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Static and dynamic thermocline evolution in the water thermocline storage tank
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Energy Procedia 158 Energy Procedia 00(2019) (2017)4471–4476 000–000 www.elsevier.com/locate/procedia
10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Static and dynamic thermocline evolution in the water thermocline The 15th International Symposium on District and Cooling Static and dynamic thermocline evolution inHeating the water thermocline storage tank storage tank Assessing the of using the heat demand-outdoor a feasibility a Zhaoyu Hea, Yijie Qian a, Chao Xuaa, Lijun Yangaa, Xiaoze Dub,a * Zhaoyufunction He , Yijiefor Qiana ,long-term Chao Xu , Lijun Yangheat , Xiaoze Dub,a* forecast temperature district demand Key Laboratory of Condition Monitoring and Control for Power Plant Equipment (North China Electric Power University), Ministry of a
Education, Beijing China Key Laboratory of Condition Monitoring and Control for Power Plant102206, Equipment (North China Electric Power University), Ministry of a,b,c a a b c c b School of Energy and Power Engineering, Lanzhou of Technology, Lanzhou 730050, China Education, BeijingUniversity 102206, China b School of Energy and Power Engineering, Lanzhou University of Technology, Lanzhou 730050, China a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Abstract Since the power load regulation capability of the combined heat and power (CHP) generation plant is weaker than that of the Since the power loadpower regulation of significance the combinedto heat and power (CHP) generation plant is weaker than that the conventional thermal plant,capability it is of great decouple the strong relation between electricity generation andofheat Abstract conventional thermalheat power plant,performance it is of great significance decouple the strong relation generation heat supply. The thermal storage of the water to thermocline heat storage tank between designedelectricity to apply to the CHPand plant is supply. TheThe thermal storagetoperformance thermocline heat storage tankextraction designed to to the is presented. tank isheat supposed charge heat of bythe heatwater exchanger with the steam turbine andapply provide hotCHP waterplant to the District heating networks commonly addressed inexchanger the literature as of of thethe most effectiveand for the presented. The is supposed to charge heat by heat with theone steam turbine extraction provideand hotdecreasing waterregular to the heat-supply gridtank directly. Byare experiments, the static and dynamic characteristics thermocline issolutions studied some greenhouse gas directly. emissions theanbuilding These systems require high investments whichmay are returned through the heat heat-supply grid Byfrom experiments, thesector. static and characteristics of the thermocline is studied and some patterns are discussed, followed by effective method todynamic reduce the thermocline expansion, which be useful for the regular design sales. Due toofthe changed climate conditions and building policies, heat demand thebefuture patterns are discussed, followed by anstorage effective method to reducerenovation the thermocline expansion, which in may usefulcould for thedecrease, design and operation the thermocline heat tank. prolonging the investment return period. and operation of the thermocline heat storage tank. The main©scope this paper to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Copyright 2018ofElsevier Ltd.isAll rights reserved. ©forecast. 2019 TheThe Authors. Published by Elsevier Ltd. district of under Alvalade, located inofLisbon (Portugal), was used case study. The district on is consisted of 665 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review responsibility the scientific committee of theas10ath International Conference Applied Energy This is an open access article under the CC period BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) buildings that vary in both construction andscientific typology. Three weather scenarios (low, medium, high) threeEnergy district th International Selection and peer-review under responsibility of the committee of the 10 Conference on and Applied (ICAE2018). Peer-review under responsibility of the scientific committee of ICAE2018 – estimate The 10th the International Conference on Applied Energy. renovation scenarios were developed (shallow, intermediate, deep). To error, obtained heat demand values were (ICAE2018). compared with results from a dynamic heat demand model, previously developed and validated by the authors. Keywords: Thermal energy storage (TES); Thermocline; Single-tank; Experimental study; Combined heat and power (CHP) The results showed thatstorage when (TES); only weather change is considered, the margin error could be acceptable for some applications Keywords: Thermal energy Thermocline; Single-tank; Experimental study;of Combined heat and power (CHP) (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.scenarios, Introduction The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1. Introduction decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and To enhance the power load regulation capability of combined heat and power (CHP) generation plant, employing renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the Tostorage enhance the power load regulation capability of [1]. combined heat and (CHP) generation employing heat is widely considered to be a valid By charging heatpower when heat demand is lowplant, and discharging coupled scenarios). The values suggested could measure be used to modify the function parameters for the scenarios considered, and heat is widely considered to be a valid measure [1]. By charging heat when heat demand is low and discharging toimprove thestorage heat-supply network during peak period, electric power generation can be decoupled from the heat supply. the accuracy of heat demand estimations.
to the heat-supply network during peak period, electric power generation can be decoupled from the heat supply. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +86(10)61773923; fax: +86(10)61773918.
address:author. [email protected] * E-mail Corresponding Tel.: +86(10)61773923; fax: +86(10)61773918. Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection peer-review under responsibility the scientific 1876-6102and Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.766
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Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
At present, sensible heat storage is the most mature heat storage technology with the widest utilizations [2]. Based on the formation of the thermocline inside the tank, single-tank heat storage is able to store both hot and cold working fluid at the same time. It is widely agreed that the single-tank heat storage owns a series of merits including higher utilization ratio of the volume, less land occupation, simpler system and less investment [3]. The thermocline, which appears during the charging/discharging in the single-tank, is a region with large temperature gradient. Because of the existence of the thermocline, the heat convection between working fluid with large temperature difference can be avoided, which is the most significant condition to guarantee the effective operation of the charging/discharging inside the single-tank. Key technologies of thermocline storage tank include reducing the thermocline thickness, preventing the heat convection and increasing the heat storage quantity, etc. In fact, to ensure efficient operation, it is important to stabilize the thermocline during the charging/discharging, especially the required temperature of hot water supplied to the customers. It is found that the formation of the thermocline is determined by the geometry of the tank, the inlet, the hydrodynamics and the thermal characteristics of the flow in the tank [4]. As discussed in [5], both the flow rate and the temperature difference of the working fluid inside the tank can affect the thermocline thickness and the influence of the former is more obvious. Meanwhile, it is proved that such influence can be weakened when using appropriate water distributors, which is also confirmed by the simulation studies of Altuntop et al. [6,7]. As for the evaluation indexes for single-tank heat storage, Castell et al. [4] have studied several dimensionless numbers to characterize the stratification in water tanks and pointed out that the Richardson number is the best one, while a work of Fernández-Seara et al. [8] has analyzed a full-scale domestic electric hot water storage tank from the perspective of exergy efficiency. Although many researches have been conducted on the single-tank heat storage, most of them focus on high temperature heat storage with molten salt or thermal oil applied to solar energy plants. More detailed performance of normal or medium temperature thermocline heat storage remains to be explored, which will be discussed in the present experimental study. 2. Experimental system and data reduction
Fig. 1. Schematic diagram of the experimental system
Fig. 2. Thermocouples distribution inside the tank, in mm
The experimental structure is depicted in Fig. 1. The cylindrical part of the tank is 1.1 m high and 0.9 m in diameter, which makes it possible to store more than 0.75 m3 of water along with the conical part at the bottom. The storage tank and all pipes are surrounded by 25 mm thermal insulation layer to reduce the heat loss. In addition, in order to better distribute the inlet water from the top of the tank, a spiral nozzle is installed at the end of the inlet water pipe inside the tank. To simulate the steam turbine extraction, a steam generator is used as heat source of the charging. During the charging, pump A operates and circulates water from the bottom to the top of the tank after being heated to the required temperature. As for the discharging, tap water is used as cold source simulating the cycling water of heat-supply net. Hot water is extracted from the top of the tank, cooled down by the heat exchanger and finally goes back to the tank from the bottom.
Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
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In the present study, two types of performance of water thermocline storage tank is discussed: the dynamic characteristics and the static characteristics. The dynamic experiments focus on the thermal performance when either charging or discharging is operating. On the contrary, the evolution of thermocline in static state after charging or discharging pauses halfway are defined as the static characteristics. In this study, all of the charging and discharging begin when the initial temperature of the whole tank reaches 40 ℃ and 80 ℃, respectively. To observe the distribution of the temperature in the tank, 33 K-type thermocouples with accuracy of ±0.5 ℃ are fixed at different heights as depicted in Fig. 2. Additionally, there are four measuring points which are set to monitor the inlet and outlet water temperatures of the tank as well as the ambient temperatures. It should be noted that since no obvious difference in the radial temperature distributions, which can be confirmed later in Fig. 4, all the temperatures are obtained by the mean of the three measuring points at the same height. In this research, two methods are used to calculate the thickness of thermocline. For detailed quantitative analysis for the dynamic characteristics, the thermocline thickness is calculated as follows: δ=∆τQ/S
(1)
where δ and Q refer to the thermocline thickness and volume flow rate, respectively. ∆τ is the duration that the thermocline passes through a certain level of thermocouples, and S is the cross-sectional area of the tank whose value is 0.636 m2 in this experiment. Consequently, the thermocline thickness obtained from the previous formula is the mean value between the two moments selected for the calculation. As for the analysis for the static characteristics, since the water is stationary inside the tank, Equation 1 is no longer effective to calculate the thermocline thickness. It would be more appropriate to confirm the thermocline thickness with the axial temperature gradient, arc tangent functions are fitted to represent the temperature-height profiles so that the temperature gradient inside the tank can be calculated precisely by calculating the slope of the function. 3. Experiment results 3.1. The static characteristics of water thermocline The static experiments mainly study the evolution of the thermocline after the charging/discharging stops operating halfway, also known as the stand-by status. In this section, the thermocline is defined where the axial temperature gradient is larger than 0.2 ℃/cm. The temperature-height profiles of different initial thermocline positions during an 8-hour settling are presented in Fig. 3b-d. For comparison, a heat dissipation profile with the initial average temperature of 80 ℃ is shown additionally in Fig. 3a. As presented in Fig. 3c, the initial thermocline range is from 48 cm to 74 cm with about 26 cm thickness. After 4 and 8 hours, its thickness increases by 38.5% and 53.8% compared to the initial one. Thus, when a thermocline exists, it continually extends but the expansion rate decreases with time. The maximum axial temperature gradient declines from 3.9 ℃/cm to 1.2 ℃/cm in 8 hours and the center of the thermocline slightly moves down. Above the thermocline, under the combined effect of higher heat loss and the heat conduction to the water below, the average temperature drops from 78.3 ℃ to 68.7 ℃. On the contrary, due to smaller temperature difference from the environment, the heat loss is smaller per unit time, and being heated by the upper liquid, water under the thermocline has a smaller temperature decline. Therefore, the thermocline can exist for a long time when the water is stationary, but its thickness constantly increases, which makes the temperature-height profiles tend to be flatter. As presented in Fig. 3a, because of heat dissipation, the whole temperature inside the tank falls 6.4 ℃ from 80.2 ℃ in 8 hours. However, according to Fig. 3b-d, under the condition of similar ambient temperature, the temperature decline of the hot end side in 8 hours is 8.4 ℃, 9.6 ℃ and 13.7 ℃ with the initial thermocline range of 28-51 cm, 4874 cm and 69-89 cm, respectively. When a thermocline exists, because of the large axial temperature gradient, the heat of the water on the hot end side is not only dissipated radially to the environment but also conducted axially to the lower height of water, so water in a certain height range will be heated to cause a temperature increase. Moreover, with similar thermocline thickness, the heat delivered between the hot and cold end is almost equal in the same period. Therefore, the temperature of the hot water drops greater if the initial thermocline position is higher.
Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
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According to the analysis of the static characteristics, when the water thermocline heat storage is applied, long time in static state should be avoided especially when the thermocline exists inside the tank, or the efficient heat storage capacity and the heat-supply temperature will drop sharply. The heat dissipation can be reduced when the heat storage tank is equipped with proper heat insulation, but it does not help slowdown the thermocline expansion caused by the axial temperature gradient inside the tank. Furthermore, the hot end temperature will decrease more rapidly if the initial position of thermocline is higher.
(a)
(b)
(c) (d) Fig. 3. (a) Heat dissipation profile; Thermocline variations with initial position of (b) 28-51 cm; (c) 48-74 cm; (d) 69-89 cm
3.2. The dynamic characteristics of water thermocline When charging/discharging operates with certain backwater temperature, a stable thermocline forms and grows inside the tank until it is completely charged or discharged. A series of 2-D temperature fields of different moments during a charging is shown in Fig. 4, which is drawn with Matlab by interpolation according to the experimental data. At the beginning, the thermocline forms at the top inside the tank. Along with the charging, the amount of hot water increases while that of cold water decreases, with the thermocline moving downward at a steady velocity. During the charging, the thermocline expands. At the end of the charging, the thermocline reaches the bottom and its thickness starts to decline. At this stage, the outlet temperature climbs until it rises to 80 ℃, which means that the water inside the tank has totally reached 80 ℃ and the entire charging finishes. The time varying curves of the thermocline thickness with different flow rates are demonstrated in Fig. 5. C and D in the legend are the abbreviations of charging and discharging, respectively. The abscissa is a dimensionless time, τ*, which is the ratio of the time of the charging/discharging to the moment when the outlet temperature begins to climb/drop, representing the extent of the process. For different flow rates, the thermocline positions are almost the same when τ* is identical. It is necessary to point out that the temperature range of the thermocline grows from zero
Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
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to a certain preset value during its formation in each charging/discharging. In this section, to better analyze the thermocline variations after it completely forms, the thermocline thickness is not shown until it reaches the temperature range from 43 ℃ to 77 ℃. It is presented in Fig. 5 that the differences of thermocline thickness between different flow rates are distinct. Similar to what is illustrated in [5], it shows smaller thermocline thickness when the flow rate is smaller. Once the thermocline forms, it keeps stable but expanding as the discharging proceeds, which is similar to the static characteristics. At the flow rate of 0.30 m3/h, the thermocline thickness grows more obviously than any other flow rates presented here. It can be explained that under a small flow rate, it needs more time to finish the discharging, which also means that the thermocline gets more time to expand. However, rather than the expansion after it completely forms, the distinction of the thermocline thickness between different flow rates comes from the initial thermocline thickness, which can be represented by the first points of each curve in Fig. 5. When the flow rate is larger, the initial thermocline forms later, and the initial thermocline thickness becomes larger. The influence of the flow rate on the initial thermocline thickness is much larger than that on the thermocline after it totally forms, i.e., to a large extent, the thermocline thickness at any moment depends on its initial thickness. For single-tank heat storage, the efficient discharging capacity is defined as the total released heat amount which is qualified to the users. Since the water temperature is usually lower than the required one within the thermocline, the existence and expansion of the thermocline will inevitably make the efficient heat storage capacity less than the maximum heat storage capacity. As mentioned above, during the charging/discharging, once a thermocline forms, it will keep extending while being stable until the end of the process. Besides, it is proved that the thermocline thickness is largely determined by its initial thickness. Consequently, if a smaller initial thermocline thickness is obtained, the thickness will maintain relatively small during the whole charging/discharging, and in this way, the efficient discharging capacity can be improved.
Fig. 4. 2-D temperature fields at different moments during a charging (40 ℃/ 80 ℃, 0.35 m3/h)
Fig. 5. Thermocline variations at different flow rates
Fig. 6. Thermocline evolutions with different flow rates and initial inlet temperatures
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Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
In a 40 ℃/80 ℃ discharging, when the bottom water temperature declines to 43 ℃, the area between the isothermal surfaces of 43 ℃ to 87 ℃ is defined as the initial thermocline. Its thickness as well as the time of occurrence is determined by the descent speed of the bottom temperature. Hence, an effective method to reduce it is to lower the inlet temperature at the early stage, making the bottom water temperature descend to the required temperature more quickly. Similarly, raising the inlet temperature at the early stage can help to reduce the thermocline thickness in a charging. Three pairs of thermocline thickness profiles are shown in Fig. 6. The extra temperature marked in the parentheses in the legend represents the average initial inlet temperature. The thermocline thickness can be reduced when the inlet temperature is adjusted at the beginning compared to the corresponding flow rate with constant inlet temperature. Consequently, by raising the initial inlet temperature of charging or lowering that of the discharging, the thermocline heat storage tank is able to keep relatively high efficient discharging capacity under a large flow rate, which is significant to enhance the charging/discharging power. Different from this experiment, in actual operation, it is not practical to adjust the inlet temperature by controlling the heating or cooling intensity of the heat source or heat customers. The purpose of regulating the inlet water temperature can be satisfied if an auxiliary heat exchanger is employed. 4. Conclusions An experimental study on the static and dynamic thermocline evolution of the water thermocline storage tank is presented. The main conclusions are summarized as follows, Once a thermocline forms, it can stably exist for a long time while it expands continually, but its expansion slows down with time. It shows positive correlation between the thermocline expansion rate and the axial temperature gradient. Even though the thermal insulation is strong, long period of settle (the stand-by status) when the thermocline exists should be avoided. During the charging/discharging, the all-time thermocline thickness is strongly related to the initial thermocline thickness. It remains smaller if the corresponding initial one is smaller. Small thickness of the initial thermocline can be built by increasing (decreasing) the inlet temperature at the early stage of the charging (discharging). Acknowledgements The financial supports for this research project from the National Natural Science Foundation of China (No. 51676069), the Technical Supporting Program of China (No. 2014BAA06B01) and the State Grid Science and Technology Program (No. GTLN201706-KJXM001) are gratefully acknowledged. References [1] Wang K, Tian HM, Jia J. Expanding the peak regulation margin of heating units by using heat storage technology. Energy Conservation Technology 2012; 174: 399-341. [2] Wu YT, Ren N, Ma CF. Research and application of molten for sensible heat storage. Energy Storage Science and Technology 2013; 6: 586592. [3] Hatte S, Mira-Hernández C, Advaith S, et al. Short and long-term sensitivity of lab-scale thermocline based thermal storage to flow disturbances. Applied Thermal Engineering 2016; 109: 936-948. [4] Castell A, Medrano M, Solé C, Cabeza L F. Dimensionless numbers used to characterize stratification in water tanks for discharging at low rates. Renewable Energy 2010; 35: 2192-2199. [5] Tang JL, Ouyang ZR, Qiu WJ. Analysis of affecting factors on water storage system thermocline of steady high magnetic field facilities. Cryogenics 2013; 193: 35-37. [6] Altuntop N, Arslan M, Ozceyhan V, Kanoglu M. Effect of obstacles on thermal stratification in hot water storage tanks. Applied Thermal Engineering 2005; 25: 2285-2298. [7] Erdemir D, Altuntop N. Improved thermal stratification with obstacles placed inside the vertical mantled hot water tanks. Applied Thermal Engineering 2016; 100: 20-29. [8] Fernández-Seara J, Uhía F J, Sieres J. Experimental analysis of a domestic electric hot water storage tank. Part Ⅰ: Static mode of operation. Applied Thermal Engineering 2017; 27: 129-136.
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Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
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www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia 158 Energy Procedia 00(2019) (2017)4471–4476 000–000 www.elsevier.com/locate/procedia
10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Static and dynamic thermocline evolution in the water thermocline The 15th International Symposium on District and Cooling Static and dynamic thermocline evolution inHeating the water thermocline storage tank storage tank Assessing the of using the heat demand-outdoor a feasibility a Zhaoyu Hea, Yijie Qian a, Chao Xuaa, Lijun Yangaa, Xiaoze Dub,a * Zhaoyufunction He , Yijiefor Qiana ,long-term Chao Xu , Lijun Yangheat , Xiaoze Dub,a* forecast temperature district demand Key Laboratory of Condition Monitoring and Control for Power Plant Equipment (North China Electric Power University), Ministry of a
Education, Beijing China Key Laboratory of Condition Monitoring and Control for Power Plant102206, Equipment (North China Electric Power University), Ministry of a,b,c a a b c c b School of Energy and Power Engineering, Lanzhou of Technology, Lanzhou 730050, China Education, BeijingUniversity 102206, China b School of Energy and Power Engineering, Lanzhou University of Technology, Lanzhou 730050, China a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Abstract Since the power load regulation capability of the combined heat and power (CHP) generation plant is weaker than that of the Since the power loadpower regulation of significance the combinedto heat and power (CHP) generation plant is weaker than that the conventional thermal plant,capability it is of great decouple the strong relation between electricity generation andofheat Abstract conventional thermalheat power plant,performance it is of great significance decouple the strong relation generation heat supply. The thermal storage of the water to thermocline heat storage tank between designedelectricity to apply to the CHPand plant is supply. TheThe thermal storagetoperformance thermocline heat storage tankextraction designed to to the is presented. tank isheat supposed charge heat of bythe heatwater exchanger with the steam turbine andapply provide hotCHP waterplant to the District heating networks commonly addressed inexchanger the literature as of of thethe most effectiveand for the presented. The is supposed to charge heat by heat with theone steam turbine extraction provideand hotdecreasing waterregular to the heat-supply gridtank directly. Byare experiments, the static and dynamic characteristics thermocline issolutions studied some greenhouse gas directly. emissions theanbuilding These systems require high investments whichmay are returned through the heat heat-supply grid Byfrom experiments, thesector. static and characteristics of the thermocline is studied and some patterns are discussed, followed by effective method todynamic reduce the thermocline expansion, which be useful for the regular design sales. Due toofthe changed climate conditions and building policies, heat demand thebefuture patterns are discussed, followed by anstorage effective method to reducerenovation the thermocline expansion, which in may usefulcould for thedecrease, design and operation the thermocline heat tank. prolonging the investment return period. and operation of the thermocline heat storage tank. The main©scope this paper to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Copyright 2018ofElsevier Ltd.isAll rights reserved. ©forecast. 2019 TheThe Authors. Published by Elsevier Ltd. district of under Alvalade, located inofLisbon (Portugal), was used case study. The district on is consisted of 665 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review responsibility the scientific committee of theas10ath International Conference Applied Energy This is an open access article under the CC period BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) buildings that vary in both construction andscientific typology. Three weather scenarios (low, medium, high) threeEnergy district th International Selection and peer-review under responsibility of the committee of the 10 Conference on and Applied (ICAE2018). Peer-review under responsibility of the scientific committee of ICAE2018 – estimate The 10th the International Conference on Applied Energy. renovation scenarios were developed (shallow, intermediate, deep). To error, obtained heat demand values were (ICAE2018). compared with results from a dynamic heat demand model, previously developed and validated by the authors. Keywords: Thermal energy storage (TES); Thermocline; Single-tank; Experimental study; Combined heat and power (CHP) The results showed thatstorage when (TES); only weather change is considered, the margin error could be acceptable for some applications Keywords: Thermal energy Thermocline; Single-tank; Experimental study;of Combined heat and power (CHP) (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.scenarios, Introduction The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1. Introduction decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and To enhance the power load regulation capability of combined heat and power (CHP) generation plant, employing renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the Tostorage enhance the power load regulation capability of [1]. combined heat and (CHP) generation employing heat is widely considered to be a valid By charging heatpower when heat demand is lowplant, and discharging coupled scenarios). The values suggested could measure be used to modify the function parameters for the scenarios considered, and heat is widely considered to be a valid measure [1]. By charging heat when heat demand is low and discharging toimprove thestorage heat-supply network during peak period, electric power generation can be decoupled from the heat supply. the accuracy of heat demand estimations.
to the heat-supply network during peak period, electric power generation can be decoupled from the heat supply. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +86(10)61773923; fax: +86(10)61773918.
address:author. [email protected] * E-mail Corresponding Tel.: +86(10)61773923; fax: +86(10)61773918. Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection peer-review under responsibility the scientific 1876-6102and Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.766
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Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
At present, sensible heat storage is the most mature heat storage technology with the widest utilizations [2]. Based on the formation of the thermocline inside the tank, single-tank heat storage is able to store both hot and cold working fluid at the same time. It is widely agreed that the single-tank heat storage owns a series of merits including higher utilization ratio of the volume, less land occupation, simpler system and less investment [3]. The thermocline, which appears during the charging/discharging in the single-tank, is a region with large temperature gradient. Because of the existence of the thermocline, the heat convection between working fluid with large temperature difference can be avoided, which is the most significant condition to guarantee the effective operation of the charging/discharging inside the single-tank. Key technologies of thermocline storage tank include reducing the thermocline thickness, preventing the heat convection and increasing the heat storage quantity, etc. In fact, to ensure efficient operation, it is important to stabilize the thermocline during the charging/discharging, especially the required temperature of hot water supplied to the customers. It is found that the formation of the thermocline is determined by the geometry of the tank, the inlet, the hydrodynamics and the thermal characteristics of the flow in the tank [4]. As discussed in [5], both the flow rate and the temperature difference of the working fluid inside the tank can affect the thermocline thickness and the influence of the former is more obvious. Meanwhile, it is proved that such influence can be weakened when using appropriate water distributors, which is also confirmed by the simulation studies of Altuntop et al. [6,7]. As for the evaluation indexes for single-tank heat storage, Castell et al. [4] have studied several dimensionless numbers to characterize the stratification in water tanks and pointed out that the Richardson number is the best one, while a work of Fernández-Seara et al. [8] has analyzed a full-scale domestic electric hot water storage tank from the perspective of exergy efficiency. Although many researches have been conducted on the single-tank heat storage, most of them focus on high temperature heat storage with molten salt or thermal oil applied to solar energy plants. More detailed performance of normal or medium temperature thermocline heat storage remains to be explored, which will be discussed in the present experimental study. 2. Experimental system and data reduction
Fig. 1. Schematic diagram of the experimental system
Fig. 2. Thermocouples distribution inside the tank, in mm
The experimental structure is depicted in Fig. 1. The cylindrical part of the tank is 1.1 m high and 0.9 m in diameter, which makes it possible to store more than 0.75 m3 of water along with the conical part at the bottom. The storage tank and all pipes are surrounded by 25 mm thermal insulation layer to reduce the heat loss. In addition, in order to better distribute the inlet water from the top of the tank, a spiral nozzle is installed at the end of the inlet water pipe inside the tank. To simulate the steam turbine extraction, a steam generator is used as heat source of the charging. During the charging, pump A operates and circulates water from the bottom to the top of the tank after being heated to the required temperature. As for the discharging, tap water is used as cold source simulating the cycling water of heat-supply net. Hot water is extracted from the top of the tank, cooled down by the heat exchanger and finally goes back to the tank from the bottom.
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In the present study, two types of performance of water thermocline storage tank is discussed: the dynamic characteristics and the static characteristics. The dynamic experiments focus on the thermal performance when either charging or discharging is operating. On the contrary, the evolution of thermocline in static state after charging or discharging pauses halfway are defined as the static characteristics. In this study, all of the charging and discharging begin when the initial temperature of the whole tank reaches 40 ℃ and 80 ℃, respectively. To observe the distribution of the temperature in the tank, 33 K-type thermocouples with accuracy of ±0.5 ℃ are fixed at different heights as depicted in Fig. 2. Additionally, there are four measuring points which are set to monitor the inlet and outlet water temperatures of the tank as well as the ambient temperatures. It should be noted that since no obvious difference in the radial temperature distributions, which can be confirmed later in Fig. 4, all the temperatures are obtained by the mean of the three measuring points at the same height. In this research, two methods are used to calculate the thickness of thermocline. For detailed quantitative analysis for the dynamic characteristics, the thermocline thickness is calculated as follows: δ=∆τQ/S
(1)
where δ and Q refer to the thermocline thickness and volume flow rate, respectively. ∆τ is the duration that the thermocline passes through a certain level of thermocouples, and S is the cross-sectional area of the tank whose value is 0.636 m2 in this experiment. Consequently, the thermocline thickness obtained from the previous formula is the mean value between the two moments selected for the calculation. As for the analysis for the static characteristics, since the water is stationary inside the tank, Equation 1 is no longer effective to calculate the thermocline thickness. It would be more appropriate to confirm the thermocline thickness with the axial temperature gradient, arc tangent functions are fitted to represent the temperature-height profiles so that the temperature gradient inside the tank can be calculated precisely by calculating the slope of the function. 3. Experiment results 3.1. The static characteristics of water thermocline The static experiments mainly study the evolution of the thermocline after the charging/discharging stops operating halfway, also known as the stand-by status. In this section, the thermocline is defined where the axial temperature gradient is larger than 0.2 ℃/cm. The temperature-height profiles of different initial thermocline positions during an 8-hour settling are presented in Fig. 3b-d. For comparison, a heat dissipation profile with the initial average temperature of 80 ℃ is shown additionally in Fig. 3a. As presented in Fig. 3c, the initial thermocline range is from 48 cm to 74 cm with about 26 cm thickness. After 4 and 8 hours, its thickness increases by 38.5% and 53.8% compared to the initial one. Thus, when a thermocline exists, it continually extends but the expansion rate decreases with time. The maximum axial temperature gradient declines from 3.9 ℃/cm to 1.2 ℃/cm in 8 hours and the center of the thermocline slightly moves down. Above the thermocline, under the combined effect of higher heat loss and the heat conduction to the water below, the average temperature drops from 78.3 ℃ to 68.7 ℃. On the contrary, due to smaller temperature difference from the environment, the heat loss is smaller per unit time, and being heated by the upper liquid, water under the thermocline has a smaller temperature decline. Therefore, the thermocline can exist for a long time when the water is stationary, but its thickness constantly increases, which makes the temperature-height profiles tend to be flatter. As presented in Fig. 3a, because of heat dissipation, the whole temperature inside the tank falls 6.4 ℃ from 80.2 ℃ in 8 hours. However, according to Fig. 3b-d, under the condition of similar ambient temperature, the temperature decline of the hot end side in 8 hours is 8.4 ℃, 9.6 ℃ and 13.7 ℃ with the initial thermocline range of 28-51 cm, 4874 cm and 69-89 cm, respectively. When a thermocline exists, because of the large axial temperature gradient, the heat of the water on the hot end side is not only dissipated radially to the environment but also conducted axially to the lower height of water, so water in a certain height range will be heated to cause a temperature increase. Moreover, with similar thermocline thickness, the heat delivered between the hot and cold end is almost equal in the same period. Therefore, the temperature of the hot water drops greater if the initial thermocline position is higher.
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According to the analysis of the static characteristics, when the water thermocline heat storage is applied, long time in static state should be avoided especially when the thermocline exists inside the tank, or the efficient heat storage capacity and the heat-supply temperature will drop sharply. The heat dissipation can be reduced when the heat storage tank is equipped with proper heat insulation, but it does not help slowdown the thermocline expansion caused by the axial temperature gradient inside the tank. Furthermore, the hot end temperature will decrease more rapidly if the initial position of thermocline is higher.
(a)
(b)
(c) (d) Fig. 3. (a) Heat dissipation profile; Thermocline variations with initial position of (b) 28-51 cm; (c) 48-74 cm; (d) 69-89 cm
3.2. The dynamic characteristics of water thermocline When charging/discharging operates with certain backwater temperature, a stable thermocline forms and grows inside the tank until it is completely charged or discharged. A series of 2-D temperature fields of different moments during a charging is shown in Fig. 4, which is drawn with Matlab by interpolation according to the experimental data. At the beginning, the thermocline forms at the top inside the tank. Along with the charging, the amount of hot water increases while that of cold water decreases, with the thermocline moving downward at a steady velocity. During the charging, the thermocline expands. At the end of the charging, the thermocline reaches the bottom and its thickness starts to decline. At this stage, the outlet temperature climbs until it rises to 80 ℃, which means that the water inside the tank has totally reached 80 ℃ and the entire charging finishes. The time varying curves of the thermocline thickness with different flow rates are demonstrated in Fig. 5. C and D in the legend are the abbreviations of charging and discharging, respectively. The abscissa is a dimensionless time, τ*, which is the ratio of the time of the charging/discharging to the moment when the outlet temperature begins to climb/drop, representing the extent of the process. For different flow rates, the thermocline positions are almost the same when τ* is identical. It is necessary to point out that the temperature range of the thermocline grows from zero
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to a certain preset value during its formation in each charging/discharging. In this section, to better analyze the thermocline variations after it completely forms, the thermocline thickness is not shown until it reaches the temperature range from 43 ℃ to 77 ℃. It is presented in Fig. 5 that the differences of thermocline thickness between different flow rates are distinct. Similar to what is illustrated in [5], it shows smaller thermocline thickness when the flow rate is smaller. Once the thermocline forms, it keeps stable but expanding as the discharging proceeds, which is similar to the static characteristics. At the flow rate of 0.30 m3/h, the thermocline thickness grows more obviously than any other flow rates presented here. It can be explained that under a small flow rate, it needs more time to finish the discharging, which also means that the thermocline gets more time to expand. However, rather than the expansion after it completely forms, the distinction of the thermocline thickness between different flow rates comes from the initial thermocline thickness, which can be represented by the first points of each curve in Fig. 5. When the flow rate is larger, the initial thermocline forms later, and the initial thermocline thickness becomes larger. The influence of the flow rate on the initial thermocline thickness is much larger than that on the thermocline after it totally forms, i.e., to a large extent, the thermocline thickness at any moment depends on its initial thickness. For single-tank heat storage, the efficient discharging capacity is defined as the total released heat amount which is qualified to the users. Since the water temperature is usually lower than the required one within the thermocline, the existence and expansion of the thermocline will inevitably make the efficient heat storage capacity less than the maximum heat storage capacity. As mentioned above, during the charging/discharging, once a thermocline forms, it will keep extending while being stable until the end of the process. Besides, it is proved that the thermocline thickness is largely determined by its initial thickness. Consequently, if a smaller initial thermocline thickness is obtained, the thickness will maintain relatively small during the whole charging/discharging, and in this way, the efficient discharging capacity can be improved.
Fig. 4. 2-D temperature fields at different moments during a charging (40 ℃/ 80 ℃, 0.35 m3/h)
Fig. 5. Thermocline variations at different flow rates
Fig. 6. Thermocline evolutions with different flow rates and initial inlet temperatures
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Zhaoyu He et al. / Energy Procedia 158 (2019) 4471–4476 Zhaoyu He et al./ Energy Procedia 00 (2018) 000–000
In a 40 ℃/80 ℃ discharging, when the bottom water temperature declines to 43 ℃, the area between the isothermal surfaces of 43 ℃ to 87 ℃ is defined as the initial thermocline. Its thickness as well as the time of occurrence is determined by the descent speed of the bottom temperature. Hence, an effective method to reduce it is to lower the inlet temperature at the early stage, making the bottom water temperature descend to the required temperature more quickly. Similarly, raising the inlet temperature at the early stage can help to reduce the thermocline thickness in a charging. Three pairs of thermocline thickness profiles are shown in Fig. 6. The extra temperature marked in the parentheses in the legend represents the average initial inlet temperature. The thermocline thickness can be reduced when the inlet temperature is adjusted at the beginning compared to the corresponding flow rate with constant inlet temperature. Consequently, by raising the initial inlet temperature of charging or lowering that of the discharging, the thermocline heat storage tank is able to keep relatively high efficient discharging capacity under a large flow rate, which is significant to enhance the charging/discharging power. Different from this experiment, in actual operation, it is not practical to adjust the inlet temperature by controlling the heating or cooling intensity of the heat source or heat customers. The purpose of regulating the inlet water temperature can be satisfied if an auxiliary heat exchanger is employed. 4. Conclusions An experimental study on the static and dynamic thermocline evolution of the water thermocline storage tank is presented. The main conclusions are summarized as follows, Once a thermocline forms, it can stably exist for a long time while it expands continually, but its expansion slows down with time. It shows positive correlation between the thermocline expansion rate and the axial temperature gradient. Even though the thermal insulation is strong, long period of settle (the stand-by status) when the thermocline exists should be avoided. During the charging/discharging, the all-time thermocline thickness is strongly related to the initial thermocline thickness. It remains smaller if the corresponding initial one is smaller. Small thickness of the initial thermocline can be built by increasing (decreasing) the inlet temperature at the early stage of the charging (discharging). Acknowledgements The financial supports for this research project from the National Natural Science Foundation of China (No. 51676069), the Technical Supporting Program of China (No. 2014BAA06B01) and the State Grid Science and Technology Program (No. GTLN201706-KJXM001) are gratefully acknowledged. References [1] Wang K, Tian HM, Jia J. Expanding the peak regulation margin of heating units by using heat storage technology. Energy Conservation Technology 2012; 174: 399-341. [2] Wu YT, Ren N, Ma CF. Research and application of molten for sensible heat storage. Energy Storage Science and Technology 2013; 6: 586592. [3] Hatte S, Mira-Hernández C, Advaith S, et al. Short and long-term sensitivity of lab-scale thermocline based thermal storage to flow disturbances. Applied Thermal Engineering 2016; 109: 936-948. [4] Castell A, Medrano M, Solé C, Cabeza L F. Dimensionless numbers used to characterize stratification in water tanks for discharging at low rates. Renewable Energy 2010; 35: 2192-2199. [5] Tang JL, Ouyang ZR, Qiu WJ. Analysis of affecting factors on water storage system thermocline of steady high magnetic field facilities. Cryogenics 2013; 193: 35-37. [6] Altuntop N, Arslan M, Ozceyhan V, Kanoglu M. Effect of obstacles on thermal stratification in hot water storage tanks. Applied Thermal Engineering 2005; 25: 2285-2298. [7] Erdemir D, Altuntop N. Improved thermal stratification with obstacles placed inside the vertical mantled hot water tanks. Applied Thermal Engineering 2016; 100: 20-29. [8] Fernández-Seara J, Uhía F J, Sieres J. Experimental analysis of a domestic electric hot water storage tank. Part Ⅰ: Static mode of operation. Applied Thermal Engineering 2017; 27: 129-136.