Climatic applicability of downdraught evaporative cooling in the United States of America

Accepted Manuscript Climatic applicability of downdraught evaporative cooling in the United States of America Pablo Aparicio Ruiz, Rosa Schiano-Phan, José Manuel Salmerón-Lissén PII: S0360-1323(18)30174-4 DOI: 10.1016/j.buildenv.2018.03.039 Reference: BAE 5373 To appear in: Building and Environment Received Date: 14 February 2018 Revised Date: 23 March 2018 Accepted Date: 24 March 2018 Please cite this article as: Aparicio Ruiz P, Schiano-Phan R, Salmerón-Lissén JoséManuel, Climatic applicability of downdraught evaporative cooling in the United States of America, Building and Environment (2018), doi: 10.1016/j.buildenv.2018.03.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Elsevier. Licencia Creative Commons CC BY-NC-ND MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Climatic applicability of downdraught evaporative cooling in the United States of America The potential for application of downdraught cooling in the United States of America (U.S.) depends on its climatic characteristics. However, due to the large geographic span of the country, it varies due to differences in latitude, and a range of geographic features influencing climate, including altitude, topography and terrain. This study describes the development of climatic applicability maps of downdraught cooling in the U.S., which can aid designers in the initial identification of the correct cooling strategy for the geographic area of interest. The proposed approach is based on a set of maps, which are derived from two related climatic indexes: dry bulb temperature to wet bulb temperature depression (DBT−WBT), representing the climatic opportunity, and 26ºC minus wet bulb temperature (26ºC−WBT), representing the climatic opportunity against the theoretical cooling requirement for each location. The downdraught cooling strategy and degree of applicability is classified in the map, based on the aforementioned climatic and cooling parameters. Finally, four representative buildings in four different regions with different climatic conditions were selected for climatic analysis. This resulted in the identification of some climate zones for downdraught cooling application in the U.S. and the suggestion of appropriate design strategies for each of them. Keywords: climatic applicability; downdraught cooling; dry bulb temperature; passive downdraught evaporative cooling; wet bulb temperature 1. Introduction The building sector accounts for a significant part of the global energy consumption. For decades, space heating and cooling (space conditioning) accounted for more than half of all the residential energy consumption [1]. In recent years, further progress has MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT been made in the identification and implementation of energy demand reduction strategies in buildings. This trend was created by an increased adoption of more efficient equipment, better insulation, more efficient windows, and population shifts to warmer climates [2]. This shift in how energy is consumed in homes has seen that even if per-household energy consumption has steadily declined, more homes are using airconditioning than in the past. According to the U.S. Energy Information Administration (EIA), the Residential Energy Consumption Survey [3], nearly 9 out of 10 U.S. homes are air conditioned by central units, individual units, or both. On the other hand, other solutions, including use of fans, dehumidifiers, and pool pumps, also increase summer electricity use in homes. In the U.S., the monthly electricity consumption peaks are in July and August when temperatures and cooling demand are the highest. The EIA estimates that 18% of annual household electricity use are for air conditioning. Three-quarters of all air-conditioned homes use central equipment, but individual air-conditioning units are more common in the cold to very cold climate regions in the northern United States and the marine climate region along the West Coast. In Figure 1 the use of this systems is shown by climatic regions. Figure 1 (Left) Percentage of homes and HVAC use in climatic regions of the United States, (Right) Percentage of buildings with energy sources use in regions of the United MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT States (Source: U.S. Energy Information Administration, Residential Energy Consumption Surveys) [4]. Various techniques have been implemented to improve the building energy efficiency. Traditional air-conditioning methods, such as heat pumps and boilers, are mostly active strategies. However, air conditioning is recognized as a significant factor in global warming and climate change [5]. On the other hand, there is a growing interest in utilizing passive and low-energy systems for cooling buildings, both residential and commercial. Moreover, according to the U.S. Energy Information Administration (EIA), the Commercial Buildings Energy Consumption Survey [6], the country has 5,557 thousand of Commercial buildings. Divided in four census regions, where all the building use heating energy sources, and a high percentage of buildings with cooling energy sources (Figure 2). Figure 2 Total electricity consumption in commercial buildings. Energy Information Administration (EIA), the Commercial Buildings Energy Consumption Survey [6] HVAC are conventional means of creating thermal comfort, however, they are energy intensive and less ecological. Passive cooling can be adopted as a viable alternative, because these technics can remove heat sensibly if the air is cooler or by evaporating water if it is dry [7]. In the search for alternatives, passive downdraught 0 50 100 150 200 250 300 350 400 Northeast Midwest South West trillion Btu Total electricity consumption (trillion Btu) Space heating Cooling Ventilation MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT evaporative cooling (PDEC) is proving to be both technically and economically viable in different parts of the world [5]. Following the theoretical and experimental work by Baruch Givoni in Israel, and by Cunningham and Thompson in Arizona, a number of pioneering buildings adopting this innovative technique have emerged around the world [8]. In the last years, the Passive and Hybrid Downdraught Evaporative Cooling (DEC) is a viable alternative to conventional mechanical cooling in buildings. These firstgeneration buildings demonstrate the technical applicability of Passive and Hybrid DEC as part of a climatically responsive approach to design and to the provision of comfort [8]. Different simplified simulation models of a PDEC have been developed and compared. The results could help designers in choosing amid different calculation models [9]. The downdraught cooling solutions are classified into three types: (1) The Passive Downdraught Evaporative Cooling (PDEC): when downdraught is achieved through the evaporation of water within an air stream. The passive cooling refers to the exploitation of an ambient heat sink to achieve cooling. The idea is based on the fact that the latent heat of water evaporation is absorbed from the passing hot-dry air stream. The process is an adiabatic humidification process in which part of the sensible heat of the air stream is transferred to latent heat. Therefore the sensible heat of the air stream decreases and its DBT decreases, while on the contrary, its latent heat increases, and as a result, the air supplied is not only cooler, but is also more humid. Due to this, it is used as a passive cooling system or part of a complex cooling system. This system works as a complete passive cooling, because in general, no active parts are added (such as pumps or fans). However, when there is no wind and the downdraught MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT airflow relies solely on buoyancy forces, this solution is not feasible, and in this case needs to use fans to enhance the air distribution. (2) The active downdraught cooling (ADC): when the cooling is achieved by using chilled water cooling coils or panels, driving air over evaporative cooling pads directly into the building, which means the strategy relies on mechanical cooling. This technique is an alternative to conventional air conditioning that can contribute to reduce the energy consumption because only the mechanical system needed for the air-cooling is responsible of this energy consumption. (3) Hybrid downdraught cooling (HDC): when it combines both ‘passive’ and ‘active’ downdraught cooling techniques. This technique is an alternative to conventional air conditioning that can contribute to reduce the energy consumption because only needs alternatively, the mechanical system needed for the air cooling, or the fans required for the air circulation and distribution. These techniques have a good potential to provide an alternative to conventional air conditioning systems offering comparable comfort levels with reduced energy consumption and therefore reduced greenhouse gas emissions. Evaporative cooling techniques have been proved feasible both from economic and technical stand points through numerous studies, nevertheless their efficiency can dramatically be reduced in the case of hot humid climates [10]. The systems use evaporative cooling in hot and dry conditions and chilled water cooling coils in warm and humid conditions [11]. In the case of the U.S., it is possible to derive in which parts of the country these techniques can be applied, and the potential of the application, taking into account the climatic data of the meteorological databases known in this country. The high potential for application of these techniques in buildings could reduce their energy impact substantially. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT In this article the applicability of the above-mentioned innovative solutions is going to be studied within the United States of America. 2. General climate classification in U.S. The United States is the world's fourth largest nation by total area, with its large size and geographic variety, it includes most climate types. The specific climatology of each county is shown in Figure 3, according to the Köppen climate classification, Table 1, [12]. Table 1 Köppen-Geiger classes included in the U.S. Climate class Climate name Climate class Climate name Af Tropical rainforest climate Cwa Monsoon-influenced humid subtropical climate Am Tropical monsoon climate Cwb Subtropical highland climate or Monsooninfluenced temperate oceanic climate Aw Tropical wet and dry Dfa Warm/Humid continental climate Bwh Warm desert climate Dfb Temperate/Humid continental climate BSh Warm semi-arid climate Dfc Cool continental climate / Subarctic climate BWk Cold desert climate Dwa Warm/Humid continental climate BSk Cold semi-arid climate Dwb Temperate/Humid continental climate Csa Warm Mediterranean climate Dwc Cool continental climate / Subarctic climate Csb Temperate Mediterranean climate Dsa Warm/Mediterranean continental climate Cfa Warm oceanic climate/ Humid subtropical climate Dsb Temperate /Mediterranean continental climate Cfb Temperate oceanic climate Dsc Mediterranean-influenced subarctic climate Cfc Subpolar oceanic climate ET Tundra climate MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Figure 3 Climate regions of the U.S. according to the Köppen climate classification. Approximately, to the east of the 100th meridian, the climate ranges from humid continental in the north to humid subtropical in the south. The Great Plains west of the 100th meridian is semi-arid. Much of the Western mountains have an alpine climate. The climate is arid in the Great Basin, desert in the Southwest, Mediterranean in coastal California, and oceanic in coastal Oregon and Washington and southern Alaska. Most of Alaska is subarctic or polar. Hawaii and the southern tip of Florida are tropical, as being the populated territories in the Caribbean and the Pacific. This study will identify the different zones of applicability of evaporative cooling in maps of the U.S., without taking into account the proposed climatic zones, but will use climatological data to develop maps at the same high-resolution level (county) as the Building America program map. The Building America climatological map will be used for validation purposes only and to verify the consistency of the results later on. However, this paper, uses the same sub-division level: the county. The county is an administrative or political subdivision of a state and it is a region having specific boundaries and usually some level of governmental authority. It is analyzed a MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT total of 3,142 counties and county-equivalents in the United States. According to this subdivision, each administrative unit has been represented by the climate of a location for which the applicability assessment procedure was illustrated. Following this, four case study buildings using PDEC were identified in the states of Arizona, Utah and California, for which a post-occupancy study was previously undertaken [13] and further detail applicability analysis illustrated. 3. Map of downdraught cooling The following applicability maps are conceived to give architects and engineers an overview of the appropriate downdraught cooling strategies during the initial conceptual stage of the design. The methodology followed has been described previously in chapter 6 of a manual on downdraught cooling [11]. Following that publication [11], more detailed applicability studies were developed for Europe and China, but never for the U.S.. Other authors purpose to assess the applicability of an indirect evaporative passive cooling system in houses across the Brazilian territory [14] or in India [15]. Analysis of conditions at a given location must be based on long-term climatic averages, such as the Typical Meteorological Year (TMY) [16]. In this study the synthetic climatic data obtained from National Solar Radiation Data Base (NSRDB) archives has been used. As they are based on more recent and accurate data, these new data sets are named TMY3 [17]. The TMY3 data set contains data for 1020 locations, representative of 761 counties. The source data is available for download from the National Renewable Energy Laboratory [17]. This data set is available for download in EnergyPlus weather format and include hourly values of solar radiation and meteorological variables for a 1-year period. Their intended use is for computer simulations of solar energy and building systems in order to facilitate performance MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT 5. Map of cooling degree days and its relation to downdraught cooling design strategies Cooling degree days represent the number of degrees and days where the outside temperature is higher than the base temperature. Cooling degree days can be used to help assess or compare different potential sites for development of cooling strategies. They can also be used as a way of normalising weather between different sites, allowing a comparison of the performance of different buildings. The use of degree days should be treated with caution as part of a broader process of analysis, providing a general indicator for order of magnitude assessments rather than for accurate, detailed comparisons. In the United States, a simplified method is used to calculate both heating and cooling degree days [30]. The mean (high temperature plus low temperature divided by two) daily temperature in Celsius and a nominal temperature of 18.3°C (65°F) are used. If the mean daily temperature is 18.3°C, no degree days are counted. If the mean daily temperature is below 18.3°C, the mean degrees below 18.3°C are counted as the heating degree day. If the mean daily temperature is above 18.3°C, the mean degrees above 18.3°C are counted as the cooling degree day. In Figure 8 the number of cooling degree days is shown. According to Figure 7 and Figure 8, the hot and dry climate regions show good potential for the use of PDEC, during the summer period. This strategy should be integrated with sun shading, high thermal mass with night ventilation and natural ventilation. Conventional air conditioning would only be required in summer under extreme hot conditions for a short period of time. However, if the neutral comfort temperature is calculated by an adaptive model [24–27], Passive and Hybrid DEC, together with other applicable passive strategies, will obtain thermal comfort on a longer period of time and possibly 100% of the occupied. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT In hot and moderately humid climates, in some periods (the hottest or high humidity days) the cooling produced by PDEC may not be sufficient to cover 100% of the cooling requirements. In these circumstances, a back-up system in conjunction with PDEC, a hybrid downdraught cooling system (HDC), has to be operated to meet the cooling load. For the hot and high humid climates, PDEC techniques cannot be operated at all, because the outdoor relative humidity is too high. However, the ADC strategy with cooling coils or panels can be proposed in this climate to cover the cooling requirements, and could save 25–35% of the electrical energy required in the building by relying on buoyancy-driven airflow [28]. Figure 8 Cooling degree days in the summer. The applicability map has been obtained by overlapping the categories of intervention and the cooling degree days maps following the rules given in Table 3. It aims to give a useful, simple and accurate method for designers, engineers and architects in order to assess the potential application of downdraught cooling systems in the U.S. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT shows that a high number of counties in the west of the 100 th meridian belong to the high or very high classes of applicability. Table 3 Downdraught cooling applicability. CDD >10000 8000-10000 6000-8000 4000-6000 2000-4000 <2000 Level of intervention VH VH VH VH H M M H VH VH H H M L M VH H H M M L L M M M M L L VL L L VL VL VL VL *VL – very low, L – low, M –medium, H – high, VH – very high applicability Figure 9 Downdraught cooling applicability maps. In Table 4 shows the number of counties in each climatic zone, according to the Köppen classification, and the level of applicability found in each climate zone. Based on the five main climate groups in the Köppen climate classification, the wider applicability is presented in arid climate (B), moderate in temperate climate, low in continental climate (D) and no applicability or very low in tropical climate (A) and polar climate (E). Note that in Table 4 is based on counties, which have unequal sizes, as shown on the map, high applicability counties are larger in size. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Table 4 Number of counties classified by level of intervention in each Köppen climate Level of intervention Level of intervention K ö ppen classification VH H M L VL - K ö ppen classification VH H M L VL - Af 0 0 0 0 0 1 Cwa 0 0 1 0 0 0 Am 0 0 0 0 0 2 Cwb 0 0 1 0 0 0 Aw 0 0 0 0 0 4 Dsa 0 0 2 2 0 0 BWh 9 2 0 0 0 0 Dsb 0 1 7 11 1 0 BWk 4 5 6 1 0 0 Dsc 0 1 3 1 1 3 BSh 2 2 5 6 1 2 Dwa 0 0 0 0 5 0 BSk 11 36 143 91 3 0 Dwb 0 0 0 3 1 0 Csa 3 6 10 3 1 0 Dwc 0 0 0 0 1 0 Csb 3 5 11 21 34 12 Dfa 0 0 15 12 410 0 Cfa 2 1 171 27 1414 101 Dfb 2 4 20 33 311 9 Cfb 0 0 1 5 62 7 Dfc 0 1 7 14 7 4 Cfc 0 0 0 0 0 2 ET 0 0 0 1 1 4 6. Applicability analysis in real buildings In order to validate the mapping approach and verify the applicability predictions, a more detailed analysis of the climatic applicability of downdraught cooling in relation to the performance of buildings that have implemented this type of system in different climatic conditions was undertaken. Four pairs of buildings and counties were selected: Sonoma (building 1) and Stanford (building 2) in the state of California, Washington (building 3) in Utah and Maricopa (building 4) in Arizona. The buildings represent the different conditions of three distinct climates. California is defined by DOE-BAP as a Hot-Dry region, however, it benefits from a Mediterranean climate in the greater San Francisco area, characterized by mild wet winters and dry sunny summers (mean max temperatures of 28ºC and afternoon RH of 35%). This is experienced in buildings 1 and 2: Kenilworth High School, in MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Petaluma, in the county of Sonoma, and the Global Ecology Research Centre, Stanford, in the county of Santa Clara. Petaluma enjoys a mild Mediterranean climate. The dry summer is characterized by typically warm days and cool nights. Standford is warm (but not hot) with dry summers. According to the Köppen Climate Classification system, Stanford has a warm-summer Mediterranean climate (Csb) [12]. Utah is defined by DOE-BAP as a Cold region, the climate of South Utah is hot and dry in summer (with mean max temperatures of 32ºC and RH of 20%) but the microclimate of the Zion National Park (building 3: Visitor Centre, Zion National Park) is influenced by the canyon system, which provides greater annual rainfall and smaller diurnal swings. Maricopa county in Arizona is defined by DOE-BAP as a Hot-Humid region, but in the more extreme climate of south Arizona, it is characterized by mild winters and very hot and dry summers (mean max temperatures of 41ºC in August and afternoon RH of 20%). This climate is experienced in building 4, the Sandra Day O’Connor Federal Courthouse, in Phoenix. According to the classification proposed by Ford [11], these buildings (Table 5, Figure 10) fall under the following typologies: (a) downdraught cooling (DC) system in large atrium for building 4, (b) DC tower attached to adjacent spaces for buildings 1, 2 and 3. The description of these four buildings and an extensive study of their postoccupancy evaluation was published in 2012, the surveys and the questionnaire mention the occupant satisfaction [8]. This study revealed that PDEC systems in increasing occupants’ satisfaction with their thermal environment. The study evaluated the occupants’ satisfaction for the four buildings respect to temperature and air during the MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT summer and winter, lighting, noise, comfort, design, needs, health, image to visitors and productivity. The best solutions were in Global Ecology Research Centre (Stanford) and Visitor Centre (Springdale) where the post-occupancy evaluation (POE) is positive in overall aspect. On the contrary, the life-style and cultural expectation affect the POE in the Federal Courthouse (Phoenix), because air-conditioned courtrooms and offices are kept at a considerably lower temperature (21ºC-23ºC). The POE it is an emerging research topic, in [31], the authors conducted a comprehensive and critical review and suggest five directions for future POE development and applications. Table 5 Information about the building locations and climatic characteristics. Buildings 1 2 3 4 Kenilworth High School Global Ecology Research Centre Visitor Centre, Zion National Park Sandra Day O’Connor Federal Courthouse Location City Petaluma Stanford Springdale Phoenix County Sonoma Santa Clara Washington Maricopa State California California Utah Arizona FIPS 6097 6085 49053 4013 Latitude: 38.52 37.62 37.08 33.45 Altitude: -122.82 -122.40 -113.60 -111.98 Weather file: Santa Rosa, CA, USA (TMY3) San Francisco Intl. Ap., CA, USA (TMY3) Saint George, UT, USA (TMY3) Phoenix Sky Harbour Intl. Ap., AZ, USA (TMY3) SUMMER June – September Avg. DBT (ºC) 18.09 18.71 29.42 33.07 Avg. DPT (ºC) 11.47 11.92 4.05 9.47 Avg. RH (%) 70 67 22 26.5 Avg. WBT (ºC) 14.00 14.48 15.46 19.15 DBT – WBT (ºC) 4.09 4.23 13.95 13.98 26-WBT (ºC) 12.06 11.52 10.54 6.86 Nº Hours where DBT>25º 478 273 2200 2713 CDH (degreehour) 1611 445 13586 21268 Köppen climates [32] Csb Warm-summer Mediterranean climate Csb Warm-summer Mediterranean climate BSk Cold semi-arid climate BWh Hot desert climate Categories of priority of intervention Very low Very low Very high Very high Downdraught cooling applicability Very low Very low Very high Very high MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Figure 10 Building in the U.S. with downdraught cooling systems. Regarding the systems through which PDEC is provided, buildings 2 and 4 use water misting devices while buildings 1 and 3 employ wetted cellulose mats. Both devices provide a source of direct evaporative cooling but the main difference consists in the former injecting nebulized water directly into the air stream, whereas the latter uses a cellulose porous media to absorb and then release the water. The relationship between the temperature of the air and its moisture content is presented graphically in the psychrometric chart. A typical psychrometric chart can be considered as shown in Figures 11, 12, 13 and 14, which plot the bioclimatic strategies which are climatically applicable in a building. In these figures, the humidity ratio is shown as the ordinate versus dry bulb temperature as the abscissa, both on linear scales. The humidity ratio, also referred to as the moisture content, is expressed in International System of Units (SI) as grams of water per kilogram of dry air, and dry bulb temperature in degrees Centigrade. The curved lines on the chart are lines of equal relative humidity, and the diagonal ones are lines of constant wet bulb temperature. Air cannot be cooled by evaporation to a temperature that is lower than its wet bulb temperature. In practice, even this theoretical limit is rarely attained, and the output Sonoma, CA Santa Clara, CA Washington, UT Maricopa, AZ 1.Kenilworth High School, Petaluma, CA. 2.Global Ecology Research Centre, Stanford, CA. 3.Visitor Centre, Zion National Park, UT. 4. Sandra Day O’Connor Federal Courthouse, Phoenix , AZ. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT of most evaporative coolers is at least 2°C warmer than the ambient wet bulb. This implies that even in very dry conditions, where evaporation can cool the air substantially, extremely high ambient temperatures may result in wet bulb temperatures that are theoretically too high for human thermal comfort but could still be perceived as comfortable compared to the dry bulb. Building bioclimatic charts (BBCCs) [33] were used to assess evaporative cooling design strategy and the hourly values for June to September are plotted along with the direct evaporative cooling boundary as well as other passive design strategies. According to this analysis, each climatic typology was examined based on the applicable downdraught cooling strategy and the building exemplification. 6.1. Applicability analysis in Warm Summer Mediterranean climate Building 1: Kenilworth High School The Mediterranean climate experienced in Petaluma, allows the use of PDEC during the hot and dry periods in the summer in combination with convective cooling by natural ventilation at periods of high humidity. The PDEC strategy is implemented in the design of the Kenilworth High School Building by using a cellulose mat system integrated at the top of the downdraught cooling towers. The environmental design strategies adopted in the school building were mainly focused on energy efficiency but poor design of the airflow patterns in many of the spaces and maintenance difficulties, meant that the PDEC system was not very effective and rarely operational [34]. As shown in Figure 11, the climate of Petaluma has a good potential for evaporative cooling, except for few hours when the temperature and the relative humidity are both very high. From the histograms in Figure 16 and Figure 17, it is apparent that during the summer months (June to September) whilst the wet bulb depression is mostly MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT gravitating in the medium to low priority of intervention zones, with the greatest frequency of hours in the range of 1 to 7°C (78%), the cooling demand reduction potential represented by (26°C – WBT) is comparatively higher with the highest frequency found for 14°C and with values of above 10°C for 73% of the time. This highlights the point that where there is low cooling need as the climate is milder and the cooling degree days are lower, there is also a greater potential for the evaporative cooling to meet most of these needs, given the low values of WBT (Figure 15). Figure 11 Psychrometric chart for the climate of Santa Rosa, used to characterise conditions for the Kenilworth High School building in Petaluma, CA. Building 2: Global Ecology Research Centre The Global Ecology Research Centre in Stanford uses night sky radiant cooling as the main cooling system and, as a secondary system, a PDEC tower employing water MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT misting devices. As shown in the psychrometric chart for San Francisco as representative of Stanford, California (Error! Reference source not found.), evaporative cooling is an effective strategy in dealing with the hottest and driest conditions. However, from the frequencies of wet bulb depression and cooling potential indicators (Figure 16 and Figure 17) it is apparent that even if the priority of intervention is low, the cooling potential is high due to WBTs well below the 24ºC threshold (Figure 15). In fact, the frequency of hours when the (DBT-WBT) depression is below 7°C is 85% and the (26°C-WBT) indicator is above 10ºC for 76% of the time. It was found that the PDEC system was only marginally used for the conditioning of the lobby area only during extreme hot weather conditions. This is in line with the output of the mapping which show low strategic opportunity for intervention based on the climate, low cooling needs (see CDD map, Figure 8) but high cooling opportunity during periods of high need. The level of applicability in this climate is very low, as it was mentioned in the section ‘Map of intervention categories’, the potential for cooling the air (DBT−WBT) is prioritized to assign the global level of applicability, and in this case, it is very low. This coincides with the conclusion that it is can be obtain graphically from the psychrometric diagram [35,36]. Since the meteorological conditions are located outside the preferred zone of action, zone 1, corresponding to evaporative cooling. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT indicator of the climatic opportunity. For the hot-dry climates with high DBT the relatively higher WBTs come closer to the 26°C threshold and therefore show low potential for cooling at 26°C. For these locations it is suggested that the indoor comfort temperature should be reconsidered and, following the adaptive comfort theory, it should be chosen as the upper limit of the comfort band for each climatic profile (i.e. often greater than 26°C) in order to maximise the evaporative cooling potential given by the local climate. Also, greater accuracy could be obtained by creating maps that can isolate daytime hours from night time hours, in order to identify the cooling opportunity during times of greater need. Similarly, in future developments the reference indoor design temperature could be varied depending on the climate in order to map with more accuracy the effective cooling opportunity. Nevertheless, the categories of intervention envisaged by applying the proposed methodology are congruent and in line with the results of the post-occupancy evaluation and the level of use of the strategies and systems in the buildings. However, as outlined above, limitations are encountered in the difficulty for these maps to represent local climatic conditions and site microclimate. Therefore, following the initial climatic feasibility analysis through maps, it is important to undertake a more detailed climate and site analysis to fully understand the seasonal and diurnal strategic potential of the local climate. 7. Conclusion New applicability maps and a climate classification of the potential for downdraught cooling in U.S. have been presented in this paper. The developed maps and classification have been based on (DBT–WBT), cooling degree hours and (26ºC-WBT) indexes. Using a bioclimatic approach, the potential use of downdraught cooling was analyzed in four counties and the results showed that the map of intervention categories developed concurred with the symmetric analysis of Givoni for evaporative cooling, validating the usefulness of the proposed maps. MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Four regions were investigated, covering the very hot and dry summers as well as the Mediterranean climate. The mapped areas show the potential for the use of different variants of downdraught cooling systems. The results indicate the applicability of different PDEC systems in varying climatic regions and provide a preliminary evaluation of the opportunity of using downdraught cooling systems as an alternative or a complement to conventional airconditioning systems. The study also highlights that in certain climates where the need for cooling is low, the opportunity for cooling provided by direct evaporation can be high, due to WBTs being sufficiently low to produce a cooling effect and potentially deal with the totality of the cooling loads; where the need for cooling is high, the cooling opportunity can result low, due to WBTs being very close to the set indoor design temperature of 26ºC. The applicability maps and classification are intended to give designers and decision makers a useful and quick method to assess the potential applicability of downdraught cooling systems in the U.S. Although this climate classification is developed for downdraught cooling strategies and it is not possible to consider all the climate parameters for all applications and all situations, it is envisaged that future developments could include varying upper threshold indoor temperatures for the calculation of the cooling opportunity currently represented by the index (26ºC-WBT). Additionally, as previous studies show, the design of the PDEC system has a direct impact on the cooling performance. The type of the water supply system, dimensions of the cooling tower and the outdoor wind speed are another factors that influence airflow rate supplied to the indoor, thus, impacting the overall cooling MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT effectiveness of the PDEC and eventually indoor thermal comfort. These are future research activities that could be carried out regarding to this topic. 8. References [1] Energy Information Administration (EIA), Annual Energy Outlook 2017, 2017. http://www.eia.gov/outlooks/aeo/pdf/0383(2017).pdf. [2] U.S. DOE, Quadrennial Technology Review: An Assessment of Energy Technologies and Research Opportunities, (2015) 1–505. http://energy.gov/sites/prod/files/2015/09/f26/Quadrennial-Technology-Review- 2015_0.pdf. 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MANUS CRIP T ACCEP TED ACCEPTED MANUSCRIPT Highlights • New applicability maps of the potential for downdraught cooling in U.S. • Classification has been based on DBT–WBT, cooling degree hours and 26ºC-WBT indexes. • Four buildings showed that the applicability categories and maps are validated. • Climate zones for downdraught cooling application in the US is defined.