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Multi-mode control method for the existing domestic hot water storage tanks with district heating supply
Energy xxx (xxxx) xxx
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Energy journal homepage: www.elsevier.com/locate/energy
Multi-mode control method for the existing domestic hot water storage tanks with district heating supply Tao Huang a, Xiaochen Yang b, c, *, Svend Svendsen b a
Department of Civil Engineering, Technical University of Denmark, Lyngby, 2800, Denmark Section of Building Energy and Services, Department of Civil Engineering, Technical University of Denmark, Lyngby, 2800, Denmark c Department of Environmental Science and Engineering, Tianjin University, Yaguan Road 135, Tianjin, 300350, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2019 Received in revised form 5 November 2019 Accepted 7 November 2019 Available online xxx
The hot water tank is widely used for domestic hot water (DHW) preparation. When supplied by district heating (DH), the operation of DHW tanks directly influences the DH return temperature, thereby affecting the DH system efficiency, supply capacity and the realization of the low temperature district heating. However, the conventional charging method often results in high DH return temperature. This study develops a new charging method aiming at reducing the DH return temperature without violating the comfort or hygiene requirements. The concept uses the multi-mode charging method considering the periodical characteristics of the load pattern. Multi scenarios are simulated by dynamic models using the practical DHW load profiles from a case study. Moreover, the impacts of the tank configuration, the location of the temperature sensor, and the distribution heat loss are investigated. The results show that the new control method can reduce the primary return temperature by 5e8 C compared to the conventional control method. The distribution heat loss imposes great impact on the DH return temperature. In addition, the tank with the external heat exchanger performs better than the tank with the internal heating coil if the circulation heat loss is less than 50% of the DHW demand. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Domestic hot water tank District heating return temperature Tank control method Distribution heat loss Modelica models
1. Introduction District heating (DH) is undergoing a rapid development widely since it is considered as a cost-effective way of heat supply and can give more access to renewable energy as heat sources. Within the European Union, more than 5000 DH systems are in operation today. In Scandinavian countries, a major proportion of energy in the building sector is consumed by heating and 70e90% of the heating needs are covered by district heating system (DH) in the city area [1]. It is estimated that this proportion will keep increasing in the future. The Fourth-Generation District Heating (4GDH) has emerged intending to achieve more efficient and sustainable heat production [2]. 4GDH is also known as the low-temperature district heating (LTDH). According to Refs. [3e5], the supply temperature of LTDH can be reduced to be 50e55 C. Therefore, LTDH is able to reduce the distribution heat loss effectively, which contributes to 10e30% of the heat delivery on average [6,7]. This proportion is
* Corresponding author.Section of Building Energy and Services, Department of Civil Engineering, Technical University of Denmark, Lyngby, 2800, Denmark. E-mail address: [email protected] (X. Yang).
even larger in the low-heat-density area [8]. However, how to deal with the peak load and improve the return temperature are still an issue before the LTDH can be widely applied. In Demark, the DH system covers both space heating and domestic hot water (DHW) demands. In Great Copenhagen area, the heat consumers usually take the indirect connection to the DH distribution grid through the in-building substations. The space heating loop uses a heat exchanger for heat exchanging with DH, while the DHW circuit often uses storage tanks. Depending on how the heating element is connected, the DHW tank can be divided into two types, i.e. DHW storage tanks with internal heating coils and DHW storage tanks with external heat exchangers (HEX). In the Danish Standard, the comfort temperature of the DHW for kitchen use is 45 C and 40 C for the shower. While concerning the hygiene requirements, the DHW supply temperature should be around 55 C and at least no less than 50 C during the peak load including the circulation loop [9,10]. Such requirements impose great difficulty on the peak shaving and reducing the return temperature of DH. A number of studies have been performed aiming at alleviating the DH peak load issue. One emphasis is to implement the heat
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storage units [11,12], while another is to manage the operation on the demand side by utilizing the building thermal mass capacity [13,14]. However, previous studies have primarily concentrated on the operation of the space heating system or the entire heating demand control. Few attentions are paid on the DHW system which has great potential to be improved. Bøhm et al. [15] investigated several buildings constructed between 1906 and 1996, and found that DHW heat demand amounts to 25e35% of the total heat demand. Over past 20 years, DHW consumption has increased nearly 50%. For future buildings, this fraction is likely to rise with the improvement of the building insulation standards [16,17]. Therefore, the improvement of the DHW heating loop is of great importance to the whole heating system. The hot water tank is widely used for the DHW preparation. Some of the previous researches related to the storage tanks are focused on the factors that can destroy the stratification inside the tank, such as the heat losses to the ambient or mixing caused by charging/discharging phases [18,19]. Yang et al. [20] found that the correct design of the control valve size, the settings of the temperature sensor (for DHW temperature control), and the reduction of the heating coil fouling growth rate are of significance in achieving low primary return temperature. Nabavitabatabayi et al. [21] investigated the impact of the phase change materials (PCMs) on hot water tank and indicated that PCMs could shift the power demand to the off-peak and enhance the heat transfer rate. In recent years, researchers started paying more attention on the modification of the control strategies of DHW production system. Prud’homme and Gillet [22] improved the structure of the electric heater by replacing one single electric element by three smaller ones, and implemented a predictive control strategy in conjunction with weather forecasts as well as user’s demand to control the heaters. The simulation results show good improvement in energy czy-Víg and Farkas [23] employed an artificial performance. Ge neural network modelling to control the tank stratification. The results show good control accuracy between the measured and simulated data. Kepplinger et al. simulated [24] and tested [25] an optimal heating strategy for an electric DHW tank based on the expected demand and the pseudo cost function. The results show great improvement in terms of the energy consumption and heat bill. It can be found that these control strategies are mainly datadriven. Therefore, they are based on the prior knowledge and information such as tariff data which may be difficult to obtain. At the meantime, such control algorithms may lead to substantially computational consumption. In this light, the commercial application of these methods remains doubts. Furthermore, it appears from the aforementioned studies that none of them focused on improving the practical charging control strategies of the DH water tank with minimal cost and maximal applicability, and few studies can be found to provide detailed analyses with respect to the impact on the primary return temperature. In practice, most DHW storage tanks are designed with the consideration of safety, which can lead to oversize. The charging valve is normally opened more than necessary since the charging controller tends to use large power to heat the DHW to the setpoint temperature rapidly. Such operation results in insufficient cooling if the DHW demand is tiny, for instance, in the nighttime. Moreover, the DH supply also needs to cover the distribution heat loss by warming up the circulation water to be above 50 C, which makes the cooling of the DH flow more difficult. Some researchers have started employing supplemental techniques to improve this issue. For instance, Yang et al. [26] propose to apply a micro heat pump to reutilize the heat from the primary return water to enable the DHW system to adapt to low-temperature district heating. However, the potential in optimizing the charging operation of the DHW tank to achieve better cooling effect has not yet been
investigated sufficiently. This work aims at optimizing the conventional control strategy of charging DHW storage tank under the premise of the thermal comfort and hygiene, in order to reduce the primary return temperature and shave the extreme peak for DHW preparation. A new concept for charging control strategy is therefore proposed. The multi-mode control is designed for the new control strategy to deal with the fluctuated DHW load profile, so to improve the system performances. Different scenarios are simulated by the dynamic models. Measurements from an existing building are used for dimensioning the optimal charging flowrate. The performances of the new charging control method and the conventional method are compared. The influence of the tank structure is also investigated by comparing the simulation results of two types of the DHW storage tank. Moreover, the performances of the new tank control strategy considering different levels of distribution heat losses are compared, so to clarify the influence by the heat losses. The results of this study are of significance for improving the overall DH system efficiency by lowering the return temperature and shaving off the peak load, which also paves the road for the application of LTDH. 2. Data and method 2.1. Simulation method Compared to the conventional charging concept, the new method intends to gradually charge the DHW storage tank by small constant flowrate, so that to achieve large temperature difference and stabilize the operation. The simulations are conducted by Modelica models. Modelica [27] is an object-oriented, equation-based language. It can be used to simulate complex engineering process. In recent years, it has been applied in more and more research works in the DH field [28e30]. The buildings library and self-developed component of the storage tank are used to emulate the practical system in the case study. The simulation environment in this study is Dymola [31], which is used to transformed the Modelica code by graphical interface and their logical connection. The systems of the tank with internal heating coil and the tank with external heat exchanger are considered for the simulation. Their performances are compared to investigate the impact of the tank geometry. The schematics of the two tank layouts are shown in Fig. 1. 2.1.1. Model 1- New charging concept for the DHW tank with internal heating coil The flow chart of the control logic of Model 1 is describe in Fig. 2. The DH supply temperature is assumed to be 80 C based on the data from the DH supply company. The ambient temperature and the domestic cold water temperature are assumed to be 20 Cand 10 C, respectively. Two temperature sensors are required by the new charging concept. They are assumed to be installed at different heights. The sensor situated on the upper part of the tank indicates the DHW supply temperature, while the sensor located on the bottom part is used to show the level of the DH return temperature. So the lower sensor is situated at the same height as the exit of the internal heating coil. The DH return temperature is expected to be reduced to below 30 C, therefore the limit for the lower temperature sensor is set to be 20 C considering the temperature difference between the fluid inside and outside the heating coil. Regarding the fluctuated DHW load profiles, the whole charging scheme is divided into 3 modes. 1. The slow charging mode: when the temperature of the upper temperature sensor is no lower than 55 C and the temperature of the lower sensor is below 20 C, the slow charging mode is
Please cite this article as: Huang T et al., Multi-mode control method for the existing domestic hot water storage tanks with district heating supply, Energy, https://doi.org/10.1016/j.energy.2019.116517
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Fig. 1. Schematics of DHW storage tank with internal heating coil (left) and DHW storage tank with external HEX (right).
Fig. 2. Control logic of the new charging concept for the tank with internal coil.
activated. The small charging flowrate heats the necessary thermal storage inside the tank on daily basis. The calculation method is introduced by Equations (1)e(3) in the following section. The lower temperature sensor indicates whether the whole tank is fully charged. If the temperature detected by the lower temperature sensor is above 20 C, the valve for charging will be switched off since the necessary thermal storage is reached and the surplus heating should be avoided to maintain the low return temperature on the DH primary side. 2. The safety charging mode: when the temperature detected by the upper temperature sensor is below 55 C, it indicates that the necessary thermal storage for the peak load has been used up. Therefore, the safety charging mode will be activated, which allows large but necessary flowrate to charge the tank rapidly until the set point of the upper sensor is reached, so to guarantee the comfort DHW temperature for the consumers. This control loop has the first priority than the others concerning the thermal requirements and supply safety.
3. Pause mode: the charging valve is switched off when the temperature detected by the upper temperature sensor reaches 60 C, so to avoid scaling and scalding problems. The heat balance between the DH supply and the overall heat consumption for DHW is shown by Eq. (1). The heat loss from the storage tank is neglected since it is well insulated, and the magnitude is much smaller compared to the circulation heat loss.
ðt qDH dt ¼ 0
ðt
qDHW þ qc; loss dt
(1)
0
The constant charging flowrate is designed to be just able to cover the overall heat load on a daily basis. Eq. (1) can be simplified to Eq. (2):
qDH;ave ¼ qDHW;ave þ qc;loss
(2)
where, qDH, ave, qDHW, ave and qc, loss are the daily average heat supply
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(kW), the daily average heat demand of DHW (kW), and the circulation heat loss (kW), which can be referenced from the measurements. The small charging flowrate ms (kg/s) thereby can be calculated by Eq. (3),
ms ¼
q DH;ave cw $ TDH;s TDH;r
(3)
where, cw is the specific heat capacity of water (4.18 kJ/(kg$K)), TDH, is the DH supply temperature and TDH, r is the desired return temperature (K), respectively. The safety charging mode is activated if the heat load has dramatic increase compared to the average load, i.e. during the morning/evening peak hours. Accordingly, the charging flowrate of the safety mode can be calculated by Eq. (4),
s
qDH;peak ¼ qDHW;max þ qc;loss
(4)
where, qDH, peak is DH supply heat of the safety mode (kW). Since the daily DHW load of the existing consumers follows certain pattern, substantial statistical investigations are conducted to identify the normal DHW peak. The safety charging flowrate (msafe) can be calculated using Eq. (5),
msafe ¼
q DH; peak cw $ TDH;s TDH; r
(5)
2.1.2. Model 2- New charging concept for the DHW tank with external heat exchanger Regarding the DHW storage tank with external HEX, the stratification inside the tank can be better retained. Therefore, the water
layer at the tank bottom is supposed to have lower temperature compared to layers above. When DHW draw-off occurs, the bottom layers are firstly pushed out of the tank and exchanges heat with the DH flow. As a result, the DH return temperature can be better cooled down. Model 2 is developed to analyse the performance of the tank with external HEX, and confirm the more efficient tank layout by comparison with Model1. In this model, heat transfer effectiveness of the HEX is assumed to 1.0. The inlet temperature on the secondary side of the HEX is assumed to be the same as the domestic cold water temperature until the tank is fully charged. The water flow on the secondary side is supposed to be heated to 60 Cand mix with DHW of the top layers inside the tank (set to 50 C), in order to maintain the required DHW supply temperature (no lower than 55 C). If the temperature from the top sensor is below 50 C, the safety mode will be switched on. On the primary side, a motorized valve module with PI control is developed to heat the DHW to the set point temperature automatically. The control logic is shown in Fig. 3. The small charging flowrate is determined based on the same theory as Model 1. Regarding the DHW flowrate from the tank to the HEX, it can be calculated as Eq. (6).
ms; 2nd ¼
qDH;ave cw $ T2nd;out T2nd; in
(6)
where, T2nd, out, T2nd, in are the water temperatures at the outlet and inlet of the HEX on the secondary side (K). The water flowrate to the HEX of the safety mode (msafe) can be calculated by Eq.(7),
msafe; 2nd ¼
qDH; peak cw $ T2nd;out T2nd; in
(7)
Fig. 3. Control logic of the new charging concept for the tank with external heat exchanger.
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Fig. 4. DH supply (Avg. Tsup) and return temperature (Avg. Tret) of the case study.
2.2. Case study and the dynamic load profile An existing multistory building is selected for the research project. The building is located in Copenhagen, Denmark. It has 5 floors with 3 apartments on each floor. Both the space heating demand and domestic hot water demand are supplied by the local district heating company. The substation is located in the basement of the building and equipped with remote controllers and heat meters. The heat meter can measure and transmit the data of the total heat consumption, water flow, supply/return temperature on the DH primary side. The flow-averaged DH supply temperature and return temperature during the measuring period are shown in Fig. 4. The distinct valley in the red line is due to the system renovation. Traditionally, there is no limit of the charging power for the DHW tank, and the charging process is determined mainly by the DHW supply temperature. As a result, the DHW tank is fully charged in short period by the large DH flow. Thereby, as shown in Fig. 4, the average DH return temperature can reach beyond 50 C after the night period and just before the peak DHW load in the morning. In order to develop the new charging method, extra smart meters are installed as Fig. 5 shows. The meter M1 measures the heat supplied to the DHW tank, the supply/return temperature from the DHW loop and the DH flowrate. The measurements from M2 indicate the actual DHW tapping profile of the 15 consumers. The meter M3 measures the energy
consumption and temperatures of the circulation loop. To set up the dynamic simulation of the new tank control strategy, the tapping profile is measured by the time step of 1 h. The measurements of 8 days (192 h) are selected as the input to the model. Fig. 6 shows the dynamic tapping profile of the case building during the period. An extremely high DHW consumption can be found on the 60th hour, which is caused by the regular thermal treatment for Legionella. From the profile, the demand during the morning/evening peak hours is much larger compared to the other period of the day. The average daily DHW demand of the case building is approximately 2 kW, while the peak DHW demand can reach to 10 kW. During the night period, there is almost no draw-off. The heat from DH supply is mainly used to cover the circulation heat loss. Since the circulation flow around 50 C is circulating all the time, the tank is gradually fully charged. Consequently, the DH return temperature increases and reaches to the peak just before the first tapping in the morning. Sensitivity analyses are conducted to investigate the impacts of several influential factors. The optimal tank control strategies are determined by dynamic simulations and applied in the case study afterwards. To build a fair comparison, the heat capacity and the tank dimension of the two tank layouts are assumed to be the same, which are identical to the DHW tank installed in the case study. The actual DHW storage tank in the case study is shown in Fig. 7. Table 1 lists the main dimension parameter inputs to the self-developed tank component in the Modelica model. In the model, the water volume inside the tank is divided into 40 layers to facilitate numerical calculations. The lower temperature sensor is located on the 35th layer from the tank top (0.2 m height from the tank bottom), which is the same height as the exit of the heating coil. The length of the distribution pipes for DHW is approximately 100 m in the case building.
2.3. Sensitivity investigations Three scenarios with different levels of the circulation heat losses are investigated, including:
Fig. 5. Installations of the extra meters in the case substation.
1. Low heat loss scenario (LHLS): the circulation heat loss of 0.5 kW, which is equal to 25% of the average daily DHW demand. 2. Moderate heat loss scenario (MHLS): the circulation heat loss of 1.0 kW, which is equal to 50% of the average daily DHW demand. 3. High heat loss scenario (HHLS): the circulation heat loss of 1.5 kW, which is equal to 75% of the average daily DHW demand.
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Fig. 6. Measured DHW load profile of the case building.
In addition to these, the investigation on the circulation heat losses is extended to more different levels, so that to identify its impact on the DH return temperature in general. The extended circulation heat loss investigated are 2.0 kW, 2.5 kW and 3.0 kW, which are equal to 100%, 125% and 150% of the DHW demand. In addition to the circulation heat loss, the impact of the location of the upper temperature sensor is also investigated. The location of the upper temperature sensor determines the on/off of the charging process, thereby influencing the stability of the stratification inside the tank and the DH return temperature. The height of 1.2 m (the 10th layer) and the height of 0.8 m (the 20th layer) are considered for the comparison in all the simulated scenarios. Considering the differences between the model inputs and the reality caused by the rounding error in the measurements and the technical assumptions, the adjustments of ±20% on the calculated ms are tested together. The simulation results with the optimal performances from Model 1 are applied in Model 2 afterwards for
Fig. 7. DHW tank in the case study.
comparisons. In order to evaluate the performance of the system, some important index parameters are investigated in the results section, such as the energy based primary return temperature (TDH, r), DHW supply temperature (TDHW), circulation water temperature (Tcir), as well as the DH charging flowrate (mDH). Furthermore, the range of the DHW supply temperature (from the minimum to the maximum temperature) is also analysed to make sure the comfort and hygiene requirements are met. The results of the first 24 h are excluded due to the initialization deviation. 3. Results 3.1. Performances of Model1 under different scenarios considering the heat loss ratio The results of the reference scenario (with conventional charging strategy) are compared to the scenarios with new charging strategy considering different heat loss levels. According to the settings of different scenarios and the regulations, the required small constant charging flowrate and safety charging flowrate of Model1 (DHW tank with the internal heating coil) are shown in Table 2. 3.1.1. Scenario 1 e Model1 under low heat loss scenario (LHLS) Fig. 8 shows the performances of the new control method for the system with a tank using internal heating coil, considering different locations of the upper temperature sensor. From the results, the new charging method improves the system performance of the LHLS (heat loss equals to 25% of the DHW demand). Both the DH return temperature and the required DH charging flowrate are substantially reduced compared to the scenario using conventional charging method (refers to the “Reference” in the figures). Moreover, the mean DHW supply temperature is within the range of 50e60 C and the minimum DHW temperature (TDHW) is above 50 C, which meets the requirements. As Fig. 8a shows, for the system in which the upper temperature sensor at higher location (10th layer), the lowest possible DH return temperature (TDH, r) is 22.0 C. It is 6.2 C lower compared to the reference case. The corresponding charging flowrate is the theoretical value minus 20% (0.8 ms). Along with the increasing of the charging flowrate, the DH return temperature also increases. For example the case with charging flowrate of 1.2 ms increases the return temperature by 1.4 C compared to the case with the charging flowrate of 0.8 ms. Regarding the scenarios which the upper temperature sensor is located on the 20th layer, the DH return temperatures are higher compared to the corresponding scenarios with the upper sensor located on the 10th layer (see Fig. 8b).
Please cite this article as: Huang T et al., Multi-mode control method for the existing domestic hot water storage tanks with district heating supply, Energy, https://doi.org/10.1016/j.energy.2019.116517
T. Huang et al. / Energy xxx (xxxx) xxx Table 1 Dimension of DHW storage tank in the case building. Parameter Tank volume Height of the tank Thickness of the insulation Specific heat conductivity of the insulation Position of the inlet of the circulation loop Exterior diameter of the heat exchanger pipe
750 L 1.59 m 70 mm 0.045 W/(m$K) 15th layer 32 mm
Table 2 Required charging flowrates of Model 1. System
LHLS
MHLS
HHLS
Circulation heat loss (qc, loss) [kW] Circulation flowrate (mc) [L/s] Calculated small charging flowrate (ms) [L/s] Safety charging flowrate (msafe) [L/s]
0.5 0.024 0.012 0.050
1.0 0.048 0.014 0.053
1.5 0.072 0.017 0.055
Among all the scenarios with the lower sensor position, the lowest DH return temperature is 24.3 C with the theoretical charging
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flowrate (ms). In terms of the mass flowrate (mDH) of the DH flow, the new charging concept requires less DH water for the DHW preparation and distribution. The system with higher location of the upper sensor has the average DH heating flow reduced to 20%, while for the system with the upper sensor located on the lower level, the reduction is around 6%. 3.1.2. Scenario 2 e Model1 under moderate heat loss scenario (MHLS) Fig. 9 shows the results of the MHLS Scenario. The conventional charging strategy results in the average DH return temperature at 32.7 C, and the daily mean DH charging mass flowrate around 0.021 kg/s. As Fig. 9a presents, under the situation that the distribution heat loss equals to 50% of the DHW demand, the new charging method reduces the DH return temperature to 27.3 C to the lowest with the upper sensor on the 10th layer. The corresponding mean DH charging flowrate is about 0.018 kg/s (mDH). The optimal charging flowrate is 0.8 ms. The system performs worse along with the charging flowrate increasing. For example, 20% increment of the charging flowrate can lead to 1 C increase of the DH return temperature. Therefore, the over dimension for the
Fig. 8. Comparison of the system performance between the new charging concept and the conventional charging method for the LHLS scenario (ms ¼ 0.012 kg/s).
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charging flowrate should be avoided. The DHW supply temperature and the circulation temperature are within the required range of the comfort and hygiene regulations. In terms of the system with the upper sensor located on the 20th layer, there is little difference in respect to the DH return temperature. But more mass flow from DH is required to provide the equivalent demand of DHW.
the DHW tank. For system with larger distribution heat losses, sufficient thermal storage capacity can help to cover the distribution heat loss stably by avoiding using the large safety charging flowrate too frequently.
3.2. Impact of the heat loss ratio on the system performance 3.1.3. Scenario 3 e Model1 under high heat loss scenario (HHLS) The results of the scenario with the heat loss equaling to 75% of the DHW demand are shown in Fig. 10. The DH return temperature of the case using conventional charging method is 35.8 C. By using the new charging method, the DH return temperature can be reduced by 4e5 C. However, comparing the performances of the scenarios according to the locations of the upper temperature sensors, the scenarios with lower position of the upper sensor result in both lower DH return temperature and less demand of the average DH charging flow. The reason is that the lower the upper sensor is located, the larger thermal content that can be stored in
To investigate the more general relation between the circulation heat loss of the system and the DH return temperature, extra scenarios regarding the heat loss ratio are simulated. The systems with the circulation heat loss of 2 kW, 2.5 kW and 3 kW are also analysed. Fig. 11 shows the results of the system with the theoretical calculated charging flowrate, and the uppers sensor is located on the 10th layer. A conspicuous uptrend of the DH return temperature can be observed with the increase of the circulation heat loss. The trend is dramatic for the cases that the circulation heat loss is less than the DHW demand. Until the circulation heat loss reached the same level as the DHW demand, such relation continues but with
Fig. 9. Comparison of the system performances between the new charging concept and the conventional charging method for the MHLS scenario (ms ¼ 0.014 kg/s).
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Fig. 10. Comparison of the system performances between the new charging concept and the conventional charging method for the HHLS scenario (ms ¼ 0.017 kg/s).
less significant increment. Therefore, it demonstrates that the circulation heat loss has the direct impact on the DH return temperature, and such impact is more severe for the systems with small relative heat loss. Theoretically the DH return temperature can be as high as the circulation temperature if the distribution heat loss is much magnificent compared to the DHW demand. Therefore, in order to cool the DH return temperature more efficiently, minimization of the circulation heat loss plays an important role. 3.3. Comparison between Model1 and Model2 by different scenarios
Fig. 11. Relation between the circulation heat loss and DH return temperature.
Table 3 presents the calculated charging flowrates on both the primary side and secondary side of the standalone heat exchanger of Model2 with externa HEX. Fig. 12 presents the parallel comparisons between the system with the internal coil and the system with the external HEX. The configurations that lead to the optimal performances of the two systems are selected for comparisons. For LHLS, the system equipped with the external HEX shows better performance compared to the system using a storage tank with internal heating coil. The DH return temperature is only 16.9 C,
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Table 3 Required charging flowrate of Model 2. System
LHLS MHLS HHLS
Circulation heat loss (qc, loss) [kW]
Charging flowrate for primary side [kg/s] Small charging flowrate (ms)
Output range of PI controller
Small charging flowrate (ms,2nd)
Safety charging flowrate (msafe,2nd)
0.5 1.0 1.5
0.009 0.011 0.013
0.2e2.1 0.34e2.2 0.41e2.5
0.012 0.015 0.017
0.024 0.035 0.043
which shows great reduction compared to Model1 (21.3 C) under the equivalent condition. The required DH charging flow of Model2 is also less. While for the MHLS and HHLS scenarios, the results are different. From Fig. 12b and c, the DH return temperature from the system with external heat exchanger are 2 C higher to maintain the required DHW supply temperature. These manifest that the layout with the external HEX has little effect of reducing the DH return temperature if the distribution heat loss of the system beyond a certain level.
4. Discussion 4.1. Improvements of the control method for Model1 with practical application A phenomenon is observed when looking at the detailed results of HHLS and MHLS. The DH return temperature always started increasing from the midnight when the circulation heat loss dominates the total heat load. The larger the distribution heat loss, the higher the average DH return temperature. In the case study, the distribution heat loss is 2.5 times as much as the DHW demand, and the average DH return temperature is almost 45 C. This is because that by covering the circulation heat loss,the temperature of the upper part of the tank is gradually cooled down below the set point of the upper temperature sensor, while the tank bottom is still maintained warm since no heat exchanging with the domestic cold water occurred for DHW production. According to the control logic of Model1, the small charging flowrate would never be activated. Instead, the safety charging flow would be utilized as long as the set point of the upper sensor (55 C) is reached. Therefore, the safety charging flow at large flowrate is switched on/off frequently to maintain the temperature of the top tank between 55 and 60 C. To improve the control of Model1, we further developed the control strategy during the non-tapping period in the night period, which is named as Model 1.1. Fig. 13 shows the logic of the improved control method. When the water temperature detected by the lower sensor is above 20 C, the controller would switch to a tiny charging flowrate in order to just compensate the distribution heat losses during the non-tapping period. Fig. 14 shows the comparison of the DH return temperature between Model 1 and the improved Model 1.1. The DH return temperatures under the two control strategies are similar for the low heat loss scenario. However, a reduction about 2 C can be observed for MHLS and HHLS by the improved Model1.1. The overall reduction of the DH return temperature compared to the reference scenario can reach to 6e8 C. Nevertheless, the DH return temperature for high heat loss scenario (HHLS) is still above 30 C from the simulation results. This indicates that, it is too difficult to cool down the DH return temperature of the system with large distribution heat loss to large extent by only improving the charging strategy. In the case study, we limited the maximum charging power to the storage tank to just necessary. Fig. 15 shows the measurements of one-week period
Charging flowrate for secondary side [kg/s]
before and after the adjustment. The new charging concept can reduce the DH return temperature, but the effect is very tiny, which is average 4 C on weekly basis. Therefore, combining extra supplementary heating devices such as heat pump or electric heater cooperatively should be considered as alternative solutions. Admittedly, the operation of heat pump or electric heater can incur extra electricity consumption. However, they also improve the system performance by saving the distribution heat losses and reduce the DH return temperature. The differences between the saved energy and consumed energy vary according to the size and the layout of the system. Therefore, specific feasibility analyses considering energy and economy benefits are necessary in the planning phase. In the case study, sequential experiments are performed to test the impact by the heat pump on reducing the DH return temperature. We installed a micro heat pump in the circulation loop for the case building, which is supposed to cover at least half of the circulation heat loss. As a result, the DH return temperature can be controlled in the range of 20e25 C based on a 90-day observation. 4.2. Limitation of the system performance by the distribution heat loss As required from the regulations, the temperature of the DHW preparation system should be above 50 C including the circulation water to avoid the Legionella growth. Therefore, for systems with great distribution heat losses, large amount of the circulation flow is necessary. As the circulation flow normally is reinjected back to the top of the DHW tank, it can enhance the mixing effect and break the thermocline inside the tank. Such phenomenon can leave severe impact on the tank with external HEX system. In the simulations it is hard to find a way to control the primary return temperature at a stable level. The fluctuation of the DH return temperature always exists. Moreover, the new charging concept intends to use small charging flowrate to realize stable constant charging process, so that to achieve more efficient cooling of the DH flow. As a result, the circulation flowrate in the simulations is 3.2 times higher than the small charging flowrate for MHLS and 4.2 times higher in HHLS. It is hard to maintain a stable stratification inside the tank, and the DH return temperature cannot be lowered effectively. That is why for the MHLS and HHLS, the system performances of the tank with external HEX are not as good as expected. But for LHLS, the circulation flowrate is not so large compared to the small charging flowrate required by the new charging concept, thus it is easier to keep the dynamic thermal balance inside the DHW tank as well as the thermocline. Thereby, the performance of the system with external HEX is better as expected. 5. Conclusion In this study, an innovative multi-mode charging strategy is developed for the DHW tank to improve the charging process and the district heating return temperature. Multi scenarios are built to
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Fig. 12. Comparisons of the performance between the system with internal heating coil (int_HEX) and the system with external HEX (ext_HEX) within different scenarios considering the heat loss ratio.
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Fig. 13. Diagram of the control logic of Model 1.1.
Fig. 14. Comparisons of the performance of Model 1 and Model 1.1 regarding systems with different heat losses.
test the performance of the new control strategy compared to the conventional one. The impact of the heat loss ratio, the location of the temperature sensor, and the system configuration are investigated by dynamic models with the measured DHW load profile. From the results, it can be concluded that: The new control concept is effective to reduce the DH return temperature by 5e8 C for the system with circulation heat loss less than 75% of the DHW demand in comparison to the conventional charging strategy. The higher location of the temperature sensor for charging control imposes positive impact on the system performance for the system with small heat loss; while for the case with large
heat loss, lower location of the temperature sensor helps to stabilize the charging process and achieve better performance. The tank with external heat exchanger (HEX) performs better than the tank with internal coil for the low heat loss scenario (heat loss is less than 25% of DHW demand). The circulation heat loss has negative impact on the performance of the DHW preparing system. The improvements by the reduction of the heat loss is more significant for the cases with the heat loss less than the DHW demand. It is difficult to reduce the DH return temperature by large extent only by improving the charging control. Alternative solution can be the combination with supplementary heating devices.
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Fig. 15. Comparison of the return temperatures in the case study before (Tret _be) and after (Tret_af) limiting the supplied heating power to the tank.
Acknowledgement This research project is funded by Danish EUDP (Energy Technology Development and Demonstration); project title: Energy Lab Nordhavn - Smart components in integrated energy systems, project number: 64015-0055. Moreover, the authors would also like to thank HOFOR e Greater Copenhagen Utility, Danfoss for their great support in order to conduct the research. References [1] Gong M, Werner S. Exergy analysis of network temperature levels in Swedish and Danish district heating systems. Renew Energy 2015;84:106e13. https:// doi.org/10.1016/j.renene.2015.06.001. [2] Lund H, Werner S, Wiltshire R, Svendsen S, Thorsen JE, Hvelplund F, et al. 4th generation district heating (4GDH). Energy 2014. https://doi.org/10.1016/ j.energy.2014.02.089. [3] Frederiksen S, Werner S. District heating and cooling. 2013. [4] Li H, Svendsen S, Werner S, Persson U, Ruehling K, Felsmann C, et al. Annex X final report toward 4 th generation district heating: experience and potential of low-temperature district heating short title: toward 4 th generation district heating. 2014. [5] Kaarup Olsen P. Guidelines for low-temperature district heating. EUDP 2010-II full-scale demonstr low-temperature dist heat exist build. 2014. p. 1e43. [6] Centralkomunernes Transmissionsselskab. Technical key figures [Tekniske nøgletal] [in danish], http://www.ctr.dk/teknik.aspx. [Accessed 11 April 2018]. [7] Fjernvarme Dansk. Benchmarking, statistics 2017 [Benchmarking, statistik 2017] [in danish], http://www.danskfjernvarme.dk/viden-om/statistiksubsection/aarsstatistik?. [Accessed 11 April 2018]. [8] Yang X, Svendsen S. Ultra-low temperature district heating system with central heat pump and local boosters for low-heat-density area: analyses on a real case in Denmark. Energy 2018. https://doi.org/10.1016/ j.energy.2018.06.068. [9] DS 439:2009. Norm for vandinstallationer [Code of Practice for domestic water supply] (in Danish). 2009. p. 75. [10] European Committee for Standardization. Recommendations for prevention of Legionella growth in installations inside buildings conveying water for human consumption. CEN/TR 2012;16355:2012. [11] Arteconi A, Hewitt NJ, Polonara F. Domestic demand-side management (DSM): role of heat pumps and thermal energy storage (TES) systems. Appl Therm Eng 2013. https://doi.org/10.1016/j.applthermaleng.2012.09.023. [12] Stinner S, Huchtemann K, Müller D. Quantifying the operational flexibility of building energy systems with thermal energy storages. Appl Energy 2016. https://doi.org/10.1016/j.apenergy.2016.08.055. K, Bindner HW. Demand side management [13] Cai H, Ziras C, You S, Li R, Honore in urban district heating networks. Appl Energy 2018. https://doi.org/10.1016/ j.apenergy.2018.08.105. [14] Finck C, Li R, Zeiler W. Economic model predictive control for demand flexibility of a residential building. Energy 2019. https://doi.org/10.1016/ j.energy.2019.03.171. [15] Bøhm B, Schrøder F, Bergsøe NC. Varmt Brugsvand (Domestic Hot Water). 1.
Denmark: Statens Byggeforskningsinstitut; 2009. [16] Bøhm B. Production and distribution of domestic hot water in selected Danish apartment buildings and institutions. Analysis of consumption, energy efficiency and the significance for energy design requirements of buildings. Energy Convers Manag 2013;67:152e9. https://doi.org/10.1016/ j.enconman.2012.11.002. [17] United Kingdom. Home Care Association. Guidance controlling scalding risks from bathing and showering. 2012. [18] Shin MS, Kim HS, Jang DS, Lee SN, Lee YS, Yoon HG. Numerical and experimental study on the design of a stratified thermal storage system. Appl Therm Eng 2004. https://doi.org/10.1016/S1359-4311(03)00242-4. [19] Jordan U, Furbo S. Thermal stratification in small solar domestic storage tanks caused by draw-offs. Sol Energy 2005. https://doi.org/10.1016/ j.solener.2004.09.011. [20] Yang L, Bøhm B, Paulsen O, Frederiksen S. Evaluation OF the dynamic performance OF a hot water tank with built-in heating coil. Int J Energy Res 1997;21:265e74. https://doi.org/10.1002/(SICI)1099-114X(199703)21: 3<265::AID-ER252>3.0.CO;2-0. [21] Nabavitabatabayi M, Haghighat F, Moreau A, Sra P. Numerical analysis of a thermally enhanced domestic hot water tank. Appl Energy 2014. https:// doi.org/10.1016/j.apenergy.2014.04.081. [22] Prud’Homme T, Gillet D. Advanced control strategy of a solar domestic hot water system with a segmented auxiliary heater. Energy Build 2001. https:// doi.org/10.1016/S0378-7788(00)00107-9. czy-Víg P, Farkas I. Neural network modelling of thermal stratification in a [23] Ge solar DHW storage. Sol Energy 2010. https://doi.org/10.1016/ j.solener.2010.02.003. [24] Kepplinger P, Huber G, Petrasch J. Autonomous optimal control for demand side management with resistive domestic hot water heaters using linear optimization. Energy Build 2015. https://doi.org/10.1016/ j.enbuild.2014.12.016. [25] Kepplinger P, Huber G, Petrasch J. Field testing of demand side management via autonomous optimal control of a domestic hot water heater. Energy Build 2016. https://doi.org/10.1016/j.enbuild.2016.06.021. [26] Yang X, Li H, Svendsen S. Alternative solutions for inhibiting Legionella in domestic hot water systems based on low-temperature district heating. Build Serv Eng Technol 2016;37:468e78. https://doi.org/10.1177/ 0143624415613945. [27] Association M. Modelica, n.d, https://www.modelica.org. [Accessed 30 August 2019]. € finger M, Basciotti D, Schmidt RR, Meissner E, Doczekal C, Giovannini A. [28] Ko Low temperature district heating in Austria: energetic, ecologic and economic comparison of four case studies. Energy 2016;110:95e104. https://doi.org/ 10.1016/j.energy.2015.12.103. [29] Kauko H, Kvalsvik KH, Rohde D, Nord N, Utne Å. Dynamic modeling of local district heating grids with prosumers: a case study for Norway. Energy 2018. https://doi.org/10.1016/j.energy.2018.03.033. [30] Schweiger G, Larsson PO, Magnusson F, Lauenburg P, Velut S. District heating and cooling systems e framework for Modelica-based simulation and dynamic optimization. Energy 2017. https://doi.org/10.1016/ j.energy.2017.05.115. [31] Modelon. Dymola. n.d, http://www.modelon.com/products/dymola/. [Accessed 30 August 2019].
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Multi-mode control method for the existing domestic hot water storage tanks with district heating supply Tao Huang a, Xiaochen Yang b, c, *, Svend Svendsen b a
Department of Civil Engineering, Technical University of Denmark, Lyngby, 2800, Denmark Section of Building Energy and Services, Department of Civil Engineering, Technical University of Denmark, Lyngby, 2800, Denmark c Department of Environmental Science and Engineering, Tianjin University, Yaguan Road 135, Tianjin, 300350, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2019 Received in revised form 5 November 2019 Accepted 7 November 2019 Available online xxx
The hot water tank is widely used for domestic hot water (DHW) preparation. When supplied by district heating (DH), the operation of DHW tanks directly influences the DH return temperature, thereby affecting the DH system efficiency, supply capacity and the realization of the low temperature district heating. However, the conventional charging method often results in high DH return temperature. This study develops a new charging method aiming at reducing the DH return temperature without violating the comfort or hygiene requirements. The concept uses the multi-mode charging method considering the periodical characteristics of the load pattern. Multi scenarios are simulated by dynamic models using the practical DHW load profiles from a case study. Moreover, the impacts of the tank configuration, the location of the temperature sensor, and the distribution heat loss are investigated. The results show that the new control method can reduce the primary return temperature by 5e8 C compared to the conventional control method. The distribution heat loss imposes great impact on the DH return temperature. In addition, the tank with the external heat exchanger performs better than the tank with the internal heating coil if the circulation heat loss is less than 50% of the DHW demand. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Domestic hot water tank District heating return temperature Tank control method Distribution heat loss Modelica models
1. Introduction District heating (DH) is undergoing a rapid development widely since it is considered as a cost-effective way of heat supply and can give more access to renewable energy as heat sources. Within the European Union, more than 5000 DH systems are in operation today. In Scandinavian countries, a major proportion of energy in the building sector is consumed by heating and 70e90% of the heating needs are covered by district heating system (DH) in the city area [1]. It is estimated that this proportion will keep increasing in the future. The Fourth-Generation District Heating (4GDH) has emerged intending to achieve more efficient and sustainable heat production [2]. 4GDH is also known as the low-temperature district heating (LTDH). According to Refs. [3e5], the supply temperature of LTDH can be reduced to be 50e55 C. Therefore, LTDH is able to reduce the distribution heat loss effectively, which contributes to 10e30% of the heat delivery on average [6,7]. This proportion is
* Corresponding author.Section of Building Energy and Services, Department of Civil Engineering, Technical University of Denmark, Lyngby, 2800, Denmark. E-mail address: [email protected] (X. Yang).
even larger in the low-heat-density area [8]. However, how to deal with the peak load and improve the return temperature are still an issue before the LTDH can be widely applied. In Demark, the DH system covers both space heating and domestic hot water (DHW) demands. In Great Copenhagen area, the heat consumers usually take the indirect connection to the DH distribution grid through the in-building substations. The space heating loop uses a heat exchanger for heat exchanging with DH, while the DHW circuit often uses storage tanks. Depending on how the heating element is connected, the DHW tank can be divided into two types, i.e. DHW storage tanks with internal heating coils and DHW storage tanks with external heat exchangers (HEX). In the Danish Standard, the comfort temperature of the DHW for kitchen use is 45 C and 40 C for the shower. While concerning the hygiene requirements, the DHW supply temperature should be around 55 C and at least no less than 50 C during the peak load including the circulation loop [9,10]. Such requirements impose great difficulty on the peak shaving and reducing the return temperature of DH. A number of studies have been performed aiming at alleviating the DH peak load issue. One emphasis is to implement the heat
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storage units [11,12], while another is to manage the operation on the demand side by utilizing the building thermal mass capacity [13,14]. However, previous studies have primarily concentrated on the operation of the space heating system or the entire heating demand control. Few attentions are paid on the DHW system which has great potential to be improved. Bøhm et al. [15] investigated several buildings constructed between 1906 and 1996, and found that DHW heat demand amounts to 25e35% of the total heat demand. Over past 20 years, DHW consumption has increased nearly 50%. For future buildings, this fraction is likely to rise with the improvement of the building insulation standards [16,17]. Therefore, the improvement of the DHW heating loop is of great importance to the whole heating system. The hot water tank is widely used for the DHW preparation. Some of the previous researches related to the storage tanks are focused on the factors that can destroy the stratification inside the tank, such as the heat losses to the ambient or mixing caused by charging/discharging phases [18,19]. Yang et al. [20] found that the correct design of the control valve size, the settings of the temperature sensor (for DHW temperature control), and the reduction of the heating coil fouling growth rate are of significance in achieving low primary return temperature. Nabavitabatabayi et al. [21] investigated the impact of the phase change materials (PCMs) on hot water tank and indicated that PCMs could shift the power demand to the off-peak and enhance the heat transfer rate. In recent years, researchers started paying more attention on the modification of the control strategies of DHW production system. Prud’homme and Gillet [22] improved the structure of the electric heater by replacing one single electric element by three smaller ones, and implemented a predictive control strategy in conjunction with weather forecasts as well as user’s demand to control the heaters. The simulation results show good improvement in energy czy-Víg and Farkas [23] employed an artificial performance. Ge neural network modelling to control the tank stratification. The results show good control accuracy between the measured and simulated data. Kepplinger et al. simulated [24] and tested [25] an optimal heating strategy for an electric DHW tank based on the expected demand and the pseudo cost function. The results show great improvement in terms of the energy consumption and heat bill. It can be found that these control strategies are mainly datadriven. Therefore, they are based on the prior knowledge and information such as tariff data which may be difficult to obtain. At the meantime, such control algorithms may lead to substantially computational consumption. In this light, the commercial application of these methods remains doubts. Furthermore, it appears from the aforementioned studies that none of them focused on improving the practical charging control strategies of the DH water tank with minimal cost and maximal applicability, and few studies can be found to provide detailed analyses with respect to the impact on the primary return temperature. In practice, most DHW storage tanks are designed with the consideration of safety, which can lead to oversize. The charging valve is normally opened more than necessary since the charging controller tends to use large power to heat the DHW to the setpoint temperature rapidly. Such operation results in insufficient cooling if the DHW demand is tiny, for instance, in the nighttime. Moreover, the DH supply also needs to cover the distribution heat loss by warming up the circulation water to be above 50 C, which makes the cooling of the DH flow more difficult. Some researchers have started employing supplemental techniques to improve this issue. For instance, Yang et al. [26] propose to apply a micro heat pump to reutilize the heat from the primary return water to enable the DHW system to adapt to low-temperature district heating. However, the potential in optimizing the charging operation of the DHW tank to achieve better cooling effect has not yet been
investigated sufficiently. This work aims at optimizing the conventional control strategy of charging DHW storage tank under the premise of the thermal comfort and hygiene, in order to reduce the primary return temperature and shave the extreme peak for DHW preparation. A new concept for charging control strategy is therefore proposed. The multi-mode control is designed for the new control strategy to deal with the fluctuated DHW load profile, so to improve the system performances. Different scenarios are simulated by the dynamic models. Measurements from an existing building are used for dimensioning the optimal charging flowrate. The performances of the new charging control method and the conventional method are compared. The influence of the tank structure is also investigated by comparing the simulation results of two types of the DHW storage tank. Moreover, the performances of the new tank control strategy considering different levels of distribution heat losses are compared, so to clarify the influence by the heat losses. The results of this study are of significance for improving the overall DH system efficiency by lowering the return temperature and shaving off the peak load, which also paves the road for the application of LTDH. 2. Data and method 2.1. Simulation method Compared to the conventional charging concept, the new method intends to gradually charge the DHW storage tank by small constant flowrate, so that to achieve large temperature difference and stabilize the operation. The simulations are conducted by Modelica models. Modelica [27] is an object-oriented, equation-based language. It can be used to simulate complex engineering process. In recent years, it has been applied in more and more research works in the DH field [28e30]. The buildings library and self-developed component of the storage tank are used to emulate the practical system in the case study. The simulation environment in this study is Dymola [31], which is used to transformed the Modelica code by graphical interface and their logical connection. The systems of the tank with internal heating coil and the tank with external heat exchanger are considered for the simulation. Their performances are compared to investigate the impact of the tank geometry. The schematics of the two tank layouts are shown in Fig. 1. 2.1.1. Model 1- New charging concept for the DHW tank with internal heating coil The flow chart of the control logic of Model 1 is describe in Fig. 2. The DH supply temperature is assumed to be 80 C based on the data from the DH supply company. The ambient temperature and the domestic cold water temperature are assumed to be 20 Cand 10 C, respectively. Two temperature sensors are required by the new charging concept. They are assumed to be installed at different heights. The sensor situated on the upper part of the tank indicates the DHW supply temperature, while the sensor located on the bottom part is used to show the level of the DH return temperature. So the lower sensor is situated at the same height as the exit of the internal heating coil. The DH return temperature is expected to be reduced to below 30 C, therefore the limit for the lower temperature sensor is set to be 20 C considering the temperature difference between the fluid inside and outside the heating coil. Regarding the fluctuated DHW load profiles, the whole charging scheme is divided into 3 modes. 1. The slow charging mode: when the temperature of the upper temperature sensor is no lower than 55 C and the temperature of the lower sensor is below 20 C, the slow charging mode is
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Fig. 1. Schematics of DHW storage tank with internal heating coil (left) and DHW storage tank with external HEX (right).
Fig. 2. Control logic of the new charging concept for the tank with internal coil.
activated. The small charging flowrate heats the necessary thermal storage inside the tank on daily basis. The calculation method is introduced by Equations (1)e(3) in the following section. The lower temperature sensor indicates whether the whole tank is fully charged. If the temperature detected by the lower temperature sensor is above 20 C, the valve for charging will be switched off since the necessary thermal storage is reached and the surplus heating should be avoided to maintain the low return temperature on the DH primary side. 2. The safety charging mode: when the temperature detected by the upper temperature sensor is below 55 C, it indicates that the necessary thermal storage for the peak load has been used up. Therefore, the safety charging mode will be activated, which allows large but necessary flowrate to charge the tank rapidly until the set point of the upper sensor is reached, so to guarantee the comfort DHW temperature for the consumers. This control loop has the first priority than the others concerning the thermal requirements and supply safety.
3. Pause mode: the charging valve is switched off when the temperature detected by the upper temperature sensor reaches 60 C, so to avoid scaling and scalding problems. The heat balance between the DH supply and the overall heat consumption for DHW is shown by Eq. (1). The heat loss from the storage tank is neglected since it is well insulated, and the magnitude is much smaller compared to the circulation heat loss.
ðt qDH dt ¼ 0
ðt
qDHW þ qc; loss dt
(1)
0
The constant charging flowrate is designed to be just able to cover the overall heat load on a daily basis. Eq. (1) can be simplified to Eq. (2):
qDH;ave ¼ qDHW;ave þ qc;loss
(2)
where, qDH, ave, qDHW, ave and qc, loss are the daily average heat supply
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(kW), the daily average heat demand of DHW (kW), and the circulation heat loss (kW), which can be referenced from the measurements. The small charging flowrate ms (kg/s) thereby can be calculated by Eq. (3),
ms ¼
q DH;ave cw $ TDH;s TDH;r
(3)
where, cw is the specific heat capacity of water (4.18 kJ/(kg$K)), TDH, is the DH supply temperature and TDH, r is the desired return temperature (K), respectively. The safety charging mode is activated if the heat load has dramatic increase compared to the average load, i.e. during the morning/evening peak hours. Accordingly, the charging flowrate of the safety mode can be calculated by Eq. (4),
s
qDH;peak ¼ qDHW;max þ qc;loss
(4)
where, qDH, peak is DH supply heat of the safety mode (kW). Since the daily DHW load of the existing consumers follows certain pattern, substantial statistical investigations are conducted to identify the normal DHW peak. The safety charging flowrate (msafe) can be calculated using Eq. (5),
msafe ¼
q DH; peak cw $ TDH;s TDH; r
(5)
2.1.2. Model 2- New charging concept for the DHW tank with external heat exchanger Regarding the DHW storage tank with external HEX, the stratification inside the tank can be better retained. Therefore, the water
layer at the tank bottom is supposed to have lower temperature compared to layers above. When DHW draw-off occurs, the bottom layers are firstly pushed out of the tank and exchanges heat with the DH flow. As a result, the DH return temperature can be better cooled down. Model 2 is developed to analyse the performance of the tank with external HEX, and confirm the more efficient tank layout by comparison with Model1. In this model, heat transfer effectiveness of the HEX is assumed to 1.0. The inlet temperature on the secondary side of the HEX is assumed to be the same as the domestic cold water temperature until the tank is fully charged. The water flow on the secondary side is supposed to be heated to 60 Cand mix with DHW of the top layers inside the tank (set to 50 C), in order to maintain the required DHW supply temperature (no lower than 55 C). If the temperature from the top sensor is below 50 C, the safety mode will be switched on. On the primary side, a motorized valve module with PI control is developed to heat the DHW to the set point temperature automatically. The control logic is shown in Fig. 3. The small charging flowrate is determined based on the same theory as Model 1. Regarding the DHW flowrate from the tank to the HEX, it can be calculated as Eq. (6).
ms; 2nd ¼
qDH;ave cw $ T2nd;out T2nd; in
(6)
where, T2nd, out, T2nd, in are the water temperatures at the outlet and inlet of the HEX on the secondary side (K). The water flowrate to the HEX of the safety mode (msafe) can be calculated by Eq.(7),
msafe; 2nd ¼
qDH; peak cw $ T2nd;out T2nd; in
(7)
Fig. 3. Control logic of the new charging concept for the tank with external heat exchanger.
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Fig. 4. DH supply (Avg. Tsup) and return temperature (Avg. Tret) of the case study.
2.2. Case study and the dynamic load profile An existing multistory building is selected for the research project. The building is located in Copenhagen, Denmark. It has 5 floors with 3 apartments on each floor. Both the space heating demand and domestic hot water demand are supplied by the local district heating company. The substation is located in the basement of the building and equipped with remote controllers and heat meters. The heat meter can measure and transmit the data of the total heat consumption, water flow, supply/return temperature on the DH primary side. The flow-averaged DH supply temperature and return temperature during the measuring period are shown in Fig. 4. The distinct valley in the red line is due to the system renovation. Traditionally, there is no limit of the charging power for the DHW tank, and the charging process is determined mainly by the DHW supply temperature. As a result, the DHW tank is fully charged in short period by the large DH flow. Thereby, as shown in Fig. 4, the average DH return temperature can reach beyond 50 C after the night period and just before the peak DHW load in the morning. In order to develop the new charging method, extra smart meters are installed as Fig. 5 shows. The meter M1 measures the heat supplied to the DHW tank, the supply/return temperature from the DHW loop and the DH flowrate. The measurements from M2 indicate the actual DHW tapping profile of the 15 consumers. The meter M3 measures the energy
consumption and temperatures of the circulation loop. To set up the dynamic simulation of the new tank control strategy, the tapping profile is measured by the time step of 1 h. The measurements of 8 days (192 h) are selected as the input to the model. Fig. 6 shows the dynamic tapping profile of the case building during the period. An extremely high DHW consumption can be found on the 60th hour, which is caused by the regular thermal treatment for Legionella. From the profile, the demand during the morning/evening peak hours is much larger compared to the other period of the day. The average daily DHW demand of the case building is approximately 2 kW, while the peak DHW demand can reach to 10 kW. During the night period, there is almost no draw-off. The heat from DH supply is mainly used to cover the circulation heat loss. Since the circulation flow around 50 C is circulating all the time, the tank is gradually fully charged. Consequently, the DH return temperature increases and reaches to the peak just before the first tapping in the morning. Sensitivity analyses are conducted to investigate the impacts of several influential factors. The optimal tank control strategies are determined by dynamic simulations and applied in the case study afterwards. To build a fair comparison, the heat capacity and the tank dimension of the two tank layouts are assumed to be the same, which are identical to the DHW tank installed in the case study. The actual DHW storage tank in the case study is shown in Fig. 7. Table 1 lists the main dimension parameter inputs to the self-developed tank component in the Modelica model. In the model, the water volume inside the tank is divided into 40 layers to facilitate numerical calculations. The lower temperature sensor is located on the 35th layer from the tank top (0.2 m height from the tank bottom), which is the same height as the exit of the heating coil. The length of the distribution pipes for DHW is approximately 100 m in the case building.
2.3. Sensitivity investigations Three scenarios with different levels of the circulation heat losses are investigated, including:
Fig. 5. Installations of the extra meters in the case substation.
1. Low heat loss scenario (LHLS): the circulation heat loss of 0.5 kW, which is equal to 25% of the average daily DHW demand. 2. Moderate heat loss scenario (MHLS): the circulation heat loss of 1.0 kW, which is equal to 50% of the average daily DHW demand. 3. High heat loss scenario (HHLS): the circulation heat loss of 1.5 kW, which is equal to 75% of the average daily DHW demand.
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Fig. 6. Measured DHW load profile of the case building.
In addition to these, the investigation on the circulation heat losses is extended to more different levels, so that to identify its impact on the DH return temperature in general. The extended circulation heat loss investigated are 2.0 kW, 2.5 kW and 3.0 kW, which are equal to 100%, 125% and 150% of the DHW demand. In addition to the circulation heat loss, the impact of the location of the upper temperature sensor is also investigated. The location of the upper temperature sensor determines the on/off of the charging process, thereby influencing the stability of the stratification inside the tank and the DH return temperature. The height of 1.2 m (the 10th layer) and the height of 0.8 m (the 20th layer) are considered for the comparison in all the simulated scenarios. Considering the differences between the model inputs and the reality caused by the rounding error in the measurements and the technical assumptions, the adjustments of ±20% on the calculated ms are tested together. The simulation results with the optimal performances from Model 1 are applied in Model 2 afterwards for
Fig. 7. DHW tank in the case study.
comparisons. In order to evaluate the performance of the system, some important index parameters are investigated in the results section, such as the energy based primary return temperature (TDH, r), DHW supply temperature (TDHW), circulation water temperature (Tcir), as well as the DH charging flowrate (mDH). Furthermore, the range of the DHW supply temperature (from the minimum to the maximum temperature) is also analysed to make sure the comfort and hygiene requirements are met. The results of the first 24 h are excluded due to the initialization deviation. 3. Results 3.1. Performances of Model1 under different scenarios considering the heat loss ratio The results of the reference scenario (with conventional charging strategy) are compared to the scenarios with new charging strategy considering different heat loss levels. According to the settings of different scenarios and the regulations, the required small constant charging flowrate and safety charging flowrate of Model1 (DHW tank with the internal heating coil) are shown in Table 2. 3.1.1. Scenario 1 e Model1 under low heat loss scenario (LHLS) Fig. 8 shows the performances of the new control method for the system with a tank using internal heating coil, considering different locations of the upper temperature sensor. From the results, the new charging method improves the system performance of the LHLS (heat loss equals to 25% of the DHW demand). Both the DH return temperature and the required DH charging flowrate are substantially reduced compared to the scenario using conventional charging method (refers to the “Reference” in the figures). Moreover, the mean DHW supply temperature is within the range of 50e60 C and the minimum DHW temperature (TDHW) is above 50 C, which meets the requirements. As Fig. 8a shows, for the system in which the upper temperature sensor at higher location (10th layer), the lowest possible DH return temperature (TDH, r) is 22.0 C. It is 6.2 C lower compared to the reference case. The corresponding charging flowrate is the theoretical value minus 20% (0.8 ms). Along with the increasing of the charging flowrate, the DH return temperature also increases. For example the case with charging flowrate of 1.2 ms increases the return temperature by 1.4 C compared to the case with the charging flowrate of 0.8 ms. Regarding the scenarios which the upper temperature sensor is located on the 20th layer, the DH return temperatures are higher compared to the corresponding scenarios with the upper sensor located on the 10th layer (see Fig. 8b).
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T. Huang et al. / Energy xxx (xxxx) xxx Table 1 Dimension of DHW storage tank in the case building. Parameter Tank volume Height of the tank Thickness of the insulation Specific heat conductivity of the insulation Position of the inlet of the circulation loop Exterior diameter of the heat exchanger pipe
750 L 1.59 m 70 mm 0.045 W/(m$K) 15th layer 32 mm
Table 2 Required charging flowrates of Model 1. System
LHLS
MHLS
HHLS
Circulation heat loss (qc, loss) [kW] Circulation flowrate (mc) [L/s] Calculated small charging flowrate (ms) [L/s] Safety charging flowrate (msafe) [L/s]
0.5 0.024 0.012 0.050
1.0 0.048 0.014 0.053
1.5 0.072 0.017 0.055
Among all the scenarios with the lower sensor position, the lowest DH return temperature is 24.3 C with the theoretical charging
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flowrate (ms). In terms of the mass flowrate (mDH) of the DH flow, the new charging concept requires less DH water for the DHW preparation and distribution. The system with higher location of the upper sensor has the average DH heating flow reduced to 20%, while for the system with the upper sensor located on the lower level, the reduction is around 6%. 3.1.2. Scenario 2 e Model1 under moderate heat loss scenario (MHLS) Fig. 9 shows the results of the MHLS Scenario. The conventional charging strategy results in the average DH return temperature at 32.7 C, and the daily mean DH charging mass flowrate around 0.021 kg/s. As Fig. 9a presents, under the situation that the distribution heat loss equals to 50% of the DHW demand, the new charging method reduces the DH return temperature to 27.3 C to the lowest with the upper sensor on the 10th layer. The corresponding mean DH charging flowrate is about 0.018 kg/s (mDH). The optimal charging flowrate is 0.8 ms. The system performs worse along with the charging flowrate increasing. For example, 20% increment of the charging flowrate can lead to 1 C increase of the DH return temperature. Therefore, the over dimension for the
Fig. 8. Comparison of the system performance between the new charging concept and the conventional charging method for the LHLS scenario (ms ¼ 0.012 kg/s).
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charging flowrate should be avoided. The DHW supply temperature and the circulation temperature are within the required range of the comfort and hygiene regulations. In terms of the system with the upper sensor located on the 20th layer, there is little difference in respect to the DH return temperature. But more mass flow from DH is required to provide the equivalent demand of DHW.
the DHW tank. For system with larger distribution heat losses, sufficient thermal storage capacity can help to cover the distribution heat loss stably by avoiding using the large safety charging flowrate too frequently.
3.2. Impact of the heat loss ratio on the system performance 3.1.3. Scenario 3 e Model1 under high heat loss scenario (HHLS) The results of the scenario with the heat loss equaling to 75% of the DHW demand are shown in Fig. 10. The DH return temperature of the case using conventional charging method is 35.8 C. By using the new charging method, the DH return temperature can be reduced by 4e5 C. However, comparing the performances of the scenarios according to the locations of the upper temperature sensors, the scenarios with lower position of the upper sensor result in both lower DH return temperature and less demand of the average DH charging flow. The reason is that the lower the upper sensor is located, the larger thermal content that can be stored in
To investigate the more general relation between the circulation heat loss of the system and the DH return temperature, extra scenarios regarding the heat loss ratio are simulated. The systems with the circulation heat loss of 2 kW, 2.5 kW and 3 kW are also analysed. Fig. 11 shows the results of the system with the theoretical calculated charging flowrate, and the uppers sensor is located on the 10th layer. A conspicuous uptrend of the DH return temperature can be observed with the increase of the circulation heat loss. The trend is dramatic for the cases that the circulation heat loss is less than the DHW demand. Until the circulation heat loss reached the same level as the DHW demand, such relation continues but with
Fig. 9. Comparison of the system performances between the new charging concept and the conventional charging method for the MHLS scenario (ms ¼ 0.014 kg/s).
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Fig. 10. Comparison of the system performances between the new charging concept and the conventional charging method for the HHLS scenario (ms ¼ 0.017 kg/s).
less significant increment. Therefore, it demonstrates that the circulation heat loss has the direct impact on the DH return temperature, and such impact is more severe for the systems with small relative heat loss. Theoretically the DH return temperature can be as high as the circulation temperature if the distribution heat loss is much magnificent compared to the DHW demand. Therefore, in order to cool the DH return temperature more efficiently, minimization of the circulation heat loss plays an important role. 3.3. Comparison between Model1 and Model2 by different scenarios
Fig. 11. Relation between the circulation heat loss and DH return temperature.
Table 3 presents the calculated charging flowrates on both the primary side and secondary side of the standalone heat exchanger of Model2 with externa HEX. Fig. 12 presents the parallel comparisons between the system with the internal coil and the system with the external HEX. The configurations that lead to the optimal performances of the two systems are selected for comparisons. For LHLS, the system equipped with the external HEX shows better performance compared to the system using a storage tank with internal heating coil. The DH return temperature is only 16.9 C,
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Table 3 Required charging flowrate of Model 2. System
LHLS MHLS HHLS
Circulation heat loss (qc, loss) [kW]
Charging flowrate for primary side [kg/s] Small charging flowrate (ms)
Output range of PI controller
Small charging flowrate (ms,2nd)
Safety charging flowrate (msafe,2nd)
0.5 1.0 1.5
0.009 0.011 0.013
0.2e2.1 0.34e2.2 0.41e2.5
0.012 0.015 0.017
0.024 0.035 0.043
which shows great reduction compared to Model1 (21.3 C) under the equivalent condition. The required DH charging flow of Model2 is also less. While for the MHLS and HHLS scenarios, the results are different. From Fig. 12b and c, the DH return temperature from the system with external heat exchanger are 2 C higher to maintain the required DHW supply temperature. These manifest that the layout with the external HEX has little effect of reducing the DH return temperature if the distribution heat loss of the system beyond a certain level.
4. Discussion 4.1. Improvements of the control method for Model1 with practical application A phenomenon is observed when looking at the detailed results of HHLS and MHLS. The DH return temperature always started increasing from the midnight when the circulation heat loss dominates the total heat load. The larger the distribution heat loss, the higher the average DH return temperature. In the case study, the distribution heat loss is 2.5 times as much as the DHW demand, and the average DH return temperature is almost 45 C. This is because that by covering the circulation heat loss,the temperature of the upper part of the tank is gradually cooled down below the set point of the upper temperature sensor, while the tank bottom is still maintained warm since no heat exchanging with the domestic cold water occurred for DHW production. According to the control logic of Model1, the small charging flowrate would never be activated. Instead, the safety charging flow would be utilized as long as the set point of the upper sensor (55 C) is reached. Therefore, the safety charging flow at large flowrate is switched on/off frequently to maintain the temperature of the top tank between 55 and 60 C. To improve the control of Model1, we further developed the control strategy during the non-tapping period in the night period, which is named as Model 1.1. Fig. 13 shows the logic of the improved control method. When the water temperature detected by the lower sensor is above 20 C, the controller would switch to a tiny charging flowrate in order to just compensate the distribution heat losses during the non-tapping period. Fig. 14 shows the comparison of the DH return temperature between Model 1 and the improved Model 1.1. The DH return temperatures under the two control strategies are similar for the low heat loss scenario. However, a reduction about 2 C can be observed for MHLS and HHLS by the improved Model1.1. The overall reduction of the DH return temperature compared to the reference scenario can reach to 6e8 C. Nevertheless, the DH return temperature for high heat loss scenario (HHLS) is still above 30 C from the simulation results. This indicates that, it is too difficult to cool down the DH return temperature of the system with large distribution heat loss to large extent by only improving the charging strategy. In the case study, we limited the maximum charging power to the storage tank to just necessary. Fig. 15 shows the measurements of one-week period
Charging flowrate for secondary side [kg/s]
before and after the adjustment. The new charging concept can reduce the DH return temperature, but the effect is very tiny, which is average 4 C on weekly basis. Therefore, combining extra supplementary heating devices such as heat pump or electric heater cooperatively should be considered as alternative solutions. Admittedly, the operation of heat pump or electric heater can incur extra electricity consumption. However, they also improve the system performance by saving the distribution heat losses and reduce the DH return temperature. The differences between the saved energy and consumed energy vary according to the size and the layout of the system. Therefore, specific feasibility analyses considering energy and economy benefits are necessary in the planning phase. In the case study, sequential experiments are performed to test the impact by the heat pump on reducing the DH return temperature. We installed a micro heat pump in the circulation loop for the case building, which is supposed to cover at least half of the circulation heat loss. As a result, the DH return temperature can be controlled in the range of 20e25 C based on a 90-day observation. 4.2. Limitation of the system performance by the distribution heat loss As required from the regulations, the temperature of the DHW preparation system should be above 50 C including the circulation water to avoid the Legionella growth. Therefore, for systems with great distribution heat losses, large amount of the circulation flow is necessary. As the circulation flow normally is reinjected back to the top of the DHW tank, it can enhance the mixing effect and break the thermocline inside the tank. Such phenomenon can leave severe impact on the tank with external HEX system. In the simulations it is hard to find a way to control the primary return temperature at a stable level. The fluctuation of the DH return temperature always exists. Moreover, the new charging concept intends to use small charging flowrate to realize stable constant charging process, so that to achieve more efficient cooling of the DH flow. As a result, the circulation flowrate in the simulations is 3.2 times higher than the small charging flowrate for MHLS and 4.2 times higher in HHLS. It is hard to maintain a stable stratification inside the tank, and the DH return temperature cannot be lowered effectively. That is why for the MHLS and HHLS, the system performances of the tank with external HEX are not as good as expected. But for LHLS, the circulation flowrate is not so large compared to the small charging flowrate required by the new charging concept, thus it is easier to keep the dynamic thermal balance inside the DHW tank as well as the thermocline. Thereby, the performance of the system with external HEX is better as expected. 5. Conclusion In this study, an innovative multi-mode charging strategy is developed for the DHW tank to improve the charging process and the district heating return temperature. Multi scenarios are built to
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Fig. 12. Comparisons of the performance between the system with internal heating coil (int_HEX) and the system with external HEX (ext_HEX) within different scenarios considering the heat loss ratio.
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Fig. 13. Diagram of the control logic of Model 1.1.
Fig. 14. Comparisons of the performance of Model 1 and Model 1.1 regarding systems with different heat losses.
test the performance of the new control strategy compared to the conventional one. The impact of the heat loss ratio, the location of the temperature sensor, and the system configuration are investigated by dynamic models with the measured DHW load profile. From the results, it can be concluded that: The new control concept is effective to reduce the DH return temperature by 5e8 C for the system with circulation heat loss less than 75% of the DHW demand in comparison to the conventional charging strategy. The higher location of the temperature sensor for charging control imposes positive impact on the system performance for the system with small heat loss; while for the case with large
heat loss, lower location of the temperature sensor helps to stabilize the charging process and achieve better performance. The tank with external heat exchanger (HEX) performs better than the tank with internal coil for the low heat loss scenario (heat loss is less than 25% of DHW demand). The circulation heat loss has negative impact on the performance of the DHW preparing system. The improvements by the reduction of the heat loss is more significant for the cases with the heat loss less than the DHW demand. It is difficult to reduce the DH return temperature by large extent only by improving the charging control. Alternative solution can be the combination with supplementary heating devices.
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Fig. 15. Comparison of the return temperatures in the case study before (Tret _be) and after (Tret_af) limiting the supplied heating power to the tank.
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