Chapter 6. Case Studies

6.1. Introduction

General engineering design principles that promote sustainability have begun to emerge (Abraham and Nguyen, 2003; Anastas and Zimmerman, 2003; see Chapter 5 of this text). These principles give general design guidance to engineers, but the art and science of designing for sustainability have not yet matured to the point where there is a well-defined set of commonly applied calculation methodologies and mathematical tools.

This current state of development of engineering for sustainability can be compared to the evolution of other fields of engineering. Consider the case of chemical engineering, which emerged late in the 19th century as a field of applied or industrial chemistry (Peppas, 1989). Chemical engineers at the turn of the 20th century studied individual industrial technologies. They learned almost exclusively through case studies. By the middle of the 20th century, however, it became apparent that most chemical processes had common parts, or unit operations. Chemical engineers began to study the design of reactors, distillation columns, and other unit operations, rather than specific processes. By the end of the 20th century sophisticated mathematical tools for modeling chemical reactions, transport phenomena, and thermodynamics had been developed, and these sophisticated design tools were applied at spatial scales from molecular (e.g., modeling the properties of nano-materials), to the scale of unit operations and chemical processes, to global scales (e.g., modeling global atmospheric chemical reactions). The sophisticated analytical tools available to chemical engineers spanned a wide range of spatial scales, as well as a range of temporal scales, from nanoseconds to decades.

Engineering tools for improving sustainability are a mixture of these three stages. While the field is no longer restricted to just examining case studies, case studies are still revealing new insights. Through the examination of many case studies, common principles have begun to emerge, and they have been the topic of this book. Risk analysis frameworks, life-cycle frameworks, methods for selecting sustainable materials, and a collection of other general principles have gained general acceptance. Some of these principles even have sophisticated analytical tools.

This final chapter of our introduction to engineering for sustainability examines how the principles of Green Engineering can be applied to case studies. Case studies have value in identifying new principles, and in serving as examples of how emergent general principles and analysis tools can be applied. This chapter presents three case studies: (1) biofuels for transportation; (2) transportation, logistics, and supply chains; and (3) sustainable built environments. Questions requiring responses that range from simple calculations to complex, open-ended analyses are embedded in each of the case studies, replacing end-of-chapter problems. Additional case studies that are available in the public domain are listed at the end of the chapter.

6.2. Biofuels for Transportation

Although liquid biofuels for transportation have received increased attention recently, they are not new. A century ago the Ford Model T was designed to run on gasoline, ethanol, and blends of these fuels, becoming a forerunner of modern flexible-fueled vehicles. Rudolf Diesel, the originator of the diesel engine, tested his engine on peanut oil at the 1900 World’s Fair in Paris (Bozbas, 2008). The motivation for biofuels a century ago was availability in an era in which petroleum-based fuels were just beginning to be widely available. In contrast, motivations for considering the use of biofuels today include mitigating the emissions of greenhouse gases, rural economic development, domestic jobs, energy security, and balance of trade. In addition, biofuels today include a wider variety of chemical structures, including not only ethanol, but other alcohols, and not only nut oils, but structures derived from organisms such as algae. These new biofuels are possible because of advances in biochemical and thermochemical processing technology, which have also increased the potential sources of biomass feedstocks.

In the United States, federal and state policies are increasingly promoting the use of biofuels. The Energy Independence and Security Act of 2007 (EISA, 2007) set a target of 36 billion gallons per year of biofuel use by 2022. To put this goal in context, in 2010, approximately 140 billion gallons of gasoline and 40 billion gallons per year of diesel were used in the United States. So the goal for biofuel use by 2022 is roughly 20% of the U.S. transportation fuel supply. The European Union is also promoting the use of biofuels, particularly biodiesel (Bozbas, 2008). As these fuels become widely deployed, their sustainability should be carefully considered. This case study will relate the concepts and analysis methods presented in this text to the evaluation of transportation biofuels. The potential environmental benefits and burdens of biofuels derived from a variety of biomass types and through various processing routes will be considered.

6.2.1. The Carbon Cycle and Biofuels

One of the primary motivations for using biofuels is their potential for reducing greenhouse gas (GHG) emissions. Reflecting this, the Renewable Fuel Standard, outlined in the Energy Independence and Security Act (EISA, 2007), defines renewable fuels and advanced biofuels based, in part, on their life-cycle GHG emissions.

Biofuels have the potential to reduce life-cycle GHG emissions because the growth of biomass withdraws carbon dioxide from the atmosphere, through photosynthesis:

CO₂ + H₂O + sunlight energy → CHO (biomass) + O₂

The molecular formula for biomass (CHO) is simplified here by neglecting minor elements such as nitrogen (N), phosphorus (P), and sulfur (S), which constitute DNA, protein, and other cellular components of biomass. In the process of bio-mass growth, photosynthesis removes CO₂ from the atmosphere, takes up water, fixes carbon and water into solid biomass, and releases oxygen (O₂) to the atmosphere. When biomass dies, organic material in the solid biomass is oxidized either microbially or thermally to form gaseous CO₂ and H₂O (mineralization) with the release of energy, reversing the process of photosynthesis. Figure 6-1 illustrates carbon cycling on a global scale, showing the cycles of photosynthesis and mineralization over terrestrial and marine environments. The carbon reservoirs (atmosphere, ocean, fossil organic carbon, plants, and soil) are shown in gigatons of carbon (GtC), and numbers associated with arrows are estimates of carbon fluxes between reservoirs (GtC/yr). On land, for example, there is a relatively rapid exchange of carbon with the atmosphere that, globally and in the absence of human influences, would be in balance, but this balance may be disrupted locally because of human activity such as deforestation, reforestation, or other land management activity. Coal, natural gas, and petroleum are large stocks of carbon derived from CO₂ sequestered in biomass and other primary producers (plankton) from tens to hundreds of millions of years ago. When fossil fuels are combusted, the CO₂ release rate is much greater than the rate at which CO₂ is sequestered again into fossil resources. This imbalance causes an accumulation of CO₂ in the atmosphere from fossil fuel combustion of roughly 3.2 GtC/yr (Figure 6-1). When biofuels produced from biomass are combusted in transportation vehicles, the CO₂ released from biofuels replaces CO₂ recently sequestered into biomass. Thus, biofuels have the potential to be carbon neutral while providing energy services such as vehicular transportation. However, this view of carbon neutrality of biofuels is overly simplistic in that land management practices and land use change induced by biofuels production may cause additional CO₂ emissions from the land on which the biomass feedstock is grown that would otherwise have not occurred.

Figure 6-1. The global carbon cycle showing the global carbon reservoirs (atmosphere, ocean, fossil organic carbon, plants, and soil) in petagrams (Pg) of carbon (10¹⁵ g = 1 GtC = 10¹² kg) and the annual fluxes and accumulation rates in gigatons of carbon per year, prior to the Industrial Revolution and since the Industrial Revolution (in lighter font). The values shown are approximate, and uncertainties exist as to some of the flow values. (From the National Oceanic and Atmospheric Administration, reported in U.S. Climate Change Science Program, 2007); NPP = net primary production (photosynthesis))

In the most recent report from the IPCC, CO₂ was listed as the most important anthropogenic greenhouse gas, and fossil fuel combustion was identified as the most common anthropogenic source (IPCC, 2007). The concentration of CO₂ has increased from a preindustrial value of 280 ppm to 390 ppm in 2010 (NOAA, 2010), the highest level in the last 650,000 years, and average global temperature increased 0.76°C (from ~13.75°C to 14.5°C) over the same time period (IPCC, 2007). The IPCC projects that global warming will continue to increase at a rate of +0.2°C per decade for the next 20 years and then, even if all man-made GHG emissions are eliminated, warming will continue for centuries because of the long timescales of climate processes.

6.2.2. Feedstocks for Biofuels

Currently, most biofuels used as replacements for gasoline are produced by fermentation of cornstarch, or sugar extracted from cane, producing ethanol as the gasoline replacement. Biodiesel is produced from various plant oils. However, recent technological advancements have made possible a much wider array of biomass feedstocks for production of liquid transportation fuels from biomass. In 2005, a U.S. government study estimated that 1 billion tons of biomass, primarily biomass that is currently viewed as waste, is available on a sustainable annual basis in the United States (Perlack et al., 2005).

Figure 6-2 shows estimates of annual production of biomass from forest and agricultural lands. Of all anticipated feedstocks, over 90% is lignocellulosic (woody) biomass as opposed to crop grains such as corn, sugar, and soybeans. Biomass from forest lands ranges from low-productivity logging residues (0.25 to 0.50 dry tons/acre/yr) to high-productivity energy crops such as hybrid poplar and willow (up to 10 dry tons/acre/yr). On agricultural lands, productivities range from lower-productivity crop residues such as corn stover (2 dry tons/acre/yr) to higher-productivity perennial energy crops such as switchgrass and miscanthus (10 dry tons/acre/yr).

Figure 6-2. A summary of currently used and potential resources at $60 per dry ton or less (U.S. Department of Energy, 2011b)

The estimates in Figure 6-2 are to be interpreted with caution as the values are affected by study assumptions, some of which include projected increases in crop productivity per acre, modifications to crop cultivation practices, fertilizer application rates, and crop residue collection efficiency. The study also assumed that all of the identified biomass would actually be available for conversion to biofuels, but availability is very sensitive to local economic conditions and landowner choices for the use of these resources. Feedstock estimates should be interpreted as bio-mass production potential as opposed to actual availability. In addition to these study limitations, other forms of biomass were not included in the feedstocks considered. Algae is gaining interest as a highly productive source of bio-oil, which can be grown in contained ponds using seawater or water from deep saline aquifers.

Per-acre productivities of algae can be very high, from 10 to 100 times greater than terrestrial energy crop productivities (Greenwell et al., 2010). Other energy crops were not covered in the “billion-ton study update” (U.S. DOE, 2011b) report such as jatropha, camelina, and castor, which are suitable for cultivation on either marginal lands or as rotation crops among food crops such as winter wheat (Shonnard et al., 2010) and require lower inputs of water and fertilizer than conventional crops. However, in order to take advantage of all of these biomass feedstocks, new technology must be developed to convert the more recalcitrant woody and plant oil biomass into liquid transportation fuels.

6.2.3. Processing Routes for Biomass to Biofuels

Processing routes for conversion of biomass to biofuels have traditionally been organized into two categories, depending on the agents for transformation and reaction conditions: biochemical and thermochemical. Biochemical conversion processes employ biological catalysts such as enzymes at mild temperatures to produce sugars from the original biomass and then ferment the sugars into oxygenated biofuels using microorganisms (Houghton et al., 2006). The choice of microorganism should take into account the types of structures in the original bio-mass, the sugars fermented, and the specific fermentation products produced. Thermochemical conversion processes employ chemical catalysts and are, for the most part, carried out at higher temperatures and pressures (NSF, 2008). These high-temperature reactions exhibit much shorter reaction times than biochemical conversion processes but often yield a wide variety of products rather than specific chemicals, such as ethanol, that can result from fermentation.

Figure 6-3 is an overview diagram showing the main conversion steps, feed-stocks, intermediates, and products for conventional biofuels (ethanol from starch crops and cane, biodiesel from triglycerides in soybeans) and for advanced biofuels. The dry mill corn ethanol process produces an intermediate glucose product and a final product of ethanol plus dry distiller grain solids (DDGS), which are marketed as animal feed. The overall fermentation reaction is given by C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. Two carbons from the sugar molecule are lost as carbon dioxide, but the energy content of the two ethanol molecules is substantially higher than that of the sugar feedstock. Process energy for corn ethanol production is typically from natural gas for steam production, and electricity is from the local grid. Because of the use of these process energy resources, corn ethanol has a relatively large fossil energy demand (ratio of fossil energy required for all processing steps per unit of energy in ethanol produced) of approximately 0.4 (Shapouri et al., 2010) or higher. The production rate of corn grain ethanol in the United States in 2009 was approximately 12 billion gal/yr (RFA, 2010a), and from sugarcane in Brazil (in 2008) was about 6.6 billion gal/yr (RFA, 2010b).

Figure 6-3. Conventional and advanced process technologies for biofuels production

Biodiesel is a methyl ester of fatty acids derived from plant oils. The biodiesel reaction can be simply described as triglyceride = methanol → 3 fatty acid methyl esters + glycerol (CH₂OH-CHOH-CH₂OH). Methanol is almost exclusively produced from natural gas (fossil origin), and co-products of biodiesel production include glycerol and a residue from the oil extraction step (soymeal, for example), which is often marketed as an animal feed. The key intermediate is a plant oil obtained from the oil extraction step.

Advanced biofuels are produced biochemically or through thermochemical conversion processing, but there are large differences from conventional biofuels processes. Hydrolysis of woody biomass is more difficult than for crop starch and will yield a mixture of 5-and 6-carbon sugars (Houghton et al., 2006), yet not many naturally occurring microorganisms are able to readily ferment 5-carbon sugars. Recent advances in metabolic engineering and systems biology of microorganisms have created unique metabolic pathways within microorganisms so that mixtures of sugars obtained from lignocellulosic biomass can be fermented into oxygenated biofuels, such as ethanol and butanol, and also into hydrocarbon fuels (Steen et al., 2010). In 2010, over 20 cellulosic ethanol commercial or demonstration projects were in development or construction with maximum production capacity of greater than 400 million gallons of cellulosic ethanol per year (RFA, 2010c). Thermochemical processing of triglycerides from plants and algae consists of mechanical and solvent extraction to recover crude plant bio-oil followed by catalytic upgrading through hydrotreatment to hydrocarbon biofuels: green gasoline, green diesel, and renewable jet. Although these hydrocarbon fuels are similar to petroleum fuels, they often have superior materials compatibility, stability, combustion, and emission properties compared to conventional biofuels and to fossil fuels (Kalnes et al., 2007). Thermochemical conversion of woody (lignocellulosic) biomass starts with gasification or pyrolysis, both being high-temperature and—pressure thermal decomposition processes. Intermediate synthesis gas (gasification) and pyrolysis bio-oil (Py-Oil/HydroPy-Oil) are upgraded catalytically in the presence of hydrogen to hydrocarbon biofuels.

There are advantages and limitations for each conventional and advanced biofuel conversion route, some being more technologically feasible and economical and others possibly being more environmentally benign (lower GHG emissions over the life cycle). Productivities per amount of biomass feedstock can also vary widely. Ultimately, the success of biofuels will depend on many factors, including social acceptability, compatibility with existing vehicular and processing infrastructure, economics, and environmental performance. The next section describes recent comparisons of environmental performance for conventional fossil fuels and future advanced fuels: biofuels and fossil-based.

6.2.4. Biofuel Life Cycles

Life-cycle assessment (LCA) is a tool that is particularly well suited to the characterization of the environmental impacts of biofuels. LCA has been used to guide research and development toward more productive synthesis routes and will continue to be used to refine approaches for biomass feedstock cultivation, harvesting, and transportation, as well as for production and use of biofuel products. This section presents a life-cycle comparison of conventional and advanced fuels, both fossil-and bio-based (Koers et al., 2009). Additional analyses are available at www.utexas.edu/research/cem/projects/epa_report.html.

Figure 6-4 shows key stages in the production of biodiesel, green diesel, petroleum diesel, and synthesis diesel. Important inputs, products, and co-products are shown at various life-cycle stages. A functional unit was chosen as 1 MJ of energy in the fuel product in order to develop life-cycle inventory data. Inventory data for each product system were obtained from reports and other sources, as described in Koers et al. (2009). Key study assumptions and inventory data sources are shown in Table 6-1. Scope for each fuel product is “cradle to grave,” from extraction from nature to product combustion in engines. One study (Hill et al., 2006) stands out from the others by including effects beyond the product systems shown in Figure 6-4, including the impacts of fuel use to drive to and from the farm and impacts of manufacture of farm equipment and buildings. Impacts contributing to global warming were assessed using global warming potentials (GWPs) of greenhouse gases relative to CO₂ (GWPs: CO₂ = 1, CH₄ = 21, N₂O = 300, refrigerants = 1000s).

Figure 6-4. Life-cycle diagrams for conventional and advanced fuels (Koers et al., 2009; reprinted with permission of John Wiley and Sons)

A comparison of GHG emissions expressed in CO₂ equivalents for the conventional and advanced fuels is shown in Figures 6-5 and 6-6. Based on this assessment, green diesel is comparable to or lower in GHG emissions than biodiesel, and results from the DOE study (Sheehan et al., 1998) data are much lower, suggesting that neglecting N₂O emissions from soybean production introduces a large error. Impacts of petroleum diesel are for the most part over 50% higher than those of green diesel and biodiesel. Synthesis diesel from coal is estimated to have over twice the impact on GHG emissions as petroleum diesel, synthesis diesel from natural gas is about 20% greater than petroleum diesel, whereas synthesis diesel from wood emits approximately 10% of the GHG emissions compared to petroleum diesel. An analysis such as this should include sensitivity studies, identifying key variables and assumptions and using alternative values for these parameters.

Figure 6-5. Greenhouse gas emissions (SBO = soybean oil, RSO = rapeseed oil, GD = green diesel) (Koers et al., 2009; reprinted with permission of John Wiley and Sons)

Figure 6-6. Greenhouse gas emissions (NG = natural gas, GD = green diesel). Displacement allocation used for all substitute diesel fuels. (Koers et al., 2009; reprinted with permission of John Wiley and Sons)

6.2.5. Cautionary Tales and Biofuels

The results presented in Section 6.2.4 show GHG emission advantages for biodiesel, green diesel, and synthesis diesel from wood compared to conventional petroleum diesel and synthesis diesel from fossil resources. In these analyses, biomass carbon was considered as climate neutral, and therefore only fossil-derived CO₂ emissions were accounted for. However, there is a growing concern that not only fossil-derived but also biomass-derived CO₂ emissions must be included in life-cycle assessments of bio-based products, and that CO₂ emissions from changes in land management are important (Searchinger et al., 2009). Furthermore, the use of fertilizers for bio-mass energy crop cultivation can contribute to nutrient loadings of streams, lakes, and rivers. Finally, the use of water in biomass production can be significant. The concluding sections of this case study will introduce these consequences of large-scale biofuel production.

Land Use Change: Direct and Indirect Effects

Land use change has occurred often throughout history, most often when natural landscapes are cleared of native vegetation and come under agricultural cultivation. Today, deforestation of tropical forests is the most important example of this (Figure 6-1), resulting in a significant flux of man-made CO₂ into the atmosphere (1 GtC/yr). As biofuels production increases significantly, there is great potential for natural landscapes to come under energy crop cultivation. When this occurs, often the amount of biomass on the land after conversion to energy crops can be much less than before. Figure 6-7 shows the evolution of cultivation on land in the Brazilian rainforest between 1975 and 1992. During land conversion, vegetation is often cleared and biomass burned, releasing a large flux of above-and below-ground carbon as CO₂ to the atmosphere. The conversion of land into fuel crop production is referred to as a direct land use change. If agricultural lands are converted from food production to fuel crop production, an indirect land use change occurs. Indirect land use change emissions of CO₂ come about as a consequence of large-scale biofuel production on lands previously used for agriculture because demand for food is inelastic (i.e., must always be met). Any lands previously used for food production that are converted to non-food uses (such as energy cropping for biofuel) will cause land use change elsewhere in the world from a natural state to agricultural cultivation. When this conversion occurs, often a large flux of CO₂ from land clearing will result. Predicting these future land conversions is very complicated, requiring an understanding of market forces, economic drivers, and societal behavior in countries around the world. A modeling approach adopted by the U.S. EPA in a final rule-making document for the federal Renewable Fuel Standards (U.S. EPA, 2010) estimates that indirect land use change emissions will equal 0.030 kg CO₂ eq./MJ corn ethanol, 0.041 kg CO₂ eq./MJ soybean biodiesel, and 0.014 kg CO₂ eq./MJ switchgrass ethanol.

Figure 6-7. Satellite images of Rondonia, in Brazil, showing the increase in cultivation between 1975 left) and 1992 (right) (USGS, 2011c)

Comparing these values to the results reported in Figures 6-5 and 6-6 demonstrates that indirect land use change emissions can be a significant, and at times a dominant, contribution to a biofuel’s GHG emissions.

Eutrophication from Nutrient Runoff and Leaching

Growing biomass requires macronutrients such as nitrogen, phosphorus, and potassium (NPK) to satisfy elemental requirements of biomass components (DNA, proteins, ATP, etc.). These nutrients are added to fields during crop cultivation. Some energy crops are heavily reliant on fertilizers, pesticides, and soil conditioners for high-productivity cultivation (corn), whereas other feedstocks are provided without external inputs (logging residues). There is a broad range of required nutrient inputs depending on biomass type, soil quality, and other factors. A portion of the fertilizers added to fields will leach into groundwater or run off fields into streams, rivers, and lakes. Depending on fertilizer type, climate, and soil conditions, as much as 50% of applied nitrogen fertilizer may run off from fields into streams or leach into groundwater. An emission factor of 30% for N fertilizers is recommended by the Intergovernmental Panel on Climate Change (IPCC, 2006) in one method. These pollutants stimulate the growth of naturally occurring algae, which degrades the clarity of water and causes oxygen depletion when the algae die and are mineralized by microbial action. These nutrients also exert negative impacts at the mouths of major rivers on all continents of the world, creating zones of oxygen depletion termed dead zones. Figure 6-8 shows the Mississippi River basin feeding fertilizer runoff to the Gulf of Mexico and depressed readings of oxygen content in ocean water. Solutions to dead zones include building wetlands and buffer zones to capture runoff and to create slow-release fertilizer formulations that deliver nutrients to plants more accurately. High costs impede the adoption of these and other responses to eutrophication.

Figure 6-8. Dead zone formation can be a result of fertilizer runoff. (http://ecowatch.ncddc.noaa.gov/hypoxia)

Water Use

Growing biofuels requires water. In some regions water is provided by rainfall, but as increasingly marginal lands are used to satisfy both food and fuel crop needs, it is likely that increasing amounts of irrigation would be required. Historically, data on water use have been difficult to obtain; however, as water becomes an increasingly scarce resource, accounting for life-cycle water use will become increasingly important.

When water is accounted for, a distinction is generally made between withdrawals and consumption. For growing biofuel crops, withdrawal is a result of irrigation, while consumption includes evapo-transpiration and the conversion of water into biomass. Table 6-2 shows estimates of water use for a variety of crops, separated into withdrawal and consumption. These estimates indicate that water use for biofuel crops can amount to thousands of gallons per gallon of fuel produced. In comparison, the extraction of crude oil has a relatively small water footprint, although there is a great deal of uncertainty regarding the amount of water used in the production of crude oil.

Table 6-2. Water Use for Different Feedstocks during Crop Growing

6.2.6. Summary of Sustainability of Biofuels

Biomass is a potential feedstock that is created by photosynthesis, in a process that removes CO₂ from the atmosphere. Combustion of biofuels from biomass returns carbon to the atmosphere in a closed cycle. As much as 1 billion dry tons of biomass are available per year in the United States, a quantity that could replace a significant fraction of current petroleum use (2005 data), and most of this biomass is of the woody type. Conventional and advanced biofuels processing technologies are under development with the goal of overcoming technical and economic barriers to large-scale commercial biofuels production. Studies of the environmental benefits of conventional and advanced biofuels are ongoing and have encouraging features, but challenges exist on managing biomass production to ensure that CO₂ emissions from land use change are not severe, that water consumption can be accommodated, and that water quality is protected from excessive nutrient runoff from energy crop production.

6.3. Transportation, Logistics, and Supply Chains

Logistics is the management of the flow of goods and services within a production and distribution system between points of origin and points of use. Logistics has origins in military applications, stemming from the need to supply troops on the move with ammunition, food, and other items. Outside of military uses, logistics figures prominently in manufacturing, distribution of produced goods to retail outlets, and managing business projects. In manufacturing logistics, the focus of effort is to ensure that the manufacturing and assembly systems are supplied with the right components at the right time and in the right order. In retail logistics, supply chain choices involve levels of inventory, transportation, locations of warehouses, tracking of goods, and management. Logistics is a part of all engineering disciplines but is most important in the fields of industrial engineering, mechanical engineering, and transportation studies.

The case studies in this section explore sustainability issues in transportation logistics. Because these choices involve supply chains, life-cycle thinking is implicit in the design decisions.

6.3.1. A Limited Life-Cycle Assessment of Garment Design, Manufacture, and Distribution

The first case study in logistics that will be considered in this section (Hopkins et al., 1994) examines the transport of the materials for a fleece pullover and final garments along their supply chains. Studies of garment life cycles have generally not found transportation to be a significant factor because road and rail transport are often used as the primary transport modes (see Table 6-3). For example, it was found that a 0.123 lb polyester woman’s blouse consumed 10,528 BTU for the manufacturing step (Franklin Associates, 1993) and that transportation contributed only about 5% to this total. The importance of transportation can change significantly, however, if air transport is used to achieve just-in-time deliveries. This suggests that optimum logistics will involve balancing the impacts of rapid transportation with the impacts of warehousing and storage of inventory along the supply chain.

Table 6-3. Energy Use by Transportation Mode

The first step in this case study will be to examine the impacts of transportation of the garments. The garment weight is 1.28 lb and the energy required for garment manufacture is assumed to be approximately 100,000 BTU.

6.3.2. Alternatives for Garment Transport Logistics

Two scenarios will be evaluated for the transport of fabric to the location of garment assembly, and then transport of the finished garment to warehousing. In scenario 1, fabric is transported from a mill in Massachusetts to a sewing factory in San Francisco by truck (road), and the finished garment is then shipped to Ventura, California, again by road transport. In scenario 2, fabric is transported by rail from Massachusetts to Miami, and then by ship to the location of sewing, Jamaica. Finished garments are transported from Jamaica to Miami by air freight, and then to Ventura by road. Table 6-4 shows transportation distances and modes for fabric and garment transport.

Table 6-4. Data for Fabric and Garment Transport from Massachusetts to Ventura via San Francisco or Jamaica

Table 6-4 maps the transportation required to move a garment to a product warehouse, but the garment still needs to be transported to the consumer. Many products are sold to customers through retail outlets, and there are multiple modes of transportation that could be used. One pathway is to ship garments to retail outlets in cardboard boxes with 18 to 20 garments per box. Garments are also shipped directly to the final customer through mail order, and there are several shipping pathways, each with unique cost and time-of-delivery features. Two scenarios for the transport of a garment from the headquarters in Ventura, California, to the final customer are described in Table 6-5: to a retail outlet in Boston, Massachusetts, or directly to a customer through mail order to Denver, Colorado.

Table 6-5. Data for Garment Transport to Retail Outlet or Direct to Customer

6.3.3. Life Cycles, Materials Use, and Transportation Logistics of Running Shoes

A second logistics case study (Morris, 2011) examines global transport of shoe components and the controversial use of a material in the sole of running shoes. This case will examine GHG emissions over the life cycle of the shoes, comparing the impacts of transport to the impacts of the selection of one critical material.

The system boundaries include the extraction of virgin materials, the transportation of the materials to their place of assembly, the manufacture of the shoes, and the shipping of finished shoes to the United States. Data for the case study were derived from sourcemap.com,¹ an open-access site devoted to tracing distances and materials of products. Data were also available on GreenXchange,² the Web site dedicated to sharing green design innovations pioneered in 2008 by shoe manufacturers.

6.3.4. Sustainability and Logistics

In these transportation logistics case studies, the analyses and impacts focused on environmental issues as opposed to broader sustainability issues, such as economic factors and societal concerns. Nevertheless, the cases demonstrated that the extent and mode of transportation used in supply chains can be very important in determining a product’s footprint. The use of global supply chains will have implications for environmental performance, costs, and employment and social structures around the world. Although it was not possible to perform a full sustainability analysis for these case studies, it is becoming clear that engineers of all disciplines will become increasingly involved in making decisions about supply chains, and those decisions will have global environmental, economic, and societal impacts.

6.4. Sustainable Built Environments

Buildings are familiar and important elements of modern life, yet the impacts of buildings on the environment and on human health and well-being are not well understood by most building users and are often not reflected in society’s approaches to protecting the environment and human health. For example, many environmental regulations are based on outdoor conditions, yet most citizens of the United States and other developed countries spend more than 90% of their time in buildings (U.S. EPA, 1987). While environmental conditions in buildings are to a small degree dependent on pollutants that are brought in from outside, many pollutants are found at greater concentrations indoors than outdoors, because of emissions released from materials within the building (e.g., walls, carpets, furniture) or activities done in the building (e.g., cooking, smoking).

In addition to buildings being an important living environment, their construction and operation are directly and indirectly responsible for the consumption of significant fractions of national energy and material flows. This consumption has environmental, economic, and social impacts.

The case study presented in this section explores the importance of buildings for the use of energy resources and materials and how a variety of sustainability criteria can be used to evaluate building performance. The use of U.S. statistics will dominate the presentation, but the interpretations derived from the U.S. data are expected to be relevant to other developed and developing countries. This case study will also introduce methods and tools that can be used to evaluate the sustainability of building designs and will briefly explore how trade-offs are often encountered (e.g., improving indoor air quality versus reducing energy losses by recirculating indoor air) in building design decisions.

6.4.1. Energy Consumed for Building Operation

Residential and commercial buildings are important end users of energy in the United States, accounting for approximately 20% of end-use national consumption (see Figure 1-3). Table 6-6 shows energy use data for residential and commercial buildings, by application and by energy type. Most energy used in buildings is in the form of natural gas and electricity, with minor contributions from biomass and petroleum. Because a substantial fraction of the energy use in buildings comes from electricity, a more comprehensive view of building energy use would include the energy losses associated with generating electricity and producing and delivering natural gas. If these energy losses are accounted for, then buildings in the United States consume about 40% of total primary energy (94.6 quadrillion BTU; U.S. DOE EIA, 2010). This includes 72% of U.S. electricity consumption and 39% of annual U.S. CO₂ emissions.

Table 6-6. 2010 U.S. Buildings Energy End-Use Splits, by Fuel Type (Quadrillion BTU)

Space heating, lighting, and space cooling are the largest building energy consumers, constituting together about 37% of total building energy use. Of less importance are water heating, refrigeration, electronics, and ventilation. Ventilation, although the smallest category, contributes indirectly to larger energy use categories such as space heating and cooling by exporting building air to the environment and importing outside air into the building, both of which require inputs of primary energy for heating and cooling, depending on the season.

Knowing the major energy uses in buildings can inform the energy-efficient design of buildings. For example, if a building is in close proximity to an industrial facility, sharing industrial waste heat could offset fossil energy resources for building heating. Energy for building heating can also be saved through greater use of insulation materials in walls and windows. Energy impacts of lighting can be reduced through the use of natural light.

6.4.2. Materials Use for Building Construction and Maintenance

In addition to requiring substantial amounts of energy for their operation, construction of new residential and commercial buildings accounts for a significant fraction of the non-energy-related materials that are produced, imported, and consumed each year. For example, approximately 3 billion tons of natural aggregates (primarily crushed stone, sand, and gravel) are used each year in the United States (Figure 1-5). In 2006, approximately 60% of natural aggregates were used in road and highway construction and 40% in construction of buildings (Sullivan, 2006), indicating that 1.2 billion tons of aggregates are used for building construction in the United States (4 tons per person per year).

Metals are another important category of materials use in buildings. According to the American Iron and Steel Institute (USGS, 2011b), 21% to 24% (depending on year) of iron and steel is shipped to the construction industry, and approximately 60% of the value of new construction put in place in 2011 is for residential and nonresidential buildings (USDC, 2011). Thus, about 15% of annual U.S. iron and steel consumption is for building construction. Iron and steel, in turn, account for 95% of metals production in the United States and in the world (USGS, 2011a). Both primary (new extraction) and recycled sources of metal are used, with roughly 50% of metals production from each (USGS, 2009). If the degree to which metals are recycled is to be substantially increased, methods for the reuse of building materials will be needed.

Building construction and use also drive other types of materials use. Water use in buildings consumes approximately 13% of potable water supplies, mostly for residential as opposed to commercial applications (U.S. EPA, 2009; USGS, 1995). Approximately 60% of forest products, including lumber, plywood/veneer, pulp products, and fuel wood, are used in building construction and operation (U.S. Forest Service, 2003). Table 6-7 summarizes the importance of buildings to the consumption of materials. Overall, it is clear that buildings represent one of the most important targets for sustainable engineering design.

6.4.3. Design of Buildings for Sustainability

In order to design buildings for sustainability, additional considerations beyond energy consumption, CO₂ emissions, and materials use must be included. Issues such as indoor air quality, water use efficiency, worker productivity, and even factors such as biodiversity and control of urban warming by buildings become important design considerations. In addition, beyond the direct interaction of people with residential and commercial buildings, there is a complex interplay between buildings and societal structure. For example, the presence or absence of a public transportation infrastructure can affect decisions on commercial or residential building location as well as decisions on the use of public transportation by the building’s occupants.

The remainder of this case study will explore selected aspects of sustainable building design. To guide this exploration, design criteria from the U.S. Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED) certification program will be used (USGBC, 2011). The LEED program is an internationally recognized building certification program for architects, contractors, owners, and operators. It provides a framework for building design, construction, operation, and maintenance to achieve reductions in environmental impacts and improvement in conditions for building occupants.

Leadership in Energy and Environmental Design (LEED)

The LEED rating system is meant for new and existing commercial, institutional, and residential buildings, and also for entire neighborhood developments. There are currently 8000 LEED-qualified buildings in the Unites States (USGBC, 2011a). After the release of LEED Green Building Rating System Version 1.0 in 1998, several updates to the rating system have occurred, in 2000, 2002, and finally with the most recent Version 2.2 in 2005 (USGBC, 2011b). There are separate rating systems for different building types and project scopes, including

• LEED for Existing Buildings: Operation and Maintenance

• LEED for New Construction

• LEED for Core and Shell

• LEED for Schools

• LEED for Neighborhood Development

• LEED for Retail

• LEED for Health Care

• LEED for Homes

• LEED for Commercial Interiors

Each rating system listed includes five environmental categories: Sustainable Sites, Water Efficiency, Energy and Atmosphere, Material Resources, and Indoor Environmental Quality. Two additional nonenvironmental human effects categories are also included in the rating system: Innovation in Design and Regional Priority.

In the LEED rating system, each category is subdivided into “credit” subcategories, and one or more points are assigned to each credit subcategory. Total possible points in each ranking system for the five environmental categories is 100 base points; however, 10 additional points can be awarded for the Innovation in Design and Regional Priority categories. The allocation of points among the credit subcategories is based on an analysis of potential environmental impacts and human benefits relative to a set of impact indicators. These impact indicators include GHG emissions, fossil fuel use, toxins and carcinogens, air and water pollutants, and indoor environment. The assessment of potential environmental impacts uses the TRACI impact assessment model from the U.S. EPA (see Chapter 5, Section 5.4) and then applies additional weightings to each subcategory credit from the National Institute of Standards and Technology (NIST), similar to the way that weightings are applied in life-cycle assessment (see Chapter 5 and the “valuation” discussion). The decision on how many points to assign to each credit subcategory is based on a consensus process with a committee of sustainability experts.

The USGBC LEED program certifies that building projects achieve certain levels of desired outcomes according to a numerical scale. For example, in retail applications, the following point scale applies to both design and construction phases:

The distribution of these points among the major building categories is dependent on application, but insights into the workings of the LEED rating system can be gained by closer inspection of one application: Retail buildings (LEED, 2010). Here the points are distributed as follows:

The greatest number of points, and therefore the greatest emphasis in green building design and operation, relates to energy consumption/atmosphere protection and sustainable sites. The following examples will highlight the energy savings opportunities from implementing some of the actions in the LEED design and operation program.

High-Efficiency Lighting

One of the credit subcategories within the Energy and Atmosphere category for LEED Retail buildings is Optimize Energy Performance, where up to 19 points may be awarded. Table 6-6 indicates that lighting is the second most important energy use category in U.S. buildings, contributing 13.4% to the annual building energy consumption total of 40.40 quadrillion BTU/yr, or 5.7% of the annual U.S. energy consumption of 94.6 quadrillion BTU/yr.

Stackhouse and Fan (2009) investigated the replacement of T12 fluorescent bulbs and fixtures with the more efficient T8 version. Existing T12 systems are generally 34-watt lamps driven by an energy-efficient magnetic ballast. A two-lamp one-ballast T12 fixture typically operates at 72 watts (34 watts for each lamp and 4 watts for the ballast) and puts out 2350 lumens. T8 lamps are thinner than T12 lamps. High-performance T8 lighting consists of high-lumen long-life lamps (2250 lumens) coupled with electronic ballasts. This combination provides nearly the same amount of light as a standard dual 34-watt T12 system but uses only 48 watts.

Functional Unit/System Boundaries. Because both bulbs last 20,000 hours and put out about the same light intensity, a suitable functional unit for comparison would be the light output over the life of a pair of bulbs/fixture units. System boundaries for this analysis are from “cradle to use” but do not include impacts of disposal. Issues can arise in the disposal of fluorescent lamps, but in this analysis it will be assumed that the lamps have comparable disposal issues.

Inventory. The inventory of inputs for each bulb system is shown in Table 6-8. The materials identified in Table 6-8 are also the names of ecoprofiles in the ecoinvent database in the LCA software tool SimaPro 7.2. The input amounts in the second column include mathematical formulas for the distribution of total mass between metals and other materials for ballast, bulb, and fixture, lens, and packaging. Electricity demand over the life of the bulb is included in the inputs table, assuming average U.S. grid electricity.

Table 6-8. Inputs of Materials and Utilities to a Pair of T12 and T8 units

Impact Assessment. Greenhouse gas emissions were converted to equivalents of CO₂ emissions by using global warming potentials (GWPs) from the IPCC 2007 100a method in SimaPro 7.2, where GWPs for CO₂, CH₄, and N₂O are 1, 25, and 298, respectively. GWPs for refrigerants and certain chlorinated and brominated solvents were also included in the analysis. Energy consumption by primary energy type was included using the Cumulative Energy Demand method in SimaPro 7.2.

LCA Results. Figure 6-9 shows the total emission of greenhouse gases expressed in CO₂ equivalents for the life of the pairs of T12 and T8 bulbs/fixtures. Most of the emissions, over 99%, are due to electricity use during the life of the bulb, ballast, and fixture. Production of the bulb systems, including packaging, is 0.1% to 0.2% of total life-cycle emissions. Savings of GHG emissions for the same functional unit for the T8 compared to the T12 is 33.3% = ((1953.8 = 1302.9)/1953.8) *100. Figure 6-10 shows the total energy demand, expressed in megajoule equivalents, reported by primary energy type for the T12 and T8 bulb systems. Most energy is provided by fossil resources, followed by nuclear, with very small contributions by renewable energy sources. Savings of energy for the same functional unit for the T8 compared to the T12 is also 33.3% = ((32,035.7 = 21,365.8)/32,035.7) *100.

Figure 6-9. Greenhouse gas emissions expressed in CO₂ equivalents for T12 and T8 bulbs

LCA Interpretation. Significant savings of energy resources and GHG emissions can be achieved through innovation in lighting systems for buildings. Other types of high-efficiency lighting besides the fluorescent systems discussed here include light-emitting diodes (LEDs), which achieve even higher energy efficiencies than fluorescent bulbs. Also, the effective use of natural light often can enhance artificial lighting systems, such as the use of ceiling skylights in many big box stores (look up the next time you are in one).

Simulating Energy Use in Buildings

Besides using high-efficiency lighting in buildings, another approach to improving building energy performance is to use energy balance simulations to optimize the effects of thermal insulation, air exchange rates, and other building design elements and operating procedures. These types of analyses can identify opportunities for improvements in the energy-related impacts of building operation.

Energy Balance Simulation. Energy balance models for buildings are valuable to engineers and architects because these models allow for a better understanding of the major causes of energy demand and can point to design decisions that reduce building energy consumption. There exist a large number of software tools for modeling building energy demand and environmental/economic impacts, and a summary of tools is available at a U.S. DOE Buildings Energy Software Tools Directory Web site (U.S. DOE, 2011a). Some of these software tools are free to download and use, others are to be used online, and others still are commercial products. One such online tool is hosted at the University of Texas, Austin, in the College of Engineering and is called the Building Mass and Energy Balance (BMEB) model (www.ce.utexas.edu/bmeb/). This model can be used to predict the change in energy use rate for residential buildings for changes in window design, ceiling and insulation, air exchange rate, and internal sources of heat from cooking. The model assumes a building location in either Austin or Chicago; the Chicago structure includes a basement that doubles the residence volume compared to the Austin structure.

The model includes conductive heat exchange through ceiling, walls, and floor between the building interior and outside environment through a series of thermal resistances caused by materials, and also in parallel through wall support structures (studs, for example). The conduction component of the model is set up to calculate the maximum heating and cooling loads that a structure must be designed to accommodate, and so the temperature differences in the heat load calculation use the maximum and minimum outside temperatures as opposed to the annual average value. Both sensible and latent heat (differences in air humidity) effects are taken into account. The effects of outside and inside wall convective resistances are also taken into account in the conduction component. Air exchange between the inside and outside of the residence is taken into account, factoring in the heat capacity of the air, rate of air exchange, and temperature difference. Internal sources of heat that can modify building energy loads from people and cooking activities are also taken into account in the model. A detailed description of all energy balance mechanisms, including equations, parameters, and input data, is available on the Web site and should be reviewed prior to using the model. Figure 6-11 shows the model interface and input fields for choices to be made in the model. Results for heating and cooling loads are also displayed, reflecting the choices made to model inputs in the parameter fields.

Figure 6-11. Building Mass and Energy Balance model interface (Corsi, 2011)

The effects of changing the air exchange rate on building energy loads are displayed in Table 6-9 for the Austin location. Building cooling loads are larger than heating loads for this location, and changes in these loads are strongly affected by air changes per hour (ACH, the air flow into/out of the building divided by the volume of the building). ACH increases cooling loads more strongly than heating loads, mostly because of the large effect of latent heat, which in this model applies only to cooling loads, as described on the model Web site.

Table 6-9. Energy Load Changes from Changes in Air Exchange in Austin, TX (kBTU = 10³ BTU)

Impacts of Building Materials

As this case study has illustrated, building energy use and the materials used in constructing buildings can have significant environmental footprints. Materials are also used in the routine operation of buildings, and this material use can have impacts as well. Refrigerants are an example of these types of impacts. Refrigerants used in building cooling can be potent greenhouse gases, and their leakage can be a factor in the overall environmental performance of a building.

The LEED framework allows credit points for selecting environmentally preferable refrigerants for air conditioning, refrigeration, and ventilation that eliminate emission of compounds that contribute to ozone depletion and climate change. The base building equipment must comply with the following formula which sets maximum thresholds for combined ozone depletion and global warming potentials (USGBC, 2009):

Sustainable Buildings Sites

Up to 10 points out of 100 possible points for LEED Retail buildings is allowed for encouraging alternative transportation by building occupants. This touches on sustainability by potentially affecting personal behaviors, shifting from use of automobiles to higher rates of use of public transportation. Whereas most LEED credits involve the direct effects of building construction and operation on environmental impacts, this component of the LEED program emphasizes the importance of a building’s indirect effects on energy and the environment. Points are allotted for proximity of the building’s main entrance to commuter rail, light rail, or subway stations, to two or more bus stops, for providing for bicycle parking and for rider change/shower rooms, for preferred parking for low-emitting and fuel-efficient vehicles, for public transportation and bicycle subsidies, and for transportation education programs. The importance of transportation for building occupants can be estimated from national statistics for household passenger vehicle use and the knowledge that most people work in a building of some sort. In 2006, passenger cars consumed 75 billion gallons of fuel out of a total consumption for all highway vehicles of 175 billion gallons (U.S. DOT, 2011). In a recent household transportation survey, approximately 25% of household vehicle miles traveled were to and from work (U.S. DOT, 2009).

6.4.4. Conclusions on Sustainability of Buildings

This case study highlighted the importance of residential and commercial buildings in energy consumption and materials use on a national scale in the United States. This materials and energy use causes direct and indirect impacts on the environment through emission of pollutants that have global, regional, and local consequences. The design of individual buildings and the manner in which buildings are integrated into other societal infrastructures such as transportation have significant effects on economic, environmental, and societal impacts. In response to the importance of buildings to economic viability and environmental performance on a national scale, a rating system for building construction and operation from the U.S. Green Buildings Council was developed (LEED). This case study explored some of the features of this building rating system with respect to lighting, building energy management, use of materials for refrigeration, and indirect effects of buildings on transportation fuel use by building occupants. Buildings are certainly very important for sustainable engineering because of the influence of people’s values and attitudes toward their design and operation, which may span a large range from energy and resource intensive to very low-impact structures. Engineers of all types will continue to develop sustainable technologies and analysis tools for improving the economic, environmental, and societal performance of residential and commercial buildings.

6.5. Additional Case Studies

The case studies presented in this chapter are just a few examples of case studies that could be used to illustrate principles from this text. Additional examples can be drawn from the published literature. Some of these case studies are formatted as problems, while others are reports that could serve as the basis for problems. The following list includes examples that have been formatted for use as problems:

• The text Pollution Prevention: Homework and Design Problems for Engineering Curricula, published by the American Institute of Chemical Engineers, contains more than 20 case studies related to life cycles, design of materials, thermodynamics, and transport phenomena (Allen et al., 1992).

• A case study of selecting alternative battery materials for electric vehicles is provided by Allen and Steele (1994).

• A case study examining alternative methods of transporting the materials required for a fleece pullover garment, through the manufacturing process, is provided by Hopkins et al. (1994).

• A case study involving a material flow analysis of silver entering San Francisco Bay is provided by Kimbrough et al. (1995).

• A case study of the selection of alternative solvents is provided by Allen (1997a).

• A case study of the development of a system for tracking corporate environmental performance is provided by Allen (1997b).

More case studies, formatted for classroom use, have been developed by participants in faculty workshops sponsored by the Center for Sustainable Engineering (www.csengin.org). The case studies are freely available, without copyright restrictions. Examples include, but are not limited to, estimating the environmental impacts of concrete; water system design, including the use of reclaimed water; methods for incorporating social dimensions of sustainability; landfill power generation; accounting for environmental costs and benefits in a semiconductor facility; assessing environmental product claims; electric power generation; and recycling systems for vehicles.

Finally, additional examples can be drawn from the Green Engineering Web site of the U.S. EPA (www.epa.gov/oppt/greenengineering) and the links available at that site.

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