Domestic Hot Water Systems Research Articles

Energy, economy and exergy evaluations of the solutions for supplying domestic hot water from low-temperature district heating in Denmark

District heating in Denmark is going through the transition from 3rd generation (80/40°C) to 4th generation (50–55°C/25°C) systems in preparation for district heating based completely on renewable fuels by 2035. However, concern about Legionella growth and reduced comfort with low-temperature domestic hot water supply may be discouraging the implementation of low-temperature district heating. Aimed at providing possible solutions, this study modelled various proposals for district heating systems with supply temperatures of 65°C, 50°C and 35°C and for two different building topologies. Evaluation models were built to investigate the energy, economy and exergy performances of the proposed domestic hot water systems in various configurations. The configurations of the devised domestic hot water substations were optimised to fit well with both low and ultra-low-temperature district heating and to reduce the return temperature to district heating. The benefits of lower return temperatures were also analysed compared with the current district heating situation. The evaluation results show that the decentralized substation system with instantaneous heat exchanger unit performed better under the 65°C and 50°C district heating scenarios, while the individual micro tank solution consumed less energy and cost less in the 35°C district heating scenario.

Domestically Acquired Legionnaires' Disease: Two Case Reports and a Review of the Pertinent Literature.

Legionella species may colonize in home water systems and cause Legionnaires' disease (LD). We herein report two cases of sporadic LD associated with the solar energy-heated hot water systems of the patients' houses. A 60-year-old woman with chronic bronchitis and diabetes mellitus presented with a high fever, abdominal pain, and diarrhea. Physical examination revealed rales, and her chest radiograph showed a homogeneous density in the left lung. The Legionella urinary antigen test was positive, and an indirect fluorescent antibody test revealed a serum antibody titer of 1/520 for L. pneumophila serogroup 1. In the second case, a 66-year-old man with diabetes mellitus was treated for pneumonia at another hospital. After the patient's general condition worsened and he required mechanical ventilation, he was referred to our hospital. The Legionella urinary antigen test was positive. Neither of the patients had been hospitalized or travelled within the previous month. Both patients used hot water storage tanks heated by solar energy; both also used an electrical device in the bathroom to heat the water when solar energy alone was insufficient. The hot water samples from the residences of both patients were positive for L. pneumophila sero-group 1. These cases show that domestic hot water systems heated by solar energy must be considered a possible source of community-acquired LD.

Experimental photovoltaic-thermal Power Plants based on TESPI panel

Solar energy can be used to produce thermal energy, by means of thermal solar panels and electrical energy, using photovoltaic (PV) modules. In urban and suburban areas, because of a limited number of surfaces apt for installation of solar systems, the full development of both technologies can be limited.The integration of solar, thermal, and PV modules into a single unit, the so-called photovoltaic thermal (PV-T) module, could be a possible solution to generate electrical energy and heat simultaneously on the same surface.Many technical solutions are currently available; however, in this study, the analysis is limited to liquid-cooled nonconcentrating PV-T collectors where c-Si cells are used. In particular, the experimental results of two pilot plants, based on patented panels, named TESPI – thermal electric solar panel integration, are presented. The main advantage of these modules is that they represent a retrofit of existing PV plants. In fact, they can be installed on top of conventional PV modules as they are based on the infrared filtering effect of water on solar radiation. A standard test for hybrid PV-T systems for developing new solutions and measuring their performance was designed. In order to improve the design of modules and check their behavior in typical domestic hot water systems, two different PV-T solutions, but with identical PV modules, were analyzed without the heat transfer system. The prototypes are made of two systems (PV and PV-T) installed on two Italian sites, Enna (Sicily) and Pisa (Tuscany), characterized by typical Mediterranean climate. In this study, PV-T systems were analyzed at a PV string level. Different operating conditions were tested and electrical and thermal critical operating conditions were detected.

Life cycle assessment of domestic hot water systems: a comparative analysis

On average, hot water is responsible for 18% of residential energy consumption and corresponding greenhouse gas (GHG) emissions. Several domestic hot water systems (DHWSs) are commonly used but their life cycle impacts are yet to be established comprehensively. This is due to those impacts varying significantly within the context and the system boundaries of the assessment. This article reports findings from a comparative cradle-to-grave life cycle assessment (LCA) of five DHWSs in the UK context. Primary data acquired from a case study contributed to achieving accurate life cycle inventories that were then modelled in SimaPro through the ecoinvent database. Global Warming Potential (GWP) is the impact assessment method used. Amongst the five types, solar heater with electric backup appears to be the least damaging alternative. The study also reinforces the importance of adopting a cradle-to-grave approach if LCA results are to accurately reflect environmental impacts holistically and lead to better, more informed decisions.

Limiting Odour Emission from Piggery through Application of Heat Recovery System

Abstract The aim of the study was to determine the effect of manure cooling by the use of the heat recovery system on odour emission from a deep litter piggery. Annual comparative research was carried out in a twin-room deep litter piggery located in Wielkopolska Voivodeship. The recovered heat was transferred to central heating and domestic hot water systems of a residential building. The study showed that the average odour emission rate from the room, where heat was recovered (0.192±0.083 ouE·s−1·kg m.c.−1) was lower than in the room without heat recovering (0.273±0.138 ouE·s−1·kg m.c. −1) (p<0.05). The difference was 27%. It was also found that there was statistically significant strong correlation (r =0.86) between the amount of the recovered heat and the percentage odour emission reduction (p <0.05). This relationship was described by the logarithmic regression line y=12.5ln(x)-21.5 (r2=0.74)

Dynamic modeling approach for determining buildings technical system energy performance

This paper presents new approach for determining buildings technical system energy performance. This new methodology describes a mathematical model for accurately predicting indoor temperature and heat losses of the space heating and domestic hot water system components. The entire model is described by system of ordinary differential equations which can be solved using standard numerical techniques. The innovative aspect of the method is the integral approach in mathematical modeling of buildings energy needs and technical systems heat loss, taking into account heat accumulation in all considered parts (building envelope+technical system). Such approach allows a detailed insight of the system behavior for chosen working conditions. This model can serve for energy performance calculations in a wide variety of buildings types and their technical systems. The calculation example is given for family house, equipped with conventional space heating and domestic hot water heating system, with the time step of 1min and for characteristic day of each month within a year. The results are compared against those obtain from EN ISO 13790 and standard series EN 15316. The comparison shows significant differences in determination of the annual delivered energy to the heating system (33%), as a consequence of difference in estimation of the energy need for heating (15%) and calculation of the technical systems recoverable heat losses utilization factor, which seems to be underestimated. The delivered energy to the space heating and domestic hot water heating system differs 25%, while the energy delivered to the generation system differs 4%.

Operation analysis and performance prediction for a GSHP system compounded with domestic hot water (DHW) system

A ground-source heat pump (GSHP) system located in Chongqing, a cooling dominated region in China, was taken as an experimental system. In order to explore the operating characteristics of the GSHP system, the GSHP system was monitored with a data acquisition system. The measured data were analyzed, including the inlet/outlet water temperatures, power consumptions, EER/COP and also the ground temperature variations. The measured system EER of 2013 and 2014 are 3.01 and 2.91, respectively, while the system COP of 2013 is 2.59. A TRNSYS model was established based on the studied GSHP system. The accuracy of the model was validated by the comparison between the measured and the simulated outlet water temperatures, and also the ground temperatures. The model was then used to predict the long-term performance of the GSHP system. It has been found that after 20-years running of the system, the heat accumulation under the ground and the deterioration of system efficiency aggravate: the ground temperature would rise up to 30.5°C from the initial value of 20°C, while the ASPF value would descend to 2.72 from the initial value of 3.12. As a remedy for the problem, the domestic hot water (DHW) system was supposed to be operated in the following years, and the simulation showed that the operation of DHW system could effectively reduce the ground temperature and improve the system performance.

Experimental investigation of graphene nanoplatelets nanofluid-based volumetric solar collector for domestic hot water systems

Today, with growth of population and increasing dependence of industry and technology on fossil energy, all communities and countries will face the challenge of energy in the future. Among all types of renewable energies, solar energy is a proper alternative to fossil fuels. For it is available in most parts of the world. In this study, the experimental investigation of volumetric solar collector’s performance which is applied in domestic hot water has been shown for the first time, by making a laboratory sample of a volumetric collector, using innovative application of graphene nanoplatelets/deionized water with weight fractions of 0.0005, 0.001 and 0.005. The experiments were carried based on EN 12975-2 at different inlet temperatures and three mass flow rate of 0.0075, 0.015 and 0.225kg/s. The results showed that the collector efficiency increases by the nanofluid weight fraction increase. According to the results, the maximum collector efficiency is obtained at 0.015kg flow rate for both the base fluid and nanofluids. The zero-loss efficiencies using nanofluids with weight fractions of 0.0005, 0.001 and 0.005 were 83.5%, 89.7% and 93.2%, respectively; whereas the zero-loss efficiency was 70% using the base fluid.

Performance analysis of solar cogeneration system with different integration strategies for potable water and domestic hot water production

A novel solar thermal cogeneration system featuring the provision of potable water with membrane distillation in combination with domestic hot water supply has been developed and experimentally analyzed. The system integrates evacuated tube collectors, thermal storage, membrane distillation unit, and heat exchangers with the overall goals of maximizing the two outputs while minimizing costs for the given design conditions. Experiments were conducted during one month’s operation at AURAK’s facility in UAE, with average peak global irradiation levels of 650W/m2. System performance was determined for three integration strategies, all utilizing brackish water (typical conductivity of 20,000μs/cm) as a feedstock: Thermal store integration (TSI), which resembles a conventional indirect solar domestic hot water system; Direct solar integration (DSI) connecting collectors directly to the membrane distillation unit without thermal storage; and Direct solar with thermal store integration (DSTSI), a combination of these two approaches. The DSTSI strategy offered the best performance given its operational flexibility. Here the maximum distillate productivity was 43L/day for a total gross solar collector area of 96m2. In terms of simultaneous hot water production, 277kWh/day was achieved with this configuration. An economic analysis shows that the DSTSI strategy has a payback period of 3.9years with net cumulative savings of $325,000 during the 20year system lifetime.

Greenhouse gas emissions from domestic hot water: heat pumps compared to most commonly used systems

AbstractWe estimate the emissions of the two most important greenhouse gasses (GHG), carbon dioxide (CO2) and methane (CH4), from the use of modern high‐efficiency heat pump water heaters compared to the most commonly used domestic hot water systems: natural gas storage tanks, tankless natural gas demand heaters, electric resistance storage tanks, and tankless electric resistance heaters. We considered both natural gas‐powered electric plants and coal‐powered plants as the source of the electricity for the heat pumps, the thermal electric storage tanks, and the tankless electric demand heaters. The time‐integrated radiative forcing associated with using a heat pump water heater was always smaller than any other means of heating water considered in this study across all time frames including at 20 and 100 years. The estimated amount of CH4 lost during its lifecycle was the most critical factor determining the relative magnitude of the climatic impact. The greatest net climatic benefit within the 20‐year time frame was predicted to be achieved when a storage natural gas water heater (the most common system for domestic hot water in the United States) fueled by shale gas was replaced with a high efficiency heat pump water heater powered by coal‐generated electricity; the heat pump system powered by renewable electricity would have had an even greater climatic benefit, but was not explicitly modeled in this study. Our analysis provides the first assessment of the GHG footprint associated with using a heat pump water heater, which we demonstrate to be an effective and economically viable way of reducing emissions of GHGs.

Wind Assisted Circulation Vs Forced Circulation Domestic Solar Hot Water Systems with Co2Filled Collector

The main focus of this study is to escalate the performance of the solar collector panel with minimal modification in a domestic solar hot water system. The conventional solar collector panel is filled with CO2 gas and the experiments were conducted for both forced circulation system (FCS) and wind assisted circulation system (WACS). The results were compared to the same systems when they are operated without filling CO2 in the solar collector panel. The hourly average efficiency seemed to be responding with respect to incident solar radiation and it corresponds to increased circulation rate of water across the collector. It was evident that the daily average system efficiency was enhanced by 3.27% and 3.6% for FCS and WACS mode of operation with CO2 filled collector panel. Comparative study between FCS and WACS modes of operation with and without CO2 reveal that WACS with CO2 gas filled in the collector plane under normal atmospheric pressure (WACS-CO2) appears be a better alternative to replace the FCS and WACS since WACS-CO2 does not require electricity for its operation and can be installed in remote areas with limited or no supply of electricity.

Investigation and Modelling of the Centralized Solar Domestic Hot Water System in Residential Buildings

The solar domestic hot water system is currently widely applied as a building energy saving technique. Various forms of the system exist, which should be properly applied in practical engineering. It is significant to study the adaptability of the solar domestic hot water system as it accounts for the building energy consumption in a non-negligible proportion, which is still scarcely studied based on field measurements in current research. This study figured out the performance of the centralized solar domestic hot water system through field measurements, as well as built a sub-hourly simulation model, which has been validated with the field measurement. The model was used to analyse the influence of several factors on the energy consumption of the system, such as the water supply strategy, the use modes of occupants, the thermal insulation of pipes, and the forms of the system. The adaptability of the system was concluded.

Effect of thermal stratification on energy and exergy in vertical mantled heat exchanger

In the present study, effect of thermal stratification on energy and exergy storage amount in vertical mantled hot water storage tanks, which are used common in solar domestic hot water systems, has been investigated. Placing obstacle inside the tank has enhanced thermal stratification and so energy and exergy of the tank. A commercial vertical mantled hot water tank was selected for experimental study. Four different obstacles were placed in four different heights from the tank bottom. Effect of the obstacle types and positions on energy and exergy storage capacity of tank has been researched experimentally. As a result, placing obstacles inside tank has enhanced the energy and exergy of tank. The best increasing has been seen for Obstacle A in Y/H = 0.250 position. In this situation energy amount increased 1.86 times and exergy amount increased 2.02 times higher than ordinary tank.

Experimental investigation on heat transfer characteristics and pressure drop of BPHE (brazed plate heat exchanger) using TiO2–water nanofluid

In this research, the effect of using TiO2–water nanofluid on heat transfer enhancement and pressure drop in a Brazed Plate Heat Exchanger used in domestic hot water system is investigated experimentally. TiO2 nanoparticles with 20nm diameter and 99+% purity are used for making the nanofluid at 0.3%, 0.8% and 1.5% weight concentration of suspended nanoparticles, in such experiments. This study is done on the brazed plate heat exchanger under turbulent flow condition. The effects of Reynolds number and weight concentration of nanoparticles on the heat transfer characteristics are investigated experimentally. This results in a significant increase in convective heat transfer coefficient through adding nanoparticles to distilled water. The results show that, the convective heat transfer coefficient and the overall heat transfer coefficient of TiO2–water nanofluid is increased with enhancement of Reynolds number and at a specified Reynolds number, heat transfer characteristics is enhanced by increasing weight concentration of nanoparticles in brazed plate heat exchanger. The maximum enhancement of the nanofluid convective heat transfer coefficient at 0.3%, 0.8% and 1.5% weight fraction of nanoparticles are about 6.6%, 13.5% and 23.7% respectively. Also the maximum enhancement of the nanofluid overall heat transfer coefficient at mentioned weight concentrations of nanoparticles are about 2.2%, 4.6% and 8.5% respectively. Finally, the nanofluid pressure drop at various levels of nanoparticles is measured experimentally. According to these results, increment in system pressure drop is negligible compared to the heat transfer improvement.

The effect of measurement time resolution on the peak time power demand reduction potential of domestic solar hot water systems

The electrical resistance showerhead is the water heating technology used in over 70% of the Brazilian dwellings. These high power (5–8 kW are typical) instant heating devices contribute significantly to the demand peak in the early evening, and are a major burden to distribution utilities in Brazil. The objective of this paper is to define consumer showering patterns, and analyze the influence of the power demand measurement time resolution on the potential of peak time power demand reduction provided by Domestic Solar Hot Water (DSHW) systems. Results show that the measurement of electrical power demand in a 15-min interval is not adequate to evaluate and verify the benefits provided by the use of DSHW systems, because it heavily underestimates the active power demand reduction at peak hours. This work suggests that a 1-min time resolution can be considered appropriate to assess the potential impacts caused by the use of DSHW systems on the peak hours demand reduction.

How significant is the stability of the radiative regime when the best operation of solar DHW systems is evaluated?

A typical Solar Domestic Hot Water (SDHW) system consisting of solar collectors and a water storage tank operating in fully mixed regime is considered. Optimal control operation is assumed. The objective is to maximize the net daily heat provided by the solar collectors. Meteorological data measured during year 2009 in Timisoara (Romania, Southeastern Europe) are used. Several days with more or less stable radiative regime are selected. Most results correspond to summer and spring days with daily relative sunshine larger than 0.4. The pumping energy increases by decreasing the daily relative sunshine. During more stable days the pump in the primary circuit operates a longer period of time than during less stable days. The opportunity to use the SDHW system for replacing a classical energy source depends on the stability of the radiative regime. The conclusion is that the dependence of SDHW systems' performance on the stability of the radiative regime is a complicate function of the specific performance indicator and the available amount of solar energy.

Energy use consequences of ventilating a net-zero energy house

A Net-Zero Energy Residential Test Facility (NZERTF) has been constructed at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, to demonstrate that a home similar in size, aesthetics, and amenities to those in the surrounding communities can achieve net-zero energy use over the course of a year while meeting the average electricity and water use needs of a family of four in the United States. The facility incorporates renewable energy and energy efficient technologies, including an air-to-air heat pump system, a solar photovoltaic system, a solar thermal domestic hot water system, and a heat recovery ventilation system sized to meet American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) Standard 62.2-2010 ventilation requirements. The largest energy end use within the home was space conditioning, which included heat loss through the building envelope, ventilation air supplied by the heat recovery ventilator (HRV), and internal loads. While HRVs are often described as being able to save energy when compared to ventilating without heat recovery, there have been no studies using a full year of measured data that determine the thermal load and energy impacts of HRV-based ventilation on the central heating and cooling system. Over the course of a year, continuous operation of the HRV at the NZERTF resulted in an annual savings of 7% in heat pump energy use compared with the hypothetical case of ventilating without heat recovery. The heat pump electrical use varied from an increase of 5% in the cooling months to 36% savings in the heating months compared with ventilation without heat recovery. The increase in the cooling months occurred when the outdoor temperature was lower than the indoor temperature, during which the availability of an economizer mode would have been beneficial. Nevertheless, the fan energy required to operate the selected HRV at the NZERTF paid for itself in the heat pump energy saved compared with ventilation without heat recovery.

Transient Analysis of a Solar Domestic Hot Water System Using Two Different Solvers

In the present work the unsteady numerical simulation of a solar domestic hot water (DHW) system composed of two flat plate collectors, a water tank for heat storage, and a coil heat exchanger is addressed. The simulations have been performed using two different solvers, namely a home-made code written in Matlab, and TRNSYS 17. In the first part of the paper, the analytical models used in the Matlab code, and the TRNSYS case are reported in detail. Successively, the results of the simulations realized by means of the two solvers are presented and compared.

Alternative solutions for inhibiting Legionella in domestic hot water systems based on low-temperature district heating

District heating is a cost-effective way of providing heat to high heat density areas. Low-temperature district heating (LTDH) is a promising way to make district heating more energy-efficient and adaptable to well-insulated buildings with low heating demand in the future. However, one concern is the multiplication of Legionella due to insufficient temperature elevation with low-temperature supply. The aim of this study was to find optimal solutions to this dilemma for specific situations. The solutions were of two types: alternative system designs and various methods of sterilization. The alternative design approach can eliminate the risk factors of Legionella by local temperature boosting and restricting system volume. Decentralized substations, micro heat pumps, electric heating elements and electric heat tracing are all investigated as alternative designs. With regard to sterilization methods, thermal treatment, ionization, chlorine, chlorine dioxide, ultraviolet light, photocatalysis and filtration are discussed as the most frequently used methods in hot water systems. The characteristics, efficacy and operation methods of LTDH using the solutions investigated are documented and compared. Finally, recommendations are given for their use in practice. Practical application: The methods for inhibiting Legionella should fit into applicable situations according to their characteristics. This study aims to recommend optimum solutions for inhibiting Legionella in LTDH scenarios, and includes a comprehensive overview of their efficiency, installation, operation and costs, so as to give adequate information for selecting appropriate solutions. In addition to sterilization methods, alternative system design implemented with new technologies can also help prevent Legionella in hot water systems. They have the additional benefit of reducing the heat loss of the hot water system. The alternative design solutions both enrich our options for water sanitation and improve the energy efficiency of our energy systems.

A Numerical Model of a Solar Domestic Hot Water System Integrating Hybrid Photovoltaic/Thermal Collectors

A Photovoltaic-Thermal (PV-T) module is a solar hybrid collector which converts part of incident solar energy into electricity and recovers a fraction of the remainder dissipated as heat.A new high efficiency, covered, water PV-T component was developed in the context of a partnership between CETHIL, Fraunhofer ISE and EDF R&D. Tested under controlled conditions, the module demonstrated unparalleled electrical and thermal efficiencies. Within the framework of the project PHOTOTHERM an array of modules was integrated into a domestic hot water system installed on the BESTLab laboratory at EDF R&D, France, so that the performance of a complete system could be monitored under real conditions. In parallel, a numerical model was developed taking into account the heterogeneity of PV cell temperature, and the geometrical complexity of the Fractherm® heat exchanger developed by Fraunhofer ISE. The model, developed in TRNYS, was validated for steady state and transient conditions. The aim of the work now is to assess the energetic and exergetic performance of systems integrating this new PV-T module.The PV-T prototype, as well as the entire domestic hot water system, and the complete TRNSYS model are presented in this paper. Model validation was performed using measurements taken under real conditions. The model allows a rapid energetic analysis of the domestic hot water system.