PSI - Issue 55

Francesca Frasca et al. / Procedia Structural Integrity 55 (2024) 32–38 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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T data were used to calculate the number of tropical nights, summer days and frost days in past and future climate, as follows: tropical nights occur when daily T min > 20°C; summer days occur when daily T min > 25°C; frost days occur when daily T min < 0°C. In addition, T data were used to compute the total energy demands due to both heating and cooling process. Results were then compared to put in evidence challenges and perspectives in managing indoor climate conditions in museums of the two cities. 2.2. Energy demand in museums The energy demand of a building required to control indoor climate conditions is directly related to the difference between an outdoor climate variable and the corresponding expected indoor value. The greater the difference between outdoor climate conditions and the desired indoor threshold, the higher is the energy needed for heating/cooling (Hao et al., 2022) . The “degree - days” indicator (eq. 1), indicated as DD, represents the number of °C that the outdoor temperature (T out ) must increase or decrease to reach a specific threshold value (T thresh ): As (EN 16893, 2018) provides both minimum and maximum value of T in accordance with the vulnerability of specific materials, we separately computed heating degree-days (HDD) and cooling degree-days (CDD), based on the equations reported by the UK Met Office (CIBSE, 2006). In this case, T thresh for HDD was set equal to 16°C to ensure thermal comfort of visitors/staff in wintertime, whereas T thresh for CDD was set equal to 20°C as precautionary T to limit thermal-induced risks in vulnerable materials suffering from chemical degradation (e.g., paper). The cooling setpoint was chosen by the necessity to prioritize the preservation of collections rather than the thermal comfort of visitors/staff in summertime (T thresh = 26°C, which is valid for occupants engaged in near sedentary physical activities). The calculation of the energy demand (ED) is based on a straightforward assumption that considers only the conduction heat transfer through the opaque components in the total heat balance of the building. Under this assumption, convection and radiation heat transfers as well as other thermal gains and losses can be considered negligible. It means that transparent components are small sized and/or covered by shutters, and ex/infiltration are limited. For this reason, the total energy demand (ED tot ) in kWh∙m -3 ·year -1 can be computed as the sum of the heating energy demand (ED heat ) and cooling energy demand (ED cool ) given by eq. 2 (Frasca et al., 2023; Las-Heras-Casas et al., 2021): ( ) 1000 tot heat cool value h A ED ED ED U HDD CDD V = + =    + (2) where is the total surface area of the building in m -2 ; is the total volume of the building space in m -3 ; A/V is the surface-area-to-volume ratio in m -1 , i.e., the larger the A/V, there is more surface area per unit volume through which material can exchange heat; ℎ is the number of hour in a day (24 h); U value is the thermal transmittance in W·m -2 ·K -1 of opaque components. Starting from HDD and CDD, it is possible to estimate the total ED in a building under different thermal insulations. Here, we considered the un-retrofitted case of buildings with a A/V = 1 built before 1945 all around Europe (Pohoryles et al., 2020) with a U value = 2.5 W·m -2 ·K -1 . In addition, the same building with two different thermal insulation retrofitting options (U value, retrofit 1 = 1.6 W·m -2 ·K -1 and U value, retrofit 2 = 0.85 W·m -2 ·K -1 ) was also considered in the evaluation. , out i i DD T T = = −  1 N thresh (1)