Page 13 - Zero Net Energy Case Study Buildings-Volume 1

As noted in the Source ZNE discussion, accurate source energy multipliers vary regionally and over time. In a similar manner, the hourly economic multipliers of TDV as applied to site energy change over time, reflecting changes to the time and duration of system peaks and to changes to the load shape of the utility grid. Although the changes from code cycle to code cycle have been fairly small to date, the cumulative impact of changes over several code cycles going forward could be substantial, in turn creating changes to the most appropriate and cost-effective building design solutions.
Summary Comparison of the Three Principal ZNE Metrics
The table below summarizes the principal characteristics of the three ZNE metrics discussed in this Introduction and used in evaluation of building energy performance and ZNE. It is interest- ing to note that the size of an on-site solar photovoltaic system required to “zero out” the energy demand over the course of year will be different depending on the energy-use metric used.
Table: Comparison of Metrics Used in Evaluation of Building Energy Performance and ZNE
INTRODUCTION ZERO NET ENERGY CASE STUDY BUILDINGS, VOL. 1
Energy Use Metric
Energy Accounting
Current Use
Solar PV System Sizing
Benefits
Issues
Site Energy
Total energy con- sumed at building site
Common metric for building designers and most ZNE build- ing measurement methodologies
Largest
• Easily understood by both public and professionals
• Can be directly metered
• No multiplier–sta- ble over time
• Fails to account for grid losses
• Unless coupled with man- datory efficiency measures, can lead to large, costly PV systems
Source Energy
Total energy con- sumed at building site multiplied by “source multiplier” to account for utility grid losses
Certain building rating methodolo- gies such as Energy Star®
Mid
• Easily understood by professionals
• Accounts for source losses
• Used nationally and internationally
• Not easily understood by the public
• Not directly linked to meter
• “Source multiplier” highly
variable by region and over time–variability may impact building design solutions
TDV Energy (Code, California)
Total energy con- sumed at building site multiplied by “economic multiplier” to account for vari- able costs of grid operation
Energy code metric (currently used in California)
Smallest
• Accounts for societal costs as- sociated with peak demand on utility grid
• Smaller PV systems reduce overall building cost
• Not easily understood
• Not directly linked to meter
• Code only, not used outside
California.
• TDV hourly multipliers vary
over time–variability may impact building design solu- tions.
Zero Net Energy Case Study Buildings: Volume 1 xi