Heating Coefficient Of Performance Research Articles

Performance analysis of a solar-assisted ground source heat pump with a single vertical U-tube ground heat exchanger

Owing to higher heat loads than cooling loads in cold climates, the performance of ground source heat pumps (GSHPs) can significantly degrade over time, leading to eventual system failure for systems not properly sized or when there is insufficient regeneration. Solar-assisted ground source heat pump (SAGSHP) systems are receiving increasing interest as a means of enhancing heating performance and maintaining ground thermal balance in cold regions. However, a critical gap remains in the literature: no study has comprehensively analyzed configurations of SAGSHP systems to enhance their overall performance considering long-term performance and realistic operating conditions. For this purpose, a comprehensive investigation of the long-term performance of single U-tube and double U-tube ground heat exchangers (GHEs) for solar-assisted ground source heat pump systems (SAGSHPs) was carried out and compared with the conventional GSHP. The effect of collector tilt angles (0–90°) on the performance of the single U-tube SAGSHP was also examined. A finite volume computational fluid dynamic model was developed and coupled with an actual building energy load profile and a solar irradiation profile based on Calgary’s weather conditions using a novel custom-coded thermodynamic model. Results show that the yearly average heating coefficient of performance (COP) increases by a factor of 1.21 and 1.18 for the single U-tube SAGSHP and double U-tube SAGSHP, respectively, when compared with the conventional GSHP. However, the average annual cooling COP reduces by 0.65 for the single U-tube and 0.72 for the double U-tube SAGSHPs. Moreover, considering the effect of tilt angle on the single U-tube SAGSHP, it is possible to improve the cooling COP when the collector tilt angle is 90°, at the expense of lower heating COP. However, the highest heating COP and annual energy savings are achieved with a tilt angle of 45°. This study provides practical insight into SAGSHP performance enhancements in cold climates.

Thermodynamic and economic analysis of a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle with dual evaporators

This paper proposes a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle with dual evaporators (SEFVIC), designed for heat pump drying application. Compared to the standard flash tank vapor injection heat pump cycle (FVIC), the SEFVIC uses an additional high-temperature evaporator to further minimize irreversible losses during the heat transfer process. Moreover, it integrates solar energy to improve the drying quality and heating capacity of the cycle. Meanwhile, the introduction of an ejector improves the system's performance by reducing throttling loss. The advantages of SEFVIC over FVIC are evaluated using theoretical models based on the first and second laws of thermodynamics. Results show that SEFVIC reveals a 34.1 % improvement in the heating coefficient of performance and a 192.8 % enhancement in volumetric heating capacity compared to FVIC under typical operating condition. Notably, SEFVIC maintains superior performance even at lower evaporating temperatures. Moreover, exergy research indicates that 77.1 % of the total exergy destruction in SEFVIC is attributed to the solar collector. Economic analysis indicates that the integration of solar energy into the SEFVIC is economically beneficial. These findings highlight the prospective application of SEFVIC and guide combining the multi-temperature heat pump drying system and auxiliary solar source.

Prediction research on the contribution degree of solar energy to the coefficient of heating performance of carbon dioxide heat pump

The carbon dioxide heat pump (CDHP) offers efficient heating over a wide temperature range, but its heating energy efficiency under low-temperature conditions decreases obviously due to a correlation with the evaporation temperature, limiting the widespread use of CDHP in cold regions. To address this, we implemented a passive solar house with short-term heat storage capacity to raise the evaporation temperature and studied it by experimental and theoretical analysis. We introduced the concept of contribution degree (K) of solar energy to the heating performance coefficient of CDHP. The prediction models for the evaporation temperature, heating performance coefficient, and K were constructed gradually. The study optimized the operation mode of solar-assisted CDHP using the evaporation temperature partition theory and presented a method to calculate the limit distribution range of K. The result shows that the order of importance level of the significant items on evaporation temperature was ambient temperature (Ta) > direct solar irradiance on the inclined surface (Gb) > scattering solar irradiance on the inclined surface (Gr) > GḃTa > Ta2 > Gb2. The predicted limit distribution range of K was [0.00, 8.17]. The study optimized the operation mode of solar-assisted CDHP into three categories. It was observed that K changes positively with any single variable according to the law of a composite function composed of a natural exponential function and a quadratic function. Our study suggests a feasible development direction and offers a new theoretical and engineering basis for overcoming the large-scale application limitations of CDHP in cold regions.

Process comparison and performance evaluation of different entrainers for pressure swing extractive distillation refining gasoline additives based on multi-objective optimization and process intensification

As a new gasoline additive, methyl tertiary amyl ether (TAME) can improve the antiknock performance and quality of gasoline. This study uses an energy-saving process for separating TAME/methanol (MeOH)/H2O by pressure-swing extractive distillation with different entrainers based on multi-objective optimization and 4E analysis. The 1entrainers are screened by the difference in relative volatility of components under different entrainer conditions, and the electrostatic potential of the molecular surface of the system and entrainers are analyzed. Multi-objective optimization of the pressure swing extractive distillation process with 5 different entrainers is carried out based on the generation non-dominated sorting genetic algorithm (NSGA-Ⅱ) optimization scheme, and the Pareto frontier is optimized by the minimum distance method. The results indicate that the extraction schemes of 1,3-propanediol (PDO) and dimethyl sulfoxide (DMSO) have excellent economic and environmental benefits through comprehensive analysis. As an effective means of process intensification, the application of heat pump technology can further reduce process costs and environmental pollution. The coefficient of heating performance (COP) is used to find the most suitable pairing route for heat pump-assisted processes, achieving the goal of reducing gas emissions and energy use. Then, different processes are evaluated in terms of economy, energy consumption, environment, and exergy loss. The results show that the process intensification scheme based on heat pump technology can effectively improve the economic and environmental benefits of the process, reducing the annual total cost (TAC) and gas emissions by 9.19% and 23.55%, respectively. This study provides the selection and reference for azeotrope separation of TAME and recycling of wastewater.

4E assessment of ejector-enhanced R290 heat pump cycle with a sub-cooler for cold region applications

Currently, the worldwide implementation of the Montreal Protocol has accelerated the development of low GWP refrigerants. R290 as an alternative refrigerant with low GWP has great advantages over R134a for heat pump applications. However, the use of expansion valve throttling in the heat pump cycle has a large irreversible loss. Therefore, an ejector-enhanced sub-cooler vapor injection heat pump cycle (ESVIC) with R290 for water heating is proposed in this paper to improve the energy efficiency by applying ejector. The thermodynamic models are established based on the 4E (energy, exergy, economic and environmental) analysis to compare its performance with the basic ejector-enhanced heat pump cycle (BEEC) and the sub-cooler vapor injection heat pump cycle (SVIC). The energy analysis results show that compared with the BEEC and SVIC, the volumetric heating capacity of the ESVIC is improved by 42.0 % and 21.2 %, and the heating coefficient of performance (COPh) of the ESVIC is improved by 27.6 % and 5.3 %, respectively. The exergy analysis results indicate that in BEEC, SVIC and ESVIC the exergy destruction of the expansion valves is 0.3 %, 13.5 % and 0.8 %, respectively. It is indicated that the adoption of ejector can significantly decrease the exergy destruction of expansion valves. Moreover, the environmental analysis results reveal that the carbon emissions of ESVIC is 21.5 % and 5.0 % lower than those of BEEC and SVIC, respectively. Besides, for the ESVIC system, the carbon emissions of the life cycle climate performance when using R290 are 9.9 % lower than that of using R134a, indicating that the R290 is a potential substitute for R134a in heat pump applications. The economic analysis results demonstrate that the unit exergy production costs of ESVIC is 21.6 % and 5.1 % lower than BEEC and SVIC, respectively.

Experimental and numerical investigation of a novel high-temperature heat pump for sensible and latent heat delivery

This paper investigates, both experimentally and numerically, the performance of a high-temperature heat pump (HTHP) prototype for steam production and thermal energy storage applications using R-1233zd(E) as a refrigerant. The main novelties of this prototype are the incorporation of an external subcooler to separate the sensible and latent heat production. Other novel points are the testing of a new variable-speed piston compressor that endures a discharge temperature of up to 160 °C, and the development of an advanced numerical model based on the components’ specifications. In this temperature range there are very few detailed studies up-to-date. The proposed HTHP was tested experimentally in 50 different working points selected based on the compressor’s limits and on the general requirements for industrial applications requiring heat at high temperatures, up to 150 °C. The results show that the proposed HTHP can deliver a total heat of 38.6 kW at 148.5 °C, within a temperature lift of 66.8 K, with an electric power consumption of 10.7 kW, and with a heating coefficient of performance (COP) of 3.6. Moreover, using the external subcooler resulted in a substantial increase in the COP, which can reach 33 % compared with similar HTHPs without a subcooler. After successfully validating the HTHP model with the experimental results, extended performance maps were developed based on more than 500 possible operating conditions covering the most common steam generation and energy storage applications. Finally, four performance correlations were obtained, with an accuracy of ±10 %, hereby providing a black-box model to ease the integration of such HTHPs into dynamic simulations of industrial processes.

Thermodynamic and economic analyses of modified ejector enhanced solar-air composite dual-source heat pump system in residential buildings

This research utilizes an ejector enhanced solar-air composite dual-source heat pump (EDHP) system for water heating in residential buildings. The main advantage of the EDHP system is the use of an ejector and dual-heat source to efficiently utilize solar energy, thereby enhancing heating performance. This ejector operates by utilizing a refrigerant stream from the solar collector line with higher pressure to elevate the suction pressure of the compressor. This feature enables the EDHP system to maintain stable heating performance even in adverse weather scenarios with low temperatures or limited solar radiation. Thermodynamic modeling using energetic and exergetic analysis methods is employed to theoretically evaluate the conventional air-source heat pump and EDHP systems. The energy analysis of the EDHP system using R134a refrigerant has shown significant improvements in both heating coefficient of performance (COP) and exergy performance (ECOP). Under specific conditions, such as an ambient temperature of −5 °C and an irradiance level of 250 W∙m2, and utilizing a 20 m2 PVT (Photovoltaic and Thermal) collector for a heating supply of 5 kW, the EDHP system exhibits a 23.64 % increase in COP and a 25.83 % increase in ECOP. Furthermore, when the electricity generated by the PV system is incorporated into the EDHP system, the demand for electricity from the grid can be reduced by up to 51.96 % under the same conditions. In the Beijing area and considering the current energy price system, the heating operating cost of the EDHP system is 0.206 Yuan/kWh. Compared to a standard air-source heat pump system, the payback period for EDHP system ranges approximately from 10 to 12 years, taking into account an annual heating period of 5 months. Among the alternative refrigerants to R134a in the EDHP system, R152a is found to be the most suitable. It shows COP improvements of 24.92 %–30.0 % and ECOP improvements of 24.89 %–29.96 %. This innovative approach opens up new possibilities for the application of heat pump technology under adverse weather conditions in residential and commercial buildings.

Performance assessment and working fluid selection for heat recovery ejector heat pump system in cold climatic conditions

The paper aims to analyze the performance and identify suitable working fluids for a new heat recovery ejector heat pump (HREHP) system. This system introduces a flue gas waste exergy recover circuit combined with the traditional ejector heat pump. Considering the factors of performance, environmental impacts, flammability, and fluid type, four working fluids are selected (i.e., R245fa, R365mfc, RE245fa2, and R1233zd(E)). Comparative study initially conducted to demonstrate the advantages of the proposed heat recovery ejector heat pump facility. Subsequently, the effects of the operational parameters on the novel system performance are investigated to derive design and operation criteria through energy and exergy analysis. Finally, sensitive analysis of generation temperature (Tg), flue gas temperature (Tfg,out), and supply water temperature (Tw) on the proposed system is carried out. Compared with traditional air source heat pump, the application of the HREHP shows higher energy efficiency and lower operating costs at lower ambient temperatures. The results indicate that the primary energy efficiency of the proposed system is 27.0 % and 35.56 % higher than that of conventional ejector heat pump cycle and air source heat pump cycle, respectively, at an ambient temperature of −10 °C. Besides, Tg has a stronger effect on the entrainment ratio and heating coefficient of performance; The primary energy efficiency is more sensitive to Tw variation. Lastly, the R365mfc and R1233zd(E) show good potential as the alternative for the R245fa. The design methods, evaluation methods, and operational characteristics obtained in this study can provide reference and guidance for the application of heat recovery ejector heat pump technology in practical engineering.

Water-free surface silanization on composite zeolite 13X/MgSO4 in a direct-contact adsorption heat pump for stable steam generation

Direct-contact method has been utilized in an open-loop adsorption heat pump based on composite adsorbent for high-temperature steam generation. Deliquescence problem arisen from the loss of salt in adsorbent under high humidity environment limits the stable performance during adsorption process. The composite zeolite was functionally modified with octyltrimethoxysilane (OTMOS) agent during water-free process based on liquid chemical grafting method. Measurement results from XRF (X-Ray Fluorescence) confirm the stable amount of salt is kept inside the modified zeolite. Characterization results from BET (Brunauer-Emmett-Teller) reveal the facts that pore volume and diameter are reduced simultaneously with the decrease in surface area. During regeneration dry air at 130 °C is used for removing water out of the packed bed filled with S8-7.5 (the composite adsorbent modified with 7.5 % OTMOS). During generation process superheated steam at 200 °C is obtained directly from hot water at 72 °C. GTL (Gross Temperature Lift) reaches 115 °C. COPh (coefficient of performance for heating) and SHP (specific heating power) for steam generation are increased by 16.2 % and 10.4 %, respectively after modification. The corresponding increases in system indicators stem from the promotion of steam mass, as adsorption abilities for composite zeolite are maintained well during direct contact with liquid water after modification.

Development and performance testing of a polyvalent heat pump for hot and cold water production

This research investigates a polyvalent heat pump that simultaneously produces hot and cold water and uses natural refrigerants. The novelty of using a 48 V direct current compressor driven by solar photovoltaic (PV) energy and the staged retrofitting improvements to this heat pump are discussed. Empirical testing has indicated that using a suction line heat exchanger yields a 30% improvement in the heating Coefficient of Performance (COP). Rising ambient temperatures could positively and negatively impact its heating and cooling performance, respectively. In addition, an average COP of 3.8 was obtained after a one-hour simultaneous mode operation, with the average hot tank temperature rising from 32.7 to 58.9°C and the average cold tank temperature dropping from 18 to 14.4°C. Being capable of operating the polyvalent heat pump under renewable energy sources and utilizing all its generated heating and cooling energy contributes further to the electrification and decarbonization of residential buildings.

Effect of drying air supply temperature and internal heat exchanger on performance of a novel closed-loop transcritical CO2 air source heat pump drying system

A novel closed-loop transcritical CO2 air-source heat pump drying (HPD) system with a relatively simple structure, higher drying temperature, and internal heat exchanger (IHX) for the drying of medical equipment was proposed and built. With or without the IHX in the system, the effects of different drying air supply temperatures on the performance of transcritical CO2 HPD system were studied experimentally. The results showed that the maximum drying air supply temperature (DAST) in the designed system could reach 72.9 °C. As the DAST raised, whether with or without the IHX in the system, the coefficient of heating performance (COPh), coefficient of system performance (COPsys), and specific moisture extraction rate (SMER) decreased gradually and moisture extraction rate (MER) increased continuously. When the DAST was 70 °C, COPh, COPsys, MER and SMER were respectively 3.68, 6.07, 3.27 kg/h and 1.68 kg/(kW·h) in the HPD system without the IHX, and were respectively 4.00, 6.89, 3.67 kg/h and 1.97 kg/(kW·h) in the HPD system with the IHX. In contrast, the compressor discharge pressure decreased 4.26 %–5.99 %, COPh increased 1.39 %–8.65 %, COPsys increased 1.89 %–13.57 %, MER increased 4.68 %–14.01 %, SMER increased 6.93 %–17.18 % in the HPD system with the IHX.

Performance evaluation of a solar-assisted ejector-enhanced vapor injection heat pump cycle

In this study, a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle (SEE-FVIC) is proposed. Compare to the flash tank vapor injection heat pump cycle (FVIC), the modified cycle introduces solar energy and ejector. On the one hand, the heat from the solar energy is used. On the other hand, the booster function of the ejector effectively reduces the refrigerant mass flowing into the low-pressure stage of the compressor, leading to reducing power consumption of the compressor and improving cycle performance. The cycle performance is studied under typical operating conditions. In particular, a comprehensive analysis of the optimum intermediate pressure Pio of the compressor is also carried out. The results show that the optimum Pio of the SEE-FVIC is higher than that of FVIC for the same operating conditions, which is 1100 kPa and 1300 kPa, respectively. Compared to the FVIC, the heating coefficient of performance (COPh) and volumetric heat capacity (Qcv) of the SEE-FVIC are increased by 38.4 % and 89.2 %, respectively, at the corresponding optimum Pio. Besides, the change of Pio has a significant effect on the volume ratio (β) and vapor injection ratio (δ) of the compressor. It is indicated that the δ varies from 0.48 to 0.67 and the corresponding β from 0.90 to 1.16 when the COPh of the SEE-FVIC reaches its maximum value for a given operating conditions.

Experimental study on thermal performance of a closed heat pump drying system

The compressor speed, expansion valve opening and drying temperature are critical to the thermal performance improvement and control logic optimization of the closed heat pump drying (CHPD) system. This paper investigates the effects of the three parameters on the thermal performance of the CHPD system by the single factor experiment method. According to the experimental findings, as the compressor speed of the system increased, the heating capacity grew by 11.4%, but the coefficient of performance (COP) declined by 41.3%. As the expansion valve opening of the system increased, the heating capacity and COP grew by 73.5% and 82.9%, respectively. As the drying temperature of the system rose, the heating capacity grew by 3.1%, but the COP dropped by 25.3%. It can be concluded that reducing the compressor speed and drying temperature, and increasing the expansion valve opening could effectively enhance the thermal performance of the CHPD system.

High-Temperature Heat Pump Using CO2-Based Mixture for Simultaneous Heat and Cold Energy Reservation

To leverage temperature glide in evaporation, a transcritcal heat pump using a CO2-based mixture is investigated from a perspective of simultaneous heat and cold energy storage. Coefficient of performance for heating (COPh) and exergy efficiency are used to evaluate system performance. A parametric investigation on the heat pump is conducted, and the coupling behavior of the cycle with thermal energy storage (TES) material is investigated in view of stored exergy of TES. Optimization and comparative studies are carried out among various mixtures. The results reveal that maximum cycle temperature is mainly affected by high pressure and superheating degree, while minimum cycle temperature, as well as cold exergy, is highly dependent on evaporating temperature glide, with little influence from high pressure. The total exergy efficiency can reach up to 60%. The temperature of low temperature TES could reach as low as −32.4 °C for CO2/R601, providing the largest proportion of cold exergy to total exergy, up to 30.1%.

Energy, exergy and economic evaluation of a solar enhanced ejector expansion heat pump cycle

Recently, heat pump technology has become the preferred method in drying field. However, most existing heat pump cycles use the single-stage vapor compression with expansion valve, which has the problem of large throttling loss. The use of ejector in the heat pump cycle can recover part of the expansion work and enhance cycle performance. Therefore, this paper proposes a solar-enhanced ejector expansion heat pump cycle (SEEHP) for drying application. Based on the conventional ejector expansion heat pump cycle (EEHP), the introduction of solar energy could not only improve the heating capacity of the cycle, but also improve the performance of the ejector. The energy, exergy and economic evaluation methods are used to theoretically analyze the EEHP and SEEHP. Compared to the EEHP, the energy analysis shows that the SEEHP with R134a refrigerant has a 23.64%–39.82% increase in heating coefficient of performance (COPh) and 32.79%–33.96% increase in volumetric heating capacity (Qcv) for the given condensing temperature range. Exergy analysis results reveal that the exergy destruction of SEEHP is mainly concentrated in the solar collector, indicating that the solar collector has the greatest potential for optimization in practical application. Economic analysis shows that investing in solar energy in the SEEHP is beneficial.

Optimization of an integrated system including a photovoltaic/thermal system and a ground source heat pump system for building energy supply in cold areas

The photovoltaic (PV) collectors tend to overheat in summer, which reduces the efficiency of power generation. The unbalanced heating and cooling loads cause the ground source heat pump (GSHP) system to operate unstably in severe cold areas. To solve the above two problems simultaneously, an integrated system including a photovoltaic/thermal system and a ground source heat pump system (PVT-GSHP) was proposed, which had an extremely high energy utilization efficiency. However, the complex variation of cooling and heating loads was influenced by meteorological conditions over time. The demand of efficient operation and good economy of the system challenged the optimal configuration and control strategy among multiple energy sources, which was investigated in this study. A dynamic model of the PVT-GSHP system based on TRNSYS was developed and the corresponding control strategy was formulated. The genetic algorithm was used to optimize the matching of buried pipe lengths and PVT collector areas. The effects of different matching on the system parts and the whole performance were analyzed. The results show that increasing the PVT collector area can increase the ground temperature and shorten the buried pipe length, but it is not good for summer cooling, thus there is an optimal match. Compared with two independent systems of the traditional GSHP system and PV generation system, the PVT-GSHP system has excellent advantages and application prospects because it has 48.3% reduction in the total buried pipe length, 68.89% reduction in the footprint area of ground heat exchangers, 10% increase in the heating coefficient of performance (COP) and 72.3% reduction in life circle cost. This study provides theoretical support for the optimal design of the PVT-GSHP systems and similar complementary multi-energy systems.

Process steam generation from low-grade waste heat in a direct- contact adsorption heat pump based on superhydrophobic surface-modified Zeolite 13X

ABSTRACT Adsorption heat pump (AdHP) is a potential technology to recover and storage low-temperature waste heat. This paper presents an experimental research on direct low-pressure process steam generation from hot water based on superhydrophobic surface-modified zeolite 13X. The measured water contact angle of the modified zeolite is larger than 150°, which restricts free water entering the pores inside the zeolite particles. Cyclic experiments are conducted for performance evaluation. High-temperature steam (205–229°C, 0.1–0.3 MPa) is generated from hot water (72°C) in the AdHP driven by dry air (130°C). The temperature of generated steam is promoted with increasing preset steam pressure, among which the temperature is raised by about 23°C at 0.3 MPa. As the boiling point of water rises simultaneously, the sensible heat demand is increased. Thus, the mass of the steam is accordingly decreased. GTL (gross temperature lift) reaches 130°C. When the preset pressure is increased, COPh (coefficient of performance for heating) and SHP (specific heat power) for direct steam generation are decreased by 36% and 39%, respectively. One feasible option to elevate these two indicators is to increase the inlet water to near boiling water to match the change in the preset pressure. However, the GTL will be decreased based on the option. Balance should be made based on the choice of design for practical application.

Experimental investigation of an air source heat pump with multigroup heat pipe radiators

As a clean and sustainable electrified heating technology, air source heat pump (ASHP) has been widely used in household in China. However, some problems occurred with the commonly used water loop system, such as low efficiency, complicated structure, more likely to be frozen and burst as well as high electric energy consumption, etc. In this paper, a new air source heat pump with multigroup heat pipe radiators (ASHPMP) was presented. In ASHPMP system, the condenser and heat pipe were coupled together to form a new type of heat radiation terminal, which was named as heat pipe radiators, multiple terminals driven by the compressor provided heat for several rooms. An experimental apparatus of the ASHPMP system was developed and the experiment was carried out in residential buildings also, and R410A was used in the heat pump and R134a was used in heat pipes. The influence of temperature, pressure of heat pump and heat pipe on the heating performance of ASHPMP under frosting/defrosting condition was analysed. The results showed that the new system can operate stably under outdoor temperatures of −12.7 °C to 6.5 °C. As the indoor temperature was set at 20 °C, the heating coefficient of performance (heating COP) could reach up to 3.15 with outdoor temperature −12.7 °C, and it was 6.73 with outdoor temperature 6.5 °C.

Large heat pumps with hot water store in local heating systems – Investigation of operation strategies

In the context of the German energy transition, heat pump technology utilizing heat and electricity from renewable sources is an alternative to traditional fossil fuel based boilers, which enables a sustainable heat supply. Therefore, the transformation of a residential area in Germany into a zero-emissions district employing a heat-pump-based local heating system with hot water store is investigated. The heat pumps are supposed to use the return flow of an existing district heating network as heat source. Most commercially available heat pumps have a standard temperature spread from 3 K to 5 K in the evaporator. In this case, the given limited source flow rate leads to a relatively low heat capacity of the heat pumps (output), which requires an unusual large dimension of the hot water store and impairs the economic efficiency of the heating system. To ensure an optimal store dimension, a possible solution is to install a mixing circuit on the heat source side of the heat pump (with a 5 K temperature spread in the evaporator) to increase the temperature spread of the heat source by remixing the return water, whereby the coefficient of performance for heating becomes smaller. Another solution is to raise the temperature spread in the evaporator directly (design of heat exchanger), also in this case, the coefficient of performance for heating diminishes. Here, we present the theoretical investigation of two heat pump configurations as well as operating strategies: a) Installation of a mixing system in the heat pumps with standard temperature spread in the evaporator (5 K) and b) constant water flow through the evaporator with varying temperature spread up to 15 K. The heat pumps were simulated using the program EBSILON®Professional. The simulations show that the strategy a) achieves higher coefficient of performance εh, HP within low heating capacity range (300 kW to 750 kW), while the strategy b) has better coefficient of performance εh, HP within high heating capacity range as well as a larger total heating capacity range up to 1750 kW. The result is a better annual coefficient of performance for heating of the strategy b) in the system simulations (heat pumps and hot water store system operation) using the Program TRNSYS. The hot water store decouples heat pump operation from heat load to ensure that the heat pumps always work at the optimum operating points.

The influences of the oil circulation ratio on the performance of a vapor injection scroll compressor in heat pump air conditioning system intended for electrical vehicles

Heat pump air conditioning (AC) based on a vapor injection scroll compressor has proven to be a promising solution to the current driving range limitations of electric vehicles (EV), especially in freezing climates. The lubricating oil inside the compressor is critical in promoting sealing and cooling, but it also exhibits a low performance under extreme working conditions. In addition, the effects of oil retention on compressor behavior and the optimal charge amount at low temperatures require further investigation. This paper adapts a second refrigerant calorimeter method to study the effects of different injection pressures on compressor performance. The results indicate that the mass flow rate, discharge temperature, compressor power consumption, and volume efficiency performance would increase significantly compared with the non-vapor injection (NV). The effects of oil circulation rate (OCR) on the performance of a vapor injection scroll compressor under two low-temperature operating conditions were also studied. The flow and heat transfer parameters were analyzed, and heating capacity was found to increase and decrease with enlarged OCR. The highest coefficient of heating performance (COP) was obtained at an OCR of 3.5% with a vapor injection pressure of 0.28 MPa. Compared with the non-vapor injection (NV) condition, the heating capacity and COP of the system at the same pressure of 0.28 MPa were increased by 6.2% and 1.8%, respectively. At extremely low temperatures, the heating capacity and COP were maximized at an OCR of 5.5% and an injection pressure of 0.45 MPa. Compared with that of NV, the heating capacity and COP of the heating system at the injection pressure were increased by 6.7% and 2.6%, respectively. The compressor discharge temperature decreased obviously with the increasing OCR. An empirical formula was devised to calculate the heating capacity for varying OCR.