Heat wave resilience in open Spaces: A case study of a Self-Sufficient cooling shelter

Energy & Buildings 320 (2024) 114626 Available online 3 August 2024 0378-7788/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Heat wave resilience in open Spaces: A case study of a Self-Sufficient cooling shelter Paz Montero-Guti´ errez a , Jos´ e S´ anchez Ramos a , MCarmen Guerrero Delgado a , * , Teresa Palomo Amores a , Alberto Cerezo-Narv´ aez b , Servando ´ Alvarez Domínguez a a Grupo Termotecnia, Escuela Superior de Ingeniería, University of Seville, Spain b Grupo Termotecnia, Escuela Superior de Ingeniería, University of Cadiz, Spain ARTICLE INFO Keywords: Climate shelter Digital model Bus Stop Outdoor Thermal Comfort Radiant panel Falling-Film ABSTRACT Climate change intensifies urban uninhabitability due to increasingly frequent heat waves. To alleviate it, climatic shelters emerge as an initiative for the research community, which should deepen the integration of technologies that use natural resources for cooling. This work aims to evaluate a novel climatic shelter by characterizing its thermal behavior through an experimental study at a bus stop. It has the capacity to alleviate the thermal stress of its occupants thanks to cooling modules. The results allow to determine the capacity of the natural water-cooling system and the level of thermal storage, allowing to quantify the impact on the citizen by means of the COMFA comfort index. It is corroborated that the chilled water temperature reduces the surface temperature by 15 ◦C. The installation reduces overheating caused by solar radiation by 75 % compared to traditional shelters. It also demonstrates energy and electrical self-sufficiency even during high intensity heat waves. The shelter maintains daytime water temperature between 18 and 25 ◦C, even in heat waves exceeding 7 days and 45 ◦C air temperature. The COMFA index has been reduced to values between 70 and 120 W/m 2 in 90 % of the worst weather conditions in Csa climates by K¨ oppen Geiger, for the months of July and August 2023. Abbreviations PV Photovoltaic panel UHI Urban Heat Island PET Physiological Equivalent Temperature COMFA Heat Load Comfort Index TMY Typical Meteorological Year CComfort –Rating scale SW Slightly Warm –Rating scale WWarm –Rating scale HHot –Rating scale VH Very Hot –Rating scale Symbols ˙ mwWater mass flow rate circulating inside the radiant modules [kg/s] CpwSpecific heat of water [kJ/kgK] TwWater temperature [◦C] QCV Convective heat transfer [W] QRAD Radiative heat transfer [W] TREP Representative fluid temperature along the length Lof the radiant module [◦C] TLIntermediate representative temperature [◦C] LLinear length of radiant modules [m] T∞Equilibrium temperature associated with the radiant system [◦C] T0Water temperature at the inlet of the radiant modules [◦C] UOverall transfer coefficient [W/m 2 K] Amodule Radiant module equivalent area [m 2 ] ρ Water density [kg/m 3 ] ˙ mmodule Water flow rate through each module [kg/s] AiTotal area of the top or the back envelope [m 2 ] NCTotal number of modules installed in top or back envelope [-] ˙ miTotal mass flow through the top or back envelope [kg/s] hCV Convective transfer coefficient outside of the radiant modules [W/m 2 K] * Corresponding author. E-mail addresses: [email protected] (P. Montero-Guti´ errez), [email protected] (J. S´ anchez Ramos), [email protected] (M. Guerrero Delgado), [email protected] (T. Palomo Amores), [email protected] (A. Cerezo-Narv´ aez), [email protected] (S. ´ Alvarez Domínguez). Contents lists available at ScienceDirect Energy &Buildings journal homepage: www.elsevier.com/locate/enb https://doi.org/10.1016/j.enbuild.2024.114626 Received 10 May 2024; Received in revised form 27 July 2024; Accepted 29 July 2024 Energy & Buildings 320 (2024) 114626 2 ARadiant modules transfer area [m 2 ] TaTemperature of air surrounding the radiant modules [◦C] hRADsurroundings Radiative transfer coefficient outside the modules [W/ m 2 K] Fsurroundings Form factor of radiant modules with surrounding surfaces [-] Tsurroundings Temperature of surrounding surfaces [◦C] σ Stefan-Boltzmann constant [W/m 2 K 4 ] ε Emissivity of radiant modules [-] Q COMFA Heat Load Comfort Index [W/m 2 ] Cres Respiratory sensible heat losses [W/m 2 ] Eres Respiratory latent heat losses [W/m 2 ] M Metabolic energy of the person [W/m 2 ] W Work performed from metabolic activity [W/m 2 ] C Convective heat transfer between the person and the air [◦C] ΔRLRadiative heat transfer between the person and the environment [◦C] Ersw Heat flow due to evaporation of sweat [W/m 2 ] ΔRCShortwave radiation absorbed by the person due to solar radiation [W/m 2 ] HRaRelative ambient humidity [%] PV(Ta)Water vapour pressure at Ta[kPa] hcConvective heat transfer coefficient of the person with the outside air [W/m 2 K] fcl Ratio of clothing area to skin area [-] Tcl Outside clothing temperature [◦C] V s Outdoor air velocity [m/s] hrRadiative heat transfer coefficient of the person with the environment [W/m 2 K] TRM Mean radiant temperature around the person [◦C] ε cl Emissivity of clothing [-] Rcl Thermal resistance of clothing [m 2 K/W] ΔTTemperature increases due to solar radiation absorbed by the person [◦C] TLW Long wavelength radiant temperature of the person with the environment [◦C] α clo Absorptivity of clothing [-] Rabs Solar energy absorbed by clothing [W/m 2 ] Fsi Form factor of the person with the surrounding surface i[-] TiTemperature of the surface isurrounding the person [◦C] H/W Urban canyon aspect ratio relating the height (H) to the width (W) TSK−REQ Skin temperature of the person [◦C] 1. Introduction 1.1. Context –Urban climate shelters Over the last few years, climate change has significantly affected the global climate system, altering several meteorological variables [1,2]. During the summer of 2023, southern Europe recorded 4 heat waves, totalling 24 days of extreme heat [3]. Understanding and mitigating the effects of Climate Change and Urban Heat Island (UHI) phenomena is essential to adapt their impact on citizens and promote more resilient and sustainable cities in the face of heat waves. The concepts of habitability and thermal comfort revitalise urban spaces, increasing research on the energy impact of outdoor facilities to identify optimal operating conditions [4–7]. Recent studies use the term oasis to refer to urban spaces providing relief from heat waves [8–10]. Urban planning studies sustainable strategies to promote energy and electricity self-sufficiency, aiming for a favourable social impact [11]. Self-sufficient cooling techniques are applied to ensure environment preservation and satisfactory comfort levels. In this context, the term climate shelter also arises, referring to public spaces where citizens can take refuge for long and medium-term periods, adapted to cope with the harsh environmental conditions of their surroundings [12]. Heat stress in these shelters is mitigated through air-conditioned facilities and/or outdoor adaptation [13–16]. Effective measures include installing pergolas, planting vertical vegetation and using high reflectivity draining pavements [17–20]. Air pretreatment and water use to cool the space through radiant energy exchange techniques stand out in air-conditioned installations (Palomo Amores et al., 2023). Short-term outdoor climate shelters represent a new challenge for the researchers due to the increasing need for urban cooling [22,23]. Natural techniques are now recognised as a proven solution to reduce people’s heat stress in a short period of time [24–26]. Transport stops are considered potential climate refuges due to their wide distribution in cities [27,28]. Moreover, self-sufficiency is considered a fundamental variable in the pursuit of environmental sustainability and the efficient use of energy and electricity resources. In this line, photovoltaic (PV) solar panels are the most widely used technology for generating clean energy. This energy produced is often used to serve pedestrians using bus stops in terms of LED lighting, Wi-Fi network and USB chargers [29]. However, even though the electrical service is solved, it does not create an energy system able to acclimatise the shelters’living Haga clic o pulse aquí para escribir texto.space. 1.2. Literature review The authors of this article have built on their previous work to address climate shelter methodology. They prototyped a scaled shelter with surface-heated modules and chilled water circulation [31]. The fluid is cooled by the Falling-Film evaporative-radiant cooling technology, which chills water through PV panels at night. During the day, these panels generate four times the energy needed for the shelter’s minimum electrical demand [32,33]. In this first paper, the authors reviewed the location and climate of these shelters, analysing technologies for water cooling and experimental processing [34]. The review emphasizes that thermal comfort at bus stops requires quality shade and surface cooling. To achieve this, the authors suggest design changes in materials, cooling techniques and renewable energy systems. Table 1 presents the current literature review, analyzing the authors’decision-making processes regarding climate shelter geometry, storage system sizing, and energy production for self-sufficiency. Reference Climate shelter or geometry Selfsufficiency Comfort Comments Ou, H. et al. [35] Study different geometries of bus stops and H/W ratios to analyse incident radiation on the person. −Mean radiant temperature study and verification with RayMan model. Selfsufficiency is not proposed. Storage volume and dissipation area not suggested. Mokhtari, R. et al. (2022) [21] Climate shelter with confinement. Study of various climate shelter Radiant modules walls. Daytime radiative cooling to UTCI index. Storage volume and dissipation area not suggested (continued on next page) Table 1 Simplified model proposal. Model U=hCV +hRAD T∞=hCV •Ta+hRAD •Tsurroundings hCV +hRAD Back envelope Top envelope hRAD =4.5W m2KhCV =2.7W m2KhRAD =4.5W m2KhCV =3.9W m2K P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 3 (continued) Reference Climate shelter or geometry Selfsufficiency Comfort Comments types and their view factors. reduce water temperature. Guerrero- Delgado, M. et al. (2020) [33] −Falling-Film for cold water production during the night. −Storage volume and convectiveradiant transfer area with the sky to reduce are suggested. Zhao, D. et al. (2019) [36] −Radiative water-cooling technology using the sky during the day. −Storage volume and dissipation area are suggested. Alikhanova, A. et al. (2019) [30] Closed climate shelter proposed at bus stops. Use of solar, wind and geothermal energy for thermal generation. −Storage volume and dissipation area not suggested. Comfort index not study. Iqbal, S. (2019) [28] Closed climate shelter. Blocking the solar radiation by means of solar blades. Underfloor heating to provide confort. −UTCI index. Selfsufficiency is not proposed. Storage volume and dissipation area not suggested. Bus stops are studied with a high form factor living area, using radiative surfaces and technologies [21,28,30]. This generates a cold confinement that improves indoor comfort conditions. The geometry and orientation of the shelter are crucial to efficiently block solar radiation [35]. Climate shelters with a lower form factor towards the sky (better shielded) reduce direct solar radiation and promote comfortable spaces. In hot and semi-arid climates (BSk −K¨ oppen-Geiger Climate), these shelters are considered as reference models, capable of reducing the COMFA index (Heat Load Factor Index) by 10 ◦C[21]. From an energy perspective, radiant surface cooling technologies are implemented, where water circulates pre-cooled by external radiative panels [21,31]. Despite the progress they represent, the installation of these systems in the urban environment still faces several limitationsHaga clic o pulse aquí para escribir texto. For example, determining its optimal operating point is complicated and tedious. Due to the innovative nature of the bus shelter with the Falling-Film system and the surface radiant system, the information necessary to evaluate the potential of this work is lacking. Therefore, if a pre-assessment methodology to aid in evaluating the proposal is absent, designers, architects, urban planners, and engineers will be deterred from the task of replicability for the following reasons: •Need to dimension the energy storage system (in volume) and the collection area necessary to produce electricity (in surface area). •Lack of tools and methodologies to detail the implementation of these modules in bus shelter geometries different from the prototyped proposal. •Challenge of optimizing design for very arid and high temperature climate zones, for which the operating days are reduced due to the set-point water temperatures achieved by the Falling-Film system. •Impact of incorrect design and dimensioning on pedestrian comfort. Common methodologies for implementing impactful urban solutions are being proposed [32,33,37,38]. Theoretical and experimental studies lay the foundation for innovative ideas and informed decision-making [39].Experimentation leads to the development of computational that replicate reality [40,41].Both prototyping and virtualisation help to quantify the effectiveness of the climate shelter for occupant. The study of outdoor comfort is essential for designing spaces that improve the well-being of citizens and promote sustainable cities. The literature uses the Physiological Equivalent Temperature Comfort Index (PET) [42–45] and Heat Load Comfort Index (COMFA) [19] to analyse how people adapt to climate change through energy analysis of implemented adaptation measures. However, the application of these indices to self-sufficient radiant technologies is limited [21]. Proposed by Brown et al. [46] and later reformulated as COMFA+by Angelotti et al. [47], COMFA is of great importance as it is a direct index that can be calculated by basic environmental measurements, considering the energy balance. In contrast, PET and HSI require considering physiological and psychological responses as well as environmental variables. Its usefulness and validity have been demonstrated by authors such as Kenny et al. [48,49] and Vanos et al. [50]. Further research is needed to understand how short-duration climate shelters impact pedestrians in order to improve infrastructure planning and design. 1.3. Aims of the paper The main objective of this study is to develop the fundamental methodologies and identify the key variables for designing and implementing climate-controlled shelters in real environments. This work proposes the adaptation of bus shelters for their replicability in cities. To this end, a scale prototype was built, experimental work was carried out and a thermal characterisation model has been validated. This model helps to establish the technical and energy parameters to assess the impact of shelters in real operating conditions. The integration of technologies in the characterisation model is essential to assess the potential of the self-sufficient installation, analyzing the production and storage of chilled water using the Falling-Film technique and PV panels. Representative case studies assess the energy and electricity impact under real climate conditions. Efficient decision making in the design phase favours an optimal climate shelter installation. The comfort index analysis quantifies the cooling effectiveness of the radiant system and its impact on occupants. This work aims to establish a knowledge base for climate shelters by focusing on three main aspects that generate social impact (Fig. 1). First, it details a simplified design model based on theoretical principles and validated through experimental data obtained from a prototype tested in a warm climate simulation [31]. Second, it assesses the self-sufficient energy impact through case studies using the ENVI-met software and post-processing of the LEONARDO software. Energy decisions related to storage tank sizing, solar collector area dimensioning, and operating period definition are also made with the implementation of the simplified model in the TRNSYS software. Thirdly, the methodology is validated using the COMFA comfort index, quantifying the reduction of thermal stress and the high efficiency of the shelter. The aim of these objectives is to advance climate shelters in urban areas through results and discussion that are replicable in other working conditions, building resilience to heat waves and addressing UHI problems. 2. Methodology 2.1. Climate shelter characterisation The experimental work of Falling-Film [33] establishes the basis for the electricity production of the climate shelter. The pumps responsible for transporting water to both the Falling-Film system and the radiant system consume part of the energy produced during the day. The roof of the climate shelter consists of 7 m 2 , capable of producing up to 400 % of the energy required, with this generation never being less than 200 % of P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 4 that needed. The panels selected are Solar Sun Power 10XSPR-P6-410- BLK-E8-AC with Enphase IQ8M-72–2-US microinverters. The established inclination is 10◦, a requirement imposed by the Falling-Film system. The lighting consumption is estimated at 30 W for 10 h/day. For water pumping, 250 W are considered for 5 h in the summer months, while for the spring and autumn months it is reduced to 40 W for 4 to 12 h. The unused energy is fed back into the electricity grid. In the experiments carried out in the climate chamber, the Falling-Film is replaced by a chiller capable of providing the same energy performance [31]. The theoretical basis of the radiant surface cooling system is based on the energy balance of Equation (1) [51]. This equation details the energy variation of the water inside the radiant modules. It is assumed that the radiant modules are completely insulated at the rear for the application of this formula. As detailed in the experimental work [31], the modules work as a heat exchanger where the main fluids are the air inside the chamber and the water circulating inside the radiant modules. The presence of both fluids causes a convective-radiant heat exchange to take place with high temperature air and elements around the climate shelter. −˙ mw•Cpw•dTw dx =QCV +QRAD (1) Where ˙ mwis the mass flow of water circulating inside the modules, Cpwis the specific heat of water, Twis the water temperature, QCV is the convective heat transfer, and QRAD, is the radiative heat transfer. 2.1.1. Digital model of the radiant system The theoretical energy principles enable the development of the simplified model described below. The aim is to support the decision making of the technical staff for the correct application of these climate shelters, adjusting to the real data obtained through the experiments. This characterisation algorithm is obtained using the experimental parameters tested in the climate chamber. Therefore, the main parameters of the model are identified by inverse characterisation of the experimental data, following the recommendations and step-by-step work offered by Guerrero-Delgado, M. et al. [33]. The simplified model applies to both the top and back envelope modules of the climate shelter. The modules are composed of coiled pipes. Water repeatedly flows through the pipes to increase their temperature. This rationale is based on the theoretical expression for the cooling of a liquid along a length [33,52,53], which is formulated in Equation (2): TREP =T∞− (T∞−T0) • e−U•Amodule ρ •˙ mmodule•Cp•L(2) Where TREP is the representative fluid temperature over the radiant module length L, T∞is the equilibrium temperature associated with the radiant modules, T0is the temperature of the cooling fluid at the inlet of Fig. 1. Methodological outline of the process. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 5 the modules, U is the overall transfer coefficient, Amodule is the equivalent module area, Cpis the specific heat of water, ρ is the density of water, y ˙ mmodule is the mass flow or flow rate of water per module. The heat transfer between the water circulating through the modules and the air surrounding the modules takes place at an intermediate representative temperature, which can be referred to as the effective or stagnation temperature [54]. Equation (3) formulates the integral according to the theoretical expression in Equation (2), which allows the intermediate representative temperature TLto be obtained (see Equation (4). TL=1 L∫L 0 T∞− (T∞−T0) • e−U•Amodule ρ •˙ mmodule•Cp•xdx (3) TL=T∞+T∞−T0 L• ρ •˙ mmodule•Cp U•Amodule • (e−U•Amodule ρ •˙ mmodule•Cp•L−1)(4) Similarly, using Equation (4), the maximum dissipated power (Q) per climate shelter module is governed by Equation (5) and Equation (6). The experimental characterisation of the equilibrium temperature associated with the radiating modules (T∞) and the overall transfer coefficient (U) is required to estimate this power. Q=T∞− (T∞−T0) • e−U•Amodule ρ •˙ mmodule•Cp•L(5) Q=˙ mmodule •Amodule •Cp• (T0−T∞) • ⎛ ⎜ ⎜ ⎜ ⎝ 1−e−U•Amodule ρ •˙ mmodule•Cp•L⎞ ⎟ ⎟ ⎟ ⎠ (6) The equivalent module area (Amodule) is specified to consider the common sections where piping is present. Equation (7) shows the mentioned area, relating the total area of the back or top envelope (Ai) of the climate shelter to the total number of modules (Nc) present in each one of them. Equation (8) also shows the flow rate through each of the modules ( ˙ mmodule) as the ratio of the total mass flow through each of the shelter envelopes ( ˙ mi) to the total number of modules (Nc) present in each of them. Amodule =Ai NC(7) ˙ mmodule = ˙ mi NC(8) Following the mathematical development, the total cooling power (Q) dissipated by the radiant modules of the climate shelter can be related to two types of heat transfer: convective heat transfer with the outside air surrounding the climate shelter (QCV) and radiant heat transfer from the modules to the surrounding elements or surfaces (QRAD)[52] (see Equation (9) −Fig. 2). Q=QCV +QRAD (9) The cooling power referred to the convective heat transfer is defined in Equation (10) [52]. Likewise, the cooling power referred to the radiative heat transfer is defined in Equation (11) and Equation (12) [52]. Since the climate shelter is to be installed on city streets, both the back and top envelope are greatly influenced by the surrounding urban elements, such as asphalt, pavements, and adjacent building facades. Consequently, the calculation of the form factor of both envelopes with the surrounding surfaces is of crucial importance. QCV =hCV •A• (Tw−Ta)(10) Where hCV is the convective transfer coefficient outside the radiant modules, A is the transfer area of the modules and Tais the ambient air temperature where the climate shelter is located. QRAD =hRADsurroundings •Fsurroundings • (T4 w−T4 surroundings)(11) hRADsurroundings =4• σ • ε •(Tw+Tsurroundings 2)3 (12) Where hRADsurroundings is the radiant transfer coefficient, Fsurroundings is the shape factor of the radiant modules for the back and top envelope with the sky and surrounding surfaces, σ is the Stefan-Boltzmann constant, ε is the surface emissivity of the radiant modules, Tsurroundings is the temperature of the surrounding surfaces and Twis the water temperature in the modules. After defining these equations, the simplified inverse characterisa- Fig. 2. Short-term climate shelter prototype. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 6 tion model is shown in Equation (13): Q=hCV •A• (Tw−Ta) + hRADsurroundings •Fsurroundings • (T4 w−T4 surroundings) (13) This expression can be reformulated as shown in Equation (14): Q= (hCV +hRAD) • A•(Tw−hCV •Ta+hRAD •Tsurroundings hCV +hRAD )(14) Where the inverse characterisation parameters obtained by experimentation can be observed. In this way, the overall transfer coefficient (U −Equation (15) and the equilibrium temperature associated with the modules are defined (T∞- Ecuaci´ on 16). U=hCV +hRAD (15) T∞=hCV •Ta+hRAD •Tsurroundings hCV +hRAD (16) Thanks to this demonstration, it is observed that the derivation of the experimental parameters depends exclusively on the convective and radiative transfer coefficients (hCV, hRAD), considering that the overall transfer coefficient is invariant in the time in which the experiments are performed. 2.1.2. Model identification The parameters of the described model are derived from the experiments conducted by the authors. Identification within the scientific community is achieved through theoretical contributions [33,55,56]. The experimental prototype installed in the climate chamber, shown in Fig. 2, considers a variable water flow in each experiment, which circulates through a total of 24 modules distributed between the back and top envelope of the climate shelter. The experimental process has been performed considering measurements of the water temperature stored in the tank, which varies between 18 and 22 ◦C. In addition, climate data are recorded by means of a climate station located at the School of Engineering of the University of Seville during the summer of 2022 and 2023. Therefore, the only values to be determined in the model are the convective transfer coefficient and the radiant film coefficient (hCV, hRAD). The prototype has been designed for high ambient temperatures, which cause the urban elements around it to store and emit energy. For this purpose, a climate chamber is used to increase the experimental ambient temperature by using a radiant floor that simulates concrete and asphalt in the sun. Consequently, the convective energy component must be reduced to increase the radiant energy exchange. Radiation is closely linked to the feeling of thermal comfort of the person inside the shelter. The greater the temperature difference between the radiant cooling modules and the surfaces surrounding the person, the greater the feeling of comfort. Fig. 3 shows the favourable behaviour of the shelter under continued operation of the climate chamber for 19 and 20 July 2022. The period of operation of the radiant modules and the chiller during the day and night are shown in blue and orange background, respectively. The blue line shows the air temperature, the yellow line shows the water temperature, the green line shows the temperature reached by the cooling modules in the back envelope, the violet line shows the temperature reached by the cooling modules in the top envelope, the orange line shows the temperature reached by the underfloor heating modules outside the shelter, and the grey line shows the temperature reached by the underfloor heating modules inside the shelter. The hours of operation of the radiant modules vary depending on the air temperature of the climate chamber and the setpoint temperature reached by the water, obtaining experimental cases that are faithful to reality. To this end, the climate chamber technologies are active throughout the day, automated so that the modules run cycles of approximately 8 h of operation to simulate a typical hot day. Fig. 4 also shows the convective and radiative exchange provided by the climate shelter during 19 and 20 July 2022. The blue line shows the convective heat flux exchanged by the top envelope, the grey line shows the radiant heat flux emitted by the top envelope, the orange line shows the convective heat flux exchanged by the back envelope, and the yellow line shows the radiant heat flux emitted by the back envelope. The radiant heat flux predominates during the operation of the shelter. The back envelope emits 65 % of radiant heat flux, compared to 38 % emitted by the top envelope. This is mainly due to the difference in size between the back and top envelope, with a larger transmission area in the back envelope. As a result, it is shown that the climate shelter in operation carries out a predominantly radiant exchange, given the severe conditions inside the climate chamber. Furthermore, all the abovementioned components suffer a sharp decrease during the operation of the chiller, due to the existence of a smaller temperature difference with the water. The first parameter to be obtained is hRAD. Equation (12) of section 2.1.1 is applied. To this end, an emissivity value ε equal to 0.6 is established, tested by means of thermography both on the back envelope and on the top one, inside the climate shelter. The radiant transfer Fig. 3. Operation of the climate chamber and climate shelter for 19–20/07/2022. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 7 coefficient values for both surfaces range between 3.7 and 5.4 W/m 2 K, proposing an average value of 4.5 W/m 2 K. The convective transfer coefficient hCV is then defined, using Equation (10) of section 2.1.1.Fig. 5 shows the representation of the convective heat fluxes of both the back and top envelope versus the temperature difference between water and air in the climate chamber for the sampled data. The experimental values for the back envelope are shown in blue, while the experimental values for the top envelope of the climate shelter are shown in orange. The linear trends for both surfaces can be seen. In all the experiments performed, the transfer coefficient for the top envelope takes a minimum and maximum value of 1.9 and 5.3 W/m 2 K, establishing a mean value of 3.9 W/m 2 K. Meanwhile, for the back envelope, a minimum and maximum value of 1.55 and 3.35 W/ m 2 K is obtained, with a mean value of 2.7 W/m 2 K. The back envelope has a lower convective heat transfer coefficient value compared to the top one, due to its increased radiant exchange. It is also justified that the low values of transfer coefficients are due to laminar flow for horizontal and vertical flat plates, characteristic of the McAdams correlation [52]. 2.1.3. Model proposal and validation The simplified model by inverse characterisation is summarised in Table 1, where the previously justified parameter values are shown. Fig. 6 also shows the validation of the model using experimental data. To carry out this validation, 10 % of the experimental data not used to establish the identification of hCV and hRAD are employed. Fig. 6.A shows the comparison of the experimental convective heat flux with the model performance. Fig. 6.B compares the value of the experimental convective transfer coefficient with the value obtained in the model performance. Fig. 7 corroborates that the error between the experiments and the proposed model is less than 10 % for most of the sampled data. 2.2. Energy impact analysis method using case studies In addition to the validation of individual models, a calculation tool is needed to establish the connection between the modelled elements and predict the behaviour of the climate shelter. For this purpose, its real impact is studied, thanks to prototyping and virtual modelling. The back and top envelope of the climate shelter are ideal locations for the installation of the radiant system, due to their energy transmission area. Likewise, the roof of the shelter is suitable for the installation of the Falling-Film system for two main reasons. The first is the visibility of the sky due to its high altitude, which is essential for the radiant energy exchange during the night, as it allows the water used by the modules to be cooled[21,33,36]. The second is the implementation of photovoltaic (PV) collectors to generate an electrically self-sufficient shelter in those places favoured by solar radiation [33].Fig. 8 summarises the different technologies necessary for the correct operation of the shelter: •Radiant modules (ID1): Modules that cool the short stay while people are waiting for public transport. These are in both top and back envelopes, perfectly insulated at their rear. Fig. 4. Radiative and convective surface exchange during 19–20/07/2022. Fig. 5. Variation of the convective heat flux from the temperature difference of water and air in the experiments conducted. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 8 •Water storage tank (ID2): Underground insulated tank containing the water volume to be cooled. •Falling-Film natural cooling system (ID3): Descending blade cooling system that lowers the temperature of the water by radiant exchange with the sky at night. •PV collectors (ID4): Photovoltaic collectors that allow the shelter to be self-sufficient, on which Falling-Film technology is installed. •Falling-Film water transport (ID5): Hydraulic pump and pressure and return pipes that circulate the water from the tank to the Falling- Film system. •Radiant modules water transport (ID6): Hydraulic pump and pressure and return pipes that circulate the water from the tank to the modules. •Collector (ID7): Valves for monitoring the water flow through the radiant modules. The operation of the climate shelter brings together the different knowledge on radiant energy exchange, dissipation area and urban comfort thanks to the theoretical and experimental contributions of other authors. The ability of radiant modules to provide lowtemperature water cooling has been studied, highlighting their high tube density which increases the heat transfer area of the refrigerant, and using TRNSYS to verify the system [57]. Energy strategies in urban spaces have also been investigated to improve habitability and develop climate shelters [58]. In addition, diurnal radiative techniques have been applied to cool the water inside the shelters, helping to establish the hours of operation and the sizing of the storage volume [21]. Finally, the energy basis for cooling water naturally has been presented, determining the areas needed for night-time dissipation and solar collection, which is crucial for the mathematical modelling of the shelter and its energy self-sufficiency [33]. 2.2.1. Simulation of urban canyon climate with ENVI-met software The first calculation tool used is ENVI-met [59].Fig. 9 shows the Fig. 6. Comparison Experiment vs. Model: Convective Heat Flow and Convective Transfer Coefficient. Fig. 7. Comparison Experiment vs. Model: Temperature of water leaving the climate shelter. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 9 calculation procedure and data analysis using this software. A total of 4 different cases are tested, considering canyon dimension, urban climate and public transport stop typology. The analysis of the output data of the simulation process is fundamental. For this purpose, LEONARDO, a tool belonging to ENVI-met, is in charge of graphing and analysing the air temperature, surface temperatures and incident solar radiation. The public transport stop is tested under different premises and its geometry is studied and modelled to evaluate its performance, following the recommendations of Mokhtari, R et al. [21] and Guerrero-Delgado. M et al. [33]. The first stop under study refers to the conventional one installed in most cities in southern Europe, model JCDecaux Line II by Grimshaw Architects [60] with dimensions of 4.2 m long, 1.56 m wide and 2.5 m high. Meanwhile, the second stop modelled and tested refers to the one that includes radiant modules in its back and top envelope, eliminating glazing and including the materials and insulation required for proper operation. On the other hand, an urban canyon is tested to study the behaviour of the climate shelter in a real environment, modelling an avenue or street that can be easily located and replicated in other cities. Specifically, the H/W aspect ratio is analysed, which is a determining factor in the urban microclimate [61]. This ratio influences the distribution of solar radiation, street ventilation and, consequently, surface temperatures. Torneo’s Avenue in Seville serve as an illustrative example. The public transport stop is in the sun all day long and is influenced by the heat energy retained and emitted by the asphalt of its 5 lanes and by the continuous traffic flow. The aspect ratio H/W is high and greater than 1, because its width is sufficiently large compared to the height of the surrounding buildings [61,62]. The avenue considered for the study complies with the North-South orientation, as it is one of the most unfavourable in terms of the number of hours it is in the sun. Hot areas classified as Csa by K¨ oppen Geiger are used as reference areas to develop the case studies. This selection represents at least 10 % of the global distribution with a high risk of overheating problems due to climate change. The hottest days are tested for the summer months in these areas. Seville is ranked as the European city that has most often reached a maximum temperature of 45 ◦C in summer months, so the climate is ideal for the study. As shown in Fig. 10, the months tested are July and August 2023, for recording up to 4 heat waves. The climate data used were recorded at the climate station located at the School of Engineering of the University of Seville [60–62]. Fig. 11 shows the study geometries defined using the ENVI-met Spaces tool as detailed in reference works such as those of Palomo- Amores, R et al [19] and Araque, K et al. [58]. For a detailed analysis of the environment, a three-dimensional grid composed of equal 1 ×1× 1 m cells is established. The correct definition of the vertical grid is crucial to ensure the consistency of the results. The cells in the vertical grid have the same dimension, eliminating the need to establish the telescopic function. The height of the realised models is twice the height present in space. Similarly, the coordinates referring to the city of Seville are defined: latitude 37.24 degrees (+N,-S) and longitude −5.58 degrees (−W,+E). The reference time zone is GMT+2 since typical summer days are simulated. The material of the modelled urban elements has been selected using the database available in the software itself. As shown in Fig. 11, the pavements selected were grey concrete for the pavement, with a reflectivity equal to 0.5, and black asphalt, with a reflectivity equal to 0.1. The facades located on both sides of the canyon have been considered for residential use and a maximum height of 5 m. The conventional urban transport stop is modelled by surfaces that have been assigned a material with the properties of glass. This means that the occupant is affected by incident solar radiation from both the top and back envelope due to the transparent glass. Meanwhile, for reasons of the installation of the radiant modules and their rear insulation on both surfaces, they are modelled with an opaque metallic material with low absorptivity on the backside and high emissivity on the underside, where the occupants are located. Fig. 8. Technologies installed at the self-sufficient public transport stop. Fig. 9. Calculation procedure with ENVI-met and climate data analysis with LEONARDO. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 16 energy harvesting area for a specific geometry. These data are essential for the replicability of the shelter in various cities and climates, providing a reliable methodology for technicians. In addition, a comfort index is studied to analyze the thermal stress of a person based on these parameters, observing how the performance of the shelter and comfort varies with changes in volume and dissipation area. Finally, an energy study of the shelter has been carried out, demonstrating that its energy and comfort self-sufficiency is one week. The novelty of this work lies in the scarcity of similar studies, which highlights its objectives as innovative. This study, together with the review of existing literature, highlights the importance of discussing the conditions that allow establishing the bases for its correct construction. These include the H/W aspect ratio and orientation of the urban canyon, the geometry of the shelter, and the degree of confinement of the stop. These considerations inform urban planning and policy, as well as the architectural design of new climate shelters. Fig. 17. Comparison of conventional shelter vs self-sufficient shelter: COMFA values 19–25/08/2023. Table 4 Influence of thermal load factors on comfort for conventional and self-sufficient shelter. Day and time: 22/08/2023 – 14:00 h (W/m 2 ) Conventional transport shelter Self-sufficient transport shelter Metabolism, perspiration and respiration 54.28 54.28 Direct radiation 72.33 0.00 Diffuse radiation 31.19 18.20 Reflected radiation 18.27 18.27 Convective air exchange 13.33 22.56 Radiative exchange 41.88 −14.60 Heat Load Index (Sum of factors) 231.27 98.71 Fig. 18. Accumulated frequency of acceptable COMFA values in warm areas. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 17 •Orientation and H/W ratio: Both are essential to ensure proper electrical efficiency and performance. North-South orientations and low H/W ratios ensure an acceptable form factor of the Falling-Film system with the sky [32,33]. In this work, an urban canyon has been studied whose orientation is North-South and the presence of urban obstacles is reduced, not affecting the creation of shadows on the collectors. The installation of this solution in avenues with different orientations should be studied according to the needs of the site. The relocation of the climate shelter on the avenue or street must be considered in each adaptation project. One of the limitations of this study lies in the study of the impact of other H/W aspect ratios of the urban canyon on the climate and thermal variables studied [43]. •Geometry, sizing and confinement: They are essential to reduce the thermal stress of the person. Nowadays, the typologies of transport shelters are very varied and combine a design in line with the aesthetics of cities. It is important to increase surfaces that block solar radiation and therefore geometries that reduce the form factors of the person with the surrounding urban elements [21]. These urban elements are often at high temperatures due to high solar radiation [71]. Improving the form factor of the person with the climate shelter improves comfort, especially if the walls are used for the installation of radiation techniques. TLW will be reduced by increasing the low temperature surfaces surrounding the occupant. The current climate shelter proposal has a form factor of 0.79 due to the dimensions of the front and ceiling. Table 5 Accumulated frequency analysis for COMFA. Instants of time (Ta >32 ◦C) Conventional transport stop Self-sufficient transport stop Instants (%) Accumulated Instants (%) Accumulated C13 0.34 0.34 860 22.62 22.62 SW 380 9.99 10.34 2119 55.73 78.35 W484 12.73 23.07 792 20.83 99.18 H520 13.68 36.74 31 0.82 100.00 VH 2405 63.26 100.00 0 0.00 100.00 Total 3802 3802 Fig. 19. Proposed climate shelter geometries. P. Montero-Guti´ errez et al. Energy & Buildings 320 (2024) 114626 18 The literature review shows that other authors analyse different typologies and form factors for different geometries. Partial and total confinement are on the rise, not only improving the radiant variable TLW but also the convective variable with the reduction of air temperature [21,28,30]. Confinement makes it possible to avoid the climate variable of wind speed, which is the main parameter affecting the thermal sensation of the pedestrian. In addition to the geometries proposed by the literature review, the authors propose the creation of new geometries that improve the confinement and reduce the external person-space form factor, as shown in Fig. 19.Fig. 19.A proposes a climate shelter with a larger number of cold surfaces inside, opting for a geometry with different roof pitches, potentially usable for installing the Falling-Film technique. The form factor of the person with the outside area takes a value equal to 0.81. Meanwhile, Fig. 19.B proposes another climate shelter with double overhangs and open front entrances. Radiant techniques would be installed on the sides and roof. This climate shelter would have a form factor of 0.74. Public policies should promote the integration of these shelters into urban development plans, ensuring that factors such as the reduction of solar radiation and the use of sustainable technologies that promote energy self-sufficiency of the infrastructure are considered [15,16,22]. They should also ensure that these guidelines are analysed for their correct location in areas with high pedestrian traffic density and locations that maximise their use and effectiveness [28,72,73]. The pavements installed around bus stops also play a crucial role in the thermal comfort of users [71]. Paved surfaces that absorb and retain heat, such as asphalt, can significantly increase the temperature around bus stops, exacerbating the urban heat island (UHI) effect. Conversely, the use of reflective or permeable pavements can help reduce surface temperatures and improve thermal comfort. Cool pavements reflect more sunlight and absorb less heat, which contributes to a cooler environment around bus stops. In addition, permeable pavements allow water infiltration and can reduce temperatures through evaporative cooling [17,71]. Building roofs are suitable locations for the Falling-Film technique. This would allow the creation of a chilled water network [74] that supplies chilled water to the climate shelters in the streets, thus improving their efficiency and performance. These modifications and considerations ensure a more robust and adaptable design of the climate shelters, optimising their functionality in various urban and climate conditions. These aspects emphasise the urgent need for sustainable urban planning to address environmental degradation, manage natural disasters and improve public health infrastructures. It also explores social and security challenges, such as housing discrimination and public safety, highlighting the need for inclusive policies to ensure equitable access to urban resources. This analysis is relevant to the creation of climate shelters in urban settings, demonstrating the importance of comprehensive and sustainable urban policies in diverse geographic regions [75]. Although the results are correct, this digital model is limited by the need to install the new climate shelter prototype in climates with a high UHI potential. Once installed, it is necessary to study the actual thermal sensation of people through satisfaction surveys and by focusing on climate variables such as wind speed and its effect on the confinement of the space. So far, it has been possible to corroborate that the experiments carried out inside the climate chamber and the proof of concept have been useful, being able to define a precise niche and step on this type of shelters. 5. Conclusions Thanks to this work, the operating conditions of a short-term climate shelter have been modelled, advancing urban adaptation to heat waves and climate change. Previous experimentation established an inverse characterisation algorithm, validating the proposed model through multiple results. Case studies in ENVI-met software obtained surface temperature and solar radiation values,implemented in TRNSYS software, optimising the accumulation and dissipation areas. The COMFA index indicated a comfort level for the occupants.The results showed that the climate shelter could cool the water up to 20 ◦C during severe heat waves, ensuring occupant comfort. The ocuppants’TRM was reduced by 15 ◦C TLW and 75 % of overheating due to solar blocking. The shelter confirmed its energy self-sufficiency during high intensity heat waves with air temperatures of 43 ◦C and up to 7 days duration. All these aspects are manifested in a favourable impact on occupant comfort. The COMFA index has been reduced to values between 70 and 120 W/m2 in 90 % of the worst weather conditions on the days tested for the months of July and August 2023 in Seville.Finally, the strengths and weaknesses of the results obtained have been detailed, providing an overview of the factors and parameters to be considered for a correct implementation in the urban environment. These future lines will allow a correct development of climate shelters in cities, being adapted to the climate, streets and avenues of any place in the world. CRediT authorship contribution statement Paz Montero-Guti´ errez: Writing –original draft, Methodology, Investigation, Data curation. Jos´ e S´ anchez Ramos: Validation, Supervision, Methodology, Investigation, Funding acquisition, Conceptualization. MCarmen Guerrero Delgado: .Teresa Palomo Amores: Visualization, Software, Formal analysis, Data curation. Alberto Cerezo-Narv´ aez: Writing –review &editing, Visualization, Formal analysis, Conceptualization. Servando ´ Alvarez Domínguez: Validation, Supervision, Methodology, Investigation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgments This study has been funded by the projects “CONSTANCY −Resilient urbanisation methodologies and natural conditioning using imaginative nature-based solutions and cultural heritage to recover the street life” (Grant Agreement PID2020-118972RB-I00) and the project “NATURBEAM −Lighting the way to a greener future to restore urban habitability through nature-based solutions”(Grant Agreement TED2021- 130416B-I00) by the Spanish Ministry of Science and Innovation. 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