Thermodynamic Analysis of a Hybrid Trigenerative Compressed Air Energy Storage System with Solar Thermal Energy

entropy

Article Thermodynamic Analysis of a Hybrid Trigenerative Compressed Air Energy Storage System with Solar Thermal Energy

Xiaotao Chen 1, Xiaodai Xue 2, Yang Si 1,2, Chengkui Liu 3,4, Laijun Chen 1,*, Yongqing Guo 1 and Shengwei Mei 1 1 Qinghai Key Lab of Eﬃcient Utilization of Clean Energy (New Energy Photovoltaic Industry Research Center), Qinghai University, Xining 810016, China; [email protected] (X.C.); [email protected] (Y.S.); [email protected] (Y.G.); [email protected] (S.M.) 2 China State Key Laboratory of Power System and Generation Equipment, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China; [email protected] 3 Qinghai Building and Materials Research Co, Ltd., Xining 810008, China; [email protected] 4 The Key Lab of Plateau Building and Eco-community in Qinghai, Xining 810008, China * Correspondence: [email protected]; Tel.: +86-0971-511-4914

Received: 29 May 2020; Accepted: 6 July 2020; Published: 13 July 2020

Abstract: The comprehensive utilization technology of combined cooling, heating and power (CCHP) systems is the leading edge of renewable and sustainable energy research. In this paper, we propose a novel CCHP system based on a hybrid trigenerative compressed air energy storage system (HT-CAES), which can meet various forms of energy demand. A comprehensive thermodynamic model of the HT-CAES has been carried out, and a thermodynamic performance analysis with energy and exergy methods has been done. Furthermore, a sensitivity analysis and assessment capacity for CHP is investigated by the critical parameters effected on the performance of the HT-CAES. The results indicate that round-trip efficiency, electricity storage efficiency, and exergy efficiency can reach 73%, 53.6%, and 50.6%, respectively. Therefore, the system proposed in this paper has high efficiency and flexibility to jointly supply multiple energy to meet demands, so it has broad prospects in regions with abundant solar energy resource.

Keywords: hybrid T-CAES; solar energy; solar adsorption chiller; performance analysis; HTF ratio for heating and cooling

1. Introduction Nowadays, the balance between cooling, heating and power supply and demand has become a key issue in many countries, due to the increasing penetration of intermittent renewable energy source (RES) and distributed generation (DG) [1,2]. The RES, such as wind and sun, are greatly subject to local environmental conditions and unpredictable weather, which will cause inconvenience for energy utilization. Besides, the inherent intermittency of renewable DG will also affect the reliability, efficiency and safety of the distributed energy system (DES) [3]. Therefore, exploring a safe, reliable, efficient, and economical DES which combines cooling, heating and power, has been the focus in the DES research field. Presently, the combined cooling, heating and power (CCHP) system and integrated with the energy storage system are the major multi-carrier energy hub technologies to cope with this problem [4]. It would serve to store the low quality (fluctuating and intermittent) energy, and provide high quality (smooth) and dispatchable cooling heating and power (CHP) based on consumption needs [5]. Among various energy storage and CCHP technologies, compressed air energy storage (CAES) may

Entropy 2020, 22, 764; doi:10.3390/e22070764 www.mdpi.com/journal/entropy Entropy 2020, 22, 764 2 of 19 act as trigenerative systems (T-CAES), which can meet the various forms energy demand of users by recovering and supplying heating by stored compressed heat and cooling energy during the expansion [6]. Three types of CAES systems have been extensively investigated in the literature: diabatic (D-CAES), isothermal (I-CAES), and adiabatic (A-CAES) [7–10]. Only two existing D-CAES plants have been constructed until now: the Huntorf plant of 290 MW, Germany, was constructed in 1978; it is worth mentioning that this plant was expanded to 321 MW in 2007. The McIntosh of 110 MW, USA, was built in 1991. However, D-CAES relies on natural gas as an external heat source which can cause certain carbon emissions [11,12]. For these reasons, significant efforts have been devoted to I-CAES and A-CAES, which address the drawbacks of D-CAES by avoiding the use of fossil fuels and recycle compression heat for enhancing their performance. Recently, most of the literature related to A-CAES is focused on utility-scale applications [13–15], which are large-scale power plants located close to energy production. Nevertheless, a small-scale A-CAES (micro-CAES) may act as a trigenerative CAES (T-CAES) that is placed close to the user side and possess a tighter coupling between the DG and the energy demand, has received great attention in DES [16,17]. Andrea L. Facci et al. [18] introduce the concept of a trigenerative compressed air storage (T-CAES), which is based on a micro-CAES. However, constrained by the limited capacity and effectiveness of the heat transfer in the micro-CAES, thermal energy storage (TES) that only depended on compression heat cannot reach a high temperature, which restricted the performance and energy density of micro-CAES systems [19–21]. Apart from micro-CAES acting as CCHP, another promising solution for improving the performance of the above system is hybrid external solar thermal energy and CAES (ST-CAES), which could notably enhance the storage heat temperature and capacity [22]. Besides, as a high-performance CCHP, ST-CAES can also enhance the comprehensive utilization level of clean energy [23–27]. Therefore, scholars and engineers have recently focused on the essential technologies of HT-CAES, such as process design, efficiency analysis, and key parameter optimization, for improving its comprehensive energy utilization efficiency. Chen et al. [22] presented a novel solar thermal-assisted A-CAES to attain stable high-grade thermal energy and analyzed its thermodynamic characteristic. Xu et al. [23] proposed a novel scheme of a wind energy complementary A-CAES and analyzed its thermodynamic performance. Ji et al. [28] proposed a gas turbine combined solar energy and CAES system and focused on process design and efficiency analysis. Mohammadi et al. [29] analyzed an integrated micro gas turbine, compressed air energy storage, and solar dish collector system. Semprini et al. [30] investigated a hybrid-ACAES (HA-CAES) design scheme to optimize cycle efficiency and efficiency for microturbines and solar dish collectors. Yang et al. [31] analyzed the CCHP integrated solar thermal energy for its cogeneration characteristics, and better energy-saving and CO2 reduction performance is obtained. Li et al. [32] proposed an HA-CAES based multi-carrier clean energy hub to realize the comprehensive consumption of renewable energy. Mei et al. [33] studied an HA-CAES as a clean energy hub in a smart micro energy grid based on solar energy. The previous studies for the HA-CAES mainly focused on the theoretical analysis and simulation of the system design and optimization. Few studies have worked on assessing the efficiency and capacity of T-CAES with stable solar thermal resources for providing CHP. As we know, no public analysis data have been elaborated on the CHP performance of the HA-CAES system. In this research, a hybrid solar thermal trigenerative CAES is proposed and thermodynamic analysis for its efficiency and capacity of CHP has been carried out. The major contributions of this paper are: (1) A novel hybrid T-CAES with a solar thermal energy collection field and absorption chiller for supplying CHP was presented. (2) The HT-CAES using VP-1 as the thermal energy storage working medium was built and the thermodynamic performance and CHP capacity were investigated. (3) HT-CAES can provide stable high-grade solar thermal energy in the discharging process, compared with A-CAES in [10], which can greatly simplify the regenerative system and enhance its efficiency and flexibility. Entropy 2020, 22, x FOR PEER REVIEW 3 of 19

CAES. This is greatly related to the critical parameters and the impact of the CHP capacity assessment on system performance in Section 4, followed by conclusions in Section 5.

2. System Description Figure 1Figure 1 shows the schematic diagram of the proposed HT-CAES system. To clearly Formatted: Font: Not Bold describe and analyze the system, all the components and streams have been numbered. The bottom part of Figure 1Figure 1. is a small-scale AA-CAES system where compression heat generated in the Formatted: Font: Not Bold charging process is stored in the high-temperature water tank (HWT) to provide heat for residents. An HT-CAES is mainly composed of five units, i.e., compression air storage unit (COM), air turbine and generator unit (TUR), solar thermal collecting and storage unit (STS), and solar absorption chiller unit (SAC). The COM unit contains a motor (M), air compressor train (AC1–AC4), Entropy 2020, 22,four 764 heat exchangers (HE1–HE4) in series, a hot-water tank (HWT), a low-temperature water tank 3 of 19 (LWT), a compression heat radiator (CHR) and an air storage chamber (ASC). The TUR unit includes a throttle valve (TV), an air turbine chain (AT1, AT2 and AT3), a preheat regenerator (PHR) and three The restheat of paperexchangers is organized(HE5, HE6 and as HE7). follows. The STS Sectionunit consists2 describes of a parabolic the trough overall collector design (PTC), processa and cold oil tank (COT), a hot oil tank (HOT), and heat transfer fluid (HTF). The HTF used in the absorber thermodynamictube model was Therminol of HT-CAES.®VP-1, which Section is widely3 elaborates used in STS system the energy [34]. It andhas an exergy optimal analysisusage range of HT-CAES. This is greatlywhen related in liquid to phase the critical from 12 °C parameters to 400 °C [35]. and Therefore, the impact Therminol of®VP-1 the is CHP quite capacitysuitable for assessmentthe on system performanceproposed inHT-CAES. Section The4, SAC followed unit comprises by conclusions refrigeration air-conditioning in Section5. (RAC), a chiller machine (CM), a cooling water tank (CWT) and LWT. Although the COM and TUR subsystems are greatly similar to the A-CAES reported in many 2. System Descriptionstudies [10–15], the HT-CAES does possess several distinct characteristics. Firstly, compression heat energy is used to supply heating to users. Secondly, high-temperature VP-1 heated by the PTC can Figure1 showssupply sufficient the schematic and high-grade diagram stable ofheat the reso proposedurces for SAC; HT-CAES VP-1 in HOT system. and compressed To clearly air can describe and analyze the system,provide cooling, all the heating, components and power. and Thirdly, streams the compressed have been air numbered. is preheated by The the exhaust bottom gas part of of Figure1 is a small-scalethe AA-CAES last stage of the system expander where before compression entering the turbo heat expander generated and is further in the heated charging by the processhigh- is stored temperature VP-1 of the heat exchanger (HE5–HE7) to enhance the comprehensive efficiency of HT- in the high-temperatureCAES. water tank (HWT) to provide heat for residents.

Figure 1. Schematic of the Hybrid trigenerative compressed air energy storage (HT-CAES) system. Figure 1. Schematic of the Hybrid trigenerative compressed air energy storage (HT-CAES) system.

An HT-CAES3. Thermodynamic is mainlycomposed Analysis Model of five units, i.e., compression air storage unit (COM), air turbine and generator unitTo (TUR),analyze the solar performance thermal of collectingthe hybrid T- andCAES, storage a comprehensive unit (STS), thermodynamic and solar model absorption is chiller unit (SAC). Theestablished COMunit in this contains section based a motor on mass (M), and ener airgy compressor balance. The trainclassical (AC1–AC4), Peng–Robinson four equations heat exchangers (HE1–HE4) in series, a hot-water tank (HWT), a low-temperature water tank (LWT), a compression heat radiator (CHR) and an air storage chamber (ASC). The TUR unit includes a throttle valve (TV), an air turbine chain (AT1, AT2 and AT3), a preheat regenerator (PHR) and three heat exchangers (HE5, HE6 and HE7). The STS unit consists of a parabolic trough collector (PTC), a cold oil tank (COT), a hot oil tank (HOT), and heat transfer fluid (HTF). The HTF used in the absorber tube was Therminol®VP-1, which is widely used in STS system [34]. It has an optimal usage range when in liquid phase from ® 12 ◦C to 400 ◦C[35]. Therefore, Therminol VP-1 is quite suitable for the proposed HT-CAES. The SAC unit comprises refrigeration air-conditioning (RAC), a chiller machine (CM), a cooling water tank (CWT) and LWT. Although the COM and TUR subsystems are greatly similar to the A-CAES reported in many studies [10–15], the HT-CAES does possess several distinct characteristics. Firstly, compression heat energy is used to supply heating to users. Secondly, high-temperature VP-1 heated by the PTC can supply sufficient and high-grade stable heat resources for SAC; VP-1 in HOT and compressed air can provide cooling, heating, and power. Thirdly, the compressed air is preheated by the exhaust gas of the last stage of the expander before entering the turbo expander and is further heated by the high-temperature VP-1 of the heat exchanger (HE5–HE7) to enhance the comprehensive efficiency of HT-CAES.

3. Thermodynamic Analysis Model To analyze the performance of the hybrid T-CAES, a comprehensive thermodynamic model is established in this section based on mass and energy balance. The classical Peng–Robinson equations of the state were selected as the property package. The analysis was carried out using Thermoﬂow software. To facilitate the analysis, some assumptions are made as follows: Entropy 2020, 22, 764 4 of 19

1. Potential and kinetic energies of all units are negligible; 2. All gases in the system are treated as the ideal gas, and the Joule–Thomson eﬀect is negligible; 3. Compression in the compressor and expansion in the turbine is regarded as an isentropic process; 4. In the charging or discharging process, the air storage chamber is considered as an isothermal process; 5. Compressed air can be stored at constant volume, and its temperature in ASC is the same as ambient temperature; 6. The pressure loss in all pipes and heat exchangers is ignored.

3.1. STS The STS includes PTC and a thermal storage tank. The PTC thermodynamic model is shown in [22]. The thermal storage tanks are modeled by dynamic mass and energy balances for two tanks. The mass balance for a tank is: dVHTF . . ρHTF = m mout, (1) dt in − where VHTF is the volume of heat transfer ﬂuid (HTF) in the tank, and ρHTF refers to the density of HTF. The energy balance for each tank is:

d(VHTFT) . . ρHTFCHTF = CHTF T m T mout UAt(T T ), (2) dt in in − in − − 0 where U is the overall heat transfer coeﬃcient for the tank walls and At is the surface area of the tank subject to the heat transfer. We assume that no heat transfer occurs from the top or bottom of either tank because the volumes of HTF (VHTF) in the tanks are not constant.

3.2. SAC The solar absorption chiller is driven by the collected heat of STS. The heat of Therminol oil VP-1 (VP-1) supplied to the vapor generator of the absorption chiller (Qab(t)) can be written as:

. . Q (t) = c m (t)(T T ), (3) ab p,O O,ab O13 − O14 . where mO,ab(t) is the mass ﬂow rate of hot oil (VP-1) injecting into the absorption chiller. TO13 is the temperature of stream O13. TO14 is the temperature of stream O14. The cooling load production by the . absorption chiller (Qcl,ab(t)) can be obtained by,

. . Q (t) = COP Q (t), (4) cl,ab ab · ab where COPab is the coeﬃcient of performance of the absorption chiller.

3.3. T-CAES

3.3.1. Charging Process During the charging process, the power produced by PV or wind can be used to drive the compressor of T-CAES. 4 . X . i i WCOM(t) = mac,T CAES(hout h ), (5) − − in i=1 . . where WCOM(t) is the compressor power consumption and mac,T CAES is the mass ﬂow rate of the i i − compressor, which is produced by wind or solar energy. hin and hout are the compressor inlet and outlet enthalpy of each stage. Entropy 2020, 22, 764 5 of 19

In the compression process, the parameters of stream A1 are the same as the ambient, and the parameters of air outlet the compressor (stream A2, A4, A6 and A8) are described as follows:

p = β p , (6) COM,i · 0 κ 1 − T0(β κ 1) Ti = − + T0, (7) ηc,T CAES − where β is the pressure ratio of the compressor in the T-CAES. T0 and p0 are the ambient temperature and pressure. ηc,T CAES is the isentropic eﬃciency of the compressor in the T-CAES and κ is the − polytropic index. The air mass ﬂow rate of the compressor in the T-CAES can be calculated by:

. . WCOM(t) mac,T CAES(t) = , (8) − cp,a(T T ) 6 − 0 where cp,a is the air-speciﬁc heat at constant pressure. The high-pressure air is cooled down through the side of the COM heat exchanger. The heat . recovered in the charging process (Qcp,HT AES(t)) and can be written as: − 9 . X . Qcp,T AES(t) = cp,amac,T CAES(t) (TAi TAi+1), (9) − − · − i=2

After cooling, the stream A9 ﬂows into the air storage chamber (ASC). The temperature of the compressed air storage tank stays the same as the ambient temperature T0. The ﬁnal air mass and pressure of compressed air inside ASC are determined as follows:

p0,ASCVASC m0,ASC = , (10) RaT0

R τch . (m0,ASC + mac,HT CAES(t)dt)RaT0 0 − pd,ASC = , (11) VASC where τch is the charging time, m0,ASC is the initial air mass of ASC before the charging process, and p0,ASC is the initial air pressure of the compressed air chamber before charging process. pd,ASC is the ﬁnal pressure of the compressed air chamber after charging process. VASC is the volume of the compressed air reservoir. Ra is the air gas constant.

3.3.2. Discharging Process During the discharging process, the volume of compressed air chamber keeps constant. As the discharging process goes on, the inside pressure of compressed air chamber drops. The temperature of stream A10 is the same as ambient temperature T0, and the pressure of ASC (stream A10) can be calculated as follows: R τdch . (m0,ASC + mat,HT CAES(t)dt)RaT0 0 − pA10 = , (12) VASC ( 0 t τdch R τdch . ≤ ≤ R τch . (13) 0 mat,T CAES(t)dt) m0,ASC + mac,T CAES(t)dt) ≤ 0 − ≤ 0 − . where mac,T CAES is the air mass flow rate of turbine in the TUR. τdch is the discharging time. This research − only considers the regulation of air flow rate by adjusting the regulating valve and the Joule–Thomson effect is negligible. Before entering each stage of turbine, the stream A11 is first preheated by the third stage expander, and then, heated by stored solar thermal energy in the HOT. The temperature of stream Entropy 2020, 22, 764 6 of 19

A13 can be calculated according to energy conversion equation, and the pressure of stream A13 equals to the pressure of stream A11 and A12.

. cp,OmO(t)(TO6 TO7) TA13 = . − + TA12, (14) cp,amat,HT CAES(t) − . where mO(t) is the mass ﬂow rate of VP-1 using to heat stream A12. TO6 and TO7 are the temperatures of stream O6 and O7, respectively. The turbine outlet air temperature in the expansion process can be written as follows: Pin = TUR,i πi,TUR out , (15) PTUR,i

1 κ Tout = Tin η Tin 1 π −κ (16) TUR TUR − en,i TUR − i,TUR where πi,TUR is the pressure ratio of the turbine. ηen,i is the isentropic eﬃciency of each stage turbine in the T-CAES. The output power of the turbine in the T-CAES is presented as follows:

3 . X . out,i in,i WTUR(t) = mat,T CAES(t)(h h ) (17) − TUR − TUR j=1

3.4. Exergy Analysis Model An exergy analysis based on the second law of thermodynamics can be performed to decide the exergy destruction of each subsystem [36]. Generally, the enthalpy exergy of the state i can be calculated by . Ex = m [(h h ) T (s so)], (18) i i i − 0 − 0 i − . where mi is the mass flow rate, h is the specific enthalpy, s is the specific entropy, and subscripts i and 0 represent state i and ambient conditions, respectively. For each subsystem j, the exergy destruction and exergy efficiency can be calculated as follows:

L = E E , (19) j j,in − j,out

Ej,out ηEXE,j = , (20) Ej,in where the subscripts in and out represent the input and output states, respectively, of subsystem j. The expressions for input and output exergy of each subsystem are listed in Table1. It is worth noting that the abbreviations in Table1 are list in the part of Nomenclature.

Table 1. Expressions of input and output exergy of each subsystem.

Subsystem Exin Exout

AC WCOM + ExA3 + ExA5 + ExA7 ExA2 + ExA4 + ExA6 + ExA8 Ex + Ex + Ex + Ex + Ex + Ex + Ex + Ex + Ex + Ex + HE of COM A2 A4 A6 A8 W1 A3 A5 A7 A9 W2 ExW3 + ExW5 + ExW7 ExW4 + ExW6 + ExW8 STS Ex Ex Ex Qu O3 − O2 AT ExA12 + ExA14 + ExA16 WTUR + ExA13 + ExA15 + ExA17 Ex + Ex + Ex + Ex + Ex + Ex + Ex + Ex + HE of TUR A10 A17 A11 A13 A11 A18 A12 A14 ExA15 + ExO5 ExA16 + ExO12 Absorption Chiller ExO13 + ExW11 ExO14 + ExW12 ASC ExA9 ExA10 Entropy 2020, 22, 764 7 of 19

3.5. Performance Criteria The proposed HT-CAES system has three modes of operation: energy storage, idle, and energy release mode. Unlike conventional CAES, energy storage modes can be divided into air storage and solar thermal collection and storage processes. It is worth noting that the process of collecting and storing solar thermal energy is independent of the air compression process. Therefore, these two processes can be performed simultaneously. In an idle mode, high-pressure air is stored in the ASC at the design pressure, and high-temperature VP-1 is stored in the HOT at the design temperature. In the energy release mode, the high-pressure air and the thermal energy are released at the same time. The compressed air is heated by the high-temperature VP-1 in the HX5–HX7 to drive the turbine to generate electrical energy. Electric storage efficiency (ESE), round-trip efficiency (RTE), and exergy efficiency (EXE) are key indicators for analyzing the performance of the proposed system. ESE is defined as the amount of electricity generated during discharging divided by the power consumption during charging. The power of the water pump and oil pump is ignored because of their low value. Therefore, ESE can be expressed as . WTUR τdch ηESE = . · . (21) W τ COM · ch RTE is defined as the ratio of total thermal and electrical energy output to total solar and electrical energy input in a full charge/discharge cycle. It can be expressed as

WTUR τdch + Qheat τhs + QCS τcs ηRTE = · · · . (22) W τ + Qu τc COM · ch · The exergy eﬃciency can be represented as

WTUR τdch + Exheat τhs + ExCS τcs ηEXE = · · · , (23) W τ + ExHTF τc COM · ch · where Qheat is the thermal energy supply to the heat load, QCS is the cooling energy that generated by absorption chiller supply to the residents collected and stored solar thermal energy, Exheat is enthalpy exergy of Qheat, ExHTF is the HTF enthalpy exergy absorbed by the PTC, and ExCS is enthalpy exergy of QCS.

4. Results and Discussion In this section, the performance of the hybrid T-CAES under typical operational conditions is analyzed and discussed. Table2 lists the design parameters of the system. In this case, the duration of charging and discharging time was 5 h and 1.4 h, respectively. The mass flow rate of the compression and expansion process was 0.33 kg/s and 1.17 kg/s. The isentropic efficiency of the air compressors was 90%. The pressure of the outlet air of the throating valve (state A11), which is determined by the expansion ratio of each stage. The minimum ASC pressure is approximately equal to the pressure of the inlet air of the first-stage air turbine (state A13). From the designed parameters presented in this section, the compressor outlet pressure and expander inlet pressures were selected as 8 MPa and 3 MPa. The effects of the compression and expansion pressures on system performance are discussed in Section 4.2. In the TUR subsystem, the isentropic efficiency of the turbines was selected at 85% and the other parameters of the STS were selected by previous studies. Entropy 2020, 22, 764 8 of 19

Table 2. Design parameters of the Hybrid trigenerative CAES (HT-CAES) system.

Parameters Units Values Parameters Units Values COM TUR Ambient pressure MPa 0.1 Inlet pressure of air turbine MPa 3 Ambient temperature ◦C 20 Inlet temperature of air turbine ◦C 200 Compression stage (i) / 4 Expansion stage (j) / 3 Isentropic efficiency of compressor % 90 Isentropic efficiency of turbine % 85 Compression ratio / 3 Expansion ratio / 2.65 τch h 5 τdch h 1.4 Mass flow rate of compressor kg/s 0.33 Mass flow rate of turbine kg/s 1.17 Temperature of recovery water ◦C 30 Range of pressure with ASC MPa 3~8 Volume of air store chamber m3 100 Efficiency of generator % 94.8

Temperature of hot water tank ◦C 60 SAC /// Cooling water supplying duration h 5 Temperature of low water tank ◦C 20 COP of absorption chiller / 0.67 Hot water supplying duration h 8 Mass flow rate of cooling water kg/s 0.63 STS Range of temperature with hot Direct normal irradiance W/m2 841.1 C 250~252 oil tank ◦ Mass flow rate of VP-1 kg/s 0.336 Solar thermal storage duration h 6 (charging process) Mass flow rate of VP-1 Efficiency of heat collection % 63.8 kg/s 1.44 (discharging process) Reflectivity of collector mirrors % 94 Collector area m2 405.8

4.1. Typical Operational Conditions Table3 lists the main simulation results of the HT-CAES system under the designed conditions. Tables4 and5 list the stream thermodynamic parameters of air, water, and Therminol VP-1 respectively. As shown in Table3, the total compressed power consumption was 190 kW and the heat energy collected from solar was 100.6 kW. The output power of the turbine was 352 kW. The ESE, RTE, and exergy efficiency of the proposed system were 53.6%, 73%, and 50.6%, respectively. As listed in Table4, the temperatures of supply and return water for the heating load were 60 ◦C and 20 ◦C. Under average solar irradiation operational conditions, the system needed about 6 h to raise 7.2 tons of VP-1 from 100 ◦C to 250 ◦C. The output power of the turbine is greatly influenced by the VP-1 temperature provided by the collected and stored solar energy, which is discussed in Section 4.2. The performance of the A-CAES depends on the regenerative system, which is to recover and store compression heat in thermal energy storage (TES). This is greatly constrained by the structure of the compressor and multi-stage heat exchanger effectiveness. The adopted solar thermal energy can avoid the high-temperature limit of the compressor and complex heat regeneration subsystem, which can greatly simplify the structure of the A-CAES [22]. For a 4 h discharge, the total power generation capacity was 955.4 kWh.

Table 3. Simulation results of the HT-CAES system under typical operational conditions.

Parameters Unit Values Compressor power consumption kW 190 Air turbine electricity generation kW 352 Collection power of PTC kW 100.6 Production the mass of cooling water ton 15 Production the mass of hot water ton 20 Electricity storage efficiency % 53.6 Round-trip efficiency % 73 Exergy efficiency % 50.6 Entropy 2020, 22, 764 9 of 19

Table 4. Thermodynamic parameters of air stream.

Stream T ( C) P (MPa) h (kJ/kg) s (kJ/kg K) Ex (kJ/kg) m (kg/s) ◦ · A1 20 0.101 419.41 3.8644 0 0.33 A2 143.7 0.309 544.15 3.8983 114.81 0.33 A3 45 0.303 444.17 3.6302 93.403 0.33 A4 178.8 0.924 579.49 3.6648 218.6 0.33 A5 45 0.906 442.98 3.3124 185.4 0.33 A6 177.3 2.729 576.64 3.3461 309.18 0.33 A7 45 2.675 439.58 2.9916 276.05 0.33 A8 179.6 8.16 575.76 3.0246 402.53 0.33 A9 45 8 430.23 2.6485 367.26 0.33 A10 19.9 2.903 412.82 2.8806 281.82 1.17 A11 65 2.903 460.02 3.0304 285.1 1.17 A12 200 2.846 600.13 3.3848 321.32 1.17 A13 90 0.949 488.73 3.4336 195.62 1.17 A14 200 0.93 601.28 3.71 227.12 1.17 A15 90.2 0.31 489.85 3.758 101.65 1.17 A16 200 0.304 601.68 4.0324 133.02 1.17 A17 90.3 0.101 490.26 4.081 7.3474 1.17 A18 34.5 0.1 308.59 6.89 0.35 1.17

Table 5. Thermodynamic parameters of Therminol VP-1 and water stream.

Stream T ( C) P (Mpa) h (kJ/kg) s (kJ/kg K) Ex (kJ/kg) m (kg/s) ◦ · O1 100 0.101 579.85 7.2 10.21 0.34 − O2 100.3 0.436 580.29 2.77 14.90 0.34 − O3 251 0.101 879.18 1.48 273.06 0.34 − O4 250 0.1014 877 2.03 144.64 1.44 − O5 250 0.103 877 1.50 268.91 1.44 − O6 250 0.103 877 1.50 268.91 0.55 − O7 100 0.101 579.85 2.76 10.54 0.55 − O8 250 0.103 877 1.50 268.91 0.45 − O9 100 0.101 579.85 2.65 21.99 0.45 − O10 250 0.103 877 1.50 268.91 0.44 − O11 100 0.101 579.85 2.64 22.22 0.44 − O12 100 0.101 579.85 2.69 17.87 1.44 − O13 250 0.12 375.20 1.02 144.64 0.9 − O14 150 0.101 14.30 2.06 14.90 0.9 − W1 20 0.103 84.01 0.30 0.00 0.2 W2 60 0.101 251.25 0.83 10.47 0.2 W3 20 0.103 84.01 0.30 0.00 0.27 W4 60 0.101 251.25 0.83 10.47 0.27 W5 20 0.103 84.01 0.30 0.00 0.27 W6 60 0.101 251.25 0.83 10.47 0.27 W7 20 0.103 84.01 0.30 0.00 0.28 W8 60 0.101 251.25 0.83 10.47 0.28 W9 60 0.403 251.50 0.83 10.77 1.02 W10 20 0.403 84.29 0.30 0.30 1.02 W11 60 0.130 251.27 0.83 10.50 0.63 W12 30 0.101 125.82 0.44 0.70 0.63 W13 4 0.2 17.01 0.061 2.0 0.4 W14 19.6 0.2 82.43 0.29 0.098 0.4

To evaluate the performance of each subsystem, the efficiency and exergy destruction were calculated from Equations (19) and (20) and Table1. Figure2 illustrated the exergy e fficiency of each subsystem. Due to the basic characteristics of STS, the exergy efficiency of the STS subsystem is much lower than that of other subsystems, except TUR’s HE. In the STS, the absorption energy of the heat Entropy 2020, 22, 764 10 of 19 medium wasEntropy provided 2020, 22, byx FOR stored PEER REVIEW Therminol VP-1 in the HOT, which was greatly affected10 of 19 by the PTC’s mirror field area,energy mass of the heat flow medium rate, weather,was provided optical by stored effi Therminolciency, VP-1 and in tracking the HOT, which accuracy was greatly [37]. In addition, due to the largeEntropyaffected temperature 2020 by, 22 the, x FOR PTC’s PEER di mirror ffREVIEWerence field between area, mass cold flow andrate, hotweather, fluids, optical the efficiency, HE of TUR and tracking exergy10 of 19 efficiency is accuracy [37]. In addition, due to the large temperature difference between cold and hot fluids, the the lowest in the discharging process. Commented [M1]: We didn’t see the citation of ref. [36] energyHE of TUR of the exergy heat mediumefficiency was is the provided lowest byin thestored discharging Therminol process. VP-1 in the HOT, which was greatly before [37], please add. affected by the PTC’s mirror field area, mass flow rate, weather, optical efficiency, and tracking accuracy [37]. In addition, due to the large temperature difference between cold and hot fluids, the Commented [M1]: We didn’t see the citation of ref. [36] HE of TUR exergy efficiency is the lowest in the discharging process. before [37], please add. 80

60 80

40 60 Exergy efficiency / % efficiency Exergy

20 40 Exergy efficiency / % efficiency Exergy 0 20 AC HE of COM AT HE of TUR STS ASC SAC FigureFigure 2. Exergy 2. Exergy eﬃ efficienciesciencies of of each each subsystem. subsystem. 0 Figure 3Figure 3 shows theAC exergy HE of COM destruction AT HE of of TUR each STS subsystem. ASC In SAC the STS system, the exergy Formatted: Font: Not Bold Figure3 shows the exergy destruction of each subsystem. In the STS system, the exergy destruction destruction is 14% because ofFigure the irreversible 2. Exergy efficiencies loss caused of eachby the subsystem. external heat conduction, convection is 14% becauseand ofthe theradiation irreversible of the parabolic loss mirror caused and byabsorber the externaltube surface heat [38,39]. conduction, The STS requires convection a long and the radiation of thetime parabolicFigure to heat 3Figure the mirrorVP-1 3 shows from and ambient the absorberexergy temperature destruction tube to surface of the each design subsystem. [38 temperature,39]. TheIn the STS beforeSTS system, requires the operation. the aexergy long In timeFormatted: to heat Font: Not Bold destructionthis process, is the14% exergy because destruction of the irreversible of the system loss caused does notby thetake external into account heat conduction, the exergy convectionanalysis of the VP-1 fromandthe ambient HT-CAES.the radiation temperatureDue of to the the parabolic high temperature to mirror the designand differentia absorber temperaturel tubeheat surfacetransfer, before[38,39]. the exergy The the destructionSTS operation. requires of a AT long In in this process, the exergy destructiontimeTUR tois theheat largest the of VP-1 the of all from system subsystems, ambient does temperaturewh notich results take to in intothe irreversible design account temperature heat the loss. exergy before the analysis operation. of In the HT-CAES. Due to the highthis temperatureprocess, the exergy diﬀ destructionerential heatof thetransfer, system does the not exergy take into destruction account the exergy of AT analysis in TUR of is the largest the HT-CAES. Due to the high AC temperature differential heat transfer, the exergy destruction of AT in of all subsystems,TUR is the which largest results of all subsystems, inHEirreversible of COM which results heat in loss. irreversible heat loss. AT ACHE of TUR 18% 14% HESTS of COM ATASC 4% SAC HE of TUR 18% 14% STS ASC 4% SAC 20% 15%

12% 20% 17% 15%

Figure 3. Exergy destruction of each subsystem. 12% 17% To further clarify the performance of the proposed HT-CAES, a comparison of the HT-CAES with the HA-CAES inFigure [29] wasFigure 3. illustratedExergy 3. Exergy destruction in destruction Table 6Table of ofeach 6 each .subsystem. In the subsystem. comparative case, the RTE and Formatted: Font: Not Bold EXE of each system were 73% and 76.5%, and 50.6% and 53.4%, respectively. The inlet temperature To furtherof the clarifyTo gas further turbine the clarify performance was the900 performance °C, which of thewas of proposedthedue prop to thosede combination HT-CAES, HT-CAES, of a thecomparison a comparison gas turbine of thewith ofHT-CAES the the heat HT-CAES with withcollector the HA-CAESand combustion in [29] chamber.was illustrated Since thesein Table types 6Table of HA-CAES 6 . In the docomparative not allocate case, TES, the the RTE output and Formatted: Font: Not Bold the HA-CAESEXEpower in of [eachof29 gas] system was turbo-generator illustrated were 73% and fluctuates in 76.5%, Table andwith6. 50.6% changes In the and comparativein53.4%, solar resp radiation.ectively. case, Although The theinlet the RTEtemperature RTE and and EXE of each system wereofexergy 73% the gas efficiency and turbine 76.5%, are was higher 900 and °C, than 50.6% which the HT-CAES, was and due 53.4%, to th isth typee combination respectively. of HA-CAES of thegreatly The gas turbinedepends inlet temperaturewith on fossil the heat fuel of the gas turbine was 900collectorsupplies,C, and which combustion leads was to due certainchamber. to carbon the Since combination emission these typess. Compared of HA-CAES of the with gas do the turbinenot above allocate HA-CAES, with TES, thethe the output heat HT- collector and power◦ of gas turbo-generator fluctuates with changes in solar radiation. Although the RTE and combustionexergychamber. efficiency Since are higher these than types the HT-CAES, of HA-CAES this typedo of HA-CAES not allocate greatly TES, depends the on output fossil fuel power of gas turbo-generatorsupplies, fluctuates which leads with to certain changes carbon in emission solar radiation.s. Compared Althoughwith the above the HA-CAES, RTE and the exergy HT- efficiency are higher than the HT-CAES, this type of HA-CAES greatly depends on fossil fuel supplies, which leads to certain carbon emissions. Compared with the above HA-CAES, the HT-CAES with STS can provide a high-grade stable external heat source, which can greatly improve the stability and flexibility of HT-CAES. Entropy 2020, 22, x FOR PEER REVIEW 11 of 19

CAES with STS can provide a high-grade stable external heat source, which can greatly improve the stability and flexibility of HT-CAES.

Table 6. The comparison of performance of the ST-CAES to the hybrid A-CAES.

Parameters Unit HT-CAES Hybrid A-CAES Compressor power consumption kWh 925 152.1 Charging time h 5 6.59 Discharging time h 1.4 5.13 With/without TES / Yes no Expansion train working period load peak hours Irradiation peak hours EntropyInlet 2020temperature, 22, 764 of air turbine °C 200 900 11 of 19 Air turbine electricity generation kWh 385.2 228.54 RoundTable trip efficiency 6. The comparison of performance % of the ST-CAES 73 to the hybrid A-CAES. 76.5 Exergy efficiency % 50.6 53.4 Parameters Unit HT-CAES Hybrid A-CAES 4.2. SensitivityCompressor Analysis power consumption kWh 925 152.1 Charging time h 5 6.59 To investigateDischarging the effect time of technique hparameters on 1.4 the performance 5.13of the HT-CAES, a With/without TES / Yes no sensitivity analysisExpansion was train conducted. working period These parameters included load peak hours inlet temperature Irradiation peakand hourspressure of the compressors Inletand temperature turbine. The of air sensitivity turbine analysis◦C was conducted 200 by varying one 900 parameter, which caused affiliatedAir turbine parameters electricity to generation vary correspo kWhndingly, while 385.2 others were kept constant. 228.54 Round trip efficiency % 73 76.5 Exergy efficiency % 50.6 53.4 4.2.1. Inlet Temperature of the Compressor 4.2. Sensitivity Analysis Figure 4a shows the effect of inlet temperature of the compressor on the WCOM, WTUR, and pressure insideTo investigate the ASC the ( ePffASCect). of The technique inlet temperature parameters on theof the performance compressor of the slightly HT-CAES, affects a sensitivity WCOM and PASC, withanalysis WTUR was kept conducted. constant. These With parameters the increasing included inlet inlet temperature temperatureand of the pressure compressor, of the compressors therefore, the ambientand temperature turbine. The sensitivityalso increased, analysis which was conducted augmented by varying the W oneCOM parameter, calculated which by Equation caused affi liated(5). Thus, when parametersthe inlet temperature to vary correspondingly, of the compressors while others increases were kept from constant. 15 °C to 35 °C, the energy consumption of WCOM4.2.1. increases Inlet Temperature by 8 kWh. of theMeanwhile, Compressor based on the ideal gas equation and constant volume of ASC, PASC increases by 0.15 MPa. Figure4a shows the e ffect of inlet temperature of the compressor on the WCOM,WTUR, and pressure Figure 4b shows that the ESE, RTE, and ηex all decline with increasing ambient temperature. As inside the ASC (PASC). The inlet temperature of the compressor slightly affects WCOM and PASC, with illustrated in Figure 4a, WCOM increases but WTUR stays constant. Furthermore, the supplied heat load WTUR kept constant. With the increasing inlet temperature of the compressor, therefore, the ambient and heat energy provided by the STS remains constant. Therefore, when the ambient temperature temperature also increased, which augmented the WCOM calculated by Equation (5). Thus, when the ex increasesinlet from temperature 15 to 35 of °C, the compressorsESE, RTE, and increases η all from decrease 15 ◦Cto by 35 1.1,◦C, 4, the and energy 2, respectively. consumption ofIn W general,COM a lower increasesambient bytemperature 8 kWh. Meanwhile, is more basedbeneficial on the to ideal system gas equationperformance. and constant volume of ASC, PASC increases by 0.15 MPa.

8.4 75

900 70

65 800

8.2 % ex η

/ kWh / 60 & / MPa

TUR 700 ASC ESE p ,

&W 55 W COM COM 600 8.0 RTE W 50 WTUR pASC RTE 500 45 ESE η ex 7.8 40 15 20 25 30 35 10 15 20 25 30 35 40 Inlet temperature of first-stage of COM T / ℃ Inlet temperature of first stage of COM T/ ℃

(a) (b)

FigureFigure 4. Effect 4. E ﬀofect inlet of inlet temperature temperature of of compressors (a )(a) on on WCOM, WCOM, WTUR, WTUR, and PASC; and (PASC;b) on ESE, (b) RTE,on ESE, and η . RTE, and ηex.

Figure4b shows that the ESE, RTE, and ηex all decline with increasing ambient temperature. 4.2.2. Inlet Temperature of the Turbine As illustrated in Figure4a, W COM increases but WTUR stays constant. Furthermore, the supplied heat load and heat energy provided by the STS remains constant. Therefore, when the ambient temperature increases from 15 to 35 ◦C, ESE, RTE, and ηex all decrease by 1.1, 4, and 2, respectively. In general, a lower ambient temperature is more beneﬁcial to system performance.

4.2.2. Inlet Temperature of the Turbine

Figure5a shows the e ﬀect of air turbine inlet temperature on WCOM,WTUR, and mass ﬂow of VP-1. As revealed in Figure5a, the increasing inlet air temperature of the turbine increases the power Entropy 2020, 22, x FOR PEER REVIEW 12 of 19

Figure 5a shows the effect of air turbine inlet temperature on WCOM, WTUR, and mass flow of VP-

1. As Entropyrevealed2020, in22, 764Figure 5a, the increasing inlet air temperature of the turbine increases 12the of 19power generation of the turbine system, owing to a greater decrease in the enthalpy of the air, which can be calculated by Equation (10). As the inlet temperature of the air turbine increases, the VP-1 mass flow also increasesgeneration because of the turbine the turbine system, needs owing more to a greater heat decreaseenergy to in theenhance enthalpy inlet of theair air,temperature. which can be When calculated by Equation (10). As the inlet temperature of the air turbine increases, the VP-1 mass ﬂow the inlet temperature of the air turbine reaches 220 °C, the maximum WTUR and mass flow of also increases because the turbine needs more heat energy to enhance inlet air temperature. When the Therminol VP-1 reach 956 kWh and 2.7 kg/s, respectively. No compressor parameters were changed, inlet temperature of the air turbine reaches 220 ◦C, the maximum WTUR and mass ﬂow of Therminol so WCOM remained the same. VP-1 reach 956 kWh and 2.7 kg/s, respectively. No compressor parameters were changed, so WCOM remained the same.

1.7 75

900 70 1.6

800 65 %

1.5 ex / kWh 700 / kg/s 60 TUR VP-1 &W 600 1.4 M 55 COM RTE, ESE η & RTE, W 500 50 WCOM 1.3 ESE W RTE TUR 45 400 MVP-1 ηex 1.2 170 180 190 200 210 220 230 170 180 190 200 210 220 230 Inlet temperature of TUR T/ ℃ Inlet temperature of TUR T / ℃ (a) (b)

FigureFigure 5. Effect 5. Eﬀ ofect air of airturbine turbine inlet inlet temperature temperature on on (a )( WCOM,a) WCOM, WTUR, WTUR, and VP-1 and mass VP-1 ﬂow; mass (b )flow; on ESE, (b) on ESE, RTE,RTE, and η ηex.ex.

Figure5b illustrates that the ESE, RTE, and ηex increased with air turbine inlet temperature. Figure 5b illustrates that the ESE, RTE, and ηex increased with air turbine inlet temperature. As As shown in Figure5a, W TUR increased while WCOM remained unchanged, which resulted in greater TUR COM shownESE in accordingFigure 5a to, W Equation increased (21), while whileQ heatW was remained constant due unchanged, to the parameter which beingresulted keep in constant greater ESE accordingduring to the Equation compression (21), stage.while Finally,Qheat was by constant a comprehensive due to the calculation parameter using being Equations keep (22)constant and (23), during the compressionRTE and ηex increasestage. Finally, with the by air a turbine comprehensive inlet temperature. calculation using Equations (22) and (23), RTE and ηex increaseFor example, with the when air the turbine air turbine inlet inlet temperature. temperature increases from 180 ◦C to 220 ◦C under design Forinlet example, air pressure, when ESE, the RTE, air turbine and ηex increase inlet temperature by 5.21%, 4.43%, increases and 4.38%, from 180 respectively. °C to 220 Therefore, °C under all design the efficiencies increase with the increasing air turbine inlet temperature. Moreover, the increments inlet air pressure, ESE, RTE, and ηex increase by 5.21%, 4.43%, and 4.38%, respectively. Therefore, all in the ESE, RTE, and η are linear; the increasing rate of total output energy also increases with the the efficiencies increase withex the increasing air turbine inlet temperature. Moreover, the increments increasing inlet air turbine pressure. in the ESE, RTE, and ηex are linear; the increasing rate of total output energy also increases with the increasing4.2.3. Inletinlet Pressureair turbine of the pressure. Turbine Figure6a shows the variation of operation time and turbine power with inlet pressure of turbine. 4.2.3. ToInlet eliminate Pressure throttling of the Turbine loss, we defined the minimum ASC pressure as equal to the inlet air turbine Figurepressure. 6a Ashows greater the turbine variation inlet of pressure operation increases time theand output turbine power power of the with turbine. inlet However,pressure of when turbine. To eliminateconstrained throttling by the ASC loss, maximum we defined pressure, the minimum increasing ASC the ASC pressure operation as equal pressure to leadsthe inlet to reduced air turbine system operation time. Meanwhile, the effect of ASC exerts minimum pressure on ESE, RTE, and η , pressure. A greater turbine inlet pressure increases the output power of the turbine. However,ex when as shown in Figure6b. The decrease in the operation time reduced the amount of hot water and constrainedVP-1 consumption. by the ASC maximum pressure, increasing the ASC operation pressure leads to reduced system operationAs shown time. in Figure Meanwhile,6b, when the inlet effect pressure of ASC of turbine exerts increasesminimum from pressure 4.9 MPa on to ESE, 8.8 MPa RTE, and and ηex, as shownthe discharge in Figure duration 6Figure time 6b. The decreases decrease from in 6.9 the h tooperation 1.4 h, the ti outputme reduced power the increases amount by 29.7of hot kW. water Formatted: Font: Not Bold and VP-1Correspondingly, consumption. ESE, RTE, and ηex increase by 8%, 5%, and 2.4%, respectively. Therefore, the combination of all these effects amplifies the ESE; RTE and ηex increase with the increase in inlet pressure of the turbine.

Entropy 2020, 22, x FOR PEER REVIEW 13 of 19

250 10 75

245 70 8 240 65 ex/ %

235 6 η 60 time / h time/ g

230 g 55 Output power 4 225

dischar Discharing time RTE, ESE and 50 OutpowerkWof TUR / ESE 220 2 45 RTE Entropy 2020, 22, x FOR PEER REVIEW ηex 13 of 19 Entropy 202021522 , , 764 40 13 of 19 0 456789 56789 Inlet pressure of first stage TUR P / MPa Inlet pressure of first stage TUR P / MPa 250 10 75 (b) 245 (a) 70 8 240 Figure 6. Effect of inlet pressure on (a) PTUR and operation65 time; (b) on ESE, RTE, and ηex. ex/ %

235 6 η 60 time / h time/ g

As shown in Figure 6Figure 6b, when inlet pressurein of turbine increases from 4.9 MPa to 8.8 MPa Formatted: Font: (Asian) 宋体, Not Bold 230 g 55 Output power 4 and the discharge225 duration time decreases from 6.9 h to 1.4 h, the output power increases by 29.7 kW.

dischar Discharing time RTE, ESE and 50 OutpowerkWof TUR / Correspondingly, ESE, RTE, and ηex increase by 8%, 5%, and 2.4%, respectively. ESE Therefore, the 220 2 45 RTE combination of all these effects amplifies the ESE; RTE and ηex increase with the ηex increase in inlet 215 40 0 456789 pressure of the turbine.56789 Inlet pressure of first stage TUR P / MPa Inlet pressure of first stage TUR P / MPa 4.2.4. Exhaust Pressure of the Compressor(a) (b)

FigureFigure 7FigureFigure 6. Effect 6. 7Eaff of ectshows inlet of inlet pressure pressurethe variation on on (a) (a )PTUR PTUR of an andthed operationoperationcompressor’s time; time; (b )(b) onpower on ESE, ESE, RTE,consumption RTE, and ηandex. ηex. Pcom and Formatted: Font: Not Bold operation time with exhaust pressure of the compressor. Increasing the exhaust of the compressor 4.2.4. Exhaust Pressure of the Compressor meansAs that shown the incompression Figure 6Figure train 6b, needs when to inlet consume pressure more of turbine energy increases to compress from the4.9 MPaair to to a 8.8 higher MPa Formatted: Font: (Asian) 宋体, Not Bold P pressure.and the discharge ThisFigure also7a showsduration increases the time variation the decreasessystem of the operation compressor’sfrom 6.9 time.h to power 1.4 As h, shown consumptionthe output in Figure powercom 7Figureand increases operation 7a and by time 29.7 b when kW. Formatted: Font: Not Bold with exhaust pressure of the compressor. Increasing the exhaust of the compressor means that the theCorrespondingly, exhaust pressure ESE, of theRTE, compressor and ηex increasesincrease byfrom 8%, 8 MPa5%, andto 12 2.4% MPa, andrespectively. the mass flowTherefore, rate of theair compression train needs to consume more energy to compress the air to a higher pressure. This also fluid is 2 t/h, the charge duration time increases from 1.4 h to 6.9 h, and the consumption power of combinationincreases of the all system these operationeffects amplifies time. As the shown ESE; in RTE Figure and7a,b ηex when increase the exhaust with the pressure increase of the in inlet compressorspressurecompressor of the increases turbine. increases by from34.2 kW. 8 MPa Correspondingly, to 12 MPa and the ESE, mass RTE, flow and rate ofηex air decrease fluid is 2by t/ h,7.36%, the charge 4.2%, and 4%, respectively.duration time Theref increasesore, from ESE, 1.4 RTE, h to 6.9and h, η andex decrease the consumption with the power increasing of compressors exhaust increases pressure by of the compressor.4.2.4. 34.2Exhaust kW. Furthermore, Correspondingly, Pressure of the the ESE, Compressor increasing RTE, and massη ex decrease flow rate by 7.36%, of compressors 4.2%, and 4%, leads respectively. to a gradual Therefore, increase in compressionESE, RTE, and powerηex decrease consumption. with the increasing exhaust pressure of the compressor. Furthermore, the Figure 7Figure 7a shows the variation of the compressor’s power consumption Pcom and Formatted: Font: Not Bold increasing mass flow rate of compressors leads to a gradual increase in compression power consumption. operation time with exhaust pressure of the compressor. Increasing the exhaust of the compressor means that350 the compression train needs to consume10 more energy to compress the air to a higher 75 pressure. This also increases the system operation time. As shown in Figure 7Figure 7a and b when Formatted: Font: Not Bold 70 the exhaust340 pressure of the compressor increases8 from 8 MPa to 12 MPa and the mass flow rate of air 65 / %

fluid is 2 t/h, the charge duration time increases fromex 1.4 h to 6.9 h, and the consumption power of

η 60 compressors330 increases by 34.2 kW. Correspondingly,6 ESE, RTE, and ηex decrease by 7.36%, 4.2%, and /kW 55

4%, respectively.COM Therefore, ESE, RTE, and ηex decrease with the increasing exhaust pressure of the P charging time / h 50 compressor.320 Furthermore, the increasing mass 4flow rate of compressors leads to a gradual increase ESE, RTE and 45 ESE in compression power consumption. P COM RTE 310 τ 2 40 OP ηex 35 8 9 10 11 12 8 9 10 11 12 350 10 Maximum exhaust pressure of COM P / MPa 75 Maximum exhaust pressure of COM / MPa (a) 70 340 8 (b) 65 / %

FigureFigure 7. Effect 7. E ﬀofect maximum of maximum exhaust exhaust pressure pressure of of COM COMex ((a)a) onon PCOM PCOM and and operation operation time; time; (b) on (b) ESE, on ESE, η 60 330RTE, and ηex. 6 /kW

RTE, and ηex. 55 COM P 4.3. Capability Analysis of CCHP charging time / h 50 320 4

4.3. Capability Analysis of CCHP ESE, RTE and The HT-CAES that equips the solar thermal and45 refrigeration subsystems has a more ﬂexible ESE P TheCCHP HT-CAES capacity. that In order equips to fully the excavate solar thermal theCOM capability and refrigeration of the HT-CAES, subsystems this section has carries a more RTE out theflexible 310 τ 2 40 analysis of its multi-energy supply characteristics.OP In order to facilitate the analysis, the following ηex CCHP capacity. In order to fully excavate the capability35 of the HT-CAES, this section carries out the assumptions8 are made: 9 10 11 12 8 9 10 11 12 Maximum exhaust pressure of COM P / MPa Maximum exhaust pressure of COM / MPa 1. Thermal energy storage temperature in the hot tank is equal to supply heating and SAC; (a) 2. The temperature in the ASC is equal to the ambient temperature; (b) Figure3. The 7. Effect cooling of watermaximum temperature exhaust andpressure evaporation of COM temperature (a) on PCOM of theand SAC operation are constant; time; (b) on ESE, RTE, and ηex.

4.3. Capability Analysis of CCHP The HT-CAES that equips the solar thermal and refrigeration subsystems has a more flexible CCHP capacity. In order to fully excavate the capability of the HT-CAES, this section carries out the

Entropy 2020, 22, 764 14 of 19

4. The PTC and SAC eﬃciency is constant; 5. The inlet thermodynamic parameters of the TUR and COM stay constant; 6. The minimum power supply time of the TUR is 0.5 h and at least 36% of the HTF is consumed.

The output electric power in the HT-CAES is shown in Section 3.3.2, and the compressed heat energy output for heating can be expressed as:

Q = m (h h ). (24) TES O O3 − O2 where mO is the mass of VP-1, hO2 and hO3 represent the speciﬁc enthalpy in the HOT and LOT, respectively. The high-grade thermal energy of the HT-CAES is provided by VP-1 in the HOT, which can be used for heating/cooling and the output heating and cooling energy can be expressed respectively as:

Q = X Q . (25) HQ · TES Q = Y COP Q . (26) CS · ab· HQ where X is the proportion of heat stored in the HOT for heating, Y is the proportion of cooling, and COPab is deﬁned in Section 3.2. Table7 lists the operation characteristics of the HT-CAES, with the heating proportion change from 0 to 64%. With the heating proportion gradually increased to 64%, the high-temperature heat transfer oil used for energy release and power generation is reduced, so the power supply time is shortened.

Table 7. The relationship between the operating characteristics and the heating proportion.

VP-1 Mass for Regenerated The Range Pressure Heating Ratio τ τ Generating Power Hot Water in ASC ch dch X% h h T T MPa 0 7.2576 18.3 8.0~3.0 5.0 1.4 14 6.24154 15.4 8.0~3.7 4.2 1.2 24 5.51578 13.5 8.0~4.2 3.7 1.0 34 4.79002 11.7 8.0~4.7 3.2 0.9 44 4.06426 10.2 8.0~5.2 2.7 0.7 54 3.3385 8.4 8.0~5.7 2.2 0.6 64 2.61274 6.6 8.0~6.2 1.7 0.5

Under the premise of constant mass ﬂow at the inlet of the turbine expander, the quality of high-pressure air consumed by the energy release power generation is reduced, which shows that the pressure range of the ASC is reduced from 8.0~3.0 MPa to 8.0~6.2 MPa, thus reducing the compression time and power consumption in the next cycle. Taking 24% of the heating proportion as an example, in the next compressed gas storage cycle, the air compressor only needs to pressurize the gas storage pipe from 4.2 MPa to 8.0 MPa, that is, the compression time is shortened, and the hot water recovered in the compression process is reduced. Figure8a shows the relationship between the output energy of the HT-CAES and the heating proportion X. With the change of heating proportion, in the next round cycle, the electric energy, heating energy and power supply energy consumed in energy storage decrease with the increase in heating proportion X and the external heating energy increases. Entropy 2020, 22, x FOR PEER REVIEW 15 of 19

Entropy 2020, 22, x FOR PEER REVIEW 15 of 19 Entropy 2020, 22, 764 15 of 19 1000 X% Supply X%Supply Heating energy/60℃ 400 heating cooling Heating energy/250℃ 64% 64% 350 800 Output electric power 54% 54% 1000 Power consumption by HT-CAES X% Supply 44% X%Supply 44% Heating energy/60℃ 400300 heating 34% cooling 34% Heating energy/250℃ 600 350250 24%64% 64%24% 800 Output electric power 14%54% 54%14% Power consumption by HT-CAES 300200 44% 44% 34% 34% 400 600 250150 24% 24% 14% 14% 200100

Output energy(kWh) Output 200 400 15050 0 0 /kWh energy Cooling heating/ Supply 100

Output energy(kWh) Output 200 -5050 0 10203040506070 0 20406080100 0 0The ratio of VP-1 in HOT for heating in each round trip X(%) /kWh energy Cooling heating/ Supply The ratio of VP-1 in HOT supplied for cooling Y / % -50 0 20406080100 0 10203040506070(a) (b) The ratio of VP-1 in HOT for heating in each round trip X(%) The ratio of VP-1 in HOT supplied for cooling Y / % Figure 8. (a) Relationship between the output energy and the heating ratio X. (b) Relationship between (a) (b) heating/cooling and heating/cooling ratio X and Y. FigureFigure 8. (a) 8.Relationship(a) Relationship between between the the output output energy energy and thethe heating heating ratio ratio X. X. (b) (b) Relationship Relationship between between heating/coolingFurther,heating to /ensurecooling and andheating/coolingthe heating power/cooling generation ratio ratio X X and andcapaci Y. ty, the relationship between the heating/cooling capacity andFurther, the cooling to ensure ratio the powerY and generationthe heating capacity, ratio X the is analyzed, relationship as between shown thein Figure heating /8b.cooling Further,Whencapacity the andto cooling ensure the cooling ratio the power ratiois fixed, Y andgeneration the the heating heating capaci capacity ratioty, X isthe increases analyzed, relationship linearly as shown between with in Figure heating the8 b.heating/cooling ratio X. When thecapacity cooling andWhen ratio the the coolingY is cooling 0 and ratio ratio the Y heating isand fixed, the ratio theheating heating X is ratio 64%, capacity X the is analyzed, maximum increases as linearly heating shown with incapacity Figure heating is 8b. ratio 385.7 X. kwh, the regulationWhenWhen the cooling Ycooling is 100%, ratio ratio and Y isis 0 thefixed, and maximum the the heating heating cooling ratio capacity X is capacity 64%, increases the maximum is 258.4 linearly kwh. heating with capacity heating is 385.7ratio kwh, X. When the coolingFigurethe regulation ratio9 shows Y Y is isthe 0 100%, and relationship the and heating the maximum between ratio coolingX the is 64%,efficiency capacity the maximum isof 258.4 the HT-CAES, kwh. heating CCHPcapacity and is the385.7 heating kwh, ratiothe regulation X andFigure cooling Y9 isshows 100%, ratio the andY. relationship When the maximum all the between heat cooling thesources e ffi capacityciency in the of ishigh-temperature the 258.4 HT-CAES, kwh. CCHP oil and tank the are heating used for ratio X and cooling ratio Y. When all the heat sources in the high-temperature oil tank are used energyFigure release 9 shows power the generation relationship (i.e., between x = 0), the the efficiency system’s of combined the HT-CAES, cooling CCHP and andheating the heating power for energy release power generation (i.e., x = 0), the system’s combined cooling and heating power efficiencyratio X and is cooling80.4%; when ratio Y.the When heating all proportionthe heat sources is x = 64%,in the and high-temperature the refrigeration oil proportion tank are used is Y =for 0, efficiency is 80.4%; when the heating proportion is x = 64%, and the refrigeration proportion is Y = 0, energy release power generation (i.e., x = 0), the system’s combined cooling and heating power and theand system the system does does not notsupply supply cooling cooling to to the the outside, outside, but thethe e efficiencyfficiency reaches reaches the the maximum maximum value value ofefficiency 85%;of 85%;when is 80.4%; when the heating the when heating theproportion proportion heating isproportion is X X == 64%,64%, and andis x the= 64%, refrigerationrefrigeration and the proportionrefrigeration proportion is Y proportionis= 100%,Y = 100%, and is alland Y = all 0, theand high-grade thethe system high-grade doesheat heat energynot energysupply that that cooling can can be be deployedto deployed the outside, in in the but systemsystem the efficiency is is used used for forreaches absorption absorption the refrigeration,maximum refrigeration, value ofunder 85%;under this when energy this the energy heating supply supply proportionmode, mode, the the lowest is lowest X = 64%,efficiency efficiency and the isis 72.6%. 72.6%. refrigeration proportion is Y = 100%, and all the high-grade heat energy that can be deployed in the system is used for absorption refrigeration, under this energy supply mode, the lowest Y=0 efficiency is 72.6%. 84 Y=10% Y=20% Y=40% Y=0 82 Y=60% 84 Y=10%Y=80% Y=20%Y=100% 80 Y=40% 82 Y=60% Y=80% 78 Y=100% 80

76 78 Efficiency of CCHP (%) CCHP of Efficiency 74 76

Efficiency of CCHP (%) CCHP of Efficiency 72 74 0 10203040506070 The ratio of VP-1 in HOT supplied for heating X / % 72 FigureFigure 9. Relationship 9. Relationship between between0 the the efficiency 10203040506070 eﬃciency of of the the HT-CAES CCHP CCHP and and the the heating heating ratio ratio X and X and The ratio of VP-1 in HOT supplied for heating X / % coolingcooling ratio ratioY. Y. Figure5. Conclusions 9. Relationship between the efficiency of the HT-CAES CCHP and the heating ratio X and 5. Conclusionscooling ratio Y. In this paper, a novel hybrid T-CAES system, i.e., a HT-CAES based on the utilization of solar Inthermal this paper, energy, a isnovel proposed hybrid and T-CAES analyzed. system, Stable i.e., high-grade a HT-CAES hot VP-1 based stored on inthe a HOTutilization was used of solar 5. Conclusions thermalto provideenergy, cooling is proposed heating and and analyzed. power, which Stable greatly high-grade improves hot the VP-1 performance stored in of a theHOT HT-CAES. was used to provideInTo this evaluatecooling paper, theheating a system novel and performance, hybrid power, T-CAES which energy system, greatly and exergy i.e.,improves analyses a HT-CAES the are conductedperformance based on on the the of system.utilizationthe HT-CAES. Finally, of solar To thermalevaluate energy, the system is proposed performance, and analyzed. energy and Stable exer high-gradegy analyses hot are VP-1 conducted stored on in thea HOT system. was Finally,used to providea sensitivity cooling analysis heating and and assessment power, whichcapacity greatly for CHP improves is conducted the performance by the key of parameters the HT-CAES. effected To evaluateon performance the system of the performance, HT-CAES. Theenergy main and conclusions exergy analyses are summarized are conducted as follows: on the system. Finally, a sensitivity analysis and assessment capacity for CHP is conducted by the key parameters effected on performance of the HT-CAES. The main conclusions are summarized as follows:

Entropy 2020, 22, 764 16 of 19 a sensitivity analysis and assessment capacity for CHP is conducted by the key parameters eﬀected on performance of the HT-CAES. The main conclusions are summarized as follows:

1. Charging and discharging times of the proposed system under design conditions are 5 h and 1.4 h, respectively. During these two modes, the system generates 498 kW h and consumes 940 kW h by the compressor and turbine. In addition, it generates 20 tons of hot water. In this situation, the ESE, RTE and exergy efficiency of the system are 53.6%, 73%, and 50.6%, respectively. 2. A sensitivity analysis has indicated that the turbine inlet temperature and pressure are the critical parameters affecting the performance of the proposed HT-CAES system. When the increasing inlet temperature from 180 ◦C to 220 ◦C under design inlet air pressure, ESE, RTE, and ηex increase by 5.21%, 4.43%, and 4.38%. 3. Finally, the ratio of heating and cooling VP-1 was discussed to evaluate the CHP capacity of the proposed HT-CAES. When the heating proportion of VP-1 is x = 64%, and the refrigeration proportion is Y = 0, the RTE reaches the maximum value of 85%. Therefore, the proposed hybrid T-CAES can act as essential components in smart energy grid and cities, owing to the high efficiency and ability to accommodate renewables.

Author Contributions: Conceptualization, X.C. X.X. and S.M.; Data curation, Y.S.; Formal analysis, C.L.; Methodology, L.C.; Validation, X.X.; Visualization, Y.G.; Supervision, X.X. and S.M.; Writing–original draft, X.C.; Writing–review & editing, X.C. All authors have read and agreed to the published version of the manuscript. Funding: This work has been supported by special fund project of The Key Lab of Plateau Building and Eco-community in Qinghai, China (KLKF-2019-004), Young Research Fund Project of Qinghai University (2019-QGY-1), and The Scientific and Technological Project of Qinghai Province (2018-ZJ-774). Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

CCHP Combined cooling heating and power HT-CAES Hybrid trigenerative compressed air energy storage system T-CAES Trigenerative compressed air energy storage system RES Renewable energy sources DG Distributed generation DES Distributed energy system CHP Cooling heating and power CAES Compressed air energy storage D-CAES Diabatic compressed air energy storage A-CAES Adiabatic compressed air energy storage TES Thermal energy storage ST-CAES solar thermal CAES HA-CAES Hybrid adiabatic compressed air energy storage COM Air compressor stage unit TUR Air turbine and generator unit HWT Hot water tank LWT Low-temperature water tank CWT Cooling water tank CM Chiller machine SAC Solar absorption chiller RAC Refrigeration air-conditioning AC Air compressor train CHR Compression heat radiator ASC Air storage chamber HE Heat exchanger TV Throttle valve Entropy 2020, 22, 764 17 of 19

PHR Preheat regenerator STS Solar thermal collecting and storage nit PTC Parabolic trough collector HOT High-temperature oil tank LOT Low-temperature oil tank HTF Heat transfer fluid ESE Electricity storage efficiency EXE Exergy efficiency RTE Round-trip efficiency OP Oil pump WP Water pump COP Coefficient of performance VP-1 heat of Therminol oil VP-1 Greek symbols η efficiency (%) ρ Density of fluid β Compression ratio κ Polytropic index π pressure ratio of turbine τ time (h) Symbols Ex exergy (kW) P Power (kW) s specific entropy (kJ/kg K) · k polytropic index h specific enthalpy (kJ/kg) Q energy (kW) . m Mass flow (kg/s) m Mass (kg) X Proportion of heat stored in the HOT for heating Y Proportion of heat stored in the HOT for cooling p Pressure (MPa) T temperature (◦C) U heat transfer coefficient R gas constant Subscripts 0 ambient condition i, j state point s isentropic u useful in input out output ab absorber ch charging dch discharging c collecting hour hs heat supply CS Cooling supply HQ Heat quantity Streams A air stream O oil stream W water stream Entropy 2020, 22, 764 18 of 19

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