Clean Power

Published on November 12th, 2012 | by Nicholas Brown


Effective Solar Energy Storage System Developed

November 12th, 2012 by  

Researchers at the University of Arkansas have developed a solar energy storage system that stores the thermal energy (heat) absorbed by solar panels at a lower cost than conventional energy storage methods.

The most common conventional methods for such a system involves storing the heat absorbed by solar thermal power plants in tanks of molten salt or in beds of packed rock or oils. This is, in general, thermal energy storage.

Panneer Selvam, center, Micah Hale, left, and Matt Strasser display the thermocline energy storage test system outside the Engineering Research Center in south Fayetteville.

Beds of packed rock are currently the cheapest and most efficient type of thermal energy storage, and they do work, but, as is the case with everything, there is room for improvement.

Beds of packed rock expand and contract as their temperature rises and falls, respectively. This was called “thermal ratcheting,” which stresses the walls of storage tanks because the rocks expand beyond their normal size (the size they were when they were fitted into the tank) — similar to how ice expands and can break glass in your freezer or pipes in winter.

“The most efficient, conventional method of storing energy from solar collectors satisfies the U.S. Department of Energy’s goal for system efficiency,” said Panneer Selvam, professor of civil engineering. “But there are problems associated with this method. Filler material used in the conventional method stresses and degrades the walls of storage tanks. This creates inefficiencies that aren’t calculated and, more importantly, could lead to catastrophic rupture of a tank.”


The New Method

The new method from the University of Arkansas researchers is a structured thermocline system in which there are parallel plates of concrete with steel pipes running through them. The steel pipes transfer heat absorbed by solar panels into the concrete, which stores it until it is needed to boil water and produce steam (which is usually the case), or supply heat to other heat-powered generators such as Stirling engines or thermoelectric modules.

This thermocline concept survived temperatures up to 600 degrees Celsius (1,112 degrees Fahrenheit) and absorbed heat at an efficiency of 93.9%.

It has an impressively low cost of $0.78 per kWh, far less than the U.S Department of Energy’s goal of $15 per kWh.

To give you a better idea of how this compares to batteries: Lead-acid batteries cost upwards of $25 per kWh, lithium-ion batteries cost $50 to $100 per kWh. Lithium-ion batteries can last 4 times longer than lead-acid batteries depending on the type and usage.

So, this should lead to further growth of the solar industry, which thoroughly benefits from reduced energy storage costs. Solar market penetration can be very high when using energy storage.

Source: Science Daily
Photo Credit: University of Arkansas

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About the Author

writes on CleanTechnica, Gas2, Kleef&Co, and Green Building Elements. He has a keen interest in physics-intensive topics such as electricity generation, refrigeration and air conditioning technology, energy storage, and geography. His website is:

  • bcmarshall

    Many of the comments relate to turbine generation. I think it’s really cool for solar space heating! Where can I get more information about the specifics of this?

  • Bassam

    I would like to see some example

  • JimmyCraqcksCapricorns

    There are also Liquid Metal batteries – large extremely cheap batteries that can be recharged infinitely.

    • Bob_Wallace

      Well, more accurately (unless you’ve heard something I haven’t) there may be large extremely cheap liquid metal batteries in the future.

      Right now reports are that smaller prototype versions are being tested. If you’ve got some information that a large scale version has been built, please share.

      It does seem that, if they work at large scale, they should be very cheap and I haven’t seen anyone identify anything about them that would ‘wear out’.

  • At temperatures needed for generating steam, heat-loss is pretty severe, so these tanks need to be extremely well insulated.

    I don’t really see why this is much cheaper than molten salt storage.

    • Ronald Brak

      The storage part is a lot cheaper. It’s just steel and concrete. But of course molten salt can be heated and then stored in a tank while heat has to be transferred into this system, so the overall cost will determine which is used. Perhaps molten salt will be better for solar thermal because the medium is liquid and so can be pumped around, while this system might be best for simple storage of heat from electrical resistance heating. If renewables frequently drop the price of electricity down towards zero it could be profitable to use cheap but low efficiency energy storage.

  • saurdigger

    It’s always nice to compare apples to apples in articles like this. Quoting the estimated “round-trip” efficiency is a better marker. Still looks like an interesting method, but 62-66% (as per “dynamo.joe” link) much more realistic.

    Their research reminds me of the Drake Landing solar thermal underground storage system (

  • While 93+% is an excellent efficiency figure, it’s not the one that matters. What matters is the round-trip efficiency, meaning how much electricity they get out of 1 kWh of heat stored.

    Most likely, the heat will be turned into electricity via a steam turbine. Current molten salt systems get somewhere between 15-20% for roundtrip efficiency, but judging by the temperatures cited here, it could be a lot higher for this solution. Nonetheless, it’s a far cry from 93%.

    Kind regards,

    Kimi Arima
    Wärtsilä Power Plants

    • Your 15-20% roundtrip efficiency is unrealistically low for molten salt. Isn’t this the loss, so that the roundtrip efficiency is 80-85% ?

      Nobody in their right mind would use molten-salt if your efficiency figure was correctl. Flow-batteries give > 65% roundtrip efficiency and they are likely comparable in cost to molten-salt.

      • CB

        Kimi is probably about right… maybe a little low. Thermal engines in this temperature range max out at about 50% efficiency. You can get more with recapturing, but I think the best that’s out there is 60% in the two-phase natural gas generators.

  • Bill Badrick

    The 93%+ is astounding, well done team.
    If you have a sec, chk out my proposal for a energy-generating molten salt 50ft h. artwork in New York City.

  • Guest

    Can you give more info on how long the energy can be stored and how much energy can be stored

    • dynamo.joe

      I didn’t see anything that listed the time, but insulation is pretty well understood, no reason it shouldn’t be able to store energy for the 10-16 or so hours the sun would be down.

      Here is a presentation with some info you might find interesting:

    • Ronald Brak

      I think current molten salt systems can store thermal energy for about a week. This can be increased, but more money will have to be spent building a better storage tank. But generally speaking, currently most energy storage usually operates over a period of an evening or a weekend, so being able to store energy for a couple of days is fine. As for how much energy can be stored, well, the storage system can be built as large as you like it. A kilo of concrete can hold nearly a kilojoule of energy for each degree celcius you raise its temperature. This means a lot of concrete is required, but as mentioned, it’s pretty cheap.

  • Can you please provide details on length of storage and scaleability?

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