Hot Water Storage Tank Research Articles

Optimal design and operation of a syngas-fuelled SOFC micro-CHP system for residential applications in different climate zones in China

a b s t r a c t Fuel cell based micro-CHP systems are expected to be one of the most promising technologies for imple- mentation in the residential sector. Since the design and operation of such CHP systems are greatly dependent upon the seasonal atmospheric conditions, it is important to evaluate their performance under difference climate conditions to ensure that it is well matched with the local heat-to-power ratio. The aim of this study is to investigate the optimal design and operation of a syngas-fuelled SOFC micro-CHP system for small households located in five different climate zones in China. The ability of the micro-CHP to cover the heat and electricity demand of a 70 m2 single-family apartment with an average number of occupants of 3 is evaluated. A detailed model of the micro-CHP unit coupled with a hot water storage tank and an auxiliary boiler is developed. System design trade-offs are discussed to determine the optimal match between the energy demand of the household for different climates across China and the energy supply of the micro-CHP during the whole year. Moreover, criteria for sizing the system components of the micro-CHP are specifically addressed. The developed methodology can be applied to different types

Reverse flow in natural convection heat pipe solar water heater

System performance of a natural convection (thermosyphon) solar water heater depends on design and setup of collector and storage tank as well as environmental factors such as solar intensity, ambient temperature and wind conditions. The relative height separating the tank and collector mainly influences the magnitude of the thermosyphon flow rates, including both forward and reverse flow at night. In this experimental investigation, an array of evacuated tube heat pipe solar collectors was connected to an insulated hot water storage tank. The effect of the separation height between tank and collectors was investigated and reported. Thermosyphon water flow rates were measured using a dye-injection procedure in both forward and reverse flow directions. The results showed that reverse flow always occurred in the evenings and was about 5–11 times less than forward flow. The overnight mean water temperature drop was independent of the height separating the collectors and storage tank and ranged between 2 and 10°C. The temperature drop was greater when the night was cooler.

Simulation and analysis of a solar assisted heat pump system with two different storage types for high levels of PV electricity self-consumption

The incentives for PV-systems in Europe is being gradually lowered or ended. This makes a higher level of self-consumption interesting for owners of PV-systems.Sweden has an incentive of 35% of the investment cost for PV-systems. Unfortunately not all consumers can get this incentive. Therefore a high level of self-consumption will be necessary if the PV-systems are to be profitable in Sweden.A reference system with two different energy storage technologies is investigated in this paper. One system with 48kWh of batteries and one system with a hot water storage tank where the electricity is stored as heat.The research questions in this paper are:Which storage system gives the highest level of PV electricity self-consumption?Are the storage systems profitable with the assumptions made in this paper?What are the levelized costs of electricity (LCOE) for the reference system with different storage system?The system with batteries has a self-consumption of 89% of the annual PV-electricity output and the system with a hot water storage tank has 88%.The system with batteries has a levelized cost of electricity two times higher than the system with a hot water storage tank.

Optimal management proposal for hybrid water heating system

Abstract This paper presents the dynamic modeling of a domestic hybrid water heating system. The system is composed of a solar collector, a heat pump water heater, a wind turbine, a battery and a hot water storage tank. Both air-source and geothermal-heat pumps are investigated. Detailed mathematical dynamic models of the individual components are presented and validated. Simulations for typical days in summer and winter for Beirut and Cedars, two Lebanese locations with different meteorological and demographic conditions, are conducted using Matlab software. A renewable coverage factor (RCF) is defined, representing the renewable energy share with respect to the total delivered energy. Results reveal that the proposed hybrid system is capable of securing all hot water needs in all case studies and RCF is always above 63%. Furthermore, an energetic, economic optimal management model is developed for the proposed hybrid system. It is applied to the considered case studies, where results illustrate the optimum size of each of the system components as well as the optimum energy-flow distributions among them over two investigated time periods, five and ten years. It is noticed that the initial cost of the hybrid system is acceptable and important annual savings are obtained.

Energy Labeling and Advanced Insulation for Thermal Energy Storages in Solar Applications

In order to reduce CO2 emissions related to heating purposes within the European Union, the EU-Commission is currently preparing ecodesign and energy-labeling requirements for water heaters, storage tanks, and space and combination heaters. Especially this also applies for solar assisted space heating and hot water preparation systems. In particular, there will be a new energy efficiency classification for solar hot water storage tanks, which will be rated (A+, A, B, C, …, G) according to their standing losses. We present a test set-up and procedure that is capable of determining the standing losses conform to the EU-directive and show various measurements of some state-of-the-art storage configurations. The results reveal the need for better insulation in order to achieve an A+/A-rating for state-of-the-art storage tanks. We compare the effects of employing an better insulated storage tank compared to state-of-the-art technology for a solar thermal systems by performing annual system simulations via TRNSYS. The system comprises of a 100% renewable space heating and domestic hot water pellet-solar combi-system suitable for residential buildings.

A Modified Efficiency Equation of Solar Collectors

This paper describes the derivation of a modified equation for solar collector efficiency that is expressed using the heating load term instead of the inlet fluid temperature term from the currently used linear collector efficiency equation. The parameters in the modified equation are estimated using test data measured for 14 days. In evaluation of the equation's validity, the calculated daily collector efficiency agrees well with the measured daily collector efficiency, with a correlation coefficient of 0.9110. The equation is also be expressed in another form by including the term for the shape of the hot water storage tank in the solar heating system. Collector efficiencies with parametric changes are calculated with the estimated parameters and compared with different global solar irradiance on solar collectors, daily average ambient temperature and heating loads per collector area. It would be necessary to estimate the parameters for better performance of the efficiency equation with more data from long-term system simulations at various operating conditions.

Compact Hot Water Storage Systems Combining Copper Tube with High Conductivity Graphite and Phase Change Materials

The low thermal conductivity of wax-based phase-change material has limited its application in latent heat thermal energy storage (TES) systems. This paper compares two ways to increase the thermal conductivity of wax by a factor of more than 100. The first involves embedding a modified copper tube/aluminum fin heat exchanger of the type typically used in room air conditioners in a container filled with wax-based phase change material. The second and more unconventional approach is to embed copper tube circuits into a high-conductivity graphite-wax composite. Both types of systems demonstrated excellent performance, however the prototype graphite-wax composite TES system offers the added advantages of leak-free operation and greater flexibility in the shape of the TES unit.This paper covers the design, performance and economics for using a TES unit to replace a conventional hot water storage tank in a heat pump water heater and a solar thermal water heater. TES unit optimization for a HPWH allowed for lower return temperature to the compressor, constant exit water temperature, faster charging time and reduced unit size by as much as 60% compared to a conventional water tank. TES unit optimization for a solar thermal application permits replacement of large water tanks with lighter weight TES units that can be placed on interior or exterior walls.Overall, this paper describes how optimized designs of thermal energy storage systems for domestic hot water delivery now in development are compact, have flexible form factor for applications in buildings where space is at a premium, deliver hot water at a constant output temperature, operate for a longer time for the same volume, or have increased efficiency over conventional systems.Some additional applications of copper heat exchangers embedded in PCM will also be mentioned, including air and ground source heat pump space heating and cooling in which the use of a compact TES unit offers higher COP due to condenser and evaporator temperature optimization.

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.

A new inlet device that enhances thermal stratification during charging in a hot water storage tank

Solar thermal energy systems are generally provided with a water storage tank. In this tank, thermal stratification takes place naturally, although it is considerably affected by the turbulence and mixing created by the water inflow and outflow. Consequently, thermal stratification is directly influenced by the inlet configuration. This study compares the effect of two water inlet devices in a hot water storage tank during a thermal charge process: a sintered bronze conical diffuser (SBCD) and a conventional inlet elbow (E). The evolution of the temperature recorded by the thermocouples, the 1-MIX number, as well as the thermocline evolution and other related parameters (shape, thickness and height of midpoint) are used to quantify the performance of the stratification. The results obtained show that the SBCD favours water stratification during thermal charging with both low and high flows. The evolution of the dimensionless height of the midpoint of the thermocline can be used to quantify the volume of water affected by the mixing produced by the turbulence generated by the water flow through the inlet device.

Thermal analysis of a novel fibre-reinforced plastic solar hot water storage tank

A fibre-reinforced plastic (FRP) hot water storage tank was developed in the present study. The main object for the development of the tank was to provide a long term storage unit that can efficiently retain and deliver stored hot water from solar collectors, and several other sources of hydrothermal fluids. A specific object was to use low-cost materials which can be easily moulded into a variety of shapes without requiring highly specialized industrial processes, as well as reduce the weight of the tank while making it withstand pressures substantially above the ambient. Results confirmed a superior storage performance over time with minimal losses. A thermal analysis was performed which showed that ambient and geometric parameters as well as the tank insulation material properties can significantly impact the storage performance.

A mixed-integer nonlinear program for the optimal design and dispatch of distributed generation systems

Maturing distributed generation (DG) technologies have promoted interest in alternative sources of energy for commercial building applications due to their potential to supply on-site heat and power at a lower cost and emissions rate compared to centralized generation. Accordingly, we present an optimization model that determines the mix, capacity, and operational schedule of DG technologies that minimize economic and environmental costs subject to the heat and power demands of a building and to the performance characteristics of the technologies. The technologies available to design the system include lead-acid batteries, photovoltaic cells, solid oxide fuel cells, heat exchangers, and a hot water storage tank. Modeling the acquisition and operation of discrete technologies requires integer restrictions, and modeling the variable electric efficiency of the fuel cells and the variable temperature of the tank water introduces nonlinear equality constraints. Thus, our optimization model is a nonconvex, mixed-integer nonlinear programming (MINLP) problem. Given the difficulties associated with solving large, nonconvex MINLPs to global optimality, we present convex underestimation and linearization techniques to bound and solve the problem. The solutions provided by our techniques are close to those provided by existing MINLP solvers for small problem instances. However, our methodology offers the possibility to solve large problem instances that exceed the capacity of existing solvers and that are critical to the real-world application of the model.

An effect of a horizontal buoyant jet on the temperature distribution inside a hot water storage tank

The hot water storage tank (for stratified thermal storage) with a heat pump draws a lot of attention nowadays due to its high performance. In Japan, reheating of the bath is commonly used, and as this mode, the jet injects horizontally at the middle of the tank, so the temperature distribution of the tank changes complexly with time. Hence a model is needed to simulate this phenomenon, precisely. Additionally, in the process of designing a hot water storage system, it is necessary to simulate temperature distribution quickly, since a test run itself is a time consuming process.In this study, visualization experiments were performed using tracer particles and thermo-sensitive liquid crystals. Experiments were also carried out to find the unsteady temperature distribution in a tank when the positively or negatively buoyant jet was injected horizontally in the middle of the tank whose size is limited and has an influence from the opposite wall.If the momentum effect of the buoyant jet is stronger than that of buoyancy, the buoyant jet impinge against the opposite wall of the tank, and a vortex was observed near the opposite wall. Empirical formulas were proposed to predict the height of the vortex “Zb” under various conditions, such as the momentum and the buoyancy of the buoyant jet, and the Prandtl number of the tank water. Furthermore, the 3D-CFD was carried out to supplement the 3D behavior of the inner tank fluid.A one dimensional model, “uniformly distributed injection model”, for simulating temperature distribution was proposed. The performance of the model was verified by comparing the results with the unsteady temperature distribution obtained experimentally. The model was also compared with the measurements obtained using a commercially available hot water storage system. Both results showed good agreements. Hence adequacy of the model was clarified.

Three-dimensional unsteady CFD simulations of a thermal storage tank performance for optimum design

This paper presents the results of three-dimensional (3D) unsteady Computational Fluid Dynamics (CFD) simulations to investigate the influence of several design and operating parameters during charging operation on the flow behaviour, thermal stratification and performance of a hot water storage tank installed in solar thermal energy systems. Validation of the unsteady flow computation results with experimental data found in the literature has shown a good agreement. Different computation test cases were run to systematically analyse the effects of key geometrical and operating parameters on the system performance.The CFD results confirmed the importance of combined effects on the performance of thermal storage tanks and showed that an appropriately designed storage tank can provide improved stratification conditions. Moreover, 3D transient CFD simulations can be used as an effective tool to optimise thermal storage tank parameters at early design stages, thus it may add to the value of the storage tank performance and efficiency, by optimising the whole solar thermal energy storage system design and size.

STUDY OF THE PERFORMANCE OF A SOLAR ADSORPTION COOLING SYSTEM

This article presents a transient model of a solar adsorption cooling system. A computer program has been developed to simulate the operation of a two bed silica gel- water adsorption cooling system as well as flat plate collectors and the hot water storage tank. This program is then utilized to simulate the performance of a sample solar adsorption cooling system used for cooling a set of rooms that comprises an area of 52 m2 located in Ahwaz city in Iran. The system has been simulated with typical weather data of solar radiation and ambient temperatures of Ahwaz. The results include the temperature profiles of hot, cooling and chilled water in addition to Temperature profiles of adsorption/desorption beds, evaporator and condenser. Also the effects of the cycle time, switching time and hot water temperature on COP and refrigeration capacity are studied. Furthermore, the effects of solar collectors’ surface area and the storage tank volume on total cost of the system are investigated in order to determine their optimum values able to maximize the overall thermo-economic performance of the system under analysis.

Thermoeconomic design optimization of a thermo-electric energy storage system based on transcritical CO2 cycles

The conceptual design of a thermo-electric energy storage (TEES) system for large scale electricity storage is discussed in this work by showing the results of the thermoeconomic optimization of three different system configurations that were identified in previous works. The system is based on transcritical CO2 cycles, water storage and salt-water ice storage and is designed for a capacity of 2 h discharge and equal charge and discharge power of 50 MW. A two-step optimization procedure is used. The system intensive design parameters are optimized at the master level through a genetic algorithm. The optimal cycle mass flow rates are calculated in a nested linear programming step where the heat integration between the cycles is optimized subject to the heat transfer feasibility imposed through Pinch Analysis cascade calculations. The synthesis of the heat exchanger network and of the storage tank systems was solved through a set of heuristic rules. Equipment purchasing costs were estimated by means of cost functions that were built upon vendors quotations. The results are discussed by showing the Pareto fronts of the three optimization cases and the trends of the decision variables along each optimal front. Nine solutions are discussed in more detail by showing the values of the design parameters and the process flow diagrams including storage and heat exchanger layouts. Design guidelines are then formulated which can be used in future works for detail plant design. The topological features that are found to maximize the system performance at the minimum costs are: superheating before the CO2 heat pump, two independent systems of hot water storage tanks above and below the ambient temperature, and air cooling at the heat pump side. The design parameters that affect significantly the costs and performances are the cycle pressures. These are in fact directly associated with temperature differences both at the cold and hot storage sides which should be carefully optimized to obtain the best trade-off between exergy losses and costs for heat exchangers. Due to the change in specific heat of the supercritical CO2 along the temperature range of hot water storage, a system of multiple storage tanks was used. Intermediate storage tanks help reduce significantly the temperature differences at the hot storage side and therefore their number represent another critical design parameter that must be optimized to achieve the best trade-off between costs and performances.

Performance Estimation and Optimal Operation of a CO2 Heat Pump Water Heating System

The daily performance of a CO2 heat pump water heating system with a hot water storage tank is affected by the history of daily hot water demand and heat pump operating conditions. To attain the maximum system performance, it is important to estimate the daily changes in the system performance values accurately in relation to those in hot water demand and heat pump operating conditions, and determine the operating conditions optimally based on the estimation. In this paper, neural network models are used for this estimation, and the values of model parameters are identified by a global optimization method. In addition, the outlet water temperature for during operation and the inlet water temperature for shutdown are determined to maximize the system efficiency subject to a lower limit for the volume of unused hot water. The validity and effectiveness of this approach are ascertained by a numerical study using a simulated hot water demand.

Investigation of a new tube-in-tube helical flow distributor design to improve temperature stratification inside hot water storage tanks operated with coiled-tube heat exchangers

Steady and unsteady heat transfer process and the developed flow and temperature field inside hot water storage tanks have been studied with a helical tube-in-tube flow distributor design. The studied flow distributor consists of two helically coiled pipes. The inner pipe is a common helically coiled tube heat exchanger and the outer pipe is a helically coiled flow distributor which separates the hot water from the significantly colder bulk water of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the heated water to a helical flow path along the tube axis which induces additional secondary flow in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different heat exchanger side flow rates, inlet temperatures and two gap size between the outer wall of the helical tube and the flow distributor have been investigated to describe the impact of the modification of these parameters for the developed velocity and temperature field. Comparison of the flow and temperature fields of hot water storage tank with common helical tube and the tube-in-tube flow distributor configuration are discussed. Numerical calculations have been carried out to examine different flow and temperature fields in laminar flow regimes. Calculated results of the inner side heat transfer intensity (Nu) have been compared to existing empirical formulas to test the validity of the numerical results in case of common helical tube heat exchanger. The proposed helical flow distributor ensures a highly stratified temperature field of storage tanks operating with helically coiled heat exchangers. In addition to this the suggested flow distributor geometry makes it possible to apply different heat transfer enhancement tools like spiral tape inserts, different kind of corrugations further improve if it is possible the poor outer side heat transfer rate of common helically coiled tube heat exchangers.

Hourly energy analysis and feasibility study of employing a thermocline TES system for an integrated CHP and DH network

The employment of a TES system to improve the performance of a DH network integrated with a CHP station is investigated. Thermal energy demand fluctuations is the main issue threatening the viability of CHP/DH projects; therefore, TES facility can be coupled with CHP plant by a smart operation strategy to match the production and consumption profiles, and result in better consequences. To evaluate the system performance and energy saving potential, the hourly simulation of a thermocline hot water storage tank coupled with CHP/DH network is carried out, considering annual thermal load of the DH network. Optimal TES size is investigated from energetic, environmental and economical viewpoints. Results depict that by coupling five 1100m3 hot water storage tanks with CHP/DH plant, the annual CHP efficiency increases by 1.12%, and annual fuel consumption rate and CO2 emissions decrease by 2.6%.

A feasibility integrated approach in the promotion of solar house design

SUMMARY In many countries, although solar energy is plentiful, due to technical and economic issues, micro-scale solar energy technologies have not properly found their places in the market. The present work demonstrates the convenience of employing economic feasibility assessments together with engineering analyses before applying solar strategies during the design stages, in order to convince house owners that these solar technologies can be economically viable. In a case study, this approach is tested for a real house project developed in North Cyprus (NC) in which solar energy applications for electricity generation, hot water preparation, space heating and passive cooling are investigated. It is reported that grid-connected photovoltaic (PV) systems would be economically feasible with savings-to-investment ratio (SIR) values of 2.9 to 3.4. A thermal performance analysis is carried out for a thermal storage wall (TSW) together with a direct solar gain (DSG) window in the living room. It is discovered that the temperature range of the living room was within 18–22 °C throughout the majority of winter days. This application is also proved to be economically feasible with an SIR of 1.3, compared to installing an 18,000 Btu/h heat pump. It is determined that both locally produced solar water heaters and imported solar water heaters (ISWHs) are economically viable compared to using a 3-kW storage-type electric water heater. Their SIR values are estimated to be 7.5 and 3.7, respectively. A solar-assisted stack-effect ventilation (SEV) system is verified to have techno-economic feasibility and is also useful in concealing the unattractive cold and hot water storage tanks on the roof. The prospective house owner decides to invest 3840 EUR in installing a grid-connected PV system of 1 kWp, an ISWH, a DSG window, building a TSW for space heating and a SEV system with an overall SIR value of 3.2. This procedure can be developed into a government policy where all these assessments are made mandatory to increase the awareness of the home builders before they make their decisions about the final designs of their houses. The present work demonstrates the convenience of employing economic feasibility assessments together with engineering analyses before applying solar strategies during the design stages, in order to convince house owners that these solar technologies can be economically viable. In a case study, this approach is tested for a real house project developed in NC in which solar energy applications for electricity generation, hot water preparation, space heating and passive cooling are investigated. Copyright © 2013 John Wiley & Sons, Ltd.

제로카본 그린홈 구현을 위한 하이브리드 열공급 시스템의 열성능 분석

This study was carried out to evaluate thermal performance of the renewable hybrid heat supply system with solar thermal system and wood pellet boiler for Zero Carbon Green home of apartment houses. The hybrid heat supply system was set up at Korea Institute Energy Research in 2011. The system was comprised of the wood pellet boiler unit with heat capacity designed as 20,000kcal/hr, a hot water storage tank for space heating, a evacuated tubular solar collector of aperture area at the install angle, a hot water storage tank. Thermal performance tests for one-house of apartment house were carried out by hot water load and heating load in winter season through the hybrid heat supply system. As a result, hot water energy supplied by the hybrid heat supply system was 11kWh in a day. Solar thermal energy portion was 2.99kWh which is 27% of the total hot water energy supply. wood pellet boiler supply portion was 8.017kWh which is 73% of the total hot water energy supply.