Domestic Hot Water Heating Research Articles

EXPERIMENTAL INVESTIGATION OF AIR-SOURCE HEAT PUMP IN DIFFERENT REAL OPERATIONAL CONDITIONS FOR HEATING DISTINCTIVE REGIMES

In a time of permanent care for environmental protection, energy consumption and conservation of resources, heat pumps are becoming increasingly important as a technology for reducing greenhouse gas emissions while preservation thermal comfort in build environment. This paper presents the experimental results of air-source heat pump in different real operating conditions, as follows: for space heating by low-temperature underfloor heating when the supply water temperature was set at 38°C, or for domestic hot water heating in the storage tank volume of 180 liters at a temperature of 46°C. In the low-temperature underfloor heating mode, the efficiency of the air-source heat pump could be increased if the maximum temperature at the end of the compression process and the condensing temperature were reduced in the initial phase of the heat pump operation. In the domestic hot water heating mode, the efficiency of the air-source heat pump could be increased if the maximum condensation temperature in the final phase of the heat pump operation was limited.

Exergoeconomic and environmental analyses of a novel multi-generation system including five subsystems for efficient waste heat recovery of a regenerative gas turbine cycle with hybridization of solar power tower and biomass gasifier

This study aims to recover the waste heat from a previously introduced regenerative gas turbine cycle (GTC) when it is driven by a hybrid heat source composed of solar power tower (SPT) and biomass gasification. The system consists of electricity, cooling, heating, and hydrogen production subsystems. Therefore, the system is converted into a novel multi-generation system which is examined through energy, exergy, exergoeconomic, and environmental analyses. In the proposed system, to recover the waste energy of a topping GTC, a steam Rankine cycle (SRC) by employing a thermoelectric generator (TEG) instead of a conventional condenser, a domestic hot water heat exchanger (DHWHX), and an LiCl-H2O absorption refrigeration system (ARS) are utilized. The power recovered by the TEG is employed to produce hydrogen by a proton exchange membrane (PEM) electrolyzer. In the base case, the proposed system with the multi-generation exergy efficiency of 43.11%, can produce 98.2 MW of electricity which 24.7 MW of this production is owing to the waste heat recovery of GTC in SRC. Heating, cooling, and hydrogen production rates are 13 MW, 10.5 MW, and 4.1 kg/h, respectively. Due to the waste heat recovery of GTC, total exergy efficiency is increased by 9.04 percentage points. The total cost rate of the proposed system is 7799 $/h, and the unit cost of multi-generation is 8.26 $/GJ. Although the increase of direct normal irradiance (DNI) brings about a slight drop in the thermodynamic performance of the system, it improves the performance of the system from economic and environmental standpoints. By increasing the number of mirrors, which induces the reduction of fuel consumption and improvement of the environmental performance of the system, it is accompanied by weakening the performance of the system from thermodynamic and economic perspectives. By raising the pressure ratio of the GTC compressor, the heating production rate declines and the performance of the system decreases from economic and environmental aspects, while the net power output and multi-generation exergy efficiency have their maximum values at pressure ratios of 14.6 and 11.2, respectively. The only positive effect of increasing the pinch temperature difference of heat recovery steam generator (HRSG) is that it can significantly increase the heating production rate.

Single case study photovoltaic panels within water heating system

Abstract The submitted article offers one of the possible options of a usage of photovoltaic panels for a domestic hot water preparation with an option to extra heating a heating system, and following recalculation of a produced electric energy amount in regards of a total return of the assembled system. An electric heating boiler combined with a heating option working on solid propellant was chosen for the experiment. The most frequently used combined electric storage tank was chosen as a classic option to heat domestic hot water.

Energy, exergy and environmental-based design and multiobjective optimization of a novel solar-driven multi-generation system

In this study, energy, exergy and environmental-based design and multiobjective optimization of a novel solar-driven gas turbine-based multi-generation system is performed. For this purpose, a novel multi-generation system composed of a solar tower-driven gas turbine cycle, a Kalina cycle, an organic Rankine cycle, a single effect absorption refrigeration cycle, an electrolyzer and two domestic hot water heaters is developed. The system can produce electricity, heating and cooling for residential applications, domestic hot water, hydrogen and swimming pool heating, simultaneously from a single renewable energy source. A novel performance indicator, exergetic quality factor (EQF), for performance comparison and multiobjective optimization of multi-generation systems is also introduced. The system is analyzed with energy, exergy, EQF, environmental and exergoenvironmental measures. A detailed parametric study is also performed to analyze the effect of varying design parameters on the performance of the proposed system. The results show that the proposed system has 55.57%, 39.45% and 50.83% of energy efficiency, exergy efficiency, and EQF values, respectively; and inclusion of EQF into multiobjective optimization improves the multi-generation system performance.

Integration of Micro-Cogeneration Units and Electric Storages into a Micro-Scale Residential Solar District Heating System Operating with a Seasonal Thermal Storage

A micro-scale district heating network based on the operation of solar thermal collectors coupled to a long-term borehole thermal storage is modeled, simulated and investigated over a period of five years. The plant is devoted to covering the domestic hot water and space heating demands of a district composed of six typical residential buildings located in Naples (southern Italy). Three alternative natural gas-fueled back-up auxiliary systems (condensing boiler and two different technologies of micro-cogeneration) aiming at balancing the solar energy intermittency are investigated. The utilization of electric storages in combination with the cogeneration systems is also considered with the aim of improving the self-consumption of cogenerated electric energy; heat recovery from the distribution circuit is also evaluated to pre-heat the mains water for domestic hot water production. The performances of the proposed plant schemes are contrasted with those of a typical Italian decentralized heating plant (based on the utilization of natural gas-fueled non-condensing boilers). The comparison highlighted that the proposed configurations can decrease the primary energy consumption (up to 11.3%), the equivalent emissions of carbon dioxide (up to 11.3%), and the operation costs (up to 14.3%), together with an acceptable simple pay-back period (about 4.4 years).

The benefits of 4th generation district heating in a 100% renewable energy system

District heating is a well-established system for providing energy efficient space and domestic hot water heating in dwellings in particularly in temperate and cold climate zones. Research has shown that going from the current 3rd generation district heating (3GDH) systems towards 4th generation district heating (4GDH) systems can facilitate a better integration between energy sectors, reduce grid losses and assist the integration of renewable energy sources. This article investigates the economic and energy effects of going from 3GDH to 4GDH for the specific case of Aalborg Municipality, Denmark based on overall hourly energy systems simulations. The analyses include effects from changes in excess heat potentials, changes in grid losses, and changes in efficiencies of conversion units in the district heating. Altogether, the analyses of the Aalborg case reveal that going from 3GDH to 4GDH decreases the primary energy consumption of the entire energy system by around 4.5% and the costs of the system by 2.7%.

Research on waste heat recovery from gas engine for auxiliary heating: An emerging operation strategy to gas engine-driven heat pump

Abstract Gas engine-driven heat pump (GEHP) is attracting increasing interest owing to the high heating efficiency and energy-saving. However, the waste heat from engine is mostly used to directly heat domestic hot water. In this study, a new type of waste heat utilization mode of GEHP was proposed: auxiliary heating. The heating capacity, gas consumption, waste heat recovery and compression ratio of water-source GEHP system with waste heat auxiliary heating mode was studied. In addition, the effects of two utilization modes of engine waste heat domestic hot water (mode Ⅰ) and auxiliary heating (mode Ⅱ) on the thermodynamic parameters were compared. The results indicated that the system efficiency was decrease with engine speed. Even in high speed operation, the primary energy efficiency (PER) of the system was more than 1.55. Under the same range of condenser water inlet temperature, the COP of mode Ⅰ was in the range of 4.71–4.95, and that of mode Ⅱ was in the range of 5.17–5.67. The energy saving and emission saving were also researched. Compared with mode Ⅰ, mode Ⅱ had higher efficiency and less pollution to the environment.

Selecting the model and influencing variables for DHW heat use prediction in hotels in Norway

Domestic hot water heat use prediction modelling is an important instrument for increasing energy efficiency in many buildings. This article addressed hourly domestic hot water heat use prediction, using a Norwegian hotel as a case study. Since the information available for buildings may vary, two widespread situations with different input variables were studied. For the first situation, the prediction is based only on data obtained from historical measured domestic hot water heat use. For the second situation, additional variables that affect domestic hot water heat use were applied. These variables were determined using the Wrapper approach. The Wrapper approach showed that factors related to the guests presence have the most significant influence on the domestic hot water heat use in the hotel. Nevertheless, daily data about the number of guests booked at the hotel did not appear to be informative enough for precise hourly modelling. Therefore, to improve the accuracy of the prediction, it was proposed to use an artificial variable. This artificial variable explained the hourly intensity of the guests domestic hot water use. In order to select the best model for the domestic hot water heat use prediction, ten advanced time series and machine learning techniques were tested based on the criteria of models adequacy. For both considered situations, the Prophet model showed the best results with R2 equal to 0.76 for the first situation, and 0.83 the second situation.

Effect of cycle-induced crack formation on the hydration behaviour of K2CO3 particles: Experiments and modelling

Thermochemical energy storage using salt hydrates is a promising concept to bridge the gap between supply and demand for solar thermal energy in residential buildings. Using a suitable thermochemical material such as a salt hydrate, a thermal energy storage device, also known as a heat battery, can be created to supply low-temperature thermal energy during colder periods. To generate adequate power from a heat battery for the production of domestic hot tap water or space heating, the hydration rate of the salt hydrate needs to be sufficiently fast. It is hypothesized that the hydration rate of the material increases over multiple charge and discharge cycles due to crack formation and volume increase of the salt hydrate particles. This hypothesis is tested by performing two kinds of experiments: optical microscopy experiments using a micro-climate chamber to evaluate the particle size, and Thermo Gravimetric Analysis (TGA) experiments to determine the hydration rate of the particles. The hydration rate and particle size are input for a nucleation and growth model that takes into account crack formation and particle growth. Optical microscopy experiments show a particle expansion of approximately 30 % over 12 cycles. Typical hydration rates are increased by a factor 15 comparing the first and the 12th TGA cycle. It is shown that particle growth and crack formation significantly contribute to the improvement of the hydration rate. Finally, taking into account crack formation and particle growth in the numerical model results in a good agreement between model and experiments. Such a numerical model can be used for heat battery design.

Modeling and experimental validation and thermal performance assessment of a sun-tracked and cooled PVT system under low solar irradiation

The PV panels cooling allows one to increase the photovoltaic conversion efficiency, as well as to obtain the waste heat, which can be utilized for various purposes. This paper presents the model of the radiator used for photovoltaic (PV) panels cooling. The cooling liquid is a water-glycol mixture, the PV panel electrical power output is 280 W. The radiator consists of a various number of segments (U-tube), that are located at the PV panel width. The three-dimensional control volume method formulation is used to calculate the temperature distribution within the PV panel surface (glass, silica layer, protective foil), and aluminum radiator. The one-dimensional energy equation is used to calculate the cooling liquid outlet temperature. The effect of an increasing number of radiator segments and the mass flow rate is studied to optimize the heat transfer performance and decrease the manufacturing costs. The simulations are conducted for various conditions, including the cloudy and sunny days. The simulation results are compared with experimental data collected at a stand installed at the laboratory of the Department of Energy at the Cracow University of Technology. The measurement results and the numerical simulation shows quite good agreement. For the studied case, it is shown that lowering the temperature of the panels increases the gross efficiency of the entire system. Moreover, the waste heat obtained from the PV cooling may be utilized as low-temperature heat for domestic hot water preheating or heat pump system applications.

A Sustainable Trigeneration System for Residential Applications

Abstract This paper features the integration of two renewable energy sources, making a new trigeneration system for residential applications. The system is primarily powered by solar photovoltaic-thermal (PVT) along with geothermal energy. This trigeneration system consists of a ground source heat pump, solar system, high-grade and low-grade heat exchangers, a heat pump system, and a water storage tank (WST). The objective of this system is to provide the main commodities for residential use including domestic hot water (DHW), electricity, and space heating. The system is analyzed energetically and exergetically using thermodynamic-based concepts. The overall energy and exergy efficiencies of the proposed system are found to be 86.9% and 74.7%, respectively. In addition, the energy and exergy efficiencies of the PVT system are obtained to be 57.91% and 34.19%, respectively. The exergy destructions at the high-grade heat exchanger and the water storage tank add up to 36.9 kW, which makes up 80% of the total exergy destruction of the system. Additionally, parametric studies are conducted to evaluate the degree of impact that various important parameters have on the overall system performance.

Development and analysis of hourly DHW heat use profiles in nursing homes in Norway

Representative profiles for domestic hot water (DHW) heat use are the main instruments for improvement in operation and design of DHW systems in buildings. To improve the existing method for DHW heat use profiles development and analysis, investigations in the three nursing homes in Norway were conducted. Statistical methods to assess the similarities of the profiles by days of the week and seasons were proposed. The analysis allowed us to identify two seasons of DHW heat use: the warm season from June to October, and the cold season including the rest of the year. In addition, it was investigated that the DHW heat use in the working days was significantly different from the weekends. According to these results, unified profiles for the months and days of the week with similar characteristics of the DHW heat use were developed. After, the method for statistical grouping of the DHW hourly heat use was applied to recognize the timing of the peak, average, and low heat use. Finally, the profiles for the DHW heat use obtained for the nursing homes were compared with profiles in the national and international standards. The drawbacks of the standards were identified.

Analysis of reliable and safe heat supply of residential buildings using waste water after DHW heat exchangers

A modified scheme of heat supply of residential buildings with dependent connection to external heating networks, providing reliability of heat supply and the necessary comfort in the premises due to mixing of waste water after domestic hot water (DHW) heaters at an outdoor temperature exceeding the temperature graph break point, is considered. A review of possible ways to improve the reliability and safety of heat supply, improve the efficiency of heat sources, as well as regulate heat supply and prevent overheating near the beginning and end of the heating period is carried out. Calculations are carried out to determine the actual amplitude of temperature fluctuations in residential buildings with a daily variation in water flow rate after hot water heaters in the conditions of application of the considered scheme of connecting buildings to the heating network. Analysis of obtained results is given, and conclusions are drawn concerning the expediency of application of the above scheme. It is established that, given the thermal stability of enclosing structures, the daily flow rate fluctuations have no significant effect on the stability of the temperature regime of residential buildings and the comfort of their indoor microclimate at high outdoor air temperatures. It is shown that, in terms of reliability of heat supply of the main group of residential buildings and ensuring the life safety, the offered scheme is not inferior to the standard two-stage scheme of connection of DHW heat exchangers, with restriction of the total consumption of network water and with dependent control of heat supply to DHW, heating and ventilation. It is noted that in the scheme under consideration, the daily average consumption of network water to domestic hot water heaters over most of the heating period is higher or equal to the estimated consumption for heating and ventilation systems, which minimizes the time of using the mixing pump and provides for the regulation of heat supply only at the expense of the wastewater circuit.

Special Section: Memorial Festschrift for the Late Professor Frank Kreith Sunrise Delayed but the Sun Shines Brightly: The Energy-Sustainability Quandary and Legacy of Professor Frank Kreith

Special Section: Memorial Festschrift for the Late Professor Frank Kreith Sunrise Delayed but the Sun Shines Brightly: The Energy-Sustainability Quandary and Legacy of Professor Frank Kreith

Heat pumps and solar energy: A review with some insights in the future

Abstract A large fraction of the energy demand is due to space heating. Direct solar heating might reduce the need of fossil fuels. However the poor solar collector efficiency when outside temperature and solar radiation are low, as in the heating season, limit most of solar collectors application to domestic hot water heating. Similarly air source heat pumps are penalized just when the heating demand is higher. Then a possible solar contribution to the outside air as a heat pump cold source was first analyzed, evaluating different integration modes of the two sources. Subsequently the coupling of a ground source and a solar section appeared a more favourable application, also because solar heat could recharge the ground in periods of low or no heating demand. At the same time the solar section might reduce the length of the expensive boreholes. Solar assisted absorption heat pumps were successfully experimented. Recently studies were devoted to a solar assistance of heat pumps by PV/T collectors, that offer both a fraction of the electricity to drive the heat pump and a solar assistance to the heat pump cold source, be it the ground or the outside air.

A comparative analysis of a heat pump application with grey wastewater source for domestic hot water preparation in hotels

System for domestic hot water (DHW) preparation generally represents a considerable portion of the energy and water consumption in hotels worldwide. As a consequence, a vast amount of heat is wasted through the drainage system from rooms and kitchens. Also, for hotel-type buildings, demand for DHW can vary according to their occupancy. Researchers and practitioners have a very challenging task to select the optimal and most sustainable solution. This paper analyses a DHW heating system with a heat pump system that uses grey wastewater (GWW) and rainwater as an energy source. The application is developed in combination with experimental site measurements (real system operation) and required standards. The research deals with the subject system in bivalent operation (basic and advanced configuration of heat pump system assisted with gas boiler). In order to evaluate the application of the advanced heat pump DHW heating system with GWW source, a comprehensive assessment is done with a gas boiler system only (base case) and solar plate collector system. For the advanced heat pump system, the study has determined the two key operation elements: hotel occupancy rate and system regulation. The financial assessment showed very promising economic metrics (PBP = 6.54 years, POP = 7.9 years IRR = 9.40%, MIRR = 17.00%) compared to the base case. The study provided results of a sensitive cost-effective system assessment for periods with lower than average hotel occupancy rates. Also, when the total operation and investment costs are compared with the data calculated on an average annual hotel occupancy rate of 68%, the final conclusion is that the investment in the advanced heat pump system is justified only if its gas consumption share is less than 14.27%. The analysis confirmed the main advantages of this heat pump system which are lowered energy consumption and reduced total operating costs.

Alternative storage options for solar thermal systems

The efficient implementation of solar systems in buildings depends on the storage of energy yielded, as it can both increase the solar system's autonomy and make it a feasible solution for zero energy buildings, and make storage vessels more compact, reducing precious space requirements. This is of particular important in places with reduced time of sunshine, where solar systems are less effective, because of the deviation between solar radiation and the demand. The traditional storage options use water, which is practical, safe and low-cost, especially when the storage requirements are small. However, when larger storage is needed, limits concerning the use of water exist, mainly due to the need for larger installation space and the increased thermal losses. The use of phase change materials (PCM) for thermal energy storage seems an upcoming technology. The main idea is the substitution of water with PCM, which feature larger specific energy storage capacity compared to other conventional materials. In the context of the specific paper, a combined solar thermal system used for the preparation of domestic hot water (DHW) and space heating (Solar Combi System) with two different types of storage is studied, for two Greek cities. The aim is to find out which is the most efficient way of storing energy with respect to the autonomy of the system, for a solar combi system. This is being achieved by determining the comparative autonomy of PCM and water storage system for various climates. It was proven that using PCM is advantageous, as it can extend the autonomy duration of the solar system for 2 to 8 hours, depending on the season and the climatic conditions. However, it was also seen that in solar combi systems used throughout the whole year, PCM are inefficient during summer period.

Performance assessment of a solar thermal combisystem in different climate zones

As one of the largest energy consumers, the residential buildings have a worthy potential for energy saving. Solar thermal combisystems, which provide the required energy for both domestic hot water and space heating, play a key role in reducing the building energy consumption. The purpose of this paper is to investigate the effect of the climatic conditions on the thermal performance of a solar combisystem using a dynamic simulation by TRNSYS. To this aim, the system performance is considered in five different climate zones including Hot-Dry, Cold-Dry, Moderate-Humid, Hot-semi-Humid, and Hot-Humid. Based on the obtained results, the required energy for domestic hot water is supplied by solar energy in all climate zones, except Moderate-Humid and Cold-Dry climate zones. For providing the required energy for space heating, it is necessary to use the auxiliary energy in all climate zones and all months, except Hot-semi-Humid and Hot-Humid climate zones, which has the small space heating loads because of the high ambient temperature in cold months. The annually total solar fraction of 74%, 61%, 47.8%, 87.9%, and 92% is obtained in Hot-Dry, Cold-Dry, Moderate-Humid, Hot-semi-Humid, and Hot-Humid, respectively. The findings confirm that the building energy consumption decreases considerably using the solar combisystem as a heating system, which resulted in significant energy and economic savings.

Thermodynamic analysis of ultralow-temperature district heating system with shared power heat pumps and triple-pipes

An ultralow-temperature district heating system that uses neighborhood-scale heat pumps was recently proposed and investigated. Although the system was found more efficient than any existing solution, there is still potential in this concept for achieving better efficiency and cost-effectiveness. One of the main origins of losses here is through the enlarged pipes. In this work, triple-pipes are proposed to be used instead of twin-pipes to have individual domestic hot water and space heating pipes, reducing the size of heat pumps and decreasing thermal loss rates. The proposed concept is designed, sized and thermodynamically analyzed for a case study in Denmark. The performance is compared with the previous version of this concept as well as the low-temperature district heating system, which is another important competitor to the proposed solution. The results prove that the proposed system outperforms both of the competitors in terms of energy efficiency. For three consecutive days in typical warm/moderate/cold weather in the case study, the proposed solution results in the rates of heat loss of 4.5/10.9/16.5%, respectively. The heat loss rates, for the conventional configuration and the low-temperature design, are 4.9/13.8/19.6% and 5.6/17.8/33.1%, respectively.

Experimental validation of a model-based method for separating the space heating and domestic hot water components from smart-meter consumption data

Smart meters are currently being rolled out in European district heating (DH) systems at a large scale to enable time-varying district heating tariffs and improve consumer awareness about their own consumption. Smart-meter data can also be used in more advanced applications, e.g. for establishing model-based control schemes for demand response purposes and data-driven building energy performance labeling schemes. Many of these applications require separate measurements of the consumption for space heating (SH) and preparation of domestic hot water (DHW); however, smart meters often only provide the total DH energy consumption (SH+DHW) in truncated units (e.g. whole kWh on an hourly basis). Typical approaches for separating these two components of DH consumption require measurements with a high temporal and numerical resolution and are therefore not applicable to smart-meter data. New methods suitable for disaggregating the combined DH demand are therefore needed. This paper presents a validation of a model-based method for disaggregating DH consumption using ground truth data from 44 residential buildings.