jueves, 27 de enero de 2011

For Excess Solar Flow, a Veg-O-Matic for Voltage

Until very recently, the solar industry was mainly concerned with getting a toehold in the production of electricity at a utility-level scale. Now a New Jersey company is looking for its niche in a different field — how to handle a system that is saturated with solar energy, sometimes enough to destabilize the electric grid.
Electronics on a panel made by Petra Solar regulate voltage so that a solar overdose does not overwhelm power lines.Petra Solar
The company, Petra Solar, has a highly visible product: it is under contract to supply 200,000 panels that Public Service Electric & Gas will attached to utility poles around New Jersey. Around 75,000 are already up.
About six feet wide and four feet high, the panels make 200 watts when in full sun, and newer models will make 225 watts, the company says. On a mild day, one could almost meet the needs of an entire house in the daytime; on a hot day, four or five would run a window air conditioner.
On the top side of the panel, the side angled toward the sun, is the obvious attention-getter, the polycrystalline solar cell. What is different is the electronics bolted to the bottom, which are meant to prevent a solar power overdose. They could turn out to be crucial, according to company executives.

At the moment, in most places there are only a few solar panels scattered across a utility’s service area, and they make very little difference to the flow of electricity. If enough of them were concentrated in one small area, however, they could produce more energy than neighborhood power lines could handle.
The reason is that those lines were sized to import the amount required for the customers in the neighborhood, and with enough rooftop solar installations, local production can amount to more kilowatts than local consumption was ever anticipated to reach.
And New Jersey is getting itself set up for relatively high concentrations of solar power in distributed locations. The state’s current plan is to have 22 percent of its energy generated from renewable sources by 2020. Of total consumption, 2.5 percent is supposed to come from solar. But that is a year-round average. For about 14 hours a day the sun is either below the horizon or too low to be useful, and some days are of course cloudy.
That means that to hit an average of 2.5 percent, solar will sometimes have to be 6 or 7 percent of the total amount of power generated.
Joe DeLuca, vice president for development and product management at Petra, said the peak percentage for solar would come each May, when days are long but the air-conditioning season has not really gotten under way and demand is therefore low.
When local production of power exceeds the combination of local consumption and the ability to export the power, voltage rises and so does frequency: while the electrons are supposed to dance back and forth 60 times per second, they can go faster. (As a practical matter, the pace is measured down to a thousandth of a cycle, and the system is almost always between 59.9 and 60.1.)
This is where the electronics box on the bottom of the panel comes in. It is a sort of Veg-O-Matic of the electrical world, slicing and dicing.
The typical function for the electronics attached to a panel is to convert the direct current produced by the solar cells into alternating current, at 60 cycles. But Petra’s box, a device about the size of a home wireless router but hardened to survive the variations in temperature and humidity outdoors, also produces something called reactive power, a type of electricity product that can regulate voltage. Voltage is a property of electricity sometimes compared to pressure in a water pipe, and reactive power can be thought of as taking up space in the pipe and thus increasing pressure. The Petra box incorporates circuitry that can make reactive power or destroy it.
The box is also set up to manipulate frequency, although that is not a problem that solar power poses yet.
The box, which Petra calls a Smart Energy Module, is designed to regulate voltage autonomously and to act in intervals of less than one-sixtieth of a second in response to local conditions. It could in theory send signals back to a central computer that would make such decisions and relay them back to the box, but “by that time it would be too late,’’ said Hussam Alatrash, an engineer at the company who wrote the algorithm to allow the box to function autonomously.
The widespread adoption of electric cars would also have an impact in voltage, Mr. DeLuca said. Plugging in a single one to recharge could easily double a home’s energy demand, and if three or four such cars were in a single cluster of houses served by a single transformer, that could cause overloads. “It’s going to be like the Fifties when everybody was adding air-conditioners,’’ he said.
Making or destroying reactive power takes energy. That can come from the solar cell itself, or the box can take the energy off the grid. The box does “talk” to a central controller that funnels information back to utility control rooms, giving an instantaneous picture of voltage and frequency.
It can also talk to nearby streetlights, or it may be mounted on a street light pole. Most streetlights have sensors that turn the light on at dusk and off at dawn; with some ingenuity, the light could also be dimmed in the middle of the night, when there is little traffic, or alternate lights could be turned off to save energy. The box can also monitor consumption of power by the light emitted, and tell a worker when the lamp has burned out.
The Petra concept is inherently more expensive per kilowatt of capacity because each panel requires its own inverter to make DC power into AC power. But its “smart” functions could make it a better investment, Petra says.
So far the panels have performed well in the sun, rain and snow, although one was on a pole that was knocked down in a car crash, Mr. DeLuca said. When emergency workers arrived, he said, “they saw our name on the panel and called us to ask what they should do.’’