Interview: Molten Salt Storage is Safe + Could Store Heat for 2 Months

April 24th, 2012 by Susan Kraemer

Last week I had the chance to talk on Skype with Kevin Smith, the CEO of SolarReserve, which is building the world’s largest concentrating solar power plant in Tonopah, Nevada. It is the same technology as one that came on line in Spain last year that can ship power 15 hours a day by using molten salt storage – both as the carrier and as the storage. It was developed by (literally) rocket scientists in the 90s with a pilot project that proved it works.

First, I had to know:

SK: I’ve had a commenter who thinks molten salt could explode… ?

KS: There’s a tremendous amount of study work thats been done on molten salt. It’s a combination of potassium and sodium nitrate. It’s a very safe compound. It’s kind of similar to the kind of garden fertilizer that you’d spread around in your garden and at these temperatures – the only problem is it’s very hot.

But theres no risk of explosion… theres no… It’s not considered a hazardous substance by the U.S. government. It’s no more hazardous than the high temperature steam, which obviously the high temperature; it’s hot…you have to treat it with that kind of caution – but theres no issues with it being hazardous.

SK: How hot do you get at the top?

KS: We’ll heat up to 1,000 degrees which is comparable to the temperatures that you’ll see in a regular power plant. The big difference with utilizing molten salt, is that at 1,000 degrees Fahrenheit it’s still a liquid which means we can store it in a tank. Its low pressure so… it’s not like it’s a high pressure tank or a high pressure system, because it’s a liquid: it’s all still at low pressures, then once we take it to the power project the steam is also at 1,000 degrees.

SK: How about any danger of heating up the surrounding air?

KS: No its completely insulated. the way the system works is what you have is there is a heat exchanger at the top of the tower. The tower is about six hundred feet tall – 550 ft ft tall – its surrounded by huge field of mirrors, and the mirrors heat up the salt as it runs through this heat exchanger and then once its heated up to full temperature it drops back down the tower and its stored in an insulated tank, its a continuos loop like the radiator in your car.

When it comes out the other end it’s at high temperature and then we put it in this tank at the base of the tower – that also is a heavily insulated tank, and its stored in that tank for when we want to generate electricity. Then when we want to generate electricity it goes through another heat exchanger that takes water and heats it into steam, then it goes through a conventional steam turbine just like a regular power plant. So our back end looks just like a conventional power plant with a steam turbine – steam that turns the blade on the turbine.

SK: The 15 hour Gemasolar plant is the same as yours, right? (Along with everything else I learned in this talk, BTW, is the correct Spanish pronunciation: it’s “hemma-solAAR”)

KS: We’ll generate similar to Gemasolar in Spain which uses up all the heat in the salt every single day. Our facility is a similar technology to theirs other than that ours is about six times as large as their project. So their project is really more of a pilot project. They’re I think 17 MW, we’re 110 MW. Ours is the first commercial utility-sized plant.

And we’ll store energy for ten hours. We’ll run during the day, and then we’ll have an additional ten hours that we could run.

SK: Could you store power for more than ten hours? KS: The molten salt in the tank loses less than one percent of it’s heat overnight. It’s a heavily insulated tank. If we filled the tank with the hot salt and it just sat there it would stay there in its molten salt state for two months. SK: Two months!!!??? KS: Now, that’s not the way we use it! We use it on a daily basis because we’re generating electricity here every day. But because its in a heavily insulated tank and because salt is a bit self-insulating: the salt in the middle sees the salt next to it is hot… only at the exterior walls do you get a little bit of heat that escapes. As a practical matter though, the heat will be used daily.

Storage means the company can produce for exactly the peak times that a utility needs to cover. For late risers in Las Vegas, who are out till midnight, that can mean a customized generation to suit their needs.

For the Nevada project we’ll only run 12 noon till 11-12 oclock at night. With Las Vegas the big load center for Nevada, they are looking for their peak requirements on the utility, which go up to 11, 12 o’clock at night. But we won’t run overnight.

SK: So, Gemasolar’s is 15 hours and Las Vegas wants just those 12 – but you could run solar 24 hours?

KS: Yeah, we could run it overnight but the utility doesn’t really want it overnight.

SK: Any things you can say about the BrightSource non-IPO?

BrightSource is a good company. They’ve got strong projects in construction. They are a different technology than ours. Clearly the markets right now are not very strong, either for Initial Public Offerings, or even just the business climate is weak right now.

Its improving, we continue to see an improvement in the economy. Our decision was not to look at IPO activity in 2012, we didn’t think the market makes sense right now and its clear from BrightSource’s activity that it’s not.

You know, they’re a good company, they’ve got a nice pipeline of projects and we expect they’ll continue to be successful – we have a different business model, on IPO we’ll probably look at that next year.

(I also had some questions about Desertec too: his answers here.)











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