Zero-Energy Buildings Defined

Aug. 1, 2007

Discover what "zero energy" means and which facilities have the most potential to achieve that status

Imagine a facility that generates enough energy to meet all its own needs: This is the philosophy behind zero-energy buildings (ZEBs). The concept isn't new, but the implementation is. Zero-energy homes are more the norm than zero-energy commercial buildings, but that may soon be changing. Companies such as San Jose, CA-based Integrated Design Associates Inc. (which is building the Z2 Design Facility highlighted in Building a Zero-Energy Commercial Office) are striving for net zero energy and zero carbon emissions.

As the idea gains traction, researchers work to develop innovative technologies that can make ZEBs a widespread possibility for all future new construction projects. Despite the fact that the application is somewhat limited today, the quest for zero energy remains one of the most dramatic means for reducing greenhouse-gas emissions attributed to commercial buildings.

While the term "zero-energy building" has many definitions, it is most often defined as a building that produces as much energy on-site as it consumes on an annual basis. They are actually referred to as net-zero-energy buildings because they do use energy; however, the supply from on-site generation is equal to (or greater than) the facility's demand.

How to Achieve Net Zero Energy
A ZEB is only possible if three things happen: 1) the goal is set early and the project team makes integrated decisions, 2) energy consumption is cut dramatically, and 3) an investment in on-site power generation is made.

1. INTEGRATE DECISIONS EARLY. All members of the project team (especially the architect and mechanical and electrical engineers) must understand the impact of their decisions and design the building and its systems together. "An integrated approach - rather than just addressing systems or subsystems in isolation - is essential," explains Philip Haves, leader of the commercial building systems group at Berkeley, CA-based Lawrence Berkeley National Laboratory.

David Kaneda, principal of Integrated Design Associates (IDeAs) Inc. and owner of the Z2 Design Facility (a ZEB project scheduled for completion this month) agrees: "It's not quite ‘design as usual.' Having a team that knows how to work together and [individuals who] understand how what they do affects what the rest of the team does is important." In many cases, trade-offs will be made. Every decision and how it will affect energy consumption must be considered. "The details are really important," says Paul Torcellini, team leader for commercial buildings research, National Renewable Energy Laboratory, Golden, CO, about examining everything from daylighting to what kind of transformers are being used.

2. INCREASE ENERGY EFFICIENCY. Building a ZEB without drastically reducing energy consumption is impossible. Torcellini recommends shooting for 20,000 to 25,000 BTUs per square foot per year, or roughly 50- to 70-percent more efficient than the ASHRAE 90.1-2004 standard. "The national average [for commercial buildings] today is somewhere between 70,000 and 80,000 BTUs per square foot," he says. Look at the systems and equipment that consume the most energy first - this is where the biggest efficiencies can be reaped. "The three big energy consumers in a building are the lights, the air-conditioning, and the equipment that's plugged into the walls," says Kaneda.

Daylighting is the No. 1 energy-saving strategy for commercial buildings, according to Torcellini. "Approximately 80 percent of the commercial floor area in this country is within 15 feet of an exterior surface (i.e. the roof, windows, or walls). There is a huge potential to offset lighting loads with daylighting," he explains. Additional energy-saving strategies include natural or mixed-mode ventilation, radiant heating/cooling, evaporative cooling, use of a dedicated outside air system (i.e. separate ventilation and thermal conditioning), ground source heat pumps, and passive solar strategies.

3. INVEST IN ON-SITE POWER GENERATION. According to the National Renewable Energy Laboratory, one of the easiest supply-side technologies to implement is a rooftop photovoltaic (PV) system. While the cost of PVs is still relatively high, available tax credits, financial incentives, and attractive loan terms can help with the initial expense. According to an article published on GreenBiz.com, when a family-owned Pepsi-Cola business in Klamath Falls, OR, pursued a goal of net zero energy in 2004, it received more than $1.6 million dollars from incentives, tax credits, and a loan to help cover the expense of its new solar electric system.

The Ideal ZEB
Not all facilities can be ZEBs. According to the U.S. Department of Energy (DOE) and the National Renewable Energy Laboratory (NREL) in a June 2006 conference paper titled Assessment of the Technical Potential for Achieving Zero-Energy Commercial Buildings, 22 percent of buildings today have the potential to be ZEBs. Through advancements in technology, an estimated 64 percent of buildings could be ZEBs by 2025.

New construction projects offer the greatest opportunity to achieve zero energy. "In a new building, you have a lot more opportunity to think about how the building systems interact. If you're doing a retrofit of an existing building, it's often difficult to put more insulation in the walls or on the roof," explains Drury B. Crawley, team leader, commercial buildings R&D, Office of Building Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C. The new construction plans for a ZEB can incorporate passive solar design and a building orientation conducive to north- or south-facing windows. "Most energy savings come at little or no additional cost from the initial design of the building," says Torcellini.

 Low-rise buildings are ideal ZEBs. "There's more opportunity to use daylighting through skylights or other openings in the roof," Crawley explains. Additionally, enough PVs can be placed on the building's roof to serve the facility's energy needs. "After more than 3 or 4 stories, there isn't enough roof area to place sufficient photovoltaics because your energy densities compared with the footprint of the building are so high," says Torcellini. Warehouses and standalone retail stores have wonderful ZEB potential for this reason. "If the mandate was to make all buildings as low energy as possible and to cover half the roof with photovoltaics, warehouses would be zero energy, but medium- to high-rise offices would never make it," he concludes.

Climate is also a factor in the feasibility of ZEBs. For example, installing a daylighting system may not be as cost effective in West Virginia (where only 40 percent of the days per year are sunny) as it would be in Arizona (where the sun is out 90 percent of the days per year). Natural ventilation is another strategy with widely varying benefits. "It can make a significant dent in your HVAC loads, but it is very climate specific," says Haves. "It's easy in San Francisco, but needs to be supplemented by mechanical cooling for a significant fraction of the year in Houston." Using the climate zones designated in ASHRAE 90.1-2004, the DOE/NREL conference paper reports that the best prospects for ZEBs are buildings located in zones 1, 2, and 3 (see the map below).

The Future of ZEBs
Commercial buildings' contribution to the problem of global warming is increasing. "The amount of commercial floor space that we are adding is growing faster than the [implementation of] energy-efficiency features that we are putting into buildings. The consequences of that are more resource depletion and emissions, and strain on the infrastructure," says Torcellini. Increasing energy efficiency and encouraging the production of on-site power generation will significantly reduce the negative impact that commercial buildings have on the environment.

Low- and zero-energy buildings are healthier, more comfortable, and pollute less. They are also less expensive to operate. Striving for the ultimate in energy efficiency may also give the building a competitive marketing or leasing advantage. "If building owners/developers want to keep up with their competition, they need to stay on top of energy-saving strategies. Energy costs in buildings have already doubled over the last 10 years, and, in most places in the country, they are going to continue to rise. Energy efficiency is becoming more and more of a factor when tenants are looking for space. They want to know what the energy bill is going to be," says Peter Rumsey, principal, Rumsey Engineers Inc., Oakland, CA.

Jana J. Madsen ([email protected]) is managing editor at Buildings magazine.

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