Conventional rooftop solar photovoltaic (PV) systems increase roof load and can compromise roof integrity and void the warranty. A building integrated photovoltaic (BIPV) roof can function as a heat reflective roof, provide renewable energy, and potentially cost less than a conventional roof and PV system installed separately.
The objectives of this project were to demonstrate and validate whether BIPV roofs can endure weather conditions as well as conventional roofs, and to verify whether an integrated rooftop PV system can result in an energy efficient roofing system. This project also investigated whether a BIPV roof system is structurally sound, how the system is expected to perform over 20 years under normal operation, and its effectiveness in providing on-site renewable energy generation. Demonstrations involved an existing BIPV roof at Luke Air Force Base (Site I) and new systems at Naval Air Station Patuxent River (Site II) and Marine Corps Air Station Yuma (Site III).
The form of BIPV roof demonstrated in this project utilizes amorphous silicon (a-Si) PV laminates (PVLs) factory adhered to an ENERGY STAR qualified polyvinyl-chloride (PVC) carrier sheet. ENERGY STAR qualified roofing products are more reflective than non-qualified products. The PVLs and PVC carrier sheet forms the BIPV panel. The BIPV roofing system consists of the thin-film PV laminate bonded to the PVC carrier sheet. The carrier sheet is installed over a base of insulation, located near the conduit runs.
Roof integrity evaluated using the ROOFER Engineered Management System showed that Sites I and III both had little change in roof condition indices; whereas, Site II had a significant reduction in membrane condition index due to mold. Note that ROOFER does not account for PV components. The ROOFER system also lowered the index of some of the project roofs due to what was deemed insufficient flashing height. This is not a fault of the BIPV system. American Society for Testing and Materials (ASTM) D 4434 tests on field weathered PVC samples from two parts of the Site III system indicated that different environmental conditions did not definitively impact longevity.
Roof reflectivity was spot measured during the three-year study. Sites I and III had up to a 29% reduction in reflectivity due to desert soiling. Site II had a 24% reduction due to mold. Desert soiling may be removed by rain, but the mold growth will only worsen. While the data show an overall decrease of reflectivity, they were still better than that of many conventional, dark roofs.
Roof temperature was studied extensively at Site III. Data shows that the BIPV roof reduces building heat gain. However, the poor building envelope interface at the attic and malfunctions with the air conditioning equipment made it impossible to correlate the facility energy use to roof temperature. Computer models simulated the BIPV impact to a prototypical office building as if it was in Phoenix, AZ; San Diego, CA; Seattle, WA; Norfolk, VA; and Jacksonville, FL. These sites were selected to represent different climates and common U.S. Department of Defense (DoD) locations within the United States. The simulations showed that BIPV roofs can result in a net positive energy savings at each location.
Actual versus expected energy output was used to assess PV performance. Site I had only two months of data due to problems with the manufacturer’s monitoring system. Data indicated that the PV system met 80% of the expected output and was likely impacted by desert soiling. Site II often experienced cloudy weather, but performed roughly 30% better than expected likely because a-Si PV works relatively well under diffused sunlight. Site III suffered from desert soiling, but not to the same extent as Site I, likely due to the facility’s small size and simpler roof design. Data shows that the Site III BIPV roof has a relatively steady power conversion efficiency and met renewable energy generation expectations.
At least three BIPV roofs experienced failed PV adhesives. The manufacturer applied a surface tape, but parts did not endure and caused other problems. A few sites had problems with occasional pin-size holes. The recommended repair procedure required a small flame, but one site lacked qualified, local personnel to perform it. Mold was a problem on several BIPV roofs in coastal/humid locations, but attempting to remove it would likely cause more damage than leaving it alone. Evidence of water ponding was found in several locations and indicates a poorly designed and/or poorly installed BIPV system or problems with the previous roof that were not resolved prior to BIPV roof installation.
The BIPV roof type demonstrated in this project is no longer available due to adhesion problems and better design practices, but adhered PV approaches are still being used and often considered to minimize roof penetrations or weight loading. In some systems, thermoplastic-olefin replaced PVC to be more adhesive-compatible; other flexible PV materials have been used because of higher conversion efficiencies; conduit became surface-mounted to be more firefighter friendly.
In spite of the improvements, the problems identified by this project may still occur with new adhered systems. The National Electric Code addresses some PV safety concerns, but fire and firefighter safety standards still need development, so base safety personnel should be consulted before and during the design phase. Improper water drainage can reduce roof longevity and may be remedied with a thorough review of the design by a roofing specialist, using a rigorous quality assurance/control plan, and performing a BIPV roof assessment before the workmanship warranty expires. In the case of a retrofit, problems with the existing roof need to be remedied prior to BIPV roof installation. Mold growth can reduce roof reflectivity even if it does not reduce roof longevity so ensure that the manufacturer and installer warranties address this aspect. PV adhesives may still fail and improperly tested solutions may worsen the situation by making other remedies more difficult to implement. A comprehensive warranty may mitigate risk, but is ineffective if the warrantor goes out of business as was the case during this project. Third-party solutions may be available, but may void any remaining warranties. Various acquisition vehicles can mitigate the technical risks, but contracting complexity, costs, and risk must be balanced.
The concerns with BIPV roofs can be mitigated, so DoD personnel in charge of rooftop solar projects need to determine whether or not the cost and benefits outweigh those of conventional rooftop PV systems. It is recommended that DoD personnel interested in BIPV roofs be aware of the issues, consult with a roofing specialist, and obtain training and/or consultation from experienced personnel prior to the design and construction phases.