The Department of Defense (DoD) has nearly 2 billion square feet (ft2) of building space under roof that require energy for heating, i.e., space heat, water heat, and equipment heat. The two largest recurring expenses for DoD facility operations are annual heating energy bills [$2.4 billion (2006) out of a $4 billion total energy bill] and infrequent, but expensive, re-roofing of buildings (about $640 million per year).
To reduce the impact of energy used to heat buildings and reduce roofing expenses, American Solar, Inc. (ASI) demonstrated the benefits of using a solar air heating metal roof over continued repair and replacement of built up roofs (BURs) at the Gaffney Fitness Center, located at Fort Meade, Maryland. The system uses conventional metal roofing in a traditional, code-approved manner to provide a long life roof as well as a solar air heating collector. The system’s dual use as both a weather barrier and a solar thermal collector reduces the cost of collecting solar energy for heating.
With the solar air heating metal roof system, the metal roof panels heat to approximately 80 degrees Fahrenheit above ambient temperatures during daylight hours. The air directly below the metal panel is then heated to a temperature close to the panel temperature. With the application of conventional fans and ducts, the solar heated air is pulled from beneath the metal roof and delivered to the building heating and plumbing systems.
This demonstration project:
- Re-roofed a badly worn and patched BUR with a long life metal roof
- Provided insulation to keep the old, covered roof warmer in winter and cooler in summer
- outdoor air preheating of ventilation air during the heating season
- direct space heating of the gym during the heating season
- domestic water preheating for showers and sinks on a year round basis.
Temperature sensors were installed within and around the new solar roof to measure air and water temperatures in order to validate the solar heating performance. Over 400,000 temperature readings were taken; one set of readings every 15 minutes, from late June 2012 through early January 2013, along with indicators of fan and pump run times. Local weather stations with time stamped solar insolation, wind speed, and ambient temperature data were used to track environmental conditions.
The solar roof demonstrated the capacity to provide weather protection comparable to other long life metal roofs, to keep the building warmer in winter and cooler in summer, as well as to consistently provide solar heat to the outside air intakes, directly to the building, and to the hot water system. The combined energy and cost savings provide a net positive savings for the Fitness Center compared to the continued installation of a series of several BURs. The solar air metal roof saves over $5000 annually and $189,000 over the 30-year life of the 9275 ft2 roof. In the best case, the cost savings are enough to pay for the re-roofing, which cannot be said for any other non-solar roof. A “first of its kind” analytical performance model was developed that enables the prediction of solar roof temperatures and heat energy flows from solar heated air and water. This model can be applied to other buildings using local solar and weather data.
The solar air heating metal roof involves a standard metal re-roof over an existing worn out roof. The HVAC and plumbing systems are also standard systems using conventional materials and design approaches. There is no special permitting required beyond the standard design review required for any re-roof or HVAC or plumbing modification.
One of the challenges in installing solar air heating roofs and re-roofs is that few of the public works designers, roofing managers, or the architects and engineers they hire are familiar with the solar roofing and re-roofing approach. Often an inquiry about solar re-roofing occurs after the building has been designed and initial cost estimates and budgets are in place and detail design is underway. It is important to inject the solar roof or re-roof approach during the early planning stages. This will increase the likelihood that the full energy savings available from the solar roof can be captured over its lifetime.