AEC - Architectural Energy Corporation

Net-zero is a hot topic in our industry and for good reason — it is presently the pinnacle of energy efficient design.  It is also a target, just a decade or two hence, for a number of building energy efficiency policies and programs around the country.  While some buildings are being constructed right now with a net-zero energy design, they are still the rare example.  For the majority of buildings that are not net-zero energy, how close are they to that target?  And as for the energy codes that are moving to a net-zero goal, how can they plot a clear path to that target? 

Up until now, efforts to compare the energy efficiency of buildings have almost always pointed back to  our nation’s energy codes. Here is the catch: there have been at least six major commercial energy codes on the books at any given time in the United States since 2000. Comparing the energy efficiency of buildings by referencing their “percent savings beyond code” can often create more confusion than clarity. “Which code?” “What year?” ASHRAE 90.1-2007, Title 24 2005, IECC 2003, Title 24 2008 . . . and the list goes on. As an answer to this problem, AEC has developed the Zero Energy Performance Index (zEPI) to bring clarity to how commercial building efficiency levels are measured and compared.

How the zEPI works

The zEPI is an absolute scale with two key points:

• A building with modeled energy use equal to that of an average building in the year 2000 receives a score of “100”. 
• A modeled building with net-zero energy use receives a score of “0”.

The zEPI scale extends in a linear fashion between, above, and below those two points. Therefore, a building that uses twice as much energy as an average building receives a score of “200”.  A building that uses half as much energy as an average building receives a score of “50”. The simple relationship between zEPI scores can be readily understood by code makers, architects and engineers as well as non-technical building owners and tenants. The zEPI scale can also measure buildings that go beyond net-zero and produce more energy than they consume; these buildings receive negative scores.

Benefits of moving to the zEPI

Charting the course to net-zero

By setting an absolute scale, the zEPI can help code making bodies chart a clear course to net-zero energy goals.  If an average building under a current energy code receives a zEPI score of “48”, and the code setting body would like to move their code to net-zero energy in twelve years, then the performance target for each of the next four code cycles could be “36”, “24”, “12”, and “0” on the zEPI scale to gradually move towards that goal.  Energy codes will, of necessity, include a renewable energy component at those lower zEPI target levels.

Comparing existing buildings’ energy efficiency

The zEPI scale can be used to evaluate the energy efficiency of existing buildings.  The information needed to assess the zEPI score of an existing building is the same as needed for a new building, only it will often be pulled from measurements and observations of the actual structure rather than from building plans.  zEPI ratings could be used for marketing purposes – to give a buyer a sense of comparative efficiency levels between buildings – or as a metric for energy efficiency incentive programs.   Use of the zEPI is currently being considered by a number of prominent commercial building efficiency rating programs, filling a role similar to that served by HERS programs for residential buildings.

Simplifying the Development of Energy Modeling Tools

By setting a fixed, universal baseline under zEPI — the average attributes of America’s building stock as of 2000 — energy modeling tools and energy modeling protocols will not need to be continually reengineered to adapt to each code revision.  This will reduce modeling tool development costs while speeding up code implementation times.

The Next Step

AEC is working with a number of partners across the country to bring zEPI to the building design and efficiency evaluation market.  In parallel AEC is developing the COMNET Modeling Guidelines and Procedures that will standardize the energy modeling process for a variety of energy modeling purposes, whether it is code compliance or federal tax credits.  COMNET will set forth the technical protocols for calculating a zEPI score.