When a building produces all the energy it consumes from renewable sources, it is referred to as a “net-zero energy” or a “zero energy” building (NZEB or ZEB). Such facilities typically feature a combination of aggressive efficiency efforts (both passive and active) and a large on-site photovoltaic (PV) solar power generation system.
The architecture of such facilities may also involve solar orientation for daylighting, large triple-glazed windows, and extra building mass to retain solar heat. Together, such options may have to cut total energy use by 75% so that the remaining needs may be provided by on-site renewable sources. In the struggle to cut emissions and foster sustainability, are NZEBs the way to go for new buildings?
Primarily, it is important to clarify what a NZEB is not. A NZEB is not off-the-grid: it uses grid-based power and/or natural gas whenever its on-site systems do not produce enough energy. Their on-site systems later produce a temporary surplus to make up the difference. While many facilities that have solar panels at times produce more kilowatt-hours (kWh) than they are consuming, they are not automatically NZE unless their total annual energy use (including that used for heating, cooling, process loads, etc.) has been countered by their own systems. Some so-called “passive” houses are designed to capture and store solar heat to minimize or avoid the use of other energy sources for heating, but they too are not automatically NZEBs unless they produce on-site electricity. Finally, a building that secures renewable energy from off-site sources by buying “green” power from the grid is also not NZE, regardless of how much clean energy it buys.
Buildings that are significantly more efficient than average, but not NZE, may instead be called “high performing.” Many of them have been chronicled in High Performing Buildings, a free American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) publication is available at www.hpbmagazine.org.
The U.S. Department of Energy (DOE) maintains a database of NZEBs at www.energy.gov/eere/buildings/zero-energy-buildings. Most are new structures and/or relatively small: the database lists none over 25,000 square feet, with most below 10,000 square feet. While most U.S. NZEBs are in California, many others are found across the world.
How realistic is net-zero as a concept for urban areas, or as a general goal for cutting energy use? A study by a DOE lab found that, unless significant ground-based PV panels are in use, NZEBs may be limited to five stories in height. Any taller involves greater total building consumption than may be provided by renewable sources located within the building’s footprint. To reach that height, the entire roof (and perhaps an overhang) must be covered by PV panels to produce power. A geothermal energy system must be deployed to exchange heat with the earth for HVAC service. Daylight dimming control, plus high-efficiency pumps, fans, lighting, and plug loads, are also essential in such facilities. Taller NZEBs require the use of ground-based PV systems such as solar gardens or PV-covered parking areas in addition to those mounted on the building.
To achieve and maintain NZEB status also involves installing, commissioning, and successfully operating layers of controls to minimize energy use. As some NZEBs have found, failure to do so may make NZEB status as tough to catch as the famed White Rabbit.
In a 2009 New York City presentation by a DOE official, five ostensibly NZEBs were reviewed. In all cases, getting them to actually net out their entire energy usage was described as “quite challenging.” While the buildings provided comfortable working conditions, energy bills failed to drop to zero. The problems that almost turned the DOE guy into a Mad Hatter were mostly related to their sophisticated controls: heating and cooling systems sometimes fought each other, and automatic dimming systems did not work consistently (or occasionally not at all). In addition, plug loads which may account for 50% of total NZEB kWh use, remained stubbornly higher than projected. Many NZEBs depend heavily on occupant training and discipline to work as expected, and such behavior was often inconsistent.
All five had been commissioned prior to occupation but, over time and varying climate conditions, settings drifted or simply refused to work as planned. After many contractor callbacks, additional metering and monitoring, and a great deal of hard work, all eventually were close to or achieved net-zero. However, keeping them there involved a sustained presence by an engineering staff who are not typically maintained on-site to manage buildings.
More recent accounts of the types of controls used in NZEBs also questioned how well they work over the long term. In a 2015 article in Lighting Design and Application, published by the Illuminating Engineering Society (IES), a survey of advanced lighting control installations, such as daylight dimming, by a specialist with Southern California Edison found that about 90% of the controls were inoperative “because there was no one onboard skilled enough to maintain them.”
In late 2015, the Continental Automated Buildings Association (CABA) issued a report called, Zero Net Energy Building Controls, and a presentation that reviewed similar problems. While focused on ways to overcome them, the study found that out of the NZEBs reviewed, only half (11 out of 23) actually attained that status. As the Red Queen told Alice, “it takes all the running you can do, to keep in the same place; if you want to get somewhere else, you must run at least twice as fast as that!” As for those building managers who did not meet the net-zero goals, “Off with their heads!”