Experimental study of an Organic Rankine Cycle with HFO-1336mzz-Z as a low global warming potential working fluid for micro-scale low temperature applications

An experimental evaluation of HFO-1336mzz-Z as a low global warming potential working fluid for ORC systems in micro-scale low temperature applications has been conducted. The energy performance in a fully monitored ORC module has been analyzed varying the heat source temperatures between 140ºC and 160ºC and heat sink temperatures between 25ºC and 40ºC. The ORC module uses a regenerative configuration allowing heat recovery not only from the heat source but also from the expanded vapor, thus improving the cycle thermal and electrical efficiency. The maximum gross electrical power generated was 1,100 W, while the net electrical efficiency ranged from 5.5% to 8.3%. The volumetric expander performance was analyzed by means of the filling factor, while deviations of expander operation from ideal performance were evaluated by means of the isentropic and overall expander-generator efficiency. Net electrical efficiency, isentropic expander efficiency and volumetric expander performance obtained with HFO-1336mzz-Z in this work are higher than those obtained with HFC-245fa in a previous work using the same experimental facility.


Introduction
Due to environmental constraints, the Organic Rankine Cycle (ORC) has been attracting increasing attention over the past decades [1].Unlike the conventional steam Rankine cycle, an ORC uses an organic working fluid to enable cost-effective use of low temperature heat sources for both power and Combined Heat and Power (CHP) applications [2].It also enables cost-effective micro-scale applications [3], i.e. applications generating less than 15 kWe of electrical power [4].Besides other advantages [5,6], the ORC technology is considered mature relative to other techniques for the direct conversion of heat to power [7].Several ORC systems have been installed for recovering waste heat from industrial processes (e.g. in the cement [8], ceramic [9] or oil industry [10]) or from power systems (e.g.internal combustion engines [11]).ORC systems have also been used for converting renewable solar [12], biomass [13] and geothermal [14] energy into power.
This wide range of potential applications has motivated research to provide suitable ORC solutions.Expander technology, working fluid choice and thermodynamic cycle configuration have a major influence on ORC system performance and economics.Volumetric or positive displacement expanders have been extensively investigated for micro-scale, low temperature ORC applications [15].They are more effective for smallscale units, characterized by lower flow rates, higher pressure ratios, and significantly lower rotational speeds than turbomachines.Moreover, they tolerate some liquid content, e.g.drops carried from the evaporator or forming through condensation during expansion [16].Working fluid selection has been also widely investigated; it has been found to be highly dependent on the target application [17] and it must balance various trade-offs, as no universally optimal fluid has been identified.Quoilin et al. [16] have found that HFC-245fa is a common working fluid in commercial ORC installations, mainly used in waste heat recovery from low temperature heat sources.They further observed that, at the present time, most commercial ORC plants follow a simple architecture: sub-critical working conditions, single-component working fluids, single evaporation pressure, and possible use of a recuperator heat exchanger.
Attending to environmental issues, HFC-245fa has a GWP of 858 [18].Low-GWP working fluids have been recently proposed as potential replacements for HFC-245fa in various applications, including ORC systems [19].HFO-1336mzz-Z (cis-CF 3 CH=CHCF 3 ; 1,1,1,4,4,4-hexafluoro-2-butene; CAS No. 692-49-9; previously referred to as DR-2) is a hydrofluoroolefin (HFO) with a GWP of 2 [18] and zero ODP [20,21].Molés et al. [22] computed attractive performance of ORC systems using HFO-1336mzz-Z as the working fluid for low temperature heat recovery.They also found that system efficiency was benefitted substantially by the use of a recuperator.Table 1 shows the main thermophysical properties of HFC-245fa and HFO-1336mzz-Z.HFO-1336mzz-Z has recently received a safety classification of A1 (lower toxicity, no flammability) according to ASHRAE Standard 34, with an Occupational Exposure Limit (OEL) of 500 ppm.Fig. 1 shows the temperature-entropy diagrams and vapor pressure curves for both fluids.HFO-1336mzz-Z generates substantially lower pressures than HFC-245fa, especially at high temperatures.It would, therefore, allow higher evaporating temperatures than HFC-245fa, when the maximum permissible evaporating temperature is limited by the maximum permissible pressure of the available equipment.Extensive HFO-1336mzz-Z pressure-temperature-density measurements have recently been reported by Tanaka et al. [23].They were used to develop an equation of state explicit in the Helmholtz free energy for HFO-1336mzz-Z by Akasaka and Lemmon [24].
Table 1.Thermophysical properties of HFC-245fa and HFO-1336mzz-Z.To the authors' knowledge, there is a lack of experimental data for ORC systems using non-flammable, low GWP working fluids.Datla and Brasz [25] compared the performance of HCFO-1233zd-E and HFC-245fa in a 75 kW standard ORC system with a radial inflow turbine at a single operating point.They demonstrated 8.7% higher net system efficiency with HCFO-1233zd-E.This work reports an experimental study of an ORC system with HFO-1336mzz-Z as a low GWP working fluid for micro-scale, low temperature applications.It covers a wide range of operating conditions (e.g.heat source and heat sink temperatures) not previously reported.The rest of the paper is organized as follows: Section 2 describes the experimental setup and methodology; Section 3 reports and discusses the main results; finally, Section 4 summarizes the main conclusions.

ORC system
A monitored commercial ORC module [26] was used as a test bench for the experiments carried out in this work.It uses a regenerative configuration, shown in Fig. 2, that allows not only recovering the thermal energy from the heat source but also the waste heat from the expander outlet, thus improving the system electrical efficiency.Other features of the commercial ORC module are listed in Table 2. Table 2. Commercial ORC module features.

Heating and cooling loops
The test bench is completed with two secondary circuits, a heat sink cooling water loop and a heat source thermal oil loop.The heat sink cooling water loop consists of a closed-type cooling system, which allows controlling the cooling water temperature.
The heat source thermal oil loop includes an electric boiler, which allows controlling the thermal oil temperature.

Measurements and calculations
The temperature and pressure at the inlet and outlet of each basic equipment component of the test facility are measured using K-type thermocouples and piezoelectric pressure gauges, respectively.The thermodynamic states of the working fluid are calculated using REFPROP [29] and HFO-1336mzz-Z property file provided by Chemours.The working fluid mass flow rate is measured by means of a Coriolis effect mass flowmeter, the heat sink cooling water loop volumetric flow rate is measured with an electromagnetic flowmeter and the heat source thermal oil loop volumetric flow rate is measured with a Vortex flowmeter.The electrical power generated by the expander and the electrical power consumed by the pump are obtained with two digital wattmeters.
All measurements are gathered with a data acquisition system and monitored through a personal computer.A summary of the measured parameters and the sensors used in this work is presented in Table 3, indicating the uncertainty associated with each measurement.
Table 3. Measured parameters, measuring instruments and measurement uncertainty.
For the analysis of the experimental data obtained during steady-state tests, various equations have been used.Input heat rate is defined as the rate of heat transfer to the working fluid at the evaporator and it is obtained as the product of the working fluid mass flow rate and the enthalpy difference between the evaporator inlet and outlet, as shown in Eq. 1.Similarly, output heat rate is defined as the rate of heat rejected by the working fluid at the condenser and it is obtained through Eq. 2.
  The rate of heat recovered at the regenerator is calculated through Eq. 3. The regenerator effectiveness is obtained through Eq. 4, where maximum heat rate is the heat rate leading to a pinch point equal to zero.
, ,, The net electrical power output is calculated from the measured electrical power generated by the expander and the measured electrical power consumed by the pump using Eq. 5.The net electrical efficiency of the system is obtained through Eq. 6. , Expander performance is quantified in terms of various ratios.The expander isentropic efficiency is obtained through Eq. 7, as the ratio of the enthalpy difference in the real expansion process to the isentropic enthalpy difference.The ratio of the measured electrical power generated by the expander to the maximum electrical power that could be ideally obtained in an isentropic expansion process is defined as the overall expander-generator efficiency through Eq. 8.The volumetric performance of the expander is represented by Eq. 9, as the ratio of the calculated volumetric flow rate at the expander inlet to the theoretical volumetric flow rate displaced by the expander, named the filling factor [30].Another parameter used for expander performance analysis is the specific volume ratio in the expander, calculated through Eq. 10.
,, , , , , ,, 3600 , , The uncertainties propagated to calculated parameters from uncertainties in measured parameters are estimated using the RSS method [31] and shown in Table 4.
Table 4. Uncertainties for calculated parameters.

Methodology of experiments
HFO-1336mzz-Z performance in the commercial ORC module was tested over an operating range expected using low grade temperature heat sources.The degree of vapor superheat at the expander inlet was maintained approximately constant, at values near 10ºC.The cooling water and thermal oil volumetric flow rates were kept approximately constant, at values shown in Table 5.The cooling water inlet temperature was set at three values, nominally 25, 32 and 40 ºC.The thermal oil inlet temperature was varied from about 140ºC to more than 155ºC.The experimental data consist of 100 steady-state tests, which are represented in Fig. 3. Table 5. Range of operating conditions in the experimental steady-state tests.
The system state is considered steady when fluctuations of all measured parameters, sampled every 1 s, remain below 1% for at least 15 min (900 measurements of each parameter).Each test is obtained, once the steady state is reached, averaging over 10 minutes all the measured parameters.

Data validation
In order to check the accuracy of the measurements, data validation is conducted comparing the input and output heat rates measured from the working fluid side and the secondary fluids side.In this way, the rate of heat transferred to the working fluid at the evaporator is compared to the rate of heat transferred from the thermal oil.Similarly, the heat rate rejected by the working fluid at the condenser is compared to the heat rate removed by the cooling water.Both energy balances are presented in Fig. 4. The heat rate supplied by the thermal oil at the evaporator is obtained through Eq. 11 using the thermal oil volumetric flow rate and its evaporator inlet and outlet temperatures.Similarly, the rate of heat removed by the cooling water at the condenser is obtained through Eq. 12.
 

Results and discussion
This section presents the results of the analysis conducted from the experimental data obtained during the tests.This analysis has been focused on the overall behavior of the ORC module and expander performance, and finally, comparing the performance obtained with HFO-1336mzzZ with that obtained with HFC-245fa in the same ORC module.
The mass flow rate of the ORC working fluid is shown in Fig. 5a.It increases with the thermal oil inlet temperature, while it is not significantly affected by the cooling water inlet temperature.The mass flow rate of the working fluid is related to its density at the expander inlet, due to the constant superheating at the volumetric expander inlet.The vapor density at the expander inlet is presented in Fig. 5b.Evaporating and condensing temperatures are shown in Fig. 5c and Fig. 5d, respectively.As expected, evaporating temperatures are increasing with the thermal oil inlet temperatures, while condensing temperatures depend on the cooling water inlet temperatures.Input heat rate, presented in Fig. 6a, ranges from 7,000 W to 11,000 W. It increases with the thermal oil inlet temperature and slightly decreases with the cooling water inlet temperature.Input heat rate is related to the mass flow rate of the working fluid and the evaporating and condensing temperatures.Output heat rate, shown in Fig. 6b, ranges from about 6,500 W to about 9,500 W. It shows trends with heat source and sink temperatures similar to those observed for the input heat rate.As mentioned before, the ORC uses a regenerative configuration that allows recovering heat not only from the heat source but also the expanded vapor.The rate of heat recovery from the vapor exiting the expander by the regenerator is presented in Fig. 6c; it ranges from 2,100 W to 3,500 W and follows trends similar to those observed for the input and output heat rates.The regenerator effectiveness is about 74%, as it can be seen in Fig. 6d.Moreover, as it is used a dry working fluid, condensation is occurring only in the condenser.Fig. 7a shows the electrical power generated by the expander-generator.The maximum electrical power generated is 1,100 W. As expected, electrical power generated increases for higher thermal oil inlet temperatures and lower cooling water inlet temperatures, due to the higher mass flow rate of the working fluid and the greater difference between evaporating and condensing temperatures.The electrical power consumed by the pump, presented in Fig. 7b, ranges from 145 W to 210 W; it increases with thermal oil inlet temperature and it is mainly related to the mass flow rate and working fluid pressure ratio.The net electrical power output of the ORC module, shown in Fig. 7c, reaches a maximum value of 900 W. The net electrical power output of the ORC module depends on the electrical power generated by the expander-generator and the electrical power consumed by the pump; it increases with the thermal oil inlet temperature and decreases with the cooling water inlet temperature.The net electrical efficiency, shown in Fig. 7d, ranges from 5.5% to 8.3%.As expected, it increases with the thermal oil inlet temperature and decreases with the cooling water inlet temperature.Various expander performance indicators are shown in Fig. 8.The volume ratio through the expander, presented in Fig. 8a, ranges between 6.5 and 10.5.The volume ratio is mainly influenced by the evaporating and condensing temperatures, and therefore it increases with the thermal oil inlet temperature and decreases with the cooling water inlet temperature.The filling factor, shown in Fig. 8b, quantifies expander volumetric performance; it takes values around 1.25.The isentropic efficiency is presented in Fig. 8c.It shows a maximum value of 87% and it decreases with the thermal oil inlet temperature due to the increase in the volume ratio through the expander and the related under-expansion energy losses.The overall expander-generator efficiency, shown in Fig. 8d, ranges from 48.5% to 57.5%.It shows different trends with thermal oil and cooling water temperatures from the isentropic efficiency because it is affected by the part-load alternator electrical efficiency and its maximum electrical efficiency, among other energy losses.Regarding the expander performance, a comparison has been made between the results obtained in this work and the results obtained in previous works.In a previous experimental analysis [26], an experimental evaluation of the working fluids HFC-245fa and HCFO-1233zd-E in ORC systems for low temperature heat sources has been conducted, using the same experimental facility and similar operating conditions as in this work.The filling factor for HFC-245fa and HCFO-1233zd-E presents values around 1.375, as shown in Fig. 9a.Comparing the filling factor of the expander for the different working fluids, the values obtained with HFO-1336mzz-Z in this work are lower than those obtained with HFC-245fa and HCFO-1233zd-E.The isentropic efficiency for HFC-245fa and for HCFO-1233zd-E has a maximum value of 70% and 75%, respectively, as shown in Fig. 9b.Those values are lower than those obtained with HFO-1336mzz-Z in this work.The overall expander-generator efficiency of the expander has similar values for all the working fluids, as shown in Fig. 9c.Regarding the system behavior, in a previous theoretical analysis [22] an evaluation of the low GWP fluid HFO-1336mzz-Z as an alternative to HFC-245fa in ORC systems for low temperature heat sources was carried out.The results obtained indicate that the ORC systems working with HFO-1336mzz-Z can achieve higher values of net electrical efficiency than those working with HFC-245fa, with this increase accentuated for high condensing temperatures.The experimental results for HFC-245fa obtained in the previous experimental analysis [26] show a net electrical efficiency ranging from 5% to 9.7%.Net electrical efficiency at the highest cooling water inlet temperature obtained in this work with HFO-1336mzz-Z is higher than the net electrical efficiency obtained with HFC-245fa, as expected from the previous theoretical analysis.

Conclusions
In this paper, a regenerative ORC experimental system with a volumetric expander and HFO-1336mzz-Z as a low GWP working fluid has been tested for micro-scale, low temperature applications.This work contributes to mitigate the lack of experimental data for ORC systems using non-flammable low GWP working fluids, presenting an experimental analysis of the behavior of one of this type of working fluids, HFO-1336mzz-Z, and comparing it with that observed for other working fluids in the same experimental facility in previous works.
An experimental campaign has been conducted, with the inlet heat source temperature varied from about 140ºC to more than 155ºC and performance data collected from 100 steady-state tests.The ORC module uses a regenerative configuration with regenerator effectiveness values obtained around 74%. System parameters such as heat rates, electrical powers, and expander performance indicators have been investigated.
Based on the experimental analysis the following conclusions can be drawn: -Maintaining a constant value of 10ºC at the expander inlet superheating degree, it has been observed that the working fluid mass flow rate and evaporating temperature increase with the thermal oil inlet temperature.The input heat rate, related to the mass flow rate and evaporating and condensing temperatures, increase with the thermal oil inlet temperature and slightly decrease with the cooling water inlet temperature.
-Expander performance has been characterized by means of various indicators.The volumetric performance of the expander is analyzed by means of the filling factor, with values around 1.25.The isentropic expander efficiency has a maximum value of 87%, while the overall expander-generator efficiency of the expander ranges from 48.5% to 57.5%.The overall expander-generator efficiency shows different trends from the isentropic efficiency, as it is affected by the alternator electrical efficiency operating at partial loads, among other energy losses.
-A comparison has been made between the results obtained in this work and the results obtained in previous works using HFC-245fa and HCFO-1233zd-E.
Regarding the expander performance, the isentropic efficiency obtained with HFO-1336mzz-Z in this work is higher than those obtained with HFC-245fa and HCFO-1233zd-E, while the filling factor values obtained with HFO-1336mzz-Z in this work are lower than those obtained with HFC-245fa and HCFO-1233zd-E.Similar values of the overall expander-generator efficiency are obtained for all the working fluids.
-Regarding the system behavior, the net electrical efficiency obtained in this work ranges from 5.5% to 8.3%.For the highest cooling water inlet temperatures, net electrical efficiency obtained with HFO-1336mzz-Z in this work is higher than the net electrical efficiency obtained with HFC-245fa in a previous work using the same experimental facility, as expected from the previous theoretical analysis.
acknowledge the University Jaume I of Spain for the financial support under the Ph.D. grant PREDOC/2013/28, project P1-1B2015-38.

Fig. 3 .
Fig. 3. Thermal oil and cooling water inlet temperatures specified during steady-state tests.

Fig. 4 .
Fig. 4. Data validation comparing working fluid and secondary fluids heat transfer rates at the evaporator and condenser.

Fig. 7 .
Fig. 7. (a) Electrical power generated by the expander, (b) electrical power consumed by the pump, (c) net electrical power output and (d) net electrical efficiency.

Fig. 3 .
Fig. 3. Thermal oil and cooling water inlet temperatures specified during steady-state tests.

Fig. 7 .
Fig. 7. (a) Electrical power generated by the expander, (b) electrical power consumed by the pump, (c) net electrical power output and (d) net electrical efficiency.

Table 2 .
Commercial ORC module features.

Table 3 .
Measured parameters, measuring instruments and measurement uncertainty.

Table 4 .
Uncertainties for calculated parameters.

Table 5 .
Range of operating conditions in the experimental steady-state tests.