Page 11 - Zero Net Energy Case Study Homes-Volume 2

INTRODUCTION ZERO NET ENERGY CASE STUDY HOMES
Metrics of ZNE Residential Projects
1. Basic Technical Metrics
Depending on how the accounting of energy use over the course of a year is done, there are three distinct technical definitions of what is meant by a zero net energy or ZNE project currently used in practice: Site ZNE, Source ZNE and TDV ZNE. (“TDV” or “Time-Dependent Valuation” is the definition used in California’s building code.)
As a practical matter, all three definitions have several aspects in common. First, the accepted time frame for ZNE accounting is one calendar year: a project is ZNE if the energy consump- tion equals renewable energy production over a one-year period. Second, the dominant form of renewable energy production selected is solar photovoltaic (PV) energy; there are other sources that would meet the renewables definition, but they are rarely selected as the most cost-effective and practical solution at the building level. Third, “at scale” deployment of ZNE residential projects presumes that the buildings are grid-connected. Though not a definitional requirement, grid-connectivity provides the most practical and cost-effective means of meeting ZNE performance targets.
A Site ZNE building has an on-site renewable energy supply, and 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. It is the one ZNE metric that can be directly metered and measured.
The Source ZNE metric recognizes that there are large energy losses attributable to the gen- eration 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. For individual projects where energy performance is measured and recorded, the absence of the ability to meter the source energy at any particular time makes the use of this metric impractical.
Finally, California uses a hybrid energy metric in its building code known as Time-Dependent Valuation or TDV, with hourly economic multipliers applied to the site energy consumption (and production) as modeled for that building by energy simulation software. This software is used to document energy code compliance at the time of permitting the project. The “economic multipli- ers” can be used by code officials to account for time-of-day costs of energy generation. TDV- ZNE represents a code path to ZNE in California, the future objective for the state energy code for building projects. However, like the Source ZNE metric, TDV-ZNE cannot be determined for a built project because there is no measured “TDV” data—it is only a useful metric before the project is constructed as a code required standard. Even then, TDV-ZNE is not a metric of a par- ticular energy performance; rather, it is a metric of the value of a particular energy performance.
Since the emphasis in ZNE performance is on actual measured data over the course of a par- ticular year, Site Energy is the only practical metric to use. Site ZNE is therefore the criterion used in this book for verifying zero-net-energy performance of residential projects. That is, all of the case study projects have measured site energy data for at least one full year that accounts for all energy use, both electric and gas, with the gas energy use shown to be offset by the on- site renewable energy system.
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