Note: Descriptions are shown in the official language in which they were submitted.
WO93/~4t2 2 ~ 31 PCI~/US!~2/0!;368
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I~RO~:D COOLIN~ P~EI)IlJM FOR IJ8~ l A
.,
T~R~ ENERGY 8TORAG~ ~Y8?EM
~ This application is a continuatisn-in-part of
`,5 Application Serial No. 722,428, filed on June 27, 1992.
. B~ck~Qun~ Q~_~h~ Invent on
Thermal energy storage 5ystems contain a`cooling
~ medium, which is frozen during the off peak, evening
r hours. During ~he daytime, heat from the s~rrounding
.~ 1~ area is us~d to ~elt the cooling mediu~. The ramoval
of heat to drive the decomposition causes the
surrounding ar~a to b~come cool~r.
U.S. Patent No. 4,540,501 discloses a the ~ al
energy stor~ge system which u~es clathrates as the
cooling ~ediu~. Clathrates are hydrates which use a
non-sto~chio~etric number o~ uater molecules per guest
molecule. Th~ guest moleculQ f~lls th~ interior of the
lattice, stabilizing the clathrat~. This stabilization
allows the watex lattic~ ~ructure to form at
te~peratures si~nificantly higher than the temperature
of ice fo~mation (0C). The ~uest ~ol~cul~ must be
highly insoluble in water, and must have a molecular
~ize which is 1~8 than 7 ~.
The halogenated hydrocarbons which are used as ~he
gue~t ~ol~cul~ are not water ~i~cible. Clathrates
will not for~ unle~s the guest ~nd ~os~ (lat~ice)
co~psund3 ~r~ in contact. In an ~ttempt to bri~g the
gUB~t mol~cul~ and wat~r into closer contact, v~rious
~ur~n~tant~ bava been add~d. U.S. Patent No. 4,540,501
di~close~ using a nonionic ~luoro~ur~actant having the
chemical ~onmula F(CF2CF2) ~-~CH2cH20 (CH2~H20) 0~ when the
gue3t molecule i~ a refrigerant cho~en fro~ brominated,
chlorinated ~n~ ~luorinated hydrocarbsns including
CCl2F2, CCl3P, CBr~3, CHClzF~ CHClF2, CH~ClF and CH3CClF2.
U.S. Patent No. 4,821,79~ disclo~es the ~SR Or Zonyl~
Plsrosur~actants in amounts between 1 to 5000 ppm
gener lly, and th~ use o~ Zonyl FSN with
~ W~93/ ~ 12 PCT/US92/0~
...
3 1
trichlorofluoromethane in the ~mount of about 200 to
300 ppm. Zonyl9 Fluorosurfactant and Unidyne DS-401
have been added to water-l,l,l-tetrafluoro~thane
clathrate forming thermal energy storage medium in
- 5 Foxma~ion of Gas Hydrate or I~e ~y ~i~ect-Contact
Eva~ora~ion of CFC Alternatives, F. Isobe and Y.H.
Mori, Int. J. Re~rig., vol. 15, No. 3 (1992~, pgs.
137 - 142. Proc. Inter. SQC. Ener~y Convers. ~nq.
.~ Con~!, Akiya et al., 1991, 26th~6) 115-119 used two
unspecifie~ surfactants in concentrations up to 500 ppm
to enhance the rate of formation of the clathrate from
a water ~ dichloro-1-fluoro~thane cooling medium.
., How~v~r, for the known guQst molecules
relativ~ly l~rge quantities of ~ur~actant have been
u~Qd (up ts and in eXcQss of 1000 ppm) and so~e of the
guest ~olecule will a~sociat~ with the urfaetant
inst2ad o~ forming a clathrat~ wlth ~atex. This
decr~as~ khe ef~iciency of ~ha th~rmal en~rgy storage
system.
~rior to the present inv~nti~n ther~ has been no
teaching in the art of how to sQlect a surfactant for a
particular cooling medium or how to determine the
amsunt of ~urf~ctant which will in~urQ opti~u~ mixing
~ with a mlni~u~ of gue~t molecul~ a~ oci~tion.
t~ , 25 Furth~rmore, many of th9 guest mol~cul~ presently
b~ing u~d ~re CFC ~uch as trichloro~luoromethane
(CYC-ll). Th~ use o~ the~ eo~pound~ i~ beeoming
di ~avor~d b¢eau~e o~ the detr~m~ntal ef~eet to the
ozone layor. Thu~ it i9 a goal o~ th~ pre~ent
invention to ~ind a ~oolng mediu~ wh~eh pO3~5 less of
a thr~at to th~ ozone }ayer. Halohydroearbons such as
HCFC-141~b) whieh eontain hydrog~n, and are believed to
po8e le3s o~ ~ threat to the ozon~ layer, and are thus
propo ed a8 ~he guest ~olecule in elathra~e formation
aeeording to the present invention.
~,,
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~ WO93/~Y2 PCT/~S92/~
2 ~ 3 ~
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Descrip~i~n_Qf the Fi~u~e
FIGURE 1 shows the relationship between the
surfa~tant concentration and the sur~ace tension of
, wat~r for the surfactant D~SC~.
- 5 Figure 2 shows the relation~hip between surfactant
concentra~ion (DRSC~) and the interfacisl tension for
l,l-dichlo~o-~-fluoroethane/wat~r 801ution.
- Detailed D~s~iDtion of the nv~ntion
.
` ~The pre nt invention pro~des a cooling medium
for ug2 in a ~her~al en~rgy tor~ge system comprising
wat~r, a gu~t ~olecule ~nd a ~urfactant having a
critical micelle concentration in an ~ount les~ than
about ~wic~ th~ cr~tical micell~ concentration.
Pr~ferably ~h~ eriti~al ~icell~ conc~ntration i less
than about lxlO'~ and mor~ pre~r~bly b~ween lx10-4M
and lxlO'q~. A thermal energy ~tor~ge uni~ which uses
the cooling ~diu~ and a proG~s ~or u~ing the thermal
; energy storag~ unit ara also di~clo~ed.
The gu~st ~olecules Or ~he present in~ention may be any
compound capabl~ o~ forming a cl~thra~ with ~ter.
Suitabl~ gues~ moleculas g~nerally hav~ an average
diameter o~ le~s than about 7 A. Pr~f~rably, ~he guest
molQcul~ re~rigerant salQctQd fro~ the group
consistin~ o~ hydrochlorofluorocarbon~,
hydro~luorocarbon~, and mixtur~3 th~reo~. Exa~ples of
pr~orr~d ~ydrochloro~luorocarbon gu~3t molecules
includs l-~luoro-1,1-dichloro~thane, 2nd
chlsrodi~luoromethane. Exa~plQs o~ prQ~erred
hydro~luorocarbon gue3t mol~cul~ include ~,1,1,2-
tetra~luoroethane, 1,1,l-tri luoro~than~,
di~luororm~th~ne, p~ntaflusro~thane, ~nd 1,?-
dl~luoro~thana. ~he con~iguration o~ th~ th~r~al
~n~rgy storag~ gy~t~ o~ thB pr~s~nt inv~ntion is the
similar to ~hat o~ U.S. patsn~ No. 4,540,501.
~ W0~3/~X P~T/~2~
.....
~ ~lii3~1
~ . 4
;~ To form a clathrate the guest molecule and water
must be dissimilar and be in contact with each other.
~` the more intimate the contact, the more efficient the
clathrate formation will be. Accordingly, emulsions of
water and the guest molecule are highly desirable~
Cla~hra~es of the pre3ent in~en~ion ~re formed from the
guest molecule, and water. Depending on the size of
the guest molecule between 5 to 17 water molecules per
-1 guest molecule are needed to ~orm a clathrate.
Preferably the amount of sach the guest molecùle and
water is at 12ast equal to the ratio necessary to form
clathrate. ~ore pr~farably, an 8XC~SS of water is used
to ~aintain a -~lurry, and en~ure con~inuous a~d
e~ficient h~t transfer. ~or exampl~, where HCFC-
141(b~ is u~ed, 20 moles of wat~r is used for each 1mol~ o~ HCFC-141(b).
The concentration of fre~ surfactant in water
af~ects th~ properties of th~ ~ter and particularly
the ~uxfacQ tenRion, which i shown ~or DRSC~ in Figure
1. For mu}tipha~ syst~s, such ~s th~ guest
molecule/w~ter mixtur~s which ar~ u ed in thermal
energy storage systems th~ intQr~acial tansion between
the gue3t molocule and water ~ay b~ mea~ured. The
er~ect o~ DRSC at varying concentrations on a 1,1-
, 25 d~chloro~ luoroethane/water i8 shown in Fiyure 2.
Th~ pr~psrtie~ of both solution chang~ rapidly b~tween
about 25 ppm and a~out 125 ppm. B~yond that rang~ of
conc~ntration, propQr~ies changa ~ora gradually. This
narrow concentratîon range in which prop2rtie~ change
rapidly is c~lled th~ critical ~icells concentration or
cmc. A~t~r the cmc has be~n Qxc~Qd~d w~ter surface
tension dQcreasQ only ~lightly by adding more
sur~actant~ Because water-guest moleculs mixing
increas~3 a3 the ~urf ace tension o~ water decreases,
maximum water-guest molecule mixing i8 achiev~d n~ar
:`'~ `i
;~: .l,
.` 1 WO 93/00~1~ Prr/u~g2/o53~i8
3 ~
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the cmc. Moreover, after about 200 ppm (twice the cmc~
~-i of the DRSC9 surfactant has been added there is
virtually no change in ei~her the surface or
~: interfacial tension. Thus, regar~less of what
. 5 surfactant i5 used, the optimum concentra~ion of
surfactant necessary ~o provide maximum ~ixing is not
gxeater th~n about twice ~ha critical micslle
concentra~ion, preferably l~ss th~n about 1.5 times the
c~c and most prefer~bly le~s th~n about the cmc. By
. 10 - limitin~ th~ amount of surfactant u~d to less than
, . ..
. about twice the cmc it is possible to use small amounts~s o~ surfactants, e~pecially if the critical micelle
. concentration is s~all (les~ ~h~n abou~ 10-3M).
Each ~ur~actant ha~ a uniqu~ cm~, whi~h depends
upon its ~tru~turQ. G~nQrally ~urfactants ~ith longer
. hydrocarbon chains havs lower critic~l micell~
conc~n~ration~. The low~r th~ c~c, the 1~ surfactant
i~ neeassary to achi~ve ~axi~um mixing. Thu~,
pr~f~rably surfactants of ~h~ pr~l~nt invention have
critical mic~ concsntrations w~ich ~r~ be}ow lxlO-
~M, and pr2ferab1y..b4tw~n ~xlO-~ and.lxlO~~M. Critical
:~ micelle conc~ntrations ~or many ~urf~ctants are listed
; in "cr~tical Mic~lls Concentrations o ~queous
Suxf~ctant Syste~ by MuX~r~ and My3~1~ [NAt. Stand.
R~f. D~ta S~r., Nat. Bur. ~tand. (U.S) 36, Feb. 1971.
Many way~ o~ d~t~rmining th~ c~c o~ 3urfactant~ are
; d~cribed by Mukerje~ et al. ~oraover, b~c use the
. pres~nt invent$on providQ~ a l~rg~ number o~ ~uitable
surf~ct~nt3, surfaet~ntQ which produc~ only gradual
~ 30 chang~s in sur~ac~ or inter~acial tension ov~x varying
.~ sur~actan~ concentratisn3, ~nd thu~ ha~e poorly definedCmC-Q are no~ preferred. An example o~ a surfactan~
having a poorly de~ined c~c is Zonyl~ FSN.
Mos~ prs~erably, a ~urfactant having a cmc less
than lxlO-qM is u~ed in an amount about equal to or
~,4,
:`g
~ W093/~12 PCT/~S92/05
8 ~ ~
slightly in excess of the cmc. By choosing surfactants
ha~ing low ~mcs and limiting the amount of surfactant
to the c~c, inefficiencies du2 to gu~st molec~le
association with the aggregated surfactant and
S competition b~tween th~ surfactan~ and guest molecule
~ay be mini~iz~d. ~hen any non-polar substance is in
contact with water, the wat~r ~o}~cules beco~e arranged
or organiz~d in a clu~tçr around ~he non-polar ~oi~ty.
It is belieYed that clathrates are formed by th~
crystallization of this clust~r. Si~ilarly, ~hen a
~urfactant is pr@sent, water ~olecules clus~er around
the sur~actant, forming surfactant aggr~gat~s. This
co~petition between the potential guest molecules and
surfacta~t molQcules for wat~r ~olacul~s d~crea es the
amount of cl~hrat~ which can ~ for~ad with a given
: amount o~ w~ter ~olecule~. ThUB ~t iS pr~ferable t~
~ini~izs the a~o~t o~ ~ur~ac~nt u~d.
In ~o~ ~ituation8, it ~ny b~ de~irabla to exc~ed
the c~c in order ~o acheiva ~ particul~r ~f~ct. When
the cmc is ~ow, the concentration of sur~actant may
~till be qui*e low ~s co~pared to convention~l
sur~ctant conc~ntration even though it is above the
cmc. Thus, thQ competition b2~w~en the sur~actant and
tha gu~t mol~cule is proportionataly l~s~ ~ev~re, even
: , 25 nt conc~ntr~tions ~hich are ~bov~ th~ cmc. Thus,
aur~ctant~ with cr$tical mic~ concentrations below
~bout lXlO'k ar~ prefe~ed.
An Qx~ple o~ a surf actant species uhic:h has been
~ound partic~larly e~fecti~a ~n ~nhancing emul~ion
formation where HCFC-141(b) is u~ed as t~ guest
molecule i~ surfactant DRSC (alkyl dimethyl benzyl
ammonium salt Or oc~aphenyl pho~phoric acid ~
commQrcially aYailabl~ fro~ Allied-Signal, Inc.). The
physical propQrtie~ of DRSCo ar~ 8hOWrl in Table 1,
3 5 below.
~' W~ ~3/00412 PCrlUS92/~36
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Table 1
I .; _ -- - . _ _.
Surf actant BP Specif ic Vapor Sol O in H20
.j~ ( F) Gravity Press . at 2 5 C
. . ~ 2 5 - C mm Hgê 2 5 - C
DRSC~ 180~ 0 ~ 95 <1 Sol .
~ o ~ Hg ~ - , . .
The cmc for DRSC~ is between about 50 ppm and
about 125 ppm, which was determined by measuring the
sur~ace tension of water as increa~ing amounts o~ DRSC~
were added! Thus, less than a~out 200 pp~ of the DRSC~
is raquired to insure emulsion formation between water
and ~he chosen guest molecul~. Prefer~bly less than
100 ppla DRSC 19 is used. The losses of the gu~st
ms~lecule (HC:P'C 141(b)) due to as. ociation with the
sur~ ac~ant d~ ::rea-ce as the aDlount of suriEactant us d is
..
dec~!aas~d~ ther~by incraasing t~ ~f~ici~ncy o~
clathrate formation, and ~ ther~al energy storage
system.
Agitation is not required ts ensure clathrate
formation of th~ cooling mediu~ o~ the ~resent
invention. However, agitation may be used to further
2 0 encourag~ clathrate or~ation ~
Emul~ion~ f or~ed ~ccording to the present
inYention ~r~ 3tabl~ at roo~a t~mp~rature, and remain
eDIul~ d ~or as long 213 two d~ys ~ith mini~um
lnag~. Th~ clathrat0 is form~d in a ~ rage
tank/crystall~z~r. The pr~sure in the c:~stallizer i5
decre~d by me~ns of a co~pre~sor, ~ described in
moxe d~tail in U.S. Patent No. 4,5~0,501, and heat is
removed unt~l the temperature o~ ~ormation f or the
clathratg is reach~d. The prsssure ~nd te~perature are
3 0 maintained until ~11 sf t~e clathrate i5 f ormed . The
clathrate is circulated through the heat exchanger via
the recirculatisn loop. Clathrate i5 circulated
through the heat exchanger, decompos~d, and the water
WQg3~12 PCT~US92/~5
`-;
~ 3 ~ 8
and guest molecule mixture is returned to the
crystallizer.
Example
. 5A solu~ion of DRSC~ ha~ing a concentration of 25
ppm was ~ade by adding 0.025 ml of DRSC~ to 1 li~er of
w~ter~ 300 ml o~ the surfactant solution was poured
into a 500 ml jar, and 30 ml of 141(b) was added. The
jar was capped and shaXen vigorously ~or 1 minute. An
emulsion f~r~ed in the jar, which W~5 stable and
r~main~d emulsi~ied for two days without noticeable
drainage.
The se led jar was placed in a fr~ezer at 40F. A
considerablQ amount of snow~lak~-like cryst~ls (the
~5 ~lathrat~) wa3 observsd in the ~ar a~ter 1.0 hour. The
jar was lef~ in the ~r~ezQr overnight. By morning
cry~tal$ had ~or~d in ~he ~ar, indicating that
cla~hrate had ~orm~d.
,Accordingly, DRSCo, which h~ a lo~ emc, is a
suitabl~ aid for clatbrat~ formation, for~ing clath-;ate
.~ at low surfactant concentration~. Becau~ only a small
amount of sur~actant i5 required (tWiCQ the cmc or
1~8s ), thers i~ less surfactant to as~ociate with the
guQst molecule (here HCFC-141(b)), and thu~ the
clathrat~ ~ormation proc~ss, and the ther~al en~rqy
storag~ syst~m are mor~ e~ici~nt.
