Development of a Commercial Building Site Selection Framework for Minimizing Greenhouse Gas Emissions and Energy Consumption Brent A. Weigel Ph.D. Candidate Georgia Institute of Technology USA firstname.lastname@example.org Summary In urbanized areas, building and transportation systems generally comprise the majority of greenhouse gas (GHG) emissions and energy consumption. The locations of building sites have significant influence on the built environment’s energy and GHG emissions efficiencies. Thus, the decision point of site selection and schematic/conceptual design for buildings represents a critical opportunity for minimizing life cycle GHG emissions and energy consumption. Green building design and rating frameworks provide some guidance and incentive for the development of more efficient building and transportation systems. However, current frameworks are based primarily on prescriptive, component criteria, rather than performance-based, whole-building evaluations. This paper presents the initial development of a commercial building site selection evaluation framework for the minimization of GHG emissions and energy consumption of transportation and building systems. The framework development examines multiple aspects of building site location efficiencies, including thermal loads on the building envelope, lighting loads, material use/reuse, and transportation supporting building operation. The expected findings are that whole-building energy and GHG emissions are sensitive to building site location, and that site-related transportation is a significant component of performance. The framework and findings will be used to support the development of quantitative performance evaluations in green building design and rating systems. Keywords: site selection, energy efficiency, green building rating systems, sustainable transportation, location efficiency, building energy use intensity. 1. Introduction 1.1 Background and Motivation Growing international concerns over climate change and sustainable development have highlighted the need for major reductions in anthropogenic greenhouse gas (GHG) emissions, particularly in urbanized areas which now contain more than half of the world’s population. In the U.S., for example, the buildings and transportation sectors account for approximately 38 percent and 34 percent respectively of direct domestic CO2 emissions [1,2]. Thus, to achieve substantial reductions in GHG emissions, improved energy efficiencies are needed in urban buildings and urban transportation systems. In terms of energy consumption and GHG emissions, the buildings and transportation sectors share not only similar levels of impact, but also notable efficiency interdependencies. Such interdependencies are evident in the energy consumption and GHG emission levels of different types of urban form. Urban form (the spatial arrangement of building types and transportation networks) influences transportation distances, transportation mode splits, building envelope heat transfer, and building material use, and each of these factors affect the energy consumption and GHG emissions of the built environment. It is generally understood among urban planning researchers and practitioners that on a per capita basis, the efficiency of the built environment correlates with the development density and land-use mix [3, 4, 5, 6, 7]. Awareness of this relationship may be helpful in planning new urban development that is more energy efficient; however, traditional transportation and land-use planning mechanisms (e.g. land-use regulation and transportation infrastructure programming) are inadequate for capitalizing on improved efficiency opportunities as they currently (and foreseeably) exist in the built environment. Much of the built environment (where populations and economic activities are concentrating) is already built, and much of the new development to come is likely to follow the trend of low-density urban sprawl – the result of enormous economic and institutional inertia (in the case of the developed world, notably the U.S.) and aspiration (in the case of the developing world). Thus, efforts to substantially regulate more compact, mixed-use development or to build large-scale alternative transportation infrastructures face considerable physical, political, and financial obstacles. To leverage effectively opportunities for improved efficiencies in the built environment, new strategies are needed to complement and/or supplant traditional building and transportation planning strategies. In the U.S., given a large existing building stock where at least half of the existing buildings will still be standing by mid-century , and a largely built-out transportation system where for the last three decades public spending on transportation operations and maintenance has exceeded public spending on transportation capital , much of the opportunity for improving the efficiencies of the built environment of today and tomorrow exists in the efficient utilization of transportation and building infrastructures. Efficient utilization of the built environment is basically an exercise in matching system users (user needs) to system infrastructure (system services). In the case of commercial development, the opportunity for matching system users to system infrastructure occurs at the key decision point of building site selection. The selection of a building site can influence the potential for energy and GHG emissions efficiency through site-dependent variables (e.g. transportation distances, transportation mode splits, building envelope heat transfer, and building material use), and through the increased utilization of the building stock. A building occupant’s selection of a building site is a decision constrained by discrete alternatives in the marketplace – the potential for achieving the most efficient utilization of the built environment is limited by the buildings/locations available at the moment of selection. The limiting effect of available site alternatives may apply not only to the selection of existing buildings, but also to the design and construction of new buildings (see section 2.2). To identify building site locations that maximize the potential for efficiency in the built environment, a performance-based building site selection evaluation framework is needed. Sustainable building design and rating systems, such as LEED® (Leadership in Energy and Environmental Design), offer a reference framework for evaluating and incentivizing greater efficiency in the built environment. Such rating systems have helped to create a green building market transformation, thereby increasing the market value of sustainable buildings. Importantly, sustainable building design and rating systems reward both building-related and transportationrelated energy efficiency measures in green building projects. However, much of the evaluation of site-dependant energy efficiency measures is prescriptive-based rather than performance-based. For example, LEED® 2009 for New Construction and Major Renovations encourages urban density/connectivity and alternative transportation in the building site through the use of prescriptive criteria, yet the rating system does not include evaluation of transportation energy savings or GHG emission reduction. Such prescriptive standards may result in site selection decisions that fail to effectively reduce energy consumption and GHG emissions. Although some portions of green building rating systems do employ a performance-based approach (e.g. Energy and Atmosphere Credit 1 of LEED®), these performance-based evaluations do not account for the relative energy efficiency performance potential of alternative building sites/locations – each project is evaluated against a site-specific baseline design, after the site has been selected. Considering the many possible variations in existing conditions and design constraints imposed by different building sites (existing envelope construction, adjoining conditioned spaces, solar ground reflection and shading, building footprint limits, available utilities, accommodation of onsite renewable energy infrastructure etc.), it should be expected that different building sites will achieve different levels of baseline performance, and that different sites will support different levels of final design performance. Thus, research is needed to develop a quantitative performance evaluation framework of building site alternatives – a framework that may be used to evaluate the potential energy consumption and GHG emissions performance of commercial development. 1.2 Research Objective The objective of the research presented in this paper is to develop a commercial building site selection evaluation framework that leads to minimization of whole-building energy consumption and GHG emissions. This research will develop analysis methods for estimating the sitedependent energy consumption and GHG emissions associated with: 1. Architectural energy systems: a. Heating, ventilating, and air-conditioning b. Lighting c. Conveyances 2. Transportation energy systems: a. Tenant journey-to-work trips b. Other work-based trips 3. Utility and on-site energy sources 4. Building material use/reuse (future research) The framework will enable the evaluation of the energy and GHG emissions of architectural and transportation-related building site aspects, at the key decision point of site selection and conceptual/schematic design. The framework will eventually be applied to case studies of commercial office development in the Atlanta, GA metropolitan region. The development and application of the framework will be limited to the context of commercial office buildings in order to maintain consistency in the building functions and the site evaluations. It is anticipated that the results of this research will help develop performance-based standards and rating systems for sustainable building site selections that enable greater whole-building energy efficiency and GHG emission reductions. The evaluation framework is intended to help inform commercial development incentives (green building certifications and/or financial instruments) aimed at the efficient development and utilization of the built environment. 2. Initial Framework Development The development of a site selection evaluation framework should be based on the academic research and professional practices already undertaken to address the energy and GHG performance of building sites. Currently, no whole-building site selection evaluation framework exists for quantifying energy consumption and GHG emissions from transportation and building systems. However, various existing analysis and calculation methods may be adapted and refined to develop such a framework. Candidate methods may be derived from research and practices in sustainable building rating systems, building design energy standards, building energy modelling, traffic impact analyses, travel demand modelling, transportation and land-use interaction research, and building life cycle analysis studies. Before synthesizing the detailed calculation procedures needed for the evaluation framework, the relevant aspects of site selection contexts faced by building occupants need to be understood. 2.1 Understanding Site Selection Contexts Figure 1 outlines the site selection alternatives context from the occupant perspective. The first question in defining the site selection context is: How does the potential new development relate spatially to the occupant’s existing development/operations? The answer to this question may have significant implications for how efficiently the occupant utilizes existing resources (if any). For example, an owner may have existing property and operations within a given urbanized area, and is considering expanding the workforce, either by adding an expansion to an existing facility or by developing another property in the region. If the owner’s planned operations require physical correspondence between the existing facility and the new expansion, then the role of transportation energy efficiency is considerably different in this context than if the new facility is completely independent. Similarly, a facility relocation context can present very unique transportation efficiency challenges compared to a new location development, since existing facilities attract trips from an established set of origins (e.g. existing employee commute shed) whereas new locations do not have an established set of origins. Location Scenario New Regional Location Regional Relocation Regional Expansion Space Requirements / Considerations New Construction Multi-tenant Renovation Single-tenant Existing fit-out Functions / Facilities Size (floor area) Market Availability Fig. 1 Outline of site selection alternatives context (occupant perspective). The context of site selection is a product of the needs of the occupant, and the availability of site alternatives in the marketplace. Considering first the needs of the occupant, perhaps the most important criteria are the amount and type of space required. The amount of required space of course dictates the scale of the commercial property and supporting infrastructures. Developments of different scale will potentially have different economies of scale (e.g. different wall to floor area ratios, centralized vs. de-centralized mechanical systems, etc.). It should be noted that economies of scale depend upon the normalizing metric of performance (see section 2.4). If the normalizing metric is building floor area (kWh/SM), then a larger building may be the most efficient choice. However, if the normalizing metric is building occupancy (kWh/person-hrs), then a smaller building may be the most efficient choice. The type of space and structure desired or considered (e.g. multitenant high-rise vs. single-tenant low-rise, renovation vs. new construction) affects the potential variability in energy consumption between potential sites. The needs/requirements must ultimately be matched to the alternatives available in the marketplace. When considering a renovation, or simply occupation, of an existing building, clearly the alternatives available in the marketplace will impact the potential for energy efficiency. In the case of constructing a new building on an undeveloped site, it may seem as though the potential for energy efficiency is limited only by available budget, technology, and expertise. However, even an undeveloped site may impose constraints on the potential energy efficiency of a new building design (e.g. available utility energy, onsite power generation feasibility, site boundary and topography constraints on building geometry, zoning laws, transportation mode shares, etc.). This outline of the site selection alternatives context helps to approach the task of identifying a site with the greatest energy efficiency potential, but a more detailed framework is needed to begin calculating energy and GHG emissions performance. The next task in developing the evaluation framework is to identify the building and transportation variables affected by the development site/location. Thus, a conceptual framework of the whole-building energy system elements influenced by site/location is needed. 2.2 A Conceptual Framework of Whole-Building Energy System Elements, As They Relate to a Building Site. Figure 2 illustrates a conceptual framework of whole-building energy system elements, as they relate to a building site. In the first three columns of the figure, the energy system elements that are potentially constrained by the site and potentially variable between different sites are divided between design, operational, and external (non-design/non-operational) elements. The external elements include the available utility energy sources that influence fuel-cycle energy efficiency, and environmental factors that influence loading on the building envelope. The environmental factors are considered to be beyond the control of the occupant, although some of these elements (e.g. ground cover and trees) could be within the property boundary and design control of the occupant. The design elements (2nd column) include aspects of the building architecture and mechanical/electrical systems that play a role in several building energy systems: space-cooling, space heating, ventilation, lighting, and materials (embodied energy). The operational elements (3rd column) include site-dependent aspects of using the building – primarily transportation access to/from the building. The location of a building within a given transportation network and land-use context can influence not only trip distances and mode shares for home-based work (HBW), nonhome-based work (NHBW), and non-home-based other (NHBO) trips, but also the rate or number of trips taken for NHBO trips (e.g. social and dining trips taken during break times). The exterior lighting schedule has been included in this category since the operation of common area lighting may not be controlled by the building occupant (e.g. walkway and parking garage lighting for a multi-tenant building). The conceptual framework accounts for energy system elements that are generally not dependent on the building site/location (4th column). The element in this category that has the most extensive impact across whole-building energy systems is the building occupancy. Building occupancy impacts space-cooling, space-heating, ventilation, lighting, and plug/process loads, and transportation activity. Furthermore, building occupancy affects the scale of the development considered in the decision context as well as the per person-hour normalization of building energy performance. It is assumed here that the estimated occupancy demands of the tenant are independent of the available site alternatives. External Elements Potentially Variable and Constrained by Site Design Elements Potentially Variable and Constrained by Site Operational Elements Potentially Variable and Constrained by Site Elements Independent of Site Building Occupancy Utility Electricity Utility Fossil Fuel Onsite Alternative Power Generation Energy Systems Building Orientation Surface Area to Volume Ratio Cooling Schedule / Setpoints Ground Reflectance Heating Schedule / Setpoints Glazing Area Glazing Conductance Misc. Equip. / Processes Glazing SC/SHGC Glazing Exterior Shading Glazing Interior Shading SpaceCooling SpaceHeating Plug / Process Loads Ventilation Schedule / Setpoints Exterior Wall Conductance Ventilation Exterior Wall Heat Capacitance Exterior Wall Absorptance Roof Conductance Roof Heat Capacitance Roof Heat Absorptance Adjacent Structures (Solar Shading) Ground Cover (Solar Reflectance) Cooling System Type / Efficiency Heating System Type / Efficiency Trip Rates (NHBO) Trip Distances (HBW, NHBW, NHBO) Mode Shares Trees (Solar Shading) Congestion / Efficiency (Actual Fuel Economy) Ambient Temperature Teleworking (Trip Rates) Transportation Vehicle Energy Efficiency (Base Fuel Economy) Ventilation System Type / Efficiency Interior Lighting Efficiency Daylighting Interior Lighting Power Density Exterior Lighting Schedule Interior Lighting Schedule Lighting Exterior Lighting Efficiency Exterior Lighting Power Density Structural Materials Interior Materials Materials Fig. 2 Conceptual framework of whole-building energy system elements, as they relate to a building site/location. This conceptual frame provides a point of departure for analyzing and quantifying the relative performance of building site alternatives. The conceptual framework identifies the energy system variables that may be affected by the building site, and the framework provides a delineation of how these variables may be controlled (by design, by operation, or by external factors beyond the occupant’s control – save by site selection). Different site alternatives considered by an occupant will likely be constrained in different ways. For example, an office space in a multi-tenant building may have constrained glazing conductance and shading coefficients (window upgrades are not an option) whereas purchase of a single-tenant building may have unconstrained glazing properties (replacement is an option), but potentially no access to natural gas service for heating. The “design” elements may be controlled either before occupancy (e.g. energy-saving design features for a new construction building) or post-occupancy (energy-saving retrofits). Similarly, the operational elements may be controlled before occupancy (e.g. selecting a site with public transit access) or post-occupancy (teleworking and carpooling programs). The fact that many of the energy system elements may be controlled or adjusted after occupancy does not necessarily neutralize the variability existing between sites before occupancy. In other words, post-occupancy measures for energy efficiency may not be capable of achieving the energy efficiency of an alternative site. For example, in terms of transportation, if an occupant’s operations will allow only a maximum of 20% employee teleworking, then an exurban site with 20% employees teleworking and the remainder (80%) commuting primarily by single occupant vehicles may not achieve the same transportation energy efficiency as a central business district site with only 50% of employees commuting by single occupant vehicles and the remainder commuting by public transit, ridesharing, or non-motorized transportation. The constraints placed on the energy system elements by the site/location can have a lasting impact on the final-design, post-occupancy, efficiency potential. The purpose of the evaluation framework under development is to quantify and compare these impacts of the site alternatives before an alternative is selected. 2.3 Accounting for Uncertainty In a context where the final energy efficiency of a site is affected by post site selection factors, an evaluation framework’s ability to convey meaningful and representative performance values requires consideration of performance uncertainty. The motivation for accounting for uncertainty is derived from an understanding that the uncertainty in the whole-building energy performance of a given site alternative may exceed the variation in estimated mean performance between site alternatives. In simple terms, the precision of the performance estimates may, in some cases, be too wide to determine the relative performance between a pair of potential sites. Whether or not the precision of the estimates are too wide, some capability for visualizing the range or distribution of estimated mean differences in energy consumption between pairs of sites could help the decision maker determine if the degree of uncertainty is acceptable for making a determination of the most energy efficient site. The uncertainty of the estimated whole-building energy performance (or mean difference in energy performance between sites) is a result of the propagation of uncertainty of the calculation inputs. The impact of uncertainty on results calculated in the evaluation framework will be quantified through sensitivity analysis of the propagation of uncertainty. 2.4 Energy Use Intensity (EUI) and GHG Intensity (GHGI) Effective evaluation and comparison of building energy performance between sites/buildings of unequal size requires normalized performance metrics or functional units. Normalized building energy use is typically expressed in terms of the amount of annual energy consumed per unit of floor area, and is referred to as the energy use intensity (EUI) . Several building design and operation energy standards and programs utilize EUIs [10, 11, 12, 13], but the measurement methods vary: different types of energy use are included or omitted (such as plug loads), some standards measure end use energy while others measure primary energy, and some standards vary with respect to the building floor area included (e.g. gross area vs. conditioned floor area). The U.S. Department of Energy (DOE) is working to standardize the measurement of building energy performance . Since no standard set of performance metrics are yet defined in the architecture/engineering/construction industries, multiple performance metrics will be utilized in the framework to analyze how different normalizing units influence performance evaluation. 2.5 The Role of Transportation Access in Whole-Building Energy Consumption The inclusion of transportation energy and GHGs in this framework development is inspired in part by literature exploring the magnitude of transportation energy with respect to whole building energy consumption. Currently, the literature offers some evidence of the proportion of whole building energy consumption related to transportation activity to/from a building site, and the importance of building location and mode choice on reducing transportation energy. Wilson conducted a study of the transportation energy intensity of buildings by comparing “driving energy” to “site energy” . Based on national data from the U.S DOE and the U.S. Environmental Protection Agency (EPA) on driver commute distance and vehicle efficiency, and building energy consumption data from the U.S. DOE, the study found that for an average office building transportation energy use exceeds building operation energy use by around 30%. For buildings built to meet applicable energy codes, transportation energy use is estimated to exceed building operation energy use by 137%. Wilson advocates the use of transportation energy intensity metrics and the development of benchmarks for performance. Researchers at the University of Toronto found that for low density residential developments, transportation accounted for 61% and 31% of GHG emissions and energy use, respectively; whereas for high density development, transportation accounted for 43% and 18% of GHG emissions and energy use, respectively . Furthermore, the researchers found that “Transportation requirements for low density suburban development are nearly 4x as energy and GHG emissions intensive as high-density urban core development per capita” and “Transportation requirements for low density suburban development are 2x as energy and GHG emissionsintensive as high-density city core development per unit of living space” . The framework under development will first examine the energy and GHGs associated with commuter access to the building site (home-based work trips), since this is generally the largest single transportation activity for the applied context of commercial office buildings. 3. Discussion of Framework Application The site selection evaluation framework will be applied to case studies of commercial office sites in the Atlanta, GA metropolitan area. The case studies to be included will explore the variation in estimated whole-building energy performance between different types of sites: suburban vs. downtown, multi-tenant vs. single tenant, new construction vs. existing fit-out. In the process of exploring the variations in energy and GHG emissions performance between different types of commercial office sites, the evaluations may be conducted within three distinct frames of reference: 1. Comparison of schematic designs; 2. Comparison of baselines; or 3. Comparison of baseline to schematic designs (energy reductions). Comparison of schematic design performance (reference frame #1) is focused on evaluation of site alternatives in terms of total end-use energy consumption. This comparison accommodates the most explicit evaluation of total energy consumption, but it is highly sensitive to the uncertainty of final building design. Thus, a comparison of schematic design performance between site alternatives may not clearly indicate, under uncertainty, which potential site best supports the best energy performance (at least not during the early conceptual or schematic design phases). Alternatively, a comparison of baseline performance between sites (reference frame #2) avoids the uncertainty of design evolution and final design, and may provide a clearer distinction in the relative performance of sites. The second reference frame (comparison of baseline performance) could provide the most targeted identification of the best performing site when utilized in conjunction with the LEED® energy performance rating system. Currently, the LEED® energy performance rating system (EA Credit 1) measures building energy performance it terms of percent reduction (of cost) against a baseline design for the given site. Although the LEED® rating system rewards improvements made in the energy efficiency of the building stock, the rating system does not necessarily reward development or occupancy of the most energy efficient building/site that meets the occupant’s needs. For example, an occupant may select an existing commercial building with an EUI that far exceeds the national average for the building classification. Proposed schematic designs for renovating the building may be estimated to achieve 30 – 40 percent energy reductions against the existing building baseline. Energy reductions of 30 – 40 percent would of course result in lower levels of final design energy consumption if the baseline design energy consumption were lower. This fact invites consideration of which site alternative provides the lowest baseline performance. If the energy reduction percentage intended by the building owner and designers is independent of the given baseline (percent reduction predetermined to meet points goal), then selection of the existing building with the lowest baseline energy consumption will result in development of a building with the lowest final design energy consumption. Furthermore, selection of the site with the lowest baseline energy consumption may help to reduce the uncertainty of achieving the best possible energy performance in the final design. If a proposed energy reduction of 40 percent is eventually changed (due to unforeseen cost or technical barriers) to a more modest 25 percent, then selection of the site with the lowest baseline energy consumption would help to preserve or “lock-in” a lower level of final design energy consumption throughout the evolution of the building design/renovation. The third reference frame (comparison of energy reductions), essentially follows the current paradigm of the LEED® rating system, in which more points are awarded for proposed buildings that reduce a higher percentage of energy relative to their own baselines (albeit in terms of energy cost rather than units of energy). Evaluation of the percentage energy reduction against a single site-specific baseline does not properly account for the opportunity to select the most efficient site available. Evaluation of the most efficient site available requires either establishing a baseline from among available site alternatives or deriving one (an average or median performance) from the existing building stock. 4. Conclusion Realization of a more sustainable built environment that meets the challenges of climate change mitigation and energy conservation will require greater efficiencies in transportation and building systems. While the deployment of new technologies will hopefully provide much of the efficiency gains needed, significant gains in efficiency must still be leveraged from the efficient utilization of new and existing transportation and building systems. The site selection evaluation framework under development can help to improve the efficient utilization of transportation and building systems by helping commercial building occupants satisfy their space and mobility needs with the most efficient transportation and building infrastructure available in the marketplace. Application of the site selection evaluation framework to case studies of commercial office development will provide insight into the opportunities for more efficiently utilizing transportation and building systems in the built environment. 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