What is a Zero Net Energy Building, Campus, District or City?
Zero Net Energy buildings (ZNEB) are a new green building trend. We are likely to see more buildings achieving ZNEB status each year, first by the dozens and soon by the hundreds, as the cost for onsite solar power continues to decrease and as more building owners see the benefit of showing their commitment to reducing carbon emissions to zero.
How many buildings could become zero net energy? Estimates range widely, from very few, to as many as 70 percent, depending on the definition of “zero net energy.” Obviously, many high-rise buildings would have a problem achieving zero net energy results, since the available collection area for solar power systems is small relative to energy demand.
Defining Zero Net Energy
In 2007, the U.S. National Renewable Energy Laboratory developed the first widely accepted definition for categorizing a zero net energy building (ZNEB):
A. A building footprint renewables ZNEB. This most restrictive definition requires all renewable energy systems, usually solar, to be sited within the building footprint, mostly on a building’s roof.
B. A building site renewables ZNEB. All renewable systems, particularly solar, are located anywhere on a building site, but not necessarily within the building footprint. (On a college campus, it might be worthwhile to include solar energy generated on top of adjacent buildings, as long as the system(s) are part of the “project.”)
C. An imported renewables ZNEB. This could include biomass-fueled boilers using renewable fuels such as wood pellets, ethanol, biodiesel, etc., that are imported onto the site.
D. An off-site purchased renewables ZNEB. A project commits to purchase enough renewable energy, from an audited supplier, to equal its annual onsite energy use.
In 2015, the U.S. Department of Energy released an updated consensus definition for a zero net energy building as: “An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable [exported] energy.” This definition can also be expanded to include energy use for a multi-building campus (e.g., corporate, government, school or university), multi-location building portfolio, or even an entire urban district or city.
Zero Net Energy for New Buildings
Of course, ZNEB’s require a building to be designed and operated as efficiently as possible, before adding the renewables contribution. For new buildings, in my book The World’s Greenest Buildings, I documented that a feasible target for energy consumption, prior to deducting the contribution from onsite renewables, would be about 30 BTU/sq.ft./year (less than 95 kWh/sq.m./year). Most well designed and operated new office (and similar) buildings can easily meet this target without significant construction cost increases, using principles of integrated design.
Zero Net Energy Retrofits
Retrofitting buildings to be zero net energy might be easier than you think. One project I like is in Singapore, with the Zero Energy Building (ZEB), located on the Building and Construction Academy’s campus. Imagine trying to take a three-story conventional classroom/office building and making it zero net energy — that’s exactly what Singapore’s government did, completing the $8 million retrofit in 2009.
Now imagine trying to do this in a cloudy tropical climate, near the equator, where temperature and relative humidity are on the order of 90F/90% almost daily, and you’ll get a picture how challenging that is. Employing many clever strategies for such important functions as daylighting and natural ventilation, the building consumes only 42 kWh/sq.m./year, about 13 BTU/sq.ft./year. Using a 190-kW rooftop PV system, the ZEB became a net positive energy contributor upon completion of the retrofit in September 2009.
This 50,000-sq.ft. building serves 80 staff and receives about 200 visitors per day. A display in the lobby shows visitors energy production and consumption since the building’s opening day, demonstrating a positive net energy production from PV from the outset.
Retrofitting existing buildings to be zero net energy is a challenge, but one worth undertaking, if we are to come close to meeting a key goal of the 2030 Challenge – by the year 2030, existing buildings as a group should consume only 50% of the energy used by similar buildings in 2003.