New Journal and we have not received input yet 32 (2022) 101303

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Thermal Science and Engineering Progress

journal homepage: www.sciencedirect.com/journal/thermal-science-and-engineering-progress

Investigation of an organic Rankine cycle (ORC) incorporating a heat

recovery water-loop: Water consumption assessment
Julbin Paul Njock a, d, *, Max Ndame Ngangue b, Alain Christian Biboum c, Olivier Thierry Sosso d,
Robert Nzengwa a
a
Laboratory of Energy, Materials, Modelling and Methods, Higher National Polytechnic School of Douala, University of Douala, P.O. BOX 2701, Douala, Cameroon
b
Laboratory of Technology and Applied Sciences, University Institute of Technology of Douala, University of Douala, P.O. BOX 8698, Douala, Cameroon
c
National Advanced School of Engineering of Yaoundé, University of Yaoundé I, BP 8390 Yaoundé, Cameroon
d
Laboratory of Thermal and Environment, Advanced Teacher’s Training College for Technical Education of Douala, University of Douala, P.O. BOX 1872, Douala,
Cameroon

A R T I C L E I N F O A B S T R A C T

Keywords: This paper presents a thermodynamic analysis of an organic Rankine cycle with a water-loop (ORC-WL) as an
Thermodynamic analysis efficient means of heat regeneration and saving Water Consumption (WC). The ORC-WL was simulated with wet,
ORC with water-loop dry and isentropic refrigerants such as R717, R600a and R1234yf, respectively, in order to compare its technical
Water consumption
performance to the conventional ORC (C-ORC) and ORC with internal heat exchanger (ORC-IHE). The ther­
Energy efficiency
Exergy efficiency
modynamic analysis developed in the EES software was used to carry out the Energy Efficiency (EnE), Exergy
Net power output Efficiency (ExE), Net Power Output (NPO), and WC as comparison performance parameters. A specific study has
shown that the ORC-IHE, due to its remarkable contribution to the degradation of available energy and with a
high WC compared to that of C-ORC, can be disqualified in favour of the ORC-WL which considerably improves
these parameters thanks to the water-loop. For the same operating conditions and with reference to R600a, When
C-ORC exhibited an EnE of 8.70 %, an ExE of 67.01 %, NPO of 8.57 kW and WC of 0.71 kg.s− 1, those of ORC-IHE
were 10.08 %; ExE of 64.87 %; NPO of 9.11 kW and WC of 5.30 kg.s.-1, respectively, whereas those of ORC-WL
were 9.95 %, 84.85 %, 9.95 kW and 0.82 kg.s− 1, respectively. The study on monitoring of evaporating tem­
perature, superheating, and pinch showed that the ORC-WL can operate with evaporating temperature and
superheating above 90 ◦ C and 15 ◦ C, respectively. A pinch of no more than 3 ◦ C, on the other hand, is ideal.
Finally, wet refrigerants are not suitable for ORC-WL, and dry refrigerants offer better performance than isen­
tropic refrigerants,

electricity market, due to Covid-19 in Africa, emissions intensity is ex­

pected to decline with the increasing substitution of fossil fuels by
1. Introduction renewable energy and gas [4]. But the recovery and growth in the in­
dustrial and residential sectors could be the main rebound factors for
According to the United Nations, people worldwide, estimated at these emissions in the next years.
7.35 billion in 2015 is forecast to increase by more than 15 % by 2030 The seventh sustainable development goal aims to transform lives,
and 32 % by 2050 [1]. The International Energy Agency’s (IEA) recent the economy and the planet through sustainable energy while
2021 report projects growth by some 2 billion people to 2050 [2]. Africa responding the challenges of climate and environmental degradation
is the fastest growing continent which the annual growth rate is esti­ [5]. To effectively ensure a genuine energy transition from fossil fuels to
mated by 2.5 % per year [1]. The IEA indicates that from such high renewable energy, as is the goal of sustainable development, a step-by-
density, Africa’s energy consumption is forecast to double and increase step approach with constructive critical analysis of novel technologies
global energy demand by 60 % by 2040 [3]. According to an IEA report developed in the electricity power sector and whose primary energy
entitled “Global Energy & CO2 Status”, about 75 % world’s electricity source is renewable is required. Organic Rankine Cycle (ORC) is a well-
generated in 2017 was from fossil fuels, contributing to a likely increase known technology able to meet this challenge.
of 1.4 % in global CO2 emissions [3]. In its recent 2021 report on the

* Corresponding author.
E-mail address: julbinpaulnjock2@gmail.com (J. Paul Njock).

https://doi.org/10.1016/j.tsep.2022.101303
Received 4 August 2021; Received in revised form 3 January 2022; Accepted 12 April 2022
Available online 22 April 2022
2451-9049/© 2022 Elsevier Ltd. All rights reserved.
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

Nomenclature W Mechanical or electrical power (kW)

Acronyms Greek symbols

A, B In the representation of fluids η For efficiency
ASHRAE American Society of Heating, Refrigerating and Air- υ For specific volume (dm3.kg− 1)
Conditioning Engineering ψ For specific exergy (kJ.kg− 1)
CO2 Carbone dioxide Subscripts
C-ORC Conventional ORC a Air
EES Engineering Equation Solver am Ambient
EnE Energy efficiency al Alternator
ExE Exergy efficiency c Critical
GWP Global Warming Potential cond Condenser
IEA International Energy Agency dest Destroy
IHE Internal Heat Exchanger H For the temperature of the source
NPO Net power output IHE Internal heat exchanger
ODP Ozone Depletion Potential II In exergy efficiency calculation
ORC Organic Rankine Cycle in Inlet
ORC-IHE ORC with Internal Heat Exchanger is Isentropic
ORC-WL ORC with Water Loop k For the heat source
R1234yf 2,3,3,3-tetrafluoropropene net Net
R152a 1,1-difluoroethane 0 For environment temperature
R22 Chlorodifluoromethane out Outlet
R600a Isobutane pump Pump
R717 Ammonia TG Thermal generator
SO2 Sulfur dioxide th Thermal
WC Water Consumption turb Turbine
Symbols 1 to 4 Common Thermodynamic states of the refrigerant in the C-
a, b, c For the thermodynamic states of water in the water-loop ORC, ORC-IHE and ORC-WL
h Enthalpy (kJ.kg− 1) 23 Thermodynamic state of the refrigerant in the ORC-IHE
I Irreversibilities (kW) and ORC-WL
M Specific mass flow rate (kg.s− 1) 41 Thermodynamic states of the refrigerant in the ORC-IHE
P Pressure (bar or MPa) or Power 2 s, 4 s Isentropic state of the refrigerant at the outlet of pump and
Q Heat (kW) turbine, respectively
s Specific entropy (kJ.kg− 1.s− 1) 3′′ In the saturated steam state of the refrigerant
T Temperature (◦ C or K)

ORC technology has demonstrated its importance as a clean tech­ turbine outlet which can be achieved by using a wet condenser, a dry
nology for sustainable energy resources [6]. Originally designed for cooling tower, or a hybrid system [14]. However, the efficiency and
waste heat recovery as cogeneration systems in industrial thermal capacity of the power plants are observed reduced with the dry cooling
plants, this technology is nowadays oriented towards the exploitation of system due to the presence of the fans that require electricity and which
energy from geothermal sources, biomass energy, solar energy and the the velocity can be raised to increase the air mass flow rate in order to
waste heat recovery at low temperature [7,8]. Furthermore, these sys­ ensure the condensation when the ambient air temperature increases
tems are considered as reliable technologies, due to their similarity with [14]. Turchi et al. [15], investigated the effect of replacing a wet cooling
conventional water Rankine technologies [6,9] with the particularity of system with a dry cooling system on a concentrating solar power plant
using a working refrigerant with a lower boiling point than the water (CSP). According to the location of the CSP, the results indicated an
[8,10]. increase in electricity cost closely related to the presence of fans in the
The potential for solar energy in Africa is unlimited, with sunshine dry cooling system. Zhai and Rubin [16] investigated on the water re­
levels twice as high as in Germany [11]. This solar potential is fairly quirements of wet and dry cooling systems for pulverized coal power
distributed, with over 80 % of African land receiving more than 2 MWh. plants with and without carbon capture systems. The results showed that
m− 2 annually [12]. In Central Africa, the amount of sunshine is com­ the makeup water requirements in the wet cooling system are affected
parable to or even greater than in many other regions of the world, by the average ambient air temperature. In contrast, the performance
which have made solar energy a significant source of their energy con­ and cost of a dry cooling system are extremely sensitive to local air
sumption. The deployment of solar ORC plants in this part of Africa temperature, respectively. According to Çengel et al. [17], more the heat
could thus contribute to solving the issue of insufficient electricity capacity of the fluid “A” is greater than the fluid “B”, more its capacity to
supply. However, there are various challenges related to this technol­ give up or store energy is better compared to the fluid “B”. This analysis
ogy’s implementation in Central Africa, including particularly the na­ shows that the wet cooling is more preferred over the dry cooling due to
ture of the working refrigerants; most of them are not eco-friendly. In the inferior thermal cooling properties of the air over the water. How­
addition, the climatic conditions are characterized by high average ever, the use of wet condensers is specifically challenged by the risk of
ambient temperatures with minimal variation [13], restricting the disruption of local water resources [18], increased water treatment costs
working fluid’s condensation stage. [19,20] and environmental restrictions [19].
Indeed, deploying solar ORC plants in Central Africa means locating Many investigations are proposing solutions for water saving in
them in sunny regions, while ensuring the condensation of steams at the power plants, whether solar thermal, coal-fired or others. Indeed, water

2
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

saving is crucial because as water is treated, water treatment plant Table 1

projects are also crucial in view of the infrastructure to be put in place Relevant highlights of each reference related to the water consumption issue.
and which the cost is important [21]. Houzhang et al. [22] presented a Authors Title Relevant highlights Ref.
new architecture for water saving in coal-fired power plants. The system
Marugan-Cruz Towards zero water The use of a dry Heller [11]
implements two fluorine plastic heat exchangers located downstream et al. consumption in solar tower cooling system in the
and upstream of a flue gas desulfurization system and recovers heat and power plants concentrated solar power
flue gas condensate from the flue gas. The results showed that this sys­ is able to reduce almost 1
tem can significantly reduce water consumption of power plants. A million cubic meters of
water per year.
similar study was done by Chen et al. [23], and led to similar results. Turchi et al. Water Use in Parabolic Water consumption for [12]
Marugan-Cruz et al. [14], studied an indirect dry cooling system inte­ Trough Power Plants: electric power generation
grating a Heller system with a direct contact water jet condenser. The Summary Results from is undergoing increasing
results showed a water saving of 1.4 × 106 m3 per year with an increase WorleyParsons’ Analyses, scrutiny, with more
NREL Technical report. emphasis being placed on
in energy production by almost 6 % compared to a mechanically draft
NREL/TP-5500–49468 low-water-use
dry cooling system and a declination of the annual energy production of technologies. In all cases,
less than 3.7 % compared to a wet cooling system. Domenichini et al. the transition to dry
[24], studied five different alternatives bituminous-coal-fired power cooling will reduce water
plant configurations without and with capture of the CO2 located in an consumption by over
90%.
area where water supply could be severely limited. The results show that Zhai and Performance and cost of wet Thermoelectric power [13]
water consumption can be limited but this limitation is not without Rubin and dry cooling systems for plants require significant
negative consequences on net electrical efficiency and specific CO2 pulverized coal power quantities of water.
emissions, which decrease and increase respectively. Zhuo et al. [25], plants with and without Increasing the plant
carbon capture and storage efficiency can decrease
analyzed the water consumption of five thermal plants in Beijing. The
cooling system size and
results show that, the power generation approaches and regions affect cost as well as
the water consumption significantly which is very different by direct consumptive water use.
cooling, water cooling and air cooling in thermal plants. Five water Otieno and An analysis of key Concentrated Solar Power [15]
saving countermeasures were proposed based on the results of the Loosen environmental and social projects have impacts on
risks in the development of local environment and
analysis, highlighting the technology and monitoring of water con­
concentrated solar power social conditions. The risk
sumption on the one hand and electricity generation and demand on the projects of disruption of local
other. The relevant highlights of each reference are presented in Table 1. water resources was found
To the best of author’s knowledge, although studies carried out so far to represent the highest
risk before and after
nowadays are a positive step forward in improving water consumption,
mitigation with a score of
they are not typically applied to ORC plants and none of them so far moderate-high and
presents the use of a water-loop as an effective means of limiting water moderate respectively.
consumption in ORC plants. The main focus of this study, contrary to Davis et al. Comparison of Alternate Cooling system power [16]
existing works, is to evaluate the effect of a water-loop on the technical Cooling Technologies for requirements for dry
California Power Plants: systems are four to six
performance of an ORC. Among other things, it will highlight the
Economic, Environmental, times those for wet
negative character of the internal heat exchanger (IHE), commonly used and Other Tradeoffs systems. A common
to improve the performance of the conventional ORC, in the degradation metric for water use in
of the energy quality and in excessive water consumption. electric power production
used, for example, in the
The remaining work was divided in four sections. A description of the
Environmental
suggested configuration will be made in section 2. A mathematical Performance Report of
modelling of each ORC configuration will be made in section 3 in which California’s Electric
the mass, energy and exergy balance equations will be presented, and Generation Facilities
the choice of working fluids will be justified. The operational conditions (CEC, 2001a) is gallons of
water per megawatt-hour
and the methodology used to produce the different results will be done
(gal/MWh) of energy
in section 4. Finally, the most relevant results of the study are summa­ generated. The amount of
rized and the main conclusions are drawn in section 5 before concluding water used depends not
in section 6. only on the type of cooling
system used but also on
the type of plant.
2. Description of the ORC configuration with water-loop Zhou et al. A cost model approach for The operating pressure, [17]
RO water treatment of water flow, water
A C-ORC as illustrated in Fig. 1.a, consists specifically of four com­ power plant concentration, inlet water
ponents, a thermal generator for steam generation, a turbine coupled to temperature has the
strong linear correlation
an alternator for electrical power, a condenser for steam condensation with water flux, and also
and a pump for supplying liquid refrigerant to the thermal generator. has the linear correlation
The working refrigerant, changes state according to the temperature and with energy consumption.
pressure levels at the thermal generator and condenser while following Marzouk and Estimating water treatment Water treatment plants [18]
Elkadi plants costs using factor aids in promoting
the same process as steam in a traditional Rankine cycle [26]. The heat
analysis and artificial sustainable practices by
recovery system generally adopted consists of combining the C-ORC neuronal networks providing clean and safe
with an IHE as shown in Fig. 1.b; generally referred to ORC-IHE [17]. water. The cost drivers
The role of IHE is to allow the recovery of some of the heat still present in influence construction
the working refrigerant at the turbine outlet before it enters the costs of water treatment
plants
condenser. The benefits are quite different from improving the perfor­ Houzhang et Proposal and techno- Considerable coal and [19]
mance of the C-ORC by reducing the amount of the heat source and al. economic analysis of a water saving rates can be
increasing the thermal efficiency [17]. (continued on next page)
In the suggested configuration, the heat transfer between the steams

3
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

Table 1 (continued )
Authors Title Relevant highlights Ref.

novel system for waste heat obtained with a short

recovery and water saving payback period.
in coal-fired power plants: A
case study
Chen et al. A novel technical route In the process of flue gas [20]
based on wet flue gas purification for coal-fired
desulfurization process for boiler, wet flue gas
flue gas deshumidification, desulfurization (WFGD) is
water and heat recovery one of the most important
sections. The emission of
the untreated wet flue gas
coming out of the WFGD
system can cause not only
the waste of a large
amount of water and heat
but some environmental
issues. The recovered
water from flue gas could
supply 47.2% and 44.8%
of the water demand for
the WFGD systems in 660
MW and 330 MW power
plants, respectively.
Domenichini Evaluation and analysis of The use of water is a [21]
et al. water usage and loss of critical aspect in the
power in plants with CO2 design, engineering and
capture operation of any fossil-
fuel-fired power plant and
it is an important element
in any environmental
assessment and life cycle
analysis. A large amount
of water is needed to
generate electricity from
any fossil-fuel-fired power
plant. The water
consumption can be
limited but this limitation
is not without negative
consequences on net
electrical efficiency and
specific CO2 emissions,
which decrease and
increase respectively.
Zhuo et al. Analysis on water Five power plants in [22]
consumption and water Beijing consume over 70%
saving countermeasures of of the total water supply
thermal power industry in to the whole electrical
Beijing power industry. The
reduction of this water
consumption can be
reduce in the regard of the
water saving
countermeasures
proposed
Fig. 1. (a) C-ORC and (b) ORC-IHE configurations.

leaving the turbine (4) and the refrigerant leaving the pump (2) takes
place through the cooling water-loop (a-b-c) as shown in Fig. 2. The 3. Modelling
thermodynamic evolution of the working refrigerant in the ORC
configuration with water-loop (ORC-WL) follows the thermodynamic 3.1. Mathematical models
evolution observed in an ORC-IHE. The cooling water (a), after cooling
and condensing the working refrigerant, is conveyed to the IHE (b) The assumptions made about the operation of each configuration were as
where it gives up heat to the liquid refrigerant leaving the pump before follows:
being discharged to the main body of the cooling source or to a cooling
tower (c). • The operating regime is permanent;
• Energy loss and pressure drops along the pipe of each cycle are
neglected;
• Heat is transferred only to the cooling refrigerant. This assumes that
there is no heat loss to the environment from the components or
pipes;
• The effect of hydrostatic pressure at the pump is such that it’s rela­
tively small to be neglected;

4
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

The specific exergy transmitted ψ per unit mass entering or leaving a

component is calculated by the (eq.4):
ψ = (h − h0 ) − T0 (s − s0 ) (4)
Where s is the specific entropy, s0 and h0 are the specific entropy and
enthalpy of the external environment, respectively.
As each component in the ORC configuration is considered as a
control volume, Eqs. (1) to (4) have been applied and listed in Table 2.
Irreversibilities at the turbine and pump are taken into account by
defining an isentropic efficiency for each of these components respec­
tively.
h3 − h4s
ηis,turb = (5)
h3 − h4

h1 − h2
ηis,pump = (6)
h1 − h2s
Where h is the specific enthalpy; the index 3, 4 and 4 s represent the
inlet, the real outlet and the isentropic outlet of the turbine, respectively;
and the index 1, 2 and 2 s represent the inlet, the real outlet and the
isentropic outlet of the pump, respectively.
The thermal effectiveness of the IHE was defined as follows:
Fig. 2. ORC-WL configuration. Max{(T23 − T2 ), (T4 − T41 ) }
ηIHE,ORC− IHE = (7)
T4 − T2
• All heat exchangers used are counter-current and insulated, and their
thermal efficiencies are assumed constant during the operating. Max{(T23 − T2 ), (Tb − Ta ) }
ηIHE,ORC− WL = (8)
Tb − T2
The total mass balance is written as: Finally, the performance of each configuration is measured by en­
∑ ∑
min = mout (1) ergy efficiency, exergy efficiency, and net power output as follows:
Pnet,i = P ́ l, i − Wpump,i − Wpumpwater ,i (9)
The energy balance is written as follows: e

∑ ∑
Qk − Wk + (mh)in = (mh)out (2) Pnet,i
ηth,i = (10)
Qg
The exergy balance is written as follows:
( ) Wturb,i
∑ T0 ηII,i = (11)
1− Qk − W + m(ψ in − ψ out ) = Xdest (3) Wturb,i + Wpumpwater ,i + Idest,total,i
Tk
The mass flow rate is represented by m. Qk is the amount of heat Where in the ith ORC configuration,i ∈{C-ORC, ORC-IHE, ORC-
transmitted across the boundary to point k at the temperature Tk ; h is the WL},Pnet,i , ηth,i and ηII,i are the net power output, energy efficiency and
specific enthalpy, W is the work exchanged between the component and exergy efficiency of a configuration, respectively; Wpump,i and Wpumpwater ,i
the external environment, ψ is the exergy related to the flow of a unit of are the mechanical power of the ORC pump and water pump respec­
mass, T0 is the temperature of the environment in which the ORC con­ tively; and P ́ l, i is the electrical power at the output of electrical
e
figurations are immersed, and Xdest is the exergy destroyed at a compo­ generator. Idest,total,i , is the total exergy destroyed calculated as follows:
nent. The subscripts in and out indicate the input and output limits of the
control volume, respectively.

Table 2
Mass, energy and exergy balance for each ORC configuration.
Configuration Component Mass balance Energy balance Exergy balance

C-ORC Pump m1 = m2 Wpump = m1 (h2 − h1 ) Xdest,pump = m1 T0 (s2 − s1 )

]
Thermal generator m3 = m2 Qg = m1 (h3 − h2 ) [ h3 − h2
Xdest,TG = m1 T0 s3 − s2 −
TH
Turbine m4 = m3 Wturb = m1 (h3 − h4 ) Xdest,turb = m1 T0 (s3 − s4 )
Wet condenser m1 = m4 ; ma = mb m1 (h4 − h1 ) = ma (hb − ha ) Xdest,cond = T0 [m1 (sb − sa ) − m1 (s1 − s4 ) ]
ORC-IHE Pump m1 = m2 Wpump = m1 (h2 − h1 ) Xdest,pump = m1 T0 (s2 − s1 )
Internal Heat Exchanger m23 = m2 ; m4 = m41 h4 − h41 = h23 − h2 Xdest,IHE = m1 T0 [(s23 − s2 ) − (s41 − s4 ) ]
]
Thermal generator m3 = m2 Qg = m1 (h3 − h2 ) [ h3 − h2
Xdest,TG = m1 T0 s3 − s2 −
TH
Turbine m41 = m3 Wturb = m1 (h3 − h4 ) Xdest,turb = m1 T0 (s3 − s4 )
Wet condenser m1 = m41 ; ma = mb m1 (h41 − h1 ) = ma (hb − ha ) Xdest,cond = T0 [m1 (sb − sa ) − m1 (s1 − s41 ) ]
ORC-WL Pump m1 = m2 Wpump = m1 (h2 − h1 ) Xdest,pump = m1 T0 (s2 − s1 )
Internal Heat Exchanger m23 = m2 ; mb = mc ma (hb − hc ) = m1 (h23 − h2 ) Xdest,IHE = T0 [m1 (s23 − s2 ) − ma (sc − sb ) ]
]
Thermal generator m3 = m2 Qg = m1 (h3 − h2 ) [ h3 − h2
Xdest,TG = m1 T0 s3 − s2 −
TH
Turbine m4 = m3 Wturb = m1 (h3 − h4 ) Xdest,turb = m1 T0 (s3 − s4 )
Wet condenser m1 = m4 ; ma = mb m1 (h4 − h1 ) = ma (hb − ha ) Xdest,cond = T0 [m1 (sb − sa ) − m1 (s1 − s4 ) ]

5
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

∑
n configuration.
Idest,total,i = Xdest,j (12)
j=1
5. Results and discussions
j, is the index corresponding to the component of an ORC
configuration. 5.1. Specific case study
The water consumption is evaluated from the heat released at the
condenser as follows: For the same 100 kW heat source and the same operating parameters
considered, Table 5 shows that the dry refrigerant R600a is well suited
Qcond,i
ma,i = (13) for the operation of the configurations studied. Based on the thermo­
hb − ha
dynamic properties at different states of R600a, the improvements in the
The mass flow rate of the cooling water is represented by ma,i . Qcond,i , C-ORC provided by the ORC-IHE are superior to those of the ORC-WL.
is the heat released at the condenser; and ha and hb are the specific en­ For example, with R600a refrigerant in the C-ORC, the temperature at
thalpies at the inlet and outlet of the condenser, respectively. the thermal generator inlet is 35.83 ◦ C. This temperature is 57.43 ◦ C in
the ORC-IHE and 55.73 ◦ C in the ORC-WL, a difference of 1.7 ◦ C
compared to the ORC-IHE. Furthermore, in the C-ORC, the temperature
3.2. Refrigerants selected and properties
of the cooling water discharged was initially observed at 59.24 ◦ C is
33 ◦ C in the ORC-IHE and 55.02 ◦ C in the ORC-WL. With a temperature
In addition to the strict energy performance requirements for ORC
drop of 26.24 ◦ C of the cooling water between the ORC-IHE and the C-
operation, there are practical and safety environmental constraints to
ORC compared to 4.22 ◦ C between the ORC-WL and the C-ORC, with the
consider when selecting the working refrigerants. Many researches have
low temperature difference observed previously, positive effects can a
been conducted to select appropriate ORC working refrigerants. Quoilin
priori be envisaged in the ORC-WL with dry refrigerants.
et al. [27], established a list of 13 general criteria for refrigerant selec­
Table 6 shows that the refrigerant R717 by its nature as a wet
tion. Wang et al. [28], investigated the energy performance of an ORC
refrigerant does not guarantee the operation of the ORC-IHE and ORC-
cycle according to the temperature of the heat source. A synthesis shows
WL for the defined operating conditions. On closer inspection, in these
that at low temperatures, low latent heat and high density of the
two configurations the refrigerant temperature at the turbine outlet is
working refrigerant are preferable to increase the turbine inlet mass flow
35 ◦ C (ORC-IHE) and the water temperature at the condenser outlet is
rate and improve ORC performance [29]. At high temperatures, the
33 ◦ C (ORC-WL), while the temperature at the pump outlet is 36.31 ◦ C.
thermostability of the working refrigerant is the main limiting factor for
This situation is such that it is the refrigerant at the pump outlet that
ORC applications [30]. Also, the environmental impact (ODP, GWP) has
transfers heat, which does not satisfy the IHE operating constraints in
to be taken into account as well as cost, safety (flammability and
both cases as indicated by Jannot [33]. During the simulations with the
toxicity) and compatibility with the ORC components (corrosion, vis­
same operating conditions, other wet refrigerants such as R22, R152a
cosity, and thermal conductivity or condensate formation during steam
and SO2 [32] were used. No compatibility of these wet refrigerants was
expansion) [31].
observed except that of R152a with the ORC-IHE. This result motivated
According to the above selection principles, three common organic
us to set aside the wet refrigerants including the R717; the aim being to
working refrigerants, R1234yf, R717, and R600a, are selected for
have refrigerants which are necessarily compatible with the three ORC
simulation even though for most of them the thermal efficiency of the
configurations.
ORC machine is lower than that offered by the non-eco-friendly fluids.
Table 7 shows that the isentropic refrigerant R1234yf is also suited
Therefore, they are more eco-friendly refrigerants with a low GWP and
for the operation of the configurations studied. Similar observations to
zero ODP and also have good acceptable performance for ORC, as pre­
those for R600a are made for R1234yf. A cooling water temperature
sented by Dijoux et al. [32]. The type of refrigerant was also varied to
drop of 15.7 ◦ C between the ORC-IHE and the C-ORC, and 1.9 ◦ C be­
take into account the influence of the wet, dry and isentropic nature of
tween the ORC-WL and the C-ORC were observed. The temperature of
the refrigerant on the performance of each configuration. The basic
35.83 ◦ C at the generator inlet of C-ORC is 57.43 ◦ C in the ORC-IHE and
physical parameters of these working refrigerants are shown in Table 3.
55.73 ◦ C in the ORC-WL, a difference of 1.7 ◦ C compared to the ORC-
IHE. In fact, positive effects can a priori also be envisaged in the ORC-
4. Operating parameters and methodology used WL with isentropic refrigerants.
Fig. 4 shows the temperature scale in the T-s diagram for each
The thermal source is assumed to be 100 kW and the rest of the configuration and as a function of R600a and R1234yf. As shown in
operating parameters used in this work are reported in Table 4. Note that Fig. 4, the slope of heat recovery by water is greater in the C-ORC (Fig. 4.
T3′′ is the temperature of the working fluid at steam saturation and is a and 4.d corresponding to R600a and R1234yf, respectively) and ORC-
equal to the evaporation temperature. The methodology used as illus­ WL (Fig. 4c and 4.f corresponding to R600a and R1234yf, respectively)
trated in Fig. 3 is such that a specific case has been studied in order to configurations compared to that observed in the ORC-IHE configuration
present the different thermodynamic states of the refrigerant at each (Fig. 4.b and 4.e corresponding to R600a and R1234yf, respectively).
inlet and outlet level of the different components of an ORC configura­ This is necessarily due to the fact that in the ORC-IHE configuration, the
tion, as well as the performance of each of these components and of the working fluid at the turbine outlet before being cooled first gives up
ORC configurations. Subsequently, the ORC configurations are simu­ some of its calories to the fluid leaving the ORC pump.
lated showing how the evaporating temperature, superheating and It should also be noted that since the water evolves at a constant
pinch influence the performance and water consumption in each ORC

Table 3
Properties of the fluids selected for this work according to Santos et al. [27] and Dijoux et al. [28].
Rfrigerant Name Molecular weight (g. Tc (K) Pc (MPa) Latent heat (kJ. Type of fluid ASHRAE Safety GWP ODP
mol− 1) kg− 1) Group

R1234yf 2,3,3,3- 114.04 367.85 3.38 180.25 Isentropic A2L 4 0

tetrafluoropropène
R600a Isobutane 58.12 407.81 3.63 303.44 Dry A3 3 0
R717 Ammonia 17.03 405.40 11.33 1064.38 Wet B2L 0 0

6
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

Table 4
Operating parameters used in this work.
Tam (◦ C) Ta (◦ C) Pa (bar) T3 (◦ C) T1 (◦ C) T3 − T3˝ (◦ C) T4 − Tb = T41 − Tb (◦ C) ηis,pump ηis,turb ηIHE ηal

32 29 3 90 35 10 2 0.85 0.80 0.85 0.90

Fig. 3. The flow chart of the methodology used in this work.

7
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

Table 5
Thermodynamic properties at different states of the dry refrigerant R600a in each ORC configuration.
Configuration Point T(◦ C) P(bar) h(kJ.kg− 1) s(kJ.kg− 1.K− 1) v(dm3.kg− 1) State of the refrigerant

C-ORC 1 35.00 4.65 284.3 1.288 1.863 Saturated liquid

2 35.83 16.40 286.9 1.290 1.857 Compressed liquid
3 100.0 16.40 694.4 2.444 24.140 Superheated steam
4 61.24 4.65 652.2 2.475 93.840 Superheated steam
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 59.24 3.00 248.2 0.822 1.002 Liquid
ORC-IHE 1 35.00 4.65 284.3 1.288 1.863 Saturated liquid
2 35.83 16.40 286.9 1.290 1.857 Compressed liquid
23 57.43 16.40 342.7 1.464 1.968 Compressed liquid
3 100.0 16.40 694.4 2.444 24.140 Superheated steam
4 61.24 4.65 652.2 2.475 93.840 Superheated steam
41 35.00 4.65 596.3 2.301 82.010 Liquid/Vapour
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 33.00 3.00 138.5 0.478 1.005 Liquid
ORC-WL 1 35.00 4.65 284.3 1.288 1.863 Saturated liquid
2 35.83 16.40 286.9 1.290 1.857 Compressed liquid
23 55.73 16.40 338.2 1.450 1.959 Compressed liquid
3 100.0 16.40 694.4 2.444 24.14 Superheated steam
4 61.24 4.65 652.2 2.475 93.840 Superheated steam
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 59.24 3.00 248.2 0.822 1.017 Liquid
c 55.02 3.00 230.6 0.768 1.014 Liquid

Table 6
Thermodynamic properties at different states of the wet refrigerant R717 in each ORC configuration.
Configuration Point T(◦ C) P(bar) h(kJ.kg− 1) s(kJ.kg− 1.K− 1) v(dm3.kg− 1) State of the refrigerant

C-ORC 1 35.00 13.51 366.1 1.567 1.702 Saturated liquid

2 36.31 51.16 373.6 1.570 1.679 Compressed liquid
3 100.0 51.16 1515 4.772 25.45 Superheated steam
4 35.00 13.51 1386 4.877 87.06 Superheated steam
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 33.00 3.00 138.5 0.478 1.005 Liquid
ORC-IHE 1 35.00 13.51 366.1 1.567 1.702 Saturated liquid
2 36.31 51.16 373.6 1.570 1.679 Compressed liquid
23 35.20 51.16 368.3 1.553 1.692 Compressed liquid
3 100.0 51.16 1515.0 4.772 25.45 Superheated steam
4 35.00 13.51 1386.0 4.877 87.06 Superheated steam
41 35.00 13.51 1391.0 7.894 87.51 Liquid/Vapour
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 33.00 3.00 138.5 0.478 1.005 Liquid
ORC-WL 1 35.00 13.51 366.1 1.567 1.702 Saturated liquid
2 36.31 51.16 373.6 1.570 1.679 Compressed liquid
23 33.50 51.16 360.1 1.526 1.684 Compressed liquid
3 100.0 51.16 1515.0 4.772 25.45 Superheated steam
4 35.00 13.51 1386.0 4.877 87.06 Superheated steam
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 33.00 3.00 138.5 0.478 1.005 Liquid
c 33.50 3.00 138.7 0.478 1.005 Liquid

pressure of 3 bars, the evaporation temperature at this pressure is about justified by an exergy efficiency of the ORC-WL of 84.85 % with R600a
134 ◦ C. This temperature level gives the certainty that in the process of and 66.29 % with R1234yf, compared to 64.87 % with R600a and 58.52
cooling the working fluid, the water will remain in the state of com­ % with R1234yf for the ORC-IHE, respectively. Another negative point
pressed liquid and the law of its boiling point. Thus, the representation of the ORC-IHE is its excessive consumption of cooling water. It is 5.30
of its evolution in the T-s diagrams of R600a and R1234yf can only kg/s with R600a and 5.11 kg/s with R1234yf. The ORC-WL consumes
highlight the quantities of heat involved. 0.82 kg/s with R600a and 1.20 kg/s with R1234yf. Under the same
Table 8 shows the energy flow for each component and the perfor­ operating conditions, the C-ORC consumes 0.71 kg/s with R600a and
mance of each ORC configuration corresponding to the considered 1.09 kg/s with R1234yf. These results show that for the case of a solar
operational conditions. For the reasons of non-compatibility of R717 ORC project, the ORC-WL is the most appropriate configuration in terms
previously indicated (Table 7), the energy flow for each component and of water savings and which the net power output is not far from that of
the performance of each ORC configuration related to the effect of this the ORC-IHE.
wet refrigerant were not listed. Table 8 shows that the electrical output Another observation made is that when the pumping power is not
with the ORC-WL configuration, in the case of dry and isentropic re­ taken into account, the net power output of the ORC-IHE configuration
frigerants, is approximately equal to that of the ORC-IHE within a few without the pumping power is 10.08 kW higher than that of the ORC-WL
watts. For the operating conditions considered, when the net power configuration observed at 9.95 kW. When the pumping power is taken
output of the ORC-IHE is 10.08 kW and 8.79 kW for R600a and R1234yf into account, the net power output of the ORC-IHE configuration
respectively, that of the ORC-WL is 9.95 kW and 8.65 kW, respectively, a observed at 9.11 kW becomes lower than that of the ORC-WL configu­
difference of 130 W and 140 W respectively. The available energy ration observed at 9.81 kW.
quality of the ORC-WL is higher than that of the ORC-IHE. This is

8
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

Table 7
Thermodynamic properties at different states of the isentropic refrigerant R1234yf in each ORC configuration.
Configuration Point T(◦ C) P(bar) h(kJ.kg− 1) s(kJ.kg− 1.K− 1) v(dm3.kg− 1) State of the refrigerant

C-ORC 1 35.00 8.95 247.2 1.161 0.949 Saturated liquid

2 36.36 30.8 249.7 1.162 0.942 Compressed liquid
3 100.0 30.8 420.5 1.649 5.109 Superheated steam
4 50.91 8.95 402.8 1.663 22.020 Superheated steam
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 48.91 3.00 205.0 0.690 1.012 Liquid
ORC-IHE 1 35.00 8.95 247.2 1.161 0.949 Saturated liquid
2 36.36 30.8 249.7 1.162 0.942 Compressed liquid
23 48.73 30.8 267.0 1.217 0.985 Compressed liquid
3 100.0 30.8 420.5 1.649 5.109 Superheated steam
4 50.91 8.95 402.8 1.663 22.02 Superheated steam
41 35.21 8.95 385.5 1.608 19.910 Liquid/Vapour
a 29.00 3.00 121.8 0.422 1.004 Liquid
b 33.21 3.00 139.4 0.481 1.005 Liquid
ORC-WL 1 35.00 8.95 247.2 1.161 0.949 Saturated liquid
2 36.36 30.8 249.7 1.162 0.942 Compressed liquid
23 47.03 30.8 264.6 1.210 0.979 Compressed liquid
3 100.0 30.8 420.5 1.649 5.109 Superheated steam
4 50.91 8.95 402.8 1.663 22.020 Superheated steam
a 29.00 3.00 121.8 0.423 1.004 Liquid
b 48.91 3.00 205.0 0.690 1.012 Liquid
c 47.01 3.00 197.1 0.665 1.011 Liquid

5.2. Effect of the evaporating temperature, superheating and pinch, increase of about 15.49 % compared to the C-ORC, and a saving of 4.48
respectively kg/s at the ORC-IHE.

5.2.1. Effect of the evaporating temperature T3′′ 5.2.2. Effect of the superheating T3 - T3′′
Fig. 5 shows the variation of energy and exergy efficiencies of the Fig. 8 shows the influence of superheating (T3 - T3′′ ) on the energy
three studied ORC configurations according to the evaporating tem­ and exergy performance of each configuration. As shown in this figure,
perature, for R600a and R1234yf respectively. Firstly, the dry refrig­ the dry refrigerant R600a offers significantly better energy and exergy
erant R600a has higher energy and exergy efficiencies than the performance than R1234yf for each configuration. As also shown in
isentropic refrigerant R1234yf. Thus, compared to R600a, an improve­ Fig. 8, R600a is suitable for each configuration in the superheating
ment in the energy efficiency of C-ORC is observed when an IHE is range, whereas the isentropic nature of R1234yf makes it not suitable for
associated with it (ORC-IHE). But with ORC-IHE, the exergy efficiency ORC-IHE and ORC-WL for superheating below 3 ◦ C. Indeed, for a su­
becomes lower than that of C-ORC. This shows that due to the IHE, the perheat below 3 ◦ C, the temperature of the fluid at the turbine outlet
irreversibilities are higher than those observed in the C-ORC. By remains lower than that of the fluid at the pump outlet. This condition,
changing the location of the IHE and implementing a water-loop be­ as mentioned above, does not allow for the preheating of the fluid at the
tween the condenser and the IHE (ORC-WL), an improvement in the pump outlet. In this temperature range the T23 temperature in the ORC-
energy efficiency of the C-ORC, slightly less than 0.13 % of that obtained IHE is higher than the T4 temperature, and the Tb temperature in the
with the ORC-IHE was observed at T3′′ = 90 ◦ C. Although this result is ORC-WL is lower than the T2 temperature. As the case of R717, this
rather low, it demonstrates the positive aspect of the water loop in heat situation does not satisfy the IHE operating constraints in both cases.
recovery. From an exergy point of view, ORC-WL significantly decreases From a superheating of 3 ◦ C, the ORC-IHE offers a higher energy per­
the irreversibilities of C-ORC as shown in Fig. 5. This result shows that formance than C-ORC, but slightly higher than ORC-WL, still with an
the use of water as a heat recovery medium improves the energy quality absolute difference of 0.13 % is observed. From an exergy point of view,
compared to C-ORC and ORC-IHE. the energy quality of ORC-IHE remains roughly equal to that of C-ORC,
Fig. 6 shows the variation of the net power output of each configu­ while it is significantly higher with ORC-WL over the entire superheat
ration with evaporating temperature. This net power output for each range. Fig. 8 also shows that with increasing superheating, the energy
configuration increases with increasing evaporating temperature. Once quality of ORC-IHE and C-ORC degrades further, while it increases
again, the net power output obtained with R600a is higher than that considerably in ORC-WL.
obtained with R1234yf. However, with both refrigerants, as shown in Fig. 9 shows the variations of the net power output according to the
Fig. 6, when the pumping power is taken into account the net power superheating (T3 - T3′′ ). As shown in this figure, the net power output
output obtained with ORC-IHE is slightly lower than that obtained with obtained with R600a is higher than that obtained with R1234yf.
ORC-WL in the considered evaporating temperature range. An absolute Increasing superheating has no significant effect on the C-ORC with
difference of 693 W was observed at T3′′ = 90 ◦ C between the two R600a, whereas with R1234yf the net power output of the C-ORC in­
challenged configurations. creases with increasing superheating. On the other hand, a considerable
Fig. 7 shows how the evaporating temperature affects the cooling increase in net power output in the ORC-IHE and ORC-WL with super­
water consumption in each configuration. R600a shows lower water heat is observed. At 90 ◦ C, the difference of net power output between
consumption than R1234yf. For each configuration the cooling water the ORC-IHE and the ORC-WL is 140 W, as previously observed.
consumption decreases slightly with increasing evaporating tempera­ Fig. 10 shows how the superheating (T3 - T3′′ ) influences the cooling
ture. Maintaining a constant pinch of 2 ◦ C between the turbine outlet (4) water consumption. Overall, the water consumption is very high in the
and the water outlet (b), the ORC-IHE shows with R600a at T3′′ = 90 ◦ C, ORC-IHE with R600a or R1234yf. With R1234yf, increasing the super­
a significant cooling water consumption of about 5.30 kg/s compared to heating from 3 ◦ C to 11 ◦ C leads to an increase in water consumption in
that of the C-ORC of 0.71 kg/s; i.e. a drastic increase of about 645.48%. the ORC-IHE. Over the superheating of 11 ◦ C, this water consumption
Using a heat regeneration water-loop, the specific cooling water con­ becomes more or less constant, indicated by a constant limit tempera­
sumption under the same operating conditions is about 0.82 kg/s; i.e. an ture T41 of 35 ◦ C. However, R600a has lower water consumption than

9
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

R1234yf
150
ORC-IHE with R1234yf
125

1
09
Water cooling with R1234yf

0, 0

19
01

g
3/k
42
0, 0
3

0, 0
100

0, 0

9m
9
0, 0
0, 1
Temperature [°C]
30,8 bar
75

50 23 4
2 41
1 8,95 bar
b
25 a

-25 0,2 0,4 0,6 0,8

-50
0,75 1,00 1,25 1,50 1,75 2,00 2,25
-1 -1
Entropy [kJ.kg .K ]
(e)
R1234yf
150
ORC-WL with R1234yf
125
1
09

Water cooling with R1234yf

9

0, 0

19
01

g
3/k
42

3
0, 0
0, 0

100
0, 0

9m
9
0, 0
0, 1
Temperature [°C]

30,8 bar
75
b
50 c 2
23 4
8,95 bar
1 b
25
a
0

-25 0,2 0,4 0,6 0,8

-50
0,75 1,00 1,25 1,50 1,75 2,00 2,25
-1 -1
Entropy [kJ.kg .K ]
(f)
Fig. 4. T-s diagrams with R600a and R1234yf and cooling water temperature evolution. (a): C-ORC with R600a; (b): ORC-IHE with R600a; (c): ORC-WL with R600a;
(d): C-ORC with R1234yf; (e): ORC-IHE with R1234yf; and (f): ORC-WL with R1234yf.

R1234yf. As shown in Fig. 10, the water consumption in the ORC-WL the fluid load of the ORC and thus a decrease in the water mass for
remains considerably lower than that observed in the ORC-IHE. cooling the fluid at the condenser.
As also shown in Fig. 10 the increase in superheat leads to a decrease
in water consumption in the ORC-WL configuration. Indeed, with 5.2.3. Effect of the pinch T4 – Tb
increasing in superheat, the enthalpy difference between the superheat Fig. 11 shows the influence of pinch (T4 - Tb) on the energy and
point and the generator inlet becomes large, which leads to a decrease in exergy performance of the three configurations studied. Keeping the

10
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

R600a
250 R600a
250
C-ORC with R600a

2
ORC-IHE with R600a

06

2
06
0,0
200

0,0
200

66
Water cooling with R600a

66
Water cooling with R600a

0, 0

3/k

g
1

0, 0

3/k
0, 2

8m

1
0, 7

0, 2
2

8m
2, 3

0, 7

2
7, 5
150

2, 3
7, 5
Temperature [°C]
150

Temperature [°C]
3
100 16,4 bar
100 16,4 bar
3

23
4
50 1 2 4,65 bar 50 4
1 2 4,65 bar
b 41
b
a a
0 0

-50 -50
0,2 0,4 0,6 0,8
0,2 0,4 0,6 0,8

-100 -100
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
Entropy [kJ.kg-1.K-1] Entropy [kJ.kg-1.K-1]
(a) (b)

R600a
250 R1234yf
150
ORC-WL with R600a
2
0,0
06
C-ORC with R1234yf
200 125
66

1
Water cooling with R600a

09
g
Water cooling with R1234yf
0, 0

3/k

9
1

0,0

19
0, 2

01

g
1

8m

3/k
42
3

0,0
0, 7

0, 0
2, 3
100

0,0
7, 5

9m
150

9
Temperature [°C]

0,0
0,1
Temperature [°C]
30,8 bar
3 75
100 b
16,4 bar

c 23 50 2 4
4
50 2 4,65 bar 1
8,95 bar
1 b
b
25
a
0
a 0
-50
0,2 0,4 0,6 0,8 -25 0,2 0,4 0,6 0,8

-100 -50
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0,75 1,00 1,25 1,50 1,75 2,00 2,25
Entropy [kJ.kg-1.K-1] Entropy [kJ.kg-1.K-1]
(c) (d)

Fig. 4. (continued).

Table 8
Energy flow for each component and performance of the C-ORC, ORC-IHE and ORC-WL configurations in the specific case study.
Refrigerant R600a R1234yf
C-ORC ORC-IHE ORC-WL C-ORC ORC-IHE ORC-WL

Thermal generator (kW) 100.0 100.0 100.0 100.0 100.0 100.0

Condenser (kW) 90.26 88.72 103.3 91.06 90.06 99.76
Pump (kW) 0.63 0.73 0.72 1.42 1.59 1.56
Water pump (kW) 0.13 0.97 0.15 0.20 0.93 0.22
Turbine (kW) 10.37 12.01 11.86 10.36 11.53 11.35
Recovery (kW) 0.00 15.88 14.41 0.00 11.27 9.55
Rejected (kW) 90.26 88.72 88.86 91.06 90.06 90.21
Refrigerant consumption (kg.s− 1) 0.245 0.284 0.28 0.59 0.65 0.64
Water consumption (kg.s− 1) 0.71 5.30 0.82 1.09 5.11 1.20
Net power output without water pump (kW) 8.70 10.08 9.95 7.90 8.79 8.65
Net power output with water pump (kW) 8.57 9.11 9.81 7.70 7.86 8.44
Total irreversibilities (kW) 5.10 6.51 2.12 7.29 8.17 5.77
Carnot efficiency (%) 17.42 17.42 17.42 17.42 17.42 17.42
Energy efficiency (%) 8.70 10.08 9.95 7.90 8.79 8.65
Exergy efficiency (%) 67.01 64.87 84.85 58.7 58.52 66.29

evaporating temperature and superheating constant, Fig. 11 shows that it is more observed in ORC-IHE than in ORC-WL. As shown in Fig. 11, the
the energy and exergy performance of R600a is higher than that of increase in pinch leads in each configuration to an increase in irre­
R1234yf. From the energy point of view, with R600a or R1234yf, the versibilities, which are manifested at the condenser in the case of the C-
sensitivity of the pinch on the energy performance of C-ORC and ORC- ORC and ORC-IHE configurations, and at the condenser and the IHE
IHE is observed to be negligible, while its increase leads to a decrease exchanger in the case of ORC-WL.
in the energy performance of ORC-WL. Indeed, in the case of the C-ORC Fig. 12 shows the variations of the net power output of the studied
and the ORC-IHE, the state (2) of the refrigerant at the thermal generator ORC configurations according to the pinch (T4 - Tb). The net power
inlet remains independent of the pinch, while in the case of the ORC-WL, output is much better with the R600a than with the R1234yf. For both
this state remains closely linked to it. As a result, the definition of a low refrigerants used, the increase in pinch is observed to be negligible on
pinch appears to be required for improved ORC-WL performance. the net power outputs of C-ORC and ORC-IHE. While this increase leads
From the exergy point of view, although the state (2) of refrigerant is the decrease of the net power output of ORC-WL, but remains compa­
not dependent on the pinch in the case of C-ORC and ORC-IHE, the rable to that of ORC-IHE. When the pinch is low, less than 3 ◦ C, the
quality of the energy in each of the three configurations is degraded and difference between the ORC-IHE and the ORC-WL is even more

11
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

18 90
= 0.80
is,turb T4 - Tb = T41 - Tb = 2 °C
16 is,pump = 0.85 T3 - T3" = 10 °C
IHE = 0.85 Ta = 29 °C 80

[%]
[%]
14
al = 0.90

II
th

T1 = 35 °C
12
70

Exergy efficiency
Energy efficiency

10

60
8

6
50
4
Carnot /C-ORC/ORC-IHE/ORC-WL
2 40
48 52 56 60 64 68 72 76 80 84 88 92
Evaporating temperature T3" [°C]
th, C-ORC/R1234yf th, ORC-IHE/R1234yf th, ORC-WL/R1234yf
th, C-ORC/R600a th, ORC-IHE/R600a th, ORC-WL/R600a
II, C-ORC/R1234yf II, ORC-IHE/R1234yf II, ORC-WL/R1234yf
II, C-ORC/R600a II, ORC-IHE/R600a II, ORC-WL/R600a

Fig. 5. Energy and exergy efficiency of ORC configurations according to the evaporating temperature for R1234yf and R600a refrigerants.

11
Pnet /C-ORC/R1234yf
10
Pnet /ORC-IHE/R1234yf
9 Pnet /ORC-WL/R1234yf
Net power output Pnet [kW]

Pnet /C-ORC/R600a
8
Pnet /ORC-IHE/R600a
7 Pnet /ORC-WL/R600a

5 = 0.80
is,turb T4 - Tb = T41 - Tb = 2 °C
is,pump = 0.85 T3 - T3" = 10 °C
4
IHE = 0.85 Ta = 29 °C
3 al = 0.90
T1 = 35 °C
2

48 52 56 60 64 68 72 76 80 84 88 92
Evaporating temperature T3" [°C]
Fig. 6. Net power output of ORC configurations according to the evaporating temperature for R1234yf and R600a refrigerants.

12
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

6
5,5

Water consumption WC [kg.s-1] 5

WC,C-ORC/R1234yf
T4 - Tb = T41 - Tb = 2 °C
4,5 WC,ORC-IHE/R1234yf T3 - T3" = 10 °C
4 WC,ORC-WL/R1234yf Ta = 29 °C
3,5 WC,C-ORC/R600a T1 = 35 °C
3 WC,ORC-IHE/R600a is,turb = 0.80

2,5 WC,ORC-WL/R600a is,pump = 0.85

IHE = 0.85
2
al = 0.90
1,5
1
0,5
48 52 56 60 64 68 72 76 80 84 88 92
Evaporating temperature T3" [°C]
Fig. 7. Water consumption of ORC configurations according to the evaporating temperature for R1234yf and R600a refrigerants.

11 92
T4 - Tb = T41 - Tb = 2 °C T1 = 35 °C
10,5 86,5
T3" = 90 °C Ta = 29 °C
[%]

[%]
10
81
th

II
9,5 = 0.80 IHE = 0.85
Energy efficiency

Exergy efficiency
is,turb
75,5
is,pump = 0.85 al = 0.90
9
70
8,5
64,5
8

7,5 59

7 53,5
0 1,5 3 4,5 6 7,5 9 10,5 12 13,5 15 16,5
Superheating T3 - T3" [°C]
th, C-ORC/R1234yf th, ORC-IHE/R1234yf th, ORC-WL/R1234yf
th, C-ORC/R600a th, ORC-IHE/R600a th, ORC-WL/R600a
II, C-ORC/R1234yf II, ORC-IHE/R1234yf II, ORC-WL/R1234yf
II, C-ORC/R600a II, ORC-IHE/R600a II, ORC-WL/R600a

Fig. 8. Energy and exergy of ORC configurations according to the superheating for R1234yf and R600a refrigerants.

13
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

11
Pnet /C-ORC/R1234yf Pnet /C-ORC/R600a
10,5 Pnet /ORC-WL/R1234yf Pnet /ORC-WL/R600a

Net power output Pnet [kW]

Pnet /ORC-IHE/R1234yf Pnet /ORC-IHE/R600a
10
T4 - Tb = T41 - Tb = 2 °C
9,5
Ta = 29 °C
9

8,5 T3" = 90 °C
T1 = 35 °C
8

= 0.80 IHE = 0.85

7,5 is,turb
is,pump = 0.85 al = 0.90

7
0 1,5 3 4,5 6 7,5 9 10,5 12 13,5 15 16,5
Superheating T3 - T3" [°C]
Fig. 9. Net power output of ORC configurations according to the superheating for R1234yf and R600a refrigerants.

6
Water consumption WC [kg.s-1]

WC,C-ORC/R1234yf
WC,ORC-IHE/R1234yf
4 Ta = 29 °C T1 = 35 °C WC,ORC-WL/R1234yf
T4 - Tb = T41 - Tb = 2 °C WC,C-ORC/R600a
WC,ORC-IHE/R600a
T3" = 90 °C
WC,ORC-WL/R600a

2 is,turb = 0.80 IHE = 0.85

is,pump = 0.85 al = 0.90

0
0 1,5 3 4,5 6 7,5 9 10,5 12 13,5 15 16,5
Superheating T3 - T3" [°C]
Fig. 10. Water consumption of ORC configurations according to the superheating for R1234yf and R600a refrigerants.

noticeable. IHE.
Fig. 13 shows the variation in the cooling water consumption of the
studied configurations according to the pinch (T4 - Tb). As shown in this 6. Conclusions
figure, the water consumption in ORC-IHE increases drastically with the
pinch. While water consumption, in the ORC-WL and C-ORC is nearly The performance of the ORC machine with a water-loop as a means
insensitive to pinch. However, the water consumption of R1234yf re­ of heat regeneration for solar power projects in climatic conditions
mains higher than that of R600a in ORC-WL and C-ORC, while in ORC- similar to Central Africa was studied in comparison with the ORC with
IHE the water consumption of R600a remains slightly higher than that of internal heat exchanger and with reference to the conventional ORC.
R1234yf. In terms of water consumption, ORC-WL is better than ORC- The comparison parameters under the same operating conditions were

14
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

10,5 88

84
10

[%]
[%]
80
Ta = 29 °C T1 = 35 °C
9,5

II
th
T4 - Tb = T41 - Tb = 2 °C = 0.80 IHE = 0.85
is,turb 76

Exergy efficiency
Energy efficiency

T3 - T3" = 10 °C is,pump = 0.85 al = 0.90

9 72

68
8,5
64
8
60

7,5 56
0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5
Pinch T4 - Tb [°C]
th, C-ORC/R1234yf th, ORC-IHE/R1234yf th, ORC-WL/R1234yf
th, C-ORC/R600a th, ORC-IHE/R600a th, ORC-WL/R600a
II, C-ORC/R1234yf II, ORC-IHE/R1234yf II, ORC-WL/R1234yf
II, C-ORC/R600a II, ORC-IHE/R600a II, ORC-WL/R600a

Fig. 11. Energy and exergy efficiency of ORC configurations according to the pinch for R1234yf and R600a refrigerants.

12
Pnet /C-ORC/R1234yf Pnet /C-ORC/R600a
11 Pnet /ORC-IHE/R1234yf Pnet /ORC-IHE/R600a
Net power output Pnet [kW]

Pnet /ORC-WL/R1234yf Pnet /ORC-WL/R600a

10

8
T4 - Tb = T41 - Tb = 2 °C = 0.80 = 0.85
is,turb IHE
Ta = 29 °C
7
T3 - T3" = 10 °C is,pump = 0.85 al = 0.90
T1 = 35 °C
6
0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5
Pinch T4 - Tb [°C]
Fig. 12. Net power output of ORC configurations according to the pinch for R1234yf and R600a refrigerants.

energy efficiency, exergy efficiency, net power and cooling water con­ fluids were R600a, R717 and R1234yf. The simulation results showed
sumption. These configurations were simulated in an environment that:
where the average ambient temperature is 32 ◦ C, the initial cooling
water temperature is 29 ◦ C and the heat source is constant at 100 kW. • Wet fluids such as R717 present unsuitable operating conditions for
The condensing temperature was assumed to be 35 ◦ C relative to the the ORC-IHE and ORC-WL configurations. Indeed, for these two
cooling water temperature. The refrigerants used as working candidate configurations and for the operating conditions considered, the gas

15
J. Paul Njock et al. Thermal Science and Engineering Progress 32 (2022) 101303

22
20

Water consumption WC [kg.s

WC,C-ORC/R1234yf

]
18 WC,ORC-IHE/R1234yf
-1 16 WC,ORC-WL/R1234yf
WC,C-ORC/R600a
14
WC,ORC-IHE/R600a
12 WC,ORC-WL/R600a Ta = 29 °C
10 T3 - T3" = 10 °C
8 T1 = 35 °C
6 = 0.85
is,turb = 0.80 IHE
4
T4 - Tb = T41 - Tb = 2 °C is,pump = 0.85 al = 0.90
2
0
0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5
Pinch T
4 - Tb [°C]

Fig. 13. Water consumption of ORC configurations according to the pinch for R1234yf and R600a refrigerants.

temperature at the turbine outlet is observed at 35 ◦ C, which does not CRediT authorship contribution statement
allow the preheating of the working fluid at the pump outlet, whose
temperature is observed at 36.31 ◦ C. Julbin Paul Njock: Conceptualization, Methodology, Validation,
• The internal heat exchanger in the ORC-IHE improves the efficiency Formal analysis, Investigation, Writing – review & editing, Writing –
of the C-ORC. However, this technique contributes considerably to original draft. Max Ndame Ngangue: Conceptualization, Methodology,
the degradation of the available energy, and we observe a drastic Supervision, Writing – review & editing, Validation, Writing – original
increase in cooling water consumption almost 7 times that of the C- draft. Alain Christian Biboum: Conceptualization, Methodology, Su­
ORC; pervision, Writing – review & editing, Validation, Writing – original
• The ORC-WL improves the energy efficiency of the C-ORC in a way draft. Olivier Thierry Sosso: Conceptualization, Methodology, Valida­
that is close to the improvement of the ORC-IHE. But the presence of tion, Formal analysis, Investigation, Writing – review & editing, Writing
the water loop in the ORC-WL significantly increases the quality of – original draft. Robert Nzengwa: Conceptualization, Methodology,
the available energy. The water consumption of the ORC-WL is Supervision, Writing – review & editing, Writing – original draft.
slightly higher than that of the C-ORC. Its net power output is only a
few watts (130 to 140 W) less than that of the ORC-IHE; Declaration of Competing Interest
• Dry refrigerants are the most suitable for the operation of the ORC-
WL. Their influence on the performance of the three configurations The authors declare that they have no known competing financial
studied is greater than that of isentropic refrigerants. interests or personal relationships that could have appeared to influence
• In the evaporating temperature range of 50–90 ◦ C, for each config­ the work reported in this paper.
uration studied and for the different working refrigerants selected,
the increase in evaporating temperature contributes to the increase References
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