Editor's Note: Scientific American's George Musser will be chronicling his experiences installing solar panels in Solar at Home (formerly 60-Second Solar). Read his introduction here and see all posts here.
In the first installment of this post, Arnold Mckinley of Xslent Energy Technologies described how "reactive power" -- that is, power stored momentarily by electrical appliances and then released -- destabilizes the electrical grid. Here he explains how home solar arrays can help.
Electricity has traditionally been distributed using a wheel and spoke grid: power travels from a large central generator to loads distributed around it. In some cases, energy travels very long distances, perhaps 500 to 1,000 miles, before being used. That model is changing. Since solar and wind inject energy at numerous local points, the grid is coming to look more like a network than like a wheel -- making it even harder than it already is to keep power flowing smoothly. Two recent developments promise to help. The first is a new generation of microinverters, and the second is the growth of the interconnected smart grid.
A solar panel generates DC power, which gets converted to AC power by a device known as an inverter. Most inverters require a certain minimum threshold voltage to work. Therefore the panels must be wired together in electrical series to raise the voltage high enough. Experience has shown that this setup is less than optimally efficient, as an earlier Solar at Home post talked about. A cloud shading a single panel reduces the efficiency of the entire string. Moreover, each panel has slightly different electrical characteristics, creating a mismatch that reduces the power generated. Finally, if the voltage from the string is too low, the inverter never turns on; so on rainy or foggy days, the system generates no power at all. The solution to all three problems is to fit each panel with its own low-voltage inverter, or microinverter. It turns on as soon as light falls on the panel and automatically compensates for the panels' electrical differences.
But if microinverters were also able to produce reactive power, they could ship the excess over the local load consumption back into the grid, as they do with active power now, and help out the utilities.
The figure at left gives an example of a basic setup where household appliances draw 1000 watts of active power and 600 volt-amps of reactive power. If a solar array can generate 1200 W of DC power, then it is capable of producing 1200 W of active AC power and 1200 VA of reactive AC power. That is enough not only to power the house but also to feed some active and reactive power into the grid. All it requires is the right microinverter.
When the ordinary inverters and microinverters were first developed, the designers paid no attention to reactive power generation. Because consumers pay only for active power, the goal was to produce as much of that as possible. Today it is clear that reactive-producing solar can help stabilize the grid, and microinverters are being designed to produce both. In fact, physics is helping us out here. Since no energy is required to produce reactive power, an inverter can produce it without sacrificing active power or requiring more solar panels.
When I first learned that reactive power can be produced without affecting the active component, I was surprised. To see that this is reality and not fantasy, the figure at the right shows two days of power production at a typical solar facility. On the first day, the microinverter was set to produce both active power (green line) and reactive power (red line); on the second, it was set to produce only active power. The switch did not affect the active power production at all. My company's website has more details on this issue.
In the past, the big problem preventing microinverters from producing reactive power was the need for weighty capacitors to store the energy temporarily. But new designs pull off the trick simply by changing the shape of the AC wave. This significantly reduces the cost and the size of the devices.
What is more, microinverters are also evolving to communicate with other grid devices, much as smart meters are already doing. Networked microinverters can report data for display on internet browsers, but some also have two-way communication, allowing operators to control their active/reactive power generation mix. Eventually, on-board intelligence will adjust the mix on the fly, providing the best economic benefit to consumers based on their rate structures (which will eventually include reactive power pricing). Such intelligence will allow these distributed networks to separate from the main continental grid and form localized microgrids, so that all the electricity we need is generated where we need it.
Photo and Diagram courtesy of Arnold Mckinley