Beyond the Solar Panel

The government tests cars for gas mileage. Now it's testing roof tiles for wattage.

Homeowners have long been able to partially power their homes with sunlight, but it meant clumsily mounting photovoltaic (PV) panels on the roof. Now the latest generation of PV panels look and act much like ordinary roofing tiles or shingles. And the National Institute of Standards and Technology (NIST) is evaluating nine of these commercial PV roofing products in hopes of providing an easy way for consumers to judge the panels' power potential.

"A lot of people are considering the use of PV products on their homes and businesses, and in order to make decisions on whether it's a worthwhile investment you need to predict their performance," says Hunter Fanney, head of NIST's Heat Transfer and Alternative Energy Systems Group in Gaithersburg, MD. "We are collecting detailed performance data to validate those models."

The roofing materials, which use various types of solar-to-electricity conversion, are being tested for 15 months. Fanney hopes to use the data to build a computer program and database with, among other things, average flat-surface solar radiation readings for neighborhoods across the United States (as measured by the weather service at the nearest airport). Punch in the performance characteristics of the roofing product you want to use, plus your location, roof orientation and slope, and other data, and -- bingo -- you'll know what kind of wattage you can expect from your roof.

According to Fanney, roofing tiles and shingles with embedded solar converters have been on the market for about three years. They look like regular roofing materials, keep out the sun and rain, and can be installed in much the same way. But by generating electricity, these tiles and shingles save consumers money.

Around 500 square feet of PV tiles can produce three kilowatts of electricity, according to Subhendu Guha, president and chief operating officer of United Solar Ovonic, a maker of PV shingles in Auburn Hills, MI -- and most roofs are several times that size. His company's version is dark blue and can blend with ordinary shingles of a similar shade. Or a builder might devote an entire sunny section to PV materials.

"A south-facing roof on a three-bedroom home could supply 20 to 30 percent of the home's electrical needs," says Paul Maycock, a consultant and head of PV Energy Systems in

Without subsidies and incentives, such as those in California, PV power costs about twice as much as utility power, says Thomas Leyden, vice president of east coast operations for PowerLight, a PV systems integrator in Berkeley, CA. That difference, however, is shrinking. "PV hardware prices have gone down tenfold in the last 15 years, thanks to new technologies, better manufacturing techniques, and more efficient use of materials," Leyden says. Prices are currently falling by about 5.5 percent yearly, he says, so they should come down another 50 percent in a little more than a decade -- and become fully competitive with utility power.

Maycock is even more optimistic, projecting that the installed price will fall from today's $8 or so per watt to $4 by 2014. That would make solar power "fully economic in the Sunbelt," he notes.

Meanwhile, Guha maintains that PV roofing is already economical at certain times of the day, in places where utilities charge extra for peak daytime usage. There, he says, it can be used to avoid paying those surcharges, a practice called "peak shaving."

Historically, the biggest market for residential PV roofing has been in Japan, which gets about half the sunlight that California does and the average residential user derives only a kilowatt. But government incentives, low mortgage interest rates, and high utility power rates have made residential PV popular there, says Maycock.

Source - MIT Technology Review