Cascade Air Source Heat Pump Research Articles

Experimental study on operating performances of the low and high temperature stages for cascade air source heat pumps

Previous studies on cascade air source heat pumps (CASHPs) mainly focused on overall performances, while impacts of the low temperature stage (LTS) and high temperature stage (HTS) on overall performances were paid less attention to. Therefore, operating performances of a CASHP in LTS and HTS were investigated in this paper, to provide a solution for the optimization of the CASHP. A CASHP unit with a variable speed compressor in LTS and a constant speed compressor in HTS was adopted, and its operating performances were investigated under 26 cases. Results showed that the pressure ratio and COP in LTS and HTS were affected differently by the compressor speed. Specifically, as the speed of the compressor in LTS increased from 30 rps to 90 rps, PrL increased by 151 % ∼ 178 %, and PrH reduced by 38 % ∼ 54 %. Meanwhile, COP in LTS reduced by 21 % ∼ 52 %, but the COP in HTS rose by 54 % ∼ 96 %. Besides, it was found that adjustments of the compressor speed in LTS could improve heating performances of the CASHP. However, when the ambient air temperature and the supply water temperature were consistent, COP changes of the CASHP were limited, only by 0.3 % ∼ 9 %. Furthermore, empirical models of the COP in LTS and HTS were established to predict the COP of the CASHP on different operating conditions. Results of this study were helpful in promoting the configurations and operation control optimization of CASHPs.

Experimental investigation and industrial application of a cascade air-source high temperature heat pump

For industrial thermal scenarios where there is no waste heat to recycle, the Air-source High Temperature Heat Pump (ASHTHP) is the only choice to replace the traditional industrial boiler and decrease energy consumption. A cascade ASHTHP has been designed and developed to support 125 °C hot water, which utilizes R410A and R245fa as working mediums. It is not only theoretically simulated and experimentally investigated, but also applied in an electroplating factory in Ningbo, China to provide 120 °C hot water. When the ambient temperature is 20 °C and the output temperature is 125 °C, the experimental heating capacity of the unit is 121.5 kW with a COP of 1.71. During the experimental investigation, the heating capacity attenuation remains within 19.25 %. Through the comparison and verification of the experimental data, the system simulation results have high accuracy with the error controlled within 5 %. When used in the electroplating factory to provide hot water at 120 °C, the cascade ASHTHP units can provide a heating capacity of 480 kW with a COP of 1.8 at the ambient temperature of around 25 °C, which results in annual savings of approximately $123,200 and a payback period of 1.65 years.

Experimental and theoretical investigation on the energetic and economic performance of low-temperature cascade air-source heat pump considering the effects of cascade heat exchanger

Air-source heat pump (ASHP) is wide used for its carbon neutrality. In cold climates, to ensure good efficiency and stability, cascade ASHP is a promising technology for high-temperature water application, such as space heating boilers retrofit. Cascade heat exchanger is a key component in cascade ASHP. However, there are few experimental studies on the effects of its area on system energetic, economic, and environmental performance under low ambient temperature and high water-supply temperature, which is essential for system improvement and application. Focusing on this research gap, this paper carries out experiment of cascade ASHP system performance and conducts analysis on system suitability based on recurrent Neural Network. To fairly compare system performance in different cold climate cities with different cascade heat exchanger areas, conversion factor γ is proposed and used in the analysis. The results show total energy consumption of compressors decreases the most in New York (21498 MJ) and least in Shenyang (9414 MJ) as cascade heat exchanger areas increases from 0.6 to 1.6 m2. Similarly, the system in New York shows the most significant reductions in carbon emissions (2701 kg/year) and total annual cost (225$), while that in Shenyang shows the smallest drop of 1277 kg/year and 118$, indicating it’s more meaningful to increase cascade heat exchanger area in a city with longer heating period. The results also show the COP varies significantly across cities, strongly correlated with average ambient temperature. The increase in COP slows down when cascade heat exchanger area reaches 1.6 m2, indicating even larger area is not recommended. These findings provide a foundation for improving and applying cascade ASHP in cold climates.

Performance analysis of a R134a/CO2 cascade heat pump in severe cold regions of China

To achieve efficient and reliable heating in severe cold regions below −20 °C, this paper investigates a R134a/CO2 cascade air source heat pump (ASHP) system. Firstly, a simulation model is constructed and used for energy performance analysis. Then experiments are carried out for model verification and performance investigation. Finally, its performance is evaluated in detail with the meteorological data of Harbin under different working conditions for energy performance forecast. Simulation results show that the heating coefficient of performance (COP) increases with higher ambient temperature and lower water supply temperature. At the same time, the modeling errors on coefficient of performance (COP) are all within ±0.2%. The experimental results show that the proposed R134a/CO2 ASHP can supply water above 50 °C with high COP up to 3.07 and 1.60 when the ambient air temperature is −5 °C and −45 °C, respectively. Furthermore, under four working conditions, the heating seasonal performance factors (HSPF) are all above 2.30 and the best is 2.39 for the −25 °C nominal operating temperature and the typical working condition of civil buildings (15 °C). Both the experimental data and the simulation results show that the proposed R134a/CO2 cascade air source heat pump system can provide efficient and reliable heating in severe cold regions.

Thermodynamic analysis of the heat pump steam system with medium-low temperature heat source

Heat Pump Steam technology driven by electric power has an important application value in the industrial thermal field. A cascade air source heat pump steam system (CASGHP) using in thermal workshop is proposed in this paper. A numerical model is established by EES software. The thermal performances of several feasible refrigerants in the cascade system are analyzed comparatively. The influences of intermediate cooling temperature (Tintc), the injection temperature (Tinje), the evaporating temperature (Tevap), the outlet steam temperature (Tout), gas-injection ratio (βg) and specific rate of compressors pressure ratio (PRR) on system power consumption of generating a ton steam (Wper) and system COP are studied. The conclusions are as follows: When R134a is used in low temperature section of the cascade system, R245fa, R1234ze(Z) and R1336mzz(Z) used in high temperature section respectively can meet the requirements of steam generation. The COP of the CASGHP adopted R1234ze(Z) is the largest, followed by R245fa and R1336mzz(Z). There exists a maximum COP, which is 2.268, 2.286 and 2.211, respectively. When Tevap is 10 °C and Tout is 120 °C, the optimal Tintc of CASGHP with these high temperature refrigerants is 53 °C, 50 °C and 53 °C, the optimal Tinje is 79 °C, 78.5 and 85.5, the optimal βg is 0.207, 0.18 and 0.165, and the optimal PRR is 0.77, 0.71 and 0.972, respectively. The COP increases by 18.4%, 18.2% and 17.9% respectively when Tevap increases from 10 °C to 30 °C, but it decreases by 14.4%, 14.2% and 10.2% respectively when Tout increases from 100 °C to 120 °C.

Temperature stage matching and experimental investigation of high-temperature cascade heat pump with vapor injection

High-temperature cascade heat pump has remarkable development potential for industrial heat generation due to its superiority in energy-saving and environment friendly. But the growing demand for higher temperature difference heating challenges the traditional cascade heat pump. One of practical solutions is the combination of cascade system and vapor injection technique. However, the introduction of vapor injection increases the complexity of the system, and the interaction between branches and the distributions of branches are to be studied to give full play to the best performance of the vapor injected cascade system. This paper focuses on temperature stage matching between the heat exchange branches of a cascade air source heat pump with vapor injection to get optimal heating performance. The optimal configurations of intermediate temperature and vapor injection branch temperature are obtained based on the optimal method. A cascade heat pump prototype with refrigerant R245fa and R410A is developed and investigated experimentally to verify the optimization method. The cascade air source heat pump could realize 140 °C heat supply and the coefficient of performance is from 1.16 to 1.58 under the ambient temperature of from −10 °C to 20 °C.

Experimental and simulation study of air source heat pump for residential applications in northern China

Air source heat pumps are vital for the implementation of coal-to-electricity policies in northern China. However, majority of the low-temperature air source heat pumps operate under a single and fixed mode, which maynotbesuitablefor residential applications in regions with variedclimates. Therefore, herein, a multimode single-fluid cascade air source heat pump (MSC-ASHP) is proposed, which includes the following modes: cascade compression heating, single-stage compression heating, and single-stage compression regeneration modes. To investigate variable-speed characteristics and control strategy of the two heating modes, the experiments and simulations are performed at different evaporating temperature and lower-stage compressor speed. The results demonstrate that the heating capacity in two modes can be enhanced effectively by adding the lower-stage compressor speed. The coeffinient of performance (COP) of cascade compression heating mode increases first and subsequently decreases along with the increasing of the lower-stage compressor speed. Under a condensing temperature of 46 °C and evaporating temperature of −35 °C, the COP is higher than 1.8, the discharge temperature is lower than 120 °C, and the compression ratio in each stage does not exceed 5.0, indicating this system functions safely and steadily in cold climates. The control strategies for the compressor speed and mode switch are summarized synthetically based on the simulation and experimental results. New heat pump system prolongs the year-round applications period of the low-temperature air source heat pump.

The intermediate temperature optimization for cascade refrigeration system and air source heat pump via extreme seeking control

Abstract The cascade refrigeration system and cascade air source heat pump have both been a well-known advanced technology for providing cooling and heating under certain circumstances. The intermediate temperature has been proven as a key factor in affecting the operating performance for both the refrigeration and heat pump system. However, the optimal intermediate temperature varies along with the change of the ambient temperature, cooling/heating demand. So achieving the real-time optimal intermediate temperature in a realistic operating circumstance is critical for improving the performance of cascade refrigeration or heat pump system. In this paper, the extreme seeking control strategy is proposed to optimize the coefficient of performance via seeking the real-time optimal intermediate temperature. A cascade refrigeration system model and a cascade air source heat pump water heater model are built based on Modelica, and the ASHP water heater model is validated by experimental data. The simulations are conducted under fixed and variable operating conditions to validate the effectiveness of the proposed strategy.

Real-time efficiency optimization of a cascade heat pump system via multivariable extremum seeking

In this paper, the Extremum Seeking Control (ESC) is proposed as a model-free real-time optimizing control strategy for efficient operation of a cascade air source heat pump (CASHP). ESC is a model-free real-time optimization framework, which can search for the unknown and slowly varying input setpoint for optimizing specific performance index. The ESC strategy is realized for the scenarios of power minimization and COP maximization, respectively. For the power minimization, the ESC optimizes the setpoints of intermediate temperature and the superheat setpoints for the high-temperature and low temperature cycles; while for the COP maximization, the manipulated inputs are the compressor speeds of the high-temperature and low-temperature cycles and the evaporator fan speed. To validate the effectiveness of the proposed ESC strategy, a Modelica based dynamic simulation model of a CASHP water heater is built with Dymola and TIL Library. The CASHP is designed to provide the discharge water temperature of 85 °C, with R404A and R245fa as the refrigerants for the low temperature cycle and high temperature cycle, respectively. Under both fixed and realistic ambient temperature profiles, the simulation results show satisfactory convergence performance for both steady-state and transient characteristics. Thus, the feasibility of the proposed CASHP control strategy is validated.

Thermal energy storage based (TES-based) reverse cycle defrosting control strategy optimization for a cascade air source heat pump

Thermal energy storage based (TES-based) reverse cycle defrosting method is a feasible way to reduce energy requirements for defrosting of cascade air source heat pumps (CASHPs). The energy stored in the phase change material based heat exchanger (PCM-HE) is the heat source for both the high stage cycle (HSC) and low stage cycle (LSC) during the TES-based reverse cycle defrosting process, thus the heat provided to the HSC and LSC are coupled, affecting the defrosting performance in LSC and heating performance in HSC. To shorten the defrosting time and maintain the indoor thermal environment, the heat distribution between HSC and LSC during TES-based reverse cycle defrosting was explored. It was found that the heat distribution pattern was mainly influenced by the refrigerant flowrate. Therefore, controlling the refrigerant flowrate of HSC and LSC during the TES-based reverse cycle defrosting is a possible way to regulate the heat distribution between HSC and LSC. In this paper, an experimental investigation of the control strategy for TES-based reverse cycle defrosting of CASHPs is presented. The defrosting performance and indoor heating performance during TES-based defrost operation with LSC compressor frequencies of 60 Hz, 75 Hz, 90 Hz, and 105 Hz were tested. Then HSC EEV openings of 80%, 90% and 100% at the optimized LSC compressor frequency of 90 Hz were considered. It was concluded that the optimal TES-based reverse cycle defrosting strategy is to maintain the LSC compressor frequency at 90 Hz and the HSC EEV opening at 80% for the equipment considered in this study. The defrosting time was 440 s and the average indoor heating capacity was 5.23 kW. The heat provided to the HSC and LSC accounted for 38.7% and 61.3% of the total heat released by PCM-HE, respectively.

Experimental analysis of heat coupling during TES based reverse cycle defrosting method for cascade air source heat pumps

Adopting thermal energy storage (TES) based reverse cycle defrosting (RCD) for cascade air source heat pumps (ASHPs) is a feasible way to tackle the defrosting problem when the cascade ASHP is operated in heating process. However, during TES-based RCD, the phase change material (PCM) undergoes multi-mode heat discharging process, resulting from different operating conditions in high temperature cycle (HTC) and low temperature cycle (LTC) of a cascade ASHP unit. The heat provided to HTC and LTC is therefore different, thus influences the performances of space heating and defrosting. In this paper, the heat coupling relations between the HTC and LTC during TES-based RCD were experimentally investigated. Five cases with the same amount of frost under different outdoor conditions were conducted. The total heat output to indoor space in HTC, the total heat consumption in LTC for defrosting and the heat provided to HTC and LTC by the PCM based heat exchanger (PCM-HE) were reported, and a discussion on the factors that influence heat coupling relations was then given. Contributions of this study are supposed to be used for adjusting the control strategy for TES-based RCD of a cascade ASHP unit.

Experimental investigation on the multi-mode heat discharge process of a PCM heat exchanger during TES based reverse cycle defrosting using in cascade air source heat pumps

Thermal energy storage (TES) based reverse cycle defrosting method has been proposed to solve the defrosting problem for cascade air source heat pumps (CASHPs), with the benefits of shortening defrosting duration and continuously supplying heat to indoor space. During TES based reverse cycle defrosting, the high temperature (HT) cycle and low temperature (LT) cycle of a CASHP unit share the same heat source, i.e., the PCM-heat exchanger (PCM-HE). Considering the different operating conditions in HT and LT cycle, the PCM will undergo multi-mode heat discharge process. In this paper, RT 10 is used as PCM in a finned-tube heat storage unit, and the multi-mode heat discharge process was experimentally investigated under the frosting condition of −12 °C and 85% RH. The results suggested that the heat transfer rate to LT cycle was higher than that to HT cycle, caused by higher temperature difference between LT cycle and PCM and LT refrigerant flowrate. The calculated heat provided to HT and LT cycle accounted for 37.7% and 62.3% of the heat released by the PCM-HE, respectively. Furthermore, the distribution pattern of the heat provided to the HT/TL cycle was explored under different outdoor frosting conditions with the same amount of frost.

Improving defrosting performance of cascade air source heat pump using thermal energy storage based reverse cycle defrosting method

To encourage a wider application of air source heat pumps (ASHPs) to colder areas due to the advantage of a higher energy efficiency, adopting cascade air source heat pumps (CASHPs) is a promising option. When CASHPs are operated in heating mode, frosting/defrosting has been problematic. However, the defrosting methods commonly used by conventional ASHPs cannot be directly applied to CASHPs. Therefore, a thermal energy storage (TES) based reverse cycle defrosting method for CASHPs has been proposed and an experimental study on its operating performances was carried out. Comparative tests when using both the standard hot gas by-pass defrosting method and the TES based reverse cycle defrosting method were carried out. The results suggested that when using the TES based reverse cycle defrosting method, defrosting duration was shortened by 71.4–80.5%, and defrosting energy consumption reduced by 65.1–85.2%, as compared to those when using the standard hot gas by-pass defrosting method. In addition, the thermal energy stored can also become a source for indoor space heating, and 37.7% of the normal heating capacity can be provided to a heated indoor environment when using the TES based reverse cycle defrosting method.

An Experimental Study on the Improvement of Reverse Cycle Defrosting Performances for a Cascade Air Source Heat Pump using Thermal Energy Storage Based Defrosting Method

Applying cascade air source heat pumps (CASHPs) in colder areas is a promising option to encourage a wider application of air source heat pumps (ASHPs) due to their advantage of a higher energy efficiency. When CASHPs are operated in heating mode, frosting/defrosting has been problematic. This paper proposed a novel thermal energy storage (TES) based reverse cycle defrosting method for CASHPs which can provide heat to defrosting and continuously supply heat to the indoor space. The comparative experiments on its operating performances was carried out by comparing standard hot gas by-pass defrosting method. The test results suggested that when using the TES based reverse cycle defrosting method, defrosting duration was shortened by ~64.3%, and defrosting energy consumption reduced by ~30.3%, as compared to those when using the standard hot gas by-pass defrosting method, and ~60% of the normal heating capacity can be provided to the indoor space during defrosting.

Experimental study of a control strategy for a cascade air source heat pump water heater

Cascade air source heat pump water heater is suitable for provide high temperature water under a wide range of outdoor air temperature. Most previous studies focused on the operating characteristics of cascade heat pump system, performance improvement, and the selection of optimal immediate temperature. However, no reported studies on the control strategies for cascade heat pump may be identified. This paper proposed a control strategy for cascade air source heat pump water heater to adjust the load variation. The principle that two stage compression ratio is approximately same is used to determine the intermediate pressure. The intermediate pressure, degree of superheat (DS) and the evaporating temperature were regulated by the compressor, electronic expansion valves (EEV) and the evaporator fan of the low temperature (LT) cycle, respectively, through Proportion Integration Differentiation (PID) controller or proportional controller. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. The results showed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.

Transient behavior and dynamic performance of cascade heat pump water heater with thermal storage system

At low ambient temperature, air-source heat pump water heater suffers from decrease of both heating capacity and coefficient of performance, and increase in compressor’s pressure ratio and discharge temperature. A cascade air-source heat pump water heater with phase change material (PCM) for thermal storage application was designed to ensure the reliable operation under various weather conditions and enhance the system performance at low ambient temperature. Dynamic experiments were carried out under various operating conditions in accordance with China National Standard GB/T23137-2008. Transient operating characteristics were adopted to analyze the performance of cascade heat pump system. Dynamic performance of the heat pump water heater in single stage mode and cascade mode was compared and discussed. The heating COP values in single stage mode ranged from 1.5 to 3.05, while in cascade mode, the heating COP values ranged from 1.74 to 2.55. Based on the transient heating COP values, critical switching curve from single stage mode to cascade mode was founded for the code of the system controller. Furthermore, energy performance between water tanks with and without PCM was compared to clarify the contribution of PCM.