Keeping it hot, late into the night

Functioning solar collection station after sundown? Pppffffttt!

Or, maybe not.

A Tower of Molten Salt Will Deliver Solar Power After Sunset

Crescent Dunes, due to come on line by the end of this year, uses over 17,000 mirrors to focus sunlight on a heat receiver atop a 165-meter-high tower—a layout resembling California’s massive Ivanpah solar power tower. However, while Ivanpah’s receiver heats steam and pipes it directly to turbine generators, SolarReserve’s heats a molten mixture of nitrate salts that can be stored in insulated tanks and withdrawn on demand to run the plant’s steam generators and turbine when electricity is most valuable. Smith expects that NV Energy, the Las Vegas–based utility contracted to buy Crescent Dunes’ output, will want it mostly during the utility’s unusually late demand peak, which the Vegas Strip’s nightlife routinely stretches toward midnight.

Mark Mehos, thermal systems group manager at the National Renewable Energy Laboratory (NREL), says molten salt towers akin to SolarReserve’s are “the next-generation technology” for solar thermal power. Plants without storage may never be able to compete with PV, says Mehos. And while molten salt storage is often added to trough-style plants, which use hectares of parabolic mirrors to heat synthetic oil flowing through pipes suspended above them, salt towers are cheaper and more efficient, he says.

Eliminating the heat exchange between oil and salts trims energy storage losses from about 7 percent to just 2 percent. The tower also heats its molten salt to 566 °C, whereas oil-based plants top out at 400 °C. That temperature boost squeezes 5 to 6 percent more power from the plant’s steam turbines and enables a tank of salt to hold two to three times as much energy. The temperature advantage could grow: In September, ­SolarReserve won a US $2.4 million grant from the U.S. Department of Energy to develop a ceramic receiver that can withstand 732 °C.

OK, engineers. Have at it.

Will this work?

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5 Responses to Keeping it hot, late into the night

  1. NotClauswitz says:

    Will it work, or will it put grant-writers to work scouring the bottom of the Govt.’s Alt-Energie money-pot? Solyndra or Saltcylindra?

  2. Work? Sure.

    Work well? Maybe, maybe not. No solar or wind-based power plant works worth a darn without some sort of storage, so molten salts is a good thing. On the other hand, this also adds a conversion stage to introduce more power loss. So I expect it’ll run considerably below that 110 MW nameplate capacity.

    And with any solar plant… they tend to run far lower than predicted. Ivanpah was running about 1 quarter of predicted output, due to weather conditions, although they claim they have it up to around 70% (which I don’t believe).

    Compared to a gas-fired plant, it’s still not great. A gas plant can come online to meet increased load in as little as thirty minutes. Ivanpah (direct conversion) takes more like six HOURS. Crescent Dunes pretty much has to take longer than that since it’s an indirect storage system.

    Also, Ivanpah takes up some 3,500 acres, for 392 MW. Crescent Dunes is 269 acres, for 110 MW. The Lausward gas combined cycle plant is rated at 595 MW, in just @10 acres.

  3. Correction to above. I realized I plugged a wrong number into my area calculator. Lausward is closer to 40 acres.

  4. Joe Wooten says:

    One of those was built in Southern California back in 1981. It was a nominal 10 MW station that sometimes produced that amount. It did not operate all that well and was shut down in 1986. I heard it was re-built in the 90’s to use molten salts, and it still did not operate that well and shut down after 3-4 years. It could only go about 3 hours after the sun set.

    I do not expect this one to operate any better. I might make rated output for a few hours on a clear day with clean mirrors, but it will still be more expensive that natural gas/coal/nuclear and will be a maintenance headache. It is another attempt to farm subsidies from uncle Sam.

  5. Petey says:

    Siemens’ division in Switzerland has been using thermal energy storage silos filled with pellets in power generation, but I haven’t heard any success stories, yet.

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