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A NEW ICE-MAKER HEAT PUMP
Session 4.3.
2082
A NEW ICE-MAKER HEAT PUMP
M.A. Paradis, Ph.D., Professor G. Faucher, Ph.D., Professor Dept Génie mécanique Université Laval Sainte-Foy, Qc, Canada, G1K 7P4
ABSTRACT A new type of ice-maker heat pump based on water supercooling is being developed. A prototype has been built and tested. It has been possible to operate the water-to-air heat pump with entering water temperatures of about 0,5 °C and exit liquid (supercooled) water temperatures of about -1,5 °C. The supercooled water changes to ice outside of the evaporator, in the storage reservoir. KEY WORDS Heat pump; Ice maker heat pump; water supercooling; ice making. INTRODUCTION An Ice-Maker Heat Pump (IMHP) is basically a vapor compression heat pump unit that extracts energy from water by freezing the liquid to ice. A large number of references exist on the subject and the problems associated with their use are rather well known. Indeed, the IMHP technology has not changed much during the last 30 or 40 years. The two basic types are still the coil-in-bin arrangement, either direct or indirect, and the plate-above-bin, for which periodic ice harvesting is necessary. All these systems have a relatively poor efficiency, are complex and costly and they can only be justified economically for some large commercial applications. Thus, it seems that a major breakthrough is needed to make the smaller system attractive, especially for northern climates. That breakthrough could now be in the making, however. For example, the intensive research and development performed on clathrate systems could result in major improvements to existing systems. Another alternative is the supercooling IMHP being developed by the authors. WATER SUPERCOOLING It has been observed that phase changes are often accompanied by an "instability" by which a substance tends to keep its initial phase for
Session 4.3.
2083
temperatures well beyond phase equilibrium temperatures. Water supercooling is one of those "instabilities": water stays in the liquid phase down to temperatures well below the so-called "freezing-point". Actually, it seems that instead of talking about instabilities, one should rather differentiate between two different notions: the nucleation temperature (that at which an initial ice crystal is formed within the water) and the phase equilibrium temperature of 0 °C. For absolutely pure water, the nucleation temperature seems to be below -20 °C, maybe even something like -40 °C (Dorsey,1948; Gilpin,1977). The only fool-proof method for initiating freezing of pure supercooled water (at any temperature above its natural nucleation point) is the introduction of an ice crystal in the liquid. Freezing will then begin immediately at the surface of that ice crystal and propagate quickly until enough latent energy has been liberated to increase the water temperature to 0 °C. The ice thus formed is not of the solid type normally seen. It grows instead as dendritic ice, i.e. as thin plate-like crystals interspersed in the water. Freezing can also be initiated by a number of methods: by mechanical or thermal shocks, by the introduction of nucleants (like silver iodide), etc. In ordinary static tap water, the multitude of impurities present act as nucleants and nucleation will normally occur at temperatures of the order of -5 °C (Gilpin, 1977). Other parameters will also affect the nucleation temperature: heat transfer rate, type of container, movement, etc. If freezing supercooled water is easy, preventing it from freezing is more difficult. Water is more readily supercooled (Mousson, 1858) if it is - in the form of fine droplets - squeezed between two glass plates well clamped together - in a fine capillary Thus, it would seem that everything that impedes a rearrangement of the molecules will facilitate supercooling. For the case of water at rest, supercooling has been studied by a large number of researchers, mainly in connection with water pipe freezing and with water droplets freezing in the atmosphere. For the case of water flowing in pipes, however, information is rather scarce. The study most pertinent to the present work is the one by Mukushi and Takahashi (1982) in which the nucleation temperature of tap water flowing in small diameter pipes was studied experimentally as a function of the Reynolds number and pipe diameter. The main conclusions were that supercooling is possible in capillary tubes, that the exact nucleation temperature is difficult to predict but also that it is easy to determine with some certainty the conditions for which freezing always occurs and those for which it seldom occurs. The parameter used to define those regions was L/d, the ratio of tube length to internal diameter, as shown in fig. 1. The Reynolds number is important in determining the maximum value of the supercooling AT S possible in the pipe. As shown in fig. 2, values of the nucleation temperature as low as -5 °C can be obtained (or ÄÔ 5 m a x = 5 °C) at low Reynolds numbers. The IMHP 0 200 400 600 800 1000 L/d described below is based on the „Ë . ,. ðFl. ðx fact that supercooling can be ^ - " ^ " T supercooling K y as a function of L/d.
2084
Session
4.3.
quasi-stable for water flowing in small diameter pipes. THE SUPERCOOLING IMHP The main characteristic of an IMHP based on water supercooling is its capacity to extract energy from liquid water entering the evaporator at temperatures approaching 0 °C. When coming out 5000 10000 of the evaporator, the water is still in the liquid phase Fig. 2. Maximum supercooling and at a temperature well as a function of Re. below the phase equilibrium temperature, i.e. supercooled. If this water is fed to a reservoir, the contact of the jet with any part of the reservoir (mechanical shock) will normally result in a phase change: water will separate in a liquid and a solid part, separation which will put the mixture back to 0 °C. Considering the value of the phase change energy, one can anticipate 1,25 % of solid particles per degree of supercooling. using water super Figure 3 shows schematically a solar-assisted IMHP cooling. The usual components can be seen: compressor, condenser, expansion valve and evaporator, reservoir and solar collectors. The design of the evaporator, however, is somewhat unusual: water flows through small diameter tubes before falling in the storage reservoir where the liquid-solid separation occurs. In figure 3, solar energy is used to melt the floating ice crystals. As presented in fig. 2, the results of Mukushi and Takahachi are difficult to use for designing a supercooling evaporator. All the results of fig. 2, however, can be represented with little error by the equation 1000 ÄÔ 2 ( L / d 2 ) " 0 ' 2 5 =
310 - 5 Re 0 ' 4
(1) where L and d are in meters. With it,one can easily verify if a certain choice of dimensions, based on the usual heat transfer calculations, will result in the desired value of air supercooling. cond. compr EXPERIMENTAL APPARATUS TXV An experimental rig has been constructed in order to verify if an IMHP based on water supercooling would work. After taking into account the heat pump components already available in the laboratory as well as the usual evaporator design criteria (desired capacity, mass flows, heat transfer rates, pressure losses, etc.) and the new design constraints (desired Re,L/d and
ò
(X)
evap.
=f^\
fe=—
^ slush
reservoir
Fig. 3 Solar assisted supercooling heat pump
Session 4.3. Ä Ô 5 ) , it was decided to use 20 diameter and 1,9 m effective length (1), one finds a maximum value for ÄÔ 5 more than sufficient and should leave a
2085
capillaries with a 2,93 mm internal for water circulation. Using equation of about 4 °C for such a case, which is good safety margin.
The evaporator was vertical and consisted of an exterior polycarbonate tube (38 mm I.D.) for the refrigerant flow and, inside, a bundle of 20 capillary tubes for the circulation of water. A central steel rod kept the refrigerant flow area under control and ensured a good rigidity. The water (10 to 20 L/min) entered at the top and was distributed uniformly to each capillary tube. The refrigerant entered at the bottom and the type of flow present (bubbly, slug, dry, etc) could be easily determined by looking through the transparent outer tube. The compressor was of the open type: the advantage was that the compression power could easily be varied (from 0,5 to about 1,5 hp) ; but the resulting COP was lower than with a hermetic compressor. The storage reservoir contained approximately 1000 L of water during the experiments. Temperatures were measured throughout with carefully calibrated thermistors. A more thourough description of the system has been given by Mercier (1986). PRELIMINARY RESULTS As a water-to-air heat pump, the system worked well from the beginning. But when the reservoir temperature dropped below 2 °C approximately, the evaporator froze solid. After weeks of minor modifications all aimed at eliminating mechanical and thermal shocks on the water side, it was possible to obtain water supercooling. The slush ice formed in the reservoir was very soft to the touch. Values for some of the variables in a typical test were as follows: - water temp., evap. inlet 0,56 °C evap. exit -1,48 °C - refrig. temp., evap. inlet -4,12 °C evap. exit -1,71 °C cond. exit 23,86 °C valve inlet 18,36 °C - water flowrate in evaporator 0,291 kg/s - evap. capacity 2,42 kW - cond. heat rejection 2,87 kW -global COP 2,2 At those conditions, the supercooling, and ice fabrication, was quite stable. But when the reservoir temperature dropped below 0,5 °C approximately, there was a tendency for the evaporator to freeze, and the system had to be stopped. The minimum nucleation temperature obtained during the test period was-2,12 °C. Figure 4 shows the variation of capacity of the evaporator as a function of the evaporator entrance temperatures (water and refrigerant sides) for 4 typical tests. The small temperature difference between water and refrigerant should be noticed: 4 °C approximately. In conventional systems, this difference is often more than twice as large because of the ice buildup on the evaporator walls. In fig. 5, which shows the approximate evolution of the temperatures along the evaporator, only the inlet and outlet values were obtained directly from measurements. The saturation temperature of the refrigerant was deduced from pressure measurements. The shape of the curves were deduced from calculations.
Session 4 . 3 .
2086
a / /
1
i
/D
i/ / /
-H
I
/
I
/o
/ / I / I / D
/ / //
v7
O
-·
-J->
0 10 20 30 Temperature (°C)
Fig. 4. Evaporator capacity versus water and refrigerant temperatures.
-6
H>
-4-2 0 1 Temperature (°C)
Fig. 5. Approximate temperature evolution in the evaporator for test #74.
CONCLUSION Preliminary experiments have been performed on a prototype of a supercooling IMHP and it has proved possible to operate it in a stable way within a restricted range of operation. Preliminary results are encouraging and the new heat pump concept seems promising. The next step in the development of the new IMHP will be to try increasing the range of stable operation of the system. A new version is now being constructed and should be tested within the coming year. Low temperature solar collectors of very simple construction are also being developed which will efficiently melt the ice formed by the heat pump. If it can be fully developed, this type of heat pump could have many applications other than the one envisaged at the beginning of the project. For example, water to air heat pumps operating with river, lake or ocean water could work all year long very efficiently. REFERENCES Dorsey, N.E. (1948). The freezing of supercooled water. Trans. Am. Phil. Soc. 38, 245. Gilpin, R.R. (1977). The effects of dendritic ice formation in water pipes. Int. J. Heat Transferf 20, 693-699. Pergamon Press. Mercier C. (1986). Simulation et expérimentation d'un évaporateur à glace opérant en régime continu. MSc thesisf Dept Mech. Eng., Université Laval, Québec, Canada. Mousson, A. (1858). Ann, d. Phvsik (POQQ.) 105, 161-174. Mukushi, T., Takahachi, S. (1982). Supercooling of liquids in forced flow in pipes and the growth of ice crystals. Hitachi Research Laboratoryf Hitachi Ltd, Scripta Pubi. Co.
2082
A NEW ICE-MAKER HEAT PUMP
M.A. Paradis, Ph.D., Professor G. Faucher, Ph.D., Professor Dept Génie mécanique Université Laval Sainte-Foy, Qc, Canada, G1K 7P4
ABSTRACT A new type of ice-maker heat pump based on water supercooling is being developed. A prototype has been built and tested. It has been possible to operate the water-to-air heat pump with entering water temperatures of about 0,5 °C and exit liquid (supercooled) water temperatures of about -1,5 °C. The supercooled water changes to ice outside of the evaporator, in the storage reservoir. KEY WORDS Heat pump; Ice maker heat pump; water supercooling; ice making. INTRODUCTION An Ice-Maker Heat Pump (IMHP) is basically a vapor compression heat pump unit that extracts energy from water by freezing the liquid to ice. A large number of references exist on the subject and the problems associated with their use are rather well known. Indeed, the IMHP technology has not changed much during the last 30 or 40 years. The two basic types are still the coil-in-bin arrangement, either direct or indirect, and the plate-above-bin, for which periodic ice harvesting is necessary. All these systems have a relatively poor efficiency, are complex and costly and they can only be justified economically for some large commercial applications. Thus, it seems that a major breakthrough is needed to make the smaller system attractive, especially for northern climates. That breakthrough could now be in the making, however. For example, the intensive research and development performed on clathrate systems could result in major improvements to existing systems. Another alternative is the supercooling IMHP being developed by the authors. WATER SUPERCOOLING It has been observed that phase changes are often accompanied by an "instability" by which a substance tends to keep its initial phase for
Session 4.3.
2083
temperatures well beyond phase equilibrium temperatures. Water supercooling is one of those "instabilities": water stays in the liquid phase down to temperatures well below the so-called "freezing-point". Actually, it seems that instead of talking about instabilities, one should rather differentiate between two different notions: the nucleation temperature (that at which an initial ice crystal is formed within the water) and the phase equilibrium temperature of 0 °C. For absolutely pure water, the nucleation temperature seems to be below -20 °C, maybe even something like -40 °C (Dorsey,1948; Gilpin,1977). The only fool-proof method for initiating freezing of pure supercooled water (at any temperature above its natural nucleation point) is the introduction of an ice crystal in the liquid. Freezing will then begin immediately at the surface of that ice crystal and propagate quickly until enough latent energy has been liberated to increase the water temperature to 0 °C. The ice thus formed is not of the solid type normally seen. It grows instead as dendritic ice, i.e. as thin plate-like crystals interspersed in the water. Freezing can also be initiated by a number of methods: by mechanical or thermal shocks, by the introduction of nucleants (like silver iodide), etc. In ordinary static tap water, the multitude of impurities present act as nucleants and nucleation will normally occur at temperatures of the order of -5 °C (Gilpin, 1977). Other parameters will also affect the nucleation temperature: heat transfer rate, type of container, movement, etc. If freezing supercooled water is easy, preventing it from freezing is more difficult. Water is more readily supercooled (Mousson, 1858) if it is - in the form of fine droplets - squeezed between two glass plates well clamped together - in a fine capillary Thus, it would seem that everything that impedes a rearrangement of the molecules will facilitate supercooling. For the case of water at rest, supercooling has been studied by a large number of researchers, mainly in connection with water pipe freezing and with water droplets freezing in the atmosphere. For the case of water flowing in pipes, however, information is rather scarce. The study most pertinent to the present work is the one by Mukushi and Takahashi (1982) in which the nucleation temperature of tap water flowing in small diameter pipes was studied experimentally as a function of the Reynolds number and pipe diameter. The main conclusions were that supercooling is possible in capillary tubes, that the exact nucleation temperature is difficult to predict but also that it is easy to determine with some certainty the conditions for which freezing always occurs and those for which it seldom occurs. The parameter used to define those regions was L/d, the ratio of tube length to internal diameter, as shown in fig. 1. The Reynolds number is important in determining the maximum value of the supercooling AT S possible in the pipe. As shown in fig. 2, values of the nucleation temperature as low as -5 °C can be obtained (or ÄÔ 5 m a x = 5 °C) at low Reynolds numbers. The IMHP 0 200 400 600 800 1000 L/d described below is based on the „Ë . ,. ðFl. ðx fact that supercooling can be ^ - " ^ " T supercooling K y as a function of L/d.
2084
Session
4.3.
quasi-stable for water flowing in small diameter pipes. THE SUPERCOOLING IMHP The main characteristic of an IMHP based on water supercooling is its capacity to extract energy from liquid water entering the evaporator at temperatures approaching 0 °C. When coming out 5000 10000 of the evaporator, the water is still in the liquid phase Fig. 2. Maximum supercooling and at a temperature well as a function of Re. below the phase equilibrium temperature, i.e. supercooled. If this water is fed to a reservoir, the contact of the jet with any part of the reservoir (mechanical shock) will normally result in a phase change: water will separate in a liquid and a solid part, separation which will put the mixture back to 0 °C. Considering the value of the phase change energy, one can anticipate 1,25 % of solid particles per degree of supercooling. using water super Figure 3 shows schematically a solar-assisted IMHP cooling. The usual components can be seen: compressor, condenser, expansion valve and evaporator, reservoir and solar collectors. The design of the evaporator, however, is somewhat unusual: water flows through small diameter tubes before falling in the storage reservoir where the liquid-solid separation occurs. In figure 3, solar energy is used to melt the floating ice crystals. As presented in fig. 2, the results of Mukushi and Takahachi are difficult to use for designing a supercooling evaporator. All the results of fig. 2, however, can be represented with little error by the equation 1000 ÄÔ 2 ( L / d 2 ) " 0 ' 2 5 =
310 - 5 Re 0 ' 4
(1) where L and d are in meters. With it,one can easily verify if a certain choice of dimensions, based on the usual heat transfer calculations, will result in the desired value of air supercooling. cond. compr EXPERIMENTAL APPARATUS TXV An experimental rig has been constructed in order to verify if an IMHP based on water supercooling would work. After taking into account the heat pump components already available in the laboratory as well as the usual evaporator design criteria (desired capacity, mass flows, heat transfer rates, pressure losses, etc.) and the new design constraints (desired Re,L/d and
ò
(X)
evap.
=f^\
fe=—
^ slush
reservoir
Fig. 3 Solar assisted supercooling heat pump
Session 4.3. Ä Ô 5 ) , it was decided to use 20 diameter and 1,9 m effective length (1), one finds a maximum value for ÄÔ 5 more than sufficient and should leave a
2085
capillaries with a 2,93 mm internal for water circulation. Using equation of about 4 °C for such a case, which is good safety margin.
The evaporator was vertical and consisted of an exterior polycarbonate tube (38 mm I.D.) for the refrigerant flow and, inside, a bundle of 20 capillary tubes for the circulation of water. A central steel rod kept the refrigerant flow area under control and ensured a good rigidity. The water (10 to 20 L/min) entered at the top and was distributed uniformly to each capillary tube. The refrigerant entered at the bottom and the type of flow present (bubbly, slug, dry, etc) could be easily determined by looking through the transparent outer tube. The compressor was of the open type: the advantage was that the compression power could easily be varied (from 0,5 to about 1,5 hp) ; but the resulting COP was lower than with a hermetic compressor. The storage reservoir contained approximately 1000 L of water during the experiments. Temperatures were measured throughout with carefully calibrated thermistors. A more thourough description of the system has been given by Mercier (1986). PRELIMINARY RESULTS As a water-to-air heat pump, the system worked well from the beginning. But when the reservoir temperature dropped below 2 °C approximately, the evaporator froze solid. After weeks of minor modifications all aimed at eliminating mechanical and thermal shocks on the water side, it was possible to obtain water supercooling. The slush ice formed in the reservoir was very soft to the touch. Values for some of the variables in a typical test were as follows: - water temp., evap. inlet 0,56 °C evap. exit -1,48 °C - refrig. temp., evap. inlet -4,12 °C evap. exit -1,71 °C cond. exit 23,86 °C valve inlet 18,36 °C - water flowrate in evaporator 0,291 kg/s - evap. capacity 2,42 kW - cond. heat rejection 2,87 kW -global COP 2,2 At those conditions, the supercooling, and ice fabrication, was quite stable. But when the reservoir temperature dropped below 0,5 °C approximately, there was a tendency for the evaporator to freeze, and the system had to be stopped. The minimum nucleation temperature obtained during the test period was-2,12 °C. Figure 4 shows the variation of capacity of the evaporator as a function of the evaporator entrance temperatures (water and refrigerant sides) for 4 typical tests. The small temperature difference between water and refrigerant should be noticed: 4 °C approximately. In conventional systems, this difference is often more than twice as large because of the ice buildup on the evaporator walls. In fig. 5, which shows the approximate evolution of the temperatures along the evaporator, only the inlet and outlet values were obtained directly from measurements. The saturation temperature of the refrigerant was deduced from pressure measurements. The shape of the curves were deduced from calculations.
Session 4 . 3 .
2086
a / /
1
i
/D
i/ / /
-H
I
/
I
/o
/ / I / I / D
/ / //
v7
O
-·
-J->
0 10 20 30 Temperature (°C)
Fig. 4. Evaporator capacity versus water and refrigerant temperatures.
-6
H>
-4-2 0 1 Temperature (°C)
Fig. 5. Approximate temperature evolution in the evaporator for test #74.
CONCLUSION Preliminary experiments have been performed on a prototype of a supercooling IMHP and it has proved possible to operate it in a stable way within a restricted range of operation. Preliminary results are encouraging and the new heat pump concept seems promising. The next step in the development of the new IMHP will be to try increasing the range of stable operation of the system. A new version is now being constructed and should be tested within the coming year. Low temperature solar collectors of very simple construction are also being developed which will efficiently melt the ice formed by the heat pump. If it can be fully developed, this type of heat pump could have many applications other than the one envisaged at the beginning of the project. For example, water to air heat pumps operating with river, lake or ocean water could work all year long very efficiently. REFERENCES Dorsey, N.E. (1948). The freezing of supercooled water. Trans. Am. Phil. Soc. 38, 245. Gilpin, R.R. (1977). The effects of dendritic ice formation in water pipes. Int. J. Heat Transferf 20, 693-699. Pergamon Press. Mercier C. (1986). Simulation et expérimentation d'un évaporateur à glace opérant en régime continu. MSc thesisf Dept Mech. Eng., Université Laval, Québec, Canada. Mousson, A. (1858). Ann, d. Phvsik (POQQ.) 105, 161-174. Mukushi, T., Takahachi, S. (1982). Supercooling of liquids in forced flow in pipes and the growth of ice crystals. Hitachi Research Laboratoryf Hitachi Ltd, Scripta Pubi. Co.