Heat Pump Hot Water Research Articles

Experimental characterization and numerical modelling of an air to water CO2 heat pump under different outdoor and operating conditions for domestic hot water generation

This work assesses the influence of different outdoor and operating conditions on the performance of an air-to-water CO2 heat pump for domestic hot water (DHW) generation. For that purpose, an air to water CO2 heat pump with internal heat exchanger (IHX) was developed and characterized in an experimental campaign under well-controlled lab conditions. The system includes an in-house control system implemented to operate at the optimal gas cooler pressure. The heat pump and the control system are tested for DHW generation. The heat pump uses a finned-tubes heat exchanger as evaporator and plate heat exchangers as gas cooler and internal heat exchanger. The numerical model used a 1dimensional, cell by cell discretization method to characterize the performance of both, the gas-cooler and the evaporator under a wide range of values of their key variables. The IHX is also modelled with the same strategy. ARHI equations are considered to describe the compressor performance. The experimental data obtained are used to validate a numerical model of the heat pump under different air velocities, air temperatures and air humidity at the evaporator. The effectiveness of the control system proposed is experimentally confirmed during the experimental tests. Results show a good agreement between the numerical simulations and the experimental data what permits to validate the model and to assess the performance of the transcritical heat pump under different climate conditions.

Optimal control of a heat pump-based energy system for space heating and hot water provision in buildings: Results from a field test

This paper presents and discusses results from a field test that uses model predictive control (MPC) to optimise the operation of a multi-source heat pump-based energy system for space heating and hot water provision in a building. The objective of the optimisation is to minimise the electricity consumption of the heat pump using flexibility from the heat sources, the space heating and domestic hot water tanks while ensuring end-user comfort and system constraints. The field test was performed in the context of the RES4BUILD project. The integrated system consists of photovoltaic-thermal (PVT) collectors, an advanced vapour-compression multi-source heat pump and water buffer tanks for space heating and hot water needs. The heat pump utilises a solar buffer, a borehole thermal energy storage (BTES) and air as heat sources. The solar buffer is supplied with heat from the PVT collectors. The optimal operation of the system is obtained by monitoring and controlling the interactions between the different system components using MPC, taking into account the weather and heat load forecasts. Results have shown that MPC has a potential and added value for the optimal operation of multi-source heat pumps in real-life while considering system constraints and user behaviour to ensure thermal comfort. However, significant effort and expert knowledge are needed to develop the sufficiently accurate system models required by this control approach. The outcomes and conclusions of this work are therefore a basis for further development of such control approaches to improve their replicability and feasibility on a large scale, considering the diverse nature of energy system components in buildings.

Energy, exergy, and exergoeconomic analyses of an air source transcritical CO2 heat pump for simultaneous domestic hot water and space heating

The transcritical CO2 heat pump system experiences significant throttling losses in space heating operation due to the higher temperature of returning water. This paper presents an energy, exergy, and exergoeconomic analyses of an air source transcritical CO2 heat pump integrating a tri-partite gas cooler, which is an effective method to match CO2 temperature glide with water for simultaneous domestic hot water and space heating (DHW+SH) production. A pinch point-based numerical model is developed to find the optimal discharge pressure. This validated model is then utilized to investigate the impacts of water inlet temperature and ambient temperature. Two configurations are examined: one with only DHW supply and the other with DHW+SH supply. The results indicate that combining SH with DHW enhances COP by 7.5% with a 7.9% reduction in discharge pressure at 10 °C ambient temperature and 10 °C water inlet temperature. At a water temperature of 10 °C, exergy efficiency improves by 4%. The compressor accounts for 54%–60% of the total exergy loss. The DHW+SH system exhibits an average exergy destruction reduction of 7.6%. With each 5 °C rise in ambient temperature, the DHW+SH system’s total exergy destruction cost rate decreases on average 7.7% compared to the DHW system. Furthermore, the combined exergy destruction cost rate of GC2 and GC3 in DHW+SH is significantly lower (by 48.2%) than only GC3 in DHW. The exergoeconomic factors of the compressor, gas cooler 2, and gas cooler 3 emphasize the need to decrease their costs to enhance cost-effectiveness.

Preparation and thermal property enhancement of sodium acetate trihydrate-lithium chloride-potassium chloride expanded graphite composite phase change materials

Phase change materials (PCMs) present promising solutions for enhancing energy efficiency in solar heat pump systems. In our study, we have innovatively developed a composite PCMs (CPCMs) by combining sodium acetate trihydrate (CH3COONa·3H2O, SAT), lithium chloride (LiCl), potassium chloride (KCl), and incorporating expanded graphite (EG). This addition of high thermal conductivity EG was aimed at bolstering the CPCM's thermal stability. Our findings reveal that SAT-LiCl-KCl/EG CPCMs, with 14 wt% EG, significantly enhances thermal energy storage performance while preserving the fundamental phase change characteristics. The resulting CPCMs demonstrates a phase transition temperature of 49.3 °C and a latent heat of 196.4 J/g, ideally suited to meet the demands of hot water heat pump systems operating in the 45–50 °C range. Notably, the thermal conductivity of this CPCMs, following the incorporation of 14 wt% EG, registers at 4.54 W/(m K), representing a remarkable 10.6-fold increase compared to the SAT-LiCl-KCl ternary eutectic salt. To ensure long-term stability, secondary encapsulation with a thermally conductive sealant was applied. Subsequent tests involved subjecting the CPCM to 3000 heating/cooling cycles, resulting in minimal deviations, with the phase transition temperature shifting by approximately 1 °C and a modest 6 % reduction in latent heat of phase transition. These outcomes underscore the robustness and reliability of our developed composite phase change materials.

The combined application technology of solar heat utilization and ground source heat pump

Energy issues play an increasingly important role in national security and economic development. The country has put forward the policy of building energy-saving and land-saving buildings. The application of the comprehensive utilization of renewable energy in the construction field has gradually become a need for technical progress and industry development. With the official promulgation and implementation of the Renewable Energy Law in 2006, the application of solar energy, geothermal energy, and other related renewable energy in the construction industry has attracted more and more attention. However, the scope of practical application is currently limited to solar energy alone to supply domestic hot water, power generation, or ground source heat pump system heating and cooling. The actual engineering cases of the combined operation of the two are relatively few, and the unreasonable joint operation mode leads to low system efficiency. In this paper, the combined application system of solar heat utilization and ground source heat pump is elaborated and analyzed in detail, from the technical feasibility, system efficiency, and other aspects of the analysis. It emphasizes the necessity and development prospect of the comprehensive utilization of renewable energy.

Modeling and experimental investigation on a gas engine-driven heat pump for space-heating and sanitary hot water

Utilization of gas engine-driven heat pumps (GEHP) can reduce the heavy loads on the power grid when electric heat pumps operate during the heating season. This study is to examine the performance of GEHP as a water heater with different engine speeds and water flow rates providing space-heating and sanitary hot water. When the engine speed rises from 1400 rpm to 1900 rpm, the temperature rise of water increases by 37% from 25.7 °C to 35.3 °C. Moreover, the system heat capacity rises from 71.8 kW to 97.7 kW with an increase of 36%, while the COP decreases from 3.6 to 3.0. As the inlet water flow rate an increase from 2.5 m3/h to 4.0 m3/h, the temperature rise decreases from 44.3 °C to 24.1 °C, with decreasing heat capacity from 128.8 kW to 112.1 kW. Additionally, the primary energy ratio (PER) initially increases from 1.46 to 1.50 at the engine speed of 1800 rpm, followed by a slight decrease to 1.48 when the engine speed reaches 1900 rpm. With an increasing water flow rate, the PER achieves its highest value of 1.64 at 3 m3/h and its lowest value of 1.49 at 3.5 m3/h.

Research on Energy Savings of an Air-Source Heat Pump Hot Water System in a College Student’s Dormitory Building

Centralized hot water systems are commonly installed in college student dormitories, representing a typical application for such systems. To achieve sustainable and environmentally friendly heating solutions, air-source heat pump hot water systems have gained attention for their high efficiency and energy-saving characteristics. By implementing heat pump technology, China could make significant progress towards achieving its carbon neutrality goals by reducing energy consumption and associated emissions. In this study, the heating performance of an air-source heat pump hot water system was tested in the field over the course of a year at a university in Chongqing, China. A simulation model was constructed using TRNSYS software, and a time-sharing control strategy was proposed to analyze the system’s operating characteristics and energy-saving performance. Results showed a 6% increase in unit annual average Coefficient of Performance (COP) and annual electricity savings of 10,027 kW·h, with an energy-saving rate of 8.77% after time-sharing control. The study highlights the significant economic and environmental benefits of adopting sustainable energy solutions, particularly in the context of increasing global greenhouse gas emissions.

Thermodynamic performance of heat pump with R1234ze(E)/R1336mzz(E) binary refrigerant

In order to solve the temperature mismatch issue of cold and hot fluids in evaporation and condensation processes for heat pump system, binary refrigerant R1234ze(E)/R1336mzz is recommended in the present research due to its large temperature glide. Through theoretical analysis, multiple configurations under conditions of three different temperature heat sources (70/50 °C, 60/40 °C and 50/30 °C) and two different heat targets (steam above 100 °C and hot water with 70 °C) are constructed adopted Ebsilon code. The optimum mass proportion is obtained as 0.3/0.7 for R1234ze(E)/R1336mzz(E), because of its maximum temperature glide (can reach 21.52 °C) and excellent thermal performance (large COP, ECOP and exergy efficiency). COP of steam high temperature heat pump with R1234ze(E)(0.3)/R1336mzz(E)(0.7) binary refrigerant and the ratio of transferred heat of 110 °C condenser to all output heat (containing 110 °C and 100 °C steam intercooler and condensers) respectively increase up to 67.25% and 335.16% under some certain conditions, compared with that with pure R1336mzz(E). COP of hot water heat pump with R1234ze(E)(0.3)/R1336mzz(E)(0.7) binary refrigerant are respectively 10.31 and 6.89 under 60/40 °C and 50/30 °C heat source conditions, which greatly increases compared with that of heat pump with pure R1234ze(E) (6.67 and 5.03) and R1336mzz(E) (6.74 and 4.54). The present research may lay a foundation for binary refrigerant adoption to replace the scheme of multi-stage evaporator or multi-stage condenser, which could reduce the irreversible loss of phase change equipment and improve thermodynamic performance of heat pumps.

Experimental validation of a state-of-the-art model predictive control approach for demand side management with a hot water heat pump

Hot water heat pumps are well suited for demand side management, as the heat pump market faced a rapid growth in the past years with the trend to decentralized domestic hot water use. Sales were accelerated through wants and needs of energy conservation, energy efficiency, and less restrictive rules regarding Legionella. While in literature the model predictive control potential for heat pumps is commonly shown in simulations, the share of experimental studies is relatively low. To this day, experimental studies considering solely domestic hot water use are not available. In this paper, the realistic achievable model predictive control potential of a hot water heat pump is compared to the standard hysteresis control, to provide an experimental proof. We show for the first time, how state-of-the-art approaches (model predictive control, system identification, live state estimation, and demand prediction) can be transferred from electric hot water heaters to hot water heat pumps, combined, and implemented into a real-world hot water heat pump setup. The optimization approach, embedded in a realistic experimental setting, leads to a decrease in electric energy demand and cost per unit electricity by approximately 12% and 14%, respectively. Further, an increase in efficiency by approximately 13% has been achieved.

Mock-up experimental study on the performance of a combined cooling-domestic hot water-ground source heat pump system

A ground source heat pump (GSHP) delivers heat from a condenser to the ground when it operates with cooling mode. However, the ground temperature increases when the GSHP system operates for a long time. The increased ground temperature can deteriorate the GSHP’s coefficient of performance (COP). To maintain the balance between the ground temperature and COP, the condensation heat from the cooling GSHP can be used for other heating systems before it is transferred to the ground. This study proposes a combined GSHP system connecting a three RT cooling GSHP with a 1.5 RT domestic hot water (DHW) heat pump, and the performance of the system was evaluated through mock-up experiments. In the combined GSHP system, the condensation heat of the cooling system was used as the heat source of the DHW system. Therefore, the ground temperature could be reduced, and the performances of both the GSHP cooler and DHW heater pump could be enhanced. Mock-up experiments for performance evaluation were conducted with cooling-only, DHW-only, and cooling-DHW operational modes. The results showed that cooling-DHW operation slowed down the change in heat source temperature. In comparison with those of the cooling-only and DWH-only heat pumps, the COPs of the cooling heat pump and DHW heat pump of the combined system were increased by 12.93% and 15.47%, respectively. Moreover, the total COP of the cooling-DHW combined GSHP system increased by 4.4% and 29.55% in comparison with those of the cooling-only GSHP and DHW-only GSHP systems, respectively. Practical application A combined GSHP system was proposed by connecting a GSHP for space cooling with a heat pump for DHW to mitigate the ground temperature changes due to the long-term operation of the GSHP system. The combined system reuses the condensation heat generated by the cooling heat pump as the heat source of the DHW heat pump for ensuring the system performance and building energy saving. Therefore, this system is suitable for buildings with large energy use and heavy hot water demand, especially in summer, such as hospital and hotel buildings.

Development of Sky-source Heat Pump System

In order to realize a decarbonized society, which is an urgent global issue, the introduction of renewable energy is being expanded. Currently, renewable energy is mainly used to create electricity, such as via photovoltaic cells and wind turbines, while the high introductory costs of other renewable energy sources, such as solar and geothermal, is an impediment to its popularization. We have developed a high-efficiency, low-cost “Sky-sourced Heat Pump SystemⓇ (SSHPⓇ)” that utilizes renewable heat, and is operating at a demonstration site in eastern Japan. Unlike conventional air conditioning systems, our SSHPⓇ system is a unique water heat-sourced heat pump system which connects various renewable energy heat sources, such as solar, air, and geothermal heat, with air conditioning units and hot water heat pumps via a heat source water loop. Based on measurements taken from March 19 to August 28, 2022, and system simulations, primary energy consumption and CO2 emissions were significantly reduced by about 22%, or 32% when compared to typical electric heat pumps. The results indicate that the introduction of SSHP is highly effective.

Estimation of energy savings resulting from the cooperation of an air to water heat pump with a solar air heater

The heating efficiency of a heat pump is strongly dependent on the difference between condensation and evaporation temperature. This article analyzes the combination of a solar air heater (SAH) and an air-to-water heat pump (AWHP) to reduce this temperature difference. A new method for assessing the energy consumption of an air-source heat pump for heating domestic hot water has been developed. It was created on the basis of a model from the EnergyPlus software, simulation results made in the DesignBuilder software environment, and mathematical equations describing the increase in the air temperature in SAH as a function of solar radiation intensity. The authors determined the thermal characteristics of SAH on the basis of experimental measurements carried out in the period from April to July 2022. The test results were interpolated and used in the development of the computational model. Two series of calculations taking into account the preheating of the air in SAH and the supply of AWHP only with ambient air were performed. As a result of the computational analysis, it was found that the maximum reduction in energy consumption by AWHP resulting from cooperation with SAH may reach approximately 16 %.

Long-Term Performance Analysis of Direct Photovoltaic Thermal-Assisted Heat Pump Water Heater Using Computational Model

The photovoltaic thermal system has a limitation in supplying hot water at the required temperature since it requires a supplementary heating system. One such system can be an air source heat pump operated by electricity from a PV/T system, which can be used to raise the temperature of the water preheated by PV/T to the required level. Such type system can also be called a hybrid PV/T-heat pump system, and it delivers thermal (hot water) and electrical energy. The performance of the hybrid PV/T-heat pump system was analyzed using the computational model for two Ethiopian cities, namely, Addis Ababa and Dire Dawa, which represent the highland and lowland regions, respectively. The simulation was conducted by inputting climatic data, daily hot water demand, and hourly hot water consumption patterns. Also, system design parameters such as PV/T area and peak Watt, PV/T warm water tank volume, air source heat pump water heater COP as a function of ambient temperature and circulating water temperature, and heat pump hot water tank volume. Besides, the effect of hot water consumption patterns (variability effect in three dissimilar cases constant, restaurant, and motel) on the system performance was examined. In addition, the effect of electric energy supply to the compressor of the heat pump using battery storage on the system performance was also investigated. In those diverse situations, the system in most of the cases generated hot water above 55°C and scored a COP of 3. The maximum value of hot water end-use efficiency of 66% was also obtained by making the hot water tank capacity about 50% of the daily hot water consumption size.

Simulation and analysis of hot water system with comprehensive utilization of solar energy and wastewater heat

With global interest in energy saving and emission reduction, the combination of waste water energy recycling and solar energy has attracted great attention. In this work, a solar-assisted waste water source heat pump hot water system was built in TRNSYS. The sensitivity analysis of different factors on the annual heating performance coefficient of the system was conducted and the optimal parameters were selected to improve the basic scheme. The results revealed that the COPsys and COPHP were highly related to the pump flowrate and the storage capacity of heat exchange tank, but the azimuth and inclination angle of solar collector have slight influence on them. Moreover, the increase of heating capacity of solar collector will lead to the decrease of COPHP, while resulting in the decrease of the heat pump power consumption. The suitable pump flowrate is 4.6 m3/h and the appropriate volume of the heat exchange tank is 1 m3. The best azimuth angle and inclination angle of the solar collector are 4° south by east and 42°, respectively. The improved system has more stable hot water temperature in winter with significant decrease of the energy consumption.

Simultaneous thermodynamic and economic enhancement of heat pumps based on a new method for avoidable irreversibility assessment

The paper presents a new method aimed at the simultaneous thermodynamic and economic enhancement of air-source and water-source heat pumps. The novel approach allows estimating the avoidable parts of the exergy destruction and investment expenditures by avoiding the need for ideal process introduction, being the most critical issue associated with the application of the existing advanced exergy-based methodology. In fact, within the proposed new method the values of the avoidable parts of investment expenditures of the kth component belonging to the considered heat pumps are determined assuming a greatly efficient component rather than not an extremely inefficient one. The proposed approach was applied to domestic hot water heat pumps using R410A and R134a to evaluate its potential. The results obtained suggested that at the investigated operation conditions the condenser and the evaporator are components for which minimising irreversibilities can provide simultaneous thermodynamic and economic enhancement.

Case Study of Load Matching and Energy Cost for Net-Zero Energy Houses in Korea

Over the past 20 years, net-zero energy house (NZEH) construction costs have steadily decreased because of many reasons, such as technical progress, energy-saving design obligations, and dramatic cost reductions in renewable energy systems, especially solar power systems. Currently, the costs of NZEH are estimated to be about 5% higher than similar-sized houses. These additional costs are mainly for installing PV systems, which can be offset by energy saving costs. This study assessed energy performance and load matching through remote monitoring systems, and energy costs were analyzed for two-family houses. The two houses were all-electric houses and different in both size and location. A 6 kWp grid-connected PV system and 16 kW air source heat pump for space heating and domestic hot water were equally implemented. After data analysis, 100% of the energies were supplied through the PV system for 3 years, thus achieving net-zero energy. According to the Korean residential electricity tariff system, the annual electricity charges were, on average, between USD 105.1 and USD 121.4 after adding demand charges and value-added tax for import electricity charges. The energy cost reduction rate, compared to the same house without a PV system, was about 95%, and the simple payback period of the 6 kW PV system in NZEH was about 6 years. In addition, the annual load cover factor and supply cover factor as load-match indices between electricity generation and the load were in a range of 0.39–0.49 and 0.37–0.42, respectively.

Research on Hot Water System of PVT Coupled Heat Pump High-speed Service Area Based on TRNSYS

The high-speed service area is located outside of the urban energy system. If there is no efficient energy saving technology, it will lead to a large amount of energy consumption. As an important ancillary facility along the expressway, the service area has a large demand for hot water due to the high density of human flow. Renewable energy should be fully utilized to make domestic hot water. In this paper, a water-cooled photo-voltaic thermal (PV/T) coupled heat pump hot water system model is proposed. The TRNSYS model for the system operation is built, and an operating experimental platform is set up to verify the model experimentally. The comprehensive performance of typical days in winter and summer was evaluated, and it was found that the average COP of the system was as high as 7.9 and 8.1 respectively. The photoelectric conversion efficiency is as high as 17.5% and 19.5%, respectively. This system has good technical adaptability in the high-speed service area, providing certain guidance for building the green and ecological high-speed service area.

Air Source Low Temperature Heat Pump Hot Water Technology

Air Source Low Temperature Heat Pump Hot Water Technology

Rainwater for residential hot water supply: Managing microbial risks

There is growing interest in using alternative water sources such as rainwater harvesting and treatment systems to supplement traditional sources and secure a stable supply. For such systems, it is important to ensure adequate water quality, as microbial contamination can be a risk factor in rainwater. The primary objective of this study was to provide proof-of-concept for the microbial treatment capacity of a residential scale rain-to-hot-water treatment system to be installed in Melbourne, Australia. The system consists of a filtration, UV and heat-pump hot water unit, and disinfects roofwater prior to use of the hot water for bath, shower and laundry. The system's efficiency was evaluated using long-term challenge tests investigating the full system and each of the separate components. The microbial treatment performance was assessed based on the systems' ability to treat high levels of E. coli, E. faecalis, Campylobacter, Salmonella and MS2 phage under challenging conditions; with varying flow rates (10-40 L/min) and microbe concentrations (104-105unit/L), and in scenarios of a power outage. Over a compressed year of operation, the full rain-to-hot-water treatment system was extremely efficient at reducing concentrations of E. coli, E. faecalis, Campylobacter, Salmonella and FRNA phages, with log reductions ranging from the lowest average of 2.1log reductions for Salmonella to a maximum of >5.1log for E. coli. Most of the treatment was provided by the UV system, and any remaining microbes present after this point were deactivated by the heat-pump system (provided that the water was given enough time to warm up). Additional modelling work showed that UV intensity, UV transmissivity and contact time (for the UV system) and temperature (for the heat-pump hot water system) could be used as parameters to predict microbial treatment performance of the system, indicating that these easily-measurable parameters could assist with ongoing operation optimisation and maintenance of such systems.

Predictive energy management of residential buildings while self-reporting flexibility envelope

In recent years, renewable energy resources have been increasingly embedded in the distribution grids, raising new issues such as reverse power flows, and challenging the traditional distribution system operation. In order to mitigate these issues, it has been proposed to operate the distribution system more flexibly. For instance, residential buildings are ideal candidates to offer energy flexibility locally and defer avoidable and expensive system expansions. Due to advances in smart meter technologies and trends towards digitalization, it becomes more and more common that electrical appliances in residential buildings are equipped with remote communication and control capabilities. This paper aims at quantifying a flexibility envelope of actively controlled flexible buildings and at analyzing the sensitivity of flexibility levels with respect to system configuration, control strategy and objective function settings. We consider rooftop photovoltaic units, air-sourced heat pumps for space heating and domestic hot water, thermal energy storage and electric vehicles. The results show that an optimal control aiming at minimizing energy costs while limiting peak power can lead to savings of up to 25% compared to the existing rule-based control. When carbon emissions are considered in the cost function, the optimized controller leads to an emission reduction of up to 21%. The time-dependent quantification of the flexibility envelope further reveals that high and low power levels can only be sustained for a limited period, whereas medium power levels can be sustained the longest.