Thermal energy storage compositions with nucleating agents

A 41.degree. F. melting/freezing point NaOH in water solution employs a nucleating agent selected from the group consisting essentially of CaCO.sub.3, Fe.sub.3 O.sub.4, FeO.TiO.sub.2, SnO.sub.2, TeO.sub.2, LiAl(SiO.sub.4) and mixtures thereof. Xanthan gum may be used as a dispersant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally refers to the field of thermal energy 
storage, which may be abbreviated as TES, and more particularly to the 
storage of such energy by means of phase change materials, often referred 
to hereinafter as PCM's. By PCM's I refer to certain chemical compositions 
that store heat energy in one phase, which energy is then either released 
in the form of heat or which acquire heat from an ambient fluid as the PCM 
changes phase. 
2. The Prior Art 
The use of certain chemical compositions for "cool" thermal energy storage 
is an art that has yet to achieve its full market potential, although 
there are at present a plurality of such "cool" storage installation in 
successful commercial operation in this country. The use of PCM's to store 
coolness, i.e., to freeze at a predetermined temperature, usually below 
about 50.degree. F., has successfully been carried out, particularly where 
time-of-use rates make such applications commercially practical. 
Commercial feasibility occurs where the differential between peak and 
off-peak electric energy rates, as well as the application of a so-called 
demand charge for the use of electricity during peak periods, makes the 
installation of a TES system viable, e.g., the cost of the system, as 
measured against the savings, provides a pay back period of perhaps three 
years. 
Using a cool storage TES system with a PCM having a melting/freezing point 
plateau of somewhat below 50.degree. F., the charging cycle is 
automatically timed to take place during off-peak hours. By the provision 
of suitable piping between a chiller and a tank holding containerized 
PCM's, cold water from the chiller is circulated to the tank, into contact 
with the containers of PCM's, and then circulated from the tank back to 
the chiller for recooling. This procedure continues until the PCM's have 
been frozen, i.e., until they have changed their state from liquid to 
solid, thereby transferring their heat of fusion to the cold water that 
has chilled them, or another cold fluid used for that purpose. That is the 
charging cycle. 
In the discharge cycle, line water at a temperature in excess of the 
melting point of the PCM is circulated through the tank of frozen, 
containerized composition, and thereby chilled approximately to the 
temperature of the frozen PCM's which, in a manner of speaking, may be 
said to have stored coolness. As the relatively warm water or other fluid 
is passed into thermal contact with the frozen PCM's, the latter melt, and 
in so doing use the heat energy of the relatively warm water to satisfy 
their heat of fusion, thereby lowering the temperature of the water, which 
is then circulated to the water-to-air heat exchangers in the structure to 
be cooled. This procedure continues until substantially all of the 
coolness stored in the frozen PCM's has been exhausted by the melting of 
the PCM's and, if desirable, by the use of the specific heat differential 
between the PCM's in liquid form and the warmer water. 
By properly sizing the TES unit, with knowledge of the extent of the peak 
period the TES system can supply all of the cooling requirements of a 
building for the entirety of the on-peak period, thereby making it 
unnecessary to use the high electricity consumption chiller during on-peak 
periods. Alternatively, the TES system can supply only a part of the 
on-peak load, which would permit the use of a smaller chiller. In either 
mode of use, a PCM thermal energy storage system can effect substantial 
savings to the owner and provide a desirable leveling effect to the 
24-hour load profile of an electric utility during summer months when 
electricity use for air conditioning is maximal. 
Any PCM utilized for thermal storage will have generic requirements. These 
include maintenance of a near constant temperature throughout the phase 
change cycle, also referred to as a melting/freezing point plateau, a 
relatively high latent heat of fusion, and a relatively high density. Such 
a PCM will be able to store large quantities of coolness, so as to make 
its use practical in commercial installations. Of course, the PCM must 
maintain its desirable thermal energy storage capacity over thousands of 
freeze-thaw cycles, indeed almost indefinitely, and must be abundantly 
available and relatively inexpensive. Such compositions are available and 
now in use. 
From a commercial point of view, the composition that has thus far found 
greatest application as a PCM is a eutectoid salt composition based on 
sodium sulfate decahydrate. This salt, like most materials, including 
water, has a tendency to supercool; however, many years ago Dr. Maria 
Telkes, a pioneer in the field, discovered that sodium tetraborate would 
ameliorate the problem of supercooling of this type of PCM. Unfortunately, 
the lowest temperature at which a sodium sulfate decahydrate eutectoid 
salt mixture has been found to freeze is 47-48.degree. F. As a 
consequence, in some situations where a full storage system design is 
employed or for other reasons, and it is requisite that chilled water be 
supplied from the PCM tank at less than 47.degree. F., a sodium sulfate 
decahydrate PCM is not appropriate. 
More recently, it has been learned that a specific NaOH/H.sub.2 O solution 
can be employed as a PCM for cool storage. The applicant has become aware 
that an aqueous solution of about 46-47% NaOH in water has a 
freezing/melting point plateau at approximately 41.degree. F., a highly 
propitious temperature for a PCM. At this temperature the water solution 
of NaOH melts congruently, i.e., when it melts, it exists as a stable 
solid in equilibrium with a liquid of the same composition. With PCM's 
that do not melt congruently, e.g., sodium sulfate-based eutectoid salts, 
a thickening or gelling agent may be used to maintain this stable 
solid/liquid equilibrium. Since at that particular concentration, rather 
than as some other, random concentration, the NaOH/H2O solution has a 
melting/freezing point plateau, it is exceptionally well suited for 
employment as a PCM. 
It will be apparent that at a random freezing/melting point on the liquidus 
line of a phase diagram of a NaOH/H.sub.2 O binary system, freezing or 
melting will only begin. As, due to such partial freezing or melting, the 
concentration of the sodium hydroxide changes, so does the 
freezing/melting point, and there is no freezing/melting point plateau. In 
a sodium hydroxide-water system or, indeed, any system that will be 
adapted for employment as a PCM, it is requisite that there be a 
substantial plateau at the freezing/melting point; otherwise, on melting, 
for example, a varying melting point would cause the fluid flowing from 
the storage tank to do so at varying temperatures, making that fluid 
unsuitable for use in air conditioning a building because of its 
non-constant exit temperatures. 
With all the benefits of a 41.degree. F. melting/freezing point NaOH in 
water solution, one serious drawback to its use is that significant 
supercooling tends to occur. In the laboratory this can be remedied by 
seeding, agitation, etc.; in a commercial installation where many 
thousands of containers of the PCM are stored in a tank and are not 
subject to easy access, the problem cannot be easily rectified. 
Experimentation has confirmed that the means for overcoming supercooling 
in eutectoid salt compositions--sodium tetraborate decahydrate--is 
ineffective in performing that function in a 41.degree. F. melting point 
NaOH/H2O solution. 
As a consequence, it is a primary object of the present invention to 
provide a supercooling inhibitor for a 47-48% NaOH/water solution so that 
when the temperature of such solution is lowered below the 41.degree. F. 
level, crystallization of the solution is initiated without additional 
seeding or agitation. 
It is another object of this invention to provide such a supercooling 
inhibitor, which may also be termed a nucleating agent, which will be 
economically effective, abundantly available, and which will perform its 
function in small quantities relative to the remainder of the NaOH/H.sub.2 
solution, since whatever percentage of the total PCM is occupied by the 
nucleating agent, that percent is a part of the entire solution that does 
not per se freeze and, therefore, does not itself store cool energy by its 
heat of fusion. 
SUMMARY OF THE INVENTION 
The basis of the present invention, as presently understood, is a 
composition of matter suitable for use in storing cooling capacity by its 
heat of fusion, which comprises a major amount of a sodium hydroxide-water 
solution in which the ratio of NaOH to H.sub.2 O is about 46-47 to 54-53 
by weight. For convenience, in the claims and elsewhere in the 
specification, that ratio will be referred to as about 47 to 53, NaOH to 
H.sub.2 O. Further, the solution contains a nucleating agent in an amount 
sufficient to inhibit supercooling of the solution. The nucleating agent 
is selected from the group consisting essentially of CaCO.sub.3, Fe.sub.3 
O.sub.4, FeO.TiO.sub.2, SnO.sub.2, TeO.sub.2 and LiAl(SiO.sub.4), and 
mixtures thereof. 
A highly preferred nucleating agent is calcium carbonate, if for no other 
reason than because it is effective and so readily available at a 
reasonable cost. Indeed, it is effective in its commercial forms as are 
the other nucleating agents named hereinbefore. Thus, calcium carbonate 
may be used in the form of granular limestone, or it may be used as 
commercially available whiting, in which it has been ground to a powder. 
Both of these forms of calcium carbonate are in wide, commercial use. As 
stated, all of the agents may be used in commercial form as agglomerates, 
e.g., the Fe.sub.3 O.sub.4 as magnetite and the LiAl(SiO.sub.4) as 
petalite. 
In addition, the composition of the present invention may also include a 
dispersant for maintaining the nucleating agent in suspension in the 
solution. The preferred dispersant is xanthan gum, which has been found to 
be particularly effective in maintaining petalite agglomerates in 
suspension.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
As has been stated, one preferred embodiment of the invention includes 
calcium carbonate as a nucleating agent. Limestone and whiting are so 
economic to use in these relatively small amounts that their use adds only 
a fraction of a penny to the cost of the PCM on a per pound basis. 
Moreover, a highly refined limestone is not necessarily required; even a 
commercial form will be effective for its use as a nucleating agent. 
Indeed, a granular material has been found to be well adapted under 
certain circumstances. 
The amount of calcium carbonate to be utilized may be determined on a trial 
and error basis, using an amount sufficient to accomplish its purpose, but 
recognizing that the more nucleating agent that is used, the less solution 
will be available to freeze for coolness storage. As a consequence, it has 
been determined that the addition of about 3 to 5 parts by weight of 
calcium carbonate is an effective amount to inhibit supercooling by 
nucleation, with 3 parts of the chemical commercially available as 
limestone from Van Water and Rodgers, Inc., of Los Angeles, Calif., having 
been subjected to testing and having been found to be effective. In the 
sample tested, the agglomerates appear to have a mean diameter of about 10 
mm. 
With respect to the other nucleating agents, the operable Fe.sub.3 O.sub.4 
was purchased from Westwood Ceramic Supply Company of City of Industry, 
Calif., as granular magnetite. The FeO.TiO.sub.2 was also purchased from 
Westwood Ceramic under the name, ilmenite, which has a chemical 
composition of FeTiO.sub.3, or FeO.TiO.sub.2. The LiAl(SiO.sub.4) was 
purchased as petalite. The magnetite appeared to have a mean diameter of 
about 50 mm in its granular form, as did the ilmenite. The tin oxide was 
less agglomerated, but the petalite had larger agglomerates, varying from 
about 100 mm to 400 mm. 
When the nucleating agent tends to "crust-over" along the bottom of the 
container, it has been found preferable to utilize a dispersing agent to 
keep the nucleating agent readily available at any location within the 
body of the solution at which crystallization on freezing is most likely 
to occur. The use of a dispersant is perhaps not necessary in many 
instances, such as when calcium carbonate is used, although further 
cycling tests may indicate that it should be utilized. 
Where it has been determined that it is preferable to use a dispersant, the 
dispersant of choice is a seaweed extract known as xanthan gum. Such gum 
is a natural high-molecular weight, branched polysaccharide. It functions 
as a hydrophilic colloid to thicken, suspend and stabilize water-based 
systems, and has been found to be particularly effective in a highly 
alkaline environment, such as that of the present invention. It is 
important to select the proper dispersant for a particular medium in which 
dispersion is to take place. In the aqueous NaOH medium in which a 
nucleating agent is to be dispersed according to the present invention, 
xanthan gum has been found to be significantly superior to other 
dispersants. 
In actual practice, four parts of calcium carbonate in the form of whiting 
were added to 47 parts of NaOH and 53 parts of water, and mixed rapidly, 
then containerized. The mixture in containers was thereafter subjected to 
cooling in a water tank maintained at 36.degree. F. The temperature of the 
solution dropped to about 36.5.degree. F., then rapidly ascended after a 
period of about 2 hours to a temperature of 40.0.degree. to 40.5.degree. 
F., where it leveled off for about 5 hours. The PCM solution appeared to 
be completely frozen after approximately 9 hours of thermal contact with 
the 36.degree. F. water. On further cooling with the water, the 
temperature of the frozen solution dropped to about 36.degree. F. over the 
next two hours. 
When the same freezing curves were plotted with about 5 parts of granular 
magnetite (Fe.sub.3 O.sub.4) added to the NaOH/H.sub.2 O solution, the 
results were similar except that the time required to have the temperature 
of the solution rise to its freezing point, measured at approximately 
40.8.degree. F., was about 1 hour longer than with the calcium carbonate. 
Such time was 6 hours, and freezing was completed after 10 hours. The 
temperature of the PCM using magnetite as a nucleating agent reached the 
temperature of the coolant, 36.degree. F., after approximately 13 total 
hours. 
Similar test results were obtained with the other materials used as 
nucleating agents, with variations. Both the ilmenite and tin oxide had 
sharply rising curves that indicated when their effect as nucleating 
agents occurs. Such nucleating effectiveness occurred earlier with the 
ilmenite than the tin oxide, and the freezing point of the 
ilmenite-nucleated solution reached 41.degree. F. Of all the materials 
used, the calcium carbonate appeared to be as good or better than the 
rest, although all were effective. Thus, at present the most preferred 
agent is calcium carbonate in the form of limestone or whiting, which may 
have a mean diameter of about 10 mm and in an amount of about 4 parts by 
weight added to 47 parts NaOH and 53 parts H.sub.2 O. No xanthan gum has 
been determined to be necessary for use in conjunction with the calcium 
carbonate nucleating agent. However, when use of xanthan gum as a 
dispersant is indicated, it was used in about 1 to 3 parts, preferably 2 
parts by weight, in addition to 3 parts petalite, 47 parts NaOH and 53 
parts H.sub.2 O. Generally, petalite and tellurite (TeO.sub.2) appear most 
likely to require a dispersant, and are not preferred. 
While this specification has been written with respect to preferred 
embodiments of my invention, certain modifications and alterations thereof 
will be apparent to those of skill in this art. As to all such obvious 
modifications and alterations, it is desired that they be included within 
the scope of the present invention, which is to be limited only by the 
purview, including equivalents, of the following, appended claims.