Material Performance of Vertical Surfaces and its Effects on Outdoor Thermal Comfort: Case of Hot and Dry Regions

: The thermal comfort of outdoor open spaces in hot and dry regions is an important aspect that needs to be considered when designing neighborhoods. Jaisalmer town which experiences harsh conditions during long summers is selected as a study area. This study analyses the material performance of vertical surfaces and their effects on outdoor thermal comfort. The existing site and its surface materials were studied, and on-site measurements were taken in June 2022. The was then compared with other scenarios which were simulated using Envi-MET 5.0.1. The scenarios were developed with the morphology exactly the same as the base case and variation in terms of surface materials. Four different configurations were developed including the base case. The results showed that the Case 1 scenario performed poorly, whereas case 4 showed the lowest temperatures. A strong correlation is observed between surface temperature and thermal parameters as well as albedo and thermal parameters. Making efforts in reducing the surface temperature and albedo can reduce the PET and Tmrt significantly. East-west oriented scenario showed the highest values for all the thermal parameters due to the longer exposure direct solar radiation. This study is an effort to emphasize the attention towards the efficient and appropriate material use which is also in harmony with the climate of the town. The traditional Jaisalmer limestone showed good positive results for all the analyses performed in this paper.


INTRODUCTION
The thermal comfort of outdoor open spaces in hot and dry regions is an important aspect that needs to be considered when designing neighborhoods.These open spaces have to bear harsh weather conditions, especially during summer.A user belonging to these regions uses the open spaces for several important day-to-day activities.Some of these activities cannot be ignored even though the weather conditions do not permit to use.In order to promote the use of outdoor spaces and improve the functioning of the town it is necessary to improve the comfort level of the user.Outdoor thermal comfort is one of the critical issues that need to be addressed.
The urban environment is influenced by several entities such as built from its geometry [1]- [3], materials, natural surroundings [4], etc.The influence of form and material was earlier studied in several studies.[5] differentiated the open spaces and their effects of shading, vegetation, water, and material properties on microclimate development and pedestrian thermal comfort.[6] pointed out the differences among the materials due to their physical properties, like albedo, thermal capacity.and density.[7] [8] highlighted that surface materials have an important role in shaping the microclimate.
Jaisalmer lies in the Thar deserts of Rajasthan.It belongs to a hot and dry climatic zone [9].The town experiences extreme weather conditions.As per [10], the minimum temperature of Jaisalmer drops to 2.8°C in December and the maximum reached 47.8°C in June.The summers are lengthy and typically harsh.The most intense heat waves were recorded in May 2016 when the maximum temperature was recorded at 52.4°C [11].The study by [12], is the foremost and only study that explored the possibility of developing a design methodology and criteria for climatically responsive buildings and settlements.Although the old city of Jaisalmer including Jaisalmer fort has been enriched with limestone materials for its buildings, there is a lack of knowledge when it comes to the thermal performance and comparison of this material with the modern developments in the town.This study analyses the material performance of vertical surfaces and their effects on outdoor thermal comfort.The study is focused on the extreme summer season.

MATERIALS AND METHODS
In this study, existing site and its surface materials were studied, on site measurements were taken during June 2022.The was then compared with other scenarios which were simulated using Envi-MET 5.0.1.The scenarios were developed with the morphology exactly same as base case and variation done in terms of surface materials.Four different configurations were developed including the base case.The materials selected for these configurations are exactly similar to the materials which are typically found on the Jaisalmer buildings and their adjacent streets and open spaces.Following are the four cases developed and compared in this paper.Case 1 is a base case which is an existing site, Case 2 and 3 are the conditions typically found in the lated developments of the town.Case 4 condition is taken from the old city where Jaisalmer yellow limestone is used extensively.

SITE AND CLIMATE
Figure 1: Site selected as a base case The selected site (Base case/ Case 1) is located at 26°55' N and 70°55'E.When compared to traditional settlement in Jaisalmer, The selected site is characterized by medium to narrow streets, regular straight oriented pathways and buildings, the orientation is mainly E-W, N-S, and NW-SE.The buildings are more or less of similar heights.The stretches with N-S and E-W orientations were taken as a base case study.II shows the physical properties of the materials used for various scenarios.This data is not directly used for the analysis whereas it is used as an input for the simulations.The output of the simulation is taken in the form of various thermal parameters such as surface temperature (Ts), albedo (A), mean radiant temperature (Tmrt) and physiological equivalent temperature (PET).Figure 2 shows the hourly variation for all the cases.The variation is also observed in the N-S oriented and E-W oriented scenarios.Moderate variation in Ts could be seen (Figure 2 a), as the day progresses the variation increases after 1 pm.The base case scenario with E-W orientation showed the higher Ts, followed by C2 E-W.Since the E-W oriented streets receive the direct solar radiation for the longer duration compared to N-S thermal conditions higher temperature could be observed.The similar results are seen in the other studies [14], [15], [16], [17].From N-S oriented scenarios case 1 showed the highest Ts, which shows the surfaces materials at this scenario are sensitive to the solar radiation and may contribute in the temperature of other thermal parameters.Lowest Ts is observed at case 4 where traditional styled materials are used.

RESULTS AND DISCUSSION
From Figure 2b it can be noticed some interesting observations.The value of albedo starts increasing at different times of the day at all the scenarios, and it also reaches at its peak at different times.It can also be seen that once the values reach the peak, the values are not declining.It can be noticed that, at N-S oriented scenarios the albedo increased earlier than the E-W oriented scenarios.The reason can be due to the presence of E facing surfaces which are available at N-S scenario are exposed early in a daytime to the direct solar radiation.It can be said that the low albedo materials having minimum duration of exposure to direct solar radiation may help reduce the thermal impact on the other thermal parameters.The study [18] also recommends to use low albedo materials to improve thermal comfort level.Moderate differences could be observed between the scenarios throughout the day for PET (Figure 2c) and Tmrt (Figure 2d).Case 1 scenario performed poorly, whereas case 4 showed the lowest temperatures.Figure 3 shows the output for the relationship of Ts and Tmrt for the coefficient of determination (R 2 ) at all the scenarios.The strong positive relationship observed at all the cases.R 2 at case 1 is 0.87, at case 2 it is 0.89, at case 3 it is 0.93 and at case 4 it is 0.88.at all the scenarios it can be seen that the making efforts in reducing the surface temperature can reduce the Tmrt significantly.Since Tmrt is one of the predictors of outdoor thermal comfort the attention should be given to control its value.Figure 4 shows the R 2 values at all cases between albedo and Tmrt.Unlike figure 3 where E-W and N-S oriented scenarios did not show the prominent difference in the relationship, Figure 4 shows the variation in the scenarios as per orientations.A strong positive relationship could be seen between albedo and Tmrt.For all the cases the E-W scenarios showed the higher values.The maximum difference occurred for the lowr albedo values, as the value of albedo increased the variation among the orientations reduced.

EFFECT OF SURFACE TEMPERATURE (TS) ON PHYSIOLOGICAL EQUIVALENT TEMPERATURE (PET)
Figure 5 shows the output for the relationship of Ts and PET for the coefficient of determination (R 2 ) at all the scenarios.The strong positive relationship observed at all the cases.R 2 at case 1 is 0.90, at case 2 it is 0.91, at case 3 it is 0.92 and at case 4 it is 0.86.at all the scenarios it can be seen that the making efforts in reducing the surface temperature can reduce the PET significantly.

EFFECT OF ALBEDO (A) ON PHYSIOLOGICAL EQUIVALENT TEMPERATURE (PET)
(a) (b) (c) (d) Figure 6: Relationship between A and PET Figure 6 shows the R 2 values at all cases between albedo and PET.Similar to figure 4 where E-W and N-S oriented scenarios showed the prominent difference in the relationship, A strong positive relationship could be seen between albedo and PET.For all the cases the E-W scenarios showed the higher values.The maximum difference occurred for the lower albedo values, as the value of albedo increased the variation of PET among the orientations reduced.

CONCLUSION
This study analyzed the material performance of vertical surfaces and its effects on outdoor thermal comfort.The study focused on the extreme summer season at hot and dry region of Jaisalmer.The existing site and its surface materials were studied, and on-site measurements were taken in June 2022.The was then compared with other scenarios which were simulated using Envi-MET 5.0.1.The scenarios were developed with the morphology exactly the same as the base case and variation in terms of surface materials.Four different configurations were developed including the base case.There are several conclusions that can be drawn from the study.The This study is an effort to emphasize the attention towards the efficient and appropriate material use which is also in harmony with the climate of the town.The traditional Jaisalmer limestone showed good positive results for all the analyses performed in this paper.

Figure 5 :
Relationship between Ts and PET the town which does not responds to the climate in terms of forms, orientations and materials performed poorly.The use of traditional locally available materials may help improve the thermal comfort level.Making efforts in reducing the surface temperature and albedo can reduce the PET and Tmrt significantly.East-west oriented scenario showed the highest values for all the thermal parameters due to the longer exposure to direct solar radiation.The efforts should be made in the future developments where the maximum mutual shading is done.

Table I :
Physical properties of the wall materials Wall

Table II :
Albedo and emissivity of the ground surface materials

Table I ,
and Table