Conversations on ZNE

The June 2017 issue of the ASHRAE Journal includes a forum with myself and other leaders of the Zero Net Energy (ZNE) movement. The following are some of my contributions. See the article for perspectives from Paul Torcellini, Smita Gupta, Jon McHugh, Mike Rosenberg and others.

Q1: What is a Zero Net Energy Building?

The United States DOE came up with a common definition of a ZNE, with input from several ASHRAE members. For ZNE buildings “the sum of all energy that is delivered to the property line must be less than the energy that is exported from the property”. All energy is included in the accounting, but energy used to charge electric vehicles driven off-site is considered “exported energy”. DOE recommends using source energy as the currency for adding up energy from multiple fuels as it considers upstream impacts of electricity generation.

While the common boundary for ZNE accounting is the property line, the DOE “Common Definition” recognizes that the boundary can be extended to portfolios of buildings (same owner, but different sites) and campuses (a contiguous group of buildings) and communities. 

Q5: A decade ago California set ambitious goals around zero net energy, i.e., all newly constructed residential buildings to be ZNE by the year 2020 and commercial buildings by 2030. Can you provide an overview of the progress, how we arrive there and some lessons learned?

The California Energy Commission has the goal to require that all low-rise residential buildings be zero net-energy by 2020, but at the same time its enabling legislation (the Warren Alquist Act) requires that its building standards “be cost-effective when taken in their entirety and when amortized over the economic life of the structure compared with historic practice”.  The challenge is to meet the ZNE goal while passing the cost-effectiveness test required by the legislature.

The CEC has always taken a societal perspective with regard to cost-effectiveness, showing that the standards are cost-effective for the people of California as opposed to individual investors. As result, California uses a type of real-time pricing in its cost effectiveness analysis called Time Dependent Valuation (TDV). When TDV was first implemented it gave more credit to renewable energy production since it aligned with higher real-time prices on hot summer afternoons, however, as California has brought more renewable energy from solar onto the grid, the TDV peak has shifted from the afternoon to the early evening, which has had a negative impact on the cost-effectiveness of solar. In addition, the CEC has always encouraged the use of gas for space heating and water heating in homes and achieving ZNE requires that homes be a net exporter of electricity. Under the current tariff structures, the excess energy is credited at a lower rate, further eroding the cost effectiveness of ZNE.

With these challenges, the current plan is for the 2020 California energy standards to require enough PV to offset the electricity in a gas home, but not enough to achieve ZNE. For all electric homes, the quantity of PV required would be the same is if the home had gas space and water heating. 

Q7: Improving building energy efficiency and deploying PVs were identified as the primary tools to achieve the ZNE goals a decade ago.  Given the advance of technologies and better understanding of individual PV’s impact with grid harmonization, should we rethink our strategy?

I believe that ZNE is still a goal we should pursue in all our buildings. It’s something that each of us as engineers and architects can pursue to address the staggering environmental problems we face. We can pursue this goal no matter who is running the government or the budget allocation to EPA and DOE. However, ZNE by itself is not enough; our ZNE buildings need to place a minimal burden on the grid and the grid needs to be modernized to accommodate a growing number of ZNE buildings. The goal of “grid harmonization” presents opportunities on both sides of the meter. Buildings should implement renewable energy only after the maximum energy efficiency is achieved and incorporate controls and storage. Utilities should increase interconnectivity, provide the right price signals, incorporate storage and treat each customer as both a supplier and consumer of electricity.