SOLAR PANELS AND WHERE TO PUT THEM

MRS. FLANNERY HAS MOVED ON to meet with another project team, and Paul is eager to tell me more about different ways we can use solar energy.

“Just now I was talking about how to use passive solar energy,” he reminds me. “The other way of using the power of the sun is called active solar. Active means that the sun is doing more than just heating the air, like it does in your living room with all the windows, or in a greenhouse. Instead, the energy from the sun is actively making something happen—like heating water for washing or making electricity to power your lights and your refrigerator.”

José is listening carefully. He asks, “Let’s say we want to heat water at my house using the sun. How would we do it?”

Paul turns back to the computer and types solar hot water heaters into the search bar, then chooses “images.” He clicks on one.

“See?” He points at the house on the screen. “On your roof you’d have a rectangular box with a glass top. The box is painted black inside because dark colors absorb heat. In the box are pipes with water running through them. The pipes bring cold water up from your water supply into the box.”

A Solar Hot Water Heater
The sun heats water as it runs through pipes inside the heater on the roof.

 
On the picture, Paul traces the blue pipes with his finger. “So, the sun shining through the glass top heats up the box, and that heat energy in the box warms the water in these pipes. And see the red pipes over here? The hot water in those pipes goes back down into your water tank, where you can store it or use it to take a shower and wash things.”

“I see,” says José. “The sun’s heating up the air in the box, and the hot air is heating the water in the pipes. Pretty slick!”

“Okay, Paul,” I say. “You’ve done a nice job of explaining how we can use the sun to make hot water. What about electricity? How can the sun make electricity?”

Paul looks at his teammates. It’s Tom who answers. “We’ve been learning about that together,” he says. “First you have to know what electricity really is and how it works. José has been doing that part.”

José sighs, looks away, and closes his eyes for a minute to concentrate. Then he opens them and turns to face us. “The first thing you need to understand is that everything is made of tiny little things called atoms,” he explains. “And I mean everything! Atoms are so tiny you can’t see them. But scientists discovered years ago that atoms actually have three parts. Here, let’s look at a picture.”

Everything on Earth is made up of tiny things called atoms.

José brings up a new picture on the computer screen. “Look,” he points. “In the middle of the atom are incredibly tiny things called protons and neutrons. Together they make up the nucleus.”

The Structure of an Atom
In the center of an atom is a nucleus made of extremely tiny protons and neutrons stuck together. Flying around the outer edge of the atom are electrons.

 
He indicates the outer edge of the atom. “Flying around them are electrons. We’re interested in the electrons, because they’re what make electricity happen.”

I hold my hand out in front of me. “So you’re telling me that my hand is made of millions of little atoms, and each one of those atoms has even smaller things called protons and neutrons and electrons inside it?”

I knew the boys had studied the structure of the atom in school the year before. Now their project was giving them a good reason to remember it.

“Right,” José affirms. “Atoms are kind of like Lego bricks. They’re the building blocks for everything. Here’s a picture of a real atom taken with a super-powerful microscope. You can see the nucleus in the middle and the electron orbiting around it.”

Sun, Wind, and Water by Mon Cochran
Photograph of an Atom
This image was taken with a really powerful microscope.

 
Tom has been checking the time on his phone. “It’s lunch time in twenty minutes,” he says. “I think we have time to tell Gramps what atoms have to do with electricity.”

“Okay.” José turns to me. “Like I said earlier, it’s the electrons we’re interested in. Electricity is just a stream of electrons that flows along a wire to wherever you want to use it— like to make light in a light bulb.

“First you need to break electrons loose from the atoms they’re flying around in,” he continues. “Then those electrons will travel along the metal in a wire.”

Across from us, Paul starts tapping rhythmically on the table and Tom is chanting, “Yes, yes, yes!”

Electricity is just a stream of electrons.

“What’s so exciting?” I ask.

“We can use the sun to break electrons loose!” Paul announces.

“That’s how solar cells work!” Tom adds. “Inside each cell, the energy from the sun is breaking electrons loose and sending them through wires into our homes.”

“Whoa!” I hold up my hands. “You’re going too fast. How does the sun break electrons loose, and where do they go?”

Tom takes over the presentation. “Here’s a picture of a solar cell,” he begins, bringing up the image.

“This cell would be part of a solar panel. There are lots of solar cells in each solar panel. The panel sits up on your roof, where it can collect sunlight. Or somewhere else out in the open, like in a field.”

A Solar Cell
A solar cell contains a “sandwich” made of different kinds of silicon.

 
“I’ve seen lots of solar panels on people’s roofs.” I look more closely at the solar cell. “It looks kind of like the layers in a sandwich,” I suggest.

“That’s right.” Tom points to the picture. “In the middle of the sandwich are two layers of silicon. The top layer is colored blue and the bottom layer is green.” Tom shoots me a glance to see if I know what silicon is.

“The beach in front of my house is covered with silicon … it’s the main ingredient in sand!”

Tom nods. “Yeah, I’m always getting it in my sandwich when my sister is running around down there!”

I want him to continue. “But how do you turn sand into the slice of silicon that goes into the solar cell sandwich?”

José jumps in. “You melt the sand down in a really hot fire until the pure silicon comes out. Then you let it cool down. Once it’s cool, you can slice it really thin with a diamond saw.”

Tom goes on. “The thing about silicon is that the electrons in its atoms are pretty loose already. That’s why it’s good for making electricity.”

He points to the picture again. “Here’s what happens. See, the light from the sun—solar radiation—shines through the glass on top of the cell. It hits the upper silicon layer and knocks electrons free from that layer. They bounce over to the bottom layer.

“Connected to the bottom layer is a wire that collects those free electrons and sends them out from the solar cell as electricity. All the cells in that solar panel are linked to each other by wires. And those wires join up into a bigger wire, combining the electricity from all the cells. Then it flows through that big wire into the house to run your lights and furnace and whatever.”

“Nice job, Tom,” I tell him. Then I notice something else in the picture. “See where it calls one silicon layer ‘n-doped’ and the other layer ‘p-doped?’ What does that mean?”

Paul takes this question. “Yeah, it took us a while to understand that,” he admits. “It’s all about how those electrons that are knocked loose from the top silicon layer get into the bottom layer. They need some way to flow down there. Scientists know that different kinds of atoms have more electrons than others. And the ones that have less can pull electrons away from atoms that have more.”

I’m obviously looking puzzled, so Paul plunges on. “For instance, phosphorus atoms have more electrons than silicon atoms do. And boron atoms have fewer electrons than silicon atoms. So people figured out how to put some phosphorus in the top layer of silicon and boron in the bottom layer. That process of adding the other elements—phosphorus and boron—is called doping the silicon.”

“Keep going,” I urge him.

“Then when you put the two layers together, the extra phosphorus electrons in the top layer are attracted by the boron atoms in the bottom layer. Those electrons flow into ‘holes’ made by the missing electrons in the boron. That stream of electrons also carries the electrons knocked loose from the silicon by the sun.”

Tom wants us to watch a video about how solar cells work, so Paul finds it on the computer.

 
How Photoelectric Solar Cells Work
Video courtesy of the US Department of Energy Photovoltaics Program

“That’s a good summary you could use in your presentation,” I say when the video is over. “How many solar cells are there in a solar panel?”

“A lot of panels have sixty cells in them,” Paul responds. “Here, take a look at this.” He switches to another picture.

“The panel is a rectangle, and each of those little squares inside it is a solar cell. I counted them, and there are six cells across the top and ten cells down the side. Six times ten makes sixty.”

“How much electricity can each panel make in a day?” José wonders aloud. In his e-mail to me, he’d said that the typical American home uses about 30,000 watt-hours of energy every day.

“I saw somewhere that each panel can produce about 250 watt-hours of electricity per hour during the day,” Tom replies.

Paul whips out his phone and chooses the calculator. “Okay, let’s say the sun shines on the panel for six hours each day,” he murmurs to himself. “Six hours times 250 watts per hour makes 1,500 watts each day for one panel.

A Solar Panel
A solar panel is many solar cells wired together.

 
“We need 30,000 watts each day to run everything in the house. So we divide that 30,000 by the 1,500 each panel makes, and we get twenty.” He looks up from the screen. “We need twenty solar panels to make enough electricity to run all the lights and the fridge and the computers and stuff in the house.” Paul looks pleased with his math skills.

“I’m really impressed by how much you guys have learned about capturing the energy of the sun!” I lean back and clasp my hands behind my head.

“One last question,” I add. “Some people can put twenty solar panels on their roof. But other people live in homes where they can’t do that. They have to get their electric power from a power company. Suppose you guys decided to start a company that produces electricity from solar energy. If you were going to build a solar power plant to make all the electricity for a whole city, where in the United States would you put it?”

“We checked that out as part of our project,” Tom answers quickly, turning back to the computer. “Here’s a map we found.”

National Solar Energy Map
In this country, solar energy is most abundant in the Southwest.

 
“You can see that the best places are in the south, especially down near the Mexican border, where the sun shines a lot,” José explains, pointing to the dark red part of the map.

“Just having a lot of sunshine isn’t enough, though,” Paul thinks aloud. “You’d want lots of space that isn’t being used for anything else. I’d build my giant solar plant out on the desert somewhere.”

Tom suddenly pivots back to the computer. “When I was doing research, I ran across a picture of a huge solar farm. Check this out!”

A Very Large Solar Farm
In 2016, the world’s largest solar farm was in India. It can provide electricity for 150,000 homes.

 
Just as the solar farm appears on Tom’s screen, the bell rings for lunch. “Can you have lunch with us?” José asks me, as he stuffs his notebook into his backpack. “I want to show you a picture I found of a solar car.”

“Sure, lead the way. I’m hungry!”

Down in the cafeteria, we all get in the ordering line. Suddenly I hear a familiar voice calling, “Gramps!”

It’s Julie, waving at me from a table where she’s sitting with a couple of friends. They hop up and head toward us. When I give her a big hug, she looks both embarrassed and pleased.

“What are you doing here, Gramps?”

I point to the guys ahead of me in line. “The solar energy team has been teaching me what they’ve been learning about how to capture the sun’s power. Have you had your lunch?” I ask her.

“We just finished,” she says. “I remember now—Tom told me you were coming.”

“What about you wanting to study water power?” I ask. “Have you been working on it?”

“Yes, we have!” she proudly reports. “My teacher Ms. Rogers said I could do a project on hydroelectric power for science if I could find two more students in the class to work on it with me.”

Julie turns to the two girls beside her. “Grandpa,” she announces very formally, “these are my friends Natasha and Amaya. Together we’re the water power team!”

“Nice to meet both of you,” I say. “Has Julie been telling you about Gaia and the water cycle and things like that?”

“Yes, she has,” Amaya answers cheerfully. “And now we’re learning all about how to make clean energy using water!”

“Gramps,” Julie begins, as I take a tray for my food, “I was going to call and ask you for a big favor … but since you’re here today, maybe …?” She looks at me hopefully.

“Sure, go ahead,” I say. “I promised to have lunch with the guys, but we have time.”

“Oh, good,” she says. “So, when we were doing research on hydroelectric power, we read about this amazing place out in Northfield, Mass. It’s called Northfield Station, and there’s a big dam across the Connecticut River that makes electricity. We were hoping maybe you could drive us out there so we could see how it works—for our project?”

All three girls gaze at me pleadingly.

“Of course!” I say immediately. I love field trips to learn new stuff, and this would be a wonderful chance to learn more about hydroelectric power with the eager water power team. “It sounds perfect. When can we go?”

Smiles burst forth on three faces, and the girls shout “Yay!” in unison. Mission accomplished, they troop off to recess. On her way through the cafeteria door, Julie shouts back, “I’ll give you a call and we’ll make a plan!”

“Sounds good!” The server behind the counter passes me a plate of lasagna and broccoli, and I catch up to the boys.

“This actually looks good!” I say to José. “Let’s find seats, and you can tell me about that solar car.”