Domestic Hot Water Storage Tank Research Articles

Experimental Performance and Cost-Effectiveness of a Combined Heating System under Saharan Climate

The solar water heater can be integrated into future residential buildings as the main energy source, which will subsequently reduce the energy cost of water heating. An original configuration for an efficient Domestic Hot Water (DHW) storage tank is developed and experimentally evaluated under Saharan climate. This novel DHW configuration includes a hybrid (solar and electric) energy system with a flat plate solar collector coupled with an electric heater. Additionally, a phase change material (PCM) mixture that is composed of paraffin wax and animal fat with a melting temperature between 35.58 °C and 62.58 °C and latent heat between 180 and 210 kJ/kg is integrated into this novel tank configuration. The experimental results indicated that hot water production by using latent heat storage could be economically attractive. By evaluating the cost of water heating expressed in Algerian dinar per liter (DZD/L), it was found that one liter of hot water may cost around 0.1362 DZD/L (i.e., 0.00096 USD/L) compared to 0.4431 DZD/L for the conventional water heater, an average energy cost savings of 69.26%. On a yearly basis, the average energy cost savings may reach up to 80.25% if optimal tilt for the solar collector is adopted on a monthly basis. The flat plate collector may be vulnerable to convective heat transfer; therefore, other solar collectors, such as vacuum tube collectors, may provide enhanced energy performance.

Why and which opacifier for perlite based vacuum insulation panels (VIPs) in the average temperature range of 10–70 °C

Little is known on the effect of opacifiers in perlite core vacuum insulation panels at lower temperatures in the range of 10–70 °C, which are important for applications like refrigerators, transport boxes, buildings, and domestic hot water storage tanks. This paper shows that the effect of opacifier on perlite core Vacuum Insulation Panels (VIPs) is radically different than that on fumed silica VIPs. Identifying an optimum proportion of opacifiers for perlite core VIPs is not a trivial task because of a concomitant rise in solid conductivity as the radiative conductivity decreases. Further, the effect of opacifiers on temperature dependent thermal conductivity of VIPs has been resolved. Opacifiers in perlite are shown to yield clear benefits by reducing the overall thermal conductivity for insulation exposed to ≥70 °C. This was not the case for ≤20 °C. Several VIPs were manufactured using a low-density expanded perlite and 3 different proportions of 4 opacifiers i.e., carbon black, graphite and two variants of silicon carbide (SiC I, SiC II). These VIPs were tested at various combinations of sealing pressure and four different average temperatures i.e., 10 °C, 20 °C, 40 °C and 70 °C. A novel methodology has been developed to derive the evacuated thermal conductivity of perlite core VIPs by curve fitting technique, in order to eliminate the effect of gaseous conduction. The results were also compared with fumed silica core vacuum insulation panels with different proportions of carbon black as opacifier. Among the opacifiers tested (carbon black, graphite, SiC I and SiC II) in this research, Silicon carbide I with a mean particle size of 20 μm was found out to be the best opacifier, with one of its composite resulting in a 50% lower thermal conductivity at 70 °C in comparison to pristine perlite core VIPs.

Staged control of domestic hot water storage tanks to support district heating efficiency

Storage tanks are commonly used for domestic hot water (DHW) preparation in large buildings supplied by district heating (DH), especially to cope with peak demand. The charging control of DHW tank systems is often suboptimal, increasing return temperatures and harming the overall DH operation efficiency. This paper presents two novel control concepts to optimize DHW tank charging, satisfying comfort and hygienic requirements without leading to excessive DH flows. The first, more complex control concept employs the smart energy meter sometimes used for DHW billing. It inspired the development of a second, broadly implementable control concept employing a staged proportional gain with an added temperature sensor. The authors tested and refined this staged-gain concept using a validated Modelica model of a real DHW system in a Danish multistory residential building. The authors subsequently implemented the staged-gain control concept in the field, successfully reducing the energy-weighted DH return temperature by 7 °C and the total DH flow by 23.6% compared to the conventional thermostatic control. This analysis accounted for the variation in DHW tapping, DHW temperature, DH supply temperature, and cold water temperature. Furthermore, the performance was robust to relaxed settings of the valve constraints, demonstrating minimal configuration requirements for new implementations.

Experimental and simulation analysis on thermal stratification characteristics in solar storage tanks with phase change materials

Among the various ways to improve energy storage and utilization in solar thermal energy storage systems, the water tank is often considered as an effective heat storage utilization. In this study, sodium acetate trihydrate (SAT) is coupled with a solar domestic hot water (DHW) storage tank as a phase change material (PCM). The thermal stratification of latent heat storage water tanks with different locations of PCM units and inlet flows was researched experimentally and numerically. The results show that in the charging process, as the inlet flow increases, the thickness of the thermocline gradually increases, with the degree of mixing in the tank aggravation. The Richardson number gradually increases as the PCM units are moved closer to the inlet. The RMSE between the experimental and simulated values reached the minimum at a flow rate of 3 L/min. The system has taken as the optimal working condition with better thermal stratification when the PCM units are placed at PCM1, and the flow rate is 3 L/min. This study provides theoretical and experimental guidance for the optimal design of DHW storage systems.

Diagnostics of a Domestic Hot Water Storage Tank under Operating Conditions

This paper presents a new method for the diagnostics of a hot water storage tank under operating conditions. Depending on the operating point of the tank, the method enables determination of thermal conductivity coefficients of the coil heat exchanger, which allows us to determine the intensity of heat transfer between the transfer medium and water in the tank as well as of tank walls, which consequently enables determination of heat losses to the environment. Furthermore, the dynamic properties of the tank may also be determined by applying this method. The advantage of this method is possibility of analyzing changes in the material constants of the coil heat exchanger, tank walls, and dynamic properties of the tank as a function of mass flow of the medium supplying the coil heat exchanger. The possibility of determining coefficients of thermal conductivity as well as the inertia of tank and exchanger, based on temperature measurements acquired in operating conditions is a novelty in this paper. Knowing the variability of material constants and of dynamic properties of the tank as a function of medium flow rate allows multicriteria optimization to be performed which, with a conventional design of the tank, results in a reduction of up to 10% in the time taken to prepare domestic hot water.

Solar assisted heat pump pilot plant management and troubleshooting by means of numerical modelling: a case study

A solar assisted heat pump integrated with fossil burners provides the space heating and Domestic Hot Water (DHW) demand of the sport palace Palacus, Genoa. The plant complexity requires a Data Acquisition System (DAS) to perform diagnoses for problem troubleshooting. A DAS with a set of sensors able to measure every key parameter of the plant loses economic feasibility as the installation complexity increases. This compulsory lack of information can be handled with numerical modelling calibrated on the collected data. In the case study, a net variation in the thermal losses during night (where all the plant is off and the facility is closed) within the DHW storage tanks has been measured. Without any apparent reason, the temperatures reached within the tank at 8.00 a.m. can be either 33 °C or 50 °C. According to the approach of inverse heat transfer problems, a numerical simulation in TRNSYS17 is used to enquire the most likely causes of failure and to establish new regulation criteria. Thermal stratification and its destruction are studied to better understand the temperature trend in the DHW tanks.

Approximating the solution of the discharging process in a domestic hot water storage tank

ABSTRACT In this article a new approximation procedure for the temperature during the discharging process in a domestic hot water storage tank is developed. The main idea of this procedure is the mathematical description of the dynamic behaviour of a boundary layer that appears inside the storage tank. This leads to an approximate solution of a simple structure but a high accuracy throughout the whole discharging process for the temperature distribution inside the storage tank. For validation the approximate solution is compared with the exact solution that is constructed with help of generalized Fourier coefficients and a numerical solution that is obtained by a semi-discretization method. Adapting the method leads also to an approximation procedure for charging processes.

Energy flexibility potential of domestic hot water systems in apartment buildings

Domestic Hot Water (DHW) storage tanks are identified as a main source of flexible energy use in buildings. As a basis for energy management in apartment buildings, this paper describes the aggregated DHW use in a case building, and analyses the potential for DHW energy flexibility by simulating different control options. The case study for the work is an apartment building in Oslo with 56 apartments and a shared DHW system. Energy measurements are available for consumed hot water, hot water circulation, and energy supplied to the DHW tanks. The measurements are presented with minute, hourly and daily values. Aggregated daily energy use for the consumed hot water is in average 362 kWh, while the energy supplied is 555 kWh. The potential for energy flexibility is analysed for a base case and for four different rule-based control options: Power limitation, Spot price savings, Flexibility sale and Solar energy. Economic consequences of the control options are compared. With the Norwegian tariff structure, maximum hourly power use has the main impact on the cost. Control systems that aim to reduce the maximum power use may be combined with spot price savings or to offer end-user flexibility services to the grid.

A Parametric Study of a Hybrid Photovoltaic Thermal (PVT) System Coupled with a Domestic Hot Water (DHW) Storage Tank

Photovoltaic-thermal panels are hybrid systems that combine the two types of conventional solar energy technologies (photovoltaic and thermal panels) and simultaneously generate both thermal and electrical energy in a micro-cogeneration system. Like any co-generation system, there is an optimal balance that can be achieved between the thermal and electrical energy produced. For this reason, it is important to establish the relationship and inter-connection between the two. Limited research is available on the cogeneration interaction in a PVT system, so the novelty of this article lies in the consideration of the entire energy system connected to the PVT panel, including the storage tank and the consumer demand curve, and the investigation of the thermal parametric variation. This study analyses the impact of the variation of some thermal parameters of a domestic hot water tank on the electrical efficiency of a photovoltaic-thermal panel. A model of a system of photovoltaic-thermal panels is built in a transient systems simulation program (TRNSYS) and a one-factor-at-a-time analysis is carried out for the cold-water main temperature, tank size, tank outlet flow and consumer demand curve. The results show that the variation of the outlet flow to the consumer has the highest impact on the electrical efficiency, of about 6.8%. The next highest impact factor is the size of the tank with a variation of 4.7%. Matching the profile of the consumer is also an important aspect. It was observed that the peak electrical efficiency occurs during peak consumer demand. Finally, the instantaneous variation of the thermal and electrical power of the system was analysed as a function of the temperature at the inlet of the photovoltaic-thermal panel.

Performance Modelling and Parametric Analysis of Domestic Hot Water Tanks in Discharging Mode

Thermal energy storage (TES) is the central component of any renewable energy system (RES) and can considerably effect its overall efficiency. A good storage tank should enhance the stratification by restricting inlet mixing. In this paper, an experimental study is presented to evaluate the performance of domestic hot water storage tanks in discharging mode. Discharging of tank was conducted with different inlet flow rate to assess the effect of inlet mixing on thermal stratification. The study was conducted to visualize the combined results of temperature distribution, MIX number and Richardson number to predict the behavior of tank under dynamic mode. It was concluded that discharging rate of 800L/hour has the maximum mixing and thus the worst stratification for current scenario.

Numerical prediction of the stratification performance in domestic hot water storage tanks

An efficient storage retains thermal stratification and improves the discharging performance. Turbulent mixing between hot and cold water is the prime source of stratification destruction. In this paper quantification of turbulent missing was achieved on the basis of temperature profile, MIX number, and Richardson number. The evaluated parameters include flow rate, ΔT, and diffuser design, henceforth a direct interdependence between each was thus established. Various CFD models were developed and experimentally validated on the test rig in order to find the optimal working conditions in discharge mode. The results proved numerically that the tank working conditions can be optimized by proper selection of inlet device. For instance, slotted type inlet device sustained maximum stratification even in as adverse a condition as of turbulent inflow & low ΔT. Perforated and simple inlet devices were capable of delivering best discharge efficiency only at low flow rate of 200 l/h and were showing insignificant dependency on ΔT. However, as flow rate is increased, ΔT dependency increased. Seeing the compounded benefits of slotted inlet devices and decreased ΔT, it was concluded that slotted inlet device delivered comparatively better thermal performance at both adverse conditions i.e. high flow & low ΔT and high flow & high ΔT, however, failed to outshine the rest of the inlet devices at low flow rate & low ΔT, and low flow rate & high ΔT. These research findings can serve as guidelines to optimize the storage tank design – more specifically, inlet device based design integrated with heating system, as thermal stratification and COP of heating system – heat pumps, for example, are inherently correlated. Heat pumps are high flow rate and low ΔT devices, while, solar systems are low flow rate and high ΔT devices, Thus, opting for accurate choice of inlet device for a particular operating condition is critical.

Analytical analysis on influence of dimensionless numbers in the thermal stratification of storage tank

An analytical analysis has been carried out to understand the influence of dimensionless numbers in the thermal stratification of solar domestic hot water storage (SDHW) tank. Thermal stratification is the phenomena to maintain cold water layer at the bottom of the storage tank and above the cold water maintaining of hot water layer. In between cold and hot water layer one barrier layer is considered, which is called a thermocline layer. From the thermocline layer the heat transfer exchanging starts in between hot and cold water layer. The thermocline layer thickness starts increasing, which is undesired and non-beneficial from maintaining of thermal stratification of the storage tank. This degradation of thermal stratification of the storage tank cannot be eliminated fully, but only it can be reduced down somehow. Hence this analytical study analysis is about to know the influence of dimensionless numbers like Reynold’number, Nusselt number, Prandtl number on the thermal stratification of storage Tank.

Multi-mode control method for the existing domestic hot water storage tanks with district heating supply

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 5–8 °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.

Performance of a demonstration solar PVT assisted heat pump system with cold buffer storage and domestic hot water storage tanks

The design and demonstration of a solar PVT assisted - heat pump system with a cold buffer storage tank on the source side of the heat pump and a hot storage tank for domestic hot water preparation is presented. The system was subject to real weather conditions and hot water draw offs. Measurements over nine months are presented and analyzed. The focus was on the thermal dynamic of the system and the interplay between the heat pump, the storage tanks and the thermal absorber of the uninsulated PVT panel. The hot water demand was almost completely covered by the thermal part of the PVT collector in sunny summer periods. In less sunny periods, the brine-water heat pump recharged the hot water tank after draw offs and discharged the buffer storage tank. The uninsulated PVT collector worked as an energy absorber and was able to extract heat form the ambient air and recharge the buffer storage tank to the ambient air temperature when no solar irradiance was available. In the less sunny and colder periods, the PVT added a significant amount of energy to the cold storage tank.

NUMERICAL AND EXPERIMENTAL ANALYSIS OF THE STRENGTH OF TANKS DEDICATED TO HOT UTILITY WATER

The focus of this paper are experimental and numerical strength tests of domestic hot water storage tanks. The tests involved the verification of the minimum wall thickness for the assumed operating parameters while meeting all safety standards. The authors presented numerical and experimental analyses for the verification of strength parameters of axial cylindrical tanks due to the lack of methodological guidelines for this type of equipment. In order to verify the conducted theoretical considerations and calculations, experimental tests of samples of front welds produced with austenitic steel as well as a pressure test for the whole tank were conducted using a research test stand.

Dynamic modeling of a sensible thermal energy storage tank with an immersed coil heat exchanger under three operation modes

In this paper we consider control-oriented modeling of a sensible thermal energy storage (TES) tank with a helical immersed heat exchanger (IHX) coil. A key focus of the modeling approach is to minimize the number of dynamic states required to adequately describe the system dynamics. The resulting model is well-suited for model-based control design, real-time simulation, and hardware-in-the-loop testing aimed at intelligent operation of TES systems. We use a discretized approach to model the temperature dynamics of the water within the storage tank. We use a quasi-steady approach to model the IHX coil dynamics, thereby limiting computational complexity. In simulation, the model runs up to 1200× faster than real-time. A simulated case study of model-based feedback control demonstrates the utility of the modeling approach. The model contains four tuning parameters that are empirically determined using experimental data collected from a commercially available domestic hot water storage tank. The model is then validated, both temporally and spatially, against data collected during the simultaneous charge/discharge mode. Finally, we quantify the trade-off between model fidelity and increased control volume discretization, showing that a 60 node model yields a RMSE value under 4.5%.

Numerical study of thermal stratification in hot water storage tank

Thermal stratified storage tanks equipped with stratification devices are an effective tool to improve the efficiency of large-scale solar thermal systems. Stratification improves the overall performance of the systems by increasing the efficiency of solar collectors. The modelling of thermal stratification is an important topic for the optimisation of storage tanks in thermal systems. The paper presents the results of numerical and experimental analyses of temperature and velocity fields in a domestic hot water storage tank equipped with three helical heating coils and a cylindrical stratification device during the heating process. The main aim of the paper is to improve the thermal stratification level in a small domestic hot water storage tank by using an appropriately shaped stratification device. CFD numerical simulations were used to analyse the flow characteristics and to determine the working conditions of the stratification device which can reduce recirculation and mixing inside the tank. A 2D-axisimmetric computational fluid flow model was performed. The standard k-e turbulence model with buoyancy effects and default turbulent constants were used. Numerical simulations were compared with experimental results obtained on the test stand.

Numerical study of thermal stratification in hot water storage tank

Thermal stratified storage tanks equipped with stratification devices are an effective tool to improve the efficiency of large-scale solar thermal systems. Stratification improves the overall performance of the systems by increasing the efficiency of solar collectors. The modelling of thermal stratification is an important topic for the optimisation of storage tanks in thermal systems. The paper presents the results of numerical and experimental analyses of temperature and velocity fields in a domestic hot water storage tank equipped with three helical heating coils and a cylindrical stratification device during the heating process. The main aim of the paper is to improve the thermal stratification level in a small domestic hot water storage tank by using an appropriately shaped stratification device. CFD numerical simulations were used to analyse the flow characteristics and to determine the working conditions of the stratification device which can reduce recirculation and mixing inside the tank. A 2D-axisimmetric computational fluid flow model was performed. The standard k-ε turbulence model with buoyancy effects and default turbulent constants were used. Numerical simulations were compared with experimental results obtained on the test stand.

CFD analysis of thermal stratification in domestic hot water storage tank during dynamic mode

In this paper, the thermal behavior of a vertical domestic hot water storage tank during the dynamic mode was studied. A 3D computational fluid dynamic (CFD) model was performed using the commercial software package Fluent v6.3. The proposed model was validated with experimental data obtained from the literature. The results of the simulation served to analyze the flow characteristics and the thermal stratification evolution during charging and discharging phases. This study was further extended to explore the effect of varying tank position (horizontal/vertical) on the system performance. A numerical comparison between the two configurations was carried out and performance parameters such as discharge efficiency, Richardson number and stratification efficiency were calculated. A major finding of the above analysis was the presence of two regions of increased thermal mixing in the horizontal tank. The contribution of these two recirculation zones on the destruction of the thermal stratification resulting in a decrease of the overall performance was demonstrated.

Domestic Hot Water Storage Tank: Design and Analysis for Improving Thermal Stratification

Experimental designs for a solar domestic hot water storage system were built in efforts to maximize thermal stratification within the tank. A stratified thermal store has been shown by prior literature to maximize temperature of the hot water drawn from the tank and simultaneously minimize collector inlet temperature required for effective heat transfer from the solar panels, thereby improving the annual performance of domestic solar hot water heating systems (DSHWH) by 30–60%. Our design incorporates partitions, thermal diodes, and a coiled heat exchanger enclosed in an annulus. The thermal diodes are passive devices that promote natural convection currents of hot water upward, while inhibiting reverse flow and mixing. Several variations of heat exchanger coils, diodes and partitions were simulated using ansys Computational Fluid Dynamics, and benchmarked using experimental data. The results revealed that the optimum design incorporated two partitions separated by a specific distance with four diodes for each partition. In addition, it was discovered that varying the length and diameter of the thermal diodes greatly affected the temperature distribution. The thermal diodes and partitions were used to maintain stratification for long periods of time by facilitating natural convective currents and taking advantage of the buoyancy effect. The results of the experiment and simulations proved that incorporating these elements into the design can greatly improve the thermal performance and temperature stratification of a domestic hot water storage tank.