Search

Carbon dioxide (CO2) heat pumps are not causing ozone layer depletion, while offering high temperature water. Typical control (TC) can realize normal operation of water-to-water CO2 heat pump. However, the approach for developing model-based optimal control (MC) for further improving the performance of water-to-water CO2 heat pump is still lacking. Hence, this study presents MC approaches for water-to-water CO2 heat pump. The developed model-based controller was composed of the optimization strategy and system models. The objective of optimization strategy was to maximize system coefficient of performance by identifying optimal setpoint values of discharge pressure and outlet water temperature at the gas cooler side. System models of compressor power and heat rate at the gas cooler side were established using large amount of data from virtual CO2 heat pump. The reliability of virtual CO2 heat pump was validated by experimental data. Results of case studies demonstrated that the system coefficient of performance with MC could be improved by 7.8 %–14.6 % in comparison with that with TC. This study therefore gave a guideline for optimal control of water-to-water CO2 heat pump.

South Africa currently has the highest carbon emission intensity per kilowatt of electricity generation globally, and its government intends to reduce it. Some of the measures taken by the government include a reduction of emissions in the building sector using solar water heating (SWH) systems. However, there is currently no study in the country that comprehensively assesses the technical, economic, and environmental impact of SWH systems across the country. This study therefore used the System Advisor Model (SAM) to model two different technologies of SWH systems (i.e., flat plate (FPC) and evacuated tube (EPC) SWH) at five different locations (i.e., Pretoria, Upington, Kimberley, Durban, and Cape Town) strategically selected across the country. According to the study, the optimum azimuth for both the evacuated tube and flat plate SWH system in South Africa is 0°. Installing FPC and EPC at the different locations would yield payback periods of 3.2 to 4.4 years and 3.5 to 4.3 years, respectively. Comparably, levelized cost of energy for the FPC and EPC will range from 7.47 to 9.62 cents/kWh and 7.66 to 9.24 cents/kWh, respectively, based on where the SWH system is located. Depending on where the facility is located, the annual cost savings for the FPC system would be between $486 and $625, while the EPC system would save between $529 and $638. Using SWHs can reduce CO2 emissions by 75–77% for the evacuated tube system and 69–76% for the flat plate system annually, depending on the location.

A novel Integrated Collector Storage Solar Water Heater (ICSSWH) has been developed, in a variety of 3 sizes, and investigated at outdoor conditions, in the Greek climate. The devices consist of flat-plate collectors integrated with latent heat storage tanks at their back, filled with a Phase Change Material (PCM). A conventional paraffin wax PCM was applied, with a melting point of 53 °C. Two heat exchanger circuits, a closed loop collector-storage circuit for charging and an open loop (service water) for discharging process are immersed in the PCM tank. The heat charging and discharging processes of the storage tank are investigated, for various charging and discharging flow rates. The three device storage sizes have total volumes of 0.620, 0.808 and 1.148 m3, with corresponding solar collector areas of 1.51, 2.02 and 2.92 m2. During discharging, the stored heat in the PCM is delivered to tap water. A minimum acceptable water temperature of 38 °C is considered, according to international standards. Under normal insolation conditions, daily stored heat of about 4.2, 6.2 and 9 kWh were measured for the three mentioned sizes, respectively. Of this amount, about 3, 4.8 and 7 kWh were transferred to tap water, respectively. The heat transferred to the storage tank was above 41 % of the daily incident solar energy on the collector plane, for all studied cases. The daily hot water production was measured up to 88.4 ℓ, 137 ℓ and 190 ℓ for each mentioned size. Details of the Heat Transfer Fluid (HTF) and tap water temperature profiles are also extracted and presented.

This study presents the development and performance evaluation of an innovative thermal energy storage (TES) system utilizing a commercially available bioderived organic phase change material (PCM) for domestic hot water production. The primary objective of this research is to enhance the efficiency and effectiveness of thermal energy storage solutions by macro-encapsulating the PCM-expanded graphite (EG) compressed modules in a multi-pass tube arrangement. A comprehensive experimental setup was employed to investigate the thermal performance of the proposed TES unit, focusing on charging and discharging cycles. Key findings reveal that conduction is the dominant mode of heat transfer, with the system achieving a significant maximum average charging power of 1440 W and a discharging power of 1990 W. The thermal energy storage capacity reached an impressive 12.6 MJ, enabling the discharge of 90 % of stored energy within 90 min. Furthermore, the exergy analysis indicated high exergy efficiencies, with charging efficiencies reaching 98 % and overall exergy efficiency at 18 %.The implications of this research are significant, demonstrating the feasibility of using bioderived organic PCM for sustainable energy applications. It highlights the potential of the modular structure of the system to integrate with heat pump and solar energy systems, thereby enhancing efficiency and sustainability in domestic hot water applications. This work significantly contributes to the advancement of sustainable thermal energy storage technologies and establishes a solid foundation for future studies aimed at optimizing TES systems for domestic hot water production.

The present study aims to assess the technical feasibility of a novel Metal Hydride-based Solar Thermal Collector (MH-STC) system. This system includes a flat-plate solar collector integrated with a metal hydride bed, a hydrogen compressor, a photovoltaic panel, a hydrogen tank, and a water storage tank. This system combines the principles of a conventional solar thermal collector with the energy storage capabilities of metal hydrides, enabling the storage of surplus thermal energy. The LaNi5 alloy was used as heat storage material. A 3D-mathematical model is established and a numerical code written in Fortran-90 is developed to simulate the dynamic behavior of the proposed solar thermal collector. It was shown that the developed 3D-model produces computational results that are consistent with experimental data. In addition, to check the accuracy of the mathematical model and numerical approach, the relative errors in the mass balance and energy balance were calculated and values of 0.054 % and 0.18 %, respectively, were obtained.The simulation results offer insights into the system's technical feasibility, revealing an overall efficiency of 90.6 %, the water temperature at the collector outlet can reach 357.3 K, and a water volume heated of 230.4 L under the considered operating conditions. Moreover, the results indicate that the proposed MH-STC system exhibits significant potential as a viable alternative for thermal storage in domestic hot water systems.

This study analyzes research trends on photovoltaic–thermal (PVT) systems used in buildings. The applications of PVT systems are also classified into system design, material and fluid types, and operating methods. Although many studies proposed new types of PVT systems and applications to improve system performance, there are few reports on the design and operating methods for the system, which consists of the PVT system and hot water storage tank in real-scale buildings. Therefore, our research team conducted a quantitative evaluation of a small household with a PVT system and hot water storage tank installed, according to design factors. The results of the parametric study showed that electricity production was more sensitive to changes in system capacity than those in heat production. By contrast, the change in heat production was dominant in response to the change in the installation angle and load profile of domestic hot water. Through energy simulation, it was found that the PVT system could provide 95% of the annual thermal energy for the domestic hot water consumption of the target buildings.

The seamless coexistence of distributed photovoltaics (PV) with the utility network may be maintained by focusing on higher energy autonomy of micro-grids.This may be approached by seeking any opportunities to increase local energy consumption, especially when legal regulations are cutting-down the benefits for energy overproduction.The microgrids combining PV with electric storage and heat pumps largely fail at satisfying local energy needs autonomously in winter periods and create excessive load of power network. Although the all-electric microgrid is a long term target, any improvements to the existing residential infrastructure with mixed-supply sources are worth revisiting as low-cost steps towards more conscious energy utilization.The concept of using Domestic Hot Water (DHW) heating to dump locally the excess of PV-energy is affordable method to improve energy autonomy of residential microgrids with minimal interference to their existing configuration and the advantage of no additional energy demand.The use of PV for DHW heating was studied in various configurations, but in contrast to former works, this paper presents the parametric analysis focusing on increasing the local consumption (goal A), minimizing the interference with utility grid (goal B) and optimal adjustment to net-metering accounting rules (goal C).The case study is the residential-type micro-grid in climate of Central Europe. The study uses both simulated (energy consumption profiles) and recorded (solar irradiance) data. The simulation is based on microgrid proportions and provides the results beyond reach of single experiments and conclusions applicable to similar cases, regardless the absolute size.

Solar photovoltaic/thermal building integrated power/heat technology (BIPV/T) can provide both electricity and heat to buildings, which is important for improving the efficiency of solar energy utilization and reducing building energy consumption. In this paper, a BIPV/T system with water-cooled wall is proposed. The water-cooled wall heat exchanger is not in direct contact with the PV module, and uses air to dissipate heat from the PV module, which in turn heats domestic water, and can provide hot water or hot air to the building as needed. This paper experimentally investigates the system’s ability to produce domestic hot water and power generation performance, with an average power generation efficiency of 12.6% for the whole day and providing 35 °C domestic hot water for more than 7 h. The results of this paper have some guiding significance for the efficient operation of BIPV/T.

In this study, the sizing of primary side components for preparation of domestic hot water are analysed, both qualitatively and based on measurements of domestic hot water demand in one multi-family building with 268 apartments. The collected data spans a period of 18 days during the end of 2020 and is collected in 15-min, 1-min, and 6-s intervals. An overview of the historic development for the design of domestic hot water flow in Sweden is also presented. There is a long-standing argument in Sweden, that the current district heating guidelines result in an overdesign of the flow for domestic hot water. The consequence of this is oversizing heat exchangers and valves in the substations. This study assesses, qualitatively, the issues related to overdesigned primary side valves for preparation of domestic hot water. A revised design for domestic hot water flow for the Swedish context is also conceptualised. The study suggests that an improved design flow for domestic hot water in multi-family buildings can be derived based on empirical measurements. The 15-min intervals are observed to tone down information of peaks to a degree where it is unsuitable to use as basis for a new design flow. The 1-min data does appear to preserve information to a degree where it can be used to assess a design flow when related to data with a 6-s interval. The 6-s data is expected to constitute a resolution that may be less available, and in this study, it constitutes a representation of the real domestic hot water demands. However, to find a fitted curve to empirical data, for the design flow based on number of apartments per multi-family building, the population of datasets needs to be increased.

While domestic hot water (DHW) production represents approximately 17% of the energy consumption of the Canadian residential sector, studies aiming to reduce this proportion are scarce. Model predictive control (MPC) can improve the performance of DHW systems by relying on demand forecasting models to operate the system. However, DHW predictions are often inaccurate for single-family or small systems, which makes the viability of MPC uncertain in these cases. To validate the applicability of MPCs relying on machine-learning forecasting models, we assessed their performance through the simulation of 40 different DHW profiles, based on measurements in the same number of single-family residential units. For the sake of comparison, four controllers are developed and tested in this paper: a rule-based controller (current situation), an optimal controller, an MPC with perfect demand forecast, and an MPC with real forecast from machine-learning models (i.e., MPC with neural networks). Furthermore, we considered 100 combinations of relative importance allocated to energy saving versus to the respect of temperature constraints in the controllers. We found that MPCs with perfect and imperfect forecasts could reach a similar performance in most cases, validating the applicability of the machine-learning models. The MPC relying on a machine-learning forecasting model yielded energy savings from 4% to 8% when compared with traditional control. The ability of the MPC to keep the stored DHW temperature closer to its minimal temperature constraint reduced heat losses and caused these savings. In comparison with optimal control, the inability to act sufficiently in advance of imperfect-forecast MPCs was identified as the main impact of forecasting inaccuracies and resulted in more temperature constraint violations.

This paper studied the performance of a compressor-assisted thermochemical sorption energy storage (CATSES) system with a solar photovoltaic-thermal collector (PV/T) to support the domestic space and hot water heating. The heat from the PV/T drives endothermic desorption, whilst the electricity from the PV/T powers the compressor to assist the low-temperature desorption. The main aim of this study was to demonstrate that the integrated system can flexibly and maximally utilise solar energy, and store solar energy in a high energy–density system with minimum loss over long-term storage. The parametric investigation on the CATSES system using SrCl2/NH3 working pair was conducted for a case study in the city of Newcastle upon Tyne in the UK, which has long wintertime with high heating demand. Two different system operation modes (Case 1 and Case 2) with different strategies of solar energy usage (direct usage / storage) were studied. By using 30 m2 PV/T collector with the CATSES reactor that contains 22 m3 (450 kg/m3) composite adsorbent and a compressor with 11.5 compression ratio, the system that operated Case 1 could achieve 100% solar fraction of annual heating demand. The achieved material-based energy storage density was around 0.6 GJ/m3 and the storage efficiency was 0.88 with the net electricity consumption of 180 kWh (around 5% average consumption of an ordinary UK household). The system that operated Case 2 stored less heat than that of the Case 1 but was able to output more electricity.

The integration of hybrid renewable energy systems, such as hybrid solar-biomass, to address thermal energy needs in various applications is a promising, efficient and eco-friendly solution that can be successfully applied for traditional baths in Morocco. Indeed, these systems are not widespread in Morocco, since traditional firewood furnaces are still commonly used for heating purposes in many different applications. Accordingly, the main objective of the study is to introduce a new hybrid solar-biomass system designed for space heating and hot water supply in a public bathhouse in Marrakesh, Morocco. This hybrid system includes small-scale pellet boilers and small dimension parabolic trough solar collectors (PTC), which is the first small-size PTC facility proposed for low temperature applications (less than 100 °C) in Morocco. Moreover, the preliminary simulation results of this hybrid solar-biomass system for space heating – case study – using TRNSYS software are presented. The use of such a hybrid system as a new design to retrofit conventional firewood boilers that are widely used for space and water heating in public bathhouses will lead to significant savings in firewood, which will contribute in particular to reducing deforestation in Morocco and will also result in substantial reductions in CO2 emissions.

With limited roof space available, the selection of the most suitable solar energy systems to meet hot water and electricity demands for a household in a cost-effective way is always a challenge. This paper presents a comparative study of solar energy collection methods to meet Australian household's energy demands using both solar thermal collectors and photovoltaic (PV) systems. Numerical models are developed for various system configurations and two different scenarios are analysed in this research. In the first scenario, only PV system is used to meet both hot water and electricity demands. In the second, both the solar thermal collectors and solar PV system are used in combination for the same roof area. Based on the modelling results, economic benefits are determined and compared. The combined system was found to be a better option, providing greater benefits in cost and emission reduction.

The data article describes detailed measurements of domestic hot water (DHW) consumption in 12 Norwegian buildings. Included in this study are 4 hotels, 4 nursing homes, and 4 apartment buildings in the greater Oslo region. Flow and temperature measurements were performed on the DHW production system in each building's heating plant, for a duration of at least 6 weeks. The measurements were conducted with an interval of 1 s, and then averaged for 2 s before analysis in order to reduce data noise and measurement uncertainties. The data set includes flow rates, temperatures, energy for consumed hot water (CHW) and distribution losses in the hot water circulation (HWC). Reuse potentials consist of analyses related to flow rates, energy use and distribution losses, e.g. in peak flow rates analysis or DHW energy flexibility simulations. The measurements were performed within the research project “Energy for domestic hot water in the Norwegian low emission Society” (VarmtVann2030).

Thermal energy storage (TES) is extensively applied in production and daily life. As a basic work, we designed a single tank phase change TES domestic hot water system using night valley power. Based on the single tank TES experimental platform, a 2D numerical model of the system was established. To optimize the performance of a single tank TES system, the effect of heat transfer fluid (HTF) inlet velocity and temperature on the heat release of the storage tank was analyzed based on simulation. The enhanced heat transfer effect of the radial cascade system and axial cascade system was also compared. The results showed that 0.8 m/s is the optimal HTF inlet velocity in the calculation range. The rule of the heat release about the HTF inlet temperature just meets the different hot water requirements in winter and summer. The heat release capacity of the radial cascade system and axial cascade system is higher than that of the conventional single-stage system, and the proportion of latent heat release in radial cascade is larger. This study will provide theoretical guidance for the optimal design of the single tank TES domestic hot water system in the future.

In this work, technologies related to the storage of solar energy, utilizing the latent heat content of phase change materials for the production of domestic hot water are reviewed. Many researchers have been involved in this field in order to accomplish the targets of environmentally friendly solutions and higher efficiency. For domestic use, materials with melting temperature between 40 and 80 °C are commonly studied, with paraffins, fatty acids, salt hydrates and alcohols being the most popular. For harvesting the solar radiation, usually flat plate or evacuated tubes solar collectors are used, either commercial ones or modified. The storage unit may include only phase change material or it can have a hybrid form combined with water. The outcome of the most studies, is that the addition of phase change materials in comparison to systems without latent storage, increases the duration of heat release towards the domestic water at the end of the day and also increases the solar collector's efficiency because it does not experience large temperature fluctuations. However, difficulties emerge during the selection of the appropriate storage material as this must have a high melting temperature in order to provide hot enough domestic water, but not higher than the temperature that the solar collector can produce, in order to fully melt the material. Moreover, investigation is necessary for the selection of the optimum water flow rate that minimizes the charging duration and maximizes the system efficiency and hot water amount. Another challenge that researchers face is the low thermal conductivity of many phase change materials. For this purpose, methods for improving the performance of the systems have been examined and they are also reported.

COVID-19 made considerable changes in the lifestyle of people, which have led to a rise in energy use in homes. So, this study investigated the relationship between COVID-19 and domestic hot water demands. For this purpose, a nondimensional and principal component analysis were conducted to find out the influencing factors using demand data before and after COVID-19 from our study site. Analysis showed that the COVID-19 outbreak affected the daily peak time and the amount of domestic hot water usage, the active case number of COVID-19 was a good indicator for correlating the changes in hot water demand and patterns. Based on this, a machine learning model with an artificial neural network was developed to predict hot water demand depending on the severity of COVID-19 and the relevant correlation was also derived. The model analysis showed that the increase in the number of active cases in the region affected the hot water demand increased at a certain rate and the maximum demand peak in morning during weekdays and weekends decreased. Furthermore, if the number of active cases reached more than 4000, the peak in morning moved to afternoon so that the energy use patterns of weekdays and weekends are assimilated., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2021 The Author(s).)