Energy and Buildings

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1 Energy and Buildings 42 (2010) Contents lists available at ScienceDirect Energy and Buildings journal homepage: A two-stage system of nocturnal radiative and indirect evaporative cooling for conditions in Tehran Moien Farmahini Farahani a, Ghassem Heidarinejad a,, Shahram Delfani b a Department of Mechanical Engineering, Tarbiat Modares University, P O Box , Tehran, Iran b Building and Housing Research Center (BHRC), P O Box , Tehran, Iran article info abstract Article history: Received 18 February 2010 Accepted 5 July 2010 Keywords: Nocturnal radiative cooling Indirect evaporative cooling Cooling coil Two-stage cooling systems In this paper, the results of an investigation on a two-stage cooling system have been studied. This system consists of a nocturnal radiative unit, a cooling coil, and an indirect evaporative cooler. During the night in summer, requisite chilled water for a cooling coil unit is provided by nocturnal radiative cooling and is stored in a storage tank. During the next day, the water in the tank provides chilled water for the cooling coil unit and hot outdoor air passes through two-stages: the cooling coil unit and an indirect evaporative cooler. Three sources provide secondary air for the indirect evaporative cooler. The sources are outdoor air, the air leaving from the cooling coil, and the air leaving from the indirect stage (regenerative). The investigation has been conducted in weather conditions in the city Tehran. The results obtained demonstrate that the first stage of the system increases the effectiveness of the indirect evaporative cooler. Also, the regenerative model provides the best comfort conditions. Therefore, this environmentally friendly and energy-efficient system can be considered as an alternative to the mechanical vapor compression systems Elsevier B.V. All rights reserved. 1. Introduction Due to great consumption of energy in buildings, there are increasing demands to design energy-efficient heating, ventilation, and air-conditioning (HVAC) equipments and systems for buildings. In fact, among the HVAC components and systems, cooling systems consume the largest amount of electrical energy. Climatic changes due to global warming, the depletion of fossil fuel resources, and demands for reducing pollution are the motivations for replacing conventional energy resources with natural resources. Methods of passive cooling such as, radiative cooling and evaporative cooling can be economical alternatives, or can be used as a pre-cooler in conventional systems. Also, these methods are known to have zero pollution, low energy consumption, and produce good indoor air quality [1 5]. Passive cooling resources are the natural heat sinks of planet earth. Three heat sinks of nature are the sky, the atmosphere, and the earth. Heat dissipation techniques are based on the transfer of excess heat to a lower temperature natural sinks. Heat dissipation from a building to the sky occurs by long-wave radiation, a process called radiative cooling. In fact, the only means by which the Corresponding author. Tel.: ; fax: addresses: gheidari@modares.ac.ir, gheidari@alum.mit.edu (G. Heidarinejad). earth loses heat is radiative cooling. The sky equivalent temperature is usually lower than the temperature of most bodies on earth; therefore, any ordinary surface that interacts with the sky has a net long-wave radiant loss [1 3]. Indirect evaporative cooling (IEC) provides low energy cost airconditioning. An IEC system consists of two impervious separate air passages: a dry, primary air passage and a wet secondary air passage. In the primary passage, outdoor airflow is sensibly cooled without adding water, while the secondary air and water flow in the secondary passage. The evaporation of water in the secondary passage decreases the wall temperature. As a result, the cold wall decreases the primary air temperature and obviously its wet-bulb temperature. A proportion of the air leaving, which has lower wet-bulb, can be used as secondary air. Use of the air leaving as secondary air is a process known as regenerative indirect evaporative cooling [6,7]. Cooling coils exchange the cooling load of hot air to chilled water loop by pushing airflow through the coil. Also, Cooling Coil Unit (CCU) can be utilized as pre-cooler systems to decrease both dry and wet-bulb temperature of hot air and to increase efficiency. Totally, taking advantage of cooling coils has a positive effect on performance of HVAC systems [8,9]. Several research papers have studied the issues of nocturnal cooling. Berdhal and Martin [10], Argiriou et al. [11], Ali et al. [12], Mihalakakou et al. [13], Al-Nimr et al. [14], Spronken-smith [15], Erell and Etzion (1999, 2000) [16,17], Meir et al. [3], Bagior /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.enbuild

2 2132 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) Nomenclature A i, A o inner and outer surface area of tubes (m 2 ) C p specific heat of the fluid (J/kg C) D out, D in outer and inner tube diameter (m) F radiator efficiency factor (dimensionless) h o, h i heat-transfer coefficient of air side and tube side of the coil, respectively (W/m 2 C) h m mass transfer coefficient (kg/m 2 C) K thermal conductivity (W/mk) L passage length (m) ṁ mass flow rate (kg/s) n number of parallel tubes in the radiator structure S emitted radiative energy to sky (W/m 2 ) T a ambient air temperature ( C) T dp dew point temperature ( C) T f outlet fluid temperature ( C) T fi fluid temperature at radiator inlet ( C) T sky sky equivalent temperature (K) T ws wet-bulb temperature of the secondary air ( C) U overall heat loss coefficient (W/m 2 C) w distance between the tubes (m) y tubes length (m) saturation effectiveness s finned surface efficiency Subscripts a ambient dp dew point p primary air s secondary air w wet-bulb gas and Mihalakakou [2], Bassindowa et al. [18], and Farmahini Farahani et al. [19]. These papers investigate experimental and theoretical applications of long-wave radiance, nocturnal radiative cooling, its potential in different climate conditions, and effects of various parameters on this passive method. Al-Juwayhel et al. [20], San Jose Alonso et al. [21], El-Dessouky et al. [22], Madhawa Hettiarachchi et al. [23], and Heidarinejad et al. [24,25] propose formulations and modeling to investigate efficiency, summarize results of experimental studies on indirect evaporative cooling, and examine two-stage indirect/direct evaporative cooling. Only Heidarinejad et al. [26] investigated a hybrid system of nocturnal radiative cooling and direct evaporative cooling. Their results show that this hybrid system can be an efficient replacement for conventional cooling systems. In addition, Heidarinejad et al. [27] propose cooling systems macrozonation for a multi-climate country. The suggested systems in this paper cover almost all zones in such a multi-climate country. This research and other research [27,26] complement investigations of feasibility of different cooling systems in different climates. To the best of authors knowledge, a two-stage system of nocturnal cooling and indirect evaporative cooling has not been sufficiently investigated. Thus, lack of knowledge about its feasibility and implementation are the motivations to study this new hybrid system. This research takes advantage of the natural sinks as follows: the water in a storage tank is cooled by means of water circulating through two flat-plate radiators during the night (nocturnal radiative cooling). During the next day, the cold water in the storage tank is pumped into a cooling coil unit where the chilled water decreases the temperature of the outdoor air, as air passes through the CCU. This process is called pre-cooling. Then, the pre-cooled air passes through an indirect evaporative unit to meet a desirable temperature. This system is shown in Fig. 1. The pre-cooling process is the key factor in augmenting efficiency of the whole cooling system. The chilled water is obtained from a renewable and pollutant-free process, which consumes low energy. The performance and feasibility of such a cooling system have been analyzed in this paper. 2. Mathematic modeling As Fig. 1 shows, the two-stage system consists of four parts: radiators, a storage tank, a cooling coil unit, and an indirect evaporative cooling unit. Formulations and modeling of each part have been concisely described in the following subsections Formulation of the flat-plate radiator and sky equivalent temperature Due to the structural similarity between solar collectors and flatplate radiators, the same mathematical formulation can be used. Eq. (1) displays temperature distribution at any desirable point along the flat-plate radiator [4,17]. T f T a + S/U T fi T a + S/U = e( U LnwFy/ṁC p) (1) where, T f is the outlet fluid temperature, T fi is the fluid temperature at the radiator inlet, T a is the ambient air temperature, n is the number of parallel tubes in the radiator structure, w is the distance between the tubes, y is the length of the tubes, F is the radiator efficiency factor, ṁ is the mass flow rate through the radiator, and C p is the specific heat of the fluid. If the time interval is kept reasonably small, this steady state expression predicts accurate outlet temperature. The overall heat loss coefficient U is the sum of the heat losses around the radiator, such as convection on top of the radiator and conduction under and on the sides of the radiator. Due to conduction beneath and on the sides of the radiator, the heat losses are the proportion of thermal conductivity of the insulations to the thickness of the insulation. The highest amount of heat loss occurs on Fig. 1. A schematic diagram of the two-stage system of radiative cooling: a cooling coil, and an indirect evaporative cooler.

3 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) the top of the radiator. The top loss is estimated by considering convection heat transfer from the upward surface of the radiator. The wind speed is accounted for top heat loss of the radiator. Usually, top loss is a linear function of wind speed. S is the emitted radiative energy from surface of the radiator to the sky. In order to calculate the emitted radiative energy, surface temperature of the radiator and sky temperature are necessary. In this investigation, the radiator temperature is an average of the inlet and outlet water temperature of the radiator. The sky temperature is defined as the temperature of a black body radiator emitting the same amount of radiative power as the sky. Eq. (2) relates the effective sky temperature to the ambient temperature, and dew point temperature [11]. ) ) 2 ( ) Tdp Tdp T sky = T a [ ( ( ] tm cos (2) where T a and T sky are the ambient air temperature and sky equivalent temperature, respectively, T dp is dew point temperature and t m is the number of hours from midnight in solar time. In Eq. (2), both T a and T sky are measured in Kelvin, but T dp is measured in C. Surrounding conditions such as ambient temperature, wind speed, and sky equivalent temperature affect the cooling performance and outlet temperature. The difference between ambient temperature and sky equivalent temperature demonstrates the potential of nocturnal cooling Stratification water tank Due to low mass flow rate during the night (for radiative cooling) water temperature layers are stratified, consequently a stratification modeling is necessary. Also, in practical applications many tanks show some degree of stratification. In a thermally stratified situation, the temperature of the contained liquid varies from the bottom to the top, being cooler at the bottom and warmer at the top. According to Fig. 2, a tank can be modeled as though it were divided into N sections, with energy balance for each section of the tank. This results in a set of N differential equations that can be solved for temperature of each section as a function of time [28]. The schematic pattern of stratified storage tank is shown in Fig. 2. Outlet temperature from the radiator decreases because of low mass flow rate. As a result, a higher degree of thermal stratification appears in the water tank. Also, the temperature at lower layers of the tank will be cooler. Moreover, in a highly stratified tank the inlet temperature to the radiator will be higher, which derives more absorbed energy. Research on solar systems shows that, energy storage efficiency increases up to 6% [29]. In this investigation, the mass flow rate is chosen as water layers are stratified in the tank Modeling of cooling coil Because input temperatures of both fluids are given and output temperatures are required to be calculated, the ε-ntu method is chosen. A sensible cooling process only exists when the outer surface temperature of the coil is equal to or higher than the dew point of the entering air. A sensible cooling process is indicated by a horizontal line towards the saturation curve on the psychrometric chart. In other words, the humidity ratio is always constant [30]. The outside surface area of the coil (A o ) multiplied by the overall heat-transfer coefficient based on the outside surface area of the coil (U O ) can be calculated as: [ ( Dout 2A i K tube ln ( Dout D in )) + 1 h i A i ] 1 1 U o A o = + (3) s h o A o where, s is finned surface efficiency, which depends on the area of the fins, the outside surface area, and the fin efficiency; D out and Fig. 2. A schematic diagram of a stratified storage tank. D in are the outer and inner tube diameter, respectively; K tube is the thermal conductivity of the metal of the tubes; A i is the inner surface area of the tubes; and, h o, h i are heat-transfer coefficients of the air side of the coil and the tube side of the coil, respectively. On the airside, the Zukauskas air heat-transfer coefficient, which is based on Reynolds number through the narrowest cross section, has been used [31]. For chilled water at turbulent flow inside the tubes, the inner surface heat-transfer coefficient can be calculated using the Dittus Boelter equation [32]. Further details about the aforementioned modeling such as, stratified storage tanks or radiators can be found in literature [16,26] Modeling of indirect evaporative cooling As a result of a primary air stream (supply air), a secondary air stream (working air), and a water stream, a non-adiabatic process occurs in an indirect heat exchanger. In the indirect evaporative cooler, parallel plates form a series of primary and secondary passages. In a cross flow configuration, the primary air flows perpendicularly to the secondary air and water in alternative passages. Water is sprayed on top of the heat exchanger, and the secondary air flows upward along the wall surfaces of the secondary passages (wet passages). Fig. 3 shows a schematic diagram of IEC. The balance of energy and mass of three streams is an important issue that should be considered in the modeling. Some assumptions are made in order to develop a mathematical model: The indirect evaporative cooler has no heat transfer with its surroundings. Diffusion in the cooler is insignificant. Lewis number is considered unity. The water is distributed uniformly over the all passages.

4 2134 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) Fig. 3. A schematic diagram of the indirect evaporative cooler. Temperature of the wall, bulk water, and air/water interface are equal. According to the above assumptions, a set of differential equations can be derived by applying the principles of energy and mass conservation. For the primary air, the governing energy balance equation is formulated below. dt p ṁ p C pp dx = h pl p (T w T p ) (4) where, ṁ is mass flow rate, C pp is the air specific heat at constant pressure, h p is the heat-transfer coefficient, and L p is the width of the primary passages. For conservation of energy and moisture in the secondary air, the following equations can be derived: ṁ s C ps dt s dy = h sl s (T w = T s ) (5) dω s ṁ s dy = h ml s (ω w ω s ) (6) where, h s and h m are the heat and mass transfer coefficients, respectively and L s is the width of the secondary passage. The energy balance equation for the water streams in the cooler is: ṁ p C pp dt p + ṁ s C ps dt s + ṁ w C pw dt w = 0 (7) By using multi step numerical integration, the coupled differential Eqs. (4), (5) and (7) can be simultaneously solved [33] Saturation effectiveness Fig. 4. The validation of the radiative cooling part by Meir et al. [3] at May 19 20, A/V = m 2 /L. temperature during the hottest month in Tehran, from July 23th to August 22nd, The ambient temperatures, dew point temperatures, and wind speed are derived from the Iran meteorological organization s internet site [34]. The water tank was divided into five sections to analyze temperature changes of the liquid in the water tank. In order to make the cooling process more efficient, during the nights, the output water exited from the top of the tank, where the water is warmer, and input water entered from the radiators at the bottom of the tank, where the water is colder. However, during the next day, the colder water at the bottom of the tank flowed towards the CCU and returned back at the top of the water tank. The radiative cooling part has been validated by results of Meir et al. experimental investigation [3]. Fig. 4 compares this modeling and their experimental results. The theoretical modeling of the cooling coil along with the accuracy of the indirect evaporative cooling modeling have been verified by using the results of the experimental setup test located in the Building and Housing Research Center (BHRC). Fig. 5 shows the experimental and theoretical results of the cooling coil. Table 1 compares the theoretical and experimental results for different climate conditions of cities [25]. As Figs. 4 and 5 and Table 1 show, all parts have been precisely modeled and output temperatures are accurately calculated. Two uncovered (unglazed) flat-plate radiators are used for nocturnal radiative cooling. The dimensions of each flat-plate radiator The cooling effectiveness of the CCU and IEC can be calculated by the following equation: = T a,in T a,out (8) T a,in T ws,in where is the cooling effectiveness, T a,in and T a,out are the inlet and outlet dry-bulb temperatures of the air stream, respectively, and T ws,in is the inlet wet-bulb temperature of the secondary air stream. The effectiveness of the stand-alone IEC unit is lower than unity. However, for a combined system, because the outlet dry-bulb temperature of the air stream can be lower than the inlet wet-bulb temperature effectiveness may be greater than unity. 3. Results and discussion The theoretical investigation of the two-stage system of nocturnal cooling, cooling coil, and indirect evaporative cooling has been studied for conditions in the city of Tehran (Longitude 51.4, Latitude 35.7 ). The study has been performed using the hourly average Fig. 5. The verification of cooling coil modeling, air volume flow rate is 530 CFM and water mass flow rate is 0.2 kg/s.

5 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) Fig. 6. Schematic structures of three providing sources of the secondary air. are m 2, and each of them consists of eight copper tubes. Each tube has a diameter of 10 mm and a center-to-center distance of m. The 200 L water tank is insulated with a 20-cm thick glass wool, k = 0.05 W/m C. The volume of the water storage tank is large enough to adequately store the required cold water for use during the next day. An effective ratio that impacts the temperature of the output water and consequently the temperature of the water in the tank is the proportion of the radiative surface area to the water storage tank volume. For this research, this ratio is A/V = m 2 /L. It is worth mentioning that the mass flow rate does not significantly influence the cooling process in a closed circulation system. In order to maintain stratification and obtain the best results, the water mass flow rate, ṁ w = kg/s, is chosen. Moreover, all thermodynamic properties of water (in the radiative part) and air (in the CCU and IEC) vary by temperature. Table 1 Comparison of theoretical modeling of indirect evaporative cooling and experimental data for three cities. Cities Humidity ratio (%) Inlet temperature ( C) Experimental outlet temperature ( C) a a Theoretical outlet temperature ( C) b Difference b a Tehran Yazd Bam a The accuracy and resolution of the temperature sensors were ±0.3 C and 0.01 C, respectively.

6 2136 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) Table 2 Geometrical parameters of the cooling coil unit. Height width mm 2 Number of rows of tubes 4 Number of tubes per row 12 Fin pitch 394/m Fin efficiency 0.85 Fin thickness mm Vertical tube spacing 38.1 mm Horizontal tube spacing 33.2 mm Tube outside diameter mm Tube wall thickness mm The cooling coil unit has corrugated aluminum fins with staggered copper tubes. A water volume flow rate of 1.9 L/s and air volume flow rate of 800 CFM (air face velocity in SI is 1.81 m/s) are chosen. These flow rates are sufficient for a room to be airconditioned. Table 2 lists other geometrical specifications. An indirect evaporative heat exchanger with dimensions of m 3 and plate spacing of 7 mm is used. The primary air has the same mass flow rate as the exiting air from the CCU. Mass flow rate of the secondary air depends on its sources. In this research, three supply sources for secondary air are evaluated: outdoor air (Model A), the air leaving the cooling coil (Model B), and the air leaving from the indirect stage (Model C). Model C is also known as a regenerative cycle. Fig. 6 demonstrates the schematic structure of the three models. The first part of the two-stage system is nocturnal radiative cooling which supplies cold water for the cooling coil unit. Fig. 7 shows the average temperature of the water, which is based on the temperature of five sections of water in the tank, from 9:00 PM to 6:00 AM for the city of Tehran. All the initial temperatures, including water in the tank and initial inlet water temperature are assumed to be 29 C. Later results prove that initial values were selected precisely. As shown in Fig. 7, radiation towards the sky effectively reduces the temperature of the water, which passes through the radiators, so that the water can be used in the cooling coil unit. The cold water, which is obtained and saved during the night, is used during the eight hours of the next day as the chilled water in the cooling coil unit to reduce the temperature of the outdoor air. These eight hours are the usual office hours from 9:00 AM to 5:00 PM. Fig. 8 depicts the exit temperature from the cooling coil unit and the three models for conditions in Tehran. Fig. 8. The temperature differences after the cooling coil unit and three models for conditions in Tehran. Fig. 8 shows that the hot outdoor air is pre-cooled by means of the cooling coil unit and then, pre-cooled air flows through passages of the IEC. The average temperature difference of the air entering and leaving the CCU is 8.26 C. Through the indirect evaporative cooling process, specific humidity of air is constant, but its sensible temperature drops. For condition in Tehran, the average temperature difference for air entering and leaving the indirect evaporative cooling for Model A, Model B and Model C are 4.38 C, 6.46 C and 8.3 C, respectively. As shown in Fig. 8, the air temperature decreases more in Model C than the other Models. Due to increases in outdoor air temperature and water in the storage tank, the temperature of output air increases but still meets the comfort conditions. Fig. 9 illustrates four last points of Fig. 8 on the psychrometric chart. As shown in Fig. 9, the two-stage system can meet comfort condition in Tehran. During both the night and the day, the temperature of the water in the different layers varies in the tank. The warmer layer is always at the top of the tank while the coldest one is at the bottom of the tank. This situation is shown in Fig. 10 for conditions in Tehran. As shown in Fig. 10, during the cooling process by the radiators, from 9:00 PM to 6:00 AM, the temperature distribution is completely stratified. From 6:00 AM to 9:00 AM, although the temperature of Fig. 7. The average temperature of water storage tank and the temperature of ambient for conditions in Tehran. Fig. 9. The two-stage cooling process on a psychrometric chart for conditions in Tehran.

7 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) ing coil unit remarkably augments the effectiveness of an indirect evaporative cooling stage. As shown in Fig. 11, the effectiveness of Model C (Regenerative) is higher than other models and Model B, which uses air leaving from the cooling coil as the secondary air, has better effectiveness than Model A. 4. Conclusion Fig. 10. The Temperature of different stratified layers of water tank for conditions in Tehran. all layers slightly increases because of the warmer surroundings, the water in the tank is stratified. During use of the cold water in the tank from 9:00 AM to 5:00 PM, because of higher mass flow rate the stratification disappears, in other words, all sections have the same temperature. Increases in temperature of higher layers during this period is slower than for the lower layers, but after a period of time, all layers lose the temperature differential. Once again, from 5:00 PM to 9:00 PM, temperatures in layers of water increase in the tank due to the warmer surrounding. Indeed, demand for more accurate calculations necessitate stratification modeling of the water tank. Also, Fig. 10 reveals that the assumed initial temperature is admissible because the temperature of the water in the storage tank at the last point (9:00 PM) is near 29 C. The tubes lengths between the storage tank and the radiators are assumed short and the lengths of the tubes through each radiator are short too. Also, during the day time, the cooling coil is used instead of the radiators. Thus, the pressure drop in tubes is so low during both the night time and the day time. However, the pumping power may insignificantly influence the water temperature by approximately 1 C. Fig. 11 compares the effectiveness of three models with and without the cooling coil unit for Tehran. It shows that adding a cool- Fig. 11. The effectiveness of different models with and without the cooling coil unit for conditions in Tehran. The performance of the two-stage system of nocturnal radiative and indirect evaporative cooling has been investigated for conditions in the city Tehran. Three models are proposed for providing the secondary air for the indirect evaporative cooler. The results show that all three models of the two-stage system are capable of providing comfort conditions. In addition, the effectiveness of all models of the hybrid system in comparison to their stand-alone equivalent indirect evaporative coolers is considerably higher. Taking advantage of the sky as a renewable source of passive cooling, the hybrid cooling system can be considered an environmentally friendly and energy-efficient system. Thus, this system can be used as a replacement for mechanical vapor compression systems, leading to lower electrical energy consumption. References [1] J. Cook, Passive Cooling, MIT Press, Cambridge, [2] H.S. Bagioras, G. Mihalakakou, Experimental and theoretical investigation of a nocturnal radiator for space cooling, Renewable Energy 33 (2008) [3] M.G. Meir, J.B. Rekstad, O.M. Lovvik, A study of a polymer-based radiative cooling system, Solar Energy 73 (6) (2003) [4] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, John Wiley & Sons, [5] M. Santamouris, D.N. Asimakopoulos, Passive Cooling of Buildings, James & James, [6] J.F. San Jose Alonso, F.J. Rey Martinez, E. Velasco Gomez, Simulation model of an indirect evaporative cooler, Energy and Buildings 29 (1998) [7] P.J. Erens, A. Dreyer, Modeling of indirect evaporative air coolers, International Journal of Heat and Mass Transfer 36 (1) (1993) [8] X. Yu, J. Wen, T.F. Smith, A model for the dynamic response of a cooling coil, Energy and Buildings 35 (2005) [9] G.-Y. Jin, W.-J. Cai, Y.-W. Wang, Y. Yao, A simple dynamic model of cooling coil unit, Energy Conversion and Management 47 (15 16) (2006) [10] P. Berdhal, M. Martin, Emissivity of clear skies, Solar Energy 32 (1984) [11] A. Argiriou, M. Santamouris, D.N. Assimakopoulos, Assessment of the radiative cooling potential of a collector using hourly weather data, Energy 19 (1994) [12] A.H.H. Ali, I.M.S. Taha, I.M. Ismail, Cooling of water flowing through a night sky radiator, Solar Energy 55 (4) (1995) [13] G. Mihalakakou, A. Ferrante, J.O. Lewis, The cooling potential of a metallic nocturnal radiator, Energy and Buildings 28 (3) (1998) [14] M.A. Al-Nimr, Z. Kodah, B. Nassar, A theoretical and experimental investigation of a radiative cooling system, Solar Energy 63 (6) (1998) [15] R.A. Spronken-smith, T.R. Oke, Scale modeling of nocturnal cooling in urban parks, Boundary-Layer Meteorology 93 (1999) [16] E. Erell, Y. Etzion, Radiative cooling of buildings with flat-plate solar collectors, Building and Environment 35 (4) (2000) [17] E. Erell, Y. Etzion, Analysis and experimental verification of an improved cooling radiator, Renewable Energy 16 (1 4) (1999) [18] H. Bassindowa, S. Al-Faidi, M.A. Bahafzallah, M.M. Al-Edini, A. Al-Ayiashi, O.M. Al-Rabghi, et al., An experimental investigation on night radiative cooling, in: Proceeding of the Seventh Saudi Engineering Conference, Riyadh, Saudi Arabia, December, [19] M. Farmahini Farahani, G. Heidarinejad, S. Delfani, Theoretical investigation of potential of nocturnal cooling in Iran, in: Proceeding conference of Irannian Society of Mechanical Engineering (ISME), Tehran, Iran, [20] F.I. Al-Juwayhel, A.A. Al-Haddad, H.I. Shaban, H.T.A. El-Dessouky, Experimental investigation of the performance of two-stage evaporative cooler, Heat Transfer Engineering 18 (1997) [21] J.F. San Jose Alonso, F.J. Rey Martinez, E. Velasco Gomez, M.A. Alvarez-Guerra Plasencia, Simulation model of an indirect evaporative cooler, Energy and Buildings 29 (1998) [22] H. El-Dessouky, H. Ettouney, A. Al-Zeefari, Performance analysis of twostage evaporative coolers, Chemical Engineering Journal 102 (3) (2004) [23] H.D. Madhawa Hettiarachchi, M. Golubovic, W.M. Worek, The effect of longitudinal heat conduction in cross flow indirect evaporative air coolers, Applied Thermal Engineering 27 (2007)

8 2138 M. Farmahini Farahani et al. / Energy and Buildings 42 (2010) [24] G. Heidarinejad, M. Bozorgmehr, Modelling of indirect evaporative air coolers, in: 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September, Crete island, Greece, [25] G. Heidarinejad, M. Bozorgmehr, S. Delfani, J. Esmaeelian, Experimental investigation of two-stage indirect/direct evaporative cooling system in various climatic conditions, Building and Environment 44 (10) (2009) [26] G. Heidarinejad, M. Farmahini Farahani, S. Delfani, Investigation of a hybrid system of nocturnal radiative cooling and direct evaporative cooling, Building and Environment 45 (6) (2010) [27] G. Heidarinejad, M. Heidarinejad, S. Delfani, J. Esmaeelian, Feasibility of using various kinds of cooling systems in a multi-climates country, Energy and Buildings 40 (2008) [28] S.P. Sukhatme, Solar Energy Principles of Thermal Collection and Storage, McGraw Hill, [29] Y.M. Han, R.Z. Wang, Y.J. Dai, Thermal stratification within the water tank, Renewable and Sustainable Energy Reviews 13 (5) (2009) [30] W.P. Jones, Air Conditioning Engineering, fifth edition, Edward Arnold, [31] A.A. Zukauskas, Heat transfer from tubes in cross flow, Advances in Heat Transfer 8 (1972) [32] P.W. Dittus, L.M.K. Boelter, Heat transfer in automobile radiators of the tubular type, Heat and Mass Transfer 12 (1985) [33] M. Bozorgmehr, Mathematical modeling of one dimensional heat and mass transfer in evaporative cooling and performance evaluation of indirect/direct cooling systems, PhD thesis, [34]