Theoretical and experimental analysis of a novel low emissivity water pond in summer

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Solar Energy 86 (2012) 3331–3344 www.elsevier.com/locate/solener

Theoretical and experimental analysis of a novel low emissivity water pond in summer Artemisia Spanaki a,⇑, Dionysia Kolokotsa b, Theocharis Tsoutsos b, Ilias Zacharopoulos a a

National Technical University of Athens, School of Architecture, Department of Architectural Technology, 42 Patission Street, 10682 Athens, Greece b Technical University of Crete, Environmental Engineering Department, 73100 Chania, Crete, Greece Received 30 March 2012; received in revised form 7 August 2012; accepted 29 August 2012 Available online 2 October 2012 Communicated by: Associate Editor Matheos Santamouris

Abstract The present study focuses on the research of a new passive roof cooling technique, based on the combination of low emissivity materials and water. A novel roof pond is chosen as the most advantageous in terms of both energy efficiency and less maintenance or functional demands. The pond – referred as “Roof Pond with Gunny Bag” (RPWGB) – is covered by a cloth floating on water level, encouraging evaporative heat losses. A mathematical model describing the energy flux through the RPWGB is developed. The following sensitivity analysis marks the parameters that reduce bottom pond temperature thus improving the efficiency of the system. The experimental study analyses alternative ways to reduce bottom pond temperature. For this purpose, the low emissivity material is placed in different positions, above, below and floating on water level. Heat dissipation occurs by means of radiation losses and water evaporation. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Roof pond; Passive cooling; Low emissivity; Energy efficiency

1. Introduction Roofs are widely considered as attractive elements for the application of passive cooling technologies in the built environment. During the last decades various roofs’ oriented technologies are developed and tested versus their contribution to the energy efficiency as well as their potential in mitigating the urban heat island. Those technologies include cool materials and cool roofs (Santamouris et al., 2011, 2012), green roofs (Zinzi and Agnoli, in press) and roof ponds. Roof ponds are considered a controversial passive cooling technology. The main benefits of roof ponds are their contribution to the improvement of indoor summer comfort conditions and the reduction of energy demand ⇑ Corresponding author. Address: W.B.18, Vrahokipos, Kokkini Hani, GR71500 Heraklion, Crete, Greece. Tel./fax: +30 2810326779. E-mail address: [email protected] (A. Spanaki).

0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.08.017

for cooling, according to experimental and numerical researches (Rincon et al., 2001). A number of theoretical models (Yadav and Rao, 1983) have been developed in order to predict the thermal performance of a building with roof ponds; Givoni (2011) generated formulas, expressing the indoor temperatures of the building as a function of the outdoor temperatures. The utilisability of the roof should be also taken into account when applying roof ponds since the accessible roof space can be considerably decreased. Various configurations are proposed to overcome this issue since it is a critical drawback comparing to other roof oriented passive cooling techniques (i.e. green roofs, roof shading, cool roofs, etc.). For example walkable roof ponds are proposed by Givoni (1994). Another critical topic is the roof pond’s aesthetics. The integration of the roof pond with the overall building should be carefully performed by the architect while taking into account the building owner’s perception since aesthetics is a very subjective issue. A detailed review

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Nomenclature a A C Cp h

H0 h1 H1 h2 H2 hc hcw hi

hwr I Ih k n P Pa

absorptance of the roof roof area, m2 specific heat specific heat, J/kg K heat transfer coefficient between the inner roof surface and the indoor air including radiative heat thickness of wetted gunny bags, m thickness of each layer of water water depth, m thickness of each roof layers, m thickness of roof, m convective heat transfer coefficient, W/m2 K convective heat transfer coefficient between gunny bags and top water layer, W/m2 K heat transfer coefficient between the inner roof surface and the indoor air including radiative heat transfer, W/m2 K convective coefficient between the roof and the bottom water layer, W/m2 K incident solar radiation on a horizontal surface, W/m2 incident solar radiation on a horizontal surface heat conductivity, W/m K number of water and roof layers saturated vapor pressures of air at the temperature of water level, N/m2 saturated vapor pressures of air at ambient air temperature, N/m2

and analysis of the benefits and drawbacks of the various roof pond configurations is performed in previous researches (Spanaki et al., 2011). The contribution of roof ponds in the urban heat island mitigation via evaporative cooling is also an important issue (Rizwan et al., 2008). Lowering the temperature in the upper level of the roof, decreases to the heat penetrating into the building; this contributes to the decrease of the ambient air temperature as the heat convection from a cooler level is lower (Synnefa and Santamouris, in press). Conclusively, roof ponds improve microclimate acting as a heat sink. Although various studies (Naticchia et al., 2007; Kharrufa and Yahyah, 2008; Raeissi and Taheri, 2008) indicate the cooling effect of roof ponds, there is a restriction on their wide applicability due to increased loads in the construction and maintenance requirements. In order to overcome the maintenance issues, Tang et al. (2004) suggested a Roof Pond with negligible maintenance. The Roof Pond with Gunny Bags (RPWGB) is simply constructed by keeping gunny bags or cloth floating on water level. The textile is supported by polystyrene strips or other floatable

Qe Qe r T Tr U

heat loss due to water evaporation from a free water level heat loss due to water evaporation from a wetted surface density time, s temperature of roof’s external surface, °C U-value, W/m2 K

Greek symbols a absorptivity e emissivity v wind speed over the water level, m/s u relative humidity r Stefan–Boltzmann-constant r = 5.67  108 W m2 K4 q density, kg/m3 Subscripts a ambient air dp dew point g gunny bags i indoor air r roof w water Abbreviations RPWGB Roof Pond with Gunny Bags RPWMI Roof Pond with Movable Insulation

material attached to it underneath. The wet gunny bags or cloths capture the solar radiation, and dissipate the heat through water evaporation, convection and radiation losses. Moreover water absorbs the heat gains from the building and dissipate them. According to simulation results (Tang & Etzion, 2004) RPWGB have a better cooling performance compared to a roof covered with wet gunny bags which had been considered an efficient roof cooling technique. The better cooling performance refers to both indoor air temperature and heat flux through the roof. Furthermore according to another simulation (Tang & Etzion, 2005) RPWGB performs slightly better than Roof Ponds with Movable insulation that is also considered as an efficient evaporative roof cooling technique. The reason for the increased efficiency of the system is attributed to the creation of water’s thermal stratification inside the pond. Concluding, Roof Ponds with gunny bags is more effective compared to specific evaporative roof cooling techniques, while the system does not demand maintenance or daily operation. To this end, the aim of the present research is to enhance the performance of the RPWGB using alternative roof

A. Spanaki et al. / Solar Energy 86 (2012) 3331–3344

pond configurations, thus proposing alternative roof pond solutions. For this purpose, the present paper includes an analytical and an experimental part. In the framework of the analytical part the parameters affecting the performance the RPWGB configuration are examined. A mathematical model that describes the heat flux through RPWGB is developed. The results of the parametric study are developed in the experimental set-up. Various roof ponds configurations using different coverings in alternative positions are experimentally tested targeting on the reduction of water overheating while encouraging heat dissipation by means of both evaporation and radiation losses. Finally, the experimental results are discussed and issues of further research are also proposed. 2. Theoretical analysis

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The assumptions of the procedure are the following:  Due to the small thickness H0 of the wet gunny bag is considered to be a single layer having the heat capacity of water.  The upper surface of the gunny bag is considered to thermally represent the whole gunny bag.  The water inside the pond is uniformly divided into n layers while the center of each layer (the thickness of each layer is h1 = H1/n) is thermally represents the whole layer.  The temperature distribution inside the pond with water depth of H1 is assumed to be one dimensional and fully thermally stratified along the depth of pond.  Heat transfer along the envelope of the pond is disregarded. Water thickness H0 and H1 are kept as constants (Tang et al., 2004).

2.1. Mathematical model RPWGB is consisted by three materials; from top to bottom the gunny bags (wet textile), water and the concrete roof are laid. The temperature distribution throughout the layers of the system is both affected by ambient and indoor air conditions, as shown in Fig. 1. The heat flux through the examined system is due to convection, evaporation, and radiation. The heat loss Qe from a wetted surface can be expressed by the empirical equation (Tang et al., 2004). Qe ¼ ð0:7581 þ 0:4257vÞðP g  uP a Þ

0:7

ð1Þ

where Pg and Pa (in N/m2) represent the saturated vapor pressures at the water temperature Tg (in °C) in the gunny bags (textile) and ambient air temperature Ta, respectively given by Tang et al. (2004) 0:5

P ¼ 3385:5  exp½8:0929 þ 0:97608 ðT þ 42:607Þ 

ð2Þ

Eq. (2) in case of Pa and Pg is respectively modified as follow: 0:5

A number of differential equations give the temperature distribution throughout the layers of the pond. Each layer is thermally represented by an energy equation of three variables: the temperature in the center of the layer and the temperatures below and above it representing the boundary conditions. The boundary conditions of the system are the following:  Ambient temperature is directly affecting the gunny bag which is upper layer of RPWGB.  The indoor air temperature is directly affecting the bottom concrete layer, which is the lowest layer of RPWGB. The interaction between the examined system and ambient and indoor air is outlined in Fig. 1. The temperature of the gunny bag Tg, that is both affected by the ambient temperature Ta and the temperature of the top water layer Tw,1, is given by the following formula: AI h ag ¼ Ahc ðT g  T a Þ þ Að0:7581 þ 0:4257vÞðP g  uP a Þ

P a ¼ 3385:5  exp½8:0929 þ 0:97608 ðT a þ 42:607Þ  ð2aÞ

0:7

þ AU gp ðT g  T w;1 Þ þ AH o Cqw

P g ¼ 3385:5  exp½8:0929 þ 0:97608 ðT g þ 42:607Þ0:5 

ð2bÞ

4

þ Aeg r½ðT g þ 273Þ  T 4sky  dT g t d

where Eq. (4) (Sodha et al., 1981; Tang et al., 2003)

Fig. 1. Interaction between the examined system and ambient and indoor air.

ð3Þ

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( U gw ¼ max

1 )   kw H0 1 ; þ ðH 0 þ 0:5h1 Þ k w hcw

ð8Þ

are fully mix. The overall procedure is repeated in order to reach a descending or uniform temperature distribution along the depth of the pond. The roof with thickness H2 consisting of a single construction material is divided into n layers. The center of each layer (thickness h2 = H2/n) is considered to thermally represent the layer. For the top roof layer (i = 1) the expression of energy equation is:

ð9Þ

U ðT x  T r;1 Þ þ k r ðT r;2  T r;1 Þ=h2 ¼ h2 qr Cpr

ð4Þ

T sky ¼ e0:25 sky ðT a þ 273Þ

ð5Þ

esky ¼ 0:754 þ 0:0044T dp

ð6Þ

T dp ¼ 26:137 þ 16:888a þ 1:0496a2

ð7Þ

a ¼ lnðuÞ  8:0929 þ 0:97608 ðT a þ 42:607Þ hc ¼ 2:8 þ 3 V

0:5

dT r;1 dt

ð14Þ

The temperature distribution throughout the water is also given by a number of differential equations of temperature derived to time (Tang et al., 2004). Tw,i represents the water temperature in layer i, while the Tw,1 represents the top water layer and Tw,n the bottom water layer. For the top water layer (i = 1) inside the pond the expression of energy equation is:   kw dT w;1 U gw ðT g  T w;1 Þ þ ð10Þ ðT w;2  T w;1 Þ ¼ h1 qw C p h1 dt

For any roof layer i (2 6 i 6 n + 1) the corresponding energy equation is as follows:

For any middle water layer i (2 6 i 6 n + 1), the corresponding energy equation is as follows:   kw dT w;1 ð11Þ ðT w;i1 þ T w;iþ1  2T w;1 Þ ¼ h1 qw C p h1 dt

The indoor air temperature of indoor air is represented by Tin. A FORTRAN code including the aforementioned differential equations is written in order to calculate the temperature distribution in the examined system. The code represents a new component that is coupled to the TRNSYS software. The FORTRAN code takes as input the climatic data file which is extracted using the [Meteonorm software]. Furthermore, the code takes also as inputs a number of parameters defined by the user that are detailed analyzed in the following chapter. During the simulation, an interaction is performed between the PRWGB FORTRAN Code and TRNSYS: The FORTRAN code gives the temperature of the internal surface of the roof Tr,n to the TRNSYS software representing the building and takes as an input the indoor air temperature Tin. The TRNSYS software takes as an input the roof temperature Tr,n and gives as an output the Indoor air temperature Tin. This “give and take” procedure between the FORTRAN component and the TRNSYS is depicted in Fig. 2. The Flow cart of the FORTRAN component and the interaction with the TRNSYS software is

For the bottom water layer (i = n), the following equation holds:   kw ðT w;n1 þ T w;n  2T w;n Þ þ U wr ðT r;1  T w;n Þ h1 dT w;n ð12Þ ¼ h1 qw C p dt ( 1  1 ) 0:5h2 0:5h1 0:5h2 1 U wr ¼ max þ ; þ ð13Þ hcw kr kw kr As shown on Eq. (12) the bottom water layer is affected by the temperature on the top of concrete slab Tr,1. At any time step a comparison of the temperatures in adjacent water layers is performed following the calculations of the previous time step. If the temperature of one layer is lower than that of the layer below, the water of the layers

AMBIENT CONDITIONS (component: type 109-TM2)

FILE *.tm2

k r ðT r;i1 þ T r;iþ1  T r;i Þ=h2 ¼ h2 qr Cpr

dT r;i dt

ð15Þ

For the bottom roof layer (i = n) the corresponding energy equation is the following: U ri ðT in  T r;i Þ þ kr ðT r;i1  T r;i Þ=h2 ¼ h2 qr Cpr

Roof Temperature on the external surface (ΤROOF)

dT r;i dt

Indoor Air Temperature (ΤINDOOR)

BUILDING (component: TYPE 56a)

Fig. 2. Simplified representation of the thermal coupling between building and investigated Roof cooling technique (RPWGB).

ð16Þ

A. Spanaki et al. / Solar Energy 86 (2012) 3331–3344

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2.2. Parametric study

ations for effective evaporative and radiative cooling have been analyzed in various studies (Cook, 1985). The present study focus on the parameters able to improve the efficiency of RPWGB. According to the mathematical model, the parameters affecting the temperature distribution of the RPWGB are the following:

Passive cooling is both affected by parameters related to the building (properties of envelope, ventilation rate, occupancy, etc.) and the climate. The analysis of the effect of building parameters to passive cooling has been investigated by a number of researches (Santamouris and Asimakopoulos, 1996). Furthermore, the climatic consider-

 The thicknesses of the layers of the RPWGB (gunny bag, water and roof).  The optical properties of the gunny: emissivity and absorptivity.  The climatic conditions; solar radiation, wind speed, ambient air temperature and relative humidity.

shown on Fig. 3. The discernible of the continuous model of differential equations is based on the finite elements method.

FLOW CHART OF SIMULATION START

CLIMATIC DATA FILE READING (*.TM2)

TIME ΔΤ=0 NO

YES INLITIAL TEMPERATURE DISTRIBUTION = AMBIENT

TEMPORARILY SAVING OF CLIMATIC DATA TRNSYS

CALCULATING Τi, q INDOOR TEMPERATURE Τin YES

NO Τi≥Ti+1

hi(Ti-Tri)

SAVING hi(Ti-Tri), Ti, q

YES Qday = Σhi (Ti-Tri)

NO COMPLETITION OF DAY (24

NO Qday / Qday,temp ≤ 1%

YES SAVING VALUES FOR THE DAY

NO

COMPLETITION OF SIMULATING PERIOD

YES OUTPUT RESULTS (FILE *.plt)

END

Fig. 3. Flow cart describing the both function of model representing energy performance of RPWGB and the thermal coupling between model and building.

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Table 1 Parameters used for simulations. Component parameters Roof thermal conductivity coefficient Roof density Roof specific heat r aw Climatic data Simulation parameters Time step Simulation period

7.92 W/m K 2400 kg/m3 840 J/kg K 5.67  108 1.46  107 Input file (.tm2) created by Meteonorm [22] 1h 8th of July to 11th of August 2010

The effect of aforementioned parameters in the thermal performance of RPWGB is assessed in the following parametric study. The effectiveness of the tested parameters is estimated by water temperature at the bottom of the pond which is practically equal to the outer roof surface temperature. The external surface of an exposed roof experiences high temperatures during summer, depending inter alia on the construction materials. The reduction of roof surface temperature results in heat flux reduction, and thus cooling demand reduction. Consequently, the lowest the bottom pond temperature is, the highest the effectiveness of the tested roof pond is considered to be. 2.2.1. Simulation results The quantity accessed for the impact in passive cooling is the bottom pond temperature. The aim of the parametric study is to establish the parameters that critically affect the bottom pond temperature. For this purpose, each of the examined parameters has altered by ±50%. The simulation parameters are listed in Table 1. The site refers to the city of Chania, Crete, Greece (Altitude 5 m, Longitude 24.0156, Latitude 35.5122). The climatic data file created using Meteonorm software. Table 2 summarizes the tested parameters and the results. 2.2.1.1. Effect of climate. The present parametric study is referred to a typical Mediterranean climate. Heat dissipation on system mainly occurs due to radiative and evaporative losses. Heat transfer by means of radiation is affected

by cloudiness and relative humidity. Since Mediterranean climates enjoy clear sky throughout the cooling season, relative humidity is the critical parameter affecting both heat losses by radiation and evaporation. The impact of relative humidity to the bottom pond temperature is assessed by two scenarios: the “warm humid” and the “warm arid” climate representing a constant relative humidity in 70% and 30% respectively. The relative humidity in the “Base Case” scenario varies throughout the day from 50% to 90%. The bottom pond temperature for the different climates is tabulated on Fig. 4a. According to the simulation results, the “warm arid” climate results a mean bottom pond temperature drop by 3.2 °C, compared to the “Base Case” scenario. The “warm humid” scenario, gives bottom pond temperature 2.9 °C greater compared to the “Base Case” scenario. As shown on Fig. 4a, the temperature of water in the warm humid scenario often exceeds the peak ambient air temperature as expected, verifying that evaporative cooling is unfavorable in humid climates. On the other hand, the temperature of water in the warm arid scenario sometimes during the night exceeds ambient temperature. As a result, the heated water during the night inhibits heat dissipation from the concrete roof to the night sky. Since the ambient temperature at night is commonly below comfort limits, the indoor temperature can be reduced by night ventilation compensating the predicament caused by the pond. 2.2.1.2. Effect of layers thickness. The examined system is consisted by three materials: the gunny bag, the Water and the Roof referred as scenarios named G, W and R respectively. The tested scenarios refer to the increase and decrease by terns of each layer thickness by 50%. The effect of gunny thickness to the bottom pond temperature is represented by the scenarios “G = 3 mm” and “G = 8 mm” representing respectively a decrease and increase of gunny thickness by 50% compared to the “Base Case scenario” (gunny thickness: 5 mm). The effect of water depth to the bottom pond temperature is represented by the scenarios “W = 5 cm” and “W = 20 cm” representing respectively a decrease and increase of water depth by 50% compared to the “Base Case scenario” (water depth: 10 cm). The impact of the concrete slab thickness on bottom pond

Table 2 Deviation between daily minimum, maximum and average bottom pond temperatures and the corresponding values of the Base scenarios, for the period between 4th and 5th of September 2010. Textile thickness

Pond (water) depth

Textile absorptivity

Textile emissivity

Base case values Deviation of tested value compared to the Base Case scenario Tested value

0.005 m 50%

+50%

0.100 m 50%

+50%

0.40 50%

+50%

0.93 50%

+6%

0.003

0.01

0.05

0.20

0.60

0.20

0.47

0.99

Daily min Daily max Daily average

Deviation of bottom pond temperatures compared 0.07 0.16 1.67 2.41 0.07 0.15 1.95 1.04 0.01 0.02 0.25 0.41

to the “Base Case” scenario 0.00 0.01 5.13 0.01 0.00 2.38 0.00 0.00 3.27

0.29 0.60 0.40

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Fig. 4a. The effect of relative humidity on Bottom pond temperature. The scenarios RH = 30% and RH = 70% respectively represent Constant values of Relative Humidity in 30% and 70%. The “Base Case” scenario is referred to the Relative Humidity values to the city of Hania varying from 50% to 90% throughout the day.

temperature is also examined, despite the fact it is usually imposed by building regulations; the effect of Roof thickness is assessed by the scenarios “R = 15 cm” and “R = 35 cm” representing respectively a decrease and increase by 50% compared to the “Base Case” scenario in

which the roof is 20 cm thick. The bottom pond temperatures of the different thicknesses scenarios are tabulated on Fig. 4b. The gunny bag is a slight layer since the tested values vary from 3 to 8 mm. Consequently, the thickness of the

Fig. 4b. Bottom pond temperatures for a variety of layer thickness. The symbols G, W and R respectively represent gunny bag, Water and Roof. The thickness of each layer on the “Base Case” scenario is the following: gunny bag 3 mm. Water 10 cm and Roof 20 cm.

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Fig. 4c. The effect of gunny’s optical properties on bottom pond temperature: The scenario “E” represents emissivity. while “A” represents absorptivity. The corresponding values for the “Base Case” scenarios are A = 0.40 and E = 0.93.

gunny bag is barely affects water temperature; altering the gunny thickness by 50% results to daily average temperature difference of bottom pond temperature between 0.02 and 0.01 °C, compared to the “Base Case” scenario. Decreasing roof thickness from 25 to 15 cm, results to a water temperature decrease of 0.12 °C, while increasing thickness of concrete slab to 35 cm results decrease of bottom pond temperature by 0.19 °C. The generally proposed water depth of RPWGB is 0.20 m. Nevertheless, the present study considers water depth of 0.10 m to avoid restrictions due to increased static loads by Greek anti-seismic regulation. According to the results of the parametric study, water depth seems to be a critical parameter. Reducing the water depth to 5 cm results to an average increase of the maximum daily bottom pond temperature by 1.95 °C. Increasing water depth to 20 cm results an average decrease of daily maximum water temperature by 1.27 °C. 2.2.1.3. Properties of gunny bag. The gunny bag floating on water level acts like a ‘thermal diode’: it effectively resists heat transfer between the covering material and water near the bottom of the pond, keeping the water relatively cool. In turn, this facilitates heat transfer from the interior of the building. The present section accesses the effect of the optical properties of gunny bag (absorptivity and emissivity) to the bottom water temperature. The scenarios A = 0.20 and A = 0.60 respectively represent the decrease and increase of absorptivity by 50%, compared to the “Base Case” scenario in which absorptivity is 0.40. The emissivity in the “Base Case” scenario is 0.93. Since the maximum

emissivity value cannot exceed 0.99, the maximum tested emissivity represents an increase by 6%. The scenario E = 0.47 represents the decrease of emissivity by 50% compared to the “Base Case” scenario. The tested values in parametric study are listed on Table 1. The bottom pond temperatures in the tested scenarios are shown on Fig. 4c. According to the simulation results, the emissivity has a major effect to bottom pond temperature. As shown in Fig. 4c, decreasing gunny’s emissivity by 50% causes an average reduction of daily maximum water temperature by 2.38 °C. Decreasing the gunny’s absortivity by 50% results to a decrease of daily maximum water temperature by 0.01 °C. 2.2.2. Theoretical improvement of pond’s cooling effectiveness The most critical parameters strongly reducing bottom pond temperature are the water depth and the emissivity of the textile floating on water level, according to the parametric study. The increase of water depth also increases the static loads, thus demands increased amplification of the roof. For this reason, the improvement of the system focuses on the properties of the floating textile. According to the parametric study, keeping a low emissivity textile afloat reduces bottom pond temperature. Low emissivity materials are usually comprised of a metal, as shown in Table 3, and thus are not expected to “function” as textiles allowing evaporative cooling through it. Nevertheless, a theoretical analysis took place using the lower emissivity values, which are underlined in Table 3. The analysis is theoretical, and represents the ability of passive cooling combining water with low emissivity materials.

A. Spanaki et al. / Solar Energy 86 (2012) 3331–3344 Table 3 Values of solar absoption (a) and infrared emittance (e) for a variety of materials The underlined values were analytically tested (Greek Regulation of Energy Efficient Buildings, University of Missouri Official Website, Omega Engineering official website). Material

Solar absorption

Infrared emittance

Aluminum paint (bright) Copper treated with NaCIO2 and NaOH Copper, aluminum, or nickel plate with CuO coating

0.30–0.50 0.87 0.93

0.40–0.60 0.13 0.09–0.21

The former part of the present study, will experimentally investigate ways to protect water with low emissivity materials, while encouraging water evaporation. The tested low emissivity values result a respectable bottom pond temperature reduction able to exceed 10 °C, as shown in Fig. 5. The use of low emissivity materials on water temperature results a mean reduction of bottom pond temperature varies from 3.83 to 6.35 °C, as shown in Table 4.

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In spite that the analysis gave very favorable results, there is a considerable uncertainty related to the morphology of the floating low emissivity material. The simulating model is referred to a textile floating on water level, allowing heat dissipation by means of water evaporation. Consequently, the ideal material for reducing water temperature is a low emissivity material that also permits water evaporation through it, thus “functions” as a textile. Since low emissivity textile does not exist, the aim of the following experimental analysis is to use low emissivity materials for water protection, while encouraging heat dissipation by means of water evaporation.

3. Experimental analysis 3.1. Experimental set-up The experimental analysis aims to propose ponds protected by low emissivity layers, while encouraging heat losses by means of evaporation and radiation, following

Fig. 5. Bottom pond temperatures for three alternative simulation scenarios: (i) Base Case (water protected with gunny bag (a = 0.40 & e = 0.93); Aluminium paint (bright) (a = 0.30 & e = 0.40); (ii) Cooper treated with NaClO2 and NaOH (a = 0.87 & e = 0.13); (iii) Cooper, aluminium, or nickel plate with CuO coating (a = 0.08 & e = 0.09). Table 4 Min, max and average bottom pond temperature differences between tested materials and “Base Case” scenario. Water Protection materials (instead of textile)

Aluminum paint Copper treated with NaCIO2 and NaOH Copper, aluminum, or nickel plate with CuO coating

Variation from bottom pond temperature of the “Base Case” scenario (°C) Min

Max

Average

2.77 3.93 4.50

6.01 9.67 10.28

3.83 6.00 6.35

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Fig. 6. Experimentally tested devices (a) Reference Pond (RPWGB- Roof Pond with Gunny Bags). (b) Low emissivity layer keeping afloat on water level (POND 1). (c) Low emissivity layer below water level (POND 2). (d) Low emissivity layer 0.15 m above water level (POND 3).

Table 5 Tested periods of the examined ponds. Testing period

RPWGB

Position of aluminum layer Pond 1

Pond 2

Pond 3

Below

Above

Water level Floating 30th July–6th August (7 days) 6th–22th August (16 days) 22th August–6th September (15 days)

the results of the parametric study. For this purpose three identical shallow ponds of 1.00 m  1.00 m  0.16 m are constructed by stainless galvanism metal. The perimeter of three ponds is insulated with 50 mm polystyrene panels, while a polyurethanes layer (naylon) waterproofs the containment of the pond. The ponds are bottomless in order to achieve thermal coupling between the cool water and the upper surface of the concrete slab. Each pond has a separate floater keeping water level constant at 0.10 m, while the water supplied by a separate water tank. Three T-Logg 100E (Greisinger electronic GmbH – D-93126) temperature data loggers are positioned at the center of the bottom of each pond. The experimental procedure took place at Vrahokipos (Latitude 35°190 60N Longitude 25°150 0E, Elevation 25 m), a rural area of Heraklion city, on Crete island in Greece. The meteorological data are taken from two weather stations; the Heraklion International Airport (Latitude 35°190 000 Longitude 25°100 000 elevation 39 m) and Technological Educational Institute of

Crete (Latitude N 35°000 0000 , Longitude E 25°000 0000 , Elevation 50 m) (Hellenic National Meteorological Service, Global Gazetteer). A variety of materials values were experimentally tested by keeping them afloat on water level, i.e. the textile used in ironing board, aluminum layer of 5 mm thick, etc. Some of the tested devices and materials did not prove to cool water. The present study presents the devices that resulted water temperature reduction, compared to the RPWGB, referred as “Reference System”. The water depth is kept constant at 0.10 m. The proposed ponds were protected by aluminum foil, a cheap low emissivity material the following ways, as shown on Fig. 6: 1. Pond 1 – Low emissivity layer keeping afloat on water level. 2. Pond 2 – Low emissivity layer below water level. 3. Pond 3 – Low emissivity layer 0.15 m above water level.

A. Spanaki et al. / Solar Energy 86 (2012) 3331–3344

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Fig. 7. Experimental measurements of Pond temperatures of the examined systems and Ambient air temperature from 3rd to 4th August 2010 for the first testing period (30th July–6th August).

The metal protection allows radiative losses, while the free water level also encourages evaporative losses. Since the aluminum foil is scratched when exposed to wind, the protection of the Pond 3 comprises of a layer of aluminum of 5 mm that is externally covered by aluminum foil. The system is similar to the “ventilated” pond which is protected by a secondary concrete roof inhibiting radiative losses. Since the area of the tested ponds is small (1 m2 each), the examined devices are not expected to have an impact in the indoor air. The influence of the pond temperature in the indoor temperature can be assessed in future experiments, in which a big portion of roof will be covered by the proposed ponds. Three pairs of comparative experiments performed between 30th of July and 6th September 2010, are compendiously presented in Table 5. 4. Results and discussion According to the experimental results, the proposed low emissivity ponds result to lower bottom layer water temperatures compared to the RPWGB. Pond 3 (low emissivity layer above water level) experiences lower temperatures fluctuation compared to other tested devices, for the 1st set of measurements (between 30th of July and 6th August 2010). The bottom temperature on Pond 3 is almost parallel to temperature fluctuations of Pond 1, as shown on Fig. 7 referred to the pond temperatures from 3rd to 4th August 2010. The average maximum temperature of Pond

3 is 28.3 °C, while the corresponding values for RPWGB and Pond 1 (low emissivity layer floating on water level) are 34.8 °C, and 32.2 °C respectively. Concluding, Pond 3 results to lower peak temperatures on the bottom, compared to the both Pond 1 and RPWGB. However, the mean minimum temperature of the RPWGB (24.9 °C) is similar to the corresponding value of the 3rd Pond (23.6 °C). In the second set of measurements, aluminum foil is placed below water level (Pond 2) in order to enhance water evaporation. The experiment lasts for 6 days, between 6th and 12th of August 2010. Fig. 8 tabulates the record temperatures for 9 and 10th of August, 2010. The temperature fluctuation on the bottom of Pond 2 is parallel to that of the RPWGB; the mean maximum temperature is 4.1 °C (=33.3–29.2 °C) lower compared to the RPWGB, while the mean minimum is 1.2 °C (=23.8– 22.7 °C) lower than the corresponding values of the RPWGB. The low minimum temperatures of water indicate effective evaporative cooling. The average mean temperature of water in the bottom of the pond is 5.1 °C lower than the corresponding value of the RPWGB (=30.6–25.7 °C). Construction difficulties encountered in Pond 2; the surface of the foil was altered by a concentration of salts and dust. The third set of measurements assesses the temperature distribution of the systems that gave the lowest bottom pond temperatures. The measurements lasted for 10 days, between the August 22th and the September 6th, 2010. Fig. 9 tabulates the recorded temperatures for the 24th to 25th of August, 2010. The average maximum water

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Fig. 8. Experimental measurements of Pond temperatures of the examined systems and Ambient air temperature from 9th to 10th August for the second testing period (6th–22th August 2010).

Fig. 9. Experimental measurements of Pond temperatures of the examined systems and Ambient air temperature from 24th to 25th August for the third testing period (22th August–6th September).

temperature at the bottom of Pond 3 is 26.1 °C, thus 2.8 °C lower than the corresponding value of Pond 2 and 6.2 °C lower than the corresponding value of the RPWGB. The

minimum water temperature at the bottom of the three examined ponds differ by more than 0.5 °C, demonstrating the similarity of the behavior of three systems during the

A. Spanaki et al. / Solar Energy 86 (2012) 3331–3344

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Table 6 Mean maximum, mean minimum and mean values of Ponds 2 and 3 and differences from the reference system values.

Average min Average max Mean

Pond 2 (aluminum foil below water surface)

Pond 3 (aluminum layer above water surface)

Temperature

Divergence compared to the ref. syst.

Temperature

Divergence compared to the ref. syst.

21.6 28.1 24.6

0.5 3.4 1.8

22.3 26.1 24.3

0.2 5.4 2.0

Table 7 Comparative assessment between Pond 2 (low emissivity layer below water level) and Pond 3 (low emissivity layer above water level): j: bad, jj: good and jjj: very good.

Energy efficiency Easiness to be constructed Easiness to large scale construction Durability of covering material

Pond 2

Pond 3

jj jj jj j

jjj jjj j jj

Table 8 Deviation between minimum and maximum bottom pond temperatures of simulation and experiments for the 2 tested days (4th and 5th September 2010). Bottom pond temperatures according to Experimental results 4th September 2010 Minimum 20.70 Maximum 31.60 5th September 2010 Minimum 21.80 Maximum 32.00

The proposed techniques do not demand daily operation and can be constructed easily, with widely available materials. On the other hand, the major disadvantage of the proposed roof ponds can be attributed to issues related to design considerations and materials’ properties for large scale application. The aluminum foil used in Pond 2 can be easily damaged while its optical properties are also misquoted by the water. Therefore, long-lived materials must be sought for this purpose. Pond 3 is protected by a thicker aluminum layer that is not altered by the wind. Nevertheless, in the case that Pond 3 is applied to larger area, adequate ventilation between water and the aluminum layer should be achieved. A comparison between Ponds 2 and 3 is summarized on Table 7.

Deviation (%)

5. Testing the reliability of the simulating model

Simulation results 19.59 29.72

5.36 5.95

20.25 30.29

7.11 5.34

night, that heat dissipation by means of radiation is maximized. The average minimum temperatures in the bottom of Pond 3 are 22.3 °C and 21.7 °C, while the corresponding values are 22.1 °C for the RPWGB and 21.6 °C for Pond 2. The temperatures of the examined systems are summarized in Table 6. The difference between the minimum temperatures on the bottom of Ponds 2 and 3 and the minimum temperatures on the bottom of the RPWGB is negligible. The minimum temperatures occurred during the night, when the heat losses by radiation are maximized. The mean maximum temperature in the bottom of Ponds 2 and 3 is respectively by 3.39 and 5.37 °C lower, compared to the corresponding value of the RPWGB. Heat losses during the day occur due to evaporation, while the low emissivity layer prevents water overheating by reflecting direct solar radiation. In conclusion, the experiments demonstrate that Ponds 2 and 3 result improved cooling effectiveness compared to the RPWGB. Concluding, the tested roof pond variants experiences low temperatures, in comparison to the surface temperatures of conventional materials and techniques. This can lead to further improvement of the microclimatic conditions. The wet upper surface of roof ponds acts as a heat sink, decreasing the ambient temperature while increasing ambient relative humidity.

The mathematical model quantitatively assess the parameters that affect most the bottom level water temperature. The findings of the parametric study are further developed in the experimental analysis. As a result, the simulation model utilizes as a screening tool. Furthermore, a comparison between the calculated and experimental bottom pond temperatures is also implemented. The deviation between the experimental and simulated Maximum bottom pond temperatures, reached 5.95%, as shown in Table 8. The corresponding value of the Minimum water temperature reached 7.11%. 6. Conclusions Roof ponds achieve efficient passive cooling, especially in hot dry climates, using widely available and inexpensive materials. The increased thermal capacity of the water together with heat losses by means of radiation and water evaporation result to a reduction of peak temperatures and temperature fluctuations. Roof pond with gunny bag is protected by a textile that is kept afloat on water level. The system does not demand daily operation or maintenance. It has increased efficiency in comparison to roof pond with movable insulation, without any functional demand. The parametric analysis comprising the first part of the present study, investigates the parameters that affect the bottom pond temperature of RPWGB. The analysis proved that the most critical parameters affecting system’s efficiency in terms of bottom pond temperature reduction, is the emissivity of the floating material and the depth of

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water; decreasing textile’s emissivity by 50% results a decrease on bottom pond temperature by 2.6 °C. Furthermore, doubling the water depth from 0.10 to 0.20 m reduces water temperature by almost 1.3 °C. Deeper ponds results extra loads to the concrete slab, limiting the application of the system to tolerant roofs. In order to optimize system effectiveness, alternative materials used for water protection were theoretically tested. Covering the pond with low emissivity layers can theoretically result bottom pond temperatures 10 °C lower compared to RPWGB. Consequently, the ideal material for water protection is a low emissivity textile, in order to permit water evaporation through it. The experimental analysis investigates ways in order to utilize low emissivity layers for water protection, while encouraging water evaporation. Placing an aluminum foil layer below water level results a mean maximum temperature reduction by 3.4 °C, according to the experiments. Furthermore, placing a low emissivity layer above water level of the ponds, results mean maximum temperature reduction in the bottom by 5.37 °C. As a result, the experiments demonstrated that the combined use of water and low emission layers can reduce the surface temperature of the roof. Further experiments could access the influence of the system to both indoor air temperature and thus cooling demand reduction. Acknowledgements We thank Theodoros Katsaounis, Associate Professor in the Department of Applied Mathematics of University of Crete in Greece, for the creation of the FORTRAN code describing the thermal performance of the RPWGB. This investigation is a part of a Ph.D. study supported by the Greek State Scholarships Foundation. References Cook, J., 1985. Passive Cooling. MIT Press, Cambridge, MA. Givoni, B., 1994. Passive and Low Energy Cooling of Buildings. Givoni, B., 2011. Indoor temperature reduction by passive cooling systems. Solar Energy 85, 1692–1726. Kharrufa, Sahar N., Yahyah, Adil, 2008. Roof pond cooling of buildings in hot arid climates. Building and Environment 43, 82–89. Naticchia, B., Fernandez-Gonzalez, A., Carbonari, A., 2007. Bayesian network model for the design of roofpond equipped buildings. Energy and Buildings 39, 258–272. Raeissi, S., Taheri, M., 2008. Skytherm: an approach to year-round thermal energy sufficient houses. Renewable Energy 19, 527–543. Rincon, J., Almao, N., Gonzalez, E., 2001. Experimental and numerical evaluation of a solar passive cooling system under hot and humid climatic conditions 71 (1), 71–80. Rizwan, A.M., Dennis, L.Y.C., Liu, C., 2008. A review on the generation, determination and mitigation of urban heat island. Journal of Environmental Sciences 20 (1), 120–128. Santamouris, M., Asimakopoulos, D., 1996. Passive Cooling of Buildings. James & James.

Santamouris, M., Gaitani, N., Spanou, A., Saliari, M., Giannopoulou, K., Vasilakopoulou, K., et al., 2012. Using cool paving materials to improve microclimate of urban areas – design realization and results of the Flisvos project. Building and Environment 53 (5–6), 128–136. Santamouris, M., Synnefa, A., Karlessi, T., 2011. Using advanced cool materials in the urban built environment to mitigate heat islands and improve thermal comfort conditions. Solar Energy 85 (12), 3085–3102. Sodha, M.S., Kumar, A., Tiwari, G.N., Tyagi, R.C., 1981. Solar Energy 26, 127. Spanaki, A., Tsoutsos, T., Kolokotsa, D., 2011. On the selection and design of the proper roof. Renewable and Sustainable Energy Reviews 15, 3523–3533. Synnefa, A., Santamouris, M., in press. Advances on technical, policy and market aspects of cool roof technology in Europe: The Cool Roofs project. Energy and Buildings. Tang, R., Etzion, Y., 2005. Cooling performance of roof ponds with gunny bags floating on water surface as compared with a movable insulation. Renewable Energy 30, 1373–1385. Tang, R., Etzion, Y., Erell, E., 2003. Experimental studies on a novel roof pond configuration for the cooling of buildings. Renewable Energy 28, 1513–1522. Tang, R.S., Etzion, Y., 2004a. On thermal performance of improved roof pond for cooling buildings. Build Environment 39, 201–209. Tang, R.S., Etzion, Y., 2004b. Comparative studies on the water evaporation rate from a wetted surface and that from a free water surface. Building Environment 39, 77–86. Tang, R.S., Etzion, Y., Meir, I.A., 2004. Estimates of clear night sky emissivity in the Negev Highlands, Israel. Energy Conversion and Management 45, 1831–1843. Yadav, R., Rao, D.F., 1983. Digital simulation of indoor temperatures of buildings with roof ponds. Solar Energy 31 (2), 205–215. Zinzi, M., Agnoli, S., in press. Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the mediterranean region. Energy and Buildings.

Further reading Crowther, K., Melzer, B., 1979. The thermosiphoning cool pool: a natural cooling system. Proc. Ill National Passive Solar Conference. American Section of ISES, San Jose, CA. Global Gazetteer Version 2.2. Site Coordinates (accessed February 2012.). Greek Anti-seismic regulation. Published by the Organisation for the Antiseismic Design and Protection Greek Regulation of energy efficient buildings (KENAK). Hellenic National Meteorological Service. Climatic data from Heraklion International Airport, Kazantzakis. Incropera, F.P., Dewitt, D.P., 1996. Fundamentals of Heat Transfer. Wiley, New York. METEONORM 6.1.0.9, 2009. Global Meteorological Database for Engineers, Planners and Education. Nahar, N.M., Sharma, P., Puurohit, M.M., 1999. Studies on solar passive cooling techniques for arid areas. Energy Conversion and Management 40, 89–95. Norton, B., Probert, S.D., 1983. Recent advances in natural circulation, solar energy water heater designs. Applied Energy 15, 15. Omega engineering. (accessed May 2012.). The Engineering Toolbox. (accessed December 2011.). TRNSYS 16.1, 2009. The Transient Energy System Simulation Tool. Solar Energy, Madison. University of Missouri. .