Using the sky as heat sink: Climatic applicability of night-sky based natural cooling techniques in Europe

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es This is an Accepted Manuscript of an article published by Elsevier in Energy Conversion and Management, Vol. 225, on December 2020, available at: https://doi.org/10.1016/j.enconman.2020.113424 Copyright 2020 Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND 1 Using the sky as heat sink: Climatic applicability of night-sky based 1 natural cooling techniques in Europe 2 MCarmen Guerrero Delgadoa*, José Sánchez Ramosb, Servando Álvarez Domíngueza 3 a Grupo de Termotecnia, Escuela Superior de Ingenieros, Universidad de Sevilla, Camino de los Descubrimientos s/n. 4 41092, Seville, Spain. 5 b Máquinas y Motores Térmicos, Escuela Superior de Ingeniería, Av. Universidad de Cádiz, 10, 11519, Puerto Real, 6 Spain 7 *Corresponding author. mail address: mgd[email protected] (MCarmen Guerrero Delgado) 8 9 Abstract 10 Climate change and environmental degradation are an existential threat faced by Europe 11 and the rest of the world. To overcome these challenges, Europe requires a new growth 12 strategy that transforms the Union into a modern economy that uses resources efficiently 13 and competitively. The use of natural technology for cooling buildings is attracting 14 attention due to the growing interest in energy and environmental conservation. 15 Therefore, given the great cooling potential from using the sky as a heat sink and to 16 promote the integration of nocturnal dissipation systems that use this environmental heat 17 sink, the authors present a study to assess the potential of radiative water-based cooling 18 systems that use a radiant panel or flat surface and water supplied to an air treatment unit 19 through a storage tank. The method implemented in the study is based on the 20 characterization of supply and demand and the subsequent linking of both through the 21 natural cooling technique. This method presents the definition of performance indicators 22 that allow to determine if the application of natural cooling techniques are of interest in a 23 specific place. The method may be applied directly to assess the energy savings achieved 24 by incorporating these systems into specific design and operating conditions, and 25 inversely, by evaluating the design and operating systems required by the systems studied 26 to obtain a given saving. The results of the study of their potential use in Europe show 27 that the use of natural radiative and falling-film technology is of great interest. From the 28 results obtained, it stands out that the integration of the falling-film system can provide 29 savings of more than 60% in extremely hot regions and much greater in areas with less 30 cooling needs. From the studies carried out, it is concluded that the integration of the 31 falling-film means that practically no part of Europe requires an additional cooling 32 system, making the continent passive in terms of cooling and facilitating the achievement 33 of the objectives set by the European Union. 34 Keywords: natural cooling technique; night-sky; falling-film; radiative cooling; climatic 35 applicability 36 37 38 39 40 41 42 2 Nomenclature 1 Variable Description Units UHI Urban Heat Island - NCT Natural Cooling Technique - CCP Climatic Cooling Potential - DEC Direct Evaporative Cooling - TMY Typical Meteorological Year - DBT Dry Bulb Temperature ºC WBT Wet Bulb Temperature ºC KPIs Key Performance Indicators - NCSF Natural Cooling Saving Fraction % saving 𝑚 󰇗 Mass flow of water kg/s 𝐶𝑝 Specific heat of water J/kgK 𝑇𝑖𝑛𝑙𝑒𝑡 Temperature of the water at the inlet of the dissipation system ºC 𝑇𝑜𝑢𝑡𝑙𝑒𝑡 Temperature of the water at the outlet of the dissipation system ºC DH Degree hours ºC 𝜀𝑠𝑖𝑛𝑘 Efficiency of the sink - 𝜀𝑒𝑥𝑐ℎ Efficiency of the heat transfer - 𝑄 Cooling energy KWh U Heat transfer coefficient of the dissipation system W/m2K A Transfer area of dissipation system m2 𝑇𝑤 Water temperature ºC 𝑇𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 Stagnation temperature ºC 𝐷𝐻𝑇𝑤 Cooling degree days of water Degree days F Utilisation factor - GCP Gross Cooling Potential Degree days NCP Net Cooling Potential Degree days ACP Available Cooling Potential Degree days 3 L Lenght of dissipator m 𝜌 Density of water Kg/m3 P Width of the dissipation system m SECP Specific Energy Cooling Potential kWh/m2 year SCSI Climatic severity index for the summer - IDW Inverse Distance Weighting - Da Dissipation system area m2 Ca Conditionated area m2 1 4 1. Introduction 1 Climate change and environmental degradation are an existential threat faced by Europe 2 and the world as a whole. To overcome these challenges, Europe requires a new growth 3 strategy that transforms the Union into a modern economy that uses resources efficiently 4 and competitively. In this regard, Europe has established the European Green Deal [1], 5 based on the roadmap to provide the European Union with a sustainable economy and 6 make the region climate neutral in 2050. Making these aims more difficult to achieve, the 7 Urban Heat Island (UHI) phenomenon is an ever-increasing problem worldwide [2]. As 8 a cause of this effect, the surface of the city tends to heat up on hot days and during 9 heatwaves, leading to an increase in air temperatures. Furthermore, the problem of 10 overheating facing urban areas is exacerbated by climate change, which has rapidly led 11 to more hot days, extreme weather conditions and a drastic increase in the cooling load 12 on buildings [3]. This problem is focused on the lack of innovation in the built 13 environment in Europe, which makes it difficult to eradicate energy poverty, mitigate the 14 urban heat island and local climate change [4]. The greater demand for cooling leads to 15 increased energy consumption, resulting in the emission of more greenhouse gases. 16 Recent studies have shown that, due to the expected increase in future temperatures, the 17 worldwide energy demand for cooling in the residential sector will increase by up to 34% 18 in 2050 and by 61% in 2100 [5], the latter figure rising even higher in studies focussing 19 on the Mediterranean region[6]. In this sense, the use of natural cooling techniques (NCT) 20 in buildings is attracting attention due to the growing interest in energy saving and the 21 protection of the environment. 22 1.1. The sky as a nocturnal heat sink 23 The common characteristic of natural cooling technique (NCT) is the existence of a 24 natural and renewable resource (environmental heat sink) such as air, water, the sky or 25 the ground temperature, which is lower than that of the air inside the building. The sky, 26 with its low temperature and unlimited availability as a renewable resource, is of great 27 interest for use as an environmental heat sink [7]. The sky's potential as a heat sink has 28 been confirmed experimentally in a study previously carried out by the authors of the 29 present work [8] . Therefore, the present study focuses on the use of this environmental 30 resource. Apart from the kind of sink, the latest natural cooling technology is classified, 31 according to the type of cooling performed in the building, into direct technology, in 32 which the heat sink promotes the direct cooling of the structure of the building and/or the 33 air inside it; and indirect technology, whereby the sink first cools a working fluid 34 (generally air or water) that absorbs in a second phase (with or without intermediate 35 storage) the residual heat of the building [9]. The present study focuses on indirect cooling 36 techniques using water as the intermediate fluid to cool the air inside the building (see 37 Table 1). This method is based on the use of a panel or flat radiative surface, technology 38 that has been assessed by the authors of this paper in an earlier study [8]. The main 39 differences between the two techniques are the heat transfer between the dissipation fluid 40 (water) and the outside environment. In radiative cooling systems, the water does not 41 make direct contact with the outside environment, and consequently, the nocturnal 42 cooling of the water is large due to convective transfer with the air and radiative transfer 43 with the sky. However, in the falling-film system, water flows over a flat surface and 44 makes direct contact with the air, so the water film is cooled by convection with the air, 45 radiation with the sky and evaporation due to the air and water coming into direct contact. 46 47 48 5 Table 1. Radiative cooling systems based on water involving a radiative panel or flat surface. 1 System Sink Effective environmental Thermal Dissipator Working fluid Hot Medium Radiative Sky Stagnation temperature [10] F(Tair, Tsky) Radiator temperature Intermediate fluid (water) Storage temperature Falling-Film Sky Stagnation temperature [10] F(Twet-bulb, Tsky) Flat surface Intermediate fluid (water) Storage temperature A review of applications in the literature reveals that the technology described in Table 1 2 can be incorporated into buildings in different ways. In both, the cold water used as the 3 dissipation fluid must be stored before being used to mitigate the heat inside the building. 4 In this sense, the storage incorporated can be conventional, such as in water storage tanks, 5 while another option is the use of the thermal mass of the building itself to store the cold 6 to be supplied to the building. 7 Table 2 shows a review of the development and application of the systems described in 8 Table 1 in buildings. Also this table 2 summarizes the most relevant results of existing 9 literature. 10 6 Table 2. Review of the development and application of the systems described in Table 1 in buildings. Reference Year System Heat Medium Type of study Summary [11] 2001 Radiative Storage temperature (water tank) Experimental Study of a building in Japan in which a nocturnal radiative cooling system is installed involving a collector on the roof top of 15.2 m2 and an underground tank of 31 m3. The cooling loads obtained were from 40-80 W/m2 [12] 2006 Radiative Storage temperature (thermal mass) Experimental Experimental assessment of the use of rooftops in Greece through the use of an experimental prototype involving the integration of a radiative water-based system. The system constructed has a dissipation area of 2.7mx4.9m. The cooling loads obtained with this system range from 40-70 W/m2 for the nominal flow tested. The results show the importance of the system's working flow to ensure the system reaches maximum efficiency. [13] 2011 Radiative Storage temperature (water tank and thermal mass) Experimental Photovoltaic-thermal collector (PVT) to generate electricity and cooling energy. Modules with a dissipation area of 2.8 m2 were used in a residential building in Madrid. The cooling loads measured were between 60-65 W/m2 when the thermal dissipator is used to cool a water tank and between 40-45 W/m2 when used to cool the roof of the building directly [14] 2012 Radiative Storage temperature (water tank) Simulation A concept was proposed involving a thermosyphon heat pipe radiator to extract heat from water in a storage tank to generate cooling water during the night. The results of the study of a similar building in different parts of Asia and Australia show that the cooling load of the building is higher when the dissipation area is greater and the water temperature lower. It also highlights that the increase in the V/A ratio (storage capacity/dissipation area) results in a higher fraction of the load, but above a certain value, this fraction increases slightly until reaching the established maximum. [15] 2014 Radiative Storage temperature (water tank) Simulation The study presents radiative cooling potential using climate data from 10 different locations in Malaysia. The study found that radiative cooling can decrease cooling needs by 11%. The same value was found for the 10 locations in this country. [16] 2015 Radiative Storage temperature (water tank) Experimental Experimental and analytical study of a nocturnal radiative cooling system with flat plate solar collectors in Beijing. The maximum cooling power reached was 26 W/m2. [17] 2018 Radiative Storage temperature (water tank) Experimental Experimental study of a SH-RC system. Validation of a theoretical model of this system with the results of the experimental evaluation. Afterwards, the cooling potential was analysed in four locations: Beijing, Hefei, Fuzhou and Urumqi. The annual net gain from radiative cooling in the four cities were 862.44 MJ / m2, 831.54 MJ / m2, 741.63 MJ / m2 and 775.34 MJ / m2. 7 [18] 2018 Radiative Storage temperature (water tank) Simulation This paper estimates energy savings from daytime radiative cooling, based specifically on photonic materials. The results of the simulation showed that the photonic radiative cooling system resulted in savings in cooling energy of 45% and 68% with regard to the VAV system for an office building located in five parts of the USA. [19] 2019 Radiative Storage temperature (water tank) Experimental Development of a 24-hour radiative system. The system was modelled and studied experimentally. The mean cooling potential of the system was estimated to be 80 W/m2. Applying the detailed model developed in a typical meteorological year (in Phoenix, Arizona) shows that the system could generate up to 350 kWh of cooling (or 26 kWh/m2) with a drop in the ambient temperature of 4-5 º C at a low flow rate of 0.1 L / (min - m2) during a typical summer month. [20] 2019 Radiative Storage temperature (water tank) Experimental Hybrid photo-thermal and radiative cooling collector using black acrylic paint as the panel coating. The net radiative cooling flux of the collector reached 55.1 W/m2 on a clear night. [21] 2020 Radiative Storage temperature (water tank) Simulation Evaluation of thermoelectric radiative cooling technology. The analysis showed that the daytime and nocturnal cooling of the radiative cooling sub-system contributed 55.0% and 45% of the annual cooling output (heat dissipation), respectively. The RSC-TEC proved to be a potential solution for adopting thermoelectric cooling technology on a large scale. [8] 2020 Falling- Film Storage temperature (water tank) Experimental The empirical findings obtained by the authors of this paper showed that the falling-film configuration improves cooling, delivering up to five times the dissipation power obtained with a radiative cooling system. The experiment was performed in Seville during the summer of 2019. [22] 2020 Falling- Film Storage temperature (water tank) Simulation Study of potential savings from integrating the falling film system with a storage tank in residential buildings. The natural dissipation technology is shown to be able to satisfy up to 80-90% of the cooling load in the climate zones and types of residential buildings studied. 8 As observed, most of the theoretical and experimental studies assessed radiative cooling 1 technology. This is because developing and verifying the increase in the cooling output 2 of falling film technology was performed recently for the first time by the authors of the 3 present study (summer 2019) [8]. Furthermore, a review of the state of the art highlights 4 the use of a water tank as the thermal storage system due to the maturity of the technology 5 and because it is easy to incorporate into a conventional cooling system. 6 1.2. Climatic applicability 7 As seen in the review of the bibliography performed in Section 1.2, the high degree of 8 development of natural cooling technology has been reported in the scientific and 9 technical literature, which includes many theoretical and experimental studies that have 10 reported successful results (see Table 2). Despite the expected decrease in the cooling 11 energy load (requirements) from using natural cooling techniques, their incorporation into 12 the building sector has been limited or practically non-existent in most countries within 13 the European Union and completely irrelevant in the rest [23]. The use of the natural 14 cooling technique (NCT) is limited as a result of a combination of factors. The main 15 reason is that the use of passive cooling strategies is largely dependent on climate 16 conditions. In order to propose a suitable passive strategy in a building for a given climate, 17 it is necessary to perform studies to assess its cooling potential and climatic applicability, 18 which makes it possible to determine whether the bioclimatic cooling system is of interest 19 taking into consideration the climate conditions. 20 In this regard, many researchers have performed climatic studies associated with different 21 kinds of natural cooling techniques. Bhamare et al. [24] developed a tool to analyse the 22 bioclimatic cooling potential of passive strategies for different climate zones in India 23 based on bioclimatic tables. The results of the analysis were assessed using an energy 24 simulation of a case study of a residential building. The technology studied by these 25 authors were natural ventilation, thermal mass, thermal mass with night ventilation and 26 direct evaporative cooling. Campaniço et al. [25] defined the climatic cooling potential 27 (CCP) associated with natural ventilation techniques and evaporative cooling and used it 28 to assess the cooling potential associated with the techniques for the whole of the Iberian 29 peninsula. Giacomo Chiesa et al. [26] analysed the geo-climatic applicability of passive 30 direct evaporative cooling (DEC) to improve thermal comfort inside buildings in the 31 Mediterranean region. To this end, they used three methods based on the definition of 32 KPIs defined by climate analysis and mapped cooling potential according to typical 33 meteorological years (TMY). Finally, they assessed the potential savings by running 34 EnergyPlus simulations of an office building selected as a representative building. 35 Aparicio-Ruiz et al. [27] analysed the potential applicability of downdraught cooling in 36 the USA. Their approach was based on a set of maps derived from two related climatic 37 indexes: dry bulb temperature to wet bulb temperature depression (DBT−WBT), 38 representing the climatic opportunity; and 26 °C minus wet bulb temperature 39 (26 °C−WBT), representing the climatic opportunity against the theoretical cooling 40 requirement for each location. Finally, for the climate analysis, four buildings were 41 selected representing four regions with different climate conditions. Lastly, Alvarez, S. 42 et al. [9] developed a general method based on calculating cooling potential maps and 43 particularized the results obtained for using the ground as an environmental heat sink. 44 1.3. Aims 45 To promote the integration of nocturnal dissipation systems that use the sky as an 46 environmental heat sink, the authors of this study perform a study to assess the climatic 47 potential within Europe of water-based radiative cooling systems involving a radiative 48 15 1 Figure 7. Available temperature and available cooling potential (falling-film system) 2 Specific energy cooling potential 3 The indicator of energy cooling potential is defined according to Equation 11: 4 𝑄=𝑈∙ 𝜀𝑒𝑥𝑐ℎ ∙𝐷𝐻𝑇𝑤 (11) 5 Where U is the heat transfer coefficient, 𝜀𝑒𝑥𝑐ℎ the exchange efficiency, and 𝐷𝐻𝑇𝑤 the sum 6 of the degrees of water cooling per hour. 7 The heat transfer coefficient values proposed to estimate this KPI were developed and 8 assessed by the authors in an earlier experimental study [22]. The values proposed are: U 9 (radiative) =10 W/m2·K and U (falling-film) =38.11 W/m2·K. Finally, as the specific 10 energy cooling potential (SECP) indicator depends on the design of the dissipator (𝜀𝑒𝑥𝑐ℎ), 11 a mean exchange efficiency of 0.7 for the falling-film system and 0.9 for the radiative 12 system (in accordance with Figures 5 and 6) is considered to determine the mean cooling 13 potential that the system is able to supply the building with. If the designer wishes to 14 make a more accurate estimation depending on the design, the graphs provided in Section 15 2.2.3 can be used. The sections below provide a detailed analysis of how the specific 16 energy cooling potential indicator is obtained considering the hypotheses mentioned. 17 Section 3 provides a detailed description of how the daily SECP is obtained by using the 18 frequency tables showing available cooling for a set of representative locations in Europe 19 (described in Section 3), and it provides data for the seasonal energy cooling potential 20 obtained for the whole of Europe. 21 Natural cooling saving fraction 22 Finally, as stated in Section 2.1.2, there is a dependence between cold production and 23 natural cooling technology. The KPI that characterizes this dependence is defined as the 24 natural cooling saving fraction (NCSF). The NCSF provides the contribution of the 25 natural cooling technology to the cooling needs of the building and depends on the fit 26 between the cooling load and the cooling generated by the dissipation system (Equation 27 12). 28 𝑁𝐶𝑆𝐹= 𝑄(S𝐸𝐶𝑃)∙𝐹 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡𝑠 (12) 29 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 Porcentage of Hours (%) Temperature (°C) T referencewater Available (falling-film) 16 Calculating the cooling energy used to reduce cooling needs requires its utilization factor 1 to be determined. The utilization factor depends on the supply, the demand of the building 2 and the technology that integrates the cold produced by the natural dissipation system 3 with the building. In this study, an assessment is made of the anticipated savings from 4 incorporating natural radiative and falling film systems into residential buildings by 5 means of a TES tank that stores the cold water produced during the night, which is then 6 used in an air treatment unit to supply the building with cold air. As stated in the review 7 of the state of the art in Section 1, the main advantage of using a water tank as the thermal 8 storage system is the maturity of the technology and because it is easy to incorporate into 9 a conventional cooling system. 10 The characterization of the utilization factor associated with the integration of the system, 11 shown in Figure 8, was obtained by the authors of the present study in a previous paper. 12 It shows that the utilization factor (F) depends on the inertia (storage capacity) and the 13 ratio of the maximum usable energy and the demand of the building (gamma), which is 14 formulated in Equation 13. As stated in the paper in which this concept is developed, this 15 factor must be obtained monthly. Direct seasonal characterization allows for interaction 16 between the supply and demand in periods, making it possible to compensate for 17 unrealistic demand. 18 𝛾=𝑄(S𝐸𝐶𝑃)∙𝑑𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑖𝑜𝑛 𝑎𝑟𝑒𝑎 [𝑘𝑊ℎ] 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑛𝑒𝑒𝑑𝑠 [𝑘𝑊ℎ] (13) 19 20 Figure 8. Utilisation factor 21 3. Application to European locations 22 In this section, the method described is applied to Europe. First, as applying the method 23 calls for the understanding of the cooling needs of the building in question, Section 3.1 24 proposes the use of a reference cooling demand. Then, it has been developed frequency 25 distribution tables for nocturnal cooling in Section 3.2. The frequencies of the nocturnal 26 cooling potential are calculated in degree hours for the nights corresponding to cooling 27 months. Furthermore, this section 3.2 presents the results of climate assessment maps 28 throughout Europe of the KPIs defined in Section 2 and the results of the study of the 29 17 climate potential and energy potential. Finally, Sections 3.3, 3.4 and 3.5 present a study 1 of the natural cooling saving fraction (NCSF) performed directly and inversely. 2 3.1. Cooling needs 3 To apply the described methods, and in particular of the natural cooling saving fraction 4 indicator, it is necessary to know the cooling needs of the building in concern, the 5 utilization factor defined above and the dissipation area of the natural cooling system. As 6 the aim of this study is to perform a global assessment of cooling potential, the parameters 7 required to calculate the indicator must be generalizable, enabling any city in Europe to 8 assess whether the use of this natural cooling technology is interesting. Furthermore, it is 9 necessary to establish the cooling needs associated with the whole of Europe. Thus, the 10 present study proposes the use of a reference cooling load, obtained as detailed below. 11 The spatial cooling load in the member states of the European Union is often not well 12 established or even unknown. This lack of knowledge about cooling is a source of 13 uncertainty for related sectors. It has inspired studies such as the one by Jakubcionis et al. 14 [30], which develops a method using a geo-referenced approach to establish a relationship 15 between the seasonal cooling load in residential buildings and cooling degree days. The 16 cooling needs of a building depend on its location and its constructive and operational 17 characteristics. Buildings are constructed in very different ways depending on the kind of 18 climate in that location. Furthermore, the existing buildings in a specific location present 19 a wide variety of cooling loads, mainly due to the existence of old and new buildings. 20 Thus, to take into account all the demand situations mentioned and to be able to obtain a 21 generalizable and practical reference standard that is representative of every location, this 22 study presents the energy simulation of a sample of residential buildings with different 23 levels of compactness (volume/transfer area) (0.5-2) and percentages and distribution of 24 glazing (ranging from buildings with between 5% and 30% of the glazed area facing 25 south). This range is considered reasonable for single-family residential buildings, as 26 shown by Potrč Obrecht. et al. [31]. 27 Likewise, the building quality considered corresponds with the demands arising from the 28 transposition of the energy performance of buildings directive (EPBD) in Spain for 29 buildings constructed from 2019 [32]; that is, taking the definition of a nearly zero energy 30 building (NZEB) in the country [33]. This considers ranges of mean thermal 31 transmittance, the value for which varies between 0.4 and 0.8 according to the climate 32 zone. Likewise, the overall airtightness of the building [n50] varies from 3 to 6 h-1 33 depending on the compactness and night-time cooling ventilation was established as 4 h- 34 1 (airflow of 4 air changes per hour). Finally, a value of 4W/m2 was taken for internal 35 gains (typical value for this kind of single-family residential building) 36 The energy simulation is performed in the unified tool LIDER-CALENER (HULC), the 37 official tool for calculating the thermal energy efficiency of buildings in Spain for 38 assessing mínimum energy requirements and energy certification [34]. This tool has been 39 widely used and referenced by several papers in the literature [35]. The simulations of 40 each of the cases mentioned are performed in the set of 894 locations corresponding to 41 the meteorological seasons of the NUTS-3 region, the climate data for which are obtained 42 from Meteonorm [36]. The set of cases calculated makes it possible to determine the 43 minimum and maximum cooling energy needs for every European location, and the 44 cooling needs in the months of June, July, August and September for each one, these 45 monthly values being necessary to determine the utilization factor for calculating the 46 NCSF indicator. Finally, among these levels of cooling needs, the present study proposes 47 18 an assessment reference value, which is the mean value within the range obtained for each 1 location. Figure 9 shows the seasonal demand obtained in each of the simulations 2 performed and the value taken as the reference demand for the different cities distributed 3 in the whole climate range in Europe (represented by its climatic severity index for the 4 summer period [31]). 5 6 7 Figure 9. Demand reference for the assessment of the NCSF indicator. 8 The estimated reference cooling needs, shown in Figure 10, divide Europe into three 9 clearly distinct zones: a cold zone, with low cooling demand (<5 kWh/m2), comprising 10 northern, western and eastern Europe; a temperate zone, comprising the north of southern 11 Europe, with an estimated cooling demand between 10-20 kWh/m2; and a warm zone 12 corresponding to the south of southern Europe, where cooling needs may reach 40 13 kWh/m2. 14 15 Figure 10. Estimation of the reference cooling needs in Europe [kWh/m2·year] 16 3.2. Cooling potentials 17 19 The results are shown below of the frequency distribution tables of night-time cooling 1 and how they are used to make a rough estimate of the daily cooling energy that can be 2 supplied to the building according to the month in question (3.2.1). This section also 3 shows the results of the climate assessment maps for the whole of Europe from the KPIs 4 defined in Section 2, in addition to the energy and climate potential maps (3.2.2). 5 Frequency distribution tables of nocturnal cooling in degree-hours 6 The frequency distribution for nocturnal cooling is defined as the frequencies of the 7 nocturnal cooling potential in degree-hours for nights (0 to 8h) corresponding to cooling 8 months. This corresponds with the accumulated difference between the water temperature 9 and the effective temperature on summer nights. These tables enable designers to make 10 an approximate estimate of the daily cooling energy that can be supplied to the building 11 during the month in question in order to implement this assessment method in future 12 studies. To create the tables, 24 countries were selected that are representative of the 13 European climate zones. The selection was made taking into account the most highly 14 represented climate zones in Europe and the variability in climate severity for each 15 climate zone. Table 3 below shows the set of cities chosen and their Köppen-Geiger 16 climate classification [37] and its climatic severity index for the summer period (SCSI). 17 Table 3. Selected climates 18 N City Country Köppen- Geiger SCSI N City Country Köppen- Geiger SCSI 1 Toledo Spain Bsk 1.59 13 Craiova Romania Cfa 0.75 2 Teruel Spain Bsk 0.62 14 Aberdeen UK Cfb 0.00 3 Madrid Spain Bsk 1.00 15 Bordeaux France Cfb 0.38 4 Montpelier France Csa 0.67 16 Stuttgart Germany Cfb 0.12 5 Athens Greece Csa 1.95 17 Paris France Cfb 0.15 6 Granada Spain Csa 1.32 18 Bucharest Romania Dfa 0.63 7 Rome Italy Csa 1.58 19 Helsinki Finland Dfb 0.00 8 Seville Spain Csa 1.87 20 Sofia Bulgaria Dfb 0.46 9 La Coruña Spain Csb 0.00 21 Ostrava Czech Rep. Dfb 0.08 10 Leon Spain Csb 0.38 22 Oulu Finland Dfc 0.00 11 Torino Italy Cfa 0.53 23 Clermont- Ferrand France Dfc 0.17 12 Bologna Italy Cfa 1.01 24 Tampere Finland Dfc 0 Table 4 shows an example of the histograms obtained, in this case corresponding to the 19 month of June in Seville (Spain) for the radiative system. Appendix A includes the 20 frequency distribution histograms for the months of June to September in all the cities 21 included in Table 3 for the two systems studied (radiative and falling film). These tables 22 allow approximate estimates to be made of the daily cooling energy required by the 23 building according to the month in question. Below, an example is given of how these 24 histograms are applied (see Table 4). 25 A radiative system can supply the building with a defined maximum cooling energy, 26 according to Equation 11 (𝑄=𝑈∙ 𝜀𝑒𝑥𝑐ℎ∙𝐷𝐻𝑇𝑤). According to Table 4, on a specific 27 day in June, the most probable range of nocturnal cooling in degree-hours corresponds 28 with ID 8, in which the nocturnal degree-hours are between 70≤DHTw<80; thus, the daily 29 cooling energy, that it can be supplied to the building for this day in June, is estimated 30 according to Equation 14 for a exchange efficiency of 0.9 and a heat transfer coefficient 31 of 10 W/m2·K, considering the hypotheses detailed in Section 2: 32 20 𝑄=𝑈∙ 𝜀𝑒𝑥𝑐ℎ ∙𝐷𝐻𝑇𝑤=10∙0.9∙75=675𝑊·ℎ m2 (14) 1 Likewise, with the frequency tables provided, it is possible to estimate a weighted average 2 day in June instead of the most likely day since there may be months in which the choice 3 of the most likely day is not obvious as there is a homogeneous distribution in different 4 ranges. 5 In this case, for the example above, considering the mean value of each interval, the 6 cooling degree-hours are calculated according to Equation 15. 7 𝐷𝐻𝑇𝑤=0.03∙15+0.07∙25+0.07∙35+0.07∙45+0.13∙55+0.13∙65+0.33∙75+ 8 0.13∙85+0.03∙95=62 𝑑𝑒𝑔𝑟𝑒𝑒 − ℎ𝑜𝑢𝑟𝑠. (15) 9 Performing the calculation by estimating the typical day, the value of the daily cooling 10 energy would be the result of Equation 16. 11 𝑄=𝑈∙ 𝜀𝑒𝑥𝑐ℎ∙𝐷𝐻𝑇𝑤=10∙0.9∙62=558𝑊·ℎ 𝑚2 (16) 12 Finally, note that this estimation was performed assuming a mean efficiency as 13 indicated in Section 2.1.4, but if the designer wishes to make a more accurate estimation 14 depending on the design, the graphs provided in Section 2.1.3 can be used. 15 Table 4. Frequency distribution of nocturnal cooling in degree hours (radiative system) 16 ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 The maps of the next sections show the results of the studies described in Section 2.2 for 17 the two dissipation systems studied (radiative and falling film). All the cooling potential 18 maps are calculated for the nocturnal use of the sink (from midnight to 8h) and the values 19 are shown correspond to the whole summer period (June to September). These maps make 20 it possible to evaluate whether applying natural cooling technology is of interest by 21 assessing the climate and energy potential associated with the climate conditions of the 22 zone in question. 23 Energy and climate potential maps 24 The cooling potential maps were created using 894 meteorological stations from the 25 NUTS-3 region. For the study zones of the north of Scandinavia were omitted due to the 26 low cooling needs of the buildings and the low density of the population [38]. The climate 27 data were obtained from Meteonorm [36] and the maps were developed using the ArcMap 28 spatial analysis tool [39], which maps the whole continent by interpolating the 894 29 21 climate stations. For the interpolation, a raster surface was generated by means of Kriging 1 interpolation with ArcGIS, the spatial analysis tool. The Kriging interpolation method is 2 a geostatistical interpolation method that generates an estimated surface from scattered 3 set points with z values. The authors selected this method since it is more accurate and 4 efficient than other interpolation methods such as IDW [40]. 5 3.2.2.1. Gross cooling potential 6 First, the gross cooling potential of the sky in Europe is shown in Figure 11, values 7 ranging between 5000 and 22000 degree-hours. The lowest values are mainly found for 8 the temperate climate zones of the Mediterranean region, while the highest values 9 correspond to the cold climates in the north of Europe. Therefore, since, in addition to 10 radiant heat exchange with the sky, the falling-film system presents heat transfer by air 11 convection, it considers that the wet-bulb temperature participates as one of its 12 environmental heat sinks. Figure 12 shows the gross cooling potential of the wet-bulb 13 temperature as a heat sink, values ranging from 1000 to 12000 degree-hours. It shows 14 that the sky presents a much greater gross cooling potential as an environmental heat sink 15 than the wet-bulb temperature. 16 17 Figure 11. Gross cooling potential (GCP) 18 22 1 Figure 12. Gross cooling potential (GCP) 2 3.2.2.2. Net cooling potential and available cooling potential 3 As stated in the section defining the KPIs, the gross cooling potential of the heat sink is 4 not totally accessible; in other words, ideal heat exchange conditions and accessibility do 5 not exist. In this regard, Section 2 defines the KPIs of the net and available cooling 6 potential. Below, Figure 13 shows the net and available cooling potential in the two 7 systems studied (radiative and falling-film). 8 23 Figure 13. Net cooling potential (NCP) and available cooling potential (ACP) 24 The net cooling potential ranges between 2500 and 17000 degree-hours for the radiative 1 system and from 3000 to 14000 degree-hours for the falling-film system. Likewise, the 2 available cooling potential ranges between 2200 and 15000 degree-hours for the radiative 3 system and from 2200 to 10000 degree-hours for the falling-film system. As stated in 4 Section 2, these differences are due to the variation in the efficiency of the sink as a result 5 of the different heat transfer processes taking place in each system (stagnation 6 temperature) and the heat exchange efficiency of the heat sink, which depends on the heat 7 transfer between the working fluid / dissipator, and between the dissipator / environment. 8 3.2.2.3. Specific energy cooling potential 9 The indicator of energy cooling potential is shown in Figures 14 and 15. As Figure 14 10 shows, the maximum cooling energy using the radiative natural cooling technology 11 ranges from 20 to 150 kWh/m2 during the whole of the summer, the lowest values 12 corresponding to the Mediterranean region and the highest to the north of Europe, which 13 has a colder climate. Figure 15 shows the maximum cooling energy values when using 14 the natural falling-film cooling technology. In this case, the cooling energy ranges from 15 90 to 400 kWh/m2, much higher values than with the radiative technology. This 16 underlines that falling-film technology is of great interest as it presents great cooling 17 potential throughout the whole of Europe. 18 19 Figure 14. Specific energy cooling potential (radiative) 20 31 Radiative Falling-Film Figure 21. Dissipation area/conditioned area (m2/m2)-Passivhaus reference 1 4. Conclusions 2 The use of natural cooling technology (NCT) in buildings is attracting attention due to 3 the growing interest in energy saving and the protection of the environment. Furthermore, 4 radiative cooling (RC) serves as a novel and promising approach to prevent cities from 5 overheating and becoming urban heat islands that is economical and environmental- 6 friendly. Using the sky as a heat sink offers great potential—confirmed by the authors in 7 a previous study—and with the aim of promoting the integration of nocturnal dissipation 8 systems that use this environmental heat sink, the authors of this study present a study to 9 assess the potential of radiative water-based cooling systems that use a radiant panel or 10 flat surface and water supplied by an air treatment unit through a TES storage tank. The 11 main conclusions of the study performed are as follows: 12 • The method developed enables designers to assess easily the applicability of the 13 natural cooling techniques studied (radiative and falling film) in a specific 14 location and under a range of design and operating conditions. The method may 15 be applied directly, thereby evaluating the energy savings made by integrating 16 these systems under specific design and operating conditions, and inversely, by 17 analysing the design and operating conditions required by the systems studied to 18 achieve a specific saving. 19 • The estimated savings are a testament to the interest in the use of natural cooling 20 technology in residential buildings; for the design cases assessed with the smallest 21 dissipation area and lowest storage volume, these savings range from 21% in hot 22 areas to 74% in cold areas using the radiative system. These savings increase if 23 the falling-film system is used, percentages ranging from 43% to 97%. 24 • For any given dissipation area, a larger storage capacity results in a higher 25 percentage of the demand satisfied. Likewise, for any given storage volume, 26 increasing the dissipation area results in more of the demand being satisfied. There 27 is, however, a water storage volume-dissipation area ratio beyond which the 28 percentage of the demand satisfied remains practically constant, this depending 29 on the case under study (climate zone and cooling demand of the building). 30 • For the cooling demand estimated as the reference value for the building stock in 31 Europe, it is possible to achieve buildings that do not require air conditioning 32 systems by using the radiative system and with designs associated with dissipation 33 area/conditioned area (m2/m2) ratios below or equal to 0.5 in the temperate zones 34 32 of southern Europe. Incorporating the system with ratios between 0.5 and 1 is 1 possible in warm zones of southern Europe. It would not be possible to achieve 2 this objective in zones with extreme climates, but achieving buildings with 3 cooling demands that meet the Passivhaus standard would be possible. Using the 4 falling-film system enables the objective to be reached in practically the whole of 5 Europe. Moreover, it shows how the ratio of the area required decreases due to 6 the increase in the cooling potential of this technology. 7 • If the reference value for the cooling demand studied refers to buildings that 8 comply strictly with the Passivhaus standard (cooling demand of 15 kWh/m2), 9 reaching the proposed objective (buildings not requiring an air conditioning 10 system) would be possible in the whole of Europe using the radiative system, the 11 maximum dissipation area ratio of 0.8 being required in hot zones. Using the 12 falling-film system, as in the previous case, would reduce the ratios required to a 13 maximum of 0.5. In this situation, it is interesting to study which technology 14 would be better, as the radiative system costs less to operate as it has a lower flow, 15 but it more expensive to install as a larger area is required to achieve the same 16 saving. 17 The study of cooling potential performed focuses on implementing natural technology in 18 single-family residential buildings, but the method allows for its incorporation to be 19 assessed in any kind of building and/or public space with specific cooling needs. A 20 highlight of this study is the huge interest in the use of these natural dissipation systems 21 for cooling spaces, and that, in addition to presenting great cooling potential, the falling- 22 film system can be integrated with photovoltaic technology, this being extremely 23 necessary for reaching European goals regarding reducing energy consumption. Its 24 integration only requires a low-cost storage tank, nozzles for generating the water film, 25 and a collector for night-time use. It costs very little to incorporate into a building because 26 a starting hypothesis is that the buildings have a dual flow ventilation system as this is 27 one of the most common solutions in residential buildings because it is one of the five 28 principles of the Passivhaus standard. 29 5. Acknowledgements 30 This study was funded by European Commission under the project LIFE WATERCOOL 31 - Water efficient systemic concept for the climate change adaptation in urban areas 32 (LIFE18 CCA/ES/001122) and MedEcoSuRe - Mediterranean University as Catalyst for 33 Eco-Sustainable Renovation (A_B.4.3_0218). It has been co-financed by the European 34 Regional Development Funds (ERDF) and the University of Seville under its Research 35 Plan. 36 6. 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Selected climates N City Country Köppen- Geiger SCSI N City Country Köppen- Geiger SCSI 1 Toledo Spain Bsk 1.59 13 Craiova Romania Cfa 0.75 2 Teruel Spain Bsk 0.62 14 Aberdeen U.K Cfb 0.00 3 Madrid Spain Bsk 1.00 15 Bordeaux France Cfb 0.38 4 Montpellier France Csa 0.67 16 Stuttgart Germany Cfb 0.12 5 Athens Greece Csa 1.95 17 Paris France Cfb 0.15 6 Granada Spain Csa 1.32 18 Bucharest Romania Dfa 0.63 7 Roma Italy Csa 1.58 19 Helsinki Finland Dfb 0.00 8 Sevilla Spain Csa 1.87 20 Sofia Bulgaria Dfb 0.46 9 La Coruña Spain Csb 0.00 21 Ostrava Czech Rep. Dfb 0.08 10 Leon Spain Csb 0.38 22 Oulu Finland Dfc 0.00 11 Torino Italy Cfa 0.53 23 Clermont- Ferrand France Dfc 0.17 12 Bologna Italy Cfa 1.01 24 Tampere Finland Dfc 0.00 37 Table A.8. Frequency distribution of night cooling in degrees-hours (radiative) Location: Toledo ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 38 Table A.9. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Toledo ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 39 Table A.10. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Teruel ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 40 Table A.11. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Teruel ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 47 Table A.18. Frequency distribution of night cooling in degrees-hours (radiative) Location: Granada ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 48 Table A.19. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Granada ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 49 Table A.20. Frequency distribution of night cooling in degrees-hours (radiative) Location: Rome ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 50 Table A.21. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Rome ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 51 Table A.22. Frequency distribution of night cooling in degrees-hours (radiative) Location: Seville ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 52 Table A.23. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Seville ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 53 Table A.24. Frequency distribution of night cooling in degrees-hours (radiative) Location: La Coruña ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 54 Table A.25. Frequency distribution of night cooling in degrees-hours (falling-film) Location: La Coruña ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 55 Table A.26. Frequency distribution of night cooling in degrees-hours (radiative) Location: Leon ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 56 Table A.27. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Leon ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 63 Table A.34. Frequency distribution of night cooling in degrees-hours (radiative) Location: Aberdeen ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 64 Table A.35. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Aberdeen ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 65 Table A.36. Frequency distribution of night cooling in degrees-hours (radiative) Location: Bordeaux ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 66 Table A.37. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Bordeaux ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 67 Table A.38. Frequency distribution of night cooling in degrees-hours (radiative) Location: Stuttgart ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 68 Table A.39. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Stuttgart ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 69 Table A.40. Frequency distribution of night cooling in degrees-hours (radiative) Location: Paris ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 70 Table A.41. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Paris ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 71 Table A.42. Frequency distribution of night cooling in degrees-hours (radiative) Location: Bucharest ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 72 Table A.43. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Bucharest ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 79 Table A.50. Frequency distribution of night cooling in degrees-hours (radiative) Location: Oulu ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 80 Table A.51. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Oulu ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 81 Table A.52. Frequency distribution of night cooling in degrees-hours (radiative) Location: Clermont-Ferrand ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 82 Table A.53. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Clermont-Ferrand ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 83 Table A.54. Frequency distribution of night cooling in degrees-hours (radiative) Location: Tampere ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 84 Table A.55. Frequency distribution of night cooling in degrees-hours (falling-film) Location: Tampere ID Description 1 DHTw<10 2 10≤ DHTw <20 3 20≤ DHTw <30 4 30≤ DHTw <40 5 40 ≤ DHTw<50 6 50 ≤ DHTw<60 7 60 ≤ DHTw<70 8 70 ≤ DHTw<80 9 80 ≤ DHTw<90 10 90 ≤ DHTw<100 11 100 ≤ DHTw<110 12 110 ≤ DHTw<120 13 120 ≤ DHTw<130 14 130 ≤ DHTw<140 15 140 ≤ DHTw<150 16 150 ≤ DHTw<160 17 160 ≤ DHTw<170 18 170 ≤ DHTw<180 19 180 ≤ DHTw<190 20 190 ≤ DHTw<200 21 200 ≤ DHTw<210 22 210 ≤ DHTw<220 23 220 ≤ DHTw<230 24 230 ≤ DHTw<240 25 240 ≤ DHTw<250 26 DHTw ≥ 250 85 Appendix B: Maps of estimated savings assuming a volume of 50 (l/m2) for the different dissipation area/conditioned area ratios assessed for the two technologies. Radiative Falling-Film Figure B.1. Energy savings fraction [%] for the design ratios: 0.25 Da/Ca 86 Radiative Falling-Film Figure B.2. Energy savings fraction [%] for the design ratios: 0.5 Da/Ca 87 Radiative Falling-Film Figure B.3. Energy savings fraction [%] for the design ratios: 1 Da/Ca