Thermal Energy Storage Device

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Hi Marcin had some ideas about an energy storage system that I wanted to share with you all at OSE. The system would be able to store energy as heat which can then be used to drive a steam engine for electricity production. The system is solid state and has no moving parts like pumps etc. Renewable energy is not used widely as the main problem is energy storage. I saw that you have a huge battery but it is expensive to manufacture recycle etc,. The device is nothing exotic; it works using eddy current induction to heat a completely sealed vessel which contains molten salt whose heat is in turn used to drive steam enines etc. The vessel is completely sealed as there is no need of pumps to transport heated salts or fluids as it works by induction. This makes it quite inexpensive and would have a high energy density. The molten salt is saltpetre and has been used extensively in various industries and so good working knowledge exists. This molten salt system is also used in solar thermal power generation stations like Ausra. This device does not have a patent and I have not applied for one anywhere. This can also be used to store energy generated by various sources like wind turbines etc as only electricity is required by the device. Do let me know what you think and let us see if this is a feasibile method.



Thank you for bringing this up. There are several points about this that could actuallly make this work - so I would like to pursue feasibility analysis with you. This is certainly not a crazy idea - but it does require serious technical due diligence. Also, this would be another motivation to get the steam engine into completion.

Please send me further technical details on the design. Let's aim to specify components and the design, and let's come up with a realistic bill of materials and cost. If we can do this, and if we can peer review the design itself, I propose that we fund this and build it in the near term.

Let me know more about your background, and how you can help make this happen. Are you familiar with induction circuitry?

This is definitely worth exploring - as molten salt storage is certainly on our radar - and this could really put us forward on the solar turbine project.



Response from Tony Sirna

I think a molten salt system might be a good choice but I think it will probably be worth using some heat exchanger rather than an induction coil. If the electricity is coming from the steam engine it will have been created from heat at 30% efficiency. I think a heat exchanger could be much more efficient at transferring heat to the storage device, rather than going to electricity first.

Lead-Acid batteries are about 90% efficient at giving back the energy you put into them. The system described above might be at best 10% efficient (30% * 30%) for the steam engine system. Perhaps if the electricity is coming from some other source, but even still 30% efficiency is not that great.

"Lead acid batteries are only about 70% in real life in a photovoltaic system (similar power output curve) due to their 50% or so efficiency at high charge regimes which is where they spend a lot of time. Nife gets about 60-65% apparently over the whole state of charge range. Nife batteries are about $60 peak kWh when the leave the factory to manufacture. Zn Br is only $25 per kWh and about 75- 80%.

Personally I like the idea of a renegerator made of a pebble bed. If you use air as the transfer medium at 1.27 J/(K*Litre) and you have a countercurrent heat exchanger to interface to the steam gen with a 10 degree temperature drop (not sure if that would work out but example) you'd need about 1259 liters (1624 grams) per second, 49 cfs or so, for 16 kW the p32 outputs so if the flow goes through a cross sectional area of 50 cm by 50 cm (25 sq decimeters) it'd need 5 m/s so 25*1.625/2= 20 watts for airflow , and the bed can be a regenerator made from gravel. Maybe steam generator coils (narrow and large in number) coudl be embedded in the regen instead to simplify things, instead of using a fan to transfer gas around convection alone might do it?

Also, don't think efficency is optional because the energy is "free" because it aint free. If you waste energy, that quickly increases the capital cost per watt delivered of the system, which is why I am partial to Zn br now."Gregor 07:55, 24 June 2011 (PDT)

-Abdul Hamdy's addition- Why not research the reconditioning of Lead Acid batteries, I live in a third world country where they are sold for about 5usd each when they are spent.. but I know that they are taken apart and "repaired" then sold second hand to poorer people. I am sure in the USA many salvage yards will have many batteries of varying conditions. I remember reading that a good percentage of batteries can be reconditioned by charging them in a specific manner. this can be used to build a trove of lead batteries which will store power during the day and the lax restriction on number can make for limiting the charge speed in order to retain the maximum efficiency of 90%(probably less due to not being new). This is concerning the car Pb battery. there is another option which are the deep cycle batteries for trains and forklifts. they are larger which makes the effort per repair much more worthwhile. here is a link to a how it's made video clip on youtube that has sparked this idea for me.(no copyright worries as the video is posted by the copyright holder)

Input from Tony Sirna of Dancing Rabbit Ecovillage

Thermal Storage

  • Hot water - limited to 100C
  • Steam Accumulator
  • Hot vegetable oil - limited to 200C without removing oxygen which causes breakdown
  • Mineral Oil and Thermic Fluids - up to 600C (I am pretty sure that is a mistake, it is 600 F not C., about 310 C)
  • Pebble Bed (rock, glass, etc.) and Oil
  • Concrete
  • Molten-salt

Hot Water

For storing low grade heat for space heating and domestic hot water an insulated, unpressurized tank will generally suffice. Copper tube heat exchangers are common. Examples [1]

Steam Accumulator

A Steam accumulator is an insulated steel pressure tank containing hot water and steam under pressure. Supposedly you can find old versions available that are no longer in use from fireless locomotive systems or other industrial applications. This might be the best option for the tinytech style engine and collector as it would integrate well with the steam based system and would be easily fabricated with the technology at FactorE. [2] and [3]

Hot Vegetable Oil

Tamera Ecovillage in Portugal is doing a solar CHP project using sunflower oil as the storage medium. Their engineer told me that 200C was the max temp before thermal breakdown in normal oxygen atmosphere (higher under nitrogen atmosphere). They chose sunflower oil beecause it is readily available around the world. [4]

Mineral Oil and Thermic Fluids

Specialized mineral oils have been developed as heat transfer and/or storage fluids. This page compares the properties of a bunch of products on the market [5] Prices seem to be about $20/gallon.

Pebble Bed and Oil

To reduce the amount of oil needed in the system you can fill a tank with rock or glass beads. Oil is then pumped through the rocks as a heat transfer fluid (also providing some of the storage). Here's a simple description [6]


Papers seem to say that storing heat in concrete may be the cheapest method. Some transfer fluid or gas is pumped through small tubes embedded in concrete. Discussion of Concrete as a Thermal Energy Storage Media on the National Renewable Energy Laboratory site


Molten salts provide higher temperature storage and heat transfer but they freeze at 110-220C so are solid at ambient temperatures. This means they must always be kept hot. Probably higher tech than desired for our project. [7]


  • Storage of Thermal Energy [8]
  • Overview on Thermal Storage Systems [9]
  • Paper from NREL [10]