Measured Performance of Building Integrated Photovoltaic Panels: Round 2

Architects, building designers, and building owners presently lack sufficient resources for thoroughly evaluating the economic impact of building integrated photovoltaics (BIPV). The National Institute of Standards and Technology (NIST) is addressing this deficiency by evaluating computer models used to predict the electrical performance of BIPV components. To facilitate this evaluation, NIST is collecting long-term BIPV performance data that can be compared against predicted values. The long-term data, in addition, provides insight into the relative merits of different building integrated applications, helps to identify performance differences between cell technologies, and reveals seasonal variations. This paper adds to the slowly growing database of longterm performance data on BIPV components. Results from monitoring eight different building-integrated panels over a twelve-month period are summarized. The panels are installed vertically, face true-south, and are an integral part of the building’s shell. The eight panels comprise the second set of panels evaluated at the NIST test facility. Cell technologies evaluated as part of this second round of testing include single crystalline silicon, polycrystalline silicon, and two thin film materials: tandem-junction amorphous silicon (2-a-Si) and copper-indium-diselenide (CIS). Two 2-a-Si panels and two CIS panels were monitored. For each pair of BIPV panels, one was insulated on its backside while the backside of the second panel was open to the indoor conditioned space. The panel with the backside thermal insulation experienced higher midday operating temperatures. The higher operating temperatures caused a greater dip in maximum power voltage. The maximum power current increased slightly for the 2-a-Si panel but remained virtually unchanged for the CIS panel. Three of the remaining four test specimens were custom-made panels having the same polycrystalline solar cells but different glazings. Two different polymer materials, Tefzel and Kynar, were tested along with 6 mm-thick, low-iron float glass. The two panels having the much thinner polymer front covers consistently outperformed the panel having the glass front. When compared on an annual basis, the energy production of each polymer-front panel was 8.5% higher than the glass-front panel. Comparison of panels of the same cell technology and comparisons between panels of different cell technologies are made on daily, monthly, and annual bases. Efficiency based on coverage area, which excludes the panel’s inactive border, is used for most “between” panel comparisons. Annual coverage-area conversion efficiencies for the vertically-installed BIPV panels range from a low of 4.6% for the 2-a-Si panels to a high of 12.2% for the two polycrystalline panels having the polymer front covers. The insulated single crystalline panel only slightly outperformed the insulated CIS panel, 10.1% versus 9.7%.