Analysis of the impact of thermally heterogeneous enclosing structures on the energy performance of a public building

Abstract

Energy efficiency in buildings is a key priority for modern Ukraine, as it directly influences not only the comfort and safety of occupants but also the country’s economic and environmental situation. Public sector buildings, which may represent up to 20% of the total building stock in Ukraine, can serve as models for enhancing thermal insulation and energy efficiency for the wider population. Among the measures for improving building energy efficiency, significant attention is given to improving the thermal envelope and reducing heat losses through various types of thermal bridges. Dynamic modeling is an effective tool for assessing the impact of thermally heterogeneous elements on a building’s overall energy performance and thermal comfort. Currently, in Ukraine, various methods exist to assess the magnitude and impact of thermal bridges on a building’s total heat transmission losses, ranging from simplified approaches to more detailed ones that require significant time investment. Therefore, the necessity of complex calculations is questioned, especially when simpler methods may suffice.

One of the most critical factors affecting a building’s energy demand is the heat transmission coefficient, which measures the amount of heat that passes through the building envelope per unit time per unit temperature difference. This coefficient depends on the material type, thickness, and surface characteristics of the construction. Linear and point thermal bridges significantly affect heat transmission, with simplified methods for evaluating their impact often resulting in higher transmission losses. In contrast, more detailed assessments provide greater accuracy, particularly when considering specific design elements such as window and door frames. This study aims to develop and test an energy model for a building using DesignBuilder software to account for dynamic environmental and internal conditions, and their effect on heating and cooling energy demands. A comparative analysis of thermal bridges in the building’s external walls is conducted, with heat transfer coefficients evaluated using different methodologies.

The object of the study is a typical school building in Kyiv, constructed in the mid-20th century. The building consists of multiple sections with different heights and serves approximately 600 students and staff. The energy modeling was conducted in DesignBuilder/EnergyPlus, using hourly weather data for Kyiv. Several scenarios were analyzed, including a base scenario without thermal bridges and additional scenarios that account for thermal bridges with various levels of detail.

The results show that ignoring thermal bridges can lead to an underestimation of the building’s heating and cooling energy demands. Accounting for linear and point thermal bridges increases the heating demand by 8.3 % compared to the base scenario. The simplified method for evaluating thermal bridges leads to the highest increase of 13.3 %. The difference between the most detailed and the simplified methods is approximately 5 %. Similar trends are observed for cooling demand, where well-insulated envelopes slow down the natural cooling process, leading to higher energy consumption for air conditioning.

The study shows that thermal bridges significantly affect the building’s annual energy demand, with heating demand differences ranging from 8.3% to 13.3% depending on the assessment method. Simplified methods that use correction factors tend to overestimate the energy demand compared to more detailed evaluations. The research highlights the importance of accounting for thermal bridges when assessing energy efficiency in buildings. Future studies should focus on more detailed CFD modeling of critical construction elements with thermal bridges to better understand their impact on energy consumption.

Key words: energy efficiency, building energy modeling, thermal bridges, heating and cooling energy demand, nearly zero-energy buildings