Saturated Water

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HintLightbulb.png Hint: For exciting work in this direction, take a look at the only company that we know of in the space of off-grid energy - Terrajoule

HintLightbulb.png Hint: Summary: saturatated water at 18 bar provides 20 kWhr per cubic meter of energy storage. Extracted at 10% by a Variable Cutoff Steam Engine, and this a practical way to store electrical energy


Basic Numbers

  • This page explores the concept of Saturated Water, as well as its various applications towards OSE's goals
  • It is liquid water at above its boiling point by being compressed under low pressure, providing heat storage that appears to be superior to any known thermal or electrochemical energy storage method, outside of chemical storage (fuels like Gasoline or Hydrogen)
  • This paper [1] shows 20-30 kWhr/m3 (about 1000 kWhr for 38 cubic meters) storage capacity which considers the pressure drop or about 1/4 of the theoretical value. Use this with a Variable Cutoff Steam Engine. Point: take 20 kWhr/m3, extract at 10% efficiency - and you have 2 kWhr from a cubic meter storage vessel! This is very attractive. For example, using a 18" steel pipe, 6 meters long - gets you this storage volume, and can be used for combined heat and power.


To 2 significant figures:

  • Definition of Saturated Water - "saturated liquid, noun in Thermodynamics -a liquid whose temperature and pressure are such that any decrease in pressure without change in temperature causes it to boil.
  • Water boiling temperature depends on pressure.
  • If you increase pressure, water will boil above 212F.
  • Calculator - give it pressure - and it tells you what temperature water boils at and its energy content at that temperature - [2]
  • 18 bar water is at 209C and 900kJ/kg, at 1 bar it is 100C and 420kJ/kg. At 18 bar, density is 860 kg/m3
  • The difference beween the two is 460 kJ/kg that can be released as steam energy. This appears to be the stored energy that can be extracted by a steam engine
    • Note: 1 MJ ~ 0.3 kWhr. ]
    • Thus, on a kilogram basis - one kg of saturated water contains about 0.15 kWhr of usable energy that can be extracted by a steam engine - as this water can release steam readily.
    • If this steam engine is 10% efficient - then we have 0.015kWhr/kg usable motion energy, convertible to electrical power
    • Thus, a 250 gallon tank (about 1 cu m) produces about 15kWhr of usable energy. However, there is a 20% factor here somewhere because the water is used up - and pressure drops. What is that reduction factor? Huh?
    • Let's go back. Heat pumped in is 460 kJ/kg, or 460 MJ/cu m. About 150 kWhr. It makes not a lot of sense to use solar to generate this heat - about 25kW array or $8k of PV would be needed. However, if this is extracted at 10% - for 15kWhr of usable energy - $8k for electrical autonomy would be great. If that figure drops to only 25% by other? losses - then it's back to biogas or charcoal. However, if solar thermal could do it at a cheaper cost than PV, that would be workable.
    • Question: Why is the thermal storage only 20kWhr/m3 if we pumped in about 150kWhr? Where did all the energy go?
  • Thus, a 10,000 gallon (38 cubic meter) tank at 18 bar contains 38*860 kg or 33,000 kg
  • Energy releasable from this is 33,000*480kJ = 16 GJ.
  • 1 GJ = 280 kWhr
  • Thus 4400 kWhr.
  • Extracted at 15% efficiency via a steam engine, that is 590 kWhr. But 1/4 the value for effective extraction due to pressure loss through discharge - and have 150 kWhr. Examine this for a system under OSE conditions of 1000 gallon tank + 1/4 HP steam engine.
  • Acceptable.
  • This means that a 1000 gallon propane tank gets you about 15 kWhr of storage when extracted with a steam engine. At $2500 tank cost - this is $170/kwHr. 2x cheaper than Lithium Ion batteries, and about same as lead acid batteries. But longer life for propane tank.
  • This may be revolution and end of battery storage on a path to ecological means of storage via solar-heated saturated water, wherever available.
  • DiY weld of 5x10 steel yields more favorable results. At $5/sf or $250/sheet - that comes to 35 cubic feet of storage - or about 250 gallons. So $1/gallon volume costs. $1000 per 1000 gallon tank, which is 1/2 or less the cost of industry standards for energy storage. Once we consider lifetime design of properly sealed metal - cost is 10x less compared to battery storage.


  • "The volume specific thermal energy density depends strongly on the variation of the saturation temperature resulting from the pressure drop during discharge, characteristic values are in the range of 20–30 kW h/m3" - [3]
  • Saturated water temperature and pressure tables - [4]

Sample System Design

  • 1000 gallon propane tank - [5] - $4k
  • Volume - 3.8 cu m. Should yield ample storage, 8 kWhr for a night

See Also

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