# Solar energy: the hope of mankind?

They love solar energy on Habré: here Google builds solar power plants ( 1 2 3 4 5 6 ), here Germany once generated a third of the current energy consumption in solar power plants ...

Comments are divided into 2 categories: “Well done, we only burn oil” and “ EROEI "The production of solar cells requires more energy than they produce!"

An arrogant reader will probably think: How does it produce less than is required for production? He set them up - they work, they don’t ask for porridge, 10 years, 50 years, 100 years - that means the total energy produced is equal to infinity, and they should be profitable at any cost of construction ...

Everything is really like, what are the approaches to solar generation, which limits the efficiency of solar cells, what ingenious ideas have already been implemented and why solar energy somehow does not actively capture the world - under the cut.

# How much energy do we get from the sun?

For every square meter from the sun comes 1367 watts of energy (solar constant). About 1020 watts (at the equator) reaches the earth through the atmosphere. If we have a solar cell efficiency of 16%, then from a square meter we can get at best 163.2 watts of electricity. But we have the weather, the sun is not at its zenith, sometimes there is night (of different durations) - how to calculate all this?

Annual insolation takes all this into account, including the type of installation of the solar battery (parallel to the earth, at an optimal angle, tracking the sun) and makes us understand how much electricity can be generated per year on average (in kW * h / m 2 , excluding Solar efficiency):
 City / Installation Type Horizontally At an optimum angle Sun tracking Astrakhan 1371 1593 2200 Vladivostok 1289 1681 2146 Moscow 1020 1173 1514 Sochi 1365 1571 2129
Those. we see that if we take 1 km 2 of solar panels, set it at the optimal angle in Moscow (40.0 °), then in a year we can generate 1173 * 0.16 = 187.6 GW * h. At a price of 3 rubles per kWh, the _conditional_ cost of the generated energy will be 561 million rubles. Why conditional - find out below.

# The main approaches to obtaining energy from the sun

Solar Thermal Emissions
A huge field of rotated mirrors reflects the sun to the solar collector, where the heat is converted into electricity by a Stirling engine , or by heating water, and then ordinary steam turbines as in a thermal power plant. Efficiency - 20-30%.

There is also an option with a linear parabolic mirror (you need to rotate only around one axis):

What is the price of the issue? If you look at the Ivanpah power plant (392 MW), which Google indirectly invested in, the cost of its construction was \$ 2.2 billion, or \$ 5612 per kW of installed capacity. Wikipedia even joyfully says that it is even more expensive than coal-fired power plants, but supposedly cheaper than nuclear ones.

However, there are a couple of nuances - 1 kW of installed capacity at a nuclear power plant actually costs \$ 2000-4000 (depending on who is building), i.e. Ivanpah actually already turns out to be more expensive than nuclear power plants. Then, if you look at the annual estimate of electricity production - 1079 GW * h, and divide by the number of hours per year, then the average annual capacity is 123.1 MW (after all, the station we generate only in the afternoon).

This brings the “average” construction cost to \$ 17,871 / kW, which is not only expensive, but fantastically expensive. Probably only electricity in space can generate electricity. Conventional gas-fired power stations cost \$ 500-1000 / kW, i.e. 18-36 times cheaper , and always work, and not as lucky.

And the last - batteries are not included in the construction cost, in general. If you add batteries (about them below) or the construction of a pumped storage power station, the cost will come out through the roof.

Solar thermal power plants have the opportunity to generate electricity around the clock, using a large amount of coolant heated per day. Such stations also exist, but they try not to write down their cost, apparently so as not to scare anyone.

Semiconductor photocells (photovoltaics, PV) - the idea is very simple, take a large area semiconductor diode. When a quantum of light flies into the pn junction, an electron-hole pair is generated that creates a voltage drop across the terminals of this diode (about 0.5V for a silicon photocell).

The efficiency of silicon solar cells is about 16%. Why so few?

The formation of an electron-hole pair requires a certain energy, no more and no less. If a quantum of light arrives with less energy than necessary, then it cannot cause the generation of a pair, and passes through silicon like through glass (because silicon is transparent to infrared light beyond 1.2 microns). If a quantum of light arrives with energy greater than necessary (green light and shorter) - a couple is generated, but excess energy is lost. If the energy is even higher (blue and ultraviolet light), the quantum may simply not have time to reach the depth of the pn junction.

In addition, the light can reflect off the surface - in order to avoid this, an anti-reflective coating is applied to the surface (as on lenses in photo lenses), and can be made into a surface in the form of a comb (then after the first reflection the light will have another chance).

You can increase the efficiency above 16% for photocells by combining several different photocells (based on other semiconductors, and accordingly with a different energy required to generate an electron-hole pair) - first we set the one that efficiently absorbs blue light, and green, red and IR transmit , then green, and at the end red and IR. It is on such 3-step elements that record performance indicators of 44% and higher are achieved .

Unfortunately, 3-stage photocells are very expensive, and now ordinary cheap single-stage silicon photocells rule the ball - it’s precisely because of the very low price that they are pulled ahead in terms of Watt / \$, The cost of one watt for silicon photocells with the introduction of giant productions in China has fallen up to ~ 0.5 \$ / Watt (i.e. for \$ 500 you can buy solar cells per 1000 watts).

The main types of silicon elements are monocrystalline (more expensive, slightly higher efficiency) and polycrystalline (cheaper to manufacture, literally 1% less efficiency). It is polycrystalline solar panels that now give the lowest cost of 1 watt of generated power.

Of the problems - solar panels are not eternal. Even if dust and dirt are not taken into account (we hope for rain and wind), due to photodegradation over the 20 years of operation, the best silicon elements lose ~ 15% of their power. Perhaps further degradation slows down, but this still needs to be taken into account. Let’s take a

look now at the main attempts to increase economic efficiency:

And let's take a small high-efficiency photocell and a parabolic mirror
This is called concentrated photovoltaics. The idea is not bad in principle - the mirror is cheaper than the solar battery, and the efficiency can be 40% and not 16 ... The only problem is that now we need (unreliable) mechanics to track the sun, and our huge swivel plate should be strong enough, to resist gusts of wind. Another problem - when the sun sets behind not too dense clouds - energy production drops to zero, because the parabolic mirror cannot focus the diffused light (in conventional solar cells, the output, of course, drops, but not to 0).

With the fall in prices for silicon solar cells, this approach turned out to be too expensive (both in terms of installation cost and maintenance)

And let's make the solar cells round, place them on the roof, and paint the roof white
This was done by the now infamous Solyndra company , which, with the filing of Barack Obama, received a state guarantee on a \$ 535 million loan from the US Department of Energy ... and suddenly declared bankruptcy. Round solar panels were made by sputtering a layer of a semiconductor (in their case, Copper indium gallium (di) selenide) on glass tubes. The efficiency of solar panels was 8.5% (yes, it turned out worse than simple and cheap silicon).

A vivid example of how American capitalism, with due lobbying, is capable of inertia pumping enormous resources into fundamentally inefficient technologies. As a result of the work, no one was imprisoned.

Now, after this riot of continuous improvement of technology, we open a sad page of history. Solar power plants generate electricity during the day, and it is most needed in the evening: This means that if we do not have batteries, we will still have to build power plants for the evening peak consumption, and during the day some should be turned off and some should be in hot reserve so that if the clouds gather above the solar power station - instantly replace the fallen solar generation. It turns out that if we oblige us to buy electricity from solar power plants at the usual price when it is generated from them, we will actually redistribute profits from existing classical generating capacities, which are forced to stand idle in reserve in favor of solar power during the day.

There is also such an interesting option - if somewhere evening peak consumption - somewhere on earth is the height of the day. Can build a solar power station there, and transmit electricity through power lines? This is possible, but it requires energy transfer over distances of about 5-8 thousand km, which also requires huge capital costs (at least until we switched to superconductors) and coordination with a bunch of countries. The Desertec project developed roughly in this direction — generation in Africa, transmission to Europe.

# Batteries

So, a 1W solar battery costs \$ 0.5. For a day, it will generate, say, 8W * h of electricity (in 8 sunny hours). How do we save this energy until the evening when it will be most needed?

Chinese lithium batteries cost about \$ 0.4 per Wh *, respectively, for 1 W of solar battery (at a price of \$ 0.5), we need batteries for \$ 3.2, i.e. the battery is 6 times more expensive than a solar battery! In addition, it must be borne in mind that after 1000-2000 charge-discharge cycles, the battery will have to be replaced, and this is only 3-6 years of service. Can eat batteries cheaper?

The cheapest ones are lead-acid (which naturally are far from “green”), their wholesale price is \$ 0.08 per Wh * h, respectively, to save daily production we need batteries at \$ 0.64, which is again more than the cost of the solar panels themselves. Lead batteries also die quickly, 3-6 years of service in this mode. Well, for dessert - the efficiency of lead-acid batteries is 75% (i.e., a quarter of the energy is lost in the charge-discharge cycle).

There is also an option with pumped storage power plants (during the day we pump water “up” with a pump, at night we work like a conventional hydroelectric power station) - but their construction is also expensive and not always possible (efficiency - up to 90%).

Due to the fact that the batteries are more expensive than the solar power plant itself, they are not provided for in large power plants, selling electricity to the distribution network immediately as they are generated, relying on ordinary power plants at night and in the evening.

# What is the fair price for unregulated solar generation?

Take Germany, for example, as a leader in the development of solar energy. Each kW generated by solar power plants is bought back at 12.08-17.45 euro cents per kWh, despite the fact that they generate a daily minimum consumption. All they achieve with this is the saving of Russian gas, as gas power plants should still be built and be in a hot reserve (and all their other expenses remain unchanged - salaries, loans, maintenance).

From an economic point of view, it would be fair if solar power plants received exactly as much as they allow gas power stations to save on fuel.

Let's say the cost of Russian gas is \$ 450 per 1 thousand m 3. From this volume, we can generate 39,000 GJ ≈10.8 * 0.4 GWh ≈ 4.32 GWh of electricity (with a generation efficiency of 40%), respectively, for 1 kWh of solar electricity we save Russian gas by \$ 0.104 = 7.87 euro cent. This is exactly what the fair value of unregulated solar generation should be, and it seems Germany is gradually moving towards this figure, but at the moment, solar energy in Germany is 50% subsidized.

# Summary

Polycrystalline solar panels provide the cheapest solar electricity, about \$ 0.5 / watt, other methods are much more expensive.

The problem of solar energy is not in the efficiency of solar cells, not in EROEI (it is really endless in theory), and not in their price - but in the fact that the generated energy is very expensive to store until the evening. Those. the main problem is batteries, which are now more expensive than solar panels and at the same time have a short service life (3-6 years).

At the moment, large-scale solar generation without batteries can only be considered as a way to save a small part of fossil fuels during the day, it cannot fundamentally reduce the number of required classical power plants (gas, coal, nuclear power plants, hydro) - they should still be in reserve by day, and fully take the load into the evening peak consumption.

If in the future, with the help of (cruel) tariffs, it is possible to shift the peak of consumption by the day, the construction of solar power plants will make more sense (for example, if the tariffs are such that it would be beneficial to include the electrolysis of aluminum and hydrogen only during the day).

The cost of “unregulated” solar generation cannot be compared with the cost of generation in classical power plants - as they generate when they succeed, and not when necessary. The fair value of unregulated solar electricity should be equal to the cost of saved fossil fuels, and no more - for gas at \$ 450, the fair price of solar generation is not higher than \$ 0.1 per 1 kW * h (accordingly, in Germany, solar generation is subsidized by ~ 50%).

“Honest” solar energy (with batteries) today can be economically justified only in remote areas where there is no way to connect to the network (as for example, in the case of a remote, lonely-standing cellular base station).

The biggest problem with solar energy is that fossil fuels are too cheap for solar generation to be economically viable.

Update: For further study, we can recommend an article on the problems of German energy in connection with solar and wind generation . There are beautiful development schedules, and in general I recommend reading Already_Yet's other articles.