Solar Thermal + Molten Salt Storage

[station240]

At a basic level, you point lots of mirrors at a central tower, which has molten salt pumped through it. The salt is used in the same way you pump water through conventional heat exchanger.
Now there are two tanks for the molten salt, a 'cold' and a hot tank. The superheated salt goes in the hot tank, the warm salt goes in the cold tank.
A heat exchanger converts the stored heat from hot salt into steam, which then drives a conventional turbine generator.

Able to store up to 24 hours of power (design option), which makes it baseload solar.
Ideally you would store all it's power, for night time/peak demand when PV solar isn't enough.

http://www.solarreserve.com/en/technology
Company uses the term concentrating solar-thermal power (CSP), so you need to look in the CSP section of their projects.

[DenzilPenberthy]

It will be very interesting to see how the new one in North Africa performs.

Do you know if there's any plans to trial this in Aus? You'd have some perfect locations for it I'd say.  More so than the typical 10-15 deg C overcast weather which is typical here

[Kilrah]

There's something in me that for some reason wants to see those several million liters of matter stored at 500+°C... 

[tszaboo]

While it is definitely interesting technology wise, I dont think ultimately this is the future. It is using moving parts, the salt is used to make water steam, turn turbine, like the 20 century. Engineering wise it is trying to get as much as possible from conventional techniques. The combined efficiency is also only 2.6% according to wiki. Most of the space is wasted between the mirrors.
You can as well store PV energy in salt, just dump the energy into salt. There is also not so much losses, because your big resistor can be insulated, while with the concentrated, it is exposed, heating the air.
I guess this technology works best for countries where there are big deserts, without sand.

[station240]

It will be very interesting to see how the new one in North Africa performs.

Do you know if there's any plans to trial this in Aus? You'd have some perfect locations for it I'd say.  More so than the typical 10-15 deg C overcast weather which is typical here

One is supposed to be under construction in South Australia, uncertainty created by the current luddite federal government has sabotaged things to some extent.
Seemingly by accident the same political party have been donated a lot of money from AGL who own a lot of coal fired power stations.
http://arena.gov.au/project/port-augusta-solar-thermal-feasibility-study/
https://alintaenergy.com.au/about-us/power-generation/port-augusta-solar-thermal      ( click link for SA)
http://www.businessspectator.com.au/news/2015/9/23/solar-energy/alinta-gives-solar-thermal-project

The "energy market conditions" relate to the government sabotaging any proven clean technology, and negative changes to the prices paid for clean energy.
Yes this means if you want money for a big project, it needs to be experimental 
No idea why the Solar Roadways mobs haven't tried to cash in.

[Kilrah]

You can as well store PV energy in salt, just dump the energy into salt.

Do you really think adding a ~0.3 factor of efficiency in the chain is a good idea? Not even counting the higher cost of the equipment.
The whole reason for direct thermal plants is the much higher efficiency.

[Kleinstein]

Under favorable conditions the efficiency of solar thermal systems can actually be higher than typical non - concentrated PV. AFIAK the version of concentrated thermal and heat engine already reach about 25% efficiency (counting the light on mirror area only) with a little room for improvements.

It is true that some of the area between the mirrors is lost to solar energy, but it may still be useful leaving some room for plants to grow.  In the dry areas suitable for concentrated solar power, land is usually not that scare - it's essentially dessert. For the same reason efficiency is not that important, high efficiency just help to keep the costs down.
This is a little different for PV in the more populated areas: here you save on the long distribution lines but unused areas like roofs are limited.

I problem with thermal power plants in such regions might be getting a heat sink, like cold water.

[tszaboo]

You can as well store PV energy in salt, just dump the energy into salt.

Do you really think adding a ~0.3 factor of efficiency in the chain is a good idea? Not even counting the higher cost of the equipment.
The whole reason for direct thermal plants is the much higher efficiency.

The situation is not that clear.
With PV, better coverage of land is possible.
The thermal transfer has has smaller losses.
The PV works with scattered light. The mirror system needs one clear direction of the light, so clouds kill the generation quickly. Here, nimbostratus (I think it is called that way, it's when the entire sky is covered by thin cloud) is usual, that would decrease the output of the mirror to nil.
And the mirror only works in the desert.
And while it seems economically better, this is what wiki said:

As of 9 September 2009, the cost of building a CSP station was typically about US$2.50 to $4 per watt

The same number for PV is around US$0.3/W + large scale inverter. PV is even economically more viable. PLease, if you find better numbers, share.

[eas]

You can as well store PV energy in salt, just dump the energy into salt.

Do you really think adding a ~0.3 factor of efficiency in the chain is a good idea? Not even counting the higher cost of the equipment.
The whole reason for direct thermal plants is the much higher efficiency.

The other issue is that storing PV in thermal form still requires most of a thermal generation plant to get the electricity back out again.

[bills]

This one was build in 1982 and ran until 1999. One of the first test plants.
https://en.wikipedia.org/wiki/The_Solar_Project

[Someone]

Do you know if there's any plans to trial this in Aus? You'd have some perfect locations for it I'd say.  More so than the typical 10-15 deg C overcast weather which is typical here

There is a long term demonstrator/research platform that tried to jump to commercialisation:
https://www.google.com.au/maps/place/ANU+Big+Dish/@-35.282005,149.1143404,17.07z/data=!4m2!3m1!1s0x0000000000000000:0xf91e604da5904a33
But the storage technology isn't fully mature/demonstrated yet. Without storage they claim very impressive efficiency numbers, so even with poor efficiency in the storage it will be a very interesting technology to watch.

[GamerAndds]

Okay this topic confuses me...

Is/are these "molten salts" pure salt or are they diluted? I read that salt melts at roughly 800 degrees c? This confuses me because people usually says the cold salt tank is around 200 degrees??? So they cant be pure salt?

[DenzilPenberthy]

Pure speculation on my part, I suspect a Wikipedia or Google search would be useful:

I suspect that by mixing different salts you could get a eutectic mixture that melts at a lower temperature than either of the pure ingredients.  e.g. 60/40 Lead-Tin solder melts at a lower temperature than either pure lead or pure tin.

[Brutte]

Okay this topic confuses me...

Is/are these "molten salts" pure salt or are they diluted? I read that salt melts at roughly 800 degrees c? This confuses me because people usually says the cold salt tank is around 200 degrees??? So they cant be pure salt?

Where have you found that sodium or potassium nitrate melts in around 800 deg. C?? Reference please.

[kaz911]

Okay this topic confuses me...

Is/are these "molten salts" pure salt or are they diluted? I read that salt melts at roughly 800 degrees c? This confuses me because people usually says the cold salt tank is around 200 degrees??? So they cant be pure salt?

Where have you found that sodium or potassium nitrate melts in around 800 deg. C?? Reference please.

Google Search for Sodium Chloride melting point???


Sodium chloride
Chemical Compound
Sodium chloride /?so?di?m ?kl??ra?d/, also known as salt or halite, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. Wikipedia
Formula: NaCl
Molar mass: 58.44 g/mol
Melting point: 801 °C
Boiling point: 1,413 °C
IUPAC ID: Sodium chloride
Density: 2.16 g/cm³
Soluble in: Water, Methanol, Formic acid, Glycerol, Propylene glycol, Formamide, Ammonia

[f5r5e5d]

its not trivial engineering - the molten salts are corrosive, you have to figure out how to start/stop with the salt mix solidifying in your pipes if too cool, and if you go for phase change on the heat extraction side you can have supercooling problems

[djacobow]

Molten salt storage solar power plants are not new. For example, Gemasolar is a 20 MW solar tower plant that has been operating in Spain since 2011. (https://en.wikipedia.org/wiki/Gemasolar_Thermosolar_Plant). Gemasolar, like the SolarReserve plants, heats the salts directly from sunlight. More common are molten salt "trough" based plants, such as Andasol, also Spain, 150MW and 7.5 hours of storage (https://en.wikipedia.org/wiki/Andasol_Solar_Power_Station) and Solana, near Pheonix AZ, USA, at 250MW x 6 hours (https://en.wikipedia.org/wiki/Solana_Generating_Station). These trough plants use a heat transfer fluid like Therminol in the troughs, which in turn heats molten salt via a heat exchanger (hx).

Also, molten salt energy storage has been used for decades in the chemical and process industries, as well, as a medium to store useful energy from one reaction and use it in another. There were also solar thermal plants with energy storage that used other materials, such a oil for a heat transfer material and for storage. The SEGS-1 plant in Daggett, CA USA, built in 1984 had such a system. However, that machines storage system was destroyed in a fire, and it was never rebuilt. (Plant continues to operate today, without storage.)  (https://en.wikipedia.org/wiki/Solar_Energy_Generating_Systems). There are ideas to use other materials for thermal storage, too, such as ceramic or graphite honeycomb (http://www.graphiteenergy.com/) (heat xfer fluid would be air or co2) and even sand http://www.energymatters.com.au/renewable-news/sand-batteries-solar-em5272/).

The salts in these molten salt plants are a mixture of sodium and nitrate salts. The mix varies for different reasons; it is not necessarily a eutectic mix. The highest usable temperature is as important as the freezing temperature, and various mixes have different properties in terms of their chemical reactivity at high temps. If you can get a 100C higher with a given mix before the salts start tobreak down or react with the atmosphere, requiring that the tanks be inerted, then that might be worth a 20C higher freezing point. Of course, another consideration is that sodium and nitrate do not cost the same, and you are buying a fair bit. Here's a 2008 paper from Sandia National Lab (USA) that describes some of the considerations: http://energy.sandia.gov/wp-content/gallery/uploads/ES2008-54174-molten-salt-for-troughs.pdf

As someone else pointed out, keeping the salts molten is not a trivial problem. Remember, these plants are built in typically desert environments, that can get quite cold at night and thermal radiation to the sky can be high. Also, during winter there can be many consecutive days without sun, even in the sunniest places on earth. Typically, all the piping needs to be heavily insulated, heat traced (that is, electrically heatable) or both, or a certain amount of hot tank energy must be reserved from energy production for the purpose of circulating through the system to keep everything molten, or a small gas-fired burner can be used to add heat to the system. A tower based plant would likely have the salt tanks at the bottom of the tower, and so the only real piping would be up and down the tower to/from the tanks, steam generator/hx on the ground, and the solar receiver on the tower. You don't see trough based systems directly heating the salt in the troughs because such machines typically have miles of receiver pipe exposed to the sky.

There are other ways to skin this cat, with tradeoffs. For example, you could built a tower based plant with a steam generating receiver at the top rather than direct heating of the salts. Then you bring steam down the tower, and use an hx to heat the salt. Later, when you want to make power from the salt storage, you use the same or a different hx to make steam, then run a turbine. The extra conversions (steam-salt-steam) costs you in efficiency when operating from storage but during the day you can operate with no conversions (steam directly to turbine), so it could be a wash, efficiency-wise depending on when you plan to make power. Direct steam also might be attractive if it simplifies the molten salt piping or if you know how to make a steam solar receiver but not a molten salt solar receiver, of course.

The economics of solar with thermal energy storage are interesting. On a straight energy basis, PV has far outpaced the cost of solar thermal energy. If you don't need or value storage (for example, your power system doesn't already have much solar on it and can manage the diurnal and weather variation just fine) then these systems are probably not interesting. On the other hand, if you already have a lot of PV on your system and your ability to deal with additional variation from a solar resource is limited, storage becomes very interesting because such machines are in theory dispatchable. That is, you run them when you need them, because power generation and solar collection are uncoupled. In that case, one might support compare PV + battery to CSP + thermal storage. In this case, the latter is still favorable, particularly at scale. Consider the 250MW x 6 hr machine mentioned above. That's 5.4 TJ. The largest battery in the US is a 32MWh (0.11 TJ) unit owned by SCE. (http://www.greentechmedia.com/articles/read/The-Biggest-Battery-in-North-America-Gets-Unveiled-By-SCE-Today).

(By the way, someone suggested storing PV energy thermally. Of course, that would be a great waste to take a low entropy energy type like electricty, and convert it to heat, just so you can convert it back to steam and electricity again. Efficiency going from electric to heat is near 100%, but the best you could do is maybe 35% on the return trip. And that's not even counting the fact that PV is only 20%+ efficient, whereas merely collecting solar heat can be much better. On the other hand, if PV is _*REALLY*_ cheap and you have more than you know what to do with, there could be some circumstance where doing this makes some economic sense!))

All that said, in most parts of the world, it will be some time before storage is "needed," which will give the battery folks plenty of time to work their way down the price / capacity curve. Of course, the thermal energy storage people will continue to progress, too, but I strongly suspect that batteries will cross over at some point.

[ PS. I wrote my Masters thesis on CSP+TES, and worked for a CSP company for several years. ]

[ PPS. Here is a tome on the economics of CSP+TES. I am a coauthor. http://www.csp-alliance.org/wp-content/uploads/2014/09/The_Economic_and_Reliability_Benefits_of_CSP_with_Thermal_Storage-2014_09_09-FINAL.pdf ]

-- dave

[Someone]

(By the way, someone suggested storing PV energy thermally. Of course, that would be a great waste to take a low entropy energy type like electricty, and convert it to heat, just so you can convert it back to steam and electricity again. Efficiency going from electric to heat is near 100%, but the best you could do is maybe 35% on the return trip. And that's not even counting the fact that PV is only 20%+ efficient, whereas merely collecting solar heat can be much better. On the other hand, if PV is _*REALLY*_ cheap and you have more than you know what to do with, there could be some circumstance where doing this makes some economic sense!))

That someone was Someone Thermal storage of excess solar energy makes a lot of sense at the point of use for the consumer, while there is a cheap source of electricity you heat up your hot water and cool down your freezers (up to a point) and then they don't need to use the more expensive energy to do the same later. Off peak electricity powered storage hot water systems are routine in Australia and use a tarrif usually less than half the normal rate, when applied to solar energy which varies in price even more it becomes a clear economic benefit to do some thermal storage. What isn't catching up is installers or manufacturers who will put the intelligence into the system for you, so its all DIY for now.

[djacobow]

(By the way, someone suggested storing PV energy thermally. Of course, that would be a great waste to take a low entropy energy type like electricty, and convert it to heat, just so you can convert it back to steam and electricity again. Efficiency going from electric to heat is near 100%, but the best you could do is maybe 35% on the return trip. And that's not even counting the fact that PV is only 20%+ efficient, whereas merely collecting solar heat can be much better. On the other hand, if PV is _*REALLY*_ cheap and you have more than you know what to do with, there could be some circumstance where doing this makes some economic sense!))

That someone was Someone Thermal storage of excess solar energy makes a lot of sense at the point of use for the consumer, while there is a cheap source of electricity you heat up your hot water and cool down your freezers (up to a point) and then they don't need to use the more expensive energy to do the same later. Off peak electricity powered storage hot water systems are routine in Australia and use a tarrif usually less than half the normal rate, when applied to solar energy which varies in price even more it becomes a clear economic benefit to do some thermal storage. What isn't catching up is installers or manufacturers who will put the intelligence into the system for you, so its all DIY for now.

Ah, I was talking about thermal storage of PV energy for later power generation. Thermal storage of PV energy for refrigeration can make reasonable sense, particularly if you are on a time-varying electric tariff, as it allows you to time-arbitrage the electric price. Use of PV to make hot water is more dubious, and really should only make sense if you already have the excess PV installed and your alternative is electric hot water. (I'm always thinking from a US perspective, from which domestic hot water heating is pretty uncommon and usually a bad idea.) In that case, the PV is already paid for, and any use you can find for the energy is better than no use. (providing the power company won't take it at a reasonable price.)

If you were building a house or did not have the excess PV already installed, then if you want to make hot water, I'm pretty sure that you'd be served by installing a solar hot water system than installing a PV system to do the same, regardless of whether you have electric hot water normally.
 

By the way, it deserves clarification: what is economic for you has everything to do with your own specific circumstances, which can vary a LOT. The level and structure of your electric tariff, your method of water heating, what investments are sunk or would need to be made, what options are and are not practical in your circumstance, etc. When I'm speaking off-the-cuff, I usually default to thinking about economics from the level of the Universe, which is not a typical perspective. I should be much more disciplined in conversation -- particularly on this forum of engineerins -- of stating my assumptions with regard to perspective.

[Someone]

Ah, I was talking about thermal storage of PV energy for later power generation. Thermal storage of PV energy for refrigeration can make reasonable sense, particularly if you are on a time-varying electric tariff, as it allows you to time-arbitrage the electric price. Use of PV to make hot water is more dubious, and really should only make sense if you already have the excess PV installed and your alternative is electric hot water. (I'm always thinking from a US perspective, from which domestic hot water heating is pretty uncommon and usually a bad idea.) In that case, the PV is already paid for, and any use you can find for the energy is better than no use. (providing the power company won't take it at a reasonable price.)

If you were building a house or did not have the excess PV already installed, then if you want to make hot water, I'm pretty sure that you'd be served by installing a solar hot water system than installing a PV system to do the same, regardless of whether you have electric hot water normally.
 

By the way, it deserves clarification: what is economic for you has everything to do with your own specific circumstances, which can vary a LOT. The level and structure of your electric tariff, your method of water heating, what investments are sunk or would need to be made, what options are and are not practical in your circumstance, etc. When I'm speaking off-the-cuff, I usually default to thinking about economics from the level of the Universe, which is not a typical perspective. I should be much more disciplined in conversation -- particularly on this forum of engineerins -- of stating my assumptions with regard to perspective.

Yes its all very dependent on the local situation, and if any work is pending. There are some interesting analysis online from people where people were replacing major appliances/services and adding more solar panels was more cost effective than the added cost of more efficient equipment! The returns are very different if you are pricing the replacement of appliances/services compared to at install time or when they require replacement anyway, and with each major purchase/change the current circumstances need to be looked at.

For reference typical electricity in Australia is around 20c/kWh, 10c/kWh off peak, and 5-10c/kWh feed in for residential solar, so its very attractive to make use of any excess solar power.

An aware fridge reduce operating costs by 30% or more, even for those without solar.

[EEVblog]

While it is definitely interesting technology wise, I dont think ultimately this is the future. It is using moving parts, the salt is used to make water steam, turn turbine, like the 20 century. Engineering wise it is trying to get as much as possible from conventional techniques.

But that's how most major power generation happens, with steam and turbines.
How do you want to convert the heat? Peltiers?

[djacobow]

While it is definitely interesting technology wise, I dont think ultimately this is the future. It is using moving parts, the salt is used to make water steam, turn turbine, like the 20 century. Engineering wise it is trying to get as much as possible from conventional techniques.

But that's how most major power generation happens, with steam and turbines.
How do you want to convert the heat? Peltiers?

Well, I think he was rejecting the thermal approach entirely. Some people think the future is PV and batteries driving inverters (and the battery charged from the grid.) And some people have more ambitious dreams of an all DC system.

Personally, I think such wholesale transition of the grid is far off. Spinning machines are amazingly efficient and reliable, and modern steam cycles come surprisingly close to the theoretical Carnot limits. Rotating synchronous generators have some other nice features, too. They have mass, so provide some inertia, and they can generally handle faults more gracefully than solid state equipment. Basically, if you don't cook the machine, it can probably be used again. Another example of the advantage of a spinning generator is that it can make reactive power, even under a  low voltage situation. This is helpful for "voltage support" under a fault or black start situation. Solid state equipment to generate reactive power, like an inverter, STATCOM or SVC can't do that trick. (This paper from Oak Ridge National Lab has a table that compares their capabilities. Ch 3. http://web.ornl.gov/sci/ees/etsd/btric/eere_research_reports/power_systems/reactive_power/nait_convention/nait_convention.pdf)


Also, even in a 100% renewable future, we're going to have wind and hydro, and there's no conceivable reason to dump spinning machines there.

[djacobow]

Yes its all very dependent on the local situation, and if any work is pending. There are some interesting analysis online from people where people were replacing major appliances/services and adding more solar panels was more cost effective than the added cost of more efficient equipment! The returns are very different if you are pricing the replacement of appliances/services compared to at install time or when they require replacement anyway, and with each major purchase/change the current circumstances need to be looked at.

For reference typical electricity in Australia is around 20c/kWh, 10c/kWh off peak, and 5-10c/kWh feed in for residential solar, so its very attractive to make use of any excess solar power.

An aware fridge reduce operating costs by 30% or more, even for those without solar.

Thanks for the clarification. That makes perfect sense to me now. You are right, at those rates, you have every incentive to find a load for your PV output at home!

In CA, USA we do not have a FiT, we have "net metering," which essentially allows you to zero out your net load (and net bill), so the $/kWh of PV generation is worth exactly the $/kWh of your prevailing tariff. But we also have a strange tiered bill, where the more you use, your rate goes up. So when you are sizing your PV system, you usually size it to reduce your bill out of the higher tiers (Where the rate is high) but as you make the system bigger and work down the tiers, the rate goes lower and you get diminishing returns. It's a strange system; there are reasons, historical and political.

[CatalinaWOW]

Yes its all very dependent on the local situation, and if any work is pending. There are some interesting analysis online from people where people were replacing major appliances/services and adding more solar panels was more cost effective than the added cost of more efficient equipment! The returns are very different if you are pricing the replacement of appliances/services compared to at install time or when they require replacement anyway, and with each major purchase/change the current circumstances need to be looked at.

For reference typical electricity in Australia is around 20c/kWh, 10c/kWh off peak, and 5-10c/kWh feed in for residential solar, so its very attractive to make use of any excess solar power.

An aware fridge reduce operating costs by 30% or more, even for those without solar.

Thanks for the clarification. That makes perfect sense to me now. You are right, at those rates, you have every incentive to find a load for your PV output at home!

In CA, USA we do not have a FiT, we have "net metering," which essentially allows you to zero out your net load (and net bill), so the $/kWh of PV generation is worth exactly the $/kWh of your prevailing tariff. But we also have a strange tiered bill, where the more you use, your rate goes up. So when you are sizing your PV system, you usually size it to reduce your bill out of the higher tiers (Where the rate is high) but as you make the system bigger and work down the tiers, the rate goes lower and you get diminishing returns. It's a strange system; there are reasons, historical and political.

Many more complications here in the states.  The tiered pricing system is widespread, but there are many other variations.  Some areas do have residential rates that depend on the time of day (essentially demand pricing).  Net metering is available in many locales, but not in all.  In almost all areas net metering is limited to some short time period, frequently the monthly billing cycle.  So excess energy generated in high solar months cannot be used to balance energy consumption in low solar months.  In some areas sophisticated meters are installed that can track direction and time of power flows.  In these cases solar sourced power is often paid at the lowest tariff rate, even if it is supplied during a high tariff period, and even if the customer supplying solar power is billed at tariff rates that vary over the day.

[tszaboo]

While it is definitely interesting technology wise, I dont think ultimately this is the future. It is using moving parts, the salt is used to make water steam, turn turbine, like the 20 century. Engineering wise it is trying to get as much as possible from conventional techniques.

But that's how most major power generation happens, with steam and turbines.
How do you want to convert the heat? Peltiers?

I showed that PV is so much more cheaper that concentration is not the future (opinion). I think what will happen: we use excess power to generate hydrogen. Cars will run on it. No exhaust gas, and they clean the air (because they have a large capacity air filter, since they require large amount of clean air for the process to work). Hydrogen generation is not really efficient now, but can ultimately solve the energy issue.
We are already at the point, where the solar inverters have to be switched off in the summer, because the voltage level of the grid is too much. Efficiency is secondary concern.
I'm still just learning the PV and renewable industry (new job), but I hope I will be able to shine some more light (pun intended) on the topic.

[Kleinstein]

Currently PV is cheaper than electricity from concentrated solar if you just count the energy. But the concentrated solar-thermal plants often do offer some degree of storage too. PV gives most of there power over something like 3 hours around noon and little over the rest of the day. In central Europe there is already so much PV installed that for these times when PV is really working the price (utility level) for electricity is down (sometimes all the way to 0)  and the grids are nearly saturated with PV. This is a little different in hot countries, where air conditioning is a major demand and peak rates are still paid on hot sunny days.

Also development of concentrated solar is a little behind PV. So there is more room for lowering prices compared to PV.

PV will need some storage like hydrogen or batteries, but this will add to costs and reduces efficiency. Using energy when it is available helps a little, but it does not work for everything may not be practical for all the small things. So if the climate allows it concentrated solar can be more useful than PV.  Concentrated systems can deliver there power to a later time and thus get a better price. They also have the option to use alternative fuel (e.g. gas) to deliver a backup for days without sun.

[djacobow]

I showed that PV is so much more cheaper that concentration is not the future (opinion).

You will get no argument from me on that point. I think CSP has mostly run its course. It /could/ have been different, the industry itself was unable to deliver on promises and the PV industry, in part aided by massive investment, really delivered more than was promised. (Look at $/kWh cost projections from the 2000-2005 era. NREL had a bunch of papers. CSP looked good at the time.) In the end, CSP is far behind on raw cost. CSP also has some limitations that gave it an uphill battle, like it could really only be cost effective at huge scale, meaning huge, difficult to fund projects, and of course, it requires DNI (direct normal irradiance) which means it has to be only in the sunniest places. So it will probably be a niche technology, though I'd be surprised to see it completely disappear.

Also, let's not forget that wind eats PV's lunch cost-wise -- at least in windy places!

I think what will happen: we use excess power to generate hydrogen.

This I'm a bit more dubious about. Yes, PV energy is cheap in sunny places, but to convert that energy to hydrogen (say, 60-70% efficiency electrolysis) then convert it back to electricity (say 40% in a combined-cycle plant or 40-60% in a fuel cell) is a pretty big hit -- you just increased the $/kWh cost of your energy by 4x. Now, you could say, well PV will be cheap enough, but I think PV is not going to get all _that_ much cheaper.

I gather you're in the industry. If you have access to a detailed (or non detailed) model of a new PV project, do the experiment of zeroing out the panel cost entirely. I think that will set an interesting near-term lower bound on PV energy prices. Land, insurance, racking, wire, labor, etc, are not going to get much cheaper.

Cars will run on it. No exhaust gas, and they clean the air (because they have a large capacity air filter, since they require large amount of clean air for the process to work).

H2 also has some issues a a motor fuel, doesnt it? Namely that to store a reasonable amount of it in a vehicle you need to compress it to insanely high pressures. Is that feasible cost wise and safety wise? Also, replacing gas as a motor fuel requires complete replacement of the fueling infastructure. (EVs do not have this problem to the same degree, because you can at least charge you daily driver at home.) These are surmountable barriers, but barriers.

Hydrogen generation is not really efficient now, but can ultimately solve the energy issue.

I agree, hydrogen as a storage medium with renewables as the energy source can solve our energy issues. But I am unconvinced that it will be the solution. I see a landscape too complex to want to make that prediction. I think we can agree that hydrogen is off to a slow start relative to battery EVs, though. This is important, because to a degree investment is destiny. It could be that H2 is the better technical solution, but if the $billions are pouring instead into batteries, it might not happen.

We are already at the point, where the solar inverters have to be switched off in the summer, because the voltage level of the grid is too much. Efficiency is secondary concern.

You say that as if it were a good thing, but it's actually awful. What it says is that on electrical grids that already have a lot of PV on them relative to total demand, that the value of incremental PV is zero. That doesn't make adding more PV cheap, it makes it exceedingly expensive. You have to find something to do with that energy, or you do not want to pay for it. If you can time shift it, you have solved the problem, but all the time shifting available today (battery, hydrogen, thermal storage) is expensive and/or inefficient. What it means is that PV energy during non-solar hours is actually expensive.

Each situation is different, but in most cases, PV is a great up to a point, then its marginal value starts to tank. Most places around the world are not near this point. In some places, they are at it, maybe past it. You do not have to take my word for it.

2012 LBNL high PV penetration study: https://emp.lbl.gov/sites/all/files/lbnl-5445e.pdf

Or check out the entire research output of Paul Denholm at NREL: http://www.nrel.gov/analysis/staff/p_denholm.html . These papers mostly are about the value of energy storage, but they show are calibrated in terms of penetration level of wind and solar. The value of storage is low at first (don't need it) and then it goes up, showing the same effect I'm talking about. You can also see it in the price duration curves. In low penetration scenarios there are fewer hours where the system price of energy drops to or below zero, in high penetration scenarios there are a lot more. These represent overgen periods; when PV output is potentially less than worthless.

I'm still just learning the PV and renewable industry (new job), but I hope I will be able to shine some more light (pun intended) on the topic.

Enjoy yourself, it's a fun and fascinating industry. Let me humbly suggest that you also look outside the  industry to get a rounder picture of what PV can and cannot accomplish: utilities, regional transmission operators and independent system operators, public utilities commissions, other renewable technology developers, national labs, etc. Yes, they all absolutely have agendas -- this is a space where bullshit abounds, so set your engineering FUD detector  to high, but there is truth and good people mixed in.