h a l f b a k e r y
Ask your doctor if the Halfbakery is right for you.
add, search, annotate, link, view, overview, recent, by name, random
news, help, about, links, report a problem
or get an account
Water is odd. Seawater freezes at about -2C, and then it
floats, which is odd. Water is densest at 4C so in polar
regions you have a strange temperature gradient with the
cold ice on top, the interface between water and ice at
-2C and then a gradual increase with depth to around 2-4C
very bottom. These are the sorts of conditions below
your typical ice sheet, but the temperature gradients get
more extreme in the air above. Above the Ross ice sheet
surface air temperatures average about -32C and dip to
-60C <link> through the winter months. Furthermore the
wind speeds are a very healthy average of 10 knots with
occasionally brutal 60+ knot periods. The ice shelf
thickness over the ocean varies a fair bit but is generally a
few meters, very drillable.
Now, using temperature gradients to generate power is
baked. Theres one in Japan <link> that operates using the
20-23C difference between surface water and deep oceanic
water. Thats 10C worse than my average and 50+C off the
best case scenario. The biggest losses derive from the
1000s of metres of piping to access the cold water at
tremendous depths AND the fact that their temperature
gradient is upside down, they have to lift all that dense
cold water. So lets beat that system.
Take a large steel tank and insert it below the ice shelf in
all that balmy 0C water. Some optimization for surface
area and ice shedding will have to be done but thats what
engineers are for. Fill this tank with some ammonia. At 1
atmosphere, this boils at -30C so the sub-shelf water will
get a nice vigorous boil going in no time, especially
considering the excellent heat properties of water
combined with a nice bit of state-change enthalpy. Your
boiling ammonia will emit gaseous ammonia, conveniently
at the top of the tank, so here you can connect an
ammonia turbine, these are baked <link>. While you could
just keep pouring ammonia in, thats a little wasteful, so
lets have a huge air-cooled condenser on top of the shelf.
The healthy winds and low winter temperatures should
have that ammonia condensed in a jiffy. Conveniently,
condensed liquid ammonia will run right back down to the
sub-shelf tank all on its own. The whole thing can be done
with essentially one moving part, the turbine. If the
surface winds get above/below -30C then its pretty trivial
to mess with the pressure of the system to manipulate the
condensation point to either keep it running in warmer
times or extract more energy from colder winds.
One pitfall is that ice shelves move. So, the clever thing to
do here is to build the ice penetrating section very tough,
anchor it well and then pump a little of the warm sub-shelf
water around the leading edge in a kind of hot-knife
manner. Or live with relocating the station every few
There, simpler than prior art. More efficient, and its
extracting thermal energy from arctic region, which should
cheer up some people. Just need to work out the grid hook
[bs0u0155, Jan 03 2019]
Oceanic Thermal Energy Conversion
[bs0u0155, Jan 03 2019]
Ice Shelf Wind/Temp Data
[bs0u0155, Jan 03 2019]
||// Conveniently, condensed liquid ammonia will run right back down to the sub-shelf tank all on its own. The whole thing can be done with essentially one moving part, the turbine. //
||Thankyou. Please sit down.
||Now, class, can anyone see the fundamental flaw in the design ?
||Yes, that's right. The condenser and the evaporator are linked by the liquid return pipe. Since the turbine presents resistance to the flow of gas, the pressure in the evaporator will tend to push liquid back up the return line (assuming it enters at the bottom of the tank) to the condenser, until the pressures equalize, due to the mass of the liquid column.
||Now, since the pressure in the condenser will be very low, because any gas entering it condenses into liquid, the height of the system must be large compared to the back pressure to allow liquid to return to the tank purely by gravity. This can be calculated based on the density of liquid ammonia and the value of gravitational acceleration.
||The solution to improve efficiency is to tap power from the turbine and use it to pump liquid back into the evaporator.
||//tap power from the turbine and use it to pump liquid back
into the evaporator.//
||Could do. Again, that's the sort of details that have already
been worked out. Could also do it mechanically. Lots of
||// Some optimization for surface area and ice shedding will have to be done but thats what engineers are for. //
||Oh really ? We'll tell you what engineers are for. They're for telling you what you can't have, and why, and how much it will cost not to have it, and why that will take a lot longer than you are prepared to wait.
// Lots of wind around. //
||Yes, and it seems to be issuing from someone's Lower Rear Orifice ...
||And you could keep this perpetual motion device going until all the thermodynamic energy of the ice float has been converted to electricity and the device floats away. But I do like the thought process
||I see two problems with this plan:
||1. Pure ammonia is terribly toxic stuff, so if it leaks you'll kill any humans or wildlife nearby. This happened pretty often with early refrigeration machines. Maybe you could have a secondary coolant loop with something less toxic like propylene glycol so the ammonia is safely sequestered.
||2. Ice shelves tend to be far away from permanent human settlements, so without hundreds of miles of transmission lines the demand will be pretty low. Still, it could be useful for remote Inuit villages or polar research stations that normally rely on diesel generators.