Method and apparatus for thermal transportation using polyvinyl alcohol

Abstract: A thermal transportation method utilizes polyvinyl alcohol which is added to a liquid, and the liquid is cooled to form an ice slurry that is pumped through a pipe to effect the thermal transportation.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to a method and apparatus for thermal transportation utilizing ice slurry moved through a pipe. Description of the Prior Art: Prior art methods of thermal transportation and storage using ice slurry include adding antifreeze protein to a liquid and cooling the liquid to produce an ice slurry that is then stored, or transported by being pumped through a pipe. Methods involving simply cooling a liquid utilize just the sensible heat of the liquid, so the quantity of heat that is transported and stored is relatively small. Also, in the case of methods that just use solid ice, it is difficult to pump the ice through pipes. Moreover, in the case of the prior art methods in which ice is produced from a liquid, since latent heat is accumulated during the phase change from liquid to ice, a large quantity of heat can be transported, and the good flow properties of the ice slurry, which exhibits good response property during thermal utilization, enables it to be pumped through piping. However, making the ice readily subjectable to secondary recrystallization (growth of the ice crystals) degrades the response property exhibited when the ice slurry is to be dissolved for thermal utilization. Because this can cause clogging of the piping, this method is not suitable for thermal storage for extended periods or for long-distance transportation applications. In the prior art methods described above the addition of antifreeze protein to the liquid enables recrystallization of the ice to be suppressed, making the methods also suitable for thermal storage for extended periods and for long-distance transportation. Antifreeze protein is protein obtained from fish in waters close to the polar regions. It is this protein that prevents the blood of the fish from freezing. However, antifreeze protein is very costly. This means that it is expensive to use ice slurry in which recrystallization is suppressed for thermal transportation. The present inventors have already proposed a method and apparatus for thermal transportation and storage using a silane coupling agent, alkane thiol or fatty acids in place of antifreeze protein [Papers (B) of The Japan Society of Mechanical Engineers, vol. 65, No. 630, pp 53 - 58 (1999-2)]. However, the method and apparatus using synthetic high polymers had a number of problems, including cost, toxicity, corrosiveness and solubility in water, which were enough to keep the method and apparatus from being practicable. More specifically, with respect to the silane coupling agent, the cost was not low enough, in addition to which the material was toxic and not soluble enough in water. Similarly, the corrosiveness of the fatty acids was a problem, as was their toxicity and lack of sufficient solubility in water. With respect also to the alkane thiol, solubility in water posed a problem, and there were also questions relating to the toxicity and corrosiveness of the substance. This invention was accomplished to overcome the foregoing problems and has as an object to provide a method and apparatus for thermal transportation that lowers the cost of thermal transportation using ice slurry in which recrystallization is suppressed, and also uses materials having good properties in terms of toxicity, corrosiveness and solubility in water. SUMMARY OF THE INVENTION For attaining this object, the present invention provides a thermal transportation method comprising adding an antifreeze agent to a liquid, cooling the liquid to which the antifreeze agent has been added to form an ice slurry, and pumping the ice slurry through a pipe, said method being characterised in that the antifreeze agent is polyvinyl alcohol, wherein the polyvinyl alcohol has an average molecular weight of more than 2000. The above object is also attained by an apparatus for thermal transportation comprising a liquid to which an antifreeze agent has been added, freezing means (3) for cooling the liquid to which the antifreeze agent has been added to form an ice slurry, a pipe (1) for transporting the liquid containing the ice slurry, and a pump (2) for moving the liquid in the pipe, said apparatus being characterized in that the antifreeze agent is polyvinyl alcohol, wherein the polyvinyl alcohol has an average molecular weight of more than 2000. The above object is also attained by the use of polyvinylalcohol as an antifreeze agent in a liquid, which liquid is cooled to form an ice slurry, and is contained in a thermal transportation apparatus, wherein polyvinylalcohol has a molecular weight of more than 2000. Thus, to achieve its object, the invention uses polyvinyl alcohol instead of the antifreeze protein, silane coupling agent, alkane thiol or fatty acids used in the prior art. The polyvinyl alcohol has the effect of suppressing recrystallization of the ice slurry and is extremely cheap, being about 1/20,000 the price of antifreeze protein. Moreover, polyvinyl alcohol is neither toxic nor corrosive, and exhibits excellent solubility in water. Thus, it enables thermal transportation using ice slurry in which recrystallization is suppressed that is highly stable and highly reliable, and low in cost. Further features of the invention, its nature and various advantages will become more apparent from the accompanying drawings and following detailed description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1(a) is the general formula of polyvinyl alcohol. Figure 1(b) illustrates the reaction between the polyvinyl alcohol and the surface of ice. Figure 2 shows the general system of a thermal transportation apparatus according to a first embodiment of the invention. Figure 3 shows the general system of a thermal transportation apparatus according to a second embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to thermal transportation comprising forming an ice slurry by cooling a liquid containing polyvinyl alcohol, and pumping the ice slurry through a pipe. The general formula of polyvinyl alcohol is (CH 2 C(OH)H)n, as shown in Figure 1(a). -OH is the hydrophilic group and -CH 2 - is the hydrophobic group. The hydrophilic group -OH of the polyvinyl alcohol molecule forms hydrogen bond with the surface of the ice, as shown in Figure 1(a). Since -CH 2 - is hydrophobic, secondary growth of the ice is suppressed, suppressing ice to ice bonding. Thus, when polyvinyl alcohol is added to water and the solution cooled, recrystallization of the ice is suppressed. As a result, ice slurry is obtained in which recrystallization is suppressed, enabling the ice slurry to be stored for extended periods and conveyed extended distances through pipes. Studies have shown that with polyvinyl alcohol having a molecular weight of 2000 or more, the above suppression of recrystallization is well exhibited. In the case of antifreeze liquid in general use, it has been necessary to add not less than 10% by weight of a substance that lowers the freezing point of the liquid. However, even when a very small amount of polyvinyl alcohol is added to liquid, 0.1% by weight, for example, the recrystallization is suppressed, enabling the ice slurry to be obtained. Water, which is easy to obtain, may be used as the liquid. However, the liquid is not limited to water, and may be any liquid that dissolves the polyvinyl alcohol. The thermal transportation apparatus using liquid containing polyvinyl alcohol in accordance with this invention will now be described. Figure 2 shows a first embodiment of the apparatus. This thermal transportation apparatus includes a pipe 1 provided with a pump 2, a freezing means 3 and a heat absorption means 4. Sealed in the pipe 1 is a liquid, such as for example water, to which polyvinyl alcohol has been added (hereinafter also referred to as "polyvinyl alcohol solution"). As explained in the foregoing, when the water is cooled, the polyvinyl alcohol serves to suppress ice recrystallization. The freezing means 3 is constituted by a freezer or the like provided for cooling the polyvinyl alcohol solution to produce ice slurry. The pump 2 is used to forcibly circulate the polyvinyl alcohol solution and ice slurry through the pipe 1. Thermal transportation is effected by the apparatus as follows. The ice slurry in the pipe 1 formed by cooling the polyvinyl alcohol solution at the freezing means 3 is pumped to the heat absorption means 4 by the pump 2. At the heat absorption means 4 the ice slurry absorbs its heat. Upon absorbing its heat the ice slurry reverts to a polyvinyl alcohol solution which is conveyed to the freezing means 3 where it is again cooled to an ice slurry. This repetitive process of cooling and thermal absorption constitutes the thermal transportation process. The addition of the polyvinyl alcohol to the water suppresses the recrystallization of the ice of the ice slurry. By thereby preventing clogging of the pipe 1, thermal transportation for extended periods is enabled. Figure 3 shows the arrangement of a thermal transportation apparatus according to a second embodiment of the invention. This apparatus comprises a freezing means 3 having a tank 11, a pipe 1 containing a polyvinyl alcohol solution, a first pump 2a, a heat maintenance means 5 having a tank 12, a second pump 2b, and a heat absorption means 4. The heat maintenance means 5 holds the ice slurry and delivers it to the heat absorption means 4 as required. Operation of the thermal transportation apparatus of Figure 3 will now be described. Polyvinyl alcohol solution in the tank 11 of the freezing means 3 is cooled to an ice slurry. From the tank 11, the ice slurry is pumped into the tank 12 of the heat maintenance means 5 by the first pump 2a. The heat maintenance means 5 serves to maintain the temperature of the ice slurry. When required, the ice slurry in the heat maintenance means 5 is delivered to the heat absorption means 4 by the second pump 2b. Since polyvinyl alcohol has been added to the water, recrystallization of ice in the slurry is suppressed, enabling the ice slurry to be maintained for an extended period in the heat maintenance means 5. In the foregoing, the embodiments have been described with reference just to the addition of polyvinyl alcohol to the liquid used. However, the invention is not limited to the addition of just polyvinyl alcohol, it being possible to promote ice formation by also adding ice nucleus bacteria. With respect to the bacteria, there can be used Pseudomonas syringae or other such known bacterium that is already used in artificial snow-making systems and the like. The type of ice bacteria and the added amount is determined according to what the ice slurry is to be used for. Experiments in which ice recrystallization and ice surfaces were observed will now be described. Experiment 1 Observation of ice recrystallization The aim of the experiment was to investigate the effect that the polyvinyl alcohol had on the recrystallization of the ice. In the experiment the recrystallization was observed of fine crystals of ice produced using a polyvinyl alcohol solution. Five polyvinyl alcohols were used, having the following average molecular weights: (1) 500, (2) 2000, (3) 31000 to 50000, (4) 85000 to 146000, and (5) 124000 to 186000. For comparison, the recrystallization of ice crystal samples formed using pure water, an ethylene glycol solution, and an antifreeze protein solution was also observed. For the experiment, drops of the polyvinyl alcohol solutions were dropped from a height of about two meters onto glass plates that had been cooled to -100°C, to allow a thin film of ice (having a polyvinyl alcohol content of 0.1% by weight) to be formed from the fine particle-shaped crystals. The ice film thus formed on the glass plate was left for 14 hours at -7°C to observe recrystallization of the ice crystals. The same procedures as in the experiment were carried out for the comparative examples. The results were as follows. (1) Polyvinyl alcohol having an average molecular weight of 500: Recrystallized ice crystals several hundred micrometers in diameter were observed. (2) Polyvinyl alcohol having an average molecular weight of 2000: Recrystallized ice crystals several hundred micrometers in diameter were observed. (3) Polyvinyl alcohol having an average molecular weight of 31000 to 50000: Non-recrystallized fine ice crystals no larger than 50 micrometers in diameter were observed. (4) Polyvinyl alcohol having an average molecular weight of 85000 to 146000: Non-recrystallized fine ice crystals no larger than 50 micrometers in diameter were observed. (5) Polyvinyl alcohol having an average molecular weight of 124000 to 186000: Non-recrystallized fine ice crystals no larger than 50 micrometers in diameter were observed. In the case of the comparative examples obtained using pure water and those obtained using an ethylene glycol solution, in each case recrystallized ice crystals several hundred micrometers in diameter were observed. In the case of the comparative examples obtained using an antifreeze protein solution, non-recrystallized fine ice crystals no larger than 50 micrometers in diameter were observed. Based on these results, it was possible to determine the following. The polyvinyl alcohol samples that exhibited a recrystallization suppressing effect each had an average molecular weight of not less than 31000. In contrast, the polyvinyl alcohol samples that did not exhibit a recrystallization suppressing effect each had an average molecular weight of 2000 or less. From this, it could be determined that in order to suppress ice recrystallization, it was necessary to use polyvinyl alcohol having an average molecular weight of more than 2000. Experiment 2 Observation of ice surface The aim of the experiment was to elucidate the mechanism of the adsorption of the polyvinyl alcohol on the ice surface. A scanning tunneling microscope (STM) was used to observe the surface of pure water ice and the ice surface on which polyvinyl alcohol having an average molecular weight of 89000 to 98000 was adsorbed. The results are as follows. In overall terms, the surface of the pure water ice was flat. However, where the polyvinyl alcohol was adsorbed on the ice surface, grooving was observed. Based on the results of the two experiments, the following considerations could be drawn. In the case of polyvinyl alcohol having an average molecular weight of more than 2000, the hydrogen bonding by the hydrophilic group in the molecules causes the adsorption on the ice surface, and this adsorption suppresses the crystal growth effect of the hydrophobic group in the molecules. In contrast, polyvinyl alcohol having an average molecular weight of 2000 or less lacks a sufficient number of hydrophilic groups for the hydrogen bonding to result in surface adsorption. Therefore, recrystallization of ice crystals cannot be effectively suppressed. Based on the foregoing, the present invention has the following effects. In accordance with the invention, a liquid to which polyvinyl alcohol is added is cooled to form an ice slurry. This means that compared to the prior art in which antifreeze protein, silane coupling agents or other such substances are added, an apparatus can be realized that at a much lower cost enables thermal transportation to be effected using ice slurry in which recrystallization is suppressed. Moreover, formation of the ice slurry can be promoted by also adding ice nucleus bacteria to the liquid to which the polyvinyl alcohol has been added. As described in the above, polyvinyl alcohol is low in cost, is neither toxic nor corrosive, and is readily soluble in water. Therefore, adding polyvinyl alcohol to a liquid forms an ideal medium for a method and apparatus for safe and reliable thermal transportation.

Claims: A thermal transportation method comprising adding an antifreeze agent to a liquid, cooling the liquid to which the antifreeze agent has been added to form an ice slurry, and pumping the ice slurry through a pipe, said method being characterised in that the antifreeze agent is polyvinyl alcohol, wherein the polyvinyl alcohol has an average molecular weight of more than 2000. A method according to claim 1, further comprising adding an ice nucleus bacterium to the liquid to which the polyvinyl alcohol has been added. An apparatus for thermal transportation comprising a liquid to which an antifreeze agent has been added, freezing means (3) for cooling the liquid to which the antifreeze agent has been added to form an ice slurry, a pipe (1) for transporting the liquid containing the ice slurry, and a pump (2) for moving the liquid in the pipe, said apparatus being characterized in that the antifreeze agent is polyvinyl alcohol, wherein the polyvinyl alcohol has an average molecular weight of more than 2000. An apparatus according to claim 3, further comprising a heat absorption means (4) for discharging heat from the ice slurry. An apparatus according to claim 3, further comprising an ice nucleus bacterium added to the liquid to which the polyvinyl alcohol has been added. Use of polyvinylalcohol as an antifreeze agent in a liquid, which liquid is cooled to form an ice slurry, and is contained in a thermal transportation apparatus, wherein polyvinylalcohol has a molecular weight of more than 2000.

Inventors: GRANDUM SVEIN, INADA TAKAAKI, LU SHU-SHEN, YABE AKIRA, GRANDUM, SVEIN, INADA, TAKAAKI, LU, SHU-SHEN, YABE, AKIRA, Inada, Takaaki Agency of Ind. Science &amp; Technology, Lu, Shu-Shen Agency of Ind. Science &amp; Technology, Yabe, Akira Agency of Ind. Science &amp; Technology
Applicants: NAT INST OF ADVANCED IND SCIEN, NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE AND TECHNOLOGY
Application year: 2000
Main classification: C09K5
IPC codes: F25C1, B65G53, C09K5, F25D17, F24F5