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Performance improvement by using dual heaters in a storage-type domestic electric water-heater
APPLIED ENERGY
Applied Energy 81 (2005) 291–305
www.elsevier.com/locate/apenergy
Performance improvement by using dual heaters in a storage-type domestic electric water-heater I. Sezai *, L.B.Y. Aldabbagh, U. Atikol, H. Hacisevki Department of Mechanical Engineering, Eastern Mediterranean University, Magusa, Mersin 10, Turkey Received 16 June 2004; revised 26 July 2004; accepted 31 July 2004 Available online 28 October 2004
Abstract In many designs of storage-type domestic electric water-heaters (EWHs) the internal heating elements are mounted at the bottom of the tank. Energy-utilization efficiencies of such EWHs are not always high since the whole tank of water is heated for even a quick shower, where the hot-water requirement is only a small fraction of the total tank capacity. In this study, performance of employing a secondary heating-element near the top part of a standard-size storage tank was experimentally investigated for energy conservation. Data were obtained for two draw-off rates of 5 and 10 L/min, and by locating a standard heating-element at three different positions; mounted vertically at the bottom and horizontally on the lateral surface 380 and 600 mm from the bottom surface. It is found that, with the heater located on the lateral surface of the storage tank, only the water above the heater can be heated while the water below the heater remains almost unaffected by the heating process. For the heater located at a height of 600 mm from the bottom, 85% of the stored energy can be utilized to supply almost 50 L of warm water, which is enough for one person to take a shower. Then, it is possible to design a tank with dual heaters, giving the users the chance of switching between the elements depending on the amount of hot-water required. This will facilitate the rational use of energy in domestic hot water preparation. Considering that the extra cost
*
Corresponding author. Fax: +90 392 365 3715. E-mail address: [email protected] (I. Sezai).
0306-2619/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2004.07.011
292
I. Sezai et al. / Applied Energy 81 (2005) 291–305
of producing an EWH with an auxiliary heating element is less than US$50, the application of dual heaters is worth considering by the manufacturers. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Electric water-heaters; Energy conservation; Thermal storage; Thermal stratification
1. Introduction Storage-type electric water-heaters (EWHs) are the choice of many homes for generating hot water due to their easy operation and maintenance. The thermal performance of storage tanks depends on the rate of mixing between the incoming cold water and the storage hot-water. For this reason, Minguez [1] proposed the use of two EWHs connected in series and having half the capacity and power rating of the single tankheater to overcome the problem of mixing during the process of charging and discharging. Kar and Kar [2], on the other hand, discovered that a dual-tank EWH of the same volume and power rating as a single-tank EWH, where the second tank has 25% of the total volume and 75% of the total power rating, provides more hot water and reduces electricity consumption. The configuration of two EWHs, however, has the disadvantage of occupying more room and looking less aesthetic than the single-tank EWHs. As an alternative option, thermal stratification can be utilized for delaying the mixing process in EHWs. Thermal stratification in storage tanks has been the subject of several studies [3,4]. The occurrence of natural stratification in such tanks causes a separation of the hot and cold water masses, without needing any physical partitioning. The phenomenon employs the mechanism of buoyancy to move and keep the low-density hot water above the higher density cold water. A region of steep temperature gradient, called the ÔthermoclineÕ, is formed between the hot and cold water masses. The intensity of mixing between the inlet cold and the hot water in storage tanks depends on the rate of degradation of the initial stratified thermocline layer during the process of charging and discharging. In the literature, it is shown that the rate of degradation is influenced by such factors as thermal losses, mixing introduced during charging/discharging processes, geometry of inlet and outlet ports and the aspect ratio of the tank [5–11]. In a recent study, Hegazy et al. [11] introduced a modified design of inlet diffuser, improving the discharging efficiencies due to better thermal-stratification inside the heater storage-tank. The proposed diffuser (a wedged pipe) was placed horizontally near the tank bottom to direct the inflow of water towards the floor of the tank. This design prevented direct vigorous mixing of charging cold water with the hot water mass. Also, thermal performance was enhanced with increasing tank aspect-ratio and decreasing draw-rate. Tully [12] investigated the influence of electrical backup element size on the performance of a solar thermosyphon domestic hot-water system (DHW). It was found that the solar DHW system will not provide adequate service without back-up and with a 1 kW element, it would be a very competitive option, in terms of a demand-
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side management program, compared to a heat-pump installation for Pretoria in South Africa. In north Cyprus, EWHs are manufactured at a 120 L standard size. Especially in winter, when solar-energy availability is low, the internal electric element, which is situated at the bottom of the tank, is used to heat the water. Although the electrical heaters are equipped with thermostats, people are accustomed to turn their heaters on just prior to their hot water needs in order to save energy. However, energy utilization efficiency is not always high since the whole tank of water is heated for even a small demand for hot water, such as washing the dishes or taking a quick shower. The energy contained in the unused hot water is eventually lost to surroundings through the insulation or to the cold water at the bottom and thus lower fraction of the storage tank can be utilized as hot water. The object of this paper is to investigate the feasibility of utilizing a secondary healing element near the top part of the standard-size storage tank mentioned here, to allow smaller volumes of hot water preparation and storage when needed. By this way it is aimed to improve the thermal performance of the domestic EWHs for the cases when demand arises for relatively small volumes of water.
2. Test apparatus A typical electric water heater (EWH), which is available in the local market in Cyprus, is used in the experiments (Fig. 1). It consists of a cylindrical tank of 121 L capacity. The tank has a height of 700 mm and an internal diameter of 470 mm, yielding an aspect ratio of 1.49. The tank is made of 2 mm galvanized-steel sheet and insulated with 35 mm thick fiberglass on all sides. The cold water enters the tank from the lateral surface, 6 cm above the bottom of the tank. The hot-water outlet port is located at the top surface. Both the inlet and outlet ports are 1/2 in standard steel coupling, flush welded to the surface. A vent pipe is located on the lateral surface 40 mm below the top in order to prevent pressure build-up in the tank. Water is heated with a 3 kW rating, immersed electric-resistance type heating element. The heating element and the thermostat, which regulates the water temperature, are manufactured as one unit, so that it can be easily fixed on flanges welded on the tank. Performance tests were carried out for three different positions of the heating element. At position A (Fig. 1), the heater is placed vertically at the bottom surface of the tank, while at positions B and C, it is mounted horizontally on the side of the tank, at heights of 380 and 600 mm, respectively. The cold water was supplied from a constant-head elevated tank to ensure steady-flow conditions. The flow rates of the cold and the hot water are the same since the tank behaves as an open system. Temperature distribution in the storage tank was measured using 35 equally spaced, K-type thermocouples. The thermocouples were fixed on a non-metallic bar, inserted vertically from a sealed opening at the top of the tank close to its axis, as shown in Fig. 1. The distance between the junctions of the thermocouples were 2 cm. Additionally, two thermocouples were placed at the inlet and outlet ports to monitor the water temperatures flowing into and out of the tank. The temperature
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Fig. 1. Cross-sectional view of the storage tank.
readings were collected by a data-acquisition system interfaced to a personal computer. Temperature data were collected at 5s intervals during each test. A calibration test showed that the accuracies of the thermocouple readings were within ±0.15 °C.
3. Test procedure The flow rate was set to the desired value by using the adjusting valve attached after the outlet valve, while keeping the inlet and outlet valves fully open during testing. The draw-off rate was measured using a calibrated tank and a stopwatch. Tests were carried out for two draw-off rates of 5 and 10 L/min for each of the three heater positions A, B, and C. The same heater is used for all tests, each time replacing a dummy heater with the test heater at one of the three positions. Before starting a new test, water in the tank is emptied. Then, the outlet valve is closed and the inlet valve opened so that the storage tank becomes full with cold water at a uniform temperature. The water is heated by switching on the electric heater until the temperature at the top section of the tank reaches 80 °C. At this point, the heater is switched off and data recording is started. At the same time, the outlet valve is opened and the discharging process starts, while charging the tank at the same rate with cold water. During the discharging/charging process, the thermocou-
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ple readings are recorded for subsequent data processing. Each test ends when the temperature of the outlet water drops down to 40 °C, which is the minimum temperature for a comfortable shower. 4. Performance parameters The initial temperature profile of the water in the tank was recorded immediately before drawing off hot water and the initial energy stored in the tank, relative to the temperature of the cold inlet water temperature Tin, is Est ¼
35 X ðqVC p Þj ðT j T in Þ;
ð1Þ
j¼1
where q, V and Cp are the density, volume and specific heat of layer j, corresponding to thermocouple j. It is assumed that the temperature Tj measured by thermocouple j prevails over the layer j. The energy contained in the water leaving the storage tank through the outlet valve up to time t is calculated from Z t Eout ¼ qVC p ðT out ðtÞ T in Þ dt; ð2Þ 0
where Tout(t) is the temperature of water measured in the pipe near the outlet valve at time t. The energy calculated in this way is relative to the inlet-water temperature. The performance of the storage tank for three cases, where the heater is installed at positions A, B, and C, is evaluated by calculating the discharging efficiency. It is defined as the fraction of the energy extracted by the time the temperature of the discharged water drops to a specified temperature. In the present study, this temperature is taken to be 40 °C. Hence, the discharging efficiency is calculated from g¼
Eout : Est
ð3Þ
The discharging efficiency, defined above, refers to the energy which can be utilized above a given water-temperature. This definition is different from that of Hegazy et al. [11] and Mavros et al. [13], who considered one tank volume of discharged water. Here the performance assessment on the basis of one tank volume of discharged water is not appropriate since the amount of hot water discharged above 40 °C is a fraction of the total storage capacity when the heater is installed at positions B or C. The maximum theoretical discharging efficiency is 100%, which corresponds to a perfectly-stratified tank in which the cold fluid pushes the hot fluid upward in a plug-flow manner without mixing and with negligible heat losses to the environment and the cold fluid. However, destruction of the stratification as a result of mixing reduces the discharging efficiency, and thus the fraction of energy that can be extracted from the storage tank by the time the discharged-water temperature falls below a specified value. Achieving a high discharging efficiency is the aim of the present design
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improvement when a small volume of water is heated by installing a secondary heater at the upper section of the storage tank.
5. Results and discussion Experiments have been performed to determine the performance of the storage tank when the electric heater is installed at three different positions, A, B or C and for two different flow rates; namely, 5 and 10 L/min. The temperature profile in the storage tank is presented in terms of the dimensionless height z/H and temperature T*, defined by T ¼
T ðz; tÞ T in ; T max T in
ð4Þ
where T(z, t) is the local temperature of the water at height z and time t and Tmax is the maximum temperature, which is also equal to the temperature of the discharged water measured at the beginning of the discharging process; that is Tmax = Tout(t = 0). The initial temperature distributions in water inside the storage tank after the heating process are shown in Fig. 2, for heater positions A, B and C. For the conventional design, where the heater is located vertically at the bottom center of the tank (position A), all the water in the tank is heated to a rather uniform temperature except the region adjacent to the tank bottom, where the temperature drops sharply. This is partly due to the inefficient heating of the bottom layers with a vertical heater and partly due to the conduction heat-loss through the metal pipe work and the supporting rods since both act as cooling fins. On the other hand, when the heater is lo-
Fig. 2. Temperature distributions in the storage tank for heater positions A, B and C prior to discharging.
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cated horizontally at the lateral wall (positions B or C), two distinct temperature regions form in the storage tank after heating. The hot and the cold water masses are separated by a rather thin layer, across which the temperature drops sharply. At the end of the heating process, the water above the heating element is heated to a rather uniform temperature while the bottom layers remain almost unaffected by the heating process. Such stratification is useful if a small volume of hot water is required since otherwise the energy stored in the lower unused part of the water would be eventually lost to the surroundings. The temperature distributions in the storage water at each 50s intervals during the discharging/charging process are shown in Figs. 3(a)–(c), separately for each of the three heater-positions and for a discharge rate of 5 L/min. For the case where the heater is positioned vertically at the bottom of the tank, the initial temperature distribution is almost uniform except in the region at the bottom of the tank where a negative temperature-gradient exists. This temperature gradient at the bottom of the hot fluid is preserved during the discharging process and forms the upper portion of the rising thermocline as observed at the high temperature end of the curves in Fig. 3(a). As a result, the thermocline region separating the cold and the hot fluids is rather thick. However, for the cases where the auxiliary heater is installed at B or C, the initial temperature gradient in the thermocline is higher due to the horizontal positioning of the heater (Figs. 3(b)–(c)). As a result, the thermocline is thinner compared with the case of the bottom-heated tank. As time passes, the thermocline region moves up and eventually the water in the thermocline starts discharging. During this time, the discharged water temperature drops since there is a temperature gradient in the thermocline region. The temperature history Tout(t) of the water leaving the storage tank is expressed in terms of a dimensionless temperature h¼
T out ðtÞ T in ; T out jt¼0 T in
ð5Þ
called the draw-off temperature and a dimensionless time, t*, defined as t ¼ t=ttotal ;
ð6Þ
where, ttotal is the total time required for discharging/charging one tank volume of water at a given flow-rate. The dimensionless time is also equal to the fraction of the storage water withdrawn from the tank. The total time can be calculated from ttotal ¼ V st =Q;
ð7Þ
where Vst is the volume of water stored in the tank and Q is the volume flow rate of water during the discharging/charging process. Fig. 4 displays the draw-off temperature variations for the three heater-positions and for flow rates of 5 and 10 L/min. The experimental curves are discontinued when the temperature at the outlet port drops below 40 °C. For the conventional design, where the heater is installed at position A, the volume of the hot water withdrawn is only slightly less than the total volume of the storage tank, since t* represents the fraction of the storage water withdrawn from the tank. For the heater at B or C, the fraction of the storage water withdrawn is
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Fig. 3. Transient temperature distributions in the storage tank for the heater (a) at position A, (b) at position B and (c) at position C, during discharging at 5 L/min.
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Fig. 4. Draw-off temperature variations for different heater positions and flow rates.
smaller. However, the rate of temperature drop during the late fast cooling periods is approximately the same for all heater-positions. When the heater is installed at position A, a large fraction of the water is discharged at nearly constant temperature, sharply dropping at the end of the discharging process. For the cases where the heater is installed at B or C, a smaller fraction is discharged at constant temperature. For the heater at positions A and B, the dimensionless time t* elapsed for the temperature of the withdrawn water to drop to 40 °C is larger for 5 L/min compared with that of 10 L/min, indicating that a higher fraction of the total stored water volume can be withdrawn at lower flow-rates. However, for the heater at C, the draw-off temperature profiles are approximately the same for the two flow rates tested. The fraction of the storage water residing above the heater and the fraction that can be discharged above 40 °C after the heating process are shown in Table 1. It is observed that, for the heater mounted on the side wall, the fraction of the storage water that can be discharged above 40 °C is approximately equal to the fraction of the water that is stored above the heater. Noting that the fraction of the water, Table 1 Fraction of the storage water heated and discharged Heater position
Fraction of the storage water above the heating element
Fraction of the storage water discharged above 40 °C 5 (L/min)
10 (L/min)
A B C
1.00 0.46 0.15
0.94 0.47 0.18
0.92 0.44 0.17
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which resides above the heater is equal to (1 z/H), then the heater position for a particular amount of hot-water requirement can be determined easily. The time history of the temperature distribution in the storage water is shown in Fig. 5 (a)–(c) at each 50s interval, for a discharge rate of 10 L/min. For the case where the heater is installed at A, the temperature profiles are distorted at the bottom of the thermocline region during the initial stages of the discharging/charging process, indicating mixing between the cold fluid at the bottom and the hot fluid at the top as a result of convective motion induced by the in-flowing water. However, these temperature oscillations die out when about half of the tank is charged with cold water, since, at this time, the thermocline region is far away from the jet induced by the inlet water. No such distortions in the temperature profile are observed when the heater is placed at positions B or C (Fig. 5 (b) and (c)), since the thermocline region is far away from the inlet water at all times. When the heater is installed at position C, the effects of the incoming water is absent, resulting in a thinner thermocline region, which is preserved throughout the discharging/charging processes as observed from Fig. 5(c). The relative performance of the storage tank, with the heater located at different heights, is presented in Fig. 6 in terms of the discharging efficiency. It is observed that the discharging efficiency decreases with increasing flow-rate regardless of the position of the heating element. For the conventional design, where the heater is located at the tank bottom, the reduction of the discharging efficiency with discharge flow rate is in line with the previous experimental studies on thermal storage-tanks [8,11,14–16]. The reduction of the discharging efficiency with an increase in the flow rate is due to increased mixing between the hot and the cold water bodies induced by the jet of the incoming cold water. The discharging efficiency drops when the heating element is installed at higher elevations, to withdraw small quantities of hot water. However, the drop of the discharging efficiency when low quantities of hot water withdrawal is acceptable. For example, using a flow rate of 5 L/min, the discharging efficiency is as high as about 85% for the extreme case where the heater is located at C. That is, installing an auxiliary heater at position C would enable a small fraction of the water to be heated and withdrawn at a relatively high discharging efficiency. However, without an auxiliary heater, the whole tank volume would have to be heated and after withdrawing a small quantity of water most of the energy would be left in the tank. Eventually, this energy would be transferred to the cold water below, so degrading the thermal stratification in the tank. In that case the discharging efficiency would be very small. The degradation of the thermal stratification inside the storage water, after withdrawing a small quantity of water (21.4 L) from a conventional storage tank with the heating element at A, is shown in Fig. 7. This amount of hot water, which is extracted at 80 °C can be used to prepare about 50 L of warm water by mixing with the cold water around 10 °C, which is enough to take a shower. This quantity is equal to the amount of water which could be drawn from the storage tank heated by the auxiliary heater positioned at C before the temperature of the extracted water falls below 40 °C. Immediately after the withdrawing process (t = 0), there is a strong stratification inside the tank where the hot and cold water bodies are separated by a
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Fig. 5. Transient temperature distributions in the storage tank for the heater (a) at position A, (b) at position B and (c) at position C, during discharging at 10 L/min.
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Fig. 6. Discharging efficiency for different heater positions and flow rates.
Fig. 7. Degradation of thermal stratification after withdrawing 21.4 L of hot water from the tank with the heater located at A.
sharp temperature gradient. At this stage, the thermocline region is very thin ( 150 mm). As time passes, the thickness of the thermocline region increases with a resultant smoothing of the temperature gradient between the hot and the cold fluids. After
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Fig. 8. The decay of thermal stratification after withdrawing 21.4 L of hot water from the tank with the heater located at A.
48h the temperature inside the storage tank becomes almost uniform, so indicating a mixed fluid in the tank. The rate of decay of thermal stratification is expressed in terms of stratification number R [17]. It is defined as the ratio of the mean of the temperature gradient at any time to that at the beginning R¼
ðoT =oZÞt ðoT =oZÞt¼0
:
ð8Þ
The mean temperature gradient is calculated from N 1 oT 1 X oT ¼ ; oZ N 1 n¼1 oZ
ð9Þ
where N equals 35, which is the number of thermojunctions used along the vertical direction in the storage tank. The decay of the thermal stratification in the storage tank, after withdrawing 21.4 L of hot water, is presented in Fig. 8. The rate of decay of stratification in the storage tank is rather high during the initial stages and falls to about 50% of its initial value after 12 h.
6. Conclusions A typical EWH, which is available in the local market in north Cyprus, was tested for three different positions of the heating element in order to investigate the
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feasibility of utilizing a secondary heater for the preparation of a small quantity of hot water. The results indicate that when the heater is located horizontally at the side wall (positions B and C), two distinct temperature regions form in the tank after heating. The hot water occupies the volume above the heater while the cold water remains almost unaffected at the bottom. A rather thin layer, across which the temperature changes sharply, separates the hot and the cold fluids. In cases where the heater is located on the side wall, it is found that the fraction of storage water, which can be discharged above 40 °C, is approximately equal to the fraction of the water residing above the heater. As a result the position of the secondary heater can be determined easily depending on the hot water requirement. The discharging efficiency of the secondary heater located on the lateral wall is above 85% for a discharge rate of 5 L/min. This value is only a little less than that of a heater located at the bottom of the tank, which is 93%.The discharging efficiency decreases only by 2.72–8.26% by doubling the flow rate regardless of the position of the heating element. The heater in position C, can supply approximately 50 L of warm water after mixing with cold mains water. According to the Turkish Standards [18], 50 L of water at 40 °C is enough for a 6 min-long one-time use in a shower. The same amount of water at 55 °C is sufficient for a 5 min-long one-time use in a kitchen sink. A new design of EWH, with dual elements, one positioned at A and the other at C, would give the EWH users the chance to switch between the elements depending on the amount of hot water required. In this way, it will be possible to save energy. Considering that the extra cost of manufacturing an EWH with a secondary heating-element is approximately US$50, the application is worthwhile for a country like north Cyprus where the electricity rates are US$0.08/kWh.
References [1] Minguez JM. Int J Energy Res 1987;11:145. [2] Kar AK, Kar U. Optimum design and selection of residential storage-type electric water-heaters for energy conservation. Energy Convers Manage 1996;37(9):1445–52. [3] Kandari MK. Thermal stratification in hot storage-tanks. Appl Energy 1990;35:299–315. [4] Al-Najem NM, Al-Marafie AM, Ezuddin KY. Analytical and experimental investigation of thermal stratification in storage tanks. Int J Energy Res 1993;17:77–88. [5] Hollands KGT, Lightstone MF. A review of low-flow, stratified tank solar water-heating systems. Solar Energy 1989;43(2):97–106. [6] Zurigat YH, Ghajar AJ, Moretti PM. Stratified thermal storage-tank inlet mixing characterization. Appl Energy 1988;30:99–111. [7] Al-Marfie A, Moustafa SM, Al-Kandarie A. Factors affecting static stratification of thermal waterstorage. Energy Source 1989;11:183–99. [8] Zurigat YH, Liche PR, Ghajar AJ. Influence of inlet geometry on mixing in thermocline thermalenergy storage. Int J Heat Mass Trans 1991;34(l):115–25. [9] Ismail AR, Leal JFB, Zanardi MA. Models of liquid-storage tanks. Energy Int J 1997;22(83): 805–15. [10] Berkel JV. Mixing in thermally-stratified energy stores. Solar Energy 1996;58(4–6):203–11. [11] Hegazy AA, Diab MR. Performance of improved design for storage-type domestic electric waterheaters. Appl Energy 2002;71(4):287–306.
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[12] Tully N. The influence of electrical back-up element size on the performance of a solar thermosyphon DHW system. Energy Int J 1995;20(3):209–17. [13] Mavros P, Belessiotis V, Haralambopoulos D. Stratified energy-storage vessels: characterization of performance and modeling of mixing behaviour. Solar Energy 1994;52(4):327–36. [14] Wildin MW, Truman CR. Performance of stratified vertical cylindrical thermal storage-tanks, part 1: scale model tank. ASHRAE Trans 1989;95(1):1086–95. [15] Abu-hamdan MG, Zurigat YH, Ghajar AJ. An experimental study of a stratified thermal store under variable inlet temperature for different inlet designs. Int J Heat Mass Transfer 1992;35(8):1927–34. [16] Eames PC, Norton B. The effect of tank geometry on thermally-stratified sensible heat storage subject to low Reynolds number flows. Int J Heat Mass Transfer 1998;41(14):2131–42. [17] Shyu RJ, Hsieh CK. Unsteady natural convection in enclosures with stratified medium. J Solar Energy Eng 1987;109:127. [18] ISISAN-series 147. Saitary plumbing systems, Istanbul, 1997 (in Turkish).
Applied Energy 81 (2005) 291–305
www.elsevier.com/locate/apenergy
Performance improvement by using dual heaters in a storage-type domestic electric water-heater I. Sezai *, L.B.Y. Aldabbagh, U. Atikol, H. Hacisevki Department of Mechanical Engineering, Eastern Mediterranean University, Magusa, Mersin 10, Turkey Received 16 June 2004; revised 26 July 2004; accepted 31 July 2004 Available online 28 October 2004
Abstract In many designs of storage-type domestic electric water-heaters (EWHs) the internal heating elements are mounted at the bottom of the tank. Energy-utilization efficiencies of such EWHs are not always high since the whole tank of water is heated for even a quick shower, where the hot-water requirement is only a small fraction of the total tank capacity. In this study, performance of employing a secondary heating-element near the top part of a standard-size storage tank was experimentally investigated for energy conservation. Data were obtained for two draw-off rates of 5 and 10 L/min, and by locating a standard heating-element at three different positions; mounted vertically at the bottom and horizontally on the lateral surface 380 and 600 mm from the bottom surface. It is found that, with the heater located on the lateral surface of the storage tank, only the water above the heater can be heated while the water below the heater remains almost unaffected by the heating process. For the heater located at a height of 600 mm from the bottom, 85% of the stored energy can be utilized to supply almost 50 L of warm water, which is enough for one person to take a shower. Then, it is possible to design a tank with dual heaters, giving the users the chance of switching between the elements depending on the amount of hot-water required. This will facilitate the rational use of energy in domestic hot water preparation. Considering that the extra cost
*
Corresponding author. Fax: +90 392 365 3715. E-mail address: [email protected] (I. Sezai).
0306-2619/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2004.07.011
292
I. Sezai et al. / Applied Energy 81 (2005) 291–305
of producing an EWH with an auxiliary heating element is less than US$50, the application of dual heaters is worth considering by the manufacturers. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Electric water-heaters; Energy conservation; Thermal storage; Thermal stratification
1. Introduction Storage-type electric water-heaters (EWHs) are the choice of many homes for generating hot water due to their easy operation and maintenance. The thermal performance of storage tanks depends on the rate of mixing between the incoming cold water and the storage hot-water. For this reason, Minguez [1] proposed the use of two EWHs connected in series and having half the capacity and power rating of the single tankheater to overcome the problem of mixing during the process of charging and discharging. Kar and Kar [2], on the other hand, discovered that a dual-tank EWH of the same volume and power rating as a single-tank EWH, where the second tank has 25% of the total volume and 75% of the total power rating, provides more hot water and reduces electricity consumption. The configuration of two EWHs, however, has the disadvantage of occupying more room and looking less aesthetic than the single-tank EWHs. As an alternative option, thermal stratification can be utilized for delaying the mixing process in EHWs. Thermal stratification in storage tanks has been the subject of several studies [3,4]. The occurrence of natural stratification in such tanks causes a separation of the hot and cold water masses, without needing any physical partitioning. The phenomenon employs the mechanism of buoyancy to move and keep the low-density hot water above the higher density cold water. A region of steep temperature gradient, called the ÔthermoclineÕ, is formed between the hot and cold water masses. The intensity of mixing between the inlet cold and the hot water in storage tanks depends on the rate of degradation of the initial stratified thermocline layer during the process of charging and discharging. In the literature, it is shown that the rate of degradation is influenced by such factors as thermal losses, mixing introduced during charging/discharging processes, geometry of inlet and outlet ports and the aspect ratio of the tank [5–11]. In a recent study, Hegazy et al. [11] introduced a modified design of inlet diffuser, improving the discharging efficiencies due to better thermal-stratification inside the heater storage-tank. The proposed diffuser (a wedged pipe) was placed horizontally near the tank bottom to direct the inflow of water towards the floor of the tank. This design prevented direct vigorous mixing of charging cold water with the hot water mass. Also, thermal performance was enhanced with increasing tank aspect-ratio and decreasing draw-rate. Tully [12] investigated the influence of electrical backup element size on the performance of a solar thermosyphon domestic hot-water system (DHW). It was found that the solar DHW system will not provide adequate service without back-up and with a 1 kW element, it would be a very competitive option, in terms of a demand-
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side management program, compared to a heat-pump installation for Pretoria in South Africa. In north Cyprus, EWHs are manufactured at a 120 L standard size. Especially in winter, when solar-energy availability is low, the internal electric element, which is situated at the bottom of the tank, is used to heat the water. Although the electrical heaters are equipped with thermostats, people are accustomed to turn their heaters on just prior to their hot water needs in order to save energy. However, energy utilization efficiency is not always high since the whole tank of water is heated for even a small demand for hot water, such as washing the dishes or taking a quick shower. The energy contained in the unused hot water is eventually lost to surroundings through the insulation or to the cold water at the bottom and thus lower fraction of the storage tank can be utilized as hot water. The object of this paper is to investigate the feasibility of utilizing a secondary healing element near the top part of the standard-size storage tank mentioned here, to allow smaller volumes of hot water preparation and storage when needed. By this way it is aimed to improve the thermal performance of the domestic EWHs for the cases when demand arises for relatively small volumes of water.
2. Test apparatus A typical electric water heater (EWH), which is available in the local market in Cyprus, is used in the experiments (Fig. 1). It consists of a cylindrical tank of 121 L capacity. The tank has a height of 700 mm and an internal diameter of 470 mm, yielding an aspect ratio of 1.49. The tank is made of 2 mm galvanized-steel sheet and insulated with 35 mm thick fiberglass on all sides. The cold water enters the tank from the lateral surface, 6 cm above the bottom of the tank. The hot-water outlet port is located at the top surface. Both the inlet and outlet ports are 1/2 in standard steel coupling, flush welded to the surface. A vent pipe is located on the lateral surface 40 mm below the top in order to prevent pressure build-up in the tank. Water is heated with a 3 kW rating, immersed electric-resistance type heating element. The heating element and the thermostat, which regulates the water temperature, are manufactured as one unit, so that it can be easily fixed on flanges welded on the tank. Performance tests were carried out for three different positions of the heating element. At position A (Fig. 1), the heater is placed vertically at the bottom surface of the tank, while at positions B and C, it is mounted horizontally on the side of the tank, at heights of 380 and 600 mm, respectively. The cold water was supplied from a constant-head elevated tank to ensure steady-flow conditions. The flow rates of the cold and the hot water are the same since the tank behaves as an open system. Temperature distribution in the storage tank was measured using 35 equally spaced, K-type thermocouples. The thermocouples were fixed on a non-metallic bar, inserted vertically from a sealed opening at the top of the tank close to its axis, as shown in Fig. 1. The distance between the junctions of the thermocouples were 2 cm. Additionally, two thermocouples were placed at the inlet and outlet ports to monitor the water temperatures flowing into and out of the tank. The temperature
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Fig. 1. Cross-sectional view of the storage tank.
readings were collected by a data-acquisition system interfaced to a personal computer. Temperature data were collected at 5s intervals during each test. A calibration test showed that the accuracies of the thermocouple readings were within ±0.15 °C.
3. Test procedure The flow rate was set to the desired value by using the adjusting valve attached after the outlet valve, while keeping the inlet and outlet valves fully open during testing. The draw-off rate was measured using a calibrated tank and a stopwatch. Tests were carried out for two draw-off rates of 5 and 10 L/min for each of the three heater positions A, B, and C. The same heater is used for all tests, each time replacing a dummy heater with the test heater at one of the three positions. Before starting a new test, water in the tank is emptied. Then, the outlet valve is closed and the inlet valve opened so that the storage tank becomes full with cold water at a uniform temperature. The water is heated by switching on the electric heater until the temperature at the top section of the tank reaches 80 °C. At this point, the heater is switched off and data recording is started. At the same time, the outlet valve is opened and the discharging process starts, while charging the tank at the same rate with cold water. During the discharging/charging process, the thermocou-
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ple readings are recorded for subsequent data processing. Each test ends when the temperature of the outlet water drops down to 40 °C, which is the minimum temperature for a comfortable shower. 4. Performance parameters The initial temperature profile of the water in the tank was recorded immediately before drawing off hot water and the initial energy stored in the tank, relative to the temperature of the cold inlet water temperature Tin, is Est ¼
35 X ðqVC p Þj ðT j T in Þ;
ð1Þ
j¼1
where q, V and Cp are the density, volume and specific heat of layer j, corresponding to thermocouple j. It is assumed that the temperature Tj measured by thermocouple j prevails over the layer j. The energy contained in the water leaving the storage tank through the outlet valve up to time t is calculated from Z t Eout ¼ qVC p ðT out ðtÞ T in Þ dt; ð2Þ 0
where Tout(t) is the temperature of water measured in the pipe near the outlet valve at time t. The energy calculated in this way is relative to the inlet-water temperature. The performance of the storage tank for three cases, where the heater is installed at positions A, B, and C, is evaluated by calculating the discharging efficiency. It is defined as the fraction of the energy extracted by the time the temperature of the discharged water drops to a specified temperature. In the present study, this temperature is taken to be 40 °C. Hence, the discharging efficiency is calculated from g¼
Eout : Est
ð3Þ
The discharging efficiency, defined above, refers to the energy which can be utilized above a given water-temperature. This definition is different from that of Hegazy et al. [11] and Mavros et al. [13], who considered one tank volume of discharged water. Here the performance assessment on the basis of one tank volume of discharged water is not appropriate since the amount of hot water discharged above 40 °C is a fraction of the total storage capacity when the heater is installed at positions B or C. The maximum theoretical discharging efficiency is 100%, which corresponds to a perfectly-stratified tank in which the cold fluid pushes the hot fluid upward in a plug-flow manner without mixing and with negligible heat losses to the environment and the cold fluid. However, destruction of the stratification as a result of mixing reduces the discharging efficiency, and thus the fraction of energy that can be extracted from the storage tank by the time the discharged-water temperature falls below a specified value. Achieving a high discharging efficiency is the aim of the present design
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improvement when a small volume of water is heated by installing a secondary heater at the upper section of the storage tank.
5. Results and discussion Experiments have been performed to determine the performance of the storage tank when the electric heater is installed at three different positions, A, B or C and for two different flow rates; namely, 5 and 10 L/min. The temperature profile in the storage tank is presented in terms of the dimensionless height z/H and temperature T*, defined by T ¼
T ðz; tÞ T in ; T max T in
ð4Þ
where T(z, t) is the local temperature of the water at height z and time t and Tmax is the maximum temperature, which is also equal to the temperature of the discharged water measured at the beginning of the discharging process; that is Tmax = Tout(t = 0). The initial temperature distributions in water inside the storage tank after the heating process are shown in Fig. 2, for heater positions A, B and C. For the conventional design, where the heater is located vertically at the bottom center of the tank (position A), all the water in the tank is heated to a rather uniform temperature except the region adjacent to the tank bottom, where the temperature drops sharply. This is partly due to the inefficient heating of the bottom layers with a vertical heater and partly due to the conduction heat-loss through the metal pipe work and the supporting rods since both act as cooling fins. On the other hand, when the heater is lo-
Fig. 2. Temperature distributions in the storage tank for heater positions A, B and C prior to discharging.
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cated horizontally at the lateral wall (positions B or C), two distinct temperature regions form in the storage tank after heating. The hot and the cold water masses are separated by a rather thin layer, across which the temperature drops sharply. At the end of the heating process, the water above the heating element is heated to a rather uniform temperature while the bottom layers remain almost unaffected by the heating process. Such stratification is useful if a small volume of hot water is required since otherwise the energy stored in the lower unused part of the water would be eventually lost to the surroundings. The temperature distributions in the storage water at each 50s intervals during the discharging/charging process are shown in Figs. 3(a)–(c), separately for each of the three heater-positions and for a discharge rate of 5 L/min. For the case where the heater is positioned vertically at the bottom of the tank, the initial temperature distribution is almost uniform except in the region at the bottom of the tank where a negative temperature-gradient exists. This temperature gradient at the bottom of the hot fluid is preserved during the discharging process and forms the upper portion of the rising thermocline as observed at the high temperature end of the curves in Fig. 3(a). As a result, the thermocline region separating the cold and the hot fluids is rather thick. However, for the cases where the auxiliary heater is installed at B or C, the initial temperature gradient in the thermocline is higher due to the horizontal positioning of the heater (Figs. 3(b)–(c)). As a result, the thermocline is thinner compared with the case of the bottom-heated tank. As time passes, the thermocline region moves up and eventually the water in the thermocline starts discharging. During this time, the discharged water temperature drops since there is a temperature gradient in the thermocline region. The temperature history Tout(t) of the water leaving the storage tank is expressed in terms of a dimensionless temperature h¼
T out ðtÞ T in ; T out jt¼0 T in
ð5Þ
called the draw-off temperature and a dimensionless time, t*, defined as t ¼ t=ttotal ;
ð6Þ
where, ttotal is the total time required for discharging/charging one tank volume of water at a given flow-rate. The dimensionless time is also equal to the fraction of the storage water withdrawn from the tank. The total time can be calculated from ttotal ¼ V st =Q;
ð7Þ
where Vst is the volume of water stored in the tank and Q is the volume flow rate of water during the discharging/charging process. Fig. 4 displays the draw-off temperature variations for the three heater-positions and for flow rates of 5 and 10 L/min. The experimental curves are discontinued when the temperature at the outlet port drops below 40 °C. For the conventional design, where the heater is installed at position A, the volume of the hot water withdrawn is only slightly less than the total volume of the storage tank, since t* represents the fraction of the storage water withdrawn from the tank. For the heater at B or C, the fraction of the storage water withdrawn is
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Fig. 3. Transient temperature distributions in the storage tank for the heater (a) at position A, (b) at position B and (c) at position C, during discharging at 5 L/min.
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Fig. 4. Draw-off temperature variations for different heater positions and flow rates.
smaller. However, the rate of temperature drop during the late fast cooling periods is approximately the same for all heater-positions. When the heater is installed at position A, a large fraction of the water is discharged at nearly constant temperature, sharply dropping at the end of the discharging process. For the cases where the heater is installed at B or C, a smaller fraction is discharged at constant temperature. For the heater at positions A and B, the dimensionless time t* elapsed for the temperature of the withdrawn water to drop to 40 °C is larger for 5 L/min compared with that of 10 L/min, indicating that a higher fraction of the total stored water volume can be withdrawn at lower flow-rates. However, for the heater at C, the draw-off temperature profiles are approximately the same for the two flow rates tested. The fraction of the storage water residing above the heater and the fraction that can be discharged above 40 °C after the heating process are shown in Table 1. It is observed that, for the heater mounted on the side wall, the fraction of the storage water that can be discharged above 40 °C is approximately equal to the fraction of the water that is stored above the heater. Noting that the fraction of the water, Table 1 Fraction of the storage water heated and discharged Heater position
Fraction of the storage water above the heating element
Fraction of the storage water discharged above 40 °C 5 (L/min)
10 (L/min)
A B C
1.00 0.46 0.15
0.94 0.47 0.18
0.92 0.44 0.17
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which resides above the heater is equal to (1 z/H), then the heater position for a particular amount of hot-water requirement can be determined easily. The time history of the temperature distribution in the storage water is shown in Fig. 5 (a)–(c) at each 50s interval, for a discharge rate of 10 L/min. For the case where the heater is installed at A, the temperature profiles are distorted at the bottom of the thermocline region during the initial stages of the discharging/charging process, indicating mixing between the cold fluid at the bottom and the hot fluid at the top as a result of convective motion induced by the in-flowing water. However, these temperature oscillations die out when about half of the tank is charged with cold water, since, at this time, the thermocline region is far away from the jet induced by the inlet water. No such distortions in the temperature profile are observed when the heater is placed at positions B or C (Fig. 5 (b) and (c)), since the thermocline region is far away from the inlet water at all times. When the heater is installed at position C, the effects of the incoming water is absent, resulting in a thinner thermocline region, which is preserved throughout the discharging/charging processes as observed from Fig. 5(c). The relative performance of the storage tank, with the heater located at different heights, is presented in Fig. 6 in terms of the discharging efficiency. It is observed that the discharging efficiency decreases with increasing flow-rate regardless of the position of the heating element. For the conventional design, where the heater is located at the tank bottom, the reduction of the discharging efficiency with discharge flow rate is in line with the previous experimental studies on thermal storage-tanks [8,11,14–16]. The reduction of the discharging efficiency with an increase in the flow rate is due to increased mixing between the hot and the cold water bodies induced by the jet of the incoming cold water. The discharging efficiency drops when the heating element is installed at higher elevations, to withdraw small quantities of hot water. However, the drop of the discharging efficiency when low quantities of hot water withdrawal is acceptable. For example, using a flow rate of 5 L/min, the discharging efficiency is as high as about 85% for the extreme case where the heater is located at C. That is, installing an auxiliary heater at position C would enable a small fraction of the water to be heated and withdrawn at a relatively high discharging efficiency. However, without an auxiliary heater, the whole tank volume would have to be heated and after withdrawing a small quantity of water most of the energy would be left in the tank. Eventually, this energy would be transferred to the cold water below, so degrading the thermal stratification in the tank. In that case the discharging efficiency would be very small. The degradation of the thermal stratification inside the storage water, after withdrawing a small quantity of water (21.4 L) from a conventional storage tank with the heating element at A, is shown in Fig. 7. This amount of hot water, which is extracted at 80 °C can be used to prepare about 50 L of warm water by mixing with the cold water around 10 °C, which is enough to take a shower. This quantity is equal to the amount of water which could be drawn from the storage tank heated by the auxiliary heater positioned at C before the temperature of the extracted water falls below 40 °C. Immediately after the withdrawing process (t = 0), there is a strong stratification inside the tank where the hot and cold water bodies are separated by a
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Fig. 5. Transient temperature distributions in the storage tank for the heater (a) at position A, (b) at position B and (c) at position C, during discharging at 10 L/min.
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Fig. 6. Discharging efficiency for different heater positions and flow rates.
Fig. 7. Degradation of thermal stratification after withdrawing 21.4 L of hot water from the tank with the heater located at A.
sharp temperature gradient. At this stage, the thermocline region is very thin ( 150 mm). As time passes, the thickness of the thermocline region increases with a resultant smoothing of the temperature gradient between the hot and the cold fluids. After
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Fig. 8. The decay of thermal stratification after withdrawing 21.4 L of hot water from the tank with the heater located at A.
48h the temperature inside the storage tank becomes almost uniform, so indicating a mixed fluid in the tank. The rate of decay of thermal stratification is expressed in terms of stratification number R [17]. It is defined as the ratio of the mean of the temperature gradient at any time to that at the beginning R¼
ðoT =oZÞt ðoT =oZÞt¼0
:
ð8Þ
The mean temperature gradient is calculated from N 1 oT 1 X oT ¼ ; oZ N 1 n¼1 oZ
ð9Þ
where N equals 35, which is the number of thermojunctions used along the vertical direction in the storage tank. The decay of the thermal stratification in the storage tank, after withdrawing 21.4 L of hot water, is presented in Fig. 8. The rate of decay of stratification in the storage tank is rather high during the initial stages and falls to about 50% of its initial value after 12 h.
6. Conclusions A typical EWH, which is available in the local market in north Cyprus, was tested for three different positions of the heating element in order to investigate the
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feasibility of utilizing a secondary heater for the preparation of a small quantity of hot water. The results indicate that when the heater is located horizontally at the side wall (positions B and C), two distinct temperature regions form in the tank after heating. The hot water occupies the volume above the heater while the cold water remains almost unaffected at the bottom. A rather thin layer, across which the temperature changes sharply, separates the hot and the cold fluids. In cases where the heater is located on the side wall, it is found that the fraction of storage water, which can be discharged above 40 °C, is approximately equal to the fraction of the water residing above the heater. As a result the position of the secondary heater can be determined easily depending on the hot water requirement. The discharging efficiency of the secondary heater located on the lateral wall is above 85% for a discharge rate of 5 L/min. This value is only a little less than that of a heater located at the bottom of the tank, which is 93%.The discharging efficiency decreases only by 2.72–8.26% by doubling the flow rate regardless of the position of the heating element. The heater in position C, can supply approximately 50 L of warm water after mixing with cold mains water. According to the Turkish Standards [18], 50 L of water at 40 °C is enough for a 6 min-long one-time use in a shower. The same amount of water at 55 °C is sufficient for a 5 min-long one-time use in a kitchen sink. A new design of EWH, with dual elements, one positioned at A and the other at C, would give the EWH users the chance to switch between the elements depending on the amount of hot water required. In this way, it will be possible to save energy. Considering that the extra cost of manufacturing an EWH with a secondary heating-element is approximately US$50, the application is worthwhile for a country like north Cyprus where the electricity rates are US$0.08/kWh.
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