Natural Working Fluids Research Articles

Experimental analysis of the R290 variable geometry ejector with a spindle

The aim of this work was to experimentally analyze the variable geometry gas ejector with a spindle designed for operation with the natural working fluid propane (R290). The performance of the ejector was evaluated based on the mass entrainment ratio and critical temperature, being the most crucial parameters for optimizing the cooling capacity in ejector refrigeration systems, as well as the ejector efficiency using common literature notation. Additionally, local pressure drop measurements allowed for the determination of pressure profiles inside for different spindle positions used for capacity regulation. The experimentally defined ejector efficiency curves demonstrated the ability to control the ejector capacity by means of the spindle, irrespective of the unfavorable operating conditions. By decreasing the effective throat area using spindle, the ejector mass entertainment ratio increased by 35%. Spindle movement allowed for the decrease of the motive nozzle flow rate by up to 65%, while still maintaining the suction of the secondary flow of the ejector. Moreover, the behavior of a slight increase in suction nozzle flow with a decrease in motive nozzle flow has been observed for the initial movement of the spindle followed by a huge drop for further reduction of the effective throat area, confirming the previous conclusions shown in the literature.

Synthesis of heat-integrated water networks: Integration of heat pumps

A typical heat-integrated water network (HIWN) synthesis problem aims to minimise freshwater and utility consumption while establishing trade-offs between operating and investment costs by exploring different water and heat integration opportunities. Heat integration between hot and cold water streams is performed to save external heating and cooling utilities required to achieve the target temperatures of these streams. This paper proposes a mathematical programming approach to integrate water-to-water heat pumps into the HIWNs. A simple cascade heat pump system with isobutane as a natural working fluid enables a larger temperature lift between the heat pump source and the sink. The objective function of the proposed mixed-integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater, heating/cooling utilities, electricity, and investment in heat exchangers and heat pumps. A sensitivity analysis is performed to evaluate the economic feasibility of the system by varying the heating/electricity cost ratio and depreciation period for the capital investment. The results show that additional utility savings can be obtained by integrating heat pumps into the HIWNs. For threshold problems, a heat pump is feasible for heating/electricity price ratios from 1.175, and heating utility is replaced by electricity for a 1:1 ratio. However, for pinched problems, the feasibility of the heat pump is shifted to lower heating/electricity price ratios, from 0.3 to 0.5, depending on the annualisation factor for the investment.

Selection of pure and binary working fluids for high-temperature heat pumps: A financial approach

High-temperature heat pumps are increasingly being recognized for decarbonizing industrial heat supply up to 200 °C. In the open literature, the operating conditions, working fluids and configurations of high-temperature heat pumps are typically optimized for achieving a maximum Coefficient of Performance (COP). This work however presents a method that optimizes the operating conditions, working fluid and configuration of the heat pump, based on a combined financial and technical appraisal. In this regard a financial model to calculate the levelized cost of heat (LCOH) of a heat pump unit is developed. The model screens a large set of working fluids and therefore allows for subcritical, transcritical and supercritical operation and includes (zeotropic) binary mixtures. The methodology is applied to a large set of generic temperature profiles, with process temperatures between 160 °C and 200 °C and heat source temperatures between 80 °C and 120 °C. The results indicate that designing a heat pump solely based on maximum COP may lead to a sub-optimal financial solution. Overall, binary mixtures, either zeotropic or azeotropic, of natural working fluids often performed best. The benefits of zeotropic mixtures are twofold: improved temperature matching and flexibility in their thermophysical properties. However, for industrial processes with high temperature glides, pure fluids operating in the transcritical regime proved to be the most promising. The study also highlights the sensitivity of the financial performance to the electricity price, annual operating hours and heating capacity.

A new cold cooling system using krypton for the future upgrade of the LHC after the long shutdown 4 (LS4)

Future silicon detectors for High Energy Physics Experiments will require operation at lower temperatures to cope with radiation damage of the sensors and consequent increase of the dark current, beyond the limit of the current CO2 evaporative cooling system. This, together with many other requirements such as mass minimization and high radiation hardness, pushes the need of a new advanced cooling technology. The new coolant shall be able to approach ultra-low temperatures below −60 °C, withstand high radiation levels while having cooling lines diameters comparable to the currently achieved with the CO2 technology. Among different natural working fluids, krypton appears as a promising coolant for the thermal management of future detectors in high-irradiated environments. The thermodynamic properties of krypton do not allow the use of a pumped loop cycle but rather impose a need of a novel cooling technology. A new ejector-supported krypton cycle is presented, highlighting the cycle dynamics involved due to the different temperature levels normally encountered during the detector lifetime.

Experimental investigation on CO2-based zeotropic mixture composition-adjustable system

CO2 is a promising natural working fluid for its environment-friendly, property-stable and low-cost, and CO2-based zeotropic mixture could enhance the comprehensive performance of combined cooling and power cycle (CCP). Generally, CCP operates in a wide temperature range, and the fixed operating composition could not realize the optimal match for two sub-cycles. Zeotropic mixture composition adjustment could realize targeted thermal match of sub-cycles and further enhance the performance of CCP. However, systematic research on composition-adjustable CCP is limited in theoretical simulation, and there is still a lack of experimental evidence for the performance enhancement of composition adjustment on CCP. Besides, the feasibility of composition adjustment on CCP has not been fully verified through experimental method yet. In this study, an experimental prototype of CO2-based composition-adjustable CCP is established, the approach to realize composition adjustment is investigated, and the systematic test is carried out. Experimental results show that the introduction of composition adjustment is beneficial to the performance of CCP. Under the same boundary conditions and similar refrigeration conditions, composition-adjustable CCP could export 13.72 % more net power than composition-fixed CCP when the initial charged CO2 mass fraction equals 0.246, and the improvement is more apparent when the initial charged composition gets larger.

Thermodynamic analysis of a high-temperature heat pump using low GWP natural working fluids for upgrading district heating to process heating

Thermodynamic analysis of a high-temperature heat pump using low GWP natural working fluids for upgrading district heating to process heating

Exergy, carbon footprint and cost lifecycle evaluation of cascade mechanical subcooling CO2 commercial refrigeration system in China

The concept of cascade mechanical subcooling is recommended to widen the application of natural working fluid CO2, improve the efficiency and reduce the carbon footprint of CO2 refrigeration system applied in commercial supermarket. Three potential configurations for CO2 system with integrated multi-stage dedicated mechanical subcooling (MS-DMS) are proposed as prospect solutions. Life cycle assessment is conducted from the viewpoint of exergy, carbon footprint and cost performance for the proposed refrigeration systems, and compared with baseline CO2 booster refrigeration system (BASE) and traditional R404A system. 8 representative cities located in diverse climatic zones across China are chosen as scenarios and discussed in detail. The results indicate the irreversible loss is efficiently reduced with the utilization of MS-DMS. The annual exergy efficiency of CO2 system integrated with triple-stage DMS (TS-DMS) is 11.92–18.48% higher than BASE. Life cycle climate performance (LCCP) is reduced by 6.87% when the TS-DMS is employed in Haikou in contrast to BASE from the perspective of carbon emissions. Additionally, the life cycle cost (LCC) is remarkably reduced by 6.8% compared with BASE. Consequently, MS-DMS CO2 systems are more suitable for the application in warm or hot regions in contrast to BASE due to its better emission reduction performance as well as lower life cycle cost.

Analysis of the possibilities of increasing the energy efficiency of absorption refrigeration appliances through the use of refrigerating accumulators

One of the most important tasks of modern society is the solution of environmental and energy problems in power engineering and, in particular, refrigeration technology. At the same time, in the field of artificial cooling systems, it is necessary to solve the problems of reducing the impact on both the ozone layer and the greenhouse effect. An effective approach here can be absorption refrigeration systems with a natural working fluid (water-ammonia solution), which does not adversely affect the environment. For the effective use of absorption refrigeration systems, it is necessary to solve the problems of increasing their energy efficiency, in particular, through the use of cold accumulators. Thus, the object of research is absorption-type cooling systems with cold accumulators.
 The paper analyzes cold accumulators with different physical nature. It is shown that melting substances can be the most effective for solving problems of low-temperature cooling. An analysis of the thermal scheme of an absorption freezer of the «chest» type, which is the most problematic in terms of providing cooling modes at a level of (–18)–(–24) °С, was carried out. Optimization thermal calculations for typical absorption freezers up to 200 dm3 have been performed. It is shown that when the chamber is initially loaded with a product at an ambient temperature, the cooling capacity of the installed absorption refrigeration units is not enough – no more than 50 % of the required one. For an absorption freezer of the «chest» type, the most suitable cold-storage materials are a eutectic aqueous solution of sodium chloride or propylene glycol, since these solutions have the desired melting point of the order of –18 °C and a fairly high melting heat. The result of optimizing the weight and size characteristics of the internal volume of the absorption freezer is the following recommendations:
 – the optimal size of wire baskets for placing products is 315´370´240 mm;
 – the gaps between the basket and the cabinet wall, as well as between the baskets themselves, should be 10 mm to ensure normal convection conditions;
 – it is not advisable to place fans inside the volume of the freezer at this stage, since the freezing time is reduced by a maximum of 30 %, but this results in additional heat generation, energy consumption and increases the shrinkage of the products stored in the chamber.

Resurrection of carbon dioxide as refrigerant in solar thermal absorption cooling systems

The existing air conditioning and cold storage systems use conventional compressor based systems, compelling more electricity and greenhouse gasses (GHG) emissions. The incumbent cooling system uses synthetic refrigerants (CFCs, HCFC, and HFCs) that outperform natural refrigerants but are banned or under time bared permission due to their harmful effects. The global community of (196 parties till 2017) has ratified Paris Accord to limit GHG emissions and use low Global Warming Potential (GWP) refrigerants, and after the ban on existing synthetic refrigerants, quested for suitable natural working fluids and retrofitting in the existing system. Among ASHRAE envisaged natural refrigerants, CO2 has resurrected as an emerging refrigerant after the availability of high pressure technologies. The proposed design of solar assisted absorption chiller employing CO2 as a heat transfer fluid for a commercial dwelling is simulated for a dwelling in the hot and humid, moderate and sun adverse region (Lahore, 31.5204° N, 74.3587° E) to assess the thermal properties of the proposed design. A thermal storage tank with immersed heat exchangers augmented to meet the intermittency of solar energy. A solar evacuated glass tube collector (EGTC) with U-shaped copper tubes is used to collect solar heat energy. Integration of renewable energy (RE) systems is inevitable due to the persistent energy crisis and climate change situations. Solar energy is a promising source of energy abundantly available in hot areas. CO2 is a natural refrigerant that outperforms ASHRAE envisaged natural refrigerants due to the low critical point. A solar thermal cooling system employing a 35.2 kW absorption chiller driven via heat energy harnessed with EGTC using R-744 supported by an auxiliary furnace is simulated in a TRNSYS® Simulation environment. The simulated system covers the cooling requirements of a large three-room dwelling in Lahore, Pakistan. The proposed design comprises an R-744-based solar heating system combined with a hot water-fired absorption chiller. The results were dynamically simulated for the hot climate of Lahore, Pakistan, with average yearly maintained temperatures of 23 °C, 26 °C, and 21 °C for the three rooms and 0.21 solar fraction for the whole year.

Numerical study on two-phase supersonic expansion refrigeration in novel CO2 refrigeration technology

The serious environmental problems have promoted the development of CO2 natural working fluid refrigerants. The supersonic two-phase expander with supersonic nozzle as the key component is proposed. A novel CO2 supersonic two-phase expansion refrigeration cycle is constructed and its cycle performance is also calculated. Then, the numerical simulation of the flow condensation and liquefaction process of CO2 in the nozzle is carried out. The results show that the inlet parameters of the supersonic two-phase expander have an important effect on the cycle performance. With the increase of inlet pressure, COP first increases and then decreases. The optimal inlet pressure of the system is 5.3 MPa with the corresponding maximum COP by 2.50, and the Relative Carnot Efficiency decreases from 0.809 to 0.755. With the inlet temperature increase, the maximum COP and the Relative Carnot Efficiency are 2.49 and 0.826, respectively. The established model can reasonably describe the supersonic expansion refrigeration characteristics and condensation phase transition characteristics of CO2. With the increase of nozzle inlet pressure and the decrease of temperature, the inlet degree of supercooling increases, while the initial nucleation position moves forward further and the peak nucleation rate decreases. Meanwhile, the droplet number decreases, both the droplet radius and the liquid mass fraction increases. The ideal cycle performs well, which provides a valuable reference for CO2 utilization.

To the issue of optimising the performance of a scroll compressor as part of a CO2 booster refrigerating machine in order to increase its efficiency

Objective. With the decreasing use of refrigerants with a high global warming potential, increasing the efficiency of refrigeration machines using natural working fluids is a very urgent task. The CO2 booster refrigerant cycle may be optimal for the fields of commercial refrigeration, climate engineering and heat pumps, but needs to be adapted to regions with hot climates. The purpose of the study is also to systematize possible ways to improve the spiral technology, as a guarantee of increasing the efficiency of booster refrigeration machines.Method. Such solutions have already been tested by increasing the transcritical cycle efficiency through the use of an ejector and parallel compression. However, the use of a scroll compressor in the transcritical cycle may be a good alternative to these methods. This solution, a little bit improves technical and economic indexes of the refrigerating machine, in the most part reducing capital and operational costs. When considering the prospect of improving the efficiency of a booster system with scroll compressor, it is necessary to address the issue of compressor performance optimisation. An issue of this kind can be referred to improving the efficiency of the booster system, at the expense of the internal characteristics of the refrigeration machine.Result. Thus, in this paper, a conceptual approach is considered to solve this problem based on a system analysis of the interconnection of the compressor as the main energy-intensive element of a refrigeration machine.Conclusion. Development of conceptual model, allows to identify influence of various factors on scroll compressor operation and to build adequate mathematical model, to choose or develop necessary calculation methods.

Two-stage evaporator for R744 heat pumps using greywater as heat source

Heat pumps are pinpointed as a key technology in decarbonizing buildings' heat supply, which is currently dominated by fossil-fuel based systems. To minimize the environmental impact of heat pumps over their lifetime, selecting natural working fluids (refrigerants) is the only viable long-term choice. Independent of the refrigerant, the efficiency of a heat pump is closely linked to temperature of the applied heat source, and to this end, wastewater is a promising candidate that is abundant in all urban environments. This experimental study investigates a novel two-stage, ejector-supported, brazed plate evaporator for its use in R744 (CO2) heat pumps with greywater as a heat source and compares it to more conventional evaporators. The work shows that splitting the evaporation process in two stages with an ejector in between allows reducing the evaporation pressure while the compressor suction pressure can be increased, in some cases significantly. Even if several possibilities for further optimization of the novel two-stage evaporator were identified, the results show heat pump efficiency improvements above 10% compared to an equivalent heat pump with a well-performing one-stage evaporator. Heat pump systems aiming at utilizing maximum temperature difference for the greywater should use a high pressure lift ejector.

The comparative analysis of the R290 heat pump system working with standard expansion valve and two-phase ejector

The heat pump systems play a significant role in the global energy transition process of household heating sources towards zero-emission. One of the key technologies to improve the efficiency of heat pump systems utilizing natural working fluids is application of the two-phase ejector, which is able to recover part of the expansion losses. The comparative experimental analysis of a novel ejector-based air-source R290 heat pump was performed. The two two-phase bypass ejectors were installed in a R290 heat pump unit and a number of modifications were introduced to the baseline system. The adaptions included implementation of the internal heat exchanger, increasing the superheat at the compressor suction port, and a liquid separator for handling the two-phase flow at the ejector outlet. The performance evaluation of the system was based on COP and system heating capacity. The comparative analysis with the R290 heat pump utilizing standard expansion valve and two-phase ejector was carried out for typical operating conditions for its domestic application during heating seasons. The system working with an ejector as a throttling device allowed for up to 33% of COP improvement over the system utilizing an expansion valve. Additionally, the ejector implementation resulted in decreased pressure ratio of the compressor, which increased the system heating capacity by up to 90%.

Energy, Exergy, and Economic Performances of Combined CO2 Power and Cooling Systems: Comparison and Multi-Objective Optimization

As a natural working fluid, carbon dioxide (CO2) has been extensively applied to produce power and cooling in thermodynamic cycles. So far, various combined CO2 power and cooling systems have been proposed to enhance the energy conversion efficiency and reduce the cost further. However, how to evaluate the performance of different combined systems for a practical application scenario is still an open question. Thus, in this work, four representative systems are considered to investigate and compare the energy, exergy, and economic performances under design conditions. The corresponding models are established, and various performance parameters are calculated. According to the obtained results, the total products of four systems are 6831.36 kW, 4421.73 kW, 6252.81 kW, and 6978.69 kW, respectively. The minimum total cost is obtained by System 2. Thereafter, in order to maximize the total product (the sum of net work and refrigerating capacity), and simultaneously minimize the total cost, key system parameters are optimized by a multi-objective optimization algorithm, namely, NSGA-Ⅱ. The optimization results show that System 1 provides the highest total product (7345.4 kW), while System 2 has the lowest cost (27.51 $/h). After comprehensive comparisons for the net work, cooling capacity, efficiencies, and total cost, System 1 is regarded as the best among the considered four systems.

Performance of binary zeotropic mixtures in organic Rankine cycles (ORCs)

Compared to pure fluids, zeotropic mixtures have the potential to lower the irreversibilities in low temperature Rankine cycles by better temperature profile matching of the working fluid with the heat source/sink. However, having a gliding temperature does not guarantee performance boost over pure fluids, as many factors influence the exergy efficiency of the cycle. In this study, 25 pure fluids and 104 binary mixtures of natural working fluids are analyzed in subcritical ORCs with heat source temperature range of 125–300 ℃ and different condensing conditions and the results are investigated within two frameworks: (1) comparing the mixtures to their pure constituents, (2) comparing the mixtures to the best performing pure fluid. In one behavior type, the performance of the mixture falls between the performance of its pure constituents for all evaporator pressure range, and the mixture provides no benefit. However, some mixtures could provide performance boost in a specific evaporator range. Therefore, the maximum allowable evaporator pressure plays an important role in the performance comparison of zeotropic mixtures to their pure constituents. Mixtures which outperform their pure constituents in the first perspective, are further analyzed in the second perspective. Finally, a screening method is presented to map the binary mixtures with performance boost compared to their pure constituents and high absolute exergy efficiency. This method is based on the key thermophysical properties of the fluids including critical temperature and normal boiling point, as well as working conditions such as heat source and heat sink temperature and PPTD in the evaporator and the condenser.

Research on the effect of flame retardants on the mildly flammable refrigerant ammonia

The natural working fluid ammonia has once again attracted widespread attention due to its environmental performance. However, the flammability of ammonia will lead to some potential safety hazards in practical applications. In this paper, the flammability characteristics of ammonia mixed with R125 and R227ea were studied experimentally. Firstly, the flammability limits of the binary mixture in dry air were measured in a 12 L spherical glass flask. The results showed that when the volume ratios of R125/NH3 and R227ea/NH3 were 0.65 and 0.63, respectively, the binary mixture reached the critical flammability ratio. And the inhibition mechanism of R125 and R227ea was discussed by the density functional theory. It was found that the flame retardants are easy to produce F and CF3 inhibition radicals. Finally, the inhibitory effects of R125 and R227ea were compared by the comprehensive analysis of environmental and safety performance. Under the premise of meeting the requirements of environmental protection, R227ea has a stronger inhibitory effect on the flammability of ammonia than R125. This paper is of great significance for the safe application of ammonia in the process industries.

Design optimization and test of the novel FeTu ‘compander’, utilising organic fluids within a closed cycle for HVACR applications

HVACR performance is influenced by the efficiency of the compression and expansion process. The ability to operate oil-free; with non-toxic, inert, natural working fluids such as air or CO2 holds high environmental value.This paper presents data of the FeTu™ compander operating in a closed cycle, for means of heating and cooling. A novel system, where compression and expansion is a continuous and simultaneous process; inherently regenerating energy to assist the compression process by expanding within a positive displacement chamber. This study documents the oil-free performance with organic working fluids pertaining to HVACR applications.Commercial 1D modelling software; GTSuite (GTS) is used to optimise cooling capacity (Q) and Coefficient of Performance (CoP). Different volume ratios (VR), pre-charge pressures, temperature inputs (ΔT) and running speeds are analysed. A multi-faceted test rig is built to compare and validate all performance data against the GTS fluid model which, once calibrated, can then be used with a high degree of certainty in predicting the performance of other applications and ΔTs. Increasing the system pre-charge pressure has a proportional impact on capacity, so this phenomenon is tested to consider the conflicting impacts of higher pressure against increased viscosity, with respect to volumetric efficiency.The CoP of the system ranges from 1 to 5 across typical HVACR ΔTs. Low VR offers highest CoP but lowest Q, so the optimal trade-off is explored. The strengths & weaknesses are then comparatively documented against incumbent vapour cycle systems.

Review of Mechanical Vapour Compression Refrigeration System Part 2: Performance Challenge

Reducing energy consumption and providing high performance for a vapour compression refrigeration system are big challenges that need more attention and investigation. This paper provides an extensive review of experimental and theoretical studies to present the vapour compression refrigeration system and its modifications that can be used to improve system’s performance and reduce its energy consumption. This paper also presents the challenges that can be considered as a gab of research for the future works and investigations. Cooling capacity, refrigerant effect, energy consumption can be improved by using vapour injection technique, natural working fluid, and heat exchanger. Based on the outcome of this paper, vapour injection technique using natural refrigerant such as water can provide ultimate friendly refrigeration system. Future vision for the vapour compression refrigeration system and its new design technique using Computational Fluid Dynamic (CFD) is also considered and presented.

Overview and outlook of research and innovation in energy systems with carbon dioxide as the working fluid

Carbon dioxide (CO2, R744) is a natural working fluid with interesting thermophysical properties that have stimulated strong attention by the academic and industrial communities for a broad range of energy applications. The technology readiness level of CO2-based energy systems is very diverse due to the increasing consideration that the fluid has been receiving since the 1990s. Hence, the state of the art in CO2 energy research spans from fundamental thermofluid and chemistry science to commercial system innovations. After a brief compendium on ongoing activities, this paper proposes a roadmap for CO2 energy research with reference to the cooling, heating and power sectors. The key knowledge gaps and the main challenges at system and component levels are critically discussed. Pathways to advance the understanding and the technological maturity of CO2 energy systems are also outlined.

A novel cryogenic condensation system based on heat-driven refrigerator without power input for volatile organic compounds recovery

Volatile organic compounds (VOCs) are important precursors of pollutants such as PM2.5 and ozone, and the development of their treatment technologies is of great significance to environmental protection and economic development. The current VOCs destruction technology, such as the combustion method, has low operating costs, but there is a large waste in the treatment of high-value gas; while the current VOCs recovery technology, such as the condensation method based on refrigerator cooling, can recover high-value gas, but it still has some unsatisfactory aspects in terms of emission concentration, refrigerant use, and power supply. In this paper, a novel cryogenic condensation system based on a heat-driven refrigerator without power input for oil gas VOCs recovery was proposed. The heat obtained by burning part of the oil gas VOCs is employed to drive a thermoacoustic refrigerator using helium as working gas to realize the recovery of the remaining oil gas VOCs, which has the advantages of not relying on external power supply, using natural working fluid, and zero-emission of oil gas. A thermodynamic calculation model was established to quantitatively calculate the recovery efficiency. The results show that the remaining 91.0% of the oil gas VOCs can be recovered by burning 9.0% of the oil gas VOCs when the refrigeration temperature is 200 K, the heat source temperature is 873 K and the concentration is 33.5% at 100 kPa. And compared with the method of burning part of the liquefied VOCs, the method of burning part of inlet gaseous VOCs to drive the refrigerator has a higher recovery efficiency (around 10%). The effects of the refrigeration temperature, the heat source temperature, VOCs concentration, VOCs inlet pressure, and refrigerator efficiency on the system were also analyzed.