Energy Storage

Published on June 6th, 2014 | by Tina Casey


Low Cost Concentrating Solar Power System Leverages Calcium Energy Storage

June 6th, 2014 by  

A new project at the Southern Research Institute is in the works that could shoot low cost concentrating solar power from an also-ran to a major player in the competitive energy marketplace. The research effort is a $1.05 million project funded partly by the Energy Department under President Obama’s SunShot Initiative. It is part of a broader program aimed at developing high-temperature thermochemical energy storage systems for CSPs.

low cost concentrating solar power

Year of CSP (cropped) courtesy of US DOE

Energy Storage And Low Cost Concentrating Solar Power

For those of you new to the topic, concentrating solar power (CSP) plants are based on the concept that it is more efficient to literally concentrate raw solar energy in one place before converting it to useful forms of energy, rather than converting it directly to electricity through an array of photovoltaic cells.

The concentrating part is accomplished with mirrors, most familiarly in the form of parabolic troughs or flat heliostats. Typically, the solar energy is transferred to a fluid such as molten salt or a specialty oil. The transfer medium is then circulated to a generating station, where it boils water for a steam turbine to generate electricity.

That’s where the competitive aspect of low cost concentrating solar power part comes in. Once the solar energy is transferred from the mirrors into heat energy, it could be stored in that form for several hours. The storage feature provides CSP plants with the ability to time-shift their electricity generation into peak hours when grid rates are higher.

That, in turn, boosts the value of the CSP supply relative to other forms of energy in the grid mix.

The Southern Research Institute CSP Project

One key goal for low cost concentrating solar power is to find the most efficient fluid or other energy storage platform. The Southern Research Institute (SRI) project consists of a high temperature solar thermal storage system that uses a calcium-based sorbent.

The technology was originally researched for use in carbon capture for coal fired power plants, so we’re intrigued to see it transferred to a zero-emission facility.

According to SRI, the calcium angle takes CSP storage up to the next level. First of all, it can operate at up to 900 degrees Celsius, compared to only 550 degrees for a molten salt system. That higher storage operating temperature enables the development of next-generation, high-efficiency solar energy conversion systems.

The SRI team also estimates that the calcium-based system will cost only about one-fourth as much as the latest molten salt system.


Not to rub salt in the wound (sorry!), but according to SRI their calcium system has a much smaller footprint than molten salt, taking only about one-sixth of the size to store the same amount of energy.

SunShot And Low Cost Concentrating Solar Power

SunShot actually got its start under the Bush Administration, and under the Obama Administration it has become the Energy Department’s signature program for pushing the cost of solar power down to a competitive level with fossil fuels.

The SRI research project is funded by a $836,697 DOE grant, with SRI chipping in $209,175. It’s part of a $10 million CSP advanced storage tech package that DOE announced back in May as part of its Year of Concentrating Solar Power festivities celebrating “a true CSP Renaissance in America.”

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About the Author

specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.

  • user31

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  • Henry WA

    “The SRI team also estimates that the calcium-based system will cost only about one-fourth as much as the latest molten salt system.”

    If this is correct it would be a major break through as it might make CSP systems with storage competitive in price

  • Matt

    Saying that at 900 degree you need less space than a 550, because of small storage. Please storage is a small portion of the area needed for a CSP plant. Also that higher temp, bring harder (more costly?) engineering issues. And this is a CSP story, about the hope/dream of getting costs down. A lot of dreams on paper don’t make the light of day. I hope they do, but …

  • dynamo.joe

    Why would salt only be able to go up to 550C? NaCl, for example, has a melting temp of 801C and a boiling point of 1413C.

    • Ronald Brakels

      The 550 degree limit is generally a limitiation of the system thermal storage is part of. To get anything hotter than I think about 560 degrees requires special materials rather than standard steel piping and that pushes the cost up. This doesn’t mean that special materials can’t be worth the expense, but it is an economic hurdle to leap.

    • PhilipKGlass

      Halide salts are too corrosive. The usual salt mixture is a eutectic of sodium and potassium nitrate, but it decomposes if heated too hot.

  • Bob_Wallace

    I also want CSP to succeed. But I’m not optimistic. I can’t see CSP competing head to head with PV solar. The market for CSP is more likely for the high demand hours just after the Sun drops too low for PV and before night time winds kick in.

    If PV drops below $1/watt and the price of generic storage drops that’s going to make it very difficult for CSP. CSP can store only heat generated by its collectors. Generic storage will be able to cycle more frequently (night wind and solar) along with doing grid firming work.

  • I really want CSP to succeed. A concern of mine is that it falls under the big plant category. These would be complicated process engineering feats of complexity like nuke plants with long project cycle times from idea to action. [and I’m a chemical/process engineer] Unlike nuke feedstock for CSP is pretty much free. Most concerning is when I read business wire style stories pitting CSP versus PV solar. This competition keeps big corporations and old-style utilities happy. Sort of a divide and conquer strategy. In the meantime PV panels get installed and generate electricity by sitting there. My suggestion is to keep CSP real simple. Like that group out of Israel is doing. Their technology is called Tulip or whatever the hebrew name is. The systems are small, but all the components are pretty much off-the-shelf. The heated medium is a synthetic oil of some kind.

    • JamesWimberley

      Abengoa are installing CSP plants on a regular basis. Since they innovate gradually, their projects have avoided the bridge-too-far problems that delayed Ivanpah, with its very high steam temperature. Any comparison with nuclear plants would be unsound; these are made ever more complicated by concerns about safety that CSP (and for that matter all other non-nuclear) generators just do not have. Steam has been managed successfully for 250 years.

      The pursuit of 24-hour generation is probably unnecessary except as a stunt to refute the “intermittency” talking point. Gemasolar did that already. In an all-renewables scenario for Australia with CSP as the main despatchable, the grid operator AEMO worked out that the optimum would be around 6 hours storage. That gets you from sunset to bedtime. There’s very little real demand in the small hours, which you can meet from hydro if there’s no wind.

      • dynamo.joe

        Since nuke plants are the other tech considering salt storage/cooling, I would say it’s fairly apt.

    • dynamo.joe

      Aora’s heated medium is air. They use a Brayton Cycle.

      I can see your concerns about complexity, but doesn’t PV just shift that from the installation site to the production facility?

  • JamesWimberley

    Calcium what? The press release doesn’t say. Calcium carbonate is super-abundant, it’s limestone and coral.

    The SunShot initiative is about to be embarrassed by reaching its advertised target ($1 per installed solar watt) too early and with existing technology. Initiatives like this one are excellent but really belong to the stage beyond that, 50c per watt. Really low-cost no-brainer solar, deep below grid parity, will trigger a tidal wave of substitution. We need this to prevent the waste of trillions ($23 trn according to the IEA) on high-cost oil and gas.

    • Omega Centauri

      I do think the next stage of solar development requires the ability to generate beyond the window of roughly two hours after sunrise and two hours before sunset. Beyond that range, some sort of storage will be needed. Perhaps this will become that. I don’t doubt that stored solar thermal energy will cost more per unit of energy than daytime PV, but thats not really the issue. Once we get enough PV that daytime power becomes cheap and abundant, we will need to tackle the evening hours.

      • David Howes

        I read recently of thermo-electric type devices that convert heat into electricity. There’s also piezoelectric crystals that convert pressure into electricity. Imagine that under your garage floor, making coverting gravity into electricity. Or under the roadways, powering the streetlights. I love the 21st century.

        • Mint

          Pressure alone doesn’t generate electrical energy. Only pressure applied through a displacement does, e.g. a car moving over it to push it down, and such energy doesn’t come for free. Every kWh you generate that way reduces MPG of the car, and at an even worse efficiency than its ICE.

          • David Howes

            ICE? If I understand you, it’s analogous to walking in sand, which requires more energy on my part, and while taking a step produces energy by the sand particles moving, once i’ve moved them with my step, they don’t make any more energy while i’m standing in place?

          • A Real Libertarian


            Internal Combustion Engine.

          • David Howes

            Ah, thanks.

          • A Real Libertarian

            No problem.

      • Mint

        Evening hours have an easy solution: batteries.

        Yes, batteries are too expensive to store solar energy from dusk till dawn, and it’ll take a technological breakthrough for that to change (not even a Terrafactory will fix that).

        But the evening peak is certainly doable, as it only needs 4-6 hours. We’re currently meeting the need with ~$1/W peakers, and $200/kWh actually has a cheaper construction cost. So as long as you charge it with energy as cheap as the natural gas you would’ve put into the peakers, you’re all good.

        I’m not sure if solar thermal can compete with that. I think it’ll only come into play if we start needing longer storage periods, e.g. by mandating 50%+ wind penetration.

        • Bob_Wallace

          It’s going to be very interesting to see if stored thermal solar will be competitive with stored PV solar.

          The advantage for PV is that its storage can be used again in the 24 hour cycle to store off-peak wind for early morning use. Cycling twice doubles revenue.

          I’m enjoying the point we’ve reached. We now know that we can replace fossil fuels on our grid without increasing the cost of electricity. Now the challenge is to find out how much we can lower the price of electricity going forward.

          • Omega Centauri

            I’m not convinced we have storage licked. certainly no to the point where 100% renewable will be cheaper than todays unsustainable solution. I am convinced that circa 80% renewables is doable at decent cost, getting the last bit is going to be a lot tougher, as we aren’t talking about storing juice for a only few hours, but rather for several days.

          • Bob_Wallace

            I am. Unless the numbers I’ve found for pump-up are way off a combination of wind, solar and pump-up would be clearly cheaper than today’s grid feed. And if vanadium flow batteries are around 8c/kWh then that’s also workable.
            Remember, we have high external costs for coal. Our coal and nuclear plants are old and will have to be replaced with something, the ‘cheap’ power they produce right now can’t continue very much longer.

          • Bob_Wallace

            A bit more, that last 20% right now is mostly NG plants that are sitting idle >70% of the time.

            Gas peakers are expensive. I suspect that adding more capacity to pump-up and an extra turbine is cheaper. Or some additional storage tanks for flow batteries.

          • dynamo.joe

            Someone mentioned Aora’s Tulip system here. Those are gas peakers and they don’t sit idle 70% of the time. Can also be used for grid stabilization.

          • Bob_Wallace

            I’m using the 2011 and 2012 CF for US natural gas plants. They run less than 30% of the time.

            And just saying that the last 20% should not be a budget buster.

            If the goal is to get carbon emissions down to 70% of 2005 levels by 2050 we can tolerate a modest amount of gas burning for deep backup. That’s a worst case solution, hopefully we’ll see storage become cheaper than peakers.

          • Omega Centauri

            I think we are decades early for worrying about the last twenty percent. Our job is to push for the first 80!

            I forsee a lot of problems.Mostly due to the combination of politics, and spent costs. I.E. a lot of stranded fossil fuel assets both above and below ground, and a society totally vulnerable to misinformation, which also worships the owners of capital. Any percieved price increase or grid instability will be demagoged, as due to renewables. And I expect cost increases, if renewables (or utility rate payers, or even taxpayers), are forced to make the owners of the stranded carbon assets whole.

          • Bob_Wallace

            Sure, fossil fuel interests are going to make a lot of noise and do what they can to slow the transition, but that will be over fairly soon. Already investment banks are talking about the lack of a future for coal. The same will happen for oil as soon as we have an acceptable alternative (moderate range, affordable EVs).

            Coal has been unsuccessful in stopping solar and wind. They’ve slowed things down a bit, especially in Australia, but the transition is now market driven and the game decided.

            As renewables increase their market share they gain political power. Wind and solar are soon going to be able to lobby just as hard as coal. And every day more people learn that wind and solar work. Once they’ve learned that it’s unlikely they will unlearn it.

            When we’ve got Oklahoma selling wind-electricity to Alabama and Georgia we know we starting on the downslope.

          • Mint

            It’s pretty clear where where thermal solar has advantages over batteries and vice versa. If you want to increase the storage capacity, you just increase the mass of molten salt or oil or whatever, which is very cheap. Its problem is not cost per kWh, but cost per kW.

            Batteries, OTOH, are very cheap per kW, but expensive per kWh. They’ve only recently become cheap enough to provide 4-6 hours of continuous output at a lower construction cost than peakers can. It’ll take a technological miracle to economically stretch that to 24 hours and beyond.

            I think solar thermal’s battle is not with batteries in the evening (can’t win there, IMO), but rather with dispatchable fossil fuel, and it’ll be a tough one to win.

          • Bob_Wallace

            I just don’t have a handle on where stored thermal might end up, cost-wise. NG prices will certainly rise, perhaps not in the short term, but supplies will dwindle over time.

            Long term storage may turn out to be pump-up and/or flow batteries where additional storage only requires installing more holding tanks.

            This is an area where we can relax and wait to see what is invented. We don’t have enough renewable generation to make large scale long term storage a need at the moment. There’s a lot of dispatchable gas to turn off first.

        • Omega Centauri

          It remaiins to be seen whether PV+batteries are cheaper than CSP+thermal storage. The later solution scales better with respect to hours of storage. The former is more flexible, and might be usable like Bob suggests -i.s. it can be charged by other than PV, which might allow some extra opportunities to charge/store.

        • David Howes

          How’s graphene for said technological breakthrough? Combined with additive manufacturing, we might be able to make batteries magnitudes smaller, cheaper, and more efficient.

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