As the world seeks to reduce its dependence on fossil fuels, companies are increasingly turning to PV panels. Photovoltaic (PV) cells are semiconductor devices, usually made of silicon, that enable sunlight to be converted directly into electricity—practically for free.

People are looking for ways to incorporate these powerful cells into the construction of homes, offices, government buildings, industrial complexes and large commercial spaces. The decision to include photovoltaics in a new building design or retrofit plan is usually simple because PV cells have no liquids, corrosive chemicals or moving parts, which results in low maintenance, minimal pollution and silent operation.

There have been two acknowledged drawbacks to PV solar panels: 1. Initial cost is high and the payback slow, often taking decades for costs to be recouped, and 2. the cells are only about 10 to 15 percent efficient, and the panels produce far more heat than electricity. Visit www.EDCmag.com for a “Design Guide for the SolarWall PV/T Solar Air Heating and Electricity from PV Modules.”

For every 1˚C rise in PV panel temperature, there is 0.4 to 0.5 percent drop in PV efficiency. When you consider that many PV solar panels are placed on roofs in temperate climates, this can represent a significant loss of effectiveness. Of greater concern is that the heat build-up can damage the PV cells, shortening their useful life.

Simply by keeping the air circulating evenly around the PV cells, a lot of the heat can be dispersed, bringing the combined PV thermal system efficiency to 50 percent or more. Further, by finding ways to recoup and use the excess heat, building owners can reduce their heating costs and reduce the payback time for the system itself.

PV/T – A Total Energy System

A photovoltaic thermal system is often referred to as a “total energy system,” because in addition to converting sunlight into electricity, it collects the residual heat energy and delivers both heat and electricity in usable form.

John Hollick, Conserval Engineering president, invented the concept of marrying PV cells with a transpired solar collector to create a PV/T system that boosts panel efficiency by as much as five-fold (www.solarwall.com).

In this system, PV panels get placed over standard transpired collector panels and placed on the roof, just like any other PV installation (walls work, too, if preferred). This helps in a few ways. For starters, each square meter of transpired solar collector paneling has 2,500 perforations which allows plenty of fresh air to pass through, balancing the air flow behind each PV module, and cooling them in the process.

In the winter months, the excess heat from the PV modules is drawn though the transpired collector into a space between the building and the panels. This warmed air then enters the building’s heating and ventilation system, effectively providing a reliable source of fresh, but pre-heated make-up air. In the summer months, although the fresh air continues to cool the PV modules, special dampers ensure that only fresh ambient (not pre-heated) air gets into the HVAC system.

The strategic placement of the PV modules is also key. Traditionally, PV modules get placed contiguously up the slope of a roof. When this happens, there’s no way to dissipate the heat between the panels, and the top part of an array can overheat. Hollick’s design calls for the PV modules to be positioned so that there are small gaps between each module, further helping to prevent excessive heat build-up. This is especially important in the summer months when even airflow is essential.

Free Heat, Minimal Space

A combined photovoltaic thermal system enables PV modules to produce more electricity than they can do on their own. By increasing the electricity production while reducing the heating energy requirements, tremendous cost savings can be achieved. Further, after factoring in the federal solar tax credit, the cost of a PV thermal system is about the same as more conventional PV installations.

In a combined PV/T system, the PV panels get mounted right over rooftop transpired solar collector (TSC) panels, so no extra space is required. On the other hand, if you have already installed TSC panels, it’s easy to add PV by means of simple clips. Installation can be completed in a day or less, with energy savings showing up immediately.