Comparative study of flexibility enhancement technologies for the [601129]

Energy Conversion and Management
Manuscript Draft

Manuscript Number: ECM -D-18-05848

Title: Comparative study of flexibility enhancement technologies for the
coal-fired combine d heat and power plant

Article Type: Original research paper

Section/Category: 2. Energy Distribution and Resource Management

Keywords: Flexibility enhancement; Combined heat and power (CHP);
Operational strategy; Thermodynamic performance; Techno -economic analysis

Abstract: The flexibility enhancement of the coal -fired combined heat and
power (CHP) plant can contribute to both renewable energy integration and
decarbonization. In this research, we classify the flexibility
enhancement technologi es into the "power to heat (P2H)" mode and
"additional heat source (AHS)" mode based on their principles and
operational strategies. Then thermodynamic performances of both modes are
compared with both the designed heat load and one -day loads. After that,
the techno -economic analysis is also conducted. Results show that with
the designed heat load, the power load factor range of the CHP plant is
extended from 54.87%~78.72% to 25.20%~100% with the AHS mode, and to 0
~78.72% with the electric boiler. And comp ared to integration with the
heat-only boiler, the coal consumption of CHP plant can be increased by
2.27% with the thermal energy storage (TES) system (Electric boiler), and
reduced by 0.78%, 1.85%, 2.33%, 3.34% with the electric boiler, electric
heat pum p, TES system (Extraction steam) and waste heat recovery,
respectively. The techno -economic analysis indicates that during the
heating period, the net annual revenues of the TES system and electric
boiler are 2.14 M$ and 1.71 M$ higher than that of the hea t-only boiler.
This study gives a methodology on choosing the technology to enhance the
flexibility for the coal -fired CHP plant.

Highlights:
1. The flexibility can be improved by the P2H mode and AHS mode .
2. The operational strategies of both mode s are presented and compared .
3. Thermodynamic and techno -economic performances are investigated .
4. The CHP plant consumes 3.34% less coal with the waste heat recovery.
5. The net annual revenue of the TES system is 2.14 M$ higher.
Highlights (for review)

1
Comparative study of flexibility enhancement technologies for the coal -fired 1
combined heat and power plant 2
Shifei Zhaoa,b, Zhihua Gea*, Jian Suna, Yulong Dingb, Yongping Yanga* 3
a National Thermal Power Engineering and Technology Research Centre, North 4
China Electric Power University, Beijing, 102206, China 5
b School of Chemical Engineering, University of Birmingham, Birmingham, B152TT, 6
United Kingdom 7
*Corresponding author ：Zhihua Ge; Yongping Yang 8
Abstract : 9
The flexibility enhancement of the coal -fired combined heat and power (CHP) plant 10
can contribut e to both renewable energy integration and decarbonization . In this 11
research, we classify the flexibili ty enhancement technologies into the “power to heat 12
(P2H) ” mode and “additional heat source (AHS) ” mode based on their principles and 13
operational strategies . Then thermodynamic performances of both modes are compared 14
with the designed heat load and one -day loads . After that, the techno -economic analysis 15
is also conduct ed. Results show that with the designed heat load, the power load factor 16
range of the CHP plant is extended from 54.87%~78.72% to 25.20%~100% with the 17
AHS mode, and to 0 ~78.72% with the elect ric boiler. And compared to integration with 18
the heat -only boiler, the coal consumption of CHP plant can be increased by 2.27% with 19
the thermal energy storage ( TES) system (Electric boiler), and reduced by 0.78%, 20
1.85%, 2.33%, 3.34% with the electric boile r, electric heat pump, TES system 21
(Extraction steam) and waste heat recovery, respectively. The techno -economic analysis 22 *Manuscript
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2
indicates that during the heating period, the net annual revenues of the TES system and 23
electric boiler are 2.14 M$ and 1.71 M$ higher than that of the heat -only boiler. This 24
study gives a methodology on choosing the technology to enhance the flexibility for 25
the coal -fired CHP plant. 26
Keywords: Flexibility enhancement; Combined heat and power (CHP); Operational 27
strategy; Thermodynamic perf ormance ; Techno -economic analysis 28
Nomenclature 29
AHS Additional Heat Source
CHP Combined Heat and Power
COP Coefficient of Power
CWP Circulating Water Pump
DH District Heating
EB Electric Boiler
EC Extraction Condensing
EHP Electric Heat Pump
HB Heat-Only Boiler
HPRH High -Pressure Regenerative Heater
HPT High -Pressure Turbine
IPT Intermediate -Pressure Turbine
LHV Lower Heating Value
LPRH Low-Pressure Regenerative Heater
LPT Low-Pressure Turbine

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O&M Operation and Maintenance
P2H Power to Heat
RE Relative Error
TES Thermal Energy Storage
THA Turbine Heat Acceptance
TMCR Turbine Maximum Continuous Rating
TNH Thermal Network Heater
TNP Thermal Network Pump
Symbol 30
b Standard coal consumption of the boiler, t/h
B
Gross coal consumption, t
Nc
Nominal investment, M$/MWh (TES system), M$/MW (others)
coalc
Price of the standard coal, $/t
EC
Investment cost of the equipment, M$
iC
Annualized investment cost, M$/year
&OMC
Operation and maintenance cost difference, M$
Pf
Power load factor, %
CF
Capacity factor, %
LD Demanding load, MW
CHPP
Power output of the CHP plant, MW
MAX
CHPP
Installed power capacity of the CHP plant, MW
PD Demanding power load, MW

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HD Demanding heat load, MW
2PHP
Additional electric power for the P2H equipment
2PHH
Heat production of the P2H equipment
r Fraction interest rate per year, %
nR
Net annual revenue, M$
S Scale of the equipment, MWh (TES system), MW (others)

Lifecycle of equipment
c
Charging efficiency of the TES system, %
d
Discharging efficiency of the TES system, %
EB
Efficiency of the Electric Boiler, %
e
Generating efficiency, %
s
Storage efficiency of the TES system, %
1. Introduction 31
Large -scale integration of the variable renewable sources, particularly wind and solar, 32
in the existing power system requires increasing flexibility in the supply -side [1]. The 33
flexibility of a power plant can be evaluated mainly from three respects: the low -load 34
operation, the fast start-stop capacity, and multiple fuel compatibility. Concerning the 35
minimum power output, the low -load operation represents how much space the power 36
plant can provide for the renewable energy in the electric power grid [2]. 37
China is a country where the coal -fired combined heat and power (CHP) plant plays 38
an important role in both the electricity and district heating (DH) industries. In northern 39
China, the install capacity of the CHP unit reached to 211 GW in 2014, which accounted 40

5
for 43% of the gross install c apacity in this area, and 30% of the total install capacity 41
in China. And the share can be large r because of the reduction of the coal -fired heating 42
boiler. In China ’s DH system, the coal -fired CHP plants cover nearly half (48%) of the 43
gross heating demand [3]. The peak shaving performance enhancement of the coal -fired 44
CHP unit can significantly improve the ability of the electric grid to accept the variable 45
renewable energy. According to the object of National Energy Administration of China, 46
the C HP plant should extend its peak shaving ability by 20%, and the capacity factor 47
should reach 40%~50% in the heating period [4]. 48
For the coal-fired power plant , the low-load operation is mainly restricted by the safe 49
operation and pollutants emission , which have attracted many researchers. Wang et al. 50
[5] gave the suggested allowable oil -free minimum load for the power plant based on 51
the operational condition s of the boiler . And the oxygen -enriched ignition and plasma 52
ignition technology are considered as effective methods for the coal -fired boiler to 53
operate in the low load. As for the steam turbine, Ahmad et al. [6] pointed out that low 54
load operation would exacerbate erosion damage due to the form vortex zone at the 55
rotor blade root and static cascade top export . Zhao et al. [7] indicated that turbine 56
therma l storage could be used to improve the flexibility of the power plants. Wang et 57
al. [8] proposed a novel water -fuel ra tio control strategy based on heat storage 58
difference, which reduced the accumulation deviations of load rate command and real – 59
time load rate . Pollutants emission control technologies mainly focus ing on the NO X 60
are also concerned. Papoas et al. [9] enhanced the deNOx performance in the low- 61
temperature regime of 100~300 oC by the manganese confined titania nanotubes 62

6
(Mn/TNT). Gao et al. [10] proved that nearly 100% NOx conversion was maintained 63
at 150 ~240 °C with the novel Mn/CeO 2 microspheres. 64
For the coal -fired CHP plant, b esides the above restrictions, t he strong relations hip 65
between the heat and power loads of the CHP plant and the policy of “Ordering Power 66
by Heat ” make s the flexibility worse [11]. On the other hand, such characters also bring 67
new method s for the flexibility enhancement by regulati ng the heat and power loads, 68
such as the high-pressure steam bypass system, electric boiler and energy storage 69
system [12-14]. Bloess et al. [15] reviewed the state -of-the-art technologies for the 70
renewable energy integration, pointed out that heat pumps and thermal energy storage 71
(TES) emerged as the favourable options. Ivanova et al. [16] indicated that the benefit 72
of the electric boiler integration with the CHP plant relied on the share of intermittent 73
generation and level of biomass energy sources . Chen et al. [17] gave research on the 74
performance of the TES tank with phase change material (PCM) on middle temperature 75
suitable for the D H system. She et al. [18] improved the liquid air energy storage by 76
introducing an organic Rankine cycle in the compression process , bringing 9.6% 77
increment of the exergy efficiency. Su et al. [19] introduced the technologies for the 78
flexibility enhancement of the coal -fired power plant, including the TES and ele ctric 79
boiler , and compared the scope of application and tech -economic performance. Besides , 80
heat pumps (including the electric, chemical and absorption heat pumps) , which are 81
widely used in the heat supply side mainly to improve the heating efficiency and 82
capacity [20-23], also have the potential in the flexibility enhancement . because they 83
can also change the heat and power relationship of the CHP plant by improving heat 84

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load with or without power consumption. Song et al. [24] provided a novel mechanism 85
by using an electric heat pump to recover the waste heat of the CHP plant. Li et al. [25] 86
indicated that with the absorption heat pump, the heating capacity of the CHP plant can 87
be increased by 52% and the energy efficiency can be improved by more than 50%. 88
The chosen of the technologies should concern the extent of the peak shaving, as well 89
as the thermodynamic and economic performances. In this research, we implement a 90
comprehensive comparative study of flexibility enhancement technologies for the coal – 91
fired CHP plant , which will help the coal -fired CHP plant stay competitive in the energy 92
transition process . 93
The study starts with the introdu ction of flexibility limit of the coal -fired CHP plant . 94
And t hen, technologies for the flexibility enhancement are presented and classified 95
based on their principles. Next, the operational strategy of each mode and the 96
corresponding coal consumption are gi ven. Following this, thermodynamic analysis 97
with both the designed heat load and one -day heat and power loads are conducted to 98
compare the operational strategies and coal -saving potentials. Finally, we discuss the 99
techno -economic performances of these tech nologies. 100
2. Flexibility of the coal -fired CHP plant 101
2.1 Coal-fired CHP plant 102
The coal-fired CHP plants usually have different modes due to the heat sources for 103
the DH system : the low vacuum mode based on the exhausted steam, the extraction 104
condensing (EC) mode based on the extraction steam [26]. Among all the modes , the 105
EC mode is the most common ly used in China, whose process is shown in Fig.1. In the 106

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typical CHP p lant with EC mode, the steam extracted between the intermediate – 107
pressure turbine (IPT) and low-pressure turbine (LPT) is used to heat the water of the 108
DH system, and the drain water returns to the deaerator or the condenser in the CHP 109
plant. Except where otherw ise stated, the CHP plant in this research refers to the CHP 110
plant with EC mode. In this study, a 300MW coal -fired CHP plant is chosen as the 111
reference system, whose main thermodynamic parameters are listed in Table.1. 112
2.2 Flexibility limit and enhancement 113
The heat and power producti ons of the CHP unit are coupled and restricted by the 114
feasible operation region, which is illustrated in Fig.2 [27]. The boundaries of AB, BC, 115
CD represent the maximum fuel, the maximum heat extraction, and the minimum fuel. 116
It can be seen that with the demanding heat load ( HD), the power load rang e is limited 117
from
MAX
DP to
MIN
DP , which is more narrow than the power -only plant [28]. Thus, to 118
meet the demanding load(
( , )D D DL H P ), the most common method is to use the coal – 119
fired heat -only boiler (HB) to reduce the heat load of the CHP plant, as well as the lower 120
limit of the power load . However, such a metho d will decrease the advantage of the 121
CHP plant in the energy system. 122
Apart from the heat -only boiler, there are other technologies to achieve the lower 123
limit of the power load, which are mainly based on two mechanisms . First, like the heat- 124
only boiler, the other auxiliary heat sources can also cover the heat load gap between 125
the heat load of CHP plant and the demanding heat load , which is “auxiliary heat source 126
(AHS) ” mode . Besides, the CHP plant can convert some electric power to heat 127
production , which is “power to heat (P2H) ” mode . The role of the P2H and AHS modes 128

9
to enhance the flexibility of the CHP plant can be seen in Fig.3 . 129
The main t echnologies of each mode are shown in Fig.4 [29, 30] . In the P2H mode, 130
the electric boiler used for the large -scale DH system is often in the form of the 131
electrode boiler wi th the high voltage . And i n the AHS mode, the waste heat recovery 132
contain s the renewable energy, absorption heat pump or other waste heat recovery 133
processes . And it is considered as no addition to the coal consumption of the CHP unit 134
to show the maximum coal -saving potential of the AHS mode. 135
3. Mode lling and Operational strategy 136
3.1 Operational strategy 137
“Power to Heat (P2H) ” mode and “Auxiliary Heat Source (AHS) ” mode lead to 138
different operational strategies, and the CHP plant need s to regulate its load to match 139
up with the requirement . To reach the demanding load ( LD) out of the feasible 140
operational region, Fig.5 gives the operational points of the CHP plant with bo th modes. 141
With the P2H mode, the CHP plant should operate in point
2PH
DL with some power 142
(
2PHP ) converting to heat (
2PHH ) to reach
( , )D D DL H P , shown in Fig.4. And the 143
standard coal consumption of the boiler (
2()PH
DbL )can be given as: 144
2
22 ( ) ( + , )PH
D D P H D P Hb L b P P H H  
(1) 145
where
2PHP denotes the additional electric power for the P2H and
2PHH is the 146
heat load c onverted by
2PHP . 147
For the electric boiler, 148
2 EB 2=P H P HHP
(2) 149
where
EB is the electric boiler efficiency (99%) [12]. 150

10
And for the electric heat pump, 151
22=P H P HH P COP
(3) 152
where COP is the coefficient of the power of the electric heat pump (COP=3) [31]. 153
With the AHS mode, t he CHP plant works in the point
AHS
DL and
AHSH is the heat 154
provided by the auxiliary heat source, shown in Fig.5. So, the standard coal 155
consumption of the AHS mode (
()AHS
DbL ) can be expressed as: 156
( ) ( , )AHS
D D D AHSb L b P H H 
(4) 157
For the heat -only boiler, the additional standard coal consumption (
HBb ) can be 158
given as: 159
AHS
HB
HBHbLHV
(5) 160
where
HB is the efficiency of the coal -fired heat only boiler (
HB =70%), and LHV is 161
the lower heating value of the standard coal ( LHV =29307.6kJ/kg) [32]. 162
And for the charging process of the TES system, the additional standard coal 163
consumption (
TESb ) will be different due to the different heat sources. When the 164
electric power is used in the charging process,
TESb can be given as: 165

= ( , )- ( , )TES D TES D D Db b P P H b P H  (6) 166
And the charging load of the TES system (
TESH ) is: 167
=TES c TESHP
(7) 168
where
c is the charging efficiency of the TES system (
c =95%) [33]. 169
Similarly, when the extraction steam is used in the charging process,
TESb can be 170
given as: 171

= ( , ) ( , )TES D D TES D Db b P H H b P H   (8) 172

11
and 173
=TES c TESHH
(9) 174
For the TES sy stem, the gross discharging power should cover the demanding heat 175
load of the auxiliary heat source: 176

11=lm
ij
AHS s d TES
ijHH
 (10) 177
where
s presents the storage efficiency of the TES system (
s =99.8% ) and
d 178
denotes the discharging efficiency of the TES system (
d =95%) [33]. 179
3.2 Model of the coal -fired CHP plant 180
EBSILON Professional software is used in this study to build the thermodynamic 181
model , which can calculate the potential performance behaviour and efficiency of the 182
power plant under the wid e range of operating conditions [34]. The model details of the 183
main components selected in this study are shown in Table 2 [35]. 184
Based on the model, Fig. 6 compares the design data gi ven by the manufacturer and 185
simulation results based on the Turbine Maximum Continuous Rating (TMCR) 186
condition and Turbine Heat Acceptance (THA) conditions. It can be seen that the model 187
is precise with the relative errors (RE) lower than 0.4%. 188
3.3 Thermodynam ic index 189
Besides the coal consumption of the boiler, t he power load factor , capacity factor and 190
generating effic iency are used to evaluate the thermodynamic performance of the CHP 191
units. The calculation of the power load factor (
Pf) is given as: 192
100%CHP
P MAX
CHPPfP
(11) 193

12
where
CHPP denote the power output of the CHP plant ,
MAX
CHPP is the maximum power 194
capacity of the CHP plant . 195
The c apacity factor (
CF) is the unitize ratio of an actual power output to the 196
maximum possible power output over th e same time period, given as [36]: 197
1
1100%n
k
CHP k
k
C n
MAX
CHP k
kPt
F
Pt


(12) 198
where
kt is the duration with the power load
k
CHPP . 199
In the CHP units, the generating efficiency (
e) can be expressed as: 200
100%D
e
CHP DP
b LHV H
(13) 201
4. Results and analysis 202
4.1 Thermodynamic performance with the designed heat load 203
In this chapter, the thermodynamic performance of the CHP plant with designed heat 204
load is analyzed. Based on the operational strategy of each mode, Fig. 7 gives the 205
potential of the flexibility enhancement by different technologies. 206
With the designed heat load (320 MW), th e power production of the CHP plant can 207
change from 253.63 MW to 176.79 MW . With the AHS mode, the power load range is 208
extended from 322.20MW to 81.20MW . As for the P2H mode, the maximum power 209
output remains constant, and the minimum power production can reach 0 MW with the 210
electric boiler and 13.34 MW with the electric heat pump (COP=3). 211
Fig.8 gives the generating efficiency and standard coal consumption of the CHP plant 212
with the heat -only boiler. To reach the maximum power output (322.20MW) from 213

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253.63 MW , the standard coal consumption of the heat -only boiler increases by 55.44 214
t/h, leading to a drop in generating efficiency by 5.88 %. Meanwhile, to achieve the low 215
limit of power load of 81.20 MW, the standard coal consumption of the heat-only boiler 216
shoul d be 29.80 t/h , bringing 12.77% decrement in the generating efficiency. 217
Fig.9 shows the power load factor and standard coal consumption of the CHP plant 218
with different technologies in the designed heat load (320MW). T he power load factor 219
range is extended from 54.87%~78.72% to 25.20%~ 100% with the AHS mode . And 220
the minimum power load factor can reach 0 with the electric boiler and 4.14% with the 221
electric heat pump (COP=3). When the power load factor decreases from 54.87% to 222
25.20%, the standard coal consump tion decreases by 6.49 t/h with the heat -only boiler, 223
which is 12.58 t/h, 21.75 t/h and 36.26 t/h with the electric boiler, electric heat pump 224
and waste heat recovery, respectively. 225
4.2 Thermodynamic performance with the one -day loads 226
In this chapter, we condu ct a case study based on the given CHP plant and demanding 227
loads to compare the operational strategies and coal -saving potentials. The 24 -hour heat 228
and power loads with 1 -hour resolution are shown in Fig.10. T o reach the capacity 229
factor set by the China go vernment (lower than 50%), the new power load s (PN) are set 230
as the 70% of the original power loads (PD), and the capital factor decreases from 69.03% 231
to 48.32%. The power load factor s of the new power loads ( PN) change from 40.65 % 232
to 57.10% , which change f rom 58.07% to 81.57% of the original power load (PD). It 233
also can be seen that the off -peak period is 0:00~6:00 and 18:00~23:00, when the peak 234
period is 7:00~1 7:00. Fig.11 gives the peak shaving demand with the new power load 235

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(PN) in the day. And the peak s having demand is from 2.82MW to 35.40MW in different 236
time of the off -peak period. 237
4.2.1 Operational strategy of AHS mode 238
For the AHS mode , the CHP plant should provide the new power load ( PN) in the off – 239
peak period, and the heat load is limited to the maximum he at load with PN. And the 240
rest of the heat load is provided by the auxiliary heat sources (ΔHAHS). Fig.12 gives the 241
heat loads of the CHP unit with the AHS mode in the off -peak time . The auxiliary heat 242
sources need to provide 325.35 MWh of heat to reach HD, which is 4.97% of the gross 243
demanding heat. And the heat loads of the auxiliary heat sources ( ΔHAHS) are from 5.08 244
MW to 63.70 MW. 245
In the AHS mode , the heat -only boiler and waste heat recovery can provide ΔHAHS 246
when it is needed. However, for the TES sys tem, the charging process is essential , 247
which has an impact on the operational strategy in the peak period (7:00~17:00) . In this 248
research, the charging process happens from 7:00, and the charging power is as large 249
as possible until achieves the gross heat demand of the auxiliary heat source. 250
Fig.1 3 shows the charging process of the TES system in the peak period with 251
different heat sources. To provide the demanding heat in the off -peak period, using 252
electric power as the heat source will take 4 hours in the charging process, and the 253
power loads of the CHP plant are 62.65~101.19MW higher than the demanding power 254
load. Similarly, using extraction steam takes 3 hours more, and the heat loads of the 255
CHP plant are 24.64~73.34MW higher than the demanding heat load . 256
4.2.2 Operational strategy of P2H mode 257

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For the P2H mode , the CHP unit only needs to regulate the heat and power loads in 258
the off -peak time (0:00~6:00 and 18:00~23:00) . Fig.14 gives the operational strategies 259
of the CHP unit with the P2H mode . 260
With the P2H mod e, the CHP plant can operate with the higher power load and lower 261
heat load than the demanding loads. At 0:00, the power load of the CHP plant with the 262
electric boiler is 32.84 MW higher than the demanding power load, when the heat load 263
is 32.51 MW lower t han the demanding heat load. And with the electric heat pump, 264
both the heat and power loads of the CHP plant is lower than that with the electric boiler. 265
4.2.3 Coal consumption comparison 266
According to the operational strategy of each technology, Fig.15 gives the standard 267
coal consumption of each mode in the whole day . It can be seen that with the waste heat 268
recovery, the CHP plant has the lowest standard coal consumption all the time. CHP 269
plant with the TES system has the same standard coal consumption with that with the 270
waste heat recovery in the off -peak period, and higher standard coal consumption in the 271
peak period. And, the other technologies (electric boiler, electric heat pump and heat – 272
only boiler) shows the opposite trend. These characters also impact the gross coal 273
consumption in the day, shown in Fig.1 6. 274
To achieve the demanding loads, the CHP plant with the heat -only boiler has to 275
consume 1674 t of coal in the whole day. Compared to the P2H mode, the gross coal 276
consumption can reduce by 0.78%(13 t) with the electric boiler and 1.85% (31 t) with 277
the electric heat pump. However, among the AHS mode, not all the technologies are 278
better than the heat -only boiler in the coal -saving potential. The gross coal consumption 279

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of the CHP plant with the TES system (Elec tric boiler) will increase by 2.27% (38 t) 280
while that with the TES system (Extraction steam) will decrease by 2.33% (39 t). With 281
the waste heat recovery, the gross coal consumption can be 1618 t, 3.34% lower than 282
that with the heat -only boiler. 283
5. Techno -economic analysis 284
5.1 Annual techno -economic analysis 285
To compare the economic performances of each technology, t echno -economic 286
evaluation is also conducted in this section [37]. The investment cost is based on the 287
scale of each equipment. And the net annual revenue considers the annualized 288
investment cost, operation and maintenanc e cost and annual income from coal -saving. 289
The equipment investment cost (
EC )of the equipment can be calculated as: 290
ENC c S
(14) 291
where
EC is the equipment investment cost;
Nc presents the nominal investment , and 292
S is the scale of the equipment. 293
The annualized investment cost (
aC) can be calculated as: 294
(1 )( + )(1 ) 1a E ArrC C Cr

(15) 295
where
AC is the auxiliary fee , which is 15% of
EC ; r is the fraction interest rate per 296
year ( r =8% ), and
 denotes the life cycle of equipment (
 = 20 years) [27]. 297
Net annual revenue is defined as: 298
&an coal O MR B c C C  
(16) 299
where
B is the standard coal consumption difference between the heat -only boiler 300
and other technologies in the whole heating period;
coalc is the price of the standard 301

17
coal (
coalc =105.47 $/t) [38];
aC is the annualized investment capital cost difference 302
between the heat-only boiler and other technologie s;
&OMC refers to t he operation 303
and maintenance cost difference, which is 4% of
aC [39]. 304
5.2 Result s and discussion 305
In this section, we compare the techno -economic performances among the heat -only 306
boiler and technologies that have fewer coal consumptions ( TES system (Extraction 307
steam), electric boiler and electric heat pump). The designed scale of each technology is 308
set as more than 25% higher than the demanding capacity in this case . And the heating 309
period is 120 days. 310
Table 3 gives the equipment costs of all the technologies [40]. Among all the 311
technologies the investment cost of the TES system is lowest, which is 1.76 M$. And 312
the electric heat pump need s the highest investment cost of 32.76 M$, which is 17.56 313
M$ higher than the heat only boiler. And compared to the heat -only boiler, the investment c osts 314
of the TES system and electric boiler are 13.44 M$ and 12.74 M$ lower, respectively. 315
Table 4 lists the techno -economic performances of all the technologies . It depicts 316
that during the heating period, the TES system can save the 4723 t of coal than th e heat – 317
only boiler, which is 3191 t and 1008 t higher than the electric boiler and electric heat 318
pump, respectively. Likewise, the coal -savings by the TES system can bring 0.50 319
M$ income per year, which is 0.34 M$ and 0.11 M$ higher than the electric boile r and 320
electric heat pump, respectively. C onsidering the addition of the investment capital cost 321
and the operation and maintenan ce cost, the net annual revenue of the TES system can 322
reach 2.14 M$. However, despite the income from coal -savings of 0.39 M$, th e net 323

18
annual revenues of the electric heat pump are 1.75 M$ lower than that of the heat -only 324
boiler, mainly caused by the expensive annualized investment of 3.84 M$. 325
For the coal -saving potential, the waste heat recovery represents the lower limit of 326
coal consumption and its techno -economic performance relies on the chosen 327
technologies and process. And in the P2H mode, the electric heat pump can save more 328
coal than the electric boiler. However, the electric boiler is better when considering the 329
economic fac tor. 330
Among all the technologies for the flexibility enhancement, only the TES system and 331
electric boiler have advantages over the heat -only boiler in both coal -saving potential 332
and economic performance , of which the former is better. Nevertheless , the ele ctric 333
boiler can reach the lower power load and its operational strategy is instantaneous 334
without considering the demanding loads at other times. Hence, the chosen of 335
technologies should well balance the peak shaving demanding as well as the techno – 336
economi c performance. 337
6. Conclusions 338
The increasing share of the renewable energy in the power industry calls for the 339
flexibility enhancement of the existing power supply side. As the primary energy 340
supplier in northern China, the coal -fired CHP plant plays a vital role i n the peak 341
shaving services. In this study, we compare the technologies for the flexibility 342
enhancement considering the operational strategy , as well as the thermodynamic and 343
techno -economic performances , which gives guidance for the flexibility enha ncement 344
of the coal -fired CHP plant. 345

19
First, we discuss the thermodynamic performance with the designed heat load, 346
including the feasible operational region and coal consumption characters. Results 347
show that the power load factor range of the CHP plant is e xtended from 78.72% ~54.87% 348
to 100% ~25.20% with the AHS mode and to 78.72% ~ 0 with the electric boiler . 349
Then, we compare the operational strategies and coal consumptions of both modes 350
based on a 24 -hour heat and power loads with 1 -hour resolution. Results show that with 351
the waste heat recovery, the CHP plant has the lowest coal consumption in all the time. 352
And to achieve the daily demanding loads, the CHP plant has to consume 1674 t of coal 353
with the heat -only boiler, which will be reduced by 0.78%(13 t) wi th the electric boiler 354
and 1.85% (31 t) with the electric heat pump. However, among the AHS mode, not all 355
the technologies are better than the heat -only boiler in the coal -saving potential. The 356
gross coal consumption of the CHP plant with the TES system (E lectric boiler) will 357
increase by 2.27% (38 t) while that with the TES system (Extraction steam) will 358
decrease by 2.33% (39 t). With the waste heat recovery, the gross coal consumption can 359
be 1618 t, 3.34% lower than that with the heat -only boiler. 360
Finally, the techno -economic analysis indicates that during the heating period, the 361
TES system can save the 4723 t of coal than the heat -only boiler, which is 3191 t and 362
1008 t higher than the electric boiler and electric heat pump, respectively. And the net 363
annua l revenue of the TES system can be 2.14 M$ higher than that of the heat -only 364
boiler when that of the electric heat pump is 1.75 M$ less. 365
Acknowledgement 366
This study is supported by the National Key Technology Support Program (NO. 367

20
2014BAA06B01) , National Natural Sc ience Foundation of China (NO.51606061 ) and 368
China Sch olarship Council. 369
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Table 1 Main thermodynamic parameters of the CHP plant
Table 2 Model details of the main components
Table 3 Equipment investment costs of all the technologies
Table 4 Techno -economic performances of all the technologies
Table Captions

Table 1 Main thermodynamic parameters of the CHP plant
Items CHP plant
Main steam mass flow (t/h) 1017.92
Main steam pressure (MPa) 16.67
Main steam temperature (oC) 538
Reheat steam pressure (MPa) 3.58
Reheat steam temperature (oC) 538
Exhausted steam pressure (kPa) 14
Heating steam pressure (kPa) 400
Installed power capacity (MW) 322.20
Maximum heating steam mass flow (t/h) 600

Table 2 Model details of the main components
Components Models
Boiler Exhaust gas temperature =130 ℃. Air ratio =1.20. Slag temperature =500 ℃.
Combustion efficiency=99%.
Steam turbine Isentropic efficiency range is from 83.53% to 93.56% given by the turbine
design book. Mechanical efficiency = 0.998.
Generator Generator efficiency = 99%
Condenser Upper temperature difference = 1 oC. Pressure loss = 5 kPa
Pump Isentropic efficiencies = 85%. Mechanical efficiency = 99.80%
Heat consumer Outlet temperature=104 oC, Inlet pressure=0.4MPa

Table 3 Equipment investment costs of all the technologies
Items Capacity* Designed capacity* Nominal investment* Investment
(MW) (MW) (M$/MW) (M$)
Heat -only boiler 63.34 80 0.19 15.20
TES system 361.22 500 3.51× 10-3 1.76
Electric boiler 22.83 30 8.19× 10-2 2.46
Electric heat pump 28.68 40 0.82 32.76
* For the TES system, the unit of the capacity, designed capacity and nominal investment is MWh, MWh, M$/MW, respectively. Table(s)

Table 4 Techno -economic performances of all the technologies
Items Heat -only
boiler TES
system Electric
boiler Electric heat
pump
Initial Investment (M$) 15.20 1.76 2.46 32.76
Annualized investment (M$/year) 1.78 0.21 0.29 3.84
O&M fee (M$/year) 0.07 0.01 0.01 0.15
Coal -savings (t/year) – 4723 1532 3715
Income from coal -savings (M$/year) – 0.50 0.16 0.39
Net annual revenue (M$) – 2.14 1.71 -1.75

Fig.1 Typical coal -fired CHP plant with extraction condensing (EC) mode
Fig.2 Feasible operation region of the CHP plant
Fig.3 Flexibility enhancement of the CHP plant: (a) “Auxiliary Heat Source (AHS) ”
mode; (b) “Power to Heat (P2H) ” mode
Fig.4 Technologies for the flexibility enhancement
Fig.5 Operational strategies of the two modes
Fig.6 Comparison between simulation results and design data
Fig.7 Flexibility enhancement by different technologies
Fig.8 Efficiency and standard coal consumption with the heat -only boiler
Fig.9 Power range and coal consumption with the designed heat load (320MW)
Fig.10 The 24 -hour heat and power loads in a day
Fig.11 Peak shaving demand with the new power load ( PN)
Fig.12 Heat loads of the CHP plant and auxiliary heat source in the off -peak time
Fig13 Charging process of the TES system in the peak period
Fig.14 Operational strategies of the electric boiler and electric heat pump
Fig.15 Coal consumption comparison in the whole day
Fig.16 Gross coal consumption in the whole day
Figure Captions

LPRH – Low Pressure Regenerative HeaterDeaerator

Boiler HPT IPT LPT
Generator
Condenser
HPRH – How Pressure Regenerative HeaterHPRHLPRH
TNH
LPT – Low Pressure Turbine HPT – High Pressure Turbine IPT – Intermediat Pressure TurbineDH Station Heat Consumer
TNH – Thermal Network HeaterFWP
Condenser
Pump
TNP
FWP – Feed Water Pump TNP – Thermal Network Pump
Fig.1 Typical coal -fired CHP plant with extraction condensing (EC) mode

0 100 200 300 400 500100200300
HBH
DH
MIN
DP
D B
C 232.24A
419.79 148.0081.20120.01Power load / MW
Heat load / MW322.20
MAX
DP
'
DL
( , )D D DL H P

Fig.2 Feasible operation region of the CHP plant Figure(s)

(a)

(b)
Fig.3 Flexibility enhancement of the CHP plant: (a) “Auxiliary Heat Source (AHS) ”
mode; (b) “Power to Heat (P2H) ” mode

Flexibility enhancement
Power to heat
(P2H)Auxiliary heat source
(AHS )
Electric
boilerElectric
heat pumpTES sytemHeat -only
boilerWaste heat
recovery
Extraction steam Electric power
Fig.4 Technologies for the flexibility enhancement

0 100 200 300 400 50050100150200250300350
AHS
DL
2PH
DL
( , )D D DL H P
B Power load (MW)
Heat load (MW)
MIN
DP
MAX
DPA
DH
C D

Fig.5 Operational strategies of the two modes

200 300 400 500 600 700 800 900 1000 1100 1200 1300050100150200250300350
CHP II
RE= 0.19%CHP I
RE=-0.14%
40%THA
RE= 0.38%50%THA
RE= -0.18%THA
RE=-0.07%
75%THA
RE= -0.39%Power load (MW)
Main steam mass flow (t/h) Design data
Simulation resultsTMCR
RE=0.00%
Fig.6 Comparison between simulation results and design data

0 100 200 300 400 500 600 700 800 900 1000 1100 120050100150200250300350
(k 0)
1(k )
EB
1(k )COP
Electric boiler Electric heat pumpPower load (MW)
Heat load (MW) Electric Boiler Electric heat pump Auxiliary heat sources(AHS)
DH
A
B
MAX
DP
MIN
DP
D
C AHS

Fig.7 Flexibility enhancement by different technologies

340 300 260 220 180 140 100 60010203040506070
CHP plantGenerating efficiency (%)
Power load (MW) e, CHP
e, CHP+HB
cHB
81.20 176.79 253.63 322.20CHP plant+ Heat-only boiler
020406080100120140160180
Standard coal consumption (t/h)
Fig.8 Efficiency and standard coal consumption with the heat -only boiler

100 80 60 40 20 0020406080100120140160Standard coal consumption (t/h)
Power load factor (%)CHP plant
CHP plant+ Electric boiler CHP+Waste heat recovery
CHP+Heat-only boiler
CHP+Electric boiler
CHP+Electric heat pump
CHP plant+ AHS mode

Fig.9 Power range and coal consumption with the designed heat load (320MW)

0 5 10 15 20050100150200250300350Power load (MW)
Time (h) Heat load ( HD)
Power load range
Original power load ( PD)
New power load ( PN)
050100150200250300
Heat load (MW)
Fig.10 The 24 -hour heat and power loads in a day

0 2 4 6 8 10 12 14 16 18 20 22-60-50-40-30-20-100102030405060Off-peak PeakPower load (MW)
Time (h)Off-peak

Fig.11 Peak shaving demand with the new power load ( PN)

0 2 4 6 18 20 22050100150200250300350Heat load (MW)
Time (h) HCHP
HAHS
Fig.12 Heat loads of the CHP plant and auxiliary heat source in the off -peak time

6 8 10 12 14 16 18100150200250300350400450500Power load (MW)
Time (h) PCHP (Electric power)
PD
HCHP (Extraction steam)
HD
050100150200250300350400
Heat load (MW)

Fig13 Charging process of the TES system in the peak period

0 2 4 6 18 20 22120140160260280300Heat Load (MW) Power Load (MW)
Time / h
HD HCHP(Electric boiler) HCHP(Electric heat pump)
PN PCHP(Electric boiler) PCHP(Electric heat pump)
Fig.14 Operational strategies of the electric boiler and electric heat pump

0 2 4 6 8 10 12 14 16 18 20 22404550556065707580859095100Standard coal consumption (t/h)
Time (h) Heat-only boiler
Waste heat recovery
TES (Extraction steam)
TES (Electric boiler)
Electric boiler
Electric heat pump
Off-peak Peak Off-peak

Fig.15 Coal consumption comparison in the whole day

1661
164316351712
1674
1618
140014501500155016001650170017501800
Waste heat recovery
AHS modeGross standard coal consumption / t
Electric boiler Electric heat pump TES (Extraction steam)
TES (Electric power) Heat-only boiler Waste heat recoveryP2H modeHeat-only boiler
Fig.16 Gross coal consumption in the whole day