Thermal Energy Storage

Thermal Energy StorageYes, we’re continuing to make great progress in all aspects of energy storage via batteries,  most importantly, in terms of cost-effectiveness.  Yet it’s hard to conceive of a time at which batteries can compete with the storage of heat energy. 

This, of course, is the driver behind molten silicon as a storage medium.  Not only is it the second most common element on the Earth’s surface (behind oxygen), but its high melting point makes it ideal for balancing the glut of solar energy during the day and wind energy at night.

The article linked above explores this further, and suggests additional uses for heat storage in modern day civilization.


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4 comments on “Thermal Energy Storage
  1. Bruce Wilson says:

    Commercial air conditioning systems use ice to store cold in the air conditioning season making it at low cost off-peak times then using it when needed (usually at high cost peak energy times). There you get the benefit of the energy released when the ice melts. I have worked with systems that stored heat in water for use later. These are both low cost low tech investments. The energy needed to melt silicon is much greater and requires much more investment in equipment.
    I think that the advances in flywheel energy storage are much closer to being available in the near future. Beacon’s flywheel technology

    • Breath on the Wind says:

      I did a reasonable amount of research on Beacon Power before their bankruptcy and my articles were listed on their website at the time. They did a lot to advance the industry with a high tech system. But reading of a contrasting flywheel system brings out some of the weak points. I am sure you will enjoy this article:

      Bill Gray had/has an innovative idea with flywheels but wasn’t able to gather sufficient funding for a prototype. If his product is viable it seems like he could have used Craig’s services.

      Flywheels are like UltraCaps with a high power density. They are best at stabilizing frequency generation in the shortest amount of time. They are sometimes used as a UPS for the short period of time before a diesel generator can come up to full power.

      There have also been flywheel powered buses The article is a bit dated as more recently flywheel energy recovery systems similar to what has been used for formula 1 racing is being tested on some buses.

  2. Breath on the Wind says:

    Thanks for this Craig. The referred article is interesting but a bit short on details. Many times it refers to the latent heat so I thought I would look it up and found it here at 1790 KJ/kg. It seems to be the highest on the list.

    “1414 degrees” website is also rather sketchy with details. The constant reference to the latent heat suggests that they are transitioning the material from a solid to a liquid. Heat distribution would be a matter of pumps and heat exchangers, but what is the mechanism for transferring the energy back to electricity? And what is the storage efficiency for transferring from electricity to molten silicon and back to electricity.

    Lithium battery efficiency is reasonably high at around 90% while pumped hydro would be around 80% efficiency. Heat transfer mechanisms like this would probably be a bit lower and so would have to make up the economics on capital costs. The high temperature ranges would add to efficiency in a conventional Rankine Thermodynamic cycle, but because of the lack of details I a suspecting it is quite low, perhaps 40% or less? Perhaps they are using a different process and getting as much as 60%?

  3. Glenn Doty says:

    It looks like this is just a form of geothermal without a well. Rather than pumping air deep into the ground through fissures in hot rocks, they’re melting silicon into liquid and bubbling gas through the liquid to super-heat the gas.

    That sounds great!

    It’s pretty easy to get a reasonably well shielded enclosure that would have low heat leakage..

    But the problem boils down to several issues:

    1: You are going to have trouble getting the cycle efficiency above 60%, which is lousy for energy storage. Between carnot limits, heat loss in the pumped gas lines, imperfect efficiency of turbines, etc… you just won’t approach the efficiency of mechanical (pumped hydrostorage) or chemical (battery) energy storage solutions.

    2: Things just don’t work well when you start creeping above ~750 C. The hotter you get, the faster everything breaks down… materials weaken, and they begin to oxidize/carbonize/siliconize VERY quickly, they embrittle… they fail.

    You can’t use the cheap-and easy options like nichrome if you’re going all the way to 1414 C (!!!)… you have to have something far more inert, like tantalum or tungsten or ruthenium.

    So the choice would be to have the heater/conductor elements be EXTREMELY expensive (alloys containing noble metals like ruthenium, tantalum, and osmium), or you have to have your heater elements be non-malleable and brittle (tungsten based alloys) which must be cast, not bent. Both lend themselves to extreme cost.

    3: The shell is going to have to be some form of ceramic. Probably alumina. It would have to be spherical in order to maximize the volume per surface area of the enclosure shell, but even at that the cost for the shell is going to be very high.
    Not only is this expensive as hell, junctions for pipelines and feedthroughs for conductor/heater wires going into and out of the insulated shell will be tricky, and will not withstand high pressure, (which will be a problem if the goal is to recover energy from the liquid by passing gas through the liquid). This means that the heat recovery will require a recuperator step, so that the heat can be exchanged with gas that can become highly pressurized. More cost, more inefficiency.

    4. Silicon “icing” at the ports. It’s fine to have an enclosure of liquid silicon that you’re bubbling gas through.. but whatever energy is absorbed by that gas will result in cooling the liquid into a solid. That’s not good, especially if that occurs near the entrance port – forming a crust wall that blocks off the incoming gas – or the exit port, where the lower-temp walls of the tubes leading off towards the recuperator could face some type of splash/solidifying which could cap off the exit for the gas flow.
    These problems will require additional heaters that will have to run continuously near the inlet and outlet for the gas flows.

    5. What gas? At these temps, silicon readily will bond with oxygen to form silica crystals, it will readily bond with nitrogen to form silicon nitride crystals, and it will bond readily with hydrogen to form silane gasses that would then escape with the rest of the gas… You’d essentially be forced to use argon, which means that you not only need a recuperator step, you’d also need a closed loop system, which would have to be kept carefully sealed and at positive pressure in all portions of the loop (lest you get contamination in your gas and have to start dealing with all the aforementioned problems with silicon bonding).

    These are just the big problems… There are many others.

    It’s a neat thought excercise: high-temp liquid with high heat of fusion… but it’s just not a viable concept.

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