National Institute of Standards and Technology における超伝導研究及び生活

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2875&context=iracc

High temperature heat pumps: Market overview, state of the art, research status, refrigerants, and application potentials

Condensation heat transfer, both inside and outside horizontal tubes, plays a key role in refrigeration, air conditioning and heat pump applications. In the recent years the science of condensation heat transfer has been severely challenged by the adoption of substitute working fluids and new enhanced surfaces for heat exchangers. Well-known and widely established semiempirical correlations to predict heat transfer during condensation may show to be quite inaccurate in some new applications, and consequently a renewed effort is now being dedicated to the characterisation of flow conditions and associated predictive procedures for heat transfer and pressure drop of condensing vapours, even in the form of zeotropic mixtures. This paper critically reviews the most recent results appeared in the open literature and pertinent to thermal design of condensers for the air conditioning and refrigeration industry; both in-tube and bundle condensation are considered, related to the use of plain and enhanced surfaces.

Condensation inside and outside smooth and enhanced tubes: a review of recent research

Abstract This study reviews the current state of the art and the current research activities of high temperature heat pumps (HTHPs) with heat sink temperatures in the range of 90 to 160 °C. The focus is on the analysis of the heat pump cycles and the suitable refrigerants. More than 20 HTHPs from 13 manufacturers have been identified on the market that are able to provide heat sink temperatures of at least 90 °C. Large application potentials have been recognized particularly in the food, paper, metal and chemical industries. The heating capacities range from about 20 kW to 20 MW. Most cycles are single-stage and differ primarily in the refrigerant (e.g. R245fa, R717, R744, R134a or R1234ze(E)) and compressor type used. The COPs range from 2.4 to 5.8 at a temperature lift of 95 to 40 K. Several research projects push the limits of the achievable COPs and heat sink temperatures to higher levels. COPs of about 5.7 to 6.5 (at 30 K lift) and 2.2 and 2.8 (70 K) are achieved at a sink temperature of 120 °C. The refrigerants investigated are mainly R1336mzz(Z), R718, R245fa, R1234ze(Z), R600, and R601. R1336mzz(Z) enables to achieve exceptionally high heat sink temperatures of up to 160 °C.

High Temperature Heat Pumps: Market Overview, Stateof the Art, Research Status, Refrigerants, and Application Potentials

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Advanced Engineering Thermodynamics

Low global warming potential refrigerants R1234ze(E) and R1234ze(Z) are anticipated to be the refrigerants of choice for high-temperature heat pump systems in industrial applications. Their thermodynamic attributes are thermodynamically, experimentally, and numerically assessed in this study. The thermodynamic assessment indicates that the theoretical coefficients of performances (COP) are maximized at a condensation temperature approximately 20 K below the critical temperatures for each refrigerant. However, when the volumetric capacity is inadequate, the actual COP differs from the theoretical COP because of the large pressure drop. The breakdown of irreversible losses, which are experimentally quantified at a condensation temperature of 75 °C, results in the largest portion of the total pressure drop. The simulation results obtained at condensation temperatures of 105 and 125 °C indicate higher COPs than that at 75 °C for R1234ze(Z). The major factor is the reduction in the irreversible loss caused by the pressure drop. The above assessments demonstrate that R1234ze(Z) is suitable for high-temperature applications rather than in typical air conditioners.

Low GWP refrigerants R1234ze(E) and R1234ze(Z) for high temperature heat pumps

In the present study we have carried out the drop-in experiments of R410A, HFO-1234ze(E) and the mixture of 50mass%HFO-1234ze(E)/50mass%HFC-32 at heating mode, using a vapor compression heat pump system developed for R410A. It has found that in the case of mixture of 50mass%HFO-1234ze(E)/50mass%HFC-32 the COP value is little affected by the degree of subcooling at the condenser outlet and it is only about 7.5% lower than that of R410A at the same heating load of 2.8 kW. This reveals that mixtures of HFO-1234ze(E)/HFC-32 are considered to be applicable as low-GWP alternatives for R410A by adjusting the composition of the mixture and by reconsidering the design parameters of components of room air-conditioning system.

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2154&context=iracc

Drop-in Experiments on Heat Pump Cycle Using HFO-1234ze(E) and Its Mixtures with HFC-32

This paper describes experimental results that show the effects of mass velocity and condensation temperature difference on the local heat transfer characteristics during condensation of R407C in a horizontal microfin tube. The experiments were performed at the saturation temperature of 40 °C, the refrigerant mass velocity of 50, 100, 200 and 300 kg m −2  s −1 , and the condensation temperature difference of 1.5, 2.5 and 4.5 K. A superficial heat transfer coefficient for the vapor phase was obtained by subtracting the heat transfer resistance of condensate film estimated by using a previously developed theoretical model of film condensation of pure vapor from the overall heat transfer resistance. On the basis of the analogy between heat and mass transfer, an empirical equation for the superficial vapor phase heat transfer coefficient was developed. The heat transfer coefficient predicted by the combination of the previously developed theoretical model of film condensation of pure vapor and the empirical equation of the superficial vapor phase heat transfer coefficient agreed with the measured values with the r.m.s. error of 9.2%.

Condensation of R407C in a horizontal microfin tube

Experimental results are presented that show the effect of fin geometry on condensation of refrigerant HFC134a in a staggered bundle of horizontal finned tubes. Two types of conventional low-fin tubes and three types of three-dimensional fin tubes were tested. The refrigerant mass velocity ranged from 8 to 23 kg/m2s and the condensation temperature difference from 1.5 to 12 K. The effect of condensate inundation was more significant for the three-dimensional fin tubes than for the low-fin tubes. In most cases, the highest performance was obtained by the tube with a three-dimensional structure at the tip of low fins. In the case of high mass velocity and high condensate inundation rate, however, the highest performance was obtained by one of the low-fin tubes. The results were compared with previous results for bundles of smooth tubes and low-fin tubes.

Condensation of downward-flowing HFC134a in a staggered bundle of horizontal finned tubes: effect of fin geometry

Domestic heat pumps constitute a significant part of the global heat pump industry. R401A offers excellent performance with no influence on the ozone depletion and has been a dominant refrigerant in most domestic heat pumps. However, R410A has a significant impact on the climate due to its high global warming potential (GWP). Thus, the heat pump industry has been focusing on the development of R410A substitutes. Pure refrigerant alternates, such as HFC32, and the blends mixed with low-GWP refrigerants, such as HFO1234yf and HFO1234ze(E), are widely reported. Besides the direct impact of refrigerants (which is estimated as GWP), domestic heat pumps indirectly affect the environment from the energy consumption and manufacturing processes. Hence the life cycle climate performance (LCCP) analysis accounts for all emissions through the lifetime of a heat pump. This article reports the LCCP evaluation of various low-GWP refrigerants for R410A replacement on domestic heat pumps. Six refrigerants, i.e., HFC32, binary blends of HFC32 and HFO1234yf (with GWP values of 300, 200 and 150), and HFC32 and HFO1234ze(E) with GWP values of 300 and 200, were compared against R410A. The performance data of these refrigerants from the experimental heat pump facility were utilized to evaluate the LCCP. Among the selected refrigerants, the binary blend of HFC32/HFO1234ze(E) with GWP 300 shows the lowest LCCP. Low-GWP refrigerants would become more competitive than R410A when CO2 emission from energy generations can be reduced by the optimization of the system or the usage of renewable energy.

Nobuo Takata

Papers

Low GWP refrigerants R1234ze(E) and R1234ze(Z) for high temperature heat pumps

Drop-in Experiments on Heat Pump Cycle Using HFO-1234ze(E) and Its Mixtures with HFC-32

Condensation of R407C in a horizontal microfin tube

Condensation of downward-flowing HFC134a in a staggered bundle of horizontal finned tubes: effect of fin geometry

The life cycle climate performance evaluation of low-GWP refrigerants for domestic heat pumps.