Thermal storage composition for low energy ice harvesting, method of using same

The disclosed thermal storage composition includes water, an electrolyte or a non-electrolyte, and a nucleating surfactant. The nucleating surfactant reduces the surface tension of the thermal storage composition and simultaneously promotes nucleation sites within the thermal storage composition. The thermal storage composition is directed along at least one heat exchanger surface cooled by a refrigerant. The thermal storage composition forms ice crystals that selectively accumulate on the heat exchanger surface. Forces resulting from gravity and the interactions with the liquid of the thermal storage composition overcome the adhesion forces between at least a portion of the ice crystals and the heat exchanger surface, causing at least a portion of the ice crystals to separate from the heat exchanger surface. The nucleating surfactant inhibits ice crystal growth on the heat exchanger surface, the smaller resultant ice crystals have reduced contact area and reduced adhesive strength, thereby providing a reliable release from the heat exchanger surface to render low energy ice harvesting.

BRIEF DESCRIPTION OF THE INVENTION 
This invention relates generally to cool thermal energy storage systems. 
More particularly, this invention relates to an improved thermal storage 
composition with reduced ice crystal size, ice crystal growth rate, and 
surface adhesion. 
BACKGROUND OF THE INVENTION 
Cool thermal energy storage is increasingly being used in building air 
conditioning systems. The major advantage of cool thermal energy storage 
is that it reduces the severity of intermittent peak air conditioning 
loads. That is, off-peak electrical utility periods may be used to operate 
the cool storage equipment and thereby alleviate the severity of the peak 
air conditioning loads. The shift of electrical energy use to utility 
off-peak periods reduces the customer's demand charges, leading to a 
reduction in electric bills. In addition, the load shift reduces the 
utility's system peak demand, thereby improving operating efficiency and 
reducing costs. 
Several methods are currently used for sensible and latent cool thermal 
energy storage. Dynamic ice harvesters are one technique for latent heat 
storage. In typical ice harvesting systems, ice is formed on the exterior 
surface of a heat transfer surface and periodically removed by means of a 
defrost harvesting cycle, which melts the ice adjacent to the heat 
transfer surface, thereby allowing all of the ice to be removed from the 
heat transfer surface. Overall efficiency of the system is adversely 
effected by the defrost harvesting cycle, which may utilize up to about 
20% of the energy input to the system. Mechanical harvesting techniques, 
such as scrapping ice from a surface, may also be used. However, such 
techniques also require additional energy. 
U.S. Pat. No. 4,907,415 (the '415 patent), owned by the assignee of the 
present invention and expressly incorporated by reference herein, 
describes an improved system in which a "self release" harvesting 
technique is used to efficiently obtain ice for use in a cool thermal 
energy storage system. The apparatus of the '415 patent includes a thermal 
storage solution and a heat exchanger. 
The thermal storage solution comprises water and a mixture of electrolytes 
and/or non-electrolytes, such as a 30/70 calcium acetate-magnesium acetate 
mixture or ethylene glyclol, respectively. When processed by the heat 
exchanger, the thermal storage solution results in a mixture of liquid and 
ice crystals, forming a "slush". "Slush" is defined as a soft mass 
consisting of a mixture of ice crystal solids and liquid. The slush is 
soft compared to a solid formed from freezing substantially pure water. As 
will be described below, the solution results in low adhesion forces 
(compared to substantially crystalline ice) between the ice crystals and 
the heat exchanger surface. The term "adhere" is used as a generic term 
for all forces tending to cause a mass consisting of a mixture of solids 
and a liquid to be attracted to a surface. Low cohesion forces cause the 
slush to be soft. As used herein, the term "cohere" is used as a generic 
term for all forces tending to cause ice crystals and a liquid to be 
attracted to each other. 
FIG. 1 illustrates one embodiment of the invention disclosed in the '415 
patent. The tank 20 holds the thermal storage solution 22. The liquid 
portion of the thermal storage solution is removed from the tank 20, using 
any convenient technique, and is directed through a conduit 24 by a pump 
26. The output of the pump 26 is in turn directed by conduit 28 to 
distributors 30 and 32. 
Liquid 33, generally indicated by arrows, from the distributors 30 and 32 
is directed to flow down the substantially vertical (downwardly extending) 
heat exchanger surfaces 34 and 36. Cold refrigerant flows through each of 
the heat exchanger surfaces, 34 and 36, entering by way of input conduit 
38 and exiting by output conduit 40. 
The temperature of the refrigerant is selected such that as the liquid 33 
flows down the heat exchanger surfaces, 34 and 36, portions of the liquid 
33 solidify to form ice crystals, illustrated as reference numeral 42. The 
accumulated ice crystals 42 interact with the liquid portion 33 of the 
thermal storage solution, the heat exchanger surfaces 34 or 36, and forces 
of gravity. The magnitude of these interactions increases as the 
accumulated ice crystals 42 increase in size and/or thickness. As these 
interactions become sufficient to overcome adhesion and/or cohesion 
forces, portions or all of the accumulated ice crystals 42 release from 
the heat exchanger surfaces 34 and 36 and are accumulated in tank 20. As 
previously indicated, the tank 20 holds the thermal storage solution, 
which may be predominantly slush or liquid, depending on the operating 
history of the system. 
It would be desirable to improve upon the technology disclosed in the '415 
patent. Specifically, it would be desirable to improve the thermal storage 
solution such that accumulated ice crystals could be more readily removed 
from a heat exchanger surface. 
SUMMARY OF THE INVENTION 
The disclosed thermal storage composition includes water, an electrolyte or 
a non-electrolyte, and a nucleating surfactant. The nucleating surfactant 
reduces the surface tension of the thermal storage composition and 
simultaneously promotes nucleation sites within the thermal storage 
composition. The thermal storage composition is directed along at least 
one heat exchanger surface cooled by a refrigerant. The thermal storage 
composition forms ice crystals that selectively accumulate on the heat 
exchanger surface. Forces resulting from gravity and/or the interactions 
with the liquid of the thermal storage composition overcome the adhesion 
forces between at least a portion of the ice crystals and the heat 
exchanger surface, causing at least a portion of the ice crystals to 
separate from the heat exchanger surface. The nucleating surfactant 
inhibits ice crystal growth on the heat exchanger surface, the smaller 
resultant ice crystals have reduced contact area and reduced adhesive 
strength, thereby providing a reliable release from the heat exchanger 
surface to render low energy ice harvesting.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed toward a novel thermal storage 
composition that comprises water, an electrolyte or a non-electrolyte, and 
a nucleating surfactant. A surfactant generally refers to any soluble 
compound that reduces the interfacial tension between a liquid and a 
solid, or between two liquids. A general surfactant with surface tension 
reduction qualities cannot be successfully employed with the present 
invention. It has been observed that such a surfactant significantly 
reduces the surface tension of the thermal storage composition. As a 
result, ice crystals that form on the heat exchanger surface cannot be 
dislodged from the surface by the downwardly flowing liquid constituent of 
the thermal storage composition. 
On the other hand, a nucleating surfactant, a surfactant that reduces the 
surface tension of a thermal storage composition and simultaneously 
promotes nucleation sites within the thermal storage composition, has 
demonstrated highly desirable performance. Nucleation is the formation, in 
a crystallization process, of new crystal nuclei in supersaturated 
solutions. 
An increase in the number of nucleation active sites tends to decrease the 
equilibrium size of ice crystals. If the nucleation rate is increased, the 
mass of each crystal is decreased, and hence, the size of each crystal is 
decreased. It has been observed that the addition of nucleating 
surfactants to the thermal storage composition of the invention reduces 
ice crystal size by a factor of approximately 50 percent. Thus, the ice 
crystals formed in conjunction with the present invention are sometimes 
referred to as being relatively small. 
In the context of the present invention, the inclusion of a nucleating 
surfactant into the thermal storage composition inhibits individual ice 
crystals from growing together. As a result, smaller ice crystals are 
grown. The smaller ice crystals have reduced contact area and reduced 
adhesive strength to the heat exchanger surface, resulting in adhesive or 
cohesive failure. Consequently, ice crystals grown on the heat exchanger 
surface are in the form of a film that reliably releases from the heat 
exchanger surface. Reliable release of ice crystals from the heat 
exchanger surface allows for low energy ice harvesting. 
Nucleating surfactants successfully used in conjunction with the present 
invention include: methylamine hydrochloride (CH.sub.3 NH.sub.2 HCL), 
triethanol amine ((OHCH.sub.2 CH.sub.2).sub.3 N), monoethanol amine 
(OHCH.sub.2 CH.sub.2 N), monoethylamine hydrochloride (ClCH.sub.2 CH.sub.2 
N), and tri-ethylamine hydrochloride (Cl(OH.sub.2)(CH.sub.2 
CH.sub.2).sub.3 N). Preferably, the nucleating surfactants are used with 
water and electrolytes and/or non-electrolytes at a concentration between 
500 and 2000 ppm, preferably approximately 1000 ppm. 
Preferably, triethanol amine and monoethanol amine are neutralized with 
hydrochloric acid. The process of neutralization may be performed by 
diluting the base surfactant with water to a ratio of 2:1. HCl is then 
added until a pH of 7 is obtained. This neutralization process attaches 
the chloride ion of HCl to the surfactant, replacing one of the OH groups. 
The remaining H and OH ions form a molecule of water. Thus, the 
neutralized triethanol amine is defined as Cl(OH).sub.2 (CH.sub.2 
CH.sub.2).sub.3 N+H.sub.2 O and the neutralized monoethanol amine is 
defined as ClCH.sub.2 CH.sub.2 N+H.sub.2 O. 
A preferable electrolyte for use with the invention is a 7% solution of 
potassium acetate. The potassium acetate tends to segregate the nucleation 
and growth of the ice crystals. Other electrolytes that may be used in 
accordance with the invention include a 30/70 calcium acetate-magnesium 
acetate mixture, potassium benzoate, sodium benzoate, nickel nitrate, 
calcium nitrate, sodium acetate, stannic chloride, thorium nitrate, 
calcium chloride, potassium chloride, sodium chloride, ammonium chloride, 
beryllium nitrate, magnesium chloride, sodium nitrate, and potassium 
cyanate. 
Non-electrolytes that may be used in accordance with the invention include 
ethylene glycol, propylene glycol, urea, and sucrose. The thermal storage 
composition of the invention may also be in the form of a mixture of 
water, electrolytes and/or non-electrolytes, and a nucleator, such as 
silver iodide. Similarly, the thermal storage composition of the invention 
may be in the form of a mixture of water, electrolytes and or 
non-electrolytes, a surfactant, and a nucleator. 
It will be appreciated by those skilled in the art that the thermal storage 
composition of the invention may be used in conjunction with any of the 
heat exchangers described in the '415 patent. Similarly, the thermal 
storage composition of the invention may be used in any of the methods 
described in the '415 patent. 
FIGS. 2 and 3 illustrate the use of the thermal storage composition of the 
invention with tube-and-shell heat exchangers. The elements in FIGS. 2 and 
3 are numbered to be consistent with similar elements in FIG. 1. 
FIG. 2 depicts a tank 20A that holds the thermal storage composition 50 of 
the present invention. The liquid portion of the thermal storage 
composition is removed from the tank 20A and is directed through a conduit 
24A by a pump 26A. The output of the pump 26A is in turn directed by 
conduit 28A to a distributor assembly 52. The distributor assembly 52 is 
coupled to a vertical tube-and-shell heat exchanger 54 which includes a 
housing 56 enclosing a plurality of vertical heat exchanger tubes 58. 
Liquid 60, generally indicated by arrows, from the distributor assembly 52 
is directed to flow along the interior walls of the vertical heat 
exchanger tubes 58. Cold refrigerant circulates within housing 56, 
entering by way of input conduit 38A and exiting by output conduit 40A. 
The temperature of the refrigerant is selected such that as the liquid 60 
flows through the vertical heat exchanger tubes 58, portions of the liquid 
60 solidify to form ice crystals. The resultant ice crystals and liquid 
constitute a slurry. The term slurry is used to distinguish from the slush 
formed in the '415 patent. As used herein, a slurry is a soft mass 
consisting of a mixture of relatively small ice crystal solids and liquid. 
The accumulated ice crystals interact with the liquid portion 60 of the 
thermal storage solution, the heat exchanger tubes 58, and forces of 
gravity. As indicated above, these interactions readily dislodge the 
accumulated ice crystals from the walls of the vertical tubes 58. As a 
result, the mixture of ice crystals and liquid which form the slurry 
returns to the tank 20A. As previously indicated, the tank 20A holds the 
thermal storage solution 50, which may be predominantly slurry or liquid, 
depending on the operating history of the system. 
FIG. 3 depicts a tank 20B that holds the thermal storage composition 50 of 
the present invention. The liquid portion of the thermal storage 
composition is removed from the tank 20B and is directed through a conduit 
24B by a pump 26B. The output of the pump 26B is in turn directed by 
conduit 28B to a distributor assembly 61. The distributor assembly 61 is 
coupled to a horizontal tube-and-shell heat exchanger 62 which includes a 
housing 64 enclosing a plurality of horizontal heat exchanger tubes 66. 
Liquid 60, generally indicated by arrows, from the distributor assembly 61 
is directed to flow through the horizontal heat exchanger tubes 66. Cold 
refrigerant circulates within housing 64, entering by way of input conduit 
38B and exiting by output conduit 40B. 
The temperature of the refrigerant is selected such that as the liquid 60 
flows through the horizontal heat exchanger tubes 66, portions of the 
liquid 60 solidify to form ice crystals. The accumulated ice crystals 
interact with the liquid portion 60 of the thermal storage solution and 
the heat exchanger tubes 66. These interactions dislodge the accumulated 
ice crystals. The moving liquid portion of the thermal storage composition 
transports the dislodged ice crystals and liquid, which form the slurry, 
to a disposal conduit 68, which returns the slurry to the tank 20B. 
The foregoing descriptions of specific embodiments of the present invention 
are presented for purposes of illustration and description. They are not 
intended to be exhaustive or to limit the invention to the precise forms 
disclosed, obviously many modifications and variations are possible in 
view of the above teachings. The embodiments were chosen and described in 
order to best explain the principles of the invention and its practical 
applications, to thereby enable others skilled in the art to best utilize 
the invention and various embodiments with various modifications as are 
suited to the particular use contemplated. It is intended that the scope 
of the invention be defined by the following Claims and their equivalents.