TY - JOUR
AU - Aydin,, Devrim
AB - Abstract In domestic buildings, water is generally heated by an immersion type electric water heater, equipped with a thermostat as one unit, which is fitted at the bottom of the tank. Despite these systems are driven by electric energy, which is not favorable compared to direct solar water heaters, they are still widely used due to the practicality and low installation costs. In current use of electric water heaters, thermostat position and water set-point temperature are crucial and these parameters should be optimized for efficient and economic use of such systems. In this study, the impact of placing the thermostat at three different elevations; namely near the bottom, in the middle and near the top of an EWH is experimentally investigated. In addition, the effect of temperature setting of the thermostat near the bottom of the tank, on the performance of the EWH is experimentally investigated. Data were obtained for 5 L/min discharging rate of the heated water. The discharge efficiencies are found to be higher for the thermostat position at the bottom, while the discharge efficiencies for thermostat positions in the middle and near the top are very close but lower than that of the one near the bottom. 1. Introduction Many families all around the world uses electric water heater (EWH), for generating and storing hot water. The energy required may be provided from solar energy by using photovoltaic panels. For a single tank or an auxiliary storage tank (paralel or series) the electrical type of heater is the most used heating element for hot water storage systems [1]. Fernándes-Seara et al. [2] and McMenamy et al. [3] has shown that the heating efficiency of such systems have very high values over 85%. The only disadvantage of electric heating systems is high electric energy consumed to generate the hot water required. However with the advancement of sustainable on site electric generation (i.e with photovoltaic panels), electric heaters could find wider application potential in dwellings in close future for hot water production. In such systems, the mixing rate of the generated hot water and the incoming cold water will effect the thermal performance of the storage tank. Aviv et al. [4] studied the mixing of the input cold water with the hot water in a vertical tank. Researchers suggested using a horizontal baffle above the vertical incoming water jet at the bottom of the tank. They found that for very low flow rates (2–3 lt/min) of the incoming cold water, the baffle is not required, however for higher flow rates (5–7 lt/min) one buffle is found to be sufficient to have a uniform mixing and establishing the stratified temperature distribution. They found that placement of a baffle is a more efficient and economically feasible solution compared to the dual tank aproach. The thermal stratification can be considered as an alternate option for decreasing the effect of the mixing problem in EWHs and have been widely studied by several researchers [5]. When thermal stratification occurs, the cold and hot water are separated without the need of a physical partitioning requirement. It establishes itself automatically due to the variying densities of the hot and cold water during the heating process. The hot water due to its low density will move to the top and the cold water which is at higher density will move to the bottom part of the storage tank, establishing a thermal gradient. That thermal gradient is called thermocline and it keeps the hot water at an upper part of the denser cold water in a form of thin layers without any need of physical separation. The established thermocline is disturbed when the hot water is discharged from the top of the storage tank while the cold water enters form the bottom side of the storage tank. The rate of degradiation of the thermocline will be influenced by many factors such as, the rate of discharge of hot water in the tank, by the geometrical configurtion of outlet and inlet water ports also by the aspect ratio of heat storage tank. Hegazy [6] has shown that, the thermal stratification can be affected by the inlet port of incoming cold water and the aspect ratio of storage tank. They proposed a new design of inlet diffuser (wedged type) in which the incoming cold water does not interfere with the hot water. The new design has reduced the rate of mixing of hot and cold water by establishing a cold water disc partition at the bottom of the tank by directing the flow of the cold water toward the tank base. With this new design, the rate of disturbance of the thermocline was reduced and the drawn-off discharge efficiency improved. The thermal performance of the tank was also increased by increasing the aspect ratio of the tank and decreasing the discharge rate. Sezai et al. [7] investigated the effect of heater location on the thermal performance of a storage type EWH having 120 L of volume. They found that, when the heating element is horizontally placed on the side/lateral surface of the storage tank, only the part of water above the heating element could be heated, whereas the cold water below the heater does not affected by the heating process. For such large capacity EWHs, with the heating element installed vertically at the bottom part of the tank, they recommended to utilize a secondary heating element positioned horizontally at the tank lateral surface below the uppermost 50 L volume for energy conservation. When a small amount of hot water is required, they suggested switching on the secondary heating element, whereas for larger amount of hot water requirement, the heater at the bottom could be switched on. There are several studies in the literature on parametric optimization and dynamic control of hot water thermal storage systems. Rahman et al. [8] performed parametric analyses on a water thermal storage tank. In the study, impact of the vertical height and location of heating and cooling coils also the impact of water flow rate on the heat storage performance were investigated. Study results showed that, increasing the height of either hot and cold heat exchanger provides an increase in the outlet cold water temperature, although beyond a height of hx/h = 0.75, the gain in temperature was found low. Assari et al. [9] investigated the impact of the water inlet and outlet location of the fluid on thermal performance in a cylindrical storage tank. It was found that location of hot water inlet to the tank has a high impact on performance enhancement. With the increase in vertical height of heating location, better performance was obtained due to less mixing of hot and cold water. In a recent study Booysen et al. [10] studied three different thermostat control strategies using dynamic modelling. Investigated strategies were aiming to provide the desired output temperatures from the water tank with maximum possible energy savings. Study results showed that, with the demonstrated thermostat control strategies energy savings in the range of 8–18% could be obtained. Roux et al. [11] developed a dynamic control strategy for EWHs by consideration of hot water consumption patterns and different set point temperatures. It was found that the investigated demand-driven control method could provide 14% energy savings in water heating applications. Huang et al. [12] investigated the impact of thermostat position, heat loss ratio and system configuration in hot water storage tank performance. It was obtained that; for the hot water tank with low heat loss, higher thermostat location provides performance enhancement. However, for the case of high heat loss, lower thermostat location was found more suitable for stabilizing the charging process and achieving better performance. Fernández-Seara et al. [13] experimentally investigated four different control strategies for charging the hot water storage tank. Different control valves and different flow rates were used in the analysis. Study results showed that the control strategy of the domestic hot water production system significantly effects the thermal performance of the hot water storage tank. In north side of Cyprus, the EWHs are produced as 120 L standard volumetric size. The storage tanks are equipped with a back-up immersion type 3 kW electrical heater, which is fitted vertically at the bottom part of the heat storage tank. In such systems, water is heated by utilizing solar energy and in winter, electical heater is used to boost the water temperature when solar energy is not sufficient during daytime. In North Cyprus, such water heating units are widely used both in detached houses and apartments as illustrated in Fig. 1. Despite in summer there is no need for the auxiliary heaters due to the high solar radiation, in winter time hot water is mostly produced by operating electric heaters. This is due to the insufficient radiation or cloud effect in winter days. In this regard, any performance improvement on the electric water heating performance could provide significant energy and cost savings for the occupants. Figure 1 Open in new tabDownload slide View of the solar/electric water heating units in Cpyrus (Photo is taken by authors). Figure 1 Open in new tabDownload slide View of the solar/electric water heating units in Cpyrus (Photo is taken by authors). The preliminary aim of present study is to experimentally investigate the energy savings by using multiple thermostats (at the bottom, middle and top parts) in a storage tank instead of the conventional configuration where a single thermostat is fitted at the bottom of the tank. With this modification it is proposed to provide a better demand side management for hot water usage in buildings, by realizing a control of the volume of heated water thereby minimizing the heat losses. In most cases, for a small volume of hot water demand, the electrical heater heats up the entire water inside the storage tank as thermostat is located at the bottom, thereby after the consumption of the needed hot water, the rest of the energy in the remaining water is lost to the surroundings. Besides the advantage of saving energy, another merit of using multiple thermostats at different levels is reducing the heating time of water which is making the process more practical for the endusers. This could be achieved by setting the desired temperature on the thermostat located at the medium or top part (depending on the required hot water volume), thereby allowing only the water above a certain level of the tank to be heated. The second aim of this work is to study the thermal performance of the domestic EWHs at different thermostat temperatures. In order to reach this aim, thermostat is set to different charging temperatures and impact of thermostat set point temperature on heat storage efficiency was experimentally investigated. Despite thermal stratification and heat transfer enhancement methods have been widely investigated for hot water storage tanks, studies on thermostat configurations (i.e using multiple thermostats, using different thermostat set point temperatures) for controlling and optimizing the heat storage charging/discharging processes is missing in the literature. Accordingly, in present study, using multiple thermostats at different elevations is considered for advanced control of heat storage charging process. Besides, the impact of different thermostat set point temperatures were applied for determining optimal operating conditions of hot water storage tanks under different cases. It is worthy to mention that no relavant research was found in the literature, investigating the effect of thermostat position and its set point temperature on the performance of EHWs. Therfore, study outcomes could provide a significant contribution towards development of more efficient and economic hot water production and storage units especially in cold climates where solar energy is limited. 2. Materials and Methods 2.1 Test equipment Figure 2 shows the test equipment, which is mainly composed of a cylindrical storage tank having a volume of approximately 121 L. The tank is manufactured from zinc plated (galvanized) steel sheet metal by arc welding technique with an internal diameter (d) of 470 mm and a height (h) of 700 mm, and thus have an aspect ratio of 1.489. The cylindrical body of the tank is made by rolling the steel sheet metal, and the two lids for the top and the bottom are made by cutting two discs from similar material having a thickness of 2 mm. Cold water inlet port is located, 60 mm above the bottom of the tank. The diameter of the inlet and outlet ports are 0.5 inch. The cold water inlet port is placed on the lateral surface of the cylindrical body in the radial direction while the outlet port for hot water discharge is placed at the top surface of the storage tank. Any pressure increase inside the tank is prevented by attaching a expansion pipe on the latteral side, 40 mm distance from the top surface of the tank. The whole tank is insulated by covering it with 35 mm thick fiberglass wool. Finally to protect the insulation and to avoid the contamination of the users, the storage tank is covered with a 0.5 mm galvanized steel sheet metal from all sides. Figure 2 Open in new tabDownload slide (a) Cross-sectional, (b) 3D view of the storage tank. Figure 2 Open in new tabDownload slide (a) Cross-sectional, (b) 3D view of the storage tank. An immersion type screw plug heater, with 3 kW power rating is used to heat the water. The heating element and the thermostat socket are assembled on a common threaded plug as one unit. This unit is fixed on the threaded socket which is butt welded at the bottom side of the storage tank. The thermostat is utlized for controlling the water temperature inside the tank. For investigating the impact of the thermostat position on the performance of EWH, two more threaded sockets are butt-welded at the vertical cylindrical surface for mounting thermostats. The performance tests have been performed for three different thermostat positions; namely A, B, and C. At position A (z/H = 0), the thermostat is mounted in vertical position at the bottom part of the tank, where the heater is placed. For investigating the positions B and C, thermostat is placed horizontally on the lateral surface of storage tank, at heights of 380 mm (z/H = 0.54) and 600 mm (z/H = 0.86), respectively. A 1 m3 volume constant head-elevated tank is used for cold water supply to provide steady-flow conditions during the experiments. Accordingly, as the tank acts as a steady state control volume, flow rates of hot and cold water were same during the experiments. A total of 33 T-type thermocouples were used to measure the temperature distribution inside the tank. These thermocouples were equally spaced and fixed on a rectangular cross section of a plexiglass rod, which is placed vertically in axial direction through a sealed opening from the top part of the tank, as shown in Figure 2. The distance between each thermocouple is 2 cm. Another set of 6 thermocouples were attached to a horizontally placed non metallic rod, which was inserted through a side hole at a height of 380 mm above the bottom (z/H = 0.54) surface of the tank. The horizontal distance between each thermocouple is 5 cm. To monitor the charging and discharging water temperatures, three thermocouples were installed to the inlet and three to the outlet ports. A data-acquisition unit connected to a PC is used to read and record the temperature readings with a time interval of 3 minutes for the heating period (charging) and with a time interval of 5 seconds for the discharging period for all tests. Calibration tests were also performed to determine the errors and accuracies of the thermocouples. According to the testing results, accuracies of the thermocouples used for temperature readings were obtained as ±0.15 °C. 2.2 Exprimental procedure Experiments were performed for a drawn-off rate of 5 L/min. A flow control valve is used after the inlet port to adjust the flow rate to the set value. Required drawn-off rates were also adjusted by using a graduated burette and stop watch before starting the tests. Throughout the experiments, an electrical heater, which is vertically installed to the bottom part of the tank, is used to heat the water. In the first part of the experiments the heater was controlled by the thermostat positioned at points A, B and C, which was set to switch off the heater at a temperature of 80 °C. For the second part, the thermostat was positioned at A, of which the set-point temperature was changed by means of a thermostat regulator for switching off the heater at different temperatures. Before starting, the tank is emptied. Then, water is recirculated through the tank by opening both the outlet and the inlet valves. The recirculation process is continued for 10 minutes for the water to achive temperature uniformity inside the tank. Then the inlet and outlet valves are closed and by turning on the electrical heater water heating (charging) process is started. During the charging period, water temperature change is recorded with a time inteval of 3 minutes. In the system, as soon as the temperature of water at the top part of the tank rises to 80 °C, thermostat automatically turns-off the heater. Then the inlet and outlet valves are opened to start the discharge process at constant charging rate of 5 L/min. The discharge temperature is recorded at every 5 seconds in this discharging period of time. In each experiment, dicharging process ends once the dicharged water temperature falls to 40 °C. Such procedure is applied due to the reason that 40 °C is considered as the minimum comfortable temperature for having shower in residential buildings. 2.3 Performance analysis During the experiments, temperature of each thermocline layer inside the tank was recorded prior to the discharge process. This will give the total energy stored in the tank which is obtained by adding all the elemental energies calculated by consideration of the temperature difference between thermocline layer, Tj and cold inlet water temperature, Tin by the use of Equation 1; $$\begin{equation} {E}_{st}=\sum_{j=1}^{33}{\left(\rho V{C}_p\right)}_{\mathrm{j}}\left({T}_j-{T}_{in}\right) \end{equation}$$(1) where V is the volume, ρ is the density and Cp is the specific heat of layer j, which are recorded by thermocouple j. The energy content of outlet water from the storage tank at the time t, is determined as follows; $$\begin{equation} {E}_{out}=\underset{0}{\overset{\mathrm{t}}{\int }}\rho V{C}_p\left({T}_{out}(t)-{T}_{in}\right) dt \end{equation}$$(2) where Tout(t) represents the outlet water temperature at the time t. The calculated energy via Equation 2 represents the difference of the energy content of outlet water relative to the inlet water energy. The experiments on the EWH were carried out by consideration of three different cases. For each case, one of the thermostats installed either at position A, B or C was turned on. A performance investigation is also performed, by changing the thermostat setting temperature of the thermostat positioned at A. By calculating the discharging efficiency, η, the overall performance of the storage can be obtained. The discharge efficiency, η, is considered as the ratio of the extracted thermal energy from the storage tank (until the discharge water temperature falls to a certain set point temperature) to the measured total thermal energy of the water prior to dicharging. In present study, this set point temperature is considered as 40 °C. Accordingly, the discharging efficiency is determined as follows; $$\begin{equation} \eta =\frac{E_{out}}{E_{st}}. \end{equation}$$(3) This discharging efficiency explained in Eq. 3, represents the useful energy extracted from the heat storage tank and utilized for hot water needs. Theoretically the maximum discharging efficiency is 100%, which can be achieved by a well insulated tank with negligible losses to the surroundings. In such case heat loss from the hot fluid to cold fluid through mixing is also considered as negligible. However, in real applications both heat loss to the environment and heat loss to the cold fluid as a result of mixing of hot and cold fluid occurs. Consequently discharging efficiency drops, thereby energy input to the storage tank in charging cycle could be partially recovered in discharging cycle at above a certain temperature. 3. Results and discussion Testings were carried out to for investigating storage tank performance operating with an electric heater installed at the bottom. In the analysis three diffent thermostat locations were considered for controlling the electric heater namely; A (bottom), B (middle), and C (top). The volumetric flow rate of water is set to of 5 L/min in dicharging tests. Water temperature inside the storage tank is represented with the dimensionless temperature T*, which is obtained via Eq. 4 as follows; $$\begin{equation} {T}^{\ast }=\frac{T\left(z,t\right)-{T}_{in}}{T_{max}-{T}_{in}} \end{equation}$$(4) where T(z,t) illustrates the local temperature of water at a certain level of the tank and at any paticular time of t. Tmax shows the maximum water temperature inside the tank, which also corresponds to the outlet water temperature at the beginning of the discharging process. For the thermostat positions A, B, and C, distribution of water temperatures at the end of the charging (prior to dicharging) process is displayed in Figure 3. A uniform water temperature distribution inside the tank, which is highly desirable, is observed. The slight drop in water temperature near to the tank bottom surface is due to the heat losses through metal piping and the support bars of the electric heater via conduction. These metallic components act as cooling fins thereby cause heat losses which effects the storage tank performance in a negative manner. Figure 3 Open in new tabDownload slide Temperature distribution of water inside the storage tank for thermostat positions A, B, and C prior to discharging cycle. Figure 3 Open in new tabDownload slide Temperature distribution of water inside the storage tank for thermostat positions A, B, and C prior to discharging cycle. As seen from Figure 3, the case corresponding to thermostat position A has higher water temperatures than those of thermostat positon at B and C. As a result of these temperature differences, the charging/discharging time for thermostat position B and C is less than that of A. The discharge efficiencies are found to be higher for the thermostat position A, while the discharge efficiencies of thermostat locations B and C are very close but lower than that of A. The numerical values are 93.77% for thermostat location at A, and 83.65%—85.80% for thermostat locations B and C, respectively. These results indicate that thermal stratification is maintained better during the discharge process when the thermostat is at the bottom (location A) of the tank, resulting in lower heat losses to the bottom cold water due to mixing. The dimensionless temperature, |${T}_{out}(t)$|⁠, represents the variation of hot outlet water temperature from the heat storage tank and it is obtained as follows; $$\begin{equation} \theta =\frac{T_{out}(t)-{T}_{in}}{\left.{T}_{out}\right|t=0-{T}_{in}} \end{equation}$$(5) where θ is the drawn-off profile. Figure 4 Open in new tabDownload slide Temperature profiles in the storage tank for thermostats located at positions A and B during discharging. Figure 4 Open in new tabDownload slide Temperature profiles in the storage tank for thermostats located at positions A and B during discharging. The dimensionless time, t*, is the ratio of any particular charging/discharging duration to the total charging/discharging time. Consequently it could be also considered as the volumetric ratio of withdrawn water form the tank to the total volume of the tank. Dimensionless time is calculated with Eq. 6 $$\begin{equation} {t}^{\ast }=\frac{t}{t_{total}} \end{equation}$$(6) where, ttotal illustrates the charging/discharging duration of water tank for the considered flow-rate of water. The required time could be determined via Eq. 7; $$\begin{equation} {t}_{total}=\frac{V_{st}}{Q} \end{equation}$$(7) where Vst and Q are volumetric capacity of the tank and flow rate of water respectively. The variations of the drawn-off temperature for two thermostat positions are illustrated in Figure 4. The experimental results were illustrated for the conditions where temperature of outlet water from the tank is > 40 °C. For the case of thermostat A, withdrawn water volume was close to the tank volume whereas for the cases of thermostat positions B and C, the volume of the withdrawn water was nearly 50% and 25% respectively. This is due to the reason that the total volume of the heated water (during charging) decreases for the order of thermostats A,B,C, depending on their positions inside the tank. As a result, discharging durations for the thermostats A and B were nearly 25 and 22 mins respectively (See: Figure 4) whilst for thermostat C it was approximately 16 mins. In addition dimensionless temperature range gets more narrow for the order of thermostat positions A,B,C. For the same order of thermostat positions, the initial volume of hot water gets less, thereby temperature drop rate of hot water (due to mixing with incoming cold water) for thermostat positions B and C was higher compared to thermostat position A. However, over the late fast cooling period, temperature drop rates were found in close approximation for all considered thermostat positions. The temperature variations along the vertical axis of the tank during the heating processes corresponding to the cases of thermostats located at A and C are shown in Figure 5. The temperature of the water in the tank starts to increase from the inlet temperature (Tin) value to the maximum value (Tmax) during the heating proceses. The temperature rises nearly uniformly with time during the heating processes as observed from the very small vertical temperature gradients. The time required to heat the tank to a desired temperature is reduced by 18 min when the thermostat is positioned at B or C, instead of at A. Figure 5 Open in new tabDownload slide Temperature distribution in the storage tank during the heating period for different time intervals for thermostat located at positions A and C. Figure 5 Open in new tabDownload slide Temperature distribution in the storage tank during the heating period for different time intervals for thermostat located at positions A and C. Figure 6 shows the temperature distribution along the horizontal direction. The temperature distribution is rather uniform indicating intense recirculation in the tank during heating. Figure 7 shows the variation of discharge temperature with time for different temperature settings of the thermostat at position A. Five different thermostat settings were selected as temperature set point. Among all, four of the settings were in low temperature range varying between 40–55 °C and one of them was 80 °C. Here, the main purpose was to investigate the impact of set point temperature on (i) characteristics of drawn-off temperature profile (ii) water temperature distribution inside the storage tank and (iii) discharge efficiency of the water tank. The selection of set point both at low temperature range (40–55 °C) and at a high temperature (80 °C) enables analyzing the performance variation (i.e. discharge efficiency) of hot water tank with the increasing thermostat set point temperatures. Furthermore it was also proposed to determine the number of people to be able to take shower at different thermostat set point temperatures. Figure 6 Open in new tabDownload slide Temperature distribution in the heating period for horizontal thermocouples at different time intervals. Figure 6 Open in new tabDownload slide Temperature distribution in the heating period for horizontal thermocouples at different time intervals. Figure 7 Open in new tabDownload slide Drawn-off Profile in the storage tank during discharhing period for different temperature settings with thermostat located at position A. Figure 7 Open in new tabDownload slide Drawn-off Profile in the storage tank during discharhing period for different temperature settings with thermostat located at position A. As illustrated in Figure 7, a rather uniform drawn-off temperature is obtained for all the cases indicating that thermal stratification is also maintained at lower thermostat settings which prevents mixing of hot water at the top part of the tank with the incoming cold water. The vertical temperature distribution in the storage tank for different temperature settings of thermostat located at position A is shown in Figure 8. For all cases, the temperature in the storage tank is rather uniform at the end of the heating process. Figure 8 Open in new tabDownload slide The temperature distribution in the storage tank at different temperature settings for thermostat located at position A. Figure 8 Open in new tabDownload slide The temperature distribution in the storage tank at different temperature settings for thermostat located at position A. At higher thermostat settings, it is expected that more energy can be stored in the storage tank. As a result, the number of persons that can take a shower with water from the same storage tank will increase at higher thermostat settings (Table 1). Table 1 Discharge Efficiency of the EWHs for various thermostat settings Thermostat temperature and heating time . Discharging Efficiency . 1person . 2persons . 3persons . 4persons . 5persons . 40 °C(2880 sec) 0.3610 0.7208 45 °C(3600 sec) 0.2729 0.5456 0.8104 50 °C(4860 sec) 0.2146 0.4384 0.6521 0.8485 55 °C(6840 sec) 0.1853 0.3768 0.5679 0.7566 0.9391 Thermostat temperature and heating time . Discharging Efficiency . 1person . 2persons . 3persons . 4persons . 5persons . 40 °C(2880 sec) 0.3610 0.7208 45 °C(3600 sec) 0.2729 0.5456 0.8104 50 °C(4860 sec) 0.2146 0.4384 0.6521 0.8485 55 °C(6840 sec) 0.1853 0.3768 0.5679 0.7566 0.9391 Open in new tab Table 1 Discharge Efficiency of the EWHs for various thermostat settings Thermostat temperature and heating time . Discharging Efficiency . 1person . 2persons . 3persons . 4persons . 5persons . 40 °C(2880 sec) 0.3610 0.7208 45 °C(3600 sec) 0.2729 0.5456 0.8104 50 °C(4860 sec) 0.2146 0.4384 0.6521 0.8485 55 °C(6840 sec) 0.1853 0.3768 0.5679 0.7566 0.9391 Thermostat temperature and heating time . Discharging Efficiency . 1person . 2persons . 3persons . 4persons . 5persons . 40 °C(2880 sec) 0.3610 0.7208 45 °C(3600 sec) 0.2729 0.5456 0.8104 50 °C(4860 sec) 0.2146 0.4384 0.6521 0.8485 55 °C(6840 sec) 0.1853 0.3768 0.5679 0.7566 0.9391 Open in new tab On the other hand, discharge efficiency will also be affected from the thermostat setting. Figure 9 shows the discharge efficiency of the EWH for different thermostat settings together with the number of persons that can take shower using all the hot water stored in the storage tank. It is observed that the efficiency desreases at higher thermostat settings, although more people can take shower with the full charge of the storage tank. A thermostat regulator, fixed inside the house, can be used to adjust the thermostat temperature setting depending on the number of people willing to take shower. In this study, volumetric flow rate of water was set to 5 L/min in discharging tests. Therefore, the impact of water flow rate on drawn-off temperature profile and discharge efficiency has not been investigated. However, it is expected that; with the increase in discharge flow rate, the temperature of the withdrawn water will show a drop at an earlier stage for thermostat positions A and B. On the other hand, for thermostat position C, the drawn-off temperature profile is expected to be minimally dependent to the discharge water flow rate according to the study performed by Sezai et al. [7]. Furthermore, based on the same study, discharge efficiency for 5 L/min and 10 L/min flow rates were in close approximation for the thermostat position A. For thermostat positions B and C, discharge efficiency with 5 L/min discharge flow rate was nearly 8% higher compared to the efficiency with 10 L/min flow rate. Accordingly, it could be concluded that in real-life applications, at high flow rates, discharge performance of the hot water tank could show a slight decrease in the case where thermostat at locations B or C are used. For the case of using thermostat A, the discharge performance is expected to be similar for different flow rates. From the economical perspective, results showed that, depending on the amount of hot water requirement, using the optimum thermostat set point temperature could provide considerable energy and cost savings. For example, in the case that hot water is needed for 1 person to take shower, setting the thermostat to 40 °C instead of 55 °C could increase the efficiency from 18% to 36%. As a result, yearly economic savings between 100$-150$ could be achieved. Similarly, if hot water for two people is required for taking shower, again selecting 40 °C as set point temperature could increase the efficiency in the range of 36 → 72% and same amount of yearly cost savings could be achieved. If the number of people to take shower is 3, selecting 45 °C (optimum set temperature for 3 people) instead of 55 °C could result in 80$-120$ annual cost savings. For 4 people or more, the thermostat set point temperature should always be > 50 °C and in such case efficiency of heat storage is > 75%. Therefore achievable cost savings is very limited. It is worth to mention that the proposed method does not require any additional costs as the conventional domestic EWHs are already equipped with thermostat. Therefore, based on the obtained results, optimizing thermostat set point temperature could be a potential method to provide energy and cost savings in building applications. According to the ‘water supply regulations’ [14] minimum hot water supply temperature to shower should be 41 °C, which is in accordance with the selected minimum thermostat set point temperature in this study. However in real applications, in order to prevent the production and growth of legionella bacteria, water is heated to a minimum temperature of 60 °C. When lower thermostat set point temperatures are selected, as investigated in this study for achieving cost savings, other water treatment methods such as copper and silver ionisation or biocide treatments [15,16] should be considered. 4. Conclusions A typical EWH, which is available in the local market in north Cyprus, is used to find out the effect of the thermostat location on the performance. Three different thermostat positions namely at heights A (z/H = 0), B (z/H = 0.54), and C (z/H = 0.86) were used. In addition, the effect of the temperature setting of the thermostat at position A on the performance is investigated. The results show that the tank, with thermostat position at A, have a higher final water temperature than those of thermostat positon at B and C. For the case where the thermostats are placed at B and C, the fraction of the storage water withdrawn is smaller than that of the case corresponding to that of thermostat at A. The time required to heat the tank to a desired temperature is reduced by 18 min when the thermostat is positioned at B or C compared with that of A. Figure 9 Open in new tabDownload slide Discharge Efficiency of the EWHs for different thermostat settings. Figure 9 Open in new tabDownload slide Discharge Efficiency of the EWHs for different thermostat settings. The discharge efficiencies are found 93.77% for thermostat location at A, and 83.65%—85.80% for thermostat locations at B and C, respectively. These results indicate that thermal stratification is maintained better during the discharging process when the thermostat is at the bottom (location A) of the tank, resulting in lower heat losses to the bottom cold water due to mixing. It is found that, for the same set point temperature for all thermostats, highest average temperature, also the highest amount of thermal energy storage inside the tank is achieved with thermostat A. With thermostats B and C, the amount of stored energy is nearly 15% and 20% less when compared with thermostat A. Results also showed that; increasing thermostat set point temperature has a negative impact on discharge efficiency and depending on the number of people to take shower, optimal set point temperature should be selected. For the increase of the set point temperature between 40 → 55 °C, efficiency becomes half. For 1–2 persons, optimum set point temperature was found as 40 °C whereas for 3,4 and 5 persons, it was determined as 45, 50 and 55 °C respectively. In terms of cost savings, up to 2 people, setting the thermostat to 40 °C instead of 55 °C could provide 100–150$ cost savings per year. For 3 people thermostat could be set to minimum 45 °C (instead of 55 °C) which could also provide 80–120$ yearly savings. If the number of people to take shower is 4 or more, cost savings potential is limited. According to the study results, in domestic applications using thermostat close to the bottom of the water tank (Thermostat A), improves the thermal stratification and provides higher efficiencies (Δη = 8%). On the other hand, depending on the hot water demand, selecting optimum set point temperature enables considerable amount of annual energy (400–600 kWh) and cost savings (80–150$). 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TI - Effect of thermostat position and its set-point temperature on the performance of a domestic electric water heater
JF - International Journal of Low-Carbon Technologies
DO - 10.1093/ijlct/ctaa007
DA - 2020-08-19
UR - https://www.deepdyve.com/lp/oxford-university-press/effect-of-thermostat-position-and-its-set-point-temperature-on-the-kYP4tWEKXh
SP - 373
EP - 381
VL - 15
IS - 3
DP - DeepDyve
ER -