Operating Range for a Combined, Building-Scale Liquid Air Energy Storage and Expansion System: Energy and Exergy Analysis
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entropy Article Operating Range for a Combined, Building-Scale Liquid Air Energy Storage and Expansion System: Energy and Exergy Analysis Todd A. Howe *, Anthony G. Pollman and Anthony J. Gannon Graduate School of Engineering and Applied Sciences, Naval Postgraduate School, Monterey, CA 93943, USA; [email protected] (A.G.P.); [email protected] (A.J.G.) * Correspondence: [email protected]; Tel.: +1-813-943-1120 Received: 15 September 2018; Accepted: 5 October 2018; Published: 8 October 2018 Abstract: This paper presents the results of an ideal theoretical energy and exergy analysis for a combined, building scale Liquid Air Energy Storage (LAES) and expansion turbine system. This work identifies the upper bounds of energy and exergy efficiency for the combined LAES-expansion system which has not been investigated. The system uses the simple Linde-Hampson and pre-cooled Linde-Hampson cycles for the liquefaction subsystem and direct expansion method, with and without heating above ambient temperature, for the energy production subsystem. In addition, the paper highlights the effectiveness of precooling air for liquefaction and heating air beyond ambient temperature for energy production. Finally, analysis of the system components is presented with an aim toward identifying components that have the greatest impact on energy and exergy efficiencies in an ideal environment. This work highlights the engineering trade-space and serves as a prescription for determining the merit or measures of effectiveness for an engineered LAES system in terms of energy and exergy. The analytical approach presented in this paper may be applied to other LAES configurations in order to identify optimal operating points in terms of energy and exergy efficiencies. Keywords: liquid air energy storage; energy analysis; exergy analysis; cryogenic system 1. Introduction Liquid air energy storage (LAES) is a developing thermal electrical energy storage technology and is a promising addition to other long-term storage technologies like pumped hydroelectric storage (PHS) and compressed air energy storage (CAES) [1,2]. LAES has a higher energy density than PHS and four to six times the energy density of CAES at 200 bar [1,3]. Studies have shown LAES is capable of higher round trip efficiency than CAES [4]. In addition, LAES has an advantage over CAES and PHS due to not being constrained to geographical features [5]. Although, micro-CAES systems are not constrained to geographic features and are effective in distributed power networks [6,7]. Highview Power Storage developed a 300 kW LAES pilot plant in Slough, Scotland and have a 10 MW commercial demonstration plant planned [8]. LAES has two main subsystems, the air liquefaction subsystem and energy production subsystem. In the liquefaction subsystem, air is compressed, cooled, and expanded which produces liquid air. Multiple configurations like Linde-Hampson cycle, Claude cycle, Heylandt cycle, Collins cycle, and more, liquefy and store air [9]. The energy production system has multiple configurations like direct expansion method, indirect Rankine cycle, indirect Brayton cycle, and other variations that may be implemented [10]. Storage is commonly included as a third subsystem, although this paper includes liquid air storage in the liquefaction subsystem. Entropy 2018, 20, 770; doi:10.3390/e20100770 www.mdpi.com/journal/entropy Entropy 2018, 20, 770 2 of 17 Entropy 2018, 20, x FOR PEER REVIEW 2 of 17 There areare multiplemultiple thermodynamicthermodynamic studies on liquid air energy storage available in literature. Early researchresearch inin 19771977 compressedcompressed air toto 77 atm,atm, dehumidifieddehumidified airair andand compressedcompressed further to 49 atmatm and cooledcooled for storage; then repressurized liquid air to 80 atm before expanding through a turbine, whichwhich gavegave anan efficiencyefficiency upup toto 72%72% [[11].11]. GuizziGuizzi etet al.al. [[12]12] conductedconducted aa thermodynamicthermodynamic analysisanalysis ofof aa LAES systemsystem withwith aa multi-stagemulti-stagecompressor compressor and and turbine, turbine, and and a a storage storage subsystem, subsystem, which which resulted resulted in in a round-tripa round-trip efficiency efficiency of 54–55%.of 54–55%. A similarA similar system system and and analysis analysis conducted conducted by [by13] [13] showed showed round round trip efficienciestrip efficiencies from 45–57%from 45 when–57% adjusting when adjusting the inlet pressurethe inlet to pressure a JT valve. to Anothera JT valve. analysis Another demonstrated analysis thedemonstrated effects on LAES the effects energy on efficiency LAES whenenergy adjusting efficiency the when compressor adjusting efficiency, the compressor compressor effi dischargeciency, pressure,compressor and discharge cryogenic pressure, pump discharge and cryogenic pressure pump [14]. Additional discharge workpressure has focused[14]. Additional on the liquefaction work has subsystem.focused on Abdothe liquefaction et al. conducted subsystem. a thermodynamic Abdo et al. analysisconducted on thea thermodynamic Linde-Hampson analysis cycle, Claudeon the cycle,Linde- andHampson Collins cycle, cycle andClaude showed cycle, Claude and Collins and Collins cycle and cycles sho bothwed having Claude higher and Collins overall cycles efficiencies both thanhaving the higher Linde-Hampson overall efficiencies cycle [15 ].than Yu etthe al. Linde conducted-Hampson an exergy cycle [15]. analysis Yu et of al. a Linde-Hampson conducted an exergy cycle withanalysis an ejectorof a Linde and-Hampson showed the cycle addition with an of ejector the ejector and reducedshowed the addition total exergy of the destruction ejector reduced in the cyclethe total [16 ].exergy destruction in the cycle [16]. This paper presents the the results results of of an an ideal ideal theoretical theoretical energy energy and and exergy exergy analysis analysis for for a liquid a liquid air airenergy energy storage storage system. system. Energy Energy is conserved is conserved through through all processes all processes and systems, and systems,but this is butuntrue this for is untrueexergy [17]. for exergy Unlike [ 17exergy]. Unlike analysis, exergy energy analysis, analysis energy does analysisnot account does for not the account quality forof energy; the quality this ofmakes energy; exergy this analysis makes useful exergy when analysis searching useful for when areas searching of improvement for areas within of improvement a system [18,19 within]. The a systemquality [of18 ,energy,19]. The or quality exergy, of energy,is the “maximum or exergy, is work the “maximum which can workbe obtained which can from be a obtained given form from of a givenenergy form using of energythe environmental using the environmental parameters as parameters the reference as the state” reference [20]. state”The importance [20]. The importance of exergy ofanalysis, exergy opposed analysis, to opposed only energy to only analysis, energy analysis,is analyzing is analyzing the irreversibilities the irreversibilities in the system in the systemand its andcomponents. its components. The analysis The in analysis this paper in thisexplores paper the explores upper bounds the upper of energy bounds efficiency of energy and efficiency exergetic andefficiency exergetic of the efficiency combined of theLAES combined-expansion LAES-expansion system, highlights system, the highlightseffectiveness the of effectiveness precooling for of precoolingliquefaction, for liquefaction,heating air heatingbeyond air ambient beyond ambienttemperature temperature for expansion, for expansion, and and analysis analysis of of the systemsystem components.components. 2.2. System Description The liquidliquid airair energyenergy storagestorage systemsystem analyzedanalyzed in thisthis paperpaper investigatesinvestigates the two differentdifferent liquefactionliquefaction subsystems,subsystems, thethe simple Linde-HampsonLinde-Hampson cycle and the pre pre-cooled-cooled Linde Linde-Hampson-Hampson cycle; and two different energy production subsystems,subsystems, aa direct expansion methodmethod andand the direct expansion method with additionaladditional heatheat added.added. Figure1 1 displays displays thethe completecomplete LAESLAES systemsystem withwith alternative alternative options toto useuse eithereither typetype ofof liquefactionliquefaction subsystemsubsystem oror energyenergy productionproduction subsystem.subsystem. Makeup Air 1 Qin Compressor d c 2’ 3 2 JT Valve 7 8 5’ 5 HX-2 HX-1 4 Qout 6 Pump b a Turbine Liquid Reservoir Generator 9 HX-1' Figure 1. System diagramdiagram of a liquidliquid airair energyenergy storagestorage system.system. The simple Linde-Hampson cycle consists of a compressor, heat exchanger (HX-1), Joule-Thomson (JT) valve, and liquid reservoir. At steady state, mixing of makeup air and return air occurs prior to entering the compressor at state 1. Air is compressed to state 2 and then cooled in HX- Entropy 2018, 20, 770 3 of 17 EntropyThe 2018 simple, 20, x FOR Linde-Hampson PEER REVIEW cycle consists of a compressor, heat exchanger (HX-1), Joule-Thomson3 of 17 (JT) valve, and liquid reservoir. At steady state, mixing of makeup air and return air occurs prior to 1entering to state the3. The compressor cooled high at state-pressure 1. Air air is is compressed expanded through to state the 2 and JT thenvalve cooled to state in 4 HX-1where to it stateis a 2 3.- Thephase cooled mixture. high-pressure The liquid air reservoir is expanded stores through liquid