Reverse Cycle Defrosting Research Articles

Thermal modeling and operating tests for a gas-engine driven heat pump working as a water heater in winter

Gas-engine driven heat pump (GEHP) working as water heater has higher energy utilization efficiency and can reduce more environmental pollution than gas-fired (oil-fired, coal-fired) boiler and electric motor driven heat pump (EHP). In the present work, a thermal model of a GEHP system working as water heater was established, and the performances (heating capacity, coefficient of performance (COP) and primary energy ratio (PER) of the GEHP system were also studied both by simulation and experiment. The results showed that the heating capacity increased with the increase of the rotary speed and water flow rate, but decreased with the increase of the hot water temperature. The COP and PER increased with the increase of the water flow rate, but decreased with the increase of the rotary speed and hot water temperature. The simulation results were well agreed with experimental data. The defrost time of the GEHP system was also studied by experiment, the results showed that the defrost time of both using waste heat recovered and reverse cycle defrost (RCD) was the shortest among the three defrost methods used in this paper.

An experimental investigation on reverse-cycle defrosting performance for an air source heat pump using an electronic expansion valve

When an air source heat pump (ASHP) operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil. Frosting deteriorates the operation and energy efficiency of the ASHP and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. On the other hand, electronic expansion valves (EEVs) are commonly used in heat pump/refrigeration systems, including ASHP units, as throttle regulators of refrigerant flow.This paper reports on an experimental investigation of the performance of a reverse cycle defrost operation for a 6.5kW experimental heating capacity residential ASHP whose outdoor coil had four-parallel refrigerant circuits, and with an EEV as the refrigerant flow throttle regulator. The detailed descriptions of the experimental ASHP unit and the experimental procedures of ASHP are firstly presented. This is followed by reporting the comparative experiments of two control strategies for the EEV to investigate their effects on defrosting performance: the EEV being fully open and the EEV being regulated by a degree of refrigerant superheat (DS) controller. Experimental results revealed that when the EEV was regulated by the DS controller during defrosting, a higher defrosting efficiency and less heat wastage would be resulted in.

An experimental study on defrosting heat supplies and energy consumptions during a reverse cycle defrost operation for an air source heat pump

Abstract For a space heating air source heat pump (ASHP) unit, when its outdoor coil surface temperature is below both the air dew point temperature and the freezing point of water, frost will form on its outdoor coil surface. Frosting affects its operational performance and energy efficiency. Therefore, periodic defrosting is necessary. Currently, the most widely used standard defrosting method for ASHP units is reverse cycle defrost. The energy that should have been used for space heating is used to melt frost, vaporize the melted frost off outdoor coil surface and heat ambient air during defrosting. It is therefore necessary to study the sources of heat supplies and the end-uses of the heat supplied during a reverse cycle defrost operation. In this paper, firstly, an experimental setup is described and experimental procedures are detailed. This is followed by reporting the experimental results and the evaluation of defrosting efficiency for the experimental ASHP unit. Finally, an evaluation of defrosting heat supplies and energy consumptions during a revere cycle defrost operation for the experimental ASHP unit is presented. The experimental and evaluation results indicated that the heat supply from indoor air contributed to 71.8% of the total heat supplied for defrosting and 59.4% of the supplied energy was used for melting frost. The maximum defrosting efficiency could be up to 60.1%.

Operating performance of novel reverse-cycle defrosting method based on thermal energy storage for air source heat pump

To solve the fundamental problem of insufficient heat available during defrosting while ensuring the efficient and safe system operation for air-source heat pumps (ASHPs). A novel reverse-cycle defrosting (NRCD) method based on thermal energy storage to eliminate frost off the outdoor coil surface was developed. Comparative experiments using both the stand reverse cycle defrosting (SRCD) method and the NRCD method were carried out on an experimental ASHP unit with a nominal 2.5 kW heating capacity. The results indicate that during defrosting operation, using the NRCD method improves discharge and suction pressures by 0.24 MPa and 0.19 MPa, respectively, shortens defrosting duration by 60%, and reduces the defrosting energy consumption by 48.1% in the experimental environment, compared with those by the use of SRCD method. Therefore, using the NRCD method can shorten the defrosting duration, improve the indoor thermal comfort, and reduce the defrosting energy consumption in defrosting.

A study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an air source heat pump – Part I: Experiments

When an air source heat pump (ASHP) unit operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil which normally has multiple parallel circuits on its refrigerant side for minimized refrigerant pressure loss and enhanced heat transfer efficiency. On its airside, however, there is usually no segmentation corresponding to the number of refrigerant circuit. Frosting deteriorates the operation and energy efficiency of the ASHP unit and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. This paper, the first part of a two-part series, reports on the experimental part of a study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an experimental 6.5 kW heating capacity residential ASHP unit. Firstly the experimental ASHP unit is described and experimental procedures detailed. Secondly, the experimental results are reported. This is followed by the discussion on the effects of downwards flowing of the melted frost along a multi-circuit outdoor coil surface on defrosting performance. Finally, the evaluation of the defrosting efficiency for the experimental ASHP unit is provided. In the second part of the series, a modeling analysis on the effects of downwards flowing of the melted frost along the multi-circuit outdoor coil surface on defrosting performance of the experimental ASHP unit will be presented.

Improving reverse cycle defrosting performance of air source heat pumps using thermal storage-based refrigerant sub-cooling energy

When an air source heat pump (ASHP) unit is used for space heating at a low ambient temperature in winter, frost may be formed on its outdoor coil surface. Over time, frost accumulation on coil surface may become sufficient to restrict air passage through the coil and increase the heat transfer resistance between ambient air and coil surface, leading to performance degradation for the outdoor coil, or even the shutdown of the ASHP unit. Therefore, periodic defrosting is necessary. Currently, the most widely used standard defrosting method for an ASHP unit is reverse cycle defrosting. During a standard reverse cycle defrosting (SRCD) operation, the indoor coil in an ASHP unit actually acts as an evaporator. Because the indoor fan is usually turned off during defrosting, there is an insignificant amount of energy from indoor coil, leading to a number of associated operational problems such as a longer defrosting duration and the risk of having a lower air temperature inside a heated space during defrosting, etc. To solve the fundamental problem of insufficient heat available during defrosting while ensuring the efficient and safe system operation, a novel reverse cycle defrosting (NRCD) method which is thermal energy storage-based using sub-cooling energy of refrigerant for ASHPs has been developed. Comparative experiments using both the SRCD method and the NRCD method were carried out on an experimental ASHP unit with a nominal 2.5 kW heating capacity. Experimental results clearly suggested that when using the NRCD method, the discharge and suction pressures were increased by 0.56 and 0.12 MPa, respectively. Furthermore, the defrosting duration and heating-resumption duration were shortened by 30.8% and 25.0%, respectively, and the defrosting energy consumption was reduced by 13.9%, compared to those when using the SRCD method. Practical application: ASHPs have been widely used worldwide due to their significant energy-saving potentials. However, frost can be formed on the outdoor coil surface when an ASHP unit operates for space heating. Frost accumulation on coil surface reduces the coil’s efficiency, or causes even the shutdown of the ASHP unit. Reverse cycle defrosting is the most widely used standard defrosting method. This paper presents the reverse cycle defrosting performance for an ASHP unit with a PCM-HE acting as both a sub-cooler during heating operation and a heat source during defrosting operation, which could help to ensure the safe operation and achieve a higher defrosting efficiency of the ASHP unit.

Defrosting method adopting dual hot gas bypass for an air-to-air heat pump

A novel dual hot gas bypass defrosting (DHBD) method is developed to remove frost from the outside heat exchanger (HEX) of an air-to-air heat pump. The proposed method adopts two bypass lines of hot gas from the compressor: one is connected to the inlet of the outdoor HEX, and the other is connected to the outlet of the exchanger. We compare the dynamic performance and defrosting time of the conventional reverse cycle defrosting (RCD), hot gas bypass cycle defrosting (HGBD), and DHBD methods using a medium air-to-air 16 kW heat pump. The salient feature of the DHBD method is its ability to prevent a sharp decrease in the compressor outlet temperature at the melting frost stage after the HGBD process begins. Due to the additional bypass, the DHBD method sustained a higher compressor outlet pressure and reduced the defrosting time by 36% compared to the HGBD method. Compared to RCD, the defrosting time was comparable (126%); however, the amenity characteristics of the DHBD method were superior than those of the RCD method. The proposed DHBD method can overcome the main disadvantages of the RCD and HGBD methods, and showed excellent performance for an air-to-air heat pump in a defrosting operation.

AN EXPERIMENTAL STUDY ON THE DEFROSTING PERFORMANCE OF A PCM-BASED REVERSE-CYCLE DEFROSTING METHOD FOR AIR SOURCE HEAT PUMPS

When an ASHP unit operates in heating mode, frost can be accumulated on the surface of its outdoor coil. Currently the most widely used defrosting method for ASHPs is reverse cycle defrost. However, the fundamental problem for such a method is that there is insufficient heat available for ASHPs during defrosting. This paper reports on an experimental study of developing a novel reverse-defrosting method which was thermal energy storage (TES) based on using phase change material (PCM). Comparative experiments were carried out at two operating conditions: standard defrosting condition and the PCM-based defrosting condition. Results indicated that the use of this PCM-based reverse defrosting method could help shorten defrosting time, increase defrost COP d, enhance system reliability, reduce transition duration from defrost to normal space heating operation, and improve indoor thermal comfort for occupants during defrosting and heating resumption process while without degrading the quality of space heating provided by the ASHP during heat operation.

Improved indoor thermal comfort during defrost with a novel reverse-cycle defrosting method for air source heat pumps

When an air-source heat pump (ASHP) unit is used for space heating at a low ambient temperature in winter, frost may be formed on its outdoor coil surface. Frosting affects its operational performance and energy efficiency, and therefore periodic defrosting is necessary. Currently, the most widely used standard defrosting method for ASHP units is reverse-cycle defrost. During a standard reverse-cycle defrosting process, the indoor coil in an ASHP unit actually acts as an evaporator, therefore, no heating is provided and hence indoor air temperature in a heated space can drop. Furthermore, a longer period of time is needed before space heating can become available immediately after the completion of defrosting. Consequently, occupants’ thermal comfort may be adversely affected. To improve the indoor thermal comfort for occupants during reverse-cycle defrosting, a novel thermal energy storage (TES) based reverse-cycle defrosting method has been developed and the improvement to occupants’ thermal comfort experimentally evaluated and is reported in this paper. Comparative experiments using both the novel TES based reverse-cycle defrosting method and the standard reverse-cycle defrosting method were carried out. Experimental results and the evaluated indoor thermal comfort indexes clearly suggested that when compared to the use of standard reverse-cycle defrost, the use of the novel reverse-cycle defrosting method can help achieve improved indoor thermal comfort, with a shorter defrosting period and a higher indoor supply air temperature during defrosting.

EXPERIMENTAL STUDY ON A NOVEL DEFROST CYCLE FOR AIR SOURCE HEAT PUMP

On the basis of the hot-gas bypass defrost cycle, a new defrost method for air source heat pump is proposed. In the new defrost cycle, the outdoor heat exchanger is divided into two parts which are connected by a capillary tube. The front and rear parts of the heat exchanger are used as the evaporator and condenser respectively during defrosting, and can be defrosted orderly by using the four-way valve. The defrosting performance of the new cycle is investigated experimentally and the results are compared with that of the reverse-cycle defrost system. The experimental results indicate that the energy is used more efficiently in the new defrost cycle, therefore, the defrosting duration and losses are less than those of the reverse-cycle defrost system. Moreover, it does not extract heat from the indoor space during defrosting in terms of the new defrost cycle. Compared with the reverse-cycle defrost system, it is found that the switching times of the four-way valve in the new defrost system are the same, and the fluctuation of the discharge and suction pressures during the defrosting is much less than that in the reverse-cycle defrosting, hence, the mechanical impact on the system is much less.

Comparison between hot-gas bypass defrosting and reverse-cycle defrosting methods on an air-to-water heat pump

Frost build-up blocks the gap between fins and decreases the airflow rate, and the heating capacity and COP of air-source heat pump become lower. The reverse-cycle defrosting (RCD) and hot-gas bypass defrosting HGBD are the two common methods of frost removal. The comparison between the RCD and HGBD methods on one heat pump were not found by authors up to now. This paper compared the dynamic characteristics during the RCD and HGBD periods on one medium air-to-water heat pump of 55 kW. The feasibility of the HGBD method without heat provided to evaporate the liquid refrigerant after melting frost, which was supposed in the danger of the compressor liquid slugging, was especially discussed. The results showed the liquid refrigerant leaving the air cooled coils for the HGBD method was trapped in the suction accumulator and flashed into the saturated gas by the compressor pumping action, and no compressor liquid slugging happened in our tests. The HGBD time was much longer than the RCD time. The amenity for the HGBD method was better than that for the RCD method, due to lower refrigerant noise, smaller indoor temperature fluctuation, and no cold blowing. The suction superheat and discharge superheat for the HGBD method were lower than those for the RCD method. The HGBD could overcome the main disadvantages of the RCD method and deserved the further investigation.

05/00819 The main constructional characteristics of contemporary urban residential buildings in Greece

Recently, multi-split air source heat pumps (ASHPs) have been used increasingly for space heating in cold winter. When it operates in frosting environment, periodic defrosting is necessary to maintain a high system performance. However, researches on its defrosting were few due to its high capital and complicated controls. To solve its fundamental problem of insufficient heat available during defrosting, a novel reverse-cycle defrosting (NRCD) method based on the phase change thermal storage has been developed. In this paper, comparative experiments using both standard reverse-cycle defrosting (SRCD) method and NRCD method were carried out on a multi-split ASHP unit with a phase change material heat exchanger (PCM-HE) acting as energy accumulator during heating operation and heat source during defrosting operation. Experimental results suggested that when using the NRCD method, the system performances, such as suction pressure and temperature, defrosting and heat-resumption durations, COP during defrosting operation can be effectively improved.

Effects of fan-starting methods on the reverse-cycle defrost performance of an air-to-water heat pump

The effects of fan starting methods on the defrost performance in an air-source heat pump were investigated experimentally. Experiments were conducted on a 50 kW unitary air-to-water heat pump. The dynamic characteristics of both the coil fan pre-start and normal-start tests during the defrost cycle were discussed. The peak of discharge pressure for the fan normal-start test was up to 2595.6 kPa during defrost termination and recovery time, which was close to the discharge protection value (2650 kPa). The discharge pressure for the fan pre-start test was 742.3 kPa lower at the end of the drain time, so the peak of the discharge pressure was 687 kPa lower during the defrost termination and recovery time than that for the fan normal-start test. We found the phenomenon seemed to be related to both the small inner volume of the plate heat exchanger and larger refrigerant flow rate during the defrost termination and recovery time. Pertinent performance data (pressure, temperature, superheat, sub-cooling, etc) was plotted and discussed to determine the effects of the coil fan pre-start.

Dynamic simulation of air-source heat pump during hot-gas defrost

The dynamic simulation of hot-gas defrost is a challenge and necessary to the simulation of a heat pump. A defrost model that bases itself on the fundamental conservation equations and experimental results is presented in this paper. The distributed model is used here for the details of evaporator and condenser that are important components during the reverse cycle defrost. For validation of the model, a test was set up which contained a nominal 0.88 kW capacity residential air-to-air heat pump that uses a capillary for expansion during the defrost. The good agreement between simulations and experimental results shows that the dynamic defrost model can be used in the research of the heat pump especially for the reverse cycle defrost.

95/05224 It is time gas-fired heat pumps had a breakthrough

The dynamic simulation of hot-gas defrost is a challenge and necessary to the simulation of a heat pump. A defrost model that bases itself on the fundamental conservation equations and experimental results is presented in this paper. The distributed model is used here for the details of evaporator and condenser that are important components during the reverse cycle defrost. For validation of the model, a test was set up which contained a nominal 0.88 kW capacity residential air-to-air heat pump that uses a capillary for expansion during the defrost. The good agreement between simulations and experimental results shows that the dynamic defrost model can be used in the research of the heat pump especially for the reverse cycle defrost.

Effect of short-tube orifice size on the performance of an air source heat pump during the reverse-cycle defrost

Many air source heat pumps use the reverse-cycle defrost to eliminate frost that forms on the outdoor heat exchanger during normal winter operation. During the defrost, the heat pump is switched from the heating to the cooling mode to provide heat to the outdoor heat exchanger to melt the frost. Once the frost is melted and drained from the heat exchanger, the unit is switched back to the heating mode. The objective of this research was to characterize the effect of short-tube orifice diameter on the response of a heat pump during the reverse-cycle defrost. An experimental apparatus was constructed containing a nominal 3 ton (cooling capacity) residential air source heat pump. A manifold was constructed which allowed for the switching of different sized orifices by turning a shut-off valve. Refrigerant temperature and pressure measurements were made throughout the system as well as refrigerant flow-rates, air-side capacity, compressor/outdoor fan power and refrigerant level in the accumulator. A detailed comparison of the effect on defrost performance is provided.