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Solar power plant in space a way to solve earthly problems or vice versa

(ORDO NEWS) — A long time ago more than 80 years ago a well known science fiction writer proposed to build a huge orbital solar power station in space and from there transmit the resulting energy to the Earth by microwaves.

Its concept is still being actively discussed by space agencies and governments around the world. But there are reasons why it is unlikely to be implemented. Nevertheless, the benefits of such stations can be enormous just not for the Earth.

Salvation of mankind or another utopia?

In 1941, Isaac Asimov published the story “Logic” (Reason). In it, as a decoration for the plot, a colossal solar power plant appears, which receives electricity from photovoltaic cells. It then generates microwaves using this electricity, which are directed to the fields of receiving antennas (or rather, rectennas) on Earth.

At that time, Azimov did not have a higher education, and the story, in general, was not at all about energy (but about robotics), so there are not so many details. But it was at this moment that an idea was born that still does not lose popularity.

Yes, there is popularity. In 2008, Japan even passed a special law , which declared the construction of such a station a key national goal. China also periodically declares its intention to build something similar, the United States does not leave this topic. What is the reason for this interest?

The thing is that there are two ways in the world to solve energy problems: efficient and those that we like. In the last century, these paths have diverged significantly. Almost no one likes thermal energy, except for fossil fuel companies: too many people (hundreds of thousands) die from it every year.

The public does not like nuclear energy, and solar energy, as special scientific works show, even with the most generous assumptions, cannot cover the main part of humanity’s needs for electricity. And we didn’t even dive into the problem of heat, and yet our species spends much more energy to generate it than to generate electricity.

Solar power plants on Earth have such limited prospects for a reason. The reason is in basic physics. On our planet, night reigns half the time, and electricity is needed even when the sun does not shine.

A huge part of the daytime the sky is covered with clouds, and as a result, in a year a solar power plant in a temperate climate produces 8 times less energy than a nuclear power plant of the same capacity. Because the nuclear power plant operates more than 8000 hours a year, and the solar power plant in the area.

Advantages of space scale

Now it is easy to understand why Asimov became so interested in space solar power plants. There are no clouds in Earth orbit. If you place the station in geostationary orbit, it will work all year, except for a small number of short eclipses (72 minutes each, near the equinox, when the Earth comes between it and the Sun).

With an efficiency of solar panels of 40% this is what gallium arsenide photocells have from 1 square meter the station will generate 4,400 kilowatt hours per year. The planet consumes approximately 25 trillion kilowatt-hours, which means 6 billion square meters of solar panels will be enough to meet all its needs.

This seems to be a lot 6 billion, but in square kilometers it is only 6000 (about four squares of St. Petersburg). True, gallium arsenide is expensive, and the efficiency of silicon solar cells is lower, only 25%.

Any solar orbital power station, in addition to the photocells themselves, will need its own mini-power line and a large microwave emitter, otherwise it will not work to transfer energy to Earth

The advantages of cosmoSES seem so self evident that many scientists and science fiction writers began to imagine foreign super civilizations based precisely on solar power plants in space let’s recall the well known “Dyson sphere”.

This feeling was especially intensified after the start of the development of Starship, a large carrier, from which output prices of ~$100 per kilogram are expected.

This is dozens of times lower than modern prices for the delivery of goods into space, and it seems that this should allow the deployment of solar energy in space.

Let’s open Western scientific and pop resources, they write right there : “The time for space solar energy may finally come. The old sci-fi dream could become a reality in the next decade or so, according to a number of researchers.”

Invisible disadvantages of space energy

By itself, the solar cell can be very light literally 100 nanometers thick. But it needs a substrate to provide mechanical strength, a conductor to dissipate current, a glass coating to reduce wear, and very important in space conditions a metal surface on the back to dissipate heat in the form of infrared radiation.

If heat dissipation is insufficient, the battery will start to overheat and degrade quickly.

It follows from all this that each solar kilowatt in open space today cannot be significantly lighter than 6-7 kilograms. And taking into account the equipment transmitting to Earth (not even created yet), at least 10.

Then his withdrawal to Starship will cost $1,000 (10 x 100). Yes, this is the cost of solar panels with installation on Earth.

Since the installation accounts for half of the cost of solar power plants, then formally the price per kilowatt of space solar power plant doubles. But the return on it is growing almost tenfold, that is, commercially it is more than justified.

However, there are nuances. Here’s the first one for you: any carrier puts many times less cargo into geostationary orbit than into low near earth orbit.

This means that the cost of launching there is many times higher than the cost of delivery to low orbit. It turns out that almost ten times higher output will be covered by the cost of delivery, which will increase the price of photovoltaic cells by almost a dozen times.

It is possible to deploy solar power plants in low orbit, but then half the time they will be in the shadow of the Earth, that is, the return drops by half at once.

True, it will still be higher than on the surface of our planet. But in practice, the main problem of solar panels the intermittency of their production cannot be solved in low orbit.

In order for electricity from orbit to compete in price with electricity from terrestrial sources, it is necessary to put cargo into geostationary orbit no more than $ 200 per kilogram. This is realistic only if Starship can launch a kilogram of cargo into low orbit not for $100, but for a multiple of less money.

So far, only Elon Musk himself gives such optimistic estimates. Despite all his undoubted merits in the field of space, the implementation of such a launch cost raises serious doubts among everyone else.

The sizes of ground receivers, rectenna fields that receive microwave radiation, are so large that some projects propose to make them perforated so that crops can be grown under them. The idea is not bad, but the rectenna fields themselves, ten kilometers in size, will not become cheaper

However, let’s assume that Musk is right, and a withdrawal cost of $20 per kilogram can be achieved. Alas, even then supplying the Earth “in Asimov’s way” will not work.

Because here the second unpleasant nuance comes into play: problems with the delivery of electricity from space to Earth. Microwaves can indeed penetrate the earth’s atmosphere, they can reach the rectenna fields of a city or factory in need of electricity and supply it with energy.

But if there is moisture in the atmosphere at that moment, a significant part of the microwaves will be absorbed. And the more moisture, the more will be the loss.

This means that the power that the ground consumer will receive will depend on the weather. In severe bad weather, there will be no electricity on Earth at all, and in fact, in some parts of the planet, storms and hurricanes are not uncommon.

Other means of remote delivery are no better. Lasers in the optical range generally almost “do not penetrate” dense clouds. Microwaves were chosen by Asimov for a reason: in the case of the earth’s atmosphere, they are indeed the most efficient means of delivery available.

Okay, let’s imagine that we found some kind of magical microwaves (magic because physics is not yet familiar with such), which would be realistic to transfer energy through dense clouds and anything else. Then we will run into the third unpleasant nuance the physics of wave propagation.

The best means of sending microwaves on Earth cannot make a beam of even 0.9 degrees. Geostationary orbit 36 ​​thousand kilometers from us. This means that the spot of microwaves on our planet will have a diameter… yes, you calculated correctly.

From 10 kilometers. Question: How are we going to build receivers of this size near every major city or factory? How much is it? You can choose an orbit lower than geostationary , but still giving round the clock exposure to the sun (sun synchronous)

But what will we do when, as the Earth rotates, the rectenna field near New York moves out of sight of the orbiting solar power plant? Water the same field with another station? Who will coordinate all this? How much will it cost to duplicate orbital solar power plants for these purposes?

Finally, Tuvalu, Vanuatu and other settlements of Mirny, located far from large power lines. Are we going to build multi-kilometer rectenna fields for everyone? Wouldn’t such a reduction in price be too expensive?

Proponents of a ring solar power plant on the Moon‘s equator argue that solar panels could be made from local materials there. But who will do it and who will build lunar factories? Automata are clearly not capable of this: it is enough to recall that in half a century of attempts to dig on Mars, they could not reach depths of more than 45 centimeters. So far, completely deserted robotic factories are just a utopia, and until the advent of strong artificial intelligence, they will definitely remain so

Some of these questions have already been answered. It turned out that a gigawatt of power receiving microwave rectennas on Earth would cost 0.2 billion dollars. Or $200 per kilowatt of power. This is still back and forth for megacities, but for smaller settlements everything is bad: microwaves cannot be focused into too small a spot.

That is, we will have to build cyclopean receiving fields even near modest energy consumers, otherwise the microwaves emitted by orbital solar power plants will simply heat the soil, and not turn into electricity for earthlings.

Finally, let’s add one more point: in space, a solar battery often degrades eight times faster than on Earth. This is especially true for orbits lying above the Earth’s magnetosphere, that is, almost all orbits constantly illuminated by the Sun.

This is not surprising: there it is not protected by the Earth’s atmosphere from short electromagnetic waves that can damage semiconductors. That is, it must be changed eight times more often, and the output per unit of power in general for life will be equal to the earth. It looks like Earth’s energy problems will have to be solved on Earth.

Where might the calling of orbital power plants actually lie?

And yet, the idea of ​​large solar power plants in space is not nearly as hopeless as the first half of this text might seem. It is definitely impractical to transfer energy to the Earth in this way. But what if instead we transfer it from Earth orbit to other parts of space?

No, we are not talking about building tens of kilometers of rectenna fields to receive microwaves from orbital SPPs on Mars in order to refuel Starships returning to Earth there..

This does not make sense: the microwave divergence will be such that the rectenna fields will have to occupy almost the entire surface of the Red Planet, otherwise the bulk of the energy will simply heat the ground, passing by the recipient. Nor would it make sense to supply energy to a colony on the Moon.

Firstly, from the geostationary orbit to the moon is ten times farther than to the Earth, and the receiving fields of the rectennas there will have to be made 100 kilometers in diameter.

Secondly, the solar constant on the Moon is not much different from that in geostationary orbit. Thirdly, it has already been written above why solar panels are generally not the best way to power large bases and colonies.

However, if we transfer the consumers of energy from orbital power plants into space, then a number of seemingly insoluble problems will be simplified for us. You can refuse microwaves with their huge beam divergence.

Yes, they are ideal for carrying energy through the earth’s atmosphere, but if the recipient of energy is outside this atmosphere, it is much more logical to use lasers. They have a much lower beam divergence so much so that even millions of kilometers from an orbital solar power station, laser beams can deliver kilowatts per square meter to the addressee.

This is not a very good choice if you are going to power some relatively compact object: the ISS or even a rover on another planet or asteroid. In space, it can be very difficult with heat dissipation. Therefore, a conditional lunar rover, which we decide to “cheer up” with additional kilowatts from a laser beam, may begin to overheat, which is undesirable.

An orbital station or ship in orbit exposed to microwaves will need additional cooling radiators, which will increase their mass.

Then it is easier to equip the station (ship) with solar batteries, since the back surface of photocells in space automatically works as a cooling radiator. Well, in this direction, the benefits of orbital solar power plants are small. But there is one more way.

The traditional problem with all spacecraft is that they need to throw something back in order to move forward. Therefore, it is very, very difficult to accelerate faster than tens of kilometers per second. Meanwhile, the Saturn system is almost 10 billion kilometers from us, and 40 trillion kilometers to the nearest star.

It is easy to see that at a speed of 20 kilometers per second we will fly to Saturn’s satellite Titan for 15 years, and to Proxima Centauri more than half a thousand years. It’s been too long, can’t it be faster?

Artist’s view of solar sail in space

Can! Back in 1899, physicist Pyotr Lebedev demonstrated in an experiment that light is able to exert physical pressure on other bodies, without “recoil” in relation to the spacecraft itself, accelerated by a source of light radiation. And in 1925, Friedrich Zander suggested using this physical fact to move spaceships in space.

Before the invention of lasers, such a sail was called a solar sail: a spacecraft had to deploy a huge plate of thin material in space (so that the total mass of the ship was not too large) and then let the Sun accelerate itself.

While moving along a spiral orbit, so as not to move away from the sun too quickly, a ship with a solar sail, according to calculations, could accelerate to hundreds or even thousands of kilometers per second, depending on the size and mass of the sail.

And yet his thrust was very small. In near-Earth space, a sail of 800 by 800 meters receives from the Sun only 5 newtons of thrust directed outward of the solar system.

A revolution in this matter was the invention of the laser. It turned out that with its help it is possible to concentrate radiation into a beam that will retain a noticeable density even over 100 million kilometers.

But here, too, a problem arose: serious laser heating, according to calculations, could overheat the spacecraft that is designed to push the laser sail. And even if the main part of the space probe was covered with a particularly thin sail, it was still too massive to be quickly accelerated to reasonable speeds.

Back in the late 1990s, the NASA Institute for Advanced Study calculated that one of the best applications of laser power transmission in space could be a laser sail, and an extremely thin one at that.

And a few years ago, Philip Lubin’s team at the California Institute of Technology decided that the easiest way to solve all the problems of a laser sail was to radically rethink what the words “spacecraft” meant.

Lubin drew attention to the fact that, in fact, not much is needed from a research probe: optical or IR sensors and a communication system. However, modern electronics already makes it possible to create a matrix of mini sensors on a single chip, and integrate a communication chip into the same place.

An antenna for communication with the Earth can be placed along the perimeter of such a silicon plate with a microcircuit applied to it and that’s it, the research apparatus is ready.

Lubin believes that a probe with a mass of no more than a gram will have the optimal parameters for exploring the solar system, which is quite realistic if we recall that the typical thickness of a silicon substrate is 300–800 micrometers.

The conditional name of the concept is Directed Energy Propulsion for Interstellar Exploration (DEEP-IN). Despite the extreme lightening of the probe, it will be possible to carry out feedback with it (due to the directed emission of the radio signal by a “probe on a silicon wafer”) at a distance of several light years: the radio signal power is enough for this, although the communication speed will be extremely low.

A laser sail is noticeably more practical than a solar sail: it gives much more kilowatts per square meter of surface. However, so that the scattering of the beam with distance does not slow down acceleration, it is still better to make its trajectory spiral

The scattering of the laser beam will be noticeably reduced by using a group of lasers operating on the principle of a phased antenna array. But even so, sending a probe to Proxima Centauri would require enormous power.

If the solar orbital power plant radiates with a total power of laser transmitters of 1 gigawatt, then the “probe on a microcircuit” will reach the nearest star in 193 years. If the power of the emitters rises to 100 gigawatts, then in just 21 years. This is an unthinkable short time, unattainable by any other means today.

A serious problem of the project turned out to be that it has not yet been possible to achieve a gram mass in the transition to demonstration devices. Of course, not least here is the fact that Lubin’s group is not an electronics engineer, so their systems are far from being the most compact in the world.

But there are also objective difficulties: in order for a probe of this kind not to fail under the influence of cosmic rays, it has to be made noticeably thicker (and with duplication of many functions) than conventional electronics.

However, plates with a diameter of 10 centimeters and a mass of about 100 grams can already be made within the framework of this technology. If such a probe were to be delivered to Mars, a laser with a power of only 100 kilowatts could handle it, and the travel time would be only three weeks.

Longer flights will require more power up to 10 megawatts to Pluto. The probe will fly to the Jupiter system in 120 days, and to Pluto in three years. For comparison: the New Horizons probe flew to Pluto for nine years, three times longer.

Why does this porridge need a sun ax?

The question arises: why is a solar orbital power plant needed for all this, why not a nuclear one? In the end, it is not so difficult to create a nuclear reactor with either 1 or 10 megawatts of electrical power and can be put into orbit “assembly”.

By grouping modules of this kind, it is possible to obtain laser systems for 100 gigawatts. Why rest on solar batteries, because at high power systems they turn out to be much more material-intensive than nuclear reactors?

The latter is indeed true. One kilowatt of solar battery power now weighs at least 6-7 kilograms, that is, 10 megawatts of such power will be pulled by 60-70 tons.

It is definitely possible to create an atomic reactor with a higher specific output. For this reason, when colonizing other celestial bodies, nuclear reactors inevitably outperform solar panels. But in open space the situation is markedly different.

It is not enough to put conditional 10-megawatt reactors into orbit and assemble Lego from them to provide energy for laser emitters. Any energy source in space would have to dissipate excess heat.

A 10 megawatt reactor will give it at least 10 megawatts, which means that it will have to deploy very large cooling radiators, no less than 1000 square meters. In addition, these radiators will have to be covered with an anti-solar screen, otherwise the rays of our luminary will prevent the reactor from cooling normally.

How much will these radiators weigh? Who will monitor their correct deployment in space? But it’s enough for one plate to “go the wrong way”, and the reactor will be forbidden to turn on so as not to overheat the core.

Solar arrays in space can power spacecraft as they descend from Earth’s orbit to more distant regions of the solar system

Compare this with an orbital solar power plant. It consists of many modular systems of moderate power, and not from a 10 megawatt installation at once.

One did not turn around – you can send a new one to replace it. Cooling radiators are already integrated into each solar panel – the back side of each photocell actively emits in the IR range, cooling the device.

An atomic reactor on the surface of Mars or even the Moon can eventually be placed in the zone of eternal shadow, buried in the ground, for example.

A small radiant radiator-umbrella, stuck over such a reactor, can be heated even to red heat in the truest sense of the word; nuclear reactors of the past already had such cooling systems.

Moreover, when the temperature of the radiator doubles, its heat dissipation capacity immediately increases by 16 times, and when it is quadrupled, by 256 times.

But in outer space, we cannot get by with such an “umbrella”. In order for the radiator to remove heat from the reactor at, for example, 800 degrees, you will have to keep the outer walls of the reactor heated, and, in fact, it will all become a cooling radiator.

But laser systems also need cooling systems, and if their cooling radiators are next to the compact and red-hot radiators of the reactors that power them, then serious problems will begin.

Of course, you can place the reactor on a kilometer rod, throw a cable along it (by the way, it will also need to be cooled). Then the radiators of the reactor will interfere little with the radiators of laser installations. But all this will add complexity, material consumption, and cost to the system.

In other words, for the foreseeable future, solar orbital power stations remain the optimal choice for providing energy to lasers that “blow” into the laser sails of space probes.

And if humanity gets around to realizing all of this, our knowledge of the distant solar system and the nearest stars will be much greater already in this century.


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