Photovoltaic Panels

Photovoltaic (PV) Panels are used as a means to generate on site energy. The panels are relatively easy to integrate into the design of the house and power system, and are a means to reduce source energy consumption. One of the draw backs are that at this point in time is that the cost of PV panels, while lower than a few years ago, still does not make them cost effective from a payback point of view. The amount of energy generated takes many years to pay off the initial cost of the panels. However, as the use and demand for PV technology increases and further advances in the technology increase the performance of the panels, the costs will continue to drop, making the technology more viable financially.

Photovoltaic systems require a collector panel and an inverter in order to produce electricity that is able to be used by the home. Photovoltaic systems are either connected to a battery storage system located on site, or connected into the power grid of the community. For locations where connection to a power grid is not available or impractical, then a battery storage system is desirable. Battery storage systems however, do require maintenance to ensure that they continue to function adequately. Tying into the local power grid is generally recommended over battery storage when possible, due to the simplicity and costs. This removes the concerns with maintenance of the battery systems.

Figure 29: Schematic of a Photovoltaic System

Design Considerations

In the design of photovoltaic systems there are several aspects of the design that can affect the performance of the system. The location and angle of the collector, internal losses, shading, and temperature should all be considered in the design of the system.

The PV panels should be installed on the South side of the building. Variations up to 15 degrees of true South will create little change in the performance of the panels, however, beyond 15 degrees the performance will begin to drop off. Also, setting the tilt of the panel to maximize the summer time solar incident angle can increase the energy production of the panel over the course of the year. This can be more difficult than it seems as aesthetic issues often begin to come into play. It may not always be desirable to have the panel in a location of high visibility, and architectural design may limit the options for the collector tilt angle. If PV technologies are going to be incorporated into the design, it should be considered early on in the conceptual design stage, so that systems could be properly integrated into the aesthetic design of the building.

Most systems will experience some internal losses in the system, and only reach approximately 80% to 90% of the rated output of the panel at a maximum. The losses are from panel temperature, dirt, dust, the resistance in the wiring and losses through the inverter. This is common for most systems and should be accounted for in the design of the system.

Even the least bit of shading of the panels can dramatically decrease the performance and close attention to keeping the panels in direct sunlight is very important. This is due to the way the photosensitive cells are linked in the array. Therefore it is very important that the panels are placed in a location such that surrounding elements (such as trees and chimneys) do not cast a shadow over even a portion of the panel. Ideally, the panels would also be cleaned with some regularity of dust, leaves, snow, or any other matter that might get deposited on the solar collector. Rain tends to primarily perform the cleaning function, but periodic detergent cleaning can remove any buildup of grime.

The performance of the panels is also significantly affected by temperature. As the temperature of the panel increases, the output of the panels is reduced. Therefore it is important to try to keep the panels as cool as possible. One strategy is to install the panels slightly off the surface of the roof, to allow for some ventilation behind the panel.

Energy Model Results

The system used in the energy model is based on a 1.9 kW photovoltaic system (Similar to SunWize Packaged PV system including a Sanyo 190BA3 Solar Module and a Fronius Grid-Tie Inverter). The area of panels required for this system was equivalent to 127 square feet or 10 panels. The amount of site generated energy was able to make up 10.6% of the whole house energy consumption.

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