Page 12 - Zero Net Energy Case Study Buildings-Volume 1

ZERO NET ENERGY CASE STUDY BUILDINGS, VOL. 1 INTRODUCTION
Concept diagram: Time-Dependent Valuation (TDV) of energy saved through building design mea- sures compared to flat energy-quantity valuation of pre-2005 California energy codes. (Courtesy TRC Energy Services--formerly Heschong Mahone Group).
In the 2013 “Integrated Energy Policy Report” or “IEPR”, the CEC includes the following defini- tion of ZNE within the context of the energy code:
“A Zero-Net-Energy Code Building is one where the net amount of energy produced by on-site renewable energy resources is equal to the value of the energy consumed an- nually by the building, at the level of a single “project” seeking development entitlements and building code permits, measured using the California Energy Commission’s Time Dependent Valuation metric. A zero-net-energy code building meets an energy use in- tensity value designated in the Building Energy Efficiency Standards by building type and climate zone that reflect best practices for highly efficient buildings.” 14
Simply stated, a building in California is ZNE from a building code perspective when the TDV metric for that building calculates out to zero. It is important to note the strong emphasis on en- ergy efficiency within this definition for ZNE: high, “best practices” levels of energy efficiency are an explicit definitional requirement for ZNE in the IEPR—simply zeroing out the metric without high levels of efficiency does not meet the California code definition for ZNE.
As a hybrid metric involving both energy and cost, TDV units are expressed as a “hybrid unit”: “$–kBtu/sq.ft. per year”. (See footnote 11 on the page ix.)
The TDV metric strongly influences the size of the on-site renewable energy system required to achieve TDV=0 over the course of a year. Due to the high TDV values attributed to on-peak energy, the TDV metric requires the smallest solar photovoltaic (PV) system of the three energy metrics under discussion in order to “get to zero”. This is the result of the fact that the building’s PV system will generally produce the largest amount of electric energy during summer months during hours that correspond closely to the time of peak demand on the grid15.
14 “Integrated Energy Policy Report - 2013 IEPR”, California Energy Commission, Publication Num- ber CEC-100-2013-001-CMF, pp 34-41. This section of 2013 IEPR provides an in-depth discussion of ZNE.
15 The correlation is not exact: the air conditioining demand which drives the peak normally lags the time of peak solar production by several hours on any given day.
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Zero Net Energy Case Study Buildings: Volume 1