LIFE CYCLE ANALYSIS OF A RESIDENTIAL HOME IN MICHIGAN


1.0 INTRODUCTION

1.1 Overview

Annually, 24 % percent of the natural gas, and 35% of the electricity in the US, is consumed by the residential housing sector [10, 11]. As a result, 1.3 million metric tons of green-house gases are emitted annually. This is equal to 31% of the green-house gases emitted from electricity and natural gas consumption by all sectors in the US.

The above figures represent energy consumption and emissions data for residential utility services. Of these, natural gas and grid electricity combine for over 90% of all energy used for space and water heating, lighting, ventilation and appliances in 1990. Coal, fuel oil, wood, and liquefied propane gas account for the remainder. In 1994, CO2 emissions associated with the residential housing sector contributed approximately 19% of total CO2 emissions released by all US sectors combined [12].

What is conventionally not considered in determining residential energy consumption is the energy required to make building materials and home appliances. This is the energy required to extract the raw materials (mining, oil extraction, timber cutting), refine those resources (smelting, refining, cutting), and manufacture ready-to-use construction materials. This last item, for example includes extrusion, molding, punching and assembly of metal, plastic, and other material into usable shapes, as well as combining different materials into composite forms such as windows, doors, pre-assembled panels, floor coverings, electrical and plumbing fixtures, and the many appliances found in modern homes.

Life cycle analysis (LCA) quantifies the environmental impacts caused by the energy and material flows in all stages of a product's life cycle. Some of the impact categories widely used to compare product systems are global warming potential (GWP), ozone depletion, nutrification, acidification, and ground-level ozone creation potential. In LCA research, the product system being investigated is structured into several stages [13]. Conventionally, these are 1) raw material acquisition, 2) parts fabrication, assembly, and construction, 3) use, and 4) retirement (or end-of-life). Life cycle assessment is commonly referred to as a cradle-to-cradle analysis because it looks at all inputs and outflows in a product system over its entire life history. In a full LCA, all inputs (material, energy, water) and outflows (air and water emissions and solid wastes) are accounted for. In this project, only primary energy and the global warming potential (GWP) will be evaluated. Primary energy is the energy that is embodied in resources as they exist in nature: the chemical energy embodied in fossil fuels or biomass, the potential energy of a water reservoir, the electromagnetic energy of solar radiation, and the energy released in nuclear reactions. For the most part, primary energy is not used directly but is first converted and transformed into electricity and fuels such as gasoline, jet fuel, heating oil, or charcoal. This statement applies to raw material extraction, transportation, manufacturing and home energy consumption as well.

While quantification of resource consumption, water emissions and solid waste resulting from material manufacturing and product use are important, the project scope focused on primary energy and GWP, which are two important indicators of the overall environmental impact of home construction and use. Using a similar approach, life cycle costing is used to determine all costs in monetary terms associated with a product. The life cycle costs in this study are all costs borne by the owner. These include all finance costs associated with:

Understanding energy consumption, GWP, and cost from a life cycle perspective is essential if a systematic and comprehensive reduction of environmental impacts is desired. All three are linked. Changes to the energy intensity of building products will change the GWP. Reductions in home energy consumption will reduce utility costs and GWP. Use of building materials with lower embodied energy may or may not affect use phase energy. Accordingly, an inventory of the product's life cycle, identifying mass and energy flows, helps in understanding the complexity of the various interactions.


1.2 Purpose of Study

The goal for undertaking the project was to determine the relationship between material production/construction (pre-use) phase energy, and use phase energy, as energy efficiency strategies are applied to various home systems. It is commonly believed that to achieve higher energy efficiency, more materials are needed in the initial construction. Thicker walls are needed obtain lower thermal conductance properties (i.e., higher R values). More windows of higher quality optimize solar heat gain. Additional internal thermal mass is required to allow for temporary storage of the increased solar heat for release at night. While these energy efficient strategies lower the building's heating fuel requirements, it is not entirely intuitive whether they actually lower total life cycle energy consumption. For example, is the additional energy required to manufacture the glass for more windows recovered with lowered heating requirements?

Other research questions to be addressed in this study include:

1.3 Similar Studies

FOOTNOTES

[10] "Table 8.9 Electric Utility Retail Sales of Electricity by End-Use Sector, 1949-1997", Energy Information Administration, Office of Energy Markets and End Use, U.S. Department of Energy, <http://www.iea.ord.gov/pub/energy.overview/aer/aer0809.txt>, 8/13/1998
[11] "Table 6.6 Natural Gas Consumption by Use Sector, 1949-1997", Energy Information Administration, Office of Energy Markets and End Use, U.S. Department of Energy, <http://www.iea.ord.gov/pub/energy.overview/aer/aer0606.txt>, 8/13/1998
[12] "Annual Energy Review 1994, July 1995", DOE/EIA-0384 (94), Energy Information Administration, Office of Energy Markets and End Use, U.S. Department of Energy, Washington, DC 20585, pg. 337
[13] Life Cycle Assessment: Inventory Guidelines and Principles (EPA 600/R-92/245). Cincinnati, OH: U.S. EPA, Office of Research and Development, Risk Reduction Engineering Laboratory, February 1993



End of section 1.0 Introduction

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0.0 Abstract and Executive Summary

2.0 Methods
3.0 Results
4.0 Conclusions

Last updated October, 9th '98, National Pollution Prevention Center