Abstract
The application of natural ventilation strategies in high-rise office buildings is considered one of the most promising trends to address high energy performance and enhance the indoor thermal comfort levels in interior office spaces. In this regard, this study attempts to assess the potential of natural ventilation strategies of a specific, previously investigated, envelope design of a high-rise office building located in a temperate climate zone. Different summer natural ventilation approaches were tested using the building energy simulation program IDA ICE 4.8, evaluating thermal comfort and energy demand. The findings indicated that considerable energy savings can be achieved, compared to conventional mechanical ventilation and air conditioning systems.
1 Introduction
Buildings are considered one of the major contributors to global energy consumption, and global climate change, they account for one-third of world energy consumption and one-quarter of CO2 emissions [1]. Nevertheless, building energy efficiency can play a vital role in addressing energy shortages, carbon emissions, and their danger to the living environment [2, 3].
Buildings of the high-rise office type are characterized by a significantly elevated level of energy consumption. Due to high wind loads on the building envelope structures, most of these buildings are operated with mechanical ventilation all over the year generating a large cooling energy demand [4]. Furthermore, cooling is the primary energy-intensive indoor conditioning technology, consequently, current high-rise office buildings are not energy efficient, comfort and health problems arise [5–7], sick building syndrome, and cannot provide a sustainable solution.
The utilization of natural ventilation in high-rise office buildings can have a crucial impact in conserving energy and creating a healthy and comfortable indoor atmosphere for employees. In this regard, study [8] examined the effect of different Double Skin Façade (DSF) configurations on wind-driven ventilation at the 10 upper floors of a 40-story tall office building. Indoor airflow simulations using Computational Fluid Dynamics (CFD) analysis were performed on 16 DSF configurations, under isothermal conditions to assess: the opening size, the number of outer skin openings per floor, cavity depth, and cavity segmentation. The investigation showed that the size of the outer skin opening is the most affecting factor on indoor airflow, while the cavity depth and segmentation do not considerably affect it. Nevertheless, the size of inner skin openings and the number of outer skin openings have a significant impact on airflow distribution and the high air velocity regulation near the windows.
In further research [9] the impact of natural ventilation strategies on energy saving for cooling and mechanical ventilation for high-rise buildings in temperate climate zones was investigated. A typical 21-story tall office building mechanically ventilated design was considered. Computational fluid dynamics analysis was performed for six natural ventilation scenarios. First, the study utilized the DesignBuilder CFD software to anticipate the airflow pattern during two summer scenarios. Subsequently, the operative temperature and total fresh air changes per hour were evaluated using EnergyPlus. The findings revealed that natural ventilation methods can offer comfortable conditions for more than 90% of the summertime occupancy, resulting in significant energy savings that would otherwise be expended on operating traditional mechanical ventilation and air conditioning systems.
In order to efficiently improve the performance of natural ventilation, a further study [10] on the characteristics of window shapes in super-tall buildings was conducted. The potential of several window types on natural ventilation in super high-rise buildings was evaluated. The study revealed that narrow and long windows can deliver a better ventilation effect, the study also presented suggestions, and recommendations for the windows opening form selection for future super high-rise buildings.
For this reason, and based on previous research results [11–13] of a fenestration geometry parameters investigation and a façade morphology optimization of a high-rise office building located in a temperate climate zone, Budapest, Hungary. This paper aims to assess the role of natural ventilation strategies by experimenting with different summer natural ventilation approaches on the building-specific perforated envelope design in order to improve the building's indoor thermal comfort and energy efficiency.
2 Methodology
The purpose of the study described in this paper is to investigate the feasibility of employing natural ventilation methods, in particular, natural ventilation strategies during summer, to enhance the energy performance and indoor thermal comfort of a high-rise office building situated in a temperate climate zone. The building model, which has 22 floors and is oriented along the North-South axis, has a height of 88.0 m. see Fig. 1.
3D building design developed in IDA ICE 4.8
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
The high-rise office building model consists of a particular perforated envelope design previously investigated [8, 9]. To improve the energy efficiency and indoor thermal comfort of the building, the two large fully glazed façades facing East and West needed to be optimized. The initial thermal simulations led to the development of a specific façade typology a “double skin façade zig-zag” consisting of a vertically folded surface with two different tilted façade faces. The north-oriented faces comprised sun-protective glazing and the second south-oriented face was covered with an Insulated Sandwich Panel (ISPs), an Expanded Polystyrene Sandwich (EPS) consisting of a double-layered aluminum structure [9]. The double skin façade zig-zag design could increase the building's thermal comfort and achieve considerable energy savings. The reference zig-zag façade design is shown in Fig. 2.
The Zig-zag double-skin façade design with the orientation
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
To conduct the research, computer modeling, and experimental method of simulation techniques were applied. The IDA ICE 4.8 complex energy simulation software was used to evaluate: the thermal comfort (the number of occupancy hours with operative temperatures ranging between 20 °C and 26 °C), The Indoor Air Quality (IAQ) level (the carbon dioxide concentration ppm), and energy consumption (the heating and cooling final energy demand kWh m−2), in the interior office spaces. Thermal simulations were performed for all offices facing East and West on an intermediate floor. The nearby built neighborhood contains low-rise buildings and did not affect the results.
The period considered for the implementation of natural ventilation in the research is summer, from April 15 to October 15. For the rest of the year, the building operates with mechanical ventilation Air Handling Unit (AHU).
The research involved three main steps:
The first step consisted in defining on which side of the zig-zag double skin façade the ventilation window should open, either on the transparent glazed side, facing North, or on the opaque side of the façade (ISPs) facing South, then define the window type openings either central or lateral (sided windows) See Fig. 3.
The zig-zag double-skin façade units' detail
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
As the central window placed on the glazed side of the facade performed the best results, the two next steps consisted in applying opening controls; manual and automated.
The manual control is a manual window opening, applied during working days from 8:00 a.m. to 5:00 p.m. The aperture intensity was set to 100% and the air handling unit was turned off. The dimensions of the openings were gradually changed starting from 10% up to 100% of the surface of the window, each time window size was increased by 10% in order to define the appropriate window size that can achieve the best performance results. It should be emphasized that the manual control assessment serves as a reference case for comparison purposes.
The automated control is a motorized window opening, controlled by outdoor temperatures, when outdoor temperatures are suitable, windows open, and the air handling unit is turned off, when temperatures are not adequate, too high, or too low, the windows close automatically and the AHU resumes operation. For the implementation of this control, several temperature tests were carried out to define the appropriate control temperature range for the windows opening. Then different window sizes were assessed starting at 10% and going up to 100%, increasing the aperture size by 10% each time, similarly to the previous step. The control was operating during working days first, during the daytime from 8:00 a.m. to 5:00 p.m., and later during the day + night to take benefit of passive natural ventilation at night, cool the building, and improve occupant health and comfort. The overall research method and the follow-up steps are presented in Fig. 4.
3 Results and discussion
The results obtained from the thermal simulations are presented as follows: manual control evaluation, automated control evaluation, and final comparison.
3.1 Manual control evaluation: energy and comfort
The summer natural ventilation manual control assessment presented in Figs 5–7 illustrates heating and cooling energy demand, thermal comfort levels, the number of comfort hours, and indoor air quality, CO2 concentration ppm, for the different window opening sizes ranging from 10 to 100%.
Results: energy (cooling, heating, and total)
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
The energy evaluation, presented in Fig. 5, showed that the largest apertures had the greatest energy consumption. The 100% aperture size model was the least efficient; it had the highest energy demand. This is mainly due to the window being open for long hours of the day (working days, from 8:00 a.m. to 5:00 p.m.) regardless of the outdoor and indoor temperatures, and the air handling unit AHU being turned off. Whereas, when the window opening was reduced, the energy performance increased accordingly, in particular the heating energy demand. Smaller openings allowed more regulation and control over long hours of operation for natural ventilation in office spaces. The best result was for the 20 and 10% aperture size designs. The thermal comfort evaluation (see Fig. 6), was consistent with the energy demand assessment. The smaller openings were the best-performing patterns, unlike the larger ones. The 20%, and 10% openings size designs achieved the best thermal comfort levels, the highest number of comfort hours, and operative temperatures in accordance with European standards for thermal comfort. As for the IAQ assessment, see Fig. 7. All the model cases had high IAQ results, below 800 ppm. However, due to the very small aperture, the CO2 concentration in the 10% case was very high and the IAQ results were not appropriate. Eventually, the 20% opening size design was the best performing for the manually operated natural ventilation investigation; it could achieve the best compromise between energy performance, thermal comfort, and indoor air quality.
3.2 Automated control evaluation: energy and comfort
Heating and cooling energy demand, thermal comfort levels, and indoor air quality results for the different window apertures ranging from 10 to 100% automated control summer natural ventilation assessment for day, and day + night-time are shown in Figs 8–10, respectively.
Results: total energy (day and day + night)
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
Results: thermal comfort (day and day + night)
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
Results: IAQ (daytime and night-time)
Citation: Pollack Periodica 18, 3; 10.1556/606.2023.00839
Motorized opening control strategies in general have improved both energy performance and thermal comfort levels for all opening sizes. The general character results for automated day, and day + night-time summer natural ventilation strategies were very similar. Though, the day + night-time cases group performed the best results overall. The different window aperture sizes had only a slight effect on the performance. However, the automated control enabled passive cooling with natural ventilation. During the day, the automation controlled when the natural ventilation system would need active cooling or mechanical ventilation. In the evening, when temperatures drop, the building envelope would open up, allowing cool air to enter the building, venting excess heat, improving internal conditions, and reducing demand for energy-intensive mechanical cooling systems. The best-performing model in terms of energy demand and thermal comfort for day, and day + night automated summer natural ventilation was the 60% opening size. It could achieve the lowest energy consumption and highest number of comfort hours. Nevertheless, the performance results for all the aperture sizes ranging from 40% up to 100% can be considered efficient. Finally, the IAQ levels, CO2 concentrations, were appropriate in all case groups.
3.3 Final comparison: energy and comfort
The section below displays the final energy demand, and thermal comfort simulation results, for the best-performing summer Natural Ventilation (NV) scenarios designs from the previous analyses, see Figs 11 and 12.
NV1 represents the reference model case operating exclusively on mechanical ventilation. NV2 represents the best-performing natural ventilation manual control scenario, the 20% window opening size. NV3 and NV4 represent the best-performing scenarios for automated natural ventilation control. The 60% opening size day, and the 60% opening size day + night, respectively, see Table 1.
Natural ventilation scenarios
| Natural ventilation scenarios | Window opening | |
| NV 01 | Mechanical ventilation – Reference – | 0% |
| NV 02 | Manual natural ventilation | 20% |
| NV 03 | Automated natural ventilation Day | 60% |
| NV 04 | Automated natural ventilation Day + Night | 60% |
The results showed low differences (<5%) for the NV2 compared to the reference model case NV1, implying that the manual natural ventilation strategy has no or little effect on the building energy performance (4% savings) and thermal comfort (3.3% savings). However, the NV3 could decrease energy consumption by 20% compared to the NV2 case, and 24% compared to the NV1 reference case. Finally, the day + night automated summer natural ventilation strategy has significantly enhanced the building envelope performance and reach over 40% reduction of overall energy consumption (NV4 vs NV1), 36% compared to NV2, and 20% compared to NV3, along with maintaining high indoor air quality and thermal comfort levels.
4 Conclusion
The present paper aimed to evaluate the effectiveness of natural ventilation strategies during summer months for reducing cooling energy demand and mechanical ventilation requirements, as well as improving indoor thermal comfort for occupants in a tall office building situated in a temperate climate zone. As a first step, the study examined various window configurations and tested different window types, sizes, and orientations, then automation and controls were applied, such as manual control (working days from 8:00 a.m. – 5:00 p.m.), and automated control (motorized window opening) considering outdoor temperatures during the day and night-time as well. The building climate and energy simulation program IDA ICE 4.8 was used as a simulation tool to assess indoor thermal comfort and energy consumption.
The optimization results have shown that the implementation of natural ventilation methods in general, have effectively improved the indoor thermal comfort environment in office spaces and reduced the total building energy demand. The automated daytime natural summer ventilation represented an effective strategy; it could reduce energy consumption by 24%. Nevertheless, the automated day + night-time summer natural ventilation strategy performed the best results; it has achieved over 40% energy savings.
In conclusion, incorporating natural ventilation techniques in tall office structures not only greatly enhances the building's energy performance, but also could take advantage of passive natural ventilation at night to dissipate the stored heat, cool the building, reduce the use of mechanical cooling systems, and deliver a high indoor human comfort level for office workers. Inductive conclusions for similarly oriented office tower envelope structures can be gained for further projects. Future research may extend this work by implementing natural ventilation strategies for additional facade morphology designs and orientations.
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