Page 9 - Zero Net Energy Case Study Buildings-Volume 1

INTRODUCTION ZERO NET ENERGY CASE STUDY BUILDINGS, VOL. 1
tion at the building level. Third, “at scale” deployment of ZNE buildings presumes that the build- ings are grid-connected: though again not a definitional requirement, grid-connectivity provides the most practical and cost-effective means of meeting ZNE performance targets. Last, all three ZNE metrics typically express the energy units in kBtu. (“TDV” ZNE employs a hybrid version of this energy unit: see the detailed discussion below.)
These different energy accounting methods arise from several factors including differences in
• How the “boundary issues” between the building and the grid are considered;
• How the fuel mix of the building and of the power grid are treated;
• How, and whether or not, the cost impacts associated with the time that energy is used
(diurnally and seasonally) are accounted for.
Site ZNE
A Site ZNE building has an on-site renewable energy supply: the amount of energy used by the building over the course of a year is equal to the amount of energy supplied by the on-site system. For grid-connected buildings, the power drawn from the utility grid equals the power exported to the utility grid. This is known as Site ZNE since the line of transaction is drawn at the building site boundary. The case study buildings in this monograph are designed to meet Site ZNE criteria as individual buildings1. This metric is the simplest metric and the easiest to use in conjunction with metered on-site performance data2. It is the one ZNE metric that can be directly metered and measured.
ZNE rating systems such as the Living Building Challenge3 from the International Living Future Institute (ILFI) use a site-based metric to document actual ZNE performance after the building is occupied. The New Buildings Institute (NBI) Buildings Database4 of high performance buildings is likewise based on the Site ZNE metric as measured for at least one year.
Source ZNE
This second type of ZNE metric recognizes that there are large energy losses attributable to the generation of electric energy at the power plant as well as additional energy losses associated with its transmission and distribution to the building site. Since these losses cannot be avoided for grid-connected buildings, the Source ZNE metric accounts for these losses, attributing them to the building’s energy use. By this definition, the line of energy transaction is no longer at the building site boundary, but extends to include the grid itself so that the energy demand impact of the building on the total system is properly counted. This energy accounting metric is known as “Source” ZNE since the source of the utility’s energy supply is included in the energy-use ac- counting for the building.
A source multiplier is applied to the building’s on-site energy use of electricity drawn from the utility grid to account for the grid system losses, especially combustion losses asociated with fossil-fuel generation. A commonly used multiplier is 3.05, accounting for total system efficiencies
1 With the exception of the Science & Engineering Building and the Classroom/Office Building at the University of California, Merced, which are planned to be part of a campus of buildings that will perform together and on average as ZNE.
2 A simple arithmetic conversion of kWh into kBtus is used, where 1.0 kWh = 3.413 kBtus. Energy use by fossil fuels is added in directly based on the consumption in kBtu. The Site ZNE metric is usually expressed in kBtu per square foot per year, a site “Energy Utilization Index” (EUI).
3 http://living-future.org/lbc
4 http://buildings.newbuildings.org/
5 M. Deru and P. Torcellini, “Source Energy and Emission Factors for Energy Use in Buildings”, National Renewable Energy Laboratory Technical Report NREL/TP-550-38617 (2007). http://www.
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