Thermal energy storage composition to provide heating and cooling capabilities

The present invention relates to a thermal energy storage composition and a method for reducing rupture failures during freezing cycles of container device phase change material. The phase change composition contains about 60 to about 91 weight percent water and about 9 to about 40 weight percent of one or a mixture of water-dispersible non-ionic surfactants, particularly alcohol ethoxylates.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to thermal energy storage formulations. More 
particularly, the present invention relates to an improved water-based 
thermal energy storage formulation which disrupts ice structure and 
reduces volume expansion associated with the transformation of water to 
ice. 
Phase change materials ("PCMs") store heat during phase transition, 
typically liquid/solid phase transitions. A large amount of thermal energy 
can be stored as latent heat of fusion during the melting of the PCM. For 
this reason, PCMs are often incorporated into thermal energy storage 
apparatuses. During the operation of such an apparatus, heat from the 
surrounding air is transferred to the PCM as heat of transition, until the 
frozen PCM completely melts. Additional heat from the surrounding air is 
then stored within the PCM as sensible heat. The heat stored within the 
PCM may be discharged from the apparatus by passing relatively cool air 
past the liquid PCM. The liquid PCM transfers its heat to the air stream, 
and thus, the temperature of the air stream is raised and the PCM is 
re-cooled. 
Various attempts have been made to incorporate PCMs into heating and air 
conditioning systems, including heat pump systems, solar collection 
systems, and more conventional heating and air conditioning systems for 
homes, vehicles, and similar structures requiring heating and cooling. For 
example, U.S. Pat. No. 5,054,540 to Carr describes a cool storage 
reservoir positioned in an air duct of a vehicle on the like. Another 
example is the "heat battery" designed to provide "instant heating" to a 
vehicle cabin. (Automotive Engineering, Vol. 100, No. 2, February, 1992.) 
A variety of materials may be used as PCMs. For example, water, paraffins, 
alcohols, and salt hydrates have notably high energy densities over 
temperature ranges of practical significance. Water, however, is of 
particular interest because it is plentiful, inexpensive, and 
environmentally friendly. As a heat storage material, it has good heat 
capacity, heat transfer properties, and an acceptable density. 
Additionally, the transformation of water into ice has a heat of fusion of 
80 cal/g and occurs at 0.degree. C. 
However, the use of water as a PCM presents some difficulties. For example, 
conventional air conditioning units must be reconfigured to operate at the 
ice temperature, and, in an air heat exchanger, moisture can freeze on the 
cooling coil. In addition to refrigeration considerations, the conversion 
of water to ice is accompanied by approximately a 9% volume expansion. 
This anomalous expansion during freezing is the cause of busted water 
pipes in homes during cold weather, and broken radiators/cracked engine 
blocks in cars. Moreover, if freezing water were to crack a thermal 
storage apparatus, causing liquid PCM to leak, a user relying on the PCM 
for climate control would be put at an unnecessary risk of exposure to 
extreme cold or heat. What is needed is a formulation for a PCM, which has 
the energy storage qualities of water but lacks the destructive freeze 
characteristics associated with water's transformation to ice. 
According to the present invention, there is provided a method for reducing 
rupture failures during freezing cycles of a container device holding in a 
PCM composition comprising about 60 to 91 weight percent water by adding 
to the composition one or a mixture of non-ionic surfactants, preferably 
ethoxylates. 
Also in accordance with the present invention there is provided a 
water-based thermal energy storage formulation capable of forming a frozen 
discontinuous water phase and of significantly maintaining the energy 
storage properties of water. The composition comprises about 60 to about 
91 weight percent water and about 9 to about 40 weight percent of one or a 
mixture of water-dispersible non-ionic surfactants. 
The project was undertaken to modify the freeze characteristics of water, 
specifically the single piece characteristic of ice and the considerable 
freeze expansion. The objective was to have ice form as discrete 
particles, forming a kind of "slippery ice" and/or to reduce the total 
expansion accompanying ice formation. The composition in accordance with 
the present invention prevents the formation of a single, solid, block of 
ice and reduces freeze expansion. 
Additional objects, features, and advantages of the invention will become 
apparent to those skilled in the art upon consideration of the following 
detailed description of the preferred embodiments exemplifying the best 
mode of carrying out the invention as presently perceived.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention there is provided a method for 
reducing rupture failures during the freezing of container devices holding 
water-based phase change materials--a problem that has been particularly 
noted with respect to thermal storage apparatuses. It has been found that 
when a non-ionic surfactant, preferably one or a mixture of ethoxylates, 
is used in combination with water, the formulation is capable of forming a 
frozen discontinuous water layer while maintaining the energy storage 
properties of water. In addition, volume expansion which accompanies the 
water to ice transformation is reduced with little if any depression in 
the freezing point. 
The presence of non-ionic surfactant in water changes the physical 
characteristics of the resulting frozen mass from that of ice. Water, in 
the absence of a non-ionic surfactant, expands as it freezes and forms as 
a solid block of ice, without any indication of discrete crystals. 
Additionally, the water solidifies from the outside of the system toward 
the center core. Thus, when the center core freezes, it expands as it 
pushes against the already solid ice block surrounding freezing core. The 
direction of the expansion depends upon the strength of the surroundings 
and will follow the path of least resistance. If that path is breaking the 
container which holds the freezing water, that is the path the expansion 
will take, despite the existence of available head space above the rather 
flat surface of the ice. 
It has been found that when non-ionic surfactants are added to the 
water-based phase change material (PCM), a somewhat "slippery" ice is 
formed. This "slippery" ice is believed to consist of discrete particles 
surrounded by a layer of solidified hydrated surfactant phase. The 
particles do not interlock, but rather move individually within the 
solidified mass. So, as the center core of the PCM freezes, any expansion 
in volume causes the surrounding particles to individually move away from 
the core. If there is available head space above the surface of the PCM, 
the particles will press up through the frozen surface and into that 
available space, see FIG. 1. Thus, the resulting frozen PCM often has the 
appearance of a domed surface. Given the movement of the individual 
particles, the water-based PCM can be frozen in a wide variety of 
inflexible commercially available containers without fear of breakage, so 
long as there is available head space to accommodate slight doming of the 
solid mass therein. 
Therefore, there is also provided, as a preferred embodiment of the present 
invention, a PCM composition comprising about 60 to about 91 weight 
percent water and about 9 to about 40 weight percent of one or a mixture 
of non-ionic surfactant. The preferred compositions differ from present 
commercially available water-based PCM's particularly in that they utilize 
a non-ionic surfactant alone or in addition to standard art-accepted 
anionic surfactants. The anionic surfactant is added to the PCM 
composition to add heat stability to the solution during sensible heat 
storage. The water-based PCM compositions of the present invention can 
also include other optional ingredients such as defoamers and 
microbiocides to aid in formulation of the composition and to enhance 
customer acceptance of the product. 
A suitable non-ionic surfactant for use in this invention is selected from 
one or a mixture of non-ionic surfactants. Non-ionic surfactants which may 
be formulated into the present invention include commercially available 
alcohol ethoxylates, including alkylphenolalkoxylates, polyoxyethylene 
derivatives of sorbitan fatty acid esters of lauric acid, palmitic acid, 
oleic acid, and stearic acid, block copolymers of propylene and ethylene 
oxides, glycerol fatty esters, polyoxyethylene esters and polyoxyethylene 
fatty acid amides. Typically, the composition includes water-dispersible, 
short chain, low molecular weight linear alcohol ethoxylates. Preferably, 
the alcohol ethoxylates are represented by the general formula 
R--O--(CH.sub.2 CH.sub.2 O).sub.n H where R is an alkyl radical having 
from 8 to about 16 carbon atoms and the number of ethoxylate groups, n, is 
from about 5 to about 8. Typical ethoxylate compositions comprise a 
mixture of a linear paraffin containing 8-16 carbon atoms and about 5 to 
about 8 ethoxy groups. Most preferably, the ethoxylate mixtures comprise 
C.sub.8 -C.sub.12 alcohol ethoxylates having about 5 ethoxy groups 
attached. Commercially available non-ionic surfactants of this type are 
sold by Vista Chemical Company under the trade name Alfonic surfactants, 
with Alfonic 1012-5 being preferred for use herein. It is contemplated, 
however, that alcohol ethoxylates such as octylphenolethoxylates and 
nonylphenolethoxylates may be used herein. 
The non-ionic surfactant is present in the composition from about 9 to 
about 40 weight present. Preferred weight percentages of the non-ionic 
surfactants are dependent upon the preferred characteristics of the PCM. 
To maximize the energy storage capability of the PCM, the composition 
typically includes from about 9 to about 30 weight percent non-ionic 
surfactants and most preferably about 9 weight percent non-ionic 
surfactants. The preferred composition for use in accordance with the 
present invention maximizes energy storage capability and includes about 9 
weight percent non-ionic surfactants. If, however, the water-based PCM is 
to have substantial reduction in its freeze expansion, the composition 
typically includes from about 20 to about 40 weight percent non-ionic 
surfactants and most preferably about 40 weight percent non-ionic 
surfactants. Ideally, the water-based composition further includes an 
anionic surfactant present in the solution from about 0.5 weight percent 
to about 1.0 weight percent. It is noted that surfactants have minimal 
affect on sensible heat storage of the PCM. In fact, for every 10 weight 
percent of surfactant, there is only a corresponding decrease in sensible 
heat storage capacity of about 5 percent. 
The anionic surfactants are preferably included in the PCM compositions of 
the present invention to prevent separation of the non-ionic surfactant 
and the water into organic and water phases at increased temperatures. The 
temperature of the PCM often rises to increased temperatures as it stores 
sensible heat. Without an anionic surfactant, separation of the layers 
occurs at about 35.degree. C. to about 40.degree. C., the cloud point of 
the solution. Preferably, with the addition of the anionic surfactant, the 
PCM composition is stable in a continuous single phase when heated past 
90.degree. C. Heat stability is often important for customer acceptance of 
the product when it is to be used to store sensible heat, because once the 
phases have separated into distinct layers, it is difficult and often 
impractical to re-mix them. For example, if the PCM were used in a 
vehicular application, it would likely be subjected to extreme heating 
conditions, well-above 35.degree. C. to 45.degree. C. Moreover, if the 
surfactant and water were to separate into layers, the agitation from 
driving along a highway would likely be insufficient to force the phases 
back into a single solution. 
A wide variety of anionic surfactants suitable for use in the present 
invention include naphthalene sulfonates, sodium stearates, salts of a 
fatty acid, and surfactants having the formula R-COOM and R-OSO.sub.3 M, 
where M represents an alkali metal or ammonium and R represents an organic 
radical having more than 10 carbon atoms. Examples of suitable anionic 
surfactants are soaps, sodium lauryl sulfonate, alkyl naphthalene 
sulfonates, and sodium stearate. Preferably, the anionic surfactant is an 
alkyl naphthalene sulfonate. Commercially available anionic surfactants of 
this type are sold by Witco, Inc., under the trade name Petro surfactants, 
with Petro ULF being a preferred alkyl naphthalene sulfonate for use 
herein. 
EXAMPLE 1 
A water-based PCM was formulated in accordance with the present invention 
to include 10 weight percent non-ionic surfactant. The surfactant, Alfonic 
1216CO-7.5 was added and dissolved in warm water. Warm water was preferred 
as it facilitated the dissolution of the ethoxylate. After the ethoxylate 
surfactant was solubilized, about 1 weight percent of Petro ULF (2 volume 
percent active), was added as a high temperature stabilizer. 
The freeze characteristics of this water-based PCM solution are plotted in 
FIG. 2. As can be seen in the graph, the freeze temperature of the PCM 
composition in accordance with the present invention was essentially the 
same as that for water. Additionally, the freeze "plateau", indicative of 
the energy storage capabilities of the material, was similar to that of 
water. The time to freeze the PCM, however, was less than the time to 
freeze the equivalent weight of water. Unlike a saltwater system (see FIG. 
3), freezing point depression and "tailing" at the end of the freeze were 
not observed. 
The Alfonic 1216CO-7.5/water system, when subjected to a freeze-thaw cycle 
formed a gelatinous layer on the surface, but no further evidence of 
segregation could be observed within the sample. As can be seen in FIG. 6, 
the freeze expansion of this system was measurably less than for pure 
water. 
EXAMPLE 2 
A water-based PCM was formulated to include 9 weight percent surfactant. 
The surfactant, Alfonic 1012-5, was dissolved with about 1 weight percent 
Petro ULF (about 2 volume percent active) in water and tested for cloud 
point. The cloud point of the solution was above 90.degree. C. Upon 6 
consecutive freeze-thaw cycles, segregation of the components was minimal. 
The composition successfully disrupts the ice structure of water during a 
freeze and thus is considered to be a preferred composition in accordance 
with the present invention. 
EXAMPLE 3 
Test solutions of PCMs were prepared in accordance with the protocol of 
Example 1 to evaluate the freezing point and the resulting freeze curve of 
the compositions. 
__________________________________________________________________________ 
Freezing 
Compound(s) 
Weight % Point .degree.C. 
Freeze Curve 
Added to of Compound 
Water Relative 
See 
Water in PCM Solubility 
to Water 
Figure 
__________________________________________________________________________ 
polyethylene glycol 
5% yes slight dep. 
3 
-300 (CW300) 
10% yes -1 -- 
20% yes -3 -- 
MgCl.sub.2.6H.sub.2 O 
9% yes -3--4 3 
Alcohol ethoxylate 
R-O(CH.sub.2 CH.sub.2 O).sub.n H 
R = C.sub.12 -C.sub.16 alkyl 
5% no slight dep. 
3 
n = 2 20% no 0 
R = C.sub.10 -C.sub.12 alkyl 
10% yes 0 -- 
n = 5 20% yes slight dep. 
-- 
30% yes -1 4 
R = C.sub.8 -C.sub.10 alkyl 
30% -- -0.7 
4 
n = 7-8 
(Novel II 810-7.5) 
R = 12-16 alkyl 
30% -- -0.1 
4 
n = 7 
(Novel II 1216-7) 
R = C.sub.12 -C.sub.16 alkyl 
10% yes 0 5 
n = 7.5 20% yes slight dep. 
5 
30% yes slight dep. 
5 
40% yes (gel) 
-1.4 
7 
__________________________________________________________________________ 
The freeze curve of the polyethylene glycol (PEG), MgCl.sub.2.6H.sub.2 O, 
and Alfonic 1216CO-2 are illustrated in FIG. 3. Each of these compounds 
were dismissed as unworkable as a PCM, in accordance with the present 
invention. High levels of PEG would be necessary for adequate suppression 
of freeze expansion. At the same time, increasing levels of PEG caused 
intolerable depression of the freezing point. Secondly, the 
MgCl.sub.2.6H.sub.2 O, depressed the freezing point to -3.degree. to 
-4.degree. C. which often exceeds the minimal default requirements for an 
air-conditioning system to generate the required amount of ice in the 
allotted time slot. Also, the use of chloride salts are bad from a 
corrosion standpoint in metal containers. Finally, the C.sub.12 -C.sub.16 
alcohol ethoxylate having 2 ethoxy groups is regarded as a surfactant 
precursor and is not soluble in water. The freeze tests demonstrated 
virtually no interaction with water. 
As illustrated in the plot of FIG. 4, three ethoxylate/water solutions were 
tested against a 100% water standard. High purity alcohol ethoxylates are 
sold by Vista under the trade name The Novel II surfactants, with Novel II 
810-7.5 and Novel II 1216-7 being plotted in FIG. 6. The Novel II 810-7.5 
exhibited more freeze depression than the Novel II 1216-7 and Alfonic 
1012-5. However, note, the initial dip in temperature for each of the 
compounds was attributed to supercooling, and was not relative to the 
freeze depression of the system. Both the Novel II 1216-7 and Alfonic 
1012-5 exhibited acceptable freeze plateaus, similar to water with minimal 
depression in freezing point. 
As illustrated in FIG. 5, Alfonic 1216CO-7.5 water solutions were tested at 
a variety of weight percentages. Note, the energy storage properties of 
the solution were best when the freezing curve plateau mimics that of 
water. As shown, the 10% Alfonic demonstrated a freeze plateau closest to 
water of the four samples with no detectable depression in the freezing 
point. In fact, the freezing point of water is slightly, if at all, 
affected up to 30 weight percent of ethoxylate. 
EXAMPLE 4 
To determine the freeze/thaw and heat stability of the Alfonic 1216CO-7.5, 
the solutions of Example 1 were subjected to freezing and subsequent 
thawing. After the thaw, only slight segregation was observed. 
Additionally cycle testing through four freeze-thaw cycles with no 
agitation between cycles and room temperature as an upper temperature 
limit, failed to reveal any changes in the freeze characteristics of the 
composition. 
At elevated temperatures, turbidity was observed in the ethoxylate/water 
solution which indicated some phase separation. However, the addition of 
Petro ULF anionic surfactant stabilized the micelles, and when heated to 
90.degree. C., and only slightly turbidity was observed. Upon cooling, 
only a single phase system was visually observed. 
EXAMPLE 5 
To determine the effect of an anionic surfactant on the heat stability of 
the PCM, a 30 weight percent solution of Alfonic 1216CO-7.5 in water was 
prepared using warm water. Ivory soap, manufactured by Proctor & Gamble, 
was added in the amount of 0.5 g per 100 g of surfactant solution. When 
heated to 90.degree. C., very slight phase separation occurred, but upon 
cooling, only a single clear solution was observed. Thus, the presence of 
the anionic surfactant prevented coalescence at high temperatures. 
EXAMPLE 6 
To observe volume expansion associated with freezing water, 100 g of water 
are placed in a 125 ml Erlenmeyer flask (slight conical shape) and placed 
in a freezer. Upon freezing, the flask broke. This test was repeated 10 
times, and each time the expansion of the ice broke the Erlenmeyer flask. 
However, when ethoxylate/water solutions were frozen under the same or 
similar conditions, the Erlenmeyer flasks did not break, as demonstrated 
in Examples 7-9. 
EXAMPLE 7 
To compare the forces associated with the volume expansion of freezing 
water with those of an ethoxylate/water solution, three 125 ml Erlenmeyer 
flasks were filled to contain 100.+-.0.2 g of water. A fourth Erlenmeyer 
flask contained 100 g of a 20 weight percent Alfonic 1216CO-7.5 in water 
which included 0.5 g of sodium stearate. The flasks were allowed to freeze 
overnight in a freezer. In the morning, each of the three flasks 
containing water were broken. The flask containing the surfactant solution 
did not break and the solution pushed the surface up into the available 
head space and formed a slight dome shaped surface. 
EXAMPLE 8 
To again compare the behavior of freezing ethoxylate/water with that of 
water, 100 g of water was added to a 125 ml Erlenmeyer flask. Three other 
flasks each contained 100 g of a 10 weight percent Alfonic 1216CO-7.5 
solution. The four flasks were placed in a freezer at -23.degree. C. and 
allowed to freeze. Upon removal, only the flask containing the water was 
broken. Each of the flasks containing the surfactant/water solution pushed 
up through the surface into the available head space in the flask. The 
frozen mass included a slight domed surface. 
EXAMPLE 9 
In order to demonstrate the physical differences between ice and frozen 
water-surfactant solutions, 1 kg of water was added to a bread pan, 
measuring 5.times.9 inches and 2.5 inches high. A 1 kg solution containing 
10 weight percent Alfonic 1216CO-7.5, 1 weight percent Petro ULF, and 89 
weight percent water was added to a second bread pan. Both pans were 
placed in a freezer overnight. A small fan was also placed in the freezer 
to ensure a complete freeze of the materials. The next morning, a hammer 
was used in an attempt to drive a nail into the block of ice. The ice 
immediately shattered as expected. However, the nail was successfully 
driven into the ethoxylate/water system without difficulty. The 1 kg mass 
was picked up using the nail only, demonstrating that the nail was firmly 
embedded in the mass. This indicated that the frozen surfactant/water 
system had properties substantially different than those of pure ice. 
The frozen ethoxylate/water system is believed to have a discontinuous 
water phase rather than a continuous particulate phase as seen with water. 
The melted PCM has a micellar structure of surfactant in water. While one 
does not wish to be held to any one theory, it is believed that the act of 
freezing causes phase inversion. Thus, once frozen, the ethoxylate/water 
system, in essence, becomes solid ice particles dispersed in the organic 
phase. So, Erlenmeyer flasks don't break and nails can be driven into the 
solid mass without fracturing the mass. 
EXAMPLE 10 
Tests were conducted to determine the volume expansion, during freezing of 
the ethoxylate/water solution. All measurements were made using test tubes 
which measured 15 cm high and had an internal diameter of 2.2 cm. To make 
the measurements, stock solutions of the Alfonic 1216CO-7.5/water were 
prepared by adding the appropriate amount of surfactant to warm water 
according to Example 1. After thorough mixing with a magnetic stirrer, the 
solutions were stoppered and allowed to cool. After cooling, a 30 g 
aliquot of each solution was placed in a respective test tube, and 5 ml of 
a hydrocarbon solvent was carefully added to the top of each solution. 
Once the hydrocarbon was added, care was exercised so that the hydrocarbon 
would not become dispersed in the ethoxylate/water solution. The height 
(proportional to volume), in cm, for both the solution and the 
hydrocarbon, was measured after thoroughly cooling the contents in an ice 
bath. Then, the test tubes were carefully transferred to a freezer, and, 
with a thermocouple inserted, cooled to -5.degree. C. to -10.degree. C. 
When the desired temperature was reached, the height of the hydrocarbon 
liquid was again measured. The frozen solid produced an indeterminate 
height for measurement. The difference in the height of the hydrocarbon 
meniscus, .DELTA.h, before and after freezing, was attributed to volume 
changes in the material being tested. To calculate the percent change in 
volume due to freezing the expression, (H.sub.1 
+.DELTA.h/H.sub.1).times.100-100 was used, where H.sub.1 is the height of 
the meniscus of the test liquid at 0.degree. C., and .DELTA.h is the 
difference in height of the hydrocarbon meniscus before and after 
freezing. 
For each solution, the volume change measurement was made six times, and 
from these six measurements, both the mean and the standard deviation were 
calculated. For outlying values, the statistical 2.5 d rule was used to 
include or exclude individual data points. The results are summarized 
below. 
__________________________________________________________________________ 
FREEZE VOLUME EXPANSION IN THE 
H.sub.2 O-ALFONIC 1216CO-7.5 SYSTEM 
SOLUTION NO. TEST % EXPANSION 
COMPOSITION 
RESULTS USED 
DUE TO FREEZE 
STD. DEVIATION 
__________________________________________________________________________ 
Water 4 9.6 .+-.9.4 
10% ALFONIC 
5 6.7 0.5 
20% ALFONIC 
5 5.9 0.5 
30% ALFONIC 
5 4.0 0.3 
40% ALFONIC 
5 2.5 0.3 
100% ALFONIC 
2 -5.1 -- 
__________________________________________________________________________ 
The above data is plotted in FIG. 6. Also plotted is a calculated value for 
volume expansion in a mixed system assuming no interactions. Although this 
assumption is not valid, the calculated line and the experimental data 
show a reasonable fit. The calculated line is from the expression: 
EQU % volume expansion (freeze)={1.09.degree.(wt. % water)+0.949 (wt. % Alfonic 
1216CO-7.5)}-100 
The value, 1.09 for water, is a well known and accepted literature value. 
Although the invention has been described in detail with reference to 
certain preferred embodiments, variations and modifications exist within 
the scope and spirit of the invention as described and defined in the 
following claims.