comparative thermodynamic analysis of compressed air and ......are simulated using aspen hysys...
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Accepted Manuscript
Comparative thermodynamic analysis of compressed air and liquid air energy storagesystems
Piotr Krawczyk, Łukasz Szabłowski, Sotirios Karellas, Emmanuel Kakaras, KrzysztofBadyda
PII: S0360-5442(17)31255-0
DOI: 10.1016/j.energy.2017.07.078
Reference: EGY 11266
To appear in: Energy
Please cite this article as: Piotr Krawczyk, Lukasz Szabłowski, Sotirios Karellas, Emmanuel Kakaras,Krzysztof Badyda, Comparative thermodynamic analysis of compressed air and liquid air energy storagesystems, (2017), doi: 10.1016/j.energy.2017.07.078
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Comparative thermodynamic analysis of compressed air and liquid air energy storage
systems
Piotr Krawczyka, Łukasz Szabłowskib, Sotirios Karellasc, Emmanuel Kakarasd and Krzysztof Badydae
a,b,e Institute of Heat Engineering, Warsaw University of Technology, Warsaw, Poland c,d National Technical University of Athens, Athens, Greece
a [email protected] b [email protected]
c [email protected] d [email protected]
Abstract:
The paper presents a thermodynamic analysis of selected CAES and LAES systems. The LAES cycle is a combination of an air liquefaction cycle and a gas turbine power generation cycle. CAES and LAES systems are simulated using Aspen HYSYS software. CAES is modeled in a dynamic mode. A comprehensive thermodynamic analysis was conducted along with the comparison of storage volumes. The results indicate that both systems are characterized by high energy storage efficiency, equal to approximately 40% for the CAES and 55% for the LAES systems. One clear advantage of the LAES over the CAES is the significantly lower volume demanded for energy storage. For the considered LAES system,the liquid air tank volume is around 5000 m3, while for the CAES the cavern volume is approximately 310000 m3. Heat exchangers and combustion chambers are the main contributors to the total exergy destruction in the analyzed systems.
Keywords: Liquid air energy storage, Compressed air energy storage, energy analysis, storage efficiency.
Abbreviations
a – attraction parameter [J2/(mol2Pa)], b – van der Waals covolume [J/(mol Pa)], B – exergy [kJ], CF – correction factor [kJ/(m3Pa)], Eel c – energy consumed by compressors and pumps [kJ], Eel g – energy from generator [kJ], h – specific enthalpy of the working medium [kJ/kg], h0 – specific enthalpy of the working medium in ambient conditions [kJ/kg], h1 – specific enthalpy of working medium at the inlet [kJ/kg], h2 – specific enthalpy of the working medium at the outlet [kJ/kg], h’2 – specific enthalpy of the working medium at the outlet for the isentropic process [kJ/kg], I – moment of inertia [kg m2], k – coefficient representing the inverse of flow resistance (conductivity) [kg/(s Pa0.5)], M – molecular weight of the working medium [kg/mol]; Mr – the rotating mass [kg]; m& – mass flow [kg/s],
shellm& – flow rate of working medium on the shell side [kg/s],
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tubem& – flow rate of working medium on tube side [kg/s], n – the volume exponent,
1n& – molar flow rate at the inlet [mol/s], P – power [kW], p – pressure [Pa], p1 – inlet pressure [Pa], p2 – outlet pressure [Pa]. Q – heat transfer between shell side and tube side [kW], Qf – chemical energy of fuel [kJ], Qloss– heat losses [kW], R – gas constant [kJ/(mol K)], Rg – the radius of gyration [m], S – entropy [kJ/K] s2 – specific entropy of working fluid at outlet [kJ/(kgK)], s0 - specific entropy of the working fluid in ambient conditions [kJ/(kgK)], T – absolute temperature [K], u – specific internal energy of the working medium [kJ/kg], u2 – specific internal energy of the working medium at the outlet [kJ/kg], T0 – ambient temperature [K], v – molar volume [m3/mole], V – volume of holdup in the shell or in the tube [m3] Greek symbols ηcycle – round trip efficiency of a cycle [-], ρ – density [kg/m3], ρ’2 – density of the working medium at the outlet for the isentropic process [kg/m3], ρ1 – inlet density of the working medium [kg/m3], ω – the rotation speed [rad/s].
1. Introduction
Energy storage plays an increasingly important role in the current energy system due to the very intensive development of highly fluctuating, intermittent renewable energy sources. Currently, there are two methods of large scale energy storage: pumped storage and compressed air energy storage (CAES). The special geographical requirements involved for the application of these technologies inhibit their development, since there are relatively few places with suitable conditions for their implementation. In the case of a CAES power plant, a high volume tank is needed to store large amount of compressed air. The construction of such tanks has negative financial implications and thus only natural reservoirs appear to be currently economically viable. Instances of such reservoirs include salt caverns, aquifers, salt mine workings, mines, and limestone and other mineral formations in hard rock structures. Meanwhile, pumped-storage plants require the existence of a water reservoir, such as a lake, located at the foot of a mountain, on top of which an upper reservoir may be constructed. An alternative to these technologies is liquid air energy storage (LAES) power plants, which can store large amounts of energy at decreased storage volumes. CAES technology has been extensively studied. Currently, there are two large plants of this type: Huntorf in Germany (290 MW) and McIntosh in the USA (110 MW). The first was launched in 1978, the second in 1991. An overview of the state of the art about CAES technology is presented in [1-6].
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In [7] a hybrid system using CAES combined with renewable energy sources (e.g. wind turbines) was investigated and dynamically modeled, and the possibility of implementing it in Poland was presented [7, 8]. In [9] a simulation and thermodynamic analysis was performed for a compressed air energy storage-combined cycle (CAES-CC). The overall efficiency of the system was about 10% higher than the conventional, non-regenerative reference CAES. According to the authors, the heat obtained from the compressor intercoolers when charging the air reservoir can be used to keep the steam part of the system at high temperature at stand-by operation. In [10] a system consisting of a CAES system and a Kalina Cycle to recover waste heat was presented. The system had an efficiency higher by 4% compared to a standalone, regenerative CAES system. An analysis of the impact of selected parameters (pressure ratio, temperature, mass flow rate of air) on the performance of an adiabatic CAES system was presented in [11]. The calculations were performed on the tacit assumption of the authors that the system operates at constant pressure. In [12] a thermodynamic analysis of an adiabatic CAES system with packed bed regenerators was presented. In this case, the heat of compression was stored in a solid material, with predicted efficiencies in the range of 70.5 – 71.1%. The maximum packed bed temperature was 713 K (for a 2-stage system). A completely different approach to the adiabatic CAES system was presented in [13], where a low temperature CAES concept was developed. The roundtrip efficiency ranged from 52% to 60% and the maximum temperature in the thermal energy storage was 200°C. According to [13], this solution uses less expensive system elements and the start-up time is less than 5 minutes. In [14] the off-design operation of a CAES system integrated into a hybrid power plant connected to a wind power plant was analyzed. The authors simulated the operation of this system on the Italian Power Exchange market for one year. The cost analysis was performed using the thermo-economic approach. In [15] the concept of a small CAES combined with photovoltaic panels and operating in order to cover the needs of a radio base station was introduced. The heat generated during the air compression was consumed during the expansion stage of the turbine. An additional function of the presented solution was the production of cooling, due to the fact that the temperature of the air leaving the turbine was equal to 3°C. Despite the small scale of this CAES system, its storage efficiency was 57%. The LAES technology is not as well known and tested as CAES. Currently, in the world there is only one research facility of this kind, with a power capacity of 350 kW and a capacity of 2.5 MWh, built in 2011 in the UK. However, because of its simplified configuration it has very poor performance – its efficiency being around 8% [16]. In [17] a concept and thermodynamic analysis of the adiabatic performance of a LAES were presented. The efficiency of the proposed solution was 49%. In [18, 19] a hybrid system was presented combining CAES and LAES with the capability of converting compressed air (50 bar) into liquid air at atmospheric pressure. According to the authors, this solution is cheaper than the LAES and the CAES system (with the use of artificial compressed air tanks). A preliminary analysis [18] indicated an efficiency of 53%, but subsequent calculations [19] showed that the efficiency of such energy storage is 42%. A comparison between LAES systems based on the composition of the liquefied gases (air, nitrogen and carbon dioxide) and a CAES system was described in [20]. The authors stated that energy storage systems using liquefied gases have a promising future in large scale energy storage. An economic analysis of energy storage systems based on compressed air and liquid air for different mixes of liquid and gaseous air (from 0 to 100%) was performed in [21]. In [22] an energy storage system based on liquid CO2 operating in a closed circuit was presented. The proposed system had two storage tanks for liquid CO2 at high and low pressure. During the charging phase, the high pressure tank was being filled and the low pressure reservoir was being
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emptied, whereas during the discharging phase the opposite process occurred. The efficiency of this system was estimated equal to 56.64%. In [23] a number of cryogenic energy storage systems combined with distributed energy sources were compared. According to the authors, the best solution for air liquefaction was the Claude cycle (using an expander and a Joule-Thompson valve) and not the Linde-Hampson cycle (whicn only included a Joule-Thompson valve). On the other hand, according to the authors of the paper, the use of cold storage can alter this result, since it reduces by several times the work required to liquefy air. A part of the working agent flowing through the turbine in the Claude cycle is only used to cool the air before the Joule-Thompson valve. An interesting energy storage system has been discussed [24]. This solution uses the Rankine cycle, with air being used as the working medium. The system can operate continuously. The air is constantly liquefied and then pumped, heated and finally expanded in the turbine. This system is equipped with a cryogenic tank for liquid air, which stores energy. The efficiency of such a system was estimated to range from 20 to 50%. An analysis of the impact of selected operational parameters on the performance of an adiabatic LAES system can be found in [25].
This paper presents a comparative analysis of energy storage systems based on liquefied air (LAES) and on compressed air (CAES). For this purpose, a CAES and a LAES with generated power outputs of 290 and 270 MW and storage capacities of 1700 and 1080 MWh, respectively, are considered. Both systems use natural gas as an additional fuel. The aim of the paper is to numerically investigate the efficiency of these systems and identify the main causes of exergy destruction.
The capacity of the working medium storage tank constitutes a major design parameter and a key difference between the two systems, as it is much lower for the LAES compared to the CAES. However, the LAES technology requires additional volume for cold storage. Considering the above, the necessary storage capacity and the unit energy density for both technologies are estimated.
2. Simulation
The calculations shown in this section were done using commercial software [26, 27]. Dynamic calculations of physical phenomena are described using linear and nonlinear differential equations, which are presented in detail in [28]. In this study, only governing equations are included. The mass flow rate of the working medium through valves is described by the following formula:
pkm ∆=& (1)
The basic equation defining the characteristics of the valves takes into account the coefficient k and the pressure drop across the valve as a function of the flow resistance:
( )21
,, ppkfm =& (2)
The relationship describing the energy balance on the shell side of the heat exchanger is given by the equation:
( ) ( )dt
uVdQOhhm shell
lossshellshell2
21
⋅=+−−⋅ ρ& (3)
The relationship describing energy balance on the tube side of the heat exchanger is given by the equation:
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( ) ( )dt
uVdQhhm tube
tubetube2
21
⋅=−−⋅ ρ& (4)
Both the polytropic and isentropic power of a compressor or expander can be calculated using (4).
−
⋅
⋅⋅
−⋅⋅=
−
11
1
1
2
1
11
n
n
p
ppCF
n
nMnP
ρ& (5)
The correction factor is expressed by the following equation:
−
−
−=
1
1
2
2
12
'1
'
ρρpp
n
n
hhCF (6)
The power provied to the working medium in the compressor is described as follows:
( )12121 hhMnP −⋅⋅=− & (7)
The power taken from the working medium in the expander is expressed by the following equation:
( )21121 hhMnP −⋅⋅=− & (8)
The rotational inertia is given by the equation: 2
grRMI ⋅= (9)
The power needed to change the rotation speed is:
dt
dIPI
ωω= (10)
The round trip efficiency of the cycle is described by the following formula:
fcel
gelcycle QE
E
+=η (11)
The exergy destruction is described using the Gouy-Stodola equation [29, 30]:
∑∆= STB 0δ (12)
3. Model description and Results The Aspen HYSYS software is used for the simulation. The air composition is shown in Table 1. The
air properties are assumed to be in accordance with ISO conditions, i.e. a temperature of 15 °C, and a pressure of 1.013 bar. Both energy storage systems are charged during the valleys of the load of the power system and discharged at the peaks. Fuel is also consumed while discharging. The fuel used in both systems was methane with a LHV of 50035 kJ/kg.
3.1 CAES A schematic diagram of the modelled CAES system is shown in Fig. 1. The installation consists of the two-stage compressor with the intercooler, the underground storage with the relatively large volume (salt cavern) and the two-stage gas turbine with two combustion chambers. The air is taken from the environment and compressed into the cavern during the charging phase, during which electricity is consumed. When the system is discharging, after being partially depressurized, the air is transported to the high-pressure combustion chamber and subsequently to the turbine. The
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expanded air leaving the HPT is transported to the low-pressure part of the gas turbine. After the expansion process, the exhaust gases are discharged into the environment.
The polytropic efficiency of the rotating machines (turbines and compressors) is assumed equal to 75%. The volume of the cavern is 310000 m3, while the pressure inside it during normal operation ranges from 43 to 70 bar. The air temperature after the intercoolers is 50 °C, while the pressure drop at the intercoolers IC2 and IC2 at nominal operating conditions is 0.1 and 0.5 bar, respectively. The inlet temperature and pressure of the cooling water are 15°C and 2 bar, while the outlet temperature and pressure are 75 °C and 1.9 bar. The compressor is divided into 2 parts, each needing 30 MW of power during the charging phase of the plant.
The temperatures after the first and second combustion chambers are respectively 550 and 825 °C. The pressure at the exit of the first turbine (HPT) is assumed to be 22 bar. The Valve V2 is controlled to maintain a constant flow (416 kg/s) during the discharging phase.
It should be noted that the parameters described above, except for the efficiency of the rotary machines, are identical to those of the Huntorf power plant.
The storage charging and discharging processes in the CAES technology are transient. This means that the operating conditions of the rotating machinery are changing depending on the storage charge level.
The variation of the pressure inside the cavern as well as the injected air mass flow rate during the charging phase are shown in Fig. 2. The mass flow rate of the air released from the cavern and the V2 valve position during the discharging phase are depicted in Fig. 3. The variation of the pressure inside the cavern during the discharging phase are shown in Fig. 4.
During the charging the system, there is a non-linear variation of mass flow rate (Fig. 2). The same behavior would also be observed for the pressure, if it was lower (if the cavern would be filled starting from atmospheric pressure). The same effect can be observed for the pressure during the discharging phase of the system (Fig. 4).
As mentioned above, while the cavern is being emptied (Fig. 3), the control system maintains the mass flow rate at a constant level by adjusting the position of the valve V2. The curve showing this parameter is highly nonlinear.
The power consumption of the compressors is 60 MW, while the total energy consumed during the charging of the system is 1775 MWh.
The power generated in the discharging phase of the CAES cycle is 290 MW, while the energy generated during the discharging phase is 1723 MWh. The round trip efficiency of the cycle is approximately 39.8%, taking also into account the chemical energy of the fuel. According to [1] the round trip efficiency of the Huntorf power plant is about 42%. The difference between those values may be caused by differences in assumptions of internal efficiencies of rotating machines in the modelled system and in Huntorf CAES.
The energy balance and exergy destruction of the system are shown in Table 2.
3.2 LAES The LAES cycle contains three principal parts: a charging device, a liquid and various thermal storage equipment components, and a generation device. Each of these three main parts can be configured in different ways. The configuration of these elements has a crucial influence on the efficiency of energy storage.
During the charging phase, excess electricity from the grid is used for air liquefaction. When the demand for electricity in the grid increases, the air is vaporized and expanded in turbines to generate electricity.
This paper presents and analyzes a hypothetical LAES system. It is assumed that the charge-to-discharge time ratio is 2 (charging time – eight hours; discharging time – four hours). The
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liquefaction plant capacity is assumed at the level of 150 kg/s. The efficiency of the compressors and turbines is assumed to be 89% and 85%, respectively.
3.2.1. Charging the system In the analyzed cycle the Hampson-Linde method of air liquefaction is used. In this method, the Joule-Thomson effect is used to cool the gas. In addition, for a LAES plant it is possible to use an external source of cooling produced from the air evaporation process during the discharging phase. This is of fundamental importance for achieving high storage efficiencies. A diagram of the analyzed LAES system is shown in Fig. 5.
In the proposed system, the air is compressed to 120 bar in a three-stage compressor equipped with intercoolers. Subsequently, it flows to a multi-stream heat exchanger in which it is condensed and cooled to temperatures bdlow -180°C. After passing through the heat exchanger, the air is directed towards the Joule Thomson valve, where it expands to a pressure of 1.5 bar and is thus further cooled. The cold air is then directed to the separator, in which the separation of the liquid and gas phases takes place.
The cold air in gaseous form is directed through the multi-stream heat exchanger to the compressor inlet. Additional streams from cold storage are introduced to the multi-stream heat exchanger (Cold Box).
The power consumption to drive the compressors in the analyzed liquefaction cycle is 98.8 MW (corresponding to a specific energy consumption of 0.184 kWh/kg liquid air). Achieving such low specific energy consumption is possible only with the implementation of cooling from the air evaporation process during the discharging of stored energy, which was described above. The energy consumption during the charging phase of the system is 790.4 MWh.
3.2.2. Cold Recycle
The possibility of storing the cooling produced during the air evaporation process and utilizing it for the liquefaction process is a key feature which enables achieving high energy storage efficiencies. Among the solutions regarding cold energy storage for LAES cycles described in the literature, solid mineral material (sand, gravel) beds are mainly considered. In the presented LAES cycle, another type of cold storage is proposed, based on liquid substances. This concept is based on using two tanks ‒ a “cold” and a “warm” one. During the cooling production, when the cooling storage is being charged, i.e. when the energy storage system is being discharged, a fluid stream from the warm tank flows through the air heater to the cold tank. Similarly, during the cooling storage discharging, i.e. charging of the energy storage system, a fluid stream from the cold tank goes to the multi-stream heat exchanger where it is heated and subsequently accumulated inside the warm tank. A technical obstacle for the implementation of this concept lies in finding substances that remain liquid at such low temperatures. The only substances that meet the above condition are hydrocarbons. In the analyzed LAES cycle, a two-step cooling storage is proposed. In the first stage, liquid propane (refrigerant R290) is used as a cooling medium (Boiling point = -42°C; Freezing point = -189°C). In the second stage liquid methanol was used (Boiling point = 65°C; Freezing point = -97.6°C).
3.2.3. Power Recovery During the discharging of the energy storage system, the pressure of the liquid air is firstly increased by a pump. In the next step, the air is evaporated and superheated. The heat required for this purpose is supplied by refrigerant R290, which is cooled from -60°C (the temperature in the warm tank) to -185°C (cold tank temperature). The air ‒ in gaseous form ‒ then flows into the second superheater where its temperature is increased to about 20°C. At the same time the methanol temperature is lowered from 25°C (warm tank temperature) to - 60°C (cold tank temperature). In the next step, the air is heated in the regenerative heat exchanger and flows into the combustion
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chamber. The gas exiting the turbine is cooled in the regenerative heat exchanger. Table 3 contains the basic thermodynamic data in the selected points of the LAES cycle.
In contrast to the CAES system, in the LAES system the compressor and turbine parameters are constant during the charging and the discharging processes. Moreover, the aforementioned parameters are independent of the storage charge level.
The power generated during the discharging phase of the LAES cycle is 271.5 MW, while the energy generated during the energy recovery is 1086 MWh. The round trip efficiency of the cycle is around 55.2%, taking into account the use of the chemical energy of the fuel.
Achieving such high efficiency is possible only due to use of the cooling produced during the air evaporation process in liquefaction, since it allows attaining an air temperature before the J-T valve at a range of -180°C. In the simple liquefaction system (without cooling storage), the maximum value of this temperature is approximately -135°C. The lower the temperature of the air before the J-T valve, the lower the gas-to-liquid ratio in the air after the throttling valve. Consequently, a smaller fraction of the working medium is being recycled.
In the considered system, approximately only 10% of the air flow passing the compressors, recirculates back at the first point of the liquefaction installation. On the other hand, when the air temperature before the J-T valve equals to -135°C and the pressure equals to 120 bar, 65% to 70% of the air stream has to be recycled. In this case, in order to keep constant the performance of the liquefaction system, the air flow rate to the compressors is increased, and thus the energy expenditure of the process is higher.
Table 4 shows the exergy destruction in selected parts of the analyzed LAES cycle. Exergy destruction occurs mainly in the heat exchangers and air throttling.
3.3 Comparison of calculation results The comparison of the major parameters for both systems is presented in the Table 5.
The data presented indicates that both systems can generate electricity with relatively similar power. The CAES system is characterized by a larger capacity, which is 58% higher. However, the fuel consumption for the single storage cycle in the CAES is more than two times higher compared to that of the LAES system. The aforementioned feature is reflected in the energy efficiency of both energy storage systems.
Beyond the efficiency, the unit energy density achievable in the storage was compared for both technologies. The major data for both systems are presented in the Table 6.
In this analysis, the volume of technological devices is neglected. The only ones taken into account are: the volume of the working medium tank and the volume of the cooling storage tanks (in the LAES case). Based on the obtained results, the LAES system is characterized by approximately six times greater unit energy density in comparison with the CAES system, which is an undoubtedly important advantage of the former. The analysis also covered the comparison of the exergy destruction of these two technologies. The results are presented in Table 7 and Fig. 6.
For both systems, the largest exergy destruction occurred in the combustion chambers. According to the results, the LAES system unit exergy destruction is lower by about approximately 40% compared to that of the CAES system.
4. Conclusions The presented technologies can be applied for energy storage in large scale systems. In this paper, a comparison of the relatively well examined technology based on compressed air with the currently developing technology based on liquefied air was presented. In both described cases, the heat generated during the air compression is not stored. The CAES system has 2 air coolers (IC1 and IC2), while the LAES has 3. This heat could be used for district heating, which would increase the overall efficiency of the entire system (the energy could be
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consumed at the same time by compressors of the CAES or LAES systems). Both systems have a combustion chamber (CAES – 2 and LAES – 1). The round trip efficiency of the CAES power plant was 39.77%, whereas the efficiency of the LAES was 55.2%.
The LAES system was additionally equipped with cooling storage, which lead to a significant increase of the efficiency of the liquefaction process. However, the higher round trip efficiency of the modeled LAES compared to the CAES derived from the application of regeneration (pre-heating the air before the combustion chamber with heat from the turbine exhaust gases).
Without the regeneration, both systems (CAES and LAES) would have comparable round trip efficiency (LAES – about 40%).
The results presented in the article prove that the LAES technology may have greater benefits than the CAES solution. The major benefit is the larger density of the energy stored. As mentioned earlier, the LAES technology is not constrained by location, unlike the CAES. The storage capacities for both systems can be similar. The LAES higher efficiency also leads to lower exergy destruction.
Considering the above, in the opinion of the authors, the LAES technology is potentially the prospective alternative for the currently used CAES technology.
Acknowledgments The first two authors were supported by the European Union in the framework of the European Social Fund through the “Didactic Development Program of the Faculty of Power and Aeronautical Engineering of the Warsaw University of Technology”.
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Table 1. Mass composition of air
Component Unit Value Oxygen %wt 23.052 Nitrogen %wt 74.99
Carbon dioxide %wt 0.046 Argon %wt 1.276 Water %wt 0.636
Table 2. Results for CAES model
Parameter Unit Value Efficiency % 39.77
Energy LPC MWh/cycle 887.5 HPC MWh/cycle 887.5 HPT MWh/cycle 283.7 LPT MWh/cycle 1439.6
CCH 1 MWh/cycle 1400.5 CCH 2 MWh/cycle 1157.8
Exergy destruction LPC MWh/cycle 151.5 HPC MWh/cycle 140.0 IC 1 MWh/cycle 206.5 IC 2 MWh/cycle 247.7 V 1 MWh/cycle 0.28
Cavern MWh/cycle 7.16 V 2 MWh/cycle 94.4
CCH 1 MWh/cycle 802.3 CCH 2 MWh/cycle 401.5
V 3 MWh/cycle 0.56 V 4 MWh/cycle 2.59 HPT MWh/cycle 35.24 LPT MWh/cycle 162.6
Table 3. Thermodynamic data in the selected nodes of analyzed LEAS system
Node Pressure, bar Temperature, °C Mass flow, kg/s 1 1 15 150 2 1 3.2 166.1 3 5 186.6 166.1 4 4.9 20 166.1 5 25 219 166.1 6 24.7 30 166.1 7 120 227.8 166.1 8 119 25 166.1 9 118 -183.9 166.1 10 1.5 -190.7 166.1 11 1.5 -190.7 16.1 12 1.5 -190.7 150 13 1.4 -129.3 16.1 14 1.5 -185 230.5 15 1.4 -60 230.5 16 1.5 -60 49 17 1.4 25 49 18 1.5 -190.7 300 19 22 -189.4 300
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20 21.5 -65.6 300 21 21.2 21.1 300 22 21 550 300 23 21 1300 306 24 1.03 582 306 25 1 84 306 26 21 15 6 27 2 25 98 28 1.5 -60 98 29 2 -60 461 30 1.5 -185 461
Table 4. Exergy destruction in selected parts of the analyzed LAES cycle
LAES cycle Unit Exergy destruction Efficiency % 55.2
Charging system - liquefaction Mixer MWh/cycle 7.8
Intercooler 1 MWh/cycle 36.3 LPC MWh/cycle 22.8
Intercooler 2 MWh/cycle 48.2 MPC MWh/cycle 23.2
Intercooler 3 MWh/cycle 54.5 HPC MWh/cycle 22.4
Cold Box MWh/cycle 31.3 J-T valve MWh/cycle 64.5
Liquid air separator MWh/cycle 0.1 Power Recovery
Pump MWh/cycle 5.2 HE1 MWh/cycle 117.9 HE2 MWh/cycle 3.9
Regenerative HE MWh/cycle 36.7 Combustion chamber MWh/cycle 336.6
Turbine MWh/cycle 46.1
Table 5. The comparison of the major parameters for the analyzed storage systems Parameter CAES cycle
LAES cycle
Energy consumption during charging
the system 1775 MWh/cycle 790.4 MWh/cycle
Power generated during discharging 290 MW 271.5 MW Energy generated during energy
recovery 1722 MWh/cycle 1086 MWh/cycle
Supplied chemical energy of fuel 2557 MWh/cycle 1200 MWh/cycle Round trip efficiency of the cycle 39.8% 55.2%
Table 6. The comparison of the major storage volume required for the analyzed systems
Parameter CAES cycle LAES cycle Volume of the cavern 310 000 m3 -
Volume of the liquid air tank - 5000 m3 Low temperature cold storage – 2
tanks (warm and cold) - 2 x 12000 m3
High temperature cold storage – 2 tanks (warm and cold)
- 2x 2000 m3
Total volume of tanks 310 000 m3 33 000 m3 Unit volume density of the energy
stored 180 m3/MWh 30 m3/MWh
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Table 7. The comparison of exergy destruction for the analyzed systems Parameter CAES cycle LAES cycle
Total exergy destruction, 2250 MWh/cycle 860 MWh/cycle The energy generated during energy
recovery 1722 MWh/cycle 1086 MWh/cycle
Unit exergy destruction 1.3 MWh/MWh 0.8 MWh/MWh
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Fig. 1. Schematic of the modeled CAES power plant
Fig. 2. Changes of pressure inside the cavern and injected air mass flow during charging
Air inmax. 91 kg/s15°C, 1 bar
Water 1 in105.5 kg/s
15°C
Water 2 in122.6 kg/s
15°C
Water 1 out75°C
Water 2 out75°C
LPCHPC
50°C
50°C
V1 V2
Cavern310 000 m3
IC1 IC2
0 or 416 kg/s50°C, 43-70 bar
Fuel 10 or 4.7 kg/s15°C, 50 bar
Fuel 20 or 3.9 kg/s15°C, 50 bar
V3 V4
HPT LPT Exhaust gases338°C, 1 bar
CCH1 CCH2
550°C, 43 bar 825°C, 22 bar1
2
3
4
5
6
7
8
9 10
11
12
13
14
15
16
0
10
20
30
40
50
60
70
80
70
75
80
85
90
95
0 3 6 9 12 15 18 21 24 27
Pre
ssu
re, b
ar
Mas
s fl
ow
, kg
/s
Time, h
Mass flow Pressure
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Fig. 3. Mass flow of the air released from the cavern and V2 valve openness during discharging
Fig. 4. Changes of pressure inside the cavern during discharging
0
20
40
60
80
100
120
0
50
100
150
200
250
300
350
400
450
0 1 2 3 4 5
Val
ve o
pen
ing
, %
Mas
s fl
ow
, kg
/s
Time, h
Mass flow Valve opening
40
45
50
55
60
65
70
75
0 1 2 3 4 5 6
Pre
ssur
e, b
ar
Time, h
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Fig. 5. Schematic of the modeled LAES system
Fig. 6. Comparison of exergy destruction in CAES and LAES systems
12
9
Make up air150 kg/s
1
Mixer
2
LPC
3 4 5 6 7
8
MPCHPC
Water in Water in Water in
Water out Water out Water out
13
16.1 kg/s, -129.3°C
10
11
18
Cold box
Liquid Air Tank5000 m3
IC1 IC3IC2
166.1 kg/s25°C, 119 bar
150 kg/s-190.7°C, 1.5 bar
300 kg/s
Cold tank 1
Cold tank 2
Warm tank 2
Warm tank 1
19 300 kg/s-189.4°C, 22 bar
20
Evaporator
21
Superheater
22 23
24
Combustion chamberTurbine
26
306 kg/s, 582°C
25
2798 kg/s
28
-60°C 16
-60°C
17
25°C
29 461 kg/s, -60°C
30 -185°C
14230.5 kg/s
15 -60°C
LPC7%
HPC6%
IC 19%
IC 211%
V 10%
Cavern0%
V 24%
CCH 136%
CCH 218%
V 30%
V 40%
HPT2%
LPT7%
Exergy destruction in CAES system
Mixer1%
Intercooler 14%
LPC3%
Intercooler 26%
MPC3%
Intercooler 36%
HPC3%
Cold Box4%
J-T valve8%
Liquid air separator
0%
Pump1%
HE114%
HE20%
Regenerative HE4%
Combustion chamber
39%Turbine
5%
Exergy destruction in LAES system
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Research Highlights
• Research on compressed air and liquid air energy storage systems;
• Analysis of cold recycle for the LAES cycle;
• Comparison of two energy storage systems;