Energy Supply Options for Net-Zero Energy Buildings
Photovoltaic panels at Oberlin College's Lewis Center for Environmental Studies. The panes cover 4,682 square feet on the building's south-facing curved roof. Photo courtesy of NREL/Robb Williamson.
At the heart of the net-zero energy building definitions is the idea that buildings can meet energy requirements from low-cost, locally available, nonpolluting, renewable sources. On this page you will learn about energy supply options and how these rank.
Renewable Energy Is Necessary for Net-Zero Energy Buildings
Much focus is placed on energy efficiency as the most cost-effective way to reduce energy use in commercial buildings. However, consumption can be reduced only so much. There is a point at which the cost of adding efficiency measures is higher than that of using renewable energy such as photovoltaics (PV).
Aggressive energy efficiency strategies can reduce a building's energy consumption by 50% to 70%. Renewable energy technologies must be used to reach the goal of a net-zero energy building (NZEB).
Visit the Net-Zero Energy Buildings Database for examples of these buildings and their detailed energy use.
Supply-Side Technologies
Various supply-side renewable energy technologies are available for NZEBs. Supply-side technologies, often called energy producers, collect natural energy and transform it into a useful form. Examples of these technologies include PV, solar hot water, wind, hydroelectric, and biofuels.
Ranking of Energy Options
All renewable sources are favorable over conventional energy sources such as coal and natural gas; however, the U.S. Department of Energy recommends the following ranking for these options (the lower numbers are preferable):
| Option Number | NZEB Supply-Side Options | Examples |
|---|---|---|
| 0 | Reduce site energy use through low-energy building technologies | Daylighting, high-efficiency heating, ventilation, and air-conditioning equipment (HVAC), natural ventilation, evaporative cooling |
| On-Site Supply Options | ||
| 1 | Use renewable energy sources available within the building's footprint | PV, solar hot water, and wind located on the building |
| 2 | Use renewable energy sources available at the site | PV, solar hot water, low-impact hydroelectric, and wind located on-site, but not on the building |
| Off-Site Supply Options | ||
| 3 | Use renewable energy sources available off site to generate energy on site | Biomass, wood pellets, ethanol, or biodiesel that can be imported from off site; waste streams from on-site processes that can be used on-site to generate electricity and heat |
| 4 | Purchase off-site renewable energy sources | Utility-based wind, PV, emissions credits, or other "green" purchasing options; hydroelectric is sometimes considered |
This hierarchy is weighted toward renewable technologies within the building footprint and site. Rooftop PV and solar water heating are the most applicable supply-side technologies for NZEBs. Other supply-side technologies such as parking lot-based wind or PV systems may be available.
The goal in developing the ranking was to encourage technologies that:
- Minimize overall environmental impact by encouraging energy-efficient building designs and reducing transportation and conversion losses
- Will be available over the lifetime of the building
- Are widely available and have high replication potential for future NZEBs.
For more information, see Zero Energy Buildings: A Critical Look at the Definition (PDF 477 KB). Download Adobe Reader.