Comparison of energy-based indicators used in life cycle assessment tools for buildings

Highlights

• Comparison of energy-based indicators used in building environmental assessment tools.

• Application of EIO-LCA, Eco-LCA, Emergy and ATHENA Impact Estimator for buildings.

• Lists challenges in evaluating buildings using LCA and similar approaches.

Abstract

Traditionally, building rating systems focused on, among others, energy used during operational stage. Recently, there is a strong push by these rating systems to include the life cycle energy use of buildings, particularly using Life Cycle Assessment (LCA), by offering credits that can be used to achieve higher certification levels. As LCA-based tools are evolving to meet this growing demand, it is important to include methods that also quantify the impact of energy being used by ecosystems that indirectly contribute to building life cycle energy use. Using a case-study building, this paper provides an up-to-date comparison of energy-based indicators in tools for building assessment, including those that report both conventional life cycle energy and those that also include a wider systems boundary that captures energy use even further upstream. This paper applies two existing LCA tools, namely, an economic input–output based model, Economic Input–Output LCA, and a process-based model, ATHENA^® Impact Estimator, to estimate life cycle energy use in an example building. In order to extend the assessment to address energy use further upstream, this paper also tests the Ecologically based LCA tool and an application of the emergy methodology. All of these tools are applied to the full service life of the building, i.e., all stages, namely, raw material formation, product, construction, use, and end-of-life; and their results are compared. Besides contrasting the use of energy-based indicators in building life cycle tools, this paper uncovered major challenges that confront stakeholders in evaluating the built environments using LCA and similar approaches.

Introduction

Because of the increased emissions of wastes and the depletion of fossil fuels, research and development in building technologies and integrated design processes have sparked greater and renewed interest among stakeholders worldwide. Current research and development goes beyond the boundaries of building design and construction, and utilizes scientific knowledge from other fields to examine building performance, from physics to understand building thermodynamics (e.g., conduction, convection and radiation across the building envelope; airflow prediction using Computational Fluid Dynamics, etc.), from chemistry to develop new building material compositions (e.g., polymer technologies used for roof coatings that turn black during winter months and white in summer months, etc.), and from biology for bio-organism-based technologies (e.g., Living Machines™ for waste water recovery onsite, etc.).

To achieve sustainability, it is necessary to assess the performance of a building and its sub-components before they are built. Many kinds of building assessment methods have been developed to support environmental decision-making, Fig. 1. The first level category includes the Assessment Frameworks. These are integrated and structured assessment models that aid in the comparison of various alternatives for projects and policies. Examples include Environmental Impact Assessment and Strategic Environmental Accounting. The second first level category is composed of analytical evaluation tools that assist in decision-making or in finding potential solutions to specific problems within the framework [1]. These tools are discussed under 2 s level sub-categories - reductionist and non-reductionist tools. While reductionist tools such as Cost Benefit Analysis, evaluate performance by reducing a complex system to a smaller set of variables and integrating its measurable characteristics, non-reductionist tools such as Multi-Criteria Analysis incorporate methodological choices that are partly subjective. Finally, metrics measure the achievement of a project in sustainability terms.

To elaborate, reductionist tools use a single measurable indicator, a single dimension, a single objective, a single scale of analysis or a single time horizon [2]. There are several types of reductionist tools such as economic and monetary tools which are distinct from biophysical models, thermodynamic methods, and energy performance tools. Economic and biophysical tools are both reductionist, but have dissimilar orientations. While the former uses market currencies as a metric, the latter uses physical units. In other words, economic models rely on an anthropocentric perspective, while biophysical tools use an eco-centric perspective [1]. Economic tools such as Cost Benefit Analysis and Whole Life Costing oversimplify environmental problems by collapsing them into a monetary dimension and since environmental costs are only partly represented by market valuations, these tools are not suitable for environmental evaluation of envelope systems. Biophysical models and thermodynamic methods for the analysis of goods and services provide a direct accounting of resource costs. The critical factors are physical measures of the “natural capital” invested in the production of the good or service. Examples of biophysical models include thermo-economics, Life Cycle Assessment (LCA), embodied energy analysis, thermodynamic input–output analysis, exergy analysis, and emergy analysis. Most biophysical models allow substitution within the same form of natural capital or resource but not between different kinds or qualities. Emergy modeling is the exception, since the normalization of quality between different resource types is performed when converting resource quantities into emergy.

Three second level sub-categories are used to categorize the metrics at varied scales or measurement boundaries. They are the ecosystem, building - environment, and building scales. Examples of ecosystem scale metrics include Ecological Footprint, Surplus Biocapacity Measure, Environmental Sustainability Index, Wellbeing Index, etc. Examples of building - environment metrics, i.e., rating systems typically used in the USA are Green Building Initiative’s Green Globes [3] and Leadership in Energy and Environmental Design or LEED™ [4]. Finally, the building scale metrics include concepts such as net energy, zero energy, net zero energy, etc.

Two of the most commonly adopted rating systems in the USA are Green Globes and LEED™. Both these rating systems utilize existing standards and procedures to rate buildings. Among others, energy estimation of new or major renovation of existing buildings is an important credit component of the rating systems that employs building energy simulation tools; see for example, Green Globes’ Energy category [5]. Recently, there is a strong push by these building rating systems to include life cycle-based environmental impacts, of which life cycle energy use is one of the primary indicators. The most recent versions of Green Globes and LEED™ offer credits to go beyond typical energy estimation and to encourage study of building energy use and environmental impacts from a life cycle perspective. As LCA-based tools to quantify building energy use are evolving to meet this growing demand, it is important to include methods that also quantify the impact of energy being used by ecosystems that indirectly contribute to building life cycle energy use. Our objective is to provide an up-to-date comparison of energy-based indicators in tools for building assessment, including those that report both conventional life cycle energy and those that also include a wider systems boundary that captures energy use even further upstream. This paper applies two existing LCA tools namely, an economic input–output based model, Economic Input–Output LCA (EIO-LCA), and a process-based model, ATHENA^® Impact Estimator, to estimate life cycle energy use in an example building. In order to extend the assessment to address energy use further upstream, this paper also tests the Ecologically based LCA (Eco-LCA) tool and an application of the emergy methodology. By expanding the boundary within which building life cycle energy is accounted for, these latter two tools are able to incorporate an approximation of building impacts on ecosystem goods and services. The incorporation of ecosystem goods and services into LCAs will improve decision making in the design, construction, operation, and decommissioning of buildings in order to minimize environmental impacts and utilize natural resources in a sustainable and efficient matter. Preliminary work related to incorporation of ecosystem services in LCA is discussed in Srinivasan et al. [6], [7].

Assessing the environmental impacts and raw material consumption associated with various approaches to the built environment is currently achieved using Life Cycle Assessments. LCA emerged as a defining framework during the last two decades, largely due to the increasing awareness of environmental issues associated with the manufacturing sector, along with the waste generated by manufacturing processes. LCA was formalized by the International Standards Organization (ISO) to examine industrial systems’ performance, from the point of extraction of raw materials, through the manufacturing process and finally to product disposal. According to an International Standards Organization (ISO) document [8], which refers to principles and framework for environmental management, LCA considers the entire life cycle of a product, in terms of energy and materials used in its manufacture, as well as the end of life of that product. LCA consists of four major steps: definition of goal and scope, life cycle inventory (LCI), Life Cycle Impact Assessment (LCIA) assessment, and interpretation of results. The definition of goal and scope describes the purpose as well as the functional unit and system boundaries of the LCA. In the LCI phase, data are collected and analyzed to determine quantities of material and energy inputs and outputs of the LCA. The LCIA determines potential damage caused by the system as defined by the quantities of inputs and outputs, and finally, in the interpretation phase, the inventory and impact assessment results are reviewed and conclusions are reached and recommendations made about the environmental performance of one product relative to one or more others [8].

Economic Input–Output (EIO) and process-based are two major approaches to developing LCI. Detailed tracking of each of the diverse processes used in the system boundary is essential for developing a process-based LCI [9]. Depending on the goal and scope, this can be a lengthy and detail-intensive procedure that may lead to high cost, time, and issues related to data confidentiality and verifiability. Whereas, a top-down approach such as EIO-LCA uses available sectoral economic data and, therefore, typically the whole economy of a country is the boundary of the analysis [10]. Although robust and easy-to-use, the EIO-LCA approach has several drawbacks: (i) it uses aggregate data, and aggregate industry sectors may not provide information on the particular processes used in the manufacturing of the product under investigation; (ii) the data for the 1997 input–output benchmark model is based on the 1997 US economy, thus adding uncertainty to results from different years, although correction coefficients exist to minimize industry data variation; and (iii) data used in the EIO model are incomplete, with inherent uncertainties, thus, potentially, underestimating results such as environmental impacts [9]. Henrickson et al. provides a detailed comparison of EIO-LCA with process-based models [11]. Many studies have demonstrated the usefulness of LCA in the assessment of the life cycle energy use of manufacturing processes [12], [13], [14]. The boundaries associated with the traditional LCA starts from the extraction of raw materials, expand to the manufacturing of a product, and continue up until the end of the product’s life [15]. Other researchers have taken a more holistic approach by going beyond the initial embodied energy of materials and including a more detailed accounting of processes and environmental impacts throughout the life cycle of the building. Two such inclusions are the energy used in construction and operation of the building [16], [17]. In another study, a process-based hybrid life cycle indicator model was used to calculate the embodied energy and emissions of a high-rise education building [18]. In this study, specific data on transportation and construction activities were included in the Input–Output model used for building materials manufacturing.

Some researchers have developed approaches to utilize LCA in the built environment [19], [20], [21]. Cole [22] took an LCA approach to analyze and determine carbon dioxide emissions and the embodied energy used by the construction process at the job-site for three different types of small building structural materials. Although great efforts have been made to quantify energy expenditures as well as environmental impacts within the built environment, few studies have used a complete holistic LCA approach that considered the whole cycle from the manufacturing of building materials through demolition of the building at the end of its useful life. Such an approach quantifies the life of the building, along with its associated environmental impacts. One such study was conducted where the authors determined the energy and mass needed for a 7300 m² six story building with a life span of 75 years [23]. This study also measured environmental impacts caused by the production of primary energy used throughout the life cycle of the building (petroleum, coal, natural gas, and nuclear energy), and their contributions to global warming, ozone depletion, acidification and nitrification of soils and water, and solid waste generation. Ramesh et al. [24] demonstrated, through a compilation of studies, that the most energy used throughout the lifecycle of a typical building is during its operation. Bilec and team [25] developed temporal- and spatial- based dynamic process modeling, i.e., Dynamic LCA and applied it to assess building use phase energies with wireless sensor networks [26] and integrating Indoor Environmental Quality metrics within this dynamic framework [27].

Yet, the use of LCA for buildings has not been as ubiquitous as in the manufacturing sector, mostly due to the fact that the design and approach used for each building are often unique. Further, the life cycle approach has raised awareness of potential impacts to the services provided by ecosystems, i.e., the ecosystem goods and services directly and indirectly responsible for the creation and support of the natural systems that support the processes and economic sectors of society. It has been argued that ecosystem services are the backbone of any economy, since all raw materials come, one way or another, from biogeochemical cycles associated with a given ecosystem, both the biotic and abiotic system components [28], [29]. Yet impacts to ecosystem services are not captured with conventional LCA methodologies.

There are several needs for improved assessment of buildings that are currently being pursued. For one, the LCA boundaries for buildings need to be standardized to develop a consistent assessment protocol, and to compare among several life cycle building strategies. Also, ecosystem goods and services need to be incorporated into the LCA of the built environment to account for resource depletion, particularly since buildings consume vast amounts of natural resources and energy, most of which are non-renewable. As cities expand to meet the need of growing populations, the goods and services that ecosystems provide, inevitably, may be threatened by unsustainable practices. It is imperative that those concerned with the built environment start implementing strategies that minimize the disruption of ecosystem goods and services, as these are the foundational support for economies.

Ecosystem services are the benefits that humans are provided directly and indirectly by ecosystems, and embody the very tangible dependencies of the human population on the biosphere [30]. Ecosystem services may be provisional (e.g., harvested crops), supporting (e.g., purification of water), regulating (e.g., climate), or cultural (e.g. outdoor recreational activities) [31].

In Life Cycle Assessments, industrial-environmental systems are represented as a series of interconnected processes with inputs and outputs of materials and energy, enveloped in the larger environmental system. An ecosystem may be modeled as a system of inputs and outputs in the context of the biosphere, which is the approach often taken in systems ecology [32]. The outputs of ecosystems that are directly or indirectly supportive of human populations in these models may be considered analogous to ecosystem services [33].

Tracking the inputs and outputs of ecosystems may then be one form of modeling ecosystem services. In the field of systems ecology, an environmental accounting methodology using emergy has been developed to account for all of the inputs of ecosystems and transform them to a common energy-based unit, the solar emjoule (sej). The sum of the required available energies may be assigned to an ecosystem service as a way of approximating the total available energy supporting the provision of that service. This methodology can be equally applied to technical systems such as a building or the built environment if the processes of energy transformation underlying the materials and energy used by the system are known. If the uses of ecosystem services as inputs to technical systems can be approximated, then using emergy, it is possible to estimate the available energy from those ecosystem services that is being used in products.

The initial concept, which eventually became emergy, was first introduced by H.T. Odum in the 1960s. Emergy is based on the laws of thermodynamics, general systems theory, and ecology [34], [35], [36]. It evaluates the dependency of a product on its upstream environmental and resource energy flows using a common unit of measurement, which is the solar emjoule. Emergy is the solar emjoules embodied in a defined material or system. Non-renewable resources are considered to be more energy intensive than renewable resources. For instance, the amount of solar emjoules needed to generate a given amount of energy embodied in fossil fuel is much greater than an equivalent amount of embodied energy in biomass, e.g., wood [36].

Emergy has been viewed as a useful method for environmental evaluation of production systems [37]. One of the first efforts to integrate emergy analysis and LCA is the Emergy-based LCA (EmLCA) approach, which integrated the impact of emissions into the emergy analysis [38], [39]. In this approach, materials (as input) and emissions (as output) were obtained from existing databases used in LCA. An Ecological Cumulative Exergy Consumption (ECEC) indicator was later developed to determine the exergy consumed by the ecological processes required to produce the raw materials, to dissipate the emissions, and to sustain the operation of the industrial processes [40], [41]. Eco-indicator 99 [42] was used to study, among others, the impact of the construction of a building on human health.

As the need to consider impacts on ecosystem services became more widely acknowledged in the field of product sustainability and LCA, Zhang et al. [43], [44] described how emergy can be used to provide an approximation of ecosystem services, and further proposed the ECEC indicator in the Eco-LCA model to capture them in an LCA.

Eco-LCA provides the framework to account for ecosystem goods and services using the ECEC indicator. This approach uses the 1997 US EIO model. In addition to estimating emissions associated with the 1997 US economy, the results of Eco-LCA can provide an idea of how much of a particular ecosystem good or service was used, or consumed to create a given product. The Eco-LCA framework was designed to show data outputs in several units, namely, mass (measured in kilograms), energy (measured in joules), Industrial Cumulative Exergy Consumption or ICEC (measured in joules), and ECEC (measured in solar equivalent joules or seJ), where ECEC is closely related to emergy [36], [40].

Since Eco-LCA uses the 1997 US EIO model, it poses similar problems to those inherent in EIO-LCA. For instance, a particular sector (e.g., Iron and Steel Mills) may be composed of several industries associated with the manufacturing of a specific product (e.g., structural steel beams of a building), but the environmental impact results depict the average of all industry’s emissions within the sector.

Moreover, there were a few attempts to implement Unit Emergy Values (UEVs) in LCA software; for instance, Raugei applied UEVs to LCI results in SimaPro [45]; and Rugani et al. [46] developed the Life Cycle Impact Assessment (LCIA) indicator based on an emergy-like quantity or Solar Energy Demand (SED), which was integrated with EcoInvent processes. Methods to determine the uncertainty of UEVs have been presented by Ingwersen [47]. Besides, Ingwersen [48] also applied to emergy concepts in EcoInvent for assessing gold mining production. Recently, Rugani et al. [49] describe potential improvements to emergy evaluations that can be gained by using LCA, and Raugei et al. [50] have expounded on the added value to LCA of including emergy as an indicator. Although the Eco-LCA and other emergy-LCA synthesis models have been applied to analyze certain sectors of the economy, they have not been extensively used in assessing the built environment.