This project evaluated the design approach and operational performance of two zero energy home (ZEH) units compared to two typically designed (baseline) housing units. Net zero energy homes generate as much electricity as they consume from the grid through a combination of: (1) energy efficient design, (2) energy generation, typically with renewable energy sources, and (3) energy conservation practices by the homeowners. The benefits of ZEHs to the Department of Defense (DoD) are lower energy costs, increased energy security, and decreased pollution from energy production and use.
One net zero energy duplex consisting of two housing units was designed and constructed in the Woodlands subdivision at Fort Campbell, Kentucky, next to a baseline duplex designed and constructed according to the standard housing design in this neighborhood. These housing units were the same size, floor plan, and orientation, and housed families with similar characteristics to minimize differences between the housing units. The ZEH design included high levels of insulation, high performance windows, a ground source heat pump, an energy recovery ventilator, and low flow water fixtures. Renewable energy systems were photovoltaic panels located on the metal roof and solar hot water heating.
Performance monitoring of all four housing units was completed to compare the energy, water, operations, occupant satisfaction, and life-cycle cost of the ZEHs compared to the baseline housing units. A monitoring system was selected, installed, and calibrated to collect data from 50 monitoring points within each home. Performance was monitored for 17 months; one year of data (January 2011 through December 2011) was used for the final data analysis and results. Whole-house energy use of all four housing units was also compared to similar homes at Fort Campbell and national averages.
The ZEH and baseline home occupants were given an orientation prior to moving into the housing units to familiarize them with the unique features of the units and the project. Tips on how to reduce energy use were provided during this orientation. Real-time energy feedback devices were placed in the housing units, and detailed monthly energy reports were provided to the occupants to inform them of opportunities for improvement. Monthly phone calls were held with the occupants to receive and provide feedback, and validate any unusual data observations (e.g., lower energy usage because occupants were on vacation).
Occupant engagement contributed to 15% less energy use in the baseline homes compared to the average home in the Woodlands community. The ZEHs used on average 24% less energy than the baseline units, but did not achieve net zero energy over the study period. The figure below summarizes the monthly energy performance of the typical Woodlands home, the average baseline home, and the ZEHs. The average solar production for the ZEHs is also shown.
The ZEH unit used 51% less water per person than the baseline unit. Both the ZEH units and the baseline units had approximately the same level of emergency maintenance needs, but technologies in the ZEH units required more preventative maintenance than the baseline units. Multiple life-cycle cost (LCC) scenarios were completed, but no scenario was LCC effective for this location for a variety of reasons including low energy costs and interest paid on the capital investment loan.
Key findings and considerations for ZEHs include:
- Feedback devices (real-time or monthly) appear to lower energy use. Occupants found that the real-time feedback devices were effective in helping them use less energy and water.
- Achieving net zero energy may have been possible with more than one year of data to help occupants and maintenance staff better understand and use the systems in the housing units.
- Modeling assumptions may not reflect actual building characteristics and use, which can affect the ability to design for net zero energy.
- Cost-effective ZEHs are difficult to achieve and are most likely to be cost effective in areas where energy costs are high and renewable resources are plentiful (e.g., California).
- Incorporating a more energy-efficient envelope was the least costly design change for this duplex (compared to ground source heat pumps, solar hot water systems, or solar panels).
- Specialized maintenance costs can impact the cost-effectiveness of a project.
Lend Lease, the Fort Campbell family housing property manager, plans to apply lessons learned to the 38,000 homes they manage for DoD and the 145,000 homes they manage worldwide. Lessons learned may also be applicable for other building types such as barracks and offices.