Techno-Economic and Exergo-Economic Evaluation of a Novel Solar Integrated Waste-to-Energy Power Plant

Full text

Applied Thermal Engineering 223 (2023) 119929
Available online 30 December 2022
1359-4311/© 2022 Elsevier Ltd. All rights reserved.
Research Paper
Techno-economic and exergo-economic evaluation of a novel solar
integrated waste-to-energy power plant
Muhammad Sajid Khan
a
,
b
, Jintao Cui
a
, Yu Ni
c
, Mi Yan
a
,
*
, Mustajab Ali
d
,
e
a
Institute of Energy and Power Engineering, College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
b
Department of Mechanical Engineering, Mirpur University of Science & Technology, (MUST) Mirpur (10250), AJK, Pakistan
c
China Power Engineering Consulting Group Co.,LTD, Beijing 100120, China
d
Department of Civil Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
e
Department of Civil Engineering, Mirpur University of Science & Technology, (MUST) Mirpur (10250), AJK, Pakistan
ARTICLE INFO
Keywords:
Waste-to-energy
Municipal solid waste incineration
Solar integration
Exergo-economic
Heliostat
Sustainability
ABSTRACT
The content of municipal solid waste generation is increasing due to the industrial and population growth.
Energy generation from municipal solid waste is one of the efcient and promising method to dispose waste in a
sustainable manner, while reducing the fossil fuel energy limitations. However, conventional waste-to-energy
plants have less thermal efciency due to the higher moisture content that lowers its heating value. The solar
thermal integration is one of the proposed solutions to enhance the steam inlet temperature in the power gen-
eration system that will further increase the plant performance. In this study, an innovative solar integrated
municipal solid waste incineration plant is proposed to advance the waste-to-energy and concentrated solar
power thermal energy technologies. The system integration is carried out utilizing the useful solar heat harvested
in the heliostat tower receiver system by transferring heat to the working steam of the incineration plant in the
heat exchanger and enhance the turbine inlet temperature and the system’s performance considerably. Based on
300 t/d incineration facility, proposed system is investigated exergo-economically by varying certain inuential
parameters using engineering equation solver and performance has been compared with the conventional
reference plant. The system is integrated with high-temperature Phase Change Materials, which able to store
thermal energy enough for the light-time system operation. The results attribute that the network output of the
proposed integrated facility has substantial improved (almost 12.61 %) but with the penalty of increase in the
exergy destruction cost rate. In addition, levelized cost of electricity of the proposed plant is 13.9 % less than the
conventional one and it decreases with the rise of steam temperature. Energy efciency of the proposed plant
improves to approximately 36.34 % as direct normal irradiation increases from 650 W/m
2
to 900 W/m
2
. In
addition, Incineration boiler is the major source of exergy destruction (almost 50 %) with 175.2 $/hr exergy
destruction cost.
1. Introduction
The sustainable disposal and effective management of rapidly
increasing municipal solid waste (MSW) is required to achieve world-
wide sustainability goals [1]. It is estimated that globally MSW gener-
ation will reach approximately 9.5 billion ton by 2050 [2]. The United
Nations Sustainable Development Goal (SDG) 7 stresses the delivery of
modern and reliable energy as a measure of poverty mitigation for all.
Waste-to-Energy incineration (WtE) is a key and promising technique to
dispose and convert waste into a considerable source for useful energy
generation by saving the land areas [3]. It is the method of generating
energy in the form of heat/ electricity by combustion of waste as a fuel
dramatically reducing waste volume [4]. Moreover, from the such ad-
vantages, WtE incineration is the most considered favorable way to
sanitary landlls, specically for medium and large-sized cities that has
very limited or shortages of landlls space. In the European Union, 247
MT of MSW was treated by 2018 out of which 47 % was either simple
incinerated or treated with energy recovery incineration [5]. Being a
technological leader, WtE incineration industry in the China is increased
6.1 times, from 54 in 2004 to 330 in 2018, treating 44.67 % of collected
waste with designed annual capacity of 133.08 MMT [6].
It is worth mentioning that electrical efciency of a conventional
WtE plant is signicantly low than the fossil fuel (coal, natural gas, etc.)
* Corresponding author.
E-mail address: yanmi1985@zjut.edu.cn (M. Yan).
Contents lists available at ScienceDirect
Applied Thermal Engineering
journal homepage: www.elsevier.com/locate/apthermeng
https://doi.org/10.1016/j.applthermaleng.2022.119929
Received 21 September 2022; Received in revised form 26 November 2022; Accepted 19 December 2022

Applied Thermal Engineering 223 (2023) 119929
2
power plants because of the higher moisture content in feed-stock, low
temperature exhaust gas as well as the combined effects of technical and
economical constraints (high stack loss, limited steam parameters,
simple cycle conguration) [3]. Additionally, live steam pressure and
temperature cannot be exceeded to 4 MPa and 400 ◦C due to the surface
corrosion of WtE boiler tubes [4]. Usually, reheating of the steam is not
accomplished during the power production process, however it has its
own importance to achieve higher efciency. Meanwhile, a volume of
research has been performed to improve the WtE incineration plants
performance. Higher efciencies of WtE power plants are attributed to
the greater steam inlet parameters to the turbine and to overcome the
aggressive ue gas attack and high-temperature corrosion, numerous
efforts have been done to develop corrosion-resistant materials [7].
Therefore proposal has been made to utilize the high-temperature phase
change materials (PCM) instead of conventional refractory bricks in the
incineration chamber [7]. Moreover, efciency of the conventional
incineration plant can be enhanced by decreasing the exhaust gas ow
rate that will ultimate lowers the stack loss [8–9].
Furthermore, integrating WtE plant to other thermal systems is an
efcient method to improve the performance of WtE plants. In this re-
gard, a natural gas combined cycle has been integrated with WtE plant to
avoid the high temperature corrosion utilizing external steam super-
heater to increase the turbine inlet temperature for higher system per-
formance [10]. WtE plant combined with coal power plant and
supercritical CO
2
(sCO
2
) cycle was proposed and investigated [11].
Proposed hybrid cascade system was 8.34 % more efcient with 3.33
MW more power output.
In addition, concentrating solar power (CSP) systems integrated to
WtE incineration plants is a promising choice to boost the performance
of incineration plant as well as to handle various problems in the energy
domain [12–13]. CSP technologies (parabolic dish, parabolic trough,
solar tower with heliostat, Fresnal lens) have ability to gain maximum
temperature that makes it possible to integrate solar thermal to other
energy sources to synergistically generate power [4]. The integration of
conventional fueled power plants with solar thermal systems is a proven
technology that effectively utilize solar energy and well-designed solar
integrated power plants have superior advantages as compared to the
solar-only plants [12–14]. It has been concluded that electricity gener-
ation utilizing hybridized solar thermal energy and conventional fuel
regenerative Rankine cycle power plant, solar-assisted coal and solar-
aided combined cycle power plants have been broadly investigated
[15]. A novel conguration of solar chimney and WtE incineration plant
in Tehran, Iran has been proposed and thermodynamically examined
[12]. Results demonstrated that overall exergy and energy efciency of
the system was almost 0.12 and 0.15, respectively. MSW power plant
integrated with parabolic trough collector has been investigated exergo-
economically [16]. The incineration boiler provided a variable heating
duty to overcome solar uctuations and regulate the power output,
while the results demonstrated the 47.4 % reduction of electricity cost.
Another hybridization of solar thermal with WtE incineration plant was
designed and analyzed as a case study in Denmark by the same re-
searchers [17]. The environmental impacts of a solar integrated WtE
incineration plant was evaluated by applying Life Cycle Assessment
(LCA), where saturated steam was produced in WtE section, while
superheating took place in an external super-heater powered by
concentrated solar system or natural gas back up boiler during the solar
uctuation /night time [18]. Different congurations of renewable
energy-based hybrid WtE incineration plants have been proposed and
investigated including low and medium grade solar thermal systems,
while feedwater heating was done by ue gas condensation [19]. Khan
et al. [13] evaluated a solar thermal integrated WtE incineration plant in
a way that the steam exiting WtE boiler temperature is further heated by
solar thermal energy before entering to the turbine. In this way turbine
inlet temperature (TIT) and proposed system performance increased
sufciently than the conventional WtE system.
Nevertheless, little research work has been published on the utili-
zation of solar energy to boost the steam temperature of MSW WtE
plants for performance enhancement. Currently, heliostat with solar
tower central receiver systems are widespread for variety of purposes
due to their capability of achieving the higher temperature ≈800 ◦C
Nomenclature
Ahel Heliostat area [m
2
]
Arec Receiver area [m
2
]
˙
C Cost rate ($/hr)
c Unit cost rate ($/MJ)
Cp Specic heat capacity (kJ/kg-K)
˙
EXin Input exergy [k W]
fk Exergoeconomic factor
h Enthalpy (kJ/kg)
hin Enthalpy [kJ/kg]
hnc Heat transfer coefcient for natural convection [W/m
2
K]
hfc Forced convective heat transfer coefcient [W/m
2
K]
IR Irreversibility
Kair Thermal conductivity [W/m K]
Lf Latent heat of fusion
˙
m Mass ow rate (kg/s)
˙
Q Heat rate (kW)
˙
Qc.v Rate of heat produced [kW]
˙
Qsun Solar energy [kW]
˙
mH2 Hydrogen ow rate [kg/s]
˙
XD Rate of exergy destruction [kW]
s Entropy (kJ/kg K)
T* Sun Temperature [K]
Tsur Surface temperature [K]
˙
W Work (kW)
˙
Zk Investment cost rate of components ($/hr)
Greek letters
η
en Energy efciency [%]
η
ex Exergy efciency [%]
Acronyms
CSP Concentrated solar power [-]
CRF Capital recovery factor
DNI Direct normal irradiation
Dp Depletion factor
EES Engineering equation solver
FWH Feed water heater
HRB Heat recovery boiler
LHV Lower heating value
LOEC Levelized cost of electricity
LCA Life Cycle Assessment
MSW Municipal solid wasteO
PCM Phase change material
SI Sustainability index
SDG Sustainable development goal
s-CO
2
Supercritical carbon dioxide [-]
HTF Heat transfer uid [-]
Re Reynolds number [-]
TIT Turbine inlet temperature
TES Thermal energy storage
WtE Waste to energy
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
3
using two-axis tracking system as compared to the other CSP technolo-
gies [20–21]. The present work concentrates on how to integrate
concentrated solar power system with the WtE incineration plant in a
convenient and effective way to boost the WtE plant performance suf-
ciently without increasing the corrosion in the MSW boiler tubes
simultaneously. Therefore, keeping in mind the complete scenario, this
research focuses to integrate solar central receiver tower heliostat sys-
tem with the WtE incineration plant in an innovative way. MSW incin-
eration boiler is used to rise the steam temperature up to 395 ◦C and then
an external heat recovery boiler is utilized that takes energy from solar
thermal system to further increase the steam temperature to 510 ◦C. As a
result, steam cycle efciency and performance can be improved without
any adverse effect on the incineration boiler tubes due to the corrosion.
Based on the 300 t/d incineration facility, the integrated system
conguration is thermodynamically and exergo-economically examined
and compared with the conventional one.
2. System description
2.1. Reference WtE plant
Fig. 1a depicts the reference WtE incineration plant that is selected
for the present case study and is located in People Republic of China with
300 ton/day, used to fulll the demands of a community. The power
generation capacity of the plant is merely 6.31 MW. The plant consists of
WtE boiler (incinerator), steam generating unit, electric generator and
air pollution control system. MSW as a fuel is fed into the incinerator
without pre-sorting for burning, where it converts into the ue gas that
will transfer energy to the incoming steam through superheater and an
economizer. The superheated steam produced by the boiler has turbine
inlet temperature (TIT) of almost 395 ℃ and is supplied to the turbine to
generate power without any reheating stage. Feed water preheating is
carried out by steam extraction and simplicity of the steam cycle leads to
its lower cycle efciency [4]. The waste consists of high chlorine and
sulfur that can cause the acidic salts/ gases during combustion and ul-
timately causes corrosion in the boiler. After the exit of ue gases from
the WtE boiler section, they are treated in the ue gas treatment devices
to decrease the pollution concentrations. The reference plant is modeled
using the design data listed in Table 1 and main parameters of the main
steam are almost 4000 kPa and 395 ℃. Moreover, temperature of the
exhaust gas is relatively high to prevent low temperature corrosion.
Furthermore, the combustion air is pre-heated between state points 14
and 15 to improve the combustion in incineration chamber, while low
steam parameters, simple steam cycle and limited boiler performance
due to the corrosion problems attributes to the lower cycle efciency of
Fig. 1a. Reference WtE incineration power plant.
Table 1
Ultimate and proximate analysis of feed stock (received basis) and basic design
parameters.
Item Value
Proximate analysis (wt %) Moisture 40
Fixed carbon 10
Ash 19
Volatile matter 31
Ultimate analysis (wt %) H 7.6
N 1
N 12.78
C 19.5
S 0.12
LHV (kJ/kg) 7000
MSW input (ton/day) 300
Net power output (MW) 6.32
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
4
almost 28.66 %.
2.2. Proposed integrated plant
Fig. 1(b) is the schematic of the proposed integrated WtE plant
conguration, where modication is carried out in the reference plant in
a way that solar energy is utilized by central tower receiver to provide
additional heating to the steam in the superheater of the heat recovery
boiler (HRB) coming from MSW incineration boiler (13–4). Phase
change material (PCM) is used as thermal energy storage to overcome
any interruption due to the solar uctuation and during the nighttime.
The detailed explanation on PCM is provided in authors published work
[422], while its thermo physical properties are listed in Table 2. The
Fig. 1b. Proposed solar integrated WtE incineration power plant.
Table 2
Basic input parameters for PCM.
Parameters Values
PCM (Reference) (at melting temperature) [27]
Compound LiF(46)–44NaF2–10MgF2
Fusion latent heat 858 J/g
Melting temperature 632 ◦C
Thermal conductivity 1.20 W/m K
Density 2.24 g/cm
3
Specic heat 1.40 J/g K
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
5
exhaust steam from the turbine is pumped to the MSW incineration
boiler via economizer of the HRB, where its temperature is initially
increased, aided by solar energy. Low temperature saturated steam is fed
into the incineration boiler, where its temperature is enhanced to merely
395 ◦C and later superheater of the HRB dramatically increases the
temperature up to 495 ◦C before entering to the steam turbine to pro-
duce work output. The amount of MSW to the incineration boiler is same
for both plants.
3. Methodology
Engineering equation solver (EES) is used for the thermodynamic
modeling and analysis of the investigated systems considering steady
state conditions and following assumptions [23].
•DNI is constant, while pressure losses and humidity effect are ignored
•Turbine and pumps have 0.8 isentropic efciencies [24].
•Thermal insulation of PCM tank is supposed to be imperfect [22].
Thermodynamic relations used to model the examined systems are
presented by the energy and mass balance equations [6].
˙
Q−˙
W=(˙m•h)in −(˙m•h)out (1)
˙min −˙mout =0(2)
General form of exergy can be presented as [25].
˙
Ex=˙m[(h−h0) − T0(s−s0)] (3)
In addition, equations with detailed discussion on solar tower helio
eld analysis is available in the authors previous published literature
[22], however energy and exergy efciencies of heliostat eld will be
presented by the following relations, while solar tower system design
input parameters are also available in [22]. In central receiver system,
sCO
2
is the heat transfer uid that transfer its energy to the steam in the
heat recovery boiler. Modeling equations used for heliostat eld and
central receiver system is provided in the Appendix section. Energy ef-
ciency is the ratio between receiver useful heat and solar energy
available to the receiver.
η
en =
˙
Qrec
˙
Qsun
(4)
η
ex =
˙
Ex,rec
˙
Ex,sun
(5)
Energy and exergy efciency of the central receiver can be computed
as [26]:
η
i=
˙
Qrec,abs
˙
Qrec (6)
η
ii =
˙
Exrec,abs
˙
Exrec (7)
Exergy balance and useful exergy absorbed by HTF is presented by
the equations given below [28 29]:
˙
Exrec =˙
Exrec,abs +˙
Exrec,loss +IRrec (8)
˙
Exrec,abs =˙mco2.Cp[Tout −Tin −T0lnTout
Tin  (9)
Additionally, the proposed model is integrated with high tempera-
ture phase change material (PCM) to store thermal energy for the no sun
radiations and night. Based on the heat energy available from solar
system, the PCM should have melting temperature <700 ◦C. The
detailed search of suitable PCM has been performed and ternary salt
compound LiF(46)–44NaF
2
–10MgF
2
has been selected for this applica-
tion. Properties of PCM are given in Table 2. PCM absorbs excess heat
during charging phase and release heat during off-sun hours to continue
system operation smoothly.
Total energy absorbed by the PCM will be computed as:
EPCM =
Tm
TimCpsdT +m.Lf+
Tf
TmmCpldT (10)
PCM energy conservation is given be the following equation.
dEPCM
dt =˙
Qnet,PCM (11)
Rate of heat transfer to PCM by collector is dened as:
˙
Qnet,PCM =˙
Qr−˙
QGT (12)
Moreover, TES insulation efciency can be evaluated as:
η
insu =Edischarge
Echarge
(13)
3.1. MSW incineration plant
The WtE plant capacity in this work is approximately 300 ton/day
with LHV of 7000 kJ/kg. Thermodynamic relations for solar-aided WtE
plant are given by the following equations.
Work output by the steam turbine is as:
˙
WTur =˙m8× (h4−h8) + ˙m5× (h4−h5)(14)
where h is enthalpy and m is the mass ow rate of steam in the Rankine
cycle. Additionally, pumps work can be given as:
˙
WP1=˙m6× (h7−h6) × 1
η
p
(15)
˙
WP2=˙m9× (h10 −h9) × 1
η
p
(16)
Heat rejected by the condenser can be written as:
˙
Qcond =˙m5× (h5−h6)(17)
Energy balance for heat recovery boiler (HRB) and MSW incineration
boiler can by summarized by the equations (27) and (28), respectively.
˙
QHRB =˙m1× (h1−h3) + ˙m13 × (h4−h13) + ˙m10 × (h11 −h10)(18)
˙
QMSW,incin =˙m11 × (h11 −h13 ) + ˙m19h19 −˙m15 h15 (19)
Thermal efciency of the conventional WtE plants will be:
η
th,MSW =˙
Wnet,cycle/˙
QMSW (20)
˙
Wnet,cycle is the net power available from the cycle, while ˙
QMSW is the heat
input to the boiler, given by the equations.
˙
Wnet,cycle =˙
WTurb −˙
Wp1−˙
Wp2(21)
˙
QMSW =LHVMSW ×˙mMSW (22a)
To nd the efciency of solar integrated WtE plant, ˙
Qsun will be
added in the denominator of equation (20).
η
th,MSW =˙
Wnet,cycle/˙
(QMSW+˙
Qsun)(22b)
Exergy analysis is an important and useful tool to analyze and
combine the thermodynamic analysis of the considered system by
identifying the major sources and reason of the irreversibilities.
Equation (23) states the MSW fuel exergy, based on an empirical
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
6
relation presented by [4].
˙
ExMSW =LHVMSW ×˙mMSW 1.0064 +0.1519 H
C+0.0616 O
C+0.0429 N
C
(23)
The exergetic efciency of the reference and proposed solar-aided
MSW plant is evaluated as:
η
ex,MSW,con =
˙
Wnet,cycle
˙
ExMSW
(24)
η
ex,MSW,pros =
˙
Wnet,cycle
˙
ExMSW +˙
Exsun
(25)
3.2. Exergo-economic analysis
The exergo-economic analysis is an important tool that examine the
system’s feasibility and shows the relation and linkages between exergy
and economic formulation in a way that the system’s performance can
be considerably enhanced. The SPECP (specic exergy costing method),
is brought in to by Tsatsaronis [30] that denes and estimates the exergy
unit cost of the investigated system. Table 3 presents the cost functions
for the proposed system components. The cost rate balance with general
formulation can be dened as:
˙
Cq,k+
i
˙
Ci,k+˙
Zk=
i
˙
Ce,k+˙
Cw,k(26)
where
˙
C=c˙
Ex (27)
here c is the per unit exergy cost. Equation (36) states that the sum-
mation of the cost rates linked with the leaving exergy streams are equal
to the total investments cost, operation and maintenance costs plus the
cost rates of entering exergy streams for the kth component. The term ˙
Zk
can be evaluated as:
˙
Zk=˙
ZCI
k+˙
ZOM
k=CRF.Zk.φ
N×3600(28)
In the above equation N and φ are the annual operating hours of the
plant (7800 hrs.) and maintenance factor (1.06). Equation (38) calculate
the capital recovery factor as:
CRF =i(1+i)n
(1+i)n−1(29)
The fuel ( ˙
Cf,k) and product ˙
(CP,k)cost rates of each system compo-
nents can be examined and assessed using fuel and product (F-P) de-
nition and concept [31] expressed by exergy analysis (Table 4).
Furthermore, cost rate of exergy destruction can also be evaluated to
cover the cost rate of fuel supplied for each component because of the
irreversibilities.
cF,k=
˙
Cf,k
˙
Exf,k
(30)
cP,k=
˙
CP,k
˙
ExP,k
(31)
˙
CD,k=cF,k×˙
Exf,k(32)
Equation (33) and (34) summarizes the levelized cost of electricity
and payback period as follow:
LCOE =Cinv.CRF.φ
Enet
(33)
Pay back period =Cinv
Ccash,flow
=Cinv
Enet.LEC (34)
Sustainability index is used to measure the impact of exergy
destruction of the investigated system on the environment and dened
as [4]:
SI =1
Dp
(35)
Dp is the depletion factor dened as the ratio between total exergy
destruction and input exergy to the system.
Dp=
˙
Edest,tot
˙
Exin
(36)
Table 3
Cost functions for WtE integrated plant.
Component Cost function
Heliostat [29] Zhelio =50Ah.N
Receiver Zrec =Ar.(79.Tr−42000)
PCM [20] Ztank =Kpcm
1+Kpcm
2log10(V) +
Kpcm
3log10(V)2
Zpcm =247mpcm
Heat Recovery Boiler ZHRB =5.805 −0.1653ΔTpp +0.0153.mrec
WtE plant [1132]
Steam turbine ZSt.Turbine =6000˙
Wnet,hpt0.71
Condenser Zcond.=1773mcond.
Pumps Zpump =3540˙
Wpump0.71
Mixing chamber ZMC.=6014mMC
Feedwater heater ZFWH =66 ˙
Q1
T+c90.1
Incinerator boiler Zincin.=2567(3600mst)0.67
Air pollution control unit ZAPC =0.06Cinv,
Auxiliary systems
[33] Zaux =10 ×106×˙
Wnet
120,000

0.65
Contingency 10 % of direct cost
Engineering, procedure,
construction
15 % of direct cost
Project, land, management 3.5 % of direct cost
Table 4
F-rule, P-rule and cost balance equations.
Component F P Cost equation
Solar receiver ˙
Exhelio ˙
Ex1−˙
Ex3 ˙
C3+˙
Csun +˙
Zrec =˙
C1
PCM ˙
ExF,recv −˙
ExL,recv ˙
Ex1−˙
Ex1s ˙
C1+˙
Zpcm =˙
C1s
HRB ˙
Ex1+˙
Ex13 +
˙
Ex10
˙
Ex3+˙
Ex4 ˙
C1+˙
ZHRB +˙
C10 =˙
C3+
˙
C10
Incinerator ˙
Ex19 +˙
ExMSW +
˙
Ex11
˙
Ex15 +
˙
Ex13
˙
CMSW +˙
ZIncin.+˙
Cair =
˙
Cflue +˙
Cashcflue =cash ,
cair =0; cMSW =
0.002$/MJ
APH ˙
Ex15 −˙
Ex16 ˙
Ex19 −˙
Ex18 ˙
C18 +˙
ZAPH +˙
C15 =
˙
C16 +˙
C19
APC ˙
Ex16 ˙
Ex17 ˙
C16 +˙
ZAPC =˙
C17
Steam Turbine ˙
Ex4−˙
Ex8−˙
Ex5 ˙
Wst.turb ˙
C4+˙
Zst.tur =˙
C8+˙
C5+
˙
Cw,st.tur
Condenser ˙
Ex5−˙
Ex6 ˙
Exwi −˙
Exwo ˙
C5+˙
Zcond.+˙
Cwi =˙
C6+
˙
Cwo
Pump1 ˙
Wp1 ˙
Ex7−˙
Ex6 ˙
C6+˙
ZP1+˙
Cw,P1=˙
C7
Pump2 ˙
Wp2 ˙
Ex10 −˙
Ex9 ˙
C9+˙
ZP2+˙
Cw,P2=˙
C10
FWH ˙
Ex7+˙
Ex8 ˙
Ex9 ˙
C8+˙
Zfwh +˙
C7=˙
C9
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
7
3.3. Supercritical carbon dioxide properties
The thermo physical properties of the sCO
2
are investigated at
various pressure and temperature levels Fig. 2 (a, b). EES is used to
evaluate the data from 350 K to 850 K and pressure at 80 bar to 120 bar.
It is well known that the critical point of CO
2
is 78 bar and 305 K,
approximately [34]. Fig. 2 concludes that specic heat capacity of sCO
2
is decreased to almost 27.55 % and 104.7 %, respectively at 80 bar and
120 bar as temperature increases from 350 K to 850 K, while 233.6 %
and 322.5 % density reduction is observed at both mentioned pressures.
This signicant reduction in the density is important for utilization of sC
O
2
in the solar collectors [22]. In addition, thermal conductivity of sCO
2
is increased with rise in the inlet temperature as presented in Fig. 2(b).
3.4. Model validation
The validation of the investigated reference WtE incineration plant
and heliostat central system has been performed and results are
compared with the existing literatures. Fig. 3 (a & b)) shows the vali-
dation of solar thermal system and Rankine cycle, respectively. Heliostat
receiver thermal efciency of the proposed and the reference model is
compared according to the input conditions [35]. Thermal efciency
enhances as led efciency will rise; present model has almost 0.25 %
less efciency than the reference model, shows an accuracy of the pro-
posed model. In addition to this, thermal efciency of the steam cycle is
evaluated at different condenser pressures and design conditions listed
by Cengel and Boles [36]. Both models show reduction in the thermal
efciency with rise in the condenser pressure but the present cycle
model has almost 0.90 % less efciency than the reference model, Fig. 2
(b).
4. Results and discussion
The present part of the work demonstrates the thermodynamic and
exergo-economic ndings after the analysis of considered examined
models performed using engineering equation solver (EES). Different
operating parameters are changed to examine their effect on the per-
formance of both systems.
Fig. 4(a) is the graphical representation between turbine inlet tem-
perature (TIT) and efciencies, network output and total exergy
destruction rate of the proposed integrated plant. Increasing TIT from
650 K to 800 K, results in the gradual enhancement in the MSW energy
and exergy efciencies from 28.03 % to 33.48 % and 25.3 % to 30 %,
respectively. The enthalpy of the working uid becomes higher as TIT
will increase, which results in more work output from the turbine.
Finally, efciencies of the system increase in a linear mode.
Additionally, turbine power improves to almost 19.42 % with rise in the
TIT and exergy destruction rate of the proposed model decreases from
25784 kW to 24823 kW (almost 3.87 %) due to the improvement in the
system performance at greater TIT.
Cost rate of exergy destruction and total cost rate of the proposed
system are noticed to be reduced to almost 16.32 % and 12.27 %,
respectively with rise in the TIT due to the increase in the exergy inow
and network output as described by Fig. 4(b). Moreover, levelized
electricity cost has a decreasing trend from 0.1148 $/kWh to 0.096
$/kWh because of the improvement in the total energy output presented
by the equation (46). However, sustainability index (equation 48) is
increased to nearly 3.87 % as TIT rises from 650 K to 800 K. This is due
to the reason that at higher TIT, workout put from the cycle enhances
with less exergy destruction and simultaneously inow enthalpy and
exergy becomes higher. Exergo-economic factor is also plotted in the
same gure and it is noticed to be increased to approximately 13.89 % as
TIT will vary.
The performance comparison between reference and solar integrated
WtE plant is conducted against turbine inlet pressure (TIP), Fig. 5(a, b).
Thermal efciency of the proposed plant is increased to almost 10.81 %
as turbine inlet pressure (TIP) rises between 3000 kPa and 8000 kPa and
it has nearly 12.57 % more efciency as compared to the conventional
WtE plant at same input conditions. In addition, exergy efciency of the
proposed and conventional WtE plant is improved from 28.11 % to
31.15 % and 24.97 % to 27.45 %, respectively that depicts, proposed
model is 13.47 % more efcient than conventional plant. At higher
turbine inlet pressure, more work output is delivered by the turbine that
further boosts the system performance. The secondary y-axis of the
Fig. 5(a) shows the variation in net work output and exergy destruction
rates of the conventional and proposed WtE plants. The net work output
of the proposed cycle is increased from 6868 kW to 7612 kW and it is
almost 13.51 % greater than the conventional plant. Simultaneously,
total exergy destruction rate for both investigated plants have gradual
decreasing trend with 2.63 % reduction with increase in the turbine inlet
pressure.
On the other hand, primary y-axis of Fig. 5(b) portrays the inuence
of varying TIP on the exergetic destruction and the total cost rate, while
levelized electricity cost and sustainability index are presented on the
secondary y-axis. Exergetic destruction cost of proposed model reduces
from 894.1 $/hr to 831.8 $/hr and it is almost 53 % higher than the
conventional model and latter has very slight variation during pressure
change. It is due to less number of components in the conventional plant
as compared to the proposed plant (inclusion of solar tower, heliostat,
PCM and HRB) and all these components have considerable initial cost.
Total cost rate is the summation of investment and exergetic
destruction cost rate, shows a 5.53 % downward trend for proposed
Fig. 2. Supercritical carbon dioxide properties; (a) specic heat capacity & density; (b) dynamic viscosity & thermal conductivity.
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
8
model (Fig. 5b), while it has slightly increased for conventional plant
(almost 0.37 %). Specically, total cost rate of the conventional plant is
approximately 67.46 % lower than the integrated plant.
Levelized electricity cost of the proposed plant reduces from 0.1033
$/kWh to 0.093 $/kWh and is almost 13.96 % lower than the conven-
tional plant due to the delivery of more net work output by the proposed
plant. In addition to this, sustainability index for proposed and con-
ventional plants are increased to nearly 2.41 % and 2.62 %, respectively
because of the decrease in the total exergy destruction rates at higher
turbine inlet pressure.
Extracted steam fraction from the turbine is an important parameter,
which affects the performance of the system by reducing net work
output (Fig. 6 a). Net workout put of the proposed model is experienced
to be decreased from 7660 kW to 6480 kW, while 19.24 % net power is
reduced for the conventional plant by increasing steam fraction from
0.1 kg/s to 2 kg/s. Consequently, total exergy destruction rate for both
the systems increases; 3.83 % and 4.8 % for the proposed and reference
models, respectively. Energy efciency of the integrated and refernce
plants are reduced from 34.74 % to 29.39 % and 30.96 % to 25.96 %,
respectively, due to the less net work output as maximum portion of
steam is directed towards feed water heater.
The cost of exergetic destruction and total cost rate of the proposed
and conventional plants have minor enhancement between 0.1 kg/s and
0.48 kg/s and then gradual decline (Fig. 6 b). However, proposed plant
has approximately 53.5 % and 64.24 % more exergy destruction cost
and total cost rate than reference plant, respectively. Additionally, LEC
and SI for both systems increase linearly as shown by the secondary y-
axis.
Direct normal irradiance (DNI) has pronounced effect on the per-
formance of the solar integrated WtE proposed system [17]. Higher
amount of DNI results in more useful heat from the solar receiver that
will further enhance the greater TIT and ultimately delivers maximum
turbine net work output. Fig. 7(a) presents the inuence of DNI variation
on the energy and exergy efciencies of the proposed model. Former
Fig. 3. Validation for the proposed model; (a) Heliostat eld efciency vs receiver thermal efciency; (b) Pressure ratio vs thermal efciency.
Fig. 4. TIT inuence on (a) efciencies, net work output and total exergy destruction; (b) total cost rate, exergy destruction cost, LEC and SI of the integrated system.
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
9
increases from 24.6 % to 33.54 %, whereas later one will vary between
22.2 % and 30.27 % due to the enhancement in the net work output and
inow exergy, while reduction of exergy losses at higher DNI. Further-
more, exergo-economic factor of the integrated system increases to
nearly 23.34 % as DNI will vary.
Fig. 7(b) depicts the variation in the exergy destruction and total cost
rate of the proposed integrated model and both are reduced to almost
27.14 % and 20.50 %, accordingly with rise in the DNI. Levelized
electricity cost is observed to be 36.53 % reduction because of the
addition of net electricity at higher DNI. This phenomenon further en-
hances the sustainability due to the low exergy destruction rate [4].
Fuel and product exergy, exergy destruction rate and cost of exergy
destruction for individual components of proposed and conventional
plants are presented by bar charts in Fig. 8(a, b), respectively. In pro-
posed plant, the highest exergy destruction is found for MSW incinera-
tion boiler, 12530 kW (50 %), while its exergy destruction cost is 175.2
$/hr followed by solar receiver (20.56 %) and air pre-heater (13.26 %).
Apart from the pumps, lowest exergy destruction is observed for feed
water heater and condenser that is 175.4 kW and 333.4 kW,
respectively.
On the other hand, exergy destruction rate for the MSW boiler in
conventional plan is nearly 14723 kW with 209.5 $/hr exergetic cost
rate of destruction. Air pollution control (APC) has approximately 1487
kW of exergy destruction, while its cost rate is 199.6 $/hr followed by
the steam turbine. The lowest exergy destruction rate in this system is
also for feed water heater and condenser.
Fig. 9(a, b) illustrates the exergy ow that occur in the conventional
and proposed MSW plants as it detects the root cause of performance
boosting for the proposed integrated scheme. The exergy input of the
MSW for both schemes is identical, however solar input exergy is the
additional input for the proposed integrated plant. Exergy carried by the
main steam from boiler to steam turbine for conventional plant is 7.97
MW, while steam returned from turbine to the MSW boiler is identical
(0.592 MW) for conventional and the proposed plant. Moreover, in the
proposed case, apart from the MSW boiler, exergy from solar receiver
augments the total exergy to the heat recovery boiler (HRB). Therefore,
steam working capability is improved and net exergy from the steam
turbine is increased in a considerable manner that will further enhance
Fig. 5. TIP impact on the (a) efciencies, net work output and total exergy destruction; (b) total cost, exergy destruction cost, SI and LEC of the reference and
proposed system.
Fig. 6. Extracted mass fraction impact (a) efciencies, net work output and total exergy destruction; (b) total cost rate, exergy destruction cost, SI and LEC of the
reference and proposed system.
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
10
Fig. 7. Effect of DNI on (a) efciencies, net work output and total exergy destruction; (b) total cost rate, exergy destruction cost, SI and LEC of the proposed system.
Fig. 8. Comparison of the fuel & product exergy, exergy destruction rate and exergetic destruction cost rate of individual components between (a) proposed model;
(b) reference model.
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
11
the proposed system performance. Finally, the above presented diagram
clear portrays the exergy inow, outow and exergy losses across the
system components.
Fig. 10 shows the hourly heat absorbed from the solar source and
excess energy stored in the thermal storage unit. Hourly solar ux data
has been taken from [Reference] and calculations have been made. The
solar concentrator has reectivity of 0.92 and solar receiver is consid-
ered to has maximum 30 % heat losses to the environment at the peak
solar ux and heat losses decreased with solar ux accordingly. It can be
seen in the Fig.? that solar ux started to increased from 8 AM and
working uid started to absorbs heat. The extra heat is stored in the PCM
based thermal unit while turbine is in running condition. Heat storage
(charging phase) started from 8 AM and when solar ux decreased,
stored heat used to run the system operation. It can be seen that after
4:30 PM, the heat storage in the PCM is used and discharging phase
started. Theoretically, stored heat can be used for system operation for
about 12–13 h of the low solar ux and off-sun operations.
5. Conclusion
In this study a novel solar integrated waste-to-energy plant is pro-
posed, investigated and compared its performance with the conven-
tional waste-to-energy plant by conducting thermal, exergetic and
exergo-economic analysis. The useful energy from the solar central
receiver tower system is utilized to substantially increase the tempera-
ture of the steam coming from municipal solid waste incineration boiler
by an external heat recovery boiler. In this way, turbine inlet tempera-
ture and net power output from the cycle increases remarkably. Based
on the 300 t/d incineration facility, proposed integrated design is
evaluated and parametric analysis has been conducted to access the
Fig. 9. Exergy ow chart of (a) Reference plant; (b) Proposed plant.
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
12
system’s performance by varying the more inuencing operating pa-
rameters. The major ndings are listed as follows:
The net work output of the proposed solar integrated plant is almost
12.61 % greater than the reference plant while, thermal efciency of
proposed plant increases to 19.4 % as turbine inlet temperature will
boost from 650 K to 800 K.
The exergetic destruction cost rate of the proposed plant is around
873.1 $/hr and 53.47 % higher than the conventional design due to the
addition of solar thermal system. However, levelized electricity cost of
the former system is 13.9 % lower as compared to the latter one.
Furthermore, steam mass fraction from the turbine to the feedwater
heater has considerable reduction on the cycle power output due to the
increase in the exergy destruction rate. Sustainability index of the plant
decreases to approximately 3.8 % by rise in the steam fraction.
The performance of the proposed integrated system can be
augmented under the wide range of direct normal irradiance. Thermal
efciency of the plant rises from 24.6 % to 33.5 % as direct normal
irradiance varies between 660 W/m
2
to 900 W/m
2
with 36.5 % and
27.14 % reduction in levelized electricity cost and exergy destruction
cost rate, respectively.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
Authors are very thankful to the International Cooperation Project of
Zhejiang (2019C04026) for their nancial support.
Appendix A
Modeling equations for heliostat eld.
Solar energy ˙
Qsun =DNI ×Ahel ×Nhel
Solar radiations on the tower ˙
Qrec =˙
Qsun ×
η
hel
Heliostat eld energy ˙
Qsun =˙
Qrec +˙
Qloss
Exergy balance ˙
Ex=˙
Exrec +˙
Exloss
˙
Ex=˙
Qsun × [1−To
Tsun]
Exergy for receiver surface ˙
Exrec =˙
Qrec × [1−To
Tsun]
Energy and exergy balance equations for central cavity receiver.
Fig. 10. Hourly heat stored in the PCM based thermal storage unit.
M. Sajid Khan et al.

Applied Thermal Engineering 223 (2023) 119929
13
Energy balance ˙
Qrec =˙
Qrec,abs +˙
Qrec,loss
Heat loss ˙
Qrec,loss =˙
Qrec,conv +˙
Qrec,em +˙
Qrec,ref +˙
Qrec,rad ˙
Qrec,em =
ε
×δ× (T4
rec,sur −T4
amb) × Ahel
Cr
˙
Qrec,abs =˙
mco2cp.co2(Tout −Tin)˙
Qrec,ref =˙
Qsun ×
ρ
ref ×Fi˙
Qrec,conv =
hnc.Arec .(Tsur −Tamb) + hfc .Arec.(Tsur −Tamb )˙
Qrec,rad =
ε
.Arec.Fi.(T4
s−T4
amb)
Natural convection heat
transfer
hnc =0.81(Trec.sur −Tamb)0.426
Force convection heat
transfer hfc =Kair
L0.0287Re0.8
air.insiPr0.33
air.insi
Loss of exergy ˙
Exrec,loss =˙
Qrec,loss.[1−T0
Trec,sur]
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