RERUN: How Solar Towers Work

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 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 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 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

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 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 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 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 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 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. 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 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

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 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, 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. 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

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 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 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

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 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. 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 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 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 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 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, 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 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.

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

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 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 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 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, 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

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 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 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 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 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 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 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.

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 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 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 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, 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 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 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 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

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 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 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 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 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. 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,

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, 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. 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 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 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 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 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, 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