R1234yf and R1234ze as alternatives to R134a in Organic Rankine Cycles for low temperature heat sources

R1234yf and R1234ze as alternatives to R134a in Organic Rankine Cycles for low temperature heat sources

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Energy Procedia 142 Energy Procedia 00(2017) (2017)1192–1198 000–000 www.elsevier.com/locate/procedia

9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK

R1234yf and R1234ze as alternatives to R134a in Organic Rankine The 15th International Symposium on District Heating and Cooling Cycles for low temperature heat sources Assessing feasibility of using the heat demand-outdoor Francisco Molés*,the Joaquín Navarro-Esbrí, Bernardo Peris, Adrián Mota-Babiloni, temperature function for a long-term district heat demand forecast Carlos Mateu-Royo a,b,c a a b c c ISTENERI. Research Group. Department of Mechanical Engineering Construction,.,Campus de Riu Sec s/n, ,Universitat I, E12071, Andrić *, A. Pina , P. Ferrão , J.and Fournier B. Lacarrière O. Le Jaume Corre

Castellón (Spain) IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract a

This paper compares the predicted organic Rankine cycle performance of two low global warming potential working fluids, R1234yf and R1234ze, alternatives to R134a over a wide range of evaporating temperatures and condensing temperatures for a Abstract given thermal power input. The results show that R1234yf would require 18.3% to 25.8% higher pump power and would enable up to 13.9% lower net cycle efficiencies than R134a over the range of cycle conditions examined in this work. In the other hand, District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the R1234ze would require 15.7% to 20.2% lower pump power and would enable up to 13.8% higher net cycle efficiencies than R134a, greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat over the range of cycle conditions analyzed. Both alternative fluids net cycle efficiency is benefitted substantially by a recuperator. sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, The differences with R134a in net cycle efficiency are accentuated for high evaporating and condensing temperatures. prolonging the investment return period. © 2017 The Authors. Published by Elsevier Ltd. The main of this paper isby to Elsevier assess the feasibility of using the heat demand – outdoor temperature function for heat demand © 2017 Thescope Authors. Published Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. Peer-review under responsibility of thelocated scientific committee of the 9thwas International Applied Energy. forecast. The district of Alvalade, in Lisbon (Portugal), used as aConference case study.onThe district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords: low GWP fluids; R134a; ORC systems; low temperature heat sources. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Nomenclature (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation the errorwork value(kJ/kg) increased up to 59.5% (depending on the weather and renovation scenarios combination considered). wscenarios,specific The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 𝑚𝑚̇ mass flow rate (kg/s) decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and W electric power (kW) renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +34-964-728-134; fax: +34-964-728-106. Cooling. E-mail address: [email protected]

Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.380

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Greek symbols η

efficiency

Subscripts p x is em n

pump expander isentropic electromechanical net

Acronyms BORC CHP GWP HFC HFO ODP ORC RORC

Basic Organic Rankine Cycle Combined Heat and Power Global Warming Potential HydroFluoroCarbon HydroFluoroOlefin Ozone Depletion Potential Organic Rankine Cycle Regenerative Organic Rankine Cycle

1. Introduction Due to environmental constraints, the Organic Rankine Cycle (ORC) has been attracting increasing attention, due to its capacity of power generation from low temperature heat sources or small sizes, where important drawbacks limit the application of steam cycles. The selection of the ORC working fluid is a key parameter that has an important influence on the system efficiency. Numerous works on this subject can be found in the literature. Lai et al. [2] investigated potential single-component working fluids for high temperature ORC processes and found that siloxanes and selected hydrocarbons are promising. Shale et al. [3], Shengjun et al. [4] and Quoilin et al. [5] evaluated various working fluids for low to medium temperature applications, highlighting that hydrofluorocarbons with low critical temperatures, such as R134a and R245fa, are suitable. Moreover, Quoilin et al. [6] highlighted that R134a is a common working fluid in commercial ORC installations, mainly used in waste heat recovery from very low temperature sources. Additionally, they observed that most commercial ORC plants exhibit now 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, R134a is a hydrofluorocarbon (HFC) with zero Ozone Depletion Potential (ODP). However, the environmental impact of a working fluid, when it escapes to the atmosphere, is not limited to stratospheric ozone layer depletion. In fact, while all HFCs are harmless to the earth’s stratospheric ozone layer, some HFCs with large GWP (Global Warming Potential) could contribute significantly to climate change. HFCs were designated as greenhouse gases under the Kyoto Protocol in 1997 [7], and they are currently targeted by efforts to reduce greenhouse gas in most developed countries. As a result, alternatives are sought for high GWP HFCs, such as R134a, which has a GWP of 1430. Low GWP working fluids are being studied for their use in ORC systems. Liu et al. [8] and Luo et al. [9] present different studies on low GWP working fluids and their potential applications in ORC for power generation. They concluded that some of the presented hydrofluoroolefins (HFO) show promising performances regarding system efficiency, especially for low-to medium temperature geothermal ORC power generation. Usman et al. [10] compare the part-load operation of air cooled and cooling tower based low-medium temperature geothermal ORC systems installed at different geographical locations, using as working fluid R245fa and comparing with a newer competitor

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R1233zde for thermo-economic performance, environment-friendly and efficient system integration. The results reveal that R1233zde has potential to replace R245fa working fluid when the source temperature is higher (around 145 °C). Datla et al. [11] pointed the DR2 (R1336mzz), still no commercial, as a future low GWP alternative, although eliminating the direct possibility to transform air-conditioning compressors into ORC expanders, since it requires a larger equipment size compared to other working fluids such as R245fa. Molés et al. [12] studied the performance of ORC systems using R1233zd and R1336mzz as alternatives to R245fa for low temperature heat recovery, concluding that these fluids are predicted to have attractive performance, where the efficiency is benefitted substantially using a recuperator. Eyerer et al. [13] and Molés et al. [14] present different experimental analysis of the applicability of the new fluid R1233zd as a drop-in replacement for R245fa in existing systems, concluding that R1233zd-E can be used as a substitute for R245fa in existing ORC systems with higher thermal efficiencies. Navarro-Esbrí et al. [15] conduct an experimental evaluation of R1336mzz-Z as a low global warming potential working fluid for ORC systems in micro-scale low temperature applications, obtaining better efficiencies than those obtained with R245fa. Some low GWP working fluids are being studied to replace R134a in various applications, including ORC systems. One of them is R1234yf [16], an HFO with a GWP of 4. Another candidate to replace R134a in ORC systems is R1234ze, an HFO with a GWP of 6 and zero ODP [17]. Yamada et al. [18] conducted their study in the low GWP fluid R1234yf, also suggesting improvements in the basic cycle to enhance the benefit obtained and they concluded about similar results compared to the R134a, but not always the best among other fluids such as isopentane, R245fa or ethanol. Invernizzi et al. [19] investigate the potential replacement of HFC-134a in ORC applications by two lowGWP refrigerant fluids, namely R1234yf and R1234ze(E), concluding that considering the direct replacement of the original fluid by the two refrigerants or a completely new design simulation results show a decrease of the net power. Le et al. [20] present a system efficiency optimization scenarios of basic and regenerative supercritical ORCs using low-GWP organic compounds as working fluid with R134a as a reference. In this study the best working fluids for system efficiency optimization of basic and regenerative cycles are R32 and R152a, respectively, while the best working fluid for net electrical power optimization of the basic cycle is R1234ze. As can be seen from the literature, most studies address working fluid selection for ORC systems. Besides there is a small number of studies including low GWP working fluids, there is a lack of studies comparing common working fluids used in commercial ORC systems, like R134a, with low GWP alternatives. Therefore, in the present work, an evaluation of the low GWP fluids R1234yf and R1234ze as alternatives to R134a in ORC systems for low temperature heat sources is carried out. The rest of the paper is organized as follows: Section 2 describes the ORC systems considered; Section 3 reports and discusses the main results; finally, Section 4 summarizes the main conclusions. 2. ORC system description With the aim of studying the feasibility of using low GWP fluids as alternatives to R134a in ORC systems for low temperature heat recovery, a thermodynamic analysis has been carried out. Two cycle configurations have been considered: the basic cycle and a regenerative cycle. The Basic Organic Rankine Cycle (BORC) is the simplest configuration. It works in subcritical conditions and requires a minimum number of equipment components. The working fluid is pumped through the evaporator to take the available heat from the thermal source. The highest enthalpy of the circuit is reduced in the expander to produce mechanical power, which is usually transformed into electricity through a generator. The fluid is condensed, subcooled and pumped again to close the loop. The Regenerative Organic Rankine Cycle (RORC) is similar to the BORC, except in that it includes an internal heat exchanger as a regenerator. This configuration uses the superheat in the vapor exiting the expander to preheat the pressurized liquid entering the evaporator, reducing at the same time the thermal load on the condenser. The basic equations used to model each configuration can be found in Reference [12]. The thermodynamic properties of R134a, R1234yf and R1234ze were obtained from the Refprop software [21]. The basic operating parameters, based on previous experimental works [14,15] that determine cycle performance were specified as indicated in Table 1. When varying a parameter, the rest of them were maintained constant at the values shown in parentheses. The subcooling, the expander and pump efficiencies and the regenerator effectiveness were kept constant.

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Table 1. Operating parameters. Parameters

Numeric values

Condensing temperature

280 K – 310 K (300 K)

Evaporating temperature

330 – 360 K (350 K)

Superheat

5K

Subcooling

5K

Regenerator effectiveness

80%

Efficiencies (ηis,x, ηis,p)

85%

Efficiencies (ηem,x)

65%

Efficiencies (ηem,p)

35%

3. Results and discussion The expander power output, the required pump power input and the net cycle efficiencies for the three fluids were compared for a given heat rate supplied to the evaporator. The ORC performance using the three working fluids is compared at a wide range of evaporating temperatures, condensing temperatures and superheat values. In a first analysis, the evaporating temperature was varied according to Table 1, maintaining the condensing temperature at 300 K (27 °C). A second analysis was carried out varying the condensing temperature according to Table 1 while maintaining the evaporating temperature at 350 K (77 °C). To compare each alternative fluid performance with R134a, the results are shown regarding relative difference taking R134a as a reference, calculated as shown in Eq. 1. %𝑋𝑋 =

𝑋𝑋𝑅𝑅1234𝑦𝑦𝑦𝑦 𝑜𝑜𝑜𝑜 𝑅𝑅1234𝑧𝑧𝑧𝑧−𝑋𝑋𝑅𝑅134𝑎𝑎 𝑋𝑋𝑅𝑅134𝑎𝑎

· 100

(1)

Fig. 1 shows the relative differences, taking R134a as a reference, for expander power output varying the evaporating temperature and condensing temperature. Focusing on the BORC configuration, the relative differences are between -1.8% and -2.6% for R1234yf and between -1.0% and -1.5% for R1234ze. In this configuration, for both fluids, the expander power output is lower than that obtained using R134a for all the operating conditions considered. Attending to the RORC configuration, the relative differences are between 0.9% and 1.8% for R1234yf and between 0.5% and 3.1% for R1234ze. In this configuration, for both fluids, the expander power output is higher than that obtained using R134a for all the operating conditions considered.

(a)

(b)

Fig. 1. Relative differences, taking R134a as a reference, for expander power output varying: (a) evaporating temperature, (b) condensing temperature.

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Fig. 2 shows the relative differences, taking R134a as a reference, for pump power input varying the evaporating temperature and condensing temperature. For R1234yf the relative differences are between 18.3% and 25.8%, while for R1234ze are between -15.7% and -20.2%. For both cycle configurations, the R1234yf system consumes higher pump power, and R1234ze consumes lower pump power than R134a throughout the range of operating conditions examined. This reduction in pump power consumption for R1234ze can be explained attending to the lower working pressures relative to R134a.

(a)

(b)

Fig. 2. Relative differences, taking R134a as a reference, for pump power input varying: (a) evaporating temperature, (b) condensing temperature.

Fig. 3 shows the relative differences, taking R134a as a reference, for net cycle efficiency varying the evaporating temperature and condensing temperature. The relative differences in the net cycle efficiency for R1234yf are between -4.4% and -13.9%, and for R1234ze are between 3.2% and 13.8%. For R1234yf for both configurations, the net cycle efficiency is lower than that obtained using R134a under the range of operating conditions considered. The reduction in the net cycle efficiency for this working fluid is accentuated for high values of evaporating temperature and condensing temperature. For R1234ze for both configurations, the net cycle efficiency is higher than that obtained using R134a for all the operating conditions considered. The increase in the net cycle efficiency using this working fluid is accentuated for high values of evaporating temperature and condensing temperature.

(a)

(b)

Fig. 3. Relative differences, taking R134a as reference, for net cycle efficiency varying the: (a) evaporating temperature, (b) condensing temperature.

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4. Conclusions In this paper, a theoretical evaluation of the low GWP fluids R1234yf and R1234ze as alternatives to R134a in ORC systems for low temperature heat sources was carried out. A major difference between these fluids attending to ORC performance is the required pump power input. Throughout the range of operating conditions and configurations examined in this paper, R1234yf would consume higher pump power than R134a by 18.3% to 25.8%, while for R1234ze the pump power would be by 15.7% to 20.2% lower than R134a. It was concluded that ORC systems working with R1234ze could achieve higher values of net cycle efficiency than those working with R134a by up to about 13.8%, while ORC systems that are working with R1234yf result in lower net cycle efficiencies than R134a by up to about 13.9%. The differences to R134a in net cycle efficiency are accentuated for high evaporating and condensing temperatures. Acknowledgements The authors acknowledge the Spanish Government for the financial support under project ENE2015-70610-R. Dr. Adrián Mota-Babiloni would like to acknowledge the funding received from the Plan for the promotion of research of the University Jaume I for the year 2016 [Grant number POSDOC/2016/23]. References [1] P. Mago, A. Hueffed, L. Chamra, Analysis and optimization of the use of CHP-ORC systems for small commercial buildings, Energy and Buildings 42 (2010) 1491 – 8. [2] N.A. Lai, M. Wendland, J. Fischer, Working fluids for high-temperature organic Rankine cycles, Energy 36 (2011) 199 – 211. [3] B. Saleh, G. Koglbauer, M. Wendland, J. Fischer, Working fluids for low-temperature organic Rankine cycles, Energy 32 (2007) 1210 – 1221. [4] Z. Shengjun, W. Huaixin, G. 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[20] V.L. Le, M. Feidt, A. Kheiri, S. Pelloux-Prayer, Performance optimization of low-temperature power generation by supercritical ORCs (organic Rankine cycles) using low GWP (global warming potential) working fluids, Energy 67 (2014) 513-526. [21] E.W. Lemmon, M.L. Huber, M.O. McLinden, REFPROP, NIST Standard Reference Database 23, v.8, National Institute of Standards, Gaithersburg, MD, USA, 2007.