Solar farms are being built across Wisconsin. How do they power homes?
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Solar farms are being built across Wisconsin. How do they power homes?

Jun 11, 2023

The change in Wisconsin's rural landscape is getting harder to miss.

Interspersed with the state's seemingly endless sea of crops and red barns are row upon row of shiny panels producing energy to power thousands of Wisconsin homes.

Five massive solar farms have gone into operation since 2020, when Wisconsin's first large-scale solar system came online in Manitowoc County. Combined, the electricity they produce can power about 130,000 homes. That number will more than triple by the end of next year, when another 13 that are now under construction will have been commissioned.

Including solar arrays that are either proposed or already approved by the state Public Service Commission, utility-scale solar energy will in the relatively near future be able to power nearly one-third of the 2.8 million housing units that the U.S. Census Bureau estimates are in Wisconsin. And that doesn't include the explosion of smaller installations, from residential rooftops to community solar parks.

Although solar farms are becoming an increasingly common sight, how they convert energy from the sun to electricity for the home can be hard to understand. Here's a step by step look at how it works:

Burning at 27 million degrees, the sun throws off immense amounts of energy. We can feel it on our skin or on an asphalt road on a hot summer day.

That energy that arrives as light is composed of photons, tiny packets of energy that on their own don't have an electric charge. But certain substances are struck by light and undergo a reaction that releases electrons, which is referred to as photovoltaics.

In a solar panel, that substance is an interconnected array of silicone-based wafers that have a positive side and a negative side. That allows the electrons to flow, creating an electrical current.

How many cells are in each panel can vary based on its dimensions and whether it has cells on both sides. Two-sided, or bifacial, panels can capture light energy that's reflected off the ground.

The electricity produced by a solar panel is direct current electricity, the same type of electricity that you get from a battery. But that's not what powers our homes, schools and workplaces. That means the current needs to go through several transformations before it becomes the alternating current, or AC, power that is delivered to our homes.

Rows of solar panels are connected by string cables, wires that run the length of a row of solar panels and then feed into a combiner box, where the power is collected and then sent on to the next step, an inverter.

Steve Greidanus, generation engineering manager at Alliant Energy, uses a comparison to a tree. Each panel, he said, is like a leaf collecting the sun’s energy. The string cables tie together the power created by each panel, similar to how branches collect energy for a tree.

Welcome to the inverter, a device that the U.S. Department of Energy calls "one of the most important pieces of equipment in a solar energy system."

This is where the now combined electricity gets converted to alternating current, the type of electricity we use in our homes.

Large solar farms use multiple inverters to collect power from the string cables, convert it to low-voltage AC current, and then send it down the line via underground cables to the next step, where all of that power is combined.

The combination of power sources happens at substations, the last on-site step before it goes to the electric grid.

It's also where the AC power that flows from the inverters is boosted by transformers to a higher voltage to match that of the nearby transmission line that ties the solar farm into the electric grid.

This is an added step compared to what happens with rooftop solar arrays, where an inverter converts the battery power to an AC current that matches the power that runs through the electric meter.

Some solar projects can be tied in to an existing substation, but large developments typically require construction of a new substation, said Eric Udelhofen, vice president of development at OneEnergy Renewables in Madison.

Industry experts like to compare the electric grid to a road system that's composed of an interconnected network that's designed to carry different amounts of traffic.

Smaller solar arrays can be hooked up to local electric networks, akin to a county trunk road, but when it comes to the big solar farms the electrons are pushed to the electric "interstate," a network of high voltage lines that move electricity across the region, said Tom Dagenais, director of system planning at Pewaukee-based American Transmission Corp.

The location of these high voltage lines plays a significant role in determining where solar farms are built.

Before the electricity gets to your home, it flows through another substation that reduces the voltage to the lower and safer levels used in homes and businesses.

If someone subscribes to a utility's solar energy project, you're paying for the generation of the power, but not for the electricity it produces.

Because it is co-mingled with all of the other power sources that feed the grid, there is no way to know where or how the electricity that arrives in your home was produced.

Dagenais uses a swimming pool analogy to explain.

"It’s like using a garden hose to fill a wading pool and then it rains," he said. "Once the water is in the pool, there’s no way to separate it based on where it came from."