Perspectives

A Quarterly Information Source from Benchmark, Inc.

Volume 60 January 2008

Photovoltaics and Roofing by Kevin Ernster, RRC

Building Integrated Photovoltaics is a term that will be heard more and more as energy prices increase and the demand for electricity grows. The term refers to buildings that produce their own electricity using photovoltaic panels (PV). These panels produce electricity from sunlight; when enough of them are combined, they can supply the energy needs of a building. If you are going to supply the energy needs of a building, an abundance of space that also receives sunlight, is needed.

In some building designs the panels are integrated into the walls of a structure. This application works great if the building has the proper orientation to the sun. However, the most common area to integrate a PV system is in the roof where exposure to the sun is typically unlimited. Rooftop-mounted PV systems are becoming more common in many areas of the country, particularly where the cost of energy is high and utility, state and federal incentives are generous. In most cases, the systems are installed, tied directly to the local power grid, and the electricity is sold to the local power company. At an installed cost of $8-$10 per watt, these systems can be very expensive. For example, a 10 Kilowatt system (10 thousand watts) will cost approximately $100,000 to install. If the system receives full direct sun for 4 hours per day, and 1/2 sun for 6 hours per day, its output will be 70-kilowatt hours (4 hours at 10 kilowatts and 6 hours at 5 kilowatts). If the cost of electricity is $0.14 per kilowatt-hour, the daily output will be $9.80. At this rate, the system will take 27 years to payback its initial cost. (This is just an example based on general costs and outputs. For better accuracy a detailed assessment is necessary for your location and facility).

In order to encourage the installation of these systems, the Federal Government, local utilities and many states offer incentives that in some cases reduce the installed costs by up to 50 percent. Several websites summarize the available incentives for both commercial and residential systems. One of these sites is the Database of State Incentives for Renewables & Efficiency (www.dsireusa.org). In addition to incentives, additional financial factors that may accelerate payback include system depreciation, and avoidance of peak demand costs.

What does all this have to do with roofing? As the installation of a PV system can be very damaging to a roof system from a traffic standpoint, and penetrations need to be made in the roof to accommodate the system, the installation of a roof and PV system go hand-in-hand. In fact, some roof systems incorporate the panels into the roof itself. We have been asked on several occasions to explore the integration of a PV project with a roofing project.

There are two types of Photovoltaic panels. Silicon panels are 1”-2” in thickness and vary in length and weight depending on the output of the panels. These silicon panels can be seen on construction signs along the highway.

Because of their size, silicon panels are usually mounted on a rack that is flashed into the roof. On a large system this can require multiple penetrations for both the rack supports and entrance wiring. The advantage of the rack system is that it uses the more efficient silicon panel and the panels are typically angled to the sun for more direct exposure.

Amorphous or thin film panels are very thin as the name implies. An example of an amorphous panel is on a solar-powered yard light. Thin amorphous panels are usually adhered or seamed-in to a single-ply roof membrane such as a PVC or KEE with minimal penetrations through the roof. Due to the panel being laid flat and not direct to the sun, the efficiency is not as great and more panels are required to obtain the same output as a rack-mounted system. If you have a metal roof system, there are thin film panels designed to fit and adhere to the metal panels between the standing seams of the system.

Part of the criteria for the selection of a roof system may be dependent on the type of PV system, or the PV system may be determined by the roof system that is appropriate for your building. While each system has its advantages and disadvantages, there is a lot of additional information that must be factored in. The first step is to determine the amount of electricity that you want produced. Once that is determined, how much rooftop is available to install the system? Highly efficient panels will require less space-but at a higher cost; while less efficient panels require more space for the same output at less cost. Are there federal, state or local financial incentives available and will the local utility assist you with the interconnection?

From a strictly financial standpoint, a PV system may not be your best investment without substantial incentives. How much will energy cost in the future and will there be enough available? What is the value of the reduction of greenhouse gases for every kilowatt-hour of production?

There is a lot to be considered before a final decision can be made, but as more companies install these panels, we should be able to answer many of the unknowns.