ZNE in California
ZNE is all the rage, just as LEED was ten years ago! Zero Net Energy (ZNE) is a “big and bold” initiative. All new California homes must be ZNE by 2020; the same is true for all new California commercial buildings by 2030. These are lofty goals set by CPUC in 2007 and codified as State policy in the California Long-Term Energy Efficiency Strategic Plan adopted in 2008. Since buildings are the second largest source of greenhouse gases, these big and bold initiatives are well targeted.
There are many ways to consider a building’s footprint, and the offset of that footprint. First, what about natural gas and propane use? (California does include these in its ZNE equation.) What about embodied energy in building materials? (California does not include that!) And what about transportation energy? Must a ZNE building also offset its power for EVs? (California does not require that.) And where is the renewable generation located? Is it on site? Or is it at the source, for instance a wind farm or community solar array? In California, at least so far, ZNE generation must be on site.
By definition, ZNE buildings must produce as much as energy as they consume over the course of a year. That’s the simple definition. Today, the State of California defines ZNE when, “The societal value of energy consumed by the building over the course of a typical year is less than or equal to the societal value of the on-site renewable energy generated.”
Let’s add a bit more detail, “A ZNE building is one where the value of the energy produced by on-site renewable energy resources is equal to the value of the energy consumed annually by the building measured using the California Energy Commission’s Time Dependent Valuation (TDV) metric.”
Under TDV, energy is valued on an hourly basis that better reflects the actual cost of energy to the customers, to the utility system and to society. TDV multipliers vary for each hour of the year and by energy type, Climate Zone and by building type.
TDV values are calculated separately for the three primary fuels used in buildings – electricity, natural gas and propane. Critics cringe at the use of TDV, noting that a ZNE building can have as little as 60% of its consumption supplied by renewables.
Energy efficiency measures will also be subject to, and potentially benefit from, TDV. The TDV method encourages building designers to design buildings that perform better during periods of high-energy costs… versus offsetting 100% of their consumption. Critics of this methodology note that ZNE homes per 2016 Title 24 part 6, and building on the State’s net energy metering, are really “zero net bill” homes.
Unquestionably, efficiency is key to ZNE, and climate can help: Zero net energy is easiest to achieve in mild coastal areas with light heating and cooling loads such as Santa Monica which passed a ground-breaking, voluntary ZNE reach code in 2016.