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So we've got a backyard pool.
And my son Calvin, and his downstairs friend Otis,
love playing in it.
But sometimes the water in the pool
is too cold to play in.
It's cold.
So Otis' mom, Sarah,
heats up water in a kettle on the stove,
pours it into the pool to heat up the water.
I think we can figure out a better way
to heat up that pool water.
You've probably noticed the difference between
wearing a light shirt and a dark shirt
standing in the sun.
Which one's hotter?
The dark shirt, right?
Light colors look light because they are reflecting
a lot of light and heat.
Dark colors look dark because they're absorbing
a lot of light and heat.
We're going to try to build
like a dark shirt for the pool to wear to help it heat up.
This is what will make the dark shirt for the pool:
black plastic trash bags.
And this is what will float it on the surface of the water.
To attach the plastic bag to the plastic ring,
I'm going to use heat from an iron
to gently melt them together.
I just have to remember to clean the plastic off the iron
before I iron my next shirt.
Remember, if you're going to do a project like this,
make sure to get an adult to help.
I'm also going to want to poke
a couple holes in this
to make sure that no air gets trapped
in between the plastic and the water.
If there's air in there, it'll make an insulating layer.
Or even if the plastic gets hot,
that heat won't transfer as well to the water.
Now I just need to make enough to heat up the pool.
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I'm pretty confident the dark plastic will heat up the water,
but I want to know by how much.
So we're going to do an experiment
using two pools that are the same.
One pool will have the plastic on it, the other pool will not.
We're going to measure the temperature of each pool
to see which one is heating up more.
We're going to record some data.
First, I have to record the temperature at the start time.
I'm taking all of my measurements
in the top few inches of water in the pool.
76 degrees.
76 degrees, perfect.
I was hoping they'd be just the same.
Now, we put on the plastic.
I like it.
They kind of look like lily pads.
So the pools have been heating up in the sun for one hour.
First I'm going to check the white t-shirt pool.
It looks like it's around 80 degrees already.
The sun's pretty powerful.
After one hour, wow, it's almost 86 degrees.
That's a pretty big difference.
The black plastic lily pads are an example
of a passive solar heating set-up.
There's no mechanism, there's no machine present.
We're just collecting heat from the sun
and transferring it directly into the water.
We're now ready to record the temperature change
after two hours in the sun.
86 degrees on this thermometer.
So the temperature's actually risen about ten degrees
over the last two hours in the swimming pool
with the white t-shirt on.
That is really warm.
96!
It's ten degrees warmer where the lily pads
have been heating it up with the heat from the sun.
That is amazing.
Can you help me take out the lily pads?
CALVIN: Yeah.
Is the temperature warmer?
Little bit warm.
Well, the lily pads seemed to work.
Just a couple hours ahead of playing in the pool,
we'll set them out on the surface,
they'll collect the heat from the sun,
put the heat into the water, and warm up the pool
so the kids can play in it.
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I wonder if there's a way that we can heat up
some of that pool water
while the kids are playing in the pool at the same time.
My idea is to build a solar heat exchanger.
A heat exchanger is just a thing that takes heat from one thing
and puts it into another thing.
So I'm going to try to take heat from the sun
and put it into the water
through this copper tubing.
Copper transmits heat really, really easily.
Another example of a heat exchanger
is a radiator in a car.
It takes heat from the car's engine
and puts it out into the air that's going past the radiator.
Putting these two ten-foot coils together
gives me 20 feet of copper tubing
all squished down into a fairly small area.
Efficient.
Now there's one more important thing to address,
and that is to turn all of this black.
Because remember, black absorbs light and heat.
If I put the surface directly facing the sun,
it'll heat up the fastest.
To hold our solar heat exchanger,
we're going to use a painting easel.
I'm going to tape this funnel onto the heat exchanger
so the kids have something to pour the water into it with.
Whoa, ow, this is hot!
Copper transmits heat really, really well,
so copper,
even if it's at the same temperature as the wood,
is going to feel hotter than the wood.
This device lets me measure the surface temperature
of different things.
So when I pull the trigger and point it at the surface
of the black copper, it'll tell me how hot it is.
140 degrees Fahrenheit.
That's pretty hot.
Calvin, can you get a bucket of water from that pool?
- Yeah! Got it.
- The water in the bucket is 84 degrees.
Hey, Calvin, how does that water feel?
Warm.
Now it's 90 degrees!
Now once you take out the lily pads
and want to play in the pool,
the heat exchanger can help increase the temperature
even more.
And what's extra fun is it can become part of the play.
(children playing, splashing)
There's a lot of kids today for the pool party
because Calvin's cousins are in town.
Zack and Freya are here, along with Otis from downstairs.
If the water's feeling a little bit cold,
here's what you do:
you take a bucket of water,
and pour it into this funnel.
Let me know how it feels.
It's hot!
So our heat exchanger is an example
of an active solar setup, where there's some activity
that has to happen to make it work.
With a lot of kinds of solar heat exchangers,
you might have a pump running to continuously cycle water
through the heat exchanger.
In this case, instead of a pump,
we've got kids.
So now they've got a little solar heat exchanger.
You pour in a bucket,
and hot water comes out.
A fun and also practical new toy for the pool.
(chuckles)
Nice.
(splashing)