Evaluation of solar aided thermal power generation ...

INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2011; 35:909–922 Published online 27 July 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1748

Evaluation of solar aided thermal power generation with various power plants Qin Yan1, Eric Hu2,,y, Yongping Yang1,z,y and Rongrong Zhai1 1

School of Energy Power and Mechanical Engineering, Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing 102206, People’s Republic of China 2 School of Mechanical Engineering, The University of Adelaide, Adelaide SA5005, Australia

SUMMARY Solar aided power generation (SAPG) is an efficient way to make use of low or medium temperature solar heat for power generation purposes. The so-called SAPG is actually ‘piggy back’ solar energy on the conventional fuel fired power plant. Therefore, its solar-to-electricity efficiency depends on the power plant it is associated with. In the paper, the developed SAPG model has been used to study the energy and economic benefits of the SAPG with 200 and 300 MW typical, 600 MW subcritical, 600 MW supercritical, and 600 and 1000 MW ultra-supercritical fuel power units separately. The solar heat in the temperature range from 260 to 901C is integrated with abovementioned power units to replace the extraction steam (to preheat the feedwater) in power boosting and fuel-saving operating modes. The results indicate that the benefits of SAPG are different for different steam extracted positions and different power plants. Generally, the larger the power plant, the higher the solar benefit if the same level solar is integrated. Copyright r 2010 John Wiley & Sons, Ltd. KEY WORDS solar aided thermal power generation; solar energy to electricity efficiency; coal consumption rate Correspondence *Eric Hu, School of Mechanical Engineering, The University of Adelaide, Adelaide SA5005, Australia. y E-mail: [email protected] z Yongping Yang, School of Energy Power and Mechanical Engineering, Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing 102206, People’s Republic of China. y E-mail: [email protected] Received 3 January 2010; Revised 12 May 2010; Accepted 15 May 2010

1. INTRODUCTION Solar energy is one of the clean and renewable green resources, but its intermittent and low intensity nature, thus low efficiency, constrain its application or increase the costs significantly [1]. Therefore, the conventional coal or gas-fired power plants are still dominant way to generate base load electricity in the world for decades to come [2,3], despite the pollution, green house gas emission, and fossil fuel resource reduction draw more and more critique to the conventional power plants [4]. Although the new designed ultra-supercritical coalfired plants have improved efficiencies to nearly 50% [5], they still cannot generate ‘green’ electricity as solar and wind powers do. The conventional power industry is under huge pressure with the forthcoming renewable energy targets and carbon taxes set by Copyright r 2010 John Wiley & Sons, Ltd.

various governments. A real technical revolution is needed to help the power industry to become cleaner than before. Solar energy aided power [6] or equipment [7] have been noticed and done some groping research works in the whole world. In this paper, solar thermal energy below 3001C (as low as 901C) was assumed to be integrated with different capacities coal-fired power plants separately (200 and 300 MW typical, 600 MW subcritical, 600 MW supercritical and 600 and 1000 MW ultrasupercritical power units). In the integrations, steam extractions from steam turbines are replaced by the thermal oil from solar collectors to heat feedwater in the thermal oil heater in both power boosting and fuelsaving operating modes. The energy and economic benefits after integration will be analyzed and discussed based on the developed models. The model will

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include steam turbine stages model, condenser model, feedwater heater model, water mixer model, boiler model, evaluating model and so on.

Evaluation of solar aided thermal power generation

Table I. The system main parameters of the LS-2 system [12,13]. Name Solar collectors

2. SYSTEM DESCRIPTION 2.1. Coal-fired power plant Coal-fired power plants have been used for more than one hundred years. In a typical regenerative and reheating Rankine steam system, the boiler is composed of furnace, drum, risers, superheaters, feedwater heaters and economizer [8]. The combustion of coal takes place in the boiler. The unsaturated feed-water from condenser enters boiler after going through lowpressure feedwater heaters and a deaerator, and then enters high-pressure feedwater heaters. The superheated steam from the boiler enters the high-pressure turbine to expand i.e. generate power, after reheated in the boiler. The steam expands further through intermediate pressure and lower pressure stages of the turbine. At the end, the final exhaust steam is condensed in the condenser. To increase the thermal efficiency of the cycle, i.e. increasing the average heat input temperature, parts of the steam is extracted at the different locations of the turbine to pre-heat the feedwater in the feedwater heaters. The deaerator is actually an open type water heater in which steam mix with the feedwater to preheat the feedwater and remove the oxygen and other incoagulable gas from the water. The feedwater heaters are typical shell-andtube closed type heaters.

2.2. Solar collector systems There are two basic types of solar collectors, focusing and non-focusing. Solar tower, solar dish and parabolic trough are three kinds of typical focusing collectors. Linear parabolic concentrators in parabolic trough type are used to focus sunlight to the receiver running along the focal line of the collector. The solar energy is absorbed by the heat transfer working fluid, such as heat transfer oil or water/steam [9]. The parabolic trough collector is typical medium temperature type, which can generate temperature up to 5001C. Solar tower and solar dish are point focusing types; they can generate higher temperature solar fluid. The highest temperatures the solar tower can generate are nearly 10001C [10]. If the focusing collectors are used in a typical solar alone power station, the peak thermal-electricity efficiency is about 20% [10,11]. A typical commercial medium temperature focusing parabolic trough collector system is LS-2 in U.S., manufactured by LUZ Company. The collector system has been used for the generation for the past two decades. Table I lists the main parameters of the system. 910

Parameters

Value

Unit

Length of single collector Width of single collector Area of single collector Focal length of the collector Focal ratio of the collector Absorber external diameter Envelope external diameter

47.1 5 235 1.84 71 70 115

m m m2 m — mm mm

The vacuum tube and flat plate are non-focusing type of collectors. Compared with focusing type collectors, their investment costs are lower and almost maintenance free, as the sun tracking system is not required. However, a typical vacuum tube collector with the selective coating can generate heat at maximal temperature about 2001C, and a flat plat solar collector can generate to 1001C [14].

2.3. Solar aided thermal power generation (SAPG) Almost all combustion-based steam power plants are running regenerative Rankine cycle thermodynamically, in which part of the steam is bled-off/extracted from the turbine to be used to pre-heat the boiler feedwater from about 401C (from the condenser) to 3001C (to the boiler) [15]. By doing this, the overall cycle thermal efficiency is increased, but the power generated per unit steam passing through boiler is reduced. In the SAPG, the bled-off steam is partly or totally replaced by solar heat carried by heat transfer fluid to preheat the feedwater, as shown in Figure 1. Therefore, the saved bled-off steam continues to expand in the turbine, to generate power. In Figure 1, if the solar input cannot satisfy the demand to heat all the distributed feedwater in cloudy days, the valves 1 and 2 can be adjusted to distribute part of the feedwater to be heat by solar thermal oil. The ratio of water flow through steam heater or thermal oil heater is decided by sun radiation condition and feedwater flow rate. In the nighttime, the valve 1 will be opened and valve 2 closed, then the extracted steam can heat the feedwater in original heater again. The operating mode is back to conventional working condition without integration. Therefore, the advantages of the SAPG include: (1) various temperature levels of solar heat can be integrated into various stages of the feedwater heaters; (2) the solar to electricity conversion rate, i.e. efficiency that is the power generated by the saved bled-off steam to the solar heat input, is no longer limited by the temperature of solar fluid as the solar does not directly generate power; (3) the solar collectors using thermal oil as the heat carrier can be used and (4) the benefit to power station can come from either additional power Int. J. Energy Res. 2011; 35:909–922 r 2010 John Wiley & Sons, Ltd. DOI: 10.1002/er


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about 645 MW or much larger, 200 MW unit has about 220 MW maximum load.

generation with the same fuel consumption i.e. solar boosting mode, or fuel and emission reduction while keeping the same generating capacity i.e. fuel-saving mode, shown in Figure 2. But in power boosting operating mode, the output power exceeds the rated load to reduce coal-fired energy consumption rate. In general, the maximum safe power output (so-called T-MCR working condition for a steam turbine) is much higher than rated load, but the energy consumption rate in the above working condition is also much higher than in rated working condition. For example, the maximum safe load for a 600 MW unit is

3. MODELING AND VALIDATION OF COAL-FIRED POWER SYSTEM The mathematic simulation model to understand the efficiencies of SAPG with various coal or gas fired power plants has been developed and validated by their designed values. Power units are grouped into subcritical, supercritical and ultra-supercritical according to their designed steam parameters. The critical status of water is 22.115 MPa and 374.151C in thermodynamics [5]. In a subcritical power plant, the steam pressure and temperature from the boiler is lower than the critical condition. If the steam’s parameters are higher than the critical condition, but less than 25 MPa and 5931C, it is called supercritical. The ultra-supercritical is defined as pressure higher than 25 MPa and temperature higher than 5931C [15,16]. Increasing steam parameters to supercritical or ultra-supercritical can get higher cycle efficiency and lower pollution emission rate [17]. 200 and 300 MW subcritical units were the main generating units in China before 2005. The performance parameters of typical subcritical 200, 300 and 600 MW, supercritical 600 MW and ultra-supercritical 600 and 1000 MW are listed in Table II. The steam and water flow and thermal system parameters of a typical 600 MW supercritical plant are drawn in Figure 3. Other units’ descriptions are listed in Table III. The serial numbers in Table III are noted in Figure 3. They are all important parameters to describe and evaluate a thermal system. The ‘—’ of 200 MW unit means the vacant parameter, because the selected 200 MW unit only sets two high-pressure feedwater heaters.

Steam

1 2 Feedwater Heater

Thermal oil heater

Oil pump

Solar collector Figure 1. Schematic scheme of solar thermal energy replacing one of bled-off steams.

Power output

Power output

Solar

Solar

Fuel Fuel

0

6

12 18 Power boosting mode

24 (Hours) 0

6

12 18 Fuel saving mode

24 (Hours)

Figure 2. Two operation modes of SAPG.

Int. J. Energy Res. 2011; 35:909–922 r 2010 John Wiley & Sons, Ltd. DOI: 10.1002/er

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Table II. The main designed values of the selected units in 100% load.

Units

200 MW subcritical

300 MW subcritical

600 MW subcritical

600 MW supercritical

600 MW ultra supercritical

1000 MW ultra supercritical

— MW MPa /1C /1C t h1 kPa 1C kJ kWh1 kg kWh1

Beizhong 201.36 16.18 530 530 590.00 5.4 246.13 7985.2 2.930

— 308.404 17.0 537 537 935.00 5.5 268.84 8005.71 3.032

Hitachi 600 17.0 538 538 1810.383 5.41 271/5 7888.31 3.017

Shangqi 600 24.2 566 566 1655.897 4.9 273.8 7517 2.758

Shangqi 600 25.0 600 600 1598.77 4.9 283.7 7408 2.663

Hitachi 1000 25.0 600 600 2733.434 4.5/5.7 294.8 7354 2.734

Parameters Manufacturer Capacity Main steam parameters

Feedwater flow rate Condenser P Feedwater temp. Heat con. rate Steam con. rate

566.0 3396.0

24.2 MPa 1655.897 t/h -3185.7

566.0 3.585 MPa 3598.8

HP

IP

0.919 MPa

(1) 600.315 MW

IP

LP

LP

32.54

Boiler

2346.9

D 301.2 2965.9

To boiler

B

A

C D HG F E HTR5

(10)

(6)

174.9

137.8

Temperature C

741.1

580.4

Enthalpy kJ /kg

(7)

273.8 1199.7

B

A

E

C

F

G

H

(9)

(8)

180.2

(5)

(4)

(3)

(2)

248.3

205.6

779.0

99.98

80.80

50.95

33.63

1080.4

889.4

419.0

338.3

213.2

140.8

HTR8

HTR7

253.8 1104.5

Pressure MPa

32.54 136.3

HTR6

211.2 903.2

HTR3

HTR4

185.7

105.5 442.5

788.5

HTR2

99.04 415.0

HTR1

56.51

86.36 361.6

I

G.C

39.19 164.1

236.5

A

B

C

D

E

F

G

H

I

5.688

3.863

1.747

0.891

0.3668

0.1117

0.05466

0.01481

--

Enthalpy kJ /kg

3050.4

2965.9

3375.3

3187.3

2972.0

2730.3

2616.3

2474.7

--

Flow t/h

101.549

143.375

59.844

77.739

81.237

40.622

60.499

33.821

1.499

Figure 3. Steam and water flow and thermal system parameters of a 600 MW supercritical unit.

Table III. System parameters of other selected units.

Parameters 200 MW subcritical 300 MW subcritical 600 MW subcritical 600 MW ultra-supercritical 1000 MW ultra-supercritical

912

1

2

3

4

5

6

7

8

9

10

MW

1C

1C

1C

1C

1C

1C

1C

1C

1C

201.36 308.40 600.00 600.27 1000.00

36.45 35.69 34.90 33.67 36.00

77.20 83.12 77.50 53.97 84.90

103.92 103.34 100.00 74.09 108.10

123.32 124.52 118.10 94.89 132.50

143.55 145.75 157.70 134.80 154.70

164.61 169.23 183.50 171.00 182.30

193.07 198.31 202.10 207.80 216.40

— 242.25 246.80 248.40 258.60

246.13 268.83 271.50 283.00 294.80



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As introduced before, an integrated thermal system model is consisted of steam turbine stages model, condenser model, feedwater heater model, water mixer model, boiler model, evaluating model and so on. The basic descriptions of the submodels are provided below.

3.1. Steam turbine stages model The ideal process in a steam turbine stages is an isentropic process in thermodynamical concept, sin 5 sout. Therefore, the ideal outlet steam specific enthalpy hout can be calculated or found from steam tables when the Pout and sout are known [18]. The ideal specific enthalpy change in the stages is: Dh ¼ hin hout

ð1Þ

The real specific enthalpy change is: Dh0 ¼ estage Dh ¼ hin h0out

ð2Þ

where estage is the isentropic efficiency of the stages, %. The real power output is: Wout ¼ Fin Dh0

ð3Þ

The real outlet steam enthalpy is: h0out ¼ hin Dh0

ð4Þ

The inlet steam flow rate entering next stages is the difference of inlet flow of this stages and extraction flow Fex. All the units of above enthalpies are kJ kg1.

3.2. Condenser model Condenser is one of the main equipments for maintaining steam turbine backpressure. Its working performance affects the units’ characters directly. In theory, the condenser can be divided to pipe and shell sides, according to its heat transfer principle. In pipe side, cooling water is heated by condensed steam. The cooling effect is decided by water flow rate, pipe structure and so on. The shell side is consisted of steam section, condensed water section and non-condensing gas section [19]. (1) Steam mass in shell side X X d Ms ¼ Fsin Fsout ð5Þ dt where Fsin is inlet steam mass, kg h1; Fsout is outlet steam mass, kg h1. (2) Air mass in shell side X X d Fain Faout ð6Þ Ma ¼ dt where Fain is inlet air mass, kg h1; Faout is outlet air mass, kg h1. (3) Condensed steam mass The vacuum in condenser is caused by condensed steam, and its condensing effect also affects condenser’s working conditions. The condensed steam mass is calculated by energy conversation. Fc ¼

as Au ðTs Tc Þ Hs Hcw

ð7Þ


where as is the condensing heat transfer efficiency (W (m2 1C)1), Au is the heat transfer area in shell side (m2), Ts is the average temperature in condenser (1C), Tc is the temperature of cooling water (1C) Hs is the average steam enthalpy in condenser (kJ kg1), Hcw is the saturated water enthalpy in condenser (kJ kg1). (4) Partial steam pressure in shell side State equation for ideal gas is applied for steam, Ps V ¼ Ms Rs ðTs 1273Þ. The partial differential of the equation is Rs ðTs 1273Þ ð8Þ V where Ps is partial steam pressure in condenser (MPa), P0s is the partial steam pressure at previous calculating time (MPa), Rs is steam gas constant (J (kg K)1), V is the condenser’s volume (m3). (5) Partial air pressure in shell side As described in steam pressure, the partial air pressure in shell side is Ps ¼ P0s 1dMs

Ra ðTa 1273Þ ð9Þ V where Ma is air mass in condenser (kg h1), Pa is partial air pressure in shell side (MPa), P0a is the partial air pressure at previous calculating time (MPa), Ra is air gas constant (J (kg K)1). (6) Pressure in shell side The total pressure in shell side is the sum of partial steam and air pressures. Pa ¼ P0a 1dMa

P ¼ Ps 1Pa (7) Cooling water outlet temperature According to energy balance in condenser, d T1 1T2 ¼ Qc Dw Cw ðT2 T1 Þ Mcw Cw dt 2

ð10Þ

ð11Þ

where T1 and T2 are cooling water inlet and outlet temperature (1C), Qc is absorbed heat by cooling water (kJ), Dw is cooling water mass flow rate (kg h1), Cw is cooling water thermal capacity (kJ (kg 1C)1), Mcw is the kept cooling water in condenser (kg).

3.3. Feedwater heater model Feedwater heaters are typical close heaters, the feedwater is heated by extracted steam from steam turbine. The heater can be sorted into higher-pressure and lower-pressure heaters, according to the inlet steam parameters. The heater consists of steam side and air side in shell side, and the pipe side is feedwater side. The main mathematic descriptions are (1) Average steam enthalpy in steam side The average steam enthalpy in feedwater heaters is calculated by energy and mass balance principles. P Fsteam Hsteam 1 F Hc P Hsteama ¼ ð12Þ Fsteam 1 F where Hsteama is the average steam enthalpy (kJ kg1), Fsteam and Hsteam are mass flow rate and enthalpy of 913

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P extracted steam (kg h1 and kJ kg1), F Hc is total vaporized steam energy (kJ). (2) Condensed steam mass in steam side Fcon ¼

0:8 Kcon Fwin ð1 foul Þ Twin Twout Hsteama Hcw 2

ð13Þ

where Kcon is heat transfer efficiency in feedwater side (W (m2 1C)1), Fwin is feedwater mass flow rate (kg h1), foul is fouling factor in feedwater side, Hcw is saturated feedwater enthalpy in special pressure (kJ kg1), Twin and Twout are inlet and outlet feedwater temperature (1C). Mscoal ¼

Pr ¼ Prs 1Pra

ð14Þ

(4) Outlet feedwater temperature The outlet temperature of heated feedwater by absorbed condensed steam can be calculated by the following equation. cw Mw ðTwin 1Twout Þ dt ¼ Fwin cw ðTwin Twout Þ1Fcon ðHsteam Hcw Þ ð15Þ where cw is water specific heat capacity (kJ (kg 1C)1), Mw is water mass in pipe side (kg h1).

3.4. Deaerator model Deaerator in coal-fired power plant is used for removing oxygen in feedwater, by heating water to saturated state. Oxygen in feedwater can corrupt steel equipments and harm safety and economic characters. In this study, the deaerator is only an open water-steam heater. So the basic heat balance equation is

Hwater ¼

ð17Þ

where Mscoal is the standard coal consumption rate (kg h1), Hmsteam and Fmsteam are enthalpy and mass flow rate of main steam (kJ kg1 and kg h1), Hmwater is main feedwater enthalpy (kJ kg1), Hrsteamout and Frsteamout are outlet enthalpy and mass flow rate of reheated steam (kJ kg1 and kg h1), Hrsteamin is inlet reheated steam enthalpy (kJ kg1), qscoal is standard coal thermal value (kJ kg1).

3.6. System evaluating model The benefits in terms of efficiency and fuel saving of various levels of solar heat in various power stations are modeled in this study. To evaluate the benefit or the efficiency of the solar heat utilization, the solar to power efficiency (Zse) and solar percentage (psolar) in the above solar aided thermal power generation is defined as follows: Zse ¼

DWe 100% Qsolar DQboiler

ð18Þ

Qsolar 100% Qboiler 1Qsolar

ð19Þ

psolar ¼

where DWe is the increased power output after the solar replacement (kW), Qsolar is the solar heat

Hsteam Fsteam 1Hfeedwater Ffeedwater 1Hdrain Fdrain 1Hother Fother Fsteam 1Ffeedwater 1Fdrain 1Fother

where Hwater is outlet feedwater enthalpy (kJ kg1), Hsteam and Fsteam are enthalpy and mass flow rate of extracted steam (kJ kg1 and kg h1), Hfeedwater and Ffeedwater are feedwater enthalpy and mass flow rate from lower-pressure heaters (kJ kg1 and kg h1), Hdrain and Fdrain are drain water enthalpy and mass flow rate from higher-pressure heaters (kJ kg1 and kg h1), Hother and Fother are enthalpy and mass flow rate of other inlet water or steam (kJ kg1 and kg h1). 914

Boiler is the main equipment in thermal power plants to combust fuel to heat water or steam. Boiler is the energy resource for an integrated thermal circle. Its main characteristic parameters are superheated and reheated steam temperature, pressure, flow rate and fuel consumption rate. As described in deaerator model, boiler is considered as a ‘black box’ with input and output parameters of temperature, enthalpy, mass flow rate and pressure. The standard coal consumption rate can be calculated by energy balance principle.

ðHmsteam Hmwater Þ Fmsteam 1ðHrsteamout Hrsteamin Þ Frsteamout qscoal

(3) Feedwater heater pressure The total pressure of heater is the sum of partial steam pressure and air pressure.

d

3.5. Boiler model

ð16Þ

transferred into the feedwater heater (kW), DQboiler is the possible change of the thermal energy load in boiler after replacement, e.g. boiler reheating load would increase if the replaced bled-off is before re-heating (kW). The other impacts after the solar integration, e.g. the minor changes of pumps work for steam and oil are not included in the calculation. ‘7’ in Equation (18) means the changed boiler heat demands, it is ‘1’ if the integration adds boiler heat load, but ‘’ when decreasing boiler heat load. Int. J. Energy Res. 2011; 35:909–922 r 2010 John Wiley & Sons, Ltd. DOI: 10.1002/er


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Other criteria used in the study are specific steam consumption rate, heat consumption rate and standard coal consumption rate, which are defined as below: Specific steam consumption rate is the mass of steam (passing trough boiler) for per kWh electrical power generated (kg kWh1) defined as d¼

D W

ð20Þ

where D is boiler steam flow rate (kg h1), W is electrical energy output the plant (kWh h1). Heat consumption rate is the per kWh fuel heat consumption, for per kWh electrical power generated (kJ kWh1) defined as Q q¼ W

ð21Þ

where Q is the total heat load in boiler per hour (kJ h1). Standard coal consumption rate (generating coal consumption rate) is standard coal consumption for per kWh (g kWh1) defined as b¼

M W

ð22Þ

where M is the coal consumption (per hour) converted to the standard coal (29 298 kJ kg1), in kg h1. The feedwater heating system of the selected 200 MW subcritical unit is more complex than others, with the additional five drain water heat exchangers and two water mixers. The thermodynamic submodels developed for the conventional power plant can accurately simulate the performance of the power plants. The main sub models will be connected and debugged according to the real steam and water flow direction. Then, all the modeled power plants are validated with simulated and designed values in Table IV.

The advantages of the supercritical and ultra-supercritical units are clear. The selected six units are typical in steam flow and structure. Therefore, other similar unit in capacity has analogous characters. The error percentages in Table IV indicate the great precision between the designed and simulated results. The main error source is the neglected parts of gland leakages and make-up water flows, especially for 200 MW subcritical unit with more water mixing and hybrid. In general, the total model precision can satisfy project modeling demands.

4. ANALYSIS OF SOLAR AIDED THERMAL POWER SYSTEM As discussed above, the purpose of extracted steam from turbine is to preheat the feedwater, thus reducing the heat demand in boiler resulting the higher cycle efficiency. However, the steam passing the turbine and power generated are reduced due to the extraction. In the SAPG system, the solar heat carried by thermal oil is to replace the extraction steam to preheat the feedwater while the saved steam is allowed to continue through the turbine to generate power. In the system, medium temperature solar heat less than 3001C, from non-concentration types of solar collectors, can be used to generate power through saved extraction steam because the highest and lowest temperatures required for feedwater heaters are less than 260 and 901C respectively in normal power stations. The assumed temperature differences between thermal oil and feedwater demands are all about 151C here. The differences are normal in industry and they are easy to achieve.

Table IV. Comparison between simulated and designed values.

Parameters Output power

Feedwater temperature

Steam consumption rate

Heat consumption rate

Coal consumption rate

DV SV (error) DV SV (error) DV SV (error) DV SV (error) DV SV (error)

Units

200 MW subcritical

300 MW subcritical

600 MW subcritical

600 MW supercritical

600 MW ultrasupercritical

1000 MW ultrasupercritical

MW MW (%) 1C 1C (%) kg kWh1 kg kWh1 (%) kJ kWh1 kJ kWh1 (%) g kWh1 g kWh1 (%)

201.36 200.24 (0.56) 246.13 245.58 (0.22) 2.930 2.946 (0.55) 7985.20 8072.04 (0.01) 272.83 275.79 (0.01)

308.404 308.673 (0.087) 268.83 269.08 (0.093) 3.032 3.031 (0.033) 8005.71 8010.67 (0.062) 273.53 273.70 (0.062)

600 600.00 (0) 271.5 271.45 (0.015) 3.017 3.017 (0) 7888.31 7897.99 (0.12) 269.52 269.85 (0.12)

600.315 600.314 (0) 273.8 275.93 (0.78) 2.758 2.758 (0) 7517 7533.88 (0.22) 256.83 257.41 (0.22)

600.274 600.271 (0.005) 283.00 284.55 (0.55) 2.663 2.663 (0) 7408.00 7388.74 (0.26) 253.11 252.45 (0.26)

1000.000 1000.163 (0.02) 294.80 294.77 (0.01) 2.734 2.733 (0.04) 7354.00 7330.37 (0.32) 251.26 250.45 (0.32)

DV, designed value; SV, simulated values.


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4.1. 200 MW subcritical unit As described in Table II, the solar heat can be used to replace the steams entering the first high-pressure heater (HTR7), the second high-pressure heater (HTR6), the first low-pressure heater (HTR4) and the last low-pressure heater (HTR1), which are at 260, 215, 160 and 901C, respectively. And the temperature difference between feedwater demand and the solar thermal oil temperature of HTR7 is 141C, with outlet feedwater temperature 2461C; HTR6 is 201C, HTR4 171C and HTR1 131C. The results for integrating solar into the 200 MW subcritical power unit are shown in Table V. Four replacements are operated in power boosting and fuel-saving modes to analyze separately. In Table V, four cases of solar replacements are simulated and analyzed in two operating modes. In the case of replacing HTR7 steam, the coal consumption rate in power boosting mode is reduced by 20 g kWh1, while increasing 20 MW power output. But it is not recommended to long-term running in this mode due to safety limitation unless the unit is retrofitted. In fuelsaving mode, the coal consumption rate is reduced by 18 g kWh1 in the replacing HTR7 case, while maintaining the base load 200 MW. In the case of replacing HTR4, the original steam entering HTR4 is 13.28 t h1 extraction steam mixed with 3.65 t h1 gland leakage steam, after extraction steam is replaced by solar heat; the gland leakage steam now enters the next heater HTR3. The 901C solar heat can also achieve reduced coal consumption rate by about 3 g kWh1.

4.2. 300 MW subcritical unit The results for integrating solar energy into the 300 MW subcritical power unit are shown in Table VI. Compared with the results in Table V, there are slight differences, because of the different steam

parameters and heat demands. It should be noted that the power generation efficiency of 901C thermal oil from solar energy is higher as 13.65%, which is very difficult for any solar (alone) thermal power generating systems to achieve in industry.

4.3. 600 MW subcritical unit For the 600 MW subcritical unit in Table III, the second high-pressure heater (HTR7), the third highpressure heater (HTR6), the second low-pressure heater (HTR3) and the first low-pressure heater (HTR1) are selected to be replaced by 260, 215, 160 and 901C thermal oil in both power boosting and fuelsaving operating modes. The results are listed in Table VII. It seems that the benefits of the larger power generation unit with solar aiding are more obvious than 200 and 300 MW units listed above. The solar heat (at 2601C) to electricity efficiency is 40%. When replacing the first high-pressure HTR7, the gland leakage steam 10 mixing with extraction 8 is moved to mix with steam 9 to enter into the gland steam heater G.C., the feedwater temperature from G.C. will have a few increasing.

4.4. 600 MW supercritical unit The thermal balance diagram of a 600 MW supercritical unit is shown in Table III, which has the similar structure to the 600 MW subcritical one. The solar thermal oil is used to replace the second high-pressure heater HTR7 at 2601C, the third high-pressure heater HTR6 at 2151C, the forth low-pressure heater HTR4 at 1601C and the second low-pressure heater HTR2 at 901C separately. The simulation results are shown in Table VIII. In Table VIII, the greater reduction of heat (coal) consumption rates indicates that the solar integration

Table V. Comparisons of the 200 MW subcritical unit in different replacements. Replacing HTR7 steam Schemes Parameters Oil temp.demand Feedwater temp. Feedwater flow Condenser flow Output power Coal con. Heat con. rate Steam con. rate Coal con. rate Input solar heat Solar to power eff Solar percentage

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Units

Base case

Power boosting

Fuel saving

Replacing HTR6 steam Power boosting

Fuel saving


Fuel saving


Fuel saving

1C — 260 260 215 215 160 160 90 90 1C 245.58 245.52 245.54 249.85 251.09 245.54 246.08 245.58 246.92 t h1 589.99 589.99 541.69 589.99 577.89 589.99 584.30 589.99 584.60 t h1 434.39 496.37 448.07 453.49 441.39 447.67 441.98 434.39 429.00 MW 200.24 219.72 200.12 205.02 200.12 202.55 200.24 202.40 200.21 t h1 55.17 56.19 51.58 54.79 53.54 55.17 54.58 55.17 54.54 kJ kWh1 8072.04 7492.55 7551.45 7829.59 7838.07 7890.53 7986.14 7985.85 7980.73 kg kWh1 2.946 2.685 2.707 2.878 2.888 2.913 2.918 2.915 2.920 g kWh1 275.79 256.00 258.01 267.51 267.80 272.67 272.86 272.85 272.68 MW 0 46.88 37.53 23.56 21.60 10.34 9.05 21.14 20.85 % 0 36.58 — 25.48 — 22.33 — 10.65 — % 0 10.25 8.94 5.28 4.96 2.30 2.04 4.71 4.70



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Table VI. Comparisons of the 300 MW subcritical unit in different replacements. Replacing HTR7 steam Schemes Parameters Oil temp.demand Feedwater temp. Feedwater flow Condenser flow Output power Coal con. Heat con. Rate Steam con. Rate Coal con. rate Input solar heat Solar to power eff Solar percentage

Units

Base case

Power boosting

Fuel saving


Fuel saving


Fuel saving


Fuel saving


Table VII. Comparisons of the 600 MW subcritical unit in different replacements. Replacing HTR7 steam Schemes Parameters Oil temp. demand Feedwater temp. Feedwater flow Condenser flow Output power Coal con. Heat con. Rate Steam con. Rate Coal con. Rate Input solar heat Solar to power eff Solar percentage

Units

Base case

Power boosting

Fuel saving


Fuel saving


Fuel saving


Fuel saving


has greater advantage in the supercritical plant. The solar-to-electricity efficiency is also higher than that in the subcritical unit in Table VI, but the solar heat demand is also increasing.

4.5. 600 MW ultra-supercritical unit The thermal balance diagram of a 600 MW ultrasupercritical unit is given in Table III. The solar heat at the same temperature levels (as previous cases) is integrated. The simulated benefits in terms of power boosting and fuel saving are listed in Table IX. In the ultra-supercritical unit, the extracted steam in low-pressure heaters are all from low-pressure stages of the turbine with lower steam quality. Therefore, the benefits of using solar to replace the extracted steam Int. J. Energy Res. 2011; 35:909–922 r 2010 John Wiley & Sons, Ltd. DOI: 10.1002/er

are limited when compared with the above 600 and 300 MW sub or supercritical units.

4.6. 1000 MW ultra-supercritical unit The 1000 MW ultra-supercritical unit modeled is a product of Hitachi, in Table III. Solar heat was integrated into the third high-pressure heater (HTR6), the third and forth low-pressure heaters (HTR4 6 and HTR3) and the last heater (HTR1). From Table X it can be seen that for the 1000 MW ultra-supercritical unit, the benefits, i.e. the reduced energy consumption rate and solar-to-electricity efficiency, are smaller than that in the supercritical units. But the average coal consumption rate difference between ultra-supercritical and subcritical units are all about 15 g kWh1, with great energy-saving potential. 917

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Table VIII. Comparisons of the 600 MW supercritical unit in different replacements. Replacing HTR7 steam Schemes Parameters Oil temp. demand Feedwater temp. Feedwater flow Condenser flow Output power Coal con. Heat con. Rate Steam con. rate Coal con. Rate Input solar heat Solar to power eff Solar percentage

Units

Base case

Power boosting

Fuel saving


Fuel saving


Fuel saving


Fuel saving

1C — 260 260 215 215 160 160 90 90 1C 275.93 275.66 277.81 275.76 277.15 275.81 278.42 275.89 277.20 t h1 1655.895 1665.273 1548.732 1655.895 1618.715 1655.895 1624.595 1655.895 1644.915 t h1 1273.388 1426.141 1309.600 1333.232 1296.052 1273.388 1242.088 1273.388 1262.408 MW 600.314 654.70 600.317 617.50 600.147 614.721 600.115 605.213 600.090 t h1 154.37 158.43 146.76 154.40 150.50 154.39 150.75 154.38 153.00 kJ kWh1 7533.88 7089.88 7162.35 7325.83 7346.95 7358.49 7359.89 7473.28 7469.78 kg kWh1 2.758 2.544 2.580 2.682 2.697 2.694 2.707 2.736 2.741 g kWh1 257.41 242.24 244.71 250.30 251.02 251.42 251.46 255.34 252.22 MW 0 102.00 72.50 56.12 48.73 59.61 55.92 36.75 36.25 % 0 41.22 — 30.94 — 23.27 — 12.75 — % 0 7.91 6.07 4.44 3.98 4.72 4.56 2.91 2.91 Table IX. Comparisons of the 600 MW ultra-supercritical unit in different replacements. Replacing HTR7 steam

Schemes Parameters Oil temp. demand Feedwater temp. Feedwater flow Condenser flow Output power Coal con. Heat con. rate Steam con.rate Coal con. Rate Input solar heat Solar to power eff Solar percentage

Units

Base case

Power boosting

Fuel saving


Fuel saving


Fuel saving


Fuel saving

1C — 260 260 215 215 160 160 90 90 1C 284.55 284.55 286.722 284.55 286.43 284.55 287.14 284.55 285.24 t h1 1598.772 1598.772 1505.912 1598.722 1556.912 1598.772 1570.532 1598.771 1593.429 t h1 1206.641 1332.717 1239.857 1275.251 1233.391 1206641 1178.401 1206.641 1201.299 MW 600.271 645.512 600.279 620.655 600.265 614.028 600.272 602.866 600.264 t h1 151.38 154.18 144.63 151.38 146.85 151.38 148.01 151.38 150.70 kJ kWh1 7388.74 6997.83 7058.90 7146.07 7167.73 7223.20 7223.84 7356.93 7355.21 kg kWh1 2.663 2.477 2.509 2.576 2.594 2.604 2.616 2.652 2.655 g kWh1 252.45 239.09 241.18 244.16 244.90 246.79 246.81 251.38 251.30 MW 0 104.30 79.19 72.81 63.49 55.67 52.79 28.63 28.48 % 0 35.61 — 28.00 — 24.71 — 9.06 — % 0 8.31 6.71 5.91 5.31 4.50 4.38 2.32 2.31

Table X. Comparisons of the 1000 MW ultra-supercritical unit in different replacements. Replacing HTR6 steam Schemes Parameters Oil temp. demand Feedwater temp. Feedwater flow Condenser flow Output power Coal con. Heat con. Rate Steam con. Rate Coal con. rate Input solar heat Solar to power eff Solar percentage

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Replacing HTR4 steam



Units

Base case

Power boosting

Fuel saving

Power boosting

Fuel saving

Power boosting

Fuel saving

Power boosting

Fuel saving

1C 1C t h1 t h1 MW t h1 kJ kWh1 kg kWh1 g kWh1 MW % %

— 294.77 2733.434 2076.699 1000.163 250.24 7330.37 2.733 250.45 0 0 0

260 294.76 2733.434 2184.318 1033.53 250.24 7093.76 2.645 242.37 106.57 31.31 5.24

260 296.57 2664.434 2115.318 1000.038 242.95 7117.74 2.664 243.19 91.73 — 4.64

215 294.77 2733.434 2076.699 1016.701 250.24 7211.14 2.689 246.38 55.77 29.65 2.74

215 296.47 2699.234 2042.499 1000.102 246.29 7215.07 2.699 246.51 51.32 — 2.56

160 294.77 2733.434 2076.699 1014.407 250.24 7227.45 2.695 246.94 66.26 21.50 3.38

160 296.49 2703.834 2047.099 1000.04 246.72 7228.06 2.704 246.96 63.68 — 3.17

90 294.77 2733.434 2076.699 1013.606 250.24 7233.16 2.697 247.13 119.45 11.25 5.87

90 296.94 2705.655 2048.920 1000.123 246.70 7227.04 2.705 246.92 117.82 — 5.87


2- 300MW sub critical unit 4- 600MW super critical unit 6- 1000 MW ultra supercritical unit

5- 600MW ultra supercritical unit

Figure 4. Comparison analysis of the solar to electricity efficiencies (in power boosting mode).


7.26

3.94

3.49

5.87 (117.82) 3.17 (63.68) 2.56 (51.32) 4.64 (91.73) 3.53

11.27

7.55

5.64

2.31 (28.48) 4.38 (52.79) 5.31 (63.49) 6.71 (79.19) 1.15

12.70

6.39

5.95

2.91 (36.25) 4.56 (55.92) 3.98 (48.73) 6.07 (72.50) 5.19

15.15

4.38

2.67

4.1 (53.25) 2.81 (36.62) 2.37 (30.69) 6.36 (78.87) 3.10

14.63

6.70

4.11

4.24 (28.76) 2.64 (17.88) 4.12 (27.62) 6.77 (44.03)

breduced

g kWh1 % (MW)

psolar breduced

g kWh1 % (MW)

psolar breduced

g kWh1 % (MW)

psolar breduced

g kWh1 % (MW)

psolar (Qsolar) breduced

3.69

4.7 (20.85) 2.04 (9.05) 4.96 (21.60) 8.94 (37.53) 17.88

1- 200 MW sub critical unit 3- 600MW sub critical unit

g kWh1

600 MW ultra-supercritical unit 600 MW supercritical unit 600 MW subcritical unit 300 MW subcritical unit

260

2601C

215

7.99

160

Solar Thermal Oil Temperature ( C)

2151C

1

2.93

90

6

1601C

5

3.11

2 3 4

901C

1

% (MW)

2

3

psolar (Qsolar)

6

breduced

2

1

g kWh1

4 5 6

5

5 6

Parameters

3 4

Thermal oil temperature

3 4 1 2

200 MW subcritical unit

45 40 35 30 25 20 15 10 5 0

Table XI. Comparison analysis of the reduced coal consumption rates and solar percentages (In fuel saving mode).

Solar to electricity efficiency (%)

The solar-to-electricity (power) efficiency, the reduced fuel consumption rate and solar percentage are selected to compare the merits of solar integration among various power plants in power boosting and fuelsaving modes. The results of solar-to-electricity efficiencies for all six different units in same solar temperatures in power boosting operating mode are shown in Figure 4. Table XI shows the reduced coal consumption rate breduced and the solar percentage psolar in different integrations in fuel-saving operating modes. It can be seen that generally, the higher the solar heat temperature, more significant the benefits for the same unit. But the rates of input solar heat in different steam extraction structures and parameters are changed with different units. Input solar heat under 901C for 1000 MW unit is much more than the 2601C working condition, but the other units are the opposite. The reason for the difference of 1000 MW unit is that the designed increased temperature (heat demand) of the replaced lower-pressure heater is higher than others. To show the comparison of solar integration clearly, the results of three 600 MW units all in power boosting operating models are shown in Figure 5. Apparently, subcritical and supercritical power plants are more beneficial for solar integration, especially for low temperature solar heat, compared with ultra-supercritical plants. The solar percentages for replacing 2151C extracted stream in the 600 MW ultra-supercritical unit is higher than subcritical and supercritical units, but the reduced coal consumption rate is less than the two units. Meanwhile, the coal consumption rates in fuel-saving operating mode will increase a bit when compared with the power boosting mode. For some integrating modes, especially in replacing the higher pressure extraction steam, if the power output exceeds the long-term safe limitation in power boosting mode, then the integration system could adjust valves 1 and 2 in Figure 1 to decrease the power output

1000 MW ultra-supercritical unit

5. THERMAL AND ECONOMIC BENEFIT ANALYSIS

psolar

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% (MW)


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Solar to electricity efficiency (%) /Solar percentage (%)

45

3

35

2

1

30

3

25

3

2

20 15

2

1

40

1 2

1

3

10 5

a

b

a c

a

b

c

a

b

c

b

c

0 90

160

215

260

Solar Thermal Oil Temperature( oC) 1- 600 MW sub critical unit solar to electricity eff. a- 600MW sub critical unit solar percentage 2- 600 MW super critical unit solar to electricity eff. b- 600MW super critical unit solar percentage 3- 600 MW ultra super critical unit solar to electricity eff. c- 600MW ultra super critical unit solar percentage

Figure 5. Solar to electricity efficiencies and solar percentages in 600 MW power boosting operating modes.

or change to fuel-saving operating mode. In other ways, if the weather condition often changes, adjusting system will work frequently. In order to avoid the parameter fluctuating, a small heat storage system can be set to stabilize the temperature changes between solar collector and thermal oil heater. But the installed heat storage system will increase investment cost apparently. In addition, the solar heat demands in fuel-saving mode is smaller than the power boosting mode for same replacement in Tables V–X. But the reduced coal consumption rates are also a little small compared with the power boosting operating mode. Meanwhile, the units operated in fuel-saving mode are much safer, because the power output is always below the rated load. In economic analysis, the capital cost for the typical parabolic trough solar collector system (maximum temperature below 3501C) is about 2000 h kW1 in international market [20], less than 1500 h kW1 in China. But the coal price is about 55–80 h t1 in China. Therefore, the payback period of the integration is acceptable, especially under the encouraged policy from the government. For example, in China, the prices of electricity output from sustainable energy (including solar energy, wind energy and so on) are much higher than the conventional coal-fired power plants. The potential carbon tax will also accelerate the process of energy-saving reform. The benefits of the SAPG are much more observable in the future. In addition, the above-mentioned integrating modes studied only the effect and benefits of replacing single feedwater heater. If the solar collector system can afford enough heat demands to satisfy replacing two or more heaters, the benefits will be more remarkable.

160 and 901C in the 200 MW typical, 300 and 600 MW subcritical, 600 MW supercritical and 600 and 1000 MW ultra-supercritical power plants are modeled. Thus their benefits in terms of integrated with solar energy to preheat the feedwater both in power boosting and fuel-saving models are studied and compared. The results indicate that the SAPG technique can achieve higher solar (to power) efficiency compared to the solar alone power plant and is more suitable to be adopted in subcritical and supercritical plants than in ultra-supercritical plants. The solar energy can play a significant (up to 20%) and ecumenically competitive role in providing base load power generation once the SAPG principle. Therefore, the SAPG is proved theoretically to be the most efficient way to make use of solar energy especially at low-to-medium temperature ranges for power generation purposes.

NOMENCLATURE SAPG P temp. con. sin sout hin hout h0out

5 solar aided power generation 5 pressure (MPa) 5 temperature (1C) 5 consumption 5 inlet steam entropy (kJ kg1) 5 outlet steam entropy (kJ kg1) 5 inlet steam-specific enthalpy (kJ kg1) 5 ideal outlet steam-specific enthalpy (kJ kg1) 5 real outlet steam-specific enthalpy (kJ kg1) 5 ideal enthalpy change (kJ kg1) 5 real enthalpy change (kJ kg1) 5 the isentropic efficiency of the stages (%) 5 inlet steam mass flow rate (kg h1) 5 real power output (kW)

6. CONCLUSIONS

Dh 0 Dh estage

In this paper, using SAPG concept [21], i.e. integrating solar heat at various temperature levels of 260, 215,

Fin Wout

920



Fex Ms Fsin Fsout Ma Fain Faout Fc as Au Ts Tc Hs Hcw Ps P0s Rs V Ma Pa P0a Ra P Mcw Cw Dw Qc T1 T2 Hsteama Fsteam H Psteam F Hc Fcon Kcon

5 steam extraction flow rate (kg h1) 5 steam mass in shell side of the condenser (kg h1) 5 inlet steam mass of the condenser (kg h1) 5 outlet steam mass of the condenser (kg h1) 5 air mass in shell side of the condenser (kg h1) 5 inlet air mass of the condenser (kg h1) 5 outlet air mass of the condenser (kg h1) 5 condensed steam mass of the condenser (kg h1) 5 the condensing heat transfer efficiency (W (m2 1C)1) 5 the heat transfer area in shell side (m2) 5 the average temperature in condenser (1C) 5 the temperature of cooling water (1C) 5 the average steam enthalpy in condenser (kJ kg1) 5 the saturated water enthalpy in condenser (kJ kg1) 5 partial steam pressure in condenser (MPa) 5 partial steam pressure at previous calculating time (MPa) 5 steam gas constant (J (kg K)1) 5 condenser’s volume (m3) 5 air mass in condenser (kg h1) 5 partial air pressure in shell side of the condenser (MPa) 5 partial air pressure at previous calculating time (MPa) 5 air gas constant (J (kg K)1) 5 pressure in shell side of the condenser (MPa) 5 the kept cooling water in condenser (kg) 5 cooling water thermal capacity (kJ (kg 1C)1) 5 cooling water mass flow rate (kg h1) 5 absorbed heat by cooling water (kJ) 5 cooling water inlet temperature (1C) 5 cooling water outlet temperature (1C) 5 the average steam enthalpy (kJ kg1) 5 mass flow rate of extracted steam (kg h1) 5 enthalpy of extracted steam (kJ kg1) 5 total vaporized steam energy (kJ) 5 condensed steam mass in steam side of the feedwater heater (kg h1) 5 heat transfer efficiency in feedwater side (W (m2 1C)1)


Q. Yan et al.

Fwin foul Hcw Twin Twout Pr Pr,s Pra Twin Twout cw Mw Hwater Hsteam Fsteam Hfeedwater Ffeedwater Hdrain Fdrain Hother Fother Mscoal Hmsteam Fmsteam Hmwater Hrsteamout Frsteamout Hrsteamin qscoal Zse psolar DWe Qsolar DQboiler

5 feedwater mass flow rate (kg h1) 5 Fouling factor in feedwater side 5 saturated feedwater enthalpy in special pressure (kJ kg1) 5 inlet feedwater temperature (1C) 5 outlet feedwater temperature (1C) 5 feedwater heater pressure (MPa) 5 partial steam pressure of the feedwater heater (MPa) 5 partial air pressure of the feedwater heater (MPa) 5 inlet temperature of heated feedwater (1C) 5 outlet temperature of heated feedwater (1C) 5 water specific heat capacity (kJ (kg 1C)1) 5 water mass in pipe side of the heater (kg h1) 5 outlet feedwater enthalpy (kJ kg1) 5 enthalpy of extracted steam (kJ kg1) 5 mass flow rate of extracted steam (kg h1) 5 feedwater enthalpy from lower-pressure heaters (kJ kg1) 5 feedeater mass flow rate from lowerpressure heaters (kg h1) 5 drain water enthalpy from higherpressure heaters (kJ kg1) 5 drain water mass flow rate from higher-pressure heaters (kg h1) 5 enthalpy of other inlet water or steam (kJ kg1) 5 mass flow rate of other inlet water or steam (kg h1) 5 the standard coal consumption rate (kg h1) 5 enthalpy and mass flow rate of main steam (kJ kg1) 5 mass flow rate of main steam (kg h1) 5 main feedwater enthalpy (kJ kg1) 5 outlet enthalpy of reheated steam (kJ kg1) 5 outlet mass flow rate of reheated steam (kg h1) 5 inlet reheated steam enthalpy (kJ kg1) 5 standard coal thermal value (kJ kg1) 5 the solar to power efficiency (%) 5 the solar percentage (%) 5 the increased power output after the solar replacement (kW) 5 the solar heat transferred into the feedwater heater (kW) 5 the possible change of the thermal energy load in boiler after replacement (kW) 921

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d D W q Q b M

breduced DV SV


5 specific steam consumption rate (kg kWh1) 5 boiler steam flow rate (kg h1) 5 electrical energy output the plant (kWh h1) 5 heat consumption rate (kJ kWh1) 5 the total heat load in boiler per hour (kJ h1) 5 standard coal consumption rate (g kWh1) 5 the coal consumption (per hour) converted to the standard coal (29 298 kJ kg1) (kg h1) 5 the reduced coal consumption rate (kg kWh1) 5 designed value 5 simulated values

ACKNOWLEDGEMENTS We are grateful for the project (No. 2009CB219801), funding from National Basic Research Program of China-973 program, and China National Natural Science Fund Project (No. 50776028) to support the research.

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