Photovoltaic (PV) systems, defined as those that collect the sun’s energy and convert it into electrical energy, have been in use for many years. The efficiency rate at which they convert energy has steadily risen, and now most PV’s can deliver a 12% conversion efficiency, while some have been reported to produce electricity at an efficiency as high as 40%. PV’s generate direct current electricity and convert it into alternating current (AC) electricity, limiting the voltage to the grid that it is connected to.
The amount of energy striking the earth during the peak of summer at a latitude of 42° (Approximately the latitude of Detroit) is 190.8 mJ/cm2 (as reported in a paper entitled Calculating the Energy from Sunlight over a 12-Hour Period, written by Joseph C. Kilecki at NASA) At an efficiency of 12%, a PV would be converting around 23 mJ/cm2 into AC electricity. How does this relate to the energy use of the average US home? The US Energy Information Agency puts the average energy use for a home in the US at approximately 8,900 kWh/yr, which is a little less than 25 kWh per day.
On their FAQ page, ETA Engineering states: Electrical energy is generally measured in kilowatt-hours (kWh). Thus, if a module produces 100 Watts for 1 hours, it has produced 100 Watt-hours or 0.1 kWh. The amount of energy produced on a given day will depend on location, shading, and module orientation (direction and tilt). In a good area for solar power (such as Phoenix, Arizona), a properly oriented module which produces 100 Watts at noon on a clear day will produce an average of about 0.5 kWh/day in January and 0.8 kWh/day in May and June. (Fluctuations result from the amount of variation in direct sunlight on a typical day). In a relatively "poor" area for solar power (such as Albany, NY), the same module will still produce about 0.25 kWh/day in January and 0.6 kWh/day in July.
So, if an average home needs 25kWh per day to operate, it would take between 30 – 100 PV panels (estimating the area of the typical panel to be approximately 9.6 ft2) to do the job, depending on location and time of year. If the entire southern-facing section of the roof of a medium size house were covered with PV panels, a significant share of the electricity consumption of that house would be supplied.
While photovoltaic panels have been around for a long time, recently systems designed to be built as part of the building have come on the scene. These systems are called Building Integrated Photovoltaics (BIPV). They can consist of roof or façade systems which are added to an existing building, or similar systems designed into the building at the time of construction. Typically, these assemblies perform more functions than generating electricity. They also resist the passage of wind, water and fire as well as serve a cosmetic function. Consequently, BIPV systems must meet not only the standards required of normal PV’s, they must also meet the requirements of the normal building materials or systems they replace (roofing, curtainwall, etc).
A typical BIPV is a roofing shingle with PV capabilities built into it. The southern-facing portion of a roof clad with these PV shingles serves as a large collection of small PV panels, all feeding into an inverter to convert the energy into a usable AC form (although, of course, the power can be used as DC if desired). However, we must still be concerned that the roofing system will protect against fire and water incursion, and will not peel off in a moderate wind storm: i.e., function as a roof.
BIPV systems are now available in glazings and laminates, modules (both façade- and roofing-integrated), and in systems designed to shade constructions to limit heat input. Due a wide range of colors, shapes and sizes, architects are now able to design as they wish and still include the capability to utilize the energy of the sun.