Vapor Compression Heat Pump Cycle Research Articles

Performance assessment of residential heat pumps operating in extreme cold climates using zeotropic mixtures

Extremely cold climates subject residential heat pumps to significant temperature differences between the heat source and the ambient being heated, which may lead to system failure and reduced compressor performance. The present study considers the possibility of improving the performance of heat pump systems that are simultaneously used for residential space and domestic water heating while subjected to climates varying from -25 °C to 5 °C by exploring the use of zeotropic mixtures. CO2-based binary mixtures composed of low-GWP (global warming potential) refrigerants are considered – R32, R1234yf, and R290 – aiming to benefit from environmentally friendly and flame suppressants characteristics of CO2, as well as the improved thermal efficiency granted by the addition of a secondary refrigerant. A thermodynamic model was developed for a standard vapor-compression heat pump cycle and used to maximize the coefficient of performance limited by the minimum pinch point in the heat exchangers. Our analysis explores the effect of several parameters, such as, the mixture components, mass fractions, space and water heating demands, and heat source temperature on the heat pump's performance. For cold climates, the mixture of R32 (90%)/CO2 (10%) yields the highest COP and CO2-rich mixtures exhibit the lowest. However, by increasing the mass fraction of CO2 within the zeotropic mixture, the pressure ratio of the heat pump was improved. When considering combined performance criteria, such as the volumetric heating effect and the total heat delivered related to heat pump size, CO2-rich mixtures tend to allow more compact systems, especially in colder climates.

Multi-mode and exergoeconomic analysis of a novel combined cooling, heating, and power system applied in the geothermal field

The exploitation and utilization of geothermal resources contribute to alleviating energy crisis and reducing environmental pollution caused by the use of fossil fuels. The energy systems driven by low-grade geothermal heat sources also have the potential for energy-saving and emission reduction. Among all kinds of energy systems, multi-mode systems with characteristics of various operation modes and flexible energy output have been widely concerned. In this work, a multi-mode CCHP system with a novel structure is proposed and driven by a closed type of geothermal heat source. The proposed parallel system is composed of a vapor compression heat pump cycle, an organic Rankine cycle, and a vapor compression refrigeration cycle. The exergy efficiency and total product unit cost are selected as objective functions in multi-objective optimization processes to characterize system thermodynamic and exergoeconomic performances, respectively. The proposed system can operate in seven different modes to realize the conversion of power, heating, and cooling capacity output. The effects of main parameters, including evaporation pressure, condensing temperature, split ratio, etc, on system performances were studied. Results found that the coupling evaporator had the highest exergy destruction rate while the component turbine showed the largest capital cost. From the viewpoint of exergoeconomic performance, the turbine and coupling condenser were the most two important components. After that, the multi-objective optimization results revealed the limits of comprehensive system performance under such closed heat source conditions. Finally, the adjustable analysis found the performance range of the proposed system regulating the seven different operation modes. The exergy efficiency and total product unit cost fluctuated in the ranges of 18.68–52.64 % and 9.56–14.93 $/GJ, respectively.

Comparative analysis and optimization of pumped thermal energy storage systems based on different power cycles

As the proportion of renewable energy in the world’s energy mix gradually increasing, energy storage technologies are gaining more and more attention. Pumped thermal energy storage (PTES) technology is one of the most promising electrical energy storage technologies. In many power cycles that can be adopted as discharge subsystems of PTES, organic Rankine cycle (ORC) is widely used because of its advantages of efficiently utilizing low-temperature waste heat and converting it into electrical energy. In addition, organic flash cycle (OFC) is considered as a promising candidate for the thermal power conversion subsystem of PTES due to its higher potential performance. In the present work, by taking four different forms of OFC and one basic ORC as power cycles, five PTES systems are established, optimized and analyzed. The results show that the turbine and compressor occupy the largest percentage of the total investment cost, while the throttle valve has the largest exergy destruction. The optimized results illustrate that the PTES system combining basic vapor compression heat pump cycle with basic organic Rankine cycle (BORC-BVCHP) shows the best economic performance with the Levelized cost of storage (LCOS) value as low as 0.4413 $/kWh at the storage temperature of 403.15 K. BORC-BVCHP system also shows the optimal round-trip efficiency (31.15%) and maximum exergy efficiency (23.40%), respectively. This indicates that ORC-based PTES system has obvious advantages over the OFC-based PTES systems in terms of economic performance and thermodynamic performances, and ORC is more suitable to be applied in the study of PTES system.

Entransy transfer analysis methodology for energy conversion systems operating with thermodynamic cycles

The entransy transfer analysis methodology proposed in this article offers an alternative novel approach of thermodynamic analysis for assessment and optimization of various energy conversion systems operating with thermodynamic cycles. It is proved that TdU entransy and TdH enthalpy-entransy are not state functions for thermodynamic systems with non-ideal gas substances, so the entransy transfer analysis methodology is constructed based on entransy process functions for generic thermodynamic systems. Accordingly, the thermodynamic evaluation indices of entransy (transfer) dissipation, cyclic net entransy transfer, and entransy (transfer) efficiency are defined. From the case studies of an organic Rankine cycle and a vapor-compression heat pump cycle, lower entransy (transfer) dissipation and higher entransy (transfer) efficiency can be adopted as the optimization objectives for heat engine and compression heat pump systems. Besides, lower cyclic net entransy transfer is preferred for thermal energy storage systems given the charging/discharging heat. The core difference between entransy transfer analysis and exergy (entropy) analysis is that entransy transfer highlights the heat transfer phenomenon, while exergy and entropy focus on the heat–work conversion phenomenon. Moreover, thermodynamic cycles can be classified into forward and reverse cycles in the sense of entransy transfer according to positive and negative cyclic net entransy transfer.

Numerical simulation on solar collector and cascade heat pump combi water heating systems in Kazakhstan climates

At low ambient temperatures, the heating capacity and coefficient of performance of a single stage vapour compression heat pump cycle is significantly getting reduced. The two stage cascade heat pump cycle operating with two different refrigerants provides a sustainable solution to lift the condenser temperature above 343 K. In this work, a numerical simulation model was developed for predicting the performance of a solar collector and two stage cascade heat pump combi water heating systems under Kazakhstan climatic conditions. The numerical simulation was performed for winter climatic conditions using nine refrigerant pairs such as, R32/R290, R32/R1234yf, R32/R134a, R410A/R290, R410A/R1234yf, R410A/R134a, R744/R290, R744/R1234yf and R744/R134a. The influences of solar irradiation, ambient temperature and condenser temperature are discussed. The solar collector and two stage heat pump combi heating system has about 30% energy savings when compared to the conventional two stage cascade heat pump without integration solar collectors. The R32/R290 refrigerant pair is having maximum coefficient of performance of 2.4 at 323 K condensing temperature and 263 K evaporating temperature. The refrigerant R744/R290 pair is identified as an environment friendly sustainable option in terms of its global warming impact for two stage cascade heat pump applications.

Comparative study of two novel micro-CCHP systems based on organic Rankine cycle and Kalina cycle

With regard to the significant role of combined cooling, heating, and power (CCHP) systems in performance enhancement of power plants, two novel micro-CCHP systems are presented which are based on organic Rankine cycle (ORC) and Kalina cycle (KC) as topping cycles. Additionally, ejector refrigeration cycle (ERC) and vapor compression heat pump cycle (VCHPC) are utilized as the bottoming cycle of the power systems. To demonstrate feasibility of the recommended micro-CCHP systems, an exhaustive thermodynamic modeling and exergoeconomic analysis are employed as the most effective tools for performance evaluation of the systems. Also, to get better performance of the systems, single- and multi-criteria optimizations are carried out, using genetic algorithm. It is figured out that the KC-based micro-CCHP system has higher optimum thermal efficiency and total sum unit cost of the product (SUCP) than the ORC-based micro-CCHP system, while it had lower exergy efficiency. Regarding that, the optimum thermal efficiency for the ORC- and KC-based micro-CCHP systems are computed by 76.54% and 77.32%, respectively, whilst the optimum exergy efficiency for the ORC- and KC-based micro-CCHP systems are calculated by 48.37% and 31.2%, respectively. In addition, generator had the major exergy destruction rate among all components for both systems. Furthermore, the results of parametric study proved that higher thermal (energy) efficiency can be computed with increasing the heat source temperature, heater temperature, evaporation temperature, and terminal temperature difference of the recovery heat exchangers.

Cycle performance evaluation of various R134a/hydrocarbon blend refrigerants applied in vapor-compression heat pumps

Blend refrigerants combing hydrofluorocarbons and hydrocarbons are good substitutes to decrease the flammability of hydrocarbons while reducing the global warming potential of hydrofluorocarbons. Four hydrofluorocarbon/hydrocarbon blends (R134a/R290, R134a/R600, R134a/R600a, and R134a/R1270) with various compositions are investigated in vapor-compression heat pump cycles. The effects of hydrocarbon fraction on the blend properties, including critical temperature, critical pressure, latent heat, saturated liquid line, and azeotropic behavior, are comparatively analyzed. Thermodynamic models are established for heat pump simulation. For each R134a/hydrocarbon blend, both the cooling and heating coefficient of performances generally first decrease and then increase with the hydrocarbon mass fraction. The coefficient of performances of R134a/R600 and R134a/R600a have dramatic changes within the hydrocarbon mass fraction of 0.2–1.0, while those of R134a/R290 and R134a/R1270 have dramatic changes within the fraction of 0.0–0.4. Lower condensing or higher evaporating temperatures lead to higher coefficient of performances. In addition, the volumetric capacities first increase and then decrease with the increase of hydrocarbon fraction. R134a/R290 and R134a/R1270 show much higher volumetric capacities as compared to R134a/R600 and R134a/R600a under higher hydrocarbon fractions, which can greatly reduce the required compressor size of pure R134a. The discharge temperatures are kept in the range of 43.0°C–72.3°C for all the blends. To obtain low global warming potential R134a/hydrocarbon blends, the hydrocarbon fraction need to be greater than 0.9, at which R134a/R1270 performs the best, with cooling/heating coefficient of performances of 5.25/4.70 and cooling/heating volumetric capacities of 4.78/3.53 MJ/m3. Generally, R134a/R290 and R134a/R1270 perform much better than R134a/R600 and R134a/R600a at the low global warming potential composition. This study can contribute to the determination of hydrofluorocarbon/hydrocarbon compositions based on comprehensive considerations of cycle efficiency, volumetric capacity, and low global warming potential target.

Pressure boost thermochemical sorption heat pump cycle

Drying is a common process in industries and usually discharges much waste heat. Heat pump is an energy efficient approach to upgrading waste heat to a higher drying temperature. In this paper, a novel pressure boost thermochemical sorption heat pump (TSHP) is proposed. Different from existing salt/ammonia heat pump cycles, a compressor is installed between two reactors to drive the cycle so that only one sorbent is needed in the cycle and the exothermic process becomes continuous. Thermodynamic analysis shows that when desorption and sorption temperatures are 70 °C and 120 °C, the COP (coefficient of performance) of the new cycle with working pair SrCl2/NH3 is 6.5, which is remarkably higher than that of conventional vapor compression heat pump (VCHP) cycle under same operating conditions. Furthermore, for higher temperature rise, a hybrid cascade heat pump cycle is proposed as well. The VCHP cycle with refrigerant R134a is selected as the low stage cycle, while the TSHP cycle with sorbent SrCl2 is chosen as the high stage cycle. For the evaporating temperature 30 °C and sorption temperature 120 °C, the cascade cycle COP is 2.4, relative to 2.0 COP of conventional vapor compression cascade heat pump cycle.

Thermodynamic analysis of vapor compression heat pump cycle for tap water heating and development of CO2 heat pump water heater for residential use

The ideal vapor compression cycle for tap water heating and its coefficient of performance (COP) have been studied theoretically at first. The ideal cycle is defined as the cycle whose high temperature heat source varies temperature with constant specific heat and other processes are same as the reverse Carnot cycle. The COP upper limit of single stage compression heat pump cycle for tap water heating with various refrigerants such as fluorocarbons and natural refrigerants was calculated. The refrigerant which achieves the highest COP for supplying hot water is CO2. Next, the prototype of CO2 heat pump water heater for residential use has been developed. Its outline and experimental results are described. Finally its further possibility of COP improvement has been studied. The COP considered a limit from a technical point of view was estimated about 6.0 at the Japanese shoulder season (spring and autumn) test condition of heating water from 17°C to 65°C at 16°C heat source air temperature (dry bulb)/12°C (wet bulb).

A comparison of transient heat pump cycle models using alternative flow descriptions

This article compares the effects of two different refrigerant flow modeling assumptions on the transient performance of vapor-compression heat pump cycles. This comparison is done on a dynamic system-level model of a flash tank vapor injection cycle that includes finite-volume heat exchanger models. The effect of the flow assumptions and specific slip ratio correlations on both the equilibrium operating point and the transient behavior of the cycle are demonstrated through both simulations and experiments. It is shown that equivalent simulations with different slip ratio correlations each have different equilibrium mass inventories and that some aspects of the dynamic system behavior, such as the suction superheat or pressure transients, exhibit significant differences both in simulations and in comparison to experimental data.

Regenerated air cycle potentials in heat pump applications

Air (reversed Brayton) cycle has been utilized in the area of refrigeration and cryogenics for several decades, but its potentials in heat pump applications were longtime underestimated. In this paper, a thermodynamic model for the regenerated air heat pump cycle with practical compressor, expander and regenerated heat exchanger was developed. Based on the model, the relations between the system performance and the operating parameters were analyzed. The optimal heating COP (coefficient of performance) and the corresponding pressure ratio were derived. Then, air heat pump cycles (regenerated cycle and basic cycle) and vapor-compression heat pump cycles (CO2 trans-critical cycle and R410A subcritical cycle) were numerically compared. The results indicated that the regenerated air heat pump cycle not only gets the heating capacity in line with the heating load under different operating conditions but also achieves higher COP over trans-critical CO2 heat pump cycle in applications of large temperature difference.

Simulation and experimental study of solar-absorption heat transformer integrating with two-stage high temperature vapor compression heat pump

In this study, simulation and experiment studies of a 10kW solar H2O–LiBr absorption heat transformer (AHT) integrating with a two-stage vapor compression heat pump (VCHP) were carried out. The whole system was named as compression/absorption heat transformer (CAHT). The VCHP was used to recover rejected heat at the AHT condenser which was transferred back to the AHT evaporator at a higher temperature. The AHT unit took solar heat from a set of flat-plate solar collectors in parallel connection. R-134a and R-123 were refrigerants in the VCHP cycle. From the simulation, the total cycle coefficient (COP) of the solar-CAHT was 0.71 compared with 0.49 of the normal solar-AHT. From the experiment, the total cycle COPs of the solar-CAHT and the solar-AHT were 0.62 and 0.39, respectively. The experimental results were lower than those of the simulated models due to the oversize of the experimental compressor. The annual expense of the solar-CAHT was found to be 5113 USD which was lower than 5418 USD of the solar-AHT. So it could be concluded that the modified unit was beneficial than the normal unit in terms of energy efficiency and economic expense.

Advances on air conditioning and heat pump system in electric vehicles – A review

Abstract There is a rising interest in electric vehicle׳s climate control system including cooling and heating. Electric vehicles (EVs, including hybrid electric vehicles and full electric vehicles) have inadequate waste heat to warm up the cabin and the climate control system has a very significant effect on the energy consumption efficiency and operating mileage. Heat pump (HP) is one approach for energy consumption efficiency improvement in EVs which can supply cooling and heating capacity. A literature review was performed on the vapor compression HP cycle design, performance characteristics, and challenges for variable working fluids for electric vehicles. The non-vapor compressor HP systems were also analyzed including the applications of magnetocaloric effect and thermoelectric effect. It seems heat pump system is a reasonable and feasible option in EV׳s climate control system if some essential issues could be solved. The mobile climate control systems based on magnetocaloric effect and thermoelectric effect could be the supplementary methods in future applications.

An important feature of air heat pump cycle: Heating capacity in line with heating load

In the conventional vapor-compression heat pumps, the heating capacity and the heating load usually vary in opposite directions, which results in a mismatch of the heating capacity and the heating load at off-design conditions. Air (reversed Brayton) cycle is a potential substitute for the conventional vapor-compression cycles. This paper proved that in theory the air heat pump cycle can make the heating capacity in line with the heating load at a stable level of heating COP (coefficient of performance). A thermodynamic model for the air heat pump cycle with practical compressor and expander was developed. The optimal heating COP and the corresponding pressure ratio were derived from the model. Then the cycle performance was analytically expressed under the optimal COP conditions. The heating capacity under different operating conditions was found in line with the heating load. Comparisons between the air heat pump cycle and two typical vapor-compression heat pump cycles were numerically done for further verification. It also turned out that the energy efficiency of air heat pump is comparable to the transcritical CO2 heat pump, particularly at large temperature difference.

Experimental investigation of a Thermally Driven Heat Pump based on a double Organic Rankine Cycle and an oil-free Compressor-Turbine Unit

This paper presents the experimental investigation of a concept of Thermally Driven Heat Pump (TDHP), based on the coupling of a vapor compression heat pump cycle and an Organic Rankine Cycle (ORC). The studied concept uses a single stage centrifugal compressor directly coupled to a single stage radial inflow turbine. The shaft is rotating on gas bearings, which allows the system to be oil-free. Like most of the other TDHP's, this system has the advantage to work with a variety of fuels or heat sources like wood pellets, natural gas, solar heat, geothermal heat or waste heat. A prototype using R134a as working fluid and with a heating capacity of about 40 kW has been developed. The results of the first tests are presented.

Performance Analysis on Solar Assisted CO<sub>2</sub> Vapor Compression Heat Pump Cycle with an Ejector

The thermodynamic performances of solar assisted CO2 vapor compression heat pump cycles with an ejector are analyzed and compared with that of conventional CO2 heat pump cycle (named as A cycle). Under the given working conditions, the COP of solar assisted CO2 heat pump cycle with an ejector after compressor cycle (named as C cycle) is much higher than that of CO2 heat pump cycle with an ejector before compressor cycle (named as B cycle) and that of the A cycle because of the lower compressor power of the C cycle. The compressor volume displacement of the C cycle demanded for providing the same heat capacity of gas-cooler is the lowest among the three cycles. So the compressor size of the C cycle is very small and the cost of the C cycle is very less. In the area having rich solar energy resource, it is significant to employ the C cycle for providing space heating by optimizing design ejector and selecting compressor, thus, the performances for this C cycle can be improved greatly. In the area not having rich solar energy resource, the B cycle can be used for providing space heating; the performances can also be improved accordingly.

Optimal Allocation of Heat Exchanger Area for Vapor-Compression Heat Pump Plants

In order to optimize coefficient of heat pump, the finite time thermodynamic optimization theory is applied in this paper. By introducing area ratio of the condenser and evaporator, the analytical expressions for the optimal allocation of heat exchanger area for the irreversible Carnot heat pump with variable-temperature heat reservoirs are derived. For illustration, a real vapor compression heat pump example is utilized. Empirical results show that the optimal values for real vapor-compression heat pump cycles are obtained when area ratio is equal to 0.916. The results can guide the evaluation of existing heat pump plants or influence design practice for future plants.The results can guide the evaluation of existing heat pump plants or influence design practice for future plants.

F222 CO_2/R32混合冷媒ヒートポンプサイクル性能に及ぼす伝熱管長の影響(OS4 省エネルギー・コジェネ・ヒートポンプ)

In this research, we carried out the performance test on the vapor compression heat pump cycle using zeotropic mixtures of CO_2 and R32. The influence of heat exchanger performance on the cycle performance was evaluated by changing the length and form of evaporator. COP of CO_2 and CO_2/R32 mixtures was improved by changing smooth tube for micro-fin tube and lengthening the heat exchange tube of evaporator. The rate of the increase in COP of mixed refrigerants is small compared with CO_2. This reveals that further improvement of the performance can be expected by adjusting form and length of the heat exchanger suitable for CO_2/R32 mixture refrigerants.

A theoretical study of an innovative ejector enhanced vapor compression heat pump cycle for water heating application

This study presents a novel vapor compression heat pump cycle in which an ejector associated with a subcooler is applied to enhance the heating performance for air-source heat pump water heater application. The heating coefficient of performance (COP h) and heating capacity of the novel cycle using the non-azeotropic mixture refrigerant R417A are theoretically investigated, for the ranges of evaporating temperature (−15 to 10 °C) and condensing temperature (55–60 °C). The theoretical results show that the COP h and volumetric heating capacity of the novel cycle are better than that of the conventional heat pump cycle. It is found that for the operating conditions considered, the maximum COP h and volumetric heating capacity can be improved by up to 1.62–6.92% and 15.20–37.32% over the conventional heat pump cycle, respectively. The performance characteristics of the novel cycle show its promise in air-source heat pump water heater applications.

Modeling and economic analysis of gas engine heat pumps for residential and commercial buildings in various climate regions of Iran

There is an increasing trend in using heat pumps in air conditioning (heating/cooling) systems of residential and commercial buildings. The required power to drive the compressor of vapor compression heat pump cycles may be provided by either an electrical motor or an internal combustion engine. In this paper thermal modeling and economic analysis of gas engine heat pumps (GEHPs) are presented based on energy and mass balance equations as well as the gas engine operating parameters (such as thermal efficiency, fuel consumption and fuel mass flow rate) and heat pump operating parameters (such as evaporator and condenser capacity and compressor input power). Based on the modeling results and with estimating GEHP fuel consumption, the economic analysis of using gas engine heat pumps (in comparison with the electrical heat pumps) at various climate regions of Iran, for both residential and commercial (office) buildings, and for both cooling and heating modes, was performed. Appropriate cost functions for predicting GEHP capital investment were proposed. Three approaches including equivalent uniform annual cost (EUAC), the annual cost of energy consumption, and payback period were applied in the economic analysis.