Page 12 - Zero Net Energy Case Study Homes-Volume 2

ZERO NET ENERGY CASE STUDY HOMES INTRODUCTION
2. Other Designations of “ZNE Homes”
See the Introduction to Volume 1 for a thorough description of other “labels” that purport to be indicators that a residential project is ZNE in some form, including the U.S. Department of Energy’s Zero Energy Ready Home (ZERH) program, the State of California’s Energy Design Rating (EDR), and three such categories as defined by the private non-profit organization, Net- Zero Energy Coalition (NZEC). The federal ZERH program is currently developing a multifami- ly-specific rating that “better aligns with the recently developed ENERGY STAR Multifamily New Construction (ESMFNC) program”2.
3. Metric for Annual Energy Use: “Energy Use Intensity (EUI)”
One other metric also deserves reiterating from the Volume 1 discussion: Energy Use Intensity, or EUI, expressed in kBtu/sf per year, is a measure of the annual energy use per unit area. A State-funded technical study3 in 2012 found that ZNE was technically achievable with EUI val- ues varying (by climate zone) from 11.5 to 17.3 for single-family homes, and from 16.0 to 18.6 for low-rise multifamily homes.
Zero-Net-Energy (ZNE) and Decarbonization
The movement toward ZNE buildings is currently based on the need to reduce carbon emis- sions caused by the energy used in buildings, which totals approximately 40% of all energy used in the U.S. and 70% of all electricity generated. Almost all ZNE residential buildings have on-site solar PV systems for their source of renewable energy and are grid-connected for back- up power during periods of low solar energy collection.
As the number of ZNE or partial-ZNE buildings has grown, particularly in the past ten years, this characteristic has created a growing dilemma for the public utility electric grid, called the duck curve challenge4. Basically, when the sun is shining, a large part of California’s electrical demand is met by the on-site rooftop solar PV systems, so the demand on the utility grid is rela- tively low in the middle of the day. But then the demand increases dramatically in the evening as consumers typically rely on the grid during these hours. To meet this surge in electrical power demand, the electric utilities typically bring online fossil-fuel-fired “peaker” power plants, with the unavoidable side effect of the generation of large amounts of carbon emissions by those power plants.
Adding to this extra carbon emission effect caused by the growth of the residential solar PV market is the normal mix of energy sources for the utility electric power grid, composed of both renewable and carbon-based fuels. Because most ZNE houses rely on the grid for their back- up energy supply throughout the day, they are occasionally using carbon-based power from the grid and therefore are not zero-carbon houses, at least not with the current California Power Mix5.
2 For more details, see: https://www.energy.gov/eere/buildings/zero-energy-ready-home-multi- family-program
3 “The Technical Feasibility of Zero Net Energy Buildings in California”, Arup, et al. 2010.
4 For a complete description of this current challenging consequence, see https://www.energy.
gov/eere/articles/confronting-duck-curve-how-address-over-generation-solar-energy
5 The 2018 California Power Mix by energy source was 47% carbon-based fuel, with the remainder provided by carbon-free sources of energy. https://ww2.energy.ca.gov/almanac/ electricity_data/total_system_power.html
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Zero Net Energy Case Study Homes: Volume 2