Air Source Heat Pump Unit Research Articles

An experimental study on time-based start defrosting control strategy optimization for an air source heat pump unit with frost evenly distributed and melted frost locally drained

Defrosting operation of an air source heat pump unit is always started with a pre-set time, corresponding to a fixed mass of frost accumulation. The exact defrosting initiation time is hard to be quantitatively given due to dynamic and uneven frosting conditions, and thus resulting in series of mal-defrosting phenomena. For an air source heat pump unit with a multi-circuit outdoor coil, when the melted frost locally drained during defrosting was considered, the optimization of start defrosting control strategy becomes more complicated. Here, this fundamental problem was experimentally investigated, with frost nearly evenly distributed on the surface of the outdoor coil at the start of defrosting. Defrosting performance of the experimental air source heat pump unit at different frost accumulations with the melted frost local drainage were then comparatively analyzed. These measured and calculated physical parameters include the temperature of tube surface and melted frost, compressor suction and discharge pressures and their difference, thermal energy taken from indoor air and electricity inputs on compressor and fans during defrosting, etc. Results showed that, the defrosting efficiency reached its peak at 51.80% when frost accumulation was at 933 g. Thus, the time-based start defrosting control strategy was demonstrated to be optimized with this method. Contributions of this study could be used for adjusting the control strategy of air source heat pump units, which are also benefit for energy saving in their industry and residential applications.

Application and Analysis of Air Source Heat Pumps in Countryside Home Heating with Substituting Electricity for Coal

In the project of “Substituting Electricity for Coal” in northern rural areas, more and more air source heat pump units are being used for home heating. When the large-scale units are connected to rural distribution networks, the power quality problems should be considered because of the voltage sag and the current harmonics caused by these units. The characteristics of the air source heat pump units are compared with those of other domestic electric heating radiators. The efficiency indices, the operating circle and the types of air source heat pumps are introduced. The starting and operational characteristics of different air source heat pump units are given and compared. The power quality problems, the voltage sag and current harmonics are analyzed and discussed based on the parameters and the actual measurement data of the units in the rural distribution networks. Some solutions are presented for the problems caused by the units.

Analysis on impacts of domestic air source heat pumps to distribution networks in coal-to-electricity projects

As an air source heat pump unit can be used for building heating at a high operating efficiency, it is used more and more widely in the coal-to-electricity project. However, due to the voltage sag, the current surge and the harmonics caused by its using, the power quality problems should be considered with the large-scale units of the air source heat pump being connected to rural distribution networks. The working principles and the types of the domestic unit are introduced. The power quality problems are discussed with the parameters and the actual measurement data of the units in the distribution networks. The starting and operational characteristics of different air source heat pumps are compared. The voltage sags in a rural distribution network are simulated and analyzed during the starting process of the air source heat pump units. The corresponding solution measures are given for the problems caused by the large-scale units of the domestic air source heat pump.

Simple index for onsite operation management of ground source heat pump systems in cooling-dominant regions

Abstract In ground source heat pump (GSHP) systems, large imbalances between cooling and heating loads cause a rise or decline in ground temperature because of thermal interference between multiple ground heat exchangers (GHEs). To evaluate annual changes in ground temperature, we applied a variable temperature penalty, which was simply obtained using measured data without computer simulation. First, we examined measured data for 3 years after completion of a hybrid GSHP system that had 70 borehole-type GHEs, combined with an air source heat pump unit. In the hybrid system, the GSHP showed high efficiency (coefficient of performance > 5.0) throughout the year and had a variable contribution between years with regard to cooling/heating output and time of operation. The amount of heat rejected to the ground by cooling reached ∼4.8 times that of heat extracted from the ground by heating after 3 years of operation. This imbalance produced ground temperature increases of ∼3 °C in an internal borehole. The variable temperature penalty reproduced the measured temperature increase, suggesting that the index is appropriate for assessing long-term ground temperature changes in the operation phase. This simple index allows operational improvement onsite and will aid the sustainable operation of GSHP systems in cooling-dominant regions.

Operating performances of a space heating ASHP unit with a foul outdoor coil following a prolonged cooling operation

Fouling and frosting of the outdoor coil in an air source heat pump (ASHP) unit are common but undesirable operating problems. At an increased level of air pollution, fouling of an outdoor coil becomes increasingly severe during its space cooling operation in summer and may potentially impact its frosting operation in winter. This paper reports on a field study on the operating performances of a space heating ASHP unit with a foul outdoor coil as a result of a prolonged cooling operation. Firstly, a detailed description of the field ASHP unit with the foul outdoor coil and test cases is given. Secondly, field study results for the test cases and their analyses are presented. The results demonstrate that fouling of an outdoor coil during its space cooling operation in summer can significantly impact its frosting performance during its space heating operation in winter.

Techno-economic analysis on frosting/defrosting operations for an air source heat pump unit with an optimized multi-circuit outdoor coil

Air source heat pump (ASHP) units are used in applications around the world. After optimizing the multi-circuit outdoor coil by installing water collecting trays between circuits and adjusting the refrigerant distribution by using valves located at each circuit, system frosting/defrosting operation performances could be effectively improved. Before practical industry-scale application of trays and valves, their economic performances should be evaluated. However, in current literature, techno-economic analysis on operation performance of ASHP units is rare, which limits the development of innovative technologies. Therefore, a techno-economic analysis on frosting/defrosting operations is carried out in this study. Firstly, the frosting/defrosting experiments are introduced, followed by a series of assumptions and calculations. Then, the economic analysis results are provided in detail. Compared with a traditional ASHP unit, the total running costs of the modified unit in the heating season could save as much as 3681.75 CNY, or 10.33%, and the total cost decreased by 3516.75 CNY, or 4.67%, over 15 years of service life. The payback period of the additional initial cost is less than 8 months. Contribution of this work plays an important role in the evaluation and application of new technologies in the HVAC field.

Application Analysis of Distributed Electric Heating System

The population of the world is increasing daily and people desire a high standard of life. As a result ， distributed electric heating system, a air source heat pump and solar thermal combined system, will be widely used for space heating as an energy efficient and environment friendly way. In this paper distributed electric heating system is designed. The system can produce hot water for heating by recovering energy from solar energy and air source. Solar collector contained 50 glass vacuum tubes. And collector area is 7.7m2.. Heating capacity of air-source heat pump unit is 10.6kW. Distributed electric heating system is run for three days. the result shows that the system can meet the need of space heating.

Energy transfer procession in an air source heat pump unit during defrosting with melted frost locally drainage in its multi-circuit outdoor coil

Air source heat pump (ASHP) units are widely used in recent years, and reverse cycle defrosting becomes the most popular method to solve their undesired frosting problem. During defrosting, a transient and nonlinear heat and mass transfer procession, the metal energy stored in the indoor and outdoor coils are varying as their temperature fluctuations. On the other hand, authors have previously confirmed the effects of melted frost and metal energy storage on system defrosting performance. However, detailed energy transfer procession without melted frost influence is still not identified. This fundamental problem directly affects the development and modification of two coils in a novel ASHP unit or an existing one. Consequently, basing on frost evenly accumulated on each circuit's surface, two cases were thereby designed in this study. Experimental results show that, the heating supply of indoor air thermal energy contributed about 80% of the total energy usage for defrosting, nearly 90% of energy consumed on frost melting and ambient air heating, respectively. After the total area of outdoor coil was enlarged by 50%, the metal energy storage effect was changed from −0.44% to −3.67%. Meanwhile, defrosting efficiency was improved by 11.66%, from 47.13% to 58.79%. Contributions of this study can effectively guide the design optimization of an ASHP unit, improve occupant's thermal comfort and promote the energy saving in buildings and industry.

Techno-economic analysis on frosting and defrosting operations of an air source heat pump unit applied in a typical cold city

In recent years, air source heat pump (ASHP) units have found applications worldwide due to their advantages. For an ASHP unit with a multi-circuit outdoor coil, when the refrigerant distribution adjusted with the valves, system defrosting efficiency could be optimized. Meanwhile, adjusting the refrigerant distribution by using valves located at each circuit could improve the frosting evenness value, and system coefficient of performance and the defrosting efficiency are thereby both optimized. However, in open literature, no tech-economic analysis work on frosting/defrosting performances of ASHP units is reported, which limits the development of innovation technologies in this field. Therefore, a techno-economic analysis on frosting/defrosting operations for an ASHP unit used in typical cold regions is given, basing on previous experimental work and series of assumptions. As concluded, the total running costs of the new ASHP unit could decrease as much as 5,327.99 CNY ($ 798.87), or 7.67%, and the total cost about 5,177.99 CNY ($ 776.36), or 6.68%, in 15 years’ service life, compared with a traditional one. The payback period of additional first cost of valves is less than 1 year. Conclusions of this study might provide a new analytical tool for scholars, researchers, product developers, and policy designers, and shed new light on the designing and performance optimization of ASHP units.

An experimental study on frosting and defrosting performances of a novel air source heat pump unit with a radiant-convective heating terminal

When the outdoor air temperature is low and relative humidity is high, frost will occur and accumulate on the outdoor coil of an ASHP unit, which may decrease the heating performances of the ASHP unit to a large degree. In addition, the defrosting performances of the commonly used defrosting method (reverse cycle defrosting) are poor due to the closed indoor air fan and insufficient energy stored in a conventional indoor coil. On the other hand, the conventional indoor coil based on forced convective heat transfer may cause strong draught sensation and dry eye problem and make users feel less comfortable than radiant heating terminals. Therefore, to improve the system defrosting performances and combine the merits of the conventional indoor coil and those of radiant heating terminals, a novel radiant-convective heating terminal was developed and integrated with an ASHP unit. The frosting and defrosting performances of the novel system were experimentally investigated, and the results showed that the average values of radiant panel surface temperature, outlet air temperature for the novel heating terminal, and system COP were 36.6 °C, 29.4 °C, and 2.78, respectively, during a 66-min frosting period (air temperature is 2.0 °C and relative humidity 84.0%). In addition, the ratio of the radiant heating capacity to natural convective heating capacity to forced convective heating capacity remained at around 1.5:1.0:7.4 during the whole frosting period. Furthermore, the experimental results demonstrated that the novel heating terminal could provide sufficient energy for the defrosting, and some energy could be provided for space heating through radiant and natural convective heat transfer during defrosting. The defrosting and resuming heating periods were 105 and 65 s, respectively.

An experimental study of frost distribution and growth on finned tube heat exchangers used in air source heat pump units

Many studies have been carried out on frost distribution and growth and their impacts on the performances of finned tube heat exchangers (FTHXs), the most commonly used structure for the outdoor coils of air source heat pump units. However, in almost all the studies, ethylene glycol water solution was used as refrigerant, which made the investigating of frost distribution and growth difficult due to the uneven temperature distribution along the tube length. Furthermore, quantitative experimental data for the frost distribution and growth on FTHXs have been insufficient. Therefore, in this paper, a quantitative experimental study using R410A as refrigerant on frost distribution and growth on two FTHXs with different fin pitches is reported. The study results demonstrated that the frost accumulated on an FTHX could be divided into two parts, one accumulated on the edge of windward fins and the other on the surfaces of fins and tubes. Furthermore, the ratios of the frost mass accumulated on the edge of windward fins to that on the entire FTHX surface were 13.7% (60 min frosting duration) and 12.5% (120 min frosting duration) for the two FTHXs, respectively. The frost thickness and average frost densities in these two parts were also evaluated and compared.

Energy transfer procession in an air source heat pump unit during defrosting

Air source heat pump units have found their wide applications in recent decades due to their high efficiency and low environmental pollution. To solve their undesired frosting problem, reverse cycle defrosting is always employed. As a transient and nonlinear heat and mass transfer procession, defrosting performance directly affects the occupants’ thermal comfort. During defrosting, the metal energy storage values of indoor and outdoor coils are varied as their temperature fluctuations. It is therefore necessary to investigate the energy transfer procession in an air source heat pump unit and the effect of metal energy storage during defrosting. However, scarce of attentions were paid to this fundamental problem. In this study, two experimental cases with two-working-circuit and three-working-circuit outdoor coils were conducted basing on frost evenly accumulated on their surfaces. After four types of energy supply and five types of energy consumption during defrosting were calculated, a qualitative and quantitative evaluation on the metal energy storage effect was then given. As concluded, after the outdoor coil enlarged 50%, the metal energy storage effect can be changed from positive (0.33%) to negative (−2.18%). The percentages of energy consumed on melting frost and vaporizing retained water were both increased. Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%. Contributions of this study can effectively guide the design optimization of indoor and outdoor coils and promote the energy saving for air source heat pump units.

Termination Control Temperature Study for an Air Source Heat Pump Unit During Its Reverse Cycle Defrosting

Abstract For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, a reverse cycle defrosting (RCD) operation is always terminated when the tube surface temperature at the exit of the lowest circuit reaches a pre-set value. It is obviously that when the pre-set temperature is higher, the defrosting duration would be prolonged. Not only more energy would be consumed on heating cold ambient air, but also the occupants’ thermal comfort adversely affected. However, if the pre-set temperature is lower, more residual water would be left on the downside surface of fin in an outdoor coil, which degrades the system performance during the next frosting/defrosting cycle. In order to find a suitable DTT or its range, in this paper, an experimental investigation on RCD operation for an ASHP unit with a multi-circuit outdoor coil was conducted and reported. As concluded, DTT is suitable at the range of 20-25 o C, around 22 o C.

Improving the frosting and defrosting performance of air source heat pump units: review and outlook

ABSTRACTWhen air source heat pump (ASHP) units operate in heating mode at low temperatures in high humidity environments, frost forms and accumulates on the surface of its outdoor coils. This frost layer adversely degrades the operating efficiency of ASHP units rapidly, and can even result in sudden shutdown. Due to this operational problem, many researchers have conducted studies on frost-retardation measures. Since the frost that is present on the ASHP unit after shutdown has to be removed, defrosting becomes necessary. Among several reported defrosting methods, such as compressor shutdown defrosting, electric heating defrosting, hot water spray defrosting and hot gas bypass defrosting, the most popular method is reverse cycle defrosting. In addition, defrosting initiation and termination control play an important role in the frosting and defrosting cycle. Due to a scarcity of reviews of the literature on improving the frosting and defrosting performance of ASHP units, this paper provides a comprehensiv...

Experimental investigation on an air source heat pump unit with a three-circuit outdoor coil for its reverse cycle defrosting termination temperature

Air source heat pump units could efficiently recover low grade waste heat from ambient air for indoor air heating or hot water supplying, which makes them widely applied in recent decades. For a vertically installed multi-circuit outdoor coil, a reverse cycle defrosting operation is always used to solve its frosting problem at high humidity and cold climate. Reverse cycle defrosting operation is terminated when the tube surface temperature at exit of the lowermost circuit reaching a pre-set value. It is obviously that when the pre-set temperature is higher or lower, the defrosting duration would be prolonged or more residual water left, respectively. Both of them result in potential energy waste for an air source heat pump unit, or even adversely degrade the indoor thermal comfort. However, as reported, a wide range of 10–35°C was used as the pre-set defrosting termination temperature, without a fixed value or range given. To save energy for an air source heat pump unit, in this paper, an experimental methodology was firstly presented. Then, an air source heat pump unit with three-circuit outdoor coil was specially selected, and experimental investigation conducted. Finally, the defrosting termination temperature was concluded suitable at 20–25°C, around 22°C for this study. This methodology makes contributions to the control strategy optimization and energy saving for air source heat pump units.

Experimental investigations and operational performance analysis on an air source heat pump unit for domestic hot water supply in South China

Air source heat pump systems have been widely used for domestic hot water and residential heating in southern China because they are much more efficient than traditional heating technologies such as electric heating and coal-fired boiler heating. Experimental investigations about an air source heat pump water heater (ASHPWH) operational performance were conducted in an environmental chamber with the variation of dry bulb temperature and wet bulb temperature, in order to accurately reflect and evaluate the actual operational performance of air source heat pump system for hot water supply. The results show that with the increase of water temperature from 30℃ to 54.2℃ the COP decreased, it respectively decreased 43%,48%,51%,64%,48% in the environment with ambient temperature at -7℃, 2℃, 7℃, 20℃, 30℃, and the average COP decreases from 4.0 to 1.96 while the elapsed time increases from 85 minutes to 295 minutes as the ambient air temperature drops. Annual operational performance was analysed in three typical city in south China. The findings indicate that annual performance factor (APF) of the ASHPWH in typical meteorological conditions of Guangzhou reached 3.35, while APF in Kumming and Nanjing decreases of 11.3% and 12.2% respectively.

Experimental investigation on reverse cycle defrosting performance improvement for an ASHP unit by evenly adjusting the refrigerant distribution in its outdoor coil

For an air source heat pump (ASHP) unit, reverse cycle defrosting operations are always conducted to solve the undesired frosting problem. Previous studies verified that, uneven refrigerant distribution (URD) and melted frost downwards flowing (MFDF) along the vertically installed multi-circuit outdoor coil would degrade system defrosting performance. However, it is hardly possible to separate the URD and MFDF effects in a practical multi-circuit outdoor coil. In addition, it is a fundamental coupled problem of uneven heat and mass transfer. Consequently, to investigate the coupled effects of MFDF and URD on system defrosting performance, a sequential experimental study is carried out and reported in this paper. Firstly, a special testing facility and tailor-made three-circuit outdoor coil were introduced. This is followed by two experimental cases designed, with frost evenly accumulated on the surface of each circuit by adjusting the stop valves’ opening degrees. After the experimental results compared, basing on refrigerant evenly and unevenly distributed into each circuit, energy analysis was also given. Finally, as concluded, the coupled effects were negative on system energy performance, and defrosting efficiency increased by 7.4%, from 40.5% to 47.9%, when the refrigerant’s distribution changed from uneven to even. This study makes contributions on the optimization of system control strategies.

Numerical study on the operating performances of a novel frost-free air-source heat pump unit using three different types of refrigerant

Air-source heat pump (ASHP) is a promising device using in residential buildings because of its energy-savings. Besides, R134a and R407C as environmental friendly refrigerant have advantage to apply into heat pump system. In this study, an investigation of a novel frost-free ASHP system, integrated with dehumidification and thermal energy storage, working with R134a and R407C as an R22 alternative, was presented. A mathematical model of the system is constructed and verified by comparison with experimental data that shows the measured results are in good accordance with the numerical ones. According to the mathematical model, the dynamic performance of the system is characterized by dehumidification and regeneration efficiency, suction and discharge pressure, COP and so on at ambient temperature of 0°C and RH (relatively humidity) of 85%. The results show that the time for keeping the evaporator frost-free operation for R22, R407C and R134a are 29, 34 and 35minutes, respectively. In addition, at the end of working time, the discharge pressure of compressor for R134a is 29 and 32% lower than that for R22 and R407C. What is more, at the given ambient temperature of −10°C and RH of 85%, the average COP for R134a is 3.3 and 8.6%, respectively, which is higher than that for R22 and R407C.

Design of a heat pump water heater performance monitoring system

Purpose This paper aims to show that by using air source heat pump (ASHP) water heater in the residential sector, the energy consumption from sanitary hot water production can be reduced by more than 50 per cent. Hence, this study quantitatively and qualitatively confirms that domestic ASHP water heater is a renewable and energy efficient device for sanitary hot water production. Design/methodology/approach Design and building of a data acquisition system comprises a data logger, power meters, flow meters, temperature sensors, ambient and relative humidity sensor and an electronic input pulse adapter to monitor the ASHP water heater performance. All the sensors are accommodated by the U30-NRC data logger. The temperature sensors are installed on the inlet pipe containing a flow meter and the outlet pipe of the ASHP unit, the vicinity of both evaporator and expel cold air. An additional temperature sensor and a flow meter that cater for hot water drawn off measurements are incorporated into the data acquisition system (DAS). Findings The result from a specific monitoring split type ASHP water heater gives an average daily coefficient of performance (COP) of 2.36 and the total electrical energy of 4.15 kWh, and volume of hot water drawn off was 273 L. These results were influenced by ambient temperature and relative humidity. Research limitations/implications The cost involved in purchasing the entire sensors and data logger limits the number and categories of ASHP water heaters whose performance were going to be monitored. Pressure sensors were excluded in the data acquisition system. Practical implications The data acquisition system can easily be designed and the logger can also be easily programed. Hence, no high technical or computer skills are needed to install the DAS and to be able to read out the results. Social implications Hence, the data acquisition system can be installed on the entire domestic Eskom roll out air source heat pump water heaters to effectively determine the coefficient of performance and demand reductions. Originality/value This DAS is the first of its kind to be built in South Africa to be used to determine the performance of an ASHP water heater with high accuracy and precision. DAS is also robust.

Investigation on the ultrasonic propagation mechanism and its application on air-source heat pump defrosting

Frosting deposited on the outdoor coil of air-source heat pump (ASHP) units deteriorates the operational performance and energy efficiency. Therefore, periodic defrosting is necessary. First, the dispersion curves for the propagation mechanism of an ultrasonic guided wave in the evaporator are determined through numerical calculation. In addition, the shear stress and vibration characteristics under ultrasonic excitation are analysed using finite element method (FEM). Finally, the vibration amplitude and defrosting performance of ultrasonic vibration is analysed. The numerical calculation results indicate that three guided wave modes exist in the evaporator, including both A0 and S0 modes of the Lamb wave and SH0 mode of the SH wave, with the optimal defrosting mode being S0 of the Lamb wave. The FEM results show that the vibrational shapes of S0 mode and longitudinal mode clearly exists in the fin and tube, the torsional and flexural modes also exist in the tube, and the FEM results are consistent with the numerical calculation results. The impedance analysis and laser vibrometer results indicate that the resonance frequency shifting, electro-acoustic converting efficiency and vibration energy decrease is due to increasing external load. The ultrasonic defrosting experimental results indicate that ultrasonic vibration can suppress frost deposit on the fin surface.