Air Source Heat Pump System Research Articles

Research on renewable energy coupling system based on medium-deep ground temperature attenuation

Medium-deep geothermal has large reserves, a high temperature, a high heat flow density, and other characteristics, but the traditional medium-deep ground heat pump system have long relied on heating from the ground, and soil temperature decreases annually. Using a community building heating system as an example, from a point of view of heat accumulation or not, this article examines the design of a photovoltaic photothermal coupled medium-deep ground source heat pump system (PV/T-GSHP) and a photovoltaic-assisted medium-deep ground source heat pump coupled air source heat pump system (PVGSHP-ASHP). First, the operational performance of a typical GSHP system was compared to that of the PV/T-GSHP and PVGSHP-ASHP systems, both of which were built using TRNSYS. Second, the energy balance and electrical consumption of each system were compared. Finally, the effect of the PV/T-GSHP system’s key parameters and the different ASHP load ratios in the PVGSHP-ASHP system on soil temperature were analyzed. The results showed that after 20 years of operation, the average soil temperature decreased from 38.29 °C to 36.89 °C for the GSHP system, resulting in a decrease in the performance coefficient of the ground source heat pump and the system performance coefficient. The PV/T-GSHP system can address the issue of declining soil temperature with an increase in soil temperature of 0.09 °C. The PVGSHP-ASHP system can mitigate the issue of decreasing soil temperature. The PVGSHP-ASHP system is better than the PV/T-GSHP system in terms of electricity economy. In the PV/T-GSHP system, the larger PV/T component area of similar structures, the smaller flow rate of the heat collector pump, and the volume, inclination, and set temperature of the heat storage tank that are more suitable for the system can achieve higher soil temperatures. As the air-source load ratio increases, the rate at which the soil temperature in the PVGSHP-ASHP system decreases progressively decelerates.

Research on water–ice heat accumulator and analysis of its potential use as hybrid source heat pump for building heating

This paper presents an experimental and analytical study of the effects of the integration of a prototype heat accumulator using water–ice latent heat as an additional hybrid low-temperature heat source for an air source heat pump (HSHP). The study was conducted in two stages. The first stage involved experimental studies on the charging and discharging processes of the prototype accumulator. The second stage was the determination of the possible energy and economic effects of using the proposed hybrid heat source for a single-family building with a thermal output of 7.9 kW. The waste heat from ventilation and gray wastewater was used to charge the studied heat accumulator. A comparative analysis of the HSHP system with two classic solutions—an air source heat pump system (ASHP) and a ground source heat pump (GSHP)—was performed. The results indicated that the HSHP system had 21% and 5% lower electricity demand than ASHP and the GSHP, respectively. An additional benefit of the developed solution was the utilization of 58% of the waste heat of the building, which accounted for 33% heating demand coverage. Economic analysis suggested that the developed HSHP system was a lower cost solution than the GSHP in terms of the investment and operation costs.

Direct-expansion solar-assisted heat pump coupled with crystallisation-controlled supercooled PCM for shifting building electricity demand

This paper proposes a novel approach for improving the performance of a direct-expansion solar-assisted heat pump (DX-SAHP) system by integrating a crystallisation controllable phase change material (PCM) and building energy demand shifting strategy. The proposed system aims to reduce buildings' energy consumption while utilizing solar energy efficiently. Sodium acetate trihydrate (SAT) as the crystallisation controllable supercooled PCM has a promising feature of releasing the stored latent heat when it is externally triggered, allowing the user to control the stored heat whenever needed. In the study, the PCM can store the thermal energy generated by the DX-SAHP during peak solar hours in an efficient way. This stored energy can be released when solar energy is unavailable, but the heating is requested. This approach not only reduces the load on the heat pump system but also makes efficient use of the stored thermal energy by crystallisation controllable PCM. A detailed controlling methodology of the system is given, and heating output is controlled considering the level of solar irradiance. The proposed system was modelled and simulated using MATLAB, and the results show that the integration of crystallisation controllable PCM and building demand shifting strategy can significantly reduce the energy consumption of the buildings. On the low solar radiation day, the COP improvement compared to the air source heat pump system was found 9.4%, this improvement value can reach 77% on high solar radiation days. The system can also effectively utilize solar energy and reduce the overall carbon footprint of buildings.

Feasibility analysis of thermal storage defrosting method for air source heat pump: From energetic and economic viewpoints

Frosting issue of air source heat pump (ASHP) is a continuing concern. The conventional reverse cycle defrosting (RCD) method absorbs heat from indoor heat exchanger, thus greatly impacts the indoor thermal comfort. To solve the problem of insufficient defrosting heat supply for RCD method, thermal energy storage (TES) based defrosting method has gained increasing attention over the past years. Yet existing studies have only focused on specific prototype tests. There lacks general analysis on system characteristics and design considerations. To better address application concerns of TES-based defrosting method, this study carried out feasibility analysis for the TES-based ASHP system from both energetic and economic aspects. The energetic analysis reveals that the stable operation of TES-based ASHP relies on proper system design and control. Feasibility conditions concerned with key operating parameters are defined and deduced. Impact factors on average heating efficiency are also analyzed. Theoretical and numerical results indicate that energy saving potential of TES-based ASHP over conventional ASHP is conditional on the defrosting discount factor of conventional RCD method. TES-based ASHP enhances heating efficiency of conventional ASHP by 3.0% to 19.5% at ambient from −2°C to 2 °C due to relatively large defrosting discount factor of conventional RCD method. With respect to the economic aspects, the capital cost of TES-based ASHP is 26% higher than that of the conventional ASHP. But the total life cycle cost can be reduced by higher heating efficiency. The condensing heat storage ASHP exhibits cost advantages to the conventional ASHP when defrosting discount factor of conventional ASHP is higher than 1.13. Further prototype test with the TES-based ASHP could shed more light on the feasibility characteristics as revealed by the theoretical analysis.

Use of a residential building’s foundation as Energy Geo-Structures: A case study in the Mediterranean environment

Geothermal energy, a type of renewable energy obtained from the Earth, can be used by Ground Source Heat Pumps (GSHPs) for space heating and cooling. GSHP systems extract/reject heat from/into the earth via a network of pipes or tubes known as Ground Heat Exchangers (GHEs),. GHEs can be horizontal or vertical and come in a variety of forms, including slinky loops and vertical spiral loops for horizontal configurations, and single U-tube, double U-tube in series or parallel, coaxial pipe, and spiral/helical pipe for vertical configurations. Although GSHPs exhibit a higher performance than conventional Air Source Heat Pumps (ASHPs), the systems are less appealing as investments due to their longer payback times and greater upfront costs. A competent design of the GSHP system with the appropriate GHE sizing, including the case of employing the GHEs in the structures foundations, can minimize the initial cost. Such structures are referred to as Thermo-Active Structure systems or Energy Geo-Structures (EGS), which have already found applications as energy piles, barrette piles, diaphragm walls, shallow foundations, retaining walls, embankments, and tunnel linings [1, 2, 3].
 Residential buildings in Cyprus have a higher cooling than heating demand, with an extended payback period for the investment, especially if the system is solely utilized for heating [4]. EGS coupled with a GSHP could offer a substitute to reduce the system’s initial expenses and make it more attractive as an investment. EGS have not yet seen applications in Cyprus, therefore an initial computational analysis utilizing the COMSOL software and relevant data is taken into consideration. To determine the heating and cooling loads, a three-bedroom, two-storey house with a total floor space of 190 m2, with nearly Zero Energy Building (nZEB) characteristics, is used as the typical case for a single detached residential building in Cyprus. The TRNSYS software was used to calculate the heating and cooling loads. In order to analyze the rejected/absorbed energy from the earth, typical foundations were constructed as full sized models in COMSOL (Figure 1). With the exception of pipes, where a 1D solution is employed, the convection diffusion equation for heat transfer is utilized with 3D conservation of heat transfer for an incompressible fluid. A detailed methodology can be found in Aresti et al. [5].
 For the COMSOL compuations, the site ground temperature characteristics and the local ambient temperature were taken into account. Based on the provided loads, the inlet and outlet temperatures were hourly calculated for the highest demand months: February for winter – heating, and July for summer – cooling.
 To assess the potential of using the foundation bed as an alternative to energy piles, a comparison on the performance of the two cases is presented in Figure 2 for both summer and winter conditions. The foundation bed is seen to work in a steady manner, which is highly desired in a system. However, it performs less well, with a lower COP than the energy piles system, which exhibits greater COP values than the foundation bed system both in winter and summer. It remains that both systems exhibit high enough COPs and nearly steady conditions. The COPs vary between 4.4 and 4.8. A yearly simulation (not shown here) yielded a lowest COP value of 4.5.
 The systems were also assessed economically by being compared to a convectional ASHP system. The economic study demonstrated that the suggested foundation systems had relatively short payback times, making them a plausible alternative. More detailed discussion on the results can be found in Aresti et al. [5]. As a future objective, an environmental and Life Cycle Analysis investigation, similar to the one performed by Aresti et al. [6], could provide a deeper insight of the plausibility of using the building foundation as EGS.

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.

Advanced exergy analysis of a novel air source heat pump system coupled with the liquid receiver and gas-liquid separator

In previous studies, an air source heat pump system coupled with the liquid receiver and the gas-liquid separator was proposed, which improved the defrosting efficiency according to the improvement of the refrigerant mass flow during defrosting and shortened the defrosting time. In this paper, the exergy efficiencies and exergy loss (including endogenous and exogenous exergy loss) of the new system are studied. They are compared with the original air source heat pump system by constructing the advanced exergy study model of the system. Results show that the exergy efficiencies of the compressor, evaporator, and throttle valve of the new system are increased by 0.2%, 2.3%, and 5.9%, respectively. The endogenous exergy loss of the new system accounts for 65.16% of the total exergy loss of the system, and most of the exergy loss in the new system is caused by the components themselves.

Probabilistic economic evaluation of the most prevalent in Greek market space heating systems

In the present study, the most prevalent space heating (SH) systems are assessed by means of probabilistic economic method, for Athens (Greece) under the ongoing energy crisis. The examined SH systems are the air source heat pump, the natural gas boiler and the oil boiler. An apartment of 85 m2 living area is chosen as the standard Athenian dwelling type for the assessment of the SH systems. In addition, two economic scenarios are set related to the annual energy inflation rate, the pessimistic and the optimistic. Simulations are executed in TRNSYS for the physical system and in MATLAB for the economic analysis. The economic analysis is carried out with the combination of the Monte Carlo simulations and the Life Cycle Cost (LCC) method. It turned out that, the LCC (as net present value) for all systems is much higher than the pre-crisis cost and is not feasible under the existing obscure economic period. It was proved that, the energy cost subsidy may be a solution on the volatility of the energy market, however, this can be achieved with rather high subsidy rates. The air source heat pump system was found to obtain better economy than the conventional fuel-based systems and the pairing of the heat pump with the PV system proved as the only economic feasible solution. The developed methodology can be utilized for evaluating the investment risk associated with the uncertain of the future cost of energy.

A method for sizing air source heat pump and electric boiler considering the peak and valley electricity prices

In a combined air source heat pump and electric boiler heating system, the capacity an oversized heat pump increases investment costs but decreases operation costs, and vice versa. Most current equipment selection methods are complex and ignore the impact of the peak valley price policy on system costs. This often leads to the selection of oversized equipment for the actual project, resulting in an increase in the total cost of the system. Therefore, we propose a new calculation method. The optimal number of non-guaranteed days for the air source heat pump (OGD) was used as the reference value for the calculation. A new optimization module was developed, and a system simulation model was built using Trnsys. The variation patterns of the OGD for 12 groups of buildings in Changchun, China, were analyzed. Finally, OGD recommendations were provided for 35 important cities in China. The results showed that OGD was not affected by changes in the building type or equipment installation costs. The average error of the annual cost calculation for different building types was 0.074%, and the maximum error was 1.7% when the equipment investment cost was changed. In addition, the OGD value was affected by a combination of outdoor temperature and local electricity prices. This value gradually decreased as the climate warmed. For regions with unfavorable price policies, such as Kashgar, China, the recommended local OGD value increased by 75.8%. The proposed method can be used as a methodological complement to the calculations of such systems during the design phase.

Exergy Analysis of Transcritical CO2 Air-Source Heat Pump with Honeycomb Gas Cooler

In order to build an efficient and energy-saving CO2 heat pump system and to improve the heat transfer efficiency of the gas cooler, a novel honeycomb gas cooler with a compact structure, high heat transfer efficiency, and high pressure-bearing capacity was proposed in our previous work. To clarify the components in the system that need further optimization and to improve its performance, an exergy analysis of a transcritical CO2 air-source heat pump system with the novel honeycomb gas cooler is studied in this paper. Based on the second law of thermodynamics, the exergy model of each component in the heat pump system is established, and the irreversible loss of each component is analyzed. In addition, the degree of energy loss of the honeycomb gas cooler is clarified, and the possibility and direction of system optimization are pointed out. The results show that the exergy efficiency of the system is 35.33% under nominal operating conditions, and there is a lot of room for improvement in its energy utilization. The three components with the largest exergy destruction percentage are the compressor, throttle valve, and evaporator in the order of 36.13%, 22.90%, and 19.51%, respectively. These components with high exergy destruction percentages are the main reasons for the large irreversible losses of the system.

Investigation on the performance coupling characteristics between a compound anti-frosting method and air source heat pump system

Air source heat pump systems are prone to frosting when operating under low temperature and high humidity working conditions, leading to reduced operational stability and performance. The traditional defrosting methods suffer from defects including high energy consumption, heating interruption, and system pressure surges. To address these issues, a novel active–passive compound anti-frosting method for air source heat pump systems is introduced in this work, which involves utilizing the synergistic effect of high-speed airflow and superhydrophobic modification. To reveal the coupling characteristics between the energy consumption of the anti-frosting method and the system performance, the airflow parameter model based on droplet growth and force taken is established, the simulation algorithm for system frosting and defrosting is developed, and the system's operating characteristics and performance under different operation modes are compared within typical frosting condition. The results indicate that the active–passive compound method can realize frost-free operation and reduce the ant-frosting and defrosting energy consumption by more than 90%. Consequently, it improves the system's comprehensive performance coefficient in a single frosting-defrosting period by 9.7%∼14.3%, compared to the control group without any anti-frosting measures. The introduction of the active method further enhances the frost suppression effect of the superhydrophobic modification.

A data-driven evaluating method on the defrosting effect of the air source heat pump system in Beijing

The current demand-based defrosting initiation strategies of air source heat pumps (ASHPs) faces difficulty in achieving a balance between cost and robustness. Therefore, an efficient and innovative demand-based defrosting initiation strategy is proposed in this paper, based on the learned the degradation of heating capacity (DHC) method using data-driven model. Firstly, the DHC method is developed using the initially installed sensors without additional sensors to identify the frosty state. Subsequently, the fully connected neural network (FNN) model is established for predicting frosty state using field measured data from the ASHP system. Finally, the effectiveness of the defrosting initiation strategy is further validated through practical testing. The results demonstrate that the DHC method provides a reliable database for training the FNN model, resulting in an impressive accuracy of 91.43% for predicting the frosty state in the testing set. By adopting the innovative demand-based defrosting initiation strategies, the defrosting frequency, heating loss, and power consumption are respectively decreased by 66.3%, 2.1%, and 6.0% with the SCOP enhanced by 8.6% during a heating season. The promising results highlight that the proposed strategy effectively reduces costs while ensuring the efficiency of demand-based control.

Development of In Situ Refrigeration Cycle Measurement Method Using Air-Side Data of Air Source Heat Pump

The refrigeration cycle of an air source heat pump system is an important information that reveals critical operating data, such as the cooling capacity, power consumption, and performance of a system during operation. Operating data, such as refrigerant pressure and enthalpy in situ, can be difficult to measure. Therefore, this study developed an in situ refrigeration cycle measurement method using the airside data of an air source heat pump. A method for measuring the refrigeration cycle is proposed using the characteristics of evaporation, compression, condensation, and expansion processes. The distance function was analyzed by normalizing the difference between the refrigerant pressure and enthalpy of the existing and proposed measurement methods. In addition, the distance function for the maximum error of the pressure and enthalpy measurement devices was analyzed and compared with the distance function of the entire data used in the evaluation. All the evaluation data had low distance function values with a maximum difference of 5%, confirming the reliability of the proposed refrigeration cycle measurement method. The power consumption and calculated COP were also evaluated using the proposed method. The Mean Bias Error (MBE) of power consumption and COP were 0.15% and 0.04%, and the coefficient of variation of root-mean-square error (CvRMSE) was 8.967% and 7.14%, respectively.

Experimental analysis of an integrated system using photovoltaic–thermal and air source heat pump for real applications

To realize zero energy buildings, photovoltaic-thermal (PVT)–ground source heat pump (GSHP) systems are being developed. However, the PVT–GSHP system faces challenges in urban areas or in existing buildings owing to the complicated installation process and high initial cost. To address this problem, this study devised an integrated PVT–air source heat pump (ASHP) system that allows installation in a limited space and reduces the initial cost. Particularly, a real-scale experimental plant was constructed based on a small-scale building to evaluate the performance of the PVT–ASHP system. The measurement data of the PVT–ASHP system was collected using a monitoring system, and the heating and cooling coefficients of performance (COPs) were evaluated. The average heating and cooling COPs of the PVT–ASHP system were 3.54 and 3.31, respectively. The COP of the system improved by approximately 52% compared to the ASHP. The PVT–ASHP system indicated a performance difference of approximately 9% compared to the PVT-GSHP system. Moreover, the initial cost of the PVT–ASHP system was reduced by 44% compared to the PVT–GSHP system. PVT–ASHP systems have economic benefits and are a suitable alternative to PVT–GSHP systems.

A fuel cell range extender integrating with heat pump for cabin heat and power generation

Batteries, Heat Pumps (HPs), and fuel cells (FCs) are critical for transport decarbonization and a net zero future. However, cabin heating in extreme conditions leads to severe driving range reduction in current Electric Vehicles (EVs). The performance of the heat pump (HP) in EVs and its performance enhancement technologies are widely investigated but cannot, simultaneously, provide sufficient heat and high COP. The source and amount of the waste heat within a vehicle for the heat pump integrated system is a crucial challenge to improve performance. The structure becomes increasingly complicated, but the benefits are not significant. Therefore, in this study, a small Fuel Cell, battery and heat pump integrated energy management system for range extended EVs (FCBEEV) is designed. The cogeneration characteristic of the fuel cell and waste heat from battery pack are utilised by the heat pump to ensure a high-level of cabin comfort in extremely cold temperatures and an extension of the driving range. A numerical model was established in MATLAB and the results were analysed from energy, exergy, environment, and economic (4E) perspectives. In this study, we show that the highest COPsys of the proposed system is 5.8 and can improve the driving range (DR) by 65% to 110% compared to the reference systems. The exergy efficiency of the suggested system is 75% at −10 °C and the fuel cell and internal condenser are the primary causes of the exergy destruction. The environmental impact decreases by 13 kg/year per car compared to current EVs with a Positive Temperature Coefficient (PTC) and Air Source Heat Pump (ASHP) system, and the reduction is primarily sourced from the indirect emissions. The operating cost which includes driving and heating is 28.9% higher than cited for an ASHP and PTC system and 41% higher than the PTC baseline system. The payback duration is 300,000 km at current market prices, and it is predicted to be shorter to 100,000 km, if the cost of the fuel cell stack is estimated at £4000 and the H2 price is the same as electricity. We anticipate that the proposed system can significantly improve cabin comfort and driving range anxieties, as well as promote the decarbonization of transport.

Dynamic Energy Analysis of Different Heat Pump Heating Systems Exploiting Renewable Energy Sources

Renewable energy source-fed heat pumps (HPs) may perform up to very high-efficiency standards, offering a promising tool in the wider residential heat decarbonization effort. In this context, this paper investigates different heating configurations utilizing various renewable thermal sources in conjunction with an HP-based system in order to determine the optimal configuration in terms of efficiency, using an existing, fully functioning residential building in Zaragoza, Spain, as our case study, comprising 40 dwellings. Four different HP configurations are investigated:, (i) an air-source system, (ii) a ground-source system, (iii) a dual-source system with solar thermal collectors, and (iv) a triple-source system based on solar, geothermal, and ambient sources. For the purpose of such investigation, detailed dynamic energy simulations are conducted through the use of the INTEMA.building tool (developed in Modelica), applying a multi-objective optimization process that aims at minimizing both the annual electricity consumption and the net present cost. It is demonstrated that the renewable thermally driven HPs are more efficient than the conventional, air-source ones, with the seasonal coefficient of performance increasing by 9.98% (ground source), 4.57% (dual source), and 17.40% (triple source), compared to the air-source heat pump system. Finally, it is revealed (via integrated techno-economic analyses) that the most effective and economical design is the dual source system, while the most expensive is the ground-source configuration. These findings can guide the ongoing design efforts on green residential heat solutions at both research and commercial implementation level.

Performance improvement of air-source heat pump via optimum control based on artificial neural network

The use of air source heat pump (ASHP) systems is an economically viable strategy for building decarbonization. However, the ASHP energy performance deteriorates under severe conditions around a building. Researchers have conducted studies for optimal compressor control to overcome these limitations. Despite these efforts, users can seldom perform the optimal regulation of heat pump systems because manufacturers are reluctant to relinquish control to them. Therefore, we developed an artificial neural network (ANN)-based optimum control logic (OCL) system such that users can directly control and optimize heat pump systems without a compressor. The developed ANN-based OCL controls the secondary-side working fluid of the heat pump considering general building conditions, and its practical applicability has been verified using dynamic simulation. The improvement in the energy performance of the optimum models with respect to the conventional models was: 1.52% and 3.58% for the cooling and heating system COP, respectively, and 0.76% and 0.81% for the heat pump COP. These results demonstrate the potential for reduction in the carbon footprint of a building while maintaining comfort by enabling the energy-efficient operation and stable load response of the heating and cooling system.

TRNSYS simulation study of the operational energy characteristics of a hot water supply system for the integrated design of solar coupled air source heat pumps

To address the issues with solar water heating systems taking up a lot of space, unstable hot water supply, air source heat pumps susceptible to frost in the winter, and low energy efficiency. The TRNSYS tool is employed in this work to simulate a solar-coupled air source heat pump system. The heat pump operation is first investigated using the inverse Carnot cycle. The performance coefficient is then calculated by the second law of thermodynamics without considering the pipeline's pressure drop and heat loss. The output temperature of the hot water that the heat pump circulates is then determined. The daily hot water needs can be estimated roughly based on information about solar radiation. The heat balance equation for flat plate solar collectors was used to compute the intensity of solar diffused radiation. The Berlage calculation was used to determine the solar radiation received on the collector's surface. After a qualitative analysis of the heat from the heat source, the efficiency of the linked heat pump and the conventional air source heat pump was compared. Analyzing the water temperature change graph for each month's data reveals that the system can achieve 50 °C during the water supply time each month. The heat pump's annual energy consumption is 6252.01 kWh, while the system's annual energy consumption is 9100.47 kWh. The study findings may be used as a guide to improving the design and management of the whole system. In addition, they may improve the solar water supply system's performance.

Theoretical and experimental investigation on the rolling piston compressor with enhance vapor injection for heat pump system

The rolling piston compressor with enhanced vapor injection (EVI) is widely used in the household air-source heat pump system. To study the thermodynamic performance and the optimal injection pressure, this paper presents a mathematical method to simulate the whole working process of rolling piston compressor with EVI, which considers suction backflow process, discharge process, vapor injection process and leakage process. According to the calculation, the effects of injection pressure on discharge mass flow rate, p-V diagram and indicated power have been analyzed. In order to verify the accuracy of simulation, an experiment has been carried out to obtain the discharge mass flow rate and the errors between simulated and experimental results are within 4%, which shows that the proposed method is precise enough. Additionally, the experimental results show that EVI can not only improve the heating capacity and COP but also decrease the discharge temperature. However, the optimal injection pressure corresponding to the maximum heating capacity is different from that corresponding to the maximum COP. From the perspective of the heating capacity, the optimal injection pressure is a little higher than the geometric mean of suction pressure and discharge pressure but from the perspective of COP, the optimal injection pressure is a little lower than the geometric mean.

Theoretical Study and Experimental Validation on the Applicable Refrigerant for Space Heating Air Source Heat Pump

The air source heat pump (ASHP) is developing rapidly and is widely used for space heating due to its potential for increasing energy efficiency and reducing greenhouse gas emissions. The choice of appropriate low global warming potential (GWP) alternative refrigerants is one of the challenges that ASHP systems face. Alternative refrigerants also affect the energy performance of these systems. Thus, it is essential to evaluate the performance of ASHP using environmentally friendly refrigerants to facilitate reasonable refrigerant selection. A theoretical model for simulating ASHP performance with different refrigerants is developed in this study. Experiments are conducted to validate the theoretical model. The simulation and the experimental results are found to be in good agreement. The ASHP performance indices, such as compression ratio (CR), discharging temperature (DT) and coefficients of performance (COP), are investigated using R22, R417A, R410A, R134a, R152a, R161 and R1234yf as working fluids. The results show that R152a has an average COP of 2.7% higher than R22, and R161 has an average COP of 1.4% higher than R22. R152a and R161 also have a higher CR but a lower DT than R22 under the same design conditions. In addition, R152a and R161 have ozone depletion potentials (ODP) of zero and extremely low GWPs; thus, they can be candidates to replace R22 in ASHP heating systems. This research provides a reference on refrigerant replacements for ASHP heating systems in North China.