IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.
www.ijiset.com
ISSN 2348 – 7968
Exergy Analysis of Solar Power Tower Plants
Satyendra Chaturvedi1 Satyaveer Singh1, Rachiit Garga1
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Department of Mechanical Engineering IIMT group of institution, Greater Noida
Email: sattu8888@gmail.com , satyaveers392@gmail.com ,
32TU
U32T
32TU
U32T
Abstract: Establishing the renewable electricity contribution from solar thermal
power systems based on energy analysis alone cannot legitimately be complete
unless the exergy concept becomes a part of that analysis. This paper presents a
theoretical framework for the energy analysis and exergy analysis of the solar
power tower system using molten salt as the heat transfer fluid. Both the energy
losses and exergy losses in each component and in the overall system are evaluated
to identify the causes and locations of the thermodynamic imperfection. Several
design parameters including the direct normal irradiation (DNI), the concentration
ratio, and the type of power cycle are also tested to evaluate their effects on the
energy and exergy performance. The results show that the maximum exergy loss
occurs in the receiver system, followed by the heliostat field system, although main
energy loss occurs in the power cycle system. The energy and exergy efficiencies
of the receiver and the overall system can be increased by increasing the DNI and
the concentration ratio, but that increment in the efficiencies varies with the values
of DNI and the concentration ratio. It is also found that the overall energy and
exergy efficiencies of the solar tower system can be increased to some extent by
integrating advanced power cycles including reheat Rankine cycles and
supercritical Rankine cycles.
Introduction and Methodology
Molten-nitrate salt was the working fluid in the solar receiver. This distinguishes it
from other power tower technologies. Liquid salt at 550°F (288°C) is pumped from
a ‘cold’ storage tank through the receiver, where it is heated to 1050°F (565°C),
and then on to a ‘hot’ tank for storage. When power is needed from the plant, hot
salt is pumped to a steam generating system that produces superheated steam for
the turbine/ generator. From the steam generator, the salt is returned to the cold
tank where it is stored and eventually reheated in the receiver. Figure is a
schematic diagram of the primary flow paths. Determining the optimum storage
size to meet power-dispatch requirements is an important part of the system design
process. ( Molten Salt Power tower System) The heliostat field that surrounds the
tower is laid out to optimize the annual performance of the plant.
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IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.
www.ijiset.com
ISSN 2348 – 7968
The field and the receiver are also sized depending on the needs of the utility. In a
typical installation, solar energy collection occurs at a rate that exceeds the
maximum heat rate required to provide steam to the turbine. Consequently, the
thermal storage system can be charged at the same time that the plant is producing
Power at full capacity. With a solar multiple of ~3, a molten-salt plant located in a
high-insolation region can be designed for an annual capacity factor of ~70%.
Consequently, power towers could potentially operate at full power for 70% of the
year without the need for a back-up fuel source. By varying the size of the solar
field, solar receiver, and size of the thermal storage, plants can be designed with
annual capacity factors ranging between 20 to 70% .
(The Solar Tower Plant in Operation)
Schematic diagram of a solar tower power plant and t-s diagram
Dig 1
Test procedure
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IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.
www.ijiset.com
ISSN 2348 – 7968
The exergy 𝜑𝜑̇ associated with solar irradiation on the heliostat mirror surface 𝑄̇ can
be expressed as
𝜑𝜑̇ = 𝑄̇ (1𝑇𝑜
)…………………………………………………….. (1)
𝑇∗
𝜑𝜑̇ Exergy associated with the solar irradiation on the heliostat mirror
surface
𝑄̇ Energy associated with the solar irradiation on the heliostat mirror
surface
𝑻𝑻o Ambient Temperature
𝑻𝑻 * Apparent sun temp. as an exergy source and taken to be 4500 K
R
R
Useful exergy absorbed by the flowing molten salt is
𝜑𝜑𝑟𝑟𝑒𝑒c,ab𝑠𝑠 = 𝑚𝑚𝑚𝑚𝑠𝑠 ( (ℎ𝑏𝑏 − ℎ𝑎𝑎 ) − 𝑇𝑇0( 𝑠𝑠𝑏𝑏 – 𝑠𝑠𝑎𝑎)
)……………………………………….(2)
𝜑𝜑𝑟𝑟𝑒𝑒c,ab𝑠𝑠 = 𝑚𝑚𝑚𝑚𝑠𝑠 𝑐𝑐𝑝𝑝𝑚𝑚𝑠𝑠 (( 𝑇𝑇𝑏𝑏 – 𝑇𝑇𝑎𝑎) −
Tb
𝑇𝑇0𝑙𝑙𝑛𝑛 ))……………………………………….(3)
Ta
Result and discussion
Properties of state points in power cycle
Table 1.
State point
Temperature in ͦC
Pressure in kPa
1
45.8
10
2s
45.9
3150
2
46.0
3150
3
236.6
3150
4s
238.7
12,600
4
239.0
12,600
5
552.0
12,600
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IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.
www.ijiset.com
ISSN 2348 – 7968
6s
327.4
3150
6
353.5
3150
7s
45.8
10
7
45.8
10
8
-
-
9
-
-
Exergy analysis of the base case solar tower system
Table 2.
Subsystem
Received(kw) Delivered(kw)
Heliostat field 7478.8
5609.1
Centralreceiver 5609.1
3111.7
SGSS
3111.7
2793.5
Power cycle
2793.5
2080.6
Over all
7478.8
1830.9
Loss(kw)
1869.7
2497.4
318.2
712.9
5647.9
Efficiency(%)
75.00
55.48
89.77
74.48
24.48
From the preceding section, it is known the present analysis is based on
the energy balance and exergy balance of each subsystem. The analysis
for the SGSS heat exchangers and the Rankine power cycle depends
only on the thermodynamics properties of molten salt and steam at each
state as shown in fig. 1, which is well developed and self-evident. The
analysis for the central receiver is based on a thermal model, which is
modified from a validated model developed by Li et al. to validate the
modification; the present model was used to calculate the thermal
performance of the sandia National Laboratories molten salt electric
experiment based on the parameters provided. The calculate energy
efficiency of the receiver is 87.77%, which agrees well with the
predicted value of 87.73% from Li`s work, and the experimental average
efficiency of 87.5% from Bergan’s experiments. Therefore the
calculated results of the paper are reasonable, which are useful for
guiding the design and operation of solar power tower plants.
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IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.
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ISSN 2348 – 7968
The results of the exergy analysis of the base case system are listed in
table 2. The results of the exergy analysis show a distinct behavior. The
total exergy efficiency of the hole system is 24.5%, while the subsystem
exergy efficiencies are 75%, 55.5%, 89.8% and 74.5% for the heliostat
field, central receiver, SGSS and the power cycle, respectively. The
power cycle subsystem has relatively large exergy efficiency (74.5%),
and the corresponding percentage exergy loss is as small as 12.6%,
although main exergy loss occurs there. On the contrary, although the
central receiver has a large energy efficiency and a small energy loss
percentage (9.7%). It has the largest percentage exergy loss (44.2%).
Followed by 33.1% in the heliostat field subsystem. This is because the
fact the solar isolation is the energy of very high quality and great
irreversibility’s occur when the high quality isolation is absorbed to the
thermal energy with the temperature of about 700-900 K, as a result, the
energy loss in the central receiver has high quality, containing a lot of
exergy.
References
“Exergy and energy analysis of solar power tower plant” Xu, Zhifeng
Wang, Xin Li, Feihu sun Laboratory of solar thermal energy and
photovoltaic system, institute of electrical engineering. Chinese academy
of science, Beijing 100190,chaina.
[1] S. K. Tyagi, S. Wang, M. K. Singhal, S. C. Kaushik and S. R. Park,
“Exergy Analysis and Parametric Study of Con-centrating Type Solar
Collectors,” International Journal of Thermal Science, Vol. 46, 2007, pp.
1304-1310.
[2] C. Xu, Z. Wang, X. Li and F. Sun, “Energy and Exergy Analysis of
Solar Power Tower Plants,” Applied Thermal Engineering, Vol. 31, No.
17- 18, 2011, pp. 3904-3913.
[3] V. S. Reddy, S. C. Kaushik and S. K. Tyagi, “Exergetic Analysis and
Performance Evaluation of Parabolic Dish Stirling Engine Solar Power
Plant,” International Journalof Energy Research, 2012.
[4] I. Dincer and M. A. Rosen, “Exergy, Energy, Environ-ment and
Sustainable Development,” Elsevier, 2007.
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IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.
www.ijiset.com
ISSN 2348 – 7968
[5] G. Glatzmaier, “Summary Report for Concentrating Solar Power
Thermal Storage Workshop,” NREL/ TP-5500[6] S. C. Kaushik, V. S. Reddy and S. K. Tyagi, “Energy and Exergy
Analyses of Thermal Power Plants: A Review,”
[7] F. Cavallaro, “Fuzzy TOPSIS Approach for Assessing Thermal
Energy Storage in Concentrated Solar Power (CSP) Systems,” Applied
Energy, Vol. 87, No. 2, 2010, pp. 496-503.
[8] S. Flueckiger, Z. Yang and S. V. Garimella, “An Inte- grated
Thermal and Mechanical Investigation of Mol- ten-Salt Thermocline
Energy Storage,” Vol. 88, No. 6, 2011, pp. 2098-2105.
[9] R. Petela, “Exergy Analysis of the Solar Cylindrical Pa-rabolic
Cooker,” Solar Energy, Vol. 79, No. 3, 2005, pp. 221-233.
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