Hot Water Preparation Research Articles

Solar thermal hot water system in hospitals: Robust design methodology considering uncertainties

The energy yield of solar thermal installations is influenced by local climatic conditions, heating demand, and the efficiency of the system. These factors are affected by uncertainties arising from natural variations and a lack of knowledge. Designers typically address these uncertainties by oversizing the collector area. However, this practice usually leads to high costs, low efficiency, and short service lifetimes of the system. This paper proposes a robust sizing methodology for the solar thermal system used for service hot water preparation in a case-study hospital with a daily average hot water consumption of 8.69 m3. This novel approach incorporates uncertainties through probabilistic simulations, assessing the system’s reliability in achieving a 70% solar fraction design goal. The simulations reveal a mere 22% confidence in the initial deterministic design concerning the solar target and emphasize the importance of thermal insulation, temperature control, and collector cleaning as key factors to reduce performance variability. Although the final revised design outcome requires a 15% increase in the collector area compared to the initial scenario and stricter specifications for control components, pipe insulation, and valve maintenance, it achieves a 90% reliability. In addition, annual carbon emissions and levelized costs of hot water production can be reduced by 18% and 13%, respectively.

Booster heat pump with drop-in zeotropic mixtures applied in ultra-low temperature district heating system

The pursuit of sustainable district heating solutions has driven a growing interest in ultra-low temperature district heating (ULTDH) systems, where booster heat pumps (BHPs) play a pivotal role despite challenges posed by their efficiency limitations under large temperature glide conditions. This paper investigates the potential of drop-in R-1234yf/R-32 zeotropic mixtures in BHPs compared to a baseline R-134a system, within the context of a ULTDH framework. This study focused on the viability of the mixtures of R-1234yf/R-32 with the composition ratio of 80 %/20 % and 90 %/10 %. The investigation reveals disparities in compressor efficiency and heat exchanger pressure drop at the component level. Device-level analysis unveils increased COP for R-1234yf/R-32 mixtures, alongside with maximum second-law efficiencies reaching 0.32. A remarkable enhancement in heating capacity up to 58 % was found. System-level analysis demonstrated exergetic efficiencies and identified preferable district heating temperatures. Exergetic efficiencies of 0.47, 0.55, and 0.59 were achieved for domestic hot water preparation at district heating supply temperatures of 30 °C, 35 °C, and 40 °C, with a subsequent shift in optimal district heating temperatures as central heating station efficiency decreased. Temperature profile analysis underscored challenges stemming from excessive subcooling, highlighting the need for configuration refinements.

Considerations regarding the recovery of residual energy for the heating/cooling of buildings and the preparation of domestic hot water

Considerations regarding the recovery of residual energy for the heating/cooling of buildings and the preparation of domestic hot water

Decarbonization Potential of Energy Used in Detached Houses—Case Study

The main objectives of this study were the energy assessment of detached houses built in different periods in a central European city. A total of 236 detached houses built between 1930 and 2023 in Debrecen (Hungary) were analyzed from an energy perspective, and their CO2 emissions were measured. It was found that the net floor area of family houses built in recent years has increased but that the compactness of buildings has increased as well. The specific heat loss coefficient and the specific energy demand for heating in new buildings have decreased to 15.2% and 18.5%, respectively, over the last 90 years. Furthermore, around one third of the analyzed buildings built several decades ago must have already been renovated at least once for energy efficiency, as their heat demands are 27.6–41.4% lower than estimated. Energy consumption in six houses built in recent years was measured and studied. It was found that the occupants’ behavior may increase CO2 emissions from heating by 26%, while CO2 emissions from hot-water preparation may decrease by 38.2%. The potential of the locally available sources of renewable energy was calculated, and the costs of decarbonization packages for eight building groups were evaluated.

Multi-aspect analysis of measures to reduce the building's energy demand

Issues related to reducing the energy demand of residential buildings are still relevant in the light of the energy transition and ongoing climate change. A single-family house from the early 1990s with a heated area of approximately 177 m2, located in the north-eastern part of Poland, was used as a case study. The article contains the results of nine years of measurements that included energy consumption for heating and domestic hot water preparation. It was estimated that the annual final energy consumption was 128.2 kWh/m2 [462 MJ/m2]. After insulating the building envelope and replacing the windows with triple-glazed ones, this value reduced to 68.2 kWh/m2 [246 MJ/m2]. The measurement results were used to build a statistical model, based on multiple regression, to calculate the energy demand of a similar building to the one analysed in this article in any given location. The article presents the second, more advanced, energy simulations model made in the DesignBuilder environment, which enabled additional analysis of the operation of the heating and ventilation system. The scope of this study also included a multi-variant economic analysis of the building energy renovation, in which the Life Cycle Cost (LCC), Net Present Value (NPV) and Internal Rate of Return (IRR) values were determined.

Field Trial Evaluation of a Hybrid Heat Pump in an Existing Multi-Family House before and after Renovation

Renovation of existing buildings is fundamental to reduce greenhouse gas emissions of the building sector and to ensure the efficient operation of renewable heating systems. In multi-family houses, the suitability of heat pumps is limited by high required temperatures for the hot water preparation, which can be mitigated by hybrid heat pump systems. In this study, the energetic performance of a hybrid heat pump in a multi-family house, built in 1964, is investigated based on field data before and after a renovation. Multiple months are measured and mapped to a full year period. The combination of different renovation measures in the heating system and building envelope is rated w.r.t. their ecological and economical impact by taking into account the actual investment costs. The evaluation shows that the installation of a hybrid heat pump can achieve an accumulated greenhouse gas emissions reduction of 45%, which is similar to a building renovation to a new-build standard, which reduces the space heating demand by up to 62%. Nevertheless, only a combination of both measures can substantially reduce the emissions, which in this case are 81% lower compared to a gas boiler in 1990, which is still below the German climate target for 2040. Due to the low investment costs of a hybrid heat pump system, tenants are more likely to profit from a renting costs reduction, while a building renovation is especially economically beneficial at high energy prices. The results therefore emphasize that the insulation level should be selected carefully, as heat pumps already prepare space heating efficiently and that the heat pump must be able to support the hot water preparation to reach high emission reduction targets.

Numerical simulation and optimization of compact latent heat exchanger with micro-channel plate in shape-stabilized composite phase change material

To enable the rapid provision of domestic hot water, this study presents the independent design of a plate heat exchanger featuring a large heat exchange area and a compact structure. Utilizing a sandwich structure, the thermal storage unit integrates two shaped composite phase change materials (CPCM), namely sodium acetate trihydrate (SAT)/expanded graphite (EG) blocks of identical thickness — in conjunction with a micro-channel heat exchange plate. Multiple thermal storage units are stacked to create the heat exchanger. Experimental validation of the heat transfer performance of this thermal energy storage heat exchanger is conducted, accompanied by the establishment of a heat transfer model elucidating the interaction between the phase change material (PCM) and the fluid. The investigation explores the impact of varying thicknesses of shape-stabilized CPCM, thermo-physical parameters of CPCM, and flow rates on enhancing the thermal efficiency of the thermal energy storage heat exchanger. Results indicate a competitive relationship between the phase change enthalpy and thermal conductivity of CPCM under constant density. Increasing the mass fraction of SAT effectively boosts its heat storage density. However, this improvement is counterbalanced by a reduction in CPCM's thermal conductivity, resulting in diminished heat transfer rates within the material and reduced efficiency in transferring heat between the material and the fluid. Optimal performance is achieved when the mass fraction of sodium acetate trihydrate is 80 wt% and the CPCM thickness is 20 mm. At these parameters, the thermal energy storage heat exchanger exhibits the highest heat transfer efficiency. Furthermore, the heat exchanger is capable of producing 197.86 kg of hot water in 1365 s at an inlet flow rate of 300 L/h, achieving an impressive discharging efficiency of 82.73 %. These findings underscore the feasibility of employing the mini-channel plate thermal energy storage heat exchanger for the rapid preparation of domestic hot water.

Cross-sectional temperature field of a solar collector’s absorber in the case of annular pipe

In households, solar collectors are finding ever increasing use for hot water preparation. Although the design of solar collectors is relatively simple, the authors over many years concentrate attention on modelling the cross-sectional temperature field of the solar collector’s absorber [1-7]. In solving the cross-sectional temperature field for an annular pipe [1], the periodical cross-sectional domain is divided into three sub-domains where the first sub-domain is the plate between pipes, the second – a pipe’s wall, and the third – the liquid; as a result, the temperature field expressions have been obtained for all the three sub-domains. In works [2, 3] the temperature fields obtained were simplified. To define its variations in time, the temperature field was found by solving the Laplace equation under non-stationary time-dependent conditions [4]. The obtained results evidence that the non stationary conditions might not be taken into account in long- lasting sunny weather, while such non-stationarity changes considerably the temperature field when it is short-term (e.g. in cloudy weather). The temperature has also been found for a square-pipe absorber [7] as more technological in design. In the present work, the final temperature field model is proposed for the absorber of a round-pipe collector. As distinguished from [1], in this work the temperature of liquid is assumed to be constant over the entire pipe cross-section. Such an assumption significantly simplifies the calculation while not changing the physical essence.

Selection of a reliable energy source suppling domestic hot water (DHW) system in the kindergarten – a case study

Renewable energy sources (RESs) are used more and more frequently as energy sources for heating and domestic hot water (DHW). However, there are many factors influencing the energy efficiency, thus also ecological benefits. Before making a decision what kind of RES is useful and reliable, a comprehensive analysis should be conducted taking into account technical, financial and ecological factors. This paper discusses different variants of energy sources that could be applied in a kindergarten building for preparing hot water, in place of existing solution (district heating system, DHS). An air heat pump (AHP) with photovoltaic panels (PV) were considered the most reliable energy sources in the analysed building, in terms of economic and environmental considerations. The simple payback time (SPBT) for this investment was estimated as 14.55 years. This solution causes the lowest CO2 emissions. Another system with solar collectors supplying hot water preparation in the heat center can be also recommended. The simple payback time in this case was slightly higher (14.94 years) and what is more, a decrease in CO2 emissions was observed compared to the actual conditions.

Development and experimental investigation of full-scale phase change material thermal energy storage prototype for domestic hot water applications

The paper presents an experimental analysis of the full-scale phase change material (PCM) thermal energy storage (TES) prototype that is designed for use in domestic hot water preparation systems. The PCM-TES prototype is based on the external arrangement of organic PCM and a custom-made compact fin-and-tube type of heat exchanger. The prototype has been designed, constructed, and tested according to the Ecodesign Directive requirements for water heaters and hot water storage tanks. Several parameters have been analysed, including the heat transfer fluid (HTF) temperature and mass flow rate, heat transfer rate, accumulated and released energy, and quantity of domestic hot water (DHW) delivered. Additional attention was paid to the reliability and repeatability of the results. Experimental research allowed to determine the quantity and temperature of DHW at a constant mass flow rate of 0.1 kg/s. The prototype unit showed that it can deliver 99.1 l of hot water at 40 °C for up to 15 min and 39 s, while the use of lower temperature DHW (up to 37 °C) allows to increase the quantity of hot water delivered up to 156.2 l. Experimental results showed the viability of the prototype, areas for improvement and further possibilities for its optimization. Overall, the study results are relevant for developing latent heat thermal energy storage systems using fin-and-tube heat exchangers in tanks and the feasibility of integrating such systems into DHW systems.

Heat Recovery Variable Refrigerant Volume System Installation and Experiences from its Summer Operation

Variable refrigerant volume (VRV) systems operate on the principle of a cooling machine. They extract heat from one side and return it on the other, enabling them to function as a source of heat for both heating and hot water preparation, as well as a source of cold for cooling within a single building. These systems efficiently remove unnecessary excess heat outside and only introduce the required amount of heat from the outside into the building. With an appropriate configuration and setting, a VRV system can transfer heat in both directions within one system, eliminating the requirement for waste heat to be removed without use in the building. Instead, it is transferred to the desired locations where it is needed. This principle necessitates the adjustment of not only the refrigerant temperature but also its flow rate. Consequently, the VRV system can fulfil tasks that are otherwise handled by several individual systems in a building.A heat recovery VRV system was installed in a small retail store to extract waste heat generated by baking ovens during the baking process. This report provides a brief summary of electricity and energy consumption measurements taken during the summer period for cooling purposes. Sequential logic is observed and coherence is ensured, with active voice predominating for clearer and more direct communication. The parameters of interest include cooling setpoints, cooling outside of working hours, and capacity assessment.

Buildings made of massive timber logs with a derogation from the application of technical conditions for traditional masonry technologies

Timber log walls are a unique form of construction - erected from solid material, they are both a building structure and a thermal barrier. Walls of wooden logs have numerous advantages, especially favourable physical and mechanical properties, including insulation and thermal capacity. The technical conditions, which are the Polish interpretation of the Nearly Zero Energy Building (nZEB) building, which a newly designed building must meet, must meet 2 conditions simultaneously: the value of the EP index, which determines the annual calculated demand for non-renewable primary energy for heating, ventilation, cooling and preparation of domestic hot water for the entire building, and adequate thermal insulation of the external walls. Calculating the thermal insulation of a wood partition, relying only on thermal conductivity coefficients without considering other properties of the material, does not reflect the true insulating properties of wood. The lack of research on the insulating properties of wood, and thus the lack of national technical and installation requirements for log houses seriously limits the development of this type of construction and forces to undertake reliable research and development work in this area. The proposed introduction in this type of building of the methodology of the so-called Integrated Energy Design (IED) allows to carry out multi-criteria analysis of design variants at the stage of the virtual model in the BIM standard, in terms of the final energy efficiency of the building. Conducting various types of variant analyses and simulations allows to achieve optimal energy efficiency and to verify and correct the adopted solutions. The high energy efficiency of the building demonstrated by such simulations can be the basis for a deviation from the current regulations, which will allow the implementation of newly designed buildings in massive log technology, without the need for additional thermal insulation. As a result, it will be possible to preserve the unique architectural expression of buildings erected with this technology, with visible massive timber logs both on the exterior facades and on the interior.

A water heater that adjusts its temperature based on solar energy

The calculation method for a water distributor of a constant cross section uniformly perforated along the height for a multilayer stratified thermal storage tank used in solar heating and hot water supply systems was developed. The advantages of using self-regulating stratification water heat accumulators in solar heat supply systems are considered. It is shown that the use of stratification for short-term and long-term thermal accumulation leads to an increase in the use of solar heat by 15-20% compared to batteries with completely mixing water. In hot water systems, these batteries can provide earlier preparation of hot water with the required temperature for consumers. Based on the analysis of world experience in the development of water heat accumulator designs using temperature stratified water and the stability of the stratified flow of coolant in the accumulator volume, a new, self-regulating, water accumulator design has been proposed.

Management of domestic hot water system based on the example of a gas-fired instantaneous water heater

Purpose: The aim of the study was to present the actual efficiency of a gas-fired instantaneous water heater for the production of domestic hot water. Design/methodology/approach: The object of the study was a gas-fired instantaneous domestic hot water heater located in a flat in Krakow. Measurements with a suitable device were carried out for three basic variants of hot water preparation, defined as showering, washing hands and washing dishes. Findings: The tests and analyses carried out have revealed that the current efficiency (average efficiency based on 3 measurements – 77.61%) differs radically from that indicated on the water heater data sheet. Such a drop in efficiency may be caused e.g. by fouling of the surface of the heat exchanger on the flue gas side and scaling of the exchanger on the heated side. Research limitations/implications: The tests proved that the heater did not reach its nominal efficiency during the tests, but on the other hand, simulations of different intensities and durations of water consumption did not drastically change the efficiency of hot water preparation. Practical implications: Regular cleaning of the exchanger would probably reduce the difference between the efficiency declared by the manufacturer and the one achieved during the tests. A clean heater will also ensure a sufficiently low level of carbon monoxide in the flue gas, which has a real impact on the safety of the appliance. Social implications: The values obtained from the measurements for each operating mode can help occupants understand how to prepare water most efficiently with this particular appliance, which will have a real impact on the bills they pay. The user will be provided with information on what percentage of energy they use from the volume of gas burned. Originality/value: Gas-fired instantaneous water heaters provide an efficient and energy-saving solution, especially in situations where the demand for hot water is low, and immediate access to it is crucial. Keywords: hot water, hot water management, gas-fired instantaneous water heater. Category of the paper: Research paper.

Thermodynamic and exergoenvironmental assessment of an innovative geothermal energy assisted multigeneration plant combined with recompression supercritical CO2 Brayton cycle for the production of cleaner products

The current study examines a novel multigeneration plant that uses geothermal energy to produce clean and sustainable products. The process includes a re-compression supercritical Brayton power plant (re-sBC), an ejector cooling unit (EJCS), a humidifier dehumidifier desalination (HDH) unit, a Proton-Exchange Membrane (PEM) process, and a hot water preparation unit. The designed plant is able to generate hot water, cooling, power, hydrogen, and freshwater using the geothermal source. In the current paper, the thermodynamic analysis, CO2 emission assessment, and exergoenvironmental index evaluation are executed to determine the system efficiency and the association between the system and the environment. A thorough parametric examination is fulfilled, to show how the system indicators impact efficiency and generated products. According to analysis data, the proposed multigeneration plant is able to generate 1278 kW of heating load, 229.6 kW of refrigeration capacity, and 109.5 kW of net power rate. Additionally, the capacity of the freshwater is 0.1188 kgs−1 and the hydrogen is 0.001317 kgs−1. This developed system emits a total of 117 tonnes of CO2 emissions per year for these useful outputs. In conclusion, the energetic and exergetic performance of the suggested MG plant are 14.09 % and 23.35 %, respectively.

Increase of energy self-consumption in hybrid RES installations 1 with PV panels and air-source heat pumps

In recent years, European countries have experienced a noteworthy surge in the interest surrounding renewable energy sources, particularly the integration of photovoltaic (PV) panels with various types of heat pumps. This study aims to evaluate the energy performance of a grid19 connected hybrid installation, combining a PV array with an air-source heat pump (AHP), for domestic hot water preparation in a residential building located in Cracow, Poland. The primary focus of this evaluation is to assess the extent to which self-consumption (SC) of energy can be increased. The study utilizes Transient System Simulation Tool 18 software to construct and simulate various system models under different scenarios. These scenarios include building electricity consumption profiles, PV power systems, and the specified management of AHP. Analyses were conducted over a period of 1 year to assess the operational performance of the systems. In the considered installations, the differences in SC values between PV installation ranged from 9 to 25%. Notably, the highest SC values were observed during the winter months. AHP with operation control allows to obtain in some months of the year up to 35% higher value the SC parameter compared to systems without AHP. The highest annual 29 SC value recorded reached 83.9%. These findings highlight the crucial role of selecting an appropriate PV system size to maximize the SC parameter.

An innovative study on a geothermal based multigeneration plant with transcritical CO2 cycle: Thermodynamic evaluation and multi-objective optimization

Renewable energy-sourced integrated plants are among the novel and interesting technologies for production of the clean and effective beneficial multiple commodities. Also, these technologies are one of the main motivations for overcoming environmental troubles. The newly designed geothermal-based multigeneration plant deals with clean and sustainable power, hydrogen, cooling, and heating generation and then comprehensive thermodynamic and economic analyses. This system is driven by a geothermal source that includes a transcritical CO2 based Rankine cycle, an ejector cooling cycle, a hydrogen generation and compression unit, and a domestic hot water preparation unit. Here, a comprehensive mathematical modeling is discussed that are energy and exergy efficiency methods along with economic cost evaluation. Furthermore, to define the optimal working conditions, a multiobjective optimization method is fulfilled based on the relationship between the developed multigeneration system's cost and exergy efficiency. Additionally, a comprehensive parametric assessment is implemented to describe how effects of various indicators on the developed model’s efficiency. In outlook of these outcomes, it is crystal clear that the quantity of clean power and hydrogen generation rate is 1283 kW and 0.009374 kgs−1, respectively. The energy and exergy efficiencies of the developed MG cycle are figured as 27.60% and 22.56%, at 152 ℃ geothermal source temperature. the optimal plant total cost rate is computed as 80.48 $h−1. Finally, it is concluded that this proposed system's performance and cost rates are compatible and applicable.

Demand side flexibility for a Heat Booster Substation with ultra low temperature district heating

Ultra-low temperature district heating (ULTDH) systems improve the heat distribution efficiency by reducing thermal network losses and provide an easier access to Renewable Energy Sources (RES) by utilizing them as heat sources through Power-to-Heat solutions. In this study, Domestic Hot Water (DHW) preparation at a Heat Booster Substation (HBS) supplied with ULTDH is investigated for generating demand side flexibility. The main components of the HBS are a Hot Water Storage Tank (HWST) and an electric-driven heat pump. A genetic algorithm based control strategy is developed to optimally utilize the thermal energy storage capability of the HBS by intelligently scheduling the charging of the HWST based on time-varying electricity price signals. A multi-objective control strategy is designed to reduce energy costs while generating flexibility on the demand side. The components of the HBS are numerically modeled and experimentally validated to form a dynamic non-linear model of the HBS. Further, the data-driven seasonal ARIMA model, ARIMA(0,1,1)x(0,1,1)168, is developed to forecast the DHW consumption based on a statistical analysis of the historic consumption profiles over a two-year period. The forecast RMSE of the DHW prediction model is less than 0.03kgs−1 over the two-year test period, indicating that the prediction model is able to accurately forecast the DHW consumption of the residents. With the control-oriented HBS numerical model and the DHW prediction model serving as decision support tools, the developed control strategy is implemented at the HBS. Results show that in comparison to a rule-based control strategy, the developed control strategy demonstrates significant demand side flexibility while attaining energy cost savings of more than 91% and 84% at time resolutions of 15 and 30 min for the control interval respectively. With increased volatility in electrical prices due to higher penetration of RES, greater energy cost savings of more than 150% are observed.

Determination of the circulation flow rate in the calculation of internal hot water supply networks

Calculations of internal ring networks of buildings have always been, are and will be an urgent task. The schemes of internal hot water supply systems change, the number of floors of buildings changes, the location of heating points from which hot water begins its journey to the consumer changes, the definition of water consumption changes, and finally the calculations themselves change, but it is undoubtedly the calculation methods for centralised hot water supply systems that are a key part of creating reliable and economical hot water supply networks in buildings. Modern methods consist of three different calculations: the supply part of the system, the circulation part with the determination of pressure losses in the entire system, and the thermal disinfection mode of the system. The circulation flow rate methodology widely used in Ukraine is based on a 1985 regulatory document and borrowed part of the calculation from the 1992 Polish regulations. As a result, questions arise that are not answered in the DBN. In terms of determining the heat losses of individual sections of the hot water system, not everything is perfect either. A building is designed for a lifespan of 50 years or more with daily, round-the-clock use of heat energy, and the heat consumption for preparation and transportation of hot water is the largest in the building and requires a detailed calculation. The DBN "Internal Water Supply and Sewerage" provides two figures for all cases: 11 and 7 W/m (specific heat losses by pipes of the internal hot water supply network). The calculated surface temperature of the pipes in different parts of the network varies, the air temperature in the premises through which the pipes pass varies, and the diameters of the pipes in certain areas change, but the two figures remain unchanged. Moreover, it is recommended that, regardless of the insulation material used for pipe insulation, the insulation thickness should be at least 10 mm. This may have been appropriate in 1985, when there were no computer programs for calculations and fewer thermal insulation materials, but now it is 2023 and the situation is completely different. The article, after analysing the regulatory documents, proposes a methodology for determining the circulation flow rate and calculating the hot water supply network in the mode of passing this flow rate.

ASSESSING THE FINANCIAL BENEFITS OF USING A SHOWER DRAIN HEAT RECOVERY SYSTEM – A CASE STUDY

The production of hot water for bathing in Poland accounts for around 15% of the total energy consumption of a typical household. According to EU data, final energy consumption for lighting and appliances is similar to final energy consumption for hot water preparation. This makes it a significant contributor to housing and utility costs, exceeded only by heating expenses. Research has indicated that only about 10% of the water utilized during a shower is necessary for hygiene purposes. Consequently, around 90% of the hot water supplied to the shower is ultimately discharged into the sewage system. By harnessing the primary energy from wastewater, we can effectively conserve heat energy and reduce the overall expenditure associated with hot water. The objective of this article is to explore the utilization of heat recovery from domestic wastewater as a means to enhance the energy efficiency of residential buildings.