PV-BILD: A Life Cycle Environmental and Economic Assessment Tool for Building-Integrated Photovoltaic Installations

Abstract

An elegant application of photovoltaic (PV) technology is in building-integrated designs (BIPV), in which the PV modules become an integral part of the building envelope. BIPV systems perform the traditional architectural functions of walls and roofs (weather protection, structural, and aesthetic) while performing the additional function of generating electricity. BIPV systems displace conventional building materials and utility-generated electricity and do not require additional land area or supplementary support structures. A life cycle environmental and economic model and software tool were developed to assist in the evaluation and design of building-integrated photovoltaic installations. This tool, called the Photovoltaic-Building Integrated Lifecycle Design tool or PV-BILD, calculates a set of life-cycle environmental impacts and benefits associated with a specified BIPV system while also evaluating the combination of conventional electricity and building materials that the BIPV system displaces. The environmental data categories investigated include energy, air pollutant emissions, water use, and waste generated. PV-BILD also calculates system energy performance metrics and system economic parameters.

PV-BILD currently includes data for two BIPV products (a shingle product and standing-seam metal roofing) and one inverter, as well as insolation and electric utility parameters for 15 cities around the United States. Results were generated for a reference BIPV system (2kWp shingle system with a 20 year life) for the 15 cities in PV-BILD. The electricity production efficiency (electricity output/total primary energy input excluding insolation) for a reference system ranged from 3.42 in Portland, OR to 5.57 in Phoenix, AZ indicating a significant return on energy investment. The energy performance of this BIPV system is dramatically better than conventional electricity generation. The pollution prevention benefits of displaced conventional building material were negligible compared to the benefits from displacing conventional electricity generation for the reference system. The reference system had the greatest air pollution prevention benefits in cities with conventional electricity generation mixes dominated by coal and natural gas, not necessarily in cities where the insolation and displaced conventional electricity were greatest. Detroit had the highest mass of pollution prevention for all air emissions except methane and carbon monoxide.

The life cycle economic analysis determined the value of the air pollution prevention achieved by the BIPV system. The value of avoided air pollution without regulation of carbon emissions ranged from 1.8 cents/kWh in Detroit to 0.5 cents/kWh in Boston based on the unit damage costs incorporated in the model. With carbon regulation, the value of avoided air pollution ranged from 4.4 cents/kWh in Detroit to 1.3 cents/kWh in Boston based on a carbon compliance cost of $130/ton. The cost for the BIPV reference system was 32 cents/kWh in Detroit based on a 20 year service life. Even when avoided damage costs are included, the reference system is not economically competitive with conventional grid electricity. It should be noted that many other environmental costs associated with conventional electricity generation such as oil spills, nuclear wastes, and habitat destruction from hydroelectric dams were not included in this analysis. Even considering these factors, consumers motivated primarily by economic arguments are not likely to deploy BIPV systems until system cost comes down substantially or the cost of conventional electricity rises.