• Energy Research
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Abstract Contrarily to what happens in northern European countries, buildings in the Mediterranean region are prone to overheating. Consequently, it is important to better understand the role that the thermal transmittance of the building envelope elements has on air-conditioning consumptions. This paper analyzes the effect of different U-values on building design in the Mediterranean area. 192 000 residential buildings were randomly generated for sixteen distinct locations and the energy consumption was assessed for each. It was found that in northern Mediterranean locations, as U-values decreased, energy consumption also decreased. However, in warmer climates, low thermal transmittances tended to significantly increase energy consumption. Hence, the lower the latitude, the higher the U-values should be, in order to prevent increasing the cooling demands. Additionally, geometry-based indexes were correlated with the building’s energy performance. For high U-values, it was found that bigger buildings worsen the energy performance and larger windows tended to improve it. For low U-values, bigger north-facing windows were beneficial. There is an adequate interval of values for which the geometry has a lower impact, which is wider and higher for lower latitudes, thus meaning that not only does the building performance improve but architects are also freer to explore alternative designs.

Abstract This paper provides an overview on how phase change materials (PCMs) can be used for the thermal regulation of photovoltaic (PV) devices, and describes an experimental apparatus to assess whether the performance of 250 W STC-rated commercial polycrystalline silicon PV panels can be improved by placing movable thermal energy storage (TES) units filled with the free-form PCM RT 22 HC on the panels' back. The outdoor apparatus is located at Coimbra, Portugal. Three identical PV panels were separately installed and individually monitored: one panel was taken as reference; the other two were considered together with a TES unit each with horizontally and vertically oriented cavities, PV/PCM1 and PV/PCM2 systems, respectively. The time evolutions of the temperature of the PV panels were compared with each other to analyse the possible thermal regulation potential of the TES units. The time evolution of the power output was also assessed to compare the efficiency of the different systems. Finally, the energy produced per day by each system was evaluated. The results showed that the PV operating temperature has increased ca. 16–21 °C and 14–18 °C in the PV/PCM1 and PV/PCM2 systems, respectively, in comparison with the reference PV panel (at peak time). Moreover, the daily energy produced by the PV panel of the PV/PCM1 and PV/PCM2 systems was, respectively, 3.3–6.5% and 3.3–6.0% lower than that produced by the reference PV panel during the measured short-term summer operation period. Therefore, it was concluded that the movable TES units have a negative impact on the performance of the PV/PCM systems, and that a PCM with a higher phase change temperature must be chosen for Mediterranean climate.

Abstract High thermal mass construction is commonly used to reduce cooling energy consumption during the summer period as a passive design strategy in the Mediterranean region. Although being a generalized design practice, the benefit to the building performance is not fully consensual within the scientific community. This work explores the influence of thermal transmittance on the energy efficiency of buildings with different thermal mass levels. Hence, a statistical comparison of the buildings’ annual energy consumption for air-conditioning is carried out based on two synthetic datasets with high and low thermal mass and varying thermal transmittance for opaque and transparent elements. In addition to climate location, the results demonstrate that thermal transmittance has varying impact on the contribution of thermal mass. The locations presenting such behavior were Marseille (−0.99% to +3.89%), Istanbul (−0.73% to +4.21%), Valencia (−1.31% to +4.97%), Algiers (−2.32% to +3.81%), Malaga (−3.95% to +6.21%), Casablanca (−5.66% to +6.96%), and Tel Aviv (−1.81% to +5.44%). These findings demonstrate that the influence of thermal mass is more complex than previously thought and levels should be chosen in relation with the thermal transmittance value.

The adaptation of spaces to different usage typologies can be complex in heritage buildings. Facilities were initially planned for a specific type of use that, when changed, require additional measures to ensure a suitable indoor environment. Passive strategies—e.g., free cooling—are commonly used as an alternative without requiring equipment installation. However, its implementation often leads to unsatisfactory conditions. Therefore, it is important to clarify the main barriers to achieving thermal comfort in readapted historic buildings. The present work investigates the thermal comfort conditions reported by workers in office spaces of a historic building in the University of Coimbra. A monitoring campaign was carried out between May and September 2020 to assess indoor conditions’ quality. Due to the current pandemic of COVID-19, offices were not occupied at full capacity. A one-day evaluation of thermal comfort was made using a climate analyzer and six occupants were surveyed on 19 August 2020. The main results highlighted discomfort due to overheating of spaces. The causes were related to the combination of inadequate implementation of the free cooling actions and the building use. Furthermore, it was recommended the installation of HVAC systems in case of full capacity.

Abstract A multi-level allocation problem consists in determining the position and size of spaces on several levels according to the user's requirements and constraints. How levels are connected and how vertical circulation objects are treated may limit the practitioner's preferences or reduce the number of solutions. Literature review has shown that in the multi-level allocation problem very often stairs and elevators are considered as fixed and rigid objects during the search process and sometimes even neglected. This paper presents an approach to deal with multi-levels based on a hybrid evolutionary technique where stairs and elevators are parametric objects, which interact with other spaces during the search process. Three case studies are presented, where the algorithm is tested on handling large and complex problems, how concurrent vertical circulation objects interact, and how several levels are stacked within a building boundary. The algorithm shows to be capable of generating coherent multi-level floor plans.

Abstract The optimization and assessment study of a thermal energy adsorption storage system is presented. The system integrates an adsorption heat storage module in a conventional hot water storage tank of a solar thermal system, operating with the silica-gel/water adsorption pair. The system was modeled using TRNSYS® and MATLAB®, and was previously assessed and improved through a set of parametric tests for each main component. In this work, the GenOpt® optimization software was used to obtain the optimal performance of the whole system. It is found that a slender and lengthy adsorber with a large number of thin fins, a thick and lengthy condenser, and an evaporator with a large number of lengthy tubes improve the system’s performance, by increasing the heat transfer areas and the adsorbent mass. The performance also improves by controlling the adsorber-condenser valve only through the system’s pressure and opening the evaporator-adsorber valve at the hot water setpoint temperature. The optimized system presents a 16% saving in annual backup energy consumption compared with a similar conventional storage system, thus validating the results of the previous segregated parametric study. This optimized system operates at the highest performance with the same configuration in different locations/climates, as only the inclination of the solar collector affects the results: larger inclinations improve the system’s performance, by favoring its operation in Winter. Results present this system as a promising solution to increase the energy storage capacity of solar thermal systems, and potentially of systems using other primary energy sources.

Abstract This paper presents a prototype tool for the space planning phase, which automatically generates alternative floor plans, according to the architect's preferences and desires, and assesses their thermal performance by coupling it with dynamic simulation. A case study of a single-family house was carried out, which comprehended two design sets. The first set correspond to a single-level house and the second set is a two level house served by one stair. Each set is made up of twelve alternative floor plans that were automatically generated, assessed, and ranked according to their thermal performance. The ranking function weights and factors variability is analyzed. The results demonstrate that two level design solutions have the best thermal performance and, within each set, the difference between the best and the worst thermal performance individual may reach 17% in the first set and 35% in the second set.

Abstract The heat transfer through a vertical stack of rectangular cavities filled with phase change materials (PCMs) is experimentally analysed in terms of both melting and solidification processes. This paper provides data that are useful for benchmarking and validation of numerical models that account for natural convection in the molten PCM. Two different PCMs are investigated: the free-form PCM-Rubitherm® RT 28 HC; and the microencapsulated PCM-Micronal® DS 5001 X. In terms of practical applications, the main goal is to discuss which PCM type is better for building applications. The time required for the melting and solidification fronts to reach the mid-plane of the cavities is presented as a function of the PCM type. During charging, the control-temperature value on the hot surface of the test-sample and the period of thermal-regulation are investigated. It is shown that the free PCM is preferable for the thermal control of vertical systems as both parameters are improved due to natural convection. The use of microencapsulated PCMs allows accelerating the charging process with almost no thermal stratification. However, in this case the control-temperature effect and the thermal-regulation period are both reduced. Regarding the discharging process, subcooling plays an important role during the solidification of the free PCM and its effect cannot be neglected when modelling.