Cascaded dual-loop organic Rankine cycle with alkanes and low global warming potential refrigerants as working fluids [original]

Wameedh Khider Abbas Abbas, Jadran Vrabec

Cascaded dual-loop organic Rankine cycle with

alkanes and low global warming potential

refrigerants as working fluids

Open Access via institutional repository of Technische Universität Berlin

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publication; also known as: Author’s Accepted Manuscript (AAM), Final Draft, Postprint)

This version is available at

https://doi.org/10.14279/depositonce-15077

Citation details

Abbas, W. K. A. Vrabec, J. (2021). Cascaded dual-loop organic Rankine cycle with alkanes and low global

warming potential refrigerants as working fluids. Energy Conversion and Management, 249, 114843.

https://doi.org/10.1016/j.enconman.2021.114843.

This work is protected by copyright and/or related rights. You are free to use this work in any way permitted by

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Cascaded dual-loop organic Rankine cycle with alkanes and low global warming

potential refrigerants as working fluids

Wameedh Khider Abbas AbbasA, Jadran VrabecB

A Thermodynamics and Energy Technology, University of Paderborn, Warburger Straße 100,

33098 Paderborn, Germany

B Thermodynamics and Process Engineering, Technical University of Berlin, Ernst-Reuter-

Platz 1, 10587 Berlin, Germany

__________________________________________________________________________________________

Abstract

A cascaded dual-loop organic Rankine cycle (CD-ORC) consisting of a high-temperature ORC

(HT-ORC) and a low-temperature ORC (LT-ORC) with a shared heat exchanger is studied.

Thermal efficiency is investigated as a key indicator for system performance over a wide range

of heat source temperature ranging from 170 °C to 330 °C. The relation between the thermal

efficiency and the critical temperature of the working fluids is explored. For this purpose, six

alkanes and 31 refrigerants with a low GWP (≤150) are considered as working fluids in the HT-

ORC and the LT-ORC, respectively. Cyclohexane and cyclopentane are found to be suitable

for the HT-ORC with a maximum thermal efficiency of 19.13% and 18.03%. The thermal

efficiency of both loops is highly affected by the working fluids since it increases with the

critical temperature. As a whole, the CD-ORC may achieve a total thermal efficiency of

25.24%, 24.88% and 24.60% when using the working fluid combinations cyclohexane-

R1366mzz(Z), cyclohexane-R1233zd(E) or cyclohexane-butane. This indicates that low GWP

refrigerants are well-suited for LT-ORC. Compared to regular ORC, CD-ORC systems may

achieve a thermal efficiency that is better by around a quarter.

Keywords: Cascaded dual-loop ORC, low GWP refrigerants, thermal efficiency, critical

temperature of working fluids

___________________________________________________________________________

Corresponding author: Jadran Vrabec, tel.: +493031422755, E-mail: vrabec@tu-berlin.de

1. Introduction

The rise in population as well as urban and technology development was accompanied by a

significantly increased demand for energy, which is met by fossil fuels as the primary energy

source due to their availability and low cost. However, studies have analyzed the risks of using

fossil fuels and their derivatives which entail extensive pollution and threaten environmental

disasters in the future [1]. Environmental pollution and increased carbon dioxide emissions

were accompanied by climate change and unprecedented weather events. The IPCC reported

on August 9, 2021, that the world will reach 1.5 °C of warming within the next twenty years.

There is thus a growing need to find alternative fuels, clean power sources with less harmful

emissions and impact on the environment [2,3].

A large body of literature has grown around the organic Rankine cycle (ORC) as a technology

to generate power from low and medium temperature heat sources. The ORC is a sustainable

technology for power generation because it can utilize heat from geothermal [4], solar [5],

combined heat and power [6], biomass [7], waste heat from industrial processes [8, 9] or other

sources [10].

Many studies have investigated different ORC architectures to enhance system performance. It

was shown that multi-loop ORC theoretically can achieve a better thermal efficiency and net

power output than regular ORC systems. Moreover, they have the capacity to concurrently

recover heat from different sources [11-18]. Multi-loop ORC consist of several ORC loops,

where each loop operates under different conditions (temperature, pressure and mass flow rate).

Generally, dual-loop ORC consist of a high-temperature loop (HT-ORC) and a low-temperature

loop (LT-ORC). The HT-ORC (topping ORC) is driven by the main heat source, while the LT-

ORC loop (bottoming ORC) employs residual heat from the HT-ORC and/or is driven by a

secondary heat source. A dual-loop ORC may allow for a better temperature profile matching

between the heat source and the working fluid in the power generating cycle, which improves

thermal efficiency. The disadvantage of multi-loop systems is their higher cost since they

require more components than regular ORC. Such systems are also characterized by more

complexity in terms of devices, sensors and operational parameters that need to be orchestrated

[19].

Ayachi et al. [20] made an exergetic optimization to examine regular and dual-loop ORC

performance for heat source temperatures between 150 °C and 165 °C. The authors considered

three low GWP refrigerants, i.e. R1234yf, butane and isobutene, and reported that the critical

temperature of the working fluid is closely related to exergy efficiency. Kosmadakis et al. [21]

tested 33 refrigerants to select a suitable working fluid for the HT cycle of a dual-loop ORC,

where the operating temperature is between 130 °C and 140 °C. They discussed the

performance of selected refrigerants together with their environmental issues and found that

low GWP refrigerants may allow for good power output and thermal efficiency.

Preissinger et al. [22] studied a dual-loop ORC system driven by a wood pellet heater and

considered 35 working fluids to examine system performance. The authors presented a

relationship between the system performance and the thermophysical properties (critical

temperature, critical pressure and molar mass) of the working fluids. They reported that the

overall thermal efficiency is affected more by the LT-ORC working fluid than by the HT-ORC

working fluid.

A study of a dual-loop ORC for waste heat recovery from a catalytic membrane reactor with

hydrocarbons and low GWP refrigerants as working fluids has been presented by Fouad [23].

The author showed that the highest thermal efficiency was achieved with heptane as a working

fluid in the HT-ORC. He found that R1234ze(Z) is a suitable alternative for R245fa and

reported a maximum overall thermal efficiency of 13.39%.

Shu et al. [24] proposed a dual-loop ORC and analyzed the impact of the working fluid and

other parameters on system performance. The proposed system consists of a HT-ORC to

recover waste heat from exhaust gas and a LT-ORC to recover the residual heat from the HT-

ORC and engine coolant. The authors selected six low GWP refrigerants for the LT-ORC and

reported that the highest net power output was achieved with R1234yf. On the other hand, the

maximum thermal efficiency was obtained with butane.

Wang et al. [25] investigated sub- and supercritical dual-loop ORC to recover waste heat from

internal combustion engines. They utilized four pairs of working fluids, including two low

GWP refrigerants, i.e. R1234yf and R1233zd(E). The authors showed that these are the most

appropriate for dual-loop ORC among the considered working fluids.

Emadi et al. [26] studied working fluid selection for a dual-loop ORC, where the HT-ORC

recovers heat from a solid-oxide fuel cell, while the LT-ORC was driven by LNG cryogenic

energy. They considered 17 low GWP refrigerants and hydrocarbons to investigate performance

in terms of exergy efficiency and turbine power output. It was found that maximum exergy

efficiency and power output are achieved with a combination of hydrocarbons.

Xia et al. [27] made a theoretical investigation on working fluids for dual-loop ORC by

employing their normal boiling point as a selection criterion. The authors studied 27 candidate

pairs to choose combinations for HT-ORC and LT-ORC to maximize performance. They found

that the optimal combination is cyclohexane-butane for the HT-ORC and LT-ORC,

respectively.

Xue et al. [28] analyzed a dual-loop ORC layout to recover waste heat from LNG cryogenic

energy and a gas-steam combined cycle power plant by taking two refrigerants as working

fluids in the HT-ORC and LT-ORC. They studied the influence of operational conditions, such

as mass flow rate and turbine inlet pressure, on performance. It was found that the maximum

thermal efficiency was 25.64%.

The exergetic optimization of dual-loop ORC for waste heat recovery was studied by

Braimaikis et al. [19]. They employed seven low GWP working fluids (three refrigerants and

four hydrocarbons) to investigate the exergy efficiency for a heat source temperature between

100 °C and 300 °C. The authors compared dual-loop ORC with regular ORC and reported that

the exergy efficiency of dual-loop ORC improves by up to a quarter when using cyclopentane

and butane in the HT-ORC and LT-ORC, respectively. They noted that the dual-loop ORC is

appropriate when the difference between the critical temperature of the working fluids and the

heat source temperature is small.

This literature overview outlines that the dual-loop ORC is a promising technology for power

generation and efficient utilization of heat sources. It also demonstrates that low GWP

refrigerants are advantageous working fluids for dual-loop ORC because of environmental and

safety aspects. To improve dual-loop ORC technology, more studies are needed for further

development, which was the goal of this work. It was attempted to fill some of the gaps with a

simulative investigation of dual-loop ORC technology by looking at a wide range of heat source

temperature. Despite many studies on refrigerants, there is a lack of work that examines low

GWP (≤150) working fluids, especially zeotropic and azeotropic refrigerant mixtures. Several

studies have considered refrigerants in ORC systems, but most have focused on specific groups

only.

This paper considers six alkanes and 31 low GWP refrigerants as working fluids in the HT-

ORC and LT-ORC, respectively. The selection criteria were based solely on environmental

considerations, where the selected working fluids have a GWP value of up to 150 and zero or

negligible ODP. The first goal was to estimate performance in terms of thermal efficiency over

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