Integrated Thermoelectric Energy Storage in A
Combined Cycle Solar Power Plant (tour)
BENALI ABDELHAKIM
Laboratory of ENERGY in ARID Zones, Faculty of Sciences and Technology,
Tahri Mohammed Béchar University,
Street of Independence Béchar, Bp 417,08000, ALGERIA
Abstract: Multi megawatt-thermoelectric energy storage based on thermodynamic cycles is an ambitious solution for the
renewable energies conversion. The main advantage of this technology is the capacity of energy storage, However, ensuring
the operation of TEES stations during unfavorable weather conditions is suspended. In this article, a specific thermoelectric
energy storage system was studied «TEES», the TEES system converts electrical energy into sensible heat by means of an
electric heater that uses the joule heating effect, the system TEES converts sensible heat into electrical energy by means of a
hybrid power plant that operates on a combined cycle « Brayton and Rankine ». The hybrid power plant uses two thermal
energy sources In order to secure the station in unfavorable weather conditions for the production of solar energy. The main
idea is to using the gaz and the sensible heat stored as two thermal inputs in the gas turbine that uses a Brayton cycle, the
thermal rejection from the gas turbine was recovered and used as a thermal input in the conventional steam turbine plant that
uses a Rankine cycle. A thermodynamic analysis of the TEES system was performed in steady state, using the thermodynamic
properties of the Coolprop database, a maximum thermal efficiency or Round trip electrical efficiency of 50% has been
reached; when the heating temperature of the compressed air reaches 1100 ° C and When the isentropic efficiency of steam
turbine is 90 percent.
Keywords: TEES, Coolprop, sensible heat, thermodynamics, thermoelectric, geothermal, Brayton, Rankine.
Received: December 9, 2022. Revised: September 8, 2023. Accepted: October 6, 2023. Published: November 6, 2023.
1. Introduction
The increasing share of renewable energy sources in
the electricity market poses new challenges in terms of
reliability and control of electricity networks. Because of
their unpredictable behaviour, wind or solar photovoltaic
energy cannot always be converted into electricity,
especially when most of the energy demand is covered by
nuclear or coal-fired power plants. Alternatively, the
generated electricity excess from renewable energy sources
can be stored and used during peak periods, like in
hydroelectric plants of the traditional pump [1]. As the
available sites for the construction of such plants are
running out, engineers are looking for alternatives for
large-scale energy storage. Electricity storage technologies
differ from each other. The others differ in terms of storage
capacity and power. Although the maximum installed
power capacities of some energy storage technologies
reach tens of MW, only hydraulic pump and compressed
air energy storage systems CAES can store and deliver this
power for hours. The remaining technologies are mainly
used to improve the stability of transmission systems and
are therefore exploited for short periods [2,3]. Unlike the
hydroelectric pump, the CAES is a developing technology
and the subject of recent literature. The reader can refer to
a review by Lund H. et al. on this subject [4]. The CAES
can be improved with thermal storage, leading to Round
trip efficiency of up to 70% [5]. A highly efficient and site-
independent CAES system has recently been proposed by
Kim Y.M. [6].
In this context, thermoelectric energy storage TEES [7-
9] represents an interesting solution in the general context
of the possibility of distributing energy systems based on
renewable energies. A TEES system essentially consists of
two sensitive accumulators of heat and cold, between
which temperatures a heat engine works. The temperature
levels are then recharged by a heat pump cycle. TEES
cycles at several MW have been proposed often using a
transcritical CO2 cycle as a power cycle and each one of the
cycles proposed has a Round trip efficiency that can reach
66 % [8-11]. Another variant of TEES is to use the Brayton
cycle as a feed cycle with air [12], Argon or other rare gases
[13,14] as working fluids. Studies have been conducted on
optimizations for TEES. Peterson [15] and Henchoz et al.
[16] noted the effectiveness of TEES at an ambient
temperature. White et al. [12] have studied the
thermodynamic aspects of a TEES system and have shown
highly efficient compression and expansion processes that
are clearly needed to achieve satisfactory cycle efficiency.
In the literature, TEES systems are not widely studied,
especially when considering the whole integration process
of auxiliary thermal energy as heat input (in the charge or
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.20
Benali Abdelhakim
E-ISSN: 2769-2507
183
Volume 5, 2023
discharge cycle). Particularly in [17], a new TEES thermo-
electric storage system with thermal integration was
proposed. The main novelty was the introduction of an
auxiliary heat source, which enhances the efficiency of the
system. Thousands of mirrors reflect sunlight towards an
absorber, which in turn converts that energy into heat,
which is stored in molten salt. Having cheap solar heat as
auxiliary energy with an electric heater is beneficial for
efficiency. Incorporating thermoelectric storage into solar
turbine plants raises the heat of salt to more than Thus, the
gas became a reserve and not a permanent supplement to
the solar heat). The usual heat pump used in the TEES
installations during charge has been replaced by an electric
heating element which acts as an intermediary for the direct
conversion of electric energy into thermal energy.
2. Description and Method of Study
Figure .1 during the charge, electric energy generated
by the wind and solar photovoltaic power plants was
converted into sensible heat contained in the molten salt via
an electric heater. Figure .1during discharge, sensible heat
was converted into electric energy by a hybrid power plant
uses combined cycle « Brayton and Rankine », combined
cycle is interpreted as follows:
Brayton Cycle: the working fluid "air" is compressed by a
compressor 1-2, the compressed air is heated by a 2-3
heater uses the sensible heat contained in liquid sodium,
and then the compressed air is heated a second time through
the combustion chamber 3-4 uses the gas, compressed air
expands in turbine 4-5, producing mechanical energy to
operate compressor 1-2 and the electricity generator.
Rankine cycle: 4-1, Saturated liquid are pumped to a high
pressure 1-2, saturated steam is superheated by a recovery
device that uses the thermal rejection from the gas turbine,
The superheated steam expands in the 2-3 turbine,
producing mechanical energy to operate the electric
generator. The Steam condenses in condenser 3-4 which
uses a cooling water circuit. Finally, the liquid discharges
from the condenser is used to start a new cycle.
The impact of the parameter « steam Turbine isentropic
efficiency, ηis,st » on the TEES system operation was
evaluated. In order to assess this impact, a thermodynamic
calculation based on standard manufacturer conditions and
the actual climatic conditions. The purpose of this
thermodynamic calculation is to determine all the operating
parameters from the TEES system, namely:
Thermal efficiency..
Total power.
Figure .1: Schema of the TEES system.( Electric heater and receiver can be in series or in parallel)
3. Thermodynamic Analysis
3.1 Hypotheses
The molten salt tanks are perfectly insulated.
All transformation (Brayton Cycle: 2 to 4 and 5 to 6 | Rankine
Cycle: 1 to 2 and 3 to 4) occurs at constant pressure.
100% of electrical energy is converted to sensible heat.
Changes in thermodynamic properties and mass flow of
air are considered negligible.
3.2 molten salt Cycle:
ṁms.C ms .Tc,ms - ṁms.C ms .Th,ms + Qe +Qr= 0
ṁms.C ms.Th,ms - ṁms.C ms .Tc,ms - Q 2-3 = 0
Qr +Qe - Q 2-3 = 0
3.3 Brayton Cycle :
ṁai.h1 + W (1-2)r – ṁai.h2 = 0 ; W (1-2)r = W (1-2)is / ηis,c
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.20
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Volume 5, 2023
ṁai.h2 + Q2-3 – ṁai.h3 = 0 ; Ea,h = [T3–T2] / [Th,ms –T2] ,
Cmin [T3–T2] = Cmax [Th,ms–Tc,ms]
Cmin = ṁai.[h3–h2]/[T3–T2], Cmax = ṁms.c ms
ṁai.h3 + Q3-4 – ṁai.h4 = 0 ; Q3-4 = ṁgas.LHV. η,cc
ṁai.h4 – WGT(4-5)r – ṁai.h5 = 0 ; WGT(4-5)r = WGT(4-5)is . ηis,t
3.4 Rankine Cycle :
ṁst.h4 + W p(4-1)r – ṁst.h1 = 0 ; W p(4-1)r = W p(4-1)is / ηis,p
ṁst.h1 + Q1-2 = ṁst.h2 ; Er = [T2–T1] / [T5–T1],
Cmin .[T2–T1] = Cmax .[T5–T6]
Cmax=ṁai.[h5–h6]/[T5–T6] , Cmin = ṁst.[h2–h1] / [T2–T1]
ṁst.h2 -WST(2-3)r - ṁst.h3 = 0 ; WST(2-3)r = WST(2-3)is . ηis,st
ṁst.h3 – Q3-4 - ṁst.h4 = 0;
3.5 Efficiencies:
 
󰇩󰇛󰇛󰇜 󰇛󰇜󰇛󰇜󰇜󰇛󰇜
󰇛 󰇜 󰇪
3.6 Input parameter:
Table.1: fluid storage property [18]
Property
(7Li2BeF4~Flibe!)
Molten salt (ms)
Lower temperature limit , (°C)
459
Upper temperature limit ,(°C)
1400
Heat capacity Cso ,(KJ/ kg K)
2.34
Table.2: input operating parameters of the TEES system.
parameter
Electric heater power ; Qe
Hot molten salt tank temperature, Th,ms
Brayton Cycle
Compressor pressure ratio , Rp
Compressed air temperature ,T4
Compressor isentropic efficiency, ηis,c
Turbine isentropic efficiency, ηis,t
Air heater efficiency , Ea,h
Generator efficiency, ηg
Combustion chamber efficiency , η,cc
Calorific value of gas (LHV)
Atmospheric condition ,
Atmospheric condition ,
Rankine Cycle
Condenser pinch point , T1- Tc,o
Cold water inlet temperature , Tc,i
Saturated liquid temperature ,T6
40 C
Recuperator efficiency , Er
80 %
Turbine isentropic efficiency , ηis,t
90 %
Pompe isentropic efficiency , ηisp
85 %
Generator efficiency, ηg
98 %
Motors efficiency, ηm
98%
(Er:80 % , NTU = 4 ,Cmin /Cmax = 1 [19]).(Ea,h :90 %, NTU = 4.7
,Cmin /Cmax=0.75 [19])
. Er and Es,h : counter-flow heat exchanger.
4. Results and Interpretation
Table.3: thermodynamic state for each point in cycles.
state
T °C
P Bar
H (KJ/Kg)
S( KJ/Kg-K)
Brayton Cycle using air
1
25
1
298.38
6.86
2
344.57
10
625.96
6.95
3
1100
10
1484.28
7.85
4
1100
10
1484.28
7.85
5
592.34
1
894.79
7.97
6
147.34
1
422
7.21
Transcritical Rankine Cycle using methanol
1
35
200
424852
-5.12
2
480
200
426697.96
-1.53
3
33
0.22
425993.78
-1.27
4
30
0.22
424822
-5.13
Table.3: output operating parameters of the TEES system.
parameter
value
Electric heater power ; Qe
50 MW
Hot sodium tank temperature, Th,so
885 °C
Cold sodium tank temperature, Tc,so
520.21 °C
Mass flow of sodium, ṁso
109.13 kg/s
Brayton Cycle using air
Real work of the compressor , W (1-2)r
30.284 MW
Real work of the turbine , WGT(4-5)r
54.496 MW
Air heater power, Q2-3
50 MW
receiver power, Q3-4
29.34 MW
Mass flow of air , ṁai
92.44 kg/s
Transcritical Rankine Cycle using methanol
Recuperator power , Q1-2
43.636 MW
Condenser power , Q3-4
27.7 MW
Mass flow of steam, ṁst
23.63 kg/s
Real work of the turbine , WST(4-5)r
16.64 MW
Mass flow of cold water , ṁc,w
662.49 kg/s
Efficiencies
Round trip electric efficiency ,ηrte
50.41 %
Thermal efficiency ,ηth
50.41 %
Total Power, Pt =(WGT(4-5)r -WC(1-2)r + WST(4-5)r) .ηg
- Wp(4-1)r / ηm
40.04 MW
Table.4: output operating parameters of the TEES system.
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.20
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E-ISSN: 2769-2507
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Volume 5, 2023
ηis,t
70
75
80
85
90
Pt
(mw)
35.666
36.602
37.508
38.409
39.326
Table.5: output operating parameters of the TEES system.
ηis,t
70
75
80
85
90
ηht or ηrte
44.95%
46.13%
47.27%
48.41%
49.56%
Table.6: output operating parameters of the TEES system.
To calculate the cost of storing kilowatt-hours, we have 50
megawatts that enter as heat, and we recover 50 percent, or
25 megawatts, and the rest comes out in the form of lost
heat, and by calculating the cost of producing this heat from
a photovoltaic source, i.e. $5.875 million, if we consider
$0.23-$0.24/ watt
The station operates 7 hours at night, which is equivalent to
7 sunny hours, and therefore the cost of storing kilowatt-
hours is (($ kWh =5.875 million $ / (7 * 25 * 1000) =
33.57) and this number is low compared to US$379/usable
kWh batteries[20][21]
5. Conclusion
Solar thermal energy is an inexhaustible resource that benefits
the environment. Its integration in TEES systems is profitable
especially on sites with More sun radiant. The study shows that
the thermal efficiency of a hybrid thermal power plant using
geothermal can reach 50 %, this means that the integration of
electricity storage using sensible heat in proposed hybrid power
plant is possible. The study shows that the integration of gas in the
hybrid power plant can Ensures continuity of work of TEES
system. The numerical application is based on an electrical input
reach 50 MW, also The numerical application means that the
proposed TEES system store electricity with a roundtrip electrical
efficiency of 50%. The variance in performance of the TEES
system was evaluated as a function of changes in isentropic
efficiency of steam turbine. Consequently, in order to increase the
Round trip electrical efficiency of the TEES, it is necessary to
Choose a high efficiency turbine.
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Contribution of Individual Authors to the
Creation of a Scientific Article (Ghostwriting
Policy)
The author contributed in the present research, at all
stages from the formulation of the problem to the
final findings and solution.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The author has no conflict of interest to declare that
is relevant to the content of this article.
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International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.20
Benali Abdelhakim
E-ISSN: 2769-2507
187
Volume 5, 2023