Zero Net Energy: What it is and the Evaluation Challenges It Presents

May 3, 2016

By Isabelle Gecils

Imagine a future where buildings produce as much energy as they consume. Tenants of these buildings cannot only save on energy costs, but also boast how they are reducing their carbon footprint. Welcome to the world of zero net energy (ZNE) buildings.

The latest energy consumption values associated with commercial buildings support the need for a new approach to reduce energy usage. Total electricity consumption in commercial buildings has almost doubled in a little over 30 years. While the industry has focused on energy efficiency—and that has reduced consumption significantly–the fact remains that new types of electronic equipment, and more need to use existing electronic equipment, has led to significant increases in actual usage. Couple this with the greater need for cuts in fossil fuel usage (to address climate change), and many believe that energy efficiency needs a partner.

ZNE buildings seek to marry energy efficiency with renewable energy resources to both reduce energy usage and reduce the carbon footprint associated with a building.

In its simplest definition, a zero net energy building is one that is optimally efficient and, over the course of a year, generates energy onsite by using clean renewable resources in the same proportion to the amount the building consumes.

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The concept of ZNE buildings, and even the definition of ZNE, is still in somewhat of an infancy stage. Defining ZNE is crucial so that implementers, designers and evaluators can develop successful strategies to achieve and measure success. Some of the key questions ZNE advocates are currently wrestling with include:

  • How do you define energy? There are several definitions for ZNE based on how the energy use accounting is done – site energy, source energy, energy cost, carbon emissions or in the case of California, the Time Dependent Valuation (TDV) metric. In addition, one must consider what whether the focus should be on electricity only, or include natural gas and other energy sources.
  • Do renewables need to be onsite? ZNE pioneers assumed that renewables, often PV, would be placed on site. However, some facilities, especially multi-story commercial buildings, have limited space to allow for PV arrays to meet all their energy needs. There is not a consensus yet whether a building that may source renewables from a distant PV plan would qualify for ZNE. There is also a lack of consensus on the list of clean technologies that qualify beyond PV.
  • When does the clock start ticking? Unlike other energy savings and demand reduction targets that heavily rely on modeling and assumptions, ZNE requires an actual confirmation that renewables have met the building’s energy consumption for a given amount of time – typically measured on the first 12 months of occupancy. However, during its first year, the building may not have reached full occupancy. Conversely, a commercial facility may open its doors for after-hours classes that increase load. Anomalous weather years may also influence energy consumption. In that case, can a ZNE building’s goals be accurately verified each year? If not, will evaluators adjust actual data to reflect expected future changes?

The industry is currently debating these questions, and Opinion Dynamics is leading two seminal efforts to provide more context to the discussion:

  • The ZNE State Agency Decision Maker study looks at the challenges local and state governments face to meet ZNE goals. An Executive Order by the governor of California mandates that state agencies shall take measures toward achieving ZNE for 50% of the square footage of existing state-owned buildings by 2025. Despite its focus on existing government facilities, the ability to break down barriers to allow existing buildings to be retrofitted to meet ZNE standards has wide-ranging implications that go beyond government facilities. Our work will provide insights on how decision-makers plan to meet this Executive Order.
  • The CEC ZNE Community Study is one of the first studies that looks at a community of 50 ZNE homes. Opinion Dynamics will assess the trade-offs and rankings homebuyers make in their decision to purchase an energy efficient home, and how they perceive the effectiveness of the ZNE label. Once residents have occupied their homes, we will assess how their behavior affects their ZNE status

We look forward to sharing our results in the future.