RERUN: How Solar Towers Work

What are solar towers? How do they generate electricity? What's the future of solar power?

Learn more about your ad-choices at https://www.iheartpodcastnetwork.com

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:04):
Get in touch with technology with tech Stuff from how
stuff Works dot com. Hey, everybody, welcome to tech Stuff.
I'm Jonathan Strickland. I am your host. I'm an executive
producer with How Stuff Works in my Heart Radio, and
I love all things tech and uh. I'm currently on vacation, y'all.
And because I'm on vacation for a couple of weeks

(00:26):
out in sunny London and Paris, I thought I would
bring some classic episodes to you guys. I didn't want
to skip any episode dates, but at the same time,
I didn't quite have the time to record a whole
bunch of extra episodes. So we're gonna look at some
classic episodes in the next few shows, with the exception
of the April first show, that will be a brand

(00:47):
new episode, so make sure you tune in for that
because I recorded something special for April Fool's Day. But
today we're gonna take a look back on a classic
episode of how solar towers work. This is a method
of harnessing energy from the sun, but it's not the
same thing as solar panels, which convert solar power directly

(01:09):
to electricity. I think it's a pretty fascinating technology, and
I talk about how it works and some of the
challenges in this episode I hope you enjoy. Today. I
want to talk a little bit about solar towers, which
is a different way of harnessing the sun's energy, and
I think it's a really clever way as well because
it doesn't rely on sunlight hitting a panel. Obviously, the

(01:32):
big big drawback to that approach is when the sun
isn't hitting a panel, you're not generating any electricity, right,
So if it's super cloudy or if it's nighttime, if
the sun is not able to hit the panel, the
sunlight's not getting there, you have nothing to convert into electricity,
and your solar panels go on to be unused for

(01:54):
that duration. So that can be really rough if you
have a long stretch of overcast days, or you live
in a place where you don't get solar exposure at
your house because of maybe they're taller buildings around you,
or trees or whatever. It maybe maybe you have a
north facing house rather than a south facing house here

(02:15):
in the Northern Hemisphere. If you have a south facing
house in the Northern Hemisphere, you're going to get more
solar exposure than a north facing house, so solar panels
have that drawback. There also, there's efficiency issues with solar panels.
When we talk about efficiency, what we mean is how
much of the sun's energy can we actually convert into

(02:35):
electricity and how much of it do we lose? How
much of that energy bounces off the panel and we
do not capture it. Most commercial solar panels, the kind
that you would put on your house, the efficiencies around eleven,
meaning that you're losing a lot of that energy not
is not converting into electricity. What that means is that

(03:00):
you have to buy more solar panels to cover more space,
to collect more solar energy, to generate enough electricity to
meet your needs. Obviously, if solar panels were efficient, which
is impossible, by the way, physically impossible. People have proved
it with mass, then if you if it were a
hundred percent efficient, you wouldn't need as many solar panels

(03:22):
in order to do what you need to do. That's
just not the case in reality. So instead we often
have to buy more than what we would like cover
a larger area, and even then again you're limited to
collecting electricity or generating electricity. I should say during the
daylight hours, and electricity also is a use it or

(03:43):
lose it kind of thing, meaning that if you don't
have an immediate use for that electricity and you don't
have a way to store it, you lose that electricity.
You have to use it when it's generated, so you
need to have some sort of battery system as well,
so that in the times when you're not using the
electricity you're generating, you can still save it for later use.

(04:04):
And until recently, batteries have been really expensive for the home,
but Tesla's power wall has kind of led a revolution
in that space, and we're starting to see more affordable
versions of batteries hit the home market. Okay, all of
that's out of the way to just say that the
traditional solar panel approach has its drawbacks. Now let's talk

(04:24):
about solar towers, because they take a totally different approach
to harnessing electricity from the sun and it's really pretty clever,
and they can harness electricity or they can generate electricity
day or night. So you might wonder, how is that
possible when when the Earth rotates so that the sun
is no longer shining on a solar tower, where's that

(04:45):
electricity coming from. So here's the way it works. Now,
I'm largely going to refer to a company called Solar Reserve,
which is here in the United States. Solar Reserve is
just one company that is building struct is like these
around the world, So I don't mean to suggest they're
the only one. I'm using them as the example because

(05:07):
that so much of their information is available to actually
read about and understand. So it's a really helpful approach.
Solar towers, Well, first of all, the name kind of
gives away the main feature. There's a tower in the
center of this uh structure or really multiple structures. So
how tall are these towers will according to Solar Reserve,

(05:29):
the height of the tower and its thermal receiver. More
on that in just a second, tends to be six
d forty feet combined or about one. So you've got
this tower in a a large area. You want to
have an area that gets a lot of solar exposure,
otherwise this is not a practical way of generating electricity.

(05:51):
So imagine like a desert, nice flat desert, lots of
sunlight hitting that desert every typical day, and you have
a tower with a receiver on the top of it. Now,
that receiver is actually a series of dark panels. And
these aren't solar panels, not in the way that you
would put on top of your house. They are not

(06:12):
converting sunlight into electricity directly. Rather, they are panels that
transfer thermal energy. In other words, they're all about transmitting heat.
So these panels have sixty six thin wall straight tubes
in them. Those tubes are designed to conduct heat from

(06:33):
the outside to the inside of that tower. And the
tubes are made out of a steel alloy that's covered
in a high absorptivity black coating to maximize the amount
of energy the panel can absorb. So you've got sunlight
shining on this tower, Well, how is that enough to
generate electricity on its own. It's not. In fact, what

(06:53):
you end up doing is surrounding the tower with mirrors. Now,
Solar Reserve uses mirrors that they call heliostats. These helio
stats are mounted on arms essentially that allow them to
track the motion of the sun. That way, the mirrors
maintain the ideal angle to uh to reflect the sun's

(07:15):
light directly towards the top of that tower. At that
receiving point of the tower. On those panels, and we're
talking about a lot of mirrors. Solar Reserve has one
area it's called the Crescent Dunes. That's there. There um
tower that they have in the US, and the Crescent
Dunes towers surrounded by more than ten thousand mirrors covering

(07:38):
a fifteen hundred acre field. So this is a big operation.
You've got to have a lot of open land for
this to work. That's obviously one of the potential drawbacks,
right that you need a place that's going to have
a lot of solar exposure and you need to have
enough space to make it make sense. But assuming you
have both of those things, you can do something pretty incredible.

(08:01):
I'll have more to say about solar towers in just
a second, but first let's take a quick break to
thank our sponsor. So what these mirrors do is direct
that sunlight up at those dark panels I was talking about.
Remember I mentioned there were fourteen. These fourteen panels are

(08:22):
divided up into two groups of seven. Each group of
seven represents kind of a circuit, and that circuit is
not for electricity. It's rather for a circuit of pipes
that are circulating liquid salts. So liquid salts are pretty cool, uh,
which is a weird way of putting it when you're
talking about thermal energy. But liquid salts can hold on

(08:46):
to more heat than water can and can remain in
liquid form, so in other words, they don't vaporize into steam.
And what solar reserve does is it pumps around five thousand,
eight hundred gallons of liquid salts per minute through the
receiver circuits that run back and forth across the inside

(09:10):
of these black panels. So imagine you've got this really
tall tower. At the top of the tower, you have
these fourteen dark panels, and then let's just take seven
on one side. You've got seven of those dark pounds
on one side. On the inside of those panels, you
would see this criss crossing of a pipe that is
circulating liquid salts through the pipe. Heat from the outside

(09:33):
comes into the tower and it begins to heat those
liquid salts running through that circuit. It's a simple heat transfer.
And if you've listened to our podcasts about things like
refrigerators and air conditioners, you know about you know how
this what the principles are, how how this is based
same basic thing. You want to give as much surface area,

(09:54):
you know, you want to. You want to dedicate as
much surface area as you can to heat and have
the liquid salts cross over as much of that surface
area as is possible to heat up the liquid salts.
As the liquid salts move through the circuit, they become molten,
so incredibly high temperatures. So the low side of the
temperatures for these liquid salts is five fifty degrees fahrenheit

(10:17):
or two degrees celsius. That's the low end. That's that's
the chili site. If you want to talk about how
hot they get, they can get up to a thousand
fifty degrees fahrenheit or five hundred sixty six degrees celsius.
That's really impressive. And so you've got this massive amount
of stored thermal energy. It's all inside the molten salts.

(10:40):
So you've got a lot of heat stored up. What
good is heat, Well, you can use heat to do
the same thing that is done in power plants all
over the world. You use heat to turn water into
steam and use the steam to turn a turbine which
generates electricity. It's an incredibly simple idea. It's the basis

(11:02):
of almost every other type of power plant. With the
exception of things like solar panels that are used in
solar farms like that, that's generating electricity straight from sunlight,
as opposed to using that electricity to somehow turn water
into steam. That would be ridiculous. You would lose way
too much energy in that approach. But things like coal

(11:23):
fired plants, even nuclear power plants, you're talking about generating
electricity by heating up water into steam and using that
steam to turn a turbine to do work, and that
turbine ends up being an electric generator and you get
electricity from that. So the the purpose of the solar
tower is really just to collect heat. That's it. It's

(11:46):
not doing anything magical. It's just generating tons and tons
of heat. I know that tons is not really a
unit when it comes to heat or temperature, but you
understand what I mean. It creates an enormous amount of
heat and the molten salts and go into a big
storage tank. And it's pretty cool because that storage tank
ends up being a a way of holding onto the

(12:08):
heat for a long time. According to Solar Reserve, the
company says that the molten salt only loses one degree
of fahrenheit, or about point five, five, five, etcetera, etcetera
degrees celsius in heat per day. So, in other words,
if you have a long stretch of overcast days, you
still have this massive that really tank. It's not a vat,

(12:33):
it's a tank because it's completely enclosed of molten salts
and they hold onto that heat, which means you can
continuously pump that through your water tank in order to
heat water up to steam. And when I say pump,
you pump it through. It's a very similar circuit that
you would find at the top of the receiver. The

(12:53):
molten salts don't mix with the water. Instead, you have
a pipe that runs through the water tank. The molten
salts run through the pipe and transfer some of their
heat to the water through the material of the pipe itself.
So you're not laying the molten salts in the water
mixed together. That would be ridiculous. Instead, you're having the

(13:14):
molten salts move through a pathway, and as they move
through that pathway, they heat up the water. The water turns,
the steam turns the turbine. Then the steam goes through
a condenser to condense the steam back into water, and
it goes back into the water tank. So once you
use the molten salts to transfer some of this heat,
they they've lost that thermal energy. They're now moving into

(13:37):
a different tank. It's a tank to pump the liquid
salts back up into the top of the tower. So
the neat thing with this system is that you can
use it to deliver electricity day or night. You've got
so much stored heat. After you get the system up
and running and it's generated enough molten salt, like it's
created enough molten salt through this process to allow this

(13:59):
to happen, you can deliver electricity on demand twenty four
hours a day. And that's how all power plants work.
They deliver electricity on demand. They're not uh set to
a certain level, and then if you don't meet that level,
they you know, just that electricity goes to waste. They
base it upon what the current demand. And I don't

(14:19):
mean that as a pun, but it came out of
that way. Chris would be so proud. They don't. They
respond to whatever the demand is at that time to
produce the amount of electricity needed. So super interesting way
of doing this by using the sun as an energy
source to create the heat needed to turn water into

(14:40):
steam and turn a turbine as opposed to a fossil
fuel like coal or oil, or a nuclear fuel in
the case of nuclear fusion. Now, if we ever are fission,
I should say nuclear fission. I know all of you
were ready to write in, and you should be because
that was a silly mistake I made nuclear fusion. If
we ever crack that nut, solar towers may seem quaint

(15:02):
in comparison. But that's a very difficult problem in physics
to to solve. So we're still waiting on that. I
have a little bit more about solar towers to talk about,
but before we get there, let's take another quick break.

(15:22):
So the neat thing about this, obviously is that if
you do have that amount of space, you can really
offset a lot of the community's electricity needs with a
solar tower. And uh, the company thinks that the lifespan
of these solar towers is somewhere in the area of
about thirty years, meaning that after thirty years we would

(15:43):
have to start to replace parts because just of wear
and tear, or they would not be as uh efficient
as they had previously been. So, for example, those panels,
if the panels become less efficient at transferring heat over time,
you would definitely want to reply them because you would
be transferring less heat to generate your Your molden salts

(16:04):
will be at a lower temperature. It would require more
of them to turn water into steam, who become less
efficient overall. So you have to make sure that, uh,
you're all the parts are working very they're very few
moving parts, which is awesome, but you have to make
sure they're working throughout the lifetime of the facility and
then obviously replaced parts as needed. So the question then

(16:27):
becomes doesn't make financial sense to switch over to using
solar towers at least to offset electricity generation in a
particular area. To answer that question, you have to look
at a lot of different factors and it is way
more complicated than just a simple yes or no. For
one thing, how much solar exposure is the area getting. Obviously,

(16:51):
if you're not getting a lot, then that's not going
to be a good choice for solar towers or a
good place to about a solar tower. I should say
also not just where and and you know what time
of year do you get solar exposure? Those would be
two big things. But how much does electricity cost in
that region already. And the reason why you have to

(17:12):
ask that question is if you were to provide electricity
through the solar reserve system, would it make financial sense
to make that switch. If coal is incredibly cheap in
the area, then financially it might make more sense to
stick with coal, even though we all know there are
big environmental drawbacks to using coal. You create a lot

(17:33):
of fossil fuels, you have a big carbon footprint that way,
but it's hard to argue with the dollar cost of energy.
If that dollar cost is higher with solar reserve, that's
a tough sell because not everyone is willing to spend
more money to keep their homes, uh, you know, supplied

(17:56):
with electricity, just so that they have a lower car
been footprint with the electricity generation. It's just the truth
of the matter, and some people don't have the money
to afford the luxury. Obviously, Now, if solar reserve is
able to be significantly less expensive than whatever the alternatives are,
that's a huge win for solar towers. So it's a

(18:19):
lot of different questions along those lines. Obviously, there are
other questions to ask, like what is the carbon footprint
of actually building the solar tower, but I would argue
that whatever it is, it probably has significantly less than
the carbon footprint produced over the lifetime of a coal
plant or a gas plant or oil plant. Um I

(18:42):
think that's pretty a pretty fair assumption, but it still
could be a very large upfront uh carbon footprint creation there.
So that's kind of the approach that I wanted to
talk about, this idea of being able to generate electricity
twenty four hours a day using sun power without being
a solar panel, and I thought it was a really

(19:03):
cool approach to that, something that could really get around
a tricky problem with solar power because I know a
lot of critics point out, hey, if the sun is
not shining, then you're not making electricity. Well that's true
with your traditional solar panels, but not with solar towers,
assuming that you don't enter into some cataclysmic event where

(19:27):
you've got crazy overcast skies for a really long time,
in which case, if we do have that, we're gonna
have other problems besides where we get our electricity. Um Oh,
and you might want to know how much electricity can
one of these facilities generate. It would be good for
me to tell you that. So Solar Reserves says that
depending on the plant design, it can generate between fifty
and two hundred megawatts on electricity, and one megawatt is

(19:51):
enough power to supply around a thousand homes. So you're
talking about with one solar tower facility between fifty thousand
and two hundred tho and homes. Obviously, there are a
lot of cities that are bigger than that, and in
order to supply the electricity for those cities, you would
need multiple solar tower facilities to to do that. Uh.

(20:12):
And again that's another question is where would this be
most appropriate. Obviously a lot of desert towns that have
medium to small populations, this would be an amazing approach
to generating electricity, but it might not work for someplace
like New York City for multiple reasons, the population size,

(20:32):
the lack of land to dedicate to solar power. Obviously,
that's another issue is that if you are going to
dedicate a land to a solar tower facility, you're not
able to use that land for other stuff, at least
not easily, So that's another consideration. Obviously, you don't want
to end up going to a place where you're dedicating
land that could be otherwise used for a more productive purpose,

(20:56):
possibly not energy related. It might be food related, or
water or something along those lines. So a lot of
things to take into consideration. But I think it's a
very elegant approach to generating electricity in a using a
renewable resource the Sun's energy, and very low impact to
the environment. The modern salts are inert they're not dangerous. Uh.

(21:18):
They're obviously dangerous in their temperature. You would not want
to touch them while they're at a thousand fifty degrees fahrenheit,
but they're not dangerous to the environment. Uh. And these again,
the systems are separate. The water system and the salt
system are separate from each other. So I think it's
a really interesting approach. Now I'm curious to hear what

(21:39):
you guys think about the most promising energy sources for
the future. Do you really think that solar power is
going to become a major way to offset our electricity generation.
I'm pretty sure. I feel fairly confident it's never going
to be the primary way we generate electricity. I don't

(21:59):
think gets practical enough to be our primary but I
certainly see it as a very strong contender for a
support system, something that can offset some of our electricity needs.
But what do you think? Do you think there are
other ones that are better? Do you think wind power
is better than solar? Or maybe uh, maybe you think

(22:21):
hydro power is better. Maybe you think geothermal. Maybe you're
looking for that nuclear fusion approach. If that break, If
there is a breakthrough nuclear fusion, that would be an
enormous benefit to all of humanity because we would suddenly
be capable of going into an era of energy surplus,
which would be phenomenal. And I hope it happens, but

(22:45):
there are a lot of challenges to get out of
the way first. So maybe I'll do a full episode
about nuclear fusion in the in the future and talk
about why it's such a tricky issue. Guys, I hope
you enjoyed that classic episode of tech Stuff. I'll be
back soon recording on episodes. I can't wait to hear
from you. If you have anything you want to send
to me, maybe it's a suggestion for a future episode,

(23:07):
send me that via email. The addresses tech Stuff at
how stuff works dot com, drop on by our website
that's tech stuff podcast dot com, and I'll talk to
you again really soon. For more on this and thousands
of other topics, because it how stuff works. Dot com