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Hydrogen as heat storage? - Page 2

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Posted by Anthony Matonak on September 2, 2007, 12:45 am
 
dances_with_barkadas@yahoo.com wrote:

You're suggesting storing heat in hydrogen gas as better than
storing it in liquid water? Hydrogen gas is pretty light. It
seems to me that it'll take a heck of a lot of very light gas
to equal the heat capacity of a few gallons of water.

Why not put some numbers to it? Say you're looking to store
heat for keeping the house warm, around 77F, 25C, 298K.

http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html
at 25C specific heat is 4.181 kJ/kgK and density 997.1 kg/m^3.
This gives us heat storage of 4169 kJ/m^3K.

http://www.engineeringtoolbox.com/hydrogen-d_976.html
http://www.engineeringtoolbox.com/gas-density-d_158.html
at 298K specific heat is 14.3 kJ/kgK and density is 0.0899 kg/m^3.
Roughly speaking (since I don't want to bother with ideal gas laws
and whatever pressure those S-Mart hydrogen tanks use) this gives
us heat storage of 1.29 kJ/m^3K.

Which would you rather use? Very cheap water at 4169 kJ/m^3K or very
expensive hydrogen at 1.29 kJ/m^3K? Let me tell you a secret, one of
them stores 3200 times more than the other. :)

Anthony

Posted by hhc314@yahoo.com on September 2, 2007, 3:12 am
 
On Sep 1, 7:54 pm, dances_with_barka...@yahoo.com wrote:


It is not when the H2 is stored in a sealed cylinder that the risk
exist, it is when the H2 leaks out and mixes with atmospheric O2.
Google deeper and you'll find that hydrogen explosions are a major
hazard in facilities that employ it, like manufacturing facilities and
research labs. I recall two structures here in Boston that were
destroyed by H2 explosions during the past 10 years.


Yes, very definitely. There is no comparison. That's precisely why
facilities that employ H2 are generally equipped with sensitive
hydrogen detection alarms and automatic ventilation facilities. I
fact, H2 emission is generally a safety consideration requiring
positive forced ventillation where storage batteries are being
recharged, usually required by law here in the US.

Even in cars, I've seen 3 instance of car batteries exploding as a
result of the emitted hydrogen. My son owned one of these, and I
believe I still have photos available on-line somewhere, and if you
would like to see the resulting damage, if I can locate them I'll be
happy to mail your a photo.

Harry C.







Posted by CWatters on September 2, 2007, 10:28 pm
 

House builders don't really like installing mains pressure unvented hot
water storage cylinders for safety reasons - or rather the extra paperwork
required.



Posted by daestrom on September 3, 2007, 2:23 pm
 

Might you be talking about the UK??  In the States, most all hot water
heates are piped directly to 'mains pressure'.  A combination
temperature/pressure relief valve provides more than enough safety in just
about all cases.  The only hot-water cylinder 'explosions' I've ever heard
of in modern times have been propane gas leaks and pilot lights.

daestrom



Posted by hhc314@yahoo.com on September 3, 2007, 8:39 pm
 wrote:

Agreed, but just one nit. Building codes in the US require the hot
water tank to have a pressure capacity at least equal to the mains
pressure, and be equipped with both an approved temperature/pressurue
release valve with a pressure setting less than the burst presure
rating of the tank (which is required by law to be stamped on the
tank's label).

Also, they are required to be equipped with a pilot light sensor
(generally using a thermocouple) which turns off BOTH and main burner
valve and the pilot light gas feed if the pilot light were to be
extinguished for any reason. The same is true for gas operated home
furnaces.

Some states is the US also require a gas water heater to be equipped
with a "vacuum breaker" valve on the water feed line, which allow air
to enter the tank if a negative pressure were to develop in the water
supply line. This can happen if a fire developes in the neighborhood,
and the fire department is heavily pumping from nearby fire hydrants.

In Europe, things are quite different. There the tankless "instant hot
water" type systems are much more common. I have no idea what
regulations apply to them, but in fact believe that in a failure mode,
they could be quite dangerous.

Here in the US, most hot water tanks fail gradually, by leaking their
contents all over the floor. This generally happend withing a year or
two of the tanks warranty expiration.  :-)

I've never heard of a catastrophic burst of a hot water heater in this
country, but it does become an emergency when the tank takes of a
serious leak and begins flooding your home or basement, plus you have
no hot water.  The water leakage can be resolved by turning off the
mains flow to the heater, but you still have to take a cold shower
until you obtain a replacement.  Bummer!  :-)

Harry C.

p.s.,  Plus, it always happens on a weekend or a holiday!



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