Solar-assisted Heat Pump Water Heater Research Articles

Utilization of Solar Energy for Water Heating Application to Improve Building Energy Efficiency: An Experimental Study

The manuscript is written for flow escalation based on an experimental data for a Solar Assisted Heat Pump Water Heater (SAHPWH) increasing building energy efficiency. For the investigation, a Solar Assisted Heat Pump (SAHP) was conceived, manufactured, and tested in real time. The findings of the experiments shows that single glazing with average sun radiation of 600–750 W/m2, COP of approx. 6 can be obtained with identical heat gains. This study shows that when a flat plate collector of area 1.83 m (L) × 1.22 m (W) × 0.1 m (T) with a 0.5-mm-thick black copper plate absorber with clear glazing as a cover receives average radiation of 700 W/m2, then setup can supply 60 litres of water for residential use from 15 °C to 45 °C in approx. 70 min. In addition, the study finds that the collector efficiency factor F’ is likewise shown to have a direct connection with the absorber and an inverse relationship of tube spacing. The findings indicated that the technology has significant commercial potential, particularly in sectors such as with solar resources for improving building energy efficiency.

Optimal energy management of a solar-assisted heat pump water heating system with a storage system

Water heating is the fourth largest energy consumer in the commercial building sector, accounting for >40 % of the energy consumed in South Africa. Traditional electric storage tank water heaters (ESTWHs) have been the most frequently used systems in the past, owing to their low electricity costs and implementation. It was low enough at that time that energy costs were not considered. These systems have grown increasingly hostile to consumers as power prices have risen, particularly to the undesirable high morning and afternoon peaks.The indirect-expansion solar-assisted heat pump water heater (IDX-SAHPWH), for the commercial sector, to reduce energy consumption of hot water production while maintaining the users' thermal comfort level, has been proposed in this paper. The system is operated under an optimal energy control scheme, integrated with the load shifting by time-of-use (TOU) pricing plan. The comparison of the performance of the proposed optimally controlled system and the electric storage tank water heater (ESTWH) was performed through the simulation approach using the Solving Constraint Integer Programs (SCIP) solver, in the MATLAB interface optimization toolbox. The simulations reveal that the proposed system operates during off-peak hours, when power prices are lower, as compared to ESTWH system.The techno-economic analysis was conducted and presented for a project lifespan of 20 years. The proposed optimally controlled system demonstrates the cost savings obtained from the winter and summer seasons, as well as annual, as 76.0 %, 75.6 % and 75.8 %, respectively. The break-even point of the project is during the 9th month of the first year of the project, with a possible savings of 71.5 % at the end of the project's lifetime.

A review of solar and air-source renewable water heating systems, under the energy management scheme

Although traditional electric storage tank water heaters (ESTWHs) are the most frequently used systems in the world, they are known to be among the highest energy consumers, due to their inefficiency in converting electrical energy to thermal energy. These systems have grown increasingly hostile to consumers, as power prices have risen. Therefore, the need to reduce energy consumption of hot water production, while maintaining the users’ thermal comfort level, has led to the development of more energy-efficient technologies. In this paper, a review of renewable energy water heating systems, particularly air-source heat pump water heaters (ASHPWHs), solar water heaters (SWHs) and solar-assisted heat pump water heaters (SAHPWHs), as well as the background literature of the studies on these systems, carried out in South Africa, under the energy management schemes, is presented.

Energy, economic, and environmental (3 E) examinations of the indirect-expansion solar heat pump water heater system: A simulation-oriented performance optimization and multi-objective decision-making

In this paper, a simulation-oriented optimization integrated with the multi-objective decision-making (MODM) model for an indirect-expansion solar-assisted heat pump (IDX-SAHP) water heater is proposed and the energy, economic, and environmental (3 E) aspects of the system are examined. The developed model is verified and its performance is scrutinized in a humid continental climate region of China. Next, the sensitivity analysis is conducted via the one-way analysis of variance (ANOVA) approach. Then, the single-objective optimization (SOO) and multi-objective optimization (MOO) are conducted via the migrating birds optimization (MBO) method to maximize the coefficient of performance (COP) and the collector efficiency (CE), singly and concomitantly. The SOO results indicate the IDX-SAHP design as per the COP-oriented optimization causes the system to operate less for about 120 hours a year in contrast to the system design as per the CE-oriented optimization. Also, the results of MOO integrated with MODM indicate that MOO gives better performance in contrast to the SOOs so that although the CE of the maximized IDX-SAHP reduces a little from 51% to 46%, its COP increases greatly from 2.5 to 7. Therefore, the overall power consumption of the system decreases about 269 hours in contrast to the initial IDX-SAHP. Besides, the environmental examination illustrates the use of IDX-SAHP in Beijing buildings can prevent the burning of 2,877, 000, 000 cubic meters of natural gas per year in the fossil fuel power plants and prevent the release of 8,055,600 tons of CO2 per year into the environment. Furthermore, the economical examination illustrates that Beijing residents may save approximately one billion USD per year on the purchase of electricity if they use the IDX-SAHP. Besides, the Chinese government can achieve about 109 billion USD per year if the equivalent electricity is exported instead of domestic consumption for Electric Water Heaters (EWHs). Moreover, the external costs decrease by 928 million USD per year if the IDX-SAHP is replaced with the EWHs. This paper discloses the significance of energy, economic, and environmental (3 E) examinations, optimization, and MODM during the IDX-SAHP design to increase the system efficiency and decrease the costs and environmental impacts by opting the most appropriate design parameters of the system.

A review of heat pump research in China using bibliometric methods

The analysis of big data can reveal the macroscopic situation of a certain field from a relatively objective angle and indicate the potential development trends in the future. This study provides a novel approach to review the development of heat pump research from 1994 to 2020 in China using the bibliometric method which belongs to big data analysis. More than 15 000 references were selected, 2716 of which are from the Web of Science core collection and 13 183 of which are from China National Knowledge Infrastructure, including articles and reviews. Their knowledge maps and co-citation analysis were performed. Different major trends were identified for 15 clusters. Results showed that ground source heat pump (GSHP), air source heat pump (ASHP)-on frost, ASHP on heating, and solar-assisted heat pump are the most active research fronts. The studies on the GSHP account for the largest portion, reaching about 21%. Ground heat exchanger and thermal imbalance are the focus points. ASHP is the hottest research direction in recent years, mainly focusing on the technologies of frosting/defrosting and heating. The studies on defrosting experience a transition from the frosting/defrosting process and thermal energy storage for defrosting to frost-free ASHP. Besides, direct-expansion solar-assisted heat pump water heater was becoming more and more active. Finally, industrial heat pump showed great development potential.

Performance-based Pareto optimization and multi-attribute decision making of an actual indirect-expansion solar-assisted heat pump system

This study presents the sensitivity analysis and simulation-based Pareto optimization of an actual indirect-expansion solar-assisted heat pump (IX-SAHP) water heater. The developed IX-SAHP is validated and its performance is fully appraised in a humid continental climate zone throughout the year. Next, the one-factor-at-a-time (OFAT) method is implemented to determine the impacts of various design parameters of IX-SAHP on the system performance. Then, the impacts of several major input variables on the pressure drop of refrigerant flow in the IX-SAHP are evaluated. Additionally, the single and multiple-objective particle swarm optimizations (MOPSO) are integrated with the designed IX-SAHP to optimize the coefficient of performance (COP) and the solar collector efficiency (SCE), separately and concurrently to achieve the optimal non-dominated solutions in the form of Prato front. Afterward, the final optimum configuration is chosen through TOPSIS integrated with the MOPSO using programming under MatLab environment. The comparison of achieved solutions shows that the multi-objective optimization gives more appropriate and reliable solutions compared to the single one. Also, the Pareto optimization results show that whereas the SCE diminishes slightly from 52% to 46.4%, but the COP highly improves from 3 to 7 which lead to a significant reduction in the overall operating hour of the IX-SAHP approximately 270 h throughout the year compared to the initial IX-SAHP model.

Energy analysis of heat pump water heaters coupled with air-based solar thermal collectors in Canada and the United States

Heat pump water heaters (HPWHs) operate with two to three times the efficiency of electric water heaters, and over three times the efficiency of natural gas water heaters; however, this benefit is accompanied by a secondary space cooling effect that increases space heating costs. To mitigate this, HPWHs may be coupled with solar thermal collectors which preheat inlet air. The objective of this study was to compare the energy performance of HPWHs and solar-assisted HPWHs (SAHPWHs) across Canada and the United States, to determine where the SAHPWH could offset the space heating increase of HPWHs. This was done using an empirical model, with different modelling methods and assumptions that were assessed and compared. It was found that the SAHPWH could fully mitigate the space heating increase throughout winter, while decreasing the space heating load by 1–3%, and during the cooling season, decreasing the space cooling load by 7–15% in all climates studied. This research indicates that HPWHs and SAHPWHs have potential to lead a higher efficiency future, by reducing energy consumption for water heating beyond what is presently achievable with electric and natural gas storage tank water heaters.

Economic analysis of heat pump water heaters coupled with air-based solar thermal collectors in Canada and the United States

Heat pump water heaters (HPWHs) could reduce residential energy consumption but there are two barriers preventing their uptake: the space cooling effect which increases space heating costs in cold climates and the high capital costs. These barriers may be mitigated by coupling a HPWH with an air-based solar collector to preheat inlet HPWH air, and using a lifecycle thinking approach to evaluate HPWHs or solar-assisted HPWHs (SAHPWHs) to encompass low operating costs. The objective of this study was to economically analyze the HPWH and SAHPWH across Canada and the United States to determine locations in which the systems are economically feasible as a method for reducing residential greenhouse gas emissions. The methodology for this included simulating the HPWH and SAHPWH performance using an experimentally validated TRNSYS model. The results show that HPWHs are typically more feasible than SAHPWHs except in locations with high electricity rates or long heating seasons, or locations with fossil fuel dominant electricity grids when carbon pricing is considered. As such, each location must be studied to determine which options are economically suitable. The findings presented illustrate economical clean water heating options, and subsidies that could be introduced to further encourage uptake of low carbon water heating.

Greenhouse gas emissions analysis of heat pump water heaters coupled with air-based solar thermal collectors in Canada and the United States

Space and water heating contribute 62% and 21% of the residential greenhouse gas emissions (GHGs) in Canada, respectively, which corresponds to 11% of the GHGs within the country. Because both space and water heating are dominated by natural gas, the carbon footprint of Canada could be significantly reduced with a shift towards clean space and water heating, such as heat pump water heaters (HPWHs). HPWHs use electricity but are between two and three times more efficient than electric water heaters (EWHs). However, while they operate, HPWHs cool nearby ambient air which increases natural gas-dominant space heating loads. Solar assisted HPWHs (SAHPWHs) may allow the GHG reductions of HPWHs to be realized without increased space heating loads. The objective of this study was to assess the feasibility of HPWHs and SAHPWHs in terms of their GHG reductions across Canada and the United States. It was found that the systems reduced GHGs compared to EWHs and natural gas water heaters (NGWHs) in many locations, as indicated graphically within this paper. The locations and electricity grid mixes which have potential for the greatest GHG reductions are shown in this paper.

Thermal performance, parametric analysis, and multi-objective optimization of a direct-expansion solar-assisted heat pump water heater using NSGA-II and decision makings

Direct-expansion solar-assisted heat pump water heaters (DX-SAHPWHs) are conducive to the environment due to the use of ambient energy and solar radiation. This paper aims to design and develop a thermodynamic model and a multi-criteria optimization of thermal performance for a DX-SAHPWH using R-134a as the working refrigerant that supplies domestic hot water for a typical building throughout the whole year. The thermodynamic model of the DX-SAHPWH is developed so that the performance of the system is solved through the try and error technique with the least initial data of the thermodynamic cycle of DX-SAHPWH. After the validation of the thermodynamic model, the performance of the DX-SAHPWH is analyzed for a typical building located at the temperate climate of Iran. Then, a parametric study is conducted to identify how various design parameters may affect the performance of the DX-SAHPWH system. In the current investigation, the effects of design parameters including solar radiation intensity, ambient air temperature, outlet water temperature of the condenser, solar collector area, compressor speed, length of tube in the condenser, external diameter of the tube in collector plate, fin thickness, and thermal conductivity of collector plate on the performance of the DX-SAHPWH are investigated. Besides, single and bi-criteria optimizations are carried out using genetic algorithm (GA) to obtain the optimal solutions of design parameters, where the coefficient of performance (COP) and the solar collector efficiency (SCE) are selected as two fitness functions. The optimal solutions achieved from the bi-criteria optimization process will be given as Pareto frontier. The final optimum solution from the available solutions on the Pareto optimal frontier is selected using decision-making methods, such as LINMAP, TOPSIS, and Shannon’s Entropy. The comparison of single and bi-criteria optimization results illustrate that the bi-criteria optimization method yields more proper results than single ones, mainly because of the lower deviation index from the ideal solution. The results of bi-criteria optimization show that SCE decreases 1.6% compared to the initial model while the COP increases close to 20% that makes the optimum solution more desirable compared to the single ones. Moreover, the performance of the optimized DX-SAHPWH system is compared with the initial model in each month of the year. The results indicate that the performance of the optimized DX-SAHPWH is highly improved so that the working hours of the system decrease close to 109 h compared to the initial model during the whole year.

Comparative experimental analysis of direct-expansion solar-assisted heat pump water heaters using R134a and R290

To conduct a comparative evaluation on the performance of a R290 direct-expansion solar-assisted heat pump (DX-SAHP) system, two experimental set-ups of the DX-SAHP system have been designed, fabricated and tested for domestic hot water supply, using R134a and R290 as refrigerant, respectively. A micro-channel solar collector/evaporator was used in the R290 system. Based on them, numerous experiments were carried out under the meteorological winter conditions to compare the thermal performance of two systems. The results show that the average compressor power of the R290 system during the whole test days reaches 301.1 W, which is 7.04% lower than that of the R134a system. The corresponding average heating capacities of the R290 and R134a systems are 859.7 and 786.4 W, respectively. When 0.2 m3 water is heated from approximate 10 to 40 °C with average solar radiation intensity of 500 ± 3 W m−2, ambient temperature of −0.7 ± 0.1 °C and wind speed of 1.2 ± 0.1 m s−1, the average coefficient of performance (COP) of the R290 system reaches 2.72, which is 32% higher than that of the R134a system. In addition, under the same conditions, the average collector efficiency of the R134a system reaches 0.66 while that of the R290 system reaches 0.73 with the corresponding average compressor power of 405.9 and 291.4 W, respectively.

Influence of different regulation modes of compressor speed on the performance of direct-expansion solar-assisted heat pump water heater

A quasi-dynamic mathematical model of a direct-expansion solar-assisted heat pump water heater is presented on the basis of theory and experiments, and the system performance in various regulation modes of the compressor speed are analyzed comparatively. A lumped parameter method is used to establish the models of a compressor and an electronic expansion valve, and a uniform flow distributed parameter method is used to establish the models of an unglazed flat-plate solar collector/evaporator and a winding micro-channel condenser. The mathematical model predicts well the experimental data within a maximum relative error of 10%. Three regulation modes under constant environmental parameters and four regulation modes under variable environmental parameters are proposed, respectively. Numerical calculations as well as experimental studies show that there is little effect of the regulation mode of the compressor speed on the average system coefficient of performance under the same operating and environmental parameters. Under typical working conditions with average compressor speed of about 2900 rpm, when water temperature rise from 28.2 to 61 ± 1 °C, the average system coefficient of performance reaches above 4.6 for different regulation modes of the compressor speed. When the average compressor speed decreases from 2910 to 2650 rpm, the average system coefficient of performance increases by 7.5%. In addition, the acceleration mode of the compressor speed enables to stabilize the evaporating temperature, and the deceleration mode of the compressor speed enables to stabilize the compressor power.

Experimental investigation on a direct-expansion solar-assisted heat pump water heater using R290 with micro-channel heat transfer technology during the winter period

Abstract R290 (propane) is an environmental friendly refrigerant with zero ozone depletion potential (ODP) and low global warming potential (GWP). A direct-expansion solar-assisted heat pump (DX-SAHP) water heater system using R290 was designed and built to investigate its thermal performance, which comprised a constant-frequency rotary-type hermetic compressor with rated power of 471 W, a micro-channel solar collector/evaporator with surface area of 2.09 m2, a micro-channel condenser with effective heat-transfer area of 0.75 m2, and an electronic expansion valve. In order to evaluate the DX-SAHP system performance during the winter period, massive experiments have been carried out with a refrigerant charge of 350 or 410 g. Under these environmental parameters as follows: the average solar radiation intensity of 20–592 W m−2, average ambient temperature of −3.0–12.2 °C, and average wind speed of 0.01–1.15 m s−1, while the 200 L water at 7.2–15.3 °C was heated to 37.7–54.9 °C, the average system coefficient of performance (COP) ranged from 2.12 to 4.43. Finally, the responses of the DX-SAHP system using R290 to variations in solar radiation intensity and ambient temperature are presented. The transient variations in the refrigerant mass flow rate, the electric power input to the compressor and the electronic expansion valve opening are also analyzed under different weather conditions.

Numerical study of a direct-expansion solar-assisted heat pump water heater under frosting conditions based on experiments

A direct-expansion solar-assisted heat pump (DX-SAHP) water heater is studied numerically. A simulation model, which includes the models of components and the frost formation of the collector/evaporator, is established to research the heating performance of the system and the growth of frost. The calculation results conform well to the experiment, which verifies the accurate predictions. On the basis of the model, the influences of the meteorological parameters on the system performance under frosting conditions are analyzed. It is found the formation of frost can be effectively inhibited by decreasing the ambient temperature below a specific value. What’s more, the collector efficiency can also be improved effectively during the incipient growth of frost crystals. Besides, higher solar radiation intensity can significantly improve the heating performance and reduce the frosting rate. By contrast, the enhancement in wind speed will have the same effect, but only slightly. The system coefficient of performance (COP) reaches above 2.75 with the reliable heating performance under typical frosting conditions, and the collector/evaporator presents the superiority in delaying frosting in comparison with the traditional evaporator for air source heat pump.

Research on real-time integrated control method of PV-SHAPWH

The real-time operation control of the photovoltaic integrated solar-assisted heat pump water heater (PV-SAHPWH) can improve the system's annual performance. In this paper, the real-time variable capacity control and starting time control of photovoltaic integrated solar heat pump are simulated and experimentally researched. A real-time integrated control system is developed to achieve the hot water task in a predetermined time, reduce energy consumption and increase the net power generation of the system. A mathematical model of photovoltaic integrated solar heat pump system was established, and a photovoltaic integrated solar heat pump experimental prototype with a 4.32 m2 heat collecting area, a R22 working fluid compressor and a 150 L thermal water tank was built in Nanjing. The simulation program verified by the fixed-frequency operation test is used to construct the system running performance database and obtain the control strategy table, and adjust the system starting time according to the weather condition forecast of the day. In the real-time variable capacity test, the inverter compressor adjusts the system capacity, and the electronic expansion valve controls the evaporator outlet superheat to realize real-time variable capacity control. The research results show that the performance coefficient and photovoltaic power generation of the real-time integrated control heat pump system are improved compared to the simulation results of other control methods, and the hot water task can be completed accurately within the predetermined time.

Modelling & Simulation of a Direct-Expansion Solar Assisted Heat Pump for Water Heating Application in South Africa

Hot water heating accounts for about 7% of the energy demand in South Africa and about 30% in the residential sector. The most common heating method electrical resistance water heaters (geysers) are inherently fossil-fuel driven and thus responsible for carbon emissions. In this paper, the potential application of a direct-expansion solar-assisted heat pump water heater (DX-SAHPWH) system in South Africa was investigated. The DX-SAHPWH combines the beneficial characteristics of both the solar thermal collector and heat pump, however, the performance of a DX- SAHPWH system is significantly influenced by the changes in meteorological conditions. A quasi-steady state mathematical model was developed and theoretical simulations were conducted using meteorological data of Cape Town. The results indicated an hourly average COP of was 4.9, collector efficiency of 50 %, solar fraction and heating times of 68 % and 192 minutes for average temperature, solar radiation and wind speeds of 23 °C , 700 W/ m2 and 3.3 m/s. The annual average COP was 5.3, collector efficiency 48 %, solar fraction 66% and heating time of 127 minutes. Furthermore, a parametric study indicated that the ambient temperature, solar radiation and wind speeds influence the system performance.

Experimental studies of a variable capacity direct-expansion solar-assisted heat pump water heater in autumn and winter conditions

A variable capacity direct-expansion solar-assisted heat pump (DX-SAHP) system was studied experimentally under the meteorological condition of Qingdao located in the East China area. It was mainly made up of a solar collector/evaporator, a variable-frequency rotary-type hermetic compressor, a micro-channel condenser, and an electronic expansion valve. A series of experiments were carried out, in order to evaluate the performance of the DX-SAHP system during the autumn and winter period, including the system coefficient of performance (COP), the electric power to the compressor, the evaporating and condensing pressures, and the discharge temperature of the compressor. Experimental results showed that for the base case considered in this study, under the sunny and overcast day conditions in the autumn, the average COP was higher than 4.0 and 3.0, respectively. Even in the extreme weather of the winter conditions, the average COP was also higher than 2.5. In general, with the increase of water temperature, the compressor speed increased gradually, and the corresponding electric power of the compressor increased sharply. The condensing pressure also increases, as does the discharge temperature of the compressor, while the evaporating pressure remains a relatively low level with smaller variations.

Experimental Research of a Water-Source Heat Pump Water Heater System

The heat pump water heater (HPWH), as a portion of the eco-friendly technologies using renewable energy, has been applied for years in developed countries. Air-source heat pump water heaters and solar-assisted heat pump water heaters have been widely applied and have become more and more popular because of their comparatively higher energy efficiency and environmental protection. Besides use of the above resources, the heat pump water heater system can also adequately utilize an available water source. In order to study the thermal performance of the water-source heat pump water heater (WSHPWH) system, an experimental prototype using the cyclic heating mode was established. The heating performance of the water-source heat pump water heater system, which was affected by the difference between evaporator water fluxes, was investigated. The water temperature unfavorably exceeded 55 °C when the experimental prototype was used for heating; otherwise, the compressor discharge pressure was close to the maximum discharge temperature, which resulted in system instability. The evaporator water flux allowed this system to function satisfactorily. It is necessary to reduce the exergy loss of the condenser to improve the energy utilization of the system.

Experimental performance analysis of a direct-expansion solar-assisted heat pump water heater with R134a in summer

A direct-expansion solar-assisted heat pump (DX-SAHP) system for domestic hot water supply was built and tested, which mainly consisted of a bare flat-plate collector/evaporator with a surface area of 2.1 m2, a variable-frequency rotary-type hermetic compressor, a hot water tank with the volume of 0.2 m3 surrounded by a micro-channel condenser, and an electronic expansion valve. A series of experiments were carried out to test the system performance under typical summer weather conditions in Qingdao, China. The effects of various parameters, including solar radiation intensity of 258–634 W·m−2, ambient temperature of 28–34 °C, compressor speed of 2500–6000 rpm and final water temperature of 50–60.3 °C, on the system performance are analyzed. Experimental results show that the system coefficient of performance(COP) could be enhanced with the increase of the solar radiation intensity or the ambient temperature, and with the decrease of the compressor speed or the final water temperature.

Control strategy and experimental analysis of a direct-expansion solar-assisted heat pump water heater with R134a

A direct-expansion solar-assisted heat pump (DX-SAHP) system was designed and built in Qingdao China, which was used to supply domestic hot water. The system mainly consisted of a bare solar collector/evaporator with area of 1.56 m2, a rotary-type hermetic compressor with rated power of 400 W, an electronic expansion valve (EEV) and a micro-channel aluminum flat tube condenser with single surface area of 0.435 m2 surrounding a 0.195 m3 water tank. The system was charged with 800 g of R134a. Based on the degree of superheat at the outlet of the solar collector/evaporator, a control strategy for the system was developed and tested over a wide range of operating conditions, which proved that it could regulate the degree of superheat in range of 5–10 °C effectively. Then the thermal performance of the system was experimentally studied under various operating conditions. Experimental results showed that for most of the time in 2016, when the 0.195 m3 water was heated to the temperature between 55 °C and 60 °C, the coefficient of performance (COP) of the prototype device was higher than 3.0. As the solar radiation intensity or ambient temperature increased, the COP of the system increased, and the heating time decreased.