Assessment of the low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) for heat pump and organic Rankine cycle applications

Journal Pre-proofs Assessment of the low-gwp refrigerants r600a, r1234ze(z) and r1233zd(e) for heat pump and organic rankine cycle applications Giovanni A. Longo, Simone Mancin, Giulia Righetti, Claudio Zilio, J. Steven Brown PII: DOI: Reference:

S1359-4311(19)35076-8 https://doi.org/10.1016/j.applthermaleng.2019.114804 ATE 114804

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

23 July 2019 9 November 2019 15 December 2019

Please cite this article as: G.A. Longo, S. Mancin, G. Righetti, C. Zilio, J. Steven Brown, Assessment of the lowgwp refrigerants r600a, r1234ze(z) and r1233zd(e) for heat pump and organic rankine cycle applications, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114804

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ASSESSMENT OF THE LOW-GWP REFRIGERANTS R600a, R1234ze(Z) and R1233zd(E) FOR HEAT PUMP AND ORGANIC RANKINE CYCLE APPLICATIONS Giovanni A. LONGO1,*, Simone MANCIN1, Giulia RIGHETTI1, Claudio ZILIO1, J. Steven BROWN2 1University

of Padova, Department of Management and Engineering, I-36100 Vicenza, ITALY

2The

Catholic University of America, Department of Mechanical Engineering, Washington D.C. 20064, USA

Abstract This paper performs the thermodynamic and the heat-transfer assessment of the low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) as alternative to the traditional low-pressure HFC refrigerants, such as R245fa, for Heat Pump (HP) and Organic Rankine Cycle (ORC) applications. The thermodynamic assessment shows that R1234ze(Z) exhibits efficiency very similar to those of R245fa, while R1233zd(E) presents higher efficiency and R600a lower efficiency than those of R245fa both in HP and ORC applications. The heat transfer assessment is carried out in condensation inside a 4 mm ID horizontal smooth tube at three different saturation temperatures (30, 35, and 40 °C), at different vapour qualities and mass velocities. R600a, R1234ze(Z) and R1233zd(E) show very similar condensation heat transfer and pressure drop performances under operating conditions typical for HP and ORC applications. The results of the thermodynamic and the heat transfer assessment show that R600a, R1234ze(Z) and R1233zd(E) are valuable long-term low-GWP substitutes for the traditional low-pressure HFC refrigerants, both in HP and ORC applications.

*Corresponding Author, Phone: +39 0444 998726, Fax: +39 0444 998888, e-mail: [email protected]

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Keywords Alternative Refrigerants, Thermodynamic Assessment, Condensation, Small-Diameter Tubes

Nomenclature cp

specific heat capacity, J kg-1K-1

d

tube diameter, m

f.s.

full scale

G

refrigerant mass flux, kg m-2s-1

GWP

global warming potential

h

heat transfer coefficient, W m-2 K-1

HC

HydroCarbon

HCFO

HydroChloroFluoroOlefin

HFO

HydroFluoroOlefin

HP

heat pump

ID

inside diameter

J

specific enthalpy, J kg-1

JH

heat transfer factor

J*G

dimensionless gas velocity

k

coverage factor

L

length of the measurement section, m

MAPE

mean absolute percentage deviation

NBP

normal boiling point, K (°C)

Nu

Nusselt number

ODP

ozone depletion potential

ORC

Organic Rankine Cycle

p

pressure, Pa

2

Pr

Prandtl number

Ra

arithmetic mean roughness (ISO4271/1), m

Re

Reynolds number

Rp

roughness (DIN 4762/1), m

T

temperature, K (°C)

X

vapour quality

Xtt

Martinelli parameter

Greek symbol 

difference



density, kg m-3



dynamic viscosity, kg m-1 s-1

Subscript comp

compressor

cond

condenser / condensation

crit

critical state

eq

equivalent

evap

evaporator / evaporation

f

frictional

G

gas / vapour

h

hydraulic

in

inlet

L

liquid

LG

liquid vapour phase change

m

mean value 3

out

outlet

pump

pump

r

refrigerant

sat

saturation

tur

turbine

1. Introduction Heat Pumps (HP) and Organic Rankine Cycles (ORC) involve the use of low-pressure refrigerants, such as R245fa, R236ea and R236fa, which exhibit very high GWP ranging from 1020 of R245fa, to 1370 of R236ea to 9810 of R236fa. The Kigali amendment [1] to the Montreal Protocol [2] established the phase down of traditional HFC refrigerants with high-GWP, therefore it is crucial to identify alternative low-GWP substitutes in each specific application. Fukuda et al. [3] and Kondou and Koyama [4] analysed the application of the low-GWP refrigerants R1234ze(Z) and R1233zd(E) in High Temperature Heat Pump (HTHP) for heat recovery in industrial application: both the refrigerants seemed to be very promising as long-term low-GWP working fluids for HTHP. Vivian et al. [5] presented a selection procedure of working fluids for ORC considering the temperature level of the heat source: R600a seems to be a very interesting low-GWP ORC fluid for heat source with an inlet temperature up to 150°C. Eyerer et al. [6] experimentally investigated the application of R1233zd(E) as drop-in substitute for R245fa in ORC applications: R1233zd(E) shows boiling heat transfer performance very similar and higher cycle efficiency than that of R245fa. Moles et al. [7] performed the experimental evaluation of R1233zd(E) as R245fa replacement in ORC for low temperature heat sources: R1233zd(E) exhibits lower power output and higher global efficiency than those of R245fa.

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Bamigbetan et al. [8] carried out an extensive review concerning the use of natural refrigerants in vapour compression HTHP: hydrocarbons, in particular R600a, emerge as interesting alternatives for high temperature applications (90–120 °C) due to their environmental friendliness and their good thermodynamic properties, although the safety concerns and some technological constrains (compressor cooling) must be still resolved. Bamigbetan et al. [9] carried out also the theoretical analysis of suitable fluids for high temperature heat pumps up to 125° C heat delivery: certain hydrocarbons (R600, R600a, R601, R601a) and hydrofluoroolefins (R1234ze(Z), R1233zd(E)) are suitable low-GWP fluid candidates for this specific applications. Arpagaus et al. [10] reviews the state of the art, the market situation and the research activities of HTHP with a special focus on the analysis of the thermodynamic reference cycles and the suitable refrigerants. R1234ze(Z) is assessed as a suitable drop-in substitute for R114 and a promising alternative to R245fa in HTHP applications, while R1233zd(E) exhibits thermophysical and thermodynamic properties very close to R245fa and it is specifically recommended for HTHP applications. Giuffrida [11] theoretically investigated the use of hydrofluoroolefins refrigerants as alternative to R245fa in a micro ORC system: R1234ze(Z) and R1233zd(E) show equal and better performance than those of R245fa, respectively. Therefore the long-term low-GWP substitutes for traditional low-pressure refrigerants in HP and ORC applications include a well-known flammable refrigerant, R600a (isobutane), and two new refrigerants recently put on the market, R1234ze(Z) and R1233zd(E). R600a is a HydroCarbon (HC) refrigerant with high flammability (ASHRAE classification A3), R1234ze(Z) is a HydroFluoroOlefin (HFO) refrigerant with mild flammability (ASHRAE classification A2L), and R1233zd(E) is a HydroChloroFluoroOlefin (HCFO) refrigerant with no flammability (ASHRAE classification A1). Two of the possible substitutes exhibit mild or high flammability, therefore their use is subjected to some constrains depending on the degree of flammability and the reference

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safety standards. Flammability issues promote the use of heat exchangers with reduced charge such as brazed plate heat exchangers, small-diameter tubes and multi-ports tubes. In the open literature it is possible to find only limited experimental data on R600a, R1234ze(Z) and R1233zd(E) two-phase heat transfer inside small-diameter tubes and channels. Longo [12, 13] investigated R600a condensation and boiling inside a small commercial brazed plate heat exchanger (dh= 4 mm) and compared its thermal and hydraulic performances to those of other two hydrocarbon refrigerants, R290 (propane) and R1270 (propylene). R600a exhibits condensation heat transfer coefficients lower than those of R1270 and higher than those of R290, while its boiling performance is lower than those of the other two hydrocarbons. Longo et al. [14] measured R1234ze(Z) condensation heat transfer coefficients and pressure drops inside a small commercial brazed plate heat exchanger (dh= 4 mm) and compared these measurements with similar data points previously obtained for refrigerant R236fa, R134a, R600a, R1234ze(E). R1234ze(Z) exhibits heat transfer coefficients much higher than those of all the other considered refrigerants and frictional pressure drop similar to R600a at the same refrigerant mass flux. Chen et al. [15] investigated the effect of compressor oil concentration in the refrigerant during R600a boiling inside a 5.4 mm ID smooth tube: the effect on heat transfer coefficients depend on heat flux, mass flux and flow regime. Righetti et al. [16] experimentally investigated R600a flow boiling inside a roll-bond evaporator (dh≈ 4.5 mm) and compared its performance to those of traditional refrigerants: R600a is a good low-GWP fluid for domestic refrigerators. Yang et al. [17, 18] studied R600a flow boiling inside a 6 mm ID horizontal smooth tube and compared its performance to those of R1234ze(E). R600a boiling heat transfer coefficients and pressure drops are higher than those of R1234ze(E). Lee et al. [19] investigated R1233zd(E) boiling inside a brazed plate heat exchangers (dh= 3.9 mm) as an alternative to R245fa in ORC: R1233zd(E) shows heat transfer and hydraulic performances similar to those of R245fa. Righetti et al. [20, 21] compared R1233zd(E) and R245fa heat transfer performances in flow boiling inside a 4.3 mm ID horizontal micro-fin tube: R1233zd(E) exhibits similar heat transfer coefficients and slightly higher frictional pressure drops than those of R245fa.

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Zhang et al. [22] presented R1233zd(E) condensation heat transfer coefficients and pressure drops measurements inside a plate heat exchanger (dh= 3.4 mm): R1233zd(E) exhibits higher heat transfer coefficients than those of R245fa at the same working conditions. Kwon et al. [23] studied R1233zd(E) condensation inside a plate heat exchanger (dh= 3.88 mm): R1233zd(E) exhibits lower heat transfer coefficients than those of R245fa. Longo et al. [24] presented heat transfer and hydraulic performances of R1234ze(Z) and R1233zd(E) during flow boiling inside a small commercial brazed plate heat exchanger (dh= 4 mm): R1234ze(Z) shows higher heat transfer coefficients and lower pressure than R1233zd(E) and both the new refrigerants are valuable alternatives to traditional refrigerant R245fa. The above literature review shows that the experimental data on R600a, R1234ze(Z) and R1233zd(E) two-phase heat transfer inside small-diameter tubes and channels is limited. In particular, there is no data points concerning R1234ze(Z) condensation inside smooth or enhanced tubes, while this new refrigerant seems to be a very promising substitute for R245fa both in HP and ORC applications. Therefore there is a specific need for new and original data on in-tube boiling and condensation of the new refrigerants in order to have a consistent data bank for their assessment. This paper carried out the thermodynamic and the heat-transfer assessment of the low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) for HP and ORC applications providing a consistent amount of new and original data points on condensation of the three refrigerants inside a small-diameter smooth tube. It should be noted that the data points on R1234ze(Z) condensation is the unique set of data available in the open literature, while this new refrigerant seems to be a very promising substitute for R245fa.

2. Thermodynamic assessment The thermodynamic assessment of a refrigerant requires the knowledge of some thermodynamic properties such as critical temperature Tcrit, critical pressure pcrit, normal boiling point NBP, latent

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heat of phase change JGL at 30 °C, together with some environmental and safety information such as ODP, GWP on 100-year basis and the ASHRAE safety classification. Table 1 summarises this data for the traditional refrigerant R245fa and the innovative refrigerants R600a, R1234ze(Z), R1233zd(E) for HP and ORC applications. The reference thermodynamic cycle for the assessment in HP applications is a simple subcritical vapour compression cycle consisting of an evaporator, a compressor, a condenser and a throttling valve, under the following conditions [3, 4]: -

condensation temperature variable from 30 °C to the critical temperature

-

evaporation temperature 40°C lower than condensation temperature

-

no condenser sub-cooling

-

3 °C evaporator super-heating, minimum value to avoid wet isentropic compression

-

70% compressor isentropic efficiency

-

no evaporator, condenser and refrigerant lines pressure drop

The efficiency of the HP reference cycle is calculated by the Coefficient of Performance: COPHP = (Jcond.in – Jcond.out) / (Jcomp.out – Jcomp.in)

(1)

Figure 1 shows the COPHP of the HP reference cycle vs. condensation temperature variable from 30°C to critical temperature for the traditional and the innovative refrigerants. For each refrigerant it is possible to identify a condensation temperature that maximises the COPHP: around 75°C for R600a, 93 °C for R1234ze(Z), 94 °C for R245fa, and 106 °C for R1233zd(E). These optimum values correspond to 85-86% of the critical temperature for all the investigated refrigerants. R1233zd(E) exhibits a maximum COPHP (5.577) which is 1.8% higher than that of R1234ze(Z) (5.480), 3.9% higher than that of R245fa (5.366), and 9.5% higher than that of R600a (5.094). The comparison between the innovative refrigerants and the traditional refrigerant R245fa shows that R1234ze(Z) presents very similar maximum efficiency in the same range of condensation temperature, while R1233zd(E) exhibits a slightly higher maximum efficiency at a condensation

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temperature 10 °C above and R600a a lower maximum efficiency at a condensation temperature 20°C below. The reference thermodynamic cycle for the assessment in ORC applications is a simple subcritical cycle consisting of an evaporator, a turbine, a condenser and a pump under the following conditions [5]: -

condensation temperature constant at 30 °C

-

evaporation temperature variable from 70°C to critical temperature

-

no condenser sub-cooling

-

no evaporator super-heating,

-

70% turbine isentropic efficiency

-

75% pump isentropic efficiency

-

no evaporator, condenser and refrigerant lines pressure drop

The efficiency of the ORC reference cycle is calculated the Cycle Efficiency: ORC = [(Jturb.in – Jturb.out) – (Jpump.out – Jpump.in)] / (Jevap.out – Jevap.in)

(2)

Figure 2 shows the ORC of the ORC reference cycle vs. evaporation temperature variable from 70°C to critical temperature for the traditional and the innovative refrigerants. For each refrigerant it is possible to identify an evaporation temperature that maximises the ORC: around 124-125 °C for R600a, 144-145 °C for R1234ze(Z), 145-150 °C for R245fa, and 155-156°C for R1233zd(E). These optimum values are around 9-10 °C below the critical temperature for all the refrigerants investigated. R1233zd(E) exhibits a maximum ORC (0.140) which is 5.3% higher than that of R1234ze(Z) (0.133), 6.9% higher than that of R245fa (0.131), and 22.8% higher than that of R600a (0.114). The comparison between the innovative refrigerants and the traditional refrigerant R245fa shows that R1234ze(Z) presents similar maximum efficiency at the same evaporation temperature, while R1233zd(E) exhibits higher maximum efficiency at an evaporation temperature 10 °C above and R600a much lower maximum efficiency at an evaporation temperature 20°C below.

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The thermodynamic assessment shows that R1234ze(Z) exhibits efficiency very similar to those of R245fa, while R1233zd(E) presents higher efficiency and R600a lower efficiency than those of R245fa both in HP and ORC applications. It should be noted that the reference cycles and the relative operating conditions applied in the thermodynamic assessment are not characteristic for a specific size (small, medium or big) of HP and ORC systems and they are used only for the comparative analysis of the different refrigerants under the same operating conditions. The identification of the optimum thermodynamic reference cycle and the relative operating conditions for each refrigerant and system size goes beyond the scope of the present analysis.

3. Heat-transfer assessment The experimental heat-transfer assessment of a refrigerant requires the combined evaluation of thermal and hydraulic performances in a specific application, in this case, the condensation inside a 4 mm ID horizontal smooth tube. The experimental facility and procedures were described in details in a previous work [25] and here they are only briefly summarised. The experimental rig is based on a closed loops scheme consisting of a refrigerant circuit (primary loop), a water-glycol loop and a refrigerated / cooling water loop (supply loops). The test-section is a tube-in-tube heat exchanger with the refrigerant condensing in the inner tube and the cooling water flowing in the annulus. The inner tube is instrumented with four T-type (copper-constantan) thermocouples embedded in its wall to measure surface temperature. The geometrical characteristics of the test-section are reported in Table 2, while Table 3 shows the characteristics of the instrumentation used in the experimental circuit. The experimental results are reported in terms of condensation heat transfer coefficients hr and frictional pressure drop pf. The condensation heat transfer coefficient is computed by measuring the surface temperature of the tube wall by thermocouples, while the condensation frictional pressure drop is obtained from the total pressure drop by subtracting the inlet / outlet local pressure drop and adding the momentum pressure rise. The refrigerant properties are evaluated by the NIST Standard Reference Database REFPROP 10.0 [26]. In general, REFPROP 10

implements a dedicated equation of state for each fluid. All the EoS are fundamental equations of state (FES) explicit in the Helmholtz energy. The number of terms for each equation is depending on the fitting on available experimental data for the relevant thermophysical properties. In their recent review about thermophysical properties of HFO refrigerants, Bobbo et al. [27] analyszed the equation of state used in REFPROP and the related uncertainties in the estimations for several HFOs, including R1233zd(E) and R1234ze(Z). In the present paper, for what concerns R245fa, R600a, R1234ze(Z) and R1234zd(E) the FES of Akasaka et al. [28], Buecker and Wagner [29], Akasaka and Lemmon [30] and Mondejar et al. [31] were used respectively.

The experimental tests include three sets of saturated vapour condensation data points carried out with refrigerant R600a, R1234ze(Z) and R1233zd(E), respectively, at three different condensation temperatures, 30, 35, and 40 °C, and different vapour quality. Table 4 shows the operating conditions in the experimental test: condensation temperature Tsat and pressure psat, mean vapour quality Xm, and refrigerant mass velocity G. The operating conditions are typical for ORC applications and for low temperature HP applications. An error analysis carried out following the approach [34] evidences an average overall uncertainty of ±8.4%, ±19,9% and ±14.1% for the condensation heat transfer coefficient and an overall uncertainty within ±13.3%, ±16.5% and ±14.4% for the total pressure drop of R600a, R1234ze(Z) and R1233zd(E), respectively. Figures 3a, 3b, and 3c present the condensation heat transfer coefficient hr versus mean vapour quality Xm at different refrigerant mass velocities, respectively for R600a, R1234ze(Z) and R1233zd(E), at 30 °C condensation temperature. The same applies for figures 4a, 4b, and 4c for a condensation temperature of 40°C. Considering that R600a has a latent heat of condensation and a specific volume of the liquid phase almost 1.5-2 times higher than those of R1234ze(Z) and R1233zd(E), a fair performance comparison must be carried out at a refrigerant mass velocity about 50% higher for R1234ze(Z) and R1233zd(E) than that of R600a. For example, the heat transfer and pressure drop performances of R600a at 200 kg m-2s-1 must be compared to those of the other two refrigerants at 300 kg m-2s-1. Taking in mind the above consideration, the three different low-

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pressure and low-GWP refrigerants under investigation (R600a, R1234ze(E), and R1233zd(E)) exhibit very similar condensation heat transfer coefficients, both in trend and magnitude. This interesting experimental result is not easily predictable by a simple theoretical analysis, considering the different trends of the thermophysical and the thermodynamic properties of the three refrigerants under investigation. In fact R600a and R1234ze(Z) exhibit similar values of thermal conductivity higher than those of R1233zd(E), which promote higher condensation heat transfer coefficients, mainly in gravity dominated condensation, while, R1233zd(E) shows lower vapour density and absolute vapour pressure than those of the other two refrigerants, which can increase the heat transfer coefficients in forced convection condensation. In order to explain the above experimental results it is essential to investigate the dominant heat transfer regimes during the experimental tests. Condensation inside smooth tube is dominated by gravity and / or vapour shear effects. In forced convection condensation vapour shear drives the condensate flow and the condensation heat transfer coefficients depend mainly on mass flux and vapour quality. In gravity-dominated condensation the condensate drainage is governed by the gravity forces and the condensation heat transfer coefficients exhibit great sensitivity to the driving temperature difference. The present experimental data shows weak sensitivity to condensation temperature / pressure and remarkable sensitivity to refrigerant mass flux and average vapour quality with a positive slope, except for mass fluxes lower than 150 kg m-2s-1 (100 kg m-2s-1 for R600a), where the sensitivity to mass flux and vapour quality disappears. Probably the condensation process is dominated by gravity forces for mass flux lower than 100 – 150 kg m-2s-1 and by vapour shear effects for higher mass fluxes. In order to confirm this hypothesis the experimental data was plotted (see figure 5) in non-dimensional co-ordinates giving the heat transfer factor [35] JH = Nur / PrL1/3

(3)

Nur = hr dh / L

(4)

PrL = L cpL / L

(5) 12

versus the equivalent Reynolds number [36] Reeq = G [(1 – X) + X (L / G)1/2] dh / L

(6)

calculated at the average vapour quality, in order to properly identify the dominant heat transfer regimes. The transition from gravity-dominated to forced convection condensation occurs for an equivalent Reynolds number around 10000 which corresponds to a refrigerant mass flux around 100 - 150 kg m-2s-1. For Reeq < 10000 the heat transfer coefficients are independent of refrigerant mass flux, showing a great sensitivity only to driving temperature difference. For Reeq > 10000 the heat transfer coefficients increase almost linearly with mass flux and vapour quality. A similar transition was already observed by the authors of the present paper for R404A [25], for R410A [37], for R134a [38] and their low-GWP substitutes condensation inside the same 4 mm ID smooth tube under similar operating conditions. The experimental data was also plotted on the Breber et al. [39] map (see figure 6) that reports also the two transition lines (blue curve for hydrocarbons and red curve for the other refrigerants) for condensation inside smooth tube suggested by Cavallini et al. [40] between gravity-dominated and forced convection. The comparison between figure 5 and figure 6 shows a fair agreement for the estimation of the dominant heat transfer regimes. The experimental heat transfer coefficients were also compared against different heat transfer calculation procedures for condensation inside smooth tube including both general-purpose models and correlations developed for a specific heat transfer regimes: Table 5 shows the MAPE between experimental and calculated values. The traditional correlations for in-tube forced convection condensation of conventional diameter greatly over-predicted the present data, whereas the classical Akers et al. [36] equation for forced convection condensation shows a reasonable ability in reproducing the experimental data in forced convection condensation with a MAPE of 7.7%, 17.5%, and 19.4% for R600a, R1234ze(Z), and R1233zde(E) data, respectively. Figure 7 shows the deviation between the experimental data in forced convection condensation and the calculated data by Akers et al. [36] model. R600a data is very well predicted both in trend and magnitude, while 13

R1234ze(Z) data and R1233zd(E) data are both well predicted in trend and slightly overestimated and underestimated in magnitude, respectively. It should be noted that no general-purpose calculation procedure is able to predict with a reasonable agreement the experimental data in all the heat transfer regimes (gravity dominated and forced convection condensation). Figures 8a, 8b, and 8c plot the condensation frictional pressure drop versus refrigerant mass flux at three different condensation temperatures, 30, 35, and 40°C, for R600a, R1234ze(Z) and R1233zd(E), respectively. Taking in mind that the performance comparison must be carried out at a refrigerant mass velocity 50% higher for R1234ze(Z) and R1233zd(E) than that of R600a, the three different refrigerants present very similar trends with relatively small difference in magnitude. Also in this case the experimental results are not easily predictable by a simple theoretical analysis. In fact R600a and R1234ze(Z) exhibit lower liquid dynamic viscosity and higher vapour pressure than those of R1233zd(E), which in general promotes lower frictional pressure drops. The experimental frictional pressure drops were compared against different correlations for in-tube condensation: Table 6 shows the mean MAPE between experimental and calculated values. Friedel [52] correlation is able to process the data in all the different heat transfer regimes showing the best performance with a MAPE of 14.8%, 15.6%, and 16.7% for R600a, R1234ze(Z) and R1233zd(E), respectively. Figure 9 shows the deviation between the experimental data and the calculated data by Friedel [52] equation. The heat transfer assessment shows that R600a, R1234ze(Z) and R1233zd(E) exhibit very similar heat transfer and hydraulic performances in condensation inside a smooth tube of small diameter under operating conditions typical both for HP and ORC applications. Also these interesting results were not easily predictable by a simple theoretical analysis.

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4. Conclusions This paper performs the thermodynamic and the heat-transfer assessment of the low-pressure and low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) for HP and ORC applications. The thermodynamic assessment shows that R1234ze(Z) exhibits efficiency very similar to those of R245fa, while R1233zd(E) presents higher efficiency and R600a lower efficiency than those of R245fa both in HP and ORC applications. The experimental heat transfer assessment is carried out with reference to condensation tests inside a 4 mm ID horizontal smooth tube at three different saturation temperatures (30, 35, and 40 °C), at different vapour qualities and mass velocities to evaluate the specific contribution of refrigerant mass flux, mean vapour quality, and condensation temperature (pressure) to the heat transfer mechanisms. A transition point from gravity-dominated to forced convection condensation was found for an equivalent Reynolds around 10000 that corresponds to a mass flux around 100150 kg m-2s-1. The experimental heat transfer coefficients in the forced convection condensation regime were sufficiently well predicted by the Akers et al. [36] model, whereas the Friedel [52] correlation was able to reproduce the frictional pressure drop data in the whole experimental range. R600a, R1234ze(Z) and R1233zd(E) show similar condensation heat transfer and pressure drop performances under operating conditions typical for HP and ORC applications. The results of the thermodynamic and the heat transfer assessment show that R600a, R1234ze(Z) and R1233zd(E) are valuable long-term low-GWP substitutes for the traditional low-pressure HFC refrigerants, both in HP and ORC applications. The availability of new and original two-phase heat transfer data points greatly increases the data bank on these new refrigerants with consistent benefits, not only for their assessment, but also for their technical and commercial applications both in HP and ORC. In fact, the limited availability or the absolute unavailability of the experimental data on of the above new refrigerants have restrained up to now their extensive application in HP and ORC.

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Acknowledgement This work was carried out within the research project “Analysis of the Thermodynamic and Thermophysical Properties of Innovative Refrigerants” between the Catholic University of America and the University of Padova and completed while the first author (Giovanni A. Longo) was on sabbatical leave at the Catholic University of America, Washington D.C., USA. in the Academic Year 2019-20.

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[10] C. Arpagaus, F. Bless, M. Uhlmann, J. Schiffmann, S.S. Bertsch, High temperature heat pumps: Market overview, state of the art, research status, refrigerants, and application potentials, Energy 152 (2018) 985-1010. [11] A. Giuffrida, A theoretical study on the performance of a scroll expander in an organic Rankine cycle with hydrofluoroolefins (HFOs) in place ofR245fa, Energy 161 (2018) 11721180. [12] G.A. Longo, Heat transfer and pressure drop during hydrocarbon refrigerant condensation inside a brazed plate heat exchanger, Int. J. Refrigeration 33 (2010) 944–953. [13] G.A. Longo, hydrocarbon refrigerant vaporization inside a brazed plate heat exchanger, ASME J. Heat Transfer 134 (2012) 101801.1–10. [14] G.A. Longo, C. Zilio, G. Righetti, J.S. Brown, Experimental assessment of the low GWP refrigerant HFO-1234ze(Z) for high temperature heat pumps, Experimental Thermal And Fluid Science 57 (2014) 293–300. [15] G.M. Chen, V.P. Zhelezny, A.V. Melnyk, K.O. Shestopalov, An experimental investigation and modelling of flow boiling heat transfer of isobutane-compressor oil solution in a horizontal smooth tube, Int. J. Refrigeration 58 (2015) 137–145. [16] G. Righetti, C. Zilio, G.A. Longo, Comparative performance analysis of the low GWP refrigerants HFO1234yf, HFO1234ze(E) and HC600a inside a roll-bond evaporator, Int. J. Refrigeration, 54 (2015) 1–9. [17] Z, Yang, M, Gong, G, Chen, X. Zou, J. Shen, Two-phase flow patterns, heat transfer and pressure drop characteristics of R600a during flow boiling inside a horizontal tube, Applied Thermal Eng. 120 (2017) 654–671. [18] Z. Yang, G. Chen, Y. Yao, Q. Song, J. Shen, M. Gong, Experimental study on flow boiling heat transfer and pressure drop in a horizontal tube for R1234ze(E) versus R600a, Int. J. Refrigeration, 85 (2018) 334–352.

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[19] D.C. Lee, D. Kim, S. Park, J. Lim, Y. Kim, Evaporation heat transfer coefficient and pressure drop of R-1233zd(E) in a brazed plate heat exchanger. Applied Thermal Eng. 130 (2018) 1147–1155. [20] G. Righetti, G.A Longo, C. Zilio, R. Akasaka, S. Mancin, R1233zd(E) flow boiling inside a 4.3 mm ID microfin tube. Int. J. Refrigeration 91 (2018) 69–79. [21] G. Righetti, G.A. Longo, C. Zilio, S. Mancin, Flow Boiling of Environmentally friendly Refrigerants inside a compact enhanced tube. Int. J. Refrigeration 104 (2019) 344-355. [22] J. Zhang, M. Ryhl Kærn, T. Ommen, B. Elmegaard, F. Haglind, Condensation heat transfer and pressure drop characteristics of R134a, R1234ze(E), R245fa and R1233zd(E) in a plate heat exchanger, Int. J. Heat Mass Trans. 128 (2019) 136-149. [23] O.J. Kwon, B.H. Shon, Y.T. Kang, Experimental investigation on condensation heat transfer and pressure drop of a low GWP refrigerant R-1233zd(E) in a plate heat exchanger. Int. J. Heat Mass Transfer 131 (2019) 1009-1021. [24] G.A. Longo, S. Mancin, G. Righetti, C. Zilio, Boiling of the new low-GWP Refrigerants R1234ze(Z) and R1233zd(E) inside a small commercial brazed plate heat exchanger. Int. J. Refrigeration 104 (2019) 376-385. [25] G. A. Longo, S. Mancin, G. Righetti, C. Zilio, Saturated vapour condensation of HFC404A inside a 4 mm ID horizontal smooth tube: Comparison with the long-term low GWP substitutes HC290 (Propane) and HC1270 (Propylene). Int. J. Heat Mass Trans. 108 (2017) 2088-2099. [26] E.W. Lemmon, I.H. Bell, M.L. Huber, M.O, McLinden, NIST Reference Fluid Thermodynamic and Transport Properties Database - REFPROP, Version 10.0, National Institute of Standards and Technology, Standard Reference Data Program, Gaithersburg, 2017. [27] S. Bobbo, G. Di Nicola, C. Zilio, J.S. Brown, L. Fedele, Low GWP halocarbon refrigerants: A review of thermophysical properties., Int. J. Refrigeration, 90 (2018) 181-201.

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[28] R. Akasaka, Y. R., Zhou, E.W. Lemmon, A Fundamental Equation of State for 1,1,1,3,3Pentafluoropropane (R-245fa). J. Phys. Chem. Ref. Data, 44(2015) 013104. [29] D. Bücker, W. Wagner, Reference Equations of State for the Thermodynamic Properties of Fluid Phase n-Butane and Isobutane. J. Phys. Chem. Ref. Data, 35(2006) 929-1019. [30] R. Akasaka, Y. Higashi, A. Miyara, S. Koyama, A Fundamental Equation of State for Cis1,3,3,3-tetrafluoropropene (R-1234ze(Z)). Int. J. Refrigeration 44 (2014) 168-176. [31] M.E. Mondejar, M.O. McLinden, E.W. Lemmon, Thermodynamic Properties of trans-1chloro-3,3,3-Trifluoropropene (R1233zd(E)): Vapor Pressure, P-rho-T Data, Speed of Sound Measurements and Equation of State. J. Chem. Eng. Data, 60 (2015) 2477-2489. [32] EPA

Significant

New

Alternatives

Policy

(SNAP)

Program,

2019,

https://www.epa.gov/SNAP [33] ANSI/ASHRAE Standard 34-2019. Designation and Safety Classification of Refrigerants. [34] S.J. Kline, F.A. McClintock. Describing uncertainties in single-sample experiments. Mech.Eng. 75 (1953) 3–8. [35] D.K. Kern, Process Heat Transfer, McGraw Hill, New York, 1950, p. 50. [36] W.W. Akers, H.A. Deans, O.K. Crosser, Condensing heat transfer within horizontal tubes, Chem. Eng. Prog. Symp. Series 55 (1959) 171–176. [37] G. A. Longo, S. Mancin, G. Righetti, C. Zilio, Saturated vapour condensation of R410A inside a 4 mm ID horizontal smooth tube: Comparison with the low GWP substitute R32. Int. J. Heat Mass Trans. 125 (2018) 702-709. [38] G. A. Longo, S. Mancin, G. Righetti, C. Zilio, Saturated vapour condensation of R134a inside a 4 mm ID horizontal smooth tube: Comparison with the low GWP substitute R152a, R123yf and R1234ze(E). Int. J. Heat Mass Trans. 133 (2019) 461-473. [39] G. Breber, J.W. Palen, J. Taborek, Prediction of Horizontal Tube Side Condensation of Pure Components Using Flow regime Criteria, J. Heat Transfer 102 (1980) 471–476.

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[40] A. Cavallini, D. Del Col, L. Doretti, M. Matkovic, L. Rossetto, C. Zilio, G. Censi, Condensation in horizontal smooth tubes: a new heat transfer model for heat exchanger design, Heat Transfer Eng. 27 (2006) 31-38. [41] A. Cavallini, R. Zecchin. A dimensionless correlation for heat transfer in forced convection condensation. Proc. 5th Int. Heat Transfer Conf., Tokyo, Japan, 3, 1974, 309–313. [42] M.K. Dobson, J.C. Chato. Condensation in smooth horizontal tubes. ASME J. Heat Transfer 120 (1998) 193–213. [43] W.-W. Wang, T.D. Radcliff, R.N. Christensen. A condensation heat transfer correlation for millimeter-scale tubing with flow regime transition. Exp. Thermal Fluid Sci. 26 (2002) 473– 485. [44] S. Koyama, K. Kuwahara, K. Nakashita, K. Yamamoto. An experimental study on condensation of refrigerant R134a in a multi-port extruded tube. Int. J. Refrigeration 24 (2003) 425–432. [45] S.M. Kim, I. Mudawar. Universal approach to predicting heat transfer coefficient for condensing mini/micro-channel flow. Int. J. Heat Mass Transfer 56 (2013) 238–250. [46] M. Macdonald, S. Garimella. Hydrocarbon condensation in horizontal smooth tubes: Part II – Heat transfer coefficient and pressure drop modeling. Int. J. Heat Mass Transfer 93 (2016) 1248–1261. [47] H. Haraguchi, S. Koyama, T. Fujii, Condensation of refrigerants HCFC 22, HFC 134a and HCFC 123 in a horizontal smooth tube (2nd report), Trans. JSME (B) 60 (1994) 245–252. [48] Shao D. W., Granryd E. G., Flow Pattern, Heat Transfer and Pressure Drop in Flow Condensation Part I: Pure and Azeotropic Refrigerants, HVAC&R Research, 6, (2000), 175195. [49] H. Muller-Steinhagen, K. Heck. A simple friction pressure drop correlation for two-phase flow in pipes. Chem. Eng. Processing 20 (1986) 297–308.

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[50] C.Y. Yang, R.L. Webb. Condensation of R-12 in Small Hydraulic Diameter Extruded Aluminum Tubes with and without Micro-Fins. Int. J. Heat Mass Transfer 39(1996) 791-800. [51] K. Mishima, T. Hibiki. Some characteristics of air–water two-phase flow in small diameter vertical tubes. Int. J. Multiphase Flow 22 (1996) 703–712. [52] L. Friedel. Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow, European Two-Phase Group Meeting, 1979, Ispra, Italy, Paper E2. [53] C.C. Wang, C.S. Chiang, D.C. Lu. Visual observation of two-phase flow pattern of R-22, R134a, and R-407C in a 6.5-mm smooth tube. Exp. Therm. Fluid Sci. 15 (1997) 395–405. [54] L. Sun, K. Mishima. An evaluation of prediction methods for saturated flow boiling heat transfer in mini-channels. Int. J. Heat Mass Transfer 52 (2009) 5323-5329. [55] S.M. Kim, I. Mudawar. Review of databases and predictive methods for heat transfer. Int. J. Heat Mass Transfer 77 (2014) 627–652.

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33

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Captions to the figures Fig. 1 Coefficient of Performance COPHP vs. condensation temperature in the HP reference cycle. Fig. 2 Cycle efficiency ORC vs. evaporation temperature in the ORC reference cycle. Fig. 3a R600a condensation heat transfer coefficient vs. mean vapour quality and refrigerant mass velocity at 30°C. Fig. 3b R1234ze(Z) condensation heat transfer coefficient vs. mean vapour quality and refrigerant mass velocity at 30°C. Fig. 3c R1233zd(E) condensation heat transfer coefficient vs. mean vapour quality and refrigerant mass velocity at 30°C. Fig. 4a R600a condensation heat transfer coefficient vs. mean vapour quality and refrigerant mass velocity at 40°C. Fig. 4b R1234ze(Z) condensation heat transfer coefficient vs. mean vapour quality and refrigerant mass velocity at 40°C. Fig. 4c R1233zd(E) condensation heat transfer coefficient vs. mean vapour quality and refrigerant mass velocity at 40°C. Fig. 5 Experimental data plotted on the non-dimensional co-ordinates JH vs. Reeq. Fig. 6 Experimental data plotted on the Breber et al. (1980) [39] map. Fig. 7 Comparison between experimental condensation heat transfer coefficients and calculated by Akers et al. (1959) [36] model. Fig. 8a R600a condensation frictional pressure drop vs. refrigerant quality and mass velocity. Fig. 8b R1234ze(Z) condensation frictional pressure drop vs. refrigerant quality and mass velocity. Fig. 8c R1233zd(E) condensation frictional pressure drop vs. refrigerant quality and mass velocity. Fig. 9. Comparison between experimental and calculated frictional pressure drops by Friedel (1979) [52] correlation.

35

Table 1.

Thermodynamic properties of refrigerants for HP and ORC applications. *T

*p

crit

crit

*J LG

*NBP

**ODP

**GWP

100

***Safety

(°C)

(MPa)

(°C)

(kJ/kg)

Classification

R245fa

154.1

3.651

14.9

187.9

0

1030

B1

R600a

134.7

3.629

-11.7

323.4

0

3

A3

R1234ze(Z)

150.1

3.530

9.8

212.0

0

6

A2L

R1233zd(E)

166.5

3.623

19.0

188.5

~0

4.7 - 7

A1

The thermodynamic properties are evaluated by the NIST Standard Reference Database REFPROP 10.0 [26]. ODP, GWP100 are evaluated accordingly with EPA/SNAP Program [32] ***Safety Classification is evaluated accordingly with ANSI/ASHRAE [33] *

**

Table 2. Geometrical characteristics of the test-section. Parameter Tube inside diameter d (mm)

4.0

Measurement section length L (mm)

800.0

Pre-section length (mm)

200.0

Total section length (mm)

1300.0

Inside tube surface roughness Ra (m) (ISO 4287/1)

0.7

Inside tube surface roughness Rp (m) (DIN 4762/1)

1.8

36

Table 3. Specification of the different measuring devices Devices

Uncertainty (k=2)

Range

T-type thermocouples

0.1 K

-20 / 80 °C

T-type thermopiles

0.05 K

-20 / 80 °C

Abs. pressure transducers

0.075% f.s.

0 / 1.0 MPa

Diff. pressure transducers

0.075% f.s.

0 / 0.05 MPa

Coriolis effect flow meters

0.1%

0 / 300 kg h-1

Magnetic flow meters

0.15% f.s.

100 / 1200 l h-1

Data logger

 2.7 V

0 / 100 mV

Table 4. Operating conditions during experimental tests Fluid

Runs

Tsat (°C)

psat (MPa)

Xm

G (kg m-2s-1)

R600a

90

29.9 – 40.1

0.397 – 0.538

0.13 – 0.95

74.2 – 312.5

R1234ze(Z)

90

29.9 – 40.0

0.202 – 0.296

0.15 – 0.95

74.4 – 308.1

R1233zd(E)

90

29.9 – 40.1

0.145 – 0.223

0.16 – 0.96

73.6 – 306.6

37

Table 5. MAPE between experimental forced convection heat transfer coefficients and models Correlation

R600a

R1234ze(Z)

R1233zd(E)

Akers et al. [36]

7.7%

17.5%

19.4%

Cavallini & Zecchin [41]

79.0%

113.0%

99.9%

Dobson & Chato [42]

42.5%

116.3%

71.4%

Wang et al. [43]

35.0%

67.4%

77.3%

Koyama et al. [44]

21.7%

31.7%

20.8%

Cavallini et al. [40]

51.3%

105.6%

77.8%

Kim and Mudawar [45]

41.3%

89.1%

79.2%

Macdonald & Garimella [46]

7.8%

76.3%

66.0%

Haraguchi et al. [47]

20.4%

25.7%

20.3%

Shao and Granryd [48]

17.3%

35.8%

22.1%

38

Table 6. MAPE between experimental frictional pressure drop and models Correlation

R600a

R1234ze(Z)

R1233zd(E)

Muller-Steinhagen and Heck [49]

18.5%

14.9%

21.1%

Yang and Webb [50]

38.8%

49.9%

56.8%

Mishima and Hibiki [51]

30.9%

29.6%

27.1%

Friedel [52]

14.8%

15.6%

16.7%

Wang et al. [53]

18.4%

17.1%

17.0%

Sun and Mishima [54]

20.1%

19.7%

24.8%

Kim and Mudawar [55]

21.4%

14.3%

13.9%

Macdonald & Garimella [46]

19.2%

28.9%

41.8%

39

ASSESSMENT OF THE LOW-GWP REFRIGERANTS R600a, R1234ze(Z) and R1233zd(E) FOR HEAT PUMP AND ORGANIC RANKINE CYCLE APPLICATIONS

Highlights



Assessment of refrigerants R600a, R1234ze(Z) and R1233zd(E) for HP and ORC



R1233zd(E) shows higher efficiency than that of R245fa both in HP and ORC



R12343ze(Z) shows similar efficiency than that of R245fa both in HP and ORC



R600a shows lower efficiency than that of R245fa both in HP and ORC



R600a, R1234ze(Z) and R1233zd(E) exhibit very similar condensation performances

40