Applied Thermal Engineering

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1 Applied Thermal Engineering 8 (2012) 117e12 Contents lists available at SciVerse ScienceDirect Applied Thermal Engineering journal homepage: Increasing effectiveness of evaporative cooling by pre-cooling using nocturnally stored water Moien Farmahini-Farahani *,1, Ghassem Heidarinejad Department of Mechanical Engineering, Tarbiat Modares University, PO Box , Tehran, Iran article info abstract Article history: Received 6 November 2011 Accepted 10 January 2012 Available online 20 January 2012 Keywords: Nocturnal radiative cooling Indirectedirect evaporative cooling Pre-cooling coil Multi-step system In this paper, a multi-step system of nocturnal radiative cooling and two-stage evaporative cooling is studied. The feasibility and potential of this system is investigated for four cities which have different climatic conditions. During the night time in summer, water is circulated from a storage tank to two radiative panels. The temperature of the water from the radiative panels decreases because of radiative heat transfer between the water in panels and night sky. During the next day, the stored cold water in the storage tank is used as coolant for a cooling coil unit. Hot outdoor air is passed through the cooling coil unit and a two-stage evaporative cooler. The results obtained demonstrate that first, the multi-step system can be considered as an alternative cooling system in some hot regions that evaporative cooling cannot be used. Second, the multi-step system has higher effectiveness than conventional two-stage evaporative coolers. Third, an energy saving of the multi-step system is between 75 and 79% compared to mechanical vapor compression systems. Consequently, this environmentally-friendly and highly-efficient system can replace the mechanical vapor compression systems. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Among the heating, ventilation, and air conditioning (HVAC) systems, cooling systems consume the largest amount of electrical energy. Cooling is an essential issue in air conditioning in warm climates. The issues of global warming, the natural resources depletion, and demand for environmentally-friendly systems have led to a growth in use of natural green resources instead of conventional systems. The usage of passive cooling system has been considered to derive heat absorbing cycles to provide thermal comfort [1e]. Heat dissipation techniques are based on the transfer of excess heat to lower temperature natural sinks. Regarding the sky, heat dissipation is carried out by long-wave radiation from a building to the sky that is called radiative cooling. The sky equivalent temperature is usually lower than the temperature of most bodies on the earth; therefore, any ordinary surface that interacts with the sky has radiant loss [2,]. Direct evaporative cooling (DEC) is the oldest, and the most widespread form of cooling systems. The underlying principle of * Corresponding author. Tel.: þ ; fax: þ address: moien_farmahini@ou.edu (M. Farmahini-Farahani). 1 Present address: School of Aerospace and Mechanical Engineering, University of Oklahoma, PO Box 7019, Norman, OK, USA. DEC is the conversion of sensible heat to latent heat. Through a direct evaporative cooling system, hot outside air passes a porous wetted medium. Heat is absorbed by the water as it evaporates from the porous wetting medium, so the air leaves the system at a lower temperature. The minimum temperature that can be obtained is the Wet-Bulb Temperature (WBT) of the entering air. Indirect evaporative cooling (IEC) has high potential for providing air conditioning demands at low energy costs. An indirect evaporative cooling system consists of two impervious separate air passages, primary and secondary air passages which are dry and wet, respectively. In the primary passages, outdoor air flow is sensibly cooled without adding water, while the secondary air and water flow in the secondary passages. The surface of the secondary passages is wetted by spray water, so that water film evaporates into the secondary air and it decreases the temperature of the wall. As a result, the cold wall removes the heat from the outdoor air. Consequently, the leaving air from the primary passages has a lower wet-bulb temperature than the entering air. In a two-stage IndirecteDirect Evaporative Cooling (IDEC) cold air enters to DEC unit after IEC causes further temperature drop, as a result, effectiveness of the systems increases. Several research papers are dedicated to explore issues about nocturnal cooling including, Erell and Etzion [4,5], Meir et al. [], Bagioras and Mihalakakou [2], and Farmahini-Farahani et al. [6]. Aforementioned research studied experimental and theoretical investigations of long-wave radiance, nocturnal radiative cooling /$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi: /j.applthermaleng

2 118 M. Farmahini-Farahani, G. Heidarinejad / Applied Thermal Engineering 8 (2012) 117e12 and its potential in different conditions, and effects of different parameters on nocturnal cooling. Regarding evaporative cooling, Dai and Sumathy [7], Liao and Chiu [8], El-Dessouky et al. [9], Camargo et al. [10], Hettiarachchi et al. [11], and Heidarinejad et al. [12,1], Riangvilaikul and Kumar [14], Bruno [15], have proposed mathematical modeling and done experimental studies in order to analyze efficiency or simulate direct, indirect, two-step indirect/ direct and dew-point evaporative cooling. Eicker and Dalibard [16] used PVT collectors to cool down a warm storage tank and a ceiling. Only Heidarinejad et al. [17] investigated a hybrid system of nocturnal radiative cooling and direct evaporative cooling and also, Farmahini-Farahani et al. [18] studied a two-stage system of nocturnal radiative cooling and indirect evaporative cooling. Their results show that this hybrid system can be an efficient replacement for conventional cooling systems. Although previous study demonstrated that IDEC cannot be used in some climatic zones [19], this investigation shows expansion of two-stage evaporative cooling coverage over more climatic zones. In addition, to the best knowledge of the authors of this paper, no significant investigation has been performed on combining nocturnal cooling and two-stage evaporative cooling. Thus, lack of information about capability, effectiveness and energy efficiency ratio of this new multi-step system is the motivation of this study. In this research, the water in a storage tank is cooled by means of circulating the water through two flat-plate radiators throughout a night (nocturnal radiative cooling). During the next day, the cold water in the storage tank is used in a Cooling Coil Unit (CCU) as chilled water to decrease temperature of outdoor air (pre-cooling). Then, the pre-cooled air passes through a two-stage indirectedirect evaporative cooler (See Fig. 1). By this way, the hot outdoor air is pre-cooled through the CCU which augments effectiveness of the whole cooling system. The chilled water is obtained from a renewable and pollutant-free process which consumes low energy in comparison with conventional mechanical vapor compression systems. 2. Formulation and mathematic modeling As Fig. 1 shows, the multi-step system consists of three parts: 1 e Radiators and a storage tank, 2 e A cooling coil unit and e An indirect-direct evaporative cooling unit which is composed of an indirect and a direct cooler. Formulations and modeling of each part have been concisely described in the following subsections Formulation of the flat-plate radiator and sky equivalent temperature By considering some modification, the same mathematics formulation that is used for solar collectors can be employed for flat-plate radiators, because both have similar structures. Eq. (1) proposes temperature distribution in any desirable point along a flat-plate radiator [4]. T f T a þ S=U UnzFy T f i T a þ S=U ¼ exp _mc p where, T f is the outlet fluid temperature,t f i 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, z is distance between the tubes, y is the tubes length, F is the radiator efficiency factor, _m is the mass flow rate of fluid through the radiator, and C p is the specific heat of the fluid which is water in this study. 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 heat losses around the radiator such as, convection on top of the radiator and conduction under and on sides of the radiator. Due to conduction beneath and on sides of the radiator, the heat losses are the proportion of thermal conductivity of the insulations to thickness of the insulation. The highest amount of heat loss occurs at 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. Finally, S is the emitted radiative energy to sky from surface of the radiator. In order to calculate emitted radiative energy, the surface temperature of the radiator and sky temperature are necessary. In this investigation the radiator temperature is an average of 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 [20,21]. T Sky ¼ T a 0:711 þ 0:56 T dp =100 (1) 2þ0:01cos 0:25 2ptm þ 0:7 T dp =100 (2) 24 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 number of hour from midnight in solar time. In Eq. (2), botht a and T Sky are calculated in Kelvin, but T dp is in Celsius. Surrounding conditions such as, ambient temperature, wind speed, and sky equivalent temperature affect cooling performance and outlet temperature. The difference between ambient temperature and sky equivalent temperature demonstrates the potential of the nocturnal cooling Stratification water tank Due to low mass flow rate during night (for radiative cooling) water temperature layers are stratified, consequently a stratification Fig. 1. A schematic diagram of the hybrid system of radiative cooling, cooling coil, and two-stage indirectedirect evaporative cooling.

3 M. Farmahini-Farahani, G. Heidarinejad / Applied Thermal Engineering 8 (2012) 117e 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 less at the bottom and more at the top. According to Fig. 2, a tank can be modeled as being divided into N sections, with energy balance for each section of the tank. The result is a set of N differential equations that can be solved for the temperatures of the N sections as functions of time. 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 show that the energystorage efficiency increases up to 6% [22]. In this investigation the mass flow rate is chosen as water layers are stratified in the tank. In stratified tanks, an energy balance on each section should be developed and a set of differential equations can be solved simultaneously [2,24]. 2.. Modeling of cooling coil Since initial temperatures of both fluids are known and output temperatures are required to be found out, the NTU method is chosen. A sensible cooling process is indicated by a horizontal line toward the saturation curve on the psychrometric chart. In other words, the humidity ratio is always constant [25]. 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. For chilled water at turbulent flow inside the tubes, the inner surface heat-transfer coefficient can be calculated using the DittuseBelter equation [26]. On the air side, the Zukauskas air heattransfer coefficient, which is based on Reynolds number through the narrowest cross section, has been used [27] Modeling of indirect evaporative cooling In the indirect evaporative cooler, parallel plates form a series of primary and secondary passages. As a result of primary air stream, secondary air stream, and water stream, a non-adiabatic process occurs in an indirect heat exchanger. In counter-flow configuration, the primary air and water flow downward from top to down and the secondary air is blown upward from bottom to top. Water is sprayed on the top of the heat exchanger. Water and the secondary air flow along wall surfaces of the secondary passages (wet passages). The Primary air flows in the alternative passages. Fig. shows a schematic diagram of IEC. By considering the balance of energy and mass of the three streams, temperature of fluids can be calculated through the heat exchanger. In order to develop a mathematical model some assumptions are made: The water is uniformly sprayed over the all secondary passages. The heat exchanger has no heat transfer with its surroundings. Thermal and mass diffusion are not significant. Lewis number is unity. Temperature of the wall, bulk water, and air/water interface are equal. According to the assumptions, a set of differential equations can be derived by applying principles of energy and mass conservation. For the primary air the governing energy balance equation is formulated below. _m p C pp dt p dx ¼ h pl p Tw T p where, _m is mass flow rate, C pp air specific heat at constant pressure, h p heat-transfer coefficient, and L p width of primary passages. For conservation of energy and moisture in the secondary air, following equations can be derived: _m s C ps dt s dy ¼ h sl s ðt w T s Þ (4) _m s du s dy ¼ h ml s ðu w u s Þ (5) where, h s, h m are respectively heat and mass transfer coefficient, and L s is width of the secondary passage. The energy balance equation for the water streams in the cooler is as follows: () _m p C pp dt p þ _m s C ps dt s þ _m w C pw dt w ¼ 0 (6) By using multi-step numerical integration coupled differential Eqs. (), (4) and (6) can be simultaneously solved. Fig. 2. A schematic pattern of a stratified storage tank. Fig.. A schematic diagram of the indirect evaporative cooling.

4 120 M. Farmahini-Farahani, G. Heidarinejad / Applied Thermal Engineering 8 (2012) 117e12 ¼ T a;in T a;out T a;in T wb;in (9) 2.5. Modeling of direct evaporative cooling In a direct evaporative cooler, the transformation of the heat and mass between air and water causes decrease in the air Dry-Bulb Temperature (DBT) and increase in its humidity, while the enthalpy is basically constant in a perfect process. The minimum temperature that can be attained is the wet-bulb temperature (WBT) of the incoming air. Wet pads or porous materials equip a water surface in which the air is humidified and the pad is wetted by dripping water. Assuming the hot air flow near to a wet surface, according to Fig. 4, heat transfer occurs due to the difference in surface temperature T sf and the flow of air temperature T a. Mass transfer also occurs, because the absolute humidity (concentration) of the air close to the surface u a is different from the humidity of the wet surface u sf. The total differential heat flow is dq ¼ hh c T sf T a þ h m i vs usf u a i da (7) where, T a is air temperature, T s is surface temperature, i vs is enthalpy of vaporization of the water at surface temperature, h c is convective heat-transfer coefficient, h c is mass transfer coefficient u a absolute humidity of air, and u sf is absolute humidity of wet surface. Considering Lewis number Le¼h c /h m C pa is equal unity and based on general mathematic modeling developed by Camargo et al. [10] and integrating over the whole area the output temperature will be: T a;out T sf T a;in T sf hc A ¼ exp _m a C pa In Eq. (8), A is the area of the heat transfer surface. This parameter depends on composition of the evaporative pad and is provided by manufacturers. It is assumed that the makeup water entering the sump to replace evaporated water is at the same adiabatic saturation temperature of the incoming air. The Dowdy and Karabash introduced a correlation to establish the convective heat-transfer coefficient in a rigid cellulose paper evaporative media [28] Effectiveness Fig. 4. A schematic element of direct evaporative cooling. The saturation effectiveness of the IDEC can be calculated by the following equation: (8) where is the saturation effectiveness, T a.in and T a.out are the inlet and outlet dry-bulb temperatures of the air stream, respectively, and T wb,in is the inlet wet-bulb temperature of the secondary air stream. Effectiveness of the stand-alone IEC or DEC 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, the effectiveness may be greater than unity.. Results and discussion The theoretical investigation of the multi-step system of nocturnal cooling, cooling coil and two-stage indirect-direct evaporative cooling has been studied for conditions of four cities. The geographical positions and climatic conditions [29] of the cities are tabulated in Table 1. Finding a viable alternative cooling system with higher effectiveness was the motivation of this research. The study has been done using the hourly average temperature during the warmest period in Iran, from July 1st to 1st, The ambient temperatures, dew-point temperatures, and wind speed are derived from Iran meteorological organization s internet site. Two uncovered (unglazed) flat-plate radiators are used for nocturnal radiative cooling. The dimensions of each flat-plate radiator are m 2. Each radiator consists of eight copper tubes. The tubes have center-to-center distance of m and internal a diameter of 10 mm. The 200 L water tank used here is insulated with a 20 cm thick glass wool k ¼ 0.05 W/m C. The volume of the water storage tank is big enough to adequately store the required cold water for the next day. Therefore, the ratio of the radiative surface area to the water storage tank is A/V ¼ m 2 / L. This ratio impacts the temperature of outlet water from radiators and as a result, that of the water storage tank. In order to gain best results and stratification be maintained, the water mass flow rate of _m ¼ 0:022 kg=s is chosen. Moreover, all thermodynamic properties of water (for the radiative part) and air (for the CCU and IDEC parts) vary by temperature. The stratification water tank modeling is used to analyze temperature changes of liquid in the water tank. Therefore, the water tank is divided into five sections. At night for the radiative cooling in which the water in the storage tank is intended to be cooled, the output water, which is warmer, exits from top of the tank and input water enters from the radiators at bottom of the water tank. However, during the next day, the colder water at the bottom of the tank flows toward the cooling coil and turns back at the top of the water tank. This approach makes the cooling process more efficient. The cooling coil unit has corrugated aluminum fins with staggered copper tubes. The water volume flow rate is 1.9 L/s and air volume flow rate of 800 CFM (air face velocity in SI is 1.81 m/s) is chosen. The two-stage evaporative cooling unit consists of IEC and DEC units. An indirect evaporative heat exchanger with dimensions of m and plate spacing of 7 mm are considered. Table 1 A list of cities and their geographical positions and climatic conditions [29]. City Longitude Latitude Climatic condition Dry-bulb temperature Wet-bulb temperature Tehran Temperate and dry DBT < 40 C WBT < 2 C Kerman Hot and dry DBT > 40 C WBT < 2 C Ahvaz Semi-humid DBT > 40 C 27 C > WBT > 2 C Tabas Hot and dry DBT > 40 C WBT < 2 C

5 M. Farmahini-Farahani, G. Heidarinejad / Applied Thermal Engineering 8 (2012) 117e The primary air has the same mass flow rate exiting air from cooling coil and mass flow rate of the secondary air depends on providing sources. The air flow after pre-cooling process enters evaporative pad with same velocity. An evaporative pad with dimensions of m is considered. It is worth mentioning that width and height of the pad are equal to dimensions of the cooling coil. In other words, it is assumed that air passes through the same channel. All parts including nocturnal radiative cooling part, cooling coil unit, and two-stage evaporative part have been validated by experimental setups and are mentioned in authors previous papers [1,17,18,0]. Also, validation figures show proper accuracy of the aforementioned mathematic modeling. The nocturnal radiative cooling process starts at 9:00PM and continues until 6:00AM next day. During this period, water from the tank is circulated through two radiative panels by a small pump. The radiative heat transfer between the panels and sky decreases the temperature of water. Two small pumps are required to circulate water during night and day time. These pumps are shown in Fig. 1. Pressure drop in tubes is insignificant during these time periods, because water is circulated between the water tank and radiators during night time. Length of tubes between the tank and radiators are assumed to be short, and also, length of tubes inside the radiators is short (around 2 m in each radiator). Therefore, during night time, the circulating pump is only required to be able to produce enough pressure to overcome pressure loss and vertical head in the tubes. During day time, the situation is the same as night time, but the cooling coil is used instead of the radiators. Based on the length and other specifications of the tubes, two 0.22 kw pumps are required for water circulation, one for day time and the other for night time. Previous studies have shown that Tehran and Kerman have the capability of providing sufficient cold water for the pre-cooling [6,18,0]. Also, two-stage evaporative cooling can meet the comfort conditions in these cities [1,19]. But, two-stage evaporative cooling cannot provide the comfort conditions in Ahvaz and Tabas [19]. These cities need desiccant wheel or conventional vapor compression systems for air conditioning during hot months. So, results of the two cities of Ahvaz and Tabas are mentioned in more details in this paper. The cold water obtained at night is used during 8 h of the next day as the chilled water in the cooling coil to reduce the temperature of outdoor air. Usually offices begin at 9:00AM and continue to work up to 5:00PM. Figs. 5 and 6 depict temperature differences after the cooling coil unit and two-stage IDEC for conditions of Ahvaz and Tabas, respectively. Figs. 5 and 6 show that hot outdoor air is pre-cooled by means of the cooling coil unit and the average temperature difference between the entering and leaving air in Ahvaz and Tabas are 8.56 C and C, respectively. Then, the pre-cooled air flows through two-stage IDEC. Through the first stage (IEC) the relative humidity of air is constant, but its sensible cooling drops down. Then, the air flows through second stage (DEC) in which relative humidity of air increases, but its sensible cooling drops down. For conditions of Ahvaz and Tabas, the average temperature difference between entering and leaving air in the two-stage IDEC are C and C, respectively. For conditions of Tehran and Kerman, the cooling coil unit on average decreases the air temperature 8.8 C and C, respectively. The IDEC on average decreases the temperature of the pre-cooled air C and 8.95 C for conditions of Tehran and Kerman, respectively. Therefore, potential of nocturnal radiative cooling is highest in Kerman followed by Tabas. Fig. 7 illustrates cooling process of the multi-step system on the psychrometric chart. As shown in Figs. 5 and 6, the multi-step Fig. 5. The temperature differences after the CCU and two-stage IDEC for conditions in Ahvaz. system can meet the comfort conditions in both Ahvaz and Tabas. As it was expected, the conditioned air for Kerman and Tehran can easily pass the comfort conditions. Also, the exiting air can be mixed with outdoor air or indoor air to meet a desired condition. It is worth mentioning, two-stage IDEC cannot provide comfort conditions for two cities of Ahvaz and Tabas. However, the multistep cooling system expands the feasibility of evaporative cooling over wider areas. For further limitation of evaporative cooling please read Ref. [19]. In addition, by considering effectiveness of 75% for DEC in the hybrid system of nocturnal radiative cooling and DEC, this hybrid system is capable of providing the thermal comfort conditions. It proves the advantage of nocturnal radiative cooling in some regions over other cooling systems such as IEC. Fig. 8 compares effectiveness of the multi-step system and indirectedirect evaporative cooling for the four cities. First, it shows that the effectiveness of conventional two-stage IDEC remarkably increases by adding a cooling coil unit working with the cold water cooled by nocturnal radiative cooling. The effectiveness is increased by 9.6, 10.6, 8. and 7.6% for Ahvaz, Tabas, Tehran and Kerman. Second, the multi-step system has higher effectiveness Fig. 6. Temperature differences after the CCU and two-stage IDEC for conditions in Tabas.

6 122 M. Farmahini-Farahani, G. Heidarinejad / Applied Thermal Engineering 8 (2012) 117e12 Table 2 EER and energy saving for each city. City EER (Btu/Wh) Energy saving (%) Ahvaz Tabas Tehran Kerman Fig. 7. Multi-step cooling process on a psychrometric chart for conditions of four cities: Ahvaz (red), Kerman (green), Tehran (black), and Tabas (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) than other hybrid systems of nocturnal cooling and evaporative cooling. For instance, for conditions of Tehran, the hybrid system of nocturnal and direct evaporative cooling has effectiveness of 10% and the two-stage system of nocturnal and indirect evaporative cooling has effectiveness of 88%. But the multi-step system of nocturnal and indirect-direct evaporative cooling has effectiveness of 106% [17,18]. It is worth mentioning adding more stages to DEC and/or IEC brings higher effectiveness, but requires more power to overcome the pressure drop through stages such as cooling coil, indirect heat exchanger, and evaporative pad. Tabas has the highest effectiveness among the cities. Its low humidity increases the efficiency of both radiative panels and the direct evaporative cooling stage. Usually, both evaporative cooling and radiative cooling are more effective in arid weather conditions. Although this system seems expensive, it consumes low energy and is easy to maintain. Energy consumption of the multi-step system is 0.8 kw which is the sum of energy consumptions of the following equipment: A fan with power of 0.24 kw to overcome pressure drops after CCU (0.02 kw), IEC (0.2 kw), and DEC (0.02 kw); A fan with power of 0.14 kw blow secondary air for IEC; Water circulating pumps for CCU (0.22 kw), IEC (0.1 kw), and DEC (0.1 kw). The Energy Efficiency Ratio (EER) is a parameter generally employed to define cooling efficiencies of unitary air conditioning systems [1]. EER is the ratio of net heat removed in Btu/h to the total applied power in Watts. Higher EER means more efficient systems. EER and an energy saving for each city are tabulated in Table 2. We assume that EER is 12 for mechanical vapor compression to calculate the energy saving [1,2]. As shown in Table 2, EER of the proposed multi-step system is much higher than that of mechanical vapor compression systems. Therefore, the multi-step system provides an energy saving of between 75 and 79%. The effectiveness of two-stage IDEC can increase by supplying secondary air of the first stage of two-stage IDEC from other sources such as, output air of the CCU, output air of the IEC (regenerative), and conditioned air inside a room (recovery). By this way, the effectiveness of the whole system increases 7e20% depending on the source of the secondary air [18,0]. 4. Conclusion The potential of a multi-step cooling system is investigated. The first step is the nocturnal cooling which provides cold water for a cooling coil unit. The second step is the cooling coil unit that uses the cold water to reduce the temperature of outdoor air. The final step is a two-stage evaporative cooling which decreases the temperature of the pre-cooled air and provides conditioned air for a residential building. The multi-step system is simulated for the four cities with various weather conditions. Results show that the systems can reduce hot air temperature 24.2 C in Tabas, 20.1 Cin Ahvaz, 20.6 C in Kerman, and 20.2 C in Tehran. Also, the system can be used in the cities of Ahvaz and Tabas in which even two-stage evaporative cooling cannot be used. In addition, the effectiveness of the system is increased by 9% (average) in comparison with a stand-alone indirectedirect evaporative cooler. The energy efficiency ratio of the multi-step system varies between 48.8 and References Fig. 8. Effectiveness of conventional two-stage indirectedirect evaporative cooling and the multi-step system of nocturnal cooling and evaporative cooling. [1] J. Cook, Passive Cooling, MIT Press, [2] H.S. Bagioras, G. Mihalakakou, Experimental and theoretical investigation of a nocturnal radiator for space cooling, Renewable Energy (2008) 1220e1227. [] M.G. Meir, J.B. Rekstad, O.M. Lovvik, A study of a polymer-based radiative cooling system, Sol. Energy 6 (200) 40e417. [4] E. Erell, Y. Etzion, Analysis and experimental verification of an improved cooling radiator, Renewable Energy 16 (1999) 700e70. [5] E. Erell, Y. Etzion, Radiative cooling of buildings with flat-plate solar collectors, Build. Environ. 5 (2000) 297e05. [6] M. Farmahini-Farahani, G. Heidarinejad, S. Delfani, Theoretical investigation of potential of nocturnal cooling in Iran, in: Conference of Iranian Society of Mechanical Engineering (ISME) (2009) Tehran, Iran. [7] Y.J. Dai, K. Sumathy, Theoretical study on a cross-flow direct evaporative cooler using honeycomb paper as packing material, Appl. Therm. Eng. 22 (2002) 1417e140. [8] C. Liao, K. Chiu, Wind tunnel modeling the system performance of alternative evaporative cooling pads in Taiwan region, Build. Environ. 7 (2002) 177e187. [9] H. El-Dessouky, H. Ettouney, A. Al-Zeefari, Performance analysis of two-stage evaporative coolers, Chem. Eng. J. 102 (2004) 255e266.

7 M. Farmahini-Farahani, G. Heidarinejad / Applied Thermal Engineering 8 (2012) 117e12 12 [10] J.R. Camargo, C.D. Ebinuma, J.L. Silveira, Experimental performance of a direct evaporative cooler operating during summer in a Brazilian city, Int. J. Refrigeration 28 (2005) 1124e112. [11] H.D.M. Hettiarachchi, M. Golubovic, W.M. Worek, The effect of longitudinal heat conduction in cross flow indirect evaporative air coolers, Appl. Therm. Eng. 27 (2007) 1841e1848. [12] G. Heidarinejad, M. Bozorgmehr, Modeling evaluation and application potential of two stage indirect/direct evaporative air coolers, in: Proceedings of the 6th International Energy Conversion Engineering Conference (IECEC), Cleveland, Ohio, [1] G. Heidarinejad, M. Bozorgmehr, S. Delfani, J. Esmaeelian, Experimental investigation of two-stage indirect/direct evaporative cooling system in various climatic conditions, Build. Environ. 44 (2009) 207e2079. [14] B. Riangvilaikul, S. Kumar, An experimental study of a novel dew point evaporative cooling system, Energy Build. 42 (2010) 67e644. [15] F. Bruno, On-site experimental testing of a novel dew point evaporative cooler, Energy Build. 4 (2011) 475e48. [16] U. Eicker, A. Dalibard, Photovoltaic-thermal collectors for night radiative cooling of buildings, Sol. Energy 85 (2011) 122e15. [17] G. Heidarinejad, M. Farmahini-Farahani, S. Delfani, Investigation of a hybrid system of nocturnal radiative cooling and direct evaporative cooling, Build. Environ. 45 (2010) 1521e1528. [18] M. Farmahini-Farahani, G. Heidarinejad, S. Delfani, A two-stage system of nocturnal radiative and indirect evaporative cooling for conditions in Tehran, Energy Build. 42 (2010) 211e218. [19] G. Heidarinejad, M. Heidarinejad, S. Delfani, J. Esmaeelian, Feasibility of using various kinds of cooling systems in a multi-climates country, Energy Build. 40 (2008) 1946e195. [20] M. Martin, P. Berdahl, Characteristic of infrared sky radiation in the United States, Sol. Energy (1984) 21e6. [21] A. Argiriou, M. Santamouris, Assessment of the radiative cooling potential of a collector using hourly weather data, Energy 19 (1994) 879e888. [22] Y.M. Han, R.Z. Wang, Y.J. Dai, Thermal stratification within the water tank, Renewable Sustainable Energy Rev. 1 (2009) 1014e1026. [2] S.P. Sukhatme, Solar Energy Principles of Thermal Collection and Storage, McGraw Hill, [24] C. Cristofari, G. Notton, P. Poggi, A. Louche, Influence of the flow rate and the tank stratification degree on the performances of a solar flat-plate collector, Int. J. Therm. Sci. 42 (200) 455e469. [25] W.P. Jones, Air Conditioning Engineering, fifth ed. Edward Arnold, [26] P.W. Dittus, L.M.K. Boelter, Heat transfer in automobile radiators of the tubular type, Heat Mass Transfer 12 (1985) e22. [27] A.A. Zukauskas, Heat transfer from tubes in cross flow, Adv. Heat Transfer. 8 (1972) 9e160. [28] J.A. Dowdy, N.S. Karabash, Experimental determination of heat and mass transfer co-efficients in rigid impregnated cellulose evaporative media, ASHRAE Transaction 97 (1987) 82e95. [29] G. Heidarinejad, S. Delfani, Outdoor design condition criteria for using in HVAC systems in the cities of Iran, in, BHRC, [0] M. Farmahini-Farahani, G. Heidarinejad, S. Delfani, A hybrid system of nocturnal radiative cooling and indirect evaporative cooling to augment efficiency, in: Conference of Iranian Society of Mechanical Engineering (ISME) (2010) Tehran, Iran. [1] ASHRAE Handbook - Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA, USA, [2] J.R. Watt, Power cost comparisons: evaporative vs refrigerative cooling, ASHRAE Transaction 94 (1998) 1108e1115. Glossary A: area of the heat transfer surface C P : specific heat (J/kg C) F: radiator efficiency factor (dimensionless) h: heat-transfer coefficient (W/m 2 C) i vs : enthalpy of vaporization (J/kg) L: length (m) Le: Lewis number (dimensionless) _m: mass flow rate (kg/s) n: number of parallel tubes (dimensionless) S: emitted radiative energy to sky (W/m 2 ) T: Temperature ( C) t m : time in 24 h (dimensionless) U: overall heat loss coefficient (W/m 2 C) y: tubes length (m) z: distance between the tubes (m) Greek symbols : saturation effectiveness (dimensionless) u: humidity (kg/kg) Subscripts a: air c: convection dp: dew-point f: fluid i: initial m: mass p: primary s: secondary sf: surface w: water wb: wet-bulb Abbreviation CCU: cooling coil unit DEC: direct evaporative cooling DBT: dry-bulb temperature IDEC: indirectdirect evaporative cooling IEC: indirect evaporative cooling EER: Energy Efficiency Ratio WBT: wet-bulb temperature