validate the mathematical model developed.

REFERENCES

Bowman, N., Lomas, K., Cook, M., Eppel, H., Ford, B., Hewitt, M., Cucinella, M., Francis, E., Rodriguez, E., Gonzalez, R., Ál-varez, S., Galata, A., Lanarde, P. and Belarbi, R. (1997), Applica-tion of PDEC to non-domestic buildings, Renewable Energy 10 (2-3), 191-196.Gillet, A. C., de Laminne, J. M. and Lefebvre, S. (1991). An Il-lustration of Semi-Passive Bioclimatic Architecture: The Belgian Pavilion Project for the Seville World Fair 1992, Plea´91 Archi-tecture and Urban Space, Seville, Spain, pp. 589-594.Hicks, N. G., Thomson, T. L., Yoklic, M. R., Chalfoun, N. V. and Kent, K. J. (1991). Evaporative Cooling for Large Exhibition Spaces: A Methodology for System Design/Engineering based on Human Comfort, PLEA´91 Architecture and Urban Space, Se-ville, Spain, pp.159-164. Incropera, F. P. and De Witt, D. P. (4th ed) (1996). Fundamentals of Heat and Mass Transfer, New York, Wiley & Sons.Kundu, P. K. and Cohen, I. M. (3rd edition) (2004). Fluid Me-chanics, San Diego, Elsevier.Launder, B. E. and Spalding, D. B. (1974). The Numerical Com-putation of Turbulent Flows. Computer Methods in Applied Me-chanics and Engineering, 3:269-289.Ponter et al. (1967). Int. J. Heat Mass Transfer, 10, 349–359; Trans. Inst. Chem.Eng., London, 45, 345–352.Stainthorp and Allen (1967). Trans. Inst. Chem. Eng. London, 43, 85–91.Watanabe et al. (1975). J.Chem. Eng., Japan, 8, 75.

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