Improvements in energy end-use efficiency are expected to be achieved through increased use of electronics in a myriad of applications, such as in buildings, vehicles, and improved delivery routing [104-106]. This trend will accelerate as
GHG mitigation strategies put additional emphasis on energy efficiency, but will also accelerate the already-growing problem of waste electronics disposal. Although use of electronics can have significant positive impacts on life-cycle efficiency and GHG reduction, their disposal can create local problems associated with the toxic materials that are used in electronics manufacture [107-109]. New approaches are being developed to reduce the environmental impact of electronics disposal and increase material recycling , but these approaches are not widely used, particularly in developing economies .
Increasing end-use energy efficiency will also provide opportunities for use of new materials, and possible use of those materials on a large scale. A key example of these new materials are nanomaterials, which are being evaluated for use in construction as coatings, concrete and steel additives, components of composite materials, and glass products, among others [111, 112]. Nanomaterials are developed from modification of materials at scales of 100 nm and smaller. The environmental impacts of these materials are largely unknown, with research in this area being in its early stages. Even so, there are indications that exposure to nanomateri-als may cause adverse health impacts [113, 114]. While it is unclear whether such impacts will occur in real-world exposures, concerns have been raised about nano-materials making their way into drinking water, with little information about their potential effects [115, 116]. Although there is, and will be, considerable pressure to move these new materials into practice, the lack of clear understanding of possible impacts has raised concerns about the pace at which these new technologies are being deployed .
End-use efficiency improvements can incorporate more than direct energy use reductions to include reductions in other resources. Reducing material and water reduces the energy and, therefore, the GHG emissions associated with the production and transport of those resources. It should be noted that end-use efficiency is likely to be less effective when broadly implemented in practice than would be indicated by technology-specific efficiency improvements. This is due in part to consumers' response to the reduction in energy prices that occurs as demand falls - the "rebound effect" . It should also be noted that reductions in energy consumption do not always translate directly into the same level of GHG reductions. This is due to the tendency of energy suppliers to respond to drops in demand by reducing use of their highest-cost energy sources, which are often among the lower-carbon sources.
In the following sections, we discuss a few of the approaches for improving energy end-use efficiency in residential and commercial buildings. The particular approaches do not cover the full spectrum of available options, but are representative of some of the potential environmental impacts associated with these types of approaches.
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