Domestic Water Heating Research Articles

Exploring the theoretical minimum energy use in the U.S. office building sector

This paper explores the minimum energy that would be required to provide all energy-using services currently used the U.S. office building sector if they were all provided by thermodynamically ideal devices that do not violate physical law. Data from the U.S. DOE and the General Services Administration were used to identify office building workspace services, equipment, occupancy and schedules. Internal building loads were estimated after identifying or conservatively approximating the thermodynamically ideal power required for each service. The average thermodynamically ideal office building in the U.S. would use 2.2 kWh/m2-yr or less, which represents 1.0% of the current average end-use consumption in the sector. Such a large gap between the thermodynamic limit and today’s practice is related to the fact that the energy performance of all the internal gain producing equipment in buildings is far from ideal along with the HVAC equipment and envelope components. Ideal office buildings would use 69.7% of their energy for cooking and related domestic water heating and cooling; illumination accounts for a smaller portion, consuming 17.5%, while office equipment and cooling would use 6.9% and 5.9% of the total consumption, respectively. Dramatic reductions come from the elimination of heating energy and air distribution energy brought about by ideal adiabatic building envelopes and frictionless air distribution. Cooling energy is virtually eliminated by the combination of greatly reduced cooling loads and ideal Carnot equipment. But the thermodynamically ideal office equipment would use two orders of magnitude less energy than today as well.

Design of a positive energy district: A Nigerian case study

The potential of possible solutions for the design and implementation of a 50-house Positive Energy District (PED) in Southern Nigeria is presented in this study. Using a transient energy systems simulation software (TRNSYS 18), the energy demand of a typical building is estimated. Different passive design options considering building materials, window sizes, building orientation, roof ventilation are studied to determine the most energy efficient building envelope. TRNSYS and PVSyst (another energy simulation software), are also used to simulate the dynamics of a positive energy building and operation of solar energy systems used for electricity generation, domestic hot water heating and space cooling for Lagos weather conditions. In terms of electric energy balance, the total annual electricity demand is 598,000 kWh while the electricity generated by PV is 728,600 kWh in a year. As regards to site energy balance, annually, the district exports more electricity than it imports. The difference is 110,200 kWh. These results show that in such climatic conditions, despite economic poverty, it is possible to implement the modern PED principles. However, first, it is necessary to reduce energy consumption as much as possible, including solar passive solutions to the building architecture.

The Joint Use of a Phase Heat Accumulator and a Compressor Heat Pump

This article presents an example of the joint use of a compressor heat pump that uses propane as a natural, ecological thermodynamic medium and a phase heat accumulator that uses paraffin as a medium. Special attention has been paid to the solidification process of the phase change material, and a simple theoretical model of the solidification of this material has been proposed. Thermodynamic balance calculations were carried out for the compressor heat pump and the phase heat accumulator. This paper presents a theoretical analysis of two examples of heating using a compressor heat pump, implementing the Linde cycle for the refrigerant R290 (propane): high-temperature heating at a temperature of 80 °C and low-temperature (surface) heating at a temperature of 60 °C, with the same unit heat output of 0.376 kW taken from the lower-temperature heat source of each evaporator. This heat is generated by the solidification of the PCM. The compressor power is 77 W in the first case and 40 W in the second. The energy efficiency coefficients of the compressor heat pump for the proposed combination of a phase heat accumulator and compressor heat pump are 5.98 and 10.40. The joint use of a heat accumulator and a heat pump presented in this paper can be used in applications for the heating of domestic water or water for space heating.

A meta-analysis of the schematic design process of deep retrofit projects

Deep retrofits of the existing building stock will be necessary to meet global emissions reductions targets. One building archetype, low-rise MURBs have been neglected in terms of research and funding for deep retrofits. A meta-analysis was conducted that compares and contrasts the schematic design approach taken for six such buildings in British Columbia which are scheduled to undergo deep retrofits with the goal of reducing GHG emissions by 80%. The analysis showed that design teams had converged toward common solutions for each building while achieving the GHG reduction target. The recommended measures include electrification of space and domestic hot water heating, adding insulation through overcladding, air sealing, ventilators for each unit, and double pane windows. A life cycle cost analysis showed that the economic viability of deep retrofits were dependent on energy price forecasts, capital cost reductions through market forces and transformation, or incentives cover the non-monetizable co-benefits of deep retrofits such as improved resiliency to climate-change or reducing overheating and air quality risks. The meta-analysis can help to streamline the early-stage and schematic design process for such buildings, which is critical to increasing the retrofit rate. This process could be replicated for other building types and construction archetypes.

Transparent insulation integration in solar thermal collector: Advancing performance for domestic water heating in oceanic climates of Western Europe

Decarbonizing residential hot water demand requires efficient solar collectors. Conventional flat plate collectors are widely available but experience high heat losses in colder climates. This work investigates the performance of a modified flat plate collector that incorporates honeycomb transparent insulation to reduce convective heat loss, offering a cost-effective alternative to energy-efficient evacuated tube collectors. The thermal performance of the modified flat plate collector was obtained using the finite element method in COMSOL Multiphysics, revealing a significant reduction in convective heat loss. The obtained thermal performance was then incorporated into a transient simulation to assess the year-around performance of the modified flat plate collector in a solar domestic hot water system for Irish climate. The transient simulation accounted for variations in the transparent insulation’s transmission with incidence angle which was overlooked in previous studies. Over a year-long simulation, the modified flat plate collector generated a total useful energy of 532.45 kWh m−2, while requiring 1640 kWh of auxiliary energy to meet the hot water demand of a single-family house in Ireland. An economic analysis of solar domestic hot water system revealed that the modified flat plate collector could yield savings of 443.44 € more than the standard flat plate collector and 3281.03 € more than the evacuated tube collector for homeowners. Consequently, the simple payback period for the solar domestic hot water system decreased from 3.72 years with a standard flat plate collector and 8.37 years with an evacuated tube collectors to 3.56 years with a modified flat plate collector. In addition to its economic viability, the modified flat plate collector could result in a CO2 savings of 901.85 kgCO2 per year. The study concludes with a sensitivity analysis to identify the market conditions that maximize the profitability of the modified flat plate collector based solar domestic hot water system.

Potential for Electrical Energy Savings in AC Systems by Utilizing Exhaust Heat from Outdoor Unit

This study explores the potential of utilizing waste heat from air conditioning systems, one of the largest consumers of electrical energy. Currently, most of the waste heat generated by outdoor units is typically released into the environment without being utilized, leading to missed energy-saving opportunities. This study analyzes the potential for improving electrical energy efficiency in air conditioning (AC) systems by harnessing this waste heat. Two primary approaches are evaluated: the first is the use of waste heat for domestic water heating, and the second is the conversion of heat into electrical energy using thermoelectric generators (TEG). The results of this research indicate that both methods have the potential to improve overall energy efficiency significantly. However, challenges related to conversion efficiency and integration of these technologies with AC systems require further, more specific studies. These findings are expected to contribute to more efficient and environmentally friendly cooling systems by optimizing technology and overcoming barriers to wider implementation.

Research on the thermal balance of a novel independent heat extraction-release double helix energy pile in long-term operation

Ground source heat pump buried pipes offer potential in civil defense due to their concealment and reliability, yet face thermal imbalance issues. This paper adopts a double helix energy pile with independent heat extraction-release capacity and develops an experiment to compare the performance of independent operation versus joint operation. Furthermore, a verified 2D model facilitated extensive simulations for long-term thermal balance, integrating refrigeration load, principles of water use in an underground engineering, and heat pump model. Comparative experiments show that joint operation can effectively alleviate soil heat accumulation by more than 20%. Extended simulations reveal that as accumulated heat is removed, the unit's performance is improved, and soil heat accumulation can boost heat extraction. However, under domestic water heat extraction, the heat-extracting pipe shows low flow rates, leading to thermal accumulation, necessitating the use of water for equipment or additional feed water to improve heat extraction. Compared to uniform enhancement heat extraction, centralized enhanced heat extraction boosts overall heat extraction capacity by 35.1%, which can more effectively mitigate thermal imbalances and improve unit performance, and achieving thermal balance in the geotechnical area requires an enhanced extraction flow rate above 10.5 L/h, with over 56 m3 of water passing through the heat-extracting pipe.

Performance assessment of residential heat pumps operating in extreme cold climates using zeotropic mixtures

Extremely cold climates subject residential heat pumps to significant temperature differences between the heat source and the ambient being heated, which may lead to system failure and reduced compressor performance. The present study considers the possibility of improving the performance of heat pump systems that are simultaneously used for residential space and domestic water heating while subjected to climates varying from -25 °C to 5 °C by exploring the use of zeotropic mixtures. CO2-based binary mixtures composed of low-GWP (global warming potential) refrigerants are considered – R32, R1234yf, and R290 – aiming to benefit from environmentally friendly and flame suppressants characteristics of CO2, as well as the improved thermal efficiency granted by the addition of a secondary refrigerant. A thermodynamic model was developed for a standard vapor-compression heat pump cycle and used to maximize the coefficient of performance limited by the minimum pinch point in the heat exchangers. Our analysis explores the effect of several parameters, such as, the mixture components, mass fractions, space and water heating demands, and heat source temperature on the heat pump's performance. For cold climates, the mixture of R32 (90%)/CO2 (10%) yields the highest COP and CO2-rich mixtures exhibit the lowest. However, by increasing the mass fraction of CO2 within the zeotropic mixture, the pressure ratio of the heat pump was improved. When considering combined performance criteria, such as the volumetric heating effect and the total heat delivered related to heat pump size, CO2-rich mixtures tend to allow more compact systems, especially in colder climates.

Thermal Energy Storage With Horizontal Thermocline Tank for Sludge Drying Application

Solar thermal technologies can be an attractive option for the decarbonization of process heat applications; in particular, thermal energy storage is a high value proposition for continuous processes that may operate at a range of thermal conditions. A thermal-fluid model is developed to investigate the buoyant flow in a horizontal thermocline storage tank for a sludge-drying application. The process conditions and tank geometry considered are unique compared to the domestic water heating application typically considered in the literature and well suited for parabolic trough collectors. It was found that good separation of hot and cold fluids can be achieved with the long horizontal tank geometry depending on the inlet configuration. Uniform flow distribution along the length of the tank is required to establish a thermocline. Assuming desirable inlet conditions can be achieved, the tank outlet temperature predicted by the thermal-fluid model shows good agreement with the system-level performance model.

ინოვაციური კონსტრუქციის სამკონტურიანი გათბობა-გაგრილების ქვაბი

Boiler that is used only for heating the heat carrier is called single-circuit boiler. The heat carrier, which is heated in the boiler furnace with fuel combustion products, circulates in the heat exchangers of the heating system (radiators, heaters, etc.). Boiler, which, in addition to the heating function, has the function of heating domestic water, is called double-circuit boiler (according to the presence of heating and hot water supply schemes). In such devices, part of the heat carrier circulates in the heating system, and part heats the internal water to the required temperature. The article presents a three-circuit heating-cooling boiler of an innovative design, which can be used both for heating the building and for obtaining sanitary hot water, as well as for cooling the building in the air-conditioning mode during the warm period of the year, for which the device is equipped with a refrigeration unit. The construction is universal, compact and in one small volume there is a device that can perform all the above functions.

Impact of myristic acid on novel designed manifold assimilated with domestic solar water heater: An experimental approach

In this study, myristic acid as heat storage material was assimilated with a novel designed manifold of evacuated tube solar collector (EVTSC). The foremost advantage of this design is that the integration of myristic acid in the manifold does not affect the vacuum tube’s strength and does not occupy extra roof space. Also, the advantageous thermal and chemical properties of myristic acid improve the performance of the system. The proposed system was tested for two different charging operations of storage material i.e. half-day charging operation and whole-day charging operation for water flow of 0.066, 0.133, and 0.20 LPM (liter per minute). These charging operations were selected to optimize the performance of the proposed system for daytime and nighttime hot water demand. The heat transfer analysis of the proposed system during both charging operations is also determined and compared. In addition to this, the machine learning approach is used to predict the hot water temperature. Concerning experimental results, the daily efficiency during HD-CHO is almost 62.42 %, 69.17 %, and 79.12 % respectively for the water flow of 0.066, 0.133, and 0.20 LPM. Whereas, during WD-CHO, daily efficiency was approximately 71.49 %, 80.42 %, and 85.21 % respectively for corresponding flow rates. Furthermore, the daily efficiency of the proposed system during HD-CHO and WD-CHO is found to be 42.85 % and 61 % higher than conventional systems. The higher and lower flow rates during both charging operations are recommended in terms of high daily efficiency and high hot water temperature respectively. Hence, the proposed system is designed to improve thermal performance, fulfill the hot water demand during late evening time, and decrease the overheating concern as compared to the conventional one.

The Storage Process of Electric Energy Produced from Renewable Sources from Hydrogen to Domestic Hot Water Heating

The expansion of renewable electricity storage technologies, including green hydrogen storage, is spurred by the need to address the high costs associated with hydrogen storage and the imperative to increase storage capacity. The initial section of the paper examines the intricacies of storing electricity generated from renewable sources, particularly during peak periods, through green hydrogen. Two primary challenges arise: firstly, the complexity inherent in the storage technology and its adaptation for electricity reproduction; and secondly, the cost implications throughout the technological chain, resulting in a significant increase in the price of the reproduced energy. Electric energy storage emerges as a pivotal solution to accommodate the growing proportion of renewable energy within contemporary energy systems, which were previously characterized by high stability. During the transition to renewable-based energy systems, optimizing energy storage technology to manage power fluctuations is crucial, considering both initial capital investment and ongoing operational expenses. The economic analysis primarily focuses on scenarios where electricity generated from renewable sources is integrated into existing power grids. The subsequent part of this paper explores the possibility of localizing excess electricity storage within a specific system, illustrated by domestic hot water.

Influence of usage and model inaccuracies on the performance of smart hot water heaters: lessons learned from a demand response field test

Domestic hot water heaters are considered to be easily integrated as flexible loads for demand response. While literature grows on reproducible simulation and lab tests, real-world implementation in field tests considering state estimation and demand prediction-based model predictive control approaches is rare. This work reports the findings of a field test with 16 autonomous smart domestic hot water heaters. The heaters were equipped with a retrofittable sensor/actuator setup and a real-time price-driven model predictive control unit, which covers state estimation, demand prediction, and optimization of switching times. With the introduction of generic performance indicators (specific costs and thermal efficiency), the results achieved in the field are compared by simulations to standard control modes (instantaneous heating, hysteresis, night-only switching). To evaluate how model predictive control performance depends on the user demand prediction and state estimation accuracy, simulations assuming perfect predictions and state estimations are conducted based on the data measured in the field. Results prove the feasible benefit of RTP-based model predictive control in the field compared to a hysteresis-based standard control regarding cost reduction and efficiency increase but show a strong dependency on the degree of utilization.

Experimental and numerical investigation of a solar thermocline system for domestic water heating applications

Experimental and numerical investigation of a solar thermocline system for domestic water heating applications

Effect of Full Implementation of Domestic Solar Water Heaters on the Electricity Peak Load in Libya

Electricity plays an important role in the contemporary life, and it has become indispensable nowadays. Reducing the peak electricity load and increasing the load factor have been considered as one of the main tasks that have to be accomplished by both electricity generation-side and demand-side managements.The residential sector of Libya consumes over 31% of the total sold electricity, and 29.8% of that is delivered to the electric water heating load. This is an inefficient way of electricity utilization. Usually, the electricity supplier in Libya used to increase the local generation capacity or import electricity from neighboring countries. Both solutions did not resolve the problem. This work attempts to investigate the effect of replacing electric water heaters in the residential sector of Libya by solar water heaters on reducing the electricity peak load and increasing the load factor. The results show that on average 3% of the peak load demand can be saved. This is equivalent to 149.5 MW of reduced power. The study also revealed that the annual amount of energy saved is up to 2.55TWh, and the load factor is improved by 2% (i.e. from 65% to 67%). This saved energy is equivalent to a power plant with a nominal capacity of 448 MW considering a load factor of 0.65.

Evaluation of thermal comfort during showering with system-related temperature fluctuations

With the decarbonization of heating sector with temperature-sensitive heat pump, comfort must be considered. For users, comfort is a decisive criterion that can hinder the decision for a supply system using renewable energies. Instantaneous domestic hot water heater pose a particular challenge here, as load changes can easily lead to temperature fluctuations. Therefore, the following study aims to determine the perception of temperature fluctuations during showering and to classify them for an assessment procedure. For this purpose, we carried out tests with 120 persons in a controlled environment. The test subjects showered at their desired temperature and gave direct feedback on imposed temperature fluctuations. Positive and negative changes with different rates of change were examined. We reported clear individual differences in the set desired temperatures as well as in the noticed and tolerated temperature fluctuations. The average desired temperature was 38.5 °C, with a quite large variation of ±5.5 K. While 2 % of the test subjects already noticed fluctuations <0.5 K and found them uncomfortable, for others (≈40 %) deviations of 4 K were still comfortable. Therefore, the resulting evaluation was based on a proportion of dissatisfied subjects according to the procedure described by the ISO 7730. This study represents a first step towards the introduction of standardized assessment of instantaneous water heaters and the comfort criteria would allow a comparison of control quality for different applications.

Evaluation of the Suitability of Using Artificial Neural Networks in Assessing the Effectiveness of Greywater Heat Exchangers

The use of greywater heat exchangers (GHEs) is an effective way to reduce energy consumption for heating domestic water. However, the available characteristics of this type of device are often insufficient and consider only a few selected parameters of water and greywater, which results in the need to look for tools enabling the determination of the effectiveness of GHEs in various operating conditions with incomplete input data. The aim of this paper was to determine the usefulness of artificial neural networks (ANNs). For this purpose, comprehensive experimental tests were carried out on the effectiveness of the horizontal heat exchanger, taking into account a wide range of water and greywater flow rates and temperatures of these media, as well as the linear bottom slope of the unit, which allowed for the creation of a database of 32,175 results. Then, the feasibility of implementing the full research plan was assessed using ANNs. The analysis showed that the impact of the media temperatures on the heat exchanger effectiveness values obtained using ANNs is limited, which makes it possible to significantly reduce the number of necessary experiments. Adopting only three temperature values of at least one medium allowed the generation of ANN models with coefficient values R2 = 0.748–0.999 and RMSE = 0.077–1.872. In the case of the tested GHE, the slope and the flow rate of the mixed water are of key importance. However, even in the case of parameters of significant importance, it is possible to reduce the research plan without compromising the final results. Assuming five different values for each of the four input parameters (a total of 625 combinations) made it possible to generate an ANN model (R2 = 0.993 and RMSE = 0.311) with high generalization ability on the full research plan covering 32,175 cases. Therefore, the conducted analysis confirmed the usefulness of ANNs in assessing the effectiveness of GHEs in various operating conditions. The approach described in this paper is important for both environmental and economic reasons, as it allows for reducing the consumption of water and energy, which are necessary to carry out such scientific research.

Maximizing solar energy utilization and controlling electrical consumption in domestic water heaters by integrating with aluminum reflector

Environmental concerns and rising energy costs require innovative solutions for household water heating. This work presents an innovative strategy for an evacuated tube-based electric/solar water heater (ESWH) incorporated with an aluminum reflector, which significantly reduces dependence on grid electricity. The whole area of the solar collector is about 2 m2, 100 L of storage each, and the rated power of the electric heater is 3 kW. Compared to existing ESWHs, the developed system achieves a significant 15.4% enhancement in annual energy consumption over hybrid models and a staggering 46.2% reduction compared to conventional electric heaters. The key improvement lies in the integration of an aluminum reflector beneath the evacuated tubes. This simple yet effective modification demonstrably enhances solar energy capture, leading to substantial grid energy savings. Our research thoroughly evaluated the system in Iraq-Baghdad's climatic conditions. Three identical water heating systems operated simultaneously under varying configurations: solar-only, electric-only, ESWH without reflector, and ESWH with reflector. A full test process relating four operation scenarios across a full day provided thorough insights into the energy consumption profile and the system's performance. The findings show the significant potential of the proposed water heater design to contribute to environmental sustainability, energy security, and reduced house energy bills. This study covers the way for wider adoption of eco-friendly and cost-effective solar water heating solutions.

A state-space heating and cooling model for thermal stratification in horizontally configured electric hot water storage cylinders

Domestic water heating significantly impacts global energy demand, with electric hot water storage devices notably contributing to grid load. Efficient management of these widespread devices is contingent on a deep understanding of the internal thermal stratification occurring naturally during heating and cooling phases. While extensive research exists for vertical water heaters, horizontal heaters, more challenging to model, have been comparatively overlooked in scholarly research. This paper introduces a state-space model developed using measured datasets, aimed at accurately describing temperature distributions inside a horizontally-configured electric hot water storage cylinder during heating, thermal diffusion, and cooling stages. The model’s effectiveness was validated through various evaluation metrics and separate, unseen evaluation datasets. It achieved an accuracy of 90.6% for the dimensionless stratification factor, 94.6% for the dimensionless exergy number, and reported Root Mean Square Errors (RMSEs) of 150kJ for energy, 11kJ for exergy, and 0.8°C for temperature.

Field test and geologic-thermal-economic analysis of medium-depth borehole heat exchanger

Medium-depth ground source heat pumps with borehole heat exchanger are pivotal in energy-saving and decarbonization. The heat transfer performance of borehole is important for design, with stratum thermal conductivity significantly influencing heat exchange, and economic considerations also act as a vital constraint on technological advancements. However, there is limited on-site testing in severe cold regions, especially in the Songliao Basin. Stratum thermal conductivity usually relies on empirical values, and its diverse impacts are not integrated into borehole heat transfer model. Additionally, previous studies optimized borehole design only from the thermal perspective. To address these gaps, we conducted an inaugural test in Shenyang. Geologic-based heat transfer model explores the geological influences, heat transfer performance, and heat extraction cost. At a depth of 2500 m, the formation's thermal conductivity is 2.45 W⋅m⁻1 K⁻1. The heat extraction, at a flow rate of 26.0 m3 h−1, achieves 352.3 kW with an inlet-outlet temperature differential of 11.7 °C. The effects of saturation, porosity, and lithology on stratum thermal conductivity were also explored. Furthermore, considering heat extraction cost, optimal depths for borehole utilizing PE-RT II and stainless-steel vacuum pipe are 2790 m (10.3 CNY·W−1) and 3140 m (10.2 CNY·W−1), with critical depths, 2910 m, governing the selection of economic inner pipe. This research establishes medium-depth ground source heat pumps as the preferred choice for low-carbon transformation in building heating, domestic hot water, and industrial and agricultural heat applications.