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re: most efficient refrigerator
2 feb 2004
daestrom  wrote:

>> >> >> http://enews.lbl.gov/science-articles/archive/energy-myths3.html and
>> >> >> http://enduse.lbl.gov/info/lbnl-45862.pdf

>...both point to, "a review of measured tests with refrigerators
>showed that there was no or little evidence of improved
>efficiency from cleaning the coils (litt, megowan, and meier 1993)."
>
>a google search for the authors found...
>http://hem.dis.anl.gov/eehem/93/930108.html
>the study was performed on 27 units, average age of 16 years.  the
>*maintenance* performed included replacing *all* gaskets and
>cleaning the coils once over the year.  when doing the maintenance, only 10
>of the units actually needed their coils cleaned... in five cases,
>the air flow was severely restricted.
>
>a small decrease appreaed in summer energy use after the maintenance, but
>the annual averge increased by 50 kwh (2.5%).

nice research. if 50 kwh was 2.5% of the annual average, each one
used 2000 kwh/year? some new fridges use about 400 kwh/year...

>...the third unit, one that had a 'plugged' condenser showed
>a marked drop in energy usage.

aha!

>> >> i wonder what we can agree on. suppose the 2'x3'x5' empty unit has 62
>> >> ft^2 of interior walls, and the full one has 5 wire shelves with 24
>> >> vertical 3" diameter x 10" tall 1-liter soda bottles on a 6" grid on
>> >> each shelf. surrounded by 3" of air, the bottles would introduce minimal
>> >> flow resistance, but they would add 90 ft^2 of cold surface with no
>> >> insulation behind it, giving the full fridge 152 vs 62 ft^2 of cold
>> >> surface, about 2.5x more...
>>
>> ...the bottle surfaces are much stiffer cool sources than the walls, no?
>> they are thin and backed by liquids, vs a 1/16" plastic fridge wall with
>> insulation behind it. the bottles would be more effective condensers,
>> after a few seconds.
>
>congratulations, you've contrived a very specific construction/loading
>pattern to *maximize* your argument.

all's fair in love and refrigeration.

>we could just as easily assume glass shelves, boxes and rectangular 
>containers stacked with no intervening space and a few other details
>to maximize my argument.

sure. we might also ask how most people pack fridges, or agree
to stop arguing about that and discuss one definite situation.

>> >> >> >those food items that aren't touching form channels with a high
>> >> >> >hydraulic diameter resulting in laminar conditions and much lower
>> >> >> >flow.
>> >>
>> >> i'd estimate that a 1/2" gap would provide minimal airflow resistance.

based on steve baer's observation that cool air is "sluggish" when
it naturally flows through channels less than about 1/4" wide...

>> what would you estimate, in this case? might be a simple duct calc.
>
>simple duct?  maybe.  wire-mesh or glass shelf so air does/doesn't flow
>evenly down the entire cross-section of the 'duct'?

i'd say "wire mesh" and ignore it, along with details like the tapered tops
of the bottles.

>duct between successive shelves line up forming longer channel,
>or staggered so flow is diverted from side to side?

lined up, like 5' vertical cylinders.

>> >> time    wall temp  pickle jar temp
>> >> (sec)   (f)        (f)
>> >>
>> >> 0       40.0       39.0
>> >> 15      41.5       39.0
>> >> 30      42.0       39.0
>> >> 45      43.5       39.0
>> >> 60      44.0       39.5
>> >> 75      45.0       39.5
>> >> 90      46.0       39.5
>> >> 105     46.0       39.5
>> >> 120     46.0       39.5

>...i saw no condensation on the mirror. the wall (vs pickle jar)
>temp would increase a lot faster with condensation, no?
>>
>> no?

i'd say yes...

>> >your data doesn't support or refute any position.
>>
>> imo, it indicates that a square foot of fridge wall would absorb less heat
>> than a square foot of bottled liquid, when the door is open.
>
>that was never a question, but yes it's true.  it also takes less to cool
>the same wall back down again.

i'm not sure how the latter matters.

an empty massless fridge might exchange exactly one volume of air when
the door is opened, with no further flow, since the temperatures inside
and outside would then be equal. the penalty for opening the door would
be the energy needed to cool that single volume of air and condense its
vapor to the dew point.

>a 'full' refrigerator can maintain its average temperature low for
>a longer time with the door open.

which makes for a continuous temp diff and a continuous flow and
more sensible and latent heat gain while the door is open.

>if we leave the door open long enough for this to impact
>the energy usage, then maybe we need to learn how to shut the door ;-)

it seems to me that even a few seconds has a significant impact.
how fast does moisture from room air condense? full fridges packed
as above are better condensers... 

>> i like the idea of heating a house with a licl pool that absorbs water
>> vapor that diffuses upstairs from a damp basement floor, in the presence
>> of air, and cooling a house with a water fountain or fogger inside and
>> a licl pool on the roof that absorbs water vapor from below, in the
>> presence of air...
>
>an interesting idea, but how do you 'recharge' the licl?

one way is to trickle it over a dark roof. these guys used a latent
heat exchanger with no air and no compressor to recover the coolth.
i don't quite understand this paper. you might, and estimate how big
an open-air pond must be to produce a given heating or cooling power.

***

"unglazed collector/regenerator performance for a solar assisted open cycle
absorption cooling system" by m. n. a. hawlader, k. s. novak, and b. d. wood
of the center for energy system research, college of engineering and applied
sciences, arizona state university, tempe, az 85287-5806 usa, in solar energy,
vol. 50, pp 59-73, 1993 describes: "an ordinary black shingled roof... used
as a collector/regenerator for the evaporation of water to obtain a strong
solution of [lithium chloride] absorbent... experimental results [using a
36' x 36' roof] show a regeneration efficiency varying between 38 and 67%.
cooling capacities ranged from 31 to 72 kw (8.8 to 20 tons)", ie about 1 ton
per 100 square feet of roof area.

in the house "water [the refrigerant] is sprayed into an evaporator, evacuated
to about 5 mmhg of pressure, where it immediately flashes into vapor... cold
water, pumped from the bottom of the evaporator, flows through a fan coil...
that blows cool air into the conditioned space. the absorber acts as a vapor
compressor and condenser for the system. water vapor from the evaporator flows
over the absorber where it is absorbed by the concentrated absorbent. the
continuous absorption of water vapor maintains a low pressure in the system
and permits flashing of water in the evaporator... the product of the 
absorption process, a weak absorbent solution, collects at the bottom of
the absorber to be pumped [up over the roof] for concentration."

"the dilute licl solution was delivered to the collector surface through
a spray header spanning the top of the roof and made from 50.8 mm (2 in)
diameter cpvc pipe fitted with 35 evenly spaced brass nozzles. the
concentrated solution collected at the bottom... in a pvc rain gutter, and
returned via gravity feed to a 1608 l (425 gallon) fiberglass tank... in
the event of of a rain, fluid flowing off the collector could be manually 
diverted to a 946 l (250 gallon) wash tank or to a roof drain. during the
initial phase of the rain, residual salt would be washed from the roof
and collected in the wash tank to be stored for later regeneration. after
sufficient rainfall, the rainwater is diverted to the roof drain."

***

>iirc, you would have to heat it to drive off the moisture it absorbs.

yes... 130 f seems like enough, in practical terms. the heating might
happen in a multi-effect rooftop solar still with transparent trays
arranged so evaporation from lower trays condenses on trays above...

>but perhaps this 'charge'/'discharge' cycle could make an effective energy
>storage mechanism for an intermittent energy source such as
>solar thermal?

sounds like a great idea, compared to 180 f water. no loss of heat over time, 
even with no insulation, and up to 10x less volume for the same heat storage,
since a pound of licl can absorb about 10 pounds (10k vs 1k btu) of water...  
 
>>        -1/2
>>   sh re     = betaxsc/(1-omega), where omega = wg  /wg
>>     x  x                                         oo   i.
>>
>> way over my head...
>
>a short 'blurb' on some of these 'dimensionless numbers' (watch the link, it
>will probably wrap badly)

http://www.fuelcellknowledge.org/research_and_analysis/heat_and_mass_transfer/dimensionless_groups/dimensionless_groups.pdf
 
thanks. i'll go look.

>> >for the heat transfer across a unit area used in dehumidification where
>> >the partial pressure of vapor is small compared to the total, the heat
>> >transfer through the liquid film must be equal to the heat transfer
>> >through the gas film plus the latent heat of condensation.  since the
>> >exact conditions at the gas-liquid film interface are unknown, the
>> >following equation must be solved iteratively through trial and error.

i almost understand that.

>> >hl*(ti - tl) = hg*(tg - ti) + lambda*k*(yg - yi)
>> >
>> >hl => liquid film heat transfer coefficient
>> >hg => gas film heat transfer coefficient
>> >tl => bulk liquid layer temperature
>> >tg=> bulk gas temperature
>> >ti=> temperature at the gas-liquid interface
>> >lambda => heat of vaporization/condensation
>> >k=>gas-phase mass-transfer coefficient
>> >yg => concentration of vapor in gas (bulk)
>> >yi=> concentration of vapor in gas at the gas-liquid interface
>> >(subscript 'g' is in the gas, 'i' is at the gas-liquid interface,
>> >and l is the liquid).
>>
>> also way over my head. how many pounds of water vapor per hour would
>> condense on a 1 ft^2 vertical surface at 36 f, if it were exposed to
>> 70 f still air at 50% rh?
>>
>> tg = 70 f? hl = 60 btu/h-f-ft^2? tl = 36 f? lambda = 1000 btu/lb?
>> yg = w = 0.00792 pounds of water vapor per pound of dry air?
>> what are hg and k and yi? use ti = (tl+tg)/2 to start?
>
>hmm... "see reference 2: a. p. colburn and o.a. hougen, 'ind. eng. chem.'
>vol 26 (1934) pp 1178-1182" ;-)

i don't think i have vol 26 of ind. eng. chem.

>most of the work in this area is experimental correlations.  there are many
>studies covering air-coolers and tubed condenser systems.  i suspect it
>matters a lot on the exact nature of the plate and many variables like
>surface roughness, 'wettability' of the surface, mixture flow and more.

maybe perfectly smooth, perfectly wet, and perfectly mixed is simpler.

>i'll look in some references i have at work on monday if i get a minute.

thanks.

nick




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