POWER FROM A CONVECTION LOOP:
this is a scaled down version of the solar tower
updraft system planned for austrailia, or the SHPEGS
project with water/silica jell adsorption instead of
ammonia absorption, or the water spray downdraft
'culvert on a hillside', proposed for hot dry
climates.... or is it a solar hot, creek water heat
sink, stirling motor with gravity/adsorption/psi
assist?
MY SMALL SCALE PLAN:
I have a 10 acre property in bc, canada, with a very
cold, (7C seasonal avg), creek water intake ~ 80'
uphill, reaches my waterbox at ~40 psi, the overall
grade is about 35%.
a wind turbine, (kicks in at 5m/s, out put at 8m/s =
10Kw), with 6m span is the basic buy;
a blown in place cement loop, consisting of two ~
500m3 chambers and two 30 meter long x 2 meter diam ducts,
(see monolithicdome.com), and a very large solar batch heater are the basic builds:
the top chamber is under ground, sloping downhill. in
the 2 meter duct preceding this chamber 20, or so,
cold creek water misters, pointing down hill, spray cool the dry, hot air. excess water is collected, and supplies the domestic hot water tank. as humid air
is lighter than dry air the next step is a silica gell
adsorbent wheel. the air is warmed by this, so a
copper pipe heat exchanger, filled with cold creek
water, recools the airstream to 10C before it falls
into the upper chamber.
a buried 2 meter air duct runs 50m down, (drop = 12m)
to the wind turbine, which sits in the mouth of
the hot air chamber, (at the lowest point therein).
HEATING CHAMBER:
a blown in place cement tube,(nickel or carbon black
outside surface, selective thermal topcoat), contains
another blown in place cement tube. this forms a one
meter 'jacket' filled with heat transfer fluid, the
smaller diam inside inclosure is the lower hot air
chamber. this section runs back uphill, smoothly
enters a 2m air duct that connects with the spray down
and desiccant wheel duct. this hot return section is spray
foam insulated and buried in a perlite trench.
this large volume of htf enables:
domestic heat,(did i mention that the system will
provide all domestic heating and removal of heat?).
fluid to circulate in the interior heat pipes: to stay
out of the airflow, these are put, infloor heating
style, on the bottom of the inner cylinder, (this adds
to seasonal thermal storage as well).
diurnal storage: i have sized the htf volume and
aperture for 7 hrs exposure = the btu's it takes to
heat my house for 24 hours when it is 20c below, PLUS
the amount of heat loss from 24 hr wind tunnel
operation. these kind of cupped cylinder, black body
collectors convert a healthy percentage of solar
energy into btu's. even if i only get 50% of the
energy falling in 7 hrs on 100 square meters, (10
meter by 10 meter collector area @ ~~1000 watts/sq
meter) into btu i have lots. even in this dead of
winter, worst case scenario, there is a cold air bonus
available. by use of a venturi valve the cold water at
50psi can draw in outside air: you can tune these as
to how much air but, due to nucleation issues, the
water would not freeze until minus 7C, or colder.(i
have tried to make snow this way...it does not work).
thus, when solar is at low ebb, some delta t can be
maintained.
seasonal thermal storage: sizing this big will give
tons of heat in the summer. the black tube sits very
close to bed rock so, with little effort and a minor
cost, shallow, vertical heat pipes are bored directly
below the hot section, (ten meters deep x one every
few feet). very small retrieval cost as the tube sits
right in the hot plume.
the look of the entire solar collector is very close
to a scaled up solar batch heater. anodized aluminum
parabolic troughs cup the black cylinder and bring the
width of apertue out to 10m. a cable tensioned ETFE
membrane covers the 10x10 meter aperture. a second
desiccant wheel draws geo temp
air into the bottom of this envelope and hot dry air
is vented at the top of the ETFE skin. although this is a parasitic drain on the collector, this space must be temp and humidity controled, (the envelope must be vented in any case), this hot dry air can be put to use regenerating the desiccant wheel.
if it seems all this water removal is costing to many btu, consider this: the the htf CANNOT impart more than a fraction of its energy to the air in the heating chamber, no matter the surface area of heat exchange. by moving some work to desiccant regeneration, MORE of my btu can be brought to bear ON the objective.
this is NOT a million dollar build:
100 sq meters ETFE foil = $15,000
cable tensioned ETFE support structure = $10,000
10 Kw wind turbine = $20,000
200 feet of 2 meter blown in place air duct, trench,
perlite = $50,000
many gallons of heat transfer fluid = $10,000
two, (well, three really), blown in place cement
chambers = $100,000.
10 6" by 10 meters deep boreholes; 60 meters @
$300/meter = $18,000
GRAND TOTAL $223,000
my stirling question is this: if this thing just laid
flat, ie - no drop at all, it looks like a sterling
setup with potential,
but no potentiator. the air would still want to move
from the high pressure to the low pressure chamber,
but would probably form loops in both air ducts; ie it
has no direction. but, aided by that 50' drop and
nudged again by the nozzles, flow is established and
no air goes the wrong way. therefore, one should get
at least the output predicted by stirling formulas
that take swept volumes and delta T as inputs. i have
seen this formula somewhere online, plug and play
style. my own Fermi estimate is that, at summer solar
max, with the hot air at ~ 160C and cold at 10C, there
may be too much wind for a 10Kw turbine.
assume 500m3 air in each chamber.
cold dry air at 10C has a density of ~ 1.2 Kg/m3
hot dry air at 150C has a density of ~ .8 Kg/m3
so, we have a 600 Kg air chamber displacing a 400Kg
air chamber. to do so, it falls 12 meters. it seems to
me that you have the potential energy of 200 Kg falling 12
meters, or 200 x 9.8 x 12 = 23520 ...... but are these mega joules or what?am i on the right track at all here? how to translate that in to Kw? it
seems to me that you would need at least a 10Kw motor
to lift 200Kg 12 meters in the time it takes for any object to fall 12 meters.
i am aware that the friction loss needs to be accounted for, this is one reason for the large diam connecting ducts, i may need help quantifying this later but, as a 10 kw only needs 5-7 m/s flow to run, and is useless beyond 10m/s, i will probably have to size down. this is only because i cannot find a suitable air motor that would take higher velocity air from a smaller duct...ie the wind turbine is off the shelf.
ok, let the torrents of 'yeah but's loose. i am
posting here to put this idea up against rigorous
thinking, but help full suggestions very welcome too.
regards duke
