A study for carbon neutrality: the impact of decisions, design and energy.
Transforming decisionmaking processes regarding energy efficiency can affect the design of an education building. Many academic institutions have committed to adopt sustainable practices that both save energy and use material resources more wisely.
Many factors affect the carbon dioxide (CO2) footprint of a building, and several steps and considerations are required during the design, construction and life cycle of a building to achieve carbon neutrality. A carbon-neutral building is focused primarily on low operational energy use and the embodied energy of the building materials. As such, a carbon-neutral building must mitigate the carbon emissions released in the materials’ fabrication, construction and continued operations of the building by generating more energy than it consumes over its life span through renewable resources. It is important to realize that carbon neutrality is not achieved the day the building opens; it is achieved over the life of the building.
The latest design approach recommends a paradigm shift in which the top decision drivers no longer are simply "reduce, reuse and recycle," but are encompassed by larger planning concepts. These concepts are represented in an inverted pyramid diagram that illustrates decisionmaking based on the constant assessment and measurement of the CO2 consequences, shown in Figure 1.
The greatest potential impact on a building’s carbon emissions and energy load is in the optimization phase at the top of the diagram. The decisions made in this phase are critical—they set the framework for the design team and will influence the decisions made in each subsequent phase. For example, the less carbon emissions resulting from decisions made in the first three phases, the less energy must then be offset in the fourth phase. As the inverted pyramid narrows, the interventions become less impactful and more costly. It is therefore essential that careful assessment precedes each decision throughout the design process.
Sidebar: Residence hall at Roger Williams University
To test sustainable strategies for achieving carbon neutrality in a residence hall, a study focused on a site at Roger Williams University in Bristol, R.I. The site will accommodate housing incorporating 512 beds, 128 of which are included in the study.
Passive strategies were studied first, and began with the optimization of surface-to-volume ratios that considered efficient building shape, solar orientation, efficient location of circulation cores, total square footage and efficient floor plate with adequate program fit-outs. Controlling heat exchange, dealing with sun exposure and shading, and water retention and extraction were important in balancing energy requirements throughout the seasons. The study minimized site disturbance, created an appropriate site density, provided shading with landscape that incorporates native plants, pervious pavement, and zoned land for future growth and geo-exchange wells. Some key strategies:
•Water-management strategies included capturing site stormwater runoff in bioswales.
•Ventilation strategies considered seasonal prevailing winds; the building’s staggered pattern avoids wind blockage.
•Southern summer winds were captured through operable windows in the lounges, creating an efficient airflow path.
•Students in each suite can control cross-ventilation through units by opening windows.
•Four-season porches and heat chimneys assist in removing warm air from interior spaces.
•Solar angles were studied to minimize heat gain during summer and capture heat during winter.
•Exterior shading with integrated photovoltaic panels were incorporated in the south facade to prevent heat gain during summer.
•In winter, the lounges capture and retain solar heat within the thermal mass of masonry walls and concrete floors to moderate extreme temperature fluctuations by slow distribution.
Daylight optimization considered space proportions for appropriate daylight levels, and integral light shelves at exterior windows to transfer light deep within rooms.
Material selection considered a material’s embodied carbon footprint during the manufacturing process, optimization and reduction of material use, effects on occupants, quality and durability performance, regional availability, and end-of-life potential for reuse, recyclability and deconstructability. A balance of these considerations is represented in the lounge spaces: The Forest-Certified glue laminated wood sequesters CO2; the argon-filled glazing controls heat gain; reclaimed/salvaged bricks represent waste redirected from landfills; the furniture contains high recycled content; the concrete contains high percentages of fly ash and local or modified aggregates; latex water-based paint throughout maintains air quality; and demountable detailed connections allow for future material reuse.
The materials’ embodied CO2 and energy were measured using "Athena Impact Estimator for Buildings." A baseline design compared with the study’s CO2-neutral design embodied far more CO2 than the CO2-neutral design. The greatest differentials are represented in the wall and beam-and-column assessments (CO2 design wood vs. baseline steel structure). This result directly illustrates the impact of material selection and reduced material use toward making a significant difference in a building’s overall carbon footprint.
Active strategies included a super-insulated building envelope with attention to air leakage in assembly details; geoexchange wells for heating the building (radiant floors) and for domestic hot water; 75 percent lighting load reductions with efficient light fixtures; daylighting controls and occupancy plug load control; and ENERGY STAR appliances.
Several scenarios were studied to reduce energy consumption and assess energy production options. Through energy modeling and analysis, the team calculated the building’s carbon footprint, or total CO2 amount, that the building would consume over its lifetime. To offset these emissions with clean energy production, it was necessary to produce positive energy. In the baseline design, carbon emissions were produced throughout the building’s lifetime, while the carbon-neutral design exhibits these emissions only during the manufacturing and construction process. The remaining stages of the carbon-neutral design show the electric loads offset by clean energy production. For clean power generation, two methods were considered: photovoltaic panels and wind turbines. These were further explored in combinations that tested wind turbines of different sizes and various amounts of photovoltaic panels.
A successful CO2-neutral building requires a monitoring system that facilitates efficient operations and optimal building performance. Each institution’s commitment to sustainability plays an important part in assuring these strategies are executed successfully.
Sidebar: Some relevant definitions
•CO2e is the universal unit of measurement used to indicate the global warming potential of each greenhouse gas. Carbon dioxide (CO2) is a naturally occurring gas that is a byproduct of burning fossil fuels and biomass, land-use changes and other industrial processes. CO2 emissions are reported in CO2e; the standard unit is Mt-CO2e or metric tons or tons of carbon dioxide equivalent.
•Carbon footprint is a measure of the exclusive total amount of carbon dioxide emissions that is directly and indirectly caused by an activity or is accumulated over the life stages of a product.
•Zero-energy design is mainly concerned with the reduction of the operating energy requirements for a building, focusing on the operating use of zero fossil energy. By definition, a carbon-neutral design incorporates zero-energy design strategies.
•Carbon neutrality is the equivalent of having a net-zero (neutral) carbon dioxide (CO2) footprint, which requires balancing a measured amount of released carbon emissions with an equivalent amount of sequestered or offset carbon emissions.
De Angel is a project architect and designer at Perkins+Will's Boston office with a focus on higher-education environments and the integration of carbon neutral/sustainable strategies in academic buildings. She can be reached at firstname.lastname@example.org.