Air-source Heat Pump Water Heater Research Articles

Thermodynamic analyses and performance improvement on a novel cascade-coupling-heating heat pump system for high efficiency hot water production

In the background of global warming and reducing carbon emission, air-source heat pump water heater is used as an energy efficient technology to improve the efficiency of building hot water production, which may account for 21–41% of building energy cost. However, there are still space for the energy efficiency improvement of heat pump water heater, as there is large temperature difference between water and refrigerant, which may limit the energy performance (especially at low ambient temperature) but receives little attention. Therefore, a novel cascade-coupling-heating heat pump system with less water-refrigerant temperature different is proposed in this paper, where part of the heat is first provided at the low-temperature stage, reducing the heat load at the high-temperature stage and the high-temperature stage compressor power, and increasing system energy efficiency. To study the system, a theoretical model was established and verified through experiments. Experimental and theoretical results show that: in the ambient temperature range of 243.15 K ∼ 323.15 K, the average and maximum improvement in the coefficient of performance of the new system reach 34.8% and 107.7%, respectively, comparing with cascade heat pump; and in the ambient temperature range of 248.15 K ∼ 323.15 K, when comparing with single-stage heat pump, the new system reveals average and maximum improvement in the coefficient of performance as high as 25.8% and 55.6%, respectively. This study provide a promising potential technology for efficiency water heating, and lay the basis for further study and application of the new system.

Assessment of Optimal Operating Range and Case Verification of a Waste Heat Air-Source Heat Pump Water Heater Based on a Semiempirical Parametric Model

Waste heat air-source heat pump water heater (WH-ASHPWH) systems have been widely used to provide water heating in buildings due to their advantages of high efficiency and low environmental impact. It is important to determine reasonable operating conditions for WH-ASHPWH systems to improve their heating efficiency, especially for systems using industrial waste heat. In this study, a semiempirical parametric model of a WH-ASHPWH system was established to simulate heating performance under different working conditions and determine the optimum operating range of the system. A new discrete model for a tube-in-tube condenser was also generated. Taking the equipment of a pharmaceutical factory as an example, the heat exchanger model was first calibrated; then, the global model was used to study the effects of different air-side and condenser-side waste heat conditions on the heating performance of the system. According to the simulation results, the coefficient of performance (COP) reached a maximum value of 4.5 under the optimum working conditions. The deviation between the COP simulation data and experimental values was less than 8.28%, and the simulation time was less than 8 min. These metrics indicate that the model is reasonable and efficient.

A novel testing and theoretical approach for air-source heat-pump water heater with flash tank vapor-injection

Air-source heat-pump (ASHP) water heater provides an important low-carbon solution for domestic water heating, which regains interest in the context of pursuing global carbon neutrality. In this study, a novel testing and theoretical approach is proposed for ASHP water heater with flash tank vapor-injection by retrofitting the testing methods of state points. Five flash tank vapor-injections with different temperature levels are tested and verified the testing method for ASHP water heater. On basis of the tested compressor suction temperature, exhaust temperature, electronic expansion valve pre-throttle temperature, electronic expansion valve throttling temperature and vapor-injection temperature, a thermodynamic analysis is conducted to determine the system performances. The relationship between the vapor-injection pressure ratio, system heating capacity and COP is identified by adjusting the expansion valve opening degree of the system. The results show that the COP of the system firstly decreases and subsequently increases with the increase in vapor-injection pressure ratio, and the minimum COP appears when the vapor-injection pressure ratio is 0.3. This study provides new insights into the advanced ASHP water heater.

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.

Design and energy flexibility analysis for building integrated photovoltaics-heat pump combinations in a house

This paper considers three configurations of air source heat pumps and building-integrated photovoltaic (BIPV) systems in a solar house. This study evaluates energy efficiency and flexibility when interacting with a smart grid. The reference case is a 5 kW BIPV system on the roof with a separate air-source heat pump water heater (HPWH). In the two more novel options we have: first, a 5 kW BIPV/thermal (BIPV/T) roof system where the heated air from the BIPV/T system is ducted to the air source of the HPWH, which also contains integrated thermal storage (hot water); in the second case we have an attached solarium with a 5 kW semi-transparent photovoltaic façade, with the solar heated air in the solarium connected to the air source of the HPWH with integrated water thermal storage. The three cases are modelled with an explicit finite difference thermal network model, and energy performance is determined and compared over a typical heating season in Montreal, Canada. The energy flexibility potential of the options is compared for different scenarios, such as heating the thermal water storage during the daytime (e.g. using the solar heat in the novel options) and using it for space heating during the time that the grid is under stress (and may have price incentives). Results show that the second novel case utilizing the solarium air as the source of the heat pump resulted in an over 80% energy use reduction relative to the reference. In comparison, the BIPV/T configuration had around 20% reduction compared to the reference case. The tank volume and solarium size had the highest impact on the flexibility of the system. Optimal sizes for the tank volume were typically between 300 and 600 L for the house with a floor area of 116 m2.

Using statistical tests to compare the coefficient of performance of air source heat pump water heaters

The study compared the coefficient of performance (COP) of two residential types of air source heat pump (ASHP) water heaters using statistical tests. The COPs were determined from the controlled volume of hot water (150, 50 and 100 L) drawn off from each tank at different time of use (morning, afternoon and evening) periods during summer and winter. Power meters, flow meters, and temperature sensors were installed on both types of ASHP water heater to measure the data needed to determine the COPs. The results showed that the mean COPs of the split and integrated type ASHP water heaters were 2.965 and 2.652 for summer and 2.657 and 2.202 for winter. In addition, the p-values of the groups COPs for the split and integrated type ASHP water heaters during winter and summer were 7.09 x 10-24 and 1.01 x 10-11, based on the one-way ANOVA and the Kruskal-Wallis tests. It can be concluded that, despite the year-round performance of both the split and integrated type ASHP water heaters, there is a significant difference in COP at 1% significance level among the four groups. Furthermore, both statistical tests confirmed these outcomes in the comparisons of the mean COPs among the four groups based on the multiple comparison algorithm.

Performance analysis on parallel condensing air-source heat pump water heater system

Based on the technology of heat pumps, a parallel condensing air-source heat pumps water heater systems providing domestic hot water is proposed to improve the energy efficiency of common ASHPWH system. Considering the climate in southern China, two operating modes of system are designed to adapt to the different water temperature conditions. the simulation model was built to analyzed the factors affecting system performance. Taking Changsha, China as an example, the COP and the energy consumption of system in each month are analyzed. We can find that the parallel condensing air-sources heat pumps water heater system can effectively disperse the total heating quantity by steps heating; the average COP in summer mode increases from 5.42 to 6.44, approaching 19.00%; the average COP in winter mode increases from 4.05 to 5.45, approaching 34.68%. The system has excellent energy-saving effect especially in winter.

The Impact of an Air to Water Heat Pump in the Residence of a University Campus

The implementation of an energy efficiency intervention in the students’ residence of the university campus may lead to a reduction in the energy consumed and electricity cost. The study focused on retrofitting a 1000 L, 12 kW boiler, with a 4.0 kW Air Source Heat Pump (ASHP) unit. A data acquisition system was built and deployed, to monitor the baseline performance of the electric boiler and the actual performance of the installed ASHP water heater (which was used to retrofit the electric boiler during the assessment period). The results show an annual electrical energy saving of 34805.94 kWh and load factor reduction of 0.124 due to the replacement of the electric boiler with the ASHP unit. The payback period of the ASHP system was 1.7 years, using the method of net present value of money. Wilcoxon rank sum test was employed to compare both the daily volume of water and energy consumed by the electric boiler and the ASHP water heater to test if their difference was of any significance. We concluded that, there exists a significant difference in the average daily energy consumed by the boiler and the ASHP water heater in both summer and winter season with the utilization of the Wilcoxon rank sum test. We could conclude that, a rollout of the ASHP units to retrofit the existing electric boiler in the students’ residence in the University campus is economically viable and calls for such an intervention is imperative.

Investigating heat transfer and frosting performance of air source heat pumps with the impact of particulate fouling

Due to superior energy efficiency and emission-reducing potential, air source heat pumps (ASHPs) have been widely used in space heating and domestic water heating. However, the heating seasons usually coincides with the serious air pollution, hence particulate fouling appears on ASHPs' outdoor coils repeatedly, despite being de-fouled periodically. In this study, four levels of fouling, namely none fouling (0 g/4 m2), mild fouling (15 g/4 m2), moderate fouling (40 g/4 m2) and severe fouling (70 g/4 m2), were concretized on a finned tube heat exchanger successively, followed by experimentally investigating the negative effects of fouling on the heat transfer performance of an air source heat pump water heater (ASHPWH). Results demonstrated that the effects of fouling on the performance of ASHPs in heating mode would be exacerbated with decreased air temperature. In addition, the frosting performances at different levels of fouling were evaluated quantitatively, by depicting the frosting evaluation plots. The frosting evaluating plots suggested that fouling would expand the air temperature and relative humidity (RH) ranges where frosting appears, and would hasten frosting process. As verification, signal factor experiments were carried at different levels of fouling. The experimental results confirmed that the increase of fouling level would result in faster frosting process, but smaller final frost accumulation mass.

Model predictive control for the operation of a transcritical CO2 air source heat pump water heater

This paper presents a model predictive control (MPC) approach to optimize the operation of a transcritical CO2 air source heat pump (ASHP) water heater. Past findings have revealed that the heat rejection pressure is the key factor affecting the system performance of the ASHP. However, the currently used PI controls based on correlations and other control strategies reported in the literature have some limitations. In this paper, we applied the MPC strategy in an ASHP water heater to optimize its operational performance in real-time. Different from the current control strategy aiming to search for the optimal heat rejection pressure, MPC predicts the future operation condition and calculates the optimal inputs based on the control-oriented model and objective function. A high-fidelity physical model is first built in Dymola to produce the operational data, then a data-driven control-oriented model for MPC is derived from the data source via a dynamical system identification. MPC is designed to maximize the coefficient of performance of the ASHP water heater under the state-space equations and constraints. The evaluation is carried out under several cases including fixed ambient temperature, realistic ambient temperature, and variable water outlet temperature. The simulation results prove the control effectiveness of MPC.

Observational evaluation of outdoor cooling potential of air-source heat pump water heaters

Heat pump water heaters are highly efficient hot water supply systems that effectively utilize the heat of outdoor air via heat pump technology. Many studies have been conducted to optimize the design and operation of heat pump water heaters from the perspective of climate change mitigation. Air-source heat pump water heaters, which absorb heat from the outdoor air and emit cold exhaust, can also be expected to alleviate the urban heat island effect; however, this has not been studied extensively. To estimate the impact of cold exhaust on building-scale climate, we conducted a multipoint measurement of the outdoor thermal environment around a low-rise apartment building equipped with air-source heat pump water heaters, in both summer and winter. Observations showed a substantial cooling effect that decreased air temperatures by 1 °C within the site boundary on summer nights when multiple heat pump water heaters operated concurrently. The analysis revealed that the sensitivity of the ambient temperatures to cold exhaust depends strongly on local atmospheric conditions. The most influential factor was the wind direction: the sensitivity increased significantly when the exhaust outlet location was at the lee side of the building. Naturally, the wind speed also affected the sensitivity, which tended to be higher when the wind speed was lower. The convective stability near the ground surface, however, showed no significant influence over the sensitivity.

Investigating the Suitability of a Heat Pump Water-Heater as a Method to Reduce Agricultural Emissions in Dairy Farms

The performance of an air-source heat pump water-heater (ASHPWH) system manufactured by Kronoterm was benchmarked in this study for the application of dairy farming in Ireland. The COP of the system was calculated to be 2.27 under normal operating conditions. The device was able to supply water at 80 °C, however a full tank at this temperature was not achieved or deemed necessary for the dairy application. Litres per kWh was used as a performance metric for the device and the usable water per unit of energy for the system was found to be 397 L when using both electric heaters and 220 L when using just the top heater both in conjunction with the heat pump. The performance of the heat pump system in terms of its cost to run and efficiency was also compared with five other water heaters. The heat pump is seen to be very efficient, however due to the carbon intensity of the Irish grid electricity and high water temperatures required, the solar water heater with gas backup was found to be the best performing under energy efficiency and carbon emissions per litre of usable water. In conclusion, although the heat pump was not the best-performing system under these metrics, the cost and complexity of the solar-gas system may be a deterrent for dairy farmers and for this reason, the heat pump is considered a cost-effective, efficient and viable option for dairy farmers trying to reduce their carbon footprint and energy bills.

Linear Regression Analysis and Techno-Economic Viability of an Air Source Heat Pump Water Heater in a Residence at a University Campus

This study quantifies the potential of a 4.0 kW air source heat pump (ASHP) unit retrofitted to a 12.0 kW, 1000 L electric boiler coupled to a 1000 L storage tank. A data acquisition system was built to monitor the performance of the electric boiler and the ASHP water heater. The annual electrical energy saving and the load factor reduction from the electric boiler because of the ASHP unit retrofit was 34,805.94 kWh and 0.124. The net present value payback period of the ASHP system was 1.60 years. A Wilcoxon rank sum test was employed to compare both the volumes of hot water and electrical energy consumed by the two systems. Linear regression models of the daily volumes of hot water and electrical energy consumed by both systems were established. The results should be of great value to the management of universities that are considering energy-efficient interventions with a significant return on investment.

Experimental Investigation on an Ultra-High Temperature Air Source Heat Pump Water Heater

Abstract To generate boiling water beyond 100 °C via heat pump technology, the prototype of an ultra-high temperature air source heat pump water heater (ASHPWH) based on a single-stage compression cycle of R134a was established, and an experimental investigation on it was conducted under an environment temperature of 25 °C. Then, thermodynamic analyses were carried out on the basis of the experimental results, especially when the prototype produced 95.9 and 100.3 °C water. The experimental and analytical results indicate that water beyond 100 °C was achieved through the prototype. When producing 100.3 °C water, the discharge temperature and compression ratio of the compressor of the prototype are only 108.4 °C and 4.07, respectively, which are in moderate levels. Correspondingly, the work input of the compressor is 0.622 kW, the heating capacity is 2.786 kW, and the heating coefficient of performance is 4.48. In addition, when producing 95.9 and 100.3 °C water, the system exergy efficiencies of the prototype are 50.76% and 49.73%, which are larger than those of the existing ASHPWHs, demonstrating that dividing the condensing process into two parts of high-grade exergy and low-grade exergy and utilizing them separately is effective. That is the essential reason of generating boiling water beyond 100 °C as expected only through the single-stage compression cycle.

Evaluation of the coefficient of performance of an air source heat pump unit and an air to water heat pump

Air source heat pump (ASHP) water heaters are efficient devices for sanitary hot water heating. The coefficient of performance (COP) of the air to water heat pump (AWHP) is constantly lower than that of the corresponding ASHP unit. The study focused on determining the COP of both the ASHP unit and the AWHP. This was achieved by the implementation of both experimental and simulation methods, with the help of a data acquisition system and the REFPROP software. The system comprised of a 1.2 kW split type ASHP unit and a 150 L high pressure geyser. A power meter, flow meters, temperature sensors, pressure sensors, ambient temperature and relative humidity sensor were installed at precise locations on the split type AWHP. Controlled volumes of 150, 50 and 100 L were drawn off from the AWHP during the morning, afternoon and evening for a year. The average COP for the summer and winter, in terms of the input electrical and output thermal energies of the AWHP were 3.02 and 2.30. The COPs of the ASHP unit, in terms of the change in the enthalpies of the refrigerant at the inlet and the outlet of the condenser and the evaporator, were 3.52 and 2.65 respectively. The study showed that the difference between the COP of the ASHP unit and that of the AWHP could be ascribed to the electrical energy consumed by the fan and the water circulation pump during the vapour compression refrigeration cycles. The work provides an energy optimisation opportunity to the manufacturers of this technology, helping to enhance the efficiency and COP of ASHP water heaters.
 Highlights
 
 The COPt of the ASHP unit was higher than the COPe of the AWHP.
 The COPe of the AWHP was the ratio of the input electrical energy consumed and the output thermal energy gained by the stored water.
 The COPt of the ASHP unit was enthalpies-dependent and a function of inlet and outlet enthalpies of the evaporator and condenser.
 The inlet and outlet refrigerant temperatures profiles of the condenser confirmed thermal energy dissipation.

Failure analysis of micro-channel condenser of air source heat pump water heater

Air source heat pump water heaters (ASHPWH) fail during service, due to the leakage of the micro-channel condensers. Failure analysis of the condenser was carried out through a series of characterization methods, including energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Results reveal that the failure of these condensers results from the corrosion of aluminium alloy in acidic aqueous medium, which is constructed by external introduced water and the decomposition products of refrigerant during service. Additionally, the deterioration of refrigeration oil, the abnormal operation of compressor, and the internal defects of condensers also contribute to the failure of the condensers. Finally, a series of countermeasures are proposed for avoiding the corrosion failure of such condensers.

Performance evaluation and multi-objective optimization of a low-temperature CO2 heat pump water heater based on artificial neural network and new economic analysis

For the advantages of high efficiency and low impact to the environment, CO2 air source heat pump water heater (ASHPWH) is applied to produce domestic water, which also reveals good potential in cold regions. In order to boost the system performance and practicability under low ambient temperature, optimization for CO2 ASHPWH is conducted using non-dominated sorting genetic algorithm (NSGA-II). A validated artificial neural network (ANN) predicts energy parameters for the optimization. And an economic model provides economic and environmental parameters, which considers the influence of housing price, tank volume, and on/off-peak electricity price, rarely taken into account in published studies. Then the optimizing progress is conducted under −20 °C ambient temperature and 9–65 °C water temperature, in which four optimized variables are selected: gas cooler outlet temperature (Tgc), heat rejection pressure (Pgc), compressor displacement (qvh) and water tank volume (Vwt). The final solution of Tgc = 15 °C, Pgc = 8294.1 kPa, Vwt = 0.3647 m3, qvh = 401.33 mL/s results in two objectives (CO2 emission and total annual cost) of 8599.4 kg and 1626.9 $/year, revealing advantages both in energy and economy. It is noteworthy that the cost of the space occupied by system is the fourth important factor in capital cost. These results lay solid foundation for further studies and system application.

Performance assessment of an air source heat pump water heater from exergy aspect

In this study, thermodynamic inefficiencies of the components and the whole system are quantified by exergy destruction. The performance of an air source heat pump water heater (ASHPWH) is examined experimentally under varying ambient conditions from the exergy destruction aspect. The exergy destruction is calculated for each moment through the entire heating period with regard to collected temperature and pressure data under various ambient conditions. The exergy destructions are examined continuously in a sequence from the beginning to the end of the water heating process. This approximation allows the designer and the manufacturer to see the progress of exergy destruction through the entire heating period to evaluate the performance of every component for avoiding irreversibilities and improving the ASHPWH. During these observations, three individual zones are observed in which the exergy destruction characteristics have changed. Furthermore, the impact of the ambient conditions on these zones is identified either. The thermodynamic properties and exergy destruction values are computed for all data collected by a MATLAB code. The results would be helpful to picture out the true evolution of the exergy destruction from component aspect either solely or combined effect of them during the water heating period.

Performance analysis and comprehensive comparison between CO2 and CO2/ethane azeotropy mixture as a refrigerant used in single-stage and two-stage vapor compression transcritical cycles

Abstract In this study, the performance of a two-stage compression transcritical cycle with an expander (TCE) and a conventional single-stage compression transcritical cycle with a throttling valve (SCT) using pure CO2 and CO2/ethane (0.78/0.22, mass fraction) azeotropy mixture as a refrigerant in an air-source heat pump water heater (ASHPWH) system is investigated. From the point of the first and second laws of thermodynamics, the theoretical analysis of cycle characteristics based on the two systems with both refrigerants are carried out and compared in detail. The results show that the COP of TCE cycle with CO2 and CO2/ ethane are at least 57.4% and 81.6% higher than those of the SCT cycle under the studied conditions. The main attractiveness of the CO2/ethane azeotropy refrigerant is connected with its lower discharge temperature compared with the pure CO2, which can prolong the compressor lifetime. In addition, the CO2/ethane azeotropy refrigerant can obviously reduce the evaporation temperature, which is conducive to the ASHPWH system in cold region.

The feasibility of solar thermal-air source heat pump water heaters in renewable energy shortage regions

Abstracts Despite the extensive literature on solar-assisted heat pump systems, including optimizations of systems configurations and operation strategies, there are few works of literature on the potential of applying solar thermal-air source heat pump water heaters (SA-WH) in solar shortage regions and none that compares alternative system improvements for the same boundary conditions and with the same level of comfort to the end-user. This work examines the feasibility of the SA-WH in solar shortage regions from a performance perspective and compares its performance to the solar water heater (SWH) and the air-source heat pump water heater (ASHP-WH). Experimental studies were carried out from March 22, 2014 to April 21, 2014, which covered spring-summer transition seasons of these particular areas. The parameter η was proposed and a reference ηbase of 2.4 was defined to rate the performance of these three systems. The results showed nearly 80% of working conditions of the SA-WH had a η which was higher than 2.4, wherein 70% were above 3. Furthermore, this study presents a regression model verified by measured data to predict the potential performance of the SA-WH in renewable energy shortage regions. This prediction model provides an optimal method of fully applying solar energy for the domestic hot water (DHW) and even heating by targeting better system performance. Then, three solar energy shortage cites of China were used as cases to illustrate the use of the prediction model. To meet the DHW demand in transitional seasons of these regions, the air-source heat pump of the SA-WH is suggested to be switched on 1 h before using when the average ambient temperature is below 20 °C; and be switched on at the hour with the highest temperature of the day when the average ambient temperature is above 20 °C and operate 1 h.