NREL (National Renewable Energy Laboratory) researchers had two houses built in Florida, one as a control and the other built in compliance with standards of the Building America program. Even though the control home met the high Energy Efficiency standards of new homes (e.g., 30R insulation in the attic), the test house showed significantly better energy efficiency as a result of:
- A high efficiency refrigerator
- High efficiency lighting
- Over-sized interior-mounted ducts
- Greater exterior wall insulation (10R rather than 4R)
- Advanced solar-controlled, double-glazed windows
- A white tile roof with three-foot overhangs.
The test house was a ZEH (Zero Energy Home). ZEHs connect to the Grid and the connection is two-way, i.e., there is a “grid tie”. As previously noted, the integration of on-site power systems with the local distributed network is an essential co-generation component. It also is essential to sustainable energy systems.
A Zero Energy Home consumes energy. It also produces it. The idea is that such homes, by means of intelligent design, achieve “net zero” energy consumption. A grid tie enables an owner to sell any excess energy produced. Often such sustainable homes use more than one renewable energy technology.
The three, principle, renewable energy sources used to achieve “net zero” are 1) solar, and 2) geothermal, with some people using 3) wind. The NREL Center for Buildings and Thermal Systems focuses primarily on solar and geothermal, in addition to the development of advanced energy efficient building materials.
The Building America program focuses on the building site, envelope, mechanical systems, and energy-use factors. This Department of Energy program also emphasizes affordable housing, i.e. energy-saving strategies at no or little extra cost. Building America homes optimize the following, energy efficiency and renewable energy features:
- Climate-specific design
- Passive solar heating and cooling
- Natural day-lighting
- Energy-efficient construction
- Energy-efficient appliances and lighting
- Solar thermal and solar electric systems.
An axiom is that the less energy lost (or the more energy conserved), the greater the initial investment and the longer period required for recovering the cost. Yet some authors are challenging this axiom, as “green building” becomes more mainstream. “The most significant correlates, in our experience,” states Alex Steffen writing for World Changing, “are whether greening — and stakeholder inclusion — are integrated into the project from the very beginning, or whether they’re added in (or slapped on) later.”
Environpundit, whose spouse is a licensed engineer, a LEED designer and also has training in economics, notes that throughout “green building” articles and reports there are several consistent themes:
- “Green building” can cost between 0-2% more than the cost of a “normal” building
- Because of decreased utility costs, the ROI (Return On Investment) can be recouped within a short a period as two years
- To achieve the highest energy savings, energy efficient design coordination should begin at the outset of the design process
- Studies show that in both offices and schools that are “green buildings” there were higher rates of worker productivity and lower rates of absences.
Architects of model “Zero Energy” homes, e.g., the Million Solar Roofs Initiative, often include photovoltaics and solar thermal systems, passive solar design, high efficiency windows and radiant barriers. Such choices are popular among builders of new “green” homes, whether or not they are striving for “net zero”. One chief difference is in the capability of the power electronics. i.e., whether or not the residence has a grid tie. (And, how difficult or easy it is to establish one.) Another important difference is the degree of efficiencies used.
Note: The term “net zero” also is used to describe grid-source, renewable energy on a net-zero pollution basis. In transportation, the equivalent term is ZEV (Zero Emissions Vehicle). Nevertheless, as hard-core treehuggers are quick to tell you, this a “flexible” term. Solar and wind are clean, renewable energy sources in that emissions are absent in the direct production of electricity, but there is environmental impact even with the manufacture of these “green power” systems.
While Zero Energy Homes focus on solar thermal and solar electric systems, micro CHP is another way that a residence might achieve sustainability. NREL also conducts research into two other forms of renewable energy: biomass and hydrogen (fuel cells).