Evaluation of Cooling Demand in Danish Residential Buildings: A Case Study of Copenhagen Apartments

Abstract: This study explores the growing cooling demand in Danish residential buildings, which is becoming more significant due to climate change and modern energy-efficient building design. The research involves monitoring temperature in eleven apartments in Copenhagen during the summer of 2023, supplemented by building energy simulations to predict future cooling needs for 2050 and 2090. The findings show that while natural ventilation can reduce the cooling demand in some scenarios, it will not be sufficient to maintain requirements for thermal comfort in the coming decades. Interestingly, the projected cooling demand for 2050 and 2090 is lower than the levels calculated for 2023. This outcome may result from the methodologies used to estimate future climate files, which are based on historical data and could potentially underestimate air temperatures and solar radiation levels. This article provides insights into mitigation strategies, including the importance of shading techniques and occupant behavior in optimizing thermal comfort and energy efficiency.

Keywords: cooling demand, thermal comfort, low-energy buildings, climate change, Denmark, natural ventilation

Introduction

The increasing focus on sustainability and energy efficiency in building design has led to the widespread adoption of low-energy homes across Denmark. However, overheating is an unintended consequence of these advances, becoming a significant concern, particularly as climate change exacerbates the intensity and frequency of heat waves. This study synthesizes the causes, consequences, and mitigation strategies related to overheating in Danish low-energy homes, particularly those built according to stringent Danish Building regulations like BR10 and BR15.

Causes of Overheating in Low-Energy Homes in Denmark: Overheating occurs when indoor temperatures exceed comfort thresholds. It is often due to a combination of high insulation and airtight construction (low ventilation) - and the widespread use of large, south-facing windows designed to maximize passive solar heating in the winter all design features intended to reduce energy use in winter. However, these same features prevent heat from escaping during warmer months. Without adequate shading, these features caused significant heat gain in low-energy houses during the summer. A survey conducted in 2015 revealed that 19% of Danish homeowners living in low-energy homes experienced daily overheating during the summer, while 32% encountered it weekly. These findings underscore the challenge of achieving thermal comfort in modern homes, which, while energy-efficient, are prone to significant temperature fluctuations during hotter periods (Knudsen et al., 2015). In urban areas, the Urban Heat Island (UHI) effect further exacerbates the overheating problem, where densely built environments retain more heat than rural surroundings. This effect, combined with smaller living spaces and single-sided dwellings (not allowing cross ventilation) commonly found in cities, leads to higher internal temperatures. In these urban environments, natural ventilation strategies, like opening windows, are often limited by concerns over noise, pollution, and security (Knudsen & Kragh, 2014).

Impact on Health: The health implications of overheating are particularly concerning for vulnerable populations, including the elderly, children, and those with pre-existing conditions. Prolonged exposure to high indoor temperatures can exacerbate respiratory and cardiovascular conditions, leading to heat-related illnesses such as heat exhaustion or heatstroke. In Denmark, the increasing frequency and intensity of heatwaves pose a significant public health risk, especially as the global population ages (Larsen, 2011).

Mitigation Strategies for Overheating: To address the issue of overheating, various mitigation strategies have been explored. External shading devices, such as blinds or shutters, have proven effective in reducing solar heat gain, particularly for homes with large south-facing windows. Improved ventilation strategies, including both natural and mechanical systems, are also critical in preventing overheating. Cross-ventilation, which allows air to flow through a building, can significantly improve thermal comfort but is more challenging to implement in urban areas due to noise and security concerns. Mechanical ventilation with heat recovery systems is another valuable strategy. These systems must be properly designed, balanced, and user-friendly for homeowners to ensure optimal performance (Knudsen et al., 2015). Thermal mass, which absorbs and stores heat during the day and releases it at night, can also help stabilize indoor temperatures. However, this approach requires careful balancing with adequate ventilation to prevent overheating (Brunsgaard et al., 2012).

Regulatory Responses and the Evolution of Building Codes: Denmark has taken a proactive approach to regulating overheating in homes. The transition from BR10 to BR15 involved stricter energy efficiency standards and adjustments to indoor temperature limits. These Danish Building Regulations aim to reduce excessive overheating by limiting the number of hours indoor temperatures can exceed 26°C or 27°C annually. These adjustments were reached by ongoing research and feedback from the building industry, demonstrating Denmark’s commitment to refining building standards based on empirical evidence and real-world conditions (Knudsen & Kragh, 2014). Despite these efforts, the problem of overheating persists, indicating a need for further improvements in building design and the inclusion of user-friendly technical systems that homeowners can easily manage. Ongoing research is focusing on developing new materials, technologies, and design strategies to improve the resilience of buildings to future climatic conditions.

This study assesses the current cooling demand in Danish residential buildings through indoor conditions monitoring and building energy simulations, projecting how cooling needs will evolve by 2050 and 2090. The aim is to support the development of more resilient building designs and cooling strategies for future climatic conditions.

Methodology

Monitoring of Temperature:The monitoring activity was carried out in eleven apartments in Copenhagen over the summer of 2023. These apartments varied in orientation, floor area, and natural ventilation potential. Indoor climate conditions—air temperature, relative humidity, and CO₂ concentration—were measured continuously at 5-minute intervals. The data collection period spanned from June to September, coinciding with the hottest months of the year.

The apartments relied on mechanical ventilation systems for air renewal. These systems were operated in free-cooling mode, i.e., bypassing heat recovery. The indoor and outdoor data were aligned using a resampling technique to create a comparable time series at a 5-minute interval.

The indoor air temperature data revealed that several apartments exceeded the comfort threshold of 26°C during the hottest days in June. The highest temperatures were recorded in apartment L20, where the tenant kept the windows closed for most of the summer (Figure 1).

Figure 1. Indoor temperature distributions for the monitored apartments (L07-L20) in summer 2023.

Building Energy Simulations:Energy simulations were conducted to evaluate the cooling demand in the apartments under various climate scenarios. The simulations were performed using the EUReCA Urban Building Energy Modeling (UBEM) tool (Prataviera, 2021), which applies a resistance-capacitance (RC) thermal network approach for dynamic thermal modeling.

The apartments were modeled as single thermal zones using the 5R1C method, which includes five thermal resistances and one thermal capacitance to represent heat exchanges through walls, windows, and ventilation. The model accounted for internal and solar heat gains, mechanical ventilation, and natural ventilation (when windows were opened). The airflow through open windows was calculated using the orifice equation, considering wind and stack effects.

The study simulated the apartments' cooling needs for three scenarios:

·         Summer 2023: Based on actual climatic conditions.

·         2050 scenario: Climate projections for mid-century based on IPCC data.

·         2090 scenario: Late-century climate predictions.

Each scenario incorporated the potential of natural ventilation (windows opened at strategic times) and mechanical ventilation. The simulations aimed to assess how future climate conditions would affect the cooling demand in residential buildings in Denmark. External climatic data, such as air temperature, humidity, wind speed, and solar irradiance, were retrieved from the Danish Meteorological Institute (DMI).

Results

Cooling Demand Simulations for 2023, 2050, and 2090: The energy simulations for summer 2023 showed that cooling demand varied between apartments, largely influenced by their orientation and window-to-wall ratio. It can be noticed that scenarios at 26°C without NV lead to specific sensible cooling demand between 6.03 (unit L15) and 19.82 kWh/m² (units L17/L18). The demand significantly decreases in the scenario at 26°C setpoint with NV, ranging between 0.30 and 5.23 kWh/m², highlighting the benefits deriving from the exploitation of windows’ opening by the occupants (Figure 2).

Figure 2. Cooling demand distribution for different apartments (L07-L20) for different cooling set-point (SP) in summer 2023.

Interestingly, the projected cooling demand for 2050 and 2090 is lower than the levels calculated for 2023. In the 2050 climate scenario, cooling demand is estimated at up to 13.16 kWh/m², with projections indicating 10.33 kWh/m² by 2090. This outcome may be influenced by the methodologies used to estimate future climate data, which rely on historical trends and may potentially underestimate future air temperatures and solar radiation levels.

Natural Ventilation and Occupant Behavior:Simulations showed that natural ventilation could reduce cooling loads by up to 97% in some cases. However, this was highly dependent on occupant behavior and window-opening patterns. In apartment L20, where windows remained closed, the cooling demand was significantly higher than in apartments where natural ventilation was used effectively. This highlights the need for developing easy-to-use systems and better occupant education on how to manage ventilation to optimize indoor comfort.

Furthermore, the simulations confirmed that natural ventilation alone would not be sufficient to maintain comfort under future climate scenarios, particularly in apartments with large glazed areas exposed to direct sunlight. Occupants’ behavior played a crucial role in determining the effectiveness of natural ventilation as a cooling strategy.

Discussion

The study findings suggest that Denmark’s current building designs, optimized for winter energy savings, are not sufficient to handle the increasing cooling demands driven by climate change. Although natural ventilation can alleviate some of the cooling needs, particularly during the nights/evenings when outdoor temperatures drop, mechanical cooling systems are necessary to maintain thermal comfort in most urban and suburban apartments.

The study also emphasizes the importance of integrating external shading techniques into future building designs. Blinds or shutters can significantly reduce solar heat gain, particularly for south- and west-facing apartments. In the absence of mechanical cooling systems, these design features will be critical to minimizing energy consumption and improving occupant comfort.

Moreover, the research highlights a clear need for occupant education. Many tenants were unaware of how to optimize their mechanical ventilation systems or use natural ventilation effectively. Educational campaigns targeting this gap could greatly reduce indoor overheating.

Conclusion

This study highlights the growing importance of cooling systems in Danish residential buildings, driven by the combined effects of climate change and energy-efficient building designs. Projected cooling demand for 2050 and 2090 is lower than the levels calculated for 2023. This outcome may result from the methodologies used to estimate future climate files, which are based on historical trends and could potentially underestimate air temperatures and solar radiation levels. Future building designs must incorporate shading techniques, natural ventilation strategies, and occupant education to optimize energy efficiency and thermal comfort.

Acknowledgment

The Landsbyggefonden and Martha og Paul Kerrn-Jespersen Fonden in Denmark fund this project.

References

Brunsgaard, C., Heiselberg, P., Knudstrup, M.-A., & Larsen, T. S. (2012). Evaluation of the Indoor Environment in the Comfort Houses: Qualitative and Quantitative Approaches. Indoor and Built Environment, 21(3), 432-451.

Dengel, A., & Swainson, M. (2012). Overheating in New Homes: A Review of the Evidence. NHBC Foundation.

Knudsen, H. N., & Kragh, J. (2014). Evaluation of energy classes 2015 and 2020 in BR10: Experiences among owners of new low-energy single-family houses and experience of stakeholders in the construction industry. SBi 2014: 7.

Knudsen, H. N., Mortensen, L. H., & Kragh, J. (2015). Satisfaction with indoor climate in new Danish low-energy houses. Proceedings of the Passivhus Norden 2015, Copenhagen.

Larsen, T. S. (2011). Assessment of the indoor climate in previous low-energy buildings. Aalborg University.

Prataviera E, Romano P, Carnieletto L, Pirotti F, Vivian J, Zarrella A. (2021). EUReCA: An open-source urban building energy modelling tool for the efficient evaluation of cities energy demand. Renewable Energy, Volume 173, Pages 544-560.