Page 10 - Zero Net Energy Case Study Buildings-Volume 1

ZERO NET ENERGY CASE STUDY BUILDINGS, VOL. 1 INTRODUCTION
in the range of 30-35% from the power plant to the building site. In reality, source losses from electric power generation vary widely on a regional and seasonal basis. A “true” multiplier for a power grid with high penetration of renewable energy sources, hydroelectric energy and nuclear energy—generation sources without combustion losses—would be substantially lower (as much as 50% lower) than for a power system dominated by fossil fuel generation.
Despite these regional and seasonal differences in generation mixes, there is nonetheless a legitimate reason for the use of a single “average” source multiplier. In the U. S. Environmental Protection Agency’s EnergyStar® Portfolio Manager® building benchmarking program, a single figure is used nationally so that building performance is evaluated on a normalized basis without penalizing (or advantaging) any given building based on the utility’s generation mix6.
The value of a source energy metric is also referenced by ASHRAE in its Vision 2020 document, albeit for different reasons:
“[Source ZNE] tends to be a better representation of the total energy impact compared to a site definition. It is challenged, however, by difficulties in acquiring site-to-source conversions and by the limitations of these conversions.” 7
It should likewise be noted that the source energy metric is a somewhat better proxy for approxi- mating reductions in grid-related greenhouse gas (GHG) emissions than the site energy metric8.
In terms of practical impacts, the use of a source metric (with its source multiplier) penalizes inef- ficient uses of electricity (e.g., electric resistance heat and incandescent lighting) and places a premium on highly efficient electric end uses (e.g., high performance heat pumps and LED light- ing). There is also a source multiplier for on-site use of fossil fuels, but it is typically much small than for electricity. (ASHRAE9 cites a source multiplier of 1.1 for natural gas).
Note that Source ZNE is a calculated metric based on “source kBtus in-and-out” and by ap- plication of a chosen source multiplier. As such, the impact for the fossil fuel source multiplier (approximately 1.1) is much smaller than for electricity (approximately 3.0). Irrespective of how source multipliers are obtained and applied, it is clear that use of a source metric requires a cal- culation, in contrast to a site metric, which can be measured directly.
Finally, if Source ZNE is used as a metric over the long term, changes in the electric power generation mix–not just regionally but also over time–would force changes in the source multipli- ers. If it is used as a building performance metric, these changes will in turn create changes in building design.
nrel.gov/docs/fy07osti/38617.pdf.
6 For a discussion of the purpose of determining a national average for the source multiplier, see
EnergyStar® Portfolio Manager® Technical Reference: “Source Energy”, https://portfoliomanager.
energystar.gov/pdf/reference/Source%20Energy.pdf?325e-437c. The EPA has determined that
source energy is the most equitable unit of evaluation of building energy efficiency for its EnergyS-
tar® score: http://www.energystar.gov/buildings/facility-owners-and-managers/existing-buildings/use-
portfolio-manager/understand-metrics/difference.
7 “ASHRAE Vision 2020 / Producing Net Zero Energy Buildings”, American Society of Heating,
Refrigerating and Air-Conditioning Engineers, January 2008.
The document nonetheless goes on to point out that “site energy measurements” are used to define
Net Zero Energy Buildings (NZEB) in Vision 2020 “through an agreement of understanding between
ASHRAE, The American Institute of Architects (AIA), the U. S. Green Building Council (USGBC) and
the Illuminating Engineering Society of North America (IESNA)”, precisely because of the complica-
tions involved in using other methods including source energy.
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M. Deru and P. Torcellini, “Source Energy and Emission Factors for Energy Use in Buildings”.
“ASHRAE Vision 2020 / Producing Net Zero Energy Buildings”.