Low-temperature Cycle Research Articles

Exergy Analysis of Two-Stage Organic Rankine Cycle Power Generation System.

Organic Rankine cycle (ORC) power generation is an effective way to convert medium and low temperature heat into high-grade electricity. In this paper, the subcritical saturated organic Rankine cycle system with a heat source temperature of 100~150 °C is studied with four different organic working fluids. The variations of the exergy efficiencies for the single-stage/two-stage systems, heaters, and condensers with the heat source temperature are analyzed. Based on the condition when the exergy efficiency is maximized for the two-stage system, the effects of the mass split ratio of the geothermal fluid flowing into the preheaters and the exergy efficiency of the heater are studied. The main conclusions include: The exergy efficiency of the two-stage system is affected by the evaporation temperatures of the organic working fluid in both the high temperature and low temperature cycles and has a maximum value. Under the same heat sink and heat source parameters, the exergy efficiency of the two-stage system is larger than that of the single-stage system. For example, when the heat source temperature is 130 °C, the exergy efficiency of the two-stage system is increased by 9.4% compared with the single-stage system. For the two-stage system, analysis of the four organic working fluids shows that R600a has the highest exergy efficiency, although R600a is only suitable for heat source temperature below 140 °C, while other working fluids can be used in systems with higher heat source temperatures. The mass split ratio of the fluid in the preheaters of the two-stage system depends on the working fluid and the heat source temperature. As the heat source temperature increases, the range of the split ratio becomes narrower, and the curves are in the shape of an isosceles triangle. Therefore, different working fluids are suitable for different heat source temperatures, and appropriate working fluid and split ratio should be determined based on the heat source parameters.

Comparative study on the effects of oil viscosity on typical coatings for automotive engine components under simulated lubrication conditions

Current emphasis on automotive engine has evoked the desire to improve tribological performance by both reducing friction and making starting easier in the automotive industry. The present study was conducted to determine the effects of viscosity of fully-formulated oil on tribological properties of the typical coatings for automotive engine components, e.g. ta-C, CrN and Cr-diamond coatings, under simulated lubrication conditions. Results showed that compared to Cr-diamond and CrN coating, the ta-C coating exhibited superior tribological properties with the lowest friction coefficients and wear rates under various testing conditions. Low-viscosity oil 0W-20 could significantly improve the tribological properties of ta-C coatings at different testing temperatures. Furthermore, in cyclic start/stop tests, ta-C coatings have been proven to possess excellent low temperature cycle starting performance when lubricated by low-viscosity oil. A synergistic effect of the low-viscosity oil and a nanoscale adaptive tribofilm was speculated for ta-C coatings to be responsible for achieving lower friction and wear simultaneously. The lubrication adaptability of low-viscosity oil to ta-C coating is technologically important and provides a high potential of industrial application for lubrication and wear prevention under actual conditions.

Lithium plating on the anode for lithium-ion batteries during long-term low temperature cycling

Occurrence of lithium plating on the anode is a severe side reaction in the lithium-ion batteries, which brings cell capacity degradation and reduces the cell safety. This paper focuses on 37Ah commercial lithium-ion batteries and clarifies the evolution of lithium plating during long-term low temperature (−10 °C) cycling. The tested cells are analyzed at different degradation stages, named “fixed-point analysis”, to evaluate the evolution process of lithium plating. It is found that the capacity fade and the internal resistance increase exhibit a decelerated trend with a turning point around 450 cycles. The loss of cyclable lithium caused by lithium plating is deemed to be the main reason behind the battery degradation. Post-mortem analysis including scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) is conduced to reveal the mechanism. The thickness and morphology change of plated lithium, and the plated lithium covered by solid electrolyte interface film, are disclosed by the surface and cross-section SEM images from the fixed-point analysis. XPS analysis further reveals the composition of solid electrolyte interface film, and the plated lithium existing. In a nutshell, lithium plating is inhomogeneous and has high spatial dependence.

(Invited) Predication of High-Rate and Low-Temperature Battery Performance By Ion Transport Obtained from Impedance Using Symmetric Cells

The expansion of lithium-ion battery (LIB) usage from small applications such as portable electronic devices to large applications such as electric vehicles and stationary power supply1 necessitates the improvement of high-rate LIB performance especially at the low-temperature region, due to the growing frequency of outside use.2 , 3 The major reason of LIB performance degradation during low-temperature operation is considered to be non-uniform reaction occurrence, which is due to a decrease in Li ion transport for charge compensation inside porous electrode during charge/discharge. In order to quantitatively determine this Li ion transport at the LiNiO2-based positive electrodes with various loading weight, porosity, and conductive carbon ratio, electrochemical impedance spectroscopy using symmetric cells (EIS-SC)4-9 was performed. In this presentation, the feasibility of the obtained ion transport was investigated in terms of parameters of predicting high-rate and low-temperature battery performance.10 The high-rate and low-temperature cycle performance of LIB was performed using a cylindrical battery composed of LiNiO2-based positive/graphite negative electrodes with LiPF6-based electrolyte at a rate of 5C corresponding 0.2 h charging/discharging and a temperature of 0 °C. The capacity retention was calculated from the capacity after 200 cycles relative to the initial capacity at a temperature of 25 °C. The electrochemical cell for impedance measurement consists of two uncharged LiNiO2-based positive electrodes via a separator containing the same Li-based electrolyte. The Nyquist plots obtained are straight line with a slop of 45° and vertical straight lines with respect to the real axis in the high and low-frequency regions, respectively, indicative of typical blocking behavior in porous electrode. Using the Nyquist plots and transmission line mode theory to investigate electrochemical processes in porous electrodes,11 the ionic resistances inside the porous positive electrodes (R ion) were quantitatively determined as ion transport parameter. The reciprocal of ionic resistance (R ion −1) for all electrodes of various configurations exhibits nicely positively correlation with capacity retention of low-temperature (0 °C) battery cycling; that is, the capacity retention of the low-temperature cycle test increases as R ion −1 increases. This result suggested that for a porous electrode, high ionic resistance (which corresponds to low ion transport) results in reaction non-uniformity, which causes the deterioration of high-rate and low-temperature cycling lifetime.Regarding the conventional cycle-test procedure for practical LIBs, there are problems of material and time consumption since it requires long testing after producing many large batteries. In this work, we subject symmetrical cells with small-sized and real-life electrodes to impedance analysis, and electrochemically quantify ion transport in practical porous electrodes employing a simple protocol that can be used to estimate high-rate and low-temperature LIB performance in a short time without finally producing many large batteries. Therefore, this criterion allows us the possibility of simple prediction for such high-rate and low-temperature LIB performance, and therefore contributes to the realization of an energy storage system that meets the demands for high-rate and low-temperature performances such as fast-charging in the future.References S. Chu and A. Majumdar, Nature, 488, 294-303 (2012). M.-T. F. Rodrigues, G. Babu, H. Gullapalli, K. Kalaga, F. N. Sayed, K. Kato, J. Joyner and P. M. Ajayan, Nat. Energy, 2, 17108 (2017). R. Schmuch, R. Wagner, G. Hörpel, T. Placke and M. Winter, Nat. Energy, 3, 267-278 (2018). N. Ogihara, Y. Itou, S. Kawauchi, C. Okuda, Y. Takeuchi and Y. Ukyo, Meeting s, MA2012-02, 723 (2012). N. Ogihara, S. Kawauchi, C. Okuda, Y. Itou, Y. Takeuchi and Y. Ukyo, J. Electrochem. Soc., 159, A1034-A1039 (2012). N. Ogiahra, Y. Itou, S. Kawauchi, T. Kobayashi and Y. Takeuchi, Meeting s, MA2013-02, 947 (2013). Y. Itou, N. Ogihara, C. Okuda, T. Sasaki, H. Nakano, T. Kobayashi and Y. Takeuchi, Meeting s, MA2014-04, 611 (2014). N. Ogihara, Y. Itou, T. Sasaki and Y. Takeuchi, J. Phys. Chem. C, 119, 4612-4619 (2015). Y. Itou, N. Ogihara and S. Kawauchi, J. Phys. Chem. C, 124, 5559-5564 (2020). N. Ogihara, Y. Itou and S. Kawauchi, J. Phys. Chem. Lett., 10, 5013-5018 (2019). R. de Levie, Electrochim. Acta, 9, 1231-1245 (1964).

(Invited) Corrosion Process of Metals below the Freezing Point

Atmospheric corrosion has been intensively investigated using an acceleration test using a temperature/humidity cycling in a laboratory to investigate the corrosion mechanism and the outdoor exposure test to evaluate the actual corrosion progress in a real environment. Corrosion of metals in a cold district is recently paid attention because a considerable amount of anti-freezing agents are sparged to the driveway in the winter season and cause severer corrosion of vehicles and structures. Repetition of freezing/melting of water also induces physical stress to the material and influences, for example, the mass-transportation process related to the corrosion reaction. In this study, therefore, the corrosion behavior of metals in the freezing condition was investigated both the outdoor exposure test and the laboratory test. In the laboratory test, low-temperature cycling was applied to the galvanic couples of Zn-steel plate cold commercial (SPCC) pair. A pair was set in a sample holder with a gap distance of ca. 0.1 mm between two metal plates and placed in a small box to avoid the direct blowing in a temperature-controlled chamber. Basic temperature cycling was composed of 4-stages as (i) low temperature (T L = –20°C, 1 h), (ii) transition from T Lto T H (1 h) (iii) high temperature (e.g., T H = 0, 1 h), and (iv) transition from T H to T L (1 h),. Relative humidity (RH) in a chamber was not actively controlled. The T and RH presented in this paper were measured by using a T & RH sensor close to the sample. At T L, 0.1% NaCl solution of ca. 1 cm3 was dropped at the gap that was immediately frozen. Fig. 1(a) shows the typical galvanic current i g in a single T-cycle observed at the gap covered with ice. In this condition, the ice was not almost melted at T H. The i g increased with T but not dependent on RH because the current flowed mainly under or around the ice. The considerably large i g observed under the ice proposed that the ionic conductivity was maintained even at the freezing temperature as –20 °C. Probably a thin liquid layer of concentrated NaCl solution was formed at the interface between the metals and ice due to the exclusion of salt from the solid ice phase during the freezing process. The presence of the liquid phase was also investigated by the different experiments of the impedance measurement between two Pt electrodes in the freezing condition. After many T-cycles, T H was raised to 2 °C to accelerate the melting of ice and, thus, the evaporation of melted water. After the many cycling, the surface was almost dried and covered with a mixture of corrosion products and salt. Fig. 1(b) shows the i g in this condition. The i g flowed at T L where the RH was rather high at stage i, increased with increasing T at stage iv, and dropped with lowering RH at T H in stage iii. This behavior was similar to that observed in the ordinal wet, and dry test conducted higher than the melting point. From these results, the atmospheric corrosion process in the cold district includes the ordinal atmospheric corrosion on the dry area and wet-corrosion in the quasi-solution under the ice. Figure 1

Exceptional sulfur and iron isotope enrichment in millimetre-sized, early Palaeozoic animal burrows

Pyrite-δ34S and -δ56Fe isotopes represent highly sensitive diagnostic paleoenvironmental proxies that express high variability at the bed (< 10 mm) scale that has so far defied explanation by a single formative process. This study reveals for the first time the paleoenvironmental context of exceptionally enriched pyrite-δ34S and -δ56Fe in bioturbated, storm-reworked mudstones of an early Ordovician storm-dominated delta (Tremadocian Beach Formation, Bell Island Group, Newfoundland). Very few studies provide insight into the low-temperature sulfur and iron cycling from bioturbated muddy settings for time periods prior to the evolution of deep soil horizons on land. Secondary ion mass spectroscopy (SIMS) analyses performed on Beach Formation muddy storm event beds reveal spatially distinct δ34S and δ56Fe values in: (a) tubular biogenic structures and trails (δ34S ~ +40‰; δ56Fe ~ −0.5‰), (b) silt-filled Planolites burrows (δ34S ~ +40‰; δ56Fe ~ +0.5 to + 2.1‰), and (c) non-bioturbated mudstone (δ34S ~ +35‰; δ56Fe ~ +0.5‰). δ34S values of well above + 40.0‰ indicate at least some pyrite precipitation in the presence of a 34S-depleted pore water sulfide reservoir, via closed system (Raleigh-type) fractionation. The preferential enrichment of 56Fe in Planolites burrows is best explained via microbially-driven liberation of Fe(II) from solid iron parent phases and precipitation from a depleted 54Fe dissolved Fe(II) reservoir. Rigorous sedimentological analysis represents a gateway to critically test the paleoenvironmental models describing the formation of a wide range of mudstones and elucidates the origins of variability in the global stable S and Fe isotope record.

First and Second Law Analysis of Cascade Refrigeration System Using Refrigerant Pair R744-R1234ze(E)

— The performance of a two stage cascade refrigeration system using environment friendly refrigerant pair R744 in low temperature cycle and R1234ze(E) in high temperature cycle has been examined by mathematical simulation in EES ( Engineering Equation Solver). The operating parameters considered in this study are evaporator temperature and condenser temperature. The results shows that with increase in condenser temperature, total exergy loss is increasing and total COP is decreasing. The similar effect has been seen with increase with evaporator temperature but rate of increase of total exergy loss and rate of decrease of total COP with evaporator temperature is low.

Influence of cycling profile, depth of discharge and temperature on commercial LFP/C cell ageing: post-mortem material analysis of structure, morphology and chemical composition

The paper presents post-mortem analysis of commercial LiFePO4 battery cells, which are aged at 55 °C and − 20 °C using dynamic current profiles and different depth of discharges (DOD). Post-mortem analysis focuses on the structure of the electrodes using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the chemical composition changes using energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). The results show that ageing at lower DOD results in higher capacity fading compared to higher DOD cycling. The anode surface aged at 55 °C forms a dense cover on the graphite flakes, while at the anode surface aged at − 20 °C lithium plating and LiF crystals are observed. As expected, Fe dissolution from the cathode and deposition on the anode are observed for the ageing performed at 55 °C, while Fe dissolution and deposition are not observed at − 20 °C. Using atomic force microscopy (AFM), the surface conductivity is examined, which shows only minor degradation for the cathodes aged at − 20 °C. The cathodes aged at 55 °C exhibit micrometer size agglomerates of nanometer particles on the cathode surface. The results indicate that cycling at higher SOC ranges is more detrimental and low temperature cycling mainly affects the anode by the formation of plated Li.Graphic abstract

Comparative evaluation of solarized triple combined cycle for different ORC fluids

Triple combined cycles have the synergetic operation of three power cycles for utilizing the heat which is otherwise lost for improving the overall cycle performance. The integration of Brayton cycle with the Rankine cycle run on steam and organic Rankine cycle (ORC) run on different working fluids yields a triple combined cycle. This paper presents a comparative evaluation of the triple combined cycle powered by a heliostat based central solar receiver system for different working fluids in the low-temperature cycle. The influence of pressure ratio, steam bleeding, and working fluid in ORC has been studied upon the performance of the triple combined cycle under consideration. The organic Rankine cycle is run on different working fluids namely R141b, R245fa, and R236ea. The study concludes that the combined cycle has the maximum thermal efficiency and overall specific work output as 33.86% and 338.62 kJ/kg respectively at 24 cycle pressure ratio, 20% steam bleeding, steam generation of 50 bar & 20 bar at 600 K in high pressure steam turbine & low pressure steam turbines, and R236ea as ORC working fluid. It is seen that the R236ea working fluid offers best combined cycle performance in comparison to R141b and R245fa working fluid.

Rankine Cycle Efficiency under Maximum Power Generation Condition as Applied to Low-Temperature Power Plant Using a Cryoproduct as the Working Substance

The “carnotized” Rankine cycle efficiency is studied under conditions of maximum power generation in a low-temperature power plant using a cryoproduct as the working substance. A method is proposed for evaluating the thermodynamic efficiency of the cycles that make efficient use of cold energy. A generalized thermal efficiency equation, ηtN = 1 − (TC/Th)m , is derived for low-temperature cycles, which can be used to increase the accuracy of determination of the efficiency and the amount of additional power generation by 20–40 %. The amount of power generated in the low-temperature power plant is estimated based on the criterion of maximum performance. This method of calculation of the characteristics of low-temperature cycles is applicable for any energy generating systems. It is shown that utilization of cold energy of a cryogenic fuel (liquefied natural gas) is a promising direction of development of the idea of cogeneration and trigeneration in power plants.

Performance evaluation of external fired hybrid solar gas-turbine power plant in Colombia using energy and exergy methods

Hydro and thermal generation power systems dominate the Colombian electricity sector. In 2017, Colombia installed electrical generation capacity was 16.8 GW. Renewable energy sources represent at least 85% of the total generation, being hydro the principal source. Several alternatives had been evaluated through the years to improve the Colombian energy matrix and capacity, including solar photovoltaic and wind plants; despite that, no consensus about the appropriate solution in terms of the available resource, energy demand, and energy mix has been attained. Thermosolar power plants arise as an alternative to produce energy in sites where nearly constant solar irradiance throughout the year is available, which is the case for most Colombian cities. This work concerned the evaluation of a single-stage hybrid Central Solar Power (CSP) plant at a location on the Caribbean Colombian coast. The study is focused on establishing the effect of local environmental conditions (ambient temperature and solar resource availability), as well as some operational cycle parameters (heat exchanger effectiveness and the system pressure ratio) on the CSP plant performance. Additionally, site emplacement conditions, i.e., proximity to the power grid, presence of conventional thermal power plants, proximity to principal cities, and availability of natural gas), are also considered to attain the factors that might constrain the plant optimal operating conditions. The CSP plant and the Direct Normal Irradiance (DNI) model results obtained fitted in good agreement the experimental data from the literature used for validation. Results have shown a global plant efficiency of 35% without solar resource which is reduced to 30% when solar contribution attains its maximum value at midday. Additionally, fuel-saving per day varies between 9.21% and 6.3% during the months of maximum and minimum global radiation, respectively. Finally, that the combustion chamber, its associated heat exchanger and the one that is in direct exchange with the surroundings, are the components with the most exergy destruction, as expected. From the above, it is sensible to explore alternatives regarding different working fluids that could be used in lower temperature cycles and other applications for heat recovery.

Enabling thermal efficiency improvement and waste heat recovery using liquid air harnessed from offshore renewable energy sources

A novel approach using decoupled processes to harness offshore renewable energy, from marine hydrokinetics, ocean waves, wind, and solar to produce liquid air, is presented in this paper. Offshore renewables interconnection using submarine medium- and high-voltage direct current technologies are used to produce liquid air that can be transported to end-use locations using repurposed liquefied natural gas tankers. Two important possibilities arise from using the proposed technology. The first possibility allows the integration with conventional thermal cycles to improve efficiency. This approach can be used to leverage efficiencies of thermal systems that have already reached a plateau in the maximum achievable efficiency via design and operation optimization. The second possibility is related to the incorporation of low temperature cycles to recover waste heat from other thermal processes. This is important considering that waste heat accounts for more than 60% of the consumed energy in the United States. A detailed technical description of the complete cycle from cryogen generation to end-use of energy is provided. Estimation of efficiency enhancement for thermal plants using liquid air including waste heat recovery is presented.

Development, characterization and modification study of eutectic fatty alcohol for cold energy storage application

Phase change energy storage technology is an important technology to solve the contradiction between energy supply and demand and improve energy efficiency. In the fields of fruit and vegetable preservation, cold chain logistics, chemical industry, medicine and other fields, low-temperature phase change materials below 0 °C have great application space, but many phase change materials have large supercooling, low thermal conductivity, mechanical properties and heat. Poor stability restricts its application in practice. In this study, using the principles of phase balance and phase law and a combination of theory and experiment, an efficient, stable, non-toxic and safe composite phase change material was developed. Using decyl alcohol (DA) and lauryl alcohol (LA) as substrates, adding covalently modified hydroxylated multi-walled carbon nanotubes (MWCNT-OH) with different mass fractions and adding sodium dodecyl benzene sulfonate (SDBS) are used as a dispersant to promote the nanoparticles to be better dispersed in the solution through ultrasonic vibration and to explore the thermal performance and thermal cycle stability of composite phase change materials. The results show that there are no supercooling and no phase separation after DA–LA eutectic. After adding MWCNT-OH and SDBS, the thermal conductivity of the composite phase change material is effectively improved and has little effect on the phase change enthalpy and phase change temperature. DA–LA/MWCNT-OH/SDBS still has good thermal stability after 200 high- and low-temperature cycle.

Efficient Low-Temperature Cycling of Lithium Metal Anodes by Tailoring the Solid-Electrolyte Interphase

Operation of Li-ion batteries below −20 °C is hindered by low electrolyte conductivity and sluggish solid-state diffusion in electrodes. Li metal anodes show promise for low-temperature operation, ...

Modelling and performance assessment of a thermally-driven cascade adsorption cycle suitable for cooling applications

Adsorption chillers can provide energy efficient cooling and have large potential for performance increase and cost reduction compared to conventional chillers. Among the different R&D activities currently in progress in the field, the development of advanced cascading adsorption cycles is an effective way to improve the performance of standard adsorption units, making this technology especially interesting in applications where waste heat for driving the adsorption chiller is a widely available, such as many industrial processes, cogeneration plants, I.C. engines, district heating networks. In this paper, a novel modelling tool able to simulate complex adsorption cycles is presented and validated with literature data. The simulation tool is used to investigate numerically the performance of a cascade adsorption cycle consisting of a twin adsorber high-temperature cycle with heat recovery coupled with an intermittent adsorber low-temperature cycle. A parametric analysis is carried out showing the optimization potential in terms of Coefficient Of Performance (COP) and specific cooling power (SCP) with varying cycle periods, step time ratios and adsorbent mass ratios. COP of 0.97 with SCP of 142 W/kg are found for water-zeolite 4A (high-temperature) and water-CaCl2/Silica gel (low-temperature cycle). These results are in line with previous findings reported in literature. Finally, useful recommendations for further performance improvement are provided.

Doping of titania with manganese for improving cycling and rate performances in lithium-ion batteries

Titanium dioxide have received a much attention for lithium-ion batteries due to safety as anode upon fast and low temperature cycling as well as appropriate stability during insertion and extraction of guest ions. However, TiO2 has low conductivity and sluggish diffusion of Li+. In order to eliminate these shortcomings, reducing of particle size and doping are considered as promising approaches. Herein, we investigate the effect of doping with manganese (atomic ratios of Mn/Ti = 0.05; 0.1; 0.2) on characteristics of anatase titania having nanoparticulate morphology. As found, Mn/Ti = 0.05 is optimal dopant concentration in terms of battery performance of titania: the capacity of 113 mAhg−1 was still maintained after 118 cycles at 1C that is rather higher as compared to undoped anatase. Such improved electrochemical behavior is associated with anatase lattice expansion due to Mn3+ incorporation and enhanced conductivity.

Experimental investigation on the pull-down performance of a -80°C ultra-low temperature freezer

Abstract This paper presents an experimental investigation on the pull-down performance of a -80°C ultra-low temperature freezer. Experiments were conducted in a two-stage cascade refrigeration system with an inner recuperative heat exchanger in low temperature cycle (LTC). In addition, the pull-down performances of the freezer were discussed on different startup temperatures ranging from -20°C to 24°C when the ambient temperature was fixed at 26°C. In this paper, the cooling process was divided into early start-up phase, rapid cooling phase and stable cooling phase, and it was found that thermal parameters changed obviously with the startup temperatures, especially in the rapid cooling phase. The experimental results showed that there were maximum discharge temperatures of LTC compressor during pull-down process, and the maximum values increased with the rise of startup temperatures. When the startup temperature was -20°C, the maximum discharge temperature of LTC compressor was 8.1% (11.6°C) lower than that when the startup temperature was 24°C. As two factors affecting the discharge temperature of LTC significantly, the superheating degree was the main influencing factor in rapid cooling phase, while the decrease of the evaporation and condensation pressures had a more significant effect in the stable cooling phase. Besides, the volumetric efficiency of high temperature cycle (HTC) compressor deteriorated more obviously during the pull-down process, and had higher potential for performance improvement compared with that of LTC.

Numerical analysis of working fluids for large scale centrifugal compressor driven cascade heat pumps upgrading waste heat

Heat pump technology has the capability to upgrade unusable low-temperature waste heat and make the upgraded heat usable for different types of industrial and domestic heating applications. Heat upgrading could allow to reach significant energy savings and reduced emissions when compared to other heat production methods. In this paper, upgrading waste heat into a higher temperature with large-scale centrifugal compressor driven cascade heat pumps was studied numerically. A combined method for heat pump thermodynamic analysis and compressor design analysis was developed and implemented in order to investigate the effect of different working fluid combinations and the effect of the cascade heat exchanger temperature level on the heat pump performance. Seven potential fluid candidates were included and condenser temperature of 90 °C, evaporator temperature of 20 °C, and evaporator heat rate of 1 MW were used in the analysis. It was concluded, that the implemented method can provide an efficient tool for evaluating suitable working fluids and design operating conditions for centrifugal compressor driven high temperature cascade heat pumps. The highest coefficient of performance of 3.08 was simulated by using R601 as the fluid in the low-temperature cycle and R245fa in the high-temperature cycle. These fluids also resulted in feasible compressor geometries and rotational speeds, as well as allowed to reach high compressor efficiencies of well above 80% based on the enthalpy loss models. Coefficient of performances of over 3.0 were simulated also with the fluid combinations R601/R600, R245fa/R245fa and R600/R245fa.

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

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

Comparative energy, exergy and economic analysis of a cascade refrigeration system incorporated with flash tank (HTC) and a flash intercooler with indirect subcooler (LTC) using natural refrigerant couples

In this paper, comparative energy, exergy and economic analysis of natural refrigerant couples working in a cascade refrigeration system incorporated with a flash tank in its higher temperature cycle and a flash intercooler with indirect sub-cooler in its lower temperature cycle have been done. An analysis is conducted using seventeen refrigerant couples from eight natural refrigerants, namely R717, R290, R600a, R744, R744a, R170, R1150 and R1270. Comparison based on COP, exergy efficiency, system cost rates, and exergy destruction rate, to identify the best alternative refrigerant couple, is performed. Thermodynamic optimization has also been carried out with evaporator temperature, condenser temperature, LTC condenser temperature, cascade temperature difference, de-superheating and subcooling parameter of LTC intercooler, as six design variables. To evaluate the economic accountability of refrigerant couples a simple payback analysis has been done at the thermodynamic optimal conditions of the system. Results of analysis and optimization show that R717-R290 is the best refrigerant couple for this system from thermodynamic as well as economic points of view. The maximum COP and exergy efficiency obtained is 1.917 and 39.14% respectively and the corresponding total annualized system cost is 836,395 $/yr for R717-R290 refrigerant couple.