Domestic Hot Water Tank Research Articles

Impact of phase change materials types and positioning on hot water tank thermal performance: Using measured water demand profile

This paper addresses a numerical investigation of the performance of a domestic hot water tank (HWT) integrated with phase change materials (PCM). The predictions made by three TRNSYS types were compared and the most appropriate one was selected as reference model. Then, the existing TRNSYS model for HWT with PCM was validated with the available data. The validated model was then used to investigate the impact of PCM type, its amount and its location within the tank on the system performance using the three real hot water withdrawal profiles; low, medium and high. An improvement in energy storage resulted from the combined use of PCM and sensible heat in a HWT with sodium acetate trihydrate +10% graphite showing the highest storage potential with reduce charging time compared to an industrial grade granulated paraffin wax and RT58-Rubitherm.

Validation of a Single Tank, Multi-mode Solar-assisted Heat Pump TRNSYS Model

This work explores the operation and performance of a solar-assisted heat pump (SAHP) system which employs a single domestic hot water (DHW) tank. The solar heat source is connected to the storage tank via a heat pump and heat exchanger, which operate in parallel paths, providing multiple modes of operation. A TRNSYS (TRaNsient SYstem Simulations Tool) model of this SAHP system is validated using a purpose-built experimental test apparatus. The main goals of the validation are to: benchmark accuracy, identify strengths and weaknesses, and build confidence in the model. The model will be used to conduct more advanced studies supporting the development of similar SAHP system configurations. Both the system and model are operated and monitored for representative days from typical meteorological year (TMY) data for Ottawa, Ontario, Canada. The agreement between simulation and experiment was found to be very strong, with typical differences in average DHW tank temperatures of less than 1 C°. The DHW tank model does bring forth some issues, as simulated stratification can often differ appreciably from experimental stratification.

Thermal behaviour of a modular storage system when subjected to variable charge and discharge sequences

This paper presents the results of an experimental study conducted on a multi-tank thermal storage. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to standard draw profiles. The experimental setup consisted of a charge loop to simulate a solar collector input, three commercially available 270L domestic hot water tanks and three side-arm, natural convection heat exchangers. The system was charged using two 3kW electric heaters which produced a variable input power to simulate the variation in output of a typical fixed orientation solar collector. Realistic hot water draws were conducted according to the specifications of the Canadian Standard Association, CSA-F379.1, for Solar Domestic Hot Water Systems. Experimental tests were conducted over 2days for three different plumbing configurations, and compared with simulation results produced in the TRNSYS simulation environment. The plumbing configurations which were considered include: series charge and series discharge, parallel charge and parallel discharge, and series charge and parallel discharge. Comparisons between system configurations were based on stratification levels, hot water delivery temperatures, and solar fractions. Results of the study showed that charging and discharging the system in parallel produced the highest solar fraction when compared to the other multi-tank configurations, with solar fractions of 0.62 and 0.67 from experimental testing and TRNSYS simulation, respectively. When compared to a single tank configuration, the parallel charge and parallel discharge configuration achieved 97.7% of the delivered solar energy of a stratified single tank system with the same storage volume and heat exchanger performance.

Feasibility of using a Stirling engine-based micro-CHP to provide heat and electricity to a recreational sailing boat in different European ports

This study analyses the behaviour of a Stirling engine micro-cogeneration system that supplies a sailing boat and the corresponding weather conditions and sea surface temperatures (SST) along fifteen European ports. The micro combined heating and power (micro-CHP) system was bench tested to provide information for tuning the model. The simulations of the engine and of the whole system were conducted in the Trnsys simulation environment. It was found that the Stirling-based cogeneration system is a unique source of heat for the yacht, while the produced electricity is stored in batteries and used whenever it is needed. The engine was fully integrated within the HVAC system of the boat such that the domestic hot water (DHW) tank, the heating installation and the bank of batteries can obtain energy from the cogeneration system. The conclusions of this work are based on the annual system performance.

Analysis and solution for renewable energy load matching for a single-family house

This paper conducts analyses and proposes solutions for reducing mismatch between renewable energy (RE) production and electrical demand from a 150m2 single-family house. The RE options are photovoltaic (PV) and micro-wind turbine. This paper mainly focuses on the situation when grid feed-in is not available, but a brief economic analysis with grid feed-in is also conducted at the end of the paper. This paper specially investigates the influence of electrical storage in batteries and thermal storage in a domestic hot water (DHW) tank with suitable RE-DHW recharging strategies. The simulation results show that the recharging strategy of excess renewable electricity to a DHW storage tank with one day's DHW volume is more technically and economically effective than using electrical battery in reducing annual mismatch. Even without battery, the improvement in reducing mismatch can be 12.7–23.3% simply by using the RE-DHW recharging strategy. Furthermore, PV with 2-axis solar tracking strategy is not economically feasible in the Nordic climate, whereas south-facing PV with a 45° fixed tilt angle is recommended. Moreover, from both the energy and economic points of view, the increased hub height with multiple turbines is preferable for the application with a 6.64–16.2% improvement in reducing mismatch.

Thermal energy storage with phase change material—A state-of-the art review

Abstract Recently, thermal energy storage (TES) has received increasing attention for its high potential to meet cities’ need for effective and sustainable energy use. Traditionally, energy was stored in the form of sensible heat which requires large volume of storage material. The storage volume can be significantly reduced if energy is stored in the form of latent heat and thus can benefit enormously practical applications. The existing approaches in the design, integration and application of phase change materials (PCMs) in domestic hot water tanks (HWT) and transpired solar collector (TSC) using water/air as the heat transfer media are reviewed. Crucial influencing factors are considered, including thermo-physical properties of different PCMs, different configurations of PCMs in HWT and TSC, and the limitations of each technique. This paper also discusses the existing simulation, design tools and experimental studies related to PCMs usage in HWT and central thermal storage.

Charge and discharge strategies for a multi-tank thermal energy storage

This paper presents the results of an experimental study conducted on a multi-tank thermal storage for solar hot water heating applications. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to constant temperature charging and constant volume hourly draws. The experimental setup consisted of a charge loop to simulate a solar collector input, three commercially available 270L domestic hot water tanks and three side-arm, natural convection heat exchangers. Tests were performed over 8h periods for three different plumbing configurations and two different hourly draw volumes. Simulations were conducted using the TRNSYS simulation environment, and the results showed that the TRNSYS model was in good agreement with the experimental results, where the discrepancies between data were found mainly in the regions of high temperature gradients within the storage tanks. Preliminary simulations were also conducted using realistic charge and draw profiles to illustrate the thermal behaviour of the tanks under non-ideal operating conditions.

Negative buoyant plume model for solar domestic hot water tank systems incorporating a vertical inlet

Thermal stratification in solar energy storage tanks plays an important role in enhancing the performance of solar domestic hot water systems. The mixing that occurs when hot fluid from the solar collector enters the top of the tank is detrimental to the stratification. Mathematical models that are used for system analysis must therefore be able to capture the effects of this inlet jet mixing in order to accurately predict system performance. This paper presents a computational study of the heat transfer and fluid flow in a thermal storage tank of a solar domestic hot water system with a vertical inlet under negative buoyant plume conditions. The effects of parameters such as the fluid inlet velocity and temperature as well as inlet pipe diameter on the thermal mixing were considered. The work culminated in the development of a one-dimensional empirical model capable of predicting the transient axial temperature distribution inside the thermal storage tank. Predictions from the new model were in good agreement with both experimental data and detailed computational fluid dynamics predictions.

Theoretical model and experimental validation of a direct-expansion solar assisted heat pump for domestic hot water applications

This paper has shown the development of a theoretical model to determine the operating parameters and consumption of a domestic hot water (DHW) installation, which uses a direct-expansion solar assisted heat pump (DXSAHP) with refrigerant R-134a, a compressor with a rated capacity of 1.1kW and collectors with a total area of 5.6m2. The model results have been compared and validated the experimental results obtained with the equipment installed at the University Carlos III, South of Madrid. The analysis was conducted over the course of a year, and the results have been represented depending on the meteorological and process variables of several representative days. Taking into account the thermal losses of the installation and the dependency on the operating conditions, the acquired experimental coefficient of performance is between 1.7 and 2.9, while the DHW tank temperature over the course of the study is 51°C.

Methodology for the design of energy production and storage systems in buildings: Minimization of the energy impact on the electricity grid

Human life requires energy. Moreover, people spend around 90% of their time in buildings while about 40% of primary energy needs are due to buildings. That is why the present paper deals with a methodology allowing identifying and assessing the energy impact of a building on the electricity grid. Thanks to both the building models we developed and fuzzy logic contribution (used to control ventilation and develop occupancy scenarios related to human habits and lifestyle), the results we obtained in simulation validate the proposed impact indicator. Different insulation levels were considered as well as energy production (solar photovoltaic and thermal panels and a vertical axis windmill) and storage (a domestic hot water tank) systems. These results highlighted the pertinence of such an indicator for optimizing the design of the just-mentioned systems and minimizing the amount of energy exchanged by buildings and the electricity grid. One can promote energy injection or take into account the status of the electricity grid when designing these systems. As a key result, the produced renewable energy is partially self-consumed, what allows for a more efficient and rational use of energy in buildings.

The Effect of Discharge Configurations on the Thermal Behaviour of a Multi-tank Storage System

This paper presents the results of an experimental study conducted on a multi-tank thermal storage. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to standard draw profiles. The experimental set up consisted of a charge loop to simulate a solar collector input, three commercially available 270 L domestic hot water tanks and three side-arm, natural convection heat exchangers. The system was charged using two 3kW electric heaters which produced a variable input power to simulate the variation in output of a typical fixed orientation solar collector. Realistic hot water draws were conducted according to the specifications of the Canadian Standard Association, CSA-F379.1, for Solar Domestic Hot Water Systems. Tests were performed over 2 days for three different plumbing configurations, and compared with simulated results produced in the TRNSYS simulation environment. Comparisons between system configurations were based on stratification levels and hot water delivery temperatures.

Utilization of phase change materials in solar domestic hot water systems

Thermal energy storage systems which keep warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. This effect is known as thermal stratification, and has been studied experimentally thoughtfully. This system stores sensible heat in water for short term applications. Adding PCM (phase change material) modules at the top of the water tank would give the system a higher storage density and compensate heat loss in the top layer because of the latent heat of PCM. Tests were performed under real operating conditions in a complete solar heating system that was constructed at the University of Lleida, Spain. In this work, new PCM-graphite compounds with optimized thermal properties were used, such as 80:20 weight percent ratio mixtures of paraffin and stearic acid (PS), paraffin and palmitic acid (PP), and stearic acid and myristic acid (SM). The solar domestic hot water (SDHW) tank used in the experiments had a 150 L water capacity. Three modules with a cylindrical geometry with an outer diameter of 0.176 m and a height of 0.315 m were used. In the cooling experiments, the average tank water temperature dropped below the PCM melting temperature range in about 6–12 h. During reheating experiments, the PCM could increase the temperature of 14–36 L of water at the upper part of the SDHW tank by 3–4 °C. This effect took place in 10–15 min. It can be concluded that PS gave the best results for thermal performance enhancement of the SDHW tank (74% efficiency).

Residential Total Energy System Installation at the Canadian Centre for Housing Technology

Distributed cogeneration in single households may provide a vi-able alternative to the construction of new central power plants in thecoming years. A key issue in residential cogeneration is how to size andintegrate the required technologies to satisfy the total energy needs ofthe household, consisting of electricity, domestic hot water, space heatingand space cooling. An interesting pathway to a more sustainable futureinvolves the use of the earth surrounding the home as both a source anda sink for energy, especially if it enables the recycling of summertimewaste heat from the generator.This demonstration project was planned and implemented at theCanadian Centre for Housing Technology (CCHT) in 2006. The CCHT,located on the campus of the National Research Council in Ottawa,Ontario, Canada maintains two detached, single-family houses thathave the capacity to assess energy and building technologies with dailysimulated occupancy effects.This article describes the residential total energy system installedin one of the houses at the CCHT, consisting of two one-ton ground source heat pumps, an air handler for supplemental/backup heating, anatural-gas-fired hot water tank, an indirect domestic hot water tank anda multistage thermostat. The bore-field consists of three vertical wellsarranged to suit a typical suburban landscape. Two of the wells servethe heat pumps and the third well is arranged between the other two tosink the waste heat from a cogeneration unit scheduled to be installed inMay 2007. The heat pump system was sized to satisfy the cooling loadin Canada’s heating dominated climate, leaving room in the operationof the system to accept waste heat from the cogeneration unit, eitherdirectly or indirectly by recycling the heat through the ground to theheat pumps.Following an earlier paper that introduced the installation anddescribed initial ground thermal response testing, this paper presents,summarizes and discusses operational results of the heat pump systemin heating mode over a continuous 47-day period ending December 21,2006. The article will also describe the con figuration planned for therecovery of heat from the cogeneration system.

Exergetic modeling and assessment of solar assisted domestic hot water tank integrated ground-source heat pump systems for residences

The present study deals with the exergetic modeling and performance evaluation of solar assisted domestic hot water tank integrated ground-source heat pump (GSHP) systems for residences for the first time to the best of the author's knowledge. The model is applied to a system, which mainly consists of (i) a water-to-water heat pump unit (ii) a ground heat exchanger system having two U-boreholes with an individual depth of 90 m, (iii) a solar collector system composing of rooftop thermal solar collectors with a total surface area of 12 m 2, (iv) a domestic hot water tank with a electrical supplementary heater, and (v) a floor heating system with a surface of 154 m 2, and (vi) circulating pumps. Exergy relations for each component of the system and the whole system are derived for performance assessment purposes, while the experimental and assumed values are utilized in the analysis. Exergy efficiency values on a product/fuel basis are found to be 72.33% for the GSHP unit, 14.53% for the solar domestic hot water system and 44.06% for the whole system at dead (reference) state values for 19 °C and 101.325 kPa. Exergetic COP values are obtained to be 0.245 and 0.201 for the GSHP unit and the whole system, respectively. The greatest irreversibility (exergy destruction) on the GSHP unit basis occurs in the condenser, followed by the compressor, expansion valve and evaporator.

Modelization of a water tank including a PCM module

The reduction of CO 2 emissions is a key component for today’s governments. Therefore, implementation of more and more systems with renewable energies is necessary. Solar systems for single family houses or residential buildings need a big water tank that many times is not easy to locate. This paper studies the modelization of a new technology where PCM modules are implemented in domestic hot water tanks to reduce their size without reducing the energy stored. A new TRNSYS component, based in the already existing TYPE 60, was developed, called TYPE 60PCM. After tuning the new component with experimental results, two more experiences were developed to validate the simulation of a water tank with two cylindrical PCM modules using type 60PCM, the cooldown and reheating experiments. Concordance between experimental and simulated data was very good. Since the new TRNSYS component was developed to simulate full solar systems, comparison of experimental results from a pilot plant solar system with simulations were performed, and they confirmed that the type 60PCM is a powerful tool to evaluate the performance of PCM modules in water tanks.

Residential CO 2 heat pump system for combined space heating and hot water heating

A theoretical and experimental study has been carried out for a residential brine-to-water CO 2 heat pump system for combined space heating and hot water heating. A 6.5 kW prototype heat pump unit was constructed and extensively tested in order to document the performance and to study component and system behaviour over a wide range of operating conditions. The CO 2 heat pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW. The CO 2 heat pump was tested in three different modes: space heating only, DHW heating only and simultaneous space heating and DHW heating. The heat pump unit gave off heat to a floor heating system at supply/return temperatures of 33/28, 35/30 or 40/35 °C, and the set-point temperature for the DHW was 60, 70 or 80 °C. Most tests were carried out at an evaporation temperature of −5 °C, and the average city water temperature was 6.5 °C. The experimental results proved that a brine-to-water CO 2 heat pump system may achieve the same or higher seasonal performance factor (SPF) than the most energy efficient state-of-the-art brine-to-water heat pump systems as long as: (1) the heating demand for hot water production constitutes at least 25% of the total annual heating demand of the residence, (2) the return temperature in the space heating system is about 30 °C or lower, (3) the city water temperature is about 10 °C or lower and (4) the exergy losses in the DHW tank are small.

Discharge mode for encapsulated PCMs in storage tanks

This paper is aimed at analysing the behaviour of encapsulated salt hydrates, used as latent energy storage in a heat transfer system of a domestic hot water tank. The salt is a eutectic mixture of hydrate nitrates of ammonium and magnesium, with low melting temperature, already tested for latent heat storage in domestic applications. In the discharge mode, cold water enters the tank and flows on the encapsulated melted PCM, which is cooled and solidified. In the initial condition the PCM is at its melting temperature. Suddenly its external surface is cooled to a constant temperature T 0; the duration of the solidification represents the time in which the latent heat is released to water. The discharge process of the phase change material (PCM) is analyzed analytically and its effectiveness is assessed, for constant surface temperature conditions, in three different geometrical configurations, i.e. considering the PCM encapsulated in slab, cylindrical or spherical polyethylene containers. The focus is on a model of the moving boundary within the phase-change material during the discharging mode, and the duration of the phenomenon. Results shown include transient position of the moving surface, temperature distribution, amount of solid PCM, energy released, and duration of complete solidification. The influence of the geometry and the Jacob number on the ending time of solidification is investigated. Among different geometrical configurations of the PCM, it is found that the shortest time for complete solidification is matched for small spherical capsules, with high Jacob numbers and thermal conductivity.

Tepidimonas aquatica sp. nov., A New Slightly Thermophilic β-Proteobacterium Isolated from a Hot Water Tank

A bacterial isolate, with an optimum growth temperature of about 50 degrees C, was recovered from a domestic hot water tank in Coimbra. Phylogenetic analysis using 16S rRNA gene sequence indicated that strain CLN-1T is a member of the beta-Proteobacteria and represents a new species of the genus Tepidimonas. The major fatty acids of strain CLN-1T are 16:0, 17:0 cyclo and 16:1 omega7c. Ubiquinone 8 is the major respiratory quinone, the major polar lipids are phosphatidylethanolamine, and phosphatidylglycerol. The new isolate is aerobic and facultatively chemolithoheterotrophic. Thiosulfate and tetrathionate are oxidized to sulfate in the presence of a metabolizable carbon source. Strain CLN-1T grows on amino acids and organic acids, but this organism does not assimilate carbohydrates. Glycerol is the only polyol assimilated. Resinic acids, namely abietic acid, dehydroabietic acid and isopimaric acid are not degraded. On the basis of the phylogenetic analyses, physiological and biochemical characteristics, we propose that strain CLN-1T represents a new species for which we offer the name Tepidimonas aquatica.

Thermal performance of a residential desuperheater/water heater system

A residential desuperheater/water heater system was tested in a laboratory under controlled environmental conditions. The desuperheater, integrated to a thermal storage system, was installed in the Dual-Air Loop Test Facility at The Center for Energy Studies, The University of Texas at Austin. The major components of the system consisted of the refrigerant compressor, domestic hot water (DHW) desuperheater, thermal storage tank with evaporator/condenser coil, DHW storage tank, DHW circulating pump, space conditioning water circulation pump, and indoor heat exchanger. Although measurements were made to quantify space heating, space cooling, and domestic water heating, this paper only emphasizes the desuperheater/water heater performance of the unit. The desuperheater system was tested over a range of outdoor conditions to determine parameters such as desuperheater rate and DHW temperatures. The desuperheater rate decreased with entering water temperature, since the heat transfer between the water and refrigerant decreased as the entering water temperature increased. In addition, the desuperheater coefficients of performance decreased with entering water temperature. The DHW tank first-hour rating, F, was 52.9 gal/h. The recovery efficiency and energy factor were 0.74 and 0.77, respectively.

Mechanical properties and durability of polypropylene fiber reinforced high-volume fly ash concrete for shotcrete applications

Complete thermal characterization was performed to the binary eutectics capric-myristic (CA:MA), lauric-myristic (LA:MA) and palmitic-stearic (PA:SA). The obtained thermophysical properties included theoretical calculus along with experimental measurements as phase change temperatures, latent heats, specific heat capacities and degradation temperatures, through modulated differential scanning calorimetry (MDSC) and thermogravimetry analysis (TGA). Likewise, vacuum-assisted method was successfully employed to produce novel shape-stabilized phase change materials (SS-PCMs), with a porous clay mineral as a support, using different impregnation percentages (25%, 35% and 40%) of CA:MA, LA:MA and PA:SA. Thermal properties were obtained in the same manner as for the binary eutectic mixtures, besides, scanning electron microscope (SEM) was used to study the SS-PCMs morphologies and leakage test performed. From the MDSC measurements, empirical equations of specific heat capacity as a function of temperature were proposed for the binary eutectics and SS-PCMs. The results of this work revealed that the optimal percentage of CA:MA, LA:MA and PA:SA was 35%, in terms of low leakage (<2%), latent heats around 45 kJ kg−1 and average heat capacities of 1.4 J g−1°C−1. As the new composites presented a wide range of phase change temperatures then SS-CAMA (23°C), SS-LAMA (36°C) and SS-PASA (55°C) can be used for diverse applications as in construction, food industry and domestic hot water tanks.