Page 123 - Zero Net Energy Case Study Homes-Volume 1

OBSERVATIONS ZERO NET ENERGY CASE STUDY HOMES
Low-Rise Multifamily Residential
The one multifamily housing project utilizes many of the same systems and features of the single-family case study projects. The air-tightness characteristic was recognized as important by the designers, but testing was not possible in this case because of shared walls between units. This design feature needs further study in the case of multifamily residential construction because of its importance in the size of heating/cooling loads.
Multifamily residential projects generally have a common solar PV system, with individual units assigned a share of the energy production according to some formula devised by the managing or regulatory body. If the individual unit is credited with the dollar amount corresponding to this share, then there is an incentive to reduce energy consumption, perhaps even to achieve ZNE at the unit level. Future case studies of market-type multifamily residential projects may demon- strate this.
Modeling and Measurement
As shown in these case studies, for individual single-family houses, energy modeling is usually done only when the builder is motivated to have performance information during the design phase or simply to right-size the solar PV system for ZNE performance. (California’s Title-24 code compliance models the house energy performance in terms of the TDV metric but does not produce actual energy-use results.) In other cases when intended for sale, the builder provides the initial solar PV system based on assumed loads with the capacity to add panels in the future if necessary to achieve ZNE performance. As seen in the case studies in this book, the latter often results in houses or units that achieve ZNE performance and many that do not.
Energy monitoring equipment is usually not installed for cost reasons, but the owner is informed of the monthly energy performance through the utility net meter data contained in the monthly utility statement and the solar PV production data. With these two sources of data information, it is possible to determine if the house is performing at ZNE overall.
The Imminent 2020 California Benchmark Goal of ZNE Design for New Residential Con- struction
The five diverse types of residential construction in these case studies illustrate many of the technical and marketing-related issues pertaining to the goal of ZNE design for all new residen- tial construction starting in 2020, a little more than one year away. Much study and discussion of these issues has occurred during the past decade and the California Energy Commission has arrived at an approach to meeting that goal in principle, which is to be discussed and evaluated further in the interim.
This approach is best described by referring to the discussion on page viii of the Introduction to this book, about the EDR rating of a residential project as prescribed by Title-24 energy stan- dards. This rating on the HERS Index scale will be set at a number (not zero) corresponding to 100%-offset of the electric energy use of a mixed-fuel residential project (gas and electricity). See the illustration on the following page of the EDR rating requirement as described on the HERS Index scale, which will be in effect in the 2020 code. The mixed-fuel residential project is assumed to be approximately 50% electric and 50% natural gas. An all-electric home would be required to meet the same EDR rating.
This 2020 code requirement is, strictly speaking, not a ZNE standard for new residential con- struction. However, local agencies may adopt “reach codes” that require an EDR=0 rating per the HERS Index scale.
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