Friday, November 12th, 2010
By Eric Carbonnier, AIA, LEED AP / Environmental Analyst
Integrating photovoltaic (PV) panels on architectural projects is a common solution to generate electricity while contributing to environmental awareness and reducing greenhouse gases. However, practitioners integrate PVs without fully understanding that they have an optimal installation condition and deviating from this diminishes the value and performance, and promotes inefficiency. Surprisingly, practitioners defend this inefficiency because any contribution to lower greenhouse gases represents progress.
In brief, PV panels convert incoming solar radiation into electricity through variety of technologies, equipment, and distribution systems. According to the National Renewable Energy Laboratory (NREL), on an optimal day, these systems capture between 15%-18% of the sun’s energy using PV panels. Optimal is the operative word because the efficiency is based on Standard Test Conditions (STC) that manufacturers use as an industry standard to validate the performance of a PV panel. STCs occur in a controlled laboratory by producing a constant irradiance of 1000 w/m2 and an operating temperature of 25c (77f)—meaning a day with no clouds or atmospheric variation at solar noon. How often does that happen?
A perfect day or a STC rarely exists, thus your optimal 15%-18% efficiency is compromised from the start. Having said that, one would expect that the objective is to minimize inefficiencies rather than increase it. However, this does not seem to be the case and practitioners are placing PV panels facing north, in the shade, or in a less than optimal orientation. To optimize performance, practitioners need to recognize that proper design integration of PV panels can minimize inefficiencies rather than promote it.
When integrating PV panels into a project, practitioners have many options that could affect electrical production. Panels can be placed flat on a roof, at optimal azimuth tilt, or can be mechanized to track the solar path. The following exercise uses the NREL-IMB Solar Electricity web calculator with the following parameters: Location is Pasadena, Calif., 4kW system, de-rating of 0.77, $0.12/kWh and uses average solar data for the region.
The results illustrate the impact of panel orientation on electrical production. The immediate impact of changing the tilt from 34˚ to laying panels flat on a roof is a 12% loss of electrical production. Situating PV panels on anything but a southerly orientation has even greater consequences, with inefficiencies ranging from 19% to 46%.
The question is simple: what is your threshold to performance inefficiency? Is 12% acceptable and if it is, then is it acceptable to adopt the same logic of inefficiency across the board of building performance? Alternatively, knowing that tracking the sun provides a 42% increase in production questions our judgment of providing the most effective use of a PV system.
Having said this, is increasing PV inefficiencies through misguided design integration a valid argument knowing that we can increase PV production by informed design strategies?Tags: Eric Carbonnier, photovoltaic, solar Integrating design strategies to improve green technology performance