Performance Assessment of Hybrid Solar Energy and ...

Accepted Manuscript Performance Assessment of Hybrid Solar Energy and Coal-Fired Power Plant Based on Feed-water Preheating

Hui Hong, Shuo Peng, Hao Zhang, Hongguang Jin PII:

S0360-5442(17)30602-3

DOI:

10.1016/j.energy.2017.04.050

Reference:

EGY 10690

To appear in:

Energy

Received Date:

24 November 2016

Revised Date:

04 April 2017

Accepted Date:

09 April 2017

Please cite this article as: Hui Hong, Shuo Peng, Hao Zhang, Hongguang Jin, Performance Assessment of Hybrid Solar Energy and Coal-Fired Power Plant Based on Feed-water Preheating, Energy (2017), doi: 10.1016/j.energy.2017.04.050

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ACCEPTED MANUSCRIPT Highlights: ► Thermodynamic Performance of assessment of hybrid solar/coal plant is studied. ► An explicit correlation of solar-to-power efficiency is derived by the energy level. ► The explicit correlation of efficiency is validated by two hybrid solar/coal plants. ► The off-design performance is disclosed with the variation of operation parameters.

ACCEPTED MANUSCRIPT 1

Performance Assessment of Hybrid Solar Energy and Coal-Fired Power Plant

2

Based on Feed-water Preheating

3

Hui HONGa, b,*, Shuo PENGc, Hao Zhanga, b and Hongguang JINa, b

4

a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, P.R. China

5

b

University of Chinese Academy of Sciences, Beijing 100049, PR China

6

c

Huaneng Clean Energy Research Institute, Beijing, 102209, P.R. China

7

* Corresponding author, Phone: +86-10-82543158; Fax: +86-10-82543019; E-mail: [email protected]

8

Abstract:

9

Hybridizing solar energy and coal-fired steam power plant is one of most attractive approaches of cost-

10

efficient solar electricity in the present. By using the concentrated solar heat at around 300 oC to replace the bleed

11

steam of the turbine for preheating feed-water of coal-fired steam cycle, higher solar-to-power efficiency is

12

possibly achieved in that the conversion of solar to power can utilize higher-temperature steam cycle. In this

13

paper, with the aid of exergy methodology, we derive expressions of the conversion of solar energy into power

14

for such kind of solar hybrid plant, especially an explicit correlation is obtained for explaining solar-to-power

15

efficiency. By using the derived expressions, we examine a typical hybrid solar system with 330 MW coal-fired

16

power plant and evaluate thermal performance of solar-to-power. In addition, the influences of key operation

17

parameters on the solar thermal performance are disclosed such as solar irradiation, incident angle and turbine

18

load. The results obtained here would be expected to provide a possibility for designing and evaluating practical

19

hybrid solar and coal-fired power plant.

20

Key words: Solar hybrid coal-fired power system; Net solar power output; Exergy destruction; Energy level;

21

Nomenclature: A

Energy level

C

Specific heat

DNI

Direct normal insolation

E

Exergy

∆EXL

Exergy destruction

FWH

Feed water heater 1

ACCEPTED MANUSCRIPT

1

2

3

H

Enthalpy

m

Flow rate

P

Pressure

Q

Heat duty

S

Total aperture area of solar field

△S

Entropy change

SWH

Solar feed water heater

T

Temperature

THA

Turbine heat-rate acceptance power

Greek symbols: η

Efficiency

θ

Incident angle

Subscripts and superscripts: 0

Ambient

bleed

Bleed steam

coal

Coal-fired power plant

col

Collector

ea

Energy accepter

ed

Energy donor

ex

exergy

h

Heater

heater

Feed water heater

hyb

Solar hybrid coal-fired power plant

in

Inlet

out

Outlet

sol

Solar

solar

Collected solar heat

tur

Turbine

w

work

1. Introduction

4

Concentrating solar thermal power generation is one of exciting candidates for use of renewable energy to

5

large-scale production of electricity. In the present, the concentrating solar thermal power generation can be

6

divided into two categories of both solar-only system and hybrid solar/fossil fuel system. From the viewpoint of

7

cost-effective solar electricity, the hybrid solar and conventional fossil- fueled power plant has a potential of 2


alleviating the difficulty of higher cost which solar–only power generation have to face [1].

2

The hybrid solar heat and coal-fired power plant is a viable application and promising technology, especially

3

for some countries like China, whose primary energy is coal. Integrating solar heat into coal-fired steam cycle

4

not only reduces the coal consumption for slowing down CO2 emissions of the coal-fired power plant, but also

5

employs large-scale equipment of steam turbine to accomplish the conversion of solar energy into power with

6

higher efficiencies. For such kind of hybrid solar/coal power plant, there are various ways to use the concentrated

7

solar energy to heat the feed-water, superheating/reheating of steam and air preheating [2]. Several researchers

8

have investigated the utilization of the solar heat to preheat the air of the boiler. Deng have proposed an attractive

9

system which uses solar heat to preheat the secondary air of the existing air preheater of a boiler and gave the

10

comprehensive analysis of thermal performance [3].

11

From the viewpoint of economy and feasibility, the solar-feed-water system is practically accomplished,

12

and a number of researchers have made more efforts. The concentrated solar heat at below 300°C can replace the

13

turbine bleed steam of the coal-fried steam cycle. Through this process, the bleed steam that was to be extracted

14

can efficiently expand in the steam turbine to further generate electricity, leading to increasing the output work

15

of the steam cycle. The first experimental hybrid solar/coal plant was built in Colorado in 2010, which integrated

16

a previously existing 44MW coal-fired power plant and a 4MW CSP installation [4]. Considerable researches

17

have pointed out that substitution of high-pressure turbine bleed steam leads to higher performance in contrast to

18

replacement of low-pressure bleed steam [5-10]. Facing the rapidly development of the concentrating solar

19

thermal power technology, this kind of solar-feed-water approach is promisingly directing into engineering

20

application. Up to now, more paramount efforts have been achieved, however, most investigations have focused

21

on the analysis of the typical hybrid solar/coal fired system or various system configurations proposed. There is

22

lack of a generally explicit expression of solar-to-power efficiency for evaluating on the thermodynamic

23

performance for this kind of hybrid solar/coal-fired plant. At the same time, there have been few publications

24

reported on the thermodynamic performance under the variation condition of solar irradiation and off-design

25

operation of steam turbine. These issues are urgently related to the design real solar/coal-fired plant.

26

In this study, this paper is to derive correlations for deeply understanding the solar thermal performance

27

with the aid of the exergy methodology. By applying the derived correlation into a typical hybrid solar system

28

with 330 MW coal-fired power plant, we are to disclose the performance behaviors under the off-design 3


conditions.

2

2. Thermodynamic modeling and explicit expression of solar-to-electricity efficiency

3

For assessing thermal performance of hybrid solar/coal fired power plant, the thermodynamic modeling of

4

the integration of solar heat and 330MW coal-fired plant is considered. Fig.1 shows a schematic diagram of

5

system equipped with parabolic trough solar collector. It is divided into three subsystems: (1) middle-temperature

6

solar feed-water system (SWH), (2) boiler systems with reheating processes, (3) steam turbine system.

7

The concentrated solar heat at around 300oC is collected by the parabolic trough collector and substitutes

8

the highest-pressure turbine bleed steam to preheat the feed-water from 249oC to 273oC. The flow of the feed

9

water from the water heater H2 is separated into two parts: one part is introduced into the next highest-pressure

10

water heater H1, the other flows into the solar feed water heaters (SWH). We determine the conversion profile

11

of the solar-to-power, the relevant governing equations for energy and exergy balances for the main sections of

12

this solar hybrid plant are described in the following. The input and output enthalpies, exergy flows and exergy

13

destructions are shown in Fig.2. In addition, for examining the contribution of solar heat to the output work of

14

the hybrid plant, the balances of both energy and exergy of the coal-fired steam cycle are considered and

15

compared.

16

2.1. Energy and exergy balances

17

Assumption. As shown in Figure 2 (a), the hybrid solar/coal system mainly concerns with three energy

18

conversions: solar heat transformed into the feed water in the SWH, the energy transformation in the boiler, heat

19

to work in the steam turbine. In the process of the SWH, there is the energy-level difference between the solar

20

heat and the feed water. In the energy transformation of the boiler, since the parameters for the feed-water of the

21

boiler and the main steam keep nearly no change after the hybridization of the solar heat. Thus, the model for the

22

boiler is considered as the single input and single output model. Furthermore, these processes including the air

23

pre-heater, the re-heater and the exhaust gas discharging in the boiler are neglected. In the steam turbine, since

24

the highest-pressure bleed steam is only replaced by the solar heat, the turbine efficiency variation and the

25

influence of the mass flow on the turbine are not considered before and after hybrid solar/coal process. In addition,

26

for both the super-heated steam and re-heat steam at the inlet and outlet turbine, their variations in the kinetic and

27

potential energies are neglected. 4


Exergy balance of the hybrid solar/coal system.

2

process can be expressible as:

As shown in Fig. 2, the exergy balance of the hybrid solar/coal

∆𝐸𝑐𝑜𝑎𝑙 + ∆𝐸𝑠𝑜𝑙𝑎𝑟 + ∆𝐸𝑤 = 𝑊ℎ𝑦𝑏 + ∆𝐸𝑋𝐿𝑆𝑊𝐻 + ∆𝐸𝑋𝐿𝑏𝑜𝑖𝑙𝑒𝑟 + ∆𝐸𝑋𝐿𝑡𝑢𝑟

(1)

3

where ∆Ecoal and ∆Esolar are, respectively, the exergies of the coal and the solar heat, while ∆Ew is the exergy input

4

of the feed water. Whyb is the power output of the hybrid solar/coal system. ∆EXLSWH is the exergy destruction of

5

the solar-feed-water heater, ∆EXLboiler is the exergy destruction of the boiler and ∆EXLtur is the exergy destruction

6

of the steam turbine.

7

At the same time, the energy balance, as shown in Fig. 2(a), is given as: ∆𝐻𝑐𝑜𝑎𝑙 + 𝑄𝑠𝑜𝑙𝑎𝑟 + ∆𝐻𝑤 = ∆𝐻𝑡𝑢𝑟

(2)

8

where ∆Hcoal is the input heat from the coal and Qsolar is the solar heat input. ∆Hw and ∆Htur are respectively, the

9

enthalpies of the feed water and the steam.

10 11

For the individual coal-fired power plant, as shown in Fig. 2(b), the energy and exergy balances are written as: ∆𝐻𝑐𝑜𝑎𝑙 + ∆𝐻𝑒𝑥 + ∆𝐻𝑤 = ∆𝐻𝑡𝑢𝑟

(3)

∆𝐸𝑐𝑜𝑎𝑙 + ∆𝐸𝑤 = 𝑊𝑐𝑜𝑎𝑙 + ∆𝐸𝑋𝐿𝐹𝑊𝐻 + ∆𝐸𝑋𝐿𝑏𝑜𝑖𝑙𝑒𝑟 + ∆𝐸𝑋𝐿𝑡𝑢𝑟

(4)

12

∆Hex is the enthalpy change of the bleed steam which heats the feed-water. ∆EXLFWH is the exergy destruction of

13

the feed-water heater. Wcoal is the output work of the only coal-fired power system.

14

2.2. Net solar-to-power of hybrid power plant

15

For this kind of hybrid solar power plant, the net solar-to-power of Wsol,hyb is defined as the difference in the

16

output works between the hybrid power plant (Whyb) and only coal power plant (Wcoal) [11]. That is: Wsol,hyb =

17

Whyb - Wcoal. Here, we consider that the exergy destructions in the boiler of the hybrid plant is the same as that of

18

the coal power plant, since any energy conversion occurring in the boiler has not changed. At the same time, both

19

the hybrid and individual coal plant have the same temperatures and the flow rates of the feed water and

20

superheated steam. Furthermore, we consider that the variation of the steam flow in the steam turbine is neglected

21

between the hybrid and the individual coal system, thereby the exergy destructions (∆EXLsteam) of the steam

22

turbine remains unchanged in equations (1) and (4). 5


By using the above-derived equations form (1) to (4), we can obtain 𝑄𝑠𝑜𝑙𝑎𝑟 = ∆𝐻𝑡𝑢𝑟

(5)

𝑊𝑠𝑜𝑙,ℎ𝑦𝑏 = ∆𝐸𝑠𝑜𝑙𝑎𝑟 + (∆𝐸𝑋𝐿𝐹𝑊𝐻 ‒ ∆𝐸𝑋𝐿𝑆𝑊𝐻)

(6)

2

Equation (6) shows the dependence of the net solar-to-power on the exergy destruction difference between

3

the solar-feed water heater and the feed-water heater of coal. Larger destruction difference will bring about higher

4

net solar-to-power. Furthermore, according to Appendix, the exergy destructions of ∆EXLFWH and ∆EXLSWH are,

5

respectively, written as: ∆EXLFWH = ∆Htur(Asteam - Awater), ∆EXLSWH = Qsolar(Asolar - Awater). Then equation (6) is

6

rewritten as: 𝑊𝑠𝑜𝑙,ℎ𝑦𝑏 = ∆𝐸𝑠𝑜𝑙𝑎𝑟 + 𝑄𝑠𝑜𝑙𝑎𝑟(𝐴𝑠𝑡𝑒𝑎𝑚 ‒ 𝐴𝑠𝑜𝑙𝑎𝑟)

(7)

7

where Asteam is the energy level of the bleed steam, Awater is the energy level of the feed water, and Asolar is the

8

energy level of the solar heat. The values of Asteam and Asolar are, respectively, dependent on the temperatures of

9

the bleed steam and the collected solar heat. We found from equation (7) the correlation of the net solar-to-power

10

with the energy-level difference between the bleed steam and the solar heat.

11

2.3. Theoretical net solar-to-electricity efficiency of hybrid power plant

12

The net solar-to-power efficiency is the core indicator which evaluates the performance of this kind of hybrid

13

power plant. Based on the obtained expression (7), the net theoretical solar-to-power efficiency of ηsol,hyb is given

14

as: 𝜂𝑠𝑜𝑙,ℎ𝑦𝑏 =

𝑊𝑠𝑜𝑙,ℎ𝑦𝑏 𝐼𝑆

=

𝑊𝑠𝑜𝑙𝑎𝑟 𝐼𝑆

+

𝑄𝑠𝑜𝑙𝑎𝑟(𝐴𝑠𝑡𝑒𝑎𝑚 ‒ 𝐴𝑠𝑜𝑙𝑎𝑟) 𝐼𝑆

(8)

15

Where DNI is the direct normal insolation, S is the aperture area of the parabolic trough mirrors. According to

16

the addressed reference [12], the exergy ∆Esolar of the collected solar heat is expressed as ∆Esolar = IS × ηcol ×

17

ηcarnot. Here, ηcol is the collector efficiency of converting concentrated solar energy into heat and ηCarnot is the

18

Carnot efficiency corresponding to the receiver temperature. Then, equation (8) is rewritten as: 𝜂𝑠𝑜𝑙,ℎ𝑦𝑏 = 𝜂𝑐𝑜𝑙 × 𝜂𝑐𝑎𝑟𝑛𝑜𝑡 + 𝜂𝑐𝑜𝑙(𝐴𝑠𝑡𝑒𝑎𝑚 ‒ 𝐴𝑠𝑜𝑙𝑎𝑟)

19 20

(9a)

It is noted that the term of 𝜂𝑐𝑜𝑙 × 𝜂𝑐𝑎𝑟𝑛𝑜𝑡 represents the theoretical solar-to-power efficiency ηsol,only in the solar-only power system [13]. Then, equation (9a) is rewritten as: 𝜂𝑠𝑜𝑙,ℎ𝑦𝑏 = 𝜂𝑠𝑜𝑙,𝑜𝑛𝑙𝑦 + 𝜂𝑐𝑜𝑙(𝐴𝑠𝑡𝑒𝑎𝑚 ‒ 𝐴𝑠𝑜𝑙𝑎𝑟) 6

(9b)


It can be seen that the energy level difference (Asteam -Aabs) plays critical role in ηsol,hyb. If Asteam > Asolar, meaning

2

that relatively lower grade of the solar heat replaces high-level of bleed steam, ηsol,hyb of the hybrid plant obtained will

3

be higher than that of solar-only plant (ηsol.only). On the contrary, if Asteam < Asolar, the net solar-to-power efficiency

4

(ηsol,hyb) will be lower than that of the solar-only plant. In contrast to previous efforts, the derived equation (9 ) further

5

stems from the energy level to derive the solar-to-power efficiency of hybrid solar/coal fired plant. It indicates that

6

obtaining higher efficiency is dependent on the energy level difference between the solar heat and the bleed steam.

7

The higher level of the bleed stem of the turbine is replaced by using middle-temperature solar heat, the better solar-

8

to-power efficiency will be obtained. Thus, equation (9) shows the theoretically explicit correlation and

9

provides a new insight on evaluating the hybrid solar/coal power plant.

10

By using equation (9), the feature of the net solar-to-power efficiency of the hybrid system with solar-feed

11

water can be understood. It can be also observed from Figure 3 that at a given solar irradiation of 600W/m2 and

12

a concentration ratio of 80 (a ratio of mirror area to receiver area), ηsol,hyb in the hybrid plant exhibits obvious

13

advantage over that in the solar-only power plant. It is emphasized that a black solar collector is here considered.

14

Furthermore, replacing middle-temperature solar heat at 300-400oC for the bleed steam of the turbine has higher

15

efficiency than other ranges of the solar heat. In addition, we also see that the relatively larger difference of (Asteam

16

–Asolar) brings about higher value of efficiency.

17

3. Expression of actual net solar-to-power efficiency of hybrid power plant

18

By applying equations (7) and (9) into the practical hybrid solar/coal plant, several actual conditions such

19

as the isentropic efficiency and the exergy destruction of the steam turbine, need to be considered. It is due to the

20

fact that in contrast to the individual coal-fired power plant, the bleed steam in the hybrid plant is increased. In

21

this case, according to the isentropic efficiency ηtur, the exergy destruction of the turbine for the actual hybrid and

22

coal plants are, respectively, written as: ∆𝐸𝑋𝐿𝑡𝑢𝑟,𝑐𝑜𝑎𝑙 = (

1 𝜂𝑡𝑢𝑟,𝑐𝑜𝑎𝑙

∆𝐸𝑋𝐿𝑡𝑢𝑟,ℎ𝑦𝑏 = (

1 𝜂𝑡𝑢𝑟,ℎ𝑦𝑏 7

‒ 1)𝑊𝑐𝑜𝑎𝑙 ‒ 1)𝑊ℎ𝑦𝑏

(10a) (10b)


where ηtur,coal and ηtur,hyb denotes respectively the turbine efficiencies of the coal-fired plant and the hybrid plant.

2

Then, equation (7) is rewritten as 𝑊𝑠𝑜𝑙,𝑎𝑐𝑡𝑢𝑎𝑙 = Δ𝐸𝑠𝑜𝑙𝑎𝑟 + 𝑄𝑠𝑜𝑙𝑎𝑟(𝐴𝑠𝑡𝑒𝑎𝑚 ‒ 𝐴𝑠𝑜𝑙𝑎𝑟) + (

3

𝜂𝑡𝑢𝑟,ℎ𝑦𝑏

(11)

‒ 1)𝑊𝑐𝑜𝑎𝑙 𝜂𝑡𝑢𝑟,𝑐𝑜𝑎𝑙 Correspondingly, the actual net solar-to-electricity efficiency is expressed as

𝜂𝑠𝑜𝑙,𝑎𝑐𝑡𝑢𝑎𝑙 = 𝜂𝑐𝑜𝑙,𝑎𝑐𝑡𝑢𝑎𝑙 × 𝜂𝑐𝑎𝑟𝑛𝑜𝑡 + 𝜂𝑐𝑜𝑙,𝑎𝑐𝑡𝑢𝑎𝑙(𝐴𝑠𝑡𝑒𝑎𝑚 ‒ 𝐴𝑠𝑜𝑙𝑎𝑟) + = 𝑓(𝜂𝑐𝑜𝑙,𝑎𝑐𝑡𝑢𝑎𝑙, 𝐴𝑏𝑙𝑒𝑒𝑑𝑠𝑡𝑒𝑎𝑚, 𝐴𝑠𝑜𝑙𝑎𝑟,

(

𝜂𝑡𝑢𝑟,ℎ𝑦𝑏 𝜂𝑡𝑢𝑟,𝑐𝑜𝑎𝑙

)

‒ 1 𝑊𝑐𝑜𝑎𝑙

𝐼𝑆

(12)

𝜂𝑡𝑢𝑟,ℎ𝑦𝑏

,𝑊 ) 𝜂𝑡𝑢𝑟,𝑐𝑜𝑎𝑙 𝑐𝑜𝑎𝑙

4

Here, the efficiency of ηcol,actual for an actually parabolic trough collector is closely related to the solar

5

irradiation of DNI and incident angle and heat loss. The incident angle  is an important factor which seriously

6

affects the parabolic collector efficiency. Therefore, the equation (13) can be rewritten as: 𝜂𝑠𝑜𝑙,𝑎𝑐𝑡𝑢𝑎𝑙 = 𝑓(𝐷𝑁𝐼,𝜗,𝑇𝑏𝑙𝑒𝑒𝑑𝑠𝑡𝑒𝑎𝑚,𝑇𝑠𝑜𝑙𝑎𝑟,𝑚 )

(13)

7

Thus, expression (12) can be regarded as the net solar-to-power efficiency of the actual hybrid plant. Two

8

typical hybrid solar with coal plants are considered to be evaluated by adopting equations (9) and (12). Figure 4

9

presents the comparison of the theoretical net solar-to-power efficiency and the actual one. Figure 4 (a) is for the

10

330 MW hybrid solar plant in China and Fig. 4 (b) is for a 500 MW hybrid solar power plant in India [14]. The

11

solar heat at around 292 oC is utilized to replace the high temperature and high pressure bleed steam of the coal

12

fired power plant. Here, the replaced bleed steam of 330MW has a temperature and pressure of 352 oC and 4.18

13

Mpa, corresponding to the energy level of 0.49. The replaced bleed steam from the 500 MW has a temperature

14

and pressure of 339 oC and 4.41 Mpa. corresponding to the energy level of 0.49. In this case, the energy-level

15

difference between the bleed steam and the solar heat are set as 0.02 and 0.04 for the 330 MW and 500 MW

16

plant, separately. Compared with the theoretical net solar-to-power efficiency, the actual net solar-to-power

17

efficiency is lower and shows the similar behavior. In addition, with the increasing of the collector temperature,

18

the difference of the net solar-to-power efficiency between the theoretical and the actual value first climbs up and

19

then goes down.

20

4. Results and discussion

8


When we use Equation (12) to examine a practical hybrid solar plant, as shown in Fig. 1, the effects of the key

2

operation parameters on the thermal performance are explained. These operation parameters not only involve in

3

the solar parameters including solar irradiation and incident angle, but also relate to the operation parameters of

4

the steam turbine. It is worthy noted that their parameters have interactions each other. In this section, we mainly

5

discuss the effects of solar irradiation, incident angle, and flow rate of the working steam on the thermal

6

performance. In addition, other parameters corresponding to different turbine loads are listed in table 1, in which

7

these operation parameters are taken from real 300MW coal-fired plant located in Xinjiang Province of China.

8

The steam turbine in this practical hybrid solar plant is made by Shanghai Turbine Company and works at the

9

subcritical condition. The steam turbine is a condensing steam turbine consist by a high-pressure cylinder and a

10

mid-pressure cylinder. The high-pressure cylinder and the mid-pressure cylinder are combined together.

11

Meanwhile, the single reheating and dual exhaust sub-systems are adopted. The main steam parameters for the

12

steam turbine are 16.70 MPa/538 oC/538 oC. Three high-pressure heaters, three low-pressure heaters and a

13

deaerator are employed as the heater for the feed-water. The solar heat at around 300 oC only replaces the highest-

14

pressure bleed steam, resulting in the little variation of the flow rate of the turbine, so the turbine efficiency

15

suffers very little. The turbine efficiencies of the hybrid plant have, respectively, the value of 0.828, 0.825 and

16

0.797 according to the turbine load of 100%, 75% and 50%, while the individual coal plant has values of

17

0.83,0.824 and 0.796.

18

4.1. Effects of solar irradiation and incident angle on the flow rate of the steam

19

By applying equation (12), we firstly discuss the influences in the collector efficiency and the turbine

20

efficiency. In this study, the parabolic trough collector is employed, in which the solar irradiation and the incident

21

angle are mainly decisive with the collector efficiency. At the same time, the flow rate of the flow rate of the

22

bleed steam directly touches to the steam turbine efficiency. It is due to the fact that the bleed steam re-flowing

23

into the turbine which increases the main flow rate as a function of the turbine efficiency [15]. For this kind of 9


solar-feed water system, the flow rate of the bleed steam can be varied with the solar irradiation (DNI) and the

2

incident angle. This is because that the variation of DNI and the incident angle seriously affects the amount of

3

the collected solar heat, further having an effect of the amount of the bleed steam replaced by the solar heat.

4

Figure 5 shows the variation of the ratio of the bleed steam (mbleed) to the main steam (mmain) with the solar

5

irradiation (DNI) and incident angle. At a given turbine load, when DNI rises firstly, the ratio of mbleed/mmain is

6

increased and then reaches to a peak value and keeps constant. This is because that the amount of the absorbed

7

solar heat rises with the increase of DNI, meaning more extracted steam flow rate entering into the steam turbine

8

to generate power. It can also be seen that mback/mmain under the condition of 50% turbine load rises more sharply

9

than that under the condition of 75% and 100% turbine load. It results from the smaller amount of the bleed steam

10

provided in the case of 50% turbine load.

11

On the other hand, at a given turbine load, the ratio of the mbleed/mmain is increased as the incident angle

12

decreases first, then reaches to a peak value and keeps constant. The variation of the mbleed/mmain with the variation

13

of incident angle is similar to that with the DNI.

14

the incident angle decreases, increasing the amount of the absorbed solar heat and decreasing of the bleed steam

15

flow rate. Therefore, the ratio of mbleed/mmain correspondingly rises for a certain steam flow rate. With the

16

continuing decrease of the incident angle, the concentrated solar heat can entirely replace the bleed steam.

The reason is that the cosine loss of solar field is decreased as

17

When the solar irradiation of DNI and the incident angle varies, correspondingly the flow rate of the bleed

18

steam will be changed and influences the turbine efficiency. Figure 6 shows the variation of the turbine efficiency

19

with DNI and the incident angle under different turbine load. Under the condition of 100% turbine load, the

20

turbine efficiency is decreased as DNI rises at first, and then keeps constant. 75% and 50% load has lower flow

21

rate of the main steam than 100% load has in the individual coal power plant. On the contrary, in the hybrid

22

system, the steam flow rate at 75% and 50% will approach to the rated one. Similarly, with the decease of the

23

incident angle, the turbine efficiency at 100% turbine load is firstly decreased and then trends to the constant,

24

resulting in the deviation of the steam flow rate from the rated one. For 75% and 50% turbine load, hybridizing

25

ways makes the steam flow rate approach to the rated flow, thereby the turbine efficiency being increased.

26

4.2. Effect of the turbine load on solar collector efficiency

27

Figure 7 illustrates the variation of solar collector efficiency with turbine load at a given DNI and an incident 10


angle. With the rise of the output work, the collector efficiency has an optimum value. For example, at DNI of

2

800W/m2 and incident angle θ of 10°, the solar heat can entirely replace the bleed steam and the solar collector

3

efficiency is increased. The reason is that the feed water flow rate rises, as the output work increases. In this case,

4

the amount of the absorbed solar heat is increased. When solar heat input partly replaces the bleed steam

5

(I=400W/m2 and θ = 40°, as shown in Fig. 7), with the increase of the turbine load, the solar collector efficiency

6

is decreased. Here, the collector efficiency can be obtained according to the reference (8).

7

4.3. Behavior of performance of solar-to-power

8

4.3.1. Contribution of solar to power

9

By hybridizing, the amount of the output work is increased in comparison with the coal-fired power system.

10

Here, the contribution of solar power increased to the power of the hybrid system is defined as the solar share,

11

which is Wsolar/Whyb. Figure 8 shows the trends of the solar share with the variation of the turbine load. It can be

12

seen that at higher solar irradiation such as DNI of 800W/m2 and smaller incident angle (such as θ= 10°), the

13

collected solar heat at around 300oC can replace higher-level bleed-steam. In this case, according to the derived

14

equation (10), the energy-level difference between the bleed steam and the solar heat increases and brings about

15

the increase of the solar power output with the rise of the output work of the system. If the solar heat operating

16

at relatively low DNI and incident angle (such as DNI of 400W/m2 and θ of 40°), the solar share declines as the

17

increase of the turbine load. It is due to the fact that the amount of the solar heat does not meet well with the need

18

of the feed-water preheated, decreasing the amount of solar-to-power and the solar share with the increase of

19

turbine load.

20

4.3.2. Net solar-to-power efficiency

21

Figure 9 shows the behavior of the net solar-to-power efficiency. At a given turbine load, the net solar-to-

22

power efficiency has also a peak value with the rise of the solar irradiation (DNI). The reason is that on one hand,

23

high solar irradiation makes the collected solar heat to replace the bleed steam for preheating feed-water; on the

24

other hand, solar irradiation continues to rise, the amount of the collected solar heat becomes more than that of

25

the need for the feed-water preheated. Thus, the solar collector efficiency becomes worse. On the base of the

26

derived equation (11), the net solar-to-power efficiency can be lowered down.

27

In addition, Fig. 9 explains the effect of the incident angle on the solar-to-power efficiency. At a given DNI 11


of 600 W/m2, for the incident angle of 30°, the net solar-to-power efficiency firstly keeps constant, and then

2

decreases. This behavior is similar to that of solar-to-power.

3

5. Conclusions

4

This paper gives the theoretically and actually correlation of the solar–to-power efficiency with the exergy

5

destruction of solar hybridization process. The derived explicit expressions explain the reason of the solar-to-

6

power efficiency of hybrid system is advantageous over that of solar-only power system. By applying the derived

7

equations into a hybrid system with 330 MW coal-fired power plant, we disclose the effects of three key

8

parameters, on the thermal performances including solar irradiation, incident angle and turbine load. At a given

9

turbine load, the net solar-to-electricity efficiency has a peak value. As the increase of incident angle, the net

10

solar-to-electricity efficiency firstly keeps constant, and then decreases. This study can provide a principle for

11

further designing and understanding the actual hybrid solar and coal-fired power plant.

12

Acknowledgments:

13

The authors gratefully acknowledge the support of the Natural Scientific Foundation of China (Grant Nos.

14

51236008).

15

Appendix A:

16

For each energy transformation process, there exist an energy donor and an energy acceptor. The exergy

17

destruction can be illustrated by the concept of energy level [15,16]. Energy level A is defined as the ratio of the

18

exergy change and the enthalpy change in the process: 𝐴=1‒

Δ𝐸 Δ𝑆 = 1 ‒ 𝑇0 × Δ𝐻 Δ𝐻

(A1)

19

where △S denotes the entropy change in the process, △E represents the exergy change in the same process, △H

20

represents the enthalpy change in the process, and T0 is ambient temperature.

21 22

For a heat transfer process, the released heat of the energy donor is equal to the absorbed heat of the energy acceptor. In this way, the exergy destruction of this process can be illustrated as: Δ𝐸𝑋𝐿 = Δ𝐻𝑒𝑑(𝐴𝑒𝑑 ‒ 𝐴𝑒𝑎)

23

(A2)

Appendix B:

24

The heat release process of extracted steam could be divided into two parts: the cooling process of steam

25

and the phase change process. According to Ishida [16,17], the enthalpy change and the entropy change of the 12


cooling process (constant-pressure) can be denoted as: Δ𝐻1 = 𝐶𝑝(𝑇𝑠𝑡𝑒𝑎𝑚 ‒ 𝑇𝑤𝑎𝑡𝑒𝑟) 𝑇𝑠𝑡𝑒𝑎𝑚 Δ𝑆1 = 𝐶𝑝𝐿𝑛( ) 𝑇𝑤𝑎𝑡𝑒𝑟

2

where Tsteam and Ts are separately the steam temperature at the inlet and outlet of feed water heater.

3 4

(B1) (B2)

For the phase change process, the enthalpy change is equal to the latent heat, which is related to steam pressure: Δ𝐻1 = 𝑟(𝑃𝑠𝑡𝑒𝑎𝑚) (B3) Δ𝑆2 = 𝑟(𝑃𝑠𝑡𝑒𝑎𝑚)/𝑇𝑤𝑎𝑡𝑒𝑟 (B4) According to equation (A1), the average energy level of energy donor or energy acceptor can be denoted as:

5

( ) 𝑇𝑠𝑡𝑒𝑎𝑚

𝑟(𝑃𝑠𝑡𝑒𝑎𝑚)

𝑇0[𝐶𝑝𝐿𝑛 + ] 𝑇𝑤𝑎𝑡𝑒𝑟 𝑇𝑤𝑎𝑡𝑒𝑟 Δ𝐸 Δ𝑆 𝐴𝑠𝑡𝑒𝑎𝑚 = = 1 ‒ 𝑇0 × =1‒ Δ𝐻 Δ𝐻 𝐶𝑝(𝑇𝑠𝑡𝑒𝑎𝑚 ‒ 𝑇𝑤𝑎𝑡𝑒𝑟) + 𝑟(𝑃𝑠𝑡𝑒𝑎𝑚) 6

where Tsteam and Psteam are separately the temperature and pressure of the extracted steam.

7

References:

8

[1]

9

Reviews 20, pp. 71-81. [2]

12 13

[3]

[4]

Deng, S., 2014. “Hybrid Solar and Coal-Fired Steam Power Plant Based on Air Preheating.” Trans. ASME, J. of Solar Energy Engineering, 136, pp 021012-1 – 021012-2.

[5]

18 19

National renewable energy laboratory, 2010, “First Hybrid CSP-Coal Power Plant is Fired Up in Colorado.” http://www.nrel.gov/csp/news/2010/870.html.

16 17

Hu, E., Yang, Y., Nishimura, A., Yilmaz, F., Kouzani, A., 2010. “Solar thermal aided power generation.” Applied Energy, 87, pp. 2881-2885.

14 15

Jamel, M.S., Rahman, A., Shamsuddin, A.H., 2013. “Advances in the integration of solar thermal energy with conventional and non-conventional power plants.” Renewable and Sustainable Energy

10 11

(B5)

Yan, Q., Yang, Y., Nishimura, A., Kouzani, A., Hu, E., 2010. “Multi-point and Multi-level Solar Integration into a Conventional Coal-Fired Power Plant.” Energy Fuels, 24 (7), pp. 3733-3738.

[6]

Yang, Y., Yan, Q., Zhai, R., et al, 2011. “An Efficient Way to Use Medium-or-Low Temperature Solar

20

Heat for Power Generation-Integration into Conventional Power Plant.” Applied thermal engineering

21

31, pp. 157-162. 13


[7]

2 3

Hou, H., Mao, J., Yang, Y., Luo, N..2012. “Solar-Coal Hybrid Thermal Power Generation- an Efficient Way to Use Solar Energy in China,” International Journal of Energy Engineering 2 (4), pp. 137-142.

[8]

Peng, S., Hong, H., Wang, Y., Wang, Z., Jin, H., 2014. “Off-design Thermodynamic Performances on

4

typical days of a 330 MW Solar-hybrid Coal-fired Power Plant in China.” Applied Energy, 130, pp.

5

500-509.

6

[9]

7 8

Coal-Fired Power Plant.” Entropy 15 (3), pp. 1014-1034. [10]

9 10

Peng, S., Wang, Z., Hong, H., Xu, D., Jin, H., 2014. “Exergy Evaluation of a Typical 330 MW Solar Hybrid Coal-fired Power Plant.” Energy Conversion and Management, 85, pp. 848-855.

[11]

11 12

Zhai, R., Zhu, Y., Yang, Y., Tan, K., Hu, E., 2013. “Exergetic and Parametric Study of a Solar Aided

Tamme, R., Buck, R., Epstein, M., Fisher, U., and Sugarmen, C., 2001, “Solar Upgrading of Fuels for Generation of Electricity,” ASME J. Sol. Energy Eng., 123, pp. 160-163.

[12]

13

Steinfeld A., 2005. “Solar Thermochemical Production of Hydrogen – a Review.” Solar Energy, 78: 603-615.

14

[13]

Fletcher, E.A., Moen, R.L., 1977. “Hydrogen- and Oxygen from Water,” Science, 4308, pp. 1050-1056.

15

[14]

Suresh M, Reddy K S, Kolar A K. 4-E (Energy, Exergy, Environment, and Economic) analysis of solar

16 17

thermal aided coal-fired power plants[J]. Energy for sustainable development, 2010, 14(4): 267-279. [15]

Montes MJ, Abanades A, Martinez-Val JM, Valdes M. 2009. “Solar multiple optimization for a solar-

18

only thermal power plant, using oil as heat transfer fluid in the parabolic trough collectors.” Solar energy,

19

83, pp. 2165-2176.

20

[16]

Ishida, M., Kawamura, K., 1982. “Energy and exergy analysis of a chemical process system with

21

distributed parameters based on the energy-direction factor diagram.” Industrial Engineering and

22

Chemistry Process Design and Development 21, pp. 690–695.

23

[17]

Ishida, M., 2002. “Thermodynamics made comprehensible.” Nova Science Pub Inc.

24 25

14


Figures:

2 3

Figure 1 Schematic diagram of hybrid plant with solar-feed-water

4

5 6

(a) Hybrid solar power plant

7 8

(b) Individual coal plant

9

Figure 2 Schematic of energy and exergy flow 15


2 3

Figure 3 Theoretically net solar-to-power efficiency of hybrid solar plant

4 5

(a) Hybrid solar/330 MW coal fired plant

(b) Hybrid solar/500 MW coal fired power plant

Figure 4 Net solar-to-power efficiency with the exergy destruction 6

16

ACCEPTED MANUSCRIPT

(a) Variation of mback/mmain with DNI

(b) Variation of mback/mmain with incident angle

Figure 5 Behaviors of mback/mmain at different operation conditions 1 2

(a) Effect of solar irradiation DNI

(b) Effect of incident angle

Figure 6 Behaviors of the turbine efficiency at different operation conditions 3 4 5

17

ACCEPTED MANUSCRIPT

1 2

Figure 7 Variation of solar collector efficiency versus turbine load

3 4 5

6 7

Figure 8 Variation of solar share at different turbine load

8 9 10

18

ACCEPTED MANUSCRIPT

(a) Influences of DNI

(b) Influences of incident angle θ

Figure 9 Behaviors of the net solar-to-power efficiency at different operation conditions 1 2

19

1

Tables:

2

Table 1 Parameters for different turbine loads 100% THA Temperature Pressure (oC)

Flow rate

75% THA Enthalpy Temperature Pressure

(MPa)

(kg/h)

(kJ/kg)

(oC)

50% THA

Flow rate

Enthalpy

Temperature

Pressure

(MPa)

(kg/h)

(kJ/kg)

(oC)

Flow rate Enthalpy

(MPa)

(kg/h)

(kJ/kg)

s1

538

16.7

1034601

3536.5

538

16.7

753340

3545.3

538

13.5

499300

3553.5

s2

538

3.472

875791

3396.9

538

2.581

647691

3396.9

538

1.751

435982

3432.6

s3

54

0.015

688877

2441.8

54

0.015

525083

2479.2

54

0.015

367555

2539.1

s4

54

0.015

822023

226

54

0.015

613155

226

54

0.015

418291

226

s5

380.1

5.7114

61830

3134.6

351.8

4.1829

38790

3095.4

344

2.8226

21686

3107.4

s6

329.7

3.8574

75873

3046.9

305.6

2.868

50319

3013.3

299.4

1.9459

28926

3025.3

s7

451

1.9557

36195

3360.6

452

1.461

24053

3369.2

453.9

0.9993

12774

3379.2

s8

359.9

1.0278

33892

3178.9

361.8

0.7714

23344

3187.7

364.8

0.5301

14351

3198.2

s9

290.8

0.5969

40170

3043.1

293.3

0.4498

27602

3052.2

296.9

0.3104

17012

3063

s10

208

0.2558

43210

2884.7

210.9

0.2036

30283

2893

215.1

0.1411

19129

2904

s11

106

0.0873

48378

2689.9

108.1

0.00693

28831

2696.6

111.4

0.0478

13144

2705.3

s12

55.4

1.724

822027

231.8

55.7

1.724

613157

233

56.4

1.724

418294

236.2

s13

93.1

1.724

822027

391.1

85.6

1.724

613157

359.6

76.2

1.724

418294

320.2

s14

125.4

1.724

822027

527.7

116.4

1.724

613157

489.5

105.2

1.724

418294

442.3

s15

153.9

1.724

822027

649.6

143.2

1.724

613157

603.9

130.2

1.724

418294

548

s16

182.7

20.35

1034601

758.3

170.1

20.35

753340

705.6

155.8

20.35

499300

642

s17

209.7

20.35

1034601

904

195.6

20.35

753340

840.8

178.5

20.35

499300

764.2

s18

246.4

20.35

1034601

1070.2

229.7

20.35

753340

992.9

209.4

20.35

499300

900.2

s19

272.1

20.35

1034601

1192

252.8

20.35

753340

1100

230.4

20.35

499300

995.2

20