Energy and Exergy Analysis of Solar Thermal Energy-based

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Energy and Exergy Analysis of Solar Thermal Energy-based

Transcript Of Energy and Exergy Analysis of Solar Thermal Energy-based

Thomas S., et al. / International Energy Journal 18 (2018) 243 – 256

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Energy and Exergy Analysis of Solar Thermal Energy-based Polygeneration Processes for Applications in Rural India
Sanju Thomas*, Ajith Kumar G.*, Sudhansu S. Sahoo#,1, and Shinu Varghese$

Abstract – The objective of this paper is to analyze the energy and exergy of a solar thermal based polygenearation process used for rural application in India. Linear Fresnel reflector (LFR) is used as solar thermal energy generation source for polygeneration process. The LFR system design is slightly modified to bring down the cost of the solar field. Electric power, cooling and generation of hot water for process applications are various polygeneric applications discussed in this work. The system is modeled to be experimented for application in rural sector of India, where various processes can function in parallel to cater to livelihood enhancements. Such, stand alone units are useful for community based refrigeration units, especially during the season of harvest. Vapour absorption machine (VAM) and adsorption chiller are selected for consideration in cooling applications, while pasteurizing unit is explained as a hot water utilization method. Molten salt-based storage for short time duration is also investigated in the current study. Thermodynamic analysis and solar field optimization are done for the polygeneration process to identify various operating parameters. Key input parameters are varied to identify the changes in outputs of various components. Energy and exergy analysis shows that there is very less loss in most of the components, while LFR and VAM are found to be high exergy loss equipment. There is a scope of scaling up of such systems which can be operated as standalone units for applications in the rural regions.

Keywords – energy analysis, exergy analysis, linear Fresnel reflector (LFR), polygeneration, solar thermal.

1 1. INTRODUCTION
In a growing economy like India where energy demand is increasing continuously over the years, the power consumption for process heat applications by industries are significantly large [1]. India has an average monthly global horizontal irradiance (GHI) of 4 to 5.5 kWh/m2/day [2]. Efficient energy conversion systems have grown of importance for research due to increase in energy demand [3]. There is need for further study and research in using polygenerative systems for decentralised energy supply, nuances in transmission and distribution networks, reliability and reduction of greenhouse gases (GHG) emissions [4]. Newer research has included renewable energy (RE) sources for consideration in polygenerative process applications [5]. Solar thermal energy was used as a RE source for polygeneration application to analyse the advantages of using different processes together than operating individually [6]. Investigations on using solar thermal as a hybrid source for polygenerative applications such as electricity; space cooling and distillation have proved encouraging. The past two decadal developments in this regard have been encouraging and it is expected that in
*Department of Mechanical Engineering, COE, CUSAT, Ernakulum
682022, India.
#Department of Mechanical Engineering, CET, Bhubaneswar, Ghatikia, Bhubaneswar, 751003, India.
$Empereal-KGDS Renewable Energy Pvt. Ltd., Coimbatore 641035, India
1
Corresponding author; Tel: + 91 9337645056, Fax: + 674-2386182. E-mail: [email protected]

near future polygeneration process can be a competitive process for production of electricity and industrial applications [1]. Earlier research shows that, linear Fresnel reflector (LFR) can be implemented in the same thermodynamic process as a parabolic trough collector (PTC) and the same annual yield can be attained [7]. However, LFR has advantages such as low cost collectors, simplified receiver tubes and direct steam generation applications [8]. Small scale (<500m2) linear Fresnel collectors are suitable, among concentrating collectors, for cheap industrial process heat applications [9].
Solar heating and cooling (SHC) using absorption and adsorption methods are probably one of the best choices that have application in heat and power generation, especially in standalone applications. Solar refrigeration systems are very optimal, particularly in summer operation mode, when the maximum demand for cooling coincides with the maximum availability of solar radiation [10]. Energy diversification and environmental friendliness make absorption cooling system advantageous than vapour compression systems [11]. The absorption cooling systems are integrated to power generation system to decrease the waste heat and to improve the performance [12]. Research studies were conducted with integration of triple effect absorption refrigeration system for heating, cooling and hot water production [12].
Justification of performing exergetic and energetic analyses together can give a complete depiction of system characteristics. Such a comprehensive analysis is a convenient approach for the performance evaluation and determination of the steps towards improvement [13]-[15]. A number of researchers have performed energy and exergy analysis of absorption refrigeration system. Thermodynamic analysis of a single effect

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vapor absorption system has been analyzed extensively [16]-[19]. Some relative works on double effect vapor absorption refrigeration system considering the analysis of the first law of thermodynamics and the second law of thermodynamics are also available [20]-[21]. Exergy analysis of concentrators has been presented by researchers during previous works [22]. Beside thermal energy analysis, an exergy analysis can lead to understand better the energy conservation potential [23]. Influence of incorporation of solar thermal power generation on a steam power plant has been investigated in Nigeria [24]. Energy and exergy analysis of solar air heaters with double flow corrugated absorber plates has been analyzed [25]. Exergy efficiency analysis was conducted on multigenerational energy system which generates power, water and cooling simultaneously [26].
While lot of work has been focused on combined heating, cooling and power, very less work was found in literature on polygenerative process. Use of solar thermal, particularly LFR system based polygeneration is rarely found in literature.
After reviewing the earlier work done by researchers and considering the importance of standalone solar thermal operated polygeneration system, an attempt is made to integrate multi utility process heat applications for livelihood enhancement at a village level. With this motivation, the objectives of this paper are designed which are as follows:
• To design a renewable energy (solar thermal) based polygenerative system for power, heating and cooling applications at village level.
• To investigate the energy, exergy and entropy analysis of the components of the system described above.
• To conduct sensitivity analysis of the system by varying key input parameters for the system
2. SYSTEM DESCRIPTION
The system is modeled to operate during sunshine hours (0800-1800 h) for the weather conditions for coastal regions of the state of Odisha, India as a standalone unit. LFR being cost effective and with potential to produce direct steam, is considered as the heat source. Slight modifications are done on the LFR to reduce the size of the solar field and for the system to be cost effective. Molten salt is considered as a storage medium to take care of any effect of the passing clouds on the mirrors. The storage mechanism considered in the study is only as a support during the lean hours of sunshine, with a storage capacity for two hours. A single stage LiBr-H2O vapor absorption machine (VAM), adsorption chiller and a pasteurization unit are considered for application to utilize process heat.
The schematic layout of the system considered for the investigation is as shown in Figure 1 which is divided into four major sub systems.
• Solar energy steam generation • Thermal energy storage • Power generation • Process application

The major components present in the solar-based polygeneration system are as follows:
• Solar collector field (Modified LFR) • Steam drum • Thermal energy storage • Steam turbine • Single effect lithium bromide – water (LiBr-
H2O) Vapour absorption unit • Closed loop cooling tower for removing heat
from VAM • Pasteurization unit • Vapour adsorption chiller • Closed loop cooling tower to remove heat from
adsorption chiller • Process heater
The description of major components are mentioned below.
2.1 Linear Fresnel Reflector (LFR) Solar Energy Collector
In order to reduce the cost of existing LFR system some modifications in the system design has been introduced. The height of the LFR receiver is brought down from a typical value of about 10 m (commonly used) to about 5m above the plane of the mirrors. Two blocks of LFR system with six numbers of mirrors, each with length and a width of 48 m and 1.05 m respectively are considered to obtain the required mirror area. The distance between nearby mirrors is chosen as 0.4m so as to minimize the shadowing effect. One block of LFR is incorporated with multiple tube receivers while the other block is given a single tube receiver. Ray tracing of the proposed LFR system were done with coding. The block having multiple tubular cavities is shown in Figure 2 along with the ray tracing of the system at zero degree sun incidence. This novel LFR block results in reduced concentration but, it is sufficient to produce process steam necessary for the turbine input. The reduced height of receiver tubes makes them accessible from the ground for easy cleaning and maintenance.
Steam (wet) at designed pressure and temperature leave LFR at point 1, which enters the steam drum. Pure steam is taken at point 2 for storage and turbine after separation in steam drum. Saturated water along with feed water streams return to LFR system at point 5.
2.2 Thermal Storage System
Phase changing material (PCM) storage containing KNO3 and NaNO3 are considered as the storage medium. The storage capacity is sized for providing 2 hours of thermal storage along with a steam accumulator used to provide smooth steam flow during solar intermittency, i.e. during no sun periods and for early starts for the day. The objective of low sizing is to reduce the cost and optimize to the climatic conditions. Table 1 represents the properties of the storage system used.
Part of dry steam from steam drum enters thermal storage at point 4 and after giving up heat to PCM materials at point 21.

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2.3 Steam Turbine
Steam at 30 bar and 234°C enters to the turbine and is expanded to 2 bar pressure to produce 18 kW of electric power. Power production capacity is based on the auxiliary consumption of all power equipment required for the system to operate. Steam leaving at 2 bar is used for process heating applications. Isentropic efficiency is assumed as 60% for turbine energy calculations.
Pure steam from LFR enters the turbine to produce electricity at point 3. Fluid exiting from turbine enters process applications.
2.4 Process Heat Applications
The outlet steam from turbine drives a 15 TR single effect vapor absorption machine (VAM). LiBr+H2O is used for the present case. The chilled water from VAM is circulated in air handling unit to provide refrigeration effect for the agro-perishables. At point 9, hot water enters VAM and chilled water leave at point 7. Heat released in condenser is taken by condensate fluid

to cooling tower and after cooling, cold fluid enters VAM condenser at point 10 for heat extraction. The proposed VAM is typically sized to cool 7000 sq. ft space, which is ideal for storing perishables from harvest from a typical village community (Typical farm products being tomatoes, potatoes and onions). The return hot water at point 11 from VAM at 100°C is used for the heat needed for pasteurizing milk produced by the local farming community. The fluid exits from pasteurization unit at point 12. The temperature requirement for pasteurising milk is from the range of 62.8°C to 71.7°C. Hot water after pasteurization is available at 80°C which enters to this chiller at point 14. A 9 TR adsorption chiller is used for cooling application, points 17 and 15 being the inlet and the outlet of the chiller unit. After releasing heat to generator of chiller unit, the fluid exits at 19 which enter to process heater. The heat available at this point can be used for other process heat applications including industrial laundry and distillation. The warm water leaves at point 20 and mixes with fresh intake in feed water tank.

Table 1. Properties of molten salt considered for storage system.

Type of the Molten Salt

KNO3 and NaNO3

Density

2100 kg/m3

Specific heat

1822J/kgK

Melting temperature

223°C

Latent heat

106 kJ/kg

Fig. 1. Schematic layout of the considered system.
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Fig. 2. Ray tracing of LFR system.

3. MODELLING AND ANALYSIS
Energy and exergy analysis for the major components of the polygeneration system are presented in this section. Data considered for the analysis are mentioned in Table 2. To calculate the properties of working fluids, in-house code was used for calculation at each state points. The relevant energy balances are mentioned in the following sections.
3.1 Energy Analysis
Assuming steady flow with negligible potential and kinetic energy, energy equations can be written as follows:
∑ m i hi = + Q ∑ m o ho + Wnet (1)
Where m is mass flow rate, h is the specific enthalpy.
Q and Wnet are heat and net work transfer rate respectively. Subscripts i and o are inlet and outlet of steady flow apparatuses used.
Energy balance equations of major components of the system considered are presented in Table 3. The Hottel–Whillier [27] equation for the actual useful heat gain (Qu) of a concentrating solar collector system like LFR system may be used as following,
= Qu FR AA C ( ρRα A ) IS −UL (T − Ta ) − εσ (T 4 − Ta4 ) (2)
Where AA is the area of the absorber surface, UL is the overall heat loss coefficient, C is the concentration ratio, σ is the Stefan-Boltzmann constant and ρR, αA, ε and FR are the reflectivity, absorptivity, emissivity and heat removal factor of the collector respectively. Is the solar intensity, T is the temperature of the collector and

Ta is the ambient temperature. The heat removal factor FR is defined as [27].

mC  AAULF ' 

= FR

p 1 − e mCp 

AU 



(3)

A L 



Where m and F’ are mass flow rate and heat removal coefficient. Overall heat loss coefficient, UL for LFR system is dependent on many factors such as cavity geometry, number of tubes, emissivity of tubes and glass cover, wind velocity induced convective heat transfer coefficient. The approach to get UL in this paper is adopted as described in [28], [29].

3.2 Exergy Analysis
Exergy balance equations can be written in many ways. In this paper, entropy balance and exergy balance [30] which are related to each other are mentioned as below

∑ m i si + Sgen ∑ =m o so

(4)

∑ m iψ i +ψ=Q ∑ m oψ o +ψWnet + Ta Sgen

(5)

Where s and ψ are entropy and exergy respectively. Sgen is entropy generation in sub system and TαSgen is exergy loss or destruction energy.
ψQ is the exergy rate due to heat transfer across the control volume at temperature, T and is written as

ψ Q= (1− Ta / T )Q

(6)

Similarly, ψWnet is exergy rate due to shaft work and can be written as

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ψ Wnet = Wnet (7)

The entropy balance and exergy balance equations are written in Table 4 and Table 5 using the above mentioned procedures.

3.3 Energy and Exergy Efficiency
Energy and exergy efficiency are otherwise called as 1st law and 2nd law efficiency [30], respectively.

ηII = ηI ηrev

(8)

COPII = COPI COPrev

(9)

4. RESULTS AND DISCUSSION
For obtaining results, certain fixed data were assumed which are listed in Table 2. The salient point data obtained using Equations mentioned earlier are listed Table 6.
Figure 3 shows the mass flow rate of steam generated from LFR system with respect to DNI and solar collector area.
Two parameters such as direct solar radiation and collector areas are varied between 4-6 kWh/m2 and 400800m2 for the analysis of the system. The data varied are considered based on the solar data available across India so that the system can be scaled and replicated elsewhere.

Fig. 3. Mass flow rate generated with variation in DNI and solar reflector area.

Fig. 4. Thermal power content of steam generated with variation in DNI and solar reflector area. www.rericjournal.ait.ac.th

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Table 2. Data considered for system analysis. Items Solar area DNI Efficiency VAM capacity ADC Capacity Steam input pressure Steam input temperature Feed water temperature Operating hours Mass flow rate from solar steam generator Mass flow rate to thermal energy storage system Mass flow rate to steam turbine Inlet enthalpy to turbine Outlet steam pressure from turbine Isentropic enthalpy Isentropic efficiency Actual enthalpy change Turbine power output Actual enthalpy at the outlet of turbine Temperature at the outlet of turbine COP of VAM Temperature at the outlet of VAM Mass flow rate of steam required for VAM Temperature at the outlet of pasteurization Thermal power for VAM Thermal power output for pasteurization COP of ADC Mass flow rate available for ADC from steam turbine outlet Temperature at the outlet of ADC Enthalpy at the inlet of ADC Mass flow rate for ADC Thermal power required for ADC Temperature at the outlet of ADC Thermal power available for process steam

Value 600
5 57% 15
9 30 234 40 8 291 58 233 2803 2 2336 60% 278 18 2525 120 0.65 100 139 80 81 3 0.611 94 100 1219 233 52 99.6 16

Units m2
kWh / m2 /day
TR TR bar °C °C hours kg / h kg / h kg / h kJ / kg bar kJ / kg
kJ / kg kWe kJ / kg °C
°C kg / h
°C kW kWe
kg / h °C
kJ / kg kg / h kW
°C kW

Table 3. Energy balance equations of sub-systems of integrated systems

LFR

m5h5 + Qu = m1h1

Steam drum

m1h1 + m22h22 =m2h2 + m5h5

Steam turbine

m3h3 =m6h6 + m13h13 + W

VAM

m6h6 + m9h9 + m10h10 = m7h7 + m8h8 + m11h11

Pasteurisation unit

m11h11 − Qpstrztn = m12h12

Adsorption chiller

m14h14 + m17 h17 + m18h18 = m15h15 + m16h16 + m19h19

Process heater

m19h19 − Qprocess = m20h20

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Table 4. Entropy balance equations of sub-systems of integrated systems.

LFR

∫T1 δ Q

m5s5 +

LFR + Sgen,LFR = m1h1

T

T5

Steam drum

m1s1 + m22 s22 + Sgen,s _ drum =m2 s2 + m5s5

Steam turbine

m3s3 + Sgen;turbine = m6 s6 + m13s13

VAM

m6 s6 + m9 s9 + m10 s10 + Sgen;vam = m7 s7 + m8s8 + m11s11

Pasteurisation unit

∫ δ T12
m11s11 +
T11

Qpn + Sgen, pn = m12 s12 T

Adsorption chiller

m14 s14 + m17 s17 + m18s18 + Sgen;chiller = m15s15 + m16 s16 + m19 s19

Process heater

T20
∫ δ m19s19 + T19

Qprocess + Sgen, process = m20 s20 T

Table 5. Exergy balance equations of sub-systems of integrated systems.

LFR

m5ψ 5

+ Qu

1 −

Ta

 

=

m1ψ1 + Ta Sgen,LFR

 T

Steam drum

m1ψ1 + m22ψ 22 = m2ψ 2 + m5ψ 5 + Ta Sgen,s _ drum

Steam turbine

m3ψ=3 m6ψ 6 + m13ψ13 + W + Ta Sgen,turbine

VAM

m6ψ 6 + m9ψ 9 + m10ψ10 = m7ψ 7 + m8ψ 8 + m11ψ11 + Ta Sgen;vam

Pasteurisation unit Adsorption chiller



−Q

 1−

Ta

 =

11 11

pn


Tcv 

m12ψ12 + Ta Sgen, pn

m14ψ14 + m17ψ17 + m18ψ18 = m15ψ15 + m16ψ16 + m19ψ19 + Ta Sgen,chiller

Process heater

m ψ − Q (1− Ta =)

19 19

process

T

cv

m20ψ 20 + Ta Sgen, process

Table 6. Properties at each state point in the current system.

Salient points

Pressure (bar)

Temperature (C)

Mass flow rate (kg/h)

1

30

234

582

2

30

234

291

3

30

234

233

4

30

234

58

5

30

140

582

6

2

120

139

7

1

7

9018

8

1

41

12906

9

2

12

9018

10

3

32

12906

11

1

100

139

12

1

80

139

13

2

120

94

14

2

120

233

15

1

7

5411

16

1

41

8034

17

2

12

5411

18

3

32

8034

19

1

99.6

233

20

1

40

233

21

1

40

58

22

30

40

291

Enthalpy (kJ/kg) 1905.818 2803.264 2803.264 2803.264 589.281 2525.153 29.523 171.801 50.602 134.373 417.436 334.990 2525.153 1218.567 29.523 171.801 50.602 134.373 417.436 167.623 167.623 170.191

Entropy (kJ/kgK)
4.415 6.185 6.185 6.185 1.732 6.666 0.106 0.585 0.180 0.464 1.302 1.075 6.666 3.344 0.106 0.585 0.180 0.464 1.302 0.572 0.572 0.571

Thermal Power (kW)
308 227 181 45 95 97 74 616 127 482 16 13 66 79 44 383 76 300 27 11
14

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It is seen that mass flow rate of steam generated is more for high DNI and high solar reflector area. The data sets are generated for 30 bar, 140°C inlet to the solar field and 30 bar, 234°C inlet to the turbine. Similarly, Figure 4 depicts the thermal power output of steam from solar block with variation in DNI and solar reflector area. It is seen that the power content (thermal) is more for high DNI and high reflector area. It means that with increase in demand, the area of LFR is to be increased so as to increase the mass flow rate and thermal power output. Higher DNI at selected locations will be useful as DNI is a factor for better output.

Figure 5 shows the mass flow rate of steam available for heating storage medium with variation in DNI. It is seen that with high mass flow rate resulted in high thermal power stored in storage medium. Although the system has been designed for 2 hour back up time during no sun period but due to high mass flow rate circulated, thermal power storage increases as well which may lead to more back up time. Sensitivity analysis shows that late afternoon cloud cover for periods of more than 45 minutes have largely affected storage efficiency and early morning start operations.

Mass flow rate of steam for heating the storage media, kg/h Thermal power stored in storage media, kW

280

280

P,T of fluid entering the storage system: 30bar, 234C

240

240

200

200

160

160

120

120

80

80

40

40

0

0

4

5

6

7

8

DNI, kWh/m2/day

Fig. 5. Mass flow rate of steam sent for heating the storage media and thermal power stored with respect to DNI.

As expected, the turbine power output from turbine and thermal power at the end of turbine increases with increase in mass flow rate as shown in Figure 6. The conditions of pressure and temperature at inlet of the turbine are maintained at 30 bar and 234°C. The exit pressure has been fixed at 2 bar as the fluid at this pressure will be required for heat requirement to VAM machine and adsorption chiller.

Analysis show that COP of the VAM is decreasing with increase in mass flow rate to generator of the VAM while the thermal power increases with the mass flow rate, which is depicted in Figure 7. The evaporator section is undisturbed as it absorbs same heat irrespective of heat supplied to generator. The COP of the adsorption chiller is also decreasing with increase in mass flow rate to generator while the thermal power increases. The same is shown in Figure 8.

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Thermal power at the exit of turbine. kW

Turbine power output, kWe

50

500

P,T at entry: 30 bar, 234 C

P,T at exit: 2 bar,120 C

450

40 Isentropic efficiciency: 60%

400

30

350

Turbine output

300

20

250

200 10

150

Thermal power at the exit of turbine

0

100

200

300

400

500

600

Mass flow rate in Turbine, kg/s

Fig. 6. Turbine power output and thermal power at the exit of the turbine with respect to mass flow rate.

Thermal power for VAM, kW

COP of VAM

VAM capacity: 15TR

90

1.0

Evaporator Temperature: 7C

Condenser Temperature: 32C

85

Generator Temperature: 120C

0.9 COP 80

75

0.8 70

65
0.7 Thermal power
60

0.6

55

50

90

100

110

120

130

140

150

Mass flow rate of hot fluid for generator of VAM (kg/s)

Fig. 7. COP of VAM and thermal power with respect to mass flow rate of hot fluid for generator.

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Thermal Power of Adsorption Chiller,kW

COP of adsorption chiller

95

Adsorption chiller capacity: 9TR

0.80

Evaporator temperature: 7C

Condenser Temeprature: 32C

90

0.75

Generator Temperature: 110C

85

0.70

80

0.65

75

0.60

70

65 0.55

60 0.50

55

140

160

180

200

220

240

260

Mass flow rate of hot fluid for generator of chiller (kg/h)

Fig. 8. COP of adsorption chiller and thermal power with respect to mass flow rate of hot fluid for generator.

Irreversibility of all components is shown in Figure 9. The majority of the components have less exergy losses. This benefits the system because there is less heat losses in these parts. It is seen that LFR and VAM experiences much irreversibility compared to other and hence these components need to be focused more during operation.

Energetic and exergetic efficiency values are mentioned in Figure 10. It is seen that Exegetic efficiency of the turbine is more compared to energetic efficiency. Similarly the energetic COP of chiller and VAM are found to be more than exergetic COP.

18

16

14

Irreversibility (kW)

12

10

8

6

4

2

0

LFR drum Steam

Turbine

VAM risation asteru
P

Chiller heater rocess
P

Fig. 9. Irreversibility associated with differeAnt components of the considered system.

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PowerMass Flow RateSteamLfrVam