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Thermal Battery Pair

for zero thermal footprint Stirling engines
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The "battery" consists of separate hot and cold thermal bottles; place one on each end of a Stirling engine and the heat flows from hot>cold producing work and not heating up the surrounding area.

*

Applications: planetbound applications would include mining and stealth, but it's a natural for off-Earth where you can have both solar energy and deep-space radiation right next to each other to charge the batteries.

More efficent than electrical batteries in that respect; may even have a better energy:weight ratio.

FlyingToaster, Jun 10 2008

Thermopiles - a neglected technology ? http://www.dself.ds.../thermoelectric.htm
Fascinating ... [8th of 7, Jun 10 2008]

Eight times more power per pound http://www.electric...stirling_robert.htm
When space-age techniques of thermal insulation are employed in conjunction with these salts, it has been found that a very good thermal storage battery can be made from which to run a Stirling engine. This is similar to the electric motor-storage battery system, except heat is stored instead of electricity. The thermal battery-Stirling system is superior, however, in that it can be recharged (electrically reheated) much faster and it can store over eight times more shaft power per pound than a lead-acid battery. [brite_eye, Jun 24 2008]

[link]






       It doesn't need to be a stirling engine, just a closed-circuit cycle like the steam turbines in a ship.   

       Sunward side: large collector array of some sort absorbs incident solar radiation, causing a working fluid to be converted from a cold liquid to a hot gas.   

       Energy convertor: Hot, high-pressure vapour is used to drive either a reciprocating or turbine mechanism to convert thermal energy to mechanical energy and thence to any desired form.   

       Shadow side: Hot, low pressure gas passes into a condenser, and sheds heat to space by radiation; by the end of the condenser path, the gas is once more a liquid. This is returned to the evaporator by an injector pump driven by power tapped off the energy convertor.   

       All the technology is Baked, it's merely a question of selecting a working fluid suitable for near space. Hydrogen or helium would be best; hydrogen remains liquid down to 14 K and it's cheap and plentiful.   

       [FT], this is awfully close to getting the dreaded Mike Foxtrot Delta for not-an-invention/ widely-known-to-exist ...... unless others do not share this opinion ....
8th of 7, Jun 10 2008
  

       [8th]The keywords are "battery" and "zero footprint"
FlyingToaster, Jun 10 2008
  

       mumble mumble.... Entropy...... mumble mumble .... Laws of Thermodynamics ........ mumble mutter sigh......   

       The stirling engine is going to produce mechanical energy. Can you please clarify how this equates to "battery" ? A battery is an energy storage device; this is a "generator" is it not ?   

       <later>   

       Thermos ... by that, do you mean a thermos vessel (Dewar) of hot material, and a similar vessel of cold material ? The energy flows from hot to cold, doing work in the process ? The whole thing enclosed in insulation so there is no "spillage" into the environment ?   

       If so, why not use the Seebeck effect to produce electricity directly from heat ? (Which is, by the way, Baked).
8th of 7, Jun 10 2008
  

       lol, yeah that would be it... apologies to Mister Thermos. for deprecating his trademark. <post edited>   

       Producing electricity ? Why ? And yes I know "battery" defines multi-cell, but I couldn't think of a more descriptive catchphrase.
FlyingToaster, Jun 10 2008
  

       // better energy:weight ratio. //   

       The term you're looking for is "energy density".   

       The measure you should apply is Watt-hours per kilogramme. How many watts can the device deliver, for how long, per unit mass ?   

       A good starting point would perhaps be a SLAB weighing one kilogramme. That approximates to 12V @ 2.5 Amp-hours (assuming the C/10 discharge rate). That's 30 watt-hours per kilogramme.   

       Primary Lithium-thionyl-chloride batteries can do MUCH better than that ......   

       <break>   

       A 300W Honda generator weighs 18.5 Kg fully fuelled and will run for 6 hours on a full tank. That's 1800 Watt hours, 97 watt-hours per kilogramme. And that's not an "ultra-lightweight" unit.... only a small proportion of the mass is fuel, too. If you doubled the fuel capacity, you'd only add another 4Kg or so to the mass; approximating to 163 Watt-hours per kg.   

       Yes, it has an appaling thermal signature; but the bar is already set high to achieve your "better energy:weight ratio" .........   

       <later>   

       Example: the device consists of two Dewars, one containing 1Kg of pure water at 1K and one containing the same amount at 372K. The equilibrium temperature, at which energy will cease to flow betwwn the two vessels, is 185K. At this point, the energy which will have passed through the interface will have been 4.2 x 1000 x 185 Joules = 777KJ.   

       One watt hour is 3600 J, therefore this is an energy movement of 215 Watt-hours; The mass of the system cannot be less than two kilogrammes, so the energy density is a useful 108 W/hr per Kg.   

       Question is, how much of that energy can you make available to do useful work ? At what point during the equilibrium process does the decreasing temerature differential (it will decay exponentially) make energy extraction so "uneconomic" to be useless ?
8th of 7, Jun 10 2008
  

       [8th] do you even *read* a post before you anno it ? I'm collecting my own feces in a jar as we speak, in anticipation of your next anno.   

       off-topic: i vaguely recall reading about a thermopile put on a truck's exhaust stack... 2 watts output or something.   

       In the meantime, my kg of plutonium beats your Honda generator. Note that applications may differ somewhat.
FlyingToaster, Jun 10 2008
  

       Sp: faeces   

       How full is the jar now ?   

       // do you even *read* a post //   

       What is this "read" of which you hu-mons speak ? your words are strange to us.   

       // kg of plutonium //   

       Active thermal source ?
8th of 7, Jun 10 2008
  

       /Example: the device consists of two Dewars, one containing 1Kg of pure water at 1K and one containing the same amount at 372K. The equilibrium temperature, at which energy will cease to flow betwwn the two vessels, is 185K. At this point, the energy which will have passed through the interface will have been 4.2 x 1000 x 185 Joules = 777KJ./   

       You are assuming that enthalpy and temperature have a linear relationship in order to arrive at 185K. You are then assuming that specific heat is constant right accross the temperature range, or at least that the effective average is 4.2.   

       Two swings, two misses.
Texticle, Jun 10 2008
  

       No, merely a conveneint approximation to illustrate the general nature of the problem, and elicit informed comment. The purely textual nature of the 1/2B precludes unambiguous expression of the appropriate algebraic notation.   

       Using the absolutely correct physical constants will not change the approximation by a significant proportion, certiabnly not by an order of magnitude; but if you are prepared to provide the precise figures, please do so, with our thanks and blessings.
8th of 7, Jun 10 2008
  

       Consider how easily a peltier can separate hot and cold. You do not get a lot of power when you bring them together.   

       Yes you can generate electricity in space this way. But there isn't much reason to have the themal bottles. You would be better off with a hot plate and a cold plate.   

       Nuclear batteries are the space power source to beat.
Bad Jim, Jun 10 2008
  

       Uhhh, no. Beating nuclear batteries is a really bad idea...... you might break something...... just put them down gently, and leave them be.
8th of 7, Jun 10 2008
  

       //Sp: faeces// too much american TV.   

       //Active thermal source ?// why not? might take awhile to measure, though, but just as practical.   

       This was designed as an adjunct to my Lunar Railway (sidereal track, solar train) (currently in rewrite) such that you could have the solar-train constantly filling hot bottles and a dark-side train "filling" cold bottles... just drop a bunch off where there's autonomousish heavy-equipment in need (and for base heating and cooling of course); obviates the need for bulky solar panels and radiators that can only be used half a month, anyways.   

       ...anybody *not* think you could get more joules out of a solar-heating panel than solar-electrics, or *not* think you could dump enough heat on the real dark side of the moon to get a pretty big quantity of stuff down to a couple degrees K ?
FlyingToaster, Jun 10 2008
  

       Since I have all you exspurts on the line, I have a query:   

       How much power could I get from, say, 1 kg of molten tungsten (at about 4000 celcius) if I used that as the hot side of a small Stirling engine, and the atmosphere as the cool side.   

       I'm not saying heat the Stiring to 4000, as it would melt, just wondering how 4000 degree tungsten compares to a lithium battery?
TIB, Jun 11 2008
  

       Can a thermal battery-Stirling system really store eight times as much power as a lead-acid battery per pound? See above link with partial extract from artilce.
brite_eye, Jun 24 2008
  

       Say you have a 1kg "water battery" which goes from almost-boiling to almost-ice... covering the complete range of the liquid phase, you would get about 2.6 MegaJoules which is about 0.7 kw/h.   

       So a 225lb battery if magically used at 100pct efficiency would give you 100hp for an hour: not too shabby (hmm, okay you want a thermos for it so say 300lbs), at which point the plug-in car that runs on water looks too good to be true (are my figures right? it's too early in the morning for me)
FlyingToaster, Jun 28 2008
  

       /are my figures right?/   

       Not really.   

       1kg of water, undergoing a 100°C temperature drop, must release 0.42MJ of energy. This is 0.117kWh (note unit).   

       A 225lb (100kg) water battery at 100% efficiency would therefore give 15.7hp (11.7kW) for an hour.
Texticle, Jul 01 2008
  

       //1kg of water, undergoing a 100°C temperature drop, must release 0.42MJ of energy// and (almost) through the phase change ?
FlyingToaster, Jul 01 2008
  

       I was assuming that it was entirely liquid phase throughout.   

       Add 0.334MJ/kg if the water freezes, or add 2.26MJ/kg if the water condenses as part of that temperature drop. Neither situation is very practical for thermal battery applications though.   

       New calculation is required if the water is well above 100°C or well below 0°C.
Texticle, Jul 01 2008
  
      
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