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re: cool tower alternatives
4 nov 2005
sqlit  wrote:

> wrote:

>> nader chalfoun and christopher trumble had an interesting tucson cool
>> tower story in the spring 2005 u oregon "connector" architecture newsletter.

[about architectural students designing and testing cool towers.]

>> it's nice to avoid the energy of a swamp cooler blower. can we also avoid
>> the large structure (sacrificing architectural drama) and use less water,
>> based on weather conditions, with constant comfort and better controls?
>>
>> how about testing an alternative? evaporate water inside a house and
>> also run a small exhaust fan as needed to remove water vapor from
>> the house. the most efficient corner for evaporative cooling in the
>> ashrae 55-2004 comfort zone is at 80 f and w = 0.012, approximately.
>>
>> we (not me, with recent flooding in pa :-) might turn on a small indoor
>> swamp cooler with a thermostat when the indoor temp rises to 80 f and
>> turn on an exhaust fan with a humidistat when the indoor rh rises to
>> 54% (w = 0.012 at 80 f.)
>>
>> with enough green plants in a house, the cooler might seldom turn on.
>> with enough air leaks, the exhaust fan might seldom turn on.
>>
>> or run a soaker hose with pressurized water from a solenoid valve (which
>> might come from a dead washing machine) over a floorslab in an existing
>> house or under a floorslab with a vapor barrier under the hose in a new
>> house. the slab's thermal mass might store coolth for more efficient
>> cooling with cooler night air below the comfort temp near the floor.
>>
>> during the day, a slow ceiling fan with a room temp thermostat and an
>> occupancy sensor could provide efficient cooling as needed. the fan could
>> provide comfort cooling and raise the acceptable rh all the way to 100% at
>> 81 f with v = 0.5 m/s, according to ashrae-55's basic program, altho that
>> might cause mold, on a continuous basis. the slab could also lower the
>> mean radiant temp. a low-e ceiling and walls could radiate less to the
>> slab when nobody's home, conserving stored coolth.
>>
>> nrel says tucson has an average humidity ratio wo = 0.0054 in june, with
>> a 67.9/99.6 f daily min/max. an 80 f house with a 400 btu/h-f thermal
>> conductance and 4k btu/h of internal gain might need (99.6-80)400+4k
>> = 8240 btu/h of cooling at 3 pm.
>-----------scientific drival ----------------------------------------

drivel restored for the literati:

evaporating p lb/h of water makes 1000p btu/h, and cooling c cfm from 99.6
to 80 f to make up for required exhaust air takes about (99.6-80)c btu/h, and
1000p = 8240+(99.6-80)c with 0.075 lb/ft^3 air and p = 60c0.075(wi-0.0054)
and wi = 0.0120 makes p = 0.0297c and 29.7c = 8240 + 19.6c, so c = 816 cfm
and p = 24.2 lb/h of water, with a net cooling of 8240/24.2 = 340 btu/lb.

how many pounds of water per hour would a cool tower need to achieve
the same 80 f at 54% rh inside this house?

ps = e^(17.863-9621/(460+80)) = 1.047 "hg at 80 f and 100% rh, so a ft^2
of 80 f damp floorslab in 80 f air at 54% rh might evaporate 0.1aps(1-0.54) 
= 0.048a lb/h of water (mis)using an ashrae swimming pool formula, ie
502 ft^2 of slab might evaporate 24.2 lb/h.

at 81 f and 100% rh indoors, 1000p = (99.6-81)400+(99.6-81)c and wi = 0.0233
and p = 0.0808c, so 80.8c = 7440 + 18.6c, so c = 120 cfm and p = 9.7 lb/h
with 7440/9.7 = 770 btu/lb of net cooling. this could work even in august,
when conventional swamp cooling wouldn't, with wo = 0.0117 and tdp = 61 f.
it might be a fast way to cool a slab.

an 80 f slab under 67.9 f air with wo = 0.0054 and pa = 29.921/(0.62198/wo+1)
= 0.257 "hg might evaporate 0.1a(ps-pa) = 0.0789a lb/h and lose (80-67.9)1.5a
= 18.2a btu/h of sensible heat, for a total of 97.1a btu/h. with enough air,
a 1000 ft^2 slab might lose 24hx8240btu/h = 198k btu in 198k/97100 = 2 hours
on a june night, with 198k/158 = 1255 btu/lb of net cooling.

>the cooling tower idea has been around arizona since the padre's came from
>mexico and california...

but that's irrelevant, when considering _alternatives_ to cool towers, no? 

>... coolers will not work over a 40 f dew point.

that's disproved in the drivel above :-)

>... it is hard to install something that is good for 60-90 days a year.

this indoor scheme is cheap, and some variations can work all year.

>i have had swamp coolers in the past...

but this scheme is not a swamp cooler.

>... for a few dollars more i could have a/c all of the time.

they can work together. a rational person who understood drivel might use
this scheme when water costs less than running an ac, depending on weather
and water and electrical costs. for instance, if water costs 0.2 cents/gallon
(one phoenix site mentions $1.50/1000 gallons), he might use ac in the last
case if the water cooling cost ($0.002/(8x1255) = $2x10^-7/btu) were less
than the cost of an ac with a cop of 3 with an electrical cost of $c/kwh
($c/(3x3412) = $cx10^-4/btu), ie if c < $0.002, ie if electricity costs less
than 0.2 cents per kwh, ie "use ac if the cost/kwh is less than the cost/gal."
but electricity typically costs 10 cents/kwh, ie 50 times more.

nick




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