Can we harness solar energy from other stars?

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1:05Hello, my name is Najah Salehi, and I'm speaking to you from Baghdad, Iraq. I'm standing outside looking at the night sky. This is Eileen Craigie. I'm in Vienna, Virginia. I can see a few stars in the sky and there's a lot of fireflies out here. As the night falls all over the world, people are looking up to the night sky. Cyprus is uniquely positioned for stargazing. Our sky in Nicaragua is one of the best in Central America. Some see a thick covering of cloud, some the faint glow of a distant city on the horizon. I have a nice view south of the mountains in front of us, but there's too much light pollution to see much of the Milky Way.

1:58But some, lots in fact, see the twinkling of thousands, millions, no, billions of stars. Scorpius is beautiful and Cygnus is swan. Ursa Major, Drago, Woods, Leo. The stars are big and bright. All famous constellations are visible. It's so pretty to look at the stars here. I swear the longer you stare, the more you see. This is CrowdScience from the BBC World Service. I'm Alex Lathbridge and this week I've been spending a lot of time pondering what I see in the night sky. This nocturnal curiosity has been piqued by a question from one of our listeners in Uganda. Hi CrowdScience, I am Dixon Mokisa, a nutritionist from Uganda.

2:44My question is about solar energy. Are there stars other than the sun known to provide solar energy? What are the opportunities to harness energy from the stars today or in the future, other than the sun? Thanks CrowdScience. Stars aren't just the tiny twinkles that we here on Earth see in the night sky. Every single one of them is a monumentally huge ball of raw energy, just like our Sun. But compared to our Sun, other stars are a long way away. So can we harness any of that immense power for ourselves?

3:29I'm being led through a dim room, lit up only by red light bulbs and what sounds Sounds like a bank of computers and other machinery humming faintly in the corner. Okay, so this is the control room of what we call our C14 telescope. But just past the control room is what I'm really here for. We're into the dome space now. What? This is a telescope! Okay, look, that sounds silly when I say it because we've come to an observatory, of course there's going to be a telescope. But this is not what I expected. This looks like a cannon. Probably the reason it's not quite looking like your classic idea of the telescope is maybe it's not quite long enough and thin enough.

4:12And this is because it incorporates mirrors instead of lenses, this particular one. Right now, I'm at the University College London Observatory in the UK, and I'm inside one of the telescope domes. There we go, system is initialised. Picture a round room with a circular metal platform in the middle, and the beautiful curved dome of the ceiling stretching above our heads. The telescope takes up most of the space in here, but there's just room for us to edge our way around it to the control panel. Take the eyepiece cover off and there you have the eyepiece. Showing me around is astronomer professor Steve Fossey.

4:54He's the director here and he's going to help me work out just how much of the light of the stars actually reaches us here on Earth. Or, in other words, how bright the stars are. But what I do need to do is open the dome so we can actually look at something. So here we go. I think we're ready. Open observatory. The dome is opening very dramatically. It's like a single slice is being pulled out very slowly. And we're seeing, oh, the night sky. So how do we measure the brightness of stars? Well, there's a system that we use using a peculiar unit called magnitudes. And these magnitudes are slightly weird because the scale is inverted, seems back to front.

5:48this goes back to antiquity when astronomers used to look at the sky and they would rank the stars so they would say there is a star of the first magnitude the first rank if you like there is a star of the second rank which is fainter and there is a star of the third rank and so on but it meant that the brightest stars had numerically smaller magnitudes so basically all these astronomers are going well my star is better than your star my star is number one yours is number two. Correct, exactly right. One of the brightest stars in the northern summer sky is called Vega and Vega is actually what we call a standard star but this is what we use as a reference star to measure brightnesses of other sources.

6:32Vega is actually zeroth magnitude, it's got magnitude zero okay. My star is even better than the number one star, it's number zero. Okay No, it gets better because the brightest star in the sky is Sirius. And Sirius is actually brighter than Vega. And if you're brighter than Vega, who's zero, where do you go? Well, you go negative. I'm leaving. Sirius is magnitude minus 1.4. Do you ever think that the astronomers are just, everyone sit down in the room and go, OK, we need to come up with a new system because my star is better than your star has made things very complicated. We're in negative numbers now.

7:16If that wasn't confusing enough, the scale is also not a linear one. It's logarithmic. So each step, the increase in brightness, is proportionally bigger the further up you go, or down. It's confusing, but there's a good reason for it. The logarithmic side actually makes it easy to compress a huge brightness range into a set of numbers that we can all Talk about without having tons of zeros at the end of the number So one star a magnitude brighter than another is two and a half times brighter First thing we have to do is take a look at our reference star So I'm gonna point to Vegas So let me set the telescope and what you will see happen is the telescope and the dome in a perfect ballet of motion

8:10So this is the wonderful Vega magnitude zero. This is incredible. Basically, I can see lots of tiny faint stars around it. But there's this one perfect point. You'll see it's twinkling. Yeah, it is. It's sort of, there's that slight pulse. But that's amazing. So I can see that with my naked eye. but I can't see the tiny little stars around it that I can just make out on the telescope. And that goes to show the magnitude, just how bright it is. Exactly. And so I suppose that's why people used it as a standard. It's there. It's there. It's easy for you to feed those photons into your telescope system and do your calibrations.

8:56So, Vega is very pretty, very bright and very important. But let's get back to Dixon's question. He wants to know if we can use the light from other stars in the same way that we use the light from the one at the centre of our solar system. So, simple question, how bright is starlight compared to the brightness of the Sun? We actually have a magnitude for the Sun and it's about minus 27. And we can compare that to, say, the brightest star in the sky, which is Sirius. and Sirius is 25 magnitudes fainter than the sun and 25 magnitudes is about a factor of 10 billion. So we're talking about 10 billion times less light energy to work with compared to the sun.

9:46Okay, well that's not great but Sirius isn't the only star in the sky. There's trillions of them. They might add up to light that we could use. So the cumulative output of the night sky turns out to be about 100 million times less bright than the sun. So you're saying there's still a chance? Well, let's say, theoretically, we would consider building a solar panel large enough to collect some power. we would probably have to build a panel that's about 500 meters on a side. For reference that's a solar panel the size of let's say 35 football fields. And this would generate one watt of power which is just about enough to power a very small LED.

10:45It's not looking good Dixon I'm sorry. I think it's going to require some very serious engineering. Sorry, was that a star pun? Serious engineering? OK, Dixon, I'm not going to give up just yet. If Steve thinks that getting energy from the stars requires some serious engineering, then I need to speak to a serious engineer. My name is Henry Snaith. I'm the Binks Professor of Renewable Energy at Oxford University. Sorry, did I say serious engineer? I meant serious professor of physics. I work on photovoltaic materials and devices. I work on discovery materials that could make it more sustainable and more efficient than it is today.

11:29If there's 100 million times less light coming from the night sky than from the sun, we're going to need some really good technology to pick it up. Something really efficient. Solar panels convert sunlight into electricity. today 95 % of them are made from silicon and they've all basically over time got more and more efficient but they're reaching or have reached pretty much their fundamental limit your highest panel efficiency is a bit under 25%. That's disappointing at that rate we're going to need another 70 football fields worth of panels just to power a small led. The most proven way to get more efficiency is to go to what we call multi-junction cells.

12:16So instead of absorbing all the sunlight in just a single material like silicon, we absorb it in different materials stacked on top of each other that absorb different sections of the solar spectrum. One of the materials that we've been working on is a material called perovskite, and that can be tuned to absorb just the visible part of the spectrum. And then the infrared light still transmitted through that top cell into silicon behind it. And just by putting one extra solar cell on top of the silicon, we could increase the theoretical efficiency up to 45 percent, going up to around eight junctions, eight different materials on top of each other, we could get up to around 60 percent efficiency.

12:58Are there technologies that are available to get solar power in low light? so an application where we need solar power in low light is actually for powering objects indoors we've had that for quite a long time i think most people probably have used a solar panel powered calculator what you're telling me that actually was real like in school that the little calculator that had the solar panel that actually was legit i always thought there was just a battery inside than that solar panels will show. I believe the panel keeps charging the little battery that is inside so that is legit that's a real thing.

13:36Actually lots of the new photovoltaic materials and devices being researched actually generate much more efficiency indoors so we are likely to see new technologies take the lead there. So solar technologies are getting more efficient every day and there are already devices out there designed to work specifically in low light. I can't believe that I carried one about in my pencil case at school. This is sounding promising. Is there anything out there that could make use of starlight? A bright sunny day you'll have about a kilowatt per square metre of sunlight. The sort of brightness of the night sky only gives about a microwatt per square metre.

14:19The solar cell itself will sense that light. You could choose the right electronics. they would generate power and voltage from that very low level of light. So the answer is yes we can but it's at such a lower intensity it's hard to perceive the practical use of that. All right look a micro watt isn't going to boil a kettle or toast a sandwich. You'd need tens of thousands of them just to run that solar powered calculator. But you know what power is power. one microwatts is better than no microwatts, you have to take what you can get. But I still think Henry might be holding out on us. Time to get hypothetical.

15:08We're going to imagine right now, that underneath your desk is a briefcase. It contains about 500 billion pounds. And your job is to harness the light energy of the stars. You've got all the money you need, all the resources you need, how would you approach this problem? And I have to be specific there, because I always tell scientists, you have X amount of money to do this. They always tell me I'd quit academia and go open a bakery. I actually wouldn't. I actually would not quit academia. So I would certainly be putting my solar array in space. I would be working on research on lightweight, very high efficiency, multi-junction perovskite solar cells and other materials.

15:53I'd then build the solar arrays in space, and I'd have some pointing towards the brightest parts of the universe so they can still generate some power when they're on the dark side of the Earth, beaming back to Earth, probably with large microwave lasers, in a very safe manner. So where does that leave us? We can detect light from the stars, although not very much. and it could theoretically power something if that something required almost no power. I reckon we've exhausted all of our star-based energy gathering potential here on Earth. Next, it's time to take this question to where the stars actually live.

16:39Space. You're listening to CrowdScience from the BBC World Service, the show that finds powerful answers in the dimly lit corners of the universe. I'm Alex Lathbridge and I'm trying to find the answer to listener Dixon's question, can we harness the energy of the stars? So, let's say we could make a hypothetical futuristic solar panel, no, stellar panel, which could capture the light of the stars, one microwatt at a time. Henry's suggestion of putting them in space is a great idea. There's loads of room up there in case we need to make them the size of a small city and we can get away from some of the problems of Earth-bound systems too like clouds and light pollution or that pesky day-night cycle.

17:32If we imagine that we put a solar panel around Earth orbit that is already 1.3 kilowatts of energy per square meter. That's a lot of energy already. And of course, the more panels you get, the more energy you get. This is Anders Sandberg from the Institute of Future Studies in Sweden. I'm a philosopher and futurist who've been working on the question about the long-term future of humanity for rather many years. There are, of course, solar panels in space right now, but they're mostly just powering the satellites that they're mounted to. We haven't yet figured out exactly how to get that energy back down to Earth but it could be done.

18:16The idea people typically talk about is actually using lasers and microwavelinks. So you take your light from the sun, convert it to electricity, send it over to where it's needed and then you can power whatever you want. This concept of putting a huge fleet of solar panels in orbit around the sun, beaming power back down to Earth, may sound very futuristic. But the idea has actually been knocking around since the 1960s. It's known as a Dyson sphere, or Dyson swarm. Freeman Dyson was an American and British physicist who did amazing work. And in 1960, he suggested these spheres that bear his name.

19:00The idea with a Dyson sphere or a Dyson swarm is simply you put up solar panels orbiting a star and collect all the starlight. Okay, so let me get this straight. We actually put those panels up close to the star and basically extract all the energy that way. Yep, but the interesting part is, of course, that it's not a solid sphere. You have a lot of free-floating panels and you can angle them so they let the sunlight pass when they're between Earth and the Sun. Right. So is this all completely like in the realms of science fiction, super far in the future? Or could we build like a Dyson structure around our star?

19:40So right now we have a kind of starter Dyson sphere in the form of satellites orbiting the sun. So you could say that Humanity has already started. Although this is kind of the kids' first little Dyson set, it's not very much. In order to build something that takes 100 % of starlight, you need to have a lot of panels. They need to literally cover the entire star. So we're talking about an enormous surface area. So that's just our sun, our local star. In order for us to build a Dyson swarm around another star, we'd actually have to get to that other star. And the nearest star is about four light years away.

20:23And that would take us, what, tens of thousands of years to travel. So how could we humans actually do that? So you can imagine using self-replicating machines. You might send your robot to drift there for 10 ,000 years and then build solar panels and more robots and then start building a Dyson Sphere and then send a happy message home. Okay, we've done it for you. Where do you want the energy? That might take a long while, of course, but the fun part is once you have a Dyson Sphere, you have a lot of energy, including energy to power spacecraft. If you have an entire Dyson Sphere pushing, you can actually send things at near the speed of light.

21:02Wow. Yeah. So that's in the realm of possibility. It's in the realm of possibility. We're not going to do it over the next few decades. So I think current energy concerns are going to have to deal with more conventional energy sources. But in the long run, I think advanced civilizations are going to do things like this. So with a Dyson sphere engulfing an entire star light years away, an advanced civilization could harness the energy from a distant star. But none of us are going to be around to see it, that's for sure. Sorry, Dixon. Let's have one final try at answering your question, though. I wonder if we've been too focused on the power of light, on those photons travelling through space to our eyes and our telescopes and our solar panels.

21:53You see, light is not actually the only potential source of energy that comes from the sun, or from any star, in fact. there's also a subtle and mysterious force that you might not have heard of, known as solar wind. Solar wind is a stream of charged particles and mainly protons and electrons that comes out from the sun in all directions. The boiling soup of ionised plasma on the surface of a star is constantly emitting tiny protons and electrons spezing out into space. The solar wind is like 400 kilometers per second, so it's much faster than anything else, including our spacecraft. Pekka Janhonen is a physicist with the Finnish Meteorological Institute and an inventor.

22:41He's invented a kind of spacecraft which can use the power of this solar wind. Maybe, just maybe, it could use the wind from the stars too. It's called the e-sail. The e-sail or the electric solar wind sail, that's the full name of it. It's a technical device for using the solar wind for propulsive purposes. So how is this technology different from traditional solar panels? Well, at least the purpose is very different. So the purpose of a solar panel is to make electricity, to make electric power. The purpose of the e-sail is to produce a small force, which we call thrust. and this stream of protons and electrons that comes from the sun acts as the momentum in the electric sail.

23:30If you're a fan of space travel, you might have heard of a solar sail before. Solar sails are huge, thin sheets of reflective material that unfold in space and catch the energy of the sunlight, which propels it forward. The bigger the sail, the faster it goes. PECA's e-sail is different. It doesn't even have a physical sail at all. Just a huge long trailing wire on the back of the spacecraft called a tether. This tether spins and as it spins it generates an enormous electromagnetic field. It's this electromagnetic field which acts as the sail. Those protons and electrons hit it, bounce off as if being repelled by a giant magnet and the spacecraft is propelled forward.

24:19This electromagnetic force can be over a million times wider than the tether itself. So for its size, this e-sail could be incredibly powerful. So the electric solar wind sail is simply more efficient than the photonic solar wind sail. This one-dimensional wire or tether can be made extremely lightweight, whereas this kind of two-dimensional membrane that reflects sunlight, Even though it's very thin, it still weighs more per unit of thrust produced. Here's the really interesting part. If PECA's e-sail were to speed out towards the edge of our solar system, the solar wind would get less and less powerful the further you travel from the sun.

25:04Eventually, it drops off completely. But that's not game over, because there's more wind out there. Interstellar wind. Yes, that's right. Energy originating from the stars. There is the interstellar wind, which is the combined solar wind of all the other stars of the universe. So in principle, we could use that interstellar wind also after exiting from the solar system. Okay, so theoretically it would still work, but the thrust generated would be a lot smaller due to the low intensity of that background solar wind compared to what we'd have in our own solar system. Yes, exactly. Our listener question was, can we harness energy from the stars?

25:53What would your answer be? I mean, the technically correct answer is that no, we can't because the stellar light is simply too weak to make any practical use of the photons that come from the distant stars. But yes, with these technologies that I described, you can sort of use it if you go out of the solar system and then you can start to sail with interstellar wind. It's not at the centre of what we are doing, but it's one edge case of what could be done with our technology.

26:26That might not be the answer you're looking for, Dixon, but you've got to appreciate where it's taken us. The light from the stars that arrives on Earth does so after a journey through the depths of space. Those photons represent something more wonderful than just energy. They bring knowledge. Each photon brings a message from a distant world, from places that we can never visit and which might no longer even exist. That's better than powering even the best toaster. Hopefully, Dixon, you agree. Take it away with the credits. That's all from this episode of CrowdScience from the BBC World Service.

27:08This week's question was from me, Dixon Mokisa from Uganda. The show was presented by Alex Leftbridge and the producer was Emily Knight. Thanks, Dixon. If you have a question on any science subject and you want the team to investigate, why not email crowdscience at bbc.co.uk. Bye.

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