Heat Transfer Engineering, 21:67–73, 2000 Copyright ° C 2000 Taylor & Francis 0145–7632/00 $12.00 + .00
The Influence of Return Loop Flow Rate on Stratification in a Vertical Hot Water Storage Tank Connected to a Heat Pump Water Heater ¨ J. P. MEYER, P. J. A. RAUBENHEIMER, and E. KRUGER Department of Mechanical and Manufacturing Engineering, Rand Afrikaans University, Auckland Park, South Africa
A temperature-controlled hot water heat pump was simulated using heating in a vertical, domestic hot water storage tank. The in uence of the return loop ow rate on strati cation was investigated experimentally. The return loop is the water line that supplies a long line of consumers with hot water, and returns colder water to the middle of the hot water storage tank. The return temperature is a function of the length of the loop, insulation, and ambient conditions. Temperatures were measured as a function of time at different vertical locations on the centerline of the storage tank. The temperature distributions in the tank were compared for different return ow rates. A return ow rate of three tank volumes per day was identi ed as preferable, although good results were also obtained for less than three tank volumes per day.
Thermal energy storage is an integral part of domestic hot water storage tanks, solar and heat pump systems. Thermal energy storage devices are usually built out of a tank in which mixing occurs. The storage of water in tanks can be optimized by separating the cold water from the hot water. By increasing the temperature gradient in the tank, the strati cation (or reduced mixing)
is also increased. Different methods exist by which the cold and hot water can be separated from each other, for example, using multiple tanks, a single tank with baf es, or a single tank where the buoyant forces separate the water with different temperatures. In a single tank, cold water is usually drawn from the bottom, circulated through the heating system, and returned to the storage tank without inducing mixing of the uid layers with different temperatures. The heating system used is usually an electric element, a solar collector, or a heat pump condenser. There are a number of ways in which loss of strati cation can take place [1], for example, conduction between hot and cold uid layers, natural or forced convective mixing. With the use of only one tank the
This project was a nal-year project at the Department of Mechanical and Manufacturing Engineering at the Rand Afrikaans University and was supported nancially by the university, Eskom, and the Foundation for Research Development. Address correspondence to Prof. J. P. Meyer, Research Group for Cooling and Heating Technology, Department of Mechanical and Manufacturing Engineering, Laboratory for Energy, Rand Afrikaans University, PO Box 524, Auckland Park, 2006, South Africa. E-mail:
[email protected]
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conduction between hot and cold uid layers is dif cult to prevent. Mixing in the tank can, however, be prevented to maintain the optimum thermal gradient and strati cation. There are a few factors that have an in uence on the mixing that occurs in a storage tank, for example, the design of the tank, the operating conditions, the temperature and ow rate of the returning water into the tank, and the temperature distribution in the tank. When the ow rate of the entering stream into the tank is high, forced-convection mixing occurs because of the momentum of the uid. When the ow rate of the returning water is low and also has a lower temperature than the surrounding uid, natural-convection mixing occurs. The factors that in uence the natural convection are the temperature of the incoming uid, the position of the inlet, and the temperature distribution within the tank. Natural convection exists not only between uid layers with different temperatures, but also between the wall of the tank and water on the inside. The water closest to the tank walls is heated and cooled faster than water deeper inside the tank. This temperature gradient on the horizontal plane causes a natural-convection circulation. By reducing the thermal conductivity of the tank wall to that of the uid, the mixing caused by the wall conduction can be reduced [2, 3]. Quite a lot of research has been done on strati cation in storage tanks where the water is heated through solar panels [4–7]. Proposed methods of increased strati cation (or reduced mixing) in storage tanks include the placement of manifolds in the tanks, where the orlon manifold produces the best results [6, 13–15]. Other methods for increased strati cation are variable volume storage [7], the constant-temperature method [11, 12], the use of momentum diffusers to reduce mixing owing to jet entrainment [1], and baf e plates. The method which holds the most promise, however, is the single-pass strategy. Using this method the return ow rate is low enough to circulate only one tank volume per day. With such a low ow rate, mixing is restricted to a minimum, but it was found that with such low ow rates thermal losses and reduced energy transfer rates are increased in the return loop as a result of the loss of water by consumers who need hotter water [8–10]. The theoretical advantages of storage tanks with manifolds, are that they provide tank strati cation without any other modi cation in system design or operation. Strati cation manifolds also reduce the vertical momentum component of entering water and thus allow buoyancy forces to direct the collector uid to the location in the tank where the temperatures of the two uids are equal. There are different manifolds available to enhance strati cation, for example, fabric manifolds, conventional drop-tube inlets, and rigid porous manifolds [16]. 68
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Figure 1 Schematic representation of a hot water storage tank with a return loop heated with a temperature-controlled heat pump water heater.
The heating loop through the heating system and the supply loop to the consumers are the two uid ow loops circulated through a storage tank. The supply loop regulates the water supply to the consumer, while the heating loop controls the heat-collecting performance of the system. The in uence of these two loops in a storage tank where mixing has to be restricted to a minimum has been researched under different conditions [1–16]. Figure 1 shows a supply loop extended to a return loop, something that has only recently been considered [17, 18]. This return loop is often used in hot water heat pump installations, where a temperature-controlled heat pump is used for heating the water. In the heating loop the water is drawn from the bottom of the tank, heated to 65±C, and released in the top of the tank. In the heat pump condenser, the product of the mass ow through the heat pump and the temperature rise is a constant by controlling the water ow rate as well as the maximum condensing temperature. Therefore, if the temperature difference over the condenser is very large, it has a very slow mass ow. However, as the temperature difference decreases the mass ow increases. Water is constantly circulated through the return loop from the top of the tank past all the possible consumers and back to the middle of the tank. This type of con guration is typically used in hotels, motels, and dormitories where there are a large number of consumers that might be far, in pipe distance, from the storage tank. If a return loop is not incorporated into the storage system, the consumers close to the end of the water line have to wait longer for hot water, since the heat losses cause a drop in hot water temperature. This happens especially late at night or early in the morning, when water was not drawn for a relative long period of time and the water in the pipe had time to drop in temperature. The biggest advantage of a return loop is that it ensures that the last vol. 21 no. 2 2000
consumer on the water line has hot water available almost immediately. Although the return loop is usually insulated, it still experiences a drop in original collected temperature as a result of heat losses to the atmosphere. For this reason the return loop inlet is not placed at the top of the tank where the water is drawn, but farther down. In applications the inlet is usually placed halfway between the feed water inlet and the hot water outlet. This is also the con guration that was used in previous work [17], except that only one ow rate of 20 liters/h on a 150-liter storage tank was tested. Furthermore, the water of the return loop was unheated to simulate the worst possible condition. In practice, the rule of thumb is that the return ow rate should be as small as possible. In practice these ow rates are up to 18 tank volumes per day when hot water heat pumps are used for water heating. The purpose of this article is to investigate the in uence of the return loop ow rate on strati cation in a vertical hot water storage tank connected to a hot water heat pump. The question can now be asked, if a minimum suggested ow rate of one tank volume per day is suggested in the literature [8–10], why is this study needed? The answer is that the ow eld in the storage tank is more complicated. Although the ow rate in the return loop is kept constant, the ow rate through the heating section varies from very low when heating starts and accelerates to a high value before heating is stopped. The outline of this article is as follows: in the next section the experimental apparatus and procedure are discussed. It consists of the facility used and the testing procedure that was followed. The generality of the experimental method is veri ed, the results are given and discussed, whereafter the article is concluded. EXPERIMENTAL APPARATUS AND PROCEDURE All tests were completed using the same experimental setup (Figure 2). It consisted of a nominal 100-liter hot water storage tank, a heating loop, and a return loop. In the heating loop, water was extracted from the bottom of the tank by a pump and heated with an electric resistance element of 2 kW before owing through a ow meter and back to the top of the tank. The ow rate in the heating loop was controlled manually to ensure an outlet temperature of 65±C with an error of less than § 5±C for more than 90–95% of the time. This large error occurred when the experiment was started and occurred when the water ow rate through the heating loop was very low and therefore dif cult to control. After a few minutes it was possible to reduce the error to §1±C. The heating loop simulates a temperature-controlled, hot water heat pump [17]. heat transfer engineering
Figure 2
Schematic representation of experimental setup.
In the return loop, the hot water was extracted from the top of the tank and pumped through a water-cooled counter ow tube-in-tube heat exchanger where the temperature was reduced to 40±C (which is the temperature of the return water). The colder return water was fed back halfway between the bottom and top of the storage tank. The ow rate of the return loop was also controlled manually. The return loop simulated a long line of consumers which caused a temperature drop. The measured capacity of the hot water storage tank was 101.2 liters with an internal diameter of 382 mm and a height (H ) of 925 mm on the vertical centerline. The top and bottom of the storage tank were conical in shape. Water was extracted 143 mm (y / H = 0.15) from the bottom of the tank for heating and returned 170 mm (y / H = 0.82) from the top of the tank. No special jets or diffusers were used where the return loop enters the storage tank. The wall thickness was 1.3 mm, and 70-mm glass wool was used for insulation. More detailed dimensions on the hot water storage tank are given by Raubenheimer [19]. The ow meters were variable-area meters with an error of § 3% on full scale. Two data loggers were used for measuring temperatures. They were set to record temperatures every minute and were connected to K-type thermocouples which were calibrated to an accuracy of § 2±C. The rst thermocouple was placed 60 mm from the bottom of the storage tank and another nine at intervals of 75, 80, 80, 85, 80, 80, 75, 75, and 70 mm on the centerline of the hot water storage tank. The thermocouples were kept in position with a hollow ceramic rod. All the thermocouple wires were positioned axially through the ceramic rod while the front measuring part was exposed to the water outside the ceramic rod at a distance of 5 mm from the rod, by drilling a small, 3-mm hole tangentially to the rod at the correct vertical distance. Ceramic was used to prevent conduction of heat from the hot water at the top to the colder water at the bottom. vol. 21 no. 2 2000
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Four additional thermocouples measured the temperatures before and after the heating element in the heating loop and in the return loop before the heat exchanger and at the entrance of the return loop (16 mm ID) just before the hot water storage tank. These four thermocouples were connected to a Fluke microprocessor-based digital thermometer which was, together with the readout device, calibrated to an accuracy of § 0.3±C. At the beginning of each experiment the tank was lled with cool water of approximately 17±C, and heated while being mixed to ensure an unstrati ed tank at 20±C before starting any tests. The test was completed when the temperature at the inlet of the heating loop rose to such a value that the outlet temperature could no longer be controlled at 65±C. The following return ow rates were considered: 0, 2, 3, 4, 5, and 6 tank volumes per day; they correspond to ow rates of 0, 8.4, 12.7, 16.9, 21.1, and 25.3 liters/h. Results were presented in a nondimensional form. The nondimensional temperature is given as T (t) ¡ Ti (Tav ¡ Ti )
Td =
(1)
and the nondimensional time as td =
t tt
(2)
The target time was the time needed to heat the water in the storage tank from the initial water temperature at the start of the test to the average hot water storage temperature. The average temperature was taken as 62.5±C, which was the average between the maximum temperature of 65±C at the top and approximately 60±C at the bottom of the storage tank. The target time to reach a fully heated tank was calculated from the following equation, where the heat losses from the storage tank to the environment were assumed negligible and the heat losses from the return loop were subtracted: Q¡
Qr =
M C p (Tav ¡ tt
Ti )
(3)
Figure 3 Effective heating capacity (Q ¡ return ow rate.
Q r ) as a function of
When the tests were started the temperature at the outlet of the return loop was 20±C; but it increased quickly to 40±C (Tr ) as soon as hot water was available at the top of the tank, which took approximately half a minute. This represents less than 1% of the nondimensional time. RESULTS The measurements of the temperature as function of time are given in Figures 4–9 for return loop ow rates of 0, 2, 3, 4, 5, and 6 tank volumes per day. For no ow through the return loop (Figure 4), the hot water rises to the top of the storage tank, which causes a temperature gradient with the colder water at the bottom of the tank. These results are as expected [17] and show that once heating starts at a speci c location (e.g., y / H = 0.5) in the storage tank, the temperature increases almost linearly as function of time. This occurs until the temperature gets close to the target temperature before it starts to decline monotonically. Furthermore, at the lowest measuring point (y / H = 0.06), heating is extremely low, since this point is lower than the inlet to the heating loop. Conduction is therefore the only mechanism of heat transfer, since the convection heat transfer is negligible. When a return loop is used at different ow rates (Figures 5–9) of between 1 and 6 tank volumes per
where Q r = m r C p (Tav ¡
Tr )
(4)
Although the storage tank could be charged more quickly if the return ow is shut off, this is not done in practice as no water would be available to consumers. The return ow extracts heat from the storage tank which corresponds to effective heating capacities (Q ¡ Q r ) shown in Figure 3. 70
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Figure 4 Temperature as a function of time for different vertical positions in the storage tank without any ow through the return loop.
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Figure 5 Temperature as a function of time with a return ow rate of 2 tank volumes per day.
Figure 6 Temperature as a function of time with a return ow rate of 3 tank volumes per day.
Figure 7 Temperature as a function of time with a return ow rate of 4 tank volumes per day.
Figure 8 Temperature as a function of time with a return ow rate of 5 tank volumes per day.
Figure 9 Temperature as a function of time with a return ow rate of 6 tank volumes per day.
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Figure 10 Time taken to reach a temperature of 40±C as a function of ow rate in the return loop for different vertical positions in the storage tanks.
day (all with the same return temperature of 40±C), the strati cation in the storage tank is in uenced, as could be expected. A discontinuity occurs in the temperature gradient at approximately the same vertical height to the storage tank as the entrance of the return loop. This is observed best in Figure 9. It appears as if the return ow separates the storage tank into upper and lower halves (Figures 7–9). In the upper half the temperature gradients are much higher than in the lower half. However, this effect is not prominent at low ow rates through the return loop (Figures 5 and 6). In Figures 10–12 the times are given before temperatures of 40±C, 50±C, and 60±C, respectively, are reached in the storage tank for the different measuring points as function of return loop ow rates. The general tendency of all three graphs is the same. It shows that if the ow rate of the return loop is 3 tank volumes per day, higher temperatures are reached at the top of the storage tank sooner than for any other return ow rate. This ow rate increases the temperature gradient (Figure 6) in the top third of the tank (y / H = 0.66, y / H = 0.75, and y / H = 0.82) more than any other return loop ow rate, while it is about the same for the storage tank without any return loop (Figure 4). It can therefore be concluded that if a return loop is used, the recommended ow rate through the return loop should be 3 tank volumes per day. However, the advantages are marginal at ow rates less than 3 tank volumes per day, while ow rates higher than 3 are
Figure 11 Time taken to reach a temperature of 50±C as a function of ow rate in the return loop for different vertical positions in the storage tanks.
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rate is higher than 5 tank volumes per day, a discontinuity occurs in the vertical temperature gradient. The discontinuity is caused by the high return ow rate. NOMENCLATURE
±
Figure 12 Time taken to reach a temperature of 60 C as a function of ow rate in the return loop for different vertical positions in the storage tanks.
detrimental to the time needed to reach high temperatures quickly at the top of the storage tank. GENERALITY OF RESULTS To ensure that the results are general and could be used for larger hot water storage tanks, measurements were also taken in a vertical 1,000-liter hot water storage tank with the same height-to-diameter ratio as the storage tank used in this study. The setup was the same as in Figure 2 except that only three temperatures were measured on the centerline, at y / H values of 0.33, 0.5, and 0.67. A return temperature of 40±C was again used. Similar return ow rates to those in the smaller storage tank were considered, namely, 0, 2, 3, 4, 5, and 6 tank volumes per day, which correspond to 0, 83, 125, 167, 208, and 250 liters/h. When the results were compared nondimensionally to the smaller tank, the results agreed very well with the 100-liter tank. The same was found when the results of this study with a 100-liter storage tank were compared to that of a 150-liter storage tank, considered previously by Wood and Meyer [17]. It can therefore be concluded that the results are general and are applicable to larger storage tanks. SUMMARY AND CONCLUSIONS Six different return loop ow rates of a vertical hot water storage tank were investigated. Temperatures were measured as a function of time on an experimentally simulated heat pump water heating system. The purpose was to determine the in uence of the ow rate in the return loop on strati cation. The position of the return loop was kept constant, halfway between the top and bottom of the storage tank. It was found that if a return loop is used, the recommended ow rate is 3 tank volumes per day. This ensures that higher temperatures are reached at the top of the storage tank sooner than with any other return ow rate. The advantages between zero and 3 tank volumes are, however, marginal. Furthermore, if the ow 72
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Cp H mr M Q Qr t td tt T Tav Td Ti Tr y
speci c heating capacity of water, J/kg K height of storage tank from bottom to top along the centerline, m ow rate in return loop, kg/s mass of water in storage tank, kg heating capacity (2,058 W) of heat pump or electrical resistance element, W heat transfer from return loop, W time, s nondimensional time target time, s temperature, ±C average temperature in hot water storage tank after heating, ±C nondimensional temperature initial uniform tank water temperature at start of test, ±C temperature of water at outlet of return loop, ±C axis along storage tank centerline with origin at bottom of tank, m
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to Characterize Mixing in Solar Water Storage Tanks, ASME J. Solar Energy Eng., vol. 116, pp. 94–99, 1994. Jularia, Y., and Gupta, S. K., Decay of Thermal Strati cation in a Water Body for Solar Energy Storage, Solar Energy, vol. 28, pp. 137–143, 1982. Shyu, R. J., and Hsieh, C. K., Unsteady Natural Convection in Enclosure with Strati ed Medium, ASME J. Solar Energy Eng., vol. 109, pp. 127–133, 1987. Lavan, Z., and Thompson, J., Experimental Study of Thermally Strati ed Hot Water Storage Tanks, Solar Energy, vol. 19, pp. 519–524, 1977. Sharp, M. K., and Loehrke, R. I., Strati ed Thermal Storage in Residential Solar Energy Applications, J. Energy, vol. 3, no. 2, pp. 106–113, 1979. Loehrke, R. I., Holzer, J. C., Gari, H. N., and Sharp, M. K., Strati cation Enhancement in Liquid Thermal Storage Tanks, J. Energy, vol. 3, no. 3, pp. 129–130, 1979. Wuestling, M. D., Klein, S. A., and Duf e, J. A., Promising Control Alternatives for Solar Water Heating Systems, ASME J. Solar Energy Eng., vol. 107, pp. 215–221, 1985. Van Koppen, C. W. J., Low Flow or Single Pass, the Heart of the Matter, 1991 Solar World Congr. Proc. Biennial Congr. Int. Solar Energy Society, vol. 2, no. 2, pp. 1363–1368, 1991. Esbensen, T., Low-Flow Solar Hot Water System, 1991 Solar World Congr. Proc. Biennial Congr. Int. Solar Energy Society, 1991, vol. 2, no. 2, pp. 1369–1371, 1991.
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[10] Furbo, S., Low-Flow Solar Heating Systems—Theory and
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Practice, 1991 Solar World Congr. Proc. Biennial Congr. Int. Solar Energy Society, 1991, vol. 2, no. 2, pp. 1374–1379, 1991. Sliwinski, B. J., Mech, A. R., and Shih, T. S., Strati cation in Thermal Storage during Charging, Sixth Int. Heat Transfer Conf., Toronto, vol. 4, pp. 149–154, 1978. Smith, C., Lof, G. O. G., and Abbud, I., Solar Space Heating with Isothermal Collector Control, Proc. Second ASME-JSESJSME Int. Conf., Reno, NV, pp. 1–5, 1991. Gari, H. N., and Loehrke, R. I., A Controlled Buoyant Jet for Enhancing Strati cation in a Liquid Storage Tank, ASME J. Fluids Eng., vol. 104, pp. 475–481, 1982. Fanny, A. H., and Klein, S. A., Thermal Performance Comparisons for Solar Water Systems Subjected to Various Collector and Heat Exchanger Flowrates, Solar Energy, vol. 40, no. 1, pp. 1–11, 1988. Davidson, J. H., Carlson, W. T., and Duff, W. S., Impact of Component Selection and Operation on Thermal Ratings of Drain-Back Solar Water Heaters, ASME J. Solar Energy Eng., vol. 114, pp. 209–216, 1992. Davidson, J. H., and Adams, D. A., Fabric Strati cation Manifolds for Solar Water Heating, ASME J. Solar Energy Eng., vol. 116, pp. 130–136, 1994. Wood, C. W., and Meyer, J. P., Unsteady Temperature Distributions in Vertical Storage Tanks Heated with Heat Pumps, Heat Transfer Eng., vol. 19, no. 3, pp. 43–52, 1998. Davis, E., and Bartera, R., Strati cation in Solar Water Heater Storage Tanks, Proc. Workshop on Solar Storage Subsystems for the Heating and Cooling of Buildings, Charlottesville, VA, April 16–18, pp. 38–42, 1975. Raubenheimer, P. J. A., The In uence of the Return Loop Flowrate on Strati cation in Vertical Hot Water Storage Tank Connected to a Heat Pump, Final-year project, Rand Afrikaans University, Johannesburg, 1997.
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Josua Petrus Meyer was born in Pretoria, South Africa, and obtained the B.Eng. (Mech.) degree with distinction at the University of Pretoria in 1984. He was employed as an Associate at the Laboratory for Advanced Engineering from January 1984 to January 1988, during which time he obtained the M.Eng. degree (with distinction) in 1986 and the Ph.D. degree in 1988. Thereafter he completed two years of National Military Service at the Military Academy whereafter he was appointed in 1990, rst as an associate professor and later in 1991 as Professor at the Potchefstroom University. He joined the Rand Afrikaans University in 1994 and is at present a Professor and the Chairperson of the Department of Mechanical and Manufacturing Engineering. Pieter Jacobus Adriaan Raubenheimer was born in Johannesburg, South Africa, and matriculated from Linden High School in 1993. He enrolled at the Rand Afrikaans University in 1994. The work described in this article formed part of his nal-year project in 1997 and contributed toward his degree in the Department of Mechanical and Manufacturing Engineering. He is currently working for SASOL, and enrolled for his Master’s Degree in Engineering Management. Eu’odia Kr¨u ger was born in Phalaborwa, South Africa, and matriculated from Noordheuwel High School in 1994. She enrolled atthe Rand Afrikaans University in 1995. The work described in this article formed part of her nal-year project in 1998 and contributed toward her degree in the Department of Mechanical and Manufacturing Engineering. She joined the Research Group for Cooling and Heating Technology in 1999, where she is currently enrolled for her Master’s Degree in Engineering under the supervision of Prof. J. P. Meyer.
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