Using multi-objective optimization to understand the impact of design decisions towards zero-energy high-rise buildings
CHALLENGE - In a drastically urbanizing environment in which 66% of the world’s population is projected to be urban by 2050, the need to reduce global CO2 emissions is becoming apparent. Currently in the EU nearly 40% of final energy consumption and 36% of greenhouse gas emissions are attributed to buildings. In order to achieve the EU’s 2020 targets in the EPBD Directive, but also to meet the longer term objectives of the climate strategy of the low carbon economy roadmap 2050, improved strategies in designing nearly Zero Energy Buildings (nZEBs) and high-rise nZEBs need to be developed. This research aimed to create a new integrated decision-making tool in designing ZEBs with the use of multi-objective optimization of building design and construction parameters for minimizing energy use and maximizing adaptive thermal comfort. This proposed strategy aims to offer an alternative to the current stepped strategies such as the Trias Energetica and the New Stepped Strategy that will lead to more optimized, energy efficient and thermally comfortable buildings. By using this new strategy, the research has shown which are the most influential parameters that should be improved in order to design zero-energy high-rise office buildings in the hot-dry climate of Athens, Greece.
SOLUTION - As a starting point, a case study high-rise office building in the hot-dry climate of Athens was used. Two multi-objective optimization rounds of design and construction parameters that can have conflicting impact on cooling, lighting and heating energy loads were implemented. 1000 different high-rise designs/ constructions were simulated for their annual energy use and levels of adaptive thermal comfort. Energy and daylight simulations were run with EnergyPlus through Rhino and Grasshopper software via the plug-ins Honeybee and Ladybug. The optimization was driven by modeFRONTIER with the genetic algorithm NSGA-II (Non-dominated Sorting Genetic Algorithm). The parameters optimized for the first optimization round were are the window to wall ratio, the wall U-value, the glazing construction U-value, the glazing G-value and the air-tightness of the facade. For the second optimization round, the parameters of the window to wall ratio, shading area and PV surface area adapted to each facade orientation (North, South, West, East) were optimized. Data analysis through charts for the various energy loads and adaptive thermal comfort levels for 1000 building designs was implement. Analyzing the graphs and implementing a sensitivity analysis indicated the impact of the various facade parameters on the energy use and adaptive thermal comfort performance of the building. For the floor plan layout optimization, Design Builder was used for a small number of energy use simulations that served also as benchmark for the results of the optimization through EnergyPlus coupled with modeFRONTIER.
BENEFITS - For the climate of Athens, Greece, the range of optimal buildings chosen through data analysis refer to buildings with window to wall ratio of 20% to 30%, wall U value 0.1 W/m2K, glazing U value 1.8 W/m2K, solar heat gain coefficient 0.3, infiltration rate 0.1 air-changes per hour and cooling set-point of 26 oC. The buildings have PV panels integrated in the facade walls. Through this integrated optimization, the building’s energy performance is reduced by 33% (from 109.12 kWh/m2 to 73.13kWh/m2) from the starting point of the current regulations in Greece. When applying improved energy use to office equipment, the total annual energy load falls to 44.9 kWh/m2 and refers to a further improvement of 38.6%. Towards minimizing the energy use, the sensitivity analysis shows that the cooling set-point has the highest influence, followed by SHGC (solar heat gain coefficient) and window to wall ratio. Glazing U value, infiltration rate and wall U value appear to have minimized influence on this objective. Towards a potentially nearly zero energy building, this research has shown that parameters like improved natural ventilation through facade and atrium, building management systems (BMS) and energy production systems with higher efficiencies need to be implemented. Also more efficient office equipment can lead to greatly reduced energy demands. This integrated optimization approach has shown that the presence of active systems (HVAC and PV panels) has overshadowed the effect of several passive design variables. For example, improved natural ventilation and setting the cooling set-point of the HVAC systems at 26 oC tends to eliminate the need for glazing constructions with very low U value (below 1.8 W/m2K). Also, the integrated optimization of window to wall ratio and shading area has overshadowed the effects of adaptive shading area per orientation. Moreover, the window to wall ratio of all the facade orientations, but especially for the south facade are proven more influential than the shading area optimization. The results indicate that the proposed integrated methodology for optimizing simultaneously passive and active systems leads to different building designs than the existing stepped strategies such as the Trias Energetica or the New Stepped Strategy. This is due to the fact that the proposed integrated strategy, enabled by optimization through modeFRONTIER, has allowed to simulate a large number of design alternatives in an automated and time-efficient way and has allowed to extract concrete conclusions by extensive data analysis. Optimization has allowed to find the trade-off solutions to several conflicting design issues affecting energy performance and thermal comfort levels.