If you concentrate sunlight and shine it directly into the water or on items absorbing sunlight which are inside the water, apart from refraction, reflection, and lens heating losses, all energy from sunlight should be transferred into the water. You lose some more from radiation of heat from the water vessel and the pipes.

In this case I think your efficiency is mainly ruled by your turbine efficiency: around 65 to 90 %, bigger turbines and hotter steam tend to be more efficient.

For the solar panels on the other hand, we look at 17 to 25ish %.

I guess steam power is still good.

I do have no idea about cost efficiency. The steam solution will have much higher maintenance costs and probably also setup costs.

You might also want to look at molten salt solar solutions.

That’s called concentrated solar power, and it uses a lot of mirrors to focus the sun’s heat onto a bunch of salt to melt it down and use it to generate electricity.

Apparently its efficiency is similar to that of solar panels. But it should be cheaper since you don’t need expensive materials for all the solar panels, only normal mirrors.

Why isn't it more hyped and spread?

It doesn’t scale down well. 1 solar panel can generate power, but for the CSP it needs a very large central heat engine and lots of mirrors surrounding it to focus light at the center.

It’s also very water intensive, which is seen as a waste because it needs to be put in places with a lot of sun, which tend to be dry and water scarce.

One big disadvantage of solar thermal is that it doesn't work when it's cloudy. Conventional (photovoltaic) solar panels work fine with the diffuse light from an overcast sky, but you cannot focus diffuse light to create high temperature.

It's also mechanically complicated. Not only do you need a turbine (or Stirling engine) but you also need a tracking device to adjust the mirror angle continuously. Whereas conventional solar panels have no moving parts at all.

All thermal power plants (solar and fossil fuel) have significant inefficiency related to the difficulty of converting heat into work. So the turbine contributes to the inefficiency but isn't the biggest factor. Solar thermal power generation looks less good considering this.

For things where you really need heat, like cooking or heating water, you can avoid that inefficiency of converting light or heat to power; for example, lots of people heat their hot water by collecting heat directly from collectors on their roof. However, despite the increased efficiency, this isn't necessarily cost-effective compared to other means of reducing fossil fuel energy use--electricity is just a really practical way to move energy around.

I would imagine CSP plants to be operating as ORC Organic rankine cycle) plants as they can achieve evaporation in lower temperatures and dont need water in their circulation.

CSP plants also have added benefit that can be used in some sort of heat storage solutions which would enable production in cloudy/ night times. I remember reading that the material heated in focal towers is some salt that actually melts there.

https://en.m.wikipedia.org/wiki/Concentrated_solar_power

It seems they want to use steam for some reason.

> converting heat into work

This article (https://www.sciencedaily.com/releases/2012/11/121119140627.htm) mentions an overall efficiency for solar steam of around 24%. Probably a bit more now.

But less than I expected!

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There was a lot of interest and excitement about it a while ago, maybe 15 years ago. What made people generally turn away from it was simply that silicon solar cells and panels got less expensive really fast, so even though when solar thermal electric power projects looked like they'd be competitive, by the time they were finished, the silicon solar cells were so much cheaper that they were no longer competitive.

However, the need for storage is leaving to renewed interest: storing heat can be cheaper than storing electricity.

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