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Comment by rgbrenner | Hacker News Reader

"All night" isn't quite true. It has 10 hours of storage. And if you want full capacity, you have to start tapping into that in the evening.

Also what's cool about this plant, is that it doesn't use natural gas in the morning to heat up the molten salt (like other plants). The salt never cools completely during the lifetime of the plant.

Expensive though.. on a 25 year contract at 13.5 cents per kwh. That'll be renegotiated after the 25 years at a much lower rate. Utilities pay higher prices for new plants (gas, oil, etc) to pay off the cost of building it. After that, it drops to a more reasonable price.

https://en.wikipedia.org/wiki/Crescent_Dunes_Solar_Energy_Pr...

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Doesn't "hours of storage" necessarily imply a rate of discharge? If you don't take any steam off it, doesn't it last more than 10 hours? It seems like they should just state the storage in terms of extractable energy stored.

And why sodium? It does not have a spectacular specific heat. Is it just convenient due to its high boiling point?

I believe it's because of the large temperature range between solid and gaseous phases, there is a ~800 degrees Kelvin difference between the boiling and melting points, which at a specific molar heat of 29 J/mol/K is greater than the energy to heat water to boiling for a ~100 degree difference at ~56 J/mol/K

Yes.. they do state the storage in terms of energy. When plants state hours, it's at full capacity. So 110MW capacity x 10 = 1.1 GWH of storage. Of course it will operate longer if you use less than full capacity.

Molten salt is used because it's liquid at atmospheric pressure, is low-cost, its operating temperatures are compatible with the most efficient steam turbines, and it is non-flammable and nontoxic.

Expensive indeed. I wonder how concentrated solar thermal compares with a simple photovoltaic plant these days? Given the precipitous drop in price for PV panels, even if solar thermal is currently cheaper, how much longer until it's just not competitive?

PV is down to around 5 cents/kwh. I seem to remember a new plant recently that was 2-3 cents, but can't find it.

The only problem with PV is its variability. When there's cloud cover, in the evening, etc.. power drops. Which means you need some kind of storage to even it out and probably a gas plant for the evening/night time.

In comparison, CSP has a steady output, which lets them make adjustments easier.

Sounds like a hybrid plant would be the best approach, with a large molten salt reservoir which is 'charged' while the sun is shining and used to run a steam generator when PV isn't available. Of course, the way battery storage prices are going, I wouldn't be surprised if battery backup becomes more economical than all that messy pipework in the near future anyway.

I don't share your optimism. It'll be a while before we can produce useful, grid sized batteries for this kind of use.

The hybrid approach you describe would require twice the surface area and probably wouldn't double the output.

I think I prefer the idea of having large plants like these, with 'night'-time capabilities and a relatively steady output. That sounds like it would work pretty well in combination with decentralized roof top solar + smallish home batteries.

The whole point is that solar thermal power costs 2x - 5x what solar photovoltaic power costs, so you use solar photovoltaic when possible and back it up with solar thermal. It does increase the area required but that's the price you pay for fully renewable baseload power. You could just use thermal but it'd be much more expensive and still require the same footprint for similar power output.

Of course, there's nothing wrong with the photovoltaic part being distributed in the form of rooftop solar, which is what's already increaslingly happening. Home storage batteries for load leveling are picking up nicely too, which is good because they're going to become mandatory within the next decade if we want metropolitan power grids to survive tens of megawatts of sometimes-rapidly-fluctuating solar power being dumped into them.

> the way battery storage prices are going,

And we didn't even start playing with Sodium as a replacement for Lithium in those batteries.

Batteries have two things going for improvements in the batteries themselves and more energy efficient homes and appliances. PC won't replace utilities in densely inhabited regions, but batteries may help evening out variable sources.

A hybrid plant isn't possible. Apart from both producing electricity from sunlight, a PV plant has nothing in common with concentrated solar thermal.

"A hybrid car isn't possible. Apart from both producing torque, an electric motor has nothing in common with an internal combustion motor."

Hybrid just means that it uses both approaches. I'm suggesting a solar thermal plant which is generally lightly loaded or unloaded during the day, storing the heat in a large reservoir while a neighboring PV plant much more cheaply covers demand. Then when the PV plant can't generate due to cloud cover or night time, the thermal plant steps up and uses the stored heat to generate power.

It's no different in principle to a hybrid car where a battery electric drive system cheaply covers as much driving as it can, and an internal combustion engine kicks in when required.

There has been at least one combined PV + CSP scheme proposed [1][2] in Chile.

This article [3] analyzes some of the merits of this approach.

[1] (Partial paywall) http://social.csptoday.com/technology/integration-csp-and-pv...

[2] https://en.wikipedia.org/wiki/Copiap%C3%B3_Solar_Project

[3] http://spie.org/newsroom/6018-photovoltaics-and-concentratin...

Much of the variability problem could be eliminated by having real-time pricing of the power. Lower prices when the sun is shining, higher prices when cloudy. Then, build into usage devices a monitor for the price, and those that can shift usage (such as charging the car and running the electric water heater) can not suck power when prices are higher and draw power when it is lower.

I.e. solve the problem on the demand side rather than the supply side.

I think this is going to be a massive part of the new energy economy. I'm picturing the power grid becoming less of an 'energy retailer' and more of an 'energy broker', with each property's power control module placing bids on energy when it needs to import and offering sale of energy when it has a surplus. There will then be a flat rate (or close to, probably tiered based on peak consumption) for access to the grid, and the power utility company becomes a power grid maintenance company rather than a generation company.

> "All night" isn't quite true.

Ok, we've replaced that title with representative language from the article.

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