Optimal operation of evaporative cooling pads: A review

Renewable and Sustainable Energy Reviews 151 (2021) 111632 Available online 6 September 2021 1364-0321/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Optimal operation of evaporative cooling pads: A review A. Tejero-Gonz´ alez a , * , A. Franco-Salas b a Department of Energy Engineering and Fluidmechanics, School of Engineering, Universidad de Valladolid, Paseo del Cauce 59, 47011, Valladolid, Spain b ETSIA, University of Sevilla, Ctra. de Utrera Km, 1, 41013, Sevilla, Spain ARTICLE INFO Keywords: Direct evaporative cooling Evaporative cooling pad Air conditioning Cooling efficiency Saturation effectiveness Pressure drop ABSTRACT Direct evaporative cooling is widely known for being an energy efficient air conditioning solution for arid and/or semi-arid climates. Commercialized evaporative cooling pads vary in material and construction characteristics, while existing research proposes several alternative wetted media and configurations. The most studied factors on the evaporative cooling pads operation are the air velocity or air mass flow rate, air psychrometric conditions; the pad thickness, its geometric characteristics and configuration, and the water flow rate supplied. The pads performance is commonly characterized through its saturation effectiveness, pressure drop, temperature drop and humidity increase achieved in the treated air, water evaporation and consumption, cooling capacity, coefficient of performance and the heat and mass transfer coefficients. Present work conducts a critical review on the existing experimental and theoretical research on commercial and alternative wetted media, identifies the gaps in the literature, proposes uniform nomenclature and methodologies, and provides a critical view on the optimal operating conditions from wetted-surface evaporative coolers. 1. Introduction Due to nowadays concerns on reducing energy consumption in the building sector, passive cooling techniques are regaining our attention. Among these passive cooling strategies is evaporative cooling [1]. As it spontaneously occurs when water comes into contact with non-saturated air, evaporative cooling is a natural phenomenon and is applied by most animals and plants to control their temperature. Since the ancient civilizations, humankind has used this simple, economic cooling technique to reduce the ambient temperature down to comfort conditions [2]. When evaporatively cooled, humid air has its Dry Bulb Temperature (DBT) decreased close to its wet bulb temperature (WBT) as it gets saturated, due to the heat and mass transfer involved in the phenomenon. To achieve this, evaporative cooling can be implemented either from a humid surface or by spraying water directly in the air. The former option bases on the use of wetted porous media, commonly called “evaporative cooling pads”. It is to be noted that performing the evaporative cooling effect from a humid media can enhance the water evaporation rate, hence the cooling effect achieved, compared to directly spraying the water within the airflow [3]. When evaporatively cooled air is directly driven to the occupied spaces, it is called Direct Evaporative Cooling (DEC). DEC systems are widely used in applications such as greenhouses [4], intensive livestock farming [5], industrial facilities [6] and urban outdoor spaces [7,8]. 1.1. Wetted porous mediums for evaporative cooling In evaporative cooling pads, air is usually forced through the media counterflow to water supplied over the media from an upper distributor. The wetted pad then enables the heat and mass transfer between air and water. The ideal characteristics and operation expected in DEC pads are: - Large, completely wetted water-to-air contact surfaces to achieve maximum air saturation. - Minimum pressure drop. - Rigid media that can be easily assembled and dismantled. - Easy cleaning and maintenance. Evaporative cooling pads can be classified according to their material and configuration in fiber pads, rigid media pads, and packages or fill pads (Fig. 1). Fiber pads frequently consist of vegetable fibers, specially aspen [9], but they can also be made of synthetic fibers. Traditional pads are of this type, due to their simplicity; however, they require careful processing and packaging. Current interest on these pads relies on the use of locally available fibers to build cheap and sustainable systems [10]. * Corresponding author. E-mail address: [email protected] (A. Tejero-Gonz´ alez). Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser https://doi.org/10.1016/j.rser.2021.111632 Received 15 July 2021; Accepted 28 August 2021 Renewable and Sustainable Energy Reviews 151 (2021) 111632 2 Rigid media pads are made of corrugated plates assembled with resin adhesives to form two different flute angles from the horizontal. Common pad thickness are in the range between 5 and 30 cm. Most rigid media pads are made of cellulose; alternative materials are plastic PVC or polyethylene, glass fiber, carbon fiber, or metallic plates. They enable large water-to-air contact areas, avoid fiber carryout and have longer lifespans than fiber panels [11,12]. Packages or fill pads refer to all porous, inorganic materials either natural, such as volcanic stones [13], or artificial, like expanded clay [14]. As they use less structured materials, they require a case to contain the wetted media and distribute the air and water flows within. Trickle fills are fill pads made of plastic or metal mesh, which present less incrustations and admit larger air and water flows with limited pressure drop and high saturation effectiveness [15,16]. 1.2. Justification and objectives Evaporative cooling from wetted media has been subject of different review articles, generally as part of a wider approach covering different evaporative cooling technologies and sometimes focused on their applicability in buildings, specially intensive farm buildings, and greenhouses [4,6,17–20]. Materials used in direct evaporative cooling systems are also the target of existing review works [21–23]. The present review article focuses on the evaporative cooling from wetted media, though does not aim at simply reviewing the existing materials or configurations. The main objective is to provide insight on the key factors affecting the performance of evaporative cooling wetted media and enhance the scientific impact of future research on the topic, through the following: 1) Serve as reference for all future researchers on DEC media to quickly identify existing literature on the same type of media and parameters they aim to study. This would help researchers to compare their results to existing ones. 2) Develop a critical revision of existing results and determine the optimal operating conditions of different DEC media, identifying the main gaps in the literature to guide and enhance future research conducted towards these issues. 3) Propose a uniform nomenclature and procedures of characterization of DEC media. Towards these targets, this review article is structured and follows the methodology described in the next section 2. 2. Methodology A systematic revision of existing research on the behavior of Direct Evaporative Cooling media was conducted, disregarding the application. Databases used for searching the existing literature were Scopus, Web of Science and Google Scholar. Because the present work focuses on the performance of wetted media rather than on their applicability, searches performed were not limited to the recent developments. Articles with limited international impact have been included when approached new materials or configurations. The present review focuses only on published research meeting the following criteria: 1) It covers all DEC systems based on evaporation from humid surfaces, disregarding the application and including both commercial pads and new prototypes. Consequently, humidification media used in some of the articles reviewed do not correspond to the commonly Abbreviations COP Coefficient of Performance DBT Dry Bulb Temperature DEC Direct Evaporative Cooling HVAC Heating, Ventilation and Air Conditioning IEC Indirect Evaporative Cooling WBD Wet Bulb Depression WBT Wet Bulb Temperature Nomenclature A pad contact area (m 2 ) As specific area or pad contact area per unit volume (m 2 ⋅m -3 ) CC cooling capacity (W) Cp specific heat (J⋅kg -1 ⋅K -1 ) l pad thickness (m) le pad characteristic length (m) m mass flow rate (kg⋅s -1 ) RH Relative Humidity (%) T temperature (◦C) v air velocity (m⋅s -1 ) V evaporative cooling pad volume (m 3 ) w humidity ratio (kg⋅kg -1 ) W power (W) Subscripts a air ew evaporated water f fan in system inlet airflow out system outlet airflow p pump w water wb wet bulb Fig. 1. Example of wetted porous mediums, (a) fiber pads, (b) rigid media pads and (c) packages or fill pads. A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 3 called “pad”, but to different designs or configurations. Spray systems are excluded. 2) Articles must not necessarily focus on DEC applications, but report data on the behavior of direct humidification media. 3) Porous media operating as what is commonly called “semi-indirect” evaporative cooling are not included, unless they provide insight on the role of the operating factors targeted in the present review. 4) Articles that only reported the comfort conditions achieved at a target space were also excluded, unless the air conditions at the pad inlet and outlet were provided. 5) Results analyzed belong to research works either based on mathematical models or experimental results. Those articles proposing mathematical models that are validated with original experimental results are classified as both “mathematical models” and “experimental research”. 6) Due to limitations in its length, the present work only approaches the main performance parameters and influencing factors. Other less studied parameters and factors will be subject to future revision. Once selected the target works, the performance factors and parameters studied are identified. Special attention is given to those articles proposing optimal operating conditions. To ease the readers identifying the sort of system they are interested in, reviewed articles are first classified depending on the “types of evaporative cooling pads” (section 3). Then, section 4 “Performance parameters” presents the parameters and factors studied through mathematical models and experimental research. Section 5 “DEC pads operating factors. Optimal performance” develop a comprehensive, critical technical review of the published research, providing global insight on the effect that the main factors studied have on the performance of DEC pads. Results derived from this analysis are then gathered in last section 7 “Conclusions”. For clarity, parameters defined in the references cited are reexpressed here with the nomenclature used in this review work, when possible. Particularly, nomenclature widely varies concerning the most used performance parameter in DEC: the temperature drop achieved to the maximum temperature drop achievable in adiabatic systems, being the latter the difference between the DBT and the WBT of ambient air (or “Wet Bulb Depression” WBD). Indeed, many articles use different terms indistinctively, sometimes when referring to previous results in existing research where a different nomenclature had been used. This lack of a uniform nomenclature may limit the scientific impact of the existing research. Fig. 2 gathers all terms used in the published research reviewed. Because this parameter expresses a temperature drop, it is commonly called “cooling efficiency/effectiveness”; however, as the psychrometric evolution approaches saturation of air, it is also usually called “saturation efficiency/effectiveness”. The terms “efficiency” and “effectiveness” are indeed indistinctively used, sometimes on purpose [17,24,25]. However, the use of the term “efficiency” may be misleading, and “effectiveness” could be more rigorous instead, as no conversion of energy forms is involved in the evaporative cooling phenomenon. Heating, Ventilation and Air Conditioning (HVAC) handbooks dealing with evaporative cooling systems also use different terms: ASHRAE refers to this parameter either as “Wet Bulb Depression Efficiency” (WBDE) [26] or “direct saturation efficiency” ( ε e) [27]; Wang [28] name it simply “effectiveness”, and Watt [2] use the term “saturating efficiency”. This incurs in different keywords in the literature for the same concept. It is believed that a uniform nomenclature would enhance the impact of the published research on DEC pads. Despite widely used, neither “cooling efficiency” or “cooling effectiveness” evidence the different maximum temperature drop achievable in adiabatic systems and one stage Indirect Evaporative Cooling (IEC) systems compared to dew-point Indirect Evaporative Cooling equipment. Indeed, it is often simply referred to as the system (cooler, pad …) “efficiency” or “effectiveness”. Khosravi et al. [29] avoid misunderstanding by using the term “cooling effectiveness” to refer to the performance of either adiabatic or dew point evaporative cooling systems, specifying it as “wet-bulb effectiveness” for the former and “dew-point effectiveness” for the latter. Al-Mogbel et al. [30] distinguish “Effectiveness of DEC cooler” from “Effectiveness of IEC cooler”. Some authors [10,31–36] define the “cooling efficiency” clarifying that it is the saturation efficiency/effectiveness. Al-Badri and Al-Waaly [37] specify that “the effectiveness of DEC was expressed by the saturation efficiency”. Given the previous reasoning, the authors recommend the use of “saturation effectiveness” because of its rigorousness and given its wide Fig. 2. Nomenclature used in published research for the parameter “saturation effectiveness". A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 4 usage. 3. Types of evaporative cooling pads This section will enable the reader to identify the type of wetted porous medium (or evaporative cooling pad) studied in the research works reviewed. Among the type of evaporative cooling pads classified in the introduction, rigid media pads studied in the literature are gathered in Table 1, while Table 2 presents the fiber and fill pads. Pad description provided is limited to the information given in the referenced work. 4. Performance parameters The most studied performance parameters of DEC pads are the saturation effectiveness, pressure drop, humidity increase, water evaporation, temperature drop, cooling capacity, Coefficient of Performance (COP), and the heat and mass transfer coefficients. As introduced in section 2, the saturation effectiveness relates the temperature drop achieved to the maximum temperature drop achievable in adiabatic systems, which is the difference between the DBT and the WBT of ambient air: Table 1 Rigid media pad in the literature: materials and geometric characteristics. Material Type of pads tested Geometric characteristics References Angle Thickness pad Specific area Cellulose KUUL pad – 12” (30 cm) – [38] Kraft and NSSC corrugated 45◦–15◦with 3 flute sizes: 2.5, 3.5 y 4.5 mm 75 cm – [39] G&R manufacture 45◦–45◦100 mm 391.1 m 2 /m 3 [15,40,41] Corrugated cellulose by Wadi Group Company 45◦–45◦35, 70, 105, and 140 mm 360 m 2 /m 3 [31,42] Star Cool 7090 45◦–45◦100, 150 and 200 mm – [43] Munters-CELdek 7060-15 45◦–15◦100 mm – [43,44] 150, 200 mm – [43] Munters-CELdek®7060 45◦–15◦100, 300 mm – [45,46] Munters-CELdek®7060 45◦–15◦100 mm +100 mm gap +100 mm – [46] Munters-CELdek7060 45◦–15◦100, 200 and 300 mm 364.7 m 2 /m 3 [47,48] Munters-CELdek Cellulose 7060 – 300 mm – [49] Munters-CELdek 45◦–45◦100 mm 347.1 m 2 /m 3 [15,40,41] Munters-CELdek 60◦–30◦50 mm 556.75 m 2 /m 3 [15,40,41] Munters-CELdek 60◦–30◦100 mm 361.5 m 2 /m 3 [15,40,41, 50] Munters-CELdek 60◦–30◦50 mm – [13,51] Munters-CELdek 60◦–30◦100 and 150 mm – [13] Munters-CELdek® 7090–15 60◦–30◦100, 150 and 200 mm – [43,52] Munters-CELdek® 7090–15 – 300 mm – [52] Munters-CELdek®7090 135◦–45◦75, 100, 150 mm – [53] Munters-CELdek 7090 45◦groove angle 100 mm 632 m 2 /m 3 [54] Munters-CELdek®5090 135◦–45◦75, 100, 150 mm – [53] Munters-CELdek®5090 Varied from 45◦to 60◦and from 15◦to 45◦, respectively Studied in the range from 0.05 to 0.3 m Studied in the range from 320 to 530 m 2 /m 3 [55] Rigid cellulose – 0.15 m 370 m 2 /m 3 [56] Corrugated paper – 0.15 m 400 m 2 /m 3 [56] Corrugated cellulose (not specified) 45◦–15◦100, 200 and 300 mm 636 m 2 /m 3 [48] 100, 150, 200 mm – [57] 300 mm – [58] 45◦–45◦10 cm – [59] – 5 cm 100 m 2 /m 3 [60] – 152 mm 370 m 2 /m 3 [61] – 3 cm – [37] – 7.5 cm – [62] – 42 mm – [63] – 100 mm – [3] – 150 mm – [64] – 0.15 m – [65] – – – [66,67] Mixed media Mixed: corrugated cellulose (CC) and Aspen Fiber (AF) – Three thicknesses are used: a) 1 cm AF+2 cm CC b) Two CC layers (4 cm) c) Two AF Layers (2 cm) – [30] Plastic coated cellulose Plastic coated cellulose rigid cooling pad 45◦–15◦138 mm – [36] Glass-Fiber Munters- GLASdek Wave angle 90◦138 mm – [34,35] Glass-Fiber pad 45◦–15◦300 mm – [58] PVC PCV Corrugated media 30◦–30◦, respectively 100, 200 and 300 mm 211.9 m 2 /m 3 respectively [48] PVC 1200 – 300 mm – [49] Trickle Trickle100 and Trickle125 – 300 mm – [49] Metallic 0.3 mm thick galvanized sheets Vertical alignment of the sheets with 7.5 mm gap 0.15, 0.30 y 0.45 m – [68] 0.7 mm thick aluminium sheets The air flows between layers that consist of 2.00 mm longitudinal fins (No. of fins in the air stream is 45) 90 mm 549 m 2 /m 3 [69] A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 5 Table 2 Fiber and alternative packages pads in the literature: materials and geometric characteristics. Type of fiber/fill Material Geometric characteristics Thickness pad References Vegetable fibers Aspen Cooling pads of two different configurations called as Type 1 (25 g fibers) and Type 2 (50 g fibers) distributed inside wire mess frame of size 17 cm × 17 cm 60 cm [24,70] – 7.5 cm [62] Aspen swamp pad 2.5 cm [59] Aspen wood pad, 85 ×73 ×73 cm 3 4 cm [71] Configurations: rectangular, cylindrical, and hexagonal pad. Contact area: 503.6 m 2 /m 3 0.15 m [56] Configurations: Three side, triangle, square, pentagon, hexagon, octagon pad. Contact area: 503.7 m 2 /m 3 0.15 m [72] Jute Length/Diameter of fiber: 900/0.1 mm 5 cm [73] Packing density: 20–22 kg/m 3 30 mm [74] Packing density: 62.75 kg/m 3 50, 75 and 100 mm [25] Eucalypt Density: 495 kg/m 3 ; Water absorption capacity: 4.3 g/g; Thermal Conductivity: 0.0368–0.0374 W/(m⋅◦C) 65 mm [29] – – [10,75] Coconut Coir fiber material pad with a wetted surface area of 250 m 2 /m 3 5, 10, and 15 cm [76] Coconut fiber sheets 2 cm thick, forming an angle of 6◦at 2 and 10 cm wide. 7 cm [77] Cooling pads of two different configurations called as Type 1 (25 g fibers) and Type 2 (50 g fibers) distributed inside wire mess frame of size 17 cm × 17 cm 60 cm [24,70] Semi-circular, 68.5 cm diameter pad, 83 cm height. [78] Packing density: 25.14 kg/m 3 50, 75 and 100 mm [25] Arranged in a 100 mm ×100 mm x 100 mm frame with mesh. Packing density: 200 kg/m 3 100 mm [54] Palm Length/Diameter of fiber: 255/0.52 mm 5 cm [73] Packing density: 20–22 kg/m 3 30 mm [74] Luffa 380/0.83 (mm) 5 cm [73] Loofah sponge (Luffa cylindrica) with an average density equal to 19.12 ± 0.89 kg/m 3 5, 7.5, 10 and 12.5 cm [79] Palash fibers and Khus roots Cooling pads of two different configurations called as Type 1 (25 g fibers) and Type 2 (50 g fibers) distributed inside wire mess frame of size 17 cm × 17 cm 60 cm [24,70] Khus Semi-circular, 68.5 cm diameter pad, 83 cm height. – [78] Straw Straw fiber contained in plastic nets 10 cm [11] Rice husk pads Local rice husk with average width of 2.02 ±0.69 mm and length of 10 ± 0.23 Mm 25.4 and 50.8 mm [80] Textile fibers and woven fabrics Nonwoven fabric A pad made of nonwoven fabric with pin-hole-sized holes (perforated pad in 5 mm diameter of pinhole and total pin-hole area is about 1/4 of the crosssectional area of test pad). Wetted surface area: 130 m 2 /m 3 5, 10, and 15 cm [76] Curtain fabric and raw cotton fabric – – [81] 100% cotton fiber Weight 25 g/m 2 0.1 mm sheets [82] Cotton fabric 25 ×1600 cm 2 piece of cotton fabric, arranged on a wire structure 25 cm [83] Sack cloth Packing density: 36.62 kg/m 3 50, 75 and 100 mm [25] Synthetic fibers Synthetic surfactant non-woven and mixed Synthetic/Natural (50% cotton fiber, 30% viscose, 20% polyester). Weight: 50 g/m 2 0.2 mm sheets [82] Paper Kraft paper Resin treated Kraft paper, Rigid Kraft Paper and Original Kraft paper. Weight 75–125 g/m 2 0.1 mm sheets [82] Wood fibers Bamboo Semi-circular, 68.5 cm diameter pad, 83 cm height. – [78] Wood chips Sliced wood contained in wire nets 10–15 cm [11] Density: 380 kg/m 3 ; Water absorption capacity: 1.3–1.5 g/g; Thermal Conductivity: 0.08–0.014 W/(m⋅◦C) 65 mm [29] Wood shaving Arranged in a 100 mm ×100 mm x 100 mm frame with mesh. Packing density: 200 kg/m 3 100 mm [54]s Plastic PVC sponge mesh Coarse fabric PVC sponge mesh2.5 mm diameter in pinhole and fine fabric PVC sponge mesh in 7.5 mm diameter pinhole 5, 10 and 15 cm [33] Plastic mesh High density polyethylene folded by means of thermal welding to form triangular flutes arranged vertically. Specific contact surface: 51.77 m 2 /m 3 and porosity: 0.969 m 3 /m 3 250 mm [15] Cylindrical recycled High-Density Polyethylene (HDPE). Diameter of 3.29 ± 0.62 mm and length of 4.13 ±0.33 mm 25.4 and 50.8 mm [80] The pad type used is a C&V RF-200 pad with dimensions of 492 ×712 mm. Grid or mesh made with polyethylene yarn, which is 0.7 mm in diameter; 4 ×4 mm ribs are made by thermofusing the material. Pad with a compactness of 117.2 m 2 /m 3 80, 160 and 240 mm [84] Shading net Sun shading ratio of shading net is 0.50 50, 100 and 150 mm [13] High density polythene packing Wetted surface area: 420 m 2 /m 3 0.15 m [56] Polymer hollow fiber 5 fiber bundles (100 fibers each). Tunnel cross section diameter 0.15 m. Packing density 10.67 m 2 /m 3 – [85,86] Stone Yellow Stone, Density: 2880 kg/m 3 ; Water absorption capacity: 0.024–0.0916 g/g; Thermal Conductivity: 0.6–2.5 W/(m⋅◦C) 65 mm [29] Piece with holes drilled – [10] Pumice 65 mm [29] (continued on next page) A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 6 ε =Tin −Tout Tin −Twb in (1) Air humidity increase through the pad is usually studied in terms of the humidity ratio: Δw=wout −win (2) Though sometimes authors simply compare the air relative humidity at the pad outlet to the air conditions at the pad inlet. If the previous equation (2) is multiplied to the air mass flow, it is usually referred as the water evaporation rate: ˙mew =˙ma⋅(wout −win)(3) The term “specific water consumption” is sometimes used to refer to the rate of evaporated water related to the pad frontal area [40,42,55] or the pad volume [55], and the temperature drop achieved [40,42]. The cooling capacity multiplies the temperature drop achieved through the pad to the air specific heat Cp a and the air mass flow: CC =˙ma⋅Cpa⋅(Tin −Tout)(4) whereas the Coefficient of Performance for evaporative cooling systems can be defined as the cooling capacity (equation (4)) related to the electric input power due to the pump (Wp) and fan (Wf): COP =CC ˙ Wp+˙ Wf (5) Finally, the heat and mass transfer coefficients, namely h c and h m , depend on numerous factors and thus it is convenient to work with empirical correlations of dimensionless numbers. Existing research propose different correlations to evaluate their results. These parameters are gathered in Table 3, related to the influencing factors studied: air velocity (v) or air flow rate ( ˙ ma), air DBT, air humidity (WBT or RH), the pad thickness (l), the pad material, its configuration or other geometric characteristics and the water flow rate (˙ mw). Literature is separated in experimental studies and those based on theoretical models. It includes all works giving results on the relation between parameters and factors, even though they do not always provide a detailed discussion. Despite approached in a few works, the air DBT and humidity have no effect on the pressure drop, which is indeed demonstrated by the results shown in these works cited. 5. DEC pads operating factors. Optimal performance Results obtained in the literature for these operating factors are reviewed and discussed next to provide insight on the optimal performance of DEC from wetted surface. 5.1. Inlet air psychrometric conditions Most works gathered in Table 3 as providing the operating hygrothermal air conditions (DBT and WBT or RH) do not actually control these factors, and simply measure their variation with time [30,38,51, 58–61,63,64,68,73,87–91,94]. Indeed, only a few deliberate on their effect on the DEC behavior. It is generally accepted that hot, dry conditions are more favorable to the operation of DEC systems [94], while they are put into question for humid climates and for locations where water is scarce [88]. In more humid locations, combination with desiccation [29] or Indirect Evaporative Cooling (IEC) [97] are preferable solutions. Despite the key effect of air hygrothermal properties on the water evaporation and hence on the performance of the system, perhaps above the remaining factors [98], only a few works in the existing literature Table 2 (continued) Type of fiber/fill Material Geometric characteristics Thickness pad References Density: 250 kg/m 3 ; Water absorption capacity: 0.024–0.0916 g/g; Thermal Conductivity: 0.186–0.433 W/(m⋅◦C) Pumice stones coarse and fine. Particle size: 20–40 and 10–30 respectively. Dry density: 340–360 kg/m 3 50, 100 and 150 mm [13] Vermiculite Density: 172 kg/m 3 ; Water absorption capacity: 3.5–5.9 g/g; Thermal Conductivity: 0.104–0.153 W/(m⋅◦C) 65 mm [29] Vermiculite Porous pads made from expanded clay (Cinasita). Two granulometries equivalent to Brita 1 and Brita 2 60, 85 or 100 mm [14] Volcanic tuff Particle size: 15–25. Dry density: 295 kg/m 3 50, 100 and 150 mm [13] Cyprus marble Cut into shapes to fit in the tunnel – [10] Ceramics Porous ceramics tubes Matrix of Porous ceramic pipes arranged vertically with a height of 1200 mm and spaced 58 mm. Outer and inner diameter: 48 and 28 mm, respectively. 600 mm [87,88] Matrix of ceramic tubes consists of 39 ceramic tubes. Material: membrane element made of Al2O3, 1/6 monochanneldesign, 0.2 μ m pore diameter, 300 mm length. The outer and inner diameter: 10 - 6 mm respectively. 28 cm [89] Matrix of Porous ceramic pipes arranged vertically with a height of 25 cm and spaced 13.5 cm. Outer and inner diameter: 7 and 2.8 cm, respectively. Pore diameter: 5–60 μ m, the pore volume:20–40%, the density in completely dry condition: 1300 kg/m 3 3 tubes in staggered-array and parallel arrangement [90] Porous ceramic cylinders 25 cm long, 2 cm diameter tubular rods – [91] – – [10] Porous ceramic evaporator prototype 315 mm ×165 mm x 35 mm Low and medium porosity – [92] Porous ceramic plates Vertical channel that consisted of two parallel walls separated by a distance 10 cm, on which the external left wall is covered by a ceramic porous layer of different thicknesses (1, 4, 8, 10 mm), and porosities (0.38 and 0.5) – [93] Mixed Ceramic plates covered with jute fiber 15 plates arranged in five columns, separated 4.6 cm. Total clay surface area 504 cm 2 and jute fiber area 516 cm 2 . – [94] Metallic Copper metal foam pad Average porosity: 0.82 3 cm [62] Others Dry bulrush basket – – [10] Laterite, Fine sand, cement and rice byproducts water-permeable material 600 mm length tubes. Outer and inner diameter: 50.8 and 25.4 mm, respectively. 100 mm ×125 mm x 250 mm blocks. – [95] Wood charcoal Packing density: 20–22 kg/m 3 30 mm [74] Pine charcoal with equivalent diameters 30 mm and 50 mm. Each size fraction of the charcoal material was filled separately into galvanized steel frames 100, 200 and 300 mm [96] A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 7 actually control both temperature and relative humidity in their experiments. Experimental results from Camargo et al. [61] show that the saturation effectiveness improves for higher DBT, when “more cooling is necessary to propitiate thermal comfort”; however, no information is given about the air humidity. Similarly, Nada et al. [31,42] observe that larger inlet DBT result into also larger humidity increase and consequent temperature drop, cooling capacity and coefficient of performance, as well as into an increase in the saturation effectiveness, but do not provide information on the inlet air humidity during the tests. Sheng and Nnanna [63] also provide experimental results of the saturation effectiveness increase with the DBT, though observe the relevance of air humidity and propose the study of the saturation effectiveness variation in terms of the “humidity ratio increase” (in g/kg). Chen et al. [85,86] obtain larger temperature drops, as relative humidity decreases, for a given DBT. He et al. [48] refer to the WBD for its significance to the evaporative cooling, but do not control it in their experiments. Velasco-G´ omez et al. [83] demonstrate that larger WBD yield larger humidity increase of treated air. As an alternative to the RH or the WBD, Ibrahim et al. [92] resort to the “ambient to saturated vapor pressure difference” to define the force that drives water evaporation from the wetted media. Simulation studies usually investigate further the effect of hygrothermal conditions. Results from mathematical models agree in that the hygrothermal conditions are determinant for the water evaporation rate. Kovaˇ cevi´ c and Sourbron [69] observe an increase in the water evaporation rate with larger inlet DBT, and He et al. [50] notice the same effect also for lower relative humidity. Consequently, Sohani et al. [55] notice that water consumption can be excessive for extremely dry conditions of up to 10–20% relative humidity. Larger humidity increase is at the same time evidence of larger temperature drops; in this sense, Sellami et al. [99] found that the outlet temperature was lower as the inlet relative humidity increased. Gilani and Poshtiri [100] indicate that smaller spacing among plates is needed as the air relative humidity increases, then provide a guide for the design of this plate spacing depending on the air conditions. However, despite the effect of the inlet air psychrometric conditions on the temperature and humidity variation, simulation results show a negligible effect on the saturation effectiveness [69,99]. In the same line, Wu et al. [35] notice no effect of the DBT or the WBT on the saturation effectiveness, for air velocities (v) of 2 m/s and pad thickness (l) of 138 mm. Beshkani and Hosseini [57] analyze the effect of the air Prandtl number, which will vary with the DBT, on the saturation effectiveness. They conclude that, for a same Reynolds number, an increase in the Prandtl number from 0.7 to 0.8 yields a 10% decrease in the saturation effectiveness; however, decreasing the Prandtl number to 0.6 shows no effect. According to the previous results, existing research coincide in the effect of higher DBT and lower RH on the increase of the air humidification, hence the temperature drop, which can be explained on the fundamentals of evaporative cooling. However, some experimental studies observe certain effect of the air DBT on the saturation effectiveness, while in the simulations it appears to be negligible. Larger air humidification evidences an also larger evaporation rate. If this evaporation rate incurs into insufficient wetting of the media due to low water flow rates, then saturation effectiveness will decrease. This will be approached in detain in section 5.5 “water flow rate”. If, on contrary, the pad stays completely wetted, inlet air hygrothermal conditions would have no effect on the saturation effectiveness. An increment in the DBT, provided a constant air humidity ratio, yields an also larger difference between this DBT and the ideally achievable WBT (namely, the WBD); as both the actual temperature drop and the maximum one increase, then the saturation effectiveness maintains. The fact that some experimental results show correlation between the DBT and the saturation effectiveness is due to limitations in the approach of the inlet air humidity (provided total wetting of the pad). The effect of air humidity is thus determinant and needs to be carefully approached, preferably controlled, in future experimental research. The DBT should never be studied separately; actually, the WBD arises as a more appropriate parameter to analyze the effect of the air hygrothermal conditions. Finally, only a few studies determine a range of air psychrometric conditions for which DEC would be practical. Gilani and Poshtiri [100] conclude that DEC is appropriate for climates where DBT varies within the range of 27–41 ◦C and RH from 10 to 60%. He et al. [49] state that it could be counterproductive to use evaporative cooling pads to precool air before natural draft dry cooling towers if outdoor air DBT is below 20 ◦C, in the case of cellulose pads, and below 24–26 ◦C for the remaining materials studied. Nada et al. [31] evaluate the DBT in the range 30◦C–50 ◦C and set the optimal operating conditions at 40 ◦C. 5.2. Air velocity/flow rate The air face velocity, or simply air velocity, corresponds to the average velocity measured at the pad inlet/outlet. This is the most studied parameter in the literature, though sometimes evaluated in terms of air flow rate. High air velocities have been repeatedly correlated to lower saturation effectiveness [16,33,40,46,48,53,61,65,69,74–76,94,96], temperature drop [42,75,91] and relative humidity variation [42,53,91], but also to larger pressure drops [15,31,39,41,46,48,96], water consumption [3,15,31,39,40,42,46,75,91], as well as heat and mass transfer coefficients [3,15,61]. However, some results obtained deserve further attention. For instance, Rong et al. [65] notice that the mass transfer coefficient related to the inlet-to-outlet density difference only depends on the air velocity, then advice that a more thorough study would be necessary. On the other hand, results on the effect of the air velocity on the “water consumption” sometimes differ depending on how this parameter is defined, as indicated in previous section 4. Greater air face velocities are intrinsically related to higher pressure drops. Malli et al. [53] justify the increase in the pressure drop for larger air velocities not only on the higher resistance to the airflow but also to the worse distribution of the flow field at the pad inlet. Another unarguably point is the effect of the air velocity on the saturation effectiveness, which is in fact defined as the “common point for all direct evaporative cooling pads” by Laknizi et al. [91]. Indeed, it is concluded from the ANOVA developed by De Melo et al. [79] that the air velocity is determinant on the saturation effectiveness. Higher air face velocities reduce the saturation effectiveness due to lower residence times, hence temperature drops. In their model, Fouda and Melikan [36] obtained shorter transient periods when operating with higher air velocities, but resulted into lower steady-state saturation effectiveness. Although Gunhan et al. [13] observe just a slight decrement of the saturation effectiveness with the air velocity, they conduct tests only for 0.6 and 1 m s −1 , with their results actually being in concordance with the remaining literature. Nonetheless, some authors observe that this effect was less significant beyond certain operating conditions. Jain and Hindoliya [24] notice that the effect on the saturation effectiveness softens significantly for airflows above 0.22 kg s −1 , while Beshkani and Hosseini [57] notice that the effect of the air velocity on the saturation effectiveness fades once the pad thickness exceeds a certain value. Similarly, Martínez et al. [16] observed the stabilization of the saturation effectiveness for air velocities above 1–1.5 m s −1 and for thicker pads. For plastic pads, which are usually of larger thickness, Franco et al. [15] obtained more stable values of the saturation effectiveness, slightly increasing as the air velocity increased from 0.25 m s −1 to 2 m s −1 but decreasing for even larger air velocities. Existing research also agree in the increase of the Cooling Capacity with greater air velocity [31,42,52,75], which demonstrates that its effect on the increased mass flow rate of treated air is greater than on the decrease of the temperature achievable. For increasing air velocities up to 0.6 m s −1 , Do˘ gramaci et al. [75] obtained a sharp increase of the defined “Specific Cooling Capacity”, while it stabilized for even larger air velocities. However, Naveenprabhu and Suresh [3] obtain a negligible influence of the air velocity on the Cooling Capacity. On contrary, A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 8 Table 3 Main operating factors and parameters of DEC pads. Parameter Factor Associated studies (models) Associated experimental studies Saturation effectiveness Air velocity (v) or air flow rate ( ˙ ma) [16,34–36,55–57,61,63, 69,72,79,85] [10,13,15,16,24,25,31–33,39,40,42,44–46,48,51,53,54,58,61, 63,65,76,77,79–81,83,85,87,88,91,94,96] Air DBT [35,49,55,61,69,85] [30,31,42,59–61,63,64,68,83,85,86,88,89,94] Air humidity (WBT or RH) [35,55,69,85] [30,59,60,64,68,83,85,86,88,89,94] Pad thickness (l) [16,34–36,55,57] [13,15,16,25,31,33,40,42,45,48,53,68,76,80] Configuration/other geometric characteristics of the pad [55–57,72] [15,30,39,40,46,53,54,70,87,96] Pad material [49,56] [10,11,13–15,24,25,29,30,33,39,48,54,59,62,70,76–78,80,81, 96] Waterflow rate ( ˙ mw) [none] [13,16,25,31,40,42,45,46,48,58,64,65,96] Pressure drop Air velocity (v) or air flow rate ( ˙ ma) [16,41,50,55,57,85] [13–16,25,31,33,38–42,45,46,48,51,53,54,58,76,77,80,83,85, 89,96] Air DBT [86] [31,58,85] Air humidity (WBT or RH) [85] [85] Pad thickness (l) [16,41,55,57] [13,14,16,31,33,40,41,45,46,48,53,76,80] Configuration/other geometric characteristics of the pad [41,55,57] [15,39–41,46,53,54,70,96] Pad material [49] [13–15,25,33,39,48,54,62,70,76,77,80,96] Waterflow rate ( ˙ mw) [41,50] [13,15,16,31,40,41,45,48,58,89,96] Humidity increase Air velocity (v) or air flow rate ( ˙ ma) [3,55,69,85] [3,10,13,16,31,38,39,42,53,54,63,74,75,77,81,85,89,91,94] Air DBT [55,59,69,85] [31,42,59,63,64,77,83,85,89,94] Air humidity (WBT or RH) [55,59,66,69,85,90] [38,53,58,59,64,69,83,85,89–91,94] Pad thickness (l) [55] [13,16,31,42,53] Configuration/other geometric characteristics of the pad [55] [39,53,54] Pad material [none] [10,13,29,39,54,58,59,77,78,81] Waterflow rate ( ˙ mw) [3] [3,16,31,42,64,89] Water evaporation/water consumption Air velocity (v) or air flow rate ( ˙ ma) [43,55,85] [10,15,16,30,31,39,40,42,45,46,48,53,54,75,85,91,96] Air DBT [50,55,66,85] [30,31,42,51,64,85,87,88] Air humidity (WBT or RH) [50,55,66,85] [30,51,64,85,87,88] Pad thickness (l) [43,55] [15,16,31,40,42,45,46,48,53] Configuration/other geometric characteristics of the pad [43,55] [15,30,39,46,53,54,96] Pad material [none] [10,15,30,39,40,48,54,62] Waterflow rate ( ˙ mw) [50] [16,31,42,48,64,96] Temperature achieved at the pad outlet/temperature drop Air velocity (v) or air flow rate ( ˙ ma) [3,36,43,55,85,100] [3,10,13,16,31,32,38,39,42,44,63,74,76,77,81,85,91,94] Air DBT [34,35,55,59,61,66,85, 100] [29,31,34,35,37,42,59–61,63,64,68,73,76,85,86,89,91,94] Air humidity (WBT or RH) [34,35,55,59,66,100] [29,34,35,59,60,64,68,73,76,86,89,94] Pad thickness (l) [36,43,55] [13,16,31,42,68,76] Configuration/other geometric characteristics of the pad [43,55,100] [39] Pad material [59] [10,11,13,24,29,39,59,70,73,74,76–78,81] Waterflow rate ( ˙ mw) [3] [3,16,31,42,64,89] Cooling capacity Air velocity (v) or air flow rate ( ˙ ma) [3,52,56,72] [3,10,31,42,54,75,91] Air DBT [none] [29,31,42,59,60,86] Air humidity (WBT or RH) [none] [29,59,60,86] Pad thickness (l) [52] [31,42] Configuration/other geometric characteristics of the pad [56,72] [54] Pad material [56,59] [10,29,54,59,62] Waterflow rate ( ˙ mw) [3] [3,31,42,64] Coefficient of Performance Air velocity (v) or air flow rate ( ˙ ma) [43,52] [10,31,42,54,75,91] Air DBT [52,59] [31,42,59] Air humidity (WBT or RH) [59] [59] Pad thickness (l) [43,52] [31,42] Configuration/other geometric characteristics of the pad [43] [54] Pad material [none] [10,54,59] Waterflow rate ( ˙ mw) [none] [31,42] Heat and mass transfer coefficients Air velocity (v) or air flow rate ( ˙ ma) [3,33,50,72] [3,15,33,40,65,77,91] Air DBT [50,85] [42,85,86] Air humidity (WBT or RH) [50,85] [85,86] Pad thickness (l) [33,72,76] [15,33,40,42,46–48,76] Configuration/other geometric characteristics of the pad [69,72] [15,40,46,77] Pad material [none] [15,77] Waterflow rate ( ˙ mw) [3] [3,48] A. Tejero-Gonz´ alez and A. Franco-Salas Renewable and Sustainable Energy Reviews 151 (2021) 111632 9 results on the COP do not always converge. Some authors [31,75] obtain better COPs as the air velocity increases; indeed, Nada et al. [42] obtain the maximum COP at air velocity 2.2 m s −1 . However, the COP may not be clearly related to the air velocity due to the additional fan needs [91], which can even result into worse COPs [52]. Many works provide recommended values for the air velocity, generally towards a balanced saturation effectiveness face to the pressure drop. Sohani et al. [55] state that an appropriate air velocity is necessary to optimize both water and electric energy consumption. However, these recommendations are as disparate as the pad types studied. Wu et al. propose 2.5 m s −1 as the optimal air face velocity [34], but recommend carefully selecting the air velocity depending on the pad thickness [35]. Similarly, Malli et al. [53] suggest that the optimal operating conditions imply a certain combination of air velocity and pad thickness, and conclude that it must correspond to lower air face velocities through wider pads. They obtain the maximum saturation effectiveness for air velocities 1.8 m s −1 and a pad thickness of 150 mm. For a 70 mm cellulose pad, Nada et al. [31] propose an optimal air velocity of 1 m s −1 , obtaining 84% saturation effectiveness and 8 Pa pressure drop when operating with inlet air temperatures of 40 ◦C, water flow rate 0.1667 kg/s and water temperature 25 ◦C. In the same line, Martínez et al. [16] propose through an exergetic analysis 1.2, 1.1 and 0.9 m s −1 as the optimal air velocities for 80, 160 and 250 mm pad thicknesses. They conclude that optimal air velocities stay far from the extreme values. Other studies identify the optimal air face velocities for diverse alternative pads. Liao and Chiu [33] develop tests on fabric PVC sponge mesh pads for air face velocities in the range from 0.5 to 2 m s −1 . They recommend air velocities between 0.75 and 1.5 m s −1 in coarse fabric PVC sponge mesh to balance the effect on the saturation effectiveness and the pressure drop, besides avoiding water carryout; however, they identify the range 1–1.5 m s −1 as the “normal” air face velocities. For nonwoven fabric and coir fiber wetting media [76], they obtained optimum air velocities between 1 and 2 m s −1 . While operating at constant 35–36 ◦C DBT and 20% RH with an evaporative cooling pad made of eucalyptus fibers, Do˘ gramaci et al. [75] identified the optimal air velocity and air mass flow rate from the cut point between the Cooling Capacity and the Specific Cooling Capacity, being this optimal conditions between 0.6 and 0.9 m s −1 or 0.06–0.08 kg s −1 in terms of air mass flow rate. Under these operating conditions, COP was 3.65, Cooling Capacity reached 0.44 kW with 0.27 g s −1 water evaporation. In a later work on five different pads [10], they identify the optimal airflow as the cut point between the CC and the saturation effectiveness, yielding the value 0.063 kg s −1 as the optimal airflow. If the cut point between the COP and the increment in the humidity ratio was considered, optimal airflow was be 0.041 kg s −1 . Ndukwu and Manuwa [74] recommend air velocities of 4 m s −1 in alternative latex foam, jute fiber, palm fiber and wood charcoal pads. However, tests are conducted only within the range 3–4.51 m s −1 and they do not analyze the effect on the pressure drop. Abdullah et al. [94] recommend the lowest air velocities of about 1 m s −1 at which they conduct tests on a jute coated ceramic DEC media, but do not optimize this selection on any other basis rather than the maximum saturation effectiveness achievable. It must be noted that conclusions derived from experimental results only apply to the type of pad studied and should not be conceived as general design recommendations. Franco et al. [40] recommend operating between 1 and 1.5 m s −1 , similar to the 1.27 m s −1 proposed in the ASABE standard ANSI/ASAE EP406.4 [9] for 100 mm thick cellulose pads, whereas a 25% lower air face velocity would be acceptable for 50 mm pads. Saturation effectiveness at these air velocities vary from 64% to 70%, pressure drop reaches 3.9–11.25 Pa and water evaporation are within the range 1.8–2.62 kg s −1 m −2 ⋅K −1 . 5.3. Pad geometric characteristics and configuration The most studied geometric characteristic is the pad thickness (l), which corresponds to the distance traversed by the airflow. Contact area between water, hence the heat and mass transfer, increases in evaporative cooling pads characterized by larger thicknesses due to larger residence times. Consequently, existing research agree in the fact that an increase in the pad thickness results into larger saturation effectiveness, water evaporation rate and temperature drop [15,35,36,39,48,53,55, 99]. Beshkani and Hosseini [57] observe that the saturation effectiveness increases with the pad thickness while Re decreases, and obtain the best result for d =30 cm and Re =300. Moreover, as seen in the previous section 5.2, the saturation effectiveness gets almost independent from the air velocity as the pad thicknesses increases [57]. According to the definition of the cooling capacity (equation (4)), larger pad thickness would also result into better cooling capacity, as observed by Nada et al. [42]. Similar results were obtained for fiber pads [13,25] and fill pads like PVC sponge [76], high-density polyethylene mesh [16], or clay [14]. However, results do not agree in terms of the COP; while in their experiments Nada et al. [31,42] obtain better COPs for larger pad thicknesses, Laknizi et al. [52] observe through simulation that the pad thickness would reduce the value of the COP. In fact, the effect of the pad thickness on the COP cannot be generalized because it depends on the power requirements of the fan (equation (5)); larger thicknesses introduce further pressure drop, hence fan power needs, that may or not be compensated by the larger pressure drop and consequent cooling capacity. In terms of exergy efficiency, Nada et al. [31] obtained slightly better results for a thicker cellulose pad (35 mm–140 mm), whereas for plastic mesh fill pads Martínez et al. [16] observe that the performance decreases for larger thicknesses, varying between around 70% and 94% for 80 mm, 160 mm and 250 mm. Because the pad thickness has two opposite effects on the exergy efficiency (the inlet exergy increases with the water evaporation rate but the exergy destruction is enhanced by the larger heat and mass transfer) [31], these two effects must be carefully studied for each particular case. On the other hand, it must be noted that larger pad thickness (l) increases the pressure drop, hence the operating costs, as well as the maintenance and investment costs. As it can be deduced from the revision of the literature, the pad thickness must be chosen to find a balance between the saturation effectiveness and the pressure drop. Moreover, larger thicknesses require longer times to reach steady state operation [36]. Among the geometric characteristics of the evaporative cooling pads is also the specific area (A), which is the heat and mass transfer area per unit volume, hence the contact area between water and air (As) related to the pad volume (V): A=As /V.(6) The pad thickness and the specific area can be related by a dimensionless geometric parameter (le/l), where the characteristic length of the pad (le) is defined by the relation of the pad volume (V) to the total contact area (As). Given previous equation (6), it would be: le l= V/As l= V/(A⋅V) l=1 l⋅A=l−1⋅A−1(7) Several research works focused on corrugated rigid media pads correlate this dimensionless parameter (le/l), together with the air and water flow rates, to the evaporative cooling pad performance in terms of saturation effectiveness, pressure drop and heat and mass transfer coefficients [15,16,33,37,42,46,48,55]. Sohani et al. [55] observed that, as the specific area (A) increases, the pressure drop also softly increases, though it soars for specific contact areas above 450 m 2 m −3 . They found that the optimal pad thickness (l) and specific area (A) where 70 mm and 420 m 2 m −3 , respectively. Other A. Tejero-Gonz´ alez and A. Franco-Salas