Display omitted
•The novel ORC-VCE combination has an average 18% COP increase.•The proposed R1234ze(E) system can increase power generation by 58% more than a conventional ORC.•The system has a wide ...operation range of ground source temperatures with higher thermal efficiency.•Additional heat exchangers increase energy efficiency and waste heat valorisation potential.
This work analyses a novel combined organic Rankine-compound ejector vapour compression cycle for power, cooling and heating production using a low-grade ground heat source as the primary heat source. Ultra-low global warming potential working fluids (R1234ze(E), R1243zf, and R1234yf) and parameters quantifying energy and exergy efficiency are considered. The system can be adapted to three operating modes, depending on the ground source temperature, ranging from 55 to 90 °C: power-cooling, power-heat pump heating, and power-ground source heating. The results indicate that this system notably increases the overall performance of all investigated refrigerants. Compared to conventional organic Rankine and vapour compression cycles (ORC and VCC), the R1234ze(E) power-cooling mode shows the highest coefficient of performance (COP) increase, 18 %. Besides, including a recapture heat exchanger for condenser waste heat recovery can increase power generation by 58 %. At ground source temperatures up to 65 ℃, power generation and thermal efficiency increased in the power-heating mode due to the absence of the compressor power consumption. The exergy efficiency follows the ground source temperatures for all modes. In power-ground source heating mode, the exergy efficiency notably increased due to the absence of the heat pump exergy destruction.
Because air conditioning and heat pump systems contribute greatly to greenhouse gas emissions, equipment with both lower global warming potential (GWP) working fluids and a higher level of ...performance should be used. R32 (difluoromethane) has been proposed to substitute R410A, particularly in residential air conditioning (RAC) systems. This study collected the most relevant and recent researches into R32 as a refrigerant so as to assess its viability in RAC systems in both Europe and the USA, as compared to R410A and other lower GWP RAC alternatives.
The R32 value of GWP is 677, which is below the F-gas regulation limit in RAC equipment (750). According to ASHRAE standard 34, R32 is less flammable than hydrocarbons, and the amount of charge permitted for R32 is above the necessary level in RAC equipment. It can be concluded that R32 has significantly good heat transfer characteristics and a level of performance that make it acceptable at low condensing temperatures, thereby avoiding overly high compressor discharge temperatures. Its performance is very similar to that of R410A across the entire operating range, and it is therefore believed that R32 will be utilized in RAC systems in the remaining countries that prioritize lower GWP fluids but are less strict in their security regulations.
To replace R410A under extreme conditions, some system modifications can be conducted, or R32 mixtures with hydrofluoroolefins (HFOs) can be used. Such mixtures achieve a lower performance than R32, but are acceptable replacements when considering their lower GWP compared to that of R32, and similar level of flammability. Finally, other (R32-based) alternative mixtures have also been developed and their behaviours studied under a wide range of operating conditions.
•HFO-1336mzz(Z), HCFO-1233zd(E) and HCFO-1224yd(Z) are numerically studied to replace HFC-245fa.•Mapping of the minimum superheat requirement to ensure the dry compression is provided.•All ...alternatives improve the COP, being benefited from the use of an IHX.•HCFO-1233zd(E) and HCFO-1224yd(Z) present similar behaviour to HFC-245fa.•The alternatives reduce the equivalent CO2 emissions.
This paper analyses the feasibility of HCFO-1224yd(Z), HCFO-1233zd(E) and HFO-1336mzz(Z), three low global warming potential (GWP) refrigerants, as alternatives to HFC-245fa in high-temperature heat pump (HTHP) systems for low-grade waste heat recovery. HTHPs are a sustainable technology that can help to mitigate climate change through the thermal valorisation of the industrial low-grade waste heat. Before presenting and analysing the results, mapping of the minimum superheat degree requirement in the operating range, and the influence of the Internal Heat Exchanger (IHX) on each alternative are studied. The simulations were carried out at condensing temperatures from 115 to 145 °C and evaporating temperatures from 45 to 75 °C, using a single-stage cycle with and without IHX. Finally, the Total Equivalent Warming Impact (TEWI) evaluation is performed to illustrate the environmental effect of each alternative. Attending to the results, HCFO-1233zd(E) improves the COP about 27% compared to HFC-245fa, whereas HFO-1336mzz(Z) and HCFO-1224yd(Z) show an improvement of approx. 21 and 17%, respectively. Although HCFO-1233zd(E) and HCFO-1224yd(Z) present similar suction volumetric flow rate to HFC-245fa, HFO-1336mzz(Z) shows a relative increment up to 80%, and therefore, higher compressor and installation size are expected for this refrigerant. Finally, the TEWI analysis presents a significant reduction of the equivalent CO2 emissions for each low GWP alternative, between 59 and 61%. HCFO-1233zd(E) shows the highest reduction in all the simulation cases, followed by HCFO-1224yd(Z) and HFO-1336mzz(Z).
•R516A, R513A, and R1234yf are compared as alternatives to R134a.•The first experimental results of the R516A blend are presented.•R513A presents the highest overall system performance by enhancement ...of 2% in cooling mode.•In cooling mode, R516A shows system COP enhancement at low evaporating temperatures ranging from (1 to 15%).
This work presents an experimental comparison for low GWP refrigerants used in vapour compression cooling and heating systems. The study compares three lower global warming potential (GWP) refrigerants (R513A, R516A, and R1234yf) as drop-in refrigerants to replace the hydrofluorocarbon (HFC) R134a. Measurements are taken from a test rig at different steady-state conditions: for the cooling mode, the evaporating temperature is −5 °C, −10 °C and −15 °C, and is combined with two condensing temperatures (32.5 °C and 40 °C), and different internal heat exchanger (IHX) effectiveness. Besides, in the heating mode, the evaporating temperature is 7.5, 15 and 22.5 °C with five condensing temperatures (55 °C to 75 °C, step of 5 °C). In the cooling mode, R513A presented the highest system COP amongst the low GWP alternatives, increasing up to 8%. R516A shows the lowest system COP at the highest evaporation temperature; however, it exhibits the highest COP and capacity at the lowest evaporation temperature. The IHX positively influences the refrigerating effect for all adopted refrigerants. Regarding the heating mode, R513A presents the highest heating capacity with an average 3% increase, whereas R516A shows the lowest results. R513A shows comparable COP to R134a, especially at higher evaporating temperatures.
High-temperature heat pumps (HTHPs) based on vapour compression can be used for industrial low-grade waste heat valorisation, which can aid in mitigating climate change. Currently, the performance of ...HTHPs operating at high-temperatures lifts is limited; therefore, advanced configurations become an opportunity for their utilisation. This paper presents an HTHP cascade with configurations of internal heat exchangers (IHXs) that uses low GWP refrigerants in both high-stage (HS) (HCFO-1233zd(E), HFO-1336mzz(Z), HCFO-1224yd(Z), and pentane) and low-stage (LS) (HFO-1234yf, HFO-1234ze(E), butane, isobutane, and propane) cycles. Prior to the analysis and presentation of results, an optimisation of the operating conditions is performed based on intermediate temperature and IHX effectiveness in both stage cycles. Results indicate that butane and isobutane appear to be the most convenient working LS fluids from the point of view of coefficient of performance (COP). The highest system performance is obtained using pentane and HFO-1336mzz(Z) in the HS cycle. Compared to third-generation refrigerants (HFC-245fa/HFC-134a), a slight COP improvement is obtained using HCFO-1233zd(E), and HCFO-1224yd(Z). A comparable or even lower volumetric flow rate at the HS compression suction is also observed. The use of pentane/butane achieved maximum COP (3.15), which is a 13% improvement compared to COP obtained when HFC-245fa/HFC-134a is employed.
•A high-temperature heat pump cascade with the IHXs is proposed and optimised.•Synthetic and natural refrigerants with low GWPs are proposed for both stages.•The higher effectiveness of both IHXs in similar proportions increases total COP.•Pentane/butane presents maximum COP and minimum HCFO-1224yd(Z)/HFO-1234yf.•A few refrigerant pairs are comparable with HFC-245fa/HFC-134a in the stages of V˙suc.
•Two-phase ejectors are expected to lead to a significantly more sustainable future.•Benefits from two-phase ejectors strongly depend on their capacity control strategy.•Capacity control mechanisms ...for two-phase ejector are comprehensively reviewed.•Two-phase ejectors can be capacity controlled in large- and medium-scale units.•Capacity control method for small-scale units still requires a major breakthrough.
The adoption of highly efficient vapour-compression heating, ventilation, air conditioning and refrigeration (HVAC&R) systems is compulsory to achieve a low-carbon society. Expansion work recovery using a two-phase ejector is widely recognized as one of the most promising measures to improve the energy efficiency of HVAC&R units. This holds true for all operation conditions provided that an effective capacity control technique is implemented. In this work a thorough critical review on the current status of the presently available capacity control strategies for two-phase ejectors was carried out. In addition, their pros and cons as well as the comparison of their performance were reported. It was concluded that two-phase ejectors can be properly capacity controlled in large- and medium-scale vapour-compression units. However, a suitable capacity control mechanism for small-scale vapour-compression solutions still requires a major breakthrough and is being intensively discussed among experts in the field.
The cooling industry involves various essential applications, such as food preservation, medicine storage and air conditioning. However, its significant direct and indirect contribution to global ...warming is bound to increase in years to come, leading to the need for highly efficient cooling units using eco-friendly working fluids. Consequently, carbon dioxide (R744) is achieving resounding success as a refrigerant for various medium- and large-capacity applications, as some of the available expansion work is recovered with the aid of two-phase ejectors. However, its adoption is being limited for small-capacity solutions (e.g. condensing units) due to the current lack of a suitable flow modulation technique for two-phase ejectors installed in these units. Therefore, the goal of this work is to bridge this knowledge gap by formulating and experimentally proving an innovative flow control mechanism for two-phase ejectors, being based on the pulse-width modulation (PWM) of the refrigerant flow through the ejector. All the experimental evaluations were carried out at the compressor speed of 50 Hz, water temperature at the gas cooler inlet of 35 °C and R744 evaporating temperature of about −5 °C.
The first experimental data revealed that the high pressure can be controlled appropriately as well as varied from about 87 bar to 112 bar, demonstrating the effectiveness of the proposed technique. In addition, the effect of the muffler volume as well as the PWM period on the ejector and system performance were investigated. It was found that the influence of both the muffler volume and the PWM period was not significant. Compared to the solution employing the passive ejector (i.e. without flow modulation technique), the unit with the PWM ejector presented enhancements in coefficient of performance (COP) by more than 5% at the optimum operation conditions. It is worth mentioning that its today’s available competitors, i.e. needle-based ejector and vortex-based ejector, feature COP enhancements by 2%–4% as contrasted with the passive ejector. As benchmarked to the standard unit (i.e. with flash gas by-pass valve and without ejector), the PWM ejector could improve the COP by more than 10% at the optimal running conditions. Also, the results obtained suggest that at present the proposed solution should operate with a PWM period of 2 s and no mufflers. Finally, the PWM ejector is characterized by low cost, simplicity, low vulnerability to clogging and no practical size or application constraints.
This paper presents a workable vapour compression system (VCS) for evaluating the performance of a refrigeration system with an integrated condenser that uses a long-term alternative refrigerant to ...halocarbon as a heat transfer medium (R600a). India's refrigeration system uses halocarbon refrigerants due to their excellent thermophysical and thermodynamic properties. Greenhouse gas emissions from halocarbon refrigerants and fossil fuel combustion contribute to global warming that engenders climate change and the deterioration of the ecosystem. The halocarbon refrigerant was discontinued based on high global warming potential. The system was investigated under various ambient temperatures of 16, 20, 24, and 28 °C (oC). The performance of the VCS was analyzed using the parameters of coefficient performance, compressor work, and pull-down time (PDT). The experimental result shows that the vapour compression system obtained its best PDT, enhanced coefficient of performance, and energy reduction when the ambient temperature was 20 °C.
This paper presents an experimental analysis of the influence of an internal heat exchanger on the performance of a vapour compression system using R1234yf as a drop-in replacement for R134a. In this ...work, we compare the energy performance of a monitored vapour compression system using both refrigerants, R134a and R1234yf, with and without the presence of an internal heat exchanger under a wide range of working conditions. A set of experimental tests are carried out varying the condensing temperature, the evaporating temperature and the internal heat exchanger use. From the experimental results, reductions in cooling capacity and COP between 6 and 13% have been observed when R134a is replaced by the drop-in fluid R1234yf, although the presence of an IHX can help to lessen these reductions between 2 and 6%. Finally, the experimental results obtained agree with the theoretical evaluations developed neglecting the pressure drops.
•We present an experimental analysis of R1234yf as a drop-in replacement for R134a.•The influence of an internal heat exchanger on the energy performance is analysed.•The analysis has been carried out by means of the main energy parameters.•Cooling capacity and COP are reduced in 6–13% when using R1234yf instead of R134a.•Using an IHX is positive to shorten the reductions in 2–6%.
This study deals with energy and irreversibility analysis of a cascade refrigeration system employing various refrigerant couples, namely R152a–R23, R290–R23, R507–R23, R234a–R23, R717–R23 and ...R404a–R23, using a computer code developed for this aim. It is assumed that the refrigeration load is 1
kW, the refrigerated space temperature is −40
°C, and the environment temperature is 300
K, while the degrees of condenser subcooling and evaporator superheat are 5 and 7
°C, respectively, for all cases. Furthermore, the polytropic efficiencies of the compressors are assumed to be equal. It has been determined that the
COP of the cascade refrigeration system increases and the irreversibility decreases with rising evaporator temperature and polytropic efficiency for all studied refrigerant couples. On the other hand, the
COP of the cascade refrigeration system decreases and the irreversibility increases on increasing the condenser temperature and the difference between the saturation temperatures of the lower and higher temperature systems in the heat exchanger (Δ
T). In all cases, the refrigerant couple R717–R23 has the highest
COP and lowest irreversibility except for the limited ranges of polytropic efficiency (50–60%) and Δ
T (13
K–16
K), while R507–R23 has the lowest
COP and highest irreversibility. The refrigerant couple R152a–R23 has been found to be an alternative couple to R717–R23 for the above mentioned ranges of polytropic efficiency and Δ
T. The refrigerant couples R134a–R23 and R290–R23 have placed in the middle range, and R404a–R23 can be considered as a replacement couple for R507–R23 in all cases.
PDF
