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

INTRODUCTION ZERO NET ENERGY CASE STUDY HOMES
The Source ZNE metric recognizes that there are large energy losses attributable to the genera- tion 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 perfor- mance 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, in its building code, California uses a hybrid energy metric 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 particular 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.
2. Broadening the Metrics of “Zero Energy Homes”
While Site ZNE is the only practical metric for a residential project to be verified as achieving ZNE performance, there are other “labels” that purport to be indicators that a residential project is ZNE. At the national level, the U.S. Department of Energy maintains a Zero Energy Ready Home (ZERH) program that offers to certify a residential project as “Zero Energy Ready” if its design meets several specific energy-efficiency standards and if it can accommodate a properly sized solar PV system that would theoretically result in ZNE performance2. Through a complex point system that rates all of these design features, a residential building is given a point total on a HERS Index3 scale. When the solar PV system is added at the correct size, the Index drops appropriately to zero and the project can be labeled as a Zero Energy Home.
An example of the HERS Index for a house with a point total of 65 is shown on the following page. Therefore, to be labeled as a Zero Energy Home, the example house would be required to have a solar PV system large enough to register enough points to tabulate a total score of zero.
Presumably, the rating system can require the size of the solar PV system to be adequate to offset the total source energy calculated to be used, but in any event, the rating is done pre- construction and the label of Zero Energy Home is not based on any kind of measured result. In the absence of any actual performance data, the building cannot be considered a verified ZNE project.
2 There are additional non-energy requirements also. Full details at https://www.energy.gov/ eere/buildings/guidelines-participating-doe-zero-energy-ready-home-program
3 The Home Energy Rating System (HERS) Index is a method of rating the energy efficiency of a house based on design features, created by the Residential Energy Services Network (RESNET). https://www.resnet.us/hers-index
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