Homes that don’t consume
The curious phenomena of zero-energy homes
Zero-energy home (ZEH) and zero-energy building (ZEB) are catchy terms frequently used in sustainable-building literature. They refer to a home or commercial building with a net energy consumption of zero during a typical year. Like most homes, a ZEH uses power from the utility grid. What’s different about a ZEH is that it puts electricity back into the grid from on-site renewable sources such as solar or wind energy. The required amount of electricity generated on-site to achieve net-zero energy depends on the ZEH definition.
There are several definitions (based on cost, site energy, source energy or carbon emissions) and understanding the differences can get pretty complicated. In my view, the best definition is one that strongly encourages energy efficiency, is easy to understand, and is verifiable. A definition based on site energy (the energy used at the home rather than what is used at the source, e.g., a power plant) fits these requirements and will also contribute to reducing both carbon emissions and a dependence on fossil fuels.
In addition, it will provide a measure of energy security against future energy crises. With this definition, a ZEH must generate electricity in an amount that equals the combined electrical and gas (or other fossil fuel) energy used for all heating, cooling, lighting, appliance and electronic needs on an annual basis. In California, solar photovoltaic (PV) systems are clearly the most popular method of on-site electricity generation.
Designing an affordable ZEH requires a holistic, integrated approach and a fair bit of building-science expertise. (If cost were not a factor, any home could achieve zero-energy by simply installing a huge solar PV array on the roof!) Fortunately, there is a plethora of information available to guide homeowners or designers interested in forming a practical zero-energy strategy. The National Association of Home Builders (NAHB) Research Center has excellent technical resources at its Web site, www.toolbase.org, including the following steps with many how-to links:
• Decrease space-heating, cooling, and water-heating energy requirements (with a highly insulating and well-sealed building envelope).
• Increase the efficiency of the furnace and air-conditioner.
• Install a solar hot-water pre-heat system, efficient backup water heater, and efficient distribution system.
• Install efficient lighting fixtures (fluorescents) and appliances (such as Energy Star).
• Install a properly sized PV system.
• Turn off lights, computers and appliances when not in use.
Pricey endeavor Attaining net zero-energy isn’t cheap, especially if it requires remodeling an older home. Even so, the ZEH model is very useful because it provides a goal and strategy for reducing energy end-use and moving toward renewable-energy resources. The U.S. Dept of Energy’s Building America Program is an industry-driven research program with a goal of achieving affordable ZEHs by 2020. Check out www.buildingamerica.gov for more resources.