Organic Rankine cycle performance evaluation and thermoeconomic assessment with various applications part I: Energy and exergy performance evaluation

Renewable and Sustainable Energy Reviews 53 (2016) 477–499

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Organic Rankine cycle performance evaluation and thermoeconomic assessment with various applications part I: Energy and exergy performance evaluation Gang Li a,b a b

Ingersoll Rand Residential Solutions, 6200 Troup Highway, Tyler, TX 75707, United States Ingersoll Rand Engineering and Technology Center-Asia Pacific, Shanghai 200051, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 23 April 2015 Received in revised form 27 May 2015 Accepted 30 August 2015

The utilization of low grade energy and renewable energy heat sources for power generation in organic Rankine cycle (ORC) system has received more attention in recent decades. In this study, working fluid candidates for various ORC applications based on the heat source temperature domains have been investigated for the thermal efficiency, exegry destruction rate and mass flow rate under different ORC configurations. The net power output from the ORC remains constant. The thermal efficiency increases as the condensing temperature diminishes, and decreases as the evaporating pressure recedes. As the condensing temperature and evaporating pressure are fixed, it can be found that as the critical temperature of the working fluid is increased, the thermal efficiency can be increased. As the heat source temperature scale increases, the operating evaporating pressure of the working fluids can be extended. The ORC with internal heat exchanger (IHX) has a higher thermal efficiency than the baseline ORC. The reheat ORC thermal efficiency is close to the baseline ORC. The regenerative ORC can achieve higher thermal efficiency than the baseline by reducing the addition of heat from the evaporator heat source. The performance of working fluid mass flow rate can reach their maximum in the low thermal efficiency region. The ORC with IHX and regenerative ORC have a lower value for exergy destruction as compared to baseline. Reheat ORC has a slightly higher exergy destruction rate. The evaporator is the largest contributor for the exergy destruction rate. In addition, the effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance has been revealed and identified. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Organic Rankine cycle Internal heat exchanger Reheat Regenerative Working fluid Cycle configuration Exergy destruction

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 Working fluid selection and cycle design configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 2.1. Working fluid selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 2.2. Cycle design configuration and modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 2.2.1. Baseline organic Rankine cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 2.2.2. Organic Rankine cycle with internal heat exchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 2.2.3. Organic Rankine reheat cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 2.2.4. Organic Rankine regenerative cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 2.2.5. Cycle configuration modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 3.1. Geothermal application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 3.1.1. Working fluid candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 3.1.2. Working fluid energy performance and exergy performance with different ORC configurations . . . . . . . . . . . . . . . . . . . . . . . . 480 3.1.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance. . . . . . . . . . . . . . 485 3.2. Low temperature solar application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

E-mail address: [email protected] http://dx.doi.org/10.1016/j.rser.2015.08.066 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

478

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

3.2.1. Working fluid candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 3.2.2. Working fluid energy performance and exergy performance with different ORC configurations . . . . . . . . . . . . . . . . . . . . . . . . 486 3.2.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance. . . . . . . . . . . . . . 489 3.3. High temperature waste heat recovery application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 3.3.1. Working fluid candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 3.3.2. Working fluid energy performance and exergy performance with different ORC configurations . . . . . . . . . . . . . . . . . . . . . . . . 491 3.3.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance. . . . . . . . . . . . . . 493 3.4. High temperature solar/biomass application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 3.4.1. Working fluid candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 3.4.2. Working fluid energy performance and exergy performance with different ORC configurations . . . . . . . . . . . . . . . . . . . . . . . . 493 3.4.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance. . . . . . . . . . . . . . 495 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

1. Introduction The utilization of waste or renewable energy has attracted much interest recently facing the shortage of fossil fuels and serious environmental pollution including global warming caused by the growing industrialization. Great efforts from both the governments and several organizations have been carried out to relieve such issues. Regarding the thermodynamic cycles, researchers have proposed several cycles including organic Rankine cycle (ORC), Kalina cycle, supper critical CO2 cycle, triangle cycle, and heat pipe technology [1–4]. Among various cycles, ORC is more practical and more widely used. It has the main advantages of simplicity and the component availability. The working fluid as the organic substance can be better adapted than water for lower heat source temperatures. In addition, the local and small scale power generation makes it possible to utilize the ORC technology, which is unlike the traditional Rankine power cycles. The high waste energy utilization can be achieved by ORC when compared with other waste heatrecovery approaches and it is easy to downsize the system volume and weight. What is more, the cost of ORC is cheaper than others such as a thermoelectric generator. Many investigations about ORC were carried out with various categories according to their application domains, such as solar energy [5], biomass heat [6], geothermal energy [7], waste heat recovery (WHR) of internal combustion (IC) engines [8], WHR of gas turbine exhausts [9] and bottoming of the water/steam Rankine cycle [1]. Regarding the geothermal power plants, one study was performed to investigate 31 pure component working fluids for sub-critical and supercritical ORCs [10]. For low-temperature solar organic Rankine cycle systems. another study by Tchanche et al. [11] comparatively accessed 20 working fluids. With waste energy sources as the heat source, Maizza and Maizza [12] investigated the thermodynamic and physical properties of 20 unconventional fluids used in organic Rankine cycles. For application in ORC biomass power and heat plants, Drescher and Bruggemann [6] developed software to find thermodynamically suitable fluids. Although an abundant literature is available on the fluid selection, there is a lack of comprehensive modeling work from both the energy and exergy aspects with various heat source levels, i.e. the various application domains. In addition, there is a lack of comprehensive performance comparison of simple ORC, ORC with internal heat exchanger (IHX), reheat ORC and regenerative ORC configurations. Therefore, the objective of the present study is to comprehensively investigate the energy and exergy performance with different working fluids under various applications. The thermodynamic models of various organic working fluids under these constrained situations were fabricated and calculated using Engineering Equation Solver (EES) software

package program [13]. The performance comparison is performed for each fluid in a feasible pre-defined operating region under the defined application domain based on the evaporating pressure and condensing temperature. The organization for this study is performed as follows: first, the background, motivation and objective are presented. Then the working fluid selection and ORC configuration are provided and described. After that the main part, “Section 3” details the energy and exergy performance comparison for various working fluids under different configurations for different application domains. The final discussion, makes the main summaries and show the main perspectives for performance and design improvements. 2. Working fluid selection and cycle design configuration 2.1. Working fluid selection The working fluid selection is critical for the system to achieve high thermal efficiencies with utilizing available heat source efficiently. There is a wide selection of the working fluids for various ORC applications. The saturation vapor curve affects the fluid applicability, cycle efficiency for power generation system. Usually a dry fluid has a positive slope while a wet fluid has a negative slope. An isentropic fluid has infinitely large slopes. It is generally accepted that the dry and isentropic fluids show better thermal efficiencies since the fluid going through the turbine do not experience condensing process while the wet fluid not. A comparison of the temperature–entropy diagram for dry, wet, and isentropic fluids is presented in Fig. 1. Good working fluids usually have the following characteristics:










Dry or isentropic fluids. High densities in both liquid and gas phase. Moderate critical parameters and high specific heat. Moderate evaporating and condensing pressures. Low toxicity, good material compatibility. Low flammability and corrosion. Good fluid stability.

Isentropic fluids such as R12 and R22 will not be investigated since they are being phased-out. The working fluids investigated are shown in Fig. 2. There are four ORC applications: geothermal application (heat source: 100 °C), low temperature solar application (heat source: 130 °C), high temperature waste heat recovery (heat source: 240 °C), and high temperature solar/biomass application (heat source: 290 °C). To have a better thermal match, applications with higher heat source temperatures usually have the working fluids with higher critical temperatures. Based on the literature review from the extensive work carried out, no working

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

s h ɛ

Nomenclature Abbreviations ORC WHR IC IHX EES

organic Rankine cycle waste heat recovery internal combustion internal heat exchanger Engineering Equation Solver

comp cond crit destr evap ex 0 p t

exergy destruction rate [kW] capacity [kW] mass flow rate [kg/s] pressure [kPa] temperature [°C]

fluid can be flagged as optical due to various operating conditions, technical maturity and match the needs of corresponding applications. All the working fluid candidates are performed to with the subcritical ORC for each profile. 2.2. Cycle design configuration and modeling In this section, the schematic of various ORC configurations, baseline ORC, ORC with IHX, reheat ORC, and regenerative ORC, is depicted. 2.2.1. Baseline organic Rankine cycle The simple ORC, which is the baseline ORC, is shown in Fig. 3. It includes the evaporator, turbine, condenser coil and the pump. The high temperature heat is absorbed in the evaporator at the high pressure from the heat source and the working fluid then shift into a saturated gas state. Then the high enthalpy saturated gas is then expanded in the turbine to produce the power output to the generator. After that the low pressure superheated gas is cooled down to the saturated liquid in the condenser. 2.2.2. Organic Rankine cycle with internal heat exchanger The ORC with IHX is delineated in Fig. 4. The thermal efficiency of the ORC system can be augmented by adding an IHX. The main difference with the baseline ORC is the IHX. The high temperature working fluid exhausted from the turbine is transported to the inlet of the low pressure side of IHX. While the low temperature working fluid exported from the pump is conveyed to the inlet of the IHX high pressure side. As a result, the heat is transferred from the low pressure side to the high pressure side. Regarding the IHX, the pressure loss and heat rejection to the environment are not considered. The effectiveness for the IHX is expressed as [14] ɛ ¼ ðT 4  T 4a Þ=ðT 4  T 2 Þ

entropy [kJ/kg K] enthalpy [kJ/kg] heat exchanger effectiveness [1/1]

Subscripts

Symbols _ Ex Q_ _ m P T

479

ð1Þ

2.2.3. Organic Rankine reheat cycle The purpose of a reheating cycle is to remove the moisture carried by the steam at the final stages of the expansion process, as shown in Fig. 5. In this variation as compared with baseline ORC, two turbines work in series. The first accepts vapor from the evaporator at high pressure. After the vapor has passed through the first turbine, it re-enters the evaporator and is reheated from the heat source before passing through a second, lower-pressure turbine. Here the low pressure at point 3a is regarded as the reheat pressure. The reheat temperatures are very close or equal to the inlet temperatures; whereas the reheat pressure needed is lower

compressor condenser critical destruction evaporator exergy reference state pump turbine

of the original evaporator pressure. Among other advantages, this prevents the vapor from condensing during its expansion and thereby damaging the turbine blades, given that more of the heat flow into the cycle occurs at higher temperature. All other components are similar to that of the baseline ORC. 2.2.4. Organic Rankine regenerative cycle The regenerative ORC, as shown in Fig. 6, is so named because after emerging from the condenser (possibly as a subcooled liquid, point 1a) the working fluid is heated by steam (point 3a) tapped from the hot portion of the cycle. The fluid at 3a is mixed with the fluid at 2a (both at the same pressure) to end up with the saturated liquid at point 2a. The regenerative can effectively raise the nominal cycle heat input temperature, by reducing the addition of heat from the evaporator heat source. This configuration can improve the cycle efficiency, as more of the heat flow into the cycle occurs at higher temperature, which indicates the process can ensures cycle economy. 2.2.5. Cycle configuration modeling The energy and exergy analysis based on the first and second laws of the thermodynamics are evaluated. The pressure drops in the evaporator, condenser, internal heat exchanger, regenerator tank and pipes are neglected for the simplicity. In addition, the assumptions are made as follows: (1) The working fluids are selected as pure dry or isentropic fluids. (2) The heat quantity provided by the heat source is sufficient to meet the ORC operation and the system is assumed at the steady state. (3) The condensing temperature is ranged from 15 °C to 30 °C. (4) The evaporating pressure is ranged from 0.4 MPa to 2 MPa. (5) The reference temperature and heat sink temperature are set to 15 °C. (6) The net power output from the ORC is set to 30 kW. (7) The efficiency of the pump and turbine is set to 0.8. (8) The IHX effectiveness is set to 0.9. (9) There is no heat loss for the regenerative tank. (10) For the reheat ORC system, the reheat pressure is the average pressure of condensing pressure and evaporating pressure. (11) For the regenerative ORC system, the intermediate pressure is the condensing pressure plus the one third of pressure difference between the condensing side and evaporating side.

400

Critical temperature ( C)

150 100 50 0

300 250 200 150

R227ea RC318 R236fa R236ea R600 R245fa

200

T [°C]

Geothermal Low temperature solar High temperature waste heat recovery High temperature solar/biomass

350

SES36 R123 R113 n-Heptane Cyclohexane Toluene

R11

250

n-Heptane Cyclohexane Ethylbenzene

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

R227ea RC318 Butene R600 R245fa SES36 R123 R113

480

100

-50 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

50

Working fluids

s [kJ/kg-K]

Fig. 2. Critical temperature of working fluids for various applications.

R22

250 200

3

2

T [°C]

150

Evaporator

100

Pump

50

Turbine

0 -50 0.50

4

1 0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

Condenser

s [kJ/kg-K] Isopentane

250

Baseline

200 Heat source

Temperature

T [°C]

150 100 50

3

2 4

1

0

Heat sink

-50 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Entropy

s [kJ/kg-K] Fig. 1. Isentropic, wet and dry working fluids. (a) Isentropic, (b) Wet, and (c) Dry.

The mathematical models for various cycle configurations are shown in Table 1.

3. Results and discussion 3.1. Geothermal application 3.1.1. Working fluid candidates Regarding the geothermal application with ORC system, the heat source temperature is set to 100 °C. The working fluid

Fig. 3. Schematics of organic Rankine baseline cycle. (a) Organic Rankine baseline cycle and (b) T–s diagram for organic Rankine baseline cycle.

candidates are shown in Fig. 7. R227ea, RC318, R236fa, R236ea, R600, R245fa with an increasing critical temperature have been depicted, as shown in Fig. 7(a). The corresponding proper operating pressure range is identified in Fig. 7(b). In the current study, the evaporating pressure and condensing temperature are served as the variables to investigate the thermal efficiency, mass flow rate and exergy destruction with various ORC configurations. 3.1.2. Working fluid energy performance and exergy performance with different ORC configurations The thermal efficiency with various configurations is shown in Fig. 8. Under a certain region for the evaporating pressure and condensing temperature, the thermal efficiency increases as the

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

3

2a Evaporator

2

2

3b Evaporator

Turbine 4 Internal heat exchanger

2

481

nd

stage

3 st 1 stage turbine

Pump 3a

Pump 4

1

4a

1

Condenser

Condenser ORC with reheat

ORC with IHX

Heat source

Temperature

Temperature

Heat source

3 2a

3

Reheat pressure

3a 2

2 4a 4

1

3b

4

1 Heat sink

Heat sink

Entropy

Entropy Fig. 4. Schematics of organic Rankine cycle with internal heat exchanger. (a) Organic Rankine cycle with internal heat exchanger and (b) T–s diagram for organic Rankine cycle with internal heat exchanger.

Fig. 5. Schematics of organic Rankine reheat cycle. (a) Organic Rankine reheat cycle, (b) T–s diagram for organic Rankine reheat cycle.

condensing temperature diminishes, and decreases as the evaporating pressure recedes. It can be found that the condensing temperature has a large effect on the thermal efficiency than the evaporating temperature. As the condensing temperature and evaporating pressure are fixed, it can be found that as the critical temperature of the working fluid is increased, the thermal efficiency can be increased. The reason may be explained from Refs. [15,16], with the simple ORC theoretical thermal efficiency expressed as (the unit for T is K here, Tm is the average temperature of evaporating and condensing temperature). 2 31 ! !n !n T nT m m 1  Tevap 1  TTcrit T T evap T cond crit crit 5 η ¼ 14 þ þ1  T crit T crit TT m 1  TT m 1  T cond 1  T cond

evaporating temperature, as shown in Fig. 7(b). Similar phenomenon can also be found in working fluid RC318, R236fa, R236ea, R600. Thermal efficiency is also compared with different ORC configurations. It can be found that the ORC with IHX has a higher thermal efficiency than the baseline ORC. The working fluid in the IHX, before entering the turbine, absorbs the heat from the working fluid exhausted from the single turbine – the enthalpy of the working fluid can be enhanced before entering the evaporator. As can be observed, under the same net power output, the heat addition of ORC with IHX is less than the baseline ORC. Therefore, the thermal efficiency can be enhanced. Regarding the reheat ORC, its thermal efficiency is quite close to the baseline ORC. The reheating cycle is usually used to remove the moisture carried by the steam at the final stages of the expansion process. If it is assumed that it has the same working fluid mass flow rate to the baseline, it can produce more useful net power output. However, it absorbs more heat at the evaporator side. As a result, under the same net power output, the reheat cycle may not achieve higher thermal efficiency than the baseline. The regenerative ORC can achieve higher thermal efficiency than the baseline. This is because the regenerative can effectively raise the nominal cycle heat input temperature, by reducing the addition of heat from the evaporator heat source. This configuration can improve the cycle efficiency, as more of the heat flow into the cycle occurs at higher temperature, which indicates the process can ensures cycle economy. The performance comparison can also be seen in Table 2.

T crit

T crit

crit

crit

ð2Þ  n¼

0:00264Lb þ0:8794 R  Tb

10 ð3Þ

As can be seen from the equations above, the thermal efficiency is dependent on the critical temperature and n values, which decide the working fluid’s molecular compositions and structures. The working fluids investigated here are dry or isentropic fluids, and the main factor influencing the thermal efficiency is the critical temperature. In the feasible working region, for working fluid R227ea, RC318, R236fa, R236ea, R600, R245fa, the thermal efficiency is increased. It can be also found that the working fluid R245fa has a narrow evaporating pressure, and this is because the saturation pressure is lower than other working fluids under same

482

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

2

2 Evaporator Pump 2

nd

stage

3 st 1 stage turbine

Regenerative tank 3a

2a

Condenser 1a

4

1

Pump1

Regenerative ORC

Temperature

Heat source

3 m

2

3a

2a

m m

1a

4

1 Heat sink

Entropy Fig. 6. Schematics of organic Rankine regenerative cycle. (a) Organic Rankine regenerative cycle and (b) T–s diagram for organic Rankine regenerative cycle.

Table 1 Mathematical models for various ORC configurations.

Component Pump Evaporator Turbine

Condenser IHX Regenerative tank

Component Pump

Performance

Baseline

IHX

Reheat

Regenerative

Energy relations _ ðh2  h1 Þ ¼ mðh _ 2s  h1 Þ=ƞp Wp ¼ m _ ðh2  h2a Þþ mðh _ 1a  h1 Þ Wp ¼ m _ ðh 3  h 2 Þ ¼m Q_

✓ ✓

✓ ✓

✓ ✓

✓ ○

○ ✓

○ ✓

○ ○

✓ ✓

evap

_ ðh 3  h 2 Þ þ m _ ðh3b  h3a Þ Q_ evap ¼ m _ ðh3  h4 Þ ¼ mðh _ 3  h4s Þ=ƞt Wt ¼ m _ ðh3  h3a Þ þ m _ ðh3b  h4 Þ Wt ¼ m _ ðh3  h3a Þ þ m _ 2 ðh3a  h4 Þ Wt ¼ m _ ðh4  h1 Þ Q_ cond ¼ m _ ðh4  h4a Þ ¼ m _ ðh2a  h2 Þ ¼m Q_

○

○

✓

○

✓ ○ ○ ✓

✓ ○ ○ ✓

○ ✓ ○ ✓

○ ○ ✓ ✓

○

✓

○

○

ɛ ¼ ðT 4  T 4a Þ=ðT 4  T 2 Þ ṁ1 h3a þ ṁ2 h1a ¼ ṁh2a _ 2 ¼m _ _ 1 þm m

○

○

○

✓

IHX

Exergy destruction rate (kW) _ 2  s1 Þ E_ xp ¼ T 0 mðs _ ðs2  s2a Þþ T 0 mðs _ 1a  s1 Þ E_ xp ¼ T 0 m

_ ððs3  s2 Þ ðh3  h2 Þ=T H  E_ xevap ¼ T 0 m _ ððs3  s2 Þ ðh3  h2 Þ=T H  þ T 0 m _ ððs3b  s3a Þ ðh3b  h3a Þ=T H  E_ xevap ¼ T 0 m

✓

✓

✓

○

○

○

○

✓

✓

✓

○

✓

○

○

✓

○

✓

✓

○

○

○

○

✓

○

○ ✓

○ ✓

○ ✓

✓ ✓

IHX

_ 4  s3 Þ E_ xt ¼ T 0 mðs _ ðs3a  s3 Þ þ T 0 mðs _ 4  s3b Þ E_ xt ¼ T 0 m Ėxt ¼ T 0 ṁðs3a  s3 Þ þ T 0 ṁ2 ðs4  s3a Þ _ ððs1  s4 Þ  ðh1  h4 Þ=T L  E_ xcond ¼ T 0 m _ ðs4a  s4 Þ þ T 0 mðs _ 2a  s2 Þ E_ xIHX ¼ T 0 m

○

✓

○

○

Regenerative tank

Ėxtank ¼ T 0 ðṁ s2a  ṁ1 s3a  ṁ2 s1a Þ

○

○

○

✓

System performance Thermal efficiency

Energy relations ƞ ¼ ðW t  W p Þ=Q_

✓ ✓

✓ ✓

✓ ✓

✓ ✓

Total power output (kW) Total exergy destruction rate (kW)

30 ¼ W t  W p P_ E_ x ¼ Exi

✓ ✓

✓ ✓

✓ ✓

✓ ✓

Evaporator Turbine

Condenser

evap

destr

ƞ indicates the thermal efficiency and E_ xdestr indicates the exergy destruction rate with unit kW; ✓ indicates the cycle configuration includes that mathematical model while ○ not.

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483

Fig. 7. T–s and P–Tsat diagram of working fluids for geothermal application. (a) T–s diagram and (b) P–Tsat diagram.

The performance of working fluid mass flow rate is shown in Fig. 9. It can be found that the values can reach their maximum in the low thermal efficiency region. As the thermal efficiency decreases, the mass flow rate increases. Regarding the effect of various configurations, the ORC with IHX has a same curve for mass flow rate to baseline. Reheat ORC has a slightly lower mass flow rate than baseline as more working fluids work at high pressure regions when they are under the same net power output. The regenerative ORC has a slightly higher mass flow rate since part of the working fluid work at the lower working region. The performance of exergy destruction rate is shown in Fig. 10. It can be found that the values can reach their maximum in the low thermal efficiency region. As the thermal efficiency decreases, the exergy destruction increases. As to the various configurations, the ORC with IHX has a lower value for exergy destruction to baseline. Reheat ORC has a slightly higher exergy destruction rate than baseline since an additional turbine is involved for exergy destruction and the thermal efficiency of reheat ORC is not higher than the baseline. The regenerative ORC has slightly lower exergy destruction than the baseline ORC since the former has a higher thermal efficiency and thus reduces the addition of heat from the evaporator heat source, through additional regenerative rank is involved. The comparison can also be seen in Table 2. The exergy

Fig. 8. Thermal efficiency of working fluids for geothermal application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

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Fig. 9. Mass flow rate of working fluids for geothermal application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

Fig. 10. Exergy destruction rate of working fluids for geothermal application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

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485

Table 2 Working fluid performance under different ORC configurations for geothermal application (Pcond ¼ 1000 kPa, Teva ¼ 20 °C). Baseline

R227ea RC318 R236fa R236ea R600 R245fa

IHX

Reheat

Regenerative

ƞ

E_ xdestr

ƞ

_ destr Ex

ƞ

_ destr Ex

ƞ

_ destr Ex

6.84E  02 8.64E  02 9.62E  02 1.09E  01 1.12E  01 1.24E  01

7.00E þ01 4.92E þ 01 4.11E þ 01 3.29E þ 01 3.10E þ 01 2.50E þ 01

7.38E  02 9.76E  02 1.03E  01 1.19E  01 1.19E  01 1.33E  01

6.26E þ01 4.01E þ 01 3.63E þ01 2.77Eþ01 2.73E þ 01 2.14E þ 01

6.68E  02 8.38E  02 9.41E  02 1.06E  01 1.10E  01 1.23E  01

7.23E þ 01 5.16E þ 01 4.27E þ 01 3.42E þ01 3.21E þ01 2.58E þ01

7.35E 02 9.50E  02 1.05E  01 1.20E  01 1.22E  01 1.37E  01

6.41E þ 01 4.34Eþ 01 3.61E þ01 2.83Eþ 01 2.69Eþ 01 2.09Eþ 01

_ destr indicates the exergy destruction rate with ƞ indicates the thermal efficiency and Ex unit kW. 30

Exergy destruction rate (kW)

Geothermal: R245fa 25

20

15

10

5

0 Baseline

IHX

Reheat

Regenerative

ORC configurations Fig. 11. Exergy distribution of R245fa for geothermal application (Pcond ¼ 1000 kPa, Teva ¼20 °C).

distribution is shown in Fig. 11. The evaporator is the largest contributor for the exergy destruction rate. For the ORC with IHX, the evaporator side exergy destruction can be reduced definitely. The reheat ORC has a larger condenser exergy destruction rate than the baseline. The regenerative ORC depicts a considerable exegry destruction rate reduction from the evaporator side. In addition, the performance of turbine power output is shown in Fig. 12. It can be found that the turbine power output increases as the condensing temperature increases, and decreases as the evaporating pressure recedes. As to the various ORC configurations, the regenerative ORC has the highest turbine power output while the reheat ORC depicts the lowest. The pump power output has similar trend to that of the turbines since the net power output is fixed, and it is not discussed here. 3.1.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance To investigate different configurations in detail, the effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance has been revealed, as shown in Fig. 13. It can be found that as the IHX effectiveness is increased, which means the working fluid in the IHX, before entering the turbine, absorbs more heat from the working fluid exhausted from the single turbine – the enthalpy of the working fluid can be enhanced before entering the evaporator. As a result, the thermal efficiency increases and exergy destruction rate decreases. As the reheat pressure increases, there are less heat absorbed from the evaporator side for the second turbine, with same net power output fixed, the thermal efficiency increases and thus the exergy destruction rate decreases. The reheat mass flow rate increases as

Fig. 12. Turbine power output for geothermal application. (a) Baseline turbine power and (b) turbine power with different configurations.

the reheat pressure increases. Regarding the regenerative ORC, as the intermediate pressure increases, more working fluid mass flow occurs at the high pressure/evaporator side, which means more heat absorbed from the heat source while the total net power output keep fixed. As a result, the thermal efficiency decreases and exergy destruction rate increase. 3.2. Low temperature solar application 3.2.1. Working fluid candidates The low temperature solar application has the heat source temperature set to 130 °C. The working fluid candidates are shown

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Fig. 13. Effect of internal heat exchanger, reheat and regenerative configurations on the thermal performance for geothermal application (Pcond ¼ 1000 kPa, Teva ¼20 °C). (a) IHX, (b) Reheat, and (3) Regenerative.

in Fig. 14. R227ea, RC318, Butene, R600, R245fa, SES36, R123, R113 with an increasing critical temperature have been depicted, as shown in Fig. 14(a). The corresponding proper operating pressure range is identified in Fig. 14(b).

3.2.2. Working fluid energy performance and exergy performance with different ORC configurations With a better thermal match at the evaporator side, the thermal efficiency of the low temperature solar application is higher than that of the geothermal application, as shown in Fig. 15. The

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Fig. 14. T–s and P–Tsat diagram of working fluids for low temperature solar application. (a) T–s diagram and (b) P–Tsat diagram.

thermal efficiency, mass flow rate and exergy destruction with various ORC configurations are investigated. The working fluids R227ea, RC318, R600, and R245fa are investigated both for geothermal application and low temperature applications. Take R245fa for example, with a higher heat source temperature, the evaporating operating pressure range becomes wider and a higher thermal efficiency can be achieved within the operating pressure range. Obviously, the R245fa can be operated under wider evaporating pressure range and can achieve a higher thermal efficiency. In addition, under the same fixed evaporating pressure and condensing temperature, the main difference is that the exergy destruction increases. Regarding the low temperature solar application, under the certain region for the evaporating pressure and condensing temperature in Fig. 15, the thermal efficiency increases as the condensing temperature diminishes, and decreases as the evaporating pressure recedes. It can be found that the condensing temperature has a large effect on the thermal efficiency than the evaporating temperature. From the working fluid R227ea to R113, as the critical temperature is increased, the thermal efficiency increases. Thermal efficiency is also compared with different ORC configurations. Similarly, it can be found that the ORC with IHX has a

Fig. 15. Thermal efficiency of working fluids for low temperature solar application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

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Fig. 16. Mass flow rate of working fluids for low temperature solar application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

Fig. 17. Exergy destruction rate of working fluids for low temperature solar application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

higher thermal efficiency than the baseline ORC. The reheat ORC thermal efficiency is quite close to the baseline ORC. The regenerative ORC can achieve higher thermal efficiency than the baseline. The performance comparison can also be seen in Table 3. The performance of working fluid mass flow rate is shown in Fig. 16. Similarly, it can be found that the values can reach their maximum in the low thermal efficiency region. As the thermal

30

Exergy destruction rate (kW)

Low temperature solar: R123 25

489

efficiency decreases, the mass flow rate increases. The ORC with IHX has a same curve for mass flow rate to baseline. Reheat ORC has a slightly lower mass flow rate than baseline. The regenerative ORC has a slightly higher mass flow rate since part of the working fluid work at the lower working region. The performance of exergy destruction rate is shown in Fig. 17. It can be found that the values can reach their maximum in the low thermal efficiency region. As the thermal efficiency decreases, the exergy destruction increases. Similarly, the ORC with IHX has a lower value for exergy destruction to baseline. Reheat ORC has a slightly higher exergy destruction rate than baseline. The regenerative ORC has slightly lower exergy destruction than the baseline ORC. The comparison can also be seen in Table 3. The exergy distribution is shown in Fig. 18. The evaporator is still the largest contributor for the exergy destruction rate. Similar conclusions can be made as compared to the geothermal application.

20

15

10

5

0 Baseline

IHX

Reheat

Regenerative

ORC configurations Fig. 18. Exergy distribution of R123 for low temperature solar application (Pcond ¼1000 kPa, Teva ¼ 20 °C).

3.2.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance To investigate different configurations in detail, the effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance has been revealed, as shown in Fig. 19. Similarly, it can be found that as the IHX effectiveness is increased, the thermal efficiency increases and exergy destruction rate decreases. As the reheat pressure increases, the thermal efficiency increases and thus the exergy destruction rate decreases. The reheat mass flow rate increases as the reheat pressure increases. As the intermediate pressure increases, more working fluid mass flow occurs at the high pressure/evaporator side, the thermal efficiency decreases and exergy destruction rate increase.

Table 3 Working fluid performance under different ORC configurations for low temperature thermal application (Pcond ¼ 1000 kPa, Teva ¼20 °C). Baseline

R227ea RC318 Butene R600 R245fa SES36 R123

IHX

Reheat

Regenerative

ƞ

_ destr Ex

ƞ

_ destr Ex

ƞ

_ destr Ex

ƞ

_ destr Ex

6.84E  02 8.64E  02 1.01E  01 1.12E  01 1.24E 01 1.46E  01 –

9.52E þ01 6.91E þ 01 5.47E þ01 4.63E þ01 3.89E þ01 2.88E þ01 –

7.38E  02 9.76E  02 1.05E  01 1.19E  01 1.33E 01 1.56E  01 –

8.60Eþ 01 5.77Eþ 01 5.19E þ01 4.18E þ 01 3.44E þ01 2.48E þ01 –

6.68E  02 8.38E  02 9.98E  02 1.10E  01 1.23E  01 1.46E  01 –

9.81E þ01 7.22E þ 01 5.58E þ01 4.77Eþ01 3.99E þ01 2.88E þ01 –

7.35E 02 9.50E  02 1.09E  01 1.22E  01 1.37E  01 1.63E  01 –

8.76E þ01 6.15E þ01 4.96E þ01 4.11E þ 01 3.35Eþ 01 2.51E þ 01 –

ƞ indicates the thermal efficiency and E_ xdestr indicates the exergy destruction rate with unit kW.

Table 4 Working fluid performance under different ORC configurations for high temperature waste heat recovery application (Pcond ¼1000 kPa, Teva ¼ 20 °C). Baseline

SES36 R123 R113 n-Heptane Cyclohexane Toluene

IHX

Reheat

Regenerative

ƞ

E_ xdestr

ƞ

_ destr Ex

ƞ

_ destr Ex

ƞ

_ destr Ex

1.46E  01 1.52E  01 1.73E 01 2.01E  01 2.13E  01 2.43E  01

6.03E þ 01 5.66E þ01 4.60E þ 01 3.55E þ 01 3.19E þ 01 2.42E þ 01

1.56E  01 1.64E  01 1.98E  01 2.66E  01 2.49E  01 3.07E 01

5.43E þ01 5.04E þ01 3.63E þ01 1.95E þ01 2.29E þ01 1.29E þ01

1.46E  01 1.51E  01 1.71E  01 1.98E  01 2.11E  01 2.43E  01

6.04E þ01 5.73E þ 01 4.67Eþ 01 3.63Eþ 01 3.24Eþ 01 2.42Eþ 01

1.63E  01 1.69E  01 1.95E  01 2.28E  01 2.37E  01 2.69E  01

5.33Eþ 01 4.91E þ01 3.93Eþ 01 3.36Eþ 01 2.77Eþ 01 2.11E þ01

ƞ indicates the thermal efficiency and E_ xdestr indicates the exergy destruction rate with unit kW.

490

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Fig. 19. Effect of internal heat exchanger, reheat and regenerative configurations on the thermal performance for low temperature solar application (Pcond ¼ 1000 kPa, Teva ¼ 20 °C). (a) IHX, (b) Reheat, and (c) Regenerative.

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491

Fig. 20. T–s and P–Tsat diagram of working fluids for high temperature waste heat recovery application. (a) T–s diagram and (b) P–Tsat diagram.

3.3. High temperature waste heat recovery application 3.3.1. Working fluid candidates The high temperature waste heat recovery application has the heat source temperature set to 240 °C. The working fluid candidates are shown in Fig. 20. SES36, R123, R113, n-heptane, cyclohexane, toluene with an increasing critical temperature have been depicted, as shown in Fig. 20(a). The corresponding proper operating pressure range is identified in Fig. 20(b). 3.3.2. Working fluid energy performance and exergy performance with different ORC configurations With a higher heating source temperature and a higher critical temperature of working fluids selected, which indicates a better thermal match at the evaporator side, the thermal efficiency of the high temperature waste heat recovery application is higher than that of the geothermal application and low temperature solar application, as shown in Fig. 21. The working fluids SES36, R123, R113 are investigated both for high temperature waste heat recovery and low temperature solar applications. With a higher heat source temperature, the evaporating operating pressure

Fig. 21. Thermal efficiency of working fluids for high temperature waste heat recovery application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

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Fig. 22. Mass flow rate of working fluids for high temperature waste heat recovery application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

Fig. 23. Exergy destruction rate of working fluids for high temperature waste heat recovery application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

30

Exergy destruction rate (kW)

High temperature waste heat recovery: Toluene 25

Regenerative tank Internal heat exchanger Pump Turbine Condenser Evaporator

20

15

10

493

investigated, as shown in Fig. 25. Similarly, it can be found that as the IHX effectiveness is increased, the thermal efficiency increases and exergy destruction rate decreases. As the reheat pressure increases, in this case for the working fluid of toluene, the thermal efficiency and the exergy destruction rate only vary quite slightly. The reheat mass flow rate increases as the reheat pressure increases. As the intermediate pressure increases, more working fluid mass flow occurs at the high pressure/evaporator side, the thermal efficiency decreases and exergy destruction rate increases. 3.4. High temperature solar/biomass application

5

0 Baseline

IHX

Reheat

Regenerative

ORC configurations Fig. 24. Exergy distribution of toluene for high temperature waste heat recovery application (Pcond ¼1000 kPa, Teva ¼ 20 °C).

range becomes wider and a higher thermal efficiency can be achieved within the operating pressure range. In addition, under the same fixed evaporating pressure and condensing temperature, the main difference is that the exergy destruction increases. Regarding the high temperature waste heat recovery application, the thermal efficiency increases as the condensing temperature diminishes, and decreases as the evaporating pressure recedes. From the working fluid SES36 to toluene, as the critical temperature is increased, the thermal efficiency increases. The comparison with different ORC configurations is performed for the thermal efficiency. Similarly, it can be found that the ORC with IHX has a higher thermal efficiency than the baseline ORC. The reheat ORC thermal efficiency is not higher than the baseline ORC. The regenerative ORC can achieve higher thermal efficiency than the baseline. The performance comparison can also be seen in Table 4. The performance of working fluid mass flow rate is shown in Fig. 22. Similarly, in the low thermal efficiency region the mass flow rate can reach their maximum. As the thermal efficiency decreases, the mass flow rate increases. The ORC with IHX has a same curve for mass flow rate to baseline. Reheat ORC has a slightly lower mass flow rate than baseline. The regenerative ORC has a slightly higher mass flow rate since part of the working fluid work at the lower working region. The performance of exergy destruction rate is shown in Fig. 23. Similarly, in the low thermal efficiency region the exergy destruction rate can reach their maximum. As the thermal efficiency decreases, the exergy destruction increases. Similarly, the ORC with IHX has a lower value for exergy destruction to baseline. Reheat ORC has a slightly higher exergy destruction rate than baseline. The regenerative ORC has slightly lower exergy destruction than the baseline ORC. The comparison can also be seen in Table 4. The exergy distribution is shown in Fig. 24. The evaporator is still the largest contributor for the exergy destruction rate. Similar conclusions can be made as compared to the aforementioned applications. 3.3.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance Regarding the high temperature waste heat recovery application, the effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance has been

3.4.1. Working fluid candidates The high temperature solar/biomass application has the heat source temperature set to 290 °C. The working fluid candidates are shown in Fig. 26. n-Heptane, cyclohexane, ethylbenzene with an increasing critical temperature have been depicted, as shown in Fig. 26(a). The corresponding proper operating pressure range is identified in Fig. 26(b). 3.4.2. Working fluid energy performance and exergy performance with different ORC configurations Similarly, the thermal efficiency of the high temperature solar/ biomass application is higher than aforementioned applications, as shown in Fig. 27. The working fluids of n-heptane, cyclohexane and ethylbenzene are investigated. With a higher heat source temperature, the evaporating operating pressure range becomes wider and a higher thermal efficiency can be achieved within the operating pressure range, such as n-heptane. Regarding the high temperature solar/biomass application, the thermal efficiency increases as the condensing temperature diminishes, and decreases as the evaporating pressure recedes. From the working fluid nheptane to ethylbenzene, as the critical temperature is increased, the thermal efficiency increases. The comparison with different ORC configurations is performed for the thermal efficiency. Similarly, it can be found that the ORC with IHX has a higher thermal efficiency than the baseline ORC. The reheat ORC thermal efficiency is not higher than the baseline ORC. The regenerative ORC can achieve higher thermal efficiency than the baseline. The performance comparison can also be seen in Table 5. The performance of working fluid mass flow rate is shown in Fig. 28. Similarly, in the low thermal efficiency region the mass flow rate can reach their maximum. As the thermal efficiency decreases, the mass flow rate increases. The ORC with IHX has a same curve for mass flow rate to baseline. Reheat ORC has a slightly lower mass flow rate than baseline. The regenerative ORC has a slightly higher mass flow rate since part of the working fluid work at the lower working region. This performance is more pronounced than aforementioned applications. The performance of exergy destruction rate is shown in Fig. 29. Similarly, in the low thermal efficiency region the exergy destruction rate can reach their maximum. As the thermal efficiency decreases, the exergy destruction increases. Similarly, the ORC with IHX has a lower value for exergy destruction to baseline and Ethylbenzene depicts lowest exergy destruction rate. Reheat ORC has a slightly higher exergy destruction rate than baseline. The regenerative ORC has surprising lower exergy destruction than the baseline ORC. The comparison can also be seen in Table 5. The exergy distribution is shown in Fig. 30. The evaporator is still the largest contributor for the exergy destruction rate and ORC with IHX and regenerative ORC have lower value than the baseline. Similar conclusions can be made as compared to the aforementioned applications.

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High temperature waste heat recovery: Toluene

High temperature waste heat recovery: Toluene 0.7

0.8

0.9

0.25 0.20 0.15

Baseline 0.10

30 25 20 15

Baseline

0.4 0.3 0.2

Baseline 0.1 0.5

0.6

0.7

0.8

0.9

400

500

600

700

800

900

0.25 0.20 0.15

Baseline 0.10

30 25 20 15

Baseline 10

0.5

Mass flow rate (kg/s)

10

0.5

Mass flow rate (kg/s)

300 0.30

1.0

Thermal efficiency

0.6

Exergy destruction rate (kW)

Thermal efficiency

0.5 0.30

1.0

Baseline

Exergy destruction rate (kW)

494

0.4 0.3 0.2 0.1 300

400

500

600

700

800

900

Reheat pressure (kPa)

Internal heat exchanger effectiveness

High temperature waste heat recovery: Toluene 400

500

600

700

800

900

0.25 0.20 0.15

Baseline 30

0.10

25 20 15

Baseline 10

Mass flow rate (kg/s)

0.5

Exergy destruction rate (kW)

Thermal efficiency

300 0.30

m

0.4 0.3 0.2 m 0.1 0.0 300

m 400

Baseline 500

600

700

800

900

Regenerative intermediate pressure (kPa) Fig. 25. Effect of internal heat exchanger, reheat and regenerative configurations on the thermal performance for high temperature waste heat recovery application (Pcond ¼ 1000 kPa, Teva ¼ 20 °C). (a) IHX, (b) Reheat, and (c) Regenerative.

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Fig. 26. T–s and P–Tsat diagram of working fluids for high temperature solar/biomass application. (a) T–s diagram, and (b) P–Tsat diagram.

3.4.3. Effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance Regarding the high temperature solar/biomass application, the effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance has been investigated, as shown in Fig. 31. Similarly, it can be found that as the IHX effectiveness is increased, the thermal efficiency increases and exergy destruction rate decreases. The reheat mass flow rate performance only varies slightly. Same phenomenon can also be found for the high temperature waste heat recovery application. It can be made the conclusion that under high heat source temperature scales, the effect of reheat pressure has little effect on the thermal efficiency, exergy destruction rate and mass flow rate. As the intermediate pressure increases, more working fluid mass flow occurs at the high pressure/evaporator side, the thermal efficiency decreases and exergy destruction rate increases. In this study, the main thermodynamic performance is evaluated for working fluids for various ORC applications. For the practical applications, the safety, low toxicity, good material compatibility, low flammability and corrosion, good fluid stability and low cost should also be considered.

Fig. 27. Thermal efficiency of working fluids for high temperature solar/biomass application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

496

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Fig. 28. Mass flow rate of working fluids for high temperature solar/biomass application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

Fig. 29. Exergy destruction rate of working fluids for high temperature solar/biomass application. (a) Baseline, (b) IHX, (c) Reheat, and (d) Regenerative.

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497

Table 5 Working fluid performance under different ORC configurations for high temperature solar/biomass application (Pcond ¼ 1000 kPa, Teva ¼20 °C). Baseline

n-Heptane Cyclohexane Ethylbenzene

IHX

Reheat

Regenerative

ƞ

_ destr Ex

ƞ

_ destr Ex

ƞ

_ destr Ex

ƞ

_ destr Ex

2.01E  01 2.13E  01 2.50E  01

4.29Eþ 01 3.89Eþ 01 2.87Eþ 01

2.66E  01 2.49E  01 3.08E  01

2.51E þ 01 2.89Eþ 01 1.76E þ01

1.98E  01 2.11E  01 2.49E  01

4.39E þ01 3.95E þ01 2.89E þ01

2.28E  01 2.37E  01 2.77E 01

4.02Eþ 01 3.40Eþ 01 2.71E þ01

ƞ indicates the thermal efficiency and E_ xdestr indicates the exergy destruction rate with unit kW.

Exergy destruction rate (kW)

30

High temperature solar/biomass: Ethylbenzene

(3)

Regenerative tank Internal heat exchanger Pump Turbine Condenser Evaporator

25

20

15

10

5

0 Baseline

IHX

Reheat

Regenerative

ORC configurations

(4)

Fig. 30. Exergy distribution of ethylbenzene for high temperature solar/biomass application (Pcond ¼1000 kPa, Teva ¼ 20 °C).

4. Conclusions (5) In this study, 14 working fluid candidates of various ORC applications based on the heat source temperature domains have been investigated for the thermal efficiency, exegry destruction rate and mass flow rate under different ORC configurations. Working fluid candidates, which are dry or isentropic fluids, are selected from the critical temperature and to ensure a better thermal match with the heat source temperature. The net power output from the ORC is constant to set to 30 kW. Both the energy and exergy performance is evaluated in an appropriate region defined by the evaporating pressure and the condensing temperature. The main conclusions are made as follows: (1) Under a certain region for the evaporating pressure and condensing temperature for various ORC applications, the thermal efficiency increases as the condensing temperature diminishes, and decreases as the evaporating pressure recedes. The condensing temperature has a large effect on the thermal efficiency than the evaporating temperature. As the condensing temperature and evaporating pressure are fixed, it can be found that as the critical temperature of the working fluid is increased, the thermal efficiency can be increased. (2) As the heat source temperature scale increases, the operating evaporating pressure can be extended and thus a higher

(6)

thermal efficiency can be achieved within the appropriate pressure region. The ORC with IHX has a higher thermal efficiency than the baseline ORC. The working fluid in the IHX, before entering the turbine, absorbs the heat from the working fluid exhausted from the single turbine – the enthalpy of the working fluid can be enhanced before entering the evaporator. Regarding the reheat ORC, its thermal efficiency is quite close to the baseline ORC. Sometimes it can be found that the reheat thermal efficiency is not higher than the baseline. This is because the reheating cycle is usually used to remove the moisture carried by the steam at the final stages of the expansion process. When employing the same mass flow rate, the reheat ORC can caused an increased net power, but at the cost of increased energy input from the evaporator side. The regenerative ORC can achieve higher thermal efficiency than the baseline by reducing the addition of heat from the evaporator heat source. The performance of working fluid mass flow rate can reach their maximum in the low thermal efficiency region. As the thermal efficiency decreases, the mass flow rate increases. The ORC with IHX has a same curve for mass flow rate to baseline. Reheat ORC has a slightly lower mass flow rate than baseline as more working fluids work at high pressure regions when they are under the same net power output. The regenerative ORC has a slightly higher mass flow rate since part of the working fluid work at the lower working region. The exergy destruction rate can reach their maximum in the low thermal efficiency region. The ORC with IHX has a lower value for exergy destruction to baseline. Reheat ORC has a slightly higher exergy destruction rate than baseline since an additional turbine is involved for exergy destruction and the thermal efficiency of reheat ORC is not higher than the baseline. The regenerative ORC has lower exergy destruction than the baseline ORC The evaporator is the largest contributor for the exergy destruction rate. The effect of IHX effectiveness, reheat pressure and regenerative intermediate pressure on system performance has been revealed to investigate different configurations in detail. It can be found that as the IHX effectiveness is increased, the thermal efficiency increases and exergy destruction rate decreases. As the reheat pressure increases, the thermal efficiency increases and thus the exergy destruction rate decreases. For the higher heat source temperature scale, the energy and exergy performance do not vary much. Regarding the regenerative ORC, as the intermediate pressure increases, more working fluid mass flow occurs at the high pressure/ evaporator side, which means more heat absorbed from the heat source while the total net power output keep fixed. As a result, the thermal efficiency decreases and exergy destruction rate increase.

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

High temperature solar/biomass: Ethylbenzene

High temperature solar/biomass: Ethylbenzene 0.7

0.8

0.9

0.3

0.2

Baseline 0.1

30 25 20 15

Baseline 10

0.5

Baseline 0.4 0.3 0.2 0.1 0.5

0.6

0.7

0.8

0.9

400

500

600

700

800

900

0.3

0.2

Baseline 0.1

30 25 20 15

Baseline 10

0.5

Mass flow rate (kg/s)

Mass flow rate (kg/s)

300 0.4

1.0

Thermal efficiency

0.6

Exergy destruction rate (kW)

Thermal efficiency

0.5 0.4

1.0

Baseline

Exergy destruction rate (kW)

498

0.4 0.3 0.2 0.1 300

400

500

600

700

800

900

Reheat pressure (kPa)

Internal heat exchanger effectiveness

High temperature solar/biomass: Ethylbenzene 400

500

600

700

800

900

0.3

0.2

Baseline 0.1

40 35 30 25 20

Baseline

10

0.5

Mass flow rate (kg/s)

15

Exergy destruction rate (kW)

Thermal efficiency

300 0.4

m

0.4 0.3

m 0.2 m

0.1

Baseline 0.0 300

400

500

600

700

800

900

Regenerative intermediate pressure (kPa) Fig. 31. Effect of internal heat exchanger, reheat and regenerative configurations on the thermal performance for high temperature solar/biomass application (Pcond ¼ 1000 kPa, Teva ¼ 20 °C). (a) IHX, (b) Reheat, and (c) Regenerative.

G. Li / Renewable and Sustainable Energy Reviews 53 (2016) 477–499

Acknowledgments No funding support. The author would like to express the deepest appreciation to Z. Li and P. Li for their endless love, support, and encouragement during the uncertainty of career path.

References [1] Wang J, Yan Z, Wang M, Ma S, Dai Y. Thermodynamic analysis and optimization of an (organic Rankine cycle) ORC using low grade heat source. Energy 2013;49:356–65. [2] Yamaguchi H, Zhang XR, Fujima K, Enomoto M, Sawada N. Solarenergy powered Rankine cycle using supercritical CO2. Appl Therm Eng 2006;26:2345–54. [3] Steffen M, Löffler M, Schaber K. Efficiency of a new triangle cycle with flash evaporation in a piston engine. Energy 2013;57:295–307. [4] Franco A, Vaccaro M. On the use of heat pipe principle for the exploitation of mediumelow temperature geothermal resources. Appl Therm Eng 2013;59 (1):189–99. [5] Wang M, Wang J, Zhao P, Dai Y. Multi-objective optimization of a combined cooling, heating and power system driven by solar energy. Energy Convers Manag 2015;89:289–97. [6] Drescher U, Bruggemann D. Fluid selection for the organic Rankine cycle (ORC) in biomass power and heat plants. Appl Therm Eng 2007;27:223–8.

499

[7] Cammarata G, Cammarata L, Petrone G. Thermodynamic analysis of ORC for energy production from geothermal resource. Energy Proc 2014;45:1337–43. [8] Capata R, Toro C. Feasibility analysis of a small-scale ORC energy recovery system for vehicular application. Energy Convers Manag 2014;86:1078–90. [9] Clemente S, Micheli D, Reini M, Taccani R. Bottoming organic Rankine cycle for a small scale gas turbine: a comparison of different solutions. Appl Energy 2013;106:355–64. [10] Saleh B, Koglbauer G, Wedland M, Fischer J. Working fluids for lowtemperature organic Rankine cycles. Energy 2007;32:1210–21. [11] Tchanche BF, Papadakis G, Lambrinos G, Frangoudakis A. Fluid selection for a low-temperature solar organic Rankine cycle. Appl Therm Eng 2009;29:2468– 76. [12] Maizza V, Maizza A. Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems. Appl Therm Eng 2001;21:381–90. [13] Engineering Equation Solver. F-Chart Software, Academic Processional Version, V.9.447; 2014. [14] Yari M. Performance analysis of the different organic Rankine cycles (ORCs) using dry fluids. Int J Exergy 2009;6:323–42. [15] Liu BT, Chien KH, Wang CC. Effect of working fluids on organic Rankine cycle for waste heat recovery. Energy 2004;29:1207–17. [16] Teng H, Regner G, Cowland C. Waste heat recovery of heavy-duty diesel engines by organic Rankine cycle part II: working fluids for WHR-ORC. SAE 2007-01-0543.